Abstract Airway hyperreactivity (AHR), the heightened responsiveness to bronchospastic agents, is a hallmark of allergic asthma. Although acute bronchospasm may be temporarily reversed by bronchodilators, no existing therapeutics effectively target AHR, which is largely unaffected by anti-inflammatory treatments including corticosteroids. The airways are densely innervated by vagal sensory afferent nerves, many of which express the capsaicin-sensitive TRPV1 and are activated by irritants and inflammation. Activation of such afferents can trigger reflex bronchospasm. House dust mite (HDM) is a major clinical cause of allergic asthma. Interestingly, exposure to HDM can evoke cough, rhinitis and wheeze prior to the development of Type 1 hypersensitivity, suggesting a direct effect of HDM components on nerves within the airways. We hypothesized that chronic HDM induces the development of AHR via the direct activation of vagal afferents, independently of IgE-mediated allergic inflammation. We analyzed lung afferent activation using multiphoton microscopy imaging of Ca2+-sensitive GCaMP6s fluorescence in vagal neurons of naïve PirtCre-GCaMP6s mice. HDM extract infused into the lungs caused robust activation of the majority of naïve airway-specific TRPV1+ neurons. HDM extract is composed of proteases and can therefore activate protease-activated receptors (PARs), a type of G-protein coupled receptor (GPCR). Using single neuron RT-PCR, we found that all airway TRPV1+ neurons expressed PAR1, whereas only 5% expressed PAR2, and PAR3 and PAR4 were not expressed. Many TRPV1+ vagal afferents release neuropeptides upon activation. We found that HDM extract evoked neuropeptide release from human tracheal rings (provided by LifeLink, from non-transplantable lungs), and this was inhibited by a PAR1 antagonist SCH79797. Lastly, we created a sensory neuronal conditional knockout of PAR1 using PirtCre mice (Pirt is only expressed in sensory afferent nerves). 13 days exposure of control PAR1-floxed mice to HDM extract (intratracheally, no adjuvant) induced lung inflammation/eosinophilia and AHR compared to vehicle exposures, as measured by histology and flexivent assessment of methacholine-induced airway resistance, respectively. Identical HDM extract exposures to PAR1CKOmice induced comparable lung eosinophilia but failed to induce AHR. Our data suggests that activation of airway vagal afferent nerves via PAR1 triggered by HDM leads to HDM-induced airway hyperreactivity, independently of allergic inflammation. This abstract is funded by: NIH, Warren Alpert Distinguished Scholar Award
Lurye et al. (Fri,) studied this question.