Dendritic cells (DC) utilize at least two pathways to process viral antigens onto MHC class I molecules. The conventional endogenous route is used to acquire antigens from both infectious and non-replicating virions. Exogenous pathways are used by DC to acquire and "cross-present" antigens derived from virus-infected donor cells that by themselves lack the ability to activate T cells directly. We analyzed the role of this pathway for antigens derived from vaccinia, a virus which inhibits DC maturation and causes extensive apoptosis of infected cells, yet is highly immunogenic. Using recombinant vaccinia virus encoding the influenza matrix protein as model vector, DC were shown to cross-present vaccinia-derived antigens from both apoptotic and necrotic infected cells to antigen-specific CD8 + T cells. Efficient cross presentation required uptake of dead cells by immature DC and exposure to maturation stimuli, especially CD40 ligand. The responding CD8 + T cells secreted IL-2 and IFN-+ , proliferated and developed into cytotoxic effectors. Quantification of the cross presentation of vaccinia-derived antigens showed this pathway to be highly efficient, corresponding to a peptide pulse of 10-100 nM. While monocytes also phagocytosed apoptotic and necrotic cells, they were far less efficient at cross-presenting vaccinia-derived antigens to CD8 + T cells. The ability of DC to cross-present vaccinia-derived antigens from infected apoptotic cells or necrotic cell lysates, bypasses the deleterious effects of direct infection of DC and provides one explanation for this pathogen's immunogenicity.