The leukocyte integrin alpha(D)beta(2) binds VCAM-1: Evidence for a binding interface between I domain and VCAM-1

The trafficking of leukocytes through tissues is supported by an interactio n between the beta(2) (CD18) integrins CD11a/CD18 (LFA-1) and CD11b/CD18 (M ac-1) and their ligand ICAM-1. The most recently identified and fourth memb er of the beta(2) integrins, alpha(D)beta(2), selectively binds ICAM-3 and does not appear to bind ICAM-1. We have reported recently that alpha(D)beta (2) can support eosinophil adhesion to VCAM-1. Here we demonstrate that exp ression of alpha(D)beta(2) in a lymphoid cell that does not express alpha(4 ) integrins confers efficient binding to VCAM-1, In addition, a soluble for m of alpha(D)beta(2) binds VCAM-1 with greater efficiency relative to ICAM- 3. The I domain of alpha D contains a binding site for VCAM-1 since recombi nant alpha(D) I domain binds specifically to VCAM-1. In addition, alpha(D) mAb that block cellular binding to VCAM-1 bind the alpha(4) I domain. Using VCAM-1 mutants we have determined that the binding site on VCAM-1 for alph a(D)beta(2) overlaps with that of alpha(4) integrins, Substitution of VCAM- 1 aspartate at position 40, D40, within the conserved integrin binding site , diminishes binding to alpha(D)beta(2) and abrogates binding to the alpha( D) I domain, The corresponding integrin binding site residue in ICAM-3 is a lso essential to alpha(D)beta(2) binding. Finally we demonstrate that alpha (D)beta(2) can support lymphoid cell adhesion to VCAM-1 under flow conditio ns at levels equivalent to those mediated by alpha(4)beta(1). These results indicate that VCAM-1 can bind to an I domain and that the binding of alpha (D)beta(2) to VCAM-1 may contribute to the trafficking of a subpopulation o f leukocytes that express alpha(D)beta(2).