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Genes and Immune Response in Alopecia Areata: Review of the Alopecia Areata Research Summit First Day Proceedings

2013; Elsevier BV; Volume: 16; Issue: 1 Linguagem: Inglês

10.1038/jidsymp.2013.3

1529-1774

David A. Norris,

melanin and skin pigmentation

alopecia areata cytotoxic T-lymphocyte antigen-4 genome-wide association studies Janus kinase natural killer natural killer group 2D The first day of this research summit reviewed the evidence that both innate and acquired immunity are involved in alopecia areata (AA). Genetic and functional immunologic studies have identified complex immunologic networks in both human and animal model AA. Natural killer group 2D ligands, IL-15, cytotoxic T-lymphocyte antigen-4, and Janus kinase signaling have emerged as attractive targets for treatment of AA using modern biologic and pharmacologic approaches. Modulation of immune privilege continues to be an intriguing target to modulate AA induction and progression. The specificity of the immune response in AA is still unknown. Pigmented anagen hair follicles appear to be the target. Whether antigen-specific activated T cells or antibodies are required for AA is not yet known. Development of a robust clinical research platform will facilitate the testing of a number of new drugs being effectively used in other autoimmune diseases. As the approaches to studying the genetics of alopecia areata (AA) have broadened from linkage studies in families, to genome-wide association studies, to deep sequencing of key areas of the genome, it has been increasingly clear that genetic control of innate and acquired immunity is the most powerful factor in determining the susceptibility to all variants of AA, from patchy disease to alopecia universalis. There are four major genetics questions that were addressed during the first day of this Research Summit:1.Which genes act as "drivers" of disease and which are merely "enablers"?2.Is there a genetic basis for disease subsets?3.What are the key genes in control of disease?4.What is the genetic relationship to other autoimmune diseases? Angela Christiano, PhD, Columbia University, reviewed the significant progress since the publication of the groundbreaking first GWAS study in AA (Petukhova et al., 2010Petukhova L. Duvic M. Hordinsky M. et al.Genome-wide association study in alopecia areata implicates both innate and adaptive immunity.Nature. 2010; 466: 113-117Crossref PubMed Scopus (546) Google Scholar). Another group has validated the genes found in the previous study, and Dr Christiano has identified further important immune response genes in a second association study: PTPN22, PTPN2, IRG, IL-13, IL-4, and IFN signature. Associated gene products in hair follicle in AA (STX 17, PRDX5, ULBP6, and ULBP3) provide important clues to possible initiation of disease, and may be therapeutic targets. Importantly, there is not much genetic overlap with either psoriasis or vitiligo, although there are five loci shared between AA and type 1 diabetes. Future genetic studies underway include combination of association and linkage studies using the 250 multiplex families in the registry, functional genomics, deep sequencing, and development of a human network plot. It is clear that complementary functional immunologic studies are necessary to characterize the pathways in AA, validating network components identified in genetics studies. The important questions now under consideration include:(1)What is the balance of innate and acquired immunity?(2)What are attractive pathways for targeted therapy?(3)What is the specificity of the immune response?(4)What is the role for loss of immune privilege in initiation and maintenance of disease?(5)What animal model best serves the research needs in AA? The seminal first GWAS study in AA (Petukhova et al., 2010Petukhova L. Duvic M. Hordinsky M. et al.Genome-wide association study in alopecia areata implicates both innate and adaptive immunity.Nature. 2010; 466: 113-117Crossref PubMed Scopus (546) Google Scholar) identified multiple genes that controlled both acquired and innate immunity. Raphael Clynes, MD, PhD, Columbia University, reported on exciting new efforts to delineate the pathways that activate innate and acquired immunity in AA, opening exciting new opportunities for targeted immunotherapy (Petukhova et al., 2011Petukhova L. Cabral R.M. Mackay-Wiggan J. et al.The genetics of alopecia areata: What's new and how will it help our patients?.Dermatol Ther. 2011; 24: 326-336Crossref PubMed Scopus (42) Google Scholar) (summarized in Table 2). He proposes that natural killer group 2D (NKG2D) ligands activate natural killer (NK) T cells (NKG2D+CD8+) to attack the cells of the hair bulb. Signaling pathways containing IL-15, IRB, and Janus kinase (JAK) 1/3 lead to the activation of cytotoxic T cells, activation of memory T cells, and downregulation of regulatory T cells. NKG2D–activating ligands and IL-15 can be found in AA hair follicle bulbs, and may provide the key response to danger signals that initiate AA. Previous studies by John P Sundberg, DVM, PhD, The Jackson Laboratory, and more recent studies by Drs Clynes and Christiano have shown impressive functional similarities between human AA and the C3H/HeJ mouse model. They have similar apparent end-organ drivers (NKG2DL (ULBP3, MICA) and IL-15 in HF), similar effectors (CD8 T-cell effectors), and similar biomarkers (IFN gene signatures), arguing for the importance of translational studies in this mouse model. In this model, IL-15 inhibitors block the development of AA, and NKG2D+CD8+T cells are the key to transfer of disease. Clynes and co-workers are effectively using the C3H/HeJ mouse to test new treatment approaches, including anti-CTLA-4 (cytotoxic T-lymphocyte antigen-4) biologics, JAK inhibitors, and IL-15 inhibitors.Table 2Inflammatory response underlying AAInnate/NKG2D response Alarms: hair follicleTNFα, IL-15, IFN-γMICA, UBLP-3, UBLP-6, Rae-1 First respondersDETC, NK, NKT, γ( )δ TAPC/sentinel Cellular sentinels: Langerhans cells, dermal dendritic cells, dermal macrophage, mast cells Determinants: HLA, TAP, IFN-γAdaptive immunity Immune cells: CD4+T cells, CD8+T cells, Treg, B cells CTLA-4, iCOS, IL-2, IL-2R, IL-2R, IL-21 Cytokine network: IL-2, IL-6, IL-17, IL-21, IFN-γ, IFN-α Downstream signaling pathways: JAK 1/2, SykAbbreviations: AA, alopecia areata; APC, antigen-presenting cell; CTLA-4, cytotoxic T-lymphocyte antigen 4; iCOS, inducible costimulator; JAK, Janus kinase; NKG2D, natural killer group 2D; TAP, transporter associated with antigen processing; TNF, tumor necrosis factor; Treg, regulatory T cells. Open table in a new tab Abbreviations: AA, alopecia areata; APC, antigen-presenting cell; CTLA-4, cytotoxic T-lymphocyte antigen 4; iCOS, inducible costimulator; JAK, Janus kinase; NKG2D, natural killer group 2D; TAP, transporter associated with antigen processing; TNF, tumor necrosis factor; Treg, regulatory T cells. Table 1AA–Associated Genes that Determine Innate and acquired immunityInnate immunity NK cell activationNKG2D ligands: ULBP6, ULBP3, MICA (expressed in AA biopsies in the hair bulb region) CytokineAcquired immunity Antigen presentation: HLA-DRA, HLA-DQA1, HLA-DQA2, HLA-DQB2, HLA-DOB9(actually there are 44 HLA genes with significant association) T cell proliferation: CTLA-4, ICOS, IL-21, IL-2, IL-2RA, IKZF4, BTNL2 T cell differentiation: Notch 4 Tregs: CTLA-4, IKZF4, IL-2, IL-2RAEnd organAntioxidant: PRDX5Premature graying: STX 17Abbreviations: AA, alopecia areata; CTLA-4, cytotoxic T-lymphocyte antigen-4; NKG2D, natural killer group 2D; PRDX5, peroxiredoxin-5; STX 17, syntaxin 17; Tregs, regulatory T cells. Open table in a new tab Abbreviations: AA, alopecia areata; CTLA-4, cytotoxic T-lymphocyte antigen-4; NKG2D, natural killer group 2D; PRDX5, peroxiredoxin-5; STX 17, syntaxin 17; Tregs, regulatory T cells. The combination of GWAS studies, functional immunology, and the mobilization of the C3H/HeJ mouse model has placed us on the threshold of revolutionary changes in the treatment of AA. John Sundberg produced additional information of the impact of animal models in AA research. GWAS studies in the C3H/HeJ mouse have shown the importance of IL-1/IL-18, CH17 cells, and TAP2/TAP1 upstream from CD8 and NK cells. Ralf Paus, University of Luebeck Germany/University of Manchester, UK, provided a thoughtful review of the current thinking on the loss of immune privilege in AA. He believes that the hair follicle bulb and bulge both enjoy immune privilege because of low expression of Class I and Class II HLA antigens, associated with high expression of α-melanocyte-stimulating hormone and transforming growth factorB2. In AA, CD8+ cells enter the hair follicle epithelial zone of the bulb, but only with the upregulation of HLA class antigen expression in the hair bulb. This is associated with five phenomena: loss of tolerance, collapse of immune privilege (IFNγ dependent), presentation of sequestered autoantigen, entry of hair follicle into anagen, and costimulatory signals from CD4+ cells and mast cells. He raises the possibility that sensitization of T cells to hair follicle antigens may NOT be necessary in AA. Dr Paus proposes that drugs that return immune privilege may be able to reverse AA, e.g. melanocyte-stimulating hormone analogs and tacrolimus. Thomas Waldmann, MD, National Institutes of Health, produced further perspectives on the great potential of IL-15 modulation in treating cancer and inflammatory diseases. IL-2 and IL-15 have contrasting role in the life of a T cell: IL-2 provides checkpoint control of T cells required for self-tolerance and prevention of autoimmunity. IL-15 favors survival of memory CD8 cells. IL-15 expands CD8 and, to a lesser extent, NK cells, and is effective in cancer models. Antibody-mediated blockade of IL-15 is an effective inhibitor of autoimmunity, e.g. in a celiac disease model. IL-15 blockade has narrow effects, whereas JAK inhibition produces broader effects and side effects. Amos Gilhar, MD, Technion-Israel Institute of Technology and Flieman Medical Center, has been responsible for some of the fundamental studies establishing AA as an autoimmune disease mediated by CD4 and CD8 T cells. His most recent animal model extends these findings, showing the importance of CD8+NK2GD+ cells in AA modeling. In his new model, peripheral blood mononuclear cell from AA patients are stimulated in vitro with high IL-2 (not hair follicle antigens), and then are injected into mice grafted with hair-bearing human skin. Hair loss occurred in both allogeneic and autologous skin grafts. Biopsies for alopecic skin showed CD4 and CD8 cells, dense NKG2D cells, and prominent NKG2D cells. This model raises questions about the effector cells in AA, and whether antigen-sensitized T cells are indeed necessary. Depletion of CD56+ cells blocked induction of alopecia. Annemieke DeJong, PhD, Columbia University, presented her preliminary studies to identify clonal T cell populations in AA patients and in the C3H/HeJ mouse model. The goal was to answer three questions:(1)Does clonal expansion occur?(2)Are pathogenic T cells in the peripheral blood?(3)Do pathogenic T cells in circulation express markers that can be used as biomarkers? Some clones in the skin were also found in peripheral blood in alopecia universalis patients, but not in patchy persistent AA. In C3H/HeJ mice engrafted with alopecic skin from other syngeneic mice, the grafted skin contained pathogenic clones, and CD8+NKG2D+ cells were found in skin draining lymph nodes. Clonal expansion was observed and clones were found in the peripheral blood. The power of parallel experiments in human and mouse AA was once again demonstrated. Ali Jabbari, MD, PhD, Columbia University, also presented an exciting recent program—development of a robust clinical research platform for clinical trials in AA. There are several problems that stand in the way of developing strong clinical research programs in AA: difficulty in recruiting patients, high and unpredictable levels of spontaneous regression of AA, and lack of biomarkers that can be used to measure response in parallel to measurement of hair regrowth. The developing research platform will include sampling of skin, serum, and peripheral blood mononuclear cell for known markers of immune response, and or skin RNA and microRNA for gene expression studies. Parallel studies in humans and mice are underway, and both systemic, topical, and intralesional drug delivery will be tested to validate response end points and biomarker usefulness. Amelia Wall Warner, PharmD, RPh, Merck Research Laboratories, finished this first day of the Summit with observations on the role of pharmacogenomics in AA drug development. Pharmacogenomics provides the opportunity to predict which patients will respond and which will not. Whole-exome and whole-genome sequencing provides the opportunity to obtain and study massive amounts of genetic data. The growth of personalized medicine promises to establish which genes are drivers of disease and which are merely enablers. Pharmacogenomics promises to provide ways to identify subsets of disease, and to predict treatments for different subsets. The first day of this Research Summit provided considerable evidence that the key questions proposed at the beginning of this summary are now being addressed with modern research techniques. We now know that(1)Both innate and acquired immunity are involved in AA.(2)Both genetic and functional immunologic studies have identified complex immunologic networks in both human and animal model AA. NKG2D ligands, IL-15, CTLA-4, and JAK signaling have emerged as attractive targets for treatment of AA using modern biologic and pharmacologic approaches.(3)Modulation of immune privilege continues to be an intriguing target to modulate AA induction and progression.(4)The specificity of the immune response in AA is still unknown. Pigmented anagen hair follicles appear to be the target. Whether antigen-specific activated T cells or antibodies are required for AA is not yet known.