Plague immunization. IV. Clinical reactions and serologic response to inoculations of Haffkine and freeze-dried plague vaccine

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.

Intranasal administration of an inactivated Yersinia pestis vaccine with interleukin-12 generates protective immunity against pneumonic plague

Inhalation of Yersinia pestis causes pneumonic plague, which rapidly progresses to death. A previously licensed killed whole-cell vaccine is presently unavailable due to its reactogenicity and inconclusive evidence of efficacy. The present study now shows that vaccination intranasally (i.n.) with inactivated Y. pestis CO92 (iYp) adjuvanted with interleukin-12 (IL-12) followed by an i.n. challenge with a lethal dose of Y. pestis CO92 prevented bacterial colonization and protected 100% of mice from pneumonic plague. Survival of the vaccinated mice correlated with levels of systemic and lung antibodies, reduced pulmonary pathology and proinflammatory cytokines, and the presence of lung lymphoid cell aggregates. Protection against pneumonic plague was partially dependent upon Fc receptors and could be transferred to naïve mice with immune mouse serum. On the other hand, protection was not dependent upon complement, and following vaccination, depletion of CD4 and/or CD8 T cells before challenge did not affect survival. In summary, the results demonstrate the safety, immunogenicity, and protective efficacy of i.n. administered iYp plus IL-12 in a mouse model of pneumonic plague.

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

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

Milestones in progression of primary pneumonic plague in cynomolgus macaques

Vaccines against primary pneumonic plague, a potential bioweapon, must be tested for efficacy in well-characterized nonhuman primate models. Telemetered cynomolgus macaques (Macaca fascicularis) were challenged by the aerosol route with doses equivalent to approximately 100 50% effective doses of Yersinia pestis strain CO92 and necropsied at 24-h intervals postexposure (p.e.). Data for telemetered heart rates, respiratory rates, and increases in the temperature greater than the diurnal baseline values identified the onset of the systemic response at 55 to 60 h p.e. in all animals observed for at least 70 h p.e. Bacteremia was detected at 72 h p.e. by a Yersinia 16S rRNA-specific quantitative reverse transcription-PCR and was detected later by the culture method at the time of moribund necropsy. By 72 h p.e. multilobar pneumonia with diffuse septal inflammation consistent with early bacteremia was established, and all lung tissues had a high bacterial burden. The levels of cytokines or chemokines in serum were not significantly elevated at any time, and only the interleukin-1beta, CCL2, and CCL3 levels were elevated in lung tissue. Inhalational plague in the cynomolgus macaque inoculated by the aerosol route produces most clinical features of the human disease, and in addition the disease progression mimics the disease progression from the anti-inflammatory phase to the proinflammatory phase described for the murine model. Defined milestones of disease progression, particularly the onset of fever, tachypnea, and bacteremia, should be useful for evaluating the efficacy of candidate vaccines.

Prevention of bubonic and pneumonic plague using plant-derived vaccines

Yersinia pestis, the causative agent of bubonic and pneumonic plague, is an extremely virulent bacterium but there are currently no approved vaccines for protection against this organism. Plants represent an economical and safer alternative to fermentation-based expression systems for the production of therapeutic proteins. The recombinant plague vaccine candidates produced in plants are based on the two most immunogenic antigens of Y. pestis: the fraction-1 capsular antigen (F1) and the low calcium response virulent antigen (V) either in combination or as a fusion protein (F1-V). These antigens have been expressed in plants using all three known possible strategies: nuclear transformation, chloroplast transformation and plant-virus-based expression vectors. These plant-derived plague vaccine candidates were successfully tested in animal models using parenteral, oral, or prime/boost immunization regimens. This review focuses on the recent research accomplishments towards the development of safe and effective pneumonic and bubonic plague vaccines using plants as bioreactors.

Yersinia pestis caf1 variants and the limits of plague vaccine protection

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

Effect of deletion of the lpxM gene on virulence and vaccine potential of Yersinia pestis in mice

Yersinia pestis undergoes an obligate flea-rodent-flea enzootic life cycle. The rapidly fatal properties of Y. pestis are responsible for the organism's sustained survival in natural plague foci. Lipopolysaccharide (LPS) plays several roles in Y. pestis pathogenesis, prominent among them being resistance to host immune effectors and induction of a septic-shock state during the terminal phases of infection. LPS is acylated with 4-6 fatty acids, the number varying with growth temperature and affecting the molecule's toxic properties. Y. pestis mutants were constructed with a deletion insertion in the lpxM gene in both virulent and attenuated strains, preventing the organisms from synthesizing the most toxic hexa-acylated lipid A molecule when grown at 25 degrees C. The virulence and/or protective potency of pathogenic and attenuated Y. pestis DeltalpxM mutants were then examined in a mouse model. The DeltalpxM mutation in a virulent strain led to no change in the LD(50) value compared to that of the parental strain, while the DeltalpxM mutation in attenuated strains led to a modest 2.5-16-fold reduction in virulence. LPS preparations containing fully hexa-acylated lipid A were ten times more toxic in actinomycin D-treated mice then preparations lacking this lipid A isoform, although this was not significant (P>0.05). The DeltalpxM mutation in vaccine strain EV caused a significant increase in its protective potency. These studies suggest there is little impact from lipid A modifications on the virulence of Y. pestis strains but there are potential improvements in the protective properties in attenuated vaccine strains.

Oral vaccination with salmonella simultaneously expressing Yersinia pestis F1 and V antigens protects against bubonic and pneumonic plague

The gut provides a large area for immunization enabling the development of mucosal and systemic Ab responses. To test whether the protective Ags to Yersinia pestis can be orally delivered, the Y. pestis caf1 operon, encoding the F1-Ag and virulence Ag (V-Ag) were cloned into attenuated Salmonella vaccine vectors. F1-Ag expression was controlled under a promoter from the caf1 operon; two different promoters (P), PtetA in pV3, PphoP in pV4, as well as a chimera of the two in pV55 were tested. F1-Ag was amply expressed; the chimera in the pV55 showed the best V-Ag expression. Oral immunization with Salmonella-F1 elicited elevated secretory (S)-IgA and serum IgG titers, and Salmonella-V-Ag(pV55) elicited much greater S-IgA and serum IgG Ab titers than Salmonella-V-Ag(pV3) or Salmonella-V-Ag(pV4). Hence, a new Salmonella vaccine, Salmonella-(F1+V)Ags, made with a single plasmid containing the caf1 operon and the chimeric promoter for V-Ag allowed the simultaneous expression of F1 capsule and V-Ag. Salmonella-(F1+V)Ags elicited elevated Ab titers similar to their monotypic derivatives. For bubonic plague, mice dosed with Salmonella-(F1+V)Ags and Salmonella-F1-Ag showed similar efficacy (>83% survival) against approximately 1000 LD(50) Y. pestis. For pneumonic plague, immunized mice required immunity to both F1- and V-Ags because the mice vaccinated with Salmonella-(F1+V)Ags protected against 100 LD(50) Y. pestis. These results show that a single Salmonella vaccine can deliver both F1- and V-Ags to effect both systemic and mucosal immune protection against Y. pestis.

Immunogenicity of the rF1+rV vaccine for plague with identification of potential immune correlates

The rF1+rV candidate sub-unit vaccine for plague, formulated by adsorption to alhydrogel, has been demonstrated to be immunogenic in the cynomolgus macaque in a clinically relevant dose-range (5-40 microg of each sub-unit) and regimen. Following two doses of vaccine, a specific IgG titre developed in a dose-related manner with predominance of the IgG1/IgG2 isotypes. Groups of macaques receiving only a single dose of vaccine at the 40 microg dose-level had a significantly reduced peak IgG response and faster decline to baseline. Serum collected at week 5 from 19 immunised animals competed with and displaced murine Mab7.3 from binding to the V antigen in vitro. By week 53 of the schedule, although absolute IgG titres had declined, 17/19 macaque sera tested contained competing antibody, indicating the durability of a functional immune response to rF1+rV in this species. Thirteen of these week 53 sera were passively transferred into groups of naive mice, and all conferred full or partial protection against subsequent challenge of the mice with plague. Generally, those sera which were most competitive with Mab 7.3 for binding to V antigen were fully protective by passive transfer, although one week-53 serum sample was fully protective by passive transfer but not active by competitive ELISA. The early development of protective immunity in macaques was also indicated from the protection conferred on naive mice by the passive transfer of immune macaque serum collected at 2-10 weeks of the immunisation schedule. Serum samples from representative macaques within this time period also inhibited the Yersinia-mediated cytotoxicity of J774 macrophages in a qualitative in vitro assay of type three secretion.

LcrV plague vaccine with altered immunomodulatory properties

Yersinia pestis, the causative agent of plague, secretes LcrV (low-calcium-response V or V antigen) during infection. LcrV triggers the release of interleukin 10 (IL-10) by host immune cells and suppresses proinflammatory cytokines such as tumor necrosis factor alpha and gamma interferon as well as innate defense mechanisms required to combat the pathogenesis of plague. Although immunization of animals with LcrV elicits protective immunity, the associated suppression of host defense mechanisms may preclude the use of LcrV as a human vaccine. Here we show that short deletions within LcrV can reduce its immune modulatory properties. An LcrV variant lacking amino acid residues 271 to 300 (rV10) elicited immune responses that protected mice against a lethal challenge with Y. pestis. Compared to full-length LcrV, rV10 displayed a reduced ability to release IL-10 from mouse and human macrophages. Furthermore, the lipopolysaccharide-stimulated release of proinflammatory cytokines by human or mouse macrophages was inhibited by full-length LcrV but not by the rV10 variant. Thus, it appears that LcrV variants with reduced immune modulatory properties could be used as a human vaccine to generate protective immunity against plague.

Human immune response to a plague vaccine comprising recombinant F1 and V antigens

The human immune response to a new recombinant plague vaccine, comprising recombinant F1 (rF1) and rV antigens, has been assessed during a phase 1 safety and immunogenicity trial in healthy volunteers. All the subjects produced specific immunoglobulin G (IgG) in serum after the priming dose, which peaked in value after the booster dose (day 21), with the exception of one individual in the lowest dose level group, who responded to rF1 only. Three subjects, found to have an anti-rV titer at screening, were excluded from the overall analysis. Human antibody functionality has been assessed by quantification of antibody competing for binding to rV in vitro and also by the transfer of protective immunity in human serum into the naive mouse. Human and macaque IgG competed for binding to rV in vitro with a mouse monoclonal antibody, previously shown to protect mice against challenge with plague, suggesting that this protective B-cell epitope on rV is conserved between these three species. Total IgG to rV in individuals and the titer of IgG competing for binding to rV correlated significantly at days 21 (r = 0.72; P < 0.001) and 28 (r = 0.82; P < 0.001). Passive transfer of protective immunity into mice also correlated significantly with total IgG titer to rF1 plus rV at days 21 (r(2) = 98.6%; P < 0.001) and 28 (r(2) = 76.8%; P < 0.03). However, no significant vaccination-related change in activation of peripheral blood mononuclear cells was detected at any time. Potential serological immune correlates of protection have been investigated, but no trends specific to vaccination could be detected in cellular markers.

Plague

Plague is a disease that has been present for thousands of years and described since the earliest medical accounts. It occurs today worldwide, and may present in a variety of clinical forms. Bubonic disease, pneumonic plague, and septicemic plague are seen in addition to a number of other less common manifestations. As an agent of bioterrorism,Yersinia pestis could pose an extreme threat if released in the appropriate form and in the appropriate environment. Presumptive diagnosis may be made with readily available techniques, but laboratory handling of specimens requires special care. When there is a strong suspicion of plague, treatment should be instituted immediately, as delaying therapy will result in increased morbidity and mortality.

Protective efficacy of a fully recombinant plague vaccine in the guinea pig

A fully recombinant sub-unit vaccine comprising the protein antigens rF1 + rV has been demonstrated to protect immunised guinea pigs against exposure to 10(5) colony-forming units (CFU) of virulent Yersinia pestis. Additionally, IgG purified from rF1 + rV-immunised guinea pig serum, protected the mouse by passive immunisation against challenge with Y. pestis whereas IgG purified from the serum of guinea pigs immunised with a licensed killed whole cell (KWC) vaccine for plague, protected less well. Guinea pigs immunised with the licensed killed whole cell vaccine developed an IgG titre for fraction 1 (F1) but not for V antigen. The differential in protection conferred on the mouse by passive immunisation with guinea pig IgG, was abrogated by the use of IgG purified from guinea pigs immunised with killed whole cell vaccine supplemented with V antigen. These findings indicate that the reduced efficacy of the licensed killed whole cell vaccine formulation previously observed in the mouse can be attributed to lack of the V antigen. Cross-protection of the mouse with guinea pig IgG suggests that the recognition of neutralising epitopes in the F1 and V proteins is conserved between these two species.

Protection conferred by a fully recombinant sub-unit vaccine against Yersinia pestis in male and female mice of four inbred strains

In this paper, we describe for the first time the use of a fully recombinant sub-unit vaccine for plague. We have compared the protection afforded by the recombinant vaccine against Yersinia pestis in male and female mice of four inbred strains. We also determined the in vivo cellular memory and antibody response after one year. The recombinant vaccine was capable of inducing protective immunity, against subcutaneous and aerosol challenge, in mice from all four strains. Although, there was some breakthrough in the CBA males challenged with 10(7) median lethal dose (MLDs) the other mice regardless of sex or strain were well protected even at this extreme challenge dose. In both male and female mice, the specific IgG titres to both antigens peaked at day 28 and 35 and in female mice these titres were maintained for >1 year.

Vaccines for preventing plague

BACKGROUND

Plague is endemic in China, Mongolia, Burma, Vietnam, Indonesia, India, large parts of Southern Africa, the United States and South America. There are three types of vaccines (live attenuated, killed and F1 fraction) with varying means of administration.

OBJECTIVES

The objective of this review was to assess the effects of vaccines to prevent plague.

SEARCH STRATEGY

We searched Medline, Embase, the Cochrane Controlled Trials Register and reference lists of articles. We handsearched the journal 'Vaccine' and contacted experts in the field.

SELECTION CRITERIA

Randomised trials comparing live and killed plague vaccines against no intervention, placebo, other plague vaccines or vaccines against other disease (control vaccines).

DATA COLLECTION AND ANALYSIS

Three reviewers assessed the eligibility of trials.

MAIN RESULTS

No trials were included.

REVIEWER'S CONCLUSIONS

There is not enough evidence to evaluate the effectiveness of any plague vaccine, or the relative effectiveness between vaccines and their tolerability. Circumstantial data from observational studies suggest that killed types may be more effective and have fewer adverse effects than attenuated types of vaccine. No evidence appears to exist on the long-term effects of any plague vaccine.

Comparison of the immunological and protective responses elicited by microencapsulated formulations of the F1 antigen from Yersinia pestis

Purified native F1 antigen from Yersinia pestis was used to assess controlled-release vaccine delivery systems in poly(lactide-co-glycolide) (PLG) microparticles and liposomes. Antigen encapsulated in PLG microparticles induced high serum titres when injected i.p. in mice: mucosal IgA was also detected. Mice immunized with F1 in Alhydrogel or PLGs were protected against subcutaneous challenge with Y. pestis. F1 antigen surface-labelled onto liposome vesicles stimulated high serum titres in Balb/c mice and also induced a mucosal response: F1-labelled liposomes protected mice against challenge with up to 1 x 10(5) organisms. These findings indicate that a significant immune response is induced by immunizing with F1 formulated in PLGs and liposomes and that protection was achieved after only one dose.

A comparison of Plague vaccine, USP and EV76 vaccine induced protection against Yersinia pestis in a murine model

The median lethal dose (MLD) of a pathogenic strain of Yersinia pestis was established by three routes of administration in three strains of mouse. There was no significant difference between the MLDs in the different strains of mouse. The MLD by the subcutaneous route in Balb/C and an outbred line was approximately 1 c.f.u.; the MLD following intraperitoneal administration was tenfold higher. There were significant differences in the mean times to death after administration of the challenge by different routes. The relative efficacy of a live attenuated vaccine strain of Y. pestis (EV76) was compared with that of the formaldehyde-killed vaccine (Plague vaccine, USP). EV76 protected against high challenge doses (up to 5.75 x 10(6) MLD), though immunized animals showed side effects of varying severity. The killed vaccine was less effective in terms of dose-protection (deaths occurred after challenge with 4000 MLD) and several of the vaccinated animals suffered sub-lethal, plague-related sequelae to the challenge.