Although significant advances have been made in characterizing the expression of IL-22 and IL-22R and the signal transduction pathways that are activated, there are conflicting reports on the biological consequences of IL-22 expression in mouse models of inflammation. For example, IL-22 production can be either pathological or tissue protective depending on the disease model examined (Zenewicz and Flavell, 2008
). The results of the present study are the first to demonstrate that IL-22 expression can promote inflammation after bleomycin-induced airway inflammation. Critically, the functional outcomes of IL-22 expression in the lung were governed by coexpression of IL-17A. When coexpressed in vivo, IL-17A and IL-22 acted synergistically to promote chemokine expression, neutrophil recruitment, and airway inflammation. Conversely, in the absence of IL-17A, IL-22 expression was no longer proinflammatory and pathological but rather conferred tissue-protective functions by promoting the integrity of the epithelial barrier. Therefore, differential spatial and temporal expression of IL-17A and IL-22 may explain the divergent functions of IL-22 reported in different models of infection or inflammation.
After bleomycin exposure, a Th17 cell response developed, characterized by expression of IL-17A and IL-22 in the lung and BAL. Previous studies have identified that many of the factors that promote the differentiation of Th17 cells have been linked in the pathogenesis of bleomycin-induced airway inflammation, including IL-6 and IL-12/23p40 (Maeyama et al., 2001
; Sakamoto et al., 2002
; Saito et al., 2008
). Consistent with a proinflammatory role for Th17 cells, neutralization of either Th17 cell–associated effector cytokine, IL-22 or IL-17A, was sufficient to provide protection against bleomycin-induced airway inflammation in WT mice, suggesting that a functional synergy between both cytokines can promote disease. Synergy between IL-22 and IL-17A has previously been observed in vitro (Liang et al., 2006
) and in vivo after infection with the pulmonary pathogen K. pneumoniae
(Aujla et al., 2008
). This synergy promoted the production of inflammatory mediators and antimicrobial peptides (Liang et al., 2006
), and was found to be beneficial for the host after pulmonary infection (Aujla et al., 2008
). However, it was also found that administration of exogenous IL-22 itself was not enough to promote neutrophil recruitment to the airway (Liang et al., 2007
), suggesting that IL-17A was required for the proinflammatory properties of IL-22. Consistent with this, we demonstrate that exogenous IL-22 is only able to promote inflammation in the airway in the presence of IL-17A. It is possible that in mouse models of psoriasis, arthritis, and protozoan infection, in which IL-22 was reported to be proinflammatory (Zheng et al., 2007
; Ma et al., 2008
; Geboes et al., 2009
; Muñoz et al., 2009
), the same synergy between IL-22 and IL-17A is operating to promote inflammation.
In the absence of IL-17A, it was found that there were increased levels of IL-22 after bleomycin instillation. This finding was consistent with previous studies that also observed increased IL-22 mRNA in the absence of IL-17A in a mouse model of colitis (O’Connor et al., 2009
) or decreased IL-22 mRNA in splenocyte cultures with the addition of exogenous IL-17A (Smith et al., 2008
; von Vietinghoff and Ley, 2009
). In in vitro studies, we demonstrated that IL-17A could suppress the expression and secretion of IL-22 from Th17 cells in a dose-dependent manner. Interestingly, it has also been reported that IL-17A can inhibit IL-17F expression by Th17 cells (von Vietinghoff and Ley, 2009
), suggesting a common pathway for IL-17A–mediated inhibition of Th17 cell effector cytokine expression. However, it has not yet been determined whether IL-17A is acting directly on the Th17 cells or through an accessory cell, and further investigation must be conducted to determine the mechanisms through which the suppression of IL-22 production occurs.
Despite elevated expression of IL-22 in the absence of IL-17A, Il17a−/−
mice were not susceptible to bleomycin-induced disease, which is consistent with a loss of the proinflammatory properties of IL-22. However, blockade of IL-22 in the absence of IL-17A exacerbated bleomycin-induced disease, indicating a tissue-protective role for IL-22 in airway inflammation in the absence of IL-17A. Consistent with this hypothesis, we found that rIL-22 could protect airway epithelial cells from bleomycin-induced apoptosis in both in vitro and in vivo assays. Further, IL-22–mediated protection from epithelial cell apoptosis was reversed in the presence of IL-17A. Previous reports proposed a constitutive tissue-protective function for IL-22 in mouse models of inflammatory bowel disease and hepatitis (Pan et al., 2004
; Radaeva et al., 2004
; Zenewicz et al., 2007
; Sugimoto et al., 2008
; Pickert et al., 2009
). However, after bleomycin-induced airway inflammation, IL-22 exhibited a constitutive proinflammatory effect in WT mice and was only tissue protective in the absence of IL-17A. One possible explanation for the constitutive proinflammatory effects of IL-22 in bleomycin-exposed WT mice in comparison to a constitutive tissue-protective role for IL-22 reported in mouse models of inflammation in the intestine or liver may be the differential coexpression of IL-17A and IL-22 in distinct tissues. For example, after bleomycin instillation the majority of IL-22–expressing cells in the lung coexpressed IL-17A and promoted inflammation. In contrast, subsets of gut-resident NK cells and skin-resident CD4+
T cells are reported to express IL-22 but do not coexpress IL-17A (Satoh-Takayama et al., 2008
; Cella et al., 2009
; Duhen et al., 2009
; Trifari et al., 2009
). Collectively, these reports support a model in which production of IL-22 by these cell populations in the absence of IL-17A may be important in promoting tissue-protective responses. Therefore, the cellular sources, anatomical location, and cytokine coexpression profile of resident and recruited cell populations may influence the functional properties of IL-22 and provide an explanation for the distinct functional roles of IL-22 in models of infection and inflammation in distinct peripheral tissues.
When anti–IL-22 mAb was administered in the absence of IL-17A, bleomycin-induced disease was comparable to that in WT mice and independent of IL-17A and IL-22. It is possible that other Th17 cell–derived cytokines such as IL-17F or TNF-α may play a significant role in this context, as well as other nonrelated inflammatory cytokines such as IFN-γ, all of which have been shown to contribute to airway inflammation in other model systems (Lukacs et al., 1995
; Segel et al., 2003
; Liang et al., 2007
; Yang et al., 2008
). Additionally, a recent report identified that in a model of bleomycin-induced fibrosis, IL-17A can act cooperatively with TGF-β to promote disease (Wilson et al., 2010
). In the present study, the influence of IL-17A and IL-22 on bleomycin-induced tissue damage and acute airway inflammation occurred independently of any significant changes in the production of TGF-β protein (unpublished data). Notwithstanding that, future studies in a model of fibrosis will be required to examine the potential functional interactions between IL-17A, IL-22, and TGF-β in the development and/or progression of disease.
Based on the in vitro and in vivo findings reported here, we propose three mechanisms by which IL-17A regulates the functional consequences of IL-22 expression. First, IL-17A regulates the in vivo and in vitro expression levels of IL-22 by inhibiting IL-22 production from Th17 cells. Second, IL-17A promotes the proinflammatory properties of IL-22 by acting in synergy to induce expression of inflammatory cytokines, chemokines, and neutrophil recruitment. Third, IL-17A prevents the tissue-protective functions of IL-22 by suppressing the antiapoptotic effects of IL-22 on epithelial cells. Therefore, this complex regulation of IL-22 by IL-17A may underlie how IL-22 can promote both pathological or tissue-protective outcomes depending on the context in which it is expressed.
The ability of IL-22 to be either pathological or protective, depending on the context in which it is expressed, is a property shared by other cytokines that signal through STAT3, including IL-6 and IL-27, which can either promote or regulate inflammation dependent on the cytokine milieu and regulation of signal transduction (Yasukawa et al., 2003
; Villarino et al., 2004
). It is probable that the interplay between IL-17A and IL-22 signaling pathways will determine the balance between proinflammatory versus tissue-protective outcomes. IL-22 is known to signal through the STAT3 and p38 mitogen-activated protein kinase pathways, whereas IL-17A signals predominantly through the NF-κB pathway (Kotenko et al., 2001
; Lejeune et al., 2002
; Gaffen, 2009
). The STAT3 and NF-κB pathways regulate a wide range of biological processes, including cell growth, differentiation, and apoptosis, and complex interactions have been reported between these two transcription factors (Alonzi et al., 2001
; Uskokovic et al., 2007
; Bollrath and Greten, 2009
). Therefore, future investigation into the interplay between the signal transduction pathways and gene targets of both IL-17A and IL-22 will likely yield further insight into the ability of IL-17A to regulate the functional consequences of IL-22 expression. Notwithstanding this, the results of the present study provide the first demonstration that IL-22 can promote disease in a model of airway inflammation, and support a model in which IL-17A regulates the levels of expression, proinflammatory properties, and tissue-protective properties of IL-22, thereby determining the functional consequences of IL-22 expression in the lung. Differential temporal and spatial coexpression of IL-17A and IL-22 may underlie the conflicting reports of the biological effects of IL-22 in distinct disease models, and may offer selective therapeutic potential in the treatment of Th17 cell–associated inflammatory diseases.