Mammalian granulocyte-macrophage colony-stimulating factor and some CpG motifs have an effect on the immunogenicity of DNA and subunit vaccines in fish

Cytokines Studied in Carp (Cyprinus carpio L.) in Response to Important Diseases

Cytokines belong to the most widely studied group of intracellular molecules involved in the function of the immune system. Their secretion is induced by various infectious stimuli. Cytokine release by host cells has been extensively used as a powerful tool for studying immune reactions in the early stages of viral and bacterial infections. Recently, research attention has shifted to the investigation of cytokine responses using mRNA expression, an essential mechanism related to pathogenic and nonpathogenic-immune stimulants in fish. This review represents the current knowledge of cytokine responses to infectious diseases in the common carp (Cyprinus carpio L.). Given the paucity of literature on cytokine responses to many infections in carp, only select viral diseases, such as koi herpesvirus disease (KHVD), spring viremia of carp (SVC), and carp edema virus disease (CEVD), are discussed. Aeromonas hydrophila is one of the most studied bacterial pathogens associated with cytokine responses in common carp. Therefore, the cytokine-based immunoreactivity raised by this specific bacterial pathogen is also highlighted in this review.

A CpG-riched plasmid as vaccine adjuvant reduce antigen dose of an inactivated Vibrio anguillarum vaccine in turbot (Scophthalmus maximus L.)

Inactivated vaccines are often applied with adjuvants in commercial fish farming. Although some mineral or non-mineral oil adjuvants show efficient improvement with inactivated vaccines, but sometimes bring side effects such as tissue adhesion and granulomatous lesion at the injection site. CpG ODN is a novel type of soluble adjuvant which has been proved to possess excellent advantages in fish vaccine development. In this study, we designed a tandem sequence of CpG ODN synthesized in plasmid pcDNA 3.1, and an inactivated Vibrio anguillarum vaccine developed in our previous work was chosen for determining the efficiency of the CpG-riched plasmids (pCpG) as an adjuvant. Results showed that pCpG we designed can offer higher immunoprotection with the vaccine. Interestingly, even below the minimum immune dosage of the vaccine, a high RPS of 84% was observed once the vaccine was administrated with the pCpG. Serum specific antibody titer, superoxide dismutase and total protein were enhanced and some immune genes related to both innate and adaptive immune response were upregulated, implying an effective auxiliary function of the pCpG. Totally, our study suggested that the pCpG is a potential and available adjuvant for turbot vaccine development.

Effect of miR-155 as a molecular adjuvant of DNA vaccine against VHSV in olive flounder (Paralichthys olivaceus)

Rhabdoviral G protein-based DNA vaccines have been recognized as a useful way to protect cultured fish from rhabdoviral diseases. In Korea, viral hemorrhagic septicemia virus (VHSV) genotype IVa has been the primary culprit of high mortalities of cultured olive flounder (Paralichthys olivaceus). In this study, we inserted a miR-155-expressing cassette into the VHSV's G protein-based DNA vaccine, and analyzed the effects of miR-155 on the antiviral activity and on the vaccine efficacy in olive flounder. Olive flounder fingerlings were intramuscularly (i.m.) immunized with 10 μg/fish (1st experiment) or 1 μg/fish (2nd experiment) of DNA vaccine plasmids. However, there were no significant differences in mortalities and serum neutralization titers between fish immunized with 1 μg and 10 μg plasmids/fish, suggesting that i.m. injection with 1 μg plasmids/fish would be enough to induce effective adaptive immune responses in olive flounder fingerlings. In survival rates, as fish immunized with just G protein expressing plasmids showed no or too low mortalities, the adjuvant effect of miR-155 was not discernible. Also, in the serum neutralization activities, although G gene or G gene plus miR-155 expressing DNA vaccines induced significantly higher activities than control vaccines (PBS and vacant vector), no significant differences were found between G gene alone and G gene plus miR-155 expressing DNA vaccines. In the serum virucidal activity, fish immunized with G gene plus miR-155 expressing DNA vaccine showed significantly higher activity against hirame rhabdovirus (HIRRV) at 3 days post-immunization (d.p.i.) compared to other groups, suggesting that miR-155 produced from the vector can enhance innate immune responses in olive flounder. The significantly enhanced serum virucidal activities against VHSV especially at 28 d.p.i. in the groups immunized with G gene alone and G gene plus miR-155 expressing DNA vaccines reflect the increased antibodies against G protein, which could activate the classical complement pathway and subsequent viral inactivation. As the available information on the DNA vaccines in olive flounder is not sufficient, more diverse researches on the protective efficacy of DNA vaccines are needed to make more practical use of DNA vaccines in olive flounder farms.

The introduction of multi-copy CpG motifs into an antiviral DNA vaccine strongly up-regulates its immunogenicity in fish

The protection conferred by antiviral DNA vaccines in fish is known to rely greatly on innate immune responses. Since oligodeoxynucleotides (ODNs) containing unmethylated CpG dinucleotides (CpG motifs) have been shown to induce potential innate immune responses, we have introduced several copies (either two or four) of a fragment containing multiple CpG sequences of known immunostimulatory effects into a DNA vaccine against viral hemorrhagic septicemia virus (VHSV). We have determined the effects of this introduction on the vaccine immunogenicity, measured as immune gene induction, serum neutralizing activity and antigen-dependent proliferation. When comparing the effects of the vaccine containing 2 copies of this CpG fragment (pVHSV-2CpG) or that containing 4 copies of the fragment (pVHSV-4CpG) with the original VHSV DNA vaccine (pVHSV), we found that the levels of expression of type I interferon (IFN) were significantly up-regulated in muscle and spleen when the CpG fragments were introduced. An up-regulation in the levels of MHC-I expression in spleen were also observed in response to the modified vaccines, whereas, the levels of transcription of interleukin 1β (IL-1β) were strongly reduced in comparison to the original vaccine. Important but very variable differences were also observed concerning the vaccine induction of IFN-γ. Moreover, the serum neutralizing capacity was strongly increased as fish were vaccinated with plasmids containing more CpG fragments. Taken together, all these results demonstrate a strongly increased immunogenicity of the VHSV DNA vaccine, through the introduction of this multicopy CpG fragment.

Unmethylated CpG oligodeoxynucleotides activate head kidney leukocytes of Atlantic cod, Gadus morhua

Bacterial DNA and synthetic oligodeoxynucleotides (ODNs) that contain unmethylated CpG motifs are strong inducers of immune response in most mammalian organisms. The use of these synthetic CpG motifs in fish, particularly in salmonids and carp, resulted in the modulation of their immune system. However, much less is known in other species of fish such as gadoids including Atlantic cod, Gadus morhua. Using head kidney (HK) leukocytes of cod in an in vitro study, we determined the effects of some established CpG-ODNs on the cellular responses of the fish immunocytes. Incubation of the HK leukocytes with 2 μM concentration of the CpG-ODNs resulted in enhanced respiratory burst. There were differential effects on the activities of acid phosphatase and cellular myeloperoxidase. Only CpG-ODN 1826 triggered a significant increase in the level of both enzymes. On the other hand, the supernatants derived from the HK leukocytes after incubation with different CpG-ODNs did not possess bactericidal activity against Vibrio anguillarum and Aeromonas salmonicida. This study has shown that CpG-ODNs at low concentrations are able to stimulate respiratory burst in cod but have minimal effects on cellular enzymatic activities and antibacterial action.

CpG oligonucleotides as adjuvants for vaccines targeting infectious diseases

Synthetic oligodeoxynucleotides (ODN) containing unmethylated CpG motifs act as immune adjuvants, accelerating and boosting antigen-specific immune responses. CpG motifs promote the induction of Th1 and pro-inflammatory cytokines and support the maturation/activation of professional antigen presenting cells (particularly plasmacytoid dendritic cells). These effects are optimized by maintaining close physical contact between the CpG ODN and the immunogen. Co-administering CpG ODN with a variety of vaccines has improved the resultant humoral and/or cellular immune responses, culminating in enhanced protective immunity in rodent and primate challenge models. Ongoing clinical studies indicate that CpG ODN are safe and well-tolerated when administered as adjuvants to humans, and that they can support increased vaccine-specific immune responses.

Immunomodulatory consequences of ODN CpG-polycation complexes

Immunostimulatory ODN CpGs have extensively been tested as adjuvants and immunotherapeutics and hold a lot of promise for human use. In our studies we took advantage of their negative charge to study their biological activities after being complexed with carbon nanotubes, a novel vector for vaccine delivery and Tat protein of HIV, a target protein for therapeutic or prophylactic intervention. In the case of carbon nanotubes, ODN CpGs were able to form stable complexes based on charge interaction and exert increased immunostimulatory activity in vitro. With regard to the Tat protein, ODN CpGs were shown to bind effectively through the basic domain of the protein representing residues 44-61. Moreover, using surface Plasmon Resonance Technology and an in vitro cellular system, ODN CpGs were shown to inhibit the interaction of Tat protein with the transactivation responsive element, a bulged RNA hairpin structure. However, when ODN CpGs were complexed with Tat they readily increased the apoptotic properties of this protein as studied in CD3-stimulated Jurkat cells. Overall, our findings together with published data support the view that for harnessing the beneficial effects of ODN CpGs a careful consideration has to be given depending on the target intervention.

NK-like and oxidative burst activities are the main early cellular innate immune responses activated after virus inoculation in reservoir fish

Viral diseases are a major problem in fish farming and a deeper knowledge of the immunological mechanisms playing a part in the antiviral defence is still important. Moreover, fish farming practices (high densities, new areas of culture and egg/larvae/adult transport) are significantly increasing the spread of viruses and the number of susceptible or reservoir fish species. In this last point, no studies have focused on the immunological mechanisms playing a part in the antiviral responses in reservoir and non-susceptible fish species. Thus, we have evaluated the very early innate immune responses of gilthead seabream (Sparus aurata) to the virus causing viral haemorrhagic septicaemia (VHSV) in salmonids since this virus has been found in seabream and neighbouring farmed marine fish species acting as a viral reservoir. The virus was detected in liver, head-kidney, spleen and peritoneal cavity suggesting that the virus reached these tissues but did not replicate as viral expression was almost absent by 72 h post-inoculation. Interestingly, VHSV provoked an influx of leucocytes to the peritoneal cavity and a redistribution of peritoneal exudate (PELs) and head-kidney (HKLs) leucocytes and their innate immune responses (non-specific cytotoxic (NCC or NK-like) activity, phagocytosis, reactive oxygen intermediate (ROI) production and myeloperoxidase (MPO) activity) were generally increased demonstrating that the immune system is activated and involved in the clearance of the virus. Strikingly, NK-like, ROI and MPO were the most enhanced by the presence of VHSV in both PELs and HKLs suggesting that these early innate immune events are crucial during early viral infection stages in non-susceptible or reservoir species. Differences in the immunological mechanisms between susceptible and reservoir species and with other particulate antigens are discussed.

Presence and role of cytosine methylation in DNA viruses of animals

Nucleotide composition varies greatly among DNA viruses of animals, yet the evolutionary pressures and biological mechanisms driving these patterns are unclear. One of the most striking discrepancies lies in the frequency of CpG (the dinucleotide CG, linked by a phosphate group), which is underrepresented in most small DNA viruses (those with genomes below 10 kb) but not in larger DNA viruses. Cytosine methylation might be partially responsible, but research on this topic has focused on a few virus groups. For several viruses that integrate their genome into the host genome, the methylation status during this stage has been studied extensively, and the relationship between methylation and viral-induced tumor formation has been examined carefully. However, for actively replicating viruses—particularly small DNA viruses—the methylation status of CpG motifs is rarely known and the effects on the viral life cycle are obscure. In vertebrate host genomes, most cytosines at CpG sites are methylated, which in vertebrates acts to regulate gene expression and facilitates the recognition of unmethylated, potentially pathogen-associated DNA. Here we briefly introduce cytosine methylation before reviewing what is currently known about CpG methylation in DNA viruses.

Construction of a recombinant plasmid containing multi-copy CpG motifs and its effects on the innate immune responses of aquatic animals

Bacterial DNA and synthetic oligodeoxynucleotides (ODNs) containing unmethylated CpG dinucleotides (CpG motifs) have been shown to induce potential immune responses. In this study, we designed a recombinant plasmid containing multi-copy CpG motifs, and observed its effects on innate immune responses of fish and prawn. The results showed that such plasmid DNA, compared to the vacant vector, can highly induce the activation of head kidney macrophages and the proliferation of peripheral blood leukocytes in Carassius auratus and Lateolabrax japonicus in vitro, as well as the activity of humoral defense proteins and the antibacterial activity of haemolymph in Litopenaeus vannamei in vivo. It implies that the multi-copy CpG motifs harboured in plasmid could contribute to these innate immunostimulatory effects. Therefore, the study suggested that the plasmid containing multi-copy CpG motifs might have its potential application in improving host resistance to pathogen insults in aquaculture, and have its notable advantages of high efficacy, economical cost and application to a broad range of aquatic species.

CpG oligodeoxynucleotides augment the immune responses of piglets to swine Pasteurella multocida living vaccine in vivo

Oligodeoxynucleotides containing unmethylated CpG motifs (CpG ODN) prevent development of T-helper type 2 (Th2) immune response and reverse established allergic responses in mouse models. However, little work on immune responses in piglets has been conducted in vivo. In this report, the ability of a porcine-specific CpG ODN to act as an immunostimulant and enhance immune responses of piglets to swine Pasteurella multocida living vaccine (SPML vaccine) was determined. The titre of IgG and IgG1/IgG2 isotype to SPML vaccine in serum, the proliferation of lymphocytes, SPML-specific interferon-gamma (IFN-gamma) and IL-6, TNF-alpha, IL-4 production of PBMCs in vitro and IFN-gamma, IL-6, TNF-alpha, IL-4, IL-10 in piglets serum were examined to identify the immune responses of the piglets. Immune responses of the piglets vaccinated with SPML and CpG ODN were significantly stronger than responses of piglets vaccinated with SPML alone. All these data summarized that immunostimulatory CpG ODN could modulate the immune response towards a Th1-like response when co-administered to piglets during SPML vaccination, which suggested that the therapeutic uses envisioned for these ODNs (as vaccine adjuvants and immunoprotective agents) may be applicable to husbandry animals.

Characterization of three distinct CpG oligonucleotide classes which differ in ability to induce IFN alpha/beta activity and cell proliferation in Atlantic salmon (Salmo salar L.) leukocytes

Oligodeoxynucleotides (ODNs) containing unmethylated CpG dinucleotides within specific sequence contexts (CpG motifs) are in humans divided into three distinct classes (A, B and C). The CpG ODNs, like baterial DNA, are recognized by the vertebrate immune system and are known to stimulate immune responses. The characterization of the different classes is based on their structure and biological activity. In this study, we report the effects of these classes of CpG ODNs as they are defined in humans on IFN alpha/beta production and cell proliferation in Atlantic salmon spleen, head kidney and blood leukocytes. These studies revealed that CpG A together with CpG C are strong inducers of IFN alpha/beta in spleen and head kidney leukocytes, whilst CpG B and CpG C had the highest capacity to stimulate cell proliferation in all three cell populations. These findings are the first to establish that the effects of the different CpG classes are comparable between fish and man.

Vaccination with Newcastle disease vaccine and CpG oligodeoxynucleotides induces specific immunity and protection against Newcastle disease virus in SPF chicken

Oligodeoxynucleotides containing unmethylated CpG motifs (CpG ODN) have been proven to be immunoprotective in mouse models. However, little work has been conducted on in vivo immune responses in chicken with CpG ODN. The objective of this study was to investigate the immunoadjuvant effects of CpG ODN to Newcastle disease (ND) vaccine and its protective effects against ND virus in SPF chicken. In this report, the titre of serum IgG to ND vaccine and the proliferation of lymphocytes were monitored in SPF chickens. The results demonstrated that the above-mentioned immune responses were significantly stronger in chickens that received CpG ODN than in the birds that received only ND vaccine. Furthermore, ND vaccine plus CpG ODN protected SPF chicken from challenge with an otherwise lethal dose of ND virus. These data suggest that CpG ODN holds considerable promise as an adjuvant for future vaccines against ND virus.

Adjuvant activity of CpG oligodeoxynucleotides

Synthetic oligodeoxynucleotides (ODNs) containing unmethylated CpG motifs directly stimulate human B cells and plasmacytoid dendritic cells (pDCs), thereby promoting the production of Th1 and proinflammatory cytokines and the maturation/activation of professional antigen-presenting cells. These activities enable CpG ODNs to act as immune adjuvants, accelerating and boosting antigen-specific immune responses by 5- to 500-fold. The CpG motifs present in bacterial DNA plasmids may contribute to the immunogenicity of DNA vaccines. Ongoing clinical studies indicate that CpG ODNs are safe and well tolerated when administered as adjuvants to humans and can improve vaccine-induced immune responses.

The effect of oral immuno-stimulation in juvenile carp (Cyprinus carpio L.)

The effect of a 2-week period of oral immuno-stimulation from the age of 2 or 6 weeks post-fertilisation (wpf; before and after reaching the ability to produce antibodies) onwards was investigated on various immune functions of the common carp, Cyprinus carpio. The immuno-stimulants Aeromonas salmonicida lipopolysaccharide, Yeast DNA (containing unmethylated CpG motifs) or high-M alginate (an extract of algae containing poly-mannuronic acid) were used. The effect of this treatment was studied on the kinetics of B cells in head kidney and peripheral blood leucocytes using flow cytometry, on the total plasma IgM level using ELISA, on cytokine and inducible nitric oxide synthase (iNOS) expression in the intestine, and acute phase protein expression in the liver, using real time quantitative PCR, and on exposure to Vibrio anguillarum. Oral administration of immuno-stimulants from 6 wpf resulted in decreased WCI12(+) (B) cell percentages in PBL (only after administration of LPS) and head kidney (all test groups), and a decreased total IgM level in plasma, suggesting that suppressive effects are strongly indicative of oral or juvenile tolerance. After administration from 2 wpf, the effects on WCI12(+) (B) cell percentages were less pronounced: the group fed with Yeast DNA showed higher percentages compared to the control group at 6 wpf, but lower percentages at 8 wpf. No changes were observed in the cytokine or iNOS expression levels in the intestine or acute phase protein expression in the liver. A challenge with V. anguillarum resulted in an initially higher cumulative mortality in the group fed with LPS, but lower mortality in the groups fed with Yeast DNA or high-M alginate compared to the control group, providing a provisional warning especially for the use of pathogen-derived immuno-stimulants, such as A. salmonicida LPS, in larval and juvenile fish.

A review of CpGs and their relevance to aquaculture

CpG oligodeoxynucleotides (ODN) have been described as functioning as natural adjuvants because they promote professional antigen presenting cell (APC) function and co-stimulate lymphocytes. The majority of studies into the immune effects of CpG ODN to date have been carried out on mammals where they are proving very successful at stimulating innate and adaptive immune responses in a variety of species as well as protecting them from bacterial, viral and protozoan pathogens. Fish also possess the ability to raise both innate and adaptive immune responses to invading pathogens and interest in the effect of CpG ODN on the piscine immune system is growing. Various studies have now been carried out to elicit the effects of CpG ODN on diverse fish species showing that 31 different B-class CpG ODN exert various immune responses both in vivo and in vitro in salmonids, cyprinids and pleuronectiformes. These responses include activation of macrophages, proliferation of leucocytes and stimulation of cytokine expression. CpG ODN have also been shown to be protective against bacterial and viral challenge as well as against pathogenic amoebae. As would be expected these effects are all dependent on not only the ODN sequence and length but on the concentration and the species in which it is being used. This review provides the first comprehensive overview of all CpG ODN tested in fish to date and brings together all the work carried out in this field.

DNA vaccination can protect Cyprinus Carpio against spring viraemia of carp virus

Several DNA constructs containing the spring viraemia of carp virus (SVCV) glycoprotein (G) gene were investigated for their ability to induce protection against SVCV following injection into myofibres. The constructs were pooled into four groups and co-injected with a plasmid encoding murine granulocyte-macrophage colony-stimulating factor. Group 1 contained one full-length and two truncated G constructs under the control of the cytomegalovirus (CMV) promoter. Group 2 contained full-length constructs with the CMV promoter, the simian virus 40 promoter and a muscle-specific promoter. Group 3 contained constructs in which the G-gene was fused with a second gene in order to improve secretion of the G-protein or to enhance destruction of transfected myocytes by T cells. Group 4 contained constructs with the CMV-Intron A promoter in plasmids with or without CpG motifs. A small-scale trial in goldfish showed that antibody responses in at least half the fish were induced by three injections of plasmids from Groups 1 and 3 whereas T-cell like responses with stimulation indices of above 3 were induced in at least half the fish by Groups 2 and 4. A single-dose of each plasmid mix was then used to protect carp in a large-scale trial. Following challenge with a heterologous strain of SVCV that killed 64% of fish, the strongest protection was observed in carp that received the full length G-gene expressed by two plasmids driven by the CMV-Intron A promoter (Group 4), with a relative percentage survival of 48% (p=0.00008).

Protective effects of a DNA vaccine expressing the infectious salmon anemia virus hemagglutinin-esterase in Atlantic salmon

Infectious salmon anemia (ISA) is a disease, caused by an orthomyxovirus, which has considerable economic impact on farming of Atlantic salmon. Here we describe the results of immunization against ISA using plasmids expressing the ISA virus hemagglutinin-esterase (HE). Immunized Atlantic salmon demonstrated moderate protection after challenge with ISA virus, with relative percent survival of 39.5 and 60.5 in two parallel groups. No protection was seen after immunization using a plasmid expressing the ISA virus nucleoprotein. Fish in the HE-immunized group had earlier onset of clearance of the virus than control fish. There was no detectable ISA virus specific humoral response after immunization. After challenge a specific humoral response could be demonstrated in the fish in all groups, but no correlation between this response and protection was found.

Porcine-specific CpG-oligodeoxynucleotide activates B-cells and increases the expression of MHC-II molecules on lymphocytes

Two CpG-oligodeoxynucleotide motifs, a mouse-specific one (CpG(mouse)) 5'-GCTAGACGTTAGCGT-3' and a porcine-specific one (CpG(pig)), 5'-TGCATCGATGCAG-3' were synthesized by two different companies and tested in vitro for their capacity to stimulate porcine peripheral blood monomorphonuclear cells (PBMC). The porcine-specific motif, consisting of a nuclease-resistant phosphorothioate guanosines at the 5' and at the 3'-end (CpG(pig)-S), enhanced significantly the proliferation of porcine PBMC in comparison with CpG(mouse). The latter motif did not induce any proliferation. Methylation of CpG(pig) diminished the proliferation. Four days of culture with CpG(pig)-S increased the percentage of B-cells as well as B-cell blasting. Moreover, CpG(pig)-S also enhanced the expression of class II MHC in most cultures while there were no changes in percentage of macrophages or in the degree of expression of the macrophage marker (monoclonal 74-22-15). In conclusion, in this study, it was confirmed that 5'-ggTGCATCGATGCAGggggg-3' is a swine-specific CpG-ODN, that activates porcine B-cells and deserves further evaluation in vivo as a potential immunostimulating adjuvant.

Immunostimulatory capacity of DNA vaccine vectors in porcine PBMC: a specific role for CpG-motifs?

With the development of DNA vaccines in pigs, the possibility was investigated that the nature and the amount of certain CpG-motifs present on plasmid DNA might have an effect on their immunostimulatory capacity. A panel of three CpG-oligodeoxynucleotides (ODN) and three eukaryotic expression vectors currently used in experimental DNA vaccines in pigs (pcDNA1, pcDNA3.1 and pCI) were screened for their immunostimulatory activity on porcine PBMC by evaluating in vitro the lymphocyte proliferative responses and cytokine profiles (IL-1alpha, IL-2, IL-4, IL-6, IL-10, IFN-gamma, TGF-beta, TNF-alpha). The vectors were chosen so that they differed in number and nature of certain CpG-motifs present on their backbone. CpG-ODN A (5'ATCGAT3') and to a lesser extend CpG-ODN C (5'AACGTT3') significantly enhanced the proliferation of porcine PBMC in contrast to CpG-ODN B (5'GACGTT3') where no effect was observed. Furthermore, CpG-ODN A significantly induced IL-6 and TNF-alpha together with elevated levels of IFN-gamma and IL-2 mRNA expression even though considerable heterogeneity was observed in the response of individual pigs. Comparison of the three vectors showed significantly increased proliferative responses for both pcDNA3.1 and pCI combined with a significant increase in IL-6 mRNA levels for pCI. For pcDNA1, proliferation was absent together with significantly decreased levels of IL-6 and IFN-gamma. CpG-ODN and plasmids both suppressed the TGF-beta and IL-1alpha mRNA expression. Taken together, these data confirm the identity of an optimal immunostimulating CpG-motif in pigs (5'-ggTGCATCGATGCAG-3') and demonstrates that the choice of the vector or the insertion of immunostimulatory motifs can be important in the future design of DNA vaccines in pigs, although further research is necessary to explore the possible link between certain CpG-motifs and the immunogenicity of DNA vaccines.

Strategies for enhancing the immunostimulatory effects of CpG oligodeoxynucleotides

Synthetic oligodeoxynucleotides (ODN) containing CpG sequences are recognized as a "danger" signal by the immune system of mammals. As a consequence, CpG ODN stimulate innate and adaptive immune responses in humans and a variety of animal species. Indeed, the potential of CpG ODN as therapeutic agents and vaccine adjuvants has been demonstrated in animal models of infectious diseases, allergy and cancer and are currently undergoing clinical trials in humans. While CpG ODN are potent activators of the immune system, their biologic activity is often transient, subsequently limiting their therapeutic application. Modifications in the CpG ODN backbone chemistry, various delivery methods including mixing or cross-linking of ODN to other carrier compounds have been shown to significantly enhance the biologic activity of ODN. However, the exact mechanisms that mediate this enhancement of activity are not well understood and may include local cell recruitment and activation, cytokine production, upregulation of receptor expression and increasing the half-life of ODN through creation of a depot. We will review the various approaches that have been used in enhancing the immunostimulatory effects of CpG ODN in vivo and also discuss the possible mechanisms that may be involved in this enhancement.

CpG oligodeoxynucleotides stimulate immune cell proliferation but not specific antibody production in rainbow trout (Oncorhynchus mykiss)

Bacterial DNA and CpG ODN have both been shown to have immunostimulatory effects in mammals, activating APCs and inducing a potent Th1 type immune response. They have also been shown to have a strong adjuvant effect and up-regulate MHC class 2 expression in murine cells, augment human and murine NK cell lytic activity, activate human B cells and induce murine B cell proliferation. However, little work has been carried out with regard to their effects on the piscine immune system. Here it is shown that various CpG ODN induce proliferation of peripheral blood leucocytes, spleen and head kidney cells from rainbow trout although, at the range of concentrations tested CpG ODN 2133 lacked the ability to induce specific antibody production to a protein antigen.

Use of CpG oligodeoxynucleotides as immunoprotective agents

Single-stranded oligodeoxynucleotides (ODNs) synthesised to express unmethylated 'CpG motifs' mimic the ability of bacterial DNA to trigger the innate immune system. CpG ODNs stimulate cells that express Toll-like receptor 9, initiating an immunomodulatory cascade resulting in the activation of B and T lymphocytes, natural killer cells, monocytes, macrophages and dendritic cells. CpG ODNs improve the host's ability to resist infection by accelerating and improving the induction of an innate and then adaptive immune response, characterised by the production of Th1 and pro-inflammatory cytokines, chemokines and polyreactive antibodies. Studies in rodents and non-human primates demonstrate that CpG ODNs can protect the host against bacterial, viral, fungal and parasitic infections. CpG-induced protection develops rapidly but persists for only a few weeks. Preclinical and clinical trials suggest that CpG ODNs can be used safely to activate the human immune system.

Efficacy of cellular vaccines and genetic adjuvants against bacterial kidney disease in chinook salmon (Oncorhynchus tshawytscha)

DNA adjuvants and whole bacterial cell vaccines against bacterial kidney disease (BKD) were tested in juvenile chinook salmon. Whole cell vaccines of either a nonpathogenic Arthrobacter spp. or an attenuated Renibacterium salmoninarum strain provided limited prophylactic protection against acute intraperitoneal challenge with virulent R. salmoninarum, and the addition of either synthetic oligodeoxynucleotides or purified R. salmoninarum genomic DNA as adjuvants did not increase protection. However, a combination of both whole cell vaccines significantly increased survival among fish naturally infected with R. salmoninarum, and the surviving fish treated with the combination vaccine exhibited reduced levels of bacterial antigens in the kidney. This is the first demonstration of a potential therapeutic effect of a whole cell vaccine against BKD.

CpG DNA induces protective antiviral immune responses in Atlantic salmon (Salmo salar L.)

Oligodeoxynucleotides (ODN) containing unmethylated CpG dinucleotides within specific sequence contexts (CpG motifs) are detected, like bacterial or viral DNA, as a danger signal by the vertebrate immune system. CpG ODN show promise as vaccine adjuvants and immunoprotective agents in animal models. Here we report that pretreatment with CpG ODN in animals induces nonspecific protection against viral infection. A panel of different synthetic CpG ODN was tested for the in vitro effects in Atlantic salmon (Salmo salar L.) leukocytes. The ODN were tested for their capacity to stimulate proliferation of peripheral blood leukocytes and to induce production of interferon-like factors in head kidney leukocytes. These studies revealed that the sequence and number of the CpG motifs as well as the lengths of the ODN contribute to their stimulatory activity. ODN with the 6-mer CpG motif (5'-GTCGTT-3') showed the highest stimulatory activity and were shown to induce protection against infectious pancreatic necrosis virus when injected in Atlantic salmon. Expression of the Mx transcript, as an indicator of alpha/beta interferon induction, was induced in the CpG-injected fish. These results suggest that CpG DNA in fish induces early, nonspecific antiviral protection.

CpG-induced immunomodulation and intracellular bacterial killing in a chicken macrophage cell line

The immunostimulatory properties of synthetic CpG oligodeoxynucleotides (ODNs) have been studied in various mammalian models including humans and mice. However, little was known about effects of CpG ODNs on immune responses of chickens, a common avian species with important economical value in the poultry industry. In the present study, two CpG ODNs, 2006 and 1826, which show immunomodulating properties for humans and mice were tested using a chicken macrophage cell line (HD11). ODN 2006, which has been reported to be an optimal stimulatory sequence for humans, showed strong immunomodulatory effects on HD11 cells, whereas ODN 1826, a CpG sequence with optimal immunostimulatory effects on mice, had weak influences on HD11 cells. ODN 2006 also induced strong IL-6 and nitric oxide secretion by HD11 cells in both dose- and time-dependent manners. Intracellular killing of Salmonella enteritidis (SE) was also increased in ODN 2006-activated HD11 cells. Furthermore, HD11 cells had reduced proliferation and underwent apoptosis, which is contradictory to the effects of ODN 2006 on human and murine cells. N(G)-monomethyl L-arginine (L-NMMA), an iNOS inhibitor, inhibited apoptosis of HD11 cells induced by ODN 2006, suggesting that this effect was likely mediated through an iNOS-dependent pathway. These results indicate that the differences in the responses of chicken HD11 macrophage cells to CpG ODNs compared to those of mammalian macrophages are species-related, and the potential of CpG ODNs as immunomodulators in poultry needs to be further explored.

Assessment of DNA vaccine potential for gilthead sea bream (Sparus aurata) by intramuscular injection of a reporter gene

Naked circular plasmid DNA containing the cytomegalovirus (CMV)-promoter-driven lacZ reporter gene (pCMV-LacZ) was injected in the epaxial muscle of gilthead sea bream (Sparus aurata). A mosaic pattern of expression of beta-galactosidase (beta-gal) in the myofibres at the site of injection was visualised by in situ histochemical staining using 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside. As measured by o-nitrophenyl-beta-D-galactopyranoside assay, beta-gal enzymatic activity was found to steadily increase for at least 50 days post injection (p.i.) in pCMV-LacZ-injected muscle. In parallel, foreign DNA was detected by polymerase chain reaction in injected muscles (but not in other tissues) up to 60 days p.i., persisting most probably in an extrachromosomal, non-replicative, circular form. Neither beta-gal activity nor pCMV-LacZ-related amplification products were found 90 days p.i. Antibodies against beta-gal were demonstrated in pCMV-LacZ-injected fish sampled 45 days p.i. The results suggest that intramuscular delivery of foreign genes represents a realistic approach for DNA vaccine technology for the prevention of infectious diseases in gilthead sea bream.

CpG oligodeoxynucleotides activate grass carp (Ctenopharyngodon idellus) macrophages

In mice and humans, B cells, antigen-presenting cells including monocytes, macrophages and dendritic cells and natural killer cells can be stimulated directly or indirectly by the bacterial DNA and oligodeoxynucleotides (ODN) containing the CpG motifs (CpG DNA). Using head kidney macrophages of grass carp (Ctenopharyngodon idellus) as an in vitro model, we investigated the effects of several CpG-ODNs on fish immunocytes. The CpG-ODNs included the optimal motifs: the ODN-1826 (GACGTT) and -2006 (GTCGTT) for the mice and humans cells, the ODN-1670 (AACGTT) used in Atlantic salmon, the ODN-D containing two repeats motif of those in 1670 and the ODN-R with an inverted CpG. The results showed that CpG has an immunomodulatory role in grass carp, and all the ODNs except the ODN-R could activate macrophages, increasing the levels of superoxide anion (O(2)(-)), hydrogen peroxide (H(2)O(2)), acid phosphatase and bactericidal activity. New evidence was provided that CpG-ODN could induce the cell-mediated immunity in fish. Interestingly, no significant differences among the ODNs tested could be found and the ODN-D was not more efficient than 1670. It suggests that the sequence which contains the unmethylated 'CG' dinucleotides could make contribute to this immunostimulatory effect. These findings indicate that CpG-ODNs could be useful tools for understanding the important anti-bacterial defense mechanism in fish. And it may have potential application in the minimizing the impact of fish diseases and enhancing the efficacy of antigen and DNA vaccines.

Biological activity of immunostimulatory CpG DNA motifs in domestic animals

Bacterial DNA contains a much higher frequency of CpG dinucleotides than are present in mammalian DNA. Furthermore, bacterial CpG dinucleotides are often not methylated. It is thought that these two features in combination with specific flanking bases constitute a CpG motif that is recognized as a "danger" signal by the innate immune system of mammals and therefore an immune response is induced when these motifs are encountered. These immunostimulatory activities of bacterial CpG DNA can also be achieved with synthetic CpG oligodeoxynucleotides (ODN). Recognition of CpG motifs by the innate immune system requires engagement of Toll-like receptor 9 (TLR-9), which induces cell signaling and subsequently triggers a pro-inflammatory cytokine response and a predominantly Th1-type immune response. CpG ODN-induced innate and adaptive immune responses can result in protection in various mouse models of disease. Based on these observations, clinical trials are currently underway in humans to evaluate CpG ODN therapies for cancer, allergy and infectious disease. However, potential applications for immunostimulatory CpG ODN in species of veterinary importance are just being explored. In this review, we will highlight what is presently known about the immunostimulatory effects of CpG ODN in domestic animals.

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.

Host responses to Renibacterium salmoninarum and specific components of the pathogen reveal the mechanisms of immune suppression and activation

During infection, Renibacterium salmoninarum survives within the pronephric macrophages of salmonid fish. Therefore, to study the initial phases of the interaction we infected macrophages with live bacteria and analysed the responses of host and pathogen. It was found that the expression of msa encoding the p57 antigen of R. salmoninarum, was constitutive, while the expression of hly and rsh, encoding haemolysins, and lysB and grp was reduced after infection. Macrophages showed a rapid inflammatory response in which the expression of interleukin-1beta (IL-1beta), major histocompatibility complex class II (MHC II), inducible cyclo-oxygenase (Cox-2), and inducible nitric oxide synthase (iNOS) was enhanced, but tumour necrosis factor-alpha (TNF-alpha) expression was greatly reduced initially and then increased. After 5 days, except for TNF-alpha and MHC II, expression returned to levels approaching those of uninfected macrophages. We propose that R. salmoninarum survives initial contact with macrophages by avoiding and/or interfering with TNF-alpha-dependent killing pathways. The effects of specific R. salmoninarum components were studied in vivo by injecting fish with DNA vaccine constructs expressing msa, hly, rsh, lysB, or grp. We found that msa reduced the expression of IL-1beta, Cox-2, and MHC II but stimulated TNF-alpha while hly, rsh and grp stimulated MHC II but down-regulated TNF-alpha. Constructs expressing hly or lysB stimulated iNOS expression and additionally, lysB stimulated TNF-alpha. The results show how p57 suppresses the host immune system and suggest that the immune mechanisms for the containment of R. salmoninarum infections rely on MHC II- and TNF-alpha-dependent pathways. Moreover, prolonged stimulation of TNF-alpha may contribute to the chronic inflammatory pathology of bacterial kidney disease.

The immune responses of common carp, Cyprinus carpio L., injected with carp interleukin-1beta gene

The function of the common carp, Cyprinus carpio L., interleukin-1beta (IL-1beta) gene was studied by DNA injection. To investigate the immune responses to IL-1beta, a plasmid construct of cytomegalovirus (CMV)-driven carp IL-1beta was injected into the epaxial muscle of carp. IL-1beta protein expressed in serum on 1, 3, and 5 days after plasmid injection was quantified by ELISA and immunohistochemistry. IL1-beta gene injection increased proliferation of the lymphocytes by phytohemagglutinin (PHA). Macrophage functions, such as production of superoxide anion and phagocytosis, also were stimulated by IL-1beta gene injection. Moreover, an increase in resistance to Aeromonas hydrophila infection was recorded in IL-1beta-injected fish compared with control fish. Thus, the cloned homolog of IL-1beta from carp has all the functional similarities to the mammalian IL-1beta gene.

Activation of nonspecific cytotoxic cells (NCC) with synthetic oligodeoxynucleotides and bacterial genomic DNA: binding, specificity and identification of unique immunostimulatory motifs

We have analyzed the effects of synthetic oligodeoxynucleotides (sODNs) and bacterial DNA (bDNA) on the in vitro activation of NCC. Teleost NCC recognition of DNA appeared to differ from that which occurs in higher vertebrates. NCC contain at least two different receptor specificities for DNA. Both oligodeoxyguanosine 20-mers (dG20) and 5'-TGCTGCTTGTGCTTGTGCTT-3' (4GC-2T) bound specifically to NCC. The existence of different receptor specificities was indicated by reciprocal cold target inhibition experiments. dG20 competed with 4GC-2T binding but sODNs composed of GpC or CpG nests did not compete with recognition by NCC of the dG20. ODN binding by NCC primarily depended on the presence of GpC or CpG nests with a preference for -G- serving as the anchor nucleotide. Secondarily, and similar to models of ODN activation in mammals, palindrome sequences of pu-pu-CpG-py-py activated NCC cytotoxicity. Additional analysis of the requirements for ODN activation indicated that guanosine could not substitute for adenosine as a purine spacer and that CpG motifs containing flanking thymidine (i.e.-GTCpGTT-) augmented the activity of the sODN containing this flanking base. Other evidence for the participation of both G and C in the recognition of specific nucleotides by NCC was that poly-dC20, dA20 or dT20 had no activating properties. Methylation of all cytosine nucleotides within an ODN abrogated activation. A canonical ODN motif of 5'-C/AT/AGCTT-3' can now be suggested for teleosts. Additional studies were done to examine the effects of in vitro treatment of NCC with bDNA. bDNA from three different disease isolates of Streptococcus iniae activated NCC cytotoxicity. Treatment of the bDNA with DNase abrogated the enhancement of cytotoxicity. Also, treatment of NCC with eukaryotic DNA had no effects on cytotoxicity. These studies suggested that NCC recognize bacterial nonmethylated DNA. The consequences of these interactions may be increased innate and acquired anti-bacterial immunity.

Immunostimulatory CpG oligodeoxynucleotides stimulate expression of IL-1beta and interferon-like cytokines in rainbow trout macrophages via a chloroquine-sensitive mechanism

Synthetic oligodeoxynucleotides (ODNs) containing unmethylated CpG motifs are known to stimulate immune responses and are potent adjuvants in higher vertebrates, but so far the effects in fish are poorly described. We here report that CpG ODNs induce IL-1beta expression and production of interferon-like cytokines in rainbow trout head-kidney macrophages, whereas ODNs with an inverted motif (GpC) have a much less stimulatory effect. We further demonstrate that endosomal maturation is essential for CpG signalling, as chloroquine, a compound known to block endosomal acidification, inhibits cytokine expression in the macrophages.

CpG motif identification for veterinary and laboratory species demonstrates that sequence recognition is highly conserved

Oligodinucleotides containing CpG motifs stimulate vertebrate immune cells in vitro, have proven efficacy in murine disease models and are currently being tested in human clinical trials as therapies for cancer, allergy, and infectious disease. As there are no known immunostimulatory motifs for veterinary species, the potential of CpG DNA as a veterinary pharmaceutical has not been investigated. Here, optimal CpG motifs for seven veterinary and three laboratory species are described. The preferential recognition of a GTCGTT motif was strongly conserved across two vertebrate phyla, although a GACGTT motif was optimal for inbred strains of mice and rabbits. In a subsequent adjuvanticity trial, the in vitro screening methodology was validated in sheep, representing the first demonstration of CpG DNA efficacy in a veterinary species. These results should provide candidate immunostimulant and therapeutic drugs for veterinary use and enable the testing of CpG DNA in large animal models of human disease.

CpG oligodeoxynucleotides and plasmid DNA stimulate Atlantic salmon (Salmo salar L.) leucocytes to produce supernatants with antiviral activity

Unmethylated CpG dinucleotides are more frequent in the genomes of bacteria and viruses than of vertebrates. We report herein that plasmid DNA and synthetic oliogodeoxynucleotides (ODNs) containing unmethylated CpG induce production of antiviral cytokine activity in Atlantic salmon leucocytes, whereas ODNs with an inverted motif (GpC) or with methylated cytosines have nearly no stimulatory effect. The adherent cell population, representing mainly macrophages, is directly activated by CpG-ODN, while the effect on the non-adherent population is weak. Since the peak antiviral activity in ODN-stimulated leucocytes is seen after 48h, this might indicate that the unmethylated DNA stimulates the adherent cells to produce co-stimulatory molecules, which in turn stimulates production of antiviral cytokines in the non-adherent cell population. The potent immune activation by CpG ODNs points to possible new applications as adjuvant in fish vaccines.

Human peripheral blood cells differentially recognize and respond to two distinct CPG motifs

Oligodeoxynucleotides (ODN) that contain unmethylated CpG dinucleotides trigger a strong innate immune response in vertebrates. CpG ODN show promise as vaccine adjuvants, anti-allergens, and immunoprotective agents in animal models. Their transition to clinical use requires the identification of motifs that are optimally stimulatory in humans. Analysis of hundreds of novel ODN resulted in the identification and characterization of two structurally distinct "clusters" of immunostimulatory CpG ODN. One cluster ("D") preferentially stimulates IFN-gamma production by NK cells, whereas the other ("K") stimulates cell proliferation and the production of IL-6 and IgM by monocytes and B cells. The distinct immunostimulatory properties of K and D ODN can improve the design of CpG-based products to achieve specific therapeutic goals.

The plasmid pcDNA3 differentially induces production of interferon-alpha and interleukin-6 in cultures of porcine leukocytes

An adjuvant effect of invertebrate DNA has been attributed to its relative high frequency of unmethylated CpG dinucleotides. Here we describe the interferon-alpha (IFN-alpha) and interleukin-6 (IL-6) inducing properties of a commonly used eukaryotic expression vector, pcDNA3, in porcine leukocytes. The magnitude of the cytokine response was compared to that induced by the synthetic ds RNA analogue poly(I):poly(C), inactivated preparations of Aujeszky's disease virus (ADV) and the Gram-negative bacteria Actinobacillus pleuropneumoniae. The plasmid, as well as poly(I):poly(C), required lipofectin to induce IFN-alpha production whereas both preparations induced IL-6 irrespective of preincubation with lipofectin. However, the nucleic acid-induced levels of IL-6 were low compared to those induced by A. pleuropneumoniae. The IFN-alpha response elicited by pcDNA3 in the presence of lipofectin was as high as, or higher than that induced by ADV. Interestingly, also A. pleuropneumoniae induced a substantial production of IFN-alpha when preincubated with lipofectin. Plasmid expression was not necessary for induction of IFN-alpha. Furthermore, the IFN-alpha inducing capacity of pcDNA3 was not reduced when the two predicted immunostimulatory sequences 5'AACGTT3' were deleted. Nor did the ability of the plasmid to induce IFN-alpha production decrease when the ampicillin resistance (ampR) gene was replaced with the kanamycin resistance (kanR) gene. However, methylation of all cytidines in CpG dinucleotides of pcDNA3 abolished the IFN-alpha inducing capacity. These in vitro results indicate an immunomodulatory role of bacterial DNA also in the pig. Unmethylated CpG dinucleotides are crucial for induction of IFN-alpha by the plasmid, but other CpG motifs than those within the 5'AACGTT3' sequences of the ampR gene contribute to this induction in porcine cells.

Assessment of DNA vaccine potential for juvenile Japanese flounder Paralichthys olivaceus, through the introduction of reporter genes by particle bombardment and histopathology

Genetic immunisation potential, following DNA bombardment for juvenile Japanese flounder, Paralichthys olivaceus was examined. GFP plasmids bombarded at two pressures, 150 and 300psi were sampled at 1, 7, 14 and 28 days, greater immunofluorescence was observed at the higher bombardment pressure. Histopathology, at 3 h post bombardment showed considerable damage to fish epithelial and dermal tissues when bombarded at pressures greater than 200 psi, with many DNA-coated gold particles present. At 150psi there was little pathology and no DNA-coated particles. Histopathology, up to 28 days again showed little pathology at 150 psi with few DNA-coated particles, whereas at 300 psi there was significant pathology observed with many DNA-coated particles seen in conjunction with the cytoplasm of inflammatory cells. By day 28 epithelial coverage was observed with tissue damage restricted to the dermal layer. Chloramphenicol acetyltransferase (CAT) assay showed long term and stable expression of the CAT protein from day 1 to day 60. The transcription activity of two promoters; pCMV-CAT and pSV2-CAT showed greater activity in the former. It was concluded that DNA vaccination potential for juvenile flounder is a viable option.

Nucleic acid vaccines: research tool or commercial reality

Polynucleotide immunization has captured the imagination of numerous researchers and commercial companies around the world as a novel approach for inducing immunity in animals. Clearly, the 'proof-of-principle' has been demonstrated both in rodents and various animal species. However, to date, no commercial veterinary vaccine has been developed, or to our knowledge, is in the licensing phase. The present review summarizes the types of pathogens and host species for which polynucleotide immunization has been tried. We have tried to identify possible barriers to commercialization of this technology and areas that need attention if this promising technology is ever to become a reality in the commercial arena.

The role of CpG motifs in innate immunity

Pattern recognition receptors of the innate immune system are able to distinguish certain prokaryotic DNAs from vertebrate DNAs by detecting unmethylated CpG dinucleotides in particular base contexts ('CpG motifs'). Recent studies have begun to define the molecular mechanisms of actions of CpG motifs and have demonstrated their stimulatory effects on leukocytes from humans and vertebrates other than mice. Oligodeoxynucleotides containing CpG motifs are highly effective Th1-like vaccine adjuvants through multiple routes of immunization and show promise as immunotherapeutic agents for cancer and allergic diseases.