Interleukin-1β and tumour necrosis factor-α as inducers of airway hyperresponsiveness and microvascular leakage.

Abstract

In asthma the development of airway hyperresponsiveness (AHR) has been linked to the presence of airway inflammation, however, the mechanism by which inflammation leads to AHR remains elusive. There is a school of thought that AHR, in vivo, is not due to an increased sensitivity of airway smooth muscle (ASM) to various contractile agonists, but rather, is a property of intact airways which have been structurally changed by chronic inflammation which then allows them to narrow excessively. The primary structural change which is considered to be important in this scenario, is an increase in airway wall thickness. Thickening of the airway wall may result from several mechanisms, one being mucosal oedema. This thesis is principally an examination of the effects of the pro-inflammatory cytokines, tumour necrosis factor-α (TNFα) and interleukin-1β (IL-1β), on two mechanisms considered to be important in the development of AHR. These being i) an increased contractility of ASM and ii) the development of microvascular leakage (MVL). The former was examined using an ovine model of in vitro contractility and the latter examined in vivo using an isolated tracheal segment (ITS) in the guinea pig. Following the detailed characterization of contractile responses of ovine ASM to various agonists, it was determined that TNFα enhances acetylcholine (ACh) contractility in ovine tracheal smooth muscle and that this effect is synergistic in the presence of IL-1β. These studies were then extended to examine mechanisms by which TNFα and IL-1β enhance ASM contractility in vitro. In this series of experiments contractile responses to neurokinin A (NKA) and histamine were also examined. Pre-incubation of ASM with TNFα and IL-1β caused a significant leftward shift in, and an increased in the magnitude of, the concentration-response curves to both ACh and NKA. Whereas, histamine contractility remained unchanged following cytokine incubation. There was no functional alteration to specific M₃ and NK₂ receptors, as reflected by antagonist affinity studies, following cytokine exposure. Neurokinin A contractility in the presence of phosphoramidon indicated that the enhanced contractility following cytokine exposure was not due to a reduction in endogenous neutral endopeptidase activity. Removal of extracellular calcium ions attenuated the contractile response to low concentrations of ACh in control and cytokine pretreated tissue. However, enhanced contractility following TNFα and IL-1β pretreatment was still present. These results demonstrate that ovine tracheal smooth muscle becomes hyperresponsive to ACh and NKA following TNFα and IL-1β exposure, via a mechanism involving intracellular calcium mobilization. In calcium-free Krebs-Henseleit solution the contractile response to histamine in tracheal muscle strips was virtually abolished. This suggests that histamine contractions in ovine ASM are predominantly mediated by extracellular calcium influx. And furthermore, the lack of an enhancement to histamine contractility following cytokine pretreatment appears to reflect agonist-specific mechanistic differences in post-receptor signalling pathways that mediate ovine ASM contraction. Finally, the effect of a combination of TNFα and IL-1β on airway MV/L was examined. Airway MVL was quantitated by determining Evans blue (EB) extravasation in an isolated segment of trachea, in anesthetised mechanically ventilated guinea pigs. The data demonstrate that TNFα and IL-1β aerosol challenge significantly increased EB extravasation compared to saline control challenge. In summary, IL-1β and TNFα are capable of enhancing ASM contractility to a variety of agonists and inducing MVL. The former mediated by a mechanism involving intracellular calcium mobilization. The results of these studies have important implications in acute exacerbations of asthma, where peribronchial oedema in conjunction with potentiated bronchoconstrictor responses, may induce rapid airway instability by unlinking the airways from the parenchyma, leading to life threatening bronchoconstriction.