Host responses to secreted Shigella virulence factors

Molecular pathogenesis of Shigella spp.: controlling host cell signaling, invasion, and death by type III secretion

Shigella spp. are gram-negative pathogenic bacteria that evolved from harmless enterobacterial relatives and may cause devastating diarrhea upon ingestion. Research performed over the last 25 years revealed that a type III secretion system (T3SS) encoded on a large plasmid is a key virulence factor of Shigella flexneri. The T3SS determines the interactions of S. flexneri with intestinal cells by consecutively translocating two sets of effector proteins into the target cells. Thus, S. flexneri controls invasion into EC, intra- and intercellular spread, macrophage cell death, as well as host inflammatory responses. Some of the translocated effector proteins show novel biochemical activities by which they intercept host cell signal transduction pathways. An understanding of the molecular mechanisms underlying Shigella pathogenesis will foster the development of a safe and efficient vaccine, which, in parallel with improved hygiene, should curb infections by this widespread pathogen.

Type III secretion systems and disease

Type III secretion systems (T3SSs) are complex bacterial structures that provide gram-negative pathogens with a unique virulence mechanism enabling them to inject bacterial effector proteins directly into the host cell cytoplasm, bypassing the extracellular milieu. Although the effector proteins vary among different T3SS pathogens, common pathogenic mechanisms emerge, including interference with the host cell cytoskeleton to promote attachment and invasion, interference with cellular trafficking processes, cytotoxicity and barrier dysfunction, and immune system subversion. The activity of the T3SSs correlates closely with infection progression and outcome, both in animal models and in human infection. Therefore, to facilitate patient care and improve outcomes, it is important to understand the T3SS-mediated virulence processes and to target T3SSs in therapeutic and prophylactic development efforts.

The regulatory protein PhoP controls susceptibility to the host inflammatory response in Shigella flexneri

The PhoP/PhoQ two-component regulatory system controls transcription of several key virulence genes essential for Salmonella survival in the host cell phagosome. Here, we determine that the PhoP/PhoQ system also regulates virulence in the aetiological agent of bacillary dysentery, Shigella flexneri, even though this pathogen escapes from the phagosome into the cytoplasm of the host cell. A phoP mutant of Shigella established infections and induced an acute inflammatory response in two different animal models. However, infections with phoP mutant bacteria were resolved more rapidly than infections with wild-type Shigella. Moreover, the Shigella phoP mutant was more sensitive than the wild-type strain to killing by polymorphonuclear leucocytes (PMNs), cationic polypeptides extracted from PMNs and other animal-derived antimicrobial peptides. The phoP mutant, however, invaded epithelial cells, spread intercellularly, induced apoptosis in macrophages and tolerated extreme acid pH as efficiently as the wild-type strain. PhoP appears to regulate Shigella susceptibility to PMNs and antimicrobial molecules that are important for the late stages of infection with this enteric bacterium.

Tripeptidyl peptidase II promotes maturation of caspase-1 in Shigella flexneri-induced macrophage apoptosis

The invasive enteropathogenic bacterium Shigella flexneri activates apoptosis in macrophages. Shigella-induced apoptosis requires caspase-1. We demonstrate here that tripeptidyl peptidase II (TPPII), a cytoplasmic, high-molecular-weight protease, participates in the apoptotic pathway triggered by Shigella. The TPPII inhibitor Ala-Ala-Phe-chloromethylketone (AAF-cmk) and clasto-lactacystin beta-lactone (lactacystin), an inhibitor of both TPPII and the proteasome, protected macrophages from Shigella-induced apoptosis. AAF-cmk was more potent than lactacystin and irreversibly blocked Shigella-induced apoptosis by 95% at a concentration of 1 microM. Conversely, peptide aldehyde and peptide vinylsulfone proteasome inhibitors had little effect on Shigella-mediated cytotoxicity. Both AAF-cmk and lactacystin prevented the maturation of pro-caspase-1 and its substrate pro-interleukin 1beta in Shigella-infected macrophages, indicating that TPPII is upstream of caspase-1. Neither of these compounds directly inhibited caspase-1. AAF-cmk and lactacystin did not impair macrophage phagocytosis or the ability of Shigella to escape the macrophage phagosome. TPPII was also found to be involved in apoptosis induced by ATP and the protein kinase inhibitor staurosporine. We propose that TPPII participates in apoptotic pathways.