Minireview Submitted: 17.7.2014 Accepted: 13.8.2014 Conflict of interest None.
Florian Sparber Institute of Microbiology, ETH Zurich, Zurich, Switzerland
DOI: 10.1111/ddg.12506
Langerhans cells: an update
Summary Langerhans cells belong to the family of dendritic cells, professional antigen- presenting cells, and populate the skin and epithelia of mammals. It was the extensive investigation of this particular dendritic cell subpopulation in earlier days, which contributed crucially to the current understanding of the regulation of antigen processing and presentation, a concept, which was termed “the Langerhans cell paradigm”. Extensive research during the last decades has revealed that Langerhans cells might not only be involved in the induction of adaptive immune responses but also in the maintenance of peripheral tolerance in order to prevent auto-immunity. In addition it appeared that Langerhans cells represent a rather extravagant dendritic cell population with a unique origin and homeostatic regulation. This review highlights the most important findings about Langerhans cell ontogeny and homeostasis as well as their function in the immune system.
Introduction Langerhans cells were discovered by Paul Langerhans, who initially believed that the network of cells rich in dendrites that he observed in the human epidermis represented nerve endings. Almost 120 years later, it was the work of the 2011 Nobel Prize Laureate Ralph Steinman, together with Gerold Schuler, which finally determined, that these Langerhans cells (LCs), belong to a group of leukocytes, the dendritic cells (DCs) [1]. DCs represent a group of “professional antigen presenting cells” (APCs), capable of bridging innate and adaptive immune system by ingesting self as well as foreign antigens and presenting peptides on MHC surface molecules to T cells in order to induce peripheral tolerance or specific immune responses. In human and mice DCs are roughly classified as (1) plasmacytoid DCs and (2) conventional DCs, which can be further subdivided into (a) lymphoid tissue-resident and (b) migratory DCs in peripheral tissues [2]. LCs belong to the migratory skin DCs and colonize the suprabasal layer of the epidermis as well as other epithelia, e. g. in the gastro-intestinal, bronchial or uro-genital tract (Figure 1).
Mouse LCs express the classical DC marker CD11c, as well as MHC-class II and the C-type lectin Langerin, whereas human LCs can be identified by the expression of HLA-DR, high levels of CD1a and Langerin [3]. LCs clearly occupy a special place within the DC family, since they possess unique developmental and homeostatic features. In the recent years our understanding of LC homeostasis and function has increased significantly.
Ontogeny and homeostasis of LCs Recent work, predominantly done with mice, has shed new light onto the cellular origin of LCs. It has become clear that LCs, unlike other DC subsets, descend from a minor population of fetal yolk sac macrophages and a major population of fetal liver monocytes, which start to seed the skin around embryonic day 7.5 and 16.5, respectively [4]. Those LC precursors rapidly proliferate in situ during the first week after birth, establishing a dense network of cells, which ultimately make up for approximately 3 % of all nucleated cells in the adult epidermis [5]. This wave of proliferation
© 2014 Deutsche Dermatologische Gesellschaft (DDG). Published by John Wiley & Sons Ltd. | JDDG | 1610-0379/2014/1212
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Minireview Langerhans cells
Figure 1 Epidermal LCs in situ. Murine LCs in an epidermal sheet expressing green fluorescent protein (GFP) (a). Human LCs in the supra-basal layer of a skin section (b). LCs (arrows) were stained for CD1a (peroxidase labeling), whereas nuclei were stained with hematoxylin).
is accompanied by the sequential expression of MHC-class II, CD11c and Langerin. By using different experimental settings, such as parabiotic mouse models and congenic bone marrow chimeras, it has become clear that the homeostatic renewal of the LC network relies on local, skin resident precursor cells, and not on the recruitment of LC precursors from the circulation [6, 7]. A recent study using a multi-color cell tracing technique suggests that the loss of LCs by migration from the epidermis in the steady state is compensated not by randomized proliferation of all LCs, but rather by division of few LCs, thereby generating terminally differentiated LC clusters [8]. However, under some circumstances, such as severe skin inflammation and rapid loss of LCs, blood-born monocytes refill the niche that has been left empty in the skin [9, 10]. The immigration of these monocytic LCs into the skin is dependent on the chemokines CCL2 and CCL20, produced by distinct cells in the bulge of hair follicles, which in turn represent the major entry site for those inflammatory LCs [9]. LCs are thought to be very long-lived cells. Intravital imaging experiments in mice suggest that murine LCs have a lifespan of 53 to 78 days [11]. Importantly, LCs in human skin transplanted onto immuno-deficient mice persisted for approximately nine weeks [12]. In line with this, a follow-up study in a patient with transplanted hands revealed that donor-derived LCs resided in the graft for more than four years [13]. Concomitantly with LC's longevity the question arises about their functional properties during aging. It is well known that with age the immune system’s function declines, which is in the context of skin, reflected by an increasing susceptibility towards malignancy and infection. This change in skin immune reactivity over time is illustrated by the fact that human skin displays different Toll-like receptor (TLR) expression patterns during its lifetime, such as significantly increased expression of TLR1 to TLR5 in prenatal and neonatal human skin compared to adult skin [14]. Previous investigations have shown that elderly people harbor
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less LCs in situ, combined with a reduced migration of LCs in response to inflammatory stimuli such as TNF-α[15, 16]. However, this age-dependent decline in LC functionality is likely to be driven by microenvironmental factors in the skin and not by changes in precursors of DCs. This is suggested by the fact that monocyte-derived LCs from elderly individuals showed comparable function to LCs derived from young individuals [17]. The migration defect of LCs in elderly humans could not be detected in mice but analysis of LCs from 12-month old mice showed an impaired capacity to prime antigen-specific T cells and different miRNA expression patterns compared to LCs isolated from mice younger than six months of age [18]. Very recently, a genome-wide sequencing approach conducted with a large cohort of individuals has revealed genes that might be associated with the “youthfulness of the skin”. Among those candidates, the gene KCND2 has been shown to be specifically expressed by LCs in the skin [19]. Still, it is not entirely clear to what extend the age-related decline in LC function contributes to the increasing prevalence of skin malignancy or infection in aged people.
Regulators of LC homeostasis The unique status of LCs within the DC family is also reflected by the requirement of distinct environmental cues and signal pathways for their homeostasis, which to a certain extent differ from those important for other conventional DCs. For example, the fms-tyrosine kinase 3 (Flt3) receptor and its corresponding ligand (Flt3L) are crucial for the expansion and homeostasis of most DC subsets in vitro and in vivo [20, 21]. Yet, it was very surprising that LCs do not require Flt3-mediated signaling for their homeostasis, since mice deficient in Flt3 signaling exhibit a normal LC network [22]. In contrast, transforming growth factor beta 1 (TGF-ß1) is an essential factor for the development and function of LCs in mice and human. Mice deficient in TGF-ß1 or its
© 2014 Deutsche Dermatologische Gesellschaft (DDG). Published by John Wiley & Sons Ltd. | JDDG | 1610-0379/2014/1212
Minireview Langerhans cells
receptor, do not possess LCs [23–25] and genetic depletion of TGF-ß-related signal molecules, such as ID2, RUNX3 or PU.1, impairs LC network formation as well [26–28]. In line with this, the bone-morphogenetic-protein 7 (Bmp7), which belongs to the TGF-ß superfamily, seems to be an instructive factor for the differentiation of LCs in mice and human [29]. Vice versa to the previously mentioned Flt3 signaling, depletion of the macrophage-colony-stimulatory factor receptor (CSF1R) in mice does not affect the development of most conventional DCs, but instead perturbs LC homeostasis [30]. Recent work, carried out independently by two groups, has identified the stroma-derived cytokine IL-34 as a high-affinity ligand of the CSF1R. Consistently, depletion of IL-34 leads to the absence of LCs as well as microglia cells, a population which develops from fetal yolk sac macrophages [31, 32], similar to LCs. In addition to the signaling pathways mentioned above, various research groups have identified additional intracellular molecules that have significant impact on the homeostasis and function of LCs. Genetic depletion of the mTOR-related RAPTOR molecule results in progressive loss of LCs in transgenic mice, suggesting that the mTORC1 signaling complex might regulate important cellular processes in LCs [33]. Recent work has confirmed the importance of mTOR-signaling in LCs by depletion of the adaptor molecule p14 (LAMTOR2), thereby abrogating mTORC1 signaling on lysosomal vesicles in DCs/LCs. As a consequence, depletion of p14 led to enhanced maturation, reduced proliferation and finally apoptosis of LCs [34, 35]. Reminiscent of the LC phenotype of p14-deficient mice, LC-specific expression of Dkk1, an antagonistic constituent of the Wnt signaling complex, causes diminished proliferation and reduced numbers of the LCs in mice [36].
Function of LCs – immunity versus tolerance Whether LCs in vivo are primarily inducers of immunity or regulators of peripheral tolerance is still a matter of debate. This discord is highlighted by studies performed with various transgenic mouse models that allow specific depletion of Langerin+ cells, including LCs. The importance of LCs in the induction of the contact hypersensitivity reaction (CHS) has been demonstrated in mice conditionally depleted of Langerin+ DCs [37], while mice constitutively depleted of LCs have shown a regulatory role of LCs in CHS [38]. Hence, the question remains unsolved, as to what degree LCs are necessary for this T cell-mediated immune response. In addition, the discovery of a second subset of dermal DCs, expressing Langerin as well, added another level of complexity to this particular questioning. Reinvestigation of former experiments in context with the newly discovered Langerin+ dermal DCs suggests, that skin DCs may display functional redundancy in CHS. Nevertheless, LCs can efficiently stimulate T cell
responses in various infectious mouse models, such as viral and bacterial infections. In a mouse model for staphylococcal scalded skin syndrome (SSSS) LCs conferred immunity by triggering an IgG-mediated, humoral immune response [39]. However, LCs also play a downregulatory role in skin infections with Leishmania major [40]. In terms of peripheral tolerance, experiments using transgenic mouse models have revealed, that expression of the model antigen ovalbumin (OVA) under the control of the keratin promotor renders these animals tolerogenic towards subsequent immunization with the OVA antigen [41]. In line with this, LCs from mice expressing OVA in the epidermis were capable of deleting OVA-specific CD8+ T cells after their adoptive transfer into the mice [42]. Recapitulating all the work on LC function, it appears that LCs might be capable of doing both, inducing immunity, as well as tolerance. This hypothesis is strengthened by work, showing that human LCs can induce regulatory T cells in the steady state, while the same LCs were able to activate effector T cells upon infectious challenge [43].
Conclusion In recent years, major progress has been made in understanding LC biology, providing us with new insights into their origins and homeostatic mechanisms, as well as their function in vivo. LCs are probably capable of eliciting both immunity and tolerance, whereby the outcome depends on the particular immunological setting. The recent discovery of the unique origin of LCs, as well as the presence of two different LC populations, steady state- as well as inflammatory LCs, sets the stage for further investigations into the regulatory mechanisms that dictate both ontogeny and homeostasis of LCs. Importantly, whether these scenarios are similar in humans remains to be confirmed.
Acknowledgment I thank Prof. Dr. Nikolaus Romani and Assoc. Prof. Dr. Patrizia Stoitzner (Department of Dermatology, Innsbruck Medical University, Austria) for proof reading the manuscript. Correspondence to Florian Sparber, PhD Vladimir-Prelog-Weg 1-5/10 8093 Zurich, Switzerland E-mail: [email protected]
References 1
Schuler G, Steinman RM. Murine epidermal Langerhans cells mature into potent immunostimulatory dendritic cells in vitro. J Exp Med 1985; 161: 526–46.
© 2014 Deutsche Dermatologische Gesellschaft (DDG). Published by John Wiley & Sons Ltd. | JDDG | 1610-0379/2014/1212
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Merad M, Sathe P, Helft J et al. The dendritic cell lineage: ontogeny and function of dendritic cells and their subsets in the steady state and the inflamed setting. Annu Rev Immunol 2013; 31: 563–604. 3 Romani N, Clausen BE, Stoitzner P. Langerhans Cells & More: Langerin-expressing dendritic cell subsets in the skin. Immunol Rev 2010; 234: 120–41. 4 Hoeffel G, Wang Y, Greter M et al. Adult Langerhans cells derive predominantly from embryonic fetal liver monocytes with a minor contribution of yolk sac-derived macrophages. J Exp Med 2012; 209: 1167–81. 5 Chorro L, Sarde A, Li M et al. Langerhans cell (LC) proliferation mediates neonatal development, homeostasis, and inflammation-associated expansion of the epidermal LC network. J Exp Med 2009; 206: 3089–100. 6 Merad M, Manz MG, Karsunky H et al. Langerhans cells renew in the skin throughout life under steady-state conditions. Nat Immunol 2002; 3: 1135–41. 7 Merad M, Hoffmann P, Ranheim E et al. Depletion of host Langerhans cells before transplantation of donor alloreactive T cells prevents skin graft-versus-host disease. Nat Med 2004; 10: 510–7. 8 Ghigo C, Mondor I, Jorquera A et al. Multicolor fate mapping of Langerhans cell homeostasis. J Exp Med 2013, 210: 1657–64. 9 Nagao K, Kobayashi T, Moro K et al. Stress-induced production of chemokines by hair follicles regulates the trafficking of dendritic cells in skin. Nat Immunol 2012; 13: 744–52. 10 Seré K, Baek JH, Ober-Blöbaum J et al. Two distinct types of Langerhans cells populate the skin during steady state and inflammation. Immunity 2012; 37: 905–16. 11 Vishwanath M, Nishibu A, Saeland S et al. Development of intravital intermittent confocal imaging system for studying Langerhans cell turnover. J Invest Dermatol 2006; 126: 2452–7. 12 Krueger GG, Daynes RA, Emam M. Biology of Langerhans cells: selective migration of Langerhans cells into allogeneic and xenogeneic grafts on nude mice. PNAS 1983; 80: 1650–4. 13 Kanitakis J, Petruzzo P, Dubernard JM. Turnover of epidermal Langerhans cells. N Engl. J Med 2004; 351: 2661–2. 14 Iram N, Mildner M, Prior M et al. Age-related changes in e xpression and function of Toll-like receptors in human skin. Development 2012; 139: 4210–9. 15 Gilchrest BA, Murphy GF, Soter NA. Effect of chronologic aging and ultraviolet irradiation on Langerhans cells in human epidermis. J Invest Dermatol 1982; 79: 85–8. 16 Bhushan M, Cumberbatch M, Dearman RJ et al. Tumour necrosis factor-alpha-induced migration of human Langerhans cells: the influence of ageing. Br J Dermatol 2002; 146: 32–40. 17 Ogden S, Dearman RJ, Kimber I, Griffiths CEM. The effect of ageing on phenotype and function of monocyte-derived Langerhans cells. Br J Dermatol 2011; 165: 184–8. 18 Xu YP, Qi RQ, Chen W et al. Aging affects epidermal L angerhans cell development and function and alters their miRNA gene expression profile. Aging (Albany NY) 2012; 4: 742–54.
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Chang AL, Atzmon G, Bergman A et al. Identification of genes promoting skin youthfulness by genome-wide association study. J Invest Dermatol 2014; 134: 651–7. Brasel K, De Smedt T, Smith JL, Maliszewski CR. Generation of murine dendritic cells from flt3-ligand-supplemented bone marrow cultures. Blood 2000; 96: 3029–39. McKenna HJ, Stocking KL, Miller RE et al. Mice lacking flt3 ligand have deficient hematopoiesis affecting hematopoietic progenitor cells, dendritic cells, and natural killer cells. Blood 2000; 95: 3489–97. Waskow C, Liu K, Darrasse-Jeze G et al. The receptor tyrosine kinase Flt3 is required for dendritic cell development in peripheral lymphoid tissues. Nat Immunol 2008; 9: 676–83. Borkowski TA, Letterio JJ, Farr AG, Udey MC. A role for endogenous transforming growth factor ß1 in Langerhans cell biology: The skin of transforming growth factor ß1 null mice is devoid of epidermal Langerhans cells. J Exp Med 1996; 184: 2417–22. Kaplan DH, Li MO, Jenison MC et al. Autocrine/paracrine TGFß1 is required for the development of epidermal L angerhans cells. J Exp Med 2007; 204: 2545–52. Kel JM, Girard-Madoux MJH, Reizis B, Clausen BE. TGF-ß is required to maintain the pool of immature Langerhans cells in the epidermis. J Immunol 2010; 185: 3248–55. Chopin M, Seillet C, Chevrier S et al. Langerhans cells are generated by two distinct PU.1-dependent transcriptional networks. J Exp Med 2014; 210: 2967–80. Fainaru O, Woolf E, Lotem J et al. Runx3 regulates mouse TGF-ß-mediated dendritic cell function and its absence results in airway inflammation. EMBO J 2004; 23: 969–79. Hacker C, Kirsch RD, Ju XS et al. Transcriptional profiling identifies Id2 function in dendritic cell development. Nat Immunol 2003; 4: 380–6. Yasmin N, Bauer T, Modak M et al. Identification of bone morphogenetic protein 7 (BMP7) as an instructive factor for human epidermal Langerhans cell differentiation. J Exp Med 2013; 210: 2597–610. Ginhoux F, Tacke F, Angeli V et al. Langerhans cells arise from monocytes in vivo. Nat Immunol 2006; 7: 265–73. Greter M, Lelios I, Pelczar P et al. Stroma-derived interleukin-34 controls the development and maintenance of langerhans cells and the maintenance of microglia. Immunity 2012; 37: 1050–60. 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: 753–60. Kellersch B, Brocker T. Langerhans cell homeostasis in mice is dependent on mTORC1 but not mTORC2 function. Blood 2013; 121: 298–307. Sparber F, Scheffler JM, Amberg N et al. The late endosomal adaptor molecule p14 (LAMTOR2) represents a novel regulator of Langerhans cell homeostasis. Blood 2014; 123: 217–27. Sparber F, Tripp CH, Komenda K et al. The late endosomal adaptor molecule p14 (LAMTOR2) regulates TGFß1-mediated homeostasis of Langerhans cells. J Invest Dermatol 2014; doi: 10.1038/jid.2014.324. [Epub ahead of print].
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36 Becker MR, Choi YS, Millar SE, Udey MC. Wnt signaling influences the development of murine epidermal langerhans cells. J Invest Dermatol 2011; 131: 1861–8. 37 Noordegraaf M, Flacher V, Stoitzner P, Clausen BE. Functional redundancy of Langerhans cells and Langerin+ dermal dendritic cells in contact hypersensitivity. J Invest Dermatol 2010; 130: 2752–9. 38 Kaplan DH, Jenison MC, Saeland S et al. Epidermal Langerhans cell-deficient mice develop enhanced contact hypersensitivity. Immunity 2005; 23: 611–20. 39 Ouchi T, Kubo A, Yokouchi M et al. Langerhans cell antigen capture through tight junctions confers preemptive immunity in experimental staphylococcal scalded skin syndrome. J Exp Med 2011; 208: 2607–13.
40 Kautz-Neu K, Noordegraaf M, Dinges S et al. Langerhans cells are negative regulators of the anti-Leishmania response. J Exp Med 2011; 208: 885–91. 41 Shibaki A, Sato A, Vogel JC et al. Induction of GVHD-like skin disease by passively transferred T cell receptor transgenic CD8+ T cells into keratin-14-ovalbumin transgenic mice. J Invest Dermatol 2004; 123: 109–15. 42 Waithman J, Allan RS, Kosaka H et al. Skin-derived dendritic cells can mediate deletional tolerance of class I-restricted self-reactive T cells. J Immunol 2007; 179: 4535–41. 43 Seneschal J, Clark RA, Gehad A et al. Human epidermal Langerhans cells maintain immune homeostasis in skin by activating skin resident regulatory T cells. Immunity 2012; 36: 873–84.
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Florian Sparber Institute of Microbiology, ETH Zurich, Zurich, Switzerland
DOI: 10.1111/ddg.12506
Langerhans cells: an update
Summary Langerhans cells belong to the family of dendritic cells, professional antigen- presenting cells, and populate the skin and epithelia of mammals. It was the extensive investigation of this particular dendritic cell subpopulation in earlier days, which contributed crucially to the current understanding of the regulation of antigen processing and presentation, a concept, which was termed “the Langerhans cell paradigm”. Extensive research during the last decades has revealed that Langerhans cells might not only be involved in the induction of adaptive immune responses but also in the maintenance of peripheral tolerance in order to prevent auto-immunity. In addition it appeared that Langerhans cells represent a rather extravagant dendritic cell population with a unique origin and homeostatic regulation. This review highlights the most important findings about Langerhans cell ontogeny and homeostasis as well as their function in the immune system.
Introduction Langerhans cells were discovered by Paul Langerhans, who initially believed that the network of cells rich in dendrites that he observed in the human epidermis represented nerve endings. Almost 120 years later, it was the work of the 2011 Nobel Prize Laureate Ralph Steinman, together with Gerold Schuler, which finally determined, that these Langerhans cells (LCs), belong to a group of leukocytes, the dendritic cells (DCs) [1]. DCs represent a group of “professional antigen presenting cells” (APCs), capable of bridging innate and adaptive immune system by ingesting self as well as foreign antigens and presenting peptides on MHC surface molecules to T cells in order to induce peripheral tolerance or specific immune responses. In human and mice DCs are roughly classified as (1) plasmacytoid DCs and (2) conventional DCs, which can be further subdivided into (a) lymphoid tissue-resident and (b) migratory DCs in peripheral tissues [2]. LCs belong to the migratory skin DCs and colonize the suprabasal layer of the epidermis as well as other epithelia, e. g. in the gastro-intestinal, bronchial or uro-genital tract (Figure 1).
Mouse LCs express the classical DC marker CD11c, as well as MHC-class II and the C-type lectin Langerin, whereas human LCs can be identified by the expression of HLA-DR, high levels of CD1a and Langerin [3]. LCs clearly occupy a special place within the DC family, since they possess unique developmental and homeostatic features. In the recent years our understanding of LC homeostasis and function has increased significantly.
Ontogeny and homeostasis of LCs Recent work, predominantly done with mice, has shed new light onto the cellular origin of LCs. It has become clear that LCs, unlike other DC subsets, descend from a minor population of fetal yolk sac macrophages and a major population of fetal liver monocytes, which start to seed the skin around embryonic day 7.5 and 16.5, respectively [4]. Those LC precursors rapidly proliferate in situ during the first week after birth, establishing a dense network of cells, which ultimately make up for approximately 3 % of all nucleated cells in the adult epidermis [5]. This wave of proliferation
© 2014 Deutsche Dermatologische Gesellschaft (DDG). Published by John Wiley & Sons Ltd. | JDDG | 1610-0379/2014/1212
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Figure 1 Epidermal LCs in situ. Murine LCs in an epidermal sheet expressing green fluorescent protein (GFP) (a). Human LCs in the supra-basal layer of a skin section (b). LCs (arrows) were stained for CD1a (peroxidase labeling), whereas nuclei were stained with hematoxylin).
is accompanied by the sequential expression of MHC-class II, CD11c and Langerin. By using different experimental settings, such as parabiotic mouse models and congenic bone marrow chimeras, it has become clear that the homeostatic renewal of the LC network relies on local, skin resident precursor cells, and not on the recruitment of LC precursors from the circulation [6, 7]. A recent study using a multi-color cell tracing technique suggests that the loss of LCs by migration from the epidermis in the steady state is compensated not by randomized proliferation of all LCs, but rather by division of few LCs, thereby generating terminally differentiated LC clusters [8]. However, under some circumstances, such as severe skin inflammation and rapid loss of LCs, blood-born monocytes refill the niche that has been left empty in the skin [9, 10]. The immigration of these monocytic LCs into the skin is dependent on the chemokines CCL2 and CCL20, produced by distinct cells in the bulge of hair follicles, which in turn represent the major entry site for those inflammatory LCs [9]. LCs are thought to be very long-lived cells. Intravital imaging experiments in mice suggest that murine LCs have a lifespan of 53 to 78 days [11]. Importantly, LCs in human skin transplanted onto immuno-deficient mice persisted for approximately nine weeks [12]. In line with this, a follow-up study in a patient with transplanted hands revealed that donor-derived LCs resided in the graft for more than four years [13]. Concomitantly with LC's longevity the question arises about their functional properties during aging. It is well known that with age the immune system’s function declines, which is in the context of skin, reflected by an increasing susceptibility towards malignancy and infection. This change in skin immune reactivity over time is illustrated by the fact that human skin displays different Toll-like receptor (TLR) expression patterns during its lifetime, such as significantly increased expression of TLR1 to TLR5 in prenatal and neonatal human skin compared to adult skin [14]. Previous investigations have shown that elderly people harbor
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less LCs in situ, combined with a reduced migration of LCs in response to inflammatory stimuli such as TNF-α[15, 16]. However, this age-dependent decline in LC functionality is likely to be driven by microenvironmental factors in the skin and not by changes in precursors of DCs. This is suggested by the fact that monocyte-derived LCs from elderly individuals showed comparable function to LCs derived from young individuals [17]. The migration defect of LCs in elderly humans could not be detected in mice but analysis of LCs from 12-month old mice showed an impaired capacity to prime antigen-specific T cells and different miRNA expression patterns compared to LCs isolated from mice younger than six months of age [18]. Very recently, a genome-wide sequencing approach conducted with a large cohort of individuals has revealed genes that might be associated with the “youthfulness of the skin”. Among those candidates, the gene KCND2 has been shown to be specifically expressed by LCs in the skin [19]. Still, it is not entirely clear to what extend the age-related decline in LC function contributes to the increasing prevalence of skin malignancy or infection in aged people.
Regulators of LC homeostasis The unique status of LCs within the DC family is also reflected by the requirement of distinct environmental cues and signal pathways for their homeostasis, which to a certain extent differ from those important for other conventional DCs. For example, the fms-tyrosine kinase 3 (Flt3) receptor and its corresponding ligand (Flt3L) are crucial for the expansion and homeostasis of most DC subsets in vitro and in vivo [20, 21]. Yet, it was very surprising that LCs do not require Flt3-mediated signaling for their homeostasis, since mice deficient in Flt3 signaling exhibit a normal LC network [22]. In contrast, transforming growth factor beta 1 (TGF-ß1) is an essential factor for the development and function of LCs in mice and human. Mice deficient in TGF-ß1 or its
© 2014 Deutsche Dermatologische Gesellschaft (DDG). Published by John Wiley & Sons Ltd. | JDDG | 1610-0379/2014/1212
Minireview Langerhans cells
receptor, do not possess LCs [23–25] and genetic depletion of TGF-ß-related signal molecules, such as ID2, RUNX3 or PU.1, impairs LC network formation as well [26–28]. In line with this, the bone-morphogenetic-protein 7 (Bmp7), which belongs to the TGF-ß superfamily, seems to be an instructive factor for the differentiation of LCs in mice and human [29]. Vice versa to the previously mentioned Flt3 signaling, depletion of the macrophage-colony-stimulatory factor receptor (CSF1R) in mice does not affect the development of most conventional DCs, but instead perturbs LC homeostasis [30]. Recent work, carried out independently by two groups, has identified the stroma-derived cytokine IL-34 as a high-affinity ligand of the CSF1R. Consistently, depletion of IL-34 leads to the absence of LCs as well as microglia cells, a population which develops from fetal yolk sac macrophages [31, 32], similar to LCs. In addition to the signaling pathways mentioned above, various research groups have identified additional intracellular molecules that have significant impact on the homeostasis and function of LCs. Genetic depletion of the mTOR-related RAPTOR molecule results in progressive loss of LCs in transgenic mice, suggesting that the mTORC1 signaling complex might regulate important cellular processes in LCs [33]. Recent work has confirmed the importance of mTOR-signaling in LCs by depletion of the adaptor molecule p14 (LAMTOR2), thereby abrogating mTORC1 signaling on lysosomal vesicles in DCs/LCs. As a consequence, depletion of p14 led to enhanced maturation, reduced proliferation and finally apoptosis of LCs [34, 35]. Reminiscent of the LC phenotype of p14-deficient mice, LC-specific expression of Dkk1, an antagonistic constituent of the Wnt signaling complex, causes diminished proliferation and reduced numbers of the LCs in mice [36].
Function of LCs – immunity versus tolerance Whether LCs in vivo are primarily inducers of immunity or regulators of peripheral tolerance is still a matter of debate. This discord is highlighted by studies performed with various transgenic mouse models that allow specific depletion of Langerin+ cells, including LCs. The importance of LCs in the induction of the contact hypersensitivity reaction (CHS) has been demonstrated in mice conditionally depleted of Langerin+ DCs [37], while mice constitutively depleted of LCs have shown a regulatory role of LCs in CHS [38]. Hence, the question remains unsolved, as to what degree LCs are necessary for this T cell-mediated immune response. In addition, the discovery of a second subset of dermal DCs, expressing Langerin as well, added another level of complexity to this particular questioning. Reinvestigation of former experiments in context with the newly discovered Langerin+ dermal DCs suggests, that skin DCs may display functional redundancy in CHS. Nevertheless, LCs can efficiently stimulate T cell
responses in various infectious mouse models, such as viral and bacterial infections. In a mouse model for staphylococcal scalded skin syndrome (SSSS) LCs conferred immunity by triggering an IgG-mediated, humoral immune response [39]. However, LCs also play a downregulatory role in skin infections with Leishmania major [40]. In terms of peripheral tolerance, experiments using transgenic mouse models have revealed, that expression of the model antigen ovalbumin (OVA) under the control of the keratin promotor renders these animals tolerogenic towards subsequent immunization with the OVA antigen [41]. In line with this, LCs from mice expressing OVA in the epidermis were capable of deleting OVA-specific CD8+ T cells after their adoptive transfer into the mice [42]. Recapitulating all the work on LC function, it appears that LCs might be capable of doing both, inducing immunity, as well as tolerance. This hypothesis is strengthened by work, showing that human LCs can induce regulatory T cells in the steady state, while the same LCs were able to activate effector T cells upon infectious challenge [43].
Conclusion In recent years, major progress has been made in understanding LC biology, providing us with new insights into their origins and homeostatic mechanisms, as well as their function in vivo. LCs are probably capable of eliciting both immunity and tolerance, whereby the outcome depends on the particular immunological setting. The recent discovery of the unique origin of LCs, as well as the presence of two different LC populations, steady state- as well as inflammatory LCs, sets the stage for further investigations into the regulatory mechanisms that dictate both ontogeny and homeostasis of LCs. Importantly, whether these scenarios are similar in humans remains to be confirmed.
Acknowledgment I thank Prof. Dr. Nikolaus Romani and Assoc. Prof. Dr. Patrizia Stoitzner (Department of Dermatology, Innsbruck Medical University, Austria) for proof reading the manuscript. Correspondence to Florian Sparber, PhD Vladimir-Prelog-Weg 1-5/10 8093 Zurich, Switzerland E-mail: [email protected]
References 1
Schuler G, Steinman RM. Murine epidermal Langerhans cells mature into potent immunostimulatory dendritic cells in vitro. J Exp Med 1985; 161: 526–46.
© 2014 Deutsche Dermatologische Gesellschaft (DDG). Published by John Wiley & Sons Ltd. | JDDG | 1610-0379/2014/1212
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