PDCD1 and cancer: Immune checkpoint inhibitors (ICIs) act specifically to block CTLA4 and PD-1 [56, 57], so we compared GzmA and GzmB expression in CD57+ CD4+ T cells in blood obtained from cancer patients before and after treatment with anti-CTLA4/PD-1 (ipilimumab/nivolumab) combination checkpoint inhibitor therapy.