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.
Copyright © 2018 by The American Association of Immunologists, Inc.