Invariant T cell subsets in immune defense of oral mucosa and skin

Innate-like T lymphocytes such as mucosa-associated invariant T (MAIT) cells and invariant natural killer T (iNKT) cells recognize antigens presented by MR1 and CD1d molecules, respectively. As MR1 presents microbial riboflavin metabolite antigens, and CD1d presents endogenous or microbial glycolipid antigens, MAIT cells and iNKT cells survey the non-protein fraction of the potential antigenic universe. Both these cell types respond in an innate-like fashion without the need for priming and clonal expansion, and are thus rapidly available to help protect the host from viral and bacterial infections. This thesis is focused primarily on the role of the innate-like T cell subsets in immune defense of the human oral mucosa and skin, two tissues that together form a large part of the body barrier to the surrounding world of microbes. 



In paper I, we establish a set of reproducible and adaptable methods to study human MAIT cell activation, cytokine production, proliferation and cytolytic function in response to microbes expressing riboflavin metabolite antigens. These novel and optimized methods establish a framework and open new possibilities to study MAIT cell immunobiology using Escherichia coli as a model antigen. 



In paper II, we study the lymphocyte population in healthy human buccal mucosa, and establish the presence of MAIT cells at this site. The buccal mucosal MAIT cell population is located both in the epithelium and in the lamina propria and can be divided into a tissue resident CD103+ subset and a tissue non-resident CD103- subset. These two subsets differ in their functional profile both between each other and compared to the circulating peripheral blood MAIT cell population. Interestingly, tissue-resident MAIT cells had a specialized polyfunctional response profile with higher IL-17 levels, and were low in the cytolytic effector molecule perforin. 



In paper III, we investigate the involvement of oral mucosal MAIT cells in apical periodontitis (AP). MAIT cells were moderately enriched in AP tissue as compared to the surrounding healthy gingival tissue, but with higher representation of the CD4+ MAIT cell subset. When the abundance of MAIT cell TCR transcript was analyzed in relation to AP microbiome data, we could identify bacterial relative abundances that negatively correlated with Vα7.2-Jα33, Cα, and IL-17A transcript expressions in AP, implying that MAIT cells could play a role in this oral disease setting. 



In paper IV, we investigate the innate immune response to herpes simplex virus 1 (HSV1) in keratinocytes. In response to HSV1 infection keratinocytes rapidly upregulate surface expression of the innate immune activating cytokine IL-15 in a TLR3-dependent manner. Interestingly, the virus actively downregulates the IL-15/IL-15Ra complex in an apparent novel mode of immune evasion. Furthermore, we show that iNKT cells can counteract this viral effect and support the maintenance of IL-15 expression on the surface of infected keratinocytes. In summary, the research covered by this thesis unravels a series of novel aspects of immunity mediated by innate-like T cells at barrier sites of the human body. The findings are of considerable importance for our understanding of immunity against bacterial and viral antigens in the oral mucosa and the skin