GP IIb/Iiia-Dependent Complement Activation Is Common In Patients with Immune Thrombocytopenic Purpura.

It is commonly accepted that antibody-mediated removal of platelets represents the major mechanism of platelet destruction in immune thrombocytopenic purpura (ITP). However, both the clinical course and the response to various treatments vary largely between ITP patients, suggesting a multi-factorial pathogenesis of thrombocytopenia in ITP. Ineffective thrombopoiesis and direct T-cell cytotoxicity have been described as additional immune-mediated mechanisms. In contrast, although some promising early studies were performed in the 1980s, the role of the complement system in ITP is still not well defined. The present pilot study investigated the possible contribution of the complement system in ITP.

We examined blood samples from 240 patients with ITP. First, all samples were assessed for the presence of free and bound platelet autoantibodies by a standard glycoprotein-specific assay (MAIPA). Second, the ability of all sera to fix complement to a panel of human platelets was investigated in a complement fixation assay. Third, fixation of C1q to immunobeads coated with platelet-derived GP IIb/IIIa as well as the generation of microparticles from test platelets were assessed by flow cytometry. The study was approved by the local research ethics committee.

Glycoprotein-specific autoantibodies were detected as platelet-bound antibodies in 129 (54%); as additional free antibodies in 26 (11%); and were undetectable in 111 (46%) of all patients. When sera from these patients where then assessed for their ability to fix complement to a panel of 5 test platelets, 103 (65%), 21 (81%), and 33 (30%) sera gave positive results. When studied in the presence of test platelets lacking either GP IIb/IIIa or GPIb/IX, 72% and 25% of all sera lost their ability to fix complement, respectively. Fixation of C1q to immunobeads covered with GP IIb/IIIa was observed in 70% of sera that had fixed complement to platelets. Half of these sera induced the production of platelet microparticles. In a group of 50 controls, platelet-reactive antibodies were undetectable, fixation of complement to platelets and fixation of C1q to immunobeads was observed in 2/50 (4%), and there were no sera inducing the production of micoparticles.

In a significant number of patients with chronic ITP, platelet autoantibodies are capable of activating the classical complement pathway. Complement fixation is even present in ITP sera without detectable autoantibodies, indicating that current techniques for autoantibody detection may be insufficient. The major target for complement-fixing autoantibodies in ITP is GP IIb/IIIa. In vitro, however, only 50% of sera that can fix complement in a GP IIb/IIIa-dependent manner are also capable of inducing platelet lysis. We conclude that complement fixation may contribute to thrombocytopenia by directly damaging platelets and/or by enhancing platelet clearance via complement-receptor mediated phagocytosis. It will thus be relevant to study the influence of complement-mediated platelet destruction on treatment responses, particularly in the light of newly developed approved and non-approved therapies, such as, thrombopoietin receptor agonists and inhibitors of complement activation.

No relevant conflicts of interest to declare.