Etanercept prevents airway hyperresponsiveness by protecting neuronal M2 muscarinic receptors in antigen-challenged guinea pigs

Background and purpose: Increased tumour necrosis factor-alpha (TNF-alpha) is associated with airway hyperreactivity in antigen-challenged animals. In human asthmatics, TNF-alpha is increased and blocking it prevents airway hyperreactivity in some asthmatic patients. However, the mechanisms by which TNF-alpha 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-alpha 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-alpha 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-alpha decreased M-2 receptor mRNA in human and guinea pig parasympathetic neurons. Conclusions and implications: Tumour necrosis factor-alpha 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.