Tuberculous pleural effusion is characterized by a T helper type 1 ( Th1) profile, but an excessive Th1 response may also cause tissue damage that might be controlled by regulatory mechanisms. In the current study we investigated the role of regulatory T cells ( Treg) in the modulation of Th1 responses in patients with tuberculous ( TB) pleurisy. Using flow cytometry we evaluated the proportion of Treg ( CD4+ CD25highforkhead box protein 3+), interferon ( IFN)-γ and interleukin ( IL)-10 expression and CD107 degranulation in peripheral blood ( PB) and pleural fluid ( PF) from patients with TB pleurisy. We demonstrated that the proportion of CD4+ CD25+, CD4+ CD25high FoxP3+ and CD8+ CD25+ cells were increased in PF compared to PB samples. Mycobacterium tuberculosis stimulation increased the proportion of CD4+ CD25low/neg IL-10+ in PB and CD4+ CD25low/neg IFN-γ+ in PF; meanwhile, CD25high mainly expressed IL-10 in both compartments. A high proportion of CD4+ CD107+ and CD8+ CD107+ cells was observed in PF. Treg depletion enhanced the in-vitro M. tuberculosis-induced IFN-γ and CD4+ and CD8+ degranulation responses and decreased CD4+ IL-10+ cells in PF. Our results demonstrated that in TB pleurisy Treg cells effectively inhibit not only IFN-γ expression but also the ability of CD4+ and CD8+ cells to degranulate in response to M. tuberculosis. [ABSTRACT FROM AUTHOR]