CD80 and neoplasm: However, under the tumor background, immunosuppressive cytokines (such as IL-10, IL-4, CX3CL, etc.)and negative costimulatory molecules (such as PD-1/PD-L1, CTLA-4/CD80/CD86, etc.)greatly limit the proliferation and activity of CTL and even cooperate with tumor extracellular matrix to exclude CTL from the tumor parenchyma, making it unable to contact tumor cells (64, 65).