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In allergic airway inflammation, dendritic cells (DCs) are required for Th2 generation, recruitment, and activation in the respiratory tract. DCs have been shown to be necessary and sufficient for the induction of Th1 immune responses. In Th2 immunity and allergic airway inflammation, the ability of a DC to function as the sole APC has not been tested. We show that CD11c/Aβb mice with MHC class II expression restricted to CD11c-expressing DCs develop airway neutrophilia rather than allergic airway inflammation. Although CD11c/Aβb mice are capable of Th2 recruitment and activation in the lung, Th2 priming in CD11c/Aβb mice results in IFN-γ production. Effective Th2 generation and allergic airway inflammation was achieved in CD11c/Aβb mice after treatment with anti-IFN-γ. These studies show that DCs alone cannot drive the development of Th2 cells but require an additional MHC class II signal to stimulate effective Th2 immunity.

IL-13 is a critical cytokine at sites of Th2 inflammation. In these locations it mediates its effects via a receptor complex, which contains IL-4Rα and IL-13Rα1. A third, high-affinity IL-13 receptor, IL-13Rα2, also exists. Although it was initially felt to be a decoy receptor, this has not been formally demonstrated and the role(s) of this receptor has recently become controversial. To define the role(s) of IL-13Rα2 in IL-13-induced pulmonary inflammation and remodeling, we compared the effects of lung-targeted transgenic IL-13 in mice with wild-type and null IL-13Rα2 loci. We also investigated the effect of IL-13Rα2 deficiency on the OVA-induced inflammatory response. In this study, we show that in the absence of IL-13Rα2, IL-13-induced pulmonary inflammation, mucus metaplasia, subepithelial fibrosis, and airway remodeling are significantly augmented. These changes were accompanied by increased expression and production of chemokines, proteases, mucin genes, and TGF-β1. Similarly, an enhanced inflammatory response was observed in an OVA-induced phenotype. In contrast, disruption of IL-13Rα2 had no effect on the tissue effects of lung-targeted transgenic IL-4. Thus, IL-13Rα2 is a selective and powerful inhibitor of IL-13-induced inflammatory, remodeling, and physiologic responses in the murine lung.

The STAT3 transcription factor is critical for cytokine signaling and the acute phase response, but its role in allergic asthma is largely undefined. To investigate the role of STAT3 in mediating allergic inflammation, we used chemical and genetic approaches to inactivate STAT3 in the airway epithelium of mice. In a murine model of chronic asthma, we demonstrate that the administration of house dust mite (HDM) leads to robust STAT3 activation in the airway epithelium, smooth muscle, and immune cells in the lungs of C57BL/6 mice. To investigate the role of STAT3 in HDM-induced airway inflammation, a conditional knockout of STAT3 in the airway epithelium was generated, e-STAT3−/−. We determined that e-STAT3−/− mice had a significant decrease in HDM-induced airway eosinophilia, lung Th2 accumulation, and chemokines compared with wild-type animals. Importantly, the e-STAT3−/− mice had a significant decrease in airway hyperresponsiveness to methacholine. The administration of two STAT kinase inhibitors diminished STAT3 activation and markedly abrogated the HDM-induced lung inflammation. These findings suggest that STAT3 acts as a novel epithelial regulator of the allergic response by altering Th2 cell recruitment and effector function, and thus, targeting this molecule may provide the basis for a novel asthma therapy.

In animals with acute airway inflammation followed by repeated exposure to inhaled Ag, inflammation wanes over time and thus limits the study of chronic airway inflammatory diseases such as asthma. We developed a model of airway inflammation and inhalational exposure to investigate regulatory pathways in the respiratory tract. We show that Th1- and Th2-induced airway inflammation followed by repeated exposure to inhaled Ag leads to a state of immunosuppression. Challenge of these animals with a marked population of TCR transgenic effector Th1 or Th2 cells results in a striking inhibition of inflammation and effector Th cells. In Th2 models, airway hyperresponsiveness, mucus, and eosinophilia are reduced. The inhibitory effects observed are Ag nonspecific, can be induced in lymphocyte-deficient mice, and are associated with a population of TGF-β1-expressing macrophages. Induction of this pathway may offer potent localized treatment of chronic T cell-mediated respiratory illnesses and provide insights into the development of such diseases.

Airway eosinophilia in asthma is dependent on cytokines secreted by Th2 cells, including IL-5 and IL-4. In these studies we investigated why the absence of IL-4 led to a reduction in airway, but not lung tissue, eosinophils. Using adoptively transferred, in vitro-generated TCR-transgenic Th2 cells deficient in IL-4, we show that this effect is independent of IL-5 and Th2 cell generation. Airway eosinophilia was no longer inhibited when IL-4−/− Th2 cells were transferred into IFN-γR−/− mice, indicating that IFN-γ was responsible for reducing airway eosinophils in the absence of IL-4. Intranasal administration of IFN-γ to mice after IL-4+/+ Th2 cell transfer also caused a reduction in airway, but not lung parenchymal, eosinophils. These studies show that IL-4 indirectly promotes airway eosinophilia by suppressing the production of IFN-γ. IFN-γ reduces airway eosinophils by engaging its receptor on hemopoietic cells, possibly the eosinophil itself. These studies capitalize on the complex counterregulatory effects of Th1 and Th2 cytokines in vivo and clarify how IL-4 influences lung eosinophilia. We define a new regulatory role for IFN-γ, demonstrating that eosinophilic inflammation is differentially regulated at distinct sites within the respiratory tract.

We investigated the role of Th1 or Th2 cells in airway hyperresponsiveness (AHR), because both IFN-γ and IL-4 and IL-5-producing CD4 T cells have been identified in the airways of asthmatics. After transfer of in vitro-generated TCR transgenic Th1 or Th2 cells and exposure to inhaled Ag, Th2 cells induced AHR and airway eosinophilia, whereas Th1 cells induced neutrophilic inflammation without AHR. Next, to determine the precise effector function of IL-4 in Th2 cell-induced AHR, we transferred IL-4−/− Th2 cells into wild-type and IL-4−/− recipient mice. After exposure to inhaled Ag, both groups of mice exhibited AHR with markedly reduced airway eosinophilia. Thus, IL-4 production by Th2 cells is not essential for the induction of AHR, but is critical for the migration of eosinophils from lung tissue into the airways.

IFN-γ produced by Th1 cells inhibits Th2-induced allergic airway disease by decreasing airway eosinophilia, mucus production and chitinase activity. To define pathways by which IFN-γ inhibits allergic airway inflammation, we generated bone marrow chimeras with IFN-γ receptor (IFN-γR) deficient mice, transferred OVA-specific Th1, Th2 or Th1+Th2 cells and exposed mice to inhaled OVA. In mice receiving Th1+Th2 cells, IFN-γR was required on both hematopoietic and non-hematopoietic cells for maximal inhibition of mucus and chitinase activity, while IFN-γR expression on hematopoietic cells was sufficient for inhibition of airway eosinophilia. We hypothesized that airway epithelial cells are essential non-hematopoietic cells required to mediate the inhibitory effects of IFN-γ during airway inflammation. We generated epi-IFN-γR mice that express IFN-γR only on airway epithelial cells. Epi-IFN-γR mice that received Th1+Th2 cells showed near total inhibition of mucus, suggesting that IFN-γ directly inhibits mucus in epithelial cells. In epi-IFN-γR mice that received Th1+Th2 cells there was partial inhibition of chitinase activity, likely due to direct effects of IFN-γ on the epithelium. In epi-IFN-γR mice, airway eosinophilia was moderately inhibited through effects on eosinophil development in the bone marrow. These studies show the importance of airway epithelial cells in the regulation of allergic airway disease and highlight the epithelium as a target for therapy in airway disease.