Immune suppression by tumor cells is regulated by USP18 expression. B16-OVA-GFP or B16-OVA-USP18 tumor cells without (A-B) or with IFN-γ sensitization (5 ng/ml) (C) were irradiated and cocultured with naïve OT-1 T cells. Cell proliferation (A) and IL-2 and IFN-γ secreteion (B-C) of the T cells were analyzed. PD-1 expression in cocultured OT-1 cells was monitored by flow cytometry (D). The CTL activity of activated OT-1 cells against B16-OVA-GFP and B16-OVA-USP18 was analyzed (E). B16-OVA-USP18 cells with or without IFN-γ (10 ng/ml) sensitization were irradiated and cocultured with naive OT-2 T cells for 72 hrs, and OT-2 T-cell proliferation (F) and cytokines IL-2 and IFN-γ secreteion (G) were analyzed.