Truncated vesicular stomatitis virus G protein improves baculovirus transduction efficiency in vitro and in vivo

Development of a stable Sf9 insect cell line to produce VSV-G pseudotyped baculoviruses

Baculoviruses have shown great potential as gene delivery vectors in mammals, although their effectiveness in transferring genes varies across different cell lines. A widely employed strategy to improve transduction efficiency is the pseudotyping of viral vectors. In this study, we aimed to develop a stable Sf9 insect cell line that inducibly expresses the G-protein of the vesicular stomatitis virus to pseudotype budded baculoviruses. It was obtained by inserting the VSV-G gene under the control of the very strong and infection-inducible pXXL promoter and was subsequently diluted to establish oligoclonal lines, which were selected by the fusogenic properties of VSV-G and its expression levels in infected cells and purified budded virions. Next, to enhance the performance of the cell line, the infection conditions under which functional pseudotyped baculoviruses are obtained were optimized. Finally, different baculoviruses were pseudotyped and the expression of the transgene was quantified in mammalian cells of diverse origins using flow cytometry. The transduction efficiency of pseudotyped baculovirus consistently increased across all tested mammalian cell lines compared with control viruses. These findings demonstrate the feasibility and advantages of improving gene delivery performance without the need to insert the pseudotyping gene into the baculoviral genomes.

Baculovirus displaying SARS-CoV-2 spike RBD promotes neutralizing antibody production in a mouse model

BACKGROUND

There is always a need for a safe and efficient vaccine platform, especially when facing a pandemic such as COVID-19. Most of the SARS-CoV-2-based vaccines are based on the full spike protein, which is presented as a trimerized protein, and many viral vector vaccines express the spike protein into the host cells and do not display it on virus surfaces. However, the spike receptor-binding domain (RBD)-based vaccines are efficient and are currently under investigation and clinical trials.

METHODOLOGY

In this study, we are testing the efficacy of the RBD displayed on a baculovirus as a mean to formulate a safe and stable carrier to induce the immune system against SARS-CoV-2. Therefore, two pseudotyped baculoviruses were constructed to display the RBD, AcRBD-sfGFP-64, and AcRBD-sfGFP-V, using two different displaying strategies based on gp64 and VSV-G envelope glycoproteins, from Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and vesicular stomatitis virus (VSV), respectively. BALB/C mice were immunized with the pseudotyped baculoviruses in a dose-optimized manner. Dot blot and Western blot were used to screen and validate the polyclonal antibodies' specificity to the SARS-CoV-2 RBD. A plaque reduction neutralization test (PRNT) was used to measure the sera neutralization capacity against a SARS-CoV-2 wild-type isolate from Egypt. ELISA was used to quantify certain cytokines for the assessment of the immune response.

RESULT

The outcome of our investigation showed that the monomeric RBD proteins were properly displayed on baculovirus and efficiently triggered the mouse immune system. The produced sera efficiently neutralized about 50% of SARS-CoV-2 in more than 100-fold serum dilution. The immunized mice showed a significant increase (p<0.01) in the levels of IL-2 and IFN-γ and a significant decrease (p<0.01) and (p<0.001) in the levels of IL-4 and IL-10, respectively, which suggest that AcRBD-sfGFP-64 and AcRBD-sfGFP-V induce Th1 cellular immune response.

CONCLUSION

The produced recombinant viruses can induce the immune response without adjuvant, which needs dose optimization and further stability tests. Neutralizing antibodies were induced without affecting the health of immunized mice. Th1 response can be attainable through the system, which is of great benefit in SARS CoV-2 infection and the system can be tested for future applications including vaccine development and polyclonal antibody production.

Baculovirus Production and Infection in Axolotls

Salamanders have served as an excellent model for developmental and tissue regeneration studies. While transgenic approaches are available for various salamander species, their long generation time and expensive maintenance have driven the development of alternative gene delivery methods for functional studies. We have previously developed pseudotyped baculovirus (BV) as a tool for gene delivery in the axolotl (Oliveira et al. Dev Biol 433(2):262-275, 2018). Since its initial conception, we have refined our protocol of BV production and usage in salamander models. In this chapter, we describe a detailed and versatile protocol for BV-mediated transduction in urodeles.

A replication-competent smallpox vaccine LC16m8Δ-based COVID-19 vaccine

Viral vectors are a potent vaccine platform for inducing humoral and T-cell immune responses. Among the various viral vectors, replication-competent ones are less commonly used for coronavirus disease 2019 (COVID-19) vaccine development compared with replication-deficient ones. Here, we show the availability of a smallpox vaccine LC16m8Δ (m8Δ) as a replication-competent viral vector for a COVID-19 vaccine. M8Δ is a genetically stable variant of the licensed and highly effective Japanese smallpox vaccine LC16m8. Here, we generated two m8Δ recombinants: one harbouring a gene cassette encoding the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) glycoprotein, named m8Δ-SARS2(P7.5-S)-HA; and one encoding the S protein with a highly polybasic motif at the S1/S2 cleavage site, named m8Δ-SARS2(P7.5-S_HN)-HA. M8Δ-SARS2(P7.5-S)-HA induced S-specific antibodies in mice that persisted for at least six weeks after a homologous boost immunization. All eight analysed serum samples displayed neutralizing activity against an S-pseudotyped virus at a level similar to that of serum samples from patients with COVID-19, and more than half (5/8) also had neutralizing activity against the Delta/B.1.617.2 variant of concern. Importantly, most serum samples also neutralized the infectious SARS-CoV-2 Wuhan and Delta/B.1.617.2 strains. In contrast, immunization with m8Δ-SARS2(P7.5-S_HN)-HA elicited significantly lower antibody titres, and the induced antibodies had less neutralizing activity. Regarding T-cell immunity, both m8Δ recombinants elicited S-specific multifunctional CD8⁺ and CD4⁺ T-cell responses even after just a primary immunization. Thus, m8Δ provides an alternative method for developing a novel COVID-19 vaccine.

Pseudotyped lentiviral vectors: Ready for translation into targeted cancer gene therapy?

Gene therapy holds great promise for curing cancer by editing the deleterious genes of tumor cells, but the lack of vector systems for efficient delivery of genetic material into specific tumor sites in vivo has limited its full therapeutic potential in cancer gene therapy. Over the past two decades, increasing studies have shown that lentiviral vectors (LVs) modified with different glycoproteins from a donating virus, a process referred to as pseudotyping, have altered tropism and display cell-type specificity in transduction, leading to selective tumor cell killing. This feature of LVs together with their ability to enable high efficient gene delivery in dividing and non-dividing mammalian cells in vivo make them to be attractive tools in future cancer gene therapy. This review is intended to summarize the status quo of some typical pseudotypings of LVs and their applications in basic anti-cancer studies across many malignancies. The opportunities of translating pseudotyped LVs into clinic use in cancer therapy have also been discussed.

Niosomal virosome derived by vesicular stomatitis virus glycoprotein as a new gene carrier

Virosomes as membranous vesicles with viral fusion protein in their membrane are versatile vehicles for cargo delivery. The vesicular stomatitis virus glycoprotein (VSV-G) is a common fusogenic protein used in virosome preparation. This glycoprotein has been used in liposomal systems so far, but in this study, we have tried to use the niosomal form instead of liposome for. Niosomes are vesicular systems composed of non-ionic surfactants. Niosomes were constructed by the thin-film hydration method. VSV-G gene in pMD2.G plasmid was expressed in the HEK293T cell line and then was reconstituted in the niosome bilayer. The formation of niosomal virosomes was confirmed with different methods such as SDS-PAGE gel, western blotting, and transmission electron microscopy (TEM). The efficiency of niosomal virosome was investigated with the pmCherry reporter gene. SDS-PAGE and western blotting proved the expression and successful insertion of protein into the bilayer. The TEM images showed the spike projection of VSV-G on the surface of niosomes. The transfection results showed high efficiency of niosomal virosomes as a novel carrier. This report has verified that niosome could be used as an efficient bilayer instead of liposome to construct virosomes.

Synthetic Virus-Derived Nanosystems (SVNs) for Delivery and Precision Docking of Large Multifunctional DNA Circuitry in Mammalian Cells

DNA delivery is at the forefront of current research efforts in gene therapy and synthetic biology. Viral vectors have traditionally dominated the field; however, nonviral delivery systems are increasingly gaining traction. Baculoviruses are arthropod-specific viruses that can be easily engineered and repurposed to accommodate and deliver large sequences of exogenous DNA into mammalian cells, tissues, or ultimately organisms. These synthetic virus-derived nanosystems (SVNs) are safe, readily customized, and can be manufactured at scale. By implementing clustered regularly interspaced palindromic repeats (CRISPR) associated protein (CRISPR/Cas) modalities into this system, we developed SVNs capable of inserting complex DNAs into genomes, at base pair precision. We anticipate a major role for SVNs as an attractive alternative to viral vectors in accelerating genome engineering and gene therapy applications in the future.

Induction of mucosal immunity against pathogens by using recombinant baculoviral vectors: Mechanisms, advantages, and limitations

Over 90% of pathogens of medical importance invade the organism through mucosal surfaces, which makes it urgent to develop safe and effective mucosal vaccines and mucosal immunization protocols. Besides, parenteral immunization does not provide adequate protective immunity in mucosal surfaces. Effective mucosal vaccination could protect local and systemic compartments and favor herd immunity. Although various mucosal adjuvants and Ag-delivery systems have been developed, none has filled the gap to control diseases caused by complex mucosal pathogens. Among the strategies to counteract them, recombinant virions from the baculovirus Autographa californica multiple nucleopolyhedrovirus (rAcMNPV) are useful vectors, given their safety and efficacy to produce mucosal and systemic immunity in animal infection models. Here, we review the immunogenic properties of rAcMNPV virions from the perspectives of mucosal immunology and vaccinology. Some features, which are analyzed and extrapolated from studies with different particulate antigens, include size, shape, surface molecule organization, and danger signals, all needed to break the tolerogenic responses of the mucosal immune tissues. Also, we present a condensed discussion on the immunity provided by rAcMNPV virions against influenza virus and human papillomavirus in animal models. Through the text, we highlight the advantages and limitations of this experimental immunization platform.

Recombinant baculovirus expressing the FrC-OVA protein induces protective antitumor immunity in an EG7-OVA mouse model

Background

The baculovirus (BV) Autographa californica multiple nuclear polyhedrosis virus has been used in numerous protein expression systems because of its ability to infect insect cells and serves as a useful vaccination vector with several benefits, such as its low clinical risks and posttranslational modification ability. We recently reported that dendritic cells (DCs) infected with BV stimulated antitumor immunity. The recombinant BV (rBV) also strongly stimulated peptide-specific T-cells and antitumor immunity. In this study, the stimulation of an immune response against EG7-OVA tumors in mice by a recombinant baculovirus-based combination vaccine expressing fragment C-ovalbumin (FrC-OVA-BV; rBV) was evaluated.

Results

We constructed an rBV expressing fragment C (FrC) of tetanus toxin containing a promiscuous MHC II-binding sequence and a p30-ovalbumin (OVA) peptide that functions in the MHC I pathway. The results showed that rBV activated the CD8⁺ T-cell-mediated response much more efficiently than the wild-type BV (wtBV). Experiments with EG7-OVA tumor mouse models showed that rBV significantly decreased tumor volume and increased survival compared with those in the wild-type BV or FrC-OVA DNA vaccine groups. In addition, a significant antitumor effect of classic prophylactic or therapeutic vaccinations was observed for rBV against EG7-OVA-induced tumors compared with that in the controls.

Conclusion

Our findings showed that FrC-OVA-BV (rBV) induced antitumor immunity, paving the way for its use in BV immunotherapy against malignancies.

Improving Baculovirus Transduction of Mammalian Cells by Incorporation of Thogotovirus Glycoproteins

Baculovirus can transduce a wide range of mammalian cells and is considered a promising gene therapy vector. However, the low transduction efficiency of baculovirus into many mammalian cells limits its practical application. Co-expressing heterologous viral glycoproteins (GPs), such as vesicular stomatitis virus G protein (VSV G), with baculovirus native envelope protein GP64 is one of the feasible strategies for improving virus transduction. Tick-borne thogotoviruses infect mammals and their GPs share sequence/structure homology and common evolutionary origins with baculovirus GP64. Herein, we tested whether thogotovirus GPs could facilitate the entry of the prototype baculovirus Autographa californica multiple multiple nucleopolyhedrovirus (AcMNPV) into mammalian cells. The gp genes of two thogotoviruses, Thogoto virus and Dhori virus, were inserted into the AcMNPV genome. Both GPs were properly expressed and incorporated into the envelope of the recombinant AcMNPVs. The transduction rates of recombinant AcMNPVs expressing the two thogotovirus GPs increased for approximately 4-12 fold compared to the wild type AcMNPV in six of the 12 tested mammalian cell lines. It seemed that thogotovirus GPs provide the recombinant AcMNPVs with different cell tropisms and showed better performance in several mammalian cells compared to VSV G incorporated AcMNPV. Further studies showed that the improved transduction was a result of augmented virus-endosome fusion and endosome escaping, rather than increased cell binding or internalization. We found the AcMNPV envelope protein GP64-mediated fusion was enhanced by the thogotovirus GPs at relatively higher pH conditions. Therefore, the thogotovirus GPs represent novel candidates to improve baculovirus-based gene delivery vectors.

Improved Baculovirus Vectors for Transduction and Gene Expression in Human Pancreatic Islet Cells

Pancreatic islet transplantation is a promising treatment for type 1 diabetes mellitus offering improved glycaemic control by restoring insulin production. Improved human pancreatic islet isolation has led to higher islet transplantation success. However, as many as 50% of islets are lost after transplantation due to immune responses and cellular injury, gene therapy presents a novel strategy to protect pancreatic islets for improved survival post-transplantation. To date, most of the vectors used in clinical trials and gene therapy studies have been derived from mammalian viruses such as adeno-associated or retrovirus. However, baculovirus BacMam vectors provide an attractive and safe alternative. Here, a novel BacMam was constructed containing a frameshift mutation within fp25, which results in virus stocks with higher infectious titres. This improved in vitro transduction when compared to control BacMams. Additionally, incorporating a truncated vesicular stomatitis virus G protein increased transduction efficacy and production of EGFP and BCL2 in human kidney (HK-2) and pancreatic islet β cells (EndoC βH3). Lastly, we have shown that our optimized BacMam vector can deliver and express egfp in intact pancreatic islet cells from human cadaveric donors. These results confirm that BacMam vectors are a viable choice for providing delivery of transgenes to pancreatic islet cells.

Baculovirus as a Tool for Gene Delivery and Gene Therapy

Based on its ability to express high levels of protein, baculovirus has been widely used for recombinant protein production in insect cells for more than thirty years with continued technical improvements. In addition, baculovirus has been successfully applied for foreign gene delivery into mammalian cells without any viral replication. However, several CpG motifs are present throughout baculoviral DNA and induce an antiviral response in mammalian cells, resulting in the production of pro-inflammatory cytokines and type I interferon through a Toll-like receptor (TLR)-dependent or -independent signaling pathway, and ultimately limiting the efficiency of transgene expression. On the other hand, by taking advantage of this strong adjuvant activity, recombinant baculoviruses encoding neutralization epitopes can elicit protective immunity in mice. Moreover, immunodeficient cells, such as hepatitis C virus (HCV)- or human immunodeficiency virus (HIV)-infected cells, are more susceptible to baculovirus infection than normal cells and are selectively eliminated by the apoptosis-inducible recombinant baculovirus. Here, we summarize the application of baculovirus as a gene expression vector and the mechanism of the host innate immune response induced by baculovirus in mammalian cells. We also discuss the future prospects of baculovirus vectors.

Baculovirus Surface Display of Immunogenic Proteins for Vaccine Development

Vaccination is an efficient way to prevent the occurrence of many infectious diseases in humans. To date, several viral vectors have been utilized for the generation of vaccines. Among them, baculovirus-categorized as a nonhuman viral vector-has been used in wider applications. Its versatile features, like large cloning capacity, nonreplicative nature in mammalian cells, and broad tissue tropism, hold it at an excellent position among vaccine vectors. In addition to ease and safety during swift production, recent key improvements to existing baculovirus vectors (such as inclusion of hybrid promoters, immunostimulatory elements, etc.) have led to significant improvements in immunogenicity and efficacy of surface-displayed antigens. Furthermore, some promising preclinical results have been reported that mirror the scope and practicality of baculovirus as a vaccine vector for human applications in the near future. Herein, this review provides an overview of the induced immune responses by baculovirus surface-displayed vaccines against influenza and other infectious diseases in animal models, and highlights the strategies applied to enhance the protective immune responses against the displayed antigens.

Gene delivery and gene expression in vertebrate using baculovirus Bombyx mori nucleopolyhedrovirus vector

The baculovirus Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) has been investigated as a possible tool for gene therapy, but its inhibition by complement proteins in human serum limits its applicability. Here, we used the baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV) to construct a gene delivery vector in which a reporter gene is driven by a cytomegalovirus IE promoter. Enhanced green fluorescent protein (EGFP) and luciferase reporter genes were used to test the efficiency of gene delivery. In vitro complement inactivation data showed that the recombinant BmNPV vector was more stable in human serum than the recombinant AcMNPV vector. The recombinant BmNPV vector successfully delivered the reporter genes into different tissues and organs in mice and chicks. These results demonstrate that the BmNPV vector is more stability against complement inactivation in human serum than the AcMNPV vector, and indicate that it may be useful as an effective gene delivery vector for gene therapy in vertebrates.

Pseudotyped baculovirus is an effective gene expression tool for studying molecular function during axolotl limb regeneration

Axolotls can regenerate complex structures through recruitment and remodeling of cells within mature tissues. Accessing the underlying mechanisms at a molecular resolution is crucial to understand how injury triggers regeneration and how it proceeds. However, gene transformation in adult tissues can be challenging. Here we characterize the use of pseudotyped baculovirus (BV) as an effective gene transfer method both for cells within mature limb tissue and within the blastema. These cells remain competent to participate in regeneration after transduction. We further characterize the effectiveness of BV for gene overexpression studies by overexpressing Shh in the blastema, which yields a high penetrance of classic polydactyly phenotypes. Overall, our work establishes BV as a powerful tool to access gene function in axolotl limb regeneration.

DAF-shielded baculovirus-vectored vaccine enhances protection against malaria sporozoite challenge in mice

Background

Previous studies have shown that the baculovirus-vectored vaccine based on the “baculovirus dual expression system (BDES)” is an effective vaccine delivery platform for malaria. However, a point of weakness remaining for use of this vaccine platform in vivo concerns viral inactivation by serum complement. In an effort to achieve complement resistance, the gene encoding the human decay-accelerating factor (hDAF) was incorporated into the BDES malaria vaccine expressing the Plasmodium falciparum circumsporozoite protein (PfCSP).

Results

The newly-developed BDES vaccine, designated BDES-sPfCSP2-Spider, effectively displayed hDAF and PfCSP on the surface of the viral envelope, resulting in complement resistance both in vitro and in vivo. Importantly, upon intramuscular inoculation into mice, the BDES-sPfCSP2-Spider vaccine had a higher protective efficacy (60%) than that of the control vaccine BDES-sPfCSP2-Spier (30%) against challenge with transgenic Plasmodium berghei sporozoites expressing PfCSP.

Conclusion

DAF-shielded BDES-vaccines offer great potential for development as a new malaria vaccine platform against the sporozoite challenge.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-2039-x) contains supplementary material, which is available to authorized users.

Construction of recombinant baculovirus vaccines for Newcastle disease virus and an assessment of their immunogenicity

Newcastle disease (ND) is a lethal avian infectious disease caused by Newcastle disease virus (NDV) which poses a substantial threat to China's poultry industry. Conventional live vaccines against NDV are available, but they can revert to virulent strains and do not protect against mutant strains of the virus. Therefore, there is a critical unmet need for a novel vaccine that is safe, efficacious, and cost effective. Here, we designed novel recombinant baculovirus vaccines expressing the NDV F or HN genes. To optimize antigen expression, we tested the incorporation of multiple regulatory elements including: (1) truncated vesicular stomatitis virus G protein (VSV-GED), (2) woodchuck hepatitis virus post-transcriptional regulatory element (WPRE), (3) inverted terminal repeats (ITRs) of adeno-associated virus (AAV Serotype II), and (4) the cytomegalovirus (CMV) promoter. To test the in vivo efficacy of the viruses, we vaccinated chickens with each construct and characterized the cellular and humoral immune response to challenge with virulent NDV (F48E9). All of the vaccine constructs provided some level of protection (62.5-100% protection). The F-series of vaccines provided a greater degree of protection (87.5-100%) than the HN-series (62.5-87.5%). While all of the vaccines elicited a robust cellular and humoral response subtle differences in efficacy were observed. The combination of the WPRE and VSV-GED regulatory elements enhanced the immune response and increased antigen expression. The ITRs effectively increased the length of time IFN-γ, IL-2, and IL-4 were expressed in the plasma. The F-series elicited higher titers of neutralizing antibody and NDV-specific IgG. The baculovirus system is a promising platform for NDV vaccine development that combines the immunostimulatory benefits of a recombinant virus vector with the non-replicating benefits of a DNA vaccine.

Baculovirus-mediated gene delivery and RNAi applications

Baculoviruses are widely encountered in nature and a great deal of data is available about their safety and biology. Recently, these versatile, insect-specific viruses have demonstrated their usefulness in various biotechnological applications including protein production and gene transfer. Multiple in vitro and in vivo studies exist and support their use as gene delivery vehicles in vertebrate cells. Recently, baculoviruses have also demonstrated high potential in RNAi applications in which several advantages of the virus make it a promising tool for RNA gene transfer with high safety and wide tropism.

Sleeping Beauty-baculovirus hybrid vectors for long-term gene expression in the eye

BACKGROUND

A baculovirus vector is capable of efficiently transducing many nondiving and diving cell types. However, the potential of baculovirus is restricted for many gene delivery applications as a result of the transient gene expression that it mediates. The plasmid-based Sleeping Beauty (SB) transposon system integrates transgenes into target cell genome efficiently with a genomic integration pattern that is generally considered safer than the integration of many other integrating vectors; yet efficient delivery of therapeutic genes into cells of target tissues in vivo is a major challenge for nonviral gene therapy. In the present study, SB was introduced into baculovirus to obtain novel hybrid vectors that would combine the best features of the two vector systems (i.e. effective gene delivery and efficient integration into the genome), thus circumventing the major limitations of these vectors.

METHODS

We constructed and optimized SB-baculovirus hybrid vectors that bear either SB100x transposase or SB transposon in the forward or reverse orientations with respect to the viral backbone The functionality of the novel hybrid vectors was investigated in cell cultures and in a proof-of-concept study in the mouse eye.

RESULTS

The hybrid vectors showed high and sustained transgene expression that remained stable and demonstrated no signs of decline during the 2 months follow-up in vitro. These results were verified in the mouse eye where persistent transgene expression was detected two months after intravitreal injection.

CONCLUSIONS

Our results confirm that (i) SB-baculovirus hybrid vectors mediate long-term gene expression in vitro and in vivo, and (ii) the hybrid vectors are potential new tools for the treatment of ocular diseases.

Baculovirus mediated transduction: analysis of vesicular stomatitis virus glycoprotein pseudotyping

The recombinant baculoviruses were constructed to investigate the necessity of VSV-G pseudotyping for mammalian cell transduction. The viruses were designed to express green fluorescent protein (GFP) gene under the control of cytomegalovirus promoter, with or without pseudotyping with VSV-G. VSV-G was placed under the control of polyhedrin promoter that is recognized by insect cells, allowing the formation of pseudotyped baculovirus. The study findings demonstrate that the pseudotyping of baculovirus significantly enhanced transduction efficiency compared to non-pseudotyped baculovirus, resulting in consequent distinction in the expression of GFP in mammalian cells. The results confirmed that pseudotyping is important for baculovirus mediated gene delivery. Further, when full-length VSV-G pseudotyping was compared with truncated VSV-G containing GED domain (G-stem of ectodomain in conjunction with the TM and CT domains of the glycoprotein), latter was relatively less efficient in transducing mammalian cells. This study demonstrated that pseudotyping with full-length VSV-G had better transduction efficiency in mammalian cells. However, at higher multiplicity of infection, both full-length and truncated VSV-G showed equivalent transduction. This study established the significance of pseudotyping of baculovirus with full-length VSV-G for efficient transduction of mammalian cells, utilizing the highly sensitive GFP marker system. These findings have significant implications in designing of baculovirus vector based antigen delivery for developing new generation vaccines.

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.

Baculovirus vector-mediated transfer of sodium iodide symporter and plasminogen kringle 5 genes for tumor radioiodide therapy

Background

Both tumor cells and their supporting endothelial cells should be considered for targeted cell killing when designing cancer treatments. Here we investigated the feasibility of combining radioiodide and antiangiogenic therapies after baculovirus-mediated transfer of genes encoding the sodium iodide symporter (NIS) and plasminogen kringle 5 (K5).

Methods

A recombinant baculovirus containing the NIS gene under control of the human telomerase reverse transcriptase (hTERT) promoter and the K5 gene driven by the early growth response 1 (Egr1) promoter was developed. Dual-luciferase reporter assay was performed to confirm the activation of hTERT transcription. NIS and K5 gene expression were identified by Western blot and Real-Time PCR. Functional NIS activity in baculovirus-infected Hela cells was confirmed by the uptake of ¹²⁵I and cytotoxicity of ¹³¹I. The apoptotic effect of ¹³¹I-induced K5 on baculovirus-infected human umbilical vein endothelial cells (HUVECs) was analyzed by a flow cytometry-based assay. In vivo, NIS reporter gene imaging and therapeutic experiments with ¹³¹I were performed. Finally, the microvessel density (MVD) in tumors after treatment was determined by CD31 immunostaining.

Results

The activation of hTERT transcription was specifically up-regulated in tumor cells. NIS gene expression markedly increased in baculovirus-infected HeLa cells, but not in MRC5 cells. The Hela cells showed a significant increase of ¹²⁵I uptake, which was inhibited by NaClO₄, and a notably decreased cell survival rate by ¹³¹I treatment. Expression of the K5 gene induced by ¹³¹I was elevated in a dose- and time-dependent manner and resulted in the apoptosis of HUVECs. Furthermore, ¹³¹I SPECT imaging clearly showed cervical tumor xenografts infected with recombinant baculovirus. Following therapy, tumor growth was significantly retarded. CD31 immunostaining confirmed a significant decrease of MVD.

Conclusion

The recombinant baculovirus supports a promising strategy of NIS-based raidoiodide therapy combined with K5-based antiangiogenic therapy by targeting both the tumor and its supporting vessels.

Optimization of eGFP expression using a modified baculovirus expression system

The baculovirus gene expression system is an efficient and safe protein expression system, since baculoviruses cannot replicate in mammalian cells. In order to improve the transduction efficiency and increase the reporter gene expression levels delivered by baculoviruses, we tested in the baculovirus expression cassette the Woodchuck hepatitis virus response element (WPRE), and AAV-derived inverted terminal repeats (ITRs) and the truncated vesicular stomatitis virus G protein (VSV-GED). The results showed that WPRE and VSV-GED have synergistic effects and could enhance the expression efficiency of enhanced green fluorescence protein (eGFP), and that ITRs effectively extended the duration of eGFP expression. We also demonstrated that the efficiency of eGFP expression varied under the control of the CMV, CBA, EF1-α or WSSV ie1 promoters in different cell lines.

Efficient viral delivery system for unnatural amino acid mutagenesis in mammalian cells

Here we report the development of a baculovirus-based delivery system that enables the efficient incorporation of unnatural amino acids into proteins in mammalian cells. We have exploited the large cargo-capacity (>30 kb) and stability of the double-stranded DNA genome of baculovirus to deliver to a variety of cell types all of the components required to genetically incorporate novel amino acids. These include the engineered tRNA/aminoacyl-tRNA synthetase pair and the nonsense mutant of the target gene. Mammalian cell transduction efficiency of baculovirus was significantly improved by incorporating genetic elements from mammalian viruses. Two polyspecific tRNA/aminoacyl-tRNA synthetase pairs were inserted into this expression system, enabling the site-specific incorporation of a variety of unnatural amino acids with novel chemical and biological properties into proteins.

Baculovirus: an insect-derived vector for diverse gene transfer applications

Insect-derived baculoviruses have emerged as versatile and safe workhorses of biotechnology. Baculovirus expression vectors (BEVs) have been applied widely for crop and forest protection, as well as safe tools for recombinant protein production in insect cells. However, BEVs ability to efficiently transduce noninsect cells is still relatively poorly recognized despite the fact that efficient baculovirus-mediated in vitro and ex vivo gene delivery into dormant and dividing vertebrate cells of diverse origin has been described convincingly by many authors. Preliminary proof of therapeutic potential has also been established in preclinical studies. This review summarizes the advantages and current status of baculovirus-mediated gene delivery. Stem cell transduction, preclinical animal studies, tissue engineering, vaccination, cancer gene therapy, viral vector production, and drug discovery are covered.

Baculovirus-mediated vascular endothelial growth factor-D(ΔNΔC) gene transfer induces angiogenesis in rabbit skeletal muscle

BACKGROUND

Occluded arteries and ischemic tissues cannot always be treated by angioplasty, stenting or by-pass-surgery. Under such circumstances, viral gene therapy may be useful in inducing increased blood supply to ischemic area. There is evidence of improved blood flow in ischemic skeletal muscle and myocardium in both animal and human studies using adenoviral vascular endothelial growth factor (VEGF) gene therapy. However, the expression is transient and repeated gene transfers with the same vector are inefficient due to immune responses.

METHODS

Different baculoviral vectors pseudotyped with or without vesicular stomatitis virus glycoprotein (VSV-G) and/or carrying woodchuck hepatitis virus post-transcriptional regulatory element (Wpre) were tested both in vitro and in vivo. VEGF-D(ΔNΔC) was used as therapeutic transgene and lacZ as a control. In vivo efficacy was evaluated as capillary enlargement and transgene expression in New Zealand White (NZW) rabbit skeletal muscle.

RESULTS

A statistically significant capillary enlargement was detected 6 days after gene transfer in transduced areas compared to the control gene transfers with baculovirus and adenovirus encoding β-galactosidase (lacZ). Substantially improved gene transfer efficiency was achieved with a modified baculovirus pseudotyped with VSV-G and carrying Wpre. Dose escalation experiments revealed that either too large volume or too many virus particles caused inflammation and necrosis in the target tissue, whereas 10(9) plaque forming units injected in multiple aliquots resulted in transgene expression with only mild immune reactions.

CONCLUSIONS

We show the first evidence of biologically significant baculoviral gene transfer in skeletal muscle of NZW rabbits using VEGF-D(ΔNΔC) as a therapeutic transgene.

Baculovirus as a vaccine vector

Baculovirus is extensively utilized as an excellent tool for production of recombinant protein in insect cells. Baculovirus infects insects in nature and is non-pathogenic to humans. In addition to insect cells, baculovirus is capable of transducing a broad range of animal cells. Due to its 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 been utilized as RNA interference mediators, gene delivery vectors, and vaccine vectors for a wide variety of applications. This article focuses on the utilization of baculoviruses as vaccine vectors to prepare antigen or subunit vaccines.

Genetic modification of baculovirus expression vectors

As a protein expression vector, the baculovirus demonstrates many advantages over other vectors. With the development of biotechnology, baculoviral vectors have been genetically modified to facilitate high level expression of heterologous proteins in both insect and mammalian cells. These modifications include utilization of different promoters and signal peptides, deletion or replacement of viral genes for increasing protein secretion, integration of polycistronic expression cassette for producing protein complexes, and baculovirus pseudotyping, promoter accommodation or surface display for enhancing mammalian cell targeting gene delivery. This review summarizes the development and the current state of art of the baculovirus expression system. Further development of baculovirus expression systems will make them even more feasible and accessible for advanced applications.

Pharmacological applications of baculovirus-mediated protein expression in mammalian cells

The development of cell-based assays for cellular receptors, ion channels, and transporters requires the delivery and expression of transgenes. Viral-mediated gene delivery is a particularly attractive approach for this purpose because of its efficiency and potential to deliver genes to a wide variety of cell types. Recombinant baculoviruses, long used to deliver genes to insect cells for overexpression, also effectively transfer genes to mammalian cells. The only required modification to the virus for this purpose is the addition of transgene expression cassettes controlled by mammalian cell-active promoters. These so-called BacMam viruses are useful for developing mammalian cell-based assays for investigating the function of recombinant proteins and for assessing the action of pharmacological modulators of their function. The use of such viruses is gaining popularity because of the ease of optimizing assay conditions, the ability to deliver multiple gene products, and of their flexibility in terms of host cells and levels of transgene expression. BacMam-mediated assays may be used for studying a wide variety of target proteins and assay technologies. Described in this unit is an example of BacMam-mediated gene delivery to configure a cell-based assay for pharmacological assessment of a G protein-coupled receptor. A protocol is also provided describing the use of a GFP-expressing BacMam to assess the susceptibility of new cell lines to transduction by the virus.

How to avoid complement attack in baculovirus-mediated gene delivery

Serum inactivation of baculovirus vectors is a significant barrier to the development of these highly efficient vectors for therapeutic gene delivery. In this review we will describe the efforts taken to avoid complement attack by passive or active measures. Evidently good targets for baculovirus-mediated gene delivery include immunoprivileged tissues, such as eye, brain and testis. Similarly baculovirus vectors have also proven their efficacy in an ex vivo setting for tissue engineering. Active measures to inhibit complement include the use of pharmacological inhibitors of complement as well as surface engineering of the baculoviral vectors through the use of synthetic polymers, pseudotyping or display of complement inhibitors. Lessons learned from these studies will significantly increase the possibility of using baculovirus vectors for therapeutic applications.

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.

Baculovirus treatment fully protects mice against a lethal challenge of FMDV

Foot-and-mouth disease virus (FMDV) causes a highly contagious and economically devastating disease that affects cattle, swine, goat and sheep among others. FMDV is able to overcome the initial host innate immune response by inhibiting the induction of antiviral molecules at both the transcriptional and the translational levels. It has been demonstrated that FMDV A/Arg/2001 causes the death of adult C57Bl/6 mice within 72h. We evaluated the capacity of Autographa californica nuclear polyhedrosis virus (AcNPV), an insect virus with potent innate immunostimulating effects, to promote early protection against FMDV A/Arg/2001 challenge in C57Bl/6 mice. Groups of 8-9 weeks old female mice were injected intravenously with AcNPV and challenged with a lethal dose of FMDV at different times post-administration. Our results showed that pretreatment of mice with a single injection of AcNPV 3h or 3 days before FMDV challenge resulted in complete abrogation of mortality and complete or partial suppression of viremia, respectively. Furthermore, no signs of disease were observed. AcNPV could be a valuable tool to improve the design of a novel vaccine that protects as an adjuvant at early times post-vaccination.

Screening of complement inhibitors: shielded baculoviruses increase the safety and efficacy of gene delivery

One of the major obstacles in the use of baculovirus vectors for in vivo gene transfer is the virus inactivation by serum complement. In this study, we investigated the effect of decay-accelerating factor (DAF), factor H (FH)-like protein-1 (FHL-1), C4b-binding protein (C4BP), and membrane cofactor protein (MCP) on protection of baculovirus vectors from the complement-mediated inactivation. Complement regulatory proteins were displayed on baculovirus surface as fusions to membrane anchor of the vesicular stomatitis virus-G (VSV-G) protein. This strategy resulted in abundant expression of recombinant proteins on the viral envelope while viral titers comparable to control virus were reached. The surface-modified vectors exhibited complement resistance in vitro, DAF showing the highest level of protection. Intraportal delivery of DAF-displaying baculovirus resulted in increased survival and enhanced gene expression in immunocompetent mice. Mice receiving DAF-displaying baculovirus also exhibited lower level of liver inflammation as evidenced by aspartate aminotransferase (AST). In line with this, macrophages treated with DAF baculovirus produced lower levels of inflammatory cytokines IL-1beta, IL-6, and IL-12p40 compared to control virus. These results suggest that DAF-display can protect the vector against complement inactivation but also reduce complement-mediated inflammation injury. In conclusion, complement shielded baculovirus vectors represent attractive tools for effective in vivo gene delivery.

Concanavalin A affinity chromatography for efficient baculovirus purification

Baculovirus has emerged as a novel gene delivery and vaccine vector, and the demand for purified baculovirus is rising due to the increasing in vivo applications. Since the baculoviral envelope protein gp64 is a glycoprotein, we aimed to develop a concanavalin A (Con A) chromatography process, which harnessed the possible affinity interaction between gp64 and Con A, for simple and effective baculovirus purification. Throughout the purification process the virus stability and recovery were assessed by quantifying the virus transducing titers [TT, defined as transducing units (TU) per milliliter] and viral particles (VP). We found that baculovirus stability was sensitive to buffer conditions and diafiltration with a tangential flow filtration system LabScale using 300 K membranes yielded recoveries of approximately 75% in TT and 82% in VP. The diafiltered baculovirus strongly bound to the Con A column as evidenced by the low virus losses to the flow through and wash fractions. The wash steps eliminated >99% of protein impurities and elution with 0.6 M alpha-D-methylmannoside at room temperature led to the recoveries of approximately 16% in VP and approximately 15.3% in TU. The resultant VP/TU ratio was as low as 41.4, attesting the high quality of the purified virus. Further elution with 1 M alpha-D-methylmannoside recovered another 6% virus TU, yielding a cumulative recovery of approximately 21.3% in TU. These data demonstrated for the first time that Con A chromatography is suitable for baculovirus purification, and may be used for the purification of other viruses with surface glycoproteins.

Autographa californica multicapsid nucleopolyhedrovirus efficiently infects Sf9 cells and transduces mammalian cells via direct fusion with the plasma membrane at low pH

The budded virus (BV) of the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) infects insect cells and transduces mammalian cells mainly through the endocytosis pathway. However, this study revealed that the treatment of the virus bound to Sf9 cells at low pH could efficiently rescue the infectivity of AcMNPV in the presence of endocytosis pathway inhibitors. A colocalization assay of the major capsid protein VP39 with the early endosome marker EEA1 showed that at low pH, AcMNPV entered Sf9 cells via an endosome-independent pathway. Using a fluorescent probe (R18), we showed that at low pH, the viral nucleocapsid entered Sf9 cells via direct fusion at the cell surface. By using the myosin-specific inhibitor 2,3-butanedione monoxime (BDM) and the microtubule inhibitor nocodazole, the low pH-triggered direct fusion was demonstrated to be dependent on myosin-like proteins and independent of microtubules. The reverse transcription-PCR of the IE1 gene as a marker for viral entry showed that the kinetics of AcMNPV in cells triggered by low pH was similar to that of the normal entry via endocytosis. The low pH-mediated infection assay and VP39 and EEA1 colocalization assay also demonstrated that AcMNPV could efficiently transduce mammalian cells via direct membrane fusion at the cell surface. More importantly, we found that a low-pH trigger could significantly improve the transduction efficiency of AcMNPV in mammalian cells, leading to the potential application of this method when using baculovirus as a vector for heterologous gene expression and for gene therapy.

Baculovirus-based display and gene delivery systems

The baculovirus expression vector system has been used extensively to produce numerous proteins originating from both prokaryotic and eukaryotic sources. In addition to easy cloning techniques and abundant viral propagation, the system's insect cell environment provides eukaryotic post-translational modification machinery. The recently established eukaryotic molecular biology tool, the baculovirus display vector system (BDVS), allows the combination of genotype with phenotype, enabling presentation of foreign peptides or even complex proteins on the baculoviral envelope or capsid. This strategy is important because it can be used to enhance viral binding and entry to mammalian cells as well as to produce antibodies against the displayed antigen. In addition, the technology should enable modifications of intracellular behavior, that is, trafficking of recombinant "nanoparticles," a highly relevant feature for studies of targeted gene or protein delivery. This article discusses the design and potential uses of insect-derived baculoviral display vectors.

Culture medium induced vimentin reorganization associates with enhanced baculovirus-mediated gene delivery

Baculoviruses can express transgenes under mammalian promoters in a wide range of vertebrate cells. However, the success of transgene expression is dependent on both the appropriate cell type and culture conditions. We studied the mechanism behind the substantial effect of the cell culture medium on efficiency of the baculovirus transduction in different cell lines. We tested six cell culture mediums; the highest transduction efficiency was detected in the presence of RPMI 1640 medium. Vimentin, a major component of type III intermediate filaments, was reorganized in the optimized medium, which associated with enhanced nuclear entry of baculoviruses. Accordingly, the phosphorylation pattern of vimentin was changed in the studied cell lines. These results suggest that vimentin has an important role in baculovirus entry into vertebrate cells. Enhanced gene delivery in the optimized medium was observed also with adenoviruses and lentiviruses. The results highlight the general importance of the culture medium in the assembly of the cytoskeleton network and in viral gene delivery.

Polyethylenimine coating to produce serum-resistant baculoviral vectors for in vivo gene delivery

Recombinant baculoviral vectors efficiently transduce many types of mammalian cells. However, their in vivo applications are hampered by the sensitivity of the virus to complement-mediated inactivation. Based on our observation that the surface charge of baculovirus is negative at neutral pH, we developed a procedure to coat baculoviral vectors with positively charged polyethylenimine 25 kDa, a commonly tested non-viral gene delivery vector, through electrostatic interaction. This coating was effective in protecting baculoviral vectors against human and rat serum-mediated inactivation in vitro, providing transduction efficiencies comparable with that generated by the control virus used under a serum-free condition. Enhanced in vivo gene expression in the liver and spleen was observed after tail vein injection of the coated viruses into mice. When injected directly into human tumor xenografts in nude mice, the coated viruses suppressed tumor development more effectively than uncoated viral vectors. These findings demonstrated the usefulness of using a simple coating method to circumvent a major obstacle to in vivo application of baculoviral vectors. The method may also serve as a flexible platform technology for improved use of the vectors, for example introducing a targeting ligand and minimizing immune responses.

(Strept)avidin-displaying lentiviruses as versatile tools for targeting and dual imaging of gene delivery

Lentiviruses have shown great promise for human gene therapy. However, no optimal strategies are yet available for noninvasive imaging of virus biodistribution and subsequent transduction in vivo. We have developed a dual-imaging strategy based on avidin-biotin system allowing easy exchange of the surface ligand on HIV-derived lentivirus envelope. This was achieved by displaying avidin or streptavidin fused to the transmembrane anchor of vesicular stomatitis virus G protein on gp64-pseudotyped envelopes. Avidin and streptavidin were efficiently incorporated on virus particles, which consequently showed binding to biotin in ELISA. These vectors, conjugated to biotinylated radionuclides and engineered to express a ferritin transgene, enabled for the first-time dual imaging of virus biodistribution and transduction pattern by single-photon emission computed tomography and magnetic resonance imaging after stereotactic injection into rat brain. In addition, vector retargeting to cancer cells overexpressing CD46, epidermal growth factor and transferrin receptors using biotinylated ligands and antibodies was demonstrated in vitro. In conclusion, we have generated novel lentivirus vectors for noninvasive imaging and targeting of lentivirus-mediated gene delivery. This study suggests that these novel vectors could be applicable for the treatment of central nervous system disorders and cancer.

Baculovirus vector as a delivery vehicle for influenza vaccines

The baculovirus vector has emerged as an efficient delivery vehicle for influenza vaccines. In addition to the ease and safety in expeditious production, recent improvements in baculovirus engineering to display foreign proteins on the surface and to express transgenes with suitable promoters in various cell lines have become milestones in the development of the baculovirus expression system. Surface-displayed and shuttle promoter-mediated baculovirus vaccines for influenza present advantages in immunogenicity and safety, as studied in several animal models. A variety of strategies, including the modification of envelope proteins for surface display, the selection of novel promoters for in vivo transductions and advancements in downstream processing, aid the improvement of baculovirus-based influenza vaccines and represent progress toward next-generation vaccines for influenza.

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.

Tumor targeting of baculovirus displaying a lymphatic homing peptide

BACKGROUND

Tumor-associated cells and vasculature express attractive molecular markers for site-specific vector targeting. To attain tumor-selective tropism, we recently developed a baculovirus vector displaying the lymphatic homing peptide LyP-1, originally identified by ex vivo/in vivo screening of phage display libraries, on the viral envelope by fusion to the transmembrane anchor of vesicular stomatitis virus G-protein.

METHODS

In the present study, we explored the specificity and kinetics of viral binding and internalization as well as in vivo tumor homing of the LyP-1 displaying virus to elucidate the applicability of baculovirus for targeted therapies.

RESULTS

We demonstrated that the LyP-1 peptide contributes to saturable binding of baculovirus in human MDA-MB-435 and HepG2 carcinoma cells and escalates the kinetics of viral internalization leading to earlier nuclear accumulation and enhanced transgene expression. The LyP-1 displaying virus also showed stronger competitiveness against transduction with wild-type baculovirus, suggesting involvement of a specific receptor in cellular attachment and entry. Following intravenous injections, the modified virus accumulated within the human MDA-MB-435 and MDA-MB-231 carcinoma xenografts in mice with higher specificity and efficiency than the control virus. Targeting of the modified virus was more specific in the MDA-MB-435 than in the MDA-MB-231 xenografts as demonstrated by higher tumor accumulation and lower distribution in nontarget organs. No apparent cytotoxicity was associated with the surface modification.

CONCLUSIONS

This first demonstration of in vivo tumor targeting of a systemically administered, tropism-modified baculoviral vector highlights the potential of baculovirus-mediated targeted therapies.

Virus production for clinical gene therapy

Gene therapy is becoming increasingly relevant for the treatment of prominent human diseases. Viral vectors are currently used in more than 50% of the gene therapy clinical trials, most of them aimed at cancer diseases. Clearly, the increasing needs of high-quality viral preparations require the elimination of process bottlenecks, streamlining the development of a viral vector into a real-world clinical tool. Virus production for clinical gene therapy can be a limiting step because many virus generation protocols rely on labor-intensive, bench-scale methods; robust, cost-effective strategies for the delivery of clinical-grade viruses are thus essential for the future of gene therapy. A comprehensive picture of key aspects on the integration of upstream and downstream processing is addressed in this chapter, by describing the case study of recombinant budded baculoviruses for gene therapy; scalable methods are described in detail as well as mandatory characterization techniques for a proper and complete quality assessment of the viral vectors.

Sustained baculovirus-mediated expression in myogenic cells

BACKGROUND

Baculovirus has emerged as a promising gene delivery vector due to its low cytotoxicity and nonreplication nature in mammalian cells. However, baculovirus-mediated expression is transient and generally lasts less than 14 days, which could restrict its application in the treatment of diseases requiring stable transgene expression.

METHODS

We transduced myoblast cell lines C2C12, Sol 8 and primary myoblasts with a baculovirus expressing the enhanced green fluorescent protein (EGFP) under the control of cytomegalovirus immediate-early promoter and measured the transduction efficiency by flow cytometry. Myogenic differentiation was induced after transduction and the longevity of EGFP expression was monitored by fluorescence microscopy. The myogenic differentiation was confirmed by reverse transcription-polymerase chain reaction (RT-PCR). The persistence of the egfp DNA and transcripts was monitored by real-time PCR and quantitative real-time RT-PCR.

RESULTS

Baculovirus efficiently transduced C2C12, Sol 8 and the primary myoblasts. The transgene expression persisted for a prolonged period of time (at least 63 days) in the cells differentiating into myotubes, but was transient in HeLa cells (<7 days). The sustained expression paralleled the myogenic differentiation and stemmed from the intracellular persistence of egfp DNA and mRNA.

CONCLUSIONS

The transgene delivered by baculovirus persists in the myotubes and endows sustained expression, which is distinct from its rapid degradation and transient expression in other cell types. These findings justify the future use of baculovirus for muscle-based gene therapy.

Targeting and purification of metabolically biotinylated baculovirus

Targeting viral entry is one of the major goals in the development of vectors for gene therapy. Ideally, the coupling of each new targeting motif would not require changes in vector structure. To achieve this, we developed novel metabolically biotinylated baculoviral vectors by displaying a small biotin acceptor peptide (BAP) fused either to different sites in the baculovirus glycoprotein gp64 or to the transmembrane anchor of vesicular stomatitis virus G protein. Baculoviral particles were biotinylated during vector production by coexpression of Escherichia coli biotin ligase (BirA). The insertion of BAP at amino acid position 283 of gp64 resulted in the most efficient biotin display. Unlike vectors with lower biotin display, these vectors also showed improved transduction when retargeted to transferrin, epidermal growth factor, and CD46 receptors overexpressed on rat glioma and human ovarian carcinoma cells. Biotinylated baculoviral vectors could also be concentrated by one-step magnetic particle-based capture to reach titers up to 10(10) plaque-forming units/ml. These results demonstrate the utility of metabolically biotinylated baculovirus for vector targeting and viral purification applications.