Molecular identification of the cross-reacting epitope on αmβ2 integrin I domain recognized by anti-αIIbβ3 monoclonal antibody 7E3 and its involvement in leukocyte adherence

The monoclonal antibody (mAb) 7E3 directed to the platelet integrin alpha(IIb)beta(3) was tested for its cross-reactivity with the homologous leukocyte integrin alpha(M)beta(2),. Nested recombinant fragments of alpha(M) I domain were expressed as glutathione S-transferase fusion proteins and analyzed for antibody recognition. In enzyme-linked immunosorbent assay, mAb 7E3 bound alpha(M) I domain fragments containing the amino-terminal sequence Cys(128)-Ser(172) whereas the carboxyl-terminal region Leu(173)-Pro(291) was ineffective. A synthetic peptide designated R1.1 and duplicating the alpha(M) sequence G(127)CPQEDSDIAFLIDGSGSIIPHDF(150) bound mAb 7E3, In contrast, the adjacent alpha(M) region F(150)RRMKEFVSTVMEQLKKSKTLFS(172) or a control peptide with a scrambled R1.1 sequence was not recognized by mAb 7E3, Binding of mAb 7E3 to alpha(M) I domain blocked monocyte and neutrophil adhesion to immobilized fibrinogen and fibrinogen-dependent leukocyte-endothelium bridging, indistinguishably from bona fide anti-beta(2) mAb IB4. In contrast, leukocyte binding to stable transfectants expressing intercellular adhesion molecule-1 was not affected by mAb 7E3, Balloon-mediated injury of iliofemoral arteries in rabbits resulted in prominent deposition of fibrinogen and increased monocyte adhesion to the injured vessel, in a reaction inhibited by mAb 7E3, but unaffected by control mAb 14E11. Through its cross-reactivity between alpha(IIb)beta(3) and alpha(M)beta(2), mAb 7E3 may initiate a new class of integrin antagonists, capable of simultaneously targeting platelet and leukocyte adhesion mechanisms in vascular injury.