Abstract
Mechanisms leading to decreased platelet count in immune thrombocytopenia (ITP) are heterogeneous. This study describes increased platelet apoptosis involving loss of mitochondrial membrane potential (ΔΨm), caspase 3 activation (aCasp3) and phosphatidylserine (PS) externalization in a cohort of adult ITP patients. Apoptosis was not related to platelet activation, as PAC-1 binding, P-selectin exposure and GPIb-IX internalization were not increased. Besides, ITP platelets were more sensitive to apoptotic stimulus in terms of aCasp3. Incubation of normal platelets with ITP plasma induced loss of ΔΨm, while PS exposure and aCasp3 remained unaltered. The increase in PS exposure observed in ITP platelets could be reproduced in normal platelets incubated with ITP plasma by adding normal CD3+ lymphocytes to the system as effector cells. Addition of leupeptin -a cathepsin B inhibitor- to this system protected platelets from apoptosis. Increased PS exposure was also observed when normal platelets and CD3+ lymphocytes were incubated with purified IgG from ITP patients and was absent when ITP plasma was depleted of auto-antibodies, pointing to the latter as responsible for platelet damage. Apoptosis was present in platelets from all patients carrying anti-GPIIb-IIIa and anti-GPIb auto-antibodies but was absent in the patient with anti-GPIa-IIa auto-antibodies. Platelet damage inversely correlated with platelet count and decreased during treatment with a thrombopoietin receptor agonist. These results point to a key role for auto-antibodies in platelet apoptosis and suggest that antibody-dependent cell cytotoxicity is the mechanism underlying this phenomenon.
Highlights
Immune thrombocytopenia (ITP) is an autoimmune condition in which defects in immune self-tolerance lead to humoral and cellular abnormal responses comprising auto-antibody production and cytotoxic effects [1]
To determine whether platelet apoptosis occurs in our cohort of adult patients with ITP, we evaluated PS exposure, ΔCm, and levels of active caspase 3 in ITP platelets in resting conditions
We demonstrate an apoptotic behavior of platelets from ITP patients, as assessed by disruption of mitochondrial membrane potential, increased PS exposure and caspase 3 activity
Summary
Immune thrombocytopenia (ITP) is an autoimmune condition in which defects in immune self-tolerance lead to humoral and cellular abnormal responses comprising auto-antibody production and cytotoxic effects [1]. Pro- and anti-apoptotic protein unbalance triggers mitochondrial outer membrane permeabilization (MOMP) that is followed by mitochondrial inner membrane potential collapse (ΔCm), efflux of cytochrome c into the cytoplasm, activation of caspase 3 and 9, phosphatidylserine (PS) externalization and microparticle shedding [12]. Since some of these events take place during platelet activation, markers of platelet apoptosis should be carefully analyzed
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