The role of immune mediators in airway inflammation
Asthma is a chronic inflammatory condition of the airways characterised by reversible airflow obstruction, airway hyper-responsiveness and inflammatory infiltrates in the airway walls containing eosinophils, T lymphocytes and mast cells. T helper (Th) lymphocyte subsets, defined by the cytokines they secrete, are thought to play a key role in the in the initiation and perpetuation of chronic airway inflammation. Th2 cells, producing interleukin (IL)-4, IL-5, IL-9 and IL-13, are thought to be of particular importance. In contrast, Thl cells producing interferon (IFN)-y may counteract the development of Th2 responses and so down-regulate the asthmatic response. The prevalence of asthma is increasing but the reasons for this are not fully understood. In addition, some patients do not respond adequately to treatment with corticosteroids, currently the most effective anti-inflammatory agents used routinely in human asthma. There is therefore continual interest in developing new therapeutic agents for asthma. A greater understanding of the regulation of inflammatory responses in asthma will assist in the identification of potential targets for therapeutic intervention. The aims of this thesis were (i) to assess the role of the cytokine IL-18 in allergic airway inflammation by determining IL-18 levels in induced sputum in asthmatic subjects in comparison to normal subjects, and by studies in a murine model of allergic asthma using IL-18 gene deficient mice and (ii) to assess the potential antiinflammatory actions of simvastatin and thymosin beta 4 sulfoxide in the murine asthma model. IL-18 is a pro-inflammatory cytokine which can promote IFN-y secretion and, in association with IL-12, enhance the development of Thl responses. However, in some circumstances it may also stimulate Th2 responses. IL-18 therefore has the potential to suppress or exacerbate allergic airway inflammation. The role of IL-18 in both clinical and experimental asthma remains unclear. Statins are inhibitors of the rate-limiting enzyme, 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, in cholesterol biosynthesis. As such they have been widely used as cholesterol lowering agents in clinical practice. They have previously been shown to have anti-inflammatory properties independent of their cholesterol-lowering ability in clinical studies of atherosclerotic disease and in animal models of Thlmediated inflammation. Thymosin beta 4 sulfoxide (T~4S0) is a 5 kDa peptide. Intracellularly its principal activity is to regulate actin polymerization. Corticosteroid treatment of monocytes in vitro induces the release of T~4S0 extracellularly, where it can inhibit neutrophil chemotaxis. Exogenous administration of T~4S0 has been shown to reduce neutrophilic inflammation in animal models. In this study it is shown that IL-18 is detectable in induced sputum fluid and IL-18 mRNA is expressed in induced sputum cells from asthmatic and nOlmal subjects. IL- 18 protein levels in induced sputum, and IL-18 mRNA expression in induced sputum cells were not significantly different between these groups. IL-18 production was localised to sputum macrophages. However, cigarette smoking significantly reduced IL-18 levels in induced sputum fluid in both asthmatic and normal subjects. In asthmatics, but not normal subjects, the reduction in IL-18 levels in sputum fluid was associated with reduced IL-18 mRNA expression in induced sputum cells. A murine model of allergic asthma, using BALB/C mice sensitised and challenged with ovalbumin (OVA), was used to examine the role of IL-18 in allergic responses in vivo. IL-18 gene knockout (ko) had significantly reduced bronchoalveolar lavage (BAL) total cell count and eosinophilia compared to wild-type (WT) mice. IL-18 ko mice had reduced IL-4 expression in thoracic lymph nodes, as assessed by quantitative peR, and significantly reduced OVA-specific IL-4 secretion from thoracic lymph node cultures assessed by ELISA. Serum OVA-specific IgG 1, IgG2a and IgE and total IgE levels were not significantly different between IL-18 ko and WT mice. The murine model of allergic asthma was also used to examine the anti-inflammatory activities of simvastatin and T~4S0 in a Th2-mediated, eosinophilic condition. Simvastatin treatment, either orally or intraperitoneally, and T~4S0 intraperitoneally reduced the total inflammatory cell infiltrate and eosinophilia in BAL fluid in response to inhaled OV A challenge. At higher doses of simvastatin intraperitoneally, a histological reduction in inflammatory infiltrates in the lungs was observed. Treatment with simvastatin intraperitoneally, but not orally, and T~4S0 were also associated with a reduction in IL-4 and IL-5 levels in BAL fluid. OVA-induced IL-4 and IL-5 secretion was reduced in thoracic lymph node cultures from both simvastatin-treated and T~4S0-treated mice. Neither simvastatin nor T~4S0 treatment altered serum total IgE or OVA-specific IgG 1 and IgG2a levels. The results described show that IL-18 can be detected in the induced sputum fluid of asthmatic and normal subjects and that cigarette smoking significantly reduces its levels. Studies in a murine model of allergic asthma suggest that IL-18 has a proinflammatory role in allergic airway inflammation, at least in part through its ability to induce IL-4 secretion. Both simvastatin and thymosin beta 4 sulfoxide had convincing anti-inflammatory properties in the murine model of asthma used, and these agents, or related compounds, may have therapeutic potential in human asthma.