Critical role for Ipaf in Pseudomonas aeruginosa-induced caspase-1 activation

Pseudomonas aeruginosa is an opportunistic Gram-negative human pathogen that is responsible for a broad range of infections in individuals with a variety of predisposing conditions. After infection, P. aeruginosa induces a marked inflammatory response in the host. However the mechanisms involved in bacterium recognition and induction of immune responses are poorly understood. Here we report that the Nod-like receptor family member Ipaf is required for optimal bacterial clearance in an in vivo model of P. aeruginosa lung infection. Further analysis showed that bacterial flagellin was essential for caspase-1 and IL-1 beta and this activity depended on Ipaf and the adaptor ASC but not TLR5. Notably, P. aeruginosa induced macrophage cell death and this event relied on flagellin and Ipaf but not on ASC. Analysis of Pseudomonas mutants revealed that different amino acid residues of flagellin were critical for sensing by Ipaf and TLR5. Finally, activation of caspase-1 and IL-1 beta secretion by P. aeruginosa required a functional type III secretion system, but not the effector molecules ExoS, ExoT and ExoY. These results provide new insight into the interaction of P. aeruginosa with host macrophages and suggest that distinct regions of flagellin are sensed by Ipaf and TLR5.