Fraction 1 capsular antigen (F1) purification from Yersinia pestis CO92 and from an Escherichia coli recombinant strain and efficacy against lethal plague challenge

Sex differences in immune protection in mice conferred by heterologous vaccines for pneumonic plague

Background

Yersinia pestis is the etiological agent of plague, which can manifest as bubonic, septicemic, and/or pneumonic disease. Plague is a severe and rapidly progressing illness that can only be successfully treated with antibiotics initiated early after infection. There are no FDA-approved vaccines for plague, and some vaccine candidates may be less effective against pneumonic plague than bubonic plague. Y. pestis is not known to impact males and females differently in mechanisms of pathogenesis or severity of infection. However, one previous study reported sex-biased vaccine effectiveness after intranasal Y. pestis challenge. As part of developing a safe and effective vaccine, it is essential that potential sex differences are characterized.

Methods

In this study we evaluated novel vaccines in male and female BALB/c mice using a heterologous prime-boost approach and monitored survival, bacterial load in organs, and immunological correlates. Our vaccine strategy consisted of two subcutaneous immunizations, followed by challenge with aerosolized virulent nonencapsulated Y. pestis. Mice were immunized with a combination of live Y. pestis pgm- pPst-Δcaf1, live Y. pestis pgm- pPst-Δcaf1/ΔyopD, or recombinant F1-V (rF1-V) combined with adjuvants.

Results

The most effective vaccine regimen was initial priming with rF1-V, followed by boost with either of the live attenuated strains. However, this and other strategies were more protective in female mice. Males had higher bacterial burden and differing patterns of cytokine expression and serum antibody titers. Male mice did not demonstrate synergy between vaccination and antibiotic treatment as repeatedly observed in female mice.

Conclusions

This study provides new knowledge about heterologous vaccine strategies, sex differences in plague-vaccine efficacy, and the immunological factors that differ between male and female mice.

Bacterial capsules: Occurrence, mechanism, and function

In environments characterized by extended multi-stress conditions, pathogens develop a variety of immune escape mechanisms to enhance their ability to infect the host. The capsules, polymers that bacteria secrete near their cell wall, participates in numerous bacterial life processes and plays a crucial role in resisting host immune attacks and adapting to their niche. Here, we discuss the relationship between capsules and bacterial virulence, summarizing the molecular mechanisms of capsular regulation and pathogenesis to provide new insights into the research on the pathogenesis of pathogenic bacteria.

Comparative Lysine Acetylome Analysis of Y. pestis YfiQ/CobB Mutants Reveals that Acetylation of SlyA Lys73 Significantly Promotes Biofilm Formation of Y. pestis

Increasing evidence shows that protein lysine acetylation is involved in almost every aspect of cellular physiology in bacteria. Yersinia pestis is a flea-borne pathogen responsible for millions of human deaths in three global pandemics. However, the functional role of lysine acetylation in this pathogen remains unclear. Here, we found more acetylated proteins and a higher degree of acetylation in Y. pestis grown under mammalian host (Mh) conditions than under flea vector (Fv) conditions, suggesting that protein acetylation could significantly change during fleabite transmission. Comparative acetylome analysis of mutants of YfiQ and CobB, the major acetyltransferase and deacetylase of Y. pestis, respectively, identified 23 YfiQ-dependent and 315 CobB-dependent acetylated proteins. Further results demonstrated that acetylation of Lys73 of the SlyA protein, a MarR-family transcriptional regulator, inhibits its binding to the promoter of target genes, including hmsT that encodes diguanylate cyclase responsible for the synthesis of c-di-GMP, and significantly enhances biofilm formation of Y. pestis. Our study presents the first extensive acetylome data of Y. pestis and a critical resource for the functional study of lysine acetylation in this pathogen. IMPORTANCE Yersinia pestis is the etiological agent of plague, historically responsible for three global pandemics. The 2017 plague epidemic in Madagascar was a reminder that Y. pestis remains a real threat in many parts of the world. Plague is a zoonotic disease that primarily infects rodents via fleabite, and transmission of Y. pestis from infected fleas to mammals requires rapid adaptive responses to adverse host environments to establish infection. Our study provides the first global profiling of lysine acetylation derived from mass spectrometry analysis in Y. pestis. Our data set can serve as a critical resource for the functional study of lysine acetylation in Y. pestis and provides new molecular insight into the physiological role of lysine acetylation in proteins. More importantly, we found that acetylation of Lys73 of SlyA significantly promotes biofilm formation of Y. pestis, indicating that bacteria can use lysine acetylation to fine-tune the expression of genes to improve adaptation.

A single-dose F1-based mRNA-LNP vaccine provides protection against the lethal plague bacterium

Messenger RNA (mRNA) lipid nanoparticle (LNP) vaccines have emerged as an effective vaccination strategy. Although currently applied toward viral pathogens, data concerning the platform's effectiveness against bacterial pathogens are limited. Here, we developed an effective mRNA-LNP vaccine against a lethal bacterial pathogen by optimizing mRNA payload guanine and cytosine content and antigen design. We designed a nucleoside-modified mRNA-LNP vaccine based on the bacterial F1 capsule antigen, a major protective component of Yersinia pestis, the etiological agent of plague. Plague is a rapidly deteriorating contagious disease that has killed millions of people during the history of humankind. Now, the disease is treated effectively with antibiotics; however, in the case of a multiple-antibiotic-resistant strain outbreak, alternative countermeasures are required. Our mRNA-LNP vaccine elicited humoral and cellular immunological responses in C57BL/6 mice and conferred rapid, full protection against lethal Y. pestis infection after a single dose. These data open avenues for urgently needed effective antibacterial vaccines.

Development of a dual antigen lateral flow immunoassay for detecting Yersinia pestis

Background

Yersinia pestis is the causative agent of plague, a zoonosis associated with small mammals. Plague is a severe disease, especially in the pneumonic and septicemic forms, where fatality rates approach 100% if left untreated. The bacterium is primarily transmitted via flea bite or through direct contact with an infected host. The 2017 plague outbreak in Madagascar resulted in more than 2,400 cases and was highlighted by an increased number of pneumonic infections. Standard diagnostics for plague include laboratory-based assays such as bacterial culture and serology, which are inadequate for administering immediate patient care for pneumonic and septicemic plague.

Principal findings

The goal of this study was to develop a sensitive rapid plague prototype that can detect all virulent strains of Y. pestis. Monoclonal antibodies (mAbs) were produced against two Y. pestis antigens, low-calcium response V (LcrV) and capsular fraction-1 (F1), and prototype lateral flow immunoassays (LFI) and enzyme-linked immunosorbent assays (ELISA) were constructed. The LFIs developed for the detection of LcrV and F1 had limits of detection (LOD) of roughly 1–2 ng/mL in surrogate clinical samples (antigens spiked into normal human sera). The optimized antigen-capture ELISAs produced LODs of 74 pg/mL for LcrV and 61 pg/mL for F1 when these antigens were spiked into buffer. A dual antigen LFI prototype comprised of two test lines was evaluated for the detection of both antigens in Y. pestis lysates. The dual format was also evaluated for specificity using a small panel of clinical near-neighbors and other Tier 1 bacterial Select Agents.

Conclusions

LcrV is expressed by all virulent Y. pestis strains, but homologs produced by other Yersinia species can confound assay specificity. F1 is specific to Y. pestis but is not expressed by all virulent strains. Utilizing highly reactive mAbs, a dual-antigen detection (multiplexed) LFI was developed to capitalize on the diagnostic strengths of each target.

Complete Protection Against Yersinia pestis in BALB/c Mouse Model Elicited by Immunization With Inhalable Formulations of rF1-V10 Fusion Protein via Aerosolized Intratracheal Inoculation

Pneumonic plague, caused by Yersinia pestis, is an infectious disease with high mortality rates unless treated early with antibiotics. Currently, no FDA-approved vaccine against plague is available for human use. The capsular antigen F1, the low-calcium-response V antigen (LcrV), and the recombinant fusion protein (rF1-LcrV) of Y. pestis are leading subunit vaccine candidates under intense investigation; however, the inability of recombinant antigens to provide complete protection against pneumonic plague in animal models remains a significant concern. In this study, we compared immunoprotection against pneumonic plague provided by rF1, rV10 (a truncation of LcrV), and rF1-V10, and vaccinations delivered via aerosolized intratracheal (i.t.) inoculation or subcutaneous (s.c.) injection. We further considered three vaccine formulations: conventional liquid, dry powder produced by spray freeze drying, or dry powder reconstituted in PBS. The main findings are: (i) rF1-V10 immunization with any formulation via i.t. or s.c. routes conferred 100% protection against Y. pestis i.t. infection; (ii) rF1 or rV10 immunization using i.t. delivery provided significantly stronger protection than rF1 or rV10 immunization via s.c. delivery; and (iii) powder formulations of subunit vaccines induced immune responses and provided protection equivalent to those elicited by unprocessed liquid formulations of vaccines. Our data indicate that immunization with a powder formulation of rF1-V10 vaccines via an i.t. route may be a promising vaccination strategy for providing protective immunity against pneumonic plague.

Plague Prevention and Therapy: Perspectives on Current and Future Strategies

Plague, caused by the bacterial pathogen Yersinia pestis, is a vector-borne disease that has caused millions of human deaths over several centuries. Presently, human plague infections continue throughout the world. Transmission from one host to another relies mainly on infected flea bites, which can cause enlarged lymph nodes called buboes, followed by septicemic dissemination of the pathogen. Additionally, droplet inhalation after close contact with infected mammals can result in primary pneumonic plague. Here, we review research advances in the areas of vaccines and therapeutics for plague in context of Y. pestis virulence factors and disease pathogenesis. Plague continues to be both a public health threat and a biodefense concern and we highlight research that is important for infection mitigation and disease treatment.

Optimised Heterologous Expression and Functional Analysis of the Yersinia pestis F1-Capsular Antigen Regulator Caf1R

The bacterial pathogen, Yersinia pestis, has caused three historic pandemics and continues to cause small outbreaks worldwide. During infection, Y. pestis assembles a capsule-like protective coat of thin fibres of Caf1 subunits. This F1 capsular antigen has attracted much attention due to its clinical value in plague diagnostics and anti-plague vaccine development. Expression of F1 is tightly regulated by a transcriptional activator, Caf1R, of the AraC/XylS family, proteins notoriously prone to aggregation. Here, we have optimised the recombinant expression of soluble Caf1R. Expression from the native and synthetic codon-optimised caf1R cloned in three different expression plasmids was examined in a library of E. coli host strains. The functionality of His-tagged Caf1R was demonstrated in vivo, but insolubility was a problem with overproduction. High levels of soluble MBP-Caf1R were produced from codon optimised caf1R. Transcriptional-lacZ reporter fusions defined the P_M promoter and Caf1R binding site responsible for transcription of the cafMA1 operon. Use of the identified Caf1R binding caf DNA sequence in an electrophoretic mobility shift assay (EMSA) confirmed correct folding and functionality of the Caf1R DNA-binding domain in recombinant MBP-Caf1R. Availability of functional recombinant Caf1R will be a valuable tool to elucidate control of expression of F1 and Caf1R-regulated pathophysiology of Y. pestis.

Impact of Toll-Like Receptor-Specific Agonists on the Host Immune Response to the Yersinia pestis Plague rF1V Vaccine

Relatively recent advances in plague vaccinology have produced the recombinant fusion protein F1-V plague vaccine. This vaccine has been shown to readily protect mice from both bubonic and pneumonic plague. The protection afforded by this vaccine is solely based upon the immune response elicited by the F1 or V epitopes expressed on the F1-V fusion protein. Accordingly, questions remain surrounding its efficacy against infection with non-encapsulated (F1-negative) strains. In an attempt to further optimize the F1-V elicited immune response and address efficacy concerns, we examined the inclusion of multiple toll-like receptor agonists into vaccine regimens. We examined the resulting immune responses and also any protection afforded to mice that were exposed to aerosolized Yersinia pestis. Our data demonstrate that it is possible to further augment the F1-V vaccine strategy in order to optimize and augment vaccine efficacy.

Epitope Binning of Novel Monoclonal Anti F1 and Anti LcrV Antibodies and Their Application in a Simple, Short, HTRF Test for Clinical Plague Detection

Mouse monoclonal antibodies were raised against plague disease biomarkers: the bacterial capsular protein fraction 1 (F1) and the low-calcium response—LcrV virulence factor (Vag). A novel tandem assay, employing BioLayer Interferometry (BLI), enabled the isolation of antibodies against four different epitopes on Vag. The tandem assay was carried out with hybridoma supernatants, circumventing the need for antibody purification. The BioLayer assay was further adopted for characterization of epitope-repetitive antigens, enabling the discovery of two unique epitopes on F1. The selected antibodies were purified and applied as “oligo-clonal” reagents for the immuno-detection of both biomarkers. The developed Homogenous Time Resolved Fluorescence (HTRF) tests were short (10 min) and simple (no washing steps), allowing for detection of 10 ng/mL F1 and 2.5 ng/mL Vag. The tests were successfully applied for detection of disease biomarkers produced by various Y. pestis strains during growth in blood culture vials.

Protection Elicited by Attenuated Live Yersinia pestis Vaccine Strains against Lethal Infection with Virulent Y. pestis

The etiologic agent of plague, Yersinia pestis, is a globally distributed pathogen which poses both a natural and adversarial threat. Due largely to the rapid course and high mortality of pneumonic plague, vaccines are greatly needed. Two-component protein vaccines have been unreliable and potentially vulnerable to vaccine resistance. We evaluated the safety and efficacy of eight live Y. pestis strains derived from virulent strains CO92 or KIM6+ and mutated in one or more virulence-associated gene(s) or cured of plasmid pPst. Stringent, single-dose vaccination allowed down-selection of the two safest and most protective vaccine candidates, CO92 mutants pgm- pPst- and ΔyscN. Both completely protected BALB/c mice against subcutaneous and aerosol challenge with Y. pestis. Strain CD-1 outbred mice were more resistant to bubonic (but not pneumonic) plague than BALB/c mice, but the vaccines elicited partial protection of CD-1 mice against aerosol challenge, while providing full protection against subcutaneous challenge. A ΔyscN mutant of the nonencapsulated C12 strain was expected to display antigens previously concealed by the capsule. C12 ΔyscN elicited negligible titers to F1 but comparable antibody levels to whole killed bacteria, as did CO92 ΔyscN. Although one dose of C12 ΔyscN was not protective, vaccination with two doses of either CO92 ΔyscN, or a combination of the ΔyscN mutants of C12 and CO92, protected optimally against lethal bubonic or pneumonic plague. Protection against encapsulated Y. pestis required inclusion of F1 in the vaccine and was associated with high anti-F1 titers.

Comparison of the transmission efficiency and plague progression dynamics associated with two mechanisms by which fleas transmit Yersinia pestis

Yersinia pestis can be transmitted by fleas during the first week after an infectious blood meal, termed early-phase or mass transmission, and again after Y. pestis forms a cohesive biofilm in the flea foregut that blocks normal blood feeding. We compared the transmission efficiency and the progression of infection after transmission by Oropsylla montana fleas at both stages. Fleas were allowed to feed on mice three days after an infectious blood meal to evaluate early-phase transmission, or after they had developed complete proventricular blockage. Transmission was variable and rather inefficient by both modes, and the odds of early-phase transmission was positively associated with the number of infected fleas that fed. Disease progression in individual mice bitten by fleas infected with a bioluminescent strain of Y. pestis was tracked. An early prominent focus of infection at the intradermal flea bite site and dissemination to the draining lymph node(s) soon thereafter were common features, but unlike what has been observed in intradermal injection models, this did not invariably lead to further systemic spread and terminal disease. Several of these mice resolved the infection without progression to terminal sepsis and developed an immune response to Y. pestis, particularly those that received an intermediate number of early-phase flea bites. Furthermore, two distinct types of terminal disease were noted: the stereotypical rapid onset terminal disease within four days, or a prolonged onset preceded by an extended, fluctuating infection of the lymph nodes before eventual systemic dissemination. For both modes of transmission, bubonic plague rather than primary septicemic plague was the predominant disease outcome. The results will help to inform mathematical models of flea-borne plague dynamics used to predict the relative contribution of the two transmission modes to epizootic outbreaks that erupt periodically from the normal enzootic background state.

Yersinia pestis can be transmitted by fleas within a few days after taking a blood meal from a highly bacteremic host, termed early-phase or mass transmission; and again after it forms a dense biofilm in the foregut of its vector that can eventually block blood feeding. The relative importance of the two transmission modes in the ecology of plague is a matter of current debate, but estimates of transmission rate, efficiency, and other parameters are limited. We compared transmission and disease progression dynamics in mice bitten by groups of fleas three days after their infectious blood meal (early-phase or mass transmission mode) and in mice bitten by individual blocked fleas. In general, a higher percentage of transmissions by blocked fleas led to terminal disease, whereas early-phase transmissions more often led to survival and an immune response, which are nonproductive infections in the sense that the bacteremia required to continue the Y. pestis life cycle did not develop and these animals would be removed from the pool of susceptibles in the host population. The data will be useful in mathematical models of plague dynamics in wild rodent populations.

The Francisella tularensis Polysaccharides: What Is the Real Capsule?

Francisella tularensis is a tier 1 select agent responsible for tularemia in humans and a wide variety of animal species. Extensive research into understanding the virulence factors of the bacterium has been ongoing to develop an efficacious vaccine. At least two such virulence factors are described as capsules of F. tularensis: the O-antigen capsule and the capsule-like complex (CLC). These two separate entities aid in avoiding host immune defenses but have not been clearly differentiated. These components are distinct and differ in composition and genetic basis. The O-antigen capsule consists of a polysaccharide nearly identical to the lipopolysaccharide (LPS) O antigen, whereas the CLC is a heterogeneous complex of glycoproteins, proteins, and possibly outer membrane vesicles and tubes (OMV/Ts). In this review, the current understanding of these two capsules is summarized, and the historical references to "capsules" of F. tularensis are clarified. A significant amount of research has been invested into the composition of each capsule and the genes involved in synthesis of the polysaccharide portion of each capsule. Areas of future research include further exploration into the molecular regulation and pathways responsible for expression of each capsule and further elucidating the role that each capsule plays in virulence.

A new recombinant F1 antigen as a cost and time-effective tool for plague diagnosis

The Yersinia pestis capsular antigen F1 is widely used in plague laboratory diagnosis. Here, we describe the production of an F1 recombinant protein within reduced time and biosafety requirements. Its evaluation in hemagglutination tests indicated that the recombinant F1 can replace the conventional F1 protein for plague diagnosis.

Plague vaccine: recent progress and prospects

Three great plague pandemics, resulting in nearly 200 million deaths in human history and usage as a biowarfare agent, have made Yersinia pestis as one of the most virulent human pathogens. In late 2017, a large plague outbreak raged in Madagascar attracted extensive attention and caused regional panics. The evolution of local outbreaks into a pandemic is a concern of the Centers for Disease Control and Prevention (CDC) in plague endemic regions. Until now, no licensed plague vaccine is available. Prophylactic vaccination counteracting this disease is certainly a primary choice for its long-term prevention. In this review, we summarize the latest advances in research and development of plague vaccines.

Targeting of the Yersinia pestis F1 capsular antigen by innate-like B1b cells mediates a rapid protective response against bubonic plague

The generation of adaptive immunity by vaccination is usually a prolonged process that requires multiple dosing over several months. Hence, vaccines are administered for disease prevention a relatively long time prior to possible infection as opposed to post-exposure prophylaxis, which typically requires rapid intervention such as antibiotic therapy. The emergence of pathogens resistant to common antibiotic treatments has prompted the search for alternative therapeutic strategies. We previously demonstrated that vaccination of mice with the F1 capsular antigen of Yersinia pestis elicits specific and effective yet, unexpectedly, rapid anti-plague immunity. Here, we show by applying genetic and immunological approaches that the F1 antigen is targeted by peritoneal innate-like B1b cells that generate a prompt T-independent (TI) anti-F1 humoral response. The rapid F1-mediated defense response was diminished in Xid (Btk^m) mice in which B1 cell numbers and activity are limited. Binding of fluorophore-labeled F1 to peritoneal B1b cells was detected as soon as 6 h post vaccination, emphasizing the high speed of this process. By assessing the ability to achieve rapid immunity with monomerized F1, we show that the natural polymeric structure of F1 is essential for (i) rapid association with peritoneal B1b cells, (ii) early induction of anti-F1 titers and (iii) rapid TI immunity in the mouse model of bubonic plague. These observations shed new light on the potential of novel as well as well-known protective antigens in generating rapid immunity and could be implemented in the rational design of future vaccines.

Rapid and sensitive detection of Yersinia pestis by lateral-flow assay in simulated clinical samples

Background

Yersinia pestis is a contributing agent to the epidemic disease, plague, which killed an estimated 200 million people during historical times. In this study, a rapid, cheap, sensitive, and specific technique, the lateral flow assay (F1 strips), has been successfully developed to detect this pathogen, by using paired monoclonal antibodies (MAbs) against Y. pestis capsule like fraction 1 (F1) protein. Compared with the polyclonal antibody (PAb) based F1 strips, the Mab-based F1 strips have a remarkable increased detection limitation (10 to 100 folds). Furthermore, besides the limitation and specificity evaluation, the application of this F1 strip on simulated clinical samples indicate the LFA can be a good candidate to detect plague.

Methods

Recombinant F1 antigen was expressed and purified from a series of works. The various anti-F1 monoclonal antibodies generated from hybridoma cells were screened with the ELISA technique. To evaluate the feasibility of this Y. pestis F1 test strip, the F1 protein/Y. pestis was spiked into simulated clinical samples such as human serum, mouse bronchoalveolar lavage fluids, and mouse blood to mimic natural infection status. Additionally, this technique was applied to detect the Y. pestis in the environment-captured rats, to evaluate the practical usefulness of the strips.

Results

By using this MAb-based-LFA technique, 4 ng/ml of recombinant F1-protein and 10³ CFU/ml of Y. pestis could be detected in less than 10 mins, which is at least 10-folds than that of the PAb format. On the other hand, although various Yersinia strains were applied to the strips, only Y. pestis strain showed a positive result; all other Yersinia species did not produce a positive signal, indicating the high efficiency and specificity of the MAb-based F1-strips.

Conclusion

Based on our findings, we suggest that the MAb-format-LFA will be valuable as a diagnostic tool for the detection of Y. pestis. This report shows that the F1 strip is sufficient to support not only the detection of plague in simulated clinical samples, but also it may be a good candidate to meet the epidemiological surveillance during an outbreak of the biological warfare.

A phase I safety and immunogenicity dose escalation trial of plague vaccine, Flagellin/F1/V, in healthy adult volunteers (DMID 08-0066)

INTRODUCTION

Intentional aerosolization of Yersinia pestis may result in pneumonic plague which is highly fatal if not treated early.

METHODS

We conducted a phase 1 randomized, double blind (within each group), placebo controlled, dose escalation trial to evaluate a plague vaccine, Flagellin/F1/V, in healthy adults aged 8 through 45years. Vaccine was administered intramuscularly on Days 0 and 28 at a dose of 1, 3, 6 or 10mcg. Subjects were observed for 4h after vaccination for cytokine release syndrome. Reactogenicity and adverse events (AE) were collected for 14 and 28days, respectively, after each vaccination. Serious AE were collected for the entire study. ELISA antibody and cytokines were measured at multiple time points. Subject's participation lasted 13months.

RESULTS

Sixty healthy subjects were enrolled; 52% males, 100% non-Hispanic, 91.7% white and mean age 30.8years. No severe reactogenicity events occurred; most AE were mild. No serious AE related to vaccine occurred. A dose response effect was observed to F1, V and flagellin. The peak ELISA IgG antibody titers (95% CI) after two 10mcg doses of vaccine were 260.0 (102.6-659.0) and 983.6 (317.3-3048.8), respectively, against F1 and V antigens. The 6mcg dose group provided similar titers. Titers were low for the placebo, 1mcg and 3mcg recipients. A positive antibody dose response was observed to F1, V and flagellin. Vaccine antigen specific serum IgE was not detected. There were no significant rises in serum or cellular cytokine responses and no significant IgG increase to flagellin after the second dose.

CONCLUSION

The Flagellin/F1/V vaccine exhibited a dose dependent increase in immunogenicity and was well tolerated at all doses. Antibody specific responses to F1, V and flagellin increased as dose increased. Given the results from this trial, testing higher doses of the vaccine may be merited.

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.

Identification and characterization of a neutralizing monoclonal antibody that provides complete protection against Yersinia pestis

Yersinia pestis (Y. pestis) has caused an alarming number of deaths throughout recorded human history, and novel prophylactics and therapeutics are necessary given its potential as a bioweapon. Only one monoclonal antibody has been identified to date that provides complete protection against Y. pestis. Here, we describe a second novel murine monoclonal antibody (F2H5) that provided complete protection against Y. pestis 141 infection when administered prophylactically to Balb/c mice (100 μg intravenously). We humanized F2H5, characterized its ability to bind to the Y. pestis F1 protein and further characterized the neutralizing epitope using computational and experimental approaches. While Western blot results suggested a linear epitope, peptide mapping using ELISA failed to identify an epitope, suggesting a conformational epitope instead. We adopted a computational approach based on Residue Contact Frequency to predict the site of antigen-antibody interaction and defined the F2H5/F1 binding site computationally. Based on computational approach, we determined that residues G104E105N106 in F1 were critical to F2H5 binding and that CDRH2 and CDRH3 of F2H5 interacted with F1. Our results show that combining computational approach and experimental approach can effectively identify epitopes.

Plague Vaccine Development: Current Research and Future Trends

Plague is one of the world’s most lethal human diseases caused by Yersinia pestis, a Gram-negative bacterium. Despite overwhelming studies for many years worldwide, there is no safe and effective vaccine against this fatal disease. Inhalation of Y. pestis bacilli causes pneumonic plague, a fast growing and deadly dangerous disease. F1/LcrV-based vaccines failed to provide adequate protection in African green monkey model in spite of providing protection in mice and cynomolgus macaques. There is still no explanation for this inconsistent efficacy, and scientists leg behind to search reliable correlate assays for immune protection. These paucities are the main barriers to improve the effectiveness of plague vaccine. In the present scenario, one has to pay special attention to elicit strong cellular immune response in developing a next-generation vaccine against plague. Here, we review the scientific contributions and existing progress in developing subunit vaccines, the role of molecular adjuvants; DNA vaccines; live delivery platforms; and attenuated vaccines developed to counteract virulent strains of Y. pestis.

Beyond Helper Phage: Using "Helper Cells" to Select Peptide Affinity Ligands

Peptides are important affinity ligands for microscopy, biosensing, and targeted delivery. However, because they can have low affinity for their targets, their selection from large naïve libraries can be challenging. When selecting peptidic ligands from display libraries, it is important to: 1) ensure efficient display; 2) maximize the ability to select high affinity ligands; and 3) minimize the effect of the display context on binding. The “helper cell” packaging system has been described as a tool to produce filamentous phage particles based on phagemid constructs with varying display levels, while remaining free of helper phage contamination. Here we report on the first use of this system for peptide display, including the systematic characterization and optimization of helper cells, their inefficient use in antibody display and their use in creating and selecting from a set of phage display peptide libraries. Our libraries were analyzed with unprecedented precision by standard or deep sequencing, and shown to be superior in quality than commercial gold standards. Using our helper cell libraries, we have obtained ligands recognizing Yersinia pestis surface antigen F1V and L-glutamine-binding periplasmic protein QBP. In the latter case, unlike any of the peptide library selections described so far, we used a combination of phage and yeast display to select intriguing peptide ligands. Based on the success of our selections we believe that peptide libraries obtained with helper cells are not only suitable, but preferable to traditional phage display libraries for selection of peptidic ligands.

Transcription of exsD is repressed directly by H-NS in Vibrio parahaemolyticus

Vibrio parahaemolyticus is a leading cause of seafood-associated diarrhea and gastroenteritis. This bacteria expresses a major virulence determinant called T3SS1. Expression of T3SS1 is tightly regulated by the ExsA-ExsC-ExsD regulatory system. The transcription of exsA and probably exsC is repressed directly by the H-NS protein. In this study, the regulation of exsD by H-NS was investigated using quantitative RT-PCR, primer extension, LacZ fusion, electrophoretic mobility shift, and DNase I footprinting assays. The results showed that His-H-NS protected a single region from 61 bp to 174 bp upstream of exsD against DNase I digestion, and a transcription start site located at 105 bp upstream of exsD was detected and its activity was repressed by H-NS. Therefore, a single H-NS-dependent promoter was transcribed for exsD in V. parahaemolyticus. Thus, all three genes in the ExsA-ExsC-ExsD regulatory system of T3SS1 are directly repressed by H-NS in V. parahaemolyticus.

Highly Effective Soluble and Bacteriophage T4 Nanoparticle Plague Vaccines Against Yersinia pestis

Plague caused by Yersinia pestis is an ancient disease, responsible for millions of deaths in human history. Unfortunately, there is no FDA-approved vaccine available. Recombinant subunit vaccines based on two major antigens, Caf 1 (F1) and LcrV (V), have been under investigation and showed promise. However, there are two main problems associated with these vaccines. First, the Yersinia capsular protein F1 has high propensity to aggregate, particularly when expressed in heterologous systems such as Escherichia coli, thus affecting vaccine quality and efficacy. Second, the subunit vaccines do not induce adequate cell-mediated immune responses that also appear to be essential for optimal protection against plague. We have developed two basic approaches, structure-based immunogen design and phage T4 nanoparticle delivery, to construct new plague vaccines that may overcome these problems. First, by engineering F1 protein, we generated a monomeric and soluble F1V mutant (F1mutV) which has similar immunogenicity as wild-type F1V. The NH2-terminal β-strand of F1 was transplanted to the COOH-terminus and the sequence flanking the β-strand was duplicated to retain a key CD4(+) T cell epitope. Second, we generated a nanoparticle plague vaccine that can induce balanced antibody- and cell-mediated immune responses. This was done by arraying the F1mutV on phage T4 via the small outer capsid (Soc) protein which binds to T4 capsid at nanomolar affinity. Preparation of these vaccines is described in detail and we hope that these would be considered as candidates for licensing a next-generation plague vaccine.

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.

MAP of F1 and V antigens from Yersinia pestis astride innate and adaptive immune response

Yersinia pestis, a causative agent of plague, has a plethora of armors to fight against major components of innate immunity and survive within host cells. Dendritic cells and macrophages are important antigen presenting cells for effective immune response. This report is focused on the changes in DC activation and TLR2 and TLR4 expression on macrophages induced by MAP of F1 and V antigens of Y. pestis. F1 and V MAPs bear potential synthetic T and B cell epitopes from F1 and V protein respectively. We evaluated these parameters in DC's isolated from spleen and lamina propria and macrophages isolated from peritoneal lavage of mice after intranasal immunization. F1 MAP and V MAP significantly increased the expression of CD80 and CD86 on CD11c(+) dendritic cells isolated from spleen and lamina propria as well as intracellular IL-12 levels. Similarly, in macrophages derived from peritoneal cavity, the above formulation enhanced TLR2 and TLR4 expression. Again after in vitro stimulation with F1 and V MAP these macrophages produced significantly high IL12 and TNFα. The study clearly indicates involvement of DC and macrophages for efficient antigen presentation to immune cells. From this study we conclude that F1MAP and VMAP ameliorate innate immune mechanism. These two synthetic constructs exert their effect via TLR2 and TLR4, leading to the production of proinflammatory cytokines by macrophages and are able to increase DC activation, that could be helpful in generation of adaptive immunity as well as is important strong immune response.

Rapid detection of Yersinia pestis recombinant fraction 1 capsular antigen

Yersinia pestis, an infectious bacterium that is a causative agent of plague, a disease which has been shown to be one of the most feared in history and which has caused millions of deaths. The capsule-like fraction 1 (F1) antigen expressed by Y. pestis is a known specific marker for the identification of the bacteria; therefore, the detection of F1 is important for Y. pestis recognition. In this study, a rapid, sensitive, and specific technique, the lateral flow assay (LFA), was successfully developed to detect Y. pestis by the recombinant F1 antigen. The assay that utilized an anti-F1 polyclonal antibody (Pab) to identify the bacteria was based on a double-antibody sandwich format on a nitrocellulose membrane. With the LFA method, 50 ng/ml of recombinant F1 protein and 10(5) CFU/mL of Y. pestis could be detected in less than 10 min. This assay also showed no cross-reaction with other Yersinia spp. or with some selected capsule-producing Enterobacteriaceae strains. Furthermore, detection of Y. pestis in simulated samples has been evaluated. The detection sensitivity of Y. pestis in various matrices was 10(5) CFU/mL, which was identical to that in PBS buffer. The results obtained suggest that LFA is an excellent tool for detection of Y. pestis contamination in an environment and hence can be used to monitor plague diseases when they emerge.

Molecualr Cloning of the capsular antigen F1 of Yersinia pestis in pBAD/gIII plasmid

Yersinia pestis which is the causative agent of pneumonic plague and distributed in all continents has led to many deaths during the history. Because of its high mortality rate, it must be diagnosed and treated at the earliest time post infection and therefore, rapid diagnostic tests are required. In the present study, we cloned the coding sequence of F1 capsular antigen of the bacteria in the pBAD/gIII plasmid for later expression and purification of the protein to produce poly and monoclonal antibodies against this antigen, and subsequently to develop rapid and efficient diagnostics tools for Y. pestis infections.

H-NS is a repressor of major virulence gene loci in Vibrio parahaemolyticus

Vibrio parahaemolyticus, a leading cause of seafood-associated diarrhea and gastroenteritis, harbors three major virulence gene loci T3SS1, Vp-PAI (T3SS1+tdh2) and T6SS2. As showing in this study, the nucleoid-associated DNA-binding regulator H-NS binds to multiple promoter-proximal regions in each of the above three loci to repress their transcription, and moreover H-NS inhibits the cytotoxicitiy, enterotoxicity, hemolytic activity, and mouse lethality of V. parahaemolyticus. H-NS appears to act as a major repressor of the virulence of this pathogen. Date presented here would promote us to gain a deeper understanding of H-NS-mediated silencing of horizontally acquired virulence loci in V. parahaemolyticus.

LcrV delivered via type III secretion system of live attenuated Yersinia pseudotuberculosis enhances immunogenicity against pneumonic plague

Here, we constructed a Yersinia pseudotuberculosis mutant strain with arabinose-dependent regulated and delayed shutoff of crp expression (araC P(BAD) crp) and replacement of the msbB gene with the Escherichia coli msbB gene to attenuate it. Then, we inserted the asd mutation into this construction to form χ10057 [Δasd-206 ΔmsbB868::P(msbB) msbB(EC) ΔP(crp21)::TT araC P(BAD) crp] for use with a balanced-lethal Asd-positive (Asd(+)) plasmid to facilitate antigen synthesis. A hybrid protein composed of YopE (amino acids [aa]1 to 138) fused with full-length LcrV (YopE(Nt138)-LcrV) was synthesized in χ10057 harboring an Asd(+) plasmid (pYA5199, yopE(Nt138)-lcrV) and could be secreted through a type III secretion system (T3SS) in vitro and in vivo. Animal studies indicated that mice orally immunized with χ10057(pYA5199) developed titers of IgG response to whole-cell lysates of Y. pestis (YpL) and subunit LcrV similar to those seen with χ10057(pYA3332) (χ10057 plus an empty plasmid). However, only immunization of mice with χ10057(pYA5199) resulted in a significant secretory IgA response to LcrV. χ10057(pYA5199) induced a higher level of protection (80% survival) against intranasal (i.n.) challenge with ~240 median lethal doses (LD50) (2.4 × 10(4) CFU) of Y. pestis KIM6+(pCD1Ap) than χ10057(pYA3332) (40% survival). Splenocytes from mice vaccinated with χ10057(pYA5199) produced significant levels of gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin-17 (IL-17) after restimulation with LcrV and YpL antigens. Our results suggest that it is possible to use an attenuated Y. pseudotuberculosis strain delivering the LcrV antigen via the T3SS as a potential vaccine candidate against pneumonic plague.

Cell density- and quorum sensing-dependent expression of type VI secretion system 2 in Vibrio parahaemolyticus

Background

Vibrio parahaemolyticus AphA and OpaR are the two master quorum sensing (QS) regulators that are abundantly expressed at low cell density (LCD) and high cell density (HCD), respectively, with a feature of reciprocally gradient production of them with transition between LCD and HCD. The type VI secretion system 2 (T6SS2) gene cluster can be assigned into three putative operons, namely VPA1027-1024, VPA1043-1028, and VPA1044-1046. T6SS2 contributes to adhesion of V. parahaemolyticus to host cells.

Methodology/Principal Findings

OpaR box-like sequences were found within the upstream promoter regions of all the above three operons, while none of AphA box-like elements could be identified for them. The subsequent primer extension, LacZ fusion, electrophoretic mobility shift, and DNase I footprinting assays disclosed that OpaR bound to the promoter regions of these three operons to stimulate their transcription, while AphA negatively regulated their transcription most likely through acting on OpaR. This regulation led to a gradient increase of T6SS2 transcription with transition from LCD to HCD.

Conclusions/Significance

V. parahaemolyticus OpaR and AphA positively and negatively regulate T6SS2 expression, respectively, leading to a gradient elevation of T6SS2 expression with transition from LCD to HCD. T6SS2 genes are thus assigned as the QS regulon members in V. parahaemolyticus.

Preclinical Safety Assessment of a Recombinant Plague Vaccine (rF1V)

A recombinant vaccine (rF1V) is being developed to protect adults 18 to 55 years of age from fatal pneumonic plague caused by aerosolized Yersinia pestis. A comprehensive series of studies was conducted to evaluate the general toxicity and local reactogenicity of the rF1V vaccine prior to first use in humans. Toxicity was evaluated in CD-1 mice vaccinated with control material and three dosage concentrations of rF1V with or without Alhydrogel® by intramuscular (IM) injection on Study Days 1, 29, 57 and 71 in a volume of 0.1 mL. Total immunizing protein given in each dose was 0, 20 or 60 μg/animal. Local reactogenicity was evaluated in mice at the dosages given and in New Zealand white (NZW) rabbits using the same injection volume and formulations (40, 80, 160 and 320 μg/mL total antigen and 0.3% (w/v) Alhydrogel®) intended for human use (0.5 mL). The rF1V vaccine produced no apparent systemic toxicity and only transient edema and erythema at the injection site. Together these results indicated a favorable safety profile for rF1V and supported its use in a Phase 1 clinical trial.

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.

Multiple antigen peptide consisting of B- and T-cell epitopes of F1 antigen of Y. pestis showed enhanced humoral and mucosal immune response in different strains of mice

Yersinia pestis is a causative agent of plague. F1 and V antigen based vaccines have shown remarkable protection in experimental animals. In order to develop epitope based immunogen, three B and one T-cell epitopes of F1 antigen with palmitate residue at amino terminal were assembled on a lysine backbone as multiple antigen peptide (MAP or F1-MAP). MAP was characterized by SDS-PAGE, immunoblot and immunoreactivity with anti F1 sera. MAP was entrapped in PLGA (polylactide-co-glycolide) microparticles and humoral, mucosal immune responses were studied after intranasal immunization with/without CpG ODN 1826 (CpG)/murabutide in different strains of mice. Serum and mucosal washes were measured for MAP specific IgG, IgA, sIgA and IgG subclasses in three strains of mice. F1-MAP showed high serum antibody and mucosal IgG and IgA peak antibody titers. MAP with CpG showed significantly high (p<0.001) peak antibody titer ranging from 102,400 to 204,800 for IgG and 6400 to 12,800 for IgA. High mucosal sIgA and its secretary component detection confirmed generation of mucosal response in intestinal and lung washes. MAP antisera also showed significant immunoreactivity with individual peptides. Moreover, antibody specific activity (IgG, IgA and sIgA) positively correlates with peak antibody titers. Predominantly IgG2a/IgG2b subclass was observed with CpG formulation but in other formulation a mixed IgG1 and IgG2a response was observed. The present study highlights the importance of multiple antigen peptide approach of F1-antigen with CpG as an alternative approach for subunit vaccine.

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.

Molecular characterization of direct target genes and cis-acting consensus recognized by quorum-sensing regulator AphA in Vibrio parahaemolyticus

BACKGROUND

AphA is the master quorum-sensing (QS) regulator operating at low cell density in vibrios. Molecular regulation of target genes by AphA has been characterized in Vibrio harveyi and V. cholerae, but it is still poorly understood in V. parahaemolyticus.

METHODOLOGY/PRINCIPAL FINDINGS

The AphA proteins are extremely conserved in V. parahaemolyticus, Vibrio sp. Ex25, Vibrio sp. EJY3, V. harveyi, V. vulnificus, V. splendidus, V. anguillarum, V. cholerae, and V. furnissii. The above nine AphA orthologs appear to recognize conserved cis-acting DNA signals which can be represented by two consensus constructs, a 20 bp box sequence and a position frequency matrix. V. parahaemolyticus AphA represses the transcription of ahpA, qrr4, and opaR through direct AphA-target promoter DNA association, while it inhibits the qrr2-3 transcription in an indirect manner. Translation and transcription starts, core promoter elements for sigma factor recognition, Shine-Dalgarno sequences for ribosome recognition, and AphA-binding sites (containing corresponding AphA box-like sequences) were determined for the three direct AphA targets ahpA, qrr4, and opaR in V. parahaemolyticus.

CONCLUSIONS/SIGNIFICANCE

AphA-mediated repression of ahpA, qrr2-4, and opaR was characterized in V. parahaemolyticus by using multiple biochemical and molecular experiments. The computational promoter analysis indicated the conserved mechanism of transcriptional regulation of QS regulator-encoding genes ahpA, qrr4, and opaR in vibrios.

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.

Vulnerabilities in Yersinia pestis caf operon are unveiled by a Salmonella vector

During infection, Yersinia pestis uses its F1 capsule to enhance survival and cause virulence to mammalian host. Since F1 is produced in large quantities and secreted into the host tissues, it also serves as a major immune target. To hold this detrimental effect under proper control, Y. pestis expresses the caf operon (encoding the F1 capsule) in a temperature-dependent manner. However, additional properties of the caf operon limit its expression. By overexpressing the caf operon in wild-type Salmonella enterica serovar Typhimurium under a potent promoter, virulence of Salmonella was greatly attenuated both in vitro and in vivo. In contrast, expression of the caf operon under the regulation of its native promoter exhibited negligible impairment of Salmonellae virulence. In-depth investigation revealed all individual genes in the caf operon attenuated Salmonella when overexpressed. The deleterious effects of caf operon and the caf individual genes were further confirmed when they were overexpressed in Y. pestis KIM6+. This study suggests that by using a weak inducible promoter, the detrimental effects of the caf operon are minimally manifested in Y. pestis. Thus, through tight regulation of the caf operon, Y. pestis precisely balances its capsular anti-phagocytic properties with the detrimental effects of caf during interaction with mammalian host.

Transcriptional regulation of opaR, qrr2-4 and aphA by the master quorum-sensing regulator OpaR in Vibrio parahaemolyticus

Background

Vibrio parahaemolyticus is a leading cause of infectious diarrhea and enterogastritis via the fecal-oral route. V. harveyi is a pathogen of fishes and invertebrates, and has been used as a model for quorum sensing (QS) studies. LuxR is the master QS regulator (MQSR) of V. harveyi, and LuxR-dependent expression of its own gene, qrr2–4 and aphA have been established in V. harveyi. Molecular regulation of target genes by the V. parahaemolyticus MQSR OpaR is still poorly understood.

Methodology/Principal Findings

The bioinformatics analysis indicated that V. parahaemolyticus OpaR, V. harveyi LuxR, V. vulnificu SmcR, and V. alginolyticus ValR were extremely conserved, and that these four MQSRs appeared to recognize the same conserved cis-acting signals, which was represented by the consensus constructs manifesting as a position frequency matrix and as a 20 bp box, within their target promoters. The MQSR box-like sequences were found within the upstream DNA regions of opaR, qrr2–4 and aphA in V. parahaemolyticus, and the direct transcriptional regulation of these target genes by OpaR were further confirmed by multiple biochemical experiments including primer extension assay, gel mobility shift assay, and DNase I footprinting analysis. Translation and transcription starts, core promoter elements for sigma factor recognition, Shine-Dalgarno sequences for ribosome recognition, and OpaR-binding sites were determined for the five target genes of OpaR, which gave a structural map of the OpaR-dependent promoters. Further computational promoter analysis indicated the above regulatory circuits were shared by several other closely related Vibrios but with slight exceptions.

Conclusions/Significance

This study gave a comprehensive computational and characterization of the direct transcriptional regulation of five target genes, opaR, qrr2–4 and ahpA, by OpaR in V. parahaemolyticus. These characterized regulatory circuits were conserved in V. harveyi and V. parahaemolyticus.

Establishment of a Swiss Webster mouse model of pneumonic plague to meet essential data elements under the animal rule

A recombinant vaccine (rF1V) is being developed for protection against pneumonic plague. This study was performed to address essential data elements to establish a well-characterized Swiss Webster mouse model for licensing the rF1V vaccine using the FDA's Animal Rule. These elements include the documentation of challenge material characteristics, aerosol exposure parameters, details of the onset and severity of clinical signs, pathophysiological response to disease, and relevance to human disease. Prior to animal exposures, an evaluation of the aerosol system was performed to determine and understand the variability of the aerosol exposure system. Standardized procedures for the preparation of Yersinia pestis challenge material also were developed. The 50% lethal dose (LD(50)) was estimated to be 1,966 CFU using Probit analysis. Following the LD(50) determination, pathology was evaluated by exposing mice to a target LD(99) (42,890 CFU). Mice were euthanized at 12, 24, 36, 48, 60, and 72 h postexposure. At each time point, samples were collected for clinical pathology, detection of bacteria in blood and tissues, and pathology evaluations. A general increase in incidence and severity of microscopic findings was observed in the lung, lymph nodes, spleen, and liver from 36 to 72 h postchallenge. Similarly, the incidence and severity of pneumonia increased throughout the study; however, some mice died in the absence of pneumonia, suggesting that disease progression does not require the development of pneumonia. Disease pathology in the Swiss Webster mouse is similar to that observed in humans, demonstrating the utility of this pneumonic plague model that can be used by researchers investigating plague countermeasures.

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.

Advanced Development of the rF1V and rBV A/B Vaccines: Progress and Challenges

The development of vaccines for microorganisms and bacterial toxins with the potential to be used as biowarfare and bioterrorism agents is an important component of the US biodefense program. DVC is developing two vaccines, one against inhalational exposure to botulinum neurotoxins A1 and B1 and a second for Yersinia pestis, with the ultimate goal of licensure by the FDA under the Animal Rule. Progress has been made in all technical areas, including manufacturing, nonclinical, and clinical development and testing of the vaccines, and in assay development. The current status of development of these vaccines, and remaining challenges are described in this chapter.

The role of immune correlates and surrogate markers in the development of vaccines and immunotherapies for plague

One of the difficulties in developing countermeasures to biothreat agents is the challenge inherent in demonstrating their efficacy in man. Since the first publication of the Animal Rule by the FDA, there has been increased discussion of potential correlates of protection in animal models and their use to establish surrogate markers of efficacy in man. The latter need to be relatively easy to measure in assays that are at least qualified, if not validated, in order to derive a quantitative assessment of the clinical benefit conferred. The demonstration of safety and clinical benefit is essential to achieve regulatory approval for countermeasures for which clinical efficacy cannot be tested directly, as is the case for example, for biodefence vaccines. 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 potential immune correlates of protection for plague, from which it may be possible to derive surrogate markers of efficacy, in order to predict the clinical efficacy of candidate prophylaxes and therapies.

Developing live vaccines against plague

Three great plague pandemics caused by the gram-negative bacterium Yersinia pestis have killed nearly 200 million people and it has been linked to biowarfare in the past. Plague is endemic in many parts of the world. In addition, the risk of plague as a bioweapon has prompted increased research to develop plague vaccines against this disease. Injectable subunit vaccines are being developed in the United States and United Kingdom.  However, the live attenuated Y. pestis-EV NIIEG strain has been used as a vaccine for more than 70 years in the former Soviet Union and in some parts of Asia and provides a high degree of efficacy against plague.  This vaccine has not gained general acceptance because of safety concerns.  In recent years, modern molecular biological techniques have been applied to Y. pestis to construct strains with specific defined mutations designed to create safe, immunogenic vaccines with potential for use in humans and as bait vaccines to reduce the load of Y. pestis in the environment.  In addition, a number of live, vectored vaccines have been reported using attenuated viral vectors or attenuated Salmonella strains to deliver plague antigens. Here we summarize the progress of live attenuated vaccines against plagu.

Two decades of plant-based candidate vaccines: a review of the chimeric protein approaches

Genetic engineering revolutionized the concept of traditional vaccines since subunit vaccines became reality. Additionally, over the past two decades plant-derived antigens have been studied as potential vaccines with several advantages, including low cost and convenient administration. More specifically, genetic fusions allowed the expression of fusion proteins carrying two or more components with the aim to elicit immune responses against different targets, including antigens from distinct pathogens or strains. This review aims to provide an update in the field of the production of plant-based vaccine, focusing on those approaches based on the production of chimeric proteins comprising antigens from human pathogens, emphasizing the case of cholera toxin/E. coli enterotoxin fusions, chimeric viruses like particles approaches as well as the possible use of adjuvant-producing plants as expression hosts. Challenges for the near future in this field are also discussed.

T cells play an essential role in anti-F1 mediated rapid protection against bubonic plague

Plague, which is initiated by Yersinia pestis infection, is a fatal disease that progresses rapidly and leads to high mortality rates if not treated. Antibiotics are an effective plague therapy, but antibiotic-resistant Y. pestis strains have been reported and therefore alternative countermeasures are needed. In the present study, we assessed the potential of an F1 plus LcrV-based vaccine to provide protection shortly pre- or post-exposure to a lethal Y. pestis infection. Mice vaccinated up to one day before or even several hours after subcutaneous challenge were effectively protected. Mice immunized one or three days pre-challenge were protected even though their anti-F1 and anti-LcrV titers were below detection levels at the day of challenge. Moreover, using B-cell deficient μMT mice, we found that rapidly induced protective immunity requires the integrity of the humoral immune system. Analysis of the individual contributions of vaccine components to protection revealed that rF1 is responsible for the observed rapid antibody-mediated immunity. Applying anti-F1 passive therapy in the mouse model of bubonic plague demonstrated that anti-F1 F(ab')(2) can delay mortality, but it cannot provide long-lasting protection, as do intact anti-F1 molecules. Fc-dependent immune components, such as the complement system and (to a lesser extent) neutrophils, were found to contribute to mouse survival. Interestingly, T cells but not B cells were found to be essential for the recovery of infected animals following passive anti-F1 mediated therapy. These data extend our understanding of the immune mechanisms required for the development of a rapid and effective post-exposure therapy against plague.