Thymic Retention of CD4+CD25^hi+FoxP3+ T Regulatory (Treg) Cells Is Responsible for Peripheral Treg Deficiency and Platelet and Megakaryocyte Destruction in Active Immune Thrombocytopenia (ITP)

Abstract 523

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder in which antibodies and/or cytotoxic T cells are directed against an individual's own platelets and/or megakaryocytes and lead to enhanced platelet destruction in the periphery and reduced platelet production within the bone marrow. It is becoming increasingly clear that the immune dysregulation leading to ITP is primarily due to T cell disturbances and recently, many reports have demonstrated that active ITP is associated with a peripheral deficiency of tolerance-inducing CD4+CD25+FoxP3+ T regulatory cells (Tregs). Using a murine model of ITP that demonstrates both antibody and T cell-mediated platelet and megakaryocyte destruction, we analyzed the role that Tregs may play in active ITP. BALB/c CD61 knockout (KO) mice were immunized against wildtype (CD61+) platelets and 1.5×10⁴ of their splenocytes were transferred ip into CB.17 severe combined immunodeficient (SCID) mice and their platelet counts, phenotype and percentage of CD4+CD25^hi+FoxP3+ Tregs were enumerated in various lymphoid tissues at various times after transfer. Results showed that within 2 weeks post-transfer, 16 of 30 transferred mice became thrombocytopenic (<300×10⁹ platelets/L) whereas the remaining mice did not. Bleeding diatheses and mortality occurred in 4 of the 16 thrombocytopenic mice. Of interest, compared with control SCID mice transferred with naïve BALB/c splenocytes or the non-thrombocytopenic SCID mice, the thrombocytopenic mice had significant elevations in thymic weights and percentages of CD4+CD25^hi+FoxP3 Tregs by 2 weeks post transfer. Splenic weights in the thrombocytopenic mice increased but at the same rate as control mice, however, the proportion of Tregs in the spleens were significantly less than control non-thrombocytopenic mice. Treatment of the thrombocytopenic mice with 2g/kg intravenous gammaglobulin (IVIg) raised the platelet counts, reduced thymic weights and normalized both the thymic and splenic Treg populations compared with controls. The results suggest that active ITP in this mouse model is associated with a peripheral (splenic) Treg deficiency due to thymic retention of the Tregs. It proposes a new mechanism of Treg deficiency in ITP; Tregs are withheld from the periphery by thymic sequestration.