Immune Checkpoint Blockade in Cancer Immunotherapy: Mechanisms, Clinical Outcomes, and Safety Profiles of PD-1/PD-L1 Inhibitors

Programmed cell death protein 1 (PD-1) and its ligand PD-L1 are critical for the regulation of T cell exhaustion and activity suppression. Tumor cells expressing immune checkpoints including PD-L1 escape monitoring of T cells from the host immune system. Checkpoint inhibitors are highly promising therapies that function as tumor-suppressing factors via modulation of tumor cell–immune cell interactions as well as boosting T cell-mediated anti-tumor immunity. Notably, PD-1 or PD-L1 monoclonal antibody (mAb) has demonstrated promising therapeutic effects in clinical studies of many types of cancer. These mAbs have caused significant tumor regression with impressive anti-tumor response rates as well as a favorable safety profile in cancer patients. Furthermore, the combination of PD-1/PD-L1 mAbs with other types of anti-tumor agents has also developed to boost the anti-tumor responses and enhance therapeutic effects in cancer patients. This review clarifies the mechanisms of PD-1/PD-L1-mediated anti-cancer immune responses and some clinical studies of mAbs targeting PD-1/PD-L1. The challenges and future of PD-1/PD-L1 blockade therapy are also discussed.