Mucosal-associated invariant T cell regulation by bacterial signalling and immunometabolism

Abstract

Mucosal-associated invariant T (MAIT) cells are a subset of antibacterial innate-like T cells that are localised to mucosal surfaces. MAIT cells are characterised by the expression of a semi-invariant T cell receptor, specific to a bacterial antigen presented on the MHC class I related protein, MR1. The bacterial ligand is derived from 5-amino-6-D-ribitylaminouracil (5-A-RU), produced as an intermediate in the bacterial riboflavin synthesis pathway. 5-A-RU undergoes non-enzymatic condensation with methylglyoxal (MG), a glycolysis by-product, to form the final ligand, 5 (2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU). Presentation of 5 OP RU to MAIT cells stimulates a robust MAIT cell response. Given the abundance of MAIT cells at mucosal surfaces and the broad range of bacteria capable of activating them, it has been hypothesised that MAIT cell activation is tightly regulated to prevent hyperactivation and immunopathology. Understanding these regulatory mechanisms may enable modulation of MAIT cells to prevent or treat human disease. Here I show that both phagocytosis of bacteria by an antigen presenting cell (APC) and enhanced glycolysis regulate MAIT cell activation. To assess this, THP-1 cells, a monocytic cell line, served as APCs. THP-1 cells were incubated with glycolysis modulators, exogenous ligand, and non-ligand producing bacteria. Primary human MAIT cells were subsequently co-cultured with THP-1 cells and activation assessed by flow cytometry. Treatment with intact bacteria and 5-A-RU was found to activate MAIT cells to a greater extent than treatment with 5-A-RU alone or with lysed bacteria. Enhancement of THP 1 glycolysis augmented MAIT cell activation to 5-A-RU alone or 5-A-RU and lysed bacteria. In contrast, a reduction in activation was not observed when THP-1 cell glycolysis or PI3K, mTORC1, and mTORC2 signalling were inhibited. Furthermore, THP-1 cells did not exhibit increased glucose uptake upon stimulation with intact bacteria. These results suggest that phagocytosis of intact bacteria may enhance glycolysis, resulting in increased production of MG and formation of 5-OP-RU. However, further research is required to confirm this. The process described provides a potential regulatory mechanism by which MAIT cell activation is regulated in response to intact bacteria but not to soluble bacteria-derived 5-A-RU.