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.

Induction of immune responses by DNA vaccines in large animals

It is generally recognized that DNA vaccines are often less effective in large animals than in mice. One possible reason for this reduced effectiveness may be transfection deficiency and the low level of expression elicited by plasmid vectors in large animals. In our attempt to enhance transfection efficiency and, thereby, enhance immune responses, we employed a variety of methods inducing gene gun delivery or suppositories as delivery vehicles to mucosal surfaces, as well as electroporation for systemic immunization. To test these different systems, we used two different antigens-a membrane antigen from bovine herpesvirus glycoprotein (BHV-1) gD and a particulate antigen from hepatitis virus B. Gene gun and suppository delivery of BHV-1 gD to the vagina resulted in the induction of mucosal immunity not only in the vagina, but also at other mucosal surfaces. These data support the contention of a common mucosal immune system. In the case of electroporation, we were able to develop significant enhancement of gene expression following electroporation with surface electrodes (non-invasive electroporation) as well as invasive electroporation using single or six-needle electrodes. Various delivery systems such as bioject or needle delivery also influenced the immune response in both the presence and absence of electroporation. These studies also demonstrated that co-administration of plasmids coding for two different antigens (BHV-1 gD and hepatitis B surface antigen (HbsAg)) did not result in significant interference between the plasmids. These studies suggest that various combinations of delivery systems can enhance immunity to DNA-based vaccines and make them practical for administration of these vaccines in large animals.

CpG-containing oligodeoxynucleotides augment and switch the immune responses of cattle to bovine herpesvirus-1 glycoprotein D

The adjuvanticity of a synthetic oligodeoxynucleotide containing unmethylated CpG motifs (CpG ODN) was determined in cattle. Calves were immunized with a truncated secreted version of glycoprotein D (tgD) of bovine herpes virus-1 (BHV-1) formulated with alum, CpG ODN, or a combination of both. BHV-1 tgD formulated with CpG ODN or with alum and CpG ODN induced a stronger and more balanced immune response than tgD in alum. This level of immunity was of sufficient magnitude to minimize weight loss and significantly reduce the duration of virus shedding after intranasal viral challenge. Local tissue reactions generated by CpG ODN were very mild and transient, whereas reactions induced by alum or a combination of CpG ODN and alum were moderate in severity and duration. These data demonstrate that CpG ODN causes minimal injection site reactions and yet acts as an effective adjuvant in cattle.

Priming by DNA immunization augments T-cell responses induced by modified live bovine herpesvirus vaccine

DNA vaccines have several advantages over conventional vaccines. One of the most important characteristics is the presentation of antigen via both MHC class I and class II receptors. Although this generally results in strong T-cell responses, antibody production and protection achieved by DNA immunization are unfortunately not always adequate. In contrast, modified live virus (MLV) vaccines usually induce adequate antibody and moderate cellular responses, whereas killed vaccines tend to elicit weak immune responses in general. A DNA prime-MLV boost regimen should result in enhanced cellular immunity and possibly improved antibody production. To test this hypothesis, plasmids encoding bovine herpesvirus-1 (BHV-1) glycoproteins B and D were delivered by gene gun to the genital mucosa of cattle prior to immunization with modified live BHV-1 vaccine. The immune responses induced were compared to those of an MLV-vaccinated group and a negative control group. Although significantly enhanced T-cell responses were induced by priming with the DNA vaccine, there was no increase in antibody titres. Similar levels of protection were induced by the MLV vaccine alone and the DNA prime and MLV boost regimen, which suggests that there is no correlation between the induction of T-cell responses and protection from BHV-1 challenge.

Suppository-mediated DNA immunization induces mucosal immunity against bovine herpesvirus-1 in cattle

Mucosal surfaces are the primary sites for the transmission of infectious agents including viruses, so effective vaccines generally should induce mucosal immunity. Furthermore, noninvasive delivery is desirable because of the ease of application, the high degree of patient compliance, and the improved safety for patients and clinicians due to the elimination of needles. Unfortunately, most of the conventional vaccines are parenterally administered and result in systemic rather than mucosal immunity. Here we present the first report of mucosal immunity by noninvasive DNA immunization in a target species. As an approach to induce mucosal immunity against bovine herpesvirus-1, cows were immunized intravaginally with suppositories containing plasmid coding for glycoprotein D. Serum IgG, as well as IgA both in the serum and in the nasal fluids, were detected, which supports the contention of a common mucosal immune system. This level of immunity was of sufficient magnitude to minimize weight loss and significantly reduce the duration of virus shedding after intranasal viral challenge, which demonstrates the efficacy of suppository-based administration of DNA vaccines to target species. As this is a very practical method of delivery, it has great potential to be applied as vaccine or therapy in a variety of species.

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.

Effects of intradermally administered plasmid deoxyribonucleic acid on ovine popliteal lymph node morphology

In the last decade it has become apparent that bacterial deoxyribonucleic acid (DNA) is recognized as a "danger signal" by the mammalian immune system. To investigate this interaction, sheep were injected intradermally two centimeters distal to the lateral prominence of the fibular head with 400 microg of purified plasmid DNA. Over a 28-day period ultrasound measurements indicated a progressive increase in size of both plasmid and saline (controls) treated popliteal lymph nodes and at Day 30 macroscopic and histological measurements of the lymph nodes were determined. Compared with the contralateral control lymph nodes, plasmid exposed lymph nodes were heavier (2.8 +/- 0.1g vs. 2.0 +/- 0.6 g) and displayed prominent histological changes in the cortex and medulla. Average medullary cord thickness (114.2 +/- 25.2 microm) and the average distance across medullary sinuses (64.4 +/- 2.5 microm) were significantly greater after plasmid exposure relative to contralateral controls (62.7 +/- 14.9 microm and 36.5 +/- 1.0 microm, respectively). Total number of germinal centers (71.4 +/- 17.7) and the total area of germinal centers (4.0 +/- 1.3 mm(2)) within the cortex of popliteal lymph nodes exposed to plasmid were also significantly greater than the controls (40.4 +/- 11.4 and 1.6 +/- 0.5 mm(2), respectively). Our results demonstrate that a single exposure to plasmid DNA has long term effects on regional lymph node weight and morphology.

Recent advances in the use of DNA vaccines for the treatment of diseases of farmed animals

DNA-based vaccination constitutes one of the most recent approaches to vaccine development. This technology is in principle one of the most simple and yet versatile methods of inducing both humoral and cellular immune responses, as well as protection against a variety of infectious agents. However, although immune responses have been induced in a number of larger species, most information on the efficacy of DNA immunization has been generated in mice. In this review the information available to date about the use of DNA vaccines in farmed animals, including cattle, pigs and poultry, is presented. The areas that need specific attention in the future to bring this technology to the market are discussed, including the issues concerning delivery, safety, compatibility of plasmids in multivalent vaccines and the potential of using immune stimulants as part of a DNA vaccine.

Sequence of the pckA gene of Escherichia coli K-12: relevance to genetic and allosteric regulation and homology of E. coli phosphoenolpyruvate carboxykinase with the enzymes from Trypanosoma brucei and Saccharomyces cerevisiae

The sequence of the pckA gene coding for phosphoenolpyruvate carboxykinase in Escherichia coli K-12 and previous molecular weight determinations indicate that this allosteric enzyme is a monomer of Mr 51,316. The protein is homologous to ATP-dependent phosphoenolpyruvate carboxykinases from Trypanosoma brucei and Saccharomyces cerevisiae. A potential ATP binding site was conserved in all three sequences. A potential binding site for the allosteric activator, calcium, identified in the E. coli enzyme, was only partially conserved in T. brucei and S. cerevisiae, consistent with the observation that the enzymes from the latter organisms were not activated by calcium. The published sequence of the ompR and envZ genes from Salmonella typhimurium is followed by a partial sequence that is highly homologous to pckA from E. coli. The order of these genes and the direction of transcription of the presumptive S. typhimurium pckA gene are the same as those in E. coli. The potential calcium binding site of the E. coli enzyme is conserved in the partial predicted sequence of the S. typhimurium phosphoenolpyruvate carboxykinase, consistent with the observation that calcium activation of the S. typhimurium phosphoenolpyruvate carboxykinase is very similar to that observed for the E. coli enzyme. A pckA mRNA transcript was observed in stationary-phase cells but not in logarithmically growing cells. The mRNA start site was mapped relative to the sequence of the pckA structural gene.