Mechanism of action for immune checkpoint inhibition targeting CTLA-4 and PD-1

(A) Inhibition of T-cell activation by interactions with tumor cells and APCs. PD-L1 and PD-L2 on tumor cells and APCs bind to PD-1 on the T cell, and B7 on APCs binds to CTLA-4 on the T cell.
(B) Antibodies to PD-1 or CTLA-4 block inhibitory interactions, allowing for positive costimulation (B7 binds CD28).