Regulation of T cell effector functions in the intestinal mucosa

Sammanfattning: T lymphocytes are a critical cellular component of the adaptive immune response. They are generated in the thymus from bone marrow derived progenitors, where they undergo commitment to the T cell lineage and differentiate and mature into naïve CD4+ and CD8+ T cells. Naïve CD4+ and CD8+ T cells activation takes place in lymphoid organs after recognition of cognate antigen in the context of major histocompatibility complex (MHC) molecules on antigen presenting cells (APCs). The effector functions of CD8+ T cells includes the MHC-I dependent recognition and lysis of target cells (e.g. virally infected cells) through the release of degranulating cytotoxic mediators but CD8+ T cells can also acquire suppressive functions (CD8+ regulatory T cells (Tregs). Activation of naïve CD4+ T cells leads to the generation of T helper cell subsets (Th, e.g. T helper (Th1), Th2, Th17 and CD4+ Tregs) and the type of Th cell that is generated during an immune response is largely dependent on the cytokine signals these cells receive during their initial activation in the lymph node.. The main function of CD4+ Th cells is to produce cytokines that that impact on the differentiation, recruitment and functionality of other immune and stromal cells, and as such these cells play a central role in shaping immune responses. Following an immune response some antigen-specific T cells remain as memory T cells. Memory T cells are long-lived and serve to protect us against re-infection with the same pathogen. The overall aim of this thesis was to study mechanisms regulating differentiation and functionality of intestinal CD4+ and CD8+ T cells. Memory CD4+ T cells is found in the circulation as well as in tissues. In addition to responding to re-infections in an antigen dependent manner these circulating memory CD4+ T cells can produce interferon (IFN)-? in response to cytokines in the absence of T cell receptor (TCR) stimulation, suggesting that they may participate in immune responses in an innate-like manner. We could show that IL-15 or tumor necrosis factor (TNF)-like cytokine 1A(TL1a) induced production of IL-5, IL-6, IL-13, IL-22, granulocyte–macrophage colony-stimulating factor (GM-CSF), IFN-? and TNF-? in a subset of memory CD4+ T cells co-expressing death receptor 3 (DR3) and IL-18R? in presence of IL-12 and IL-18. TL1a synergized with IL-15 to further enhance the levels of all cytokines measured and the cytokine-induced cytokine production was severely diminished in the absence of IL-18. The intestine is a large reservoir of memory CD4+ T cells and we could show that a large proportion of these cells co-expressed IL-18R? and DR3 in both humans and mice and these cells produced cytokines in response to cytokine stimulation. These results suggest that both human and murine memory IL-18R?+DR3+ CD4+ T cells may contribute to antigen-independent innate-like responses in the intestine. We also found that CD4+ T cells expressing IL-18R? and DR3 were present in the inflamed small intestine in patients with Crohn’s disease (CD), a chronic inflammatory disease that can affect all areas of the gastrointestinal tract, where theu co-localized with IL-18-expressing cells in intestinal lymphoid aggregates. This suggests that cytokine-activated CD4+ T cells might contribute to the high levels of proinflammatory cytokines observed in these patients. Bioactive IL-18 mRNA transcripts are increased in mucosal samples obtained from CD compared to controls. The role of IL-18 in intestinal inflammation has been thoroughly studied in various animal models of IBD and while most of these mice are protected from severe intestinal inflammation in absence of IL-18 there are also studies suggesting a protective role for IL-18. Although IL-18 is a pleotropic cytokine all of these studies was carried out by systemically blocking IL-18 signalling and did not take into account what cells that were affected. In the CD45RBhigh CD4+ T cell transfer model of IBD we could observe that all CD4+ T cells recovered from mesenteric lymph node and colon lamina propria (LP) expressed IL-18R? in the setting of colitis. To identify a potential role of IL-18 signalling in CD4+ T cells ability to induce colitis in this model we transferred il-18r-/- CD45RBhighCD4+ T cells into rag-1-/- mice. The number of IFN-? producing CD4+ T cells recovered from the colitic colon LP were significantly reduced compared to their wild type counterparts and il-18r-/- CD4+ T cells in the setting of colitis failed to produce cytokines in response to cytokine stimulation. Despite these findings il-18r-/- CD4+ T cells were equally proficient at inducing colitis as those CD4+ T cells from wild type mice. These results suggesting that IL-18 signaling in CD4+ T cells is not critical for their ability to drive disease pathology in the CD45RBhigh transfer model of colitis. The intestine is daily exposed to large amounts of antigens and is the major entry site for intracellular bacteria whose control requires the induction of protective CD8+ T cell responses. In the intestinal fatty acid-binding protein promoter truncated ovalbumin (iFABP-tOVA) mouse in which a surrogate antigen is expressed in the small intestinal epithelium we assessed mechanisms regulating mucosal CD8+ T cell priming and differentiation in the steady state and inflammatory setting. CD8+ T cells were found to differentiate into two distinct subsets, CD107a/b+ cytotoxic T cells (CTLs) and FoxP3+CD8+ T cells. FoxP3+CD8+ T cells but not CTLs required chemokine receptor 9 (CCR9) for effective homing to and expansion within the small intestinal mucosa. Further, we found that IRF8, but not IRF4, expression by intestinal dendritic cells (DCs) was critical for the development of the FoxP3+ subset in steady state but not the inflammatory setting. Collectively these findings broaden our understanding of the mechanisms regulating CD8+ T cell responses in the intestinal mucosa and have potential implications for mucosa vaccine design.