Neutrophil activation by anti‐ β 2 glycoprotein I monoclonal antibodies via Fcγ receptor II

Murine monoclonal antibodies (mAbs) to human β 2 ‐glycoprotein I ( β 2 GPI), a plasma protein required for the binding of some antiphospholipid antibodies, have been shown to possess lupus anticoagulant properties and to activate platelets via Fcγ receptor (FcγR) crosslinking. Here we investigated their ability to induce polymorphonuclear leukocyte (PMN) functional responses. The six mAbs (IgGl isotype) tested in combination with β 2 GPI led to a concentration‐dependent activation of human PMNs as appreciated by granule release, H 2 O 2 production, and cytosolic Ca 2+ increase. This activation process was accompanied by the enhancement of PMN‐mediated heparan sulfate loss from 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 β 2 GPI‐dependent manner but were devoid of activating effects. Carbamylated β 2 GPI was unable to mediate PMN‐antibody binding and subsequent activation. In addition, cationization of β 2 GPI or removal of its sialic acid groups led to higher efficiency in binding to the PMN surface and triggering activation in comparison with the untreated protein. Thus, the process of PMN activation depends on mAb binding to these cells through both Fab (via β 2 GPI) and Fc domains, as confirmed by the suppression of all responses upon treatment with an anti‐FcγRII, but not anti‐FcγRIII, antibody. Our data suggest a model of cellular activation by β 2 GPI‐dependent antiphospholipid antibodies. J. Leukoc. Biol . 57: 387–394; 1995.