USP18 expression in tumor cells with IFN-γ signaling. B16 tumor cells were stimulated in vitro with IFN-γ (10 ng/ml) (R&D systems), IFN-α (10 ng/ml) (R&D systems), or LPS (1 μg/ml) (Invivogen) for 24 hr, then harvested for qRT-PCR assay of USP18 mRNA (A) and Western blot assay (B) of USP18 and pStat1 protein expression. EMT6 and 4 T1 tumor cells were untreated or treated with IFN-γ (5 ng/ml) for 24 hr, USP18 expression was analyzed by Western blot analysis (C). B16-GFP cells were subcutaneously inoculated into C57BL/6 mice until tumor sizes reached about 5 mm. Some mice received adoptive transfer of 5 × 106 activated OT-1 cells for 48 hr. B16-GFP tumor cells were collected and qRT-PCR analysis of USP18 expression (D). B16 tumor cells were cocultured with activated OT-1 cells for 24 hr and analyzed for USP18 and pStat1 expression (E).