Targeting improves the efficacy of a DNA vaccine against Corynebacterium pseudotuberculosis in sheep

Recent advances in antigen targeting to antigen-presenting cells in veterinary medicine

Advances in antigen targeting in veterinary medicine have gained traction over the years as an alternative approach for diseases that remain a challenge for traditional vaccines. In addition to the nature of the immunogen, antigen-targeting success relies heavily on the chosen receptor for its direct influence on the elicited response that will ensue after antigen uptake. Different approaches using antibodies, natural or synthetic ligands, fused proteins, and DNA vaccines have been explored in various veterinary species, with pigs, cattle, sheep, and poultry as the most frequent models. Antigen-presenting cells can be targeted using a generic approach, such as broadly expressed receptors such as MHC-II, CD80/86, CD40, CD83, etc., or focused on specific cell populations such as dendritic cells or macrophages (Langerin, DC-SIGN, XCR1, DC peptides, sialoadhesin, mannose receptors, etc.) with contrasting results. Interestingly, DC peptides show high specificity to DCs, boosting activation, stimulating cellular and humoral responses, and a higher rate of clinical protection. Likewise, MHC-II targeting shows consistent results in enhancing both immune responses; an example of this strategy of targeting is the approved vaccine against the bovine viral diarrhea virus in South America. This significant milestone opens the door to continuing efforts toward antigen-targeting vaccines to benefit animal health. This review discusses the recent advances in antigen targeting to antigen-presenting cells in veterinary medicine, with a special interest in pigs, sheep, cattle, poultry, and dogs.

Vaccines for caseous lymphadenitis: up-to-date and forward-looking strategies

Caseous lymphadenitis (CLA) is an infectious chronic disease responsible for economic losses in sheep and goat breeding worldwide. CLA has no effective treatment, evidencing the vaccination schedule as the best control strategy. Although some commercial vaccines have been available, none of them provides total protection, which is sometimes insufficient and does not reach the same efficiency when compared in sheep and goats. They also have questionable safety levels and side effects. In light of this, several experimental vaccines are in development in order to improve safety, reproducibility, and protective immune response against the etiologic agent of CLA, Corynebacterium pseudotuberculosis. In this review, we discussed aspects as antigen, adjuvant, routes of administration, protection level, and animal models used in CLA vaccine development, as well the challenges and future perspectives. KEY POINTS: Caseous lymphadenitis (CLA) does not have an appropriate commercial vaccine. Different experimental vaccines are in development aiming to protect against Corynebacterium pseudotuberculosis. An ideal vaccine for CLA is necessary for the disease control.

The failure of a DNA prime/protein boost regime and CTLA-4 mediated targeting to improve the potency of a DNA vaccine encoding Fasciola hepatica phosphoglycerate kinase in sheep

DNA vaccination in large animals has often been associated with poor immunogenicity, consequently several approaches have been evaluated to enhance its efficacy. Here, we tested a cDNA encoding a phosphoglycerate kinase from Fasciola hepatica (cDNA-FhPGK/pCMV) as a vaccine against ovine fasciolosis and investigated whether a DNA prime/protein boost regime or CTLA-4 (cytotoxic lymphocyte antigen 4) mediated targeting improved DNA vaccine efficacy. No statistically significant differences in the cellular responses were seen in either vaccine trial when compared with the respective control groups. However, specific antibody responses were considerably enhanced in DNA primed/protein boosted sheep, but not among CTLA-4 targeted cDNA-FhPGK/pCMV vaccinated animals. Nevertheless, increased titers of specific IgG1 did not contribute to protection against infection, with no differences in liver fluke recoveries reported. If DNA vaccines against fasciolosis in target species are to reach the market one day, more research in this area is needed.

Leader gene of Corynebacterium pseudotuberculosis may be useful in vaccines against caseous lymphadenitis of goats: a bioinformatics approach

We conducted an in silico analysis to search for important genes in the pathogenesis of Caseous Lymphadenitis (CL), with prospects for use in formulating effective vaccines against this disease. For this, we performed a survey of proteins expressed by Corynebacterium pseudotuberculosis, using protein sequences collected from the NCBI GenPept database and the keywords “caseous lymphadenitis” and “Corynebacterium pseudotuberculosis” and “goats”. A network was developed using the STRING 10 database, with a confidence score of 0.900. For every gene interaction identified, we summed the interaction score of each gene, generating a combined association score to obtain a single score named weighted number of links (WNL). Genes with the highest WNL were named “leader genes”. Ontological analysis was extracted from the STRING database through Kyoto Encyclopedia of Genes and Genomes (KEGG) database. A search in the GenPept database revealed 2,124 proteins. By using and plotting with STRING 10, we then developed an in silico network model comprised of 1,243 genes/proteins interconnecting through 3,330 interactions. The highest WNL values were identified in the rplB gene, which was named the leader gene. Our ontological analysis shows that this protein acts effectively mainly on Metabolic pathways and Biosynthesis of secondary metabolites. In conclusion, the in silico analyses showed that rplB has good potential for vaccine development. However, functional assays are needed to make sure that this protein can potentially induce both humoral and cellular immune responses against C. pseudotuberculosis in goats.

Immune-Informatic Analysis and Design of Peptide Vaccine From Multi-epitopes Against Corynebacterium pseudotuberculosis

Caseous lymphadenitis (CLA) is a disease caused by Corynebacterium pseudotuberculosis bacteria that affects sheep and goats. The absence of a serologic diagnose is a factor that contributes for the disease dissemination, and due to the formation of granuloma, the treatment is very expensive. Therefore, prophylaxis is the approach with best cost-benefit relation; however, it still lacks an effective vaccine. In this sense, this work seeks to apply bioinformatic tools to design an effective vaccine against CLA, using CP40 protein as standard for the design of immunodominant epitopes, from which a total of 6 sequences were obtained, varying from 10 to 16 amino acid residues. The evaluation of different properties of the vaccines showed that the vaccine is a potent and nonallergenic antigen remaining stable in a wide range of temperatures. The initial tertiary structure of the vaccine was then predicted and a model selected. Later, the process of CP40 protein and TLR2 receptor binding was performed, presenting interaction with this receptor, which plays an important role in the activation of the immune response.

Recombinant esterase from Corynebacterium pseudotuberculosis in DNA and subunit recombinant vaccines partially protects mice against challenge

PURPOSE

We tested the efficacy of the esterase encoded by cp1002_RS09720 from Corynebacteriumpseudotuberculosis in recombinant subunit and DNA caseous lymphadenitis (CLA) vaccines. This target was predicted as one of the best CLA vaccine candidates by mature epitope density analysis.

METHODOLOGY

Gene cp1002_RS09720 was cloned into two different vectors (pAE for subunit vaccine and pTARGET for DNA vaccine). Four groups of 15 mice each were immunized with the recombinant esterase rCP09720 associated with aluminium hydroxide adjuvant (G1), pTARGET/cp09720 DNA vaccine (G2), a naked pTARGET (G3) or PBS as a negative control (G4). Immunization occurred in two doses intercalated by a 21 day interval. Twenty-one days after the last dose administration, animals were challenged with a virulent C. pseudotuberculosis MIC-6 strain.

RESULTS

G1 showed high levels of IgG1 and IgG2a on days 21 and 42 post-immunization and a significant level of IFN-γ (P<0.05), suggesting a Th1 response. The protection levels obtained were 58.3 and 16.6 % for G1 and G2, respectively.

CONCLUSION

The subunit vaccine composed of the recombinant esterase rCP09720 and Al(OH)3 is a promising antigenic formulation for use against CLA.

Bacterial Sphingomyelinases and Phospholipases as Virulence Factors

Bacterial sphingomyelinases and phospholipases are a heterogeneous group of esterases which are usually surface associated or secreted by a wide variety of Gram-positive and Gram-negative bacteria. These enzymes hydrolyze sphingomyelin and glycerophospholipids, respectively, generating products identical to the ones produced by eukaryotic enzymes which play crucial roles in distinct physiological processes, including membrane dynamics, cellular signaling, migration, growth, and death. Several bacterial sphingomyelinases and phospholipases are essential for virulence of extracellular, facultative, or obligate intracellular pathogens, as these enzymes contribute to phagosomal escape or phagosomal maturation avoidance, favoring tissue colonization, infection establishment and progression, or immune response evasion. This work presents a classification proposal for bacterial sphingomyelinases and phospholipases that considers not only their enzymatic activities but also their structural aspects. An overview of the main physiopathological activities is provided for each enzyme type, as are examples in which inactivation of a sphingomyelinase- or a phospholipase-encoding gene impairs the virulence of a pathogen. The identification of sphingomyelinases and phospholipases important for bacterial pathogenesis and the development of inhibitors for these enzymes could generate candidate vaccines and therapeutic agents, which will diminish the impacts of the associated human and animal diseases.

Corynebacterium pseudotuberculosis cp09 mutant and cp40 recombinant protein partially protect mice against caseous lymphadenitis

Background

Caseous lymphadenitis (CLA) is an infectious disease that affects small ruminants and is caused by Corynebacterium pseudotuberculosis. This disease is responsible for high economic losses due to condemnation and trim of infected carcasses, decreased leather and wool yield, loss of sales of breeding stock and deaths from internal involvement. Treatment is costly and ineffective; the most cost-effective strategy is timely immunisation. Various vaccine strategies have been tested, and recombinant vaccines are a promising alternative. Thus, in this study, different vaccine formulations using a recombinant protein (rCP40) and the CP09 live recombinant strain were evaluated. Five groups of 10 mice each were immunised with saline (G1), rCP40 (G2), CP09 (G3), a combination of CP09 and rCP40 (G4) and a heterologous prime-boost strategy (G5). Mice received two immunisations within 15 days. On day 30 after primary immunisation, all groups were challenged with a C. pseudotuberculosis virulent strain. Mice were monitored and mortality was recorded for 30 days after challenge.

Results

The G2, G4 and G5 groups showed high levels of IgG1 and IgG2a; G2 presented significant IgG2a production after virulent challenge in the absence of IgG1 and IgG3 induction. Thirty days after challenge, the mice survival rates were 20 (G1), 90 (G2), 50 (G3), 70 (G4) and 60% (G5).

Conclusions

rCP40 is a promising target in the development of vaccines against caseous lymphadenitis.

In silico analysis of Acinetobacter baumannii phospholipase D as a subunit vaccine candidate

The rate of human health care-associated infections caused by Acinetobacter baumannii has increased significantly in recent years for its remarkable resistance to desiccation and most antibiotics. Phospholipases, capable of destroying a phospholipid substrate, are heterologous group of enzymes which are believed to be the bacterial virulence determinants. There is a need for in silico studies to identify potential vaccine candidates. A. baumannii phospholipase D (PLD) role has been proved in increasing organism's resistance to human serum, destruction of host epithelial cell and pathogenesis in murine model. In this in silico study high potentials of A. baumannii PLD in elicitation of humoral and cellular immunities were elucidated. Thermal stability, long half-life, non-similarity to human and gut flora proteome and non-allergenicity were in a list of A. baumannii PLD positive properties. Potential epitopic sequences were also identified that could be used as peptide vaccines against A. baumannii and various other human bacterial pathogens.

An iron-acquisition-deficient mutant of Corynebacterium pseudotuberculosis efficiently protects mice against challenge

Caseous lymphadenitis (CLA) is a chronic disease that affects sheep and goats worldwide, and its etiological agent is Corynebacterium pseudotuberculosis. Despite the economic losses caused by CLA, there is little information about the molecular mechanisms of bacterial pathogenesis, and current immune prophylaxis against infection has been unable to reduce the incidence of CLA in goats. Recently, 21 different mutant strains of C. pseudotuberculosis were identified by random mutagenesis. In this study, these previously generated mutants were used in mice vaccination trials to develop new immunogens against CLA. Based on this analysis, CZ171053, an iron-acquisition-deficient mutant strain, was selected. After challenge with a virulent strain, 80% of the animals that were immunized with the CZ171053 strain survived. Furthermore, this vaccination elicited both humoral and cellular responses. Intracellular survival of the bacterium was determined using murine J774 cells; in this assay, the CZ171053 had reduced intracellular viability. Because iron acquisition in intracellular bacteria is considered one of their most important virulence factors during infection, these results demonstrate the immunogenic potential of this mutant against CLA.

The Larval Specific Lymphatic Filarial ALT-2: Induction of Protection Using Protein or DNA Vaccination

Genes from the infective stage of lymphatic filarial parasites expressed at the time of host invasion have been identified as potential vaccine candidates. By screening an L3 cDNA library with sera from uninfected longstanding residents of an area endemic for onchocerciasis, so-called "endemic normals" (EN), we have cloned and characterized one such gene termed the abundant larval transcript two (ALT-2). The stage specificity of ALT-2 gene transcription and protein synthesis was confirmed by PCR using genespecific primers, and by western blot analysis of protein extracts from various stages of the parasite life cycle using specific antisera. Significant differences in antibody response to the recombinant ALT-2 were observed in endemic populations with differing clinical manifestations of lymphatic filariasis with an antibody response present in sera from 18 of 25 (72%) EN subjects compared to 9 of 25 (36%) with subclinical microfilaracmia (MF) and 14 of 25 (52%) of those with chronic lymphatic obstruction (CP) (P=0.01 for comparison of EN to CP or to MF). This differential responsiveness suggests that the protective immunity postulated to account for their uninfected status might be associated with a response to this protein. When the utility of ALT-2 as a vaccine candidate was tested in a murine model using either recombinant protein or a DNA vaccine construct, statistically significant protection was observed when compared to a control filarial gene product expressed across all stages of the parasite lifecycle (SXP-1; P=0.02 for protein and P=0.01 for the DNA vaccine) or compared to adjuvant alone. This level of protection indicates that this vaccine is a promising candidate for further development.

Antigen targeting to APC: from mice to veterinary species

Antigen delivery to receptors expressed on antigen presenting cells (APC) has shown to improve immunogenicity of vaccines in mice. An enhancement of cytotoxic T lymphocyte (CTL), helper T cell or humoral responses was obtained depending on the type of APC and the surface molecule targeted. Although this strategy is being also evaluated in livestock animals with promising results, some discrepancies have been found between species and pathogens. The genetic diversity of livestock animals, the different pattern of expression of some receptors among species, the use of different markers to characterize APC in large animals and sometimes the lack of reagents make difficult to compare results obtained in different species. In this review, we summarize the data available regarding antigen targeting to APC receptors in cattle, sheep and pig and discuss the results found in these animals in the context of what has been obtained in mice.

The effect of antigen size on the immunogenicity of antigen presenting cell targeted DNA vaccine

Directing antigens to antigen presenting cells (APCs) has been demonstrated to be an efficient strategy to enhance immune responses induced by DNA vaccination. Fusion of antigens to cytotoxic T-Lymphocyte antigen 4 (CTLA4), a ligand of B7 molecules on the surfaces of APCs with strong binding affinity, enhanced the immunogenicities of antigens in various degrees. To investigate the relationship between antigen size and the immunogenicity of CTLA4 fusion DNA vaccine, we constructed CTLA4 targeted fusion anti-caries DNA vaccines containing different size of antigens. In vivo and in vitro experiments showed that CTLA4 fusion with smaller antigen induced stronger humoral immune responses and had higher affinity to B7-expressed cells than fusion with larger antigen. In conclusion, antigen size is one of the important factors regulating the potency of humoral immune response induced by CTLA4 targeted DNA vaccines.

Molecular characterization of the Corynebacterium pseudotuberculosis hsp60-hsp10 operon, and evaluation of the immune response and protective efficacy induced by hsp60 DNA vaccination in mice

Background

Heat shock proteins (HSPs) are important candidates for the development of vaccines because they are usually able to promote both humoral and cellular immune responses in mammals. We identified and characterized the hsp60-hsp10 bicistronic operon of the animal pathogen Corynebacterium pseudotuberculosis, a Gram-positive bacterium of the class Actinobacteria, which causes caseous lymphadenitis (CLA) in small ruminants.

Findings

To construct the DNA vaccine, the hsp60 gene of C. pseudotuberculosis was cloned in a mammalian expression vector. BALB/c mice were immunized by intramuscular injection with the recombinant plasmid (pVAX1/hsp60).

Conclusion

This vaccination induced significant anti-hsp60 IgG, IgG1 and IgG2a isotype production. However, immunization with this DNA vaccine did not confer protective immunity.

Homology modeling, molecular dynamics and QM/MM study of the regulatory protein PhoP from Corynebacterium pseudotuberculosis

Corynebacterium pseudotuberculosis is a facultatively intracellular Gram-positive bacterium that causes caseous lymphadenitis, principally in sheep and goats, though sometimes in other species of animals, leading to considerable economic losses. This pathogen has a TCS known as PhoPR, which consists of a sensory histidine kinase protein (PhoR) and an intracellular response regulator protein (PhoP). This system is involved in the regulation of proteins present in various processes, including virulence. The regulation is activated by PhoP protein phosphorylation, an event that requires a magnesium (Mg(2+)) ion. Here we describe the 3D structure of the regulatory response protein (PhoP) of C. pseudotuberculosis through molecular modeling by homology. The model generated provides the first structural information on a full-length member of the OmpR/PhoP subfamily. Classical molecular dynamics was used to investigate the stability of the model. In addition, we used quantum mechanical/molecular mechanical techniques to perform (internal, potential) energy optimizations to determine the interaction energy between the Mg(2+) ion and the structure of the PhoP protein. Analysis of the interaction energy residue by residue shows that Asp-16 and Asp-59 play an important role in the protein-Mg(2+) ion interactions. These results may be useful for the future development of a new vaccine against tuberculosis based on genetic attenuation via a point mutation that results in the polar residue Asp-16 and/or Asp-59 being replaced with a nonpolar residue in the DNA-binding domain of PhoP of C. pseudotuberculosis.

Current strategies for subunit and genetic viral veterinary vaccine development

Developing vaccines for livestock provides researchers with the opportunity to perform efficacy testing in the natural hosts. This enables the evaluation of different strategies, including definition of effective antigens or antigen combinations, and improvement in delivery systems for target antigens so that protective immune responses can be modulated or potentiated. An impressive amount of knowledge has been generated in recent years on vaccine strategies and consequently a wide variety of antigen delivery systems is now available for vaccine research. This paper reviews several antigen production and delivery strategies other than those based on the use of live viral vectors. Genetic and protein subunit vaccines as well as alternative production systems are considered in this review.

Targeting to porcine sialoadhesin receptor improves antigen presentation to T cells

Antibody-mediated targeting of antigen to specific antigen presenting cells (APC) receptors is an attractive strategy to enhance T cell immune responses to weak immunogenic antigens. Here, we describe the characterization of two monoclonal antibodies (mAb) against different epitopes of porcine sialoadhesin (Sn) and evaluate in vitro the potential of targeting this receptor for delivery of antigens to APC for T cell stimulation. The specificity of these mAb was determined by amino acid sequence analysis of peptides derived from the affinity purified antigen. Porcine Sn is expressed by macrophages present in the border between white and red pulp of the spleen and in the subcapsular sinus of lymph nodes, an appropriate location for trapping blood and lymph-borne antigens. It is also expressed by alveolar macrophages and monocyte-derived dendritic cells (MoDC). Blood monocytes are negative for this molecule, but its expression can be induced by treatment with IFN-alpha. MAb bound to Sn is rapidly endocytosed. MAb to sialoadhesin induced in vitro T cell proliferation at concentrations 100-fold lower than the non-targeting control mAb when using T lymphocytes from pigs immunized with mouse immunoglobulins as responder cells and IFN-alpha treated monocytes or MoDC as APC, suggesting a role of sialoadhesin in antigen uptake and/or delivery into the presentation pathway in APC.

PyNTTTTGT prototype oligonucleotide IMT504, a novel effective adjuvant of the FMDV DNA vaccine

Synthetic oligodeoxynucleotides (ODNs) such as CpG can stimulate B and plasmacytoid dendritic cells in vertebrate immune systems. Several studies showed that non-CpG ODNs could also induce strong stimulation of B and T cells. PyNTTTTGT ODNs, non-CpG ODNs, can activate and cause immunoglobulin secretion by B cells and proliferation of T cells in vivo. By using PyNTTTTGT ODNs as an adjuvant for a FMDV DNA vaccine, we found that levels of antibody production, T-cell proliferation, and CTL activity were significantly increased compared with the DNA vaccine alone. Compared with the adjuvant effects of CpG ODNs on DNA vaccination, similar levels of antibody production and T-cell proliferation, and higher levels of CTL activity and IFN-gamma expression in CD8 T cells were induced by the IMT504 ODNs. On the other hand, RT-PCR results show that IMT504 ODN may activate the DNA sensor of DAI (DNA-dependent activator of IFN regulatory factors) and partially stimulate TLR9. At this point, the PyNTTTTGT prototype IMT504 ODN can reasonably be predicted to be a good adjuvant for FMDV DNA vaccine in small animals, but its efficacy in larger animals remains to be explored.

Antigens of Corynebacterium pseudotuberculosis and prospects for vaccine development

Corynebacterium pseudotuberculosis continues to cause considerable economic losses in ovine and caprine herds worldwide, causing caseous lymphadenitis. Nevertheless, the immunology of this disease is relatively unknown. Novel antigens may provide vaccines that are more effective and improve diagnostic methods for better control of this disease. The available commercial vaccines are not able to fully protect susceptible animals, cannot be used in all host species and are not licensed for use in many countries. Recent studies on the genomics of C. pseudotuberculosis and on its molecular determinants of virulence should bring us new alternatives for more effective vaccine formulations.

Mapping and characterization of visna/maedi virus cytotoxic T-lymphocyte epitopes

CD8(+) cytotoxic T-lymphocyte (CTL) responses have been shown to be important in the control of human and simian immunodeficiency virus infections. Infection of sheep with visna/maedi virus (VISNA), a related lentivirus, induces specific CD8(+) CTL in vivo, but the specific viral proteins recognized are not known. To determine which VISNA antigens were recognized by sheep CTL, we used recombinant vaccinia viruses expressing the different genes of VISNA: in six sheep (Finnish LandracexDorset crosses, Friesland and Lleyn breeds) all VISNA proteins were recognized except TAT. Two sheep, shown to share major histocompatibility complex (MHC) class I alleles, recognized POL and were used to map the epitope. The pol gene is 3267 bp long encoding 1088 aa. By using recombinant vaccinia viruses a central portion (nt 1609-2176, aa 537-725) was found to contain the CTL epitope and this was mapped with synthetic peptides to a 25 aa region (aa 612-636). When smaller peptides were used, a cluster of epitopes was detected: at least three epitopes were present, at positions 612-623: DSRYAFEFMIRN; 620-631: MIRNWDEEVIKN; and 625-635: EEVIKNPIQAR. A DNA-prime-modified vaccinia virus Ankara (MVA)-boost strategy was employed to immunize four sheep shown to share MHC class I allele(s) with the sheep above. Specific CTL activity developed in all the immunized sheep within 3 weeks of the final MVA boost although half the sheep showed evidence of specific reactivity after the DNA-prime immunizations. This is the first report, to our knowledge, of induction of CTL by a DNA-prime-boost method in VISNA infection.

Engineering enhancement of the immune response to HBV DNA vaccine in mice by the use of LIGHT gene adjuvant

DNA vaccines could induce protective immune responses in several animal models. Many strategies have been employed to improve the effect of nucleic acid vaccines. LIGHT is a member of the TNF superfamily and functions as a co-stimulatory molecule for T cell proliferation. In the study, the immunogenicity in the induction of humoral and cellular immune responses by HBV DNA vaccine and the adjuvant effect of LIGHT were studied in a murine model. The eukaryotic expression plasmid pcDNA-L was constructed by inserting mouse LIGHT gene into the vector pcDNA3.1(+). In vitro expression of LIGHT was detected by RT-PCR and indirect immunofluorescence assay in transfected HeLa cells. MLR assay showed that LIGHT-transfected DCs induced markedly higher allogeneic lymphocyte proliferation than pcDNA-transfected DCs and untreated DCs at all dilutions. After BALB/c mice were immunized by three intramuscular injections of the HBV DNA vaccine plasmids alone or in combination with LIGHT expression plasmids, the different levels of anti-HBV immune responses were measured comparable to the control groups immunized with parent plasmid pcDNA or PBS. The HBsAg-specific splenocytes proliferation and specific cytotoxic activities of splenic CTLs in the coinoculation group were both significantly higher than those in the HBV DNA single inoculation group, and an enhancement of antibody response was also observed in the coinoculation group compared with the single inoculation group. Taken together, coimmunization of HBV DNA vaccine plasmids and LIGHT expression plasmids can elicit stronger humoral and cellular immune responses in mice than HBV DNA vaccine plasmids alone, and LIGHT may be an effective immunological adjuvant in HBV DNA vaccination.

DNA vaccines and their applications in veterinary practice: current perspectives

Inoculation of plasmid DNA, encoding an immunogenic protein gene of an infectious agent, stands out as a novel approach for developing new generation vaccines for prevention of infectious diseases of animals. The potential of DNA vaccines to act in presence of maternal antibodies, its stability and cost effectiveness and the non-requirement of cold chain have heightened the prospects. Even though great strides have been made in nucleic acid vaccination, still there are many areas that need further research for its wholesome practical implementation. Major areas of concern are vaccine delivery, designing of suitable vectors and cytotoxic T cell responses. Also, the induction of immune responses by DNA vaccines is inconclusive due to the lack of knowledge regarding the concentration of the protein expressed in vivo. Alternative delivery systems having higher transfection efficiency and the use of cytokines, as immunomodulators, needs to be further explored. Recently, efforts are being made to modulate and prolong the active life of dendritic cells, in order to make antigen presentation a more efficacious one. For combating diseases like acquired immunodeficiency syndrome (AIDS), influenza, malaria and tuberculosis in humans; and foot and mouth disease, Aujesky’s disease, swine fever, rabies, canine distemper and brucellosis in animals, DNA vaccine clinical trials are underway. This review highlights the salient features of DNA vaccines, and measures to enhance their efficacy so as to devise an effective and novel vaccination strategy against animal diseases.

A DNA vaccine against dolphin morbillivirus is immunogenic in bottlenose dolphins

The immunization of exotic species presents considerable challenges. Nevertheless, for facilities like zoos, animal parks, government facilities and non-profit conservation groups, the protection of valuable and endangered species from infectious disease is a growing concern. The rationale for immunization in these species parallels that for human and companion animals; to decrease the incidence of disease. The U.S. Navy Marine Mammal Program, in collaboration with industry and academic partners, has developed and evaluated a DNA vaccine targeting a marine viral pathogen - dolphin morbillivirus (DMV). The DMV vaccine consists of the fusion (F) and hemagglutinin (H) genes of DMV. Vaccine constructs (pVR-DMV-F and pVR-DMV-H) were evaluated for expression in vitro and then for immunogenicity in mice. Injection protocols were designed for application in Atlantic bottlenose dolphins (Tursiops truncatus) to balance vaccine effectiveness with clinical utility. Six dolphins were inoculated, four animals received both pDMV-F and pDMV-H and two animals received a mock vaccine (vector alone). All animals received an inoculation week 0, followed by two booster injections weeks 8 and 14. Vaccine-specific immune responses were documented in all four vaccinated animals. To our knowledge, this is the first report of pathogen-specific immunogenicity to a DNA vaccine in an aquatic mammal species.

A mechanistic study of immune system activation by fusion of antigens with the ligand-binding domain of CTLA4

Fusion proteins consisting of the ligand-binding domain of CTLA4 covalently attached to an antigen (Ag) are potent immunogens. This fusion strategy effectively induces Ag-specific immunity both when introduced as a DNA-based vaccine and as a recombinant protein. CTLA4 is a ligand for B7 molecules expressed on the surface of antigen-presenting cells (APCs), and this interaction is critical for the fusion protein to stimulate Ag-specific immunity. We show that interaction of the fusion protein with either B7-1 or B7-2 is sufficient to stimulate immune activity, and that T cells are essential for the development of IgG responses. In addition, we demonstrate that human dendritic cells (DCs) pulsed with CTLA4-Ag fusion proteins can efficiently present Ag to T cells and induce an Ag-specific immune response in vitro. These studies provide further mechanistic understanding of the process by which CTLA4-Ag fusion proteins stimulate the immune system, and represent an efficient means of generating Ag-specific T cells for immunotherapy.

Efficacy of particle-based DNA delivery for vaccination of sheep against FMDV

As an alternative strategy to classical inactivated viral vaccine against FMDV, naked DNA vaccine is attractive because of safety, flexibility and low cost. However DNA vaccination is usually poorly efficient in target species. Indeed we found that naked DNA plasmids encoding for P1-2A3C3D and GM-CSF proteins did not induce any detectable immunity against FMDV in sheep. Interestingly, we demonstrate herein that formulations of DNA on poly(D,L-lactide-co-glycolide) (PLG) or in lipofectin triggered divergent types of immune responses: PLG stimulated a T cell response and could elicit significant neutralising antibody titers, whereas lipofectin generated even higher antibody titers but no significant T cell response. The DNA/PLG regimen used in five sheep protected against clinical symptoms and viraemia and prevented the carrier state in four of them. Thus formulated DNA can be remarkably efficient against FMDV in a ruminant species that is usually refractory to DNA vaccination.

Immunogenicity of CTLA4 fusion anti-caries DNA vaccine in rabbits and monkeys

Enhancement of mucosal and systemic immune responses is still a challenge for the application of DNA vaccine. Here, we show anti-caries DNA vaccines, pGJA-P and pGJA-P/VAX, encoding Streptococcus mutans antigens fused to cytotoxic T lymphocyte antigen-4 (CTLA4), which binds to B7 molecule expressed on the surfaces of antigen-presenting cells. Rabbits and monkeys were immunized via intranasal or intramuscular routes. The fusion vaccine induced accelerated and increased specific antibody responses in serum and saliva compared with non-fusion DNA vaccine in rabbits. Significant specific serum IgG and salivary IgA levels could be detected in fusion vaccine-immunized monkeys. Therefore, this study demonstrates that fusing antigens to CTLA4 results in enhancing immune efficacy and strongly suggests that it may represent a promising approach to prevent dental caries or other mucosal infectious diseases. These findings also suggest that CTLA4 fusion anti-caries DNA vaccine may be effective immunogen in primates.

DNA vaccines in sheep: CTLA-4 mediated targeting and CpG motifs enhance immunogenicity in a DNA prime/protein boost strategy

DNA vaccines have proven to be an efficient means of inducing immune responses in small laboratory animals; however, their efficacy in large out-bred animal models has been much less promising. In addressing this issue, we have investigated the ability of ovine cytotoxic lymphocyte antigen 4 (CTLA-4) mediated targeting and ruminant specific CpG optimised plasmids, both alone and in combination, to enhance immune responses in sheep to the pro cathepsin B (FhCatB) antigen from Fasciola hepatica. In this study, CTLA-4 mediated targeting enhanced the speed and magnitude of the primary antibody response and effectively primed for a potent memory response compared to conventional DNA vaccination alone, which failed to induce a detectable immune response. While the CpG-augmentation of the CTLA-4 targeted construct did not further enhance the magnitude or isotype profile of the CTLA-4 induced antibody titres, it did result in the induction of significant antigen-specific, lymphocyte-proliferative responses that were not observed in any other treatment group, showing for the first time that significant cellular responses can be induced in sheep following DNA vaccination. In contrast, CpG-augmentation in the absence of CTLA-4 mediated targeting failed to induce a detectable immune response. This is the first study to explore the potential adjuvant effects of ruminant specific CpG motifs on DNA vaccine induced immune responses in sheep. The ability of CpG-augmented CTLA-4 mediated targeting to induce both humoral and cellular immune responses in this study suggests that this may be an effective approach for enhancing the efficacy of DNA vaccines in large out-bred animal models.

DNA immunization of dairy cows with the clumping factor A of Staphylococcus aureus

Blocking the primary stages of Staphylococcus aureus infection, specifically the bacterial adhesion to cell and the colonization of the mucosal surface, may be the most effective strategy for preventing infections. Clumping factor A (ClfA) is considered to be one of the most important adhesions factors of S. aureus to host cells. The present study describes the immune response of dairy cattle to a DNA vaccine against ClfA and evaluates the ability of specific genetic adjuvants, targeting sequences (granulocyte macrophage colony-stimulating factor and cytotoxic T lymphocyte antigen-4) and transporter molecules (chitosan and copolymer) to modify the immune response of cows. The results show that vaccination of cows with fibrinogen-binding region A induced a strong and specific antibody response to ClfA in comparison with a control group injected with the pCI vector alone. Although the co-expression of both genetic adjuvants and the addition copolymer transporter did not augment the overall antibody response, these approaches decreased the number of non-responsive cows. Chitosan was the only factor that did not enhance the immune response. Three months after the last DNA immunization, three cows from each of the pGM-CSF, internal ribosomal entry site (IRES), pCTLA and pCI groups were injected with 200 microg of recombinant ClfA protein in incomplete Freund's adjuvant. A strong humoral response was observed in all groups following this protein boost, with the response occurring slightly earlier in DNA-primed protein boost cows. Sera and milk samples taken from cows after the second DNA injection or after the protein boost (sera only) were analyzed for their ability to block adherence and increase phagocytosis. Pre-incubation of S. aureus with sera or milk from vaccinated cows significantly reduced the pathogen's ability to adhere to MAC-T cells relative to the sera and milk samples from the pCI-injected control cows. Similarly, pools of sera and milk from vaccinated cows increased phagocytosis of S. aureus by neutrophils. After the protein boost, sera were more efficient promoters of phagocytosis, reflecting the higher anti-ClfA antibody level of these sera. DNA-prime/protein boost regimes combined with molecular adjuvants appeared to be effective in generating a strong immune response to S. aureus antigens in cattle.

Immunization with the gene expressing woodchuck hepatitis virus nucleocapsid protein fused to cytotoxic-T-lymphocyte-associated antigen 4 leads to enhanced specific immune responses in mice and woodchucks

A number of options are available to modify and improve DNA vaccines. An interesting approach to improve DNA vaccines is to fuse bioactive domains, like cytotoxic-T-lymphocyte-associated protein 4 (CTLA-4), to an antigen. Such fusion antigens are expressed in vivo and directed to immune cells by the specific bioactive domain and therefore possess great potential to induce and modulate antigen-specific immune responses. In the present study, we tested this new approach for immunomodulation against hepadnavirus infection in the woodchuck model. Plasmids expressing the nucleocapsid protein (WHcAg) and e antigen (WHeAg) of woodchuck hepatitis virus (WHV) alone or in fusion to the extracellular domain of woodchuck CTLA-4 and CD28 were constructed. Immunizations of mice with plasmids expressing WHcAg or WHeAg led to a specific immunoglobulin G2a (IgG2a)-dominant antibody response. In contrast, fusions of WHcAg to CTLA-4 and CD28 induced a specific antibody response with comparable levels of IgG1 and IgG2a. Furthermore, the specific IgG1 response to WHcAg/WHeAg developed immediately after a single immunization with the CTLA-4-WHcAg fusion. Woodchucks were immunized with plasmids expressing WHeAg or the CTLA-4-WHcAg fusion and subsequently challenged with WHV. CTLA-4-WHcAg showed an improved efficacy in induction of protective immune responses to WHV. In particular, the anti-WHsAg antibody response developed earlier after challenge in woodchucks that received immunizations with CTLA-4-WHcAg, consistent with the hypothesis that anti-WHs response is dependent on a Th cell response to WHcAg. In conclusion, the use of fusion genes represents a generally applicable strategy to improve DNA vaccination.

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 improved formulation and delivery of DNA vaccines to veterinary target species

Interest in DNA immunization of animals continues, despite the fact that immune responses induced by DNA vaccines are generally lower than those elicited by conventional vaccines. In attempts to enhance the immune response to DNA vaccines, individuals have tried a variety of immune modulators, cytokines, and costimulatory molecules, but these only boost immune responses marginally. These results clearly demonstrate that the major challenge to improving DNA-based vaccines is to improve the transfection efficiency. Gene gun and electroporation can increase transfection and improve immune responses significantly, but these technologies have not yet advanced to the stage of routine use in livestock. Hopefully, transfection efficiency can be increased further in a user-friendly manner to ensure that the benefits of using DNA vaccines become a reality.

Approaches to enhance the efficacy of DNA vaccines

DNA vaccines consist of antigen-encoding bacterial plasmids that are capable of inducing antigen-specific immune responses upon inoculation into a host. This method of immunization is advantageous in terms of simplicity, adaptability, and cost of vaccine production. However, the entry of DNA vaccines and expression of antigen are subjected to physical and biochemical barriers imposed by the host. In small animals such as mice, the host-imposed impediments have not prevented DNA vaccines from inducing long-lasting, protective humoral, and cellular immune responses. In contrast, these barriers appear to be more difficult to overcome in large animals and humans. The focus of this article is to summarize the limitations of DNA vaccines and to provide a comprehensive review on the different strategies developed to enhance the efficacy of DNA vaccines. Several of these strategies, such as altering codon bias of the encoded gene, changing the cellular localization of the expressed antigen, and optimizing delivery and formulation of the plasmid, have led to improvements in DNA vaccine efficacy in large animals. However, solutions for increasing the amount of plasmid that eventually enters the nucleus and is available for transcription of the transgene still need to be found. The overall conclusions from these studies suggest that, provided these critical improvements are made, DNA vaccines may find important clinical and practical applications in the field of vaccination.

DNA fusion gene vaccines against cancer: from the laboratory to the clinic

Vaccination against target antigens expressed by cancer cells has now become a realistic goal. DNA vaccines provide a direct link between identification of genetic markers in tumors and vaccine formulation. Simplicity of manufacture facilitates construction of vaccines against disease subsets or even for individual patients. To engage an immune system that exists to fight pathogens, we have developed fusion gene vaccines encoding tumor antigens fused to pathogen-derived sequences. This strategy activates high levels of T-cell help, the key to induction and maintenance of effective immunity. We have dissected the immunogenic tetanus toxin to obtain specific sequences able to activate antibody, CD4+, or CD8+ T cells to attack selected fused tumor antigens. Principles established in preclinical models are now being tested in patients. So far, objective immune responses against idiotypic antigen of neoplastic B cells have been observed in patients with B-cell malignancies and in normal transplant donors. These responses provide a platform for testing physical methods to improve DNA delivery and strategies to boost responses. For cancer, demands are high, because vaccines have to activate powerful immunity against weak antigens, often in a setting of immune damage or tolerance. Vaccination strategies against cancer and against microbes are sharing knowledge and technology for mutual benefit.

DNA vaccines against human immunodeficiency virus type 1 in the past decade

This article reviews advances in the field of human immunodeficiency virus type 1 (HIV-1) and AIDS vaccine development over the last decade, with an emphasis on the DNA vaccination approach. Despite the discovery of HIV-1 and AIDS in humans nearly 20 years ago, there is no vaccine yet that can prevent HIV-1 infection. The focus has shifted toward developing vaccines that can control virus replication and disease progression by eliciting broadly cross-reactive T-cell responses. Among several approaches evaluated, the DNA-based modality has shown considerable promise in terms of its ability to elicit cellular immune responses in primate studies. Of great importance are efforts aimed at improvement of the potency of this modality in the clinic. The review discusses principles of DNA vaccine design and the various mechanisms of plasmid-encoded antigen presentation. The review also outlines current DNA-based vaccine strategies and vectors that have successfully been shown to control virus replication and slow disease progression in animal models. Finally, it lists recent strategies that have been developed as well as novel approaches under consideration to enhance the immunogenicity of plasmid-encoded HIV-1 antigen in various animal models.

Priming of piglets against enterotoxigenic E. coli F4 fimbriae by immunisation with FAEG DNA

Early vaccination is necessary to protect pigs against postweaning diarrhoea caused by enterotoxigenic Escherichia coli (ETEC). However, at present no commercial vaccine allows successful vaccination. This is partly due to the presence of maternally derived antibodies. Since DNA vaccines are suggested to be superior to protein vaccines in young animals with maternal antibodies, we determined whether the fimbrial adhesin (FaeG) of F4ac(+) ETEC could be used as a plasmid DNA vaccine to prime piglets in a heterologous prime-boost approach. Hereto, pcDNA1/faeG19 was constructed and expression of rFaeG in Cos-7 cells was demonstrated. Thereafter, pigs were immunised (days 0, 21 and 42) intramuscularly by injection or intradermally by gene gun and humoral and cellular immune responses were analysed. Even though responses were low, results demonstrated that intramuscular injection was superior to gene gun delivery for priming the humoral immune response since higher antibody titres were raised, whereas gene gun delivery better induced a cellular response, evaluated by a lymphocyte proliferation assay. Effective priming of the humoral immune response was evidenced by high IgG titres 1 week after a protein boost with purified F4. The low responses to the pcDNA1/faeG19 DNA vaccination suggest that delivery of the DNA and/or the expression of the faeG gene should be improved.

In vivo electroporation improves immune responses to DNA vaccination in sheep

In vivo electroporation was utilised to enhance plasmid DNA expression in sheep muscle to improve the immune response to DNA vaccination. DNA encoding enhanced green fluorescence protein expressed at higher levels in sheep muscle following in vivo electroporation which caused minimal muscle damage. Groups of seven sheep were then given three intramuscular injections of plasmids encoding two Haemonchus contortus Ag, with and without electroporation at 0, 3 and 7 weeks. Humoral responses were enhanced in electroporated sheep. Four weeks after vaccination, all groups were injected subcutaneously with recombinant Ag formulated in Quil A. Induction of vaccine-specific immune memory was demonstrated in DNA-vaccinated sheep.

DNA vaccines and their application against parasites--promise, limitations and potential solutions

DNA or nucleic acid vaccines are being evaluated for efficacy against a range of parasitic diseases. Data from studies in rodent model systems have provided proof of principle that DNA vaccines are effective at inducing both humoral and T cell responses to a variety of candidate vaccine antigens. In particular, the induction of potent cellular responses often gives DNA vaccination an immunological advantage over subunit protein vaccination. Protection against parasite challenge has been demonstrated in a number of systems. However, application of parasite DNA vaccines in large animals including ruminants, primates and humans has been compromised by the relative lack of immune responsiveness to the vaccines, but the reasons for this hyporesponsiveness are not clear. Here, we review DNA vaccines against protozoan parasites, in particular vaccines for malaria, and the use of genomic approaches such as expression library immunization to generate novel vaccines. The application of DNA vaccines in ruminants is reviewed. We discuss some of the approaches being evaluated to improve responsiveness in large animals including the use of cytokines as adjuvants, targeting molecules as delivery ligands, electroporation and CpG oligonucleotides.

Gene gun immunization in a preclinical model is enhanced by B7 targeting

DNA vaccines have great potential but despite the promise shown in rodent models, responses in large animals, including humans, have been disappointing. Furthermore, gene gun delivery of DNA has been used to improve these responses. However, most cells that are transfected are not the professional antigen presenting cells (APC) which are critical for generating the primary immune response. Here, we show that in the large animal model of the pig, the combination of the use of gene gun delivery and a DNA vector that targets antigen presenting cells by expressing a CTLA4-ovalbumin (OVA) fusion antigen, leads to enhanced ovalbumin specific serum IgG, IgA, IgG1 and IgG2 immune responses.

Optimization of DNA vaccination immune responses in dairy cows: effect of injection site and the targeting efficacy of antigen-bCTLA-4 complex

The objectives of this study were to evaluate the effects of immunization site and antigen presenting cell targeting on cattle immune responses to DNA immunization. Cows were vaccinated with the plasmid expression vector pCI alone, pCI encoding the bacterial antigen beta-galactosidase (pCI-beta-gal) or pCI encoding bCTLA 4 fused to beta-gal (pCI-bCTLA-hIgG-beta-gal). The plasmids were delivered by intramuscular, intradermal, intramammary gland, or intra supramammary lymph node needle-injection. Both vaccines induced significant humoral and cellular immune responses. pCI-beta-gal elicited a higher IgG response than immunization with pCI-bCTLA-hIgG-beta-gal. Cows injected intramuscularly and intramammary had higher IgG and IgG-1 humoral responses than cows immunized intradermaly or in the lymph nodes. The injection site did not significantly affect the magnitude of the IgG2 and IgM antibody responses, although a trend similar to the IgG results was observed. The lymphocyte proliferation index was higher with pCI-beta-gal but was not affected by the injection site. These results suggest that in bovine, the injection site can affect immune responses but they do not provide evidence that bCTLA-4-hIgG-antigen targeting is effective in cattle.

Targeting with bovine CD154 enhances humoral immune responses induced by a DNA vaccine in sheep

CD40-CD154 interactions play an important role in regulating humoral and cell-mediated immune responses. Recently, these interactions have been exploited for the development of therapeutic and preventive treatments. The objective of this study was to test the ability of bovine CD154 to target a plasmid-encoded Ag to CD40-expressing APCs. To achieve this, a plasmid coding for bovine CD154 fused to a truncated secreted form of bovine herpesvirus 1 glycoprotein D (tgD), pSLIAtgD-CD154, was constructed. The chimeric tgD-CD154 was expressed in vitro in COS-7 cells and reacted with both glycoprotein D- and CD154-specific Abs. Both tgD and tgD-CD154 were capable of binding to epithelial cells, whereas only tgD-CD154 bound to B cells. Furthermore, dual-labeling of ovine PBMCs revealed that tgD-CD154 was bound by primarily B cells. The functional integrity of the tgD-CD154 chimera was confirmed by the induction of both IL-4-dependent B cell proliferation and tgD-specific lymphoproliferative responses in vitro. Finally, sheep immunized with pSLIAtgD-CD154 developed a more rapid primary tgD-specific Ab response and a significantly stronger tgD-specific secondary response when compared with animals immunized with pSLIAtgD and control animals. Similarly, virus-neutralizing Ab titers were significantly higher after secondary immunization with pSLIAtgD-CD154. These results demonstrate that using CD154 to target plasmid-expressed Ag can significantly enhance immune responses induced by a DNA vaccine.

HBV DNA vaccine with adjuvant cytokines induced specific immune responses against HBV infection

AIM: To seek for an effective method to improve the immune responses induced by DNA vaccine expressing HBV surface antigen (pCR3.1-S) in Balb/c mice (H-2^d).

METHODS: The pCR3.1-S plasmid and the eukaryotic expression vectors expressing murine IL-2 (pDOR-IL-2) or IL-12 (pWRG3169) were injected into mice subcutaneously. The immune responses to pCR3.1-S and the adjuvant effect of the cytokines plasmid were studied. Meanwhile the effect of pCR3.1-S on anti-translated subcutaneous tumor of P815 mastocytoma cells stably expressing HBsAg (P815-HBV-S) was also studied. Anti-HBs in serum was detected by enzyme-linked immunoadsordent assay (ELISA) and HBsAg specific cytotoxic T lymphocytes (CTLs) activity was measured by ⁵¹Cr release assay. After three weeks of DNA immunization, the cells of P815-HBV-S were inoculated into mice subcutaneously and the tumor growth was measured every five days. The survival rate and living periods of mice were also calculated.

RESULTS: After 8 wk DNA immunization, the A 450 nm values of sera in mice immunized with pCR3.1, pCR3.1-S and pCR3.1-S codeliveried with IL-2 or IL-12 plasmids were 0.03 ± 0.01, 1.24 ± 0.10, 1.98 ± 0.17 and 1.67 ± 0.12 respectively. Data in mice codeliveried pCR3.1-S with IL-2 or IL-12 plasmids were significantly higher than that of mice injected pCR3.1 or pCR3.1-S only. The HBsAg specific CTL activities in mice coinjected with pCR3.1-S and IL-2 or IL-12 eukaryotic expression vectors were (61.9 ± 7.1)% and (73.3 ± 8.8)%, which were significantly higher than that of mice injected with pCR3.1 (10.1 ± 2.1)% or pCR3.1-S (50.5 ± 6.4)%. The HBsAg specific CTL activities in mice injected with pCR3.1, pCR3.1-S, pCR3.1-S combined with IL-2 or IL-12 eukaryotic expression vectors decreased significantly to (3.2 ± 0.8)%, (10.6 ± 1.4)%, (13.6 ± 1.3)% and (16.9 ± 2.3)% respectively after the spleen cells were treated by anti-CD8⁺ monoclonal antibody, but presented no significant change to anti-CD4⁺ monoclonal antibody or unrelated to monoclonal antibody. The HBV-S DNA vaccine (pCR3.1-S) could evidently inhibit the tumor growth, prolong the survival period of mice and improve the survival rate of mice and these effects could be improved by IL-12 gene codeliveried.

CONCLUSION: HBV DNA vaccine has a strong antigenicity in humoral and cellular immunities, which can be promoted by plasmid expressing IL-2 or IL-12. CD8⁺ cells executed the CTL activities. DNA vaccine may be useful for both prophylaxis and treatment of HBV infection.

Efficacy of DNA vaccination by different routes of immunisation in sheep

DNA vaccination, delivered through various routes, has been used extensively in laboratory animals. Few studies have focused on veterinary species and while results obtained in laboratory animals can often be extrapolated to veterinary species this is not always the case. In this study we have compared the effect of the route of immunisation with DNA on the induction of immune responses and protection of sheep to challenge with live Corynebacterium pseudotuberculosis. Intramuscular injection of plasmid DNA encoding an inactivated form of the phospholipase D (PLD) antigen linked to CTLA4-Ig resulted in the induction of a strong memory response and sterile immunity following challenge in 45% of the animals. In contrast, gene gun delivery or subcutaneous (SC) injection of the DNA vaccine induced comparatively poor responses and insignificant levels of protection. Thus, DNA vaccine efficacy in sheep is strongly influenced by the route of vaccination. Amongst intramuscular vaccinates, protected sheep had significantly elevated IgG2 responses compared to unprotected animals, while both subgroups had equivalent IgG1 levels. This suggests that the presence of IgG2 antibodies and hence a Th1-like response, induced by the DNA vaccine gave rise to protective immunity against C. pseudotuberculosis.