The Leukocyte Integrin αDβ2 Binds VCAM-1: Evidence for a Binding Interface Between I Domain and VCAM-1

Abstract

Abstract The trafficking of leukocytes through tissues is supported by an interaction between the β2 (CD18) integrins CD11a/CD18 (LFA-1) and CD11b/CD18 (Mac-1) and their ligand ICAM-1. The most recently identified and fourth member of the β2 integrins, αDβ2, selectively binds ICAM-3 and does not appear to bind ICAM-1. We have reported recently that αDβ2 can support eosinophil adhesion to VCAM-1. Here we demonstrate that expression of αDβ2 in a lymphoid cell that does not express α4 integrins confers efficient binding to VCAM-1. In addition, a soluble form of αDβ2 binds VCAM-1 with greater efficiency relative to ICAM-3. The I domain of αD contains a binding site for VCAM-1 since recombinant αD I domain binds specifically to VCAM-1. In addition, αD mAb that block cellular binding to VCAM-1 bind the αD I domain. Using VCAM-1 mutants we have determined that the binding site on VCAM-1 for αDβ2 overlaps with that of α4 integrins. Substitution of VCAM-1 aspartate at position 40, D40, within the conserved integrin binding site, diminishes binding to αDβ2 and abrogates binding to the αD I domain. The corresponding integrin binding site residue in ICAM-3 is also essential to αDβ2 binding. Finally, we demonstrate that αDβ2 can support lymphoid cell adhesion to VCAM-1 under flow conditions at levels equivalent to those mediated by α4β1. These results indicate that VCAM-1 can bind to an I domain and that the binding of αDβ2 to VCAM-1 may contribute to the trafficking of a subpopulation of leukocytes that express αDβ2.