Figure 1.

Mucosal‐associated invariant T (MAIT) cell activation and activities in the setting of infection. (a) Infected cells activate MAIT cells by two independent pathways. (1) Bacteria produce small molecule vitamin B metabolites that are loaded onto MR1 and presented on the surface of the infected cell. The semi‐invariant MAIT cell T‐cell receptor αβ (TCR ‐αβ) binds the MR1–ligand complex resulting in TCR‐dependent MAIT cell activation. (2) Alternatively, or additionally, cytokines produced by infected cells, including interleukin‐12 (IL‐12), IL‐15, IL‐18 and IL‐7 can activate MAIT cells through cytokine receptors. (b) Upon activation, MAIT cells perform anti‐bacterial effector functions. Rapidly, they release effector molecules including (1) cytokines [including interferon‐γ (IFN‐γ), tumour necrosis factor‐α (TNF‐α) and possibly IL‐17] and (2) cytolytic products (including granzymes and perforin). In the tissue environment these may result in the activation of innate immune cells, lysis of infected cells, direct killing of bacteria, and recruitment of other immune cells to the site of infection. (3) MAIT cell activation may result in local proliferation and/or (4) apoptosis of individual MAIT cell clones both of which may potentially shape the tissue‐resident TCR repertoire.