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Background Type I interferons (IFN-I) have recently emerged as key regulators of tumor response to chemotherapy and immunotherapy. However, IFN-I function in cytotoxic T lymphocytes (CTLs) in the tumor microenvironment is largely unknown. Methods Tumor tissues and CTLs of human colorectal cancer patients were analyzed for interferon (alpha and beta) receptor 1 (IFNAR1) expression. IFNAR1 knock out (IFNAR-KO), mixed wild type (WT) and IFNAR1-KO bone marrow chimera mice, and mice with IFNAR1 deficiency only in T cells (IFNAR1-TKO) were used to determine IFN-I function in T cells in tumor suppression. IFN-I target genes in tumor-infiltrating and antigen-specific CTLs were identified and functionally analyzed. Results IFNAR1 expression level is significantly lower in human colorectal carcinoma tissue than in normal colon tissue. IFNAR1 protein is also significantly lower on CTLs from colorectal cancer patients than those from healthy donors. Although IFNAR1-KO mice exhibited increased susceptibility to methylcholanthrene-induced sarcoma, IFNAR1-sufficient tumors also grow significantly faster in IFNAR1-KO mice and in mice with IFNAR1 deficiency only in T cells (IFNAR1-TKO), suggesting that IFN-I functions in T cells to enhance host cancer immunosurveillance. Strikingly, tumor-infiltrating CTL levels are similar between tumor-bearing WT and IFNAR1-KO mice. Competitive reconstitution of mixed WT and IFNAR1-KO bone marrow chimera mice further determined that IFNAR1-deficient naïve CTLs exhibit no deficiency in response to vaccination to generate antigen-specific CTLs as compared to WT CTLs. Gene expression profiling determined that Gzmb expression is down-regulated in tumor-infiltrating CTLs of IFNAR1-KO mice as compared to WT mice, and in antigen-specific IFNAR1-KO CTLs as compared to WT CTLs in vivo. Mechanistically, we determined that IFN-I activates STAT3 that binds to the Gzmb promoter to activate Gzmb transcription in CTLs. Conclusion IFN-I induces STAT3 activation to activate Gzmb expression to enhance CTL effector function to suppress tumor development. Human colorectal carcinoma may use down-regulation of IFNAR1 on CTLs to suppress CTL effector function to evade host cancer immunosurveillance. Electronic supplementary material The online version of this article (10.1186/s40425-019-0635-8) contains supplementary material, which is available to authorized users.

Tumor cells respond to IFN-γ of activated T cells to upregulate programmed death-ligand 1 (PD-L1) in the tumor microenvironment as an adaptive immune resistance mechanism. Tumor cells also express oncogene-driven PD-L1. PD-L1 is also expressed on myeloid-derived suppressor cells (MDSCs). It is known that both type I and II IFNs upregulate PD-L1 expression in MDSCs. However, the molecular mechanism underlying PD-L1 expression in MDSCs is still largely unknown. We report in this article that MDSCs exhibit constitutive STAT1 phosphorylation in vitro without exogenous IFNs, indicating a constitutive active JAK-STAT signaling pathway in mouse MDSCs in vitro. Furthermore, IFN-α and IFN-β but not IFN-γ are endogenously expressed in the MDSC cell line in vitro and in tumor-induced MDSCs in vivo. Neutralizing type I IFN or inhibiting the JAK-STAT signaling pathway significantly decreased constitutive PD-L1 expression in MDSCs in vitro. However, neither IFN-α expression level nor IFN-β expression level is correlated with PD-L1 expression level in MDSCs; instead, the level of IFN receptor type I (IFNAR1) is correlated with PD-L1 expression levels in MDSCs. Consequently, knocking out IFNAR1 in mice diminished PD-L1 expression in tumor-induced MDSCs. Therefore, we determined that 1) PD-L1 expression in MDSCs is activated by type I IFN through an autocrine manner and 2) the expression level of PD-L1 is controlled at least in part by the IFNAR1 level on MDSCs. Our data indicate that MDSCs may maintain their PD-L1 expression via autocrine type I IFN to exert their suppressive activity in the absence of IFN-γ from the suppressed T cells in the tumor microenvironment.

SUMMARY IL-10 functions as a suppressor of colitis and colitis-associated colon cancer, but it is also a risk locus associated with ulcerative colitis. The mechanism underlying the contrasting roles of IL-10 in inflammation and colon cancer is unknown. We report here that inflammation induces the accumulation of CD11b+Gr1+ myeloid-derived suppressor cells (MDSCs) that express high levels of IL-10 in colon tissue. IL-10 induces the activation of STAT3 that directly binds to the Dnmt1 and Dnmt3b promoters to activate their expression, resulting in DNA hypermethylation at the Irf8 promoter to silence IRF8 expression in colon epithelial cells. Mice with Irf8 deleted in colonic epithelial cells exhibit significantly higher inflammation-induced tumor incidence. Human colorectal carcinomas have significantly higher DNMT1 and DNMT3b and lower IRF8 expression, and they exhibit significantly higher IRF8 promoter DNA methylation than normal colon. Our data identify the MDSC-IL-10-STAT3-DNMT3b-IRF8 pathway as a link between chronic inflammation and colon cancer initiation.