CD95 (Fas) may control the expansion of activated T cells after elimination of bacteria in murine listeriosis

CD95 (Fas) is known to mediate activation-induced T-cell death by apoptosis. To understand the role of CD95 during the course of bacterial infection, we examined the kinetics of αβ and γδ T cells in the peritoneal cavities and livers of 5-week-old CD95-defective MRL/lpr mice after an intraperitoneal infection with Listeria monocytogenes. The number of bacteria in the spleen decreased to an undetectable level by day 10 after infection with 7 x l0³ Listeria cells similar to the number in MRL/+/+ mice. The number of αβ T cells expressing CD44 and CD95 reached a maximum in the peritoneal cavity on day 6 after listerial infection and thereafter decreased gradually in MRL/+/+ mice, whereas CD44⁺ αβ T cells without CD95 expression continued to increase throughout the course of listerial infection in MRL/lpr mice. Freshly isolated T cells from MRL/+/+ mice infected with L. monocytogenes 10 days previously showed DNA fragmentation with apoptosis, whereas such fragmentation was not prominent in T cells from infected MRL/lpr mice. In correlation with the increased number of CD44⁺ αβ T cells, Listeria-specific T-cell proliferation of peritoneal exudate cells was significantly greater in MRL/lpr mice than in MRL/+/+ mice on day l0 after listerial infection. In contrast to αβ T cells, γδ T cells increased in number only transiently in the peritoneal cavity and liver after listerial infection in both MRL/lpr mice and MRL/+/+ mice. These results suggest that CD95-mediated cell death with apoptosis may be involved in termination of the αβ-T-cell-mediated immune response after the battle against L. monocytogenes has been won, whereas γδ T cells may undergo apoptosis independently of CD95 during the course of listerial infection.