Mucosal-associated invariant T (MAIT) cells are a subset of the T cell population defined by an invariant T cell receptor (TCR) that is stimulated by bacterial metabolites. These cells preferentially migrate to mucosal tissues such as gut, liver and skin. MAIT cells are activated by metabolites from the riboflavin (Vitamin B2) biosynthetic pathway that are presented by MHC class 1-related (MR1) molecules expressed on numerous of cell types including antigen presenting cells (APCs) and epithelial cells. The V alpha segment of MAIT TCRs is invariant and composed of Vα7.2, whereas the V beta portion can be variable. MAIT cells are present at very low levels in neonatal blood but significantly increase in adults and show very high variance in their frequency among individuals. While there is evidence that several pathogenic bacteria can activate MAIT cells and that one of their functions is to kill cells with intracellular bacteria, it remains unknown how they discriminate among thousands of commensal bacteria that can also produce Vitamin B2 metabolites. Further, the role of the TCR variable beta region in antigen recognition is still unclear. In this thesis project, we hypothesized that MAIT cells can be stimulated by bacteria that inhabit human mucosal tissues, and that this microbiota-MAIT cell interaction is one of the driving forces in their expansion and variation in the human population. In support of this hypothesis, we observed a reduced proportion of MAIT cells in the blood of perinatally HIV-infected children, which correlated with other microbiota-associated parameters including Th17 cells and inflammatory markers of microbiota alterations. However, it is unclear whether MAIT cells can discriminate bacterial species that reside in the human microbiota, which can produce the riboflavin intermediates. To address this, we developed an in vitro functional assay using human T cells engineered for MAIT-specific TCRs (eMAIT-TCRs) stimulated by MR1-expressing B cell lines presenting the bacterial metabolites. We then screened 47 microbiota-associated bacterial species from different phyla for their eMAIT-TCR stimulatory capacities. Only bacterial species that encoded a riboflavin biosynthesis pathway were stimulatory for MAIT-TCRs. Most species that were high-stimulators belonged to Bacteroidetes and Proteobacteria phyla, although with significant variance, whereas low/non-stimulator species were either Actinobacteria or Firmicutes. Furthermore, we determined a wide range of intra-species variation in eMAIT-TCR stimulation capacities of Staphylococcus epidermidis , a skin commensal, which suggests that bacteria can modulate the production capacities of MAIT-TCR stimulatory antigens. Remarkably, we also discovered that human T cells not only express low-levels of MR1 but can also present bacterial metabolites to MAIT cells in an MR1-restricted fashion and trigger TCR signaling to induce cytokine secretion (IFNγ and TNFα), albeit at lower levels. In conclusion, our findings revealed that MAIT cells could discriminate between MR1-restricted, bacteria-derived metabolites through their TCR signaling thresholds. This knowledge paves the way to elucidate the complexity of MAIT cell recognition and its response to the human microbiota, which establishes a framework to explore mechanistic or therapeutic approaches for maintenance of a healthy immunological equilibrium.
|Commitee:||Diefenbach, Catherine, Shopsin, Bo, Torres, Victor|
|School:||New York University|
|Department:||Basic Medical Science|
|School Location:||United States -- New York|
|Source:||DAI-B 79/02(E), Dissertation Abstracts International|
|Keywords:||Aging, HIV, Human microbiota, MAIT, Mucosal immunity|
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