The Future of Smallpox Vaccination: is MVA the key?

Development of Modified Vaccinia Virus Ankara-Based Vaccines: Advantages and Applications

Modified vaccinia virus Ankara (MVA) is a promising viral vector for vaccine development. MVA is well studied and has been widely used for vaccination against smallpox in Germany. This review describes the history of the origin of the virus and its properties as a vaccine, including a high safety profile. In recent years, MVA has found its place as a vector for the creation of vaccines against various diseases. To date, a large number of vaccine candidates based on the MVA vector have already been developed, many of which have been tested in preclinical and clinical studies. We discuss data on the immunogenicity and efficacy of some of these vaccines.

Evidence for Residual Immunity to Smallpox After Vaccination and Implications for Re-emergence

INTRODUCTION

Smallpox has been eradicated but advances in synthetic biology have increased the risk of its re-emergence. Residual immunity in individuals who were previously vaccinated may mitigate the impact of an outbreak, but there is a high degree of uncertainty about the duration and degree of residual immunity. Both cell-mediated and humoral immunity are thought to be important but the exact mechanisms of protection are unclear. Guidelines usually suggest vaccine-induced immunity wanes to zero after 3-10 years post vaccination, whereas other estimates show long term immunity over decades.

MATERIALS AND METHODS

A systematic review of the literature was conducted to quantify the duration and extent of residual immunity to smallpox after vaccination.

RESULTS

Twenty-nine papers related to quantifying residual immunity to smallpox after vaccination were identified: neutralizing antibody levels were used as immune correlates of protection in 11/16 retrospective cross-sectional studies, 2/3 epidemiological studies, 6/7 prospective vaccine trials and 0/3 modeling studies. Duration of protection of >20 years was consistently shown in the 16 retrospective cross-sectional studies, while the lowest estimated duration of protection was 11.7 years among the modeling studies. Childhood vaccination conferred longer duration of protection than vaccination in adulthood, and multiple vaccinations did not appear to improve immunity.

CONCLUSIONS

Most studies suggest a longer duration of residual immunity (at least 20 years) than assumed in smallpox guidelines. Estimates from modeling studies were less but still greater than the 3-10 years suggested by the WHO Committee on International Quarantine or US CDC guidelines. These recommendations were probably based on observations and studies conducted while smallpox was endemic. The cut-off values for pre-existing antibody levels of >1:20 and >1:32 reported during the period of endemic smallpox circulation may not be relevant to the contemporary population, but have been used as a threshold for identifying people with residual immunity in post-eradication era studies. Of the total antibodies produced in response to smallpox vaccination, neutralizing antibodies have shown to contribute significantly to immunological memory. Although the mechanism of immunological memory and boosting is unclear, revaccination is likely to result in a more robust response. There is a need to improve the evidence base for estimates on residual immunity to better inform planning and preparedness for re-emergent smallpox.

Hazard Characterization of Modified Vaccinia Virus Ankara Vector: What Are the Knowledge Gaps?

Modified vaccinia virus Ankara (MVA) is the vector of choice for human and veterinary applications due to its strong safety profile and immunogenicity in vivo. The use of MVA and MVA-vectored vaccines against human and animal diseases must comply with regulatory requirements as they pertain to environmental risk assessment, particularly the characterization of potential adverse effects to humans, animals and the environment. MVA and recombinant MVA are widely believed to pose low or negligible risk to ecosystem health. However, key aspects of MVA biology require further research in order to provide data needed to evaluate the potential risks that may occur due to the use of MVA and MVA-vectored vaccines. The purpose of this paper is to identify knowledge gaps in the biology of MVA and recombinant MVA that are of relevance to its hazard characterization and discuss ongoing and future experiments aimed at providing data necessary to fill in the knowledge gaps. In addition, we presented arguments for the inclusion of uncertainty analysis and experimental investigation of verifiable worst-case scenarios in the environmental risk assessment of MVA and recombinant MVA. These will contribute to improved risk assessment of MVA and recombinant MVA vaccines.

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.

Gene Expression Driven by a Strong Viral Promoter in MVA Increases Vaccination Efficiency by Enhancing Antibody Responses and Unmasking CD8⁺ T Cell Epitopes

Viral vectors are promising tools for vaccination strategies and immunotherapies. However, CD8⁺ T cell responses against pathogen-derived epitopes are usually limited to dominant epitopes and antibody responses to recombinant encoded antigens (Ags) are mostly weak. We have previously demonstrated that the timing of viral Ag expression in infected professional Ag-presenting cells strongly shapes the epitope immunodominance hierarchy. T cells recognizing determinants derived from late viral proteins have a clear disadvantage to proliferate during secondary responses. In this work we evaluate the effect of overexpressing the recombinant Ag using the modified vaccinia virus early/late promoter H5 (mPH5). Although the Ag-expression from the natural promoter 7.5 (P7.5) and the mPH5 seemed similar, detailed analysis showed that mPH5 not only induces higher expression levels than P7.5 during early phase of infection, but also Ag turnover is enhanced. The strong overexpression during the early phase leads to broader CD8 T cell responses, while preserving the priming efficiency of stable Ags. Moreover, the increase in Ag-secretion favors the induction of strong antibody responses. Our findings provide the rationale to develop new strategies for fine-tuning the responses elicited by recombinant modified vaccinia virus Ankara by using selected promoters to improve the performance of this viral vector.

An overview of live attenuated recombinant pseudorabies viruses for use as novel vaccines

Pseudorabies virus (PRV) is a double-stranded, DNA-based swine virus with a genome approximating 150 kb in size. PRV has many nonessential genes which can be replaced with genes encoding heterologous antigens but without deleterious effects on virus propagation. Recombinant PRVs expressing both native and foreign antigens are able to stimulate immune responses. In this paper, we review the current status of live attenuated recombinant PRVs and live PRV-based vector vaccines with potential for controlling viral infections in animals.

The strategic use of novel smallpox vaccines in the post-eradication world

We still face a threat of orthopoxviruses in the form of biological weapons and emerging zoonoses. Therefore, there is a need to maintain a comprehensive defense strategy to counter the low-probability, high-impact threat of smallpox, as well as the ongoing threat of naturally occurring orthopoxvirus disease. The currently licensed live-virus smallpox vaccine ACAM2000 is effective, but associated with serious and even life-threatening adverse events. The health threat posed by this vaccine, and other previously licensed vaccines, has prevented many first responders, and even many in the military, from receiving a vaccine against smallpox. At the same time, global immunity produced during the smallpox eradication campaign is waning. Here, we review novel subunit/component vaccines and how they might play roles in unconventional strategies to defend against emerging orthopoxvirus diseases throughout the world and against smallpox used as a weapon of mass destruction.

Polyclonal antibody cocktails generated using DNA vaccine technology protect in murine models of orthopoxvirus disease

Background

Previously we demonstrated that DNA vaccination of nonhuman primates (NHP) with a small subset of vaccinia virus (VACV) immunogens (L1, A27, A33, B5) protects against lethal monkeypox virus challenge. The L1 and A27 components of this vaccine target the mature virion (MV) whereas A33 and B5 target the enveloped virion (EV).

Results

Here, we demonstrated that the antibodies produced in vaccinated NHPs were sufficient to confer protection in a murine model of lethal Orthopoxvirus infection. We further explored the concept of using DNA vaccine technology to produce immunogen-specific polyclonal antibodies that could then be combined into cocktails as potential immunoprophylactic/therapeutics. Specifically, we used DNA vaccines delivered by muscle electroporation to produce polyclonal antibodies against the L1, A27, A33, and B5 in New Zealand white rabbits. The polyclonal antibodies neutralized both MV and EV in cell culture. The ability of antibody cocktails consisting of anti-MV, anti-EV, or a combination of anti-MV/EV to protect BALB/c mice was evaluated as was the efficacy of the anti-MV/EV mixture in a mouse model of progressive vaccinia. In addition to evaluating weight loss and lethality, bioimaging technology was used to characterize the spread of the VACV infections in mice. We found that the anti-EV cocktail, but not the anti-MV cocktail, limited virus spread and lethality.

Conclusions

A combination of anti-MV/EV antibodies was significantly more protective than anti-EV antibodies alone. These data suggest that DNA vaccine technology could be used to produce a polyclonal antibody cocktail as a possible product to replace vaccinia immune globulin.

Development of Vaccinia reporter viruses for rapid, high content analysis of viral function at all stages of gene expression

Vaccinia virus is the prototypical orthopoxvirus of Poxviridae, a family of viruses that includes the human pathogens Variola (smallpox) and Monkeypox. Core viral functions are conserved among orthopoxviruses, and consequently Vaccinia is routinely used to study poxvirus biology and screen for novel antiviral compounds. Here we describe the development of a series of fluorescent protein-based reporter Vaccinia viruses that provide unprecedented resolution for tracking viral function. The reporter viruses are divided into two sets: (1) single reporter viruses that utilize temporally regulated early, intermediate, or late viral promoters; and (2) multi-reporter viruses that utilize multiple temporally regulated promoters. Promoter and reporter combinations were chosen that yielded high signal-to-background for stage-specific viral outputs. We provide examples for how these viruses can be used in the rapid and accurate monitoring of Vaccinia function and drug action.

Smallpox vaccine safety is dependent on T cells and not B cells

The licensed smallpox vaccine, ACAM2000, is a cell culture derivative of Dryvax. Both ACAM2000 and Dryvax are administered by skin scarification and can cause progressive vaccinia, with skin lesions that disseminate to distal sites. We have investigated the immunologic basis of the containment of vaccinia in the skin with the goal to identify safer vaccines for smallpox. Macaques were depleted systemically of T or B cells and vaccinated with either Dryvax or an attenuated vaccinia vaccine, LC16m8. B cell depletion did not affect the size of skin lesions induced by either vaccine. However, while depletion of both CD4(+) and CD8(+) T cells had no adverse effects on LC16m8-vaccinated animals, it caused progressive vaccinia in macaques immunized with Dryvax. As both Dryvax and LC16m8 vaccines protect healthy macaques from a lethal monkeypox intravenous challenge, our data identify LC16m8 as a safer and effective alternative to ACAM2000 and Dryvax vaccines for immunocompromised individuals.

Principles of antidote pharmacology: an update on prophylaxis, post-exposure treatment recommendations and research initiatives for biological agents

The use of biological agents has generally been confined to military-led conflicts. However, there has been an increase in non-state-based terrorism, including the use of asymmetric warfare, such as biological agents in the past few decades. Thus, it is becoming increasingly important to consider strategies for preventing and preparing for attacks by insurgents, such as the development of pre- and post-exposure medical countermeasures. There are a wide range of prophylactics and treatments being investigated to combat the effects of biological agents. These include antibiotics (for both conventional and unconventional use), antibodies, anti-virals, immunomodulators, nucleic acids (analogues, antisense, ribozymes and DNAzymes), bacteriophage therapy and micro-encapsulation. While vaccines are commercially available for the prevention of anthrax, cholera, plague, Q fever and smallpox, there are no licensed vaccines available for use in the case of botulinum toxins, viral encephalitis, melioidosis or ricin. Antibiotics are still recommended as the mainstay treatment following exposure to anthrax, plague, Q fever and melioidosis. Anti-toxin therapy and anti-virals may be used in the case of botulinum toxins or smallpox respectively. However, supportive care is the only, or mainstay, post-exposure treatment for cholera, viral encephalitis and ricin - a recommendation that has not changed in decades. Indeed, with the difficulty that antibiotic resistance poses, the development and further evaluation of techniques and atypical pharmaceuticals are fundamental to the development of prophylaxis and post-exposure treatment options. The aim of this review is to present an update on prophylaxis and post-exposure treatment recommendations and research initiatives for biological agents in the open literature from 2007 to 2009.

Modified vaccinia virus Ankara exerts potent immune modulatory activities in a murine model

Background

Modified vaccinia virus Ankara (MVA), a highly attenuated strain of vaccinia virus, has been used as vaccine delivery vector in preclinical and clinical studies against infectious diseases and malignancies. Here, we investigated whether an MVA which does not encode any antigen (Ag) could be exploited as adjuvant per se.

Methodology/Principal Findings

We showed that dendritic cells infected in vitro with non-recombinant (nr) MVA expressed maturation and activation markers and were able to efficiently present exogenously pulsed Ag to T cells. In contrast to the dominant T helper (Th) 1 biased responses elicited against Ags produced by recombinant MVA vectors, the use of nrMVA as adjuvant for the co-administered soluble Ags resulted in a long lasting mixed Th1/Th2 responses.

Conclusions/Significance

These findings open new ways to potentiate and modulate the immune responses to vaccine Ags depending on whether they are co-administered with MVA or encoded by recombinant viruses.

Can vaccinia virus be replaced by MVA virus for testing virucidal activity of chemical disinfectants?

BACKGROUND

Vaccinia virus strain Lister Elstree (VACV) is a test virus in the DVV/RKI guidelines as representative of the stable enveloped viruses. Since the potential risk of laboratory-acquired infections with VACV persists and since the adverse effects of vaccination with VACV are described, the replacement of VACV by the modified vaccinia Ankara strain (MVA) was studied by testing the activity of different chemical biocides in three German laboratories.

METHODS

The inactivating properties of different chemical biocides (peracetic acid, aldehydes and alcohols) were tested in a quantitative suspension test according to the DVV/RKI guideline. All tests were performed with a protein load of 10% fetal calf serum with both viruses in parallel using different concentrations and contact times. Residual virus was determined by endpoint dilution method.

RESULTS

The chemical biocides exhibited similar virucidal activity against VACV and MVA. In three cases intra-laboratory differences were determined between VACV and MVA - 40% (v/v) ethanol and 30% (v/v) isopropanol are more active against MVA, whereas MVA seems more stable than VACV when testing with 0.05% glutardialdehyde. Test accuracy across the three participating laboratories was high. Remarkably inter-laboratory differences in the reduction factor were only observed in two cases.

CONCLUSIONS

Our data provide valuable information for the replacement of VACV by MVA for testing chemical biocides and disinfectants. Because MVA does not replicate in humans this would eliminate the potential risk of inadvertent inoculation with vaccinia virus and disease in non-vaccinated laboratory workers.

Control of vaccinia virus skin lesions by long-term-maintained IFN-gamma+TNF-alpha+ effector/memory CD4+ lymphocytes in humans

Vaccinia virus (VV) vaccination is used to immunize against smallpox and historically was considered to have been successful if a skin lesion formed at the vaccination site. While antibody responses have been widely proposed as a correlate of efficacy and protection in humans, the role of cellular and humoral immunity in VV-associated skin lesion formation was unknown. We therefore investigated whether long-term residual humoral and cellular immune memory to VV, persisting 30 years after vaccination, could control VV-induced skin lesion in revaccinated individuals. Here, we have shown that residual VV-specific IFN-gamma+TNF-alpha+ or IFN-gamma+IL-2+ CD4+ lymphocytes but not CD8+ effector/memory lymphocytes expressing a skin-homing marker are inversely associated with the size of the skin lesion formed in response to revaccination. Indeed, high numbers of residual effector T cells were associated with lower VV skin lesion size after revaccination. In contrast, long-term residual VV-specific neutralizing antibody (NAbs) titers did not affect skin lesion formation. However, the size of the skin lesion strongly correlated with high levels of NAbs boosted after revaccination. These findings demonstrate a potential role for VV-specific CD4+ responses at the site of VV-associated skin lesion, thereby providing new insight into immune responses at these sites and potentially contributing to the development of new approaches to measure the efficacy of VV vaccination.

Studies of the suitability of fowlpox as a decontamination and thermal stability simulant for variola major

Variola major, the causative agent of smallpox, has been eradicated from nature. However, stocks still exist; thus, there is a need for relevant decontamination studies, preferably with nonpathogenic simulants. Previous studies have shown a similarity in response of vaccinia virus and variola major to various decontaminants and thermal inactivation. This study compared vaccinia and fowlpox viruses under similar conditions, using disinfectants and temperatures for which variola major data already existed. Most disinfectants showed similar efficacy against vaccinia and fowlpox, suggesting the utility of fowlpox as a decontamination simulant. Inactivation kinetics studies showed that fowlpox behaved similarly to variola major when treated with 0.1% iodine and 5.7% polyethyleneglycol nonylphenyl ether, 0.025% sodium hypochlorite, 0.05% sodium hypochlorite, and 0.1% cetyltrimethylammonium chloride and 0.05% benzalkonium chloride, but differed in its response to 0.05% iodine and 0.3% polyethyleneglycol nonylphenyl ether and 40% ethanol. Thermal inactivation studies demonstrated that fowlpox is a suitable thermal simulant for variola major between 40°C and 55°C.

Wanted: correlates of vaccine-induced protection against simian immunodeficiency virus

PURPOSE OF REVIEW

We will highlight recent advances in defining the attributes of immune responses that control AIDS virus replication using animal models, and also point out key gaps in our understanding that should be addressed in future research.

RECENT FINDINGS

Many different vaccine approaches are currently being evaluated in animal models. Almost all of them elicit strong cellular or humoral immune responses in macaques. Commonly used prime-and-boost strategies have had varying degrees of success in diminishing chronic phase virus loads in vaccinated animals, but few have shown durable reduction in replication of the CCR5-tropic simian immunodeficiency viruses (SIVs) that most closely resemble field isolates of HIV. Investigators are therefore turning to other systems, including live attenuated vaccines and cohorts of spontaneous SIV controllers, to help identify the properties of successful host responses to pathogenic SIV. These responses likely incorporate multiple coordinated effector mechanisms.

SUMMARY

There is no effective AIDS vaccine on the horizon. CD4+ and CD8+ T cell responses and antibodies have all been implicated in control of SIV replication seen in various experimental systems, but the correlates of protection against AIDS virus replication have not yet been definitively identified. Animal models will remain a necessary component of this research.

New classes of orthopoxvirus vaccine candidates by functionally screening a synthetic library for protective antigens

The licensed smallpox vaccine, comprised of infectious vaccinia, is no longer popular as it is associated with a variety of adverse events. Safer vaccines have been explored such as further attenuated viruses and component designs. However, these alternatives typically provide compromised breadth and strength of protection. We conducted a genome-level screening of cowpox, the ancestral poxvirus, in the broadly immune-presenting C57BL/6 mouse as an approach to discovering novel components with protective capacities. Cowpox coding sequences were synthetically built and directly assayed by genetic immunization for open-reading frames that protect against lethal pulmonary infection. Membrane and non-membrane antigens were identified that partially protect C57BL/6 mice against cowpox and vaccinia challenges without adjuvant or regimen optimization, whereas the 4-pox vaccine did not. New vaccines might be developed from productive combinations of these new and existing antigens to confer potent, broadly efficacious protection and be contraindicated for none.

Application of pharmacogenomics to vaccines

The field of pharmacogenomics and pharmacogenetics provides a promising science base for vaccine research and development. A broad range of phenotype/genotype data combined with high-throughput genetic sequencing and bioinformatics are increasingly being integrated into this emerging field of vaccinomics. This paper discusses the hypothesis of the 'immune response gene network' and genetic (and bioinformatic) strategies to study associations between immune response gene polymorphisms and variations in humoral and cellular immune responses to prophylactic viral vaccines, such as measles-mumps-rubella, influenza, HIV, hepatitis B and smallpox. Immunogenetic studies reveal promising new vaccine targets by providing a better understanding of the mechanisms by which gene polymorphisms may influence innate and adaptive immune responses to vaccines, including vaccine failure and vaccine-associated adverse events. Additional benefits from vaccinomic studies include the development of personalized vaccines, the development of novel vaccines and the development of novel vaccine adjuvants.

Personalized vaccines: the emerging field of vaccinomics

The next 'golden age' in vaccinology will be ushered in by the new science of vaccinomics. In turn, this will inform and allow the development of personalized vaccines, based on our increasing understanding of immune response phenotype: genotype information. Rapid advances in developing such data are already occurring for hepatitis B, influenza, measles, mumps, rubella, anthrax and smallpox vaccines. In addition, newly available data suggest that some vaccine-related adverse events may also be genetically determined and, therefore, predictable. This paper reviews the basis and logic of personalized vaccines, and describes recent advances in the field.

Immunogenicity of recombinant Modified Vaccinia Ankara following a single or multi-dose vaccine regimen in rhesus monkeys

Modified Vaccinia Ankara (MVA) is a replication-defective strain of vaccinia virus (VV) that is being investigated in humans as an alternative vaccine against smallpox. Understanding the parameters of a MVA vaccine regimen that can effectively enhance protective immunity will be important for clinical development. The present studies utilize cohorts of rhesus monkeys immunized with recombinant MVA (rMVA) or recombinant VV (rVV) vaccine vectors to investigate the magnitude, breadth, and durability of anti-VV immunity elicited by a single or multi-dose vaccine regimen. These data demonstrate that a single immunization with rMVA elicits weaker cellular and humoral immunity compared to a single inoculation with rVV. However, vaccine-elicited antibody responses, but not T cell responses, are significantly enhanced with repeated immunizations of rMVA. Importantly, only monkeys receiving up to four inoculations with rMVA generated neutralizing antibody (NAb) responses that were comparable in magnitude and durability to those elicited in monkeys receiving two inoculations with rVV. These data also show that the breadth of antibody responses against protein antigens associated with two antigenically distinct forms of infectious VV are similar in rMVA- and rVV-immunized monkeys. Together, these studies suggest that a multi-dose vaccine regimen utilizing up to four inoculations of MVA generates robust and durable antibody-mediated immunity comparable to that elicited by replication-competent VV.

A novel replication-competent vaccinia vector MVTT is superior to MVA for inducing high levels of neutralizing antibody via mucosal vaccination

Mucosal vaccination offers great advantage for inducing protective immune response to prevent viral transmission and dissemination. Here, we report our findings of a head-to-head comparison of two viral vectors modified vaccinia Ankara (MVA) and a novel replication-competent modified vaccinia Tian Tan (MVTT) for inducing neutralizing antibodies (Nabs) via intramuscular and mucosal vaccinations in mice. MVTT is an attenuated variant of the wild-type VTT, which was historically used as a smallpox vaccine for millions of Chinese people. The spike glycoprotein (S) of SARS-CoV was used as the test antigen after the S gene was constructed in the identical genomic location of two vectors to generate vaccine candidates MVTT-S and MVA-S. Using identical doses, MVTT-S induced lower levels (∼2-3-fold) of anti- SARS-CoV neutralizing antibodies (Nabs) than MVA-S through intramuscular inoculation. MVTT-S, however, was capable of inducing consistently 20-to-100-fold higher levels of Nabs than MVA-S when inoculated via either intranasal or intraoral routes. These levels of MVTT-S-induced Nab responses were substantially (∼10-fold) higher than that induced via the intramuscular route in the same experiments. Moreover, pre-exposure to the wild-type VTT via intranasal or intraoral route impaired the Nab response via the same routes of MVTT-S vaccination probably due to the pre-existing anti-VTT Nab response. The efficacy of intranasal or intraoral vaccination, however, was still 20-to-50-fold better than intramuscular inoculation despite the subcutaneous pre-exposure to wild-type VTT. Our data have implications for people who maintain low levels of anti-VTT Nabs after historical smallpox vaccination. MVTT is therefore an attractive live viral vector for mucosal vaccination.

Monkeypox virus evades antiviral CD4+ and CD8+ T cell responses by suppressing cognate T cell activation

Monkeypox virus (MPV) is a virulent human pathogen that has gained increased attention because of its potential use as a bioterrorism agent and inadvertent introduction into North America in 2003. The US outbreak also provided an important opportunity to study MPV-specific T cell immunity. Although MPV-specific CD4(+) and CD8(+) T cells could recognize vaccinia virus (VV)-infected monocytes and produce inflammatory cytokines such as IFNgamma and TNFalpha, they were largely incapable of responding to autologous MPV-infected cells. Further analysis revealed that, unlike cowpox virus (CPV), MPV did not interfere with MHC expression or intracellular transport of MHC molecules. Instead, MPV-infected cells were capable of preventing T cell receptor (TcR)-mediated T cell activation in trans. The ability to trigger a state of nonresponsiveness represents a unique MHC-independent mechanism for blocking antiviral T cell activation and inflammatory cytokine production and is likely an important attribute involved with viral dissemination in the infected host.

Substitution of vaccinia virus Elstree by modified vaccinia virus Ankara to test the virucidal efficacy of chemical disinfectants

After the eradication of variola in 1980, the smallpox vaccination was considered to be no longer required and was subsequently abandoned mainly because of possible adverse effects of vaccinia virus especially in first-time vaccinees. Despite a growing number of humans without immunity against vaccinia virus, vaccinia virus Lister Elstree (VACV) is still prescribed for testing virucidal efficacy of chemical disinfectants in the guidelines of the German Veterinary Medical Society [Deutsche Veterinärmedizinische Gesellschaft (DVG)], the German Association for the Control of Virus Diseases [Deutsche Vereinigung zur Bekämpfung der Viruskrankheiten (DVV)] and the Robert Koch Institute (RKI). To evaluate a possible substitution of VACV, with the attenuated modified vaccinia virus Ankara (MVA) the virucidal efficacy of four different DVG-listed commercially available chemical disinfectants representing different groups of chemicals was tested against these two viruses. Quantitative suspension tests and qualitative carrier tests with poplar wood and gauze were performed. Distinction of VACV and MVA was confirmed by cytopathogenic effects, such as differences in plaque morphology. No significant difference in disinfection efficacy between VACV and MVA was observed for any of the disinfectants tested. Implying that vaccinia virus poses a risk after inadvertent inoculation, our results show that MVA, which does not replicate in humans, should replace VACV in the chemical disinfectant testing guidelines.

ACAM2000: a newly licensed cell culture-based live vaccinia smallpox vaccine

BACKGROUND

Due to concern over i) expiration of currently available calf-lymph vaccine (Dryvax); ii) calf lymph as a vaccine (bovine spongiform encephalopathy [BSE], other possible contaminations and animal welfare); and iii) use of variola as a weapon for bioterrorism, a new and safer vaccinia-based smallpox vaccine derived from new cell culture-based technology was proposed. Federally funded work by Acambis, Inc. resulted in FDA approval for ACAM2000 in August 2007.

OBJECTIVES

This paper describes the development from conception to FDA approval of the new vaccinia cell cultured-based smallpox vaccine ACAM2000.

METHODS

Data were compiled from available public reports.

RESULTS/CONCLUSIONS

The studies with ACAM2000 indicate that it closely matches the safety of Dryvax in both non-clinical and clinical trials. ACAM2000 met two of the four primary surrogate efficacy end point criteria established for the Phase III clinical trials. Concern over the incidence of myopericarditis with ACAM2000 and Dryvax exists. So far the cardiac events seem to be self-limited. There are no pediatric safety data for ACAM2000. Overall, clinical trial results were sufficient to convince the FDA that ACAM2000 is a suitable replacement for Dryvax in the event of bioterrorism involving variola (smallpox).

Co-expressing GP5 and M proteins under different promoters in recombinant modified vaccinia virus ankara (rMVA)-based vaccine vector enhanced the humoral and cellular immune responses of porcine reproductive and respiratory syndrome virus (PRRSV)

The porcine reproductive and respiratory syndrome virus (PRRSV) has three major structural proteins which designated as GP5, M, and N. Protein GP5 and M have been considered very important to arouse the humoral and cellular immune responses against PRRSV infection and proposed to be the excellent candidate proteins in the design of PRRS bioengineering vaccine. There were some attempts on expressing GP5 or M in DNA vaccine and adenovirus to arouse humoral and cellular immune responses, but few papers have been reported on that the immune response can be difference because of the expression patterns of GP5 and M proteins in the recombinant virus. In this article, four recombinant viruses that expressed GP5 and M proteins of PRRSV in the modified vaccinia virus ankara (MVA) with different expression patterns were made. In these recombinant virus (rMVAs), GP5 and M proteins were expressed in MVA in the same virus but under the control of two promoters (rMVA-GP5/M), or as a fusion protein under one promoter (rMVA-GP5-M), or separately (rMVA-GP5 and rMVA-M). The humoral and cellular immune responses for the four recombinant viruses were evaluated with mouse model. Every mouse was inoculated with 5 x 10(5) TCID50 of the different rMVAs and boosted 3 weeks later. Neutralizing antibody titers for each group were detected with virus neutralization test assay weekly after the primary inoculation for 13 weeks to evaluate the humoral immune response. The production of gamma interferon (IFN-gamma), interleukin-2 (IL-2), and interleukin-4 (IL-4) was detected in splenocytes of rMVA-inoculated mice at 30, 60, and 90 days post inoculation to evaluate the cellular immune response. Results showed that rMVA-GP5 and rMVA-M cannot induce obvious humoral and cellular immune responses; rMVA-GP5-M inoculated group developed better immune responses than rMVA-GP5 and rMVA-M inoculated groups; however, mice inoculated with rMVA-GP5/M maintained the strongest cellular response against PRRS and consistently enhanced the anti-PRRSV humoral responses. The strategy of co-expressing PRRSV GP5 and M protein in MVA under the control of different promoters might be an attractive method for future PRRSV vaccine design.

Characterization and use of mammalian-expressed vaccinia virus extracellular membrane proteins for quantification of the humoral immune response to smallpox vaccines

The licensed smallpox vaccine Dryvax is used as the standard in comparative immunogenicity and protection studies of new smallpox vaccine candidates. Although the correlates of protection against smallpox are unknown, recent studies have shown that a humoral response against the intracellular mature virion and extracellular enveloped virion (EV) forms of vaccinia virus is crucial for protection. Using a recombinant Semliki Forest virus (rSFV) vector system, we expressed a set of full-length EV proteins for the development of EV antigen-specific enzyme-linked immunosorbent assays (ELISAs) and the production of monospecific antisera. The EV-specific ELISAs were used to evaluate the EV humoral response elicited by Dryvax and the nonreplicating modified vaccinia virus Ankara (MVA) in mouse vaccination experiments comparing doses and routes of vaccination. Quantitatively similar titers of antibodies against EV antigens A33R, A56R, and B5R were measured in mice vaccinated with Dryvax and MVA when MVA was administered at a dose of 10(8) plaque-forming units. Further, a substantial increase in the EV-specific antibody response was induced in mice inoculated with MVA by using a prime-boost schedule. Finally, we investigated the abilities of the EV-expressing rSFV vectors to elicit the production of polyclonal monospecific antisera against the corresponding EV proteins in mice. The monospecific serum antibody levels against A33R, A56R, and B5R were measurably higher than the antibody levels induced by Dryvax. The resulting polyclonal antisera were used in Western blot analysis and immunofluorescence assays, indicating that rSFV particles are useful vectors for generating monospecific antisera.

Evaluation of modified vaccinia virus Ankara as an alternative vaccine against smallpox in chronically HIV type 1-infected individuals undergoing HAART

The fear of malevolent use of variola virus by terrorists has led to the implementation of a health care worker vaccination program and to the consideration of vaccination for the general public. However, due to concerns about side effects of the classical smallpox vaccine, especially for immunocompromised individuals, a safer vaccine is urgently needed. We characterized the immunogenicity of modified vaccinia virus Ankara (MVA), one of the more promising alternative smallpox vaccines, in a cohort of 10 chronically HIV-1-infected individuals undergoing highly active antiretroviral therapy (HAART). Nine subjects received smallpox vaccination as children while one subject was never vaccinated against smallpox. All the subjects had CD4 counts >400 cells/mm(3) and 8 out of 10 had undetectable viral loads. MVA was able to elicit humoral and cellular immune responses in the majority of individuals. Vaccinia-specific antibodies were mainly of the IgG class while T cells specific to vaccinia were predominantly CD8(+). The immune responses were maintained over 1 year. Similar vaccinia specific humoral immune responses were observed when our cohort of HIV-1-infected individuals was compared to smallpox-vaccinated healthy subjects. The observed immune responses suggest that the highly attenuated MVA could be used as a substitute vaccine against smallpox in chronically HIV-1-infected individuals undergoing HAART.

Rapid identification of an HLA-B*1501-restricted vaccinia peptide antigen

We identified a modified vaccinia virus Ankara (MVA)-peptide recognized by T cells of a vaccinated patient, applying a modified expression-based procedure. CD8(+) T cells reactive with MVA were expanded from peripheral blood of a patient after MVA-vaccination. The restricting HLA-elements and the antigen were determined applying stably HLA-class I-transfected K562 cells that were either infected wit MVA or transfected with a panel of MVA open reading frames (ORF). The T cells recognized the ORF 28R (K7R) gene product in association with HLA-B*1501 and the peptide-coding region was localized applying truncated in vitro transcribed mRNA. The 9-mer peptide encoded (ORF 28(25-33), SIIDLIDEY) was identified that was also recognized by ex vivo CD8(+) T cells from the patient and further healthy individuals.

The potential role of fowlpox virus in rational vaccine design

The design of optimal vaccines requires detailed knowledge of how protective immune responses are generated in vivo under normal circumstances. This approach to vaccine development, where the immune correlates of protection are defined and vaccines are designed to elicit the same response, is called rational vaccine design. Poxviruses are attractive candidates for inclusion in such design strategies owing to their large genome, which allows for the inclusion of multiple heterologous genes, including those encoding antigens, co-stimulatory molecules and cytokines. Fowlpox virus, the prototypical member of the Avipoxvirus genus, is particularly suitable, as it is also incapable of replicating in mammalian cells. The potential of recombinant fowlpox virus as a safe vaccine vector is being evaluated currently in a number of clinical trials for diseases, including HIV, malaria and various types of cancer. Despite their promise, intricate details regarding how fowlpox virus interacts with the host immune system have not been resolved. In this review, the issues surrounding the use of fowlpox virus as a vaccine vector and possible strategies for enhancing its efficacy are discussed.

Expression of CCL20 and granulocyte-macrophage colony-stimulating factor, but not Flt3-L, from modified vaccinia virus ankara enhances antiviral cellular and humoral immune responses

While modified vaccinia virus Ankara (MVA) is currently in clinical development as a safe vaccine against smallpox and heterologous infectious diseases, its immunogenicity is likely limited due to the inability of the virus to replicate productively in mammalian hosts. In light of recent data demonstrating that vaccinia viruses, including MVA, preferentially infect antigen-presenting cells (APCs) that play crucial roles in generating antiviral immunity, we hypothesized that expression of specific cytokines and chemokines that mediate APC recruitment and activation from recombinant MVA (rMVA) vectors would enhance the immunogenicity of these vectors. To test this hypothesis, we generated rMVAs that express murine granulocyte-macrophage colony-stimulating factor (mGM-CSF), human CCL20/human macrophage inflammatory protein 3alpha (hCCL20/hMIP-3alpha), or human fms-like tyrosine kinase 3 ligand (hFlt3-L), factors predicted to increase levels of dendritic cells (DCs), to recruit DCs to sites of immunization, or to promote maturation of DCs in vivo, respectively. These rMVAs also coexpress the well-characterized, immunodominant lymphocytic choriomeningitis virus nucleoprotein (NP) antigen that enabled sensitive and quantitative assessment of antigen-specific CD8(+) T-cell responses following immunization of BALB/c mice. Our results demonstrate that immunization of mice with rMVAs expressing mGM-CSF or hCCL20, but not hFlt3-L, results in two- to fourfold increases of cellular immune responses directed against vector-encoded antigens and 6- to 17-fold enhancements of MVA-specific antibody titers, compared to those responses elicited by nonadjuvanted rMVA. Of note, cytokine augmentation of cellular immune responses occurs when rMVAs are given as primary immunizations but not when they are used as booster immunizations, suggesting that these APC-modulating proteins, when used as poxvirus-encoded adjuvants, are more effective at stimulating naïve T-cell responses than in promoting recall of preexisting memory T-cell responses. Our results demonstrate that a strategy to express specific genetic adjuvants from rMVA vectors can be successfully applied to enhance the immunogenicity of MVA-based vaccines.

Identification of vaccinia CD8+ T-cell epitopes conserved among vaccinia and variola viruses restricted by common MHC class I molecules, HLA-A2 or HLA-B7

Immunization with vaccinia virus results in long-lasting protection against smallpox and is an approach that has been successfully used to eliminate natural smallpox infections worldwide. Today, vaccinia virus is very important not only as a vaccine virus to protect human against smallpox, but also as an expression vector for immunization against other infectious diseases, such as HIV and cancer. In this article, we identify three new vaccinia human CD8+ T-cell epitopes conserved among vaccinia and variola viruses restricted by HLA-A2, HLA-B7, or HLA-B*3502, which belongs to the HLA-B7 supertype. Identification of these CD8+ T-cell epitopes restricted by common HLA alleles will help to quantitate human CD8+ T-cell responses to licensed and experimental smallpox vaccines and to vaccinia virus vectors. CD8+ T-cell responses specific to these epitopes can also be used to quantitate cellular immune responses, especially with new smallpox vaccines that do not induce a "take," such as the modified vaccinia virus Ankara strain. Combined with previous reports by us and others, these results show that there are some vaccinia viral proteins containing multiple epitopes restricted by different MHC molecules of humans and mice.

LC16m8: an attenuated smallpox vaccine

The frequency of moderate to severe adverse reactions associated with smallpox vaccines currently stockpiled in the US, and the continued threat of bioterrorism have prompted the development of effective vaccines with improved safety profiles. LC16m8, an attenuated, replicating smallpox vaccine derived from the Lister strain of vaccinia, is currently licensed in Japan where it was safely used in over 50,000 children in the 1970s. It has been shown to have markedly less neurotoxicity than unattenuated vaccines in nonclinical studies. LC16m8 is immunogenic after a single dose, and recent studies in two different animal models have demonstrated protective efficacy equivalent to that of the only FDA-licensed smallpox vaccine. This article reviews the history and available scientific literature regarding LC16m8 and provides comparisons to other smallpox vaccines.

Attenuated poxvirus expressing three immunodominant CMV antigens as a vaccine strategy for CMV infection

BACKGROUND

Human cytomegalovirus (CMV) infection is an important risk factor in the post-transplant (Tx) recovery phase for both hematopoietic stem cell Tx (HSCT) and solid organ Tx (SOT) recipients. CMV infection may be prevented or controlled by simultaneously inducing both CMV-specific neutralizing antibody (nAb) and cellular immunity. Soluble (s) UL55 (surface glycoprotein), UL83 (tegument protein) and UL123/e4 (nuclear protein) are immunodominant in eliciting both CMV nAb and cellular immunity. An attenuated poxvirus, modified vaccinia Ankara (MVA) was selected to develop this vaccine strategy in Tx recipients, because of its clinical safety record, large foreign gene capacity, and capability to activate strong humoral and cellular immune responses against recombinant antigens.

OBJECTIVES

A subunit vaccine that targets multiple CMV antigens will be used to gain maximal coverage and protective function against CMV infection. rMVA simultaneously expressing sUL55, UL83 and UL123/e4 will be generated, and humoral and cellular immunity it elicits will be characterized, after murine immunization and in vitro to amplify clinical recall responses.

STUDY DESIGN

rMVA will be constructed in two steps using UL123/e4-pLW22 followed by sUL55-UL83-pLW51 transfer plasmids. Western blots will be used to characterize expression levels of each antigen. Primary immunity will be evaluated in mouse models, while recall responses to the virally expressed CMV antigens will be assessed in human peripheral blood.

RESULTS

We generated CMV-MVA via homologous recombination, and demonstrated high expression levels of sUL55, UL83 and UL123/e4 by Western blot. CMV-MVA immunization potently induced both humoral and cellular immunity to sUL55, UL83 and UL123 after murine immunization, and cellular immunity to UL83 and UL123 by in vitro amplification of T cell recall responses in human PBMC.

CONCLUSIONS

rMVA promotes high level expression of three immunodominant CMV antigens, which is reflected in results of immunization studies in which high titers of UL55-specific antibodies and CD4+ T-help are detected, as well as high levels of UL83-specific and moderate levels of UL123-specific CD8+ CTL.