Recombinant alphaviruses as vectors for anti-tumour and anti-microbial immunotherapy

First-in-Human Phase I Clinical Trial of an SFV-Based RNA Replicon Cancer Vaccine against HPV-Induced Cancers

A first-in-human phase I trial of Vvax001, an alphavirus-based therapeutic cancer vaccine against human papillomavirus (HPV)-induced cancers was performed assessing immunological activity, safety, and tolerability. Vvax001 consists of replication-incompetent Semliki Forest virus replicon particles encoding HPV16-derived antigens E6 and E7. Twelve participants with a history of cervical intraepithelial neoplasia were included. Four cohorts of three participants were treated per dose level, ranging from 5 × 10⁵ to 2.5 × 10⁸ infectious particles per immunization. The participants received three immunizations with a 3-week interval. For immune monitoring, blood was drawn before immunization and 1 week after the second and third immunization. Immunization with Vvax001 was safe and well tolerated, with only mild injection site reactions, and resulted in both CD4⁺ and CD8⁺ T cell responses against E6 and E7 antigens. Even the lowest dose of 5 × 10⁵ infectious particles elicited E6/E7-specific interferon (IFN)-γ responses in all three participants in this cohort. Overall, immunization resulted in positive vaccine-induced immune responses in 12 of 12 participants in one or more assays performed. In conclusion, Vvax001 was safe and induced immune responses in all participants. These data strongly support further clinical evaluation of Vvax001 as a therapeutic vaccine in patients with HPV-related malignancies.

Semliki Forest Virus replicon particles production in serum-free medium BHK-21 cell cultures and their use to express different proteins

The production of biopharmaceuticals as vaccines in serum-free media results in reduced risk of contamination and simpler downstream processing. The production of enveloped viruses and viral vectors such as Semliki Forest Virus (SFV) typically requires lipids that are provided by supplementation with animal serum, so production under serum-free conditions is challenging. In this work, the capacity to deliver genetic material of SFV-viral replicon particles (SFV-VRPs) produced in BHK-21 cells adapted to serum-free medium (BHK/SFM) was evaluated. Three transgenes were evaluated: GFP used as a model protein, while hepatitis C virus nonstructural protein 3 protease domain (HCV-NS3p) and rabies virus glycoprotein (RVGP) were selected based on their distinct nature (enzyme and glycoprotein, respectively). BHK/SFM cells produced a sevenfold higher number of SFV-VRPs, as determined by qRT-PCR. These particles showed similar capacities of infecting BHK/FBS or BHK/SFM cells. GFP expression was evaluated by flow cytometry, HCV-NS3p activity by enzymatic assay, and RVGP expression by ELISA and Western Blot. Expression analysis revealed higher levels of GFP and HCV-NS3p in BHK/SFM, while the levels of RVGP were similar for BHK/SFM and BHK/FBS. In conclusion, the BHK/SFM cells showed increased SFV-VRP production yields, without affecting vector infectivity or heterologous gene expression, hence validating the use of BHK/SFM for industrial applications.

Potent Anti-hepatitis C Virus (HCV) T Cell Immune Responses Induced in Mice Vaccinated with DNA-Launched RNA Replicons and Modified Vaccinia Virus Ankara-HCV

Hepatitis C is a liver disease caused by the hepatitis C virus (HCV) affecting 71 million people worldwide with no licensed vaccines that prevent infection. Here, we have generated four novel alphavirus-based DNA-launched self-amplifying RNA replicon (DREP) vaccines expressing either structural core-E1-E2 or nonstructural p7-NS2-NS3 HCV proteins of genotype 1a placed under the control of an alphavirus promoter, with or without an alphaviral translational enhancer (grouped as DREP-HCV or DREP-e-HCV, respectively). DREP vectors are known to induce cross-priming and further stimulation of immune responses through apoptosis, and here we demonstrate that they efficiently trigger apoptosis-related proteins in transfected cells. Immunization of mice with the DREP vaccines as the priming immunization followed by a heterologous boost with a recombinant modified vaccinia virus Ankara (MVA) vector expressing the nearly full-length genome of HCV (MVA-HCV) induced potent and long-lasting HCV-specific CD4⁺ and CD8⁺ T cell immune responses that were significantly stronger than those of a homologous MVA-HCV prime/boost immunization, with the DREP-e-HCV/MVA-HCV combination the most immunogenic regimen. HCV-specific CD4⁺ and CD8⁺ T cell responses were highly polyfunctional, had an effector memory phenotype, and were mainly directed against E1-E2 and NS2-NS3, respectively. Additionally, DREP/MVA-HCV immunization regimens induced higher antibody levels against HCV E2 protein than homologous MVA-HCV immunization. Collectively, these results provided an immunization protocol against HCV by inducing high levels of HCV-specific T cell responses as well as humoral responses. These findings reinforce the combined use of DREP-based vectors and MVA-HCV as promising prophylactic and therapeutic vaccines against HCV.IMPORTANCE HCV represents a global health problem as more than 71 million people are chronically infected worldwide. Direct-acting antiviral agents can cure HCV infection in most patients, but due to the high cost of these agents and the emergence of resistant mutants, they do not represent a feasible and affordable strategy to eradicate the virus. Therefore, a vaccine is an urgent goal that requires efforts to understand the correlates of protection for HCV clearance. Here, we describe for the first time the generation of novel vaccines against HCV based on alphavirus DNA replicons expressing HCV antigens. We demonstrate that potent T cell immune responses, as well as humoral immune responses, against HCV can be achieved in mice by using a combined heterologous prime/boost immunization protocol consisting of the administration of alphavirus replicon DNA vectors as the priming immunization followed by a boost with a recombinant modified vaccinia virus Ankara vector expressing HCV antigens.

T Lymphocytes as Measurable Targets of Protection and Vaccination Against Viral Disorders

Continuous epidemiological surveillance of existing and emerging viruses and their associated disorders is gaining importance in light of their abilities to cause unpredictable outbreaks as a result of increased travel and vaccination choices by steadily growing and aging populations. Close surveillance of outbreaks and herd immunity are also at the forefront, even in industrialized countries, where previously eradicated viruses are now at risk of re-emergence due to instances of strain recombination, contractions in viral vector geographies, and from their potential use as agents of bioterrorism. There is a great need for the rational design of current and future vaccines targeting viruses, with a strong focus on vaccine targeting of adaptive immune effector memory T cells as the gold standard of immunity conferring long-lived protection against a wide variety of pathogens and malignancies. Here, we review viruses that have historically caused large outbreaks and severe lethal disorders, including respiratory, gastric, skin, hepatic, neurologic, and hemorrhagic fevers. To observe trends in vaccinology against these viral disorders, we describe viral genetic, replication, transmission, and tropism, host-immune evasion strategies, and the epidemiology and health risks of their associated syndromes. We focus on immunity generated against both natural infection and vaccination, where a steady shift in conferred vaccination immunogenicity is observed from quantifying activated and proliferating, long-lived effector memory T cell subsets, as the prominent biomarkers of long-term immunity against viruses and their associated disorders causing high morbidity and mortality rates.

Intracellular Delivery of HCV NS3p gene using vectored particles

Viral hepatitis caused by the hepatitis C virus (HCV) affects millions of people worldwide. The non-structural protein 3 (NS3), one of the most conserved proteins in HCV, is the target of many therapeutic studies. The NS3 protease domain (NS3p) has a range of cytotoxic T lymphocyte (CTL) epitopes, and synthesizing the protein inside the cells is the most appropriate way to present it to the immune system. We developed a tool to study this kind of presentation, using two vectored particle (VP) systems, one based on the Semliki Forest virus (SFV) and the other on HCV pseudoparticles (HCVpp), both carrying the protease domain of the NS3 gene. In addition to producing the particles, we developed a method to quantify these VPs using qRT-PCR. We produced batches of approximately 2.4 × 10⁴ SFV-NS3p/μL and 4.0 × 10² HCVpp-NS3p/μL. BHK-21 and HuH-7 cells treated with the VPs expressed the NS3 protein, thus showing the functionality of this system.

Comparative protein profiling of B16 mouse melanoma cells susceptible and non-susceptible to alphavirus infection: Effect of the tumor microenvironment

Alphavirus vectors are promising tools for cancer treatment. However, relevant entry mechanisms and interactions with host cells are still not clearly understood. The first step toward a more effective therapy is the identification of novel intracellular alterations that could be associated with cancer aggressiveness and could affect the therapeutic potential of these vectors. In this study, we observed that alphaviruses efficiently infected B16 mouse melanoma tumors/tumor cells in vivo, whereas their transduction efficiency in B16 cells under in vitro conditions was blocked. Therefore, we further aimed to understand the mechanisms pertaining to the differential transduction efficacy of alphaviruses in B16 tumor cells under varying growth conditions. We hypothesized that the tumor microenvironment might alter gene expression in B16 cells, leading to an up-regulation of the expression of virus-binding receptors or factors associated with virus entry and replication. To test our hypothesis, we performed a proteomics analysis of B16 cells cultured in vitro and of B16 cells isolated from tumors, and we identified 277 differentially regulated proteins. A further in-depth analysis to identify the biological and molecular functions of the detected proteins revealed a set of candidate genes that could affect virus infectivity. Importantly, we observed a decrease in the expression of interferon α (IFN-α) in tumor-isolated cells that resulted in the suppression of several IFN-regulated genes, thereby abrogating host cell antiviral defense. Additionally, differences in the expression of genes that regulate cytoskeletal organization caused significant alterations in cell membrane elasticity. Taken together, our findings demonstrated favorable intracellular conditions for alphavirus transduction/replication that occurred during tumor transformation. These results pave the way for optimizing the development of strategies for the application of alphaviral vectors as a potent cancer therapy.

Modification of a salmonid alphavirus replicon vector for enhanced expression of heterologous antigens

A salmonid alphavirus (SAV) replicon has been developed to express heterologous antigens but protein production was low to modest compared with terrestrial alphavirus replicons. In this study, we have compared several modifications to a SAV replicon construct and analysed their influence on foreign gene expression. We found that an insertion of a translational enhancer consisting of the N-terminal 102 nt of the capsid gene, together with a nucleotide sequence encoding the foot-and-mouth disease virus (FMDV) 2A peptide, caused a significant increase in EGFP reporter gene expression. The importance of fusing a hammerhead (HH) ribozyme sequence at the 5' end of the viral genome was also demonstrated. In contrast, a hepatitis D virus ribozyme (HDV-RZ) sequence placed at the 3' end did not augment expression of inserted genes. Taken together, we have developed a platform for optimized antigen production, which can be applied for immunization of salmonid fish in the future.

High efficiency of alphaviral gene transfer in combination with 5-fluorouracil in a mouse mammary tumor model

Background

The combination of virotherapy and chemotherapy may enable efficient tumor regression that would be unachievable using either therapy alone. In this study, we investigated the efficiency of transgene delivery and the cytotoxic effects of alphaviral vector in combination with 5-fluorouracil (5-FU) in a mouse mammary tumor model (4 T1).

Methods

Replication-deficient Semliki Forest virus (SFV) vectors carrying genes encoding fluorescent proteins were used to infect 4 T1 cell cultures treated with different doses of 5-FU. The efficiency of infection was monitored via fluorescence microscopy and quantified by fluorometry. The cytotoxicity of the combined treatment with 5-FU and alphaviral vector was measured using an MTT-based cell viability assay. In vivo experiments were performed in a subcutaneous 4 T1 mouse mammary tumor model with different 5-FU doses and an SFV vector encoding firefly luciferase.

Results

Infection of 4 T1 cells with SFV prior to 5-FU treatment did not produce a synergistic anti-proliferative effect. An alternative treatment strategy, in which 5-FU was used prior to virus infection, strongly inhibited SFV expression. Nevertheless, in vivo experiments showed a significant enhancement in SFV-driven transgene (luciferase) expression upon intratumoral and intraperitoneal vector administration in 4 T1 tumor-bearing mice pretreated with 5-FU: here, we observed a positive correlation between 5-FU dose and the level of luciferase expression.

Conclusions

Although 5-FU inhibited SFV-mediated transgene expression in 4 T1 cells in vitro, application of the drug in a mouse model revealed a significant enhancement of intratumoral transgene synthesis compared with 5-FU untreated mice. These results may have implications for efficient transgene delivery and the development of potent cancer treatment strategies using alphaviral vectors and 5-FU.

Combination of alphavirus replicon particle-based vaccination with immunomodulatory antibodies: therapeutic activity in the B16 melanoma mouse model and immune correlates

Induction of potent immune responses to self-antigens remains a major challenge in tumor immunology. We have shown that a vaccine based on alphavirus replicon particles (VRP) activates strong cellular and humoral immunity to tyrosinase-related protein-2 (TRP2) melanoma antigen, providing prophylactic and therapeutic effects in stringent mouse models. Here, we report that the immunogenicity and efficacy of this vaccine is increased in combination with either antagonist anti-CTL antigen-4 (CTLA-4) or agonist anti-glucocorticoid-induced TNF family-related gene (GITR) immunomodulatory monoclonal antibodies (mAb). In the challenging therapeutic setting, VRP-TRP2 plus anti-GITR or anti-CTLA-4 mAb induced complete tumor regression in 90% and 50% of mice, respectively. These mAbs had similar adjuvant effects in priming an adaptive immune response against the vaccine-encoded antigen, augmenting, respectively, approximately 4- and 2-fold the TRP2-specific CD8(+) T-cell response and circulating Abs, compared with the vaccine alone. Furthermore, while both mAbs increased the frequency of tumor-infiltrating CD8(+) T cells, anti-CTLA-4 mAb also increased the quantity of intratumor CD4(+)Foxp3(-) T cells expressing the negative costimulatory molecule programmed death-1 (PD-1). Concurrent GITR expression on these cells suggests that they might be controlled by anti-GITR mAbs, thus potentially explaining their differential accumulation under the two treatment conditions. These findings indicate that combining immunomodulatory mAbs with alphavirus-based anticancer vaccines can provide therapeutic antitumor immune responses in a stringent mouse model, suggesting potential utility in clinical trials. They also indicate that tumor-infiltrating CD4(+)Foxp3(-)PD-1(+) T cells may affect the outcome of immunomodulatory treatments.

Alphavirus-based vaccines encoding nonstructural proteins of hepatitis C virus induce robust and protective T-cell responses

An absolute prerequisite for a therapeutic vaccine against hepatitis C virus (HCV) infection is the potency to induce HCV-specific vigorous and broad-spectrum T-cell responses. Here, we generated three HCV vaccines based on a recombinant Semliki Forest virus (rSFV) vector expressing all- or a part of the conserved nonstructural proteins (nsPs) of HCV. We demonstrated that an rSFV vector was able to encode a transgene as large as 6.1 kb without affecting its vaccine immunogenicity. Prime-boost immunizations of mice with rSFV expressing all nsPs induced strong and long-lasting NS3-specific CD8(+) T-cell responses. The strength and functional heterogeneity of the T-cell response was similar to that induced with rSFV expressing only NS3/4A. Furthermore this leads to a significant growth delay and negative selection of HCV-expressing EL4 tumors in an in vivo mouse model. In general, as broad-spectrum T-cell responses are only seen in patients with resolved HCV infection, this rSFV-based vector, which expresses all nsPs, inducing robust T-cell activity has a potential for the treatment of HCV infections.

Genome-wide RNAi screen identifies SEC61A and VCP as conserved regulators of Sindbis virus entry

Alphaviruses are a large class of insect-borne human pathogens and little is known about the host-factor requirements for infection. To identify such factors, we performed a genome-wide RNAi screen using model Drosophila cells and validated 94 genes that impacted infection of Sindbis virus (SINV), the prototypical alphavirus. We identified a conserved role for SEC61A and valosin-containing protein (VCP) in facilitating SINV entry in insects and mammals. SEC61A and VCP selectively regulate trafficking of the entry receptor NRAMP2, and loss or pharmacological inhibition of these proteins leads to altered NRAMP2 trafficking to lysosomal compartments and proteolytic digestion within lysosomes. NRAMP2 is the major iron transporter in cells, and loss of NRAMP2 attenuates intracellular iron transport. Thus, this study reveals genes and pathways involved in both infection and iron homeostasis that may serve as targets for antiviral therapeutics or for iron-imbalance disorders.

Quantitative RT-PCR for titration of replication-defective recombinant Semliki Forest virus

Virus titration may constitute a drawback in the development and use of replication-defective viral vectors like Semliki Forest virus (SFV). The standardization and validation of a reverse transcription quantitative PCR (qRT-PCR) method for SFV titration is presented here. The qRT-PCR target is located within the nsp1 gene of the non-structural polyprotein SFV region (SFV RNA), which allows the strategy to be used for several different recombinant SFV constructs. Titer determinations were carried out by performing virus titration and infection assays with SFVs containing an RNA coding region for the rabies virus glycoprotein (RVGP) or green fluorescent protein (GFP). Results showed that the standardized qRT-PCR is applicable for different SFV constructs, and showed good reproducibility. To evaluate the correlation between the amount of functional SFV RNA in a virus lot and its infectivity in BHK-21 cell cultures, a temperature mediated titer decrease was performed and successfully quantitated by qRT-PCR. When used for cell infection at the same multiplicity of infection (MOI), the temperature treated SFV-RVGP samples induced the same levels of RVGP expression. Similarly, when different SFV-GFP lots with different virus titers, as accessed by qRT-PCR, were used for cell infection at the same MOI, the cultures showed comparable amounts of fluorescent cells. The data demonstrate a good correlation between the amount of virus used for infection, as measured by its SFV RNA, and the protein synthesis in the cells. In conclusion, the qRT-PCR method developed here is accurate and enables the titration of replication-defective SFV vectors, an essential aid for viral vector development as well as for establishment of production bioprocesses.

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.

Respiratory syncytial virus vaccine development

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract viral disease in infants and young children. Presently, there are no explicit recommendations for RSV treatment apart from supportive care. The virus is therefore responsible for an estimated 160,000 deaths per year worldwide. Despite half a century of dedicated research, there remains no licensed vaccine product. Herein are described past and current efforts to harness innate and adaptive immune potentials to combat RSV. A plethora of candidate vaccine products and strategies are reviewed. The development of a successful RSV vaccine may ultimately stem from attention to historical lessons, in concert with an integral partnering of immunology and virology research fields.

Role of regulatory T-cells in immunization strategies involving a recombinant alphavirus vector system

BACKGROUND

Regulatory T-cells (Treg) hamper immune responses elicited by cancer vaccines. Therefore, depletion of Treg is being used to improve the outcome of vaccinations.

METHODS

We studied whether an alphavirus vector-based immunotherapeutic vaccine changes the number and/or activity of Treg and if Treg depletion improves the efficacy of this vaccine against tumours. The vaccine is based on a Semliki Forest virus (SFV). The recombinant SFV replicon particles encode a fusion protein of E6 and E7 from human papillomavirus (HPV) type 16 (SFVeE6,7).

RESULTS

We demonstrated that SFVeE6,7 immunization did not change Treg levels and their suppressive activity. Depletion of Treg in mice, using the novel anti-folate receptor 4 antibody, did not enhance the immune response induced by SFVeE6,7 immunization. Both the priming and the proliferation phases of the HPV-specific response elicited with SFVeE6,7 were not affected by the immune-suppressive activity of Treg. Moreover, Treg depletion did not improve the therapeutic antitumour response of SFVeE6,7 in a murine tumour model.

CONCLUSIONS

The efficacy of the SFVeE6,7 vaccine was not hampered by Treg. Therefore, SFVeE6,7 seems a very promising candidate for the treatment of HPV-induced disease, as it may not require additional immune interventions to modulate Treg activity.

Alphavirus replicon particles expressing TRP-2 provide potent therapeutic effect on melanoma through activation of humoral and cellular immunity

Background

Malignant melanoma is the deadliest form of skin cancer and is refractory to conventional chemotherapy and radiotherapy. Therefore alternative approaches to treat this disease, such as immunotherapy, are needed. Melanoma vaccine design has mainly focused on targeting CD8⁺ T cells. Activation of effector CD8⁺ T cells has been achieved in patients, but provided limited clinical benefit, due to immune-escape mechanisms established by advanced tumors. We have previously shown that alphavirus-based virus-like replicon particles (VRP) simultaneously activate strong cellular and humoral immunity against the weakly immunogenic melanoma differentiation antigen (MDA) tyrosinase. Here we further investigate the antitumor effect and the immune mechanisms of VRP encoding different MDAs.

Methodology/Principal Findings

VRP encoding different MDAs were screened for their ability to prevent the growth of the B16 mouse transplantable melanoma. The immunologic mechanisms of efficacy were investigated for the most effective vaccine identified, focusing on CD8⁺ T cells and humoral responses. To this end, ex vivo immune assays and transgenic mice lacking specific immune effector functions were used. The studies identified a potent therapeutic VRP vaccine, encoding tyrosinase related protein 2 (TRP-2), which provided a durable anti-tumor effect. The efficacy of VRP-TRP2 relies on a novel immune mechanism of action requiring the activation of both IgG and CD8⁺ T cell effector responses, and depends on signaling through activating Fcγ receptors.

Conclusions/Significance

This study identifies a VRP-based vaccine able to elicit humoral immunity against TRP-2, which plays a role in melanoma immunotherapy and synergizes with tumor-specific CD8⁺ T cell responses. These findings will aid in the rational design of future immunotherapy clinical trials.

Acute infection with venezuelan equine encephalitis virus replicon particles catalyzes a systemic antiviral state and protects from lethal virus challenge

The host innate immune response provides a critical first line of defense against invading pathogens, inducing an antiviral state to impede the spread of infection. While numerous studies have documented antiviral responses within actively infected tissues, few have described the earliest innate response induced systemically by infection. Here, utilizing Venezuelan equine encephalitis virus (VEE) replicon particles (VRP) to limit infection to the initially infected cells in vivo, a rapid activation of the antiviral response was demonstrated not only within the murine draining lymph node, where replication was confined, but also within distal tissues. In the liver and brain, expression of interferon-stimulated genes was detected by 1 to 3 h following VRP footpad inoculation, reaching peak expression of >100-fold over that in mock-infected animals. Moreover, mice receiving a VRP footpad inoculation 6, 12, or 24 h prior to an otherwise lethal VEE footpad challenge were completely protected from death, including a drastic reduction in challenge virus titers. VRP pretreatment also provided protection from intranasal VEE challenge and extended the average survival time following intracranial challenge. Signaling through the interferon receptor was necessary for antiviral gene induction and protection from VEE challenge. However, VRP pretreatment failed to protect mice from a heterologous, lethal challenge with vesicular stomatitis virus, yet conferred protection following challenge with influenza virus. Collectively, these results document a rapid modulation of the host innate response within hours of infection, capable of rapidly alerting the entire animal to pathogen invasion and leading to protection from viral disease.

Augmentation of alphavirus vector-induced human papilloma virus-specific immune and anti-tumour responses by co-expression of interleukin-12

To enhance the efficacy of a therapeutic immunisation strategy against human papillomavirus-induced cervical cancer we evaluated the adjuvant effect of interleukin-12 (IL12) expressed by a Semliki Forest virus vector (SFV) in mice. Depending on the dose and schedule, SFV-IL12 stimulated antigen-specific CTL responses elicited upon immunisation with recombinant SFV expressing HPV16-E6E7 (SFVeE6,7). SFVeE6,7-CTL and anti-tumour activity were enhanced by a low dose of SFV-IL12 to the prime immunisation. Using higher dosages these activities were reduced. Addition of SFV-IL12 to the booster immunisation further reduced the efficacy of the SFVeE6,7 immunisation. In transgenic mice, tolerant for HPV16-E6E7, SFV-IL12 also stimulated SFVeE6,7-induced CTL responses. In conclusion, SFV-IL12 can enhance antigen-specific immune responses. Yet, prudence is called for when considering co-administration of SFV-IL12 to a vaccine, as the enhancement of cell-mediated immune responses greatly depends on dosage and schedule.

Therapeutic and prophylactic applications of alphavirus vectors

Alphavirus vectors are high-level, transient expression vectors for therapeutic and prophylactic use. These positive-stranded RNA vectors, derived from Semliki Forest virus, Sindbis virus and Venezuelan equine encephalitis virus, multiply and are expressed in the cytoplasm of most vertebrate cells, including human cells. Part of the genome encoding the structural protein genes, which is amplified during a normal infection, is replaced by a transgene. Three types of vector have been developed: virus-like particles, layered DNA-RNA vectors and replication-competent vectors. Virus-like particles contain replicon RNA that is defective since it contains a cloned gene in place of the structural protein genes, and thus are able to undergo only one cycle of expression. They are produced by transfection of vector RNA, and helper RNAs encoding the structural proteins. Layered DNA-RNA vectors express the Semliki Forest virus replicon from a cDNA copy via a cytomegalovirus promoter. Replication-competent vectors contain a transgene in addition to the structural protein genes. Alphavirus vectors are used for three main applications: vaccine construction, therapy of central nervous system disease, and cancer therapy.

Alphavirus replicon particles acting as adjuvants promote CD8+ T cell responses to co-delivered antigen

Alphavirus replicon particles induce strong antibody and CD8+ T cell responses to expressed antigens in numerous experimental systems. We have recently demonstrated that Venezuelan equine encephalitis virus replicon particles (VRP) possess adjuvant activity for systemic and mucosal antibody responses. In this report, we demonstrate that VRP induced an increased and balanced serum IgG subtype response to co-delivered antigen, with simultaneous induction of antigen-specific IgG1 and IgG2a antibodies, and increased both systemic and mucosal antigen-specific CD8+ T cell responses, as measured by an IFN-gamma ELISPOT assay. Additionally, VRP further increased antigen-specific T cell immunity in an additive fashion following co-delivery with the TLR ligand, CpG DNA. VRP infection led to recruitment of CD8+ T cells into the mucosal compartment, possibly utilizing the mucosal homing receptor, as this integrin was upregulated on CD8+ T cells in the draining lymph node of VRP-infected animals, where VRP-infected dendritic cells reside. This newly recognized ability of VRP to mediate increased T cell response towards co-delivered antigen provides the potential to both define the molecular basis of alphavirus-induced immunity, and improve alphavirus-based vaccines.

A plasmid-based reverse genetics system for animal double-stranded RNA viruses

Mammalian orthoreoviruses (reoviruses) are highly tractable experimental models for studies of double-stranded (ds) RNA virus replication and pathogenesis. Reoviruses infect respiratory and intestinal epithelium and disseminate systemically in newborn animals. Until now, a strategy to rescue infectious virus from cloned cDNA has not been available for any member of the Reoviridae family of dsRNA viruses. We report the generation of viable reovirus following plasmid transfection of murine L929 (L) cells using a strategy free of helper virus and independent of selection. We used the reovirus reverse genetics system to introduce mutations into viral capsid proteins σ1 and σ3 and to rescue a virus that expresses a green fluorescent protein (GFP) transgene, thus demonstrating the tractability of this technology. The plasmid-based reverse genetics approach described here can be exploited for studies of reovirus replication and pathogenesis and used to develop reovirus as a vaccine vector.

Viral vector-based prime-boost immunization regimens: a possible involvement of T-cell competition

Vaccination with recombinant viral vectors may be impeded by preexisting vector-specific immunity or by vector-specific immunity induced during the priming immunization. It is assumed that virus-neutralizing antibodies represent the principal effector mechanism of vector-specific immunity, while killing of infected cells by vector-specific cytotoxic T lymphocytes (CTLs) has also been suggested. Using recombinant Semliki Forest virus (rSFV) expressing E6E7 antigen from human papillomavirus, we demonstrate that secondary immune responses against E6E7 are neither affected by vector-specific antibodies nor by CTL-mediated killing of infected cells. Instead, the presence of the antigen during the prime immunization appeared to be the main determinant for the boosting efficacy. After priming with rSFVeE6,7, a homologous booster stimulated the primed E6E7-specific CTL response and induced long-lasting memory. Passively transferred SFV-neutralizing antibodies did not inhibit E6E7-specific CTL responses, although transgene expression was strongly reduced under these conditions. Conversely, in mice primed with irrelevant rSFV, induction of E6E7-specific CTLs was inhibited presumably due to vector-specific responses induced by the priming immunization. When during the priming with irrelevant rSFV, E7-protein was co-administered, the inhibitory effect of vector-specific immunity was abolished. These results suggest that, apart from vector-specific antibodies or killing of infected cells, T-cell competition may be involved in determining the efficacy of viral vector-based prime-boost immunization regimens.

Salmonella typhimurium stimulation combined with tumour-derived heat shock proteins induces potent dendritic cell anti-tumour responses in a murine model

Appropriate activation of the immune system and effective targeting of tumour cells are the primary hurdles to be overcome for cancer immunotherapy to be successful and applicable to a wide range of tumour types. Our studies have examined the ability of bacterial-stimulated dendritic cells (DCs), loaded with tumour-associated antigens, to inhibit tumour growth in a murine model. Immature murine bone marrow-derived DCs were stimulated in vitro with the cytoplasmic fraction (CM) of Salmonella typhimurium in combination with heat shock proteins (hsps) from 4T1 tumours, isolated using heparin affinity chromatography. Activated DCs were administered subcutaneously. Tumours were generated by orthotopic inoculation of 4T1 cells in Balb/c mice. Primary tumour growth was measured using Vernier calipers, while lung metastases were measured using the clonogenic assay. S. typhimurium CM induced potent tumour necrosis factor (TNF)-alpha responses from DCs accompanied by significant up-regulation of CD80 and CD86 expression. When injected into mice, bacterial-stimulated DCs loaded with 4T1 hsps inhibited the formation of new 4T1 tumours and reduced the growth rate of established tumours. In addition, the number of lung metastatic nodules was reduced significantly in the DC-treated mice (1.6 +/- 0.6 versus 245.9 +/- 55.6, P = 0.0015). DCs stimulated with CM alone, exposed to tumour hsps alone or exposed to tumour hsps from an unrelated tumour cell line did not induce a protective immune response. Dendritic cells primed with a proinflammatory bacterial stimulus and tumour-associated antigens induce a protective anti-tumour immune response in this murine model.

Evaluation of neurovirulence and biodistribution of Venezuelan equine encephalitis replicon particles expressing herpes simplex virus type 2 glycoprotein D

The safety of a propagation-defective Venezuelan equine encephalitis virus (VEEV) replicon particle vaccine was examined in mice. After intracranial inoculation we observed approximately 5% body weight loss, modest inflammatory changes in the brain, genome replication, and foreign gene expression. These changes were transient and significantly less severe than those caused by TC-83, a live-attenuated vaccinal strain of VEEV that has been safely used to immunize military personnel and laboratory workers. Replicon particles injected intramuscularly or intravenously were detected at limited sites 3 days post-administration, and were undetectable by day 22. There was no evidence of dissemination to spinal cord or brain after systemic administration. These results demonstrate that propagation-defective VEEV replicon particles are minimally neurovirulent and lack neuroinvasive potential.