Polyinosinic: polycytidylic acid (poly I: C), a synthetic analog of double-stranded viral RNA, serves as a potent adjuvant for vaccination against soluble proteins, pathogens, and tumors. Poly I: C is sensed by both TLR3 in the endosomes and melanoma differentiation-associated protein 5 (MDA5) in the cytoplasm. Although it is known that TLR3 is required for cross-priming of CD8 T cells specific for viral Ags, the role of MDA5 in inducing CD8 T cell responses is still unclear. In this study, we demonstrate that in mice lacking MDA5, the majority of CD8 T cells do not survive after primary immunization with poly I: C and Ag, impairing memory response to subsequent Ag challenge. Furthermore, bone marrow chimera experiments revealed that MDA5 expression in radiation-resistant stromal cells, but not in radiation-sensitive hematopoietic cells, is essential for establishing CD8 T cell memory. We conclude that MDA5 and TLR3 mediate substantially distinct yet complementary functions during poly I: C-mediated activation of Ag-specific CD8 T cell responses.