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The enteropathogen Y. pseudotuberculosis can survive in the harsh environment of lymphoid compartments that abounds in immune cells. This capacity is dependent on the plasmid-encoded Yersinia outer proteins (Yops) that are delivered into the host cell via a mechanism involving the Yersinia type three secretion system. We show that the virulence protein YopK has a role in the mechanism by which Y. pseudotuberculosis avoids the polymorphonuclear leukocyte (PMN, or neutrophil) defense. A yopK mutant, which is attenuated in the mouse infection model where it fails to cause systemic infection, was found to colonize Peyer's patches and mesenteric lymph nodes more rapidly than the wild-type strain. Further, in mice lacking PMNs, the yopK mutant caused full disease with systemic spread and typical symptoms. Analyses of effects on PMNs revealed that both the wild-type strain and the yopK mutant inhibited internalization and ROS production, as well as neutrophil extracellular trap formation by PMNs. However, the wild-type strain effectively avoided induction PMN death, whereas the mutant caused a necrotic-like PMN death. Taken together, our results indicate that YopK is required for the ability of Yersinia to resist the PMN defense, which is critical for the virulence of the pathogen. We suggest a mechanism where YopK functions to prevent unintended Yop delivery and thereby PMN disruption resulting in necrotic like cell death, which would enhance the inflammatory response favoring the host.

There are three human pathogenic species of the genus Yersiniae: Yersinia pestis, Yersinia enterocolitica, and Yersinia pseudotuberculosis. To cause disease, these strains inhibit several key innate defense mechanisms, including phagocytosis, the critical process for bacterial clearance. The ability of Yersinia to evade the immune defense is dependent on delivery of virulence effectors, Yersinia outer proteins (Yops), into the interacting cell by a mechanism involving the type III secretion machinery. We have shown that the virulence protein YopK plays an important role in the control of Yop effector translocation via a feedback mechanism involving another virulence protein, YopE. We also found that YopK participated in regulation of Yop effector translocation by modulating level and ratio of the pore-forming proteins YopB and YopD in the target cell membrane. Further, using a yeast two-hybrid screen with YopK as a bait, the eukaryotic protein RACK1 was identified as a target for this virulence protein. We found that RACK1 was engaged upon Y. pseudotuberculosis-mediated β1-integrin activation, where it was recruited to phagocytic cups. Downregulation of RACK1 by RNAi resulted in a reduced ability of Y. pseudotuberculosis to block phagocytosis, indicating that RACK1 is required for efficient Yersinia-mediated antiphagocytosis. Based on our data, we suggest a model where Yersinia, via YopK, targets RACK1 to ensure a directed delivery of the Yop effectors to the “right place” where they bind to and inactivate their targets, resulting in efficient inhibition of phagocytosis.  

A yopK mutant strain over-delivers Yop effectors, but is still avirulent in mice, indicating that YopK is important for the fine-tuning of effector protein delivery during infection. To analyse this, we investigated the importance of YopK during in vivo infection. We found that a yopK mutant colonized Peyer’s patches and the mesenteric lymph node more rapidly compared to wild-type Y. pseudotuberculosis, but was unable to spread systemically to liver and spleen and cause full disease in mice. Further, we showed that a yopK mutant was able to colonize liver and spleen and cause full disease in mice lacking the main phagocytes, polymorphonuclear leukocytes (PMNs). We also showed that YopK was important for Yersinia-mediated silencing of the PMN response.

To summarize, we suggest that YopK is important for Yersinia to evade the PMN defense and thereby spread systemically and cause disease. YopK is proposed to do this by allowing a controlled, directed Yop effector delivery that is just sufficient to inhibit host immune defense mechanisms. The controlled and precise delivery of virulence effectors avoids inappropriate triggering of PMNs and thereby an enhanced immune response favoring the host.

Many Gram-negative bacteria use type III secretion systems to translocate effector proteins into host cells. These effectors interfere with cellular functions in a highly regulated manner resulting in effects that are beneficial for the bacteria. The pathogen Yersinia can resist phagocytosis by eukaryotic cells by translocating Yop effectors into the target cell cytoplasm. This is called antiphagocytosis, and constitutes an important virulence feature of this pathogen since it allows survival in immune cell rich lymphoid organs. We show here that the virulence protein YopK has a role in orchestrating effector translocation necessary for productive antiphagocytosis. We present data showing that YopK influences Yop effector translocation by modulating the ratio of the pore-forming proteins YopB and YopD in the target cell membrane. Further, we show that YopK that can interact with the translocators, is exposed inside target cells and binds to the eukaryotic signaling protein RACK1. This protein is engaged upon Y. pseudotuberculosis-mediated beta1-integrin activation and localizes to phagocytic cups. Cells with downregulated RACK1 levels are protected from antiphagocytosis. This resistance is not due to altered levels of translocated antiphagocytic effectors, and cells with reduced levels of RACK1 are still sensitive to the later occurring cytotoxic effect caused by the Yop effectors. Further, a yopK mutant unable to bind RACK1 shows an avirulent phenotype during mouse infection, suggesting that RACK1 targeting by YopK is a requirement for virulence. Together, our data imply that the local event of Yersinia-mediated antiphagocytosis involves a step where YopK, by binding RACK1, ensures an immediate specific spatial delivery of antiphagocytic effectors leading to productive inhibition of phagocytosis.

Recent work has shown that a domain of YopE of Yersinia pseudotuberculosis ranging from amino acids 54 to 75 (R. Krall, Y. Zhang, and J. T. Barbieri, J. Biol. Chem. 279:2747-2753, 2004) is required for proper localization of YopE after ectopic expression in eukaryotic cells. This domain, called the membrane localization domain (MLD), has not been extensively studied in Yersinia. Therefore, an in cis MLD deletion mutant of YopE was created in Y. pseudotuberculosis. The mutant was found to secrete and translocate YopE at wild-type levels. However, the mutant was defective in the autoregulation of YopE expression after the infection of HeLa cells. Although the mutant translocated YopE at wild-type levels, it showed a delayed HeLa cell cytotoxicity. This delay was not caused by a change in GTPase activating protein (GAP) activity, since the mutant showed wild-type YopE GAP activity toward Rac1 and RhoA. The MLD mutant displayed a changed intracellular localization pattern of YopE in HeLa cells after infection, and the YopEDeltaMLD protein was found to be dispersed within the whole cell, including the nucleus. In contrast, wild-type YopE was found to localize to the perinuclear region of the cell and was not found in the nucleus. In addition, the yopEDeltaMLD mutant was avirulent. Our results suggest that YopE must target proteins other than RhoA and Rac1 and that the MLD is required for the proper targeting and hence virulence of YopE during infection. Our results raise the question whether YopE is a regulatory protein or a "true" virulence effector protein.

The bacterial pathogen Yersinia pseudotuberculosis uses a type III secretion (T3S) system to translocate Yop effectors into eukaryotic cells. Effectors are thought to gain access to the cytosol via pores formed in the host cell plasma membrane. Translocated YopE can modulate this pore formation through its GTPase-activating protein (GAP) activity. In this study, we analysed the role of translocated YopE and all the other known Yop effectors in the regulation of effector translocation. Elevated levels of Yop effector translocation into HeLa cells occurred by YopE-defective strains, but not those defective for other Yop effectors. Only Yersinia devoid of YopK exhibits a similar hyper-translocation phenotype. Since both yopK and yopE mutants also failed to down-regulate Yop synthesis in the presence of eukaryotic cells, these data imply that translocated YopE specifically regulates subsequent effector translocation by Yersinia through at least one mechanism that involves YopK. We suggest that the GAP activity of YopE might be working as an intra-cellular probe measuring the amount of protein translocated by Yersinia during infection. This may be a general feature of T3S-associated GAP proteins, since two homologues from Pseudomonas aeruginosa, exoenzyme S (ExoS) and exoenzyme T (ExoT), can complement the hyper-translocation phenotypes of the yopE GAP mutant.

Relapsing-fever borreliosis caused by Borrelia duttonii is a common cause of complications of pregnancy, miscarriage, and neonatal death in sub-Saharan Africa. We established a murine model of gestational relapsing fever infection for the study of the pathological development of these complications. We demonstrate that B. duttonii infection during pregnancy results in intrauterine growth retardation, as well as placental damage and inflammation, impaired fetal circulation, and decreased maternal hemoglobin levels. We show that spirochetes frequently cross the maternal-fetal barrier, resulting in congenital infection. Furthermore, we compared the severity of infection in pregnant and nonpregnant mice and show that pregnancy has a protective effect. This model closely parallels the consequences of human gestational infection, and our results provide insight into the mechanisms behind the complications of pregnancy that have been reported in human relapsing-fever infection.