The integrity of the intestinal microbiome and specific bacterial species are important for responses to immune checkpoint inhibitors in cancer treatment, but how this interaction is mediated is unclear. Data now published in Science Immunology show that a combination therapy of anti-PD-1 and anti-CTLA-4 antibodies enables the translocation of some intestinal bacterial species into B16 melanomas and their draining lymph nodes in mice, in a dendritic cell-dependent manner. The most abundant of these translocated species were shown to activate dendritic cells to prime T cell responses for anti-tumor activity. The researchers also found that immune checkpoint inhibition induced mesenteric lymph node remodeling, which facilitated bacterial translocation to tumors and their draining lymph nodes to ultimately control the growth of those tumors.