Targeting the IL-33/ST2 pathway in asthma : implications for development, exacerbation and treatment

Abstract

Asthma is a term that encompasses a disease spectrum with different phenotypes that vary in severity and whose common characteristic is airflow limitation. Allergic asthma is the most common phenotype of asthma and is characterized by allergen induced inflammatory responses, airway hyperresponsiveness (AHR), and remodeling. The limitations of current treatment together with the prospect of curative therapies set the incentive for identifying novel targets for asthma treatment. The epithelium-derived cytokine interleukin-33 (IL-33) and its receptor ST2 are implicated in the initiation and progression of asthma by several genetic and experimental studies. This thesis investigates the role of the IL-33/ST2 pathway in asthma development, exacerbation and treatment, using mouse models of asthma.

In paper I, we investigated the role of IL-33/ST2 signaling in promoting allergen-induced AHR, airway inflammation, and remodeling in a mouse model of asthma, in which wild-type and ST2-/- mice were exposed to intranasal instillations of house dust mite (HDM) extract. We revealed that ST2-dependent signaling is important for the development of AHR in the peripheral lung compartment and for the development of inflammatory responses including airway eosinophilia, induction of allergen-specific IgE, inflammation and goblet cell hyperplasia in the peripheral airways, and production of the cytokines IL-5, IL-13 and IL-33.

In paper II, we tested the hypothesis that IL-33/ST2-dependent mast cell responses contribute to the development of AHR and airway inflammation, using a model where the lungs of mast cell deficient mice were engrafted with either wild-type or ST2-/- bone marrow derived mast cells, and subsequently exposed to HDM. Unexpectedly, we discovered a protective role for ST2-dependent mast cell responses in the development of AHR located in the peripheral lung. This protective effect appeared to be independent of airway inflammation but was associated with elevated levels of PGE2, which has a bronchoprotective role in asthma.

In paper III, we explored the potential of IL-33 to exacerbate allergen-induced asthma responses by exposing OVA-sensitized mice to IL-33 before each antigen challenge. We found that IL-33 acted cooperatively with antigen to aggravate AHR, remodeling and several inflammatory responses, including a substantial potentiation of antigen-specific IgE antibody production, increased mast cell activity, elevated levels of the TH2 cytokines IL-4, IL-5 and IL-13, and accumulation of inflammatory cells in the airways and lung tissue, including expansion of the ILC2 population in the lungs.

In paper IV, we evaluated the effects of vaccination against IL-33 in a mouse model of HDM-induced asthma. The vaccine comprised a recombinant IL-33 protein modified to induce immunological memory response, while reducing its cytokine activity. Vaccination against IL-33 induced high titers of anti-IL-33 IgG antibodies, and attenuated several HDM-induced responses including AHR, airway eosinophilia, accumulation of inflammatory cells in the airways, and the levels of inflammatory cytokines including IL-25, IL-33 and TSLP.

In conclusion, the work presented in this thesis provides further evidence and new insights into the importance of the IL-33/ST2 pathway in the development of asthma. The studies of this thesis identify an important role for this pathway in the regulation of AHR in the peripheral lung compartment, a protective role on AHR mediated by mast cells, and a role in asthma exacerbations associated with the expansion of the ILC2 population. Finally, we demonstrate that targeting the IL-33/ST2 pathway by vaccination against IL-33 has the potential to be an effective therapeutic tool for treating asthma.