The transcriptome of the baculovirus Autographa californica multiple nucleopolyhedrovirus in Trichoplusia ni cells

BmNPV Bm60 is a key target gene used by a resistant strain of Bombyx mori to inhibit BmNPV proliferation

Bombyx mori nucleopolyhedrovirus (BmNPV) is a pathogen that causes significant losses to the silkworm industry. Numerous antiviral genes and proteins have been identified by studying silkworm resistance to BmNPV. However, the molecular mechanism of silkworm resistance to BmNPV is unclear. We analyzed the differences between the susceptible strain 871 and a near-isogenic resistant strain 871C. The survival of strain 871C was significantly greater than that of 871 after oral and subcutaneous exposure to BmNPV. Strain 871C exhibited a nearly 10,000-fold higher LD50 for BmNPV compared to 871. BmNPV proliferation was significantly inhibited in all tested tissues of strain 871C using HE strain and fluorescence analysis. Strain 871C exhibited cellular resistance to BmNPV rather than peritrophic membrane or serum resistance. Strain 871C suppressed the expression of the viral early gene Bm60. This led to the inhibition of BmNPV DNA replication and late structural gene transcription based on the cascade regulation of baculovirus gene expression. Bm60 could also interact with the viral DNA binding protein and alkaline nuclease, as well as host proteins Methylcrotonoyl-CoA carboxylase subunit alpha, mucin-2-like protein, and 30 K-8. Overexpression of 30 K-8 significantly inhibited BmNPV proliferation. These results increase understanding of the molecular mechanism behind silkworm resistance to BmNPV and suggest targets for the breeding of resistant silkworm strains and the controlling pest of Lepidoptera.

Baculovirus entry into the central nervous system of Spodoptera exigua caterpillars is independent of the viral protein tyrosine phosphatase

Neuroparasitism concerns the hostile take-over of a host's nervous system by a foreign invader, in order to alter the behaviour of the host in favour of the parasite. One of the most remarkable cases of parasite-induced host behavioural manipulation comprises the changes baculoviruses induce in their caterpillar hosts. Baculoviruses may manipulate caterpillar behaviour in two ways: hyperactivity (increased movement in the horizontal plane) and/or tree-top disease (movement to elevated levels in the vertical plane). Those behavioural changes are followed by liquefaction and death of the caterpillar. In Autographa californica multiple nucleopolyhedrovirus (AcMNPV)-infected Spodoptera exigua caterpillars, an enzymatic active form of the virally encoded protein tyrosine phosphatase (PTP) is needed for the expression of hyperactivity from 3 days post infection (dpi). Using eGFP-expressing recombinant AcMNPV strains, we show that infection of the caterpillar's central nervous system (CNS) can be observed primarily from 3 dpi onwards. In addition, we demonstrate that the structural and enzymatic function of PTP does not play a role in infection of the CNS. Instead we show that the virus entered the CNS via the trachea, progressing caudally to frontally through the CNS and that the infection progressed from the outermost cell layers towards the inner cell layers of the CNS, in a PTP independent manner. These findings help to further understand parasitic manipulation and the mechanisms by which neuroparasites infect the host nervous system to manipulate host behaviour.

A Lac Repressor-Inducible Baculovirus Expression Vector for Controlling Adeno-Associated Virus Capsid Ratios

The baculovirus expression vector (BEV) system is an efficient, cost-effective, and scalable method to produce recombinant adeno-associated virus (rAAV) gene therapy vectors. Most BEV designs emulate the wild-type AAV transcriptome and translate the AAV capsid proteins, VP1, VP2, and VP3, from a single mRNA transcript with three overlapping open reading frames (ORFs). Non-canonical translation initiation codons for VP1 and VP2 reduce their abundances relative to VP3. Changing capsid ratios to improve rAAV vector efficacy requires a theoretical modification of the translational context. We have developed a Lac repressor-inducible system to empirically regulate the expression of VP1 and VP2 proteins relative to VP3 in the context of the BEV. We demonstrate the use of this system to tune the abundance, titer, and potency of a neurospecific rAAV9 serotype derivative. VP1:VP2:VP3 ratios of 1:1:8 gave optimal potency for this rAAV. It was discovered that the ratios of capsid proteins expressed were different than the ratios that ultimately were in purified capsids. Overexpressed VP1 did not become incorporated into capsids, while overexpressed VP2 did. Overabundance of VP2 correlated with reduced rAAV titers. This work demonstrates a novel technology for controlling the production of rAAV in the BEV system and shows a new perspective on the biology of rAAV capsid assembly.

Evaluation of an inducible knockout system in insect cells based on co-infection and CRISPR/Cas9

Due to comparably high product titers and low production costs, the baculovirus/insect cell expression system is considered a versatile production platform in the biopharmaceutical industry. Its excellence in producing complex multimeric protein assemblies, including virus-like particles (VLPs), which are considered promising vaccine candidates to counter emerging viral threats, made the system even more attractive. However, the co-formation of budded baculovirus during VLP production poses a severe challenge to downstream processing. In order to reduce the amount of budded baculovirus in the expression supernatant we developed an inducible knockout system based on CRISPR/Cas9 and co-infection with two baculoviral vectors: one bringing along the Cas9 nuclease and the other one having incorporated the sequence for sgRNA expression. With our set-up high titer viruses can be generated separately, as only when both viruses infect cells simultaneously a knockout can occur. When budding essential genes gp64 and vp80 were targeted for knockout, we measured a reduction in baculovirus titer by over 90%. However, as a consequence, we also determined lower overall eYFP fluorescence intensity showing reduced recombinant protein production, indicating that further improvements in engineering as well as purification are required in order to ultimately minimize costs and timeframes for vaccine production utilizing the baculovirus/insect cell expression system.

Genome-wide nonessential gene identification of Autographa californica multiple nucleopolyhedrovirus

The Baculovirus Expression Vector System (BEVS) is an insect cell-based heterologous protein expression system that possesses powerful potential in the development of protein drugs and vaccines. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the most widely-used vector in BEVS with 151 open reading frames (ORFs) containing essential and nonessential genes. Deletion of nonessential genes has many advantages including increased foreign gene insertion. In this study, the λ red recombination system was used to knock out genes in a modified AcMNPV that carried an enhanced yellow fluorescent protein (eYFP) at the Ac126-Ac127 locus. Eighty genes were almost completely deleted respectively and 69 gene knockout AcMNPVs (KOVs) were obtained to evaluate their infection efficiency. After infecting Spodoptera frugiperda 9 (Sf9) cells, 51 KOVs including 62 genes showed similar infectivity as wide type (WT) and hence were defined as nonessential genes. However, 18 KOVs produced fewer infectious virions, indicating that these genes were influential in the production of progeny viruses. Combining our research with previous studies, a desired minimal AcMNPV genome containing 86 ORFs and all of the homologous regions (hrs) was brought up, facilitating genetic modification of baculovirus vectors and improvement of recombinant protein expression in the future.

Evaluating Novel Quantification Methods for Infectious Baculoviruses

Accurate and rapid quantification of (infectious) virus titers is of paramount importance in the manufacture of viral vectors and vaccines. Reliable quantification data allow efficient process development at a laboratory scale and thorough process monitoring in later production. However, current gold standard applications, such as endpoint dilution assays, are cumbersome and do not provide true process analytical monitoring. Accordingly, flow cytometry and quantitative polymerase chain reaction have attracted increasing interest in recent years, offering various advantages for rapid quantification. Here, we compared different approaches for the assessment of infectious viruses, using a model baculovirus. Firstly, infectivity was estimated by the quantification of viral nucleic acids in infected cells, and secondly, different flow cytometric approaches were investigated regarding analysis times and calibration ranges. The flow cytometry technique included a quantification based on post-infection fluorophore expression and labeling of a viral surface protein using fluorescent antibodies. Additionally, the possibility of viral (m)RNA labeling in infected cells was investigated as a proof of concept. The results confirmed that infectivity assessment based on qPCR is not trivial and requires sophisticated method optimization, whereas staining of viral surface proteins is a fast and feasible approach for enveloped viruses. Finally, labeling of viral (m)RNA in infected cells appears to be a promising opportunity but will require further research.

IE1 of Autographa californica Multiple Nucleopolyhedrovirus Activates Low Levels of Late Gene Expression in the Absence of Virus RNA Polymerase

Early and late gene expressions of baculoviruses have been known to rely on host RNA polymerase II and a virus-encoded RNA polymerase, separately. In this study, we found that Autographa californica multiple nucleopolyhedrovirus (AcMNPV) recombinant bacmids with the individual RNA polymerase subunit genes deleted could support low levels of expression of a reporter gene under the control of the promoter of a typical late gene, vp39, in transfected Sf9 cells. Through multistep subcloning of a genomic library of the virus and transient expression assay analysis, ie1 was identified to be the only viral gene that was responsible for activation of late gene expression in the absence of the viral RNA polymerase. Furthermore, IE1 was found to be capable of activating reporter gene expression from the promoters of additional late genes polh, p6.9, odv-e18, odv-e25, and gp41, independent of any additional viral factors. Deletion of ie1 from the virus genome eliminated late gene expression. The IE1-activated late gene expression was enhanced by the viral hr4b. It was shown to be insensitive to inhibition of α-amanitin and did not appear to have stable transcription start sites. It is proposed that IE1 may serve to recruit newly synthesized viral RNA polymerase to viral DNA by activating low levels of pretranscription of the late genes to create an appropriate DNA conformation. IMPORTANCE The late gene expression of baculovirus has been known to depend on the virus-encoded RNA polymerase, which consists of four subunits. The immediate-early gene ie1 was found to be required for viral early gene expression, late gene expression, and DNA replication. How it functions in late gene expression remains unclear. In this study, we found that AcMNPV IE1 could activate low levels of gene expression from late gene promoters independently of any additional viral factors, with nonspecific transcription start sites. This new finding will shed light on the role of IE1 in the regulation of late gene expression and the understanding of the mechanism of late gene transcription initiation.

Dissecting insect cell heterogeneity during influenza VLP production using single-cell transcriptomics

The insect cell-baculovirus expression vector system (IC-BEVS) has been widely used to produce recombinant protein at high titers, including complex virus-like particles (VPLs). However, cell-to-cell variability upon infection is yet one of the least understood phenomena in virology, and little is known about its impact on production of therapeutic proteins. This study aimed at dissecting insect cell population heterogeneity during production of influenza VLPs in IC-BEVS using single-cell RNA-seq (scRNA-seq). High Five cell population was shown to be heterogeneous even before infection, with cell cycle being one of the factors contributing for this variation. In addition, infected insect cells were clustered according to the timing and level of baculovirus genes expression, with each cluster reporting similar influenza VLPs transgenes (i.e., hemagglutinin and M1) transcript counts. Trajectory analysis enabled to track infection progression throughout pseudotime. Specific pathways such as translation machinery, protein folding, sorting and degradation, endocytosis and energy metabolism were identified as being those which vary the most during insect cell infection and production of Influenza VLPs. Overall, this study lays the ground for the application of scRNA-seq in IC-BEVS processes to isolate relevant biological mechanisms during recombinant protein expression towards its further optimization.

AC81 Is a Putative Disulfide Isomerase Involved in Baculoviral Disulfide Bond Formation

The correct formation of native disulfide bonds is critical for the proper structure and function of many proteins. Cellular disulfide bond formation pathways commonly consist of two parts: sulfhydryl oxidase-mediated oxidation and disulfide isomerase-mediated isomerization. Some large DNA viruses, such as baculoviruses, encode sulfhydryl oxidases, but viral disulfide isomerases have not yet been identified, although G4L in poxvirus has been suggested to serve such a function. Here, we report that the baculovirus core gene ac81 encodes a putative disulfide isomerase. ac81 is conserved in baculoviruses, nudiviruses, and hytrosaviruses. We found that AC81 homologs contain a typical thioredoxin fold conserved in disulfide isomerases. To determine the role of AC81, a series of Autographa californica nucleopolyhedrovirus (AcMNPV) bacmids containing ac81 knockout or point mutations was generated, and the results showed that AC81 is essential for budded virus production, multinucleocapsid occlusion-derived virus (ODV) formation, and ODV embedding in occlusion bodies. Nonreducing Western blot analysis indicated that disulfide bond formation in per os infectivity factor 5 (PIF5), a substrate of the baculoviral sulfhydryl oxidase P33, was abnormal when ac81 was knocked out or mutated. Pulldown assays showed that AC81 interacted with PIF5 and P33 in infected cells. In addition, two critical regions that harbor key amino acids for function were identified in AC81. Taken together, our results suggest that AC81 is a key component involved in the baculovirus disulfide bond formation pathway and likely functions as a disulfide isomerase. IMPORTANCE Many large DNA viruses, such as poxvirus, asfarvirus, and baculovirus, encode their own sulfhydryl oxidase to facilitate the disulfide bond formation of viral proteins. Here, we show that AC81 functions as a putative disulfide isomerase and is involved in multiple functions of the baculovirus life cycle. Interestingly, AC81 and P33 (sulfhydryl oxidase) are conserved in baculoviruses, nudiviruses, and hytrosaviruses, which are all insect-specific large DNA viruses replicating in the nucleus, suggesting that viral disulfide bond formation is an ancient mechanism shared by these viruses.

Successful Rescue of Synthetic AcMNPV with a ~17 kb Deletion in the C1 Region of the Genome

Baculoviruses have been widely used as expression vectors. However, numerous genes in the baculoviral genome are non-essential for cellular infection and protein expression, making the optimisation of baculovirus expression vectors possible. We used a synthetic biological method to reduce the number of genes in a partial region of the autograph californica multiple nucleopolyhedrovirus (AcMNPV), the most widely used baculovirus expression vector. The C1 region of the AcMNPV is 46.4 kb and is subdivided into B1, B2, and B3 fragments. We first designed modified B1, B2, and B3 fragments by deleting the non-essential genes, and then synthesised complete viral genomes containing either individual modified B fragments or joint modified B fragments through transformation-related recombination in yeast. The synthetic genomes were then transfected into Sf9 cells to rescue the progeny viruses and test their infectivity. The design-build-test cycle was repeated until the ultimately rescued virus could produce progeny viruses efficiently. Finally, AcMNPV-Syn-mC1-1.1 by deleting approximately 17.2 kb, including 20 ORFs, in the C1 region, was obtained. This is essential to the synthesis of a minimal AcMNPV genome that can generate infectious progeny viruses and can be further used to optimise the foundation of baculovirus expression vectors.

Utility of Alternative Promoters for Foreign Gene Expression Using the Baculovirus Expression Vector System

The baculovirus expression vector system (BEVS) is a widely used platform for recombinant protein production for use in a wide variety of applications. Of particular interest is production of virus-like particles (VLPs), which consist of multiple viral proteins that self-assemble in strict stoichiometric ratios to mimic the structure of a virus but lacks its genetic material, while a significant amount of effort has been spent on optimizing expression ratios by co-infecting cells with multiple recombinant BEVs and modulating different process parameters, co-expressing multiple foreign genes from a single rBEV may offer more promise. However, there is currently a lack of promoters available with which to optimize co-expression of each foreign gene. To address this, previously published transcriptome data was used to identify promoters that have incrementally lower expression profiles and compared by expressing model cytoplasmic and secreted proteins. Bioinformatics was also used to identify sequence determinants that may be important for late gene transcription regulation, and translation initiation. The identified promoters and bioinformatics analyses may be useful for optimizing expression of foreign genes in the BEVS.

Gene Expression Analysis of Adapted Insect Cells during Influenza VLP Production Using RNA-Sequencing

Adaptive laboratory evolution has been used to improve production of influenza hemagglutinin (HA)-displaying virus-like particles (VLPs) in insect cells. However, little is known about the underlying biological mechanisms promoting higher HA-VLP expression in such adapted cell lines. In this article, we present a study of gene expression patterns associated with high-producer insect High Five cells adapted to neutral pH, in comparison to non-adapted cells, during expression of influenza HA-VLPs. RNA-seq shows a decrease in the amount of reads mapping to host cell genomes along infection, and an increase in those mapping to baculovirus and transgenes. A total of 1742 host cell genes were found differentially expressed between adapted and non-adapted cells throughout infection, 474 of those being either up- or down-regulated at both time points evaluated (12 and 24 h post-infection). Interestingly, while host cell genes were found up- and down-regulated in an approximately 1:1 ratio, all differentially expressed baculovirus genes were found to be down-regulated in infected adapted cells. Pathway analysis of differentially expressed genes revealed enrichment of ribosome biosynthesis and carbohydrate, amino acid, and lipid metabolism. In addition, oxidative phosphorylation and protein folding, sorting and degradation pathways were also found to be overrepresented. These findings contribute to our knowledge of biological mechanisms of insect cells during baculovirus-mediated transient expression and will assist the identification of potential engineering targets to increase recombinant protein production in the future.

Analysis of luciferase dsRNA production during baculovirus infection of Hi5 cells: RNA hairpins expressed by very late promoters do not trigger gene silencing

The baculovirus expression vector system (BEVS) has become an important platform for the expression of recombinant proteins and is especially useful for the production of large protein complexes such as virus-like particles (VLPs). An important application for VLPs is their use as vehicles for targeted delivery of drugs or toxins which requires the development of methods for efficient loading with the intended cargo. Our research intends to employ the BEVS for the production of VLPs for the delivery of insecticidal dsRNA molecules to targeted insect pests (as "dsRNA-VLPs"). A convenient strategy would be the co-expression of long dsRNAs with viral capsid proteins and their simultaneous encapsulation during VLP assembly but the capacity of the BEVS for the production of long dsRNA has not been assessed so far. In this study, the efficiency of production of long RNA hairpins targeting the luciferase gene ("dsLuc") by the polyhedrin promoter during baculovirus infection was evaluated. However, RNAi reporter assays could not detect significant amounts of dsLuc in Hi5 cells infected with recombinant baculovirus, even in the presence of co-expressed dsRNA-binding protein B2-GFP or the employment of the MS2-MCP system. Nevertheless, dot blot analyses using anti-dsRNA antibody revealed that baculovirus-mediated expression of B2-GFP resulted in significant increases in dsRNA levels in infected cells that may correspond to hybridized complementary viral transcripts. Using B2-GFP as a genetically encoded sensor, dsRNA foci were detected in the nuclei that partially co-localized with DAPI staining, consistent with their localization at the virogenic stroma. Co-localization experiments with the baculovirus proteins vp39, Ac93, ODV-E25 and gp64 indicated limited overlap between B2-GFP and the ring zone compartment where assembly of nucleocapsids and virions occurs. Stability experiments showed that exogenous dsRNA is resistant to degradation in extracts of non-infected and infected Hi5 cells and it is proposed that strong unwinding activity at the virogenic stroma in the infected nuclei may neutralize the annealing of complementary RNA strands and block the production of long dsRNAs. Because the strong stability of exogenous dsRNA, transfection can be explored as an alternative method for delivery of cargo for dsRNA-VLPs during their assembly in baculovirus-infected Hi5 cells.

Rapid titration of recombinant baculoviruses based on NanoLuc secretion in early infection

The baculovirus expression vector system has become a powerful tool for recombinant protein production and gene delivery. However, existing titration methods for baculovirus are not economical in terms of test time and cost. A titration method based on NanoLuc secretion that allows for titration of recombinant baculoviruses at 4 h post infection (hpi) is described. In the assay, the envelop protein GP64 signal peptide-guided NanoLuc was secreted into the culture medium in proportion to the virus amount during early infection under combined control by the homologous region 5 (hr5) enhancer and the promoter of immediate early gene 1 (ie1) plus L21. Two timepoint standard curves of luciferase activity to virus titers of 5-9 logs were established with excellent linearity and correlation coefficients (slope = 1.050, R² ≥ 0.9969) using a secretory Nanoluc (secrNluc) - inserted standard baculovirus. Through the assay, the titers of three recombinant viruses prepared independently were calculated by directly measuring luciferase activity in the supernatant at 4 and 6 hpi, with greater accuracy compared to the endpoint dilution assay. These results show the efficacy of this proposed method as a streamlined assay for rapidly titrating recombinant baculoviruses.

Conserved Viral Transcription Plays a Key Role in Virus-Like Particle Production of the Parasitoid Wasp Venturia canescens

Nudiviruses are large double-stranded DNA viruses related to baculoviruses known to be endogenized in the genomes of certain parasitic wasp species. These wasp-virus associations allow the production of viral particles or virus-like particles that ensure wasp parasitism success within lepidopteran hosts. Venturia canescens is an ichneumonid wasp belonging to the Campopleginae subfamily that has endogenized nudivirus genes belonging to the Alphanudivirus genus to produce "virus-like particles" (Venturia canescens virus-like particles [VcVLPs]), which package proteic virulence factors. The main aim of this study was to determine whether alphanudivirus gene functions have been conserved following endogenization. The expression dynamics of alphanudivirus genes was monitored by a high throughput transcriptional approach, and the functional role of lef-4 and lef-8 genes predicted to encode viral RNA polymerase components was investigated by RNA interference. As described for baculovirus infections and for endogenized nudivirus genes in braconid wasp species producing bracoviruses, a transcriptional cascade involving early and late expressed alphanudivirus genes could be observed. The expression of lef-4 and lef-8 was also shown to be required for the expression of alphanudivirus late genes allowing correct particle formation. Together with previous literature, the results show that endogenization of nudiviruses in parasitoid wasps has repeatedly led to the conservation of the viral RNA polymerase function, allowing the production of viruses or viral-like particles that differ in composition but enable wasp parasitic success. IMPORTANCE This study shows that endogenization of a nudivirus genome in a Campopleginae parasitoid wasp has led to the conservation, as for endogenized nudiviruses in braconid parasitoid wasps, of the viral RNA polymerase function, required for the transcription of genes encoding viral particles involved in wasp parasitism success. We also showed for the first time that RNA interference (RNAi) can be successfully used to downregulate gene expression in this species, a model in behavioral ecology. This opens the opportunity to investigate the function of genes involved in other traits important for parasitism success, such as reproductive strategies and host choice. Fundamental data acquired on gene function in Venturia canescens are likely to be transferable to other parasitoid wasp species used in biological control programs. This study also renders possible the investigation of other nudivirus gene functions, for which little data are available.

Dual isoform sequencing reveals complex transcriptomic and epitranscriptomic landscapes of a prototype baculovirus

In this study, two long-read sequencing (LRS) techniques, MinION from Oxford Nanopore Technologies and Sequel from the Pacific Biosciences, were used for the transcriptional characterization of a prototype baculovirus, Autographa californica multiple nucleopolyhedrovirus. LRS is able to read full-length RNA molecules, and thereby distinguish between transcript isoforms, mono- and polycistronic RNAs, and overlapping transcripts. Altogether, we detected 875 transcript species, of which 759 were novel and 116 were annotated previously. These RNA molecules include 41 novel putative protein coding transcripts [each containing 5'-truncated in-frame open reading frames (ORFs), 14 monocistronic transcripts, 99 polygenic RNAs, 101 non-coding RNAs, and 504 untranslated region isoforms. This work also identified novel replication origin-associated transcripts, upstream ORFs, cis-regulatory sequences and poly(A) sites. We also detected RNA methylation in 99 viral genes and RNA hyper-editing in the longer 5'-UTR transcript isoform of the canonical ORF 19 transcript.

Multiloci Manipulation of Baculovirus Genome Reveals the Pivotal Role of Homologous Regions in Viral DNA Replication, Progeny Production, and Enhancing Transcription

The engineering of viral genomes facilitates both fundamental and applied research on viruses. However, the multiloci manipulation of DNAs of viruses with large DNA genomes, such as baculoviruses, herpesviruses, and poxviruses, is technically challenging, particularly for highly homologous or repetitive sequences. Homologous regions (hrs) have multiple copies in many large DNA viruses and play pivotal roles in the viral life cycle. Here, we used synthetic biology to investigate the fundamental function of baculoviral hrs by conducting multiloci manipulation of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) DNA that contains eight hrs scattered in the genome. Using transformation-associated recombination in yeast, we generated recombinant AcMNPV genomes in which we deleted all hrs or retained a single hr (hr1, hr2, or hr3). Infectious viruses were rescued after transfecting the synthetic viral genomes into host cells, and their replication features were characterized. The results demonstrated that deletion of all hrs severely compromised viral DNA replication and progeny production, whereas retaining only a single hr was essential for efficient viral DNA replication and progeny production. The synthetic virus with hr2 or hr3 showed a growth curve similar to that of the parental virus. Transcriptomic analysis revealed that hr1, hr2, and hr3 could enhance gene transcription within a surrounding region of 14.6 kb, 13.8 kb, and 29.8 kb, respectively. Overall, this study revealed the advantages of synthetic biology in multiloci engineering and functional studies of large DNA viruses. In addition, our findings on hrs will be helpful for the design and improvement of baculovirus-based expression vectors.

The amino acids of Autographa californica multiple nucleopolyhedrovirus P48 critical for the association with Ac93 are important for the nuclear egress of nucleocapsids and efficient formation of intranuclear microvesicles

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) undergoes a biphasic life cycle with the production of two physically and functionally distinct virions: budded virions (BVs) and occlusion-derived virions (ODVs). Nuclear egress of nucleocapsids and intranuclear microvesicle formation are critical for the morphogenesis of BVs and ODVs, respectively, but the mechanisms and details of these two processes remain unknown. Our previous studies have shown that AcMNPV p48 (ac103) gene is essential for the nuclear egress of nucleocapsids and efficient formation of intranuclear microvesicles, and protein P48 associates with Ac93, which is also involved in the above processes in virion morphogenesis. In this study, we present evidence that alanine substitution for residues N318, V319, C320, R321, and I323 of P48 disrupted the association with Ac93. Moreover, mutation of these residues blocked the nuclear egress of nucleocapsids and efficient formation of intranuclear microvesicles, and subsequent BV formation, as well as ODV envelopment and embedding of ODVs into polyhedra. These results suggested that the association between P48 and Ac93 may be important for both BV and ODV morphogenesis.

Assessing the Impact of a Viral Infection on the Expression of Transposable Elements in the Cabbage Looper Moth (Trichoplusia ni)

Most studies of stress-induced transposable element (TE) expression have so far focused on abiotic sources of stress. Here, we analyzed the impact of an infection by the AcMNPV baculovirus on TE expression in a cell line (Tnms42) and midgut tissues of the cabbage looper moth (Trichoplusia ni). We find that a large fraction of TE families (576/636 in Tnms42 cells and 503/612 in midgut) is lowly expressed or not expressed at all [≤ 4 transcripts per million (TPM)] in the uninfected condition (median TPM of 0.37 in Tnms42 and 0.46 in midgut cells). In the infected condition, a total of 62 and 187 TE families were differentially expressed (DE) in midgut and Tnms42 cells, respectively, with more up- (46) than downregulated (16) TE families in the former and as many up- (91) as downregulated (96) TE families in the latter. Expression log2 fold changes of DE TE families varied from -4.95 to 9.11 in Tnms42 cells and from -4.28 to 7.66 in midgut. Large variations in expression profiles of DE TEs were observed depending on the type of cells and on time after infection. Overall, the impact of AcMNPV on TE expression in T. ni is moderate but potentially sufficient to affect TE activity and genome architecture. Interestingly, one host-derived TE integrated into AcMNPV genomes is highly expressed in infected Tnms42 cells. This result shows that virus-borne TEs can be expressed, further suggesting that they may be able to transpose and that viruses may act as vectors of horizontal transfer of TEs in insects.

Solutions against emerging infectious and noninfectious human diseases through the application of baculovirus technologies

Abstract

Baculoviruses are insect pathogens widely used as biotechnological tools in different fields of life sciences and technologies. The particular biology of these entities (biosafety viruses 1; large circular double-stranded DNA genomes, infective per se; generally of narrow host range on insect larvae; many of the latter being pests in agriculture) and the availability of molecular-biology procedures (e.g., genetic engineering to edit their genomes) and cellular resources (availability of cell lines that grow under in vitro culture conditions) have enabled the application of baculoviruses as active ingredients in pest control, as systems for the expression of recombinant proteins (Baculovirus Expression Vector Systems—BEVS) and as viral vectors for gene delivery in mammals or to display antigenic proteins (Baculoviruses applied on mammals—BacMam). Accordingly, BEVS and BacMam technologies have been introduced in academia because of their availability as commercial systems and ease of use and have also reached the human pharmaceutical industry, as incomparable tools in the development of biological products such as diagnostic kits, vaccines, protein therapies, and—though still in the conceptual stage involving animal models—gene therapies. Among all the baculovirus species, the Autographa californica multiple nucleopolyhedrovirus has been the most highly exploited in the above utilities for the human-biotechnology field. This review highlights the main achievements (in their different stages of development) of the use of BEVS and BacMam technologies for the generation of products for infectious and noninfectious human diseases.

Key points

• Baculoviruses can assist as biotechnological tools in human health problems.

• Vaccines and diagnosis reagents produced in the baculovirus platform are described.

• The use of recombinant baculovirus for gene therapy–based treatment is reviewed.

A review of alternative promoters for optimal recombinant protein expression in baculovirus-infected insect cells

Generating recombinant proteins in insect cells has been made possible via the use of the Baculovirus Expression Vector System (BEVS). Despite the success of many proteins via this platform, some targets remain a challenge due to issues such as cytopathic effects, the unpredictable nature of co-infection and co-expressions, and baculovirus genome instability. Many promoters have been assayed for the purpose of expressing diverse proteins in insect cells, and yet there remains a lack of implementation of those results when reviewing the landscape of commercially available baculovirus vectors. In advancing the platform to produce a greater variety of proteins and complexes, the development of such constructs cannot be avoided. A better understanding of viral gene regulation and promoter options including viral, synthetic, and insect-derived promoters will be beneficial to researchers looking to utilize BEVS by recruiting these intricate mechanisms of gene regulation for heterologous gene expression. Here we summarize some of the developments that could be utilized to improve the expression of recombinant proteins and multi-protein complexes in insect cells.

Comparison of CRISPR-Cas9 Tools for Transcriptional Repression and Gene Disruption in the BEVS

The generation of knock-out viruses using recombineering of bacmids has greatly accelerated scrutiny of baculovirus genes for a variety of applications. However, the CRISPR-Cas9 system is a powerful tool that simplifies sequence-specific genome editing and effective transcriptional regulation of genes compared to traditional recombineering and RNAi approaches. Here, the effectiveness of the CRISPR-Cas9 system for gene disruption and transcriptional repression in the BEVS was compared. Cell lines constitutively expressing the cas9 or dcas9 gene were developed, and recombinant baculoviruses delivering the sgRNA were evaluated for disruption or repression of a reporter green fluorescent protein gene. Finally, endogenous AcMNPV genes were targeted for disruption or downregulation to affect gene expression and baculovirus replication. This study provides a proof-of-concept that CRISPR-Cas9 technology may be an effective tool for efficient scrutiny of baculovirus genes through targeted gene disruption and transcriptional repression.

Monobac System-A Single Baculovirus for the Production of rAAV

Large-scale manufacturing of rAAV is a bottleneck for the development of genetic disease treatments. The baculovirus/Sf9 cell system underpins the first rAAV treatment approved by EMA and remains one of the most advanced platforms for rAAV manufacturing. Despite early successes, rAAV is still a complex biomaterial to produce. Efficient production of the recombinant viral vector requires that AAV replicase and capsid genes be co-located with the recombinant AAV genome. Here, we present the Monobac system, a singular, modified baculovirus genome that contains all of these functions. To assess the relative yields between the dual baculovirus and Monobac systems, we prepared each system with a transgene encoding γSGC and evaluated vectors’ potency in vivo. Our results show that rAAV production using the Monobac system not only yields higher titers of rAAV vector but also a lower amount of DNA contamination from baculovirus.

Bombyx mori Nucleopolyhedrovirus p26 Is Associated with Viral Late Stage Replication

Bombyx mori nucleopolyhedrovirus (BmNPV) p26 is conserved among all Lepidoptera baculoviruses that have been completely sequenced thus far, and some baculoviruses even have two copies of p26, which suggested that p26 may play an important role in the virus infection cycle. This study aimed to characterize BmNPV p26. We found that BmNPV p26 transcripts were detectable as early as 3 h post-infection (hpi), and the transcript levels rapidly increased starting from 12 hpi. Western blot analysis using an anti-p26 polyclonal antibody demonstrated that the corresponding protein was also detectable from 6 hpi in BmNPV-infected cell lysates. Immunofluorescence analysis demonstrated that p26 was mainly dispersed in the infected cell cytoplasm, whereas the over-expressed fusion protein EGFP-p26 also accumulated in the nucleus. These results indicated that p26 is an early BmNPV gene and has functions both in the cytoplasm and the nucleus. RNAi-based knockdown of p26 could produce infectious virus and normal-appearing virions but decreased budded virus (BV) production in BmNPV-infected cells at 72 hpi. Moreover, the results of further quantitative PCR (Q-PCR) analysis indicated that the gp64 and p74 transcripts levels decreased significantly. These results indicated that BmNPV p26 may be associated with BmNPV replication during the late infection stage.

Systematic Analysis of 42 Autographa Californica Multiple Nucleopolyhedrovirus Genes Identifies An Additional Six Genes Involved in the Production of Infectious Budded Virus

Baculoviruses have been widely used as a vector for expressing foreign genes. Among numerous baculoviruses, Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the most frequently used and it encodes 155 open reading frames (ORFs). Here, we systematically investigated the impact of 42 genes of AcMNPV on the production of infectious budded viruses (BVs) by constructing gene-knockout bacmids and subsequently conducting transfection and infection assays. The results showed that among the 39 functionally unverified genes and 3 recently reported genes, 36 are dispensable for infectious BV production, as the one-step growth curves of the gene-knockout viruses were not significantly different from those of the parental virus. Three genes (ac62, ac82 and ac106/107) are essential for infectious BV production, as deletions thereof resulted in complete loss of infectivity while the repaired viruses showed no significant difference in comparison to the parental virus. In addition, three genes (ac13, ac51 and ac120) are important but not essential for infectious BV production, as gene-knockout viruses produced significantly lower BV levels than that of the parental virus or repaired viruses. We then grouped the 155 AcMNPV genes into three categories (Dispensable, Essential, or Important for infectious BV production). Based on our results and previous publications, we constructed a schematic diagram of a potential mini-genome of AcMNPV, which contains only essential and important genes. The results shed light on our understanding of functional genomics of baculoviruses and provide fundamental information for future engineering of baculovirus expression system.

Improvement of protein production by engineering a novel antiapoptotic baculovirus vector to suppress the expression of Sf-caspase-1 and Tn-caspase-1

The baculovirus expression vector system (BEVS) is an attractive manufacturing platform for recombinant protein production in insect cells. However, baculovirus infection commonly induces host apoptosis in 3-4 days which would subsequently terminate the protein expression. Previous studies have proved that protein production by BEVS can be elevated in apoptosis-suppressed insect cells. We also developed a baculovirus vector in our previous report to inhibit the apoptosis and improve protein production in Sf9 cells. In this study, we designed five short hairpin RNA (shRNA) expression cassettes targeting a conserved region in Spodoptera frugiperda caspase-1 (Sf-caspase-1) and Trichoplusia ni caspase-1 (Tn-caspase-1), and found that introduction of C to T mutations within the stem region of the expression cassette was beneficial for the heterologous protein expression. One of the improved shRNA expression cassettes was knocked into a bacmid with the deletion of several nonessential genes. The novel baculovirus vector demonstrated the ability to suppress cell apoptosis in both Sf9 and High Five cells, and exhibited superior recombinant protein productivity of intracellularly expressed GFP and firefly luciferase and secreted glycoprotein OD-Fc. The antiapoptotic baculovirus vector developed in this study could serve as a useful tool for the protein production in scientific research and pharmaceutical industries.

A comprehensive antigen production and characterisation study for easy-to-implement, specific and quantitative SARS-CoV-2 serotests

BACKGROUND

Antibody tests are essential tools to investigate humoral immunity following SARS-CoV-2 infection or vaccination. While first-generation antibody tests have primarily provided qualitative results, accurate seroprevalence studies and tracking of antibody levels over time require highly specific, sensitive and quantitative test setups.

METHODS

We have developed two quantitative, easy-to-implement SARS-CoV-2 antibody tests, based on the spike receptor binding domain and the nucleocapsid protein. Comprehensive evaluation of antigens from several biotechnological platforms enabled the identification of superior antigen designs for reliable serodiagnostic. Cut-off modelling based on unprecedented large and heterogeneous multicentric validation cohorts allowed us to define optimal thresholds for the tests' broad applications in different aspects of clinical use, such as seroprevalence studies and convalescent plasma donor qualification.

FINDINGS

Both developed serotests individually performed similarly-well as fully-automated CE-marked test systems. Our described sensitivity-improved orthogonal test approach assures highest specificity (99.8%); thereby enabling robust serodiagnosis in low-prevalence settings with simple test formats. The inclusion of a calibrator permits accurate quantitative monitoring of antibody concentrations in samples collected at different time points during the acute and convalescent phase of COVID-19 and disclosed antibody level thresholds that correlate well with robust neutralization of authentic SARS-CoV-2 virus.

INTERPRETATION

We demonstrate that antigen source and purity strongly impact serotest performance. Comprehensive biotechnology-assisted selection of antigens and in-depth characterisation of the assays allowed us to overcome limitations of simple ELISA-based antibody test formats based on chromometric reporters, to yield comparable assay performance as fully-automated platforms.

FUNDING

WWTF, Project No. COV20-016; BOKU, LBI/LBG.

VP39 of Spodoptera litura multicapsid nucleopolyhedrovirus cannot efficiently rescue the nucleocapsid assembly of vp39-null Autographa californica multiple nucleopolyhedrovirus

Background

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) vp39 is conserved in all sequenced baculovirus genomes. In previous studies, VP39 has been identified as the major capsid structure protein of baculoviruses and found to be essential for nucleocapsid assembly. The nucleocapsid composition and structure of Group I and II NPVs of the Alphabaculovirus genus are very similar. It is not clear whether the major capsid structure protein VP39 of Group I NPVs is functionally identical to or substitutable with the Group II NPV VP39. In this study, the function of Group II Spodoptera litura MNPV (SpltMNPV) VP39 in Group I AcMNPV was characterized.

Methods

Sequence alignment of AcMNPV VP39 and SpltMNPV VP39 was performed using Clustal X and edited with GeneDoc. To determine whether VP39 of Group I NPVs can be functionally substituted by Group II NPV VP39, a vp39-null AcMNPV (vAcvp39KO) and a vp39-pseudotyped AcMNPV (vAcSpltvp39:FLAG), in which the Group I AcMNPV vp39 coding sequence was replaced with that of SpltMNPV from Group II NPVs, were constructed via homologous recombination in Escherichia coli. Using an anti-FLAG monoclonal antibody, immunoblot analysis was performed to examine SpltMNPV VP39 expression. Fluorescence and light microscopy were used to monitor viral replication and infection. Viral growth curve analysis was performed using a fifty percent tissue culture infective dose (TCID₅₀) endpoint dilution assay. Viral morphogenesis was detected using an electron microscope.

Results

Sequence alignment indicated that the N-termini of AcMNPV VP39 and SpltMNPV VP39 are relatively conserved, whereas the C-terminus of SpltMNPV VP39 lacks the domain of amino acid residues 306–334 homologous to AcMNPV VP39. Immunoblot analysis showed that SpltMNPV VP39 was expressed in vAcSpltvp39:FLAG. Fluorescence and light microscopy showed that vAcSpltvp39:FLAG did not spread by infection. Viral growth curve analysis confirmed a defect in infectious budded virion production. Electron microscopy revealed that although masses of abnormally elongated empty capsid structures existed inside the nuclei of Sf9 cells transfected with vAcSpltvp39:FLAG, no nucleocapsids were observed.

Conclusion

Altogether, our results demonstrated that VP39 from SpltMNPV cannot efficiently substitute AcMNPV VP39 during nucleocapsid assembly in AcMNPV.

A Conserved Phenylalanine Residue of Autographa Californica Multiple Nucleopolyhedrovirus AC75 Protein Is Required for Occlusion Body Formation

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf75 (ac75) is a highly conserved gene that is essential for AcMNPV propagation. However, the key domains or residues of the AC75 protein that play a role in viral propagation have not been identified. In this study, sequence alignment revealed that residues Phe-54 and Gln-81 of AC75 were highly conserved among alphabaculoviruses and betabaculoviurses. Thus, Phe-54 and Gln-81 AC75 mutation bacmids were constructed. We found that Gln-81 was not required for viral propagation, whereas mutating Phe-54 reduced budded virus production by 10-fold and impaired occlusion body formation when compared with that of the wild-type AcMNPV. Electron microscopy observations showed that the Phe-54 mutation affected polyhedrin assembly and also occlusion-derived virus embedding, whereas western blot analysis revealed that mutating Phe-54 reduced the amount of AC75 but did not affect the localization of AC75 in infected cells. A protein stability assay showed that the Phe-54 mutation affected AC75 stability. Taken together, Phe-54 was identified as an important residue of AC75, and ac75 is a pivotal gene in budding virus production and occlusion body formation.

A Tale of Two Transcriptomic Responses in Agricultural Pests via Host Defenses and Viral Replication

Autographa californica Multiple Nucleopolyhedrovirus (AcMNPV) is a baculovirus that causes systemic infections in many arthropod pests. The specific molecular processes underlying the biocidal activity of AcMNPV on its insect hosts are largely unknown. We describe the transcriptional responses in two major pests, Spodoptera frugiperda (fall armyworm) and Trichoplusia ni (cabbage looper), to determine the host-pathogen responses during systemic infection, concurrently with the viral response to the host. We assembled species-specific transcriptomes of the hemolymph to identify host transcriptional responses during systemic infection and assessed the viral transcript abundance in infected hemolymph from both species. We found transcriptional suppression of chitin metabolism and tracheal development in infected hosts. Synergistic transcriptional support was observed to suggest suppression of immune responses and induction of oxidative stress indicating disease progression in the host. The entire AcMNPV core genome was expressed in the infected host hemolymph with a proportional high abundance detected for viral transcripts associated with replication, structure, and movement. Interestingly, several of the host genes that were targeted by AcMNPV as revealed by our study are also targets of chemical insecticides currently used commercially to control arthropod pests. Our results reveal an extensive overlap between biological processes represented by transcriptional responses in both hosts, as well as convergence on highly abundant viral genes expressed in the two hosts, providing an overview of the host-pathogen transcriptomic landscape during systemic infection.

Autographa Californica Multiple Nucleopolyhedrovirus orf13 Is Required for Efficient Nuclear Egress of Nucleocapsids

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf13 (ac13) is a conserved gene in all sequenced alphabaculoviruses. However, its function in the viral life cycle remains unknown. In this study, we found that ac13 was a late gene and that the encoded protein, bearing a putative nuclear localization signal motif, colocalized with the nuclear lamina. Deletion of ac13 did not affect viral genome replication, nucleocapsid assembly or occlusion body (OB) formation, but reduced virion budding from infected cells by approximately 400-fold compared with the wild-type virus. Deletion of ac13 substantially impaired the egress of nucleocapsids from the nucleus to the cytoplasm, while the OB morphogenesis was unaffected. Taken together, our results indicated that ac13 was required for efficient nuclear egress of nucleocapsids during virion budding, but was dispensable for OB formation.

Gene expression patterns of Cydia pomonella granulovirus in codling moth larvae revealed by RNAseq analysis

The Cydia pomonella granulovirus (CpGV) has been used as a biological control agent of codling moth (Cydia pomonella), a severe global pest on pome fruit. Despite the economic importance, our knowledge of its molecular biology is still limited and a detailed picture of its gene expression is still missing. Here, we sequenced the transcriptome of codling moth larvae infected with the Mexican isolate CpGV-M and analyzed the expression of viral genes at 12, 48, and 96 h post infection (hpi). The results showed that two genes (p6.9 and pp31/39K) related to DNA binding of virus production, were highly expressed at 48 and 96 hpi. From 48 to 96 hpi, the expression of genes associated with virus replication and dissemination decreased, whereas the expression of genes related to infectious virion production and per os infectivity increased. This study provides a comprehensive view of CpGV gene expression patterns in host larvae.

Transcriptome of Cydia pomonella granulovirus in susceptible and type I resistant codling moth larvae

The baculovirus Cydia pomonella granulovirus (CpGV) is a biocontrol agent used worldwide against the codling moth (CM), Cydia pomonella L., a severe pest in organic and integrated pome fruit production. Its successful application is increasingly challenged by the occurrence of CM populations resistant to commercial CpGV products. Whereas three types (I-III) of CpGV resistance have been identified, type I resistance compromising the efficacy of CpGV-M, the so-called Mexican isolate of CpGV, is assumed to be the most widely distributed resistance type in Central Europe. Despite the wide use of CpGV products as biocontrol agents, little information is available on gene-expression levels in CM larvae. In this study, the in vivo transcriptome of CpGV-M infecting susceptible (CpS) and resistant (CpRR1) CM larvae was analysed at 24, 48, 72, 96 and 120 hours post infection in the midgut and fat body tissue by using a newly developed microarray covering all ORFs of the CpGV genome. According to their transcript abundance, the CpGV genes were grouped into four temporal clusters to which groups of known and unknown function could be assigned. In addition, sets of genes differentially expressed in the midgut and fat body were found in infected susceptible CpS larvae. For the resistant CpRR1 larvae treated with CpGV-M, viral entry in midgut cells could be confirmed from onset but a significantly reduced gene expression, indicating that type I resistance is associated with a block of viral gene transcription and replication.

Advances in the Bioinformatics Knowledge of mRNA Polyadenylation in Baculovirus Genes

Baculoviruses are a group of insect viruses with large circular dsDNA genomes exploited in numerous biotechnological applications, such as the biological control of agricultural pests, the expression of recombinant proteins or the gene delivery of therapeutic sequences in mammals, among others. Their genomes encode between 80 and 200 proteins, of which 38 are shared by all reported species. Thanks to multi-omic studies, there is remarkable information about the baculoviral proteome and the temporality in the virus gene expression. This allows some functional elements of the genome to be very well described, such as promoters and open reading frames. However, less information is available about the transcription termination signals and, consequently, there are still imprecisions about what are the limits of the transcriptional units present in the baculovirus genomes and how is the processing of the 3′ end of viral mRNA. Regarding to this, in this review we provide an update about the characteristics of DNA signals involved in this process and we contribute to their correct prediction through an exhaustive analysis that involves bibliography information, data mining, RNA structure and a comprehensive study of the core gene 3′ ends from 180 baculovirus genomes.

Identification of miRNAs encoded by Autographa californica nucleopolyhedrovirus

Two Autographa californica nucleopolyhedrovirus (AcMNPV) encoded miRNAs, AcMNPV-miR-1 and AcMNPV-miR-3, have been reported by us in 2013 and 2019, respectively. Here, we present an integrated investigation of AcMNPV-encoded miRNAs, which include the above two miRNAs and three additional newly identified miRNAs. Six candidate miRNAs were predicted through small RNA deep sequencing and bioinformatics, of which, five were validated. Three miRNAs are located opposite the coding sequences, the other two are located in the coding sequences of viral genes. Targets in both virus and host were predicted and subsequently tested using dual-luciferase reporter assays. The validated targets were found mainly in AcMNPV, except for the targets of AcMNPV-miR-4, which are all host genes. Based on reporter assays, the five miRNAs predominantly function by down-regulating their targets. The transcription start sites of these miRNAs were bioinformatic screened based on known baculovirus promoter motifs. Our study reveals that AcMNPV-encoded miRNAs function as fine modulators of the interactions between host and virus by regulating viral and/or host genes.

Unraveling the Genome-Wide Impact of Recombinant Baculovirus Infection in Mammalian Cells for Gene Delivery

Baculovirus expression systems have been widely used to produce recombinant mammalian proteins owing to the lack of viral replication in vertebrates. Although several lines of evidence have demonstrated impacts of baculovirus infection in mammalian hosts, genome-wide effects have not been fully elucidated. Here, we provide comparative transcriptome profiles of baculovirus and host-immune response genes in recombinant baculovirus-infected mammalian and insect cells. Specifically, to decipher the impacts of baculovirus infection in mammalian cells, we conducted total RNA-seq on human 293TT cells and insect Sf9 cells infected with recombinant baculovirus. We found that baculovirus genes were rarely expressed under the control of baculoviral promoters in 293TT cells. Although some baculovirus early genes, such as PE38 and IE-01, showed limited expression in 293TT cells, baculoviral late genes were mostly silent. We also found modest induction of a small number of mammalian immune response genes associated with Toll-like receptors, cytokine signaling, and complement in baculovirus-infected 293TT cells. These comprehensive transcriptome data will contribute to improving recombinant baculovirus as tools for gene delivery, gene therapy, and vaccine development.

Baculovirus Transduction in Mammalian Cells Is Affected by the Production of Type I and III Interferons, Which Is Mediated Mainly by the cGAS-STING Pathway

The baculovirus Autographa californica multiple nucleopolyhedrovirus is an insect virus with a circular double-stranded DNA genome, which, among other multiple biotechnological applications, is used as an expression vector for gene delivery in mammalian cells. Nevertheless, the nonspecific immune response triggered by viral vectors often suppresses transgene expression. To understand the mechanisms involved in that response, in the present study, we studied the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway by using two approaches: the genetic edition through CRISPR/Cas9 technology of genes encoding STING or cGAS in NIH/3T3 murine fibroblasts and the infection of HEK293 and HEK293 T human epithelial cells, deficient in cGAS and in cGAS and STING expression, respectively. Overall, our results suggest the existence of two different pathways involved in the establishment of the antiviral response, both dependent on STING expression. Particularly, the cGAS-STING pathway resulted in the more relevant production of beta interferon (IFN-β) and IFN-λ1 in response to baculovirus infection. In human epithelial cells, IFN-λ1 production was also induced in a cGAS-independent and DNA-protein kinase (DNA-PK)-dependent manner. Finally, we demonstrated that these cellular responses toward baculovirus infection affect the efficiency of transduction of baculovirus vectors.IMPORTANCE Baculoviruses are nonpathogenic viruses that infect mammals, which, among other applications, are used as vehicles for gene delivery. Here, we demonstrated that the cytosolic DNA sensor cGAS recognizes baculoviral DNA and that the cGAS-STING axis is primarily responsible for the attenuation of transduction in human and mouse cell lines through type I and type III IFNs. Furthermore, we identified DNA-dependent protein kinase (DNA-PK) as a cGAS-independent and alternative DNA cytosolic sensor that contributes less to the antiviral state in baculovirus infection in human epithelial cells than cGAS. Knowledge of the pathways involved in the response of mammalian cells to baculovirus infection will improve the use of this vector as a tool for gene therapy.

Bombyx mori nucleopolyhedrovirus Bm46 is essential for efficient production of infectious BV and nucleocapsid morphogenesis

Bombyx mori nucleopolyhedrovirus (BmNPV) orf46 (Bm46), the orthologues of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac57, is a highly conserved gene in group Ⅰ and group Ⅱ nucleopolyhedroviruses (NPVs). However, its function in viral life cycle is unclear. Our results indicated that Bm46 transcript was detected from infected cells at 12 h post infection, while Bm46 protein was detectable from 24 to 72 h post infection. Upon the deletion of Bm46, fewer infectious BVs were produced by titer assays, but neither viral DNA synthesis nor occlusion bodies (OBs) production was affected. Electron microscopy revealed that Bm46 knockout interrupted nucleocapsid assembly and occlusion-derived virus (ODV) embedding, resulting in aberrant capsid-like tubular structures accumulated in the RZ (ring zone). Interestingly, this abnormally elongated capsid structures were consistent with the immunofluorescence microscopy results showing that VP39 assembled into long filaments and cables in the RZ. Moreover, DNA copies decreased by 30 % in occlusion bodies (OBs) produced by Bm46-knockout virus. qRT-PCR and Western blot analysis showed that the expression of VP39 was affected by Bm46 disruption. Taken together, our findings clearly pointed out that Bm46 played an important role in BV production and the proper formation of nucleocapsid morphogenesis.

A new nodavirus-negative Trichoplusia ni cell line for baculovirus-mediated protein production

Cell lines derived from Trichoplusia ni (Tn) are widely used as hosts in the baculovirus-insect cell system (BICS). One advantage of Tn cell lines is they can produce recombinant proteins at higher levels than cell lines derived from other insects. However, Tn cell lines are persistently infected with an alphanodavirus, Tn5 cell-line virus (TnCLV), which reduces their utility as a host for the BICS. Several groups have isolated TnCLV-negative Tn cell lines, but none were thoroughly characterized and shown to be free of other adventitious viruses. Thus, we isolated and extensively characterized a new TnCLV-negative line, Tn-nodavirus-negative (Tn-NVN). Tn-NVN cells have no detectable TnCLV, no other previously identified viral contaminants of lepidopteran insect cell lines, and no sequences associated with any replicating virus or other viral adventitious agents. Tn-NVN cells tested negative for >60 species of Mycoplasma, Acholeplasma, Spiroplasma, and Ureaplasma. Finally, Tn-NVN cells grow well as a single-cell suspension culture in serum-free medium, produce recombinant proteins at levels similar to High Five™ cells, and do not produce recombinant glycoproteins with immunogenic core α1,3-fucosylation. Thus, Tn-NVN is a new, well-characterized TnCLV-negative cell line with several other features enhancing its utility as a host for the BICS.

Entomopathogenic Viruses in the Neotropics: Current Status and Recently Discovered Species

The market for biological control of insect pests in the world and in Brazil has grown in recent years due to the unwanted ecological and human health impacts of chemical insecticides. Therefore, research on biological control agents for pest management has also increased. For instance, insect viruses have been used to protect crops and forests around the world for decades. Among insect viruses, the baculoviruses are the most studied and used viral biocontrol agent. More than 700 species of insects have been found to be naturally infected by baculoviruses, with 90% isolated from lepidopteran insects. In this review, some basic aspects of baculovirus infection in vivo and in vitro infection, gene content, viral replication will be discussed. Furthermore, we provide examples of the use of insect viruses for biological pest control and recently characterized baculoviruses in Brazil.

The protective and pathogenic roles of CXCL17 in human health and disease: Potential in respiratory medicine

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C-X-C motif chemokine 17 (CXCL17), plays a functional role in maintaining homeostasis at mucosal barriers.

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CXCL17 expression is associated with both disease progression and protection in various diseases.

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The multifactorial mechanistic properties of CXCL17 could be exploited as a therapeutic target

C-X-C motif chemokine 17 (CXCL-17) is a novel chemokine that plays a functional role maintaining homeostasis at distinct mucosal barriers, including regulation of myeloid-cell recruitment, angiogenesis, and control of microorganisms. Particularly, CXCL17 is produced along the epithelium of the airways both at steady state and under inflammatory conditions. While increased CXCL17 expression is associated with disease progression in pulmonary fibrosis, asthma, and lung/hepatic cancer, it is thought to play a protective role in pancreatic cancer, autoimmune encephalomyelitis and viral infections. Thus, there is emerging evidence pointing to both a harmful and protective role for CXCL17 in human health and disease, with therapeutic potential for translational applications. In this review, we provide an overview of the discovery, characteristics and functions of CXCL17 emphasizing its clinical potential in respiratory disorders.

Disrupting the association of Autographa californica multiple nucleopolyhedrovirus Ac93 with cellular ESCRT-III/Vps4 hinders nuclear egress of nucleocapsids and intranuclear microvesicles formation

The endosomal sorting complex required for transport (ESCRT) pathway is required for efficient egress of Autographa californica multiple nucleopolyhedrovirus (AcMNPV). In this study, we found that Ac93, a baculovirus core protein, contains a conserved MIM1-like motif. Alanine substitutions for six leucine residues in MIM1-like motif revealed that L142, L145, L146, and L149 are required for association of Ac93 with the MIT domain of Vps4. Mutations of these residues also blocked self-association and the association of Ac93 with ESCRT-III proteins or other viral core proteins Ac76 and Ac103, and resulted in a substantial reduction of infectious virus production, less efficient nuclear egress of progeny nucleocapsids, and the defect of intranuclear microvesicles formation. Combined with the localization of the association of Ac93 with ESCRT-III/Vps4 and other viral proteins at the nuclear membrane, we propose that the coordinated action of these viral proteins and ESCRT-III/Vps4 may be involved in remodeling the nuclear membrane.

Ac154 carried out anti-apoptotic role during AcMNPV infection process in the host insect cells

AcMNPV is the first baculovirus to be sequenced and is considered a model of baculovirus. ac154 is a later expression gene in AcMNPV genome and its function is unknown. In this study, we explored the function of Ac154 in AcMNPV infection process in host Sf9 cells. The results showed that Ac154 was distributed in both nucleus and cytoplasm. Knockout of ac154 did not affect the production of BV, but the yield of progeny virus was reduced, indicating the auxiliary function of Ac154 in virus production. MTT assay showed that Ac154 promoted the proliferation and inhibited apoptosis of Sf9 cells. Overexpression of ac154 gene significantly increased the transcription level of anti-apoptotic gene p35, and delayed the expression of the pro-apoptotic protein SfP53 and reduced its expression level, which indicated its anti-apoptotic role in the host cells. In conclusion, our results demonstrated Ac154 could delay apoptosis process in host cells by regulating the transcription of p35 gene and the expression of SfP53 protein, which provided a more favorable environment for progeny virus replication and packaging, thereby promoting the proliferation of progeny virus. So we provided a potentially improved bac-to-bac eukaryotic protein expression system and biopesticide in this work.

Improvement of baculovirus as protein expression vector and as biopesticide by CRISPR/Cas9 editing

The clustered regularly interspaced short palindromic repeats (CRISPR) system-associated Cas9 endonuclease is a molecular tool that enables specific sequence editing with high efficiency. In this study, we have explored the use of CRISPR/Cas9 system for the engineering of baculovirus. We have shown that the delivering of Cas9-single guide RNA ribonucleoprotein (RNP) complex with or without DNA repair template into Sf21 insect cells through lipofection might be efficient to produce knockouts as well as knock-ins into the baculovirus. To evaluate potential application of our CRISPR/Cas9 method to improve baculovirus as protein expression vector and as biopesticide, we attempted to knockout several genes from a recombinant AcMNPV form used in the baculovirus expression system as well as in a natural occurring viral isolate from the same virus. We have additionally confirmed the adaptation of this methodology for the generation of viral knock-ins in specific regions of the viral genome. Analysis of the generated mutants revealed that the editing efficiency and the type of changes was variable but relatively high. Depending on the targeted gene, the editing rate ranged from 10% to 40%. This study established the first report revealing the potential of CRISPR/Cas9 for genome editing in baculovirus, contributing to the engineering of baculovirus as a protein expression vector as well as a biological control agent.

Mutation of ABC transporter ABCA2 confers resistance to Bt toxin Cry2Ab in Trichoplusia ni

Insecticidal proteins from Bacillus thuringiensis (Bt) are the primary recombinant proteins expressed in transgenic crops (Bt-crops) to confer insect resistance. Development of resistance to Bt toxins in insect populations threatens the sustainable application of Bt-crops in agriculture. The Bt toxin Cry2Ab is a major insecticidal protein used in current Bt-crops, and resistance to Cry2Ab has been selected in several insects, including the cabbage looper, Trichoplusia ni. In this study, the Cry2Ab resistance gene in T. ni was mapped to Chromosome 17 by genetic linkage analyses using a whole genome resequencing approach, and was then finely mapped using RNA-seq-based bulked segregant analysis (BSA) and amplicon sequencing (AmpSeq)-based fine linkage mapping to a locus containing two genes, ABCA1 and ABCA2. Mutations in ABCA1 and ABCA2 in Cry2Ab resistant T. ni were identified by both genomic DNA and cDNA sequencing. Analysis of the expression of ABCA1 and ABCA2 in T. ni larvae indicated that ABCA2 is abundantly expressed in the larval midgut, but ABCA1 is not a midgut-expressed gene. The mutation in ABCA2 in Cry2Ab resistant T. ni was identified to be an insertion of a transposon Tntransib in ABCA2. For confirmation of ABCA2 as the Cry2Ab-resistance gene, T. ni mutants with frameshift mutations in ABCA1 and ABCA2 were generated by CRISPR/Cas9 mutagenesis. Bioassays of the T. ni mutants with Cry2Ab verified that the mutations of ABCA1 did not change larval susceptibility to Cry2Ab, but the ABCA2 mutants were highly resistant to Cry2Ab. Genetic complementation test of the ABCA2 allele in Cry2Ab resistant T. ni with an ABCA2 mutant generated by CRISPR/Cas9 confirmed that the ABCA2 mutation in the Cry2Ab resistant strain confers the resistance. The results from this study confirmed that ABCA2 is essential for the toxicity of Cry2Ab in T. ni and mutation of ABCA2 confers the resistance to Cry2Ab in the resistant T. ni strain derived from a Bt resistant greenhouse population.

Functional Characterization of the Group I Alphabaculovirus Specific Gene ac73

Baculoviridae is a family of large DNA viruses that specifically infect insects. It contains four genera, Alpha-, Beta-, Gamma-, and Deltabaculovirus. Alphabaculovirus is further divided into Group I and II, and Group I appears to be emerged most recently among all baculoviruses. Interestingly, there are 12 Group I specific genes that are only found in this lineage. Studying these genes is helpful to understand how baculoviruses evolved. Here, we reported the functional analyzing results of ac73, a function unknown Group I specific gene of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) which is the type species of baculovirus. The AC73 protein encoded by ac73 was found to be expressed during the late stage of infection and incorporated into the nucleocapsids of budded virus (BV) and occlusion-derived virus (ODV). In infected cells, AC73 resided mainly in the ring zone region of the nucleus, and appeared to be assembled into occlusion bodies (OBs). The ac73 knockout and repaired viruses were constructed and studied by in vitro and in vivo infection. Although ac73 was not essential for BV and ODV or OB formation, the BV titer and viral infectivity in insect larvae of ac73 knockout AcMNPV decreased by about 5-8 and 3-4 fold compared to those of wild type virus, respectively, suggesting ac73 contributed to infectious BV production and viral infectivity in vivo. This research provides new insight into the function of this Group I specific gene.