CTLA4 and neoplasm: CTLA-4 integrates with the costimulatory molecules CD80 (B7-1) and CD86 (B7-2) that express on the surfaces of APCs, while PD-L1 is expressed on a wide variety of cell types, including tumor-associated fibroblasts, tumor cells, APCs, etc. As a result, CTLA-4 inhibits T-cell activation within secondary lymphoid organs, but PCD-1/PD-L1 chiefly regulates T-cell function within peripheral tissues and the tumor microenvironment.