Search

Two distinct and complementary pathways, one mediated by perforin and the other dependent upon CD95 (Fas), effect cell-mediated cytotoxicity. We examined the relative roles of these pathways in host defenses against the intracellular bacterial pathogen Listeria monocytogenes by using murine listeriosis as a model system. Mice which lacked both perforin and Fas (P0L0) were generated, and their responses to primary and secondary listeriosis were compared to those of wild-type (WT), Fas-deficient (L0), and perforin knockout (P0) mice. Relative to WT mice during primary listeriosis, P0 mice exhibited a reduced capacity to clear the infection from their spleens but not their livers whereas L0 mice had elevated bacterial titers in their livers and a modestly increased titer in their spleens. In contrast, bacterial titers in P0L0 mice were increased approximately 50- to 560-fold in their spleens and 230- to 1, 000-fold in their livers; eventual clearance of listeriae from both organs was significantly delayed. Furthermore, the resistance of P0L0 mice to secondary listeriosis was significantly reduced in their spleens and livers compared to that of WT, P0, or L0 mice. In vitro experiments indicated that immune cytotoxic T lymphocytes (CTL) lysed L. monocytogenes-infected hepatocytes primarily via a Fas-dependent, perforin-independent mechanism. The absence of Fas severely abrogated the lysis of infected hepatocytes by immune CD8(+) CTL. Taken together, these results provide the first evidence for Fas-dependent CTL-mediated lysis of L. monocytogenes-infected hepatocytes and demonstrate complementary roles for Fas and perforin in host defenses against an intracellular bacterial pathogen.

We have used a perforin-less (PO) mouse to explore alternate CTL-mediated lytic pathways. PO mice are unable to overcome an infection with LCMV in vivo. Nevertheless, splenocytes from infected mice show vigorous, antigen-specific cytotoxicity that requires the presence of the Fas antigen on target cells. The Fas lytic pathway is virtually indistinguishable, in terms of kinetics and magnitude of cytotoxicity, from perforin/granzyme-mediated lysis. It is rapidly induced in CTL upon occupation of the TcR, and requires protein synthesis for full expression. Upon removal of the activating signal, the capacity for fas-mediated lysis rapidly disappears. PO mice infected with LCMV also undergo what appears to be a CD8-mediated immunopathology, and rarely live beyond one month. The precise basis of this pathology is unknown at present. Given the widespread distribution of Fas in mice, particularly on inflamed tissues, the complete failure to clear virus from any tissue or organ is surprising.