Antigen-specific T cells encountering and responding to tumor cells are regulated by a variety of mechanisms including inhibitory receptors and regulatory T cells to prevent overexposure immune responses. These regulatory mechanisms, so called ‘‘immune checkpoints’’, suppress T cell response. Thus, it prevents tissue damage and the development of autoimmunity by preventing an excessive immune response. Immune checkpoint inhibitors are increasingly used in clinical practice and have become part of the standard treatment approach for the treatment of various tumor types. These agents are antibody-based treatments targeting cytotoxic T-lymphocyte antigen-4, programmed cell death-1 and programmed cell death ligand-1. They have been used in the treatment of melanoma, renal cell carcinoma, non-small cell lung cancer, and some other tumor types. The incidence of toxicities has expanded with increasing clinical use. The side effects resulting from the immunological effects of these treatments are called ‘‘immune-related adverse events’’ and affect many organs in a wide spectrum, unlike conventional chemotherapy side effects. In order to understand the mechanisms of action and side effects of these therapies, which are more frequently encountered in clinical practice, it is necessary to understand immune checkpoints and inhibitors. In this review, receptors that function as immunological checkpoints, as well as the drugs targeting them, are described.

Keywords: PD-1, PD-L1, CTLA-4, Cancer, Immunotherapy