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Characterization of regulatory T cells identified as CD4+CD25highCD39+ in patients with active tuberculosis

Forkhead box P3 (FoxP3) is a transcription factor whose expression characterizes regulatory T cells (Treg), but it is also present on activated T cells, thus hindering correct Treg identification. Using classical markers for Treg recognition, discordant results were found in terms of Treg expansion during active tuberculosis (TB) disease. Recently CD39 has been shown to be an accurate marker for Treg detection. The objectives of this study were: (i) to identify Treg expressing CD39 in patients with TB and to compare the results with those obtained by the standard phenotypic markers; (ii) to evaluate if Treg are expanded in vitro by exogenous interleukin (IL)‐2 or by antigen‐specific stimulation; and (iii) to characterize Treg function on the modulation of antigen‐specific responses. We enrolled 13 patients with pulmonary TB and 12 healthy controls. Treg were evaluated by flow cytometry ex vivo and after antigen‐specific in vitro stimulation using CD25, FoxP3, CD127 and CD39 markers. Results indicate that CD39+ cells within the CD4+CD25high cells have Treg properties (absence of interferon‐γ production and transforming growth factor‐β1 release upon stimulation). Ex vivo analysis did not show significant differences between TB patients and controls of Treg by classical or novel markers. In contrast, a significantly higher percentage of Treg was found in TB patients after antigen‐specific stimulation both in the presence or absence of IL‐2. Depletion of CD39+ Treg increased RD1‐specific responses significantly. In conclusion, CD39 is an appropriate marker for Treg identification in TB. These results can be useful for future studies to monitor Mycobacterium tuberculosis‐specific response during TB.