PRF1 and neoplasm: Besides, the assessment of cytotoxic markers, Perforin and Granzyme B, in OT-1 CD8 + T cells showed that the upregulation of UPP1 in tumor cells significantly reduced the cytotoxicity of OT-1 CD8 + T cells, while this killing ability could be partially restored by blocking PD-L1 (Fig. 4i and Supplementary Fig. 10i).