Markers of endothelial and in vivo platelet activation in patients with essential thrombocythemia and polycythemia vera

Potential biomarkers and immune characteristics for polycythemia vera-related atherosclerosis using bulk RNA and single-cell RNA datasets: a combined comprehensive bioinformatics and machine learning analysis

Background

Polycythemia vera (PV) is a myeloproliferative disease characterized by significantly higher hemoglobin levels and positivity for JAK2 mutation. Thrombosis is the main risk event of this disease. Atherosclerosis (AS) can markedly increase the risk of arterial thrombosis in patients with PV. The objectives of our study were to identify potential biomarkers for PV-related AS and to explore the molecular biological association between PV and AS.

Methods

We extracted microarray datasets from the Gene Expression Omnibus (GEO) dataset for PV and AS. Common differentially expressed genes (CGs) were identified by differential expression analysis. Functional enrichment and protein-protein interaction (PPI) networks were constructed from the CG by random forest models using LASSO regression to identify pathogenic genes and their underlying processes in PV-related AS. The expression of potential biomarkers was validated using an external dataset. A diagnostic nomogram was constructed based on potential biomarkers to predict PV-related AS, and its diagnostic performance was assessed using ROC, calibration, and decision curve analyses. Subsequently, we used single-cell gene set enrichment analysis (GSEA) to analyze the immune signaling pathways associated with potential biomarkers. We also performed immune infiltration analysis of AS with "CIBERSORT" and calculated Pearson's correlation coefficients for potential biomarkers and infiltrating immune cells. Finally, we observed the expression of potential biomarkers in immune cells based on the single-cell RNA dataset.

Results

Fifty-two CGs were identified based on the intersection between up-regulated and down-regulated genes in PV and AS. Most biological processes associated with CGs were cytokines and factors associated with chemotaxis of immune cells. The PPI analysis identified ten hub genes, and of these, CCR1 and MMP9 were selected as potential biomarkers with which to construct a diagnostic model using machine learning methods and external dataset validation. These biomarkers could regulate Toll-like signaling, NOD-like signaling, and chemokine signaling pathways associated with AS. Finally, we determined that these potential biomarkers had a strong correlation with macrophage M0 infiltration. Further, the potential biomarkers were highly expressed in macrophages from patients with AS.

Conclusion

We identified two CGs (CCR1 and MMP9) as potential biomarkers for PV-related AS and established a diagnostic model based on them. These results may provide insight for future experimental studies for the diagnosis and treatment of PV-related AS.

Practical management of the haemorrhagic complications of myeloproliferative neoplasms

Myeloproliferative neoplasms can be associated with bleeding manifestations which can cause significant morbidities. Although haematologists are aware of the likelihood of this complication in the setting of myeloproliferative neoplasms, it may often be overlooked especially in patients with no extreme elevation of blood counts and those with myelofibrosis. Acquired von Willebrand syndrome and platelet dysfunction are the two common diagnoses to be considered in this regard. In this review article, we discuss the mechanisms for the development of these rare bleeding disorders, their diagnosis and practical management.

Clinical and Radiological Profile of Cerebrovascular Disease in Polycythemia: Analysis of Neurologic Manifestations from a Tertiary Center in South India

PURPOSE

Patients with polycythemia, either primary or secondary, are at elevated risk for thrombotic complications, including stroke. We aimed to investigate the clinical and radiological characteristics of cerebrovascular disease (CVD) in polycythemia, and describe other neurologic manifestations.

METHODOLOGY

We conducted a cross-sectional study of patients diagnosed with polycythemia between 2014 and 2019 at a tertiary care center and collected relevant medical data with a special focus on cerebrovascular disease and neurologic manifestations. We performed descriptive and inferential analyses. We have also described and analyzed the available neuroimaging features.

RESULTS

We analyzed data from 56 patients. 20 patients (35.7%) had ischemic stroke. The incidence of CVD was higher in those with primary polycythemia (43%) than in those with secondary polycythemia (8%). The most common subtype of stroke was large vessel disease, and the most common arterial territory was the anterior circulation. There was no statistically significant difference in the hematocrit level between those with or without CVD. Neuroimaging revealed multiple large vessel intracranial stenoses on MR Angiography, and hyperdense vessels on plain CT. Other neurologic manifestations included headache, seizures, dizziness, visual symptoms and papilledema, and these were significantly more common in primary polycythemia.

CONCLUSIONS

CVD is common in patients with polycythemia. The most common type observed was large vessel occlusion, predominantly in the anterior circulation. In stroke patients, multiple vessel stenosis and hyperdense vessels may be clues to polycythemia.

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.

Pathophysiology 1. Mechanisms of Thrombosis in Cancer Patients

Thrombosis is a major cause of morbidity and mortality in cancer patients. The pathogenesis of blood coagulation activation in oncological patients is complex and involves both clinical and biological factors. Abnormalities in one or more coagulation test are common in cancer patients, even without thrombotic manifestations, indicating an ongoing hypercoagulable condition. Moreover, venous thromboembolism (VTE) can be the first symptom of an occult malignancy in an otherwise healthy individual. The levels of laboratory markers of activation of blood coagulation parallel the development of malignancy, being the coagulant mechanisms important for both thrombogenesis and tumor progression. Besides general clinical risk factors for VTE, also disease-specific clinical factors, i.e., type and stage of the tumor, and anticancer therapies increase the thrombotic risk in these patients. Furthermore, biological factors, including the cancer cell-specific prothrombotic properties together with the host cell inflammatory response to the tumor, are relevant as well as unique players in the pathogenesis of the cancer-associated hypercoagulability. Cancer cells produce and release procoagulant and fibrinolytic proteins, inflammatory cytokines, and procoagulant microparticles. They also express adhesion molecules binding to the receptors of host vascular cells (i.e., endothelial cells, platelets, and leukocytes), thereby stimulating the prothrombotic properties of these normal cells, including the shed of cell-specific microparticles and neutrophil extracellular traps. Of interest, several genes responsible for the cellular neoplastic transformation drive the programs of hemostatic properties expressed by cancer tissues. A better understanding of such mechanisms will help the development of novel strategies to prevent and treat the Trousseau's syndrome (i.e., cancer-associated thrombosis).

Expression of CD markers in JAK2V617F positive myeloproliferative neoplasms: Prognostic significance

Myeloproliferative neoplasms (MPNs) are clonal stem cell disorders characterized by the presence of JAK2^V617F mutation. Thrombohemorrhagic as well as autoimmune or inflammatory phenomena are common clinical outcomes of these disorders. Recent studies have shown that abnormality in frequency and function of blood cells manifested by an alteration in CD markers' expression patterns play a key role in these complications. So, there may be a relationship between CD markers' expressions and prognosis of JAK2^V617F positive MPNs. Therefore, in this review, we have focused on these abnormalities from the perspective of changing expressions of CD markers and assessment of the relationship between these changes with prognosis of JAK2^V617F positive MPNs. It can be stated that the abnormal expression of a large number of CD markers can be used as a prognostic biomarker for clinical outcomes including thrombohememorrhagic events, as well as autoimmune and leukemic transformation in JAK2^V617F positive MPNs. Considering the possible role of CD markers' expressions in JAK2^V617F MPNs prognosis, further studies are needed to confirm the relationship between the expression of CD markers with prognosis to be able to find an appropriate therapeutic approach via targeting CD markers.

Advances and challenges in the management of essential thrombocythemia

The new World Health Organization (WHO) diagnostic criteria for essential thrombocythemia (ET) issued in 2008 made an important distinction between true ET and early myelofibrosis (MF), which has helped to identify a more homogenous population for the diagnosis with longer survival and much less transformation to overt MF. The recent finding of a new mutation (CALR), which is mutually exclusive with JAK2 and MPL mutations, adds to the characterization of ET patients, since there are important phenotypic differences between the mutation types. CALR patients are younger, have lower white blood cell counts (WBC) and a lower thrombosis incidence. A growing field of interest is the state of hypercoagulation due to dysfunction of hemostatic systems, cell-cell interaction and hereditary prothrombotic traits. Activation of platelets, WBC and endothelial cells has been found, making the whole intravascular milieu prothrombotic. Several risk score models, based on retrospective studies, have been developed lately, distinguishing patient groups with graded risk for complications and death. Even if these may be helpful in evaluating patients, they have not been validated in prospective studies and there are not enough data to support their use as a basis for treatment algorithms. The traditional risk factors age, previous thrombosis and platelets >1500 × 10(9)/l are still recommended for the distinction between high risk and low risk ET and the decision to give cytoreductive therapy. However, cardiovascular (CV) risk factors add to thrombosis risk and should be considered both for specific treatment in any risk group and for upgrading low risk patients with high CV risk to an intermediary group where active therapy with aspirin and cytoreduction may be considered. First-line cytoreductive therapy differs with age; in younger patients interferon (IFN) or anagrelide are preferable, in older patients hydroxycarbamide (HC). Second-line therapy for younger patients is HC, for older patients IFN or anagrelide (ANA). JAK2 inhibitors may be suitable in rare cases with symptoms not responding to other therapy.

Cerebral thrombosis and myeloproliferative neoplasms

Myeloproliferative neoplasms (MPN) are acquired clonal disorders characterized by the proliferation of bone marrow myeloid cells. Different somatic mutations have been recently associated with MPN, the most common being JAK-2 V617F. Among MPN, polycythemia vera and essential thrombocythemia are particularly associated with an increased risk to develop thrombotic complications, either arterial or venous. Cerebrovascular events (stroke and transient ischemic attacks) are prevalent, accounting for approximately two-thirds of all events. Also cerebral vein thrombosis can complicate MPN and can be the first manifestation of the disease. Risk factors for thrombosis in patients with MPN are related or unrelated to the disease. Among the former there are cellular risk factors, such as increased white blood cell counts, vascular cell activation, endothelial dysfunction, and plasmatic risk factors, such as increased plasma viscosity, reduced levels of protein S, increased thrombin generation. The latter include increased age and previous thrombotic events. In addition, common cardiovascular risk factors (smoking, hypertension, diabetes, dyslipidemia, obesity) contribute to the pathogenesis of arterial events, whereas circumstantial risk factors (particularly oral contraceptive use and pregnancy/puerperium) to that of venous events. Primary prevention of arterial thrombosis with antiplatelet therapy is warranted in the majority of patients with MPN, whereas primary prevention of venous thrombosis is limited to anticoagulant prophylaxis during high-risk situations. Secondary prevention includes long-term antiplatelet therapy for arterial and short- or long-term anticoagulant therapy for venous thrombosis, depending on the risk factors present at the first event.

Stalled cerebral capillary blood flow in mouse models of essential thrombocythemia and polycythemia vera revealed by in vivo two-photon imaging

BACKGROUND

Essential thrombocythemia (ET) and polycythemia vera (PV) are myeloproliferative neoplasms (MPNs) that share the JAK2(V617F) mutation in hematopoietic stem cells, leading to excessive production of predominantly platelets in ET, and predominantly red blood cells (RBCs) in PV. The major cause of morbidity and mortality in PV and ET is thrombosis, including cerebrovascular occlusive disease.

OBJECTIVES

To identify the effect of excessive blood cells on cerebral microcirculation in ET and PV.

METHODS

We used two-photon excited fluorescence microscopy to examine cerebral blood flow in transgenic mouse models that mimic MPNs.

RESULTS AND CONCLUSIONS

We found that flow was 'stalled' in an elevated fraction of brain capillaries in ET (18%), PV (27%), mixed MPN (14%) and secondary (non-MPN) erythrocytosis (27%) mice, as compared with controls (3%). The fraction of capillaries with stalled flow increased when the hematocrit value exceeded 55% in PV mice, and the majority of stalled vessels contained only stationary RBCs. In contrast, the majority of stalls in ET mice were caused by platelet aggregates. Stalls had a median persistence time of 0.5 and 1 h in ET and PV mice, respectively. Our findings shed new light on potential mechanisms of neurological problems in patients with MPNs.

Prophylaxis and management of venous thromboembolism in patients with myeloproliferative neoplasms: consensus statement of the Haemostasis Working Party of the German Society of Hematology and Oncology (DGHO), the Austrian Society of Hematology and Oncology (ÖGHO) and Society of Thrombosis and Haemostasis Research (GTH e.V.)

Patients with Philadelphia chromosome-negative myeloproliferative neoplasms (MPN) like polycythemia vera and essential thrombocythemia are at increased risk of arterial and venous thrombosis. Strategies of prevention may consist of platelet aggregation inhibitors and/or cytoreductive agents depending on the underlying disease and the individual risk. Clinical evidence for management of acute venous thromboembolic events in MPN patients is limited. Modality and duration of therapeutic anticoagulation after venous thrombosis has to be evaluated critically with special regard to the increased risk for spontaneous bleeding events associated with the underlying diseases. Both for therapy of the acute event and for secondary prophylaxis, low-molecular-weight heparins should preferentially be used. A prolongation of the therapeutic anticoagulation beyond the usual 3 to 6 months can only be recommended in high-risk settings and after careful evaluation of potential risks and benefits for the individual patient. New direct oral anticoagulants (NOAC) should not preferentially be used due to lack of clinical experience in patients with MPN and potential drug interactions (e.g. with JAK inhibitors). Consequent treatment of the underlying myeloproliferative disease and periodical evaluation of the response to therapy is crucial for optimal secondary prophylaxis of thromboembolic events in those patients.

JAK2V617F-positive endothelial cells contribute to clotting abnormalities in myeloproliferative neoplasms

The Janus kinase 2 (JAK2) V617F mutation is the primary pathogenic mutation in patients with Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs). Although thrombohemorrhagic incidents are the most common causes of morbidity and mortality in patients with MPNs, the events causing these clotting abnormalities remain unclear. To identify the cells responsible for the dysfunctional hemostasis, we used transgenic mice expressing JAK2V617F in specific lineages involved in thrombosis and hemostasis. When JAK2V617F was expressed in both hematopoietic and endothelial cells (ECs), the mice developed a significant MPN, characterized by thrombocytosis, neutrophilia, and splenomegaly. However, despite having significantly higher platelet counts than controls, these mice showed severely attenuated thrombosis following injury. Interestingly, platelet activation and aggregation in response to agonists was unaltered by JAK2V617F expression. Subsequent bone marrow transplants revealed the contribution of both endothelial and hematopoietic compartments to the attenuated thrombosis. Furthermore, we identified a potential mechanism for this phenotype through JAK2V617F-regulated inhibition of von Willebrand factor (VWF) function and/or secretion. JAK2V617F(+) mice display a condition similar to acquired von Willebrand syndrome, exhibiting significantly less high molecular weight VWF and reduced agglutination to ristocetin. These findings greatly advance our understanding of thrombohemorrhagic events in MPNs and highlight the critical role of ECs in the pathology of hematopoietic malignancies.

Abnormal P-selectin localization during megakaryocyte development determines thrombosis in the gata1low model of myelofibrosis

Patients with primary myelofibrosis have increased risk for bleeding and thrombosis. It is debated whether propensity to thrombosis is due to increased numbers of platelet microparticles and/or to pathological platelet-neutrophil interactions. Platelet neutrophil interactions are mediated by P-selectin and even though the megakaryocytes of myelofibrosis patients express normal levels of P-selectin, it remains abnormally localized to the demarcation membrane system rather than being assembled into the α-granules in platelets. Mice carrying the hypomorphic Gata1(low) mutation express the same megakaryocyte abnormalities presented by primary myelofibrosis patients, including abnormal P-selectin localization to the DMS and develop with age myelofibrosis, a disease that closely resembles human primary myelofibrosis. Whether these mice would also develop thrombosis has not been investigated as yet. The aim of this study was to determine whether Gata1(low) mice would develop thrombosis with age and, in this case, the role played by P-selectin in the development of the trait. To this aim, Gata1(low) mice were crossed with P-sel(null) mice according to standard genetic protocols and Gata1(low)P-sel(wt), Gata1(low)P-sel(null) and Gata1(WT)P-sel(null) or Gata1(wt)P-sel(wt) (as controls) littermates obtained. It was shown that platelet counts, but not hematocrit, are reduced in Gata1(low) mice. Moreover, platelet microparticles are reduced in Gata1(low) mice and P-selectin positive platelet microparticles were not found. To determine the phenotypic implications of the different mutations, bleeding time was estimated by a tail cut procedure. Mutant mice were sacrificed and presence of thrombosis was determined by immunohistological staining of organs. Gata1(low) mice with or without the P-selectin null trait had a prolonged bleeding time compared to wild type mice. However, in Gata1(low) mice significantly higher frequency of thrombotic events was seen in adult and old Gata1(low) mice compared to Gata1(low)P-sel(null) mice. Thus, presence of the P-selectin null trait rescued Gata1(low) mice from the thrombotic phenotype, but did not change the level of platelet microparticles. Taken together these data indicate that abnormal localization of P-selectin, induced by the Gata1(low) mutation, and thus, increased pathological interactions with leucocytes, is responsible for the increased presence of thrombosis seen in these mice.

Aspirin responsive platelet thrombophilia in essential thrombocythemia and polycythemia vera

Essential thrombocythemia (ET) and polycythemia vera (PV) frequently present with erythromelalgia and acrocyanotic complications, migraine-like microvascular cerebral and ocular transient ischemic attacks (MIAs) and/or acute coronary disease. The spectrum of MIAs in ET range from poorly localized symptoms of transient unsteadiness, dysarthria and scintillating scotoma to focal symptoms of transient monocular blindness, transient mono- or hemiparesis or both. The attacks all have a sudden onset, occur sequentially rather than simultaneously, last for a few seconds to several minutes and are usually associated with a dull, pulsatile or migraine-like headache. Increased hematocrit and blood viscosity in PV patients aggravate the microvascular ischemic syndrome of thrombocythemia to major arterial and venous thrombotic complications. Phlebotomy to correct hematocrit to normal in PV significantly reduces major arterial and venous thrombotic complications, but fails to prevent the platelet-mediated erythromelalgia and MIAs. Complete long-term relief of the erythromelalgic microvascular disturbances, MIAs and major thrombosis in ET and PV patients can be obtained with low dose aspirin and platelet reduction to normal, but not with anticoagulation. Skin punch biopsies from the erythromelalgic area show fibromuscular intimal proliferation of arterioles complicated by occlusive platelet-rich thrombi leading to acrocyanotic ischemia. Symptomatic ET patients with erythromelalgic microvascular disturbances have shortened platelet survival, increased platelet activation markers β-thromboglobulin (β-TG), platelet factor 4 (PF4) and thrombomoduline (TM), increased urinary thromboxane B2 (TXB2) excretion, and no activation of the coagulation markers thrombin fragments F1+2 and fibrin degradation products. Inhibition of platelet cyclooxygenase (COX1) by aspirin is followed by the disappearance and no recurrence of microvascular disturbances, increase in platelet number, correction of the shortened platelet survival times to normal, and reduction of increased plasma levels of β-TG, PF4, TM and urinary TXB2 excretion to normal. These results indicate that platelet-mediated fibromuscular intimal proliferation and platelet-rich thrombi in the peripheral, cerebral and coronary end-arterial microvasculature are responsible for the erythromelalgic ischemic complications, MIAs and splanchnic vein thrombosis. Baseline platelet P-selectin levels and arachidonic acid induced COX1 mediated platelet activation showed a highly significant increase of platelet P-selectin expression (not seen in ADP and collagen stimulated platelets), which was significantly higher in JAK2^V617F mutated compared to JAK2 wild type ET.

Thrombotic disease in the myeloproliferative neoplasms

Thrombosis is a leading cause of morbidity and mortality in patients with Philadelphia chromosome–negative myeloproliferative neoplasms (MPNs), particularly polycythemia vera and essential thrombocythemia. Mechanisms involved in the pathogenesis of the acquired thrombophilic state associated with these diseases include abnormalities of MPN clone–derived blood cells, which display prothrombotic features, and abnormalities of normal vascular cells, which become procoagulant in response to inflammatory stimuli. Ultimately, the release into the blood of elevated levels of procoagulant microparticles by platelets and vascular cells and the increase in the global thrombin generation due to an acquired activated protein C resistance result in a highly prothrombotic scenario in patients with polycythemia vera and essential thrombocythemia. The acquired point mutation in the pseudokinase domain of JAK2 (JAK2V617F) in these disorders is variably associated with thrombosis and, more consistently, with elevations in WBC counts and alterations in biomarkers of blood-clotting abnormalities. The predictive value of these biomarkers for thrombosis remains to be established to identify subsets of patients at elevated risk who may benefit from prophylaxis with antithrombotic drugs.

Direct evidence for normalization of platelet function resulting from platelet count reduction in essential thrombocythemia

Essential thrombocythemia is characterized by persistent elevation and functional disturbances of platelets. Both the platelet function analyzer-100 (PFA-100) collagen-epinephrine (CEPI) cartridge and aggregometry with epinephrine are considered sensitive and valid methods in detecting abnormal platelet function in essential thrombocythemia. We attempted to confirm that restoration of abnormal platelet function results from platelet count reduction in essential thrombocythemia, by using these two methods. Thirty-nine essential thrombocythemia patients were divided into two groups on the basis of their platelet count. Group A participants (n = 20) exhibited platelet counts greater than 500 × 10/l, whereas group B participants (n = 19) had platelet counts below this limit. Hematological parameters, plasma von Willebrand factor (vWF) antigen and activity levels were assessed. Platelet function was analyzed by the PFA-100 and light transmission aggregometry with epinephrine, collagen, and ADP. The point mutation JAK2 V617F was identified and its effect on platelet function tests was also investigated. By using logistic regression analysis, white blood cell count, vWF activity level, and the measurements of aggregation in response to epinephrine were significantly different between the two groups. Epinephrine-induced aggregation retained the statistical significance in the multivariable procedure (P : 0.002). PFA-100 CEPI closure times were lower - but not statistically significant - in group B. Neither the JAK2 V617F positivity nor different cytoreductive treatments had any influence on ex-vivo platelet function tests. Our findings demonstrate normalization of platelet function resulting from platelet count reduction in essential thrombocythemia and reinforce the concept of lowering platelet counts in these patients.

Chronic cyanosis and vascular function: implications for patients with cyanotic congenital heart disease

In patients with cyanotic congenital heart disease, chronic hypoxaemia leads to important changes in blood vessel function and structure. Some of these alterations are maladaptive and probably contribute to impaired cardiopulmonary performance and an increased incidence of thrombotic and embolic events. Recent evidence suggests that deranged endothelial function, a sequel of chronic cyanosis, could be an important factor in the pathogenesis of cyanosis-associated cardiovascular risk. In this article, we discuss the physiological and mechanical consequences of compensatory erythrocytosis and possible pathophysiological mechanisms of vascular dysfunction in chronic cyanosis.

Inhibition of tissue factor expression by hydroxyurea in polymorphonuclear leukocytes from patients with myeloproliferative disorders: a new effect for an old drug?

BACKGROUND

Polymorphonuclear leukocytes (PMN) from healthy subjects can produce and store tissue factor (TF), which is expressed on PMN surface upon in vitro stimulation with P-selectin.

RESULTS

We report here that platelets and PMN from 12 patients with myeloproliferative disorders (MPD) (six with polycythemia vera, six with essential thrombocythemia) show up regulation of P-selectin and TF, respectively, in the absence of any in vitro challenge. The number of circulating mixed platelet-PMN aggregates was also increased. PMN TF expression as well as mixed platelet-PMN aggregates, but not platelet P-selectin, were significantly reduced in six MPD patients after treatment with hydroxyurea (HU). In vitro studies performed on PMN separated from healthy donors confirmed HU effects (0-1400 microm). HU prevented both P-selectin-induced TF expression and mixed cell aggregate formation. The inhibitory effect of HU was specific for P-selectin-induced PMN activation, as it did not affect formyl-methionyl-leucyl-phenylalanine-induced PMN TF expression.

CONCLUSIONS

In MPD patients, platelet P-selectin-mediated TF expression on circulating PMN may play a role in thrombus formation and represents a novel target for the antithrombotic activity of HU.