PDCD1 and cancer: Recent studies have revealed that elevated tumor matrix stiffness not only amplifies cancer immune evasion by upregulating the expression of immune checkpoint molecules PD-L1, PD-1, and cytotoxic T lymphocyte-associated protein 4 (CTLA-4) 43, 159, but also forms physical barriers that impede T cell migration and infiltration 21, 114, while limiting the delivery and penetration of immune checkpoint inhibitors 155.