Inhibition of Pseudomonas aeruginosa-secreted protease IV reduces lung

The opportunistic pathogen Pseudomonas aeruginosa secretes numerous proteases that disrupt host defenses. Among them, the lysyl endopeptidase PrpL has been implicated in virulence, but its role in inflammatory responses has remained unclear. This study shows that purified PrpL activates the AP-1 transcription factors following instillation into mouse lungs, and drives robust production of IL-1{beta}, IL-6, and TNF, as well as clinical symptoms. A proteolytically inactive mutant of PrpL fails to elicit these responses. Structural analysis of the complex of PrpL and its natural inhibitory propeptide (PrpLPP) using X-ray crystallography revealed the inhibitory mechanism of PrpLPP, which guided the identification of a pre-existing dipeptide inhibitor (LasBi) that blocked PrpL activity and restricted AP-1-driven cytokine induction and clinical symptoms in vivo. This study shows that the alkaline protease AprA degrades PrpLPP and functions redundantly with LasB to liberate mature, active PrpL. Our findings: 1) establish PrpL as a key virulence factor that triggers AP-1-mediated inflammatory signaling, 2) provide structural and functional insights into PrpL inhibition, and 3) identify AprA as a novel upstream PrpL activator. Together, these results highlight PrpL as a promising anti-virulence therapeutic target. Given the functional redundancy of inflammatory effectors produced by P. aeruginosa, strategies aimed at mitigating P. aeruginosa-induced lung inflammation through PrpL inhibition would likely need to be combined with approaches targeting additional pro-inflammatory bacterial factors. Author SummaryWe identify PrpL as a critical Pseudomonas aeruginosa protease that activates the inflammatory transcription factor AP-1 and drives potent lung inflammation. Structural analysis of a natural PrpL inhibitor guided development of an existing dipeptide inhibitor that significantly reduced the production of proinflammatory cytokines and clinical responses in vivo. Moreover, discovery of the AprA protease as an additional PrpL activator refines the activation model and suggests potential upstream therapeutic strategies. These findings highlight PrpL as both a driver of pathogenesis and a promising therapeutic target for treating P. aeruginosa infections.