Cytotoxic T lymphocytes (CTL) represent one of the front lines of defense for the immune system, killing virus-infected and tumor-transformed cells. CTL use at least two mechanisms to induce apoptosis in their targets, one mediated by perforin and granzymes, and the other triggered by the death ligand, CD95-ligand (CD95L). Here we used an in vivo cytotoxicity assay to measure specific clearance of antigen-bearing target cells in mice that had previously been immunized with non-infectious cell-associated-antigens. We found that perforin was dispensable for efficient clearance of antigen-bearing cells from immunized mice, but only if CD95/CD95L was functional; however, there was a delay in target cell clearance in the absence of perforin. Additionally, we observed approximately 35% target cell clearance in the absence of both perforin and CD95L which was only slightly abrogated in the presence of a neutralizing anti-TNF antibody. The presence of a dominant negative Fas-Associated Death Domain (FADD) did not block target cell clearance and therefore cannot be attributed to known death receptors. Taken together these data suggest that perforin- and CD95L-dependent killing are complementary at early time points, can each compensate for the absence of the other at later time points and that there is an additional component of antigen-restricted CTL killing independent of both perforin, CD95L and TNFα.
Keywords: death receptor/ligand, perforin, antigen-restricted killing, in vivo cytotoxicity