CD274 and neoplasm: In this regard, the activation of the PD-1/PD-L1 pathway has a central role, witnessed by the expression of PD-L1 by multiple cell types within the microenvironment of OPL (tumor-associated macrophages, fibroblasts, lymphocytes) and by the fact that PD-L1 expression in epithelial and subepithelial cells is associated with malignant transformation [57].