Silkworm-baculovirus Expression Vector System Research Articles

Optimization of SARS-CoV-2 Spike Protein Expression in the Silkworm and Induction of Efficient Protective Immunity by Inoculation With Alum Adjuvants.

The newly emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is causing a spread of coronavirus disease 2019 (COVID-19) globally. In order to end the COVID-19 pandemic, an effective vaccine against SARS-CoV-2 must be produced at low cost and disseminated worldwide. The spike (S) protein of coronaviruses plays a pivotal role in the infection to host cells. Therefore, targeting the S protein is one of the most rational approaches in developing vaccines and therapeutic agents. In this study, we optimized the expression of secreted trimerized S protein of SARS-CoV-2 using a silkworm-baculovirus expression vector system and evaluated its immunogenicity in mice. The results showed that the S protein forming the trimeric structure was the most stable when the chicken cartilage matrix protein was used as the trimeric motif and could be purified in large amounts from the serum of silkworm larvae. The purified S protein efficiently induced antigen-specific antibodies in mouse serum without adjuvant, but its ability to induce neutralizing antibodies was low. After examining several adjuvants, the use of Alum adjuvant was the most effective in inducing strong neutralizing antibody induction. We also examined the adjuvant effect of paramylon from Euglena gracilis when administered with the S protein. Our results highlight the effectiveness and suitable construct design of the S protein produced in silkworms for the subunit vaccine development against SARS-CoV-2.

Production of scFv, Fab, and IgG of CR3022 Antibodies Against SARS-CoV-2 Using Silkworm-Baculovirus Expression System.

COVID-19, caused by SARS-CoV-2, is currently spreading around the world and causing many casualties. Antibodies against such emerging infectious diseases are one of the important tools for basic viral research and the development of diagnostic and therapeutic agents. CR3022 is a monoclonal antibody against the receptor binding domain (RBD) of the spike protein (S protein) of SARS-CoV found in SARS patients, but it was also shown to have strong affinity for that of SARS-CoV-2. In this study, we produced large amounts of three formats of CR3022 antibodies (scFv, Fab and IgG) with high purity using a silkworm-baculovirus expression vector system. Furthermore, SPR measurements showed that the affinity of those silkworm-produced IgG antibodies to S protein was almost the same as that produced in mammalian expression system. These results indicate that the silkworm-baculovirus expression system is an excellent expression system for emerging infectious diseases that require urgent demand for diagnostic agents and therapeutic agents.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12033-021-00373-0.

Stable trimer formation of spike protein from porcine epidemic diarrhea virus improves the efficiency of secretory production in silkworms and induces neutralizing antibodies in mice

Porcine epidemic diarrhea virus (PEDV) is a highly infectious pathogen of watery diarrhea that causes serious economic loss to the swine industry worldwide. Especially because of the high mortality rate in neonatal piglets, a vaccine with less production cost and high protective effect against PEDV is desired. The intrinsically assembled homotrimer of spike (S) protein on the PEDV viral membrane contributing to the host cell entry is a target of vaccine development. In this study, we designed trimerized PEDV S protein for efficient production in the silkworm-baculovirus expression vector system (silkworm-BEVS) and evaluated its immunogenicity in the mouse. The genetic fusion of the trimeric motif improved the expression of S protein in silkworm-BEVS. A small-scale screening of silkworm strains to further improve the S protein productivity finally achieved the yield of about 2 mg from the 10 mL larval serum. Mouse immunization study demonstrated that the trimerized S protein could elicit strong humoral immunity, including the S protein-specific IgG in the serum. These sera contained neutralizing antibodies that can protect Vero cells from PEDV infection. These results demonstrated that silkworm-BEVS provides a platform for the production of trimeric S proteins, which are promising subunit vaccines against coronaviruses such as PEDV.

Efficient production of recombinant T7 endonuclease I using silkworm-baculovirus expression vector system

• Recombinant T7E1 was expressed and purified from the silkworm-BEVS. • N-tagged rT7E1 is highly expressed in silkworm pupae. • Silkworm-derived rT7E1 sufficiently cleaved the mismatches of designed and CRISPR/Cas9-mediated DNA substrates. Recently, T7 Endonuclease I (T7E1) cleavage assay has been widely employed as an efficient approach for detecting mutations from CRISPR/Cas9 targeted samples. This enzyme is sufficient to detect single- and multiple-base mismatches from various heteroduplex DNA samples. However, T7E1 is quite expensive for researchers to use it only for screening mutations, especially in the condition of a large number of test samples. Regarding the production of this enzyme, to data, only the E. coli system has been reported and the highly overexpressed T7E1 seems toxic to the E. coli host cells. Thus, in this study, we tested whether the silkworm-baculovirus expression vector system (BEVS) is suitable to produce recombinant T7 Endonuclease I (rT7E1). The rT7E1 with N- or C-tags in cultured silkworm cells and silkworm pupae were successfully expressed. Our results demonstrated that the rT7E1-Ntag was highly expressed in silkworm pupae and we obtained rT7E1 proteins in high purity. Moreover, rT7E1 from silkworm-BEVS sufficiently recognized and cleaved the mismatches of designed and CRISPR/Cas9-mediated DNA substrates, which was equivalent to the commercial rT7E1 of the E. coli system. Taken together, our study would greatly support the genome-editing research by providing a cost-effective and active rT7E1 enzyme.

Optimal silkworm larva host for high-level production of Mus musculus IL-4 using a baculovirus expression vector system

Abstract Interleukine-4 (IL-4) is a cytokine that plays an important role in the immune system and recognized as a biological medicine. Therefore, there is a demand for the production of IL-4 with high performance. The expression of a recombinant IL-4 protein in the prokaryotic system usually results in the formation of an inclusion body. To date, the solution to obtain those active products without the refolding process remains to be established. In this study, we tried to acquire a biologically active recombinant Mus musculus IL-4 (rMmIL-4) using a silkworm-baculovirus expression vector system (silkworm-BEVS). We constructed two recombinant baculoviruses coding rMmIL-4 with the distinct location of affinity purification tags and succeeded in the expression and purification of rMmIL-4 proteins directly without the refolding process. Both purified proteins displayed comparable biological activity to the commercial proteins produced by the E. coli expression system. Besides, we performed screening of silkworm strains to seek optimal hosts for the mass-production of rMmIL-4. Intriguingly, we found that some silkworm strains showed significantly higher secretion levels of rMmIL-4 in silkworm sera. Our study provides meaningful insights into the industrial-scale production of rMmIL-4 with high productivity for pharmaceutical applications in the future.

Expression and Purification of Vaccinia Virus DNA Topoisomerase IB Produced in the Silkworm-Baculovirus Expression System.

Type IB DNA topoisomerases are enzymes to change the topological state of DNA molecules and are essential in studying replication, transcription, and recombination of nucleic acids in vitro. DNA topoisomerase IB from Vaccinia virus (vTopIB) is a 32kDa, type I eukaryotic topoisomerase, which relaxed positively and negatively supercoiled DNAs without Mg2+ and ATP. Although vTopIB has been effectively produced in E. coli expression system, no studies remain available to explore an alternative platform to express recombinant vTopIB (rvTopIB) in a higher eukaryote, where the one can expect post-translational modifications that affect the activity of rvTopIB. Here in this study, rvTopIB with N-terminal tags was constructed and expressed in a silkworm-baculovirus expression vector system (silkworm-BEVS). We developed a simple two consecutive chromatography purification to obtain highly pure rvTopIB. The final yield of rvTopIB obtained from a baculovirus-infected silkworm larva was 83.25μg. We also evaluated the activity and function of rvTopIB by the DNA relaxation activity assays using a negatively supercoiled pUC19 plasmid DNA as a substrate. With carefully assessing optimized conditions for the reaction buffer, we found that divalent ions, Mg2+, Mn2+, Ca2+, as well as ATP stimulate the DNA relaxation activity by rvTopIB. The functional and active form of rvTopIB, together with the yields of the protein we obtained, suggests that silkworm-BEVS would be a potential alternative platform to produce eukaryotic topoisomerases on an industrial scale.

Expression and purification of biologically active human granulocyte-macrophage colony stimulating factor (hGM-CSF) using silkworm-baculovirus expression vector system

Human granulocyte-macrophage colony stimulating factor (hGM-CSF) is a hematopoietic growth factor. It is widely employed as a therapeutic agent targeting neutropenia in cancer patients undergoing chemotherapy and in patients with AIDS or after bone marrow transplantation. In this study, we constructed the recombinant baculoviruses for the expression of recombinant hGM-CSF (rhGM-CSF) with two small affinity tags (His-tag and Strep-tag) at the N or C-terminus. Compared to N-tagged rhGM-CSF, C-tagged rhGM-CSF was highly recovered from silkworm hemolymph. The purified rhGM-CSF proteins migrated as a diffuse band and were confirmed to hold N-glycosylations. A comparable activity was achieved when commercial hGM-CSF was tested as a control. Considering the high price of hGM-CSF in the market, our results and strategies using silkworm-baculovirus system can become a great reference for mass production of the active rhGM-CSF at a lower cost.

Expression of the thermostable Moloney murine leukemia virus reverse transcriptase by silkworm-baculovirus expression system

Abstract Reverse transcriptase from Moloney murine leukemia virus (MMLVRT) is an RNA dependent DNA polymerase, which has been used as a fundamental tool for molecular biology and biotechnology. The secondary structures formed in the RNA templates decrease the accessibility of the reverse transcriptase to the RNA templates; it is important to unfold the RNA secondary structure by increasing the reaction temperature to perform the successful transcription. In this study, we applied silkworm baculovirus expression vector system (silkworm-BEVS) to mass-produce and purify the recombinant MMLVRTs with the N- or C- terminal tandem tags. We confirmed that both of the recombinant MMLVRT enzymes have intact DNA polymerase activity. It is notable that C- terminal tagged MMLVRT outperformed MMLVRT obtained from the E. coli expression system in terms of thermostability and sensitivity to low quantities of RNA template. Taken together, these results demonstrate that silkworm-BEVS is a promising alternative strategy to produce the functional and thermostable reverse transcriptase.

Expression, purification, and characterization of highly active endo-α-N-acetylgalactosaminidases expressed by silkworm-baculovirus expression system

Abstract The O-glycosidase, endo-α-N-acetylgalactosaminidase from Enterococcus faecalis (endoEF) catalyzes the cleavage of core 1 and core 3 type O-linked disaccharides between GalNAc and serine or threonine residues from glycoproteins. The endoEF has broad substrate specificity and thus is extensively utilized for the structural and functional analysis of the O-linked glycans. In this study, we expressed and purified the recombinant endoEF (rEndoEF) by using the silkworm-baculovirus expression vector system (Silkworm-BEVS) and confirmed the deglycosylation activity of rEndoEF targeting reporter glycoproteins, which was equivalent to the commercial O-glycosidase. Thus, our study provides important clues to produce highly active rEndoEF O-glycosidases employing silkworm-BEVS as an alternative.

Purification and characterization of immunogenic recombinant virus-like particles of porcine circovirus type 2 expressed in silkworm pupae.

Porcine circovirus type 2 (PCV2) is a primary causative agent of postweaningmultisystemic wasting syndrome (PMWS), which has a significant economic impact on the swine industry. The capsid protein (Cap) encoded by ORF2 of the viral genome has been used effectively as a vaccine against PCV2 infection. The Cap protein can spontaneously assemble into virus-like particles (VLPs) that are safe and highly immunogenic for vaccine applications. Several expression systems, including bacteria, yeast and insect cells, have been utilized to produce PCV2 VLPs. However, in some cases, the recombinant Cap (rCap) proteins produced in bacteria and yeast do not assemble spontaneously. In this study, we expressed rCap protein using a silkworm-baculovirus expression vector system (silkworm-BEVS) for mass production of PCV2 VLPs and established a simple three-step protocol for its purification from pupae: extraction by detergent, ammonium sulfate precipitation and anion exchange column chromatography. Size-exclusion chromatography (SEC) analysis and transmission electron microscope (TEM) observation showed that purified rCap proteins formed VLPs with a similar morphology to that of the original virus. Furthermore, the VLPs produced in silkworms were capable of inducing neutralizing antibodies against PCV2 in mice. Our results demonstrated that the silkworm system is a powerful tool for the production of PCV2 VLPs and will be useful for the development of a reliable and cost-effective PCV2 vaccine.

Production of a biologically active human basic fibroblast growth factor using silkworm-baculovirus expression vector system

Abstract As a therapeutic treatment, recombinant human basic fibroblast growth factor (rhbFGF) is usually employed in tissue regeneration, and as an essential component in culture medium for maintaining the induced pluripotent stem (iPS) cell and embryonic stem (ES) cell in an undifferentiated state. Therefore, a large amount of biologically active rhbFGF is required. In this study, silkworm-baculovirus expression vector system (silkworm-BEVS) is employed to achieve a high productivity of recombinant rhbFGF with two small affinity tags (His-tag and STREP-tag) at the N or C-terminus. It is observed that rhbFGF with 30 K signal peptide of silkworm were successfully expressed but are not sufficiently secreted into the culture medium of cultured insect cells. Then we purified the N- or C-tagged intracellular rhbFGF protein and obtained a yield of about 0.7 mg/larva and 1.2 mg/larva, respectively. Although the final yield of the C-tagged rhbFGF is higher than that of the N-tagged, rhbFGF with N-tag demonstrated promising and comparable biological activity, which is evaluated through a mammalian cell proliferation assay. Taken together, these results indicate that silkworm-BEVS could contribute to the mass-production of the biologically active rhbFGF for medical uses.

Characterization of Recombinant Thermococcus kodakaraensis (KOD) DNA Polymerases Produced Using Silkworm-Baculovirus Expression Vector System.

The KOD DNA polymerase from Thermococcus kodakarensis (Tkod-Pol) has been preferred for PCR due to its rapid elongation rate, extreme thermostability and outstanding fidelity. Here in this study, we utilized silkworm-baculovirus expression vector system (silkworm-BEVS) to express the recombinant Tkod-Pol (rKOD) with N-terminal (rKOD-N) or C-terminal (rKOD-C) tandem fusion tags. By using BEVS, we produced functional rKODs with satisfactory yields, about 1.1mg/larva for rKOD-N and 0.25mg/larva for rKOD-C, respectively. Interestingly, we found that rKOD-C shows higher thermostability at 95°C than that of rKOD-N, while that rKOD-N is significantly unstable after exposing to long period of heat-shock. We also assessed the polymerase activity as well as the fidelity of purified rKODs under various conditions. Compared with commercially available rKOD, which is expressed in E. coli expression system, rKOD-C exhibited almost the same PCR performance as the commercial rKOD did, while rKOD-N did lower performance. Taken together, our results suggested that silkworm-BEVS can be used to express and purify efficient rKOD in a commercial way.

High-level expression and purification of biologically active human IL-2 using silkworm-baculovirus expression vector system

Interleukin 2 (IL-2) is a pharmacologically vital cytokine secreted mainly by activated CD4+ T lymphocyte. Recombinant human IL-2 (rhIL-2) protein has already been globally applied as an immune-therapeutic reagent for various diseases. Therefore, there is great interest in developing an active form of rhIL-2 in very large amounts and with excellent purity for clinical use. In this study, we successfully mass-expressed and purified N- or C-terminal tandem tag-fused rhIL-2 in a baculovirus expression vector system (BEVS) using silkworm larvae as factories. We confirmed that the intrinsic instability of hIL-2 causes the loss and low recovery of N-tagged rhIL-2 and that C-tagged rhIL-2 is more suitable for mass production. Furthermore, the activity of purified rhIL-2s was further validated by a cell proliferation assay of a human natural killer cell line, and both rhIL-2 proteins produced in the silkworm-BEVS exhibited comparable activity to that of the commercial E.coli-derived rhIL-2. Taken together, our results and strategies could contribute greatly to the mass production of active rhIL-2.

Expression of polyhedrin-hEGF fusion protein in cultured cells and larvae of Bombyx mori

For mass production of human epidermal growth factor (hEGF), silkworm baculovirus expression vector system (BEVS) was adopted in this study. hEGF gene was in-frame fused with polyhedrin (Ph) gene under the control of Ph promoter and was used to co-transfect BmN cell with the modified Bombyx mori baculovirus DNA to obtained recombinant virus. The ELISA showed a maximum expression on day 4 in larvae and pupae. Both cellular extracts and haemolymph of silkworm larvae infected with rBacPh-EGF could all support the proliferation of Balb/c 3T3 cell. The corresponding materials from BmN cell and silkworm larvae infected with wild virus also indicated a weak effect of upregulation on 3T3 cells proliferation. The animal study showed that both pupae infected with rBacPhEGF virus and wild virus protected the gastric mucosa against ethanol-induced damage in rats, although the protection from pupae infected with wild virus was slightly weak. The mechanism is under investigating.

Expression of Cholera Toxin B Subunit and Assembly as Functional Oligomers in Silkworm

The nontoxic B subunit of cholera toxin (CTB) can significantly increase the ability of proteins to induce immunological tolerance after oral administration, when it was conjugated to various proteins. Recombinant CTB offers great potential for treatment of autoimmune disease. Here we firstly investigated the feasibility of silkworm baculovirus expression vector system for the cost-effective production of CTB under the control of a strong polyhedrin promoter. Higher expression was achieved via introducing the partial non-coding and coding sequences (ATAAAT and ATGCCGAAT) of polyhedrin to the 5' end of the native CTB gene, with the maximal accumulation being approximately 54.4 mg/L of hemolymph. The silkworm bioreactor produced this protein vaccine as the glycoslated pentameric form, which retained the GM1-ganglioside binding affinity and the native antigenicity of CTB. Further studies revealed that mixing with silkworm-derived CTB increases the tolerogenic potential of insulin. In the nonconjugated form, an insulin : CTB ratio of 100 : 1 was optimal for the prominent reduction in pancreatic islet inflammation. The data presented here demonstrate that the silkworm bioreactor is an ideal production and delivery system for an oral protein vaccine designed to develop immunological tolerance against autoimmune diabetes and CTB functions as an effective mucosal adjuvant for oral tolerance induction.