Background: Tumor-associated macrophage (TAM) is an important innate immune cell-subset in tumor microenvironment, and that is also a pivotal orchestrator of tumor-promoting inflammation and tumor progression. Evidence proved that TAMs are up-regulated in a great number of cancers, and most of them are alternative activated M2 phenotype, which greatly promote the progress of cancer diseases. Group 1 innate lymphocytes including conventional NK cells and type 1 innate lymphocytes (ILC1s), are

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... ndant in intestinal tissue, and characterized by expressing transcription factor T-bet and secreting interferon (IFN)-γ, which can promote the macrophage to classically activated anti-tumor M1 phenotype. However, the relationship between these two cell subsets remains unclear in colon cancer. Methods: Flow cytometry was used to detect the percentage of M1 phenotype macrophage, M2 phenotype macrophage and group 1 innate lymphocytes in colon cancer tissue and paracancer healthy colon tissue of AOM/DSS-induced colon cancer mice model. In vitro isolating group 1 innate lymphocytes and inducing bone marrow-derived macrophage to detect the cross-talk when co-cultured. Adoptively transfer or blocking group 1 innate lymphocytes in vivo to investigate the role of group 1 innate lymphocytes on tumor-infiltrating macrophage and the tumor growth. Results: We found that M1 phenotype macrophage and group 1 innate lymphocytes were down-regulated in colon cancer tissue, and they were positively correlated. Group 1 innate lymphocytes promoted macrophage to classically activated M1 phenotype in vitro, and that could be blocked by anti-IFN-γ. In vivo results showed that the administration of group 1 innate lymphocytes-blocking antibody anti-NK1.1 could decrease the number of M1 phenotype macrophage in tumor tissue of MC38 tumor-bearing mice and promote the tumor growth, while adoptively transferring group 1 innate lymphocytes led to tumor-inhibiting and level of M1 phenotype macrophage up-regulating in MC38 tumor-bearing mice. Conclusions: Our studies preliminarily prove that group 1 innate lymphocytes promote the alternative activation of M1 macrophage by secreting IFN-γ to inhibit the progress of colon cancer for the first time, which may provide an insight in the immunotherapy of colon cancer.