Idiopathic aplastic anemia (AA) is characterized by immune-mediated destruction of hematopoietic stem cells, leading to peripheral pancytopenia. Immune pathogenesis in AA is supported by experimental data, as well as clinical observations and may be related to the breach of peripheral or central tolerance. Regulatory T cells (Treg) constitute one of the most important mechanisms of central tolerance engaged in the down-modulation of autoreactive T cells. Tregs have been found to be reduced in several autoimmune diseases and decreased frequencies of Tregs were also reported in AA and MDS. Overexpression of the high affinity IL-2 receptor alpha chain (CD25) and the forkhead family transcription factor P3 (FoxP3), required for the development and function of Tregs, serve as phenotypic markers for Tregs. We investigated Treg levels in a cohort of AA patients (N=21) and healthy individuals (N=15); flow cytometric quantification of Treg was carried out after surface/intracellular staining of whole blood for Treg markers (CD3, CD4, CD25, FoxP3). After proper gating (light scatter properties, CD3, CD4, CD25), CD4+ T cells were subdivided into CD25−, CD25int and CD25hi populations, and the co-expression of CD25hi and Foxp3 was analyzed. In comparison to controls, AA patients (N=12) show not only lower frequencies of CD4+CD25hi+ T cells within the total lymphocyte population (median 0.07% vs. 0.21%; p=.03), but also absolute lower absolute numbers (1.31/uL vs. 5.78/uL, p=.0002). Similarly, CD4+CD25hi+FoxP3+ T cells were found to be depressed in untreated AA patients in comparison to controls (median 0.07% vs. 0.21% and 1.06/uL vs. 4.76/uL; p=.03 and p=.003). While Tregs were lower in patients with active disease unresponsive to immunosuppressive treatment (responder 0.1% vs non responder 0.07%, CD4+CD25hi Tcells, p=.02), serial testing performed in 6 patients treated with ATG/CsA did not reveal correlation between hematologic improvement and recovery of Treg numbers. When double immunohistochemical staining for CD3 and Foxp3 was performed in pre-treatment bone marrow core biopsies of AA patients (N=3) and controls (N=2) a mean of 3 CD3+Foxp3+ cells/10 high power fields (hpf) were counted (vs. mean 28/10 hpf, p<.05 in controls), suggesting that lower numbers of Tregs were also present in the bone marrow of AA patients. In conclusion, our results suggest that Tregs are decreased in blood and marrow of patients with idiopathic AA, consistent with the breach of peripheral tolerance in AA. In addition to flow cytometry, immunohistochemical staining of histologic specimens can be used for the quantitative analysis of Tregs in bone marrow failure syndromes and other immune-mediated conditions such as GvHD.

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