Recombinant factor VIIa restores aggregation of αIIbβ3-deficient platelets via tissue factor–independent fibrin generation

P-selectin expression has been shown inHelicobacter pylori-infected persons, an infection that has been clinically associated with platelet-related diseases, such as idiopathic thrombocytopenic purpura. However, the role of P-selectin expression during H pylori infection remains unclear. In this study, we hypothesized that P-selectin expression was associated with platelet aggregation during H pylori infection. Using flow cytometry, we examined the levels of adhesion between H pylori and platelets as well as the levels of P-selectin expression and platelet phosphatidylserine (PS) expression during H pylori infection. Significantly high levels of adhesion between proaggregatory bacteria and platelets were observed. We identified that H pylori IgG is required for bacteria to induce Pselectin expression and that a significant release of P-selectin is essential for H pylori to induce aggregation. In addition, cellular apoptotic signs, such as membrane blebbing, were observed in platelet aggregates. PS expression was also detected in platelets during infection with both pro-aggrogatory and nonaggregatory strains of H pylori. These results suggest that the decrease in platelet counts seen during H pylori infection is the result of P-selection-dependent platelet aggregation and PS expression induced by the bacteria. IntroductionMany diseases associated with platelet aggregation have been described as being related to Helicobacter pylori infection. For example, H pylori-infected persons have a tendency toward suffering from myocardial infarctions, 1-3 coronary heart disease, 4,5 and stroke. 6 It has also been suggested that H pylori may trigger the formation of thrombotic thrombocytopenic purpura (TTP) by inducing platelet aggregation through an interaction with the von Willebrand factor (VWF). 7 There have also been implications that chronic H pylori infection may be associated with idiopathic thrombocytopenic purpura (ITP), as eradication of the bacteria from gastric mucosa has shown improvement in some ITP patients. [8][9][10][11][12][13][14][15][16] There is ongoing interest in identifying the various H pylori virulence factors that may predict the risk of developing symptoms of ITP. Studies have primarily focused on 2 groups of putative bacterial virulence factors, the cag pathogenicity island (for which CagA is a marker) and the vacuolating cytotoxin, such as VacA 17,18 ; however, CagA and VacA have not been suggested to be causes of H pylori-induced platelet aggregation. 19 There has been evidence showing that associations exist between H pylori and platelet aggregation in vivo. Platelet aggregation was observed in rat gastric mucosal microcirculation in vivo after H pylori administration. 20 An increase in arterial thrombosis was also found in chronic H pylori-infected mice. 17 However, the mechanisms of how H pylori induces platelet aggregation are not clearly understood. Byrne et al proposed that the H pylori strain 60190 (ATCC 49503) induces platelet aggregation through interactions between H pylori, its ant...

Section: Platelet Preparationmentioning

confidence: 99%

P-selectin–dependent platelet aggregation and apoptosis may explain the decrease in platelet count during Helicobacter pylori infection

Yeh

Tsai

Wu³

et al. 2010

“…In the absence of factor VIII or IX it can be shown that at pharmacologic doses, fVIIa will bind weakly to activated platelets and can directly activate factor X to factor Xa such that in the presence of factor V, thrombin generation can be improved over and above that generated by the tissue factor/VIIa complex alone ( Figure 2). [20][21][22] The binding of rfVIIa to activated platelets may explain why rfVIIa is localized only to the site of bleeding. In this model, it is clear that the tissue factor pathway must be intact in order for the initial activation of platelets to occur by the small amount of thrombin formed at the level of the tissue factorbearing cell.…”

Section: Mechanism Of Action Of Factor Viiamentioning

confidence: 99%

The use of recombinant factor VIIa in the treatment of bleeding disorders

Roberts

Monroe

White

2004

252

207

Recombinant factor VIIa was initially developed for the treatment of hemorrhagic episodes in hemophilic patients with inhibitors to factors VIII and IX. After its introduction, it has also been used "off-label" to enhance hemostasis in nonhemophilic patients who experience bleeding episodes not responsive to conventional therapy. Evidence so far indicates that the use of factor VIIa in hemophilic patients with inhibitors is both safe and effective. Anecdotal reports also suggest that the product is safe and effective in controlling bleeding in nonhemophilic patients. However, its use in these conditions has not been approved by the FDA, and conclusive evidence of its effectiveness from controlled clinical trials is not yet available. Several questions pertaining to the use of factor VIIa require further investigation, including the mechanism of action; the optimal dose; definitive indications; ultimate safety; and laboratory tests for monitoring therapy. (Blood.

“…17 Furthermore, several studies indicated the existence of an as-yet-unidentified platelet receptor for fibrin. 18,19 Moreover, polymerized fibrin has also been reported to trigger the procoagulant activity of platelets. 20 These observations led us to investigate the role of GPVI in platelet/fibrin interactions.…”

mentioning

confidence: 99%

Platelet glycoprotein VI binds to polymerized fibrin and promotes thrombin generation

Mammadova‐Bach

Ollivier

Loyau

et al. 2015

225

170

Key Points• GPVI interaction with polymerized fibrin triggers a new loop amplifying thrombin generation and platelet recruitment at the clot surface.Fibrin, the coagulation end product, consolidates the platelet plug at sites of vascular injury and supports the recruitment of circulating platelets. In addition to integrin aIIbb3, another as-yet-unidentified receptor is thought to mediate platelet interaction with fibrin. Platelet glycoprotein VI (GPVI) interacts with collagen and several other adhesive macromolecules. We evaluated the hypothesis that GPVI could be a functional platelet receptor for fibrin. Calibrated thrombin assays using platelet-rich plasma (PRP) showed that tissue factor-triggered thrombin generation was impaired in GPVI-deficient patients and reduced by the anti-GPVI Fab 9O12. Assays on reconstituted PRP and PRP from fibrinogen-deficient patients revealed a fibrinogen-dependent enhancement of thrombin generation, which relied on functional GPVI. The effect of GPVI was found to depend on fibrin polymerization. A binding assay showed a specific interaction between GPVI-Fc and fibrin, inhibited by the Fab 9O12. This Fab also reduced platelet adhesion to fibrin at low (300 s 21) and high (1500 s 21 ) wall shear rates. Platelets adherent to fibrin displayed shape change, exposure of procoagulant phospholipids, and the formation of small clots. When hirudinated blood was perfused at 1500 s 21 over preformed fibrin-rich clots, the Fab 9O12 decreased the recruitment of platelets by up to 85%. This study identifies GPVI as a platelet receptor for polymerized fibrin with 2 major functions: (1) amplification of thrombin generation and (2) recruitment of circulating platelets to clots. These so-farunrecognized properties of GPVI confer on it a key role in thrombus growth and stabilization. (Blood. 2015;126(5):683-691)