Patients with prolonged clotting times caused by lupus anticoagulant (LAC) are at risk for thrombosis. This paradoxal association is not understood. LAC is frequently caused by anti-beta2-glycoprotein I (beta 2GPI) antibodies. Antibody-induced dimerization of beta 2GPI increases the affinity of beta 2GPI for phospholipids, explaining the observed prolonged clotting times. We constructed dimers of beta 2GPI that mimic effects of beta 2GPI-anti-beta 2GPI antibody complexes, and we studied their effects on platelet adhesion and thrombus formation in a flow system. Dimeric beta 2GPI increased platelet adhesion to collagen by 150% and increased the number of large aggregates. We also observed increased platelet adhesion to collagen when whole blood was spiked with patient-derived polyclonal anti-beta 2GPI or some, but not all, monoclonal anti-beta 2GPI antibodies with LAC activity. These effects could be abrogated by inhibition of thromboxane synthesis. A LAC-positive monoclonal anti-beta 2GPI antibody, which did not affect platelet adhesion, prevented the induced increase in platelet adhesion by beta 2GPI dimers. Furthermore, increased platelet adhesion disappeared after preincubation with receptor-associated protein, a universal inhibitor of interaction of ligands with members of the low density lipoprotein receptor family. Using co-immunoprecipitation, it was shown that dimeric beta 2GPI can interact with apolipoprotein E receptor 2 (apoER2'), a member of the low density lipoprotein receptor family present on platelets. These results demonstrate that dimeric beta 2GPI induces increased platelet adhesion and thrombus formation, which depends on activation via apoER2'.

PCC reverses anticoagulation safely, faster and with less bleeding than FFP.

The role of Gas6 in endothelial cell (EC) function remains incompletely characterized. Here we report that Gas6 amplifies EC activation in response to inflammatory stimuli in vitro. In vivo, Gas6 promotes and accelerates the sequestration of circulating platelets and leukocytes on activated endothelium as well as the formation and endothelial sequestration of circulating platelet-leukocyte conjugates. In addition, Gas6 promotes leukocyte extravasation, inflammation, and thrombosis in mouse models of inflammation (endotoxinemia, vasculitis, heart trans- IntroductionThe growth arrest-specific gene 6 (Gas6) binds to the receptor tyrosine kinases Axl, Tyro3, and Mer. 1 Gas6 is composed of a N-terminal gamma-carboxy-glutamic acid domain (Gla-domain), a loop region, 4 EGF-like repeats, and a C-terminal steroid hormone binding globulin-like (SHBG-like) domain). 1 Even though this molecule was discovered as a homologue of the anticoagulant protein S more than a decade ago, its role in vivo remains incompletely characterized. 2,3 Originally identified in fibroblasts, Gas6 is expressed in various cell types, including endothelial cells (ECs), 4 smooth muscle, 5 and bone marrow cells. 6 Gas6 and its receptors modify platelet activation and aggregation, 7-10 but the role of Gas6 in the interplay between platelets and other cell types, such as ECs and leukocytes, 11 during inflammatory conditions remains unclear.Several lines of evidence indeed suggest that Gas6 may affect ECs and leukocytes. ECs and leukocytes express Gas6 and its receptors, especially in conditions of inflammation and repair. 4,[12][13][14][15][16] Gas6 stimulates EC survival [17][18][19][20] and promotes angiogenesis by enforcing the adhesion of Axl-expressing ECs via homophilic interactions, 21-23 yet another study suggested that activation of Axl impairs tyrosine phosphorylation of vascular endothelial growth factor (VEGF) receptor-2. 24 The activity of Gas6 on leukocytes also remains incompletely understood. Indeed, genetic loss of Mer inhibits cytokine production by natural killer cells 25 while it stimulates tumor necrosis factor-␣ (TNF-␣) production by monocytes 14 and impairs clearance of apoptotic cells. 26 Loss of all 3 Gas6 receptors, on the other hand, induces lymphoproliferative disorders via hyperactivation of antigen-presenting cells, 27,28 but mice lacking Gas6 (Gas6 Ϫ/Ϫ ) do not develop autoimmune health problems (P.C., unpublished data, 2008). In humans, the plasma levels of Gas6 were found to be elevated during severe sepsis, a life-threatening condition involving increased interactions between ECs, leukocytes, and platelets. 29,30 However, exogenous Gas6 inhibits granulocyte adhesion to ECs, but only at very high doses. 31 Furthermore, the role of endogenous Gas6 in leukocyte extravasation in vivo was not studied. Here, by using our previously generated Gas6 Ϫ/Ϫ mice, 7 we studied whether Gas6 might play a role in EC activation and in the interactions between ECs, platelets, and leukocytes during inflammatory conditions. Methods MiceTh...

The hemostatic system comprises platelet aggregation, coagulation and fibrinolysis also termed primary, secondary and tertiary hemostasis. From the platelet transcriptome 6000 mRNA species and represent receptors, ion channels, signalling molecules, kinases, phosphatases, and structural, metabolic and regulatory proteins. This abundance of regulatory proteins points towards the importance of signal transduction in platelet function. First platelets adhere to collagen, this induces activation signals such as TXA(2) that induces further Ca(2+) increase. Consecutively, fibrinogen binds to the integrin alpha(IIb)beta(3) resulting in aggregation.This self-amplifying process is controlled by signals, from endothelial cells, to restrict the platelet plug to the site of vessel injury. Secondary hemostasis (coagulation) consists of an extrinsic and intrinsic pathway. Thrombin is generated via Factor Xa resulting from the extrinsic tenase reaction that is turned of by tissue factor pathway inhibitor. While thrombin generation is maintained via positive feedback mechanisms activating factors V, VIII and XI. Excess thrombin is inhibited by antithrombin or by autodownregulation via activation of protein C. Since minor injuries are common, platelets and plasma clotting factors constantly produce clots to stop bleeding. If clots remained after the tissue healing, the vascular bed would become obstructed with clots therefore this is regulated by fibrinolysis, tertiary hemostasis. Tissue-type plasminogen activator synthesised by the endothelium, converts plasminogen to plasmin, the clot lysis enzyme. Plasmin clears the blood vessels by degrading fibrin. Fibrinolysis is controlled by plasminogen activators inhibitor (PAI-1), alpha2-antiplasmin and alpha2-macroglobulin, and thrombin-activatable fibrinolysis inhibitor (TAFI).

SummaryAntibodies to factor VIII (inhibitors) are usually produced at the beginning of treatment with factor VIII and are rare in multitransfused patients. Such antibodies are deemed to be patient-related, as supported by the description of a number of associated risk factors. However, a second category of inhibitors has recently been identified, namely antibodies occurring in multitransfused patients as a result of exposure to a particular factor VIII concentrate. A first outbreak of product-related inhibitors was recently described. The present paper describes the second well-documented occurrence of such inhibitors.Eight out of 140 multitransfused patients with severe haemophilia A developed an inhibitor to factor VIII shortly after changing treatment to a double-virus inactivated plasma-derived factor VIII concentrate. In addition to solvent-detergent treatment, this concentrate was pasteurised at 63° C for 10 hours. Exposure to the pasteurised product before inhibitor detection ranged from 9 to 45 days. Inhibitor titers varied between 2.2 and 60 Bethesda Units and recovery of transfused factor VIII ranged from 0.21 to 0.68 (expressed as IU/dl factor VIII rise per IU/kg administered). In contrast to usual inhibitors in haemophilia A patients, these product-related inhibitors showed complex inhibition kinetics. They were found specific for the factor VIII light chain. The inhibitors gradually declined when exposure to the pasteurised product was stopped, despite further treatment with other factor VIII concentrates. The present data stress the importance of carefully monitored clinical studies, both in previously treated and previously untreated patients, before introduction of a new or modified clotting factor concentrate.