Liu and colleagues develop a dendritic cell vaccine loaded with the gut commensals Bacteroides fragilis (DC-Bf), which triggers CD8+ T cell-dependent antitumor immune responses via MHC-I-mediated cross-presentation and interleukin-12 secretion, and enhances the efficacy of PD-1 antibody by improving the immunosuppressive tumor microenvironment and diversifying the T cell receptor repertoire.