CTLA-4 (Cytotoxic T-lymphocyte antigen 4), also known as CD152, belongs to immunoglobulin superfamily. It is an intracellular protein expressed in resting T cells; CTLA-4 translocates to the cell surface and transmits an inhibitory signal to T cells. In addition, CTLA-4 competes with its homologous T-cell co-stimulatory protein CD28 for ligands expressed on APC, namely CD80 (B7-1) and CD86 (B7-2), thereby antagonizing CD28 and inhibiting CD28 signaling, resulting in inhibition of IL-2, IFN-γ, IL-4 cytokines production, IL-2 receptor expression and cell cycle progression. Because Tregs constitutively express CTLA-4 at high levels, anti-CTLA-4 therapy would be expected to have the greatest effect on these cells. Ipilimumab, a fully human IgG1 monoclonal antibody, is the only commercially available CTLA-4 antagonist. It has been approved by FDA for treatment of metastatic melanoma, adjuvant melanoma and advanced renal cell carcinoma, based on its significant effect of tumor suppression demonstrated in clinical trials.
