Semliki Forest virus-based DNA expression vector: transient protein production followed by cell death

An mRNA-Based Multiple Antigenic Gene Expression System Delivered by Engineered Salmonella for Severe Fever with Thrombocytopenia Syndrome and Assessment of Its Immunogenicity and Protection Using a Human DC-SIGN-Transduced Mouse Model

Currently, there are no commercial vaccines or therapeutics against severe fever with thrombocytopenia syndrome (SFTS) virus. This study explored an engineered Salmonella as a vaccine carrier to deliver a eukaryotic self-mRNA replicating vector, pJHL204. This vector expresses multiple SFTS virus antigenic genes for the nucleocapsid protein (NP), glycoprotein precursor (Gn/Gc), and nonstructural protein (NS) to induce host immune responses. The engineered constructs were designed and validated through 3D structure modeling. Western blot and qRT-PCR analyses of transformed HEK293T cells confirmed the delivery and expression of the vaccine antigens. Significantly, mice immunized with these constructs demonstrated a cell-mediated and humoral response as balanced Th1/Th2 immunity. The JOL2424 and JOL2425 delivering NP and Gn/Gc generated strong immunoglobulin IgG and IgM antibodies and high neutralizing titers. To further examine the immunogenicity and protection, we utilized a human DC-SIGN receptor transduced mouse model for SFTS virus infection by an adeno-associated viral vector system. Among the SFTSV antigen constructs, the construct with full-length NP and Gn/Gc and the construct with NP and selected Gn/Gc epitopes induced robust cellular and humoral immune responses. These were followed by adequate protection based on viral titer reduction and reduced histopathological lesions in the spleen and liver. In conclusion, these data indicate that recombinant attenuated Salmonella JOL2424 and JOL2425 delivering NP and Gn/Gc antigens of SFTSV are promising vaccine candidates that induce strong humoral and cellular immune responses and protection against SFTSV. Moreover, the data proved that the hDC-SIGN transduced mice as a worthy tool for immunogenicity study for SFTSV.

Expression and Immunogenicity of Recombinant African Swine Fever Virus Proteins Using the Semliki Forest Virus

African swine fever virus (ASFV) is a large DNA virus belonging to the Asfarviridae family that damages the immune system of pigs, resulting in the death or slaughter of millions of animals worldwide. Recent modern techniques in ASFV vaccination have highlighted the potential of viral replicon particles (RPs), which can efficiently express foreign proteins and induce robust cellular and humoral immune responses compared with the existing vaccines. In this study, we established a Semliki Forest virus (SFV) vector by producing replication-defective viral particles. This vector was used to deliver RPs expressing ASFV antigens. SFV-RPs expressing ASFV p32 (SFV-p32) and p54 (SFV-p54) were tested in baby hamster kidney (BHK-21) cells. Proteins expression was evaluated via western blotting and indirect immunofluorescence, while immunogenicity was evaluated in BALB/c mice. The resulting RPs exhibited high levels of protein expression and elicited robust humoral and cellular immune responses.

Highly feasible immunoprotective multicistronic SARS-CoV-2 vaccine candidate blending novel eukaryotic expression and Salmonella bactofection

Introduction

The emergence of SARS-CoV-2 variants has raised concerns on future vaccine efficacy as most vaccines target only the spike protein. Hence, vaccines targeting multiple SARS-CoV-2 proteins will offer broader protection and improve our preparedness to combat the pandemic.

Objectives

The study aimed to develop a novel vaccine strategy by combining a eukaryotic vector expressing multiple SARS-CoV-2 genes and Salmonella-mediated in vivo DNA delivery.

Methods

The eukaryotic vector was designed to function as a DNA-launched RNA replicon in a self-replicating and self-amplifying mRNA mechanism. By exploiting the self-cleaving peptide, P2A, we fused four SARS-CoV-2 targets, including receptor-binding domain (RBD), heptad repeat domain (HR), membrane protein (M) and epitopes of nsp13, in a single open reading frame. Western blot and immunofluorescence assays were used to determine protein expression. In mice, the vaccine's safety and immunogenicity were investigated.

Results

Western blot analysis revealed co-expression all four proteins from the vaccine construct, confirming the efficiency of Salmonella-mediated gene delivery and protein expression. The vaccine candidate was safe and elicited robust antigen-specific antibody titers in mice, and a recall response from splenocytes revealed induction of strong cell-mediated immunity. Flow cytometry demonstrated an increase in sub-populations of CD4+ and CD8+ T cells with the highest CD4+ and CD8+ T cells recorded for HR and RBD, respectively. Overall, humoral and cellular immune response data suggested the induction of both Th1 and Th2 immunity with polarization towards an antiviral Th1 response. We recorded a potent SARS-CoV-2 neutralizing antibody titers in the immunized mice sera.

Conclusions

The Salmonella bactofection ensured optimum in vivo gene delivery, and through a P2A-enabled efficient multicistronic expression, the vaccine candidate elicited potent anti-SARS-CoV-2 immune responses. These findings provide important insight into development of an effective multivalent vaccine to combat SARS-CoV-2 and its variants.

Eukaryotic expression system complemented with expressivity of Semliki Forest Virus's RdRp and invasiveness of engineered Salmonella demonstrate promising potential for bacteria mediated gene therapy

This study describes an efficient eukaryotic expression system (pJHL204) built into the Salmonella delivery system to enhance the essential efficacy and effectiveness of conventional DNA therapy. The expression system utilizes RNA-dependent RNA polymerase activity (RdRp) of Semiliki Forest Virus attributing to dramatic antigen expression by cytoplasmic mRNA amplification. Functional characterization of the pJHL204 by in vitro and in vivo transfection studies revealed the improved expression of mRNA at least 150 folds than the RdRp mutant plasmid under in vitro conditions. Using green fluorescence protein (GFP) and mCherry as bait proteins this system was extensively characterized for plasmid delivery capacity, antigen expression, and safety using in vivo and in vitro models by employing flow cytometry, fluorescence microscopy, and immunohistochemical staining. Employment of Salmonella as a carrier significantly extends plasmid in vivo survivability and prolongs the effective duration until the elimination of the Salmonella carrier strain in the host. The strategy can be easily adapted for P2A connected multiple antigen delivery in a single vector system due to the significantly high cargo capacity of Salmonella. A mouse challenge study was carried out utilizing P2A connected H1N1 hemagglutinin (HA) and neuraminidase (NA) via the Salmonella carrier strain JOL2500 significantly reduced viral activity and protected mice against the H1N1 challenge and demonstrates potential to redefine in vivo DNA therapy as a reliable and safe system to treat human diseases using useful microbes like Salmonella.

A vaccinia virus recombinant transcribing an alphavirus replicon and expressing alphavirus structural proteins leads to packaging of alphavirus infectious single cycle particles

Poxviruses and Alphaviruses constitute two promising viral vectors that have been used extensively as expression systems, or as vehicles for vaccine purposes. Poxviruses, like vaccinia virus (VV) are well-established vaccine vectors having large insertion capacity, excellent stability, and ease of administration. In turn, replicons derived from Alphaviruses like Semliki Forest virus (SFV) are potent protein expression and immunization vectors but stocks are difficult to produce and maintain. In an attempt to demonstrate the use of a Poxvirus as a means for the delivery of small vaccine vectors, we have constructed and characterized VV/SFV hybrid vectors. A SFV replicon cDNA was inserted in the VV genome and placed under the control of a VV early promoter. The replicon, transcribed from the VV genome as an early transcript, was functional, and thus capable of initiating its own replication and transcription. Further, we constructed a VV recombinant additionally expressing the SFV structural proteins under the control of a vaccinia synthetic early/late promoter. Infection with this recombinant produced concurrent transcription of the replicon and expression of SFV structural proteins, and led to the generation of replicon-containing SFV particles that were released to the medium and were able to infect additional cells. This combined VV/SFV system in a single virus allows the use of VV as a SFV delivery vehicle in vivo. The combination of two vectors, and the possibility of generating in vivo single-cycle, replicon containing alphavirus particles, may open new strategies in vaccine development or in the design of oncolytic viruses.

Immune responses in mice vaccinated with a suicidal DNA vaccine expressing the hemagglutinin glycoprotein from the peste des petits ruminants virus

Peste des petits ruminants (PPR), an acute and highly contagious disease, affects sheep, goats, and some small ruminants. The hemagglutinin (H) glycoprotein of the PPR virus (PPRV) is considered important for inducing protective immune responses. In this study, a suicidal DNA vaccine based on the Semliki Forest virus (SFV) replicon was constructed and tested for its ability to induce immunogenicity in a mouse model. For this, the H gene of PPRV was cloned and inserted into pSCA1, an SFV replicon vector. The resultant plasmid named pSCA1-H was then transfected into BHK-21 cells following which the antigenicity of the expressed protein was confirmed by Western blotting and immunofluorescence. The pSCA1-H plasmid was then injected intramuscularly into BALB/c mice thrice at 2-week intervals. To evaluate the immunogenicity of pSCA1-H, specific antibodies and neutralizing antibodies against PPRV-H were measured using an indirect enzyme-linked immunosorbent assay and a microneutralization test, respectively. Cell-mediated immune responses were also examined using a lymphocyte proliferation assay. The results showed that pSCA1-H could express the H protein in BHK-21 cells. Specific antibodies, neutralizing antibodies, and lymphocyte proliferation responses were all induced in mice. Thus, this suicidal DNA vaccine could be a promising new approach for vaccine development against PPR.

Dual specific antitumor effects of Semliki Forest virus-based DNA vector carrying suicide Escherichia coli purine nucleoside phosphorylase gene via Salmonella

The Escherichia coli purine nucleoside phospho-rylase/2-fluoro-2-deoxyadenosine (ePNP/F-dAdo) suicide system has demonstrated a powerful killing and bystander effects on tumor cells. However, several drawbacks to this approach remain to be resolved, such as the side-effects and the low efficiency of ePNP-targeted expression. A human telo-merase reverse transcriptase promoter-driven Semliki Forest virus-based DNA vector (pShT-ePNP) with high expression of the ePNP gene was constructed. Live attenuated Salmonella typhimurium 7207 (SL7207) was used initially as a vehicle to targetly transfer the large alphavirus vector into tumor cells. The in vitro quantitative analysis showed ~2-fold higher green fluorescent protein (GFP) expression for pShT-GFP than for conventional cytomegalovirus (CMV) promoter-mediated eukaryotic expression plasmids such as pIRES-GFP and the targeted expression of the ePNP gene in tumor cells was also detected by RT-PCR. After F-dAdo addition, the enzymatic conversion of F-Ado into 2-fluoroadmine (F-Ade) was tested by HPLC. Cell cytotoxicity assays showed that the significant inhibitory effect of the SL/pShT-ePNP system on tumor cells was dose- and time-dependent. Following oral administration, recombinant bacteria targetly allocated within the solid tumor and the expression of ePNP and GFP genes in vivo were detected by RT-PCR or observed by fluorescence microscopy. SL/pShT-ePNP and F-dAdo were also found to exert powerful therapeutic effects in combination against tumor growth and for prolonging the lifespan of tumor-bearing mice. These findings suggest that the SL/pShT-ePNP system may serve as a powerful strategy for tumor therapy.

A hemagglutinin-esterase-expressing salmonid alphavirus replicon protects Atlantic salmon (Salmo salar) against infectious salmon anemia (ISA)

A replicon expression system based on the salmonid alphavirus (SAV) that encodes the infectious salmon anemia virus (ISAV) hemagglutinin-esterase (HE) was constructed and found to be an efficacious vaccine against infectious salmon anemia (ISA). Following a single intramuscular immunization, Atlantic salmon (Salmo salar) were effectively protected against subsequent ISAV challenge. Additional replicons coding for the ISAV fusion glycoprotein (F) or the ISAV matrix protein (M) were created and tested in combination with the replicon that encodes the HE. The ISAV HE was confirmed as a potent antigen, but neither the F nor the M proteins were found to be essential for immunization-induced protection. Innate immune response induced at the site of vaccination illustrated the immunogenicity of the SAV-based replicon and its ability to activate antiviral responses in Atlantic salmon. The successful testing of the SAV-based replicon as a vaccine model against ISA showed that the replicon approach may represent a novel immunization technology for the aquaculture industry. It offers potential benefits in terms of safety, efficacy, flexibility, and vaccine production complexity.

Enhanced immunogenicity induced by an alphavirus replicon-based pseudotyped baculovirus vaccine against porcine reproductive and respiratory syndrome virus

Pseudotyped baculovirus has emerged as a promising vector for vaccine development and gene therapy. Alphaviruses, such as Semliki Forest virus (SFV), have also received considerable attention for use as expression vectors because of their self-replicating properties. In this study, pseudotyped baculovirus containing the hybrid cytomegalovirus (CMV) promoter/SFV replicon was used as a vector to co-express the GP5 and M proteins of porcine reproductive and respiratory syndrome virus (PRRSV). The immunogenicity of the resulting recombinant baculovirus (BV-SFV-5m6) was compared with the pseudotyped baculovirus vaccine (BV-CMV-5m6), in which the expression of GP5 and M were driven by the CMV promoter only. In vitro, BV-SFV-5m6 exhibited enhanced expression of foreign proteins and also caused apoptosis in transduced cells. After immunization in BALB/c mice, BV-SFV-5m6 induced strong GP5-specific ELISA antibodies and neutralizing antibodies against homologous and heterologous viruses, along with dose sparing. Further analysis of the cell-mediated immune response showed that BV-SFV-5m6 elicited a Th1-dominant immune response that was greater than that elicited by BV-CMV-5m6. Taken together, the results of this study indicate that a baculovirus containing the hybrid CMV promoter/alphavirus replicon can be utilized as an alternative strategy to develop an efficacious vaccine against PRRSV infection.

Alphavirus replicon vaccines

The alphavirus replicon technology has been utilized for many years to develop vaccines for both veterinary and human applications. Many developments have been made to the replicon platform recently, resulting in improved safety and efficacy of replicon particle (RP) vaccines. This review provides a broad overview of the replicon technology and safety features of the system and discusses the current literature on RP and replicon-based vaccines.

Alphavirus vectors for cancer therapy

Alphaviruses contain a single strand RNA genome that can be easily modified to express heterologous genes at very high levels in a broad variety of cells, including tumor cells. Alphavirus vectors can be used as viral particles containing a packaged vector RNA, or directly as nucleic acids in the form of RNA or DNA. In the latter case alphavirus RNA is cloned within a DNA vector downstream of a eukaryotic promoter. Expression mediated by these vectors is generally transient due to the induction of apoptosis. The high expression levels, induction of apoptosis, and activation of type I IFN response are the key features that have made alphavirus vectors very attractive for cancer treatment and vaccination. Alphavirus vectors have been successfully used as vaccines to induce protective and therapeutic immune responses against many tumor-associated antigens in animal models of mastocytoma, melanoma, mammary, prostate, and virally induced tumors. Alphavirus vectors have also shown a high antitumoral efficacy by expressing antitumoral molecules in tumor cells, which include cytokines, antiangiogenic factors or toxic proteins. In these studies induction of apoptosis in tumor cells contributed to the antitumoral efficacy by the release of tumor antigens that can be uptaken by antigen presenting cells, enhancing immune responses against tumors. The potential use of alphaviruses as oncolytic agents has also been evaluated for avirulent strains of Semliki Forest virus and Sindbis virus. The fact that this latter virus has a natural tropism for tumor cells has led to many studies in which this vector was able to reach metastatic tumors when administered systemically. Other "artificial" strategies to increase the tropism of alphavirus for tumors have also been evaluated and will be discussed.

Efficient gene delivery into mammalian cells by recombinant baculovirus containing a hybrid cytomegalovirus promoter/Semliki Forest virus replicon

BACKGROUND

Baculovirus, which is widely utilized as an excellent tool for the production of recombinant protein in insect cells, has recently emerged as a novel and attractive gene delivery vehicle for mammalian cells. Alphavirus, such as Semliki Forest virus (SFV), has also received considerable attention for use as expression vectors because of its self-replicating property. In the present study, we investigated the characterization of recombinant baculovirus incorporating a hybrid cytomegalovirus (CMV) promoter/SFV replicon.

METHODS

Recombinant baculovirus containing the hybrid CMV promoter/SFV replicon was constructed. Using enhanced green fluorescence protein (EGFP) as the reporter gene, gene delivery efficiencies and the ability to express heterogenous protein in mammalian cells were evaluated. Optimal transduction conditions, including transduction temperature, time and dose, were also investigated.

RESULTS

The obtained recombinant baculovirus, Bac-CMV/SFV-EGFP, exhibited high transduction efficiency and high-level expression of reporter protein in mammalian cells. Furthermore, this recombinant baculovirus could induce apoptosis in mammalian cells in the course of transduction, as demonstrated by the observed DNA laddering patterns and increased caspase-3 activity.

CONCLUSIONS

The developed baculovirus vector has a high transduction efficiency and the ability to mediate foreign gene expression in mammalian cells. Taken together with its pro-apoptotic properties, this baculovirus vector may provide an alternative tool for vaccine development.

Properties and use of novel replication-competent vectors based on Semliki Forest virus

Background

Semliki Forest virus (SFV) has a positive strand RNA genome and infects different cells of vertebrates and invertebrates. The 5' two-thirds of the genome encodes non-structural proteins that are required for virus replication and synthesis of subgenomic (SG) mRNA for structural proteins. SG-mRNA is generated by internal initiation at the SG-promoter that is located at the complementary minus-strand template. Different types of expression systems including replication-competent vectors, which represent alphavirus genomes with inserted expression units, have been developed. The replication-competent vectors represent useful tools for studying alphaviruses and have potential therapeutic applications. In both cases, the properties of the vector, such as its genetic stability and expression level of the protein of interest, are important.

Results

We analysed 14 candidates of replication-competent vectors based on the genome of an SFV4 isolate that contained a duplicated SG promoter or an internal ribosomal entry site (IRES)-element controlled marker gene. It was found that the IRES elements and the minimal -21 to +5 SG promoter were non-functional in the context of these vectors. The efficient SG promoters contained at least 26 residues upstream of the start site of SG mRNA. The insertion site of the SG promoter and its length affected the genetic stability of the vectors, which was always higher when the SG promoter was inserted downstream of the coding region for structural proteins. The stability also depended on the conditions used for vector propagation. A procedure based on the in vitro transcription of ligation products was used for generation of replication-competent vector-based expression libraries that contained hundreds of thousands of different genomes, and maintained genetic diversity and the ability to express inserted genes over five passages in cell culture.

Conclusion

The properties of replication-competent vectors of alphaviruses depend on the details of their construction. In the case of SFV4, such vectors should contain the SG promoter with structural characteristics for this isolate. The main factor for instability of SFV4-based replication-competent vectors was the deletion of genes of interest, since the resulting shorter genomes had a growth advantage over the original vector.

Construction, properties, and potential application of infectious plasmids containing Semliki Forest virus full-length cDNA with an inserted intron

Semliki Forest virus (SFV, genus Alphavirus) has a broad host range, high efficiency of viral protein expression, and the ability to stimulate an immune response. These properties have made SFV an attractive tool for development of expression vectors, and plasmid clones containing cDNA of the SFV genome often are used. However, instability of these plasmids resulting from cryptic expression of SFV envelope proteins in Escherichia coli represents a problem both for the development of SFV-based vectors and for SFV research. In this study, an infectious plasmid of SFV, pCMV-SFV4, was constructed; its toxic effect was eliminated by intron insertion in the capsid protein encoding region. When transfected into mammalian cells, the plasmid clone was highly infectious and produced virus with properties identical to those of wild-type SFV. The inserted intron was efficiently and properly removed from the RNA genome of SFV. Therefore, this novel and stabilized infectious SFV plasmid represents a superior tool for basic studies of SFV as well as for biotechnological applications.

Protective immune responses in guinea pigs and swine induced by a suicidal DNA vaccine of the capsid gene of swine vesicular disease virus

A suicidal DNA vaccine based on a Semliki Forest virus (SFV) replicon was evaluated for the development of a vaccine against swine vesicular disease virus (SVDV). The 1BCD gene of SVDV was cloned and inserted into pSCA1, an SFV DNA-based replicon vector. The resultant plasmid, pSCA/1BCD, was transfected into BHK-21 cells and the antigenicity of the expressed protein was confirmed using an indirect immunofluorescence assay. Immunogenicity was studied in guinea pigs and swine. Animals were injected intramuscularly three times with pSCA/1BCD at regular intervals. Anti-SVDV antibodies were detected by ELISA, the lymphocyte proliferation response was tested by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide method and neutralizing antibodies were measured by microneutralization tests. The data showed that SVDV-specific antibodies, neutralizing antibodies and lymphocyte proliferation were induced in both guinea pigs and swine. Furthermore, after three successive vaccinations with pSCA/1BCD, half of the pigs were protected against challenge with SVDV. These results should encourage further work towards the development of a DNA vaccine against SVDV.

Enhanced immunogenicity to food-and-mouth disease virus in mice vaccination with alphaviral replicon-based DNA vaccine expressing the capsid precursor polypeptide (P1)

Recently, alphavirus replicon-based DNA vaccines, also known as suicidal DNA vaccines, have emerged as an important strategy to enhance the potency of DNA vaccines. In this study, two different types of DNA vaccines encoding the capsid precursor polypeptide (P1) of foot-and-mouth disease virus (FMDV) were constructed and the immunogenicity were investigated and compared in mouse model. The first DNA vaccine, pcDP1, is a conventional plasmid DNA vaccine in which P1 was driven directly by a cytomegalovirus promoter. The second DNA vaccine, pSCAP1, is a Semliki Forest virus (SFV) replicon-based DNA vaccine encoding the same antigen. In vitro expression and characterization indicated that two vaccine vectors could correctly produce the P1 antigen. However, pSCAP1 could induce obvious apoptosis of the transfected cells. After immunization in BALB/c mice, the P1-specific ELISA antibodies, neutralizing antibodies, as well as lymphocyte proliferative responses induced by pSCAP1 were significantly higher than those obtained in mice immunized with pcDP1. Notably, mice immunized with the pSCAP1 had the determined ability of clearing virus in their sera after FMDV challenge. These results indicate that the SFV replicon-based DNA vaccine pSCAP1 are more effective than conventional DNA vaccine and it can be considered a promising approach for the development of a safety and efficacious vaccine against FMDV.

Alphaviral cytotoxicity and its implication in vector development

A great variety of viruses have been engineered to serve as expression vectors. Among them, the alphaviruses Semliki Forest virus and Sindbis virus represent promising tools for heterologous gene expression in a wide variety of host cells. Several applications have already been described in neurobiological studies, in gene therapy, for vaccine development and in cancer therapy. Both viruses trigger stress pathways in the cells they infect, sometimes culminating in the death of the host. This inherent property is either an advantage or a drawback, depending on the type of application. This review covers the development and applications of alphavirus vectors and, as our work has been mainly with Semliki Forest virus, we have focused on this virus with special emphasis on how the understanding of Semliki Forest virus cytotoxicity enables it to be manipulated and used.

Development of improved Sindbis virus-based DNA expression vector

We have constructed an improved DNA expression vector based on the Sindbis virus. Several DNA-based Sindbis virus vectors were constructed to investigate the efficiency of transgene expression. These vectors, when transfected into mammalian cells, have been used to express heterologous genes. A recombinant genome of Sindbis plasmid DNA, in which the structural genes were replaced by a polylinker cassette to allow for insertion of heterologous genes, was placed under the control of a simian virus (SV 40) promoter with a hepatitis delta virus (HDV) antigenomic ribozyme and a polyadenylation signal. Transfection of mammalian cells with this Sindbis-based plasmid vector, pSin-SV40-HDV-SV40pA, resulted in transient high-level expression of the beta-galactosidase reporter gene. The expression level of beta-galactosidase from pSin-SV40-HDV-SV40pA was more than 16-fold higher than that of pSin-Lux originally reported by Herweijer et al. In vivo expression was also detected after injection of plasmid DNA into mouse quadriceps. In vivo expression was transient and undetectable after day 14. Furthermore, we demonstrate that the transfection of cells with this Sindbis virus vector results in apoptotic death on glioma cells. We have demonstrated a high-level expression of the exogenous beta-galactosidase gene from the pSin-SV40-HDV-SV40pA construct using a Sindbis replication system.

Comparison of immune responses and protective efficacy of suicidal DNA vaccine and conventional DNA vaccine encoding glycoprotein C of pseudorabies virus in mice

In the present study, the immunogenicity and protective efficacy of a suicidal DNA vaccine (pSFVC1.5) incorporating Semliki Forest virus (SFV) replicon and expressing glycoprotein C (gC) of pseudorabies virus (PrV) was investigated and compared with a conventional plasmid DNA vaccine (pcDC) encoding the same antigen. In vitro, pSFVC1.5 could express gC protein and induce apoptosis of the transfected cells. After immunization in BALB/c mice, the gC-specific ELISA antibodies and neutralizing antibodies induced by pSFVC1.5 were relatively lower than those obtained in mice immunized with pcDC. However, mice immunized with pSFVC1.5 could confer more efficient protection than pcDC (100 and 62.5%, respectively) when challenged with the field PrV at 4 weeks after secondary immunization. Further analyses of cell-mediated immune responses showed that pSFVC1.5 induced stronger lymphocyte proliferative responses and higher levels of IFN-gamma, suggesting pSFVC1.5 could induce an enhanced Th1-type immune response. Collectively these results indicated that suicidal DNA vaccine is an alternative strategy to conventional DNA vaccine and can be considered a promising approach for the development of an efficacious vaccine against PrV.

Venezuelan Equine Encephalitis Replicon Immunization Overcomes Intrinsic Tolerance and Elicits Effective Anti-tumor Immunity to the ‘Self’ tumor-associated antigen, neu in a Rat Mammary Tumor Model

Many tumor-associated antigens (TAAs) represent 'self' antigens and as such, are subject to the constraints of immunologic tolerance. There are significant barriers to eliciting anti-tumor immune responses of sufficient magnitude. We have taken advantage of a Venezuelan equine encephalitis-derived alphavirus replicon vector system with documented in vivo tropism for immune system dendritic cells. We have overcome the intrinsic tolerance to the 'self' TAA rat neu and elicited an effective anti-tumor immune response using this alphavirus replicon vector system and a designed target antigen in a rigorous rat mammary tumor model. We have demonstrated the capacity to generate 50% protection in tumor challenge experiments (p = 0.004) and we have confirmed the establishment of immunologic memory by both second tumor challenge and Winn Assay (p = 0.009). Minor antibody responses were identified and supported the establishment of T helper type 1 (Th1) anti-tumor immune responses by isotype. Animals surviving in excess of 300 days with established effective anti-tumor immunity showed no signs of autoimmune phenomena. Together these experiments support the establishment of T lymphocyte dependent, Th1-biased anti-tumor immune responses to a non-mutated 'self' TAA in an aggressive tumor model. Importantly, this tumor model is subject to the constraints of immunologic tolerance present in animals with normal developmental, temporal, and anatomical expression of a non-mutated TAA. These data support the continued development and potential clinical application of this alphaviral replicon vector system and the use of appropriately designed target antigen sequences for anti-tumor immunotherapy.

Alphavirus vectors and vaccination

Alphaviruses are positive-stranded RNA viruses that have a broad host range and therefore are capable of replicating in many vertebrate and invertebrate cells. The single-stranded alphavirus genome is divided into two ORFs. The first ORF encodes the nonstructural proteins that are translated upon entry of the virus into the cytoplasm and are responsible for transcription and replication of viral RNA. The second ORF is under the control of a subgenomic promoter and normally encodes the structural proteins, which are responsible for encapsidation of viral RNA and final assembly into enveloped particles. Expression vectors have been engineered from at least three alphaviruses in which the structural protein gene region has been replaced by heterologous genes and have been shown to express high levels of the heterologous protein in cultured cells. These RNA vectors, known as replicons, are capable of replicating on their own but are not packaged into virus-like particles unless the structural proteins are provided in trans. Thus, replicons are single cycle vectors incapable of spreading from infected to noninfected cells. Because of these features, alphavirus replicon vectors are being developed as a platform vaccine technology for numerous viral, bacterial, protozoan and tumour antigens where they have been shown to be efficient inducers of both humoral and T cell responses. In addition, as the alphavirus structural proteins are not expressed in vaccine recipients, antivector immune responses are generally minimal, allowing for multiple effective immunisations of the same individual.

Nucleic acid vaccines: tasks and tactics

There are no adequate vaccines against some of the new or reemerged infectious scourges such as HIV and TB. They may require strong and enduring cell-mediated immunity to be elicited. This is quite a task, as the only known basis of protection by current commercial vaccines is antibody. As DNA or RNA vaccines may induce both cell-mediated and humoral immunity, great interest has been shown in them. However, doubt remains whether their efficacy will suffice for their clinical realization. We look at the various tactics to increase the potency of nucleic acid vaccines and divided them broadly under those affecting delivery and those affecting immune induction. For delivery, we have considered ways of improving uptake and the use of bacterial, replicon or viral vectors. For immune induction, we considered aspects of immunostimulatory CpG motifs, coinjection of cytokines or costimulators and alterations of the antigen, its cellular localization and its anatomical localization including the use of ligand-targeting to lymphoid tissue. We also thought that mucosal application of DNA deserved a separate section. In this review, we have taken the liberty to discuss these enhancement methods, whenever possible, in the context of the underlying mechanisms that might argue for or against these strategies.

Immunization strategy against cervical cancer involving an alphavirus vector expressing high levels of a stable fusion protein of human papillomavirus 16 E6 and E7

We are developing immunization strategies against cervical carcinoma and premalignant disease, based on the use of recombinant Semliki Forest virus (SFV) encoding the oncoproteins E6 and E7 from high-risk human papilloma viruses (HPV). Thus far, protein-based, as well as genetic immunization studies have demonstrated low to moderate cellular immune responses against E6 and E7. To improve these responses, we modified the structure and expression level of the E6 and E7 proteins produced by the SFV vector. Specifically, a construct was generated encoding a fusion protein of E6 and E7, while furthermore a translational enhancer was included (enhE6,7). Infection of cells with recombinant SFV-enhE6,7 resulted in the production of large amounts of the E6,7 fusion protein. The fusion protein was more stable than either one of the separate proteins. Immunization of mice with SFV-enhE6,7 resulted in strong, long-lasting HPV-specific cytotoxic T lymphocyte responses. Tumor challenge experiments in mice demonstrated that immunization with SFV-enhE6,7 resulted in prevention of tumor outgrowth and subsequent protection against tumor re-challenge.

Semliki Forest virus-based vaccines: persistence, distribution and pathological analysis in two animal systems

This study has examined the persistence, distribution and pathological changes following intramuscular administration of Semliki Forest virus (SFV) vaccine vectors in mice and chickens. Administration of recombinant SFV RNA particles showed persistence at the injection site of mice up to 7 days, transient detection in secondary lymphoid organs and no dissemination to distal sites. In contrast, administration of a layered SFV DNA/RNA vector and a conventional standard naked DNA vector resulted in long-term persistence at the injection site, plasmid DNA being detected at 8 months post-inoculation in mice. Plasmid DNA was found distributed throughout the body, and tissues distal from the site of injection were positive up to 3 months. A similar pattern was observed in chickens. Mild pathological changes were observed at the injection site only, and plasmid DNA or recombinant RNA was not detected in mouse foetuses. These findings indicate that SFV-based vectors have the potential to be developed as safe vaccines.

Recombinant Semliki Forest virus particles expressing louping ill virus antigens induce a better protective response than plasmid-based DNA vaccines or an inactivated whole particle vaccine

Louping ill virus (LIV) infection of mice was used as a model to evaluate the protective efficacy of Semliki Forest virus (SFV)-based vaccines in comparison to a standard DNA vaccine and a commercial chemically inactivated vaccine. The recombinant SFV-based vaccines consisted of suicidal particles and a naked layered DNA/RNA construct. The nucleic acid vaccines expressed the spike precursor prME and the nonstructural protein 1 (NS1) antigens of LIV. Three LIV strains of graded virulence for mice were used for challenge. One of these was a naturally occurring antibody escape variant. All vaccines tested induced humoral immunity but gave varying levels of protection against lethal challenge. Only recombinant SFV particles administered twice gave full protection against neuronal degeneration and encephalitis induced by two of the three challenge strains, and partial protection against the highly virulent strain, whereas the other vaccines tested gave lower levels of partial protection.

Secretion from bovine chromaffin cells acutely expressing exogenous proteins using a recombinant Semliki Forest virus containing an EGFP reporter

Acute expression of recombinant proteins throughout a population of postmitotic bovine chromaffin cells was achieved using the Semliki Forest virus expression system (P. Liljestrom and H. Garoff (1991) Biotechnology 9:1356-1361). The virus was modified to express a green fluorescent protein, which faithfully reported the expression of the recombinant proteins. Two types of reporting virus were constructed: the first included a second subgenomic element, and the second an internal ribosome entry site. Both were used to express the recombinant proteins beta-galactosidase, 5HT3 receptor, or tetanus toxin light chain. Beta-galactosidase was used to quantify the rate of expression of recombinant protein in chromaffin cells, the 5HT3 receptor to trigger secretion, and the toxin to block secretion. The experiments clearly show that infection and expression of recombinant proteins throughout a population of chromaffin cells do not, per se, affect the rate and extent of triggered exocytosis, endocytosis, or membrane recycling pathways. The catecholamine content of the cell is unaltered, and the secretory mechanism can be accessed within a few hours after infection. This noncytopathic method of acutely expressing specific proteins at physiological levels in chromaffin cells offers a powerful new tool for dissecting the roles of many proteins implicated in exo- and endocytosis.

Comparison of the protective efficacy of naked DNA, DNA-based Sindbis replicon, and packaged Sindbis replicon vectors expressing Hantavirus structural genes in hamsters

Seoul virus (SEOV) is a member of the Hantavirus genus (family Bunyaviridae) and an etiological agent of hemorrhagic fever with renal syndrome. The medium (M) and small (S) gene segments of SEOV encode the viral envelope glycoproteins and nucleocapsid protein, respectively. We compared the immunogenicity and protective efficacy of naked DNA (pWRG7077), DNA-based Sindbis replicon (pSIN2.5), and packaged Sindbis replicon vectors (pSINrep5), containing either the M or S gene segment of SEOV in Syrian hamsters. All of the vectors elicited an anti-SEOV immune response to the expressed SEOV gene products. Vaccinated hamsters were challenged with SEOV and monitored for evidence of infection. Protection from infection was strongly associated with M-gene vaccination. A small number of S-gene-vaccinated animals also were protected. Hamsters vaccinated with the pWRG7077 vector expressing the M gene demonstrated the most consistent protection from SEOV infection and also were protected from heterologous hantavirus (Hantaan virus) infection.

Alphaviruses as tools in neurobiology and gene therapy

The broad host range and superior infectivity of alphaviruses have encouraged the development of efficient expression vectors for Semliki Forest virus (SFV) and Sindbis virus (SIN). The generation of high-titer recombinant alphavirus stocks has allowed high-level expression of a multitude of nuclear, cytoplasmic, membrane-associated and secreted proteins in a variety of different cell lines and primary cell cultures. Despite the viral cytopathogenic effects, functional assays on recombinant proteins are possible for a time-period of at least 24 hours post-infection. The high percentage (80-95%) of primary neurons infected with SFV has allowed localization and functional studies of recombinant proteins in these primary cell cultures. Through multiple infection studies the interaction of receptor and G protein subunits has become feasible. Establishment of efficient scale-up procedures has allowed production of large quantities of recombinant protein. Potential gene therapy applications of alphaviruses could be demonstrated by injection of recombinant SIN particles expressing beta-galactosidase into mouse brain. Tissue/cell specific infection has been achieved by introduction of an IgG-binding domain of protein A domain into one of the spike proteins of SIN. This enabled efficient targeting of infection to human lymphoblastoid cells.