Active von Willebrand factor in thrombotic thrombocytopenic purpura and malaria

Abstract

Thrombotic thrombocytopenic purpura (TTP) and malaria are two diseases of distinct origin. TTP is a rare disorder caused by a deficiency of the von Willebrand factor (VWF) cleaving protease ADAMTS13. Malaria is a poverty-related disease caused by protozoan parasites from the genus Plasmodium. TTP and malaria share several clinical symptoms including intravascular platelet agglutination with thrombocytopenia, haemolytic anemia, neurological symptoms and fever. VWF plays an essential role in the adhesion of platelets to the injured vessel wall under conditions of blood flow. The platelet-binding site of plasma VWF, which is located in the A1 domain, is encrypted to prevent unwanted interaction with platelets. In order to arrest bleeding, VWF is converted from its latent conformation into an active conformation, a conformation in which the platelet-binding site in the A1 domain is exposed. Persistence of active VWF in the circulation would cause undesired VWF-platelet aggregate formation, resulting in thrombosis and/or bleeding from thrombocytopenia. Therefore, tight regulation of VWF activation and inactivation mechanisms is essential to prevent the occurrence of both bleeding and thrombotic events. An important regulator of VWF activity is ADAMTS13, a metalloprotease that cleaves VWF. TTP is caused by a genetic or auto-immune based deficiency or dysfunction of ADAMTS13. Aberrations in VWF have been described for both TTP and malaria. In acute TTP, when patients suffer from thrombosis and require plasma exchange, absence of ADAMTS13 results in the presence of active VWF multimers in the circulation. Consequently, microthrombi are being formed that cause occlusion of the microvasculature and organ damage. With respect to remission, when patients do not longer require plasma exchange to prevent thrombosis, no information is available about active VWF. For malaria, elevated VWF-antigen levels have been reported in field studies, but the eventual role of active VWF in the development of platelet-clumping and thrombocytopenia is unknown. This thesis aims to provide more insight into the origin of active VWF and the role of active VWF in TTP and malaria. Chapter 3 focuses on endothelial cells and reports studies on the origin of active VWF. Chapter 4 describes VWF- and ADAMTS13-related features in a cohort of TTP patients in remission and gives insight in the composition of active VWF multimers. Chapter 5 addresses the relevance of an ADAMTS13 activity assay, based on fluorescence resonance energy transfer, with regard to TTP diagnosis. Chapters 6 and 7 describe the relation between (active) VWF, ADAMTS13 and thrombocytopenia in an experimental human infection malaria study involving healthy volunteers (chapter 6) and in a field study on the Indonesian island Sumba (chapter 7). In the last chapter (chapter 8), the findings from the preceding chapters are taken together and discussed.