In this study, we will investigate the effect of tezepelumab on the epithelial barrier function of the upper and lower airways in patients suffering from severe asthma with and without chronic rhinosinusitis with nasal polyps. This will be achieved by analysis of epithelial barrier function upon challenge with various harmful substances (e.g. cigarette smoke extract, allergens) in cultured primary respiratory tract epithelial cells. Furthermore we will assess changes in clinical parameters, cellular composition and inflammatory mediators.