Neurotrophins and nitric oxide in relation to airway responses in experimental asthma in vivo

Abstract

Asthma is an inflammatory disease of allergic or non-allergic genesis characterized by intermittent reversible airway obstruction and variable degrees of persistent airway hyperresponsiveness (AHR). Increase in AHR likely arises from augmented airway inflammation, such as associated with a late airway response. Hallmarks of allergic inflammation are leukocyte accumulation in the lung, increased levels of nitric oxide (NO) in exhaled air, and changes such as airway smooth muscle contraction and oedema, leading to airflow limitation and AHR. The mechanisms behind AHR are not fully understood but involvement of various cells, including inflammatory cells and neuronal cells, and mediators, such as neurotrophins have been suggested.

The overall aim of this thesis was to identify novel targets to treat AHR and airway inflammatory diseases. A specific focus was to determine whether the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) have a role in development of AHR and, if so, how this is mediated. Another was to further understand how leukocyte recruitment to the lung develops during allergic asthma. A model for measurement of airway responses and their relationship with changes in pulmonary mechanics and exhaled NO in animals in vivo was established. By using this model in guinea pigs, the neurotrophins NGF and BDNF were shown to increase the bronchoconstricting response evoked by histamine and the tachykinin neurokinin A (NKA). The NGF- and BDNF-induced increase in airway responses in vivo was accompanied by decreased levels of NO in exhaled air. This suggests that NGF and BDNF may be mediators responsible for development of airway hyperresponsiveness. Blockade of the histamine H1-receptor by mepyramine abolished the NGF-induced AHR to NKA. When endogenous NO production was inhibited, BDNF induced an earlier histamine-evoked increase in airway responses. In sensitized rats, blockade of the cell adhesion molecule integrin á2 by inhaled antibodies prevented allergen exposure-induced recruitment of eosinophils and neutrophils to the airways, inhibited the increase in pro-inflammatory cytokines in broncheoalveolar lavage fluid and abolished inflammation-induced increase of NO in exhaled air.

The findings indicate that neurotrophins, including NGF and BDNF, which in asthma are increased in blood and airways, have an important role in development of AHR. Mast cells located in the airway mucosa are suggested to have a central role in the NGF-evoked AHR. NGF may facilitate the mast cells and make them more sensitive to NKA, resulting in NKA-induced mast cell degranulation and histamine release and an increased airway response caused by both NKA and histamine. These findings add support for an important role of neuro-immune cross talk in development of airway hyperresponsiveness in inflammatory airway diseases. Endogenous NO seems to exert a braking action on BDNF-induced enhancement of airway responses and a reduced ability to release NO may be one mechanism for AHR during elevated NGF or BDNF levels. Local administration of anti-á2 integrin antibody prevented allergic airway inflammation, likely by inhibition of leukocyte migration to the airways. Novel strategies to treat AHR and airway inflammation might evolve from further understanding of the roles of neurotrophins, NO and integrins, based on findings in this thesis.