Rabies virus glycoprotein expression in Drosophila S2 cells. I: design of expression/selection vectors, subpopulations selection and influence of sodium butyrate and culture medium on protein expression

Production of Rabies VLPs in Insect Cells by Two Monocistronic Baculoviruses Approach

Rabies is an ancient zoonotic disease that still causes the death of over 59,000 people worldwide each year. The rabies lyssavirus encodes five proteins, including the envelope glycoprotein and the matrix protein. RVGP is the only protein exposed on the surface of viral particle, and it can induce immune response with neutralizing antibody formation. RVM has the ability to assist with production process of virus-like particles. VLPs were produced in recombinant baculovirus system. In this work, two recombinant baculoviruses carrying the RVGP and RVM genes were constructed. From the infection and coinfection assays, we standardized the best multiplicity of infection and the best harvest time. Cell supernatants were collected, concentrated, and purified by sucrose gradient. Each step was used for protein detection through immunoassays. Sucrose gradient analysis enabled to verify the separation of VLPs from rBV. Through the negative contrast technique, we visualized structures resembling rabies VLPs produced in insect cells and rBV in the different fractions of the sucrose gradient. Using ELISA to measure total RVGP, the recovery efficiency of VLPs at each stage of the purification process was verified. Thus, these results encourage further studies to confirm whether rabies VLPs are a promising candidate for a veterinary rabies vaccine.

Identification and characterization of four Drosophila suzukii cellularization genes and their promoters

Background

The spotted-wing Drosophila (Drosophila suzukii) is a widespread invasive pest that causes severe economic damage to fruit crops. The early development of D. suzukii is similar to that of other Drosophilids, but the roles of individual genes must be confirmed experimentally. Cellularization genes coordinate the onset of cell division as soon as the invagination of membranes starts around the nuclei in the syncytial blastoderm. The promoters of these genes have been used in genetic pest-control systems to express transgenes that confer embryonic lethality. Such systems could be helpful in sterile insect technique applications to ensure that sterility (bi-sex embryonic lethality) or sexing (female-specific embryonic lethality) can be achieved during mass rearing. The activity of cellularization gene promoters during embryogenesis controls the timing and dose of the lethal gene product.

Results

Here, we report the isolation of the D. suzukii cellularization genes nullo, serendipity-α, bottleneck and slow-as-molasses from a laboratory strain. Conserved motifs were identified by comparing the encoded proteins with orthologs from other Drosophilids. Expression profiling confirmed that all four are zygotic genes that are strongly expressed at the early blastoderm stage. The 5′ flanking regions from these cellularization genes were isolated, incorporated into piggyBac vectors and compared in vitro for the promoter activities. The Dsnullo promoter showed the highest activity in the cell culture assays using D. melanogaster S2 cells.

Conclusions

The similarities in the gene coding and 5′ flanking sequence as well as in the expression pattern of the four cellularization genes between D. melanogaster and D. suzukii, suggest that conserved functions may be involved in both species. The high expression level at the early blastoderm stage of the four cellularization genes were confirmed, thus their promoters can be considered in embryonic lethality systems. While the Dsnullo promoter could be a suitable candidate, all reported promoters here are subject to further in vivo analyses before constructing potential pest control systems.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-020-00939-y.

Purification of rabies virus glycoprotein produced in Drosophila melanogaster S2 cells: An efficient immunoaffinity method

Most rabies vaccines are based on inactivated virus, which production process demands a high level of biosafety structures. In the past decades, recombinant rabies virus glycoprotein (RVGP) produced in several expression systems has been extensively studied to be used as an alternative vaccine. The immunogenic characteristics of this protein depend on its correct conformation, which is present only after the correct post-translational modifications, typically performed by animal cells. The main challenge of using this protein as a vaccine candidate is to keep its trimeric conformation after the purification process. We describe here a new immunoaffinity chromatography method using a monoclonal antibody for RVGP Site II for purification of recombinant rabies virus glycoprotein expressed on the membrane of Drosophila melanogaster S2 cells. RVGP recovery achieved at least 93%, and characterization analysis showed that the main antigenic proprieties were preserved after purification.

Single-cell cloning enables the selection of more productive Drosophila melanogaster S2 cells for recombinant protein expression

The generation of monoclonal cell lines is an important early process development step for recombinant protein production. Although single-cell cloning is an established method in mammalian cell lines, straightforward protocols are not yet available for insect cells. We describe a new method for the generation of monoclonal insect cells without using fetal bovine serum and/or feeder cells pretreated by irradiation or exposure to mitomycin. Highly productive clones of Drosophila melanogaster S2 cells were prepared in a two-step procedure, comprising the establishment of a polyclonal population and subsequent single cell isolation by limiting dilution. Necessary growth factors were provided by co-cultivation of single transformants with untransfected feeder cells, which were later removed by antibiotic selection. Enhanced expression of EGFP and two target peptides was confirmed by flow cytometry and dot/western blotting. Highly productive clones were stable, showed a uniform expression profile and typically a sixfold to tenfold increase in cell-specific productivity.

Optimized expression of the antimicrobial protein Gloverin from Galleria mellonella using stably transformed Drosophila melanogaster S2 cells

Antimicrobial proteins and peptides (AMPs) are valuable as leads in the pharmaceutical industry for the development of novel anti-infective drugs. Here we describe the efficient heterologous expression and basic characterization of a Gloverin-family AMP derived from the greater wax moth Galleria mellonella. Highly productive single-cell clones prepared by limiting dilution achieved a 100% increase in productivity compared to the original polyclonal Drosophila melanogaster S2 cell line. Comprehensive screening for suitable expression conditions using statistical experimental designs revealed that optimal induction was achieved using 600 µM CuSO4 at the mid-exponential growth phase. Under these conditions, 25 mg/L of the AMP was expressed at the 1-L bioreactor scale, with optimal induction and harvest times ensured by dielectric spectroscopy and the online measurement of optical density. Gloverin was purified from the supernatant by immobilized metal ion affinity chromatography followed by dialysis. In growth assays, the purified protein showed specific antimicrobial activity against two different strains of Escherichia coli.

Impact of recombinant Drosophila S2 cell population enrichment on expression of rabies virus glycoprotein

Recombinant Drosophila S2 cells have been used for the expression of many proteins of medical interest. However, membrane-attached glycoproteins, which commonly exhibit lower expression levels compared to soluble proteins, may require special procedures in order to attain high levels of expression. In this study, two S2 cell population enrichment methods (antibiotic and immunomagnetic selection) were evaluated for their ability to enhance expression of the membrane-anchored rabies virus glycoprotein (RVGP). Quantification of RVGP production and determination of its cDNA copy number in transformed cells showed that both enrichment methods increased RVGP expression without significantly affecting its gene copy number. More interestingly, RVGP mRNA levels measured after cycloheximide treatment were poorly correlated with glycoprotein levels. Both enrichment methods enhanced expression of RVGP by recombinant S2 cells, with the highest level of expression achieved using immunomagnetic selection.

Rabies vaccine development by expression of recombinant viral glycoprotein

The rabies virus envelope glycoprotein (RVGP) is the main antigen of rabies virus and is the only viral component present in all new rabies vaccines being proposed. Many approaches have been taken since DNA recombinant technology became available to express an immunogenic recombinant rabies virus glycoprotein (rRVGP). These attempts are reviewed here, and the relevant results are discussed with respect to the general characteristics of the rRVGP, the expression system used, the expression levels achieved, the similarity of the rRVGP to the native glycoprotein, and the immunogenicity of the vaccine preparation. The most recent studies of rabies vaccine development have concentrated on in vivo expression of rRVGP by viral vector transduction, serving as the biotechnological basis for a new generation of rabies vaccines.

Expression of Viral Envelope Glycoproteins in Drosophila melanogaster S2 Cells

The expression of recombinant viral envelope glycoproteins in S2 (Drosophila melanogaster) has been performed with good results. This chapter contains protocols for the utilization of this system for the expression and analysis of proteins presented in cell plasma membrane.

Expression and immunogenicity of recombinant glycoprotein D of herpes simplex virus 1 in Drosophila S2 cells

Herpes simplex virus type 1 (HSV-1) is responsible for cold sores in the general population, but also contributes to the development of other more serious diseases in some circumstances. The viral glycoprotein D (gD) is essential for virus entry into host cells. In the present study, the Drosophila melanogaster Schneider 2 (S2) expression system (DES) was evaluated for the expression of recombinant gD1. The DNA sequences encoding the full-length gD1 (369aa, FLgD1) and a truncated gD1 form corresponding to the ectodomain (314aa, EgD1) were cloned into S2 expression vector pMT/BiP/V5-HisA to generate pMT-EgD1 and pMT-FLgD1, respectively. Two forms of gD1 gene were fitted with a hexahistidine tag to facilitate their purification. Cell populations expressing the highest gD1 levels were selected by using a limiting dilution assay. Western blot, flow cytometry (FACS), and confocal immunofluoresence assay demonstrated that the full-length form is restrained in the lipid membranes of the cell and the ectodomain form is secreted into the medium. Recombinant ectodomain gD1 was scaled up and purified from the culture medium using nickel nitrilotriacetic acid affinity chromatography, and a maximum production level of 56.8 mg/L of recombinant gD1 was obtained in a shake-flask culture of S2 cells after induction with 5 µM CdCl2 for 4 days. Mice were then immunized with recombinant purified gD1 and produced high titers of antibody measured by enzyme-linked immunosorbent assay (ELISA; 1:5,120,000) as well as high plaque neutralization titer (1:320). Overall, the data indicated that stable expression in S2 cells is a practical way of synthesizing gD1 for use in structural and functional studies in the further study.

Rabies virus glycoprotein and immune response pattern using recombinant protein or recombinant RNA viral vectors

The present study shows the humoral and cellular aspects of immune response generated by a recombinant rabies virus glycoprotein (rRVGP) as compared to those generated by viral vector carrying the RNA coding for this protein (RVGP-RNA). The rRVGP was synthesized by stably transfected Drosophila melanogaster Schneider 2 (S2) cells and the RVGP-RNA was carried by a recombinant Semiliki Forest Virus (SFV-RVGP). The data show that protein as well as the RNA vaccine was capable of inducing reasonably acceptable levels of antibodies as compared to the optimized commercial whole virus vaccine. As expected, the RNA vaccine was clearly more effective than the protein vaccines in inducing a cellular immune response, as evaluated by the IgG2a/IgG1 ratio and synthesis of interferon gamma (IFNγ) and interleukin 2 (IL2). Our study supports the importance of vaccine designing taking into consideration the concept of DNA/RNA ability to induce an effective cell immune response.

Kinetic studies of recombinant rabies virus glycoprotein (RVGP) cDNA transcription and mRNA translation in Drosophila melanogaster S2 cell populations

Recombinant rabies virus glycoprotein (RVGP) was expressed in cell membranes of stably transfected Drosophila S2 cells using constitutive and inducible promoters. Although with quantitative differences of RVGP expression in both systems, the cDNA transcription, as evaluated by relative RVGP mRNA levels measured by qRT-PCR, sustained the amount of RVGP producing cells and the RVGP volumetric (ΠRVGP) productivity. At the transition to the stationary cell growth phase, once the cell culture slowed down its rate of multiplication, an accumulation of RVGP mRNA and RVGP was clearly observed in both cell populations. Nevertheless, cell cultures performed under sub-optimal temperatures indicated that an envisaged increase in the RVGP production is not only dependent on cell growth rate, but essentially on optimal cell metabolic state.

Drosophila melanogaster S2 cells for expression of heterologous genes: From gene cloning to bioprocess development

In the present review we discuss strategies that have been used for heterologous gene expression in Drosophila melanogaster Schneider 2 (S2) cells using plasmid vectors. Since the growth of S2 cells is not dependent on anchorage to solid substrates, these cells can be easily cultured in suspension in large volumes. The factors that most affect the growth and gene expression of S2 cells, namely cell line, cell passage, inoculum concentration, culture medium, temperature, dissolved oxygen concentration, pH, hydrodynamic forces and toxic metabolites, are discussed by comparison with other insect and mammalian cells. Gene expression, cell metabolism, culture medium formulation and parameters involved in cellular respiration are particularly emphasized. The experience of the authors with the successful expression of a biologically functional protein, the rabies virus glycoprotein (RVGP), by recombinant S2 cells is presented in the topics covered.

Novel monoclonal antibodies that bind to wild and fixed rabies virus strains

Ten monoclonal antibodies (MAbs) against rabies virus, including IgG3κ, IgG2aκ, IgMκ, and an IgG2bκ isotype, were produced and characterized using neutralization, ELISA, immunodot-blot, and immunofluorescence assays. MAb 8D11, which recognized rabies virus glycoprotein, was found to neutralize rabies virus in vitro. When submitted to an immunofluorescence assay, seven MAbs showed different reactivity against 35 Brazilian rabies virus isolates. Three MAbs (LIA 02, 3E6, and 9C7) only failed to recognize one or two virus isolates, whereas MAb 6H8 was found to be reactive against all virus isolates tested. MAbs were also evaluated for their immunoreactivity against fixed rabies virus strains present in human and veterinary commercial vaccines. MAbs LIA 02, 6H8, and 9C7 reacted against all vaccine strains, while the remaining MAbs recognized at least 76% of vaccine strains tested. This research provides a set of MAbs with potential application for improving existing or developing new diagnostic tests and immunoassays.

Recombinant respiratory syncytial virus F protein expression is hindered by inefficient nuclear export and mRNA processing

Studies of the fusion activity of respiratory syncytial virus (RSV) F protein are significantly hindered by low recombinant expression levels. While infection produces F protein levels detectable by western blot, recombinant expression produces undetectable to low levels of F protein. Identifying the obstacles that hinder recombinant F protein expression may lead to improved expression and facilitate the study of F protein function. We hypothesized that nuclear localization and/or inefficient RNA polymerase II-mediated transcription contribute to poor recombinant F protein expression. This study shows a combination of stalled nuclear export, premature polyadenylation, and low mRNA abundance all contribute to low recombinant F protein expression levels. In addition, this study provides an expression optimization strategy that results in greater F protein expression levels than observed by codon-optimization of the F protein gene, which will be useful for future studies of F protein function.