Immunostimulatory bacterial DNA sequences activate dendritic cells and promote priming and differentiation of CD8+ T cells

CpG Oligonucleotides as Vaccine Adjuvants

CpG Oligonucleotides (ODN) are immunomodulatory synthetic oligonucleotides specifically designed to stimulate Toll-like receptor 9. TLR9 is expressed on human plasmacytoid dendritic cells and B cells and triggers an innate immune response characterized by the production of Th1 and pro-inflammatory cytokines. This chapter reviews recent progress in understanding the mechanism of action of CpG ODN and provides an overview of human clinical trial results using CpG ODN to improve vaccines for the prevention/treatment of cancer, allergy, and infectious disease.

Therapeutic antitumor potential of endoglin-based DNA vaccine combined with immunomodulatory agents

Therapy targeting tumor blood vessels ought to inhibit tumor growth. However, tumors become refractory to antiangiogenic drugs. Therefore, therapeutic solutions should be sought to address cellular resistance to antiangiogenic therapy. In this regard, reversal of the proangiogenic and immunosuppressive phenotype of cancer cells, and the shift of the tumor microenvironment towards more antiangiogenic and immune-stimulating phenotype may hold some promise. In our study, we sought to validate the effects of a combination therapy aimed at reducing tumor blood vessels, coupled with the abrogation of the immunosuppressive state. To achieve this, we developed an oral DNA vaccine against endoglin. This antigen was carried by an attenuated Salmonella Typhimurium and applied before or after tumor cell inoculation into immunocompetent mice. Our results show that this DNA vaccine effectively inhibited tumor growth, in both the prophylactic and therapeutic settings. It also activated both specific and nonspecific immune responses in immunized mice. Activated cytotoxic T-lymphocytes were directed specifically against endothelial and tumor cells overexpressing endoglin. The DNA vaccine inhibited angiogenesis but did not affect wound healing. In combination with interleukin-12-mediated gene therapy, or with cyclophosphamide administration, the DNA vaccine resulted in reduced microvessel density and lowered the level of Treg lymphocytes in the experimental tumors. This effectively inhibited tumor growth and prolonged survival of the treated animals. Polarization of tumor milieu, from proangiogenic and immunosuppressive, towards an immunostimulatory and antiangiogenic profile represents a promising avenue in anticancer therapy.

Response of nitric oxide production to CpG oligodeoxynucleotides in turkey and chicken peripheral blood monocytes

We evaluated the innate immune response to various synthetic CpG-containing oligodeoxynucleotides (CpG ODNs) by measuring nitric oxide production in the peripheral blood monocytes from turkey poults. The results indicate that the presence of the CpG dinucleotide in ODNs was a prerequisite for activation of turkey monocytes and induction of nitric oxide (NO) synthesis. CpG motifs and sequence structure of the ODNs were also found to influence stimulatory activity greatly. The most potent CpG ODN to induce NO synthesis in turkey monocytes was human-specific CpG ODN M362, followed by CpG ODN 2006 (human), CpG ODN#17 (chicken) and CpG ODN 1826 (mouse). The optimal CpG motif for NO induction was GTCGTT. Phosphorothioate modification of CpG ODNs also significantly increased stimulatory activity. Compared with chicken monocytes, turkey monocytes appeared to be less sensitive to CpG motif variation, whereas chicken monocytes were found to respond more strictly to human-specific CpG ODNs or ODNs that contain GTCGTT motifs.

CpG oligodeoxynucleotide-induced immunity prevents growth of germinal center-derived B lymphoma cells

Therapeutic efficacy of CpG oligodeoxynucleotide (ODN) ISS 1018 was tested in a murine B cell lymphoma model. Previous studies showed that the B lymphoma cells of SJL mice stimulate vigorous proliferation of host CD4(+) TH cells that is unaccompanied by development of tumor-specific CTL. In the presence of ISS 1018, however, tumor cells stimulated high levels of CTL activity in vitro, and this cytotoxic activity was inhibited when anti-IL-12 mAb was added to the cultures. Tumor cells pre-incubated with ISS 1018 were also able to generate CTL without addition of exogenous ODN, and FACS analysis revealed that following incubation with ISS 1018 for 24 h, tumor cells exhibited upregulation of MHC I, MHC II, and co-stimulatory molecule CD80. Finally, tumor-injected mice treated with ISS 1018 showed significantly less growth of tumor cells in lymph nodes and spleen, and exhibited prolonged survival compared to mice treated with a control ODN. The documented effects of CpG ODNs to stimulate cytokines, such as IL-12, from antigen presenting cells, and to upregulate expression of MHC Class I and Class II, as well as co-stimulatory molecules on tumor cells, are also the likely mechanisms by which CTL are generated by ISS 1018 in the SJL B cell lymphoma model.

DNA vaccines for therapy of tuberculosis: Where are we now?

DNA vaccines that were being investigated in mice for prophylactic use against tuberculosis were soon found also to be surprisingly effective as treatment against established infection. The immune system was stimulated to kill the bacteria, even including the persistent latent bacteria that are otherwise refractory to the immune system and antibacterial chemotherapeutic drugs alike. Subsequent results from a range of laboratories using diverse DNA vaccines in diverse murine models of infection have been very varied, ranging from enhanced pathology, through negligible effects, to major additive benefit from combined vaccine and chemotherapy. This review summarises the data and assesses future prospects.

CpG immunostimulatory sequences enhance contact hypersensitivity responses in mice

Bacterial DNA and synthetic cytidine-phosphate-guanosine-oligodeoxynucleotides (CpG ODN) potently activate dendritic cells (DC) and therefore have been proposed as adjuvants for vaccination strategies. Although CpG ODN are considered as safe adjuvants this study shows that CpG ODN are responsible for enhanced antigen-specific skin inflammatory reactions. We used the murine model of contact hypersensitivity (CHS) to 2,4-dinitrofluorobenzene (DNFB) in which hapten-specific CD8+T cytotoxic 1 cells are effector cells. Subcutaneous injection of CpG ODN, 1 d before sensitization enhanced the CHS response to DNFB and resulted in increased recruitment of CD8+ T cells at the challenge sites, whereas control ODN injection did not have any effect. This effect was local and not systemic as it was only observed when DNFB was applied at the same site as the CpG motifs. CpG ODN-induced enhancement of CHS was due to increased antigen-presenting cell functions of DC since: (i) CpG ODN-injected skin revealed upregulated expression of major histocompatibility complex class II, CD80, and CD86 molecules and (ii) CpG ODN treatment of DNFB-derivatized DC enhanced the intensity of CHS responses after in vivo transfer. Taken together, the results show that CpG ODN may be responsible for immune side-effects such as worsening of T cell-mediated skin diseases.

Modulation of immune responses to bovine herpesvirus-1 in cattle by immunization with a DNA vaccine encoding glycoprotein D as a fusion protein with bovine CD154

The objective of this study was to determine whether a DNA vaccine encoding bovine CD154 linked to a truncated version of bovine herpesvirus-1 (BHV-1) glycoprotein D (tgD-CD154) induces enhanced tgD-specific immune responses in cattle. In vitro characterization demonstrated that tgD and tgD-CD154 both bind to cultured bovine B cells, whereas only tgD-CD154 induces interleukin-4-dependent proliferation, suggesting that tgD-CD154 specifically binds the CD40 receptor and induces receptor signalling. Calves were immunized with plasmid encoding either tgD or tgD-CD154 by intradermal injection with a needle-free device. After two immunizations, tgD-specific immune responses were observed in both vaccinated groups and after challenge with BHV-1 these responses further increased. Animals immunized with plasmid encoding tgD-CD154 had significantly higher tgD-specific serum titres of immunoglobulins G and A but significantly lower numbers of tgD-specific interferon-gamma-secreting cells than animals immunized with plasmid encoding tgD after BHV-1 challenge. This suggests that the expression of an antigen as a chimeric protein with CD154 can qualitatively alter immune responses in cattle. Since we previously showed that plasmid encoding tgD-CD154 induces significantly enhanced secondary tgD-specific antibody responses in sheep, there appear to be interspecies differences in the immune responses induced by tgD-CD154, which suggests that both proteins in the chimeric molecule may influence protein targeting and the induction of an immune response.

Identification of CpG oligodeoxynucleotide motifs that stimulate nitric oxide and cytokine production in avian macrophage and peripheral blood mononuclear cells

Unmethylated CpG dinucleotides within specific flanking bases (referred to as CpG motif) are relatively abundant in bacterial DNA and are known to stimulate innate immune responses. In this study, synthetic CpG containing oligodeoxydinucleotides (CpG-ODNs) were evaluated for their ability to stimulate nitric oxide (NO), interleukin-1beta (IL-1beta), and interferon-gamma (IFN-gamma) production using an avian macrophage cell line (HD11) and peripheral blood mononuclear cells (PBMC). Results showed ODNs containing the CpG motif can activate the HD11cells and induce NO production. The optimal CpG-ODN motif for NO induction was GTCGTT. Increasing GTCGTT motifs in CpG-ODN significantly enhanced the stimulatory effect. Deviation of flanking bases of the CpG dinucleotide diminished the stimulatory activity. We also found CpG-ODN differentially stimulated expression of cytokine genes. The most active CpG motif for NO induction was also a strong stimulant for the IL-1beta gene expression in the HD11 cells, whereas different CpG motifs were found to induce IFN-gamma gene expression in PBMC.

Immunomodulatory effects of CpG oligodeoxynucleotides on established th2 responses

CpG oligodeoxynucleotides (CpG ODNs) are known to induce type 1 T-helper-cell (Th1) responses. We have previously demonstrated that CpG ODNs administered during sensitization prevent Th2-mediated eosinophilic airway inflammation in vivo. We also reported that key Th1 cytokines, gamma interferon (IFN-gamma) and interleukin 12 (IL-12), are not necessary for this protection. Recent in vivo data suggest that CpG ODNs might also reverse established pulmonary eosinophilia. In order to clarify how CpG ODNs can inhibit established Th2 responses, we evaluated the cytokine production from splenocytes from antigen- and alum-immunized mice. Restimulation with antigen induced IL-5, which was clearly inhibited by coculture with CpG ODNs in a concentration-dependent manner. CpG ODNs also induced IFN-gamma, but in a concentration-independent manner. The inhibition of IL-5 production was not mediated through natural killer cells or via CD8(+) T lymphocytes. Although IFN-gamma plays an important role in inhibition of antigen-induced IL-5 production by CpG ODNs, IFN-gamma was not the sole factor in IL-5 inhibition. CpG ODNs also induced IL-10, and this induction correlated well with IL-5 inhibition. Elimination of IL-10 reduced the anti-IL-5 effect of CpG ODNs, although incompletely. This may be because IFN-gamma, induced by CpG ODNs, is also inhibited by IL-10, serving as a homeostatic mechanism for the Th1-Th2 balance. Overproduction of IFN-gamma was downregulated by CpG ODN-induced IL-10 via modulation of IL-12 production. These data suggest that CpG ODNs may inhibit established Th2 immune responses through IFN-gamma and IL-10 production, the latter serving to regulate excessive Th1 bias. These properties of CpG ODNs might be a useful feature in the development of immunotherapy adjuvants against allergic diseases such as asthma.

Maternal and newborn immunization with a human immunodeficiency virus-1 immunogen in a rodent model

We examined immunization with an inactivated, gp120-depleted human immunodeficiency virus (HIV) antigen in incomplete Freund's adjuvant (IFA), also containing a sequence of immunostimulatory (ISS) DNA, during the last trimester of pregnancy and neonatally in a rat model. Pregnant rats were immunized in the third trimester and their litters were immunized during the newborn period. In addition, litters of rats from non-immunized mothers were immunized during the neonatal period. As another control, pregnant rats were immunized and their litters analysed. Supernants from peripheral blood mononuclear cells (PBMCs) were assayed from newborns at 4 weeks of age for HIV-specific interferon-gamma (IFN-gamma), HIV-specific regulated on activation, normal, T-cell expressed, and secreted (RANTES), and serum for p24 antigen-specific immunoglobulin G (IgG) production. In the animals whose pregnant mothers were immunized and were also immunized during the neonatal period, we observed HIV-specific IFN-gamma production and HIV-specific RANTES production, but weak p24 IgG antibody production. Animals immunized only during the neonatal period developed the highest levels of HIV-specific IFN-gamma production, but somewhat lower levels of HIV-specific RANTES and p24 IgG antibody production. The group of animals whose mothers had received immunizations during the last trimester of pregnancy, but were not immunized during the neonatal period, developed the strongest p24 IgG antibody levels, but little or undetectable HIV-specific IFN-gamma or RANTES production. Neonatal immunization resulted primarily in cell-mediated immune responses, while animals born to mothers who were immunized during the last trimester had primarily an antibody-mediated immune response. Immunization of pregnant animals followed by neonatal immunization resulted in a mixed cell-mediated/antibody type profile in the neonatal animal. Future studies should provide insights into neonatal immunity and potential vaccine approaches to prevent neonatal infection and perinatal transmission.

CpG motifs in bacterial DNA and their immune effects

Unmethylated CpG motifs are prevalent in bacterial but not vertebrate genomic DNAs. Oligodeoxynucleotides (ODN) containing CpG motifs activate host defense mechanisms leading to innate and acquired immune responses. The recognition of CpG motifs requires Toll-like receptor (TLR) 9, which triggers alterations in cellular redox balance and the induction of cell signaling pathways including the mitogen activated protein kinases (MAPKs) and NF kappa B. Cells that express TLR-9, which include plasmacytoid dendritic cells (PDCs) and B cells, produce Th1-like proinflammatory cytokines, interferons, and chemokines. Certain CpG motifs (CpG-A) are especially potent at activating NK cells and inducing IFN-alpha production by PDCs, while other motifs (CpG-B) are especially potent B cell activators. CpG-induced activation of innate immunity protects against lethal challenge with a wide variety of pathogens, and has therapeutic activity in murine models of cancer and allergy. CpG ODN also enhance the development of acquired immune responses for prophylactic and therapeutic vaccination.

Nucleic acid vaccines: tasks and tactics

There are no adequate vaccines against some of the new or reemerged infectious scourges such as HIV and TB. They may require strong and enduring cell-mediated immunity to be elicited. This is quite a task, as the only known basis of protection by current commercial vaccines is antibody. As DNA or RNA vaccines may induce both cell-mediated and humoral immunity, great interest has been shown in them. However, doubt remains whether their efficacy will suffice for their clinical realization. We look at the various tactics to increase the potency of nucleic acid vaccines and divided them broadly under those affecting delivery and those affecting immune induction. For delivery, we have considered ways of improving uptake and the use of bacterial, replicon or viral vectors. For immune induction, we considered aspects of immunostimulatory CpG motifs, coinjection of cytokines or costimulators and alterations of the antigen, its cellular localization and its anatomical localization including the use of ligand-targeting to lymphoid tissue. We also thought that mucosal application of DNA deserved a separate section. In this review, we have taken the liberty to discuss these enhancement methods, whenever possible, in the context of the underlying mechanisms that might argue for or against these strategies.

Oral DNA vaccination: antigen uptake and presentation by dendritic cells elicits protective immunity

Melanoma differentiation antigens, such as glycoprotein 100 (gp100), have been shown to induce both cellular and humoral immune responses against melanoma in mouse and man. They are therefore considered as potential targets for melanoma immunotherapy. In this study, we have used the attenuated auxotrophic mutant strain SL7207 of Salmonella typhimurium as vehicle for a human gp100 (hgp100) DNA vaccine against melanoma. In vitro studies indicate that Salmonella/pCMV-hgp100 is efficiently scavenged by dendritic cells, resulting in the expression of the hgp100 transcription unit in the DC. In addition, oral administration of Salmonella/pCMV-hgp100 results in the expression of hgp100 RNA and protein by cells exhibiting DC-morphology in mesenteric lymph nodes as soon as 3 days after vaccination. Analysis of the efficacy of the Salmonella/pCMV-hgp100 vaccine in the B16/hgp100 model demonstrated the induction of strong anti-hgp100 CTL responses and protective immunity in 70% of the vaccinated mice, but not in control mice. Based on these data, we consider S. typhimurium as a useful vehicle for the design of recombinant DNA based anti-cancer vaccines.

Cytolytic T cells in the immune response to mycobacterium tuberculosis

Cytolytic T cells (CTL) are of paramount importance in immune defense against tumors and viruses. Work over the past decade has revealed that lysis of infected cells is also involved in protective immunity to bacteria and parasites, including Mycobacterium tuberculosis. Experiments involving gene-deleted mice and the characterization of CTL lines derived from tuberculosis patients suggest an important role of CTL in immunity to tuberculosis. More recently, the identification of an effector pathway of human CTL provided evidence for direct antimicrobial activity of CTL. This pathway involves the combined action of the pore-forming perforin and the antibacterial granulysin, both expressed in the granules of CTL. Granulysin binds to the bacterial cell surface, thereby disrupting the membrane and causing osmotic lysis. The relevance of this pathway for protection against intracellular pathogens is suggested by the expression of high amounts of granulysin in tissue from patients with tuberculoid leprosy, which are able to contain the spread of the bacilli. These findings support the current concept of designing novel vaccination strategies which elicit not only CD4 + T helper cells, but also CD8 + CTL with direct antibacterial activity.

CpG oligodeoxynucleotides enhance host defense during murine tuberculosis

Oligodeoxynucleotides (ODNs) containing unmethylated CpG motifs activate immune cells to produce cytokines. CpG ODNs protect mice against infections with intracellular bacteria by the induction of a T helper 1 (Th1) response. To determine the effect of CpG ODNs in pulmonary tuberculosis, mice were treated with CpG ODNs or control ODNs at the time of intranasal infection. CpG ODNs reduced mycobacterial outgrowth for up to 5 weeks after Mycobacterium tuberculosis infection and were associated with a decrease in inflammation in lung tissue. CpG treatment was also associated with elevated levels of gamma interferon (IFN-gamma) and decreased levels of interleukin 4 in the lungs and an increased capacity of splenocytes to secrete Th1-type cytokines. CpG ODNs given 2 weeks after infection were still able to reduce mycobacterial outgrowth and to enhance a Th1 response 5 weeks postinfection. Administration of CpG ODNs to IFN-gamma-gene-deficient mice failed to reduce mycobacterial outgrowth. These data suggest that CpG ODNs improve host defense during pulmonary tuberculosis by an IFN-gamma-dependent mechanism.

T cell immunity in neonates

Typically, neonates exhibit decreased or aberrant cellular immune responses when compared to adults, resulting in increased susceptibility to infection. However, it is clear that newborns are able to generate adult-like protective T cell responses under certain conditions. The focus of our research is to understand the deficiencies within the neonatal immune system that lead to improper cellular responses and how priming conditions can be altered to elicit the appropriate T cell response necessary to protect against development of pathogen-induced disease. With these goals in mind, we are exploring the attributes of neonatal T cells and their development, as well as the conditions during priming that influence the resulting response to immune challenge during the neonatal period.

Genetic adjuvants for DNA vaccines

The relatively low efficacy of DNA vaccines in inducing immune responses, especially in large animal species and humans, has impaired their practical use. Despite considerable effort expended on improving DNA vaccine delivery, only minute amounts of Ag are available for immune induction following DNA vaccination. Two complementary strategies have been used to improve and modulate the immune response induced by DNA vaccines: (i) supplementing DNA vaccines with plasmids encoding cytokines and (ii) targeting the Ag encoded by DNA vaccine through genetically fusing the Ag to molecules binding cell surface receptors. This paper reviews recent progress in these two areas and possible mechanisms responsible for the observed effects.

HIV-Specific CD4(+) and CD8(+) immune responses are generated with a gp120-depleted, whole-killed HIV-1 immunogen with CpG immunostimulatory sequences of DNA

We examined the adjuvant effects of a synthetic CpG oligodeoxynucleotide immunostimulatory sequence (ISS) using a whole-killed, gp120-depleted HIV antigen (HIV-1 antigen) in a Lewis rat model. We hypothesized that HIV-1-specific CD4(+) T helper (Th) immune responses could be enhanced when an ISS was combined with an HIV-1 antigen in incomplete Freund's adjuvant (IFA). We also reasoned that if such Th responses were sufficient, such a combination might also induce HIV-specific CD8(+) T cell immune responses. Here we demonstrate that the HIV-1 antigen in IFA combined with ISS stimulates both CD4(+) and CD8(+) HIV-specific immune responses as measured by interferon-gamma (IFN-gamma) in the ELISPOT assay. A strong correlation between these CD4(+) and CD8(+) responses was demonstrated. Furthermore, we found that the HIV-1 antigen in IFA with ISS as an adjuvant stimulated strong antibody responses to core antigen (p24). These studies suggest that the combination of the whole-killed, gp120-depleted HIV-1 antigen in IFA with ISS may be an ideal candidate to test in nonhuman primates and in human studies as a preventive HIV-1 vaccine.