CTLA-4
CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) is an immune checkpoint receptor that inhibits T-cell activation at the priming stage, when a T cell first encounters an antigen-presenting cell.
Function
CTLA-4 competes with the costimulatory receptor CD28 for the same ligands, CD80 and CD86, on antigen-presenting cells. CD28 binding delivers the second signal a naive T cell needs to activate; CTLA-4 binds CD80/CD86 more avidly, delivers an inhibitory signal instead, and removes the ligands from the surface by trans-endocytosis. The receptor’s job is to set the threshold for starting a response, which is why its loss produces explosive lymphoproliferation: mice and humans without CTLA-4
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