Platelet ultrastructural abnormalities in three patients with type 2B von Willebrand disease

Several reports have described the presence of giant platelets in patients with type 2B von Willebrand disease (VWD). We have now characterized the ul trastructural changes in platelets from three unrelated patients with type 2B VWD and different mutations within exon 28 of the von Willebrand factor (VWF) gene. Electron microscopy showed that each of these subjects had an i ncreased proportion of large platelets when compared with those of a patien t with type 2A VWD or control subjects. Immunogold labelling for VWF was pe rformed. Large masses detected by anti-VWF antibody were seen not only on t he platelet surface, but also inside the platelet surface-connected canalic ular system (SCCS) when ultrathin sections were labelled. This suggested tr anslocation of the abnormally bound VWF from the platelet surface. Labellin g of the or-granules was eccentric as for normal platelets. Labelling for g lycoprotein (Gr) Ib was seen on the surface and within the SCCS, suggesting co-localization with the bound VWF. However, there was no evidence for VWF in endosomes or other endocytic vesicles. The presence of platelet-bound V WF was not accompanied by high levels of platelet activation, as detected b y electron microscopy, or by using monoclonal antibodies against P-selcctin or activation-dependent determinants on GP IIb-IIIa in flow cytometry. Int riguingly, platelet ultrastructure often resembled that seen in patients wi th congenital thrombocytopathies characteristic of giant platelet syndromes .