Steady State Sickle Cell Anemia Is Associated with Increased Formation of Erythrocyte-Derived Microparticles and Acceleration of Thrombin Generation.

Patients with Sickle Cell Anemia (SCA) are at risk of thrombosis, but this clinical manifestation is variable and it is probably not associated with the frequency of vaso-occlusive crisis. Increased plasma levels of platelet- and erythrocyte-derived microparticles and hypercoagulability markers have been reported in steady state SCA patients. However, the link between hypercoagulability and SCA is not completelty understood.

We determined erythrocyte-derived (Ed-MP) and platelet-derived microparticles (Pd-MP) levels in patients with steady state SCA. We studied their relationship with hemolysis markers and their impact on thrombin generation process.

Consecutive out-patients with steady state SCA (n=78) and 20 healthy age and sex-matched controls were included. They were free of any acute episode of SCA for at least one month prior inclusion. Microparticles were assessed with standardized whole blood flow cytometry assay. Ed-MP and Pd-MP were identified using respectively anti-CD235a and anti-CD41 monoclonal antibody and annexine V. Thrombin generation (TG) in citrated platelet poor plasma was assessed with Calibrated Automated Thrombogram^® (Stago, France) using PPP-reagent 5pM^® (Thrombinoscope BV, Nederlands). The following TG parameters were analyzed: lag-time (LT), time to peak of thrombin (ttpeak), peak of thrombin (peak), mean velocity rate index of the TG (MRI) and endogenous thrombin potential (ETP).

Mean patient age was 25±8 (range 17-58 ys). In the patients group, Ed-MP and Pd-MP, expressing or not phosphatidyl-serine (PS), were significantly increased compared to the control group. Thrombogram parameters were not significantly different in both groups (Table 1). There was a slight though significant inverse correlation between Ed-MP and both LT and ttpeak (r=-0.235, r=-0.315 respectively; p<0.05). Ed-MP levels were correlated with MRI increase (r=0.241; p<0.05). Ed-MP values were inversely correlated with Hb levels and well correlated with reticulocytes count (r=-0.427, r=0.520 respectively; p<0.05). No relationship was found between Ed-MP and ETP values. The sub-population of Ed-MP/PS⁺ (expressing PS) showed also an inverse correlation with both ttPeak (r=-0.315, p<0.05) and ETP (r=-0.236; p<0.05), and a positive correlation with MRI (r=0.306 ; p<0.05). Pd-MP concentration was inversely correlated with Hb levels (r=-0.273 ; p<0.05). Only Pd-MP/PS⁺ plasma concentration was slightly by significantly correlated with ETP (r=0.225; p=0,049).

Patients with steady state SCA presented a significant increase of Ed-MP and Pd-MP plasma levels which seems to be linked to haemolysis degree. Each type of microparticles had different impact on TG process. Ed-MP induced acceleration of TG kinetics without increase of ETP whereas Pd-MP/PS⁺ affected mainly ETP. However the increase of Ed-MP and Pd-MP plasma concentration does not appear to be by itself a sufficient condition to induce a significant increase of TG.

No relevant conflicts of interest to declare.