The nonreplicating smallpox candidate vaccines defective vaccinia Lister (dVV-L) and modified vaccinia Ankara (MVA) elicit robust long-term protection

Cross-reactive immune responses to monkeypox virus induced by MVA vaccination in mice

Mpox (monkeypox) infection cases increased recently in non-Mpox outbreak areas, potentially causing an international threat. The desire to defend against a potential outbreak has led to renewed efforts to develop Mpox vaccines. In this report, mice were immunized with various doses of modified vaccinia virus Ankara (MVA) to evaluate the cross-reactive immune response of MVA immunization against protective antigens of the current monkeypox virus. We demonstrated that MVA induced specific antibodies against protective antigens (A29, A35, B6, M1, H3, and I1), mediating the neutralization abilities against the MVA and the monkeypox virus (MPXV). Moreover, recombinant protective antigens of the MPXV elicited cross-binding and cross-neutralizing activities for MVA. Hence, the MVA induced cross-reactive immune responses, which may guide future efforts to develop vaccines against the recent MPXV. Notably, compared to the other protective antigens, the predominant A29 and M1 antigens mediated higher cross-neutralizing immune responses against the MVA, which could serve as antigen targets for novel orthologous orthopoxvirus vaccine.

A chemical method for generating live-attenuated, replication-defective DNA viruses for vaccine development

The development of a chemically attenuated, replication-incompetent virus vaccine can provide protection against diseases caused by DNA viruses. In this study, we have developed a method to produce live-attenuated, replication-defective viruses using centanamycin (CM), a chemical compound that alkylates the A-T-rich minor groove of the DNA and thereby blocks DNA replication. We tested the efficacy of CM to produce live-attenuated, replication-defective human cytomegalovirus, mouse cytomegalovirus, and herpes simplex virus-2 (HSV-2), suggesting a broad application for generating live-attenuated, replication-defective DNA viruses. Mass spectrometry analysis showed that CM alkylate viral DNA at the adenine-N3 position. Moreover, mice immunization with CM-attenuated mouse cytomegalovirus (MCMV) produced a robust immune response and reduced the viral load in immunized animals against challenges with live, wild-type MCMV. Our study offers a unifying and attractive therapeutic opportunity that chemically attenuated live DNA viruses can be readily developed as new frontline vaccines.

Production of single-cycle infectious SARS-CoV-2 through a trans-complemented replicon

Single-cycle infectious virus can elicit close-to-natural immune response and memory. One approach to generate single-cycle severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is through deletion of structural genes such as spike (S) and nucleocapsid (N). Transcomplementation of the resulting ΔS or ΔN virus through enforced expression of S or N protein in the cells gives rise to a live but unproductive virus. In this study, ΔS and ΔN BAC clones were constructed and their live virions were rescued by transient expression of S and N proteins from the ancestral and the Omicron strains. ΔS and ΔN virions were visualized by transmission electron microscopy. Virion production of ΔS was more efficient than that of ΔN. The coated S protein from ΔS was delivered to infected cells in which the expression of N protein was also robust. In contrast, expression of neither S nor N was detected in ΔN-infected cells. ΔS underwent viral RNA replication, induced type I interferon (IFN) response, but did not form plaques. Despite RNA replication in cells, ΔS infection did not produce viral progeny in culture supernatant. Interestingly, viral RNA replication was not further enhanced upon overexpression of S protein. Taken together, our work provides a versatile platform for development of single-cycle vaccines for SARS-CoV-2.

The African Swine Fever Virus (ASFV) Topoisomerase II as a Target for Viral Prevention and Control

African swine fever (ASF) is, once more, spreading throughout the world. After its recent reintroduction in Georgia, it quickly reached many neighboring countries in Eastern Europe. It was also detected in Asia, infecting China, the world's biggest pig producer, and spreading to many of the surrounding countries. Without any vaccine or effective treatment currently available, new strategies for the control of the disease are mandatory. Its etiological agent, the African swine fever virus (ASFV), has been shown to code for a type II DNA topoisomerase. These are enzymes capable of modulating the topology of DNA molecules, known to be essential in unicellular and multicellular organisms, and constitute targets in antibacterial and anti-cancer treatments. In this review, we summarize most of what is known about this viral enzyme, pP1192R, and discuss about its possible role(s) during infection. Given the essential role of type II topoisomerases in cells, the data so far suggest that pP1192R is likely to be equally essential for the virus and thus a promising target for the elaboration of a replication-defective virus, which could provide the basis for an effective vaccine. Furthermore, the use of inhibitors could be considered to control the spread of the infection during outbreaks and therefore limit the spreading of the disease.

Bovine Vaccinia: Insights into the Disease in Cattle

Bovine vaccinia (BV), caused by Vaccinia virus (VACV), is a zoonosis characterized by exanthematous lesions in the teats of dairy cows and the hands of milkers and is an important public health issue. Severe VACV-induced lesions in the teats and udder of cows and buffaloes could lead to mastitis and other secondary infections, thereby reducing productivity and resulting in economic losses to the dairy industry. In Brazil, BV re-emerged in the late 1990s and is now endemic in most of the Brazilian territory. In the last 15 years, much effort has been made to know more about this disease and its epidemiology, etiologic agents, and interactions with the host and the environment. In this review, we describe the known dynamics of VACV infection in cattle and the viral shedding routes, as well as the relevance of BV for animal and public health.

Challenges and Achievements in Prevention and Treatment of Smallpox

Declaration of smallpox eradication by the WHO in 1980 led to discontinuation of the worldwide vaccination campaign. The increasing percentage of unvaccinated individuals, the existence of its causative infectious agent variola virus (VARV), and the recent synthetic achievements increase the threat of intentional or accidental release and reemergence of smallpox. Control of smallpox would require an emergency vaccination campaign, as no other protective measure has been approved to achieve eradication and ensure worldwide protection. Experimental data in surrogate animal models support the assumption, based on anecdotal, uncontrolled historical data, that vaccination up to 4 days postexposure confers effective protection. The long incubation period, and the uncertainty of the exposure status in the surrounding population, call for the development and evaluation of safe and effective methods enabling extension of the therapeutic window, and to reduce the disease manifestations and vaccine adverse reactions. To achieve these goals, we need to evaluate the efficacy of novel and already licensed vaccines as a sole treatment, or in conjunction with immune modulators and antiviral drugs. In this review, we address the available data, recent achievements, and open questions.

E3L and F1L Gene Functions Modulate the Protective Capacity of Modified Vaccinia Virus Ankara Immunization in Murine Model of Human Smallpox

The highly attenuated Modified Vaccinia virus Ankara (MVA) lacks most of the known vaccinia virus (VACV) virulence and immune evasion genes. Today MVA can serve as a safety-tested next-generation smallpox vaccine. Yet, we still need to learn about regulatory gene functions preserved in the MVA genome, such as the apoptosis inhibitor genes F1L and E3L. Here, we tested MVA vaccine preparations on the basis of the deletion mutant viruses MVA-ΔF1L and MVA-ΔE3L for efficacy against ectromelia virus (ECTV) challenge infections in mice. In non-permissive human tissue culture the MVA deletion mutant viruses produced reduced levels of the VACV envelope antigen B5. Upon mousepox challenge at three weeks after vaccination, MVA-ΔF1L and MVA-ΔE3L exhibited reduced protective capacity in comparison to wildtype MVA. Surprisingly, however, all vaccines proved equally protective against a lethal ECTV infection at two days after vaccination. Accordingly, the deletion mutant MVA vaccines induced high levels of virus-specific CD8+ T cells previously shown to be essential for rapidly protective MVA vaccination. These results suggest that inactivation of the anti-apoptotic genes F1L or E3L modulates the protective capacity of MVA vaccination most likely through the induction of distinct orthopoxvirus specific immunity in the absence of these viral regulatory proteins.

Comparative Efficacy of Intramuscular and Scarification Routes of Administration of Live Smallpox Vaccine in a Murine Challenge Model

In recent years concern has mounted regarding the possibility of a re-emergence of smallpox through biowarfare or bioterrorism. There is also concern over the incidence of human monkeypox in endemic areas and the potential for monkeypox to be accidentally transported to non-endemic areas. In the event of re-emergence of smallpox or emergence of monkeypox, the accepted route of administration for live replicating smallpox vaccine is dermal scarification, which generates a virus-shedding lesion that persists for several days at the vaccination site. The lesion is a potential source of contact transmission of vaccine to individuals who may be contra-indicated for receipt of the live vaccine. In this study, we compare dermal scarification with intramuscular vaccination for replicating smallpox vaccine in a mouse lethal challenge model. Comparisons are made over multiple vaccine and challenge doses and data recorded for lethality, disease severity, and antibody responses. Qualitative and quantitative differences between the two routes are observed, and for the intramuscular route the febrile response is not suppressed after subsequent virulent vaccinia virus challenge. However both routes generate an immune response and protect from severe disease and death. Although dermal scarification is the preferred route of vaccination for the general population, intramuscular vaccination may be an option for people who are not contraindicated for the live vaccine, but who are close contacts of people who are contraindicated for the live vaccine, in an emergency situation.

Short-term clinical safety profile of brincidofovir: A favorable benefit-risk proposition in the treatment of smallpox

Brincidofovir (BCV, CMX001) is an orally available, long-acting, broad-spectrum antiviral that has been evaluated in healthy subjects in Phase I studies and in hematopoietic cell transplant recipients and other immunocompromised patients in Phase II/III clinical trials for the prevention and treatment of cytomegalovirus and adenovirus infections. BCV has also shown in vitro activity against orthopoxviruses such as variola (smallpox) virus, and is under advanced development as a treatment for smallpox under the US FDA's 'Animal Rule'. The anticipated treatment regimen for smallpox is a total weekly dose of 200 mg administered orally for 3 consecutive weeks. To assess the benefit-to-risk profile of BCV for the treatment of smallpox, we evaluated short-term safety data associated with comparable doses from Phase I studies and from adult and pediatric subjects in the cytomegalovirus and adenovirus clinical programs. When administered at doses and durations similar to that proposed for the treatment of smallpox, BCV was generally well tolerated in both adults and pediatric subjects. The most common adverse events were mild gastrointestinal events and asymptomatic, transient, and reversible elevations in serum transaminases. The data presented herein indicate a favorable safety profile for BCV for the treatment of smallpox, and support its continued development for this indication.

Modified Vaccinia Virus Ankara: History, Value in Basic Research, and Current Perspectives for Vaccine Development

Safety tested Modified Vaccinia virus Ankara (MVA) is licensed as third-generation vaccine against smallpox and serves as a potent vector system for development of new candidate vaccines against infectious diseases and cancer. Historically, MVA was developed by serial tissue culture passage in primary chicken cells of vaccinia virus strain Ankara, and clinically used to avoid the undesirable side effects of conventional smallpox vaccination. Adapted to growth in avian cells MVA lost the ability to replicate in mammalian hosts and lacks many of the genes orthopoxviruses use to conquer their host (cell) environment. As a biologically well-characterized mutant virus, MVA facilitates fundamental research to elucidate the functions of poxvirus host-interaction factors. As extremely safe viral vectors MVA vaccines have been found immunogenic and protective in various preclinical infection models. Multiple recombinant MVA currently undergo clinical testing for vaccination against human immunodeficiency viruses, Mycobacterium tuberculosis or Plasmodium falciparum. The versatility of the MVA vector vaccine platform is readily demonstrated by the swift development of experimental vaccines for immunization against emerging infections such as the Middle East Respiratory Syndrome. Recent advances include promising results from the clinical testing of recombinant MVA-producing antigens of highly pathogenic avian influenza virus H5N1 or Ebola virus. This review summarizes our current knowledge about MVA as a unique strain of vaccinia virus, and discusses the prospects of exploiting this virus as research tool in poxvirus biology or as safe viral vector vaccine to challenge existing and future bottlenecks in vaccinology.

Modified Vaccinia Ankara Virus Vaccination Provides Long-Term Protection against Nasal Rabbitpox Virus Challenge

Modified vaccinia Ankara virus (MVA) is a smallpox vaccine candidate. This study was performed to determine if MVA vaccination provides long-term protection against rabbitpox virus (RPXV) challenge, an animal model of smallpox. Two doses of MVA provided 100% protection against a lethal intranasal RPXV challenge administered 9 months after vaccination.

Defending against smallpox: a focus on vaccines

Smallpox has shaped human history, from the earliest human civilizations well into the 20th century. With high mortality rates, rapid transmission, and serious long-term effects on survivors, smallpox was a much-feared disease. The eradication of smallpox represents an unprecedented medical victory for the lasting benefit of human health and prosperity. Concerns remain, however, about the development and use of the smallpox virus as a biological weapon, which necessitates the need for continued vaccine development. Smallpox vaccine development is thus a much-reviewed topic of high interest. This review focuses on the current state of smallpox vaccines and their context in biodefense efforts.

Smallpox Vaccination of Laboratory Workers at US Variola Testing Sites

To evaluate the need to revaccinate laboratory workers against smallpox, we assessed regular revaccination at the US Laboratory Response Network’s variola testing sites by examining barriers to revaccination and the potential for persistence of immunity. Our data do not provide evidence to suggest prolonging the recommended interval for revaccination.

Vaccinia Virus LC16m8∆ as a Vaccine Vector for Clinical Applications

The LC16m8 strain of vaccinia virus, the active ingredient in the Japanese smallpox vaccine, was derived from the Lister/Elstree strain. LC16m8 is replication-competent and has been administered to over 100,000 infants and 3,000 adults with no serious adverse reactions. Despite this outstanding safety profile, the occurrence of spontaneously-generated large plaque-forming virulent LC16m8 revertants following passage in cell culture is a major drawback. We identified the gene responsible for the reversion and deleted the gene (B5R) from LC16m8 to derive LC16m8Δ. LC16m8∆ is non-pathogenic in immunodeficient severe combined immunodeficiency (SCID) mice, genetically-stable and does not reverse to a large-plaque phenotype upon passage in cell culture, even under conditions in which most LC16m8 populations are replaced by revertants. Moreover, LC16m8∆ is >500-fold more effective than the non-replicating vaccinia virus (VV), Modified Vaccinia Ankara (MVA), at inducing murine immune responses against pathogenic VV. LC16m8∆, which expresses the SIV gag gene, also induced anti-Gag CD8⁺ T-cells more efficiently than MVA and another non-replicating VV, Dairen I minute-pock variants (DIs). Moreover, LC16m8∆ expressing HIV-1 Env in combination with a Sendai virus vector induced the production of anti-Env antibodies and CD8⁺ T-cells. Thus, the safety and efficacy of LC16m8∆ mean that it represents an outstanding platform for the development of human vaccine vectors.

Elements in the Development of a Production Process for Modified Vaccinia Virus Ankara

The production of several viral vaccines depends on chicken embryo fibroblasts or embryonated chicken eggs. To replace this logistically demanding substrate, we created continuous anatine suspension cell lines (CR and CR.pIX), developed chemically-defined media, and established production processes for different vaccine viruses. One of the processes investigated in greater detail was developed for modified vaccinia virus Ankara (MVA). MVA is highly attenuated for human recipients and an efficient vector for reactogenic expression of foreign genes. Because direct cell-to-cell spread is one important mechanism for vaccinia virus replication, cultivation of MVA in bioreactors is facilitated if cell aggregates are induced after infection. This dependency may be the mechanism behind our observation that a novel viral genotype (MVA-CR) accumulates with serial passage in suspension cultures. Sequencing of a major part of the genomic DNA of the new strain revealed point mutations in three genes. We hypothesize that these changes confer an advantage because they may allow a greater fraction of MVA-CR viruses to escape the host cells for infection of distant targets. Production and purification of MVA-based vaccines may be simplified by this combination of designed avian cell line, chemically defined media and the novel virus strain.

Utilizing poxviral vectored vaccines for antibody induction-progress and prospects

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Poxviral vectors are now regarded as robust tools for B cell and antibody induction.

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Antibody responses can be induced against the vector as well as a transgene.

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Increasing application is seen in heterologous prime–boost immunization regimes.

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Effective veterinary poxviral vaccine products are now licensed.

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Promising results of antibody induction are being reported in human clinical trials.

Over the last decade, poxviral vectors emerged as a mainstay approach for the induction of T cell-mediated immunity by vaccination, and their suitability for human use has led to widespread clinical testing of candidate vectors against infectious intracellular pathogens and cancer. In contrast, poxviruses have been widely perceived in the vaccine field as a poor choice of vector for the induction of humoral immunity. However, a growing body of data, from both animal models and recent clinical trials, now suggests that these vectors can be successfully utilized to prime and boost B cells and effective antibody responses. Significant progress has been made in the context of heterologous prime–boost immunization regimes, whereby poxviruses are able to boost responses primed by other vectors, leading to the induction of high-titre antigen-specific antibody responses. In other cases, poxviral vectors have been shown to stimulate humoral immunity against both themselves and encoded transgenes, in particular viral surface proteins such as influenza haemagglutinin. In the veterinary field, recombinant poxviral vectors have made a significant impact with numerous vectors licensed for use against a variety of animal viruses. On-going studies continue to explore the potential of poxviral vectors to modulate qualitative aspects of the humoral response, as well as their amenability to adjuvantation seeking to improve quantitative antibody immunogenicity. Nevertheless, the underlying mechanisms of B cell induction by recombinant poxviruses remain poorly defined, and further work is necessary to help guide the rational optimization of future poxviral vaccine candidates aiming to induce antibodies.

Protective efficacy of Modified Vaccinia virus Ankara in preclinical studies

Modified Vaccinia virus Ankara (MVA) is a tissue culture-derived, highly attenuated strain of vaccinia virus (VACV) exhibiting characteristic defective replication in cells from mammalian hosts. In the 1960s MVA was originally generated as a candidate virus for safer vaccination against smallpox. Now, MVA is widely used in experimental vaccine development targeting important infectious diseases and cancer. Versatile technologies for genetic engineering, large-scale production, and quality control facilitate R&D of recombinant and non-recombinant MVA vaccines matching today's requirements for new biomedical products. Such vaccines are attractive candidates for delivering antigens from pathogens against which no, or no effective vaccine is available, including emerging infections caused by highly pathogenic influenza viruses, chikungunya virus, West Nile virus or zoonotic orthopoxviruses. Other directions are seeking valuable vaccines against highly complex diseases such as AIDS, malaria, and tuberculosis. Here, we highlight examples of MVA candidate vaccines against infectious diseases, and review the efforts made to assess both the efficacy of vaccination and immune correlates of protection in preclinical studies.

A genotype of modified vaccinia Ankara (MVA) that facilitates replication in suspension cultures in chemically defined medium

While vectored vaccines, based on hyperattenuated viruses, may lead to new treatment options against infectious diseases and certain cancers, they are also complex products and sometimes difficult to provide in sufficient amount and purity. To facilitate vaccine programs utilizing host-restricted poxviruses, we established avian suspension cell lines (CR and CR.pIX) and developed a robust, chemically defined, culturing process for production of this class of vectors. For one prominent member, modified vaccinia Ankara (MVA), we now describe a new strain that appears to replicate to greater yields of infectious units, especially in the cell-free supernatant of cultures in chemically defined media. The new strain was obtained by repeated passaging in CR suspension cultures and, consistent with reports on the exceptional genetic stability of MVA, sequencing of 135 kb of the viral genomic DNA revealed that only three structural proteins (A3L, A9L and A34R) each carry a single amino acid exchange (H639Y, K75E and D86Y, respectively). Host restriction in a plaque-purified isolate of the new genotype appears to be maintained in cell culture. Processing towards an injectable vaccine preparation may be simplified with this strain as a complete lysate, containing the main burden of host cell contaminants, may not be required anymore to obtain adequate yields.

Deletion of the viral anti-apoptotic gene F1L in the HIV/AIDS vaccine candidate MVA-C enhances immune responses against HIV-1 antigens

Vaccinia virus (VACV) encodes an anti-apoptotic Bcl-2-like protein F1 that acts as an inhibitor of caspase-9 and of the Bak/Bax checkpoint but the role of this gene in immune responses is not known. Because dendritic cells that have phagocytosed apoptotic infected cells cross-present viral antigens to cytotoxic T cells inducing an antigen-specific immunity, we hypothesized that deletion of the viral anti-apoptotic F1L gene might have a profound effect on the capacity of poxvirus vectors to activate specific immune responses to virus-expressed recombinant antigens. This has been tested in a mouse model with an F1L deletion mutant of the HIV/AIDS vaccine candidate MVA-C that expresses Env and Gag-Pol-Nef antigens (MVA-C-ΔF1L). The viral gene F1L is not required for virus replication in cultured cells and its deletion in MVA-C induces extensive apoptosis and expression of immunomodulatory genes in infected cells. Analysis of the immune responses induced in BALB/c mice after DNA prime/MVA boost revealed that, in comparison with parental MVA-C, the mutant MVA-C-ΔF1L improves the magnitude of the HIV-1-specific CD8 T cell adaptive immune responses and impacts on the CD8 T cell memory phase by enhancing the magnitude of the response, reducing the contraction phase and changing the memory differentiation pattern. These findings reveal the immunomodulatory role of F1L and that the loss of this gene is a valid strategy for the optimization of MVA as vaccine vector.

Modified vaccinia virus Ankara-based vaccine vectors induce apoptosis in dendritic cells draining from the skin via both the extrinsic and intrinsic caspase pathways, preventing efficient antigen presentation

Dendritic cells (DC) are potent antigen-presenting cells and central to the induction of immune responses following infection or vaccination. The collection of DC migrating from peripheral tissues by cannulation of the afferent lymphatic vessels provides DC which can be used directly ex vivo without extensive in vitro manipulations. We have previously used bovine migrating DC to show that recombinant human adenovirus 5 vectors efficiently transduce afferent lymph migrating DEC-205(+) CD11c(+) CD8(-) DC (ALDC). We have also shown that recombinant modified vaccinia virus Ankara (MVA) infects ALDC in vitro, causing downregulation of costimulatory molecules, apoptosis, and cell death. We now show that in the bovine system, modified vaccinia virus Ankara-induced apoptosis in DC draining from the skin occurs soon after virus binding via the caspase 8 pathway and is not associated with viral gene expression. We also show that after virus entry, the caspase 9 pathway cascade is initiated. The magnitude of T cell responses to mycobacterial antigen 85A (Ag85A) expressed by recombinant MVA-infected ALDC is increased by blocking caspase-induced apoptosis. Apoptotic bodies generated by recombinant MVA (rMVA)-Ag85A-infected ALDC and containing Ag85A were phagocytosed by noninfected migrating ALDC expressing SIRPα via actin-dependent phagocytosis, and these ALDC in turn presented antigen. However, the addition of fresh ALDC to MVA-infected cultures did not improve on the magnitude of the T cell responses; in contrast, these noninfected DC showed downregulation of major histocompatibility complex class II (MHC-II), CD40, CD80, and CD86. We also observed that MVA-infected ALDC promoted migration of DEC-205(+) SIRPα(+) CD21(+) DC as well as CD4(+) and CD8(+) T cells independently of caspase activation. These in vitro studies show that induction of apoptosis in DC by MVA vectors is detrimental to the subsequent induction of T cell responses.

The attenuated NYCBH vaccinia virus deleted for the immune evasion gene, E3L, completely protects mice against heterologous challenge with ectromelia virus

The New York City Board of Health (NYCBH) vaccinia virus (VACV) vaccine strain was deleted for the immune evasion gene, E3L, and tested for its pathogenicity and ability to protect mice from heterologous challenge with ectromelia virus (ECTV). NYCBHΔE3L was found to be highly attenuated for pathogenicity in a newborn mouse model and showed a similar attenuated phenotype as the NYVAC strain of vaccinia virus. Scarification with one or two doses of the attenuated NYCBHΔE3L was able to protect mice equally as well as NYCBH from death, weight loss, and viral spread to visceral organs. A single dose of NYCBHΔE3L resulted in low poxvirus-specific antibodies, and a second dose increased levels of poxvirus-specific antibodies to a level similar to that seen in animals vaccinated with a single dose of NYCBH. However, similar neutralizing antibody titers were observed following one or two doses of NYCBHΔE3L or NYCBH. Thus, NYCBHΔE3L shows potential as a candidate for a safer human smallpox vaccine since it protects mice from challenge with a heterologous poxvirus.

Rabies virus as a research tool and viral vaccine vector

Until recently, single-stranded negative sense RNA viruses (ssNSVs) were one of only a few important human viral pathogens, which could not be created from cDNA. The inability to manipulate their genomes hindered their detailed genetic analysis. A key paper from Conzelmann's laboratory in 1994 changed this with the publication of a method to recover rabies virus (RABV) from cDNA. This discovery not only dramatically changed the broader field of ssNSV biology but also opened a whole new avenue for studying RABV pathogenicity, developing novel RABV vaccines as well a new generation of RABV-based vaccine vectors, and creating research tools important in neuroscience such as neuronal tracing.

Development of the small-molecule antiviral ST-246 as a smallpox therapeutic

Naturally occurring smallpox has been eradicated, yet it remains as one of the highest priority pathogens due to its potential as a biological weapon. The majority of the US population would be vulnerable in a smallpox outbreak. SIGA Technologies, Inc. has responded to the call of the US government to develop and supply to the Strategic National Stockpile a smallpox antiviral to be deployed in the event of a smallpox outbreak. ST-246(®) (tecovirimat) was initially identified via a high-throughput screen in 2002, and in the ensuing years, our drug-development activities have spanned in vitro analysis, preclinical safety, pharmacokinetics and efficacy testing (all according to the 'animal rule'). Additionally, SIGA has conducted Phase I and II clinical trials to evaluate the safety, tolerability and pharmacokinetics of ST-246, bringing us to our current late stage of clinical development. This article reviews the need for a smallpox therapeutic and our experience in developing ST-246, and provides perspective on the role of a smallpox antiviral during a smallpox public health emergency.

Deletion of the A35 gene from Modified Vaccinia Virus Ankara increases immunogenicity and isotype switching

We show here that the immunogenicity of the Modified Vaccinia Ankara MVA vaccine strain can be improved by deletion of the A35 gene, without diminishing the ability of the virus to replicate. Deletion of the A35 gene resulted in increased virus-specific immunoglobulin production, class switching to IgG isotypes, and virus-specific IFNγ-secreting splenocytes. The MVA35 deletion virus provided excellent protective efficacy against virulent virus challenge. These results suggest that A35 deletion mutant strains will have superior vaccine performance for poxvirus vaccines as well as platform vaccines for other infectious diseases and cancer.

Long term recall of memory CD8 T cells in mice to first and third generation smallpox vaccines

Since long-term immunity is a critical component of any effective vaccine, we compared over a 15-month period, the strength, durability and specificity of immunity of an attenuated smallpox vaccine Modified Vaccinia Ankara (MVA) to the New York City Board of Health (NYCBH) vaccine. The frequencies of CD8(+) T cells to an immunodominant CD8 T cell epitope B8R(20-27) remained remarkably stable in mice given either MVA or NYCBH. Both groups were also protected from a lethal intranasal challenge with Western Reserve strain of vaccinia virus (VACV-WR). Cytokine responses to virus-specific peptides were detectable with significant boosting upon challenge. Expression of most phenotypic markers that define antigen-specific memory CD8 T cells was similar while CD27 was differentially expressed on lung-specific T cells compared to the spleen. Our data indicate robust vaccinia-specific CD8(+) T cell recall responses to lethal secondary challenge in protected mice with no apparent effect of age on T cell pools established much earlier in life.

Third-generation smallpox vaccines: challenges in the absence of clinical smallpox

Smallpox, a disease caused by variola virus, is estimated to have killed hundreds of millions to billions of people before it was certified as eradicated in 1980. However, there has been renewed interest in smallpox vaccine development due in part to zoonotic poxvirus infections and the possibility of a re-emergence of smallpox, as well as the fact that first-generation smallpox vaccines are associated with relatively rare, but severe, adverse reactions in some vaccinees. An understanding of the immune mechanisms of vaccine protection and the use of suitable animal models for vaccine efficacy assessment are paramount to the development of safer and effective smallpox vaccines. This article focuses on studies aimed at understanding the immune responses elicited by vaccinia virus and the various animal models that can be used to evaluate smallpox vaccine efficacy. Harnessing this information is necessary to assess the effectiveness and potential usefulness of new-generation smallpox vaccines.

The poxvirus A35 protein is an immunoregulator

It was shown previously that the highly conserved vaccinia virus A35 gene is an important virulence factor in respiratory infection of mice. We show here that A35 is also required for full virulence by the intraperitoneal route of infection. A virus mutant in which the A35 gene has been removed replicated normally and elicited improved antibody, gamma interferon-secreting cell, and cytotoxic T-lymphocyte responses compared to wild-type virus, suggesting that A35 increases poxvirus virulence by immunomodulation. The enhanced immune response correlated with an improved control of viral titers in target organs after the development of the specific immune response. Finally, the A35 deletion mutant virus also provided protection from lethal challenge (1,000 50% lethal doses) equal to that of the wild-type virus. Together, these data suggest that A35 deletion viruses will make safer and more efficacious vaccines for poxviruses. In addition, the A35 deletion viruses will serve as improved platform vectors for other infectious diseases and cancer and will be superior vaccine choices for postexposure poxvirus vaccination, as they also provide improved kinetics of the immune response.

Antiviral protection following immunization correlates with humoral but not cell-mediated immunity

Smallpox was a deadly disease when it was rife yet despite its eradication more than 30 years ago, the possibility of accidental or intentional release has driven research in search of better definitions of correlates of protective immunity. Mousepox, a disease caused by ectromelia virus (ECTV), is arguably one of the best surrogate small animal models for smallpox. Correlates of protection in mousepox are well defined during primary infection, whereas those in a secondary infection, which have definite relevance to vaccination strategies, are less well understood. We previously established that neutralizing antibody (Ab), which is generated far more rapidly during a secondary infection compared with a primary infection, has a key role during a secondary virus challenge. In this study, we show that the route of immunization or the use of homologous or heterologous virus vaccines for immunization does not influence the ability of mice to control high-dose virulent ECTV challenge or to mount a substantial secondary neutralizing Ab response. In contrast, the recall cytotoxic T lymphocyte (CTL) responses generated under these regimes of immunization were varied and did not correlate with virus control. Furthermore, unlike the recall Ab response that was generated rapidly, the kinetics of the secondary antiviral CTL response was no different to a primary infection and peaked only at day 8 post-challenge. This finding further underscores the importance of Ab in conferring protection during secondary poxvirus infection. This information could potentially prove useful in the design of safer and more efficacious vaccines against poxviruses or other diseases using poxvirus vectors.

Characterization of a single-cycle rabies virus-based vaccine vector

Recombinant rabies virus (RV)-based vectors have demonstrated their efficacy in generating long-term, antigen-specific immune responses in murine and monkey models. However, replication-competent viral vectors pose significant safety concerns due to vector pathogenicity. RV pathogenicity is largely attributed to its glycoprotein (RV-G), which facilitates the attachment and entry of RV into host cells. We have developed a live, single-cycle RV by deletion of the G gene from an RV vaccine vector expressing HIV-1 Gag (SPBN-DeltaG-Gag). Passage of SPBN-DeltaG-Gag on cells stably expressing RV-G allowed efficient propagation of the G-deleted RV. The in vivo immunogenicity data comparing single-cycle RV to a replication-competent control (BNSP-Gag) showed lower RV-specific antibodies; however, the overall isotype profiles (IgG2a/IgG1) were similar for the two vaccine vectors. Despite this difference, mice immunized with SPBN-DeltaG-Gag and BNSP-Gag mounted similar levels of Gag-specific CD8(+) T-cell responses as measured by major histocompatibility complex class I Gag-tetramer staining, gamma interferon-enzyme-linked immunospot assay, and cytotoxic T-cell assay. Moreover, these cellular responses were maintained equally at immunization titers as low as 10(3) focus-forming units for both RV vaccine vectors. CD8(+) T-cell responses were significantly enhanced by a boost with a single-cycle RV complemented with a heterologous vesicular stomatitis virus glycoprotein. These findings demonstrate that single-cycle RV is an effective alternative to replication-competent RV vectors for future development of vaccines for HIV-1 and other infectious diseases.

Smallpox vaccines for biodefense

Few diseases can match the enormous impact that smallpox has had on mankind. Its influence can be seen in the earliest recorded histories of ancient civilizations in Egypt and Mesopotamia. With fatality rates up to 30%, smallpox left its survivors with extensive scarring and other serious sequelae. It is estimated that smallpox killed 500 million people in the 19th and 20th centuries. Given the ongoing concerns regarding the use of variola as a biological weapon, this review will focus on the licensed vaccines as well as current research into next-generation vaccines to protect against smallpox and other poxviruses.

The new ACAM2000 vaccine and other therapies to control orthopoxvirus outbreaks and bioterror attacks

Quarantine, case tracing and population vaccination facilitated the global eradication, in 1980, of variola virus, the causative agent of smallpox. The vaccines used during the eradication period, including Dryvax, the smallpox vaccine used in the USA, were live vaccinia and cowpoxvirus-based vaccines, which induced long-lasting and cross-protective immunity against variola and other related poxviruses. These vaccine viruses were produced by serial propagation in domesticated animals. The drawbacks to such serially propagated live viral vaccines include the level of postvaccination local and systemic reactions and contraindications to their use in immunocompromised individuals, individuals with certain skin and cardiac diseases, and pregnant women. In the latter stages of the population-based smallpox vaccination campaign, research began with ways to improve safety and modernizing the manufacture of vaccinia vaccines; however, with the eradication of variola this work stopped. Outbreaks of monkeypoxvirus in humans and the bioterrorist threat of monkeypox and variola virus renewed the need for improved vaccinia vaccines. ACAM2000 is one of the new generation of smallpox vaccines. It is produced in cell culture from a clonally purified master seed stock of vaccinia derived from the New York City Board of Health strain of vaccinia. The clonally purified master seed assures a more homogeneous vaccine without the inherent mutations associated with serial propagation and the cell culture limits adventitious and bacterial contamination in vaccine production. In preclinical and clinical trials, ACAM2000 demonstrated an immunogenicity and safety profile similar to that of Dryvax.

Expanding the repertoire of Modified Vaccinia Ankara-based vaccine vectors via genetic complementation strategies

Background

Modified Vaccinia virus Ankara (MVA) is a safe, highly attenuated orthopoxvirus that is being developed as a recombinant vaccine vector for immunization against a number of infectious diseases and cancers. However, the expression by MVA vectors of large numbers of poxvirus antigens, which display immunodominance over vectored antigens-of-interest for the priming of T cell responses, and the induction of vector-neutralizing antibodies, which curtail the efficacy of subsequent booster immunizations, remain as significant impediments to the overall utility of such vaccines. Thus, genetic approaches that enable the derivation of MVA vectors that are antigenically less complex may allow for rational improvement of MVA-based vaccines.

Principal Findings

We have developed a genetic complementation system that enables the deletion of essential viral genes from the MVA genome, thereby allowing us to generate MVA vaccine vectors that are antigenically less complex. Using this system, we deleted the essential uracil-DNA-glycosylase (udg) gene from MVA and propagated this otherwise replication-defective variant on a complementing cell line that constitutively expresses the poxvirus udg gene and that was derived from a newly identified continuous cell line that is permissive for growth of wild type MVA. The resulting virus, MVAΔudg, does not replicate its DNA genome or express late viral gene products during infection of non-complementing cells in culture. As proof-of-concept for immunological ‘focusing’, we demonstrate that immunization of mice with MVAΔudg elicits CD8+ T cell responses that are directed against a restricted repertoire of vector antigens, as compared to immunization with parental MVA. Immunization of rhesus macaques with MVAΔudg-gag, a udg⁻ recombinant virus that expresses an HIV subtype-B consensus gag transgene, elicited significantly higher frequencies of Gag-specific CD8 and CD4 T cells following both primary (2–4-fold) and booster (2-fold) immunizations as compared to the udg⁺ control virus MVA-gag, as determined by intracellular cytokine assay. In contrast, levels of HIV Gag-specific antibodies were elicited similarly in macaques following immunization with MVAΔudg-gag and MVA-gag. Furthermore, both udg⁻ and udg⁺ MVA vectors induced comparatively similar titers of MVA-specific neutralizing antibody responses following immunization of mice (over a 4-log range: 10⁴–10⁸ PFU) and rhesus macaques. These results suggest that the generation of MVA-specific neutralizing antibody responses are largely driven by input MVA antigens, rather than those that are synthesized de novo during infection, and that the processes governing the generation of antiviral antibody responses are more readily saturated by viral antigen than are those that elicit CD8+ T cell responses.

Significance

Our identification of a spontaneously-immortalized (but not transformed) chicken embryo fibroblast cell line (DF-1) that is fully permissive for MVA growth and that can be engineered to stably express MVA genes provides the basis for a genetic system for MVA. DF-1 cells (and derivatives thereof) constitute viable alternatives, for the manufacture of MVA-based vaccines, to primary CEFs – the conventional cell substrate for MVA vaccines that is not amenable to genetic complementation strategies due to these cells' finite lifespan in culture. The establishment of a genetic system for MVA, as illustrated here to allow udg deletion, enables the generation of novel replication-defective MVA mutants and expands the repertoire of genetic viral variants that can now be explored as improved vaccine vectors.

Postexposure immunization with modified vaccinia virus Ankara or conventional Lister vaccine provides solid protection in a murine model of human smallpox

BACKGROUND

Decades after the cessation of smallpox vaccination, the potential of the deliberate release of pathogenic orthopoxviruses has forced a reconsideration of using these extremely efficient human vaccines. Scenarios of sudden biothreats have prompted demand for rapidly protective vaccination. However, the feasibility of short-term vaccination (i.e., vaccination shortly before exposure) with vaccinia virus (VACV) is uncertain.

METHODS

We tested the rapid protective capacity of vaccines based on VACV strain Lister (VACV-Lister) and on modified VACV Ankara (MVA) in different mouse models, comparing lethal infections with VACV strain Western Reserve (VACV-WR) or ectromelia virus (ECTV).

RESULTS

In contrast to VACV-WR challenge, we found extended incubation periods after ECTV challenge, allowing successful therapeutic immunization with VACV-Lister and MVA when applied 2-3 days after exposure. Rapid protection from respiratory tract ECTV infection was significantly affected by vaccine dose and was associated with occurrence of poxvirus-specific antibodies. Vaccinations in type I interferon receptor-deficient mice were protective, whereas recombination activating gene 1-deficient mice lacking mature T and B cells failed to mount immunity after short-term vaccination, confirming an essential role of adaptive immune responses.

CONCLUSIONS

ECTV infection in mice models the course of human smallpox. Our data provide evidence to substantiate historical data on the usefulness of postexposure vaccination with conventional VACV and the new candidate MVA to protect against fatal orthopoxvirus infections.

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.

Smallpox vaccines for biodefense: need and feasibility

Smallpox, eradicated as a cause of natural disease through an intensive global effort in the later part of the 20th Century, has resurfaced as a possible agent of bioterrorism. For this reason, there is renewed interest in smallpox vaccines. Live vaccinia virus, an orthopoxvirus related to smallpox, has a long and successful clinical track record as an effective smallpox vaccine; however, its use is associated with uncommon yet serious adverse events. This has led to a surge of recent research into newer-generation smallpox vaccines with improved safety profiles and retained efficacy. This article will review the history of smallpox vaccines, assess the status of newer-generation vaccines and examine the overall risk-versus-benefit profile of smallpox vaccination.

Comparing the antibody responses against recombinant hemagglutinin proteins of avian influenza A (H5N1) virus expressed in insect cells and bacteria

The hemagglutinin (HA) of influenza A virus plays an essential role in mediating the entry of the virus into host cells. Here, recombinant full-length HA5 protein from a H5N1 isolate (A/chicken/hatay/2004(H5N1)) was expressed and purified from the baculovirus-insect cell system. As expected, full-length HA5 elicits strong neutralizing antibodies, as evaluated in micro-neutralization tests using HA5 pseudotyped lentiviral particles. In addition, two fragments of HA5 were expressed in bacteria and the N-terminal fragment, covering the ectodomain before the HA1/HA2 polybasic cleavage site, was found to elicit neutralizing antibodies. But the C-terminal fragment, which covers the remaining portion of the ectodomain, did not. Neutralizing titer of the anti-serum against the N-terminal fragment is only four times lower than the anti-serum against the full-length HA5 protein. Using a novel membrane fusion assay, the abilities of these antibodies to block membrane fusion were found to correlate well with the neutralization activities.

Vaccinia virus entry/fusion complex subunit A28 is a target of neutralizing and protective antibodies

The vaccinia virus entry/fusion complex (EFC) is comprised of at least eight transmembrane proteins that are conserved in all poxviruses. However, neither the physical structure of the EFC nor the immunogenicity of the individual components has been determined. We prepared soluble forms of two EFC components, A28 and H2, by replacing the transmembrane domain with a signal peptide and adding a polyhistidine tail. The proteins were expressed by baculoviruses, secreted from insect cells, purified by affinity chromatography and used to raise antibodies in rabbits. The antibodies recognized the viral proteins but only the antibody to recombinant A28 bound intact virions and neutralized infectivity. Analyses with a set of overlapping peptides revealed a neutralizing epitope between residues 73 and 92 of A28. Passive immunization of mice with IgG purified from the anti-A28 serum provided partial protection against a vaccinia virus intranasal challenge, whereas IgG from the anti-H2 serum did not.

New generation smallpox vaccines: a review of preclinical and clinical data

The recognition that smallpox is a potential threat agent of bioterrorism has engendered renewed interest in the development of improved vaccines against this pathogen. The purpose of this paper is to review current data regarding novel approaches to smallpox vaccines in comparison with traditional vaccine strategies. The method used is a literature search using overlapping search terms and citations from the relevant, published literature. Substantial animal and limited human data suggest that selected second and third generation smallpox vaccines, specifically tissue-cultured vaccinia virus and replication-competent, highly attenuated vaccinia virus possess immunogenicity and surrogate efficacy profiles similar to those of first generation New York City Board of Health and Lister vaccines. Replication-defective, attenuated vaccinia appears to be less immunogenic in both animals and humans but may have utility as a priming agent in those with contraindications to live vaccinia. There is a clear risk of myopericarditis with first and second generation products, but the relative risk of this complication among various vaccine approaches cannot as yet be determined. The incidence of other serious, albeit uncommon, adverse events of smallpox vaccines cannot be determined for newer vaccine approaches because these agents have not yet been deployed on a scale large enough to discern rare events. Since 2001, progress towards improved smallpox vaccines has been accelerated, spurred on by the threat of bioterrorism. Among newer vaccine candidates, replication-competent, highly attenuated vaccinia and tissue culture-derived live vaccinia appear to offer the greatest potential for efficacy, although it is unclear whether these products offer a safer alternative to existing first generation vaccines.

Disparity between levels of in vitro neutralization of vaccinia virus by antibody to the A27 protein and protection of mice against intranasal challenge

Immunization with recombinant proteins may provide a safer alternative to live vaccinia virus for prophylaxis of poxvirus infections. Although antibody protects against vaccinia virus infection, the mechanism is not understood and the selection of immunogens is daunting as there are dozens of surface proteins and two infectious forms known as the mature virion (MV) and the enveloped virion (EV). Our previous studies showed that mice immunized with soluble forms of EV membrane proteins A33 and B5 and MV membrane protein L1 or passively immunized with antibodies to these proteins survived an intranasal challenge with vaccinia virus. The present study compared MV protein A27, which has a role in virus attachment to glycosaminoglycans on the cell surface, to L1 with respect to immunogenicity and protection. Although mice developed similar levels of neutralizing antibody after immunizations with A27 or L1, A27-immunized mice exhibited more severe disease upon an intranasal challenge with vaccinia virus. In addition, mice immunized with A27 and A33 were not as well protected as mice receiving L1 and A33. Polyclonal rabbit anti-A27 and anti-L1 IgG had equivalent MV-neutralizing activities when measured by the prevention of infection of human or mouse cells or cells deficient in glycosaminoglycans or by adding antibody prior to or after virus adsorption. Nevertheless, the passive administration of antibody to A27 was poorly protective compared to the antibody to L1. These studies raise questions regarding the basis for antibody protection against poxvirus disease and highlight the importance of animal models for the early evaluation of vaccine candidates.

Vaccinia viruses with mutations in the E3L gene as potential replication-competent, attenuated vaccines: scarification vaccination

In this study, we evaluated the efficacy of vaccinia virus (VACV) containing mutations in the E3L virulence gene to protect mice against a lethal poxvirus challenge after vaccination by scarification. VACV strains with mutations in the E3L gene had significantly decreased pathogenicity, even in immune deficient mice, yet retained the ability to produce a potent Th1-dominated immune response in mice after vaccination by scarification, while protecting against challenge with wild type, pathogenic VACV. Initial experiments were done using the mouse-adapted, neurovirulent Western Reserve (WR) strain of vaccinia virus. Testing of the full E3L deletion mutation in the Copenhagen and NYCBH strains of VACV, which are more appropriate for use in humans, produced similar results. These results suggest that highly attenuated strains of VACV containing mutations in E3L have the potential for use as scarification administered vaccines.

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.

Targeting the vaccinia virus L1 protein to the cell surface enhances production of neutralizing antibodies

The current live-orthopoxvirus vaccine is associated with minor to serious adverse affects, and is contraindicated for use in a significant portion of the population. As an alternative vaccine, we have previously shown that a DNA subunit vaccine (4pox) based on four orthopoxvirus immunogens (L1R, B5R, A27L and A33R) can produce protective immunity against lethal orthopoxvirus challenges in mice and nonhuman primates. Because antibodies are critical for protection against secondary orthopoxvirus infections, we are now interested in strategies that will enhance the humoral immune response against vaccine targets. Here, we tested the immunogenicity of an L1R construct to which a tissue plasminogen activator signal sequence was placed in frame with the full-length L1R gene. The tPA-L1R construct produced a more robust neutralizing antibody response in vaccinated mice when the DNA vaccine was administered by gene-gun as a prime/single boost. When the tPA-L1R construct was substituted for the unmodified L1R gene in the 4pox vaccine, given as a prime and single boost, animals were better protected from lethal challenge with vaccinia virus (VACV). These findings indicate that adding a tPA-leader sequence can enhance the immunogenicity of the L1R gene when given as a DNA vaccine. Furthermore, our results demonstrate that a DNA-based vaccine is capable of establishing protection from lethal orthopoxvirus challenges when administered as a prime and single boost without requiring adjuvant.

Short- and long-term immunogenicity and protection induced by non-replicating smallpox vaccine candidates in mice and comparison with the traditional 1st generation vaccine

This study assessed three non-replicating smallpox vaccine candidates (modified vaccinia Ankara (MVA), NYVAC and HR) for their immunogenicity and ability to protect mice against an intranasal cowpox virus challenge and compared them with the traditional replicating vaccine. A single immunisation with the non-replicating vaccines induced a complete protection from death at short-term, but was not fully protective when mice were challenged 150 days post-vaccination with protection correlated with the specific neutralizing antibodies and CD4(+) T-cells responses. Prime-boost vaccination enabled effective long-term protection from death for mice vaccinated with MVA, but protection from disease and CD4(+) T-cell level were lower than the ones induced by the traditional vaccine over the long-term period. Further investigations are necessary with MVA to determine the optimal conditions of immunisation to induce at long-term immunogenicity and protection observed with the 1st generation smallpox vaccine.

Immune responses to the smallpox vaccine given in combination with ST-246, a small-molecule inhibitor of poxvirus dissemination

The re-emerging threat of smallpox and the emerging threat of monkeypox highlight the need for effective poxvirus countermeasures. Currently approved smallpox vaccines have unacceptable safety profiles and, consequently, the general populace is no longer vaccinated, leading to an increasingly susceptible population. ST-246, a small-molecule inhibitor of poxvirus dissemination, has been demonstrated in various animal models to be safe and effective in preventing poxviral disease. This suggests that it may also be used to improve the safety of the traditional smallpox vaccine provided that it does not inhibit vaccine-induced protective immunity. In this study, we compared the immune responses elicited by the smallpox vaccine alone or in combination with ST-246 in mice. Normal lesion formation following dermal scarification with the attenuated New York City Board of Health strain (Dryvax), commonly referred to as a vaccine “take”, was not inhibited although severe lesions and systemic disease due to vaccination with the virulent Western Reserve (VV-WR) strain were prevented. The vaccine given with ST-246 did not affect cellular immune responses or neutralizing antibody titers although anti-vaccinia ELISA titers were slightly reduced. Vaccination in combination with ST-246 provided equivalent short- and long-term protection against lethal intranasal challenge with VV-WR when compared to vaccine alone. These results suggest that ST-246 does not compromise protective immunity elicited by the vaccine and provide the basis for future studies examining the efficacy of ST-246 in preventing or treating adverse events due to vaccination.

A novel, killed-virus nasal vaccinia virus vaccine

Live-virus vaccines for smallpox are effective but have risks that are no longer acceptable for routine use in populations at minimal risk of infection. We have developed a mucosal, killed-vaccinia virus (VV) vaccine based on antimicrobial nanoemulsion (NE) of soybean oil and detergent. Incubation of VV with 10% NE for at least 60 min causes the complete disruption and inactivation of VV. Simple mixtures of NE and VV (Western Reserve serotype) (VV/NE) applied to the nares of mice resulted in both systemic and mucosal anti-VV immunity, virus-neutralizing antibodies, and Th1-biased cellular responses. Nasal vaccination with VV/NE vaccine produced protection against lethal infection equal to vaccination by scarification, with 100% survival after challenge with 77 times the 50% lethal dose of live VV. However, animals protected with VV/NE immunization did after virus challenge have clinical symptoms more extensive than animals vaccinated by scarification. VV/NE-based vaccines are highly immunogenic and induce protective mucosal and systemic immunity without the need for an inflammatory adjuvant or infection with live virus.

Vaccinia viruses with mutations in the E3L gene as potential replication-competent, attenuated vaccines: intra-nasal vaccination

Vaccinia virus (VACV) has been used as the vaccine to protect against smallpox, and recombinant VACVs have been used to develop vaccine candidates against numerous cancers and infectious diseases. Although relatively safe for use in humans, the strains of VACV that were used as smallpox vaccines led to several complications including, progressive infection in immune compromised individuals, eczema vaccination in individuals with a history of atopic dermatitis, and encephalitis and perimyocarditis in apparently healthy individuals. The work described in this paper focuses on attenuated strains of VACV that may have the potential for use as vaccine vectors with reduced pathogenicity. We have generated several VACV mutants in a WR background with specific mutations in the E3L gene that were at least a 1000-fold less pathogenic compared to wtVACV upon intra-nasal infection of mice. Many of these mutant viruses replicated to high titers in the nasal mucosa of mice following intra-nasal administration. Despite replication to high titers in the nose, there was little spread to other organs in infected animals. Intra-nasal vaccination with doses as low as 100-1000 pfu (plaque forming units) of these replicating VACV constructs were sufficient to protect the host from challenge with large doses of wtVACV. Similar constructs in a Copenhagen and a NYCBH background were highly attenuated, yet effective as vaccines in the mouse model. These recombinant VACV constructs may be promising vector candidates for use in vaccination strategies against smallpox and other pathogens.

Structural basis for the binding of the neutralizing antibody, 7D11, to the poxvirus L1 protein

Medical countermeasures to prevent or treat smallpox are needed due to the potential use of poxviruses as biological weapons. Safety concerns with the currently available smallpox vaccine indicate a need for research on alternative poxvirus vaccine strategies. Molecular vaccines involving the use of proteins and/or genes and recombinant antibodies are among the strategies under current investigation. The poxvirus L1 protein, encoded by the L1R open reading frame, is the target of neutralizing antibodies and has been successfully used as a component of both protein subunit and DNA vaccines. L1-specific monoclonal antibodies (e.g., mouse monoclonal antibody mAb-7D11, mAb-10F5) with potent neutralizing activity bind L1 in a conformation-specific manner. This suggests that proper folding of the L1 protein used in molecular vaccines will affect the production of neutralizing antibodies and protection. Here, we co-crystallized the Fab fragment of mAb-7D11 with the L1 protein. The crystal structure of the complex between Fab-7D11 and L1 reveals the basis for the conformation-specific binding as recognition of a discontinuous epitope containing two loops that are held together by a disulfide bond. The structure of this important conformational epitope of L1 will contribute to the development of molecular poxvirus vaccines and also provides a novel target for anti-poxvirus drugs. In addition, the sequence and structure of Fab-7D11 will contribute to the development of L1-targeted immunotherapeutics.