PRF1 and neoplasm: While CD4+ T cells kill tumor cells and recruit tumor-specific CTLs by producing IFN-γ and IL-2 21, CTLs mediate tumoricidal activity directly through the release of cytotoxic granules (perforin and granzyme) or indirectly through secretion of cytokines (IFN-γ and tumor necrosis factors) 22.