Immunization with a DNA plasmid encoding the SAG1 (P30) protein of Toxoplasma gondii is immunogenic and protective in rodents

Toxoplasma gondii vaccine candidates: a concise review

Toxoplasma gondii is an obligate intracellular parasite that causes toxoplasmosis. It has been shown that the severity of symptoms depends on the functioning of the host immune system. Although T. gondii infection typically does not lead to severe disease in healthy people and after infection, it induces a stable immunity, but it can contribute to severe and even lethal Toxoplasmosis in immunocompromised individuals (AIDS, bone marrow transplant and neoplasia). The antigens that have been proposed to be used in vaccine candidate in various studies include surface antigens and secretory excretions that have been synthesized and evaluated in different studies. In some studies, secretory antigens play an important role in stimulating the host immune response. Various antigens such as SAG, GRA, ROP, ROM, and MAG have been from different strains of T. gondii have been synthesized and their protective effects have been evaluated in animal models in different vaccine platforms including recombinant antigens, nanoparticles, and DNA vaccine. Four bibliographic databases including Science Direct, PubMed Central (PMC), Scopus, and Google Scholar were searched for articles published up to 2020.The current review article focuses on recent studies on the use and usefulness of recombinant antigens, nanoparticles, and DNA vaccines.

Insight into the current Toxoplasma gondii DNA vaccine: a review article

INTRODUCTION

Toxoplasma gondii (T.gondii) is a widespread protozoan with significant economic losses and public health importance. But so far, the protective effect of reported DNA-based vaccines fluctuates widely, and no study has demonstrated complete protection.

AREAS COVERED

This review provides an inclusive summary of T. gondii DNA vaccine antigens, adjuvants, and some other parameters. A total of 140 articles from 2000 to 2021 were collected from five databases. By contrasting the outcomes of acute and chronic challenges, we aimed to investigate and identify viable immunological strategies for optimum protection. Furthermore, we evaluated and discussed the impact of several parameters on challenge outcomes in the hopes of developing some recommendations to assist better future horizontal comparisons among research.

EXPERT OPINION

In the coming five years of research, the exploration of vaccine cocktails combining invasion antigens and metabolic antigens with genetic adjuvants or novel DNA delivery methods may offer us desirable protection against this multiple stage of life parasite. In addition to finding a better immune strategy, developing better in silico prediction methods, solving problems posed by variables in practical applications, and gaining a more profound knowledge of T.gondii-host molecular interaction is also crucial towards a successful vaccine.

Diagnostic Value of Circulating Antigens in the Serum of Piglets with Experimental Acute Toxoplasmosis

Toxoplasmosis, caused by Toxoplasma gondii, an apicomplexan parasite, infects all warm-blooded animals, including a third of the human population. Laboratory diagnosis of acute toxoplasmosis is based on the detection of anti-T. gondii IgM and IgG and T. gondii nucleic acid; however, these assays have certain limitations. Circulating Ags (CAgs) are reliable diagnostic indicators of acute infection. In this study, we established a model of acute T. gondii infection in Large White pigs. CAg levels peaked between 3 and 5 d after inoculation, and 28 CAgs were identified using an immunoprecipitation-shotgun approach, among which dolichol-phosphate-mannose synthase family protein (TgDPM), C3HC zinc finger-like protein (TgZFLP3), and ribosomal protein RPL7 (TgRPL7) were selected to further investigate their value in the diagnosis of acute toxoplasmosis. Immunofluorescence assays revealed that TgDPM and TgRPL7 were localized in the membrane surface, while TgZFLP3 was localized in the apical end. Western blotting revealed the presence of the three proteins in the serum during acute infection. Indirect ELISA results indicate that TgZFLP3 is likely to be a novel candidate for the diagnosis of acute toxoplasmosis. However, these three proteins may not be useful as candidate vaccines against toxoplasmosis owing to their low protective ability. In addition, deletion of the zflp3 gene partially attenuated virulence in Kunming mice. Collectively, we identified 28 CAgs in the serum of piglets with experimental acute toxoplasmosis and confirmed that TgZFLP3 is a potential biomarker for acute T. gondii infection. The results of this study provide data to improve the detection efficiency of acute toxoplasmosis.

Toxoplasma gondii GRA15 DNA Vaccine with a Liposomal Nanocarrier Composed of an SS-Cleavable and pH-Activated Lipid-like Material Induces Protective Immunity against Toxoplasmosis in Mice

Toxoplasma gondii affects the health of humans and livestock and causes severe illness in the fetus and immunocompromised individuals. Because of the high incidence and severe consequences of T. gondii infection, a safe and suitable vaccine is needed. We found that lipid nanoparticles (LNPs) consisting of a series of functional materials prepared with vitamin E, such as SS-cleavable and pH-activated lipid-like materials (ssPalmE), were a safe and efficient way to develop next-generation DNA vaccines. In this study, we prepared ssPalmE-LNP to encapsulate pCpG-free-T. gondii dense granule protein 15 DNA (ssPalmE-LNP_TgGRA15). Following a challenge infection with avirulent PLK strain of T. gondii, the mice immunized with ssPalmE-LNP_TgGRA15 had a significantly higher survival rate and lower clinical scores compared with unimmunized and ssPalmE-LNP_non-coding-immunized mice. Immunization of mice with the ssPalmE-LNP_TgGRA15 led to a significantly higher production of specific IgG1 and IG2c antibodies compared with unimmunized and ssPalmE-LNP_non-coding-immunized mice, while there was no statistically significant difference in the concentration of serum interferon-gamma at the acute stage of the infection. These findings indicate that ssPalmE-LNP is an effective cargo for the transportation of DNA vaccines for protozoan infections. To explore the mechanism of protective immunity induced by ssPalmE-LNP_TgGRA15, further immunological study is needed in the future.

The immune response against Toxoplasma gondii in BALB/c mice induced by mannose-modified nanoliposome of excreted/secreted antigens

This study aimed to compare the immune response against Toxoplasma gondii (T. gondii) in BALB/c mice induced by excreted/secreted (E/S) antigens and mannose-modified nanoliposome of E/S antigens. Here, E/S antigens and mannose-modified nanoliposome of E/S antigens were firstly prepared, and then BALB/c female inbred mice were separately immunized. In the next step, anti-E/S antigen antibodies and the relative expression levels of IL-10 and IL-12 mRNA were detected by ELISA and real-time PCR, respectively. After immunization, mice were intraperitoneally challenged with 102 tachyzoites of T. gondii, and the survival rate was recorded. The ELISA analysis showed significant differences between the levels of anti-E/S antigen antibodies in the mice immunized by E/S antigens and those immunized by mannose-modified nanoliposome of E/S antigens at days 7, 10, 20, 25, and 30 (P < 0.05). Real-time PCR analysis showed that the relative expression of IL-10 was significantly decreased during 20 days. Yet, the relative expression of IL-12 was significantly increased during 20 days (P < 0.05). In T. gondii challenge test, significant differences were found between the survival rates of mice immunized by E/S antigens and mice immunized by mannose-modified nanoliposome with E/S antigens. This project evidenced that mannose-modified nanoliposome of E/S antigens induced a more powerful immune response against T. gondii in BALB/c mice when compared with excreted/secreted antigens alone.

Genetic immunization against toxoplasmosis: A review article

Toxoplasma gondii is a protozoan coccidian parasite belonging to Phylum Apicomplexa and is the causative agent of toxoplasmosis as a zoonotic disease around the world. It is one of the most important protozoa which is transmitted via various routes and infects several warm-blooded animals. The seroprevalence of T. gondii infection is high worldwide and leads to clinical, psychological, and economic problems. At present, available drug therapy for toxoplasmosis has severe side effects, so the development of new anti-toxoplasma drugs or effective vaccines is mandatory. Therefore, different measures have been taken for the development of anti-toxoplasmosis vaccines, and various studies have shown that DNA vaccines could be one of the most successful approaches against the intracellular parasite, T. gondii. Many of these studies have evaluated the efficacy of immunogenicity and different aspects of the DNA vaccines for toxoplasmosis including single genes or multi-gene plasmids with or without adjuvants. Most of the literature confirms that DNA vaccines containing different antigens of the toxoplasma parasite can induce suitable immune response and protection in acute or chronic toxoplasmosis. Therefore, in this review article, we aimed to discuss the current status of DNA vaccines as a new immunization method against toxoplasmosis.

Epitope Analysis and Efficacy Evaluation of Phosphatase 2C (PP2C) DNA Vaccine Against Toxoplasma gondii Infection

Toxoplasma gondii infects almost all warm-blooded animals and negatively affects the health of a wide range of these animals, including humans. Protein phosphatase 2C (PP2C) is a T. gondii protein secreted by rhoptry organelles during host cell invasion. However, very little is known about whether this protein can induce protective immunity against T. gondii. In this study, bioinformatics analysis of PP2C revealed some useful information in the context of anti-toxoplasmosis treatments and vaccine research. In addition, the PP2C gene was amplified, and a eukaryotic expression vector (pEGFP-PP2C) was successfully constructed to express PP2C. Finally, the constructed pEGFP-PP2C was injected into mice to evaluate whether it could induce immunoprotection. Compared with the control groups, we found that immunizations with the pEGFP-PP2C plasmid could elicit specific IgG antibodies and cytokines against T. gondii infection. The survival of mice immunized with the pEGFP-PP2C plasmid was significantly prolonged compared with that of the control group mice. Based on the ability of pEGFP-PP2C to induce specific immune responses against T. gondii, we propose that PP2C merits consideration as a potential vaccine candidate against toxoplasmosis.

Evaluation of CpG-ODN-Adjuvanted Toxoplasma Gondii Virus-Like Particle Vaccine upon One, Two, and Three Immunizations

Successful vaccines against specific pathogens often require multiple immunizations and adjuvant usage. Yet, assessing the protective efficacy of different immunization regimens with adjuvanted Toxoplasma gondii vaccines remains elusive. In this study, we investigated the vaccine efficacy induced by CpG-ODN-adjuvanted T. gondii virus-like particles (VLPs) after challenge infection with T. gondii (ME49) in mice (BALB/c) upon one, two, and three immunizations. Immunization with adjuvanted T. gondii VLPs induced higher levels of T. gondii-specific IgG and/or IgA antibody responses, germinal center (GC) B cells, total B cells, and CD4⁺ and CD8⁺ T cells compared with unadjuvanted VLPs. Increasing the number of immunizations was strongly correlated with enhanced protective immunity against T. gondii in mice, with the highest protection being demonstrated in mice thrice-immunized with either adjuvanted T. gondii VLPs or VLPs alone. Notably, lesser bodyweight reductions and cerebral cyst counts were observed in mice receiving multiple immunizations with the adjuvanted VLPs, thereby confirming the effectiveness of adjuvanted boost immunizations. These results demonstrated that multiple immunizations with T. gondii VLPs is an effective approach, and the CpG-ODN can be developed as an effective adjuvant for T. gondii VLP vaccines.

Protective immunity induced by a DNA vaccine cocktail expressing TgSAG1, TgROP2, and the genetic adjuvant HBsAg against Toxoplasma gondii infection

Toxoplasma gondii is an intracellular obligate parasitic protozoon that can infect all warm-blooded animals, causing zoonotic toxoplasmosis. So far, there is no commercial toxoplasmosis vaccine for human use. In the present study, we constructed a DNA vaccine cocktail which includes the surface protein (SAG1) and the rhoptry protein ROP2 denoted as pEGFP-N1-SAG1-ROP2. In order to improve the efficacy, HBsAg was used as a genetic adjuvant to construct pEGFP-N1-HBsAg-SAG1-ROP2. Two eukaryotic plasmids were transiently transfected into HEK293T cells and the expression was examined using fluorescence microscopy and western blotting. We then immunized Kunming mice intramuscularly with the DNA vaccine. After three immunizations, the immune response was evaluated by measuring antibody levels, cytokine production, percentages of CD4⁺ and CD8⁺ T lymphocytes, and the survival times of the T. gondii RH strain challenged mice. The results showed that the two DNA vaccines stimulated Th1 responses, and had a higher antibody titer, IL-2, IL-12, and IFN-γ levels, and percentage of CD4⁺ and CD8⁺ T lymphocytes than the control group. In addition, mice immunized with the pEGFP-N1-HBsAg-SAG1-ROP2 vaccine showed increased survival times compared with pEGFP-N1-SAG1-ROP2.

Toxoplasma gondii surface antigen 1 (SAG1) as a potential candidate to develop vaccine against toxoplasmosis: A systematic review

Toxoplasma gondii is an intracellular parasite that infects a broad range of animal species and humans. As the main surface antigen of the tachyzoite, SAG1 is involved in the process of recognition, adhesion and invasion of host cells. The aim of the current systematic review study is to clarify the latest status of studies in the literature regarding SAG1-associated recombinant proteins or SAG1-associated recombinant DNAs as potential vaccines against toxoplasmosis. Data were systematically collected from six databases including PubMed, Science Direct, Web of Science, Google Scholar, EBSCO and Scopus, up to 1st of January 2019. A total of 87 articles were eligible for inclusion criteria in the current systematic review. The most common antigens used for experimental cocktail vaccines together with SAG1 were ROP2 and SAG2. In addition, the most parasite strains used were RH and ME49. Freund's adjuvant and cholera toxin have been predominantly utilized. Furthermore, regarding the animal models, route and dose of vaccination, challenge methods, measurement of immune responses and cyst burden have been discussed in the text. Most of these experimental vaccines induce immune responses and have a high degree of protection against parasite infections, increase survival rates and duration and reduce cyst burdens. The data demonstrated that SAG1 antigen has a high potential for use as a vaccine and provided a promising approach for protecting humans and animals against toxoplasmosis.

Induction of Protective Immunity against Toxoplasma gondii in Mice by Nucleoside Triphosphate Hydrolase-II (NTPase-II) Self-amplifying RNA Vaccine Encapsulated in Lipid Nanoparticle (LNP)

RNA-based vaccine represents an irresistible and safe immunization strategy with decreasing theoretical risks of genomic integration and malignant cell transformation. To our knowledge, however, there is no report about development of RNA vaccine against Toxoplasma gondii infection. We have previously demonstrated that the recombinant T. gondii nucleoside triphosphate hydrolase-II (NTPase-II) protein is able to provide protective Th1 cell-mediated immunity against T. gondii. Herein, we evaluated the immunogenic potential of a self-amplifying RNA vaccine-encoding T. gondii NTPase-II gene, RREP-NTPase-II, delivered by a synthetic lipid nanoparticle (LNP). Immunization of mice with naked RREP-NTPase-II induced a strong cellular and humoral immune response with high-IgG antibody titers and IFN-γ production. The immunized mice displayed significantly prolonged survival time and reduction in brain parasite load (46.4%) compared with control group. Furthermore, mice vaccinated with RREP-NTPase-II-encapsulated LNP displayed significantly enhanced protection against acute infection as well as chronic infection with PRU cyst, which shows 62.1% reduction in brain cyst burden in comparison to control group. These results suggest that the combination of self-amplifying RNA and LNP ion would be beneficial to the development of a safe and long-acting vaccine against toxoplasmosis.

Protective immune response in mice induced by a suicidal DNA vaccine encoding NTPase-II gene of Toxoplasma gondii

DNA-based alphaviral RNA replicon vectors, also called suicidal DNA vectors, have been employed to alleviate biosafety concerns attribution to its ability to induce apoptotic cell death of the transfected cells. Toxoplasma gondii nucleoside triphosphate hydrolase-II (TgNTPase-II), which facilitates the parasite to salvage purines from the host cell for survival and replication, have been demonstrated to be a potential vaccine candidate for toxoplasmosis. Herein, we evaluated the immunogenic potential of a suicidal DNA vaccine encoding TgNTPase-II gene, pDREP-TgNTPase-II, delivered intramuscularly in combination with electroporation. Immunization of mice with pDREP-TgNTPase-II elicited specific humoral responses, with high IgG antibody titers and a mixed IgG1/IgG2a response. The cellular immune response was associated with high level production of IFN-γ, IL-2, IL-10 cytokines and low level IL-4 production as well as the increase of the percentage of CD8+ T cells, indicating that a Th1 predominant response was elicited. Furthermore, mice vaccinated with this suicidal DNA vaccine displayed partial protection against acute infection with the virulent RH strain as well as chronic infection with PRU cyst, which shows 77.7% and 71.4% reduction in brain cyst burden in comparison to PBS and pDREP-eGFP control group, respectively. Based on the cellular and antibody responses, the suicidal DNA vaccine elicited a Th1-predominant immune response against T. gondii challenge.

Evaluation of immune responses induced by rhoptry protein 5 and rhoptry protein 7 DNA vaccines against Toxoplasma gondii

Infection with the protozoan parasite Toxoplasma gondii is widespread, and the organism can cause congenital infections in humans. The horizontal transmission of Toxoplasma is even more common than congenital. An effective vaccine strategy brings the prospect of improving Toxoplasma disease control. Rhoptry protein 5 (ROP5) and ROP7 are potential stimulators of humoral and cellular immune responses. In this study, we constructed a multi-antigenic DNA vaccine expressing ROP5 and ROP7 of T. gondii and compared the protective efficacy to single-gene vaccines and control groups. BALB/c mice were immunized intramuscularly three times. The levels of IgG antibodies and cytokines in mice immunized with the multi-antigenic DNA vaccine (pROP5/ROP7) were significantly higher than those in the control mice. Mice vaccinated with pROP5/ROP7 showed a longer survival time (16 days) than single-gene-immunized mice (11 and 12 days, respectively) or control mice (8 days) after a challenge with 1 × 10(4) tachyzoites of RH strain of T. gondii. Furthermore, after intragastric infection with 20 cysts of PRU strain of T. gondii, the number of brain cysts in mice immunized with pROP5/ROP7 was only 25% of the number in control mice. Our results showed that a DNA vaccine encoding ROP5 and ROP7 significantly enhanced protection against T. gondii challenge.

In silico and in vivo analysis of Toxoplasma gondii epitopes by correlating survival data with peptide-MHC-I binding affinities

BACKGROUND

Protein antigens comprising peptide motifs with high binding affinity to major histocompatibility complex class I (MHC-I) molecules are expected to induce a stronger cytotoxic T-lymphocyte response and thus provide better protection against infection with microorganisms where cytotoxic T-cells are the main effector arm of the immune system.

METHODS

Data on cyst formation and survival were extracted from past studies on the DNA immunization of mice with plasmids coding for Toxoplasma gondii antigens. From in silico analyses of the vaccine antigens, the correlation was tested between the predicted affinity for MHC-I molecules of the vaccine peptides and the survival of immunized mice after challenge with T. gondii. ELISPOT analysis was used for the experimental testing of peptide immunogenicity.

RESULTS

Predictions for the Db MHC-I molecule produced a strong, negative correlation between survival and the dissociation constant of vaccine-derived peptides. The in silico analyses of nine T. gondii antigens identified peptides with a predicted dissociation constant in the interval from 10nM to 40μM. ELISPOT assays with splenocytes from T. gondii-infected mice further supported the importance of the peptide affinity for MHC-I.

CONCLUSIONS

In silico analysis clearly helped the search for protective vaccine antigens. The ELISPOT analysis confirmed that the predicted T-cell epitopes were immunogenic by their ability to release interferon gamma in spleen cells.

Levels of Transforming Growth Factor-Beta After Immunization of Mice With in vivo prepared Toxoplasma gondii Excretory/Secretory Proteins

BACKGROUND

Zoonotic parasite Toxoplasma gondii has a high prevalence in human populations. A suitable vaccine for animals can stop the transmission of infection between animal and human.

OBJECTIVES

The aim of this study was to evaluate in vivo prepared excretory/secretory antigens (E/SA) as a potential candidate for immunization against the parasite and its effect on the production of transforming growth factor-beta (TGF-β).

MATERIALS AND METHODS

Toxoplasma gondii tachyzoites were inoculated in the peritoneal cavity of mice and E/SA was harvested and used in animal immunization with and without adjuvant. Serum levels of anti-E/SA antibodies and TGF-β were measured in days 0, 3, 7, 14, 28 and 56 after immunization using ELISA technique. The measurements were statistically analyzed.

RESULTS

Our results showed that the serum levels of anti-E/SA immunoglobulins significantly increased in all of the immunized groups. The differences of the serum levels of TGF-β between the groups were statistically significant at days 28 and 56 after immunization with E/SA.

CONCLUSIONS

Based on our study, in vivo prepared E/SA may be considered as a good candidate for animal immunization.

Alpha-galactosylceramide enhances protective immunity induced by DNA vaccine of the SAG5D gene of Toxoplasma gondii

BACKGROUND

Toxoplasmosis caused by the intracellular parasite Toxoplasma gondii (T. gondii) is a global epidemic parasitic disease. DNA vaccines play an important role in preventing the spread of toxoplasmosis. SAG family genes encoding particular surface proteins of T. gondii are the best candidates of DNA vaccine. As a member of SAG family genes, SAG5 gene has been proved to have better antigenic than SAG1. In addition, alpha-Galactosylceramide (α-GalCer) was used to be an adjuvant in malaria vaccine and received positive results. In this study, the effect of the DNA vaccine enhanced by α-GalCer was evaluated by immunizing BALB/c mice.

METHODS

In the present study, SAG5D gene of T. gondii was cloned, sequenced, and biologically characterized. BALB/c mice were randomly divided into five groups, including three experimental groups (pEGFP-C1-SAG5D, α-GalCer and α-GalCer/pEGFP-C1-SAG5D) and two control groups (PBS and pEGFP-C1), and were immunized intramuscularly three times. The levels of IgG antibodies and cytokine productions in mouse sera were determined by enzyme-linked immunosorbent assays (ELISA). Two weeks after the last immunization, all mice were challenged intraperitoneally with 1 × 10(4) tachyzoites of T. gondii and the survival time of mice was recorded.

RESULTS

A significant level of increase of IgG response against the soluble tachyzoite antigens (STAg) was detected by ELISA in experimental group. It revealed relatively high level of IFN-γ production by the spleen cells. There were higher productions of interleukin-4 (IL-4) in α-GalCer treated groups compared to control groups. Challenge experiment showed a longer survival period (11 days compared with 5 days in control) in SAG5D DNA vaccinated mice was found after a lethal challenge with T. gondii RH strain.

CONCLUSIONS

The present study suggested that T. gondii SAG5D was a novel and positive DNA vaccine candidate against toxoplasmosis. In addition, the adjuvant (α-GalCer) enhanced the body's cellular immune response and prolonged the survival time of mice after challenge.

Production of in-vitro refolded and highly antigenic SAG1 for development of a sensitive and specific Toxoplasma IgG ELISA

Recombinant antigens are increasingly applied to replace native antigens in serological tests. Surface antigen 1 (SAG1) is a highly immunogenic antigen and probably represents the most explored and used antigen of Toxoplasma gondii for development of serological test kits. The presence of six disulfide bridges in its structure makes SAG1 a highly conformational protein. In fact, antigenicity of SAG1 is greatly dependent on proper disulfide bonding and folding. In-vitro refolding of SAG1 inclusion bodies, produced in Escherichia coli, was reported to result in soluble and antigenic protein. We produced SAG1 in E. coli and highly purified it by a single denaturing immobilized metal affinity chromatography. Refolding of denatured SAG1 was performed by (a) dialysis in the presence of reduced/oxidized glutathione, (b) drop-wise dilution and (c) drop-wise dilution in the presence of CuSo4. Refolding in the presence of oxido-shuffling reagent was much more efficient in producing presumably correctly-folded and highly antigenic SAG1 as demonstrated by non-reducing SDS-gel electrophoresis, ELISA, Western blotting and reversed-phase HPLC. An IgG ELISA developed using SAG1 refolded in the presence of oxido-shuffling reagent displayed high sensitivity and specificity for detection of Toxoplasma IgG antibodies in pregnant women.

DNA vaccination with a gene encoding Toxoplasma gondii Deoxyribose Phosphate Aldolase (TgDPA) induces partial protective immunity against lethal challenge in mice

Background

Toxoplasma gondii is an obligate intracellular parasite that causes a pathological status known as toxoplasmosis, which has a huge impact on human and animal health. Currently, the main control strategy depends on the usage of drugs that target the acute stage of the infection, however, drawbacks were encountered while applying this method; therefore, development of an alternative effective method would be important progress. Deoxyribose Phosphate Aldolase (TgDPA) plays an important role supporting cell invasion and providing energy for the parasite.

Methods

TgDPA was expressed in Escherichia coli and the purified recombinant protein was used to immunize rats. The antibodies obtained were used to verify in vitro expression of TgDPA. The vector pVAX1 was utilized to formulate a DNA vaccine designated as pTgDPA, which was used to evaluate the immunological changes and the level of protection against challenge with the virulent RH strain of T. gondii.

Results

DNA vaccine, TgDPA revealed that it can induce a strong humoral as well as cellular mediated response in mice. These responses were a contribution of T_H1, T_H2 and T_H17 type of responses. Following challenge, mice immunized with TgDPA showed longer survival rates than did those in control groups.

Conclusions

Further investigation regarding TgDPA is required to shed more light on its immunogenicity and its possible selection as a vaccine candidate.

The effect of chitosan nanospheres on the immunogenicity of Toxoplasma lysate vaccine in mice

Toxoplasmosis, a zoonotic parasitic disease, is a huge challenge for which there is no effective vaccine up till now. In this study, chitosan nanospheres encapsulated with Toxoplasma lysate vaccine was evaluated for its ability to protect mice against both acute and chronic toxoplasmosis models of infection. Results showed that chitosan nanospheres were equally effective to Freund's incomplete adjuvant (FIA) in enhancing the efficacy of Toxoplasma lysate vaccine. The effectiveness was demonstrated by the delayed death of vaccinated mice following challenge either with virulent RH or avirulent Me49 strains, the significant decrease in parasite density in different organs, significant increase in the humoral and cellular immune response (IgG and IFN γ) with a marked reduction of pathological changes in the different organs. However chitosan nanospheres were superior to FIA due to their cost effective preparation and much less necrotic changes induced in the studied organs. The success of chitosan polymer as an alternative to commonly used adjuvants paves the way for the use of other newly developed polymers to be used in the field of vaccine development.

Intranasal immunisation of the recombinant Toxoplasma gondii receptor for activated C kinase 1 partly protects mice against T. gondii infection

Nasal vaccination is an effective therapeutic regimen for preventing certain infectious diseases. The mucosal immune response is important for resistance to Toxoplasma gondii infection. In this study, we evaluated the immune responses elicited in BALB/c mice by nasal immunisation with recombinant T. gondii receptor for activated C kinase 1 (rTgRACK1) and their protective efficacy against T. gondii RH strain during both chronic and lethal infections. Nasal vaccination with rTgRACK1 increased the level of secretory IgA in nasal, intestinal and vesical washes, and the level of IFN-γ and IL-2 in intestinal washes, indicating that rTgRACK1 vaccination promotes mucosal immune responses. The mice immunised with rTgRACK1 also displayed increased levels of rTgRACK1-specific IgA, total IgG, IgG1 and in particular IgG2a in their blood sera, increased production of IFN-γ, IL-2 and IL-4 but not IL-10 from their isolated spleen cells, and enhanced splenocyte proliferation in vitro. rTgRACK1-vaccinated mice were effectively protected against infection with T. gondii RH strain, showing over 50% reduction of tachyzoite burdens in their liver and brain tissues during a chronic infection, and also a 45% increase in their survivals during a lethal challenge. These results indicate that rTgRACK1 might represent an intriguing immunogen for developing a mucosal vaccine against toxoplasmosis.

Vaccination with Toxoplasma lysate antigen and CpG oligodeoxynucleotides: comparison of immune responses in intranasal versus intramuscular administrations

Toxoplasma gondii (T. gondii) is one of the most successful intracellular protozoan parasites on earth and highly prevalent in most warm-blooded vertebrates. There are no drugs that target the chronic cyst stage of this infection; therefore, development of an effective vaccine would be an important advance in disease control. Oligodeoxynucleotides (ODN) which contain immunostimulatory CG motifs (CpG ODN) can promote T-helper 1 (Th1) responses, an adjuvant activity that is desirable for vaccination against intracellular pathogen. In this study, we compare the immune responses of Toxoplasma susceptible C57BL/6 mice following intranasal and intramuscular vaccination with Toxoplasma lysate antigen (TLA) with or without CpG ODN as adjuvant. Immunized and control non-immunized mice were challenged with 85 cyst of the moderately virulent Beverley strain of T. gondii. Intranasal vaccination gave significantly a higher protection compared to other groups as indicated by prolonged survival and significantly reduced brain cyst burden (P < 0.01). Intranasal vaccination stimulated cellular immunity towards Th1 response characterized by significant INF-γ production (P < 0.01). Furthermore, fecal IgA antibody levels as an indicator of mucosal immune responses were significantly higher (P < 0.05) in intranasal vaccinated group before the challenge compared to all other groups. Intranasal vaccination was not able to upgrade the Th1 humoral arm. In contrast, intramuscular vaccination enhanced humoral immunity towards a type Th1 pattern characterized by a significant increase of specific IgG and Ig2a. Our results suggest that intranasal administration of CpG/TLA would provide a stable, pronounced, and effective vaccine against toxoplasmosis through stimulation of Th1 cellular immunity and mucosal IgA.

DNA prime and peptide boost immunization protocol encoding the Toxoplasma gondii GRA4 induces strong protective immunity in BALB/c mice

Background

Toxoplasma gondii is a widespread intracellular parasite, which infects most vertebrate animal hosts and causes zoonotic infection in humans. Vaccine strategy remains a promising method for the prevention and control of toxoplasmosis. T. gondii GRA4 protein has been identified as a potential candidate for vaccine development. In our study, we evaluated the immune response induced by four different immunization vaccination strategies encoding TgGRA4.

Methods

BALB/c mice were intramuscularly (i.m.) immunized four times according to specific immunization schedules. Generally, mice in experimental groups were immunized with polypeptide, pGRA4, peptide/DNA, or DNA/peptide, and mice in the control groups were injected with PBS or pEGFP. After immunization, the levels of IgG antibodies and cytokine productions were determined by enzyme-linked immunosorbent assays (ELISA). The survival time of mice was also evaluated after challenge infection with the highly virulent T. gondii RH strain.

Results

The results showed that mice vaccinated with different immunization regimens (polypeptide, pGRA4, peptide/DNA, or DNA/peptide) elicited specific humoral and cellular responses, with high levels of total IgG, IgG2a isotype and gamma interferon (IFN-γ), which suggested a specific Th1 immunity was activated. After lethal challenge, an increased survival time was observed in immunized mice (11.8 ± 4.8 days) compared to the control groups injected with PBS or pEGFP (P < 0.05). Mice injected with PBS or pEGFP died within 8 days, and there was no significant difference in the protection level in two groups (P > 0.05).

Conclusions

These results demonstrated that this DNA prime and peptide boost immunization protocol encoding the TgGRA4 can elicit the highest level of humoral and cellular immune responses compared to other immunized groups, which is a promising approach to increase the efficacy of DNA immunization.

Recombinant vaccines against T. gondii: comparison between homologous and heterologous vaccination protocols using two viral vectors expressing SAG1

The use of recombinant viral vectors expressing T. gondii antigens is a safe and efficient approach to induce immune response against the parasite and a valuable tool for vaccine development. We have previously protected mice from toxoplasmosis by immunizing the animals with an adenovirus expressing the protein SAG1 (AdSAG1) of T. gondii. We are now looking for ways to improve the vaccination strategy and enhance protection. One limitation of homologous vaccinations (sequential doses of the same vector) is induction of anti-vector immune response that blocks cell transduction, restricts transgene expression and, consequently, compromises the overall outcome of vaccination. One way to avert the effects of anti-vector response is to use different viruses in prime and boost (heterologous vaccination). Bearing this in mind, we generated a modified Vaccinia Virus Ankara encoding SAG1 (MVASAG1), to be tested as boost agent after prime with AdSAG1. Although minor differences were observed in the magnitude of the anti-SAG1 immune response induced by each vaccination protocol, the heterologous immunization with AdSAG1 followed by MVASAG1 resulted in improved capacity to control brain cyst formation in a model of chronic toxoplasmosis in C57BL/6 mice.

Interleukin-10 Serum Levels after Vaccination with In Vivo Prepared Toxoplasma gondii Excreted/Secreted Antigens

OBJECTIVES

Toxoplasma gondii is a worldwide prevalent zoonotic parasite which causes toxoplasmosis. An appropriate vaccine for animals could interrupt the circle between animals and humans. Our previous study showed that excreted/secreted antigens (E/SA), derived from the peritoneum of mice infected with T. gondii tachyzoites could be considered as a good candidate for animal vaccination. Interleukin-10 (IL-10) inhibits proliferation of B and T lymphocytes and induces homeostasis in immune system responses. However, since IL-10 has also been shown to suppress the killing of T. gondii by human macrophages, the aim of this study was to evaluate IL-10 serum levels after vaccination with T. gondii E/SA prepared in vivo.

METHODS

T. gondii tachyzoites were inoculated in the peritoneum of mice and harvested E/SA were used as a vaccine, with and without adjuvant, in T. gondii infected and un-infected mice. IL-10 serum levels were evaluated using the ELISA technique.

RESULTS

The data showed that although serum levels of IL-10 were not changed at the early phases, they were elevated at the end phases of vaccination with T. gondii E/SA.

CONCLUSION

Based on these and our previous results, it can be concluded that in vivo prepared T. gondii E/SA could be considered as a good candidate for animal vaccination.

Protective immune response in BALB/c mice induced by DNA vaccine of the ROP8 gene of Toxoplasma gondii

Toxoplasmosis in humans and other animals is caused by the protozoan parasite Toxoplasma gondii. During the process of host cell invasion and parasitophorous vacuole formation by the tachyzoites, the parasite secretes Rhoptry protein 8 (ROP8), an apical secretory organelle. Thus, ROP8 is an important protein for the pathogenesis of T. gondii. The ROP8 DNA was constructed into a pVAX-1 vaccine vector and used for immunizing BALB/c mice. Immunized mice developed immune response characterized by significant antibody responses, antigen-specific proliferation of spleen cells, and production of high levels of IFN-γ (816 ± 26.3 pg/mL). Challenge experiments showed significant levels of increase in the survival period (29 days compared with 9 days in control) in ROP8 DNA vaccinated mice after a lethal challenge with T. gondii. Results presented in this study suggest that ROP8 DNA is a promising and potential vaccine candidate against toxoplasmosis.

Compound DNA vaccine encoding SAG1/ SAG3 with A2/B subunit of cholera toxin as a genetic adjuvant protects BALB/c mice against Toxoplasma gondii

Background

Intracellular parasites, such as T. gondii, present a plurality of antigens because of the complexity of its life cycle. Compound DNA vaccines bring a new approach and hope for the treatment of toxoplasmosis. In this study, a DNA vaccine encoding two major surface antigens SAG1, SAG3 from T. gondii, with A₂/B subunit of cholera toxin as a genetic adjuvant was constructed.

Methods

BALB/c mice were immunized intramuscularly with PBS, pcDNA3.1, pSAG1, pSAG1/SAG3 and pSAG1/SAG3-CTXA₂/B three times separately. Immunized mice were tested for IgG antibody and IFN-γ and IL-4 production by ELISA. The proliferation of T cells was measured by DNA synthesis assay and the lymphocyte subsets of spleen cells by flow cytometry. All the immunized mice were challenged with 10³ highly virulent RH tachyzoites of Toxoplasma gondii intraperitoneally and the survival times were recorded.

Results

An enhanced production of IgG antibodies, antigen-specific lymphocyte proliferation and IFN-γ production from splenic cells were induced in mice immunized with pSAG1/SAG3 compared to mice immunized with pSAG1 (P<0.05). Introduction of CTXA₂/B further enhanced the Th1 cell-mediated immunity with higher levels of IFN-γ, lymphocyte proliferation activity and percentage of CD8⁺ T-cells. When challenged with lethal doses of T. gondii (1×10³), all control mice (PBS and empty plasmid group) died within 6 days. Mice immunized with pSAG1 died within 8 days. While 20% and 40% survival rate were achieved from mice immunized with pSAG1/SAG3 and pSAG1/SAG3-CTXA₂/B.

Conclusions

This study indicates the compound DNA vaccine encoding T. gondii antigens SAG1, SAG3 with CTXA₂/B gene was a promising DNA vaccine candidate against toxoplasmosis, which could effectively enhance the humoral and cellular immune response and prolong survival time in vaccinated mice.

Protective immunity induced by a DNA vaccine expressing eIF4A of Toxoplasma gondii against acute toxoplasmosis in mice

Toxoplasma gondii is an obligate intracellular protozoan parasite infecting humans, mammals and birds. Eukaryotic translation initiation factor (eIF4A) is a newly identified protein associated with tachyzoite virulence. To evaluate the protective efficacy of T. gondii eIF4A, a DNA vaccine (pVAX-eIF4A) encoding T. gondii eIF4A (Tg-eIF4A) gene was constructed. The expression ability of this recombinant DNA plasmid was examined in Marc145 cells by IFA. Then, Kunming mice were intramuscularly immunized with pVAX-eIF4A and followed by challenge infection with the highly virulent T. gondii RH strain. The results showed that vaccination with pVAX-eIF4A elicited specific humoral responses, with high IgG antibody titers and specific lymphocyte proliferative responses. The cellular immune response was associated with significant production of IFN-γ, IL-2 in Kunming mice, and a mixed IgG1/IgG2a response with predominance of IgG2a production, indicating that a Th1 type response was elicited after immunization with pVAX-eIF4A. In addition, the increase of the percentage of CD8+ T cells in lymphoid in mice suggested the activation of MHC class I restricted antigen presentation pathways. After lethal challenge, the mice vaccinated with the pVAX-eIF4A showed a significantly prolonged survival time (23.0±5.5 days) compared with control mice which died within 7 days of challenge (P<0.05). These results demonstrate that pVAX-eIF4A could elicit strong humoral, Th1-type cellular immune responses and increase survival time of immunized mice, suggesting that eIF4A is a promising vaccine candidate against acute T. gondii infection in mice.

Evaluation of protective immune responses induced by DNA vaccines encoding Toxoplasma gondii surface antigen 1 (SAG1) and 14-3-3 protein in BALB/c mice

Background

Toxoplasmosis, caused by an obligate intracellular protozoan parasite Toxoplasma gondii, has been a serious clinical and veterinary problem. Effective DNA vaccines against T. gondii can prevent and control the spread of toxoplasmosis, which is important for both human health and the farming industry. The T. gondii 14-3-3 protein has been proved to be antigenic and immunogenic and was a potential vaccine candidate against toxoplasmosis. In this study, we evaluated the immune responses induced by recombinant plasmids encoding T. gondii surface antigen 1 (SAG1) and 14-3-3 protein by immunizing BALB/c mice intramuscularly.

Methods

In the present study, BALB/c mice were randomly divided into five groups, including three experimental groups (pSAG1, p14-3-3 and pSAG1/14-3-3) and two control groups (PBS and pBudCE4.1), and were immunized intramuscularly three times. The levels of IgG antibodies and cytokine production in mouse sera were determined by enzyme-linked immunosorbent assays (ELISA). Two weeks after the last immunization, all mice were challenged intraperitoneally (i.p.) with 1×10⁴ tachyzoites of T. gondii and the survival time of mice was observed and recorded every day.

Results

Mice vaccinated with pSAG1, p14-3-3 or pSAG1/14-3-3 developed high levels of IgG2a and gamma interferon (IFN-γ) and low levels of interleukin-4 (IL-4) and interleukin-10 (IL-10) compared to control groups (PBS or pBudCE4.1), which suggested a modulated Th1 type immune response (P<0.05). After intraperitoneal challenge with 1×10⁴ tachyzoites of T. gondii (RH strain), the survival time of mice in experimental groups was longer than control groups (P<0.05). Mouse immunized with pSAG1/14-3-3 induced a higher level of IgG antibody response and significantly prolonged the survival time when compared with pSAG1 or p14-3-3 (P<0.05).

Conclusions

The study suggested that T. gondii 14-3-3 protein can induce effective immune responses in BALB/c mice and was a novel DNA vaccine candidate against toxoplasmosis, and the immune protective efficacy elicited by SAG1 gene was also demonstrated. Our results also showed multi-gene vaccine significantly enhanced immune responses and protective efficacy and was superior to the single-gene vaccine.

Protective immune response against Toxoplasma gondii elicited by a recombinant DNA vaccine with a novel genetic adjuvant

Previous immunological studies from our laboratory have demonstrated the potential role of Toxoplasma gondii antigens SAG1 and GRA2 as vaccine candidates. To further evaluate the vaccine's effects, a series of recombinant DNA vaccines pVAX1-SAG1, pVAX1-GRA2 and pVAX1-SAG1-GRA2, termed pSAG1, pGRA2 and pSAG1-GRA2, respectively, were constructed. A plasmid pVAX1-S/PreS2, termed pSPreS2 encoding hepatitis B virus (HBV) surface antigen (HBsAg) S and PreS2 as a novel genetic adjuvant, was also constructed. The expression abilities of those DNA plasmids were examined in HFF cells by Western blotting. Then BALB/c mice were intramuscularly immunized with DNA plasmids and followed by challenging with the highly virulent T. gondii RH strain. The results demonstrated that the recombinant DNA vaccine pSAG1-GRA2 was capable of eliciting high levels of antibodies, a Th1 type of immune response with significant production of IFN-γ and low levels of IL-4 or IL-10 in BALB/c mice, and partial protection against the acute phase of toxoplasmosis as compared to pSAG1, pGRA2 and controls. In addition, the adjuvant pSPreS2 formulated with DNA vaccine induced a Th1 type of immune response and therefore might be a novel genetic adjuvant to DNA vaccine for further investigation.

MyD88-dependent protective immunity elicited by adenovirus 5 expressing the surface antigen 1 from Toxoplasma gondii is mediated by CD8(+) T lymphocytes

Toxoplasma gondii is an intracellular parasite widely spread around the world. The surface antigens (SAG) 1, 2 and 3 are the main proteins expressed on the surface of T. gondii tachyzoites. Replication-defective adenovirus serotype 5 (rAd5) is one of the most potent recombinant viral vectors for eliciting T cell-mediated immunity in mice and humans. Here we show that vaccination with rAd5 expressing SAG1 (AdSAG1), but neither SAG2 nor SAG3, induces protective immunity in the highly susceptible C57BL/6 mice challenged with T. gondii. Furthermore, we evaluated different immunological components involved on viral induced protective immunity. We observed that host protection elicited by AdSAG1 is highly dependent on IL-12, IFN-γ and CD8(+) T lymphocytes. Importantly, the induction of protective immunity (T cell-derived IFN-γ) was also dependent on Myeloid Differentiation Factor 88 (MyD88), and thus, likely to involve Toll-like Receptors. We conclude that protective parasite specific-CD8(+) T cells are elicited by a mechanism that involves MyD88-dependent induction of IL-12.

Construction of Neospora caninum stably expressing TgSAG1 and evaluation of its protective effects against Toxoplasma gondii infection in mice

Toxoplasma gondii and Neospora caninum are closely related apicomplexan parasites. The surface antigen 1 of T. gondii (TgSAG1) is a major immunodominant antigen and, therefore, is considered to be a good candidate for the development of an effective recombinant vaccine against toxoplasmosis. In this study, N. caninum stably expressing the TgSAG1 gene (Nc/TgSAG1) was constructed using pyrimethamine-resistant DHFR-TS and GFP genes as double-selection markers. The expression level, molecular weight, and antigenic property of recombinant TgSAG1 expressed by the Nc/TgSAG1 were similar to those of the native TgSAG1. The mice immunized with Nc/TgSAG1 induced TgSAG1-specific Th1-dominant immune responses and protected the mice from a lethal challenge infection with T. gondii. These results indicate that N. caninum may provide a new tool for the production of a live recombinant vector vaccine against toxoplasmosis in animals. To our knowledge, this is the first report to evaluate the usefulness of N. caninum-based live vaccine.

Effect of codon optimization and subcellular targeting on Toxoplasma gondii antigen SAG1 expression in tobacco leaves to use in subcutaneous and oral immunization in mice

BACKGROUND

Codon optimization and subcellular targeting were studied with the aim to increase the expression levels of the SAG178-322 antigen of Toxoplasma gondii in tobacco leaves. The expression of the tobacco-optimized and native versions of the SAG1 gene was explored by transient expression from the Agrobacterium tumefaciens binary expression vector, which allows targeting the recombinant protein to the endoplasmic reticulum (ER) and the apoplast. Finally, mice were subcutaneously and orally immunized with leaf extracts-SAG1 and the strategy of prime boost with rSAG1 expressed in Escherichia coli was used to optimize the oral immunization with leaf extracts-SAG1.

RESULTS

Leaves agroinfiltrated with an unmodified SAG1 gene accumulated 5- to 10-fold more than leaves agroinfiltrated with a codon-optimized SAG1 gene. ER localization allowed the accumulation of higher levels of native SAG1. However, no significant differences were observed between the mRNA accumulations of the different versions of SAG1. Subcutaneous immunization with leaf extracts-SAG1 (SAG1) protected mice against an oral challenge with a non-lethal cyst dose, and this effect could be associated with the secretion of significant levels of IFN-gamma. The protection was increased when mice were ID boosted with rSAG1 (SAG1+boost). This group elicited a significant Th1 humoral and cellular immune response characterized by high levels of IFN-gamma. In an oral immunization assay, the SAG1+boost group showed a significantly lower brain cyst burden compared to the rest of the groups.

CONCLUSION

Transient agroinfiltration was useful for the expression of all of the recombinant proteins tested. Our results support the usefulness of endoplasmic reticulum signal peptides in enhancing the production of recombinant proteins meant for use as vaccines. The results showed that this plant-produced protein has potential for use as vaccine and provides a potential means for protecting humans and animals against toxoplasmosis.

Prime and boost immunization with influenza and adenovirus encoding the Toxoplasma gondii surface antigen 2 (SAG2) induces strong protective immunity

In this work, we explored an original vaccination protocol using recombinant influenza and adenovirus. We constructed recombinant influenza viruses harboring dicistronic NA segments containing the surface antigen 2 (SAG2) from Toxoplasma gondii under control of the duplicated 3' promoter. Recombinant influenza viruses were able to drive the expression of the foreign SAG2 sequence in cell culture and to replicate efficiently both in cell culture and in lungs of infected mice. In addition, mice primed with recombinant influenza virus and boosted with a recombinant adenovirus encoding SAG2 elicited both humoral and cellular immune responses specific for SAG2. Moreover, when immunized animals were challenged with the cystogenic P-Br strain of T. gondii, they displayed up to 85% of reduction in parasite burden. These results demonstrate the potential use of recombinant influenza vectors harboring the dicistronic segments in the development of vaccines against infectious diseases.

Vaccines against Toxoplasma gondii: challenges and opportunities

Development of vaccines against Toxoplasma gondii infection in humans is of high priority, given the high burden of disease in some areas of the world like South America, and the lack of effective drugs with few adverse effects. Rodent models have been used in research on vaccines against T. gondii over the past decades. However, regardless of the vaccine construct, the vaccines have not been able to induce protective immunity when the organism is challenged with T. gondii, either directly or via a vector. Only a few live, attenuated T. gondii strains used for immunization have been able to confer protective immunity, which is measured by a lack of tissue cysts after challenge. Furthermore, challenge with low virulence strains, especially strains with genotype II, will probably be insufficient to provide protection against the more virulent T. gondii strains, such as those with genotypes I or II, or those genotypes from South America not belonging to genotype I, II or III. Future studies should use animal models besides rodents, and challenges should be performed with at least one genotype II T. gondii and one of the more virulent genotypes. Endpoints like maternal-foetal transmission and prevention of eye disease are important in addition to the traditional endpoint of survival or reduction in numbers of brain cysts after challenge.