Flow cytometry for comprehensive assessment of platelet functional activity in response to ADP stimulation

OBJECTIVES

Flow cytometry with adenosine diphosphate (ADP) allows to characterize molecular changes of platelet function caused by this physiologically important activation, but the methodology has not been thoroughly investigated, standardized and characterized yet. We analyzed the influence of several major variables and chose optimal conditions for platelet function assessment.

METHODS

For activation, 2.5 μM CaCl2 , 5 μM ADP and antibodies were added to diluted blood and incubated for 15 min. We analyzed kinetics of antibody binding and effects of their addition sequence, agonist concentration, blood dilution, exogenous calcium addition and platelet fixation.

RESULTS

We tested our protocol on 11 healthy children, 22 healthy adult volunteers, 9 patients after a month on dual antiplatelet therapy after percutaneous coronary intervention (PCI), 7 adult patients and 14 children with immune thrombocytopenia (ITP). We found that our protocol is highly sensitive to ADP stimulation with low percentage of aggregates formation. The assay is also sensitive to platelet function inhibition in post-PCI patients. Finally, platelet preactivation with ITP plasma was stronger and caused increase in activation response to ADP stimulation compared to preactivation with low dose of ADP.

CONCLUSIONS

Our assay is sensitive to antiplatelet therapy and platelet preactivation in ITP patients under physiological conditions with minimal percentage of aggregates formation.

Healthy pediatric platelets are moderately hyporeactive in comparison with adults' platelets

Studies on platelet function in children older than neonatal period are few and their results are controversial. The pediatric platelets were alternatively reported to be more active or less active than adults' ones. We compared platelet function in the several age groups of children to adults and evaluated the age when platelet function reaches the adults' status. The study included 76 healthy children and 49 healthy adult volunteers. Types of platelet activation used included: collagen-related peptide (CRP) and PAR-1 activating peptide SFLLRN; SFLLRN, PAR-4 activating peptide AYPGKF and adenosine diphosphate (ADP); ADP. The parameters determined included forward (FSC) and side scatter (SSC), CD42b, CD61, CD62P, PAC-1, annexin V binding and mepacrine release levels. Resting pediatric platelets were similar to adults' platelets except for 1.2-fold decreased FSC and dense granules volume in youngest children, and 2.5-fold increased annexin V level in children aged 1-10 years. After CRP+SFLLRN stimulation, pediatric platelets had a 1.2-fold lower alpha- and 1.1-fold lower dense granule release than adults. For SFLLRN+AYPGKF+ADP stimulation, this was observed only for youngest children. The response to ADP stimulation was identical for pediatric platelets and adults. Pediatric platelets have lower granular release than adults' platelets, which persists until the age of 18.

Platelet function and blood coagulation system status in childhood essential thrombocythemia

Childhood essential thrombocythemia (ET) is a rare chronic myeloproliferative disorder. The quality of life of ET patients may decrease as a result of ischemic and hemorrhagic complications of unclear origin. Our goal was to characterize the hemostatic system in children with ET. We genotyped and investigated blood samples from 20 children with ET in a prospective case series study using platelet aggregation, functional flow cytometry (FC) assay and standard clotting assays. Three children had a JAK2V617F mutation, 4 had mutations in CALR and 13 were triple-negative. Myelofibrosis in stage 1-2 was detected in 3 children. Three patients had bleeding episodes and seven had ischemic events. Aggregation in response to collagen, adenosine diphosphate, and ristomycin was decreased in all patients. In FC, significant changes in the whole patient group compared to the healthy children control group were decrease in the resting forward scatter and PAC1 binding (activated GPIIb/IIIa) level. For the activated platelets, dense granules release (by mepacrine), PAC1, and GPIIb/IIIa levels were significantly decreased. GPIb/V/IX, P-selectin, and phosphatidylserine levels manifested only moderate differences. Forward and side scatter changes in response to stimulation (representing shape change) and dense granules release were significantly lower in the 3 patients with bleeding than in the 17 patients without hemorrhage. Activated partial thromboplastin time was slightly prolonged, prothrombin index was slightly shortened and thrombin time was normal, while fibrinogen was mildly decreased in the ET patients. It could be concluded that the observed platelet function defects could be related to bleeding in ET, and be potentially used as a marker.

[Whether fibrinogen concentrates are necessary in Russia?]

PURPOSE OF THE STUDY

To analyze an efficiency of hypofibrinogenemia treatment. In the Scientific Center for Hematology (Moscow) significant hypofibrinogenemia occurs in 3% of patients with hemoblastosis. 1000 doses of cryoprecipitate are used for a hypofibrinogenemia treatment every year (21-23 doses for each patient). Containing of fibrinogen in a one cryoprecipitate dose is from 108 mug to 711 mug (M = 276 mug). Volume of one dose is from 8 to 90 ml (M = 24 ml). Hypofibrinogenemia occurred in all patients required a cryoprecipitate transfusion (M = 1 g L(-1), min 0.5 g L(-1), max 2 g L(-1)). We fixed an increasing of fibrinogen level in plasma by 0.7 +/- 0.2 g L(-1) after the cryoprecipitate transfusion. We analyzed a world experience of the use of fibrinogen containing blood components.

CONCLUSIONS

Fresh frozen plasma transfusion cannot be a choice method of treatment for hypofibrinogenemia. Fibrinogen's concentrate has the same effectiveness as a cryoprecipitate both for congenital and acquired deficit of fibrinogen. The frequency of complications due to fibrinogen's concentrate is low. Currently clinical studies of recombinant fibrinogen are conducted. Ways of implementation of fibrinogen preparations in Russia are discussed.