CTLA4 (Cytotoxic T-Lymphocyte Associated Protein 4 / CD152 Checkpoint Gene)
CTLA4 (cytotoxic T-lymphocyte associated protein 4; HGNC:2505, NCBI Gene ID 1493) is a human locus mapped to chromosome band 2q33.2 that encodes homodimeric coinhibitory receptor CTLA-4 / CD152 (223 aa). CTLA4 acts in competitive binding & trans-endocytosis of CD80/CD86 (B7-1/B7-2) during lymph-node T-cell priming, together with PDCD1 (PD-1 Immune Checkpoint Gene), FOXP3 (Regulatory T-Cell Master Transcription Factor), and Monoclonal Antibodies. Encodes the CTLA-4 immune checkpoint receptor that outcompetes CD28 for B7 ligands to restrain early T-cell priming.
Genomic Organization & Cytogenetic Locus: 4-exon gene on chromosome 2q33.2 situated within the CD28-CTLA4-ICOS co-receptor cluster, encoding an extracellular MYPPPY-motif IgV domain (exon 2), a hydrophobic transmembrane domain (exon 3, skipped in the soluble sCTLA-4 isoform), and an clathrin AP-2-binding YVKM cytoplasmic tail (exon 4).
Cellular & Tissue Expression Profile: Constitutively expressed at high levels on FOXP3-positive CD4+ regulatory T cells (Tregs) and rapidly translocated from intracellular vesicles to the immunological synapse of conventional CD4+ and CD8+ T cells within 24–48 hours of TCR activation in secondary lymphoid organs.
Molecular Function & Pathways
Biochemical & Physiological Mechanism: Binds the shared B7 costimulatory ligands CD80 (B7-1) and CD86 (B7-2) on antigen-presenting dendritic cells with 10- to 20-fold higher avidity than the activating receptor CD28, both competitively blocking CD28 co-stimulation and physically stripping CD80/CD86 molecules off the antigen-presenting cell surface via CTLA-4-mediated trans-endocytosis for lysosomal degradation.
Clinical & Therapeutic Significance
Human Genetic Variants & Disease Phenotypes: Heterozygous loss-of-function mutations in humans cause autosomal dominant CTLA-4 haploinsufficiency with autoimmune infiltration (CHAI disease), presenting with hypogammaglobulinemia, lymphocytic infiltration of lung, brain, and gut, and autoimmune cytopenias, whereas common SNPs (such as +49A>G / rs231775) confer susceptibility to Graves disease and type 1 diabetes.
Therapeutic & Clinical Pharmacology: Pioneered as a cancer immunotherapy target by James P. Allison (2018 Nobel Prize in Physiology or Medicine) with the anti-CTLA-4 antibodies ipilimumab (Yervoy) and tremelimumab; conversely, recombinant CTLA-4-Ig fusion proteins (abatacept, belatacept) mimic CTLA-4 to treat rheumatoid arthritis, juvenile idiopathic arthritis, and CHAI syndrome.