DIFFERENTIAL LIGAND-BINDING SPECIFICITIES OF RECOMBINANT CD11B CD18 INTEGRIN I-DOMAIN/

The alpha subunits of leukocyte CD11/CD18 integrins contain an similar to 200-amino acid ''inserted'' domain (I-domain) that may be importan t for multivalent adhesive recognitions. A recombinant form of the I-d omain of CD11b/CD18 was generated and analyzed directly for interactio n with complementary integrin ligands. CD11b I-domain bound the activa tion-dependent monoclonal antibody 7E3, and the functionally blocking anti-CD11b monoclonal antibodies OKM9, 60.1, and LM2/1, but not OKM1 o r M1/70. Fibrinogen or soluble intercellular adhesion molecule-1 assoc iated with CD11b I-domain in a concentration-dependent manner. Binding of I-125-fibrinogen to recombinant CD11b I-domain was saturable, gove rned by a K-d of similar to 0.22 +/- 0.06 mu M, and fully inhibited by molar excess of unlabeled fibrinogen, or by the P1 peptide (KY)GWTVFQ KRLDGSV (IC50 similar to 2.5-5 mu M), duplicating the fibrinogen gamma chain sequence Gly(190)-Val(202). In contrast, I-125-factor X binding to CD11b I-domain was only partially inhibited (50-60%) by a molar ex cess of unlabeled factor X, and entirely unaffected by functionally bl ocking anti-CD11b monoclonal antibodies or by factor X-derived synthet ic peptidyl antagonists. We conclude that the I-domain of CD11b partic ipates in qualitative mechanisms of receptor activation and contains t he binding site(s) for the CD11b/CD18 ligands fibrinogen and intercell ular adhesion molecule-1, while it is only minimally implicated in the recognition of factor X.