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Recent clinical trials suggest that blockade of integrins is a promising strategy for the treatment of acute coronary syndromes. Administration of 7E3 monoclonal antibody (mAb) Fab fragment (c7E3 Fab) directed against platelet integrin IIb/IIIa (alpha IIb beta 3, CD41/CD61) reduces acute ischemic complications of coronary angioplasty and clinical restenosis at 6 months. However, 7E3 mAb is not selective for platelet IIb/IIIa but also cross-reacts with the leukocyte integrin Mac-1 (alpha M beta 2, CD11b/CD18) and the vitronectin receptor (alpha v beta 3, CD51/CD61). Information regarding how this mAb may affect other cells important in vascular repair is scant. Potential interactions of c7E3 Fab with inflammatory (i.e., monocytes and neutrophils), vascular smooth muscle, and endothelial cells may contribute to the in vivo actions of c7E3 Fab. In this study we explored the binding of 7E3 to monocytic cells and the functional effect of 7E3 and c7E3 Fab on Mac-1-mediated adhesion to fibrinogen (FGN) and intercellular adhesion molecule-1 (ICAM-1), ligands abundant in the injured vessel wall. Flow cytometry demonstrated that 7E3 bound to THP-1 monocytic cells and identified a subpopulation (approximately 10%) of Mac-1 that was qualitatively similar to that recognized by CBRM1/5, a mAb directed to an activation-specific neoepitope present on a subset of Mac-1 molecules. mAb 7E3 bound to K562 cells transfected with just the alpha subunit (CD11b) of Mac-1 but not to nontransfected cells, confirming a direct interaction between 7E3 and Mac-1. mAb 7E3 and c7E3 Fab blocked the adhesion of Mac-1-bearing cells to FGN (80 +/- 11% and 78 +/- 9% inhibition, respectively) and ICAM-1 (62 +/- 14% and 62 +/- 17%). Both 7E3 and c7E3 Fab significantly inhibited (70 +/- 6% and 62 +/- 26%) soluble FGN binding to human peripheral blood monocytes. Thus, c7E3 Fab cross-reacts with the CD11b subunit of Mac-1 and interrupts cell-extracellular matrix and cell-cell adhesive interactions and may thereby influence the recruitment of circulating monocytes to sites of vessel injury. Given the recent evidence that adherent and infiltrating monocyte number directly correlates with the extent of neointimal hyperplasia, inhibition of Mac-1-dependent adhesion and IIb/IIIa-dependent function by c7E3 Fab may jointly contribute to the regulation of vascular repair and to the sustained clinical benefits observed with c7E3 Fab after angioplasty.

In this study we have compared the ligand binding activity of the two closely related beta 2 integrins, Mac-1 and p150,95, which are expressed separately as receptors permanently transfected into K562 cells. Mac-1 has previously been shown to associate with Fc gamma R, particularly Fc gamma RIII, but K562 cells express only endogenous Fc gamma RIIA. We have, therefore, taken advantage of this situation to examine a possible relationship between Fc gamma RIIA with Mac-1 and p150,95 in the absence of other Fc gamma R. The main finding is that anti-Fc gamma RII mAb have a profound inhibitory effect on cell adhesion mediated by Mac-1, but not on the adhesion mediated by p150,95. Thus, in spite of the fact that Mac-1 and p150,95 bind to the same or at least a very similar selection of ligands, their association with other receptors on the cellular membrane, and therefore their mode of regulation may be different.

Unactivated peripheral blood leukocytes show little tendency to bind to other cells or matrix components, whilst, in the presence of inflammatory mediators, adhesive interactions can rapidly increase. The Leu-CAM (beta 2 integrin) family of adhesion molecules have been shown to mediate a variety of these induced adhesion events. Here we describe a monoclonal antibody against CD18, KIM185, which stimulates JY cell homotypic aggregation by a CD11 a pathway as well as inducing the adherence of neutrophils to protein-coated plastic by a CD11b-dependent mechanism. The antibody recognizes an epitope distinct from the previously described KIM127 antibody and evidence is presented that the binding of KIM185 can cause a change in the conformation of the CD18 molecule.

The Leukocytic cell-adhesion molecule (beta 2 integrin) family of adhesion molecules play a key role in the intercellular adhesive interactions necessary for normal immune cell function. In this study, we report an antibody that recognizes an epitope on the Leukocytic cell-adhesion molecule common beta-chain (CD18) and promotes both lymphocyte function-associated Ag-1- and CR3-dependent adhesion events. The antibody recognizes a temperature-sensitive epitope that is not dependent on the presence of divalent cations. It is proposed that antibody binding promotes a conformational change in both lymphocyte function-associated Ag-1 and CR3, which may mimic a natural activation mechanism, resulting in increased cellular adhesion.