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

Abstract

Their recently reported ability to rapidly relax airways has further increased their interest in the treatment of pulmonary disease. However, the mechanisms behind this effect are not fully understood. The present study, therefore, aimed to determine whether airway smooth muscle (ASM)-dependent mechanisms could be identified. TLR7 agonists were added to guinea pig airways following precontraction with carbachol in vitro or histamine in vivo. Pharmacological inhibitors were used to dissect conventional pathways of bronchodilation; tetrodotoxin was used or bilateral vagotomy was performed to assess neuronal involvement. Human ASM cells (HASMCs) were employed to determine the effect of TLR7 agonists on intracellular Ca 2ϩ ([Ca 2ϩ ]i) mobilization. The well-established TLR7 agonist imiquimod rapidly relaxed precontracted airways in vitro and in vivo. This relaxation was demonstrated to be independent of nitric oxide, carbon monoxide, and cAMP signaling, as well as neuronal activity. A limited role for prostanoids could be detected. Imiquimod induced [Ca 2ϩ ]i release from endoplasmic reticulum stores in HASMCs, inhibiting histamine-induced [Ca 2ϩ ]i. The TLR7 antagonist IRS661 failed to inhibit relaxation, and the structurally dissimilar agonist CL264 did not relax airways or inhibit [Ca 2ϩ ]i. This study shows that imiquimod acts directly on ASM to induce bronchorelaxation, via a TLR7-independent release of [Ca 2ϩ ]i. The effect is paralleled by other bronchorelaxant compounds, like chloroquine, which, like imiquimod, but unlike CL264, contains the chemical structure quinoline. Compounds with quinoline moieties may be of interest in the development of multifunctional drugs to treat pulmonary disease. asthma; bronchodilation; imiquimod; quinoline; toll-like receptor 7

The bronchodilatory capacity of imiquimod: the existence of two mechanisms

“…In addition, activation of sensory neurons by imiquimod has been shown to be TLR7 independent (6), which suggests that a TLR7-dependent neuronal mechanism of relaxation does not occur via sensory neurons. We could further demonstrate in our study that all nerve-mediated relaxation was blocked by TTX (9). NOreleasing inhibitory nonadrenergic, noncholinergic (iNANC) fibers have repeatedly been shown to be TTX sensitive (10), suggesting that our use of TTX was sufficient to evaluate and discount the possible contribution of NO-mediated neuronal relaxation, the mechanism proposed by Drake and colleagues.…”

“…TO THE EDITOR: We recently published a study investigating the bronchodilatory capacity of the Toll-like receptor 7 (TLR7) agonist imiquimod, establishing the presence of a nonneuronal, TLR7-independent mechanism of dilation (9). Our study adds new information to the previously published reports, which demonstrated that relaxation can be mediated by a TLR7-dependent release of nitric oxide (NO) from innervating nerves (2,5).…”

“…In our study, we demonstrated that administration of the neurotoxin tetrodotoxin (TTX) in vitro had no effect on imiquimod-induced bronchodilation and thus concluded that neuronal pathways were not involved in dilation (9). Due to the presence of TTX-resistant nerves, particularly those originating from the nodose ganglia (8), Drake suggests that this is not sufficient to discount neuronal involvement.…”

TLR7 agonist attenuates acetylcholine‐induced, Ca2+‐dependent ionic currents in swine tracheal submucosal gland cells

“…Although the effects of TLR9 on the activity of SERCA2 are different from ours, these findings suggested that an alternative TLR signalling pathway, in addition to the canonical TLR-nuclear factor-B axis, likely exists. In contrast, Larsson and colleagues have recently reported that R837 may induce [Ca 2+ ] i release from ER stores in human airway smooth muscle cells (ASMCs) via a TLR7-independent mechanism (Larsson et al, 2016). Although we are not fully able to explain this discrepancy, this is probably because our study was performed in a different manner.…”

The impact of asthma, toll-like receptors 7 and 8, genetic and clinical determinants on antiviral immune response

“…In addition to the Th2 modulating effect, TLR7 agonists relax human airway smooth muscle strips in vitro (Drake et al 2013) previously modelled only in animals (Drake et al 2013;Ekman, Adner & Cardell 2011;Kaufman, Fryer & Jacoby 2011). Interestingly, the mechanism of bronchodilation may be independent of TLR7 signalling and instead signify an off-target effect of the quinoline containing TLR7 agonists (Larsson et al 2016), further supporting the development of TLR7/8 agonists as asthma medications.…”