Cigarette Smoke Exposure Impairs Pulmonary Bacterial Clearance and Alveolar Macrophage Complement-Mediated Phagocytosis of Streptococcus pneumoniae

Cigarette smoke exposure increases the risk of pulmonary and invasive infections caused by Streptococcus pneumoniae, the most commonly isolated organism from patients with community-acquired pneumonia. Despite this association, the mechanisms by which cigarette smoke exposure diminishes host defense against S. pneumoniae infections are poorly understood. In this study, we compared the responses of BALB/c mice following an intratracheal challenge with S. pneumoniae after 5 weeks of exposure to room air or cigarette smoke in a whole-body exposure chamber in vivo and the effects of cigarette smoke on alveolar macrophage phagocytosis of S. pneumoniae in vitro. Bacterial burdens in cigarette smoke-exposed mice were increased at 24 and 48 h postinfection, and this was accompanied by a more pronounced clinical appearance of illness, hypothermia, and increased lung homogenate cytokines interleukin-1β (IL-1β), IL-6, IL-10, and tumor necrosis factor alpha (TNF-α). We also found greater numbers of neutrophils in bronchoalveolar lavage fluid recovered from cigarette smoke-exposed mice following a challenge with heat-killed S. pneumoniae. Interestingly, overnight culture of alveolar macrophages with 1% cigarette smoke extract, a level that did not affect alveolar macrophage viability, reduced complement-mediated phagocytosis of S. pneumoniae, while the ingestion of unopsonized bacteria or IgG-coated microspheres was not affected. This murine model provides robust additional support to the hypothesis that cigarette smoke exposure increases the risk of pneumococcal pneumonia and defines a novel cellular mechanism to help explain this immunosuppressive effect.Pneumococcal pneumonia, caused by the Gram-positive pathogen Streptococcus pneumoniae, is the most common form of community-acquired pneumonia in the United States and worldwide (24, 26). This organism can disseminate from the respiratory tract and is the leading cause of death from invasive bacterial infections, with antibiotic-resistant strains becoming increasingly more common (18, 26). Cigarette smoke (CS) exposure increases the risk of serious pneumococcal infections in humans (2, 29), although the mechanisms underlying this effect are not known. Consistent with increased risks of many infectious diseases among smokers (3), animal models have been used to demonstrate impairments in host defense against viral (13, 33), fungal (8), and bacterial (11) infections in smoke-exposed animals. To our knowledge, no reports exist which demonstrate the effects of CS exposure on host defense in a murine model of pneumococcal pneumonia, despite the clinical significance of this pathogen.The alveolar macrophage (AM) is a specifically differentiated resident phagocyte in the pulmonary alveoli that acts to maintain an environment free of pathogens and debris (27). Under normal conditions, AMs constitute the majority of immune cells within the alveolar space and act as a first line of innate host defense in the lung, using an array of receptors to recognize pathogen-associated molecular patterns (PAMPs) and to facilitate phagocytic uptake (36). Normally, AM function is tightly regulated to prevent inappropriate inflammation that could result in lung damage (1), but under conditions which overwhelm their clearance capacity, AMs play additional roles in the generation and subsequent resolution of inflammation and leukocyte recruitment (28, 37). Murine models of pulmonary pneumococcal infection have shown increased mortality (22) and bacterial burden (10) following AM depletion, indicating their importance in the innate host defense against such infections. Phagocytosis of S. pneumoniae is enhanced following opsonization with complement fragments C3b and C3bi, which adhere to the surfaces of bacteria. The critical importance of C3 in this context was recently demonstrated by studies reporting defects in host defense against pneumococcal pneumonia (19, 34).The increased susceptibility of smokers to pneumococcal pneumonia is incompletely understood, and no reports to date have assessed the effects of CS exposure on AM phagocytosis of pneumococcus, although many studies have demonstrated impairments in phagocytosis of other targets (15, 16, 21, 30, 31). Therefore, we determined the effects of CS exposure on pulmonary host defense against pneumococcal pneumonia in a murine model and assessed the effects of CS on AM phagocytosis of S. pneumoniae in vitro.