Smoke Exposure Exacerbates an Ethanol-Induced Defect in Mucociliary Clearance of Streptococcus pneumoniae

Excessive ethanol (EtOH) ingestion has long been associated with an increased risk of lower respiratory tract infections, including the most common bacterial cause of community-acquired pneumonia, Streptococcus pneumoniae (the pneumococcus). Pneumococcal infections in alcoholics often are severe and more likely to result in bacteremia, thereby increasing the mortality rate (Ruiz et al., 1999; Young et al. 1989). The effects of alcoholism on the immune system have been carefully examined through both animal models and human studies (as reviewed in Szabo 1999; and Zhang et al., 2002). Little research has been done, however, to elucidate the concurrent effects of smoke exposure and excessive EtOH ingestion, even though greater than 80% of human alcoholics also smoke cigarettes (Bien and Burge, 1990) and smoking alone is a significant risk factor for the development of invasive pneumococcal disease (Aronson et al., 1982: Nuorti et al., 2000). Because smoking and excessive alcohol ingestion are closely linked and both behaviors increase the risk of severe pneumococcal disease, it is important that their concurrent effects on the host are examined thoroughly. The first step in establishment of pneumococcal pneumonia is colonization of the bacterium in the nasopharyngeal region. The pneumococcus is obviously proficient at colonizing, as about 40% of adult Americans carry the organism during the winter months (Austrian, 1986). To produce a pulmonary pneumococcal infection, however, the organism must travel from the nasopharynx into the lower respiratory tract. To defend itself against such an attack, the host presents an elaborate mechanical defense system known as the mucociliary clearance apparatus to prevent bacteria residing in the nasopharynx from reaching the lung. This defense system is composed of ciliated columnar epithelial cells whose cilia beat in a coordinated manner to move mucous and trapped microorganisms back to the upper respiratory tract. The rate at which bacteria are removed from the airways depends in part on the pace at which the cilia beat, known as ciliary beat frequency (CBF). CBF is regulated primarily by protein kinase activity, such that it is increased by the activation of protein kinase A (PKA) and protein kinase G (PKG), and decreased by activation of protein kinase C (PKC) (Levin et al., 1997; Salathe et al., 1993; Wong et al., 1998; Wyatt et al., 1998). Using an in vitro model, acute EtOH exposure has been shown to activate PKA activity and increase CBF in bovine bronchial epithelium (Sisson et al., 1999). In contrast, chronic exposure of bovine bronchial epithelial cells to EtOH results in a return to original baseline values (Wyatt and Sisson, 2001). In vitro exposure of cultured bovine tracheal cells to cigarette smoke extract also has been shown to activate PKC and thus downregulate CBF (Kensler and Battista, 1963; Sisson and Tuma, 1994, Wyatt et al., 2000). Although these separate effects of cigarette smoke or EtOH exposure on CBF have been examined in vitro, to our knowledge no in vivo studies have been published to date looking at the concurrent effect of the two treatments on PKC activity and CBF, particularly as they relate to bacterial movement from the nasopharynx into the lungs. We postulated that a defect or alteration in CBF due to smoke exposure and ethanol ingestion would decrease the mucociliary clearance of S. pneumoniae, preventing the cilia from effectively keeping the organisms away from the lung. To test this hypothesis we utilized an in vivo model of concurrent smoke exposure and ethanol ingestion developed in our laboratory (Gentry-Nielsen et al., 2004). Through the use of computerized frequency analysis of CBF and direct PKC activity assays, we have shown that smoke exposure exacerbates an alcohol-induced defect in ciliary response that correlates with increased movement of organisms into the lungs of rats. We suggest this as one potential cause of the increased risk of pneumococcal pneumonia in the smoking, alcoholic population.