It was previously demonstrated that Esc(1–21) has a strong in vitro antipseudomonal activity, with a minimal inhibitory concentration (MIC) of 4 μM26. Before performing in vivo studies, the effect of the two esculentin derivatives on the integrity of human airway epithelium was analyzed at three selected peptide concentrations by measuring the transepithelial electrical resistance (TEER) in polarized human primary bronchial cells. Furthermore, the human cathelicidin AMP LL-37 was used as a reference. As shown in Fig. 1, when all peptides were used at a concentration corresponding to 2 × MIC (8 μM), they did not affect the epithelium integrity. When tested at a higher concentration (i.e. 20 μM), LL-37 initially decreased TEER, but this value recovered and was close to the initial one after 10 hours. Differently, when used at the higher concentration of 64 μM, it exhibited significantly detrimental effect on disturbing the epithelial cell tight junction and its negative effect persisted for a long period of time (up-triangle, broken line). In comparison, only a mild decrease in the TEER was observed for 64 μM of Esc(1–21) within 24 h (upside down triangle, broken line). Interestingly, Esc(1-21)-1c treatment had no measureable difference on TEER and did not disturb the membrane integrity at the same 64 μM concentration (square).Figure 1Effects of AMPs on the TEER of lung epithelial cells. Peptides in PBS were added to the apical compartment of the normal primary human bronchial epithelial cells maintained in ALI condition. TEER was measured at multiple time points (1, 2, 3, 5, 8, 10, 12 and 24 hours). Results are representative data from three independent experiments.