Both extracellular immunoglobin-like domains of CD80 contain residues critical for binding T cell surface receptors CTLA-4 and CD28.

The B7-related molecules CD80 and CD86 are expressed on antigen-presenting cells, bind the homologous T cell receptors CD28 and CTLA-4, and trigger costimulatory signals important for optimal T cell activation. All four molecules are immunoglobulin superfamily members, each comprising an extracellular Ig variable-like (IgV) domain, with CD80 and CD86 containing an additional Ig constant-like (IgC) domain. Despite limited sequence identity, CD80 and CD86 share similar overall receptor binding properties and effector functions. We have identified, by site-directed mutagenesis of soluble forms of CD80 and CD86, residues in both the IgV and IgC domains that are important for CTLA4Ig and CD28Ig binding. Mutagenesis in the IgV domain of CD80 identified 11 amino acids that support receptor binding. Many of these residues are conserved in the B7 family, are hydrophobic, and approximately map to the GFCC'C" beta-sheet face of an IgV fold. Mutagenesis of corresponding residues in CD86 established that some, but not all, of these residues also played a role in CD86 receptor binding. In general, mutations had a similar effect on CTLA4Ig and CD28Ig binding, thereby indicating that both receptors bind to overlapping sites on CD80 and CD86. Further, mutagenesis of several conserved residues in the ABED beta-sheet face of the IgC domain of CD80 completely ablated receptor binding. Point mutagenesis had a more pronounced effect than complete truncation of the IgC domain. Thus, full CTLA4Ig and CD28Ig binding to B7 molecules is dependent upon residues in the GFC'C" face of the IgV domain and the ABED face of the IgC domain.