CTLA4 or CTLA-4 (cytotoxic T-lymphocyte-associated protein 4), also known as CD152 (cluster of differentiation 152), is a protein receptor that, functioning as an immune checkpoint, downregulates immune responses. CTLA4 is constitutively expressed in Tregs but only upregulated in conventional T cells after activation. It acts as an `off` switch when bound to CD80 or CD86 on the surface of antigen-presenting cells.