Genetically engineered poxviruses for recombinant gene expression, vaccination, and safety

Production of recombinant proteins by vaccinia virus in a microcarrier based mammalian cell perfusion bioreactor

The HeLa cell-vaccinia virus expression system was evaluated for the production of recombinant proteins (enhanced green fluorescent protein (EGFP) and HIV envelope coat protein, gp120) using microcarriers in 1.5 L perfused bioreactor cultures. Perfusion was achieved by use of an alternating tangential flow device (ATF), increasing the length of the exponential phase by 50 h compared to batch culture and increasing the maximum cell density from 1.5x10(6) to 4.4x10(6) cell/mL. A seed train expansion method using cells harvested from microcarrier culture and reseeding onto fresh carriers was developed. EGFP was first used as a model protein to study process parameters affecting protein yield, specifically dissolved oxygen (DO) and temperature during the production phase. The highest level of EGFP, 12+/-1.5 microg/10(6) infected cells, was obtained at 50% DO and 31 degrees C. These setpoints were then used to produce glycoprotein, gp120, which was purified and deglycosylated, revealing a significant amount of N-linked glycosylation. Also, biological activity was assayed, resulting in an ID50 of 3.1 microg/mL, which is comparable to previous reports.

Intratumoral Administration of a Recombinant Canarypox Virus Expressing Interleukin 12 in Patients with Metastatic Melanoma

The aim of this study was to evaluate the tolerability and activity of intratumoral administered human interleukin 12 encoded by a vector derived from the canarypox virus (ALVAC-IL-12). Nine patients with surgically incurable metastatic melanoma who had subcutaneous nodules available for injection were enrolled. ALVAC-IL-12 was administered by intratumoral injection on days 1, 4, 8, and 11. Tumor nodules greater than 2 cm in diameter were injected with 2 x 10(6) median tissue culture infectious doses (TCID(50)), and smaller tumors were injected with 1 x 10(6) TCID(50). The total dose per patient per time point ranged from 1 x 10(6) to 4 x 10(6) TCID(50). Toxicity was mild to moderate and consisted of inflammatory reactions at the injection site and fever associated with chills, myalgia, and fatigue. No dose-limiting toxicities occurred. Increases in IL-12 mRNA, and also increases in interferon gamma mRNA, were observed in ALVAC-IL-12-injected tumors compared with saline-injected control tumors in four of the nine patients. ALVAC-IL-12-injected tumors were also characterized by T cell infiltration. Three patients demonstrated increases in serum IL-12 and in interferon gamma levels. All patients developed neutralizing IgG antibody to the canarypox vector. One patient manifested a complete response of injected subcutaneous metastases and uninjected in-transit metastases. The intratumoral injection of ALVAC-IL-12 at these dose levels and according to this schedule was well tolerated and resulted in measurable biologic response in patients with metastatic melanoma.

Infectious cDNA clones of porcine reproductive and respiratory syndrome virus and their potential as vaccine vectors

Full-length infectious cDNA clones have recently become available for both European and North American genotypes of porcine reproductive and respiratory syndrome virus (PRRSV), and it is now possible to alter the PRRSV genome and create genetically defined mutant viruses. Among many possible applications of the PRRSV infectious cDNA clones, development of genetically modified vaccines is of particular interest. Using infectious clones, the PRRSV genome has been manipulated by changing individual amino acids, deleting coding regions, inserting foreign sequences, and generating arterivirus chimeras. The limited available data suggest that all structural proteins of PRRSV are essential for replication of the virus, and that PRRSV infectivity is relatively intolerant of subtle changes within the structural proteins. The major tasks in PRRSV research are to identify virulence factors and pathogenic mechanisms, and to understand the structure–function relationships of individual viral proteins. Utilizing these infectious clones as tools, a new generation of safe and efficacious PRRS vaccines may be constructed.

Immunization with modified vaccinia virus Ankara-based recombinant vaccine against severe acute respiratory syndrome is associated with enhanced hepatitis in ferrets

Severe acute respiratory syndrome (SARS) caused by a newly identified coronavirus (SARS-CoV) is a serious emerging human infectious disease. In this report, we immunized ferrets (Mustela putorius furo) with recombinant modified vaccinia virus Ankara (rMVA) expressing the SARS-CoV spike (S) protein. Immunized ferrets developed a more rapid and vigorous neutralizing antibody response than control animals after challenge with SARS-CoV; however, they also exhibited strong inflammatory responses in liver tissue. Inflammation in control animals exposed to SARS-CoV was relatively mild. Thus, our data suggest that vaccination with rMVA expressing SARS-CoV S protein is associated with enhanced hepatitis.

Virosomes as new carrier system for cancer vaccines

HER-2/neu, a tumor-associated antigen (TAAg), plays a critical role in oncogenesis of various tumor types, and its selective overexpression by malignant tumor cells makes it an ideal target for immunotherapy. A prerequisite for clinical vaccines is the construction of safe and highly immunogenic reagents able to generate efficient immune responses against TAAg. Previous protein vaccines, consisting of the extracellular domain of HER-2/neu (pNeuECD), were shown to elicit an immune response that did not provide protection from transplantable tumors expressing HER-2/neu. Here we showed that virosomes, which consist of reconstituted viral envelopes without viral genetic material, can act as a carrier and an adjuvant for a truncated protein pNeuECD. Mice vaccinated with pNeuECD either encapsulated in virosomes or bound to the virosomal membrane (Vir-pNeuECD), generated rNeu-specific humoral and cytotoxic immune responses. In addition, Vir-p(NeuECD) induced significant tumor rejection and additionally did not lead to delayed tumor formation when compared with free pNeuECD in complete Freund's adjuvant. There was no difference between the virosomal constructs. Taken together these results suggest that virosomes, as clinically approved safe vaccines, can be used to elicit both humoral and cell-mediated responses against TAAg and induce tumor rejection. Our model is providing important preclinical data to design human vaccination trials for patients with tumors overexpressing HER-2/neu, either as a primary vaccination or as a boost in combination with other vaccines in a context of an adjuvant treatment plan.

Neutralization assay using a modified vaccinia virus Ankara vector expressing the green fluorescent protein is a high-throughput method to monitor the humoral immune response against vaccinia virus

Vaccination against smallpox is again considered in order to face a possible bioterrorist threat, but the nature and the level of the immune response needed to protect a person from smallpox after vaccination are not totally understood. Therefore, simple, rapid, and accurate assays to evaluate the immune response to vaccinia virus need to be developed. Neutralization assays are usually considered good predictors of vaccine efficacy and more informative with regard to protection than binding assays. Currently, the presence of neutralizing antibodies to vaccinia virus is measured using a plaque reduction neutralization test, but this method is time-consuming and labor-intensive and has a subjective readout. Here, we describe an innovative neutralization assay based on a modified vaccinia virus Ankara (MVA) vector expressing the green fluorescent protein (MVA-gfp). This MVA-gfp neutralization assay is rapid and sensitive and has a high-throughput potential. Thus, it is suitable to monitor the immune response and eventually the efficacy of a large campaign of vaccination against smallpox and to study the vector-specific immune response in clinical trials that use genetically engineered vaccinia viruses. Most importantly, application of the highly attenuated MVA eliminates the safety concern in using the replication-competent vaccinia virus in the standard clinical laboratory.

Local delivery of recombinant vaccinia virus expressing secondary lymphoid chemokine (SLC) results in a CD4 T-cell dependent antitumor response

Secondary lymphoid chemokine (SLC) attracts mature dendritic cells (DCs) and naïve T cells. Co-localization of these cells within local tumor environments may enhance the induction of tumor-specific T cells. However, the presence of danger signals or other DC maturation signals are required to optimize T-cell priming. We hypothesized that expression of SLC in vaccinia virus would provide local chemokine delivery and adjuvant factors. A recombinant vaccinia virus expressing murine SLC (rVmSLC) was constructed and characterized. SLC expression was confirmed by Western blot analysis and functional activity was determined by in vitro chemotaxis assay. Supernatants from rVmSLC-infected cells attracted CD4 T cells, and also induced the migration of CD8 T cells and DCs. Although poxviruses are known to express several chemokine-binding proteins, systemic injection of rVmSLC was well tolerated in mice up to a dose of 1 x 10(7) pfu and did not significantly alter vaccinia-specific T-cell immunity. Local injection of rVmSLC into established tumors derived from the murine colon cancer line, CT26, resulted in enhanced infiltration of CD4 T cells, which correlated with inhibition of tumor growth. The central role of CD4 T cells was further demonstrated by loss of anti-tumor activity in CD4 T-cell depleted mice. Intratumoral delivery of SLC using a poxviral vaccine extends the use of SLC in anti-tumor therapies and may present an effective alternative for improving the immunotherapy of cancer alone or in combination with other anti-tumor agents for clinical therapy.

Boosting T cell-mediated immunity to tyrosinase by vaccinia virus-transduced, CD34(+)-derived dendritic cell vaccination: a phase I trial in metastatic melanoma

PURPOSE

Six American Joint Committee on Cancer stage IV melanoma patients were enrolled into a Phase I study of vaccination with autologous CD34(+)-derived dendritic cells transduced with a modified vaccinia Ankara virus encoding human tyrosinase gene (MVA-hTyr).

EXPERIMENTAL DESIGN

Patients received a first intravenous injection of 1 x 10(8) MVA-hTyr-transduced dendritic cells, followed by three s.c. injections at a 14-day interval.

RESULTS

Treatment was well tolerated, except for low-grade fever (three of six patients), mild erythema at injection site (five of six), and vitiligo (two of six). A partial response, involving shrinkage of an s.c. nodule, later surgically removed, was observed in 1 patient, who then remained disease-free (>850 days). By human lymphocyte antigen tetramer analysis, significant and often long-lasting increases in frequency of T cells directed to tyrosinase(368-376) but not to gp100(209-217) were documented in periphery of 4 of 5 HLA-A*0201+ patients, a few days after vaccine administration. In addition, maturation phenotype of tyrosinase-specific T cell shifted toward the T effector memory/T terminally differentiate stages (CCR7(-)CD45RA(-/+)) in synchrony with the T-cell frequency peaks. By enzyme-linked immunospot in peripheral blood of five HLA-A*0201+ patients, we found that the vaccine could induce interferon gamma-releasing effector cells directed to HLA-A*0201/tyrosinase(368-376) and to vaccinia virus HLA-A*0201/H3L(184-192) epitopes. Moreover, an interferon gamma response after vaccination was elicited even against the HLA-DRB1-1501/tyrosinase(386-406) epitope in one out of two HLA-A* DRB1-01501+ patients.

CONCLUSIONS

These results indicate that vaccination with MVA-hTyr-transduced dendritic cells is well tolerated, can possibly produce clinical responses, and activates tyrosinase- and vaccinia virus-specific T cells in vivo. These data suggest a broad utility of the MVA vector for targeting tumor-associated antigens to dendritic cells for tumor immunotherapy.

Cellular immune responses induced in cattle by heterologous prime-boost vaccination using recombinant viruses and bacille Calmette-Guérin

The development of novel vaccine strategies to replace or supplement bacille Calmette-Guérin (BCG) is urgently required. Here we study, in cattle, the use of heterologous prime-boost strategies based on vaccination with BCG and the mycobacterial mycolyl transferase Ag85A (Rv3804c) expressed either in recombinant modified vaccinia virus Ankara (MVA85A) or attenuated fowlpox strain FP9 (FP85A). Five different vaccination schedules were tested in the first experiment: MVA85A followed by BCG (group 1); BCG followed by MVA85A (group 2); BCG followed by FP85A and then MVA85A (group 3); MVA85A followed by MVA85A and then FP85A (group 4); and FP85A followed by FP85A and then MVA85A (group 5). Vaccine-induced levels of cellular immunity were assessed by determining interferon-gamma (IFN-gamma) responses in vitro. Prime-boost protocols, using recombinant MVA and BCG in combination (groups 1-3), resulted in significantly higher frequencies of Ag85-specific IFN-gamma-secreting cells than the two viral vectors used in combination (P=0.0055), or BCG used alone (groups 2 and 3, P=0.04). The T-cell repertoires of the calves in all five groups were significantly broader following heterologous booster immunizations than after the primary immunization. In a second experiment, the effects of BCG\MVA85A heterologous prime-boost vaccination were compared with BCG\BCG homologous revaccination. The results suggested a higher Ag85A-specific response with a wider T-cell repertoire in the MVA85A-boosted calves than in the BCG\BCG-vaccinated calves. In conclusion therefore, the present report demonstrates the effectiveness of heterologous prime-boost strategies based on recombinant MVA and BCG to induce strong cellular immune responses in cattle and prioritise such vaccination strategies for rapid assessment of protective efficacy in this natural target species of tuberculosis.

A rapid and efficient method to express target genes in mammalian cells by baculovirus

AIM: To investigate the modification of baculovirus vector and the feasibility of delivering exogenous genes into mammalian cells with the culture supernatant of Spodoptera frugiperta (Sf9) cells infected by recombinant baculoviruses.

METHODS: Two recombinant baculoviruses (BacV-CMV-EGFPA, BacV-CMV-EGFPB) containing CMV-EGFP expression cassette were constructed. HepG2 cells were directly incubated with the culture supernatant of Sf9 cells infected by recombinant baculoviruses, and reporter gene transfer and expression efficiencies were analyzed by flow cytometry (FCM). The optimal transduction conditions were investigated by FCM assay in HepG2 cells. Gene-transfer and expression efficiencies in HepG2 or CV1 cells by baculovirus vectors were compared with lipofectAMINE, recombinant retrovirus and vaccinia virus expression systems. Twenty different mammalian cell lines were used to investigate the feasibility of delivering exogenous genes into different mammalian cells with the culture supernatant of infected Sf9 cells.

RESULTS: CMV promoter could directly express reporter genes in Sf9 cells with a relatively low efficiency. Target cells incubated with the 1:1 diluted culture supernatant (moi = 50) for 12 h at 37 °C could achieve the highest transduction and expression efficiencies with least impairment to cell viability. Under similar conditions the baculovirus vector could achieve the highest gene-transfer and expression efficiency than lipofectAMINE, recombinant retrovirus and vaccinia virus expression systems. Most mammalian cell lines could be transduced with recombinant baculovirus. In primate adherent culture cells the recombinant baculovirus could arrive the highest infection and expression efficiencies, but it was not very satisfactory in the cell lines from mice and suspended culture cells.

CONCLUSION: Mammalian cells incubated with the culture supernatant of infected Sf9 cells could serve as a very convenient way for rapid and efficient expression of foreign genes in mammalian cells, but it might be more suitable for primate adherent culture cells.

Multiprotein HIV type 1 clade B DNA and MVA vaccines: construction, expression, and immunogenicity in rodents of the MVA component

Recombinant modified vaccinia virus Ankara (MVA) expressing SIV or SHIV Gag-Pol and Env, alone or in conjunction with a related DNA vaccine, effectively controls immunodeficiency virus infections in nonhuman primates. Here we describe the construction, characterization, and immunogenicity of MVA/HIV 48, a candidate HIV-1 clade B Gag-Pol-Env vaccine. A novel transfer vector was designed to allow the incorporation of HIV genes regulated by vaccinia virus promoters together with a reporter gene into a single site in the MVA genome and to automatically delete the reporter after the initial isolation of the recombinant MVA. MVA/HIV 48 contains chimeric HIV-1 HXB-2/BH10 gag-pol sequences, a deletion of integrase, inactivating point mutations in reverse transcriptase, and HIV-1 ADA env sequences with a truncation of most of the cytoplasmic domain to enhance expression on the plasma membrane. Cells infected with MVA/HIV 48 expressed HIV proteins, which were processed to the expected size. The Env was inserted into the plasma membrane and was functional in a CCR5 coreceptor-dependent cell fusion assay. Moreover, virus-like particles were released into the medium and budding particles containing Env were visualized by immunoelectron microscopy. Rodents that were immunized with MVA/HIV 48 produced antibodies, which neutralized a heterologous HIV-MN strain, and Gag-specific CD8 T cells. In the accompanying paper, we show that MVA/HIV 48 provided efficient boosting of an HIV DNA vaccine.

Severe acute respiratory syndrome coronavirus spike protein expressed by attenuated vaccinia virus protectively immunizes mice

The spike protein (S), a membrane component of severe acute respiratory syndrome coronavirus (SARS-CoV) is anticipated to be an important component of candidate vaccines. We constructed recombinant forms of the highly attenuated modified vaccinia virus Ankara (MVA) containing the gene encoding full-length SARS-CoV S with and without a C-terminal epitope tag called MVA/S-HA and MVA/S, respectively. Cells infected with MVA/Sor MVA/S-HA synthesized a 200-kDa protein, which was recognized by antibody raised against a synthetic peptide of SARS-CoV S or the epitope tag in Western blot analyses. Further studies indicated that S was N-glycosylated and migrated in SDS polyacrylamide gels with an apparent mass of approximately 160 kDa after treatment with peptide N-glycosidase F. The acquisition of resistance to endoglycosidase H indicated trafficking of S to the medial Golgi compartment, and confocal microscopy showed that S was transported to the cell surface. Intranasal or intramuscular inoculations of BALB/c mice with MVA/S produced serum antibodies that recognized the SARS S in ELISA and neutralized SARS-CoV in vitro. Moreover, MVA/S administered by either route elicited protective immunity, as shown by reduced titers of SARS-CoV in the upper and lower respiratory tracts of mice after challenge. Passive transfer of serum from mice immunized with MVA/S to naïve mice also reduced the replication of SARS-CoV in the respiratory tract after challenge, demonstrating a role for antibody to S in protection. The attenuated nature of MVA and the ability of MVA/S to induce neutralizing antibody that protects mice support further development of this candidate vaccine.

Attenuated poxviruses generate clinically relevant frequencies of CMV-specific T cells

Immunotherapeutic approaches to limit cytomegalovirus (CMV) morbidity and mortality after hematopoietic stem cell transplants (HSCTs) are currently under investigation as alternatives to antiviral drugs. In this context, we have inserted full-length and ubiquitin-modified CMV phosphoprotein 65 (pp65), phosphoprotein 150 (pp150), and immediate early protein 1 (IE1) immunodominant antigens into the virulent Western Reserve strain of vaccinia virus (VV) and the highly attenuated strain, modified vaccinia Ankara (MVA). Recombinant (r) VV or rMVA stimulated vigorous expansion of CMV-specific CD8+ T cells in CMV-positive donor peripheral blood mononuclear cells (PBMCs), which showed minimal alloreactivity and high levels of HLA tetramer binding, cytokine production, and cytotoxicity. Ubiquitinated antigens had a profound effect when expressed in VV. Single antigen rMVA expressing pp65 or IE1, either ubiquitin-modified or native, stimulated both cytotoxic T lymphocyte (CTL) populations to be expanded up to 500-fold in a 60-mL blood draw from the same donor. This result demonstrates the clinical feasibility of simultaneously amplifying multiple CMV-CTL populations. Transgenic HLA A2.1 (HHD II) mice, immunized with the same rMVA as used with human PBMCs, produced a robust cytotoxic response to both CMV pp65 and IE1. The specificity of the vigorous immunologic response to rMVA, both in vitro and in vivo, makes them candidates for clinical evaluation in the context of adoptive immunotherapy for hematopoietic stem cell transplant (HSCT) recipients or donor vaccination.

Induction of potent humoral and cell-mediated immune responses by attenuated vaccinia virus vectors with deleted serpin genes

Vaccinia virus (VV) has been effectively utilized as a live vaccine against smallpox as well as a vector for vaccine development and immunotherapy. Increasingly there is a need for a new generation of highly attenuated and efficacious VV vaccines, especially in light of the AIDS pandemic and the threat of global bioterrorism. We therefore developed recombinant VV (rVV) vaccines that are significantly attenuated and yet elicit potent humoral and cell-mediated immune responses. B13R (SPI-2) and B22R (SPI-1) are two VV immunomodulating genes with sequence homology to serine protease inhibitors (serpins) that possess antiapoptotic and anti-inflammatory properties. We constructed and characterized rVVs that have the B13R or B22R gene insertionally inactivated (vDeltaB13R and vDeltaB22R) and coexpress the vesicular stomatitis virus glycoprotein (v50DeltaB13R and v50DeltaB22R). Virulence studies with immunocompromised BALB/cBy nude mice indicated that B13R or B22R gene deletion decreases viral replication and significantly extends time of survival. Viral pathogenesis studies in immunocompetent CB6F(1) mice further demonstrated that B13R or B22R gene inactivation diminishes VV virulence, as measured by decreased levels of weight loss and limited viral spread. Finally, rVVs with B13R and B22R deleted elicited potent humoral, T-helper, and cytotoxic T-cell immune responses, revealing that the observed attenuation did not reduce immunogenicity. Therefore, inactivation of immunomodulating genes such as B13R or B22R represents a general method for enhancing the safety of rVV vaccines while maintaining a high level of immunogenicity. Such rVVs could serve as effective vectors for vaccine development and immunotherapy.

Use of recombinant modified vaccinia Ankara viral vectors for equine influenza vaccination

Recombinant modified vaccinia Ankara (MVA) vectors expressing equine influenza virus genes were constructed and evaluated for use in equine vaccination. Two strains of recombinant MVA, expressing either hemagglutinin (HA) or nucleoprotein (NP) genes were constructed. Each influenza virus gene was cloned from A/equine/Kentucky/1/81 (Eq/Ky) into an MVA construction plasmid, and was introduced to the deletion III locus of the wild type MVA genome by homologous recombination. Recombinant viruses were plaque purified, and antigen expression was confirmed by immunostaining. Two ponies were primed by vaccination with 50 microg HA-DNA and two ponies were vaccinated with 50 microg NP-DNA using the PowderJect XR research device. Six and 10 weeks later, ponies were immunized with 2 x 10(9) infectious units of recombinant MVA encoding the homologous influenza antigen, equally divided between intramuscular and intradermal sites in the neck. A marked rise in influenza virus-specific IgGa and IgGb serum antibody titers was detected following administration of MVA boosters with both HA and NP antigens. Influenza virus-specific lymphoproliferative responses and IFN-gamma mRNA production were also strongly elicited by both antigens. This study demonstrates the facility with which recombinant MVA viruses expressing defined pathogen genes can be constructed, and provides preliminary evidence of the immunogenicity and safety of these vectors in the horse.

T Cells from a High Proportion of Apparently Naive Cattle Can Be Activated by Modified Vaccinia Virus Ankara (MVA)

Modified vaccinia virus Ankara (MVA) was used as a vector to express genes from bovine respiratory syncytial virus (BRSV). Using these recombinant viruses as recall antigens for cells from BRSV-immuned cattle proved to be problematic because non-recombinant MVA itself frequently stimulated high levels of T lymphocyte activation. This phenomenon was observed in a high percentage of cattle from multiple herds. Gamma delta TCR(+) T cells were more sensitive to activation by MVA than other classes of T cells. A serological assay for MVA neutralization detected low, fluctuating titers of serum virus neutralizing (SVN) activity toward MVA in some cattle, but these were lower titers than those observed in cattle that underwent MVA vaccination. T cell reactivity in non-vaccinated cattle did not correlate significantly (p > 0.05) with SVN activity, undermining the notion that any adaptive immune response was responsible for the observed T cell sensitivity. More probable explanations are that MVA has mitogenic or superantigenic properties, or that the virus induces gammadelta TCR(+) T cell activation through interactions with innate pattern recognition receptors.

Replication and expression of a swinepox virus vector delivering feline leukemia virus Gag and Env to cell lines of swine and feline origin

The host range of swinepox virus (SPV) is restricted to swine, although SPV has been shown to infect mammalian, non-swine cells, without recovery of infectious virus. SPV is a reasonable candidate for development as a non-productively replicating viral vector for use in non-swine, mammalian species, such as the cat. A novel SPV gene deletion (SPV 043) was created and found to be non-attenuating. This deletion was utilized to generate a stable recombinant virus expressing the Gag-Pro and Env proteins of feline leukemia virus (FeLV). Expression and replication of this vector was studied in embryonic swine kidney cells (ESK-4), and two feline cell lines, Crandell feline kidney cells (CRFK) and feline skin fibroblasts (FSF). Our results showed that feline cells were susceptible to infection by SPV and supported expression of foreign genes driven by synthetic poxvirus promoters, however, SPV viral DNA was not replicated in feline cells and infectious virus was not recovered. In addition, FeLV Gag virus-like particles were produced from both ESK-4 and CRFK cells and foreign antigens were incorporated into infectious SPV intracellular mature virions (IMV). These results suggest that SPV may have potential as a safe vaccine delivery vector for cats.

The genomic sequence of ectromelia virus, the causative agent of mousepox

Ectromelia virus is the causative agent of mousepox, an acute exanthematous disease of mouse colonies in Europe, Japan, China, and the U.S. The Moscow, Hampstead, and NIH79 strains are the most thoroughly studied with the Moscow strain being the most infectious and virulent for the mouse. In the late 1940s mousepox was proposed as a model for the study of the pathogenesis of smallpox and generalized vaccinia in humans. Studies in the last five decades from a succession of investigators have resulted in a detailed description of the virologic and pathologic disease course in genetically susceptible and resistant inbred and out-bred mice. We report the DNA sequence of the left-hand end, the predicted right-hand terminal repeat, and central regions of the genome of the Moscow strain of ectromelia virus (approximately 177,500 bp), which together with the previously sequenced right-hand end, yields a genome of 209,771 bp. We identified 175 potential genes specifying proteins of between 53 and 1924 amino acids, and 29 regions containing sequences related to genes predicted in other poxviruses, but unlikely to encode for functional proteins in ectromelia virus. The translated protein sequences were compared with the protein database for structure/function relationships, and these analyses were used to investigate poxvirus evolution and to attempt to explain at the cellular and molecular level the well-characterized features of the ectromelia virus natural life cycle.

Local immunotherapy of spontaneous feline fibrosarcomas using recombinant poxviruses expressing interleukin 2 (IL2)

We tested the canarypox virus vector ALVAC and the genetically attenuated vaccinia virus vector NYVAC as vehicles for achieving local immunomodulation in domestic animals bearing spontaneous tumours. Following intratumoral administration of ALVAC-, or NYVAC-luciferase in dogs with melanoma, it was demonstrated that viral recombinants remained localized along the needle track, with no virus detectable in the periphery of the tumour. Given these distribution characteristics and their well-documented safety profile, ALVAC- or NYVAC-based recombinants expressing feline or human IL2, respectively, were administered to domestic cats, in order to prevent the recurrence of spontaneous fibrosarcomas. In the absence of immunotherapy, tumour recurrence was observed in 61% of animals within a 12-month follow-up period after treatment with surgery and iridium-based radiotherapy. In contrast, only 39 and 28% of cats receiving either NYVAC-human IL2 or ALVAC-feline IL2, respectively, exhibited tumour recurrences. Based on such results, and in the context of ongoing clinical studies conducted in humans, we discuss the utilization of ALVAC- or NYVAC-based recombinants as viable therapeutic modalities for local immunotherapy or therapeutic vaccination against cancer, both in humans and companion animals.

Immunoprophylaxis to Prevent Mother-to-Child Transmission of HIV-1

Antiretroviral therapy can profoundly reduce the risk of mother-to-child transmission (MTCT) of HIV, but the drugs have a relatively short half-life and should thus be administered throughout breast-feeding to optimally prevent postnatal infection of the infant. The potential toxicities and the development of resistance may limit the long-term efficacy of antiretroviral prophylaxis, and a safe and effective active/passive immunoprophylaxis regimen, begun at birth, and potentially overlapping with interpartum or neonatal chemoprophylaxis, would pose an attractive alternative. This review draws on data presented at the Ghent Workshop on prevention of breast milk transmission and on selected issues from a workshop specifically relating to immunoprophylaxis held in Seattle in October 2002. This purpose of this review is to address the scientific rationale for the development of passive (antibody) and active (vaccine) immunization strategies for prevention of MTCT. Data regarding currently or imminently available passive and active immunoprophylaxis products are reviewed for their potential use in neonatal trials within the coming 1-2 years.

Immunogenicity of multiple gene and clade human immunodeficiency virus type 1 DNA vaccines

The ability to elicit an immune response to a spectrum of human immunodeficiency virus type 1 (HIV-1) gene products from divergent strains is a desirable feature of an AIDS vaccine. In this study, we examined combinations of plasmids expressing multiple HIV-1 genes from different clades for their ability to elicit humoral and cellular immune responses in mice. Immunization with a modified Env, gp145DeltaCFI, in combination with a Gag-Pol-Nef fusion protein plasmid elicited similar CD4(+) and CD8(+) cellular responses to immunization with either vector alone. Further, when mice were immunized with a mixture of Env from three clades, A, B, and C, together with Gag-Pol-Nef, the overall potency and balance of CD4(+)- and CD8(+)-T-cell responses to all viral antigens were similar, with only minor differences noted. In addition, plasmid mixtures elicited antibody responses comparable to those from individual inoculations. These findings suggest that a multigene and multiclade vaccine, including components from A, B, and C Env and Gag-Pol-Nef, can broaden antiviral immune responses without immune interference. Such combinations of immunogens may help to address concerns about viral genetic diversity for a prospective HIV-1 vaccine.

Vaccinia virus recombinant expressing gene of tick-borne encephalitis virus non-structural NS1 protein elicits protective activity in mice

A vaccinia virus recombinant containing the non-structural tick-borne encephalitis virus (TBEV) NS1 gene was developed. The recombinant expressed native dimeric form of the NS1 protein in infected cells and protected mice against lethal infection with TBEV.

Local delivery of poxvirus vaccines for melanoma

The development of vaccines for melanoma has been accelerated by the identification of melanoma-associated antigens, a better understanding of basic immunologic principles, and the ability to construct complex vectors for immunization. The location and context in which T-cell priming occurs significantly influences the type and magnitude of immune response. Furthermore, there is a delicate balance between the generation of tumor-specific immunity and the emergence of tumor escape variants. We have focused on the direct intra-tumoral delivery of poxvirus vaccines expressing costimulatory molecules as a strategy for overcoming local immunosuppression in the treatment of established melanoma. Poxviruses provide potent danger signals and, in the presence of costimulation, local administration provides a mechanism to prime tumor-specific T-cell responses. The clinical application of this approach will likely depend on the ability to induce systemic anti-tumor immunity following local injection and we are evaluating this in current clinical trials. These studies may have important implications for the design of vaccine strategies for melanoma and other tumors.

Poxviruses as vaccine vectors

The discovery of Jenner in 1798 founded the science of immunology and eventually led to smallpox eradication from the earth in 1980 after a world-wide vaccination campaign with vaccinia virus (another poxvirus) and paradoxically, despite the eradication of smallpox, there has been an explosion of interest in vaccinia virus in the eighties. This interest has stemmed in part from the application of molecular genetics to clone and express foreign genes from recombinant vaccinia viruses. Vaccinia is also gaining renewed interest due to bioterrorism. These recombinant viruses have multiple applications in research and vaccinology and led to the development of vectored vaccines, such as the recombinant vaccinia rabies vaccine used to eliminate rabies in Western Europe and, more recently, in the United States. Secondly, alternative poxvirus vectors, such as avipox viruses, were proved to be even safer and efficacious non-replicating vectors (suiciole vectors) when used in non-avian species.

The Orf virus E3L homologue is able to complement deletion of the vaccinia virus E3L gene in vitro but not in vivo

Orf virus (OV), the prototypic parapoxvirus, is resistant to the effects of interferon (IFN) and this function of OV has been mapped to the OV20.0L gene. The protein product of this gene shares 31% amino acid identity to the E3L-encoded protein of vaccinia virus (VV) that is required for the broad host range and IFN-resistant phenotype of VV in cells in culture and for virulence of the virus in vivo. In this study we investigated whether the distantly related OV E3L homologue could complement the deletion of E3L in VV. The recombinant VV (VV/ORF-E3L) expressing the OV E3L homologue in place of VV E3L was indistinguishable from wt VV in its cell-culture phenotype. But VV/ORF-E3L was over a 1000-fold less pathogenic than wt VV (LD(50) > 5 x 10(6) PFU, compared to LD(50) of wtVV = 4 x 10(3) PFU) following intranasal infection of mice. While wt VV spread to the lungs and brain and replicated to high titers in the brain of infected mice, VV/ORF-E3L could not be detected in the lungs or brain following intranasal infection. VV/ORF-E3L was at least 100,000-fold less pathogenic than wt VV on intracranial injection. Domain swap experiments demonstrate that the difference in pathogenesis maps to the C-terminal domain of these proteins. This domain has been shown to be required for the dsRNA binding function of the VV E3L.

Novel recombinant parapoxvirus vectors induce protective humoral and cellular immunity against lethal herpesvirus challenge infection in mice

Orf virus (ORFV; Parapoxvirus ovis) was used to develop a novel vector system for the generation of effective and safe live vaccines. Based on the attenuated ORFV strain D1701-V, recombinants were produced that express the glycoproteins gC (D1701-VrVgC) or gD (D1701-VrVgD) of the alphaherpesvirus of swine, pseudorabies virus (PRV). Expression of gC and gD was also demonstrated on the surface of recombinant virus-infected murine cells that do not produce infectious ORFV. Single or combined immunization with the ORFV recombinants protected different mouse strains of a host species nonpermissive for ORFV against a fulminant, lethal PRV challenge infection equal to immunization with PRV live vaccine. Most notably, even a single immunization with D1701-VrVgC was protective, whereas two applications of D1701-VrVgD were required for immune protection. The higher protective capacity of D1701-VrVgC correlated with the induction of a strong specific humoral immune response. This suggestion was supported by transfer experiments using sera from recombinant-immunized mice, which resulted in partial gC but not gD antibody-mediated protection of the naïve recipients. Remarkably, immunization of different immune-deficient mice demonstrated that the application of the PRV gC-expressing recombinant controlled the challenge infection in the absence of either CD4(+) or CD8(+) T cells, B cells, or an intact perforin pathway. In contrast, D1701-VrVgD-immunized mice lacking CD4(+) T cells exhibited reduced protection, whereas animals lacking CD8(+) T cells, B cells, or perforin resisted the challenge infection. The present study demonstrates the potential of these new vector vaccines to efficiently prime both protective humoral and cell-mediated immune mechanisms in a host species nonpermissive for the vector virus.

Characterisation of the substrate specificity of homogeneous vaccinia virus uracil-DNA glycosylase

The decision to stop smallpox vaccination and the loss of specific immunity in a large proportion of the population could jeopardise world health due to the possibility of a natural or provoked re-emergence of smallpox. Therefore, it is mandatory to improve the current capability to prevent or treat such infections. The DNA repair protein uracil-DNA glycosylase (UNG) is one of the viral enzymes important for poxvirus pathogenesis. Consequently, the inhibition of UNG could be a rational strategy for the treatment of infections with poxviruses. In order to develop inhibitor assays for UNG, as a first step, we have characterised the recombinant vaccinia virus UNG (vUNG) and compared it with the human nuclear form (hUNG2) and catalytic fragment (hUNG) UNG. In contrast to hUNG2, vUNG is strongly inhibited in the presence of 7.5 mM MgCl(2). We have shown that highly purified vUNG is not inhibited by a specific uracil-DNA glycosylase inhibitor. Interestingly, both viral and human enzymes preferentially excise uracil when it is opposite to cytosine. The present study provides the basis for the design of specific inhibitors for vUNG.

Development of a recombinant vaccine against Japanese encephalitis

Japanese encephalitis (JE) is the major form of viral encephalitis in much of the South-East Asia, India, and China. The disease is caused by a mosquito-borne virus known as Japanese encephalitis virus (JEV). The virus spreads in the form of epidemics, although several endemic areas for JEV activity are known. In recent years, JEV has spread to newer geographic locations such as Australia and Pakistan, and thus has become an important emerging virus infection in these areas. A mouse brain-derived, formalin-inactivated vaccine is available for immunization against JE. Because the formalin-inactivated JEV vaccine has limitations in terms of safety, availability, and cost, attempts are being made to develop improved vaccine using the recombinant DNA technology. This article reviews various attempts in this direction and summarizes the latest developments such as the recombinant yellow fever virus- or the plasmid DNA-based JEV vaccine.

Evaluation of lumpy skin disease virus, a capripoxvirus, as a replication-deficient vaccine vector

Lumpy skin disease virus (LSDV), a capripoxvirus with a host range limited to ruminants, was evaluated as a replication-deficient vaccine vector for use in non-ruminant hosts. By using the rabies virus glycoprotein (RG) as a model antigen, it was demonstrated that recombinant LSDV encoding the rabies glycoprotein (rLSDV-RG) was able to express RG in both permissive (ruminant) and non-permissive (non-ruminant) cells. The recombinant LSDV, however, replicated to maturity only in permissive but not in non-permissive cells. Recombinant LSDV-RG was assessed for its ability to generate immunity against RG in non-ruminant hosts (rabbits and mice). Rabbits inoculated with rLSDV-RG produced rabies virus (RV) neutralizing antibodies at levels twofold higher than those reported by the WHO to be protective. BALB/c mice immunized with rLSDV-RG elicited levels of RV-specific cellular immunity (T-cell proliferation) comparable with those of mice immunized with a commercial inactivated rabies vaccine (Verorab; Pasteur Merieux). Most importantly, mice immunized with rLSDV-RG were protected from an aggressive intracranial rabies virus challenge.

Gene therapy: theoretical and bioethical concepts

Gene therapy holds great promise. Somatic gene therapy has the potential to treat a wide range of disorders, including inherited conditions, cancers, and infectious diseases. Early progress has already been made in the treatment of a range of disorders. Ethical issues surrounding somatic gene therapy are primarily those concerned with safety. Germline gene therapy is theoretically possible but raises serious ethical concerns concerning future generations.

Adverse events occurring after smallpox vaccination

We reviewed the literature on adverse events reported to occur after smallpox vaccination. Nearly one-half of the United States population is vaccinia-naïve and may be at risk for development of serious adverse events. We describe the clinical features of postvaccinial central nervous system disease, progressive vaccinia, eczema vaccinatum, accidental implantations, "generalized vaccinia," and the common erythematous and/or urticarial rashes. In the 1960s, death occurred approximately once in every million primary vaccinations, with fatalities resulting from progressive vaccinia, postvaccinial encephalitis, and eczema vaccinatum. Death in revaccinees occurred less commonly and almost entirely from progressive vaccinia. In today's population, death rates might be higher because of the increased prevalence of immune deficiency and atopic dermatitis.

Human neutrophils utilize a Rac/Cdc42-dependent MAPK pathway to direct intracellular granule mobilization toward ingested microbial pathogens

Elevated levels of mitogen-activated protein kinase/extracellular regulatory kinase (MAPK/ERK) activity are frequently found in some cancer cells. In efforts to reduce tumor growth, attempts have been made to develop cancer therapeutic agents targeting the MAPK. Here, by use of biologic, biochemical, and gene manipulation methods in human polymorphonuclear neutrophils (PMNs), we have identified a key pathway important in normal cell function involving MAPK/ERK in PMNs for growth inhibition of Candida albicans. Contact with C albicans triggered MAPK/ERK activation in PMNs within 5 minutes, and blocking of MAPK/ERK activation, either by the pharmacologic reagent PD098059 or by dominant-negative MAPK kinase (MEK) expression via vaccinia viral delivery, suppressed antimicrobial activity. Rac and Cdc42, but not Ras or Rho, were responsible for this MAPK/ERK activation. Expression of dominant-negative Rac (N17Rac) or Cdc42 (N17Cdc42) eliminated not only C albicans- mediated ERK phosphorylation but also phagocytosis and granule migration toward the ingested microbes, whereas dominant-negative Ras (N17Ras) and Rho (N19Rho) did not. PAK1 (p21-activated kinase 1) activation is induced by C albicans, suggesting that PAK1 may also be involved in the Rac1 activation of MAPK/ERK. We conclude from these data that Rac/Cdc42-dependent activation of MAPK/ERK is a critical event in the immediate phagocytic response of PMNs to microbial challenge. Therefore, use of MAPK pharmacologic inhibitors for the treatment of cancer may result in the interruption of normal neutrophil function. A balance between therapeutic outcome and undesirable side effects must be attained to achieve successful and safe anticancer therapy.

Ultraviolet-irradiated vaccinia virus recombinants, exposing HIV-envelope on their outer membrane, induce antibodies against this antigen in rabbits

The construction and isolation of recombinants of vaccinia virus (IHD-J strain), bearing on their outer membrane a chimeric protein consisting of the cytoplasmic and transmembrane domains of vaccinia B5R protein and the external domain of HIV envelope, has been previously described by us. The present study aimed to investigate the potential use of such recombinants as a vaccine, following inactivation of their infectivity by ultraviolet (UV) irradiation. The minimal dose of UV irradiation, required for the complete inactivation of the infectivity of these recombinants, was determined. Injections of rabbits with the irradiated noninfectious recombinant viruses successfully induced specific antibodies against the HIV envelope antigen, in addition to those against the poxvirus.

Vector-based delivery of tumor-associated antigens and T-cell co-stimulatory molecules in the induction of immune responses and anti-tumor immunity

It has now been demonstrated in both experimental models and recent clinical trials that certain "self" antigens, which are functionally non-immunogenic in the host, can become immunogenic if presented to the immune system in a certain way. Here, we describe recombinant vaccines and vaccine strategies that have been developed to induce and potentiate T-cell responses of the host to such self-antigens. These strategies include: (a) the use of recombinant poxvirus vectors in which the tumor-associated antigen (TAA) is inserted as a transgene. Recombinant vaccinia vaccines and recombinant avipox (replication-defective) vaccines have been employed to break tolerance to a self-antigen; (b) the use of diversified prime and boost strategies using different vaccines; and (c) the insertion of multiple T-cell co-stimulatory molecules into recombinant poxvirus vectors, along with the TAA gene, to enhance T-cell immune responses to the TAA and induce anti-tumor immunity.

Laboratory acquired infection with recombinant vaccinia virus containing an immunomodulating construct

Handling of Vaccinia virus represents a risk for laboratory-acquired infections, especially in individuals without completed vaccination. We report the case of a Vaccinia infection in a previously vaccinated researcher working with various genetically modified strains. We could confirm the infection by electron microscopy, positive cell culture, virus-specific PCR, sequence analysis, and viral neutralization test. The isolated virus carried a functionally inactivated cytohesin-1 gene of human origin, which had been shown to impair leukocyte adhesion by interacting with the LFA/ICAM-1 axis. The immunomodulating nature of the inserted construct might thus have added to the infectivity of the virus. We emphasize on the necessity of Vaccinia vaccination in laboratory staff working in the field.

Overcoming immunity to a viral vaccine by DNA priming before vector boosting

Replication-defective adenovirus (ADV) and poxvirus vectors have shown potential as vaccines for pathogens such as Ebola or human immunodeficiency virus in nonhuman primates, but prior immunity to the viral vector in humans may limit their clinical efficacy. To overcome this limitation, the effect of prior viral exposure on immune responses to Ebola virus glycoprotein (GP), shown previously to protect against lethal hemorrhagic fever in animals, was studied. Prior exposure to ADV substantially reduced the cellular and humoral immune responses to GP expressed by ADV, while exposure to vaccinia inhibited vaccine-induced cellular but not humoral responses to GP expressed by vaccinia. This inhibition was largely overcome by priming with a DNA expression vector before boosting with the viral vector. Though heterologous viral vectors for priming and boosting can also overcome this effect, the paucity of such clinical viral vectors may limit their use. In summary, it is possible to counteract prior viral immunity by priming with a nonviral, DNA vaccine.

Construction of a novel set of transfer vectors to study vaccinia virus replication and foreign gene expression

Vaccinia virus (VV) is a useful expression vector for many laboratory applications. To date, approximately 60% ( approximately 120) of the VV genes remain uncharacterized. The thought of smallpox being used as a biological weapon has gained attention. In light of this, it is imperative that we continue to study the basic replication of VV, a poxvirus that is closely related to smallpox. A set of plasmid vectors were constructed to generate gene deletions (pZIPPY-NEO/GUS) in VV or for foreign gene expression (pBR-EXPRESS). The vectors contain the Escherichia coli neomycin resistance (neo) and beta-glucuronidase (gusA) genes as selectable markers to facilitate isolation of recombinant viruses. These are the first transfer vectors to use a neo/gusA selection system. We used these vectors to successfully generate a recombinant D9R deletion mutant of VV and to express E. coli lacZ gene. Results indicate that both vectors are highly suited for their designed purpose.

DNA vaccination with linear minimalistic (MIDGE) vectors confers protection against Leishmania major infection in mice

Immunization protocols based on priming with plasmid DNA and boosting with recombinants of vaccinia virus (rVV) encoding the same antigen offer great promise for the prevention and treatment of many parasitic and viral infections for which conventional vaccination has little or no effect. To overcome some of the potential problems associated to the use of plasmids, we have developed minimalistic, immunogenically defined, gene expression (MIDGE((R))) vectors. These linear vectors contain only the minimum sequence required for gene expression and can be chemically modified to increase the immune response. Here, we demonstrate that MIDGE vectors coding for the LACK antigen confer a highly effective protection against Leishmania infection in susceptible Balb/c mice. Protection is achieved at lower doses of vector compared to conventional plasmids. This efficacy could be greatly improved by the addition of a nuclear localization signal (NLS) peptide to the end of the MIDGE vector. In fact, immunization with two doses of NLS-modified MIDGE conferred similar or even better protection than that achieved by priming with plasmid DNA followed by boosting with rVV. These results demonstrate that MIDGE vectors are a good alternative to plasmid and rVV for immunization.

Armed therapeutic viruses: strategies and challenges to arming oncolytic viruses with therapeutic genes

Oncolytic viruses are attractive therapeutics for cancer because they selectively amplify, through replication and spread, the input dose of virus in the target tumor. To date, clinical trials have demonstrated marked safety but have not realized their theoretical efficacy potential. In this review, we consider the potential of armed therapeutic viruses, whose lytic potential is enhanced by genetically engineered therapeutic transgene expression from the virus, as potential vehicles to increase the potency of these agents. Several classes of therapeutic genes are outlined, and potential synergies and hurdles to their delivery from replicating viruses are discussed.

Quality control of biotechnology-derived vaccines: technical and regulatory considerations

Vaccines for human use have been produced for decades using classical manufacturing methods including culture of viruses and bacteria followed by various concentration-, inactivation-, detoxification-, conjugation production processes. Availability of techniques for molecular biology and for the complete chemical synthesis of genes provides prospects of genetic engineering of microorganisms so as to generate novel biotechnological/biological-derived vaccines. The potential large-scale availability of biotechnology-derived vaccines makes feasible their evaluation in the prevention and/or treatment of various infectious, chronic, degenerative and cancer human diseases. There are potential safety concerns that arise from the novel manufacturing processes and from the complex structural and biological characteristics of the products. These products have distinguishing characteristics to which consideration should be given in a well-defined quality control testing programme. The evaluation of their quality, safety, efficacy and stability necessitate complex analytical methods and appropriate physicochemical, biochemical and immunochemical methods for the analysis of the molecular entity. A flexible approach to the control of these novel products is being developed by regulatory authorities so that recommendations can be modified in the light of experience of research and development in vaccinology, production and use of biotechnology products and with the further development of new technologies.

Gene transfer in higher animals: theoretical considerations and key concepts

Gene transfer technology provides the ability to genetically manipulate the cells of higher animals. Gene transfer permits both germline and somatic alterations. Such genetic manipulation is the basis for animal transgenesis goals and gene therapy attempts. Improvements in gene transfer are required in terms of transgene design to permit gene targeting, and in terms of transfection approaches to allow improved transgene uptake efficiencies.

Generation of recombinant fowlpox virus using the non-essential F11L orthologue as insertion site and a rapid transient selection strategy

Avipoxviruses show an abortive replication phenotype in mammalian cells and are under evaluation as safe vectors for vaccination. Non-essential gene sequences located in highly conserved regions of virus genomes are considered particularly useful to integrate heterologous DNA. Fowlpox virus F11L orthologue is described in this paper as a suitable locus for insertion into fowlpox virus genome. Disruption of the F11L coding sequence by integration of an expression cassette for the Escherichia coli lacZ and guanine phosphoribosyltransferase marker genes resulted in the isolation of replication competent knockout viruses. Growth of F11L-knockout viruses in primary chicken embryo fibroblasts was unimpaired in comparison to wild type-virus. To test the generation of vector viruses, an insertion plasmid was constructed that contains F11L-specific sequences for homologous recombination, the E. coli lacZ and gpt genes as transient selectable marker, and the vaccinia virus early/late promoter P7.5 for transcriptional control of target gene expression. The coding sequence of the melanoma-associated antigen tyrosinase was chosen as model recombinant gene. Isolation of tyrosinase-recombinant viruses, which produced stably the insert, demonstrated the usefulness of the F11L-insertion site for the generation of fowlpox vectors. Rapid isolation of those recombinants was achieved by using a double selective system and linearising the vector plasmid before transfection.

Cloning the vaccinia virus genome as a bacterial artificial chromosome in Escherichia coli and recovery of infectious virus in mammalian cells

The ability to manipulate the vaccinia virus (VAC) genome, as a plasmid in bacteria, would greatly facilitate genetic studies and provide a powerful alternative method of making recombinant viruses. VAC, like other poxviruses, has a linear, double-stranded DNA genome with covalently closed hairpin ends that are resolved from transient head-to-head and tail-to-tail concatemers during replication in the cytoplasm of infected cells. Our strategy to construct a nearly 200,000-bp VAC-bacterial artificial chromosome (BAC) was based on circularization of head-to-tail concatemers of VAC DNA. Cells were infected with a recombinant VAC containing inserted sequences for plasmid replication and maintenance in Escherichia coli; DNA concatemer resolution was inhibited leading to formation and accumulation of head-to-tail concatemers, in addition to the usual head-to-head and tail-to-tail forms; the concatemers were circularized by homologous or Cre-loxP-mediated recombination; and E. coli were transformed with DNA from the infected cell lysates. Stable plasmids containing the entire VAC genome, with an intact concatemer junction sequence, were identified. Rescue of infectious VAC was consistently achieved by transfecting the VAC-BAC plasmids into mammalian cells that were infected with a helper nonreplicating fowlpox virus.

Immunogenicity and safety of defective vaccinia virus lister: comparison with modified vaccinia virus Ankara

Potent and safe vaccinia virus vectors inducing cell-mediated immunity are needed for clinical use. Replicating vaccinia viruses generally induce strong cell-mediated immunity; however, they may have severe adverse effects. As a vector for clinical use, we assessed the defective vaccinia virus system, in which deletion of an essential gene blocks viral replication, resulting in an infectious virus that does not multiply in the host. The vaccinia virus Lister/Elstree strain, used during worldwide smallpox eradication, was chosen as the parental virus. The immunogenicity and safety of the defective vaccinia virus Lister were evaluated without and with the inserted human p53 gene as a model and compared to parallel constructs based on modified vaccinia virus Ankara (MVA), the present "gold standard" of recombinant vaccinia viruses in clinical development. The defective viruses induced an efficient Th1-type immune response. Antibody and cytotoxic-T-cell responses were comparable to those induced by MVA. Safety of the defective Lister constructs could be demonstrated in vitro in cell culture as well as in vivo in immunodeficient SCID mice. Similar to MVA, the defective viruses were tolerated at doses four orders of magnitude higher than those of the wild-type Lister strain. While current nonreplicating vectors are produced mainly in primary chicken cells, defective vaccinia virus is produced in a permanent safety-tested cell line. Vaccines based on this system have the additional advantage of enhanced product safety. Therefore, a vector system was made which promises to be a valuable tool not only for immunotherapy for diseases such as cancer, human immunodeficiency virus infection, or malaria but also as a basis for a safer smallpox vaccine.

Viral oncolysis

Although the concept of using viruses as antineoplastic agents dates back nearly a century, recent advances in the fields of molecular biology, genetics, and virology have enabled investigators to engineer viruses with greater potency and tumor specificity. Further enhancements involve arming these viruses with therapeutic transgenes, and combining the traditional modalities of chemotherapy and radiation therapy with oncolytic viral therapy in hopes of reducing the chance of developing resistant tumor cell clones. Another means of augmenting the antineoplastic effect of these viruses involves modulating the immune response to minimize antiviral immunity, while at the same time maximizing antitumor immunity. A better understanding of mechanisms that viruses use to overcome cellular defenses to achieve robust replication within the cell will lead to development of oncolytic viruses with better tumor specificity and reduced toxicity. Initial clinical studies have shown that oncolytic viral therapy for metastatic disease is safe and well tolerated. In addition, using similar genetic modification strategies, these viruses have demonstrated antineoplastic effects in humans similar to those seen in preclinical animal models.