Toll-like receptor 5 (TLR5), IL-1 (cid:12) secretion, and asparagine endopeptidase are critical factors for alveolar macrophage phagocytosis and bacterial killing.

A de fi cit in early clearance of Pseudomonas aeruginosa ( P. aeruginosa ) is crucial in nosocomial pneumonia and in chronic lung infec- tions. Few studies have addressed the role of Toll-like receptors (TLRs), which are early pathogen associated molecular pattern receptors, in pathogen uptake and clearance by alveolar macrophages (AMs). Here, we report that TLR5 engagement is crucial for bacterial clearance by AMs in vitro and in vivo because un fl agellated P. aeruginosa or different mutants defective in TLR5 activa- tion were resistant to AM phagocytosis and killing. In addition, the clearance of PAK (a wild-type P. aeruginosa strain) by primary AMs was causally associated with increased IL-1 β release, which was dramatically reduced with PAK mutants or in WT PAK-infected pri- mary TLR5 − / − AMs, demonstrating the dependence of IL-1 β production on TLR5. We showed that this IL-1 β production was important in endosomal pH acidi fi cation and in inducing the killing of bacteria by AMs through asparagine endopeptidase (AEP), a key endosomal cysteine protease. In agreement, AMs from IL-1R1 − / − and AEP − / − mice were unable to kill P. aeruginosa . Altogether, these fi ndings demonstrate that TLR5 engagement plays a major role in P. aeru- ginosa internalization and in triggering IL-1 β formation. secreted through the syringe, either before or after engulfment into the phag- olysosome, is necessary for in fl ammasome engagement, activation of caspase-1, and maturation of pro-IL-1 β into mature IL-1 β . The latter is key in the induction of phagolysosome acidi fi cation (3). Finally, acidi fi cation triggers the activation of the cysteine lysosomal protease AEP, which is shown to be a key factor in the killing of phagocytosed bacteria (3).