The antiviral response can be subdivided into two stages. First, viruses get recognized by nucleic acid receptors that drive the expression of type I and III interferons. Subsequently, secreted IFN I/III activates the canonical transcription factor STAT1/STAT2/IRF9, which in a positive feedback loop again drives further IFN I/III expression plus additional antiviral genes like OAS1/2 or MX1 (39). Interestingly, the initial induction of IFNλ mRNA after virus infection was not altered by PAO1-CM, but nevertheless IFNλ protein and signaling was significantly reduced compared to control or Boston-CM treated cells. Of note and in line with the literature, type I IFN was not induced by RSV in bronchial epithelial cells (Figure S2). Further analysis revealed that P. aeruginosa secretes proteases degrading type III IFN and thereby inhibiting the antiviral response. Of note, we observed that also exogenous type I IFN was degraded (Figure S6) indicating that in a physiological setting type I IFN as produced by immune cells would also get inactivated. Moreover, protease activity of various P. aeruginosa CF isolates correlated significantly with the ability to degrade recombinant IFNλ. Most of the secreted proteases are under the control of the quorum sensing regulator LasR and we could demonstrate that the ability of PAO1, Boston or the longitudinal CF isolates to suppress the antiviral response was associated with functional LasR. It is well-known that LasR is subject to mutations in the course of P. aeruginosa infections in CF patients e.g., it was reported that in a CF cohort 22% of the P. aeruginosa strains have an altered LasR sequence (6, 25, 40, 41). The involvement of LasR is further supported by the fact that LasR deleted CF P. aeruginosa isolates were not able to modulate the antiviral response whereas their parental counterpart did. LasR dependent proteases contributing substantially to the virulence of P. aeruginosa are AprA, LasA, LasB, and PrpL (42, 43). A limitation of the study is that LasR complemented mutants could not be used. However, five independent targeted mutants behave exactly the same way and the loss of the ability to degrade IFNλ correlated with a mutated LasR. Using P. aeruginosa PA14 deleted of either of these proteases showed that AprA is mostly responsible for the modulation of the antiviral response. In line with this, comparison of the protein sequence of AprA of PAO1, PA14, and PA7 revealed that PA7/group 3 AprA did not cluster within PA14 or PAO1 (Figure S7). AprA, also known as serralysin or alkaline metalloprotease, is a metalloprotease regulated directly by LasR and has previously been reported to degrade complement, alpha1-proteinase inhibitor, interleukins and interferon gamma (6, 44, 45). It is secreted as an inactive zymogen, which becomes active by the cleavage of a 9-amino acid propeptide either by other proteases (LasA/B) or in an autocatalytic manner. To our knowledge this is the first study showing that AprA is also able to degrade IFNλ thereby modulating the antiviral response of epithelial cells. It is well-known that CF patients produce antibodies against several Pseudomonas antigens including AprA. Moreover, it has been shown that these antibodies are able to block AprA activity (46–49). These antibodies would therefore be able to counteract AprA dependent type III IFN degradation. However, these antibodies need to be present at high titers at the site of infection in the conducting airways. Since high titers are regularly detected only in chronically infected patients neutralizing antibodies are only present when AprA expression is decreased. In addition, anti AprA antibodies are IgG subtypes which get passively secreted in the alveolar space and subsequently transported by the mucocilliary escalator to the airways (50, 51). Considering decreased mucocilliary clearance in CF patients sufficient titers might not be reached in this condition. In line with our results, Bomberger et al. were able to show that CFTR inhibitory factor (CIF), secreted by P. aeruginosa, is able to block presentation of viral antigens on MHC class I of bronchial epithelial cells and recognition by CD8+ cells adding another layer of complexity on how P. aeruginosa is able to modulate the antiviral defense (52).