Neuronal toll-like receptor (TLR) 7 expression and function in human airways

BACKGROUND : TLR7 detects viral single-stranded RNA and triggers an innate immune response. We recently showed that TLR7 agonists relax guinea pig, mouse, and human airways within seconds. We investigated TLR7 expression and signaling in post-mortem human airways. METHODS : Human trachea and primary bronchi were immunostained with anti-TLR7 antibodies, neuronal marker PGP9.5 and nuclear stain DAPI, and were imaged by confocal microscopy (20X, 63X; 1.4 numerical aperture). TLR7 function was assessed in human airway strips suspended in an organ bath. Airways were contracted with methacholine (10 µM) and exposed to TLR7 agonist (R837; 10-300 µM). Some airways were pretreated with the TLR7 antagonist (IRS661; 100 µM) or nitric oxide synthase inhibitor (L-NMMA; 100 µM). RESULTS : TLR7 (labeled yellow) was expressed on airway nerves (labeled red), but not airway smooth muscle (labeled blue). TLR7 was expressed on nerve cell bodies located within the airway wall, consistent with parasympathetic nerve distribution. TLR7 agonist caused dose-dependent relaxation of pre-contracted human airways that was blocked by the TLR7 antagonist IRS661, and by inhibition of nitric oxide synthase. ![Figure][1] CONCLUSIONS : Neuronal TLR7 expression suggests airway nerves have a novel role in the immune response to respiratory viruses, and can induce bronchorelaxation via the production of nitric oxide. [1]: pending:yes