PRF1 and neoplasm: To determine the role of both helper and cytotoxic activities of Trp1 cells in tumor rejection, we transferred either Trp1 or perforin-1-deficient Trp1 cells (Prf1−/−Trp1) (Figure S1E) into wild-type (WT) or Ifngr1−/− hosts combined with radiation and αCTLA-4 (Figure 1D).