In order to analyze whether P. aeruginosa is able to modulate the antiviral response of bronchial epithelial cells, we prepared control medium or conditioned medium (CM) from two different strains of P. aeruginosa, PAO1 (commonly used in research) and Boston (quality control strain), respectively. CM contains soluble factors secreted by P. aeruginosa during growth. We focused on soluble factors since P. aeruginosa is mostly located intraluminally in CF lungs and direct cell-cell contacts are less common (24). Subsequently, we used CM or control medium to treat airway BEAS2B cells and thereafter infected the cells with hRV (strain RV1b) or RSV (Figure 1A). Subsequently, antiviral responses were analyzed after various incubation times. Induction of the antiviral genes MX1 and OAS1 upon virus infection did not show any significant differences between BEAS2B cells pretreated with CM of P. aeruginosa Boston strain (CM-Boston) or control medium (Figure 1B). However, cells treated with CM of P. aeruginosa strain PAO1 (CM-PAO1) showed a significant decrease in the induction of both genes after infection with hRV and RSV, which was most pronounced after 14 h of infection compared to control medium [fold induction (FI) of MX1: 285 vs. 6, p < 0.001; FI OAS1: 49 vs. 1, p < 0.001; Figure 1A]. Since effects were stronger after RSV infection all subsequent experiments were done with RSV. Of note, conditioning with P. aeruginosa medium alone did not affect MX1 or OAS1 expression (not shown).