AttenuatedSalmonellaandShigellaas Carriers for DNA Vaccines

The discovery that genes can be functionally transferred from bacteria to mammalian cells has suggested the possible use of bacterial vectors as gene delivery vehicles for vaccines. Attenuated invasive human intestinal bacteria, such as Salmonella and Shigella, have been used as plasmid DNA vaccine carriers and their potency has been evaluated in several animal models. This delivery system allows the administration of DNA vaccines together with associated bacterial immunostimulators directly to professional antigen presenting cells via human mucosal surfaces. Various strategies have been taken to improve the use of this delivery system to achieve robust immune responses at both mucosal and systemic sites of the immunized animals.

Mechanisms of action of DNA vaccines

The field of DNA vaccines can trace its inception to two papers which demonstrated that administration of plasmid DNA vectors expressing proteins resulted in expression in situ. Thereafter, the possible application of this technique to vaccine development was demonstrated through the induction of antibody responses in mice against a foreign protein, cellular immune responses against a viral antigen and protective efficacy in an infectious disease challenge model. Subsequently, the general utility of DNA vaccines in animal models of infectious and non-infectious disease has been established (for review, see [5]). Initially, most efforts were directed toward demonstration of effectiveness in particular disease models. Recently, however, more attention has been paid to gaining a better understanding of some of the underlying mechanisms of DNA vaccines. This review will focus on this new information and discuss it in the context of how it could benefit the development of more effective DNA vaccines.

Priming of CTL responses by DNA vaccines: direct transfection of antigen presenting cells versus cross-priming

DNA vaccines can induce cytotoxic T lymphocyte (CTL) responses in various species including mice, non-human primates and humans. It is now well established that antigen presenting cells (APCs) are required for induction of these responses. However, it is not yet known whether this is a function of antigen expression within or acquisition of antigen by these cells, or a combination of both. Cross-priming has been demonstrated to occur from cells (including muscle cells) to APCs in vivo. In addition, there is evidence that APCs can be transfected after DNA vaccination. Hence, efforts to facilitate cross-priming and to increase transfection of APCs will be important for increasing the potency of DNA vaccines.

Induction of potent immune responses by cationic microparticles with adsorbed human immunodeficiency virus DNA vaccines

The effectiveness of cationic microparticles with adsorbed DNA at inducing immune responses was investigated in mice, guinea pigs, and rhesus macaques. Plasmid DNA vaccines encoding human immunodeficiency virus (HIV) Gag and Env adsorbed onto the surface of cationic poly(lactide-coglycolide) (PLG) microparticles were shown to be substantially more potent than corresponding naked DNA vaccines. In mice immunized with HIV gag DNA, adsorption onto PLG increased CD8(+) T-cell and antibody responses by approximately 100- and approximately 1,000-fold, respectively. In guinea pigs immunized with HIV env DNA adsorbed onto PLG, antibody responses showed a more rapid onset and achieved markedly higher enzyme-linked immunosorbent assay and neutralizing titers than in animals immunized with naked DNA. Further enhancement of antibody responses was observed in animals vaccinated with PLG/DNA microparticles formulated with aluminum phosphate. The magnitude of anti-Env antibody responses induced by PLG/DNA particles was equivalent to that induced by recombinant gp120 protein formulated with a strong adjuvant, MF-59. In guinea pigs immunized with a combination vaccine containing HIV env and HIV gag DNA plasmids on PLG microparticles, substantially superior antibody responses were induced against both components, as measured by onset, duration, and titer. Furthermore, PLG formulation overcame an apparent hyporesponsiveness of the env DNA component in the combination vaccine. Finally, preliminary data in rhesus macaques demonstrated a substantial enhancement of immune responses afforded by PLG/DNA. Therefore, formulation of DNA vaccines by adsorption onto PLG microparticles is a powerful means of increasing vaccine potency.

Tuberculosis DNA vaccines

DNA vaccines have been the subject of intense investigation for the past 10 y, during which time several tuberculosis (TB) DNA vaccines have been shown to confer protective immunity in animal models. So far, proof of principle for priming of immune responses by a naked DNA vaccine (malaria) has been demonstrated in humans, but potency remains a significant limitation. However, new DNA vaccine formulations and delivery systems are being developed with markedly improved potency in animal models. Therefore, there is a clear path to the human clinical testing of TB DNA vaccines.

Relative potency of cellular and humoral immune responses induced by DNA vaccination

DNA vaccines can prime broad-based immune responses in small animal models. In the present study, we sought to evaluate the relative ability of DNA vaccines to induce humoral and cellular immune responses. Using a DNA vaccine encoding HIV gag in mice, we observed that CD8+ T cell responses were primed more readily than were antibody responses, particularly at low doses of DNA. These CD8+ T cell responses were detected in spleen cells, as well as at local sites such as the lung and draining lymph nodes. The potency of the HIV gag DNA vaccine used was sufficient to prime strong CTL responses in macaques, but only low to undetectable antibody responses. Therefore, DNA vaccines appear able to prime strong, broad CTL but only modest antibody responses. These results may have implications on the development of vaccines against infectious diseases where both CTL and antibody responses are desired, such as HIV.

An update on the state of the art of DNA vaccines

DNA vaccines have been extensively studied in the past ten years and much is now known about their effectiveness and mode of action in animal models. Several DNA vaccines have been tested in phase I clinical trials, with mixed results. That is, DNA vaccines appear safe and well tolerated, but lack potency. This has led to the search for technologies that will enable sufficient potency for effectiveness in humans.

Plasmid DNA adsorbed onto cationic microparticles mediates target gene expression and antigen presentation by dendritic cells

Dendritic cells (DC) play a key role in antigen presentation and activation of specific immunity. Much current research focuses on harnessing the potency of DC for vaccines, gene therapy, and cancer immunotherapy applications. However, DC are not readily transfected in vitro by traditional nonviral techniques. A novel DNA vaccine formulation was used to determine if DC are transfected in vitro. The formulation consists of plasmid DNA adsorbed on to cationic microparticles composed of the biodegradable polymer polylactide-co-glycolide (PLG) and the cationic surfactant, cetyltrimethylammonium bromide (CTAB). Using preparations of fluorescent-labeled plasmid DNA formulated on PLG-CTAB microparticles to study internalization by macrophages and dendritic cells in vitro and in vivo, we found that most, but not all, of the fluorescence was concentrated in endosomal compartments. Furthermore, uptake of plasmid DNA encoding HIV p55 gag adsorbed to PLG-CTAB microparticles by murine bone marrow-derived dendritic cells resulted in target gene expression, as detected by RT-PCR. The antigen was subsequently processed and presented, resulting in stimulation of an H-2kd-restricted, gag-specific T cell hybridoma. Activation of the hybridoma, detected by IL-2 production, was dose-dependent in the range of 0.1-20 microg DNA (10-2000 microg PLG) and was sustained up to 5 days after transfection. Thus, adsorption of plasmid DNA on PLG-CTAB microparticles provides a potentially useful nonviral approach for in vitro transfection of dendritic cells. Gene Therapy (2000) 7, 2105-2112.

Antigen presentation and DNA vaccines

There is reasonable evidence that both cross-priming and direct transfection of antigen-presenting cells (APCs) play a role in induction of immune responses by DNA vaccines. It is not known which mode is more important for priming cytotoxic T cell responses, but both are sufficient and neither alone is necessary. Hence, a rational strategy for increasing DNA vaccine potency would be to facilitate both pathways. With regard to cross-priming, a better understanding of the nature of the antigen transferred and the molecules/cells involved may suggest ways to design DNA vaccines to enhance this pathway. With respect to transfection of APCs, certain DNA formulations or delivery systems may be able to target APCs for increased DNA uptake. Other considerations include recruitment of APCs to the site of DNA injection and manipulation of these cells to ensure the proper activation state for priming immune responses. The burgeoning scientific literature in these areas indicates that much effort is currently being directed toward these goals.

Distribution of DNA vaccines determines their immunogenicity after intramuscular injection in mice

Intramuscular injection of DNA vaccines elicits potent humoral and cellular immune responses in mice. However, DNA vaccines are less efficient in larger animal models and humans. To gain a better understanding of the factors limiting the efficacy of DNA vaccines, we used fluorescence-labeled plasmid DNA in mice to 1) define the macroscopic and microscopic distribution of DNA after injection into the tibialis anterior muscle, 2) characterize cellular uptake and expression of DNA in muscle and draining lymph nodes, and 3) determine the effect of modifying DNA distribution and cellular uptake by volume changes or electroporation on the magnitude of the immune response. Injection of a standard 50-microl dose resulted in the rapid dispersion of labeled DNA throughout the muscle. DNA was internalized within 5 min by muscle cells near the injection site and over several hours by cells that were located along muscle fibers and in the draining lymph nodes. Histochemical staining and analysis of mRNA expression in isolated cells by RT-PCR showed that the transgene was detectably expressed only by muscle cells, despite substantial DNA uptake by non-muscle cells. Reduction of the injection volume to 5 microl resulted in substantially less uptake and expression of DNA by muscle cells, and correspondingly lower immune responses against the transgene product. However, expression and immunogenicity were restored when the 5-microl injection was followed by electroporation in vivo. These findings indicate that distribution and cellular uptake significantly affect the immunogenicity of DNA vaccines.

Delivery systems for gene-based vaccines

Along with the elucidation of the role of cytotoxic T lymphocytes in the immune responses against a number of pathogens and cancer, and with the increased understanding of the cellular processing mechanisms of antigens for generation of these cells, has come an increased focus on vaccines that can generate cellular immunity along with antibodies. Promising approaches based on the delivery of genes, either as plasmid DNA or by viral vectors, have been extensively evaluated pre-clinically and in early-phase clinical trials. Although the first generation of DNA plasmid vaccines were broadly effective in animal disease models, early clinical immunogenicity pointed towards the need for increased potency. This manuscript reviews recent developments for gene-based vaccines, specifically, new approaches for formulating and delivering plasmid DNA and alphaviral replicon vectors, all of which have resulted in increased potency of gene-based vaccines.

Priming of hepatitis C virus-specific cytotoxic T lymphocytes in mice following portal vein injection of a liver-specific plasmid DNA

The immunology of hepatitis C virus (HCV) infection should be studied in the context of HCV antigen expression in the liver, because HCV primarily infects this organ. Indeed, the nature, function, and fate of T cells primed after antigen expression in the liver might differ from those primed when antigens are expressed systemically or in other organs, because the nature of the antigen-presenting cells (APCs) involved may be different. In addition, the normal liver contains a resident population of lymphocytes that differ from those present at other sites. Thus, we investigated whether HCV-specific CD8(+) cytotoxic T cells (CTLs) could be elicited following portal vein (PV) injection of plasmid DNA in mice whose hepatic veins were transiently occluded. We show that PV injection of mice with "naked" DNA expressing the HCV-NS5a protein, under the control of a liver-specific enhancer/promoter, resulted in NS5a expression in the liver and the priming of HCV-specific CTLs. These results suggested that such a model might be relevant to the study of HCV-specific immune responses primed during natural infection.

Increased DNA vaccine delivery and immunogenicity by electroporation in vivo

DNA vaccines have been demonstrated to be potent in small animals but are less effective in primates. One limiting factor may be inefficient uptake of DNA by cells in situ. In this study, we evaluated whether cellular uptake of DNA was a significant barrier to efficient transfection in vivo and subsequent induction of immune responses. For this purpose, we used the technique of electroporation to facilitate DNA delivery in vivo. This technology was shown to substantially increase delivery of DNA to cells, resulting in increased expression and elevated immune responses. The potency of a weakly immunogenic hepatitis B surface Ag DNA vaccine was increased in mice, as seen by a more rapid onset and higher magnitude of anti-hepatitis B Abs. In addition, the immunogenicity of a potent HIV gag DNA vaccine was increased in mice, as seen by higher Ab titers, a substantial reduction in the dose of DNA required to induce an Ab response, and an increase in CD8+ T cell responses. Finally, Ab responses were enhanced by electroporation against both components of a combination HIV gag and env DNA vaccine in guinea pigs and rabbits. Therefore, cellular uptake of DNA is a significant barrier to transfection in vivo, and electroporation appears able to overcome this barrier.

Immunogenicity and protective efficacy of DNA vaccines encoding secreted and non-secreted forms of Mycobacterium tuberculosis Ag85A

OBJECTIVE

To determine the efficacy of Ag85A-DNA against challenge with a highly virulent human clinical isolate of Mycobacterium tuberculosis (CSU37) and to compare the potencies of two types of Ag85A-DNA vaccines; those expressing secreted and non-secreted forms of the protein.

DESIGN

Ag85A-DNA vaccinated mice were challenged with a highly virulent clinical isolate of M. tuberculosis (CSU37) in order to compare the efficacy of these vaccines. In vitro studies were also performed.

RESULTS

Enhanced humoral and cellular responses were induced in mice vaccinated with the secreted Ag85A-DNA compared to the non-secreted Ag85A-DNA. In addition, secreted Ag85A-DNA conferred protective immunity against infection with M. tuberculosis (CSU37).

CONCLUSIONS

DNA vaccines encoding M. tuberculosis Ag85A have been shown to induce potent humoral and cellular immune responses leading to protection from M. tuberculosis (Erdman) challenge in mouse models. In this study we demonstrate that Ag85A can confer protection in a rigorous challenge model using a highly virulent human clinical isolate of M. tuberculosis (CSU37). This challenge model appears able to discriminate between DNA vaccines of differing potencies, as the more immunogenic DNA construct encoding a secreted form of Ag85A was protective, whereas the less immunogenic DNA construct encoding a non-secreted form of Ag85A was not.

Quantification of the number of cytotoxic T cells specific for an immunodominant HCV-specific CTL epitope primed by DNA immunization

Priming of strong cellular immune responses to hepatitis C (HCV) is thought to be important for eradication of infection. Although productive infection of HCV occurs only reproducibly in humans and chimpanzees, definition of HCV-specific T cell epitopes in mice is necessary to screen efficiently HCV vaccine strategies for their ability to prime cellular immune responses. Out of seven strains of mice screened for immunodominant CTL epitopes against HCV-1a Core, E2, NS5a and NS5b, only one epitope (p214K9) in only one mouse strain was identified. Enumeration of p214K9-specific CD8+ cells by flow cytometry revealed that the number of epitope specific CTL primed by 'naked' DNA immunization was lower than that reported during viral infection. The p214K9 epitope described here, combined with analysis of CTL responses by flow cytometry, should be instrumental in ranking various HCV vaccine strategies for their ability to prime CTL responses.

Enhanced type I immune response to a hepatitis B DNA vaccine by formulation with calcium- or aluminum phosphate

DNA vaccines induce protective humoral and cell-mediated immune responses in several animal models. When compared with conventional vaccines, however, DNA vaccines often induce lower antibody titers. We have now found that formulation of a DNA vaccine encoding hepatitis B surface antigen with calcium- or aluminum phosphate adjuvants can increase antibody titers by 10-100-fold and decrease the immunogenic dose of DNA by 10-fold. Furthermore, boosting an HBs protein-primed response with the adjuvanted DNA vaccine resulted in a dramatic increase in the HBs-specific IgG2a response reflecting a shift towards a TH1 response. The mechanism by which aluminum phosphate exerts its adjuvant effect is not through increased expression of HBsAg in vivo; rather, the adjuvant appears to increase the number and affinity of HBs peptide antigen-specific IFN-gamma and IL-2 secreting T cells.

Enhancement of DNA vaccine potency using conventional aluminum adjuvants

The immunogenicity and protective efficacy of DNA vaccines have been amply demonstrated in numerous animal models of infectious disease. However, the feasibility of DNA vaccines for human use is not yet known. In order to investigate potential means of increasing the potency of DNA vaccines, conventional adjuvants such as aluminum salts were tested. Coadministration of these adjuvants with DNA vaccines substantially enhanced the ability of these vaccines to induce antibody responses up to 100-fold in mice and guinea pigs, and 5-10-fold in non-human primates. Effective formulations had no demonstrable effect on the levels of antigen expression in situ and consisted of adjuvants that did not form complexes with the plasmid DNA; rather they exerted their effects on antigen after expression in situ. Therefore, the potency of DNA vaccines both in laboratory rodents and in non-human primates can be substantially increased by simple formulation with conventional aluminum adjuvants.

Dose dependence of CTL precursor frequency induced by a DNA vaccine and correlation with protective immunity against influenza virus challenge

Intramuscular injection of BALB/c mice with a DNA plasmid encoding nucleoprotein (NP) from influenza virus A/PR/8/34 (H1N1) provides cross-strain protection against lethal challenge with influenza virus A/HK/68 (H3N2). CTL specific for the H-2Kd-restricted epitope NP147-155 are present in these mice and are thought to play a role in the protection. To assess the effectiveness of NP DNA immunization in comparison with influenza virus infection in the induction of CTL responses, we monitored the frequency of CTL precursors (CTLp) in mice following i.m. injection with NP DNA or intranasal infection with influenza virus and showed that the CTLp frequency in NP DNA-immunized mice can reach levels found in mice that had been infected with influenza virus. We also measured the CTLp frequency, anti-NP Ab titers, and T cell proliferative responses in mice that were injected with titrated dosages of NP DNA and documented a correlation of the CTLp frequency and the Ab titers, but not proliferative responses, with the injection dose. Furthermore, we observed a positive correlation between the frequency of NP147-155 epitope-specific CTLp and the extent of protective immunity against cross-strain influenza challenge induced by NP DNA injection. Collectively, these results and our early observations from adoptive transfer experiments of in vitro activated lymphocytes from NP DNA-immunized mice suggest a protective function of NP-specific CTLp in mice against cross-strain influenza virus challenge.

DNA vaccines against tuberculosis

DNA plasmids encoding Mycobacterium tuberculosis antigen 85 (Ag85) were tested as vaccines in animal models. Ag85 DNA induced relevant immune responses (i.e. T helper (Th) cells, Th1 cytokines and cytotoxic T lymphocytes) and was protective in mouse and guinea pig models of mycobacterial disease. Therefore, DNA vaccination holds promise as an effective means of preventing tuberculosis in humans. Furthermore, this technique is amenable to identifying the protective antigens of M. tuberculosis.

DNA vaccines for viral diseases

DNA plasmids encoding foreign proteins may be used as immunogens by direct intramuscular injection alone, or with various adjuvants and excipients, or by delivery of DNA-coated gold particles to the epidermis through biolistic immunization. Antibody, helper T lymphocyte, and cytotoxic T lymphocyte (CTL) responses have been induced in laboratory and domesticated animals by these methods. In a number of animal models, immune responses induced by DNA vaccination have been shown to be protective against challenge with various infectious agents. Immunization by injection of plasmids encoding foreign proteins has been used successfully as a research tool. This review summarizes the types of DNA vaccine vectors in common use, the immune responses and protective responses that have been obtained in animal models, the safety considerations pertinent to the evaluation of DNA vaccines in humans and the very limited information that is available from early clinical studies.

DNA vaccines

Immunization with plasmid DNA encoding antigenic proteins elicits both antibody and cell-mediated immune responses. This method of producing the protein antigens of interest directly in host cells can provide appropriate tertiary structure for the induction of conformationally specific antibodies, and also facilitates the induction of cellular immune responses. DNA immunization has provided effective protective immunity in various animal models. The immune responses induced by DNA vaccines may in some instances be preferable to those produced by immunization using conventional methods. DNA vaccination appears to be applicable to a variety of pathogens and is a useful method of raising immune responses. Thus this approach to vaccination has the potential to be a successful method of rapidly screening for antigens capable of inducing protective immunity, and of inducing protective immunity against pathogens of clinical importance.

Immunogenicity and efficacy of DNA vaccines encoding influenza A proteins in aged mice

Influenza is a leading cause of morbidity and mortality in older persons. The current influenza vaccine is only modestly successful, in part because of an age-related decline in immunogenicity and also because it induces only type-specified immunity. To overcome this, we evaluated DNA vaccines encoding A/PR8/34 haemagglutinin (HA) and nucleoprotein (NP) in young and aged BALB/c mice. Control mice were given formalin-inactivated A/PR8/34, control DNA, or a non-lethal dose of PR8. Aged mice given HA DNA developed slightly lower anti-HA serum antibodies than young mice; however, both young and aged mice were protected from a homotypic PR8 challenge. Following vaccination with NP DNA, both young and aged mice developed anti-NP bulk cytotoxic T-lymphocyte (CTL) activity and pCTL frequency similar to control animals. When challenged with a low dose of A/HK/68 (H3N2) influenza virus, both young mice and aged mice showed significant protection as measured by inhibition of weight loss. When challenged with a relatively high dose of A/HR/68 (H3N2) influenza virus, however, the anti-NP vaccine only partially protected young mice and failed to protect aged mice. These data demonstrate that DNA-based vaccines are immunogenic in aged animals, but suggest that factors other than the age-related decline in CTL activity also contribute to the increased morbidity and mortality of influenza in the elderly.

Induction of MHC class I-restricted CTL response by DNA immunization with ubiquitin-influenza virus nucleoprotein fusion antigens

DNA vaccines have been shown to be an effective means of inducing cytotoxic T-lymphocyte (CTL) responses in both young and aged mice. Better understanding of the pathways by which antigens encoded by DNA vaccines are processed and presented to CTL may allow for improvements in CTL responses in older animals. Since CTL recognize short peptides presented by MHC class I molecules, and since ubiquitin-dependent proteolysis is widely believed to be responsible for degradation of endogenously synthesized antigens and generation of these peptide ligands, we sought to use ubiquitin (Ub) conjugation to target influenza virus nucleoprotein (NP) antigen into the Ub-proteasome degradation pathway for MHC class I-restricted antigen processing and presentation. However, the addition of the Ub moiety did not affect the half-life of Ub-NP protein in transiently transfected human rhabdomyosarcoma (RD) cells. Moreover, the modifications of NP DNA vaccine with Ub conjugation did not affect their ability to induce a CTL response specific for the H-2Kd-restricted NP147-155 epitope, as assessed by both percent cytolysis in bulk CTL culture and by CTL precursor (CTLp) frequency in limiting dilution analysis (LDA). In contrast, the anti-NP antibody (Ab) responses were dramatically reduced in mice immunized with low doses (1 microgram) of Ub-NP constructs, compared with mice immunized with wild-type NP DNA. These results demonstrate that Ub conjugation alone does not guarantee targeting of endogenously synthesized antigens for rapid degradation by proteasomes. Furthermore, the ability of ubiquintination to reduce Ab responses to NP without affecting CTL responses suggests that the Ub modifications result in a lower availability of full-length NP from transfected cells in vivo. The implications of these data on antigen presentation and cross-priming are discussed.

Protective CD4+ and CD8+ T cells against influenza virus induced by vaccination with nucleoprotein DNA

DNA vaccination is an effective means of eliciting both humoral and cellular immunity, including cytotoxic T lymphocytes (CTL). Using an influenza virus model, we previously demonstrated that injection of DNA encoding influenza virus nucleoprotein (NP) induced major histocompatibility complex class I-restricted CTL and cross-strain protection from lethal virus challenge in mice (J. B. Ulmer et al., Science 259:1745-1749, 1993). In the present study, we have characterized in more detail the cellular immune responses induced by NP DNA, which included robust lymphoproliferation and Th1-type cytokine secretion (high levels of gamma interferon and interleukin-2 [IL-2], with little IL-4 or IL-10) in response to antigen-specific restimulation of splenocytes in vitro. These responses were mediated by CD4+ T cells, as shown by in vitro depletion of T-cell subsets. Taken together, these results indicate that immunization with NP DNA primes both cytolytic CD8+ T cells and cytokine-secreting CD4+ T cells. Further, we demonstrate by adoptive transfer and in vivo depletion of T-cell subsets that both of these types of T cells act as effectors in protective immunity against influenza virus challenge conferred by NP DNA.

Evaluation of new vaccines in the mouse and guinea pig model of tuberculosis

The results of this study provide the first evidence that two completely separate vaccine approaches, one based on a subunit vaccine consisting of a mild adjuvant admixed with purified culture filtrate proteins and enhanced by the cytokine interleukin-2 and the second based on immunization with DNA encoding the Ag85A protein secreted by Mycobacterium tuberculosis, could both prevent the onset of caseating disease, which is the hallmark of the guinea pig aerogenic infection model. In both cases, however, the survival of vaccinated guinea pigs was shorter than that conferred by Mycobacterium bovis BCG, with observed mortality of these animals probably due to consolidation of lung tissues by lymphocytic granulomas. An additional characteristic of these approaches was that neither induced skin test reactivity to commercial tuberculin. These data thus provide optimism that development of nonliving vaccines which can generate long-lived immunity approaching that conferred by the BCG vaccine is a feasible goal.

Vaccination with plasmid DNA encoding mycobacterial antigen 85A stimulates a CD4+ and CD8+ T-cell epitopic repertoire broader than that stimulated by Mycobacterium tuberculosis H37Rv infection

Vaccination of mice with plasmid DNA carrying the gene for the major secreted mycobacterial antigen 85A (Ag85A) from Mycobacterium tuberculosis is a powerful technique for generating robust specific Thl helper T-cell responses, CD8+-mediated cytotoxicity, and protection against M. tuberculosis challenge (K. Huygen et al., Nat. Med. 2:893-898, 1996). We have now analyzed in more detail the antigen-specific immune CD4+- and CD8+-T-cell responses induced in BALB/c mice vaccinated with Ag85A DNA and have compared these responses to those generated by intravenous infection with M. tuberculosis. T-cell-epitope mapping, as measured by interleukin-2 and gamma interferon secretion from splenic T cells restimulated in vitro with synthetic 20-mer peptides spanning the complete mature sequence of Ag85A, demonstrated that DNA vaccination stimulated a stronger and broader T-cell response than did M. tuberculosis infection. Moreover, elevated cytotoxic T lymphocyte (CTL) activity against Ag85A-transfected and peptide-pulsed P815 target cells could be generated exclusively by vaccination with plasmid DNA, not following M. tuberculosis infection. By using DNA vaccination, three Ag85A CTL epitopes with predicted major histocompatibility complex class I binding motifs were defined. One of them was previously reported as a dominant, promiscuously recognized T-cell epitope in healthy humans with primary infections. These data strengthen the potential of DNA vaccination with respect to inducing antituberculous immunity in humans.

DNA vaccines

In just a few years, injection of plasmid DNA to elicit immune responses in vivo has developed from an interesting observation to a viable vaccine strategy. DNA vaccines have been shown to elicit both cellular and humoral immune responses and to be effective in a variety of preclinical bacterial, viral, and parasitic animal models. This review will discuss the current knowledge of vector design, methods of plasmid delivery, immune responses elicited by various DNA vaccines, safety issues, and production and release of plasmid as a vaccine product. The potential of this new vaccine strategy and its future prospects is summarized.

Further protection against antigenic drift of influenza virus in a ferret model by DNA vaccination

Previously we showed that immunization of ferrets with DNA encoding the hemagglutinin (HA), nucleoprotein (NP), and matrix protein (M1) of influenza virus induced protective immune responses. A DNA vaccine encoding HA (from a 1991 strain), NP and M1 (from a 1989 strain) protected ferrets better against challenge with the antigenic drift variant A/Georgia/03/93 than did the inactivated vaccine from the 1992-93 influenza season. Here we report that the same DNA vaccine protected ferrets against a second, further divergent, drift variant (A/Johannesburg/33/94). Furthermore, the extent of protection provided by the DNA vaccine was equivalent to the homologous protection provided by an inactivated vaccine that exactly matched the challenge strain.

Immunization of non-human primates with DNA vaccines

The pre-clinical efficacy of DNA vaccines has been demonstrated in a number of animal models, but more limited data exist regarding their immunogenicity in non-human primates. The studies described below demonstrate that DNA vaccines in reasonable dosages encoding a variety of viral proteins could result in the generation of antibodies, neutralizing antibodies, or cytotoxic T lymphocytes in primates. Furthermore, these responses could be boosted by repeat administration of the DNA vaccine. In an effort to assess the safety of such vaccines sera from primates was shown to lack anti-DNA antibodies.

Priming of cytotoxic T lymphocytes by DNA vaccines: requirement for professional antigen presenting cells and evidence for antigen transfer from myocytes

BACKGROUND

MHC class I molecule-restricted cytotoxic T-lymphocyte (CTL) responses are induced following either intramuscular (i.m.) injection of a DNA plasmid encoding influenza virus nucleoprotein (NP) or transplantation of myoblasts stably transfected with the NP gene, the latter indicating that synthesis of NP by myocytes in vivo is sufficient to induce CTL. The present study was designed to investigate the role of muscle cells and involvement of professional antigen-presenting cells (APCs) in priming CTL responses following DNA vaccination.

MATERIALS AND METHODS

Parent-->F1 bone marrow (BM) chimeric mice were generated whose somatic cells include muscle cells bearing both parental MHC haplotypes, while their professional APCs express only the donor MHC haplotypes.

RESULTS AND CONCLUSIONS

Upon injection of NP DNA, or after infection with influenza virus, CTL responses generated in the chimeras were restricted to the donor MHC haplotype. Thus cells of BM lineage were definitively shown to be responsible for priming such CTL responses after infection or DNA immunization. Moreover, expression of antigen by muscle cells in BM chimeric mice after myoblast transplantation is sufficient to induce CTL restricted only by the MHC haplotype of the donor BM. This indicates that transfer of antigen from myocytes to professional APCs can occur, thus obviating a requirement for direct transfection of BM-derived cells.

Expression of a viral protein by muscle cells in vivo induces protective cell-mediated immunity

Intramuscular injection of plasmid DNA expression vectors results in transfection of myocytes in situ. To determine whether expression of antigen by myocytes is sufficient to induce protective cell-mediated immunity, stably transfected myoblasts expressing influenza nucleoprotein (NP) were transplanted into mice. These animals produced high-titer anti-NP antibodies and MHC class I-restricted cytotoxic T lymphocytes, and were protected from a cross-strain lethal challenge with influenza A virus. Therefore, antigen expression by muscle cells in vivo is sufficient to confer protective cell-mediated immunity.

Expression and immunogenicity of Mycobacterium tuberculosis antigen 85 by DNA vaccination

Plasmid DNA expression vectors encoding Mycobacterium tuberculosis antigen 85 (Ag85) were tested as vaccines in preclinical animal models. Expression of secreted and nonsecreted forms of Ag85 was observed after transient transfection of cells in vitro. In mice, both types of Ag85 DNA constructs induced strong humoral and cell-mediated immune responses, as measured by ELISA of sera and recall responses of spleen cells restimulated in vitro, respectively, Therefore, DNA vaccination is an effective means of expressing mycobacterial proteins in eukaryotic cells leading to the induction of potent immune responses.

Immunogenicity and efficacy of a tuberculosis DNA vaccine encoding the components of the secreted antigen 85 complex

BALB/c and C57BL/6 mice were injected intramuscularly with plasmid DNA encoding the three components of the immunodominant 30-32 kDa antigen 85 complex (Ag85A, Ag85B, and Ag85C) from Mycobacterium tuberculosis culture filtrate, in order to investigate the utility of nucleic acid vaccination for induction of immune responses against mycobacterial antigens. Ag85A and Ag85B encoding plasmids induced a robust Th1-like response towards native Ag85, characterized by elevated levels of interleukin (IL)-2, interferon-gamma, and TNF-alpha. Levels of IL-4, IL-6, and IL-10 were low or undetectable. Plasmid encoding Ag85C was not effective. Cytotoxic T cell activity was also generated in in vitro restimulated splenocyte cultures from Ag85A and Ag85B DNA vaccinated mice. Finally, Ag85A and Ag85B DNA vaccination conferred significant protection against mycobacterial replication in lungs from B6 mice, subsequently challenged. Therefore, this technique may be useful for the definition of protective antigens of M. tuberculosis and the development of a more effective tuberculosis vaccine.

Induction of humoral and cellular immune responses by vaccination with M. tuberculosis antigen 85 DNA

DNA vaccines have been demonstrated to be effective in inducing protective cell-mediated immune responses in animal models of infectious disease. In order to investigate this approach for potential use as a vaccine for tuberculosis, DNA constructs encoding Mycobacterium tuberculosis antigen 85A (Ag85A) were prepared. Expression of Ag85A in mammalian cells was demonstrated by transient transfection of cells in vitro. Intramuscular injection of Ag85A DNA vaccines resulted in the generation of anti-Ag85A antibodies and robust cell-mediated immune responses, as measured by lymphoproliferation of spleen cells in vitro upon specific antigen restimulation, leading to protection in animal challenge models. Therefore, the technique of DNA vaccination is effective in inducing relevant immune responses for protection against tuberculosis and may be used to identify the protective antigens of M. tuberculosis.

Protective cellular immunity: cytotoxic T-lymphocyte responses against dominant and recessive epitopes of influenza virus nucleoprotein induced by DNA immunization

DNA immunization offers a novel means to induce cellular immunity in a population with a heterogeneous genetic background. An immunorecessive cytotoxic T-lymphocyte (CTL) epitope in influenza virus nucleoprotein (NP), residues 218 to 226, was identified when mice were immunized with a plasmid DNA encoding a full-length mutant NP in which the anchor residues for the immunodominant NP147-155 epitope were altered. Mice immunized with wild-type or mutant NP DNA were protected from lethal cross-strain virus challenge, and the protection could be adoptively transferred by immune splenocytes, indicating the role of cell-mediated immunity in the protection. DNA immunization is capable of eliciting protective cellular immunity against both immunodominant and immunorecessive CTL epitopes in the hierarchy seen with virus infection.

Induction of immunity by DNA vaccination: application to influenza and tuberculosis

DNA vaccination is an effective means of inducing both humoral and cell-mediated immunity in animal models of infectious disease. Presented here are data generated in two distinct disease models; one viral (influenza) and one bacterial (tuberculosis). Specifically, plasmid DNA encoding an influenza virus antigen (nucleoprotein; NP) and a Mycobacterium tuberculosis antigen (antigen 85; Ag85) were prepared and tested as DNA vaccines in mice. In both cases, high titer antibody responses and robust cell-mediated immune responses were induced against the respective antigens. With respect to the latter, lymphocyte proliferation, Th1-type cytokine secretion, and cytotoxic T lymphocyte responses were observed upon restimulation with antigen in vitro. Furthermore, protective efficacy in animal challenge models was demonstrated in both systems. The data support the hypothesis that DNA vaccination will prove to be a broadly applicable technique for inducing immunity against various infectious diseases.

DNA vaccines

Observations in the early 1990s that plasmid DNA could directly transfect animal cells in vivo sparked exploration of the use of DNA plasmids to induce immune responses by direct injection into animals of DNA encoding antigenic proteins. This method, termed DNA immunization, now has been used to elicit protective antibody and cell-mediated immune responses in a wide variety of preclinical animal models for viral, bacterial, and parasitic diseases. DNA vaccination is particularly useful for the induction of cytotoxic T cells. This review summarizes current knowledge on the vectors, immune responses, immunological mechanisms, safety considerations, and potential for further application of this novel method of immunization.

Characterization of humoral immune responses induced by an influenza hemagglutinin DNA vaccine

We have examined in detail the characteristics of the humoral immune response and protective efficacy induced by an influenza hemagglutinin (HA) DNA vaccine. In mice injected intramuscularly with HA DNA, the magnitude of the immune responses generated, as measured by ELISA and hemagglutination inhibiting (HI) antibodies, was directly related to the amount of DNA injected and the number of doses administered. The level of anti-HA antibodies in DNA-vaccinated mice was higher than that in convalescent immune mice and was maintained for at least 1.5 years. The immunoglobulin isotype profile of the antibodies was predominantly IgG2a, similar to that induced by live virus infection but in contrast to the relative abundance of IgG1 antibodies observed after inoculation with formalin-inactivated whole virus. The presence of pre-challenge HI antibodies was found to be a good correlate of protection, in that every animal with a detectable HI titer was protected from a lethal challenge. Complete protection from a lethal dose of influenza virus (A/PR/34), as judged by 100% survival and no weight loss, was conferred by as little as 1 microgram of DNA (given twice). Furthermore, mice injected with 10 to 100 micrograms doses, when subsequently challenged with virus, showed no increase in HI titer and no production of antibodies directed against the challenge virus, suggesting a substantial inhibition of virus replication after challenge.

DNA vaccines

Immunization with plasmid DNA encoding antigenic proteins elicits both antibody and cell-mediated immune responses. This method of producing the protein antigens of interest directly in host cells can provide appropriate tertiary structure for the induction of conformationally specific antibodies, and also facilitates the induction of cellular immune responses. DNA immunization has provided effective protective immunity in various animal models. The immune responses induced by DNA vaccines may in some instances be preferable to those produced by immunization using conventional methods. DNA vaccination appears to be applicable to a variety of pathogens and is a useful method of raising immune responses. Thus this approach to vaccination has the potential to be a successful method of rapidly screening for antigens capable of inducing protective immunity, and of inducing protective immunity against pathogens of clinical importance.

Toward the development of DNA vaccines

DNA vaccination has proved to be a generally applicable technology in various preclinical animal models of infectious and noninfectious disease and several DNA vaccines have now entered phase I human clinical trials. It is too early to predict the effectiveness of DNA vaccines in humans and whether improved formulations of DNA vaccines will be required but several lines of investigation have suggested ways in which DNA vaccines may be improved, such as increases in expression, facilitation of DNA targeting or uptake, and enhancement of immune responses.

Generation of MHC class I-restricted cytotoxic T lymphocytes by expression of a viral protein in muscle cells: antigen presentation by non-muscle cells

Expression of reporter genes in muscle cells has been achieved by intramuscular (i.m.) injection of plasmid DNA expression vectors. We previously demonstrated that this technique is an effective means of immunization to elicit both antibodies capable of conferring homologous protection and cell-mediated immunity leading to cross-strain protection against influenza virus challenge in mice. These results suggested that expression of viral proteins by muscle cells can result in the generation of cellular immune responses, including cytotoxic T lymphocytes (CTL). However, because DNA has the potential to be internalized and expressed by other cell types, we sought to determine whether or not induction of CTL required synthesis of antigen in non-muscle cells and if not whether transfer of antigen to antigen-presenting cells from muscle cells may be involved. In the present study we demonstrate that transplantation of nucleoprotein (NP)-transfected myoblasts into syngeneic mice led to the generation of NP-specific antibodies and CTL and cross-strain protective immunity against a lethal challenge with influenza virus. Furthermore transplantation of NP-expressing myoblasts (H-2k) intraperitoneally into F1 hybrid mice (H-2d x H-2k) elicited NPCTL restricted by the MHC haplotype of both parental strains. These results indicate that NP expression by muscle cells after transplantation was sufficient to generate protective cell-mediated immunity and that induction of the CTL response was mediated at least in part, by transfer of antigen from the transplanted muscle cells to a host cell.

DNA vaccines

Preclinical DNA vaccine development has continued apace during the past year, with the investigation of several new infectious and non-infectious disease targets as well as advances in our understanding of some of the basic immunologic mechanisms, such as effector cells, responsible for conferring protection. The coming year promises to be at least as exciting, as initial human clinical studies have begun.

Immunogenicity and protective efficacy of a tuberculosis DNA vaccine

Tuberculosis is the most widespread and lethal infectious disease affecting humans. Immunization of mice with plasmid DNA constructs encoding one of the secreted components of Mycobacterium tuberculosis, antigen 85 (Ag85), induced substantial humoral and cell-mediated immune responses and conferred significant protection against challenge with live M. tuberculosis and M. bovis bacille Calmette-Guérin (BCG). These results indicate that immunization with DNA encoding a mycobacterial antigen provides an efficient and simple method for generating protective immunity and that this technique may be useful for defining the protective antigens of M. tuberculosis, leading to the development of a more effective vaccine.