T helper 17 cells are involved in the local and systemic inflammatory response in community-acquired pneumonia.

Background: Recent findings in mouse models suggest that T helper (Th)17 cells, characterised by production of interleukin (IL)-17A and IL-22, are involved in the immunopathogenesis of pneumonia.

Objective: In this study, we aimed to identify the involvement of Th17 cells in human community-acquired pneumonia (CAP).

Design: Within 24 h of admission, T cells from peripheral blood (n=39) and bronchoalveolar lavage (BAL, n=20) of CAP patients and of 10 healthy individuals were analysed by intracellular flow cytometry for the production of various cytokines, including IL-17A and IL-22. Peripheral blood T cells were also analysed 7 and 30 days after admission. Th17 cytokine profiles were correlated with pneumonia severity index and microbial aetiology.

Results: In the BAL of CAP patients, proportions of IL-17A and IL-22 single positive, as well as IL-17A/IL-22 double positive CD4 T cells were significantly increased compared with healthy individuals. Significantly increased proportions of IL-17A/IL-22 double positive CD4 T cells in BAL were found in non-severe and severe CAP patients, as well as in pneumococcal and non-pneumococcal CAP. In the peripheral blood of CAP patients upon admission, we found significantly increased proportions of IL-17A/IL-22 double positive CD4 T cells. One week after admission, the proportions of these double positive cells were still significantly increased in CAP patients compared with healthy individuals.

Conclusions: These data indicate that Th17 cells are engaged in the local and systemic immune response in human pneumonia. Especially, IL-17A/IL-22 double positive Th17 cells may be involved in the immunopathogenesis of CAP.