MR1-independent activation of human mucosal-associated invariant T cells by mycobacteria

Tuberculosis (TB) is the leading cause of mortality from a single infectious agent, Mycobacterium tuberculosis. Relevant immune targets of the partially efficacious TB vaccine bacille Calmette-Guérin (BCG) remain poorly defined. Mucosal-associated invariant T (MAIT) cells are MHC-related protein 1 (MR1)-restricted T cells, which are reactive against M. tuberculosis, and underexplored as potential TB vaccine targets. We sought to determine whether BCG vaccination activated mycobacteria-specific MAIT cell responses in humans. We analyzed whole blood samples from M. tuberculosis-infected South African adults who were revaccinated with BCG after a six-month course of isoniazid preventative therapy. In vitro BCG stimulation potently induced IFN-g expression by phenotypic (CD8⁺CD26⁺CD161⁺) MAIT cells, which constituted the majority (75%) of BCG-reactive IFN-g- producing CD8⁺ T cells. BCG revaccination transiently expanded peripheral blood frequencies of BCG-reactive IFN-g⁺ MAIT cells, which returned to baseline frequencies a year following vaccination. In another cohort of healthy adults who received BCG at birth, 53% of mycobacteria-reactive-activated CD8 T cells expressed CDR3a TCRs, previously reported as MAIT TCRs, expressing the canonical TRAV1-2-TRAJ33 MAIT TCRa rearrangement. CD26 and CD161 coexpression correlated with TRAV1-2⁺CD161⁺ phenotype more accurately in CD8⁺ than CD42CD82 MAIT cells. Interestingly, BCG-induced IFN-g expression by MAIT cells in vitro was mediated by the innate cytokines IL-12 and IL-18 more than MR1-induced TCR signaling, suggesting TCR-independent activation. Collectively, the data suggest that activation of blood MAIT cells by innate inflammatory cytokines is a major mechanism of responsiveness to vaccination with whole cell vaccines against TB or in vitro stimulation with mycobacteria.