Differential Expression of Markers of Apoptosis in Platelets From Children with Acute Versus Chronic Immune Thrombocytopenia

Abstract 1092

Immune thrombocytopenia (ITP) is a common hematologic disorder in children that can lead to severe bleeding symptoms. In most children with ITP, platelet counts return to normal after weeks to months (acute ITP), however, in about 10–20% of patients, the low platelet counts persist for 12 months or longer (chronic ITP). No biological markers have been identified to predict the duration and/or severity of ITP. We have previously reported enhanced platelet apoptosis at the time of diagnosis of ITP in pediatric patients that was ameliorated after intravenous immunoglobulin (IVIg) (Winkler et al, Br J Haematol 2012;156:508–15). We have now investigated differences in the expression of markers of apoptosis in platelets from children with acute vs. chronic ITP.

23 pediatric patients with acute ITP were investigated and compared to 10 children with chronic ITP. In addition, from the initial group of acute ITP, 6 children developed chronic ITP and initial- and follow up results were compared. Markers of apoptosis, including activated caspase-3, caspase-8 and caspase-9, phosphatidylserine (PS) exposure, dissipation of the mitochondrial inner membrane potential (ΔYm), as well as microparticle formation, were analyzed by flow cytometry. At ITP diagnosis, the mean platelet count was 4×10⁹/L (range: 1–14×10⁹/L) and the proportions of platelets with activated caspase-3 (median, range) (20.4%, 1.4–64%, n=23), caspase-8 (16.7%, 1.0 – 42.7%, n=12) and caspase-9 (13.1%, 5 – 59.6%, n=12) were increased. While a higher mean platelet count was found in 10 children with chronic ITP (25×10⁹/L, 4–60G/l), the proportions of platelets with activated caspase-3 (2.6%, 0.3–11.6%), caspase-8 (5.6%, 0.3–12.6%) and caspase-9 (4.3%, 0.3–15.6%) were significantly lower compared to children at diagnosis of acute ITP, but still higher compared to healthy controls (0.95%, 0 – 5.9%; 0.7%, 0.04 – 2.3% and 0.4%, 0.03 – 2.16%, respectively; n = 11) and children with thrombocytopenia due to chemotherapy (1.3%, 0.1 – 4.6%; 1.8%, 0.9 – 3.8%; and 1.8%, 0.6 – 2.9%, respectively; n = 11). Among the 6 children (26%) who developed chronic ITP from the initial cohort of 23 children, a mean platelet count of 29 (3–67×10⁹/L) at >12 months after initial presentation was found. Except for one, none of the children with chronic ITP presented with bleeding symptoms; the median bleeding score was 2.5 (range: 1–3) at diagnosis and 1 (range: 0–2.5) at follow up during chronic ITP. In 5 of the children who developed chronic ITP, caspase activation was studied at diagnosis and at follow up >12 months after. In all of them, the proportions of platelets with activated caspase-3 (1.6%, 0.3–3.3%), caspase-8 (4.8%, 0.3–6.3%) and caspase-9 (4.1%, 0.3–7%) were found to be significantly lower at follow up compared to the time at diagnosis.

In conclusion, although platelet apoptosis is enhanced at the time of diagnosis of pediatric ITP, this is not observed in platelets from patients with chronic ITP to the same degree. Further studies are needed to investigate other markers of apoptosis in platelets in the course of acute and chronic ITP.