Immune Checkpoint Proteins

Immune checkpoint proteins are the "brake molecules" of the immune system. They are predominantly expressed on the surface of immune cells such as T cells, and their core function is to regulate the intensity of immune responses, preventing excessive activation that could damage self-tissues. Under physiological conditions, receptors on T cells—such as PD-1 and CTLA-4—bind to their ligands (e.g., PD-L1, CD80/CD86) on normal cells or antigen-presenting cells, thereby suppressing T cell activation and maintaining immune homeostasis. Tumor cells often overexpress ligands such as PD-L1, which bind to PD-1 on T cells, inhibiting immune-mediated killing and enabling immune evasion.