Absence of incorporation of plasma von Willebrand factor into porcine platelet alpha-granules.

In order to study the relationship between plasma and platelet von Willebrand factor (vWF), we used an experimental model of crossed bone marrow transplantation (BMT) between SLA immunocompatible normal and homozygous von Willebrand (vWD) pigs. A normal pig received bone marrow from a vWD pig and a second pig with vWD was engrafted with marrow from a normal pig. Each recipient, after total irradiation of 10 Grays, received by a central catheter 10(10) monocellular bone marrow cells without immunosuppression. The animals were followed for 50 d and no graft rejection or graft-versus-host disease was observed. After aplasia occurring 3 weeks after BMT, white blood cells and platelets returned to normal. Before transplantation, in the vWD pig, vWFAg and vWF activity were not detected in plasma and in platelet and megakaryocyte alpha-granules. After transplantation with normal marrow, platelet vWFAg and platelet vWF activity wer normal and high molecular weight multimers and numerous tubular structures were present in alpha-granules. Before transplantation, the normal pig had normal plasma and platelet vWFAg-vWF activity, normal multimeric pattern, and the platelet and megakaryocyte alpha-granules displayed many tubular structures, eccentrically located in one of their poles, coinciding with immunogold staining vWFAg. After transplantation with homozygous vWD marrow, platelet and megakaryocyte alpha-granules lacked tubular structures. Alpha-granule immunogold staining for vWF was consistently negative, although plasma vWF was at a normal level. In conclusion, this study shows that, unlike other plasma proteins such as fibrinogen. vWF endocytosis does not occur from plasma to the platelet alpha-granules. Platelet and megakaryocyte vWF solely originates from megakaryocyte endogenous synthesis and is independent of plasma vWF.