Demonstration of the role of VWF in platelet FVIII level and activity. (A) Antigenic determination of hB-FVIII in platelet releasate compared with a standard curve using recombinant hB-FVIII.²  Platelets were from transgenic hB-FVIII/FVIII^null mice that were also either wild type (WT; +/+), heterozygote (+/-), or homozygote (-/-) for the VWF targeted gene deletion as noted in the graph. Means ± 1 standard deviation (SD) are shown. (B) WBCT studies were determined as previously described.²  Studied animals are as in panel A. Means ± 1 standard deviation (SD) are shown. WBCT in WT mice is 5.0 ± 1.0 minute and greater than 45 minutes in FVIII^null mice. (C) Carotid artery FeCl₃ injury studies as previously described.¹⁰  A representative thrombosis curve for each platelet-donor phenotype (top row) and for the FVIII^null/VWF^+/+ recipient mice (center) is shown. Similar curves for posttransfusion recipient animals are shown (bottom row). Each group was studied 5 or more times with similar results.