COX-2 Inhibitors Decrease Expression of PD-L1 in Colon Tumors and Increase the Influx of Type I Tumor-infiltrating Lymphocytes

Colon cancer is initiated under inflammatory conditions associated with upregulation of immune checkpoint proteins. We evaluated immune modulation induced by nonsteroidal anti-inflammatory agents used for colon cancer prevention. Both celecoxib and naproxen inhibited polyp growth in APC Min mice. Treatment of mice with either drug significantly decreased PD-L1 expression on polyps in a dose-dependent manner (P < 0.0001 for both). The decrease in PD-L1 was associated with an influx of CD8+ T cells into pared with lesions from untreated animals and correlated with disease control. Naproxen is a nonselective inhibitor of both COX-1 and COX-2, and we questioned the role of the different cyclooxygenases in PD-L1 regulation. Silencing either COX-2 or COX-1 RNA in the murine colon cancer cell line MC38, reduced PD-L1 expression by 86% in COX2???silenced cells (P < 0.0001) while there was little effect with COX-1 siRNA compared with control. Naproxen could inhibit the growth of MC38 in vivo. Naproxen-treated mice demonstrated a significant reduction in MC38 growth as compared with control (P < 0001). Both Tbet+ CD4 and