Abstract
CD4(+) T-cell help (CD4 help) plays a pivotal role in CD8(+) T-cell responses against viral infections. However, the role in primary CD8(+) T-cell responses remains controversial. We evaluated the effects of infection route and viral dose on primary CD8(+) T-cell responses to vaccinia virus (VACV) in MHC class II(-/-) mice. CD4 help deficiency diminished the generation of VACV-specific CD8(+) T cells after intraperitoneal (i.p.) but not after intranasal (i.n.) infection. A large viral dose could not restore normal expansion of VACV-specific CD8(+) T cells in i.p. infected MHC II(-/-) mice. In contrast, dependence on CD4 help was observed in i.n. infected MHC II(-/-) mice when a small viral dose was used. These data suggested that primary CD8(+) T-cell responses are less dependent on CD4 help in i.n. infection compared to i.p. infection. Activated CD8(+) T cells produced more IFN-γ, TNF-α and granzyme B in i.n. infected mice than those in i.p. infected mice, regardless of CD4 help. IL-2 signaling via CD25 was not necessary to drive expansion of VACV-specific CD8(+) T cells in i.n. infection, but it was crucial in i.p. infection. VACV-specific CD8(+) T cells underwent increased apoptosis in the absence of CD4 help, but proliferated normally and had cytotoxic potential, regardless of infection route. Our results indicate that route of infection and viral dose are two determinants for CD4 help dependence, and intranasal infection induces more potent effector CD8(+) T cells than i.p. infection.