PLATELET-ACTIVATING PROPERTIES OF MURINE MONOCLONAL-ANTIBODIES TO BETA-2-GLYCOPROTEIN-I

Previously developed murine monoclonal antibodies (MAbs) to human beta 2-glycoprotein I (beta2GPI), a plasma protein required for the binding of anti-phospholipid antibodies, were studied for anti-platelet react ivity and influence on platelet function. The six MAbs (IgG1 isotype) tested interacted with both intact and fixed platelets in a beta2GPI-d ependent manner. Carbamylated beta2GPI was still recognized by MAbs bu t was unable to mediate platelet-antibody binding. MAbs induced aggreg ation and secretion responses of platelets in platelet-rich plasma (PR P) and whole blood, provided subthreshold concentrations of weak agoni sts (i. e. ADP or adrenaline) were added. When aggregation in PRP was evaluated by a counting technique instead of turbidometrically, the so le addition of MAbs led to a rapid fall in single platelets. Triggerin g gel-filtered platelets with MAbs together with beta2GPI, but not its carbamylated form, led to platelet activation after a lag time, as mo nitored by aggregometry, measurements of ATP and beta-thromboglobulin secretion and calcium mobilization. F(ab')2 fragments of one of the MA bs failed to activate platelets but inhibited the responses to the who le antibody. This process thus depends on MAbs binding to platelets th rough both Fab and Fc domains, as confirmed by the suppression of plat elet responses upon pretreatment with the anti-FcgammaRII MAb IV.3. Ag gregation and secretion induced by MAbs plus beta2GPI did not require exogenous fibrinogen and were variably inhibited in the presence of ac etyl salicylic acid, apyrase or Ca2+, depending on the concentrations used for the two proteins. We propose that platelet FcgammaRII crossli nking that follows a platelet-antibody interaction via the platelet bi nding protein, beta2GPI, may be a new mechanism by which anti-beta2 GP I antibodies activate platelets and/or synergize with weak agonists.