Fc-independent cross-linking of a novel platelet membrane protein by a
monoclonal antibody causes platelet activation
S De Reys, MF Hoylaerts, M De Ley, J Vermylen and H Deckmyn
Center for Molecular and Vascular Biology, Faculty of Medicine, University
of Leuven, Belgium.
A monoclonal antiplatelet antibody (MA-13G8E1) is described that dose-
dependently induces platelet aggregation and serotonin release in an Fc-
independent fashion. Whereas platelets were equally aggregated by F(ab')2
fragments of this monoclonal antibody (MoAb), its Fab fragments, on the
other hand, were inactive, indicating that divalent interaction is an
essential requirement to induce platelet activation by MA-13G8E1. In
addition, we could show that platelet epitope cross- linking by MA-13G8E1
occurred on the same platelet. MA-13G8E1 stimulated platelet phospholipase
C (PLC) and induced activation of protein kinase C (PKC), both of which
were almost unaffected by aspirin pretreatment. Furthermore, PLC activation
appeared to be a direct antibody-mediated effect, since intracellular Ca2+
rises were not inhibited by EGTA, cytochalasin B, or aggregation-blocking
MA-16N7C2 (antiglycoprotein [anti-GP]IIb/IIa). The MA-13G8E1 antigen is
constitutively expressed on resting platelets of different species (7,100
+/- 800 molecules per human platelet), but not on other cell types tested.
Both immunoprecipitation and affinity isolation by MA- 13G8E1 showed two
low-molecular weight proteins (45 and 36 kD), having slightly acidic
isoelectric pH levels (4.5 to 5.5) and forming multimolecular complexes. In
conclusion, we found an MoAb that is able to induce platelet activation in
an Fc-independent fashion. The mechanism involves cross-linking of a
hitherto undescribed platelet membrane protein, leading to PLC and PKC
stimulation.
Volume 84,
Issue 2,
pp. 547-555,
07/15/1994
Copyright © 1994 by The American Society of Hematology
