Morphological characterization of baculovirus Autographa californica multiple nucleopolyhedrovirus

EXTL3 and NPC1 are mammalian host factors for Autographa californica multiple nucleopolyhedrovirus infection

Baculovirus is an obligate parasitic virus of the phylum Arthropoda. Baculovirus including Autographa californica multiple nucleopolyhedrovirus (AcMNPV) has been widely used in the laboratory and industrial preparation of proteins or protein complexes. Due to its large packaging capacity and non-replicative and non-integrative natures in mammals, baculovirus has been proposed as a gene therapy vector for transgene delivery. However, the mechanism of baculovirus transduction in mammalian cells has not been fully illustrated. Here, we employed a cell surface protein-focused CRISPR screen to identify host dependency factors for baculovirus transduction in mammalian cells. The screening experiment uncovered a series of baculovirus host factors in human cells, including exostosin-like glycosyltransferase 3 (EXTL3) and NPC intracellular cholesterol transporter 1 (NPC1). Further investigation illustrated that EXTL3 affected baculovirus attachment and entry by participating in heparan sulfate biosynthesis. In addition, NPC1 promoted baculovirus transduction by mediating membrane fusion and endosomal escape. Moreover, in vivo, baculovirus transduction in Npc1-/+ mice showed that disruption of Npc1 gene significantly reduced baculovirus transduction in mouse liver. In summary, our study revealed the functions of EXTL3 and NPC1 in baculovirus attachment, entry, and endosomal escape in mammalian cells, which is useful for understanding baculovirus transduction in human cells.

Membrane-based steric exclusion chromatography for the purification of a recombinant baculovirus and its application for cell therapy

The continuously increasing potential of stem cell treatments for various medical conditions has accelerated the need for fast and efficient purification techniques for individualized cell therapy applications. Genetic stem cell engineering is commonly done with viral vectors like the baculovirus. The baculovirus is a safe and efficient gene transfer tool, that has been used for the expression of recombinant proteins for many years. Its purification has been based mainly on ion exchange matrices. However, these techniques impair process robustness, if different genetically modified virus particles are applied. Here, we evaluated the membrane-based steric exclusion chromatography for the purification of insect cell culture-derived recombinant Autographa californica multicapsid nucleopolehydroviruses for an application in cell therapy. The method has already proven to be a powerful tool for the purification of Influenza A virus particles, using cellulose membranes. Aside from the aforementioned cellulose, we evaluated alternative stationary phases, such as glass fiber and polyamide membranes. The highest dynamic binding capacitiy was determined for cellulose with 5.08E + 07 pfu per cm² membrane. Critical process parameters were optimized, using a design of experiments (DoE) approach. The determined process conditions were verified by different production batches, obtaining a mean virus yield of 91% ± 6.5%. Impurity depletion was >99% and 85% for protein and dsDNA, without nuclease treatment. Due to the method's specificity, its application to other baculoviruses, with varying surface modifications, is conceivable without major process changes. The physiological buffer conditions enable a gentle handling of the virus particles without decreasing the transduction efficacy. The simple procedure with sufficient impurity removal enables the substitution of time-consuming ultra centrifugation steps and can serve as a first process unit operation to obtain higher purities.

Baculovirus for gene delivery to mammalian cells: Past, present and future

Baculovirus is an insect virus which has been used for more than thirty years for production of recombinant proteins in insect cells. However, baculovirus can also be harnessed for efficient gene delivery to mammalian cells if it is equipped with mammalian promoters. This technology is known as BacMam and has been used for gene delivery to immortalized cell lines, stem cells, and primary cells, as well as for gene delivery in animals. Baculovirus has unique features when compared to mammalian viruses. Besides the fact that it is replication-incompetent and does not integrate into the host genome, it has large capacity for foreign DNA. This capacity can for example be used to deliver multiple genes for reprogramming of stem cells, or for delivery of large homology constructs for genome editing. In this review, we provide a brief overview of baculovirus-based gene delivery and its recent applications in therapy and basic research. We also describe how baculovirus is manipulated for efficient transduction in mammalian cells and we highlight possible future improvements.

Structural Organization of Baculovirus Occlusion Bodies and Protective Role of Multilayered Polyhedron Envelope Protein

Baculoviruses are the ingenious insect pathogens. Outside the host, baculovirus occlusion bodies (OB) provide stability to occlusion-derived viruses (ODV) embedded within. The OB is an organized structure, chiefly composed of proteins namely polyhedrin, polyhedron envelope protein (PEP) and P10. Currently, the structural organization of OB is poorly understood and the role of OB proteins in conferring the stability to ODV is unknown. Here we have shown that the assembly of polyhedrin unit cells into an OB is a rapid process; the PEP forms in multiple layers; the PEP layers predominantly contribute to ODV viability. Full-grown OBs (n = 36) were found to be 4.0 ± 1.0 µm in diameter and possessed a peculiar geometry of a truncated rhombic dodecahedron. The atomic force microscopy (AFM) study on the structure of OBs at different stages of growth in insect cells revealed polyhedrin assembly and thickness of PEP layers. The thickness of PEP layers at 53 h post-transfection (hpt) ranged from 56 to 80 nm. Mature PEP layers filled up approximately one third of the OB volume. The size of ODV nucleocapsid was found to be 433 ± 10 nm in length. The zeta potential and particle size distribution study of viruses revealed the protective role of PEP layers. The presence of a multilayered PEP confers a viable advantage to the baculoviruses compared to single-layered PEP. Thus, these findings may help in developing PEP layer-based biopolymers for protein-based nanodevices, nanoelectrodes and more stable biopesticides.

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.