Induction of antibodies in mice by a recombinant baculovirus expressing pseudorabies virus glycoprotein B in mammalian cells

Pseudorabies Virus Glycoproteins E and B Application in Vaccine and Diagnosis Kit Development

Background: Pseudorabies virus (PRV) is a highly infectious pathogen that affects a wide range of mammals and imposes a significant economic burden on the global pig industry. The viral envelope of PRV contains several glycoproteins, including glycoprotein E (gE) and glycoprotein B (gB), which play critical roles in immune recognition, vaccine development, and diagnostic procedures. Mutations in these glycoproteins may enhance virulence, highlighting the need for updated vaccines. Method: This review examines the functions of PRV gE and gB in vaccine development and diagnostics, focusing on their roles in viral replication, immune system interaction, and pathogenicity. Additionally, we explore recent findings on the importance of gE deletion in attenuated vaccines and the potential of gB to induce immunity. Results: Glycoprotein E (gE) is crucial for the virus's axonal transport and nerve invasion, facilitating transmission to the central nervous system. Deletion of gE is a successful strategy in vaccine development, enhancing the immune response. Glycoprotein B (gB) plays a central role in viral replication and membrane fusion, aiding viral spread. Mutations in these glycoproteins may increase PRV virulence, complicating vaccine efficacy. Conclusion: With PRV glycoproteins being essential to both vaccine development and diagnostic approaches, future research should focus on enhancing these components to address emerging PRV variants. Updated vaccines and diagnostic tools are critical for combating new, more virulent strains of PRV.

A recombinant baculovirus vector vaccine (BacMCP) against the infectious spleen and kidney necrosis virus (ISKNV)

The infectious spleen and kidney necrosis virus (ISKNV) is a highly lethal virus, which has brought significant losses to aquaculture. Therefore, a new vaccine against ISKNV with high efficiency, safety and convenience must be developed. While baculoviruses are more commonly used as protein expression systems for vaccine antigen production, this paper used baculovirus technology to develop a live-vector vaccine, BacMCP, which contains the coding sequence of the major capsid protein (MCP) (GenBank accession no. AF371960) of ISKNV and is driven by a CMV promoter. Real-time PCR and immunofluorescence showed that the MCP gene was successfully delivered to and expressed in fish cells and tissues inoculated with BacMCP. Immune-related gene (IgM, TGF-β, IL-1, IL-8, TNF-α) expression was induced in BacMCP-treated groups of largemouth bass compared with control groups. Specific antibodies could be detected in the serum of BacMCP injection-vaccinated largemouth bass by ELISA. After injection or immersion vaccination with BacMCP for 21 days, largemouth bass were infected with ISKNV. The immune effect of the injected immunization on fish in different sizes was evaluated. The vaccine efficacy of injection-vaccinated bass was 100% in small bass and 85.7% in large bass. The vaccine efficacy of immersion-vaccinated small bass was 77.3%. This study suggested that BacMCP can be used as a vector-based vaccine candidate to prevent the diseases caused by ISKNV infection.

Bioengineered baculoviruses as new class of therapeutics using micro and nanotechnologies: principles, prospects and challenges

Designing a safe and efficient gene delivery system is required for success of gene therapy trials. Although a wide variety of viral, non-viral and polymeric nanoparticle based careers have been widely studied, the current gene delivery vehicles are limited by their suboptimal, non-specific therapeutic efficacy and acute immunological reactions, leading to unwanted side effects. Recently, there has been a growing interest in insect-cell-originated baculoviruses as gene delivery vehicles for diverse biomedical applications. Specifically, the emergence of diverse types of surface functionalized and bioengineered baculoviruses is posed to edge over currently available gene delivery vehicles. This is primarily because baculoviruses are comparatively non-pathogenic and non-toxic as they cannot replicate in mammalian cells and do not invoke any cytopathic effect. Moreover, emerging advanced studies in this direction have demonstrated that hybridizing the baculovirus surface with different kinds of bioactive therapeutic molecules, cell-specific targeting moieties, protective polymeric grafts and nanomaterials can significantly improve the preclinical efficacy of baculoviruses. This review presents a comprehensive overview of the recent advancements in the field of bioengineering and biotherapeutics to engineer baculovirus hybrids for tailored gene therapy, and articulates in detail the potential and challenges of these strategies for clinical realization. In addition, the article illustrates the rapid evolvement of microfluidic devices as a high throughput platform for optimizing baculovirus production and treatment conditions.

[Analysis of the application of host innate immune response to control and prevent infection]

The baculovirus Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) has been widely used not only to achieve a high level of foreign gene expression in insect cells but also for efficient gene transduction into mammalian cells without any replication. In addition to the efficient gene delivery, baculovirus has been shown to induce host innate immune responses in various mammalian cells and in mice. The baculovirus has abundant CpG motifs in the viral genome and is capable of inducing pro-inflammatory cytokines and interferons (IFNs) through Toll-like receptor (TLR)-dependent and -independent signaling pathways in a cell-type-specific manner. The cytoplasmic helicase proteins RIG-I (retinoic-acid-inducible protein I) and MDA5 (melanoma-differentiation-associated gene 5) have been identified as viral dsRNA detectors and the adaptor IPS-1 (IFN-beta promoter stimulator-1) interacts with RIG-I and MDA5 to facilitate type-I IFN production mediated interferon regulatory factor 3 (IRF3) and 7 (IRF7). These helicases and IPS-1, however, were not essential for the type-I IFN and inflammatory cytokine responses to baculovirus. The baculovirus also has a strong adjuvant activity, and recombinant baculoviruses encoding neutralization epitopes elicit protective immunity in mice. This review deals with the current status of our knowledge of the induction of host innate immune responses by baculovirus and discusses the future prospects for baculovirus vectors.

Induction of antigen-specific immune responses in mice by recombinant baculovirus expressing premembrane and envelope proteins of West Nile virus

Background

West Nile Virus (WNV) is an emerging arthropod-born flavivirus with increasing distribution worldwide that is responsible for a large proportion of viral encephalitis in humans and horses. Given that there are no effective antiviral drugs available for treatment of the disease, efforts have been directed to develop vaccines to prevent WNV infection. Recently baculovirus has emerged as a novel and attractive gene delivery vehicle for mammalian cells.

Results

In the present study, recombinant baculoviruses expressing WNV premembrane (prM) and envelope (E) proteins under the cytomegalovirus (CMV) promoter with or without vesicular stomatitis virus glycoprotein (VSV/G) were constructed. The recombinant baculoviruses designated Bac-G-prM/E and Bac-prM/E, efficiently express E protein in mammalian cells. Intramuscular injection of the two recombinant baculoviruses (at doses of 10⁸ or 10⁹ PFU/mouse) induced the production of WNV-specific antibodies, neutralizing antibodies as well as gamma interferon (IFN-γ) in a dose-dependent pattern. Interestingly, the recombinant baculovirus Bac-G-prM/E was found to be a more efficient immunogen than Bac-prM/E to elicit a robust immune response upon intramuscular injection. In addition, inoculation of baculovirus resulted in the secretion of inflammatory cytokines, such as TNF-α, IL-2 and IL-6.

Conclusions

These recombinant baculoviruses are capable of eliciting robust humoral and cellular immune responses in mice, and may be considered as novel vaccine candidates for West Nile Virus.

Baculovirus as a gene delivery vector: recent understandings of molecular alterations in transduced cells and latest applications

Baculovirus infects insects in nature and is non-pathogenic to humans, but can transduce a broad range of mammalian and avian cells. Thanks to the biosafety, large cloning capacity, low cytotoxicity and non-replication nature in the transduced cells as well as the ease of manipulation and production, baculovirus has gained explosive popularity as a gene delivery vector for a wide variety of applications. This article extensively reviews the recent understandings of the molecular mechanisms pertinent to baculovirus entry and cellular responses, and covers the latest advances in the vector improvements and applications, with special emphasis on antiviral therapy, cancer therapy, regenerative medicine and vaccine.

A pseudotype baculovirus expressing the capsid protein of foot-and-mouth disease virus and a T-cell immunogen shows enhanced immunogenicity in mice

Background

Foot-and-mouth disease (FMD) is a highly contagious disease of livestock which causes severe economic loss in cloven-hoofed animals. Vaccination is still a major strategy in developing countries to control FMD. Currently, inactivated vaccine of FMDV has been used in many countries with limited success and safety concerns. Development of a novel effective vaccine is must.

Methods

In the present study, two recombinant pseudotype baculoviruses, one expressing the capsid of foot-and-mouth disease virus (FMDV) under the control of a cytomegalovirus immediate early enhancer/promoter (CMV-IE), and the other the caspid plus a T-cell immunogen coding region under a CAG promoter were constructed, and their expression was characterized in mammalian cells. In addition, their immunogenicity in a mouse model was investigated. The humoral and cell-mediated immune responses induced by pseudotype baculovirus were compared with those of inactivated vaccine.

Results

Indirect immunofluorescence assay (IFA) and indirect sandwich-ELISA (IS-ELISA) showed both recombinant baculoviruses (with or without T-cell epitopes) were transduced efficiently and expressed target proteins in BHK-21 cells. In mice, intramuscular inoculation of recombinants with 1 × 10⁹ or 1 × 10¹⁰ PFU/mouse induced the production of FMDV-specific neutralizing antibodies and gamma interferon (IFN-γ). Furthermore, recombinant baculovirus with T-cell epitopes had better immunogenicity than the recombinant without T-cell epitopes as demonstrated by significantly enhanced IFN-γ production (P < 0.01) and higher neutralizing antibody titer (P < 0.05). Although the inactivated vaccine produced the highest titer of neutralizing antibodies, a lower IFN-γ expression was observed compared to the two recombinant pseudotype baculoviruses.

Conclusions

These results indicate that pseudotype baculovirus-mediated gene delivery could be a alternative strategy to develop a new generation of vaccines against FMDV infection.

The effect of Houttuynia cordata injection on pseudorabies herpesvirus (PrV) infection in vitro

CONTEXT

Pseudorabies herpesvirus (PrV) belongs to the Alphaherpesvirinae. Piglets infected with PrV die within a few days. Development of effective antiviral agents is one alternative or complementary method to prevent PrV infection. Houttuynia cordata Thunb. (Saururaceae), H. cordata, a traditional Chinese medicine, is often used to relieve lung abnormal symptoms, infectious disease, refractory hemoptysis and malignant pleural effusion in China.

OBJECTIVE

The present study aimed to investigate the effect of H. cordata injection on cell infection by PrV using Vero cells (a monkey kidney cell line) and swine testis cells (ST) as models.

MATERIALS AND METHODS

The infectivity of PrV was determined by plaque assays when H. cordata was applied to the virus, to the virus infected cells, and to the cells prior to infection. The genomic DNA copies post-drug treatment were confirmed by PCR and reverse transcription PCR. The cell apoptosis caused by the virus was analyzed.

RESULTS

H. cordata efficiently inhibited cell infection after incubating the drug with PrV. Nevertheless, H. cordata was more efficient in Vero cells than in ST cells in terms of its inhibitory effect. Low-dosage drug inhibited cell apoptosis induced by PrV; nevertheless, high-dosage drug alone resulted in cell apoptosis.

DISCUSSION AND CONCLUSION

H. cordata has a direct inhibitory activity against PrV in vitro. H. cordata may be used as an anti-PrV agent or combined with other anti-PrV agents. PrV infection induces cell apoptosis and H. cordata inhibits cell infection. The optimal administration dosage of H. cordata should be taken into account in the future, because high-dosage H. cordata alone causes cell apoptosis.

Construction and immunogenicity of a recombinant pseudotype baculovirus expressing the glycoprotein of rabies virus in mice

A pseudotype baculovirus with the glycoprotein of vesicular stomatitis virus (VSV-G) on the envelope was used as a vector for the construction of recombinant baculovirus expressing the G protein of rabies virus (RABV) under the cytomegalovirus (CMV) promoter. The generated recombinant baculovirus (BV-G) efficiently expressed the RABV G proteins in mammalian cells. Intramuscular vaccination with BV-G (10(9) PFU/mouse) induced the production of RABV G-specific neutralizing antibodies and strong T cell responses in mice. Our data clearly indicate that pseudotype baculovirus-mediated gene delivery can be utilized as an alternative strategy to develop a new generation of vaccine against RABV infection.

New baculovirus recombinants expressing Pseudorabies virus (PRV) glycoproteins protect mice against lethal challenge infection

The present study demonstrates the protective potential of novel baculovirus recombinants, which express the glycoproteins gB, gC, or gD of Pseudorabies virus (PRV; Alphaherpesvirus of swine) and additionally contain the glycoprotein G of Vesicular Stomatitis Virus (VSV-G) in the virion (Bac-G-PRV). To evaluate the protective capacity, mixtures of equal amounts of the PRV gB-, gC-, and gD-expressing baculoviruses were used for immunization. Three intramuscular immunizations with that Bac-G-PRV mixture could protect mice against a lethal PRV challenge infection. To achieve complete protection high titers of Bac-G-PRV and three immunizations were necessary. This immunization with Bac-G-PRV resulted in the induction of high titers of PRV-specific serum antibodies of the IgG2a subclass and of interferon (IFN)-gamma, indicating a Th1-type immune response. Moreover, splenocytes of immunized mice exhibited natural killer cell activity accompanied by the production of IFN-alpha and IFN-gamma. Collectively, the presented data demonstrate for the first time that co-expression of VSV-G in baculovirus recombinant vaccines can improve the induction of a protective immune response against foreign antigens.

Antigen delivery systems for veterinary vaccine development. Viral-vector based delivery systems

The recent advances in molecular genetics, pathogenesis and immunology have provided an optimal framework for developing novel approaches in the rational design of vaccines effective against viral epizootic diseases. This paper reviews most of the viral-vector based antigen delivery systems (ADSs) recently developed for vaccine testing in veterinary species, including attenuated virus and DNA and RNA viral vectors. Besides their usefulness in vaccinology, these ADSs constitute invaluable tools to researchers for understanding the nature of protective responses in different species, opening the possibility of modulating or potentiating relevant immune mechanisms involved in protection.

Targeting the porcine immune system--particulate vaccines in the 21st century

During the last decade, the propagation of immunological knowledge describing the critical role of dendritic cells (DC) in the induction of efficacious immune responses has promoted research and development of vaccines systematically targeting DC. Based on the promise for the rational design of vaccine platforms, the current review will provide an update on particle-based vaccines of both viral and synthetic origin, giving examples of recombinant virus carriers such as adenoviruses and biodegradable particulate carriers. The viral carriers carry pathogen-associated molecular patterns (PAMP), used by the original virus for targeting DC, and are particularly efficient and versatile gene delivery vectors. Efforts in the field of synthetic vaccine carriers are focussing on decorating the particle surface with ligands for DC receptors such as heparan sulphate glycosaminoglycan structures, integrins, Siglecs, galectins, C-type lectins and toll-like receptors. The emphasis of this review will be placed on targeting the porcine immune system, but reference will be made to advances with murine and human vaccine delivery systems where information on DC targeting is available.

Preparation of recombinant baculoviruses with the BVboost system

We have developed an improved transposition-based system (BVboost) for the generation of recombinant baculoviruses. This system bypasses the disadvantages of the original transposition-based generation of baculoviral genomes in Escherichia coli while remaining a simple, rapid, and convenient viral production technique. The method is based on the modified donor vector (pBVboost) and an improved selection scheme of the baculoviral genomes (bacmids) in E. coli with a mutated sacB gene. Recombinant bacmids can be generated at a frequency of ~107/μg of donor vector with a negligible background. The BVboost system also allows efficient setups for high-throughput screening and gene expression purposes. After cloning the desired gene/cDNA/library into a BVboost system-compatible donor plasmid, the recombinant baculoviral genome is prepared simply by transforming electrocompetent DH10BacΔTn7 E. coli cells with the donor. Transfer from the donor vector into the baculoviral genome (bacmid) occurs via a Tn7-mediated site-specific transposition reaction in E. coli cells. The selection scheme guarantees that virtually all colonies are correct. The recombinant baculoviral genome is subsequently extracted from E. coli culture using a modified isolation procedure for large plasmids. To generate the recombinant viruses, insect cells are transfected with the isolated recombinant bacmid. This protocol provides instructions on how to prepare recombinant baculoviruses by the BVboost system in order to express the desired gene(s) in insect and/or vertebrate cells.

Immunization with pseudotype baculovirus expressing envelope protein of Japanese encephalitis virus elicits protective immunity in mice

BACKGROUND

Japanese encephalitis (JE) is a serious infection and disease in southern and eastern Asia. The design and development of safer and more efficacious vaccines against Japanese encephalitis virus (JEV) is a high-priority target in the world. Recently, baculovirus pseudotyped with vesicular stomatitis virus glycoprotein (VSVG) was described as an attractive gene delivery vehicle in mammalian cells and a potential vector for vaccine development. In the present study, we constructed a recombinant pseudotype baculovirus encoding the JEV envelope (E) protein and demonstrated that it could elicit high protective immunity in mice.

METHODS

Recombinant pseudotype baculovirus (BV-G-E) was generated by inserting JEV E gene fragment into pFastBac-VSV/G vector. BALB/c mice were immunized with BV-G-E and challenged with JEV wild-type strain. The neutralization antibody, interferon (IFN)-gamma expression and release, and survival rate were analysed and compared with the group of immunized with inactivated vaccine and DNA vaccine (pc-E) encoding the same gene of JEV.

RESULTS

We demonstrated that intramuscular injections of BV-G-E at various doses into mice produced higher levels of JEV-specific neutralizing antibodies, IFN-gamma and better protective efficacy against a lethal challenge with JEV than that of pc-E. Furthermore, BV-G-E could elicit a higher level of cellular immunity response and provide equal protective efficacy against JEV challenge compared to inactivated vaccine.

CONCLUSIONS

Our data demonstrate that BV-G-E elicited higher levels of protective immunity compared to DNA vaccine and that pseudotype baculovirus-mediated gene delivery can be utilized as an alternative strategy to develop new generations of vaccines against JEV infection.

Baculovirus vector for gene delivery and vaccine development

The baculovirus Autographa californica multiple nucleopolyhedrovirus has been widely used not only to acheive a high level of foreign gene expression in insect cells, but also for efficient gene transduction into mammalian cells. Recombinant and pseudotyped baculoviruses possessing chimeric or foreign ligands have been constructed to improve the efficiency of gene transduction and to confer specificity for gene delivery into mammalian cells, respectively. Baculoviral DNA CpG motifs induce proinflammatory cytokines through a Toll-like receptor (TLR9)/MyD88-dependent signaling pathway. Other baculovirus components produce type I interferons via a TLR-independent pathway. Baculovirus exhibits a strong adjuvant property and recombinant baculoviruses encoding microbial antigens elicit antibodies to the antigens and provide protective immunity in mice. This review deals with recent progress in the application of baculovirus vectors to gene delivery and vaccine development, and discusses the future prospects of baculovirus vectors.

Post-transcriptional regulatory element boosts baculovirus-mediated gene expression in vertebrate cells

Baculoviruses can express transgenes in a wide range of vertebrate cells. However, in some cells transgene expression is weak. To enhance transgene expression, we studied the effect of the Woodchuck hepatitis virus post-transcriptional regulatory element (WPRE) on baculovirus (BV)-mediated gene expression of several transgenes. A significant increase in BV-mediated gene expression was detected in several cell lines. A 10-fold increase in transgene expression was observed with the WPRE as determined by the percentage of positive cells and mean fluorescence intensity (MFI). Furthermore, a combination of optimized cell culture medium and WPRE virus led to more than a 60-fold increase in gene expression. In accordance, elevated mRNA and protein levels were detected in WPRE-virus transduced cells. In HepG2 and RaaSMC, WPRE-mediated enhancement was comparable to the previously shown positive effect of sodium butyrate on BV-mediated gene expression. Thus, inclusion of the WPRE into a baculovirus vector provides a simple means to improve BV-mediated gene expression in vertebrate cells.

Baculovirus-based Vaccination Vectors Allow for Efficient Induction of Immune Responses Against Plasmodium falciparum Circumsporozoite Protein

Baculovirus vectors are able to transduce a large variety of mammalian cell types and express transgenes placed under the control of heterologous promoters. In this study, we evaluated the potential of baculovirus vectors for malaria vaccination. To induce efficient CD4(+) and CD8(+) T-cell responses, we produced a series of vectors that display the Plasmodium falciparum circumsporozoite (CS) protein in the virion envelope and/or allow for CS expression upon transduction of mammalian cells. We found that baculovirus vectors can transduce professional antigen-presenting cells and trigger their maturation, which is a prerequisite for efficient antigen presentation. Upon intramuscular injection into mice, the vector that both displayed and expressed CS induced higher anti-CS antibody titers (of the immunoglobulin (IgG)1 and IgG2a type) and a higher frequency of interferon-gamma-producing T cells specific to CS, than the vectors which either only displayed or only expressed CS. The baculovirus CS display/expression vector was also superior in inducing CS-specific CD4(+) and CD8(+) T-cell responses in vitro using human peripheral blood mononuclear cells from naive donors. This, together with the absence of pre-existing immunity to baculoviruses in humans, the absence of viral gene expression in mammalian cells, and the relative low immunogenicity of baculovirus virions, makes these vectors promising tools for vaccination. Furthermore, the ability to produce large amounts in serum-free medium at a low cost adds a further advantage to this vector system.

Inhibition of HIV-1 replication by vesicular stomatitis virus envelope glycoprotein pseudotyped baculovirus vector-transduced ribozyme in mammalian cells

The baculovirus has recently emerged as a promising vector for in vivo gene therapy. To investigate its potential as a delivery vector for an anti-virus ribozyme targeting HIV-1, we constructed recombinant baculovirus vectors bearing a ribozyme-synthesizing cassette driven by the tRNA(i)(Met) promoter with enhanced transduction efficiency by displaying vesicular stomatitis virus glycoprotein (VSV-G) on the viral envelope. Transduction of HeLa CD4(+) cells with a recombinant baculovirus delivering the HIV-1 U5 gene-specific ribozyme dramatically suppressed HIV-1 expression in this cell line. The VSV-G pseudotyped baculovirus vector-transduced ribozyme potently inhibited HIV-1 replication compared to a recombinant baculovirus vector-transduced ribozyme lacking VSV-G. The use of a baculovirus vector might be beneficial for application in gene therapy.

Baculovirus-mediated gene delivery into Mammalian cells does not alter their transcriptional and differentiating potential but is accompanied by early viral gene expression

Gene delivery to neural cells is central to the development of transplantation therapies for neurological diseases. In this study, we used a baculovirus derived from the domesticated silk moth, Bombyx mori, as vector for transducing a human cell line (HEK293) and primary cultures of rat Schwann cells. Under optimal conditions of infection with a recombinant baculovirus containing the reporter green fluorescent protein gene under mammalian promoter control, the infected cells express the transgene with high efficiency. Toxicity assays and transcriptome analyses suggest that baculovirus infection is not cytotoxic and does not induce differential transcriptional responses in HEK293 cells. Infected Schwann cells retain their characteristic morphological and molecular phenotype as determined by immunocytochemistry for the marker proteins S-100, glial fibrillary acidic protein, and p75 nerve growth factor receptor. Moreover, baculovirus-infected Schwann cells are capable of differentiating in vitro and express the P0 myelination marker. However, transcripts for several immediate-early viral genes also accumulate in readily detectable levels in the transduced cells. This transcriptional activity raises concerns regarding the long-term safety of baculovirus vectors for gene therapy applications. Potential approaches for overcoming the identified problem are discussed.

Vaccines for viral and parasitic diseases produced with baculovirus vectors

The baculovirus-insect cell expression system is an approved system for the production of viral antigens with vaccine potential for humans and animals and has been used for production of subunit vaccines against parasitic diseases as well. Many candidate subunit vaccines have been expressed in this system and immunization commonly led to protective immunity against pathogen challenge. The first vaccines produced in insect cells for animal use are now on the market. This chapter deals with the tailoring of the baculovirus-insect cell expression system for vaccine production in terms of expression levels, integrity and immunogenicity of recombinant proteins, and baculovirus genome stability. Various expression strategies are discussed including chimeric, virus-like particles, baculovirus display of foreign antigens on budded virions or in occlusion bodies, and specialized baculovirus vectors with mammalian promoters that express the antigen in the immunized individual. A historical overview shows the wide variety of viral (glyco)proteins that have successfully been expressed in this system for vaccine purposes. The potential of this expression system for antiparasite vaccines is illustrated. The combination of subunit vaccines and marker tests, both based on antigens expressed in insect cells, provides a powerful tool to combat disease and to monitor infectious agents.

Baculovirus vectors for gene therapy

Since the discovery that baculoviruses can efficiently transduce mammalian cells, baculoviruses have been extensively studied as potential vectors for both in vitro and in vivo gene therapy. This chapter reviews the history of this research area, cells permissive to baculovirus transduction, factors influencing transduction and transgene expression, efforts to improve transduction, mechanisms of virus entry and intracellular trafficking, applications for in vivo and ex vivo gene therapy, as well as advantages, limitations, and safety issues concerning use of baculoviruses as gene therapy vectors. Recent progress and efforts directed toward overcoming existing bottlenecks are emphasized.

Baculovirus as versatile vectors for protein expression in insect and mammalian cells

Today, many thousands of recombinant proteins, ranging from cytosolic enzymes to membrane-bound proteins, have been successfully produced in baculovirus-infected insect cells. Yet, in addition to its value in producing recombinant proteins in insect cells and larvae, this viral vector system continues to evolve in new and unexpected ways. This is exemplified by the development of engineered insect cell lines to mimic mammalian cell glycosylation of expressed proteins, baculovirus display strategies and the application of the virus as a mammalian-cell gene delivery vector. Novel vector design and cell engineering approaches will serve to further enhance the value of baculovirus technology.

Baculovirus as a highly efficient gene delivery vector for the expression of hepatitis delta virus antigens in mammalian cells

Baculovirus has been employed for a wide variety of applications. In this study, we further expanded the application to the high-level expression of hepatitis delta virus (HDV) antigens and the formation of virus-like particles (VLP) in transduced mammalian cells. To this end, two recombinant baculoviruses were constructed to express large hepatitis delta antigen (L-HDAg) and hepatitis B surface antigen (HBsAg) under mammalian promoters. With a simplified transduction protocol using unconcentrated virus, high transduction efficiencies were achieved in hepatoma cells, in which L-HDAg and HBsAg were expressed abundantly, allowing for easy colorimetric detection in Western blots. L-HDAg alone was nucleus-bound and HBsAg alone was secreted; formation and secretion of HDV-like particles were readily detected upon coexpression, indicating that the baculovirus-expressed proteins were processed correctly as the authentic proteins. Quantitative real-time PCR (Q-PCR) analyses quantitatively revealed that baculovirus transduction was more efficient than plasmid transfection with respect to DNA uptake and DNA transport to the nucleus. Furthermore, superinfection introduced more baculovirus DNA into cells in the long-term culture as revealed by Q-PCR, thereby enhancing and prolonging the expression. In summary, baculovirus transduction can be an attractive method as an alternative to the plasmid transfection commonly employed for HDV research thanks to the significantly higher gene delivery efficiencies as well as the abundant expression and proper processing. Baculovirus can also be envisaged as a useful tool for investigating protein-cell interactions and virus assembly.

Baculovirus as a highly efficient expression vector in insect and mammalian cells

Baculovirus has been widely used for the production of recombinant proteins in insect cells. Since the finding that baculovirus can efficiently transduce mammalian cells, the applications of baculovirus have been greatly expanded. The prospects and drawbacks of baculovirus-mediated gene expression, either in insect or in mammalian cells, are reviewed. Recent progresses in expanding the applications to studies of gene regulation, viral vector preparation, in vivo and ex vivo gene therapy studies, generation of vaccine vectors, etc are discussed and the efforts directed towards overcoming the existing bottlenecks are particularly emphasized.

Baculovirus vectors elicit antigen-specific immune responses in mice

To characterize the induction of antigen-specific immune response mediated by baculovirus, vectors expressing the E2 glycoprotein of hepatitis C virus or the carcinoembryonic antigen (CEA) under the control of the cytomegalovirus immediate-early promoter-enhancer were constructed. Additionally, a baculovirus vector encoding the E2 glycoprotein (Bac-G-E2) and expressing vesicular stomatitis virus glycoprotein (VSV-G) in the viral envelope was generated by inserting the VSV-G coding sequence downstream of the polyhedrin promoter. Mice were subjected to intramuscular, intranasal, or subcutaneous inoculations with Bac-E2 and the cellular immune response was monitored by ELISPOT and intracellular staining. Additionally, humoral response was monitored by titrating anti-E2 antibodies. Induction of a measurable anti-E2 T-cell response was observed only after intramuscular injection and was predominantly CD8(+) specific. The immunogenic properties of baculovirus as vaccine vector were not restricted to E2 because a CEA-specific CD4(+) T-cell response was observed upon intramuscular injection of Bac-CEA. Interestingly, the Bac-G-E2 vector was shown to be a more efficient immunogen than Bac-E2, in view of the 10-fold difference in the minimal dose required to elicit a measurable T-cell response upon intramuscular injection. Induction of inflammatory cytokines such as gamma interferon, tumor necrosis factor alpha, and interleukin-6 was detected as early as 6 h postinjection of Bac-G-E2. Most importantly, both vectors elicited CD8(+) T cells with effector function capable of lysing target cells loaded with a hepatitis C virus-specific epitope. Additionally, enhanced NK cytolytic activity was detected in immunized mice. Thus, these results further demonstrate that baculovirus may be considered a useful vector for gene therapy.

Recombinant baculoviruses as mammalian cell gene-delivery vectors

The baculovirus expression system has been used extensively for the expression of recombinant proteins in insect cells. Recently, recombinant baculovirus vectors engineered to contain mammalian cell-active promoter elements, have been used successfully for transient and stable gene delivery in a broad spectrum of primary and established mammalian cells. The application of modified baculoviruses for in vivo gene delivery has also been demonstrated. In contrast to other commonly used viral vectors, baculoviruses have the unique property of replicating in insect cells while being incapable of initiating a replication cycle and producing infectious virus in mammalian cells. The viruses can be readily manipulated, accommodate large insertions of foreign DNA, initiate little to no microscopically observable cytopathic effect in mammalian cells and have a good biosafety profile. These attributes will undoubtedly lead to the increased application and continued development of this system for efficient gene delivery into mammalian cells. Who said you can't teach an old dog new tricks?