The IL-23-IL-17 axis in inflammatory arthritis.

REVIEWS The IL‑23–IL‑17 axis in inflammatory arthritis Erik Lubberts Abstract | The discovery that the IL‑23–IL‑17 immune pathway is involved in many models of autoimmune disease has changed the concept of the role of T‑helper cell subsets in the development of autoimmunity. In addition to TH17 cells, IL‑17 is also produced by other T cell subsets and innate immune cells; which of these IL‑17-producing cells have a role in tissue inflammation, and the timing, location and nature of their role(s), is incompletely understood. The current view is that innate and adaptive immune cells expressing the IL‑23 receptor become pathogenic after exposure to IL‑23, but further investigation into the role of IL‑23 and IL‑17 at different stages in the development and progression of chronic (destructive) inflammatory diseases is needed. Rheumatoid arthritis (RA) and spondyloarthritis (SpA) are the two most common forms of chronic immune-mediated inflammatory arthritis, and the IL‑23–IL‑17 axis is thought to have a critical role in both. This Review discusses the basic mechanisms of these cytokines in RA and SpA on the basis of findings from disease-specific animal models as well as human ex vivo studies. Promising therapeutic applications to modulate this immune pathway are in development or have already been approved. Blockade of IL‑17 and/ or TH17-cell activity in combination with anti-TNF therapy might be a successful approach to achieving stable remission or even prevention of chronic immune-mediated inflammatory diseases. Lubberts, E. Nat. Rev. Rheumatol. advance online publication 28 April 2015; doi:10.1038/nrrheum.2015.53

Introduction

Department of Rheumatology, Erasmus MC University Medical Center, Wytemaweg 80, 3015 CN Rotterdam, Netherlands. e.lubberts@ erasmusmc.nl

IL‑17 signalling has a role in many stages of inflamma‑ tory arthritis, particularly in orchestrating tissue inflam‑ mation and destruction.1–7 For example, overexpression of IL‑17A (commonly referred to as IL‑17) in healthy mouse knee joints induced rheumatoid arthritis (RA)like pathology with features including joint inflam‑ mation, focal bone erosion and cartilage damage. 8 Furthermore, in a mouse model of streptococcal cell wall (SCW)-induced arthritis, IL‑17 receptor (IL‑17R) signal‑ ling in resident synovial cells was essential in driving selflimiting acute joint inflammation to become persistent and destructive arthritis.9–10 IL‑17A is the prototypical member of the IL‑17 family of cytokines, which consists of six members, IL‑17A– IL‑17F.11–13 IL‑17A and IL‑17F, the two isoforms that share the most homology at the amino acid level, have overlapping but also distinct effector functions in dif‑ ferent autoimmune diseases.14,15 The role of the indi‑ vidual IL‑17 family members, in particular IL‑17B, IL‑17C, IL‑17D and IL‑17E, in autoimmune inflamma‑ tory diseases, and in the context of other inflammatory cytokines, is unclear. Both IL‑17A and IL‑17F can be secreted as homodimers or as an IL‑17A–IL‑17F hetero dimer. 16,17 and signal though the receptor subunits IL‑17RA and IL‑17RC.18–20 However, other IL‑17R sub units (IL‑17RB, IL‑17RD, IL‑17RE) have been described that are essential for signalling by individual IL‑17 family members.21,22 Interestingly, IL‑17RA might be crucial for signalling by most, if not all, IL‑17 family members.20,23 Competing interests The author declares no competing interests.

The main source of IL‑17A and IL‑17F is type 17 T helper cells (TH17 cells), which also produce cytokines such as IL‑22 and IL‑21. The cytokine combinations that induce and activate TH17 cells influence the pathogenic potential of these cells.24–26 Exposure to the IL‑12 family member IL‑23 is crucial not only for the stabilization of these cells, but also for their ability to generate pathogenic TH17 cells that induce autoimmune tissue inflammation, as shown in experimental autoimmune encephalomyelitis (EAE; a mouse model of multiple sclerosis).24,27–29 IL‑23 is a heterodimeric cytokine that consists of a p40 subunit, which it shares with IL‑12, and a p19 subunit. Studies in different mouse models such as EAE and collagen-induced arthritis (CIA; a mouse model of RA) revealed that the IL‑23–IL‑17 (TH17) axis, rather than the IL‑12–IFNγ (TH1) pathway, is essential for the development of auto‑ immunity.27,30 An appreciation of the critical role of the IL‑23–IL‑17 immune pathway in the development of auto‑ immunity in many experimental models has changed the field of research regarding the role of the various subsets of T helper cells in autoimmunity.23,31–34 As well as TH17 cells, IL‑17 is also produced by other cells such as CD8 + T cells, γδ T cells, natural killer (NK) cells, NKT cells, mast cells and neutrophils.35–40 In 2013, the new group of IL‑17A-producing innate lymphoid cells (ILCs) was named group 3 ILCs. This group, which consists of lymphoid tissue-inducer (LTi) cells, NCR+ ILC3s (which express the NK cell activating receptor [NCR] NKp46) and NCR– ILC3s is defined by the capacity of cells to produce IL‑17A and/or IL‑22.41 Like TH17 cells, group 3 ILCs depend on the transcrip‑ tion factor RORγt for their development and function.41

NATURE REVIEWS | RHEUMATOLOGY

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REVIEWS Key points ■■ The IL‑23–IL‑17 axis is critically involved in the development of autoimmunity ■■ Tissue-specific IL‑17A expression exacerbates tissue damage and disease chronicity ■■ The roles of IL‑23 and IL‑17 at different stages in the development and progression of rheumatoid arthrits (RA) and spondyloarthritis needs further investigation ■■ Greater understanding of the role of the IL‑23–IL‑17 axis in RA as opposed to psoriatic arthritis and psoriasis is needed ■■ Early combination therapy neutralizing both TNF activity and the IL‑17/TH17 pathway might be a successful approach to achieving stable remission in patients with autoimmune disease and might even prevent disease development

TH1

CCR6–, CCR4–, CXCR3+, CD25–/low : IFN-γ+, IL-17A–

TH2

CCR6–, CCR4+, CXCR3–, CD25–/low : IL-4+ ‘TH17’ cells CCR6+, CCR4+, CXCR3–, CCR10–, CD25–/low : IL-17Ahi/++, IFN-γ–

TH17 CD4+ CD45RO+ (memory) T cell

CCR6+ IL-17A+ IL-17F+ IFN-γ+ IL-22+

‘Double-positive’ cells CCR6+, CCR4+, CXCR3+, CCR10–, CD25–/low : IL-17A+, IFN-γlow ‘Non-classical TH1’ or TH17.1 cells CCR6+, CCR4–/low, CXCR3+, CD25–/low : IL-17Alow, IFN-γhi/+ ‘Double-negative’ cells CCR6+, CCR4–, CXCR3–, CD25–/low : IL-17Alow, IFN-γlow

TH22

CCR6+, CCR4+, CXCR3–, CCR10+ : IL-22+, IL-17A–/low

TREG

FOXP3+, CD25+/hi

Figure 1 | CD4+ memory T‑helper cells consist of different subpopulations. Different Nature Reviews | Rheumatology CD4+ memory (CD45RO+) T‑helper cell subsets can be discriminated by combinations of chemokine receptor markers. In particular, the combination of CCR6, CXCR3 and CCR4 can help to discriminate between TH1 (CCR6–CCR4–CXCR3+), TH2 (CCR6– CCR4+CXCR3–) and TH17/TH22 (CD4+CD45RO+CCR6+) cells. This last population can be further distinguished, by CCR10 expression, as TH17 cells are CCR10– and TH22 cells are CCR10+. These populations are all CD25–/low. The CCR6+ population is heterogeneous, consisting of TH17 cells (CCR6+CCR4+CXCR3–CCR10–), doublepositive cells (CCR6+CCR4+CXCR3+), nonclassical TH1 cells also called TH17.1 (CCR6+CCR4-CXCR3+) and double-negative cells (CCR6+CCR4–CXCR3–). Note the heterogeneity in IL‑17A/IFN‑γ high and low expression in the different CCR6+ subpopulations.90,175 TREG cells can be identified as FOXP3+CD25hi. Abbreviations: CCR, CC-chemokine receptor; CXCR, CXC-chemokine receptor; FOXP3, forkhead box protein P3; TH1, type 1 T helper cell; TH17, type 17 T helper cell; TH2, type 2 T helper cell; TH22, type 22 T helper cell; TREG, regulatory T cell.

However, the exact role of IL‑17-producing cells in the immunopathogenesis of chronic destructive arthritis has not been resolved, and whether all these cells can, like TH17 cells, be pathogenic or nonpathogenic depending on the cytokine milieu in which they are active is unclear. Furthermore, the numbers of most of these innate T cells, such as γδ T cells and ILCs, are relatively low, although the percentage of IL‑17-positivity within such innate T cell population(s) might be high during arthritis, as was shown for IL‑17+ γδ T cells compared with IL‑17+CD4+ T cells.42 Interestingly, IL‑23 receptor (IL‑23R) signalling in these

innate and adaptive T cells might be essential for their pathogenic function, and could influence their stability, plasticity and migratory behaviour in vivo.24 Granulocytemacrophage colony-stimulating factor (GM‑CSF) produced by TH17 cells might also be an important patho‑ genic cytokine in IL‑23-dependent disease progression, as described in EAE.43,44 A role for GM‑CSF has also been shown in experimental models of inflammatory arthritis, and a first clinical trial of a GM‑CSF-neutralizing agent for patients with RA is underway.45–47 However, the identifica‑ tion and specific contribution of the GM‑CSF-producing myeloid cells and/or T cells to the pathogenesis of arthritis have not yet been clearly established. This Review describes the current understanding of the role of the IL‑23–IL‑17 immune pathway in chronic immune-mediated inflammatory arthritis, with a focus on RA and SpA, on the basis of findings from diseasespecific animal models as well as human ex vivo studies. Potential differences between autoimmune and auto inflammatory arthritis in relation to the IL‑23–IL‑17 axis are discussed.

IL‑23 and IL‑17 in arthritis models

Ample evidence exists from animal studies that IL‑17A contributes to the pathogenesis of arthritis.48 Although this T‑cell cytokine was already considered a target for pharmaceutical neutralization, the discovery of TH17 cells in 2005 revolutionized research in the immuno logy and rheumatology fields and helped to resolve some inadequacies of the TH1–TH2 concept of inflam‑ matory autoimmune diseases, which had dominated immunology of T cells for almost 20 years (Figure 1, Box 1, Box 2).33,34,49

Role in autoimmune arthritis models In CIA, the IL‑23–TH17 axis is clearly critical to the devel‑ opment of autoimmune arthritis. IL‑23p19-deficient (Il23a–/–) mice, which lack functional IL‑23, were com‑ pletely protected against the development of CIA, in con‑ trast to Il12a–/– mice, which lack functional IL‑12. Notably, IL‑17-producing CD4+ T cells were absent in the Il23a–/– mice despite normal induction of IFN‑γ-producing CD4+ TH1 cells,30 which suggests that the former but not the latter are critical for the initiation of CIA. Approximately 20% of Il17–/– mice develop CIA, although a milder form than seen in wild-type mice; 2 by contrast, however, Il17ra–/– mice were completely protected against the development of CIA, similar to Il23a–/– mice (Figure 2).7 This finding indicates that IL‑17RA signalling is an essen‑ tial downstream pathway in the IL‑23–IL‑17 axis for the development of autoimmune arthritis. Interestingly, mice deficient for Act1 (also known as adapter protein CIKS), which is essential for IL‑17RA signalling and is also involved in B‑cell-activating factor receptor (BAFF‑R, also known as TNF receptor superfamily member 13C) signalling,50 also did not develop CIA (Figure 3).51 As the IL‑17RA subunit is part of the receptor for many IL‑17 family members, whether IL‑17 family members other than IL‑17A are responsible for this phenomenon remains to be determined.

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REVIEWS Box 1 | Differentiation of CD4+ T cells Naïve CD4 T cells can be polarized into different subsets of T‑helper cells (Figure 1), depending on the cytokine environment of the T cells. This cytokine milieu depends, in turn, on the APC that presents antigen to the naive CD4+ T cell.176 TH1 cells are induced by the T-box transcription factor TBX21 (encoded by Tbx21) and produce IFN‑γ, IL‑2 and lymphotoxin. TH1 cells are crucial for clearing intracellular pathogens; however, exaggerated TH1 responses to selfantigens have been linked to induction of autoimmunity.177–179 TH2 cells, induced by the trans-acting T-cell-specific transcription factor GATA3, produce IL‑4, IL‑5, and IL‑13 and are involved in humoral responses and clearance of extracellular pathogens.180,181 The generation of TH17 cells requires the transcription factor RORγt (encoded by RORc),182–185 under the direction of cytokines that induce (TGF‑β, IL‑6, IL‑1), amplify (IL‑21) or stabilize (IL‑23) these cells.49,186 Other T‑cell subsets have been described, such as TH22, TFH and TREG cells, which are induced by the specific transcription factors AhR, BCL6 and FOXP3, respectively. Plasticity of T‑helper cell phenotypes by epigenetic modifications leads to further functional diversity of these T cells (see Box 2).187 +

Abbreviations: AhR, aryl hydrocarbon receptor; APC, antigen-presenting cell; BCL6, B-cell lymphoma 6 protein; FOXP3, forkhead box protein P3; GATA3, trans-acting T-cell-specific transcription factor GATA-3; RORγt, RAR-related orphan receptor γt; TFH cell, follicular helper T cell; TH1 cell, type 1 T helper cell; TH17 cell, type 17 T helper cell; TH2 cell, type 2 T helper cell; TH22 cell, type 22 T helper cell; TREG cell, regulatory T cell; T-box transcription factor TBX21.

Box 2 | Plasticity between TH17, TH1 and TREG cells in arthritis TH17 cells and TREG cells share a common inducer (TGF‑β), an overlapping chemokine receptor profile, and expression of the TH17-associated transcription factor RORγt.181 In mice, TREG cells can be converted to IL‑17-producing T cells.188–190 Human TREG cells, defined as CD4+CD25highFOXP3+CD127–CD27+, were also reported to differentiate into IL‑17 producing T cells, accompanied by upregulation of RORγt and CCR6 expression.191 In a 2014 study, TH17 cells originating from FOXP3+ T cells were shown to have a key role in the pathogenesis of autoimmune arthritis.192 Alongside this evidence of TH17–TREG cell plasticity, in mice, differentiated TH17 cells responded rapidly in vitro to IL‑12, by upregulating the expression of IFN‑γ and down regulating IL‑17 expression. Whether these cells are more closely related to TH1 cells or to the nonclassical TH1 cells (also called TH17.1 cells) needs further investigation. The development of IFN‑γ-producing effector T cells from IL‑17producing progenitor cells is inhibited in the presence of TGF‑β, which is important for the maintenance of IL‑17 expression by TH17-polarized cells.193–195 Epigenetic regulation seems to have a role in effector T‑cell plasticity. For example, the gene encoding T‑bet, the master regulator of TH1 differentiation, was found to be in an active state, according to histone methylation marks, in both TH17 and TREG cells. This implies that TH17 and TREG cells retain the potential to upregulate the expression of T‑bet and to differentiate towards TH1 cells.196 Together, these observations indicate plasticity between TREG, TH1 and TH17 subsets exist, and that differentiation does not completely restrict these subsets to separate lineages. This new concept of T‑cell plasticity could be relevant to the arthritis disease process. Abbreviations: CCR, CC-chemokine receptor; FOXP3, forkhead box protein P3; RORγt, RARrelated orphan receptor γt; TH1 cell, type 1 T helper cell; TH17 cell, type 17 T helper cell; TH2 cell, type 2 T helper cell; TH22 cell, type 22 T helper cell; TREG cell, regulatory T cell.

In line with these data from mouse studies, igurati‑ mod, a novel DMARD that predominantly targets IL‑17 signalling,52 is proving to be safe and efficacious for the treatment of RA and is in clinical use in China and Japan.52–54 Studies in mice with CIA showed that igurati‑ mod mainly disrupts Act1–TRAF5 and Act1–IKKi inter‑ actions in the IL‑17 signalling pathway in synoviocytes. In cultures of fibrobast-like synoviocytes (FLSs), igurati‑ mod suppressed the expression of various IL‑17-induced proinflammatory factors, which was linked to decreased stabilization of mRNA of related genes and reduced phosphorylation of mitogen-activated protein kinases.52

Role in nonautoimmune arthritis models In a nonautoimmune but strongly T‑cell-dependent methylated bovine serum albumin (mBSA) antigeninduced arthritis (AIA) model, IL‑23 deficiency did not prevent the onset of AIA, but did prevent the joint inflammation from progressing to destructive synovi‑ tis, similar to the arthritis developed by Il17ra–/– mice (Figure 2).42 Compared with wild-type mice with AIA, Il23a–/– arthritic mice had lower proportions of IL‑17+ γδ T cells and IL‑17+CD4+ T cells in the spleen, pop‑ liteal lymph nodes and inflamed synovium. Interestingly, although the proportions of IFN‑γ+ CD4+ T cells were similar in the spleen and lymph nodes of Il23–/– and wildtype mice after induction of AIA, the proportion of these cells was lower in the inflamed knee joint (the target organ) of Il23a–/– mice (Figure 4).42 The findings suggest IL‑23 could be required to promote TH1 and TH17 effec‑ tor responses, especially at the site of inflammation in the target organ. More-precise characterization of the effector T cells involved is needed in order to understand this phenomenon (Figure 1). Of note, in an adjuvantfree model of peptidoglycan-induced acute arthritis, Il23a–/– mice developed milder arthritis compared with wild-type mice (Lubberts et al., unpublished observa‑ tions). These data together suggests that IL‑23 has an essential role in the development of T‑cell-dependent and T‑cell-independent arthritis.

Anti-IL‑23 therapy in arthritis models Effects at different stages of arthritis In rats, treatment with anti-IL‑23 antibody after onset of CIA has been shown to reduce paw volume, although the effects of IL‑23 neutralization on synovial inflam‑ mation and the autoimmune response was unclear.55 A subsequent study in which mice were treated with antibody specific for IL‑23p19 at different stages of CIA indicated the existence of IL‑23-dependent and IL‑23independent stages of autoimmune arthritis.56 In these mice, blocking IL‑23 activity after immunization with type II collagen (CII) but before the onset of disease suppressed the severity of CIA (Figure 2), although the incidence of arthritis was not reduced and its onset was not delayed.56 By contrast, administration of antiIL‑23p19 antibody after the first clinical signs of CIA did not result in clinical improvement of the disease. Although the mechanism is not completely under‑ stood, it might be that neutralizing IL‑23 leads to loss of pathogenic behaviour of effector cells due to instabil‑ ity of these cells, or that lack of migration of the effector cells to the site of inflammation hampers induction of full-blown arthritis. Notably, during the preclinical stage of CIA, CIIspecific CD4 + IL‑17 + T cells were already present, 56 whereas in CII-immunized Il23a –/– mice, which are completely protected from CIA, 30 IL‑17-expressing CD4+ T cells were lacking. In line with the data from mouse studies, patients with RA showed no clinical improvement of RA after treatment with an IL‑12– IL‑23 inhibitor in a phase IIA randomized controlled trial (RCT).57



REVIEWS a

Initiation phase

CIA onset

Arthritis score

IL-23p19 or IL-17 deficiency fully protects against CIA7,30 IL-17A deficiency confers ~20% incidence of CIA2

of IL‑23p19 after clinical onset of arthritis might not be beneficial for patients with RA.

Effector phase

Anti-IL-17A antibody or sIL-17R-Fc just before CIA onset suppresses clinical signs of CIA1,3 Anti-IL-17A antibody after CIA onset suppresses CIA progression3 Anti-IL-23p19 antibody after CIA onset has no effect on clinical signs of CIA56

Anti-IL-23p19 antibody before CIA onset suppresses clinical signs of CIA56

Anti-CII autoreactive TH cell ICs, FcγR, complement, macrophage activation: TNF, IL-1 γδ T cell, TH cell, B cell activation

Day 0 Immunization with CII

Anti-CII IgG2a

ICs, T cells, IL-1

Day 21 Intraperitoneal injection of CII

b

Day 42

Primary arthritis sIL-17RA-Fc suppresses primary arthritis7

Arthritis score

Knockout of IL-23p19 or IL-17RA suppresses primary arthritis42

Arthritis flare Anti-IL-23p19 or anti-IL-17A antibody before antigen rechallenge suppresses arthritis flare56,94

IL‑23 in osteoclastogenesis

TH cells, γδ T cells Synoviocyte activation Cytokine production

γδ T cell, TH cell, B cell activation Day 0 Immunization with mBSA

Memory TH cells

Day 7 Intra-articular mBSA injection

Day 21

Effects on T‑cell-driven disease flare RA is characterized by alternating periods of remis‑ sion and relapse. This pattern of relapse, or flare, can be mimicked using the mBSA-induced AIA model, which is highly dependent on memory T cells. In this model, 7 days after immunization with mBSA, direct injection of mBSA into one knee joint induces a primary mono‑ articular arthritis that typically lasts for a few weeks. After these few weeks the recovered knee is very sensi‑ tive to AIA flare, which can be induced by injection of a small amount of mBSA into the joint. Mice treated with anti-IL‑23p19 antibody have been shown to experience less-severe mBSA-induced flares, suggesting a role for IL‑23 in this model of memory‑T-cell-mediated arthritis flare (Figure 2).56 Therefore, T‑cell-mediated relapses in patients with autoimmune arthritis might be controlled by anti-IL‑23p19 therapy. Further investigation is needed to understand why IL‑23 inhibition is effective during the effector stage of mBSA-induced arthritis flare but not in the effector stage of autoimmune arthritis.

Day 49 Day 56 Intra-articular mBSA injection

Figure 2 | Overview of the role of IL‑17/IL‑17RA and IL‑23 signalling during the Nature Reviews | Rheumatology pathogenesis of different experimental arthritis models. a | The effects of IL‑23p19 knock-out, IL‑17A knock-out and IL‑17RA knock-out on the development of autoimmune CIA. Therapeutic intervention with neutralization of IL‑23p19 or IL‑17A during different stages of CIA either has no effect or has beneficial effects on the progression of the disease, as assessed by clinical scores. b | The effect of IL‑23p19 knock-out and IL‑17RA knock-out on the development of a T-cell-driven monoarthritis using the mBSA AIA model. The effect of neutralizing IL‑17A or IL‑23p19 on the progression of arthritis as well as during a flare-up of the arthritis was monitored by clinical scores. Abbreviations: AIA, antigen-induced arthritis; CII, type II collagen; CIA, collagen-induced arthritis; FcγR, Fcγ receptor; IC, immune complex, IL-17RA, IL-17 receptor subunit A; mBSA, methylated bovine serum albumin; TH cell, T helper cell.

Together, these data suggest that IL‑23 is involved in driving the severity of arthritis by regulating the pathogenic behaviour of IL‑17+ T cells, which influ‑ ence loss of tolerance and activation of downstream effector pathways during autoimmunity. However, the data also suggest a limited window for the effectiveness of anti-IL‑23 treatment, and that therapeutic targeting

IL‑23 is critical for osteoclast formation and maintenance of bone mass,58 although by itself IL‑23 is insufficient to induce osteoclastogenesis.59 In FLSs, IL‑23 induces expression of receptor activator of nuclear factor κB ligand (RANKL, also known as TNF ligand superfamily member 11),60 a critical cytokine in osteoclastogenesis and osteoclast activation. As synoviocytes can express IL‑22,61 which may act as a promoter of osteoclasto genesis,62,63 it is unknown whether the IL‑23-induced expression of RANKL by these cells is mediated by IL‑22 (Figure 5). Furthermore, IL‑23 was shown in vitro to act directly on myeloid precursor cells to induce expres‑ sion of RANK (also known as TNF receptor superfamily member 11A) in an IL‑17-independent way, enhancing osteoclast formation in cooperation with RANKL.59 The effects of IL‑23 on osteoclastogenesis via T cells is less clear because both stimulatory and inhibitory effects have been described.64–66 IL‑23 induced RANKL expres‑ sion by CD4+ T cells and promoted osteoclastogenesis in a mouse model of autoimmune arthritis.66 Furthermore, IL‑23 induced osteoclastogenesis in cultures of human peripheral blood mononuclear cells (PBMCs) in the absence of osteoblasts or exogenous RANKL; this process was inhibited by osteoprotegerin, anti-IL‑17 antibody or TNF inhibition, indicating the involvement of RANKL, IL‑17 and TNF in IL‑23-induced osteoclastogenesis.55 Interestingly, in activated human T cells, IL‑23 increased the production of IL‑17 relative to the bone-protective T‑cell cytokine IFN‑γ.55 Of note, IL‑23 can be produced by osteoclasts and might enhance T‑cell-mediated osteoclastogenesis (Figure 5). Although IL‑23R can be expressed by γδ T cells and (activated) CD4+ T cells, both of which can be found in the joints of mice with CIA, TH17 cells seem to dominate with regard to bone destruction in vivo.67



REVIEWS

IL-17RA/RC

IL-1β IL-6 IL-23

Stromal/ ACT1 myeloid cell

BAFF

IL-6 BAFFR

ACT1

gp130

IL-6R BAFF

Upregulation IL-6R IL-6R

BAFFR

B cell IL-17RA/ RC

Survival Proliferation Differentiation Regulating class switch recombination Autoantibody production Selection of autoreactive B cells

IL-17RA

ACT1/TRAF3

pSTAT3 RORC

T-cell proliferation

STAT3 P

IκBα

(Activated/memory) T cell

TH17 cell differentiation

MAPK

TAK1

AP-1

NFκB

mRNA stabilization C/EBP

C-REL AP-1 C/EBP

RORC

IL-17A IL-17F

IL-17RC

IL-23R IL-12R IKKi β1 TRAF6 JAK2 Act1 TRAF2 TRAF5 TYK2 STAT3 P

IL-23R

ACT1

ACT1

IL-17A/A IL-17A/F IL-17F/F

IL-23 p40p19

Nucleus

Cell membrane

Cytoplasm

IL-17A IL-17F IL-23R

CXCL1, CXCL2, CXCL8 CCL2, CCL7 G-CSF, GM-CSF

Figure 3 | The IL‑23–IL‑17 signalling pathway in autoimmune arthritis, including ACT1 and BAFF‑R signalling. can be Nature Reviews |BAFF Rheumatology produced by two major sources: stromal cells and myeloid cells. BAFF can activate B cells via BAFF‑R to activate ACT1. This results in the survival, proliferation, differentiation and regulation of class-switching recombination, leading to autoantibody production. In addition, BAFF can stimulate myeloid cells to produce IL‑1β, IL‑6, and IL‑23. Moreover, BAFF can activate (memory) T cells, leading to increased expression of IL‑6R. The myeloid-derived IL‑1β, IL‑6 and IL‑23 can bind to their specific receptors on the stimulated (memory) T cells, inducing these cells to differentiate or activate into T H17 cells. IL‑6R and IL‑23R activation on the activated T cells will trigger STAT3 phosphorylation and RORC expression, leading to induction of IL‑17A, IL‑17F and IL‑23R expression. IL‑17A and IL‑17F can bind to their specific receptor and, via ACT1 and TH17 cells, activate AP‑1, NFκB and C/EBP. This activation results in enhanced expression of different chemokines and cytokines such as CXCL1, CXCL2, CXCL8, CCL2, CCL7, G‑CSF and GM‑CSF. Abbreviations: ACT1, nuclear factor NFκB activator 1; AP‑1, activator protein 1; BAFF, B-cell-activating factor (also known as TNF ligand superfamily member 13B); BAFF-R, BAFF receptor (also known as TNF receptor superfamily member 13C); C/EBP, CCAAT/enhancer binding protein; CCL, CC-chemokine ligand; CXCL, CXC-chemokine ligand; G-CSF, granulocyte colony-stimulating factor; GM-CSF, granulocytemacrophage colony-stimulating factor; IL‑6R, IL-6 receptor; IL-17RA, IL-17 receptor subunit A; IL-17RC, IL-17 receptor subunit C; IL‑23R, IL-23 receptor; JAK2, Janus kinase 2; MAPK, mitogen-activated protein kinase; NFκB, nuclear factor κB; STAT3, signal transducer and activator of transcription 3; TH17 cell, type 17 T helper cell; TRAF, TNF receptor associated factor; TYK2, non-receptor tyrosine-protein kinase TYK2.

IL‑22 in destructive arthritis

IL‑22 might have a dual role in synovial inflammation and the destructive process in RA. IL‑22 promotes osteo‑ clastogenesis in RA by inducing expression of RANKL in human FLSs.62,63 However, IL‑22 has no direct role in an IL‑17/T H17-mediated model of human synovial inflammation, and synovial inflammation was similar in Il22 –/– and wild-type mice in the T‑cell-mediated mBSA-induced arthritis model.68 By contrast, serum levels of IL‑22 in patients with RA are associated with erosive disease, suggesting that IL‑22 has a role in the pathophysiology of the disease.69 Interestingly, and in contrast to the nonautoimmune AIA model described above, IL‑22 has a proinflamma‑ tory role in the autoimmune CIA model.63,70 Of note, the functionality of pathogenic IL‑17-producing CD4+ T cells from wild-type mice and Il22 –/– mice do not differ, but IL‑22 has an essential role in terminal B‑cell

differentiation in CIA, as well as in germinal centre for‑ mation, plasma cell formation and autoantibody produc‑ tion.70 As anti-citrullinated protein antibodies (ACPAs) can bind osteoclast precursor cells and directly promote their differentiation into bone-resorbing osteoclasts,71 it is intriguing to speculate that IL‑22 might have a indirect role in RA by mediating osteoclast activity via germinal centre formation and B‑cell differentiation that leads to autoantibody production (Figure 5). Of note, since IL‑23 has a role in regulating the expression of IL‑22, the involvement of IL‑23 in this process can not be excluded.72–74 Whether this mechanism also accounts for human chronic inflammatory autoimmune diseases such as RA is unclear.

TH1 and TH17 cells in early RA

Autoimmune diseases such as RA have long been consid‑ ered to be associated with TH1 cells rather than TH2 cells.75



REVIEWS

CD4+

TH1 TH17

Blood Blood/lymphoid organ

(Secondary) lymphoid organ

TH22

IL-22 CD4+

TH17

B cell

Differentiation

Stromal cell

Endocrine system Neural system

TH1 IL-21

TFH TH22

Plasma cell

Memory+

TH17 Migration

Autoantibodies CD25–/low CCR6+

Inflammation

Autoantibodies IL-22

TNF IL-17A

Target organs

IL-8

IL-6

CCR6+ memory CD4+ T cell

TH17

TH22

IL-17A

CCL20

IL-22

IL-6

IL-17A IL-23

IL-17+

IL-17+

TNF

IL-23

IL-23R+ resident T cell

IL-23 responsive cell

IL-23R

RANKL TNF

Synovial fibroblast

B cell

Neutrophil

IL-26

IFN-γ

IL-23

IL-22

IL-17F

IL-22 MMPs

IL-17A TH1

TH22

CCR6 IL-17A

IFN-γ

TH17

Myeloid cell

RANKL TH1

Plasma cell

IL-1

FcγR

Autoantibodies

Keratinocyte

ICs

Osteoclast Joint

Inflammation

Inflammation

Skin

Enthesis

Nature | Rheumatology Figure 4 | Schematic overview of the role of the IL‑23–IL‑17 immune pathway in joint inflammation, skinReviews inflammation and enthesis. These are hallmarks of diseases such as RA, psoriasis, PsA and SpA. TH cells activated in the lymphoid organ migrate via the blood stream to the target organ, where they are involved in inducing or aggravating local tissue inflammation. In the joint, IL‑17-producing cells including TH17 and in particular the CD4+ memory CCR6+ T cell populations (see Figure 1) will produce TH17-related cytokines such as IL‑17A, IL‑17F, IL‑22 and IFN‑γ. Direct cell–cell interaction as well as cytokinedriven activation of tissue-specific cells (FLSs, resident macrophages and myeloid cells) leads to a boost in cytokine production and activation of enzymes such as MMPs. In addition, T cell (in)dependent B‑cell activation and T‑cell-dependent B‑cell terminal differentiation result in autoantibody production. This process creates an environment that leads to cartilage destruction by proinflammatory cytokines, IC-mediated effector pathways and MMPs, as well as bone erosion through osteoclast formation and activation. In the skin, the inflammatory process is driven by an influx of IL‑17-producing cells, including TH17 cells and TH22 cells, and activation of keratinocytes by IL‑23, IL‑22, IL‑17A and TNF. Enthesitis has been shown to be strongly IL‑23-mediated, involving resident IL‑23R+ T cells and IL‑23-responsive cells, with IL‑17A, IL‑22 and IL‑6 contributing to local inflammation. The extent of inflammation and, in particular, tissue destruction in these diseases could depend on whether the inflammation is more autoimmune or autoinflammatory by nature. The endocrine and neural systems might have a role in regulating the differentiation, migration and inflammation of these pathogenic T cells, although the mechanism is not understood. Abbreviations: CCL, CC-chemokine ligand; CCR, CC-chemokine receptor; FcγR, Fcγ receptor; FLS, fibroblast-like synoviocyte; IC, immune complex; IL-23R, IL-23 receptor; MMP, matrix metalloproteinase; PsA, psoriatic arthritis; RA, rheumatoid arthritis; RANKL, receptor activator of nuclear factor κB ligand; SpA, spondyloarthritis; TFH cell, follicular helper T cell; TH cell, T helper cell. 6 | ADVANCE ONLINE PUBLICATION


REVIEWS ?

?

IL-23

IL-22 Stromal cell

IL-23

IL-22

T cell IL-17

Monocyte/ macrophage

IL-22R IL-17

Synovial fibroblast

COX-2 PGE2 IL-6

IL-22

TNF IL-1β Cartilage damage

MMPs, IL-6 IL-1β, TNF

CXCL13

Germinal centre

ACPAs

RANKL

RANKL

T cells

RANKL

TNF, IL-17 Osteoclast

B cells

Pre-osteoclast

Plasma cell

Autoantibodies

IL-23 IL-22

?

Osteoblast

Nature Reviews | Rheumatology Figure 5 | The role and interaction of IL‑23/TH17 cytokines in relation to tissue inflammation, autoantibody production and bone erosion in the pathogenesis of autoimmune arthritis. IL‑17A is involved in the activation of different target cells. Autocrine IL‑17A production induced by the interaction of CD4+ memory CCR6+ T cells and FLSs is mediated by the COX‑2–PGE2 pathway. IL‑22 is a FLSproliferation factor but also communicates with stromal cells to mediate terminal B‑cell differentiation and production of autoantibodies that might be involved in osteoclastogenesis. RANKL is the key cytokine driving osteoclastogenesis and bone erosion. Most of the cytokines involved in bone erosion, such as IL‑1β, TNF, IL‑17, IL‑22 and IL‑23, act via activation of the RANKL–RANK–OPG system. Of note, osteoclasts can produce IL‑23, which indirectly influences osteoclastogenesis via RANKL induction by T cells and might also contribute by activating pre-osteoclasts. The direct effects of IL‑23 on osteoblasts are unknown. Abbreviations: ACPA, anticitrullinated protein antibody; COX‑2, cyclooxygenase 2; CXCL13, CXC-chemokine ligand 13; FLS, fibroblast-like synoviocyte; IL-22R, IL-22 receptor; MMP, matrix metalloproteinase; OPG, osteoprotegerin; PGE2, prostaglandin E2; RANK, receptor activator of nuclear factor κB; RANKL, RANK ligand.

However, both TH1 and TH17 cells are now recognized to have a role in RA, although which of these cell types drives disease chronicity is not fully elucidated.76 Studies published in the late 1990s showed that IL‑17A expression is increased in the joint of patients with RA compared with healthy individuals or patients with osteoarthritis (OA),77,78 and that IL‑17A is produced by some proinflammatory TH1 or nonpolarized TH0 cells isolated from the synovial membrane and synovial fluid of patients with RA.79 It later became clear that the newly discovered IL‑17A-expressing TH17 cell subset was increased in the peripheral blood of patients with RA compared with healthy controls and was associated with disease activity.5,80,81 Interestingly, a triallelic dinucleotide polymorphism of CCR6 (the gene encoding CC‑chemokine receptor [CCR] 6, a surface marker of TH17 cells) is associated with expression levels of CCR6 and with IL‑17-positivity in the sera of patients with RA.82 The CCR6 dinucleotide

polymorphism is also associated with other IL‑17driven autoimmune diseases, namely Graves disease and Crohn disease.76 In line with the association with CCR6, IL‑17Aproducing CCR6+ memory T cells have been detected in the synovial fluid of patients with RA,5,68 and a rel‑ atively high percentage of these cells were detected in PBMCs from treatment-naive patients with early RA (Figure 1).80,81 Of note, these cells co-express the TH1 cytokine IFN‑γ and the TH17 cytokine IL‑17,74 and also express IL‑22 and TNF81,83 (Figure 1). The population of IL‑22-producing CD4+CCR6+ cells also includes a subset of TH22 cells, which can be dis‑ criminated from TH17 cells on the basis of chemokine receptor markers (Figure 1). Elevated levels of IL‑22 have been found in RA, and correlated with disease activity and radiographic progression; TH22 cells have also been shown to be increased in patients with RA, in direct cor‑ relation with TH17 cells.61,68,69,84 Discriminating whether IL‑22 is produced by TH17 and/or TH22 cells could shed light on the role of TH22 cells in relation to TH17 cells in the pathogenesis of RA. Importantly, because the CCR6 + CD4 + memory T cell population is heterogeneous, consisting of TH17 (CCR4+CXCR3–CCR10–) cells, CCR4+CXCR3+ (double positive) cells, CCR4–CXCR3– (double negative) cells, and nonclassical TH1 (CCR4–CXCR3+) cells (also called TH17.1 cells), further analysis of these cells and their localization during different stages of inflammatory arthritis is needed (Figure 1).

TH1 and TH17 cells in established RA and JIA

In children with juvenile idiopathic arthritis (JIA), the proportion of T cells that are IL‑17+ is increased within the joints, compared with the peripheral blood.85 Of note, enrichment of IL‑17+ CD4+ T cells correlated with clinical phenotype, being more pronounced in patients with a more-severe subtype of JIA.85 Within the joint, numbers of IL‑17+ T cells and forkhead box protein P3 (FOXP3)-expressing T regulatory (TREG) cells were inversely correlated.85 Although TH17 cells are relatively frequent in PBMCs from treatment-naive patients with early RA,5,80,81 reports of the proportion of these cells in PBMCs and synovial fluid mononuclear cells (SFMCs) from patients with established RA are inconsistent.5,68,83,86–89 Yamada et al.83 found no difference in the frequency of TH17 cells between patients with established RA and healthy controls, and the frequency of TH17 cells in the RA patients did not correlate with 28-joint disease activity score (DAS28). In addition, TH17 cells were less abundant in the joints than in the peripheral blood of the patients with RA, whereas the frequency of TH1 cells were more abundant in the joints than in the peripheral blood.83 By contrast, other studies found elevated populations of CCR6+CD4+ memory TH17 cells in both PBMCs and SFMCs from patients with active RA,68 an increased frequency of cir‑ culating IL‑17+CD4+CD161+ T cells in patients with active disease,89 and an increased frequency of TH17 cells in SFMCs versus PBMCs in samples from patients with early



REVIEWS active RA.5 The discrepancy between these results could be attributable to differences in the disease stage or dura‑ tion studied, but could also be due to the heterogeneity of RA. Also, the definition of TH17 cells is critical, and the use of surface markers such as the chemokine receptors CCR6 and CCR4 is essential (Figure 1).90 Although T H17 cells are known to contribute to inflammation,4,70 and CCR6+ TH17 cells are preferentially recruited to inflamed joints via CC‑chemokine ligand 20 (CCL20) in both patients with RA and SKG mice (a model of T‑cell-mediated chronic arthritis,91 experi‑ mental models of IL‑17/T H17-independent, IFN‑γ/ TH1-dependent arthritis also exist.92 For example, the development of proteoglycan-induced arthritis does not require IL‑17; both IFN‑γ and IL‑17 have the potential to induce arthritis in this model, but its severity is depend‑ ent on the production of IFN‑γ.92 Moreover, in contrast to early CIA, CD4+IL‑17A+ T cells are rarely found in arthritic joints in late-stage CIA.93 Although blockade of IL‑17A in established CIA has been shown to have less clinical benefit than neutralizing this cytokine early in the disease course, IL‑17 blockade during AIA flare sup‑ pressed joint inflammation and prevented bone erosion and osteoclast-like activity.3,93,94 In conclusion, although IL‑17 has a role throughout all stages of chronic disease and might therefore contribute to RA chronicity, further studies are needed to ascertain the in vivo localization of IL‑17-producing cells at different stages of autoimmune arthritis, and to better understand IL‑17/TH17-cell biology in chronic destructive arthritis.

Memory CD4+CCR6+ TH17 cells in RA

TH17 cells have been detected in different arthropathies but their functional role in human diseases is still not fully elucidated. In fact, the functionality of the different TH cells subsets in the development of persistant joint inflammation is still unknown. It has been shown that IL‑17 and TNF activate FLSs inducing elevated produc‑ tion of IL‑6 and IL‑8.78,95–100 In experimental arthritis, an IL‑17-triggered positive-feedback loop of IL‑6 signal‑ ling in fibroblasts has been found, but whether such a mechanism exist for human TH17-mediated autoimmune diseases needs further clarification.98,101 Regarding this issue, IL‑17A+TNF+ memory T cells detected in PBMCs from treatment naive patients with early RA81 have been characterized as TH17 cells accord‑ ing to their expression of surface markers, including CCR6, IL‑17F, IL‑22, IL‑26, RORC, CCL20 and low levels of T-box transcription factor TBX21, forkhead box protein P3 (FOXP3) and IFN‑γ. In co-cultures with FLSs from patients with early RA, these primary CCR6+ TH17 cells, but not CCR6– TH1 cells, potently induced production of IL‑6, IL‑8, matrix metalloproteinase (MMP)‑1 and MMP‑3. Of interest, the TH17 cells also increased IL‑17A expression, indicating the presence of a proinflammatory feedback loop that might be important for the persistence of synovitis.81 Optimal suppression of these effects, especially with respect to MMP‑1 and MMP‑3 production, was achieved by blocking both TNF and IL‑17 signalling,81 indicating the additional value

of blocking TH17 activity as well as TNF in early RA. This study provided the first ex vivo data indicating the potential pathogenicity of TH17 cells in human arthritis.

Combined blockade of TNF and IL‑17

IL‑17A overexpression induces an RA‑like phenotype in a healthy mouse knee joint, and IL‑17A has been shown to enhance both the joint-inflammatory and tissuedestructive capacity of TNF.8,102,103 Conversely, combined blockade of IL‑17 and TNF is more effective than mono‑ therapy in CIA.102,104 Interestingly, the therapeutic effect of combined blockade of IL‑17 and TNF endures much longer after cessation of the therapy compared with monotherapy, indicating a more sustained therapeutic effect after neutralizing both TNF and IL‑17.104 Ex vivo studies using synovial cells in co-culture with T cells revealed that the interaction with FLSs promotes TH17 cell expansion through caspase 1 activation.81,105 Interestingly, the autocrine IL‑17A production result‑ ing from FLS–TH17 cell interaction is hardly dependent on IL‑6 or IL‑1β, but rather is mediated primarily by the cyclooxygenase–prostaglandin E2 pathway, independently of IL‑23.106 In line with the data showing that combined blockade of both IL‑17A and TNF is needed to suppress the proinflammatory feedback loop in co-cultures of earlyRA FLSs and CCR6+ memory TH17cells,81 it became clear that TNF blockade does not directly suppress expression of the TH17 cytokines IL‑17A, IL‑17F and IL‑22.81,107 By contrast, calcitriol (1,25[OH] 2D3, the active form of vitamin D) has direct suppressive effects on the expres‑ sion of these cytokines and on the activity of TH17 cells from patients with RA.107 Thus, therapeutic activation of vitamin D receptor signalling in addition to TNF blockade could help fully neutralize pathogenic TH17 cell activity in RA and potentially other IL‑17/TH17-mediated dis‑ eases. The data also indicate that anti-TNF therapy might be less effective in patients with RA with increased levels of IL‑17A or TH17-cell activity. Indeed, increased base‑ line frequencies of TH17 cells and circulating IL‑17 levels have been linked to subsequent inadequate response to anti‑TNF therapy in patients with RA.104,108 Of note, both in CIA and in patients with RA, a signifi cant increase in circulating TH17 cells has been observed after anti-TNF therapy, accompanied by an increase in production of the p40 subunit shared by IL‑12 and IL‑23 production.104,109 Other studies have shown that increased IL‑17 production after TNF blockade in patients with RA is accompanied by a decrease in TH17-specific CCR6 expression and inhibition of CCL20 production in the rheumatoid synovium, indicating impaired homing of IL‑17+ or TH17 cells to the site of inflammation after anti-TNF therapy. 110,111 Furthermore, TNF blockade induces IL‑10 expression in human IL‑17+CD4+ T cells,112 in dicating changes in the pathogenic phenotype of these cells.113 Together, these data suggest the potential benefit of treating TNF nonresponders with anti-IL‑17 therapy. In a phase II RCT that enrolled patients naive to bio‑ logic therapy or who had an inadequate response to TNF inhibitors, the anti-IL‑17 monoclonal antibody



REVIEWS ixekizumab was superior to placebo in improving signs and symptoms of RA in both patient populations.114 Further clinical studies are needed to examine poten‑ tial additional value of combination blockade of TNF and IL‑17 activity, as it has been shown not only that these cytokines have additive activity, but also that neutralizing TNF alone has no direct inhibitory effect on IL‑17/TH17 activity.81,107,115

Clinical trials of IL‑17 modulation in RA

Abatacept, a cytotoxic T lymphocyte-associated antigen 4 (CTLA4)–Ig fusion protein that competes with the CD28 T‑cell receptor to disrupt T‑cell co-stimulatory signals, modulates T cell effector functions in patients with RA, most strikingly in those with autoantibodypositive disease. 116 Abatacept treatment of patients with ACPA-positive RA significantly down-regulated all key T‑cell effector subsets including TH1, TH2 and TH17 cells; however, in vitro studies of RA synovial fluid samples indicated that abatacept did not increase the functional capacity of CD4+CD25hi TREG cells.116 In line with this finding, another study found that blockade of CD28 signalling by abatacept led to a decrease in IL‑17producing and IFNγ-producing T cells in peripheral blood.117 Interestingly, only levels of IL‑17F, and not IL‑17A or IL‑17A–IL‑17F, in the plasma of patients with RA were decreased after abatacept treatment.118 In studies of patients with RA, treatment with tocili zumab, a humanized anti-IL‑6-receptor antibody, resulted in improvement in DAS28, in association with a significant decrease in the percentage of TH17 cells and an increase in the percentage of TREG cells in the peripheral blood.119,120 These data suggest that tocili‑ zumab might contribute to clinical improvement of RA by correcting the imbalance between TH17 cells and TREG cell in active disease, in favour of protective TREG cells, through inhibition of IL‑6 activity.119,120

Clinical trials of IL‑17 blockade in RA

Antibodies that target the cytokine IL‑17A (ixekizumab and secukinumab) or the receptor IL‑17RA (brodalumab) have been trialled in patients with RA.114,121–125 From these studies it has become clear that these drugs have safety profiles comparable to those of other biologic agents with no unexpected safety concerns, although neutro‑ penia and leukopenia have been reported during antiIL‑17 treatment.122 These reports are in line with evidence from animal models that IL‑17 is involved in granulocyte colony-stimulating factor (G-CSF)-mediated neutrophil production, and that circulating neutrophil counts are lower in Il17ra–/– mice than wild‑type mice.126,127 In a phase I RCT of patients with RA being treated with oral DMARDs, those who received ixekizumab showed improved signs and symptoms of RA compared with those who received placebo, with a significant change in DAS28 as early as week 1. Also, the proportions of patients who fulfilled the ACR criteria for 20%, 50% and 70% improvement (ACR20, ACR50 and ACR70, respectively) were greater in the ixekizumab group than the placebo group.122 Ixekizumab also improved signs

and symptoms of RA in a phase II study that enrolled patients who were either naive to biologic therapy or had an inadequate response to TNF inhibitors.114 The ACR20 response rate was higher with ixekizumab treat‑ ment than with placebo at week 12, and DAS28 calculated with C‑reactive protein (CRP) level (DAS28-CRP), clini‑ cal disease activity index score and CRP levels at week 12 were all decreased in ixekizumab‑treated patients versus placebo-treated patients.114 In a phase I study of the anti-IL‑17A antibody secukinumab, ACR20 response rates were higher with secukinumab treatment versus placebo at week 12, and DAS28 and CRP levels decreased over time.121 In a phase II study of secukinumab, the primary efficacy endpoint (ACR20 response at week 16) was not achieved, although a greater decrease in DAS28 and a significant reduction in serum CRP levels (measured by high-sensitivity CRP [hsCRP] assay) at week 16 were observed with secukin‑ imab treatment compared with placebo.124 Interestingly, the 52-week follow-up results of this study showed that patients with improved ACR response and DAS28CRP at week 16 sustained their response through week 52. Moreover, the responses in these patients improved through week 52, with ACR50 rates rising from 45% at week 16 to 55% at week 52.125 By contrast, brodalumab, an anti-IL‑17R antibody, showed no evidence of a clinical effect in patients with RA in a phase Ib study, although this treatment was effective in patients with psoriatic arthritis (PsA) and plaque psoriasis.123,128,129 These clinical trial data show the efficacy of neutral‑ izing IL‑17A, at least in a subgroup of patients with RA. The differences in outcome achieved by targeting the ligand versus targeting the receptor underscore the need for a greater understanding of the IL‑17 pathway in RA as opposed to PsA and psoriasis.125

IL‑23 and IL‑17 in SpA

Spondyloarthritis (SpA) encompasses a heterogeneous group of disorders that includes ankylosing spondylitis (AS), PsA, reactive arthritis, and arthritis associated with inflammatory bowel disease.130 The axial skeleton is a dominant site of SpA pathology, with inflammation of the ligamentous attachments in the affected spine result‑ ing in pain, stiffness and impaired mobility.130 Like RA, SpA is a common form of chronic immune-mediated inflammatory arthritis, but whereas RA is characterized by destructive damage to the joints, the hallmark of SpA is pathologic new-bone formation.131–133 Genetic data have associated polymorphisms in the gene encoding IL‑23R with susceptibility to SpA,134–136 and several cytokine pathways, such as TNF, IL‑17 and IL‑23, have been genetically associated with AS, the proto‑ typical subtype of SpA.130,131 Levels of IL‑17A, IL‑6, TGF‑β and IFN‑γ are increased in the sera and synovial fluid of patients with reactive arthritis and undifferentiated SpA as compared with patients with RA, and increased numbers of TH17 cells have been found in the peripheral blood of patients with PsA and AS, suggesting the involvement of TH1 and TH17 in inflammation in SpA.137,138 TH17 cells



REVIEWS Autoimmune arthritis

Autoinflammatory arthritis

Antigen

Lymphoid organ (spleen, lymph node) B cell T cell

Germinal centre

Synovial B cell

Plasma cell Synovial T cell

Autoantibodies

Myeloid cell FcγR

TLR

ICs Osteoclast

Stromal cell (synovial fibroblast)

SpA, AS Development: IL-23/IL-17-induced inflammation; IL-23R SNPs

RA Development: IL-23/IL-17/TH17-induced autoimmunity

Progression: Innate-mediated immune pathway; cytokine-mediated pathways; IL-17+ and IL-23+ innate (T) cell interactions with myeloid and stromal cells (osteoblasts, endothelial cells); tissue remodelling

Progression: Cytokine-induced proinflammatory loops; proteolytic enzymes (MMPs) and other downstream antibody effector pathways; tissue damage

Joint? IL-23/IL-17/TH17-induced autoimmunity

Innate (T) cell

PsA

Skin? IL-23/IL-17/TH17 and IL-22/TH22-induced inflammation

Nature Reviews | Rheumatology

from patients with SpA are highly differentiated and are polyfunctional in terms of T‑cell receptor (TCR) activa‑ tion.138 In addition, the percentages of IL‑17+CD4+ T cells and IL‑22+CD4+ T cells were increased in the peripheral blood of patients with AS and patients with RA in com‑ parison with healthy individuals.139 Phenotypic analy‑ sis showed that the vast majority of IL‑17-producing cells expressed CD4 and CD45RO, and most of these CD4+CD45RO+ cells also expressed CCR6 and CCR4 but only half expressed IL‑23R. However, the percentage of IL‑23R+CD4+ T cells was positively correlated with the frequency of IL‑17+CD4+ or IL‑22+CD4+ T cells.139 In line with this finding, TH22 cells, TH17 cells and IL‑22 were more frequent in patients with AS or RA compared with patients with OA, implicating these T‑helper cells in the pathogenesis of AS.140 Of note, a positive correlation between IL‑22 expression and frequency of TH17 cells was found only in patients with AS and not in those with RA. Moreover, the percentages of both TH22 and TH17 cells correlated positively with disease activity only in patients with RA and not in patients with AS.134 Another study

◀

Figure 6 | The IL‑23–IL‑17 immune pathway in RA, SpA and PsA in relation to autoimmune-like or autoinflammatory pathogenesis. RA is considered an autoimmune-mediated disease in which the IL‑23–IL‑17 pathway might be critical for the development of the disease; autoantibodies are essential and contribute substantially to disease severity. Cytokine-induced proinflammatory loops have a critical role in boosting the progression of RA including downstream antibody effector pathways which might become less IL‑23/IL‑17-dependent over the course of the disease. Potential flare-up of arthritis might be strongly IL‑23/IL‑17mediated. In SpA, the discussion is ongoing as to whether AS, and potentially PsA, are autoinflammatory diseases with strong involvement of innate IL‑23+/IL‑17A+ cells. The interaction of these cells with different tissue-specific cells leads to aggressive inflammation involving activation of cytokine-mediated pathways that result in bone erosion and tissue remodelling. In PsA, involvement of both the skin and joints suggests that the disease pathogenesis might involve both autoimmune (joint) and autoinflammatory (skin) types of inflammation, but this hypothesis requires further research. Abbreviations: AS, ankylosing spondylitis; FcγR, Fcγ receptor; IL-23R, IL-23 receptor; DC, dendritic cell; IC, immune complex; MMP, matrix metalloproteinase; PsA, psoriatic arthritis; RA, rheumatoid arthritis; SNP, single nucleotide polypmorphism; SpA, spondyloarthritis; TH17 cell, type 17 T helper cell; TH22 cell, type 22 T helper cell; TLR, Toll‑like receptor.

found that the TH17-related cytokines CCL20 and IL‑23 were expressed in the joints of patients with SpA as well as patients with RA, but serum levels of IL‑23 correlated with disease activity only in those with RA.141 The degree of intimal lining layer hyperplasia was strongly associ‑ ated with levels of IL‑17, IL‑23 and CCL20 in the joints of patients with RA, but this association was not present in patients with SpA.141 These data suggest a close interrela‑ tion between IL‑23, IL‑22 and TH17 cells in RA and SpA, although the TH17 cytokine system might be differentially regulated in RA and SpA (Figure 6).141 An analysis of IL‑17-producing cells in the facet joints of patients with SpA revealed that most IL‑17+ cells were myeloperoxicase-positive and CD15+ neutrophils, whereas only a small proportion were CD3+ T cells or mast cells.142 In the synovium of patients with SpA, the number of c‑Kit+ mast cells was increased compared with the synovium from patients with RA. Mast cells were the main IL‑17+ cell population in the SpA synovium, and these mast cells expressed significantly more IL‑17 than their counterparts in the RA synovium.132 Interestingly, clinically effective anti-TNF therapy did not modulate the mast cell–IL‑17 axis in SpA.132 These studies suggest that innate immunity might be of greater relevance to SpA pathogenesis than the TH17-mediated adaptive immune response, and raise the question of whether inflammation in SpA is more autoinflammatory than autoimmune in origin.142–145 In line with this thought, the frequency of IL‑23R+ γδ T cells in the periphery was higher in patients with AS than in healthy individuals, in association with increased IL‑17 secretion.144 The role of these IL‑17producing innate cells in relation to T helper cells in the pathogenesis of SpA needs further clarification.



REVIEWS Key features of SpA are gut inflammation and enthesi‑ tis.131 Elevated mRNA of IL‑23p19 was found in the ter‑ minal ileum in patients with AS and those with Crohn disease. Moreover, infiltrating monocyte-like cells in inflamed mucosa from these patients produced IL‑23.146 Interestingly, Paneth cells produce IL‑23 as well as IL‑17A.146,147 However, in patients with AS, but not those with Crohn disease, IL‑23 was not associated with upregulation of IL‑17, IL‑6 and IL‑1β. This finding suggests that overexpression of IL‑23 is critical for subclinical gut inflammation in AS.146 Enthesitis has also been shown to be IL‑23dependent.148–150 In mice, overexpression of IL‑23 in vivo induced the development of the features of human SpA, including enthesitis and new bone formation.148 Characterization of the IL‑23-responsive cells revealed a population of RORγt+CD3+CD4–CD8– resident T cells that produced IL‑22 and IL‑17 upon exposure to IL‑23 (Figure 4).148 Interestingly, a mutation in tyrosine-protein kinase Zap‑70, which impairs T‑cell receptor signalling and leads to the production of highly autoreactive T cells, predisposed SKG mice to develop SpA and ileitis resem‑ bling Crohn disease after exposure to β‑glucan.149 Of note, arthritis and spondylitis were mediated by T‑cells and were dependent on IL‑23;149 moreover, enthesitis and ileitis also required IL‑23 and enthesitis was specifically dependent on IL‑17 and IL‑22 in this model.150

IL‑23 and IL‑17 in PsA

PsA is a chronic inflammatory arthritis commonly associ‑ ated with psoriasis. This phenotypically distinct subtype of SpA, which affects both skin and joints, is associ‑ ated with single nucleotide polymorphisms (SNPs) in the genes encoding IL‑23R and IL‑23 (Figure 4).151,152 Susceptibility to PsA is also associated with SNPs in the gene encoding ACT1 (TRAF3IP2), which directs signal‑ ling downstream of IL‑17RA.153 These data imply that the IL‑23–IL‑17 axis has a central role in the pathogenesis of PsA. This notion is further supported by evidence that expression of IL‑23p19–IL‑23R and IL‑17A–IL‑17R is elevated in psoriatic skin and synovial fluid from patients with PsA.154–157 The IL‑23–IL‑17 axis is also critical to the induction of psoriasis-like skin inflammation in mice.158–161 Increased frequencies of IL‑17 + and IL‑22 + CD4 + T cells have been found in the peripheral blood of patients with psoriasis and patients with PsA.162 However, the differential distribution of these cells at various disease sites suggests they have shared as well as distinct roles in the pathogenesis of these diseases.162 In contrast to RA, which is associated with HLA class II, PsA is associated with MHC class I. CD4+ T cells predominate over CD8+ cells in the synovial membrane of patients with PsA, whereas the opposite occurs in the synovial fluid.163 The frequency of IL‑17+CD8+ T cells found to be higher in the synovial fluid of patients with PsA than in the peripheral blood and correlated with disease activity and progressive joint erosion.164 PsA can lead to destructive bone loss, and 67% of patients with PsA have signs of erosive bone disease.165 Synovial T cells from patients with PsA induce

osteoclastogenesis and bone resorption via RANKL. Osteoclastogenesis can be triggered by TNF, IL‑23 and IL‑17.166 IL‑23 and IL‑17 might exert effects on bone loss independently of each other, and might induce bone resorption associated with PsA pathology through distinct molecular mechanisms.167

Targeting the IL‑23–IL‑17 axis in SpA

Clinical trials using antibodies that target IL‑17A (ixeki‑ zumab and secukinumab), IL‑17RA (brodalumab), both IL‑17A and TNF (bispecific antibodies and ABT-122, a dual-variable-domain immunoglobulin), the p40 subunit of IL‑12 and IL‑23 (ustekinumab and briakinumab) or the p19 subunit of IL‑23 (tildrakizumab and guselkumab) have been performed in patients with psoriasis, AS and PsA.168 Antibodies specifically targeting IL‑17A or IL‑23 have been shown to be very effective for the treatment of psoriasis. In fact, neutralizing IL‑17 has been shown to achieve better PASI (psoriasis area and severity index) scores than the current standard drug using ustekinumab and etanercept, leading to the recommendation of secuki‑ numab (anti-IL‑17A) as first-line therapy for patients with psoriasis.129,169–172 The results of IL‑17A blockade (with secukinumab) in a phase II study in patients with AS are promising.168 Clinical trials with anti-IL‑23p19 antibody and IL‑17 inhibition in inflammatory arthri‑ tis are ongoing, with new data expected soon. However, clinical studies so far have revealed that the IL‑23– IL‑17 immune pathway contribute substantially to the pathogenesis of psoriasis, AS and PsA.

Conclusions

Ample evidence implicates the IL‑23–IL‑17 axis in chronic immune-mediated inflammatory arthritis. From studies in mouse models of arthritis, it is clear that the presence of IL‑17A in the tissue exacerbates chronicity and tissue damage, and that neutralization of IL‑17A activity is more effective in suppressing disease progres‑ sion and severity when carried out early rather than later in the disease course. This suggests that the interaction of IL‑17 with other cytokines acting on different cell types to increases the release of proinflammatory mediators might trigger positive-feedback loops that become less IL‑17 dependent, or even IL‑17-independent, over time. By contrast, T‑cell mediated flares of arthritis remain strongly IL‑17-mediated. Unravelling the in vivo process of IL‑17-inducing pathway(s) over the course of inflam‑ matory arthritis will help to develop the best strategy for targeting IL‑17. In clinical trials, anti-IL‑17 therapy has variable efficacy, being highly effective in psoriasis, moderate in PsA and moderate to weak in RA.173 In RA clinical trials, a challenging but potentially important step could be to move from testing anti-IL‑17A therapy in patients with established severe disease to those with early-onset RA. Importantly, lessons learned from clinical trials of the efficacy of new cytokine inhibitors revealed distinct cytokine-driven activation profiles that define disease groups. These studies also support the ‘old’ idea of the existence of a cytokine hierarchy.174 The IL‑23–IL‑17 pathway might be critical in connecting



REVIEWS early innate responses to adaptive immune responses, thus making the inflammation harder to control and leading to persistent immune-mediated arthritis. Selective targeting of clusters of cytokines, for example by using bispecific antibodies or small-molecules inhibi‑ tors of targets such as RORγt, ACT1, STAT3 and Janus kinases, could be essential to substantially improve the treatment of patients with chronic immune-mediated arthritis, with special attention to long-term toxicity.174 The discovery of the IL‑23–IL‑17 immune pathway has boosted our knowledge regarding the immuno‑ logical mechanisms of many chronic inflammatory dis‑ eases including inflammatory arthritis. However, many

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Lubberts, E. et al. IL‑1‑independent role of IL‑17 in synovial inflammation and joint destruction during collagen-induced arthritis. J. Immunol. 167, 1004–1013 (2001). Nakae, S., Nambu, A., Sudo, K. & Iwakura, Y. Suppression of immune induction of collageninduced arthritis in IL‑17‑deficient mice. J. Immunol. 171, 6173–6177 (2003). Lubberts, E. et al. Treatment with a neutralizing anti-murine interleukin‑17 antibody after the onset of collagen-induced arthritis reduces joint inflammation, cartilage destruction, and bone erosion. Arthritis Rheum. 50, 650–659 (2004). Lubberts, E., Koenders, M. I. & van den Berg, W. B. The role of T‑cell interleukin‑17 in conducting destructive arthritis: lessons from animal models. Arthritis Res. Ther. 7, 29–37 (2005). Leipe, J. et al. Role of TH17 cells in human autoimmune arthritis. Arthritis Rheum. 62, 2876–2885 (2010). Miossec, P. & Kolls, J. K. Targeting IL‑17 and TH17 cells in chronic inflammation. Nat. Rev. Drug Discov. 11, 763–776 (2012). Corneth, O. B. et al. Absence of interleukin‑17 receptor A signalling prevents autoimmune inflammation of the joint and leads to a TH2-like phenotype in collagen-induced arthritis. Arthritis Rheum. 66, 340–349 (2014). Lubberts, E. et al. Overexpression of IL‑17 in the knee joint of collagen type II immunized mice promotes collagen arthritis and aggravates joint destruction. Inflamm. Res. 51, 102–104 (2002). Lubberts, E. et al. Requirement of IL‑17 receptor signaling in radiation-resistant cells in the joint for full progression of destructive synovitis. J. Immunol. 175, 3360–3368 (2005). Koenders, M. I. et al. Interleukin‑17 receptor deficiency results in impaired synovial expression of interleukin‑1 and matrix metalloproteinases 3, 9, and 13 and prevents cartilage destruction during chronic reactivated streptococcal cell wall-induced arthritis. Arthritis Rheum. 52, 3239–3247 (2005). Infante-Duarte, C., Horton, H. F., Byrne, M. C. & Kamradt, T. Microbial lipopeptides induce the production of IL‑17 in TH cells. J. Immunol. 165, 6107–6115 (2000). Moseley, T. A., Haudenschild, D. R., Rose, L. & Reddi, A. H. Interleukin‑17 family and IL‑17 receptors. Cytokine Growth Factor Rev. 14, 155–174 (2003). Lubberts, E. The role of IL‑17 and family members in the pathogenesis of arthritis. Curr. Opin. Investig. Drugs 4, 572–577 (2003). Kolls, J. K. & Linden, A. Interleukin‑17 family members and inflammation. Immunity 21, 467–476 (2004).

questions remain and further clarification is needed to fully understand the contribution of this important pathway, including potentially distinct roles of IL‑17 and IL‑23, and where, when and how this pathway influences the pathogenesis of these inflammation-driven tissue destructive diseases (Figure 6). In addition to target‑ ing TNF activity, novel therapeutic opportunities are available through modulation of the IL‑23–IL‑17 axis. Intelligent and rational choices of combination therapy could further improve current treatment protocols. Full understanding of disease pathogenesis in an indivi dual patient could provide the basis of a personalized treatment strategy.

15. Iwakura, Y., Ishigame, H., Saijo, S. & Nakae, S. Functional specialization of interleukin‑17 family members. Immunity 34, 149–162 (2011). 16. Wright, J. F. et al. Identification of an interleukin 17F/17A heterodimer inactivated human CD4+ T cells. J. Biol. Chem. 282, 13447–13455 (2007). 17. Liang, S. C. et al. An IL‑17F/A heterodimer protein is produced by mouse TH17 cells and induces airway neutrophil recruitment. J. Immunol. 179, 7791–7799 (2007). 18. Toy, D. et al. Cutting Edge: interleukin 17 signals through a heteromeric receptor complex. J. Immunol. 177, 36–39 (2006). 19. Wright, J. F. et al. The human IL‑17F/IL‑17A heterodimeric cytokine signals through the IL‑17RA/IL‑17RC receptor complex. J. Immunol. 181, 2799–2805 (2008). 20. Gaffen, S. Structure and signalling in the IL‑17 receptor family. Nat. Rev. Immunol. 9, 556–567 (2009). 21. Song, X. et al. IL‑17RE is the functional receptor for IL‑17C and mediates mucosal immunity to infection with intestinal pathogens. Nat. Immunol. 12, 1151–1158 (2011). 22. Chang, S. H. et al. Interleukin‑17C promotes TH17 cell responses and autoimmune disease via interleukin‑17 receptor E. Immunity 35, 611–621 (2011). 23. Patel, D. D., Lee, D. M., Kolbinger, F. & Antoni, C. Effects of IL‑17A blockade with secukinumab in autoimmune diseases. Ann. Rheum. Dis. 72, ii116–ii123 (2013). 24. Croxford, A. L., Mair, F. & Becher, B. IL‑23: one cytokine in control of autoimmunity. Eur. J. Immunol. 42, 2263–2273 (2012). 25. McGeachy, M. J. et al. TGF-β and IL‑6 drive the production of IL‑17 and IL‑10 by T cells and restrain TH‑17 cell-mediated pathology. Nat. Immunol. 8, 1390–1397 (2007). 26. Zielinski, C. E. et al. Pathogen-induced human TH17 cells produce IFN‑γ or IL‑10 and are regulated by IL‑1β. Nature 484, 514–518 (2012). 27. Cua, D. J. et al. Interleukin‑23 rather than interleukin‑12 is the critical cytokine for autoimmune inflammation of the brain. Nature 421, 744–748 (2003). 28. Awasthi, A. et al. Cutting edge: IL‑23 receptor GFP reporter mice reveal distinct populations of IL‑17‑producing cells. J. Immunol. 182, 5904–5908 (2009). 29. McGeachy, M. J. et al. The interleukin 23 receptor is essential for the terminal differentiation of interleukin 17-producing effector T helper cells in vivo. Nat. Immunol. 10, 314–324 (2009).


30. Murphy, C. A. et al. Divergent pro- and antiinflammatory roles for IL‑23 and IL‑12 in joint autoimmune inflammation. J. Exp. Med. 198, 1951–1957 (2003). 31. Oppmann, B. et al. Novel p19 protein engages IL‑12p40 to form a cytokine, IL‑23, with biological activities similar as well as distinct from IL‑12. Immunity 13, 715–725 (2000). 32. Langrish, C. L. et al. IL‑23 drives a pathogenic T cell population that induces autoimmune inflammation. J. Exp. Med. 201, 233–240 (2005). 33. Harrington, L. E. et al. Interleukin 17-producing CD4+ effector T cells develop via a lineage distinct from the T helper type 1 and 2 lineages. Nat. Immunol. 6, 1123–1132 (2005). 34. Park, H. et al. A distinct lineage of CD4 T cells regulates tissue inflammation by producing IL‑17. Nat. Immunol. 6, 1133–1141 (2005). 35. Hueber, A. J. et al. Mast cells express IL‑17A in rheumatoid arthritis synovium. J. Immunol. 184, 3336–3340 (2010). 36. Noordenbos, T. et al. Interleukin‑17‑positive mast cells contribute to synovial inflammation in spondylarthritis. Arthritis Rheum. 64, 99–109 (2012). 37. Van Baarsen, L. et al. Heterogeneous expression pattern of interleukin‑17A (IL‑17A), IL‑17F and their receptors in synovium of rheumatoid arthritis, psoriatic arthritis and osteoarthritis: possible explanation for non-response to anti‑IL‑17 therapy? Arthritis Res. Ther. 16, 426 (2014). 38. Moran, E. M. et al. IL‑17A expression is localised to both mononuclear and polymorphonuclear synovial cell infiltrates. PLoS ONE 6, e24048 (2011). 39. Appel, H. et al. Analysis of IL‑17+ cells in facet joints of patients with spondyloarthritis suggests that the innate immune pathway might be of greater relevance than the TH17-mediated adaptive immune response. Arthritis Res. Ther. 13, R95 (2011). 40. Cua, D. J. & Tato, C. M. Innate IL‑17‑producing cells: the sentinels of the immune system. Nat. Rev. Immunol. 10, 479–489 (2010). 41. Spits, H. et al. Innate lymphoid cells—a proposal for uniform nomenclature. Nat. Immunol. 13, 145–149 (2013). 42. Cornelissen, F. et al. (2009). Interleukin‑23 is critical for full-blown expression of a nonautoimmune destructive arthritis and regulates interleukin‑17A and RORγt in γδ T cells. Arthritis Res. Ther. 11, R194 (2009). 43. El-Behi, M. et al. The encephalitogenicity of TH17 cells is dependent on IL‑1‑ and IL‑23‑induced production of the cytokine GM‑CSF. Nat. Immunol. 12, 568–575 (2011).


REVIEWS 44. Codarri, L. et al. RORγt drives production of the cytokine GM‑CSF in helper T cells, which is essential for the effector phase of autoimmune neuroinflammation. Nat. Immunol. 12, 560–567 (2011). 45. Campbell, I. K. et al. Protection from collageninduced arthritis in granulocyte-macrophage colony-stimulating factor-deficient mice. J. Immunol. 161, 3639–3644 (1998). 46. Plater-Zyberk, C. et al. Combined blockade of granulocyte-macrophage colony stimulating factor and interleukin 17 pathways potently suppresses chronic destructive arthritis in a tumour necrosis factor α‑independent mouse model. Ann. Rheum. Dis. 68, 721–728 (2009). 47. Burmester, G. R. et al. Efficacy and safety of mavrilimumab in subjects with rheumatoid arthritis. Ann. Rheum. Dis. 72, 1445–1452 (2013). 48. Lubberts, E. TH17 cytokines and arthritis. Semin. Immunopathol. 32, 43–53 (2010). 49. Korn, T., Bettelli, E., Oukka, M. & Kuchroo, V. K. IL‑17 and TH17 cells. Annu. Rev. Immunol. 27, 485–517 (2009). 50. Qian, Y. et al. Act1, a negative regulator in CD40and BAFF-mediated B cell survival. Immunity 21, 575–587 (2004). 51. Pisitkun, P., Claudio, E., Ren, N., Wang, H. & Siebenlist, U. The adapter protein CIKS/ACT1 is necessary for collagen-induced arthritis, and it contributes to the production of collagen-specific antibody. Arthritis Rheum. 62, 3334–3344 (2010). 52. Luo, Q. et al. A novel disease-modifying antirheumatic drug, iguratimod, ameliorates murine arthritis by blocking IL‑17 signaling, distinct from methotrexate and leflunomide. J. Immunol. 191, 4969–4978 (2013). 53. Hara, M. et al. Safety and efficacy of combination therapy of iguratimod with methotrexate for patients with active rheumatoid arthritis with an inadequate response to methotrexate: an open-label extension of a randomized, double-blind, placebo-controlled trial. Mod. Rheumatol. 24, 410–418 (2014). 54. Li, J. et al. Efficacy and safety of iguratimod for the treatment of rheumatoid arthritis. Clin. Dev. Immunol. 13, 310628 (2013). 55. Yago, T. et al. IL‑23 induces human osteoclastogenesis via IL‑17 in vitro, and anti‑IL‑23 antibody attentuates collagen-induced arthritis. Arthritis Res. Ther. 9, R96 (2007). 56. Cornelissen, F. et al. IL‑23 dependent and independent stages of experimental arthritis: no clinical effect of therapeutic IL‑23p19 inhibition in collagen-induced arthritis. PLoS ONE 8, e57553 (2013). 57. Krausz, S. et al. Brief report: a phase IIa, randomized, double-blind, placebo-controlled trial of apilimod mesylate, an interleukin‑12/ interleukin‑23 inhibitor, in patients with rheumatoid arthritis. Arthritis Rheum. 64, 1750–1755 (2012). 58. Adamopoulos, I. E. et al. IL‑23 is critical for induction of arthritis, osteoclast formation, and maintenance of bone mass. J. Immunol. 187, 951–959 (2011). 59. Chen, L., Wei, X. Q., Evans, B., Jiang, W. & Aeschlimann, D. IL23 promotes osteoclast formation by up-regulation of receptor activator of NF‑κB (RANK) expression in myeloid precursor cells. Eur. J. Immunol. 38, 2845–2854 (2008). 60. Li, X. et al. IL‑23 induces receptor activator of NF‑κB ligand expression in fibroblast-like synoviocytes via STAT3 and NF‑κB signal pathways. Imunol. Lett. 127, 100–107 (2010). 61. Ikeuchi, H. et al. Expression of interleukin‑22 in rheumatoid arthritis: potential role as a

62.

63.

64.

65.

66.

67.

68.

69.

70.

71.

72.

73.

74.

75. 76.

77.

78.

79.

80.

proinflammatory cytokine. Arthritis Rheum. 52, 1037–1046 (2005). Kim, K. W. et al. Interleukin‑22 promotes osteoclastogenesis in rheumatoid arthritis through induction of RANKL in human synovial fibroblasts. Arthritis Rheum. 64, 1015–1023 (2012). Geboes, L. et al. Proinflammatory role of the TH17 cytokine interleukin‑22 in collagen-induced arthritis in C57BL/6 mice. Arthritis Rheum. 60, 390–395 (2009). Quinn, J. M. et al. IL‑23 inhibits osteoclastogenesis indirectly through lymphocytes and is required for the maintenance of bone mass in mice. J. Immunol. 181, 5720–5729 (2008). Kamiya, S. et al. Effects of IL‑23 and IL‑27 on osteoblasts and osteoclasts: inhibitory effects on osteoclast differentiation. J. Bone Mineral Metabol. 25, 277–285 (2007). Ju, J. H. et al. IL‑23 induces receptor activator of NF‑κB ligand expression on CD4+ T cells and promotes osteoclastogenesis in an autoimmune arthritis models. J. Immunol. 181, 1507–1518 (2008). Pöllinger, B. et al. TH17 cells, not IL‑17+ γδ T cells, drive arthritic bone destruction in mice and humans. J. Immunol. 186, 2602–2612 (2011). Van Hamburg, J. P. et al. IL‑17/TH17 mediated synovial inflammation is IL‑22 independent. Ann. Rheum. Dis. 72, 1700–1707 (2013). Leipe, J. et al. Interleukin‑22 serum levels are associated with radiographic progression in rheumatoid arthritis. Ann. Rheum. Dis. 70, 1453–1457 (2011). Corneth, O. B. J. et al. Impaired B cell immunity in IL‑22 knockout mice in collagen-induced arthritis. Ann. Rheum. Dis. 70 (Suppl. 2), A58–A59 (2011). Harre, U. et al. Induction of osteoclastogenesis and bone loss by human autoantibodies against citrullinated vimentin. J. Clin. Invest. 122, 1791–1802 (2012). Liang, S. C. et al. Interleukin (IL)‑22 and IL‑17 are coexpressed by TH17 cells and cooperatively enhance expression of antimicrobial peptides. J. Exp. Med. 203, 2271–2279 (2006). Zheng, Y. et al. Interleukin‑22, a TH17 cytokine, mediates IL‑23‑induced dermal inflammation and acanthosis. Nature 445, 648–651 (2007). Mus, A. M. et al. Interleukin‑23 promotes TH17 differentiation by inhibiting T‑bet and FoxP3 and is required for elevation of interleukin‑22, but not IL‑21, in autoimmune experimental arthritis. Arthritis Rheum. 62, 1043–1050 (2010). Firestein, G. S. Evolving concepts of rheumatoid arthritis. Nature 423, 356–361 (2003). Lubberts, E. IL‑17/TH17 targeting: on the road to prevent chronic destructive arthritis? Cytokine 41, 84–91 (2008). Kotake, S. et al. IL‑17 in synovial fluids from patients with rheumatoid arthritis is a potent stimulator of osteoclastogenesis. J. Clin. Invest. 103, 1345–1352 (1999). Chabaud, M. et al. Human interleukin‑17: a T cell-derived proinflammatory cytokine produced by the rheumatoid synovium. Arthritis Rheum. 42, 963–970 (1999). Aarvak, T., Chabaud, M., Kallberg, E., Miossec, P., Natvig, J. B. Change in the TH1/TH2 phenotype of memory T‑cell clones from rheumatoid arthritis synovium. Scand. J. Immunol. 50, 1–9 (1999). Colin, E. M. et al. 1, 25-dihydroxyvitamin D3 modulates TH17 polarization and interleukin‑22 expression by memory T cells from patients with early rheumatoid arthritis. Arthritis Rheum. 62, 132–142 (2010).


81. van Hamburg, J. P. et al. TH17 cells, but not TH1 cells, from patients with early rheumatoid arthritis are potent inducers of matrix metalloproteinases and proinflammatory cytokines upon synovial fibroblast interaction, including autocrine interleukin‑17A production. Arthritis Rheum. 63, 73–83 (2011). 82. Kochi, Y. et al. A regulatory variant in CCR6 is associated with rheumatoid arthritis susceptibility. Nat. Genet. 42, 515–519 (2010). 83. Yamada, H. et al. TH1 but not TH17 cells predominate in the joints of patients with rheumatoid arthritis. Ann. Rheum. Dis. 67, 1299–1304 (2008). 84. Zhang, L. et al. Elevated TH22 cells correlated with TH17 cells in patients with rheumatoid arthritis. J. Clin. Immunol. 31, 606–614 (2011). 85. Nistala, K. et al. Interleukin‑17‑producing T cells are enriched in the joints of children with arthritis, but have a reciprocal relationship to regulatory T cell numbers. Arthritis Rheum. 58, 875–887 (2008). 86. Gullick, N. J. et al. Enhanced and persistent levels of IL‑17+D4+ T cells and serum IL‑17 in patients with early inflammatory arthritis. Clin. Exp. Immunol. 174, 292–301 (2013). 87. Kim, J. et al. Elevated levels of T helper 17 cells are associated with disease activity in patients with rheumatoid arthritis. Ann. Lab. Med. 33, 52–59 (2013). 88. Arroyo-Villa, I. et al. Frequency of TH17 CD4+ T cells in early rheumatoid arthritis: a marker of anti-CCP seropositivity. PLoS ONE 7, e42189 (2012). 89. Miao, J. et al. Frequencies of circulating IL‑17‑producing CD4+CD161+ T cells and CD4+CD161+ T cells correlate with disease activity in rheumatoid arthritis. Mod. Rheumatol. 24, 265–570 (2014). 90. Acosta-Rodriguez, E. et al. Surface phenotype and antigenic specificity of human interleukin 17-producing T helper memory cells. Nat. Immunol. 8, 639–646 (2007). 91. Hirota, K. et al. Preferential recruitment of CCR6expressing TH17 cells to inflamed joints via CCL20 in rheumatoid arthritis and its animal model. J. Exp. Med. 204, 2803–2812 (2007). 92. Doodes, P. D. et al. Development of proteoglycaninduced arthritis is independent of IL‑17. J. Immunol. 181, 329–337 (2008). 93. Pöllinger, B. IL‑17 producing T cells in mouse models of multiple sclerosis and rheumatoid arthritis. J. Mol. Med. 90, 613–624 (2012). 94. Koenders, M. I. et al. Blocking of interleukin‑17 during reactivation of experimental arthritis prevents joint inflammation and bone erosion by decreasing RANKL and interleukin‑1. Am. J. Pathol. 167, 141–149 (2005). 95. Miossec, P. Interleukin‑17 in rheumatoid arthritis. If T cells were to contribute to inflammation and destruction through synergy. Arthritis Rheum. 48, 594–601 (2003). 96. Bombara, M. P. et al. Cell contact between T cells and synovial fibroblasts causes induction of adhesion molecules and cytokines. J. Leukoc. Biol. 54, 399–406 (1993). 97. Brennan, F. M. & McInnes, I. B. Evidence that cytokines play a role in rheumatoid arthritis. J. Clin. Invest. 118, 3537–3545 (2008). 98. McInnes, I. B., Leung, B. P. & Liew, F. Y. Cell–cell interactions in synovitis. Interactions between T lymphocytes and synovial cells. Arthritis Res. 2, 374–378 (2000). 99. Parsonage, G. et al. Prolonged, granulocytemacrophage colony-stimulating factordependent, neutrophil survival following rheumatoid synovial fibroblast activation by IL‑17 and TNFα. Arthritis Res. Ther. 10, R47 (2008).


REVIEWS 100. Tran, C. N. et al. Molecular interactions between T cells and fibroblast-like synoviocytes: role of membrane tumor necrosis factor‑α on cytokineactivated T cells. Am. J. Pathol. 171, 1588–1598 (2007). 101. Ogura, H. et al. Interleukin‑17 promotes autoimmunity by triggering a positive-feedback loop via interleukin‑6 induction. Immunity 29, 628–636 (2008). 102. Koenders, M. I. et al. Tumor necrosis factor‑interleukin‑17 interplay induces S100A8, interleukin‑1β, and matrix metalloproteinases, and drives irreversible cartilage destruction in murine arthritis: rationale for combination treatment during arthritis. Arthritis Rheum. 63, 2329–2339 (2011). 103. Zwerina, K. et al. Anti IL‑17A therapy inhibits bone loss in TNF‑α-mediated murine arthritis by modulation of the T‑cell balance. Eur. J. Immunol. 42, 413–423 (2012). 104. Alzabin, S. et al. Incomplete response of inflammatory arthritis to TNFα blockade is associated with the TH17 pathway. Ann. Rheum. Dis. 71, 1741–1748 (2012). 105. Eljaafari, A. et al. Bone marrow-derived and synovium-derived mesenchymal cells promote TH17 cell expansion and activation through caspase 1 activation: contribution to the chronicity of rheumatoid arthritis. Arthritis Rheum. 64, 2147–2157 (2012). 106. Paulissen, S. M. et al. Synovial fibroblasts directly induce TH17 pathogenicity via the cyclooxygenase/prostaglandin E2 pathway, independent of IL‑23. J. Immunol. 191, 1364–1372 (2013). 107. van Hamburg, J. P. et al. TNF blockade requires 1,25(OH)2D3 to control human TH17-mediated synovial inflammation. Ann. Rheum. Dis. 71, 606–612 (2012). 108. Chen, D. Y. et al. Increasing levels of circulating TH17 cells and interleukin‑17 in rheumatoid arthritis patients with an inadequate response to anti‑TNF‑α therapy. Arthritis Res. Ther. 13, R126 (2011). 109. Notley, C. A. et al. Blockade of tumor necrosis factor in collagen-induced arthritis reveals a novel immunoregulatory pathway for TH1 and TH17 cells. J. Exp. Med. 205, 2491–2497 (2008). 110. Aerts, N. E. et al. Increased IL‑17 production by peripheral T helper cells after tumour necrosis factor blockade in rheumatoid arthritis is accompanied by inhibition of migrationassociated chemokine receptor expression. Rheumatology 49, 2264–2272 (2010). 111. Kawashiri, S. Y. et al. Proinflammatory cytokines synergistically enhance the production of chemokine ligand 20 (CCL20) from rheumatoid fibroblast-like synovial cells in vitro and serum CCL20 is reduced in vivo by biologic diseasemodifying antirheumatic drugs. J. Rheumatol. 36, 2397–2402 (2009). 112. Evans, H. G. et al. TNF‑α blockade induces IL‑10 expression in human CD4+ T cells. Nat. Commun. 5, 3199 (2014). 113. Zielinski, C. E. et al. Pathogen-induced human TH17 cells produce IFN‑γ or IL‑10 and are regulated by IL‑1β. Nature 484, 514–518 (2012). 114. Genovese, M. C. et al. A phase II randomized study of subcutaneous ixekizumab, an anti‑interleukin‑17 monoclonal antibody, in rheumatoid arthritis patients who were naive to biologic agents or had an inadequate response to tumor necrosis factor inhibitors. Arthritis Rheum. 66, 1693–1704 (2014). 115. van den Berg, W. B. & Miossec, P. IL‑17 as a future therapeutic target for rheumatoid arthritis. Nat. Rev. Rheumatol. 5, 549–553 (2009).

116. Pieper, J. et al. CTLA4-Ig (abatacept) therapy modulates T cell effector functions in autoantibody-positive rheumatoid arthritis patients. BMC Immunol. 14, 34 (2013). 117. Scarsi, M. et al. Reduction of peripheral blood T cells producing IFN‑γ and IL‑17 after therapy with abatacept for rheumatoid arthritis. Clin. Exp. Rheumatol. 32, 204–210 (2014). 118. Jain, M. et al. Increased plasma IL‑17F levels in rheumatoid arthritis patients are responsive to methotrexate, anti-TNF, and T cell costimulatory modulation. Inflammation 38, 180–186 (2015). 119. Samson, M. et al. Brief report: inhibition of interleukin‑6 function corrects TH17/TREG cell imbalance in patients with rheumatoid arthritis. Arthritis Rheum. 64, 2499–2503 (2012). 120. Pesce, B. et al. Effect of interleukin‑6 receptor blockade on the balance between regulatory T cells and T helper type 17 cells in rheumatoid arthritis patients. Clin. Exp. Immunol. 171, 237–242 (2013). 121. Hueber, W. et al. Effects of AIN457, a fully human antibody to interleukin‑17A, on psoriasis, rheumatoid arthritis, and uveitis. Sci. Transl. Med. 2, 52ra72 (2010). 122. Genovese, M. C. et al. LY2439821, a humanized anti‑interleukin‑17 monoclonal antibody, in the treatment of patients with rheumatoid arthritis. Arthritis Rheum. 62, 929– 939 (2010). 123. Martin, D. A. et al. A phase 1b multiple ascending dose study evaluating safety, pharmacokinetics, and early clinical response of brodalumab, a human anti‑IL‑17R antibody, in methotrexate-resistant rheumatoid arthritis. Arthritis Res. Ther. 15, R164 (2013). 124. Genovese, M. C. et al. Efficacy and safety of secukinumab in patients with rheumatoid arthritis: a phase II, dose-finding, double-blind, randomised, placebo controlled study. Ann. Rheum. Dis. 72, 863–869 (2013). 125. Genovese, M. C. et al. One-year efficacy and safety results of secukinumab in patients with rheumatoid arthritis: a phase II, dose-finding, double-blind, randomized, placebo-controlled study. J. Rheumatol. 41, 414–421 (2014). 126. Ye, P. et al. Requirement of interleukin‑17 receptor signalling for lung CXC chemokine and granulocyte colony-stimulating factor expression, neutrophil recruitment, and host defence. J. Exp. Med. 194, 519–527 (2001). 127. Smith, E. et al. IL‑17A inhibits the expansion of IL‑17A‑producing T cells in mice through “shortloop” inhibition via IL‑17 receptor. J. Immunol. 181, 1357–1364 (2008). 128. Mease, P. J. et al. Brodalumab, an anti‑IL‑17RA monoclonal antibody, in psoriatic arthritis. N. Engl. J. Med. 370, 2295–2306 (2014). 129. Papp, K. A. et al. Brodalumab, an anti‑IL‑17receptor antibody for psoriasis. N. Engl. J. Med. 366, 1181–1189 (2012). 130. Sherlock, J. P., Buckley, C. D. & Cua, D. J. The critical role of interleukin‑23 in spondyloarthropathy. Mol. Immunol. 57, 38–43 (2014). 131. Hreggvidsdottir, H. S., Noordenbos, T. & Baeten, D. L. Inflammatory pathways in spondyloarthritis. Mol. Immunol. 57, 28–37 (2014). 132. Noordenbos, T. et al. Interleukin‑17‑positive mast cells contribute to synovial inflammation in spondylarthritis. Arthritis Rheum. 64, 99–109 (2012). 133. Lories, R. J. & Baeten, D. L. Differences in pathophysiology between rheumatoid arthritis and ankylosing spondylitis. Clin. Exp. Rheumatol. 27, S10–14 (2009).


134. Rueda, B. et al. The IL23R Arg381Gln non‑synonymous polymorphism confers susceptibility to ankylosing spondylitis. Ann. Rheum. Dis. 67, 1451–1454 (2008). 135. Reveille, J. D. et al. Genome-wide association study of ankylosing spondylitis identifies nonMHC susceptibility loci. Nat. Genet. 42, 123– 127 (2010). 136. Coffre, M. et al. Combinatorial control of TH17 and TH1 cell function by genetic variation at genes associated with the IL‑23 signaling pathway in spondyloarthritis. Arthritis Rheum. 65, 1510–1521 (2013). 137. Singh, R., Aggarwal, A. & Misra, R. TH1/TH17 cytokine profiles in patients with reactive arthritis/undifferentiated spondyloarthropathy. J. Rheumatol. 34, 2285–2290 (2007). 138. Jandus, C. et al. Increased numbers of circulating polyfunctional TH17 memory cells in patients with seronegative spondylarthritides. Arthritis Rheum. 58, 2307–2317 (2008). 139. Shen, H., Goodall, J. C. & Hill Gaston, J. S. Frequency and phenotype of peripheral blood TH17 cells in ankylosing spondylitis and rheumatoid arthritis. Arthritis Rheum. 60, 1647–1656 (2009). 140. Zhang, L. et al. Increased frequencies of TH22 cells as well as TH17 cells in the peripheral blood of patients with ankylosing spondylitis and rheumatoid arthritis. PLoS ONE 7, e31000 (2012). 141. Melis, L. et al. Systemic levels of IL‑23 are strongly associated with disease activity in rheumatoid arthritis but not spondyloarthritis. Ann. Rheum. Dis. 69, 618–623 (2010). 142. Appel, H. et al. Analysis of IL‑17+ cells in facet joints of patients with spondyloarthritis suggests that the innate immune pathway might be of greater relevance than the TH17-mediated adaptive immune response. Arthritis Res. Ther. 13, R95 (2011). 143. Yeremenko, N. & Baeten, D. IL‑17 in spondyloarthritis: is the T‑party over? Arthritis Res. Ther. 13, 115 (2011). 144. Kenna, T. J. & Brown, M. A. The role of IL‑17‑secreting mast cells in inflammatory joint disease. Nat. Rev. Rheum. 9, 375–379 (2013). 145. Ambarus, C., Yeremenko, N., Tak, P. P. & Baeten, D. Pathogenesis of spondyloarthritis: autoimmune or autoinflammatory? Curr. Opin. Rheumatol. 24, 351–358 (2012). 146. Ciccia, F. et al. Overexpression of interleukin‑23, but not interleukin‑17, as an immunologic signature of subclinical intestinal inflammation in ankylosing spondylitis. Arthritis Rheum. 60, 955–965 (2009). 147. Takahashi, N. et al. IL‑17 produced by Paneth cells drives TNF-induced shock. J. Exp. Med. 205, 1755–1761 (2008). 148. Sherlock, J. P. et al. IL‑23 induces spondyloarthropathy by acting on ROR-γt+ CD3+CD4‑CD8‑ entheseal resident T cells. Nat. Med. 18, 1069–1076 (2012). 149. Ruutu, M. et al. β‑glucan triggers spondylarthritis and Crohn’s disease-like ileitis in SKG mice. Arthritis Rheum. 64, 2211–2222 (2012). 150. Benham, H. et al. IL‑23‑mediates the intestinal response to microbial β‑glucan and the development of spondyloarthritis pathology in SKG mice. Arthritis Rheumatol. 66, 1755–1767 (2014). 151. Bowes, J. et al. Confirmation of TNIP1 and IL23A as susceptibility loci for psoriatic arthritis. Ann. Rheum. Dis. 70, 1641–1644 (2011). 152. Filer, C. et al. Investigation of association of the IL12B and IL23R genes with psoriatic arthritis. Arthritis Rheum. 58, 3705–3709 (2008).


REVIEWS 153. Hüffmeier, U. et al. Common variants at TRAF3IP2 are associated with susceptibility to psoriatic arthritis and psoriasis. Nat. Genet. 42, 996–999 (2010). 154. Raychaudhuri, S. P., Raychaudhuri, S. K. & Genovese, M. C. IL‑17 receptor and its functional significance in psoriatic arthritis. Mol. Cell. Biochem. 359, 419–429 (2012). 155. Mrabet, D. et al. Synovial fluid and serum levels of IL‑17, IL‑23, and CCL‑20 in rheumatoid arthritis and psoriatic arthritis: a Tunisian crosssectional study. Rheumatol. Int. 33, 265–266 (2013). 156. Tonel, G. et al. Cutting edge: A critical functional role for IL‑23 in psoriasis. J. Immunol. 185, 5688–5691 (2010). 157. Wilson, N. J. et al. Development, cytokine profile and function of human interleukin 17-producing helper T cells. Nat. Immunol. 8, 950–957 (2007). 158. Chan, J. R. et al. IL‑23 stimulates epidermal hyperplasia via TNF and IL‑20R2‑dependent mechanisms with implications for psoriasis pathogenesis. J. Exp. Med. 203, 2577–2587 (2006). 159. van der Fits, L. et al. Imiquimod-induced psoriasis-like skin inflammation in mice is mediated via the IL‑23/IL‑17 axis. J. Immunol. 182, 5836–5845 (2009). 160. Rizzo, H. L. et al. IL‑23‑mediated psoriasis-like epidermal hyperplasia is dependent on IL‑17A. J. Immunol. 186, 1495–1502 (2011). 161. Cai, Y. et al. Pivotal role of dermal IL‑17‑producing γδ T cells in skin inflammation. Immunity 35, 596–610 (2011). 162. Benham, H. et al. TH17 and TH22 cells in psoriatic arthritis and psoriasis. Arthritis Res. Ther. 15, R136 (2013). 163. Costello, P., Bresnihan, B., O’Farrelly, C. & FitzGerald, O. Predominance of CD8+ T lymphocytes in psoriatic arthritis. J. Rheumatol. 26, 1117 (1999). 164. Menon, B. et al. Interleukin‑17+CD8+ T cells are enriched in the joints of patients with psoriatic arthritis and correlate with disease activity and joint damage progression. Arthritis Rheumatol. 66, 1272–1281(2014). 165. Ory, P. A., Gladman, D. D. & Mease, P. J. Psoriatic arthritis and imaging. Ann. Rheum. Dis. 64 (Suppl. 2), ii14–17 (2005). 166. Novelli, L., Chimenti, M. S., Chiricozzi, A. & Perricone, R. The new era for the treatment of psoriasis and psoriatic arthritis: perspectives and validated strategies. Autoimmun. Rev. 13, 64–69 (2014). 167. Suzuki, E., Mellins, E. D., Gershwin, M. E., Nestle, F. O. & Adamopoulos, I. E. The IL‑23/ IL‑17 axis in psoriatic arthritis. Autoimmun. Rev. 13, 496–502 (2014). 168. Gaffen, S. L., Jain, R., Garg, A. V., Cua, D. J. The IL‑23‑IL‑17 immune axis: from mechanisms to therapeutic testing. Nat. Rev. Immunol. 14, 585–600 (2014).

169. Leonardi, C. et al. Anti-interleukin‑17 monoclonal antibody ixekizumab in chronic plaque psoriasis. N. Engl. J. Med. 366, 1190–1199 (2012). 170. Rich, P. et al. Secukinumab induction and maintenance therapy in moderate-to‑severe plaque psoriasis: a randomized, double-blind, placebo-controlled, phase II regimen-finding study. Br. J. Dermatol. 168, 402–411 (2013). 171. Papp, K. A. et al. Dose-dependent improvement in chronic plaque psoriasis following treatment with anti‑IL‑23p19 humanized monoclonal antibody (MK‑3222) [abstract]. Presented at the American Academy of Dermatology 71st Annual Meeting (2013). 172. Chiricozzi, A. & Krueger, J. G. IL‑17 targeted therapies for psoriasis. Expert Opin. Investig. Drugs 22, 993–1005 (2013). 173. Kirkham, B. W., Kavanaugh, A. & Reich, K. Interleukin‑17A: a unique pathway in immunemediated diseases: psoriasis, psoriatic arthritis and rheumatoid arthritis. Immunology 141, 133–142 (2014). 174. Schett, G., Elewaut, D., McInnes, I. B., Dayer, J. M. & Neurath, M. F. How cytokine networks fuel inflammation: toward a cytokine-based disease taxonomy. Nat. Med. 19, 822–824 (2013). 175. Paulissen, S. M. et al. Higher proportions of pathogenic CCR6+ T cell subpopulations in ACPA+ compared to ACPA– patients with early rheumatoid arthritis. Ann. Rheum. Dis. 73 (Suppl. 1), A33–A34 (2014). 176. Abbas, A. K., Murphy, K. M., Sher, A. Functional diversity of helper T lymphocytes. Nature 383, 787–793 (1996). 177. Szabo, S. J. et al. A novel transcription factor, T‑bet, directs TH1 lineage commitment. Cell 100, 655–669 (2000). 178. Szabo, S. J., Sullivan, B. M., Peng, S. L., Glimcher, L. H. Molecular mechanisms regulating TH1 immune responses. Annu. Rev. Immunol. 21, 713–758 (2003). 179. Kuchroo, V. K. et al. T cell response in experimental autoimmune encephalomyelitis (EAE): role of self and cross-reactive antigens in shaping, tuning, and regulating the autopathogenic T cell repertoire. Annu. Rev. Immunol. 20, 101–123 (2002). 180. Zheng, W., Flavell, R. A. The transcription factor GATA‑3 is necessary and sufficient for TH2 cytokine gene expression in CD4 T cells. Cell 89, 587–596 (1997). 181. Mosmann, T. R., Coffman, R. L. TH1 and TH2 cells: different patterns of lymphokine secretion lead to different functional properties. Annu. Rev. Immunol. 7, 145–173 (1989). 182. Bettelli, E. et al. Reciprocal developmental pathways for the generation of pathogenic effector TH17 and regulatory T cells. Nature 441, 235–238 (2006). 183. Mangan, P. R. et al. Transforming growth factor-β induces development of the TH17 lineage. Nature 441, 231–234 (2006).


184. Veldhoen, M. et al. TGFβ in the context of an inflammatory cytokine milieu supports de novo differentiation of IL‑17‑producing T cells. Immunity 24, 179–189 (2006). 185. Ivanov, I. I. et al. The orphan nuclear receptor RORγt directs the differentiation program of proinflammatory IL‑17+ T helper cells. Cell 126, 1121–1133 (2006). 186. Bettelli, E., Korn, T., Oukka, M., Kuchroo, V. K. Induction and effector functions of TH17 cells. Nature 453, 1051–1057 (2008). 187. Kanno, Y., Vahedi, G., Hirahara, K., Singleton, K. & O’Shea, J. J. Transcriptional and epigenetic control of T helper cell specification: molecular mechanisms underlying commitment and plasticity. Annu. Rev. Immunol. 30, 707–731 (2012). 188. Radhakrishnan, S. et al. Reprogrammed FOXP3+ T regulatory cells become IL‑17+ antigen-specific autoimmune effectors in vitro and in vivo. J. Immunol. 181, 3137–3147 (2008). 189. Xu, L., Kitani, A., Fuss, I. & Strober, W. Cutting edge: regulatory T cells induce CD4+CD25‑Foxp3‑ T cells or are self-induced to become TH17 cells in the absence of exogenous TGF‑β. J. Immunol. 178, 6725–6729 (2007). 190. Yang, X. O. et al. T helper 17 lineage differentiation is programmed by orphan nuclear receptors ROR α and ROR γ. Immunity 28, 29–39 (2008). 191. Koenen, H. J. et al. Human CD25highFoxppos regulatory T cells differentiate into IL‑17‑producing cells. Blood 112, 2340–2352 (2008). 192. Komatsu, N. et al. Pathogenic conversion of Foxp3+ T cells into TH17 cells in autoimmune arthritis. Nat. Med. 20, 62–68 (2014). 193. Lee, Y. K. et al. Late developmental plasticity in the T helper 17 lineage. Immunity 30, 92–107 (2009). 194. Lexberg, M. H. et al. TH memory for interleukin‑17 expression is stable in vivo. Eur. J. Immunol. 38, 2654–2664 (2008). 195. Martin-Orozco, N., Chung, Y., Chang, S. H., Wang, Y. H. & Dong, C. TH17 cells promote pancreatic inflammation but only induce diabetes efficiently in lymphopenic hosts after conversion into TH1 cells. Eur. J. Immunol. 39, 216–224 (2009). 196. Wei, G. et al. Global mapping of H3K4me3 and H3K27me3 reveals specificity and plasticity in lineage fate determination of differentiating CD4+ T cells. Immunity 30, 155–167 (2009). Acknowledgements The author acknowledges team members (past and present) of the Lubberts Lab and the collaborating clinicians and researchers for scientific interactions and helpful discussions regarding this topic, as well as the Dutch Arthritis Association for financial grant support in different projects related to this topic in the past 10 years.


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