Vaccines against Toxoplasma gondii: new developments and perspectives

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.

Evaluation of immuno-efficacy of a novel DNA vaccine encoding Toxoplasma gondii rhoptry protein 38 (TgROP38) against chronic toxoplasmosis in a murine model

Background

Toxoplasma gondii is an obligate intracellular parasite which can infect almost all mammalian animals, leading to toxoplasmosis. T. gondii rhoptry protein 38 (TgROP38) is an active rhoptry protein kinase which is involved in the inhibitory effect on host cell transcription by down-regulating the MAPK signaling track.

Methods

TgROP38 gene was amplified and inserted into eukaryotic vector pVAX I and formed the DNA vaccine pVAX-ROP38. Mice in the experimental group were intramuscularly immunized with pVAX-ROP38 and those injected with pVAX I, PBS or nothing were treated as controls. After three injections at two week intervals, all mouse groups were challenged intraperitoneally with 1000 tachyzoites of the virulent T. gondii RH strain (Type I, ToxoDB #10) and 10 cysts of the PRU strain (Type II, ToxoDB #1), respectively.

Results

Mice inoculated with pVAX-ROP38 vaccine had a higher level of IgG antibodies (P < 0.01) and T lymphoproliferative response. The high ratio of IgG2a/IgG1 and the increasing levels of IFN-γ and IL-2 (P < 0.05) indicated an activated Th1 cell-mediated immune responses. Furthermore, the CD4⁺ and CD8⁺ proportions in vaccinated mice were also increased significantly compared with that in mice of the three control groups (P < 0.01). In the model of acute infection, the average survival time of mice in the pVAX-ROP38 group (8.1 days ± 0.75) was no statistically different compared to that in the PBS, pVAX I and blank control groups which died within 7 days. However, in the model of chronic infection, the brain cyst reduction in the pVAX-ROP38 group reached 76.6%, compared to controls (P < 0.01).

Conclusions

The present study revealed that the pVAX-ROP38 vaccine could elicit strong humoral and cell immunity response against chronic T. gondii infection in mice, resulting in the reduction of the brain cyst formation effectively, which suggests that TgROP38 is a desirable vaccine candidate against chronic T. gondii infection.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2334-14-525) contains supplementary material, which is available to authorized users.

Protective efficacy of Toxoplasma gondii calcium-dependent protein kinase 1 (TgCDPK1) adjuvated with recombinant IL-15 and IL-21 against experimental toxoplasmosis in mice

Background

Toxoplasma gondii can infect all warm-blooded animals including humans. Infection with T. gondii is probably the leading cause of posterior uveitis in humans and the most comment route of transmission is raw and undercooked meat from infected animals. T. gondii calcium-dependent protein kinase 1 (TgCDPK1) plays a critical role in direct parasite motility, host-cell invasion, and egress.

Methods

We constructed a DNA vaccine expressing TgCDPK1 inserted into eukaryotic expression vector pVAX I and evaluated the immune protection induced by pVAX-CDPK1 in Kunming mice. Mice immunized with pVAX-CDPK1 intramuscularly and/or with a plasmid encoding IL-15 and IL-21 (pVAX-IL-21-IL-15). The immune responses were analyzed including lymphoproliferative assay, cytokine, antibody measurements, lymphocyte surface markers by flow cytometry and protective efficacy were measured as survival and cysts numbers after challenge 1 to 2 months post vaccination.

Results

Immunization with pVAX-CDPK1 or pVAX-IL-21-IL-15 alone developed strong humoral responses and Th1 type cellular immune responses, and the significantly (P < 0.05) increase of both the percentages of CD4+ and CD8+ T cells compared with all the controls (blank control, PBS, and pVAX). Co-injection of pVAX-IL-21-IL-15 significantly increased humoral and cellular immune responses compared to the group of pVAX-CDPK1 or pVAX-IL-21-IL-15. Challenge experiments showed that co-administration of pVAX-IL-21-IL-15 and pVAX-CDPK1 significantly (P < 0.05) increased survival time (19.2 ± 5.1 days) compared with pVAX-CDPK1 (17.3 ± 4.3 days) or pVAX-IL-21-IL-15 (12.0 ± 2.0 days) alone, and pVAX-IL-21-IL-15 + pVAX-CDPK1 significantly reduced the number of brain cysts (72.7%) in contrast to pVAX-ROP13 (45.7%) or pVAX-IL-21-IL-15 alone (43.6%).

Conclusions

TgCDPK1 is identified to be a promising vaccine candidate for inducing a strong humoral and cellular response against T. gondii infection, and thus synergistic of mIL-21 and mIL-15 can induce non-specific immune responses, but also facilitate specific humoral as well as cellular immune responses elicited by DNA vaccine against acute and chronic T. gondii infection in mice.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2334-14-487) contains supplementary material, which is available to authorized users.

Advantages and disadvantages of conditional systems for characterization of essential genes in Toxoplasma gondii

The dissection of apicomplexan biology has been highly influenced by the genetic tools available for manipulation of parasite DNA. Here, we describe different techniques available for the generation of conditional mutants. Comparison of the advantages and disadvantages of the three most commonly used regulation systems: the tetracycline inducible system, the regulation of protein stability and site-specific recombination are discussed. Using some previously described examples we explore some of the pitfalls involved in gene-function analysis using these systems that can lead to wrong or over-interpretation of phenotypes. We will also mention different options to standardize the application of these techniques for the characterization of gene function in high-throughput.

Veterinary vaccines against toxoplasmosis

Toxoplasma gondii is a cosmopolitan protozoan parasite that infects a wide range of mammal and bird species. Common infection leads to high economic (e.g., abortions in sheep) and human (e.g., congenital toxoplasmosis or neurotoxoplasmosis in humans) losses. With one exception (Toxovax for sheep), there are no vaccines to prevent human or animal toxoplasmosis. The paper presents the current state and challenges in the development of a vaccine against toxoplasmosis, designed for farm animals either bred for consumption or commonly kept on farms and involved in parasite transmission. So far, the trials have mostly revolved around conventional vaccines and, compared with the research using laboratory animals (mainly mice), they have not been very numerous. However, the results obtained are promising and could be a good starting point for developing an effective vaccine to prevent toxoplasmosis.

Evaluation of immune responses in mice after DNA immunization with putative Toxoplasma gondii calcium-dependent protein kinase 5

Toxoplasma gondii can cause serious public health problems and economic losses worldwide. Calcium-dependent protein kinases (CDPKs) are key mediators of T. gondii signaling pathways and are implicated as important virulence factors. In the present study, we cloned a novel T. gondii CDPK gene, named TgCDPK5, and constructed the eukaryotic expression vector pVAX-CDPK5. Then, we evaluated the immune protection induced by pVAX-CDPK5 in Kunming mice. After injection of pVAX-CDPK5 intramuscularly, immune responses, determined with lymphoproliferative assays and cytokine and antibody measurements, were monitored, and mouse survival times and brain cyst formation were evaluated following challenges with the T. gondii RH strain (genotype I) and the PRU strain (genotype II). pVAX-CDPK5 effectively induced immune responses with increased specific antibodies, a predominance of IgG2a production, and a strong lymphocyte proliferative response. The levels of gamma interferon (IFN-γ), interleukin 2 (IL-2), and IL-12(p70) and the percentages of CD3(+) CD4(+) and CD3(+) CD8(+) cells in mice vaccinated with pVAX-CDPK5 were significantly increased. However, IL-4 and IL-10 were not produced in the vaccinated mice. These results demonstrate that pVAX-CDPK5 can elicit strong humoral and cellular Th1 immune responses. The survival time of immunized mice challenged with the T. gondii RH strain (8.67 ± 4.34 days) was slightly, but not significantly, longer than that in the control groups within 7 days (P > 0.05). The numbers of brain cysts in the mice in the pVAX-CDPK5 group were reduced by ∼40% compared with those in the control groups (P < 0.05), which provides a foundation for the further development of effective subunit vaccines against T. gondii.

Comparative efficacy of a multi-epitope DNA vaccine via intranasal, peroral, and intramuscular delivery against lethal Toxoplasma gondii infection in mice

Background

Toxoplasmosis is an important zoonosis, being a cause of congenital disease and abortion in animals and humans. DNA vaccination as a promising vaccine remains a challenge for an improved delivery system.

Methods

In this study, attenuated Salmonella typhimurium BRD509 was used to deliver a DNA vaccine encoding several epitopes, derived from the tachyzoite proteins SAG1, GRA1, ROP2, GRA4 and bradyzoite proteins SAG2C, SAG2X of Toxoplasma gondii and A₂/B subunit of cholera toxin. The recombinant plasmids were electroporated into attenuated Salmonella typhimurium. Humoral and cellular immune responses were evaluated for BALB/c mice administered with this attenuated recombinant Salmonella vaccine via the oral and nasal route or by intramuscular injection with DNA plasmid directly.

Results

High IgG levels were present in the mice immunized intramuscularly, while IgA levels were higher in the oral and nasal immunization groups. Furthermore, cellular immunity was activated in oral immunization groups with 60% survival rate following challenge with high virulent RH strain.

Conclusions

The results from this study indicate that a DNA vaccine encoding multi-epitopes of T. gondii delivered by attenuated Salmonella is promising.

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.

A comparative study between excretory/secretory and autoclaved vaccines against RH strain of Toxoplasma gondii in murine models

Toxoplasma gondii is an obligate intracellular protozoan that has a major importance in public health, in addition to veterinary medicine. Therefore, the development of an effective vaccine for controlling toxoplasmosis is an important goal. Excretory/secretory antigens (ESA), were previously identified as potential vaccine candidates, proved to play important roles in the pathogenesis and immune escape of the parasite. In addition, autoclaved Toxoplasma vaccine (ATV) is a special type of killed vaccine, recently characterized. The aim of the present work was, to compare between excretory/secretory and ATV against RH strain of T. gondii in mice based on; parasitological and histopathological levels. Tachyzoites were harvested from peritoneal exudates of infected mice and were used for challenge infection and vaccine preparation. BCG was used as an adjuvant. Mice were allocated equally into five groups; they were vaccinated intradermally over the sternum. The results of this study showed that the survival time after challenge, extended up to 16 days in ESA vaccinated group and up to 15 days in autoclaved Toxoplasma vaccinated group. ESA vaccinated group exhibited a profound decrease in parasite load following parasite challenge with a higher percentage of reduction in parasite count in all examined organs than the autoclaved Toxoplasma vaccinated group. The histopathological picture of the liver in both immunized groups, revealed marked reduction in the pathological changes observed as compared to controls, especially in ESA vaccinated group. It was concluded that vaccination with ESA showed more promising results versus ATV, as demonstrated by the survival rate of vaccinated mice, tachyzoites count and histopathological examination.

Protective immunity against Toxoplasma gondii induced by DNA immunization with the gene encoding a novel vaccine candidate: calcium-dependent protein kinase 3

Background

Toxoplasma gondii can infect almost all warm-blood animals including human beings. The plant-like calcium-dependent protein kinases (CDPKs) harbored by T. gondii are involved in gliding motility, cell invasion, egress and some other developmental processes, and so have been implicated as important virulence factors.

Methods

In the present study, we constructed a DNA vaccine expressing T. gondii CDPK3 (TgCDPK3) and evaluated its protective efficacy against T. gondii infection in Kunming mice. The gene sequence encoding TgCDPK3 was inserted into the eukaryotic expression vector pVAX I, and mice were immunized with pVAX-CDPK3 intramuscularly.

Results

The results showed that mice immunized with pVAX-CDPK3 developed a high level of specific antibodies and a strong lymphoproliferative response. The significantly increased levels of IFN-γ, IL-2, IL-12 (p70) and IL-23 and high ratio of IgG2a to IgG1 antibody titers indicated that a Th1 type response was elicited after immunization with pVAX-CDPK3. Furthermore, the percentage of CD4+ T cells in mice vaccinated with pVAX-CDPK3 was significantly increased. After lethal challenge with the tachyzoites of the virulent T. gondii RH strain, the mice immunized with pVAX-CDPK3 prolonged the survival time from 10 days to 24 days (13.5 ± 4.89) compared to untreated mice or those received PBS or pVAX I which died within 7 days (P < 0.05). In chronic infection model (10 cysts of the T. gondii PRU strain), the numbers of brain cysts of the mice immunized with pVAX-CDPK3 reduced significantly when compared with those in control groups (P < 0.05), and the rate of reduction could reach to about 50%.

Conclusions

TgCDPK3 can generate protective immunity against acute and chronic T. gondii infection in Kunming mice and is a promising vaccine candidate for further development of an effective vaccine against T. gondii.