Platelet Apoptosis in Adult Immune Thrombocytopenia. Relationship with Auto-Antibodies, Platelet Function and Treatment

Similarly to nucleated cells, platelet life span is also controlled by an intrinsic apoptotic program that triggers collapse of the mitochondrial inner membrane potential, activation of caspases-3, -8 and -9, phosphatidylserine (PS) externalization and microparticle shedding. The aim of the present study was to investigate platelet apoptosis in adult patients with immune thrombocytopenia (ITP) under different treatment conditions and to search for its relationship with the type of auto-antibody and the platelet-activation status.

Twenty-four patients with chronic ITP, age 42 (21-80) years (median and range) diagnosed according to current criteria (Rodeghiero et al, 2009) were included after written informed consent in accordance with the Declaration of Helsinki. The study was approved by the Ethics Committee from Instituto de Investigaciones Medicas Alfredo Lanari. Platelet count was 38x10⁹/L (6-85). Platelet apoptosis was evaluated by phosphatidylserine (PS) exposure on the platelet surface using FITC-conjugated Anexin-V, mitochondrial electrochemical potential changes (ΔΨm) using the cell penetrating lipophilic cationic fluorochrome JC-1, and activated caspase-3 (a-casp3) measured by the cell-penetrating carboxyfluorescein-labelled fluoromethyl ketone tetrapeptide (FAM-DEVD-FMK). These parameters were studied in resting platelets and after stimulation with calcium ionophore (A23187). Platelet activation was evaluated by FITC-PAC-1 binding to activated GPIIb-IIIa and GPIb-IX internalization using PE-CD42b, in resting conditions and after stimulation either with ADP or TRAP. Apoptosis and activation parameters were evaluated by flow cytometry.

In resting conditions, platelets from ITP patients showed increased PS expression and a- casp3 and abnormal ΔΨm (table 1).

After stimulation with A23187, ITP platelets had similar levels of PS expression (p=0.305, n=20) and ΔmΨ (p=0.383, n=25) compared to normal controls. However, an increased sensitivity to the apoptotic stimulus was evidenced by elevated levels of a-casp3 at low and high A23187 concentrations (1-3 mM, p=0.097; 6-10 mM, p=0.002). Platelet apoptosis was not related to platelet activation, as PAC-1 binding was not increased in ITP platelets (basal p=0.847, ADP-induced p=0.059, TRAP-induced p=0.103, n=16). Besides, internalization of GPIb-IX after ADP and TRAP stimulation was also normal (p=NS, n=9 for both agonists). Platelets from ITP patients bearing two of the most frequently found auto-antibodies (5 with anti-GPIIb-IIIa and 1 with anti-GPIb-IX) had similar levels of PS expression and ΔmΨ at resting conditions than those who were negative for these auto-antibodies (n=14) (p=0.265 and 0.148, respectively). There were no differences in apoptosis markers either in resting platelets or after stimulation when comparing untreated patients (n=9) vs patients under any kind of treatment (n=15) (resting conditions, PS p=0.737; ΔmΨ p=0.270; stimulated, PS p=0.966; ΔmΨ p=0.987), although platelet count was similar in both groups. Normal platelets incubated during 1 hour with plasma from ITP had higher a-casp3 than those incubated with normal plasma (n=12 and 9, respectively, p=0.027), suggesting a plasmatic component could be responsible for the apoptotic stimulus.

In conclusion, increased platelet apoptosis in ITP patients could be induced by a plasmatic factor, contributing to thrombocytopenia in this entity.