Thrombopoietin Can Induce Tolerogenic Responses Via the Modulation of Thymic and Splenic T Lymphocyte Populations in a Murine Model of Immune Thrombocytopenia (ITP)

Abstract 1088

Clinical trials using thrombopoietin (TPO) mimetics have clearly shown their benefit in raising platelet counts in patients with immune thrombocytopenia (ITP), however, little is known whether TPO may affect immunologic responses. It has been demonstrated that, like other therapies, TPO treatment can rescue the peripheral CD4+CD25+FoxP3+ T regulatory cell (Tregs) deficiency that is commonly observed in patients with ITP (Bao W et al Blood 116;4639, 2010). Of interest, an earlier murine study also suggested that TPO could affect the production of T cells, particularly CD4+ T cells (Zhao JZ et al Haematologica 83:572, 1998). Using a murine model of ITP that demonstrates both antibody and T cell mediated thrombocytopenia, we studied T cell responses in ITP mice treated with TPO. ITP was induced in severe combined immunodeficient (SCID) mice by administration of 2×10⁴ splenocytes from CD61 (GPIIIa) knockout mice immunized against CD61 and the mice where either treated or not with weekly subcutaneous injections of 1ug of murine TPO. Mice were bled weekly to count platelets and obtain sera for anti-platelet antibody production and at 3 weeks after the splenocyte transfer, they were sacrificed and their spleens and thymi were harvested. Splenocytes and thymocytes were isolated and examined by flow cytometry to quantify CD4+CD25+FoxP3+ (Tregs) and CD8+CD25+FoxP3+ (Tc-regs). We observed that TPO treatment caused an expected significant increase in platelet counts, however, the treatment also caused a significant reduction in the titre of serum anti-platelet antibodies. This was associated with significant changes in the proportions of both CD4+ Tregs and CD8+ Tc-regs within the thymus and spleen. TPO treatment rescued the peripheral splenic Treg and Tc-reg deficiencies and the cells ability to functionally suppress the proliferation of conventional CD4+CD25- T cells. The data suggests that in addition to its megakaryocyte-stimulating properties, TPO treatment has additional tolerance-inducing effects associated with anti-platelet antibody reduction and CD4+ Treg and CD8+ Tc-reg stimulation.