Immunogenicity and protective immunity against bubonic plague and pneumonic plague by immunization of mice with the recombinant V10 antigen, a variant of LcrV

A novel sORF gene mutant strain of Yersinia pestis vaccine EV76 offers enhanced safety and improved protection against plague

We recently identified two virulence-associated small open reading frames (sORF) of Yersinia pestis, named yp1 and yp2, and null mutants of each individual genes were highly attenuated in virulence. Plague vaccine strain EV76 is known for strong reactogenicity, making it not suitable for use in humans. To improve the immune safety of EV76, three mutant strains of EV76, Δyp1, Δyp2, and Δyp1&yp2 were constructed and their virulence attenuation, immunogenicity, and protective efficacy in mice were evaluated. All mutant strains were attenuated by the subcutaneous (s.c.) route and exhibited more rapid clearance in tissues than the parental strain EV76. Under iron overload conditions, only the mice infected with EV76Δyp1 survived, accompanied by less draining lymph nodes damage than those infected by EV76. Analysis of cytokines secreted by splenocytes of immunized mice found that EV76Δyp2 induced higher secretion of multiple cytokines including TNF-α, IL-2, and IL-12p70 than EV76. On day 42, EV76Δyp2 or EV76Δyp1&yp2 immunized mice exhibited similar protective efficacy as EV76 when exposed to Y. pestis 201, both via s.c. or intranasal (i.n.) routes of administration. Moreover, when exposed to 200-400 LD50 Y. pestis strain 201Δcaf1 (non-encapsulated Y. pestis), EV76Δyp2 or EV76Δyp1&yp2 are able to afford about 50% protection to i.n. challenges, significantly better than the protection afforded by EV76. On 120 day, mice immunized with EV76Δyp2 or EV76Δyp1&yp2 cleared the i.n. challenge of Y. pestis 201-lux as quickly as those immunized with EV76, demonstrating 90-100% protection. Our results demonstrated that deletion of the yp2 gene is an effective strategy to attenuate virulence of Y. pestis EV76 while improving immunogenicity. Furthermore, EV76Δyp2 is a promising candidate for conferring protection against the pneumonic and bubonic forms of plague.

<i>In silico</i> Research at the Stages of Designing Modern Means for Prevention of Plague (by the Example of Subunit Vaccines)

The purpose of this review was to analyze the findings of domestic and foreign researchers on the development of modern drugs for the specific prevention of plague and to illustrate the possibilities of using bioinformatics analysis at the design stages to create an effective and safe vaccine. Work on the creation of an effective new-generation plague vaccine is hampered by several factors associated primarily with the presence of mechanisms of evasion from the immune system of the macroorganism, as well as a large number of pathogenicity determinants in the plague agent. Due to the development of approaches that are based on in silico studies, there is a progressive development of vaccine technologies oriented primarily to the use of the most important immunogens of the plague microbe (F1 and V antigen). Studies aimed at improving the antigenic properties of F1 and LcrV, as well as work on bioinformatic search and analysis of additional promising components to be included in the composition of subunit vaccines are considered as topical applications of bioinformatics data analysis in developing the tools for enhancing the effectiveness of protection through vaccination with subunit preparations.

Vaccine Potential of a Recombinant Bivalent Fusion Protein LcrV-HSP70 Against Plague and Yersiniosis

To counteract the deadly pathogens, i.e., Y. pestis, Y. enetrocolitica, and Y. pseudotuberculosis, we prepared a recombinant DNA construct lcrV-hsp70 encoding the bivalent fusion protein LcrV-HSP70. The lcrV gene of Y. pestis and hsp70 domain II DNA fragment of M. tuberculosis were amplified by PCR. The lcrV amplicon was first ligated in the pET vector using NcoI and BamHI restriction sites. Just downstream to the lcrV gene, the hsp70 domain II was ligated using BamHI and Hind III restriction sites. The in-frame and the orientation of cloned lcrV-hsp70 were checked by restriction analysis and nucleotide sequencing. The recombinant bivalent fusion protein LcrV-HSP70 was expressed in E. coli and purified by affinity chromatography. The vaccine potential of LcrV-HSP70 fusion protein was evaluated in formulation with alum. BALB/c mice were vaccinated, and the humoral and cellular immune responses were studied. The fusion protein LcrV-HSP70 induced a strong and significant humoral immune response in comparison to control animals. We also observed a significant difference in the expression levels of IFN-γ and TNF-α in LcrV–HSP70-immunized mice in comparison to control, HSP70, and LcrV groups. To test the protective efficacy of the LcrV–HSP70 fusion protein against plague and Yersiniosis, the vaccinated mice were challenged with Y. pestis, Y. enterocolitica, and Y. pseudotuberculosis separately. The bivalent fusion protein LcrV–HSP70 imparted 100% protection against the plague. In the case of Yersiniosis, on day 2 post challenge, there was a significant reduction in the number of CFU of Y. enterocolitica and Y. pseudotuberculosis in the blood (CFU/ml) and the spleen (CFU/g) of vaccinated animals in comparison to the LcrV, HSP70, and control group animals.

FPR1 is the plague receptor on host immune cells

The plague agent, Yersinia pestis, employs a type III secretion system (T3SS) to selectively destroy human immune cells, thereby enabling its replication in the bloodstream and transmission to new hosts via fleabite. The host factors responsible for the selective destruction of immune cells by plague bacteria were not known. Here we show that LcrV, the needle cap protein of the Y. pestis T3SS, binds N-formylpeptide receptor (FPR1) on human immune cells to promote the translocation of bacterial effectors. Plague infection in mice is characterized by high mortality, however N-formylpeptide receptor deficient animals exhibit increased survival and plague-protective antibody responses. We identified FPR1 p.R190W as a candidate human resistance allele that protects neutrophils from Y. pestis T3SS. These findings reveal the plague receptor on immune cells and show that FPR1 mutations provide for plague survival, which appears to have shaped human immune responses towards other infectious diseases and malignant neoplasms.

Shift from primary pneumonic to secondary septicemic plague by decreasing the volume of intranasal challenge with Yersinia pestis in the murine model

Yersinia pestis is the causative agent of pneumonic plague, a disease involving uncontrolled bacterial growth and host immunopathology. Secondary septicemic plague commonly occurs as a consequence of the host inflammatory response that causes vasodilation and vascular leakage, which facilitates systemic spread of the bacteria and the colonization of secondary tissues. The mortality rates of pneumonic and septicemic plague are high even when antibiotics are administered. In this work, we show that primary pneumonic or secondary septicemic plague can be preferentially modeled in mice by varying the volume used for intranasal delivery of Y. pestis. Low volume intranasal challenge (10μL) of wild type Y. pestis resulted in a high frequency of lethal secondary septicemic plague, with a low degree of primary lung infection and rapid development of sepsis. In contrast, high volume intranasal challenge (30μL) yielded uniform early lung infection and primary disease and a significant increase in lethality. In a commonly used BSL2 model, high volume challenge with Y. pestis lacking the pigmentation locus (pgm-) gave 105-fold greater deposition compared to low volume challenge, yet moribund mice did not develop severe lung disease and there was no detectable difference in lethality. These data indicate the primary cause of death of mice in the BSL2 model is sepsis regardless of intranasal dosing method. Overall, these findings allow for the preferential modeling of pneumonic or septicemic plague by intranasal dosing of mice with Y. pestis.

Yersinia pestis Exploits Early Activation of MyD88 for Growth in the Lungs during Pneumonic Plague

Yersinia pestis causes bubonic, pneumonic, and septicemic plague. Although no longer responsible for pandemic outbreaks, pneumonic plague continues to be a challenge for medical treatment and has been classified as a reemerging disease in some parts of the world. In the early stage of infection, inflammatory responses are believed to be suppressed by Y. pestis virulence factors in order to prevent clearance, while later, the hyperactivation of inflammation contributes to the progression of disease. In this work, we sought to identify the host factors that mediate this process and studied the role of the Toll/interleukin 1 (IL-1) receptor adapter and major inflammatory mediator myeloid differentiation primary response 88 (MyD88) in pneumonic plague. We show that pulmonary challenge of Myd88^-/- mice with wild-type (WT) Y. pestis results in significant loss of pro- and anti-inflammatory cytokines and chemokines, especially gamma interferon (IFN-γ) and KC, in the lungs compared to that in WT mice. Bacterial growth in the lungs occurred more rapidly in the WT mice, however, indicating a role for the MyD88 response in facilitating the primary lung infection. Nevertheless, Myd88^-/- mice were more sensitive to lethality from secondary septicemic plague. Together these findings indicate a central role for MyD88 during the biphasic inflammatory response to pulmonary Y. pestis infection. In the early phase, low-level MyD88-dependent chemokine expression limits initial growth but facilitates Y. pestis access to a protected replicative niche. The later hyperinflammatory phase is partially MyD88 dependent and ineffective in the lungs but controls systemic infection and reduces the progression of secondary septicemic plague.

Immunity against the Obligate Intracellular Bacterial Pathogen Rickettsia australis Requires a Functional Complement System

The complement system has a well-defined role in deterring blood-borne infections. However, complement is not entirely efficacious, as several bacterial pathogens, including some obligate intracellular pathogens, have evolved mechanisms for resistance. It is presumed that obligate intracellular bacteria evade complement attack by residing within a host cell; however, recent studies have challenged this presumption. Here, we demonstrate that the complement system is activated during infection with the obligate intracellular bacterium Rickettsia australis and that genetic ablation of complement increases susceptibility to infection. Interaction of Rickettsia australis with serum-borne complement leads to activation of the complement cascade, producing three effector mechanisms that could negatively influence R. australis. The C9-dependent membrane attack complex can lead to deposition of a bacteriolytic membrane pore on the bacteria, but this system does not contribute to control of rickettsial infection. Similarly, complement receptor (CR1/2)-dependent opsonophagocytosis may lead to engulfment and killing of the bacteria, but this system is also dispensable for immunity. Nevertheless, intact complement is essential for naturally acquired and antibody-mediated immunity to Rickettsia infection. Comparison of infection in mice lacking the central complement protein C3 with infection in their wild-type counterparts demonstrated decreases in gamma interferon (IFN-γ) production, IgG secretion, and spleen hyperplasia in animals lacking complement. The correlation between loss of secondary immune functions and loss of complement indicates that the proinflammatory signaling components of the complement system, and not membrane attack complex or opsonophagocytosis, contribute to the immune response to this pathogen.

Immunology of Yersinia pestis Infection

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

A Recombinant Trivalent Fusion Protein F1-LcrV-HSP70(II) Augments Humoral and Cellular Immune Responses and Imparts Full Protection against Yersinia pestis

Plague is one of the most dangerous infections in humans caused by Yersinia pestis, a Gram-negative bacterium. Despite of an overwhelming research success, no ideal vaccine against plague is available yet. It is well established that F1/LcrV based vaccine requires a strong cellular immune response for complete protection against plague. In our earlier study, we demonstrated that HSP70(II) of Mycobacterium tuberculosis modulates the humoral and cellular immunity of F1/LcrV vaccine candidates individually as well as in combinations in a mouse model. Here, we made two recombinant constructs caf1–lcrV and caf1–lcrV–hsp70(II). The caf1 and lcrV genes of Y. pestis and hsp70 domain II of M. tuberculosis were amplified by polymerase chain reaction. Both the recombinant constructs caf1–lcrV and caf1–lcrV–hsp70(II) were cloned in pET28a vector and expressed in Escherichia coli. The recombinant fusion proteins F1–LcrV and F1–LcrV–HSP70(II) were purified using Ni-NTA columns and formulated with alum to evaluate the humoral and cell mediated immune responses in mice. The protective efficacies of F1–LcrV and F1–LcrV–HSP70(II) were determined following challenge of immunized mice with 100 LD₅₀ of Y. pestis through intraperitoneal route. Significant differences were noticed in the titers of IgG and it’s isotypes, i.e., IgG1, IgG2b, and IgG3 in anti- F1–LcrV–HSP70(II) sera in comparison to anti-F1–LcrV sera. Similarly, significant differences were also noticed in the expression levels of IL-2, IFN-γ and TNF-α in splenocytes of F1–LcrV–HSP(II) immunized mice in comparison to F1–LcrV. Both F1–LcrV and F1–LcrV–HSP70(II) provided 100% protection. Our research findings suggest that F1–LcrV fused with HSP70 domain II of M. tuberculosis significantly enhanced the humoral and cellular immune responses in mouse model.

Hereditary Hemochromatosis Predisposes Mice to Yersinia pseudotuberculosis Infection Even in the Absence of the Type III Secretion System

The iron overload disorder hereditary hemochromatosis (HH) predisposes humans to serious disseminated infection with pathogenic Yersinia as well as several other pathogens. Recently, we showed that the iron-sulfur cluster coordinating transcription factor IscR is required for type III secretion in Y. pseudotuberculosis by direct control of the T3SS master regulator LcrF. In E. coli and Yersinia, IscR levels are predicted to be regulated by iron bioavailability, oxygen tension, and oxidative stress, such that iron depletion should lead to increased IscR levels. To investigate how host iron overload influences Y. pseudotuberculosis virulence and the requirement for the Ysc type III secretion system (T3SS), we utilized two distinct murine models of HH: hemojuvelin knockout mice that mimic severe, early-onset HH as well as mice with the Hfe^{C282Y∕C282Y} mutation carried by 10% of people of Northern European descent, associated with adult-onset HH. Hjv^−∕− and Hfe^{C282Y∕C282Y} transgenic mice displayed enhanced colonization of deep tissues by Y. pseudotuberculosis following oral inoculation, recapitulating enhanced susceptibility of humans with HH to disseminated infection with enteropathogenic Yersinia. Importantly, HH mice orally infected with Y. pseudotuberculosis lacking the T3SS-encoding virulence plasmid, pYV, displayed increased deep tissue colonization relative to wildtype mice. Consistent with previous reports using monocytes from HH vs. healthy donors, macrophages isolated from Hfe^{C282Y∕C282Y} mice were defective in Yersinia uptake compared to wildtype macrophages, indicating that the anti-phagocytic property of the Yersinia T3SS plays a less important role in HH animals. These data suggest that Yersinia may rely on distinct virulence factors to cause disease in healthy vs. HH hosts.

Plague Vaccines: Status and Future

Three major plague pandemics caused by the gram-negative bacterium Yersinia pestis have killed nearly 200 million people in human history. Due to its extreme virulence and the ease of its transmission, Y. pestis has been used purposefully for biowarfare in the past. Currently, plague epidemics are still breaking out sporadically in most of parts of the world, including the United States. Approximately 2000 cases of plague are reported each year to the World Health Organization. However, the potential use of the bacteria in modern times as an agent of bioterrorism and the emergence of a Y. pestis strain resistant to eight antibiotics bring out severe public health concerns. Therefore, prophylactic vaccination against this disease holds the brightest prospect for its long-term prevention. Here, we summarize the progress of the current vaccine development for counteracting plague.

HSP70 domain II of Mycobacterium tuberculosis modulates immune response and protective potential of F1 and LcrV antigens of Yersinia pestis in a mouse model

No ideal vaccine exists to control plague, a deadly dangerous disease caused by Yersinia pestis. In this context, we cloned, expressed and purified recombinant F1, LcrV antigens of Y. pestis and heat shock protein70 (HSP70) domain II of M. tuberculosis in E. coli. To evaluate the protective potential of each purified protein alone or in combination, Balb/C mice were immunized. Humoral and cell mediated immune responses were evaluated. Immunized animals were challenged with 100 LD50 of Y. pestis via intra-peritoneal route. Vaccine candidates i.e., F1 and LcrV generated highly significant titres of anti-F1 and anti-LcrV IgG antibodies. A significant difference was noticed in the expression level of IL-2, IFN-γ and TNF-α in splenocytes of immunized animals. Significantly increased percentages of CD4+ and CD8+ T cells producing IFN-γ in spleen of vaccinated animals were observed in comparison to control group by flow cytometric analysis. We investigated whether the F1, LcrV and HSP70(II) antigens alone or in combination can effectively protect immunized animals from any histopathological changes. Signs of histopathological lesions noticed in lung, liver, kidney and spleen of immunized animals on 3rd day post challenge whereas no lesions in animals that survived to day 20 post-infection were observed. Immunohistochemistry showed bacteria in lung, liver, spleen and kidney on 3rd day post-infection whereas no bacteria was observed on day 20 post-infection in surviving animals in LcrV, LcrV+HSP70(II), F1+LcrV, and F1+LcrV+HSP70(II) vaccinated groups. A significant difference was observed in the expression of IL-2, IFN-γ, TNF-α, and CD4+/CD8+ T cells secreting IFN-γ in the F1+LcrV+HSP70(II) vaccinated group in comparison to the F1+LcrV vaccinated group. Three combinations that included LcrV+HSP70(II), F1+LcrV or F1+LcrV+HSP70(II) provided 100% protection, whereas LcrV alone provided only 75% protection. These findings suggest that HSP70(II) of M. tuberculosis can be a potent immunomodulator for F1 and LcrV containing vaccine candidates against plague.

Multiple roles of Myd88 in the immune response to the plague F1-V vaccine and in protection against an aerosol challenge of Yersinia pestis CO92 in mice

The current candidate vaccine against Yersinia pestis infection consists of two subunit proteins: the capsule protein or F1 protein and the low calcium response V protein or V-antigen. Little is known of the recognition of the vaccine by the host's innate immune system and how it affects the acquired immune response to the vaccine. Thus, we vaccinated Toll-like receptor (Tlr) 2, 4, and 2/4-double deficient, as well as signal adaptor protein Myd88-deficient mice. We found that Tlr4 and Myd88 appeared to be required for an optimal immune response to the F1-V vaccine but not Tlr2 when compared to wild-type mice. However, there was a difference between the requirement for Tlr4 and MyD88 in vaccinated animals. When F1-V vaccinated Tlr4 mutant (lipopolysaccharide tolerant) and Myd88-deficient mice were challenged by aerosol with Y. pestis CO92, all but one Tlr4 mutant mice survived the challenge, but no vaccinated Myd88-deficient mice survived the challenge. Spleens from these latter nonsurviving mice showed that Y. pestis was not cleared from the infected mice. Our results suggest that MyD88 appears to be important for both an optimal immune response to F1-V and in protection against a lethal challenge of Y. pestis CO92 in F1-V vaccinated mice.

Remote monitoring of the progression of primary pneumonic plague in Brown Norway rats in high-capacity, high-containment housing

Development of new vaccines, diagnostics, and therapeutics for biodefense or other relatively rare infectious diseases is hindered by the lack of naturally occurring human disease on which to conduct clinical trials of efficacy. To overcome this experimental gap, the U.S. Food and Drug Administration established the Animal Rule, in which efficacy testing in two well-characterized animal models that closely resemble human disease may be accepted in lieu of large-scale clinical trials for diseases with limited natural human incidence. In this report, we evaluated the Brown Norway rat as a model for pneumonic plague and describe the natural history of clinical disease following inhalation exposure to Yersinia pestis. In high-capacity, high-containment housing, we monitored temperature, activity, heart rate, and rhythm by capturing electronic impulses transmitted from abdominal telemeter implants. Using this system, we show that reduced activity and development of fever are sensitive indications of disease progression. Furthermore, we identified heart arrhythmias as contributing factors to the rapid progression to lethality following the fever response. Together, these data validate the Brown Norway rat as an experimental model for human pneumonic plague and provide new insight that may ultimately lead to novel approaches in postexposure treatment of this devastating infection.

Mutated and bacteriophage T4 nanoparticle arrayed F1-V immunogens from Yersinia pestis as next generation plague vaccines

Pneumonic plague is a highly virulent infectious disease with 100% mortality rate, and its causative organism Yersinia pestis poses a serious threat for deliberate use as a bioterror agent. Currently, there is no FDA approved vaccine against plague. The polymeric bacterial capsular protein F1, a key component of the currently tested bivalent subunit vaccine consisting, in addition, of low calcium response V antigen, has high propensity to aggregate, thus affecting its purification and vaccine efficacy. We used two basic approaches, structure-based immunogen design and phage T4 nanoparticle delivery, to construct new plague vaccines that provided complete protection against pneumonic plague. The NH₂-terminal β-strand of F1 was transplanted to the COOH-terminus and the sequence flanking the β-strand was duplicated to eliminate polymerization but to retain the T cell epitopes. The mutated F1 was fused to the V antigen, a key virulence factor that forms the tip of the type three secretion system (T3SS). The F1mut-V protein showed a dramatic switch in solubility, producing a completely soluble monomer. The F1mut-V was then arrayed on phage T4 nanoparticle via the small outer capsid protein, Soc. The F1mut-V monomer was robustly immunogenic and the T4-decorated F1mut-V without any adjuvant induced balanced T_H1 and T_H2 responses in mice. Inclusion of an oligomerization-deficient YscF, another component of the T3SS, showed a slight enhancement in the potency of F1-V vaccine, while deletion of the putative immunomodulatory sequence of the V antigen did not improve the vaccine efficacy. Both the soluble (purified F1mut-V mixed with alhydrogel) and T4 decorated F1mut-V (no adjuvant) provided 100% protection to mice and rats against pneumonic plague evoked by high doses of Y. pestis CO92. These novel platforms might lead to efficacious and easily manufacturable next generation plague vaccines.

Plague caused by Yersinia pestis is a deadly disease that wiped out one-third of Europe's population in the 14th century. The organism is listed by the CDC as Tier-1 biothreat agent, and currently, there is no FDA-approved vaccine against this pathogen. Stockpiling of an efficacious plague vaccine that could protect people against a potential bioterror attack has been a national priority. The current vaccines based on the capsular antigen (F1) and the low calcium response V antigen, are promising against both bubonic and pneumonic plague. However, the polymeric nature of F1 with its propensity to aggregate affects vaccine efficacy and generates varied immune responses in humans. We have addressed a series of concerns and generated mutants of F1 and V, which are completely soluble and produced in high yields. We then engineered the vaccine into a novel delivery platform using the bacteriophage T4 nanoparticle. The nanoparticle vaccines induced robust immunogenicity and provided 100% protection to mice and rats against pneumonic plague. These highly efficacious new generation plague vaccines are easily manufactured, and the potent T4 platform which can simultaneously incorporate antigens from other biothreat or emerging infectious agents provides a convenient way for mass vaccination of humans against multiple pathogens.

LcrV mutants that abolish Yersinia type III injectisome function

LcrV, the type III needle cap protein of pathogenic Yersinia, has been proposed to function as a tether between YscF, the needle protein, and YopB-YopD to constitute the injectisome, a conduit for the translocation of effector proteins into host cells. Further, insertion of LcrV-capped needles from a calcium-rich environment into host cells may trigger the low-calcium signal for effector translocation. Here, we used a genetic approach to test the hypothesis that the needle cap responds to the low-calcium signal by promoting injectisome assembly. Growth restriction of Yersinia pestis in the absence of calcium (low-calcium response [LCR(+)] phenotype) was exploited to isolate dominant negative lcrV alleles with missense mutations in its amber stop codon (lcrV(*327)). The addition of at least four amino acids or the eight-residue Strep tag to the C terminus was sufficient to generate an LCR(-) phenotype, with variant LcrV capping type III needles that cannot assemble the YopD injectisome component. The C-terminal Strep tag appears buried within the cap structure, blocking effector transport even in Y. pestis yscF variants that are otherwise calcium blind, a constitutive type III secretion phenotype. Thus, LcrV(*327) mutants arrest the needle cap in a state in which it cannot respond to the low-calcium signal with either injectisome assembly or the activation of type III secretion. Insertion of the Strep tag at other positions of LcrV produced variants with wild-type LCR(+), LCR(-), or dominant negative LCR(-) phenotypes, thereby allowing us to identify discrete sites within LcrV as essential for its attributes as a secretion substrate, needle cap, and injectisome assembly factor.

Plague vaccines: current developments and future perspectives

Despite many decades of intensive studies of Yersinia pestis, the causative agent of plague, there is no safe and efficient vaccine against this devastating disease. A recently developed F1/V subunit vaccine candidate, which relies mainly on humoral immunity, showed promising results in animal studies; however, its efficacy in humans still has to be carefully evaluated. In addition, those developing next-generation plague vaccines need to pay particular attention to the importance of eliciting cell-mediated immunity. In this review, we analyzed the current progress in developing subunit, DNA and live carrier platforms of delivery by bacterial and viral vectors, as well as approaches for controlled attenuation of virulent strains of Y. pestis.

Hereditary hemochromatosis restores the virulence of plague vaccine strains

Nonpigmented Yersinia pestis (pgm) strains are defective in scavenging host iron and have been used in live-attenuated vaccines to combat plague epidemics. Recently, a Y. pestis pgm strain was isolated from a researcher with hereditary hemochromatosis who died from laboratory-acquired plague. We used hemojuvelin-knockout (Hjv(-/-)) mice to examine whether iron-storage disease restores the virulence defects of nonpigmented Y. pestis. Unlike wild-type mice, Hjv(-/-) mice developed lethal plague when challenged with Y. pestis pgm strains. Immunization of Hjv(-/-) mice with a subunit vaccine that blocks Y. pestis type III secretion generated protection against plague. Thus, individuals with hereditary hemochromatosis may be protected with subunit vaccines but should not be exposed to live-attenuated plague vaccines.

Coagulases as determinants of protective immune responses against Staphylococcus aureus

During infection, Staphylococcus aureus secretes two coagulases (Coa and von Willebrand factor binding protein [vWbp]), which, following an association with host prothrombin and fibrinogen, form fibrin clots and enable the establishment of staphylococcal disease. Within the genomes of different S. aureus isolates, coagulase gene sequences are variable, and this has been exploited for a classification of types. We show here that antibodies directed against the variable prothrombin binding portion of coagulases confer type-specific immunity through the neutralization of S. aureus clotting activity and protection from staphylococcal disease in mice. By combining variable portions of coagulases from North American isolates into hybrid Coa and vWbp proteins, a subunit vaccine that provided protection against challenge with different coagulase-type S. aureus strains in mice was derived.

The natural history and incidence of Yersinia pestis and prospects for vaccination

Plague is an ancient, serious, infectious disease which is still endemic in regions of the modern world and is a potential biothreat agent. This paper discusses the natural history of the bacterium and its evolution into a flea-vectored bacterium able to transmit bubonic plague. It reviews the incidence of plague in the modern world and charts the history of vaccines which have been used to protect against the flea-vectored disease, which erupts as bubonic plague. Current approaches to vaccine development to protect against pneumonic, as well as bubonic, plague are also reviewed. The considerable challenges in achieving a vaccine which is licensed for human use and which will comprehensively protect against this serious human pathogen are assessed.

Polymorphisms in the lcrV gene of Yersinia enterocolitica and their effect on plague protective immunity

Current efforts to develop plague vaccines focus on LcrV, a polypeptide that resides at the tip of type III secretion needles. LcrV-specific antibodies block Yersinia pestis type III injection of Yop effectors into host immune cells, thereby enabling phagocytes to kill the invading pathogen. Earlier work reported that antibodies against Y. pestis LcrV cannot block type III injection by Yersinia enterocolitica strains and suggested that lcrV polymorphisms may provide for escape from LcrV-mediated plague immunity. We show here that polyclonal or monoclonal antibodies raised against Y. pestis KIM D27 LcrV (LcrV(D27)) bind LcrV from Y. enterocolitica O:9 strain W22703 (LcrV(W22703)) or O:8 strain WA-314 (LcrV(WA-314)) but are otherwise unable to block type III injection by Y. enterocolitica strains. Replacing the lcrV gene on the pCD1 virulence plasmid of Y. pestis KIM D27 with either lcrV(W22703) or lcrV(WA-314) does not affect the ability of plague bacteria to secrete proteins via the type III pathway, to inject Yops into macrophages, or to cause lethal plague infections in mice. LcrV(D27)-specific antibodies blocked type III injection by Y. pestis expressing lcrV(W22703) or lcrV(WA-314) and protected mice against intravenous lethal plague challenge with these strains. Thus, although antibodies raised against LcrV(D27) are unable to block the type III injection of Y. enterocolitica strains, expression of lcrV(W22703) or lcrV(WA-314) in Y. pestis did not allow these strains to escape LcrV-mediated plague protective immunity in the intravenous challenge model.

Prevention of pneumonic plague in mice, rats, guinea pigs and non-human primates with clinical grade rV10, rV10-2 or F1-V vaccines

Yersinia pestis causes plague, a disease with high mortality in humans that can be transmitted by fleabite or aerosol. A US Food and Drug Administration (FDA)-licensed plague vaccine is currently not available. Vaccine developers have focused on two subunits of Y. pestis: LcrV, a protein at the tip of type III secretion needles, and F1, the fraction 1 pilus antigen. F1-V, a hybrid generated via translational fusion of both antigens, is being developed for licensure as a plague vaccine. The rV10 vaccine is a non-toxigenic variant of LcrV lacking residues 271-300. Here we developed Current Good Manufacturing Practice (cGMP) protocols for rV10. Comparison of clinical grade rV10 with F1-V did not reveal significant differences in plague protection in mice, guinea pigs or cynomolgus macaques. We also developed cGMP protocols for rV10-2, a variant of rV10 with an altered affinity tag. Immunization with rV10-2 adsorbed to aluminum hydroxide elicited antibodies against LcrV and conferred pneumonic plague protection in mice, rats, guinea pigs, cynomolgus macaques and African Green monkeys. The data support further development of rV10-2 for FDA Investigational New Drug (IND) authorization review and clinical testing.

Multi-Functional Characteristics of the Pseudomonas aeruginosa Type III Needle-Tip Protein, PcrV; Comparison to Orthologs in other Gram-negative Bacteria

Pseudomonas aeruginosa possesses a type III secretion system (T3SS) to intoxicate host cells and evade innate immunity. This virulence-related machinery consists of a molecular syringe and needle assembled on the bacterial surface, which allows delivery of T3 effector proteins into infected cells. To accomplish a one-step effector translocation, a tip protein is required at the top end of the T3 needle structure. Strains lacking expression of the functional tip protein fail to intoxicate host cells. P. aeruginosa encodes a T3S that is highly homologous to the proteins encoded by Yersinia spp. The needle-tip proteins of Yersinia, LcrV, and P. aeruginosa, PcrV, share 37% identity and 65% similarity. Other known tip proteins are AcrV (Aeromonas), IpaD (Shigella), SipD (Salmonella), BipD (Burkholderia), EspA (EPEC, EHEC), Bsp22 (Bordetella), with additional proteins identified from various Gram-negative species, such as Vibrio and Bordetella. The tip proteins can serve as a protective antigen or may be critical for sensing host cells and evading innate immune responses. Recognition of the host microenvironment transcriptionally activates synthesis of T3SS components. The machinery appears to be mechanically controlled by the assemblage of specific junctions within the apparatus. These junctions include the tip and base of the T3 apparatus, the needle proteins and components within the bacterial cytoplasm. The tip proteins likely have chaperone functions for translocon proteins, allowing the proper assembly of translocation channels in the host membrane and completing vectorial delivery of effector proteins into the host cytoplasm. Multi-functional features of the needle-tip proteins appear to be intricately controlled. In this review, we highlight the functional aspects and complex controls of T3 needle-tip proteins with particular emphasis on PcrV and LcrV.

Plague in Guinea pigs and its prevention by subunit vaccines

Human pneumonic plague is a devastating and transmissible disease for which a Food and Drug Administration-approved vaccine is not available. Suitable animal models may be adopted as a surrogate for human plague to fulfill regulatory requirements for vaccine efficacy testing. To develop an alternative to pneumonic plague in nonhuman primates, we explored guinea pigs as a model system. On intranasal instillation of a fully virulent strain, Yersinia pestis CO92, guinea pigs developed lethal lung infections with hemorrhagic necrosis, massive bacterial replication in the respiratory system, and blood-borne dissemination to other organ systems. Expression of the Y. pestis F1 capsule was not required for the development of pulmonary infection; however, the capsule seemed to be important for the establishment of bubonic plague. The mean lethal dose (MLD) for pneumonic plague in guinea pigs was estimated to be 1000 colony-forming units. Immunization of guinea pigs with the recombinant forms of LcrV, a protein that resides at the tip of Yersinia type III secretion needles, or F1 capsule generated robust humoral immune responses. Whereas LcrV immunization resulted in partial protection against pneumonic plague challenge with 250 MLD Y. pestis CO92, immunization with recombinant F1 did not. rV10, a vaccine variant lacking LcrV residues 271-300, elicited protection against pneumonic plague, which seemed to be based on conformational antibodies directed against LcrV.

Novel genetic tools for diaminopimelic acid selection in virulence studies of Yersinia pestis

Molecular studies of bacterial virulence are enhanced by expression of recombinant DNA during infection to allow complementation of mutants and expression of reporter proteins in vivo. For highly pathogenic bacteria, such as Yersinia pestis, these studies are currently limited because deliberate introduction of antibiotic resistance is restricted to those few which are not human treatment options. In this work, we report the development of alternatives to antibiotics as tools for host-pathogen research during Yersinia pestis infections focusing on the diaminopimelic acid (DAP) pathway, a requirement for cell wall synthesis in eubacteria. We generated a mutation in the dapA-nlpB(dapX) operon of Yersinia pestis KIM D27 and CO92 which eliminated the expression of both genes. The resulting strains were auxotrophic for diaminopimelic acid and this phenotype was complemented in trans by expressing dapA in single and multi-copy. In vivo, we found that plasmids derived from the p15a replicon were cured without selection, while selection for DAP enhanced stability without detectable loss of any of the three resident virulence plasmids. The dapAX mutation rendered Y. pestis avirulent in mouse models of bubonic and septicemic plague which could be complemented when dapAX was inserted in single or multi-copy, restoring development of disease that was indistinguishable from the wild type parent strain. We further identified a high level, constitutive promoter in Y. pestis that could be used to drive expression of fluorescent reporters in dapAX strains that had minimal impact to virulence in mouse models while enabling sensitive detection of bacteria during infection. Thus, diaminopimelic acid selection for single or multi-copy genetic systems in Yersinia pestis offers an improved alternative to antibiotics for in vivo studies that causes minimal disruption to virulence.

Membrane targeting and pore formation by the type III secretion system translocon

The type III secretion system (T3SS) is a complex macromolecular machinery employed by a number of Gram-negative species to initiate infection. Toxins secreted through the system are synthesized in the bacterial cytoplasm and utilize the T3SS to pass through both bacterial membranes and the periplasm, thus being introduced directly into the eukaryotic cytoplasm. A key element of the T3SS of all bacterial pathogens is the translocon, which comprises a pore that is inserted into the membrane of the target cell, allowing toxin injection. Three macromolecular partners associate to form the translocon: two are hydrophobic and one is hydrophilic, and the latter also associates with the T3SS needle. In this review, we discuss recent advances on the biochemical and structural characterization of the proteins involved in translocon formation, as well as their participation in the modification of intracellular signalling pathways upon infection. Models of translocon assembly and regulation are also discussed.

Contributions of chaperone/usher systems to cell binding, biofilm formation and Yersinia pestis virulence

Yersinia pestis genome sequencing projects have revealed six intact uncharacterized chaperone/usher systems with the potential to play roles in plague pathogenesis. We cloned each locus and expressed them in the Δfim Escherichia coli strain AAEC185 to test the assembled Y. pestis surface structures for various activities. Expression of each chaperone/usher locus gave rise to specific novel fibrillar structures on the surface of E. coli. One locus, y0561-0563, was able to mediate attachment to human epithelial cells (HEp-2) and human macrophages (THP-1) but not mouse macrophages (RAW264.7), while several loci were able to facilitate E. coli biofilm formation. When each chaperone/usher locus was deleted in Y. pestis, only deletion of the previously described pH 6 antigen (Psa) chaperone/usher system resulted in decreased adhesion and biofilm formation. Quantitative RT-PCR (qRT-PCR) revealed low expression levels for each novel chaperone/usher system in vitro as well as in mouse tissues following intravenous infection. However, a Y. pestis mutant in the chaperone/usher locus y1858-1862 was attenuated for virulence in mice via the intravenous route of infection, suggesting that expression of this locus is, at some stage, sufficient to affect the outcome of a plague infection. qRT-PCR experiments also indicated that expression of the chaperone/usher-dependent capsule locus, caf1, was influenced by oxygen availability and that the well-described chaperone/usher-dependent pilus, Psa, was strongly induced in minimal medium even at 28 °C rather than 37 °C, a temperature previously believed to be required for Psa expression. These data indicate several potential roles for the novel chaperone/usher systems of Y. pestis in pathogenesis and infection-related functions such as cell adhesion and biofilm formation.

Different strategies for preparation of non-tagged rV270 protein and its efficacy against Yersinia pestis challenge

OBJECTIVE

LcrV is an important component for the development of a subunit vaccine against plague. To reduce immunosuppressive activity of LcrV, a recombinant LcrV variant lacking amino acids 271 to 326 (rV270) was prepared by different methods in this study.

METHODS

A new strategy that produced non-tagged or authentic rV270 protein was designed by insertion of rV270-thrombin-hexahistidine fusion gene into the vector pET24a, or by insertion of hexahistidine-enterokinase-rV270 or hexahistitine-factor Xa-rV270 fusion gene into the vector pET32a. After Co(2+) affinity chromatography, a purification strategy was developed by cleavage of His tag on column, following Sephacryl S-200HR column filtration chromatography.

RESULTS

Removal of His tag by thrombin, enterokinase and factor Xa displayed a yield of 99.5%, 32.4% and 15.3%, respectively. Following Sephacryl S-200HR column filtration chromatography, above 97% purity of rV270 protein was obtained. Purified rV270 that was adsorbed to 25% (v/v) Al(OH)₃ adjuvant in phosphate-buffered saline (PBS) induced very high titers of antibody to rV270 in BALB/c mice and protected them (100% survival) against subcutaneous challenge with 10⁶ CFU of Y. pestis virulent strain 141.

CONCLUSION

The completely authentic rV270 protein can be prepared by using enterokinase or factor Xa, but they exhibited extremely low cleavage activity to the corresponding recognition site. Thrombin cleavage is an efficient strategy to prepare non-tagged rV270 protein and can be easily operated in a large scale due to its relatively low cost and high cleavage efficacy. The recombinant rV270 can be used as a key component to develop a subunit vaccine of plague.

Contribution of coagulases towards Staphylococcus aureus disease and protective immunity

The bacterial pathogen Staphylococcus aureus seeds abscesses in host tissues to replicate at the center of these lesions, protected from host immune cells via a pseudocapsule. Using histochemical staining, we identified prothrombin and fibrin within abscesses and pseudocapsules. S. aureus secretes two clotting factors, coagulase (Coa) and von Willebrand factor binding protein (vWbp). We report here that Coa and vWbp together are required for the formation of abscesses. Coa and vWbp promote the non-proteolytic activation of prothrombin and cleavage of fibrinogen, reactions that are inhibited with specific antibody against each of these molecules. Coa and vWbp specific antibodies confer protection against abscess formation and S. aureus lethal bacteremia, suggesting that coagulases function as protective antigens for a staphylococcal vaccine.

Efficacy and safety of a modified vaccinia Ankara (MVA) vectored plague vaccine in mice

The efficacy and safety of plague vaccines based on the modified vaccinia Ankara (MVA) viral vector was evaluated. MVA recombinants were constructed expressing Yersinia pestis antigens under the translational control of the encephalomyocarditis virus (EMCV) internal ribosomal entry site (IRES) and/or fused to the tissue plasminogen activator (tPA) secretory signal. A MVA/Y. pestis recombinant that expressed a truncated version of the low-calcium response V antigen (MVA/IRES/tPA/V(307)), conferred significant protection (87.5-100%) against intranasal or intraperitoneal challenge with CO92 (encapsulated) or Java 9 (non-encapsulated) strains of Y. pestis, respectively. In contrast, a MVA/Y. pestis recombinant that expressed the full-length V antigen provided only 37.5% protection against challenge with CO92 or Java 9 strains, respectively. Interestingly, a MVA/Y. pestis recombinant that expressed the capsular protein (F1) did not elicit significant antibody titers but still conferred 50% and 25% protection against CO92 or Java 9 challenge, respectively. The MVA/Y. pestis recombinant viruses did not demonstrate any mortality or morbidity in SCID mice. Based on their safety and efficacy in mice, these MVA/Y. pestis recombinants are candidates for further development as biodefense and public health vaccines.

Amino acid residues 196-225 of LcrV represent a plague protective epitope

LcrV, a protein that resides at the tip of the type III secretion needles of Yersinia pestis, is the single most important plague protective antigen. Earlier work reported monoclonal antibody MAb 7.3, which binds a conformational epitope of LcrV and protects experimental animals against lethal plague challenge. By screening monoclonal antibodies directed against LcrV for their ability to protect immunized mice against bubonic plague challenge, we examined here the possibility of additional protective epitopes. MAb BA5 protected animals against plague, neutralized the Y. pestis type III secretion pathway and promoted opsonophagocytic clearance of bacteria in blood. LcrV residues 196-225 were necessary and sufficient for MAb BA5 binding. Compared to full-length LcrV, a variant lacking its residues 196-225 retained the ability of eliciting plague protection. These results identify LcrV residues 196-225 as a linear epitope that is recognized by the murine immune system to confer plague protection.

YopR impacts type III needle polymerization in Yersinia species

A hallmark of Yersinia type III machines is the presence of needles extending from the bacterial surface. Needles perform two functions, serving as the conduits for the transport of effectors into immune cells but also acting as a sensor. The polymerized needle protein, YscF, is thought to perceive threshold levels of environmental calcium ions to trigger secretion. yopR (yscH) is a gene downstream of yscEFG, encoding the chaperones and principal building blocks of the needle. Here we investigated the contribution of YopR towards type III secretion and pathogenesis. Yersinia pestis KIM D27 mutants lacking yopR were defective for virulence in a mouse model of septicemic plague. yopR variants of Yersinia enterocolitica W22703 displayed a reduced ability to inject effectors into macrophages and required lower calcium concentrations to activate type III secretion than wild-type yersiniae. Furthermore, yopR mutants failed to assemble YscF into needle complexes and instead secreted YscF into the medium. These results imply that YopR may be involved in controlling the secretion of YscF, thereby impacting the assembly of type III machines. An alternative possibility, which YopR participates directly in the polymerization of YscF, seems less likely as YopR is not associated with purified needles.

Adhesive organelles of Gram-negative pathogens assembled with the classical chaperone/usher machinery: structure and function from a clinical standpoint

This review summarizes current knowledge on the structure, function, assembly and biomedical applications of the superfamily of adhesive fimbrial organelles exposed on the surface of Gram-negative pathogens with the classical chaperone/usher machinery. High-resolution three-dimensional (3D) structure studies of the minifibers assembling with the FGL (having a long F1-G1 loop) and FGS (having a short F1-G1 loop) chaperones show that they exploit the same principle of donor-strand complementation for polymerization of subunits. The 3D structure of adhesive subunits bound to host-cell receptors and the final architecture of adhesive fimbrial organelles reveal two functional families of the organelles, respectively, possessing polyadhesive and monoadhesive binding. The FGL and FGS chaperone-assembled polyadhesins are encoded exclusively by the gene clusters of the γ3- and κ-monophyletic groups, respectively, while gene clusters belonging to the γ1-, γ2-, γ4-, and π-fimbrial clades exclusively encode FGS chaperone-assembled monoadhesins. Novel approaches are suggested for a rational design of antimicrobials inhibiting the organelle assembly or inhibiting their binding to host-cell receptors. Vaccines are currently under development based on the recombinant subunits of adhesins.

Absence of inflammation and pneumonia during infection with nonpigmented Yersinia pestis reveals a new role for the pgm locus in pathogenesis

Yersinia pestis causes primary pneumonic plague in many mammalian species, including humans, mice, and rats. Virulent Y. pestis strains undergo frequent spontaneous deletion of a 102-kb chromosomal DNA fragment, known as the pigmentation (pgm) locus, when grown in laboratory media, yet this locus is present in every virulent isolate. The pgm locus encodes, within a high-pathogenicity island, siderophore biosynthesis genes that are required for growth in the mammalian host when inoculated by peripheral routes. Recently, higher challenge doses of nonpigmented Y. pestis were reported to cause fatal pneumonic plague in mice, suggesting a useful model for studies of virulence and immunity. In this work, we show that intranasal infection of BALB/c mice with nonpigmented Yersinia pestis does not result in pneumonic plague. Despite persistent bacterial colonization of the lungs and the eventual death of infected mice, pulmonary inflammation was generally absent, and there was no disease pathology characteristic of pneumonic plague. Iron given to mice at the time of challenge, previously shown to enhance the virulence of pgm-deficient strains, resulted in an accelerated disease course, with less time to bacteremia and lethality, but lung inflammation and pneumonia were still absent. We examined other rodent models and found differences in lung inflammatory responses, some of which led to clearance and survival even when high challenge doses were used. Together, the results suggest that the Y. pestis pgm locus encodes previously unappreciated virulence factors required for the induction of pneumonic plague that are independent of iron scavenging from the mammalian host.

Amino acid and structural variability of Yersinia pestis LcrV protein

The LcrV protein is a multifunctional virulence factor and protective antigen of the plague bacterium and is generally conserved between the epidemic strains of Yersinia pestis. We investigated the diversity in the LcrV sequences among non-epidemic Y. pestis strains which have a limited virulence in selected animal models and for humans. Sequencing of lcrV genes from 19 Y. pestis strains belonging to different phylogenetic groups (subspecies) showed that the LcrV proteins possess four major variable hotspots at positions 18, 72, 273, and 324-326. These major variations, together with other minor substitutions in amino acid sequences, allowed us to classify the LcrV alleles into five sequence types (A-E). We observed that the strains of different Y. pestis "subspecies" can have the same type of LcrV, including that conserved in epidemic strains, and different types of LcrV can exist within the same natural plague focus. Therefore, the phenomenon of "selective virulence" characteristic of the strains of the microtus biovar is unlikely to be the result of polymorphism of the V antigen. The LcrV polymorphisms were structurally analyzed by comparing the modeled structures of LcrV from all available strains. All changes except one occurred either in flexible regions or on the surface of the protein, but local chemical properties (i.e. those of a hydrophobic, hydrophilic, amphipathic, or charged nature) were conserved across all of the strains. Polymorphisms in flexible and surface regions are likely subject to less selective pressure, and have a limited impact on the structure. In contrast, the substitution of tryptophan at position 113 with either glutamic acid or glycine likely has a serious influence on the regional structure of the protein, and these mutations might have an effect on the function of LcrV. The polymorphisms at positions 18, 72 and 273 were accountable for differences in the oligomerization of LcrV.

Dual-function antibodies to Yersinia pestis LcrV required for pulmonary clearance of plague

Yersinia pestis causes pneumonic plague, a necrotic pneumonia that rapidly progresses to death without early treatment. Antibodies to the protective antigen LcrV are thought to neutralize its essential function in the type III secretion system (TTSS) and by themselves are capable of inducing immunity to plague in mouse models. To develop multivalent LcrV antibodies as a therapeutic treatment option, we screened for monoclonal antibodies (MAbs) to LcrV that could prevent its function in the TTSS. Although we were able to identify single and combination MAbs that provided the high-level inhibition of the TTSS, these did not promote phagocytosis in vitro and were only weakly protective in a mouse pneumonic plague model. Only one MAb, BA5, was able to protect mice from pneumonic plague. In vitro, MAb BA5 blocked the TTSS with efficiency equal to or even less than that of other MAbs as single agents or as combinations, but its activity led to increased phagocytic uptake. Polyclonal anti-LcrV was superior to BA5 in promoting phagocytosis and also was more efficient in protecting mice from pneumonic plague. Taken together, the data support a hypothesis whereby the pulmonary clearance of Y. pestis by antibodies requires both the neutralization of the TTSS and the simultaneous stimulation of innate signaling pathways used by phagocytic cells to destroy pathogens.

Deletion of Braun lipoprotein gene (lpp) attenuates Yersinia pestis KIM/D27 strain: role of Lpp in modulating host immune response, NF-kappaB activation and cell death

The pathogenic species of yersiniae potently blocks immune responses in host cells by using the type III secretion apparatus and its effector proteins. In this study, we characterized potential mechanisms associated with the Braun lipoprotein (Lpp) that contributed to a further attenuation of a pigmentation locus-minus Yersinia pestis KIM/D27 mutant strain and its ability to generate immune responses in mice. The lpp gene encodes one of the major outer membrane lipoproteins that is involved in inflammatory responses and septic shock. We found that sera and splenocytes from Deltalpp mutant-immunized mice, when transferred to naïve animals, provided protection to the latter against challenge with a lethal dose of the Y. pestis parental strain. Further, the Deltalpp mutant promoted ex vivo a significantly higher interleukin (IL)-2 and interferon-gamma production from T cells of immunized mice, when compared to those from animals infected with the sub-lethal dose of the parental Y. pestis KIM/D27 strain. Likewise, murine primary macrophages infected with the mutant, when compared to those infected with the parental strain in vitro, produced significantly higher IL-12 levels. Importantly, increased nuclear factor-kappa B activation and decreased apoptosis were noted in splenocytes and primary macrophages of mice challenged with the Deltalpp mutant, when compared to those in animals infected with the parental Y. pestis KIM/D27 strain. Finally, significantly higher levels of antibodies specific for the parental Y. pestis antigens were developed in mice first immunized with the Deltalpp mutant and then challenged with the parental strain, compared to the antibody levels in animals that were immunized and then infected with the parental KIM/D27 strain. To our knowledge, this is the first report of a mechanistic basis for attenuation and immunological responses associated with deletion of the lpp gene from the Y. pestis KIM/D27 strain.

Vapour-phase hydrogen peroxide inactivates Yersinia pestis dried on polymers, steel, and glass surfaces

AIMS

This study evaluated the inactivation of virulent Yersinia pestis dried on polymers, steel, and glass surfaces using vapour-phase hydrogen peroxide.

METHODS AND RESULTS

A suspension of Y. pestis CO92 (1.70 x 10(8) CFU) was dried on 10 different types of test surfaces and exposed to vapour-phase hydrogen peroxide fumigation for a contact time of 2 h. A significant reduction in the log10 CFU of Y. pestis on all 10 materials was observed between the controls evaluated after a 1 h drying time and unexposed controls evaluated after the decontamination run. Qualitative growth assessment showed that vapour-phase hydrogen peroxide exposure inactivated Y. pestis on all replicates of the 10 test materials as well as biological indicators up to 7 days postexposure.

CONCLUSIONS

Virulent Y. pestis CO92 is inactivated on polymers, steel, and glass surfaces when exposed to vapour-phase hydrogen peroxide without observable physical damage to the test materials.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides information for using vapour-phase hydrogen peroxide as a practical process for the decontamination of virulent Y. pestis in circumstances where time-dependent attenuation/inactivation orliquid/heat decontamination may not be the most suitable approach.

Pneumonic plague pathogenesis and immunity in Brown Norway rats

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

Yersinia pestis IS1541 transposition provides for escape from plague immunity

Yersinia pestis is perhaps the most feared infectious agent due to its ability to cause epidemic outbreaks of plague disease in animals and humans with high mortality. Plague infections elicit strong humoral immune responses against the capsular antigen (fraction 1 [F1]) of Y. pestis, and F1-specific antibodies provide protective immunity. Here we asked whether Y. pestis generates mutations that enable bacterial escape from protective immunity and isolated a variant with an IS1541 insertion in caf1A encoding the F1 outer membrane usher. The caf1A::IS1541 insertion prevented assembly of F1 pili and provided escape from plague immunity via F1-specific antibodies without a reduction in virulence in mouse models of bubonic or pneumonic plague. F1-specific antibodies interfere with Y. pestis type III transport of effector proteins into host cells, an inhibitory effect that was overcome by the caf1A::IS1541 insertion. These findings suggest a model in which IS1541 insertion into caf1A provides for reversible changes in envelope structure, enabling Y. pestis to escape from adaptive immune responses and plague immunity.

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.

Immunization with recombinant V10 protects cynomolgus macaques from lethal pneumonic plague

Vaccine and therapeutic strategies that prevent infections with Yersinia pestis have been sought for over a century. Immunization with live attenuated (nonpigmented) strains and immunization with subunit vaccines containing recombinant low-calcium-response V antigen (rLcrV) and recombinant F1 (rF1) antigens are considered effective in animal models. Current antiplague subunit vaccines in development for utilization in humans contain both antigens, either as equal concentrations of the two components (rF1 plus rLcrV) or as a fusion protein (rF1-rLcrV). Here, we show that immunization with either purified rLcrV (a protein at the tip of type III needles) or a variant of this protein, recombinant V10 (rV10) (lacking amino acid residues 271 to 300), alone or in combination with rF1, prevented pneumonic lesions and disease pathogenesis. In addition, passive immunization studies showed that specific antibodies of macaques immunized with rLcrV, rV10, or rF1, either alone or in combination, conferred protection against bubonic plague challenge in mice. Finally, we found that when we compared the reactivities of anti-rLcrV and anti-rV10 immune sera from cynomolgus macaques, BALB/c mice, and brown Norway rats with LcrV-derived peptides, rV10, but not rLcrV immune sera, lacked antibodies recognizing linear LcrV oligopeptides.

Vaccination of mice with a Yop translocon complex elicits antibodies that are protective against infection with F1- Yersinia pestis

Yersinia pestis, the bacterial agent of plague, secretes several proteins important for pathogenesis or host protection. The F1 protein forms a capsule on the bacterial cell surface and is a well-characterized protective antigen but is not essential for virulence. A type III secretion system that is essential for virulence exports Yop proteins, which function as antiphagocytic or anti-inflammatory factors. Yop effectors (e.g., YopE) are delivered across the host cell plasma membrane by a translocon, composed of YopB and YopD. Complexes of YopB, YopD, and YopE (BDE) secreted by Yersinia pseudotuberculosis were purified by affinity chromatography and used as immunogens to determine if antibodies to the translocon could provide protection against Y. pestis in mice. Mice vaccinated with BDE generated high-titer immunoglobulin G antibodies specific for BDE, as shown by enzyme-linked immunosorbent assay and immunoblotting, and were protected against lethal intravenous challenge with F1(-) but not F1(+) Y. pestis. Mice passively immunized with anti-BDE serum were protected from lethal challenge with F1(-) Y. pestis. The YopB protein or a complex of YopB and YopD (BD) was purified and determined by vaccination to be immunogenic in mice. Mice actively vaccinated with BD or passively vaccinated with anti-BD serum were protected against lethal challenge with F1(-) Y. pestis. These results indicate that anti-translocon antibodies can be used as immunotherapy to treat infections by F1(-) Y. pestis.

Yersinia pestis caf1 variants and the limits of plague vaccine protection

Yersinia pestis, the highly virulent agent of plague, is a biological weapon. Strategies that prevent plague have been sought for centuries, and immunization with live, attenuated (nonpigmented) strains or subunit vaccines with F1 (Caf1) antigen is considered effective. We show here that immunization with live, attenuated strains generates plague-protective immunity and humoral immune responses against F1 pilus antigen and LcrV. Y. pestis variants lacking caf1 (F1 pili) are not only fully virulent in animal models of bubonic and pneumonic plague but also break through immune responses generated with live, attenuated strains or F1 subunit vaccines. In contrast, immunization with purified LcrV, a protein at the tip of type III needles, generates protective immunity against the wild-type and the fully virulent caf1 mutant strain, in agreement with the notion that LcrV can elicit vaccine protection against both types of virulent plague strains.

Current challenges in the development of vaccines for pneumonic plague

Inhalation of Yersinia pestis bacilli causes pneumonic plague, a rapidly progressing and exceptionally virulent disease. Extensively antibiotic-resistant Y. pestis strains exist and we currently lack a safe and effective pneumonic plague vaccine. These facts raise concern that Y. pestis may be exploited as a bioweapon. Here, I review the history and status of plague vaccine research and advocate that pneumonic plague vaccines should strive to prime both humoral and cellular immunity.

Braun lipoprotein (Lpp) contributes to virulence of yersiniae: potential role of Lpp in inducing bubonic and pneumonic plague

Yersinia pestis evolved from Y. pseudotuberculosis to become the causative agent of bubonic and pneumonic plague. We identified a homolog of the Salmonella enterica serovar Typhimurium lipoprotein (lpp) gene in Yersinia species and prepared lpp gene deletion mutants of Y. pseudotuberculosis YPIII, Y. pestis KIM/D27 (pigmentation locus minus), and Y. pestis CO92 with reduced virulence. Mice injected via the intraperitoneal route with 5 x 10(7) CFU of the Deltalpp KIM/D27 mutant survived a month, even though this would have constituted a lethal dose for the parental KIM/D27 strain. Subsequently, these Deltalpp KIM/D27-injected mice were solidly protected against an intranasally administered, highly virulent Y. pestis CO92 strain when it was given as five 50% lethal doses (LD(50)). In a parallel study with the pneumonic plague mouse model, after 72 h postinfection, the lungs of animals infected with wild-type (WT) Y. pestis CO92 and given a subinhibitory dose of levofloxacin had acute inflammation, edema, and masses of bacteria, while the lung tissue appeared essentially normal in mice inoculated with the Deltalpp mutant of CO92 and given the same dose of levofloxacin. Importantly, while WT Y. pestis CO92 could be detected in the bloodstreams and spleens of infected mice at 72 h postinfection, the Deltalpp mutant of CO92 could not be detected in those organs. Furthermore, the levels of cytokines/chemokines detected in the sera were significantly lower in animals infected with the Deltalpp mutant than in those infected with WT CO92. Additionally, the Deltalpp mutant was more rapidly killed by macrophages than was the WT CO92 strain. These data provided evidence that the Deltalpp mutants of yersiniae were significantly attenuated and could be useful tools in the development of new vaccines.