PRF1 and neoplasm: ,21,22 Upon activation, this relatively rare (∼0.1% of total T cells) but powerful T cell subset has multimodal activity via bidirectional crosstalk with Ag-presenting cells (e.g., B cells, dendritic cells [DCs]) inducing mixed Th1/Th2-type cytokines triggering downstream effector cell activation, and direct lysis of CD1d+ tumor cells, tumor-associated macrophages (TAMs), and myeloid-derived suppressor cells (MDSCs) via perforin/granzyme B and Fas ligand cytotoxic pathways.10