Nitric oxide-producing myeloid-derived suppressor cells inhibit vascular E- selectin expression in human squamous cell carcinomas

Squamous cell carcinomas (SCC) are sun-induced skin cancers that are particularly numerous and aggressive in immunosuppressed individuals. SCC evade immune detection at least in part by down-regulating E-selectin on tumor vessels, thereby restricting entry of skin homing T cells into tumors. We find that nitric oxide potently suppresses E-selectin expression on human endothelial cells and that SCC are infiltrated by nitric oxide-producing iNOS(+) CD11b(+) CD33(+) CD11c(−) HLA-DR(−) myeloid-derived suppressor cells (MDSC). MDSC from SCC produced NO, TGFβ and arginase and inhibited endothelial E-selectin expression in vitro. MDSC from SCC expressed the chemokine receptor CCR2 and tumors expressed the CCR2 ligand HBD3, suggesting CCR2-HBD3 interactions may contribute to MDSC recruitment to SCC. Treatment of SCC in vitro with the iNOS inhibitor L-NNA induced E-selectin expression at levels comparable to imiquimod-treated SCC undergoing immunologic destruction. Our results suggest that local production of NO in SCC may impair vascular E-selectin expression. We show that MDSC are critical producers of NO in SCC and that NO inhibition restores vascular E-selectin expression, potentially enhancing T cell recruitment. iNOS inhibitors and other therapies that reduce NO production may therefore be effective in the treatment of SCC and their premalignant precursor lesions actinic keratoses.