General method for production and selection of infectious vaccinia virus recombinants expressing foreign genes

Transgene expression knock-down in recombinant Modified Vaccinia virus Ankara vectors improves genetic stability and sustained transgene maintenance across multiple passages

Modified vaccinia virus Ankara is a versatile vaccine vector, well suited for transgene delivery, with an excellent safety profile. However, certain transgenes render recombinant MVA (rMVA) genetically unstable, leading to the accumulation of mutated rMVA with impaired transgene expression. This represents a major challenge for upscaling and manufacturing of rMVA vaccines. To prevent transgene-mediated negative selection, the continuous avian cell line AGE1.CR pIX (CR pIX) was modified to suppress transgene expression during rMVA generation and amplification. This was achieved by constitutively expressing a tetracycline repressor (TetR) together with a rat-derived shRNA in engineered CR pIX PRO suppressor cells targeting an operator element (tetO) and 3' untranslated sequence motif on a chimeric poxviral promoter and the transgene mRNA, respectively. This cell line was instrumental in generating two rMVA (isolate CR19) expressing a Macaca fascicularis papillomavirus type 3 (MfPV3) E1E2E6E7 artificially-fused polyprotein following recombination-mediated integration of the coding sequences into the DelIII (CR19 M-DelIII) or TK locus (CR19 M-TK), respectively. Characterization of rMVA on parental CR pIX or engineered CR pIX PRO suppressor cells revealed enhanced replication kinetics, higher virus titers and a focus morphology equaling wild-type MVA, when transgene expression was suppressed. Serially passaging both rMVA ten times on parental CR pIX cells and tracking E1E2E6E7 expression by flow cytometry revealed a rapid loss of transgene product after only few passages. PCR analysis and next-generation sequencing demonstrated that rMVA accumulated mutations within the E1E2E6E7 open reading frame (CR19 M-TK) or deletions of the whole transgene cassette (CR19 M-DelIII). In contrast, CR pIX PRO suppressor cells preserved robust transgene expression for up to 10 passages, however, rMVAs were more stable when E1E2E6E7 was integrated into the TK as compared to the DelIII locus. In conclusion, sustained knock-down of transgene expression in CR pIX PRO suppressor cells facilitates the generation, propagation and large-scale manufacturing of rMVA with transgenes hampering viral replication.

Vaccines as treatments for prostate cancer

Prostate cancer is a leading cause of death in men worldwide. For over 30 years, growing interest has focused on the development of vaccines as treatments for prostate cancer, with the goal of using vaccines to activate immune cells capable of targeting prostate cancer to either eradicate recurrent disease or at least delay disease progression. This interest has been prompted by the prevalence and long natural history of the disease and by the fact that the prostate is an expendable organ. Thus, an immune response elicited by vaccination might not need to target the tumour uniquely but could theoretically target any prostate tissue. To date, different vaccine approaches and targets for prostate cancer have been evaluated in clinical trials. Overall, five approaches have been assessed in randomized phase III trials and sipuleucel-T was approved as a treatment for metastatic castration-resistant prostate cancer, being the only vaccine approved to date by the FDA as a treatment for cancer. Most vaccine approaches showed safety and some evidence of immunological activity but had poor clinical activity when used as monotherapies. However, increased activity has been observed when these vaccines were used in combination with other immune-modulating therapies. This evidence suggests that, in the future, prostate cancer vaccines might be used to activate and expand tumour-specific T cells as part of combination approaches with agents that target tumour-associated immune mechanisms of resistance.

Evaluation of T cell responses to naturally processed variant SARS-CoV-2 spike antigens in individuals following infection or vaccination

Most existing studies characterizing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific T cell responses are peptide based. This does not allow evaluation of whether tested peptides are processed and presented canonically. In this study, we use recombinant vaccinia virus (rVACV)-mediated expression of SARS-CoV-2 spike protein and SARS-CoV-2 infection of angiotensin-converting enzyme (ACE)-2-transduced B cell lines to evaluate overall T cell responses in a small cohort of recovered COVID-19 patients and uninfected donors vaccinated with ChAdOx1 nCoV-19. We show that rVACV expression of SARS-CoV-2 antigen can be used as an alternative to SARS-CoV-2 infection to evaluate T cell responses to naturally processed spike antigens. In addition, the rVACV system can be used to evaluate the cross-reactivity of memory T cells to variants of concern (VOCs) and to identify epitope escape mutants. Finally, our data show that both natural infection and vaccination could induce multi-functional T cell responses with overall T cell responses remaining despite the identification of escape mutations.

An immunodominant NP105-113-B*07:02 cytotoxic T cell response controls viral replication and is associated with less severe COVID-19 disease

NP105-113-B*07:02-specific CD8+ T cell responses are considered among the most dominant in SARS-CoV-2-infected individuals. We found strong association of this response with mild disease. Analysis of NP105-113-B*07:02-specific T cell clones and single-cell sequencing were performed concurrently, with functional avidity and antiviral efficacy assessed using an in vitro SARS-CoV-2 infection system, and were correlated with T cell receptor usage, transcriptome signature and disease severity (acute n = 77, convalescent n = 52). We demonstrated a beneficial association of NP105-113-B*07:02-specific T cells in COVID-19 disease progression, linked with expansion of T cell precursors, high functional avidity and antiviral effector function. Broad immune memory pools were narrowed postinfection but NP105-113-B*07:02-specific T cells were maintained 6 months after infection with preserved antiviral efficacy to the SARS-CoV-2 Victoria strain, as well as Alpha, Beta, Gamma and Delta variants. Our data show that NP105-113-B*07:02-specific T cell responses associate with mild disease and high antiviral efficacy, pointing to inclusion for future vaccine design.

Pursuit of chlorovirus genetic transformation and CRISPR/Cas9-mediated gene editing

Genetic and molecular modifications of the large dsDNA chloroviruses, with genomes of 290 to 370 kb, would expedite studies to elucidate the functions of both identified and unidentified virus-encoded proteins. These plaque-forming viruses replicate in certain unicellular, eukaryotic chlorella-like green algae. However, to date, only a few of these algal species and virtually none of their viruses have been genetically manipulated due to lack of practical methods for genetic transformation and genome editing. Attempts at using Agrobacterium-mediated transfection of chlorovirus host Chlorella variabilis NC64A with a specially-designed binary vector resulted in successful transgenic cell selection based on expression of a hygromycin-resistance gene, initial expression of a green fluorescence gene and demonstration of integration of Agrobacterium T-DNA. However, expression of the integrated genes was soon lost. To develop gene editing tools for modifying specific chlorovirus CA-4B genes using preassembled Cas9 protein-sgRNA ribonucleoproteins (RNPs), we tested multiple methods for delivery of Cas9/sgRNA RNP complexes into infected cells including cell wall-degrading enzymes, electroporation, silicon carbide (SiC) whiskers, and cell-penetrating peptides (CPPs). In one experiment two independent virus mutants were isolated from macerozyme-treated NC64A cells incubated with Cas9/sgRNA RNPs targeting virus CA-4B-encoded gene 034r, which encodes a glycosyltransferase. Analysis of DNA sequences from the two mutant viruses showed highly targeted nucleotide sequence modifications in the 034r gene of each virus that were fully consistent with Cas9/RNP-directed gene editing. However, in ten subsequent experiments, we were unable to duplicate these results and therefore unable to achieve a reliable system to genetically edit chloroviruses. Nonetheless, these observations provide strong initial suggestions that Cas9/RNPs may function to promote editing of the chlorovirus genome, and that further experimentation is warranted and worthwhile.

Vaccinia Virus Expressing Interferon Regulatory Factor 3 Induces Higher Protective Immune Responses against Lethal Poxvirus Challenge in Atopic Organism

Vaccinia virus (VACV) is an enveloped DNA virus from the Orthopoxvirus family, various strains of which were used in the successful eradication campaign against smallpox. Both original and newer VACV-based replicating vaccines reveal a risk of serious complications in atopic individuals. VACV encodes various factors interfering with host immune responses at multiple levels. In atopic skin, the production of type I interferon is compromised, while VACV specifically inhibits the phosphorylation of the Interferon Regulatory Factor 3 (IRF-3) and expression of interferons. To overcome this block, we generated a recombinant VACV-expressing murine IRF-3 (WR-IRF3) and characterized its effects on virus growth, cytokine expression and apoptosis in tissue cultures and in spontaneously atopic Nc/Nga and control Balb/c mice. Further, we explored the induction of protective immune responses against a lethal dose of wild-type WR, the surrogate of smallpox. We demonstrate that the overexpression of IRF-3 by WR-IRF3 increases the expression of type I interferon, modulates the expression of several cytokines and induces superior protective immune responses against a lethal poxvirus challenge in both Nc/Nga and Balb/c mice. Additionally, the results may be informative for design of other virus-based vaccines or for therapy of different viral infections.

Immune Responses in Malaria and Vaccine Strategies

Malaria is a pandemic with nearly half of global population at risk, caused by parasite Plasmodium species, particularly P. falciparum with a high morbidity and mortality, especially among children. There is an urgent need for development of population protective vaccines, such as in sub-Saharan low-income countries, where P. falciparum malaria is endemic. After years of endeavour with children and adults for safety and efficacy clinical trials, the P. falciparum circumsporozoite protein antigen, is targeted by specific antibodies induced by recombinant vaccine, called TRS,S. TRS,S has been authorized by WHO and Malawi Government to be the first malaria vaccine for up to 2 years of aged children for protection against malaria. Other malaria vaccines in clinical trials are also very promising candidates, including the original live, X-ray attenuated P-sporozoite vaccine, inducing antigen-specific T cell immunity at liver stage. Malaria parasite at blood symptomatic stage is targeted by specific antibodies to parasite-infected erythrocytes, which are important against pathogenic placenta-infected erythrocyte sequestration. Here, the demographic distribution of Plasmodium species and their pathogenicity in infected people are discussed. The role of innate phagocytic cells and malaria antigen specific T cell immunity, as well as that of specific antibody production by B cells are highlighted. The paramount role of cytotoxic CD8⁺ T cellular immunity in malaria people protection is also included.

A Novel Ebola Virus VP40 Matrix Protein-Based Screening for Identification of Novel Candidate Medical Countermeasures

Filoviruses, such as Ebola virus and Marburg virus, are of significant human health concern. From 2013 to 2016, Ebola virus caused 11,323 fatalities in Western Africa. Since 2018, two Ebola virus disease outbreaks in the Democratic Republic of the Congo resulted in 2354 fatalities. Although there is progress in medical countermeasure (MCM) development (in particular, vaccines and antibody-based therapeutics), the need for efficacious small-molecule therapeutics remains unmet. Here we describe a novel high-throughput screening assay to identify inhibitors of Ebola virus VP40 matrix protein association with viral particle assembly sites on the interior of the host cell plasma membrane. Using this assay, we screened nearly 3000 small molecules and identified several molecules with the desired inhibitory properties. In secondary assays, one identified compound, sangivamycin, inhibited not only Ebola viral infectivity but also that of other viruses. This finding indicates that it is possible for this new VP40-based screening method to identify highly potent MCMs against Ebola virus and its relatives.

Tools for the targeted genetic modification of poxvirus genomes

The potential of viruses as biotechnology platforms is becoming more appealing due to technological advances in synthetic biology techniques and to the increasing accessibility of means to manipulate virus genomes. Among viral systems, poxviruses, and their prototype member Vaccinia Virus, are one of the outstanding choices for different biotechnological and medical applications based on heterologous gene expression, recombinant vaccines or oncolytic viruses. The refinement of genetic engineering methods on Vaccinia Virus over the last decades have contributed to facilitate the manipulation of the genomes of poxviruses, and may aid in the improvement of virus variants designed for different goals through reverse genetic approaches. Targeted genetic changes are usually performed by homologous recombination with the viral genome. In addition to the classic approach, recent methodological advances that may assist new strategies for the mutation or edition of poxvirus genomes are reviewed.

Investigating Viruses During the Transformation of Molecular Biology: Part II

My scientific career started at an extraordinary time, shortly after the discoveries of the helical structure of DNA, the central dogma of DNA to RNA to protein, and the genetic code. Part I of this series emphasizes my education and early studies highlighted by the isolation and characterization of numerous vaccinia virus enzymes, determination of the cap structure of messenger RNA, and development of poxviruses as gene expression vectors for use as recombinant vaccines. Here I describe a shift in my research focus to combine molecular biology and genetics for a comprehensive understanding of poxvirus biology. The dominant paradigm during the early years was to select a function, isolate the responsible proteins, and locate the corresponding gene, whereas later the common paradigm was to select a gene, make a mutation, and determine the altered function. Motivations, behind-the-scenes insights, importance of new technologies, and the vital roles of trainees and coworkers are emphasized.

Generation of Vaccinia Virus Gene Deletion Mutants Using Complementing Cell Lines

This protocol describes how to couple two techniques, the generation of complementing cells lines and production of viral deletion mutants, to rapidly construct novel tools for poxvirus analysis. Specifically, the production and utilization of a complementing cell line expressing a poxvirus gene of interest are critical for the generation of poxvirus mutants in which essential genes are disrupted. Complementing cells are also valuable for the characterization of vaccinia genes in the absence of infection. Here, we detail the process of isolating vaccinia virus deletion mutants. Deletion mutant generation involves homologous recombination between replicating viral DNA and transfected DNA followed by selection and screening on a complementing cell line that provides the deleted gene in trans. Finally, deletion is confirmed by polymerase chain reaction, sequencing, and functional assays if available.

Modified Vaccinia Virus Ankara Can Induce Optimal CD8+ T Cell Responses to Directly Primed Antigens Depending on Vaccine Design

A variety of strains of vaccinia virus (VACV) have been used as recombinant vaccine vectors with the aim of inducing robust CD8⁺ T cell immunity. While much of the pioneering work was done with virulent strains, such as Western Reserve (WR), attenuated strains such as modified vaccinia virus Ankara (MVA) are more realistic vectors for clinical use. To unify this literature, side-by-side comparisons of virus strains are required. Here, we compare the form of antigen that supports optimal CD8⁺ T cell responses for VACV strains WR and MVA using equivalent constructs. We found that for multiple antigens, minimal antigenic constructs (epitope minigenes) that prime CD8⁺ T cells via the direct presentation pathway elicited optimal responses from both vectors, which was surprising because this finding contradicts the prevailing view in the literature for MVA. We then went on to explore the discrepancy between current and published data for MVA, finding evidence that the expression locus and in some cases the presence of the viral thymidine kinase may influence the ability of this strain to prime optimal responses from antigens that require direct presentation. This extends our knowledge of the design parameters for VACV vectored vaccines, especially those based on MVA.IMPORTANCE Recombinant vaccines based on vaccinia virus and particularly attenuated strains such as MVA are in human clinical trials, but due to the complexity of these large vectors much remains to be understood about the design parameters that alter their immunogenicity. Previous work had found that MVA vectors should be designed to express stable protein in order to induce robust immunity by CD8⁺ (cytotoxic) T cells. Here, we found that the primacy of stable antigen is not generalizable to all designs of MVA and may depend where a foreign antigen is inserted into the MVA genome. This unexpected finding suggests that there is an interaction between genome location and the best form of antigen for optimal T cell priming in MVA and thus possibly other vaccine vectors. It also highlights that our understanding of antigen presentation by even the best studied of vaccine vectors remains incomplete.

Primary Human B Cells at Different Differentiation and Maturation Stages Exhibit Distinct Susceptibilities to Vaccinia Virus Binding and Infection

Vaccinia virus (VACV), the prototypical member of the poxvirus family, was used as a live-virus vaccine to eradicate smallpox worldwide and has recently received considerable attention because of its potential as a prominent vector for the development of vaccines against infectious diseases and as an oncolytic virus for cancer therapy. Studies have demonstrated that VACV exhibits an extremely strong bias for binding to and infection of primary human antigen-presenting cells (APCs), including monocytes, macrophages, and dendritic cells. However, very few studies have assessed the interactions of VACV with primary human B cells, a main type of professional APCs. In this study, we evaluated the susceptibility of primary human peripheral B cells at various differentiation and maturation stages to VACV binding, infection, and replication. We found that plasmablasts were resistant to VACV binding, while other B subsets, including transitional, mature naive, memory, and plasma cells, were highly susceptible to VACV binding. VACV binding preference was likely associated with differential expression of chemokine receptors, particularly CXCR5. Infection studies showed that plasmablast, plasma, transitional, and mature naive B cells were resistant to VACV infection, while memory B cells were preferentially infected. VACV infection in ex vivo B cells was abortive, which occurred at the stage of late viral gene expression. In contrast, activated B cells were permissive to productive VACV infection. Thus, primary human B cells at different differentiation stages exhibit distinct susceptibilities to VACV binding and infection, and the infections are abortive and productive in ex vivo and activated B cells, respectively.IMPORTANCE Our results provide critical information to the field of poxvirus binding and infection tropism. We demonstrate that VACV preferentially infects memory B cells that play an important role in a rapid and vigorous antibody-mediated immune response upon reinfection by a pathogen. Additionally, this work highlights the potential of B cells as natural cellular models to identify VACV receptors or dissect the molecular mechanisms underlying key steps of the VACV life cycle, such as binding, penetration, entry, and replication in primary human cells. The understanding of VACV biology in human primary cells is essential for the development of a safe and effective live-virus vector for oncolytic virus therapy and vaccines against smallpox, other pathogens, and cancer.

SPI-1 is a missing host-range factor required for replication of the attenuated modified vaccinia Ankara (MVA) vaccine vector in human cells

Modified vaccinia virus Ankara (MVA) is the leading poxvirus vector for development of vaccines against diverse infectious diseases. This distinction is based on high expression of proteins and good immunogenicity despite an inability to assemble infectious progeny in human cells, which together promote efficacy and safety. Nevertheless, the basis for the host-range restriction is unknown despite past systematic attempts to identify the relevant missing viral gene(s). The search for host-range factors is exacerbated by the large number of deletions, truncations and mutations that occurred during the long passage history of MVA in chicken embryo fibroblasts. By whole genome sequencing of a panel of recombinant host-range extended (HRE) MVAs generated by marker rescue with 40 kbp segments of vaccinia virus DNA, we identified serine protease inhibitor 1 (SPI-1) as one of several candidate host-range factors present in those viruses that gained the ability to replicate in human cells. Electron microscopy revealed that the interruption of morphogenesis in human cells infected with MVA occurred at a similar stage as that of a vaccinia virus strain WR SPI-1 deletion mutant. Moreover, the introduction of the SPI-1 gene into the MVA genome led to more than a 2-log enhancement of virus spread in human diploid MRC-5 cells, whereas deletion of the gene diminished the spread of HRE viruses by similar extents. Furthermore, MRC-5 cells stably expressing SPI-1 also enhanced replication of MVA. A role for additional host range genes was suggested by the restoration of MVA replication to a lower level relative to HRE viruses, particularly in other human cell lines. Although multiple sequence alignments revealed genetic changes in addition to SPI-1 common to the HRE MVAs, no evidence for their host-range function was found by analysis thus far. Our finding that SPI-1 is host range factor for MVA should simplify use of high throughput RNAi or CRISPR/Cas single gene methods to identify additional viral and human restriction elements.

Poxvirus vectors have outstanding properties for development of vaccines against a myriad of infectious agents due to their ability to retain long segments of foreign DNA and high-level gene expression. Safety concerns led to a preference for attenuated poxviruses that lost the ability to produce infectious progeny in human cells. The most widely used poxvirus vector is modified vaccinia virus Ankara (MVA), which exhibits an extreme host-range restriction in most mammalian cells. MVA was attenuated by passaging more than 500 times in chicken embryo fibroblasts during which large deletions and numerous additional genetic changes occurred. Despite ongoing clinical testing of MVA-vectored vaccines, the basis for its host-range restriction remained unknown. Here we show that re-introduction of the SPI-1 gene into MVA or host cells increased virus spread by more than 100-fold in a human diploid cell line, providing an important insight into the mechanism responsible for the host-range restriction. This information could help design improved vectors and develop non-avian cell lines for propagation of candidate MVA vaccines.

Comparative purification and characterization of hepatitis B virus-like particles produced by recombinant vaccinia viruses in human hepatoma cells and human primary hepatocytes

This study describes the comparative expression and purification of hepatitis B surface antigen (HBsAg) particles produced upon infection of human primary hepatocytes and human hepatoma cell lines (HuH-7 and HepG2) with recombinant vaccinia viruses. The highest levels of HBsAg expression were found in HuH-7 hepatoma cells following infection with recombinant vaccinia viruses, which contain the S gene under control of a 7.5 k-promoter. Four different methods for purification of the HBsAg particles were examined: isopycnic ultracentrifugation, sucrose cushion sedimentation, isocratic column gel filtration, and binding to anti-HBs-coated microparticles. The highest degree of purity of HBsAg particles was reached by the method based on anti-HBs-coated microparticles. The resulting product was >98% pure. Biochemical analysis and characterization of purified HBsAg particles were performed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), western blotting, and electron microscopy. The HBsAg, purified from human hepatoma cell lines and from human primary hepatocytes, consisted of both the non-glycosylated (p25) and the glycosylated (gp27) form and assembled into typical 22-nm particles, and thus may be of great interest and importance for research, diagnostics, and medical treatments.

Preclinical safety evaluation of hepatic arterial infusion of oncolytic poxvirus

Purpose

Oncolytic poxvirus has shown promise in treating various solid tumors, such as liver cancer, and administration of oncolytic poxvirus via the hepatic artery may provide more survival benefits than other routes of administration. However, there is a lack of safety information to guide the application of hepatic arterial infusion (HAI) of oncolytic poxvirus in human studies. To investigate the acute and chronic toxicity of HAI administration of oncolytic poxvirus in animals and provide safety information for future human studies.

Methods

VV^tk-, a vaccinia poxvirus with inactivated thymidine kinase gene, was administered via HAI to rabbits with normal liver function under angiography (1×10⁸ or 1×10⁹ pfu), and rats with N-nitrosomorpholine-induced precancerous liver cirrhosis under open surgery (1×10⁸ pfu). Body weights and survival were monitored and blood samples were collected for hematological and biochemical tests. Distribution of A56 (a specific marker for poxvirus infection) in rabbit organs was evaluated using immunofluorescence assays.

Results

HAI of high doses of VV^tk- did not cause any acute or chronic changes in body weight, survival or in biochemical, hematological tests in the 2 animal models, and none of the changes showed dose dependency (in rabbit study), or were influenced by liver cirrhosis (in rat study). A56 was not detected in any of the major rabbit organs.

Conclusion

HAI may provide a safe alternative route of oncolytic poxvirus administration for human studies.

Baited vaccines: A strategy to mitigate rodent-borne viral zoonoses in humans

Rodents serve as the natural reservoir and vector for a variety of pathogens, some of which are responsible for severe and life-threatening disease in humans. Despite the significant impact in humans many of these viruses, including Old and New World hantaviruses as well as Arenaviruses, most have no specific vaccine or therapeutic to treat or prevent human infection. The recent success of wildlife vaccines to mitigate rabies in animal populations offers interesting insight into the use of similar strategies for other zoonotic agents of human disease. In this review, we discuss the notion of using baited vaccines as a means to interrupt the transmission of viral pathogens between rodent reservoirs and to susceptible human hosts.

Mutagenic repair of double-stranded DNA breaks in vaccinia virus genomes requires cellular DNA ligase IV activity in the cytosol

Poxviruses comprise a group of large dsDNA viruses that include members relevant to human and animal health, such as variola virus, monkeypox virus, cowpox virus and vaccinia virus (VACV). Poxviruses are remarkable for their unique replication cycle, which is restricted to the cytoplasm of infected cells. The independence from the host nucleus requires poxviruses to encode most of the enzymes involved in DNA replication, transcription and processing. Here, we use the CRISPR/Cas9 genome engineering system to induce DNA damage to VACV (strain Western Reserve) genomes. We show that targeting CRISPR/Cas9 to essential viral genes limits virus replication efficiently. Although VACV is a strictly cytoplasmic pathogen, we observed extensive viral genome editing at the target site; this is reminiscent of a non-homologous end-joining DNA repair mechanism. This pathway was not dependent on the viral DNA ligase, but critically involved the cellular DNA ligase IV. Our data show that DNA ligase IV can act outside of the nucleus to allow repair of dsDNA breaks in poxvirus genomes. This pathway might contribute to the introduction of mutations within the genome of poxviruses and may thereby promote the evolution of these viruses.

Strategies to obtain multiple recombinant modified vaccinia Ankara vectors. Applications to influenza vaccines

As a vaccination vector, MVA has been widely investigated both in animal models and humans. The construction of recombinant MVA (rMVA) relies on homologous recombination between an acceptor virus and a donor plasmid in infected/transfected permissive cells. Our construction strategy "Red-to-Green gene swapping" - based on the exchange of two fluorescent markers within the flanking regions of MVA deletion ΔIII, coupled to fluorescence activated cell sorting - is here extended to a second insertion site, within the flanking regions of MVA deletion ΔVI. Exploiting this strategy, both double and triple rMVA were constructed, expressing as transgenes the influenza A proteins HA, NP, M1, and PB1. Upon validation of the harbored transgenes co-expression, double and triple recombinants rMVA(ΔIII)-NP-P2A-M1 and rMVA(ΔIII)-NP-P2A-M1-(ΔVI)-PB1 were assayed for in vivo immunogenicity and protection against lethal challenge. In vivo responses were identical to those obtained with the reported combinations of single recombinants, supporting the feasibility and reliability of the present improvement and the extension of Red-to-Green gene swapping to insertion sites other than ΔIII.

Rapid Generation of Multiple Loci-Engineered Marker-free Poxvirus and Characterization of a Clinical-Grade Oncolytic Vaccinia Virus

Recombinant poxviruses, utilized as vaccine vectors and oncolytic viruses, often require manipulation at multiple genetic loci in the viral genome. It is essential for viral vectors to possess no adventitious mutations and no (antibiotic) selection marker in the final product for human patients in order to comply with the guidance from the regulatory agencies. Rintoul et al. have previously developed a selectable and excisable marker (SEM) system for the rapid generation of recombinant vaccinia virus. In the current study, we describe an improved methodology for rapid creation and selection of recombinant poxviruses with multiple genetic manipulations solely based on expression of a fluorescent protein and with no requirement for drug selection that can lead to cellular stress and the risk of adventitious mutations throughout the viral genome. Using this improved procedure combined with the SEM system, we have constructed multiple marker-free oncolytic poxviruses expressing different cytokines and other therapeutic genes. The high fidelity of inserted DNA sequences validates the utility of this improved procedure for generation of therapeutic viruses for human patients. We have created an oncolytic poxvirus expressing human chemokine CCL5, designated as vvDD-A34R-hCCL5, with manipulations at two genetic loci in a single virus. Finally, we have produced and purified this virus in clinical grade for its use in a phase I clinical trial and presented data on initial in vitro characterization of the virus.

Generation of Recombinant Vaccinia Viruses

This unit describes how to infect cells with vaccinia virus and then transfect them with a plasmid-transfer vector or PCR fragment to generate a recombinant virus. Selection and screening methods used to isolate recombinant viruses and a method for the amplification of recombinant viruses are described. Finally, a method for live immunostaining that has been used primarily for detection of recombinant modified vaccinia virus Ankara (MVA) is presented. © 2017 by John Wiley & Sons, Inc.

Novel Nonreplicating Vaccinia Virus Vector Enhances Expression of Heterologous Genes and Suppresses Synthesis of Endogenous Viral Proteins

Viruses are used as expression vectors for protein synthesis, immunology research, vaccines, and therapeutics. Advantages of poxvirus vectors include the accommodation of large amounts of heterologous DNA, the presence of a cytoplasmic site of transcription, and high expression levels. On the other hand, competition of approximately 200 viral genes with the target gene for expression and immune recognition may be disadvantageous. We describe a vaccinia virus (VACV) vector that uses an early promoter to express the bacteriophage T7 RNA polymerase; has the A23R intermediate transcription factor gene deleted, thereby restricting virus replication to complementing cells; and has a heterologous gene regulated by a T7 promoter. In noncomplementing cells, viral early gene expression and DNA replication occurred normally but synthesis of intermediate and late proteins was prevented. Nevertheless, the progeny viral DNA provided templates for abundant expression of heterologous genes regulated by a T7 promoter. Selective expression of the Escherichia coli lac repressor gene from an intermediate promoter reduced transcription of the heterologous gene specifically in complementing cells, where large amounts might adversely impact VACV replication. Expression of heterologous proteins mediated by the A23R deletion vector equaled that of a replicating VACV, was higher than that of a nonreplicating modified vaccinia virus Ankara (MVA) vector used for candidate vaccines in vitro and in vivo, and was similarly immunogenic in mice. Unlike the MVA vector, the A23R deletion vector still expresses numerous early genes that can restrict immunogenicity as demonstrated here by the failure of the prototype vector to induce interferon alpha. By deleting immunomodulatory genes, we anticipate further improvements in the system.

Vaccines provide an efficient and effective way of preventing infectious diseases. Nevertheless, new and better vaccines are needed. Vaccinia virus, which was used successfully as a live vaccine to eradicate smallpox, has been further attenuated and adapted as a recombinant vector for immunization against other pathogens. However, since the initial description of this vector system, only incremental improvements largely related to safety have been implemented. Here we described novel modifications of the platform that increased expression of the heterologous target gene and decreased expression of endogenous vaccinia virus genes while providing safety by preventing replication of the candidate vaccine except in complementing cells used for vector propagation.

Transient dominant host-range selection using Chinese hamster ovary cells to generate marker-free recombinant viral vectors from vaccinia virus

Recombinant vaccinia viruses (rVACVs) are promising antigen-delivery systems for vaccine development that are also useful as research tools. Two common methods for selection during construction of rVACV clones are (i) co-insertion of drug resistance or reporter protein genes, which requires the use of additional selection drugs or detection methods, and (ii) dominant host-range selection. The latter uses VACV variants rendered replication-incompetent in host cell lines by the deletion of host-range genes. Replicative ability is restored by co-insertion of the host-range genes, providing for dominant selection of the recombinant viruses. Here, we describe a new method for the construction of rVACVs using the cowpox CP77 protein and unmodified VACV as the starting material. Our selection system will expand the range of tools available for positive selection of rVACV during vector construction, and it is substantially more high-fidelity than approaches based on selection for drug resistance.

Investigating Viruses during the Transformation of Molecular Biology

This Reflections article describes my early work on viral enzymes and the discovery of mRNA capping, how my training in medicine and biochemistry merged as I evolved into a virologist, the development of viruses as vaccine vectors, and how scientific and technological developments during the 1970s and beyond set the stage for the interrogation of nearly every step in the reproductive cycle of vaccinia virus (VACV), a large DNA virus with about 200 genes. The reader may view this article as a work in progress, because I remain actively engaged in research at the National Institutes of Health (NIH) notwithstanding 50 memorable years there.

Deletion of the Chemokine Binding Protein Gene from the Parapoxvirus Orf Virus Reduces Virulence and Pathogenesis in Sheep

Orf virus (ORFV) is the type species of the Parapoxvirus genus of the family Poxviridae and infects sheep and goats, often around the mouth, resulting in acute pustular skin lesions. ORFV encodes several secreted immunomodulators including a broad-spectrum chemokine binding protein (CBP). Chemokines are a large family of secreted chemotactic proteins that activate and regulate inflammation induced leukocyte recruitment to sites of infection. In this study we investigated the role of CBP in vivo in the context of ORFV infection of sheep. The CBP gene was deleted from ORFV strain NZ7 and infections of sheep used to investigate the effect of CBP on pathogenesis. Animals were either infected with the wild type (wt) virus, CBP-knockout virus or revertant strains. Sheep were infected by scarification on the wool-less area of the hind legs at various doses of virus. The deletion of the CBP gene severely attenuated the virus, as only few papules formed when animals were infected with the CBP-knock-out virus at the highest dose (10⁷ p.f.u). In contrast, large pustular lesions formed on almost all animals infected with the wt and revertant strains at 10⁷ p.f.u. The lesions for the CBP-knock-out virus resolved approximately 5–6 days p.i, at a dose of 10⁷ pfu whereas in animals infected with the wt and revertants at this dose, lesions began to resolve at approximately 10 days p.i. Few pustules developed at the lowest dose of 10³ p.f.u. for all viruses. Immunohistochemistry of biopsy skin-tissue from pustules showed that the CBP-knockout virus replicated in all animals at the highest dose and was localized to the skin epithelium while haematoxylin and eosin staining showed histological features of the CBP-knockout virus typical of the parent virus with acanthosis, elongated rete ridges and orthokeratotic hyperkeratosis. MHC-II immunohistochemistry analysis for monocytes and dendritic cells showed greater staining within the papillary dermis of the CBP-knock-out virus compared with the revertant viruses, however this was not the case with the wt where staining was similar. Our results show that the CBP gene encodes a secreted immunodulator that has a critical role in virulence and pathogenesis.

Generation of Recombinant Vaccinia Viruses

This unit describes how to infect cells with vaccinia virus and then transfect them with a plasmid-transfer vector or PCR fragment to generate a recombinant virus. Selection and screening methods used to isolate recombinant viruses and a method for the amplification of recombinant viruses are described. Finally, a method for live immunostaining that has been used primarily for detection of recombinant modified vaccinia virus Ankara (MVA) is presented. © 2017 by John Wiley & Sons, Inc.

Recombinant Poxvirus and the Tumor Microenvironment: Oncolysis, Immune Regulation and Immunization

Oncolytic viruses (OVs) are being extensively studied for their potential roles in the development of cancer therapy regimens. In addition to their direct lytic effects, OVs can initiate and drive systemic antitumor immunity indirectly via release of tumor antigen, as well as by encoding and delivering immunostimulatory molecules. This combination makes them an effective platform for the development of immunotherapeutic strategies beyond their primary lytic function. Engineering the viruses to also express tumor-associated antigens (TAAs) allows them to simultaneously serve as therapeutic vaccines, targeting and amplifying an immune response to TAAs. Our group and others have shown that vaccinating intratumorally with a poxvirus that encodes TAAs, in addition to immune stimulatory molecules, can modulate the tumor microenvironment, overcome immune inhibitory pathways, and drive both local and systemic tumor specific immune responses.

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.

What Lies Beneath: Antibody Dependent Natural Killer Cell Activation by Antibodies to Internal Influenza Virus Proteins

The conserved internal influenza proteins nucleoprotein (NP) and matrix 1 (M1) are well characterised for T cell immunity, but whether they also elicit functional antibodies capable of activating natural killer (NK) cells has not been explored. We studied NP and M1-specific ADCC activity using biochemical, NK cell activation and killing assays with plasma from healthy and influenza-infected subjects. Healthy adults had antibodies to M1 and NP capable of binding dimeric FcγRIIIa and activating NK cells. Natural symptomatic and experimental influenza infections resulted in a rise in antibody dependent NK cell activation post-infection to the hemagglutinin of the infecting strain, but changes in NK cell activation to M1 and NP were variable. Although antibody dependent killing of target cells infected with vaccinia viruses expressing internal influenza proteins was not detected, opsonising antibodies to NP and M1 likely contribute to an antiviral microenvironment by stimulating innate immune cells to secrete cytokines early in infection. We conclude that effector cell activating antibodies to conserved internal influenza proteins are common in healthy and influenza-infected adults. Given the significance of such antibodies in animal models of heterologous influenza infection, the definition of their importance and mechanism of action in human immunity to influenza is essential.

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Functional antibodies to influenza matrix 1 and nucleoprotein are common in healthy and influenza-infected humans.

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Opsonising antibodies to matrix 1 and nucleoprotein can bind FcγRIIIa dimers and activate natural killer cells.

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Influenza infection increased natural killer cell activation to hemagglutinin but changes to the internal proteins varied

Influenza virus causes both seasonal outbreaks and global pandemics. The current influenza vaccine provides minimal protection against divergent strains of the virus not found in the vaccine. While neutralising antibodies induced by vaccination are able to confer strain-specific protection, antibodies directed against conserved influenza proteins may be able to provide some cross-protection. Animal models suggest a protective role for anti-nucleoprotein antibodies. Exploring the functional capacity of human antibodies against internal influenza proteins to engage Fc receptors and activate innate immune cells may present a unique approach in the development of a more universal influenza vaccine.

Oncolytic and immunologic cancer therapy with GM-CSF-armed vaccinia virus of Tian Tan strain Guang9

Targeted oncolytic vaccinia viruses are being developed as a novel strategy in cancer therapy. Arming vaccinia viruses with immunostimulatory cytokines can enhance antitumor efficacy. Such engineered oncolytic viruses, like JX-594, a Wyeth strain vaccinia virus modified with human granulocyte-macrophage colony-stimulating factor (GM-CSF), have shown promising results and have proceeded rapidly in clinical trials. However, the oncolytic potential of the Chinese vaccine strain Tian Tan (VTT) has not been explored. In this study, we constructed a targeted oncolytic vaccinia virus of Tian Tan strain Guang9 (VG9) expressing murine GM-CSF (VG9-GMCSF) and evaluated the antitumor effect of this recombinant vaccinia virus in a murine melanoma model. In vitro, viral replication and cytotoxicity of VG9-GMCSF was as potent as VG9; in vivo, VG9-GMCSF significantly inhibited the growth of subcutaneously implanted melanoma tumors, prolonged the survival of tumor-bearing mice, and produced an antitumor cytotoxic response. Such antitumor effect may be due to the lytic nature of virus as well as the stimulation of immune activity by GM-CSF production. Our results indicate that VG9-GMCSF induces strong tumoricidal activity, providing a potential therapeutic strategy for combating cancer.

Generation of Recombinant Vaccinia Viruses

This unit describes how to infect cells with vaccinia virus and then transfect them with a plasmid-transfer vector or PCR fragment to generate a recombinant virus. Selection and screening methods used to isolate recombinant viruses and a method for the amplification of recombinant viruses are described. Finally, a method for live immunostaining that has been used primarily for detection of recombinant modified vaccinia virus Ankara (MVA) is presented.

A marker-free system for highly efficient construction of vaccinia virus vectors using CRISPR Cas9

The current method for creation of vaccinia virus (VACV) vectors involves using a selection and purification marker, however inclusion of a gene without therapeutic value in the resulting vector is not desirable for clinical use. The Cre-LoxP system has been used to make marker-free Poxviruses, but the efficiency was very low. To obtain a marker-free VACV vector, we developed marker gene excision systems to modify the thymidine kinase (TK) region and N1L regions using Cre-Loxp and Flp-FRET systems respectively. CRISPR-Cas9 system significantly resulted in a high efficiency (~90%) in generation of marker gene-positive TK-mutant VACV vector. The marker gene (RFP) could be excised from the recombinant virus using Cre recombinase. To make a marker-free VV vector with double gene deletions targeting the TK and N1L gene, we constructed a donor repair vector targeting the N1L gene, which can carry a therapeutic gene and the marker (RFP) that could be excised from the recombinant virus using Flp recombinase. The marker-free system developed here can be used to efficiently construct VACV vectors armed with any therapeutic genes in the TK region or N1L region without marker genes. Our marker-free system platform has significant potential for development of new marker-free VACV vectors for clinical application.

Efficiently editing the vaccinia virus genome by using the CRISPR-Cas9 system

Vaccinia virus (VACV) continues to be used in immunotherapy for the prevention of infectious diseases and treatment of cancer since its use for the eradication of smallpox. However, the current method of editing the VACV genome is not efficient. Here, we demonstrate that the CRISPR-Cas9 system can be used to edit the VACV genome rapidly and efficiently. Additionally, a set of 8,964 computationally designed unique guide RNAs (gRNAs) targeting all VACV genes will be valuable for the study of VACV gene functions.

Oncolytic tanapoxvirus expressing FliC causes regression of human colorectal cancer xenografts in nude mice

Colorectal cancers are significant causes of morbidity and mortality and existing therapies often perform poorly for individuals afflicted with advanced disease. Oncolytic virotherapy is an emerging therapeutic modality with great promise for addressing this medical need. Herein we describe the in vivo testing of recombinant variants of the tanapoxvirus (TPV). Recombinant viruses were made ablated for either the 66R gene (encoding a thymidine kinase), the 2L gene (encoding a TNF-binding protein), or both. Some of the recombinants were armed to express mouse chemotactic protein 1 (mCCL2/mMCP-1), mouse granulocyte-monocyte colony stimulating factor (mGM-CSF), or bacterial flagellin (FliC). Tumors were induced in athymic nude mice by implantation of HCT 116 cells and subsequently treated by a single intratumoral injection of one of the recombinant TPVs. Histological examination showed a common neoplastic cell type and a range of immune cell infiltration, necrosis, and tumor cell organization. Significant regression was seen in tumors treated with virus TPV/Δ2L/Δ66R/fliC, and to a lesser extent the recombinants TPV/Δ2L and TPV/Δ66R. Our results suggest that oncolytic recombinants of the TPV armed with activators of the innate immune response may be effective virotherapeutic agents for colorectal cancers in humans and should be explored further to fully realize their potential.

Engineering herpes simplex viruses by infection-transfection methods including recombination site targeting by CRISPR/Cas9 nucleases

Herpes simplex viruses (HSVs) are frequent human pathogens and the ability to engineer these viruses underpins much research into their biology and pathogenesis. Often the ultimate aim is to produce a virus that has the desired phenotypic change and no additional alterations in characteristics. This requires methods that minimally disrupt the genome and, for insertions of foreign DNA, sites must be found that can be engineered without disrupting HSV gene function or expression. This study advances both of these requirements. Firstly, the use of homologous recombination between the virus genome and plasmids in mammalian cells is a reliable way to engineer HSV such that minimal genome changes are made. This has most frequently been achieved by cotransfection of plasmid and isolated viral genomic DNA, but an alternative is to supply the virus genome by infection in a transfection-infection method. Such approaches can also incorporate CRISPR/Cas9 genome engineering methods. Current descriptions of infection-transfection methods, either with or without the addition of CRISPR/Cas9 targeting, are limited in detail and the extent of optimization. In this study it was found that transfection efficiency and the length of homologous sequences improve the efficiency of recombination in these methods, but the targeting of the locus to be engineered by CRISPR/Cas9 nucleases has an overriding positive impact. Secondly, the intergenic space between UL26 and UL27 was reexamined as a site for the addition of foreign DNA and a position identified that allows insertions without compromising HSV growth in vitro or in vivo.

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.

Analysis of the anti-apoptotic activity of four vaccinia virus proteins demonstrates that B13 is the most potent inhibitor in isolation and during viral infection

Vaccinia virus (VACV) is a large dsDNA virus encoding ~200 proteins, several of which inhibit apoptosis. Here, a comparative study of anti-apoptotic proteins N1, F1, B13 and Golgi anti-apoptotic protein (GAAP) in isolation and during viral infection is presented. VACVs strains engineered to lack each gene separately still blocked apoptosis to some degree because of functional redundancy provided by the other anti-apoptotic proteins. To overcome this redundancy, we inserted each gene separately into a VACV strain (vv811) that lacked all these anti-apoptotic proteins and that induced apoptosis efficiently during infection. Each protein was also expressed in cells using lentivirus vectors. In isolation, each VACV protein showed anti-apoptotic activity in response to specific stimuli, as measured by immunoblotting for cleaved poly(ADP ribose) polymerase-1 and caspase-3 activation. Of the proteins tested, B13 was the most potent inhibitor, blocking both intrinsic and extrinsic stimuli, whilst the activity of the other proteins was largely restricted to inhibition of intrinsic stimuli. In addition, B13 and F1 were effective blockers of apoptosis induced by vv811 infection. Finally, whilst differences in induction of apoptosis were barely detectable during infection with VACV strain Western Reserve compared with derivative viruses lacking individual anti-apoptotic genes, several of these proteins reduced activation of caspase-3 during infection by vv811 strains expressing these proteins. These results illustrated that vv811 was a useful tool to determine the role of VACV proteins during infection and that whilst all of these proteins have some anti-apoptotic activity, B13 was the most potent.

T cell inactivation by poxviral B22 family proteins increases viral virulence

Infections with monkeypox, cowpox and weaponized variola virus remain a threat to the increasingly unvaccinated human population, but little is known about their mechanisms of virulence and immune evasion. We now demonstrate that B22 proteins, encoded by the largest genes of these viruses, render human T cells unresponsive to stimulation of the T cell receptor by MHC-dependent antigen presentation or by MHC-independent stimulation. In contrast, stimuli that bypass TCR-signaling are not inhibited. In a non-human primate model of monkeypox, virus lacking the B22R homologue (MPXVΔ197) caused only mild disease with lower viremia and cutaneous pox lesions compared to wild type MPXV which caused high viremia, morbidity and mortality. Since MPXVΔ197-infected animals displayed accelerated T cell responses and less T cell dysregulation than MPXV US2003, we conclude that B22 family proteins cause viral virulence by suppressing T cell control of viral dissemination.

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

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

Primary human leukocyte subsets differentially express vaccinia virus receptors enriched in lipid rafts

Poxviruses, including vaccinia virus (VV) and canarypox virus (ALVAC), do not indiscriminately infect all cell types of the primary human leukocytes (PHLs) that they encounter but instead demonstrate an extremely strong bias toward infection of monocytes and monocyte lineage cells. We studied the specific molecular events that determine the VV tropism for major PHL subsets including monocytes, B cells, neutrophils, NK cells, and T cells. We found that VV exhibited an extremely strong bias of cell surface protein-dependent binding to monocytes, B cells, and activated T cells to a similar degree and to neutrophils to a much lesser extent. Resting T cells and resting NK cells exhibited only trace amounts of VV binding. Activated T cells, however, became permissive to VV binding, infection, and replication, while activated NK cells still resisted VV binding. VV binding strongly colocalized with lipid rafts on the surfaces of all VV binding-susceptible PHL subsets, even when lipid rafts were relocated to cell uropods upon cell polarization. Immunosera raised against detergent-resistant membranes (DRMs) from monocytes or activated T cells, but not resting T cells, effectively cross-blocked VV binding to and infection of PHL subsets. CD29 and CD98, two lipid raft-associated membrane proteins that had been found to be important for VV entry into HeLa cells, had no effect on VV binding to and infection of primary activated T cells. Our data indicate that PHL subsets express VV protein receptors enriched in lipid rafts and that receptors are cross-presented on all susceptible PHLs.

Reflections on the early development of poxvirus vectors

Poxvirus expression vectors were described in 1982 and quickly became widely used for vaccine development as well as research in numerous fields. Advantages of the vectors include simple construction, ability to accommodate large amounts of foreign DNA and high expression levels. Numerous poxvirus-based veterinary vaccines are currently in use and many others are in human clinical trials. The early reports of poxvirus vectors paved the way for and stimulated the development of other viral vectors and recombinant DNA vaccines.

Molecular anatomy and number of antigen specific CD8 T cells required to cause type 1 diabetes

We quantified CD8 T cells needed to cause type 1 diabetes and studied the anatomy of the CD8 T cell/beta (β) cell interaction at the immunologic synapse. We used a transgenic model, in situ tetramer staining to distinguish antigen specific CD8 T cells from total T cells infiltrating islets and a variety of viral mutants selected for functional deletion(s) of various CD8 T cell epitopes. Twenty percent of CD8 T cells in the spleen were specific for all immunodominant and subdominant viral glycoprotein (GP) epitopes. CTLs to the immunodominant LCMV GP33-41 epitope accounted for 63% of the total (12.5% of tetramers). In situ hybridization analysis demonstrated only 1 to 2% of total infiltrating CD8 T cells were specific for GP33 CD8 T cell epitope, yet diabetes occurred in 94% of mice. The immunologic synapse between GP33 CD8 CTL and β cell contained LFA-1 and perforin. Silencing both immunodominant epitopes (GP33, GP276–286) in the infecting virus led to a four-fold reduction in viral specific CD8 CTL responses, negligible lymphocyte infiltration into islets and absence of diabetes.

Insulin-dependent type 1 diabetes (T1D) is characterized by elevated blood sugar, lymphocytic infiltration into the islets of Langerhans and T cell destruction of beta (β) cells. β cells produce insulin whose function is to maintain and regulate glucose hemostasis. However, in vivo, the numbers of antigen specific T cells that migrate to the islets to cause T1D, the engagement of such T cells with β cells at the immunologic synapse and the molecules expressed at the synapse are not clear. Using a transgenic model of virus induced T1D, a panel of viruses with CD8 T cell epitope mutations and in situ tetramer hybridization, we note of the total CD8 T cells infiltrating the islets, only 1–2% are antigen specific recognizing the immunodominant virus CD8 T cell epitope expressed on β cells. Immunohistochemical analysis of the synapse found between antigen specific CD8 T cells and β cells displays attachment by LFA-1 and presence of perforin, the molecule indicative of lytic activity.

AMP-activated protein kinase is required for the macropinocytic internalization of ebolavirus

Identification of host factors that are needed for Zaire Ebolavirus (EBOV) entry provides insights into the mechanism(s) of filovirus uptake, and these factors may serve as potential antiviral targets. In order to identify novel host genes and pathways involved in EBOV entry, gene array findings in the National Cancer Institute's NCI-60 panel of human tumor cell lines were correlated with permissivity for EBOV glycoprotein (GP)-mediated entry. We found that the gene encoding the γ2 subunit of AMP-activated protein kinase (AMPK) strongly correlated with EBOV transduction in the tumor panel. The AMPK inhibitor compound C inhibited infectious EBOV replication in Vero cells and diminished EBOV GP-dependent, but not Lassa fever virus GPC-dependent, entry into a variety of cell lines in a dose-dependent manner. Compound C also prevented EBOV GP-mediated infection of primary human macrophages, a major target of filoviral replication in vivo. Consistent with a role for AMPK in filovirus entry, time-of-addition studies demonstrated that compound C abrogated infection when it was added at early time points but became progressively less effective when added later. Compound C prevented EBOV pseudovirion internalization at 37°C as cell-bound particles remained susceptible to trypsin digestion in the presence of the inhibitor but not in its absence. Mouse embryonic fibroblasts lacking the AMPKα1 and AMPKα2 catalytic subunits were significantly less permissive to EBOV GP-mediated infection than their wild-type counterparts, likely due to decreased macropinocytic uptake. In total, these findings implicate AMPK in macropinocytic events needed for EBOV GP-dependent entry and identify a novel cellular target for new filoviral antivirals.

Poxviral vectors for cancer immunotherapy

INTRODUCTION

Poxviral vaccines have been given to over 1 billion people in the successful global eradication of smallpox. Recombinant poxviruses have been investigated extensively as a novel immunotherapy for cancer, undergoing several iterations to optimize their immunogenicity and efficacy. The current platform expressing multiple costimulatory molecules plus a tumor-associated antigen such as PSA, that is, PSA-TRICOM (PROSTVAC-V/F), is promising and is currently in a Phase III randomized, placebo-controlled clinical trial in metastatic castration-resistant prostate cancer.

AREAS COVERED

This review discusses the clinical development of poxviral-based cancer vaccines, with a particular focus on the rationale for combining vaccines with other treatment modalities, including radiotherapy, chemotherapy, hormonal therapy, other immune-based therapies and molecularly targeted therapy. We also discuss the importance of appropriate patient selection in clinical trial design.

EXPERT OPINION

Preclinical and early clinical studies employing poxviral-vector vaccines have shown promising results with this novel immunologic approach, both alone and combined with other therapies. The challenges of translating the science of immunotherapy to clinical practice include clinical trial design that includes appropriate patient selection, appropriate end points and identification of meaningful surrogate biomarkers.

Expression and cellular immunogenicity of a transgenic antigen driven by endogenous poxviral early promoters at their authentic loci in MVA

CD8(+) T cell responses to vaccinia virus are directed almost exclusively against early gene products. The attenuated strain modified vaccinia virus Ankara (MVA) is under evaluation in clinical trials of new vaccines designed to elicit cellular immune responses against pathogens including Plasmodium spp., M. tuberculosis and HIV-1. All of these recombinant MVAs (rMVA) utilize the well-established method of linking the gene of interest to a cloned poxviral promoter prior to insertion into the viral genome at a suitable locus by homologous recombination in infected cells. Using BAC recombineering, we show that potent early promoters that drive expression of non-functional or non-essential MVA open reading frames (ORFs) can be harnessed for immunogenic expression of recombinant antigen. Precise replacement of the MVA orthologs of C11R, F11L, A44L and B8R with a model antigen positioned to use the same translation initiation codon allowed early transgene expression similar to or slightly greater than that achieved by the commonly-used p7.5 or short synthetic promoters. The frequency of antigen-specific CD8(+) T cells induced in mice by single shot or adenovirus-prime, rMVA-boost vaccination were similarly equal or marginally enhanced using endogenous promoters at their authentic genomic loci compared to the traditional constructs. The enhancement in immunogenicity observed using the C11R or F11L promoters compared with p7.5 was similar to that obtained with the mH5 promoter compared with p7.5. Furthermore, the growth rates of the viruses were unimpaired and the insertions were genetically stable. Insertion of a transgenic ORF in place of a viral ORF by BAC recombineering can thus provide not only a potent promoter, but also, concomitantly, a suitable insertion site, potentially facilitating development of MVA vaccines expressing multiple recombinant antigens.

Protein B5 is required on extracellular enveloped vaccinia virus for repulsion of superinfecting virions

Vaccinia virus (VACV) spreads across cell monolayers fourfold faster than predicted from its replication kinetics. Early after infection, infected cells repulse some superinfecting extracellular enveloped virus (EEV) particles by the formation of actin tails from the cell surface, thereby causing accelerated spread to uninfected cells. This strategy requires the expression of two viral proteins, A33 and A36, on the surface of infected cells and upon contact with EEV this complex induces actin polymerization. Here we have studied this phenomenon further and investigated whether A33 and A36 expression in cell lines causes an increase in VACV plaque size, whether these proteins are able to block superinfection by EEV, and which protein(s) on the EEV surface are required to initiate the formation of actin tails from infected cells. Data presented show that VACV plaque size was not increased by expression of A33 and A36, and these proteins did not block entry of the majority of EEV binding to these cells. In contrast, expression of proteins A56 and K2 inhibited entry of both EEV and intracellular mature virus. Lastly, VACV protein B5 was required on EEV to induce the formation of actin tails at the surface of cells expressing A33 and A36, and B5 short consensus repeat 4 is critical for this induction.

Easy and efficient protocols for working with recombinant vaccinia virus MVA

Modified vaccinia virus Ankara (MVA) is a highly attenuated and replication-deficient strain of vaccinia virus that is increasingly used as vector for expression of recombinant genes in the research laboratory and in biomedicine for vaccine development. Major benefits of MVA include the clear safety advantage compared to conventional vaccinia viruses, the longstanding experience in the genetic engineering of the virus, and the availability of established procedures for virus production at an industrial scale. MVA vectors can be handled under biosafety level 1 conditions, and a multitude of recombinant MVA vaccines has proven to be immunogenic and protective when delivering various heterologous antigens in animals and humans. In this chapter we provide convenient state-of-the-art protocols for generation, amplification, and purification of recombinant MVA viruses. Importantly, we include methodology for rigid quality control to obtain best possible vector viruses for further investigations including clinical evaluation.

A selectable and excisable marker system for the rapid creation of recombinant poxviruses

BACKGROUND

Genetic manipulation of poxvirus genomes through attenuation, or insertion of therapeutic genes has led to a number of vector candidates for the treatment of a variety of human diseases. The development of recombinant poxviruses often involves the genomic insertion of a selectable marker for purification and selection purposes. The use of marker genes however inevitably results in a vector that contains unwanted genetic information of no therapeutic value.

METHODOLOGY/PRINCIPAL FINDINGS

Here we describe an improved strategy that allows for the creation of marker-free recombinant poxviruses of any species. The Selectable and Excisable Marker (SEM) system incorporates a unique fusion marker gene for the efficient selection of poxvirus recombinants and the Cre/loxP system to facilitate the subsequent removal of the marker. We have defined and characterized this new methodological tool by insertion of a foreign gene into vaccinia virus, with the subsequent removal of the selectable marker. We then analyzed the importance of loxP orientation during Cre recombination, and show that the SEM system can be used to introduce site-specific deletions or inversions into the viral genome. Finally, we demonstrate that the SEM strategy is amenable to other poxviruses, as demonstrated here with the creation of an ectromelia virus recombinant lacking the EVM002 gene.

CONCLUSION/SIGNIFICANCE

The system described here thus provides a faster, simpler and more efficient means to create clinic-ready recombinant poxviruses for therapeutic gene therapy applications.