Fig. 1

The gut microbiome interacts with the immune system locally in the gut and peripherally in the tumor microenvironment (TME). Within the gut, the gut microbiota plays an essential role in maintaining the mucosal barrier to protect the gut from pathogens. Microbes can interact with DCs directly and induce their maturation. Some microbiota-derived metabolites, such as SCFAs, inosine, and peptidoglycan, or invasive microbes can activate macrophages (Møs), T cells and B cells in the lamina propria or Peyer’s patches. Systemically, gut microbiota-derived metabolites can disseminate to distal sites, especially the TME, through the portal vein and interact with tumor-associated lymphocytes, including DCs, NK cells, Møs and T cells. CTL cytotoxic T lymphocyte