Immunostimulatory DNA sequences necessary for effective intradermal gene immunization

Reduction of antigen expression from DNA vaccines by coadministered oligodeoxynucleotides

Bacterial DNA or synthetic oligodeoxynucleotides (ODN) containing unmethylated CpG dinucleotides within the context of certain flanking bases (CpG motifs) have potent stimulatory effects on the vertebrate immune system. CpG ODN with a synthetic nuclease-resistant phosphorothioate backbone (S-ODN) can be used as an adjuvant to augment both humoral and cell-mediated immune responses against a protein antigen. It has also been shown that the presence of CpG motifs in DNA vaccines may be responsible, at least in part, for their efficacy. Here we evaluate the possibility of using CpG ODN as an adjuvant with DNA vaccines to further improve their efficacy. We show that it is not possible to directly mix S-ODN with plasmid DNA because this will result in an ODN dose-dependent reduction in gene expression from the plasmid, possibly because of competitive interference at binding sites on the surface of target cells. Although ODN with a phosphorothioate-phosphodiester chimeric backbone (SDS-ODN) do not adversely effect the level of gene expression (except when certain sequences, such as a poly G, are present), this is not useful, as SDS-ODN are apparently also not sufficiently nuclease resistant to exert a strong CpG adjuvant effect. Neither is it possible to augment responses to DNA vaccines by administering the CpG S-ODN at a different time or site than the plasmid DNA. Thus, at least for the present, it appears necessary to clone CpG motifs into DNA vaccine vectors to take advantage of their adjuvant effect.

Bacterial DNA as an evolutionary conserved ligand signalling danger of infection to immune cells

During infection, the innate limb of the immune system senses danger (pathogens) via constitutively expressed pattern-recognition receptors, and responds with activation and secretion of pro-inflammatory cytokines. Cell-wall components of gram-positive and gram-negative bacteria, such as peptidoglycan, endotoxin or lipoteichoic acid, activate via CD14, a prototypic pattern-recognition receptor for carbohydrates. This review article focuses on an alternative recognition system of the innate immune system for the recognition of bacterial DNA. Bacterial DNA differs from eukaryotic DNA in its frequency of the dinucleotides CG and its lack of methylation. These structural differences appear to be sensed by cells of the innate immune system such as antigen-presenting cells. As a consequence bacterial DNA serves as an alternate ligand to signal danger of infection. Bacterial DNA and (synthetic) oligonucleotides (ODN) derived thereof are as efficient as endotoxin in activating macrophages and dendritic cells and in triggering release of pro-inflammatory cytokines. In mice sensitized with D-galactosamine (D-GalN), high doses of bacterial DNA from either gram-positive or gram-negative pathogens induce a lethal cytokine syndrome (lethal shock). Therefore, bacterial DNA may represent a hitherto unrecognized pathophysiological entity in host-parasite interactions. Moreover, recent evidence suggests that bacterial DNA or immunostimulating ODN triggers the immunostimulation of antigen-presenting cells, and can be utilized as adjuvant to enhance immune responses of the adaptive immune system towards poorly immunogenic antigens. In fact, foreign DNA might be useful as immunotherapeutically active adjuvant to direct adaptive immune responses towards Thl-dominated immune reactions. If these findings are operative in humans, immunostimulating ODN might be used to influence Th2-dominated diseases such as allergy.

The influence of DNA sequence on the immunostimulatory properties of plasmid DNA vectors

To determine the influence of DNA sequence on immunostimulatory properties of vaccine vectors, we tested the induction of in vitro and in vivo immune responses by plasmids modified to contain extended runs of dG sequences. Studies with oligonucleotides indicate that dG sequences can directly stimulate B cells as well as enhance the activity of immunostimulatory CpG motifs because of interaction with the macrophage scavenger receptor (MSR); this receptor can bind a variety of polyanions including dG sequences. To modify vectors, we introduced stretches of 20-60 dG residues into the pCMV-beta and pSG5rab.gp vectors and measured the ability of these plasmids to induce IL-12 and IFN-gamma production by murine splenocytes. The induction of in vivo antibody responses to rabies glycoprotein was also assessed with the pSG5rab.gp vectors. In in vitro cultures, cytokine production induced by plasmids with and without dG sequences was similar. Furthermore, the addition of dG sequences to pSG5rab.gp vectors failed to enhance the anti-rabies glycoprotein response to immunization. To assess further mechanisms by which plasmids stimulate macrophages, we measured the effects of MSR ligands on in vitro cytokine induction. In in vitro cultures, poly(G), dG30, and fucoidan inhibited IL-12 induction by plasmids. IL-12 induction was also inhibited by mammalian DNA but was unaffected by polyanions that are not MSR ligands. Together, these results suggest that the addition of 20 to 60-base dG sequences to plasmids does not significantly affect their properties as immunostimulators or vaccines. Furthermore, these results suggest that MSR ligands can block cytokine induction by plasmid DNA whether or not the plasmid contains extended runs of dG.

Coadministration of IL-12 or IL-10 expression cassettes drives immune responses toward a Th1 phenotype

Cytokines are important regulators of the immune response. They influence immune expression, the development of immunologic memory, and regulation of antigen-specific and nonspecific immune activation as well as allergic responses. In a model system in mice, we have studied the effect of plasmids expressing interleukin (IL)-10 or IL-12 on the modulation of antigen-specific responses. Coadministration of IL-12 or IL-10 genes with DNA immunogens directed the antigen-specific immune response toward a T helper (Th1)-type immunity. In addition to the modulation of antigen-specific immune responses, we studied the induction of delayed-type hypersensitivity (DTH) to contact allergens as an in vivo model of the Th1 response. We found that IL-12 and IL-10 gene-containing plasmids, and not the bacterial plasmid alone, upregulate this response. Our cytokine gene delivery technique demonstrates an important level of control of the magnitude and direction of induced immune responses and could be advantageous in a wide variety of immunotherapeutic strategies.

Intranasal administration of human immunodeficiency virus type-1 (HIV-1) DNA vaccine with interleukin-2 expression plasmid enhances cell-mediated immunity against HIV-1

DNA vaccine against human immunodeficiency virus type-1 (HIV-1) can induce substantial levels of HIV-1-specific humoral and cell-mediated immunity. To develop more potent HIV-1 DNA vaccine formulations, we used a murine model to explore the immunomodulatory effects of an interleukin-2 (IL-2) expression plasmid on an HIV-1 DNA vaccine following intranasal administration of the combination. When the vaccine and expression plasmid were incorporated into cationic liposomes and administered to mice, the HIV-1-specific delayed-type hypersensitivity response and cytotoxic T lymphocyte activity were significantly increased. Restimulated immune lymphoid cells showed enhanced production of both IL-2 and interferon-gamma and reduced secretion of IL-4. The level of total antibody to HIV-1 antigen was not greatly changed by coadministration of the DNA vaccine and IL-2 expression plasmid. An analysis of serum HIV-1-specific IgG subclasses showed a significant drop in the IgG1/IgG2a ratio in the group that received the plasmid-vaccine combination. These results demonstrate that the IL-2 expression plasmid strongly enhances the HIV-1-specific immune response via activation of T helper type-1 cells.

Specific Immune Induction Following DNA-Based Immunization Through In Vivo Transfection and Activation of Macrophages/Antigen-Presenting Cells

The initiation of an adaptive immune response requires Ag presentation in combination with the appropriate activation signals. Classically, Ag presentation and immune activation occur in the lymph node and spleen, where a favorable organ architecture and rich cellular help can enhance the process. Recently, several investigators have reported the use of DNA expression cassettes to elicit cellular and humoral immunity against diverse pathogens. Although the immune mechanisms involved are still poorly understood, plasmid inoculation represents a model system for studying immune function in response to invading pathogens. In this report, we demonstrate the presence of activated macrophages or dendritic cells in the blood lymphocyte pool and peripheral tissues of animals inoculated with DNA expression cassettes. These cells are directly transfected in vivo, present Ag, and display the surface proteins CD80 and CD86. Our studies indicate that these cells function as APC and can activate naive T lymphocytes. They may represent an important first step APC in genetic immunization and natural infection.

Modulation of virus-induced delayed-type hypersensitivity by plasmid DNA encoding the cytokine interleukin-10

This report evaluates the efficacy of eukaryotic expression plasmids encoding cytokines at modulating the induction and expression of cutaneous delayed-type hypersensitivity (DTH) responses to virus infections. Mice given a single intramuscular administration of cytokine DNA were subsequently infected with either herpes simplex virus (HSV) or vaccinia virus, then tested for DTH. Responses in animals given interleukin-10 DNA were markedly suppressed for at least 5 weeks after pretreatment. Animals also expressed diminished T-cell proliferative responses and modest changes in the balance of T helper type 1 and 2 T-cell reactions. Treatment of animals already sensitized to express DTH, also showed inhibited responses, these taking 6-7 days after treatment to become apparent. Our results show the potency and convenience of plasmid DNA encoding cytokines to modulate inflammatory reactions. Advantages and risks of the cytokine DNA approach are briefly discussed.

Optimization of vaccine responses in early life: the role of delivery systems and immunomodulators

Infant immunization is a particularly important field with multiple challenges for vaccine research and development. There is, together with a high susceptibility to infections, a lower efficacy of most vaccinations in newborns and young infants, compared to those performed later in life. In the present review, the authors focus on problems arising from the attempt to vaccinate against pathogens very early in life, and on the role of selective adjuvants (i.e. antigen delivery systems or immunomodulators) that could be used to: (i) rapidly induce strong antibody responses of the appropriate isotypes; (ii) elicit sustained antibody responses extending beyond infancy; (iii) induce efficient Th1 and CTL responses in spite of the preferential Th2 polarization of early life responses; (iv) escape from maternal antibody mediated inhibition of vaccine responses; (v) show acceptable reactogenicity in early life; and (vi) allow incorporation of several vaccine antigens into a single formulation so as to reduce the number of required injections. How such objectives might be achieved by several of the vaccine formulations currently in development is illustrated by reviewing data from experimental models and clinical studies, when available.

CpG Motifs in Bacterial DNA Activate Leukocytes Through the pH-Dependent Generation of Reactive Oxygen Species

B cells and monocytes endocytose DNA into an acidified intracellular compartment. If this DNA contains unmethylated CpG dinucleotides in particular base contexts (CpG motifs), these leukocytes are rapidly activated. We now show that both B cell and monocyte-like cell line responses to DNA containing CpG motifs (CpG DNA) are sensitive to endosomal acidification inhibitors; they are completely blocked by bafilomycin A, chloroquine, and monensin. The specificity of these inhibitors is demonstrated by their failure to prevent responses to LPS, PMA, or ligation of CD40 or IgM. Acidification of endosomal CpG DNA is coupled to the rapid generation of intracellular reactive oxygen species. The CpG DNA-induced reactive oxygen species burst is linked to the degradation of IκB and the activation of NFκB, which induces leukocyte gene transcription and cytokine secretion. These studies demonstrate a novel pathway of leukocyte activation triggered by CpG motifs.

Bystander stimulation of T cells in vivo by cytokines

Immune responses to infectious agents, especially viruses, are often associated with extensive proliferation of T cells and transient enlargement of the lymphoid tissues. Since the precursor frequency of T cells for specific antigen is low, the bulk of the T cells proliferating in the primary response are presumably stimulated via non-antigen-specific mechanisms, e.g. via cytokines elicited by the infectious agent concerned. Such 'bystander' stimulation of T cells occurs in mice injected with agents that elicit production of type I interferon (IFN I). Induction of IFN I in vivo causes marked stimulation of the CD44hi subset of CD8+ T cells and is prominent after injection of live viruses or products of bacteria such as lipopolysaccharide. Cytokines elicited by infectious agents may act as adjuvants during the primary response and could serve to boost the survival of long-lived memory cells.

Vaccination with DNA encoding an immunodominant myelin basic protein peptide targeted to Fc of immunoglobulin G suppresses experimental autoimmune encephalomyelitis

We explore here if vaccination with DNA encoding an autoantigenic peptide can suppress autoimmune disease. For this purpose we used experimental autoimmune encephalomyelitis (EAE), which is an autoaggressive disease in the central nervous system and an animal model for multiple sclerosis. Lewis rats were vaccinated with DNA encoding an encephalitogenic T cell epitope, guinea pig myelin basic protein peptide 68-85 (MBP68-85), before induction of EAE with MBP68-85 in complete Freund's adjuvant. Compared to vaccination with a control DNA construct, the vaccination suppressed clinical and histopathological signs of EAE, and reduced the interferon gamma production after challenge with MBP68-85. Targeting of the gene product to Fc of IgG was essential for this effect. There were no signs of a Th2 cytokine bias. Our data suggest that DNA vaccines encoding autoantigenic peptides may be useful tools in controlling autoimmune disease.

Cationic Lipids Enhance Cytokine and Cell Influx Levels in the Lung Following Administration of Plasmid: Cationic Lipid Complexes

Administration of plasmid/lipid complexes to the lung airways may be associated, in addition to expression of transgene, with a range of other responses. We report here the induction of cytokines and cellular influx in the lung airway following intratracheal administration of an N-[1-(2–3-dioleyloxy)propyl]-N,N,N-trimethylammonium chloride/cholesterol/plasmid positively charged complex in mice. We show that 1) the appearance of the Th1-associated cytokines IFN-γ and IL-12 in bronchoalveolar lavage fluid is caused by unmethylated CpG dinucleotide sequences present within the plasmid, and is enhanced by the lipid formulation; 2) cationic lipids by themselves do not induce IL-12 or IL-12p40; 3) TNF-α is rapidly induced by cationic lipids and plasmid/lipid complex, but not by plasmid alone; 4) an acute cellular influx is induced by cationic lipid alone and by a plasmid/lipid complex, but to a much lesser extent by plasmid alone; and 5) plasmid methylation does not influence the degree of inflammatory cell influx. The induction of the innate immune responses by plasmid/lipid complexes may be advantageous to gene therapy of lung diseases. In particular, induction of the Th1 cell-promoting cytokines by plasmid/lipid complexes could, in conjunction with an expressed transgene, be used to modulate immune responses in the lung airways in disease conditions that are deficient in Th1 cell responses or that have a dominant Th2 phenotype. Alternatively, the elimination of immunostimulatory sequences in plasmids may improve the tolerability and/or efficacy of nonviral gene therapy, especially for diseases requiring chronic administration.

Genetic vaccines: strategies for optimization

Vaccination with attenuated or killed microbes, purified or recombinant subunit proteins and synthetic peptides is often hampered by toxicity, the presence of infectious agents, weak immune responses and prohibiting costs, especially in the developing world. Such problems may be circumvented by genetic immunization which has recently emerged as an attractive alternative to conventional vaccines. Numerous studies have already shown that immunization of experimental animals with plasmid DNA encoding antigens from a wide spectrum of bacteria, viruses, protozoa and cancers leads to protective humoral and cell-mediated immunity. This review deals with the background and progress made so far with DNA vaccines and their theoretical and practical advantages as well as potential risks, discusses proposed mechanisms of DNA transfection of cells and induction of immune responses to the produced vaccine antigen, and evaluates strategies for the control and optimization of such responses.

Repressor titration: a novel system for selection and stable maintenance of recombinant plasmids

The propagation of recombinant plasmids in bacterial hosts, particularly in Escherichia coli, is essential for the amplification and manipulation of cloned DNA and the production of recombinant proteins. The isolation of bacterial transformants and subsequent stable plasmid maintenance have traditionally been accomplished using plasmid-borne selectable marker genes. Here we describe a novel system that employs plasmid-mediated repressor titration to activate a chromosomal selectable marker, removing the requirement for a plasmid-borne marker gene. A modified E.coli host strain containing a conditionally essential chromosomal gene (kan) under the control of the lac operator/promoter, lac O/P, has been constructed. In the absence of an inducer (allolactose or IPTG) this strain, DH1 lackan , cannot grow on kanamycin-containing media due to the repression of kan expression by LacI protein binding to lac O/P. Transformation with a high copy-number plasmid containing the lac operator, lac O, effectively induces kan expression by titrating LacI from the operator. This strain thus allows the selection of plasmids without antibiotic resistance genes (they need only contain lac O and an origin of replication) which have clear advantages for use as gene therapy vectors. Regulation in the same way of an essential, endogenous bacterial gene will allow the production of recombinant therapeutics devoid of residual antibiotic contamination.

DNA as an adjuvant: capacity of insect DNA and synthetic oligodeoxynucleotides to augment T cell responses to specific antigen

How strong adjuvants such as complete Freund's adjuvant (CFA) promote T cell priming to protein antigens in vivo is still unclear. Since the unmethylated CpG motifs in DNA of bacteria and other nonvertebrates are stimulatory for B cells and antigen-presenting cells, the strong adjuvanticity of CFA could be attributed, at least in part, to the presence of dead bacteria, i.e., a source of stimulatory DNA. In support of this possibility, evidence is presented that insect DNA in mineral oil has even stronger adjuvant activity than CFA by a number of parameters. Synthetic oligodeoxynucleotides (ODNs) containing unmethylated CpG motifs mimic the effects of insect DNA and, even in soluble form, ODNs markedly potentiate clonal expansion of T cell receptor transgenic T cells responding to specific peptide.

Multi-plasmid DNA vaccination avoids antigenic competition and enhances immunogenicity of a poorly immunogenic plasmid

DNA immunization is a very promising approach to the formulation of multivalent vaccines. However, little information is currently available on the immunogenicity of multi-plasmid formulations. To address this issue, we immunized mice with a combination of four plasmids encoding malarial antigens and we compared antibody responses with those obtained with single-plasmid injections. We found that when four plasmids encoding Plasmodium falciparum circumsporozoite protein, thrombospondin-related anonymous protein, major merozoite surface protein (MSP)1 and Pfs25 are co-injected into mice, Ab responses against each antigen are elicited at levels at least as high as the level obtained with single-plasmid injection. The quality of antibody production, as determined by isotype analysis, was similar when single- and multi-plasmid administrations were compared, indicating the priming of the same cytokine profile for CD4+ T helper cells. The sera from mice immunized with the four-plasmid formulation specifically recognized sporozoites, blood stage schizonts and gametes, indicating that DNA immunization induced antibody responses relevant to the native conformation. Finally and of particular interest, in the case of MSP1, the antibody response appears to be strongly potentiated by the presence of additional plasmids, indicating an adjuvant effect of DNA.

Inhibitory effect of lipopolysaccharide on immune response after DNA immunization is route dependent

The DNA prepared from E. coli contained high levels of lipopolysaccharide (LPS). When antigen-encoding DNA was injected into mice, toxicity and increased IgM responses were observed. A method for purifying high yields of DNA (up to 12 mg/L of broth culture) with very low levels of LPS (0.05 ng/mg) was developed. When this purified DNA was used for immunization studies, the toxicity and increased IgM responses were abrogated. Thus, LPS was added to DNA in order to examine its influence on the IgG and cytotoxic T lymphocyte (CTL) response after intramuscular (i.m.) or intradermal (i.d.) DNA immunization. The IgG response to DNA-encoded antigen was inhibited in a dose-dependent manner by the i.d., but not the i.m., route of immunization. Surprisingly, no effect on the CTL response was observed. Therefore, the ability to produce high yields of plasmid DNA with very low levels of endotoxin contamination is advantageous for DNA immunization studies, not only for toxicologic but also for immunologic considerations. Furthermore, these results provide further evidence that immune induction occurs via different mechanisms after i.m. and i.d. DNA immunization.

IFN-gamma primes macrophage responses to bacterial DNA

Macrophages recognize and are activated by unmethylated CpG motifs in bacterial DNA. Here we demonstrate that production of nitric oxide (NO) from murine RAW 264 macrophages and bone marrow-derived macrophages (BMM) in response to bacterial DNA is absolutely dependent on interferon-gamma (IFN-gamma) priming. Similarly, arginine uptake and expression of the inducible nitric oxide synthase (iNOS) gene in response to bacterial DNA in BMM occurred only after IFN-gamma priming. In contrast, mRNA for the cationic amino acid transporter, CAT2, was induced by plasmid DNA alone, and priming with IFN-gamma had no effect on this response. Tumor necrosis factor-alpha (TNF-alpha) release from RAW 264 and BMM in response to bacterial DNA was augmented by IFN-gamma pretreatment. In a stably transfected HIV-1 long terminal repeat (LTR) luciferase RAW 264 cell line, IFN-gamma and bacterial DNA synergized in activation of the HIV-1 LTR. Bacterial DNA has been shown to induce IFN-gamma production in vivo as an indirect consequence of interleukin-12 (IL-12) and TNF-alpha production from macrophages. The results herein suggest the existence of a self-amplifying loop that may have implications for therapeutic applications of bacterial DNA.

Genetically engineered vaccines: an overview

Despite the early success demonstrated with the hepatitis B vaccine, no other recombinant engineered vaccine has been approved for use in humans. It is unlikely that a recombinant vaccine will be developed to replace an existing licensed human vaccine with a proven record of safety and efficacy. This is due to the economic reality of making vaccines for human use. Genetically engineered subunit vaccines are more costly to manufacture than conventional vaccines, since the antigen must be purified to a higher standard than was demanded of older, conventional vaccines. Each vaccine must also be subjected to extensive testing and review by the FDA, as it would be considered a new product. This is costly to a company in terms of both time and money and is unnecessary if a licensed product is already on the market. Although recombinant subunit vaccines hold great promise, they do present some potential limitations. In addition to being less reactogenic, recombinant subunit vaccines have a tendency to be less immunogenic than their conventional counterparts. This can be attributed to these vaccines being held to a higher degree of purity than was traditionally done for an earlier generation of licensed subunit vaccines. Ironically, the contaminants often found in conventional subunit vaccines may have aided in the inflammatory process, which is essential for initiating a vigorous immune response. This potential problem may be overcome by employing one of the many new types of adjuvants that are becoming available for use in humans. Recombinant subunit vaccines may also suffer from being too well-defined, because they are composed of a single antigen. In contrast, conventional vaccines contain trace amounts of other antigens that may aid in conferring an immunity to infectious agents that is more solid than could be provided by a monovalent vaccine. This problem can be minimized, where necessary, by creating recombinant vaccines that are composed of multiple antigens from the same pathogen. These issues are less of a concern with a live attenuated vaccine, since these vaccines are less costly, require fewer steps to manufacture, and elicit long-lived immunity after only a single dose. Unfortunately, live vaccines carry a higher risk of vaccine-induced complications in recipients that make their use in highly developed, litiginous countries unlikely. In lesser developed countries, where the prevalence of disease and the need for effective vaccines outweighs the risk associated with their administration, live vaccines may play an important role in human health. This review has attempted to make the reader aware of some of the current approaches and issues that are associated with the development of these vaccines. Genetically engineered vaccines hold great promise for the future, but the potential of these vaccines to improve human and animal health has yet to be fully realized.

Louis DC, Lee KP. Nucleic Acid Vaccine-Induced Immune Responses Require CD28 Costimulation and Are Regulated by CTLA4

Immunization with plasmids expressing specific genes (DNA or nucleic acid vaccination (NAV)) elicits robust humoral and cell-mediated immune responses. The mechanisms involved in T cell activation by NAV are incompletely characterized. We have examined the costimulatory requirements of NAV. CD28-deficient mice did not mount Ab or CTL responses following i.m. immunization with eukaryotic expression plasmids encoding the bacterial gene β-galactosidase (βgal). Because these mice retained their ability to up-regulate the CTLA4 receptor (a negative regulator of T cell activation), we examined CTLA4’s role in the response of wild-type BALB/c mice to NAV. Intact anti-CTLA4 mAb but not Fab fragments suppressed the primary humoral response to pCIA/βgal without affecting recall responses, indicating CTLA4 activation inhibited Ab production but not T cell priming. Blockade of the ligands for CD28 and CTLA4, CD80 (B7-1) and CD86 (B7-2), revealed distinct and nonoverlapping function. Blockade of CD80 at initial immunization completely abrogated primary and secondary Ab responses, whereas blockade of CD86 suppressed primary but not secondary responses. Simultaneous blockade of CD80 + CD86 was less effective at suppressing Ab responses than either alone. Enhancement of costimulation via coinjection of B7-expressing plasmids augmented CTL responses but not Ab responses, and without evidence of Th1 to Th2 skewing. These findings suggest complex and distinct roles for CD28, CTLA4, CD80, and CD86 in T cell costimulation following nucleic acid vaccination.

Antiviral Activities of Individual Murine IFN-α Subtypes In Vivo: Intramuscular Injection of IFN Expression Constructs Reduces Cytomegalovirus Replication

The IFN-α cytokines belong to a multigene family. However, the in vivo biological functions of each of the IFN-α subtypes is unknown. Recently, we developed an experimental model in which the tibialis anterior muscles of mice were transfected in situ with naked DNA plasmids encoding an IFN transgene. Here we use this model to investigate the in vivo effect of the expression of three murine IFN-α subtypes (A1, A4, and A9) on murine CMV replication in C57BL/6, BALB/c, and A/J mice. CMV was shown to replicate in the tibialis anterior muscles of mice for at least 6 days and induced an inflammatory infiltrate. However, mice expressing the IFN-α transgenes showed a marked reduction in the peak titers of virus replication, with less severe inflammation in the muscles compared with control mice that were inoculated with blank vectors. Moreover, mice expressing the IFN-α1 transgene had significantly lower CMV titers in the inoculated muscle than mice expressing either the IFN-α4 or the IFN-α9 transgenes. Furthermore, IFN-α/β receptor knockout mice had markedly higher levels of CMV replication in the tibialis anterior muscles than the wild-type parental strain (129/Sv/Ev) following IFN-α1 transgene inoculation, suggesting that the protection observed is due to host cell-mediated IFN signaling. These data provide the first evidence indicating that there are in vivo differences in the antiviral efficacy of the IFN-α subtypes.

Induction of CD8+ T Cell Responses to Dominant and Subdominant Epitopes and Protective Immunity to Sendai Virus Infection by DNA Vaccination

While recent studies have demonstrated that DNA vaccination induces potent CD8+ T cell memory in vivo, it is unclear whether this memory is qualitatively and quantitatively comparable with that induced by natural viral infection. In the current studies, we have investigated the induction of CD8+ memory CTL responses to Sendai virus nucleoprotein (NP) in C57BL/6 mice following gene gun vaccination. The data demonstrate that this mode of vaccination induces potent long-lived memory CTL precursors (CTLp) specific for both the dominant (NP324–332/Kb) and the subdominant (NP324–332/Db) epitopes of NP. The frequencies of T cells specific for each of these epitopes in the spleen is about 1:2000 CD8+ T cells, similar to those induced by intranasal infection with Sendai virus. Moreover, the induction of memory CTLp by DNA vaccination is independent of MHC class II molecules or Ab, as is the case for memory CTLp induction by live Sendai virus infection. CTLp specific for both epitopes are capable of migrating to the lung following Sendai virus infection and express potent cytotoxic activity at the site of infection. Consistent with this activity, DNA vaccination with Sendai virus NP induced a substantial degree of Ab-independent protection from a challenge with a lethal dose of Sendai virus. Taken together, these data demonstrate that for the parameters tested, DNA vaccination is indistinguishable from live virus infection in terms of priming functional memory CTLp with broad specificity for both dominant and subdominant T cell epitopes.

Delivery of Multiple CD8 Cytotoxic T Cell Epitopes by DNA Vaccination

Development of CD8 αβ CTL epitope-based vaccines requires an effective strategy capable of co-delivering large numbers of CTL epitopes. Here we describe a DNA plasmid encoding a polyepitope or “polytope” protein, which contained multiple contiguous minimal murine CTL epitopes. Mice vaccinated with this plasmid made MHC-restricted CTL responses to each of the epitopes, and protective CTL were demonstrated in recombinant vaccinia virus, influenza virus, and tumor challenge models. CTL responses generated by polytope DNA plasmid vaccination lasted for 1 yr, could be enhanced by co-delivering a gene for granulocyte-macrophage CSF, and appeared to be induced in the absence of CD4 T cell-mediated help. The ability to deliver large numbers of CTL epitopes using relatively small polytope constructs and DNA vaccination technology should find application in the design of human epitope-based CTL vaccines, in particular in vaccines against EBV, HIV, and certain cancers.

Plasmid DNA encoding targeted naturally processed peptides generates protective cytotoxic T lymphocyte responses in immunized animals

Genetic immunization has been widely applied in efforts to find novel and efficient mechanisms of stimulating the immune response. An effective attack against viral pathogens or tumors often requires activation of T cell-mediated immunity and the generation of cytotoxic T cells. Intramuscular immunization with plasmid DNA containing cDNAs that encode proteins results in expression and secretion of the foreign antigen by muscle cells. T cell activation occurs when peptide fragments of the exogenous protein are presented by major histocompatibility complex class I molecules on the surface of professional antigen-presenting cells. Identification of specific peptide epitopes from a protein antigen presented to T cells during an infectious process or tumor situation would provide all of the antigenic information needed to stimulate effective T cell-mediated immunity. Such peptides represent the naturally processed epitopes selected by the processing machinery of antigen presenting cells. Delivery of this information to the appropriate cells in vivo might be sufficient to stimulate T cell immunity and overcome the difficulties associated with overexpression of large protein antigens or those with potentially toxic side effects. This report describes the use of naturally processed T cell epitopes, administered in plasmid DNA vaccines, to stimulate cytotoxic T cell responses to two viral antigens effectively.

Induction of an antigen-specific, CD1-restricted cytotoxic T lymphocyte response In vivo

The majority of T cell responses are restricted to peptide antigens bound by polymorphic major histocompatibility complex (MHC) molecules. However, peptide antigens can be presented to T cells by murine non-MHC-encoded CD1d (mCD1) molecules, and human T cell lines specific for nonpeptide antigens presented on CD1 isoforms have been identified. It is shown here that antigen-specific, mCD1-restricted lymphocytes can be generated in vivo by immunizing mice with a combination of plasmids encoding chicken ovalbumin, murine CD1d, and costimulatory molecules. Splenocytes from immunized mice have CD1d-restricted, MHC- unrestricted, ovalbumin-specific cytolytic activity that can be inhibited by anti-CD1 antibodies as well as a competing CD1-binding peptide. These results suggest a physiologic role for murine CD1d to present exogenous protein antigens.

Development of Th1 and Th2 Populations and the Nature of Immune Responses to Hepatitis B Virus DNA Vaccines Can Be Modulated by Codelivery of Various Cytokine Genes

In this study, we provide direct evidence that the magnitude and nature of the immune response to a DNA vaccine can be differentially regulated by codelivery of various mouse cytokine genes. Mice immunized with a hepatitis B virus (HBV) DNA vaccine and the IL-12 or IFN-γ gene exhibited a significant enhancement of Th1 cells and increased production of anti-HBV surface IgG2a Ab, as well as a marked inhibition of Th2 cells and decreased production of IgG1 Ab. In contrast, coinjection of the IL-4 gene significantly enhanced the development of specific Th2 cells and increased production of IgG1 Ab, whereas Th1 differentiation and IgG2a production were suppressed. Coinjection of the IL-2 or the granulocyte-macrophage-CSF gene enhanced the development of Th1 cells, while the development of Th2 cells was not affected, and the production of IgG1 and IgG2a Ab were both increased. The CTL activity induced by HBV DNA vaccination was most significantly enhanced by codelivery of the IL-12 or IFN-γ gene, followed by the IL-2 or granulocyte-macrophage-CSF gene, whereas codelivery of the IL-4 gene suppressed the activity. When challenged with HBV surface Ag (HBsAg)-expressing syngeneic tumors, significant reduction of tumor growth was observed in mice that were coadministered the IL-12 gene but not the IL-4 gene. Taken together, these results demonstrate that application of a cytokine gene in a DNA vaccine formulation can influence the differentiation of Th cells as well as the nature of an immune response and may thus provide a strategy to improve its prophylactic and therapeutic efficacy.

Vaccination against enteric pathogens: from science to vaccine trials

Recent advances in scientific research and clinical trials have shown promise for vaccine development against enteric pathogens. Identification of new virulence factors, such as the two distinct shigella enterotoxins, has allowed the development of new immunogen or new attenuated strains. Improved knowledge facilitated the development of safer attenuated live microorganism and construction of multivalent vaccines. Finally, an important advantage is the use of nonreplicating plasmid DNA vectors to express protective antigens in the host.

DNA immunization against herpes simplex virus: enhanced efficacy using a Sindbis virus-based vector

Previously we reported the development of a plasmid DNA expression vector system derived from Sindbis virus (T. W. Dubensky, Jr., et al., J. Virol. 70:508-519, 1996). In vitro, such vectors exhibit high-level heterologous gene expression via self-amplifying cytoplasmic RNA replication. In the present study, we demonstrated the in vivo efficacy of the Sindbis virus-based pSIN vectors as DNA vaccines. A single intramuscular immunization of BALB/c mice with pSIN vectors expressing the glycoprotein B of herpes simplex virus type 1 induced a broad spectrum of immune responses, including virus-specific antibodies, cytotoxic T cells, and protection from lethal virus challenge in two different murine models. In addition, dosing studies demonstrated that the pSIN vectors were superior to a conventional plasmid DNA vector in the induction of all immune parameters tested. In general, 100- to 1,000-fold-lower doses of pSIN were needed to induce the same level of responsiveness as that achieved with the conventional plasmid DNA vector. In some instances, significant immune responses were induced with a single dose of pSIN as low as 10 ng/mouse. These results indicate the potential usefulness of alphavirus-based vectors for DNA immunization in general and more specifically as a herpes simplex virus vaccine.

How bacteria initiate inflammation: aspects of the emerging story

Recent studies have shown that bacteria possess an array of proinflammatory molecules in addition to the extensively studied lipopolysaccharide and superantigens. These bacterial molecules include soluble and membrane-associated inducers of cytokine release, inducers of host cell apoptosis, and immunostimulatory DNA. There is therefore much greater diversity in the class of molecules and mechanisms by which bacteria engage the host immune system than previously appreciated.

Isolation of a high-affinity stable single-chain Fv specific for mesothelin from DNA-immunized mice by phage display and construction of a recombinant immunotoxin with anti-tumor activity

Mesothelin is a differentiation antigen present on the surface of ovarian cancers, mesotheliomas, and several other types of human cancers. Because among normal tissues, mesothelin is present only on mesothelial cells, it represents a good target for antibody-mediated delivery of cytotoxic agents. In the present study mice were immunized with an eukaryotic expression vector coding for mesothelin. When high serum antibody titers were obtained, a phage display library was made from the splenic mRNA of these mice. After three rounds of panning on recombinant mesothelin, a single-chain Fv (scFv)-displaying phage was selected that bound specifically to recombinant mesothelin and mesothelin-positive cells. The scFv was used to construct an immunotoxin by genetically fusing it with a truncated mutant of Pseudomonas exotoxin A. The purified immunotoxin binds mesothelin with high affinity (Kd 11 nm), is stable for over 40 hr at 37 degrees C and is very cytotoxic to cells expressing mesothelin. It also produces regressions of tumors expressing mesothelin. This combination of selective cytotoxicity, high activity, and stability makes the immunotoxin a good candidate for development as a therapeutic agent. This work also shows that DNA immunization can be used to isolate and clone antibodies against epitopes present on human proteins in their native conformation.

The role of CpG dinucleotides in DNA vaccines

DNA vaccines can induce potent humoral and cellular immune responses without any additional adjuvant. Recent studies indicate that unmethylated CpG dinucleotides within DNA vaccines are immune stimulatory and exert an essential endogenous adjuvant activity. These CpG motifs can be added deliberately to DNA or conventional protein vaccines to enhance the Th1 immune response.

Protective efficacy of DNA vaccines against duck hepatitis B virus infection

The efficacy of DNA vaccines encoding the duck hepatitis B virus (DHBV) pre-S/S and S proteins were tested in Pekin ducks. Plasmid pcDNA I/Amp DNA containing the DHBV pre-S/S or S genes was injected intramuscularly three times, at 3-week intervals. All pre-S/S and S-vaccinated ducks developed total anti-DHBs and specific anti-S antibodies with similar titers reaching 1/10,000 to 1/50,000 and 1/2,500 to 1/4,000, respectively, after the third vaccination. However, following virus challenge, significant differences in the rate of virus removal from the bloodstream and the presence of virus replication in the liver were found between the groups. In three of four S-vaccinated ducks, 90% of the inoculum was removed between <5 and 15 min postchallenge (p.c.) and no virus replication was detected in the liver at 4 days p.c. In contrast, in all four pre-S/S-vaccinated ducks, 90% of the inoculum was removed between 60 and 90 min p.c. and DHBsAg was detected in 10 to 40% of hepatocytes. Anti-S serum abolished virus infectivity when preincubated with DHBV before inoculation into 1-day-old ducklings and primary duck hepatocyte cultures, while anti-pre-S/S serum showed very limited capacity to neutralize virus infectivity in these two systems. Thus, although both DNA vaccines induced high titers of anti-DHBs antibodies, anti-S antibodies induced by the S-DNA construct were highly effective in neutralizing virus infectivity while similar levels of anti-S induced by the pre-S/S-DNA construct conferred only very limited protection. This phenomenon requires further clarification, particularly in light of the development of newer HBV vaccines containing pre-S proteins and a possible discrepancy between anti-HBs titers and protective efficacy.

The mode of presentation and route of administration are critical for the induction of immune responses to p53 and antitumor immunity

We have examined the immune response to full-length wild-type human p53 presented by a recombinant canarypox vector (ALVAC) and by plasmid DNA. For the ALVAC recombinant, intravenous, but not subcutaneous, intramuscular or intradermal administration, induced CD8+ CTLs that lysed tumor cells transfected with human mutant p53. Intrasplenic administration also induced CTLs. Biodistribution studies showed that intravenously injected ALVAC localized primarily in the lung, liver and spleen, whereas intramuscularly injected virus remained predominantly at the injection site. Intradermal and intramuscular immunization with naked plasmid DNA encoding human wild-type p53 also induced a specific CTL response. DNA immunization induced complete protection against challenge with a mouse embryo fibroblast transfected with human mutant p53 and partial, but significant, protection against a transfected mastocytoma. The ALVAC recombinant induced partial protection in both models. These results suggest that recombinant ALVAC and DNA might be interesting presentation platforms for p53 to be tested in clinical studies.

DNA vaccines: safety and efficacy issues

DNA technology has been harnessed to produce a variety of plasmid-based vaccines designed to prevent viral, bacterial and parasitic infections. The rapid adoption and implementation of this novel vaccine strategy carries with it important safety and efficacy concerns. This review will focus on whether DNA vaccines (1) are likely to induce systemic or organ-specific autoimmune disease, (2) have the potential to induce tolerance rather than immunity, and (3) are as effective in individuals with depressed immune function as they are in healthy adults.

Immunostimulatory DNA: sequence-dependent production of potentially harmful or useful cytokines

Certain bacterial immunostimulatory (i.s.) DNA sequences containing unmethylated CpG motifs stimulate antigen-presenting cells (APC) to express a full complement of costimulatory molecules and to produce cytokines including interleukin (IL)-12 and tumor necrosis factor (TNF)-alpha. While IL-12 is key to their T helper cell (Th)1-promoting adjuvant activity, secretion of toxic levels of TNF-alpha is harmful in that it promotes toxic shock. Given the beneficial as well as harmful consequences of i.s. DNA, we investigated the possibility of identifying DNA sequences, i.e. CpG oligodeoxynucleotides (ODN) which differentially activate IL-12 versus TNF-alpha cytokine production in APC. Here, we describe an i.s. DNA sequence with these characteristics. While its potential to induce IL-12 is preserved, its ability to trigger TNF-alpha release is strongly curtailed both in vitro and in vivo. I.s. DNA could be segregated into lethal and non-lethal in a mouse toxic shock model. The non-toxic i.s. DNA was useful as an adjuvant, thus allowing cytotoxic T cell responses to the soluble protein ovalbumin and conferring a resistant Th 1 phenotype to BALB/c mice lethally infected with Leishmania major. This i.s. CpG motif may thus be prototypic for a useful immunostimulating DNA sequence that lacks harmful side effects.

Allergen gene transfer

DNA-based immunization induces a biased Th1 immune response, and offers a novel strategy for modulation of the Th2-associated response. Recent studies have provided evidence that antigen-induced allergic responses can be modulated in rodents immunized with plasmid DNAs encoding the sensitizing antigens. Further understanding of the regulatory mechanism involved and optimization of gene transfer/expression systems will assist greatly in proving the clinical utility of this process.

Promoter attenuation in gene therapy: interferon-gamma and tumor necrosis factor-alpha inhibit transgene expression

One of the major limitations to current gene therapy is the low-level and transient vector gene expression due to poorly defined mechanisms, possibly including promoter attenuation or extinction. Because the application of gene therapy vectors in vivo induces cytokine production through specific or nonspecific immune responses, we hypothesized that cytokine-mediated signals may alter vector gene expression. Our data indicate that the cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) inhibit transgene expression from certain widely used viral promoters/enhancers (cytomegalovirus, Rous sarcoma virus, simian virus 40, Moloney murine leukemia virus long terminal repeat) delivered by adenoviral, retroviral or plasmid vectors in vitro. A constitutive cellular promoter (beta-actin) is less sensitive to these cytokine effects. Inhibition is at the mRNA level and cytokines do not cause vector DNA degradation, inhibit total cellular protein synthesis, or kill infected/transfected cells. Administration of neutralizing anti-IFN-gamma monoclonal antibody results in enhanced transgene expression in vivo. Thus, standard gene therapy vectors in current use may be improved by altering cytokine-responsive regulatory elements. Determination of the mechanisms involved in cytokine-regulated vector gene expression may improve the understanding of the cellular disposition of vectors for gene transfer and gene therapy.