EP4 (PTGER4) modulates the prostate cancer immune microenvironment. YY001, an EP4 antagonist, inhibits MDSC differentiation and function while enhancing T-cell proliferation and antitumor activity. It reduces MDSC infiltration and boosts CD8+ T-cell activation, resulting in a robust antitumor immune response in clinical. Here, CD8A is linked to prostate cancer.