Toxoplasma gondii inhibits granzyme B-mediated apoptosis by the inhibition of granzyme B function in host cells.

Host defense to the apicomplexan parasite Toxoplasma gondii is critically dependent on CD8(+) T cells, whose effector functions include the induction of apoptosis in target cells following the secretion of granzyme proteases. Here we demonstrate that T. gondii induces resistance of host cells to apoptosis induced by recombinant granzyme B. Granzyme B induction of caspase-independent cytochrome c release was blocked in T. gondii-infected cells. Prevention of apoptosis could not be attributed to altered expression of the Bcl-2 family of apoptotic regulatory proteins, but was instead associated with reduced granzyme B-mediated, caspase-independent cleavage of procaspase 3 to the p20 form in T. gondii-infected cells, as well as reduced granzyme B-mediated cleavage of the artificial granzyme B substrate, GranToxiLux. The reduction in granzyme B proteolytic function in T. gondii-infected cells could not be attributed to altered granzyme B uptake or reduced trafficking of granzyme B to the cytosol, implying a T. gondii-mediated inhibition of granzyme B activity. Apoptosis and GranToxiLux cleavage were similarly inhibited in T. gondii-infected cells exposed to the natural killer-like cell line YT-1. The endogenous granzyme B inhibitor PI-9 was not up-regulated in infected cells. We believe these findings represent the first demonstration of granzyme B inhibition by a cellular pathogen and indicate a new modality for host cell protection by T. gondii that may contribute to parasite immune evasion.