The helper-dependent pathway of priming CD8(+) T cells involves "licensing" of DCs by CD40L on CD4(+) T cells. The helper-independent ("helpless") pathways elicited by many viruses, including influenza, are less widely understood. We have postulated that CD40L can be up-regulated on DCs by such viruses, and this promotes priming of CD8(+) T cells via CD40. Most studies on costimulation have been performed in the presence of CD4(+) T cells, and so the role of CD40L costimulation under helpless circumstances has not been fully elucidated. Here, we investigated such a role for CD40L using CD40L KO mice. Although the number of influenza-specific CD8(+) T cells was unaffected by the absence of CD4(+) T cells, it was markedly decreased in the absence of CD40L. Proliferation (the number of CD44(+)BrdU(+) influenza-specific CD8(+) T cells) in the primary response was diminished in CD40L KO mice at Day 8 but not at Day 5 after infection. MLR studies indicated that CD40L expression on DCs was critical for CD8(+) T cell activation. Adoptive transfer of CD40 KO CD8(+) T cells compared with WT cells confirmed that CD40 on such cells was critical for the generation of primary anti-influenza CD8(+) T cell responses. The late effect also corresponded with the late expression of CD40 by influenza-specific CD8(+) T cells. We suggest that costimulation via CD40L on DCs and CD40 on CD8(+) T cells is important in optimizing primary CD8(+) T cell responses during influenza infection.