Description
CD80 is a monomeric surface protein found on activated APCs. The extracellular region of CD80 consists of two highly glycosylated Ig- like extracellular domains, which have defined regions for binding its ligands, CD28 or CTLA-4, on T cells.One ligand of CD80, CD28, provides critical costimulatory signals to activated T cells to increase cytokine production, cell proliferation, and cell viability, while the other ligand, CTLA-4, may mediate a growth-inhibitory signal.The same regions of CD80 are involved in binding to both CD28 and CTLA-4, but CTLA-4 binds to CD80 with about a 20-fold higher avidity than CD28 does. This suggests that low levels of CTLA-4 expressed on the cell surface can compete for CD80 binding even when CD28 is expressed at much higher levels