Pathogenesis of Yersinia pestis infection in BALB/c mice: effects on host macrophages and neutrophils

Yersinia pestis subverts the dermal neutrophil response in a mouse model of bubonic plague

The majority of human Yersinia pestis infections result from introduction of bacteria into the skin by the bite of an infected flea. Once in the dermis, Y. pestis can evade the host’s innate immune response and subsequently disseminate to the draining lymph node (dLN). There, the pathogen replicates to large numbers, causing the pathognomonic bubo of bubonic plague. In this study, several cytometric and microscopic techniques were used to characterize the early host response to intradermal (i.d.) Y. pestis infection. Mice were infected i.d. with fully virulent or attenuated strains of dsRed-expressing Y. pestis, and tissues were analyzed by flow cytometry. By 4 h postinfection, there were large numbers of neutrophils in the infected dermis and the majority of cell-associated bacteria were associated with neutrophils. We observed a significant effect of the virulence plasmid (pCD1) on bacterial survival and neutrophil activation in the dermis. Intravital microscopy of i.d. Y. pestis infection revealed dynamic interactions between recruited neutrophils and bacteria. In contrast, very few bacteria interacted with dendritic cells (DCs), indicating that this cell type may not play a major role early in Y. pestis infection. Experiments using neutrophil depletion and a CCR7 knockout mouse suggest that dissemination of Y. pestis from the dermis to the dLN is not dependent on neutrophils or DCs. Taken together, the results of this study show a very rapid, robust neutrophil response to Y. pestis in the dermis and that the virulence plasmid pCD1 is important for the evasion of this response.

Yersinia pestis remains a public health concern today because of sporadic plague outbreaks that occur throughout the world and the potential for its illegitimate use as a bioterrorism weapon. Since bubonic plague pathogenesis is initiated by the introduction of Y. pestis into the skin, we sought to characterize the response of the host’s innate immune cells to bacteria early after intradermal infection. We found that neutrophils, innate immune cells that engulf and destroy microbes, are rapidly recruited to the injection site, irrespective of strain virulence, indicating that Y. pestis is unable to subvert neutrophil recruitment to the site of infection. However, we saw a decreased activation of neutrophils that were associated with Y. pestis strains harboring the pCD1 plasmid, which is essential for virulence. These findings indicate a role for pCD1-encoded factors in suppressing the activation/stimulation of these cells in vivo.

Reciprocal regulation of pH 6 antigen gene loci by PhoP and RovA in Yersinia pestis biovar Microtus

AIM

To explore the transcriptional regulation of the psaEF and psaABC loci by the RovA and PhoP regulators in Yersinia pestis.

MATERIALS & METHODS

Primer extension, LacZ fusion, gel mobility shift and DNase I footprinting assays were conducted in combination for this gene regulation study.

RESULTS

It was determined that PhoP and RovA recognized the promoter-proximal regions of psaEF and psaABC in order to repress and stimulate their transcription, respectively. The translation/transcription start sites, Shine-Dalgarno sequences (ribosomal binding site), core promoter -10 and -35 elements, PhoP and RovA sites and PhoP/RovA consensus-like sequences were identified to determine the structural organization of PhoP/RovA-dependent promoters of psaEF and psaABC.

CONCLUSION

RovA stimulated psaEF and psaABC, while PhoP repressed these two operons involving the direct association between RovA/PhoP and target promoter regions. The reciprocal regulation of psa genes by PhoP and RovA could contribute to the tightly controlled expression of the pH 6 antigen during infection.

Fibrin facilitates both innate and T cell-mediated defense against Yersinia pestis

The Gram-negative bacterium Yersinia pestis causes plague, a rapidly progressing and often fatal disease. The formation of fibrin at sites of Y. pestis infection supports innate host defense against plague, perhaps by providing a nondiffusible spatial cue that promotes the accumulation of inflammatory cells expressing fibrin-binding integrins. This report demonstrates that fibrin is an essential component of T cell-mediated defense against plague but can be dispensable for Ab-mediated defense. Genetic or pharmacologic depletion of fibrin abrogated innate and T cell-mediated defense in mice challenged intranasally with Y. pestis. The fibrin-deficient mice displayed reduced survival, increased bacterial burden, and exacerbated hemorrhagic pathology. They also showed fewer neutrophils within infected lung tissue and reduced neutrophil viability at sites of liver infection. Depletion of neutrophils from wild-type mice weakened T cell-mediated defense against plague. The data suggest that T cells combat plague in conjunction with neutrophils, which require help from fibrin to withstand Y. pestis encounters and effectively clear bacteria.

Integrating high-content imaging and chemical genetics to probe host cellular pathways critical for Yersinia pestis infection

The molecular machinery that regulates the entry and survival of Yersinia pestis in host macrophages is poorly understood. Here, we report the development of automated high-content imaging assays to quantitate the internalization of virulent Y. pestis CO92 by macrophages and the subsequent activation of host NF-κB. Implementation of these assays in a focused chemical screen identified kinase inhibitors that inhibited both of these processes. Rac-2-ethoxy-3 octadecanamido-1-propylphosphocholine (a protein Kinase C inhibitor), wortmannin (a PI3K inhibitor), and parthenolide (an IκB kinase inhibitor), inhibited pathogen-induced NF-κB activation and reduced bacterial entry and survival within macrophages. Parthenolide inhibited NF-κB activation in response to stimulation with Pam3CSK4 (a TLR2 agonist), E. coli LPS (a TLR4 agonist) or Y. pestis infection, while the PI3K and PKC inhibitors were selective only for Y. pestis infection. Together, our results suggest that phagocytosis is the major stimulus for NF-κB activation in response to Y. pestis infection, and that Y. pestis entry into macrophages may involve the participation of protein kinases such as PI3K and PKC. More importantly, the automated image-based screening platform described here can be applied to the study of other bacteria in general and, in combination with chemical genetic screening, can be used to identify host cell functions facilitating the identification of novel antibacterial therapeutics.

Autoregulation of PhoP/PhoQ and positive regulation of the cyclic AMP receptor protein-cyclic AMP complex by PhoP in Yersinia pestis

Yersinia pestis is one of the most dangerous bacterial pathogens. PhoP and cyclic AMP receptor protein (CRP) are global regulators of Y. pestis, and they control two distinct regulons that contain multiple virulence-related genes. The PhoP regulator and its cognate sensor PhoQ constitute a two-component regulatory system. The regulatory activity of CRP is triggered only by binding to its cofactor cAMP, which is synthesized from ATP by adenylyl cyclase (encoded by cyaA). However, the association between the two regulatory systems PhoP/PhoQ and CRP-cAMP is still not understood for Y. pestis. In the present work, the four consecutive genes YPO1635, phoP, phoQ, and YPO1632 were found to constitute an operon, YPO1635-phoPQ-YPO1632, transcribed as a single primary RNA, whereas the last three genes comprised another operon, phoPQ-YPO1632, transcribed with two adjacent transcriptional starts. Through direct PhoP-target promoter association, the transcription of these two operons was stimulated and repressed by PhoP, respectively; thus, both positive autoregulation and negative autoregulation of PhoP/PhoQ were detected. In addition, PhoP acted as a direct transcriptional activator of crp and cyaA. The translational/transcriptional start sites, promoter -10 and -35 elements, PhoP sites, and PhoP box-like sequences were determined for these PhoP-dependent genes, providing a map of the PhoP-target promoter interaction. The CRP and PhoP regulons have evolved to merge into a single regulatory cascade in Y. pestis because of the direct regulatory association between PhoP/PhoQ and CRP-cAMP.

Omics strategies for revealing Yersinia pestis virulence

Omics has remarkably changed the way we investigate and understand life. Omics differs from traditional hypothesis-driven research because it is a discovery-driven approach. Mass datasets produced from omics-based studies require experts from different fields to reveal the salient features behind these data. In this review, we summarize omics-driven studies to reveal the virulence features of Yersinia pestis through genomics, trascriptomics, proteomics, interactomics, etc. These studies serve as foundations for further hypothesis-driven research and help us gain insight into Y. pestis pathogenesis.

Toward a molecular pathogenic pathway for Yersinia pestis YopM

YopM is one of the six "effector Yops" of the human-pathogenic Yersinia, but its mechanism has not been defined. After delivery to J774A.1 monocyte-like cells, YopM can rapidly bind and activate the serine/threonine kinases RSK1 and PRK2. However, in infected mice, effects of Y. pestis YopM have been seen only after 24-48 h post-infection (p.i.). To identify potential direct effects of YopM in-vivo we tested for effects of YopM at 1 h and 16-18 h p.i. in mice infected systemically with 10(6) bacteria. At 16 h p.i., there was a robust host response to both parent and ΔyopM-1 Y. pestis KIM5. Compared to cells from non-infected mice, CD11b(+) cells from spleens of infected mice produced more than 100-fold greater IFNγ. In the corresponding sera there were more than 100-fold greater amounts of IFNγ, G-CSF, and CXCL9, as well as more than 10-fold greater amounts of IL-6, CXCL10, and CXCL1. The only YopM-related differences were slightly lower CXCL10 and IL-6 in sera from mice infected 16 h with parent compared to ΔyopM-1 Y. pestis. Microarray analysis of the CD11b(+) cells did not identify consistent transcriptional differences of ≥4-fold at 18 h p.i. However, at 1 h p.i. mRNA for early growth response transcription factor 1 (Egr1) was decreased when YopM was present. Bone marrow-derived macrophages infected for 1 h also expressed lower Egr1 message when YopM was present. Infected J774A.1 cells showed greater expression of Egr1 at 1 h p.i. when YopM was present, but this pattern reversed at 3 h. At 6 h p.i., Cxcl10 mRNA was lower in parent-strain infected cells. We conclude that decreased Egr1 expression is a very early transcriptional effect of YopM and speculate that a pathway may exist from RSK1 through Egr1. These studies revealed novel early transcriptional effects of YopM but point to a time after 18 h of infection when critical transitional events lead to later major effects on cytokine gene transcription.

Early sensing of Yersinia pestis airway infection by bone marrow cells

Bacterial infection of the lungs triggers a swift innate immune response that involves the production of cytokines and chemokines that promote recruitment of immune cells from the bone marrow (BM) into the infected tissue and limit the ability of the pathogen to replicate. Recent in vivo studies of pneumonic plague in animal models indicate that the pulmonary pro-inflammatory response to airway infection with Yersinia pestis is substantially delayed in comparison to other pathogens. Consequently, uncontrolled proliferation of the pathogen in the lungs is observed, followed by dissemination to internal organs and death. While the lack of an adequate early immune response in the lung is well described, the response of BM-derived cells is poorly understood. In this study, we show that intranasal (i.n.) infection of mice with a fully virulent Y. pestis strain is sensed early by the BM compartment, resulting in a reduction in CXCR4 levels on BM neutrophils and their subsequent release into the blood 12 hours (h) post infection. In addition, increased levels of BM-derived hematopoietic stem and progenitor cells (HSPC) were detected in the blood early after infection. Mobilization of both immature and mature cells was accompanied by the reduction of BM SDF-1 (CXCL-12) levels and the reciprocal elevation of SDF-1 in the blood 24 h post infection. RT-PCR analysis of RNA collected from total BM cells revealed an early induction of myeloid-associated genes, suggesting a prompt commitment to myeloid lineage differentiation. These findings indicate that lung infection by Y. pestis is sensed by BM cells early after infection, although bacterial colonization of the BM occurs at late disease stages, and point on a potential cross-talk between the lung and the BM at early stages of pneumonic plague.

The Yfe and Feo transporters are involved in microaerobic growth and virulence of Yersinia pestis in bubonic plague

The Yfe/Sit and Feo transport systems are important for the growth of a variety of bacteria. In Yersinia pestis, single mutations in either yfe or feo result in reduced growth under static (limited aeration), iron-chelated conditions, while a yfe feo double mutant has a more severe growth defect. These growth defects were not observed when bacteria were grown under aerobic conditions or in strains capable of producing the siderophore yersiniabactin (Ybt) and the putative ferrous transporter FetMP. Both fetP and a downstream locus (flp for fet linked phenotype) were required for growth of a yfe feo ybt mutant under static, iron-limiting conditions. An feoB mutation alone had no effect on the virulence of Y. pestis in either bubonic or pneumonic plague models. An feo yfe double mutant was still fully virulent in a pneumonic plague model but had an ∼90-fold increase in the 50% lethal dose (LD(50)) relative to the Yfe(+) Feo(+) parent strain in a bubonic plague model. Thus, Yfe and Feo, in addition to Ybt, play an important role in the progression of bubonic plague. Finally, we examined the factors affecting the expression of the feo operon in Y. pestis. Under static growth conditions, the Y. pestis feo::lacZ fusion was repressed by iron in a Fur-dependent manner but not in cells grown aerobically. Mutations in feoC, fnr, arcA, oxyR, or rstAB had no significant effect on transcription of the Y. pestis feo promoter. Thus, the factor(s) that prevents repression by Fur under aerobic growth conditions remains to be identified.

Roles of chaperone/usher pathways of Yersinia pestis in a murine model of plague and adhesion to host cells

Yersinia pestis and many other Gram-negative pathogenic bacteria use the chaperone/usher (CU) pathway to assemble virulence-associated surface fibers termed pili or fimbriae. Y. pestis has two well-characterized CU pathways: the caf genes coding for the F1 capsule and the psa genes coding for the pH 6 antigen. The Y. pestis genome contains additional CU pathways that are capable of assembling pilus fibers, but the roles of these pathways in the pathogenesis of plague are not understood. We constructed deletion mutations in the usher genes for six of the additional Y. pestis CU pathways. The wild-type (WT) and usher deletion strains were compared in the murine bubonic (subcutaneous) and pneumonic (intranasal) plague infection models. Y. pestis strains containing deletions in CU pathways y0348-0352, y1858-1862, and y1869-1873 were attenuated for virulence compared to the WT strain by the intranasal, but not subcutaneous, routes of infection, suggesting specific roles for these pathways during pneumonic plague. We examined binding of the Y. pestis WT and usher deletion strains to A549 human lung epithelial cells, HEp-2 human cervical epithelial cells, and primary human and murine macrophages. Y. pestis CU pathways y0348-0352 and y1858-1862 were found to contribute to adhesion to all host cells tested, whereas pathway y1869-1873 was specific for binding to macrophages. The correlation between the virulence attenuation and host cell binding phenotypes of the usher deletion mutants identifies three of the additional CU pathways of Y. pestis as mediating interactions with host cells that are important for the pathogenesis of plague.

Yersinia pestis intracellular parasitism of macrophages from hosts exhibiting high and low severity of plague

BACKGROUND

Yersinia pestis causes severe disease in natural rodent hosts, but mild to inapparent disease in certain rodent predators such as dogs. Y. pestis initiates infection in susceptible hosts by parasitizing and multiplying intracellularly in local macrophages prior to systemic dissemination. Thus, we hypothesize that Y. pestis disease severity may depend on the degree to which intracellular Y. pestis overcomes the initial host macrophage imposed stress.

METHODOLOGY/PRINCIPAL FINDINGS

To test this hypothesis, the progression of in vitro infection by Y. pestis KIM62053.1+ of mouse splenic and RAW264.7 tissue culture macrophages and dog peripheral blood-derived and DH82 tissue culture macrophages was studied using microscopy and various parameters of infection. The study showed that during the early stage of infection, intracellular Y. pestis assumed filamentous cellular morphology with multiple copies of the genome per bacterium in both mouse and dog macrophages. Later, in mouse macrophages, the infection elicited spacious vacuolar extension of Yersinia containing vacuoles (YCV), and the filamentous Y. pestis reverted to coccobacillary morphology with genomic equivalents approximately equaling colony forming units. In contrast, Y. pestis infected dog macrophages did not show noticeable extension of YCV, and intracellular Y. pestis retained the filamentous cellular morphology for the entire experiment in DH82 cells or were killed by blood-derived macrophages. In addition, during the later stage of infection, Y. pestis infected mouse macrophages exhibited cell lysis whereas dog macrophages did not.

CONCLUSION/SIGNIFICANCE

Overall, these results support our hypothesis that Y. pestis in mouse macrophages can overcome the initial intracellular stress necessary for subsequent systemic infection. However, in dogs, failure of Y. pestis to overcome macrophage imposed stress may result in mild or in apparent disease in dogs.

Opposing roles for interferon regulatory factor-3 (IRF-3) and type I interferon signaling during plague

Type I interferons (IFN-I) broadly control innate immunity and are typically transcriptionally induced by Interferon Regulatory Factors (IRFs) following stimulation of pattern recognition receptors within the cytosol of host cells. For bacterial infection, IFN-I signaling can result in widely variant responses, in some cases contributing to the pathogenesis of disease while in others contributing to host defense. In this work, we addressed the role of type I IFN during Yersinia pestis infection in a murine model of septicemic plague. Transcription of IFN-β was induced in vitro and in vivo and contributed to pathogenesis. Mice lacking the IFN-I receptor, Ifnar, were less sensitive to disease and harbored more neutrophils in the later stage of infection which correlated with protection from lethality. In contrast, IRF-3, a transcription factor commonly involved in inducing IFN-β following bacterial infection, was not necessary for IFN production but instead contributed to host defense. In vitro, phagocytosis of Y. pestis by macrophages and neutrophils was more effective in the presence of IRF-3 and was not affected by IFN-β signaling. This activity correlated with limited bacterial growth in vivo in the presence of IRF-3. Together the data demonstrate that IRF-3 is able to activate pathways of innate immunity against bacterial infection that extend beyond regulation of IFN-β production.

Type I interferons (IFN-I) broadly stimulate innate immunity against viral, bacterial and parasitic pathogens. Many bacterial pathogens induce IFN-I through phosphorylation of Interferon Regulatory Factor 3 (IRF-3) allowing it to bind promoters containing Interferon Stimulated Response Elements (ISRE) which include IFN-β and pro-inflammatory cytokines and chemokines. Secreted IFN-β is taken up by the IFN-αβ receptor (IFNAR), triggering activation of the JAK-STAT pathway which also activates ISRE-containing genes. In this work, we have discovered a novel anti-bacterial function of IRF-3. We show that the respiratory pathogen, Yersinia pestis, the causative agent of plague, activates IRF-3 and the IFN-I response and that these two events cause opposite outcomes in the host. While IRF-3 is necessary for an early stage of phagocytosis, IFNAR signaling promotes the infection and may directly contribute to neutrophil depletion during infection. These results demonstrate that an IFN-independent function of IRF-3 is important to host defense against bacterial infection.

Cell-mediated immune response to epitopic MAP (multiple antigen peptide) construct of LcrV antigen of Yersinia pestis in murine model

Yersinia pestis is the causative agent of plague. Cellular immunity seems to play an important role in defense against this disease. The subunit vaccine based on V (Lcr V) antigen has been proved to be immunogenic in animals and in humans. The multiple antigen peptide (MAP), incorporating all the relevant B and T cell epitopes is highly immunogenic in mice through intranasal route of immunization in PLGA particles containing CpG-ODN as an immunoadjuvant inducing humoral and mucosal immune response. In the present study, cell-mediated immune response using same MAP was studied in murine model. Primary and memory T cell responses were studied in outbred and inbred mice immunized intranasally with MAP in the presence of two immunoadjuvants (Murabutide and CpG-ODN). All the three compartments (Spleen, Lamina propria and Peyer's patches) of the lymphoid system showed increased lymphoproliferative response. Highest lymphoproliferative response was observed especially with CpG-ODN. Cytokine profile in the culture supernatant showed highest Th(1) and Th(17) levels. FACS analysis showed expansion of both CD4(+) and CD8(+) T cells producing gamma-interferon, perforin and granzyme-B with major contribution from CD4(+) T cells.

The in vivo extracellular life of facultative intracellular bacterial parasites: role in pathogenesis

Classically labeled facultative intracellular pathogens are characterized by the ability to have an intracellular phase in the host, which is required for pathogenicity, while capable of extracellular growth in vitro. The ability of these bacteria to replicate in cell-free conditions is usually assessed by culture in artificial bacteriological media. However, the extracellular growth ability of these pathogens may also be expressed by a phase of extracellular infection in the natural setting of the host with pathologic consequences, an ability that adds to the pathogenic potential of the infectious agent. This infective capability to grow in the extracellular sites of the host represents an additional virulence attribute of those pathogens which may lead to severe outcomes. Here we discuss examples of infectious diseases where the in vivo infective extracellular life is well documented, including infections by Francisella tularensis, Yersinia pestis, Burkholderia pseudomallei, Burkholderia cenocepacia, Salmonella enterica serovar Typhimurium and Edwardsiella tarda. The occurrence of a phase of systemic dissemination with extracellular multiplication during progressive infections by facultative intracellular bacterial pathogens has been underappreciated, with most studies exclusively centered on the intracellular phase of the infections. The investigation of the occurrence of a dual lifestyle in the host among bacterial pathogens in general should be extended and likely will reveal more cases of infectious diseases with a dual infective intracellular/extracellular pattern.

A Toll/interleukin (IL)-1 receptor domain protein from Yersinia pestis interacts with mammalian IL-1/Toll-like receptor pathways but does not play a central role in the virulence of Y. pestis in a mouse model of bubonic plague

The Toll/interleukin (IL)-1 receptor (TIR) domain is an essential component of eukaryotic innate immune signalling pathways. Interaction between TIR domains present in Toll-like receptors and associated adaptors initiates and propagates an immune signalling cascade. Proteins containing TIR domains have also been discovered in bacteria. Studies have subsequently shown that these proteins are able to modulate mammalian immune signalling pathways dependent on TIR interactions and that this may represent an evasion strategy for bacterial pathogens. Here, we investigate a TIR domain protein from the highly virulent bacterium Yersinia pestis, the causative agent of plague. When overexpressed in vitro this protein is able to downregulate IL-1β- and LPS-dependent signalling to NFκB and to interact with the TIR adaptor protein MyD88. This interaction is dependent on a single proline residue. However, a Y. pestis knockout mutant lacking the TIR domain protein was not attenuated in virulence in a mouse model of bubonic plague. Minor alterations in the host cytokine response to the mutant were indicated, suggesting a potential subtle role in pathogenesis. The Y. pestis mutant also showed increased auto-aggregation and reduced survival in high-salinity conditions, phenotypes which may contribute to pathogenesis or survival.

A transposon site hybridization screen identifies galU and wecBC as important for survival of Yersinia pestis in murine macrophages

Yersinia pestis is able to survive and replicate within murine macrophages. However, the mechanism by which Y. pestis promotes its intracellular survival is not well understood. To identify genes that are important for Y. pestis survival in macrophages, a library comprised of ∼31,500 Y. pestis KIM6+ transposon insertion mutants (input pool) was subjected to negative selection in primary murine macrophages. Genes underrepresented in the output pool of surviving bacteria were identified by transposon site hybridization to DNA oligonucleotide microarrays. The screen identified several genes known to be important for survival of Y. pestis in macrophages, including phoPQ and members of the PhoPQ regulon (e.g., pmrF). In addition, genes predicated to encode a glucose-1-phosphate uridylyltransferase (galU), a UDP-N-acetylglucosamine 2-epimerase (wecB) and a UDP-N-acetyl-d-mannosamine dehydrogenase (wecC) were identified in the screen. Viable-count assays demonstrated that a KIM6+ galU mutant and a KIM6+ wecBC mutant were defective for survival in murine macrophages. The galU mutant was studied further because of its strong phenotype. The KIM6+ galU mutant exhibited increased susceptibility to the antimicrobial peptides polymyxin B and cathelicidin-related antimicrobial peptide (CRAMP). Polyacrylamide gel electrophoresis demonstrated that the lipooligosaccharide (LOS) of the galU mutant migrated faster than the LOS of the parent KIM6+, suggesting the core was truncated. In addition, the analysis of LOS isolated from the galU mutant by mass spectrometry showed that aminoarabinose modification of lipid A is absent. Therefore, addition of aminoarabinose to lipid A and complete LOS core (galU), as well as enterobacterial common antigen (wecB and wecC), is important for survival of Y. pestis in macrophages.

Molecular characterization of transcriptional regulation of rovA by PhoP and RovA in Yersinia pestis

BACKGROUND

Yersinia pestis is the causative agent of plague. The two transcriptional regulators, PhoP and RovA, are required for the virulence of Y. pestis through the regulation of various virulence-associated loci. They are the global regulators controlling two distinct large complexes of cellular pathways.

METHODOLOGY/PRINCIPAL FINDINGS

Based on the LacZ fusion, primer extension, gel mobility shift, and DNase I footprinting assays, RovA is shown to recognize both of the two promoters of its gene in Y. pestis. The autoregulation of RovA appears to be a conserved mechanism shared by Y. pestis and its closely related progenitor, Y. pseudotuberculosis. In Y. pestis, the PhoP regulator responds to low magnesium signals and then negatively controls only one of the two promoters of rovA through PhoP-promoter DNA association.

CONCLUSIONS/SIGNIFICANCE

RovA is a direct transcriptional activator for its own gene in Y. pestis, while PhoP recognizes the promoter region of rovA to repress its transcription. The direct regulatory association between PhoP and RovA bridges the PhoP and RovA regulons in Y. pestis.

Formation and regulation of Yersinia biofilms

Flea-borne transmission is a recent evolutionary adaptation that distinguishes the deadly Yersinia pestis from its progenitor Y. Pseudotuberculosis, a mild pathogen transmitted via the food-borne route. Y. Pestis synthesizes biofilms in the flea gut, which is important for fleaborne transmission. Yersinia biofilms are bacterial colonies surrounded by extracellular matrix primarily containing a homopolymer of N-acetyl-D-glucosamine that are synthesized by a set of specific enzymes. Yersinia biofilm production is tightly regulated at both transcriptional and post-transcriptional levels. All the known structural genes responsible for biofilm production are harbored in both Y. Pseudotuberculosis and Y. Pestis, but Y. Pestis has evolved changes in the regulation of biofilm development, thereby acquiring efficient arthropod-borne transmission.

Yersinia pseudotuberculosis is resistant to killing by human neutrophils

The interaction between human neutrophils and the Gram negative gastrointestinal pathogen Yersinia pseudotuberculosis was investigated in vitro. Despite the wealth of data describing how Yersinia can affect the function of neutrophils, there are no published studies describing if neutrophil cells can affect the viability of Y. pseudotuberculosis. The wild-type IP32953 strain of Y. pseudotuberculosis was found to be resistant to killing by human neutrophils. Confocal examination and flow-cytometric analysis of this interaction revealed that bacteria were taken up.

The role of the phoPQ operon in the pathogenesis of the fully virulent CO92 strain of Yersinia pestis and the IP32953 strain of Yersinia pseudotuberculosis

At the genomic level, Yersinia pestis and Yersinia pseudotuberculosis are nearly identical but cause very different diseases. Y. pestis is the etiologic agent of plague; whereas Y. pseudotuberculosis causes a gastrointestinal infection primarily after the consumption of contaminated food. In many gram-negative pathogenic bacteria, PhoP is part of a two-component global regulatory system in which PhoQ serves as the sensor kinase, and PhoP is the response regulator. PhoP is known to activate a number of genes in many bacteria related to virulence. To determine the role of the PhoPQ proteins in Yersinia infections, primarily using aerosol challenge models, the phoP gene was deleted from the chromosome of the CO92 strain of Y. pestis and the IP32953 strain of Y. pseudotuberculosis, leading to a polar mutation of the phoPQ operon. We demonstrated that loss of phoPQ from both strains leads to a defect in intracellular growth and/or survival within macrophages. These in vitro data would suggest that the phoPQ mutants would be attenuated in vivo. However, the LD(50) for the Y. pestis mutant did not differ from the calculated LD(50) for the wild-type CO92 strain for either the bubonic or pneumonic murine models of infection. In contrast, mice challenged by aerosol with the Y. pseudotuberculosis mutant had a LD(50) value 40× higher than the wild-type strain. These results demonstrate that phoPQ are necessary for full virulence by aerosol infection with the IP32953 strain of Y. pseudotuberculosis. However, the PhoPQ proteins do not play a significant role in infection with a fully virulent strain of Y. pestis.

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

Many gram-negative bacteria use type III secretion systems to translocate effector proteins into host cells. These effectors interfere with cellular functions in a highly regulated manner resulting in effects that are beneficial for the bacteria. The pathogen Yersinia can resist phagocytosis by eukaryotic cells by translocating Yop effectors into the target cell cytoplasm. This is called antiphagocytosis, and constitutes an important virulence feature of this pathogen since it allows survival in immune cell rich lymphoid organs. We show here that the virulence protein YopK has a role in orchestrating effector translocation necessary for productive antiphagocytosis. We present data showing that YopK influences Yop effector translocation by modulating the ratio of the pore-forming proteins YopB and YopD in the target cell membrane. Further, we show that YopK that can interact with the translocators, is exposed inside target cells and binds to the eukaryotic signaling protein RACK1. This protein is engaged upon Y. pseudotuberculosis-mediated β1-integrin activation and localizes to phagocytic cups. Cells with downregulated RACK1 levels are protected from antiphagocytosis. This resistance is not due to altered levels of translocated antiphagocytic effectors, and cells with reduced levels of RACK1 are still sensitive to the later occurring cytotoxic effect caused by the Yop effectors. Further, a yopK mutant unable to bind RACK1 shows an avirulent phenotype during mouse infection, suggesting that RACK1 targeting by YopK is a requirement for virulence. Together, our data imply that the local event of Yersinia-mediated antiphagocytosis involves a step where YopK, by binding RACK1, ensures an immediate specific spatial delivery of antiphagocytic effectors leading to productive inhibition of phagocytosis.

Involvement of CD8+ T cell-mediated immune responses in LcrV DNA vaccine induced protection against lethal Yersinia pestis challenge

Yersinia pestis (Y. pestis) is the causative pathogen of plague, a highly fatal disease for which an effective vaccine, especially against mucosal transmission, is still not available. Like many bacterial infections, antigen-specific antibody responses have been traditionally considered critical, if not solely responsible, for vaccine-induced protection against Y. pestis. Studies in recent years have suggested the importance of T cell immune responses against Y. pestis infection but information is still limited about the details of Y. pestis antigen-specific T cell immune responses. In current report, studies are conducted to identify the presence of CD8+ T cell epitopes in LcrV protein, the leading antigen of plague vaccine development. Furthermore, depletion of CD8+ T cells in LcrV DNA vaccinated Balb/C mice led to reduced protection against lethal intranasal challenge of Y. pestis. These findings establish that an LcrV DNA vaccine is able to elicit CD8+ T cell immune responses against specific epitopes of this key plague antigen and that a CD8+ T cell immune response is involved in LcrV DNA vaccine-elicited protection. Future studies in plague vaccine development will need to examine if the presence of detectable T cell immune responses, in particular CD8+ T-cell immune responses, will enhance the protection against Y. pestis in higher animal species or humans.

IL-17 contributes to cell-mediated defense against pulmonary Yersinia pestis infection

Pneumonic plague is one of the world's most deadly infectious diseases. The causative bacterium, Yersinia pestis, has the potential to be exploited as a biological weapon, and no vaccine is available. Vaccinating B cell-deficient mice with D27-pLpxL, a live attenuated Y. pestis strain, induces cell-mediated protection against lethal pulmonary Y. pestis challenge. In this article, we demonstrate that prime/boost vaccination with D27-pLpxL confers better protection than prime-only vaccination. The improved survival does not result from enhanced bacterial clearance but is associated with increased levels of IL-17 mRNA and protein in the lungs of challenged mice. The boost also increases pulmonary numbers of IL-17-producing CD4 T cells. Interestingly, most of these cells simultaneously produce canonical type 1 and type 17 cytokines; most produce IL-17 and TNF-α, and many produce IL-17, TNF-α, and IFN-γ. Neutralizing IL-17 counteracts the improved survival associated with prime/boost vaccination without significantly impacting bacterial burden. Thus, IL-17 appears to mediate the enhanced protection conferred by booster immunization. Although neutralizing IL-17 significantly reduces neutrophil recruitment to the lungs of mice challenged with Y. pestis, this impact is equally evident in mice that receive one or two immunizations with D27-pLpxL, suggesting it cannot suffice to account for the improved survival that results from booster immunization. We conclude that IL-17 plays a yet to be identified role in host defense that enhances protection against pulmonary Y. pestis challenge, and we suggest that pneumonic plague vaccines should aim to induce mixed type 1 and type 17 cellular responses.

Distinct CCR2(+) Gr1(+) cells control growth of the Yersinia pestis ΔyopM mutant in liver and spleen during systemic plague

We are using a systemic plague model to identify the cells and pathways that are undermined by the virulence protein YopM of the plague bacterium Yersinia pestis. In this study, we pursued previous findings that Gr1(+) cells are required to selectively limit growth of ΔyopM Y. pestis and that CD11b(+) cells other than polymorphonuclear leukocytes (PMNs) are selectively lost in spleens infected with parent Y. pestis. When PMNs were ablated from mice, ΔyopM Y. pestis grew as well as the parent strain in liver but not in spleen, showing that these cells are critical for controlling growth of the mutant in liver but not spleen. In mice lacking expression of the chemokine receptor CCR2, wild-type growth was restored to ΔyopM Y. pestis in both organs. In spleen, the Gr1(+) cells differentially recruited by parent and ΔyopM Y. pestis infections were CCR2(+) Gr1(+) CD11b(+) CD11c(Lo-Int) MAC3(+) iNOS(+) (inducible nitric oxide synthase-positive) inflammatory dendritic cells (iDCs), and their recruitment to spleen from blood was blocked when YopM was present in the infecting strain. Consistent with influx of iDCs being affected by YopM in spleen, the growth defect of the ΔyopM mutant was relieved by the parent Y. pestis strain in a coinfection assay in which the parent strain could affect the fate of the mutant in trans. In a mouse model of bubonic plague, CCR2 also was shown to be required for ΔyopM Y. pestis to show wild-type growth in skin. The data imply that YopM's pathogenic effect indirectly undermines signaling through CCR2. We propose a model for how YopM exerts its different effects in liver and spleen.

Model systems to study plague pathogenesis and develop new therapeutics

The Gram negative bacterium Yersinia pestis can infect humans by multiple routes to cause plague. Three plague pandemics have occurred and Y. pestis has been linked to biowarfare in the past. The continued risk of plague as a bioweapon has prompted increased research to understand Y. pestis pathogenesis and develop new plague therapeutics. Several in vivo models have been developed for this research and are reviewed here.

TNFα and IFNγ contribute to F1/LcrV-targeted immune defense in mouse models of fully virulent pneumonic plague

Immunization with the Yersinia pestis F1 and LcrV proteins improves survival in mouse and non-human primate models of pneumonic plague. F1- and LcrV-specific antibodies contribute to protection, however, the mechanisms of antibody-mediated defense are incompletely understood and serum antibody titers do not suffice as quantitative correlates of protection. Previously we demonstrated roles for tumor necrosis factor-alpha (TNFα) and gamma-interferon (IFNγ) during defense against conditionally attenuated pigmentation (pgm) locus-negative Y. pestis. Here, using intranasal challenge with fully virulent pgm-positive Y. pestis strain CO92, we demonstrate that neutralizing TNFα and IFNγ interferes with the capacity of therapeutically administered F1- or LcrV-specific antibody to reduce bacterial burden and increase survival. Moreover, using Y. pestis strain CO92 in an aerosol challenge model, we demonstrate that neutralizing TNFα and IFNγ interferes with protection conferred by immunization with recombinant F1-LcrV fusion protein vaccine (p<0.0005). These findings establish that TNFα and IFNγ contribute to protection mediated by pneumonic plague countermeasures targeting F1 and LcrV, and suggest that an individual's capacity to produce these cytokines in response to Y. pestis challenge will be an important co-determinant of antibody-mediated defense against pneumonic plague.

Bacteria-induced phagocyte secondary necrosis as a pathogenicity mechanism

Triggering of phagocyte apoptosis is a major virulence mechanism used by some successful bacterial pathogens. A central issue in the apoptotic death context is that fully developed apoptosis results in necrotic cell autolysis (secondary necrosis) with release of harmful cell components. In multicellular animals, this occurs when apoptosing cells are not removed by scavengers, mainly macrophages. Secondary necrotic lysis of neutrophils and macrophages may occur in infection when extensive phagocyte apoptosis is induced by bacterial cytotoxins and removal of apoptosing phagocytes is defective because the apoptotic process exceeds the available scavenging capacity or targets macrophages directly. Induction of phagocyte secondary necrosis is an important pathogenic mechanism, as it combines the pathogen evasion from phagocyte antimicrobial activities and the release of highly cytotoxic molecules, particularly of neutrophil origin, such as neutrophil elastase. This pathogenicity mechanism therefore promotes the unrestricted multiplication of the pathogen and contributes directly to the pathology of several necrotizing infections, where extensive apoptosis and necrosis of macrophages and neutrophils are present. Here, examples of necrotizing infectious diseases, where phagocyte secondary necrosis is implicated, are reviewed.

Global gene expression profiling of Yersinia pestis replicating inside macrophages reveals the roles of a putative stress-induced operon in regulating type III secretion and intracellular cell division

Yersinia pestis, the causative agent of plague, is a facultative intracellular pathogen. Previous studies have indicated that the ability of Y. pestis to survive inside macrophages may be critical during the early stages of plague pathogenesis. To gain insights into the biology of intracellular Y. pestis and its environment following phagocytosis, we determined the genome-wide transcriptional profile of Y. pestis KIM5 replicating inside J774.1 macrophage-like cells using DNA microarrays. At 1.5, 4, and 8 h postinfection, a total of 801, 464, and 416 Y. pestis genes were differentially regulated, respectively, compared to the level of gene expression of control bacteria grown in tissue culture medium. A number of stress-response genes, including those involved in detoxification of reactive oxygen species, as well as several metabolic genes involved in macromolecule synthesis, were significantly induced in intracellular Y. pestis, consistent with the presence of oxidative stress and nutrient starvation inside Yersinia-containing vacuoles. A putative stress-induced operon consisting of y2313, y2315, and y2316 (y2313-y2316), and a previously unidentified open reading frame, orfX, was studied further on the basis of its high level of intracellular expression. Mutant strains harboring either deletion, Deltay2313-y2316 or DeltaorfX, exhibited diverse phenotypes, including reduced effector secretion by the type III secretion system, increased intracellular replication, and filamentous morphology of the bacteria growing inside macrophages. The results suggest a possible role for these genes in regulating cell envelope characteristics in the intracellular environment.

AIP56: a novel bacterial apoptogenic toxin

Photobacterium damselae subsp. piscicida (Phdp) is a Gram-negative pathogen agent of an important fish septicemia. The key virulence factor of Phdp is the plasmid-encoded exotoxin AIP56, which is secreted by exponentially growing pathogenic strains. AIP56 has 520 amino acids including an N-terminal cleavable signal peptide of 23 amino acid residues, two cysteine residues and a zinc-binding region signature HEXXH that is typical of most zinc metallopeptidases. AIP56 induces in vitro and in vivo selective apoptosis of fish macrophages and neutrophils through a caspase-3 dependent mechanism that also involves caspase-8 and -9. In vivo, the AIP56-induced phagocyte apoptosis progresses to secondary necrosis with release of cytotoxic phagocyte molecules including neutrophil elastase. Fish injected with recombinant AIP56 die with a pathology similar to that seen in the natural infection.

Neutrophils are important in early control of lung infection by Yersinia pestis

In this paper we evaluate the role of neutrophils in pneumonic plague. Splenic neutrophils from naïve BALB/c mice were found to reduce numbers of culturable Yersinia pestis strain GB in suspension. A murine, BALB/c, intranasal model of pneumonic plague was used in conjunction with in vivo neutrophil ablation, using the GR-1 antibody. This treatment reduced neutrophil numbers without affecting other leukocyte numbers. Neutrophil ablated mice exhibited increased bacterial colonisation of the lung 24h post infection. Furthermore, exposure of Y. pestis to human neutrophils resulted in a 5-fold reduction in the number of viable bacterial cells, whereas, PBMCs had no effect.

Comparative Analyses of Transcriptional Profiles in Mouse Organs Using a Pneumonic Plague Model after Infection with Wild-Type Yersinia pestis CO92 and Its Braun Lipoprotein Mutant

We employed Murine GeneChips to delineate the global transcriptional profiles of the livers, lungs, and spleens in a mouse pneumonic plague infection model with wild-type (WT) Y. pestis CO92 and its Braun lipoprotein (Deltalpp) mutant with reduced virulence. These organs showed differential transcriptional responses to infection with WT Y. pestis, but the overall host functional processes affected were similar across all three tissues. Gene expression alterations were found in inflammation, cytokine signaling, and apoptotic cell death-associated genes. Comparison of WT and Deltalpp mutant-infected mice indicated significant overlap in lipopolysaccharide- (LPS-) associated gene expression, but the absence of Lpp perturbed host cell signaling at critical regulatory junctions resulting in altered immune response and possibly host cell apoptosis. We generated a putative signaling pathway including major inflammatory components that could account for the synergistic action of LPS and Lpp and provided the mechanistic basis of attenuation caused by deletion of the lpp gene from Y. pestis in a mouse model of pneumonic plague.

Yersinia pestis two-component gene regulatory systems promote survival in human neutrophils

Human polymorphonuclear leukocytes (PMNs, or neutrophils) are the most abundant innate immune cell and kill most invading bacteria through combined activities of reactive oxygen species (ROS) and antimicrobial granule constituents. Pathogens such as Yersinia pestis resist destruction by the innate immune system and are able to survive in macrophages and neutrophils. The specific molecular mechanisms used by Y. pestis to survive following phagocytosis by human PMNs are incompletely defined. To gain insight into factors that govern Y. pestis intracellular survival in neutrophils, we inactivated 25 two-component gene regulatory systems (TCSs) with known or inferred function and assessed susceptibility of these mutant strains to human PMN granule extracts. Y. pestis strains deficient for PhoPQ, KdpED, CheY, CvgSY, and CpxRA TCSs were selected for further analysis, and all five strains were altered for survival following interaction with PMNs. Of these five strains, only Y. pestis DeltaphoPQ demonstrated global sensitivity to a panel of seven individual neutrophil antimicrobial peptides and serine proteases. Notably, Y. pestis DeltaphoPQ was deficient for intracellular survival in PMNs. Iterative analysis with Y. pestis strains lacking the PhoP-regulated genes ugd and pmrK indicated that the mechanism most likely responsible for increased resistance to killing is 4-amino-4-deoxy-l-arabinose modification of lipid A. Together, the data provide new information about Y. pestis evasion of the innate immune system.

Reduced apoptosis of mouse macrophages induced by yscW mutant of Yersinia pestis results from the reduced secretion of YopJ and relates to caspase-3 signal pathway

The virulence of the pathogenic Yersinia species depends on a plasmid-encoded type III secretion system (T3SS) that injects six Yersinia outer protein (Yop) effector proteins into the cytosol of macrophages, leading to disruption of host defence mechanisms. Here, we report that a T3SS structural protein YscW of Yersinia pestis contributed to the induction of apoptosis of murine macrophages. The apoptotic percentage of macrophages, from both mouse peritoneal cavity and spleen, and of RAW264.7 cell line, caused by the yscW mutant strain was significantly lower than that by wild type (WT) Y. pestis and yscW complemented strain. Meanwhile, detection of caspase-3 activity in macrophages, a key apoptosis-inducing protein, showed coincident results with the changes of macrophage apoptosis induced by WT, yscW mutant and complemented strains, indicating that macrophage apoptosis was related to caspase-3 signal pathways. However, ectopic expression of YscW in RAW264.7 cells cannot increase the macrophage apoptosis and death, suggesting that YscW itself could not induce macrophage apoptosis directly. To get insight into the mechanism of this phenomenon, we investigated the secretion of YopJ, which has been thought to be the only Yop effector related to apoptosis, in WT, mutant and complemented strains, respectively. Results showed that in yscW mutant strain, secretion of YopJ was decreased significantly in the supernatant than that in WT or complemented strain. This means although YscW does not induce apoptosis directly, it can indirectly affect apoptosis through reducing the secretion of YopJ.

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

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

Innate immune response during Yersinia infection: critical modulation of cell death mechanisms through phagocyte activation

Yersinia pestis, the etiological agent of plague, is one of the most deadly pathogens on our planet. This organism shares important attributes with its ancestral progenitor, Yersinia pseudotuberculosis, including a 70-kb virulence plasmid, lymphotropism during growth in the mammalian host, and killing of host macrophages. Infections with both organisms are biphasic, where bacterial replication occurs initially with little inflammation, followed by phagocyte influx, inflammatory cytokine production, and tissue necrosis. During infection, plasmid-encoded attributes facilitate bacterial-induced macrophage death, which results from two distinct processes and corresponds to the inflammatory crescendo observed in vivo: Naïve cells die by apoptosis (noninflammatory), and later in infection, activated macrophages die by pyroptosis (inflammatory). The significance of this redirected cell death for the host is underscored by the importance of phagocyte activation for immunity to Yersinia and the protective role of pyroptosis during host responses to anthrax lethal toxin and infections with Francisella, Legionella, Pseudomonas, and Salmonella. The similarities of Y. pestis and Y. pseudotuberculosis, including conserved, plasmid-encoded functions inducing at least two distinct mechanisms of cell death, indicate that comparative studies are revealing about their critical pathogenic mechanism(s) and host innate immune responses during infection. Validation of this idea and evidence of similar interactions with the host immune system are provided by Y. pseudotuberculosis-priming, cross-protective immunity against Y. pestis. Despite these insights, additional studies indicate much remains to be understood concerning effective host responses against Yersinia, including chromosomally encoded attributes that also contribute to bacterial evasion and modulation of innate and adaptive immune responses.

Evaluation of a Yersinia pestis mutant impaired in a thermoregulated type VI-like secretion system in flea, macrophage and murine models

Type VI secretion systems (T6SSs) have been identified recently in several Gram-negative organisms and have been shown to be associated with virulence in some bacterial pathogens. A T6SS of Yersinia pestis CO92 (locus YPO0499-YPO0516) was deleted followed by investigation of the phenotype of this mutation. We observed that this T6SS locus of Y. pestis was preferentially expressed at 26 degrees C in comparison to 37 degrees C suggesting a possible role in the flea cycle. However, we found that the deletion of T6SS locus YPO0499-YPO0516 in Y. pestis CO92 had no effect on the ability of this strain to infect the oriental rat flea, Xenopsylla cheopis. Nevertheless, this mutant displayed increased intracellular numbers in macrophage-like J774.A1 cells after 20 h post-infection for bacterial cells pre-grown at 26 degrees C indicating that expression of this T6SS locus limited intracellular replication in macrophages. In addition, deletion of the YPO0499-YPO0516 locus reduced the uptake by macrophages of the Y. pestis mutant pre-grown at 37 degrees C, suggesting that this T6SS locus has phagocytosis-promoting activity. Further study of the virulence of the T6SS mutant in murine bubonic and inhalation plague models revealed no attenuation in comparison with the parental CO92 strain.

Yersinia pestis can bypass protective antibodies to LcrV and activation with gamma interferon to survive and induce apoptosis in murine macrophages

Yersinia pestis, the agent of plague, uses a type III secretion injectisome to deliver Yop proteins into macrophages to counteract phagocytosis and induce apoptosis. Additionally, internalized Y. pestis can survive in the phagosomes of naïve or gamma interferon (IFN-gamma)-activated macrophages by blocking vacuole acidification. The Y. pestis LcrV protein is a target of protective antibodies. The binding of antibodies to LcrV at the injectisome tip results in neutralization of the apoptosis of Y. pestis-infected macrophages and is used as an in vitro correlate of protective immunity. The cytokines IFN-gamma and tumor necrosis factor alpha can cooperate with anti-LcrV to promote protection against lethal Y. pestis infection in mice. It is not known if these phagocyte-activating cytokines cooperate with anti-LcrV to increase the killing of the pathogen and decrease apoptosis in macrophages. We investigated how anti-LcrV and IFN-gamma impact bacterial survival and apoptosis in cultured murine macrophages infected with Y. pestis KIM5. Y. pestis KIM5 opsonized with polyclonal or monoclonal anti-LcrV was used to infect macrophages treated with or without IFN-gamma. The phagocytosis and survival of KIM5 and the apoptosis of macrophages were measured at different time points postinfection. The results show that anti-LcrV reduced apoptosis at an early time point (5 h) but not at a later time point (24 h). Polyclonal anti-LcrV was unable to inhibit apoptosis at either time point in IFN-gamma-activated macrophages. Additionally, anti-LcrV was ineffective at promoting the killing of KIM5 in naïve or activated macrophages. We conclude that Y. pestis can bypass protective antibodies to LcrV and activation with IFN-gamma to survive and induce apoptosis in murine macrophages.

Gr1+ cells control growth of YopM-negative yersinia pestis during systemic plague

YopM, a protein toxin of Yersinia pestis, is necessary for virulence in a mouse model of systemic plague. We previously reported YopM-dependent natural killer (NK) cell depletion from blood and spleen samples of infected mice. However, in this study we found that infection with Y. pestis KIM5 (YopM(+)) caused depletion of NK cells in the spleen, but not in the liver, and antibody-mediated ablation of NK cells had no effect on bacterial growth. There was no YopM-associated effect on the percentage of dendritic cells (DCs) or polymorphonuclear leukocytes (PMNs) in the early stage of infection; however, there was a YopM-associated effect on PMN integrity and on the influx of monocytes into the spleen. Ablation of Gr1(+) cells caused loss of the growth defect of YopM(-) Y. pestis in both the liver and spleen. In contrast, ablation of macrophages/DCs inhibited growth of both parent and mutant bacteria, accompanied by significantly fewer lesion sites in the liver. These results point toward PMNs and inflammatory monocytes as major cell types that control growth of YopM(-) Y. pestis. Infection with fully virulent Y. pestis CO92 and a YopM(-) derivative by intradermal and intranasal routes showed that the absence of YopM significantly increased the 50% lethal dose only in the intradermal model, suggesting a role for YopM in bubonic plague, in which acute inflammation occurs soon after infection.

Yersinia pestis endowed with increased cytotoxicity is avirulent in a bubonic plague model and induces rapid protection against pneumonic plague

An important virulence strategy evolved by bacterial pathogens to overcome host defenses is the modulation of host cell death. Previous observations have indicated that Yersinia pestis, the causative agent of plague disease, exhibits restricted capacity to induce cell death in macrophages due to ineffective translocation of the type III secretion effector YopJ, as opposed to the readily translocated YopP, the YopJ homologue of the enteropathogen Yersinia enterocolitica Oratio8. This led us to suggest that reduced cytotoxic potency may allow pathogen propagation within a shielded niche, leading to increased virulence. To test the relationship between cytotoxic potential and virulence, we replaced Y. pestis YopJ with YopP. The YopP-expressing Y. pestis strain exhibited high cytotoxic activity against macrophages in vitro. Following subcutaneous infection, this strain had reduced ability to colonize internal organs, was unable to induce septicemia and exhibited at least a 10(7)-fold reduction in virulence. Yet, upon intravenous or intranasal infection, it was still as virulent as the wild-type strain. The subcutaneous administration of the cytotoxic Y. pestis strain appears to activate a rapid and potent systemic, CTL-independent, immunoprotective response, allowing the organism to overcome simultaneous coinfection with 10,000 LD(50) of virulent Y. pestis. Moreover, three days after subcutaneous administration of this strain, animals were also protected against septicemic or primary pneumonic plague. Our findings indicate that an inverse relationship exists between the cytotoxic potential of Y. pestis and its virulence following subcutaneous infection. This appears to be associated with the ability of the engineered cytotoxic Y. pestis strain to induce very rapid, effective and long-lasting protection against bubonic and pneumonic plague. These observations have novel implications for the development of vaccines/therapies against Y. pestis and shed new light on the virulence strategies of Y. pestis in nature.

Yersinia pestis can reside in autophagosomes and avoid xenophagy in murine macrophages by preventing vacuole acidification

Yersinia pestis survives and replicates in phagosomes of murine macrophages. Previous studies demonstrated that Y. pestis-containing vacuoles (YCVs) acquire markers of late endosomes or lysosomes in naïve macrophages and that this bacterium can survive in macrophages activated with the cytokine gamma interferon. An autophagic process known as xenophagy, which destroys pathogens in acidic autophagolysosomes, can occur in naïve macrophages and is upregulated in activated macrophages. Studies were undertaken here to investigate the mechanism of Y. pestis survival in phagosomes of naïve and activated macrophages and to determine if the pathogen avoids or co-opts autophagy. Colocalization of the YCV with markers of autophagosomes or acidic lysosomes and the pH of the YCV were determined by microscopic imaging of infected macrophages. Some YCVs contained double membranes characteristic of autophagosomes, as determined by electron microscopy. Fluorescence microscopy showed that approximately 40% of YCVs colocalized with green fluorescent protein (GFP)-LC3, a marker of autophagic membranes, and that YCVs failed to acidify below pH 7 in naïve macrophages. Replication of Y. pestis in naïve macrophages caused accumulation of LC3-II, as determined by immunoblotting. While activation of infected macrophages increased LC3-II accumulation, it decreased the percentage of GFP-LC3-positive YCVs (approximately 30%). A viable count assay showed that Y. pestis survived equally well in macrophages proficient for autophagy and macrophages rendered deficient for this process by Cre-mediated deletion of ATG5, revealing that this pathogen does not require autophagy for intracellular replication. We conclude that although YCVs can acquire an autophagic membrane and accumulate LC3-II, the pathogen avoids xenophagy by preventing vacuole acidification.

Intracellular pathogenic bacteria and fungi--a case of convergent evolution?

The bacterium Yersinia pestis and the fungus Cryptococcus neoformans are the causative agents of human plague and cryptococcosis, respectively. Both microorganisms are facultatively intracellular pathogens. A comparison of their pathogenic strategies reveals similar tactics for intracellular survival in Y. pestis and C. neoformans despite their genetic unrelatedness. Both organisms can survive in environments where they are vulnerable to predation by amoeboid protozoal hosts. Here, we propose that the overall similarities in their pathogenic strategies are an example of convergent evolution that has solved the problem of intracellular survival.

Yersinia pestis and host macrophages: immunodeficiency of mouse macrophages induced by YscW

The virulence of the pathogenic Yersinia species depends on a plasmid-encoded type III secretion system (T3SS) that transfers six Yersinia outer protein (Yop) effector proteins into the cytoplasm of eukaryotic cells, leading to disruption of host defence mechanisms. It is shown in this study that Yersinia pestis YscW, a protein of the T3SS injectisome, contributes to the induction of a deficiency in phagocytosis in host macrophages and a reduction in their antigen-presenting capacity. A Y. pestis strain lacking yscW had no effect on uptake by host macrophages. In mice infected with wild-type Y. pestis, the yscW mutant or a complement strain, immunodeficiency was observed in host macrophages compared with those from uninfected mice. However, the phagocytosis and antigen presenting capacities of macrophages infected by yscW mutant strain both in vivo and in vitro were significantly higher than those by wild type strain. Consistent with this finding, when YscW was expressed in the RAW264.7 macrophage cell line, phagocytosis and antigen-presenting capacities were significantly lower than those of the control groups. These results indicate that Y. pestis YscW may directly induce immunodeficiency in murine macrophages by crippling their phagocytosis and antigen-presenting capacities. These data provide evidences to Y. pestis pathogenesis that some proteins in T3SS injectisome, such as YscW protein, might play independent roles in disrupting host defense apart from their known functions.

Antibodies and cytokines independently protect against pneumonic plague

Yersinia pestis causes pneumonic plague, an exceptionally virulent disease for which we lack a safe and effective vaccine. Antibodies specific for the Y. pestis F1 and LcrV proteins can protect mice against pulmonary Y. pestis infection. We demonstrate that neutralizing tumor necrosis factor-alpha (TNFalpha) and gamma-interferon (IFNgamma) abrogates this protection at sub-optimal levels of F1- or LcrV-specific antibody, but not at optimal levels. Moreover, we demonstrate that endogenous TNFalpha and IFNgamma confer measurable protection in the complete absence of protective antibodies. These findings indicate that antibodies and cytokines independently protect against pneumonic plague and suggest that surrogate assays for plague vaccine efficacy should consider both the level of vaccine-induced antibody and the capacity of vaccine recipients to produce TNFalpha and IFNgamma upon exposure to Y. pestis.

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

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

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.