title: The binding sites for competitive antagonistic, allosteric antagonistic, and agonistic antibodies to the I domain of integrin LFA-1.
abstract: We explore the binding sites for mAbs to the alpha I domain of the integrin alphaLbeta2 that can competitively inhibit, allosterically inhibit, or activate binding to the ligand ICAM-1. Ten mAbs, some of them clinically important, were mapped to species-specific residues. The results are interpreted with independent structures of the alphaL I domain determined in seven different crystal lattices and in solution, and which are present in three conformational states that differ in affinity for ligand. Six mAbs bind to adjacent regions of the beta1-alpha1 and alpha3-alpha4 loops, which show only small (mean, 0.8 angstroms; maximum, 1.8 angstroms) displacements among the eight I domain structures. Proximity to the ligand binding site and to noncontacting portions of the ICAM-1 molecule explains competitive inhibition by these mAbs. Three mAbs bind to a segment of seven residues in the beta5-alpha6 loop and alpha6 helix, in similar proximity to the ligand binding site, but on the side opposite from the beta1-alpha1/alpha3-alpha4 epitopes, and far from noncontacting portions of ICAM-1. These residues show large displacements among the eight structures in response to lattice contacts (mean, 3.6 angstroms; maximum, 9.4 angstroms), and movement of a buried Phe in the beta5-alpha6 loop is partially correlated with affinity change at the ligand binding site. Together with a lack of proximity to noncontacting portions of ICAM-1, these observations explain variation among this group of mAbs, which can either act as competitive or allosteric antagonists. One agonistic mAb binds distant from the ligand binding site of the I domain, to residues that show little movement (mean, 0.5 angstroms; maximum, 1.0 angstroms). Agonism by this mAb is thus likely to result from altering the orientation of the I domain with respect to other domains within an intact integrin alphaLbeta2 heterodimer.
authors.affiliation: CBR Institute for Biomedical Research, Department of Pathology, Harvard Medical School, Boston, MA 02115, USA.
authors.forename: Chafen
authors.initials: C
authors.lastname: Lu
authors.forename: Motomu
authors.initials: M
authors.lastname: Shimaoka
authors.forename: Azucena
authors.initials: A
authors.lastname: Salas
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Springer
mesh_terms: D000495:Allosteric Site / Q000235:genetics / Q000276:immunology; D000595:Amino Acid Sequence; D019943:Amino Acid Substitution / Q000235:genetics / Q000276:immunology; D000818:Animals; D000911:Antibodies, Monoclonal / Q000378:metabolism*; D001667:Binding, Competitive / Q000235:genetics / Q000276:immunology; D040881:CD11a Antigen / Q000235:genetics / Q000276:immunology / Q000378:metabolism / Q000502:physiology; D018821:CD18 Antigens / Q000235:genetics / Q000276:immunology / Q000378:metabolism / Q000502:physiology; D002450:Cell Communication / Q000235:genetics / Q000276:immunology; D002460:Cell Line; D000939:Epitopes / Q000096:biosynthesis / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D006801:Humans; D016023:Integrins / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000276:immunology* / Q000502:physiology; D016169:Lymphocyte Function-Associated Antigen-1 / Q000235:genetics / Q000276:immunology* / Q000378:metabolism* / Q000502:physiology; D051379:Mice; D008969:Molecular Sequence Data; D017434:Protein Structure, Tertiary / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000819:agonists* / Q000037:antagonists & inhibitors* / Q000276:immunology / Q000378:metabolism; D014162:Transfection
chemical_list: D000911:Antibodies, Monoclonal; D040881:CD11a Antigen; D018821:CD18 Antigens; D000939:Epitopes; D016023:Integrins; D016169:Lymphocyte Function-Associated Antigen-1; D011993:Recombinant Fusion Proteins