Project Abstract Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells (IMCs). Under physiological conditions, IMCs quickly differentiate into mature granulocytes, macrophages or dendritic cells. By contrast, under pathological conditions, a partial block of IMC differentiation into mature myeloid cells results in the expansion and activation of this population. In human cancer patients, massive accumulation of MDSCs is a hallmark of cancer progression. One key function of MDSCs is to inhibit activation of cytotoxic T lymphocytes (CTLs) through multiple suppressive mechanisms. PD-L1 has emerged as a new immune suppressive factor of MDSCs. However, the function of MDSC-expressed PD-L1 in suppression of CTL activation in the tumor microenvironment is currently controversial. Our preliminary studies determined for the first time that type I IFNs regulate constitutive PD-L1 expression in MDSCs in an autocrine manner. We further determined that IFNAR1 controls PD-L1 expression level in MDSCs in the tumor microenvironment. Therefore, type I IFNs might play a dominant role over IFNγ in up-regulating PD-L1 expression in MDSCs in the tumor microenvironment, which remains to be determined. Our central hypothesis is that type I IFNs regulate PD-L1 expression in tumor-infiltrating MDSCs and both tumor-expressed and MDSC-expressed PD-L1 contributes to CTL suppression and tumor immune evasion in human colon cancer. The objectives are: 1) elucidate the molecular mechanism underlying PD-L1 expression regulation by type I IFNs in MDSCs; 2) Determine the relative contributions of tumor-expressed and MDSC-expressed PD-L1 in suppression of CTL activation and tumor immune evasion; and 3) Test the hypothesis that type I IFN regulates PD-L1 expression in MDSCs in human colon cancer patients. Successful completion of the proposed studies will determine the function of MDSC-expressed PD-L1 in immune suppression and tumor immune evasion in human colorectal cancer patients and identify novel molecular target to enhance the efficacy of checkpoint inhibitor immunotherapy in human colon cancer.