CD28/CTLA-4 and CD80/CD86 families: signaling and function.

T cell stimulation in the absence of a second, costimulatory signal can lead to anergy or the induction of cell death. CD28 is a major T cell costimulatory receptor, the coengagement of which can prevent anergy and cell death. The CD28 receptor is a member of a complex family of polypeptides that includes at least two receptors and two ligands. Cytotoxic lymphocyte-associated molecule-4 (CTLA-4, CD152) is the second member of the CD28 receptor family. The ligands or counterreceptors for these two proteins are the B7 family members, CD80 (B7-1) and CD86 (B7-2). This article reviews the CD28/CTLA4 and CD80/CD86 families, and outlines the functional outcomes and biochemical signaling pathways recruited after CD28 ligation.