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Keywords:

  • efficacy;
  • immune correlates;
  • plague;
  • surrogate markers;
  • Yersinia pestis

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

The causative organism of plague is the bacterium Yersinia pestis. Advances in understanding the complex pathogenesis of plague infection have led to the identification of the F1- and V-antigens as key components of a next-generation vaccine for plague, which have the potential to be effective against all forms of the disease. Here we review the roles of F1- and V-antigens in the context of the range of virulence mechanisms deployed by Y. pestis, in order to develop a greater understanding of the protective immune responses required to protect against plague.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

Plague is an ancient disease which has caused millions of deaths in three great pandemics [1]. Plague is still endemic in parts of Asia, Africa and the Americas, causing approximately 3000 reported cases annually [2]. The disease is caused by the bacterium Yersinia pestis and is enzootic, being harboured by infected rodents, most notoriously the black rat. Man is an accidental host and can develop bubonic plague if bitten by an infected flea. Bubonic plague, if untreated, can develop into septicaemic plague or a secondary pneumonic plague. Alternatively, primary pneumonic plague can be contracted by inhalation of bacteria from an infected animal or human case. Either syndrome, if untreated, is potentially fatal.

To date, a number of vaccines for plague have been used in man. However, it is difficult to gain an objective view of their clinical efficacy, due to the sporadic nature of plague outbreaks and the hysteria which may surround them. Killed whole cell (KWC) vaccine formulations cannot be demonstrated to protect against experimental pneumonic plague [3-5], unlike a live attenuated EV76 vaccine. However, the efficacy data from the clinical use of the EV76 vaccine are more mixed [6]. Here we review the pathogenicity of Y. pestis, its ability to evade host innate immunity and how a growing understanding of the protective immune responses required to defeat this organism may lead to a licensable next-generation vaccine.

The Yersinia species and Y. pestis

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

Y. pestis is a Gram-negative, rod-shaped bacterium belonging to the Yersinia species within the Enterobacteriaceae family [1]. Since its discovery and isolation by Yersin in 1894 [7], this organism has been the focus of study and interest for many scientists and has become a paradigm of bacterial evolution. Y. pestis is thought to have evolved over the last 1500–20 000 years from the enteropathogen, Y. pseudotuberculosis [8], to become a flea-vectored pathogen lethal to man [9, 10]. The flea acquires Y. pestis from an infected rodent and the bacteria multiply in the mid-gut, extending eventually into the flea's oesophagus and proventriculus. The flea reaches a ‘blocked’ stage in which it can bite but cannot feed without first regurgitating the bacterial meal into a new host [9]. Additionally, in the context of the rapid spread of disease, it is thought that unblocked fleas can achieve early-phase transmission of bacteria to new hosts [11].

Y. pestis has been categorized as a facultative intracellular pathogen because on mammalian infection it gains entry to and grows in host macrophages, in which it is transported to draining lymph nodes [12]. On apoptosis of the infected macrophage, Y. pestis becomes extracellular, colonizing major organs and causing fatal systemic disease through the deployment of a range of virulence mechanisms and immune evasion strategies.

Expression of virulence factors and evasion of host innate immunity

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

In common with the enteropathogenic yersiniae, Y. pseudotuberculosis and Y. enterocolitica, Y. pestis possesses a 70 kb plasmid (pYV/pCD1) which encodes for the V-antigen and type III secretion system (T3S); unlike the other yersiniae, however, Y. pestis has also acquired a larger plasmid (100 kb) termed pFra/pMT1 and a small plasmid (9·5 kb) termed pPst/pPCP1/pPla [13-15]. These plasmids encode for a range of virulence factors which, collectively, further the survival and dissemination of Y. pestis in the mammalian host. Additionally, Y. pestis deploys a range of mechanisms to escape the host's innate defences.

The pFra/pMT1 plasmid encodes for two proteins, Fraction 1 antigen (F1-antigen) and a phospholipase D, known as murine toxin (MT), while the pPst/pPCP1/pPla plasmid encodes for a collection of proteins which facilitate the dissemination of insect-transmitted Y. pestis in the mammalian host [16-19]. These factors include pesticin, coagulase and plasminogen activator (PLA) and are involved collectively at the interface of successful transmission of Y. pestis by the flea (pesticin, coagulase) and the subsequent breaking-down of physical barriers (endothelial and cell membrane) in the host to achieve dissemination. PLA, a surface protease, is an essential virulence factor, exerting multiple effects in the host to achieve the successful dissemination of Y. pestis from peripheral infection sites by the cleavage of fibrin clots [20] through the activation of plasminogen to plasmin, and by facilitating adhesion to extracellular matrices and invasion of mammalian endothelial cells [21, 22]. When a Pla deletion mutant of Y. pestis was delivered intranasally to mice it established a local pulmonary infection, but did not disseminate from the lungs, due possibly to the entrapment of bacteria in fibrin clots [23]. Further, there is evidence that PLA inactivates innate defence mechanisms deployed by the host, such as the production of cationic anti-microbial peptides [24]. In contrast, and despite its name, the major virulence role ascribed to murine toxin is to protect Y. pestis bacteria in the flea gut from degradation by digestive enzymes [25].

F1-antigen is a capsule-associated protein which is expressed under the influence of the caf operon on the bacterial cell surface at 37°C [26]. Synthesized as a 15k Da monomer, it forms a large homopolymer (>200 kDa) on the bacterial cell surface in a stacked ring structure composed of heptamers [27]. The physical structure of F1-antigen alone was thought to deter phagocytosis of the bacteria by macrophages, thus protecting Y. pestis from the host's innate immune system [28]. However, it is now thought that F1-antigen inhibits bacterial adhesion to epithelial cells, thus assuming a role in bacterial transmission in addition to, or instead of, its anti-phagocytic effect [29-31]. Unlike PLA, however, F1-antigen is not strictly an essential virulence factor in Y. pestis, as F1-negative mutants with retained, if reduced, virulence exist in nature [32]. During its evolution to become a flea-vectored pathogen, Y. pestis has also lost or inactivated genes which are associated with its original enteropathogenic lifestyle; for example, yadA, Ifp and inv genes (involved with invasion and colonization) [32, 33], but which are no longer required or were actively detrimental in its new niches. The flea-vectored Y. pestis possesses genes which encode lipopolysaccharide (LPS), but those encoding associated O-antigens have been inactivated: such O antigen-deficient strains are often called ‘rough mutants’ [34]. Although rough mutants are considered traditionally to be attenuated due to their increased sensitivity to lysis by complement, plague is a fully virulent rough mutant and the deficiency in O-antigen allows PLA to compensate in providing protection at the bacterial surface from the host's innate immune system [34, 35]. Y. pestis also evades Toll-like receptor (TLR) activation by producing a tetra-acylated LPS which, unlike a the hexa-acylated form, is not recognized by TLR-4 receptors [36]. Additionally, and unlike Y. enterocolitica, Y. pestis is recognized only weakly by TLR-2 receptors [37], thus avoiding macrophage activation.

Y. pestis also produces surface fimbriae comprising an adhesin termed the pH 6 antigen, which is expressed from the genomic psa operon [38, 39]. Thought previously to have a role in the cell-to-cell transmission of Y. pestis in the mammalian host, pH 6 antigen is now thought to bind plasma apo-B lipoprotein (mainly low-density lipoproteins) to the bacterial cell surface [40]to provide an anti-phagocytic function [41, 42]. The pH 6 antigen strongly promotes adhesion to host cells. When the host cell adhesive properties of double- and single-deletion mutants for psa and caf in the pYV/pCD1-negative Y. pestis Kim6 were compared, the Psa+ phenotype was dominant and the anti-adhesive effect of F1-antigen was detectable only in the absence of Psa [31]. Furthermore, the multiplicity of virulence factors in Y. pestis was emphasized by the fact that the double-deletion mutant was still able to adhere to and invade human epithelial cells in vitro, revealing potential further undefined adhesins and invasins [31].

Ten other chaperone-usher systems similar to caf and psa and under genomic control have been described in the bacterium to confer surface proteins which may contribute to its virulence [43]. Y. pestis has retained the Ail adhesin, which is thought to confer resistance to the host's complement system by binding inhibitors of the lectin and classical pathways [44] and to be active in modulating host phagocytes in infected lymph nodes [45-47].

Why has Y. pestis become a lethal pathogen?

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

Uniquely among the Yersinia species, Y. pestis has evolved to become a lethal human pathogen. One of the key virulence factors involved in plague pathogenesis, the T3S encoded by pYV/pCD1, is also present in the enteropathogenic Yersiniae. As is the case in some other Gram-negative bacteria, such as those belonging to Escherichia, Salmonella, Shigella and Pseudomonas species, the T3S is a major contributory factor to virulence [48] and its loss results in severe attenuation. Possession of a T3S system, together with the other plasmid- and genome-encoded virulence factors described above and the strategies that Y. pestis has evolved to evade host innate immunity, have combined in influence to make Y. pestis a lethal pathogen [10].

Y. pestis was identified as the paradigm for T3S in the late 1990s [48]. At that time, it was known that Y. pestis produced a range of Yersinia outer proteins (Yops), among which the Virulence, or V-antigen, discovered many years previously [49, 50] and thought to be entirely intracellular, where it had a pivotal regulatory role in T3S, was also classed. The later observation of V-antigen on the bacterial surface [51] was unexpected. Subsequently, the role that surface-exposed V-antigen had in mediating host cell contact with a bacterial injectisome, through which anti-host and cytotoxic factors could be translocated into the host cell [48, 52], became clear when V-antigen was identified on the tip of the injectisome [53] in a proposed pentameric structure [54], and explained why V-antigen was an essential virulence factor.

Virulence factors as immunogens

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

Concurrent with the elucidation of the T3S system in Y. pestis, the F1- and V-antigens were identified as target proteins for inclusion in a subunit vaccine [4, 5]. Empirical observation had shown each to be immunogenic in its own right and to protect mice against challenge [4, 5, 55, 56] although, in free combination or as a genetic fusion, they are additive in effect. Furthermore, recent data has shown that the alum-adjuvanted F1+V vaccine can induce full protection in mice when given 1 day prior to subcutaneous infection with the virulent Y. pestis Kim 53 and still conferred 90% protection when given 3 days after infection [57].

Monoclonal antibodies (MAbs) to each protein conferred passive protection against whole organism challenge in naive mice, endorsing the prominence of F1- and V-antigens as protective immunogens [58]. Furthermore, at least one protective B cell epitope identified in V-antigen fitted into the proposed pentameric structure for V-antigen on the tip of the injectisome [59].

In addition to V- and F1-antigens, a range of virulence factors has been surveyed for their protective potential (Table 1). Of these, only Yersinia secretory factor F (YscF), which forms the stem of the injectisome [60], and some of the Yops, offer any positive effect as immunogens, in terms of a delayed time to death or a significant increase in the lethal dose, 50% (LD50) value in immunized mice [61-65]. Despite the clear role of Pla as a surface protease and essential virulence factor in Y. pestis, immunization with a DNA construct expressing PLA in vivo did not induce protection in BALB/c mice [66]. Of all the antigens tested, only F1 and V offer significant protection against the pneumonic model of plague infection. This is important, as the KWC vaccine formulations do not protect against inhalational exposure in experimental models [4, 55, 67] and cases of pneumonic plague have been reported in KWC-vaccinated individuals [68, 69].

Table 1. Key virulence factors from Yersinia pestis with potential as candidate subunit vaccines.
SubunitDescriptorProtective against
Y. pestis bubonic modelY. pestis pneumonic model
F1Fraction 1 capsular proteinProtective [4]Protective [96]
PlaPlasminogen activatorExtends time to death [23]Not done
pH 6 antigenFimbrial adhesinNot protective [86]Not done
Yersinia outer proteins   
VVirulence (V) antigenProtective [4]Protective [55]
YscFYersinia secretory factor F134-fold increase in LD50 in immunized mice against intravenous challenge [60]Not done
Yop DT3S-translocation YopPartially [62]Not done
YopET3S-cytotoxin effector YopNot protective [61, 62]Not done
Yop HT3S- PTPase effector YopNot protective [62]Not done
YpkAT3S-Ser/Thr kinase effector YopDelayed time to death [62]Not done
YopKT3S-regulates Yop release?Not protective [61, 62]Not done
YopMT3S-effector yopNot protective [95]Not done
YopNT3S-regulates Yop release?Not protective [61, 62]Not done

Evidence for the protective efficacy of F1 and V in animal models

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

The protective efficacy of F1 and V has been demonstrated in a range of animal models, which has been reviewed recently [70]. The range of species in which the V antigen or F1/V vaccine combination has been evaluated is constantly growing, and now includes data generated in Old World non-human primates [71-78], as evidence is collected to achieve regulatory approval. The immunogenicity and efficacy data reported to date from these models are summarized in Table 2. In order, however, to delineate the contribution that antibody and cellular immunity, respectively, are making to protective efficacy, it is useful to consider the results of passive transfer of immune sera, monoclonal antibodies or cells into a naive animal prior to challenge, to use knock-out mice lacking defined components of the cellular immune system and to use strategies to evaluate the induction of cellular memory for the vaccine components. These approaches are discussed below.

Table 2. Animal models reported to date for evaluation of F1/V vaccine.
SpeciesReferenceProtective-bubonicProtective-pneumonic
Mouse (many studies: representative studies here)   
Balb/c, C57Bl6, CBA, CB6F1[75]Not done
[96]
[97]
Swiss Webster[5]
Porton outbred[67] 
Guinea-pig[80] 
[98] 
Brown Norway rat[99] 
Black-footed ferret[100] 
Prairie dog[101]Not done
Cynomolgus macaque[71]Not done
[72]Not done
[74]Not done
[75]Not doneNot done
[76]Not done
[82]✓ by passive transfer. into miceNot done
[77]Not done
Rhesus macaque[78]Not done
African Green monkey[102]Not done
Baboons[73]Protective by passive transfer into miceNot done

Evidence for the protective effect of antibody in plague infection

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

There is now substantial evidence that antibody to F1 and V can protect animal models against Y. pestis infection. Most of these data have been generated in the mouse models of bubonic and pneumonic plague. Using the passive transfer of anti-F1 and anti-V monoclonals intratracheally into naive mice, for example, Hill et al. [58] demonstrated protection from subsequent aerosol exposure to Y. pestis. The passive transfer of polyclonal anti-sera to F1 and V, induced in a range of species such as the mouse, guinea-pig and macaque, was also able to protect naive mice following either systemic or aerosol exposure to Y. pestis [79-82]. Anti-sera to F1 and V derived from human clinical trial volunteers were also able to protect mice in a dose-dependent manner [83]. As a post-exposure therapy, the administration of polyclonal anti-F1 immunoglobulin (Ig)G at 24 h after infection of mice with Y. pestis was also fully protective [57]. However, passive protection with antibody to F1 in infected mice was reduced in C3−/– mice which lack a functional complement system, indicating the contribution of Fc-effector mechanisms to antibody-mediated protection [57].

Evidence for the role of cell-mediated immunity in protective efficacy against plague

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

While the passive transfer of antibody with specificity for the F1- or V-antigens can provide short-term protection against Y. pestis, the need for an accompanying cell-mediated immune (CMI) response is also apparent. Depletion of the proinflammatory cytokines tumour necrosis factor (TNF)-α and interferon (IFN)-γ prior to the passive transfer of the anti-V MAb7·3 into mice completely abrogated the protective effect observed in undepleted mice, indicating that a cellular proinflammatory response is also essential in protection against Y. pestis [84]. Similarly, anti-F1 IgG transferred passively into T cell receptor knock-out mice (TCR/) at 24 h after infection was unable to rescue them, whereas it fully protected μMT mice which lack functional B cells, indicating that T cells play a critical role in the protection mediated by antibody to F1-antigen [57].

Other evidence for the importance of the cellular response in protection against plague came from studies in knock-out mice in which μMT mice were demonstrated to be protected against subsequent challenge by the passive transfer of F1/V immune CD4+ T cells [85]. The transfer of F1/V immune CD4+ T cells into severe combined immunodeficient (SCID)/beige mice gave partial protection against subsequent Y. pestis challenge [86]. Signal transducer and activator of transcription (STAT)-4 knock-out mice [with a reduced T helper type 1 (Th1) response], when immunized with F1/V, were poorly protected against Y. pestis challenge, while STAT-6 knock-out mice with intact Th1 responses and diminished Th2 responses were fully protected [87]. Collectively, these data indicate the need for an F1/V-specific T cell response with a preferential Th1 polarization for protection against Y. pestis in the mouse model of infection.

T cell epitope mapping of F1 and V antigens

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

The identification of protective T cell epitopes would provide definitive evidence of the importance of a specific CMI response in protection against Y. pestis, and progress is being made towards mapping such epitopes.

Murine T cell epitopes for haplotypes H-2d and H-2k have been mapped onto the ribbon structure of F1 [88]. An H-2d (Balb/c) T cell epitope at amino acid positions 7–20 has been identified in the N-terminal sequence and was disrupted deliberately by engineering the F1 protein into a monomeric, circularly permutated structure, cpF1 [89]. To achieve cpF1, the N-terminal donor strand was excised and attached to the C-terminal sequence via a linker. When cpF1 was used to immunize mice it was found to be immunogenic but could no longer induce protective efficacy in Balb/c mice, although it remained a protective immunogen in C57Bl6 (H-2k mice). This study provided definitive evidence for a protective murine T cell epitope in the N-terminus of F1, which is H-2d-restricted [89].

Subsequently, human leucocyte antigen (HLA)-transgenic mice have been used to identify human T cell epitopes in the F1 and V antigens and an HLA DR1-restricted T cell epitope has been described in the C-terminal sequence (amino acids 141–160) of F1 [90]. The epitope mapping of V antigen is also ongoing in the mouse [91] as well as in HLA transgenic mice. If protective function can be ascribed to some of these epitopes, they hold great potential as target sequences against which to screen for recognition by ex-vivo T cells from human vaccinees.

Surrogate markers of efficacy

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

Ultimately, the aim is to achieve a safe and efficacious vaccine licensed for clinical use which protects comprehensively against all forms of plague. Naturally arising attenuated strains of Y. pestis, such as the EV series, have been used as vaccines and rationally attenuated strains have also been proposed as vaccine candidates [86]; however, the identification of a protective subunit vaccine represents a safer option for clinical use. To this end, the rF1/V candidate is in advanced development, with Phase II clinical trials in progress [92]. Because a direct demonstration of efficacy in the clinic will not be possible for ethical and practical reasons, the vaccine will be licensed using the Food and Drug Administration's Animal Rule [93] and based on surrogate markers of efficacy reported in clinical trial volunteers. Based on what we know of the complex immune responses required for protection, these surrogate markers are expected to constitute a range of biomarkers indicating the development of functional serological and cellular immune responses to the vaccine [92]. Potential biomarkers which correlate with protection (so-called immune correlates) have been reviewed recently [94], and as the vaccine progresses through regulatory approval, a demonstration of correlation between measurable immune parameters and protection will assume increasing importance.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References

In conclusion, Y. pestis is a complex organism which has evolved into a serious human pathogen with a range of potent virulence mechanisms. Great strides have been made in understanding the pathogenesis of Y. pestis and the epidemiology of plague which, in turn, have facilitated the development of vaccines, leading to current recombinant candidates in advanced development. There is now a prospect of licensing a recombinant safe, subunit vaccine which, for the first time, is expected to protect against all forms of the disease, including pneumonic plague. Licensure for clinical use of the vaccine will depend upon the satisfactory identification of immune correlates of protection and the derivation of measurable surrogate markers of efficacy, as discussed above.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. The Yersinia species and Y. pestis
  5. Expression of virulence factors and evasion of host innate immunity
  6. Why has Y. pestis become a lethal pathogen?
  7. Virulence factors as immunogens
  8. Evidence for the protective efficacy of F1 and V in animal models
  9. Evidence for the protective effect of antibody in plague infection
  10. Evidence for the role of cell-mediated immunity in protective efficacy against plague
  11. T cell epitope mapping of F1 and V antigens
  12. Surrogate markers of efficacy
  13. Conclusions
  14. Acknowledgements
  15. Disclosure
  16. References