Looking Beyond Th17 Cells: A Role for Tr1 Cells in Ankylosing Spondylitis?

Abstract

The chronic inflammatory arthritis ankylosing spondylitis (AS) is a highly heritable disease of complex genetics (1–3). The genetic association between AS and HLAB27 has been studied for almost 50 years, and over the last decade or so large-scale genomics studies have defined variants outside the HLA that confer risk of developing AS (1, 2, 4). Key among these are genes involved in T cell activation (including ERAP1, ERAP2, NPEPPS, UBE2E3, UBE2L3), immune signaling (including IL23R, IL6R, IL12B, TYK2) and various transcription factors involved in functional differentiation of immune cells (including TBX21, RUNX3 and EOMES). Functional genomic approaches have implicated several immune cell types in disease processes, and studies support a role for microbial dysbiosis in disease pathogenesis. It is now obvious that AS is not only a genetically complex but also immunologically complex disease. To date, however, much effort has focused on ‘low hanging fruit’ from genomics studies, most notably IL23R. This important work has led to advances in treatment options for AS patients through inhibiting the pathogenic effects of IL-17. But trials of the IL-23 inhibitor Ustekinumab were not successful. Potential reasons for the failure of Ustekinumab Phase 3 trials in AS are many and include difficulties with outcome measures and trial design. But the failure has also sparked the community to re-evaluate subtleties in models of AS immunopathogenesis. With many AS-associated genes implicated in various aspects of T cell biology, it is hard to pinpoint exact processes or pathways that are of critical importance in AS. Speculation needs to be supported by empirical observations from well-designed studies that push us beyond consideration of Th17 cell biology. Recently, Hanson and colleagues (5) provided evidence that AS patients exhibit significant reductions in the size of CD4 and CD8 T cell expansions globally in the peripheral blood, suggesting that perturbations in T cell survival, senescence, or regulation of clonal proliferation occur in AS patients during adaptive immune responses. This brings into focus what role regulatory T cells play in AS and indeed which populations of regulatory T cells may be of relevance to AS. Regulatory T cells were originally described as a subset of immune cells critical for negative regulation of immune-mediated inflammation and prevention of autoimmune diseases. However, Tregs are also implicated in the suppression of both innate and adaptive immune cells towards allergens, organ transplants, commensals, food, and other innocuous environmental triggers (6). FoxP3⁺ Tregs may be thymically induced (tTregs), peripherally induced (pTregs) or induced in cell culture, in response to TGF-β. The tissue environment promotes Tregs to express tissue-specific transcription factors that cooperate with FoxP3, providing a specialized function and supporting Treg cell subset homeostasis (7). Tregs regulate their immune environment by contact-dependent mechanisms, such as CD95 induction of conventional T cell apoptosis and CTLA4 downregulation of APC co-stimulatory function, as well as cytokine-mediated functions, including CD25 adsorption of IL-2 and IL-10 secretion which attenuates DC function and promotes Tr1 cell differentiation (8). Tr1 cells, another subtype of regulatory T cells, do not constitutively express the transcription factor Foxp3. Upon TCR recognition of their cognate antigen at the site of tissue inflammation, Tr1 cells secrete large quantities of IL-10, which has many immunomodulatory effects on local immune cells (9). Both Tr1 and Treg cells serve a vital role in preventing deleterious immune responses with comparable mechanisms of suppression, yet Tregs are essential in the initial stage of immune suppression at the site of inflammation, while Tr1 cells are key for the maintenance of long-term tolerance and restoration of tissue homeostasis (10). Tr1 cells were first described in severe combined immunodeficiency disease (SCID) in the early 1990’s (11). Since then, accumulating evidence implicates impaired function and reduced Tr1 cell numbers in immunopathogenesis of various immune-mediated diseases. Among these are inflammatory bowel disease (IBD), psoriasis, multiple sclerosis (MS), Grave’s disease, Hashimoto’s thyroiditis, and systemic lupus erythematosus (12–14). Broadly, IL-10 produced by Tr1 cells is a key regulator of TNF-mediated pathologies (15). Naïve CD4+ T cells acquire a Tr1 phenotype upon cytokine signaling via IL-10, IL-21, or IL-27 which promotes STAT3 activation and subsequent priming of the IL10 locus. Transcription factors that bind to IL10 include EOMES, IRF4, c-Maf, Ahr, and Blimp-1 which act through multiple pathways to induce a stable production of IL-10 (10). In contrast, Th17 cells, although necessary for host defense against extracellular pathogens, when dysregulated become major pathogenic drivers of inflammation in many immune-mediated diseases. TGF-β and IL-6 are the key cytokines for initiating Th17 differentiation, which induces IL-23R expression as well as high secretion of the pro-inflammatory cytokine IL-17 (16, 17) (Figure 1). Microarray gene expression analysis comparing Tr1 cells and Th17 cells prior to IL-23 signaling identified the most predominantly overexpressed genes in Tr1 cells to be IRF1, IRF8, PRDM1 (Blimp-1), and TBX21 (18). IL-23 is secreted by various immune cells including dendritic cells (DCs) and macrophages in response to toll-like receptor signaling (19). Under homeostatic conditions, the presence of IL-23 in the distal small bowel promotes a localized cytokine environment that targets IL-23 sensitive intestinal cells which support mucosal barrier function and intestinal immunity. LAG-3, which is expressed on natural regulatory T cells (Tregs),...