Vaccination concepts against Toxoplasma gondii

Evaluation of long-term immunity and protection against T. gondii after immunization with multivalent recombinant chimeric T. gondii proteins

Toxoplasmosis caused by the opportunistic, cosmopolitan protozoan Toxoplasma gondii is one of the most common parasitoses in the world. Although it may prove dangerous or even fatal for immunocompromised individuals, immunoprophylaxis for humans is still nonexistent. Thus, the aim of the current work was to assess the ability of two immunogenic recombinant chimeric T. gondii proteins, SAG2-GRA1-ROP1 (SGR) and SAG1-MIC1-MAG1-GRA2 (SMMG), selected in previous experiments to induce long-lasting immunity when administered with a safe adjuvant. Thus, the determination of immunological parameters and parasite challenge were performed both two weeks after the last boost injection and 6 months postvaccination. Both experimental vaccines triggered specific humoral and cellular responses in immunized C3H/HeOuJ male mice, characterized by the production of specific IgG (IgG1/IgG2a) antibodies in vivo and the synthesis of key Th1/Th2 cytokines by Toxoplasma lysate antigen-stimulated splenocytes in vitro. Although the levels of specific antibodies and cytokine release were in most cases lower six months postimmunization, the protection rates conferred by the vaccination were comparable regardless of the time after the administration of the last vaccine dose. The results indicate that both preparations induce long-lasting immunity, which makes them attractive candidates for further research aimed at boosting their immunogenicity and immunoprotective capacity.

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.

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 Immunogenic and Immunoprotective Activities of Recombinant Chimeric T. gondii Proteins Containing AMA1 Antigen Fragments

Toxoplasmosis, one of the most common parasitoses worldwide, is potentially dangerous for individuals with a weakened immune system, but specific immunoprophylaxis intended for humans is still lacking. Thus, efforts have been made to create an efficient universal vaccine for both animals and humans to overcome the shortcomings of currently used treatment methods and protect all hosts against toxoplasmosis. The current work represents a relatively new approach to vaccine development based on recombinant chimeric Toxoplasma gondii antigens. In the present research, three tetravalent chimeric proteins containing different portions of the parasite’s AMA1 antigen—AMA1^domainI-SAG2-GRA1-ROP1_L (A^NSGR), AMA1^{domainsII,III}-SAG2-GRA1-ROP1_L (A^CSGR) and AMA1^fullprotein-SAG2-GRA1-ROP1_L (A^FSGR)—were tested for their immunogenic and immunoprotective capacities. All tested proteins were immunogenic, as evidenced by the triggering of specific humoral and cellular immune responses in vaccinated C3H/HeOuJ mice, defined by the production of specific IgG (IgG1/IgG2a) antibodies in vivo and synthesis of key Th1/Th2 cytokines by Toxoplasma lysate antigen-stimulated splenocytes in vitro. Although all tested preparations provided partial protection against chronic toxoplasmosis in immunized and T. gondii-challenged mice, the intensity of the generated immunoprotection depended on the fragment of the AMA1 antigen incorporated into the chimeric antigen’s structure.

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.

A one health approach to vaccines against Toxoplasma gondii

Toxoplasmosis is a serious disease with global impact, now recognised as one of the most important food borne diseases worldwide and a major cause of production loss in livestock. A one health approach to develop a vaccination programme to tackle toxoplasmosis is an attractive and realistic prospect. Knowledge of disease epidemiology, parasite transmission routes and main risk groups has helped to target key host species and outcomes for a vaccine programme and these would be to prevent/reduce congenital disease in women and sheep; prevent/reduce T. gondii tissue cysts in food animal species and to prevent/reduce T. gondii oocyst shedding in cats. Most animals, including humans, develop good protective immunity following infection, involving cell mediated immune responses, which may explain why live vaccines are generally more effective to protect against T. gondii. Recent advances in our knowledge of parasite genetics and gene manipulation, strain variation, key antigenic epitopes, delivery systems and induction of immune responses are all contributing to the prospects of developing new vaccines which may be more widely applicable. A key area in progressing vaccine development is to devise standard vaccine efficacy models in relevant animal hosts and this is where a one health approach bringing together researchers across different disciplines can be of major benefit. The tools and technologies are in place to make a real impact in tackling toxoplasmosis using vaccination and it just requires a collective will to make it happen.

GRA24-Based DNA Vaccine Prolongs Survival in Mice Challenged With a Virulent Toxoplasma gondii Strain

Toxoplasma gondii causes infections in a wide range of intermediate hosts and remains a threatening disease worldwide because of the lack of effective drugs and vaccines. Dense granule protein 24 (GRA24) is a novel essential virulence factor that is transferred into the nucleus of host cells from the parasitophorous vacuole to regulate gene expression. In the present study, bioinformatic analysis showed that GRA24 had a high score for B-cell and T-cell epitopes compared with surface antigen 1 (SAG1), which has been studied as a promising vaccine candidate. As a DNA vaccine, pVAX1-GRA24 was injected intramuscularly into BALB/c mice and the induced immune response was evaluated. pVAX1-GRA24 induced high levels of a mixed Th1/Th2 cytokines at 6 weeks after immunization. Antibody determinations, cytokines [interferon gamma (IFN-γ), interleukin (IL)-12, IL-4, IL-10], antigen-specific lymphocyte proliferation, CD4+ and CD8+ T lymphocytes, and cytotoxic T lymphocyte activity showed that mice immunized with pVAX1-GRA24 produced specific humoral and cellular immune responses. The expression levels of interferon regulatory factor 8 (IRF8), nuclear factor kappa B (NF-κB), and T-Box 21 (T-bet) were significantly higher in the pVAX1-GRA24 immunization group than in the control groups. Survival times were prolonged significantly (24.6 ± 5.5 days) in the mice immunized with pVAX1-GRA24 compared with the mice in the control groups, which died within 7 days of T. gondii challenge (p < 0.05). The results of the present study showed that pVAX1-GRA24 induced a T. gondii-specific immune response and thus represents a promising candidate vaccine to treat toxoplasmosis.

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.

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.

Calcium-dependent protein kinases are potential targets for Toxoplasma gondii vaccine

Toxoplasma gondii belongs to the Apicomplexa phylum that caused a widespread zoonotic infection in wide range of intermediate hosts. Over one-third of the world's population are latently infected with T. gondii and carry it. The complex life cycle of T. gondii indicates the presence of a plurality of antigenic epitopes. During the recent years, continuous efforts of scientists have made precious advances to elucidate the different aspects of the cell and molecular biology of T. gondii. Despite of great progresses, the development of vaccine candidates for preventing of T. gondii infection in men and animals is still remains a challenge. The calcium-dependent protein kinases (CDPKs) belongs to the superfamily of kinases, which restricted to the apicomplexans, ciliates, and plants. It has been documented that they contribute several functions in the life cycle of T. gondii such as gliding motility, cell invasion, and egress as well as some other critical developmental processes. In current paper, we reviewed the recent progress concerning the development of CDPK-based vaccines against acute and chronic T. gondii.

Moving towards improved vaccines for Toxoplasma gondii

INTRODUCTION

Toxoplasma gondii is an intracellular parasitic protozoan that infects almost all warm-blooded animals and humans, resulting in threats to public health and economic losses. Despite continuous research efforts, there are still very few effective strategies against toxoplasmosis. In the past few years, numerous vaccination experiments have been performed to control T. gondii infection.

AREAS COVERED

In this review, the authors summarize the development of T. gondii vaccines with proper adjuvants, ranging from live or live-attenuated vaccines to protein vaccines, DNA vaccines, epitope vaccines and novel vaccines. They also highlight the challenges involved in the development of T. gondii vaccines, including specific impediments and shortcomings.

EXPERT OPINION

Moving towards the development of effective vaccines against T. gondii is not only a tedious mission but also a difficult challenge. Future studies should consider new approaches and strategies for vaccine development, particularly novel vaccines and genetic adjuvants, as well as optimizing immunization protocols and evaluation criteria.

rROP2 from Toxoplasma gondii as a potential vaccine against oocyst shedding in domestic cats

Abstract The aim of the present study was to evaluate oocyst shedding in cats immunized by nasal route with T. gondii proteins ROP2. Twelve short hair cats (Felis catus) were divided in three groups G1, G2 and G3 (n=4). Animals from G1 received 100 μg of rROP2 proteins plus 20 μg of Quil-A, G2 received 100 μg of BSA plus 20 μg of Quil-A, and the G3 only saline solution (control group). All treatments were done by intranasal route at days 0, 21, 42, and 63. The challenge was performed in all groups on day 70 with ≅ 800 tissue cysts of ME-49 strain by oral route. Animals from G1 shed less oocysts (86.7%) than control groups. ELISA was used to detect anti-rROP2 IgG and IgA, however, there were no correlation between number of oocyst shedding by either IgG or IgA antibody levels. In the present work, in spite of lesser oocysts production in immunized group than control groups, it was not possible to associate the use of rROP2 via nostrils with protection against oocyst shedding. For the future, the use of either other recombinant proteins or DNA vaccine, in combination with rROP2 could be tested to try improving the efficacy of this kind of vaccine.

Protective efficacy of recombinant canine adenovirus type-2 expressing TgROP18 (CAV-2-ROP18) against acute and chronic Toxoplasma gondii infection in mice

Background

The use of recombinant viral vectors expressing T. gondii antigens is a safe and efficient approach to induce immune responses against the parasite, as well as a valuable tool for vaccine development. We have previously prolonged the survival time of mice challenged with the RH strain of T. gondii by immunizing the mice with a eukaryotic vector expressing the protein ROP18 of T. gondii. We are now looking for ways to improve this vaccination strategy and enhance protection.

Methods

In this study, we constructed and characterized a novel recombinant canine adenovirus type 2 expressing ROP18 (CAV-2-ROP18) of T. gondii by cytopathic effect (CPE) and indirect immunofluorescence assay (IFA) following transfection into MDCK cells. Intramuscular immunization of Kunming mice with CAV-2-ROP18 was carried out to evaluate humoral and cellular immune responses.

Results

The vaccination of experimental mice with CAV-2-ROP18 elicited antibody production against ROP18, including high levels of a mixed IgG1/IgG2a and significant production of IFN-γ or IL-2, and displayed a significant bias towards a helper T cell type 1 (Th1) profile. Furthermore, the presence of T. gondii-specific IFN-γ-production and TNF-α-production T cells was elicited in both CD⁴⁺ and CD⁸⁺ T cell compartments. Significantly higher survival rates (40%) occurred in the experimental group, and a reduction in brain cyst burden was detected in vaccinated mice.

Conclusion

These results demonstrate the potential use of a CAV vector harboring the ROP18 gene in the development of a vaccine against acute and chronic toxoplasmosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-015-0815-1) contains supplementary material, which is available to authorized users.

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.

Intranasal immunisation with recombinant Toxoplasma gondii actin partly protects mice against toxoplasmosis

Toxoplasma gondii is a ubiquitous protozoan intracellular parasite, the causative agent of toxoplasmosis, and a worldwide zoonosis for which an effective vaccine is needed. Actin is a highly conserved microfilament protein that plays an important role in the invasion of host cells by T. gondii. This study investigated the immune responses elicited by BALB/c mice after nasal immunisation with a recombinant T. gondii actin (rTgACT) and the subsequent protection against chronic and lethal T. gondii infections. We evaluated the systemic response by proliferation, cytokine and antibody measurements, and we assessed the mucosal response by examining the levels of TgACT-specific secretory IgA (SIgA) in nasal, vaginal and intestinal washes. Parasite load was assessed in the liver and brain, and the survival of mice challenged with a virulent strain was determined. The results showed that the mice immunised with rTgACT developed high levels of specific anti-rTgACT IgG titres and a mixed IgG1/IgG2a response with a predominance of IgG2a. The systemic immune response was associated with increased production of Th1 (IFN-γ and IL-2), Th2 (IL-4) and Treg (IL-10) cytokines, indicating that not only Th1-type response was induced, but also Th2- and Treg-types responses were induced, and the splenocyte stimulation index (SI) was increased in the mice immunised with rTgACT. Nasal immunisation with rTgACT led to strong mucosal immune responses, as seen by the increased secretion of SIgA in nasal, vaginal and intestinal washes. The vaccinated mice displayed significant protection against lethal infection with the virulent RH strain (survival increased by 50%), while the mice chronically infected with RH exhibited lower liver and brain parasite loads (60.05% and 49.75%, respectively) than the controls. Our data demonstrate, for the first time, that actin triggers a strong systemic and mucosal response against T. gondii. Therefore, actin may be a promising vaccine candidate against toxoplasmosis.

DNA immunization with eukaryotic initiation factor-2α of Toxoplasma gondii induces protective immunity against acute and chronic toxoplasmosis in mice

Toxoplasma gondii infection is a serious health problem of humans and animals worldwide. T. gondii eukaryotic initiation factor-2α (TgIF2α) plays a crucial role in parasite viability and is an important virulence factor of T. gondii. To evaluate the vaccine potential of TgIF2α, we constructed a novel eukaryotic plasmid pVAX-IF2α expressing TgIF2α from the RH strain and validated expression and immunogenicity in vitro in the Marc145 cell expression system by indirect immunofluorescence (IFA). Administration of pVAX-IF2α intramuscularly induced specific humoral immune responses including high levels of specific TgIF2α IgG antibody and a mixed IgG1/IgG2a response with a predominance of IgG2a production. The cellular immune response was elicited, showing significant production of IFN-γ and IL-2 associated with Th1 type response, and thus strong cell-mediated cytotoxic activity with increased frequencies of IFN-γ parameters analyzed in both CD4(+) and CD8(+) T cell compartments (CD4(+) IFN-γ(+) T cells and CD8(+) IFN-γ(+) T cells). Immunization resulted in partial protection against acute and chronic toxoplamosis in outbred Kunming mice, demonstrated by a significantly prolonged survival time (15.9±4.6 days) after challenge with the virulent RH strain and significant reduction in brain cysts (44.1%) against chronic infection with PRU cyst in contrast to control mice. Our data suggested that pVAX-IF2α could be used as a DNA vaccine candidate against both acute and chronic T. gondii infection by the activation of effective humoral and cellular immune responses.

Vaccines against Toxoplasma gondii: new developments and perspectives

Toxoplasmosis caused by the protozoan Toxoplasma gondii is a major public health problem, infecting one-third of the world human beings, and leads to abortion in domestic animals. A vaccine strategy would be an ideal tool for improving disease control. Many efforts have been made to develop vaccines against T. gondii to reduce oocyst shedding in cats and tissue cyst formation in mammals over the last 20 years, but only a live-attenuated vaccine based on the S48 strain has been licensed for veterinary use. Here, the authors review the recent development of T. gondii vaccines in cats, food-producing animals and mice, and present its future perspectives. However, a single or only a few antigen candidates revealed by various experimental studies are limited by only eliciting partial protective immunity against T. gondii. Future studies of T. gondii vaccines should include as many CTL epitopes as the live attenuated vaccines.

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.

Vaccines against Toxoplasma gondii: status, challenges and future directions

Toxoplasma gondii is a ubiquitous protozoan parasite that can infect a wide range of animals including humans. This single known species in the genus Toxoplasma is considered as one of the most successful eukaryotic pathogens which is of major medical and veterinary importance. Effective vaccines may contribute toward preventing and controlling the spread of toxoplasmosis. The present communication addresses the current status of development of vaccines against T. gondii. Further discussion is made on the difficulties along with challenges, such as vaccine construct, mode of vaccine administration and standardization of immunization evaluation. Finally suggestions are made on possible directions for future research on the development of vaccines against T. gondii.

Humoral and cellular immune responses in pigs immunized intranasally with crude rhoptry proteins of Toxoplasma gondii plus Quil-A

We evaluated the humoral and cellular immune responses in pigs immunized intranasally with crude rhoptry proteins of Toxoplasma gondii plus Quil-A. The experiment used 13 mixed-breed pigs divided into the following three groups: G1 (vaccinated-challenged, n=6), which received the rhoptry vaccine (200(g/dose); G2 (adjuvant-challenged, n=4), which received PBS plus Quil-A; and G3 (unvaccinated-challenged, n=3), which was the control group. The treatments were performed intranasally at days 0, 21, and 42. Three pigs from G1 produced IgG and IgM antibody levels above the cut-off in the ELISA on the challenge day. Partial protection was observed in G1 at the chronic phase of infection when compared with G3. The preventable fractions were 41.6% and 6.5%, in G1 and G2, respectively. The results of this study suggest that rhoptry proteins plus Quil-A stimulated humoral, local, and systemic immune responses, which were able to partially protect the brain from cyst formation.