Increase of CD4⁺CD25^high Regulatory T Cells in the Peripheral Blood of Acute Myeloid Leukemia Patients.

PURPOSE: Although immunosuppression has long been recognized in acute myeloid leukemia (AML), the underlying basis for the lack of an effective immune response against the tumor remains unclear. T cells constitutively expressing both CD4 and CD25 are essential for maintenance of self-tolerance and therefore have been referred to as regulatory T cells (Treg). Experimental tumor models in mice revealed that Tregs are potent inhibitors of an antitumor immune response. The purpose of the current study was to demonstrate the possible involvement of CD4⁺CD25^high regulatory T cells in immune system impairment in patients with acute myeloid leukemia.

METHODS: The frequency and phenotypes of CD4⁺CD25^high T cells in the peripheral blood of 36 patients suffering from acute myeloid leukemia and from 15 healthy controls were determined by flow cytometry. To assess the functional activity of CD4⁺CD25^high cells, CD4⁺CD25^high or CD4⁺CD25⁻ cells were purified from PBMCs by sorting with FACSVantage. The immunoregulatory properties of CD4⁺CD25^high and CD4⁺CD25⁻ T cells were characterized by proliferation and suppression assays. Also, cytokine production after in vitro stimulation was examined in CD4⁺CD25^high and/or CD4⁺CD25⁻ T cells isolated from AML patients. In addition, the frequency of apoptotic or proliferating T cells in CD4⁺CD25^high T cells was determined by 7AAD or ki67 binding cells.

RESULTS: Compared with healthy volunteers, patients with acute myeloid leukemia had a higher proportion of CD4⁺CD25^high T cells (4.1±1.8% vs 2.0±0.5%) in peripheral blood with characteristics of Tregs, i.e., they are CD45-RA(−), CD69(−), CD45-RO(+), CD95(+), and intercellular CTLA-4(+), and secreted TGF-, TNF-α and IL-10 but did not secrete IL-2, IL-4, IL-5 or IFN-. When cocultured with CD4⁺25⁻ cells, CD4⁺CD25^high T cells potently suppressed their proliferation through a dose-dependent manner, thus behaving as Treg. In addition, In order to explain the increased prevalence of CD4⁺CD25^high T cells in AML patients, we examined the apoptosis and proliferation variations of CD4⁺CD25^high T cells in AML patients. Notably, in AML patients, both apoptosis and proliferation of CD4⁺CD25^high T cells were significantly higher than that did normal donor.

CONCLUSIONS: we provide evidence of an increased pool of CD4⁺CD25^high regulatory T cells in the peripheral blood of AML patients with potent immunosuppressive features, which maybe due to the proliferation of CD4⁺CD25^high T cells. This finding suggests that the use of immunomodulatory therapy to treat patients with AML may be an effective strategy.