Figure 2.
The concept of checkpoint antibodies in immunotherapy. The T cell receptor on CD8+ cytotoxic T cells recognizes MHC class I molecules in combination with a specific peptide (grey dot). This would activate the T cell and induce the delivery of the cytolytic hit on the tumor cell unless the response is quenched by checkpoint proteins like PD-1 on the activated cytotoxic T cells and its cognate receptor PD-L1 or PD-L2 on the tumor cell. Checkpoint antibodies bind to these proteins to inhibit their interaction and prevent the quenching of the immune responses, thus initiating unrestricted killing of tumor cells by tumor cell specific cytotoxic T cells.