PDCD1 and neoplasm: Moreover, this chemotherapy-mediated antitumor effect relies on the presence of CD8+ T cells in in vivo studies using CD8+ T cell-deficient mice bearing MC38-expressing antigen CEA2 tumors, and combined therapy with anti-PD-1 antibodies increased the efficacy of PD-1 checkpoint blockade to reduce the tumor burden in MC38-CEA2 tumor-bearing mice [43].