Granzyme A Produced by gamma(9)delta(2) T Cells Induces Human Macrophages to Inhibit Growth of an Intracellular Pathogen

Human gamma(9)delta(2) 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. gamma(9)delta(2) T cells produced soluble factors that could pass through 0.45 mu m membranes and inhibit intracellular mycobacteria in human monocytes cultured below transwell inserts. Neutralization of TNF-alpha in cocultures of infected monocytes and gamma(9)delta(2) T cells prevented inhibition, suggesting that TNF-alpha was the critical inhibitory factor produced by gamma(9)delta(2) T cells. However, only siRNA-mediated knockdown of TNF-alpha in infected monocytes, but not in gamma(9)delta(2) T cells, prevented mycobacterial growth inhibition. Investigations of other soluble factors produced by gamma(9)delta(2) 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 gamma(9)delta(2) 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-alpha 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.