Protection against lethal simian immunodeficiency virus SIVsmmPBj14 disease by a recombinant Semliki Forest virus gp160 vaccine and by a gp120 subunit vaccine

Comparative analysis of humoral immune responses to HIV type 1 envelope glycoproteins in mice immunized with a DNA vaccine, recombinant Semliki Forest virus RNA, or recombinant Semliki Forest virus particles

The Semliki Forest virus (SFV) system seems to be a useful new approach for generating effective immune responses against HIV-1 in animal models. We evaluated this system by comparing the humoral immune responses raised in mice immunized against the HIV-1 envelope with the SFV system, a DNA vaccine, and a recombinant Env glycoprotein. gp160 ELISA antibody titers (204,800) were highest in the sera from mice immunized with recombinant Semliki Forest virus particles. These sera contained antibodies to the CD4-binding site and recognized linear epitopes on gp120 and gp41 that were also recognized by a pool of sera from HIV1-infected individuals. This demonstrates that the HIV-1 envelope produced in vivo by the SFV system does not fold aberrantly. A low level of neutralizing antibodies against the HIV-1LAI strain was also detected in the serum of one mouse immunized with recombinant SFV particles, suggesting that booster injections should be given to achieve a more effective immune response. SFV recombinant particles induced the strongest humoral responses to the HIV-1 envelope of all the potential HIV env vaccines tested.

DNA-based and alphavirus-vectored immunisation with prM and E proteins elicits long-lived and protective immunity against the flavivirus, Murray Valley encephalitis virus

The immunogenicity and protective efficacy of DNA-based vaccination with plasmids encoding the membrane proteins prM and E of the flavivirus Murray Valley encephalitis virus (MVE) were investigated. Gene gun-mediated intradermal delivery of DNA encoding the prM and E proteins elicited long-lived, virus-neutralising antibody responses in three inbred strains of mice and provided protection from challenge with a high titer inoculum of MVE. Intramuscular DNA vaccination by needle injection also induced MVE-specific antibodies that conferred resistance to challenge with live virus but failed to reduce virus infectivity in vitro. The two routes of DNA-based vaccination with prM and E encoding plasmids resulted in humoral immunty with distinct IgG subtypes. MVE-specific IgG1 antibodies were always prevalent after intradermal DNA vaccination via a gene gun but not detected when mice were immunised with DNA by the intramuscular route or infected with live virus. We also tested a Semliki Forest virus replicon as vector for a flavivirus prM and E protein-based subunit vaccine. Single-cycle infections in mice vaccinated with packaged recombinant replicon particles elicited durable, MVE-specific, and virus-neutralising antibody responses.

Characterization of the Peptide Binding Motif of a Rhesus MHC Class I Molecule (Mamu-A*01) That Binds an Immunodominant CTL Epitope from Simian Immunodeficiency Virus

The majority of immunogenic CTL epitopes bind to MHC class I molecules with high affinity. However, peptides longer or shorter than the optimal epitope rarely bind with high affinity. Therefore, identification of optimal CTL epitopes from pathogens may ultimately be critical for inducing strong CTL responses and developing epitope-based vaccines. The SIV-infected rhesus macaque is an excellent animal model for HIV infection of humans. Although a number of CTL epitopes have been mapped in SIV-infected rhesus macaques, the optimal epitopes have not been well defined, and their anchor residues are unknown. We have now defined the optimal SIV gag CTL epitope restricted by the rhesus MHC class I molecule Mamu-A*01 and defined a general peptide binding motif for this molecule that is characterized by a dominant position 3 anchor (proline). We used peptide elution and sequencing, peptide binding assays, and bulk and clonal CTL assays to demonstrate that the optimal Mamu-A*01-restricted SIV gag CTL epitope was CTPYDINQM181–189. Mamu-A*01 is unique in that it is found at a high frequency in rhesus macaques, and all SIV-infected Mamu-A*01-positive rhesus macaques studied to date develop an immunodominant gag-specific CTL response restricted by this molecule. Identification of the optimal SIV gag CTL epitope will be critical for a variety of studies designed to induce CD8+ CTL responses specific for SIV in the rhesus macaque.

A minimally replicative HIV-2 live-virus vaccine protects M. nemestrina from disease after HIV-2(287) challenge

M. nemestrina immunized with an apathogenic HIV-2 molecular clone (HIV-2KR) were protected from CD4 decline and disease upon challenge with HIV-2(287), after any immunizing virus could be detected. Higher but not lower inocula of HIV-2KR were protective against intravenous inoculation of either 10(5) or 10(1) TCID50 of HIV-2(287). Protected animals displayed substantial reductions in PBMC proviral burden (1-3 logs), viral titers (1-2 logs), and plasma viral RNA (2-4 logs) compared to unprotected or naive animals as early as 1 week postinfection. Plasma viral RNA became undetectable after 24 weeks in protected animals, but remained high in unprotected animals. No viral RNA was present in the spleen of the protected animal necropsied more than a year after challenge (though viral DNA was still present). No neutralizing responses could be demonstrated, but CTL activity was detected sooner and at higher levels after challenge in protected than in unprotected macaques. In this novel HIV-2 vaccine model, protection was clearly dose-dependent, and clearance of challenge virus RNA from the plasma did not require detectable ongoing replication of the immunizing virus at the time of challenge.

Replicon-helper systems from attenuated Venezuelan equine encephalitis virus: expression of heterologous genes in vitro and immunization against heterologous pathogens in vivo

A replicon vaccine vector system was developed from an attenuated strain of Venezuelan equine encephalitis virus (VEE). The replicon RNA consists of the cis-acting 5' and 3' ends of the VEE genome, the complete nonstructural protein gene region, and the subgenomic 26S promoter. The genes encoding the VEE structural proteins were replaced with the influenza virus hemagglutinin (HA) or the Lassa virus nucleocapsid (N) gene, and upon transfection into eukaryotic cells by electroporation, these replicon RNAs directed the efficient, high-level synthesis of the HA or N proteins. For packaging of replicon RNAs into VEE replicon particles (VRP), the VEE capsid and glycoproteins were supplied in trans by expression from helper RNA(s) coelectroporated with the replicon. A number of different helper constructs, expressing the VEE structural proteins from a single or two separate helper RNAs, were derived from attenuated VEE strains Regeneration of infectious virus was not detected when replicons were packaged using a bipartite helper system encoding the VEE capsid protein and glycoproteins on two separate RNAs. Subcutaneous immunization of BALB/c mice with VRP expressing the influenza HA or Lassa virus N gene (HA-VRP or N-VRP, respectively) induced antibody responses to the expressed protein. After two inoculations of HA-VRP, complete protection against intranasal challenge with influenza was observed. Furthermore, sequential immunization of mice with two inoculations of N-VRP prior to two inoculations of HA-VRP induced an immune response to both HA and N equivalent to immunization with either VRP construct alone. Protection against influenza challenge was unaffected by previous N-VRP immunization. Therefore, the VEE replicon system was characterized by high-level expression of heterologous genes in cultured cells, little or no regeneration of plaque-forming virus particles, the capability for sequential immunization to multiple pathogens in the same host, and induction of protective immunity against a mucosal pathogen.

Outcome of immunization of cynomolgus monkeys with recombinant Semliki Forest virus encoding human immunodeficiency virus type 1 envelope protein and challenge with a high dose of SHIV-4 virus

Infection of macaques with chimeric simian-human immunodeficiency viruses (SHIVs) allows evaluation of HIV-1 envelope vaccines. SHIV-4 is based on SIVmac239 but carries the env, tat, and rev genes of HIV-1IIIB. In this study we used Semliki Forest virus (SFV) RNA vectors to express the envelope protein gp160 of HIV-1IIIB in cynomolgus macaques. Monkeys were immunized four times with recombinant suicide SFV. Whereas two of four monkeys showed T cell-proliferative responses, only one monkey had demonstrable levels of antibodies to HIV-1 gp41 and gp120 as shown by enzyme-linked immunosorbent assay (ELISA) and Western blot. The vaccinated monkeys and four control animals were challenged with 10,000 MID100 (100% minimum infectious doses) of cell-free monkey cell-grown SHIV-4 virus. As demonstrated by virus isolation, all macaques became infected after challenge. All vaccinated monkeys showed an HIV-1-specific anamnestic T cell-proliferative response. Three of four vaccines had developed HIV-1-Env-specific antibodies 2 weeks after challenge whereas none of the four controls showed any detectable immune response at this time point. Furthermore, three of four vaccinated monkeys had no demonstrable viral antigenemia and low viral load as opposed to one of the four naive control animals.

Live, attenuated simian immunodeficiency virus vaccines elicit potent resistance against a challenge with a human immunodeficiency virus type 1 chimeric virus

Three rhesus macaques, previously immunized with SIVdelta3 or SIVdelta2, each an attenuated derivative of SIVmac239, and two naive monkeys were challenged with 30,000 50% tissue culture infective doses of SHIV, an SIV/human immunodeficiency virus type 1 (HIV-1) chimeric virus bearing the dual-tropic envelope of HIV-1DH12. By several criteria, including virus isolation, serological assays, and PCR (both DNA and reverse transcriptase), SHIV levels were reduced to barely detectable levels in the circulating blood of vaccinated animals. The resistant SIV-vaccinated macaques had no preexisting neutralizing antibodies directed against SHIV, nor did they produce neutralizing antibodies at any time over a 14-month observation period following SHIV challenge. Interestingly, SIV sequences, derived from the vaccine, could be amplified from numerous tissue samples collected at the conclusion of the experiment, 60 weeks postchallenge, but SHIV-specific sequences (viz., HIV-1 env) could not. These results demonstrate that live attenuated SIV vaccines provide strong long-term protection even against challenge strains with highly divergent envelope sequences.

Studies of AIDS vaccination using an ex vivo feline immunodeficiency virus model: protection conferred by a fixed-cell vaccine against cell-free and cell-associated challenge differs in duration and is not easily boosted

Cats immunized with cells infected with a primary isolate of feline immunodeficiency virus (FIV) and fixed with paraformaldehyde were challenged with cell-free or cell-associated homologous virus obtained ex vivo. Complete protection was observed in animals challenged with cell-free virus 4 months after completion of vaccination (p.v.) or with cell-associated virus 12 months p.v. In contrast, no protection was observed in cats challenged with cell-free virus 12 or 28 months p.v. or with cell-associated virus 37.5 months p.v. Prior to the 28- and 37.5-month challenges, the animals had received a booster dose of vaccine that had elicited a robust anamnestic immune response. These results show that vaccine-induced protection against ex vivo FIV is achievable but is relatively short-lived and can be difficult to boost.

Macrophage-tropic HIV and SIV envelope proteins induce a signal through the CCR5 chemokine receptor

Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) enter target cells by forming a complex between the viral envelope protein and two cell-surface membrane receptors: CD4 and a 7-span transmembrane chemokine receptor. Isolates of HIV that differ in cellular tropism use different subsets of chemokine receptors as entry cofactors: macrophage-tropic HIVs primarily use CCR5, whereas T-cell-tropic and dual-tropic isolates use CXCR4 receptors. HIV-mediated signal transduction through CCR5 is not required for efficient fusion and entry of HIV in vitro. Here we show that recombinant envelope proteins from macrophage-tropic HIV and SIV induce a signal through CCR5 on CD4+ T cells and that envelope-mediated signal transduction through CCR5 induces chemotaxis of T cells. This chemotactic response may contribute to the pathogenesis of HIV in vivo by chemo-attracting activated CD4+ cells to sites of viral replication. HIV-mediated signalling through CCR5 may also enhance viral replication in vivo by increasing the activation state of target cells. Alternatively, envelope-mediated CCR5 signal transduction may influence viral-associated cytopathicity or apoptosis.

Alphaviruses as expression vectors

Alphavirus vectors have been used for efficient high-level expression of a variety of topologically different proteins, allowing studies of protein transport, localization and functional activity in a broad range of host cells. Complex transmembrane proteins have been produced in large quantities through the establishment of scale-up technology. Alphavirus vectors have also shown promising potential in vaccine production and preliminary gene therapy applications.

Infectious particles derived from Semliki Forest virus vectors encoding murine leukemia virus envelopes

Semliki Forest virus vectors encoding murine leukemia virus (MLV) envelope protein with a truncated cytoplasmic tail generate submicrometer, cell-associated, membranous particles that transmit replication-competent vector RNA specifically to cells bearing the MLV receptor. Such "minimal" viruses could have applications as retroviral vaccines or in the study of virus evolution.

Recombinant viruses as vaccines and immunological tools

Recombinant viruses have been investigated as candidate vaccines, and have also been used extensively as immunological tools. Recent advances in this area include the following: the construction and testing of a recombinant simian immunodeficiency virus encoding human interferon-gamma; the development of new vectors such as recombinant poliovirus; and the generation of polyepitope vaccines. Basic immunological research has benefited from the use of recombinant viruses to further understand the role of molecules such as CD40 ligand, nitric oxide and interleukin-4.

Alphavirus expression vectors and their use as recombinant vaccines: a minireview

Alphavirus vectors have become widely used in basic research to study the structure and function of proteins and for protein production purposes. Development of a variety of vectors has made it possible to deliver foreign sequences as naked RNA or DNA, or as suicide virus particles produced using helper vector strategies. Preliminary reports also suggest that these vectors may be useful for in vivo applications where transient, high-level protein expression is desired, such as recombinant vaccines. The initial studies have already shown that alphavirus vaccines can induce strong humoral and cellular immune responses with good immunological memory and protective effects.

Induction of feline immunodeficiency virus specific antibodies in cats with an attenuated Salmonella strain expressing the Gag protein

Salmonella typhimurium aroA strains (SL3261), expressing high levels of the Gag protein of feline immunodeficiency virus (FIV) fused with maltose binding protein (SL3261-MFG), were constructed using an invertible promoter system that allows the stable expression of heterologous antigens at levels toxic for bacteria. A SL3261 strain expressing the B subunit of cholera toxin by a similar system (SL3261-CtxB) served as a control in FIV-immunization experiments. Cats immunized once orally or intraperitoneally with SL3261-MFG or SL3261-CtxB all developed serum antibodies to SL3261 lipopolysaccharide and against maltose binding protein or the B subunit of cholera toxin, respectively. Two intraperitoneal immunizations with SL3261-MFG also resulted in the development of Gag specific serum antibodies. Two oral immunizations with SL3261-MFG primed for a Gag specific response, which was demonstrated upon FIV challenge. All challenged cats became infected and no significant differences in viral loads were found between SL3261-MFG and SL3261-CtxB immunized cats.

Humoral, mucosal, and cellular immunity in response to a human immunodeficiency virus type 1 immunogen expressed by a Venezuelan equine encephalitis virus vaccine vector

A molecularly cloned attenuated strain of Venezuelan equine encephalitis virus (VEE) has been genetically configured as a replication-competent vaccine vector for the expression of heterologous viral proteins (N. L. Davis, K. W. Brown, and R. E. Johnston, J. Virol. 70:3781-3787, 1996). The matrix/capsid (MA/CA) coding domain of human immunodeficiency virus type 1 (HIV-1) was cloned into the VEE vector to determine the ability of a VEE vector to stimulate an anti-HIV immune response in mice. The VEE-MA/CA vector replicated rapidly in the cytoplasm of baby hamster kidney (BHK) cells and expressed large quantities of antigenically identifiable MA/CA protein. When injected subcutaneously into BALB/c mice, the vector invaded and replicated in the draining lymphoid tissues, expressing HIV-1 MA/CA at a site of potent immune activity. Anti-MA/CA immunoglobulin G (IgG) and IgA antibodies were present in serum of all immunized mice, and titers increased after a second booster inoculation. IgA antibodies specific for MA/CA were detected in vaginal washes of mice that received two subcutaneous immunizations. Cytotoxic T-lymphocyte responses specific for MA/CA were detected following immunization with the MA/CA-expressing VEE vector. These findings demonstrate the ability of a VEE-based vaccine vector system to stimulate a comprehensive humoral and cellular immune response. The multifaceted nature of this response makes VEE an attractive vaccine for immunization against virus infections such as HIV-1, for which the correlates of protective immunity remain unclear, but may include multiple components of the immune system.

A model for the maturation of protective antibody responses to SIV envelope proteins in experimentally immunized monkeys

Studies using live attenuated virus vaccines in the simian immunodeficiency virus (SIV) rhesus macaque model have demonstrated broad protection against experimental challenge. Protection in these studies was found to be critically dependent on the length of time postvaccination, suggesting that protective immunity involves a necessary maturation of immune responses. The current study characterizes the evolution of protective envelope-specific antibody responses from monkeys inoculated with the highly attenuated SIV/17E-Cl virus vaccine. For comparison, the same antibody assays were used to characterize the properties of SIV envelope-specific antibodies elicited by inactivated whole virus and envelope subunit vaccines that failed to provide protection from experimental virus challenge. Results of these studies identify a continuous and complex maturation of envelope-specific antibody responses during the first six to eight months postinfection. Furthermore, the time required for maturation of SIV envelope-specific antibodies parallels the time required for the development of protective immunity against experimental challenge with heterologous strains of SIV. While no single immune correlate of protection has been identified, we suggest that a combination of antibody parameters may serve as prognostic indicators in the development of candidate AIDS vaccines.

Mechanisms of protection induced by attenuated simian immunodeficiency virus. IV. Protection against challenge with virus grown in autologous simian cells

Attenuated simian immunodeficiency virus (SIV) induces potent protection against infection with wild-type virus, but the mechanism of this immunity remains obscure. Allogeneic antibodies, which arise within animals as a result of SIV infection, might protect against challenge with exogenous SIV grown in allogeneic cells. To test this hypothesis, eight macaques were infected with attenuated SIV and subsequently challenged with wild-type SIV grown in autologous cells or heterologous cells. The results clearly demonstrated that animals infected with attenuated SIV are protected against wild-type SIV grown in autologous or heterologous cells. Thus, the hypothesis that live attenuated SIV protects by the induction of allogeneic antibodies is not tenable.

Alphavirus-based expression vectors: strategies and applications

Alphaviruses are positive-strand RNA viruses that can mediate efficient cytoplasmic gene expression in insect and vertebrate cells. Through recombinant DNA technology, the alphavirus RNA replication machinery has been engineered for high-level expression of heterologous RNAs and proteins. Amplification of replication-competent alpha-virus RNAs (replicons) can be initiated by RNA or DNA transfection and a variety of packaging systems have been developed for producing high titers of infectious viral particles. Although normally cytocidal for vertebrate cells, variants with adaptive mutations allowing noncytopathic replication have been isolated from persistently infected cultures or selected using a dominant selectable marker. Such mutations have been mapped and used to create new alphavirus vectors for noncytopathic gene expression in mammalian cells. These vectors allow long-term expression at moderate levels and complement previous vectors designed for short-term high-level expression. Besides their use for a growing number of basic research applications, recombinant alphavirus RNA replicons may also facilitate genetic vaccination and transient gene therapy.

Alphaviruses as vectors for gene delivery

Alphavirus vectors have become an important complement to the systems available for transient expression of cloned sequences in cell culture and in vivo. They express foreign sequences from cytoplasmically self-replicating RNA, which can be delivered into the cell either as RNA or DNA. This article outlines some of the properties of alphavirus expression systems, and discusses the pros and cons underlying their present use in basic research and for in vivo applications.