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

Vaccine development: Current trends and technologies

Despite the effectiveness of vaccination in reducing or eradicating diseases caused by pathogens, there remain certain diseases and emerging infections for which developing effective vaccines is inherently challenging. Additionally, developing vaccines for individuals with compromised immune systems or underlying medical conditions presents significant difficulties. As well as traditional vaccine different methods such as inactivated or live attenuated vaccines, viral vector vaccines, and subunit vaccines, emerging non-viral vaccine technologies, including viral-like particle and nanoparticle vaccines, DNA/RNA vaccines, and rational vaccine design, offer new strategies to address the existing challenges in vaccine development. These advancements have also greatly enhanced our understanding of vaccine immunology, which will guide future vaccine development for a broad range of diseases, including rapidly emerging infectious diseases like COVID-19 and diseases that have historically proven resistant to vaccination. This review provides a comprehensive assessment of emerging non-viral vaccine production methods and their application in addressing the fundamental and current challenges in vaccine development.

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

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

Emerging Concepts and Technologies in Vaccine Development

Despite the success of vaccination to greatly mitigate or eliminate threat of diseases caused by pathogens, there are still known diseases and emerging pathogens for which the development of successful vaccines against them is inherently difficult. In addition, vaccine development for people with compromised immunity and other pre-existing medical conditions has remained a major challenge. Besides the traditional inactivated or live attenuated, virus-vectored and subunit vaccines, emerging non-viral vaccine technologies, such as viral-like particle and nanoparticle vaccines, DNA/RNA vaccines, and rational vaccine design, offer innovative approaches to address existing challenges of vaccine development. They have also significantly advanced our understanding of vaccine immunology and can guide future vaccine development for many diseases, including rapidly emerging infectious diseases, such as COVID-19, and diseases that have not traditionally been addressed by vaccination, such as cancers and substance abuse. This review provides an integrative discussion of new non-viral vaccine development technologies and their use to address the most fundamental and ongoing challenges of vaccine development.

Fasciola hepatica Extracellular Vesicles isolated from excretory-secretory products using a gravity flow method modulate dendritic cell phenotype and activity

Parasite-released extracellular vesicles (EVs) deliver signals to the host immune system that are critical to maintaining the long-term relationship between parasite and host. In the present study, total EVs (FhEVs) released in vitro by adults of the helminth parasite Fasciola hepatica were isolated using a recently described gravity flow method that protects their structural integrity. The FhEVs molecular cargo was defined using proteomic analysis and their surface topology characterised by glycan microarrays. The proteomic analysis identified 618 proteins, 121 of which contained putative N-linked glycosylation sites while 132 proteins contained putative O-linked glycosylation sites. Glycan arrays revealed surface-exposed glycans with a high affinity for mannose-binding lectins indicating the predominance of oligo mannose-rich glycoproteins, as well as other glycans with a high affinity for complex-type N-glycans. When added to bone-marrow derived dendritic cells isolated FhEV induced a novel phenotype that was categorised by the secretion of low levels of TNF, enhanced expression of cell surface markers (CD80, CD86, CD40, OX40L, and SIGNR1) and elevation of intracellular markers (SOCS1 and SOCS3). When FhEV-stimulated BMDCs were introduced into OT-II mice by adoptive transfer, IL-2 secretion from skin draining lymph nodes and spleen cells was inhibited in response to both specific and non-specific antigen stimulation. Immunisation of mice with a suspension of FhEV did not elicit significant immune responses; however, in the presence of alum, FhEVs induced a mixed Th1/Th2 immune response with high antigen specific antibody titres. Thus, we have demonstrated that FhEVs induce a unique phentotype in DC capable of suppressing IL-2 secretion from T-cells. Our studies add to the growing immuno-proteomic database that will be an important source for the discovery of future parasite vaccines and immunotherapeutic biologicals.

Parasite-released extracellular vesicles (EVs) deliver signals to the host immune system that are critical to maintaining the long-term relationship between parasite and host. This study isolated total EVs (FhEVs) released in vitro by the adult stages of the parasitic worm Fasciola hepatica using a gravity flow method that protects the structural integrity of the vesicles. Proteomic analysis identified 618 proteins, 121 of which contained putative N-linked glycosylation sites while 132 proteins contained putative O-linked glycosylation sites while glycan arrays revealed surface-exposed glycans were predominantly oligo mannose-rich glycoproteins, and glycans with a high affinity for complex-type N-glycans. Since the EV molecular cargo can influence host immune cells, FhEVs were added to bone-marrow derived dendritic cells, inducing a novel cell phenotype that when adoptive transferred into OT-II mice inhibited IL-2 secretion from skin draining lymph nodes and spleen cells. Immunisation of mice with FhEV did not elicit significant immune responses; however, in the presence of alum, FhEVs induced a mixed Th1/Th2 immune response with high antigen specific antibody titres. This studied sheds like on the biological activity of FhEVs and added to the growing immuno-proteomic database that will be an important source for the discovery of future therapeutics.

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

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

Administration of anti-CTLA-4 monoclonal antibody augments protective immunity induced by Schistosoma japonicum glutathione-S-transferase

AIMS

The aim of this study was to evaluate the effect of anti-CTLA-4 monoclonal antibody (mAb) on 26-kDa glutathione-S-transferase (GST) vaccine-induced immunity against Schistosoma japonicum infection.

METHODS AND RESULTS

Mice immunized with GST before infection with S japonicum cercariae were injected with anti-CTLA-4 mAb. Worm reduction rate of GST was increased from 25.41% in mice with GST immunization to 52.48% in mice with GST plus anti-CTLA-4 mAb. The percentages of regulatory T cells (Tregs) were significantly higher following administration of both GST and anti-CTLA-4 mAb, or anti-CTLA-4 mAb alone. Elevated levels of IFN-γ, IL-2, IL-4 and IL-5 were observed.

CONCLUSION

These results demonstrated that CTLA-4 may inhibit the protective effect of GST vaccine, and anti-CTLA-4 mAb may be used as an adjuvant to enhance the immune protection conferred by the GST vaccine by enhancing Th1- and Th2-type immune response.

Protective efficacy of liver fluke DNA vaccines: A systematic review and meta-analysis: Guiding novel vaccine development

The immunogenicity and efficacy of Fasciola DNA vaccines have not yet been comprehensively summarised in the form of a systematic review and meta-analysis. Though multiple vaccine studies with respect to Fasciola vaccines exist, the variance in the experimental parameters has made comparison difficult. We conducted a bibliographic database search in Scopus, PubMed, Science Direct, Cochrane Library, EMBASE and Web of Science databases, limited to publications from 1998 to 2017. The key words: Liver fluke, Fasciola hepatica, Fasciola gigantica, DNA vaccination, and immunogenicity were used in combination to form search strings. A total of 4760 studies were identified after initial screening, of which 14 qualified for systematic review and 7 for meta-analysis. The mean Odds Ratio (OR) for all studies was 0.565 (95% confidence interval (CI) of 0.293 to 1.087), which means the percentage of protection in terms of decreased fluke burden in animals vaccinated with DNA vaccines was 43.5%. A moderate protective efficacy was observed for cysteine protease and phosphoglycerate kinase vaccine antigen candidates (pooled OR and 95% CI, [0.542; 0.179-1.721] and [0.616; 0.219-1.735], respectively). Vaccine effectiveness was observed in individual studies and cohorts; however, the overall pooled efficacy for all vaccine candidates was found to be non-significant. Despite multiple individual studies showing promising results for various DNA vaccine candidates against fascioliasis, the pooled studies showed the non-significant effect of the vaccine formulations against fluke burden, and displayed minimal protective efficacy against Fasciola infection. Though promising results are observed in isolated studies, further animal trials with standardised experimental parameters are required to develop new vaccine candidates effective against Fasciola.

Intranasal delivery of a formulation containing stage-specific recombinant proteins of Fasciola hepatica cathepsin L5 and cathepsin B2 triggers an anti-fecundity effect and an adjuvant-mediated reduction in fluke burden in sheep

Fasciola hepatica infection continues to be a major problem in the agriculture sector, particularly in sheep and cattle. Cathepsin L and B proteases are major components of the excretory/secretory material of the parasite, and their roles in several important aspects of parasite invasion and survival has led to their use as targets in rational vaccine design. Previous studies in rats demonstrated that the use of stage-specific antigens, cathepsin B2 and cathepsin L5, as part of a multivalent vaccine, was able to confer significant protection against challenge. In the present study, recombinant versions of cathepsin L5 and cathepsin B2 produced in yeast were used in combination to vaccinate sheep. Intramuscular and intranasal forms of administration were applied, and sheep were subsequently challenged with 150 F. hepatica metacercariae. Intramuscular vaccination was able to induce a strong systemic antibody response against both antigens, but failed to confer significant protection. Conversely, no elevated antibody response was detected against the vaccine antigens following nasal vaccination; however, a reduction in parasite egg viability (>92%) and a statistically significant (p = 0.006), predominantly adjuvant-mediated reduction in worm burdens was observed.

Development of Experimental Vaccines Against Liver Flukes

A multitude of experimental vaccines have been developed against liver flukes in the past. However, there has yet to be the development of a commercial livestock vaccine. Reasons for this may be multiple, and include the lack of identification of the best antigen(s), or the immune response induced by those antigens not being appropriate in either magnitude or polarity (and therefore not protective). Cathepsin proteases are the major component of the excretory/secretory (ES) material of liver flukes in all stages of their life cycle in the definitive host and are the primary antigens of interest for the vaccine development in many studies. Hence, this chapter presents the methodologies of using cathepsin proteases as targeted antigens in recombinant protein and DNA vaccine development to engender protective immune responses against fasciolosis.First, the experimental vaccines developed in the past and the criteria of an effective vaccine for fasciolosis are briefly reviewed. Then flowcharts for recombinant protein vaccine and DNA vaccine development are presented, followed by the detailed materials and methodologies.

Antiparasitic DNA vaccines in 21st century

Demands for effective vaccines to control parasitic diseases of humans and livestock have been recently exacerbated by the development of resistance of most pathogenic parasites to anti-parasitic drugs. Novel genomic and proteomic technologies have provided opportunities for the discovery and improvement of DNA vaccines which are relatively easy as well as cheap to fabricate and stable at room temperatures. However, their main limitation is rather poor immunogenicity, which makes it necessary to couple the antigens with adjuvant molecules. This paper review recent advances in the development of DNA vaccines to some pathogenic protozoa and helminths. Numerous studies were conducted over the past 14 years of 21st century, employing various administration techniques, adjuvants and new immunogenic antigens to increase efficacy of DNA vaccines. Unfortunately, the results have not been rewarding. Further research is necessary using more extensive combinations of antigens; alternate delivery systems and more efficient adjuvants based on knowledge of the immunomodulatory capacities of parasitic protozoa and helminths.

Evaluation of the immune response and protective efficacy of Schistosoma mansoni Cathepsin B in mice using CpG dinucleotides as adjuvant

Schistosomiasis is the most important human helminth infection due to its impact on public health. Worldwide, schistosomiasis is estimated to infect at least 200 million individuals while 700 million are at risk. The clinical manifestations are chronic and significantly decrease an individual's quality of life. Infected individuals suffer from long-term organ pathologies including fibrosis which eventually leads to organ failure. The development of a vaccine against this parasitic disease would contribute to a long-lasting decrease in disease spectrum and transmission. Our group has chosen to target Schistosoma mansoni Cathepsin B as a prospective vaccine candidate. The recombinant protein was tested in the presence of synthetic oligodeoxynucleotides containing unmethylated CpG dinucleotides, which are Toll-like receptor 9 agonists known to stimulate a Th1 response. This formulation conferred a 59% decrease in worm burden as well as a reduction in egg burden. Hepatic egg burden and intestinal egg burden were decreased by 56% and 54% respectively. Immunizations with the formulation elicited robust production of Sm-Cathepsin B specific antibodies, both IgG1 and IgG2c but with the latter predominating. Furthermore, splenocytes isolated from the immunized animals, compared to control animals, had increased secretion levels of key Th1 cytokines, IFN-γ and TNF-α, as well as the chemokine CCL5 when stimulated with recombinant Sm-Cathepsin B. These results highlight the potential of Sm-Cathepsin B/CpG as a vaccine candidate against schistosomiasis.

Improving vaccines by incorporating immunological coadjuvants

While vaccination continues to be the most successful interventionist health policy to date, infectious disease remains a significant cause of death worldwide. A primary reason that vaccination is not able to generate effective immunity is a lack of appropriate adjuvants capable of initiating the desired immune response. Adjuvant combinations can potentially overcome this problem; however, the possible permutations to consider, which include the route and kinetics of vaccination, as well as combinations of adjuvants, are practically limitless. This review aims to summarize the current understanding of adjuvants and related immunological processes and how this knowledge can and has been applied to the strategic selection of adjuvant combinations as components of vaccines against human infectious disease.

Enhancement of antimycobacterial Th1-cell responses by a Mycobacterium bovis BCG prime-protein boost vaccination strategy

Tuberculosis is a major global health problem, and the only available vaccine Bacille Calmette-Guérin (BCG) is not sufficiently effective against the disease. It is extremely urgent to develop novel vaccine approaches. Previous research demonstrated that there were several Regions of Difference (RD1-16) between the substrains of BCG and Mycobacterium tuberculosis or Mycobacterium bovis. The ORFs Rv1769 and Rv1772 are located in the RD14 deletions and have not been major targets of study. However, some studies have demonstrated that the two genes (Rv1769 and Rv1772) are excellent T cell antigens, which might induce an immune response. What kind of role these ORFs might play in anti-mycobacterial immunity, however, is still unknown. In our research we used the BCG prime-protein boost strategy to immunize BALB/c mice and evaluated its immunogenicity. Our data suggest that our novel BCG-P+PRO69 vaccine could elicit the most long-lasting and strongest Th1 type cellular immune responses. This response is characterized by a strong antibody response, the proliferation rate of splenocytes, a high percentage of CD4+ and CD8+ T cells and high levels of IFN-γ in antigen-stimulated splenocyte cultures. These results indicate that prime-boost is a potent strategy and the protein of gene Rv1769 is a potential antigen or subunit vaccine to TB for further study.

DNA vaccines: roles against diseases

Vaccination is the most successful application of immunological principles to human health. Vaccine efficacy needs to be reviewed from time to time and its safety is an overriding consideration. DNA vaccines offer simple yet effective means of inducing broad-based immunity. These vaccines work by allowing the expression of the microbial antigen inside host cells that take up the plasmid. These vaccines function by generating the desired antigen inside the cells, with the advantage that this may facilitate presentation through the major histocompatibility complex. This review article is based on a literature survey and it describes the working and designing strategies of DNA vaccines. Advantages and disadvantages for this type of vaccines have also been explained, together with applications of DNA vaccines. DNA vaccines against cancer, tuberculosis, Edwardsiella tarda, HIV, anthrax, influenza, malaria, dengue, typhoid and other diseases were explored.

Evaluation of the immune responses induced by four targeted DNA vaccines encoding the juvenile liver fluke antigen, cathepsin B in a mouse model

Background

Liver fluke can infect cattle and sheep, and is also emerging as a human pathogen in developing countries. Cathepsin B (Cat B2) is a major cysteine protease secreted by the juvenile flukes. To enhance the immune responses of Cat B2, the cDNA sequence was fused with four different DNA vaccine vectors. The induced cellular and antibody responses were compared in vaccinated mice.

Methods

The following recombinant DNA vaccine constructs were constructed: empty vector VR1012 as negative control, cytoplasmic construct pVR1012 Cat B2, secretory construct pVR1020 Cat B2, chemokine-fused construct pMCP3 Cat B2 and lymph node targeting construct pCTLA-4 Cat B2. Plasmids were constructed using standard procedures, and positive constructs screened and selected using restriction digestion analysis followed by sequence analysis. The constructs were then tested in Cos-7 cells for in vitro expression, which was analysed using immunoblotting. Subsequently, female BALB/c mice were immunised with DNA constructs as vaccines. Elicited antibody responses were measured using ELISA. The ratio between IgG1 and IgG2a antibody responses was estimated among different vaccine groups. IgG antibody avidity assay was performed and the relative avidity index was calculated. The induced cytokine production from splenocytes of vaccinated animals was estimated using ELISPOT.

Results

DNA vaccine constructs carrying Cat B2 were expressed in Cos-7 cell lines and encoded protein was recognised using western blotting using rat anti- cathepsin B antibody. DNA vaccines elicited high Cat B2- specific IgG, IgG1, IgE and also modest IgG2a antibody responses. Cat B2 specific IL-4 T cell responses were also observed in Cat B2 vaccinated mice. The comparison of immunogenic potential in each of these constructs was demonstrated as enhanced antibody responses on the lymph-node targeting vector pCTLA-4 Cat B2, the high antibody avidity of chemo-attractant pMCP3 Cat B2 and stronger T cellular responses of non-secretory DNA vaccine pVR1012 Cat B2 in vaccinated animals.

Conclusion

This study showed that the targeting DNA vaccine strategies enhanced specific immune responses to juvenile fluke Cat B2. The results of our current study have demonstrated that a gene-based vaccine as an immunotherapeutic approach to combat Fasciola infection may be feasible.

Characterization and expression of cathepsin B2 in Fasciola gigantica

Fasciola gigantica cathepsin B belongs to a family of cysteine proteases which is involved in invasion of host tissues. In this study, the recombinant cathepsin B2 (rFgCatB2), synthesized in Pichia pastoris, showed enzymatic activity on a fluorometric substrate Z-Phe-Arg-AMC and gelatin. Furthermore, this recombinant enzyme could degrade IgG and type I collagen. Mouse antiserum against rFgCatB2 reacted with the native FgCatB2 in whole body (WB) extracts of metacercariae (MET), newly excysted juveniles (NEJ) and 2week-old juveniles, but not in 3, 4 week-old juveniles and adult flukes. Immunolocalization showed the presence of cathepsin B2 only in the caecal epithelium of MET, NEJ and 2 week-old juveniles. Co-localization of FgCatB2 and a prominent antigen of NEJ, FgCatB3, revealed that these proteins were expressed at the same regions in the caecal epithelium. Anti-rFgCatB2 showed no cross reaction with the other parasites' antigens by Western blotting. These findings suggest that CatB2 is expressed only in early stages of the parasite and may be involved in digestion of host connective tissues and evasion of the host immune system during their penetration and migration. Thus, CatB2 could be considered as an immunodiagnostic and vaccine candidate for fasciolosis.

Fusion with extracellular domain of cytotoxic T-lymphocyte-associated-antigen 4 leads to enhancement of immunogenicity of Hantaan virus DNA vaccines in C57BL/6 mice

BACKGROUND

Hantaan virus (HTNV) is the causative agent of the most severe form of a rodent-borne disease known as hemorrhagic fever with renal syndrome (HFRS). A safe and effective HTNV vaccine is needed. Vaccination with DNA constructs expressing fused antigen with bioactive factors, has shown promising improvement of immunogenicity for viral agents in animal models, but the effect of fusion strategy on HTNV DNA vaccine has not been investigated.

RESULTS

DNA plasmids encoding the HTNV nucleocapsid protein (N) and glycoprotein (Gn and Gc) in fusion to the extracellular domain of cytotoxic T-lymphocyte-associated-antigen 4 (eCTLA-4) targeting to antigen presenting cells (APCs) were constructed. Intramuscular immunization of mice with plasmids expressing eCTLA-4-HTNV-N/GP fusion proteins leads to a significant enhancement of the specific antibody response as well as cytotoxic T-lymphocyte (CTL) response in C57BL/6 mice. Moreover, this effect could be further augmented when co-administered with CpG motifs.

CONCLUSIONS

Modification of viral antigen in fusion to bioactive factor will be promising to confer efficient antigen presentation and improve the potency of DNA vaccine in mice.

Current strategies for subunit and genetic viral veterinary vaccine development

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

Cathepsin B proteases of flukes: the key to facilitating parasite control?

Cysteine proteases are important virulence factors for parasites. This review will focus on the cathepsin B proteases of trematodes (also known as flukes) which are abundant in juvenile and immature flukes. Recent research, primarily in Fasciola, using inhibitors, RNA interference (RNAi) and vaccination studies indicates that cathepsin Bs play a key role in the biology of trematodes. As these proteases are largely expressed by infective parasite stages, their inactivation by chemotherapy or vaccination will greatly reduce the damage wrought by flukes as they invade host tissues. This validates cathepsin Bs as key strategic targets for fluke control.

Identification, expression and immunolocalization of cathepsin B3, a stage-specific antigen expressed by juvenile Fasciola gigantica

To identify antigens that could potentially be developed as vaccines against Fasciola gigantica, somatic antigens were analyzed by immunoprecipitation using pooled sera from rats infected with F. gigantica metacercariae. A prominent antigen of the newly excysted juveniles (NEJ), cathepsin B3 protease (FgCatB3), was identified by N-terminal sequencing and PCR screening of a cDNA library. Recombinant FgCatB3 (rFgCatB3) was expressed in Pichia pastoris, and shown to catalyse the digestion of gelatin, the fluorometric substrate Z-Phe-Arg-AMC and native fibronectin, suggesting that this enzyme may be involved in digesting host connective tissues during the fluke's penetration and migration in the host. Rabbit polyclonal sera against rFgCatB3 was produced and used to determine the distribution of the native cathepsin B3 protease in various developmental stages of F. gigantica. By Western blotting, cathepsin B3 was detected in the whole body (WB) extract of metacercariae and NEJ but not in 4-week-old juveniles or adult parasites which confirmed the stage-specific characteristics of cathepsin B3. Immunolocalization of cathepsin B3 protease in each parasite stage showed that high levels of FgCatB3 were present in the caecal epithelium of the metacercariae and NEJ. The differential distribution of FgCatB3 in the different life cycle stages suggests that this protease is functionally important for the juvenile stage of F. gigantica.

Immune responses to polyethylenimine-mannose-delivered plasmid DNA encoding a Fasciola gigantica fatty acid binding protein in mice

Fasciola gigantica fatty acid binding protein (FABP) was evaluated for evoking an immune response in mice, by delivering the gene coding for this protein with mannosylated-polyethylenimine (PEI) to peritoneal cells. Mice were immunized with 50 microg recombinant plasmid DNA (Group I) or DNA-PEI-mannose (a 22 kDa linear cationic polymer with mannose ligand) (Group II) via the intraperitoneal route. Antibody studies showed no significant humoral immune response evoked to this DNA immunization with either PEI-mannose-delivered or naked DNA. However, on protein boosting of these DNA-primed mice there was a significant enhancement of antibody titre. Flow cytometric bead array was used to measure quantities of interleukin (IL)-2, IL-4, IL-5, interferon-gamma (IFN-gamma) and tumour necrosis factor (TNF) cytokines. Overexpression of T-helper 1 (Th1) cytokines such as IFN-gamma and TNF, with a lower but significant expression of the T-helper 2 (Th2) cytokine IL-5 was detected. Gene delivery using polyethylenimine-mannose ligand showed significant expression of IFN-gamma and TNF (P < 0.05), but no significant difference in IL-2, IL-4 and IL-5 (P>0.05) cytokine expression was observed between naked-DNA- and mannosylated PEI-DNA-delivered mice. Naked- or PEI-delivered-DNA immunization produced insignificant levels of IL-2 and IL-4 (P>0.05) cytokines in both groups of mice.

Immune responses to polyethylenimine delivered plasmid DNA encoding aFasciola giganticafatty acid binding protein in mice and rabbits

Fasciola giganticafatty acid binding protein (FABP) was evaluated for evoking an effective immune response in mice and rabbits, when delivered as a DNA vaccine in muscle cells. Polyethylenimine (PEI), 25 kDa, branched cationic polymer was used as a delivery vehicle for this DNA in the muscle cells of mice and rabbits. Naked DNA evoked mixed Th1 and Th2 responses in mice. PEI condensed DNA, at amine nitrogen over DNA phosphate (N/P) ratios of 4, 6 and 8 and with various DNA concentrations, failed to evoke a significantly higher antibody response compared to naked DNA in mice. Similarly, the humoral immune response to naked DNA administration in rabbit thigh muscles was poor and no boosting of this antibody response on administration of DNA complexed to PEI was observed. On metacercarial challenge, rabbits failed to show any significant protective immune response in both the naked DNA and PEI–DNA immunized groups. Administration of PEI alone (12.5 μg) in mouse thigh muscles caused significant muscle cytotoxicity but condensation of DNA with PEI had less of a toxic effect on muscle cells, which was inversely related to the N/P ratio. Delivery of plasmid DNA encodingF. giganticaFABP by high molecular weight polyethylenimine (branched, 25 kDa) did not boost the effective immune response in both the animal species, which could either be attributed to cytotoxicity associated with this cationic polymer or muscle cells being unsuitable target cells for PEI condensed DNA delivery.

A major cathepsin B protease from the liver fluke Fasciola hepatica has atypical active site features and a potential role in the digestive tract of newly excysted juvenile parasites

The newly excysted juvenile (NEJ) stage of the Fasciola hepatica lifecycle occurs just prior to invasion into the wall of the gut of the host, rendering it an important target for drug development. The cathepsin B enzymes from NEJ flukes have recently been demonstrated to be crucial to invasion and migration by the parasite. Here we characterize one of the cathepsin B enzymes (recombinant FhcatB1) from NEJ flukes. FhcatB1 has biochemical properties distinct from mammalian cathepsin B enzymes, with an atypical preference for Ile over Leu or Arg residues at the P(2) substrate position and an inability to act as an exopeptidase. FhcatB1 was active across a broad pH range (optimal activity at pH 5.5-7.0) and resistant to inhibition by cystatin family inhibitors from sheep and humans, suggesting that this enzyme would be able to function in extracellular environments in its mammalian hosts. It appears, however, that the FhcatB1 protease functions largely as a digestive enzyme in the gut of the parasite, due to the localization of a specific, fluorescently labeled inhibitor with an Ile at the P(2) position. Molecular modelling and dynamics were used to predict the basis for the unusual substrate specificity: a P(2) Ile residue positions the substrate optimally for interaction with catalytic residues of the enzyme, and the enzyme lacks an occluding loop His residue crucial for exopeptidase activity. The unique features of the enzyme, particularly with regard to its specificity and likely importance to a vital stage of the parasite's life cycle, make it an excellent target for therapeutic inhibitors or vaccination.

Evaluation of protective effect of multi-epitope DNA vaccine encoding six antigen segments of Toxoplasma gondii in mice

To investigate the vaccine potential of multi-epitope vaccines against toxoplasmosis, a multi-epitope DNA vaccine, eukaryotic plasmid pcDNA3.1/T-ME expressing six antigen segments (SAG1(238-256), SAG1(281-320), GRA1(170-193), GRA4(331-345), GRA4(229-245), and GRA2(171-185)) of Toxoplasma gondii was constructed. We investigated the efficacy of pcDNA3.1/T-ME with or without co-administration of a CpG-oligodeoxynucleotide (CpG-ODN) as an adjuvant to protect mice (BALB/c and C57BL/6) against toxoplasmosis. High survival rates were observed in mice immunized with pcDNA3.1/T-ME when challenged with T. gondii RH strain. Lymphocyte proliferation assays, cytokine, and antibody determinations show that mice immunized with pcDNA3.1/T-ME produced stronger humoral and Th1-type cellular immune responses compared to untreated mice or those immunized with empty plasmids. However, co-immunization with CpG-ODN resulted in impaired immune responses. Our data demonstrates that multi-epitope DNA vaccination is a potential strategy for the control of toxoplasmosis and paves the way for further investigations into producing a multi-epitope anti-T. gondii DNA vaccine.

Vaccination with combination of Fit3L and RANTES in a DNA prime-protein boost regimen elicits strong cell-mediated immunity and antitumor effect

With accumulating evidence indicating the importance of cytotoxic T lymphocytes (CTLs) in the antitumor response, strategies are being pursued to elicit augmented CD8(+) T-cell responses against tumors with tumor vaccines. Here, we report the protective efficacy of vaccine-elicited antitumor immune responses with an aggressive HBc-expressing B16-HBc melanoma, which expressed HBc as a self and model antigen, tumor model. We demonstrated that the significantly better memory responses or marked inhibition on tumor growth could be achieved after coadministration of cytokine adjuvants RANTES and Flt3L in a DNA prime-protein boost regimen. Furthermore, the augmentation of DNA prime-protein boost regimens by cytokines gene was due to the improvement the immunopotency of DNA vaccine and subsequently the augmented Ag-specific and IFN-gamma mediating CD8(+) T-cell responses after protein boosting. Hence, this study demonstrates for the first time that combinatorial use of chemotactic and potent DC-specific growth factor molecules provides a useful strategy for enhancing antitumor responses.

Vaccination against fasciolosis by a multivalent vaccine of stage-specific antigens

Liver flukes produce cathepsin B and cathepsin L in their excretory-secretory material. These proteases are proposed to be key virulence factors for parasite infection, and are therefore targets for vaccination. Cathepsin B is predominately released in the juvenile stage of the life cycle, while different cathepsin L's are released throughout the cycle. Three proteases (cathepsin L5, cathepsin L1g and cathepsin B) were expressed in yeast from cDNA clones isolated from adult, metacercariae and newly excysted juvenile flukes respectively. Each was used singly or in combination to vaccinate rats that were subsequently challenged with Fasciola hepatica metercercariae. Each protein induced an immune response, and all groups vaccinated with recombinant protein yielded significantly fewer and smaller flukes than the control group. Maximal protection of 83% was seen in the group vaccinated with cathepsin B and cathepsin L5 in combination.

A distinctive repertoire of cathepsins is expressed by juvenile invasive Fasciola hepatica

Secreted cysteine proteases are relevant actors in parasite biology, taking part in critical host colonization roles such as traversing tissue barriers, immune evasion and nutrient digestion. In the trematode Fasciola hepatica, the initial step to successful infection of the mammalian host is the excystment of metacercariae and the invasion through the intestinal wall by the newly excysted juveniles (NEJ). While the cathepsin L-like cysteine proteinases secreted by the adult fluke have been extensively characterized, the cataloguing and description of the cathepsins B and L reported in the invasive stages is only sketchy. To identify the cathepsins expressed during excystment and early invasion we constructed cDNA libraries encoding NEJ cathepsins B and L. We found two cathepsin L-like cysteine proteinases (CL3, CL4) and three cathepsins B (CB1, CB2, CB3) which are predominantly expressed in NEJ. Phylogenetic analysis showed that NEJ-expressed cathepsins L constitute a well-defined clade separate from the adult enzymes. Excystment induction resulted in a significant increment in activity towards cathepsin-specific fluorogenic substrates in metacercariae homogenates, consistent with the detection of precursor and mature forms of cathepsins B and L before and after induction. In NEJ culture supernatants, protein and relative activity profiles show subtle changes during the first 48 h, with prevalence of cathepsin L-like activity, although cathepsins CB3 and CL3 were detected by mass spectrometry. Noticeably, the hydrolysis of a substrate with proline in the P2 position was predominant, a property only shared with adult CL2 and vertebrate cathepsin K among the C1A subfamily of cysteine proteases. Collectively these mRNA, protein and enzymatic data demonstrate the existence of a NEJ-specific repertoire of cathepsins expressed early in invasion, distinct to those used by other trematodes, potentially relevant for specific vaccine and chemotherapy design. The diversity of proteases employed by trematodes in the invasion process is discussed.

Biomedical applications of sheep models: from asthma to vaccines

Although rodent models are very popular for scientific studies, it is becoming more evident that large animal models can provide unique opportunities for biomedical research. Sheep are docile in nature and large in size, which facilitates surgical manipulation, and their physiology is similar to humans. As a result, for decades they have been chosen for several models and continue to be used to study an ever-increasing array of applications. Despite this, their full potential has not been exploited. Here, we review the use of sheep as an animal model for human vaccine development, asthma pathogenesis and treatment, the study of neonatal development, and the optimization of drug delivery and surgical techniques.

Current status of veterinary vaccines

The major goals of veterinary vaccines are to improve the health and welfare of companion animals, increase production of livestock in a cost-effective manner, and prevent animal-to-human transmission from both domestic animals and wildlife. These diverse aims have led to different approaches to the development of veterinary vaccines from crude but effective whole-pathogen preparations to molecularly defined subunit vaccines, genetically engineered organisms or chimeras, vectored antigen formulations, and naked DNA injections. The final successful outcome of vaccine research and development is the generation of a product that will be available in the marketplace or that will be used in the field to achieve desired outcomes. As detailed in this review, successful veterinary vaccines have been produced against viral, bacterial, protozoal, and multicellular pathogens, which in many ways have led the field in the application and adaptation of novel technologies. These veterinary vaccines have had, and continue to have, a major impact not only on animal health and production but also on human health through increasing safe food supplies and preventing animal-to-human transmission of infectious diseases. The continued interaction between animals and human researchers and health professionals will be of major importance for adapting new technologies, providing animal models of disease, and confronting new and emerging infectious diseases.

Co-delivery of plasmid-encoded cytokines modulates the immune response to a DNA vaccine delivered by in vivo electroporation

In this study, in vivo electroporation of a DNA vaccine adjuvanted with plasmids encoding different cytokines was investigated in large animals. Sheep were injected intramuscularly with a DNA vaccine encoding an antigen of Haemonchus contortus (pNPA) and plasmids encoding different cytokines followed by in vivo electroporation. Plasmids (pCI) carrying the genes of different cytokines including ovine IL-4(pCI-IL4), IL-10(pCI-IL10), GM-CSF(pCI-GMCSF), and MCP-1alpha(pCI-MCP1alpha), and pCI-IL4+pCI-GMCSF were co-delivered with pNPA. The results showed that co-delivery of pCI-GMCSF or pCI-IL4+pCI-GMCSF significantly enhanced both antibody responses and T cell proliferation responses to the antigen after two DNA immunisations compared to co-delivery of pCI. In contrast, antibody responses of the sheep that received pCI-IL10 were decreased significantly. Other cytokine expressing plasmids did not significantly alter the measured immune responses. Furthermore, co-delivery of pCI-GMCSF increased IgG2 response more than IgG1 responses, suggesting a Th1 bias. However, the increase in IgG2 over IgG1 was less apparent when co-delivery of pCI-IL4 with pCI-GMCSF. Interestingly, the co-delivery of pCI-IL4 alone did not increase the IgG1 titre, suggesting that both pCI-GMCSF and pCI-IL4 are required for optimal IgG1 production. Thus, co-delivery of plasmid-encoded cytokine genes with in vivo electroporation has the ability to effectively modulate immune responses to a DNA vaccine in a large animal.

Immunostimulatory CpG oligodeoxynucleotides enhance the induction of bovine CD4+ cytotoxic T-lymphocyte responses against the polymorphic immunodominant molecule of the protozoan parasite Theileria parva

Enhancement of the induction of cytotoxic T-cell responses by immunostimulatory CpG oligodeoxynucleotides has been described in humans and mouse models. The present study attempted to address whether CpG has a similar effect in cattle. Immunisation of cattle with a recombinant form of the polymorphic immunodominant molecule from Theileria parva emulsified with immunostimulatory CpG oligodeoxynucleotides in adjuvant had no effect on the induction of antibody responses including the isotype profile, but significantly enhanced the induction of cytolytic responses that were mediated by CD4+CD3+ T cells utilizing the perforin-granzyme pathway.