Abstract
Programmed death receptor-1 (PD-1) and T cell immunoglobulin and mucin domain-containing protein-3 (Tim-3) play important roles in tumor immune evasion. PD-1 blockade could produce an effective antitumor effect in many solid tumors except prostate cancer (PCa) because of rare programmed death ligand-1 (PD-L1) expression on PCa cells. Streptavidin (SA)-GM-CSF surface-anchored tumor cell (Anchored GM-CSF) vaccines could increase the number of tumor-infiltrating lymphocytes (TILs) and induce specific antitumor immune responses. The Anchored-GM-CSF vaccine and anti-PD-1 antibodies exerted synergistic effects in mouse models of PCa metastasis. However, the response rate was low due to the presence of other negative regulatory pathways. Tim-3 expression could be upregulated at resistance to combination therapy with anti-PD-1 antibodies and the Anchored GM-CSF vaccine. Sequential administration of anti-PD-1 and anti-Tim-3 antibodies could further improve the efficacy of the Anchored GM-CSF vaccine therapy, and tumor regression was noted in over 60% of animals. This triple therapy improved the specific cytotoxic activity, proliferation and secretion of CD8+ TILs and reduced the production of tumor-promoting cytokines. These findings indicated that this triple therapy could induce a robust antitumor immune response in mouse models of PCa.