Interleukin-26 upregulates interleukin-22 production by human CD4⁺ T cells in tuberculous pleurisy.

IL-26 is a potentially important player in host defense and may be a pathogenic factor in the chronic inflammatory disorders of humans. However, the involvement of IL-26 in tuberculous pleural effusion (TPE) has not been investigated. The concentration of IL-26 was determined in pleural fluids and sera from patients with pleural effusions. Flow cytometry was performed to identify the cell origin of IL-26. The effects of tuberculosis-specific antigen (ESAT-6/CFP-10) on IL-26 expression of CD4⁺ T cell were explored. The impacts of IL-26 on modulating CD4⁺ T cell polarization were also investigated. The concentrations of IL-26 were much higher in tuberculous, malignant, and infectious PE than those in the corresponding serum. The expression of IL-26 on CD4⁺ T cells was much higher in tuberculous PE than those in the corresponding serum, and pleural Th1 and Th17 cells might be the major cell sources of IL-26. The addition of ESAT-6/CFP-10 to CD4⁺ T cells led to increasing the number of IL-26-producing CD4⁺ T cells and IL-26 expression on Th1 and Th17 cells. IL-26 could induce the differentiation and generation of IL-22 by memory and naive CD4⁺ T cells. IL-26 also upregulated the mRNA encoding CC-chemokine ligand 20 (CCL20) and CCL22 by mononuclear cells isolated from TPE. This study implies that pleural Th1 and Th17 cells are the major cell sources of IL-26, which could induce the differentiation and generation of Th22 cells by CD4⁺ T cells, suggesting the involvement of IL-26 in the pathogenesis of human TPE. KEY IL-26 is overexpressed in TPE patients and presents a higher concentration in pleural effusion than the corresponding peripheral blood. Pleural Th1 and Th17 cells might be the major cell sources of IL-26 in TPE patients. IL-26 promotes IL-22 secretion and Th22 generation by CD4⁺ T cells isolated from TPE patients. IL-26 may play an active role in the pathogenesis of tuberculous pleurisy.

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