Elevated Red Cell Microparticles (RMP) in Hematologic and Thrombotic Disorders

Introduction. Circulating cell-derived microparticlesa (MP) from platelets (PMP), leukocytes (LMP), and endothelia (EMP) have been well-documented for their roles in hemostasis, thrombosis and inflammation but the clinical significance of RMP is less well understood. The purpose of this study is to study the relation of elevated RMP to selected hematologic and thrombotic disorders.

Methods. This is a retrospective study on RMP profiles for patients referred to University of Miami Hospital and Clinics for hematologic consultation over the last 8 years. The patient population includes 51 hemolytic anemia (HA), 459 thrombocytopenia (TP), 26 myeloproliferative disorder (MPD), 413 hypercoagulable state (HCS), and 446 thrombotic disorders (TBS). Some patients were analyzed in more than one of the above disorders. Levels of RMP were measured by flow cytometry with PE-conjugated anti-CD235a labeling as previously described [Thromb. & Haemost., 2013;110:751-60]. Levels of RMP above mean +2SD of controls (> 2,000 counts/mL) are designated as "elevated RMP".

Results. (I) Prevalence of elevated RMP in patient populartion: Elevated RMP were found in 31 of 51 HA (60.8%), 138 of 459 TP (30.1%), 20 of 26 MPD (78.1%), 175 of 413 HCS (42.4%), and 251 of 446 TBS (56.3%). (II) Association of elevated RMP with thrombosis: Of 31 HA patients with elevated RMP, 11 were positive for TBS, and the remaining 20 were negative. Levels of RMP (mean ±SD) for TBS(+) and TBS(-) were 5,824 ±3713 and 3,265 ±1,048, counts /mL, respectively (p<0.01). Of 138 TP patients with elevated RMP, 31 were TBS(+) and 107 were TBS(-). Levels of RMP for TBS(+) and TBS(-) were 4,698 ±3,208 and 3,012 ±1,503, counts/mL, respectively (p <0.001). (III) RMP in HCS: Of 175 HCS with elevated RMP, 116 were TBS(+) and 59 were TBS(-). The levels of RMP for TBS(+) and TBS(-) were 4,062 ±3,285 and 2,987 ±1,454, counts/mL, respectively (p <0.02). Among these 116 TBS(+) patients, 22 did not receive antithrombotic treatment at the time of assay (HCS-1), 84 received treatment at time of assay (HCS-2), and 10 developed recurrent TBS despite therapy (HCS-3). Levels of RMP for HCS-1, HCS-2, and HCS-3 were 5,091 ±3,804, 3,973 ±3,044, and 6,191 ±3,763, respectively. Of these, HCS-2 differed significantly from HCS-3, p < 0.05. (IV) TBS with vs. without risk factors: Of 251 TBS with elevated RMP, 226 had known hematological or coagulation disorders, or risk factors for TBS (such as HA, TP, MPD, thrombocytosis, cancers, atrial fibrillation, APS, lupus, or markers for HCS). The remaining 25 without risk factors nevertheless had TBS and elevated RMP, suggesting RMP may be a useful biomarker for thrombotic risk.

Discussion. (i) The cause of RMP elevation in HA is reasonably attributed to red cell destruction. The cause in TP/ITP is likely due to products of platelet destruction or leukocyte activation. The cause in MPD could result from clearance of the high burden of red cells and platelets. (ii) Patients who were TBS(+) had higher RMP than TBS(-), seen in HCS, HA, TP, raising the question of whether high RMP is a cause or consequence of TBS. To answer this will require further study. (iii) The finding of exceptionally high RMP levels in recurrent TBS vs. non-recurrent TBS (HCS-3 vs. HC-2) indicates that RMP levels reflect severity of TBS. (iv) Finally, these data indicate that RMP may be a useful biomarker of thrombotic risk, particularly because some TBS patients had elevated RMP, yet tested negative by all conventional markers of HCS workups.