Extracellular acidity in tumor tissue upregulates programmed cell death protein 1 expression on tumor cells via proton-sensing G protein-coupled receptors

Acidity in the tumor microenvironment has been reported to promote cancer growth and metastasis. In our study, we examined a potential relation between extracellular acidity and expression level of the immune checkpoint molecule programmed cell death protein 1 (PD-L1) in murine squamous cell carcinoma (SCC) and melanoma cell lines. PD-L1 expression in the tumor cells was upregulated by culturing in a low pH culture medium. Tumor-bearing mice were allowed to ingest sodium bicarbonate, resulting in neutralization of acidity in the tumor tissue, a decrease in PD-L1 expression in tumor cells and suppression of tumor growth in vivo. Proton-sensing G protein-coupled receptors, T-cell death-associated gene 8 (TDAG8) and ovarian cancer G-protein-coupled receptor 1 (OGR1), were upregulated by low pH, and essentially involved in the acidity-induced elevation of PD-L1 expression in the tumor cells. Human head and neck SCC RNAseq data from the Cancer Genome Atlas also suggested a statistically significant correlation between expression levels of the proton sensors and PD-L1 mRNA expression. These findings strongly suggest that neutralization of acidity in tumor tissue may result in reduction of PD-L1 expression, potentially leading to inhibition of an immune checkpoint and augmentation of antitumor immunity.