Prostate Cancer Cells Secrete PD-1 in Exosomes to Enhance Myeloid-Derived Suppressor Cell Activity and Promote Tumor Immune Evasion.

PD-1 restrains effective killing of cancer cells by the immune system and is predominantly located on the surface of T cells or other immune cells. However, cancer cells also express PD-1 to varying degrees, which is commonly associated with a poor prognosis. Here, we investigated the regulation and function of PD-1 expression in prostate cancer (PCa) and revealed the impact on the tumor microenvironment. PD-1 expression in cancer cells positively correlated with Gleason grade and metastasis but negatively correlated with CD8+ T cell infiltration in PCa patients. PCa cells secreted PD-1 in exosomes that enhanced the activity of myeloid-derived suppressor cells (MDSCs) by activating JAK/STAT3 signaling. The activated MDSCs in turn reduced the infiltration of CD8+ T cells within the tumor, promoting tumor immune evasion. The ubiquitin-specific protease USP7 induced deubiquitination and elevated the abundance of PD-1 in PCa, and USP7 inhibition sensitized PCa tumors to anti-PD-1 antibody treatment. Given the modest efficacy of current immunotherapeutic approaches for PCa, strategies to inhibit the secretion of PD-1-bearing exosomes or USP7 function may emerge as promising immunostimulatory interventions for treating PCa.