DNA vaccines: technology and application as anti-parasite and anti-microbial agents

Vaccination of goats with DNA vaccines encoding H11 and IL-2 induces partial protection against Haemonchus contortus infection

DNA vaccines expressing Haemonchus contortus H11 antigen with or without interleukin (IL)-2 were tested for protection against H. contortus infection in goats. Sixteen goats (8-10 months of age) were allocated into four trial groups. On days 0 and 14, group 1 was immunised with a DNA vaccine expressing H11 and IL-2 and group 2 was immunised with a DNA vaccine expressing H11 only. Group 3 was an unvaccinated positive control group challenged with H. contortus third stage larvae (L3). Group 4 was an unvaccinated negative control group that was not challenged with L3. Animals in groups 1-3 were challenged with 5000 infective H. contortus L3 14 days after the second immunisation. Transcription of H11 and IL-2 was demonstrated in muscle by reverse transcriptase-PCR 10 days after primary immunisation and translation of H11 was detected by Western blot analysis 7 days after the second immunisation. Following immunisation with a DNA vaccine expressing H11 and IL-2, high levels of specific serum immunoglobulin (Ig) G, non-specific serum IgA, mucosal IgA, CD4(+) T lymphocytes, CD8(+) T lymphocytes and B lymphocytes were produced. Following challenge with L3, cumulative mean faecal worm egg counts and worm burdens in group 1 were reduced by 56.6% and 46.7%, respectively, while corresponding reductions in group 2 were 44.8% and 38.0%. There was a small but significant difference in abomasal worm burdens in goats in groups 1 (395.3±37.6) and 2 (459.5±101.6) compared to group 3 (741.5±241.5; P<0.05). Use of a DNA vaccine expressing H11 and IL-2 conferred partial protection against Haemonchus contortus infection in goats.

Protective immune response in BALB/c mice induced by a suicidal DNA vaccine of the MIC3 gene of Toxoplasma gondii

To evaluate the protective efficiency of a suicidal DNA vaccine against protozoal parasite Toxoplasma gondii, the microneme protein 3 (MIC3) gene was cloned into suicidal vector pSCA1 and conventional DNA vaccine vector pcDNA3.1+ respectively, their protection against T. gondii challenge were assessed in this study. The recombinant plasmids named pSCA/MIC3 and pcDNA/MIC3 were transfected into BHK-21 cells. The expression of MIC3 in BHK-21 cells was confirmed by RT-PCR and indirect immunofluorescence test. Then BALB/c mice were immunized with pSCA/MIC3 or pcDNA/MIC3. Anti-Tg-MIC3 antibodies were detected by indirect ELISA and the cell immune response were examined by lymphocyte proliferation assay and real time RT-PCR. The results showed that the titre of anti-Tg-MIC3 antibodies, stimulation index (SI) of lymphocyte proliferation response and IFN-gamma expression level induced by pSCA/MIC3 and pcDNA/MIC3 were significantly higher than controls (P<0.05), whereas IL-4 expression level in BALB/c mice immunized with either pSCA/MIC3 or pcDNA/MIC3 was lower than that in control group. After a lethal challenge against T. gondii, survival time of the mice immunized with this suicidal DNA vaccine pSCA/MIC3 and conventional DNA vaccine pcDNA/MIC3 were significantly prolonged in comparison with the control groups (P<0.05), but the difference of protective immune response in BALB/c mice between pSCA/MIC3 and pcDNA/MIC3 was not statistically significant (P>0.05). The findings demonstrated that like conventional DNA vaccine pcDNA/MIC3, suicidal DNA vaccine pSCA/MIC3 also provided favourable efficacy, but it could improve the biosafety of conventional vaccines. This result suggested that suicidal DNA vaccine pSCA/MIC3 is a potential candidate vaccine against toxoplasmosis.

Immunogenicity of a DNA vaccine expressing the Neospora caninum surface protein NcSRS2 in mice

The immunogenicity of a DNA vaccine expressing the surface protein NcSRS2 of Neospora caninum was studied in BALB/c mice. The NcSRS2-encoding DNA was obtained by PCR amplification of the NcSRS2 ORF gene from the p43 plasmid encoding the N. caninum surface protein NcSRS2, ligated to the mammalian expression vector pcDNA3.1/Zeo(+) and propagated in E. coli DH5alpha to produce the N. caninum NcSRS2 DNA vaccine. BALB/c mice were immunised by two intramuscular injections of the DNA vaccine with or without complete Freund's adjuvant (CFA). Serum antibody titres and nitric oxide (NO) concentrations, and splenocyte proliferation and cytokine expression were measured after immunisation. The DNA vaccine induced T-cell-mediated immunity as shown by significantly increased NO concentrations, cytokine gene (IL-2 and IFN-gamma) expression, and NcSRS2 protein-stimulated lymphocyte proliferation in mice immunised with the DNA vaccine. The vaccine also induced weak humoral immunity. The immunogenicity of the DNA vaccine was slightly enhanced by CFA. The immune response was specific to NcSRS2. No immune response was observed in mice immunised with the pcDNA3.1/Zeo(+) vector alone.

Quantitative real-time PCR study on persistence of pDNA vaccine pVax-Hsp60 TM814 in beef muscles

Background

Application of plasmid DNA for immunization of food-producing animals established new standards of food safety. The addition of foreign products e.g. pDNA into the food chain should be carefully examined to ensure that neither livestock animals nor consumers develop unpredicted or undesirable side-effects.

Methods

A quantitative real-time PCR (QRTPCR) methodology was developed to study the biodistribution and persistence of plasmid DNA vaccine pDNAX (pVAX-Hsp60 TM814) in mice and beef cattle. The linear quantification range and the sensitivity of the method was found to be 10 – 10⁹ copies per reaction (500 ng/gDNA) and 3 copies per reaction, respectively.

Results

Persistence of pDNAX in mice muscle tissue was restricted to injection site and the amount of pDNAX showed delivery formulation dependent (naked pDNA, electroporation, cationic liposome complexes) and mouse age-dependent clearance form injection site but pDNAX was still detectable even after 365 days. The QRTPCR analysis of various muscle tissue samples of vaccinated beef bulls performed 242–292 days after the last revaccination proved that residual pDNAX was found only in the injection site. The highest plasmid levels (up to 290 copies per reaction) were detected in the pDNAX:CDAN/DOPE group similarly to mice model. No pDNA was detected in the samples from distant muscles and draining lymph nodes.

Conclusion

Quantitative real-time PCR (QRTPCR) assay was developed to assess the residual pDNA vaccine pVAX-Hsp60 TM814 in mice and beef cattle. In beef cattle, ultra low residual level of pDNA vaccine was only found at the injection site. According to rough estimation, consumption of muscles from the injection site represents almost an undetectable intake of pDNA (400 fg/g muscle tissue) for consumers. Residual plasmid in native state will hardly be found at measurable level following further meat processing. This study brings supportive data for animal and food safety and hence for further approval of pDNA vaccine field trials.

Genetic immunisation by liver stage antigen 3 protects chimpanzees against malaria despite low immune responses

Background

The true interest of genetic immunisation might have been hastily underestimated based on overall immunogenicity data in humans and lack of parallelism with other, more classical immunisation methods.

Principal Findings

Using malaria Liver Stage Antigen-3 (LSA-3), we report that genetic immunization induces in chimpanzees, the closest relative of humans, immune responses which are as scarce as those reported using other DNA vaccines in humans, but which nonetheless confer strong, sterile and reproducible protection. The pattern was consistent in 3/4 immunized apes against two high dose sporozoite challenges performed as late as 98 and 238 days post-immunization and by a heterologous strain.

Conclusions

These results should, in our opinion, lead to a revisiting of the value of this unusual means of immunisation, using as a model a disease, malaria, in which virulent challenges of volunteers are ethically acceptable.

Kinesin motor domain of Leishmania donovani as a future vaccine candidate

Visceral leishmaniasis (VL) is one of the important parasitic diseases, with approximately 350 million people at risk. Due to the nonavailability of an ideal drug, development of a safe, effective, and affordable vaccine could be a solution for control and prevention of this disease. The present study was carried out to examine the immunological potential of kinesin protein from the microtubule locus of Leishmania donovani as a suitable vaccine candidate. In silico analysis of this region revealed clusters of major histocompatibility complex class I and II binding epitopes in its motor domain region. A recombinant protein was expressed from this region and named rLvacc. The antigenicity and immunogenicity studies of this protein by Western blot analysis revealed that rLvacc is strongly recognized by sera from acute VL patients. To evaluate its immunogenicity, peripheral blood mononuclear cells from cured VL patients were separated, and a lymphocyte proliferation assay was carried out in the presence of rLvacc. After lymphocyte proliferation, the pooled culture supernatant was assayed for anti-rLvacc antibody titers using an enzyme-linked immunosorbent assay. The results showed that immunoglobulin G2 (IgG2) subtype antibodies were predominant, while IgG1 subtype antibodies were produced in very low titers. On the basis of these ex vivo preliminary findings, its immunogenicity was studied in BALB/c mice. Vaccination with the DNA construct generated a good cellular immune response with significant increases in gamma interferon and interleukin-2 (IL-2) cytokine levels (Th1), but no increase in IL-4 levels (Th2). Taken together, our findings suggest the kinesin motor domain region of L. donovani as a potential vaccine candidate against visceral leishmaniasis.

Leishmaniavaccines: progress and problems

Leishmaniaare protozoan parasites spread by a sandfly insect vector and causing a spectrum of diseases collectively known as leishmaniasis. The disease is a significant health problem in many parts of the world resulting in an estimated 12 million new cases each year. Current treatment is based on chemotherapy, which is difficult to administer, expensive and becoming ineffective due to the emergence of drug resistance. Leishmaniasis is considered one of a few parasitic diseases likely to be controllable by vaccination. The relatively uncomplicated leishmanial life cycle and the fact that recovery from infection renders the host resistant to subsequent infection indicate that a successful vaccine is feasible. Extensive evidence from studies in animal models indicates that solid protection can be achieved by immunisation with protein or DNA vaccines. However, to date no such vaccine is available despite substantial efforts by many laboratories. Advances in our understanding ofLeishmaniapathogenesis and generation of host protective immunity, together with the completedLeishmaniagenome sequence open new avenues for vaccine research. The major remaining challenges are the translation of data from animal models to human disease and the transition from the laboratory to the field. This review focuses on advances in anti-leishmania vaccine development over the recent years and examines current problems hampering vaccine development and implementation.

Studies on the protective immunity of Schistosoma japonicum bivalent DNA vaccine encoding Sj23 and Sj14

In order to explore the high performance bivalent DNA vaccine of Schistosoma japonicum, the fatty-acid-binding protein (Sj14) and the 23 kDa transmembrane protein (Sj23) two proteins were selected to construct the DNA-based vaccine. It was successful to construct a bivalent DNA vaccine using three strategies: the co-expression of two genes, a fusion gene expression and two kinds of plasmids in combination (cocktail vaccine). The bivalent DNA was proven to express well in vitro and in vivo by indirect immunofluorescence test (IIF) and reverse transcriptase-polymerase chain reaction (RT-PCR). The protective immunity of bivalent DNA vaccine was higher than that of univalent DNA vaccine (p<0.05). There were four groups of bivalent vaccine whose protective immunity was higher than 50%. Granuloma diameter reduction rates were in the range of 18-39%. There was no significant impact on immunity protection exerted by the four factors including dosage, inoculated times, inoculated routes and challenge time after the last immunization in three levels (p>0.05).

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.

Schistosoma japonicum: the design and experimental evaluation of a multivalent DNA vaccine

The aim of this study was to construct and evaluate the immunity efficacy of the DNA multivalent vaccine pVIVO(2)SjFABP-23. The vaccine was constructed and produced as follows. Forty BALB/c mice were divided into four groups designated pVIVO(2), pVIVO(2)Sj23, pVIVO(2)SjFABP and pVIVO(2)SjFABP-23. Each mouse was immunized with 100 mug of the corresponding plasmid DNA by intramuscular injection. 28 days post-vaccination, the mice were challenged with S. japonicum cercariae, and the worm and egg burdens were determined 42 days post-challenge. Serum samples were collected from all the mice before and after vaccination and at the end of the experiment, and used for antibody detection. The IFN-gamma and IL-4 levels were quantified in the supernatants of specifically stimulated spleen cells. The number of worms was reduced by 52%, 40% and 42% in mice respectively immunized with pVIVO(2)SjFABP-23, pVIVO(2)Sj23 or pVIVO(2)SjFABP. A respective 61%, 38% and 39% egg reduction was determined relative to those mice that only received the empty pVIVO2 plasmid. pVIVO(2)SjFABP-23 immunization increased IgG levels against SWAP and SEA. Increased IFN-gamma levels were detected in the supernatant of specific stimulated spleen cells from mice immunized with the 3 different constructs. The multivalent DNA vaccine developed induced higher levels of protection than the two monovalent tested vaccines.

Immunostimulatory oligodeoxynucleotides are potent enhancers of protective immunity in mice immunized with recombinant ORFF leishmanial antigen

Unmethylated CpG dinucleotides in bacterial DNA or synthetic oligonucleotides (ODN) have proved as promising adjuvants for promotion of T helper 1 (Th1) type immune response. The potent Th1 like immune activation by CpG-ODNs suggests a possible utility for vaccination against leishmaniasis. We therefore investigated the effect of ODN containing immunostimulatory CG motifs as adjuvant with recombinant ORFF (rORFF) leishmanial antigen. BALB/c mice were vaccinated with the rORFF with or without CpG-ODN as adjuvant and then challenged with Leishmania donovani metacyclic promastigotes. Administration of CpG-ODN alone resulted in partial protection against challenge with L. donovani in BALB/c mice. Combination of rORFF and CpG-ODN showed enhanced reduction in parasite load (84%) when compared to rORFF (56%) vaccinated mice. Immunization with rORFF alone did not induce the typical Th response whereas co-administration of rORFF with CpG-ODN resulted in enhanced production of immunoglobulin G2a and interferon gamma. Our results further demonstrate that CpG-ODN alone or in combination with rORFF resulted in a dose dependent increase of nitric oxide production in activated macrophages. These studies suggest that CpG-ODN are promising immune enhancers for vaccination against visceral leishmaniasis.

A vaccine against Asian schistosomiasis

There is continued transmission of schistosomiasis japonica in China and Philippines despite highly effective control programs that focus on the application of the highly effective drug praziquantel (PZQ). The massive Three Gorges Dam across the Yangtze River in Southern China, soon to be completed, is expected to significantly increase schistosomiasis transmission and introduce the disease into areas currently unaffected. After long-term experience it is generally accepted that PZQ chemotherapy, although the cornerstone of current control programs, does have significant limitations. Furthermore, efficient drug delivery requires a substantial infrastructure to regularly cover all parts of an endemic area. Although there is not yet clear-cut evidence for the existence of PZQ-resistant schistosome strains, decreased susceptibility to the drug has been observed in several countries. As a result, a protective vaccine represents an essential component for the long-term control of schistosomiasis. This article briefly reviews aspects of anti-schistosome protective immunity that are important in the context of vaccine development. The current status in the development of vaccines against Schistosoma japonicum will then be discussed as will new approaches that may improve on the efficacy of available vaccines, and aid in the identification of new targets for immune attack. With new and extensive data becoming available from the S. japonicum genome project, the prospects for developing an effective vaccine are encouraging. The challenges that remain are many but it is crucial that the momentum towards developing effective anti-schistosome vaccines is maintained.

Induction of protective cellular immunity against Mycobacterium tuberculosis by recombinant attenuated self-destructing Listeria monocytogenes strains harboring eukaryotic expression plasmids for antigen 85 complex and MPB/MPT51

We report here the induction of specific protective cellular immunity against Mycobacterium tuberculosis by the employment of vaccination with recombinant attenuated Listeria monocytogenes strains. We constructed self-destructing attenuated L. monocytogenes Delta 2 strains carrying eukaryotic expression plasmids for the antigen 85 complex (Ag85A and Ag85B) and for MPB/MPT51 (mycobacterial protein secreted by M. bovis BCG/mycobacterial protein secreted by M. tuberculosis) molecules. Infection of these recombinant bacteria allowed expression of the genes in the J774A.1 murine macrophage cell line. Intraperitoneal vaccination of C57BL/6 mice with these recombinant bacteria was capable of inducing purified protein derivative-specific cellular immune responses, such as foot pad reactions, proliferative responses of splenocytes, and gamma interferon production from splenocytes, suggesting the efficacy of vaccination against mycobacterial infection by use of these recombinant L. monocytogenes strains. Furthermore, intravenous vaccination with recombinant bacteria carrying expression plasmids for Ag85A, Ag85B, or MPB/MPT51 in BALB/c mice elicited significant protective responses, comparable to those evoked by a live Mycobacterium bovis BCG vaccine. Notably, this is the first report to show that MPB/MPT51 is a major protective antigen in addition to Ag85A and Ag85B, which have been reported to be major mycobacterial protective antigens.

Cytotoxic T-Lymphocyte-, and Helper T-Lymphocyte-Oriented DNA Vaccination

DNA vaccines have advantages over other types of vaccines in that they can induce strong cellular immune responses, namely cytotoxic T lymphocytes (CTL) and helper T lymphocytes (Th). DNA vaccines are therefore considered a promising alternative to attenuated live vaccines in the field of infectious diseases. So far, various DNA vaccines have been generated and tried to induce a particular cellular immune response by virtue of recombinant DNA technology. DNA vaccines have been designed for efficient transcription and translation of target genes by a variety of strategies. Also, various DNA vaccine strategies for induction of specific CTL and Th have been reported by taking into consideration antigen presentation pathways and the strategies have been shown to be effective to elicit particular T-cell responses. In this paper, we have reviewed these strategies, including our study on epitope-specific T-cell induction by DNA vaccination against Listeria monocytogenes infection. From this review, it has been surmised that, to induce strong immune responses by DNA vaccines, the immunization route and the immunization regimen, such as heterologous "prime-boost" regimen, should also be considered.

Technological advances and genomics in metazoan parasites

Molecular biology has provided the means to identify parasite proteins, to define their function, patterns of expression and the means to produce them in quantity for subsequent functional analyses. Whole genome and expressed sequence tag programmes, and the parallel development of powerful bioinformatics tools, allow the execution of genome-wide between stage or species comparisons and meaningful gene-expression profiling. The latter can be undertaken with several new technologies such as DNA microarray and serial analysis of gene expression. Proteome analysis has come to the fore in recent years providing a crucial link between the gene and its protein product. RNA interference and ballistic gene transfer are exciting developments which can provide the means to precisely define the function of individual genes and, of importance in devising novel parasite control strategies, the effect that gene knockdown will have on parasite survival.

Protection against Schistosoma mansoni utilizing DNA vaccination with genes encoding Cu/Zn cytosolic superoxide dismutase, signal peptide-containing superoxide dismutase and glutathione peroxidase enzymes

Protection against Schistosoma mansoni infection in C57BL/6 female mice was evaluated by two DNA vaccination strategies. Mice were either vaccinated by intramuscular injection with pcDNAI/Amp constructs encoding either Cu/Zn cytosolic superoxide dismutase (CT-SOD), signal peptide-containing SOD (SP-SOD), glutathione peroxidase (GPX(bb)) or a mutated form of GPX (GPX(m)), or primed with naked DNA constructs and boosted with recombinant vaccinia virus (RVV) containing the same genes. Animals were then challenged with S. mansoni and the level of protection was assessed as the reduction in worm burden. CT-SOD showed significant levels of protection compared to the control group, ranging between 44 and 60%, while SP-SOD exhibited from 22 to 45%. GPX(bb) showed levels of protection (23-55%) higher than GPX(m) (25-34%). The prime-boost strategy gave the same results as naked DNA or recombinant vaccinia virus alone except in the case of GPX, where the protection was 85%.

The MIC3 gene of Toxoplasma gondii is a novel potent vaccine candidate against toxoplasmosis

Infection with the intracellular protozoan parasite Toxoplasma gondii causes serious public health problems and is of great economic importance worldwide. The micronemal protein MIC3, which is a potent adhesin of T. gondii, could be a significant candidate vaccine against toxoplasmosis. In this study, all CBA/J mice intramuscularly vaccinated with a plasmid encoding the immature form of the MIC3 protein (pMIC3i) produced specific anti-MIC3 immunoglobulin G (IgG) antibodies, and their sera displayed high antibody titers. This response was increased by the coadministration of a plasmid encoding the granulocyte-macrophage colony-stimulating factor (pGM-CSF). Similarly, a specific and significant cellular immune response was obtained in mice immunized with pMIC3i, and this response was markedly enhanced by pGM-CSF coadministration. The cellular immune response was associated with the production of gamma interferon IFN-gamma and interleukin-2 (IL-2), indicating that this was a Th1-type response. This was confirmed by the production of large amounts of IgG2a. Mice immunized with pMIC3i displayed significant protection against an oral challenge with T. gondii 76K cysts, exhibiting fewer brain cysts than did the control mice. Coadministration of pGM-CSF enhanced this protection. In conclusion, this study describes the design of a potent DNA vaccine encoding the novel T. gondii target antigen, MIC3 protein, that elicits a strong specific immune response as well as providing effective protection against T. gondii infection. In the attempt to achieve complete protection against toxoplasmosis, MIC3 is a good candidate vaccine which could be combined with other relevant and previously described candidates, such as SAG1 and GRA4.

Vaccination with DNA encoding ORFF antigen confers protective immunity in mice infected with Leishmania donovani

The gene ORFF is part of the multigenic LD1 locus on chromosome 35 that is frequently amplified in Leishmania. The function of ORFF is unknown. The gene encoding ORFF was cloned into a eukaryotic expression vector downstream to the cytomegalovirus (CMV) promoter. BALB/c mice were injected intramuscularly with ORFF DNA and challenged with Leishmania donovani promastigotes. Vaccination with ORFF gene induced both humoral and cellular immune response against ORFF, which provided significant level of protection against challenge with L. donovani. A qualitative PCR was used to determine whether activation of Th1 cells develops selectively in response to this ORFF DNA vaccine. The results indicated that mRNA for IFN-gamma was significantly induced in immunized mice. No significant change in IL-4 mRNA expression was observed in mice immunized with ORFF DNA vaccine versus mice immunized with control plasmid. Thus, DNA immunization may offer an attractive alternative strategy against leishmaniasis.

Endosomal/lysosomal targeting of a single helper T-cell epitope of an intracellular bacterium by DNA immunisation induces a specific T-cell subset and partial protective immunity in vivo

We evaluated here the effect of the intracellular targeting of a helper T-cell (Th) epitope, literiolysin O 215-226 derived from Listeria monocytogenes, on induction of a specific Th by gene gun immunisation. Immunisation of C3H/He mice with pE215LAMP plasmid encoding the Th epitope fused with the endosomal/lysosomal targeting signal of lysosome-associated membrane protein (LAMP)-1 gave the epitope-specific proliferative responses of CD4(+) T lymphocytes. In addition, specific interferon-gamma production from the splenocytes was observed. Concomitantly, pE215LAMP-immunised mice showed moderate, but significant protective immunity against listerial challenge. These results suggest that the intracellular targeting of a Th epitope to endosomal/lysosomal compartments by DNA immunisation is useful for eliciting a specific Th subset in vivo.

Mouse antibody response to a microsporidian parasite following inoculation with a gene coding for parasite ribosomal RNA

This study found that a plasmid construct encoding the small-subunit ribosomal RNA (SSUrRNA) of the microsporidian Microgemma caulleryi generates a humoral response upon intramuscular inoculation in mice. The plasmid used was pCMV, following preliminary trials indicating efficient beta-galactosidase gene expression in mouse muscle cells transfected with pCMV/beta-Gal. The antibodies produced after inoculation with pCMV/SSUDNA recognized parasite spore antigens and reached maximum levels at 30 days postinoculation, subsequently remaining stable for at least 120 days. Due to the highly conserved sequence of the SSUrDNA in different microsporidian species, these results open up interesting prospects for broad-spectrum vaccination.

Vaccination with plasmid DNA encoding TSA/LmSTI1 leishmanial fusion proteins confers protection against Leishmania major infection in susceptible BALB/c mice

We have recently shown that a cocktail containing two leishmanial recombinant antigens (LmSTI1 and TSA) and interleukin-12 (IL-12) as an adjuvant induces solid protection in both a murine and a nonhuman primate model of cutaneous leishmaniasis. However, because IL-12 is difficult to prepare, is expensive, and does not have the stability required for a vaccine product, we have investigated the possibility of using DNA as an alternative means of inducing protective immunity. Here, we present evidence that the antigens TSA and LmSTI1 delivered in a plasmid DNA format either as single genes or in a tandem digene construct induce equally solid protection against Leishmania major infection in susceptible BALB/c mice. Immunization of mice with either TSA DNA or LmSTI1 DNA induced specific CD4(+)-T-cell responses of the Th1 phenotype without a requirement for specific adjuvant. CD8 responses, as measured by cytotoxic-T-lymphocyte activity, were generated after immunization with TSA DNA but not LmSTI1 DNA. Interestingly, vaccination of mice with TSA DNA consistently induced protection to a much greater extent than LmSTI1 DNA, thus supporting the notion that CD8 responses might be an important accessory arm of the immune response for acquired resistance against leishmaniasis. Moreover, the protection induced by DNA immunization was specific for infection with Leishmania, i.e., the immunization had no effect on the course of infection of the mice challenged with an unrelated intracellular pathogen such as Mycobacterium tuberculosis. Conversely, immunization of BALB/c mice with a plasmid DNA that is protective against challenge with M. tuberculosis had no effect on the course of infection of these mice with L. major. Together, these results indicate that the protection observed with the leishmanial DNA is mediated by acquired specific immune response rather than by the activation of nonspecific innate immune mechanisms. In addition, a plasmid DNA containing a fusion construct of the two genes was also tested. Similarly to the plasmids encoding individual proteins, the fusion construct induced both specific immune responses to the individual antigens and protection against challenge with L. major. These results confirm previous observations about the possibility of DNA immunization against leishmaniasis and lend support to the idea of using a single polygenic plasmid DNA construct to achieve polyspecific immune responses to several distinct parasite antigens.

DNA immunization against the clumping factor A (ClfA) of Staphylococcus aureus

Adhesins are considered the most important virulence factors during early phases Staphylococcus aureus infection. Antibodies induced by vaccination toward an adhesin should reduce the adherence of the pathogen and augment its phagocytosis. The present report describes the immune response of mice to a DNA vaccine directed against one of these adhesins, clumping factor A (ClfA). Injection of plasmids expressing the fibrinogen-binding region A of ClfA induced a strong and specific antibody response to ClfA in mice. In addition, splenocyte proliferation was provoked by in vitro stimulation with recombinant ClfA, thus, indicating direct implication of these cells in the immune response. Pre-incubation of S. aureus with sera of vaccinated mice reduced the pathogen's ability to bind fibrinogen by up to 92%. These pre-incubated bacteria were phagocytosed by macrophages at an increased level in vitro and were less virulent in vivo in a mouse mastitis model. However, DNA-immunized mice were not protected against an intraperitoneal challenge. Overall, the results suggest that DNA immunization against adhesins represents a new and valuable approach to combat S. aureus infections.

Comparative efficacy of the Schistosoma mansoni nucleic acid vaccine, Sm23, following microseeding or gene gun delivery

Sm23 is an integral membrane protein expressed widely in the human parasitic worm Schistosoma mansoni. Sm23 has already been shown to elicit protective immune responses following immunization with peptides or DNA constructs. In this study, we evaluated the immunogenicity and the protective efficacy of the Sm23 DNA vaccine using two different intradermal DNA delivery methods: microseeding and gene gun. Using both techniques, all mice immunized with the Sm23-pcDNA construct generated Sm23-specific immunoglobulin (Ig)G antibody, while mice immunized with the control plasmid, pcDNA, did not. Antibody isotypes analysis revealed that microseeding elicited mainly IgG2a and IgG2b antibodies, with relatively low levels of IgG1 and IgG3. The relative IgG1/IgG2a ratio was 0.03, indicative of a Th1 type immune response. In contrast, gene gun immunization resulted in significantly higher levels of IgG1 and IgG3. The relative IgG1/IgG2a ratio in this case was 11, indicative of a Th2 type immune response. No significant difference in the levels of IgG2b was observed. Coimmunization with plasmid DNA encoding either interleukin (IL)-12 or IL-4 by microseeding did not affect the levels of IgG1, while the levels of IgG2a and IgG2b were reduced. On the other hand, the levels of IgG3 were significantly increased by IL-4, but unchanged by IL-12. Importantly, in all experiments, the Sm23-pcDNA vaccine provided statistically significant levels of protection against challenge infection. Microseeding immunizations resulted in higher levels of protection (31-34% protection) than gene gun immunization (18% protection). This suggests that the Th1 type immune response elicited by microseeding immunization was responsible for the higher protection levels. However, the protective effect of the vaccine was not affected by coadministering plasmids encoding either IL-12 or IL-4 using the microseeding technique.

Immunization with plasmid DNA encoding the integral membrane protein, Sm23, elicits a protective immune response against schistosome infection in mice

Schistosomes are helminth parasites infecting at least 200 million people worldwide. In this study, we evaluated the feasibility of using a nucleic acid vaccine to induce protective immune responses to the Schistosoma mansoni integral membrane protein Sm23. C57BL/6 mice were immunized by intramuscular injection in three separate vaccination trials. ELISA and Western Blot analyses indicated that mice immunized with a DNA plasmid construct encoding Sm23 (Sm23-pcDNA) generated specific IgG for Sm23, while sera from mice immunized with the control pcDNA plasmid did not. The vaccine elicited IgG(2a), and IgG(1) antibody isotypes. We also tested the adjuvant activity of IL-12 and IL-4 on humoral responses to Sm23. Co-immunization with plasmid encoding IL-12 did not affect the level of anti-Sm23 IgG(2a), but did reduce the IgG(1) level. In contrast, co-injection with a plasmid encoding IL-4 significantly reduced the level of anti-Sm23 IgG(2a), while the level of IgG(1) was largely unchanged. Importantly, the Sm23-pcDNA vaccine provided statistically significant levels of protection against challenge infection (21-44%, P<0.001-0.02). Co-administration of plasmids encoding either IL-12 or IL-4 did not significantly enhance this protective effect.

DNA vaccination with asparaginyl endopeptidase (Sm32) from the parasite Schistosoma mansoni: anti-fecundity effect induced in mice

DNA-based vaccine technology was used to induce an immune response in mice against a schistosome cysteine proteinase, asparaginyl endopeptidase (Sm32). The cDNA coding for Sm32 was cloned in a mammalian expression vector under control of the CMV promoter/enhancer and expressed for the first time in transfected mammalian cells as well as in mice immunized with the Sm32-encoding DNA construct. These mice developed antibodies which recognized the native protein not only in homogenates of Schistosoma mansoni worms but also in the gut on cryostat sections of the parasites. This DNA vaccine led to an anti-fecundity effect: female worms of a challenge infection produced 37% less eggs than those growing in naïve mice. The results suggest that Sm32 may be a candidate antigen for the generation of an anti-pathology vaccine against schistosomes.

Schistosomiasis in the People's Republic of China: prospects and challenges for the 21st century

Schistosomiasis japonica is a serious communicable disease and a major disease risk for more than 30 million people living in the tropical and subtropical zones of China. Infection remains a major public health concern despite 45 years of intensive control efforts. It is estimated that 865,000 people and 100,250 bovines are today infected in the provinces where the disease is endemic, and its transmission continues. Unlike the other schistosome species known to infect humans, the oriental schistosome, Schistosoma japonicum, is a true zoonotic organism, with a range of mammalian reservoirs, making control efforts extremely difficult. Clinical features of schistosomiasis range from fever, headache, and lethargy to severe fibro-obstructive pathology leading to portal hypertension, ascites, and hepatosplenomegaly, which can cause premature death. Infected children are stunted and have cognitive defects impairing memory and learning ability. Current control programs are heavily based on community chemotherapy with a single dose of the drug praziquantel, but vaccines (for use in bovines and humans) in combination with other control strategies are needed to make elimination of the disease possible. In this article, we provide an overview of the biology, epidemiology, clinical features, and prospects for control of oriental schistosomiasis in the People's Republic of China.

Leishmaniasis: current status of vaccine development

Leishmaniae are obligatory intracellular protozoa in mononuclear phagocytes. They cause a spectrum of diseases, ranging in severity from spontaneously healing skin lesions to fatal visceral disease. Worldwide, there are 2 million new cases each year and 1/10 of the world's population is at risk of infection. To date, there are no vaccines against leishmaniasis and control measures rely on chemotherapy to alleviate disease and on vector control to reduce transmission. However, a major vaccine development program aimed initially at cutaneous leishmaniasis is under way. Studies in animal models and humans are evaluating the potential of genetically modified live attenuated vaccines, as well as a variety of recombinant antigens or the DNA encoding them. The program also focuses on new adjuvants, including cytokines, and delivery systems to target the T helper type 1 immune responses required for the elimination of this intracellular organism. The availability, in the near future, of the DNA sequences of the human and Leishmania genomes will extend the vaccine program. New vaccine candidates such as parasite virulence factors will be identified. Host susceptibility genes will be mapped to allow the vaccine to be targeted to the population most in need of protection.

Immunisation with DNA encoding Leishmania infantum protein papLe22 decreases the frequency of parasitemic episodes in infected hamsters

We tested in outbred golden hamsters the protective potential of highly immunogenic Leishmania infantum protein papLe22 which we recently identified. Immunisation was performed using papLe22 cDNA, administered as a single intramuscular injection. The level of antibodies directed against total leishmanial antigens was significantly decreased in the vaccinated hamsters as compared with the controls, indicating that the administration of papLe22 cDNA downregulated the Th2 type response and suggesting that the immune response was reoriented toward the cell-mediated type. The presence of the parasite kDNA in the peripheral blood was systematically detected as early as 3 weeks post infection in all mock-vaccinated hamsters. By contrast, in the vaccinated animals the occurrence of the episodes of Leishmania circulation was reduced by 50%. The immunisation presenting efficacy in this highly susceptible species which develop VL similar in gravity to human and canine disease should prove also efficient in naturally infected hosts. The marked decrease of the frequency of parasite circulation induced by papLe22 cDNA immunisation appears therefore important and potentially able to reduce transmission and thus to control the spread of the disease.

Molecular cloning and enzymatic expression of the 28-kDa glutathione S-transferase of Schistosoma japonicum: evidence for sequence variation but lack of consistent vaccine efficacy in the murine host

Glutathione S-transferases (GSTs) have long been regarded as attractive vaccine (and drug) targets in schistosomes due to their suspected role in detoxification processes. Indeed, the 28-kDa GST of Schistosoma mansoni (SmGST28) has proven efficacy as an antigen for protective immunity reducing worm burden, female fecundity and egg viability. In contrast, the vaccinating effects of the bacterial expressed homologue of Philippine S. japonicum (SjpGST28) have proved disappointing, possibly because this recombinant form was an incomplete sequence, lacking five N-terminal amino acids which may have affected its vaccination efficacy. Here we describe the cloning and functional enzymatic expression of a complete cDNA encoding SjpGST28. We report also on the immunogenicity and vaccine efficacy of this molecule as a purified recombinant protein and as a DNA plasmid vaccine in the murine model. We further describe the cloning of several complete cDNAs encoding the Chinese homologue of SjpGST28 and the identification of 3 SjcGST28 sequence variants which are probably encoded by distinct alleles.

Multi-epitope schistosome vaccine candidates tested for protective immunogenicity in mice

The major challenge in the development of anti-schistosome vaccines is to use defined antigens to stimulate an appropriate immune response that leads to resistance. Several promising candidate vaccine antigens including the glycolytic enzyme triose-phosphate isomerase (SmTPI), a 28 kDa glutathione-S-transferase (Sm28), the myofibrilar protein paramyosin (Sm97), an integral membrane protein (Sm23) and calpain (Smcalpain) have been characterised and their primary sequences derived for Schistosoma mansoni. Furthermore, sequences are available for synthetic peptides mimicking epitopes on these molecules capable of inducing schistosome-specific T- and B-cell responses. These schistosome vaccine candidates have generally been tested with varying degrees of success as single components, with only one report of the use of a multivalent antigen or multi-epitope approach. We describe the assembly of multiple defined and different epitopes of S. mansoni into a variety of single covalent structures; these included a DNA vaccine encoding different epitopes in tandem, the polyprotein itself that is encoded by this DNA and branched synthetic peptide epitope-based polymers in which the individual epitopes are pendant from an inert backbone. Each of the vaccine constructs examined, with the exception of the DNA vaccine, generated antibodies that were capable of binding to a tandem sequence of the epitopes. Although these results were encouraging, none of the constructs protected animals from subsequent challenge infection, indicating that the immune responses elicited were inadequate or inappropriate for parasite killing in vivo.

Cutaneous vaccination: the skin as an immunologically active tissue and the challenge of antigen delivery

Vaccination is one of the major achievements of modern medicine. As a result of vaccination, diseases such as polio and measles have been controlled and small pox has been eradicated. However, despite these successes there are still many microbial diseases that cause tremendous suffering because there is no vaccine or the vaccines available are inadequate. In addition, even if vaccines were available for all infectious diseases there is no guarantee that people would use them routinely. One of the major impediments to ensuring vaccine efficacy and compliance is that of delivery. Presently most vaccines are given by intramuscular administration. Unfortunately this is often traumatic, especially in infants. Thus, if it was possible to replace intramuscular immunization by mucosal (oral/intranasal) or transdermal delivery it may be possible to both enhance mucosal immunity as well as improve overall compliance rates. The transdermal route has been used by the pharmaceutical industry for the delivery of various low molecular weight drugs. Some of the approaches used for smaller compounds may also have potential for delivery of either protein or polynucleotide vaccines. However, there is a greater challenge to delivering large molecular weight molecules through the skin due to size, charge and other physicochemical properties. This review will describe the recent advances that have been made in dermal and topical delivery as related to vaccines.

Modulation of immune responses following antigen administration by mucosal route

Most microbial infections are either restricted to the mucosal membranes or the etiologic agents needed to transit the mucosa. Thus, it is desirable to stimulate a mucosal response following vaccination, to block both infection and disease development. Attenuated vaccine carriers mimic natural infections, triggering also mucosal responses. Similar results can be achieved by administering antigens with appropriate adjuvants. However, the delivery of antigens per se is not sufficient to engender a protective response. A successful immunization requires the elicitation of an appropriate type of immune response (e.g. antibodies vs. cell-mediated immunity, Th1 vs. Th2 helper pattern). Therefore, a successful vaccination strategy demands the choice of adequate antigens, and their appropriate delivery and/or formulation to promote the required quality of immune response. Different strategies to optimize the immune responses elicited following vaccine administration by the mucosal route are discussed.

The search for a vaccine against schistosomiasis--a difficult path but an achievable goal

The search for an effective vaccine against schistosomiasis, a parasitic disease currently affecting over 200 million people, remains a desirable but as yet challenging and elusive goal. Progress in the area has been relatively slow but research demonstrating the ability of humans to acquire natural immunity to schistosome infection, together with the successful use in animals of attenuated vaccines, supplemented with encouraging results obtained with defined antigens, suggests that development of a vaccine is achievable. Noteworthy also are recent immune correlate findings which shed light on the complex, putatively protective immune responses in humans, which have improved the prospects of success. With the first human clinical trial having been completed with a schistosome vaccine candidate, this review examines current progress aimed at achieving the objective of a safe and effective vaccine for widespread use against schistosomiasis. The review emphasises work undertaken in the author's laboratory and those of his chief collaborators in the search for a vaccine against schistosomiasis japonica, a disease of major public health significance in The People's Republic of China and The Philippines. Schistosomiasis vaccines should not be considered as the panacea for schistosomiasis control as, when available, it is generally envisaged that they would be used as one component of an integrated strategy complementing currently available and effective tools such as chemotherapy, improvements to sanitation, piped water supply, effective sewage draining and health education.

Genetic immunization of mice with DNA encoding the 23 kDa transmembrane surface protein of Schistosoma japonicum (Sj23) induces antigen-specific immunoglobulin G antibodies

The 23 kDa transmembrane surface protein of schistosomes is of recognized interest in studies of immune responsiveness in schistosomiasis. To examine the immunogenicity of the 23 kDa antigen of Schistosoma japonicum, Sj23, when delivered by genetic immunization, mice were immunized using a DNA construct containing the Sj23 cDNA under the control of a CMV promotor. Serological analysis of peripheral blood from immunized mice demonstrated that this construct was able to induce the production of antigen-specific IgG antibodies that recognized a schistosome antigen of 23 kDa in Western blots. Despite inducing antigen-specific antibodies, the Sj23 DNA vaccine was unable to confer protection in immunized mice subjected to challenge with S.japonicum cercariae. Appropriate engineering of the unique structure of the Sj23 kDa transmembrane protein of schistosomes may provide a novel vehicle for expressing foreign epitopes from other infectious agents or, possibly, cancer antigens, anchored to the surface of transfected cells.

Molecular cloning and functional expression of a cDNA encoding the major endoplasmic reticulum-associated calcium-binding protein, calreticulin, from Philippine strain Schistosoma japonicum

We describe the cloning of a full length calreticulin (CR)-encoding cDNA clone isolated by immunoscreening of a cDNA library prepared with mRNA from adult worms of the Philippine strain of Schistosoma japonicum, the cause of Asian schistosomiasis. The sequence of the cDNA is presented, and its molecular characterisation and functional expression as a Ca2+-binding protein described. The potential role of CR in inducing protective immunity in the schistosomes is discussed.

DNA immunization by intramuscular injection or gene gun induces specific IgG antibodies against a Schistosoma japonicum 22 kDa antigen, Sj22, when fused to the murine Ig K-chain secretory leader sequence

The 22 kDa tegumental surface membrane-associated antigen of Schistosoma japonica, Sj22, is of recognized interest in schistosomiasis vaccine research. However, previous attempts to induce antibody responses against Sj22 by DNA immunization have been unsuccessful. In this report we demonstrate that fusing the Sj22 cDNA to the murine immunoglobulin Ig kappa-chain secretory leader sequence results in the generation of antigen-specific IgG antibodies following DNA immunization. Mice were immunized into the skin with DNA-coated gold microspheres using a gene-gun, or into the quadriceps muscle by intramuscular injection. Both methods of delivery generated antigen-specific IgG antibodies against the 22 kDa schistosome antigen. The use of a secretory leader sequence, such as the murine Ig-kappa chain used in this study, may facilitate the induction of host antibody responses following DNA immunization with other parasite cDNAs.