Two types of autoantibody-mediated thrombocytopenia in patients with systemic lupus erythematosus

Objectives. To determine whether autoantibodies to two platelet-specific antigens, glycoprotein IIb/IIIa (GPIIb/IIIa) and thrombopoietin receptor (TPOR), contribute to thrombocytopenia in patients with systemic lupus erythematosus (SLE). Methods. Circulating B cells producing anti-GPIIb/IIIa antibodies and serum anti-TPOR antibodies were measured in 32 SLE patients with thrombocytopenia, 30 SLE patients without thrombocytopenia, 92 patients with idiopathic thrombocytopenia and 60 healthy controls. The megakaryocyte density in bone-marrow smears from all the patients with thrombocytopenia was evaluated. Results. Anti-GPIIb/IIIa and anti-TPOR antibody responses were more frequent in SLE patients with thrombocytopenia than in those without thrombocytopenia (88 vs 17%, P < 0.0001; and 22% vs 0%, P=0.01, respectively). The frequencies of these platelet-related antibodies were comparable between SLE patients with thrombocytopenia and patients with idiopathic thrombocytopenia. Twenty-nine (91%) SLE patients with thrombocytopenia had either anti-GPIIb/IIIa or anti-TPOR antibody, and six had both. In SLE patients with thrombocytopenia, the anti-TPOR-positive patients had significantly higher frequencies of megakaryocytic hypoplasia and poorer therapeutic responses to corticosteroids and intravenous immunoglobulin than did the anti-TPOR-negative patients, most of whom had the anti-GPIIb/IIIa antibody alone. Conclusions. Anti-GPIIb/IIIa and anti-TPOR antibodies are major factors contributing to SLE-associated thrombocytopenia, but the clinical presentations associated with these autoantibodies are different.