SUMMARY/ABSTRACT COPD is the 3rd leading cause of death and incidence continues to rise. Many people are trying to quit smoking by using electronic (e)-cigarettes, even though data suggest that e-cigarette use adversely impacts cessation rates. Patients are switching to or adding on e-cigarette use because advertisements say they are completely safe. Young adult (18-40) never smokers are vaping e-cigarettes - inhaling vapor containing nicotine and other chemicals from these drug delivery devices. Our in vivo, ex vivo and in vitro data suggest significant adverse effects on host defenses, inflammation in the lung and systemically, and bacterial virulence. Vaping may lead to long-term effects, such as development of COPD, heart disease, and kidney failure, but it will be decades before epidemiologic data can demonstrate or disprove it. We propose experiments to evaluate effects of e-cigarette vapor on primary human airway cells of host defense: macrophages (>90% of airway cells in normal human subjects) and neutrophils (elevated in the airways of cigarette smokers - the largest population using e-cigarettes). These cells are directly exposed to inhaled vapor in the airways and are the mainstays of host defense in lung and blood. Comparison to cigarette smoke is important to answer one of the most urgent questions: Are e-cigarettes safer than conventional cigarettes? We will determine whether e-cigarette vapor negatively impacts the ability of macrophages and neutrophils to kill common pathogens and examine whether e-cigarettes dysregulate inflammatory responses, as both increases and decreases in inflammation are known to cause chronic disease. We will use our established mouse model of e-cigarette vapor inhalation, such that we may study effects of e-cigarettes in the setting of common human diseases, such as gram-negative sepsis, acute lung injury, and gram-positive pneumonia and bacteremia. Finally, the incidence of invasive disease and death due to antibiotic resistant bacteria is growing rapidly. In particular, methicillin resistant Staphylococcus aureus (MRSA) is the leading cause of bloodstream, skin and soft tissue infections. MRSA colonizes the nasopharynx of 20% of the population chronically. We recently found that MRSA is harder to kill and more virulent after exposure to e-cigarette vapor as well as conventional tobacco smoke. In this proposal we will: 1. Determine whether e-cigarette vapor promotes virulence in colonizing S. aureus in vivo by isolating strains from e-cigarette users, cigarette smokers and controls; and 2. Evaluate e-cigarette effects on both host defense and bacterial virulence in a physiologic setting (mice colonized with MRSA and concomitantly inhaling e-cigarette vapor) to determine whether deleterious effects of e-cigarettes on host defenses and bacterial pathogenicity occur in a more complex setting, as would happen in human airways. These studies may have a major public health impact, laying groundwork for future studies and for making informed e-cigarette recommendations to the public at large.