Localization of the binding site on intercellular adhesion molecule-3 (ICAM-3) for lymphocyte function-associated antigen 1 (LFA-1)

Intercellular adhesion molecule 3 (ICAM-3; CD50) is the predominant counter receptor on resting T cells and monocytes for the leukocyte integrin, lymphocyte function associated antigen 1 (LFA-1; CD11a/CD18), and may play an important role in the initial stages of the T cell-dependent immune response, Deletion of individual immunoglobulin superfamily (IgSF) domains of ICAM-3 and ICAM-3 IgSF domain chimeras with CD21 showed there is a single LFA-1 binding site in ICAM-3 and that IgSF domain 1 is necessary and sufficient for LFA-1 binding. Epitope mapping and functional studies performed with 17 anti-ICAM-3 monoclonal antibodies demonstrated that only some monoclonal antibodies, with epitopes wholly within domain 1 of ICAM-3, were able to block binding of ICAM-3 bearing cells to purified LFA-1, in agreement with the data obtained from the domain deletion mutants and CD21 chimeras. Analysis of a panel of 45 point mutants of domain 1 of ICAM-3 identified five residues that may contact LFA-1 as part of the binding site, Asn(23), Ser(25), Glu(37), Phe(54), and Gln(75). These five residues are predicted by molecular modeling, based on the structure of vascular cell adhesion molecule 1 (VCAM-1), to cluster in two distinct locations on domain 1 of ICAM-3 on the BED face (Asn(23) and Ser(25)) and on the C strand or CD loop (E37), the E strand (F54), and the FG loop (Q75). The residues, Asn(23) and Ser(25), comprise a consensus N-linked glycosylation site.