Cutting edge:: Murine CD59a modulates antiviral CD4+ T cell activity in a complement-independent manner

CD59 blocks formation of the membrane attack complex Of complement by inhibiting binding of C9 to the C5b-8 complex. To investigate a role for CD59 in promoting T cell responses, we compared T cell activation in CD59a-deficient (Cd59a(-/-)) and wild-type (WT) mice after in vitro stimulation and after infection with rVV. Virus-specific CD4(+) T cell responses were significantly enhanced in Cd59a(-/-) mice compared with WIT mice. Similarly, Cd59a(-/-) T cells responded more vigorously to in vitro stimulation with CD3-specific Abs compared with WT mice. This effect of CD59a on T cell proliferation was found to be complement-independent. Collectively, these results demonstrate that CD59a down-modulates CD4(+) T cell activity in vitro and in vivo, thereby revealing another link between complement regulators and T cell activation.