The Pseudomonas aeruginosa type III secretion system plays a dual role in the regulation of caspase-1 mediated IL-1beta maturation

The type III secretion system (T3SS) is a complex nanomachine of many pathogenic Gram-negative bacteria. It forms a proteinaceous channel that is inserted into the host eukaryotic cell membrane for injection of bacterial proteins that manipulate host cell signaling. However, few studies have focused on the effector-independent functions of the T3SS. Using a murine model of acute lung infection with Pseudomonas aeruginosa, an important human opportunistic pathogen, we compared the pathogenicity of mutant bacteria that lack all of the known effector toxins ( DSTY), with mutant bacteria that also lack the major translocator protein PopB (DSTY/DPopB) and so cannot form a functional T3SS channel in the host cell membrane. Mortality was higher among mice challenged with DSTY compared to mice challenged with DSTY/DPopB mutant bacteria. In addition, mice infected with DSTY showed decreased bacterial clearance from the lungs compared to those infected with DSTY/DPopB. Infection was in both cases associated with substantial killing of lung infiltrating macrophages. However, macrophages from DSTY-infected mice died by pro-inflammatory necrosis characterized by membrane permeabilization and caspase-1 mediated IL-1b production, whereas macrophages from DSTY/DPopB infected mice died by apoptosis, which is characterized by annexin V positive staining of the cell membrane and caspase-3 activation. This was confirmed in macrophages infected in vitro. These results demonstrate a T3SS effector toxin independent role for the T3SS, in particular the T3SS translocator protein PopB, in the pathogenicity of P. aeruginosa durin