Granzyme A Produced by γ9δ2 T Cells Induces Human Macrophages to Inhibit Growth of an Intracellular Pathogen

Human γ9δ2 t cells potently inhibit pathogenic microbes,including intracellular mycobacteria,but the key inhibitory mechanism(s) involved have not been identified. we report a novel mechanism involving the inhibition of intracellular mycobacteria by soluble granzyme a. γ9δ2 t cells produced soluble factors that could pass through 0.45 μm membranes and inhibit intracellular mycobacteria in human monocytes cultured below transwell inserts. neutralization of tnf-α in co-cultures of infected monocytes and γ9δ2 t cells prevented inhibition,suggesting that tnf-α was the critical inhibitory factor produced by γ9δ2 t cells. however,only sirna- mediated knockdown of tnf-α in infected monocytes,but not in γ9δ2 t cells,prevented mycobacterial growth inhibition. investigations of other soluble factors produced by γ9δ2 t cells identified a highly significant correlation between the levels of granzyme a produced and intracellular mycobacterial growth inhibition. furthermore,purified granzyme a alone induced inhibition of intracellular mycobacteria,while knockdown of granzyme a in γ9δ2 t cell clones blocked their inhibitory effects. the inhibitory mechanism was independent of autophagy,apoptosis,nitric oxide production,type i interferons,fas/fasl and perforin. these results demonstrate a novel microbial defense mechanism involving granzyme a-mediated triggering of tnf-α production by monocytes leading to intracellular mycobacterial growth suppression. this pathway may provide a protective mechanism relevant for the development of new vaccines and/or immunotherapies for macrophage-resident chronic microbial infections. © 2013 spencer et al.