The calcium-dependent neutral protease of human blood platelets: a comparison of its effects on the receptors for von Willebrand factor and for the Fc-fragment derived from IgG.

Glycocalicin (Gc) is the large, water soluble fragment, obtained by cleavage of one of the major membrane glycoproteins, GP Ib, of human platelets by means of the endogenous, calcium-dependent neutral protease (CNP) obtained from lysed platelets. GP Ib has been proposed as the receptor for von Willebrand factor (vWF) as well as for the Fc-receptor of the platelet surface. We have investigated, whether Gc was involved in a receptor function for aggregated human IgG, which is a powerful activator of platelets. Neither Gc nor asialo-Gc inhibited the stimulation of human blood platelets by bisdiazoniumbenzidine-aggregated human IgG (BDB-IgG). Moreover, platelets, after treatment with a crude preparation of CNP, which removes Gc, could be stimulated by BDB-IgG as well as or better than control platelets, but were unreactive with bovine vWF. We conclude that the Gc-moiety of GP Ib, which is involved in the bovine vWF binding site, is not the Fc-receptor on platelets. Thus, the inhibition, by human or rabbit IgG aggregates or monomeric rabbit IgG, of vWF-induced platelet agglutination, as reported by other authors, is either due to a steric effect resulting from a vicinal position of both receptors or involves the residual part of GP Ib after cleavage of Gc.

Monomeric IgG preparations for intravenous use inhibit platelet stimulation by polymeric IgG.

Seven different immunoglobulin G (IgG) preparations for intravenous use were tested for their capacity to inhibit serotonin-release from washed human platelets induced by IgG coupled with bis-diazobenzidine. Using a reference standard consisting of the top third fraction of an ultracentrifuged gammaglobulin preparation for intramuscular use, two chemically treated preparations were less potent inhibitors than the standard while two preparations, one pH 4 treated and another albumin protected, were better inhibitors. A pepsin treated preparation was devoid of inhibitory capacity, whereas Fc fragments derived from human IgG were extremely efficient. Evidence is discussed that the inhibitory capacity is inversely correlated to the content of oligo- and/or dimeric IgG molecules.