Humoral and cellular immunities elicited by HIV-1 vaccination

Intranasal delivery of cationic PLGA nano/microparticles-loaded FMDV DNA vaccine encoding IL-6 elicited protective immunity against FMDV challenge

Mucosal vaccination has been demonstrated to be an effective means of eliciting protective immunity against aerosol infections of foot and mouth disease virus (FMDV) and various approaches have been used to improve mucosal response to this pathogen. In this study, cationic PLGA (poly(lactide-co-glycolide)) nano/microparticles were used as an intranasal delivery vehicle as a means administering FMDV DNA vaccine encoding the FMDV capsid protein and the bovine IL-6 gene as a means of enhancing mucosal and systemic immune responses in animals. Three eukaryotic expression plasmids with or without bovine IL-6 gene (pc-P12A3C, pc-IL2AP12A3C and pc-P12AIL3C) were generated. The two latter plasmids were designed with the IL-6 gene located either before or between the P12A and 3C genes, respectively, as a means of determining if the location of the IL-6 gene affected capsid assembly and the subsequent immune response. Guinea pigs and rats were intranasally vaccinated with the respective chitosan-coated PLGA nano/microparticles-loaded FMDV DNA vaccine formulations. Animals immunized with pc-P12AIL3C (followed by animals vaccinated with pc-P12A3C and pc-IL2AP12A3C) developed the highest levels of antigen-specific serum IgG and IgA antibody responses and the highest levels of sIgA (secretory IgA) present in mucosal tissues. However, the highest levels of neutralizing antibodies were generated in pc-IL2AP12A3C-immunized animals (followed by pc-P12AIL3C- and then in pc-P12A3C-immunized animals). pc-IL2AP12A3C-immunized animals also developed stronger cell mediated immune responses (followed by pc-P12AIL3C- and pc-P12A3C-immunized animals) as evidenced by antigen-specific T-cell proliferation and expression levels of IFN-γ by both CD4+ and CD8+ splenic T cells. The percentage of animals protected against FMDV challenge following immunizations with pc-IL2AP12A3C, pc-P12AIL3C or pc-P12A3C were 3/5, 1/5 and 0/5, respectively. These data suggested that intranasal delivery of cationic PLGA nano/microparticles loaded with various FMDV DNA vaccine formulations encoding IL-6 as a molecular adjuvant enhanced protective immunity against FMDV, particularly pc-IL2AP12A3C with IL-6 gene located before P12A3C gene.

Enhancement of salivary IgA response to a DNA vaccine against Streptococcus mutans wall-associated protein A in mice by plasmid-based adjuvants

A specific salivary IgA (sIgA) response was obtained in mice by intranasal immunization with a naked DNA vaccine consisting of the Streptococcus mutans wall-associated protein A gene (wapA) inserted into the mammalian expression vector pcDNA3.1/V5/His-TOPO. In the present study, the vaccine, referred to as pcDNA-wapA, was administered with or without the cationic lipid DMRIE-C. No mucosal response was observed in mice immunized with the vaccine alone, whereas a weak and temporal sIgA response was obtained when the vaccine was mixed with DMRIE-C. To investigate the use of pcDNA containing the interleukin 5 (IL-5) gene (pcDNA-il-5) or the cholera toxin B gene (pcDNA-ctb) as genetic adjuvants, these constructs were used in co-immunization studies. The enhancement effect was transient with pcDNA-il-5, but longer lasting with pcDNA-ctb, thus supporting the use of the latter as a genetic adjuvant to DNA vaccine.

Immunological analysis of a Lactococcus lactis-based DNA vaccine expressing HIV gp120

For reasons of efficiency Escherichia coli is used today as the microbial factory for production of plasmid DNA vaccines. To avoid hazardous antibiotic resistance genes and endotoxins from plasmid systems used nowadays, we have developed a system based on the food-grade Lactococcus lactis and a plasmid without antibiotic resistance genes. We compared the L. lactis system to a traditional one in E. coli using identical vaccine constructs encoding the gp120 of HIV-1. Transfection studies showed comparable gp120 expression levels using both vector systems. Intramuscular immunization of mice with L. lactis vectors developed comparable gp120 antibody titers as mice receiving E. coli vectors. In contrast, the induction of the cytolytic response was lower using the L. lactis vector. Inclusion of CpG motifs in the plasmids increased T-cell activation more when the E. coli rather than the L. lactis vector was used. This could be due to the different DNA content of the vector backbones. Interestingly, stimulation of splenocytes showed higher adjuvant effect of the L. lactis plasmid. The study suggests the developed L. lactis plasmid system as new alternative DNA vaccine system with improved safety features. The different immune inducing properties using similar gene expression units, but different vector backbones and production hosts give information of the adjuvant role of the silent plasmid backbone. The results also show that correlation between the in vitro adjuvanticity of plasmid DNA and its capacity to induce cellular and humoral immune responses in mice is not straight forward.

Pulmonary DNA vaccination: concepts, possibilities and perspectives

Mucosal immunity establishes the first line of defence against pathogens entering the body via mucosal surfaces. Besides eliciting both local and systemic immunity, mucosal vaccination strategies that are non-invasive in nature may increase patient compliance and reduce the need for vaccine application by trained personnel. A relatively new concept is mucosal immunization using DNA vaccines. The advantages of DNA vaccines, such as the opportunity to combine the genetic information of various antigen epitopes and stimulatory cytokines, the enhanced stability and ease of production make this class of vaccines attractive and suitable for mucosal application. In contrast to the area of intranasal vaccination, only a few recent studies have focused on pulmonary immunization and the involvement of the pulmonary immune system in eliciting protective immune responses against inhaled pathogens. This review focuses on DNA vaccine delivery to the lung as a promising approach to prevent pulmonary-associated diseases caused by inhaled pathogens. Attractive immunological features of the lung as a site for immunization, the mechanisms of action of DNA vaccines and the pulmonary application of such vaccines using novel delivery systems will be discussed. We also examine pulmonary diseases prone to prevention or therapeutical intervention by application of DNA vaccines.

DNA vaccines expressing different forms of simian immunodeficiency virus antigens decrease viremia upon SIVmac251 challenge

We have tested the efficacy of DNA immunization as a single vaccination modality for rhesus macaques followed by highly pathogenic SIVmac251 challenge. To further improve immunogenicity of the native proteins, we generated expression vectors producing fusion of the proteins Gag and Env to the secreted chemokine MCP3, targeting the viral proteins to the secretory pathway and to a beta-catenin (CATE) peptide, targeting the viral proteins to the intracellular degradation pathway. Macaques immunized with vectors expressing the MCP3-tagged fusion proteins developed stronger antibody responses. Following mucosal challenge with pathogenic SIVmac251, the vaccinated animals showed a statistically significant decrease in viral load (P = 0.010). Interestingly, macaques immunized with a combination of vectors expressing three forms of antigens (native protein and MCP3 and CATE fusion proteins) showed the strongest decrease in viral load (P = 0.0059). Postchallenge enzyme-linked immunospot values for Gag and Env as well as gag-specific T-helper responses correlated with control of viremia. Our data show that the combinations of DNA vaccines producing native and modified forms of antigens elicit more balanced immune responses able to significantly reduce viremia for a long period (8 months) following pathogenic challenge with SIVmac251.

Augmentation of antigen-specific immune responses using DNA-fusogenic liposome vaccine

In an attempt to enhance the immunological efficacy of genetic immunization, we investigated a new biological means for delivering antigen gene directly to the cytoplasm via membrane fusion. In this context, we investigated fusogenic liposome (FL) encapsulating DNA as a possible genetic immunization vehicle. RT-PCR analysis indicated that a FL could introduce and express encapsulating OVA gene efficiently and rapidly in vitro. Consistent with this observation, an in vitro assay showed that FL-mediated antigen-gene delivery can induce potent presentation of antigen via the MHC class I-dependent pathway. Accordingly, immunization with FL containing the OVA-gene induced potent OVA-specific Th1 and Th2 cytokine production. Additionally, OVA-specific CTL responses and antibody production were also observed in systemic compartments including the spleen, upon immunization with the OVA-gene encapsulating FL. These findings suggest that FL is an effective genetic immunization carrier system for the stimulation of antigen-specific immune responses against its encoding antigen.

Immunogenicity of HIV-1 Env and Gag in baboons using a DNA prime/protein boost regimen

OBJECTIVES

To evaluate the immunogenicity of sequence-modified HIV env and gag in baboons using DNA prime and protein boost strategy.

METHODS

Synthetic sequence-modified HIV gene cassettes were constructed that expressed three different forms of Env proteins, gp140, gp140mut and gp140TM, plus or minus a mutation in the protease-cleavage site. These plasmids were used to immunize baboons (Papio cynocephalus). A group of baboons was also immunized with both env and gag DNA followed by p55Gag virus-like particles (VLP) boost.

RESULTS

Modest antibody responses and low or no lymphoproliferative responses were observed following multiple DNA immunizations. In contrast, strong antibodies and substantial antigen-specific lymphoproliferative responses were seen following booster immunizations with oligomeric Env protein (o-gp140US4) in MF59. Neutralizing antibody responses were scored against T cell line adapted HIV-1 strains after the protein boosters, but neutralizing responses were low or absent against homologous and heterologous primary isolate strains. In the group receiving both gag and env vaccines, modest antigen-specific antibody and lymphoproliferative responses were scored after the DNA immunizations; these responses were enhanced several-fold upon boosting with the VLP preparations. The addition of Gag antigen did not interfere with Env-specific antibody responses, but there was a negative effect on the levels of Env-specific lymphoproliferation.

CONCLUSIONS

These results highlight the importance of improving the potency of HIV DNA vaccines by enhanced DNA delivery and prime-boost vaccine technologies to generate more robust immune responses in larger animal models. In addition, care must be taken when immunizations with Env and Gag antigens are performed together.

DNA vaccines against human immunodeficiency virus type 1 in the past decade

This article reviews advances in the field of human immunodeficiency virus type 1 (HIV-1) and AIDS vaccine development over the last decade, with an emphasis on the DNA vaccination approach. Despite the discovery of HIV-1 and AIDS in humans nearly 20 years ago, there is no vaccine yet that can prevent HIV-1 infection. The focus has shifted toward developing vaccines that can control virus replication and disease progression by eliciting broadly cross-reactive T-cell responses. Among several approaches evaluated, the DNA-based modality has shown considerable promise in terms of its ability to elicit cellular immune responses in primate studies. Of great importance are efforts aimed at improvement of the potency of this modality in the clinic. The review discusses principles of DNA vaccine design and the various mechanisms of plasmid-encoded antigen presentation. The review also outlines current DNA-based vaccine strategies and vectors that have successfully been shown to control virus replication and slow disease progression in animal models. Finally, it lists recent strategies that have been developed as well as novel approaches under consideration to enhance the immunogenicity of plasmid-encoded HIV-1 antigen in various animal models.

Potent CD4+ T cell responses elicited by a bicistronic HIV-1 DNA vaccine expressing gp120 and GM-CSF

Virus-specific CD4(+) T cell responses have been shown to play a critical role in controlling HIV-1 replication. Candidate HIV-1 vaccines should therefore elicit potent CD4(+) as well as CD8(+) T cell responses. In this report we investigate the ability of plasmid GM-CSF to augment CD4(+) T cell responses elicited by an HIV-1 gp120 DNA vaccine in mice. Coadministration of a plasmid expressing GM-CSF with the gp120 DNA vaccine led to only a marginal increase in gp120-specific splenocyte CD4(+) T cell responses. However, immunization with a bicistronic plasmid that coexpressed gp120 and GM-CSF under control of a single promoter led to a dramatic augmentation of vaccine-elicited CD4(+) T cell responses, as measured by both cellular proliferation and ELISPOT assays. This augmentation of CD4(+) T cell responses was selective, since vaccine-elicited Ab and CD8(+) T cell responses were not significantly changed by the addition of GM-CSF. A 100-fold lower dose of the gp120/GM-CSF bicistronic DNA vaccine was required to elicit detectable gp120-specific splenocyte proliferative responses compared with the monocistronic gp120 DNA vaccine. Consistent with these findings, i.m. injection of the gp120/GM-CSF bicistronic DNA vaccine evoked a more extensive cellular infiltrate at the site of inoculation than the monocistronic gp120 DNA vaccine. These results demonstrate that bicistronic DNA vaccines containing GM-CSF elicit remarkably potent CD4(+) T cell responses and suggest that optimal Th cell priming requires the precise temporal and spatial codelivery of Ag and GM-CSF.

Vaccine-induced immune responses in rodents and nonhuman primates by use of a humanized human immunodeficiency virus type 1 pol gene

A synthetic gene consisting of the reverse transcriptase (RT) and integrase (IN) domains of human immunodeficiency virus type 1 (HIV-1) pol was constructed using codons most frequently used in humans. The humanized pol gave dramatically improved levels of Rev-independent, in vitro protein production in mammalian cells and elicited much stronger cellular immunity in rodents than did virus-derived gene. Specifically, BALB/c mice were immunized with plasmids and/or recombinant vaccinia virus constructs expressing the synthetic gene. High frequencies of Pol-specific T lymphocytes were detected in these animals by the gamma interferon enzyme-linked immunospot assay against pools of short overlapping peptides. Characterization of the stimulatory peptides from these pools indicates that the optimized gene constructs are able to effectively activate both CD4+ and CD8+ T cells. Immunization of rhesus macaques with DNA vaccines expressing the humanized pol coupled to a human tissue plasminogen activator leader sequence led to pronounced in vitro cytotoxic T-lymphocyte killing activities and enhanced levels of circulating Pol-specific T cells, comparable to those observed in HIV-1-infected human subjects. Thus, optimizing the immunogenic properties of HIV-1 Pol at the level of the gene sequence validates it as an antigen and provides an important step toward the construction of a potent pol-based HIV-1 vaccine component.

DNA-immunization with a V2 deleted HIV-1 envelope elicits protective antibodies in macaques

Rhesus macaques immunized with the HIV-1 SF162DeltaV2 gp140 envelope using the DNA-prime plus protein-boost vaccination methodology, developed HIV envelope-specific T-cell lymphoproliferative responses and potent neutralizing antibodies. To evaluate the protective potential of these antibodies during acute infection, the animals were depleted of their CD8+ T lymphocytes using specific monoclonal antibodies and subsequently challenged intravenously with the pathogenic SHIV(SF162P4) isolate. As compared to non-vaccinated animals (one of which died from AIDS 16 weeks post-exposure) the vaccinated macaques had lower levels of peak viremia, rapidly cleared virus from the periphery and developed delayed seroconversion to SIV core antigens.

Immune response induced by immunization with Hepatitis B virus core DNA isolated from chronic active hepatitis patients

There are many mutations in the gene encoding Hepatitis B virus (HBV) core antigen of chronic active hepatitis patients, and such mutations are most likely to be related to the severity of disease. Here, we constructed plasmids containing wild-type and deletion type of HBV core gene (HBc) to develop an experimental DNA vaccine and to compare immunogenicity of two types of HBc vaccine. Twenty-nine wild-types and seven deletion types of HBc were detected in sera of 32 Korean patients with chronic active hepatitis. Four wild-types (W1, W2, W4, W6) and two deletion types (D3, D4) of HBc were cloned into the pcDNA3 vector. Intramuscular immunization with wild-type HBc efficiently increased serum anti-HBc antibody response in a dose-dependent manner. Anti-HBc antibody response in mice injected with W6 increased 14 days after immunization, and peaked after 30 days and was maintained at least up to 50 days. W6 immunization induced a specific cytotoxic T lymphocyte response to W6-transfected 3LL (3LL-W6), and reduced the sizes of tumor mass of mice challenged with 3LL-W6 or 3LL transfected with D4. However, intramuscular immunization with D3 and D4 did not show antibody response at all. D3 and D4 have 157 bp (from 331 to 491 bp) and 122 bp (from 327 to 448 bp) gene deletion, respectively, and these encode class II MHC-restricted T-cell epitope. Altogether, these results suggest that mutant virus that has deleted HBc gene may evade immune systems due to loss of T-cell epitope.

DNA vaccination for HIV-1 and SIV

Control of the worldwide AIDS epidemic will only be achieved with a safe and effective prophylactic HIV-1 vaccine. DNA vaccination has recently emerged as a promising vaccine modality that can elicit both humoral and cellular immune responses. HIV-1- and SIV-specific immune responses have been elicited by DNA vaccines in both mice and nonhuman primates. However, these immune responses have not been capable of protecting nonhuman primates against pathogenic AIDS virus challenges. A number of approaches are therefore being investigated to augment DNA vaccine-elicited immune responses.

Elicitation of high-frequency cytotoxic T-lymphocyte responses against both dominant and subdominant simian-human immunodeficiency virus epitopes by DNA vaccination of rhesus monkeys

Increasing evidence suggests that the generation of cytotoxic T-lymphocyte (CTL) responses specific for a diversity of viral epitopes will be needed for an effective human immunodeficiency virus type 1 (HIV-1) vaccine. Here, we determine the frequencies of CTL responses specific for the simian immunodeficiency virus Gag p11C and HIV-1 Env p41A epitopes in simian-human immunodeficiency virus (SHIV)-infected and vaccinated rhesus monkeys. The p11C-specific CTL response was high frequency and dominant and the p41A-specific CTL response was low frequency and subdominant in both SHIV-infected monkeys and in monkeys vaccinated with recombinant modified vaccinia virus Ankara vectors expressing these viral antigens. Interestingly, we found that plasmid DNA vaccination led to high-frequency CTL responses specific for both of these epitopes. These data demonstrate that plasmid DNA may be useful in eliciting a broad CTL response against multiple epitopes.

DNA vaccination with the human immunodeficiency virus type 1 SF162DeltaV2 envelope elicits immune responses that offer partial protection from simian/human immunodeficiency virus infection to CD8(+) T-cell-depleted rhesus macaques

DNA immunization of macaques with the SF162DeltaV2 envelope elicited lymphoproliferative responses and potent neutralizing antibodies. The animals were depleted of their CD8(+) T lymphocytes and then challenged intravenously with SHIV162P4. Compared to unvaccinated animals, the vaccinated macaques had lower peak viremia levels, rapidly cleared plasma virus, and showed delayed seroconversion.

Functional equivalency of B7-1 and B7-2 for costimulating plasmid DNA vaccine-elicited CTL responses

A costimulatory signal in addition to an Ag-specific stimulus is required for optimal activation of T lymphocytes. CD28, the primary positive costimulatory receptor on T cells, has two identified ligands, B7-1 and B7-2. Whether B7-1 and B7-2 have identical, overlapping, or distinct functions remains unresolved. In this study, we show that mice lacking B7-2 were unable to generate CTL responses following immunization with a plasmid DNA vaccine. The ability of these B7-2-deficient mice to generate CTL responses following plasmid gp120 DNA vaccination was fully reconstituted by coadministering either a plasmid expressing B7-2 or B7-1. Moreover, the ability to generate CTL responses following plasmid DNA vaccination in mice lacking both B7-1 and B7-2 could be reconstituted by administering either plasmid B7-1 or plasmid B7-2 with the vaccine construct. These data demonstrate that either B7-1 or B7-2 administered concurrently with a plasmid DNA vaccine can fully costimulate vaccine-elicited CTL responses. Functional differences between B7-1 and B7-2 observed in vivo therefore may not reflect inherent differences in the interactions of CD28 with these ligands.

Construction, biological activity, and immunogenicity of synthetic envelope DNA vaccines based on a primary, CCR5-tropic, early HIV type 1 isolate (BX08) with human codons

So far codon-optimized HIV-1 envelope genes have been investigated for the T cell line-adapted strain MN, which differs in several aspects from primary isolates. Envelopes of primary isolates may be more relevant for vaccine purposes. This article describes for the first time the engineering and characterization of four "humanized" genes encoding the secreted gp120/gp140, or the membrane-bound gp150/gp160, of a primary CCR5 tropic, clade B, clinical isolate HIV-1(BX08). The genes were built in fragments for easy cassette exchange of regions important for immunogenicity, function, and expression. The transcription and expression of the synthetic genes in mammalian cell lines were Rev independent and highly increased. Increased expression of membrane-bound gp160 induced a high cytopathic effect in U87.CD4.CCR5 cells. Gene gun and intramuscular DNA vaccination in mice induced a strong specific cytotoxic T lymphocyte response independent of the gene construct, expression level, or DNA immunization route. In contrast, the highest anti-gp120 antibody levels were induced by synthetic genes encoding the secreted glycoproteins followed by gp160/gp150. Unlike HIV-1(MN), HIV-1(BX08) V3 was not immune dominant. Despite the high antibody response only low and inconsistent neutralizing titers to the homologous HIV-1 isolate were measured. However, neutralization of SHIV89.6P could be obtained. Thus, the neutralizing epitopes on the cell line-adapted SHIV89.6P and HIV-1(MN) may be more antigenically available for the cross-neutralizing antibodies induced. In conclusion, complete "humanization" of the DNA vaccine genes failed to induce a consistent neutralizing antibody response, albeit expression and immunogenicity of the primary HIV-1 glycoproteins were greatly improved. Optimization in terms of improving neutralization may require further modifications of the DNA vaccine gene. The synthetic cassette construct described is a convenient tool developed to investigate this further.

Simian immunodeficiency virus (SIV) gag DNA-vaccinated rhesus monkeys develop secondary cytotoxic T-lymphocyte responses and control viral replication after pathogenic SIV infection

The potential contribution of a plasmid DNA construct to vaccine-elicited protective immunity was explored in the simian immunodeficiency virus (SIV)/macaque model of AIDS. Making use of soluble major histocompatibility class I/peptide tetramers and peptide-specific killing assays to monitor CD8(+) T-lymphocyte responses to a dominant SIV Gag epitope in genetically selected rhesus monkeys, a codon-optimized SIV gag DNA vaccine construct was shown to elicit a high-frequency SIV-specific cytotoxic T-lymphocyte (CTL) response. This CTL response was demonstrable in both peripheral blood and lymph node lymphocytes. Following an intravenous challenge with the highly pathogenic viral isolate SIVsm E660, these vaccinated monkeys developed a secondary CTL response that arose with more rapid kinetics and reached a higher frequency than did the postchallenge CTL response in control plasmid-vaccinated monkeys. While peak plasma SIV RNA levels were comparable in the experimentally and control-vaccinated monkeys during the period of primary infection, the gag plasmid DNA-vaccinated monkeys demonstrated better containment of viral replication by 50 days following SIV challenge. These findings indicate that a plasmid DNA vaccine can elicit SIV-specific CTL responses in rhesus monkeys, and this vaccine-elicited immunity can facilitate the generation of secondary CTL responses and control of viral replication following a pathogenic SIV challenge. These observations suggest that plasmid DNA may prove a useful component of a human immunodeficiency virus type 1 vaccine.

Augmentation of immune responses to HIV-1 and simian immunodeficiency virus DNA vaccines by IL-2/Ig plasmid administration in rhesus monkeys

The potential utility of plasmid DNA as an HIV-1 vaccination modality currently is an area of active investigation. However, recent studies have raised doubts as to whether plasmid DNA alone will elicit immune responses of sufficient magnitude to protect against pathogenic AIDS virus challenges. We therefore investigated whether DNA vaccine-elicited immune responses in rhesus monkeys could be augmented by using either an IL-2/Ig fusion protein or a plasmid expressing IL-2/Ig. Sixteen monkeys, divided into four experimental groups, were immunized with (i) sham plasmid, (ii) HIV-1 Env 89.6P and simian immunodeficiency virus mac239 Gag DNA vaccines alone, (iii) these DNA vaccines and IL-2/Ig protein, or (iv) these DNA vaccines and IL-2/Ig plasmid. The administration of both IL-2/Ig protein and IL-2/Ig plasmid induced a significant and sustained in vivo activation of peripheral T cells in the vaccinated monkeys. The monkeys that received IL-2/Ig plasmid generated 30-fold higher Env-specific antibody titers and 5-fold higher Gag-specific, tetramer-positive CD8+ T cell levels than the monkeys receiving the DNA vaccines alone. IL-2/Ig protein also augmented the vaccine-elicited immune responses, but less effectively than IL-2/Ig plasmid. Augmentation of the immune responses by IL-2/Ig was evident after the primary immunization and increased with subsequent boost immunizations. These results demonstrate that the administration of IL-2/Ig plasmid can substantially augment vaccine-elicited humoral and cellular immune responses in higher primates.

Improvement of the immune response against plasmid DNA encoding OspC of Borrelia by an ER-targeting leader sequence

The present study outlines the characterization of a DNA-based immune response against the OspC antigen, one of the most promising candidates for a Borrelia vaccine. Balb/c mice were injected intradermally with plasmid DNA encoding the OspC gene (lacking the natural leader sequence) under transcriptional control of the cytomegalovirus (CMV) promotor. Immunization with this construct elicited only a marginal response, which was drastically improved by a fusion construct containing the human tissue plasminogen activator (hTPA) signal sequence. The results indicate that for DNA-based immunization against OspC an ER-targeting signal may be necessary for both antibody production as well as cellular immune responses.

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.

DNA inoculations with HIV-1 recombinant genomes that express cytokine genes enhance HIV-1 specific immune responses

Vaccination with HIV-1 DNA sequences induce both humoral and cellular immune responses in experimental animals. However, these responses are relatively weak and are often only transient in their nature. In order to enhance the level of HIV-1 specific immunity, we have engineered HIV-1 DNA constructs which contained various cytokine genes such as interleukin-2 (IL-2), granulocyte-macrophage colony stimulating factor (GM-CSF) and interferon-gamma (IFN-gamma) gene. These constructs have deleted the tat and nmf genes of HIV-1 to eliminate their immunosuppressive effects. Immunizations with these recombinant constructs elicited moderate proliferative T cell responses but poor antibody responses in rats. However, inoculations of HIV-1 DNA that contained the GM-CSF or the IL-2 gene significantly enhanced humoral and proliferative T cell responses, respectively. Thus, recombinant HIV-1 genomes such as those described here may increase the efficacy of DNA vaccination.

DNA vaccines for viral diseases

DNA vaccines, with which the antigen is synthesized in vivo after direct introduction of its encoding sequences, offer a unique method of immunization that may overcome many of the deficits of traditional antigen-based vaccines. By virtue of the sustained in vivo antigen synthesis and the comprised stimulatory CpG motifs, plasmid DNA vaccines appear to induce strong and long-lasting humoral (antibodies) and cell-mediated (T-help, other cytokine functions and cytotoxic T cells) immune responses without the risk of infection and without boost. Other advantages over traditional antigen-containing vaccines are their low cost, the relative ease with which they are manufactured, their heat stability, the possibility of obtaining multivalent vaccines and the rapid development of new vaccines in response to new strains of pathogens. The antigen-encoding DNA may be in different forms and formulations, and may be introduced into cells of the body by numerous methods. To date, animal models have shown the possibility of producing effective prophylactic DNA vaccines against numerous viruses as well as other infectious pathogens. The strong cellular responses also open up the possibility of effective therapeutic DNA vaccines to treat chronic viral infections.

DNA vaccination affords significant protection against feline immunodeficiency virus infection without inducing detectable antiviral antibodies

To test the potential of a multigene DNA vaccine against lentivirus infection, we generated a defective mutant provirus of feline immunodeficiency virus (FIV) with an in-frame deletion in pol (FIVDeltaRT). In a first experiment, FIVDeltaRT DNA was administered intramuscularly to 10 animals, half of which also received feline gamma interferon (IFN-gamma) DNA. The DNA was administered in four 100-microg doses at 0, 10, and 23 weeks. Immunization with FIVDeltaRT elicited cytotoxic T-cell (CTL) responses to FIV Gag and Env in the absence of a serological response. After challenge with homologous virus at week 26, all 10 of the control animals became seropositive and viremic but 4 of the 10 vaccinates remained seronegative and virus free. Furthermore, quantitative virus isolation and quantitative PCR analysis of viral DNA in peripheral blood mononuclear cells revealed significantly lower virus loads in the FIVDeltaRT vaccinates than in the controls. Immunization with FIVDeltaRT in conjunction with IFN-gamma gave the highest proportion of protected cats, with only two of five vaccinates showing evidence of infection following challenge. In a second experiment involving two groups (FIVDeltaRT plus IFN-gamma and IFN-gamma alone), the immunization schedule was reduced to 0, 4, and 8 weeks. Once again, CTL responses were seen prior to challenge in the absence of detectable antibodies. Two of five cats receiving the proviral DNA vaccine were protected against infection, with an overall reduction in virus load compared to the five infected controls. These findings demonstrate that DNA vaccination can elicit protection against lentivirus infection in the absence of a serological response and suggest the need to reconsider efficacy criteria for lentivirus vaccines.

Augmentation and Suppression of Immune Responses to an HIV-1 DNA Vaccine by Plasmid Cytokine/Ig Administration

The use of cytokines has shown promise as an approach for amplifying vaccine-elicited immune responses, but the application of these immunomodulatory molecules in this setting has not been systematically explored. In this report we investigate the use of protein- and plasmid-based cytokines to augment immune responses elicited by an HIV-1 gp120 plasmid DNA vaccine (pV1J-gp120) in mice. We demonstrate that immune responses elicited by pV1J-gp120 can be either augmented or suppressed by administration of plasmid cytokines. A dicistronic plasmid expressing both gp120 and IL-2 induced a surprisingly weaker gp120-specific immune response than did the monocistronic pV1J-gp120 plasmid. In contrast, systemic delivery of soluble IL-2/Ig fusion protein following pV1J-gp120 vaccination significantly amplified the gp120-specific immune response as measured by Ab, proliferative, and CTL levels. Administration of plasmid IL-2/Ig had different effects on the DNA vaccine-elicited immune response that depended on the temporal relationship between Ag and cytokine delivery. Injection of plasmid IL-2/Ig either before or coincident with pV1J-gp120 suppressed the gp120-specific immune response, whereas injection of plasmid IL-2/Ig after pV1J-gp120 amplified this immune response. To maximize immune responses elicited by a DNA vaccine, therefore, it appears that the immune system should first be primed with a specific Ag and then amplified with cytokines. The data also show that IL-2/Ig is more effective than native IL-2 as a DNA vaccine adjuvant.

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.

A vaccine for HIV type 1: the antibody perspective

Antibodies that bind well to the envelope spikes of immunodeficiency viruses such as HIV type 1 (HIV-1) and simian immunodeficiency virus (SIV) can offer protection or benefit if present at appropriate concentrations before viral exposure. The challenge in antibody-based HIV-1 vaccine design is to elicit such antibodies to the viruses involved in transmission in humans (primary viruses). At least two major obstacles exist. The first is that very little of the envelope spike surface of primary viruses appears accessible for antibody binding (low antigenicity), probably because of oligomerization of the constituent proteins and a high degree of glycosylation of one of the proteins. The second is that the mature oligomer constituting the spikes appears to stimulate only weak antibody responses (low immunogenicity). Viral variation is another possible obstacle that appears to present fewer problems than anticipated. Vaccine design should focus on presentation of an intact mature oligomer, increasing the immunogenicity of the oligomer and learning from the antibodies available that potently neutralize primary viruses.

Potent, protective anti-HIV immune responses generated by bimodal HIV envelope DNA plus protein vaccination

It is generally thought that an effective vaccine to prevent HIV-1 infection should elicit both strong neutralizing antibody and cytotoxic T lymphocyte responses. We recently demonstrated that potent, boostable, long-lived HIV-1 envelope (Env)-specific cytotoxic T lymphocyte responses can be elicited in rhesus monkeys using plasmid-encoded HIV-1 env DNA as the immunogen. In the present study, we show that the addition of HIV-1 Env protein to this regimen as a boosting immunogen generates a high titer neutralizing antibody response in this nonhuman primate species. Moreover, we demonstrate in a pilot study that immunization with HIV-1 env DNA (multiple doses) followed by a final immunization with HIV-1 env DNA plus HIV-1 Env protein (env gene from HXBc2 clone of HIV IIIB; Env protein from parental HIV IIIB) completely protects monkeys from infection after i.v. challenge with a chimeric virus expressing HIV-1 env (HXBc2) on a simian immmunodeficiency virusmac backbone (SHIV-HXBc2). The potent immunity and protection seen in these pilot experiments suggest that a DNA prime/DNA plus protein boost regimen warrants active investigation as a vaccine strategy to prevent HIV-1 infection.

Gene gun-based nucleic acid immunization alone or in combination with recombinant vaccinia vectors suppresses virus burden in rhesus macaques challenged with a heterologous SIV

Gene gun-based DNA immunization alone or in combination with recombinant vaccinia vectors was evaluated for the ability to elicit protective immune responses in rhesus macaques challenged with a pathogenic, heterologous simian immunodeficiency virus (SIV). Six monkeys primed with seven consecutive doses of DNA encoding SIVmac239 gp120 and gp160 (DNA + DNA) were divided into two groups. Three of these animals received another DNA booster immunization and the remaining three received a booster immunization containing a homologous, live recombinant vaccinia virus expressing SIVmac251 gp160 (DNA + VAC). In addition, a group of 15 animals primed with recombinant vaccinia vectors were divided into two groups. One group of six monkeys received another immunization of vaccinia (VAC + VAC) and the other nine animals received a DNA (mac239) booster immunization (VAC + DNA). Geometric mean end-point IgG titres in the DNA + VAC and VAC + DNA groups were substantially higher than the responses seen in the VAC + VAC and DNA + DNA groups, demonstrating a synergistic relationship between DNA-based vaccines and recombinant vaccinia virus-based vaccines. All vaccinates and five naive controls were challenged 19 weeks after the final booster immunization with 10 animal infectious doses of SIVDelta/B670. The vaccines did not prevent infection. However, all vaccine groups showed significant virus load reductions from seven to 56 days post challenge when compared to controls. Although the DNA + DNA group developed the lowest prechallenge antibody responses, the most significant reduction (200-fold) in virus load was associated with this group. In addition, a significant delay in CD4+ T cell loss relative to controls was observed in the DNA + DNA group. These results demonstrate that a gene gun-based DNA vaccine provided some attenuation of infection and CD4+ T cell loss after a heterologous challenge.