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-γ)-producing lung NK cells were detected. However, only the IL-22⁺ IFN-γ⁻ lung NK subset was observed after stimulation with IL-23. IL-23 receptor (IL-23R) blocking dramatically inhibited IL-22 production, but not IFN-γ 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-γ 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.NK cells are important innate immune effectors that patrol the body for invading pathogens and tumors. Primary biological functions of NK cells include natural cytotoxicity and cytokine generation, through which NK cells directly or indirectly control infections and tumors and regulate the immune system (8). Accumulating evidence has unveiled other novel functions of NK cells that are associated with their anatomic locations. For example, in the uterus, NK cells support reproductive tissue development by providing a variety of cytokines, growth factors, and angiogenic factors (18, 26). The uterine NK cells also demonstrate a unique receptor repertoire, the Ly49 phenotype of which is strikingly different from that of spleen NK cells (39).Very recently, an NK1.1 low or negative subset of NK cells (CD3⁻ NKp46⁺) has been identified in the intestinal mucosa and found to be capable of making interleukin-22 (IL-22) (7, 24, 31, 32). IL-22 is one of the IL-10 cytokine family members that have been shown to be important in regulating mucosal epithelial cell function, maintaining barrier integrity, and protection from bacterial infections in the gut and lung (4, 43). Interestingly, gut NK cells are distinguished by an immature phenotype, as evidenced by the lack of multiple traditional NK cell markers, such as Ly49A, Ly49D, Ly49C/I, and Ly49G2, and by altered expression of several markers, such as CD122, NK1.1, CD49b (DX5), CD11b, CD27, and CD127, in comparison with spleen NK cells (24, 31, 32). Functionally, gut NK cells lack the capability of gamma interferon (IFN-γ) production and cytotoxicity (24, 31, 32). Taken together, the unique nontraditional features of gut NK cells indicate a distinct developmental process (11, 36) in which they acquire the ability to produce IL-22 and thus are crucial components against intestinal bacterial infections.In addition to the gut, the respiratory tract is an important mucosal system that can be easily invaded by microorganisms. In the lung, NK cells constitute about 10% of the total resident lymphocytes, a relatively higher percentage than that distributed in most other lymphoid tissues and nonlymphoid tissues (17), indicating potential crucial involvement of NK cells in lung infections. Indeed, lung NK cells are known to be vital for containing numerous pulmonary infections, including those caused by Mycobacterium tuberculosis, Cryptococcus neoformans, Bordetella pertussis, respiratory syncytial virus, and influenza virus (12, 16). The potential mechanism of NK cell defense in lung infections has been attributed to NK cell IFN-γ production and their cytolytic functions. However, IL-22 has been implicated in protection against respiratory infection with Gram-negative bacteria, such as Klebsiella pneumoniae, where IL-22 levels increase after infection (4). Whether lung NK cell production of IL-22 in the context of respiratory virus infection or IL-22 itself is important for viral protection has not been investigated.In this study, we investigated the phenotypes and IL-22 production potential of lung NK cells in the context of influenza virus infection. The data show that lung NK cells are phenotypically similar to spleen NK cells yet capable of producing IL-22 upon in vitro stimulation and after influenza virus infection in vivo. Unlike gut NK cells, IL-22-producing lung NK cells are capable of making IFN-γ and display cytolytic potential. After influenza virus infection, in spite of the detection of IL-22-producing NK cells in the lung, IL-22 levels actually went down, and mice treated with anti-IL-22 antibodies had reduced virus titers, with little change in disease severity. These observations show that IL-22 serves different roles in bacterial versus virus infections of the lung and suggest that it may be actively regulated to limit proliferation of cells targeted by the influenza virus.