Intraperitoneal colon cancer cells also upregulated the expression of immune checkpoints, PD-L1, PD-1, and CTLA-4, which led to the dysfunction of T cells within peritoneal tumors.45–47 Notably, T cells within peritoneal tumors displayed severe immune dysfunction compared with those within the spleen; they upregulated PD-1, as opposed to interleukin-2 and CD28, which are required for T cell activation and survival. Here, PDCD1 is linked to malignant colon neoplasm.