Only a Subset of Phosphoantigen-Responsive γ9δ2 T Cells Mediate Protective Tuberculosis Immunity

Abstract

Mycobacterium tuberculosis and Mycobacterium bovis bacillus Calmette-Guérin (BCG) induce potent expansions of human memory Vγ9+Vδ2+ T cells capable of IFN-γ production, cytolytic activity, and mycobacterial growth inhibition. Certain phosphoantigens expressed by mycobacteria can stimulate γ9δ2 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 γ9δ2 T cells potently inhibit intracellular mycobacterial growth, phosphoantigen-activated γ9δ2 T cells fail to inhibit mycobacteria, although both develop similar effector cytokine and cytolytic functional capacities. γ9δ2 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 γ9δ2 T cells. Testing this hypothesis, we compared the TCR sequence diversity of γ9δ2 T cells expanded with BCG-infected vs phosphoantigen-treated dendritic cells. BCG-stimulated γ9δ2 T cells displayed a more restricted TCR diversity than phosphoantigen-activated γ9δ2 T cells. In addition, only a subset of phosphoantigen-activated γ9δ2 T cells functionally responded to mycobacteria-infected dendritic cells. Furthermore, differential inhibitory functions of BCG- and phosphoantigen-activated γ9δ2 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 γ9δ2 T cells, and provide the first indication that γ9δ2 T cells can develop pathogen specificity similar to αβ T cells. Specific targeting of protective γ9δ2 T cell subsets will be important for future tuberculosis vaccines.

Human γδ T Cells: A Lymphoid Lineage Cell Capable of Professional Phagocytosis

“…TLR and related "sensing of nonself" molecules in endosomes and in the cytoplasm are known to be present and responsive in ␥␦ T cells (38,39), and could therefore represent an important regulating step in the encounter with an endocytosed or phagocytosed pathogen. It was recently demonstrated that mycobacteria infection activates and expands certain subsets of V␥9␦2 ␥␦ T cells that then develop greater Ag specificity by TCR selection reminiscent of ␣␤ T cells forming true memory (40). It would be interesting to address the possibility that such specific V␥9␦2 ␥␦ T cell expansion involved an initial phagocytosis event.…”

Mycobacterium-Specific γ9δ2T Cells Mediate Both Pathogen-Inhibitory and CD40 Ligand-Dependent Antigen Presentation Effects Important for Tuberculosis Immunity

“…Extracellular BCG were removed by washing. IL-2 (20 U/ml) in fresh RPMI 1640 medium with 10% human serum, 2 mM L-glutamine, and 1% penicillin-streptomycin was added every 3 or 4 days as needed to maintain the lines (28,29). ␥ 9 ␦ 2 T cell clones were generated from BCG-specific ␥ 9 ␦ 2 T cell lines as described previously (29).…”

“…␥ 9 ␦ 2 T cell clones were prepared by limiting dilution from BCG-expanded ␥ 9 ␦ 2 T cell lines as previously described (29). ␥␦ T cells were depleted from an aliquot of autologous PBMC prior to stimulation with BCG.…”

“…In addition, ␥ 9 ␦ 2 T cells can be stimulated by naturally occurring nonpeptidic antigens, such as prenyl pyrophosphates (also known as phosphoantigens), potentially broadening the host immune recognition of invading mycobacterial pathogens (26,27). However, although ␥ 9 ␦ 2 T cells can be expanded by stimulation with phosphoantigens, we have previously demonstrated that these phosphoantigen-expanded ␥ 9 ␦ 2 T cells do not provide optimal protective effects capable of inhibiting intracellular mycobacterial growth (28,29). Therefore, the specific M. tuberculosis antigens capable of inducing ␥ 9 ␦ 2 T cells relevant for TB protective immunity remain to be identified.…”

Identification of a New Tuberculosis Antigen Recognized by γδ T Cell Receptor

“…Originally, M. tuberculosis-derived nonpeptidic, phosphorylated biometabolites (phosphoantigens), such as isopentenyl pyrophosphate (IPP), were regarded as the main ␥␦ T cell receptor (TCR)-recognized antigens. However, phosphoantigen-activated ␥␦ T cells display a restricted TCR diversity, and only a subset of phosphoantigenresponsive ␥␦ T cells mediate protective immunity against M. tuberculosis (10). In contrast, ␥␦ T cells activated by M. tuberculosisderived protein antigens were reported to be able to effectively induce innate and adaptive immunity against M. tuberculosis.…”