Gut microbiota, Th17 cells, and IL-17 families. (a) The increase in IL-17 induced by gut microbiota can upregulate the expression of CD39 and CD73 in Treg cells and enhance the immunosuppressive ability of Treg cells. Meanwhile, IL-17 promotes the immunosuppressive ability of MDCS by upregulating IL-10 and IL-13, thus promoting tumor immune evasion. (b) Gut microbiota induces Th0 cells to differentiate into Th17 cells and migrate into the bone marrow. Il-17 released from Th17 cells in the bone marrow promotes MM progression through the IL-6–STAT3 axis. (c) In CRC, gut microbiota promotes the function of MDSCs, induces T-cell apoptosis, inhibits NK cell function, and promotes M2 polarization of macrophages.