The role of immune mediators in airway inflammation

McKay, Anne (2004) The role of immune mediators in airway inflammation. PhD thesis, University of Glasgow.

Abstract

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.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: R Medicine > R Medicine (General)
• Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Infection & Immunity
• Supervisor's Name: E., Prof. Lewin and N., Prof. Thomson
• Date of Award: 2004
• Depositing User: Miss Louise Annan
• Unique ID: glathesis:2004-4828
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2014 12:35
• Last Modified: 14 Jan 2014 12:35
• URI: https://theses.gla.ac.uk/id/eprint/4828