Biosafety aspects of the recombinant live oral Vibrio cholerae vaccine strain CVD 103-HgR

Immunogenicity and protective efficacy of a live, oral cholera vaccine formulation stored outside-the-cold-chain for 140 days

Background

Cholera, an acute watery diarrhoeal disease caused by Vibrio cholerae serogroup O1 and O139 across the continents. Replacing the existing WHO licensed killed multiple-dose oral cholera vaccines that demand ‘cold chain supply’ at 2–8 °C with a live, single-dose and cold chain-free vaccine would relieve the significant bottlenecks and cost determinants in cholera vaccination campaigns. In this direction, a prototype cold chain-free live attenuated cholera vaccine formulation (LACV) was developed against the toxigenic wild-type (WT) V. cholerae O139 serogroup. LACV was found stable and retained its viability (5 × 10⁶ CFU/mL), purity and potency at room temperature (25 °C ± 2 °C, and 60% ± 5% relative humidity) for 140 days in contrast to all the existing WHO licensed cold-chain supply (2–8 °C) dependent killed oral cholera vaccines.

Results

The LACV was evaluated for its colonization potential, reactogenicity, immunogenicity and protective efficacy in animal models after its storage at room temperature for 140 days. In suckling mice colonization assay, the LACV recorded the highest recovery of (7.2 × 10⁷ CFU/mL) compared to those of unformulated VCUSM14P (5.6 × 10⁷ CFU/mL) and the WT O139 strain (3.5 × 10⁷ CFU/mL). The LACV showed no reactogenicity even at an inoculation dose of 10⁴–10⁶ CFU/mL in a rabbit ileal loop model. The rabbits vaccinated with the LACV or unformulated VCUSM14P survived a challenge with WT O139 and showed no signs of diarrhoea or death in the reversible intestinal tie adult rabbit diarrhoea (RITARD) model. Vaccinated rabbits recorded a 275-fold increase in anti-CT IgG and a 15-fold increase in anti-CT IgA antibodies compared to those of rabbits vaccinated with unformulated VCUSM14P. Vibriocidal antibodies were increased by 31-fold with the LACV and 14-fold with unformulated VCUSM14P.

Conclusion

The vaccine formulation mimics a natural infection, is non-reactogenic and highly immunogenic in vivo and protects animals from lethal wild-type V. cholerae O139 challenge. The single dose LACV formulation was found to be stable at room temperature (25 ± 2 °C) for 140 days and it would result in significant cost savings during mass cholera vaccination campaigns.

Safety and immunogenicity of single-dose live oral cholera vaccine strain CVD 103-HgR in healthy adults age 18-45

The attenuated recombinant Vibrio cholerae O1 vaccine strain CVD 103-HgR, re-developed as PXVX0200, elicits a rapid serum vibriocidal antibody (SVA) response and protects against cholera diarrhea in volunteer challenge studies. We performed a phase 3, placebo controlled, double blind, multi-center study to further assess the safety, immunogenicity, and lot-to-lot consistency of PXVX0200. Adult volunteers 18-45 years of age were randomized 8:1 to receive a single dose of 1 × 10⁹ CFU of PXVX0200 from three production lots or saline placebo. Immunogenicity endpoints included SVA and anti-cholera toxin (CT) antibody levels on days 1, 11, 29, 91 and 181. Safety was assessed by comparing solicited signs and symptoms on days 1-8, unsolicited adverse events through day 29 and serious adverse events through day 181. A total of 3146 participants were enrolled, including 2795 vaccine and 351 placebo recipients. The SVA seroconversion rates at day 11 were 94% and 4% in the PXVX0200 and placebo recipients, respectively (P < .0001). Cumulative SVA seroconversion occurred among 96% of vaccine recipients. PXVX0200 SVA GMTs peaked on day 11 and remained significantly higher than placebo through day 181 while the fold-rise over baseline in PXVX0200 anti-CT antibody was significantly greater than placebo at every post-vaccination time point. Most reactogenicity was mild and resolved within 1-3 days with headache and diarrhea more frequently reported in PXVX0200 recipients. There were no differences in unsolicited adverse events and no study-related serious adverse events. Immunogenicity and safety endpoints were equivalent between the three production lots. PXVX0200 is immunogenic and well tolerated across multiple production lots.

CLINICAL TRIALS REGISTRATION

Clinicaltrials.gov NCT02094586.

Elimination of persistent vaccine bacteria of Salmonella enterica serovar Typhimurium in the guts of immunized mice by inducible expression of truncated YncE

Orally administered vaccine bacteria usually persist for a period of time in the intestinal tracts of immunized individuals, and are excreted in feces to the environment resulting in a potential biosafety issue. The releasing risk can be minimized by immediate elimination of the persistent vaccine bacteria once adequate protective immune responses have been elicited by the vaccine bacteria. In a previous study, inducible expression of truncated yncE gene (yncE*) was found lethal to host bacteria. This feature has an application potential in biosafety control. Here, we assessed the efficacy of YncE* in eliminating an attenuated strain of Salmonella enterica serovar Typhimurium in a mouse model. To this end, a pBAD-derived plasmid containing yncE* under the control of the Ara promoter was transformed into a ΔphoPQ mutant of S. Typhimurium. Our data show that the induced expression of yncE* in the presence of arabinose eliminated the vaccine bacteria both in vitro and in vivo. BALB/c mice with or without streptomycin-pretreatment were used to assess the efficacy of YncE* in vivo. Oral administration of 500 μl of 20% arabinose at 24 h postvaccination removed the vaccine bacteria from the guts of the tested mice without streptomycin-pretreatment. For streptomycin-pretreated mice, which were colonized with higher levels of Salmonella, an additional gavage of arabinose was required to completely eliminate the vaccine bacteria in the guts of the tested mice. The orally administered arabinose did not affect the persistence of bacteria that had penetrated the intestinal mucosa of the immunized mice. Furthermore, there was no significant difference in the protection rate between the routine immunization and the immunization with the arabinose treatment. The results indicate that the yncE* element improves the biosafety of the bacterial vaccine, and can be taken in consideration in future design of live bacterial vaccines.

Evidence for CVD 103-HgR as an effective single-dose oral cholera vaccine

We propose the ideal oral cholera vaccine (OCV) should be an inexpensive, single, oral dose that rapidly confers immunity for a long duration, and is well tolerated by individuals vulnerable to cholera. Vaccine trials in industrialized countries of a single oral dose of 5 × 108 colony forming units (CFU) of the live, attenuated cholera strain CVD 103-HgR have shown 88–97% serum vibriocidal antibody seroconversion rates, a correlate of protection and documented vaccine efficacy of ≥80% using volunteer challenge studies with wild-type cholera. For individuals of developing countries, a 5 × 109 CFU dose of CVD 103-HgR is necessary to elicit similar antibody responses. Presently, a reformulation of CVD 103-HgR is in late-stage clinical development for prospective US FDA licensure; making a cholera vaccine for US travelers potentially accessible in 2016. The availability of CVD 103-HgR should be a welcome addition to the currently available OCVs.

The state-of-the-art of approved and under-development cholera vaccines

Cholera remains a huge public health problem. Although in 1894, the first cholera vaccination was reported, an ideal vaccine that meets all the requirements of the WHO has not yet been produced. Among the different approaches used for cholera vaccination, attenuated vaccines represent a major category; these vaccines are beneficial in being able to induce a strong protective response after a single administration. However, they have possible negative effects on immunocompromised patient populations. Both the licensed CVD103-HgR and other vaccine approaches under development are detailed in this article, such as the Vibrio cholerae 638 vaccine candidate, Peru-15 or CholeraGarde(®) and the VA1.3, VA1.4, IEM 108 VCUSM2 and CVD 112 vaccine candidates. In another strategy, killed V. cholerae vaccines have been developed, including Dukoral(®), mORCAX(®) and Sanchol™. The killed vaccines are already sold, and they have successfully demonstrated their potential to protect populations in endemic areas or after natural disasters. However, these vaccines do not fulfill all the requirements of the WHO because they fail to confer long-term protection, are not suitable for children under two years, require more than a single dose and require a distribution chain with cold storage. Lastly, other vaccine strategies under development are summarized in this review. Among these strategies, vaccine candidates based on alternative drug delivery systems that have been reported lately in the literature are discussed, such as microparticles, proteoliposomes, LPS subunits, DNA vaccines and rice seeds containing toxin subunits. Preliminary results reported by many groups working on alternative delivery systems for cholera vaccines demonstrate the importance of new technologies in addressing old problems such as cholera. Although a fully ideal vaccine has not yet been designed, promising steps have been reported in the literature resulting in hope for the fight against cholera.

Perforin- and granzyme-mediated cytotoxic effector functions are essential for protection against Francisella tularensis following vaccination by the defined F. tularensis subsp. novicida ΔfopC vaccine strain

A licensed vaccine against Francisella tularensis is currently not available. Two Francisella tularensis subsp. novicida (herein referred to by its earlier name, Francisella novicida) attenuated strains, the ΔiglB and ΔfopC strains, have previously been evaluated as potential vaccine candidates against pneumonic tularemia in experimental animals. F. novicida ΔiglB, a Francisella pathogenicity island (FPI) mutant, is deficient in phagosomal escape and intracellular growth, whereas F. novicida ΔfopC, lacking the outer membrane lipoprotein FopC, which is required for evasion of gamma interferon (IFN-γ)-mediated signaling, is able to escape and replicate in the cytosol. To dissect the difference in protective immune mechanisms conferred by these two vaccine strains, we examined the efficacy of the F. novicida ΔiglB and ΔfopC mutants against pulmonary live-vaccine-strain (LVS) challenge and found that both strains provided comparable protection in wild-type, major histocompatibility complex class I (MHC I) knockout, and MHC II knockout mice. However, F. novicida ΔfopC-vaccinated but not F. novicida ΔiglB-vaccinated perforin-deficient mice were more susceptible and exhibited greater bacterial burdens than similarly vaccinated wild-type mice. Moreover, perforin produced by natural killer (NK) cells and release of granzyme contributed to inhibition of LVS replication within macrophages. This NK cell-mediated LVS inhibition was enhanced with anti-F. novicida ΔfopC immune serum, suggesting antibody-dependent cell-mediated cytotoxicity (ADCC) in F. novicida ΔfopC-mediated protection. Overall, this study provides additional immunological insight into the basis for protection conferred by live attenuated F. novicida strains with different phenotypes and supports further investigation of this organism as a vaccine platform for tularemia.

Immunization with Vibrio cholerae outer membrane vesicles induces protective immunity in mice

The gram-negative bacterium Vibrio cholerae releases outer membrane vesicles (OMVs) during growth. In this study, we immunized female mice by the intranasal, intragastric, or intraperitoneal route with purified OMVs derived from V. cholerae. Independent of the route of immunization, mice induced specific, high-titer immune responses of similar levels against a variety of antigens present in the OMVs. After the last immunization, the half-maximum total immunoglobulin titer was stable over a 3-month period, indicating that the immune response was long lasting. The induction of specific isotypes, however, was dependent on the immunization route. Immunoglobulin A, for example, was induced to a significant level only by mucosal immunization, with the intranasal route generating the highest titers. We challenged the offspring of immunized female mice with V. cholerae via the oral route in two consecutive periods, approximately 30 and 95 days after the last immunization. Regardless of the route of immunization, the offspring was protected against colonization with V. cholerae in both challenge periods. Our results show that mucosal immunizations via both routes with OMVs derived from V. cholerae induce long-term protective immune responses against this gastrointestinal pathogen. These findings may contribute to the development of "nonliving," OMV-based vaccines against V. cholerae and other enteric pathogens, using the oral or intranasal route of immunization.

A Vibrio cholerae serogroup O1 vaccine candidate against CTX ET Phi infection

Cholera is a severe diarrheal disease that may spread rapidly. Vaccination is considered a valid measure against it. We developed a new vaccine candidate, IEM109, against Vibrio cholerae. To generate this candidate, a chromosomal fragment containing the TLC element, attB of the CTX Phi integration site, and RTX cluster responsible for the cytotoxic activity for mammalian cells was deleted through homologous recombination from the previously described El Tor biotype, IEM101. The protective genes ctxB and rstR, which establish resistance to CTX Phi infections, were inserted into that same location on the chromosome of IEM109 to enhance the safety and genetic stability of the vaccine candidate and to prevent horizontal gene transfer. In in vivo tests, cell cultures showed that the cytotoxic effect of IEM109 on Hep-2 was negative. Furthermore, the infection rate of El Tor biotype CTX Phi to that of IEM109 in the rabbit intestine is 3000-fold lower than that of IEM101. Intraintestinal vaccination of rabbits with a single dose of IEM109 elicits high titers of anti-CTB IgG and vibriocidal antibodies. When challenged with 0.5-2 microg CT and 10(5) to 10(8)CFU of four wild toxigenic strains of different biotypes and serogroups, IEM109 conferred full protection. Thus, IEM109 is a stable vaccine candidate that evokes not only antitoxic and vibriocidal immunities, but also resistance to the El Tor biotype CTX Phi infection.

Biological safety concepts of genetically modified live bacterial vaccines

Live vaccines possess the advantage of having access to induce cell-mediated and antibody-mediated immunity; thus in certain cases they are able to prevent infection, and not only disease. Furthermore, live vaccines, particularly bacterial live vaccines, are relatively cheap to produce and easy to apply. Hence they are suitable to immunize large communities or herds. The induction of both cell-mediated immunity as well as antibody-mediated immunity, which is particularly beneficial in inducing mucosal immune responses, is obtained by the vaccine-strain's ability to colonize and multiply in the host without causing disease. For this reason, live vaccines require attenuation of virulence of the bacterium to which immunity must be induced. Traditionally attenuation was achieved simply by multiple passages of the microorganism on growth medium, in animals, eggs or cell cultures or by chemical or physical mutagenesis, which resulted in random mutations that lead to attenuation. In contrast, novel molecular methods enable the development of genetically modified organisms (GMOs) targeted to specific genes that are particularly suited to induce attenuation or to reduce undesirable effects in the tissue in which the vaccine strains can multiply and survive. Since live vaccine strains (attenuated by natural selection or genetic engineering) are potentially released into the environment by the vaccinees, safety issues concerning the medical as well as environmental aspects must be considered. These involve (i) changes in cell, tissue and host tropism, (ii) virulence of the carrier through the incorporation of foreign genes, (iii) reversion to virulence by acquisition of complementation genes, (iv) exchange of genetic information with other vaccine or wild-type strains of the carrier organism and (v) spread of undesired genes such as antibiotic resistance genes. Before live vaccines are applied, the safety issues must be thoroughly evaluated case-by-case. Safety assessment includes knowledge of the precise function and genetic location of the genes to be mutated, their genetic stability, potential reversion mechanisms, possible recombination events with dormant genes, gene transfer to other organisms as well as gene acquisition from other organisms by phage transduction, transposition or plasmid transfer and cis- or trans-complementation. For this, GMOs that are constructed with modern techniques of genetic engineering display a significant advantage over random mutagenesis derived live organisms. The selection of suitable GMO candidate strains can be made under in vitro conditions using basic knowledge on molecular mechanisms of pathogenicity of the corresponding bacterial species rather than by in vivo testing of large numbers of random mutants. This leads to a more targeted safety testing on volunteers and to a reduction in the use of animal experimentation.

Biosafety evaluation of recombinant live oral bacterial vaccines in the context of European regulation

Live bacterial vaccines represent a highly valid preventive strategy in the fight against infectious disease. However, the road from research to market is peppered with hurdles, one of which is the requirement for high biosafety characteristics, which the candidate vaccine has to display. In Europe, the European Agency for the evaluation of medicinal products (EMEA) is the relevant authority regulating the licensure of genetically engineered vaccines. For this purpose, the agency may rely on several directives and guidelines defined in the past 15 years. As for live vaccines containing genetically modified organisms (GMOs) susceptible to be released into the environment, Directive 2001/18/EC determines the framework and principles of an environmental risk assessment (ERA) process, the results of which constitute an important section of the vaccine registration package submitted to registration authorities. In this article, we address the implications of current European regulations for the approval of live oral bacterial vaccines with emphasis on the assessment of potential risks associated with environmental release. Biosafety aspects of already registered and some promising live bacterial vaccine strains will be briefly discussed.

Vaccines against typhoid fever

Because of high infectivity and significant disease burden, typhoid fever constitutes a major global health problem. Implementation of adequate food handling practices and establishment of safe water supplies are the cornerstone for the development of an effective prevention program. However, vaccination against typhoid fever remains an essential tool for the effective management of this disease. Currently, there are two well tolerated and effective licensed vaccines. One is based on defined subunit virulence (Vi) polysaccharide antigen and can be administered either intramuscularly or subcutaneously and the other is based on the use of live attenuated bacteria for oral administration. The advantages and disadvantages of the various approaches taken in the development of a vaccine against typhoid fever are discussed, along with the potential for future vaccine candidates.