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

Advancement in the development of DNA vaccines against Trypanosoma brucei and future perspective

Trypanosomes are the extracellular protozoan parasites that cause human African trypanosomiasis disease in humans and nagana disease in animals. Tsetse flies act as a vector for the transmission of the disease in African countries. Animals infected with these parasites become useless or workless, and if not treated, disease can be fatal. There are many side effects associated with old treatments and some of them result in death in 5% of cases. There is a major surface glycoprotein in the parasite known as variant surface glycoprotein. The immune system of the host develops antibodies against this antigen but due to antigenic variation, parasites evade the immune response. Currently, no vaccine is available that provides complete protection. In murine models, only partial protection was observed using certain antigens. In order to develop vaccines against trypanosomes, molecular biology and immunology tools have been used. Immunization is the sole method for the control of disease because the eradication of the vector from endemic areas is an impossible task. Genetic vaccines can carry multiple genes encoding different antigens of the same parasite or different parasites. DNA immunization induces the activation of both cellular immune response and humoral immune response along with the generation of memory. This review highlights the importance of DNA vaccines and advances in the development of DNA vaccines against T. brucei.

Stage-Specific Class I Nucleases of Leishmania Play Important Roles in Parasite Infection and Survival

Protozoans of the genus Leishmania are the causative agents of an important neglected tropical disease referred to as leishmaniasis. During their lifecycle, the parasites can colonize the alimentary tract of the sand fly vector and the parasitophorous vacuole of the mammalian host, differentiating into distinct stages. Motile promastigotes are found in the sand fly vector and are transmitted to the mammalian host during the insect blood meal. Once in the vertebrate host, the parasites differentiate into amastigotes and multiply inside macrophages. To successfully establish infection in mammalian hosts, Leishmania parasites exhibit various strategies to impair the microbicidal power of the host immune system. In this context, stage-specific class I nucleases play different and important roles related to parasite growth, survival and development. Promastigotes express 3’-nucleotidase/nuclease (3’-NT/NU), an ectoenzyme that can promote parasite escape from neutrophil extracellular traps (NET)-mediated death through extracellular DNA hydrolysis and increase Leishmania-macrophage interactions due to extracellular adenosine generation. Amastigotes express secreted nuclease activity during the course of human infection that may be involved in the purine salvage pathway and can mobilize extracellular nucleic acids available far from the parasite. Another nuclease expressed in amastigotes (P4/LmC1N) is located in the endoplasmic reticulum of the parasite and may be involved in mRNA stability and DNA repair. Homologs of this class I nuclease can induce protection against infection by eliciting a T helper 1-like immune response. These immunogenic properties render these nucleases good targets for the development of vaccines against leishmaniasis, mainly because amastigotes are the form responsible for the development and progression of the disease. The present review aims to present and discuss the roles played by different class I nucleases during the Leishmania lifecycle, especially regarding the establishment of mammalian host infection.

Vaccination with Messenger RNA: A Promising Alternative to DNA Vaccination

The first proof-of-concept studies about the feasibility of genetic vaccines were published over three decades ago, opening the way for future development. The idea of nonviral antigen delivery had multiple advantages over the traditional live or inactivated pathogen-based vaccines, but a great deal of effort had to be invested to turn the idea of genetic vaccination into reality. Although early proof-of-concept studies were groundbreaking, they also showed that numerous aspects of genetic vaccines needed to be improved. Until the early 2000s, the vast majority of effort was invested into the development of DNA vaccines due to the potential issues of instability and low in vivo translatability of messenger RNA (mRNA). In recent years, numerous studies have demonstrated the outstanding abilities of mRNA to elicit potent immune responses against infectious pathogens and different types of cancer, making it a viable platform for vaccine development. Multiple mRNA vaccine platforms have been developed and evaluated in small and large animals and humans and the results seem to be promising. RNA-based vaccines have important advantages over other vaccine approaches including outstanding efficacy, safety, and the potential for rapid, inexpensive, and scalable production. There is a substantial investment by new mRNA companies into the development of mRNA therapeutics, particularly vaccines, increasing the number of basic and translational research publications and human clinical trials underway. This review gives a broad overview about genetic vaccines and mainly focuses on the past and present of mRNA vaccines along with the future directions to bring this potent vaccine platform closer to therapeutic use.

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.

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.

Identification of Cytauxzoon felis antigens via protein microarray and assessment of expression library immunization against cytauxzoonosis

Background

Cytauxzoonosis is a disease of felids in North America caused by the tick-transmitted apicomplexan parasite Cytauxzoon felis. Cytauxzoonosis is particularly virulent for domestic cats, but no vaccine currently exists. The parasite cannot be cultivated in vitro, presenting a significant limitation for vaccine development.

Methods

Recent sequencing of the C. felis genome has identified over 4300 putative protein-encoding genes. From this pool we constructed a protein microarray containing 673 putative C. felis proteins. This microarray was probed with sera from C. felis-infected and naïve cats to identify differentially reactive antigens which were incorporated into two expression library vaccines, one polyvalent and one monovalent. We assessed the efficacy of these vaccines to prevent of infection and/or disease in a tick-challenge model.

Results

Probing of the protein microarray resulted in identification of 30 differentially reactive C. felis antigens that were incorporated into the two expression library vaccines. However, expression library immunization failed to prevent infection or disease in cats challenged with C. felis.

Conclusions

Protein microarray facilitated high-throughput identification of novel antigens, substantially increasing the pool of characterized C. felis antigens. These antigens should be considered for development of C. felis vaccines, diagnostics, and therapeutics.

Electronic supplementary material

The online version of this article (10.1186/s12014-018-9218-9) contains supplementary material, which is available to authorized users.

Coadminstration of L. major amastigote class I nuclease (rLmaCIN) with LPD nanoparticles delays the progression of skin lesion and the L. major dissemination to the spleen in BALB/c mice-based experimental setting

Human cutaneous leishmaniasis is a disease caused by eukaryotic single-celled Leishmania species, the developmental program of which relies upon blood-feeding adult female sand flies and their dominant mammal blood sources, namely wild rodents in area where human beings exert more or less transient activities. The recourse to model rodents - namely laboratory mice such as C57BL/6 mice - has allowed extracted the immune signatures that account for the healing of the transient cutaneous lesion that develops at the site where Leishmania major promastigotes were delivered. Indeed, if the latter mice are exposed to a second inoculum of L. major promastigotes, no lesion will develop in the secondary skin site remodeled as a niche for a low size intracellular L. major amastigote population. Moreover, IFN-γ dominates over IL-10 in the supernatant of cultures of PBMCs -prepared from blood sampled from human beings who healed from a cutaneous lesion- and incubated with L. major class I Nuclease LmaCIN, a protein highly expressed in the cell-cycling amastigote population which is dominant by macrophages. Altogether, these datasets were strong incentive to promote research aimed to design and monitor efficacy of L. major amastigote protein-based vaccines in pre-clinical settings. Using L. major enzyme class I nuclease (LmaCIN) expressed in the L. major cell-cycling amastigote population hosted by macrophages, BALB/c mice were immunized three times with either rLmaCIN plus LPD nanoparticles (LPD-rLmaCIN), or rLmaCIN-CpG DNA or free rLmaCIN and dextrose. The following parameters: footpad swelling, splenic L. major load, L. major binding IgGs and cytokine profiles of rLmaCIN- reactive T lymphocytes were then compared. Once coadminstered with LPD, rLmaCIN allow BALB/c mice to display delayed onset of skin lesion at the challenge inoculation site and delayed L. major dissemination from the challenged site to the spleen. Thus, the LPD-rLmaCIN is shown to display some promising features out of three formulations inoculated to the BALB/c mouse immunization.

Immune responses in rats and sheep induced by a DNA vaccine containing the phosphoglycerate kinase gene of Fasciola hepatica and liver fluke infection

Immune responses of rats and sheep following vaccination with cDNA encoding phosphoglycerate kinase of Fasciola hepatica (cDNA-FhPGK/pCMV) and F. hepatica infection were investigated in the present study. cDNA-FhPGK/pCMV vaccinated female Sprague-Dawley rats were better protected by vaccination than their male counterparts - 48% reduction in fluke burden for females and no protection for males when compared with appropriate infection control groups. Moreover, male rats developed marked leukocytosis during the study with higher neutrophil, eosinophil and monocyte responses than females. Additionally, dynamics of eosinophil and monocyte responses varied between sexes. Increased titres of anti-FhPGK IgG1 and IgG2a correlated with the protective effect of vaccination that was observed among female rats. In the case of male sheep, no differences in worm burdens and in the course of the immune response were observed following vaccination. Titres of specific antibodies detected were low, and cellular responses were not significant. Apparently, sheep immune responses induced by cDNA-FhPGK/pCMV vaccination are not effective at controlling F. hepatica infection. Poor immunogenicity of DNA vaccines in large animals is still a major obstacle of this technology that has to be overcome.

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.

Immune responses and protective efficacy of a novel DNA vaccine encoding outer membrane protein of avian Pasteurella multocida

Avian Pasteurella multocida is a causative agent of fowl cholera. Two proteins OmpH and OmpA are the major immunogenic antigens of avian P. multocida, which play an important role in inducing immune responses that confer resistance against infections. In the present study, we used pcDNA3.1(+) as a vector and constructed DNA vaccines with the genes encoding the two antigens mentioned above. These DNA vaccines include monovalent (pcDNA-OMPH, pOMPH and pcDNA-OMPA, pOMPA), divalent combination (pcDNA-OMPH+pcDNA-OMPA, pOMPH+pOMPA) and fusion of two gene vaccines (pcDNA-OMPH/OMPA, pOMPHA). The immune responses to these DNA vaccines were evaluated by serum antibody titers, lymphocyte proliferation assay and titers of a cytokines, IFN-γ. The protective efficacy after challenging with a virulent avian P. multocida strain, CVCC474, was evaluated by survival rate. A significant increase in serum antibody levels was observed in chickens vaccinated with divalent combination and fusion DNA vaccines. Additionally, the lymphocyte proliferation (SI value) and the levels of IFN-γ were both higher in chickens immunized with divalent combination and fusion DNA vaccines than in those vaccinated with monovalent DNA vaccines (P<0.05). Furthermore, the protection provided by divalent combination and fusion DNA vaccines was superior to that provided by monovalent DNA vaccines after challenging with the avian P. multocida strain CVCC474. And the protective efficacy in chickens immunized three times with the fusion DNA vaccine was equivalent to the protective efficacy in chickens vaccinated once with the attenuated live vaccine. This suggests that divalent combination and fusion DNA vaccines represent a promising approach for the prevention of fowl cholera.

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.

Selection and identification of malaria vaccine target molecule using bioinformatics and DNA vaccination

Following a genome-wide search for a blood stage malaria DNA-based vaccine using web-based bioinformatic tools, 29 genes from the annotated Plasmodium yoelii genome sequence (www.PlasmoDB.org and www.tigr.org) were identified as encoding GPI-anchored proteins. Target genes were those with orthologues in P. falciparum, containing an N-terminal signal sequence containing hydrophobic amino acid stretch and signal P criteria, a transmembrane-like domain and GPI anchor motif. Focusing on the blood stage, we extracted mRNA from pRBCs, PCR-amplified 22 out of the 29 selected genes, and eventually cloned nine of these into a DNA vaccine plasmid, pVAX 200-DEST. Biojector-mediated delivery of the nine DNA vaccines was conducted using ShimaJET to C57BL/6 mice at a dose of 4 μg/mouse three times at an interval of 3 weeks. Two weeks after the second booster, immunized mice were challenged with P. y. yoelii 17XL-parasitized RBCs and the level of parasitaemia, protection and survival was assessed. Immunization with one gene (PY03470) resulted in 2-4 days of delayed onset and level of parasitaemia and was associated with increased survival compared to non-immunized mice. Antibody production was, however, low following DNA vaccination, as determined by immunofluorescence assay. Recombinant protein from this gene, GPI8p transamidase-related protein (rPyTAM) in PBS or emulsified with GERBU adjuvant was also used to immunize another set of C57BL/6 mice with 10-20 μg/mouse three times at 3-week interval. Higher antibody response was obtained as determined by ELISA with similar protective effects as observed after DNA vaccination.

Quantitation of cytokine mRNA by real-time RT-PCR during a vaccination trial in a rabbit model of fascioliasis

Use of the rabbit as disease model has long been hampered by a lack of immunological assays specific to this species. In the present study we developed a SYBR Green-based, real-time RT-PCR protocol to quantitate cytokine mRNA in freshly harvested rabbit peripheral mononuclear cells. The method was validated in the course of a vaccination trial in which animals vaccinated with the recombinant antigen FhSAP2 were challenged with Fasciola hepatica metacercariae. Changes in the levels of rabbit interleukin (IL)-2, IL-4, IL-6, IL-10, tumor necrosis factor-alpha (TNFalpha), and interferon-gamma (IFNgamma) mRNA were determined. Messenger RNA from the universally expressed housekeeping gene GAPDH was used as an amplification control and allowed for correction of variations in the efficiencies of RNA extraction and reverse transcription. Rabbits vaccinated with FhSAP2 showed an 83.3% reduction in liver fluke burden after challenge infection when compared to non-vaccinated controls. All cytokine mRNAs were found at detectable levels; however, the levels of IFNgamma, TNFalpha, IL-2 and IL-10 were significantly higher in the vaccinated group compared to the non-vaccinated group. These results suggest that protection conferred by FhSAP2 protein could be associated with a mixed Th1/Th2 immune response in which Th1 cytokines are dominant. The real-time RT-PCR method described herein can be a useful tool for monitoring changes in basic immune functions in the rabbit model of fascioliasis and may also aid in studies of human diseases for which the rabbit is an important experimental model.

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.

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.

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.

Vaccination with a Plasmodium chabaudi adami multivalent DNA vaccine cross-protects A/J mice against challenge with P. c. adami DK and virulent Plasmodium chabaudi chabaudi AS parasites

A current goal of malaria vaccine research is the development of vaccines that will cross-protect against multiple strains of malaria. In the present study, the breadth of cross-reactivity induced by a 30K multivalent DNA vaccine has been evaluated in susceptible A/J mice (H-2a) against infection with the Plasmodium chabaudi adami DK strain and a virulent parasite subspecies, Plasmodium chabaudi chabaudi AS. Immunized A/J mice were significantly protected against infection with both P. c. adami DK (31-40% reduction in cumulative parasitemia) and P. c. chabaudi AS parasites, where a 30-39% reduction in cumulative parasitemia as well as enhanced survival was observed. The 30K vaccine-induced specific IFN-gamma production by splenocytes in response to native antigens from both P. c. chabaudi AS and P. c. adami DK. Specific antibodies reacting with surface antigens expressed on P. c. adami DS and P. c. chabaudi AS infected red blood cells, and with opsonizing properties, were detected. These results suggest that multivalent vaccines encoding conserved antigens can feasibly induce immune cross-reactivity that span Plasmodium strains and subspecies and can protect hosts of distinct major histocompatibility complex haplotypes.

Mucosal delivery of a transmission-blocking DNA vaccine encoding Giardia lamblia CWP2 by Salmonella typhimurium bactofection vehicle

In this study, we investigated the use of Salmonella typhimurium (STM1 strain) as a bactofection vehicle to deliver a transmission-blocking DNA vaccine (TBDV) plasmid to the intestinal immune system. The gene encoding the full length cyst wall protein-2 (CWP2) from Giardia lamblia was subcloned into the pCDNA3 mammalian expression vector and stably introduced into S. typhimurium STM1. Eight-week-old female BALB/c mice were orally immunized every 2 weeks, for a total of three immunizations. Vaccinated and control mice were sacrificed 1 week following the last injection. Administration of the DNA vaccine led to the production of CWP2-specific cellular immune responses characterized by a mixed Th1/Th2 response. Using ELISA, antigen-specific IgA and IgG antibodies were detected in intestinal secretions. Moreover, analysis of sera demonstrated that the DNA immunization also stimulated the production of CWP2-specific IgG antibodies that were mainly of the IgG2a isotype. Finally, challenge infection with live Giardia muris cysts revealed that mice receiving the CWP2-encoding DNA vaccine were able to reduce cyst shedding by approximately 60% compared to control mice. These results demonstrate, for the first time, the development of parasite transmission-blocking immunity at the intestinal level following the administration of a mucosal DNA vaccine delivered by S. typhimurium STM1.

Parasite vaccines: The new generation

Parasites cause some of the most devastating and prevalent diseases in humans and animals. Moreover, parasitic infections increase mortality rates of other serious non-parasitic infections caused by pathogens such as HIV-1. The impact of parasitic diseases in both industrialised and developing countries is further exacerbated by the resistance of some parasites to anti-parasitic drugs and the absence of efficacious parasite vaccines. Despite years of research, much remains to be done to develop effective vaccines against parasites. This review focuses on the more recent vaccine strategies such as DNA and viral vector-based vaccines that are currently being used to develop vaccines against parasites. Obstacles yet to be overcome and possible advantages and disadvantages of these vaccine modalities are also discussed.

Enhancement of antibody and cellular immune responses to malaria DNA vaccines by in vivo electroporation

We evaluated the effectiveness of in vivo electroporation (EP) for the enhancement of immune responses induced by DNA plasmids encoding the pre-erythrocytic Plasmodium yoelii antigens PyCSP and PyHEP17 administered intramuscularly and intradermally to mice. EP resulted in a 16- and 2-fold enhancement of antibody responses to PyCSP and PyHEP17, respectively. Immunization with 5 microg of DNA via EP was equivalent to 50 microg of DNA via conventional needle, thus reducing by 10-fold the required dose to produce a given effect. Moreover, IFN-gamma responses were increased by approximately 2-fold. Data demonstrate the potential of EP to enhance immune responses to DNA vaccines against infectious agents.

Analytical and biological characterization of supercoiled plasmids purified by various chromatographic techniques

Supercoiled plasmids are an important component of gene-based delivery vehicles. A number of production methods for clinical applications have been developed, each resulting in very high-quality product with low levels of residual contaminants. There is, however, no consensus on the optimal methods to characterize plasmid quality, and further, to determine if these methods are predictive of either product stability or biological activity. We have produced two plasmids using four production purification methodologies based on PolyFlo and hydrophobic interaction chromatography (HIC), either alone or in tandem processes. In each case, the product was analyzed using standard molecular biological methods. We also performed a number of biophysical analyses such as dynamic light scattering (DLS), circular dichroism (CD), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). Minimal differences were detected among the preparations based on the more standard molecular biological methods. Some small differences were detected, however, using biophysical techniques, particularly FTIR and DSC, which may reflect small variations in plasmid tertiary structure and thermal stability. Stability after heat exposure at 60 degrees C, exposure to fetal bovine serum and long-term storage at 4 degrees C varied between plasmids. One plasmid showed no difference in stability depending on the production process, but the other showed significant differences. Evaluation in vivo in models for gene immunization and gene therapy showed significant differences in the response depending on the method of purification. Preparations using a tandem process of PolyFlo used in two separation modes provided higher biological activity compared to a tandem HIC/PolyFlo process or either resin used alone in a single column process. These data indicate that the process by which supercoiled plasmids are made can influence plasmid stability and biological activity and emphasize the need for more rigorous methods to evaluate supercoiled plasmids as gene-delivery vehicles.

Infection with Plasmodium berghei boosts antibody responses primed by a DNA vaccine encoding gametocyte antigen Pbs48/45

An important consideration in the development of a malaria vaccine for individuals living in areas of endemicity is whether vaccine-elicited immune responses can be boosted by natural infection. To investigate this question, we used Plasmodium berghei ANKA blood-stage parasites for the infection of mice that were previously immunized with a DNA vaccine encoding the P. berghei sexual-stage antigen Pbs48/45. Intramuscular immunization in mice with one or two doses of DNA-Pbs48/45 or of empty DNA vaccine as control did not elicit detectable anti-Pbs48/45 antibodies as determined by enzyme-linked immunosorbent assay. An infection with P. berghei ANKA 6 weeks after DNA vaccination elicited comparable anti-Pbs48/45 antibody levels in mice which had been primed with DNA-Pbs48/45 or with empty DNA vaccine. However, a repeat infection with P. berghei ANKA resulted in significantly higher anti-Pbs48/45 antibody levels in mice which had been primed with the DNA-Pbs48/45 vaccine than the levels in the mock DNA-vaccinated mice. In parallel and as an additional control to distinguish the boosting of Pbs48/45 antibodies exclusively by gametocytes during infection, a separate group of mice primed with DNA-Pbs48/45 received an infection with P. berghei ANKA clone 2.33, which was previously described as a "nongametocyte producer." To our surprise, this parasite clone too elicited antibody levels comparable to those induced by the P. berghei gametocyte producer clone. We further demonstrate that the nongametocyte producer P. berghei clone is in fact a defective gametocyte producer that expresses Pbs48/45, much like the gametocyte producer clone, and is therefore capable of boosting antibody levels to Pbs48/45. Taken together, these results indicate that vaccine-primed antibodies can be boosted during repeat infections and warrant further investigation with additional malaria antigens.

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.

Identification and characterization of Neospora caninum cyclophilin that elicits gamma interferon production

Gamma interferon (IFN-gamma) response is essential to the development of a host protective immunity in response to infections by intracellular parasites. Neosporosis, an infection caused by the intracellular protozoan parasite Neospora caninum, is fatal when there is a complete lack of IFN-gamma in the infected host. However, the mechanism by which IFN-gamma is elicited by the invading parasite is unclear. This study has identified a microbial protein in the N. caninum tachyzoite N. caninum cyclophilin (NcCyP) as a major component of the parasite responsible for the induction of IFN-gamma production by bovine peripheral blood mononuclear cells (PBMC) and antigen-specific CD4(+) T cells. NcCyP has high sequence homology (86%) with Toxoplasma gondii 18-kDa CyP with a calculated molecular mass of 19.4 kDa. NcCyP is a secretory protein with a predicted signal peptide of 17 amino acids. Abundant NcCyP was detected in whole-cell N. caninum tachyzoite lysate antigen (NcAg) and N. caninum tachyzoite culture supernatant. In N. caninum tachyzoite culture supernatant, three NcCyP bands of 19, 22, and 24 kDa were identified. NcAg stimulated high levels of IFN-gamma production by PBMC and CD4(+) T cells. The IFN-gamma-inducing effect of NcAg was blocked by cyclosporine, a specific ligand for CyP, in a dose-dependent manner. Furthermore, cyclosporine abolished IFN-gamma production by PBMC from naïve cows as well as PBMC and CD4(+) T cells from infected/immunized cows. These results indicate that the N. caninum tachyzoite naturally produces a potent IFN-gamma-inducing protein, NcCyP, which may be important for parasite survival as well as host protection.

Malaria vaccines in development

The recent infusion of public and private funding for malaria vaccine development has greatly accelerated the pace at which candidate malaria vaccines are entering the clinic. Recent promising results from vaccine trials carried out in malaria-naive and -endemic populations have revealed important insights into what will be required of a successful vaccine. Significant challenges lie ahead, not the least of which is insuring access of a malaria vaccine to the populations that need it most. Creative strategies, strong partnerships with developing countries, industry-like approaches to product development, and political vision and leadership on the part of wealthy nations will be critical to the successful implementation of this important new tool to reduce the intolerable burden of malaria.