The leukocyte integrin α(D)β₂ binds VCAM-1: Evidence for a binding interface between I domain and VCAM-1

The trafficking of leukocytes through tissues is supported by an interaction between the β₂ (CD18) integrins CD11a/CDl8 (LFA-1) and CD11b/CD18 (Mac-1) and their ligand ICAM-1. The most recently identified and fourth member of the β₂ integrins, α(D)β₂, selectively binds ICAM-3 and does not appear to bind ICAM-1. We have reported recently that α(D)β₂ can support eosinophil adhesion to VCAM-1. Here we demonstrate that expression of α(D)β₂ in a lymphoid cell that does not express α₄ integrins confers efficient binding to VCAM-1. In addition, a soluble form of α(D)β₂ binds VCAM-1 with greater efficiency relative to ICAM-3. The I domain of αD contains a binding site for VCAM-1 since recombinant α(D) I domain binds specifically to VCAM-1. In addition, α(D) mAb that block cellular binding to VCAM-1 bind the α(D) I domain. Using VCAM-1 mutants we have determined that the binding site on VCAM-1 for α(D)β₂ overlaps with that of α₄ integrins. Substitution of VCAM-1 aspartate at position 40, D40, within the conserved integrin binding site, diminishes binding to α(D)β₂ and abrogates binding to the α(D) I domain. The corresponding integrin binding site residue in ICAM-3 is also essential to α(D)β₂ binding. Finally, we demonstrate that α(D)β₂ 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 α(D)β₂ to VCAM-1 may contribute to the trafficking of a subpopulation of leukocytes that express α(D)β₂.