Macrophage activation is a key feature of inflammatory reactions occurring during bacterial infections, immune responses and tissue injury. We previously demonstrated that human macrophages of different origin express the tyrosine kinase receptor recepteur d'origine nantaise, the human receptor for MSP (RON) and produce superoxide anion (O(2)(-)) when challenged with macrophage-stimulating protein (MSP), the endogenous ligand for RON. This study was aimed to evaluate the role of MSP in alveolar macrophages (AM) isolated from healthy volunteers and patients with interstitial lung diseases (sarcoidosis, idiopathic pulmonary fibrosis), either smokers or non-smokers, by evaluating the respiratory burst, cytokine release and nuclear factor-kappa B (NF-kappaB) activation. MSP effects were compared with those induced by known AM stimuli, for example, phorbol myristate acetate, N-formyl-methionyl-leucyl-phenylalanine, lipopolysaccharide.MSP evokes O(2)(-) production, cytokine release and NF-kappaB activation in a concentration-dependent manner. By evaluating the respiratory burst, we demonstrate a significantly increased O(2)(-) production in AM from healthy smokers or smokers with pulmonary fibrosis, as compared to non-smokers, thus suggesting MSP as an enhancer of cigarette smoke toxicity. Besides inducing interleukin-1 beta (IL-1beta) and interleukin-10 (IL-10) production, MSP triggers an enhanced tumor necrosis factor-alpha release, especially in healthy and pulmonary fibrosis smokers. On the contrary, MSP-induced IL-10 release is higher in AM from healthy non-smokers. MSP activates the transcription factor NF-kappaB; this effect is more potent in healthy and fibrosis smokers (2.5-fold increase in p50 subunit translocation). This effect is receptor-mediated, as it is prevented by a monoclonal anti-human MSP antibody. The higher effectiveness of MSP in AM from healthy smokers and patients with pulmonary fibrosis is suggestive of its role in these clinical conditions.