A Plasmodium falciparum candidate vaccine based on a six-antigen polyprotein encoded by recombinant poxviruses

Malaria vaccines: the 60-year journey of hope and final success-lessons learned and future prospects

BACKGROUND

The world has made great strides towards beating malaria, although about half of the world population is still exposed to the risk of contracting malaria. Developing an effective malaria vaccine was a huge challenge for medical science. In 2021 the World Health Organization (WHO) approved the first malaria vaccine, RTS,S/AS01 vaccine (Mosquirix™), for widespread use. This review highlights the history of development, and the different approaches and types of malaria vaccines, and the literature to date. It covers the developmental stages of RTS,S/AS01 and recommends steps for its deployment. The review explores other potential vaccine candidates and their status, and suggests options for their further development. It also recommends future roles for vaccines in eradicating malaria. Questions remain on how RTS,S vaccine will work in widespread use and how it can best be utilized to benefit vulnerable communities.

CONCLUSION

Malaria vaccines have been in development for almost 60 years. The RTS,S/AS01 vaccine has now been approved, but cannot be a stand-alone solution. Development should continue on promising candidates such as R21, PfSPZ and P. vivax vaccines. Multi-component vaccines may be a useful addition to other malaria control techniques in achieving eradication of malaria.

Meningococcal PorB induces a robust and diverse antigen specific T cell response as a vaccine adjuvant

Vaccines formulated with adjuvant have been effective against numerous infectious diseases, almost always due to induction of functional antibodies that recognizes the pathogen of interest. There is an unmet clinical need for vaccine adjuvants that induce T cells responses to potentially enhance protection against malignancies and intracellular pathogens, where a humoral response, alone, may not be adequate for protection. In this study, we demonstrate that a TLR2 ligand-based adjuvant, meningococcal PorB, has broad immunostimulatory activity with the ability to induce a robust and diverse vaccine antigen specific T cell response. We demonstrate that a vaccine formulated with PorB admixed with ovalbumin induces a wide variety of antigen specific antibody subclasses and effector molecules (MIG, MCP-1, IP-10, MIP-1α, KC & IL-2) with known roles for inducing T cell responses, along with elevated levels of Th1 and Th2 type cytokines upon antigen stimulation. We confirmed production of these cytokines by examining the antigen-specific T cells induced by PorB in vivo. After two immunizations with vaccine formulated with PorB/OVA, antigen-specific CD4 and CD8 T cells were significantly increased in numbers and produced IL-4 or IFN-γ upon ex vivo antigen re-stimulation. Finally, in a Listeria mouse infection model, vaccine formulated with PorB significantly reduced the bacterial burden upon a low dose infection and increased survival upon a high dose infection with recombinant Listeria monocytogenes engineered to express OVA (rLmOVA), a pathogen that requires OVA-antigen specific cytotoxic CD8 T cells for clearance. In summary, PorB is able to induce antigen specific broad B and T cell responses, illustrating its potential as a potent and new vaccine adjuvant.

Identification of Immunodominant Responses to the Plasmodium falciparum Antigens PfUIS3, PfLSA1 and PfLSAP2 in Multiple Strains of Mice

Malaria, caused by the Plasmodium parasite, remains a serious global public health concern. A vaccine could have a substantial impact on eliminating this disease, alongside other preventative measures. We recently described the development of three novel, viral vectored vaccines expressing either of the antigens PfUIS3, PfLSA1 and PfLSAP2. Each vaccination regimen provided high levels of protection against chimeric parasite challenge in a mouse model, largely dependent on CD8⁺ T cells. In this study we aimed to further characterize the induced cellular immune response to these vaccines. We utilized both the IFNγ enzyme-linked immunosorbent spot assay and intracellular cytokine staining to achieve this aim. We identified immunodominant peptide responses for CD4⁺ and CD8⁺ T cells for each of the antigens in BALB/c, C57BL/6 and HLA-A2 transgenic mice, creating a useful tool for researchers for subsequent study of these antigens. We also compared these immunodominant peptides with those generated from epitope prediction software, and found that only a small proportion of the large number of epitopes predicted by the software were identifiable experimentally. Furthermore, we characterized the polyfunctionality of the induced CD8⁺ T cell responses. These findings contribute to our understanding of the immunological mechanisms underlying these protective vaccines, and provide a useful basis for the assessment of these and related vaccines as clinical constructs.

The evolution of poxvirus vaccines

After Edward Jenner established human vaccination over 200 years ago, attenuated poxviruses became key players to contain the deadliest virus of its own family: Variola virus (VARV), the causative agent of smallpox. Cowpox virus (CPXV) and horsepox virus (HSPV) were extensively used to this end, passaged in cattle and humans until the appearance of vaccinia virus (VACV), which was used in the final campaigns aimed to eradicate the disease, an endeavor that was accomplished by the World Health Organization (WHO) in 1980. Ever since, naturally evolved strains used for vaccination were introduced into research laboratories where VACV and other poxviruses with improved safety profiles were generated. Recombinant DNA technology along with the DNA genome features of this virus family allowed the generation of vaccines against heterologous diseases, and the specific insertion and deletion of poxvirus genes generated an even broader spectrum of modified viruses with new properties that increase their immunogenicity and safety profile as vaccine vectors. In this review, we highlight the evolution of poxvirus vaccines, from first generation to the current status, pointing out how different vaccines have emerged and approaches that are being followed up in the development of more rational vaccines against a wide range of diseases.

Fowlpox virus as a recombinant vaccine vector for use in mammals and poultry

Live vaccines against fowlpox virus, which causes moderate pathology in poultry and is the type species of the Avipoxvirus genus, were developed in the 1920s. Development of recombinant fowlpox virus vector vaccines began in the 1980s, for use not only in poultry, but also in mammals including humans. In common with other avipoxviruses, such as canarypox virus, fowlpox virus enters mammalian cells and expresses proteins, but replicates abortively. The use of fowlpox virus as a safe vehicle for expression of foreign antigens and host immunomodulators, is being evaluated in numerous clinical trials of vaccines against cancer, malaria, tuberculosis and AIDS, notably in heterologous prime-boost regimens. In this article, technical approaches to, and issues surrounding, the use of fowlpox virus as a recombinant vaccine vector in poultry and mammals are reviewed.

DNA vaccines against tropical parasitic diseases

Parasitic diseases caused by protozoan and helminth parasites are among the leading causes of morbidity and mortality in tropical and subtropical regions of the world. Unfortunately, at present, there is no vaccine against any human parasitic disease. Conventional vaccine methods have largely failed against parasitic infections. This is due, in part, to the complexity of the parasite life cycle, the ability of the parasite to evade the immune system, and difficulties in identifying and eliciting the desired protective immune responses. The discovery of DNA vaccines has renewed hope for vaccine development against parasites. In the last decade, DNA vaccines were successful in inducing at least partial protection against several parasitic diseases. This review discusses the latest developments in DNA vaccines against tropical parasitic diseases.

Malaria vaccine adjuvants: latest update and challenges in preclinical and clinical research

There is no malaria vaccine currently available, and the most advanced candidate has recently reported a modest 30% efficacy against clinical malaria. Although many efforts have been dedicated to achieve this goal, the research was mainly directed to identify antigenic targets. Nevertheless, the latest progresses on understanding how immune system works and the data recovered from vaccination studies have conferred to the vaccine formulation its deserved relevance. Additionally to the antigen nature, the manner in which it is presented (delivery adjuvants) as well as the immunostimulatory effect of the formulation components (immunostimulants) modulates the immune response elicited. Protective immunity against malaria requires the induction of humoral, antibody-dependent cellular inhibition (ADCI) and effector and memory cell responses. This review summarizes the status of adjuvants that have been or are being employed in the malaria vaccine development, focusing on the pharmaceutical and immunological aspects, as well as on their immunization outcomings at clinical and preclinical stages.

High-capacity adenoviral vectors circumvent the limitations of ΔE1 and ΔE1/ΔE3 adenovirus vectors to induce multispecific transgene product-directed CD8 T-cell responses

BACKGROUND

The ability to induce cytotoxic T lymphocyte (CTL) responses that are multispecific is considered to comprise an essential feature for an efficacious genetic vaccine against many pathogens including HIV and hepatitis C virus. ΔE1Ad vectors are promising vectored vaccines but have been shown to induce antigen-specific CTLs with only limited multispecificity. In the present study, we investigated the applicability of gene-deleted high-capacity adenovirus (HC-Ad) vectors and focused on the induction of multispecific CTL responses.

METHODS

We generated Δ E1 and HC-Ad vectors expressing hepatitis B virus small surface antigen (HBsAg). We comparatively analyzed the CTL profiles against various transgene product- and vector-derived epitopes in several mouse strains and HBsAg- and vector-directed antibody responses.

RESULTS

HC-Ad vectors efficiently induced multispecific HBsAg-directed CTLs. By contrast, ΔE1Ad vectors mainly primed CTLs against one immunodominant epitope of HBsAg. This absence of multispecific CTL responses correlated with the induction of CTLs against viral epitopes generated by de novo expression of Ad genes from the ΔE1Ad vector. However, Ad-specific CTLs induced in trans did not impair HC-AdS-induced multispecific CTL responses against HBsAg. Finally, HC-Ad vectors also induced higher HBsAg antibody titers compared to ΔE1Ad vectors.

CONCLUSIONS

De novo expression of viral genes from ΔE1Ad vector genomes restricts the multispecificity of transgene product-specific CTLs by immunodominance effects. HC-Ad vectors devoid of Ad genes are favorable for the induction of both multispecific CD8 T-cell responses and high antibody responses. Our results suggest the deletion of Ad genes as an important means for developing potent Ad-based vectored vaccines.

Safety and immunogenicity of heterologous prime-boost immunisation with Plasmodium falciparum malaria candidate vaccines, ChAd63 ME-TRAP and MVA ME-TRAP, in healthy Gambian and Kenyan adults

BACKGROUND

Heterologous prime boost immunization with chimpanzee adenovirus 63 (ChAd63) and Modified vaccinia Virus Ankara (MVA) vectored vaccines is a strategy recently shown to be capable of inducing strong cell mediated responses against several antigens from the malaria parasite. ChAd63-MVA expressing the Plasmodium falciparum pre-erythrocytic antigen ME-TRAP (multiple epitope string with thrombospondin-related adhesion protein) is a leading malaria vaccine candidate, capable of inducing sterile protection in malaria naïve adults following controlled human malaria infection (CHMI).

METHODOLOGY

We conducted two Phase Ib dose escalation clinical trials assessing the safety and immunogenicity of ChAd63-MVA ME-TRAP in 46 healthy malaria exposed adults in two African countries with similar malaria transmission patterns.

RESULTS

ChAd63-MVA ME-TRAP was shown to be safe and immunogenic, inducing high-level T cell responses (median >1300 SFU/million PBMC).

CONCLUSIONS

ChAd63-MVA ME-TRAP is a safe and highly immunogenic vaccine regimen in adults with prior exposure to malaria. Further clinical trials to assess safety and immunogenicity in children and infants and protective efficacy in the field are now warranted.

TRIAL REGISTRATION

Pactr.org PACTR2010020001771828 Pactr.org PACTR201008000221638 ClinicalTrials.gov NCT01373879 NCT01373879 ClinicalTrials.gov NCT01379430 NCT01379430.

A review of malaria vaccine clinical projects based on the WHO rainbow table

Development and Phase 3 testing of the most advanced malaria vaccine, RTS,S/AS01, indicates that malaria vaccine R&D is moving into a new phase. Field trials of several research malaria vaccines have also confirmed that it is possible to impact the host-parasite relationship through vaccine-induced immune responses to multiple antigenic targets using different platforms. Other approaches have been appropriately tested but turned out to be disappointing after clinical evaluation.

As the malaria community considers the potential role of a first-generation malaria vaccine in malaria control efforts, it is an apposite time to carefully document terminated and ongoing malaria vaccine research projects so that lessons learned can be applied to increase the chances of success for second-generation malaria vaccines over the next 10 years.

The most comprehensive resource of malaria vaccine projects is a spreadsheet compiled by WHO thanks to the input from funding agencies, sponsors and investigators worldwide. This spreadsheet, available from WHO's website, is known as "the rainbow table". By summarizing the published and some unpublished information available for each project on the rainbow table, the most comprehensive review of malaria vaccine projects to be published in the last several years is provided below.

Vaccines against malaria

There is no licenced vaccine against any human parasitic disease and Plasmodium falciparum malaria, a major cause of infectious mortality, presents a great challenge to vaccine developers. This has led to the assessment of a wide variety of approaches to malaria vaccine design and development, assisted by the availability of a safe challenge model for small-scale efficacy testing of vaccine candidates. Malaria vaccine development has been at the forefront of assessing many new vaccine technologies including novel adjuvants, vectored prime-boost regimes and the concept of community vaccination to block malaria transmission. Most current vaccine candidates target a single stage of the parasite's life cycle and vaccines against the early pre-erythrocytic stages have shown most success. A protein in adjuvant vaccine, working through antibodies against sporozoites, and viral vector vaccines targeting the intracellular liver-stage parasite with cellular immunity show partial efficacy in humans, and the anti-sporozoite vaccine is currently in phase III trials. However, a more effective malaria vaccine suitable for widespread cost-effective deployment is likely to require a multi-component vaccine targeting more than one life cycle stage. The most attractive near-term approach to develop such a product is to combine existing partially effective pre-erythrocytic vaccine candidates.

O-GlcNAc modification of the anti-malarial vaccine candidate PfAMA1: in silico-defined structural changes and potential to generate a better vaccine

The complex life cycle of plasmodial parasites makes the selection of a single subunit protein a less than optimal strategy to generate an efficient vaccinal protection against malaria. Moreover, the full protection afforded by malarial proteins carried by intact parasites implies that immune responses against different antigens expressed in different phases of the cycle are required, but also suggests that native malarial antigens are presented to the host immune system in a manner that recombinant proteins do not achieve. The malarial apical membrane antigen 1 (AMA1) represents a suitable vaccine candidate because AMA1 is expressed on sporozoites and merozoites and allows them to invade hepatocytes and erythrocytes, respectively. Anti-AMA1 antibodies and cytotoxic T-cells are therefore expected to interfere both with the primary invasion of hepatocytes by sporozoites and with the later propagation of merozoites in erythrocytes, and thus efficiently counteract parasite development in its human host. AMA1 bears potential glycosylation sites and the human erythrocytic O-linked N-acetylglucosamine transferase (OGT) could glycosylate AMA1 through combinatorial metabolism. This hypothesis was tested in silico by developing binding models of AMA1 with human OGT complexed with UDP-GlcNc, and followed by the binding of O-GlcNAc with the hydroxyl group of AMA1 serine and threonine residues. Our results suggests that AMA1 shows potential for glycosylation at Thr517 and Ser498 and that O-GlcNAc AMA1 may constitute a conformationally more appropriate antigen for developing a protective anti-malarial immune response.

A human Phase I/IIa malaria challenge trial of a polyprotein malaria vaccine

We examined the safety, immunogenicity and efficacy of a prime-boost vaccination regime involving two poxvirus malaria subunit vaccines, FP9-PP and MVA-PP, expressing the same polyprotein consisting of six pre-erythrocytic antigens from Plasmodium falciparum.

Following safety assessment of single doses, 15 volunteers received a heterologous prime-boost vaccination regime and underwent malaria sporozoite challenge. The vaccines were safe but interferon-γ ELISPOT responses were low compared to other poxvirus vectors, despite targeting multiple antigens. There was no vaccine efficacy as measured by delay in time to parasitaemia. A number of possible explanations are discussed, including the very large insert size of the polyprotein transgene.

Control of important helminthic infections vaccine development as part of the solution

Among the tools available for the control of helminth infections, chemotherapy has come to totally dominate the field. In the veterinary field, development of drug resistance has appeared but this is not (yet) a problem in the control of human diseases. Although there is no vaccine commercially available for any human parasitic infection yet, recent progress in vaccine development is making this a future possibility for several diseases. The goal of chemotherapy is to alleviate infection and morbidity in the definitive host, or reduce transmission, while the effect of available vaccine candidates would mainly be to influence transmission through targeting the intermediate or reservoir host, when the infection is zoonotic. Apart from this general scheme, there are also vaccine candidates targeting the parasites in the definitive host, in particular the early developmental stages, which should reduce the risk of drug failure. Since the biological targets in most cases are different, vaccination would be synergistic with drug therapy. This review covers diseases caused by helminthes in both humans and animals and includes examples of diseases caused by cestodes, nematodes and trematodes. The focus is on infections for which vaccine development has been undertaken for a long time, resulting in products that could realistically become integrated into control strategies in the near future.

Immune requirements of post-exposure immunization with modified vaccinia Ankara of lethally infected mice

Current prophylactic vaccines work via the induction of B and T cell mediated memory that effectively control further replication of the pathogen after entry. In the case of therapeutic or post-exposure vaccinations the situation is far more complex, because the pathogen has time to establish itself in the host, start producing immune-inhibitory molecules and spread into distant organs. So far it is unclear which immune parameters have to be activated in order to thwart an existing lethal infection. Using the mousepox model, we investigated the immunological mechanisms responsible for a successful post-exposure immunization with modified vaccinia Ankara (MVA). In contrast to intranasal application of MVA, we found that intravenous immunization fully protected mice infected with ectromelia virus (ECTV) when applied three days after infection. Intravenous MVA immunization induced strong innate and adaptive immune responses in lethally infected mice. By using various gene-targeted and transgenic mouse strains we show that NK cells, CD4 T cells, CD8 T cells and antibodies are essential for the clearance of ECTV after post-exposure immunization. Post-exposure immunization with MVA is an effective measure in a murine model of human smallpox. MVA activates innate and adaptive immune parameters and only a combination thereof is able to purge ECTV from its host. These data not only provide a basis for therapeutic vaccinations in the case of the deliberate release of pathogenic poxviruses but possibly also for the treatment of chronic infections and cancer.

Comparing human T cell and NK cell responses in viral-based malaria vaccine trials

Vaccination with viral-based vaccines continues to hold promise for the prevention of malaria. Whilst antigen-specific T cell responses are considered a major aim of such an approach, a role for induced NK cells as anti-malarial effector cells, or in shaping T cell responses, has received less attention. In this study naïve human volunteers were vaccinated in a prime-boost vaccination regimen comprising recombinant viral vectors fowlpox (FP9) and modified vaccinia Ankara (MVA) encoding liver-stage antigens, or a virosome vaccine. Significant T cell responses specific for the vectored vaccine antigens were demonstrated by IFNgamma ELISPOT and intracellular cytokine staining (ICS) for IFNgamma and IL-2, the ICS being associated with increased time to parasitaemia following subsequent challenge. Numbers of CD56(bright) lymphocytes increased significantly following vaccination, as did CD3(+) CD56(+) lymphocytes, whilst CD56(dim) cells did not. No such increases were seen with the virosome vaccine. There was no significant correlation of these CD56(+) populations with the antigen-specific T cell responses nor time to parasitaemia. To investigate pathways of immune activation that could contribute to these lymphocyte responses, viral vectors were shown in vitro to efficiently infect APCs but not lymphocytes, and stimulated inflammatory cytokines such as type I interferons. In conclusion, measuring antigen-specific T cells is more meaningful than NK cells in these vaccination regimens.

Protective CD8 T cells against Plasmodium liver stages: immunobiology of an 'unnatural' immune response

SUMMARY

Immunization with high doses of irradiated sporozoites delivered by the bites of infected mosquitoes has been shown to induce protective responses against malaria, mediated in part by CD8(+) T cells. In contrast, natural transmission involving low exposure to live sporozoite antigen fails to elicit strong immunity. In this review, we examine how irradiated sporozoite immunization breaks the natural host-parasite interaction and induces protective CD8(+) T cells. Upon biting, the malaria-infected mosquitoes deposit parasites in the skin, many of which eventually exit to the bloodstream and infect hepatocytes. However, certain antigens, including the circumsporozoite (CS) protein, remain in the skin and are presented in the draining lymph node. These antigens prime specific CD8(+) T cells, which migrate to the liver where they eliminate parasitized hepatocytes. We discuss the relevance of the different tissue compartments involved in the induction and effector phases of this response, as well as the cellular requirements for priming and memory development of CD8(+) T cells, which include a complete dependence on dendritic cells and a near absolute need for CD4(+) T-cell help. Finally, we discuss the impact of the immunodominant CS protein on this protection and the implications of these findings for vaccine design.

Antibodies to pre-erythrocytic Plasmodium falciparum antigens and risk of clinical malaria in Kenyan children

BACKGROUND

IgG antibodies to pre-erythrocytic antigens are involved in prevention of infection and disease in animal models of malaria but have not been associated with protection against disease in human malaria.

METHODS

Levels of IgG antibodies to circumsporozoite protein (CSP), liver-stage antigen type 1 (LSA-1), and thrombospondin-related adhesive protein (TRAP) were measured in 86 children in a malaria-holoendemic area of Kenya. The children were then monitored for episodes of clinical malaria for 52 weeks.

RESULTS

Children with high levels of IgG antibodies to CSP, LSA-1, and TRAP had a decreased risk of clinical malaria (adjusted hazard ratio, 0.29; 95% confidence interval 0.10-0.81; P = .02), a lower incidence of clinical malaria (P=.006), protection from clinical malaria with a parasite level of > or =4000 parasites/microL (P= .03), and a higher hemoglobin level at enrollment (P= .009), compared with children with lower antibody levels. Protection against malaria morbidity was associated primarily with antibodies to CSP and LSA-1.

CONCLUSIONS

Kenyan children with high levels of IgG antibodies to the pre-erythrocytic antigens CSP, LSA-1, and TRAP have a lower risk of developing clinical malaria than children without high levels of these antibodies. The decreased risk of clinical malaria may be mediated in part by prevention of high-density parasitemia.

Impact of recombinant adenovirus serotype 35 priming versus boosting of a Plasmodium falciparum protein: characterization of T- and B-cell responses to liver-stage antigen 1

Prime-boost vaccination regimens with heterologous antigen delivery systems have indicated that redirection of the immune response is feasible. We showed earlier that T-cell responses to circumsporozoite (CS) protein improved significantly when the protein is primed with recombinant adenovirus serotype 35 coding for CS (rAd35.CS). The current study was designed to answer the question whether such an effect can be extended to liver-stage antigens (LSA) of Plasmodium falciparum such as LSA-1. Studies with mice have demonstrated that the LSA-1 protein induces strong antibody response but a weak T-cell immunity. We first identified T-cell epitopes in LSA-1 by use of intracellular gamma interferon (IFN-gamma) staining and confirmed these epitopes by means of enzyme-linked immunospot assay and pentamer staining. We show that a single immunization with rAd35.LSA-1 induced a strong antigen-specific IFN-gamma CD8(+) T-cell response but no measurable antibody response. In contrast, vaccinations with the adjuvanted recombinant LSA-1 protein induced remarkably low cellular responses but strong antibody responses. Finally, both priming and boosting of the adjuvanted protein by rAd35 resulted in enhanced T-cell responses without impairing the level of antibody responses induced by the protein immunizations alone. Furthermore, the incorporation of rAd35 in the vaccination schedule led to a skewing of LSA-1-specific antibody responses toward a Th1-type immune response. Our results show the ability of rAd35 to induce potent T-cell immunity in combination with protein in a prime-boost schedule without impairing the B-cell response.

Parasite vaccines: The new generation

Parasites cause some of the most devastating and prevalent diseases in humans and animals. Moreover, parasitic infections increase mortality rates of other serious non-parasitic infections caused by pathogens such as HIV-1. The impact of parasitic diseases in both industrialised and developing countries is further exacerbated by the resistance of some parasites to anti-parasitic drugs and the absence of efficacious parasite vaccines. Despite years of research, much remains to be done to develop effective vaccines against parasites. This review focuses on the more recent vaccine strategies such as DNA and viral vector-based vaccines that are currently being used to develop vaccines against parasites. Obstacles yet to be overcome and possible advantages and disadvantages of these vaccine modalities are also discussed.

An accelerated vaccine schedule with a poly-antigenic hepatitis C virus MVA-based candidate vaccine induces potent, long lasting and in vivo cross-reactive T cell responses

We designed and evaluated in HLA-class I transgenic mouse models a hepatitis C virus (HCV) T cell-based MVA vectored vaccine expressing three viral antigens known to be targets of potent CD8+- and CD4+-mediated responses. An accelerated (3 week-based) vaccination induced specific CD8+ T cells harboring two effector functions (cytolytic activity - both in vitro and in vivo- and production of IFNgamma) as well as specific CD4+ T cells recognizing all three vaccine antigens. Responses were long lasting (6 months), boostable by a fourth MVA vaccination and in vivo cross-reactive as demonstrated in a surrogate Listeria-based challenge assay. This candidate vaccine has now moved into clinical trials.

Viral vector vaccines make memory T cells against malaria

Vaccines that comprise attenuated viral vectors encoding antigens from target pathogens generate potent T-cell responses. One such pathogen is malaria, and in particular the liver stage of its life cycle. Immunogenicity and efficacy studies in animals and humans have revealed the generation of memory T cells of both the central and effector phenotypes, depending on the viral vectors used in the malaria vaccination regime (viral species and serotype, combination and sequence for prime-boost) and suggest a divergence in their protective role. Being able to influence the memory T-cell make-up in a rational manner may allow us to develop more efficacious vaccines.

Genus Avipoxvirus

Poxviruses identified in skin lesions of domestic, pet or wild birds are assigned largely by default to the Avipoxvirus genus within the subfamily Chordopoxvirinae of the family Poxviridae. Avipoxviruses have been identified as the causative agent of disease in at least 232 species in 23 orders of birds. Vaccines based upon attenuated avipoxvirus strains provide good disease control in production poultry, although with the large and intensive production systems there are suggestions and real risks of emergence of strains against which current vaccines might be ineffective. Sequence analysis of the whole genome has revealed overall genome structure and function resemblance to the Chordopoxvirinae; however, avipoxvirus genomes exhibit large-scale genomic rearrangements with more extensive gene families and novel host range gene in comparison with the other Chordopoxvirinae. Phylogenetic analysis places the avipoxviruses externally to the Chorodopoxvirinae to such an extent that in the future it might be appropriate to consider the Avipoxviruses as a separate subfamily within the Poxviridae. A unique relationship exists between Fowlpox virus (FWPV) and reticuloendothelosis viruses. All FWPV strains carry a remnant long terminal repeat, while field strains carry a near full-length provirus integrated at the same location in the FWPV genome. With the development of techniques to construct poxviruses expressing foreign vaccine antigens, the avipoxviruses have gone from neglected obscurity to important vaccine vectors in the past 20 years. The seminal observation of their utility for delivery of vaccine antigens to non-avian species has driven much of the interest in this group of viruses. In the veterinary area, several recombinant avipoxviruses are commercially licensed vaccines. The most successful have been those expressing glycoprotein antigens of enveloped viruses, e.g. avian influenza, Newcastle diseases and West Nile viruses. Several recombinants have undergone extensive human clinical trials as experimental vaccines against HIV/AIDS and malaria or as treatment regimens in cancer patients. The safety profile of avipoxvirus recombinants for use as veterinary and human vaccines or therapeutics is now well established.

A DNA prime-modified vaccinia virus ankara boost vaccine encoding thrombospondin-related adhesion protein but not circumsporozoite protein partially protects healthy malaria-naive adults against Plasmodium falciparum sporozoite challenge

The safety, immunogenicity, and efficacy of DNA and modified vaccinia virus Ankara (MVA) prime-boost regimes were assessed by using either thrombospondin-related adhesion protein (TRAP) with a multiple-epitope string ME (ME-TRAP) or the circumsporozoite protein (CS) of Plasmodium falciparum. Sixteen healthy subjects who never had malaria (malaria-naive subjects) received two priming vaccinations with DNA, followed by one boosting immunization with MVA, with either ME-TRAP or CS as the antigen. Immunogenicity was assessed by ex vivo gamma interferon (IFN-gamma) enzyme-linked immunospot assay (ELISPOT) and antibody assay. Two weeks after the final vaccination, the subjects underwent P. falciparum sporozoite challenge, with six unvaccinated controls. The vaccines were well tolerated and immunogenic, with the DDM-ME TRAP regimen producing stronger ex vivo IFN-gamma ELISPOT responses than DDM-CS. One of eight subjects receiving the DDM-ME TRAP regimen was completely protected against malaria challenge, with this group as a whole showing significant delay to parasitemia compared to controls (P = 0.045). The peak ex vivo IFN-gamma ELISPOT response in this group correlated strongly with the number of days to parasitemia (P = 0.033). No protection was observed in the DDM-CS group. Prime-boost vaccination with DNA and MVA encoding ME-TRAP but not CS resulted in partial protection against P. falciparum sporozoite challenge in the present study.

Viral vectors for malaria vaccine development

A workshop on viral vectors for malaria vaccine development, organized by the PATH Malaria Vaccine Initiative, was held in Bethesda, MD on October 20, 2005. Recent advancements in viral-vectored malaria vaccine development and emerging vector technologies were presented and discussed. Classic viral vectors such as poxvirus, adenovirus and alphavirus vectors have been successfully used to deliver malaria antigens. Some of the vaccine candidates have demonstrated their potential in inducing malaria-specific immunity in animal models and human trials. In addition, emerging viral-vector technologies, such as measles virus (MV), vesicular stomatitis virus (VSV) and yellow fever (YF) virus, may also be useful for malaria vaccine development. Studies in animal models suggest that each viral vector is unique in its ability to induce humoral and/or cellular immune responses. Those studies have also revealed that optimization of Plasmodium genes for mammalian expression is an important aspect of vaccine design. Codon-optimization, surface-trafficking, de-glycosylation and removal of toxic domains can lead to improved immunogenicity. Understanding the vector's ability to induce an immune response and the expression of malaria antigens in mammalian cells will be critical in designing the next generation of viral-vectored malaria vaccines.

Advances in vaccine development against the pre-erythrocytic stage of Plasmodium falciparum malaria

With approximately 2.4 billion people at risk, Plasmodium falciparum malaria infection caused by an infectious bite of an Anopheles mosquito continues to be a major cause of mortality and morbidity, mainly in the tropics and subtropics. Measures to control the mosquito vector on a broad scale are expensive and need to be maintained continuously. The rapid emergence of parasite strains that are resistant to affordable drugs highlights the urgent need for a cheap and effective vaccine. Candidate vaccines that have been developed to date target different stages of the parasite life cycle. This review describes the recent advances in the development of a vaccine that aims to terminate the infection at its first stage in the liver. The candidate vaccines that are currently under clinical evaluation are introduced and the results from recent trials discussed. The review aims to explain the immunologic challenges a successful vaccine has to meet, as well as the different strategies that are currently employed in an attempt to induce a protective immune response. Furthermore, an outline of available options to be tested in the near future will be presented.

Anti-CD25 Antibody Enhancement of Vaccine-Induced Immunogenicity: Increased Durable Cellular Immunity with Reduced Immunodominance

An efficacious vaccine strategy must be capable of inducing strong responses of an appropriate phenotype that are long lasting and sufficiently broad to prevent pathogen escape mechanisms. In the present study, we use anti-CD25 mAb to augment vaccine-induced immunity in mice. We demonstrate that coformulation of Ab and poxviral- or adenoviral-vectored vaccines induces significantly increased T cell responses to a malaria Ag; prior anti-CD25 Ab administration was not required for this effect. Furthermore, this vaccination approach subverts immunodominant epitope hierarchies by enhancing responses to subdominant epitopes induced by recombinant modified vaccinia virus Ankara immunization. Administration of anti-CD25 with a vaccine also induces more durable immunity compared with vaccine alone; significantly higher T cell responses were observed 100 days after the primary immunization. Enhanced immunogenicity is observed for multiple vaccine types with enhanced CD4+ and CD8+ T cell responses induced by bacillus Calmette-Guérin and a recombinant subunit protein vaccine to hepatitis B virus and with multiple Ags of tumor, viral, bacterial, and parasitic origin. Vaccine strategies incorporating anti-CD25 lead to improved protection against pre-erythrocytic malaria challenge. These data underpin new strategies for the design and development of more efficacious vaccines in clinical settings.

Protective efficacy of an oral vaccine to reduce carriage of Borrelia burgdorferi (strain N40) in mouse and tick reservoirs

Lyme disease is caused by the spirochete Borrelia burgdorferi, which is transmitted through the bite of infected Ixodes ticks. Vaccination of mice with outer surface protein A (OspA) of B. burgdorferi has been shown to both protect mice against B. burgdorferi infection and reduce carriage of the organism in feeding ticks. Here we report the development of a murine-targeted OspA vaccine utilizing Vaccinia virus to interrupt transmission of disease in the reservoir hosts, thus reducing incidence of human disease. Oral vaccination of mice with a single dose of Vaccinia expressing OspA resulted in high antibody titers to OspA, 100% protection of vaccinated mice from infection with B. burgdorferi, and significant clearance of B. burgdorferi from infected ticks fed on vaccinated animals. The results indicate the vaccine is effective and may provide a manner to reduce incidence of Lyme disease.

Eimeria tenella: identification of secretory and surface proteins from expressed sequence tags

To identify new vaccine candidates, Eimeria tenella expressed sequence tags (ESTs) from public databases were analysed for secretory molecules with an especially developed automated in silico strategy termed DNAsignalP. A total of 12,187 ESTs were clustered into 2881 contigs followed by a blastx search, which resulted in a significant number of E. tenella contigs with homologies to entries in public databases. Amino acid sequences of appropriate homologous proteins were analysed for the occurrence of an N-terminal signal sequence using the algorithm signalP. The resulting list of 84 entries comprised 51 contigs whose deduced proteins showed homologies to proteins of apicomplexan parasites. Based on function or localisation, we selected candidate proteins classified as (i) secreted proteins of Apicomplexa parasites, (ii) secreted enzymes, and (iii) transport and signalling proteins. To verify our strategy experimentally, we used a functional complementation system in yeast. For five selected candidate proteins we found that these were indeed secreted. Our approach thus represents an efficient method to identify secretory and surface proteins out of EST databases.

Building better T-cell-inducing malaria vaccines

Since malaria continues to account for millions of deaths annually in endemic regions, the development of an effective vaccine remains highly desirable. The life cycle of malaria poses a number of challenges to the immune response since phases of the cycle express varying antigen profiles and have different locations, thus requiring differing antigenic targets and effector mechanisms. To confer sterile immunity, a vaccine would have to target the pre-erythrocytic stages of infection. Since at this stage the parasite is hidden within liver cells, the host defence predominantly requires cell-mediated immunity, chiefly T cells, to eliminate infected hepatocytes. The development of such vaccines has progressed from irradiated sporozoites, through recombinant proteins, to recombinant DNA and viral vectors. Some of the experimental vaccination regimens that explore various combinations of vaccines for priming and boosting, together with numbers of vaccinations, interval between them, and the vaccination site, are revealing strong immunogenicity and evidence of efficacy in human challenge studies and in field trials. Such approaches should lead to deployable vaccines that protect against malarial disease.

Induction of strain-transcending immunity against Plasmodium chabaudi adami malaria with a multiepitope DNA vaccine

A major goal of current malaria vaccine programs is to develop multivalent vaccines that will protect humans against the many heterologous malaria strains that circulate in endemic areas. We describe a multiepitope DNA vaccine, derived from a genomic Plasmodium chabaudi adami DS DNA expression library of 30,000 plasmids, which induces strain-transcending immunity in mice against challenge with P. c. adami DK. Segregation of this library and DNA sequence analysis identified vaccine subpools encoding open reading frames (ORFs)/peptides of >9 amino acids [aa] (the V9+ pool, 303 plasmids) and >50 aa (V50+ pool, 56 plasmids), respectively. The V9+ and V50+ plasmid vaccine subpools significantly cross-protected mice against heterologous P. c. adami DK challenge, and protection correlated with the induction of both specific gamma interferon production by splenic cells and opsonizing antibodies. Bioinformatic analysis showed that 22 of the V50+ ORFs were polypeptides conserved among three or more Plasmodium spp., 13 of which are predicted hypothetical proteins. Twenty-nine of these ORFs are orthologues of predicted Plasmodium falciparum sequences known to be expressed in the blood stage, suggesting that this vaccine pool encodes multiple blood-stage antigens. The results have implications for malaria vaccine design by providing proof-of-principle that significant strain-transcending immunity can be induced using multiepitope blood-stage DNA vaccines and suggest that both cellular responses and opsonizing antibodies are necessary for optimal protection against P. c. adami.

Enhancement of antigen acquisition by dendritic cells and MHC class II-restricted epitope presentation to CD4+ T cells using VP22 DNA vaccine vectors that promote intercellular spreading following initial transfection

Induction of immune responses against microbial antigens using DNA is an attractive strategy to mimic the immunity induced by live vaccines. Although DNA vaccines are efficacious in murine models, the requirement for multiple immunizations using high doses in outbred animals and humans has hindered deployment. This requirement is, in part, a result of poor vaccine spreading and suboptimal DC transfection efficiency. Incorporation of a signal that directs intercellular spreading of a DNA-encoded antigen is proposed to mimic live vaccine spreading and increase dendritic cell (DC) presentation. Bovine herpes virus 1 tegument protein, BVP22, is capable of trafficking to surrounding cells. To test the hypothesis that BVP22 enhances spreading and antigen presentation to CD4+ T cells, a DNA construct containing BVP22, fused in-frame to a sequence encoding a T cell epitope of Anaplasma marginale, was generated. A construct with reversed BVP22 sequence served as a negative control. Immunocytometric analysis of transfected primary keratinocytes, human embryonic kidney 293, COS-7, and Chinese hamster ovary cells showed that BVP22 enhanced intercellular spreading by > or = 150-fold. Flow cytometric analysis of antigen-presenting cells (APCs) positively selected from cocultures of transfected cells and APCs showed that 5% of test APCs were antigen-positive, compared with 0.6% of control APCs. Antigen-specific CD4+ T cell proliferation demonstrated that BVP22 enhanced DC antigen presentation by > or = 20-fold. This first report of the ability of BVP22 to increase DNA-encoded antigen acquisition by DCs and macrophages, with subsequent enhancement of major histocompatibility complex class II-restricted CD4+ T cell responses, supports incorporating a spreading motif in a DNA vaccine to target CD4+ T cell-dependent immunity in outbred animals.

Process development and analysis of liver-stage antigen 1, a preerythrocyte-stage protein-based vaccine for Plasmodium falciparum

Plasmodium falciparum liver-stage antigen 1 (LSA-1) is expressed solely in infected hepatocytes and is thought to have a role in liver schizogony and merozoite release. Specific humoral, cellular, and cytokine immune responses to LSA-1 are well documented, with epitopes identified that correlate with antibody production, proliferative T-cell responses, or cytokine induction. With the goal of developing a vaccine against this preerythrocyte-stage protein, we undertook the good manufacturing practices (GMP) manufacture of a recombinant LSA-1 construct, LSA-NRC, incorporating the N- and C-terminal regions of the protein and two of the centrally placed 17-amino-acid repeats. To improve the protein yield, a method of codon harmonization was employed to reengineer the gene sequence for expression in Escherichia coli. A 300-liter GMP fermentation produced 8 kg of bacterial cell paste, and a three-step column chromatographic method yielded 8 mg of purified antigen per g of paste. The final bulk protein was >98% pure, demonstrated long-term stability, and contained <0.005 endotoxin units per 50 microg of protein. To accomplish the initial stages of evaluation of this protein as a human-use vaccine against malaria, we immunized rabbits and mice with LSA-NRC in Montanide ISA 720. New Zealand White rabbits and A/J (H-2K) mice produced high-titer antibodies that recognized liver-stage parasites in infected cultured human hepatocytes. Gamma interferon-producing cells, which have been associated with LSA-1-mediated protection, were detected in splenocytes harvested from immunized mice. Finally, sera taken from people living in a region where malaria is holoendemic recognized LSA-NRC by Western blotting.

Enhanced T cell-mediated protection against malaria in human challenges by using the recombinant poxviruses FP9 and modified vaccinia virus Ankara

Malaria is a major global health problem for which an effective vaccine is required urgently. Prime-boost vaccination regimes involving plasmid DNA and recombinant modified vaccinia virus Ankara-encoding liver-stage malaria antigens have been shown to be powerfully immunogenic for T cells and capable of inducing partial protection against experimental malaria challenge in humans, manifested as a delay in time to patent parasitemia. Here, we report that substitution of plasmid DNA as the priming vector with a specific attenuated recombinant fowlpox virus, FP9, vaccine in such prime-boost regimes can elicit complete sterile protection that can last for 20 months. Protection at 20 months was associated with persisting memory but not effector T cell responses. The protective efficacy of various immunization regimes correlated with the magnitude of induced immune responses, supporting the strategy of maximizing durable T cell immunogenicity to develop more effective liver-stage vaccines against Plasmodium falciparum malaria.

MVA as a vector for vaccines against HIV-1

A vaccine against HIV Type 1 (HIV-1) is urgently needed. Modified vaccinia virus Ankara is an attenuated smallpox vaccine which can be adapted to express HIV-1 antigens. In this review, we discuss the features which make modified vaccinia virus Ankara an attractive vector for genetic vaccines and have put it, together with several other recombinant viral vectors, at the forefront of HIV-1 vaccine development. Many candidate vaccines including those vectored by modified vaccinia virus Ankara are now entering human trials, the results of which will become available in the coming years.

DNA vaccines: designing strategies against parasitic infections

The complexity of parasitic infections requires novel approaches to vaccine design. The versatility of DNA vaccination provides new perspectives. This review discusses the use of prime-boost immunizations, genetic adjuvants, multivalent vaccines and codon optimization for optimal DNA vaccine design against parasites.

A randomised, double-blind, controlled vaccine efficacy trial of DNA/MVA ME-TRAP against malaria infection in Gambian adults

BACKGROUND

Many malaria vaccines are currently in development, although very few have been evaluated for efficacy in the field. Plasmodium falciparum multiple epitope (ME)- thrombospondin-related adhesion protein (TRAP) candidate vaccines are designed to potently induce effector T cells and so are a departure from earlier malaria vaccines evaluated in the field in terms of their mechanism of action. ME-TRAP vaccines encode a polyepitope string and the TRAP sporozoite antigen. Two vaccine vectors encoding ME-TRAP, plasmid DNA and modified vaccinia virus Ankara (MVA), when used sequentially in a prime-boost immunisation regime, induce high frequencies of effector T cells and partial protection, manifest as delay in time to parasitaemia, in a clinical challenge model.

METHODS AND FINDINGS

A total of 372 Gambian men aged 15-45 y were randomised to receive either DNA ME-TRAP followed by MVA ME-TRAP or rabies vaccine (control). Of these men, 296 received three doses of vaccine timed to coincide with the beginning of the transmission season (141 in the DNA/MVA group and 155 in the rabies group) and were followed up. Volunteers were given sulphadoxine/pyrimethamine 2 wk before the final vaccination. Blood smears were collected weekly for 11 wk and whenever a volunteer developed symptoms compatible with malaria during the transmission season. The primary endpoint was time to first infection with asexual P. falciparum. Analysis was per protocol. DNA ME-TRAP and MVA ME-TRAP were safe and well-tolerated. Effector T cell responses to a non-vaccine strain of TRAP were 50-fold higher postvaccination in the malaria vaccine group than in the rabies vaccine group. Vaccine efficacy, adjusted for confounding factors, was 10.3% (95% confidence interval, -22% to +34%; p = 0.49). Incidence of malaria infection decreased with increasing age and was associated with ethnicity.

CONCLUSIONS

DNA/MVA heterologous prime-boost vaccination is safe and highly immunogenic for effector T cell induction in a malaria-endemic area. But despite having produced a substantial reduction in liver-stage parasites in challenge studies of non-immune volunteers, this first generation T cell-inducing vaccine was ineffective at reducing the natural infection rate in semi-immune African adults.

An adenoviral type 5 vector carrying a type 35 fiber as a vaccine vehicle: DC targeting, cross neutralization, and immunogenicity

Substituting the coat proteins of adenoviral vector serotype 5 (Ad5) can alter vector tropism and circumvent vector neutralization. Here we report that an Ad5 vector carrying a part of the fiber molecule of human subgroup B adenovirus serotype 35 (Ad5.Fib35) transduces cultured human dendritic cells (DC) and circulating myeloid derived DC with approximately 10-fold greater efficiency than Ad5 in vitro. The improved DC transduction results in increased T-cell activation ex vivo. In vivo however, immunogenicity of the vectors in mice and non-human primates did not correlate with in vitro DC tropism. Ad5.Fib35 was less immunogenic in monkeys than Ad5, despite the improved primate DC tropism of Ad5.Fib35. In mice with high Ad5 vector-specific immunity, Ad5.Fib35 showed no significant difference in anti-insert immunity over Ad5 indicating that fiber exchange alone does not evade pre-existing Ad5 immunity. We thus conclude that, for ex vivo vaccination, Ad5.Fib35 shows promise as vector for loading of DC but is unable to circumvent anti-Ad5 immunity limiting its in vivo utility.

Cellular immune responses induced in cattle by heterologous prime-boost vaccination using recombinant viruses and bacille Calmette-Guérin

The development of novel vaccine strategies to replace or supplement bacille Calmette-Guérin (BCG) is urgently required. Here we study, in cattle, the use of heterologous prime-boost strategies based on vaccination with BCG and the mycobacterial mycolyl transferase Ag85A (Rv3804c) expressed either in recombinant modified vaccinia virus Ankara (MVA85A) or attenuated fowlpox strain FP9 (FP85A). Five different vaccination schedules were tested in the first experiment: MVA85A followed by BCG (group 1); BCG followed by MVA85A (group 2); BCG followed by FP85A and then MVA85A (group 3); MVA85A followed by MVA85A and then FP85A (group 4); and FP85A followed by FP85A and then MVA85A (group 5). Vaccine-induced levels of cellular immunity were assessed by determining interferon-gamma (IFN-gamma) responses in vitro. Prime-boost protocols, using recombinant MVA and BCG in combination (groups 1-3), resulted in significantly higher frequencies of Ag85-specific IFN-gamma-secreting cells than the two viral vectors used in combination (P=0.0055), or BCG used alone (groups 2 and 3, P=0.04). The T-cell repertoires of the calves in all five groups were significantly broader following heterologous booster immunizations than after the primary immunization. In a second experiment, the effects of BCG\MVA85A heterologous prime-boost vaccination were compared with BCG\BCG homologous revaccination. The results suggested a higher Ag85A-specific response with a wider T-cell repertoire in the MVA85A-boosted calves than in the BCG\BCG-vaccinated calves. In conclusion therefore, the present report demonstrates the effectiveness of heterologous prime-boost strategies based on recombinant MVA and BCG to induce strong cellular immune responses in cattle and prioritise such vaccination strategies for rapid assessment of protective efficacy in this natural target species of tuberculosis.

Progress in DNA-based heterologous prime-boost immunization strategies for malaria

An effective vaccine against malaria is urgently required to relieve the immense human suffering and mortality caused by this parasite. A successful subunit vaccine against the liver stage of malaria will require the induction of high levels of protective T cells. Despite success in small animal models, DNA vaccines fail to induce strong cellular immune responses in humans. However, DNA vaccines can induce a T-cell response that can be strongly boosted by recombinant viral vectors. We have evaluated this heterologous prime-boost approach using the Plasmodium berghei mouse model for immunogenicity and protective efficacy against malaria challenge using combinations of plasmid DNA, recombinant modified vaccinia virus Ankara, fowlpox virus, and non-replicating adenovirus. We have proceeded to test immunogenicity and efficacy of successful heterologous prime-boost vaccines in phase I/IIa trials in malaria naïve subjects in the UK and in semi-immune individuals in The Gambia. In these clinical trials, remarkably high levels of effector T-cell responses have been induced and significant protection documented in a human sporozoite challenge model. We summarize the preclinical design and development of these heterologous prime-boost vaccines and discuss the encouraging results that have been observed in vaccinated humans.