Studies have shown that T-cell-dendritic cell (DC) interaction is required for efficient DC maturation. However, the identities of the molecules that mediate the interaction in vivo are largely unknown. Here, we show that maturation of DCs as well as CD8 T-cell responses were impaired in B7-H1-deficient (B7-H1(-/-)) mice to influenza virus infection. Both defects were restored by transferring B7-H1-expressing naïve T cells into B7-H1(-/-) mice. Similarly, transferring DCs from wild-type mice or from RAG1(-/-) mice that had been injected with B7-H1-expressing naïve T cells also restored CD8 T-cell responses in B7-H1(-/-) mice. These results demonstrate that B7-H1 on naïve T cells is required to condition immature DCs to undergo efficient maturation when they encounter microbial infection. In return, the mature DCs stimulate a robust T-cell response against the infecting pathogen.