Viral regulation of CD95 expression and apoptosis in T lymphocytes.

T lymphocyte apoptosis has been observed in vivo in association with viral infections. One mechanism known to mediate T cell apoptosis is the CD95 (Fas/APO-1) Ag apoptotic pathway. CD95 is a cell surface protein that is expressed on activated T cells and is capable of relaying an intracellular apoptotic signal upon binding with CD95 ligand. CD95-dependent apoptosis has been shown to be involved in the homeostatic control of peripheral T cell numbers; however, the functional significance of the CD95 apoptotic pathway in the context of viral infections has not been clearly elucidated. We show that exposure to a variety of viral pathogens causes enhanced CD95 expression in naive peripheral blood cell T lymphocytes without leading to enhanced susceptibility to CD95-dependent apoptosis. Productive viral infection was not required for this effect. Furthermore, using a T cell line and the K562 tumor cell line, we demonstrate that cells normally resistant to CD95-mediated killing become susceptible when productively infected with virus in a manner that does not rely on increased CD95 surface expression. These findings demonstrate a regulatory influence of viral infection on CD95 expression and activity and suggest that in addition to having a role in T cell homeostasis, the CD95 apoptotic pathway might also function to eliminate virus-infected T cells.