Intradermal immunization of rats with plasmid DNA encoding Schistosoma mansoni 28 kDa glutathione S-transferase

Recent Advances and Methodological Considerations on Vaccine Candidates for Human Schistosomiasis

Schistosomiasis remains a neglected tropical disease of major public health concern with high levels of morbidity in various parts of the world. Although considerable efforts in implementing mass drug administration programs utilizing praziquantel have been deployed, schistosomiasis is still not contained. A vaccine may therefore be an essential part of multifaceted prevention control efforts. In the 1990s, a joint United Nations committee promoting parasite vaccines shortlisted promising candidates including for schistosomiasis discussed below. After examining the complexity of immune responses in human hosts infected with schistosomes, we review and discuss the antigen design and preclinical and clinical development of the four leading vaccine candidates: Sm-TSP-2 in Phase 1b/2b, Sm14 in Phase 2a/2b, Sm-p80 in Phase 1 preparation, and Sh28GST in Phase 3. Our assessment of currently leading vaccine candidates revealed some methodological issues that preclude a fair comparison between candidates and the rationale to advance in clinical development. These include (1) variability in animal models - in particular non-human primate studies - and predictive values of each for protection in humans; (2) lack of consensus on the assessment of parasitological and immunological parameters; (3) absence of reliable surrogate markers of protection; (4) lack of well-designed parasitological and immunological natural history studies in the context of mass drug administration with praziquantel. The controlled human infection model - while promising and unique - requires validation against efficacy outcomes in endemic settings. Further research is also needed on the impact of advanced adjuvants targeting specific parts of the innate immune system that may induce potent, protective and durable immune responses with the ultimate goal of achieving meaningful worm reduction.

Schistosomiasis and hookworm infection in humans: Disease burden, pathobiology and anthelmintic vaccines

Helminth diseases are the ancient scourges of humans and their damages are 'silent and insidious'. Of the helminth infections, schistosomiasis and hookworm infection have a great impact. This review covers information regarding vaccine candidates against schistosomiasis and hookworms that reached at least up to the phase-1 trial and literatures regarding other vaccine candidates have been excluded. For clinical manifestations, all available literatures were included, and for epidemiology and global burden of the diseases (GBD), literatures only within 2000-2019 were included. Literatures were searched surfing various databases including PubMED, Google Scholar, and Science Direct and overall over 150 literatures were identified. Globally ~250 million people are suffering from schistosomiasis, resulting 1430 thousand DALY (disability adjusted life year) per year. On the other hand, about 1.3 billion people are infected with hookworm (HW), and according to WHO, ~878 million school-age children (SAC) are at risk. HW is estimated to cause 65,000 deaths annually, accounts for 845 thousand DALYs as well as to cause 6-35.3% loss in productivity. Despite tremendous efforts, very few anthelmintic vaccine candidates such as Na-GST-1, Na-APR-1 and Na-ASP-2 against HW, and Sm28GST/Sh28GST, Sm-p80, Sm14 and Sm-TSP-1/SmTSP-2 against schistosomiasis reached up to the clinical trials. More efforts are needed to achieve the WHO targets taken against the maladies.

Toll-Like Receptor 9 Activation Rescues Impaired Antibody Response in Needle-free Intradermal DNA Vaccination

The delivery of plasmid DNA to the skin can target distinct subsets of dermal dendritic cells to confer a superior immune response. The needle-free immunization technology offers a reliable, safe and efficient means to administer intradermal (ID) injections. We report here that the ID injection of DNA vectors using an NF device (NF-ID) elicits a superior cell-mediated immune response, at much lesser DNA dosage, comparable in magnitude to the traditional intramuscular immunization. However, the humoral response is significantly impaired, possibly at the stage of B cell isotype switching. We found that the NF-ID administration deposits the DNA primarily on the epidermis resulting in a rapid loss of the DNA as well as the synthesized antigen due to the faster regeneration rate of the skin layers. Therefore, despite the immune-rich nature of the skin, the NF-ID immunization of DNA vectors may be limited by the impaired humoral response. Additional booster injections are required to augment the antibody response. As an alternative and a viable solution, we rescued the IgG response by coadministration of a Toll-like receptor 9 agonist, among other adjuvants examined. Our work has important implication for the optimization of the emerging needle-free technology for ID immunization.

Protective Potential of Antioxidant Enzymes as Vaccines for Schistosomiasis in a Non-Human Primate Model

Schistosomiasis remains a major cause of morbidity in the world. The challenge today is not so much in the clinical management of individual patients, but rather in population-based control of transmission in endemic areas. Despite recent large-scale efforts, such as integrated control programs aimed at limiting schistosomiasis by improving education and sanitation, molluscicide treatment programs and chemotherapy with praziquantel, there has only been limited success. There is an urgent need for complementary approaches, such as vaccines. We demonstrated previously that anti-oxidant enzymes, such as Cu-Zn superoxide dismutase (SOD) and glutathione S peroxidase (GPX), when administered as DNA-based vaccines induced significant levels of protection in inbred mice, greater than the target 40% reduction in worm burden compared to controls set as a minimum by the WHO. These results led us to investigate if immunization of non-human primates with antioxidants would stimulate an immune response that could confer protection as a prelude study for human trials. Issues of vaccine toxicity and safety that were difficult to address in mice were also investigated. All baboons in the study were examined clinically throughout the study and no adverse reactions occurred to the immunization. When our outbred baboons were vaccinated with two different formulations of SOD (SmCT-SOD and SmEC-SOD) or one of GPX (SmGPX), they showed a reduction in worm number to varying degrees, when compared with the control group. More pronounced, vaccinated animals showed decreased bloody diarrhea, days of diarrhea, and egg excretion (transmission), as well as reduction of eggs in the liver tissue and in the large intestine (pathology) compared to controls. Specific IgG antibodies were present in sera after immunizations and 10 weeks after challenge infection compared to controls. Peripheral blood mononuclear cells, mesenteric, and inguinal node cells from vaccinated animals proliferated and produced high levels of cytokines and chemokines in response to crude and recombinant antigens compared with controls. All together, these data demonstrate the potential of antioxidants as a vaccine in a non-human primate model.

Eliminating Schistosomes through Vaccination: What are the Best Immune Weapons?

The successful development of vaccines depends on the knowledge of the immunological mechanisms associated with the elimination of the pathogen. In the case of schistosomes, its complex life cycle and the mechanisms developed to evade host immune system, turns the development of a vaccine against the disease into a very difficult task. Identifying the immunological effector mechanisms involved in parasite attrition and the major targets for its response is a key step to formulate an effective vaccine. Recent studies have described some promising antigens to compose a subunit vaccine and have pointed to some immune factors that play a role in parasite elimination. Here, we review the immune components and effector mechanisms associated with the protective immunity induced by those vaccine candidates and the lessons we have learned from the studies of the acquired resistance to infection in humans. We will also discuss the immune factors that correlate with protection and therefore could help to evaluate those vaccine formulations in clinical trials.

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.

Computational vaccinology: an important strategy to discover new potential S. mansoni vaccine candidates

The flatworm Schistosoma mansoni is a blood fluke parasite that causes schistosomiasis, a debilitating disease that occurs throughout the developing world. Current schistosomiasis control strategies are mainly based on chemotherapy, but many researchers believe that the best long-term strategy to control schistosomiasis is through immunization with an antischistosomiasis vaccine combined with drug treatment. Several papers on Schistosoma mansoni vaccine and drug development have been published in the past few years, representing an important field of study. The advent of technologies that allow large-scale studies of genes and proteins had a remarkable impact on the screening of new and potential vaccine candidates in schistosomiasis. In this postgenomic scenario, bioinformatic technologies have emerged as important tools to mine transcriptomic, genomic, and proteomic databases. These new perspectives are leading to a new round of rational vaccine development. Herein, we discuss different strategies to identify potential S. mansoni vaccine candidates using computational vaccinology.

Vaccination against Schistosoma japonicum infection by DNA vaccine encoding Sj22.7 antigen

To observe the in vitro expression of DNA vaccine pcDNA3-Sj22.7 and its immunological effect in mice, the recombinant plasmid pcDNA3-Sj22.7 was used to transfect HeLa cells with liposome-mediated method and the expression of Sj22.7 mRNA and protein was examined using reverse transcription-polymerase chain reaction, sodium dodecylsulfate-polyacrylamide gel electrophoresis and Western blot. Then, the ability of pcDNA3-Sj22.7 to protect against Schistosoma japonicum challenge infections was analyzed according to worm reduction rate and egg reduction rate after vaccination of mice. The serum levels of specific IgG antibody and T lymphocyte proliferation response were also determined. After the challenge infection, Sj22.7-driven interferon (IFN)-gamma and interleukin (IL)-4 was also quantified. Results showed that pcDNA3-Sj22.7 could express Sj22.7 mRNA and protein in vitro. Immunization resulted in a worm reduction rate of 29.70%, egg reduction rate of 47.25% (liver) and 51.73% (intestine), and egg reduction rate of 25.90% (eggs per female), suggesting induction of significant anti-fecundity in the pcDNA3-Sj22.7 group. Enzyme-linked immunosorbent assay and Western blot analysis indicated that immunized mice generated specific IgG against Sj22.7. T lymphocytes from mice immunized with pcDNA3-Sj22.7 showed a significant proliferation response to rSj22.7. The culture of spleen cells showed that secretion of IFN-gamma increased but IL-4 decreased. The results indicate hat DNA vaccination by pcDNA3-Sj22.7 is sufficient to elicit significant levels of protective immunity against S. japonicum infection. The DNA vaccine could induce significant cellular and humoral immune response, and display predominant T helper cell type 1 type immune responses, which contribute to the protective immunity against challenge infection in mice.

Vaccination with DNA encoding cysteine proteinase confers protective immune response to rats infected with Clonorchis sinensis

Cysteine proteinases of C. sinensis are important virulence factors that induce pathological changes associated with larval migration and localized biliary epithelial destruction. This study investigated the immunogenicity and protective efficacy of a DNA vaccine encoding Clonorchis sinensis cysteine proteinase (CsCP). The CsCP cDNA sequence displays significant homology to the mammalian or trematode cathepsin L. Plasmid DNA carrying the CsCP gene (pcDNA3.1-CsCP) was injected into Sprague-Dawley (SD) rats intradermally. Animals injected with pcDNA3.1-CsCP developed CsCP-specific antibodies, which exhibited an IgG2a dominance in sera. In addition, the DNA vaccine elicited the production of IFN-gamma, but not IL-4 in splenocytes, suggesting the induction of a typical Th-1 dominated immune response in rats. The pcDNA3.1-CsCP induced a significant level of protection (31.5%, p<0.05) in SD rats challenged with C. sinensis metacercariae. These results indicate that pcDNA3.1-CsCP induces both humoral and cellular immune responses. The CsCP gene may be a good candidate for use in future studies of vaccination against clonorchiasis.

Recombinant tegumental protein Shistosoma japonicum very lowdensity lipoprotein binding protein as a vaccine candidate against Schistosoma japonicum

A polyhistidine-tagged recombinant tegumental protein Schistosoma japonicum very lowdensity lipoprotein binding protein (SVLBP) from adult Schistosoma japonicum was expressed in Escherichia coli. The affinity purified rSVLBP was used to vaccinate mice. The worm numbers and egg deposition recovered from the livers and veins of the immunized mice were 33.5% and 47.6% less than that from control mice, respectively (p<0.05). There was also a marked increase in the antibody response in vaccinated mice: the titer of IgG1 and IgG2a, IgG2b in the vaccinated group was significantly higher than that in the controls (>1:6,400 in total IgG). In a comparison of the reactivity of sera from healthy individuals and patients with rSVLBP, recognition patterns against this parasite tegumental antigen varied among different groups of the individuals. Notably, the average titres of anti-rSVLBP antibody in sera from faecal egg-negative individuals was significantly higher than that in sera from the faecal egg-positives, which may be reflect SVLBP-specific protection. These results suggested that the parasite tegumental protein SVLBP was a promising candidate for further investigation as a vaccine antigen for use against Asian schistosomiasis.

Fasciola hepatica: humoral and cytokine responses to a member of the saposin-like protein family following delivery as a DNA vaccine in mice

The humoral and cellular responses to DNA vaccination of BALB/c mice with a novel antigen from the Fasciola hepatica saposin-like protein family (FhSAP-2) have been investigated. Two constructs were produced containing the FhSAP-2 DNA sequence, one intended for extracellular secretion of FhSAP-2 protein, and one expressing FhSAP-2 in the cytoplasm of a transfected cell. The constructs were tested in HEK 293T cells, with the secretory construct producing less detectable FhSAP-2 relative to cytoplasmic construct when observed by fluorescence. The size of expressed protein was confirmed by Western blot of cell lysate, but FhSAP-2 was undetectable in cell supernatants. Both, secretory and cytoplasmic constructs as well as FhSAP-2 recombinant protein were tested in mice. The antibody response elicited in mice vaccinated with the rFhSAP-2 induced high levels of IgG(1), IgG(2), and IgE as well as high levels of IL-10 and IFNgamma indicating a mixed Th1/Th2 response. Vaccination of mice intramuscularly with the cytoplasmic FhSAP-2 construct resulted in a dominant IgG(2a) isotype antibody as well as a dominant IFNgamma cytokine, with significant IgE, IgG(1), and IL-10 responses also present, suggesting a mixed Th1/Th2 profile. Isotype and cytokine profiles elicited by the FhSAP-2 secretory construct were similar to those obtained with the cytoplasmic construct but at levels that were significantly lower. The results demonstrate that FhSAP-2 can be delivered as a DNA vaccine construct and induces a stronger Th1 response than the recombinant protein alone. This could result in an improvement in the immunoprophylactic potential of this candidate vaccine against F. hepatica.

Nucleic acid vaccination with Schistosoma mansoni antioxidant enzyme cytosolic superoxide dismutase and the structural protein filamin confers protection against the adult worm stage

Schistosomiasis remains a worldwide endemic cause of chronic and debilitating illness. There are two paradigms that exist in schistosome immunology. The first is that the schistosomule stages are the most susceptible to immune killing, and the second is that the adult stage, through evolution of defense mechanisms, can survive in the hostile host environment. One mechanism that seems to aid the adult worm in evading immune killing is the expression of antioxidant enzymes to neutralize the effects of reactive oxygen and nitrogen species. Here, we challenge one paradigm by targeting adult Schistosoma mansoni worms for immune elimination in an experimental mouse model using two S. mansoni antioxidants, cytosolic superoxide dismutase (SmCT-SOD) and glutathione peroxidase (SmGPX), and a partial coding sequence for a structural protein, filamin, as DNA vaccine candidates. DNA vaccination with SmCT-SOD induced a mean of 39% protection, filamin induced a mean of 50% protection, and SmGPX induced no protection compared to controls following challenge with adult worms by surgical transfer. B- and T-cell responses were analyzed in an attempt to define the protective immune mechanism(s) involved in adult worm killing. SmCT-SOD-immunized mice presented with a T1 response, and filamin-immunized mice showed a mixed T1-T2 response. We provide evidence for natural boosting after vaccination. Our results demonstrate that adult worms can be targeted for immune elimination through vaccination. This represents an advance in schistosome vaccinology and allows for the development of a therapeutic as well as a prophylactic vaccine.

Methylated CpG-Containing Plasmid Activates the Immune System

Bacterial DNA differs from mammalian DNA by the presence of unmethylated cytosine-phosphate-guanosine (CpG) motifs. The immunostimulatory properties of a DNA vaccine have been suspected to be associated with these motifs. The aim of this study was to assess the inactivation of the immunostimulatory potential of a plasmid after methylation of its CpG motifs. We constructed two identical non-coding plasmids, and one of these was de novo methylated on its CG sequences. A single administration of recombinant antigen with methylated or unmethylated CpG-containing plasmid was performed in mice. As expected, only unmethylated CpG-containing plasmid enhanced the specific immune response. However, a study of in vivo activation of Langerhans' cells and analysis of mRNA synthesis indicated that both the plasmids promoted cell emigration and cytokine induction. These data highlight that a methylated CpG-containing plasmid is not inert and carries immunomodulatory properties. The results further emphasize the necessity to definitively identify the mode of action of plasmids used for DNA vaccination.

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%.

Expression of a 28-kilodalton glutathione S-transferase antigen of Schistosoma mansoni on the surface of filamentous phages and evaluation of its vaccine potential

A cloning and expression system that allows display of proteins on the surface of filamentous phages was exploited to display a 28-kDa glutathione S-transferase (Sm28GST) antigen of the human parasite Schistosoma mansoni. The phage-displayed Sm28GST (pdGST) was immunoreactive and was recognized by immune sera, suggesting that the Sm28GST protein displayed on the surface of phages potentially maintains native conformation. Subsequent immunization studies showed that mice can develop high titers of antibodies against pdGST and do not require any additional adjuvant for immunization. Isotype analysis suggested that the pdGST immunization predominantly induced immunoglobulin G2b (IgG2b), IgG3, and IgM anti-GST antibodies in mice. Furthermore, the pdGST immunization was found to confer about 30% protection after a challenge infection with 100 cercariae of S. mansoni in BALB/c mice. These findings suggest that phage display is a simple, efficient, and promising tool to express candidate vaccine antigens for immunization against infectious agents.

Protective effect of a naked DNA vaccine cocktail against lethal toxoplasmosis in mice

BALB/c mice were intramuscularly immunized with low doses (25-50microg) of DNA cocktail containing plasmids encoding the full-length SAG1/P30 and the 196-561 terminal sequence of ROP2 genes. This immunization resulted in a Th1-type response with predominance of IgG2a and a specific T-cell proliferation with high levels of interferon-gamma (IFN-gamma) secretion, whereas no IL-4 was detected. Moreover, DNA cocktail immunization induced a long-lasting protection against a lethal challenge with the highly virulent Toxoplasma gondii RH strain, whereas low doses of single genes were not protective. These results support further investigations to achieve a multigene anti-T. gondii DNA vaccine.

The contribution of molecular biology to the development of vaccines against nematode and trematode parasites of domestic ruminants

Rapid developments in molecular biology have had an enormous impact on the prospects for the development of vaccines to control the major nematode and trematode infestations of livestock. Vaccine candidates are purified using conventional protein chemistry techniques but the limitations imposed by the scarcity of parasite material provide an insurmountable barrier for commercial vaccine production by this means. The ability to purify mRNA from different parasite life-cycle stages and to prepare cDNA expression libraries from it has proven central to the identification of immunogenic parasite proteins. Potentially, protective parasite antigens can now be produced in recombinant form in a variety of vectors and this represents a key breakthrough on the road to commercial vaccine production. The contribution of molecular biology to this process is discussed using several examples, particularly in vaccine development against the pathogenic abomasal nematode of sheep and goats, Haemonchus contortus, and the liver fluke of sheep and cattle, Fasciola hepatica. The difficulties of producing recombinant proteins in the correct form, with appropriate post-translational modification and conformation, are discussed as well as emerging means of antigen delivery including DNA vaccination. The opportunities offered by genome and expressed sequence tag analyses programmes for antigen targeting are discussed in association with developing microarray and proteomics technologies which offer the prospect of large scale, rapid antigen screening and identification.

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.

Features of the antibody response attributable to plasmid backbone adjuvanticity after DNA immunization

DNA vaccination induces antigen-specific immune responses with characteristics distinct from other vaccination modes. In the present study, the contribution of the plasmid backbone adjuvant effect to the quality of the DNA-raised antibody response was investigated. For this purpose, three intradermal primings were compared in mice using: (1) the recombinant Schistosoma haematobium glutathione S-transferase antigen (rSh28GST): (2) rSh28GST supplemented with a non-coding plasmid; and (3) a Sh28GST-encoding plasmid. In contrast to immunization with the protein, DNA immunization elicited a very stable antibody (Ab) response over a prolonged period of time. This feature was attributed to the plasmid backbone, because co-administration of the non-coding plasmid with rSh28GST allowed the maintenance of the specific Ab response. A strong anamnestic Ab response was induced after intradermal boost with rSh28GST only in the mice primed with pMSh. This indicated that the selective ability of DNA vaccination to induce memory humoral response was independent of the plasmid backbone. In contrast the plasmid backbone was found to strongly participate in the preferential IgG2a Ab production observed. These results suggest that, following DNA immunization, the Th1-biased profile and the maintenance of the long-lived Ab response could be attributed to an adjuvant effect of the plasmid backbone during priming, whereas the strength of B-cell memory was independent of this effect.

Immunostimulatory effect of IL-18-encoding plasmid in DNA vaccination against murine Schistosoma mansoni infection

In vivo delivery of DNA encoding antigens is a simple tool to induce immune responses against pathogens. This approach to vaccination also offers the possibility to codeliver plasmids encoding immunomodulatory molecules in order to drive immune responses towards optimal protective effects. In the murine model of Schistosoma mansoni infection, vaccination inducing a Th1 profile has been shown to be protective. In this study, we used a plasmid encoding the Th1-promoting cytokine IL-18, since we observed that percutaneous infection of Balb/c mice strongly induced the production of IL-18 mRNA in the skin. Intradermal injection of the IL-18-encoding plasmid prior to infection did not interfere with parasite migration through the skin although it led to a local and transient cellular infiltration. When the IL-18-encoding plasmid was codelivered with a S. mansoni glutathione S-transferase (Sm28GST)-encoding plasmid, a 30-fold increase of antigen-specific IFN-gamma secretion by spleen cells was observed in comparison to spleen cells from mice that had received only the Sm28GST-encoding plasmid. This immunostimulatory effect was related to a significant protective effect (28% reduction in egg laying and 23% reduction in worm burden) which was attributed to a cooperative effect between both plasmids. Therefore, this study shows that codelivery of an IL-18-encoding plasmid with an antigen-encoding plasmid can stimulate specific cellular responses and induce protective effects against S. mansoni infection.

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.

DNA immunisation against the CFA/I fimbriae of enterotoxigenic Escherichia coli (ETEC)

The CFA/I fimbria promotes the attachment of enterotoxigenic Escherichia coli (ETEC) to the surface of human enterocytes. The generation of a protective immune response requires the induction of antibodies able to block the CFA/I-mediated binding of ETEC to receptors located on the small intestine epithelium or on the surface of human red blood cells, in hemagglutination tests. An eukaryotic expression plasmid, pBLCFA, encoding the CFA/I gene under the control of the human cytomegalovirus major immediate-early promoter was constructed as a prototype DNA vaccine against ETEC. pBLCFA-tranfected BHK-21 cells secreted a peptide cross-reacting with a monoclonal antibody raised against CFA/I subunits. BALB/c mice immunized intramuscularly with one or two doses of purified pBLCFA developed CFA/I-specific serum antibodies for at least 52 weeks, composed predominantly of the IgG1 subclass. pBLCFA-induced antibodies bind mainly to epitopes exposed on the surface of intact CFA/I fimbriae and do not react with immune recessive epitopes found in other ETEC fimbra sharing amino acid homologies with CFA/I. Furthermore, pBLCFA-induced antibodies were able to block the adhesive properties of the CFA/I fimbriae, as evaluated by the ability to inhibit the hemagglutination promoted by CFA/I-expressing ETEC cells. These results suggest that secretion of CFA/I encoded by pBLCFA preserves important conformational epitopes required for the generation of protective antibodies against the adhesive properties of the CFA/I fimbriae and open new perspectives for the development of DNA vaccines against enteric bacterial pathogens.

Genetic vaccination against malaria infection by intradermal and epidermal injections of a plasmid containing the gene encoding the Plasmodium berghei circumsporozoite protein

The circumsporozoite protein (CSP) from the surface of sporozoite stage Plasmodium sp. malaria parasites is among the most important of the malaria vaccine candidates. Gene gun injection of genetic vaccines encoding Plasmodium berghei CSP induces a significant protective effect against sporozoite challenge; however, intramuscular injection does not. In the present study we compared the immune responses and protective effects induced by P. berghei CSP genetic vaccines delivered intradermally with a needle or epidermally with a gene gun. Mice were immunized three times at 4-week intervals and challenged by a single infectious mosquito bite. Although 50 times more DNA was administered by needle than by gene gun, the latter method induced significantly greater protection against infection. Intradermal injection of the CSP genetic vaccine induced a strong Th1-type immune response characterized by a dominant CSP-specific immunoglobulin G2a (IgG2a) humoral response and high levels of gamma interferon produced by splenic T cells. Gene gun injection induced a predominantly Th2-type immune response characterized by a high IgG1/IgG2a ratio and significant IgE production. Neither method generated measurable cytotoxic T lymphocyte activity. The results indicate that a gene gun-mediated CS-specific Th2-type response may be best for protecting against malarial sporozoite infection when the route of parasite entry is via mosquito bite.

Protective immunity induced by the full-length cDNA encoding paramyosin of Chinese Schistosoma japonicum

The full-length cDNA encoding paramyosin of Chinese S. japonicum (Sjc97) has been cloned and sequenced for the first time. The homology of the nucleotide sequence of paramyosin and the deduced amino acid sequence between Chinese, Philippine, Japanese strains and S. mansoni (Sm97) were 99.7, 99.8 and 96% at amino acid level, respectively; 99.4, 99.2 and 91% at nucleotide level, respectively. The immunogenicity and protective efficacy of a DNA vaccine encoding Sjc97 was evaluated in C57BL/6 mice. Mice immunized intramuscularly with pCMV-Sjc97 resulted in predominantly IgG2a and IgG2b immune responses, and immune sera were able to mediate antibody-dependent macrophage mediated cytotoxicity in vitro. Cytokine profiles in immunized C57BL/6 mice demonstrated Th1 bias, with IFN-gamma and IL-2 production and lack of IL-4 and IL-5. Immunization with pCMV-Sjc97 conferred a significant level of protection against cercariae in C57BL/6 mice. These results demonstrate that the nucleic acid encoding Sjc97 was able to induce a Th1 type immune response and confer protective efficacy in C57BL/6 mice when administrated via the intramuscular route.

Immunogenicity of plasmid DNA encoding the 62 kDa fragment of Schistosoma japonicum myosin

The recombinant Schistosoma mansoni 62 kDa myosin fragment, rIrV-5, is highly protective in experimental animals, however, vaccination of mice and rats with the recombinant Schistosoma japonicum homologue, rSj62, did not induce significant resistance against S. japonicum infection. To explore alternative ways of presenting this antigen, we further constructed a plasmid (VRSj62) which encodes Sj62 using the VR1020 vector and tested it in vaccination experiments. Four immunisations with 10 microg VRSj62 DNA alone were sufficient to induce high and progressively increasing levels of IgG antibodies against rSj62 with increasing numbers of injections in CBA/Ca mice (IgG titre > or =1:25000), and three injections with 50 microg VRSj62 DNA alone induced significant IgG responses in C57Bl/6 mice (IgG titre, 1:1600). However, vaccination with plasmid DNA entrapped in cationic liposomes or together with pUC19 DNA as a source of CpG motifs, both of which have been reported to enhance immune responses, did not enhance specific antibody production. In spite of the stimulation of specific antibodies against rSj62 with the naked DNA construct no resistance to challenge was demonstrated.

DNA vaccination with genes encoding Toxoplasma gondii antigens GRA1, GRA7, and ROP2 induces partially protective immunity against lethal challenge in mice

C57BL/6, C3H, and BALB/c mice were vaccinated with plasmids encoding Toxoplasma gondii antigens GRA1, GRA7, and ROP2, previously described as strong inducers of immunity. Seroconversion for the relevant antigen was obtained in the majority of the animals. T. gondii lysate stimulated specific T-cell proliferation and secretion of gamma interferon (IFN-gamma) in spleen cell cultures from vaccinated BALB/c and C3H mice but not in those from control mice. Although not proliferating, stimulated splenocytes from DNA-vaccinated C57BL/6 mice also produced IFN-gamma. No interleukin-4 was detected in the supernatants of lysate-stimulated splenocytes from DNA-vaccinated mice in any of the mouse strains evaluated. As in infected animals, a high ratio of specific immunoglobulin G2a (IgG2a) to IgG1 antibodies was found in DNA-vaccinated C3H mice, suggesting that a Th1-type response had been induced. For BALB/c mice, the isotype ratio of the antibody response to DNA vaccination was less polarized. The protective potential of DNA vaccination was demonstrated in C3H mice. C3H mice vaccinated with plasmid encoding GRA1, GRA7, or ROP2 were partially protected against a lethal oral challenge with cysts of two different T. gondii strains: survival rates increased from 10% in controls to at least 70% after vaccination in one case and from 50% to at least 90% in the other. In vaccinated C3H mice challenged with a nonlethal T. gondii dose, the number of brain cysts was significantly lower than in controls. DNA vaccination did not protect BALB/c or C57BL/6 mice. Our results demonstrate for the first time in an animal model a partially protective effect of DNA vaccination against T. gondii.

DNA vaccination strategies against infectious diseases

DNA immunisation represents a novel approach to vaccine and immunotherapeutic development. Injection of plasmid DNA encoding a foreign gene of interest can result in the subsequent expression of the foreign gene products and the induction of an immune response within a host. This is relevant to prophylactic and therapeutic vaccination strategies when the foreign gene represents a protective epitope from a pathogen. The recent demonstration by a number of laboratories that these immune responses evoke protective immunity against some infectious diseases and cancers provides support for the use of this approach. In this article, we attempt to present an informative and unbiased representation of the field of DNA immunisation. The focus is on studies that impart information on the development of vaccination strategies against a number of human and animal pathogens. Investigations that describe the mechanism(s) of protective immunity induced by DNA immunisation highlight the advantages and disadvantages of this approach to developing vaccines within a given system. A variety of systems in which DNA vaccination has resulted in the induction of protective immunity, as well as the correlates associated with these protective immune responses, will be described. Particular attention will focus on systems involving parasitic diseases. Finally, the potential of DNA immunisation is discussed as it relates to veterinary medicine and its role as a possible vaccine strategy against animal coccidioses.

Humoral responses in mice following vaccination with DNA encoding glutathione S-transferase of Fasciola hepatica: effects of mode of vaccination and the cellular compartment of antigen expression

The humoral responses in mice following vaccination with DNA constructs encoding Fasciola hepatica glutathione S-transferase (GST) have been evaluated. GST47 cDNA was subcloned into two DNA vaccine vectors, VR1012 and VR1020, which direct expression to the cytoplasmic and extracellular compartments, respectively. Expression was confirmed by transfection into COS 7 cells. Groups of mice were vaccinated with these constructs, by either intramuscular injection with the VR1012-or VR1020-based constructs, or intradermal vaccination (with a gene gun) with the VR1020-based construct. Vaccination with the construct designed for secretion resulted in an increased humoral response compared to vaccination with the nonsecretory construct. The level of the total humoral response after vaccination with the secretion construct was not dependent on the route of vaccination. However, the isotype profile of the response differed between the groups; intramuscular vaccination with the construct directing cytoplasmic expression yielded an immuoglobulin (Ig)G2a dominant (Th1-type) response, intradermal vaccination with the secretory construct a IgG1/IgE dominant (Th2-type) response, and intramuscular vaccination with the secretory construct a mixed isotype response. These results demonstrate that the immunogenicity of a DNA vaccine based on Fasciola GST, as well as the isotype of the response against GST, is determined by the mode of vaccine administration.

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

DNA vaccines have been termed The Third Generation of Vaccines. The recent successful immunization of experimental animals against a range of infectious agents and several tumour models of disease with plasmid DNA testifies to the powerful nature of this revolutionary approach in vaccinology. Among numerous advantages, a major attraction of DNA vaccines over conventional vaccines is that they are able to induce protective cytotoxic T-cell responses as well as helper T-cell and humoral immunity. Here we review the current state of nucleic acid vaccines and cover a wide range of topics including delivery mechanisms, uptake and expression of plasmid DNA, and the types of immune responses generated. Further, we discuss safety issues, and document the use of nucleic acid vaccines against viral, bacterial and parasitic diseases, and cancer. The early potential promise of DNA vaccination has been fully substantiated with recent, exciting developments including the movement from testing DNA vaccines in laboratory models to non-human primates and initial human clinical trials. These advances and the emerging voluminous literature on DNA vaccines highlight the rapid progress that has been made in the DNA immunization field. It will be of considerable interest to see whether the progress and optimism currently prevailing can be maintained, and whether the approach can indeed fulfil the medical and commerical promise anticipated.

Glutathione S-transferases of 28kDa as major vaccine candidates against schistosomiasis

For the development of vaccine strategies to generate efficient protection against chronic infections such as parasitic diseases, and more precisely schistosomiasis, controlling pathology could be more relevant than controlling the infection itself. Such strategies, motivated by the need for a cost-effective complement to existing control measures, should focus on parasite molecules involved in fecundity, because in metazoan parasite infections pathology is usually linked to the output of viable eggs. In numerous animal models, vaccination with glutathione S-transferases of 28kDa has been shown to generate an immune response strongly limiting the worm fecundity, in addition to the reduction of the parasite burden. Recent data on acquired immunity directed to 28GST in infected human populations, and new development to draw adapted vaccine formulations, are presented.

DNA vaccines for viral diseases

DNA vaccines, with which the antigen is synthesized in vivo after direct introduction of its encoding sequences, offer a unique method of immunization that may overcome many of the deficits of traditional antigen-based vaccines. By virtue of the sustained in vivo antigen synthesis and the comprised stimulatory CpG motifs, plasmid DNA vaccines appear to induce strong and long-lasting humoral (antibodies) and cell-mediated (T-help, other cytokine functions and cytotoxic T cells) immune responses without the risk of infection and without boost. Other advantages over traditional antigen-containing vaccines are their low cost, the relative ease with which they are manufactured, their heat stability, the possibility of obtaining multivalent vaccines and the rapid development of new vaccines in response to new strains of pathogens. The antigen-encoding DNA may be in different forms and formulations, and may be introduced into cells of the body by numerous methods. To date, animal models have shown the possibility of producing effective prophylactic DNA vaccines against numerous viruses as well as other infectious pathogens. The strong cellular responses also open up the possibility of effective therapeutic DNA vaccines to treat chronic viral infections.