DNA vector constructs that prime hepatitis B surface antigen-specific cytotoxic T lymphocyte and antibody responses in mice after intramuscular injection

Tipping the Proteome with Gene-Based Vaccines: Weighing in on the Role of Nanomaterials

Since the first generation of DNA vaccines was introduced in 1988, remarkable improvements have been made to improve their efficacy and immunogenicity. Although human clinical trials have shown that delivery of DNA vaccines is well tolerated and safe, the potency of these vaccines in humans is somewhat less than optimal. The development of a gene-based vaccine that was effective enough to be approved for clinical use in humans would be one of, if not the most important, advance in vaccines to date. This paper highlights the literature relating to gene-based vaccines, specifically DNA vaccines, and suggests possible approaches to boost their performance. In addition, we explore the idea that combining RNA and nanomaterials may hold the key to successful gene-based vaccines for prevention and treatment of disease.

Quantitation of plasmid DNA deposited on gold particles for particle-mediated epidermal delivery using ICP-MS

DNA-plasmid-based vaccines are a promising class of next generation therapeutics. Particle-mediated epidermal delivery is an attractive method for the administration of DNA plasmid vaccines. This technology utilizes minute quantities of DNA plasmid which have been deposited onto the surface of 2-3-microm gold particles, and so the development of this technology requires the use of analytical methods that can accurately quantitate the amount of the DNA on the particle. Spectroscopic methods are generally insufficient for this task due to interference from the gold particle. ICP-MS circumvents this issue while allowing for the sensitive, reproducible, and accurate determination of the quantity of DNA on the particle surface. This report will detail the development and application of such a method.

The immunogenicity of adenovirus vectors limits the multispecificity of CD8 T-cell responses to vector-encoded transgenic antigens

We investigated whether the immunogenicity of antigens delivered by recombinant E1-deleted adenovirus (Ad) is impaired by the concomitant priming of specific immunity to protein antigens of the vector. A comparative evaluation of the immunogenicity of hepatitis B surface antigen (HBsAg or S) or ovalbumin (OVA) was carried out in mice injected with either the antigen-encoding Ad vector or a corresponding plasmid DNA vaccine. Recombinant Ad, but not the plasmid DNA vaccine, induced long lasting, specific CD8 T-cell immunity to immunodominant epitopes of the two antigens. In contrast, the HBsAg-encoding pCI/S DNA, but not the Ad/S vaccine, was shown to prime CD8 T-cell responses to subdominant HBsAg epitopes. Ad/S-primed CD8 T-cell responses to immunodominant epitopes of vector-encoded or capsid-delivered Ad proteins apparently suppressed CD8 T-cell priming to subdominant HBsAg epitopes. In B-cell-deficient mice, the established, Ad-specific T-cell immunity induced by vaccination with an irrelevant Ad vector impaired the priming of HBsAg-specific CD8 T-cell responses by Ad/S. It is clear, therefore, that a T-cell immunity specific for Ad proteins (either delivered with the Ad capsid or transcribed from the Ad genome) is efficiently primed by vaccination with Ad vectors, and can limit the immunogenicity (particularly of subdominant epitopes) of Ad vector-encoded transgenic antigens.

Nucleic acid immunizations

This unit provides protocols for two broadly used methods of DNA immunization: saline injections and gene gun deliveries of DNA. Saline injections deliver DNA into extracellular spaces; gene gun deliveries bombard DNA directly into cells. Support protocols present methods for preparation of DNA-coated gold beads, creation of cartridges containing these beads, and optimization of gene gun parameters. Issues relating to plasmid vectors, DNA preparation and doses, and immunization regimens are also discussed. Expression library immunizations, genetic and conventional adjuvants, alternative boosts, neonatal immunizations, and approaches to mucosal delivery of DNA are covered in the commentary.

Mouse monoclonal antibodies in biological research: strategies for high-throughput production

Mouse monoclonal antibodies have become key components in basic research as well as in the clinical laboratory. Being invaluable tools in many biological assays, they continue to be the primary choice in the research field, although the conventional technology used for hybridoma generation and screening is a still lengthy, time-consuming and low-throughput process. With the advent of genetic immunisation and the application of automation and microarray to the traditional biological assays, the monoclonal antibody field has been revolutionised. Here, we will briefly review the most relevant strategies which have made the manufacture of murine monoclonal antibodies a faster and high-throughput technology.

Priming Protective CD8 T Cell Immunity by DNA Vaccines Encoding Chimeric, Stress Protein-Capturing Tumor-Associated Antigen

DNA vaccines encoding heat shock protein (hsp)-capturing, chimeric peptides containing antigenic determinants of the tumor-associated Ag (TAA) gp70 (an envelope protein of endogenous retrovirus) primed stable, specific, and tumor-protective CD8 T cell immunity. Expression of gp70 transcripts was detectable in most normal tissues but was particularly striking in some (but not all) tumor cell lines tested (including the adenocarcinoma cell line CT26). An approximately 200 residue gp70 fragment or its L(d)-binding antigenic AH1 peptide cloned in-frame behind an hsp-capturing (cT(272)) or noncapturing (T(60)) N-terminal large SV40 tumor Ag sequence was expressed as either hsp-binding or -nonbinding chimeric Ags. Only hsp-capturing, chimeric fusion proteins were expressed efficiently in transfected cell lines and primed TAA-specific CD8 T cell immunity. This immunity mediated protection in the CT26 and mKSA models. A vaccination strategy based on delivering antigenic, hsp-associated TAA fragments can thus prime protective CD8 T cell immunity even if these TAA are of low intrinsic immunogenicity.

Translation from Cryptic Reading Frames of DNA Vaccines Generates an Extended Repertoire of Immunogenic, MHC Class I-Restricted Epitopes

To test whether simple expression units used in DNA vaccines can generate immunogenic, MHC class I-binding epitopes by translating other than the primary open reading frame (ORF), we constructed a vector (pCI/SX) that encodes the small hepatitis B surface Ag in the primary ORF, and a C-terminal fragment (residue 344-832) of the polymerase (Pol) in an alternative (out-of-frame) reading frame. pCI/SX efficiently primed multispecific, HLA-A2-restricted CD8+ T cell responses to epitopes of hepatitis B surface Ag and of Pol (Pol3, Pol(803-811)). Pol3-containing products generated from pCI/SX were detected only by T cell assays, but not by biochemical assays. Priming Pol-specific T cell responses to epitopes generated from alternative ORFs depended on promoter sequences that drive transcription in the DNA vaccine (human CMV-derived promoter sequences being more efficient than SV40-derived promoter sequences). Human CMV promoter-driven Pol constructs encoding different Pol fragments in primary or alternative reading frames elicited comparable levels of Pol3-specific T cell responses. We confirmed efficient T cell priming to epitopes from alternative ORFs by constructing DNA vaccines that encode an SV40-derived cT(1-272) protein fused either in frame or out of frame with an immunogenic OVA fragment (OVA(18-385)). Similar OVA-specific CD8+ T cell responses were primed by both alternative vaccine constructs. Hence, DNA vaccine-stimulated T cell responses to epitopes generated from alternative ORFs seem to be a regular event, although its biological role and risks are largely unexplored.

Type I IFN negatively regulates CD8+ T cell responses through IL-10-producing CD4+ T regulatory 1 cells

Pleiotropic, immunomodulatory effects of type I IFN on T cell responses are emerging. We used vaccine-induced, antiviral CD8(+) T cell responses in IFN-beta (IFN-beta(-/-))- or type I IFN receptor (IFNAR(-/-))-deficient mice to study immunomodulating effects of type I IFN that are not complicated by the interference of a concomitant virus infection. Compared with normal B6 mice, IFNAR(-/-) or IFN-beta(-/-) mice have normal numbers of CD4(+) and CD8(+) T cells, and CD25(+)FoxP3(+) T regulatory (T(R)) cells in liver and spleen. Twice as many CD8(+) T cells specific for different class I-restricted epitopes develop in IFNAR(-/-) or IFN-beta(-/-) mice than in normal animals after peptide- or DNA-based vaccination. IFN-gamma and TNF-alpha production and clonal expansion of specific CD8(+) T cells from normal and knockout mice are similar. CD25(+)FoxP3(+) T(R) cells down-modulate vaccine-primed CD8(+) T cell responses in normal, IFNAR(-/-), or IFN-beta(-/-) mice to a comparable extent. Low IFN-alpha or IFN-beta doses (500-10(3) U/mouse) down-modulate CD8(+) T cells priming in vivo. IFNAR- and IFN-beta-deficient mice generate 2- to 3-fold lower numbers of IL-10-producing CD4(+) T cells after polyclonal or specific stimulation in vitro or in vivo. CD8(+) T cell responses are thus subjected to negative control by both CD25(+)FoxP3(+) T(R) cells and CD4(+)IL-10(+) T(R1) cells, but only development of the latter T(R) cells depends on type I IFN.

DNA vaccines prime CD8+ T?cell responses to epitopes of viral antigens produced from overlapping reading frames of a single coding sequence

A hepatitis B virus (HBV)-derived sequence that encodes the 832-residue polymerase (Pol) protein of HBV in the primary open reading frame (ORF), and the three (large, middle and small) hepatitis B surface antigen (HBsAg) variants in an alternative ORF was used. This sequence was cloned into expression vectors in which Pol was expressed under heterologous (HCMV, SV40 or metallothionin) promoter control. Some Pol-encoding vectors coexpressed Pol as well as readily detectable amounts of HBsAg. Efficient HBsAg expression depended on endogenous HBV promoter sequences but was apparently also facilitated by heterologous promoter sequences located upstream of the HBV Pol sequence. DNA immunization of mice efficiently coprimed CD8(+) T cell responses to epitopes of Pol and HBsAg. Over expression of Pol (using an hsp73-facilitated expression system) did not correlate with the immunogenicity of the K(d)/Pol(140-148) epitope. Immunodominant L(d)-restricted CD8(+) T cell responses to HBsAg down-modulated priming of CD8(+) T cell responses to other HBsAg epitopes but not to the K(d)/Pol(140-148) epitope. Different antigens transcribed from alternative reading frames of a single sequence in a DNA vaccine can thus efficiently prime multispecific T cell responses.

The efficacy of electroporated plasmid vaccines correlates with long-term antigen production in vivo

We sought to determine whether in vivo electroporation might overcome the requirement for strong transcriptional promoters in plasmid vaccines. We tested plasmid vaccines containing viral promoters that exhibited a broad range of transcriptional activities by in vitro assay. In contrast to the in vitro observations, no difference in gene expression was measured in vivo 24h following injection when the plasmids were introduced via electroporation. Despite the similarities in gene expression in vivo, the cellular and humoral responses elicited by these plasmid vaccines were dramatically different and correlated more closely with the promoter activity measured in vitro. When antigen production in vivo was measured over a longer period following injection, significant differences in gene expression became apparent after 2-3 days and the differences in gene expression at day 7 correlated well with plasmid immunogenicity. These studies reconfirm the importance of antigen production for effective plasmid vaccination and demonstrate that the duration of gene expression should also be considered when designing plasmid vaccines.

Different sources of ?help? facilitate the antibody response to hepatitis D virus ? antigen

Repeated injections of hepatitis D antigen (HDAg) delivered either as a recombinant protein, or expressed from a DNA vaccine elicited no (or only very low) antibody responses in inbred mouse strains. Codelivery of oligonucleotides (ODN) with immune-stimulating sequences (ISS) with the protein antigen, or ISS in DNA vaccines (encoding HDAg) did not overcome the low intrinsic immunogenicity of this small viral antigen for B cells. In contrast, codelivery of immunogenic, heterologous proteins (either mixed to recombinant HDAg as recombinant proteins, or fused to HDAg sequences as chimeric antigens expressed from DNA vaccines) provided specific, CD4+ T cell-dependent "help" that supported efficient priming of antibody responses to HDAg. Chimeric proteins in which selected HDAg fragments were fused in frame with immunogenic, heterologous protein fragments produced by DNA vaccines allowed the mapping of antibody-binding HDAg domains of the viral antigen. The described approach thus facilitates induction of serum antibody responses against native viral antigens with low immunogenicity for B cells.

The hybrid cytomegalovirus enhancer/chicken beta-actin promoter along with woodchuck hepatitis virus posttranscriptional regulatory element enhances the protective efficacy of DNA vaccines

DNA vaccines represent a novel and powerful alternative to conventional vaccine approaches. They are extremely stable and can be produced en masse at low cost; more importantly, DNA vaccines against emerging pathogens or bioterrorism threats can be quickly constructed based solely upon the pathogen's genetic code. The main drawback of DNA vaccines is that they often induce lower immune responses than traditional vaccines, particularly in nonrodent species. Thus, improving the efficacy of DNA vaccines is a critical issue in vaccine development. In this study we have enhanced the efficacy of DNA vaccines by adopting strategies that increase gene expression. We generated influenza-hemagglutinin (HA)-encoding DNA vaccines that contain the hybrid CMV enhancer/chicken beta-actin (CAG) promoter and/or the mRNA-stabilizing post-transcriptional regulatory element from the woodchuck hepatitis virus (WPRE). Mice were immunized with these DNA vaccines, and the influenza-HA-specific cellular and humoral immune responses were compared with a conventional, HA-encoding DNA vaccine whose gene expression was driven by the CMV immediate-early promoter (pCMV-HA). CAG promoter-driven DNA vaccines elicited significantly higher humoral and cellular immune responses compared with the pCMV-HA vaccine. DNA vaccines consisting of both CAG and WPRE elements (pCAG-HA-WPRE) induced the highest level of protective immunity, such that immunization with 10-fold lower DNA doses prevented death in 100% of the mice upon lethal viral challenge, whereas all mice immunized with the conventional pCMV-HA vaccine succumbed to influenza infection.

Novel peptide-based vaccines efficiently prime murine "help"-independent CD8+ T cell responses in the liver

Vaccines for the prophylactic and/or therapeutic immunization against hepatotropic pathogens (e.g., hepatitis B and hepatitis C virus) should establish long-lasting, specific antiviral effector/memory CD8+ T cell immunity in the liver. We describe a novel peptide-based vaccine in which antigenic major histocompatibility complex Class I-binding peptides are fused to a cationic (e.g., human immunodeficiency virus tat-derived) domain and complexed to immune-stimulating oligonucleotides. This vaccine formulation efficiently primes liver-homing, Class I-restricted CD8+ effector/memory T cell responses. In different antigen systems, this formulation was more potent in priming liver-homing CD8+ T cell responses than DNA-based vaccines delivering the same epitopes. CD8+ T cell priming was independent of CD4+ T cell "help" but submitted to regulatory control by CD25+ CD4+ T cells. The vaccine efficiently primed memory/effector CD8+ T cells detectable in the liver for more than 3 months after a single injection. With increasing time after priming, the phenotype of these specific memory CD8+ T cells shifted from an effector memory to a central memory type. The vaccine could override T cell tolerance in mice expressing the relevant antigen from a transgene in the liver. The CD8+ T cell immunity in the liver primed by this peptide formulation could be boosted by challenge injections. In conclusion, we describe a simple and potent vaccine formulation that has the potential to generate or reconstitute specific CD8+ T cell immunity to hepatotropic pathogens in the liver.

Antigenic epitopes fused to cationic peptide bound to oligonucleotides facilitate Toll-like receptor 9-dependent, but CD4+ T cell help-independent, priming of CD8+ T cells

A priority in current vaccine research is the development of adjuvants that support the efficient priming of long-lasting, CD4(+) T cell help-independent CD8(+) T cell immunity. Oligodeoxynucleotides (ODN) with immune-stimulating sequences (ISS) containing CpG motifs facilitate the priming of MHC class I-restricted CD8(+) T cell responses to proteins or peptides. We show that the adjuvant effect of ISS(+) ODN on CD8(+) T cell priming to large, recombinant Ag is enhanced by binding them to short, cationic (arginine-rich) peptides that themselves have no adjuvant activity in CD8(+) T cell priming. Fusing antigenic epitopes to cationic (8- to 10-mer) peptides bound to immune-stimulating ISS(+) ODN or nonstimulating NSS(+) ODN (without CpG-containing sequences) generated immunogens that efficiently primed long-lasting, specific CD8(+) T cell immunity of high magnitude. Different MHC class I-binding epitopes fused to short cationic peptides of different origins showed this adjuvant activity. Quantitative ODN binding to cationic peptides strikingly reduced the toxicity of the latter, suggesting that it improves the safety profile of the adjuvant. CD8(+) T cell priming supported by this adjuvant was Toll-like receptor 9 dependent, but required no CD4(+) T cell help. ODN (with or without CpG-containing sequences) are thus potent Th1-promoting adjuvants when bound to cationic peptides covalently linked to antigenic epitopes, a mode of Ag delivery prevailing in many viral nucleocapsids.

Peptides containing antigenic and cationic domains have enhanced, multivalent immunogenicity when bound to DNA vaccines

We explored strategies to codeliver DNA- and peptide-based vaccines in a way that enhances the immunogenicity of both components of the combination vaccine for T cells. Specific CD8(+) T cell responses to an antigenic peptide are primed when the peptide is fused to a cationic peptide domain that is bound to plasmid DNA or oligonucleotides (ODN; with or without CpG motifs). Plasmid DNA mixed with antigenic/cationic peptides or histones forms large complexes with different biological properties depending on the molar ratios of peptide/protein and polynucleotide. Complexes containing high (but not low) molar ratios of cationic peptide to DNA facilitate transfection (DNA uptake and expression of the plasmid-encoded product) of cells. In contrast, complexes containing low (but not high) molar ratios of cationic peptide to DNA prime potent multispecific T cell responses after a single intramuscular injection of the complexes. The general validity of this observation was confirmed mixing different antigenic/cationic peptides with different DNA vaccines. In these vaccine formulations, multispecific CD8(+) T cell responses specific for epitopes of the peptide- as well as the DNA-based vaccine were efficiently coprimed, together with humoral antibody responses to conformational determinants of large viral antigens encoded by the DNA vaccine. The data indicate that mixtures of DNA vaccines with antigenic, cationic peptides are immunogenic vaccine formulations particularly suited for the induction of multispecific T cell responses.

Specific, functional effector/memory CD8+ T cells are found in the liver post-vaccination

BACKGROUND

The liver efficiently eliminates activated CD8+ T blasts. It is unknown if vaccine-primed CD8+ T blasts migrate to and establish functional CD8+ T cell immunity in the liver post-immunization.

AIMS

We tested, if functional CD8+ T cell populations can be detected in the liver post-vaccination.

METHODS

Murine CD8+ T cells with different epitope/restriction specificities were primed by intramuscular injection of protein- or DNA-based vaccines. The kinetics of appearance in the liver, as well as the surface phenotype and functional competence of intrahepatic, specific CD8+ T cell populations was tested.

RESULTS

High numbers of specific CD8+ T cells appear in the liver after vaccination that are activated (CD69+ CD44+), express effector functions (CD27lo/CD28lo phenotype, interferon gamma secretion, specific cytolytic reactivity), but show no evidence of apoptosis (annexin V-, B220lo, similar numbers/kinetics in primed, congenic lpr/lpr mice). Specific CD8+ T cells from the liver adoptively transferred into a naïve, syngeneic host successfully reconstitute specific CD8+ T cell immunity.

CONCLUSIONS

Specific, functionally competent CD8+ effector/memory T cell populations are established in the liver for months post-vaccination.

Codelivery of a DNA vaccine and a protein vaccine with aluminum phosphate stimulates a potent and multivalent immune response

The study explores the possibility of efficiently codelivering DNA vaccines and protein-based vaccines by formulation with aluminum phosphate (AlPO4). When mixed with aluminum adjuvants, plasmid DNA bound tightly onto aluminum hydroxide [Al(OH)3] but not to AlPO4. Different doses of DNA vaccines formulated with AlPO4 [but not Al(OH)3] induced enhanced humoral responses and supported priming of MHC class I restricted cellular immunity. Different proteins mixed with the plasmid DNA vaccine in the AlPO4 formulation did not impair its immunogenicity. Coinjection of two different vaccine-relevant antigens in the same AlPO4 formulation, one as a DNA vaccine and the other as a recombinant protein, elicited polyvalent, humoral, and cellular immune responses to all antigens delivered. The isotype profiles of the induced humoral responses and the cytokine profiles of the specifically primed T cell responses indicated that the combined vaccines supported copriming of Th1- and Th2-biased as well as balanced responses. These findings indicate that the AlPO4 adjuvant, a widely accepted adjuvant in human vaccination practice, can be used to combine protein- and DNA-based vaccination to prime an enhanced and balanced specific immunity.

Production of antibodies by single DNA immunization: comparison of various immunization routes

DNA immunization is a recent vaccination method that induces humoral and cellular immune responses in a range of hosts. Different immunization routes induce a different degree of the immune response. In the present report, we demonstrate that multiple intramuscular immunizations of plasmid DNA encoding various leukocyte surface molecules induced a specific antibody response. In contrast, a single intramuscular immunization could not induce antibody production. To study the induction of antibody response after a single immunization of plasmid DNA, mice were single-dose intramuscularly, intraperitoneally, intravenously and intrasplenically immunized, simultaneously, with the same preparation of plasmid DNA encoding CD147 membrane protein. We observed that only the intrasplenic route induced specific antibody production. The induction of antibody by intrasplenic immunization was confirmed by using plasmid DNA encoding CD54 molecule. By this single-dose DNA intrasplenic immunization, the generated antibodies could be detected in mice up to 6 months. These results suggest that the injected DNA is expressing the relevant protein antigen in the spleen for several months after injection. Our results demonstrate that direct immunization of antigen-encoding DNA into the spleen is a more effective method for induction of antibody production. This finding may support future investigations of DNA vaccination strategies that specifically promote the uptake of plasmid by splenocytes. Intrasplenic immunization may also be helpful in the understanding of the early events of the immune response to DNA vaccine and be useful as an effective route for the induction of immune responses.

Priming biologically active antibody responses against an isolated, conformational viral epitope by DNA vaccination

The immunodominant, conformational "a" determinant of hepatitis B surface Ag (HBsAg) elicits Ab responses. We selectively expressed the Ab-binding, glycosylated, native a determinant (residue 120-147) of HBsAg in a fusion protein containing C-terminally the HBsAg fragment SII (residue 80-180) fused to a SV40 T-Ag-derived hsp73-binding 77 aa (T(77)) or non-hsp-binding 60 aa (T(60)) N terminus. A DNA vaccine encoding non-hsp-binding secreted T(60)-SII fusion protein-stimulated murine Ab responses with a similar efficacy as a DNA vaccine encoding the secreted, native, small HBsAg. A DNA vaccine encoding hsp73-binding, intracellular T(77)-SII fusion protein-stimulated murine Ab responses less efficiently but comparable to a DNA vaccine encoding the intracellular, native, large HBsAg. HBsAg-specific Abs elicited by either the T(60)-SII-expressing or the T(77)-SII-expressing DNA vaccine suppressed HBsAg antigenemia in transgenic mice that produce HBsAg from a transgene in the liver; hence, a biologically active B cell response cross-reacting with the native, viral envelope epitope was primed by both DNA vaccine constructs. HBsAg-specific Ab and CTL responses were coprimed when an S(20-50) fragment (containing the immunodominant, L(d)-binding epitope S(28-39)) of HBsAg was fused C-terminally to the pCI/T(77)-SII sequence (pCI/T(77)-SII-L(d) DNA vaccine). Chimeric, polyepitope DNA vaccines encoding conformational, Ab-binding epitopes and MHC class I-binding epitopes can thus efficiently deliver antigenic information to different compartments of the immune system in an immunogenic way.

The immunodominant, Ld-restricted T cell response to hepatitis B surface antigen (HBsAg) efficiently suppresses T cell priming to multiple Dd-, Kd-, and Kb-restricted HBsAg epitopes

MHC-I-restricted CTL responses of H-2(d) (L(d+) or L(d-)) and F(1) H-2(dxb) mice to hepatitis B surface Ag (HBsAg) are primed by either DNA vaccines or HBsAg particles. The D(d)/S(201-209) and K(d)/S(199-208) epitopes are generated by processing endogenous HBsAg; the K(b)/S(208-215) epitope is generated by processing exogenous HBsAg; and the L(d)/S(28-39) epitope is generated by exogenous as well as endogenous processing of HBsAg. DNA vaccination primed high numbers of CTL specific for the L(d)/S(28-39) HBsAg epitope, low numbers of CTL specific for the D(d)/S(201-209) or K(d)/S(199-208) HBsAg epitopes in BALB/c mice, and high numbers of D(d)/S(201-209)- and K(d)/S(199-208)-specific CTL in congenic H-2(d)/L(d-) dm2 mice. In F(1)(dxb) mice, the K(d)-, D(d)-, and K(b)-restricted CTL responses to HBsAg were strikingly suppressed in the presence but efficiently elicited in the absence of L(d)/S(28-39)-specific CTL. Once primed, the K(d)- and D(d)-restricted CTL responses to HBsAg were resistant to suppression by immunodominant L(d)/S(28-39)-specific CTL. The L(d)-restricted immunodominant CTL reactivity to HBsAg can thus suppress priming to multiple alternative epitopes of HBsAg, independent of the processing pathway that generates the epitope, of the background of the mouse strain used, and of the presence/absence of different allelic variants of the K and D MHC class I molecules.

Coadministration of gamma interferon with DNA vaccine expressing woodchuck hepatitis virus (WHV) core antigen enhances the specific immune response and protects against WHV infection

DNA vaccinations are able to induce strong cellular immune responses in mice and confer protection against infectious agents. However, DNA vaccination of large animals appears to be less effective and requires repeated injections of large amounts of plasmid DNA. Enhancement of the efficiency of DNA vaccines may be achieved by coapplication of cytokine-expressing plasmids. Here we investigated, with woodchucks, whether coadministration of an expression plasmid for woodchuck gamma interferon (IFN-gamma), pWIFN-gamma, can improve DNA vaccination with woodchuck hepatitis virus core antigen (WHcAg). Animals were immunized with pWHcIm (a plasmid expressing WHcAg) alone or with a combination of pWHcIm and pWIFN-gamma using a gene gun. Six weeks postimmunization, all animals were challenged with 10(5) genome equivalents of woodchuck hepatitis virus (WHV). The antibody and lymphoproliferative immune responses to WHV proteins were determined after immunization and after challenge. Vaccination with pWHcIm and pWIFN-gamma led to a pronounced lymphoproliferative response to WHcAg and protected woodchucks against subsequent virus challenge. Two of three animals vaccinated with pWHcIm alone did not show a detectable lymphoproliferative response to WHcAg. A low-level WHV infection occurred in these woodchucks after challenge, as WHV DNA was detectable in the serum by PCR. None of the pWHcIm-vaccinated animals showed an anti-WHcAg antibody response after DNA vaccination or an anamnestic response after virus challenge. Our results indicate that coadministration of the WIFN-gamma gene with pWHcIm enhanced the specific cellular immune response and improved the protective efficacy of WHV-specific DNA vaccines.

Vaccination with glutamic acid decarboxylase plasmid DNA protects mice from spontaneous autoimmune diabetes and B7/CD28 costimulation circumvents that protection

The nonobese diabetic (NOD) mouse develops spontaneous T-cell-dependent autoimmune diabetes. We tested here whether vaccination of NOD mice with a plasmid DNA encoding glutamic acid decarboxylase (GAD), an initial target islet antigen of autoimmune T cell repertoire, would modulate their diabetes. Our results showed that vaccination of young or old female NOD mice with the GAD-plasmid DNA, but not control-plasmid DNA, effectively prevented their diabetes, demonstrating that GAD-plasmid DNA vaccination is quite effective in abrogating diabetes even after the development of insulitis. The prevention of diabetes did not follow the induction of immunoregulatory Th2 cells but was dependent upon CD28/B7 costimulation. Our results suggest a potential for treating spontaneous autoimmune diabetes via DNA vaccination with plasmids encoding self-Ag.

Revealing the potential of DNA-based vaccination: lessons learned from the hepatitis B virus surface antigen

DNA-based vaccination is a novel technique to efficiently stimulate humoral (antibody) and cellular (T cell) immune responses to protein antigens. In DNA-based vaccination, immunogenic proteins are expressed in in vivo transfected cells of the vaccine recipients in their native conformation with correct posttranslational modifications from antigen-encoding expression plasmid DNA. This ensures the integrity of antibody-defined epitopes and supports the generation of protective (neutralizing) antibody titers. Plasmid DNA vaccination is furthermore an exceptionally potent strategy to stimulate CD8+ cytotoxic T lymphocyte (CTL) responses because antigenic peptides are efficiently generated by endogenous processing of intracellular protein antigens. These key features make DNA-based immunization an attractive strategy for prophylactic and therapeutic vaccination against extra- and intracellular pathogens. In this brief review, we summarize the current state of expression vector design, DNA delivery strategies, priming immune responses to intracellular or secreted antigens by DNA vaccines and unique advantages of DNA- versus recombinant protein-based vaccines using the hepatitis B surface antigen (HBsAg) as a model antigen.

Production of polyclonal and monoclonal antibodies against CD54 molecules by intrasplenic immunization of plasmid DNA encoding CD54 protein

DNA immunization, in theory, is of great interest as a source of specific antibodies against different antigens. In an attempt to produce polyclonal and monoclonal antibodies against cell surface molecules by using the DNA immunization strategy, intramuscular and intrasplenic routes of DNA injection were compared. Two to five, but not a single, intramuscular DNA immunizations induced anti-CD54 and anti-CD147 antibody production. In contrast, a single intrasplenic immunization of CD54-encoding DNA could induce anti-CD54 antibody production. To produce monoclonal antibody (mAb), spleen cells obtained from an intrasplenic CD54-encoding DNA immunized mouse were fused with myeloma cells using the standard hybridoma technique. A hybridoma secreting specific mAb to CD54 was established. The generated mAb reacted to CD54 protein expressed on transfected COS cells and various cell types, the same as using standard CD54 mAb MEM-111. Our results demonstrated that direct immunization of antigen-encoding DNA into spleen is an effective route for production of both polyclonal and monoclonal antibodies to cell surface molecules. This finding is very useful for the production of antibodies to cell surface molecules where the protein antigen is not available or difficult to prepare, but cDNA encoding the corresponding protein is available.

Targeting murine immune responses to selected T cell- or antibody-defined determinants of the hepatitis B surface antigen by plasmid DNA vaccines encoding chimeric antigen

By exchanging sequences from the middle-surface (MS) and small-surface (S) Ag of hepatitis B virus (HBV) with corresponding sequences of the MS Ag of woodchuck hepatitis virus, we constructed chimeric MS variants. Using these constructs as DNA vaccines in mice, we selectively primed highly specific (non-cross-reactive) Ab responses to pre-S2 of the HBV MS Ag and the "a" determinant of the HBV S Ag, as well as L(d)- or K(b)-restricted CTL responses to HBV S epitopes. In transgenic mice that constitutively express large amounts of HBV surface Ag in the liver we could successfully suppress serum antigenemia (but not Ag production in the liver) by adoptive transfer of anti-pre-S2 or anti-"a" immunity but not CTL immunity. DNA vaccines greatly facilitate construction of chimeric fusion Ags that efficiently prime specific, high-affinity Ab and CTL responses. Such vaccines, in which sequences of an Ag of interest are exchanged between different but related viruses, are interesting tools for focusing humoral or cellular immunity on selected antigenic determinants and elucidating their biological role.

Effect of different promoters on immune responses elicited by HIV-1 gag/env multigenic DNA vaccine in Macaca mulatta and Macaca nemestrina

pCMV-NL(Deltapol) and pAKV-NL(Deltapol) expressed human immunodeficiency virus type 1 (HIV-1) gag and env under the regulation of the human cytomegalovirus (CMV) immediate-early (IE) promoter/enhancer and the endogenous AKV murine leukemia viral long terminal repeat (LTR), respectively. Analysis of the immune responses elicited by direct DNA injection of pCMV-NL(Deltapol) and pAKV-NL(Deltapol) in macaques indicated that generation of the humoral and T-cell proliferative responses correlated directly with the promoter strength of the vaccine DNAs. In Macaca mulatta, pCMV-NL(Deltapol) generated stronger humoral responses and T-cell proliferative responses to Gag and Env using less DNA and fewer number of injections than pAKV-NL(Deltapol). Similarly, in Macaca nemestrina pCMV-NL(Deltapol) elicited high humoral responses, which persisted long-term and were boostable. Injection of large amounts of pAKV-NL(Deltapol), in general, failed to produce antibody levels comparable to pCMV-NL(Deltapol). However, injection of a control animal with large amounts of vector DNA produced a generalized enzyme-linked immunosorbent assay (ELISA) reactivity to HIV-1. The results indicated that generation of high immune responses to HIV-1 cannot be achieved by increasing the vaccine DNA dose and may require high protein expression from the DNA by including a strong promoter or by the use of other boosting agents. Furthermore, safety concerns may arise with increasing the DNA dose that could need additional investigation.

Ongoing murine T1 or T2 immune responses to the hepatitis B surface antigen are excluded from the liver that expresses transgene-encoded hepatitis B surface antigen

Different protein- or DNA-based vaccination techniques are available that prime potent humoral and cellular, T1 or T2 immune responses to the hepatitis B surface Ag (HBsAg) in mice. T1 and T2 are immune responses with isotype profile indicating Th1 and Th2 immunoregulation. We tested whether HBsAg-specific immune responses can be established in transgenic mice that express HBsAg in the liver (HBs-tg mice) using either these different vaccination techniques or an adoptive transfer system. HBsAg-specific responses could not be primed in HBs-tg mice with the established, potent vaccine delivery techniques. In contrast, adoptive transfers of T1- and T2-type HBsAg-immune spleen cells into congenic HBs-tg hosts (that were not conditioned by pretreatment) suppressed HBsAg antigenemia and gave rise to HBsAg-specific serum Ab titers. The establishment of continuously rising anti-HBsAg serum Ab levels with alternative isotype profiles (reflecting T1 or T2 polarization) in transplanted HBs-tg hosts required donor CD4+ T cell-dependent restimulation of adoptively transferred immune cells by transgene-derived HBsAg. Injections of HBsAg-specific Abs into HBs-tg mice did not establish stable humoral immunity. The expanding T1 or T2 immune responses to HBsAg in HBs-tg hosts did not suppress transgene-directed HBsAg expression in the liver and did not induce liver injury. In addition to priming functional antiviral effector cells, the conditioning of the liver microenvironment to enable delivery of antiviral effector functions to this organ are therefore critical for effective antiviral defense. A major challenge in the development of a therapeutic vaccine against chronic hepatitis B or C virus infection is thus the efficient targeting of specifically induced immune effector specificities to the liver.

DNA and RNA-based vaccines: principles, progress and prospects

DNA vaccines were introduced less than a decade ago but have already been applied to a wide range of infectious and malignant diseases. Here we review the current understanding of the mechanisms underlying the activities of these new vaccines. We focus on recent strategies designed to enhance their function including the use of immunostimulatory (CpG) sequences, dendritic cells (DC), co-stimulatory molecules and cytokine- and chemokine-adjuvants. Although genetic vaccines have been significantly improved, they may not be sufficiently immunogenic for the therapeutic vaccination of patients with infectious diseases or cancer in clinical trials. One promising approach aimed at dramatically increasing the immunogenicity of genetic vaccines involves making them 'self-replicating'. This can be accomplished by using a gene encoding RNA replicase, a polyprotein derived from alphaviruses, such as Sindbis virus. Replicase-containing RNA vectors are significantly more immunogenic than conventional plasmids, immunizing mice at doses as low as 0.1 microg of nucleic acid injected once intramuscularly. Cells transfected with 'self-replicating' vectors briefly produce large amounts of antigen before undergoing apoptotic death. This death is a likely result of requisite double-stranded (ds) RNA intermediates, which also have been shown to super-activate DC. Thus, the enhanced immunogenicity of 'self-replicating' genetic vaccines may be a result of the production of pro-inflammatory dsRNA, which mimics an RNA-virus infection of host cells.

Alternative pathways for processing exogenous and endogenous antigens that can generate peptides for MHC class I-restricted presentation

The concept of distinct endogenous and exogenous pathways for generating peptides for MHC-I and MHC-II-restricted presentation to CD4+ or CD8+ T cells fits well with the bulk of experimental data. Nevertheless, evidence is emerging for alternative processing pathways that generate peptides for MHC-I-restricted presentation. Using a well characterized, particulate viral antigen of prominent medical importance (the hepatitis B surface antigen), we summarize our evidence that the efficient, endolysosomal processing of exogenous antigens can lead to peptide-loaded MHC-I molecules. In addition, we describe evidence for endolysosomal processing of mutant, stress protein-bound, endogenous antigens that liberate peptides binding to (and presented by) MHC-I molecules. The putative biological role of alternative processing of antigens generating cytotoxic T-lymphocyte-stimulating epitopes is discussed.

Priming MHC-I-Restricted Cytotoxic T Lymphocyte Responses to Exogenous Hepatitis B Surface Antigen Is CD4+ T Cell Dependent

MHC-I (Ld)-restricted, S28–39-specific CTL responses are efficiently primed in H-2d BALB/c mice injected with low doses of native hepatitis B surface Ag (HBsAg) lipoprotein particles without adjuvants. Priming of this CTL response by exogenous HBsAg required CD4+ T cell “help” and IL-12: this CTL response could be neither induced in mice depleted of CD4+ T cells by in vivo Ab treatment, nor in (CD4+ T cell-competent or CD4+ T cell-depleted) IL-12-unresponsive STAT4−/− knockout BALB/c mice. Codelivery of oligonucleotides (ODN) with immunostimulating CpG sequences (ISS) with exogenous HBsAg reconstituted the CTL response to exogenous HBsAg in CD4+ T cell-depleted normal mice and in CD4+ T cell-competent and CD4+ T cell-depleted STAT4−/− BALB/c mice. Injection (by different routes) of “naked” pCI/S plasmid DNA encoding HBsAg into IL-12-responsive or –unresponsive BALB/c mice efficiently primed the MHC-I-restricted, HBsAg-specific CTL response. CTL priming was not detectable when CD4+ T cell-depleted animals were subjected to genetic immunization. In vivo priming of the well-characterized CD8+ CTL response to HBsAg in “high responder” BALB/c mice either by exogenous surface lipoprotein particles or by DNA vaccination is thus CD4+ T cell dependent. CTL priming by exogenous HBsAg, but not by genetic immunization, is IL-12 dependent. The dependence of CTL priming by exogenous HBsAg on CD4+ T cells can be overcome by codelivering ODN with ISS motifs, and this “adjuvants effect” operates efficiently in IL-12-unresponsive mice. The data characterize a feature of the adjuvant effect of ISS-containing ODN on CTL priming that may be of major interest for the design of CTL-stimulating vaccines with efficacy in immunodeficiency conditions.

Truncated or chimeric endogenous protein antigens gain immunogenicity for B cells by stress protein-facilitated expression

Truncated variants of the SV40 large T antigen (T-Ag) with an intact N terminus are as efficiently expressed in eukaryotic transfectants as wild-type (wt) T-Ag. Coprecipitation of N-terminal T-Ag fragments with the constitutively expressed, cytosolic stress protein hsp73 suggests that this chaperone stabilized expression of the truncated T-Ag fragments. In contrast to T-Ag, the 163-residue N-terminal preS domain of the hepatitis B surface antigen (HBsAg) is difficult to express. When the preS domain is C-terminally fused to a hsp73-binding cytoplasmic T-Ag (cT-Ag) fragment its stable expression as a chimeric cT-preS protein is obtained. DNA-based vaccination with plasmid DNA encoding either wt or hsp-associated mutant T-Ag elicited potent MHC class I-restricted, T-Ag-specific T cell responses. In contrast, DNA vaccination with hsp73-binding (mutant or chimeric) T-Ag variants, but not with wt T-Ag elicited T-Ag-specific antibody responses. Furthermore, vaccination with cT-preS-encoding plasmid DNA induced antibodies binding to the preS domain of the large HBsAg. Hence, hsp73-bound endogenous antigens efficiently stimulate antibody responses. These findings may be relevant for tumor immunology and autoimmunity.

DNA vaccines: technology and application as anti-parasite and anti-microbial agents

DNA vaccines have been termed The Third Generation of Vaccines. The recent successful immunization of experimental animals against a range of infectious agents and several tumour models of disease with plasmid DNA testifies to the powerful nature of this revolutionary approach in vaccinology. Among numerous advantages, a major attraction of DNA vaccines over conventional vaccines is that they are able to induce protective cytotoxic T-cell responses as well as helper T-cell and humoral immunity. Here we review the current state of nucleic acid vaccines and cover a wide range of topics including delivery mechanisms, uptake and expression of plasmid DNA, and the types of immune responses generated. Further, we discuss safety issues, and document the use of nucleic acid vaccines against viral, bacterial and parasitic diseases, and cancer. The early potential promise of DNA vaccination has been fully substantiated with recent, exciting developments including the movement from testing DNA vaccines in laboratory models to non-human primates and initial human clinical trials. These advances and the emerging voluminous literature on DNA vaccines highlight the rapid progress that has been made in the DNA immunization field. It will be of considerable interest to see whether the progress and optimism currently prevailing can be maintained, and whether the approach can indeed fulfil the medical and commerical promise anticipated.

Enhancing the immunogenicity of exogenous hepatitis B surface antigen-based vaccines for MHC-I-restricted T cells

Vaccination with either exogenous hepatitis B surface antigen (HBsAg) lipoprotein particles without adjuvants, or plasmid DNA encoding secreted small HBsAg stimulate long-lasting, potent antibody responses in H-2d (BALB/c) and C57Bl/6 (H-2b) mice. Vaccination with exogenous HBsAg primes MHC-I restricted cytotoxic T lymphocyte (CTL) responses to HBsAg in H-2d but not H-2b mice, while DNA vaccination primes HBsAg-specific CTL responses in both mouse strains. We defined vaccination strategies that could elicit CTL responses to exogenous HBsAg in 'low responder' C57Bl/6 mice. We found that the bacterial plasmid DNA itself, synthetic oligodeoxynucleotides containing immunostimulating sequences, or recombinant Th1 cytokines (IL12, IFNgamma) efficiently support priming of CTL responses to exogenous HBsAg in 'low responder' H-2b mice, but have only minor effects on CTL priming in 'high responder' H-2d mice in the high dose range tested. These molecularly well defined adjuvants can thus efficiently support priming of anti-viral T cell responses under 'low responder' conditions.

Vaccination with trypomastigote surface antigen 1-encoding plasmid DNA confers protection against lethal Trypanosoma cruzi infection

DNA vaccination was evaluated with the experimental murine model of Trypanosoma cruzi infection as a means to induce antiparasite protective immunity, and the trypomastigote surface antigen 1 (TSA-1), a target of anti-T. cruzi antibody and major histocompatibility complex (MHC) class I-restricted CD8(+) cytotoxic T-lymphocyte (CTL) responses, was used as the model antigen. Following the intramuscular immunization of H-2(b) and H-2(d) mice with a plasmid DNA encoding an N-terminally truncated TSA-1 lacking or containing the C-terminal nonapeptide tandem repeats, the antibody level, CTL response, and protection against challenge with T. cruzi were assessed. In H-2(b) mice, antiparasite antibodies were induced only by immunization with the DNA construct encoding TSA-1 containing the C-terminal repeats. However, both DNA constructs were efficient in eliciting long-lasting CTL responses against the protective H-2Kb-restricted TSA-1515-522 epitope. In H-2(d) mice, inoculation with either of the two TSA-1-expressing vectors effectively generated antiparasite antibodies and primed CTLs that lysed T. cruzi-infected cells in an antigen-specific, MHC class I-restricted, and CD8(+)-T-cell-dependent manner. When TSA-1 DNA-vaccinated animals were challenged with T. cruzi, 14 of 22 (64%) H-2(b) and 16 of 18 (89%) H-2(d) mice survived the infection. The ability to induce significant murine anti-T. cruzi protective immunity by immunization with plasmid DNA expressing TSA-1 provides the basis for the application of this technology in the design of optimal DNA multicomponent anti-T. cruzi vaccines which may ultimately be used for the prevention or treatment of Chagas' disease.

Modulation of alloimmunity to major histocompatibility complex class I by cotransfer of cytokine genes in vivo

Major histocompatibility complex (MHC) class I antigen is a potent stimulus for alloimmune responses and is the principal immunologic target mediating acute cellular rejection of allografts. Using a method of direct in vivo gene transfer of cDNA encoding donor type MHC class I, we showed in a rat model that recipient muscle could express the transferred MHC class I cDNA, resulting in alloimmunization of the recipient. This was most graphically demonstrated by accelerated rejection of cardiac allografts expressing the same MHC class I as encoded by the immunizing cDNA. We now report the use of the particle-mediated gene transfer via a gene gun (Geneva, Middleton, WI, USA) to transfer MHC class I, as well as cytokine gene expression vectors, into rat skin. Compared to intramuscular injection, gene gun transfer to skin resulted in more efficient immunization. Donor-specific cytotoxic T lymphocyte (CTL) responsiveness and antibody levels increased. Furthermore, coexpression of certain cytokine genes with the MHC class I cDNA modulated the immune response. Specifically, coimmunization with IL-10 cDNA abrogated immunity to allo-MHC class I, while coimmunization with GM-CSF cDNA enhanced it. The influence of expression of these genes in skin was demonstrated by alteration of donor cardiac allograft survival. This model is useful for induction and modulation of alloimmune responses and may be used to develop gene therapy strategies to modify them.

Increased immune response elicited by DNA vaccination with a synthetic gp120 sequence with optimized codon usage

DNA vaccination elicits humoral and cellular immune responses and has been shown to confer protection against several viral, bacterial, and parasitic pathogens. Here we report that optimized codon usage of an injected DNA sequence considerably increases both humoral and cellular immune responses. We recently generated a synthetic human immunodeficiency virus type 1 gp120 sequence in which most wild-type codons were replaced with codons from highly expressed human genes (syngp120). In vitro expression of syngp120 is considerably increased in comparison to that of the respective wild-type sequence. In BALB/c mice, DNA immunization with syngp120 resulted in significantly increased antibody titers and cytotoxic T-lymphocyte reactivity, suggesting a direct correlation between expression levels and the immune response. Moreover, syngp120 is characterized by rev-independent expression and a low risk of recombination with viral sequences. Thus, synthetic genes with optimized codon usage represent a novel strategy to increase the efficacy and safety of DNA vaccination.

Processing of exogenous hepatitis B surface antigen particles for Ld-restricted epitope presentation depends on exogenous beta2-microglobulin

Processing of exogenous hepatitis B surface antigen (HBsAg) particles in an endolysosomal compartment generates peptides that bind to the major histocompatibility complex (MHC) class I molecule Ld and are presented to CD8+ cytotoxic T lymphocytes. Surface-associated 'empty' MHC class I molecules associated neither with peptide, nor with beta2-microglobulin (beta2m) are involved in this alternative processing pathway of exogenous antigen for MHC class I-restricted peptide presentation. Here, we demonstrate that internalization of exogenous beta2m is required for endolysosomal generation of presentation-competent, trimeric Ld molecules in cells pulsed with exogenous HBsAg. These data point to a role of endocytosed exogenous beta2m in the endolysosomal assembly of MHC class I molecules that present peptides from endosomally processed, exogenous antigen.