Validation of flow cytometric analysis of platelet function in patients with a suspected platelet function defect

Essentials The diagnosis of mild platelet function disorders (PFDs) is challenging. Validation of flow cytometric testing in patients with suspected PFDs is required. Flow cytometry has added value to light transmission aggregometry (LTA) in diagnosis of PFDs. There is fair agreement in diagnosing PFDs between LTA and flow cytometry.

SUMMARY

Background Light transmission aggregometry (LTA) is the most commonly used test for the diagnosis of platelet function disorders (PFDs), but has moderate sensitivity for mild PFDs. Flow cytometry has been recommended for additional diagnostics of PFDs but is not yet standardized as a diagnostic test. We developed a standardized protocol for flow cytometric analysis of platelet function that measures fibrinogen binding and P-selectin expression as platelet activation markers in response to agonist stimulation. Objectives To determine the additional value of flow cytometric platelet function testing to standard LTA screening in a cross-sectional cohort of patients with a suspected PFD. Methods Platelet function was assessed with flow cytometry and LTA in 107 patients suspected of a PFD in whom von Willebrand disease and coagulation factor deficiencies were excluded. Both tests were compared in terms of agreement and discriminative ability for diagnosing patients with PFDs. Results Out of 107 patients, 51 patients had an elevated bleeding score; 62.7% of the patients had abnormal platelet function measured with flow cytometry and 54.2% of the patients were abnormal based on LTA. There was fair agreement between LTA and flow cytometry (κ = 0.32). The discriminative ability of flow cytometric analysis in patients with an elevated bleeding score was good (AUC 0.82, 0.74-0.90), but moderate for LTA (AUC 0.70, 0.60-0.80). Both tests combined had a better discriminative ability (AUC 0.87, 0.80-0.94). Conclusion Flow cytometric analysis of platelet function has added value in diagnostics of PFDs in patients with unexplained bleeding tendency.

Agonist-induced platelet reactivity correlates with bleeding in haemato-oncological patients

BACKGROUND AND OBJECTIVE

Prophylactic platelet transfusions are administered to prevent bleeding in haemato-oncological patients. However, bleeding still occurs, despite these transfusions. This practice is costly and not without risk. Better predictors of bleeding are needed, and flow cytometric evaluation of platelet function might aid the clinician in identifying patients at risk of bleeding. This evaluation can be performed within the hour and is not hampered by low platelet count. Our objective was to assess a possible correlation between bleeding and platelet function in thrombocytopenic haemato-oncological patients.

MATERIALS AND METHODS

Inclusion was possible for admitted haemato-oncology patients aged 18 years and above. Furthermore, an expected need for platelet transfusions was necessary. Bleeding was graded according to the WHO bleeding scale. Platelet reactivity to stimulation by either adenosine diphosphate (ADP), cross-linked collagen-related peptide (CRP-xL), PAR1- or PAR4-activating peptide (AP) was measured using flow cytometry.

RESULTS

A total of 114 evaluations were available from 21 consecutive patients. Platelet reactivity in response to stimulation by all four studied agonists was inversely correlated with significant bleeding. Odds ratios (OR) for bleeding were 0·28 for every unit increase in median fluorescence intensity (MFI) [95% confidence interval (CI) 0·11-0·73] for ADP; 0·59 [0·40-0·87] for CRP-xL; 0·59 [0·37-0·94] for PAR1-AP; and 0·43 [0·23-0·79] for PAR4-AP. The platelet count was not correlated with bleeding (OR 0·99 [0·96-1·02]).

CONCLUSION

Agonist-induced platelet reactivity was significantly correlated to bleeding. Platelet function testing could provide a basis for a personalized transfusion regimen, in which platelet transfusions are limited to those at risk of bleeding.

[Platelet function tests: tailored antiplatelet therapy for vascular patients?]

- Platelet aggregation inhibitors, also known as antiplatelet therapy (APT), are prescribed for the prevention of secondary cardiovascular events (CVE) after endovascular revascularization procedures.- Platelet aggregation inhibitors are not equally effective in all patients. The phenomenon of high residual platelet reactivity despite APT is called 'high on-treatment platelet reactivity' (HTPR); it bears an increased risk of secondary CVE.- Platelet function tests (PFT) can be used to diagnose HTPR. There are various tests available; of those, light transmission aggregometry (LTA) is considered the gold standard. Some tests are only suitable for determining the effect of a certain category of APT.- Research into the usefulness of PFTs to optimise treatment with APT has not yet produced an unambiguous conclusion.- Currently there is not yet an indication for routine use of PFT in clinical practice. However, for the treatment of certain categories of patients with thromboembolic disease - such as those with renal failure or a history of kidney transplant - PFT can be considered.

Hematopoietic α7 nicotinic acetylcholine receptor deficiency increases inflammation and platelet activation status, but does not aggravate atherosclerosis

BACKGROUND

The autonomic nervous system attenuates inflammation through activation of the α7 nicotinic acetylcholine receptor (α7nAChR), a pathway termed the cholinergic anti-inflammatory reflex. Interestingly, α7nAChR is expressed on immune cells and platelets, both of which play a crucial role in the development of atherosclerosis.

OBJECTIVE

To investigate the role of hematopoietic α7nAChR in inflammation and platelet function in atherosclerotic ldlr(-/-) mice and to identify its consequences for atherosclerotic lesion development.

METHODS

Bone marrow from α7nAChR(-/-) mice or wild-type littermates was transplanted into irradiated ldlr(-/-) mice. After a recovery period of 8 weeks, the mice were fed an atherogenic Western-type diet for 7 weeks.

RESULTS

Hematopoietic α7nAChR deficiency clearly increased the number of leukocytes in the peritoneum (2.6-fold, P < 0.001), blood (2.9-fold; P < 0.01), mesenteric lymph nodes (2.0-fold; P < 0.001) and spleen (2.2-fold; P < 0.01), indicative of an increased inflammatory status. Additionally, expression of inflammatory mediators was increased in peritoneal leukocytes (TNFα, 1.6-fold, P < 0.01; CRP, 1.8-fold, P < 0.01) as well as in the spleen (TNFα, 1.6-fold, P < 0.01). The lack of α7nAChR on platelets from these mice increased the expression of active integrin αIIb β3 upon stimulation by ADP (1.9-fold, P < 0.01), indicating increased activation status, while incubation of human platelets with an α7nAChR agonist decreased aggregation (-35%, P < 0.05). Despite the large effects of hematopoietic α7nAChR deficiency on inflammatory status and platelet function, it did not affect atherosclerosis development or composition of lesions.

CONCLUSIONS

Hematopoietic α7nAChR is important for attenuation of inflammatory responses and maintaining normal platelet reactivity, but loss of hematopoietic α7nAChR does not aggravate development of atherosclerosis.

Platelet signaling induced by lipoproteins

Cardiovascular disease is the main cause of death and disability in the Western society. Lipoproteins play an important role in the development of this disease and affect different cell types involved in atherosclerosis and thrombosis. Based on their density, five classes of lipoproteins have been identified which all influence cells via distinct mechanisms. Modification turns lipoproteins into atherogenic particles with a prominent role in atherogenesis. The interaction of lipoproteins with platelets has been under investigation for a number of years. Especially the role of LDL in platelet signaling has been studied intensively as platelets of hypercholesterolemic patients are hyperreactive and show hyperaggregability in vitro and enhanced activity in vivo, suggesting that LDL enhances platelet responsiveness. Several signaling pathways induced by LDL have been revealed in vitro, such as signaling via p38 mitogen-activated protein kinase (p38MAPK) and p125 focal adhesion kinase (p125FAK). HDL opposes the activating properties of LDL on platelets, whereas the effects of chylomicrons, VLDL or IDL on platelet function are controversial. Modification of lipoproteins is associated with the generation of new constituents with new signaling properties. In particular, the platelet-activating properties of lysophosphatidic acid, which is a constituent of atherosclerotic plaques and is generated upon oxidation of LDL, have been investigated intensively. This review provides a summary of the activation of signaling pathways after platelet-lipoprotein interactions, with special emphasis on the role of these interactions in the development of thrombosis and atherosclerosis.