alpha(M)beta(2) (CD11b/CD18, Mac-1) integrin activation by a unique monoclonal antibody to alpha(M) I domain that is divalent cation-sensitive

The beta (2) (CD18) leukocyte integrins play a key role in normal and infla mmatory immune responses, In resting leukocytes, these receptors do not bin d ligands. However, when leukocytes are exposed to an appropriate agonist, high-affinity ligand binding is achieved, presumably as a result of conform ational changes in the integrin. In this study, we describe a novel monoclo nal antibody, mAb 6C1, directed against the alpha (M) subunit, which direct ly induces adhesion of alpha (M)beta (2)-transfected CHO cells to fibrinoge n, ICAM-1, and iC3b, Induction of binding could also be accomplished by mon ovalent Fah fragments of mAb 6C1 at concentrations similar to that observed with intact IgG, demonstrating stimulation of adhesion was not because of receptor cross-linking at the cell surface. The binding of mAb 6C1 induces conformational changes in the receptor, as evidenced by die expression of a n "activation reporter" epitope recognized by mAb 24. The binding of mAb 6C 1 is modulated by divalent cations, Mn2+ promoted high. levels of 6C1 bindi ng, and Mg2+ supported low levels of binding, however Ca2+ failed to suppor t binding, A unique distinction of mAb 6C1 is localization of its epitope t o the alpha (M) I domain. The alpha (M) I domain is essential for ligand bi nding, can directly bind divalent cations, and participates in the regulati on of alpha (M)beta (2) ligand-binding affinity, Thus, these studies have i dentified a novel alpha (M) I domain activation epitope of alpha (M)beta (2 ) and support the idea that the I domain modulates the activational state o f the beta (2) integrins.