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 co unter receptor on resting T cells and monocytes for the leukocyte inte grin, lymphocyte function associated antigen 1 (LFA-1; CD11a/CD18), an d may play an important role in the initial stages of the T cell-depen dent immune response, Deletion of individual immunoglobulin superfamil y (IgSF) domains of ICAM-3 and ICAM-3 IgSF domain chimeras with CD21 s howed there is a single LFA-1 binding site in ICAM-3 and that IgSF dom ain 1 is necessary and sufficient for LFA-1 binding. Epitope mapping a nd functional studies performed with 17 anti-ICAM-3 monoclonal antibod ies demonstrated that only some monoclonal antibodies, with epitopes w holly 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 f rom the domain deletion mutants and CD21 chimeras. Analysis of a panel of 45 point mutants of domain 1 of ICAM-3 identified five residues th at may contact LFA-1 as part of the binding site, Asn(23), Ser(25), Gl u(37), Phe(54), and Gln(75). These five residues are predicted by mole cular modeling, based on the structure of vascular cell adhesion molec ule 1 (VCAM-1), to cluster in two distinct locations on domain 1 of IC AM-3 on the BED face (Asn(23) and Ser(25)) and on the C strand or CD l oop (E37), the E strand (F54), and the FG loop (Q75). The residues, As n(23) and Ser(25), comprise a consensus N-linked glycosylation site.