YopH of Yersinia pseudotuberculosis interrupts early phosphotyrosine signalling associated with phagocytosis

Pathogenicity and virulence of Yersinia

The genus Yersinia includes human, animal, insect, and plant pathogens as well as many symbionts and harmless bacteria. Within this genus are Yersinia enterocolitica and the Yersinia pseudotuberculosis complex, with four human pathogenic species that are highly related at the genomic level including the causative agent of plague, Yersinia pestis. Extensive laboratory, field work, and clinical research have been conducted to understand the underlying pathogenesis and zoonotic transmission of these pathogens. There are presently more than 500 whole genome sequences from which an evolutionary footprint can be developed that details shared and unique virulence properties. Whereas the virulence of Y. pestis now seems in apparent homoeostasis within its flea transmission cycle, substantial evolutionary changes that affect transmission and disease severity continue to ndergo apparent selective pressure within the other Yersiniae that cause intestinal diseases. In this review, we will summarize the present understanding of the virulence and pathogenesis of Yersinia, highlighting shared mechanisms of virulence and the differences that determine the infection niche and disease severity.

Citric Acid Controls the Activity of YopH Bacterial Tyrosine Phosphatase

Purpose

Citric acid (CA) is a tricarboxylic acid with antioxidant and antimicrobial properties. Based on previous studies, the small compound with its three carboxylic groups can be considered a protein tyrosine phosphatase inhibitor. YopH, a protein tyrosine phosphatase, is an essential virulence factor in Yersinia bacteria.

Materials and Methods

We performed enzymatic activity assays of YopH phosphatase after treatment with citric acid in comparison with the inhibitory compound trimesic acid, which has a similar structure. We also measured the cytotoxicity of these compounds in Jurkat T E6.1 and macrophage J774.2 cell lines. We performed molecular docking analysis of the binding of citric acid molecules to YopH phosphatase.

Results

Citric acid and trimesic acid reversibly reduced the activity of YopH enzyme and decreased the viability of Jurkat and macrophage cell lines. Importantly, these two compounds showed greater inhibitory properties against bacterial YopH activity than against human CD45 phosphatase activity. Molecular docking simulations confirmed that citric acid could bind to YopH phosphatase.

Conclusion

Citric acid, a known antioxidant, can be considered an inhibitor of bacterial phosphatases.

Yersinia deploys type III-secreted effectors to evade caspase-4 inflammasome activation in human cells

IMPORTANCE

Yersinia are responsible for significant disease burden in humans, ranging from recurrent disease outbreaks (yersiniosis) to pandemics (Yersinia pestis plague). Together with rising antibiotic resistance rates, there is a critical need to better understand Yersinia pathogenesis and host immune mechanisms, as this information will aid in developing improved immunomodulatory therapeutics. Inflammasome responses in human cells are less studied relative to murine models of infection, though recent studies have uncovered key differences in inflammasome responses between mice and humans. Here, we dissect human intestinal epithelial cell and macrophage inflammasome responses to Yersinia pseudotuberculosis. Our findings provide insight into species- and cell type-specific differences in inflammasome responses to Yersinia.

Host-directed therapy for bacterial infections -Modulation of the phagolysosome pathway

Bacterial infections still impose a significant burden on humanity, even though antimicrobial agents have long since been developed. In addition to individual severe infections, the f fatality rate of sepsis remains high, and the threat of antimicrobial-resistant bacteria grows with time, putting us at inferiority. Although tremendous resources have been devoted to the development of antimicrobial agents, we have yet to recover from the lost ground we have been driven into. Looking back at the evolution of treatment for cancer, which, like infectious diseases, has the similarity that host immunity eliminates the lesion, the development of drugs to eliminate the tumor itself has shifted from a single-minded focus on drug development to the establishment of a treatment strategy in which the de-suppression of host immunity is another pillar of treatment. In infectious diseases, on the other hand, the development of therapies that strengthen and support the immune system has only just begun. Among innate immunity, the first line of defense that bacteria encounter after invading the host, the molecular mechanisms of the phagolysosome pathway, which begins with phagocytosis to fusion with lysosome, have been elucidated in detail. Bacteria have a large number of strategies to escape and survive the pathway. Although the full picture is still unfathomable, the molecular mechanisms have been elucidated for some of them, providing sufficient clues for intervention. In this article, we review the host defense mechanisms and bacterial evasion mechanisms and discuss the possibility of host-directed therapy for bacterial infection by intervening in the phagolysosome pathway.

Yersinia Type III-Secreted Effectors Evade the Caspase-4 Inflammasome in Human Cells

Yersinia are gram-negative zoonotic bacteria that use a type III secretion system (T3SS) to inject Yersinia outer proteins (Yops) into the host cytosol to subvert essential components of innate immune signaling. However, Yersinia virulence activities can elicit activation of inflammasomes, which lead to inflammatory cell death and cytokine release to contain infection. Yersinia activation and evasion of inflammasomes have been characterized in murine macrophages but remain poorly defined in human cells, particularly intestinal epithelial cells (IECs), a primary site of intestinal Yersinia infection. In contrast to murine macrophages, we find that in both human IECs and macrophages, Yersinia pseudotuberculosis T3SS effectors enable evasion of the caspase-4 inflammasome, which senses cytosolic lipopolysaccharide (LPS). The antiphagocytic YopE and YopH, as well as the translocation regulator YopK, were collectively responsible for evading inflammasome activation, in part by inhibiting Yersinia internalization mediated by YadA and β1-integrin signaling. These data provide insight into the mechanisms of Yersinia-mediated inflammasome activation and evasion in human cells, and reveal species-specific differences underlying regulation of inflammasome responses to Yersinia .

Yersinia pseudotuberculosis YopE prevents uptake by M cells and instigates M cell extrusion in human ileal enteroid-derived monolayers

Many pathogens use M cells to access the underlying Peyer's patches and spread to systemic sites via the lymph as demonstrated by ligated loop murine intestinal models. However, the study of interactions between M cells and microbial pathogens has stalled due to the lack of cell culture systems. To overcome this obstacle, we use human ileal enteroid-derived monolayers containing five intestinal cell types including M cells to study the interactions between the enteric pathogen, Yersinia pseudotuberculosis (Yptb), and M cells. The Yptb type three secretion system (T3SS) effector Yops inhibit host defenses including phagocytosis and are critical for colonization of the intestine and Peyer's patches. Therefore, it is not understood how Yptb traverses through M cells to breach the epithelium. By growing Yptb under two physiological conditions that mimic the early infectious stage (low T3SS-expression) or host-adapted stage (high T3SS-expression), we found that large numbers of Yptb specifically associated with M cells, recapitulating murine studies. Transcytosis through M cells was significantly higher by Yptb expressing low levels of T3SS, because YopE and YopH prevented Yptb uptake. YopE also caused M cells to extrude from the epithelium without inducing cell-death or disrupting monolayer integrity. Sequential infection with early infectious stage Yptb reduced host-adapted Yptb association with M cells. These data underscore the strength of enteroids as a model by discovering that Yops impede M cell function, indicating that early infectious stage Yptb more effectively penetrates M cells while the host may defend against M cell penetration of host-adapted Yptb.

Macrophages in Microbial Pathogenesis: Commonalities of Defense Evasion Mechanisms

Macrophages are key arsenals of the immune system against invaders. After compartmental isolation of a pathogen in phagosomes, the host immune response attempts to neutralize the pathogen. However, pathogens possess the ability to subvert these assaults and can also convert macrophages into their replicative niche. The multiple host defense evasion mechanisms employed by these pathogens like phagosome maturation arrest, molecular mimicry through secretory antigens, interference with host signaling, active radical neutralization, inhibition of phagosome acidification, alteration of programmed cell death and many other mechanisms. Macrophage biology as a part of the host-pathogen interaction has expanded rapidly in the past decade. The present review aims to shed some light upon the macrophage defense evasion strategies employed by infecting pathogens. We have also incorporated recent knowledge in the field of macrophage dynamics during infection and evolutionary perspectives of macrophage dynamics.

Peroxiredoxin AhpC1 protects Pseudomonas aeruginosa against the inflammatory oxidative burst and confers virulence

Pseudomonas aeruginosa is an opportunistic bacterium in patients with cystic fibrosis and hospital acquired infections. It presents a plethora of virulence factors and antioxidant enzymes that help to subvert the immune system. In this study, we identified the 2-Cys peroxiredoxin, alkyl-hydroperoxide reductase C1 (AhpC1), as a relevant scavenger of oxidants generated during inflammatory oxidative burst and a mechanism of P. aeruginosa (PA14) escaping from killing. Deletion of AhpC1 led to a higher sensitivity to hypochlorous acid (HOCl, IC₅₀ 3.2 ± 0.3 versus 19.1 ± 0.2 μM), hydrogen peroxide (IC₅₀ 91.2 ± 0.3 versus 496.5 ± 6.4 μM) and the organic peroxide urate hydroperoxide. ΔahpC1 strain was more sensitive to the killing by isolated neutrophils and less virulent in a mice model of infection. All mice intranasally instilled with ΔahpC1 survived as long as they were monitored (15 days), whereas 100% wild-type and ΔahpC1 complemented with ahpC1 gene (ΔahpC1 attB:ahpC1) died within 3 days. A significantly lower number of colonies was detected in the lung and spleen of ΔahpC1-infected mice. Total leucocytes, neutrophils, myeloperoxidase activity, pro-inflammatory cytokines, nitrite production and lipid peroxidation were much lower in lungs or bronchoalveolar liquid of mice infected with ΔahpC1. Purified AhpC neutralized the inflammatory organic peroxide, urate hydroperoxide, at a rate constant of 2.3 ± 0.1 × 10⁶ M^-1s^-1, and only the ΔahpC1 strain was sensitive to this oxidant. Incubation of neutrophils with uric acid, the urate hydroperoxide precursor, impaired neutrophil killing of wild-type but improved the killing of ΔahpC1. Hyperuricemic mice presented higher levels of serum cytokines and succumbed much faster to PA14 infection when compared to normouricemic mice. In summary, ΔahpC1 PA14 presented a lower virulence, which was attributed to a poorer ability to neutralize the oxidants generated by inflammatory oxidative burst, leading to a more efficient killing by the host. The enzyme is particularly relevant in detoxifying the newly reported inflammatory organic peroxide, urate hydroperoxide.

Manipulation of Focal Adhesion Signaling by Pathogenic Microbes

Focal adhesions (FAs) serve as dynamic signaling hubs within the cell. They connect intracellular actin to the extracellular matrix (ECM) and respond to environmental cues. In doing so, these structures facilitate important processes such as cell-ECM adhesion and migration. Pathogenic microbes often modify the host cell actin cytoskeleton in their pursuit of an ideal replicative niche or during invasion to facilitate uptake. As actin-interfacing structures, FA dynamics are also intimately tied to actin cytoskeletal organization. Indeed, exploitation of FAs is another avenue by which pathogenic microbes ensure their uptake, survival and dissemination. This is often achieved through the secretion of effector proteins which target specific protein components within the FA. Molecular mimicry of the leucine-aspartic acid (LD) motif or vinculin-binding domains (VBDs) commonly found within FA proteins is a common microbial strategy. Other effectors may induce post-translational modifications to FA proteins through the regulation of phosphorylation sites or proteolytic cleavage. In this review, we present an overview of the regulatory mechanisms governing host cell FAs, and provide examples of how pathogenic microbes have evolved to co-opt them to their own advantage. Recent technological advances pose exciting opportunities for delving deeper into the mechanistic details by which pathogenic microbes modify FAs.

Delivery of Heterologous Proteins, Enzymes, and Antigens via the Bacterial Type III Secretion System

The Type III Secretion System (T3SS) is a multimeric protein complex composed of over 20 different proteins, utilized by Gram-negative bacteria to infect eukaryotic host cells. The T3SS has been implicated as a virulence factor by which pathogens cause infection and has recently been characterized as a communication tool between bacteria and plant cells in the rhizosphere. The T3SS has been repurposed to be used as a tool for the delivery of non-native or heterologous proteins to eukaryotic cells or the extracellular space for a variety of purposes, including drug discovery and drug delivery. This review covers the methodology of heterologous protein secretion as well as multiple cases of utilizing the T3SS to deliver heterologous proteins or artificial materials. The research covered in this review will serve to outline the scope and limitations of utilizing the T3SS as a tool for protein delivery.

Yersinia pseudotuberculosis YopH targets SKAP2-dependent and independent signaling pathways to block neutrophil antimicrobial mechanisms during infection

Yersinia suppress neutrophil responses by using a type 3 secretion system (T3SS) to inject 6–7 Yersinia effector proteins (Yops) effectors into their cytoplasm. YopH is a tyrosine phosphatase that causes dephosphorylation of the adaptor protein SKAP2, among other targets in neutrophils. SKAP2 functions in reactive oxygen species (ROS) production, phagocytosis, and integrin-mediated migration by neutrophils. Here we identify essential neutrophil functions targeted by YopH, and investigate how the interaction between YopH and SKAP2 influence Yersinia pseudotuberculosis (Yptb) survival in tissues. The growth defect of a ΔyopH mutant was restored in mice defective in the NADPH oxidase complex, demonstrating that YopH is critical for protecting Yptb from ROS during infection. The growth of a ΔyopH mutant was partially restored in Skap2-deficient (Skap2KO) mice compared to wild-type (WT) mice, while induction of neutropenia further enhanced the growth of the ΔyopH mutant in both WT and Skap2KO mice. YopH inhibited both ROS production and degranulation triggered via integrin receptor, G-protein coupled receptor (GPCR), and Fcγ receptor (FcγR) stimulation. SKAP2 was required for integrin receptor and GPCR-mediated ROS production, but dispensable for degranulation under all conditions tested. YopH blocked SKAP2-independent FcγR-stimulated phosphorylation of the proximal signaling proteins Syk, SLP-76, and PLCγ2, and the more distal signaling protein ERK1/2, while only ERK1/2 phosphorylation was dependent on SKAP2 following integrin receptor activation. These findings reveal that YopH prevents activation of both SKAP2-dependent and -independent neutrophilic defenses, uncouple integrin- and GPCR-dependent ROS production from FcγR responses based on their SKAP2 dependency, and show that SKAP2 is not required for degranulation.

Pathogenic Yersinia species carry a virulence plasmid encoding a type 3 secretion system that translocates 6–7 effector Yops into host cells. We demonstrate that YopH protects Yersinia pseudotuberculosis from neutrophil-produced reactive oxygen species (ROS) and degranulation by interfering with signaling pathways downstream of three major receptor classes in neutrophils. We show that a previously identified target of YopH, SKAP2, controls some of the pathways essential for YopH to inactivate during infection. SKAP2 is essential in mediating ROS production downstream of two major receptors; however, it is dispensable for degranulation from the three major receptors tested. Our study illustrates that YopH protects Y. pseudotuberculosis by blocking both SKAP2-dependent and independent signaling pathways that regulate several neutrophil functions.

Thinking Outside the Bug: Molecular Targets and Strategies to Overcome Antibiotic Resistance

Since their discovery in the early 20th century, antibiotics have been used as the primary weapon against bacterial infections. Due to their prophylactic effect, they are also used as part of the cocktail of drugs given to treat complex diseases such as cancer or during surgery, in order to prevent infection. This has resulted in a decrease of mortality from infectious diseases and an increase in life expectancy in the last 100 years. However, as a consequence of administering antibiotics broadly to the population and sometimes misusing them, antibiotic-resistant bacteria have appeared. The emergence of resistant strains is a global health threat to humanity. Highly-resistant bacteria like Staphylococcus aureus (methicillin-resistant) or Enterococcus faecium (vancomycin-resistant) have led to complications in intensive care units, increasing medical costs and putting patient lives at risk. The appearance of these resistant strains together with the difficulty in finding new antimicrobials has alarmed the scientific community. Most of the strategies currently employed to develop new antibiotics point towards novel approaches for drug design based on prodrugs or rational design of new molecules. However, targeting crucial bacterial processes by these means will keep creating evolutionary pressure towards drug resistance. In this review, we discuss antibiotic resistance and new options for antibiotic discovery, focusing in particular on new alternatives aiming to disarm the bacteria or empower the host to avoid disease onset.

Identification of specific sequence motif of YopN of Yersinia pseudotuberculosis required for systemic infection

Type III secretion systems (T3SSs) are tightly regulated key virulence mechanisms shared by many Gram-negative pathogens. YopN, one of the substrates, is also crucial in regulation of expression, secretion and activation of the T3SS of pathogenic Yersinia species. Interestingly, YopN itself is also targeted into host cells but so far no activity or direct role for YopN inside host cells has been described. Recently, we were able show that the central region of YopN is required for efficient translocation of YopH and YopE into host cells. This was also shown to impact the ability of Yersinia to block phagocytosis. One difficulty in studying YopN is to generate mutants that are not impaired in regulation of the T3SS. In this study we extended our previous work and were able to generate specific mutants within the central region of YopN. These mutants were predicted to be crucial for formation of a putative coiled-coil domain (CCD). Similar to the previously described deletion mutant of the central region, these mutants were all impaired in translocation of YopE and YopH. Interestingly, these YopN variants were not translocated into host cells. Importantly, when these mutants were introduced in cis on the virulence plasmid, they retained full regulatory function of T3SS expression and secretion. This allowed us to evaluate one of the mutants, yopN_GAGA, in the systemic mouse infection model. Using in vivo imaging technology we could verify that the mutant was also attenuated in vivo and highly impaired to establish systemic infection.

Macrophage-microbe interaction: lessons learned from the pathogen Mycobacterium tuberculosis

Macrophages, being the cornerstone of the immune system, have adapted the ancient nutrient acquisition mechanism of phagocytosis to engulf various infectious organisms thereby helping to orchestrate an appropriate host response. Phagocytosis refers to the process of internalization and degradation of particulate material, damaged and senescent cells and microorganisms by specialized cells, after which the vesicle containing the ingested particle, the phagosome, matures into acidic phagolysosomes upon fusion with hydrolytic enzyme-containing lysosomes. The destructive power of the macrophage is further exacerbated through the induction of macrophage activation upon a variety of inflammatory stimuli. Despite being the end-point for many phagocytosed microbes, the macrophage can also serve as an intracellular survival niche for a number of intracellular microorganisms. One microbe that is particularly successful at surviving within macrophages is the pathogen Mycobacterium tuberculosis, which can efficiently manipulate the macrophage at several levels, including modulation of the phagocytic pathway as well as interfering with a number of immune activation pathways that normally would lead to eradication of the internalized bacilli. M. tuberculosis excels at circumventing destruction within macrophages, thus establishing itself successfully for prolonged times within the macrophage. In this contribution, we describe a number of general features of macrophages in the context of their function to clear an infection, and highlight the strategies employed by M. tuberculosis to counter macrophage attack. Interestingly, research on the evasion tactics employed by M. tuberculosis within macrophages not only helps to design strategies to curb tuberculosis, but also allows a better understanding of host cell biology.

YopN Is Required for Efficient Effector Translocation and Virulence in Yersinia pseudotuberculosis

Type III secretion systems (T3SSs) are used by various Gram-negative pathogens to subvert the host defense by a host cell contact-dependent mechanism to secrete and translocate virulence effectors. While the effectors differ between pathogens and determine the pathogenic life style, the overall mechanism of secretion and translocation is conserved. T3SSs are regulated at multiple levels, and some secreted substrates have also been shown to function in regulation. In Yersinia, one of the substrates, YopN, has long been known to function in the host cell contact-dependent regulation of the T3SS. Prior to contact, through its interaction with TyeA, YopN blocks secretion. Upon cell contact, TyeA dissociates from YopN, which is secreted by the T3SS, resulting in the induction of the system. YopN has also been shown to be translocated into target cells by a T3SS-dependent mechanism. However, no intracellular function has yet been assigned to YopN. The regulatory role of YopN involves the N-terminal and C-terminal parts, while less is known about the role of the central region of YopN. Here, we constructed different in-frame deletion mutants within the central region. The deletion of amino acids 76 to 181 resulted in an unaltered regulation of Yop expression and secretion but triggered reduced YopE and YopH translocation within the first 30 min after infection. As a consequence, this deletion mutant lost its ability to block phagocytosis by macrophages. In conclusion, we were able to differentiate the function of YopN in translocation and virulence from its function in regulation.

Pseudomonas aeruginosa Effector ExoS Inhibits ROS Production in Human Neutrophils

Neutrophils are the first line of defense against bacterial infections, and the generation of reactive oxygen species is a key part of their arsenal. Pathogens use detoxification systems to avoid the bactericidal effects of reactive oxygen species. Here we demonstrate that the Gram-negative pathogen Pseudomonas aeruginosa is susceptible to reactive oxygen species but actively blocks the reactive oxygen species burst using two type III secreted effector proteins, ExoS and ExoT. ExoS ADP-ribosylates Ras and prevents it from interacting with and activating phosphoinositol-3-kinase (PI3K), which is required to stimulate the phagocytic NADPH-oxidase that generates reactive oxygen species. ExoT also affects PI3K signaling via its ADP-ribosyltransferase activity but does not act directly on Ras. A non-ribosylatable version of Ras restores reactive oxygen species production and results in increased bacterial killing. These findings demonstrate that subversion of the host innate immune response requires ExoS-mediated ADP-ribosylation of Ras in neutrophils.

Immunology of Yersinia pestis Infection

As a pathogen of plague, Yersinia pestis caused three massive pandemics in history that killed hundreds of millions of people. Yersinia pestis is highly invasive, causing severe septicemia which, if untreated, is usually fatal to its host. To survive in the host and maintain a persistent infection, Yersinia pestis uses several stratagems to evade the innate and the adaptive immune responses. For example, infections with this organism are biphasic, involving an initial "noninflammatory" phase where bacterial replication occurs initially with little inflammation and following by extensive phagocyte influx, inflammatory cytokine production, and considerable tissue destruction, which is called "proinflammatory" phase. In contrast, the host also utilizes its immune system to eliminate the invading bacteria. Neutrophil and macrophage are the first defense against Yersinia pestis invading through phagocytosis and killing. Other innate immune cells also play different roles, such as dendritic cells which help to generate more T helper cells. After several days post infection, the adaptive immune response begins to provide organism-specific protection and has a long-lasting immunological memory. Thus, with the cooperation and collaboration of innate and acquired immunity, the bacterium may be eliminated from the host. The research of Yersinia pestis and host immune systems provides an important topic to understand pathogen-host interaction and consequently develop effective countermeasures.

Site-Directed Mutagenesis and Its Application in Studying the Interactions of T3S Components

Type III secretion systems are a prolific virulence determinant among Gram-negative bacteria. They are used to paralyze the host cell, which enables bacterial pathogens to establish often fatal infections-unless an effective therapeutic intervention is available. However, as a result of a catastrophic rise in infectious bacteria resistant to conventional antibiotics, these bacteria are again a leading cause of worldwide mortality. Hence, this report describes a pDM4-based site-directed mutagenesis strategy that is assisting in our foremost objective to better understand the fundamental workings of the T3SS, using Yersinia as a model pathogenic bacterium. Examples are given that clearly document how pDM4-mediated site-directed mutagenesis has been used to establish clean point mutations and in-frame deletion mutations that have been instrumental in identifying and understanding the molecular interactions between components of the Yersinia type III secretion system.

The Amino-Terminal Part of the Needle-Tip Translocator LcrV of Yersinia pseudotuberculosis Is Required for Early Targeting of YopH and In vivo Virulence

Type III secretion systems (T3SS) are dedicated to targeting anti-host effector proteins into the cytosol of the host cell to promote bacterial infection. Delivery of the effectors requires three specific translocator proteins, of which the hydrophilic translocator, LcrV, is located at the tip of the T3SS needle and is believed to facilitate insertion of the two hydrophobic translocators into the host cell membrane. Here we used Yersinia as a model to study the role of LcrV in T3SS mediated intracellular effector targeting. Intriguingly, we identified N-terminal lcrV mutants that, similar to the wild-type protein, efficiently promoted expression, secretion and intracellular levels of Yop effectors, yet they were impaired in their ability to inhibit phagocytosis by J774 cells. In line with this, the YopH mediated dephosphorylation of Focal Adhesion Kinase early after infection was compromised when compared to the wild type strain. This suggests that the mutants are unable to promote efficient delivery of effectors to their molecular targets inside the host cell upon host cell contact. The significance of this was borne out by the fact that the mutants were highly attenuated for virulence in the systemic mouse infection model. Our study provides both novel and significant findings that establish a role for LcrV in early targeting of effectors in the host cell.

Otopathogenic Pseudomonas aeruginosa Enters and Survives Inside Macrophages

Otitis media (OM) is a broad term describing a group of infectious and inflammatory disorders of the middle ear. Despite antibiotic therapy, acute OM can progress to chronic suppurative otitis media (CSOM) characterized by ear drum perforation and purulent discharge. Pseudomonas aeruginosa is the most common pathogen associated with CSOM. Although, macrophages play an important role in innate immune responses but their role in the pathogenesis of P. aeruginosa-induced CSOM is not known. The objective of this study is to examine the interaction of P. aeruginosa with primary macrophages. We observed that P. aeruginosa enters and multiplies inside human and mouse primary macrophages. This bacterial entry in macrophages requires both microtubule and actin dependent processes. Transmission electron microscopy demonstrated that P. aeruginosa was present in membrane bound vesicles inside macrophages. Interestingly, deletion of oprF expression in P. aeruginosa abrogates its ability to survive inside macrophages. Our results suggest that otopathogenic P. aeruginosa entry and survival inside macrophages is OprF-dependent. The survival of bacteria inside macrophages will lead to evasion of killing and this lack of pathogen clearance by phagocytes contributes to the persistence of infection in CSOM. Understanding host-pathogen interaction will provide novel avenues to design effective treatment modalities against OM.

Yersinia pseudotuberculosis Blocks Neutrophil Degranulation

Neutrophils are essential components of immunity and are rapidly recruited to infected or injured tissue. Upon their activation, neutrophils release granules to the cell's exterior, through a process called degranulation. These granules contain proteins with antimicrobial properties that help combat infection. The enteropathogenic bacterium Yersinia pseudotuberculosis successfully persists as an extracellular bacterium during infection by virtue of its translocation of virulence effectors (Yersinia outer proteins [Yops]) that act in the cytosol of host immune cells to subvert phagocytosis and proinflammatory responses. Here, we investigated the effect of Y. pseudotuberculosis on neutrophil degranulation upon cell contact. We found that virulent Y. pseudotuberculosis was able to prevent secondary granule release. The blocking effect was general, as the release of primary and tertiary granules was also reduced. Degranulation of secondary granules was also blocked in primed neutrophils, suggesting that this mechanism could be an important element of immune evasion. Further, wild-type bacteria conferred a transient block on neutrophils that prevented their degranulation upon contact with plasmid-cured, avirulent Y. pseudotuberculosis and Escherichia coli Detailed analyses showed that the block was strictly dependent on the cooperative actions of the two antiphagocytic effectors, YopE and YopH, suggesting that the neutrophil target structures constituting signaling molecules needed to initiate both phagocytosis and general degranulation. Thus, via these virulence effectors, Yersinia can impair several mechanisms of the neutrophil's antimicrobial arsenal, which underscores the power of its virulence effector machinery.

Two host clades, two bacterial arsenals: evolution through gene losses in facultative endosymbionts

Bacterial endosymbiosis is an important evolutionary process in insects, which can harbor both obligate and facultative symbionts. The evolution of these symbionts is driven by evolutionary convergence, and they exhibit among the tiniest genomes in prokaryotes. The large host spectrum of facultative symbionts and the high diversity of strategies they use to infect new hosts probably impact the evolution of their genome and explain why they undergo less severe genomic erosion than obligate symbionts. Candidatus Hamiltonella defensa is suitable for the investigation of the genomic evolution of facultative symbionts because the bacteria are engaged in specific relationships in two clades of insects. In aphids, H. defensa is found in several species with an intermediate prevalence and confers protection against parasitoids. In whiteflies, H. defensa is almost fixed in some species of Bemisia tabaci, which suggests an important role of and a transition toward obligate symbiosis. In this study, comparisons of the genome of H. defensa present in two B. tabaci species (Middle East Asia Minor 1 and Mediterranean) and in the aphid Acyrthosiphon pisum revealed that they belong to two distinct clades and underwent specific gene losses. In aphids, it contains highly virulent factors that could allow protection and horizontal transfers. In whiteflies, the genome lost these factors and seems to have a limited ability to acquire genes. However it contains genes that could be involved in the production of essential nutrients, which is consistent with a primordial role for this symbiont. In conclusion, although both lineages of H. defensa have mutualistic interactions with their hosts, their genomes follow distinct evolutionary trajectories that reflect their phenotype and could have important consequences on their evolvability.

Structural insight into effector proteins of Gram-negative bacterial pathogens that modulate the phosphoproteome of their host

Invading pathogens manipulate cellular process of the host cell to establish a safe replicative niche. To this end they secrete a spectrum of proteins called effectors that modify cellular environment through a variety of mechanisms. One of the most important mechanisms is the manipulation of cellular signaling through modifications of the cellular phosphoproteome. Phosphorylation/dephosphorylation plays a pivotal role in eukaryotic cell signaling, with ∼ 500 different kinases and ∼ 130 phosphatases in the human genome. Pathogens affect the phosphoproteome either directly through the action of bacterial effectors, and/or indirectly through downstream effects of host proteins modified by the effectors. Here we review the current knowledge of the structure, catalytic mechanism and function of bacterial effectors that modify directly the phosphorylation state of host proteins. These effectors belong to four enzyme classes: kinases, phosphatases, phospholyases and serine/threonine acetylases.

A new class of salicylic acid derivatives for inhibiting YopH of Yersinia pestis

Previously, we identified a class of salicylic acid derivatives that display inhibitory activity against the protein tyrosine phosphatase YopH from Yersinia pestis. Because docking study suggested that the large phenyl ring attaching to the salicylic acid core might be exposed to the solvent and might not contribute significantly to binding, we have developed a new class of compounds that no longer contain this phenyl ring. We first devised a synthetic scheme for the compounds and then developed an automated computational screening model surrounding this synthetic scheme to help select a small number of compounds for synthesis and experimental testing. Based on this computational screening model and the analysis of the structure-activity relationship of our previous class of compounds, we have synthesized eight compounds and found five that yield micromolar activity. When applying in a larger scale, the synthetic scheme and the computational screening model developed here should help to identify even more potent inhibitors in the future.

Does bacterial infection cause genome instability and cancer in the host cell?

Research of the past several decades suggests that bacterial infection can lead to genome instability of the host cell often resulting in cancer development. However, there is still a substantial lack of knowledge regarding possible mechanisms involved in the development of genomic instability. Several questions remain unanswered, namely: Why has the causative relationship between the bacterial infection and cancer been established only for a small number of cancers? What is the mechanism responsible for the induction of genome instability and cancer? Is the infection process required to cause genome instability and cancer? In this review, we present a hypothesis that the bacterial infection, exposure to heat-killed bacteria or even some bacterial determinants may trigger genome instability of exposed and distal cells, and thus may cause cancer. We will discuss the mechanisms of host responses to the bacterial infection and present the possible pathways leading to genome instability and cancer through exposure to bacteria.

Bacillus cereus Certhrax ADP-ribosylates vinculin to disrupt focal adhesion complexes and cell adhesion

Bacillus cereus is often associated with mild to moderate gastroenteritis; however, some recent isolates cause inhalational anthrax-like diseases and death. These potential emerging human pathogens express multiple virulence factors. B. cereus strain G9241 expresses anthrax toxin, several polysaccharide capsules, and the novel ADP-ribosyltransferase, Certhrax. In this study, we show that Certhrax ADP-ribosylates Arg-433 of vinculin, a protein that coordinates actin cytoskeleton and extracellular matrix interactions. ADP-ribosylation of vinculin disrupted focal adhesion complexes and redistributed vinculin to the cytoplasm. Exogenous vinculin rescued these phenotypes. This provides a mechanism for strain G9241 to breach host barrier defenses and promote bacterial growth and spread. Certhrax is the first bacterial toxin to add a post-translational modification to vinculin to disrupt the actin cytoskeleton.

Synthetic chalcones and sulfonamides as new classes of Yersinia enterocolitica YopH tyrosine phosphatase inhibitors

YopH plays a relevant role in three pathogenic species of Yersinia. Due to its importance in the prevention of the inflammatory response of the host, this enzyme has become a valid target for the identification and development of new inhibitors. In this work, an in-house library of 283 synthetic compounds was assayed against recombinant YopH from Yersinia enterocolitica. From these, four chalcone derivatives and one sulfonamide were identified for the first time as competitive inhibitors of YopH with binding affinity in the low micromolar range. Molecular modeling investigations indicated that the new inhibitors showed similar binding modes, establishing polar and hydrophobic contacts with key residues of the YopH binding site.

Context-dependent protein folding of a virulence peptide in the bacterial and host environments: structure of an SycH-YopH chaperone-effector complex

Yersinia pestis injects numerous bacterial proteins into host cells through an organic nanomachine called the type 3 secretion system. One such substrate is the tyrosine phosphatase YopH, which requires an interaction with a cognate chaperone in order to be effectively injected. Here, the first crystal structure of a SycH-YopH complex is reported, determined to 1.9 Å resolution. The structure reveals the presence of (i) a nonglobular polypeptide in YopH, (ii) a so-called β-motif in YopH and (iii) a conserved hydrophobic patch in SycH that recognizes the β-motif. Biochemical studies establish that the β-motif is critical to the stability of this complex. Finally, since previous work has shown that the N-terminal portion of YopH adopts a globular fold that is functional in the host cell, aspects of how this polypeptide adopts radically different folds in the host and in the bacterial environments are analysed.

Insight into bacterial virulence mechanisms against host immune response via the Yersinia pestis-human protein-protein interaction network

A Yersinia pestis-human protein interaction network is reported here to improve our understanding of its pathogenesis. Up to 204 interactions between 66 Y. pestis bait proteins and 109 human proteins were identified by yeast two-hybrid assay and then combined with 23 previously published interactions to construct a protein-protein interaction network. Topological analysis of the interaction network revealed that human proteins targeted by Y. pestis were significantly enriched in the proteins that are central in the human protein-protein interaction network. Analysis of this network showed that signaling pathways important for host immune responses were preferentially targeted by Y. pestis, including the pathways involved in focal adhesion, regulation of cytoskeleton, leukocyte transendoepithelial migration, and Toll-like receptor (TLR) and mitogen-activated protein kinase (MAPK) signaling. Cellular pathways targeted by Y. pestis are highly relevant to its pathogenesis. Interactions with host proteins involved in focal adhesion and cytoskeketon regulation pathways could account for resistance of Y. pestis to phagocytosis. Interference with TLR and MAPK signaling pathways by Y. pestis reflects common characteristics of pathogen-host interaction that bacterial pathogens have evolved to evade host innate immune response by interacting with proteins in those signaling pathways. Interestingly, a large portion of human proteins interacting with Y. pestis (16/109) also interacted with viral proteins (Epstein-Barr virus [EBV] and hepatitis C virus [HCV]), suggesting that viral and bacterial pathogens attack common cellular functions to facilitate infections. In addition, we identified vasodilator-stimulated phosphoprotein (VASP) as a novel interaction partner of YpkA and showed that YpkA could inhibit in vitro actin assembly mediated by VASP.

Type III secretion system-dependent translocation of ectopically expressed Yop effectors into macrophages by intracellular Yersinia pseudotuberculosis

Yersinia pseudotuberculosis is a Gram-negative bacterial pathogen. Virulence in Y. pseudotuberculosis requires the plasmid-encoded Ysc type III secretion system (T3SS), which functions to translocate a set of effectors called Yops into infected host cells. The effectors function to antagonize phagocytosis (e.g., YopH) or to induce apoptosis (YopJ) in macrophages infected with Y. pseudotuberculosis. Additionally, when antiphagocytosis is incomplete and Y. pseudotuberculosis is internalized by macrophages, the bacterium can survive in phagosomes. Previous studies have shown that delivery of effectors into host cells occurs efficiently when Yersinia is extracellular. However, it is not clear whether the T3SS can be utilized by intracellular Y. pseudotuberculosis to translocate Yops. This possibility was investigated here using Y. pseudotuberculosis strains that express YopJ or YopH under the control of an inducible promoter. Bone marrow-derived murine macrophages were infected with these strains under conditions that prevented the survival of extracellular bacteria. Effector translocation was detected by measuring apoptosis or the activities of Yop-β-lactamase fusion proteins. Results showed that macrophages underwent apoptosis when YopJ expression was induced prior to phagocytosis, confirming that delivery of this effector prior to or during uptake is sufficient to cause cell death. However, macrophages also underwent apoptosis when YopJ was ectopically expressed after phagocytosis; furthermore, expression of the translocator YopB from intracellular bacteria also resulted in increased cell death. Analysis by microscopy showed that translocation of ectopically expressed YopH- or YopJ-β-lactamase fusions could be correlated with the presence of viable Y. pseudotuberculosis in macrophages. Collectively, our results suggest that the Ysc T3SS of Y. pseudotuberculosis can function within macrophage phagosomes to translocate Yops into the host cytosol.

Expression hierarchy in the Yersinia type III secretion system established through YopD recognition of RNA

The Yersinia type III secretion system (T3SS) is environmentally responsive to enable its rapid induction upon contact with host cells and is necessary for Yersiniae to establish a replicative niche and cause disease. YopD, a translocator protein, represses the expression of T3SS genes until signalled by environmental cues, a mechanism known as the low calcium response. In this work, we investigated recognition of target genes by Yersinia pestis YopD. Expression of all genes of the T3SS was induced in a yopD mutant, though not to the same degree, with effector Yops most affected. Two, short AU-rich sequence elements up- and downstream of start codons of target genes were necessary but not sufficient for YopD mediated repression. Purified YopD-LcrH bound specifically to target RNAs in vitro with different relative affinities, with effector Yops having greater affinity. Together, the data suggest YopD binds to T3SS transcripts where it may prevent ribosome binding causing accelerated mRNA degradation. This regulatory mechanism may ensure an expression hierarchy during the low calcium response as low affinity YopD targets such as chaperones would be translated prior to high affinity targets such as effector Yops allowing the bacteria another layer of control over Yop translocation during infection.

The RACK1 signaling scaffold protein selectively interacts with Yersinia pseudotuberculosis virulence function

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 β1-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.

Beta1 integrin-dependent engulfment of Yersinia enterocolitica by macrophages is coupled to the activation of autophagy and suppressed by type III protein secretion

Autophagy is a central lysosomal degradation process that is essential for the maintenance of cellular homeostasis. Autophagy has furthermore emerged as integral part of the host immune response. Autophagic processes promote the separation and degradation of intracellular microorganisms which contributes to the development of innate and adaptive immunity. Some pathogenic microbes have therefore evolved mechanisms to evade or impede autophagy. We analyzed the effects of the enteropathogenic bacterium Yersinia enterocolitica on autophagy in macrophages. Yersiniae use a number of defined adhesins and secreted proteins to manipulate host immune responses. Our results showed that Y. enterocolitica defective in type III protein secretion efficiently activated autophagy in macrophages. Autophagy was mediated by the Yersinia adhesins invasin and YadA and particularly depended on the engagement of beta(1) integrin receptors. Several autophagy-related events followed beta(1) integrin-mediated engulfment of the bacteria including the formation of autophagosomes, processing of the marker protein LC3, redistribution of GFP-LC3 to bacteria-containing vacuoles, and the segregation of intracellular bacteria by autophagosomal compartments. These results provide direct evidence for the linkage of beta(1) integrin-mediated phagocytosis and autophagy induction. Multiple microbes signal through integrin receptors, and our results suggest a general principle by which the sensing of an extracellular microbe triggers autophagy. Owing to the importance of autophagy as host defense response, wild-type Y. enterocolitica suppressed autophagy by mobilizing type III protein secretion. The subversion of autophagy may be part of the Y. enterocolitica virulence strategy that supports bacterial survival when beta(1) integrin-dependent internalization and autophagy activation by macrophages are deleterious for the pathogen.

Cell type-specific effects of Yersinia pseudotuberculosis virulence effectors

One important feature of Yersinia pseudotuberculosis that enables resistance against the host immune defence is delivery of the antiphagocytic effectors YopH and YopE into phagocytic cells. The tyrosine phosphatase YopH influences integrin signalling, and YopE impairs cytoskeletal dynamics by inactivating Rho GTPases. Here, we report the impact of these effectors on internalization by dendritic cells (DCs), which internalize antigens to orchestrate host immune responses. We found that this pathogen resists internalization by DCs via YopE. YopH that is important for blocking phagocytosis by macrophages and neutrophils and which is also present inside the DCs does not contribute to the resistance. However, the YopH targets Fyb and p130Cas show higher expression levels in macrophages than in DCs. Furthermore, live cell microscopy revealed that the cells internalize Y. pseudotuberculosis in different ways: the macrophages utilize a locally restricted receptor-mediated zipper mechanism, whereas DCs utilize macropinocytosis involving constitutive ruffling that randomly catches bacteria into membrane folds. We conclude that YopH impacts early phagocytic signalling from the integrin receptor to which the bacterium binds and that this tight receptor-mediated stimulation is absent in DC macropinocytosis. Inactivation of cytoskeletal dynamics by YopE affects ruffling activity and hence also internalization. The different modes of internalization can be coupled to the major functions of these respective cell types: elimination by phagocytosis and antigen sampling.

Targeting of immune signalling networks by bacterial pathogens

Host defence against microbial pathogens requires appropriate coordination of multiple signalling pathways. These pathways are triggered by innate immune recognition of conserved microbial molecules, and initiate an inflammatory cascade that involves recruitment of leukocytes to the site of infection, activation of antimicrobial effector mechanisms and induction of an adaptive immune response that promotes clearance of infection and long-term immune memory. Microbial pathogens possess specialized proteins termed virulence factors, which interfere with host defence at several levels. Many virulence factors from diverse pathogens have been identified in recent years and their functions linked to disruption of essential processes of immune defence, from signalling to phagocytosis. Although the diversity of pathogens and virulence factors is immense, common themes have emerged with regard to how microbial pathogens interfere with immune responses. Here we discuss recent advances in our understanding of how virulence factors target innate and adaptive immune responses, focusing on bacterial pathogens. We also propose that pathogens responsible for causing acute infection tend to target central components (hubs) of cellular signalling pathways, causing global disruption of the host response. By contrast, pathogens that cause chronic or persistent infections tend to target more peripheral signalling network components (nodes) to promote pathogen persistence.

Pneumonic plague pathogenesis and immunity in Brown Norway rats

The Brown Norway rat was recently described as a bubonic plague model that closely mimics human disease. We therefore evaluated the Brown Norway rat as an alternative small animal model for pneumonic plague and characterized both the efficacy and potency of vaccine candidates. When infected by intranasal instillation, these rats rapidly developed fatal pneumonic plague within 2 to 4 days of infection. Plague disease was characterized by severe alveolar edema and vascular hemorrhage in the lung in addition to fulminant necrotizing pneumonia caused by massive bacterial replication and inflammation. Twenty-four hours before death, animals developed systemic disease with an apparent delayed inflammatory response. We evaluated the ability of the protective antigen, LcrV, and a mutant derivative, V10, to protect these rats from pneumonic plague. Both were highly effective vaccines because complete protection was observed at challenge doses of 7500 LD(50). Antibody analyses suggested stronger potency of V10 immune sera compared with LcrV in the passive transfer of immunity to bubonic plague, with multiple neutralizing epitopes in LcrV. Taken together, these data demonstrate the effectiveness of inhibiting type III secretion in the prevention of pneumonic plague in rats and reveal critical contributions from both the cellular and humoral immune systems. Thus, the Brown Norway rat is an appealing alternative small animal model for the study of pneumonic plague pathogenesis and immunity.

Characterization of new substrates targeted by Yersinia tyrosine phosphatase YopH

YopH is an exceptionally active tyrosine phosphatase that is essential for virulence of Yersinia pestis, the bacterium causing plague. YopH breaks down signal transduction mechanisms in immune cells and inhibits the immune response. Only a few substrates for YopH have been characterized so far, for instance p130Cas and Fyb, but in view of YopH potency and the great number of proteins involved in signalling pathways it is quite likely that more proteins are substrates of this phosphatase. In this respect, we show here YopH interaction with several proteins not shown before, such as Gab1, Gab2, p85, and Vav and analyse the domains of YopH involved in these interactions. Furthermore, we show that Gab1, Gab2 and Vav are not dephosphorylated by YopH, in contrast to Fyb, Lck, or p85, which are readily dephosphorylated by the phosphatase. These data suggests that YopH might exert its actions by interacting with adaptors involved in signal transduction pathways, what allows the phosphatase to reach and dephosphorylate its susbstrates.

Horizontal gene transfer: sustaining pathogenicity and optimizing host-pathogen interactions

Successful host-pathogen interactions require the presence, maintenance and expression of gene cassettes called 'pathogenicity islands' (PAIs) and 'metabolic islands' (MAIs) in the respective pathogen. The products of these genes confer on the pathogen the means to recognize their host(s) and to efficiently evade host defences in order to colonize, propagate within the host and eventually disseminate from the host. Virulence effectors secreted by type III and type IV secretion systems, among others, play vital roles in sustaining pathogenicity and optimizing host-pathogen interactions. Complete genome sequences of plant pathogenic bacteria have revealed the presence of PAIs and MAIs. The genes of these islands possess mosaic structures with regions displaying differences in nucleotide composition and codon usage in relation to adjacent genome structures, features that are highly suggestive of their acquisition from a foreign donor. These donors can be other bacteria, as well as lower members of the Archaea and Eukarya. Genes that have moved from the domains Archaea and Eukarya to the domain Bacteria are true cases of horizontal gene transfer. They represent interdomain genetic transfer. Genetic exchange between distinct members of the domain Bacteria, however, represents lateral gene transfer, an intradomain event. Both horizontal and lateral gene transfer events have been used to facilitate survival fitness of the pathogen.

A novel autotransporter adhesin is required for efficient colonization during bubonic plague

Many proteins secreted by the type V secretion system (autotransporters) have been linked to virulence in gram-negative bacteria. Several putative conventional autotransporters are present in the Yersinia pestis genome, but only one, YapE, is conserved in the other pathogenic Yersinia species. Here, we introduce YapE and demonstrate that it is secreted via a type V mechanism. Inactivation of yapE in Y. pestis results in decreased efficiency in colonization of tissues during bubonic infection. Coinfection with wild-type bacteria only partially compensates for this defect. Analysis of the host immune response suggests that YapE is required for either efficient colonization at the inoculation site or dissemination to draining lymph nodes. YapE also demonstrates adhesive properties capable of mediating interactions with bacteria and eukaryotic cells. These findings support a role for YapE in modulating host-pathogen interactions that are important for colonization of the mammalian host.

Impassable YscP substrates and their impact on the Yersinia enterocolitica type III secretion pathway

Yersinia type III machines secrete protein substrates across the bacterial envelope and, following assembly of their secretion needles, transport effector Yops into host cells. According to their destination during type III secretion, early, middle, and late secretion substrates can be distinguished; however, the signals and mechanisms whereby these proteins are recognized and transported by the secretion machine are not understood. Here, we examine several hybrids between secretion substrates and the impassable reporter protein glutathione S-transferase (GST). YscP-GST and YopR-GST blocked type III secretion; however, YscF-, YopD-, YopN-, and LcrV-GST did not. Unlike YopR-GST, which can block type III machines only during their assembly, expression of YscP-GST led to an immediate and complete block of all secretion. The secretion signal of YscP was mapped to its first 10 codons or amino acids; however, YscP(Delta 2-15)-GST, lacking this secretion signal, imposed a partial blockade. YscP-GST copurified with the type III ATPase complex (YscN, YscL, and YscQ) and with YscO, suggesting that the association of specific machine components with the impassable substrate may cause the block in type III secretion.

Yersinia outer proteins: Yops

The pathogenic bacteria Yersinia spp. contain a virulence plasmid that encodes a type III secretion system and effectors. During infection, four of the effectors target the actin cytoskeleton, crippling the phagocytic machinery in the infected cell. The remaining two effectors dampen the innate immune response by targeting important signalling pathways. Although the biochemical activity for each of these effectors is known, the mechanisms involved in their ordered secretion and delivery remain elusive.

Yersinia as oral live carrier vaccine: influence of Yersinia outer proteins (Yops) on the T-cell response

Attenuated enteropathogenic Yersinia strains are attractive candidates for the development of oral live carrier vaccines. Yersiniae colonize the small intestine and invade lymphoid tissue of the terminal ileum where they replicate extracellularly. Yersiniae can be engineered to secrete or translocate heterologous antigens into the cytosol of antigen-presenting cells by their type 3 secretion system (T3SS). This results in the induction of both cellular and humoral immune responses to heterologous antigens of viral, bacterial and parasitic origin. In this review, we summarize the progress in developing Yersinia-based vaccine carrier strains by mutating the T3SS effector proteins of Yersinia called Yops (Yersinia outer proteins) to both attenuate the strains and to modulate the T-cell response.

An injected bacterial effector targets chromatin access for transcription factor NF-kappaB to alter transcription of host genes involved in immune responses

Phosphorylation of histone H3 at Ser10 increases chromatin accessibility to transcription factor NF-kappaB on a subset of genes involved in immune responses. Here we report that a bacterial pathogen abrogated phosphorylation of histone H3 to 'shape' the transcriptional responses of infected host cells. We identify the Shigella flexneri protein effector OspF as a dually specific phosphatase that dephosphorylated mitogen-activated protein kinases in the nucleus, thus preventing histone H3 phosphorylation at Ser10 in a gene-specific way. That activity of OspF enabled shigella to block the activation of a subset of NF-kappaB-responsive genes, leading to compromised recruitment of polymorphonuclear leukocytes to infected tissues. S. flexneri has thus evolved the capacity to precisely modulate host cell epigenetic 'information' as a strategy for repressing innate immunity.

Integrin traffic

Cell adhesion, migration and the maintenance of cell polarity are all processes that depend on the correct targeting of integrins and the dynamic remodelling of integrin-containing adhesion sites. The importance of the endo/exocytic cycle of integrins as a key regulator of these functions is increasingly recognized. Several recent publications have provided mechanistic insight into how integrin traffic is regulated in cells. Increasing evidence suggests that small GTPases such as Arf6 and members of the Rab family control integrin internalization and recycling back to the plasma membrane along microtubules. The fine tuning of these trafficking events seems to be mediated by specific guanine-nucleotide-exchange factors (GEFs) and GTPase-activating proteins (GAPs). In addition, several kinases regulate integrin traffic. The identification of their substrates has demonstrated how these kinases regulate integrin traffic by controlling small GTPases or stabilizing cytoskeletal tracks that are crucial for efficient traffic of integrins to the plasma membrane.

The type III secretion injectisome

The type III secretion injectisome is a complex nanomachine that allows bacteria to deliver protein effectors across eukaryotic cellular membranes. In recent years, significant progress has been made in our understanding of its structure, assembly and mode of operation. The principal structural components of the injectisome, from the base located in the bacterial cytosol to the tip of the needle protruding from the cell surface, have been investigated in detail. The structures of several constituent proteins were solved at the atomic level and important insights into the assembly process have been gained. However, despite the ongoing concerted efforts of molecular and structural biologists, the role of many of the constituent components of this nanomachine remain unknown.