Defect of regulatory T cells in patients with Omenn syndrome

Background: Omenn syndrome (OS) is an autosomal-recessive disorder characterized by severe immunodeficiency and T-cell-mediated autoimmunity. The disease is caused by hypomorphic mutations in recombination-activating genes that hamper the process of Variable (V) Diversity (D) Joining (J) recombination, leading to the generation of autoreactive T cells. We have previously shown that in OS the expression of autoimmune regulator, a key factor governing central tolerance, is markedly reduced. Objective: Here, we have addressed the role of peripheral tolerance in the disease pathogenesis. Methods: We have analyzed forkhead box protein P3 (FOXP3) expression in peripheral blood T cells of 4 patients with OS and in lymphoid organs of 8 patients with OS and have tested the suppressive activity of sorted CD4⁺ CD25^high peripheral blood T cells in 2 of these patients. Results: We have observed that CD4⁺CD25^highT cells isolated ex vivo from patients with OS failed to suppress proliferation of autologous or allogenic CD4⁺ responder T cells. Moreover, despite individual variability in the fraction of circulating FOXP3⁺ CD4 cells in patients with OS, the immunohistochemical analysis of FOXP3 expression in lymph nodes and thymus of patients with OS demonstrated a severe reduction of this cell subset compared with control tissues. Conclusion: Overall, these results suggest a defect of regulatory T cells in OS leading to a breakdown of peripheral tolerance, which may actively concur to the development of autoimmune manifestations in the disease.