Ultralarge complexes of PF4 and heparin are central to the pathogenesis of heparin-induced thrombocytopenia.
Heparin-induced thrombocytopenia and thrombosis (HITT) is a severe complication of heparin therapy caused by antibodies to complexes between unfractionated heparin (UFH) and platelet factor 4 (PF4) that form over a narrow molar range of reactants and initiate antibody-induced platelet activation. We observed that UFH and tetrameric PF4 formed ultralarge (> 670 kDa) complexes (ULCs) only over a narrow molar range with an optimal ratio of PF4 to heparin of approximately 1:1. These ULCs were stable and visible by electron microscopy, but they could be dissociated into smaller complexes upon addition of heparin. ULCs formed inefficiently when PF4 was incubated with low-molecular-weight heparin, and none formed with the pentasaccharide fondaparinux sodium. In addition, mutation studies showed that formation of ULCs depended on the presence of PF4 tetramers. The ULCs were more reactive as determined by their capacity to bind to a HITT-like monoclonal antibody and showed greater capacity to promote platelet activation in an antibody- and FcgammaRIIA-dependent manner than were the smaller complexes. The capacity of PF4 to form ULCs composed of multiple PF4 tetramers arrayed in a lattice with several molecules of UFH may play a fundamental role in autoantibody formation, antibody-dependent platelet activation, and the propensity for thrombosis in patients with HITT.
Division of Hematology, Children's Hospital of Philadelphia, PA, USA.
Lubica
L
Rauova
Mortimer
M
Poncz
Steven E
SE
McKenzie
Michael P
MP
Reilly
Gowthami
G
Arepally
John W
JW
Weisel
Chandrasekaran
C
Nagaswami
Douglas B
DB
Cines
Bruce S
BS
Sachais
D000818:Animals; D000911:Antibodies, Monoclonal; D002851:Chromatography, High Pressure Liquid; D006493:Heparin / Q000737:chemistry* / Q000494:pharmacology* / Q000648:ultrastructure; D006801:Humans; D046529:Microscopy, Electron, Transmission; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D015539:Platelet Activation / Q000187:drug effects / Q000276:immunology; D010978:Platelet Factor 4 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D013552:Swine; D013921:Thrombocytopenia / Q000139:chemically induced* / Q000378:metabolism*
D000911:Antibodies, Monoclonal; D010978:Platelet Factor 4; D006493:Heparin