19Mucosal-associated invariant T (MAIT) cells can be activated by viruses through a cytokine-20 dependent mechanism, and thereby protect from lethal infection. Given this, we reasoned 21 MAIT cells may have a critical role in the immunogenicity of replication-incompetent 22 adenovirus vectors, which are novel and highly potent vaccine platforms. In vitro, ChAdOx1 23 (Chimpanzee Adenovirus Ox1) induced potent activation of MAIT cells. Activation required 24 transduction of monocytes and plasmacytoid dendritic cells to produce IL-18 and IFN-a, 25 respectively. IFN-a-induced monocyte-derived TNF-a was identified as a novel intermediate 26 in this activation pathway, and activation required combinatorial signaling of all three 27 cytokines. Furthermore, ChAdOx1-induced in vivo MAIT cell activation in both mice and 28 human volunteers. Strikingly, MAIT cell activation was necessary in vivo for development of 29ChAdOx1-induced HCV-specific CD8 T cell responses. These findings define a novel role for 30