Profile of Cell-Derived Microparticles (C-MP) in ITP: Red Cell Microparticles (RMP) Correlate with Severity of ITP.

BACKGROUND: The roles of cell-derived microparticles (C-MP) released from platelets (PMP), endothelial cells (EMP), leukocytes (LMP) and red cells (RMP) in hemostasis, thrombosis and inflammation have been appreciated in recent years. Although PMP were shown to be hemostatically active, the roles of other C-MP, especially RMP, have not been studied in ITP. We investigated C-MP in patients with ITP.

MATERIAL AND METHODS: One-hundred-six patients with ITP were studied. They consisted of 73 pts with active ITP (ITP-A, 30M/43F, mean age 53.8 yr) and 33 pts in remission (ITP-R, 4M/29F, mean age 53.4 yr). ITP-A was defined by plt count <140,000 for >3 months; ITP-R was defined by plt count >140,000 for >3 months. Mean plt counts was 78,000 for ITP-A and 224,000 for ITP-R. The two groups were similar in ages. CBC with plt count, aPTT and activities of FVIII, FIX, FXI, were measured. Using flow cytometry, PMP were identified by CD41+, EMP by CD31+/CD41−, LMP by CD45+, and RMP by glycophorin+. Effects of intravenous immunoglobulins (IVIG) on C-MP were also evaluated.

RESULTS: There was a strong inverse correlation between plt counts and RMP (p=0.002). RMP were higher when plt counts were lower. A significant correlation was found between platelet counts and PMP (p<0.0001) and EMP (p<0.0001), but not LMP (p>0.05). The mean value of RMP was higher in ITP-A than ITP-R (p= 0.009). Conversely, PMP and EMP were higher in ITP-R (P<0.0001). No difference was found in LMP. FVIII and IX were higher in ITP-A than ITP-R (p=0.006, p=0.001, respectively).The aPTT was shorter and FXI higher in ITP-A but they did not reach statistical significance. Only RMP (not PMP, EMP or LMP) correlated with elevated factors VIII, IX and XI (P=0.005, 0.001 and 0.005 respectively) and inversely correlated with aPTT (p=0.009). Infusion of IVIG reduced RMP (RMP pre/post=1519/1018), not the other C-MP, in 9 pts

DISCUSSION/CONCLUSIONS: (i) RMP inversely correlated with plt counts and appear to be a sensitive marker of severity of ITP. However mechanisms of RMP generation, their reduction following IVIG and their roles remain to be elucidated. (ii) Among C-MP, only RMP, not other C-MP, were associated with shortening aPTT and elevated FVIII, IX, XI. (iii) RMP are positive for annexin V, providing anionic phospholipids for clotting factors and generating thrombin (data not shown). These findings suggest that RMP associated with these factors are involved in hemostasis to prevent bleeding in severe ITP.