Yost, Bethany L.

M2muscarinic receptors limit acetylcholine release from the pulmonary parasympathetic nerves. M2receptors are dysfunctional in antigen-challenged guinea pigs, causing increased vagally mediated bronchoconstriction. Dysfunction of these M2receptors is due to eosinophil major basic protein, which is an antagonist for M2receptors. Histamine-induced bronchoconstriction is composed of a vagal reflex in addition to its direct effect on airway smooth muscle. Because hyperreactivity to histamine is seen in antigen-challenged animals, we hypothesized that hyperreactivity to histamine may be due to increased vagally mediated bronchoconstriction caused by dysfunction of M2receptors. In anesthetized, antigen-challenged guinea pigs, histamine-induced bronchoconstriction was greater than that in control guinea pigs. After vagotomy or atropine treatment, the response to histamine in antigen-challenged animals was the same as that in control animals. In antigen-challenged animals, blockade of eosinophil influx into the airways or neutralization of eosinophil major basic protein prevented the development of hyperreactivity to histamine. Thus hyperreactivity to histamine in antigen-challenged guinea pigs is vagally mediated and dependent on eosinophil major basic protein.

Control of airway smooth muscle is provided by parasympathetic nerves that release acetylcholine onto M3 muscarinic receptors. Acetylcholine release is limited by inhibitory M2 muscarinic receptors. In antigen-challenged guinea pigs, hyperresponsiveness is due to blockade of neuronal M2 receptors by eosinophil major basic protein (MBP). Because exposure of guinea pigs to ozone also causes M2dysfunction and airway hyperresponsiveness, the role of eosinophils in ozone-induced hyperresponsiveness was tested. Animals were exposed to filtered air or to 2 parts/million ozone for 4 h. Twenty-four hours later, the muscarinic agonist pilocarpine no longer inhibited vagally induced bronchoconstriction in ozone-exposed animals, indicating M2 dysfunction. M2 receptor function in ozone-exposed animals was protected by depletion of eosinophils with antibody to interleukin-5 and by pretreatment with antibody to guinea pig MBP. M2 function was acutely restored by removal of MBP with heparin. Ozone-induced hyperreactivity was also prevented by antibody to MBP and was reversed by heparin. These data show that loss of neuronal M2 receptor function after ozone is due to release of eosinophil MBP.

Asthma exacerbations, many of which are virus induced, are associated with airway eosinophilia. This may reflect altered inflammatory response to viruses in atopic individuals. Inhibitory M2 muscarinic receptors (M2Rs) on the airway parasympathetic nerves limit acetylcholine release. Both viral infection and inhalational antigen challenge cause M2R dysfunction, leading to airway hyperresponsiveness. In antigen-challenged, but not virus-infected guinea pigs, M2R dysfunction is due to blockade of the receptors by the endogenous antagonist eosinophil major basic protein (MBP). We hypothesized that sensitization to a nonviral antigen before viral infection alters the inflammatory response to viral infection, so that M2R dysfunction and hyperreactivity are eosinophil mediated. Guinea pigs were sensitized to ovalbumin intraperitoneally, and 3 wk later were infected with parainfluenza. In sensitized, but not in nonsensitized animals, virus-induced hyperresponsiveness and M2R dysfunction were blocked by depletion of eosinophils with antibody to interleukin (IL)-5 or treatment with antibody to MBP. An additional and unexpected finding was that sensitization to ovalbumin caused a marked (80%) reduction in the viral content of the lungs. This was reversed by the antibody to IL-5, implicating a role for eosinophils in viral immunity.