Azithromycin is used in long-term, low-dose treatment of airway diseases where airway wall remodelling is present. Since it improves total score symptom and respiratory function of such patients, we hypothesise that azithromycin's additional clinical benefits are due to an inhibition of airway smooth muscle cell (SMC) proliferation.Rabbit tracheal SMCs were treated with azithromycin (10 -5 to 10 -6 M) in the presence or absence of 10% fetal bovine serum (FBS). The proliferation was estimated using the Cell Titer 961 AQ ueous One Solution Assay (Promega, Madison, WI, USA). Cell viability was assessed with Trypan blue staining and flow cytometry after 7-aminoactinomycin D (7-AAD) staining. Induction of autophagy was studied by indirect immmunofluorescence and/or Western blotting with antibodies against human smooth muscle a-actin, beclin 1, light chain 3 and caspase 3. The involvement of the phosphoinositide 3-kinase pathway was investigated with the inhibitors LY294002 and wortmannin. Incubation with azithromycin for 72 h in the presence of FBS reduced SMC proliferation and viability in a dose-dependent manner. Azithromycin treatment was accompanied by the formation of cytoplasmic vacuoles, characteristic of autophagy. All these effects were reversible after azithromycin removal and prevented by the autophagy inhibitor, 3-methyladenine, or LY294002, but not by wortmannin.In conclusion, azithromycin reduces proliferation and causes autophagy of airway SMCs.KEYWORDS: Airway smooth muscle cells, autophagy, azithromycin, proliferation M acrolide antibiotics are widely used for the treatment of airway diseases. Lowdose, long-term macrolide therapy has been reported to be very effective in patients with chronic airway diseases, such as diffuse panbronchiolitis, chronic bronchitis and bronchial asthma [1][2][3][4][5]. In many of these diseases, airway wall remodelling is present. Airway wall remodelling includes thickening of the reticular membrane, proliferation of the smooth muscle cells (SMCs) and increase in both number and size of vessels [6,7].The improvement of pulmonary function, total score symptom and quality of life with macrolides is mainly attributed to their antimicrobial and anti-inflammatory activity. However, data available from studies on airway smooth muscle show an effect of macrolides on the contractility of airway smooth muscle. Specifically, erythromycin inhibits cholinergic neuroeffector transmission in the airways [8,9] and azithromycin has a direct relaxant effect on precontracted rabbit airway smooth muscle [10].It is possible that azithromycin has an additional effect in airway SMC proliferation. As has been shown previously, erythromycin inhibits hypertrophic and metaplastic changes of goblet cells in rat nasal epithelium [11], roxithromycin inhibits proliferation of human coronary artery SMCs [12], and rapamycin and its analogue SAR943 inhibit proliferation of human epithelial and SMCs [13], while clarithromycin and azithromycin induce apoptosis of activated lymphocytes [14]. In o...