Airway hyperresponsiveness and viral recognizing toll-like receptors

Abstract

Common colds caused by viruses are, in healthy individuals, usually self-limited and with relatively mild symptoms. In asthmatic individuals, respiratory viruses can burst the defense immune system and trigger asthma exacerbations, including causing airway hyperresponsiveness (AHR). AHR is defined as the predisposition of the airways to contract excessively in response to stimuli that would produce little or no effect in healthy persons. The mechanism by which infections alter AHR is unclear. Toll-like receptors (TLRs) are at the forefront of our microbial defense and studies suggest that TLRs might have a role in the development of AHR. The present thesis investigates the impact of TLR ligands (TLR 3, 4, 7, and 9).

The presented data are derived from experiments using TLR ligands in mice and guinea pig models of AHR with and without concomitant local inflammation. The latter was induced by ovalbumin (OVA) exposure. The result is presented in five papers. The first paper explored the relationship between microbial stimulation and the development of AHR by applying either poly(I:C) activating TLR3 or LPS triggering TLR4, representing viral and bacterial-induced interactions, respectively, intranasally in vivo. The second paper repeated these experiments in mice with pre-established, OVA-induced allergic lung inflammation. The activation of TLR3 or TLR4, caused AHR, with local inflammatory profiles characterized by inflammatory cell recruitment and cytokine release that appeared to be receptor specific. Paper III explored the effects of concomitant TLR3 and TLR4 stimulation. Four days of consecutive treatment in naïve mice triggered an AHR along with an increased influx of inflammatory cells and enhanced release of proinflammatory cytokines including TNFα in bronchioalveolar lavage fluid (BALF). In mice, with OVA-induced allergic airway inflammation, the combined TLR3 and TLR4 stimulation caused a further increase of AHR in the peripheral lung. Treatment with TNFα-blocking antibody infliximab given to allergic mice blunted tissue damping (G) and tissue elastance (H) without affecting the influx of the cells in BALF. This inhibitory effect in the peripheral airways was not found in naïve mice. Paper IV evaluated how in vivo administration of the viral TLR agonists TLR7 or TLR9, given to mice with allergic lung inflammation, affects AHR and airway inflammation. The challenge with the TLR7 agonist reduced AHR and airway inflammation in asthmatic mice, whereas TLR9 activation showed the opposite effect with a more profound inflammation. Paper V used an in vivo model to investigate whether the TLR7 agonist Imiquimod could induce bronchorelaxation by acting directly on airway smooth muscle. Imiquimod was shown to relax guinea pig airways pre-contracted with histamine within seconds. Unexpectedly, the bronchodilatory effect was found to be independent of TLR7. A quinolone moiety in imiquimod also found in other bronchodilatory compounds, like quinine and chloroquine, may contribute to the bronchodilatory property.

To conclude; Challenge with TLR agonists can increase AHR in allergic airways which is confirming that TLRs can play a pivotal role in the responsiveness following microbial infection in asthmatics. An exacerbated AHR does not need to be associated with increased inflammation and the results suggest that at least part of the effect can be a direct-acting effect on the ASM. Blocking of TNFα showed a therapeutic effect on peripheral airways in allergic animals during exposure to viral and bacterial TLR agonists. This indicates TNFα blockade as a treatment option. Challenge with a viral TLR7 agonist reduced AHR and airway inflammation in asthmatic mice. Opposite effects of viral TLR9 agonist call attention to the complexity of TLR interaction. TLR7 agonists with quinoline moieties can be of interest in the development of drugs to treat asthma as they can reduce AHR and induce bronchodilatation.