Spontaneous excision of BAC vector sequences from bacmid-derived baculovirus expression vectors upon passage in insect cells

Baculovirus-insect cell expression systems

In the early 1980s, the first-published reports of baculovirus-mediated foreign gene expression stimulated great interest in the use of baculovirus-insect cell systems for recombinant protein production. Initially, this system appeared to be the first that would be able to provide the high production levels associated with bacterial systems and the eukaryotic protein processing capabilities associated with mammalian systems. Experience and an increased understanding of basic insect cell biology have shown that these early expectations were not completely realistic. Nevertheless, baculovirus-insect cell expression systems have the capacity to produce many recombinant proteins at high levels and they also provide significant eukaryotic protein processing capabilities. Furthermore, important technological advances over the past 20 years have improved upon the original methods developed for the isolation of baculovirus expression vectors, which were inefficient, required at least some specialized expertise and, therefore, induced some frustration among those who used the original baculovirus-insect cell expression system. Today, virtually any investigator with basic molecular biology training can relatively quickly and efficiently isolate a recombinant baculovirus vector and use it to produce their favorite protein in an insect cell culture. This chapter will begin with background information on the basic baculovirus-insect cell expression system and will then focus on recent developments that have greatly facilitated the ability of an average investigator to take advantage of its attributes.

Generation of baculovirus vectors for the high-throughput production of proteins in insect cells

The baculovirus expression system is one of the most popular methods used for the production of recombinant proteins but has several complex steps which have proved inherently difficult to adapt to a multi-parallel process. We have developed a bacmid vector that does not require any form of selection pressure to separate recombinant virus from non-recombinant parental virus. The method relies on homologous recombination in insect cells between a transfer vector containing a gene to be expressed and a replication-deficient bacmid. The target gene replaces a bacterial replicon at the polyhedrin loci, simultaneously restoring a virus gene essential for replication. Therefore, only recombinant virus can replicate facilitating the rapid production of multiple recombinant viruses on automated platforms in a one-step procedure. Using this vector allowed us to automate the generation of multiple recombinant viruses with a robotic liquid handler and then rapidly screen infected insect cell supernatant for the presence of secreted proteins.

Low multiplicity of infection in vivo results in purifying selection against baculovirus deletion mutants

The in vivo fate of Autographa californica multiple nucleopolyhedrovirus deletion mutants originating from serial passage in cell culture was investigated by passaging a population enriched in these mutants in insect larvae. The infectivity of polyhedra and occlusion-derived virion content per polyhedron were restored within two passages in vivo. The frequency of occurrence of deletion mutants was determined by real-time PCR. The frequency of the non-homologous region origin (non-HR ori) of DNA replication was reduced to wild-type levels within two passages. The frequency of the polyhedrin gene did not increase and remained below wild-type levels. A low m.o.i. during the initial infection in insect larvae, causing strong purifying selection for autonomously replicating viruses, could explain these observations. The same virus population used in vivo was also passaged once at a different m.o.i. in cell culture. A similar effect (i.e. lower non-HR ori frequency) was observed at low m.o.i. only, indicating that m.o.i. was the key selective condition.

Production of a recombinant antibody fragment in whole insect larvae

Infection of insect cells with baculovirus expression constructs is commonly used to produce recombinant proteins that require post-translational modifications for their activity, such as mammalian proteins. However, technical restraints limit the capacity of insect cell-based culture systems to be scaled up to produce the large amounts of recombinant protein required for human pharmaceuticals. In this study, we designed an automated insect rearing system and whole insect baculovirus expression system (PERLXpress) for the expression and purification of recombinant proteins on a large scale. As a test model, we produced a recombinant mouse anti-botulinum antibody fragment (Fab) in Trichoplusia ni larvae. A recombinant baculovirus co-expressing the Fab heavy and light chains together with N-terminal sequences from the silkworm hormone bombyxin, to direct proteins into the secretory pathway, was constructed. Fifth instar larvae were reared and infected orally with recombinant (pre- occluded) baculovirus using the automated system and harvested approximately after 4 days. The total yield of recombinant Fab was 1.1 g/kg of larvae, resulting in 127 mg of pure Fab in one production run. The Fab was purified to homogeneity using immobilized metal affinity chromatography, gel filtration, and anion exchange chromatography. The identity of the purified protein was verified by Western blots and size-exclusion chromatography. Purified recombinant Fab was used to detect botulinum toxin in ELISA experiments, demonstrating that the heavy and light chains were properly assembled and folded into functional heterodimers. We believe that this is the first demonstration of the expression of a recombinant antibody in whole insect larvae. Our results demonstrate that a baculovirus-whole larvae expression system can be used to express functionally active recombinant Fab fragments. As the PERLXpress system is an automated and linearly scalable technology, it represents an attractive alternative to insect cell culture for the production of large amounts of human pharmaceuticals.

Production of recombinant adeno-associated vectors using two bioreactor configurations at different scales

The conventional methods for producing recombinant adeno-associated virus (rAAV) rely on transient transfection of adherent mammalian cells. To gain acceptance and achieve current good manufacturing process (cGMP) compliance, clinical grade rAAV production process should have the following qualities: simplicity, consistency, cost effectiveness, and scalability. Currently, the only viable method for producing rAAV in large-scale, e.g. > or =10(16) particles per production run, utilizes baculovirus expression vectors (BEVs) and insect cells suspension cultures. The previously described rAAV production in 40 L culture using a stirred tank bioreactor requires special conditions for implementation and operation not available in all laboratories. Alternatives to producing rAAV in stirred tank bioreactors are single-use, disposable bioreactors, e.g. Wave. The disposable bags are purchased pre-sterilized thereby eliminating the need for end-user sterilization and also avoiding cleaning steps between production runs thus facilitating the production process. In this study, rAAV production in stirred tank and Wave bioreactors was compared. The working volumes were 10 L and 40 L for the stirred tank bioreactors and 5 L and 20 L for the Wave bioreactors. Comparable yields of rAAV, approximately 2E+13 particles per liter of cell culture were obtained in all volumes and configurations. These results demonstrate that producing rAAV in large scale using BEVs is reproducible, scalable, and independent of the bioreactor configuration.

The Homingbac baculovirus cloning system: An alternative way to introduce foreign DNA into baculovirus genomes

An in vitro baculovirus cloning system has been developed for direct cloning of foreign DNA into baculovirus genomes. This system is called the "Homingbac system" because it uses homing endonucleases. The Homingbac system was engineered into the baculoviruses AcMNPV, BmNPV, PxMNPV, RoMNPV, HaSNPV and HzSNPV. All Homingbac viruses were designed to retain the polyhedra phenotype so that they could be inoculated per os to insects. This is the first time a common in vitro baculovirus cloning system has been made for multiple baculovirus species that include both groups I and II nucleopolyhedroviruses (NPVs). In this study, the Homingbac system was demonstrated by directly cloning a PCR-amplified beta-glucuronidase gene cassette into a parent Homingbac virus. This new collection of groups I and II NPV Homingbac viruses are a significant expansion of in vitro cloning technology and are new tools for making recombinant baculoviruses.

High cell density fed batch and perfusion processes for stable non-viral expression of secreted alkaline phosphatase (SEAP) using insect cells: Comparison to a batch Sf-9-BEV system

The development of insect cells expressing recombinant proteins in a stable continuous manner is an attractive alternative to the BEV system for recombinant protein production. High cell density fed batch and continuous perfusion processes can be designed to maximize the productivity of stably transformed cells. A cell line (Sf-9SEAP) expressing high levels of the reporter protein SEAP stably was obtained by lipid-mediated transfection of Sf-9 insect cells and further selection and screening. The expression of the Sf-9SEAP cells was compared with the BEVS system. It was observed that, the yield obtained in BEVS was similar to the batch Sf-9SEAP at 8 and 7 IU/mL, respectively. The productivity of this foreign gene product with the stable cells was enhanced by bioprocess intensification employing the fed-batch and perfusion modes of culture to increase the cell density in culture. The fed batch process yielded a maximum cell density of 28 x 10(6) cells/mL and 12 IU/mL of SEAP. Further improvements in the productivity could be made using the perfusion process, which demonstrated a stable production rate for extended periods of time. The process was maintained for 43 days, with a steady-state cell density of 17-20 x 10(6) cells/mL and 7 IU/mL SEAP. The total yield obtained in the perfusion process (394 IU) was approximately 22 and 8 times higher than that obtained in a batch (17.6 IU) and fed batch (46.1 IU) process, respectively.

Economized large-scale production of high yield of rAAV for gene therapy applications exploiting baculovirus expression system

BACKGROUND

The versatility of recombinant adeno-associated vector (rAAV) as a gene delivery system is due to the vector's ability to transduce different cell types as well as dividing and non-dividing cells. Large-scale production of rAAV remains one of the major challenges for continued development of pre-clinical and clinical studies, and for its potential commercialization. The baculovirus expression vectors (BEVS) and insect cells represent a potential method to produce rAAV economically at large scale. This technology uses three different BEVs (Bac-Rep, Bac-GFP, and Bac-VP) each at a multiplicity of infection (MOI) of 3. We reported previously the production of rAAV at 40 L scale using a stirred-tank bioreactor (STB). However, production in larger volumes is limited by the stability of the BEVs and amount of BEVs needed to achieve the target MOI of 3 per BEV. Here, the production parameters were optimized and the baculovirus stability was determined.

METHODS

The stability of the three types of baculovirus used to produce rAAV was determined for six expansion passages by protein expression analysis. To economize baculovirus, MOI and cell density at time of infection (TOI) were evaluated initially at small scale and then applied to the 10 L scale.

RESULTS

An MOI = 0.03 and TOI cell density of 1 x 10(6) cells/mL produced high titer rAAV without comprising yield. To confirm the scalability of the process, rAAV was produced in a 10 L STB using the optimized parameters obtaining a 10x increase in yield ( approximately 1 x 10(14) rAAV DNAse-resistant particles per liter).

CONCLUSION

These findings contribute to the process development for large-scale production of rAAV for gene therapy applications and its commercialization.

A suite of parallel vectors for baculovirus expression

The expression of proteins using recombinant baculoviruses is a mature and widely used technology. However, some aspects of the technology continue to detract from high throughput use and the basis of the final observed expression level is poorly understood. Here, we describe the design and use of a set of vectors developed around a unified cloning strategy that allow parallel expression of target proteins in the baculovirus system as N-terminal or C-terminal fusions. Using several protein kinases as tests we found that amino-terminal fusion to maltose binding protein rescued expression of the poorly expressed human kinase Cot but had only a marginal effect on expression of a well-expressed kinase IKK-2. In addition, MBP fusion proteins were found to be secreted from the expressing cell. Use of a carboxyl-terminal GFP tagging vector showed that fluorescence measurement paralleled expression level and was a convenient readout in the context of insect cell expression, an observation that was further supported with additional non-kinase targets. The expression of the target proteins using the same vectors in vitro showed that differences in expression level were wholly dependent on the environment of the expressing cell and an investigation of the time course of expression showed it could affect substantially the observed expression level for poorly but not well-expressed proteins. Our vector suite approach shows that rapid expression survey can be achieved within the baculovirus system and in addition, goes some way to identifying the underlying basis of the expression level obtained.

Vaccines for viral and parasitic diseases produced with baculovirus vectors

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

Production of adeno-associated viral vectors in insect cells using triple infection: Optimization of baculovirus concentration ratios

The production of viral vectors or virus-like particles for gene therapy or vaccinations using the baculovirus expression system is gaining in popularity. Recently, reports of a viral vector based on adeno-associated virus (AAV) produced in insect cells using the baculovirus expression vector system have been published. This system requires the triple infection of cells with baculovirus vectors containing the AAV gene for replication proteins (BacRep), the AAV gene for structural proteins (BacCap), and the AAV vector genome (BacITR). A statistical approach was used to investigate the multiplicities of infection of the three baculoviruses and the results were extended to the production of AAVs containing various transgenes. Highest AAV yields were obtained when BacRep and BacCap, the baculovirus vectors containing genes that code for proteins necessary for the formation of the AAV vector, were added in equal amounts at high multiplicities of infection. These combinations also resulted in the closest ratios of infectious to total AAV particles produced. Overexpression of the AAV structural proteins led to the production of empty AAV capsids, which is believed to overload the cellular machinery, preventing proper encapsidation of the AAV vector transgene, and decreased the viability of the insect cells. Delaying the input of BacCap, to reduce the amount of capsids produced, resulted in lower infectious AAV titers then when all three baculoviruses were put into the system at the same time. The amount of BacITR added to the system can be less than the other two without loss of AAV yield.

Stabilized baculovirus vector expressing a heterologous gene and GP64 from a single bicistronic transcript

The efficient scale-up of recombinant protein production in insect-cell bioreactors using baculovirus expression vectors is hampered by reductions in yield with increasing viral passage, the so-called passage effect. This phenomenon is characterized by the generation and subsequent accumulation of defective interfering baculoviruses (DIs), which interfere with the replication of genomically intact virus. A novel baculovirus expression vector is presented equipped with a bicistronic expression cassette that allows the simultaneous expression of the recombinant gene (GFP, first cistron) and an essential baculovirus gene (GP64, second cistron) from a single messenger RNA (mRNA). The translation of GP64 is mediated by an internal ribosome entry site (IRES) element from Rhopalosiphum padi virus (RhPV) while the native GP64 gene is deleted. In this way, a dominant selection pressure is placed on the entire bicistronic mRNA and hence on the maintenance of the foreign gene. The bicistronic expression vector was superior to the control baculovirus vector in that GFP expression remained at much higher levels upon continued virus passage. The versatility of this stabilized vector was demonstrated by its ability to propagate in a number of cell lines including Sf21, Sf9 and High Five cells. This novel baculovirus vector is especially valuable for large-scale recombinant protein production in insect-cell bioreactors where the number of viral passages is high.

Successful production of pseudotyped rAAV vectors using a modified baculovirus expression system

Scalable production of rAAV vectors remains a major obstacle to the clinical application of this prototypical gene therapy vector. A recently developed baculovirus-based production protocol (M. Urabe et al., 2002, Hum. Gene Ther. 13, 1935-1943) found limited applications due to the system's design. Here we report a detailed analysis of the stability of the original baculovirus system components BacRep, BacVP, and transgene cassette-containing BacGFP. All of the baculovirus helpers analyzed were prone to passage-dependent loss-of-function deletions resulting in considerable decreases in rAAV titers. To alleviate the instability and to extend the baculovirus platform to other rAAV serotypes, we have modified both Rep- and Cap-encoding components of the original system. The modifications include a parvoviral phospholipase A2 domain swap allowing production of infectious rAAV8 vectors in vivo. Alternatively, an infectious rAAV8 (or rAAV5) vector incorporating the AAV2 VP1 capsid protein in a mosaic vector particle with AAV8 capsid proteins was produced using a novel baculovirus vector. In this vector, the level of AAV2 VP1 expression is controlled with a "riboswitch," a self-cleaving ribozyme controlled by toyocamycin in the "ON" mode. The redesigned baculovirus system improves our capacity for rAAV manufacturing by making this production platform more applicable to other existing serotypes.

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

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

Evaluation of baculovirus expression vectors with enhanced stability in continuous cascaded insect-cell bioreactors

Continuous protein production with baculovirus expression vectors in insect-cell bioreactors is characterized by a dramatic drop in heterologous protein production within a few weeks. This is mainly due to the spontaneous deletion of the heterologous gene(s) from the baculovirus genome and/or to the rapid accumulation of defective interfering baculoviruses (DIs). Cell culture experiments with bacmid-derived baculoviruses showed that spontaneous deletions in the foreign bacterial artificial chromosome (BAC) sequences readily occurred. These deletions correlated with a low density of baculovirus homologous (repeat) regions (hrs), which are located dispersed throughout the baculovirus genome and are believed to act as origins of viral DNA replication (oris). To test the hypothesis that deletions are more likely to occur in regions with a low ori density, the properties of bacmid-derived baculoviruses with an additional hr in the unstable BAC sequences were compared to the standard bacmid-derived baculovirus in a continuous cascaded insect-cell bioreactor configuration. All viruses were equipped with a green fluorescent protein (GFP) gene and a gene encoding the classical swine fever virus E2 glycoprotein (CSFV-E2). The insertion of an extra hr in the BAC vector led to improved genetic stability of adjacent sequences, resulting in prolonged protein expression. The maintenance of the BAC sequences appeared to be dependent on the orientation of the inserted hr. The advantages of the utilization of hrs to improve the stability of baculovirus expression vectors for the large-scale protein production in insect-cell bioreactors are discussed.

From gene to protein: a review of new and enabling technologies for multi-parallel protein expression

In the post-genomic era, increasingly greater demands and expectations are being placed on protein production laboratories to produce more proteins and in faster timelines. This has been coupled with an exponential increase in the number of requests for the production of proteins which lack structural and functional information. No longer can groups use literature available in the public domain solely to drive their expression strategy, and moreover current expression and concomitant purification strategies clearly do not meet modern-day demands for protein production. This review will therefore attempt to provide a definitive review of current 'best in class' cloning, expression and purification systems, and the adaptations and developments that have been made by laboratories, both academic and industrial, to enhance protein production throughput.