NEUTROPHIL ACTIVATION BY ANTI-BETA(2) GLYCOPROTEIN-I MONOCLONAL-ANTIBODIES VIA FC-GAMMA RECEPTOR-II

Murine monoclonal antibodies (mAbs) to human beta(2)-glycoprotein I (b eta(2)GPI), a plasma protein required for the binding of some antiphos pholipid antibodies, have been shown to possess lupus anticoagulant pr operties and to activate platelets via Fc gamma receptor (Fc gamma R) crosslinking. Here we investigated their ability to induce polymorphon uclear leukocyte (PMN) functional responses. The six mAbs (IgG1) isoty pe) tested in combination with beta(2)GPI led to a concentration-depen dent activation of human PMNs as appreciated by granule release, H2O2 production, and cytosolic Ca2+ increase. This activation process was a ccompanied by the enhancement of PMN-mediated heparan sulfate loss fro m the endothelial cell line EA.hy 926 without evidence for cell lysis or detachment. F(ab')(2) fragments of one of the mAbs bound to PMNs in a beta(2)GPI-dependent manner but were devoid of activating effects. Carbamylated beta(2)GPI was unable to mediate PMN-antibody binding and subsequent activation. In addition, cationization of beta(2)GPI or re moval of its sialic acid groups led to higher efficiency in binding to the PMN surface and triggering activation in comparison with the untr eated protein. Thus, the process of PMN activation depends on mAb bind ing to these cells through both Fab (via beta(2)GPI) and Fc domains, a s confirmed by the suppression of all responses upon treatment with an anti-Fc gamma RII, but not anti-Fc gamma RIII, antibody. Our data sug gest a model of cellular activation by beta(2)GPI-dependent antiphosph olipid antibodies.