High efficiency gene transfer into mammalian kidney cells using baculovirus vectors

Recovery Infectious Enterovirus 71 by Bac-to-Bac Expression System in vitro and in vivo

Enterovirus 71 (EV71) is one of the most important etiological agents for hand–foot–mouth disease. Compared with coxsackievirus A16 infection, EV71 infection is often associated with severe central nervous system complications, such as encephalitis, encephalomyelitis, and acute flaccid paralysis in infants and young children. In this study, we constructed a recombinant baculovirus with T7 ribonucleic acid polymerase under the control of a cytomegalovirus promoter and simultaneously engineered the T7 promoter upstream of a full-length EV71 complementary deoxyribonucleic acid. After transduction into mammalian cells, typical cytopathic effects (CPEs) and VP1 signals were detected in cells transfected with recombinant baculovirus. Additionally, viral particles located in the cytoplasm of human rhabdomyosarcoma cells (Rd) and Vero cells were observed by electron microscope, indicating that EV71 was recovered using a Bac-to-Bac expression system in vitro. After four passages, the rescued virus had a growth curve and plaque morphology similar to those of the parental virus. Furthermore, the Vp1 gene and the protein from the mouse brain were detected by reverse transcription polymerase chain reaction and immunohistochemistry after intracerebral injection of purified recombinant baculovirus. Typical CPEs were observed after inoculation of the supernatant from mouse brain to Rd cells, revealing a reconstruction of EV71 in vivo. Thus, we established a new approach to rescue EV71 based on a baculovirus expression system in vitro and in vivo, which may provide a safe and convenient platform for fundamental research and a strategy to rescue viruses that currently lack suitable cell culture and animal models.

Expression Screening of Human Integral Membrane Proteins Using BacMam

This chapter describes the step-by-step methods employed by the Structural Genomics Consortium (SGC) for screening and producing proteins in the BacMam system. This eukaryotic expression system was selected and a screening process established in 2016 to enable production of highly challenging human integral membrane proteins (IMPs), which are a significant component of our target list. Here, we discuss our recently developed platform for identifying expression and monodispersity of IMPs from 3 mL of HEK293 cells.

The Pacific White Shrimp β-actin Promoter: Functional Properties and the Potential Application for Transduction System Using Recombinant Baculovirus

A newly isolated Pacific white shrimp (Litopenaeus vannamei) beta-actin promoter SbaP and its derivative compact construct SbaP (ENX) have recently been demonstrated to promote ectopic gene expression in vitro and in vivo. To further explore the potential transduction application, this newly isolated shrimp promoter SbaP was comparatively tested with cytomegalovirus (CMV), simian virus 40 (SV40), polyhedrin (Polh), and white spot syndrome virus immediate early gene 1 (WSSV ie1) four constitutive promoters and a beta-actin promoter (TbaP) from tilapia fish to characterize its promoting function in eight different cell lines. Luciferase quantitation assays revealed that SbaP can drive luciferase gene expression in all eight cell lines including sf21 (insect), PAC2 (zebrafish), EPC (carp), CHSE-214 (chinook salmon), GSTEF (green sea turtle), MS-1 (monk seal), 293T (human), and HeLa (human), but at different levels. Comparative analysis revealed that the promoting activity of SbaP was lower (≤10-fold) than CMV but higher (2-20 folds) than Polh in most of these cell lines tested. Whereas, SbaP mediated luciferase expression in sf21 cells was over 20-fold higher than CMV, SV40, Polh, and TbaP promoter. Compared to the SbaP, SbaP (ENX), which was constructed on the basis of SbaP by deletion of two "negative" regulatory elements, exhibited no significant change of promoting activity in EPC and PAC2 cells, but a 5 and 16 % lower promoting effect in 293T and HeLa cells, respectively. Additionally, a recombinant baculovirus was constructed under the control of SbaP (ENX), and efficient promoter activity of newly generated baculoviral vector was detected both in vitro of infected sf21 cells and in vivo of injected indicator shrimp. These results warrant the potential application of SbaP, particularly SbaP (ENX) in ectopic gene expression in future.

Autographa californica multiple nucleopolyhedrovirus PK-1 is essential for nucleocapsid assembly

PK-1 (Ac10) is a baculovirus-encoded serine/threonine kinase and its function is unclear. Our results showed that a pk-1 knockout AcMNPV failed to produce infectious progeny, while the pk-1 repair virus could rescue this defect. qPCR analysis demonstrated that pk-1 deletion did not affect viral DNA replication. Analysis of the repaired recombinants with truncated pk-1 mutants demonstrated that the catalytic domain of protein kinases of PK-1 was essential to viral infectivity. Moreover, those PK-1 mutants that could rescue the infectious BV production defect exhibited kinase activity in vitro. Therefore, it is suggested that the kinase activity of PK-1 is essential in regulating viral propagation. Electron microscopy revealed that pk-1 deletion affected the formation of normal nucleocapsids. Masses of electron-lucent tubular structures were present in cell transfected with pk-1 knockout bacmid. Therefore, PK-1 appears to phosphorylate some viral or cellular proteins that are essential for DNA packaging to regulate nucleocapsid assembly.

AcMNPV enhances infection by ThorNPV in Sf21 cells and SeMNPV in Hi5 cells

An expression cassette containing the DsRed2 gene, which encodes the red fluorescent protein (RFP), was inserted into the wide-host-range Autographa californica M nucleopolyhedrovirus (AcMNPV) at the polyhedrin locus (vAcDsRed2). An expression cassette containing the enhanced green fluorescent protein (EGFP) gene was inserted at the gp37 locus of the narrow-host-range Thysanoplusia orichalcea MNPV (ThorMNPV) and the p10 locus of Spodoptera exigua MNPV (SeMNPV) to produce vThGFP and vSeGFP, respectively. vThGFP and vSeGFP are poor at infecting Sf21 and Hi5 cells, respectively, whereas vAcDsRed2 is highly infectious to both cell lines. During co-infection, vAcDsRed2 enhanced vThGFP infection in Sf21 cells by approximately 20-fold, and it enhanced vSeGFP infection in Hi5 cells by more than 300-fold, as detected by fluorescence measurements. In contrast, vThGFP reduced vAcDsRed2 infection by 5.4-fold in Sf21 cells, while vSeGFP reduced vAcDsRed2 by 3.2-fold in Hi5 cells. Plaque assay data did not suggest viral recombination, but vThGFP plaques surrounded by vAcDsRed2 plaques were observed. A viral DNA replication assay performed by real-time quantitative PCR suggested that the detected fluorescence correlated with virus replication. Sf21 cells infected with vAcDsRed2 were resistant to superinfection by viruses of the same type expressing EGFP (vAcGFP). These results demonstrated that AcMNPV could enhance replication of ThorMNPV and SeMNPV in non-permissive cells without recombination.

Baculoviral expression of telomerase in primary human fibroblasts to rejuvenate cells for tissue engineering

Tissue engineering involves the use of synthetic or natural materials as a scaffold to support the growth of replacement tissue or organs. The use of autologous cells to populate the scaffold avoids problems associated with rejection; however, a major limitation of this approach is the finite lifespan of primary cells in culture. This finite lifespan is due to the shortening of telomeres, short repetitive sequences of DNA located at the ends of eukaryotic chromosomes. Ectopic expression of telomerase reverse transcriptase (hTERT) is able to reconstitute telomerase activity and maintain the length of telomeres. This study investigated an alternative gene delivery vector, baculovirus, for the expression of hTERT in primary human cells. A recombinant baculovirus was used to efficiently deliver the hTERT gene to primary fibroblasts and the telomerase enzyme was found to be active. Although no increase in telomere length was detected, expression of hTERT in primary fibroblasts resulted in a significant extension of replicative lifespan. To our knowledge this is a novel attempt to use a recombinant baculovirus for the extension of cellular lifespan by exogenous expression of telomerase.

Efficient gene delivery into fish cells by an improved recombinant baculovirus

Studies on transduction of mammalian cells have shown that baculovirus can be used as an effective vector for gene delivery. However, previous studies have found the gene delivery efficiency to be very low in differentiated fish cells. In this study, an improved recombinant baculovirus, containing cytomegalovirus immediate-early (CMV-IE) promoter and an enhanced green fluorescent protein (EGFP) gene as the reporter gene, was constructed. The transduction efficiency of recombinant baculovirus in several fish cell lines was measured by flow cytometry (FCM) and the persistence of EGFP was monitored by fluorescence microscopy. The results demonstrated that baculovirus can mediate the high efficiency of gene delivery into differentiated fish cells. Furthermore, it was found that growth medium is superior to PBS as the surrounding solution to enhance the transduction efficiency in some fish cells. In addition, transgene expression can persist for a lengthy period in fish cells, and attained highest level several days later after transduction. This study suggest that the improved recombinant baculovirus can be an excellent gene delivery vector for fish cells and also providing new insights into the transduction of vertebrate cells with baculovirus.

Baculovirus vector-mediated transfer of NIS gene into colon tumor cells for radionuclide therapy

AIM: To investigate the feasibility of radionuclide therapy of colon tumor cells by baculovirus vector-mediated transfer of the sodium/iodide symporter (NIS) gene.

METHODS: A recombinant baculovirus plasmid carrying the NIS gene was constructed, and the viruses (Bac-NIS) were prepared using the Bac-to-Bac system. The infection efficiency in the colon cancer cell line SW1116 of a green fluorescent protein (GFP) expressing baculovirus (Bac-GFP) at different multiplicities of infection (MOI) with various concentrations of sodium butyrate was determined by flow cytometry. An in vitro cytotoxicity assay was also conducted after infection of SW1116 cells with Bac-NIS. Iodine uptake of Bac-NIS infected SW1116 cells and inhibition of this uptake by sodium perchlorate was examined, and the effect of Bac-NIS-mediated ¹³¹I in killing tumor cells was evaluated by cell colony formation tests.

RESULTS: Infection and transgene expression in SW1116 with Bac-GFP were significantly enhanced by sodium butyrate, as up to 72% of SW1116 cells were infected with the virus at MOI of 400 and sodium butyrate at 0.5 mmol/L. No obvious cytotoxicity was observed under these conditions. Infection of SW1116 with Bac-NIS allowed uptake of ¹³¹I in these tumor cells, which could be inhibited by sodium perchlorate. The viability of SW1116 cells infected with Bac-NIS was significantly lower than with Bac-GFP, suggesting that NIS gene-mediated ¹³¹I uptake could specifically kill tumor cells.

CONCLUSION: Baculovirus vector-mediated NIS gene therapy is a potential approach for treatment of colon cancer.

Efficient gene delivery into mammalian cells by recombinant baculovirus containing a hybrid cytomegalovirus promoter/Semliki Forest virus replicon

BACKGROUND

Baculovirus, which is widely utilized as an excellent tool for the production of recombinant protein in insect cells, has recently emerged as a novel and attractive gene delivery vehicle for mammalian cells. Alphavirus, such as Semliki Forest virus (SFV), has also received considerable attention for use as expression vectors because of its self-replicating property. In the present study, we investigated the characterization of recombinant baculovirus incorporating a hybrid cytomegalovirus (CMV) promoter/SFV replicon.

METHODS

Recombinant baculovirus containing the hybrid CMV promoter/SFV replicon was constructed. Using enhanced green fluorescence protein (EGFP) as the reporter gene, gene delivery efficiencies and the ability to express heterogenous protein in mammalian cells were evaluated. Optimal transduction conditions, including transduction temperature, time and dose, were also investigated.

RESULTS

The obtained recombinant baculovirus, Bac-CMV/SFV-EGFP, exhibited high transduction efficiency and high-level expression of reporter protein in mammalian cells. Furthermore, this recombinant baculovirus could induce apoptosis in mammalian cells in the course of transduction, as demonstrated by the observed DNA laddering patterns and increased caspase-3 activity.

CONCLUSIONS

The developed baculovirus vector has a high transduction efficiency and the ability to mediate foreign gene expression in mammalian cells. Taken together with its pro-apoptotic properties, this baculovirus vector may provide an alternative tool for vaccine development.

The feasibility of using a baculovirus vector to deliver the sodium-iodide symporter gene as a reporter

PURPOSE

To evaluate the efficiency of baculovirus vectors in transducing FTC-133 cells and to examine the feasibility of using baculovirus vectors for the delivery of the sodium-iodide symporter (NIS) gene as a reporter through co-transduction to monitor the expression of the target gene.

METHOD

Two recombinant baculoviruses were constructed to express NIS and green fluorescent protein (GFP) respectively. FTC-133, 8050C, SW1116, A549 cells, were infected with Bac-GFP. The infection efficiency of Bac-GFP and the intensity of fluorescence, in either the presence or absence of sodium butyrate, were monitored by flow cytometry. The iodine uptake by FTC-133 cells infected with Bac-NIS was measured using a gamma counter. FTC-133 cells were infected with a mixture of equal amounts of Bac-NIS and Bac-GFP at different setting of multiplicity of infection (MOI). The changes of GFP fluorescence intensity and iodine uptake were monitored 24 h after infection in the coinfected cells.

RESULTS

We have successfully constructed recombinant baculoviruses carrying NIS and GFP under the control of the cytomegalovirus IE-1 promoter. We found that transduced efficiency of baculovirus in 8505C, SW1116, A549 cells are low in absence of sodium butyrate. Yet Bac-GFP infects FTC-133 cells at a high efficiency, 77.67%, 85.57% and 93.23% with MOI of 100, 200 and 400, respectively. The fluorescence intensity of the Bac-GFP infected tumor cells correlated positively with the MOI of the virus. Sodium butyrate induction increased both the infection efficiency and the fluorescence intensity, but increase of infection efficiency was insignificant in FTC-133 cells. Reporter gene (GFP) expression in FTC-133 is stable within 7 days after infection. The radioactivity incorporated by the tumor cells infected with Bac-NIS correlated positively with the MOI of Bac-NIS as well. In tumor cells co-infected with Bac-NIS and Bac-GFP, the amount of radioactivity incorporated significantly correlated with the GFP fluorescence intensity (r=0.922).

CONCLUSION

Baculovirus vectors are powerful vehicles for studying FTC-133 tumor cells in gene delivery. It is feasible to use a baculovirus vector to deliver NIS as a reporter gene to monitor the expression of target genes. This is therefore an effective approach for the detection of target gene expression in gene therapy.

Transduction of vertebrate cells with Spodoptera exigua multiple nucleopolyhedrovirus F protein-pseudotyped gp64-null Autographa californica multiple nucleopolyhedrovirus

Budded virions of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) can enter a variety of non-host cells. The capacity of GP64, AcMNPV's endogenous envelope fusion protein, and SeF, the fusion protein from a gp64(-) baculovirus, to mediate baculovirus entry into vertebrate cells was examined by comparing the transduction efficiencies of engineered AcMNPV variants with either of the two envelope proteins into 17 vertebrate cell lines. At an m.o.i. of 500, GP64-expressing viruses transduced all cell lines with varying efficiencies. Transduction efficiencies of SeF-pseudotyped gp64-null AcMNPV into all cell lines were lower than those of GP64-expressing viruses, and were undetectable in seven cell lines. At an m.o.i. of 50, transduction of all mammalian cell lines transducible by the SeF-pseudotyped gp64-null AcMNPV at an m.o.i. of 500 was no longer detectable. An amplifiable SeF-pseudotyped gp64-null AcMNPV vector with greatly reduced tropism for vertebrate cells may have applications in engineering AcMNPV for targeted transduction.

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.

Effects of temperature and shear force on infectivity of the baculovirus Autographa californica M nucleopolyhedrovirus

Virus stability and infectivity during stressful conditions was assessed to establish guidelines for future virus filtration experiments and to contribute to the body of knowledge on a widely used virus. A recombinant baculovirus of Autographa californica M nucleopolyhedrovirus (AcMNPV), vHSGFP, was incubated at 15-65 degrees C. A 2-log decrease in virus infectivity occurred after virus incubation above 45 degrees C. The activation energy of virus deactivation was circa 108 kJ/mol. Dynamic light scattering revealed an increase in apparent virus particle size from 150+/-19 to 249+/-13 nm at 55 degrees C. Protein and DNA concentrations in solution correlated well with virus aggregation as temperature was increased. Infectivity of vHSGFP stored for 5 months at 4 degrees C or exposed to shear stress from stirring (100 rpm, 1.02x10(-5) psi) and pumping (50-250 ml/min, 1.45x10(-5) to 7.25x10(-5) psi) did not change with time. Unlike temperature variations, cold storage and shear stress appeared to have little impact on infectivity.

The histone deacetylase inhibitor sodium butyrate inhibits baculovirus-mediated transgene expression in Sf9 cells

Recent studies have indicated that histone deacetylase inhibitors (HDACis) could enhance and prolong expression of exogenous genes delivered by various viral vehicles in mammalian cells, including baculovirus vectors. In this study, the effects of HDACis on expression of a baculovirus-mediated eGFP reporter gene under control of baculovirus late promoter p10 in Sf9 cells were evaluated. It was found that sodium butyrate (NaBu) decreased the expression level of the target gene driven by p10 promoter by four to fivefold. Moreover, addition of NaBu increased DNaseI-sensitivity of transgene p10 promoter region and did not influence viral DNA replication. FACS assay has shown that both NaBu and fluorodeoxyuridine (FdUrd) blocked Sf9 cells at G1 phase and inhibited the target gene expression. Another HDACi, trichostatin, had little effects on both cell cycle and Ac-p10-eGFP expression, strongly suggesting that cell cycle arrest accounts for the mechanisms by which NaBu inhibits Ac-p10-eGFP expression. The inhibiting effects of NaBu on baculovirus transgene expression in Sf9 cells are promoter specific since the enhancement of NaBu on transgene expression in insect and mammalian cells are mediated by baculovirus harboring a murine cytomegalovirus (mCMV) immediate early promoter. This study was aimed at improving the productivity of the recombinant proteins and providing a better understanding of the epigenetic regulation of baculovirus gene expression.

Non-invasive Imaging in Gene Therapy

Several methods are available for non-invasive imaging of gene delivery and transgene expression, including magnetic resonance imaging (MRI), single photon emission tomography (SPECT)/positron emission tomography (PET), and fluorescence and bioluminescence imaging. However, these imaging modalities differ greatly in terms of their sensitivity, cost, and ability to measure the signal. Whereas MRI can produce a resolution of approximately 50 mum, optical imaging achieves only 3-5 mm but outperforms MRI in terms of the cost of the imaging device. Similarly, SPECT and PET give a resolution of only 1-2 mm but provide for relatively easy quantitation of the signal and need only nanograms of probe, compared with the microgram or milligram levels required for MRI and optical imaging. To develop safer and more efficient gene delivery vectors, it is essential to perform rigorous in vivo experiments, to image particle biodistribution and transduction patterns, and to quantify the transgene expression profile. Differences between modalities have a significant effect on the resultant imaging resolution for gene therapy. This review describes the methodologies in use and highlights recent key approaches using the latest imaging modalities in gene therapy. Future trends in gene therapy imaging are also discussed.

SPECT/CT imaging of baculovirus biodistribution in rat

Non-invasive imaging provides essential information regarding the biodistribution of gene therapy vectors and it can also be used for the development of targeted vectors. In this study, we have utilized micro Single-photon emission computed tomography to visualize biodistribution of a (99m)Tc-polylys-ser-DTPA-biotin-labelled avidin-displaying baculovirus, Baavi, after intrafemoral (i.f.), intraperitoneal (i.p.), intramuscular (i.m.) and intracerebroventricular (i.c.v.) administration. The imaging results suggest that the virus can spread via the lymphatic network after different administration routes, also showing accumulation in the nasal area after systemic administration. Extensive expression in the kidneys and spleen was seen after i.p. administration, which was confirmed by reverse transcriptase-polymerase chain reaction and immunohistochemistry. Additionally, transduction of kidneys was seen with i.m. and i.f. administrations. We conclude that baculovirus may be beneficial for the treatment of kidney diseases after i.p. administration route.

Group I but not group II NPV induces antiviral effects in mammalian cells

Nucleopolyhedrovirus (NPV) is divided into Group I and Group II based on the phylogenetic analysis. It has been reported that Group I NPVs such as Autographa californica multiple NPV (AcMNPV) can transduce mammalian cells, while Group II NPVs such as Helicoverpa armigera single NPV (HaSNPV) cannot. Here we report that AcMNPV was capable of stimulating antiviral activity in human hepatoma cells (SMMC-7721) manifested by inhibition of Vesicular Stomatitis virus (VSV) replication. In contrast, the HaSNPV and the Spodoptera exigua multiple NPV (SeMNPV) of group II had no inhibitory effect on VSV. Recombinant AcMNPV was shown to induce interferons alpha/beta even in the absence of transgene expression in human SMMC-7721 cells, while it mediated transgene expression in BHK and L929 mammalian cells without an ensuing antiviral activity.

Enhanced baculovirus-mediated transduction of human cancer cells by tumor-homing peptides

Tumor cells and vasculature offer specific targets for the selective delivery of therapeutic genes. To achieve tumor-specific gene transfer, baculovirus tropism was manipulated by viral envelope modification using baculovirus display technology. LyP-1, F3, and CGKRK tumor-homing peptides, originally identified by in vivo screening of phage display libraries, were fused to the transmembrane anchor of vesicular stomatitis virus G protein and displayed on the baculoviral surface. The fusion proteins were successfully incorporated into budded virions, which showed two- to fivefold-improved binding to human breast carcinoma (MDA-MB-435) and hepatocarcinoma (HepG2) cells. The LyP-1 peptide inhibited viral binding to MDA-MB-435 cells with a greater magnitude and specificity than the CGKRK and F3 peptides. Maximal 7- and 24-fold increases in transduction, determined by transgene expression level, were achieved for the MDA-MB-435 and HepG2 cells, respectively. The internalization of each virus was inhibited by ammonium chloride treatment, suggesting the use of a similar endocytic entry route. The LyP-1 and F3 peptides showed an apparent inhibitory effect in transduction of HepG2 cells with the corresponding display viruses. Together, these results imply that the efficiency of baculovirus-mediated gene delivery can be significantly enhanced in vitro when tumor-targeting ligands are used and therefore highlight the potential of baculovirus vectors in cancer gene therapy.

Analysis of baculovirus aggregates using flow cytometry

Aggregation of viral particles represents a significant problem for baculoviral stock processing and storage. Aggregation may also affect the results of viral particle counting. A method using flow cytometry was previously developed in our lab to measure the concentration of baculovirus particles produced in insect cell cultures. In the present study, the use of the flow cytometry method was extended to the detection of baculovirus aggregates. Flow cytometry analysis of freshly prepared baculovirus stocks, stained with SYBR Green, generally exhibited a single unimodal distribution; while, baculovirus stocks stored at 4 degrees C for a few months exhibited a bimodal distribution of the fluorescent intensity signal. The bimodal distribution was associated with a decrease in the size of the original viral population and an emergence of a new viral population with a high fluorescence intensity. Treatment of these samples with an endonuclease (Benzonase) confirmed that the new population observed in the flow cytometry analysis is not free cellular DNA. Filtration through 0.22 and 0.45 microm membranes of the stored samples prior to flow cytometry analysis confirmed that the high fluorescence intensity population involved particles larger than a single baculovirus. Exposing freshly amplified baculovirus stocks with a unimodal distribution to a pH of 5.3, a condition known to induce aggregation, showed the emergence of a second population with a bimodal distribution. These results suggest that flow cytometry analysis could be used to detect baculovirus aggregates. The aggregates were associated with high fluorescence intensity populations and the mean green fluorescence intensity of these populations could be used as an indicator of the mean aggregate size.

A novel recombinant virus-like particle vaccine for prevention of porcine parvovirus-induced reproductive failure

A novel vaccine against porcine parvovirus (PPV), composed of recombinant virus-like particles (PPV-VLPs) produced with the baculovirus expression vector system (BEVS) at industrial scale, was tested for its immunogenicity and protective potency. A formulation of submicrogram amounts of PPV-VLPs in a water-in-mineral oil adjuvant evoked high serum antibody titres in both guinea pigs, used as reference model, and target species, pigs. A single immunisation with 0.7microg of this antigen yielded complete foetal protection against PPV infection after challenge with a virulent strain of this virus. Furthermore, also in the presence of mild adjuvants the protective action of these PPV-VLPs is excellent. This recombinant subunit vaccine overcomes some of the drawbacks of classical PPV vaccines.

Transduction of avian cells with recombinant baculovirus

Transduction of chicken and duck cells was examined by Ac-CMV-eGFP, a recombinant baculovirus capable of expressing an eGFP reporter gene under the control of the CMV promoter. The results showed that chicken and duck cells were transducible, as demonstrated by a flow cytometry assay. The transduction efficiency of duck cells was higher than that of chicken cells. The addition of histone deacetylase inhibitor sodium butyrate enhanced the expression levels of eGFP both in chicken and duck cells. Ac-CMV-eGFP is capable of transducing genes into a variety of chicken cells in organs such as liver, lung and kidney. Of three cells from different organs tested, the highest transduction was observed in lung cells (49.8%), followed by kidney cells (44%) and liver cells (43%). Only in chicken liver cells AcMNPV transduction was in a dose-dependent manner. It also showed that baculovirus enters the avian cells by endocytosis and is released into the cytoplasm by acid-induced fusion.

Efficient and stable gene expression in rabbit intervertebral disc cells transduced with a recombinant baculovirus vector

STUDY DESIGN

A recombinant baculovirus vector expressing GFP (Ac-CMV-GFP) was tested in rabbit intervertebral disc cells cultured in monolayer in vitro. Direct infection of intervertebral disc cells was then assessed in vivo.

OBJECTIVES

To test the efficacy of the baculovirus-mediated marked gene transfer to rabbit intervertebral disc cells, and the expression of the transgene in vitro and in vivo.

SUMMARY OF BACKGROUND DATA

The potential application of gene therapy for the treatment of degenerative disc disease has been an area of exciting and active research. Many efforts have been made toward the construction of delivery vectors using mammalian viruses, such as adenovirus, adeno-associated virus, retrovirus, and herpesvirus. However, recently, baculovirus-derived vectors have emerged as possible tools for gene transfer into mammalian cells in vitro and in vivo but, to our knowledge, have never been tested in an intervertebral disc cell.

METHODS

The intervertebral disc cells cultured in monolayer were treated with 6 different doses of baculovirus carrying the green fluorescence protein gene (Ac-CMV-GFP). Fluorescence microscopy and flow cytometry were used to analyze transgene expression. The Autographa californica nucleopolyhedrovirus/GFP virus was then injected directly into the intervertebral discs of 8 rabbits at 7, 13, 20, and 28 days after injection. The nucleus pulposus tissues of injected discs were evaluated immediately by fluorescence microscopy for GFP expression.

RESULTS

A dose of Ac-CMV-GFP at a multiplicity of infection of 200 achieved the highest transduction ratio (approximately 87% of nucleus pulposus cells) and long-term expression without any toxicity to the cells. In vivo assay showed that Ac-CMV-GFP could also mediate GFP expression in rabbit intervertebral disc cells without inducing any symptoms. The GFP expression level at 7 days after transduction was significantly higher than at 21 and 28 days after treatment.

CONCLUSIONS

Baculovirus can transfer exogenous genes into rabbit nucleus pulposus cells at a high efficiency, and safely both in vitro and in vivo. The results suggest that baculoviruses might be useful tools as gene therapy vectors for intervertebral disc diseases.

Characterization of baculovirus Autographa californica multiple nuclear polyhedrosis virus infection in mammalian cells

The baculovirus Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) is used as a vector in many gene therapy studies. Wild-type AcMNPV infects many mammalian cell types in vitro, but does not replicate. We investigated the dynamics of AcMNPV genomic DNA in infected mammalian cells and used flow cytometric analysis to demonstrate that recombinant baculovirus containing a cytomegalovirus immediate early promoter/enhancer with green fluorescent protein (GFP) expressed high levels of GFP in Huh-7 cells, but not B16, Raw264.7, or YAC-1 cells. The addition of butyrate, a deacetylase inhibitor, markedly enhanced the percentage of GFP-expressing Huh-7 and B16 cells, but not Raw264.7 and YAC-1 cells. The addition of 5-aza-2'-deoxycytidine, a DNA methylation inhibitor, had no enhancing effect. Polymerase chain reaction analysis using AcMNPV-gp64-specific primers indicated that AcMNPV infected not only Huh-7 and B16 cells, but also Raw264.7 and YAC-1 cells in vitro. The genomic DNA was detected in Huh-7 and B16 cells 96 h after infection. Genomic AcMNPV DNA in YAC-1 cells was not transported to the nucleus. Luciferase assay indicated that AcMNPV p35 gene mRNA and p35 promoter activity were clearly expressed only in Huh-7 and B16 cells. These results suggest that viral genomic DNA expression is restricted by different host cell factors, such as degradation, deacetylation, and inhibition of nuclear transport, depending on the mammalian cell type.

Comparison between recombinant baculo- and adenoviral-vectors as transfer system in cardiovascular cells

The development of effective gene-therapeutic applications for cardiovascular disorders is in part limited by the lack of appropriate delivery systems. In an attempt to overcome this deficiency, we investigated the ability of baculoviral vectors to transduce human cardiovascular cells, for which data are missing in literature. Additionally, baculovirus ability to transduce target cells was compared to that of an adenoviral vector, a well characterized and widely used viral vector. Transduction experiments, performed using baculo/adenoviral vectors expressing the enhanced green fluorescence protein, revealed that, under the experimental condition considered, baculoviruses but not adenoviruses efficiently transduce human coronary smooth muscle cells (hCSMC); an opposite behavior was noticed for human coronary endothelial cells (hCEC). Thus, baculoviral vectors are potentially indicated as transfer system in the treatment of coronary restenosis, where growth inhibitory genes should reach hCSMC but not hCEC. When used to transduce human cardiomyocytes and fibroblasts, both vectors behaved similarly. Finally, studies on cellular DNA replication revealed a more prolonged and pronounced negative effect on cells transduced by adenoviral compared to baculoviral vectors. Our data indicate that baculoviruses represent an attractive alternative to adenoviruses as transfer vectors in cardiovascular cells and that baculovirus have the potential to be used as gene transfer system in cardiovascular diseases such as restenosis.

Evaluation of Cell-Based Assays for Steroid Nuclear Receptors Delivered by Recombinant Baculoviruses

The authors describe the use of modified baculoviruses containing mammalian expression cassettes (BacMam technology) in steroid nuclear receptor reporter assays designed for screening and profiling agonist and antagonist compounds. Baculo-viruses were constructed that express full-length human genes for mineralocorticoid receptor (MR), glucocorticoid receptor (GR), progesterone receptor A (PR-A), and progesterone receptor B (PR-B) from the cytomegalovirus immediate early promoter. A virus carrying the mouse mammary tumor virus–firefly luciferase (MMTV-Luc) cassette was generated to provide a suitable reporter construct. Feasibility studies with BacMam-MR in single-dose tests of 1000 compounds showed high correlation to the standard transfection-based assay results. Likewise, in dose-response experiments, BacMam-based assays for GR and PR-B produced potency and efficacy values similar to transfection assay results. At various receptor/reporter ratios, the BacMam assays showed good flexibility, demonstrating consistent signal-to-background (S/B) ratios and compound potencies. Increasing transduction time from 24 to 48 h provided no benefit, actually reducing overall assay performance as measured by S/B and Z′ values. The BacMam technology was applied in studies of isoforms PR-A and PR-B, which showed similar responses to a series of agonists. Taken together, the results demonstrate the utility of steroid nuclear receptor BacMam constructs for compound screening procedures with high reproducibility, reduced turnaround time, and lower cost.

Baculovirus as versatile vectors for protein expression in insect and mammalian cells. Nat Biotechnol. 2005;23:567-575. [PMID: 15877075 DOI: 10.1038/nbt1095]

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.

The GP64 protein of Autographa californica multiple nucleopolyhedrovirus rescues Helicoverpa armigera nucleopolyhedrovirus transduction in mammalian cells

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) belonging to the group I nucleopolyhedroviruses (NPVs) and expressing the envelope-fusion glycoprotein GP64 transduces a variety of mammalian cells to express foreign genes under the control of mammalian promoters. In contrast, the group II Helicoverpa armigera single NPV (HaSNPV) encoding a different envelope protein, the F protein, shows no detectable infectivity towards mammalian cells. This limitation was overcome by expressing AcMNPV GP64 in HaSNPV. Although the transduction ratios were lower overall, the range of mammalian cell types transduced by HaSNPV was consistent with those transduced by AcMNPV. These findings indicate that the F protein functions only in insect cells, whereas the GP64 protein works in both insect and mammalian cells.