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

Mechanisms and applications of immune stimulatory CpG oligodeoxynucleotides

Immune stimulation has been widely recognized as an undesirable side effect of certain antisense oligodeoxynucleotides (ODN) which can interfere with their therapeutic application. It is now clear that these dose-dependent immune stimulatory effects primarily result from the presence of an unmethylated CpG dinucleotide in particular base contexts ('CpG motif). The sequence-specific immune activation is not just an experimental artifact, but is actually a highly evolved immune defense mechanism whose actual 'goal' is the detection of microbial nucleic acids. In contrast to vertebrate DNA, in which CpG dinucleotides are 'suppressed' and are highly methylated, microbial genomes do not generally feature CpG suppression or methylation [1]. Immune effector cells such as B cells, macrophages, dendritic cells, and natural killer cells appear to have evolved pattern recognition receptors (PRR) that by binding the microbe-restricted structure of CpG motifs, trigger protective immune responses. Although the specific immune activation appears to have a variety of potential therapeutic applications, it is generally undesirable in antisense ODN. Immune stimulation may be avoided in antisense oligos by the selection of CpG-free target sequences, by the use of ODN backbones that do not support immune stimulation, or by selective modifications of the cytosine in any CpG dinucleotides.

Importance of nucleotide sequence and chemical modifications of antisense oligonucleotides

The antisense approach is conceptually simple and elegant; to design an inhibitor of a specific mRNA, one needs only to know the sequence of the targeted mRNA and an appropriately modified complementary oligonucleotide. Of the many analogs of oligodeoxynucleotides explored as antisense agents, phosphorothioate analogs have been studied the most extensively. The use of phosphorothioate oligodeoxynucleotides as antisense agents in various studies have shown promising results. However, they have also indicated that quite often, biological effects observed could be solely or partly non-specific in nature. It is becoming clear that not all phosphorothioate oligodeoxynucleotides of varying length and base composition are the same, and important consideration should be given to maintain antisense mechanisms while identifying effective antisense oligonucleotides. In this review, I have summarized the progress made in my laboratory in understanding the specificity and mechanism of actions of phosphorothioate oligonucleotides and the rationale for designing second-generation mixed-backbone oligonucleotides.

Th1/Th2 cells

A large body of evidence indicates the existence of functionally polarized CD4+ T-cell responses based on their profile of cytokine secretion. Type 1 T helper (Th1) cells produce interferon-gamma, interleukin (IL)-2, and tumour necrosis factor (TNF)-beta, which activate macrophages and are responsible for cell-mediated immunity and phagocyte-dependent protective responses. By contrast, type 2 Th (Th2) cells produce IL-4, IL-5, IL-10, and IL-13, which are responsible for strong antibody production, eosinophil activation, and inhibition of several macrophage functions, thus providing phagocyte-independent protective responses. Th1 cells mainly develop following infections by intracellular bacteria and some viruses, whereas Th2 cells predominate in response to infestations by gastrointestinal nematodes. Polarized Th1 and Th2 cells not only exhibit different functional properties, but also show the preferential expression of some activation markers and distinct transcription factors. Several mechanisms may influence the Th cell differentiation, which include the cytokine profile of "natural immunity" evoked by different offending agents, the nature of the peptide ligand, as well as the activity of some costimulatory molecules and microenvironmentally secreted hormones, in the context of the individual genetic background. In addition to playing different roles in protection, polarized Th1-type and Th2-type responses are also responsible for different types of immunopathological reactions. Th1 cells are involved in the pathogenesis of organ-specific autoimmune disorders, Crohn's disease, Helicobacter pylori-induced peptic ulcer, acute kidney allograft rejection, and unexplained recurrent abortions. In contrast, allergen-specific Th2 responses are responsible for atopic disorders in genetically susceptible individuals. Moreover, Th2 responses against still unknown antigens predominate in Omenn's syndrome, idiopathic pulmonary fibrosis, and progressive systemic sclerosis. Finally, the prevalence of Th2 responses may play some role in a more rapid evolution of human immunodeficiency virus infection to the full-blown disease. The Th1/Th2 paradigm also provides the rationale for the development of new types of vaccines against infectious agents and of novel strategies for the therapy of allergic and autoimmune disorders.

Repeated administration of synthetic oligodeoxynucleotides expressing CpG motifs provides long-term protection against bacterial infection

Synthetic oligodeoxynucleotides (ODN) expressing unmethylated CpG motifs stimulate an innate immune response characterized by the production of polyreactive immunoglobulin M antibodies and immunomodulatory cytokines. This immune response has been shown to protect mice from challenge by Listeria monocytogenes and Francisella tularensis for up to 2 weeks. By repeatedly administering CpG ODN two to four times/month, we found that this protection could be maintained indefinitely. Protection was associated with a significant increase in the number of spleen cells that could be triggered by subsequent pathogen exposure to secrete gamma interferon and interleukin-6 in vivo (P < 0.01). ODN-treated animals remained healthy and developed neither macroscopic nor microscopic evidence of tissue damage or inflammation. Thus, repeated administration of CpG ODN may provide a safe means of conferring long-term protection against infectious pathogens.

Advances in vaccine adjuvants

Currently, aluminum salts and MF59 are the only vaccine adjuvants approved for human use. With the development of new-generation vaccines (including recombinant subunit and mucosal vaccines) that are less immunogenic, the search for more potent vaccine adjuvants has intensified. Of the novel compounds recently evaluated in human trials, immunostimulatory molecules such as the lipopolysaccharide derived MPL and the saponin derivative QS21 appear most promising, although doubts have been raised as to their safety in humans. Preclinical work with particulate adjuvants, such as the MF59 microemulsion and lipid-particle immune-stimulating complexes (Iscoms), suggest that these molecules are also potent elicitors of humoral and cellular immune responses. In addition, preclinical data on CpG oligonucleotides appear to be encouraging, particularly with respect to their ability to selectively manipulate immune responses. While all these adjuvants show promise, further work is needed to better define the mechanisms of adjuvant action. Ultimately, the development of more potent adjuvants may allow vaccines to be used as therapeutic, rather than prophylactic, agents.

Plasmid DNA vaccines are effective in the absence of IFNgamma

Intramuscular injection of bacterially derived plasmid DNA results in the development of both humoral and cellular immune responses against plasmid-encoded antigens. Immunostimulatory CpG sequences within bacterial DNA are thought to enhance this process by stimulating the secretion of proinflammatory cytokines such as interferon gamma (IFNgamma) by cells of the innate immune system. Although IFNgamma induction by CpG elements within plasmid DNA has been documented in vitro and more recently in vivo, and coimmunization with plasmids expressing IFNgamma has been shown to enhance DNA-immunization-induced immune responses, it is unclear if IFNgamma is necessary for successful DNA immunization. To address this issue, we compared humoral and cellular immune responses in wild-type and IFNgamma-deficient mice vaccinated with a plasmid (pCMVNP) expressing the nucleoprotein gene from the arenavirus lymphocytic choriomeningitis virus (LCMV). IFNgamma-positive (BALB/c) and IFNgamma-negative (GKO) mice responded to DNA vaccination by the development of antigen-specific CD8(+) T cells, which were detectable directly ex vivo by intracellular cytokine staining and comprised 0.7-2.5% of all CD8(+) T cells in the vaccine. DNA vaccines also induced virus-specific cytotoxic T lymphocytes (CTL), even in the absence of IFNgamma. DNA vaccination of both mouse strains also was associated with a significant reduction in viral titers after LCMV challenge, indicating that, at least in the presence of other immune effector mechanisms, IFNgamma is not required for induction of protective anti-viral immunity by DNA immunization. No quantitative differences were observed in antiviral IgG levels among GKO and BALB/c vaccinees, although GKO mice did exhibit a significant reduction of the IgG2a:IgG1 ratio, in agreement with the previously documented requirement for IFNgamma in isotype switching to IgG2a. Immunized BALB/c mice produced similar levels of both IgG1 and IgG2a, indicating a mixed Th1/Th2 response to intramuscular immunization with pCMVNP. These results show that IFNgamma induction by bacterially derived plasmid DNA does not contribute to the magnitude of the antibody response and is not required for the induction or short-term maintenance of DNA-induced CTL. However, IFNgamma is necessary for the development of IgG2a antibodies that may be crucial for protection against some pathogens.

CpG DNA as mucosal adjuvant

We have previously found synthetic oligodeoxynucleotides (ODN) containing immunostimulatory CpG motifs to be a potent adjuvant to protein administered by intramuscular injection or intranasal inhalation to BALB/c mice. Herein we have further evaluated the potential of CpG ODN as a mucosal adjuvant to purified hepatitis B surface antigen (HBsAg) when administered alone or with cholera toxin (CT). CpG ODN and CT both augmented systemic (humoral and cellular) and mucosal immune responses against HBsAg, and these could be further enhanced with higher doses of adjuvant or boosting. Overall, antibody isotypes with CT alone were predominantly IgG1 (Th2-like) whereas they were predominantly IgG2a (Th1-like) with CpG ODN alone or in combination with CT. Results from this study indicate that stimulatory CpG ODN are promising new adjuvants for mucosal vaccination strategies, whether used alone or in combination with other mucosal adjuvants.

Synthetic oligodeoxynucleotides containing CpG motifs enhance immunogenicity of a peptide malaria vaccine in Aotus monkeys

Synthetic peptide and recombinant protein vaccines are optimally immunogenic when delivered with an effective adjuvant. Candidate vaccines currently insufficiently immunogenic may induce a protective immunity if they could be delivered with more effective adjuvants. For example, immunogens that induce promising responses when administered to mice with complete and incomplete Freund's adjuvants perform less well in primate animal models where complete Freund's adjuvant is not used. We report the use of synthetic oligodeoxynucleotides containing CpG motifs, the sequences of which are based on immunostimulatory bacterial DNA sequences, to enhance the immune response in Aotus monkeys to a synthetic peptide malaria vaccine. Monkeys were immunized with the synthetic peptide PADRE 45, a synthetic peptide containing amino acid sequences derived from the circumsporozoite protein (CSP) from Plasmodium falciparum, and delivered in an emulsion of saline and Montanide 720, a mannide oleate in oil solution, that also contained one of three oligodeoxynucleotides. The animals receiving oligodeoxynucleotides containing either three or four CpG motifs produced antibodies that bound a recombinant CSP as measured in ELISA, and reacted with P. falciparum sporozoites in a sporozoite immunofluorescent test. These responses were significantly greater than those seen in animals receiving the oligodeoxynucleotide without CpG motifs. These data indicate that oligodeoxynucleotides containing CpG motifs improve immunogenicity of peptide immunogens in non-human primates, and may be immunopotentiators useful in humans.

CpG DNA: a potent signal for growth, activation, and maturation of human dendritic cells

DNA molecules containing unmethylated CpG-dinucleotides in particular base contexts ("CpG motifs") are excellent adjuvants in rodents, but their effects on human cells have been less clear. Dendritic cells (DCs) form the link between the innate and the acquired immune system and may influence the balance between T helper 1 (Th1) and Th2 immune responses. We evaluated the effects of CpG oligodeoxynucleotides alone or in combination with granulocyte-macrophage colony-stimulating factor (GMCSF) on different classes of purified human DCs. For primary dendritic precursor cells isolated from human blood, CpG oligonucleotides alone were superior to GMCSF in promoting survival and maturation (CD83 expression) as well as expression of class II MHC and the costimulatory molecules CD40, CD54, and CD86 of DCs. Both CD4-positive and CD4-negative peripheral blood dendritic precursor cells responded to CpG DNA which synergized with GMCSF but these DCs showed little response to lipopolysaccharide (LPS). In contrast, monocyte-derived DCs did not respond to CpG, but they were highly sensitive to LPS, suggesting an inverse correlation between CpG and LPS sensitivity in different subsets of DCs. Compared with GMCSF, CpG-treated peripheral blood DCs showed enhanced functional activity in the mixed lymphocyte reaction and induced T cells to secrete increased levels of Th1 cytokines. These findings demonstrate the ability of specific CpG motifs to strongly activate certain subsets of human DCs to promote Th1-like immune responses, and support the use of CpG DNA-based trials for immunotherapy against cancer, allergy, and infectious diseases.

Mucosal immunity and tolerance: relevance to vaccine development

The mucosal immune system of mammals consists of an integrated network of lymphoid cells which work in concert with innate host factors to promote host defense. Major mucosal effector immune mechanisms include secretory antibodies, largely of immunoglobulin A (IgA) isotype, cytotoxic T cells, as well as cytokines, chemokines and their receptors. Immunologic unresponsiveness (tolerance) is a key feature of the mucosal immune system, and deliberate vaccination or natural immunization by a mucosal route can effectively induce immune suppression. The diverse compartments located in the aerodigestive and genitourinary tracts and exocrine glands communicate via preferential homing of lymphocytes and antigen-presenting cells. Mucosal administration of antigens may result in the concomitant expression of secretory immunoglobulin A (S-IgA) antibody responses in various mucosal tissues and secretions, and under certain conditions, in the suppression of immune responses. Thus, developing formulations based on efficient delivery of selected antigens/tolerogens, cytokines and adjuvants may impact on the design of future vaccines and of specific immunotherapeutic approaches against diseases associated with untoward immune responses, such as autoimmune disorders, allergic reactions, and tissue-damaging inflammatory reactions triggered by persistent microorganisms.

Bacterial DNA or Oligonucleotides Containing Unmethylated CpG Motifs Can Minimize Lipopolysaccharide-Induced Inflammation in the Lower Respiratory Tract Through an IL-12-Dependent Pathway

To determine whether the systemic immune activation by CpG DNA could alter airway inflammation, we pretreated mice with either i.v. bacterial DNA (bDNA) or oligonucleotides with or without CpG motifs, exposed these mice to LPS by inhalation, and measured the inflammatory response systemically and in the lung immediately following LPS inhalation. Compared with non-CpG oligonucleotides, i.v. treatment with CpG oligonucleotides resulted in higher systemic concentrations of polymorphonuclear leukocytes, IL-10, and IL-12, but significantly reduced the concentration of total cells, polymorphonuclear leukocytes, TNF-α, and macrophage inflammatory protein-2 in the lavage fluid following LPS inhalation. The immunoprotective effect of CpG-containing oligonucleotides was dose-dependent and was most pronounced in mice pretreated between 2 and 4 h before the inhalation challenge, corresponding to the peak levels of serum cytokines. bDNA resulted in a similar immunoprotective effect, and methylation of the CpG motifs abolished the protective effect of CpG oligonucleotides. The protective effect of CpG oligonucleotides was observed in mice with either a disrupted IL-10 or IFN-γ gene, but release of cytokines in the lung was increased, especially in the mice lacking IFN-γ. In contrast, CpG DNA did not protect mice with a disrupted IL-12 gene against the LPS-induced cellular influx, even though CpG DNA reduced the release of TNF-α and macrophage inflammatory protein-2 in the lung. These findings indicate that CpG-containing oligonucleotides or bDNA are protected against LPS-induced cellular airway inflammation through an IL-12-dependent pathway, and that the pulmonary cytokine and cellular changes appear to be regulated independently.

CpG DNA and LPS induce distinct patterns of activation in human monocytes

A specific set of immune functions is switched on in response to DNA containing unmethylated CpG dinucleotides in particular base contexts ('CpG motifs'). Plasmids, viral vectors and antisense oligodeoxynucleotides used for DNA vaccination, gene replacement or gene blockade contain immunostimulatory CpG motifs which may have independent biological activity. Although the immune stimulatory effects of CpG motifs on murine cells are well established, the evaluation of their possible effects on human cells is complicated by the higher LPS sensitivity of human leukocytes compared with those in mice. To address this issue, we analyzed CpG- and LPS-mediated immune activation of human PBMC. The biologic activity of LPS could be detected within 4 h using intracellular TNF staining of monocytes with flow cytometry at concentrations just one-twentieth (0.0014 Eu/ml) of the lower detection limit for the routinely used LAL assay (0.03 EU/ml). In contrast to the rapid LPS response, CpG DNA-stimulated TNF and IL-6 synthesis in human monocytes was not detectable until 18 h. E. coli DNA induced IL-6 synthesis in a concentration-dependent manner (30 micrograms/ml E. coli DNA; 409 pg/ml +/- 75 pg/ml, n = 7, IL-6 ELISA), but calf thymus DNA did not (< 10 pg/ml). Likewise, the CpG oligodeoxynucleotides 1760 (phosphorothioate) and 2059 (unmodified) induced IL-6 synthesis, but the corresponding control oligonucleotides 1908 and 2077 did not CpG DNA and LPS enhanced IL-6 synthesis synergistically. ICAM-1-expression of monocytes was increased 4.6-fold by E. coli DNA, 3.5-fold by 1760 and three-fold by 2059, compared with 3.6-fold by a maximal LPS stimulus and no change with non-CpG DNA. In conclusion, CpG-motifs induce TNF, IL-6 and ICAM-1 expression in human monocytes, but the kinetics of this differ from that induced by LPS, which makes it possible to distinguish immune activation by these agents. These results have important implications for the clinical development of therapeutic DNA in humans.

Immunostimulatory CpG motifs trigger a T helper-1 immune response to human immunodeficiency virus type-1 (HIV-1) gp 160 envelope proteins

Bacterial DNA sequences containing unmethylated CpG motifs have recently been proposed to exhibit immunostimulatory effects on B-, T- and NK cells, leading to the induction of humoral and cell-mediated immune responses. In the present study we investigated the immunomodulatory effects of a CpG-containing oligodeoxynucleotide (CpG ODN) to the HIV-1 gp 160 envelope (Env) protein in the BALB/c mouse model. Priming and boosting of mice with gp 160 adsorbed to aluminium hydroxide (Alum) induced a typical T helper-2 (Th2)-dominated immune response with high titers of gp 160-specific immunoglobulin (Ig)G1 isotypes but a weak IgG2a response. Specifically re-stimulated splenocytes from these mice predominantly secreted interleukin (IL)-5 but only minute amounts of interferon-gamma (IFN-gamma) upon specific re-stimulation. In contrast, a boost immunisation of gp 160/Alum primed mice with a gp 160/Alum/CpG combination resulted in a seven times higher production of IgG2a antibodies, without affecting the titers of IgG1 isotypes. Furthermore, approximately 10-fold increased levels of IFN-gamma, but significantly reduced amounts of IL-5, were secreted from gp 160-restimulated splenic cells. A further greater than 30-fold increase in the levels of specific IgG2a responses and a substantially elevated secretion of IFN-gamma were observed when the mice received gp160/Alum/CpG combinations for priming and boost injections. Thus, CpG ODNs are useful as an adjuvant to induce a typical Th0/Th1 response to HIV gp 160 proteins. However, despite the induction of a more Th1-like immune response, gp 16O/Alum/CpG combinations were not sufficient to prime an Env-specific cytotoxic T-cell (CTL) response.

CpG Oligodeoxynucleotides Can Circumvent the Th2 Polarization of Neonatal Responses to Vaccines But May Fail to Fully Redirect Th2 Responses Established by Neonatal Priming

Neonatal murine responses to a panel of conventional vaccines differ qualitatively from adult responses by a particular polarization toward a Th2 pattern and a frequent limitation of the Th1 and CTL responses required for protection against intracellular microorganisms. In contrast, DNA vaccines induce adult-like Th1/CTL neonatal responses against the same vaccine Ags. In this report, we show that this can be related to their content in unmethylated CpG motifs. Oligodeoxynucleotides (ODN) containing CpG motifs activate neonatal APCs to produce IL-12 in vitro and induce adult-like Th1 responses to tetanus toxoid and measles Ags in vivo, with production of IgG2a-specific Abs and adult-like secretion of IFN-γ and IL-5 by Ag-specific T cells. However, in spite of their capacity to trigger neonatal B cell proliferation in vitro, CpG-ODN only partially enhanced early life Ab responses. Finally, using Th1-driving CpG-ODN with the boosting dose of a protein vaccine was sufficient to redirect adult but not neonatally primed Th2 responses. These observations could be important for the development of novel vaccines that will have to be effective early in life.

CpG DNA can induce strong Th1 humoral and cell-mediated immune responses against hepatitis B surface antigen in young mice

Successful neonatal immunization of humans has proven difficult. We have evaluated CpG-containing oligonucleotides as an adjuvant for immunization of young mice (1-14 days old) against hepatitis B virus surface antigen. The protein-alum-CpG formulation, like the DNA vaccine, produced seroconversion of the majority of mice immunized at 3 or 7 days of age, compared with 0-10% with the protein-alum or protein-CpG formulations. All animals, from neonates to adults, immunized with the protein-alum vaccine exhibited strong T helper (Th)2-like responses [predominantly IgG1, weak or absent cytotoxic T lymphocytes (CTL)]. Th2-type responses also were induced in young mice with protein-CpG (in 1-, 3-, and 7-day-old mice) and protein-alum-CpG (in 1- and 3-day-old mice) but immunization carried out at older ages gave mixed Th1/Th2 (Th0) responses. DNA vaccines gave Th0-like responses when administered at 1 and 7 days of age and Th1-like (predominantly IgG2a and CTL) responses with 14-day-old or adult mice. Surprisingly, the protein-alum-CpG formulation was better than the DNA vaccine for percentage of seroconversion, speed of appearance, and peak titer of the antibody response, as well as prevalence and strength of CTL. These findings may have important implications for immunization of human infants.

Cutting Edge: Rapid Induction of Mitogen-Activated Protein Kinases by Immune Stimulatory CpG DNA

Unmethylated CpG motifs in bacterial DNA or synthetic oligodeoxynucleotides (CpG DNA) rapidly activate B cells and monocyte-derived cells; however, the intracellular signaling pathways involved in this process are unclear. Here, we demonstrate that CpG DNA induces the activation of c-Jun NH2-terminal kinase and p38 but does not activate the extracellular receptor kinase in murine B and monocyte-like cell lines. CpG DNA also induces the phosphorylation of activating transcription factor-2, c-Jun, and mitogen-activated protein kinase (MAPK)-activated protein kinase 2 as well as the activation of activator protein-1 (AP-1) DNA binding. Inhibition of p38 led to the suppression of CpG DNA-induced AP-1 DNA-binding activity and cytokine production, indicating that the p38 pathway is required for mediating these immune stimulatory effects of CpG DNA. Chloroquine, an endosomal acidification inhibitor, selectively abolished CpG DNA-mediated MAPK activation. Our results indicate that CpG DNA activates the p38 and c-Jun NH2-terminal kinase MAPK and leads to the activation of AP-1 via a pathway which is sensitive to chloroquine.

Cutting Edge: CpG DNA Is a Potent Enhancer of Systemic and Mucosal Immune Responses Against Hepatitis B Surface Antigen with Intranasal Administration to Mice

Mucosal immunity is difficult to induce with subunit vaccines unless such vaccines are administered with a mucosal adjuvant such as cholera toxin (CT); however, CT is toxic in humans. Synthetic oligodeoxynucleotides containing immunostimulatory CpG motifs (CpG) are potent adjuvants for the induction of Th1-like systemic immune responses against parenterally delivered proteins. Here, we show in mice that intranasal delivery of hepatitis B surface Ag, which alone has no effect, elicits good immune responses when given with CpG oligodeoxynucleotides and/or CT. Overall, CpG is superior to CT for the induction of humoral and cell-mediated systemic immunity as well as mucosal immune responses (IgA) at local (lung) and distant (feces) sites. Furthermore, CpG and CT act synergistically, giving stronger responses than those observed with 10 times more of either adjuvant alone. Ab isotypes were predominantly IgG1 (Th2-like) with CT, mixed IgG1/IgG2a (Th0) with CpG, and predominantly IgG2a (Th1-like) with CpG and CT together.

Sequence motifs in adenoviral DNA block immune activation by stimulatory CpG motifs

Unmethylated CpG dinucleotides in particular base contexts (CpG-S motifs) are relatively common in bacterial DNA but are rare in vertebrate DNA. B cells and monocytes have the ability to detect such CpG-S motifs that trigger innate immune defenses with production of Th1-like cytokines. Despite comparable levels of unmethylated CpG dinucleotides, DNA from serotype 12 adenovirus is immune-stimulatory, but serotype 2 is nonstimulatory and can even inhibit activation by bacterial DNA. In type 12 genomes, the distribution of CpG-flanking bases is similar to that predicted by chance. However, in type 2 adenoviral DNA the immune stimulatory CpG-S motifs are outnumbered by a 15- to 30-fold excess of CpG dinucleotides in clusters of direct repeats or with a C on the 5' side or a G on the 3' side. Synthetic oligodeoxynucleotides containing these putative neutralizing (CpG-N) motifs block immune activation by CpG-S motifs in vitro and in vivo. Eliminating 52 of the 134 CpG-N motifs present in a DNA vaccine markedly enhanced its Th1-like function in vivo, which was increased further by the addition of CpG-S motifs. Thus, depending on the CpG motif, prokaryotic DNA can be either immune-stimulatory or neutralizing. These results have important implications for understanding microbial pathogenesis and molecular evolution and for the clinical development of DNA vaccines and gene therapy vectors.

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.

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.