T lymphocytes mediate protection against Yersinia enterocolitica in mice: characterization of murine T-cell clones specific for Y. enterocolitica

Lung Resident Memory T Cells Activated by Oral Vaccination Afford Comprehensive Protection against Pneumonic Yersinia pestis Infection

A growing body of evidence has shown that resident memory T (TRM) cells formed in tissue after mucosal infection or vaccination are crucial for counteracting reinfection by pathogens. However, whether lung TRM cells activated by oral immunization with Yptb1(pYA5199) play a protective role against pneumonic plague remains unclear. In this study, we demonstrated that lung CD4+ and CD8+ TRM cells significantly accumulated in the lungs of orally Yptb1(pYA5199)-vaccinated mice and dramatically expanded with elevated IL-17A, IFN-γ, and/or TNF-α production after pulmonary Yersinia pestis infection and afforded significant protection. Short-term or long-term treatment of immunized mice with FTY720 did not affect lung TRM cell formation and expansion or protection against pneumonic plague. Moreover, the intratracheal transfer of both lung CD4+ and CD8+ TRM cells conferred comprehensive protection against pneumonic plague in naive recipient mice. Lung TRM cell-mediated protection was dramatically abolished by the neutralization of both IFN-γ and IL-17A. Our findings reveal that lung TRM cells can be activated via oral Yptb1(pYA5199) vaccination, and that IL-17A and IFN-γ production play an essential role in adaptive immunity against pulmonary Y. pestis infection. This study highlights an important new target for developing an effective pneumonic plague vaccine.

Remarkably Robust Antiviral Immune Response despite Combined Deficiency in Caspase-8 and RIPK3

Caspase-8 (Casp8)-mediated signaling triggers extrinsic apoptosis while suppressing receptor-interacting protein kinase (RIPK) 3-dependent necroptosis. Although Casp8 is dispensable for the development of innate and adaptive immune compartments in mice, the importance of this proapoptotic protease in the orchestration of immune response to pathogens remains to be fully explored. In this study, Casp8^-/-Ripk3^-/- C57BL/6 mice show robust innate and adaptive immune responses to the natural mouse pathogen, murine CMV. When young, these mice lack lpr-like lymphoid hyperplasia and accumulation of either B220 ⁺ CD3⁺ or B220^-CD3⁺CD4⁺ and CD8⁺ T cells with increased numbers of immature myeloid cells that are evident in older mice. Dendritic cell activation and cytokine production drive both NK and T cell responses to control viral infection in these mice, suggesting that Casp8 is dispensable to the generation of antiviral host defense. Curiously, NK and T cell expansion is amplified, with greater numbers observed by 7 d postinfection compared with either Casp8^+/-Ripk3^-/- or wild type (Casp8^+/+Ripk3^+/+ ) littermate controls. Casp8 and RIPK3 are natural targets of virus-encoded cell death suppressors that prevent infected cell apoptosis and necroptosis, respectively. It is clear from the current studies that the initiation of innate immunity and the execution of cytotoxic lymphocyte functions are all preserved despite the absence of Casp8 in responding cells. Thus, Casp8 and RIPK3 signaling is completely dispensable to the generation of immunity against this natural herpesvirus infection, although the pathways driven by these initiators serve as a crucial first line for host defense within virus-infected cells.

IL-12/23p40 overproduction by dendritic cells leads to an increased Th1 and Th17 polarization in a model of Yersinia enterocolitica-induced reactive arthritis in TNFRp55-/- mice

Dendritic cells (DCs) play critical functions in the initiation of immune responses. Understanding their role in reactive arthritis (ReA) will help delineate the pathogenesis of this arthropathy. In early studies, we detected IL-12/23p40 deregulation in Yersinia entercolitica (Ye)-induced ReA in TNFRp55-deficient (TNFRp55^-/-) mice. In this study, we assessed the contribution of DCs in this overproduction. First, greater levels of IL-12/23p40, IFN-γand IL-17A were confirmed in supernatants of lipopolysaccharide (LPS)-stimulated TNFRp55^-/-splenocytes obtained on arthritis onset (day 14 after Ye infection). Later, DCs were identified as a precise source of IL-12/23p40 since increased frequency of splenic IL-12/23p40⁺DCs was detected in TNFRp55^-/- mice. After robust in vivo amplification of DCs by injection of Fms-like tyrosine kinase 3-Ligand (Flt3L)-transfected BL16 melanoma, DCs were purified. These cells recapitulated the higher production of IL-12/23p40 under TNFRp55deficiency. In agreement with these results, TNFRp55^-/- DCs promoted Th1 and Th17 programs by co-culture with WT CD4⁺lymphocytes. A mechanistic study demonstrated that JNK and p38 MAPK pathways are involved in IL-12/23p40 overproduction in purified TNFRp55^-/- DCs as well as in the JAWS II cell line. This deregulation was once again attributed to TNFRp55 deficiency since CAY10500, a specific inhibitor of this pathway, compromised TNF-mediated IL-12/23p40 control in LPS-stimulated WT DCs. Simultaneously, this inhibition reduced IL-10 production, suggesting its role mediating IL-12/23p40 regulation by TNFRp55 pathway. These results provide experimental data on the existence of a TNFRp55-mediated anti-inflammatory circuit in DCs. Moreover, these cells may be considered as a novel target in the treatment of ReA.

Yersinia pseudotuberculosis supports Th17 differentiation and limits de novo regulatory T cell induction by directly interfering with T cell receptor signaling

Adaptive immunity critically contributes to control acute infection with enteropathogenic Yersinia pseudotuberculosis; however, the role of CD4⁺ T cell subsets in establishing infection and allowing pathogen persistence remains elusive. Here, we assessed the modulatory capacity of Y. pseudotuberculosis on CD4⁺ T cell differentiation. Using in vivo assays, we report that infection with Y. pseudotuberculosis resulted in enhanced priming of IL-17-producing T cells (Th17 cells), whereas induction of Foxp3⁺ regulatory T cells (Tregs) was severely disrupted in gut-draining mesenteric lymph nodes (mLNs), in line with altered frequencies of tolerogenic and proinflammatory dendritic cell (DC) subsets within mLNs. Additionally, by using a DC-free in vitro system, we could demonstrate that Y. pseudotuberculosis can directly modulate T cell receptor (TCR) downstream signaling within naïve CD4⁺ T cells and Tregs via injection of effector molecules through the type III secretion system, thereby affecting their functional properties. Importantly, modulation of naïve CD4⁺ T cells by Y. pseudotuberculosis resulted in an enhanced Th17 differentiation and decreased induction of Foxp3⁺ Tregs in vitro. These findings shed light to the adjustment of the Th17-Treg axis in response to acute Y. pseudotuberculosis infection and highlight the direct modulation of CD4⁺ T cell subsets by altering their TCR downstream signaling.

Proinflammatory microenvironments within the intestine regulate the differentiation of tissue-resident CD8⁺ T cells responding to infection

We report that oral infection with Yersinia pseudotuberculosis (Yptb) results in development of two distinct populations of pathogen-specific CD8 tissue-resident memory T (T_RM) cells in the lamina propria (LP). CD103^– T cells did not require transforming-growth factor-β (TGF-β) signaling, but were true resident memory cells. Unlike CD103⁺ CD8 T cells, which were TGF-β-dependent and scattered in the tissue, CD103^– T cells clustered with CD4 T cells and CX3CR1⁺ macrophages and/or dendritic cells around areas of bacterial infection. CXCR3-dependent recruitment to inflamed areas was critical for development of the CD103^– population and pathogen clearance. These studies have identified the preferential development of CD103^– LP T_RM cells in inflammatory microenvironments within the LP and suggest that this subset plays a critical role in controlling infection.

Insights how monocytes and dendritic cells contribute and regulate immune defense against microbial pathogens

The immune system protects from infections primarily by detecting and eliminating invading pathogens. Beside neutrophils, monocytes and dendritic cells (DCs) have been recently identified as important sentinels and effectors in combating microbial pathogens. In the steady state mononuclear phagocytes like monocytes and DCs patrol the blood and the tissues. Mammalian monocytes contribute to antimicrobial defense by supplying tissues with macrophage and DC precursors. DCs recognize pathogens and are essential in presenting antigens to initiate antigen-specific adaptive immune responses, thereby bridging the innate and adaptive immune systems. Both, monocytes and DCs play distinct roles in the shaping of immune response. In this review we will focus on the contributions of monocytes and lymphoid organ DCs to immune defense against microbial pathogens in the mouse and their dynamic regulation from steady state to infection.

Influence of PhoP and intra-species variations on virulence of Yersinia pseudotuberculosis during the natural oral infection route

The two-component regulatory system PhoP/PhoQ has been shown to (i) control expression of virulence-associated traits, (ii) confer survival and growth within macrophages and (iii) play a role in Yersinia infections. However, the influence of PhoP on virulence varied greatly between different murine models of infection and its role in natural oral infections with frequently used representative isolates of Y. pseudotuberculosis was unknown. To address this issue, we constructed an isogenic set of phoP⁺ and phoP⁻ variants of strain IP32953 and YPIII and analyzed the impact of PhoP using in vitro functionality experiments and a murine oral infection model, whereby we tested for bacterial dissemination and influence on the host immune response. Our results revealed that PhoP has a low impact on virulence, lymphatic and systemic organ colonization, and on immune response modulation by IP32953 and YPIII, indicating that PhoP is not absolutely essential for oral infections but may be involved in fine-tuning the outcome. Our work further revealed certain strain-specific differences in virulence properties, which do not strongly rely on the function of PhoP, but affect tissue colonization, dissemination and/or persistence of the bacteria. Highlighted intra-species variations may provide a potential means to rapidly adjust to environmental changes inside and outside of the host.

Yersinia pseudotuberculosis enterocolitis mimicking enteropathic γδ T-cell lymphoma with abnormal clonality

BACKGROUND

Yersinia pseudotuberculosis generally infects the gastrointestinal tract and causes enteropathy symptoms suggesting infection. Y. pseudotuberculosis infections are often complicated with intraceliac lymphoadenopathy mimicking malignant lymphoma. This is a first case of Yersinia pseudotuberculosis enteropathy mimicking enteropathic γδ T-cell lymphoma. This case highlighted the γδ T-cell reaction to Yersinia enterocolitis sometimes mimicking malignant lymphoma clinically.

CASE PRESENTATION

A 72-year-old female was referred to our institute due to abdominal pain with skin rush, fever and diarrhea. Computed tomography (CT) scanning revealed mucosal swelling of the cecum with enlargement of regional lymph nodes. Laboratory data showed elevated CRP (7.74 mg/dL), an increased level of soluble interleukin-2 receptor (sIL-2R 3095 IU/mL), and CD3+ γδ T-cell circulation in peripheral blood and bone marrow (10.9% and 3.9%, respectively). Increased proportions of γδ T-cells supported the diagnosis of malignant lymphoma. Colonoscopy demonstrated hemorrhagic mucosal erosion with partial ulceration, and the subsequent pathological findings at the inflammation site suggested malignant lymphoma histopathology in the colon. These objective findings were entirely consistent with enteropathic γδ T-cell lymphoma. Thereafter, however, the microbiological results of the patient's stool at admission showed Yersinia pseudotuberculosis, and she was diagnosed as having Yersinia enterocolitis. All abnormal findings including subjective symptoms were in remission or mitigated within 2 weeks after her onset. Even the γδ T-cell circulation disappeared (0.04% in peripheral blood), and we speculate that those cells were a reaction to the Yersinia infection.

CONCLUSION

In this case, a differential diagnosis included infectious enterocolitis from other immunogenic or malignant diseases. Although a measurement of sIL-2R is critical in differentiating malignant lymphoma in patients suffering with lymph adenopathy, that is not confirmative. This patient's case indicates that T cells expressing the γδ T-cell receptor might be associated with the acute and late phase reactions, in which T cells play a role in the construction of granulomas and the establishment of sequelae.

The gut mucosal immune system in the neonatal period

Invasive sepsis in the newborn period is a major cause of childhood morbidity and mortality worldwide. The infant immune system undoubtedly differs intrinsically from the mature adult immune system. Current understanding is that the newborn infant immune system displays a range of competencies and is developing rather than deficient. The infant gut mucosal immune system is complex and displays a plethora of phenotypic and functional irregularities that may be clinically important. Various factors affect and modulate the infant gut mucosal immune system: components of the intestinal barrier, the infant gut microbiome, nutrition and the maternal-infant hybrid immune system. Elucidation of the phenotypic distribution of immune cells, their functional significance and the mucosa-specific pathways used by these cells is essential to the future of research in the field of infant immunology.

Kinetics of innate immune response to Yersinia pestis after intradermal infection in a mouse model

A hallmark of Yersinia pestis infection is a delayed inflammatory response early in infection. In this study, we use an intradermal model of infection to study early innate immune cell recruitment. Mice were injected intradermally in the ear with wild-type (WT) or attenuated Y. pestis lacking the pYV virulence plasmid (pYV(-)). The inflammatory responses in ear and draining lymph node samples were evaluated by flow cytometry and immunohistochemistry. As measured by flow cytometry, total neutrophil and macrophage recruitment to the ear in WT-infected mice did not differ from phosphate-buffered saline (PBS) controls or mice infected with pYV(-), except for a transient increase in macrophages at 6 h compared to the PBS control. Limited inflammation was apparent even in animals with high bacterial loads (10(5) to 10(6) CFU). In addition, activation of inflammatory cells was significantly reduced in WT-infected mice as measured by CD11b and major histocompatibility complex class II (MHC-II) expression. When mice infected with WT were injected 12 h later at the same intradermal site with purified LPS, Y. pestis did not prevent recruitment of neutrophils. However, significant reduction in neutrophil activation remained compared to that of PBS and pYV(-) controls. Immunohistochemistry revealed qualitative differences in neutrophil recruitment to the skin and draining lymph node, with WT-infected mice producing a diffuse inflammatory response. In contrast, focal sites of neutrophil recruitment were sustained through 48 h postinfection in pYV(-)-infected mice. Thus, an important feature of Y. pestis infection is reduced activation and organization of inflammatory cells that is at least partially dependent on the pYV virulence plasmid.

Mitogen-activated protein kinase-dependent interleukin-1α intracrine signaling is modulated by YopP during Yersinia enterocolitica infection

Yersinia enterocolitica is a food-borne pathogen that preferentially infects the Peyer's patches and mesenteric lymph nodes, causing an acute inflammatory reaction. Even though Y. enterocolitica induces a robust inflammatory response during infection, the bacterium has evolved a number of virulence factors to limit the extent of this response. We previously demonstrated that interleukin-1α (IL-1α) was critical for the induction of gut inflammation characteristic of Y. enterocolitica infection. More recently, the known actions of IL-1α are becoming more complex because IL-1α can function both as a proinflammatory cytokine and as a nuclear factor. In this study, we tested the ability of Y. enterocolitica to modulate intracellular IL-1α-dependent IL-8 production in epithelial cells. Nuclear translocation of pre-IL-1α protein and IL-1α-dependent secretion of IL-8 into the culture supernatant were increased during infection with a strain lacking the 70-kDa virulence plasmid compared to the case during infection with the wild type, suggesting that Yersinia outer proteins (Yops) might be involved in modulating intracellular IL-1α signaling. Infection of HeLa cells with a strain lacking the yopP gene resulted in increased nuclear translocation of pre-IL-1α and IL-1α-dependent secretion of IL-8 similar to what is observed with bacteria lacking the virulence plasmid. YopP is a protein acetylase that inhibits mitogen-activated protein kinase (MAP kinase)- and NF-κB-dependent signal transduction pathways. Nuclear translocation of pre-IL-1α and IL-1α-dependent secretion of IL-8 in response to Yersinia enterocolitica infection were dependent on extracellular signal-regulated kinase (ERK) and p38 MAP kinase signaling but independent of NF-κB. These data suggest that Y. enterocolitica inhibits intracellular pre-IL-1α signaling and subsequent proinflammatory responses through inhibition of MAP kinase pathways.

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.

Pathogenesis of Y. enterocolitica and Y. pseudotuberculosis in Human Yersiniosis

Yersiniosis is a food-borne illness that has become more prevalent in recent years due to human transmission via the fecal-oral route and prevalence in farm animals. Yersiniosis is primarily caused by Yersinia enterocolitica and less frequently by Yersinia pseudotuberculosis. Infection is usually characterized by a self-limiting acute infection beginning in the intestine and spreading to the mesenteric lymph nodes. However, more serious infections and chronic conditions can also occur, particularly in immunocompromised individuals. Y. enterocolitica and Y. pseudotuberculosis are both heterogeneous organisms that vary considerably in their degrees of pathogenicity, although some generalizations can be ascribed to pathogenic variants. Adhesion molecules and a type III secretion system are critical for the establishment and progression of infection. Additionally, host innate and adaptive immune responses are both required for yersiniae clearance. Despite the ubiquity of enteric Yersinia species and their association as important causes of food poisoning world-wide, few national enteric pathogen surveillance programs include the yersiniae as notifiable pathogens. Moreover, no standard exists whereby identification and reporting systems can be effectively compared and global trends developed. This review discusses yersinial virulence factors, mechanisms of infection, and host responses in addition to the current state of surveillance, detection, and prevention of yersiniosis.

Immune evasion by Yersinia enterocolitica: differential targeting of dendritic cell subpopulations in vivo

CD4(+) T cells are essential for the control of Yersinia enterocolitica (Ye) infection in mice. Ye can inhibit dendritic cell (DC) antigen uptake and degradation, maturation and subsequently T-cell activation in vitro. Here we investigated the effects of Ye infection on splenic DCs and T-cell proliferation in an experimental mouse infection model. We found that OVA-specific CD4(+) T cells had a reduced potential to proliferate when stimulated with OVA after infection with Ye compared to control mice. Additionally, proliferation of OVA-specific CD4(+) T cells was markedly reduced when cultured with splenic CD8α(+) DCs from Ye infected mice in the presence of OVA. In contrast, T-cell proliferation was not impaired in cultures with CD4(+) or CD4(-)CD8α(-) DCs isolated from Ye infected mice. However, OVA uptake and degradation as well as cytokine production were impaired in CD8α(+) DCs, but not in CD4(+) and CD4(-)CD8α(-) DCs after Ye infection. Pathogenicity factors (Yops) from Ye were most frequently injected into CD8α(+) DCs, resulting in less MHC class II and CD86 expression than on non-injected CD8α(+) DCs. Three days post infection with Ye the number of splenic CD8α(+) and CD4(+) DCs was reduced by 50% and 90%, respectively. The decreased number of DC subsets, which was dependent on TLR4 and TRIF signaling, was the result of a faster proliferation and suppressed de novo DC generation. Together, we show that Ye infection negatively regulates the stimulatory capacity of some but not all splenic DC subpopulations in vivo. This leads to differential antigen uptake and degradation, cytokine production, cell loss, and cell death rates in various DC subpopulations. The data suggest that these effects might be caused directly by injection of Yops into DCs and indirectly by affecting the homeostasis of CD4(+) and CD8α(+) DCs. These events may contribute to reduced T-cell proliferation and immune evasion of Ye.

Alternative endogenous protein processing via an autophagy-dependent pathway compensates for Yersinia-mediated inhibition of endosomal major histocompatibility complex class II antigen presentation

Extracellular Yersinia pseudotuberculosis employs a type III secretion system (T3SS) for translocating virulence factors (Yersinia outer proteins [Yops]) directly into the cytosol of eukaryotic cells. Recently, we used YopE as a carrier molecule for T3SS-dependent secretion and translocation of listeriolysin O (LLO) from Listeria monocytogenes. We demonstrated that translocation of chimeric YopE/LLO into the cytosol of macrophages by Yersinia results in the induction of a codominant antigen-specific CD4 and CD8 T-cell response in orally immunized mice. In this study, we addressed the requirements for processing and major histocompatibility complex (MHC) class II presentation of chimeric YopE proteins translocated into the cytosol of macrophages by the Yersinia T3SS. Our data demonstrate the ability of Yersinia to counteract exogenous MHC class II antigen presentation of secreted hybrid YopE by the action of wild-type YopE and YopH. In the absence of exogenous MHC class II antigen presentation, an alternative pathway was identified for YopE fusion proteins originating in the cytosol. This endogenous antigen-processing pathway was sensitive to inhibitors of phagolysosomal acidification and macroautophagy, but it did not require the function either of the proteasome or of transporters associated with antigen processing. Thus, by an autophagy-dependent mechanism, macrophages are able to compensate for the YopE/YopH-mediated inhibition of the endosomal MHC class II antigen presentation pathway for exogenous antigens. This is the first report demonstrating that autophagy might enable the host to mount an MHC class II-restricted CD4 T-cell response against translocated bacterial virulence factors. We provide critical new insights into the interaction between the mammalian immune system and a human pathogen.

Transforming growth factor beta and CD25 are important for controlling systemic dissemination following Yersinia enterocolitica infection of the gut

Infection of the gut by invasive bacterial pathogens leads to robust inflammatory responses that if left unchecked can lead to autoimmune disease and other sequelae. How the immune system controls inflammation and limits collateral damage to the host during acute bacterial infection is poorly understood. Here, we report that antibody-mediated neutralization of transforming growth factor beta (TGF-beta) prior to infection with the model enteric pathogen Yersinia enterocolitica reduces the mean time to death by 1 day (P=0.001), leads to rapid colonization of the liver and lung, and is associated with exacerbation of inflammatory histopathology. During Yersinia enterocolitica infection CD4+ cells are the source of de novo TGF-beta transcription in the Peyer's patches, mesenteric lymph nodes, and spleen. Correspondingly there is both antigen-specific and -independent expansion of CD4+ CD25+ Foxp3+ and TGF-beta+ T-regulatory cells (T-regs) after Yersinia infection that is reduced in ovalbumin T-cell receptor-restricted OT-II mice. Functional inactivation of CD25 by anti-CD25 treatment results in more rapid death, dissemination of the bacteria to the liver and lungs, and exacerbated inflammatory histopathology, similar to what is seen during TGF-beta neutralization. Altogether, these data suggest that TGF-beta produced by T-regs is important in restricting bacteria during the acute phase of invasive bacterial infection of the gut. These data expand the roles of T-regs to include tempering inflammation during acute infection in addition to the well-established roles of T-regs in chronic infection, control of immune homeostasis, and autoimmune disease.

Yersinia enterocolitica promotes robust mucosal inflammatory T-cell immunity in murine neonates

Mucosal immunity to gastrointestinal pathogens in early life has been studied only slightly. Recently, we developed an infection model in murine neonates using the gastroenteric pathogen Yersinia enterocolitica. Here, we report that oral infection of neonatal mice with low doses of virulent Y. enterocolitica leads to vigorous intestinal and systemic adaptive immunity. Y. enterocolitica infection promoted the development of anti-LcrV memory serum IgG1 and IgG2a responses of comparable affinity and magnitude to adult responses. Strikingly, neonatal mesenteric lymph node CD4(+) T cells produced Yersinia-specific gamma interferon (IFN-gamma) and interleukin-17A (IL-17A), exceeding adult levels. The robust T- and B-cell responses elicited in neonates exposed to Y. enterocolitica were associated with long-term protection against mucosal challenge with this pathogen. Using genetically deficient mice, we found that IFN-gamma and CD4(+) cells, but not B cells, are critical for protection of neonates during primary Y. enterocolitica infection. In contrast, adults infected with low bacterial doses did not require either cell population for protection. CD4-deficient neonatal mice adoptively transferred with CD4(+) cells from wild-type, IFN-gamma-deficient, or IL-17AF-deficient mice were equally protected from infection. These data demonstrate that inflammatory CD4(+) T cells are required for protection of neonatal mice and that this protection may not require CD4-derived IFN-gamma, IL-17A, or IL-17F. Overall, these studies support the idea that Y. enterocolitica promotes the development of highly inflammatory mucosal responses in neonates and that intestinal T-cell function may be a key immune component in protection from gastrointestinal pathogens in early life.

Trimer stability of YadA is critical for virulence of Yersinia enterocolitica

Yersinia adhesin A (YadA) is a trimeric autotransporter adhesin with multiple functions in host-pathogen interactions. The aim of this study was to dissect the virulence functions promoted by YadA in vitro and in vivo. To accomplish this, we generated Yersinia enterocolitica O:8 mutants expressing point mutations in YadA G389, a highly conserved residue in the membrane anchor of YadA, and analyzed their impact on YadA expression and virulence functions. We found that point mutations of YadA G389 led to impaired transport, stability, and surface display of YadA. YadA G389A and G389S mutants showed comparable YadA surface expression, autoagglutination, and adhesion to those of wild-type YadA but displayed reduced trimer stability and complement resistance in vitro and were 10- to 1,000-fold attenuated in experimental Y. enterocolitica infection in mice. The G389T, G389N, and G389H mutants lost trimer stability, exhibited strongly reduced surface display, autoagglutination, adhesion properties, and complement resistance, and were avirulent (>10,000-fold attenuation) in mice. Our data demonstrate that G389 is a critical residue of YadA, required for optimal trimer stability, transport, surface display, and serum resistance. We also show that stable trimeric YadA protein is essential for virulence of Y. enterocolitica.

Interaction of Yersinia with the gut: mechanisms of pathogenesis and immune evasion

Yersinia entercolitica and Yersinia pseudotuberculosis are human foodborne pathogens that interact extensively with tissues of the gut and the host's immune system to cause disease. As part of their pathogenic strategies, the Yersinia have evolved numerous ways to invade host tissues, gain essential nutrients, and evade host immunity. Technological advances over the last 10 years have revolutionized our understanding of host-pathogen interactions. The application of these new technologies has also shown that even well-understood pathogens such as the Yersinia have many surprises waiting to be revealed. The complex interaction with the host has made Yersinia a paradigm for understanding bacterial pathogenesis and the host response to invasive bacterial infections. This review examines the mechanisms of immune evasion employed by the Yersinia and highlights recent advances in understanding the host-pathogen interaction.

CD8(+) T cells restrict Yersinia pseudotuberculosis infection: bypass of anti-phagocytosis by targeting antigen-presenting cells

All Yersinia species target and bind to phagocytic cells, but uptake and destruction of bacteria are prevented by injection of anti-phagocytic Yop proteins into the host cell. Here we provide evidence that CD8⁺ T cells, which canonically eliminate intracellular pathogens, are important for restricting Yersinia, even though bacteria are primarily found in an extracellular locale during the course of disease. In a model of infection with attenuated Y. pseudotuberculosis, mice deficient for CD8⁺ T cells were more susceptible to infection than immunocompetent mice. Although exposure to attenuated Y. pseudotuberculosis generated T_H1-type antibody responses and conferred protection against challenge with fully virulent bacteria, depletion of CD8⁺ T cells during challenge severely compromised protective immunity. Strikingly, mice lacking the T cell effector molecule perforin also succumbed to Y. pseudotuberculosis infection. Given that the function of perforin is to kill antigen-presenting cells, we reasoned that cell death marks bacteria-associated host cells for internalization by neighboring phagocytes, thus allowing ingestion and clearance of the attached bacteria. Supportive of this model, cytolytic T cell killing of Y. pseudotuberculosis–associated host cells results in engulfment by neighboring phagocytes of both bacteria and target cells, bypassing anti-phagocytosis. Our findings are consistent with a novel function for cell-mediated immune responses protecting against extracellular pathogens like Yersinia: perforin and CD8⁺ T cells are critical for hosts to overcome the anti-phagocytic action of Yops.

Pathogenic Yersinia are bacteria that cause diverse diseases such as gastroenteritis and plague. Yersinia binds to specialized immune cells called macrophages, which attempt to engulf and destroy the bacteria. The bacteria resist destruction by injecting proteins called Yops into macrophages, which stops the engulfment process. Yersinia thus survives as attached but extracellular bacteria to cause disease. Yersinia disease can be prevented by immunization. In this study, we identified one mechanism of protective immunity—that host cells called CD8⁺ T lymphocytes are important to restrict Yersinia infection. This observation is unusual because CD8⁺ T cells generally protect against intracellular pathogens: T cells destroy the host cell harboring the pathogen, thus preventing the pathogen's replication. We present data consistent with the model that CD8⁺ T cells can also restrict extracellular bacteria by showing that T cells target host cells with extracellularly attached Yersinia, thus allowing the host cells and associated bacteria to be engulfed and removed by neighboring macrophages.

Kinetics of B cell Response DuringYersinia enterocoliticaInfection in Resistant and Susceptible Strains of Mice

In this study we analyze the B-cell response in murine yersiniosis. To this end, we determined whether polyclonal activation of B-lymphocytes occurs during infection of susceptible (BALB/c) and resistant (C57BL/6) mice with Y. enterocolitica O:8 and compared the immunoglobulin (Ig) isotypes produced in response to the infection by the two strains. The number of splenic cells secreting nonspecific and specific immunoglobulins was determined by ELISPOT. The presence of anti-Yersinia antibodies in serum was detected by ELISA. In both strains, the number of specific Ig-secreting cells was relatively low. Polyclonal B-cell activation was observed in both strains of mice, and the greatest activation was observed in the BALB/c mice, mainly for IgG1- and IgG3- secreting cells. The C57BL/6 mice showed a predominance of IgG2a-secreting cells. The peak production of anti-Yersinia IgG antibodies in the sera of BALB/c mice was seen on the 28th day after infection. The greatest increase in IgM occurred on the 14th day. A progressive increase of specific IgG antibodies was observed in C57BL/6 mice up to the 28th day after infection while IgM increased on the 21st day after infection. The production of specific IgA antibodies was not detected in either BALB/c or C57BL/6 mice. We conclude that polyclonal activation of B lymphocytes occurs in both the Yersinia-resistant and Yersinia-susceptible mice and that the more intense activation of B lymphocytes observed in the susceptible BALB/c mice does not enhance their resistance to Y. enterocolitica infection.

Immune defense against pneumonic plague

SUMMARY

Yersinia pestis is one of the world's most virulent human pathogens. Inhalation of this Gram-negative bacterium causes pneumonic plague, a rapidly progressing and usually fatal disease. Extensively antibiotic-resistant strains of Y. pestis exist and have significant potential for exploitation as agents of terrorism and biowarfare. Subunit vaccines comprised of the Y. pestis F1 and LcrV proteins are well-tolerated and immunogenic in humans but cannot be tested for efficacy, because pneumonic plague outbreaks are uncommon and intentional infection of humans is unethical. In animal models, F1/LcrV-based vaccines protect mice and cynomolgus macaques but have failed, thus far, to adequately protect African green monkeys. We lack an explanation for this inconsistent efficacy. We also lack reliable correlate assays for protective immunity. These deficiencies are hampering efforts to improve vaccine efficacy. Here, I review the immunology of pneumonic plague, focusing on evidence that humoral and cellular defense mechanisms collaborate to defend against pulmonary Y. pestis infection.

Macrophage activation redirects yersinia-infected host cell death from apoptosis to caspase-1-dependent pyroptosis

Infection of macrophages by Yersinia species results in YopJ-dependent apoptosis, and naïve macrophages are highly susceptible to this form of cell death. Previous studies have demonstrated that macrophages activated with lipopolysaccharide (LPS) prior to infection are resistant to YopJ-dependent cell death; we found this simultaneously renders macrophages susceptible to killing by YopJ⁻Yersinia pseudotuberculosis (Yptb). YopJ⁻Yptb-induced macrophage death was dependent on caspase-1 activation, resulting in rapid permeability to small molecules, followed by membrane breakdown and DNA damage, and accompanied by cleavage and release of proinflammatory interleukin-18. Induction of caspase-1-dependent death, or pyroptosis, required the bacterial type III translocon but none of its known translocated proteins. Wild-type Yptb infection also triggered pyroptosis: YopJ-dependent activation of proapoptotic caspase-3 was significantly delayed in activated macrophages and resulted in caspase-1-dependent pyroptosis. The transition to susceptibility was not limited to LPS activation; it was also seen in macrophages activated with other Toll-like receptor (TLR) ligands and intact nonviable bacteria. Yptb infection triggered macrophage activation and activation of caspase-1 in vivo. Y. pestis infection of activated macrophages also stimulated caspase-1 activation. These results indicate that host signaling triggered by TLR and other activating ligands during the course of Yersinia infection redirects both the mechanism of host cell death and the downstream consequences of death by shifting from noninflammatory apoptosis to inflammatory pyroptosis.

Pathogenic Yersinia are bacteria capable of interacting with host immune cells and inhibiting their function. Macrophages are potent antimicrobial immune cells that eliminate invading microbes, and represent a major target for Yersinia during infection. Yersinia triggers death of resting macrophages by apoptosis, a process thought to be advantageous for Yersinia growth during early stages of infection because important host cells are eliminated without perturbing the surrounding tissue. However, activated macrophages with enhanced antimicrobial activity play a crucial role in controlling Yersinia infection. To elucidate the mechanisms involved in successful defense against infection, the authors investigated the response of activated macrophages to Yersinia, which revealed induction of a proinflammatory cell death pathway termed pyroptosis. Unlike apoptosis, pyroptosis unleashes inflammatory mediators capable of enhancing immune responses and clearing bacteria. Macrophage activation and pyroptosis was observed in infected host tissue. Thus, regulating the mechanism of cell death is important for effective responses to Yersinia infection: activated macrophages resisting apoptosis are redirected to utilize pyroptosis, an inflammatory process facilitating host resistance.

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.

Modulation of dendritic cell differentiation and function by YopJ ofYersinia pestis

Yersinia pestis evades immune responses in part by injecting into host immune cells several effector proteins called Yersinia outer proteins (Yops) that impair cellular function. This has been best characterized in the innate effector cells, but much less so for cells involved in adaptive immune responses. Dendritic cells (DC) sit at the crossroads between innate and adaptive immunity, and can function to initiate or inhibit adaptive immune responses. Although Y. pestis can target and inactivate DC, the mechanism responsible for this remains unclear. We have found that injection of Y. pestis YopJ into DC progenitors disrupts key signal transduction pathways and interferes with DC differentiation and subsequent function. YopJ injection prevents up-regulation of the NF-kappaB transcription factor Rel B and inhibits MAPK/ERK activation--both having key roles in DC differentiation. Furthermore, YopJ injection prevents costimulatory ligand up-regulation, LPS-induced cytokine expression, and yields differentiated DC with diminished capability to induce T cell proliferation and IFN-gamma induction. By modulating DC function through YopJ-mediated disruption of signaling pathways during progenitor to DC differentiation, Yersinia may interfere with the adaptive responses necessary to clear the infection as well as establish a tolerant immune environment that leads to chronic infection/carrier state in the surviving host.

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

Enteric Yersinia spp. invade Peyer's patches, disseminate to lymphoid tissues, and induce mucosal and systemic immune responses. Many virulence factors of Yersinia enterocolitica have been investigated in detail and were found to act on host cells involved in innate and adaptive immunity. Recent work explored as to whether attenuated Y. enterocolitica or recombinant components of Y. enterocolitica can be used as tools for vaccination. We and others have tested whether by means of the type three secretion system in attenuated Y. enterocolitica strains antigens might be delivered to antigen-presenting cells in order to induce CD8 and CD4 T cell responses. Alternatively, recombinant components of Y. enterocolitica such as invasin protein which binds to beta1 integrins of host cells have been tested for their ability to target antigen along with microparticles (fused to invasin) to antigen-presenting cells and to act as adjuvant. The work summarized in this article demonstrates that Y. enterocolitica and its components might be useful tools for novel vaccination strategies; in fact, invasin when fused to antigen and coated to microparticles might induce both CD4 and CD8 T cell responses. Likewise, attenuated Y. enterocolitica live carrier strains were reported to induce both CD8 and some CD4 T cell responses. However, we need to know more about how Y. enterocolitica subverts functions of antigen-presenting cells in order to design mutants with optimized antigen delivery features and deletion in those virulence factor that contribute to subversion of innate or adaptive immune responses.

General and specific host responses to bacterial infection in Peyer's patches: a role for stromelysin-1 (matrix metalloproteinase-3) during Salmonella enterica infection

Salmonella enterica serovar Typhimurium (S. typhimurium) and Yersinia enterocolitica are enteric pathogens capable of colonizing and inducing inflammatory responses in Peyer's patches (PPs) and mesenteric lymph nodes (MLNs). Although the tissue colonization pattern is similar between these two pathogens, their pathogenic lifestyles are quite different. For example, while S. typhimurium is primarily an intracellular pathogen, Y. enterocolitica survives primarily extracellularly. We determined and compared the transcriptional changes occurring in response to S. typhimurium and Y. enterocolitica colonization of PP using Affymetrix GeneChip technology. Both pathogens elicited a general inflammatory response indicated by the upregulation of cytokines and chemokines. However, specific differences were also observed, most notably in the transcriptional regulation of gamma interferon (IFN-gamma) and IFN-gamma-regulated genes in response to S. typhimurium but not Y. enterocolitica. Of particular note, a group of genes encoding matrix metalloproteinases (MMPs) had increased transcript numbers in the PPs following infection with both pathogens. The experiments described here compare oral S. typhimurium or Y. enterocolitica infection in stromelysin-1 (MMP-3)-deficient mice (mmp-3(-/-)) with mice possessing functional MMP-3 (mmp-3(+/+)). There was little difference in the survival of MMP-3-deficient mice infected with Y. enterocolitica when compared with littermate controls. Surprisingly though, mmp-3(-/-) mice were markedly more resistant to S. typhimurium infection than the control mice. S. typhimurium was able to colonize mmp-3(-/-) mice, albeit in a delayed fashion, to equivalent levels as mmp-3(+/+) mice. Nevertheless, significantly lower levels of inflammatory cytokines were detected in tissues and serum in the mmp-3(-/-) mice in comparison with mmp-3(+/+) mice. We hypothesize that MMP-3 is involved in initiating an early and lethal cytokine response to S. typhimurium colonization.

Yersinia enterocolitica YopP inhibits MAP kinase-mediated antigen uptake in dendritic cells

Yersinia enterocolitica (Ye) targets mouse dendritic cells (DCs) and inhibits their ability to trigger T cell activation. Here we have investigated whether Ye might interfere with antigen presentation in DCs. Infection of DCs with the Ye wild-type strain reduced OVA uptake by DCs as demonstrated by flow cytometry and confocal laser scan microscopy. In contrast, DCs infected with Yersinia outer protein P (YopP)-deficient mutant strain rapidly internalized OVA. Furthermore, transfection of DCs with YopP, but not with a cysteine protease deficient YopP-C172A mutant, reduced uptake of OVA. This finding suggests that YopP, a virulence factor of Ye, inhibits OVA uptake by DCs. By the use of MAPK inhibitors we provide evidence that YopP mediates reduction of OVA uptake by its ability to block MAPK signalling pathways in host cells. Using transferrin (Tf) as specific marker for clathrin-mediated endocytosis (CME) and lucifer yellow (LY) as specific marker for macropinocytosis (MP) we could show that YopP inhibits CME, whereas other Yops inhibit MP. In keeping with these data, activation and proliferation of OVA-specific T cells was reduced when DCs were treated with MAPK inhibitors. Together, our data demonstrate that (i) MAPK play an important role in antigen uptake by CME in DCs, and (ii) that YopP inhibits this pathway of antigen uptake in DCs, which might contribute to evasion of adaptive immunity.

The Yersinia enterocolitica invasin protein promotes major histocompatibility complex class I- and class II-restricted T-cell responses

Yersinia enterocolitica invasin (Inv) protein confers internalization into and expression of proinflammatory cytokines by host cells. Both events require binding of Inv to beta1 integrins, which initiates signaling cascades including activation of focal adhesion complexes, Rac1, mitogen-activated protein kinase, and NF-kappaB. Here we tested whether Inv might be suitable as a delivery molecule and adjuvant if used as a component of a vaccine. For this purpose, hybrid proteins composed of Inv and ovalbumin (OVA) were prepared, applied as a coating to microparticles, and used for vaccination. Fusion of OVA to Inv did not significantly disturb the ability of Inv to promote host cell binding, internalization, and interleukin-8 (IL-8) secretion when applied as a coating to microparticles. The microparticles were used for vaccination of mice adoptively transferred with OVA-specific T cells from OT-1 or DO11.10 mice. Administration of OVA-Inv-coated microparticles induced OVA-specific T-cell responses. OVA-specific CD4 T cells produced both gamma interferon (IFN-gamma) and IL-4 as determined by enzyme-linked immunosorbent assay. Likewise, pronounced OVA-specific CD8 T-cell responses associated with IFN-gamma production were observed. Together, these results suggest that Inv might be an attractive tool in vaccination as it confers both host cell uptake and adjuvant activity by engagement of beta1 integrins of host cells, which leads to CD4 as well as CD8 T-cell responses.

Yersinia's stratagem: targeting innate and adaptive immune defense

In contrast to Salmonella and Shigella, enteropathogenic Yersinia species are extracellular multiplying Gram-negative bacteria. This life style requires a sophisticated anti-host strategy, which is implemented by the Yersinia virulence plasmid. This plasmid encodes the type 3 secretion system (injectisome), at least six microinjected anti-host effector proteins, a trimeric coiled coil outer membrane protein (Yersinia adhesin) with cell adhesin and protective functions against complement and defensins, and the released V antigen, which has Toll-like receptor 2 agonist activity.

Cell-mediated protection against pulmonary Yersinia pestis infection

Pulmonary infection with the bacterium Yersinia pestis causes pneumonic plague, an often-fatal disease for which no vaccine is presently available. Antibody-mediated humoral immunity can protect mice against pulmonary Y. pestis infection, an experimental model of pneumonic plague. Little is known about the protective efficacy of cellular immunity. We investigated the cellular immune response to Y. pestis in B-cell-deficient microMT mice, which lack the capacity to generate antibody responses. To effectively prime pulmonary cellular immunity, we intranasally vaccinated microMT mice with live replicating Y. pestis. Vaccination dramatically increased survival of microMT mice challenged intranasally with a lethal Y. pestis dose and significantly reduced bacterial growth in pulmonary, splenic, and hepatic tissues. Vaccination also increased numbers of pulmonary T cells, and administration of T-cell-depleting monoclonal antibodies at the time of challenge abrogated vaccine-induced survival. Moreover, the transfer of Y. pestis-primed T cells to naive microMT mice protected against lethal intranasal challenge. These findings establish that vaccine-primed cellular immunity can protect against pulmonary Y. pestis infection and suggest that vaccines promoting both humoral and cellular immunity will most effectively combat pneumonic plague.

Induction of CD8+ T cell responses by Yersinia vaccine carrier strains

Yersinia enterocolitica employs a type III secretion system (TTSS) to target virulence factors (e.g. YopE) into the cytosol of the host cells. We utilized the TTSS to introduce a recombinant antigen directly into the cytosol of host cells and to investigate the potential of Y. enterocolitica and Y. pseudotuberculosis as live carrier for vaccines. The model antigen ovalbumin (Ova) was fused to defined secretion or translocation domains of the Yersinia effector protein YopE and introduced into attenuated mutant strains of Y. enterocolitica and Y. pseudotuberculosis. In vitro experiments showed secretion and translocation of YopE-Ova hybrid proteins into host cells. To investigate the resulting immune responses, mice expressing transgenic Ova-specific T cell receptors were used. Both Y. enterocolitica and Y. pseudotuberculosis mutants induced efficaciously Ova-specific CD8+ T cell responses. The translocation domain of YopE was required for induction of CD8+ T cell responses in vivo, but not for T cell responses induced in vitro. The in vivo frequency of Ova-specific splenic T cells was up to six-fold higher in mice immunized with YopE-Ova-translocating Y. enterocolitica/Y. pseudotuberculosis mutants than in control mice. The Ova-specific T cells were shown to produce high amounts of IFN-gamma. We did not observe significant Ova-specific CD4+ T cell or antibody responses upon vaccination with either of the strains. In conclusion, Yersinia live carrier vaccine strains are suitable to target antigens into the MHC class I pathway and stimulate CD8+ T cell responses and thus, might be useful in vaccine approaches against intracellular pathogens.

Involvement of the virulence gene products of Yersinia enterocolitica in the immune response of infected mice

The virulence of Yersinia enterocolitica is known to be highly dependent on its virulence plasmid. However, it remains unclear whether the virulence plasmid is engaged also in the induction of cell-mediated immunity that is essential for protective immunity in the host. In this study, we have compared the induction of type 1 helper T cell immunity against Y. enterocolitica using a virulent strain (P+) harboring the pYV plasmid and an avirulent strain (P-) harboring no pYV. Spleen cells from both groups of mice immunized with 1/10 LD50 of P+ strain and those with 1/10 LD50 of P- strain produced a high level of gamma interferon (IFN-gamma) upon stimulation with heat-killed bacteria, and CD4+ T cells were exclusively responsible for IFN-gamma production. When crude Yersinia outer proteins (Yops) were used for antigenic stimulation, IFN-gamma response of immune spleen cells against crude Yops was observed only in mice immunized with P+ strain. Flowcytometric analysis revealed a significant level of increase in IFN-gamma-producing CD8+ T cells as well as the increase in IFN-gamma-producing CD4+ T cells against crude Yops. These results suggest that the virulence plasmid of Y. enterocolitica is involved in the induction of Th1-type of possibly protective T cells in infected mice.

Y. enterocolitica translocated Yops impair stimulation of T-cells by antigen presenting cells

As T helper cells play a crucial role in the defense of the mouse immune system against Yersinia enterocolitica, an effective subversion strategy for the pathogen would be the inhibition of T-cell activation. In this study, we investigated whether Y. enterocolitica impairs this process on the level of antigen presentation. For this purpose, we used T-cells to measure the antigen presentation capacity of dendritic cells after they had been incubated with different types of Yersinia mutants. We could show that Y. enterocolitica impairs the processing of antigens by dendritic cells, that this effect is dependent on factors translocated by the pathogenicity-plasmid-encoded type III secretion system and that the most important factor appears to be YopP. The YopP effect is partly mediated by the killing of APCs, but in addition to this there appears to be an alternative way of action that results in the inhibition of antigen processing. The YopP effect is not mediated by soluble factors. In contrast to antigen processing, antigen presentation was only weakly affected by pathogenicity plasmid encoded factors in dendritic cells, but obviously in A20.J B-cells.

Yersinia enterocolitica induces apoptosis and inhibits surface molecule expression and cytokine production in murine dendritic cells

Yersinia enterocolitica evades innate immunity by expression of a variety of pathogenicity factors. Therefore, adaptive immunity including CD4(+) T cells plays an important role in defense against Y. enterocolitica. We investigated whether Y. enterocolitica might target dendritic cells (DC) involved in adaptive T-cell responses. For this purpose, murine DC were infected with Y. enterocolitica wild-type and mutant strains prior to incubation with ovalbumin (OVA) as antigen and 5-(6)-carboxyfluorescein diacetate N-succinimidyl ester-labeled OVA-specific T cells from DO11.10 mice. While T-cell proliferation was partially affected by infection of DC with plasmid-cured and YopP-deficient Yersinia mutant strains, no T-cell proliferation occurred after infection of DC with wild-type Y. enterocolitica. Infection of DC with Y. enterocolitica wild type resulted in decreased up-regulation of major histocompatibility complex class II, CD54 (intercellular adhesion molecule 1), CD 80, and CD86 expression. Experiments with plasmid-cured Y. enterocolitica or a YopP-deficient mutant strain revealed that YopP accounts for inhibition of surface molecule expression. Wild-type Y. enterocolitica suppressed the release of KC, tumor necrosis factor alpha, interleukin-10 (IL-10), and IL-12 by DC, while infection of DC with plasmid-cured Y. enterocolitica or with the YopP-deficient mutant resulted in the production of these cytokines. Moreover, infection with wild-type Y. enterocolitica induced apoptosis in DC mediated by YopP. Apoptosis occurred despite translocation of NF-kappaB to the nucleus, as demonstrated by electromobility shift assays. Together, these data demonstrate that Y. enterocolitica targets functions of murine DC that are required for T-cell activation. This might contribute to evasion of adaptive immune responses by Y. enterocolitica.

Protective role of interleukin-6 during Yersinia enterocolitica infection is mediated through the modulation of inflammatory cytokines

Yersinia enterocolitica is a gram-negative enteric pathogen responsible for a number of gastrointestinal disorders. A striking feature of the pathology of a Y. enterocolitica infection is inflammation. Recently, we demonstrated a role for interleukin-1alpha (IL-1alpha) in the establishment of intestinal inflammation in response to a Y. enterocolitica infection. A cytokine directly affected by IL-1 levels is IL-6. A previous report suggested that IL-6 plays an anti-inflammatory role during Y. enterocolitica infection, and in other systems IL-6 has been shown to be proinflammatory. Therefore, a closer examination of the roles of IL-6 and inflammatory cytokines in the control of Y. enterocolitica infection in IL-6(-/-) mice was undertaken. Y. enterocolitica organisms were more virulent in the IL-6(-/-) mice (60-fold decreased 50% lethal dose) and colonized systemic tissues more rapidly and to a higher level than in the wild-type mice. One role of IL-6 during a Y. enterocolitica infection may be the downmodulation of the inflammatory response. The IL-6(-/-) mice have a more robust T(H)1 T-cell response, as well as hyperinflammatory pathologies. These phenotypes appear to be due to the misregulation of tumor necrosis factor alpha, monocyte chemotactic protein 1, IL-10, transforming growth factor beta1, and gamma interferon in the IL-6(-/-) mouse. These data provide further insight into the intricate cytokine signaling pathways involved in the regulation of inflammatory responses and the control of bacterial infections.

The rovA mutant of Yersinia enterocolitica displays differential degrees of virulence depending on the route of infection

Yersinia enterocolitica is an invasive enteric pathogen that causes significant inflammatory disease. Recently, we identified and characterized a global regulator of virulence (rovA). When mice are infected orally with the rovA mutant they are attenuated by 50% lethal dose (LD(50)) analysis and have altered kinetics of infection. Most significantly, mice orally infected with the rovA mutant have greatly reduced inflammation in the Peyer's patches compared to those infected with wild-type Y. enterocolitica. However, we present data here indicating that when the rovA mutant bacteria are delivered intraperitoneally (i.p.), they are significantly more virulent than when delivered orally. The i.p. LD(50) for the rovA mutant is only 10-fold higher than that of the wild-type Y. enterocolitica, and there are significant inflammatory responses to the rovA mutant that are evident in the liver and spleen. Altogether, these data suggest that the RovA regulon may be required for the early events of the infection that occur in the Peyer's patches. Furthermore, these data suggest that the RovA regulon may be dispensable for Y. enterocolitica systemic disease and inflammatory responses if the Peyer's patches are bypassed.

Attenuated Yersinia enterocolitica mutant strains exhibit differential virulence in cytokine-deficient mice: implications for the development of novel live carrier vaccines

Yersinia enterocolitica mutant strains, including mutants deficient in the chaperone SycH resulting in a functional deficiency in tyrosine phosphatase (YopH), Mn-cofactored superoxide dismutase (SodA), iron-repressive protein 1 (IRP-1), and Yersinia adhesin A (YadA), were demonstrated to be highly attenuated in wild-type C57BL/6 mice. TNFRp55(-/-), IL-12p40(-/-), and IL-18(-/-) mutant mice, in which the Yersinia wild-type strain causes severe systemic infections, were used to investigate whether these Yersinia mutant strains would be attenuated in immunodeficient hosts. A plasmid-cured Yersinia mutant strain was unable to colonize any of the mutant mice tested. A SycH-deficient mutant strain colonized intestinal tissues of these mice but was attenuated for systemic infection in all of the mutant mice. Both YadA- and Irp-1-deficient Yersinia mutants were still attenuated in IL-12(-/-) and IL-18(-/-) mice but were pathogenic in TNFRp55(-/-) mice. By contrast, a Yersinia sodA mutant was highly pathogenic for TNFRp55(-/-) and IL-12p40(-/-) mice while interleukin-18 (IL-18) was dispensable. This finding demonstrates that certain virulence factors enable yersiniae to compete with distinct cytokine-dependent host defense mechanisms. Moreover, while gamma interferon mRNA expression did not reflect protective host responses in cytokine-deficient mice, IL-10 expression coincided with a heavy splenic bacterial load and was associated with progressive infection courses. We can thus segregate minor (SodA), intermediate (YadA and IRP-1), and major (YopH) virulence factors of Y. enterocolitica. Finally, we demonstrate that, even in immunocompromised hosts, Yersinia sycH and, with some restrictions, irp-1 mutants may be suitable for use as live carrier vaccines.

Evidence of a leading role for VEGF in Bartonella henselae-induced endothelial cell proliferations

Bartonella henselae causes the vasculoproliferative disorders bacillary angiomatosis (BA) and bacillary peliosis (BP). The pathomechanisms of these tumorous proliferations are unknown. Our results suggest a novel bacterial two-step pathogenicity strategy, in which the pathogen triggers growth factor production for subsequent proliferation of its own host cells. In fact, B. henselae induces host cell production of the angiogenic factor vascular endothelial growth factor (VEGF), leading to proliferation of endothelial cells. The presence of B. henselae pili was associated with host cell VEGF production, as a Pil- mutant of B. henselae was unable to induce VEGF production. In turn, VEGF-stimulated endothelial cells promoted the growth of B. henselae. Immunohistochemistry for VEGF in specimens from patients with BA or BP revealed increased VEGF expression in vivo. These findings suggest a novel bacteria-dependent mechanism of tumour growth.

Differential stimulation of helper and cytotoxic T cells by dendritic cells after infection by Yersinia enterocolitica in vitro

Dendritic cells (DC) are antigen-presenting cells crucial for initiating immune responses like sensitization of T cells to foreign antigens. We have previously shown that infection of DC by enteropathogenic Yersinia enterocolitica in vitro leads to a transient suppression in the immunostimulatory capacity for autologous enriched total T cells. In this study, we found that killed Yersinia could replace live bacteria in this aspect, and that yersinial lipopolysaccharide (LPS)-antigen could be detected intracellularly over a time course of 8 days. A suppressive effect on T cell proliferation after stimulation with Yersinia-infected compared to uninfected DC was seen for CD4+ T cells isolated by immunomagnetic separation techniques over the whole time course of 8 days, whereas CD8+ T cells followed to exhibit a suppressed proliferation rate starting on day 5 post infection till the end of the time course. In contrast, enriched total T cells stimulated by Yersinia-infected DC showed weaker proliferation till day 6 post infection compared to stimulation by uninfected DC, but not thereafter. Mixing of purified CD4+ and CD8+ T cells at day 8 post infection could reconstitute the effect seen for enriched total T cells. Thus, helper in concert with cytotoxic T cells might contribute to the immune responses, that are necessary for control of Yersinia-infections.

Arthritis after experimental infection with Yersinia enterocolitica 0:3 in rabbits

Arthritis in rabbits was caused after experimental oral infection with Yersinia enterocolitica (serotype 0:3, biotype 4, pYV+). Clinical and laboratory signs, bacterial dissemination to the viscera, immune response and morphological findings were studied from day 1 to day 40 post-infection (p.i.). Augmentation of body temperature and erythrocyte sedimentation rate occurred on day 1, and on day 8 p.i. was accompanied by leucopenia. The number of alveolar macrophages was increased up to the 15th day p.i., in contrast to peritoneal macrophage numbers. Extensive bacterial colonization of the internal organs was detected at necropsy until the end of the experiment. Analysis of the cell immune response revealed activation of B cells in peripheral blood, spleen and thymus as well as augmentation of T-cell number in the lymphoid organs examined on days 15, 28 and 40 p.i. Histological changes typical of a generalized infection, such as purulent meningoencephalitis, catarrhal pneumonia and lymphadenitis, were observed. Clinical and morphological manifestations of arthritis were also established. The results obtained show that Y. enterocolitica (serotype 0:3, pYV+) induces a generalized, non-lethal infection in Chinchilla rabbits, complicated by arthritis.

Conjugation of hydroxyethyl starch to desferrioxamine (DFO) modulates the dual role of DFO in Yersinia enterocolitica infection

The iron chelator desferrioxamine (DFO) B is widely used in the therapy of patients with iron overload. As a side effect, DFO may favor the occurrence of fulminant Yersinia infections. Previous work from our laboratory showed that this might be due to a dual role of DFO: growth promotion of the pathogen and immunosuppression of the host. In this study, we sought to determine whether conjugation of DFO to hydroxyethyl starch (HES-DFO) may prevent exacerbation of Yersinia infection in mice. We found HES-DFO to promote neither growth of Yersinia enterocolitica nor mitogen-induced T-cell proliferation and gamma interferon production by T cells in vitro. Nevertheless, in vivo HES-DFO promoted growth of Y. enterocolitica possibly due to cleavage of HES and release of DFO. The pretreatment of mice with DFO resulted in death of all mice 2 to 5 days after application of a normally sublethal inoculum of Y. enterocolitica, while none of the mice pretreated with HES-DFO died within the first 7 days postinfection. However, some of the HES-DFO-treated mice died 8 to 14 days postinfection. Thus, due to the delayed in vivo effect HES-DFO failed to trigger Yersinia-induced septic shock, which accounts for early mortality in DFO-associated septicemia. Moreover, our data suggest that DFO needs to be taken up by host cells in order to exert its immunosuppressive action. These results strongly suggest that HES-DFO might be a favorable drug with fewer side effects than DFO in terms of DFO-promoted fulminant infections.

Cell surface presentation of recombinant (poly-) peptides including functional T-cell epitopes by the AIDA autotransporter system

For the efficient surface presentation and release of virulence factors especially pathogenic Gram-negative bacteria have developed several distinct secretion mechanisms. An increasing number of pathogens in various species employs a mechanism denoted the 'autotransporter' pathway. This pathway is characterised by an outer membrane translocator module representing the C-terminal domain of the transported protein itself. An intriguing potential application of such systems involves the transport and surface expression of recombinant proteins or peptides, like e.g. the presentation of antigens for the generation of live oral vectors as vaccine carriers. Here we report on the incorporation of heterologous (poly-) peptides in permissive sites of the translocator module of the adhesin-involved-in-diffuse-adherence (AIDA) autotransporter system. We demonstrate the presentation of the B subunit of the heat labile enterotoxin of Escherichia coli (LTB) as well as of functional T-cell epitopes of Yersinia enterocolitica heat-shock protein 60 (Y-hsp60) on the surface of E. coli.

Tumor necrosis factor receptor p55-deficient mice respond to acute Yersinia enterocolitica infection with less apoptosis and more effective host resistance

Tumor necrosis factor (TNF) has generally been regarded as a protective cytokine in host defense against bacterial infections. In the present study, we evaluated the role of TNF in the acute phase of infection by Yersinia enterocolitica by using mice rendered genetically deficient in TNF receptor p55 (TNFRp55(-/-)). Unexpectedly, TNFRp55(-/-) mice showed more effective resistance to the bacteria, reflected in enhanced bacterial clearance and less tissue damage, than did control C57BL/6 mice. C57BL/6 mice showed evidence of extensive apoptosis in the spleen accompanied by a selective decrease in the CD4(+)-T-cell population of splenocytes, whereas TNFRp55(-/-) mice were spared these changes. The splenocytes from TNFRp55(-/-) mice also maintained a robust gamma interferon IFN-gamma response to mitogenic stimulation, while the comparable response in C57BL/6 mice was impaired. In addition, splenocytes harvested from infected mice demonstrated lower production of interleukin-10 IL-10 in TNFRp55(-/-) mice than in C57BL/6 mice. These findings suggest that Yersinia can induce TNFRp55-mediated apoptosis of splenocytes in the acute phase of the infection and that alteration of T-cell-generated cytokines can dramatically alter the early events in host defense against this pathogen.

Interferon consensus sequence binding protein confers resistance against Yersinia enterocolitica

Interferon consensus sequence binding protein (ICSBP)-deficient mice display enhanced susceptibility to intracellular pathogens. At least two distinct immunoregulatory defects are responsible for this phenotype. First, diminished production of reactive oxygen intermediates in macrophages results in impaired intracellular killing of microorganisms. Second, defective early interleukin-12 (IL-12) production upon microbial challenge leads to a failure in gamma interferon (IFN-gamma) induction and subsequently in T helper 1 immune responses. Here, we investigated the role of ICSBP in resistance against the extracellular bacterium Yersinia enterocolitica. ICSBP(-/-) mice failed to produce IL-12 and IFN-gamma, but also IL-4, after Yersinia challenge. In addition, granuloma formation was highly disturbed in infected ICSBP(-/-) mice, leading to multiple necrotic abscesses in affected organs. Consequently, ICSBP(-/-) mice rapidly succumbed to acute Yersinia infection. In vitro treatment of spleen cells from ICSBP(-/-) mice with recombinant IL-12 (rIL-12) or rIL-18 in combination with a second stimulus resulted in IFN-gamma induction. In experimental therapy of infected ICSBP(-/-) mice, we observed that administration of rIL-12 induced IFN-gamma production which was associated with improved resistance to Yersinia. In contrast, treatment with rIL-18 failed to enhance endogenous IFN-gamma production but nevertheless reduced bacterial burden in ICSBP(-/-) mice. Although cytokine therapy with rIL-12 or rIL-18 ameliorated the course of Yersinia infection in ICSBP(-/-) mice, both cytokines failed to completely restore impaired immunity. Taken together, the results indicate that the transcription factor ICSBP is essential for efficient host immune defense against Yersinia. These results are important for understanding the complex host immune responses in bacterial infections.

Suppression of T and B lymphocyte activation by a Yersinia pseudotuberculosis virulence factor, yopH

The acquired immune responses are crucial to the survival of Yersinia-infected animals. Mice lacking T cells are sensitive to Yersinia infection, and a humoral response to Yersinia can be protective. Diverse mechanisms for Yersinia to impair and evade the host innate immune defense have been suggested, but the effects of Yersinia on lymphocytes are not known. Here, we demonstrate that after a transient exposure to Y. pseudotuberculosis, T and B cells are impaired in their ability to be activated through their antigen receptors. T cells are inhibited in their ability to produce cytokines, and B cells are unable to upregulate surface expression of the costimulatory molecule, B7.2, in response to antigenic stimulation. The block of lymphocyte activation results from the inhibition of early phosphorylation events of the antigen receptor signaling complex. Through the use of Y. pseudotuberculosis mutants, we show that the inhibitory effect in both T cells and B cells is dependent on the production of Yersinia outermembrane protein (Yop) H, a tyrosine phosphatase. Our results suggest a mechanism by which the pathogenic bacteria may modulate a wide range of T and B cell-mediated immune responses.

Rational live oral carrier vaccine design by mutating virulence-associated genes of Yersinia enterocolitica

Three different Yersinia enterocolitica serotype O8 strains harboring mutations in virulence-associated genes coding for Yersinia adhesin A (YadA), Mn-cofactored superoxide dismutase (SodA), and high-molecular-weight protein 1 were analyzed for their ability to colonize and persist in tissues after orogastric immunization of C57BL/6 mice. We demonstrated that all three Yersinia mutant strains were markedly impaired in their ability to disseminate into the spleens and livers of immunized mice but were able to colonize the Peyer's patches for at least 12 days, resulting in the induction of significant antibody titers against Yersinia outer proteins (Yops) and in the priming of Yersinia antigen-specific CD4+ Th1 cells isolated from spleens. The high level of attenuation did not diminish the immunogenic properties of the mutant strains. In fact, mice immunized with a single oral dose of any of the mutant strains were protected against a lethal oral-challenge infection with wild-type Y. enterocolitica. Moreover, adoptive transfer of Yersinia-specific antibodies from sera of mice immunized with the mutant WAP-314 sodA revealed that this protection could be mediated by Yersinia-specific immunoglobulins.