PRF1 and hepatocellular carcinoma: Granzyme B, perforin and IFNγ secretion by CD8 T cells generates potent anti-tumour activity but high expression of PD-1 on exhausted T cells contributes to ineffective effector T cell function, and selective in vitro depletion of these immunosuppressive cells results in improvement of T effector cell function in HCC patients (128, 129).