CTLA-4

null Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4, CD152) is a founding immune checkpoint receptor of the CD28 superfamily, expressed on activated conventional T cells and constitutively on regulatory T cells (Tregs). CTLA-4 shares its ligands B7-1 (CD80) and B7-2 (CD86) with the co-stimulator CD28 but binds them with markedly higher affinity, competitively stripping B7 molecules off antigen-presenting cells and thereby terminating CD28 co-stimulation; CTLA-4 additionally delivers cell-intrinsic inhibitory signals through its cytoplasmic YVKM motif recruiting AP-2 and SHP-2, and its trans-endocytosis of B7 depletes APC co-stimulatory capacity. CTLA-4 is essential for peripheral tolerance: germline CTLA4 haploinsufficiency causes autoimmune lymphoproliferative syndrome. Ipilimumab, the first checkpoint-blocking antibody approved in cancer (melanoma), unleashes antitumor T cells by freeing B7 for CD28 engagement, with Treg depletion in tumors contributing to efficacy.