Toll-like Receptor 7 Rapidly Relaxes Human Airways

Abstract

Rationale: Toll-like receptors (TLRs) 7 and 8 detect respiratory virus single-stranded RNA and trigger an innate immune response. We recently described rapid TLR7-mediated bronchodilation in guinea pigs. Objectives: To characterize TLR7 expression and TLR7-induced airway relaxation in humans and in eosinophilic airway inflammation in guinea pigs. To evaluate the relaxant effects of other TLRs. Methods: Human airway smooth muscle strips were contracted with methacholine in vitro, and responses to TLR7 and TLR8 agonists were assessed. TLR7-mediated nitric oxide production was measured using a fluorescent indicator, and TLR7 expression was characterized using immunofluorescence. TLR7 signaling was also evaluated in ovalbumin-challenged guinea pigs. Measurements and Main Results: The TLR7 agonist imiquimod (R837) caused rapid dose-dependent relaxation of methacholine-contracted human airways in vitro. This was blocked by the TLR7 antagonist IRS661 and by inhibiting nitric oxide production but not by inhibiting prostaglandin production. TLR7 activation markedly increased fluorescence of a nitric oxide detector. TLR7 was expressed on airway nerves, but not airway smooth muscle, implicating airway nerves as the source of TLR7-induced nitric oxide production. TLR7-mediated relaxation persisted in inflamed guinea pigs airways in vivo. The TLR8 agonists polyuridylic acid and polyadenylic acid also relaxed human airways, and this was not blocked by the TLR7 antagonist or by blocking nitric oxide or prostaglandin production. No other TLRs relaxed the airways. Conclusions: TLR7 is expressed on airway nerves and mediates relaxation of human and animal airways through nitric oxide production. TLR7-mediated bronchodilation may be a new therapeutic strategy in asthma.Keywords: Toll-like receptor 7; imiquimod; nitric oxide; asthma; nerve Toll-like receptor (TLR) 7 is a pattern recognition receptor in the lungs (1-3) that detects single-stranded RNA genomes, common to many respiratory viruses (4). Respiratory viruses cause airway hyperreactivity (an abnormal tendency to contract) (5, 6) and asthma exacerbations (7). A role for TLR7 in the development of virus-induced airway hyperreactivity has been proposed, because TLR7 activation causes a proinflammatory type 1 T-helper cell (Th1) immune response (8) and because TLR7 polymorphisms have been associated with asthma (9). In patients with asthma, however, TLR7 responses were reduced (10). Furthermore, in mouse models of allergic airway inflammation, the TLR7 imidazoquinoline agonists imiquimod (R837) and resiquimod (R848) reduced airway hyperreactivity, airway eosinophilia, goblet cell hyperplasia, and smooth muscle hypertrophy (11-14), whereas TLR7 knockout mice were prone to the development of asthma-like airway pathology after pneumovirus infection (15). These studies suggest that TLR7 may be protective against the development of allergic type 2 T-helper cell airway inflammation, possibly by altering the Th1 versus Th2 immune balance (16). Similarly, a TLR7 agonist is currently i...

The TLR7 agonist imiquimod induces bronchodilation via a nonneuronal TLR7-independent mechanism: a possible role for quinoline in airway dilation

“…Their recently reported ability to rapidly relax airways (11,22) has increased their interest in the treatment of pulmonary disease. The present study verified the ability for imiquimod to induce a strong and rapid dilatory effect in isolated guinea pig airways.…”

“…Using our in vitro and in vivo models, we next examined molecules and associated signaling pathways involved in bronchodilation, including the role of NO and prostanoids, previously implicated in imiquimod-mediated bronchodilation (11,22), CO, cGMP, PKG, and PKA.…”

“…TLR7 agonists are of particular interest as novel studies have demonstrated that the prototypical agonist imiquimod (R-837) also harbors the ability to induce rapid airway relaxations in vitro, as well as in vivo (11,22). This further highlights a role for the use of these agonists in the treatment of asthma.…”

Update in Asthma 2013

“…Three studies were published in the Journal in 2013 that identified new factors that may affect airway smooth muscle contraction and relaxation (55)(56)(57). Two of the studies applied multiple modalities to mouse, guinea pig, and human tissues and identified transmembrane protein 16A, a calcium-activated chloride channel, and Toll-like receptor 7, respectively, as factors that modify the response of airway smooth muscle to contractile agents (56,57).…”

“…Two of the studies applied multiple modalities to mouse, guinea pig, and human tissues and identified transmembrane protein 16A, a calcium-activated chloride channel, and Toll-like receptor 7, respectively, as factors that modify the response of airway smooth muscle to contractile agents (56,57). The third study combined laser capture microdissection of human airway biopsies and RNA sequencing to identify two genes (FAM129A and SYNPO2) that were significantly changed in expression with corticosteroids in asthma and which correlated with degree of airway hyper-responsiveness (55).…”