A PLATELET ACTIVATION-SPECIFIC MONOCLONAL-ANTIBODY THAT RECOGNIZES A RECEPTOR-INDUCED BINDING-SITE ON CANINE FIBRINOGEN

An activation-specific monoclonal antibody (MoAb) termed ''Canine Acti vated Platelet 1'' (CAP1) has been developed and partially characteriz ed. Flow cytometric studies of isolated canine platelets, using adenos ine diphosphate (ADP) and platelet activating factor (PAF) as agonists , demonstrated that CAP1 binding site number was proportional to agoni st strength and agonist concentration. MoAb CAP1 binding was diminishe d by ethylenediamine-tetraacetic acid, suggesting that the antigen was either stabilized by calcium or antigen binding to the platelet surfa ce was mediated by calcium. ADP-activated gel-filtered platelets also demonstrated reduced binding of MoAb CAP1 even in the presence of 1 mM CaCl2. Binding of MoAb CAP1 could be partially restored by activating gel-filtered platelets with PAF, suggesting that the antigen was eith er present within platelet granule membranes or was exposed after bind ing of released protein(s) with a platelet receptor. A monoclonal anti body to human platelet glycoprotein IIIa (GPIIIa), which cross-reacts with canine platelet GPIIIa regardless of platelet activation status, did not inhibit binding of MoAb CAP1. MoAb CAP1 bound to isolated cani ne fibrinogen captured on polystyrene microtiter plates in the absence of platelet proteins. Immunoblots indicated that MoAb CAP1 recognizes nonreduced fibrinogen as well as a plasmin digest of isolated canine fibrinogen. Results of the present studies suggest that MoAb CAP1 reco gnizes a receptor-induced binding site on canine fibrinogen.