The expansion of CD8+ T cells in response to Ag can be characterized as either dependent or independent of CD4+ T cells. The factors that influence this dichotomy are poorly understood but may be dependent upon the degree of inflammation associated with the Ag. Using dendritic cells derived from MHC class II-deficient mice to avoid interaction with CD4+ T cells in vivo, we have compared the immunogenicity of peptide-pulsed dendritic cells stimulated with molecules associated with infection to those stimulated via CD40. In the absence of CD4+ T cell help, the expansion of primary CD8+ T cells after immunization with TNF-α-or poly (I: C)-stimulated dendritic cells was minimal. In comparison, LPS-or CpG-stimulated dendritic cells elicited substantial primary CD8+ T cell responses, though not to the same magnitude generated by immunization with CD40L-stimulated dendritic cells. Remarkably, mice immunized with any stimulated dendritic cell population generated fully functional recall CD8+ T cells without the aid of CD4+ T cell help. The observed hierarchy of immunogenicity was closely correlated with the expression of CD70 (CD27L) on the stimulated dendritic cells, and Ab-mediated blockade of CD70 substantially prevented the CD4+ T cell-independent expansion of primary CD8+ T cells. These results indicate that the expression of CD70 on dendritic cells is an important determinant for helper-dependence of primary CD8+ T cell expansion and provide an explanation for the ability of a variety of pathogens to stimulate primary CD8+ T cell responses in the absence of CD4+ T cells.