Characterization of ICAM‐4 binding to the I domains of the CD11a/CD18 and CD11b/CD18 leukocyte integrins

Intercellular adhesion molecule‐4 (ICAM‐4, LW blood group antigen), a member of the immunoglobulin superfamily expressed on red cells, has been reported to bind to CD11a/CD18 and CD11b/CD18 leukocyte integrins. The location of the ICAM‐4 binding sites on CD11a/CD18 and CD11b/CD18 are not known. CD11/CD18 integrin I domains have been found to act as major binding sites for physiological ligands and a negatively charged glutamic acid in ICAMs is considered important for binding. ICAM‐4 lacks such a residue, which is replaced by an arginine. However, we demonstrate here that ICAM‐4 in red cells and transfected fibroblasts interacts specifically with the I domains of CD11a/CD18 and CD11b/CD18 integrins. The binding was inhibited by anti‐I domain and anti‐ICAM‐4 antibodies and it was dependent on divalent cations. Interestingly, ICAM‐4 negative red cells were still able to bind to the CD11b/CD18 I domain but the binding of these cells to the CD11a/CD18 I domain was clearly reduced. Using a solid phase assay, we were able to show that isolated I domains directly and specifically bind to purified recombinant ICAM‐4 in a cation dependent manner. Competition experiments indicated that the binding sites in ICAM‐4 for the CD11a and CD11b I domains are different. However, the ICAM‐4 binding region in both I domains seems to overlap with the regions recognized by the ICAM‐1 and ICAM‐2. Thus we have established that the I domains contain an ICAM‐4 binding region in CD11a/CD18 and CD11b/CD18 leukocyte integrins.