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.