CD152 (CTLA-4 (cytotoxic T-lymphocyte associated protein 4)) and CD28 are structurally homologous molecules that are members of the immunoglobulin (Ig) gene superfamily. Both CD152 and CD28 are composed of a single Ig V-like extracellular domain, a transmembrane domain and an intracellular domain.
CD152 and CD28 are both expressed on the cell surface as homodimers or as monomers. CD152 was originally identified as a gene that was specifically expressed by cytotoxic T lymphocytes. However, CD152 transcripts have since been found in both Th1 and Th2, and CD4⁺ and CD8⁺ T cell clones. Whereas, CD28 expression is constitutive on the surfaces of 95% of CD4⁺ T cells and 50% of CD8⁺ T cells and is down regulated upon T cell activation, CD152 expression is upregulated rapidly following T cell activation and peaks approximately 24 hours following activation. Although both CD152 and CD28 can bind to the same ligands, CD152 binds to B71 and B72 with 20-100-fold higher affinity than CD28.

CD152 (CTLA-4) and CD28, together with their ligands B7-1 and B7-2 (cluster of differentiation 80 and 86 respectively), constitute one of the dominant costimulatory pathways that regulate T and B cell responses. CTLA-4 (cytotoxic T-lymphocyte associated protein 4) is known to prevent immune responses, and its deficiency causes fatal lymphoproliferation in mouse models. Overexpression of the gene is observed in rheumatoid arthritis. Mutation in CTLA-4 is associated with insulin-dependent (type 1) diabetes mellitus, Grave′s disease and Hashimoto′s thyroiditis.

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The extracellular domain of human CD152 [CTLA-4] (aa 37-160) is fused to the N-terminus of the Fc region of a mutant human IgG1.