Introduction Pseudomonas aeruginosa is an important opportunistic respiratory pathogen responsible for ventilator-associated pneumonia, acute lower respiratory tract infections in immunocompromised patients and chronic respiratory infections in cystic fibrosis. Antibody-mediated adaptive immune responses exist in cystic fibrosis patients but these lack protective opsonic activity. The role of CD4⁺T cell responses to Pseudomonas remains unclear. Novel T helper (Th) cell subsets have also recently been defined; Th22 cells produce IL-22, whereas Th17 cells produce IL-17 cytokines in addition to IL-21 and IL-22. The properties of IL-22-producing CD4⁺ cells in humans remains to be defined, but include an important respiratory anti-microbial defense function. Immunopathological roles of Th17/Th22 cells may also exist. We assessed T-helper cell responses to Pseudomonas in healthy adults.

Methods CD14⁺ monocytes and memory CD4⁺CD45RO⁺T cells were isolated from peripheral blood using magnetic-activated cell sorting. Monocyte-derived dendritic cells (DCs) were generated by culture in the presence of IL-4 and GM-CSF. DCs were stimulated with live Pseudomonas aeruginosa strain PA103. DC activation markers, CD40 and CD86, were measured via flow cytometry. Autologous T cells were co-cultured with activated DCs, or their supernatants, for 6-days. T cell phenotype was analysed via flow cytometry and ELISA of supernatants for secreted cytokines. T cell incorporation of carboxyfluorescein-succinimidyl-ester was utilised to demonstrate cell multiplication.

Results We demonstrate that monocyte-derived DCs from healthy adults are readily activated by Pseudomonas. Co-culture of autologous memory CD4⁺T cells with Pseudomonas-activated DCs resulted in CD4⁺ T cells primarily producing IFN-gamma, as well as T cell subsets expressing both IFN-gamma and IL-22, or IL-22 alone. No Th17 response to Pseudomonas was evident. The response was MHC-restricted and specific T cell proliferation was demonstrated. Reproducibility of the response within individuals was also shown.

Conclusions These data demonstrate that healthy adults possess Th1 and Th22 memory cell responses to Pseudomonas aeruginosa. The results may have important implications for patients with deficits in cellular immunity and the pathogenesis of chronic pseudomonas infection in cystic fibrosis. Further work is required to ascertain cross-reactivity of response, antigenic triggers, and the role these cellular responses play in protection vs pathogenesis in Pseudomonal pulmonary infections.