Cytotoxic T-Lymphocyte-Associated molecule-4 (CTLA-4), also known as cluster of differentiation 152 (CD152) is located on band q33-q34 on chromosome 2 in humans, and band C of chromosome 1 in mice (1). CTLA-4 has over 100 single nucleotide polymorphisms (SNPs) associated with autoimmunity and cancer. SNPs in the CTLA-4 promoter have been correlated with a variety of cancers including breast cancer, cervical cancer, colorectal cancer, head and neck cancer, hepatocellular cancer, melanoma, non-small-cell lung carcinoma (NSCLC), renal cancer and more. The 223 amino acid (aa) protein product consists of several domains including: a 36 aa leader peptide, a 116 aa extracellular Ig V-like ligand binding domain, a 37 aa transmembrane domain and a 34 aa cytoplasmic domain. The cytoplasmic domain contains tyrosine motifs, at Y201 VKM and Y218 FIP. Several intracellular proteins bind Y201 VKM including: phosphatidyl-inositol 3 kinase (PI3-K), Src homology 2 domain containing protein tyrosine phosphatase (SHP-2), the serine threonine phosphatase PP2a and clathrin adaptor proteins activator proteins (AP1, AP2).The mature type 1 transmembrane glycoprotein has a native molecular weight of 33-37 kDa (2,3).
CTLA-4 plays a role in immune self-tolerance, which is important for avoiding autoimmune diseases. Unfortunately, this mechanism can be co-opted in order to suppress immune responses in the context of cancer. CTLA-4's highly studied role in the T cell response has been well described (4). CTLA-4 was originally identified as a gene that was specifically expressed by cytotoxic T lymphocytes. However, CTLA-4 transcripts have since been found in both T helper type 1 and 2 (Th1 and Th2) cells, and CD4+ and CD8+ T cell clones. CTLA-4 is a negative costimulatory molecule that is upregulated over the course of 2-3 days in T cells in response to T cell receptor (TCR) engagement with peptide major histocompatibility complex (pMHC) on antigen presenting cells. CTLA-4 recruitment to the immunological synapse is positively correlated with TCR signal strength. CTLA-4 outcompetes CD28, a positive costimulatory molecule, for the ligands B7-1 (CD80) and B7-2 (CD86) in order to attenuate T cell activation. CTLA-4 has a much higher affinity and avidity for B7-1 and B7-2 than CD28. CTLA-4 and CD28 are structurally homologous molecules that are members of the immunoglobulin (Ig) gene superfamily. CTLA-4 and CD28 are both expressed on the cell surface as disulfide-linked homodimers or as monomers. Unlike CTLA-4, 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. CTLA-4 negative co-stimulation suppresses T cell activation, thereby facilitating immune evasion. CTLA-4's effects occur via cell intrinsic and cell extrinsic mechanisms. Cell intrinsic mechanisms include inhibition of protein translation, recruitment of phosphatases, activation of ubiquitin ligases, inhibition of cytokine receptor signaling, and inhibition of lipid domain formation at membrane of T cells. Cell extrinsic mechanisms include stimulating the release of the suppressive indoleamine (2,3) deoxygenase (IDO), modulation of regulatory T cell function and the aforementioned competitive inhibition of CD28 costimulatory signaling (5).
CTLA-4 is a well-known immune checkpoint target. The human monoclonal CTLA-4 antibody Ipilimumab was approved by the food and drug administration (FDA) to treat melanoma in 2011 (4). Since then, it has been approved for colorectal cancer in children 12 and older, hepatocellular carcinoma, NSCLC, and renal cell carcinoma. The Ipilimumab binding epitope overlaps with the B7 binding domain and blocks the interaction via steric hinderance of B7(4). Ipilimumab allows for B7-CD28 positive costimulatory signaling, which is an important signal for T cell activation (5).