Role of P30 in host immunity and pathogenesis of T. gondii infection

Enhancement of immune responses by vaccine potential of three antigens, including ROP18, MIC4, and SAG1 against acute toxoplasmosis in mice

Toxoplasma gondii (T. gondii) causes considerable financial losses in the livestock industry and can present serious threats to pregnant women, as well as immunocompromised patients. Therefore, it is required to design and produce an efficient vaccine for controlling toxoplasmosis. The present study aimed to evaluate the protective immunity induced by RMS protein (ROP18, MIC4, and SAG1) with Freund adjuvant, calcium phosphate nanoparticles (CaPNs), and chitosan nanoparticles (CNs) in BALB/c mice. The RMS protein was expressed in Escherichia coli (E. coli) and purified using a HisTrap HP column. Thereafter, cellular and humoral immunity was assessed by injecting RMS protein on days 0, 21, and 35 into four groups [RMS, RMS-chitosan nanoparticles (RMS-CNs), RMS-calcium phosphate nanoparticles (RMS-CaPNs), and RMS-Freund]. Phosphate buffered saline (PBS), CNs, CaPNs, and Freund served as the four control groups. The results displayed that vaccination with RMS protein and adjuvants significantly elicited the levels of specific IgG antibodies and cytokines against toxoplasmosis. There were high levels of total IgG, IgG2a, and IFN-γ in vaccinated mice, compared to those in the control groups, especially in the RMS-Freund, indicating a Th-1 type response. The vaccinated and control mice were challenged intraperitoneally with 1 × 10³ tachyzoites of the T. gondii RH strain four weeks after the last injection, and in RMS-Freund and RMS-CaPNs groups, the highest increase in survival time was observed (15 days). The RMS can significantly increase Th1 and Th2 responses; moreover, multi-epitope vaccines with adjuvants can be a promising strategy for the production of a vaccine against toxoplasmosis.

Protective efficacy by a novel multi-epitope vaccine, including MIC3, ROP8, and SAG1, against acute Toxoplasma gondii infection in BALB/c mice

Toxoplasma gondii is an intracellular apicomplexan parasite, which can cause a serious infectious disease in pregnant women and immunocompromised individuals. Therefore, the development of a polyvalent vaccine consisting of all stages of the parasite life cycle using the epitopes from tachyzoites, bradyzoites, and sporozoites is likely to be required for complete protective immunity. In this study, we designed protein vaccine candidate based on the prediction of specific epitopes (i.e., B cell and T cell) from three Toxoplasma gondii antigens. The MRS protein (MIC3: 30-180, ROP8: 85-185, and SAG1: 85-235) was expressed in Escherichia coli, and purification was performed using a HisTrap HP column and then we evaluated immunogenicity and protective property in BALB/c mice. Seventy-two mice were randomly divided into six groups, including three vaccinations (i.e., MRS, MRS-Freund, and MRS-Calcium Phosphate Nanoparticles (MRS-CaPNs)) and three control (i.e., Phosphate-buffered saline, Freund, and CaPNs) groups. All groups were immunized three times via subcutaneous injection within three-week intervals. In the vaccination groups, the BALB/c mice were injected with 20 μg of MRS protein for the first time and 10 μg of MRS for the next two times. Antibodies, cytokines, and splenocytes proliferation in the immunized mice were assayed using the enzyme-linked immunosorbent assay. Protective efficacy was analyzed by challenging the immunized mice with T. gondii of RH strain. Antibody, cytokine, and lymphocyte proliferation assays showed that the mice immunized with MRS induced stronger humoral and T helper type 1 cell-mediated immune responses, compared to the control mice. However, co-immunization with adjuvants (i.e., Freund and CaNPs) resulted in impaired immune responses. Effective protection against the parasite achieved an increase in survival time in the immunized mice, especially in the MRS-CaNPs group. The obtained results of the present study demonstrated that multi-epitope protein vaccination, MRS, is a potential strategy against toxoplasmosis infection. In addition, the vaccine co-delivered with CaPNs could provide an important key for vaccine candidate to control T. gondii infection.

SAG4 DNA and Peptide Vaccination Provides Partial Protection against T. gondii Infection in BALB/c Mice

Toxoplasma gondii can lead to congenital infections in human. Surface antigen protein 4 (SAG4) of T. gondii is a potential stimulator for humoral and cellular immune responses. In the present study, a DNA vaccine encoding SAG4 from T. gondii was constructed and used to immunize BALB/c mice with peptide to evaluate the protective efficacy of the vaccine. The productions of IgG antibodies and cytokines (gamma interferon) from the vaccine (pSAG4/peptide) group were significantly higher than pSAG4 or peptide groups. After a lethal challenge by 1 × 10⁴ tachyzoites from the I strain (RH), the survival time of mice immunized by pSAG4/peptide was longer than that of pSAG4 or peptide immunized mice or control mice. Moreover, after challenging by 20 cysts of the II strain (PRU) of T. gondii, the number of brain cysts from pSAG4/peptide vaccinated mice was only 31% of the number in PBS injected mice. The findings suggested the SAG4 DNA vaccine with peptide led significant immune responses and improved the protection against T. gondii challenges.

Research progress on surface antigen 1 (SAG1) of Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasitic protozoan that has a wide host range and causes a zoonotic parasitosis called toxoplasmosis. This infection causes significant morbidity, costs for care and loss of productivity and suffering. The most effective measures to minimize this parasite’s harm to patients are prompt diagnosis and treatment and preventing infection. A parasite surface antigen, SAG1, is considered an important antigen for the development of effective diagnostic tests or subunit vaccines. This review covers several aspects of this antigen, including its gene structure, contribution to host invasion, mechanisms of the immune responses and its applications for diagnosis and vaccine development. This significant progress on this antigen provides foundations for further development of more effective and precise approaches to diagnose toxoplasmosis in the clinic, and also have important implications for exploring novel measures to control toxoplasmosis in the near future.

Transcriptomic analysis of toxoplasma development reveals many novel functions and structures specific to sporozoites and oocysts

Sexual reproduction of Toxoplasma gondii occurs exclusively within enterocytes of the definitive felid host. The resulting immature oocysts are excreted into the environment during defecation, where in the days following, they undergo a complex developmental process. Within each oocyst, this culminates in the generation of two sporocysts, each containing 4 sporozoites. A single felid host is capable of shedding millions of oocysts, which can survive for years in the environment, are resistant to most methods of microbial inactivation during water-treatment and are capable of producing infection in warm-blooded hosts at doses as low as 1–10 ingested oocysts. Despite its extremely interesting developmental biology and crucial role in initiating an infection, almost nothing is known about the oocyst stage beyond morphological descriptions. Here, we present a complete transcriptomic analysis of the oocyst from beginning to end of its development. In addition, and to identify genes whose expression is unique to this developmental form, we compared the transcriptomes of developing oocysts with those of in vitro-derived tachyzoites and in vivo-derived bradyzoites. Our results reveal many genes whose expression is specifically up- or down-regulated in different developmental stages, including many genes that are likely critical to oocyst development, wall formation, resistance to environmental destruction and sporozoite infectivity. Of special note is the up-regulation of genes that appear “off” in tachyzoites and bradyzoites but that encode homologues of proteins known to serve key functions in those asexual stages, including a novel pairing of sporozoite-specific paralogues of AMA1 and RON2, two proteins that have recently been shown to form a crucial bridge during tachyzoite invasion of host cells. This work provides the first in-depth insight into the development and functioning of one of the most important but least studied stages in the Toxoplasma life cycle.

Reactivity of Synthetic SAG1 (p30) Peptide Sequences with RH, S273 and Beverley Strain-Induced Anti-Toxoplasma gondii Antibodies

OBJECTIVES

We compared the reactivity of IgG1 and IgG2a antibodies in mouse sera after infection with virulent RH and low-virulent S273 and Beverley strains of Toxoplasma gondii against RH SAG1 recombinant p30 (rp30) and synthetic SAG1 peptides.

METHODS

Infected mouse serum samples were collected 9 days after infection, and the level of total IgG, IgG1 and IgG2a against the RH SAG1 rp30 protein and twenty peptides of the RH SAG1 protein were assessed. The glycosylphosphatidylinositol (GPI) modification site, the hydrophilic-hydrophobic structure, the transmembrane region and the secondary structure of the SAG1 sequence of virulent and low-virulent strains were analyzed using software.

RESULTS

The virulent strain-infected mice produced a higher level of IgG1 but a lower IgG2a against the rp30 antigen, while the low-virulent strain-infected mice produced a higher level of IgG2a than the virulent strain. The difference in the secondary structure of SAG1 protein between the virulent and low-virulent strain was largely confined to amino acid positions 291-336, showing mutations and GPI anchor site.

CONCLUSION

The difference in the reactivity of IgG against the rp30 antigen and synthetic peptides between virulent and low-virulent strains points to the importance of the primary and secondary structure assumed by antigens in the activation of Th cells and, subsequently, in the induction of IgG and its subclasses.

The surface of Toxoplasma tachyzoites is dominated by a family of glycosylphosphatidylinositol-anchored antigens related to SAG1

Toxoplasma gondii is an Apicomplexan parasite with a complex life cycle that includes a rapidly dividing asexual stage known as the tachyzoite. The tachyzoite surface has been reported to comprise five major antigens, the most abundant of which is designated SAG1 (for surface antigen 1). At least one of the other four (SAG3) and another recently described minor antigen (SRS1 [for SAG1-related sequence 1]) have previously been shown to be structurally related to SAG1. To determine if further SAG1 homologs exist, we searched a Toxoplasma expressed sequence tag (EST) database and found numerous ESTs corresponding to at least three new genes related to SAG1. Like SAG1, these new SRS genes encode apparently glycosylphosphatidylinositol-anchored proteins that share several motifs and a set of conserved cysteine residues. This family appears to have arisen by divergence from a common ancestor under selection for the conservation of overall topology. The products of two of these new genes (SRS2 and SRS3) are shown to be expressed on the surface of Toxoplasma tachyzoites by immunofluorescence. We also identified strain-specific differences in relative expression levels. A total of 10 members of the SAG1 gene family have now been identified, which apparently include three of the five major surface antigens previously described and one antigen expressed only in bradyzoites. The function of this family may be to provide a redundant system of receptors for interaction with host cells and/or to direct the immune responses that limit acute T. gondii infections.

Seroprevalences to Toxoplasma gondii in privately-owned dogs in Taiwan

The prevalences of IgG and IgM antibodies to Toxoplasma gondii in pet dogs in Taiwan were measured by using both a kinetics-based enzyme-linked immunosorbent assay and immunoblotting. A logistic regression model with five factors (sex, age, weight, breed, domain) was analyzed. Pet dogs (n = 658) had seroprevalence of 7.9%, and had IgG and IgM geometric mean titers of 1:50 and 1:31, respectively. Older or heavier pet dogs had higher odds of seropositivity than younger or lighter dogs. Also, mixed-breed dogs had higher odds of seropositivity than pure-bred dogs.

Safe haven: the cell biology of nonfusogenic pathogen vacuoles

Our understanding of both membrane traffic in mammalian cells and the cell biology of infection with intracellular pathogens has increased dramatically in recent years. In this review, we discuss the cell biology of the host-microbe interaction for four intracellular pathogens: Chlamydia spp., Legionella pneumophila, Mycobacterium spp., and the protozoan parasite Toxoplasma gondii. All of these organisms reside in vacuoles inside cells that have restricted fusion with host organelles of the endocytic cascade. Despite this restricted fusion, the vacuoles surrounding each pathogen display novel interactions with other host cell organelles. In addition to the effect of infection on host membrane traffic, we focus on these novel interactions and relate them where possible to nutrient acquisition by the intracellular organisms.

Genetic analysis in Toxoplasma: gene discovery with expressed sequence tags and rapid mapping of natural polymorphisms

Genetic analysis of the protozoan parasite Toxoplasma gondii has undergone a rapid expansion in recent years. This is due to effort in a number of laboratories that have worked on the development of molecular genetic techniques. It is also due, however, to the natural biology of this system (including a well-described sexual cycle) that makes possible genetic mapping of the F1 progeny from a cross. In this article, we present a detailed methodology for rapidly mapping natural polymorphisms between the ME49 and CEP strains for which extensive restriction fragment length polymorphism analysis has already been performed. The example we present shows that the failure to detect expression of bradyzoite-specific surface antigens in the CEP strain under conditions that promote differentiation in vitro is not a result of a general failure to express such genes; instead, it is apparently due to antigenic polymorphism in the gene products concerned. This conclusion was reached rapidly and definitively by genetic mapping, whereas molecular approaches would have taken considerably longer. We also show how the recent effort to create an extensive database of expressed sequence tags for this parasite can promote the very rapid discovery of genes that reveal much about the biology of Toxoplasma. The example presented deals with the expression of a family of closely related surface antigens in the tachyzoite stage.

[Immunological properties of the 30 kDa antigen of toxoplasma gondii]

The molecular weight 30 kDa membrane protein of Toxoplasma gondii (Toxoplasma 30 kDa) apparently conserved in most strains of T. gondii and sera of infected hosts. The present study aimed to elucidate Toxoplasma 30 kDa as a useful diagnostic antigen for serodiagnosis of toxoplasmosis by ELISA and for induction of protective immunity. Murine spleen cells immunized with the membrane antigen of T. gondii were fused with mouse Sp2/O-Ag14 myeloma cells. Out of 8 clones selected, five were IgG2b, the others belonged to IgG1 and IgG2a. The 30 kDa antigen was distributed mainly on the surface membrane of tachyzoites by indirect fluorescence method. Murine peritoneal macrophages which were activated by 30 kDa antigen produced more amounts of NO2 compared with crude antigen-treated group, however there were no significant differences in toxoplamacidal activity between the two groups. Higher specificity of Toxoplasma 30 kDa antigen was recognized for serodiagnosis of toxoplasmosis than the crude antigen. From these results, Toxoplasma 30 kDa antigen enhances the cytotoxic effect of macrophages as well as a more reliable means for the serodiagnosis of toxoplasmosis by ELISA.

Toxoplasma gondii: the role of parasite surface and secreted proteins in host cell invasion

The potential role of the 5 surface proteins of Toxoplasma gondii tachyzoites in host cell invasion was investigated using an in vitro neutralization assay. Supporting earlier findings, TG05.54, a monoclonal antibody recognizing the major surface protein SAG 1, was shown to cause a consistent and significant blockade of invasion into bovine kidney cells, indicating a functional role for this protein in host cell invasion. The neutralizing effect was only seen with certain anti-SAG 1 monoclonal antibodies, suggesting the presence of a functional ligand within the molecule. A second surface protein, SAG 2 was also shown to be involved in the invasion process. Anti-SAG 2 antibodies prevented parasite reorientation, leaving zoites immobilized on the host cell membrane and resulting in increased internalization of tachyzoites. Antibodies recognizing other surface, rhoptry, dense granule and microneme molecules had no effect on invasion.

A ubiquitous intracellular parasite: the cellular biology of Toxoplasma gondii

Toxoplasma gondii shares many features with other apicomplexan parasites but is unusual in its extremely broad host and tissue specificity. The parasite exhibits typical 'zoite' morphology, its highly polar structure being dictated by the complex cytoskeleton. Molecules on the surface of the zoite are prime candidates for interaction with the host cell and in vitro assays have implicated 2 of the 5 tachyzoite surface molecules in invasion: SAG1 as a ligand mediating host cell invasion, and SAG2 in enabling reorientation prior to invasion. The functional roles of other molecules, secreted from internal organelles during invasion and intracellular development, are also becoming clear through immuno-EM and biochemical studies, and from sequence data. Molecules from the rhoptries including the penetration enhancing factor ROP1 are secreted at the point of invasion and are integral to the newly formed parasitophorous vacuole membrane. Release of the dense granule molecules GRA 1-6, appears to be calcium regulated and occurs within 10 min of invasion leading to formation of the tubular membranous network and stabilization of the vacuole. The interaction between Toxoplasma and the host cell is stage specific. The tachyzoite divides rapidly and synchronously forming rosettes and causing host cell lysis, while the bradyzoite exhibits slow asynchronous division secreting a granular matrix and becoming enclosed within a cyst wall. This altered phenotype is a reflection of changes in gene expression. Bradyzoite specific molecules are found internally, on the parasite surface, and in the cyst matrix while important tachyzoite proteins such as SAG1 and SAG2 are downregulated. Differentiation between the 2 stages is reversible and is influenced by immunomodulatory agents. However a strong genetic element is involved and it is notable that virulent strains show a very low frequency of cyst production.

Impairment of the cellular immune response in acute murine toxoplasmosis: regulation of interleukin 2 production and macrophage-mediated inhibitory effects

Depression of the cellular immune response to Toxoplasma gondii has been reported in both mice and humans. The present study was undertaken to determine the kinetics and mechanism of the observed downregulation of interleukin 2 (IL-2) production during experimental murine toxoplasmosis. For these investigations, the cell-mediated immune response to the wild type (PTg) was compared with that to the less-virulent mutant parasite (PTgB), which is deficient in the major surface antigen, p30 (SAG-1). Spleen cells from infected A/J mice failed to proliferate in response to Toxoplasma antigens during the first week of infection. Both PTg- and PTgB-infected A/J mice exhibited a significant reduction in the concanavalin A (Con A)-induced lymphoproliferative response. Further, the response of splenocytes from mice infected with the wild-type parasite was significantly diminished compared with that of mice infected with PTgB. The lymphoproliferative response to Con A reached its nadir at day 7 and remained below control levels for at least 14 days postinfection. By day 21 postinfection, the response to Con A and to Toxoplasma antigens was restored to the level observed prior to day 7. Con A-stimulated culture supernatants of spleen cells from mice on day 7 postinfection contained significantly less IL-2 than normal mice. There was no significant difference in the numbers of binding sites or capacity of high-affinity IL-2 receptors between infected and normal mouse splenocytes as determined by Scatchard analysis. Exogenous IL-2 at different concentrations failed to restore the proliferative response of lymphocytes from infected mice to Con A. Adherent macrophages from 7-day-infected mice were able to suppress IL-2 production by normal splenocytes following stimulation with Con A. The inhibitory activity mediated by infected cells was reversed by the antibody to IL-10 but not transforming growth factor beta. There were insignificant levels of nitric oxide production in both infected and normal splenocytes. These results indicate that during acute murine toxoplasmosis, there is a well-defined period (day 7) during which both the T-cell mitogen and parasite antigen-associated lymphoproliferative response are reduced. Further, there is a reduction in the production of IL-2 and an increase in IL-10, which appear to mediate, in part, the observed downregulation of immunity to T. gondii.