Elevated Circulating Platelet-Derived Microparticles in Ph1 Negative Chronic Myeloproliferative Disorders.
Authors: Tintillier-Colin Véronique , Cambier Nathalie , Bourgeois-Petit Emmanuelle , Rose Christian , Forzy Gérard , Demory Jean-Loup , Charpentier Agnès 
Published: October 2019

Abstract
Abstract 1906
Poster Board I-929
Microparticles (MP) are plasma membrane vesicles bearing potent procoagulant proteins and released into the circulation by various blood and endothelial cells during cellular activation and apoptosis. Their concentration has been shown markedly increased in vascular and thromboembolic diseases and in several types of cancer. Essential Thrombocythemia (ET), Polycythemia Vera (PV) and Primary Myelofibrosis (PM) are Bcr-Abl-negative chronic myeloproliferative disorders (MPD) with common biological and clinical features, especially thrombocytosis and more or less effective megakaryocytic proliferation. ET, PV and PMF are also characterized by an increased risk of thrombotic complications. Given the involvement of megacaryocytopoiesis in this group of diseases, we chose to focus on circulating platelet-derived microparticles (PMP) whose count can be established by flow cytometry. Our goals were to look for an increased release of PMP in MPD and if so, try to establish a correlation between thrombotic events recorded retrospectively and PMP level.

Patients and methods:
Plasma samples were collected from 85 patients with MPD (40 ET, 28 PV, 17 PMF) at any time of the disease course except blastic phase and 31 healthy age-paired controls ; the quantity of PMP (positive for the platelet marker CD41 and the microparticle marker AnnexinV) was measured by flow cytometric analysis using the FC500 cytometer from Beckmann-Coulter™. Pre-analytic and testing procedures complied with the recommendations of the ISTH Standardization Subcommittee and besides, our laboratory participates in a multicenter program to standardize the enumeration of cellular microparticles.

Results:
The number of PMP is significantly higher in ET, PV and MFP patients (median: 4862 MPP/mL, 3289 MPP/mL and 6114 MPP/mL respectively) than in controls (median 1310 MPP/mL) [p=0.000018, p=0.0013, p=0.0009]. Neither in MPD patients nor in control group PMP correlates with platelet count and appears thus as an independent parameter. ROC method allows to fix a cut-off at 3121 MPP/mL to discriminate MPD from controls. This value has a sensibility of 75 % and a specificity of 94%. Interestingly, the total number of PMP is not different between untreated patients (mean=7246 MPP/mL) and those who received myelosuppressive therapy (mean=6320 MPP/mL). Regarding thrombotic events, we failed to demonstrate any significant difference between patients with previous thrombosis and those without; however, the small size of the series and the retrospective assessment are possible bias.

Conclusion:
Patients with Phi negative MPDs have a high number of circulating PMP, irrelevant to platelet count or treatment by Hydroxy-urea or aspirin. A role for microparticles in the development of these diseases and in the occurrence of thrombotic complications can be hypothesized but requires further studies -;preferably prospective- on larger cohorts of patients.

Disclosures:
Charpentier: Schering-Plough: Research Funding.