Effect of recombinant von Willebrand factor reproducing type 2B or type 2Mmutations on shear-induced platelet aggregation

The aim was to better understand the function of von Willebrand factor (vWF ) A1 domain in shear-induced platelet aggregation (SIPA), at low (200) and high shear rate (4000 seconds(-1)) generated by a Couette viscometer, We re port on 9 fully multimerized recombinant vWFs (rvWFs) expressing type 2M or type 2B von Willebrand disease (vWD) mutations, characterized respectively by a decreased or increased binding of VWF to GPlb in the presence of rist ocetin, We expressed 4 type 2M (-G561A, -E596K, -R611H, and -I662F) and 5 t ype 2B (rvWF-M540MM, -V551F, -V553M, -R578Q, and -L697V), SIPA was strongly impaired in all type 2M rvWFs at 200 and 4000 seconds(-1). Decreased aggre gation was correlated with ristocetin binding to platelets. In contrast, a distinct effect of botrocetin was observed, since type 2M rvWFs (-G561A, -E 596K, and -I662F) were able to bind to platelets to the same extent as wild type rvWF (rvWF-WT), Interestingly, SIPA at 200 and 4000 seconds-l confirm ed the gain-of-function phenotype of the 5 type 2B rvWFs, Our data indicate d a consistent increase of SIPA at both low and high shear rates, reaching 95% of total platelets, whereas SIPA did not exceed 40% in the presence of rvWF-WT. Aggregation was completely inhibited by monoclonal antibody 6D1 di rected to GPlb, underlining the importance of vWF-GPlb interaction in type 28 rvWF, Impaired SIPA of type 2M rvWF could account for the hemorrhagic sy ndrome observed in type 2M vWD, Increased SIPA of type 28 rvWF could be res ponsible for unstable aggregates and explain the fluctuant thrombocytopenia of type 28 vWD. (C) 2000 by The American Society of Hematology.