The ecological balance of intestinal microbiota plays an important role in digestion,absorp-tion,metabolism,immunity and protection against pathogens.Intestinal microbiota can not only regulate the innate immunity of the body,but also can stimulate the immune response by the bacteria itself and its metabo-lites.The imbalance intestinal microbiota may lead to abnormal immune mechanism,and then participate in the occurrence and development of the tumor,especially colorectal cancer.

The tumor microenvironment is closely related to the occurrence and development of tumor. Hypoxia is considered to be one of the most important factors in tumor microenvironment.Formation of hypoxic microenvironment can be found in most of malignant tumors,which can inhibit the anti-tumor immune response. Recent studies have indicated that immunosuppressive cells,tumor stem cells and circulating tumor cells in hypoxic tumor microenvironment can mediate immune suppression and immune tolerance,and then promote development of tumor.The new immune therapy will focus on normalizing tumor vasculature,reconstructing the tumor microenvironment,avoiding immune suppression and averting tumor immune tolerance.