Kunjin virus replicon vaccine vectors induce protective CD8+ T-cell immunity

Analysis of adaptive mutations in Kunjin virus replicon RNA reveals a novel role for the flavivirus nonstructural protein NS2A in inhibition of beta interferon promoter-driven transcription

The establishment of persistent noncytopathic replication by replicon RNAs of a number of positive-strand RNA viruses usually leads to generation of adaptive mutations in nonstructural genes. Some of these adaptive mutations (e.g., in hepatitis C virus) increase the ability of RNA replication to resist the antiviral action of alpha/beta interferon (IFN-alpha/beta); others (e.g., in Sindbis virus) may also lead to more efficient IFN production. Using puromycin-selectable Kunjin virus (KUN) replicon RNA, we identified two adaptive mutations in the NS2A gene (producing Ala30-to-Pro and Asn101-to-Asp mutations in the gene product; for simplicity, these will be referred to hereafter as Ala30-to-Pro and Asn101-to-Asp mutations) that, when introduced individually or together into the original wild-type (wt) replicon RNA, resulted in approximately 15- to 50-fold more efficient establishment of persistent replication in hamster (BHK21) and human (HEK293 and HEp-2) cell lines. Transfection with a reporter plasmid carrying the luciferase gene under the control of the IFN-beta promoter resulted in approximately 6- to 7-fold-higher luciferase expression in HEp-2 cells stably expressing KUN replicon RNA with an Ala30-to-Pro mutation in the NS2A gene compared to that observed in HEp-2 cells stably expressing KUN replicon RNA with the wt NS2A gene. Moreover, cotransfection of plasmids expressing individual wt or Ala30-to-Pro-mutated NS2A genes with the IFN-beta promoter reporter plasmid, followed by infection with Semliki Forest virus to activate IFN-beta promoter-driven transcription, showed approximately 7-fold inhibition of luciferase expression by the wt but not by the Ala30-to-Pro-mutated NS2A protein. The results show for the first time a role for the flavivirus nonstructural protein NS2A in inhibition of IFN-beta promoter-driven transcription and identify a single-amino-acid mutation in NS2A that dramatically reduces this inhibitory activity. The findings determine a new function for NS2A in virus-host interactions, extend the range of KUN replicon vectors for noncytopathic gene expression, and identify NS2A as a new target for attenuation in the development of live flavivirus vaccines.

Heat shock protein 10 inhibits lipopolysaccharide-induced inflammatory mediator production

Heat shock protein 10 (Hsp10) and heat shock protein 60 (Hsp60) were originally described as essential mitochondrial proteins involved in protein folding. However, both proteins have also been shown to have a number of extracellular immunomodulatory activities. Here we show that purified recombinant human Hsp10 incubated with cells in vitro reduced lipopolysaccharide (LPS)-induced nuclear factor-kappaB activation and secretion of several inflammatory mediators from RAW264.7 cells, murine macrophages, and human peripheral blood mononuclear cells. Induction of tolerance by contaminating LPS was formally excluded as being responsible for Hsp10 activity. Treatment of mice with Hsp10 before endotoxin challenge resulted in the reduction of serum tumor necrosis factor-alpha and RANTES (regulated upon activation, normal T cell expressed and secreted) levels and an elevation of serum interleukin-10 levels. Hsp10 treatment also delayed mortality in a murine graft-versus-host disease model, where gut-derived LPS contributes to pathology. We were unable to confirm previous reports that Hsp10 has tumor growth factor properties and suggest that Hsp10 exerts anti-inflammatory activity by inhibiting Toll-like receptor signaling possibly by interacting with extracellular Hsp60.

A totally synthetic vaccine of generic structure that targets Toll-like receptor 2 on dendritic cells and promotes antibody or cytotoxic T cell responses

A simple generic peptide-based vaccine structure that targets Toll-like receptor 2-expressing dendritic cells and causes their activation is described. The vaccines are totally synthetic, serve as their own adjuvant, and are composed of (i) a single helper T cell epitope, (ii) a target epitope that is either recognized by CD8+ T cells or B cells, and (iii) a Toll-like receptor 2-targeting lipid moiety, S-[2,3-bis(palmitoyloxy)propyl]cysteine, that is situated between the peptide epitopes to form a branched configuration. The different CD8+ T cell epitopes examined were from (i) influenza virus, (ii) the intracellular bacterium Listeria monocytogenes, and (iii) ovalbumin as a model tumor antigen. Vaccines containing a B cell epitope from gastrin or luteinizing hormone-releasing hormone as a B cell epitope were also examined for their ability to elicit antibody against the parent hormones. Each of the vaccines was capable of inducing either CD8+ T cell or antibody-mediated immune responses. The lipidated vaccines, but not the nonlipidated vaccines, were able to mediate protection against viral or bacterial infection and mediate prophylactic and therapeutic anticancer activity. The two hormone-based vaccines induced high antibody titers, which in the case of luteinizing hormone-releasing hormone resulted in abrogation of reproductive function. These results highlight the utility of simple, totally synthetic, epitope-based vaccines.

ISCOM based vaccines for cancer immunotherapy

Immunostimulating complex (ISCOM) vaccines are particulate antigen delivery vehicles composed of saponin, cholesterol, phospholipid and immunogen. Here we illustrate that ISCOM-based vaccines represent an attractive modality for the development of anti-cancer vaccines. Using murine models and a model cancer antigen, ISCOM vaccines were shown to induce potent CD8 T cell responses, to mediate protection in three different tumor models, to promote Th1-biased immunity, and to induce CD8 T cell responses in the absence of CD4+ T cell help. The former three activities were also found to be substantially improved when the vaccine antigen was associated with the ISCOM structure. Furthermore, the presence in vivo of pre-existing antibodies against the vaccine antigen did not inhibit CD8 T cell induction by the ISCOM vaccine. Although vaccination was effective against challenge with vaccine-antigen expressing tumors, no activity against neighboring vaccine-antigen negative tumor cells was observed, indicating that determinant spreading or bystander activity does not lead to significant anti-cancer activity.

Recombinant Kunjin virus replicon vaccines induce protective T-cell immunity against human papillomavirus 16 E7-expressing tumour

The persistence of the E7 oncoprotein in transformed cells in human papillomavirus (HPV)-associated cervical cancer provides a tumour-specific antigen to which immunotherapeutic strategies may be directed. Self-replicating RNA (replicon) vaccine vectors derived from the flavivirus Kunjin (KUN) have recently been reported to induce T-cell immunity. Here, we report that inclusion of a CTL epitope of HPV16 E7 protein into a polyepitope encoded by a KUN vector induced E7-directed T-cell responses and protected mice against challenge with an E7-expressing epithelial tumour. We found replicon RNA packaged into virus-like particles to be more effective than naked replicon RNA or plasmid DNA constructed to allow replicon RNA transcription in vivo. Protective immunity was induced although the E7 CTL epitope was subdominant in the context of other CTL epitopes in the polyepitope. The results demonstrate the efficacy of the KUN replicon vector system for inducing protective immunity directed towards a virally encoded human tumour-specific antigen, and for inducing multi-epitopic CTL responses.

Kunjin RNA replication and applications of Kunjin replicons

The Kunjin virus (KUNV) has provided a useful laboratory model for Flavivirus RNA replication. The synthesis of progeny RNA(+) strands occurs via asymmetric and semiconservative replication on a template of recycling double-stranded RNA (dsRna) or replicative form (RF). Kinetics of viral RNA synthesis indicated a cycle period of about 15 min during which, on average, a single nascent RNA (+) strand displaces the pre-existing RNA(+) strand in the replicative intermediate. Data on the composition of the replication complex (RC) in KUNV-infected cells were obtained from several sources, including analyses of the partially-purified still active RC, immunogold labeling of cryosections using monospecific antibodies to the nonstructural proteins and to the dsRNA, radioimmunoprecipitations of cell lysates using antibodies to dsRNA and to an RC-associated cell marker, and pull-down assays of cell lysates using fusion proteins GST-NS2A and GST-NS4A. These results yeilded a consensus composition of NS1, NS2A, NS3, NS4A, and NS5 strongly associated with the dsRNA template. The RC was located in induced membranes described as vesicle packets. The RNA-dependent RNA polymerase activity late in infection did not require continuing protein synthesis. Replication of genomic RNA was completely dependent on the presence of conserved complementary or cyclization sequences near the 5' and 3' ends. Assembly of the RC during translation in cis and the relationships, particularly those of NS1 and NS5 among the components, were deduced from an extensive set of complementation experiments in trans involving mutations/deletions in all the nonstructural proteins and use of KUN or alphahavirus replicons as helpers. The KUN replicon has found useful applications also as a noncytopathic vector for the continuing expression of foreign genes, delivered either as packaged RNA or as plasmid DNA.

Mimicking live flavivirus immunization with a noninfectious RNA vaccine

Flaviviruses are human pathogens of world-wide medical importance. They have recently received much additional attention because of their spread to new regions (such as West Nile virus to North America), highlighting their potential as newly emerging disease agents. Using tick-borne encephalitis virus, we have developed and evaluated in mice a new genetic vaccine based on self-replicating but noninfectious RNA. This RNA contains all of the necessary genetic information for establishing its replication machinery in the host cell, thus mimicking a natural infection. However, genetic modifications in the region encoding the capsid protein simultaneously prevent the assembly of infectious virus particles and promote the secretion of noninfectious subviral particles that elicit neutralizing antibodies. These characteristics demonstrate that a new generation of flavivirus vaccines can be designed that stimulate the same spectrum of innate and specific immune responses as a live vaccine but have the safety features of an inactivated vaccine.

Tetracycline-inducible packaging cell line for production of flavivirus replicon particles

We have previously developed replicon vectors derived from the Australian flavivirus Kunjin that have a unique noncytopathic nature and have been shown to direct prolonged high-level expression of encoded heterologous genes in vitro and in vivo and to induce strong and long-lasting immune responses to encoded immunogens in mice. To facilitate further applications of these vectors in the form of virus-like particles (VLPs), we have now generated a stable BHK packaging cell line, tetKUNCprME, carrying a Kunjin structural gene cassette under the control of a tetracycline-inducible promoter. Withdrawal of tetracycline from the medium resulted in production of Kunjin structural proteins that were capable of packaging transfected and self-amplified Kunjin replicon RNA into the secreted VLPs at titers of up to 1.6 x 10(9) VLPs per ml. Furthermore, secreted KUN replicon VLPs from tetKUNCprME cells could be harvested continuously for as long as 10 days after RNA transfection, producing a total yield of more than 10(10) VLPs per 10(6) transfected cells. Passaging of VLPs on Vero cells or intracerebral injection into 2- to 4-day-old suckling mice illustrated the complete absence of any infectious Kunjin virus. tetKUNCprME cells were also capable of packaging replicon RNA from closely and distantly related flaviviruses, West Nile virus and dengue virus type 2, respectively. The utility of high-titer KUN replicon VLPs was demonstrated by showing increasing CD8(+)-T-cell responses to encoded foreign protein with increasing doses of KUN VLPs. A single dose of 2.5 x 10(7) VLPs carrying the human respiratory syncytial virus M2 gene induced 1,400 CD8 T cells per 10(6) splenocytes in an ex vivo gamma interferon enzyme-linked immunospot assay. The packaging cell line thus represents a significant advance in the development of the noncytopathic Kunjin virus replicon-based gene expression system and may be widely applicable to the basic studies of flavivirus RNA packaging and virus assembly as well as to the development of gene expression systems based on replicons from different flaviviruses.

DNA vaccine coding for the full-length infectious Kunjin virus RNA protects mice against the New York strain of West Nile virus

A plasmid DNA directing transcription of the infectious full-length RNA genome of Kunjin (KUN) virus in vivo from a mammalian expression promoter was used to vaccinate mice intramuscularly. The KUN viral cDNA encoded in the plasmid contained the mutation in the NS1 protein (Pro-250 to Leu) previously shown to attenuate KUN virus in weanling mice. KUN virus was isolated from the blood of immunized mice 3-4 days after DNA inoculation, demonstrating that infectious RNA was being transcribed in vivo; however, no symptoms of virus-induced disease were observed. By 19 days postimmunization, neutralizing antibody was detected in the serum of immunized animals. On challenge with lethal doses of the virulent New York strain of West Nile (WN) or wild-type KUN virus intracerebrally or intraperitoneally, mice immunized with as little as 0.1-1 microg of KUN plasmid DNA were solidly protected against disease. This finding correlated with neutralization data in vitro showing that serum from KUN DNA-immunized mice neutralized KUN and WN viruses with similar efficiencies. The results demonstrate that delivery of an attenuated but replicating KUN virus via a plasmid DNA vector may provide an effective vaccination strategy against virulent strains of WN virus.

DNA vaccination: the answer to stable, protective T-cell memory?

Distinct populations of both CD4(+) and CD8(+) memory T cells have been identified on the basis of their location (lymphoid versus non-lymphoid tissues) and function. There have been several recent advances in our understanding of the factors that govern the generation and maintenance of T-cell memory in vivo. Utilising this knowledge could contribute to the refinement of DNA vaccine technology to induce more robust and longer-lasting protective T-cell based immunity.

Kunjin virus replicon vectors for human immunodeficiency virus vaccine development

We have previously demonstrated the ability of the vaccine vectors based on replicon RNA of the Australian flavivirus Kunjin (KUN) to induce protective antiviral and anticancer CD8+ T-cell responses using murine polyepitope as a model immunogen (I. Anraku, T. J. Harvey, R. Linedale, J. Gardner, D. Harrich, A. Suhrbier, and A. A. Khromykh, J. Virol. 76:3791-3799, 2002). Here we showed that immunization of BALB/c mice with KUN replicons encoding HIV-1 Gag antigen resulted in induction of both Gag-specific antibody and protective Gag-specific CD8+ T-cell responses. Two immunizations with KUNgag replicons in the form of virus-like particles (VLPs) induced anti-Gag antibodies with titers of > or =1:10,000. Immunization with KUNgag replicons delivered as plasmid DNA, naked RNA, or VLPs induced potent Gag-specific CD8+ T-cell responses, with one immunization of KUNgag VLPs inducing 4.5-fold-more CD8+ T cells than the number induced after immunization with recombinant vaccinia virus carrying the gag gene (rVVgag). Two immunizations with KUNgag VLPs also provided significant protection against challenge with rVVgag. Importantly, KUN replicon VLP vaccinations induced long-lasting immune responses with CD8+ T cells able to secrete gamma interferon and to mediate protection 6 to 10 months after immunization. These results illustrate the potential value of the KUN replicon vectors for human immunodeficiency virus vaccine design.

Traditional and novel approaches to flavivirus vaccines

Yellow fever, dengue, Japanese encephalitis and tick-borne encephalitis viruses are the medically most important members of the Flavivirus genus composed primarily of arboviruses. In this paper, we review the commercially available traditional flavivirus vaccines against yellow fever, Japanese encephalitis, and tick-borne encephalitis, as well as modern approaches to flavivirus vaccines. Formalin inactivation technology has been employed to produce killed vaccines. Flaviviruses have been attenuated by multiple passages in animal tissues and cell cultures to produce empirical live attenuated vaccines. The use of traditional methods is being pursued to develop vaccines against other flavivirus diseases, such as dengue, and to improve existing vaccines, such as for Japanese encephalitis. With the recent development of infectious clones, rational approaches to attenuated flavivirus vaccines have employed the introduction of specific mutations into wild type viruses and chimerisation between different viruses. Novel methods for delivery of live vaccines, such as inoculation of infectious DNA or RNA, have been described. Other approaches, such as the construction of protein subunit, expression vector-based and naked DNA vaccines, have been proposed to create alternate vaccine candidates.

Generation of CTL responses using Kunjin replicon RNA

The Kunjin replicon was used to express a polytope that consisted of seven hepatitis C virus cytotoxic T lymphocyte epitopes and one influenza cytotoxic T lymphocyte epitope for vaccination studies. The self-replicating nature of, and expression from, the ribonucleic acid was confirmed in vitro. Initial vaccinations with one dose of Kun-Poly ribonucleic acid showed that an influenza-specific cytotoxic T lymphocyte response was elicited more efficiently by intradermal inoculation compared with intramuscular delivery. Two micrograms of ribonucleic acid delivered in the ear pinnae of mice was sufficient to elicit a detectable cytotoxic T lymphocyte response 10 days post-vaccination. Further vaccination studies showed that four of the seven hepatitis C virus cytotoxic T lymphocyte epitopes were able to elicit weak cytotoxic T lymphocyte responses whereas the influenza epitope was able to elicit strong, specific cytotoxic T lymphocyte responses following three doses of Kun-Poly ribonucleic acid. These studies vindicate the use of the Kunjin replicon as a vector to deliver encoded proteins for the development of cell-mediated immune responses.