Smallpox DNA vaccine protects nonhuman primates against lethal monkeypox

A case of mistaken identity? Vaccinia virus in a live camelpox vaccine

Live-attenuated (LA), and inactivated adjuvant (IA) camelpox virus (CMLV) vaccines are produced in several countries worldwide. A tissue culture attenuated CMLV isolated (Jouf-78) is used to produce an LA vaccine in Saudi Arabia (Hafez et al., 1992). DNA extracts from the Saudi LA vaccine were used as positive controls for a routine ATIP PCR produced fragments longer than 881 bp. PCR-amplified ATIP sequences were similar to vaccinia virus (VACV) Lister strain. PCR and sequence analysis of two extracellular enveloped virus (EEV)-specific (A33R and B5R), and two intracellular mature virus (IMV) (L1R and A27L) othrologue genes from the vaccine DNA extracts confirmed the finding. CMLV sequences were not detected in vaccine DNA extracts. A VACV Lister strain imported from Switzerland was used in control experiments during initial testing of the Saudi LA vaccine. High antigenic similarity between VACV and CMLV, and a possible contamination event during production may have caused this issue. Environmental and health impact studies were recommended because early VACV vaccines produced in some European countries contained nonhighly attenuated strains that were not adequately screened for adventitious agents.

Epitope mapping by random peptide phage display reveals essential residues for vaccinia extracellular enveloped virion spread

BACKGROUND

A33 is a type II integral membrane protein expressed on the extracellular enveloped form of vaccinia virus (VACV). Passive transfer of A33-directed monoclonal antibodies or vaccination with an A33 subunit vaccine confers protection against lethal poxvirus challenge in animal models. Homologs of A33 are highly conserved among members of the Orthopoxvirus genus and are potential candidates for inclusion in vaccines or assays targeting extracellular enveloped virus activity. One monoclonal antibody directed against VACV A33, MAb-1G10, has been shown to target a conformation-dependent epitope. Interestingly, while it recognizes VACV A33 as well as the corresponding variola homolog, it does not bind to the monkeypox homolog. In this study, we utilized a random phage display library to investigate the epitope recognized by MAb-1G10 that is critical for facilitating cell-to-cell spread of the vaccinia virus.

RESULTS

By screening with linear or conformational random phage libraries, we found that phages binding to MAb-1G10 display the consensus motif CEPLC, with a disulfide bond formed between two cysteine residues required for MAb-1G10 binding. Although the phage motif contained no linear sequences homologous to VACV A33, structure modeling and analysis suggested that residue D115 is important to form the minimal epitope core. A panel of point mutants expressing the ectodomain of A33 protein was generated and analyzed by either binding assays such as ELISA and immunoprecipitation or a functional assessment by blocking MAb-1G10 mediated comet inhibition in cell culture.

CONCLUSIONS

These results confirm L118 as a component of the MAb-1G10 binding epitope, and further identify D115 as an essential residue. By defining the minimum conformational structure, as well as the conformational arrangement of a short peptide sequence recognized by MAb-1G10, these results introduce the possibility of designing small molecule mimetics that may interfere with the function of A33 in vivo. This information will also be useful for designing improved assays to evaluate the potency of monoclonal and polyclonal products that target A33 or A33-modulated EV dissemination.

Side-by-side comparison of gene-based smallpox vaccine with MVA in nonhuman primates

Orthopoxviruses remain a threat as biological weapons and zoonoses. The licensed live-virus vaccine is associated with serious health risks, making its general usage unacceptable. Attenuated vaccines are being developed as alternatives, the most advanced of which is modified-vaccinia virus Ankara (MVA). We previously developed a gene-based vaccine, termed 4pox, which targets four orthopoxvirus antigens, A33, B5, A27 and L1. This vaccine protects mice and non-human primates from lethal orthopoxvirus disease. Here, we investigated the capacity of the molecular adjuvants GM-CSF and Escherichia coli heat-labile enterotoxin (LT) to enhance the efficacy of the 4pox gene-based vaccine. Both adjuvants significantly increased protective antibody responses in mice. We directly compared the 4pox plus LT vaccine against MVA in a monkeypox virus (MPXV) nonhuman primate (NHP) challenge model. NHPs were vaccinated twice with MVA by intramuscular injection or the 4pox/LT vaccine delivered using a disposable gene gun device. As a positive control, one NHP was vaccinated with ACAM2000. NHPs vaccinated with each vaccine developed anti-orthopoxvirus antibody responses, including those against the 4pox antigens. After MPXV intravenous challenge, all control NHPs developed severe disease, while the ACAM2000 vaccinated animal was well protected. All NHPs vaccinated with MVA were protected from lethality, but three of five developed severe disease and all animals shed virus. All five NHPs vaccinated with 4pox/LT survived and only one developed severe disease. None of the 4pox/LT-vaccinated animals shed virus. Our findings show, for the first time, that a subunit orthopoxvirus vaccine delivered by the same schedule can provide a degree of protection at least as high as that of MVA.

Recovery and molecular characterization of live Camelpox virus from skin 12 months after onset of clinical signs reveals possible mechanism of virus persistence in herds

Potentially pathogenic orthopoxviruses (OPVs) persist in nature and re-emerge for reasons we do not fully understand. New information pertaining to Orthopoxvirus (OPV) persistence in nature would significantly improve surveillance and control programs. In a recent investigation of a Camelpox virus (CMLV) outbreak in Eastern Saudi Arabia, atypical minute pox-like skin lesions (AMPL) persisted on 42.9% of convalescent camels (8.8% of herd) for more than a year after the onset of clinical signs. In order to investigate whether AMPL were related to CMLV infection, AMPL homogenates were inoculated on the chorioallantoic membranes (CAM) of specific-pathogen-free (SPF) embryonating chicken eggs (ECE). Live CMLV was recovered from AMPL homogenates. The sequences of the ATIP gene of viruses isolated in the beginning of the outbreak, and one year later from AMPL were identical, and similar to the Kazakhstan isolate CMLV M-96. Virus identity was confirmed by sequence analysis of the CMLV A33R, A27L, B5R, and L1R orthologue genes. Uninfected adult camels that came in contact with animals showing AMPL became infected within two weeks. Since AMPL were easily missed by veterinarians and camel drivers, it was concluded that CMLV survival in persistent skin lesions may be a key mechanism in maintaining the virus in previously infected camel herds during inter-epizootic periods.

The myristate moiety and amino terminus of vaccinia virus l1 constitute a bipartite functional region needed for entry

Vaccinia virus (VACV) L1 is a myristoylated envelope protein which is required for cell entry and the fusion of infected cells. L1 associates with members of the entry-fusion complex (EFC), but its specific role in entry has not been delineated. We recently demonstrated (Foo CH, et al., Virology 385:368-382, 2009) that soluble L1 binds to cells and blocks entry, suggesting that L1 serves as the receptor-binding protein for entry. Our goal is to identify the structural domains of L1 which are essential for its functions in VACV entry. We hypothesized that the myristate and the conserved residues at the N terminus of L1 are critical for entry. To test our hypothesis, we generated mutants in the N terminus of L1 and used a complementation assay to evaluate their ability to rescue infectivity. We also assessed the myristoylation efficiency of the mutants and their ability to interact with the EFC. We found that the N terminus of L1 constitutes a region that is critical for the infectivity of VACV and for myristoylation. At the same time, the nonmyristoylated mutants were incorporated into mature virions, suggesting that the myristate is not required for the association of L1 with the viral membrane. Although some of the mutants exhibited altered structural conformations, two mutants with impaired infectivity were similar in conformation to wild-type L1. Importantly, these two mutants, with changes at A4 and A5, undergo myristoylation. Overall, our results imply dual differential roles for myristate and the amino acids at the N terminus of L1. We propose a myristoyl switch model to describe how L1 functions.

Outbreaks of human monkeypox after cessation of smallpox vaccination

The recent observation of a surge in human monkeypox in the Democratic Republic of the Congo (DRC) prompts the question of whether cessation of smallpox vaccination is driving the phenomenon, and if so, why is re-emergence not universal throughout the historic geographic range of the virus? Research addressing the virus's mechanisms for immune evasion and induction, as well as that directed at elucidating the genes involved in pathogenesis in different viral lineages (West African vs Congo Basin), provide insights to help explain why emergence appears to be geographically limited. Novel vaccines offer one solution to curtail the spread of this disease.

Intrabronchial inoculation of cynomolgus macaques with cowpox virus

The public health threat of orthopoxviruses from bioterrorist attacks has prompted researchers to develop suitable animal models for increasing our understanding of viral pathogenesis and evaluation of medical countermeasures (MCMs) in compliance with the FDA Animal Efficacy Rule. We present an accessible intrabronchial cowpox virus (CPXV) model that can be evaluated under biosafety level-2 laboratory conditions. In this dose-ranging study, utilizing cynomolgus macaques, signs of typical orthopoxvirus disease were observed with the lymphoid organs, liver, skin (generally mild) and respiratory tract as target tissues. Clinical and histopathological evaluation suggests that intrabronchial CPXV recapitulated many of the features of monkeypox and variola virus, the causative agent of smallpox, infections in cynomolgus macaque models. These similarities suggest that CPXV infection in non-human primates should be pursued further as an alternative model of smallpox. Further development of the CPXV primate model, unimpeded by select agent and biocontainment restrictions, should facilitate the development of MCMs for smallpox.

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.

Passive immunotherapies protect WRvFire and IHD-J-Luc vaccinia virus-infected mice from lethality by reducing viral loads in the upper respiratory tract and internal organs

Whole-body bioimaging was employed to study the effects of passive immunotherapies on lethality and viral dissemination in BALB/c mice challenged with recombinant vaccinia viruses expressing luciferase. WRvFire and IHD-J-Luc vaccinia viruses induced lethality with similar times to death following intranasal infection, but WRvFire replicated at higher levels than IHD-J-Luc in the upper and lower respiratory tracts. Three types of therapies were tested: licensed human anti-vaccinia virus immunoglobulin intravenous (VIGIV); recombinant anti-vaccinia virus immunoglobulin (rVIG; Symphogen, Denmark), an investigational product containing a mixture of 26 human monoclonal antibodies (HuMAbs) against mature virion (MV) and enveloped virion (EV); and HuMAb compositions targeting subsets of MV or EV proteins. Bioluminescence recorded daily showed that pretreatment with VIGIV (30 mg) or with rVIG (100 μg) on day -2 protected mice from death but did not prevent viral replication at the site of inoculation and dissemination to internal organs. Compositions containing HuMAbs against MV or EV proteins were protective in both infection models at 100 μg per animal, but at 30 μg, only anti-EV antibodies conferred protection. Importantly, the t statistic of the mean total fluxes revealed that viral loads in surviving mice were significantly reduced in at least 3 sites for 3 consecutive days (days 3 to 5) postchallenge, while significant reduction for 1 or 2 days in any individual site did not confer protection. Our data suggest that reduction of viral replication at multiple sites, including respiratory tract, spleen, and liver, as monitored by whole-body bioluminescence can be used to predict the effectiveness of passive immunotherapies in mouse models.

Cowpox virus infection of cynomolgus macaques as a model of hemorrhagic smallpox

Hemorrhagic smallpox was a rare but severe manifestation of variola virus infection that resulted in nearly 100% mortality. Here we describe intravenous (IV) inoculation of cowpox virus Brighton Red strain in cynomolgus macaques (Macaca fascicularis) which resulted in disease similar in presentation to hemorrhagic smallpox in humans. IV inoculation of macaques resulted in a uniformly lethal disease within 12 days post-inoculation in two independent experiments. Clinical observations and hematological and histopathological findings support hemorrhagic disease. Cowpox virus replicated to high levels in blood (8.0-9.0 log(10) gene copies/mL) and tissues including lymph nodes, thymus, spleen, bone marrow, and lungs. This unique model of hemorrhagic orthopoxvirus infection provides an accessible means to further study orthopoxvirus pathogenesis and to identify virus-specific and nonspecific therapies. Such studies will serve to complement the existing nonhuman primate models of more classical poxviral disease.

The pathology of experimental poxvirus infection in common marmosets (Callithrix jacchus): further characterization of a new primate model for orthopoxvirus infections

Zoonotic orthopoxvirus (OPV) can induce severe disease in man and the virus has potential for use in bioterrorism. New vaccines and therapeutics against OPV infections must be tested in animal models. The aim of this study was to characterize the clinical course and pathology of a new OPV isolate, calpox virus, which is infectious in marmosets. Infection experiments were performed with 28 common marmosets (Callithrix jacchus) exposed to different challenge doses of calpox virus by the intravenous, oropharyngeal and intranasal (IN) routes. The median marmoset IN infectious dose corresponded to 8.3 × 10(2)plaque forming units of calpox virus. Infected animals developed reproducible clinical signs and died within 4-15 days post infection. Characteristic pox-like lesions developed in affected organs, particularly in the skin, mucous membranes, lymph nodes, liver and spleen. Calpox virus disease progression and pathological findings in the common marmoset appear to be consistent with lethal OPV infections in man and in other non-human primate (NHP) models. IN inoculation with low virus doses mimics the natural route of the human variola virus infection. Thus, the marmoset model of calpox virus infection can be considered to be relevant to investigation of the mechanisms of OPV pathogenesis and pathology and for the evaluation of new vaccines and antiviral therapies.

Antibody against extracellular vaccinia virus (EV) protects mice through complement and Fc receptors

Protein-based subunit smallpox vaccines have shown their potential as effective alternatives to live virus vaccines in animal model challenge studies. We vaccinated mice with combinations of three different vaccinia virus (VACV) proteins (A33, B5, L1) and examined how the combined antibody responses to these proteins cooperate to effectively neutralize the extracellular virus (EV) infectious form of VACV. Antibodies against these targets were generated in the presence or absence of CpG adjuvant so that Th1-biased antibody responses could be compared to Th2-biased responses to the proteins with aluminum hydroxide alone, specifically with interest in looking at the ability of anti-B5 and anti-A33 polyclonal antibodies (pAb) to utilize complement-mediated neutralization in vitro. We found that neutralization of EV by anti-A33 or anti-B5 pAb can be enhanced in the presence of complement if Th1-biased antibody (IgG2a) is generated. Mechanistic differences found for complement-mediated neutralization showed that anti-A33 antibodies likely result in virolysis, while anti-B5 antibodies with complement can neutralize by opsonization (coating). In vivo studies found that mice lacking the C3 protein of complement were less protected than wild-type mice after passive transfer of anti-B5 pAb or vaccination with B5. Passive transfer of anti-B5 pAb or monoclonal antibody into mice lacking Fc receptors (FcRs) found that FcRs were also important in mediating protection. These results demonstrate that both complement and FcRs are important effector mechanisms for antibody-mediated protection from VACV challenge in mice.

Establishment of the black-tailed prairie dog (Cynomys ludovicianus) as a novel animal model for comparing smallpox vaccines administered preexposure in both high- and low-dose monkeypox virus challenges

The 2003 monkeypox virus (MPXV) outbreak and subsequent laboratory studies demonstrated that the black-tailed prairie dog is susceptible to MPXV infection and that the ensuing rash illness is similar to human systemic orthopoxvirus (OPXV) infection, including a 7- to 9-day incubation period and, likely, in some cases a respiratory route of infection; these features distinguish this model from others. The need for safe and efficacious vaccines for OPVX in areas where it is endemic or epidemic is important to protect an increasingly OPXV-naïve population. In this study, we tested current and investigational smallpox vaccines for safety, induction of anti-OPXV antibodies, and protection against mortality and morbidity in two MPXV challenges. None of the smallpox vaccines caused illness in this model, and all vaccinated animals showed anti-OPXV antibody responses and neutralizing antibody. We tested vaccine efficacy by challenging the animals with 10(5) or 10(6) PFU Congo Basin MPXV 30 days postvaccination and evaluating morbidity and mortality. Our results demonstrated that vaccination with either Dryvax or Acambis2000 protected the animals from death with no rash illness. Vaccination with IMVAMUNE also protected the animals from death, albeit with (modified) rash illness. Based on the results of this study, we believe prairie dogs offer a novel and potentially useful small animal model for the safety and efficacy testing of smallpox vaccines in pre- and postexposure vaccine testing, which is important for public health planning.

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.

Comparative analysis of monkeypox virus infection of cynomolgus macaques by the intravenous or intrabronchial inoculation route

Monkeypox virus (MPXV) infection has recently expanded in geographic distribution and can be fatal in up to 10% of cases. The intravenous (i.v.) inoculation of nonhuman primates (NHPs) results in an accelerated fulminant disease course compared to that of naturally occurring MPXV infection in humans. Alternative routes of inoculation are being investigated to define an NHP model of infection that more closely resembles natural disease progression. Our goal was to determine if the intrabronchial (i.b.) exposure of NHPs to MPXV results in a systemic disease that better resembles the progression of human MPXV infection. Here, we compared the disease course following an i.v. or i.b. inoculation of NHPs with 10-fold serial doses of MPXV Zaire. Classical pox-like disease was observed in NHPs administered a high virus dose by either route. Several key events were delayed in the highest doses tested of the i.b. model compared to the timing of the i.v. model, including the onset of fever, lesion appearance, peak viremia, viral shedding in nasal and oral swabs, peak cytokine levels, and time to reach endpoint criteria. Virus distribution across 19 tissues was largely unaffected by the inoculation route at the highest doses tested. The NHPs inoculated by the i.b. route developed a viral pneumonia that likely exacerbated disease progression. Based on the observations of the delayed onset of clinical and virological parameters and endpoint criteria that may more closely resemble those of human MPXV infection, the i.b. MPXV model should be considered for the further investigation of viral pathogenesis and countermeasures.

Multivalent smallpox DNA vaccine delivered by intradermal electroporation drives protective immunity in nonhuman primates against lethal monkeypox challenge

The threat of a smallpox-based bioterrorist event or a human monkeypox outbreak has heightened the importance of new, safe vaccine approaches for these pathogens to complement older poxviral vaccine platforms. As poxviruses are large, complex viruses, they present technological challenges for simple recombinant vaccine development where a multicomponent mixtures of vaccine antigens are likely important in protection. We report that a synthetic, multivalent, highly concentrated, DNA vaccine delivered by a minimally invasive, novel skin electroporation microarray can drive polyvalent immunity in macaques, and offers protection from a highly pathogenic monkeypox challenge. Such a diverse, high-titer antibody response produced against 8 different DNA-encoded antigens delivered simultaneously in microvolumes has not been previously described. These studies represent a significant improvement in the efficiency of the DNA vaccine platform, resulting in immune responses that mimic live viral infections, and would likely have relevance for vaccine design against complex human and animal pathogens.

Smallpox vaccines: targets of protective immunity

The eradication of smallpox, one of the great triumphs of medicine, was accomplished through the prophylactic administration of live vaccinia virus, a comparatively benign relative of variola virus, the causative agent of smallpox. Nevertheless, recent fears that variola virus may be used as a biological weapon together with the present susceptibility of unimmunized populations have spurred the development of new-generation vaccines that are safer than the original and can be produced by modern methods. Predicting the efficacy of such vaccines in the absence of human smallpox, however, depends on understanding the correlates of protection. This review outlines the biology of poxviruses with particular relevance to vaccine development, describes protein targets of humoral and cellular immunity, compares animal models of orthopoxvirus disease with human smallpox, and considers the status of second- and third-generation smallpox vaccines.

Capturing the natural diversity of the human antibody response against vaccinia virus

The eradication of smallpox (variola) and the subsequent cessation of routine vaccination have left modern society vulnerable to bioterrorism employing this devastating contagious disease. The existing, licensed vaccines based on live vaccinia virus (VACV) are contraindicated for a substantial number of people, and prophylactic vaccination of large populations is not reasonable when there is little risk of exposure. Consequently, there is an emerging need to develop efficient and safe therapeutics to be used shortly before or after exposure, either alone or in combination with vaccination. We have characterized the human antibody response to smallpox vaccine (VACV Lister) in immunized volunteers and isolated a large number of VACV-specific antibodies that recognize a variety of different VACV antigens. Using this broad antibody panel, we have generated a fully human, recombinant analogue to plasma-derived vaccinia immunoglobulin (VIG), which mirrors the diversity and specificity of the human antibody immune response and offers the advantage of unlimited supply and reproducible specificity and activity. The recombinant VIG was found to display a high specific binding activity toward VACV antigens, potent in vitro VACV neutralizing activity, and a highly protective efficacy against VACV challenge in the mouse tail lesion model when given either prophylactically or therapeutically. Altogether, the results suggest that this compound has the potential to be used as an effective postexposure prophylaxis or treatment of disease caused by orthopoxviruses.

Development of CMX001 for the Treatment of Poxvirus Infections

CMX001 (phosphonic acid, [[(S)-2-(4-amino-2-oxo-1(2H)-pyrimidinyl)-1-(hydroxymethyl)ethoxy]methyl]mono[3-(hexadecyloxy)propyl] ester) is a lipid conjugate of the acyclic nucleotide phosphonate, cidofovir (CDV). CMX001 is currently in Phase II clinical trials for the prophylaxis of human cytomegalovirus infection and under development using the Animal Rule for smallpox infection. It has proven effective in reduction of morbidity and mortality in animal models of human smallpox, even after the onset of lesions and other clinical signs of disease. CMX001 and CDV are active against all five families of double-stranded DNA (dsDNA) viruses that cause human morbidity and mortality, including orthopoxviruses such as variola virus, the cause of human smallpox. However, the clinical utility of CDV is limited by the requirement for intravenous dosing and a high incidence of acute kidney toxicity. The risk of nephrotoxicity necessitates pre-hydration and probenecid administration in a health care facility, further complicating high volume CDV use in an emergency situation. Compared with CDV, CMX001 has a number of advantages for treatment of smallpox in an emergency including greater potency in vitro against all dsDNA viruses that cause human disease, a high genetic barrier to resistance, convenient oral administration as a tablet or liquid, and no evidence to date of nephrotoxicity in either animals or humans. The apparent lack of nephrotoxicity observed with CMX001 in vivo is because it is not a substrate for the human organic anion transporters that actively secrete CDV into kidney cells. The ability to test the safety and efficacy of CMX001 in patients with life-threatening dsDNA virus infections which share many basic traits with variola is a major advantage in the development of this antiviral for a smallpox indication.

Development of ST-246® for Treatment of Poxvirus Infections

ST-246 (Tecovirimat) is a small synthetic antiviral compound being developed to treat pathogenic orthopoxvirus infections of humans. The compound was discovered as part of a high throughput screen designed to identify inhibitors of vaccinia virus-induced cytopathic effects. The antiviral activity is specific for orthopoxviruses and the compound does not inhibit the replication of other RNA- and DNA-containing viruses or inhibit cell proliferation at concentrations of compound that are antiviral. ST-246 targets vaccinia virus p37, a viral protein required for envelopment and secretion of extracellular forms of virus. The compound is orally bioavailable and protects multiple animal species from lethal orthopoxvirus challenge. Preclinical safety pharmacology studies in mice and non-human primates indicate that ST-246 is readily absorbed by the oral route and well tolerated with the no observable adverse effect level (NOAEL) in mice measured at 2000 mg/kg and the no observable effect level (NOEL) in non-human primates measured at 300 mg/kg. Drug substance and drug product processes have been developed and commercial scale batches have been produced using Good Manufacturing Processes (GMP). Human phase I clinical trials have shown that ST-246 is safe and well tolerated in healthy human volunteers. Based on the results of the clinical evaluation, once a day dosing should provide plasma drug exposure in the range predicted to be antiviral based on data from efficacy studies in animal models of orthopoxvirus disease. These data support the use of ST-246 as a therapeutic to treat pathogenic orthopoxvirus infections of humans.

The inability of vaccinia virus A33R protein to form intermolecular disulfide-bonded homodimers does not affect the production of infectious extracellular virus

The orthopoxvirus protein A33 forms a disulfide-bonded high molecular weight species that could be either a homodimer or a heteromultimer. The protein is a major target for neutralizing antibodies and the majority of antibodies raised against A33 only recognize the disulfide-bonded form. Here, we report that A33 is present as a disulfide-bonded homodimer during infection. Additionally, we examined the function of intermolecular disulfide bonding in A33 homodimerization during infection. We show that the cysteine at amino acid 62 is required for intermolecular disulfide bonding, but not dimerization as this mutant was still able to form homodimers. To investigate the role of disulfide-bonded homodimers during viral morphogenesis, recombinant viruses that express an A33R with cysteine 62 mutated to serine were generated. The recombinant viruses had growth characteristics similar to their parental viruses, indicating that intermolecular disulfide-bonded homodimerization of A33 is not required for its function.

Cytosolic disulfide bond formation in cells infected with large nucleocytoplasmic DNA viruses

Proteins that have evolved to contain stabilizing disulfide bonds generally fold in a membrane-delimited compartment in the cell [i.e., the endoplasmic reticulum (ER) or the mitochondrial intermembrane space (IMS)]. These compartments contain sulfhydryl oxidase enzymes that catalyze the pairing and oxidation of cysteine residues. In contrast, most proteins in a healthy cytosol are maintained in reduced form through surveillance by NADPH-dependent reductases and the lack of sulfhydryl oxidases. Nevertheless, one of the core functionalities that unify the broad and diverse set of nucleocytoplasmic large DNA viruses (NCLDVs) is the ability to catalyze disulfide formation in the cytosol. The substrates of this activity are proteins that contribute to the assembly, structure, and infectivity of the virions. If the last common ancestor of NCLDVs was present during eukaryogenesis as has been proposed, it is interesting to speculate that viral disulfide bond formation pathways may have predated oxidative protein folding in intracellular organelles.

Single-shot immunization with recombinant adenovirus encoding vaccinia virus glycoprotein A27L is protective against a virulent respiratory poxvirus infection

Significant safety issues have emerged concerning the general use of DRYVAX vaccine. Vaccination with replication-defective recombinant adenovirus (rAd) vaccines may offer a safer and effective alternative to live vaccinia virus (VV) vaccination. Six individual poxvirus glycoproteins: A33R, A34R, A36R, B5R, A27L or L1R that are normally expressed on the surface of infectious vaccinia virus were encoded in rAd vaccines and tested in mice in this study. A single-shot intramuscular injection of rAd encoding A27L protected mice against a lethal intranasal challenge with VV at 4 weeks post-vaccination. By 10 weeks post-vaccination, a significant decrease in post-challenge morbidity was observed that correlated with potent neutralizing antibody responses and the emergence of specific polyfunctional T cell responses. The immunogenicity and protective efficacy of rAd-A27L immunization persisted for at least 35 weeks post-vaccination. This study is the first demonstration that a single-shot subunit vaccine encoding a poxvirus protein confers protection against the mortality and morbidity associated with poxvirus infection.

Immunogenicity and protective efficacy of a DNA vaccine against Venezuelan equine encephalitis virus aerosol challenge in nonhuman primates

A study to evaluate the immunogenicity and protective efficacy of a Venezuelan equine encephalitis virus (VEEV) DNA vaccine in an aerosol model of nonhuman primate infection was performed. Cynomolgus macaques vaccinated with a plasmid expressing the 26S structural genes of VEEV subtype IAB by particle-mediated epidermal delivery (PMED) developed virus-neutralizing antibodies. No serum viremia was detected in two out of three macaques vaccinated with the VEEV DNA after aerosol challenge with homologous virus, while one displayed a low viremia on a single day postchallenge. In contrast, all three macaques vaccinated with empty vector DNA developed a high viremia that persisted for at least 3 days after challenge. In addition, macaques vaccinated with the VEEV DNA had reduced febrile reactions, lymphopenia, and clinical signs of disease postchallenge as compared to negative control macaques. Therefore, although the sample size was small in this pilot study, these results indicate that a VEEV DNA vaccine administered by PMED can at least partially protect nonhuman primates against an aerosol VEEV challenge.

Evaluating the orthopoxvirus type I interferon-binding molecule as a vaccine target in the vaccinia virus intranasal murine challenge model

The biological threat imposed by orthopoxviruses warrants the development of safe and effective vaccines. We developed a candidate orthopoxvirus DNA-based vaccine, termed 4pox, which targets four viral structural components, A33, B5, A27, and L1. While this vaccine protects mice and nonhuman primates from lethal infections, we are interested in further enhancing its potency. One approach to enhance potency is to include additional orthopoxvirus immunogens. Here, we investigated whether vaccination with the vaccinia virus (VACV) interferon (IFN)-binding molecule (IBM) could protect BALB/c mice against lethal VACV challenge. We found that vaccination with this molecule failed to significantly protect mice from VACV when delivered alone. IBM modestly augmented protection when delivered together with the 4pox vaccine. All animals receiving the 4pox vaccine plus IBM lived, whereas only 70% of those receiving a single dose of 4pox vaccine survived. Mapping studies using truncated mutants revealed that vaccine-generated antibodies spanned the immunoglobulin superfamily domains 1 and 2 and, to a lesser extent, 3 of the IBM. These antibodies inhibited IBM cell binding and IFN neutralization activity, indicating that they were functionally active. This study shows that DNA vaccination with the VACV IBM results in a robust immune response but that this response does not significantly enhance protection in a high-dose challenge model.

Ectromelia virus infections of mice as a model to support the licensure of anti-orthopoxvirus therapeutics

The absence of herd immunity to orthopoxviruses and the concern that variola or monkeypox viruses could be used for bioterroristic activities has stimulated the development of therapeutics and safer prophylactics. One major limitation in this process is the lack of accessible human orthopoxvirus infections for clinical efficacy trials; however, drug licensure can be based on orthopoxvirus animal challenge models as described in the "Animal Efficacy Rule". One such challenge model uses ectromelia virus, an orthopoxvirus, whose natural host is the mouse and is the etiological agent of mousepox. The genetic similarity of ectromelia virus to variola and monkeypox viruses, the common features of the resulting disease, and the convenience of the mouse as a laboratory animal underscores its utility in the study of orthopoxvirus pathogenesis and in the development of therapeutics and prophylactics. In this review we outline how mousepox has been used as a model for smallpox. We also discuss mousepox in the context of mouse strain, route of infection, infectious dose, disease progression, and recovery from infection.

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.

A recombinant flagellin-poxvirus fusion protein vaccine elicits complement-dependent protection against respiratory challenge with vaccinia virus in mice

Bacterial flagellin is a potent adjuvant that enhances adaptive immune responses to a variety of protein antigens. The vaccinia virus antigens L1R and B5R are highly immunogenic in the context of the parent virus, but recombinant forms of the proteins are only weakly immunogenic. Therefore we evaluated the humoral response to these antigens in mice when flagellin was used as an adjuvant. Flagellin-L1R and flagellin-B5R fusion proteins were more potent than flagellin, L1R, and B5R as separate proteins. At least three immunizations with flagellin-L1R and flagellin-B5R fusion proteins were required to confer protection in mice against challenge with vaccinia virus. Immune mice exhibited only limited signs of disease following challenge. Additionally, virus neutralization titers correlated with protection. Depletion of complement using cobra venom factor resulted in a marked decrease in the survival of immunized mice after challenge with vaccinia virus. Our results are consistent with the conclusion that flagellin-L1R and flagellin-B5R fusion proteins are effective in eliciting protective immunity against vaccinia virus that is dependent, in large part, on complement.

Animal models of orthopoxvirus infection

Smallpox was one of the most devastating diseases known to humanity. Although smallpox was eradicated through a historically successful vaccination campaign, there is concern in the global community that either Variola virus (VARV), the causative agent of smallpox, or another species of Orthopoxvirus could be used as agents of bioterrorism. Therefore, development of countermeasures to Orthopoxvirus infection is a crucial focus in biodefense research, and these efforts rely on the use of various animal models. Smallpox typically presented as a generalized pustular rash with 30 to 40% mortality, and although smallpox-like syndromes can be induced in cynomolgus macaques with VARV, research with this virus is highly restricted; therefore, animal models with other orthopoxviruses have been investigated. Monkeypox virus causes a generalized vesiculopustular rash in rhesus and cynomolgus macaques and induces fatal systemic disease in several rodent species. Ectromelia virus has been extensively studied in mice as a model of orthopoxviral infection in its natural host. Intranasal inoculation of mice with some strains of vaccinia virus produces fatal bronchopneumonia, as does aerosol or intranasal inoculation of mice with cowpox virus. Rabbitpox virus causes pneumonia and fatal systemic infections in rabbits and can be naturally transmitted between rabbits by an aerosol route similar to that of VARV in humans. No single animal model recapitulates all known aspects of human Orthopoxvirus infections, and each model has its advantages and disadvantages. This article provides a brief review of the Orthopoxvirus diseases of humans and the key pathologic features of animal models of Orthopoxvirus infections.

A protein-based smallpox vaccine protects non-human primates from a lethal monkeypox virus challenge

Concerns about infections caused by orthopoxviruses, such as variola and monkeypox viruses, drive ongoing efforts to develop novel smallpox vaccines that are both effective and safe to use in diverse populations. A subunit smallpox vaccine comprising vaccinia virus membrane proteins A33, B5, L1, A27 and aluminum hydroxide (alum) ± CpG was administered to non-human primates, which were subsequently challenged with a lethal intravenous dose of monkeypox virus. Alum adjuvanted vaccines provided only partial protection but the addition of CpG provided full protection that was associated with a more homogeneous antibody response and stronger IgG1 responses. These results indicate that it is feasible to develop a highly effective subunit vaccine against orthopoxvirus infections as a safer alternative to live vaccinia virus vaccination.

A novel highly reproducible and lethal nonhuman primate model for orthopox virus infection

The intentional re-introduction of Variola virus (VARV), the agent of smallpox, into the human population is of great concern due its bio-terroristic potential. Moreover, zoonotic infections with Cowpox (CPXV) and Monkeypox virus (MPXV) cause severe diseases in humans. Smallpox vaccines presently available can have severe adverse effects that are no longer acceptable. The efficacy and safety of new vaccines and antiviral drugs for use in humans can only be demonstrated in animal models. The existing nonhuman primate models, using VARV and MPXV, need very high viral doses that have to be applied intravenously or intratracheally to induce a lethal infection in macaques. To overcome these drawbacks, the infectivity and pathogenicity of a particular CPXV was evaluated in the common marmoset (Callithrix jacchus).A CPXV named calpox virus was isolated from a lethal orthopox virus (OPV) outbreak in New World monkeys. We demonstrated that marmosets infected with calpox virus, not only via the intravenous but also the intranasal route, reproducibly develop symptoms resembling smallpox in humans. Infected animals died within 1-3 days after onset of symptoms, even when very low infectious viral doses of 5x10(2) pfu were applied intranasally. Infectious virus was demonstrated in blood, saliva and all organs analyzed.We present the first characterization of a new OPV infection model inducing a disease in common marmosets comparable to smallpox in humans. Intranasal virus inoculation mimicking the natural route of smallpox infection led to reproducible infection. In vivo titration resulted in an MID(50) (minimal monkey infectious dose 50%) of 8.3x10(2) pfu of calpox virus which is approximately 10,000-fold lower than MPXV and VARV doses applied in the macaque models. Therefore, the calpox virus/marmoset model is a suitable nonhuman primate model for the validation of vaccines and antiviral drugs. Furthermore, this model can help study mechanisms of OPV pathogenesis.

Safety and immunogenicity of modified vaccinia Ankara (ACAM3000): effect of dose and route of administration

BACKGROUND

We conducted a clinical trial of the safety and immunogenicity of modified vaccinia Ankara (MVA) to examine the effects of dose and route of administration.

METHODS

Seventy-two healthy, vaccinia virus-naive subjects received 1 of 6 regimens of MVA (ACAM3000) or placebo consisting of 2 administrations given 1 month apart.

RESULTS

MVA was generally well tolerated at all dose levels and by all routes. More pronounced local reactogenicity was seen with the intradermal and subcutaneous routes than with intramuscular administration. Binding antibodies to whole virus and neutralizing antibodies to the intracellular mature virion and extracellular enveloped virion forms of vaccinia virus were elicited by all routes of MVA administration and were greater for the higher dose by each route. Similar levels of neutralizing antibodies were seen at a 10-fold-lower dose given intradermally (1 x 10(7) median tissue culture infective doses [TCID(50)]), compared with responses after 1 x 10(8) TCID(50) given intramuscularly or subcutaneously. T cell immune responses to vaccinia virus were detected by an interferon gamma enzyme-linked immunospot assay but had no clear relationship to dose or route.

CONCLUSIONS

These data suggest that intradermal immunization with MVA provides a dose-sparing effect by eliciting antibody responses similar in magnitude and kinetics to those elicited by the intramuscular or subcutaneous routes but at a 10-fold-lower dose.

Effect of vaccination with modified vaccinia Ankara (ACAM3000) on subsequent challenge with Dryvax

BACKGROUND

Despite the success of smallpox vaccination, the immunological correlates of protection are not fully understood. To investigate this question, we examined the effect of immunization with modified vaccinia Ankara (MVA) on subsequent challenge with replication-competent vaccinia virus (Dryvax).

METHODS

Dryvax challenge by scarification was conducted in 36 healthy subjects who had received MVA (n = 29) or placebo (n = 7) in a previous study of doses and routes of immunization. Subjects were followed up for clinical take, viral shedding, and immune responses.

RESULTS

MVA administration attenuated clinical takes in 21 (72%) of 29 subjects, compared with 0 of 7 placebo recipients (P = .001). Attenuation was most significant in MVA groups that received 1 x 10(7) median tissue culture infective doses (TCID(50)) intradermally (P = .001) and 1 x 10(7) TCID(50) intramuscularly (P = .001). Both duration and peak titer of viral shedding were reduced in MVA recipients. Peak neutralizing antibody responses to vaccinia virus or MVA previously induced by MVA immunization were associated with attenuated takes (P = .02) and reduced duration (P = .001) and titer (P = .005) of viral shedding.

CONCLUSIONS

MVA immunization results in clinical and virologic protection against Dryvax challenge. Protection is associated with prior induction of neutralizing antibodies to MVA or vaccinia virus. MVA administered intradermally has protective and immunologic responses similar to those of a 10-fold-higher dose given subcutaneously.

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.

Recombinant A27 protein synergizes with modified vaccinia Ankara in conferring protection against a lethal vaccinia virus challenge

Highly attenuated modified vaccinia virus Ankara (MVA) is being considered as a safer alternative to conventional smallpox vaccines such as Dryvax or ACAM 2000, but it requires higher doses or more-frequent boosting than replication-competent Dryvax. Previously, we found that passive transfer of A27 antibodies can enhance protection afforded by vaccinia immune globulin (VIG), which is derived from Dryvax immunized subjects. Here we investigated whether protective immunity elicited by MVA could be augmented by prime-boost or combination immunizations with a recombinant A27 (rA27) protein. We found that a prime/boost immunization regimen with rA27 protein and MVA, in either sequence order, conferred protection to mice challenged with a lethal dose of vaccinia virus strain Western Reserve (VV-WR), compared to no protection after immunizations with a similar dose of either MVA or rA27 alone. Moreover, protection was achieved in mice primed simultaneously with combination of both MVA and rA27 in different vaccination routes, without any boost, even though MVA or rA27 alone at the same dose gave no protection. These findings show that rA27 can synergize with MVA to elicit robust protection that has a dose-sparing effect on MVA and can accelerate protection by eliminating the need for a booster dose.

The structure of the poxvirus A33 protein reveals a dimer of unique C-type lectin-like domains

The current vaccine against smallpox is an infectious form of vaccinia virus that has significant side effects. Alternative vaccine approaches using recombinant viral proteins are being developed. A target of subunit vaccine strategies is the poxvirus protein A33, a conserved protein in the Chordopoxvirinae subfamily of Poxviridae that is expressed on the outer viral envelope. Here we have determined the structure of the A33 ectodomain of vaccinia virus. The structure revealed C-type lectin-like domains (CTLDs) that occur as dimers in A33 crystals with five different crystal lattices. Comparison of the A33 dimer models shows that the A33 monomers have a degree of flexibility in position within the dimer. Structural comparisons show that the A33 monomer is a close match to the Link module class of CTLDs but that the A33 dimer is most similar to the natural killer (NK)-cell receptor class of CTLDs. Structural data on Link modules and NK-cell receptor-ligand complexes suggest a surface of A33 that could interact with viral or host ligands. The dimer interface is well conserved in all known A33 sequences, indicating an important role for the A33 dimer. The structure indicates how previously described A33 mutations disrupt protein folding and locates the positions of N-linked glycosylations and the epitope of a protective antibody.

Heavily isotype-dependent protective activities of human antibodies against vaccinia virus extracellular virion antigen B5

Antibodies against the extracellular virion (EV or EEV) form of vaccinia virus are an important component of protective immunity in animal models and likely contribute to the protection of immunized humans against poxviruses. Using fully human monoclonal antibodies (MAbs), we now have shown that the protective attributes of the human anti-B5 antibody response to the smallpox vaccine (vaccinia virus) are heavily dependent on effector functions. By switching Fc domains of a single MAb, we have definitively shown that neutralization in vitro--and protection in vivo in a mouse model--by the human anti-B5 immunoglobulin G MAbs is isotype dependent, thereby demonstrating that efficient protection by these antibodies is not simply dependent on binding an appropriate vaccinia virion antigen with high affinity but in fact requires antibody effector function. The complement components C3 and C1q, but not C5, were required for neutralization. We also have demonstrated that human MAbs against B5 can potently direct complement-dependent cytotoxicity of vaccinia virus-infected cells. Each of these results was then extended to the polyclonal human antibody response to the smallpox vaccine. A model is proposed to explain the mechanism of EV neutralization. Altogether these findings enhance our understanding of the central protective activities of smallpox vaccine-elicited antibodies in immunized humans.

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.

Expansion, reexpansion, and recall-like expansion of Vgamma2Vdelta2 T cells in smallpox vaccination and monkeypox virus infection

Little is known about the in vivo kinetics of T-cell responses in smallpox/monkeypox. We showed that macaque Vgamma2Vdelta2 T cells underwent 3-week-long expansion after smallpox vaccine immunization and displayed simple reexpansion in association with sterile anti-monkeypox virus (anti-MPV) immunity after MPV challenge. Virus-activated Vgamma2Vdelta2 T cells exhibited gamma interferon-producing effector function after phosphoantigen stimulation. Surprisingly, like alphabeta T cells, suboptimally primed Vgamma2Vdelta2 T cells in vaccinia virus/cidofovir-covaccinated macaques mounted major recall-like expansion after MPV challenge. Finally, Vgamma2Vdelta2 T cells localized in inflamed lung tissues for potential regulation. Our studies provide the first in vivo evidence that viruses, despite their inability to produce exogenous phosphoantigen, can induce expansion, reexpansion, and recall-like expansion of Vgamma2Vdelta2 T cells and stimulate their antimicrobial cytokine response.

Molecular smallpox vaccine delivered by alphavirus replicons elicits protective immunity in mice and non-human primates

Naturally occurring smallpox was eradicated as a result of successful vaccination campaigns during the 1960s and 1970s. Because of its highly contagious nature and high mortality rate, smallpox has significant potential as a biological weapon. Unfortunately, the current vaccine for orthopoxviruses is contraindicated for large portions of the population. Thus, there is a need for new, safe, and effective orthopoxvirus vaccines. Alphavirus replicon vectors, derived from strains of Venezuelan equine encephalitis virus, are being used to develop alternatives to the current smallpox vaccine. Here, we demonstrated that virus-like replicon particles (VRPs) expressing the vaccinia virus A33R, B5R, A27L, and L1R genes elicited protective immunity in mice comparable to vaccination with live-vaccinia virus. Furthermore, cynomolgus macaques vaccinated with a combination of the four poxvirus VRPs (4pox-VRP) developed antibody responses to each antigen. These antibody responses were able to neutralize and inhibit the spread of both vaccinia virus and monkeypox virus. Macaques vaccinated with 4pox-VRP, flu HA VRP (negative control), or live-vaccinia virus (positive control) were challenged intravenously with 5 x 10(6)pfu of monkeypox virus 1 month after the second VRP vaccination. Four of the six negative control animals succumbed to monkeypox and the remaining two animals demonstrated either severe or grave disease. Importantly, all 10 macaques vaccinated with the 4pox-VRP vaccine survived without developing severe disease. These findings revealed that a single-boost VRP smallpox vaccine shows promise as a safe alternative to the currently licensed live-vaccinia virus smallpox vaccine.

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.

Diverse recognition of conserved orthopoxvirus CD8+ T cell epitopes in vaccinated rhesus macaques

Vaccinia virus (VACV) induces a vigorous virus-specific CD8+ T cell response that plays an important role in control of poxvirus infection. To identify immunodominant poxvirus proteins and to facilitate future testing of smallpox vaccines in non-human primates, we used an algorithm for the prediction of VACV peptides able to bind to the common macaque MHC class I molecule Mamu-A*01. We synthesized 294 peptides derived from 97 VACV ORFs; 100 of these peptides did not contain the canonical proline at position three of the Mamu-A*01 binding motif. Cellular immune responses in PBMC from two vaccinia-vaccinated Mamu-A*01+ macaques were assessed by IFNgamma ELISPOT assays. Vaccinated macaques recognized 17 peptides from 16 different ORFs with 6 peptides recognized by both macaques. Comparison with other orthopoxvirus sequences revealed that 12 of these epitopes are strictly conserved between VACV, variola, and monkeypoxvirus. ELISPOT responses were also observed to eight epitopes that did not contain the canonical P3 proline. These results suggest that the virus-specific CD8+ T cell response is broadly directed against multiple VACV proteins and that a subset of these T cell epitopes is highly conserved among orthopoxviruses.

Application of bioluminescence imaging to the prediction of lethality in vaccinia virus-infected mice

To find an alternative endpoint for the efficacy of antismallpox treatments, bioluminescence was measured in live BALB/c mice following lethal challenge with a recombinant WR vaccinia virus expressing luciferase. Intravenous vaccinia immunoglobulin treatments were used to confer protection on a proportion of animals. Using known lethality outcomes in 200 animals and total fluxes recorded daily in live animals, we performed univariate receiver operating characteristic (ROC) curve analysis to assess whether lethality can be predicted based on bioluminescence. Total fluxes in the spleens on day 3 and in the livers on day 5 generated accurate predictive models; the area under the ROC curve (AUC) was 0.91. Multiple logistic regression analysis utilizing a linear combination of six measurements: total flux in the liver on days 2, 3, and 5; in the spleen on days 1 and 3; and in the nasal cavity on day 4 generated the most accurate predictions (AUC = 0.96). This model predicted lethality in 90% of animals with only 10% of nonsurviving animals incorrectly predicted to survive. Compared with bioluminescence, ROC analysis with 25% and 30% weight loss as thresholds accurately predicted survival on day 5, but lethality predictions were low until day 9. Collectively, our data support the use of bioimaging for lethality prediction following vaccinia virus challenge and for gaining insight into protective mechanisms conferred by vaccines and therapeutics.

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.

Statistical approach to estimate vaccinia-specific neutralizing antibody titers using a high-throughput assay

Because of the bioterrorism threat posed by agents such as variola virus, considerable time, resources, and effort have been devoted to biodefense preparation. One avenue of this research has been the development of rapid, sensitive, high-throughput assays to validate immune responses to poxviruses. Here we describe the adaptation of a beta-galactosidase reporter-based vaccinia virus neutralization assay to large-scale use in a study that included over 1,000 subjects. We also describe the statistical methods involved in analyzing the large quantity of data generated. The assay and its associated methods should prove useful tools in monitoring immune responses to next-generation smallpox vaccines, studying poxvirus immunity, and evaluating therapeutic agents such as vaccinia virus immune globulin.

Comparative evaluation of the immune responses and protection engendered by LC16m8 and Dryvax smallpox vaccines in a mouse model

The immune response elicited by LC16m8, a candidate smallpox vaccine that was developed in Japan by cold selection during serial passage of the Lister vaccine virus in primary rabbit kidney cells, was compared to Dryvax in a mouse model. LC16m8 carries a mutation resulting in the truncation of the B5 protein, an important neutralizing target of the extracellular envelope form of vaccinia virus (EV). LC16m8 elicited a broad-spectrum immunoglobulin G (IgG) response that neutralized both EV and the intracellular mature form of vaccinia virus and provoked cell-mediated immune responses, including the activation of CD4+ and CD8+ cells, similarly to Dryvax. Mice inoculated with LC16m8 had detectable but low levels of anti-B5 IgG compared to Dryvax, but both Dryvax and LC16m8 sera neutralized vaccinia virus EV in vitro. A truncated B5 protein (approximately 8 kDa) was expressed abundantly in LC16m8-infected cells, and both murine immune sera and human vaccinia virus immunoglobulin recognized the truncated recombinant B5 protein in antigen-specific enzyme-linked immunosorbent assays. At a high-dose intranasal challenge (100 or 250 50% lethal doses), LC16m8 and Dryvax conferred similar levels of protection against vaccinia virus strain WR postvaccination. Taken together, the results extend our current understanding of the protective immune responses elicited by LC16m8 and indicate that the relative efficacy in a mouse model rivals that of previously licensed smallpox vaccines.