Evaluation of Venezuelan Equine Encephalitis (VEE) replicon-based Outer surface protein A (OspA) vaccines in a tick challenge mouse model of Lyme disease

Self-replicating alphavirus RNA vaccines

Recombinant nucleic acids are considered as promising next-generation vaccines. These vaccines express the native antigen upon delivery into tissue, thus mimicking live attenuated vaccines without having the risk of reversion to pathogenicity. They also stimulate the innate immune system, thus potentiating responses. Nucleic acid vaccines are easy to produce at reasonable cost and are stable. During the past years, focus has been on the use of plasmid DNA for vaccination. Now mRNA and replicon vaccines have come into focus as promising technology platforms for vaccine development. This review discusses self-replicating RNA vaccines developed from alphavirus expression vectors. These replicon vaccines can be delivered as RNA, DNA or as recombinant virus particles. All three platforms have been pre-clinically evaluated as vaccines against a number of infectious diseases and cancer. Results have been very encouraging and propelled the first human clinical trials, the results of which have been promising.

Borrelia burgdorferi BBA52 is a potential target for transmission blocking Lyme disease vaccine

The surface-exposed antigens of Borrelia burgdorferi represent important targets for induction of protective host immune responses. BBA52 is preferentially expressed by B. burgdorferi in the feeding tick, and a targeted deletion of bba52 interferes with vector-host transitions in vivo. In this study, we demonstrate that BBA52 is an outer membrane surface-exposed protein and that disulfide bridges take part in the homo-oligomeric assembly of native protein. BBA52 antibodies lack detectable borreliacidal activities in vitro. However, active immunization studies demonstrated that BBA52 vaccinated mice were significantly less susceptible to subsequent tick-borne challenge infection. Similarly, passive transfer of BBA52 antibodies in ticks completely blocked B. burgdorferi transmission from feeding ticks to naïve mice. Taken together, these studies highlight the role of BBA52 in spirochete dissemination from ticks to mice and demonstrate the potential of BBA52 antibody-mediated strategy to complement the ongoing efforts to develop vaccines for blocking the transmission of B. burgdorferi.

A host-restricted viral vector for antigen-specific immunization against Lyme disease pathogen

Newcastle disease virus (NDV) is an avian virus that is attenuated in primates and is a potential vaccine vector for human use. We evaluated NDV as a vector for expressing selected antigens of the Lyme disease pathogen Borrelia burgdorferi. A series of recombinant NDVs were generated that expressed intracellular or extracellular forms of two B. burgdorferi antigens: namely, the basic membrane protein A (BmpA) and the outer surface protein C (OspC). Expression of the intracellular and extracellular forms of these antigens was confirmed in cultured chicken cells. C3H or Balb/C mice that were immunized intranasally with the NDV vectors mounted vigorous serum antibody responses against the NDV vector, but failed to mount a robust response against either the intracellular or extracellular forms of BmpA or OspC. By contrast, a single immunization of hamsters with the NDV vectors via the intranasal, intramuscular, or intraperitoneal route resulted in rapid and rigorous antibody responses against the intracellular or extracellular forms of BmpA and OspC. When groups of hamsters were separately inoculated with various NDV vectors and challenged with B. burgdorferi (10⁸ cells/animal), immunization with vector expressing either intracellular or extracellular BmpA was associated with a significant reduction of the pathogen load in the joints. Taken together, our studies highlighted the importance of NDV as vaccine vector that can be used for simple yet effective immunization of hosts against bacterial infections including Lyme disease.

Development of a vaccinia virus based reservoir-targeted vaccine against Yersinia pestis

Yersinia pestis, the causative organism of plague, is a zoonotic organism with a worldwide distribution. Although the last plague epidemic occurred in early 1900s, human cases continue to occur due to contact with infected wild animals. In this study, we have developed a reservoir-targeted vaccine against Y. pestis, to interrupt transmission of disease in wild animals as a potential strategy for decreasing human disease. A vaccinia virus delivery system was used to express the F1 capsular protein and the LcrV type III secretion component of Y. pestis as a fusion protein. Here we show that a single dose of this vaccine administered orally, generates a dose-dependent antibody response in mice. Antibody titers peak by 3 weeks after administration and remain elevated for a minimum of 45 weeks. Vaccination provided up to 100% protection against challenge with Y. pestis administered by intranasal challenge at 10 times the lethal dose with protection lasting a minimum of 45 weeks. An orally available, vaccinia virus expressed vaccine against Y. pestis may be a suitable vaccine for a reservoir targeted strategy for the prevention of enzootic plague.

Recombinant vectors as influenza vaccines

The antiquated system used to manufacture the currently licensed inactivated influenza virus vaccines would not be adequate during an influenza virus pandemic. There is currently a search for vaccines that can be developed faster and provide superior, long-lasting immunity to influenza virus as well as other highly pathogenic viruses and bacteria. Recombinant vectors provide a safe and effective method to elicit a strong immune response to a foreign protein or epitope. This review explores the advantages and limitations of several different vectors that are currently being tested, and highlights some of the newer viruses being used as recombinant vectors.

Development of a vaccine vector based on a subgenomic replicon of porcine reproductive and respiratory syndrome virus

In this study, a DNA-launched subgenomic replicon of porcine reproductive and respiratory syndrome virus (PRRSV) was developed for use as a vaccine vector. This replicon plasmid contained a PRRSV subgenome without structural genes ORF2-ORF6, and was under the transcriptional control of the immediate-early promoter of cytomegalovirus (CMV). Using enhanced green fluorescent protein (EGFP) as a reporter gene, the DNA-launched subgenomic replicon of PRRSV, named pOK-Clone20-rep, could express heterologous genes in vitro. After direct inoculation of pOK-Clone20-rep, mice developed antibody responses that were specific for both the EGFP and the N gene in a dose-dependent manner. Furthermore, mice immunized with pOK-Clone20-rep at a dose of 100microg showed significantly enhanced levels of IFN-gamma compared with those inoculated with 100microg of pcD-EGFP, a conventional DNA vaccine that encodes EGFP. In summary, the results show that the DNA-launched subgenomic replicon of PRRSV could not only mediate foreign gene expression in vitro but also induced an immune response in vivo. Similarly, expression and immunogenicity of the N gene also strengthened the potential of the replicon to serve as a vaccine vector expressing multiple genes. It therefore provides a useful tool for vaccine development and the study of the transcription and replication of PRRSV.

Venezuelan equine encephalitis virus replicon particles encoding respiratory syncytial virus surface glycoproteins induce protective mucosal responses in mice and cotton rats

Respiratory syncytial virus (RSV) is an important viral pathogen that causes severe lower respiratory tract infection in infants, the elderly, and immunocompromised individuals. There are no licensed RSV vaccines to date. To prevent RSV infection, immune responses in both the upper and lower respiratory tracts are required. Previously, immunization with Venezuelan equine encephalitis virus replicon particles (VRPs) demonstrated effectiveness in inducing mucosal protection against various pathogens. In this study, we developed VRPs encoding RSV fusion (F) or attachment (G) glycoproteins and evaluated the immunogenicity and efficacy of these vaccine candidates in mice and cotton rats. VRPs, when administered intranasally, induced surface glycoprotein-specific virus neutralizing antibodies in serum and immunoglobulin A (IgA) antibodies in secretions at the respiratory mucosa. In addition, fusion protein-encoding VRPs induced gamma interferon (IFN-gamma)-secreting T cells in the lungs and spleen, as measured by reaction with an H-2K(d)-restricted CD8(+) T-cell epitope. In animals vaccinated with F protein VRPs, challenge virus replication was reduced below the level of detection in both the upper and lower respiratory tracts following intranasal RSV challenge, while in those vaccinated with G protein VRPs, challenge virus was detected in the upper but not the lower respiratory tract. Close examination of histopathology of the lungs of vaccinated animals following RSV challenge revealed no enhanced inflammation. Immunization with VRPs induced balanced Th1/Th2 immune responses, as measured by the cytokine profile in the lungs and antibody isotype of the humoral immune response. These results represent an important first step toward the use of VRPs encoding RSV proteins as a prophylactic vaccine for RSV.

A Novel Alphavirus Vaccine Encoding Prostate-Specific Membrane Antigen Elicits Potent Cellular and Humoral Immune Responses

PURPOSE

Prostate-specific membrane antigen (PSMA) is an attractive target for active immunotherapy. Alphavirus vaccines have shown promise in eliciting immunity to tumor antigens. This study investigated the immunogenicity of alphavirus vaccine replicon particles (VRP) that encode PSMA (PSMA-VRP).

EXPERIMENTAL DESIGN

Cells were infected with PSMA-VRP and evaluated for PSMA expression and folate hydrolase activity. Mice were immunized s.c. with PSMA-VRP or purified PSMA protein. Sera, splenocytes, and purified T cells were evaluated for the magnitude, durability, and epitope specificity of the anti-PSMA response. Antibodies were measured by flow cytometry, and cellular responses were measured by IFN-gamma enzyme-linked immunospot and chromium release assays. Cellular responses in BALB/c and C57BL/6 mice were mapped using overlapping 15-mer PSMA peptides. A Good Laboratory Practice-compliant toxicology study was conducted in rabbits.

RESULTS

PSMA-VRP directed high-level expression of active PSMA. Robust T-cell and B-cell responses were elicited by a single injection of 2 x 10(5) infectious units, and responses were boosted following repeat immunizations. Anti-PSMA responses were detected following three immunizations with 10(2) infectious units and increased with increasing dose. PSMA-VRP was more immunogenic than adjuvanted PSMA protein. Responses to PSMA-VRP were characterized by Th-1 cytokines, potent CTL activity, and IgG2a/IgG2b antibodies. T-cell responses in BALB/c and C57BL/6 mice were directed toward different PSMA peptides. Immunogenic doses of PSMA-VRP were well tolerated in mice and rabbits.

CONCLUSIONS

PSMA-VRP elicited potent cellular and humoral immunity in mice, and specific anti-PSMA responses were boosted on repeat dosing. PSMA-VRP represents a promising approach for immunotherapy of prostate cancer.

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.

Distinct contributions of vaccine-induced immunoglobulin G1 (IgG1) and IgG2a antibodies to protective immunity against influenza

Vaccination represents the most effective form of protection against influenza infection. While neutralizing antibodies are typically measured as a correlate of vaccine-induced protective immunity against influenza, nonneutralizing antibodies may contribute to protection or amelioration of disease. The goal of this study was to dissect the individual contributions of the immunoglobulin G1 (IgG1) and IgG2a antibody isotypes to vaccine-induced immunity against influenza virus. To accomplish this, we utilized an influenza vaccine regimen that selectively enhanced IgG1 or IgG2a antibodies by using either DNA or viral replicon particle (VRP) vectors expressing influenza virus hemagglutinin (HA) (HA-DNA or HA-VRP, respectively). After HA-DNA vaccination, neutralizing antibodies were detected by both in vitro (microneutralization) and in vivo (lung viral titer) methods and were associated with increased IgG1 expression by enzyme-linked immunosorbent assay (ELISA). Vaccination with HA-VRP did not strongly stimulate either neutralizing or IgG1 antibodies but did induce IgG2a antibodies. Expression of IgG2a antibodies in this context correlated with clearance of virus and increased protection against lethal influenza challenge. Increased induction of both antibody isotypes as measured by ELISA was a better correlate for vaccine efficacy than neutralization alone. This study details separate but important roles for both IgG1 and IgG2a expression in vaccination against influenza and argues for the development of vaccine regimens that stimulate and measure expression of both antibody isotypes.

Vaccination of mice with gonococcal TbpB expressed in vivo from Venezuelan equine encephalitis viral replicon particles

We investigated the immunogenicity of gonococcal transferrin binding protein B (TbpB) expressed with and without a eukaryotic secretion signal from a nonpropagating Venezuelan equine encephalitis virus replicon particle (VRP) delivery system. TbpB was successfully expressed in baby hamster kidney (BHK) cells, and the presence of the eukaryotic secretion signal not only apparently increased the protein's expression but also allowed for extracellular localization and glycosylation. Mice immunized with VRPs produced significant amounts of serum antibody although less than the amounts produced by mice immunized with recombinant protein. The response of mice immunized with VRPs encoding TbpB was consistently more Th1 biased than the response of mice immunized with recombinant protein alone. Boosting with recombinant protein following immunization with TbpB VRPs resulted in higher specific-antibody levels without altering the Th1/Th2 bias. Most of the immunization groups produced significant specific antibody binding to the intact surface of the homologous Neisseria gonorrhoeae strain. Immunization with TbpB VRPs without a eukaryotic secretion signal generated no measurable specific antibodies on the genital mucosal surface, but inclusion of a eukaryotic secretion signal or boosting with recombinant protein resulted in specific immunoglobulin G (IgG) and IgA in mucosal secretions after TbpB VRP immunization. The TbpB VRP system has potential for an N. gonorrhoeae vaccine.

Protective efficacy of an oral vaccine to reduce carriage of Borrelia burgdorferi (strain N40) in mouse and tick reservoirs

Lyme disease is caused by the spirochete Borrelia burgdorferi, which is transmitted through the bite of infected Ixodes ticks. Vaccination of mice with outer surface protein A (OspA) of B. burgdorferi has been shown to both protect mice against B. burgdorferi infection and reduce carriage of the organism in feeding ticks. Here we report the development of a murine-targeted OspA vaccine utilizing Vaccinia virus to interrupt transmission of disease in the reservoir hosts, thus reducing incidence of human disease. Oral vaccination of mice with a single dose of Vaccinia expressing OspA resulted in high antibody titers to OspA, 100% protection of vaccinated mice from infection with B. burgdorferi, and significant clearance of B. burgdorferi from infected ticks fed on vaccinated animals. The results indicate the vaccine is effective and may provide a manner to reduce incidence of Lyme disease.

Heterologous gene expression by infectious and replicon vectors derived from tick-borne encephalitis virus and direct comparison of this flavivirus system with an alphavirus replicon

The flavivirus tick-borne encephaltis virus (TBEV) was established as a vector system for heterologous gene expression. The variable region of the genomic 3′ non-coding region was replaced by an expression cassette consisting of the reporter gene enhanced green fluorescent protein (EGFP) under the translational control of an internal ribosomal entry site element, both in the context of an infectious virus genome and of a replicon lacking the genes of the surface proteins prM/M and E. The expression level and the stability of expression were measured by fluorescence-activated cell-sorting analysis and compared to an established alphavirus replicon vector derived from Venezuelan equine encephaltis virus (VEEV), expressing EGFP under the control of its natural subgenomic promoter. On the first day, the alphavirus replicon exhibited an approximately 180-fold higher expression level than the flavivirus replicon, but this difference decreased to about 20- and 10-fold on days 2 and 3, respectively. Four to six days post-transfection, foreign gene expression by the VEEV replicon vanished almost completely, due to extensive cell killing. In contrast, in the case of the TBEV replicon, the percentage of positive cells and the amount of EGFP expression exhibited only a moderate decline over a time period of almost 4 weeks. The infectious TBEV vector expressed less EGFP than the TBEV replicon at all times. Significant expression from the infectious vector was maintained for four cell-culture passages. The results indicate that the VEEV vector is superior with respect to achieving high expression levels, but the TBEV system may be advantageous for applications that require a moderate, but more enduring, gene expression.

Future vaccines against emerging encephalitides

The vaccine strategies available for control of emerging encephalitides range in a continuum from traditional approaches to those utilizing new technologies. In this report, we explore the use of live attenuated vaccines where the attenuating mutations have been selected in a rational way to improve attenuation without sacrificing effectiveness. A strategy for paired lethal and resuscitating mutations is presented that will greatly reduce the possibility of reversion to virulence. Finally, we describe an example of a vaccine vector system that could be rapidly adapted for use against these virus diseases as they emerge.