Expression of muscarinic receptors by human macrophages

Abstract

Macrophages increase in number and are highly activated in chronic obstructive pulmonary disease (COPD). Muscarinic receptor antagonists inhibit acetylcholine-stimulated release of neutrophilic chemoattractants, suggesting that acetylcholine may regulate macrophage responses. Therefore, expression and function of components of the non-neuronal cholinergic system in monocyte-macrophage cells was investigated. RNA was isolated from monocytes, monocyte-derived macrophages (MDMs), lung and alveolar macrophages from nonsmokers, smokers and COPD patients, and expression of the high-affinity choline transporter, choline acetyltransferase, vesicular acetylcholine transporter and muscarinic receptors (M1-M5) ascertained using real-time PCR. M2 and M3 receptor expression was confirmed using immunocytochemistry. Release of interleukin (IL)-8, IL-6 and leukotriene (LT)B4 were measured by ELISA or EIA. All monocyte-macrophage cells expressed mRNA for components of the non-neuronal cholinergic system. Lung macrophages expressed significantly more M1 mRNA compared with monocytes, and both lung macrophages and alveolar macrophages expressed the highest levels of M3 mRNA. Expression of M2 and M3 protein was confirmed in MDMs and lung macrophages. Carbachol stimulated release of LTB4 from lung macrophages (buffer 222.3±75.1 versus carbachol 1,118±622.4 pg·mL-1 n=15, p<0.05) but not IL-6 or IL-8. LTB4 release was attenuated by the M3 antagonist, 1,1-dimethyl-4- diphenylacetoxypiperidinium iodide (4-DAMP; half maximal effective concentration 5.2±2.2 nM; n=9). Stimulation of macrophage M3 receptors promotes release of LTB4, suggesting that antimuscarinic agents may be anti-inflammatory. Copyright©ERS 2012.