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

Comparison of the immune response and protection against the experimental Toxoplasma gondii infection elicited by immunization with the recombinant proteins BAG1, ROP8, and BAG1-ROP8

Toxoplasmosis is one of the most dangerous zoonotic diseases, causing serious economic losses worldwide due to abortion and reproductive problems. Vaccination is the best way to prevent disease; thus, it is imperative to develop a candidate vaccine for toxoplasmosis. BAG1 and ROP8 have the potential to become vaccine candidates. In this study, rTgBAG1, rTgROP8, and rTgBAG1-rTgROP8 were used to evaluate the immune effect of vaccines in each group by detecting the humoral and cellular immune response levels of BABL/c mice after immunization and the ability to resist acute and chronic infection with Toxoplasma gondii (T. gondii). We divided the mice into vaccine groups with different proteins, and the mice were immunized on days 0, 14, and 28. The protective effects of different proteins against T. gondii were analysed by measuring the cytokines, serum antibodies, splenocyte proliferation assay results, survival time, and number and diameter of brain cysts of mice after infection. The vaccine groups exhibited substantially higher IgG, IgG1, and IgG2a levels and effectively stimulated lymphocyte proliferation. The levels of IFN-γ and IL-2 in the vaccine group were significantly increased. The survival time of the mice in each vaccine group was prolonged and the diameter of the cysts in the vaccine group was smaller; rTgBAG1-rTgROP8 had a better protection. Our study showed that the rTgBAG1, rTgROP8, and rTgBAG1-rTgROP8 recombinant protein vaccines are partial but effective approaches against acute or chronic T. gondii infection. They are potential candidates for a toxoplasmosis vaccine.

Toxoplasma rhoptry proteins that affect encephalitis outcome

Toxoplasma gondii, a widespread obligate intracellular parasite, can infect almost all warm-blooded animals, including humans. The cellular barrier of the central nervous system (CNS) is generally able to protect the brain parenchyma from infectious damage. However, T. gondii typically causes latent brain infections in humans and other vertebrates. Here, we discuss how T. gondii rhoptry proteins (ROPs) affect signaling pathways in host cells and speculate how this might affect the outcome of Toxoplasma encephalitis.

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.

Toxoplasmosis vaccines: what we have and where to go?

Despite recent major advances in developing effective vaccines against toxoplasmosis, finding new protective vaccination strategies remains a challenging and elusive goal as it is critical to prevent the disease. Over the past few years, various experimental approaches have shown that developing an effective vaccine against T. gondii is achievable. However, more remains unknown due to its complicated life cycle, difficulties in clinical translation, and lack of a standardized platform. This minireview summarizes the recent advances in the development of T. gondii vaccines and the main obstacles to developing a safe, effective and durable T. gondii vaccine. The successes and failures in developing and testing vaccine candidates for the T. gondii vaccine are also discussed, which may facilitate the future development of T. gondii vaccines.

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.

REVIEW OF DNA VACCINE APPROACHES AGAINST THE PARASITE TOXOPLASMA GONDII

Toxoplasma gondii is an apicomplexan parasite that affects both humans and livestock. Transmitted to humans through ingestion, it is the second-leading cause of foodborne illness-related death. Currently, there exists no approved vaccine for humans or most livestock against the parasite. DNA vaccines, a type of subunit vaccine which uses segments of the pathogen's DNA to generate immunity, have shown varying degrees of experimental efficacy against infection caused by the parasite. This review compiles DNA vaccine efforts against Toxoplasma gondii, segmenting the analysis by parasite antigen, as well as a review of concomitant adjuvant usage. No single antigenic group was consistently more effective within in vivo trials relative to others.

Immunoinformatic analysis of immunogenic B- and T-cell epitopes of MIC4 protein to designing a vaccine candidate against Toxoplasma gondii through an in-silico approach

Purpose

Toxoplasmosis, transmitted by Toxoplasma gondii, is a worldwide parasitic disease that affects approximately one-third of the world's inhabitants. Today, there are no appropriate drugs to deter tissue cysts from developing in infected hosts. So, developing an effective vaccine would be valuable to avoid from toxoplasmosis. Considering the role of microneme antigens such as microneme protein 4 (MIC4) in T. gondii pathogenesis, it can be used as potential candidates for vaccine against T. gondii.

Materials and Methods

In this study several bioinformatics methods were used to assess the different aspects of MIC4 protein such as secondary and tertiary structure, physicochemical characteristics, the transmembrane domains, subcellular localization, B-cell, helper-T lymphocyte, cytotoxic-T lymphocyte epitopes, and other notable characteristic of this protein design a suitable vaccine against T. gondii.

Results

The studies revealed that MIC4 protein includes 59 potential post-translational modification sites without any transmembrane domains. Moreover, several probable epitopes of B- and T-cells were detected for MIC4. The secondary structure comprised 55.69% random coil, 5.86% beta-turn, 19.31% extended strand, and 19.14% alpha helix. According to the Ramachandran plot results, 87.42% of the amino acid residues were located in the favored, 9.44% in allowed, and 3.14% in outlier regions. The protein allergenicity and antigenicity revealed that it was non-allergenic and antigenic.

Conclusion

This study gives vital basic on MIC4 protein for further research and also established an effective vaccine with different techniques against acute and chronic toxoplasmosis.

In Silico Prediction of T and B Cell Epitopes of SAG1-Related Sequence 3 (SRS3) Gene for Developing Toxoplasma gondii Vaccine

: Toxoplasmosis is a worldwide infection that can lead to serious problems in immune-compromised individuals and fetuses. A DNA vaccine strategy would be an ideal tool against Toxoplasma gondii. One of the necessary measures to provide an effective vaccine is the selection of proteins with high antigenicity. The SAG1-related sequence 3 (SRS3) protein is a major surface antigen in T. gondii that can be used as a vaccine candidate. In the present study, bioinformatics and computational methods were utilized to predict protein characteristics, as well as secondary and tertiary structures. The in silico approach is highly suited to analyze, design, and evaluate DNA vaccine strategies. Hence, in silico prediction was used to identify B and T cell epitopes and compare the antigenicity of SRS3 and other candidate genes of Toxoplasma previously applied in the production of vaccines. The results of the analysis theoretically showed that SRS3 has multiple epitopes with high antigenicity, proposing that SRS3 is a promising immunogenic candidate for the development of DNA vaccines against toxoplasmosis.

Majid R. A Review on the Prevalence of Toxoplasma gondii in Humans and Animals Reported in Malaysia from 2008-2018

Toxoplasmosis is a disease caused by the protozoan parasite Toxoplasma gondii (T. gondii). Human toxoplasmosis seroprevalence in Malaysia has increased since it was first reported in 1973 as shown in previous reviews of 1991 and 2007. However, over a decade since the last review, comprehensive data on toxoplasmosis in Malaysia is lacking. This work aimed at reviewing articles on toxoplasmosis research in Malaysia in order to identify the research gaps, create public awareness, and efforts made so far and proffer management options on the disease. The present review examines the available published research articles from 2008 to 2018 related to toxoplasmosis research conducted in Malaysia. The articles reviewed were retrieved from nine credible databases such as Web of Science, Google Scholar, ScienceDirect, PubMed, Scopus, Springer, Wiley online library, Ovid, and Cochrane using the keywords; Malaysia, toxoplasmosis, Toxoplasma gondii, toxoplasma encephalitis, seroprevalence, human immunodeficiency virus (HIV) patients, pregnant women, genotype strain, anti-toxoplasma antibodies, felines, and vaccine. The data highlighted seropositive cases from healthy community members in Pangkor Island (59.7%) and among migrant workers (57.4%) at alarming rates, as well as 42.5% in pregnant women. Data on animal seroprevalence were limited and there was no information on cats as the definitive host. Genetic characterization of Toxoplasma gondii from HIV patients; pregnant women, and domestic cats is lacking. This present review on toxoplasmosis is beneficial to researchers, health workers, animal health professionals, and policymakers. Therefore, attention is required to educate and enlighten health workers and the general public about the risk factors associated with T. gondii infection in Malaysia.

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.

In silico analysis and expression of a novel chimeric antigen as a vaccine candidate against Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that causes one of the most common parasitic infections in humans and other warm-blooded animals. Currently, there are no effective treatments for inhibiting the formation of chronic tissue cysts in infected hosts. Thus, the development of a vaccine to protect against toxoplasmosis is an attractive option for avoiding infection. The aim of this study was to design an epitope-based vaccine for T. gondii. In the present study, an in silico approach was used to predict and analyze B-cell and T-cell epitopes and the transmembrane domain of proteins SAG1, MIC3, and ROP8. We also predicted the antigenicity, allergenicity, secondary and tertiary structures, and physicochemical characteristics of a chimeric protein. Next, codon optimization and mRNA structure prediction were conducted using bioinformatics tools, and the designed construct was chemically synthesized and cloned into the pET28a vector. SAG1 (amino acid positions 85-235), MIC3 (30-180), and ROP8 (85-185) were found to have several strong immunodominant epitopes that were joined with a rigid linker A(EAAAK)₂A. Although the resultant protein called MRS (MIC3, ROP8, and SAG1) did not turn out to be an allergen, its antigenicity was estimated to be 0.7983. Additionally, MRS was selected as the best vaccine candidate on the basis of its secondary and tertiary structures. The number of amino acids, molecular weight, and numbers of negatively and positively charged residues of MRS were 427 and 45,661.31 Da, 45, and 50, respectively. ΔG of the best-predicted structure was -413.0 kcal/mol, and the first nucleotides at the 5' end did not form a stable hairpin or pseudoknot. Finally, successful expression and verification of the expressed MRS protein showed that in silico analysis was almost accurate. This vaccine candidate selected by in silico tools should be validated in experimental studies.

A systematic review on efficiency of microneme proteins to induce protective immunity against Toxoplasma gondii

Toxoplasma gondii is an intracellular parasite infecting almost all warm-blooded animals. Many studies on vaccination have been performed previously, and micronemal proteins (MICs) have crucial importance in this regard. The current review aims to reveal the efficiency of MICs as target antigen, adjuvants, animal models (species/strain), T. gondii strains for challenge infection, and routes of vaccine to prevent Toxoplasma infection. A comprehensive literature search was performed on April 18, 2018, in several known databases. Studies were included when evaluating vaccines based on MIC against T. gondii compared to that of a control group. Two independent researchers done the search process, study choice, and data extraction. A total of 28 articles published were selected for further analysis. Among them, 57.03% of the studies focused on MIC3 and its epitopes. SAG1 was further used in cocktail vaccines compared to other antigens. GM-CSF and Freund's complete were the predominant adjuvants used. BALB/c mice have been introduced as a proper model for lethal challenge. Virulent T. gondii (RH) was utilized more than other strains for challenge. Among MICs, the results of vaccination with MIC1-4, MIC6, and PLP1 demonstrated significantly strong humoral and cellular immunity, increased survival time, and reduced cyst burden in the mice. This review summarizes the latest results on MIC-based vaccines and presents that the most effective vaccination procedure is the administration of the cocktail vaccines. Our survey can serve as a basis for further studies to develop more efficient novel vaccines against T. gondii for animals and humans.

Rhoptry antigens as Toxoplasma gondii vaccine target

Toxoplasmosis is a cosmopolitan zoonotic infection, caused by a unicellular protozoan parasite known as Toxoplasma gondii that belongs to the phylum Apicomplexa. It is estimated that over one-third of the world's population has been exposed and are latently infected with the parasite. In humans, toxoplasmosis is predominantly asymptomatic in immunocompetent persons, while among immunocompromised individuals may be cause severe and progressive complications with poor prognosis. Moreover, seronegative pregnant mothers are other risk groups for acquiring the infection. The life cycle of T. gondii is very complex, indicating the presence of a plurality of antigenic epitopes. Despite of great advances, recognize and construct novel vaccines for prevent and control of toxoplasmosis in both humans and animals is still remains a great challenge for researchers to select potential protein sequences as the ideal antigens. Notably, in several past years, constant efforts of researchers have made considerable advances to elucidate the different aspects of the cell and molecular biology of T. gondii mainly on microneme antigens, dense granule antigens, surface antigens, and rhoptry proteins (ROP). These attempts thereby provided great impetus to the present focus on vaccine development, according to the defined subcellular components of the parasite. Although, currently there is no commercial vaccine for use in humans. Among the main identified T. gondii antigens, ROPs appear as a putative vaccine candidate that are vital for invasion procedure as well as survival within host cells. Overall, it is estimated that they occupy about 1%–30% of the total parasite cell volume. In this review, we have summarized the recent progress of ROP-based vaccine development through various strategies from DNA vaccines, epitope or multi epitope-based vaccines, recombinant protein vaccines to vaccines based on live-attenuated vectors and prime-boost strategies in different mouse models.

Protective Efficacy Against Acute and Chronic Toxoplasma gondii Infection Induced by Immunization With the DNA Vaccine TgDOC2C

Toxoplasma gondii is a ubiquitous intracellular apicomplexan parasite that can cause zoonotic toxoplasmosis. Effective vaccines against T. gondii infection are necessary to prevent and control the spread of toxoplasmosis. The present study analyzed the B-linear epitopes of T. gondii DOC2 (TgDOC2) protein and then cloned the C-terminus of the TgDOC2 gene (TgDOC2C) to construct the pVAX-TgDOC2C eukaryotic vector. After intramuscular injection of pVAX-TgDOC2C, immune responses were monitored. Two weeks after the last immunization, the protective effects of pVAX-TgDOC2C against acute and chronic toxoplasmosis were evaluated by challenges with T. gondii RH tachyzoites (genotype I) and PRU cysts (genotype II). The DNA vaccine elicited strong humoral and cellular immune responses with high levels of IgG antibody, IL-2 and IFN-γ production compared to those of the controls. The percentage of CD4+ and CD8+ T cells in mice immunized with pVAX-TgDOC2C was significantly increased compared to that of mice injected with empty pVAX I or PBS. After acute infection with 10³ lethal tachyzoites, mice immunized with pVAX-TgDOC2C survived longer (12.5 days) than mice treated with pVAX I (8 days) and PBS (7.5 days). Mice immunized with pVAX-TgDOC2C had significantly less brain cysts (1600.83 ± 284.61) compared to mice immunized with pVAX I (3016.67 ± 153.84) or PBS (3100 ± 246.98). Together, these results demonstrated that TgDOC2C confers protective immunity against T. gondii infection and may be a promising candidate antigen for further development of an effective multicomponent vaccine for veterinary use against toxoplasmosis in livestock animals.

A systematic review of Toxoplasma gondii antigens to find the best vaccine candidates for immunization

At present, there is not any available accepted vaccine for prevention of Toxoplasma gondii (T. gondii) in human and animals. We conducted literature search through English (Google Scholar, PubMed, Science Direct, Scopus, EBSCO, ISI Web of Science) scientific paper databases to find the best vaccine candidates against toxoplasmosis among T. gondii antigens. Articles with information on infective stage, pathogenicity, immunogenicity and characterization of antigens were selected. We considered that the ideal and significant vaccines should include different antigens and been expressed in all infective stages of the parasite with a high pathogenicity and immunogenicity. Evaluation within this systematic review indicates that MIC 3, 4, 13, ROP 2, RON 5, GRA 1, 6, 8, 14 are expressed in all three infective stages and have pathogenicity and immunogenicity. MIC 5, ROM 4, GRA 2, 4, 15, ROP 5, 16, 17, 38, RON 4, MIC 1, GRA 10, 12, 16, SAG 3 are expressed in only tachyzoites and bradyzoites stages of T. gondii with pathogenicity/immunogenicity. Some antigens appeared to be expressed in a single stage (tachyzoites) but have high pathogenicity and induce immune response. They include enolase2 (ENO2), SAG 1, SAG5D, HSP 70, ROM 1, ROM 5, AMA 1, ROP 18, RON2 and GRA 24. In conclusion, current vaccination against T. gondii infection is not satisfactory, and with the increasing number of high-risk individuals, the development of an effective and safe specific vaccine is greatly valuable for toxoplasmosis prevention. This systematic review reveals prepare candidates for immunization studies.

Vaccination against Toxoplasma gondii using rhoptry antigens: a systematic review

Toxoplasma gondii is an obligate intracellular parasitic protozoan that infects a wide variety of vertebrates as intermediate hosts. The aim of the current systematic review study is to clarify the latest status of studies in the literature regarding rhoptry-associated recombinant proteins or rhoptry-associated recombinant DNAs as potential vaccines against toxoplasmosis. The search was performed systematically in 8 databases, four in English and four in Persian, up to February 2017. Overall, ROP2 was the most commonly used ROPs in DNA vaccines (27.27%) and protein vaccines (6.81%). Furthermore, regarding the type of adjuvants, route and dose of vaccination, animal models, challenge methods, and measurement of immune responses has been discussed in the text. It is hoped that this article help researchers to conduct more effective studies in the field of immunization against T. gondii.

A Gene-Based Positive Selection Detection Approach to Identify Vaccine Candidates Using Toxoplasma gondii as a Test Case Protozoan Pathogen

Over the last two decades, various in silico approaches have been developed and refined that attempt to identify protein and/or peptide vaccines candidates from informative signals encoded in protein sequences of a target pathogen. As to date, no signal has been identified that clearly indicates a protein will effectively contribute to a protective immune response in a host. The premise for this study is that proteins under positive selection from the immune system are more likely suitable vaccine candidates than proteins exposed to other selection pressures. Furthermore, our expectation is that protein sequence regions encoding major histocompatibility complexes (MHC) binding peptides will contain consecutive positive selection sites. Using freely available data and bioinformatic tools, we present a high-throughput approach through a pipeline that predicts positive selection sites, protein subcellular locations, and sequence locations of medium to high T-Cell MHC class I binding peptides. Positive selection sites are estimated from a sequence alignment by comparing rates of synonymous (dS) and non-synonymous (dN) substitutions among protein coding sequences of orthologous genes in a phylogeny. The main pipeline output is a list of protein vaccine candidates predicted to be naturally exposed to the immune system and containing sites under positive selection. Candidates are ranked with respect to the number of consecutive sites located on protein sequence regions encoding MHCI-binding peptides. Results are constrained by the reliability of prediction programs and quality of input data. Protein sequences from Toxoplasma gondii ME49 strain (TGME49) were used as a case study. Surface antigen (SAG), dense granules (GRA), microneme (MIC), and rhoptry (ROP) proteins are considered worthy T. gondii candidates. Given 8263 TGME49 protein sequences processed anonymously, the top 10 predicted candidates were all worthy candidates. In particular, the top ten included ROP5 and ROP18, which are T. gondii virulence determinants. The chance of randomly selecting a ROP protein was 0.2% given 8263 sequences. We conclude that the approach described is a valuable addition to other in silico approaches to identify vaccines candidates worthy of laboratory validation and could be adapted for other apicomplexan parasite species (with appropriate data).

Evaluation of the Protective Effect of Deoxyribonucleic Acid Vaccines Encoding Granule Antigen 2 and 5 Against Acute Toxoplasmosis in BALB/c Mice

AbstractToxoplasma gondii infects a broad range of warm-blooded hosts, including humans. Important clinical manifestations include encephalitis in immunocompromised patients as well as miscarriage and fetal damage during early pregnancy. Toxoplasma gondii dense granule antigen 2 and 5 (GRA2 and GRA5) are essential for parasitophorous vacuole development of the parasite. To evaluate the potential of GRA2 and GRA5 as recombinant DNA vaccine candidates, these antigens were cloned into eukaryotic expression vector (pcDNA 3.1C) and evaluated in vaccination experiments. Recombinant DNA vaccines constructed with genes encoding GRAs were validated in Chinese hamster ovary cells before evaluation using lethal challenge of the virulent T. gondii RH strain in BALB/c mice. The DNA vaccines of pcGRA2 and pcGRA5 elicited cellular-mediated immune response with significantly higher levels of interferon-gamma, interleukin-2 (IL-2), IL-4, and IL-10 (P < 0.05) compared with controls. A mixed T-helper cell 1 (Th1)/Th2 response was associated with slightly prolonged survival. These findings provide evidence that DNA vaccination with GRA2 and GRA5 is associated with Th1-like cell-mediated immune responses. It will be worthwhile to construct recombinant multiantigen combining full-length GRA2 or/and GRA5 with various antigenic proteins such as the surface antigens and rhoptry antigens to improve vaccination efficacy.

Vaccination with toxofilin DNA in combination with an alum-monophosphoryl lipid A mixed adjuvant induces significant protective immunity against Toxoplasma gondii

BACKGROUND

A widely prevalent disease, toxoplasmosis poses serious health threats to both humans and animals; therefore, development of an ideal DNA vaccine against Toxoplasma gondii is needed eagerly. The purpose of the present study is to assess the protective efficacy of a DNA vaccine encoding the T. gondii toxofilin gene (pEGFP-toxofilin). In addition, toxofilin DNA vaccine combined with the individual adjuvants, alum or monophosphoryl lipid A (MPLA), or a mixture of alum-MPLA adjuvant were screened for their ability to enhance antibody responses.

METHODS

Using bioinformatics, we analyzed the gene and amino acid sequences of the toxofilin protein, recognizing and identifying several potential linear B and T helper (Th)-1 cell epitopes. BALB/c mice were immunized three times with either toxofilin DNA vaccine alone or in combination with the adjuvants such as alum, MPLA or an alum-MPLA mixture. The systemic immune response was evaluated by cytokine, the percentage of CD4 (+) and CD8 (+) T cells and specific antibody measurement. Two weeks after the last immunization, protective efficacy was evaluated by challenging intraperitoneally with 1 × 10⁴ tachyzoites of T. gondii or intragastrically with 20 cysts of T. gondii PRU strain.

RESULTS

All experimentally immunized mice developed strong humoral and cellular immune responses compared with the control groups. Moreover, by comparison with the non-adjuvant toxofilin DNA vaccine group, adding alum adjuvant to toxofilin DNA vaccine resulted in an increase in humoral response and a skewed Th2 response. However, the MPLA adjuvant with toxofilin DNA vaccine induced significantly enhanced humoral and Th1-biased immune responses. Importantly, the co-administration of alum-MPLA adjuvant in combination with the toxofilin DNA vaccine shifted the Th2 immune response to a Th1 response compared with the alum-toxofilin group, and elicited the strongest humoral and Th1 responses among all the groups. At the same time, a longer survival time and less cyst amounts against T. gondii infection were also observed in the alum-MPLA-toxofilin group in comparison with single or no adjuvant groups.

CONCLUSIONS

Toxoplasma gondii toxofilin is a promising vaccine candidate that warrants further development. Co-administration of the alum-MPLA adjuvant mixture with DNA vaccine could effectively enhance immunogenicity and strongly skew the cellular immune response towards a Th1 phenotype.

Recombinant canine adenovirus type-2 expressing TgROP16 provides partial protection against acute Toxoplasma gondii infection in mice

We previously demonstrated that the survival time of BALB/c mice challenged with Toxoplasma gondii RH strain was prolonged by immunising the mice with a eukaryotic vector expressing the protein ROP16 of T. gondii. Building upon previous findings, we are exploring improved vaccination strategies to enhance protection. In this work, a novel recombinant canine adenovirus type 2 expressing ROP16 (CAV-2-ROP16) of T. gondii was constructed and identified to express ROP16 in Madin-Darby canine kidney cells (MDCK) cells by western blot (WB) and indirect immunofluorescence (IFA) assays. Intramuscular immunisation of BALB/c mice with CAV-2-ROP16 was performed to evaluate the humoral and cellular immune responses. This vaccination triggered significant humoral and cellular responses, including ROP16-stimulated lymphoproliferation (P<0.05). Compared to control groups, the CAV-2-ROP16 immunised mice had high production of IFN-γ, IL-2 and IL-12 (P<0.05), with a predominance of IgG2a production, but not IL-10 (P>0.05), revealing that a predominant Th1-type response had developed. The cell-mediated cytotoxic activity with high levels of IFN-γ and TNF-α was significantly increased in both CD4⁺ and CD8⁺ T-cell compartments in the mice immunised with CAV-2-ROP16 (P<0.05), compared to three control groups. In addition, when immunised mice were challenged with the RH strain of T. gondii, they showed a significantly increased survival rate (25%) 80days post infection compared with control mice that all died within seven days (P<0.05). The 25% protection rate elicited by the recombinant virus CAV-2-ROP16 has not been achieved in the field of anti-T. gondii vaccination until now. Our work presents the successful use of recombinant virus CAV-2-ROP16 in vaccination protocols to protect against intraperitoneal challenge with the virulent RH strain of T. gondii. This system was shown to be extremely efficient in eliciting humoral and cellular immune responses that led to a significant improvement in survival time in mice.

Induction of Protective Immunity against Toxoplasmosis in BALB/c Mice Vaccinated with Toxoplasma gondii Rhoptry-1

Toxoplasma gondii is the causative agent for toxoplasmosis. The rhoptry protein 1 (ROP1) is secreted by rhoptry, an apical secretory organelle of the parasite. ROP1 plays an important role in host cell invasion. In this study, the efficacy of ROP1 as a vaccine candidate against toxoplasmosis was evaluated through intramuscular or subcutaneous injection of BALB/c mice followed by immunological characterization (humoral- and cellular-mediated) and lethal challenge against virulent T. gondii RH strain in BALB/c mice. Briefly, a recombinant DNA plasmid (pVAX1-GFP-ROP1) was expressed in CHO cells while expression of recombinant ROP1 protein (rROP1) was carried out in Escherichia coli expression system. Immunization study involved injection of the recombinant pVAX1-ROP1 and purified rROP1 into different group of mice. Empty vector and PBS served as two different types of negative controls. Results obtained demonstrated that ROP1 is an immunogenic antigen that induced humoral immune response whereby detection of a protein band with expected size of 43 kDa was observed against vaccinated mice sera through western blot analysis. ROP1 antigen was shown to elicit cellular-mediated immunity as well whereby stimulated splenocytes with total lysate antigen (TLA) and rROP1 from pVAX1-ROP1 and rROP1-immunized mice, respectively, readily proliferated and secreted large amount of IFN-γ (712 ± 28.1 pg/ml and 1457 ± 31.19 pg/ml, respectively) and relatively low IL-4 level (94 ± 14.5 pg/ml and 186 ± 14.17 pg/ml, respectively). These phenomena suggested that Th1-favored immunity was being induced. Vaccination with ROP1 antigen was able to provide partial protection in the vaccinated mice against lethal challenge with virulent RH strain of tachyzoites. These findings proposed that the ROP1 antigen is a potential candidate for the development of vaccine against toxoplasmosis.

DNA vaccination with a gene encoding Toxoplasma gondii Rhoptry Protein 17 induces partial protective immunity against lethal challenge in mice

Toxoplasma gondii is an obligate intracellular apicomplexan parasite that affects humans and various vertebrate livestock and causes serious economic losses. To develop an effective vaccine against T. gondii infection, we constructed a DNA vaccine encoding the T. gondii rhoptry protein 17 (TgROP17) and evaluated its immune protective efficacy against acute T. gondii infection in mice. The DNA vaccine (p3×Flag-CMV-14-ROP17) was intramuscularly injected to BALB/c mice and the immune responses of the vaccinated mice were determined. Compared to control mice treated with empty vector or PBS, mice immunized with the ROP17 vaccine showed a relatively high level of specific anti-T. gondii antibodies, and a mixed IgG1/IgG2a response with predominance of IgG2a production. The immunized mice also displayed a specific lymphocyte proliferative response, a Th1-type cellular immune response with production of IFN-γ and interleukin-2, and increased number of CD8(+) T cells. Immunization with the ROP17 DNA significantly prolonged the survival time (15.6 ± 5.4 days, P < 0.05) of mice after challenge infection with the virulent T. gondii RH strain (Type I), compared with the control groups which died within 8 days. Therefore, our data suggest that DNA vaccination with TgROP17 triggers significant humoral and cellular responses and induces effective protection in mice against acute T. gondii infection, indicating that TgROP17 is a promising vaccine candidate against acute toxoplasmosis.

Protective efficacy of pVAX-RON5p against acute and chronic infections of Toxoplasma gondii in BALB/c mice

Toxoplasma gondii can infect all the warm-blooded animals and humans and causes serious diseases especially in immuno-compromised patients and pregnant women. Rhoptry neck proteins (RONs) play an important role in the formation of moving junction, which mediates the invasion of this parasite. A recombinant plasmid pVAX-RON5p, which can express part of RON5 protein in the eukaryocyte, was generated and used to immune BALB/c mice for evaluating the protective efficacy against the acute and chronic infections of T. gondii. Both humoral and cellular immune responses were evoked in mice by pVAX-RON5p immunization, and a slightly prolonged survival period was detected in the immunized group (7.6 ± 3.31 days) compared to the blank control (4.9 ± 0.32 days) after acute T. gondii infection (P < 0.05). For chronic infection of T. gondii, the number of cysts in the brain of pVAX-RON5p-immunized mice decreased 25.8% compared to blank control (P < 0.05). Our data suggested that RON5p DNA vaccine can induce partial protective immunity against acute and chronic T. gondii infections.

Immunity in the spleen and blood of mice immunized with irradiated Toxoplasma gondii tachyzoites

Toxoplasma gondii infection induces a strong and long-lasting immune response that is able to prevent most reinfections but allows tissue cysts. Irradiated, sterilized T. gondii tachyzoites are an interesting vaccine, and they induce immunity that is similar to infection, but without cysts. In this study, we evaluated the cellular immune response in the blood and spleen of mice immunized with this preparation by mouth (v.o.) or intraperitoneally (i.p.) and analyzed the protection after challenge with viable parasites. BALB/c mice were immunized with three i.p. or v.o. doses of irradiated T. gondii tachyzoites. Oral challenge with ten cysts of the ME-49 or VEG strain at 90 days after the last dose resulted in high levels of protection with low parasite burden in the immunized animals. There were higher levels of specific IgG, IgA and IgM antibodies in the serum, and the i.p. immunized mice had higher levels of the high-affinity IgG and IgM antibodies than the orally immunized mice, which had more high-affinity IgA antibodies. B cells (CD19(+)), plasma cells (CD138(+)) and the CD4(+) and CD8(+) T cell populations were increased in both the blood and spleen. Cells from the spleen of the i.p. immunized mice also showed antigen-induced production of interleukin-10 (IL-10), interferon gamma (IFN-γ) and interleukin 4 (IL-4). The CD4(+) T cells, B cells and likely CD8(+) T cells from the spleens of the i.p. immunized mice proliferated with a specific antigen. The protection was correlated with the spleen and blood CD8(+) T cell, high-affinity IgG and IgM and antigen-induced IL-10 and IL-4 production. Immunization with irradiated T. gondii tachyzoites induces an immune response that is mediated by B cells and CD4(+) and CD8(+) T cells, with increased humoral and cellular immune responses that are necessary for host protection after infection. The vaccine is similar to natural infection, but free of tissue cysts; this immunity restrains infection at challenge and can be an attractive and efficient model for vaccine development in toxoplasmosis.

Protective immunity against acute toxoplasmosis in BALB/c mice induced by a DNA vaccine encoding Toxoplasma gondii elongation factor 1-alpha

BACKGROUND

Toxoplasma gondii can infect almost all warm-blood animals including human beings. The high incidence and severe damage that can be caused by T. gondii infection clearly indicates the need for the development of a vaccine. T. gondii elongation factor 1-alpha (TgEF-1α) plays an important role in pathogenesis and host cell invasion for this parasite. The aim of this study was to evaluate the immune protective efficacy of a DNA vaccine encoding TgEF-1α gene against acute T. gondii infection in mice.

METHODS

A DNA vaccine (pVAX-EF-1α) encoding T. gondii EF-1a (TgEF-1α) gene was constructed and its immune response and protective efficacy against lethal challenge in BALB/c mice were evaluated.

RESULTS

Mice inoculated with the pVAX-EF-1α vaccine had a high level of specific anti-T. gondii antibodies and produced high levels of IFN-gamma, interleukin (IL)-4, and IL-17. The expression levels of MHC-I and MHC-II molecules as well as the percentages of both CD4(+) and CD8(+) T cells in mice vaccinated with pVAX-EF-1α were significantly increased (p < 0.05), compared with those in all the mice from control groups (blank control, PBS, and pVAXI). Immunization with pVAX-EF-1α significantly (p < 0.05) prolonged mouse survival time to 14.1 ± 1.7 days after challenge infection with the virulent T. gondii RH strain, compared with mice in the control groups which died within 8 days.

CONCLUSIONS

DNA vaccination with pVAX-EF-1α triggered strong humoral and cellular responses and induced effective protection in mice against acute T. gondii infection, indicating that TgEF-1α is a promising vaccine candidate against acute toxoplasmosis.

Recent advances in developing vaccines against Toxoplasma gondii: an update

Toxoplasma gondii, a significant public health risk, is able to infect almost all warm-blooded animals including humans, and it results in economic losses in production animals. In the last three years, a large number of vaccination experiments have been performed to control T. gondii infection, with the target of limiting the acute infection and reducing or eliminating tissue cysts in the intermediate hosts. In this paper, we summarize the latest results of the veterinary vaccines against T. gondii infection since 2013. Immunization with live-attenuated whole organisms of non-reverting mutants has been shown to induce remarkably potent immune responses associated with control of acute and chronic toxoplasmosis. The non-cyst-forming mutants are promising new tools for the development of veterinary vaccines against T. gondii infection.

Recent advances in Toxoplasma gondii immunotherapeutics

Toxoplasmosis is an opportunistic infection caused by the protozoan parasite Toxoplasma gondii. T. gondii is widespread globally and causes severe diseases in individuals with impaired immune defences as well as congenitally infected infants. The high prevalence rate in some parts of the world such as South America and Africa, coupled with the current drug treatments that trigger hypersensitivity reactions, makes the development of immunotherapeutics intervention a highly important research priority. Immunotherapeutics strategies could either be a vaccine which would confer a pre-emptive immunity to infection, or passive immunization in cases of disease recrudescence or recurrent clinical diseases. As the severity of clinical manifestations is often greater in developing nations, the development of well-tolerated and safe immunotherapeutics becomes not only a scientific pursuit, but a humanitarian enterprise. In the last few years, much progress has been made in vaccine research with new antigens, novel adjuvants, and innovative vaccine delivery such as nanoparticles and antigen encapsulations. A literature search over the past 5 years showed that most experimental studies were focused on DNA vaccination at 52%, followed by protein vaccination which formed 36% of the studies, live attenuated vaccinations at 9%, and heterologous vaccination at 3%; while there were few on passive immunization. Recent progress in studies on vaccination, passive immunization, as well as insights gained from these immunotherapeutics is highlighted in this review.

Toxoplasma gondii: protective immunity induced by rhoptry protein 9 (TgROP9) against acute toxoplasmosis

Toxoplasma gondii rhoptry protein 9 (ROP9) is involved in the early stages of host invasion, and contains B cell epitopes. The aim of this study was to evaluate the immune protective efficacy of a DNA vaccine encoding TgROP9 gene against acute T. gondii infection in mice. A DNA vaccine (pVAX-ROP9) encoding TgROP9 inserted into eukaryotic expression vector pVAX I was constructed, and the efficacy of intramuscular vaccination of Kunming mice with pVAX-ROP9 was analyzed. Mice immunized with pVAX-ROP9 induced a high level of specific anti-T. gondii antibodies, as well as a mixed IgG1/IgG2a response with predominance of IgG2a production. Also, injection of pVAX-ROP9 induced a specific lymphocyte proliferative responses and Th1-type cellular immune response with production of IFN-γ and interleukin-2. The percentages of CD4+ and CD8+ T cells were significantly increased in mice immunized with pVAX-ROP9, compared to empty vector, PBS or blank controls. Immunization with pVAX-ROP9 significantly (P<0.05) prolonged survival time (12.9±2.9days) after challenge infection with the virulent T. gondii RH strain (Type I), compared with the control groups which died within 6days. DNA vaccination with pVAX-ROP9 triggered strong humoral and cellular responses, and induced effective protection in mice against acute T. gondii infection, indicating that TgROP9 is a promising vaccine candidate against acute toxoplasmosis.

Mechanisms of cellular invasion by intracellular parasites

Numerous disease-causing parasites must invade host cells in order to prosper. Collectively, such pathogens are responsible for a staggering amount of human sickness and death throughout the world. Leishmaniasis, Chagas disease, toxoplasmosis, and malaria are neglected diseases and therefore are linked to socio-economical and geographical factors, affecting well-over half the world's population. Such obligate intracellular parasites have co-evolved with humans to establish a complexity of specific molecular parasite-host cell interactions, forming the basis of the parasite's cellular tropism. They make use of such interactions to invade host cells as a means to migrate through various tissues, to evade the host immune system, and to undergo intracellular replication. These cellular migration and invasion events are absolutely essential for the completion of the lifecycles of these parasites and lead to their for disease pathogenesis. This review is an overview of the molecular mechanisms of protozoan parasite invasion of host cells and discussion of therapeutic strategies, which could be developed by targeting these invasion pathways. Specifically, we focus on four species of protozoan parasites Leishmania, Trypanosoma cruzi, Plasmodium, and Toxoplasma, which are responsible for significant morbidity and mortality.

Protective immunity induced by a DNA vaccine-encoding Toxoplasma gondii microneme protein 11 against acute toxoplasmosis in BALB/c mice

Toxoplasma gondii is one of the most prevalent intracellular parasites and is threatening the health of both humans and animals, therefore causing incalculable economic losses worldwide. Vaccination is thought to be an efficient way of controlling toxoplasmosis. T. gondii microneme protein 11 (MIC11) is a soluble microneme protein which is presumably considered facilitating the early stage of cell invasion. To evaluate the protective efficacy of T. gondii MIC11, in the present study, a new DNA vaccine-encoding the α-chain of T. gondii MIC11 was constructed using the pcDNA3.1 vector. Expression of MIC11 from this vector was confirmed by indirect immunofluorescence assay following transfection into baby hamster kidney (BHK) cells. Intramuscular immunization of BALB/c mice with pcDNA/MIC11 was carried out to evaluate the immune responses by serum antibodies titers, lymphoproliferation assay, and cytokines assay. The protective efficacy was evaluated by survival rate in mice after challenging with highly virulent strain of T. gondii. The results demonstrated that this vaccination elicited significant humoral responses and T. gondii lysate antigen (TLA)-stimulated lymphoproliferation (p < 0.05). Compared to controls, the pcDNA/MIC11 immunized mice had high production of IFN-γ, IL-12, and IL-2 (p < 0.05), but not IL-4 (p > 0.05), indicating that a predominant Th1 type response was developed. The vaccination also increased the survival rate of immunized mice when they were challenged with a lethal dose of tachyzoites of T. gondii RH strain. These data suggest that T. gondii MIC11 is a reasonable vaccine candidate deserving further studies, and pcDNA/MIC11 is a potential strategy for the control of toxoplasmosis.