PDCD1 and malignant colon neoplasm: Indeed, in models of pancreatic cancer, colon cancer, and breast cancer, blockade of CXCR2 suppresses the recruitment of MDSCs, triggering antitumor immunity, increasing T cell numbers, and sensitizing tumors to anti-PD-1 therapy.57 The above studies clearly demonstrate that PD-1/PD-L1 checkpoint blockade may indirectly drive checkpoint resistance by expanding and recruiting MDSCs.