Production of anti-PF4 antibodies in antiphospholipid antibody-positive patients is not affected by COVID-19 vaccination.

BACKGROUND: Antibodies against cationic platelet chemokine, platelet factor 4 (PF4/CXCL4), have been described in heparin-induced thrombocytopenia (HIT), but also in patients positive for antiphospholipid antibodies (aPL) even in the absence of heparin treatment and HIT-related clinical manifestations. Anti-PF4 antibodies have been recently described also in subjects who developed thrombosis with thrombocytopenia syndrome (TTS) in association with adenoviral vector-based, but not with mRNA-based, COVID-19 vaccines. OBJECTIVE: To investigate whether COVID-19 vaccination affects the production of anti-PF4 antibodies in aPL-positive patients and in control groups. METHODS: Anti-PF4 immunoglobulins were detected in patients' and controls' serum samples by ELISA and their ability to activate normal platelets was assessed by the platelet aggregation test. RESULTS: Anti-PF4 were found in 9 of 126 aPL-positive patients, 4 of 50 patients with COVID-19, 9 of 49 with other infections, and 1 of 50 aPL-negative patients with systemic lupus erythematosus. Clinical manifestations of TTS were not observed in any aPL patient positive for anti-PF4, whose serum failed to cause platelet aggregation. The administration of COVID-19 vaccines did not affect the production of anti-PF4 immunoglobulins or their ability to cause platelet aggregation in 44 aPL-positive patients tested before and after vaccination. CONCLUSIONS: Heparin treatment-independent anti-PF4 antibodies can be found in aPL-positive patients and asymptomatic carriers, but their presence, titre as well as in vitro effect on platelet activation are not affected by COVID-19 vaccination.

Current management of cancer-associated venous thromboembolism in patients with thrombocytopenia: a retrospective cohort study.

Optimal management of venous thromboembolism (VTE) in cancer patients with thrombocytopenia is uncertain. We described current management and clinical outcomes of these patients. We retrospectively included a cohort of cancer patients with acute VTE and concomitant mild (platelet count 100,000-150,000/mm3), moderate (50,000-99,000/mm3), or severe thrombocytopenia (< 50,000/mm3). Univariate and multivariate logistic regression analyses explored the association between different therapeutic strategies and thrombocytopenia. The incidence of VTE and bleeding complications was collected at a 3-month follow-up. A total of 194 patients of whom 122 (62.89%) had mild, 51 (26.29%) moderate, and 22 (11.34%) severe thrombocytopenia were involved. At VTE diagnosis, a full therapeutic dose of LMWH was administered in 79.3, 62.8 and 4.6% of patients, respectively. Moderate (OR 0.30; 95% CI 0.12-0.75), severe thrombocytopenia (OR 0.01; 95% CI 0.00-0.08), and the presence of cerebral metastasis (OR 0.06; 95% CI 0.01-0.30) were independently associated with the prescription of subtherapeutic LMWH doses. Symptomatic VTE (OR 4.46; 95% CI 1.85-10.80) and pulmonary embolism (OR 2.76; 95% CI 1.09-6.94) were associated with the prescription of full therapeutic LMWH doses. Three-month incidence of VTE was 3.9% (95% CI 1.3-10.1), 8.5% (95% CI 2.8-21.3), 0% (95% CI 0.0-20.0) in patients with mild, moderate, and severe thrombocytopenia, respectively. The corresponding values for major bleeding and mortality were 1.9% (95% CI 0.3-7.4), 6.4% (95% CI 1.7-18.6), 0% (95% CI 0.0-20.0) and 9.6% (95% CI 5.0-17.4), 48.2% (95% CI 16.1-42.9), 20% (95% CI 6.6-44.3). In the absence of sound evidence, anticoagulation strategy of VTE in cancer patients with thrombocytopenia was tailored on an individual basis, taking into account not only the platelet count but also VTE presentation and the presence of cerebral metastasis.

Platelet Adhesion and Thrombus Formation in Microchannels: The Effect of Assay-Dependent Variables.

Microfluidic flow chambers (MFCs) allow the study of platelet adhesion and thrombus formation under flow, which may be influenced by several variables. We developed a new MFC, with which we tested the effects of different variables on the results of platelet deposition and thrombus formation on a collagen-coated surface. METHODS: Whole blood was perfused in the MFC over collagen Type I for 4 min at different wall shear rates (WSR) and different concentrations of collagen-coating solutions, keeping blood samples at room temperature or 37 °C before starting the experiments. In addition, we tested the effects of the antiplatelet agent acetylsalicylic acid (ASA) (antagonist of cyclooxygenase-1, 100 µM) and cangrelor (antagonist of P2Y12, 1 µM). RESULTS: Platelet deposition on collagen (I) was not affected by the storage temperature of the blood before perfusion (room temperature vs. 37 °C); (II) was dependent on a shear rate in the range between 300/s and 1700/s; and (III) was influenced by the collagen concentration used to coat the microchannels up to a value of 10 µg/mL. ASA and cangrelor did not cause statistically significant inhibition of platelet accumulation, except for ASA at low collagen concentrations. CONCLUSIONS: Platelet deposition on collagen-coated surfaces is a shear-dependent process, not influenced by the collagen concentration beyond a value of 10 µg/mL. However, the inhibitory effect of antiplatelet drugs is better observed using low concentrations of collagen.

Evaluation of platelet function in essential thrombocythemia under different analytical conditions.

Background. Studies of platelet aggregation (PA) in essential thrombocythemia (ET) reported contrasting results, likely due to differences in analytical conditions.Objective. We investigated platelet aggregation using different techniques and analytical conditions.Patients and Methods. PA was studied by light-transmission aggregometry (LTA) in platelet-rich plasma (PRP) and impedance aggregometry in PRP and whole blood (WB). ADP, collagen, thrombin receptor activating peptide (TRAP-14) and adrenaline were used as agonists. Since ET patients (n = 41) were on treatment with aspirin (100 mg/d), healthy controls (n = 29) were given aspirin (100 mg/d) for 5 days before testing: therefore, thromboxane A2-independent PA was tested in all subjects. Blood samples were collected in citrate (C) [low Ca2+] or lepirudin (L) [physiological Ca2+]; platelet count was adjusted to 250 x 109/L in a set of C-PRP (adjusted C-PRP) and left unmodified in the other samples.Results. Results of PA in 17 ET patients who were poor responders to aspirin (high serum thromboxane B2 levels) were not included in the analysis. With LTA, PA in ET was lower than in controls in adjusted C-PRP and normal in native C-PRP and L-PRP. With impedance aggregometry, PA in L-PRP and L-WB tended to be higher in ET than in controls. Platelet serotonin and ADP contents were reduced in ET. The percentages of circulating platelets expressing P-selectin and platelet-leukocyte hetero-aggregates were higher in ET.Conclusions. Analytical conditions dramatically affect in vitro PA of ET patients, which appears defective under the least physiological conditions and normal/supranormal under conditions that are closer to the physiological.

Abnormal proplatelet formation and emperipolesis in cultured human megakaryocytes from gray platelet syndrome patients.

The Gray Platelet Syndrome (GPS) is a rare inherited bleeding disorder characterized by deficiency of platelet α-granules, macrothrombocytopenia and marrow fibrosis. The autosomal recessive form of GPS is linked to loss of function mutations in NBEAL2, which is predicted to regulate granule trafficking in megakaryocytes, the platelet progenitors. We report the first analysis of cultured megakaryocytes from GPS patients with NBEAL2 mutations. Megakaryocytes cultured from peripheral blood or bone marrow hematopoietic progenitor cells from four patients were used to investigate megakaryopoiesis, megakaryocyte morphology and platelet formation. In vitro differentiation of megakaryocytes was normal, whereas we observed deficiency of megakaryocyte α-granule proteins and emperipolesis. Importantly, we first demonstrated that platelet formation by GPS megakaryocytes was severely affected, a defect which might be the major cause of thrombocytopenia in patients. These results demonstrate that cultured megakaryocytes from GPS patients provide a valuable model to understand the pathogenesis of GPS in humans.

Megakaryocytic emperipolesis and platelet function abnormalities in five patients with gray platelet syndrome.

The gray platelet syndrome (GPS) is a rare congenital platelet disorder characterized by mild to moderate bleeding diathesis, macrothrombocytopenia and lack of azurophilic α-granules in platelets. Some platelet and megakaryocyte (MK) abnormalities have been described, but confirmative studies of the defects in larger patient cohorts have not been undertaken. We studied platelet function and bone marrow (BM) features in five GPS patients with NBEAL2 autosomal recessive mutations from four unrelated families. In 3/3 patients, we observed a defect in platelet responses to protease-activated receptor (PAR)1-activating peptide as the most consistent finding, either isolated or combined to defective responses to other agonists. A reduction of PAR1 receptors with normal expression of major glycoproteins on the platelet surface was also found. Thrombin-induced fibrinogen binding to platelets was severely impaired in 2/2 patients. In 4/4 patients, the BM biopsy showed fibrosis (grade 2-3) and extensive emperipolesis, with many (36-65%) MKs containing 2-4 leukocytes engulfed within the cytoplasm. Reduced immunolabeling for platelet factor 4 together with normal immunolabeling for CD63 in MKs of two patients demonstrated that GPS MKs display an alpha granule-specific defect. Increased immunolabeling for P-selectin and decreased immunolabeling for PAR1, PAR4 and c-MPL were also observed in MKs of two patients. Marked emperipolesis, specific defect of MK alpha-granule content and defect of PAR1-mediated platelet responses are present in all GPS patients that we could study in detail. These results help to further characterize the disease.

Impact of gene polymorphisms, platelet reactivity, and the SYNTAX score on 1-year clinical outcomes in patients with non-ST-segment elevation acute coronary syndrome undergoing percutaneous coronary intervention: the GEPRESS study.

OBJECTIVES: The aim of this study was to investigate the association between high on-treatment platelet reactivity (HPR) and the SYNTAX (Synergy Between Percutaneous Coronary Intervention With Taxus and Cardiac Surgery) score (SS) for risk prediction of major adverse cardiovascular events (MACE) in patients with non-ST-segment elevation acute coronary syndrome (NSTEACS) undergoing percutaneous coronary intervention (PCI). BACKGROUND: Platelet function testing may be used to optimize antiplatelet therapy in high-risk patients, but identification of this category of patients remains challenging. METHODS: The GEPRESS (Gene Polymorphism, Platelet Reactivity, and the Syntax Score) study was a prospective, multicenter, observational study enrolling 1,053 patients with NSTEACS undergoing PCI and treated with clopidogrel. The platelet reactivity index (PRI) was measured at 3 time points: before PCI, at hospital discharge, and 1 month after PCI. Genetic variants of clopidogrel metabolism were determined in 750 patients. Patients were stratified by the presence of HPR (PRI >50%) and by tertile of the SS (upper SS tertile ≥15). The primary objective of this study was the risk of MACE in the period between 1 month and 1 year. RESULTS: Between 1 month and 1 year, 1-month HPR was an independent predictor of MACE in patients with an SS ≥15, but not in those with an SS <15, displaying a 5-fold increase in event rates (10.4% vs. 2.5%; p < 0.0001). CYP2C19*2 was the only single nucleotide polymorphism associated with HPR, but it was not associated with MACE. Although there was a significant variability in the PRI across the 1-month period, predischarge HPR and SS effectively stratified the risk of subsequent MACE up to 1-year follow-up. CONCLUSIONS: In clopidogrel-treated patients with NSTEACS undergoing PCI, HPR was independently associated with an increased risk of MACE only in the presence of a high SS.

Congenital defects of platelet function.

Congenital abnormalities of platelet function disorder (PFD) are associated with the heightened risk for bleeding. Typically, patients with PFDs have mucocutaneous bleeding of variable severity and excessive hemorrhage after surgery or trauma. The diagnostic laboratory assessment appropriate for the evaluation of suspected inherited PFD should be based on a two-step diagnostic strategy: the first step, based on screening tests, helps raising a diagnostic hypothesis, which should then be tested in the second step, which is based on the use of specific tests. The first step should include: complete blood cell count, examination of the peripheral blood smear, and assessment of platelet aggregation. Although light transmission aggregometry (LTA) is the most widely used platelet function test, it is relatively insensitive to defects of platelet secretion; for this reason, laboratory tests that measure platelet aggregation and secretion simultaneously, such as lumi-aggregometry, should be preferred to traditional LTA. The second step includes specific tests (e.g., flow cytometry, Western blotting, DNA analysis). Platelet transfusions should be used only to treat severe bleeding episodes. Recombinant Factor VIIa can be used in patients with severe bleeding episodes who do not respond to platelet transfusion because of alloimmunization. Fibrinolytic inhibitors or the vasopressin analogue desmopressin (DDAVP) should be used in all other circumstances.

Clearance of circulating activated platelets in polycythemia vera and essential thrombocythemia.

Essential thrombocythemia (ET) and polycythemia vera (PV) are characterized by persistent platelet activation. The mechanisms involved in their clearance are poorly characterized. In the present study, we report that leukocytes were actively involved in platelet disposal in 51 patients with ET and 30 with PV, but not in 70 age- and sex-matched controls. The fraction of circulating neutrophils and monocytes that had phagocytosed platelets, as assessed by flow cytometry, was significantly higher in patients with PV or ET, independently of hydroxyurea treatment, than in controls. Platelet phagocytosis by circulating leukocytes was confirmed by confocal and electron microscopy. The lack of effect of hydroxyurea, which disrupts the P-selectin/P-selectin glycoprotein ligand 1 (PSGL-1) interaction, suggests a P-selectin-independent mechanism. This hypothesis was confirmed in an ad hoc animal model based on the in vivo injection of activated platelets from P-selectin(+/+) and P-selectin(-/-) mice. P-selectin expression was associated with an earlier and effective clearance of platelets by neutrophils. A second delayed, P-selectin-independent phase actively involved monocytes. Our results suggest that phagocytic clearance of platelets by leukocytes occurs in PV and ET, possibly involving P-selectin-dependent and -independent pathways, thus representing a novel mechanism to remove activated platelets from the circulation.