Yersinia enterocolitica: subversion of adaptive immunity and implications for vaccine development

Sleep enhances numbers and function of monocytes and improves bacterial infection outcome in mice

Sleep strongly impacts both humoral and cellular immunity; however, its acute effects on the innate immune defense against pathogens are unclear. Here, we elucidated in mice whether sleep affects the numbers and functions of innate immune cells and their defense against systemic bacterial infection. Sleep significantly increased numbers of classical monocytes in blood and spleen of mice that were allowed to sleep for six hours at the beginning of the normal resting phase compared to mice kept awake for the same time. The sleep-induced effect on classical monocytes was neither caused by alterations in corticosterone nor myelopoiesis, bone marrow egress or death of monocytes and did only partially involve Gαi-protein coupled receptors like chemokine receptor 2 (CCR2), but not the adhesion molecules intercellular adhesion molecule 1 (ICAM-1) or lymphocyte function-associated antigen 1 (LFA-1). Notably, sleep suppressed the expression of the clock gene Arntl in splenic monocytes and the sleep-induced increase in circulating classical monocytes was abrogated in Arntl-deficient animals, indicating that sleep is a prerequisite for clock-gene driven rhythmic trafficking of classical monocytes. Sleep also enhanced the production of reactive oxygen species by monocytes and neutrophils. Moreover, sleep profoundly reduced bacterial load in blood and spleen of mice that were allowed to sleep before systemic bacterial infection and consequently increased survival upon infection. These data provide the first evidence that sleep enhances numbers and function of innate immune cells and therewith strengthens early defense against bacterial pathogens.

Assessment of the efficacy of an autogenous vaccine against Yersinia pseudotuberculosis in young Merino sheep

AIMS To assess the efficacy of an autogenous vaccine against Yersinia pseudotuberculosis III in preventing clinical disease and deaths due to yersiniosis in young Merino sheep, and to determine the effect of vaccination on the prevalence of faecal shedding of pathogenic Yersinia spp., daily liveweight gain, and development of antibodies to Yersinia spp. following vaccination and natural exposure. METHODS In six groups (three groups each from two farms) of young Merino sheep, 148-150 animals were systematically allocated to be vaccinated twice with an autogenous, formalin- killed bacterin vaccine containing Y. pseudotuberculosis serotype III or to remain non-vaccinated. All vaccinated and non-vaccinated sheep were run together in their original groups throughout the trial. Faecal and blood samples were collected, and liveweight measured, at the time of vaccination and subsequently over a 6-month period to determine faecal shedding of Y. enterocolitica and Y. pseudotuberculosis, seroprevalence of antibodies to Yersinia outer membrane proteins (YOP) and changes in liveweight. RESULTS None of the six trial groups experienced an outbreak of clinical yersiniosis during the study period. On Farm A, the prevalence of shedding of either or both Yersinia spp. was <40% on all but one sampling occasions. On Farm B the prevalence of shedding of both Yersinia spp. peaked at 98%, 96 days after vaccination. Mean liveweight and daily liveweight gain at the end of the study were similar in vaccinated and non-vaccinated groups on both farms (p>0.1), as was the prevalence of faecal shedding of Yersinia spp. (p>0.2), and the proportion of animals that became seropositive for antibodies to YOP following vaccination (p>0.1). CONCLUSIONS AND CLINICAL RELEVANCE This vaccine had, at most, limited effects on seroconversion and, under the conditions of this study, had no demonstrable impact on liveweight, mean daily liveweight gain or faecal shedding of Yersinia spp. Further studies are needed to determine the efficacy of this vaccine during outbreaks of yersiniosis or following experimental challenge with pathogenic Yersinia spp..

Invasin-functionalized liposome nanocarriers improve the intracellular delivery of anti-infective drugs

Liposomes containing gentamicin and surface-functionalized with InvA497 showed a reduced infection load of both cytosolic and vacuolar intracellular bacteria.

Co-expression of the C-terminal domain of Yersinia enterocolitica invasin enhances the efficacy of classical swine-fever-vectored vaccine based on human adenovirus

The use of adenovirus vector-based vaccines is a promising approach for generating antigen-specific immune responses. Improving vaccine potency is necessary in other approaches to address their inadequate protection for the majority of infectious diseases. This study is the first to reconstruct a recombinant replication-defective human adenovirus co-expressing E2 and invasin C-terminal (InvC) glycoproteins (rAd-E2-InvC). rAd-E2-InvC with 2 x 10(6) TCID50 was intramuscularly administered two times to CSFV-free pigs at 14 day intervals. No adverse clinical reactions were observed in any of the pigs after the vaccination. The CSFV E2-specific antibody titer was significantly higher in the rAd-E2-InvC group than that in the rAdV-E2 group as measured by NPLA and blocking ELISA. Pigs immunized with rAd-E2-InvC were completely protected against lethal challenge. Neither CSFV RNA nor pathological changes were detected in the tissues after CSFV challenge. These results demonstrate that rAd-E2-InvC could be an alternative to the existing CSF vaccine. Moreover, InvC that acts as an adjuvant could enhance the immunogenicity of rAdV-E2 and induce high CSFV E2-specific antibody titer and protection level.

Bacteriomimetic invasin-functionalized nanocarriers for intracellular delivery

Intracellular bacteria invade mammalian cells to establish an infectious niche. The current work models adhesion and subsequent internalization strategy of pathogenic bacteria into mammalian cells to design a bacteriomimetic bioinvasive delivery system. We report on the surface functionalization of liposomes with a C-terminal fragment of invasin (InvA497), an invasion factor in the outer membrane of Yersinia pseudotuberculosis. InvA497-functionalized liposomes adhere to mammalian epithelial HEp-2 cell line at different infection stages with a significantly higher efficiency than liposomes functionalized with bovine serum albumin. Covalent attachment of InvA497 results in higher cellular adhesion than liposomes with physically adsorbed InvA497 with non-specific surface protein alignment. Uptake studies in HEp-2 cells indicate active internalization of InvA497-functionalized liposomes via β1-integrin receptor-mediated uptake mechanism mimicking the natural invasion strategy of Y. pseudotuberculosis. Uptake studies in Caco-2 cells at different polarization states demonstrate specific targeting of the InvA497-functionalized liposomes to less polarized cells reflecting the status of inflamed cells. Moreover, when loaded with the anti-infective agent gentamicin and applied to HEp-2 cells infected with Y. pseudotuberculosis, InvA497-functionalized liposomes are able to significantly reduce the infection load relative to non-functionalized drug-loaded liposomes. This indicates a promising application of such a bacteriomimetic system for drug delivery to intracellular compartments.

Human and animal isolates of Yersinia enterocolitica show significant serotype-specific colonization and host-specific immune defense properties

Yersinia enterocolitica is a human pathogen that is ubiquitous in livestock, especially pigs. The bacteria are able to colonize the intestinal tract of a variety of mammalian hosts, but the severity of induced gut-associated diseases (yersiniosis) differs significantly between hosts. To gain more information about the individual virulence determinants that contribute to colonization and induction of immune responses in different hosts, we analyzed and compared the interactions of different human- and animal-derived isolates of serotypes O:3, O:5,27, O:8, and O:9 with murine, porcine, and human intestinal cells and macrophages. The examined strains exhibited significant serotype-specific cell binding and entry characteristics, but adhesion and uptake into different host cells were not host specific and were independent of the source of the isolate. In contrast, survival and replication within macrophages and the induced proinflammatory response differed between murine, porcine, and human macrophages, suggesting a host-specific immune response. In fact, similar levels of the proinflammatory cytokine macrophage inflammatory protein 2 (MIP-2) were secreted by murine bone marrow-derived macrophages with all tested isolates, but the equivalent interleukin-8 (IL-8) response of porcine bone marrow-derived macrophages was strongly serotype specific and considerably lower in O:3 than in O:8 strains. In addition, all tested Y. enterocolitica strains caused a considerably higher level of secretion of the anti-inflammatory cytokine IL-10 by porcine than by murine macrophages. This could contribute to limiting the severity of the infection (in particular of serotype O:3 strains) in pigs, which are the primary reservoir of Y. enterocolitica strains pathogenic to humans.

Bacterial invasion factors: tools for crossing biological barriers and drug delivery?

The oral route is the preferential route of drug delivery in humans. However, effective delivery through the gastrointestinal tract is often hampered by the low permeability of the intestinal epithelium. One possibility to overcome this problem is the encapsulation of drugs inside nanoparticulate systems, containing targeting moieties with cell invasive properties. The bioinvasive features of the delivery system could be provided by the attachment of bacterial invasion factors, which promote efficient uptake into host cells and mediate rapid transcytosis of the pathogen through the intestinal epithelium. This review gives an overview of bacterial invasion systems. The molecular structure and function of suitable bacterial invasins, their relative values as targeting agents and possible pitfalls of their use are described. The potential of bioinvasive drug delivery systems is mainly presented on the basis of the well-characterized Yersinia invasin protein, which enters M cells to gain access to subepithelial layers of the gastrointestinal tract, but alternative approaches and future prospects for oral drug delivery are also discussed.

Edwardsiella tarda Eta1, an in vivo-induced antigen that is involved in host infection

Edwardsiella tarda, a Gram-negative bacterium, is a severe fish pathogen that can also infect humans. In this study, we identified, via in vivo-induced antigen technology, an E. tarda antigen, Eta1, and analyzed its function in a Japanese flounder (Paralichthys olivaceus) model. Eta1 is composed of 226 residues and shares homology with putative bacterial adhesins. Quantitative real-time reverse transcriptase (RT)-PCR analysis indicated that when cultured in vitro, eta1 expression was growth phase dependent and reached maximum at mid-logarithmic phase. During infection of flounder lymphocytes, eta1 expression was drastically increased at the early stage of infection. Compared to the wild type, the eta1-defective mutant, TXeta1, was unaffected in growth but exhibited attenuated overall virulence, reduced tissue dissemination and colonization capacity, and impaired ability to invade flounder lymphocytes and to block the immune response of host cells. The lost virulence of TXeta1 was restored when a functional eta1 gene was reintroduced into the strain. Western blot and immunodetection analyses showed that Eta1 is localized to the outer membrane and exposed on the surface of E. tarda and that recombinant Eta1 (rEta1) was able to interact with flounder lymphocytes. Consistent with these observations, antibody blocking of Eta1 inhibited E. tarda infection at the cellular level. Furthermore, when used as a subunit vaccine, rEta1 induced strong protective immunity in flounder against lethal E. tarda challenge. Taken together, these results indicate that Eta1 is an in vivo-induced antigen that mediates pathogen-host interaction and, as a result, is required for optimal bacterial infection.

Inv1: an Edwardsiella tarda invasin and a protective immunogen that is required for host infection

Invasin is an outer membrane protein that is known to mediate entry of enteric bacteria into mammalian cells. In this study, we analyzed the function and immunoprotective potential of the invasin Inv1 from Edwardsiella tarda, a serious fish pathogen that can also infect humans. In silico analysis indicated that Inv1 possesses a conserved N-terminal DUF3442 domain and a C-terminal group 1 bacterial Ig-like domain. Subcellular localization analysis showed that Inv1 is exposed on cell surface and could be recognized by specific antibodies. Mutation of inv1 had no effect on bacterial growth but attenuates overall bacterial virulence and impaired the ability of E. tarda to attach and invade into host cells. Consistent with these observations, antibody blocking of Inv1 inhibited E. tarda infection of host cells. To examine the immunoprotective potential of Inv1, recombinant Inv1 (rInv1) corresponding to the DUF3442 domain was purified and used to vaccinate Japanese flounder (Paralichthys olivaceus). The results showed that rInv1 induced strong protection against lethal-dose challenge of E. tarda. ELISA analysis showed that rInv1-vaccinated fish produced specific serum antibodies that could enhance the serum bactericidal activity against E. tarda. Taken together, these results indicate that Inv1 is a surface-localized virulence factor that is involved in host infection and can induce effective immunoprotection when used as a subunit vaccine.

A protective epitope in type III effector YopE is a major CD8 T cell antigen during primary infection with Yersinia pseudotuberculosis

Virulence in human-pathogenic Yersinia species is associated with a plasmid-encoded type III secretion system that translocates a set of Yop effector proteins into host cells. One effector, YopE, functions as a Rho GTPase-activating protein (GAP). In addition to acting as a virulence factor, YopE can function as a protective antigen. C57BL/6 mice infected with attenuated Yersinia pestis generate a dominant H2-Kb-restricted CD8 T cell response to an epitope in the N-terminal domain of YopE (YopE69-77), and intranasal vaccination with the YopE69-77 peptide and the mucosal adjuvant cholera toxin (CT) elicits CD8 T cells that are protective against lethal pulmonary challenge with Y. pestis. Because YopE69-77 is conserved in many Yersinia strains, we sought to determine if YopE is a protective antigen for Yersinia pseudotuberculosis and if primary infection with this enteric pathogen elicits a CD8 T cell response to this epitope. Intranasal immunization with the YopE69-77 peptide and CT elicited a CD8 T cell response that was protective against lethal intragastric Y. pseudotuberculosis challenge. The YopE69-77 epitope was a major antigen (∼30% of splenic CD8 T cells were specific for this peptide at the peak of the response) during primary infection with Y. pseudotuberculosis, as shown by flow cytometry tetramer staining. Results of infections with Y. pseudotuberculosis expressing catalytically inactive YopE demonstrated that GAP activity is dispensable for a CD8 T cell response to YopE69-77. Determining the features of YopE that are important for this response will lead to a better understanding of how protective CD8 T cell immunity is generated against Yersinia and other pathogens with type III secretion systems.

DNA vaccines for targeting bacterial infections

DNA vaccination has been of great interest since its discovery in the 1990s due to its ability to elicit both humoral and cellular immune responses. DNA vaccines consist of a DNA plasmid containing a transgene that encodes the sequence of a target protein from a pathogen under the control of a eukaryotic promoter. This revolutionary technology has proven to be effective in animal models and four DNA vaccine products have recently been approved for veterinary use. Although few DNA vaccines against bacterial infections have been tested, the results are encouraging. Because of their versatility, safety and simplicity a wider range of organisms can be targeted by these vaccines, which shows their potential advantages to public health. This article describes the mechanism of action of DNA vaccines and their potential use for targeting bacterial infections. In addition, it provides an updated summary of the methods used to enhance immunogenicity from codon optimization and adjuvants to delivery techniques including electroporation and use of nanoparticles.

YopJ-promoted cytotoxicity and systemic colonization are associated with high levels of murine interleukin-18, gamma interferon, and neutrophils in a live vaccine model of Yersinia pseudotuberculosis infection

Several Yersinia species have been utilized as live attenuated vaccines to prime protective immunity against yersiniae and other pathogens. A type III secretion system effector known as YopJ in Y. pseudotuberculosis and Y. pestis and YopP in Y. enterocolitica has been shown to regulate host immune responses to live Yersinia vaccines. YopJ/P kills macrophages and dendritic cells, reduces their production of tumor necrosis factor alpha (TNF-alpha) and interleukin-12 (IL-12), and promotes systemic colonization in mouse models of intestinal Yersinia infection. Furthermore, YopP activity decreases antigen presentation by dendritic cells, and a yopP mutant of a live Y. enterocolitica carrier vaccine elicited effective priming of CD8 T cells to a heterologous antigen in mice. These results suggest that YopJ/P activity suppresses both innate and adaptive immune responses to live Yersinia vaccines. Here, a sublethal intragastric mouse infection model using wild-type and catalytically inactive yopJ mutant strains of Y. pseudotuberculosis was developed to further investigate how YopJ action impacts innate and adaptive immune responses to a live vaccine. Surprisingly, YopJ-promoted cytotoxicity and systemic colonization were associated with significant increases in neutrophils in spleens and the proinflammatory cytokines IL-18 and gamma interferon (IFN-gamma) in serum samples of mice vaccinated with Y. pseudotuberculosis. Secretion of IL-18 accompanied YopJ-mediated killing of macrophages infected ex vivo with Y. pseudotuberculosis, suggesting a mechanism by which this effector directly increases proinflammatory cytokine levels in vivo. Mice vaccinated with the wild-type strain or the yopJ mutant produced similar levels of antibodies to Y. pseudotuberculosis antigens and were equally resistant to lethal intravenous challenge with Y. pestis. The findings indicate that a proinflammatory, rather than anti-inflammatory, process accompanies YopJ-promoted cytotoxicity, leading to increased systemic colonization by Y. pseudotuberculosis and potentially enhancing adaptive immunity to a live vaccine.

Yersinia enterocolitica Yop mutants as oral live carrier vaccines

Attenuated enteropathogenic yersiniae that translocate heterologous antigens into the cytosol of antigen presenting cells via their type three secretion system (TTSS) are considered promising candidates for the development of live oral vaccine carrier strains that induce CD8 T cell responses. Wild type Yersinia enterocolitica of serotype O:8 however efficiently suppresses the immune response of the host by translocating effector proteins called Yersinia outer proteins (Yops) into the cytosol of immune cells. We therefore tested immunogenicity, protective efficacy, and virulence ofyop mutants that translocate the model antigen Listeriolysin (LLO) of Listeria monocytogenes in a mouse model. A deltayopP mutant-based vaccine carrier strain induced the highest numbers of LLO91-99-specific CD8 T cells and effectively protected mice against a lethal challenge with Listeria whereas deltayopPT, deltayopPV(K42Q), and deltayopPO mutants of Y. enterocolitica induced fewer CD8 T cells and conferred only partial protection. The deltayopPH, deltayopPE, deltayopPM, and deltayopPQ mutants induced the weakest CD8 T cell response and did not significantly protect mice against Listeria presumably due to the strong attenuation of these strains in the mouse model. Even though a Y. enterocolitica strain WA-C(pTTSS), which translocated only LLO (but not Yops), induced superior MHC class I-restricted antigen presentation in DC compared to the deltayopP mutants in vitro, this strain was not able to significantly colonize mouse tissue or to induce CD8 T cell responses in vivo. The success in designing a Yersinia oral vaccine carrier is therefore dependent to a great extent on the subtle balance between immunogenicity and attenuation.