From www.bloodjournal.org by guest on September 11, 2016. For personal use only.
associated with reduced infiltration by neutrophils and macrophages and reduced levels of proinflammatory cytokines, such as interleukin-6 (IL-6), IL-1b, and tumor necrosis factor a after CIA. Consistent with the role of fXIII in stabilizing fibrin matrices, fXIIIA2/2 mice had diffused and limited fibrin deposition in the joint after CIA. These findings provide compelling evidence that the proinflammatory effects of fXIII in rheumatoid arthritis are fibrindependent and suggest that fibrin cross-linking in vivo is required for the activation of the inflammatory response (see figure). In a second set of experiments, the group made an unanticipated discovery for a fibrin-independent function of fXIII in the joint, leading to direct damage to the cartilage and bone. Surprisingly, knee joints of CIA-challenged fXIIIA2/2 mice had dramatically reduced numbers of osteoclasts, which are responsible for cartilage and bone resorption (see figure). Intriguingly, osteoclast reduction associated with reduced levels of osteoclast regulators, such as receptor activator of nuclear factor kB ligand (RANKL) and osteoprotegerin, was uniquely associated with fXIII deficiency because genetic loss of fibrinogen did not impact the osteoclast numbers after CIA. Osteoclast maturation of CD11b1 progenitor cells derived from fXIIIA2/2 mice showed defects in osteoclast differentiation in vitro. Fibrininduced stimulation of CD11b1 cells induces robust activation of inflammatory and oxidative stress pathways.7,8 Although the effects of fibrin on osteoclastogenesis in vitro were not tested in the study, the findings strongly support a fibrin-independent role of fXIII in bone destruction in the inflamed joint (see figure). This exciting study raises several important questions. Does fXIII play a role in other autoimmune models of inflammation and tissue destruction? Although it is likely that fXIII regulates inflammation via stabilizing fibrin or other substrates in other disease settings, it is unknown whether its effects in tissue destruction are uniquely linked to osteoclast differentiation in the joint. Although this study supports prior findings demonstrating a critical role for coagulation in the activation of innate immunity, the role of fibrin and the coagulation cascade in the regulation of adaptive immunity remains largely unknown. Depletion of fXIII in CIA does not reduce the numbers of T and B cells in peripheral immune organs.1
420
However, it remains unknown whether fXIII-mediated, fibrin-induced activation of resident innate immune cells regulates local T-cell activation in tissues. Finally, this study, together with compelling evidence for the contribution of coagulation in rheumatoid arthritis patients,3 not only opens new avenues for repurposing anticoagulant therapy to limit both inflammation and bone erosion but also fuels enthusiasm for the development of new drugs to selectively target components of the coagulation cascade in autoimmune diseases. The study by Raghu et al1 adds fXIII as a key player in rheumatoid arthritis with a dual role in inflammation and bone destruction. This exciting new finding has the potential to launch a new line of research to determine the pleiotropic effects of fXIII in inflammatory induced tissue damage and develop novel therapeutic strategies for autoimmune and inflammatory diseases. Conflict-of-interest disclosure: The author declares no competing financial interests. n
REFERENCES 1. Raghu H, Cruz C, Rewerts CL, et al. Transglutaminase factor XIII promotes arthritis through mechanisms linked to inflammation and bone erosion. Blood. 2015;125(3):427-437. 2. Davalos D, Akassoglou K. Fibrinogen as a key regulator of inflammation in disease. Semin Immunopathol. 2012;34(1):43-62. 3. Hoppe B, D¨orner T. Coagulation and the fibrin network in rheumatic disease: a role beyond haemostasis. Nat Rev Rheumatol. 2012;8(12):738-746. 4. Flick MJ, LaJeunesse CM, Talmage KE, et al. Fibrin(ogen) exacerbates inflammatory joint disease through a mechanism linked to the integrin aMb2 binding motif. J Clin Invest. 2007;117(11):3224-3235. 5. Busso N, P´eclat V, Van Ness K, et al. Exacerbation of antigen-induced arthritis in urokinase-deficient mice. J Clin Invest. 1998;102(1):41-50. 6. Flick MJ, Chauhan AK, Frederick M, et al. The development of inflammatory joint disease is attenuated in mice expressing the anticoagulant prothrombin mutant W215A/E217A. Blood. 2011;117(23):6326-6337. 7. Davalos D, Ryu JK, Merlini M, et al. Fibrinogeninduced perivascular microglial clustering is required for the development of axonal damage in neuroinflammation. Nat Commun. 2012;3:1227. 8. Adams RA, Bauer J, Flick MJ, et al. The fibrin-derived gamma377-395 peptide inhibits microglia activation and suppresses relapsing paralysis in central nervous system autoimmune disease. J Exp Med. 2007;204(3):571-582. © 2015 by The American Society of Hematology
l l l IMMUNOBIOLOGY
Comment on Milne et al, page 470
Langerhans cells: straight from blood to skin? ----------------------------------------------------------------------------------------------------Nikolaus Romani1 and James W. Young2 CANCER CENTER
1
MEDICAL UNIVERSITY OF INNSBRUCK; 2MEMORIAL SLOAN KETTERING
In this issue of Blood, Milne et al1 report a developmental pathway for human Langerhans cells from circulating CD142CD1c1 dendritic cell (DC) precursors in peripheral blood. The Langerhans cell progeny, expressing CD207 (langerin) and bearing the hallmark Birbeck granules, result from culture in granulocyte macrophage colony-stimulating factor (GM-CSF) and transforming growth factor-b1 (TGF-b1) and/or bone morphogenetic protein 7 (BMP7). A similar study by Mart´ınez-Cingolani et al,2 also published recently in Blood, identified the same circulating CD142CD1c1 Langerhans cell precursors. These authors used a different anti-CD1c monoclonal antibody (BDCA-1). In that study, TGF-b1 synergized with thymic stromal lymphopoietin (TSLP) to yield similar Langerhans cell progeny. Both groups used fetal calf serum (FCS), however, which affects differentiation and maturation by unknown mechanisms and is not physiologically comparable to human plasma in vivo. Controls with FCS-containing medium, but without cytokines, indicated that FCS was not the driver of the conversion. However, Milne et al1 could not generate Langerhans cell progeny by culture in X-Vivo medium without FCS, despite inclusion of GM-CSF, TGF-b1, and BMP7, all of which, together with TSLP, are epithelial cell–derived cytokines under physiologic conditions. BLOOD, 15 JANUARY 2015 x VOLUME 125, NUMBER 3
From www.bloodjournal.org by guest on September 11, 2016. For personal use only.
R
esidence in stratified epithelia, CD207 expression, and Birbeck granules all define bona fide Langerhans cells in humans (see figure). Indeed, the progeny of CD1c1 blood DCs express substantial CD207 after culture with either GM-CSF/TGF-b1/ BMP71 or TGF-b1/TSLP.2 Birbeck granule morphology by electron microscopy is also convincing. Neither group evaluated the capacity to populate the epidermis, for which several methods exist using artificial skin equivalents, even though their Langerhans cell progeny express a number of skin-homing receptors like CCR6,2 EpCAM,1 or E-cadherin.1 Mouse studies have shown that Langerhans cell precursors only enter the epidermis before they express CD207, however. Hence the CD207expressing CD1c1 progeny reported by these authors may already be too developed for such homing. The work by Milne et al1 addresses another important issue. So-called monocytederived Langerhans cells, first reported by Geissmann et al,3 used negatively selected rather than positively selected precursors, which would have included the CD1c1 blood DC precursors now described by both groups. In fact, Milne et al1 demonstrate that positively selected CD141 or CD161 blood monocytes cultured under comparable conditions to CD1c1 DCs yield progeny with far less expression of CD1a, CD207, or Birbeck granules compared with Langerhans
cell progeny derived from CD1c1 DCs. These data are also useful in explaining how a CD142 monocyte, intermediate between CD341 hematopoietic progenitor cells and the final Langerhans cell progeny, can yield Langerhans cells. This avoids invoking a direct precursor–progeny connection with macrophages, as suggested by prior reports of CSF1 receptor (macrophage colonystimulating factor receptor)-positive monocytic Langerhans cell precursors in mice, where the relevant cytokine proved to be interleukin-34 binding to CSF1-receptor, not macrophage colony-stimulating factor.4 Langerhans cells function as prototypic DCs, both in the induction of immunity5,6 and in the maintenance of tolerance.7 Murine Langerhans cells share a common embryonal origin from fetal liver and yolk sac with macrophages, microglia, and Kupffer cells.8 Human Langerhans cells were observed in human fetal epidermis as early as 9 weeks’ estimated gestational age.9 This is before the beginning of hematopoiesis in the bone marrow at 10.5 weeks’ estimated gestational age, suggesting that human Langerhans cells also derive from fetal hematopoietic tissues. Langerhans cells are very long-lived and maintain their steadystate population density (approximately 600/mm2) in the epidermis by low-level proliferation from local precursors. In inflamed mouse skin, however, Langerhans cell replenishment of those
Langerin expression and Birbeck granules are hallmarks of Langerhans cells. (A) Conventional fluorescence microscopy of an epidermal sheet prepared from healthy human skin. Langerhans cells are immunolabeled with anti-CD207/langerin monoclonal antibody (Miltenyi Biotec). Scale bar, 30 mm. (B) Detail from a transmission electron micrograph of a human epidermal Langerhans cell in situ featuring 2 tennis racket–shaped Birbeck granules. Scale bar, 60 nm.
BLOOD, 15 JANUARY 2015 x VOLUME 125, NUMBER 3
that have migrated from the epidermis to draining lymph nodes occurs in 2 phases. Blood monocyte precursors provide only the first wave, whereas hematopoietic precursors comprise the second, more durable repletion of the long-lived local precursors and their Langerhans cell progeny.10 It remains elusive, however, under which circumstances CD1c1 DCs would differentiate in vivo into epidermal Langerhans cells in humans. It seems unlikely that they contribute to the initial seeding of human epidermis with Langerhans cells. Because they do not express CD14, it also seems unlikely that they correspond to the monocytic precursors in the first phase of Langerhans cell repopulation in inflamed murine epidermis.10 In contrast, the cytokines supporting human CD1c1 DC conversion into Langerhans cells are all epithelial cell derived, and the Langerhans cell progeny express skin-homing receptors. CD1c1 DCs are therefore plausible candidate precursors responsible for the second phase of epidermal repopulation by Langerhans cells after an inflammatory insult. The identification of a phenotypically defined Langerhans cell precursor in blood could provide a more exportable methodology for generating this highly potent DC subtype in vitro, rather than the current standard methodology of starting with CD341 hematopoietic progenitor cells.5 To this end, however, more rigorous sideby-side functional rather than phenotypic comparisons are required between different candidate Langerhans cells. Although technically and logistically demanding, Langerhans cells isolated from the epidermis will remain the “gold standard.” Human Langerhans cells are the most potent DC subtype for the stimulation of CTLs, which they accomplish without interleukin-12p705; human Langerhans cells are the most potent cross-presenting DC subtype.5,6 In this respect, the 2 studies discussed here present investigators with some unresolved paradoxes. Mart´ınez-Cingolani et al2 describe a Th2 profile stimulated by their Langerhans cells progeny, which would not support potent CD81 CTL generation. Moreover, because CD1411 (BDCA-31) and CD1c1 (BDCA11) mark mutually exclusive DC subsets, in which CD1411 (BDCA31) DCs are the most potent crosspresenting DCs, then both Milne et al1
421
From www.bloodjournal.org by guest on September 11, 2016. For personal use only.
and Mart´ınez-Cingolani et al2 present investigators with another ambiguity. Absent functional data, it remains unclear how they would reconcile the BDCA-11/CD1c1 (BDCA-32/CD1412) profile of the DC precursors they describe with the resultant Langerhans cell progeny that should have potent cross-presenting capacity, nominally associated with the converse CD1411 (BDCA31) phenotype. Detailed functional data will again therefore prove essential in extending these groups’ very compelling and novel findings. Conflict-of-interest disclosure: The authors declare no competing financial interests. n REFERENCES 1. Milne P, Bigley V, Gunawan M, Haniffa M, Collin M. CD1c1 blood dendritic cells have Langerhans cell potential. Blood. 2015;125(3):470-473. 2. Mart´ınez-Cingolani C, Grandclaudon M, Jeanmougin M, Jouve M, Zollinger R, Soumelis V. Human blood BDCA-1 dendritic cells differentiate into Langerhans-like cells with thymic stromal lymphopoietin and TGF-b. Blood. 2014;124(15):2411-2420. 3. Geissmann F, Prost C, Monnet JP, Dy M, Brousse N, Hermine O. Transforming growth factor beta1, in the presence of granulocyte/macrophage colony-stimulating factor and interleukin 4, induces differentiation of human peripheral blood monocytes into dendritic Langerhans cells. J Exp Med. 1998;187(6):961-966. 4. Wang Y, Szretter KJ, Vermi W, et al. IL-34 is a tissue-restricted ligand of CSF1R required for the development of Langerhans cells and microglia. Nat Immunol. 2012;13(8):753-760. 5. Ratzinger G, Baggers J, de Cos MA, et al. Mature human Langerhans cells derived from CD341 hematopoietic progenitors stimulate greater cytolytic T lymphocyte activity in the absence of bioactive IL-12p70, by either single peptide presentation or cross-priming, than do dermal-interstitial or monocytederived dendritic cells. J Immunol. 2004;173(4): 2780-2791.
l l l MYELOID NEOPLASIA
Comment on Gambacorti-Passerini et al, page 499
Mutations of ETNK1 in aCML and CMML ----------------------------------------------------------------------------------------------------Olivier Kosmider
PARIS DESCARTES UNIVERSITY
In this issue of Blood, Gambacorti-Passerini et al describe, for the first time, mutations of an ethanolamine kinase called ETNK1, which is physiologically involved in the first step of the phosphatidylethanolamine biosynthesis pathway.1 The authors describe 2 recurrent point mutations of ETNK1 in 9% of atypical chronic myeloid leukemias (aCMLs) and show that these mutations are associated with the impaired catalytic activity of the kinase.
A
typical CML is an entity defined in the World Health Organization 2008 classification that belongs to the group of myeloproliferative/myelodysplastic disorders. Since 2013, next-generation sequencing approaches conducted on aCML patient samples have described recurrent mutations in CSF3R and SETBP1, genes whose mutations are able to activate cellular proliferation. Long known in oncogenesis, the activating mutations of the granulocyte colony-stimulating factor receptor, also called CSF3R, were first described in 25% to 50% of aCML cases as well as in 3% of chronic myelomonocytic leukemia (CMML) cases in 2013.2 In aCML, a hot spot affecting codon 618 is frequently affected and could be a therapeutic target in
response either to Src kinase or to Janus kinase inhibitors.2,3 Initially described in more than 90% of patients with Schinzel-Giedion syndrome,4 SETBP1 mutations were identified in 24.3% of aCML samples tested by Piazza et al.5 At the same time, SETBP1 mutations have been identified in other hematological malignancies, specifically in 4% to 14% of CMMLs, demonstrating one more time that the mutational spectrum between aCML and CMML is very close. SETBP1 mutations were characterized as conferring a proliferative advantage to mutated cells5-7 and as closely related to a negative effect on overall survival in patients with aCML. As suggested by Makishima et al,6 the presence of SETBP1 mutations could allow the use of transforming
6. Klechevsky E, Morita R, Liu M, et al. Functional specializations of human epidermal Langerhans cells and CD141 dermal dendritic cells. Immunity. 2008;29(3): 497-510. 7. Seneschal J, Clark RA, Gehad A, Baecher-Allan CM, Kupper TS. Human epidermal Langerhans cells maintain immune homeostasis in skin by activating skin resident regulatory T cells. Immunity. 2012;36(5): 873-884. 8. Guilliams M, Ginhoux F, Jakubzick C, et al. Dendritic cells, monocytes and macrophages: a unified nomenclature based on ontogeny. Nat Rev Immunol. 2014; 14(8):571-578. 9. Schuster C, Vaculik C, Fiala C, et al. HLA-DR1 leukocytes acquire CD1 antigens in embryonic and fetal human skin and contain functional antigen-presenting cells. J Exp Med. 2009;206(1):169-181. 10. Ser´e K, Baek JH, Ober-Bl¨obaum J, et al. Two distinct types of Langerhans cells populate the skin during steady state and inflammation. Immunity. 2012; 37(5):905-916. © 2015 by The American Society of Hematology
422
Codons of ETNK1 affected by mutations are indicated by the red and blue circles in aCML and CMML, respectively. See Figure 1A in the article by Gambacorti-Passerini et al that begins on page 499.
BLOOD, 15 JANUARY 2015 x VOLUME 125, NUMBER 3
From www.bloodjournal.org by guest on September 11, 2016. For personal use only.
2015 125: 420-422 doi:10.1182/blood-2014-11-610667
Langerhans cells: straight from blood to skin? Nikolaus Romani and James W. Young
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associated with reduced infiltration by neutrophils and macrophages and reduced levels of proinflammatory cytokines, such as interleukin-6 (IL-6), IL-1b, and tumor necrosis factor a after CIA. Consistent with the role of fXIII in stabilizing fibrin matrices, fXIIIA2/2 mice had diffused and limited fibrin deposition in the joint after CIA. These findings provide compelling evidence that the proinflammatory effects of fXIII in rheumatoid arthritis are fibrindependent and suggest that fibrin cross-linking in vivo is required for the activation of the inflammatory response (see figure). In a second set of experiments, the group made an unanticipated discovery for a fibrin-independent function of fXIII in the joint, leading to direct damage to the cartilage and bone. Surprisingly, knee joints of CIA-challenged fXIIIA2/2 mice had dramatically reduced numbers of osteoclasts, which are responsible for cartilage and bone resorption (see figure). Intriguingly, osteoclast reduction associated with reduced levels of osteoclast regulators, such as receptor activator of nuclear factor kB ligand (RANKL) and osteoprotegerin, was uniquely associated with fXIII deficiency because genetic loss of fibrinogen did not impact the osteoclast numbers after CIA. Osteoclast maturation of CD11b1 progenitor cells derived from fXIIIA2/2 mice showed defects in osteoclast differentiation in vitro. Fibrininduced stimulation of CD11b1 cells induces robust activation of inflammatory and oxidative stress pathways.7,8 Although the effects of fibrin on osteoclastogenesis in vitro were not tested in the study, the findings strongly support a fibrin-independent role of fXIII in bone destruction in the inflamed joint (see figure). This exciting study raises several important questions. Does fXIII play a role in other autoimmune models of inflammation and tissue destruction? Although it is likely that fXIII regulates inflammation via stabilizing fibrin or other substrates in other disease settings, it is unknown whether its effects in tissue destruction are uniquely linked to osteoclast differentiation in the joint. Although this study supports prior findings demonstrating a critical role for coagulation in the activation of innate immunity, the role of fibrin and the coagulation cascade in the regulation of adaptive immunity remains largely unknown. Depletion of fXIII in CIA does not reduce the numbers of T and B cells in peripheral immune organs.1
420
However, it remains unknown whether fXIII-mediated, fibrin-induced activation of resident innate immune cells regulates local T-cell activation in tissues. Finally, this study, together with compelling evidence for the contribution of coagulation in rheumatoid arthritis patients,3 not only opens new avenues for repurposing anticoagulant therapy to limit both inflammation and bone erosion but also fuels enthusiasm for the development of new drugs to selectively target components of the coagulation cascade in autoimmune diseases. The study by Raghu et al1 adds fXIII as a key player in rheumatoid arthritis with a dual role in inflammation and bone destruction. This exciting new finding has the potential to launch a new line of research to determine the pleiotropic effects of fXIII in inflammatory induced tissue damage and develop novel therapeutic strategies for autoimmune and inflammatory diseases. Conflict-of-interest disclosure: The author declares no competing financial interests. n
REFERENCES 1. Raghu H, Cruz C, Rewerts CL, et al. Transglutaminase factor XIII promotes arthritis through mechanisms linked to inflammation and bone erosion. Blood. 2015;125(3):427-437. 2. Davalos D, Akassoglou K. Fibrinogen as a key regulator of inflammation in disease. Semin Immunopathol. 2012;34(1):43-62. 3. Hoppe B, D¨orner T. Coagulation and the fibrin network in rheumatic disease: a role beyond haemostasis. Nat Rev Rheumatol. 2012;8(12):738-746. 4. Flick MJ, LaJeunesse CM, Talmage KE, et al. Fibrin(ogen) exacerbates inflammatory joint disease through a mechanism linked to the integrin aMb2 binding motif. J Clin Invest. 2007;117(11):3224-3235. 5. Busso N, P´eclat V, Van Ness K, et al. Exacerbation of antigen-induced arthritis in urokinase-deficient mice. J Clin Invest. 1998;102(1):41-50. 6. Flick MJ, Chauhan AK, Frederick M, et al. The development of inflammatory joint disease is attenuated in mice expressing the anticoagulant prothrombin mutant W215A/E217A. Blood. 2011;117(23):6326-6337. 7. Davalos D, Ryu JK, Merlini M, et al. Fibrinogeninduced perivascular microglial clustering is required for the development of axonal damage in neuroinflammation. Nat Commun. 2012;3:1227. 8. Adams RA, Bauer J, Flick MJ, et al. The fibrin-derived gamma377-395 peptide inhibits microglia activation and suppresses relapsing paralysis in central nervous system autoimmune disease. J Exp Med. 2007;204(3):571-582. © 2015 by The American Society of Hematology
l l l IMMUNOBIOLOGY
Comment on Milne et al, page 470
Langerhans cells: straight from blood to skin? ----------------------------------------------------------------------------------------------------Nikolaus Romani1 and James W. Young2 CANCER CENTER
1
MEDICAL UNIVERSITY OF INNSBRUCK; 2MEMORIAL SLOAN KETTERING
In this issue of Blood, Milne et al1 report a developmental pathway for human Langerhans cells from circulating CD142CD1c1 dendritic cell (DC) precursors in peripheral blood. The Langerhans cell progeny, expressing CD207 (langerin) and bearing the hallmark Birbeck granules, result from culture in granulocyte macrophage colony-stimulating factor (GM-CSF) and transforming growth factor-b1 (TGF-b1) and/or bone morphogenetic protein 7 (BMP7). A similar study by Mart´ınez-Cingolani et al,2 also published recently in Blood, identified the same circulating CD142CD1c1 Langerhans cell precursors. These authors used a different anti-CD1c monoclonal antibody (BDCA-1). In that study, TGF-b1 synergized with thymic stromal lymphopoietin (TSLP) to yield similar Langerhans cell progeny. Both groups used fetal calf serum (FCS), however, which affects differentiation and maturation by unknown mechanisms and is not physiologically comparable to human plasma in vivo. Controls with FCS-containing medium, but without cytokines, indicated that FCS was not the driver of the conversion. However, Milne et al1 could not generate Langerhans cell progeny by culture in X-Vivo medium without FCS, despite inclusion of GM-CSF, TGF-b1, and BMP7, all of which, together with TSLP, are epithelial cell–derived cytokines under physiologic conditions. BLOOD, 15 JANUARY 2015 x VOLUME 125, NUMBER 3
From www.bloodjournal.org by guest on September 11, 2016. For personal use only.
R
esidence in stratified epithelia, CD207 expression, and Birbeck granules all define bona fide Langerhans cells in humans (see figure). Indeed, the progeny of CD1c1 blood DCs express substantial CD207 after culture with either GM-CSF/TGF-b1/ BMP71 or TGF-b1/TSLP.2 Birbeck granule morphology by electron microscopy is also convincing. Neither group evaluated the capacity to populate the epidermis, for which several methods exist using artificial skin equivalents, even though their Langerhans cell progeny express a number of skin-homing receptors like CCR6,2 EpCAM,1 or E-cadherin.1 Mouse studies have shown that Langerhans cell precursors only enter the epidermis before they express CD207, however. Hence the CD207expressing CD1c1 progeny reported by these authors may already be too developed for such homing. The work by Milne et al1 addresses another important issue. So-called monocytederived Langerhans cells, first reported by Geissmann et al,3 used negatively selected rather than positively selected precursors, which would have included the CD1c1 blood DC precursors now described by both groups. In fact, Milne et al1 demonstrate that positively selected CD141 or CD161 blood monocytes cultured under comparable conditions to CD1c1 DCs yield progeny with far less expression of CD1a, CD207, or Birbeck granules compared with Langerhans
cell progeny derived from CD1c1 DCs. These data are also useful in explaining how a CD142 monocyte, intermediate between CD341 hematopoietic progenitor cells and the final Langerhans cell progeny, can yield Langerhans cells. This avoids invoking a direct precursor–progeny connection with macrophages, as suggested by prior reports of CSF1 receptor (macrophage colonystimulating factor receptor)-positive monocytic Langerhans cell precursors in mice, where the relevant cytokine proved to be interleukin-34 binding to CSF1-receptor, not macrophage colony-stimulating factor.4 Langerhans cells function as prototypic DCs, both in the induction of immunity5,6 and in the maintenance of tolerance.7 Murine Langerhans cells share a common embryonal origin from fetal liver and yolk sac with macrophages, microglia, and Kupffer cells.8 Human Langerhans cells were observed in human fetal epidermis as early as 9 weeks’ estimated gestational age.9 This is before the beginning of hematopoiesis in the bone marrow at 10.5 weeks’ estimated gestational age, suggesting that human Langerhans cells also derive from fetal hematopoietic tissues. Langerhans cells are very long-lived and maintain their steadystate population density (approximately 600/mm2) in the epidermis by low-level proliferation from local precursors. In inflamed mouse skin, however, Langerhans cell replenishment of those
Langerin expression and Birbeck granules are hallmarks of Langerhans cells. (A) Conventional fluorescence microscopy of an epidermal sheet prepared from healthy human skin. Langerhans cells are immunolabeled with anti-CD207/langerin monoclonal antibody (Miltenyi Biotec). Scale bar, 30 mm. (B) Detail from a transmission electron micrograph of a human epidermal Langerhans cell in situ featuring 2 tennis racket–shaped Birbeck granules. Scale bar, 60 nm.
BLOOD, 15 JANUARY 2015 x VOLUME 125, NUMBER 3
that have migrated from the epidermis to draining lymph nodes occurs in 2 phases. Blood monocyte precursors provide only the first wave, whereas hematopoietic precursors comprise the second, more durable repletion of the long-lived local precursors and their Langerhans cell progeny.10 It remains elusive, however, under which circumstances CD1c1 DCs would differentiate in vivo into epidermal Langerhans cells in humans. It seems unlikely that they contribute to the initial seeding of human epidermis with Langerhans cells. Because they do not express CD14, it also seems unlikely that they correspond to the monocytic precursors in the first phase of Langerhans cell repopulation in inflamed murine epidermis.10 In contrast, the cytokines supporting human CD1c1 DC conversion into Langerhans cells are all epithelial cell derived, and the Langerhans cell progeny express skin-homing receptors. CD1c1 DCs are therefore plausible candidate precursors responsible for the second phase of epidermal repopulation by Langerhans cells after an inflammatory insult. The identification of a phenotypically defined Langerhans cell precursor in blood could provide a more exportable methodology for generating this highly potent DC subtype in vitro, rather than the current standard methodology of starting with CD341 hematopoietic progenitor cells.5 To this end, however, more rigorous sideby-side functional rather than phenotypic comparisons are required between different candidate Langerhans cells. Although technically and logistically demanding, Langerhans cells isolated from the epidermis will remain the “gold standard.” Human Langerhans cells are the most potent DC subtype for the stimulation of CTLs, which they accomplish without interleukin-12p705; human Langerhans cells are the most potent cross-presenting DC subtype.5,6 In this respect, the 2 studies discussed here present investigators with some unresolved paradoxes. Mart´ınez-Cingolani et al2 describe a Th2 profile stimulated by their Langerhans cells progeny, which would not support potent CD81 CTL generation. Moreover, because CD1411 (BDCA-31) and CD1c1 (BDCA11) mark mutually exclusive DC subsets, in which CD1411 (BDCA31) DCs are the most potent crosspresenting DCs, then both Milne et al1
421
From www.bloodjournal.org by guest on September 11, 2016. For personal use only.
and Mart´ınez-Cingolani et al2 present investigators with another ambiguity. Absent functional data, it remains unclear how they would reconcile the BDCA-11/CD1c1 (BDCA-32/CD1412) profile of the DC precursors they describe with the resultant Langerhans cell progeny that should have potent cross-presenting capacity, nominally associated with the converse CD1411 (BDCA31) phenotype. Detailed functional data will again therefore prove essential in extending these groups’ very compelling and novel findings. Conflict-of-interest disclosure: The authors declare no competing financial interests. n REFERENCES 1. Milne P, Bigley V, Gunawan M, Haniffa M, Collin M. CD1c1 blood dendritic cells have Langerhans cell potential. Blood. 2015;125(3):470-473. 2. Mart´ınez-Cingolani C, Grandclaudon M, Jeanmougin M, Jouve M, Zollinger R, Soumelis V. Human blood BDCA-1 dendritic cells differentiate into Langerhans-like cells with thymic stromal lymphopoietin and TGF-b. Blood. 2014;124(15):2411-2420. 3. Geissmann F, Prost C, Monnet JP, Dy M, Brousse N, Hermine O. Transforming growth factor beta1, in the presence of granulocyte/macrophage colony-stimulating factor and interleukin 4, induces differentiation of human peripheral blood monocytes into dendritic Langerhans cells. J Exp Med. 1998;187(6):961-966. 4. Wang Y, Szretter KJ, Vermi W, et al. IL-34 is a tissue-restricted ligand of CSF1R required for the development of Langerhans cells and microglia. Nat Immunol. 2012;13(8):753-760. 5. Ratzinger G, Baggers J, de Cos MA, et al. Mature human Langerhans cells derived from CD341 hematopoietic progenitors stimulate greater cytolytic T lymphocyte activity in the absence of bioactive IL-12p70, by either single peptide presentation or cross-priming, than do dermal-interstitial or monocytederived dendritic cells. J Immunol. 2004;173(4): 2780-2791.
l l l MYELOID NEOPLASIA
Comment on Gambacorti-Passerini et al, page 499
Mutations of ETNK1 in aCML and CMML ----------------------------------------------------------------------------------------------------Olivier Kosmider
PARIS DESCARTES UNIVERSITY
In this issue of Blood, Gambacorti-Passerini et al describe, for the first time, mutations of an ethanolamine kinase called ETNK1, which is physiologically involved in the first step of the phosphatidylethanolamine biosynthesis pathway.1 The authors describe 2 recurrent point mutations of ETNK1 in 9% of atypical chronic myeloid leukemias (aCMLs) and show that these mutations are associated with the impaired catalytic activity of the kinase.
A
typical CML is an entity defined in the World Health Organization 2008 classification that belongs to the group of myeloproliferative/myelodysplastic disorders. Since 2013, next-generation sequencing approaches conducted on aCML patient samples have described recurrent mutations in CSF3R and SETBP1, genes whose mutations are able to activate cellular proliferation. Long known in oncogenesis, the activating mutations of the granulocyte colony-stimulating factor receptor, also called CSF3R, were first described in 25% to 50% of aCML cases as well as in 3% of chronic myelomonocytic leukemia (CMML) cases in 2013.2 In aCML, a hot spot affecting codon 618 is frequently affected and could be a therapeutic target in
response either to Src kinase or to Janus kinase inhibitors.2,3 Initially described in more than 90% of patients with Schinzel-Giedion syndrome,4 SETBP1 mutations were identified in 24.3% of aCML samples tested by Piazza et al.5 At the same time, SETBP1 mutations have been identified in other hematological malignancies, specifically in 4% to 14% of CMMLs, demonstrating one more time that the mutational spectrum between aCML and CMML is very close. SETBP1 mutations were characterized as conferring a proliferative advantage to mutated cells5-7 and as closely related to a negative effect on overall survival in patients with aCML. As suggested by Makishima et al,6 the presence of SETBP1 mutations could allow the use of transforming
6. Klechevsky E, Morita R, Liu M, et al. Functional specializations of human epidermal Langerhans cells and CD141 dermal dendritic cells. Immunity. 2008;29(3): 497-510. 7. Seneschal J, Clark RA, Gehad A, Baecher-Allan CM, Kupper TS. Human epidermal Langerhans cells maintain immune homeostasis in skin by activating skin resident regulatory T cells. Immunity. 2012;36(5): 873-884. 8. Guilliams M, Ginhoux F, Jakubzick C, et al. Dendritic cells, monocytes and macrophages: a unified nomenclature based on ontogeny. Nat Rev Immunol. 2014; 14(8):571-578. 9. Schuster C, Vaculik C, Fiala C, et al. HLA-DR1 leukocytes acquire CD1 antigens in embryonic and fetal human skin and contain functional antigen-presenting cells. J Exp Med. 2009;206(1):169-181. 10. Ser´e K, Baek JH, Ober-Bl¨obaum J, et al. Two distinct types of Langerhans cells populate the skin during steady state and inflammation. Immunity. 2012; 37(5):905-916. © 2015 by The American Society of Hematology
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Codons of ETNK1 affected by mutations are indicated by the red and blue circles in aCML and CMML, respectively. See Figure 1A in the article by Gambacorti-Passerini et al that begins on page 499.
BLOOD, 15 JANUARY 2015 x VOLUME 125, NUMBER 3
From www.bloodjournal.org by guest on September 11, 2016. For personal use only.
2015 125: 420-422 doi:10.1182/blood-2014-11-610667
Langerhans cells: straight from blood to skin? Nikolaus Romani and James W. Young
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