Vaccination against parasitic diseases: some alternative concepts for the definition of protective antigens

Effect of alginate nanoparticles on the immunogenicity of excretory-secretory antigens against acute toxoplasmosis in murine model

Efficacy of alginate nanoparticles (Alg-NPs) as vaccine delivery for the excretory-secretory antigens (ESAs) against the virulent strain of Toxoplasma gondii was evaluated. Swiss albino mice were intraperitoneally immunized with three doses of either in vivo and in vitro-prepared ESA vaccines, 20 µg each, at 2-week intervals, then were challenged with 2500 tachyzoites of the RH HXGPRT (-) Toxoplasma gondii strain, four weeks later. Mice mortality, tachyzoite number in both peritoneal fluid and impression smear, and viability, ultrastructural tachyzoite changes, measuring immunological markers, and histopathological changes of both liver and spleen were studied, in comparison to alum adjuvanted ESAs and infected control subgroups. The in vivo-prepared Alg-NPs loaded ESAs vaccinated subgroups induced significant reduction in mice mortality, tachyzoite count in both peritoneal fluid and impression smears and viability. Scanning electron microscopy revealed tachyzoites deformities with multiple irregularities, while transmission electron microscopy showed tachyzoites distortion, disrupted plasma membranes, loss of nuclear integrities, and absence of dense granules with extensive vacuolations. A statistically significant increase in the level of both IFN-γ and anti-Toxoplasma IgG was noted. Histopathological results recorded amelioration of the pathological changes induced by Toxoplasma infection in both liver and spleen, with scanty parasites. Therefore, Alg-NPs proved its effectiveness in enhancing the ESAs antigencity, and recommended to test its potentiality as drugs carrier for anti-Toxoplasma agents to enhance their therapeutic efficacy.

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.

Evaluation of Immunoprotection Conferred by the Subunit Vaccines of GRA2 and GRA5 against Acute Toxoplasmosis in BALB/c Mice

Toxoplasmosis is a foodborne disease caused by Toxoplasma gondii, an obligate intracellular parasite. Severe symptoms occur in the immunocompromised patients and pregnant women leading to fatality and abortions respectively. Vaccination development is essential to control the disease. The T. gondii dense granule antigen 2 and 5 (GRA2 and GRA5) have been targeted in this study because these proteins are essential to the development of parasitophorous vacuole (PV), a specialized compartment formed within the infected host cell. PV is resistance to host cell endosomes and lysosomes thereby protecting the invaded parasite. Recombinant dense granular proteins, GRA2 (rGRA2) and GRA5 (rGRA5) were cloned, expressed, and purified in Escherichia coli, BL21 (DE3) pLysS. The potential of these purified antigens as subunit vaccine candidates against toxoplasmosis were evaluated through 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. Results obtained demonstrated that rGRA2 and rGRA5 elicited humoral and cellular-mediated immunity in the mice. High level of IgG antibody was produced with the isotype IgG2a/IgG1 ratio of ≈0.87 (p < 0.001). Significant increase (p < 0.05) in the level of four cytokines (IFN-γ, IL-2, IL-4, and IL-10) was obtained. The antibody and cytokine results suggest that a mix mode of Th1/Th2-immunity was elicited with predominant Th1-immune response inducing partial protection against T. gondii acute infection in BALB/c mice. Our findings indicated that both GRA2 and GRA5 are potential candidates for vaccine development against T. gondii acute infection.

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

Background

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

Methods

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

Results

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

Conclusions

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

T cell clones raised from chronically infected healthy humans by stimulation with Toxoplasma gondii excretory-secretory antigens cross-react with live tachyzoites: characterization of the fine antigenic specificity of the clones and implications for vaccine development

Excreted-secreted Ags (ESA) of Toxoplasma gondii (Tg) play an important role in the stimulation of the host immune system in both acute and chronic infections. To identify the parasite Ag(s) involved in the maintenance of T cell-mediated long term immunity, 40 ESA-specific T cell clones were derived from three chronically infected healthy subjects. All the clones were CD4+ and recognized both ESA and live tachyzoites in a HLA-DR-restricted manner. Conversely, CD4+ tachyzoite-specific T cell clones from the same subjects proliferated in response to ESA, pointing to shared immunodominant Ags between ESA and Tg tachyzoites. By T cell blot analysis using SDS-PAGE-fractionated parasite extracts, the following patterns of reactivity were detected. Of 25 clones, 6 recognized Tg fractions in the 24- to 28-kDa range and proliferated to purified GRA2, 5 reacted with Tg fractions in the 30- to 33-kDa range; and 4 of them proved to be specific for rSAg1. Although surface Ag (SAg1) is not a member of ESA, small amounts of this protein were present in ESA preparation by Western blot. Of 25 clones, 8 responded to Tg fractions in the 50- to 60-kDa range but not to the 55-kDa recombinant rhoptries-2 parasite Ag, and 6 did not react with any Tg fraction but proliferated in response to either ESA or total parasite extracts. In conclusion, CD4+ T cells specific for either ESA (GRA2) or SAg1 may be involved in the maintenance of long term immunity to Tg in healthy chronically infected individuals.

Partial protection of mice against Trypanosoma cruzi after immunizing with the TcY 72 antigenic preparation

A 72 kDa Trypanosoma cruzi glycoprotein recognized by the 164C11 monoclonal antibody (IgM isotype) was purified by preparative electrophoresis. The antigenic preparation obtained, named TcY 72, was used to immunize C57Bl/10 mice. The following results were observed after immunization: (1) induction of higher titres of IgG than IgM antibodies, as evaluated by indirect immunofluorescence; (2) significant DTH after injection of epimastigotes in mice footpads; (3) peak parasitemia in immunized mice was significantly reduced and animals were negative by 13 days post-infection, although the mice still succumb to infection; (4) the phenotypic analysis of spleen cell populations showed a decrease in the CD4/CD8 ratio in immunized mice. Taken as a whole, these findings indicate that TcY 72 is immunogenic and potentially important for protective immunity.

Immune response of lambs to vaccination with Ostertagia circumcincta surface antigens eliciting bile antibody responses

Immune responses to surface antigens of infective larvae of Ostertagia circumcincta recognized by bile antibodies of sheep immune to challenge were studied in 5-month-old Finn-Dorset male lambs. The sheep were vaccinated subcutaneously with 2 doses of 25 micrograms/kg body weight of surface proteins immunoprecipitated by bile antibodies derived from protected lambs. These antigens were purified from immune complexes by affinity chromatography and then injected with beryllium hydroxide as an adjuvant. The immunized lambs were challenged with 5 x 10(4) L3 and the worm burdens evaluated on day 21 post challenge. These were significantly (P < 0.01) lower in the vaccinated group than in the challenged controls (72% protection). The mucosal and bile IgM, recognizing the L3 surface, showed significantly higher levels in the vaccinated lambs compared to the challenge controls. Mucosal and bile IgA antibody levels against the same antigens were low and no significant differences were observed between vaccinated and control lambs.

Surface and excretory/secretory antigens of fourth-stage larvae and adult Ostertagia circumcincta

With the use of an enzyme-linked immunosorbent assay (ELISA), it was revealed that surface antigens of both adult and fourth-stage larvae (L4) of Ostertagia circumcincta induced high levels of serum IgG antibodies, while serum IgA antibody levels were low but increased significantly (P < 0.01) after infection. Immunofluorescence studies on the surface of viable L4 and adult nematodes showed that the IgG response was stage-specific only in animals vaccinated with adult surface extracts. The results of Western blot analysis using these antibodies suggested that at least eight polypeptides were shed from the L4 surface to the environment and that infection induced (or boosted) IgG antibody against a further four polypeptides. A comparison of reactivity of pre- and post-infection sera of sheep vaccinated with adult nematode surface antigens suggested that only one of the antigens stripped from the nematode surface was immunogenic and/or present in a concentration sufficient to induce an IgG response following parenteral vaccination. Infection boosted the IgG antibodies to a further four polypeptides. Only one polypeptide of 63 kDa seems to be shed in vivo from the adult nematode surface. Ten to eleven antigens were recognised in adult excretory/secretory products by serum IgG of multiple-infected sheep.

Molecular structure of a Toxoplasma gondii dense granule antigen (GRA 5) associated with the parasitophorous vacuole membrane

The P21 antigen of Toxoplasma gondii, defined by the monoclonal antibody TG17-113, has been described as a dense granule component, secreted in the parasitophorous vacuole during host cell invasion. The present work reports the cloning of the gene encoding the P21 antigen, for which we propose the name GRA 5. A cDNA library was screened with a rat antiserum raised against an HPLC fraction enriched in the P21 antigen. cDNA clones encoding GRA 5 were selected by antibody selection on the recombinant proteins. All these clones were incomplete at the 5' end. The 5' fragment of the longest cDNA clone isolated by this first screening was used as a probe in secondary screenings of cDNA and genomic DNA libraries. A genomic fragment containing the P21 gene and nearly full-length cDNAs have been isolated and sequenced. The gene encoding GRA 5 is 834 bp long and does not contain any intron. The deduced amino acid sequence of an open reading frame encoding 133 amino acids perfectly matched that of 5 peptides microsequenced from the native antigen. A N-terminal hydrophobic region was found to possess the characteristics of a signal peptide of 25 amino acids. A second hydrophobic domain, bordered by two hydrophilic regions strongly suggests a transmembrane region. This molecular structure is supported by ultrastructural studies showing the association of the P21 antigen with the parasitophorous vacuole membrane.

Molecular characterization of a dense granule antigen (Gra 2) associated with the network of the parasitophorous vacuole in Toxoplasma gondii

The monoclonal antibody (mAb) TG17.179 recognizes an excreted-secreted antigen (ESA) of 28.5 kDa named Gra 2, which is stored in the dense granules of Toxoplasma cells and secreted into the parasitophorous vacuole after host cell invasion. Screening of an expression cDNA library with TG17.179 led to the isolation of several clones, the longest one (clone L) being of 1030 bp. Clone L cDNA was found to be homologous to a previously described composite cDNA encoding a P28 protein of Toxoplasma gondii. Characterization of one genomic clone indicates that the complete GRA 2 gene is about 1.3 kb in length, including an intron of 241 bp. Northern blot and primer extension analyses confirmed the size of the mature messenger (1.1 kb). Amino acid partial sequencing of the native antigen purified by HPLC and metabolic radiolabelings of ESAs perfectly matched the primary amino acid structure deduced from the clone L cDNA. This primary translation product consists of an 185 amino acid polypeptide (19.8 kDa) including a 23 amino acid signal sequence. The presence of many serine and threonine residues may indicate an O-glycosylation. The predicted mature polypeptide shows an internal helical domain with 2 amphipathic alpha-helices. These might be involved in the association of Gra 2 with the membranous network within the parasitophorous vacuole.

Immune responses of sheep to surface antigens of infective larvae of Ostertagia circumcincta

Immune responses to surface antigens of infective larvae of Ostertagia circumcincta were studied in 5-month old Finn-Dorset male lambs. The sheep were vaccinated subcutaneously with 2 doses of 25 micrograms/kg body weight of infective larvae surface or somatic extracts and Freund's or beryllium hydroxide adjuvants. It was found that only in lambs vaccinated with L3 surface extracts and beryllium hydroxide as an adjuvant did worm burdens differ significantly (P less than 0.01) from those of challenge controls (71.7% protection). Sheep vaccinated with the same antigenic preparation but administered with Freund's adjuvant or with beryllium adjuvant and somatic extracts demonstrated rather poor protection (32.4 and 30.5% respectively). All the vaccinated sheep showed a high IgG response to L3 surface antigens while significant levels of serum and bile IgA reacting with the surface extracts were only detected in those lambs vaccinated with L3 surface extracts and beryllium hydroxide. In immunofluorescence tests serum IgA of this group reacted with the whole surface of exsheathed larvae or with the site of opening of excretory pore while IgG antibodies reacted strongly with the anterior and posterior parts of the infective larvae. Both IgA and IgG responses to surface antigens were stage specific.

A comparison of sporozoite and cyst merozoite surface proteins of Sarcocystis

Surface labeling of Sarcocystis muris and S. suicanis sporozoites with N-hydroxysuccinimide biotin led to the detection of major membrane proteins with relative molecular weights of 29 and 30 kDa, respectively. Immunoblots of Sarcocystis sporozoite proteins probed with sera from infected hosts or with polyclonal monospecific antibodies generated against membrane antigens of cyst merozoites (noncorresponding stages) showed cross-reactivity between the two developmental stages (cyst merozoites and sporozoites) as well as between the species S. muris and S. suicanis. Two-dimensional gel electrophoresis resulted in the identification of isoforms of the sporozoite membrane antigens, with isoelectric points ranging from pH 4.7 to pH 6.4 for S. muris and from pH 4.7 to pH 5.2 for S. suicanis. The molecular masses, the charge heterogeneity, and the immunological reactivity of the surface proteins of Sarcocystis sporozoites were similar to those of cyst merozoites of both species.

Immunization with Trypanosoma cruzi epimastigote antigens incorporated into iscoms protects against lethal challenge in mice

An immunoglobulin G3 monoclonal antibody obtained by immunizing mice with a cell membrane preparation of epimastigotes of Trypanosoma cruzi was shown to agglutinate live epimastigotes, lyse blood-form trypanosomes, and partially protect mice by passive transfer. The main antigens recognized by the monoclonal antibody were located in the flagella of epimastigotes and blood-form trypanosomes. Antigens of epimastigotes, purified by affinity chromatography with the monoclonal antibody, were shown to be highly glycosylated and revealed a wide band with an Mr between 45,000 and 68,000 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. Immunization of mice with a small concentration of the affinity purified antigens incorporated into an antigen delivery system prepared with Quil A (Isotec AB, Lulea, Sweden), a saponin derivative, induced strong antibody and cell-mediated immune responses and protected 100% of the immunized animals against death due to challenge with 100 100% lethal doses of blood form trypanosomes. Protection was due to the use of the antigen delivery system, since mice immunized with equal concentrations of antigens alone or in combination with saponin had 100% mortality. The results suggest that small concentrations of epimastigote antigens obtained by biochemical methods and incorporated into the proper antigen delivery system may serve as a vaccine against Chagas' disease.

Isotypic profiles of antibody responses to Toxoplasma gondii infection in rats and mice: kinetic study and characterization of target antigens of immunoglobulin A antibodies

The antibody responses to Toxoplasma gondii were investigated in rat and mouse experimental models. The immunoglobulin A (IgA) antibody response was of particular interest because acquisition of Toxoplasma gondii is usually by the oral route. The rat model was used because the natural resistance of rats to the parasite is similar to the natural resistance exhibited by adult humans. There was an early and simultaneous rise in IgA and IgM antibody responses. The IgA antibody response was maximal around day 40. IgA antibodies from Fischer rats were mainly directed against soluble and membrane antigens of 28.5, 29, 30, 35, and 38 kilodaltons (kDa). In mice, however, a major antigen of 29 kDa was recognized by IgA antibodies. Moreover, in orally infected rats, an intense IgE antibody response against the major surface antigen, P30, was observed. An IgA antibody response was also observed in rats and mice immunized with Toxoplasma excreted-secreted antigens, even without adjuvant. This response was mainly directed against 28.5- and 34-kDa antigens in rats. Serum IgA from infected rats tested against the excreted-secreted antigens bound to 28.5- 34-, and 39-kDa antigens, whereas sera from infected mice recognized only the 34-kDa antigen.

Protection of nude rats against Toxoplasma infection by excreted-secreted antigen-specific helper T cells

In the present work we demonstrate the implication of excreted-secreted antigens in eliciting the protective cell-mediated immunity developed by rats toward Toxoplasma gondii. We first showed that 10(4) specific T cells from T. gondii-infected rats conferred to nude rats the ability to resist an infection by the highly virulent RH strain of T. gondii. In a second series of experiments, the role of excreted-secreted antigens in this protection was demonstrated. After the adoptive transfer to nude rats of various doses (10(3), 10(4), 10(5)) of excreted-secreted antigen-specific helper T cells (propagated in vitro during one month), significant protection toward T. gondii was induced. Moreover, these cells were responsible for a specific antibody response in nude rats, which are normally unable to develop any specific humoral response. The specificity of these antibodies was directed toward different molecules with molecular masses of 104, 97, 57, 39, 30, 21, and 18 kilodaltons; some of these have been previously characterized as major excreted-secreted antigens.

Toxoplasma gondii: characterization and localization of antigens secreted from tachyzoites

Since we had previously demonstrated the protective role played by Toxoplasma excreted-secreted antigens, the aim of the present work was to produce monoclonal antibodies directed against these antigens in order to determine if their localization in the parasite is compatible with a mechanism of excretion or secretion. Western immunoblotting analysis revealed three monoclonal antibodies (TG17-179, TG17-43, and TG17-113) raised against excreted-secreted antigens of 28.5, 27, and 21 kDa, respectively. The TG17-179 which reacts with antigens isolated by Concanavalin A affinity chromatography is directed against a glycosylated 28.5-kDa component. Colloidal immunogold labeling showed the ultrastructural localization of the 21-, 27-, and 28.5-kDa antigens in the matrix of the dense granules of tachyzoites and associated with the microvilli network of the parasitophorous vacuole, after host cell invasion. These observations suggest the following mechanism of Toxoplasma secretion: secreted antigens are first stored in tachyzoite-dense granules and are then released inside the parasitophorous vacuole. Among the secretory molecules characterized here, the native 27-kDa antigen recognized by TG17-43 is a calcium-binding protein found to be intermixed with the 21- and 28.5-kDa antigens inside the dense granules and hence could play a role in the packaging of secretory products. In addition, the 21- and 28.5-kDa antigens were also located beneath the parasite plasma-lemma. This particular location could reflect a transient step characteristic of T. gondii secretion.

Identification and biochemical characterization of antigens of tachyzoites and bradyzoites of Toxoplasma gondii with cross-reactive epitopes

The aim of the present work was the identification and biochemical characterization of antigens from the tachyzoite and bradyzoite stages of Toxoplasma gondii that share cross-reactive epitopes. Our previous work has demonstrated the induction by tachyzoite excreted-secreted antigens of both a humoral and a cell-mediated protective response. We investigated the question as to whether some bradyzoite and tachyzoite (excreted-secreted, soluble or membrane) antigens share cross-reactive epitopes. Using immunoprecipitation techniques, we identified four tachyzoite antigens with molecular weights of 63, 43, 39, and 28.5 kDa, which were recognized both by sera raised against tachyzoite antigens and by chronic-phase human sera with residual IgG antibodies. In an attempt to define the biochemical nature of these antigens, we show that the 43- and 28.5-kDa antigens seem to be glycosylated since they bind to concanavalin A, as does a 37-kDa tachyzoite antigen.

Molecular basis of host-parasite relationship: towards the definition of protective antigens

In spite of some remarkable progress in our understanding of the immune response to parasites and in the molecular cloning of dozens of genes encoding for potentially protective proteins, no definitive step has yet been made towards operational vaccines against major human parasitic diseases. The reasons for our present failures are no longer attributable to the lack of appropriate tools but rather to our rather primitive knowledge of the basic mechanisms governing host-parasite relationship. Mainly on the basics of our personal observations, we have attempted to review and discuss some of the prominent factors in host-parasite interactions, such as molecular mimicry, phyletic convergence, molecular mechanisms of infectivity and lures of cell communication. In many respects, the development of efficient vaccines applicable to humans appears closely dependent on a better understanding of the basic biological mechanisms underlying the natural history of parasitic diseases. In this context, the development of new concepts regarding the definition of potentially protective antigens based on the study of non-surface molecules, cross-reactive stage antigens and antibody isotype selection might represent promising alternatives.

Molecular characterization of a 23-kilodalton major antigen secreted by Toxoplasma gondii

The strategy chosen for cloning potential vaccine antigens of Toxoplasma gondii was based on the hypothesis that the definitive protection observed in natural infection is due to the presence of encysted bradyzoite forms in host tissues throughout life. The antigens released by the bradyzoites would maintain an immune response against the invading tachyzoites. This led us to identify in tachyzoite in vitro translation products a polypeptide of 24 kDa that is an excreted-secreted antigen (ESA) and is cross-reactive with bradyzoites. In addition, the detection of anti-P24 IgG antibodies is correlated with the chronic infection in man. The gene encoding P24 has been isolated, sequenced, and expressed in Escherichia coli and eukaryotic cells. The recombinant proteins were immunogenic in mice, producing anti-native P23 antibodies. Immunocytochemical analysis located the native antigen in the dense granules of both tachyzoite and bradyzoite forms and showed that it is secreted within host-cell-modified phagosome. Moreover 45Ca2+ labeling as well as regional homologies indicate that this protein has Ca2+-binding properties, suggesting its physiological importance in host-cell invasion. P23 is of diagnostic interest as a marker of chronic toxoplasmosis and is proposed as a vaccine component.

Induction of a protective antibody-dependent response against toxoplasmosis by in vitro excreted/secreted antigens from tachyzoites of Toxoplasma gondii

Toxoplasma gondii is a worldwide protozoan parasite which causes severe disease in congenitally infected children and in immunocompromised patients. Besides the well-defined cytoplasmic and membrane antigens of tachyzoites, we felt that excreted/secreted antigens could play a major role in the immune response. We first report the development of a well-controlled procedure for obtaining tachyzoite excreted/secreted antigens (E/SA) in cell-free incubation media. The E/SA immunogenic in human, rat and mouse toxoplasmosis were then characterized. The major E/SA recognized by human sera from the chronic phase of toxoplasmosis had molecular weights of 108, 97, 86, 69, 60, 57, 42, 39, 28.5, 27 and 26 kD. When injected into +/+ Fischer rats, E/SA elicited high antibody titres. In addition, passive transfer of these sera to highly susceptible nu/nu littermates induced a significant degree of protection towards the virulent RH strain of T. gondii. This work, which demonstrates the key role played by E/SA in the protective immune response, suggests that these antigens should be of value both for diagnostic purposes and for the development of new strategies for immunization against toxoplasmosis.