Detection of Anti-Thrombopoietin Antibodies in Patients with Immune Thrombocytopenia

Background: Immune thrombocytopenia (ITP) is an autoimmune disease characterized by the presence of autoantibodies against platelet membrane glycoproteins, which cause the autoantibody-mediated destruction of platelets and impaired platelet production. Thrombopoietin (TPO) binds to its receptor on the surface of hematopoietic stem cells and megakaryocytes and induces their maturation and proliferation. Patients with thrombocytopenia due to aplastic anemia have drastically elevated plasma levels of TPO, whereas patients with ITP have normal or slightly elevated plasma levels of TPO despite their low platelet count. Furthermore, based on the existence of a multitude of autoantibody reactivities in ITP, including antibodies against platelets and TPO receptors, the presence of anti-TPO antibodies in patients with ITP may be suspected.

Objective: We developed assay systems to detect plasma anti-TPO antibodies and screen patients with ITP. We examined the clinical characteristics associated with anti-TPO antibodies and their pathogenic roles in patients with ITP.

Methods: Plasma anti-TPO antibodies from 101 patients with ITP and 72 healthy controls were measured by enzyme-linked immunosorbent assay (ELISA) using recombinant human TPO (rhTPO) as an antigen. The specificity of anti-TPO antibody reactivity was confirmed by ELISA competition assay. The presence of anti-TPO antibodies was further examined using immunoprecipitation and immunoblotting using rhTPO. To investigate whether anti-TPO antibodies inhibited functional interactions between TPO and TPO receptors, we examined extracellular signal-regulated kinases (ERKs), downstream signals induced by TPO. The binding of TPO to TPO receptors induced the phosphorylation of ERK in TPO receptor-expressing UT-7/TPO cells.

Results: The level of anti-TPO antibodies measured by ELISA was significantly greater in the samples from patients with ITP than in those from healthy controls (2.91 ± 3.64 units versus 1.45 ± 0.67 units, P < 0.001). Samples were classified as positive or negative for anti-TPO antibody, as determined by immunoprecipitation and immunoblotting. Thus, the ELISA positive-cutoff value was considered to be the mean plus 3.5 standard deviation (SD) of 72 healthy control plasma samples. Plasma anti-TPO antibodies were detected in twenty-four ITP patients (23.8%), but in none of the healthy controls. By ELISA competition assay, anti-TPO antibody reactivity was inhibited dose-dependently by preincubation of patient plasma with rhTPO. In addition, anti-TPO antibody-positive plasma samples inhibited the phosphorylation of ERK in UT-7/TPO cells. In contrast, healthy control plasma had no inhibitory effect. Furthermore, the number of megakaryocytes was decreased relatively in the anti-TPO antibody-positive ITP patients. There was no difference in the TPO levels in plasma between ITP patients with anti-TPO antibodies and patients without anti-TPO antibodies (63.6 ± 79.7 pg/ml versus 45.2 ± 49.3 pg/ml).

Conclusion: Our results have thus demonstrated the presence of anti-TPO autoantibodies in patients with ITP. The ELISA using rhTPO was specific for the detection of anti-TPO antibodies and thus allows their easy and rapid measurement in clinical settings. These findings suggest that functional anti-TPO antibodies cause impaired megakaryocyte proliferation and platelet production in patients with ITP.