PRF1 and neoplasm: Activation of PD-1/PD-L1 pathway (up-regulation of PD-L1 level on tumor cells and/or increase of PD-1-expressing CD8 T cells) reduces the release of anti-tumor cytokines (e.g. IFN-γ) or cytotoxins (e.g. perforin or granzyme B) by CD8 T cells resulting in inhibition of T cell anti-tumor cytotoxic function (7).