Background: The use of electronic cigarettes (ECIGs) is increasing, but the impact of ECIG-vapor on cellular processes like inflammation or host defense are less understood. The aim of the present study was to compare the acute effects of traditional cigarettes (TCIGs) and ECIG-exposure on host defense, inflammation, and cellular activation of cell lines and primary differentiated human airway epithelial cells (pHBE). Methods: We exposed pHBEs and several cell lines to TCIG-smoke or ECIG-vapor. Epithelial host defense and barrier integrity were determined. The transcriptome of airway epithelial cells was compared by gene expression array analysis. Gene interaction networks were constructed and differential gene expression over all groups analyzed. The expression of several candidate genes was validated by qRT-PCR. Results: Bacterial killing, barrier integrity and the expression of antimicrobial peptides were not affected by ECIG-vapor compared to control samples. In contrast, TCIGs negatively affected host defense and reduced barrier integrity in a significant way. Furthermore ECIG-exposure significantly induced IL-8 secretion from Calu-3 cells but had no effect on NCI-H292 or primary cells. The gene expression based on array analysis distinguished TCIG-exposed cells from ECIG and room air-exposed samples. Conclusion: The transcriptome patterns of host defense and inflammatory genes are significantly distinct between ECIG-exposed and TCIG-treated cells. The overall effects of ECIGs on epithelial cells are less in comparison to TCIG, and ECIG-vapor does not affect host defense. Nevertheless, although acute exposure to ECIG-vapor induces inflammation, and the expression of S100 proteins, long term in vivo data is needed to evaluate the chronic effects of ECIG use. Background The contribution of exposure to cigarette smoke (CS) to the development and progression of chronic obstructive pulmonary disease (COPD), cancer, and cardiovascular diseases is widely recognized [1, 2]. Electronic cigarettes (ECIGs) are commercially available since 2004, but patents for similar devices reach back to 1965 [3, 4]. ECIGs are devices that produce a vapor by heating a liquid [3] that usually contains a mixture of glycerol, propylene-glycol, water, flavors, and different concentrations of nicotine. The flavors cover a broad range of tastes from fruit or spices, to different brands of tobacco. An intense scientific and political discussion about the toxicity and potential harm reduction of ECIGs is ongoing. Glycerol-propylene-glycol-water mixtures have been in use for a long time as artificial fog in aviation emergency training and entertainment business, but only a few studies about possible side effects exist before the emergence of ECIGs [5, 6]. Several studies have analyzed the effects of ECIGs on lung cells with the goal to evaluate toxic effects on cells and tissues [7]. A number of negative outcomes on different tissue culture systems (cell death, impaired repair, oxidative stress) have been reported [7-9]. In parallel, several ...