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Background: Both regulatory T cells (Tregs) and T helper IL-17-producing cells (Th17 cells) have been found to be involved in human malignancies, however, the possible implication of Tregs in regulating generation and differentiation of Th17 cells in malignant pleural effusion remains to be elucidated.Methods: The numbers of both CD39(+)Tregs and Th17 cells in malignant pleural effusion and peripheral blood from patients with lung cancer were determined by flow cytometry. The regulation and mechanism of Tregs on generation and differentiation of Th17 cells were explored.Results: Both CD39(+)Tregs and Th17 cells were increased in malignant pleural effusion when compared with blood, and the numbers of CD39(+)Tregs were correlated negatively with those of Th17 cells. It was also noted that high levels of IL-1β, IL-6, and TGF-β1 could be observed in malignant pleural effusion when compared the corresponding serum, and that pleural CD39(+)Tregs could express latency-associated peptide on their surface. When naïve CD4(+) T cells were cocultured with CD39(+)Tregs, Th17 cell numbers decreased as CD39(+)Treg numbers increased, addition of the anti-latency-associated peptide mAb to the coculture reverted the inhibitory effect exerted by CD39(+)Tregs.Conclusions: Therefore, the above results indicate that CD39(+)Tregs inhibit generation and differentiation of Th17 cells via a latency-associated peptide-dependent mechanism. [ABSTRACT FROM AUTHOR]