Only a subset of phosphoantigen-responsive γ9δ2 T cells mediate protective tuberculosis immunity

Mycobacterium tuberculosis and Mycobacterium bovis bacillus Calmette-Guerin (BCG) induce potent expansions of human memory V gamma V-+(9)delta(+)(2) T cells capable of IFN-gamma production, cytolytic activity, and mycobacterial growth inhibition. Certain phosphoantigens expressed by mycobacteria can stimulate gamma(9)delta(2) T cell expansions, suggesting that purified or synthetic forms of these phosphoantigens may be useful alone or as components of new vaccines or immunotherapeutics. However, we show that while mycobacteria-activated gamma(9)delta(2) T cells potently inhibit intracellular mycobacterial growth, phosphoantigen-activated gamma(9)delta(2), T cells fail to inhibit mycobacteria, although both develop similar effector cytokine and cytolytic functional capacities. gamma(9)delta(2) T cells receiving TLR-mediated costimulation during phosphoantigen activation also failed to inhibit mycobacterial growth. We hypothesized that mycobacteria express Ags, other than the previously identified phosphoantigens, that induce protective subsets of gamma(9)delta(2), T cells. Testing this hypothesis, we compared the TCR sequence diversity of gamma(9)delta(2), T cells expanded with BCG-infected vs phosphoantigen-treated dendritic cells. BCG-stimulated gamma(9)delta(2), T cells displayed a more restricted TCR diversity than phosphoantigen-activated gamma(9)delta(2), T cells. In addition, only a subset of phosphoantigen-activated gamma(9)delta(2). T cells functionally responded to mycobacteria-infected dendritic cells. Furthermore, differential inhibitory functions of BCG- and phosphoantigen-activated gamma(9)delta(2), T cells were confirmed at the clonal level and were not due to differences in TCR avidity. Our results demonstrate that BCG infection can activate and expand protective subsets of phosphoantigen-responsive gamma(9)delta(2) T cells, and provide the first indication that gamma(9)delta(2), T cells can develop pathogen specificity similar to alpha beta T cells. Specific targeting of protective gamma(9)delta(2), T cell subsets will be important for future tuberculosis vaccines.