Regulation of inflammatory responses in allergic asthma

Camporota, Luigi (2006) Regulation of inflammatory responses in allergic asthma. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Allergic asthma is the result of a Th2-mediated immune response against allergens, with Th2 cytokines, such as IL-4, IL-5 and II,-13 playing a crucial role. T helper cells are thought to be able to cross-regulate each other and a number of possible treatments for allergic disorders have been tested in order to either potentiate Thl responses or block Th2 cytokines or their receptors. In my thesis I have sought to inhibit Th2 cytokine responses and, thereby, modulate allergic responses, using either cytokine inhibitors or microbial agents which are known to down-regulate cytokine responses.

I have tested a soluble form of IL-4 receptor (sIL-4R) with the aim of blocking IL-4 and preventing the signalling cascade leading to cell activation and gene transcription. Recombinant human IL-4 caused an increase in IL-5 production by PBMC which peaked at 20 nM. sIL-4R caused a significant concentration-dependent inhibition of IL-5 secretion. The extent of inhibition with sIL-4R was comparable to that achieved by using anti IL-4 and IL-4R antibodies. Th2 down-regulation was not secondary to reduced survival of PBMC as tested in proliferation assays. Soluble IL-4R exerted a differential effect on Th2 and Th1 cytokines at low concentrations. It caused a significant (p<0.05) inhibition of Der p-induced release of IL-5 at 1 µg/ml, without affecting IFN-γ production. A 2.6-fold increase in IFN-γ concentration was observed only when sIL-4R was used at 10 µg/ml, suggesting that inhibition of Th2 cytokines is not necessarily associated with promotion of Thl responses.

To test the hypothesis that the non-pathogenic Mycobacterium vaccae could reduce airway inflammation and the asthmatic reaction in vivo following allergen challenge and the Th2 response of ex vivo challenged PBMC, I conducted a clinical trial where an M. vaccae extract, SRL172, was administered intradermally. M. vaccae was able to cause a reduction in the late asthmatic response (LAR). During the LAR, the mean maximum fall in FEV₁ was 35.8% [16.5-66.7%] in subjects receiving SRL172 and 29.5% [16.9-39.4%] in subjects receiving placebo. In a subgroup of patients with mild asthma, M. vaccae caused a mean 47.2% relative reduction in the AUC of the LAR (p=0.026). However, the difference between treatment groups did not achieve statistical significance. Similarly, no difference was found in the early asthmatic response, (expressed as maximum % fall in either FEV₁ or AUC) and in PC₂₀FEV₁ between the two treatment groups (p=0.98). Sputum analysis showed no difference in the differential cell count or in the levels of ECP and tryptase. Studies on peripheral blood mononuclear cells (PBMC) of these patients showed showed a trend towards reduction in IL-5 synthesis in vitro and serum IgE levels three weeks post-treatment with M. vaccae (p=0.07) but not placebo.