Interleukin-22 (IL-22) Production by Pulmonary Natural Killer Cells and the Potential Role of IL-22 during Primary Influenza Virus Infection

We set out to test the hypothesis that interleukin-22 (IL-22), a cytokine crucial for epithelial cell homeostasis and recovery from tissue injury, would be protective during influenza virus infection. Recent studies have identified phenotypically and functionally unique intestinal NK cells capable of producing the cytokine IL-22. Unlike gut NK cells that produce IL-22, the surface phenotypes of lung NK cells were similar to those of spleen NK cells and were characteristically mature. With mitogen stimulation, both single and double IL-22- and gamma interferon (IFN-gamma)-producing lung NK cells were detected. However, only the IL-22(+) IFN-gamma(-) lung NK subset was observed after stimulation with IL-23. IL-23 receptor (IL-23R) blocking dramatically inhibited IL-22 production, but not IFN-gamma production. Furthermore, we found that NK1.1(+) or CD27(-) lung NK cells were the primary sources of IL-22. After influenza virus infection, lung NK cells were quickly activated to produce both IFN-gamma and IL-22 and had increased cytotoxic potential. The level of IL-22 in the lung tissue declined shortly after infection, gradually returning to the baseline after virus clearance, although the IL-22 gene expression was maintained. Furthermore, depletion of NK cells with or without influenza virus infection reduced the protein level of IL-22 in the lung. Anti-IL-22 neutralization in vivo did not dramatically affect weight loss and survival after virus clearance. Unexpectedly, anti-IL-22-treated mice had reduced virus titers. Our data suggest that during primary respiratory viral infection, IL-22 seems to a play a marginal role for protection, indicating a differential requirement of this cytokine for bacterial and viral infections.