The I domain is a major recognition site on the leukocyte integrin Mac-1 (CD11b/CD18) for four distinct adhesion ligands

Despite the identification and characterization of several distinct ligands for the leukocyte integrin (CD11/CD18) family of adhesion receptors, little is known about the structural regions on these molecules that mediate ligand recognition. In this report, we use α subunit chimeras of Mac-1 (CD11b/CD18) and p150,95 (CD11c/CD18), and an extended panel of newly generated and previously characterized mAbs specific to the α chain of Mac-1 to map the binding sites for four distinct ligands for Mac-1: iC3b, fibrinogen, ICAM-1, and the as-yet uncharacterized counterreceptor responsible for neutrophil homotypic adhesion. Epitopes of mAbs that blocked ligand binding were mapped with the chimeras and used to localize the ligand recognition sites because the data obtained from functional assays with the Mac-1/p150,95 chimeras were not easily interpreted. Results show that the I domain on the a chain of Mac-1 is an important recognition site for all four ligands, and that the NH₂-terminal and perhaps divalent cation binding regions but not the COOH-terminal segment may contribute. The recognition sites in the I domain appear overlapping but not identical as individual Mac1-ligand interactions are distinguished by the discrete patterns of inhibitory mAbs. Additionally, we find that the a subunit NH₂-terminal region and divalent cation binding region, despite being separated by over 200 amino acids of the I domain, appear structurally apposed because three mAbs require the presence of both of these regions for antigenic reactivity, and chimeras that contain the NH₂ terminus of p150,95 require the divalent cation binding region of p150,95 to associate firmly with the β subunit.