In the healthy intestinal mucosa, homeostasis between the immune system and commensal microflora prevents detrimental inflammatory responses. Infection with acute enteropathogens such as Salmonella enterica serovar Typhimurium disturbs this homeostasis and triggers inflammation, but the underlying mechanisms are poorly understood. We found that bacterial delivery or ectopic expression of the S. Typhimurium type III effector protein SopE, a known activator of host cellular Rho GTPases, led to proinflammatory caspase-1 activation and consequent maturation and secretion of the cytokine IL-1beta. In vivo, SopE triggered mucosal inflammation in wild-type but not caspase-1(-/-), IL-1R(-/-), or IL-18(-/-) mice. Bone marrow chimeras indicated that caspase-1 was more important in stromal cells, most likely enterocytes, than in bone marrow-derived cells. SopE-mediated caspase-1 activation in vitro was mediated by cellular Rho GTPases Rac-1 and Cdc42. These findings implicate SopE-driven Rho GTPase-mediated caspase-1 activation in stromal cells as a mechanism eliciting mucosal inflammation during S. Typhimurium infection.
Pseudomonas aeruginosa is an opportunistic bacterial pathogen that forms a serious problem for immunocompromised patients and also the leading cause of mortality in cystic fibrosis. The overall importance of a functional Type III secretion system (T3SS) in P. aeru inosa virulence has been well established, but the underlying mechanisms are still unclear. Using in vitro infected macrophages as w as a murine model of acute lung infection, we show that the Caspase-1 mediated maturation and secretion of IL-1β needs a translocation competent T3SS and Flagellin, but not the Type III effector proteins ExoS, ExoT and ExoY. However, ExoS was found to negative regulate the P. aeruginosa induced IL-1β maturation by a mechanism that is dependent on its ADP ribosyltransferase activity. Moreov ExoS deficiency also switched the mode of macrophage death from apoptosis to pro-inflammatory pyroptosis. Altogether, these da demonstrate a dual role for the P. aeruginosa T3SS in the regulation of Caspase-1 mediated IL-1β production and provide new insigh into the mechanisms of immune evasion by this pathogen.
Pseudomonas aeruginosa is a dangerous pathogen particularly because it harbors multiple virulence factors. It causes several types of infection, including dermatitis, endocarditis, and infections of the urinary tract, eye, ear, bone, joints and, of particular interest, the respiratory tract. Patients with cystic fibrosis, who are extremely susceptible to Pseudomonas infections, have a bad prognosis and high mortality. An important virulence factor of P. aeruginosa, shared with many other gram-negative bacteria, is the type III secretion system, a hollow molecular needle that transfers effector toxins directly from the bacterium into the host cell cytosol. This complex macromolecular machine works in a highly regulated manner and can manipulate the host cell in many different ways. Here we review the current knowledge of the structure of the P. aeruginosa T3SS, as well as its function and recognition by the immune system. Furthermore, we describe recent progress in the development and use of therapeutic agents targeting the T3SS.
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