Blood cell activation in myeloproliferative neoplasms

Endothelial Damage in JAK2V617F Myeloproliferative Neoplasms with Splanchnic Vein Thrombosis

BACKGROUND

 JAK2V617F-mutated myeloproliferative neoplasms (MPN) exhibit abnormal proliferation of bone marrow progenitors and increased risk of thrombosis, specifically in splanchnic veins (SVT). The contribution of the endothelium to the development of the prothrombotic phenotype was explored.

MATERIAL AND METHODS

 Plasma and serum samples from JAK2V617F MPN patients with (n=26) or without (n=7) thrombotic debut and different treatments, were obtained (n=33). Cultured endothelial cells (ECs) were exposed to serum samples from these patients and from healthy donors as controls. Changes in markers of inflammation (VCAM-1, ICAM-1), cell permeability (VE-cadherin), production of VWF, extracellular matrix (ECM) reactivity, and activation of intracellular signaling pathways related to stress, proliferation, inflammation (Akt, p44/42, IkBa), and JAK2/STAT3 pathway, were assessed by immunofluorescence, flow adhesion, SDS-PAGE and immunoblot. Additionally, circulating markers of endothelial activation and damage (VWF, sVCAM-1, sTNFRI, thrombomodulin, angiopoietin-2, a2-antiplasmin activity, PAI-1) were evaluated in Patients' plasma.

RESULTS

 The in vitro studies showed that EC exposure to MPN thrombotic patients' sera resulted in increased VCAM-1 and ICAM-1, and reduced VE-cadherin expression (p<0.05) at the cell surface. Production and release of VWF to the ECM were higher (p<0.05), with increased platelet adhesion after perfusing whole blood, being more noticeable in response to sera from non-treated patients. Furthermore, intracellular activation of Akt, p44/42, IkBa and JAK2/STAT3 was observed. Moreover, plasma levels of VWF, TNF-R1, VCAM-1, thrombomodulin, and angiopoietin-2 were higher in JAK2V617F+ MPN patients with thrombosis.

CONCLUSION

 The present findings suggest that circulating factors in MPNs with SVT debut induce endothelial proinflammatory and prothrombotic phenotypes, which are modulated in vitro with MPN treatment.

Genomic classification and outcomes of young patients with polycythemia vera and essential thrombocythemia according to the presence of splanchnic vein thrombosis and its chronology

To elucidate the role of splanchnic vein thrombosis (SVT) and genomic characteristics in prognosis and survival, we compared patients with polycythemia vera (PV) or essential thrombocythemia (ET) presenting SVT at diagnosis (n = 69, median age 43 years) or during follow-up (n = 21, median age 46 years) to a sex- and age-matched control group of PV/ET without SVT (n = 165, median age 48 years). The majority of patients presenting with SVT at diagnosis were classified as myeloproliferative neoplasm with heterozygous JAK2 mutation (87% of cases vs. 69% in PV/ET control group, p < 0.05), characterized by low JAK2 allele burden and no high-risk mutations. Despite this lower molecular complexity, patients presenting with SVT showed a higher risk of death (HR 3.0, 95% CI 1.5-6.0, p = 0.003) and lower event-free survival (HR 3.0, 95% CI 1.9-4.8, p < 0.001) than age- and sex-matched PV/ET controls. In patients presenting with SVT, molecular high-risk was associated with increased risk of venous re-thrombosis (HR 5.8, 95% CI 1.4-24.0, p = 0.01). Patients developing SVT during follow-up were more frequently allocated in molecular high-risk than those with SVT at diagnosis (52% versus 13%, p < 0.05). In the whole cohort of patients, molecular classification identified PV/ET patients at higher risk of disease progression whereas DNMT3A/TET2/ASXL1 mutations were associated with higher risk of arterial thrombosis. In conclusion, clinical and molecular characteristics are different in PV/ET patients with SVT, depending on whether it occurs at diagnosis or at follow-up. Molecular characterization by NGS is useful for assessing the risk of thrombosis and disease progression in young patients with PV/ET.

Thrombosis in patients with myeloproliferative neoplasms. Case report

Thrombotic complications are the most significant factors determining the prognosis in myeloproliferative neoplasms. Markers for assessing the risk of thrombosis are the number of leukocytes, platelets, hemoglobin level, hematocrit, age, molecular status, history of thrombosis, obesity, arterial hypertension, hyperlipidemia, hereditary or acquired thrombophilia. The pathogenesis of thrombosis in patients with myeloproliferative neoplasms is complex and multifactorial. In most cases, the etiological factor remains unknown. Currently, antiplatelet and anticoagulant therapy is carried out on an individual basis. The algorithm for primary and secondary (after thrombosis) prevention requires development and testing. We present a clinical case of repeated arterial and venous thrombotic complications in a patient with primary myelofibrosis.

Beyond cells: The extracellular circulating 20S proteasomes

Accumulating evidence arising from numerous clinical studies indicate that assembled and functional 20S proteasome complexes circulate freely in plasma. Elevated levels of this core proteolytic complex have been found in the plasma of patients suffering from blood, skin and solid cancers, autoimmune disorders, trauma and sepsis. Moreover, in various diseases, there is a positive correlation between circulating 20S proteasome (c20S) levels and treatment efficacy and survival rates, suggesting the involvement of this under-studied c20S complex in pathophysiology. However, many aspects of this system remain enigmatic, as we still do not know the origin, biological role or mechanisms of extracellular transport and regulation of c20S proteasomes. In this review, we provide an overview of the current understanding of the c20S proteasome system and discuss the remaining gaps in knowledge.

Mass Cytometry Phenotyping of Human Granulocytes Reveals Novel Basophil Functional Heterogeneity

Basophils, the rarest granulocyte, play critical roles in parasite- and allergen-induced inflammation. We applied mass cytometry (CyTOF) to simultaneously asses 44 proteins to phenotype and functionally characterize neutrophils, eosinophils, and basophils from 19 healthy donors. There was minimal heterogeneity seen in eosinophils and neutrophils, but data-driven analyses revealed four unique subpopulations within phenotypically basophilic granulocytes (PBG; CD45+HLA-DR-CD123+). Through CyTOF and fluorescence-activated cell sorting (FACS), we classified these four PBG subpopulations as (I) CD16lowFcεRIhighCD244high (88.5 ± 1.2%), (II) CD16highFcεRIhighCD244high (9.1 ± 0.4%), (III) CD16lowFcεRIlowCD244low (2.3 ± 1.3), and (IV) CD16highFcεRIlowCD244low (0.4 ± 0.1%). Prospective isolation confirmed basophilic-morphology of PBG I-III, but neutrophilic-morphology of PBG IV. Functional interrogation via IgE-crosslinking or IL-3 stimulation demonstrated that PBG I-II had significant increases in CD203c expression, whereas PBG III-IV remained unchanged compared with media-alone conditions. Thus, PBG III-IV could serve roles in non-IgE-mediated immunity. Our findings offer new perspectives in human basophil heterogeneity and the varying functional potential of these new subsets in health and disease.

Unexpected low expression of platelet fibrinogen receptor in patients with chronic myeloproliferative neoplasms: how does it change with aspirin?

This study was conducted to evaluate the expression of fibrinogen receptors on platelets of Philadelphia‐negative chronic myeloproliferative neoplasm (MPN) patients. We collected blood samples from 40 consecutive MPN patients and healthy volunteers. We performed flow cytometry analysis of P‐selectin expression and integrin beta‐3, activation of glycoprotein (GP) IIb/IIIa and fibrinogen receptor exposure (PAC‐1 binding). Surprisingly, we found a very low PAC‐1 binding capacity in MPN patients; however, the expression of PAC‐1 was almost completely recovered with aspirin intake. We hypothesize that the hypercoagulable states observed in MPN patients could depend on a primarily plasma‐driven impairment of fibrin turnover and thrombin generation.

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.

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.

JAK2-V617F promotes venous thrombosis through β1/β2 integrin activation

JAK2-V617F-positive chronic myeloproliferative neoplasia (CMN) commonly displays dysfunction of integrins and adhesion molecules expressed on platelets, erythrocytes, and leukocytes. However, the mechanism by which the 2 major leukocyte integrin chains, β1 and β2, may contribute to CMN pathophysiology remained unclear. β1 (α4β1; VLA-4) and β2 (αLβ2; LFA-1) integrins are essential regulators for attachment of leukocytes to endothelial cells. We here showed enhanced adhesion of granulocytes from mice with JAK2-V617F knockin (JAK2+/VF mice) to vascular cell adhesion molecule 1- (VCAM1-) and intercellular adhesion molecule 1-coated (ICAM1-coated) surfaces. Soluble VCAM1 and ICAM1 ligand binding assays revealed increased affinity of β1 and β2 integrins for their respective ligands. For β1 integrins, this correlated with a structural change from the low- to the high-affinity conformation induced by JAK2-V617F. JAK2-V617F triggered constitutive activation of the integrin inside-out signaling molecule Rap1, resulting in translocation toward the cell membrane. Employing a venous thrombosis model, we demonstrated that neutralizing anti-VLA-4 and anti-β2 integrin antibodies suppress pathologic thrombosis as observed in JAK2+/VF mice. In addition, aberrant homing of JAK2+/VF leukocytes to the spleen was inhibited by neutralizing anti-β2 antibodies and by pharmacologic inhibition of Rap1. Thus, our findings identified cross-talk between JAK2-V617F and integrin activation promoting pathologic thrombosis and abnormal trafficking of leukocytes to the spleen.

Phosphatidylserine-exposing blood and endothelial cells contribute to the hypercoagulable state in essential thrombocythemia patients

The mechanisms of thrombogenicity in essential thrombocythemia (ET) are complex and not well defined. Our objective was to explore whether phosphatidylserine (PS) exposure on blood cells and endothelial cells (ECs) can account for the increased thrombosis and distinct thrombotic risks among mutational subtypes in ET. Using flow cytometry and confocal microscopy, we found that the levels of PS-exposing erythrocytes, platelets, leukocytes, and serum-cultured ECs were significantly higher in each ET group [JAK2, CALR, and triple-negative (TN) (all P < 0.001)] than those in controls. Among ET patients, those with JAK2 mutations showed higher levels of PS-positive erythrocytes, platelets, neutrophils, and serum-cultured ECs than TN patients or those with CALR mutations, which show similar levels. Coagulation function assays showed that higher levels of PS-positive blood cells and serum-cultured ECs led to markedly shortened coagulation time and dramatically increased levels of FXa, thrombin, and fibrin production. This procoagulant activity could be largely blocked by addition of lactadherin (approx. 70% inhibition). Confocal microscopy showed that the FVa/FXa complex and fibrin fibrils colocalized with PS on ET serum-cultured ECs. Additionally, we found a relationship between D-dimer, prothrombin fragment F1 + 2, and PS exposure. Our study reveals a previously unrecognized link between hypercoagulability and exposed PS on cells, which might also be associated with distinct thrombotic risks among mutational subtypes in ET. Thus, blocking PS-binding sites may represent a new therapeutic target for preventing thrombosis in ET.

Crucial factors of the inflammatory microenvironment (IL-1β/TNF-α/TIMP-1) promote the maintenance of the malignant hemopoietic clone of myelofibrosis: an in vitro study

Along with molecular abnormalities (mutations in JAK2, Calreticulin (CALR) and MPL genes), chronic inflammation is the major hallmark of Myelofibrosis (MF). Here, we investigated the in vitro effects of crucial factors of the inflammatory microenvironment (Interleukin (IL)-1β, Tumor Necrosis Factor (TNF)-α, Tissue Inhibitor of Metalloproteinases (TIMP)-1 and ATP) on the functional behaviour of MF-derived circulating CD34⁺ cells.

We found that, regardless mutation status, IL-1β or TNF-α increases the survival of MF-derived CD34⁺ cells. In addition, along with stimulation of cell cycle progression to the S-phase, IL-1β or TNF-α ± TIMP-1 significantly stimulate(s) the in vitro clonogenic ability of CD34⁺ cells from JAK2^V617 mutated patients. Whereas in the JAK2^V617F mutated group, the addition of IL-1β or TNF-α + TIMP-1 decreased the erythroid compartment of the CALR mutated patients. Megakaryocyte progenitors were stimulated by IL-1β (JAK2^V617F mutated patients only) and inhibited by TNF-α. IL-1β + TNF-α + C-X-C motif chemokine 12 (CXCL12) ± TIMP-1 highly stimulates the in vitro migration of MF-derived CD34+ cells. Interestingly, after migration toward IL-1β + TNF-α + CXCL12 ± TIMP-1, CD34⁺ cells from JAK2^V617F mutated patients show increased clonogenic ability.

Here we demonstrate that the interplay of these inflammatory factors promotes and selects the circulating MF-derived CD34⁺ cells with higher proliferative activity, clonogenic potential and migration ability. Targeting these micro-environmental interactions may be a clinically relevant approach.

Risk of thrombosis according to need of phlebotomies in patients with polycythemia vera treated with hydroxyurea

Hematocrit control below 45% is associated with a lower rate of thrombosis in polycythemia vera. In patients receiving hydroxyurea, this target can be achieved with hydroxyurea alone or with the combination of hydroxyurea plus phlebotomies. However, the clinical implications of phlebotomy requirement under hydroxyurea therapy are unknown. The aim of this study was to evaluate the need for additional phlebotomies during the first five years of hydroxyurea therapy in 533 patients with polycythemia vera. Patients requiring 3 or more phlebotomies per year (n=85, 16%) showed a worse hematocrit control than those requiring 2 or less phlebotomies per year (n=448, 84%). There were no significant differences between the two study groups regarding leukocyte and platelet counts. Patients requiring 3 or more phlebotomies per year received significantly higher doses of hydroxyurea than the remaining patients. A significant higher rate of thrombosis was found in patients treated with hydroxyurea plus 3 or more phlebotomies per year compared to hydroxyurea with 0-2 phlebotomies per year (20.5% vs. 5.3% at 3 years; P<0.0001). In multivariate analysis, independent risk factors for thrombosis were phlebotomy dependency (HR: 3.3, 95%CI: 1.5-6.9; P=0.002) and thrombosis at diagnosis (HR: 4.7, 95%CI: 2.3-9.8; P<0.0001). The proportion of patients fulfilling the European LeukemiaNet criteria of resistance/intolerance to hydroxyurea was significantly higher in the group requiring 3 or more phlebotomies per year (18.7% vs. 7.1%; P=0.001) mainly due to extrahematologic toxicity. In conclusion, phlebotomy requirement under hydroxyurea therapy identifies a subset of patients with increased proliferation of polycythemia vera and higher risk of thrombosis.

Increasing procoagulant activity of circulating microparticles in patients with Philadelphia-negative myeloproliferative neoplasms: a single-centre experience

Microparticles are small membrane fragments with dimension between 0.1 and 1 μm, which are released during cell activation or apoptosis, exposing the phospholipid phosphatidylserine and membrane antigens typical for cellular origin. Philadelphia-negative myeloproliferative neoplasms (MPNs) are characterized by an increased risk of thrombosis. Data from literature suggest an association between thrombosis and the procoagulant activity of microparticles. Association between the procoagulant activity of microparticles and the incidence of thrombosis was assesed in a group of 126 patients with Philadelphia-negative MPNs. Measurement of microparticles procoagulant activity was performed using a functional assay, namely the Zymuphen MP-activity (Hyphen Biomed, Neuville-sur-oise, France). A total of 539 samples were analysed within this group of patients, regardless of patients' state of health. A significantly higher circulating microparticles procoagulant activity was found in MPN patients as compared with the control group (P < 0.001). A pathological level of procoagulant activity was observed more frequently in patients with polycythaemia vera (88%, P = 0.002) than groups of patients with essential thrombocythaemia (73.2%) and primary myelofibrosis (68.3%); the same result was confirmed in patients with a history of venous thrombosis in comparison with patients without thrombosis (84.7 vs. 73.2%, P = 0.029). Patients without cytoreductive treatment had a higher activity of microparticles (P = 0.010). Furthermore, presence of JAK2 V617F mutation was associated with an increased procoagulant activity (P = 0.007), as well as the higher JAK2 V617F allele burden (P = 0.001). Further prospective clinical studies will be necessary to evaluate the clinical relevance of microparticles in the prediction hypercoagulable state in these patients.

[Treatment of essential thrombocythemia]

Essential thrombocythemia is a chronic myeloproliferative neoplasm characterized by sustained thrombocytosis, bone marrow megakaryocytic hyperplasia and an increased risk of thrombosis and hemorrhage. The goal of treatment is to prevent the development of vascular complications without increasing the risk of transformation. Patients aged>60 years or a history of thrombosis have a high risk of thrombosis while those with a platelet count>1,500 x 10(9)/l have a higher risk of hemorrhage. Patients with low-risk essential thrombocythemia can be managed appropriately with low-dose of acetylsalicylic acid or even observation only, while patients with a high-risk disease are candidates to receive cytoreductive treatment, hydroxyurea being the first choice therapy. Anagrelide is the most suitable option for patients with resistance or intolerance to hydroxyurea. All patients must be submitted to a rigorous control of cardiovascular risk factors.

The JAK2 V617F mutation in patients with cerebral venous thrombosis

BACKGROUND

It is currently unclear whether or not cerebral venous thrombosis, such as splanchnic venous thrombosis, can be the first manifestation of an underlying myeloproliferative neoplasm.

OBJECTIVE

To determine the prevalence of the JAK2 V617F mutation in patients with a first episode of cerebral venous thrombosis.

PATIENTS

In this retrospective cohort study, patients with cerebral venous thrombosis were tested for the JAK2 V617F mutation and were followed until the development of a myeloproliferative neoplasm or censored at the end of follow-up.

RESULTS

Ten of 152 patients (6.6%) carried the JAK2 V617F mutation. Three of them had known acquired risk factors for thrombosis, and five had thrombophilia. Six patients met the diagnostic criteria for myeloproliferative neoplasm at the time of cerebral venous thrombosis, and three additional patients developed the disease during the follow-up (median duration 7.8 years, range 6 months to 21.3 years), giving an annual incidence of 0.26% patient-years (95% confidence interval 0.05-0.64). The last patient has no evidence of disease after 3 years of follow-up. Patients without the JAK2 V617F mutation at the time of cerebral venous thrombosis were retested at the end of the follow-up and remained negative, with normal blood counts (log-rank test χ(2) : 159 [P<0.0001]).

CONCLUSIONS

Cerebral venous thrombosis can be the first symptom of a myeloproliferative neoplasm. Patients with cerebral venous thrombosis can carry the JAK2 V617F mutation, irrespective of blood count.

Special issues in myeloproliferative neoplasms

Special issues in myeloproliferative neoplasms (MPN) comprise clinical conditions with high relevance for the duration and quality of the patient's life, but with limited evidence to support sound diagnostic and therapeutic recommendations and a low probability of being solved by the current standard of clinical research. These issues include MPN in pregnancy and in children, abdominal vein thrombosis, bleeding complications, surgery, pruritus, and leukemic transformation. Practical suggestions to guide clinical decisions in these settings remain largely empirical, but recently developed guidelines based on experts' consensus may help to tackle these problems. This article reviews the state of the art regarding these issues, with special emphasis on experts' consensus recommendations.

Management of Essential Thrombocythemia

Essential thrombocythemia (ET) is a Philadelphia chromosome (Ph)–negative myeloproliferative neoplasm (MPN) characterized by thrombocytosis and megakaryocytic hyperplasia of the bone marrow, with presence of the JAK2 V617F mutation in 50%-60% of patients. ET evolves to myelofibrosis in a minority of cases, whereas transformation to acute leukemia is rare and increases in association with the use of certain therapies. Survival of ET patients does not substantially differ from that of the general population. However, important morbidity is derived from vascular complications, including thrombosis, microvascular disturbances, and bleeding. Because of this, treatment of ET must be aimed at preventing thrombosis and bleeding without increasing the risk of transformation of the disease. Patients are considered at high risk of thrombosis if they are older than 60 years or have a previous history of thrombosis and at high risk of bleeding if platelet counts are > 1500 × 109/L. Patients with low-risk ET are usually managed with low-dose aspirin, whereas treatment of high-risk ET is based on the use of cytoreductive therapy, with hydroxyurea as the drug of choice and IFN-α being reserved for young patients or pregnant women. For patients resistant or intolerant to hydroxyurea, anagrelide is recommended as second-line therapy. Strict control of coexistent cardiovascular risk factors is mandatory for all patients. The role in ET therapy of new drugs such as pegylated IFN or the JAK2 inhibitors is currently under investigation.

Efficacy of NS-018, a potent and selective JAK2/Src inhibitor, in primary cells and mouse models of myeloproliferative neoplasms

Aberrant activation of Janus kinase 2 (JAK2) caused by somatic mutation of JAK2 (JAK2V617F) or the thrombopoietin receptor (MPLW515L) plays an essential role in the pathogenesis of myeloproliferative neoplasms (MPNs), suggesting that inhibition of aberrant JAK2 activation would have a therapeutic benefit. Our novel JAK2 inhibitor, NS-018, was highly active against JAK2 with a 50% inhibition (IC₅₀) of <1 n, and had 30–50-fold greater selectivity for JAK2 over other JAK-family kinases, such as JAK1, JAK3 and tyrosine kinase 2. In addition to JAK2, NS-018 inhibited Src-family kinases. NS-018 showed potent antiproliferative activity against cell lines expressing a constitutively activated JAK2 (the JAK2V617F or MPLW515L mutations or the TEL–JAK2 fusion gene; IC₅₀=11–120 n), but showed only minimal cytotoxicity against most other hematopoietic cell lines without a constitutively activated JAK2. Furthermore, NS-018 preferentially suppressed in vitro erythropoietin-independent endogenous colony formation from polycythemia vera patients. NS-018 also markedly reduced splenomegaly and prolonged the survival of mice inoculated with Ba/F3 cells harboring JAK2V617F. In addition, NS-018 significantly reduced leukocytosis, hepatosplenomegaly and extramedullary hematopoiesis, improved nutritional status, and prolonged survival in JAK2V617F transgenic mice. These results suggest that NS-018 will be a promising candidate for the treatment of MPNs.

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.

Splanchnic vein thrombosis, the onset manifestation in JAK positive Chronic Myeloproliferative Disorders Neoplasms

BACKGROUND

Patients with Myeloproliferative Neoplasms-(MPN) have a high risk of thrombotic complications. Portal vein thrombosis is a severe complication, which in many cases, appears at the onset of the disease; the risk factors are related to the presence of qualitatively altered thrombocytes and leucocytes, leading to their activation and appearance of leukocytes-platelet-aggregates; anomalies of portal vein endothelial cells are also implicated. The presence of JAK2V617F mutation increases the risk for splahnic thrombosis.

METHODS AND RESULTS

We present three patients with portal vein thrombosis and Budd Chiari syndrome, who were further diagnosed with MPN-the thrombosis was the onset event of the disease.

CONCLUSION

Patients were diagnosed with thrombosis of the portal vein before being diagnosed with MPN. Splenectomy was not associated with risk of thrombosis for the two cases in which it was performed; for one case, splenectomy was a therapeutic method to resolve portal hypertension. All patients had homozygous JAK2 mutation, which is associated in recent studies with increased risk of portal, mesenteric thrombosis. The high number of platelet was difficult to control for all patients. Bone marrow biopsy and determination of JAK status are valuable investigations for patients who have splenoportal thrombosis, with no apparent identifiable cause.