Background: Human MAIT cells are an evolutionarily conserved, innate-like T cell population characterised by the semi-invariant Vα7.2-Jα33 chain of the TCR, which restricts them to non-polymorphic, major histocompatibility complex class I-related protein 1, MR1. The MAIT cell population is also able to respond to cytokines IL-12 and IL-18 in a TCR-independent manner. However, how these TCR-dependent and TCR-independent signals may combine to induce triggering has not been defined.

Aims: We aimed to dissect the relationship between TCR signaling and cytokine signaling in MAIT cells, and explore whether one signal would enhance the other.

Results: We performed functional assays on selected CD8+ T cells, and stimulated them with triggers including TCR (anti-CD3/28), and cytokines (IL-12, 18, 23, and type I IFN, α and β). Overnight, single-cytokine signaling synergistically enhances TCR signaling in a dose-dependent manner, with the effect of IL-12 being the most pronounced. Type I IFNs, including IFN α and β also enhance TCR signalling, but the effect is most marked over a shorter timecourse. Combinations between IL-12, 18 and 23 all enhanced TCR signaling - with up to 70% cells producing IFNγ, whilst TCR-only trigger induced 5%. Similarly, we discovered a synergistic effect on GrB production associated with Type I IFNs. We have further confirmed the synergistic effect of TCR and cytokines using an infection model, with THP1 cells as APCs, an E. coli trigger and blocking of MR-1 and/or cytokines

Conclusion: TCR and cytokine signals synergise in MAIT cells mounting immune responses to bacteria. We have identified a variety of cytokines, which alone or in combination could enhance MAIT cell activation with TCR triggers, each with slightly different impacts. These data indicate that optimal activation of MAIT cells may require combinatorial signals in vivo, and indicate potential crosstalk between these signals to tune and regulate MAIT cell functions.