Ty virus-like particles, DNA vaccines and Modified Vaccinia Virus Ankara; comparisons and combinations

Recombinant modified vaccinia virus Ankara-based malaria vaccines

A safe and effective malaria vaccine is a crucial part of the roadmap to malaria elimination/eradication by the year 2050. Viral-vectored vaccines based on adenoviruses and modified vaccinia virus Ankara (MVA) expressing malaria immunogens are currently being used in heterologous prime-boost regimes in clinical trials for induction of strong antigen-specific T-cell responses and high-titer antibodies. Recombinant MVA is a safe and well-tolerated attenuated vector that has consistently shown significant boosting potential. Advances have been made in large-scale MVA manufacture as high-yield producer cell lines and high-throughput purification processes have recently been developed. This review describes the use of MVA as malaria vaccine vector in both preclinical and clinical studies in the past 5 years.

The influence of delivery vectors on HIV vaccine efficacy

Development of an effective HIV/AIDS vaccine remains a big challenge, largely due to the enormous HIV diversity which propels immune escape. Thus novel vaccine strategies are targeting multiple variants of conserved antibody and T cell epitopic regions which would incur a huge fitness cost to the virus in the event of mutational escape. Besides immunogen design, the delivery modality is critical for vaccine potency and efficacy, and should be carefully selected in order to not only maximize transgene expression, but to also enhance the immuno-stimulatory potential to activate innate and adaptive immune systems. To date, five HIV vaccine candidates have been evaluated for efficacy and protection from acquisition was only achieved in a small proportion of vaccinees in the RV144 study which used a canarypox vector for delivery. Conversely, in the STEP study (HVTN 502) where human adenovirus serotype 5 (Ad5) was used, strong immune responses were induced but vaccination was more associated with increased risk of HIV acquisition than protection in vaccinees with pre-existing Ad5 immunity. The possibility that pre-existing immunity to a highly promising delivery vector may alter the natural course of HIV to increase acquisition risk is quite worrisome and a huge setback for HIV vaccine development. Thus, HIV vaccine development efforts are now geared toward delivery platforms which attain superior immunogenicity while concurrently limiting potential catastrophic effects likely to arise from pre-existing immunity or vector-related immuno-modulation. However, it still remains unclear whether it is poor immunogenicity of HIV antigens or substandard immunological potency of the safer delivery vectors that has limited the success of HIV vaccines. This article discusses some of the promising delivery vectors to be harnessed for improved HIV vaccine efficacy.

Clinical development of Modified Vaccinia virus Ankara vaccines

The smallpox vaccine Vaccinia was successfully used to eradicate smallpox, but although very effective, it was a very reactogenic vaccine and responsible for the deaths of one or two people per million vaccinated. Modified Vaccinia virus Ankara (MVA) is a replication-deficient and attenuated derivative, also used in the smallpox eradication campaign and now being developed as a recombinant viral vector to produce vaccines against infectious diseases and cancer. Many clinical trials of these new vaccines have been conducted, and the findings of these trials are reviewed here. The safety of MVA is now well documented, immunogenicity is influenced by the dose and vaccination regimen, and information on the efficacy of MVA-vectored vaccines is now beginning to accumulate.

Immune responses elicited by co-immunization of Plasmodium vivax and P. falciparum MSP-1 using prime-boost immunization strategies

Carboxy-terminus of merozoite surface protein-1 (MSP-1(19) ) is the major protein on the surface of the plasmodial merozoite that acts as one of the most important blood-stage vaccine candidates. The present investigation was designed to evaluate the immune responses when either two recombinant antigens (rPvMSP-1(19) + rPfMSP-1(19)) or two plasmid constructs (pcDNA3.1 hygro-PvMSP-1(19) + pcDNA3.1 hygro-PfMSP-1(19)) were administered in combination at a single site in mice by using different immunization strategies (protein/protein, DNA/DNA and DNA/protein) at weeks 0, 5 and 8. All mice were monitored for the level of MSP-1(19) -specific antibody for up to 40 weeks. The inclusion of both recombinant antigens in a vaccine mixture could not inhibit induction of antibodies to the other antigen when the two recombinant antigens were combined in immunization formulation. Interestingly, antisera from immunized mice with either recombinant antigen failed to cross-react with heterologous antigen. Moreover, the results of this study showed that co-immunization with both antigens at a single site generated a substantial PvMSP-1(19) - and PfMSP-1(19) -specific antibody responses and also IFN-γ cytokine production (Th1 response) in DNA/protein prime-boost immunization strategies. The increased humoral response to PvMSP-1(19) and PfMSP-1(19) lasted nearly a year after immunization. Therefore, the results of this study are encouraging for the development of multi-species malaria vaccine based on MSP-1(19) antigen.

Partially randomized, non-blinded trial of DNA and MVA therapeutic vaccines based on hepatitis B virus surface protein for chronic HBV infection

BACKGROUND

Chronic HBV infects 350 million people causing cancer and liver failure. We aimed to assess the safety and efficacy of plasmid DNA (pSG2.HBs) vaccine, followed by recombinant modified vaccinia virus Ankara (MVA.HBs), encoding the surface antigen of HBV as therapy for chronic HBV. A secondary goal was to characterize the immune responses.

METHODS

Firstly 32 HBV e antigen negative (eAg(-)) participants were randomly assigned to one of four groups: to receive vaccines alone, lamivudine (3TC) alone, both, or neither. Later 16 eAg(+) volunteers in two groups received either 3TC alone or both 3TC and vaccines. Finally, 12 eAg(-) and 12 eAg(+) subjects were enrolled into higher-dose treatment groups. Healthy but chronically HBV-infected males between the ages of 15-25 who lived in the western part of The Gambia were eligible. Participants in some groups received 1 mg or 2 mg of pSG2.HBs intramuscularly twice followed by 5×10(7) pfu or 1.5×10(8) pfu of MVA.HBs intradermally at 3-weekly intervals with or without concomitant 3TC for 11-14 weeks. Intradermal rabies vaccine was administered to a negative control group. Safety was assessed clinically and biochemically. The primary measure of efficacy was a quantitative PCR assay of plasma HBV. Immunity was assessed by IFN-γ ELISpot and intracellular cytokine staining.

RESULTS

Mild local and systemic adverse events were observed following the vaccines. A small shiny scar was observed in some cases after MVA.HBs. There were no significant changes in AST or ALT. HBeAg was lost in one participant in the higher-dose group. As expected, the 3TC therapy reduced viraemia levels during therapy, but the prime-boost vaccine regimen did not reduce the viraemia. The immune responses were variable. The majority of IFN-γ was made by antigen non-specific CD16(+) cells (both CD3(+) and CD3(-)).

CONCLUSIONS

The vaccines were well tolerated but did not control HBV infection.

TRIAL REGISTRATION

ISRCTN ISRCTN67270384.

Why functional pre-erythrocytic and bloodstage malaria vaccines fail: a meta-analysis of fully protective immunizations and novel immunological model

BACKGROUND

Clinically protective malaria vaccines consistently fail to protect adults and children in endemic settings, and at best only partially protect infants.

METHODOLOGY/PRINCIPAL FINDINGS

We identify and evaluate 1916 immunization studies between 1965-February 2010, and exclude partially or nonprotective results to find 177 completely protective immunization experiments. Detailed reexamination reveals an unexpectedly mundane basis for selective vaccine failure: live malaria parasites in the skin inhibit vaccine function. We next show published molecular and cellular data support a testable, novel model where parasite-host interactions in the skin induce malaria-specific regulatory T cells, and subvert early antigen-specific immunity to parasite-specific immunotolerance. This ensures infection and tolerance to reinfection. Exposure to Plasmodium-infected mosquito bites therefore systematically triggers immunosuppression of endemic vaccine-elicited responses. The extensive vaccine trial data solidly substantiate this model experimentally.

CONCLUSIONS/SIGNIFICANCE

We conclude skinstage-initiated immunosuppression, unassociated with bloodstage parasites, systematically blocks vaccine function in the field. Our model exposes novel molecular and procedural strategies to significantly and quickly increase protective efficacy in both pipeline and currently ineffective malaria vaccines, and forces fundamental reassessment of central precepts determining vaccine development. This has major implications for accelerated local eliminations of malaria, and significantly increases potential for eradication.

Clinical experience with plasmid DNA- and modified vaccinia virus Ankara-vectored human immunodeficiency virus type 1 clade A vaccine focusing on T-cell induction

Candidate human immunodeficiency virus type 1 (HIV-1) vaccines focusing on T-cell induction, constructed as pTHr.HIVA DNA and modified vaccinia virus Ankara (MVA).HIVA, were delivered in a heterologous prime-boost regimen. The vaccines were tested in several hundred healthy or HIV-1-infected volunteers in Europe and Africa. Whilst larger trials of hundreds of volunteers suggested induction of HIV-1-specific T-cell responses in <15 % of healthy vaccinees, a series of small, rapid trials in 12-24 volunteers at a time with a more in-depth analysis of vaccine-elicited T-cell responses proved to be highly informative and provided more encouraging results. These trials demonstrated that the pTHr.HIVA vaccine alone primed consistently weak and mainly CD4(+), but also CD8(+) T-cell responses, and the MVA.HIVA vaccine delivered a consistent boost to both CD4(+) and CD8(+) T cells, which was particularly strong in HIV-1-infected patients. Thus, whilst the search is on for ways to enhance T-cell priming, MVA is a useful boosting vector for human subunit genetic vaccines.

Protective and pathogenic roles of CD8+ T cells during malaria infection

CD8+ T cells play a key role in protection against pre-erythrocytic stages of malaria infection. Many vaccine strategies are based on the idea of inducing a strong infection-blocking CD8+ T cell response. Here, we summarize what is known about the development, specificity and protective effect of malaria-specific CD8+ T cells and report on recent developments in the field. Although work in mouse models continues to make progress in our understanding of the basic biology of these cells, many questions remain to be answered - particularly on the roles of these cells in human infections. Increasing evidence is also emerging of a harmful role for CD8+ T cells in the pathology of cerebral malaria in rodent systems. Once again, the relevance of these results to human disease is one of the primary questions facing workers in this field.

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.

Approaches to enhance the efficacy of DNA vaccines

DNA vaccines consist of antigen-encoding bacterial plasmids that are capable of inducing antigen-specific immune responses upon inoculation into a host. This method of immunization is advantageous in terms of simplicity, adaptability, and cost of vaccine production. However, the entry of DNA vaccines and expression of antigen are subjected to physical and biochemical barriers imposed by the host. In small animals such as mice, the host-imposed impediments have not prevented DNA vaccines from inducing long-lasting, protective humoral, and cellular immune responses. In contrast, these barriers appear to be more difficult to overcome in large animals and humans. The focus of this article is to summarize the limitations of DNA vaccines and to provide a comprehensive review on the different strategies developed to enhance the efficacy of DNA vaccines. Several of these strategies, such as altering codon bias of the encoded gene, changing the cellular localization of the expressed antigen, and optimizing delivery and formulation of the plasmid, have led to improvements in DNA vaccine efficacy in large animals. However, solutions for increasing the amount of plasmid that eventually enters the nucleus and is available for transcription of the transgene still need to be found. The overall conclusions from these studies suggest that, provided these critical improvements are made, DNA vaccines may find important clinical and practical applications in the field of vaccination.

The hybrid cytomegalovirus enhancer/chicken beta-actin promoter along with woodchuck hepatitis virus posttranscriptional regulatory element enhances the protective efficacy of DNA vaccines

DNA vaccines represent a novel and powerful alternative to conventional vaccine approaches. They are extremely stable and can be produced en masse at low cost; more importantly, DNA vaccines against emerging pathogens or bioterrorism threats can be quickly constructed based solely upon the pathogen's genetic code. The main drawback of DNA vaccines is that they often induce lower immune responses than traditional vaccines, particularly in nonrodent species. Thus, improving the efficacy of DNA vaccines is a critical issue in vaccine development. In this study we have enhanced the efficacy of DNA vaccines by adopting strategies that increase gene expression. We generated influenza-hemagglutinin (HA)-encoding DNA vaccines that contain the hybrid CMV enhancer/chicken beta-actin (CAG) promoter and/or the mRNA-stabilizing post-transcriptional regulatory element from the woodchuck hepatitis virus (WPRE). Mice were immunized with these DNA vaccines, and the influenza-HA-specific cellular and humoral immune responses were compared with a conventional, HA-encoding DNA vaccine whose gene expression was driven by the CMV immediate-early promoter (pCMV-HA). CAG promoter-driven DNA vaccines elicited significantly higher humoral and cellular immune responses compared with the pCMV-HA vaccine. DNA vaccines consisting of both CAG and WPRE elements (pCAG-HA-WPRE) induced the highest level of protective immunity, such that immunization with 10-fold lower DNA doses prevented death in 100% of the mice upon lethal viral challenge, whereas all mice immunized with the conventional pCMV-HA vaccine succumbed to influenza infection.

Control of simian/human immunodeficiency virus viremia and disease progression after IL-2-augmented DNA-modified vaccinia virus Ankara nasal vaccination in nonhuman primates

A successful HIV vaccine may need to stimulate antiviral immunity in mucosal and systemic immune compartments, because HIV transmission occurs predominantly at mucosal sites. We report here the results of a combined DNA-modified vaccinia virus Ankara (MVA) vaccine approach that stimulated simian/human immunodeficiency virus (SHIV)-specific immune responses by vaccination at the nasal mucosa. Fifteen male rhesus macaques, divided into three groups, received three nasal vaccinations on day 1, wk 9, and wk 25 with a SHIV DNA plasmid producing noninfectious viral particles (group 1), or SHIV DNA plus IL-2/Ig DNA (group 2), or SHIV DNA plus IL-12 DNA (group 3). On wk 33, all macaques were boosted with rMVA expressing SIV Gag-Pol and HIV Env 89.6P, administered nasally. Humoral responses were evaluated by measuring SHIV-specific IgG and neutralizing Abs in plasma, and SHIV-specific IgA in rectal secretions. Cellular responses were monitored by evaluating blood-derived virus-specific IFN-gamma-secreting cells and TNF-alpha-expressing CD8+ T cells, and blood- and rectally derived p11C tetramer-positive T cells. Many of the vaccinated animals developed both mucosal and systemic humoral and cell-mediated anti-SHIV immune responses, although the responses were not homogenous among animals in the different groups. After rectal challenge of vaccinated and naive animals with SHIV89.6P, all animals became infected. However a subset, including all group 2 animals, were protected from CD4+ T cell loss and AIDS development. Taken together, these data indicate that nasal vaccination with SHIV-DNA plus IL-2/Ig DNA and rMVA can provide significant protection from disease progression.

Boosting with recombinant vaccinia increases HPV-16 E7-Specific T cell precursor frequencies and antitumor effects of HPV-16 E7-expressing Sindbis virus replicon particles

Immunotherapy using the heterologous prime-boost regimen has emerged as an attractive approach for generating antigen-specific T-cell-mediated immune responses against tumors and infectious diseases. We have previously linked the Mycobacterium tuberculosis heat-shock protein 70 (HSP70) to the HPV-16 E7 antigen creating a chimera, E7/HSP70. We found that nucleic acid vaccines encoding E7/HSP70 can generate strong antitumor immunity. Recently, replication-defective Sindbis virus replicon particle vaccines have been considered as an important vector system for vaccine development. In this study, we assessed whether the combination of E7/HSP70 Sindbis virus replicon particles (SINrep5-E7/HSP70) and E7/HSP70 vaccinia (Vac-E7/HSP70) can further enhance E7-specific immune responses using sequential vaccination. We found that priming with SINrep5-E7/HSP70 and boosting with Vac-E7/HSP70 generated the highest number of E7-specific CD8(+) T cells and best antitumor effect compared to other combinations. Moreover, our data showed that at the dosage and route of immunization used in this study, mice treated with the Sindbis virus replicon particle prime-vaccinia boost regimen generated stronger antitumor responses compared to mice treated with the DNA prime-vaccinia boost vaccine regimen. Our results encourage the use of the Sindbis virus replicon particle prime-vaccinia boost regimen in future clinical trials.

Heterologous priming-boosting immunization of cattle with Mycobacterium tuberculosis 85A induces antigen-specific T-cell responses

Heterologous priming-boosting vaccination regimens involving priming with plasmid DNA antigen constructs and inoculating (boosting) with the same recombinant antigen expressed in replication-attenuated poxviruses have recently been demonstrated to induce immunity, based on CD4(+)- and CD8(+)-T-cell responses, against several diseases in both rodents and primates. We show that similar priming-boosting vaccination strategies using the 85A antigen of Mycobacterium tuberculosis are effective in inducing antigen-specific gamma interferon-secreting CD4(+) and CD8(+) T cells, detected by a bovine enzyme-linked immunospot assay, in Bos indicus cattle. T-cell responses induced by priming with either plasmid DNA or fowlpox virus 85A constructs were enhanced by boosting with modified vaccinia virus Ankara expressing the same antigen administered intradermally. On the basis of the data, it appears that intradermal priming was more effective than intramuscular delivery of the priming dose for boosting with the modified vaccinia virus Ankara strain in cattle. Using either fowlpox virus or DNA priming, there was a significant bias toward induction of CD4(+)- rather than CD8(+)-T-cell responses. These data illustrate the general applicability of priming-boosting vaccination strategies for induction of antigen-specific T-cell responses and suggest that the method may be useful for development of veterinary vaccines.

Prime-boost strategies for malaria vaccine development

Malaria is an intracellular pathogen, for which an effective vaccine is likely to require induction of cell-mediated immunity. Immunisation approaches that stimulate strong and persistent levels of effector T-cells are being sought by many researchers. DNA vaccines, recombinant protein and viral vectors were amongst the vaccine delivery systems that appeared promising for the generation of cellular immunity, and in some initial studies in small animals this goal was achieved. However, clinical trials of these candidate vaccines when used alone or in repeated homologous boosting regimes have been disappointing, with short-lived low levels of induced specific T-cell responses. Recent years have seen the development of immunisation strategies using a combination of different antigen delivery systems encoding the same epitopes or antigen, delivered at an interval of a few weeks apart. This sequential immunisation approach with different vectors is known as heterologous prime-boosting and is capable of inducing greatly enhanced and persistent levels of CD8+ T-cells and Th1-type CD4+ T-cells compared to homologous boosting. This review will summarise the key pre-clinical studies of prime-boost strategy and outline recent progress in clinical trials of this approach. Possible mechanisms of action and potential improvements to existing delivery systems will be discussed. The prime-boost approach represents an encouraging step towards establishing an effective preventative vaccine to one of the world's greatest killers.

Current status of malaria vaccine development

There is an urgent need to develop an effective vaccine against malaria--a disease that has approximately 10% of the world population at risk of infection at any given time. The economic burden this disease puts on the medico-social set-up of countries in Sub-Saharan Africa and South East Asia is phenomenal. Increasing drug resistance and failure of vector control strategies have necessitated the search for a suitable vaccine that could be integrated into the extended program of immunization for countries in the endemic regions. Malaria vaccine development has seen a surge of activity in the last decade or so owing largely to the advances made in the fields of genetic engineering and biotechnology. This revolution has brought sweeping changes in the understanding of the biology of the parasite and has helped formulate newer more effective strategies to combat the disease. Latest developments in the field of malaria vaccine development will be discussed in this chapter.

Comparison of virus production in chicken embryo fibroblasts infected with the WR, IHD-J and MVA strains of vaccinia virus: IHD-J is most efficient in trans-Golgi network wrapping and extracellular enveloped virus release

Modified vaccinia virus Ankara (MVA) is an attenuated strain derived from vaccinia virus (VV) Ankara that grows efficiently in primary chicken embryo fibroblasts (CEFs) and baby hamster kidney cells only. MVA produces significantly more of the enveloped forms of VV in infected CEFs compared with VV strain Copenhagen. In the present study, production of the different infectious forms of VV was compared in CEFs infected with MVA or with two well-characterized replication-competent VV strains, WR and IHD-J. In a time-course experiment, the infectivity associated with the extracellular enveloped virus (EEV), the cell-associated enveloped virus (CEV) and intracellular mature and enveloped viruses was determined. Further, the production of the different viral forms was quantified by electron microscopy (EM). The data collectively indicate that IHD-J is most efficient in producing all of the trans-Golgi network-wrapped forms and releases the highest titres of EEVs into the extracellular medium, with WR being least efficient. MVA initially replicated with faster kinetics, resulting in more intracellular virus and CEVs between 8 and 24 h post-infection (p.i.). As assessed by EM, the faster growth kinetics of MVA resulted in 3.5-fold more CEVs at the cell surface at 24 h p.i., compared with both WR and IHD-J. Accordingly, we found that despite the presence of two in-frame deletions in the A36R gene of MVA, this virus was able to make actin tails in CEFs.

Aspects of cancer immunotherapy

Cancer immunotherapy has traditionally undergone a 'revolution' every decade, from the use of Bacille Calmette-Guérin by scarification in the 1970s, to interleukin-2 therapies in the 1980s, and monoclonal antibody treatments in the early 1990s. Usually the early reports on the use of such agents were encouraging, but when more patients were studied in multiple centres, the initial promising results could not be confirmed. Now in a new century, we have more reagents and methods available than ever before - indeed, with such a plethora of reagents it is difficult to envisage them being fully and appropriately tested within the next decade, by which time there will be even more reagents to test. However, there have been three major advances which should lead to substantial progress in cancer immunotherapy: (1) the widespread use of genetic engineering, enabling identification of candidate vaccine proteins and manipulation of their sequences; (2) the production of antigens, antibodies and cytokines in large amounts by recombinant technologies, and (3) an understanding of the mode of presentation of peptides by major histocompatibility complex Class I and Class II molecules and their recognition by T cells. Despite these advances, there are major problems facing cancer immunotherapy, such as the ability of tumours to mutate and evade the immune system and the difficulty of precisely defining the interactions of effector cells in mediating 'rejection' or destruction of a tumour. There are clearly immunological similarities with diseases such as malaria and schistosomiasis, where the invading foreign organisms can use a variety of strategies to resist an elicited immune response. The failure to find a suitable vaccine for these diseases must lead to some pessimism for the development of immunotherapy for an autologous tumour. However, there are promising studies now in progress which should give an indication of the most important directions to follow. This review provides a commentary on aspects of cancer immunotherapy and in particular will deal with: (1) the selection of antigens as vaccine components; (2) the modes of presentation of antigens, particularly by major histocompatibility complex Class I molecules; and (3) new modes of delivery of vaccine immunogens.

Tumor growth inhibition elicited by different vaccines and correlation with antigen specific cytotoxic T-cell frequencies determined by intracellular interferon-gamma staining

Different vaccines based on naked DNA and the modified vaccinia virus Ankara (MVA) were compared for their efficiency to protect mice against tumors bearing the model antigen beta-galactosidase (beta-Gal) and for their potential to induce an antigen specific cellular immune response. Mice were immunized with the LacZ gene applied as naked DNA. In accordance with the observed beta-Gal-specific T-cell frequency, only 20% of mice boosted with LacZ naked DNA developed tumors whereas all mice boosted with MVA expressing LacZ developed a tumor. Mice vaccinated with mock DNA or mock virus developed tumors in 60 or 100%, respectively. MVA vaccination led to strong and long-lasting CD4- and CD8-T-cell responses against viral antigens but not against beta-Gal.

Exoerythrocytic malaria vaccine development: understanding host-parasite immunobiology underscores strategic success

Malaria imposes an enormous health burden on people living in the tropics and effective control measures are urgently needed. The vast majority of deaths in humans from malaria are caused by one species of the protozoan, Plasmodium falciparum. An efficacious and cost-effective vaccine against this parasite is considered a holy grail of modern molecular medicine. A vaccine that targets liver-stage parasites would prevent infection from reaching the blood and causing clinical disease. Among around 40 known Plasmodium falciparum antigens, only a few are expressed exclusively by mosquito-transmitted sporozoites or infected hepatocytes. Studies in humans have consistently related immune responses to these antigens with resistance to infection or disease, providing a powerful rationale for the development of pre-erythrocytic vaccines. By dissecting the mechanism(s) of immunity to these antigens, we can best evaluate in different delivery systems epitopes associated with protection as components of a focused and coordinated multiantigen malaria vaccine strategy.

The block in assembly of modified vaccinia virus Ankara in HeLa cells reveals new insights into vaccinia virus morphogenesis

It has previously been shown that upon infection of HeLa cells with modified vaccinia virus Ankara (MVA), assembly is blocked at a late stage of infection and immature virions (IVs) accumulate (G. Sutter and B. Moss, Proc. Natl. Acad. Sci. USA 89:10847-10851, 1992). In the present study the morphogenesis of MVA in HeLa cells was studied in more detail and compared to that under two conditions that permit the production of infectious particles: infection of HeLa cells with the WR strain of vaccinia virus (VV) and infection of BHK cells with MVA. Using several quantitative and qualitative assays, we show that early in infection, MVA in HeLa cells behaves in a manner identical to that under the permissive conditions. By immunofluorescence microscopy (IF) at late times of infection, the labelings for an abundant membrane protein of the intracellular mature virus, p16/A14L, and the viral DNA colocalize under permissive conditions, whereas in HeLa cells infected with MVA these two structures do not colocalize to the same extent. In both permissive and nonpermissive infection, p16-labeled IVs first appear at 5 h postinfection. In HeLa cells infected with MVA, IVs accumulated predominantly outside the DNA regions, whereas under permissive conditions they were associated with the viral DNA. At 4 h 30 min, the earliest time at which p16 is detected, the p16 labeling was found predominantly in a small number of distinct puncta by IF, which were distinct from the sites of DNA in both permissive and nonpermissive infection. By electron microscopy, no crescents or IVs were found at this time, and the p16-labeled structures were found to consist of membrane-rich vesicles that were in continuity with the cellular endoplasmic reticulum. Over the next 30 min of infection, a large number of p16-labeled crescents and IVs appeared abruptly under both permissive and nonpermissive conditions. Under permissive conditions, these IVs were in close association with the sites of DNA, and a significant amount of these IVs engulfed the viral DNA. In contrast, under nonpermissive conditions, the IVs and DNA were mostly in separate locations and relatively few IVs acquired DNA. Our data show that in HeLa cells MVA forms normal DNA replication sites and normal viral precursor membranes but the transport between these two structures is inhibited.

Enhanced CD8 T cell immunogenicity and protective efficacy in a mouse malaria model using a recombinant adenoviral vaccine in heterologous prime-boost immunisation regimes

Recombinant replication-defective adenovirus expressing the CS gene from Plasmodium berghei (Ad-PbCS) was found to induce a strong CD8(+) T cell response after intra-dermal or -muscular immunisation. Boosting of an adenovirus-primed immune response with the replication-impaired poxvirus, modified vaccinia virus Ankara (MVA) led to enhanced immunogenicity and substantial protective efficacy. The recombinant adenoviral vaccine was capable of boosting to protective levels a CD8(+) T cell response primed by either a plasmid DNA vaccine, a recombinant Ty virus-like particle vaccine or recombinant MVA each expressing the same epitope or antigen. Complete protective efficacy after intradermal immunisation was observed with the adenovirus prime-MVA boost regime. This study identifies recombinant replication-defective adenovirus as an alternative to recombinant replication-defective poxviruses as boosting agents for the induction of strong protective CD8(+) T cell responses.

Malaria vaccines

Malaria kills one child in Africa every 30 s. After summarising the burden of malaria, the life-cycle of this parasite in humans and female Anopheles mosquitoes is outlined. Important differences between natural immunity and that induced by current candidate vaccines are discussed. In the main part of the review, the recent rapid expansion in evaluation of candidate malaria vaccines in clinical trials across the world is discussed. Subunit vaccine technologies are progressing rapidly with new delivery systems, vectors and antigens under evaluation as well as new polyepitope approaches. Combination vaccination regimens, improved adjuvants and genetic engineering of antigens are all improving the immunogenicity of candidate vaccines. We also discuss particular difficulties in vaccination against malaria, the conduct of field trials of malaria vaccines in non-industrialised countries and the need for even greater co-operation between researchers. Finally, the important concept of iterative vaccine development is raised and the prospects for effective malaria vaccination are discussed.

DNA-based vaccines against malaria: status and promise of the Multi-Stage Malaria DNA Vaccine Operation

The introduction of DNA vaccine technology has facilitated an unprecedented multi-antigen approach to developing an effective vaccine against complex pathogens such as the Plasmodium spp. parasites that cause malaria. We have established the capacity of DNA vaccines encoding Plasmodium antigens to induce CD8(+) cytotoxic T lymphocyte and interferon-gamma responses in mice, monkeys and humans. However, like others, we have found that the first or second generation DNA vaccines on their own are not optimal, and have demonstrated the potential of heterologous prime/boost immunisation strategies involving priming with DNA and boosting with poxvirus or recombinant protein in adjuvant. In this review, we summarise the current status and promise of our programmatic efforts to develop a DNA-based vaccine against malaria, our Multi-Stage Malaria DNA Vaccine Operation, and illustrate the transition of promising developments in the laboratory to clinical assessment in humans.

Boosting with recombinant vaccinia increases HPV-16 E7-specific T cell precursor frequencies of HPV-16 E7-expressing DNA vaccines

We have previously linked the sorting signals of the lysosome-associated membrane protein-1 (LAMP-1) to HPV-16 E7 antigen, creating a chimera, Sig/E7/LAMP-1. We found that both Sig/E7/LAMP-1-containing recombinant vaccinia virus (Vac-Sig/E7/LAMP-1) and Sig/E7/LAMP-1 DNA can generate strong antitumor immunity. To determine whether combination of Sig/E7/LAMP-1 DNA and Vac-Sig/E7/LAMP-1 can further enhance immune responses, sequential vaccination with Sig/E7/LAMP-1 DNA and Vac-Sig/E7/LAMP-1 was given. We found that priming with Sig/E7/LAMP-1 DNA and boosting with Vac-Sig/E7/LAMP-1 generated the strongest E7-specific CD8(+) T cell responses. Our results encourage the use of the DNA prime/vaccinia booster regimen in future clinical trials.