• Vaccine Name: rF1 + rV
• Target Pathogen: Yersinia pestis
• Target Disease: Plague
• Vaccine Ontology ID: VO_0000830
• Type: Subunit vaccine
• Adjuvant:
  • Adjuvant name:
  • VO adjuvant ID: VO_0001241
  • Description: This vaccine utilizes a combination of the Fraction 1 antigen (F1) and the V antigen of Y. pestis in an optimum molar ratio. The F1 and V antigens, in recombinant form in this vaccine, are natural virulence factors of Y. pestis. Fraction 1 antigen, the major protein component of the capsule surrounding Y. pestis cells, is expressed only at 37 °C and it is believed to have anti-phagocytic activity. High anti-F1 titres have been correlated with survival following plague infection. Recombinant F1 antigen (rF1) has been produced by cloning the caf operon from Y. pestis into Escherichia coli and the protection provided by highly purified native F1 and recombinant F1 against Y. pestis has been demonstrated not to differ. The V antigen of Y. pestis is a secreted protein that is thought to act both as a regulatory protein and as a virulence factor. The V antigen has a key role in the Type III secretion process utilised by Y. pestis to translocate cytotoxic and anti-phagocytic Yersinia outer proteins (Yops) into the host cell. Supporting evidence for this role has been gained and the V antigen has been visualised on the bacterial cell surface (Jones et al., 2003).
    Both the rF1 and rV proteins, administered in alhydrogel, have been demonstrated to be highly immunogenic and protective against virulent plague in a number of animal models: mice, guinea pigs, and cynomolgus macaques (unpublished data). Further, the combination of rF1 plus rV is additive in the protection conferred on the vaccinee. In the mouse, the combined immunoglobulin G1 (IgG1) titer to rF1 plus rV has been shown to correlate with protection against challenge. Further, protection against plague in the mouse has been demonstrated by the passive transfer of antiserum specific for rF1 plus rV from immunized BALB/c mice into naïve SCID/beige mice (Williamson et al., 2005).
• Preparation: The rF1 and rV antigens were produced in Escherichia coli from the expression systems previously described, under Good Manufacturing Practice conditions. The vaccine was formulated by adsorption to 20% (vol/vol) adjuvant at the required concentrations of each protein such that concentrations in the range 10 µg rF1 + 10 µg rV per ml up to 80 µg rF1 + 80 µg rV per ml were achieved in a final concentration of 0.26% (wt/vol) alhydrogel to achieve a molar ratio for rF1 to rV of 2:1. The formulated vaccine was designated rYP002 (Williamson et al., 2005).
• Virulence:
• Description: This vaccine utilizes a combination of the Fraction 1 antigen (F1) and the V antigen of Y. pestis in an optimum molar ratio. The F1 and V antigens, in recombinant form in this vaccine, are natural virulence factors of Y. pestis. Fraction 1 antigen, the major protein component of the capsule surrounding Y. pestis cells, is expressed only at 37 °C and it is believed to have anti-phagocytic activity. High anti-F1 titres have been correlated with survival following plague infection. Recombinant F1 antigen (rF1) has been produced by cloning the caf operon from Y. pestis into Escherichia coli and the protection provided by highly purified native F1 and recombinant F1 against Y. pestis has been demonstrated not to differ. The V antigen of Y. pestis is a secreted protein that is thought to act both as a regulatory protein and as a virulence factor. The V antigen has a key role in the Type III secretion process utilised by Y. pestis to translocate cytotoxic and anti-phagocytic Yersinia outer proteins (Yops) into the host cell. Supporting evidence for this role has been gained and the V antigen has been visualised on the bacterial cell surface (Jones et al., 2003).
  Both the rF1 and rV proteins, administered in alhydrogel, have been demonstrated to be highly immunogenic and protective against virulent plague in a number of animal models: mice, guinea pigs, and cynomolgus macaques (unpublished data). Further, the combination of rF1 plus rV is additive in the protection conferred on the vaccinee. In the mouse, the combined immunoglobulin G1 (IgG1) titer to rF1 plus rV has been shown to correlate with protection against challenge. Further, protection against plague in the mouse has been demonstrated by the passive transfer of antiserum specific for rF1 plus rV from immunized BALB/c mice into naïve SCID/beige mice (Williamson et al., 2005).

Guinea pig Response

• Host Strain: Six- to eight-week-old Dunkin Hartley female guinea pigs (Charles River Laboratories, Margate, Kent, UK).
• Vaccination Protocol: Guinea pigs (18, divided into three groups of six) were immunised with 50 μg rF1+50 μg rV adsorbed to 25% (v/v) Alhydrogel in 0.3 ml PBS. Individual guinea pigs were randomly selected and used as untreated controls for the challenge and immune response analysis. Guinea pigs were boosted at day 21, exactly as for the priming immunisation.
  In order to raise antiserum for the passive immunisation of mice, a further group of 6 guinea pigs was immunised with 50 μg rF1+50 μg rV in 0.3 ml of 25% (v/v) Alhydrogel in PBS on days 1, 21 and 115. Additional groups of six guinea pigs were immunised with either 0.3 ml USP KWCV or the same volume of USP KWCV supplemented with 50 μg of rV antigen. These animals also received boosting immunisations on days 21 and 115.
  Three groups of six guinea pigs immunised with rF1+rV vaccine were challenged subcutaneously on day 90 with 0.1 ml aliquots of Y. pestis strain GB at various cell densities between 105 and 108 colony-forming units (CFU)/ml. A naïve control group of six guinea pigs was also challenged. The passively immunised Balb/c mice were challenged subcutaneously with either 10 or 1000 MLD Y. pestis strain GB and were observed for 8 days post-challenge (Jones et al., 2003).
• Persistence: Immunised guinea pigs were challenged at day 90 and were fully protected at the lowest challenge dose (105 CFU), surviving for 26 days post-challenge. However, there was loss of protection as the challenge dose was increased so that five of six animals were protected against 106 CFU and only three of six against 107 CFU. All of the naïve control animals had succumbed to a challenge of 104 CFU (Jones et al., 2003).
• Side Effects: Buboe development occurred although protection against challenge was achieved in rF1+rV-immunised guinea pigs (Jones et al., 2003).
• Efficacy: 50% of the guinea pigs had responded strongly to rF1 after two doses of vaccine and the remainder had responded less well. While at the lowest challenge level, the IgG response to F1 was adequate to protect; it may be that it was insufficient against the highest challenge level, so that 50% of the animals succumbed to infection. Too high and too frequent dosing with F1 antigen or whole killed plague bacilli in the guinea pig is not efficacious (Jones et al., 2003).
• Description: The efficacy of the rF1+rV vaccine in protecting guinea pigs against subcutaneous challenge with a virulent Y. pestis strain was assessed and compared with that of the USP KWCV. Previously, deficiencies in efficacy of KWCV formulations have been attributed to the lack of V antigen in these formulations and so the effect on the protective efficacy of the KWCV of supplementing it with rV antigen has been determined by passive transfer of immune guinea pig sera into naive mice with subsequent challenge of the mice with a virulent plague strain (Jones et al., 2003).

Mouse Response

• Host Strain: Six to eight week-old male and female BALB/c, CBA, CB6F1 and C57BL mice, raised under specific-pathogen-free conditions (Charles River Laboratories, Margate, Kent, UK).
• Vaccination Protocol: Mice were divided into groups of 10 for immunisation that consisted of a total of 0.1 cm^3 primary immunising dose of 10 μg of rF1 and 10 μg of rV adsorbed to 25% v/v Alhydrogel in phosphate-buffered saline. The immunising dose was equally divided between intramuscular sites in each hind leg. Individuals were randomly selected from each strain and used as untreated controls for the challenge and immune response analysis. At day 21 or 28 mice were boosted receiving doses as described above (Jones et al., 2000).
• Persistence: All strains of mice were protected against challenge with Y. pestis GB following immunisation. Male CBA and CB6F1 mice were less well protected against s.c. challenge than either the BALB/c or C57BL6. Nevertheless, the mean time to death for CBA following s.c. challenge was 13.5 days at 4.9×10^7 CFU and 14 days at 4.9×10^5 CFU; significantly delayed compared with the 3.1 days mean time to death seen in unimmunised control mice. A similar difference was apparent in CB6F1 mice and for both strains following aerosol challenge. Mortalities in both the CBA and CB6F1 mice after aerosol challenge were dose-independent. All survivors tested for Y. pestis after 15 days were found to have no bacteria in the spleen, blood, lung or liver (Jones et al., 2000).
• Side Effects: No side effects noted.
• Efficacy: The recombinant vaccine is capable of inducing protective immunity in all four strains of mice representing H-2b, H-2d, H-2k, haplotypes and a H-2b/H-2d F1 hybrid. Although breakthrough in protection was observed in male mice, both sexes raised good antibody responses to the vaccine and these were maintained for greater than 1 year in the female mice.
  The challenge data from the first trial suggested that CBA and CB6F1 males were less well protected than female mice but the BALB/c and C57BL6 data showed males of those strains were as well protected as females (Jones et al., 2000).
  
• Description: Significant differences in the ability to develop an antibody response to the foreign antigen were observed between various strains of mouse depending on their H-2 genes and were related to the ability of each strain of mouse to raise antibodies after immunisation with low doses of antigen. To investigate the possibility that potent immune responses seen in BALB/c female mice were a function of their H-2 haplotype, this study immunised males and female of four strains of mice each with different haplotypes; BALB/c (H-2d), CBA (H-2k). C57BL6 (H-2b) and CB6F1 (the F1 hybrid from a cross between BALB/c and C57BL6). CB6F1 mice co-express both the H-2d and H-2b forms of the MHC molecule (Jones et al., 2000) .

Human Response

• Host Strain: Healthy adult males.
• Vaccination Protocol: Twenty-four healthy adult males were studied in a double-blind, ascending-dose design, such that groups of six individuals received the vaccine at dose levels of 5 µg,10 µg, 20 µg, and 40 µg of each subunit in a dose volume of 0.5 ml; however, data was reported for only 20 of the 24. Attached to each dose group were two individuals who were administered placebo (alhydrogel in PBS). The vaccine or placebo was administered to individuals in a two-dose intramuscular regimen, with the priming dose on day 1 and the booster dose on day 21. For any one individual, the dose level given for priming and boosting was identical. Equal aliquots of individual serum samples, collected at day 35 from volunteers in all the vaccine dose groups, were pooled by dose group prior to dilution in PBS. The pooled samples were used to passively immunize groups of five BALB/c female mice (Charles River United Kingdom) intraperitoneally (Williamson et al., 2005).
• Persistence: Antibodies to rV were produced within two weeks of the first dose at day 1, with greater levels observed after the booster dose at day 21. Across the dose range studied, at least 50% of the maximum mean anti-rV titer was retained for up to 3 months following the first dose. Antibodies to rF1 were also generated within two weeks of the first dose at day 1, with greater levels observed after the booster dose at day 21. Across the dose range studied, at least 50% of the maximum mean anti-rF1 titer was retained for up to 3 months following the first dose (Williamson et al., 2005).
• Immune Response: The number of individuals determined to have a titer of competing antibody increased to a maximum of 18/20 at day 28, seven days after administration of the second dose. The data gained indicate that the vaccine was immunogenic in recipients at all dose levels tested, although data from only three subjects were available at the 20-µg dose level and the lowest dose level (5 µg) was suboptimal.The titer of specific IgG developed by individuals at 21 and 28 days post-initial vaccination and 7 days postboost correlated with the development of a titer of antibody competing for binding to the rV antigen and with transferable protective immunity (Williamson et al., 2005).
• Side Effects: No side effects noted.
• Description: This is a Phase I safety and immunogenicity trial in healthy volunteers.