A surface-modified baculovirus vector with improved gene delivery to B-lymphocytic cells

Architecture of the baculovirus nucleocapsid revealed by cryo-EM

Baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) has been widely used as a bioinsecticide and a protein expression vector. Despite their importance, very little is known about the structure of most baculovirus proteins. Here, we show a 3.2 Å resolution structure of helical cylindrical body of the AcMNPV nucleocapsid, composed of VP39, as well as 4.3 Å resolution structures of both the head and the base of the nucleocapsid composed of over 100 protein subunits. AcMNPV VP39 demonstrates some features of the HK97-like fold and utilizes disulfide-bonds and a set of interactions at its C-termini to mediate nucleocapsid assembly and stability. At both ends of the nucleocapsid, the VP39 cylinder is constricted by an outer shell ring composed of proteins AC104, AC142 and AC109. AC101(BV/ODV-C42) and AC144(ODV-EC27) form a C14 symmetric inner layer at both capsid head and base. In the base, these proteins interact with a 7-fold symmetric capsid plug, while a portal-like structure is seen in the central portion of head. Additionally, we propose an application of AlphaFold2 for model building in intermediate resolution density.

Advances in CRISPR-Cas9 for the Baculovirus Vector System: A Systematic Review

The baculovirus expression vector systems (BEVS) have been widely used for the recombinant production of proteins in insect cells and with high insert capacity. However, baculovirus does not replicate in mammalian cells; thus, the BacMam system, a heterogenous expression system that can infect certain mammalian cells, was developed. Since then, the BacMam system has enabled transgene expression via mammalian-specific promoters in human cells, and later, the MultiBacMam system enabled multi-protein expression in mammalian cells. In this review, we will cover the continual development of the BEVS in combination with CRPISPR-Cas technologies to drive genome-editing in mammalian cells. Additionally, we highlight the use of CRISPR-Cas in glycoengineering to potentially produce a new class of glycoprotein medicines in insect cells. Moreover, we anticipate CRISPR-Cas9 to play a crucial role in the development of protein expression systems, gene therapy, and advancing genome engineering applications in the future.

Functional Implications of Epstein-Barr Virus Lytic Genes in Carcinogenesis

Epstein-Barr virus (EBV) is associated with a diverse range of tumors of both lymphoid and epithelial origin. Similar to other herpesviruses, EBV displays a bipartite life cycle consisting of latent and lytic phases. Current dogma indicates that the latent genes are key drivers in the pathogenesis of EBV-associated cancers, while the lytic genes are primarily responsible for viral transmission. In recent years, evidence has emerged to show that the EBV lytic phase also plays an important role in EBV tumorigenesis, and the expression of EBV lytic genes is frequently detected in tumor tissues and cell lines. The advent of next generation sequencing has allowed the comprehensive profiling of EBV gene expression, and this has revealed the consistent expression of several lytic genes across various types of EBV-associated cancers. In this review, we provide an overview of the functional implications of EBV lytic gene expression to the oncogenic process and discuss possible avenues for future investigations.

Baculoviruses in Gene Therapy and Personalized Medicine

This review will outline the role of baculoviruses in gene therapy and future potential in personalized medicine. Baculoviruses are a safe, non-toxic, non-integrative vector with a large cloning capacity. Baculoviruses are also a highly adaptable, low-cost vector with a broad tissue and host tropism due to their ability to infect both quiescent and proliferating cells. Moreover, they only replicate in insect cells, not mammalian cells, improving their biosafety. The beneficial properties of baculoviruses make it an attractive option for gene delivery. The use of baculoviruses in gene therapy has advanced significantly, contributing to vaccine production, anti-cancer therapies and regenerative medicine. Currently, baculoviruses are primarily used for recombinant protein production and vaccines.  This review will also discuss methods to optimize baculoviruses protein production and mammalian cell entry, limitations and potential for gene therapy and personalized medicine. Limitations such as transient gene expression, complement activation and virus fragility are discussed in details as they can be overcome through further genetic modifications and other methods. This review concludes that baculoviruses are an excllent candidate for gene therapy, personalized medicine and other biotherapeutic applications.

The Major Hurdle for Effective Baculovirus Transduction into Mammalian Cells Is Passing Early Endosomes

Baculoviruses, although they infect insects in nature, can transduce a wide variety of mammalian cells and are therefore promising gene therapy vectors. However, baculovirus transduction into many mammalian cells is very inefficient, and the limiting stages and factors remain unknown. An important finding is that a short-duration trigger with low pH can significantly enhance virus transduction efficiency, but the mechanism is poorly understood. Herein, we performed a detailed comparative study on entry mechanisms of the prototypical baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) into insect and mammalian cells. The results showed that AcMNPV could be internalized into mammalian cells efficiently, but fusion in early endosomes (EEs) appeared to be the major obstacle. Measurement of endosomal pH suggested that virus fusion might be restricted under relatively high-pH conditions in mammalian cells. Interestingly, mutations of the major viral fusion protein GP64 that conferred decreased fusogenicity did not affect virus infection of insect cells, whereas virus transduction into mammalian cells was severely impaired, suggesting a more stringent dependence on GP64 fusogenicity for AcMNPV entry into mammalian cells than into insect cells. An increase in the fusogenicity of GP64 mutants resulting from low pH triggered the rescue of fusion-deficient recombinant virus transduction efficiency. Based on the above-described findings, the pH of EEs was specifically reduced with a Na⁺/K⁺-ATPase inhibitor, and the AcMNPV transduction of many mammalian cells indeed became highly efficient. This study not only revealed the roadblocks to mammalian cell entry of baculovirus but also provides a new strategy for improving baculovirus-based gene delivery and therapy.IMPORTANCE Baculoviruses can transduce a wide variety of mammalian cells but do so with low efficiency, which greatly limits their practical application as potential gene delivery vectors. So far, the understanding of baculovirus entry into mammalian cells is obscure, and the limiting stages and factors are unclear. In this study, by comparatively analyzing the mechanisms of baculovirus entry into mammalian and insect cells, virus fusion during the early stage of endocytosis was revealed as the major obstacle for efficient baculovirus transduction into mammalian cells. A higher fusogenicity of the major viral fusion protein GP64 was found to be required for virus entry into mammalian cells than for entry into insect cells. Interestingly, by decreasing the pH of early endosomes with a specific agent, virus transduction of a wide range of mammalian cells was greatly enhanced. This study uncovers the roadblocks to mammalian cell entry of baculoviruses and presents mechanisms to overcome the roadblocks.

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.

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.

Viral-mediated gene delivery for cell-based assays in drug discovery

BACKGROUND

Adenovirus, retrovirus and lentivirus-based vectors, originally engineered and optimized for in vivo and ex vivo gene therapy, have become increasingly useful for viral-mediated gene delivery to support in vitro cell-based assays. Viral vectors underpin functional genomics screening of cDNA, shRNA and aptamer libraries, are used for a variety of target validation studies and importantly, for high-throughput cell-based drug discovery and compound profiling assays. The baculovirus/insect cell expression system had gained prevalence as a tool for recombinant protein production when it was observed that recombinant baculovirus vectors too could serve as efficient gene delivery vehicles for a wide range of mammalian cells. Although the use of baculovirus vectors in vivo has lagged behind retroviral, adenoviral and lentiviral vectors, they have gained prominence for development of in vitro cell-based assays due to the ease of generation, broad host range and excellent biosafety profile. There is an increasing emphasis on cell-based assays in high-throughput automated drug discovery laboratories and a variety of commercially available viral-vectors can be used for supporting these assays.

OBJECTIVE

We compare and contrast the current viral-mediated gene delivery vector systems and highlight their suitability for cell-based drug discovery assays.

CONCLUSION

Viral-mediated gene delivery is increasingly being used in support of genome scale target validation studies and cell-based assay development for specific drug target genes such as ion channels, G protein-coupled receptors and intracellular enzymes. The choice of a delivery system over another for a particular application is largely dictated by the cell types and cell lines in use, virus cellular tropism, assay throughput, safety requirements and ease/cost of reagent generation.

Characterization of the immune responses elicited by baculovirus-based vector vaccines against influenza virus hemagglutinin

AIM

To compare the specific immune responses elicited by different baculovirus vectors in immunized mice.

METHODS

We constructed and characterized two recombinant baculoviruses carrying the expression cassette for the H5N1 influenza virus hemagglutinin (HA) gene driven by either an insect cell promoter (vAc-HA) or a dual-promoter active both in insect and mammalian cells (vAc-HA-DUAL). Virus without the HA gene (vAc-EGFP) was used as a control. These viruses were used to immunize mice subcutaneously and intraperitoneally. The production of total and specific antibodies was determined by ELISA and competitive ELISA. Cytokine production by the spleen cells of immunized mice was studied using the ELISPOT assay.

RESULTS

Both the vAc-HA and vAc-HA-DUAL vectors expressed HA proteins in insect Sf9 cells, and HA antigen was present in progeny virions. The vAc-HA-DUAL vector also mediated HA expression in virus-transduced mammalian cell lines (BHK and A547). Both vAc-HA and vAc-HA-DUAL exhibited higher transduction efficiencies than vAc-EGFP in mammalian cells, as shown by the expression of the reporter gene egfp. Additionally, both vAc-HA and vAc-HA-DUAL induced high levels of HA-specific antibody production in immunized mice; vAc-HA-DUAL was more efficient in inducing IFN-γ and IL-2 upon stimulation with specific antigen, whereas vAc-HA was more efficient in inducing IL-4 and IL-6.

CONCLUSION

Baculovirus vectors elicited efficient, specific immune responses in immunized mice. The vector displaying the HA antigen on the virion surface (vAc-HA) elicited a Th2-biased immune response, whereas the vector displaying HA and mediating HA expression in the cell (vAc-HA-DUAL) elicited a Th1-biased immune response.

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.

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.

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 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.