Etanercept prevents airway hyperresponsiveness by protecting neuronal M ₂ muscarinic receptors in antigen-challenged guinea pigs

Background and purpose: Increased tumour necrosis factor-α (TNF-α) is associated with airway hyperreactivity in antigen-challenged animals. In human asthmatics, TNF-α is increased and blocking it prevents airway hyperreactivity in some asthmatic patients. However, the mechanisms by which TNF-α mediates hyperreactivity are unknown. Airway hyperreactivity can be caused by dysfunction of neuronal M ₂ muscarinic receptors that normally limit acetylcholine release from parasympathetic nerves. Here we test whether blocking TNF-α receptors with etanercept prevents M ₂ receptor dysfunction and airway hyperreactivity in antigen-challenged guinea pigs. Experimental approach: Ovalbumin-sensitized guinea pigs were challenged by inhalation of antigen. Some animals received etanercept (3 mg kg ^-1 i.p.) 3 h before challenge. 24 h after challenge, airway hyperreactivity and M ₂ receptor function were tested. Inflammatory cells in bronchoalveolar lavage, blood and lung were counted. TNF-α and its receptors were detected by real-time RT-PCR and immunocytochemistry in parasympathetic nerves from humans and guinea pigs and in human neuroblastoma cells. Key results: Antigen-challenged animals were hyperreactive to vagal stimulation and neuronal M ₂ receptors were dysfunctional. Both M ₂ receptor dysfunction and airway hyperreactivity were prevented by etanercept. Etanercept reduced eosinophils around airway nerves, and in blood, bronchoalveolar lavage and airway smooth muscle. Also, TNF-α decreased M ₂ receptor mRNA in human and guinea pig parasympathetic neurons. Conclusions and implications: Tumour necrosis factor-α may contribute to M ₂ receptor dysfunction and airway hyperreactivity directly by decreasing receptor expression and indirectly by promoting recruitment of eosinophils, containing major basic protein, an M ₂ antagonist. This suggests that etanercept may be beneficial in treatment of allergic asthma.