Immunostimulatory DNA sequences function as T helper-1-promoting adjuvants

Control of immune responses by gene immunization

The use of plasmid DNA to elicit immune responses has greatly increased our ability to skew the desired immune response to a particular antigen. DNA immunization elicits potent cell-mediated responses including humoral immunity as well as cytolytic T-lymphocyte immunity. This review will first discuss the overall immune response induced by naked DNA vaccination and will then summarize recent advances in basic research on DNA immunization, which have furthered our understanding of the role of DNA as an adjuvant as well as a carrier of genetic material. Subsequently, we will consider the possible mechanisms by which DNA immunization is able to induce such immune responses and how DNA immunization may be useful in both basic science research and also in future vaccine development in various disease processes. Finally, we will examine the advantages and disadvantages of DNA vaccines as well as safety issues. In conclusion, DNA vaccination shows promise in a number of areas including infectious diseases, allergy and cancer immunotherapies.

Application of lipids and plasmid design for gene delivery to mammalian cells

Cationic lipids are widely used for in vitro gene transfer due to their efficiency. The major challenges for the improvement of in vivo cationic lipid-mediated gene delivery reside in the design of more biocompatible lipoplexes mimicking viral-mediated gene delivery and in understanding the fate of the lipoplexes within the cells.

Activation of Cutaneous Dendritic Cells by CpG-Containing Oligodeoxynucleotides: A Role for Dendritic Cells in the Augmentation of Th1 Responses by Immunostimulatory DNA

Genetic vaccination depends at least in part on the adjuvant properties of plasmids, properties that have been ascribed to unmethylated CpG dinucleotides in bacterial DNA. Because dendritic cells (DC) participate in the T cell priming that occurs during genetic vaccination, we reasoned that CpG-containing DNA might activate DC. Thus, we assessed the effects of CpG oligodeoxynucleotides (CpG ODN) on Langerhans cell (LC)-like murine fetal skin-derived DC (FSDDC) in vitro and on LC in vivo. Treatment with CpG ODN as well as LPS induced FSDDC maturation, manifested by decreased E-cadherin-mediated adhesion, up-regulation of MHC class II and costimulator molecule expression, and acquisition of enhanced accessory cell activity. In contrast to LPS, CpG ODN stimulated FSDDC to produce large amounts of IL-12 but only small amounts of IL-6 and TNF-α. Injection of CpG ODN into murine dermis also led to enhanced expression of MHC class II and CD86 Ag by LC in overlying epidermis and intracytoplasmic IL-12 accumulation in a subpopulation of activated LC. We conclude that immunostimulatory CpG ODN stimulate DC in vitro and in vivo. Bacterial DNA-based vaccines may preferentially elicit Th1-predominant immune responses because they activate and mobilize DC and induce them to produce large amounts of IL-12.

CpG DNA Induces Sustained IL-12 Expression In Vivo and Resistance to Listeria monocytogenes Challenge

Vertebrates have evolved innate immune defense mechanisms that recognize and respond to structural patterns that are specific to microbial molecules. One such pattern recognition system is based on unmethylated CpG dinucleotides in particular sequence contexts (CpG motifs); these motifs are common in bacterial DNA but are under-represented (“CpG suppression”) and methylated in vertebrate DNA. Mice that are injected with bacterial DNA or synthetic oligodeoxynucleotides (ODNs) containing CpG motifs respond with a rapid production of IL-12 and IFN-γ. The serum levels of IL-12 were increased for at least 8 days after a single injection of CpG ODNs, but IFN-γ levels returned to baseline within 24 h. This Th1-like cytokine response to CpG motifs induces a state of resistance to infection by Listeria monocytogenes in susceptible specific pathogen-free BALB/c mice. Resistance developed within 48 h of pretreatment with CpG ODNs, persisted for at least 2 wk, and was dependent upon IFN-γ secretion. These data support the hypothesis that CpG DNA motifs are a “danger signal” that activates protective innate immune defenses and may have therapeutic potential.

Future directions for allergen immunotherapy

Over the last 30 years several approaches to modify immunotherapy have been tested, including allergoids, alum precipitation, and most recently peptides. However, none of these have replaced the traditional regimens. Over the same period our scientific understanding of allergic disease has been transformed. Today it is possible to identify and monitor changes occurring during treatment and to target many different aspects of the immune system. Recombinant technology provides a powerful technique both for sequencing proteins and producing allergens in commercial quantities. The recombinant proteins can be modified by site-directed mutagenesis so as to decrease their reactivity with IgE antibodies while maintaining reactivity with T cells. Knowledge of the tertiary structure of allergens will make it simpler to identify and change surface epitopes. A completely different approach is to use plasmids to introduce the genes for an allergen. The strength of this technique is that the plasmid can be designed to control expression and also to influence the cytokine profile of the response or the isotype of antibodies produced. Finally, different adjuvants can be used with proteins to alter the response. These include IL-12, immunostimulatory sequences of DNA, and bacterial proteins such as those used in HibVax. It is now possible to identify the cells that control the immune response to allergens and to design treatments that will either downregulate or change the response of T cells. The challenge is to transform this information into an effective treatment for allergic disease.

DNA immunization targeting the skin: molecular control of adaptive immunity

DNA-based immunization represents a novel approach for vaccine development. Recombinant DNA techniques are used to clone DNA sequences encoding antigens of choice into eukaryotic expression plasmids, which are readily and economically amplified in bacteria and recovered with a high degree of purity. For immunization, plasmid DNA is either coated onto microscopic gold particles and bombarded into skin using a gene gun or injected into skin or muscle. Expression of administered genes results in the induction of humoral and cellular immune responses against the encoded antigen. DNA immunization is capable of inducing protective immunity in a number of animal models of infectious disease and cancer. Recent studies suggest that antigen-specific cytotoxic T lymphocyte induction occurs through the presentation of appropriate peptides in the context of major histocompatibility complex molecules on bone marrow-derived professional antigen presenting cells. Following DNA inoculation into the skin, Langerhans cells and/or dermal dendritic cells are believed to acquire the newly synthesized antigen, either through direct transfection or via antigen uptake from transfected keratinocytes, and migrate to regional lymph nodes where they stimulate primary T cell responses. The nature of the immune response depends on the route, method, and timing of DNA delivery and can also be influenced by co-delivery of plasmids encoding immunomodulating cytokines like IFN-alpha, IL-2, or IL-12 and costimulatory molecules like B7-1. While many aspects of the biology of cutaneous DNA immunization remain unknown, the skin appears to offer unique potential as a target for DNA-based 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.

Specific inhibition of interleukin-10 production in murine macrophage-like cells by phosphorothioate antisense oligonucleotides

The effects of phosphorothioate antisense oligonucleotides (AS-S-oligos) directed against murine interleukin-10 (IL-10) mRNA on IL-10 production in RAW264.7 cells, a murine macrophage-like cell line, when stimulated by lipopolysaccharide (LPS) were examined. Of the six AS-S-oligos used, AS-S-oligos directed against the 3'-untranslated region (3'-UTR) of IL-10 mRNA (AS6-S-oligo) showed the strongest inhibitory effect on IL-10 production, and this inhibition was dose and time dependent. Reverse transcription-polymerase chain reaction (RT-PCR) revealed that the antisense effect originated from a specific reduction of target IL-10 mRNA by hybridization with AS6-S-oligo. In addition, AS6-S-oligo did not affect tumor necrosis factor-alpha (TNF-alpha) production in cells stimulated by LPS, and S-oligos with control sequences did not affect IL-10 production. These findings suggested that AS6-S-oligo most powerfully inhibited IL-10 production in macrophages by an antisense mechanism.

Experimental vaccine strategies for cancer immunotherapy

Recently, cancer immunotherapy has emerged as a therapeutic option for the management of cancer patients. This is based on the fact that our immune system, once activated, is capable of developing specific immunity against neoplastic but not normal cells. Increasing evidence suggests that cell-mediated immunity, particularly T-cell-mediated immunity, is important for the control of tumor cells. Several experimental vaccine strategies have been developed to enhance cell-mediated immunity against tumors. Some of these tumor vaccines have generated promising results in murine tumor systems. In addition, several phase I/II clinical trials using these vaccine strategies have shown extremely encouraging results in patients. In this review, we will discuss many of these promising cancer vaccine strategies. We will pay particular attention to the strategies employing dendritic cells, the central player for tumor vaccine development.

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.

Immuno-stimulatory effects of bacterial-derived plasmids depend on the nature of the antigen in intramuscular DNA inoculations

The CpG motifs of bacterial-derived plasmids augment antigen-specific immune responses and steer those responses towards the T helper 1 (Th1) type. In this study, we have addressed the immuno-stimulatory effect of intramuscular co-administration of CpG motifs containing vector DNA on the modulation of immune responses to the haemagglutinin (HA) and the nucleoprotein (NP) proteins of influenza virus. The co-administration of vector DNA with a HA-encoding plasmid DNA showed a significant enhancement in the total IgG response, the generation of cytotoxic T lymphocyte (CTL), and the T-cell proliferative response. In the case of NP-encoding plasmid DNA inoculations, the co-administration of vector DNA slightly decreased the total IgG response, although the IgG2a/IgG1 ratio and the CTL responses to NP were significantly increased. These observations suggest that the immuno-stimulatory effects of bacterial-derived plasmids depend upon the nature of the co-administered antigen.

Modulation of immune responses to hepatitis C virus envelope E2 protein following injection of plasmid DNA using single or combined delivery routes

Different delivery routes of plasmid DNA may result in the induction of differential humoral and cellular immunity. We have studied the influence of two main routes of plasmid injection, performed intramuscularly and intraepidermally using a gene gun, for the induction of immune responses specific to hepatitis C virus (HCV) envelope protein E2. Three plasmids expressing different immunogenic domains of E2 (amino acids [aa] 384443, aa 504-555, and aa 384-746) were injected into BALB/c mice according to five different protocols using various combinations of intramuscular (i.m.) or intraepidermal (i.e.) primary and booster injections. Seroconversion rates, antibody titers and isotypes, epitope recognition, and T-helper (Th) release cytokine profiles were analyzed. Antibody titers and epitope recognition were linked to either or both the nature of the immunogen expressed and the delivery route chosen. In all cases, the lowest antibody titers were obtained using single i.m.-based protocols. Independently of the antibody titers generated, only some specific i.e.-combined delivery routes induced antibodies able to recognize determinants located in the N-terminal of E2 (aa 384411 and aa 411437) and mimicked by synthetic peptides. By contrast, the antibody isotypes and the splenic cytokine production identified were independent of the plasmids used and the delivery route implemented. All conditions resulted in Th-1 like responses suggested by the exclusive detection of IgG2a and 2b antibodies and the production of interferon gamma (INF-gamma) but no interleukin-4 (IL-4). Overall, our results suggest that the combination of i.m. and i.e. delivery routes provides the most efficient way to induce a broad immune response against HCV-E2.

CpG DNA, a novel immune enhancer for systemic and mucosal immunization with influenza virus

Bacterial DNA causes B cell proliferation, immunoglobulin secretion, and Th1-like cytokine secretion, due to unmethylated CpG dinucleotides in particular base contexts (CpG motifs), which are far more common in bacterial DNA than in vertebrate DNA. Synthetic oligodeoxynucleotides (ODN) containing CpG motifs also trigger immune activation, suggesting possible utility as vaccine enhancers. Mice systemically primed with formalin-inactivated influenza virus mixed with CpG ODN, generated virus-specific serum antibodies at titres approximately seven times higher than mice immunized without CpG; the titres were further increased following an identical second injection. To determine whether CpG could be absorbed through mucosae and enhance vaccination responses, mice were immunized intranasally (IN) with the same preparation of virus with or without CpG ODN or Escherichia coli DNA. Following IN immunization, CpG ODN or E. coli DNA promoted increased production of influenza-specific antibodies in serum, saliva and the genital tract, compared with the control groups. These studies indicate that stimulatory CpG ODN are promising new immune enhancers for vaccination applications.

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.

Bacterial DNA and immunostimulatory CpG oligonucleotides trigger maturation and activation of murine dendritic cells

Bacterial DNA and immunostimulatory (i.s.) synthetic CpG-oligodeoxynucleotides (ODN) act as adjuvants for Th1 responses and cytotoxic T cell responses to proteinaceous antigens. Dendritic cells (DC) can be referred to as "nature's adjuvant" since they display the unique capacity to sensitize naive T cells. Here, we demonstrate that bacterial DNA or i.s. CpG-ODN cause simultaneous maturation of immature DC and activation of mature DC to produce cytokines. These events are associated with the acquisition of professional antigen-presenting cell (APC) function. Unfractionated murine bone marrow-derived DC and FACS-fractionated MHC class IIlow (termed immature DC) or MHC class IIhigh populations (termed mature DC) were stimulated with bacterial DNA or i.s. CpG-ODN. Similar to lipopolysaccharide, i.s. CpG-ODN caused up-regulation of MHC class II, CD40 and CD86, but not CD80 on immature and mature DC. In parallel both DC subsets were activated to produce large amounts of IL-12, IL-6 and TNF-alpha. CpG-ODN-activated DC displayed professional APC function in allogeneic mixed lymphocyte reaction and in staphylococcal enterotoxin B-driven naive T cell responses. We interpret these findings to mean that bacterial DNA and i.s. CpG-ODN cause maturation (first step) and activation (second step) of DC to bring about conversion of immature DC into professional APC.

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.

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.

Cutting Edge: CpG Oligodeoxynucleotides Trigger Protective and Curative Th1 Responses in Lethal Murine Leishmaniasis

Synthetic oligodeoxynucleotides containing CpG dinucleotides (CpG-ODN) mimic the immunostimulatory qualities of bacterial DNA. We asked whether immunostimulation by CpG-ODN predisposes for a commitment toward a Th1 vs a Th2 response in Leishmania major infection, a model for a lethal Th2-driven disease, in BALB/c mice. CpG-ODN induced Th1 effector T cells in vitro and conveyed protective immunity to disease-prone BALB/c mice in vivo. Conversion to a Th1-driven resistant phenotype was associated with IL-12 production and maintained the expression of IL-12R β2-chains. Most strikingly, CpG-ODN were even curative when given as late as 20 days after lethal L. major infection, indicating that CpG-ODN revert an established Th2 response. These findings imply an important role of bacterial DNA and CpG-ODN in the instruction of adaptive immune responses. They also point to the therapeutic potential of CpG-ODN in redirecting curative Th1 responses in Th2-driven disorders.

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.

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.

Differential induction of immunoglobulin G subclasses by immunization with DNA vectors containing or lacking a signal sequence

The route and method used to immunize mice with antigen-expressing DNA plasmids have an impact on the resulting T-helper cell response and IgG subclass distribution. Previous findings further indicate that the intracellular targeting of expressed antigens influences the differentiation of naive T-cells into either a Th1 or a Th2 type of response. In the present study, we analyzed the levels of IgG1 and IgG2a antibodies, as correlates of Th2 and Th1 responses, respectively, after intramuscular injection of mice with plasmids encoding a chimeric protein containing a Plasmodium falciparum blood stage antigen expressed in two different forms. One plasmid expresses the antigen in a secreted form as it is preceded by a signal sequence while expression from the other plasmid, lacking this sequence, results in cytoplasmic localization of the antigen. Mice immunized with the plasmid encoding secreted antigen responded with predominantly IgG1 antibodies. In contrast, sera from mice immunized with the plasmid expressing cytosolic protein displayed a mixed IgG1/IgG2a profile. In line with previous findings, our results suggest that the intracellular targeting of proteins expressed by DNA plasmids is an important factor for the differentiation of Th cells and the resulting subclass pattern of IgG responses.

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.

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.

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.