The role of IL-33 and ST2 in allergic airways disease

Asthma is a chronic disease characterised by variable airflow obstruction, bronchial hyperresponsiveness and airways inflammation. At an immunological level Th2 inflammation and the presence of activated eosinophils and mast cells are key features of asthma. ST2, the receptor for the novel cytokine IL-33, is expressed upon Th2 lymphocytes and mast cells but its role in clinical and experimental asthma remains unclear. IL-33 has been shown to induce local and systemic eosinophilia when administered to the peritoneum of mice. In this thesis I have set out to test the hypothesis that the activation of mast cells by IL-33 acting on cell surface ST2 plays a critical role in allergic airways inflammation. I began by studying the function of ST2 on mast cells in vitro. I found that ST2 was expressed at an early stage of development, and correlated closely with the expression of the stem cell factor receptor (c-kit), a marker present on mast cells from a progenitor stage. Despite this mast cells generated form ST2 gene deleted mice proliferated and matured normally. When mast cells were activated by IL-33, acting in an ST2-dependent manner, pro-inflammatory cytokines and chemokines were released that have potential roles in asthma, specifically IL-6, IL-13, MIP-1α and MCP-1. To extend these findings I looked at the role of ST2 in allergic airways inflammation. I first optimised and validated an ovalbumin and adjuvant based ‘short’ twelve day model of murine asthma and demonstrated that ST2 gene deletion results in attenuated eosinophilic inflammation. In addition to being ST2 dependent it is possible that this adjuvant based short model is mast cell dependent, unlike longer adjuvant based models which are mast cell and ST2 independent. Therefore I went on to study an adjuvant-free model of asthma which has been demonstrated to be mast cell dependent. In this adjuvant-free model of asthma the airway inflammation was attenuated in ST2 gene deficient mice compared with wild type mice, while AHR was unaffected. There was an associated reduction in IgE production and thoracic lymph node recall Th2 cytokine responses. I then examined the effect of ST2 activation in the lungs. When IL-33 was administered directly to the airways of naive mice it induced the features of experimental asthma. There was extensive eosinophilic inflammation within the lung tissue and airspaces. The Th2 cytokines IL-5 and IL-13, and the eosinophil chemoattractant chemokines eotaxin-1 and eotaxin-2 were detected at increased concentrations. Significant airways hyperresponsiveness was also generated. Using ST2 gene deleted mice I demonstrated that these effects were ST2 specific. Although I have shown that mast cells are activated by IL-33 in vitro, I used mast cell deficient mice to demonstrate that the eosinophilic inflammation generated by IL-33 is unaffected by the absence of mast cells. These data show that IL-33 can induce in the lungs the cardinal pathological characteristics of asthma, and that it appears to act upstream of other important mediators such as IL-13 and the eotaxins. Furthermore the IL-33 receptor ST2 is required in an adjuvant free model of asthma, which is more akin to human disease. Placing these findings in the context of recent evidence that IL-33 is released by structural cells in response to damage or injury suggests that IL-33 may play a key role in initiating the immunological features of clinical asthma. As a consequence of this position in the hierarchy of inflammation IL-33 offers a promising direct target for novel biological therapies in asthma.