Figure 1.

(A) APC presents tumor-associated antigens to circulating T cells via MHC complex binding to TCR. However, a co-stimulatory signal, provided upon the binding of CD28/B7 is required for T cell activation. Following T-cell stimulation, PD-1 expression occurs. PD-L1 and PD-L2 expressed on tumor cells can interact with PD-1 on T cells in the tumor microenvironment causing T-cell down regulation and tumor immune evasion. (B) Anti-PD-1 inhibits PD-1 binding to PD-L1 and PD-L2 allowing the immune system to recognize tumors as foreign and induce immune-mediated tumor cellular death.

Abbreviations: APC, antigen-presenting cell; MHC, major histocompatibility complex; TCR, T-cell receptor; CD, cluster designation; PD-1, programmed death-1; PD-L, programmed cell death ligand.