Analysis of ligand-induced and ligand-attenuated epitopes on the leukocyte integrin alpha4beta1: VCAM-1, mucosal addressin cell adhesion molecule-1, and fibronectin induce distinct conformational changes.
The leukocyte integrin alpha4beta1 is a receptor for both cell surface ligands (VCAM-1 and mucosal addressin cell adhesion molecule-1 (MAdCAM-1)) and extracellular matrix components (fibronectin). Through regulated interactions with these molecules, alpha4beta1 mediates leukocyte migration from the vasculature at sites of inflammation. Regulation of integrin activity plays a key role in controlling leukocyte-adhesive events and appears to be partly determined by changes in integrin conformation. Several mAbs that recognize ligand-induced binding site epitopes on integrins have been characterized, and a subset of these mAbs are capable of stimulating integrin-ligand binding. Conversely, some mAbs recognize epitopes that are attenuated by ligand engagement and allosterically inhibit ligand binding. To gain insight into ligand-specific effects on integrin conformation, we have examined the ability of different ligands to modulate the binding of four distinct classes (A, B1, B2, and C) of anti-alpha4 Abs to alpha4beta1. VCAM-1 attenuated B (antifunctional) class epitopes via an allosteric mechanism and also allosterically inhibited the binding of the function-blocking anti-beta1 mAb 13. Additional alpha4beta1 ligands (fibronectin fragments, MAdCAM-1, and the CS1 peptide) also inhibited mAb 13-integrin binding; however, the epitopes of the class B anti-alpha4 mAbs were attenuated by the fibronectin fragments, but not by MAdCAM-1 or the CS1 peptide. Of the two anti-alpha4 class A mAbs examined, one recognized an epitope that was induced uniquely by VCAM-1. Taken together, these data suggest that overlapping but distinct binding mechanisms exist for different alpha4beta1 ligands and that distinct conformational changes are induced upon integrin engagement by different ligands.
Newham
P
P
Wellcome Trust Centre for Cell-Matrix Research, School of Biological Sciences, University of Manchester, United Kingdom.
Craig
S E
SE
Clark
K
K
Mould
A P
AP
Humphries
M J
MJ
D000818:Animals; D015815:Cell Adhesion Molecules; D002460:Cell Line; D000939:Epitopes / Q000276:immunology / Q000378:metabolism; D005353:Fibronectins / Q000276:immunology / Q000378:metabolism*; D006801:Humans; D007136:Immunoglobulins / Q000276:immunology / Q000378:metabolism*; D039041:Integrin alpha4beta1; D016023:Integrins / Q000276:immunology / Q000378:metabolism*; D007962:Leukocytes / Q000276:immunology* / Q000378:metabolism; D008024:Ligands; D051379:Mice; D009088:Mucoproteins / Q000276:immunology / Q000378:metabolism*; D011487:Protein Conformation; D016201:Receptors, Lymphocyte Homing / Q000276:immunology / Q000378:metabolism*; D015398:Signal Transduction / Q000276:immunology; D019010:Vascular Cell Adhesion Molecule-1 / Q000276:immunology / Q000378:metabolism*
D015815:Cell Adhesion Molecules; D000939:Epitopes; D005353:Fibronectins; D007136:Immunoglobulins; D039041:Integrin alpha4beta1; D016023:Integrins; D008024:Ligands; C504265:MADCAM1 protein, human; C489082:Madcam1 protein, mouse; D009088:Mucoproteins; D016201:Receptors, Lymphocyte Homing; D019010:Vascular Cell Adhesion Molecule-1