Identification of two alleles in the GRA4 locus of Toxoplasma gondii determining a differential epitope which allows discrimination of type I versus type II and III strains

Bioinformatics analysis of single and multi-hybrid epitopes of GRA-1, GRA-4, GRA-6 and GRA-7 proteins to improve DNA vaccine design against Toxoplasma gondii

Toxoplasma gondii, is a causative agent of morbidity and mortality in immunocompromised and congenitally-infected individuals. Attempts to construct DNA vaccines against T. gondii using surface proteins are increasing. The dense granule antigens are highly expressed in the acute and chronic phases of T. gondii infection and considered as suitable DNA vaccine candidates to control toxoplasmosis. In the present study, bioinformatics tools and online software were used to predict, analyze and compare the structural, physical and chemical characters and immunogenicity of the GRA-1, GRA-4, GRA-6 and GRA-7 proteins. Sequence alignment results indicated that the GRA-1, GRA-4, GRA-6 and GRA-7 proteins had low similarity. The secondary structure prediction demonstrated that among the four proteins, GRA-1 and GRA-6 had similar secondary structure except for a little discrepancy. Hydrophilicity/hydrophobicity analysis showed multiple hydrophilic regions and some classical high hydrophilic domains for each protein sequence. Immunogenic epitope prediction results demonstrated that the GRA-1 and GRA-4 epitopes were stable and GRA-4 showed the highest degree of antigenicity. Although the GRA-7 epitope had the highest score of immunogenicity, this epitope was instable and had the lowest degree of antigenicity and half-time in eukaryotic cell. Also, the results indicated that GRA4-GRA7 epitope and GRA6-GRA7 had the highest degree of antigenicity and immunogenicity among multi-hybrid epitopes, respectively. Totally, in the present study, single epitopes showed the highest degree of antigenicity compared with multi-hybrid epitopes. Given the results, it can be concluded that GRA-4 and GRA-7 can be powerful DNA vaccine candidates against T. gondii.

Genetic diversity of bovine Neospora caninum determined by microsatellite markers

Neospora caninum is one of the most significant parasitic organisms causing bovine abortion worldwide. Despite the economic impact of this infection, relatively little is known about the genetic diversity of this parasite. In this study, using Nc5 and ITS1 nested PCR, N. caninum has been detected in 12 brain samples of aborted fetuses from 298 seropositive dairy cattle collected from four different regions in Tehran, Iran. These specimen (Nc-Iran) were genotyped in multilocus using 9 different microsatellite markers previously described (MS4, MS5, MS6A, MS6B, MS7, MS8, MS10, MS12 and MS21). Microsatellite amplification was completely feasible in 2 samples, semi-completely in 8 samples, and failed in 2 samples. Within the two completely performed allelic profiles of Nc-Iran strains, unique multilocus profiles were obtained for both and novel allelic patterns were found in the MS8 and MS10 microsatellite markers. The Jaccard's similarity index showed significant difference between these two strains and from other standard isolates derived from GenBank such as Nc-Liv, Nc-SweB1, Nc-GER1, KBA1, and KBA2. All samples originating from the same area showed identical allelic numbers and a correlation between the number of repeats and geographic districts was observed.

Analyzing and identifying novel B cell epitopes within Toxoplasma gondii GRA4

Background

The identification of specific epitopes targeted by the host antibody response is important for understanding the natural response to infection and for the development of epitope-based marker vaccines and diagnostic tools for toxoplasmosis. In this study, Toxoplasma gondii GRA4 epitopes were identified using software-based prediction and a synthetic peptide technique.

Methods

The complete GRA4 gene sequence was obtained from T. gondii of the Gansu Jingtai strain of tachyzoites. The potential B cell epitopes of GRA4 was predicted using the PROTEAN subroutine in the DNASTAR software package. The peptides with good hydrophilicity, high accessibility, high flexibility and strong antigenicity were chemically synthesized and assessed by ELISA using pig sera from different time points after infection.

Results

The potential B cell epitopes of GRA4 predicted by bioinformatics tools focused on six regions of GRA4, 52–77 aa, 93–112 aa, 127–157 aa, 178–201 aa, 223–252 aa and 314–333 aa. Eleven shorter peptides from the six regions were synthesized and assessed by ELISA using pig sera from different time points after infection. Three of the eleven peptides (amino acids 62–77, 233–252 and 314–333) tested were recognized by all sera.

Conclusions

We precisely located the T. gondii GRA4 epitopes using pig sera collected at different time points after infection. The identified epitopes may be useful for additional studies of epitope-based vaccines and diagnostic reagents.

DNA prime and peptide boost immunization protocol encoding the Toxoplasma gondii GRA4 induces strong protective immunity in BALB/c mice

Background

Toxoplasma gondii is a widespread intracellular parasite, which infects most vertebrate animal hosts and causes zoonotic infection in humans. Vaccine strategy remains a promising method for the prevention and control of toxoplasmosis. T. gondii GRA4 protein has been identified as a potential candidate for vaccine development. In our study, we evaluated the immune response induced by four different immunization vaccination strategies encoding TgGRA4.

Methods

BALB/c mice were intramuscularly (i.m.) immunized four times according to specific immunization schedules. Generally, mice in experimental groups were immunized with polypeptide, pGRA4, peptide/DNA, or DNA/peptide, and mice in the control groups were injected with PBS or pEGFP. After immunization, the levels of IgG antibodies and cytokine productions were determined by enzyme-linked immunosorbent assays (ELISA). The survival time of mice was also evaluated after challenge infection with the highly virulent T. gondii RH strain.

Results

The results showed that mice vaccinated with different immunization regimens (polypeptide, pGRA4, peptide/DNA, or DNA/peptide) elicited specific humoral and cellular responses, with high levels of total IgG, IgG2a isotype and gamma interferon (IFN-γ), which suggested a specific Th1 immunity was activated. After lethal challenge, an increased survival time was observed in immunized mice (11.8 ± 4.8 days) compared to the control groups injected with PBS or pEGFP (P < 0.05). Mice injected with PBS or pEGFP died within 8 days, and there was no significant difference in the protection level in two groups (P > 0.05).

Conclusions

These results demonstrated that this DNA prime and peptide boost immunization protocol encoding the TgGRA4 can elicit the highest level of humoral and cellular immune responses compared to other immunized groups, which is a promising approach to increase the efficacy of DNA immunization.

Evaluation of protective effect of multi-epitope DNA vaccine encoding six antigen segments of Toxoplasma gondii in mice

To investigate the vaccine potential of multi-epitope vaccines against toxoplasmosis, a multi-epitope DNA vaccine, eukaryotic plasmid pcDNA3.1/T-ME expressing six antigen segments (SAG1(238-256), SAG1(281-320), GRA1(170-193), GRA4(331-345), GRA4(229-245), and GRA2(171-185)) of Toxoplasma gondii was constructed. We investigated the efficacy of pcDNA3.1/T-ME with or without co-administration of a CpG-oligodeoxynucleotide (CpG-ODN) as an adjuvant to protect mice (BALB/c and C57BL/6) against toxoplasmosis. High survival rates were observed in mice immunized with pcDNA3.1/T-ME when challenged with T. gondii RH strain. Lymphocyte proliferation assays, cytokine, and antibody determinations show that mice immunized with pcDNA3.1/T-ME produced stronger humoral and Th1-type cellular immune responses compared to untreated mice or those immunized with empty plasmids. However, co-immunization with CpG-ODN resulted in impaired immune responses. Our data demonstrates that multi-epitope DNA vaccination is a potential strategy for the control of toxoplasmosis and paves the way for further investigations into producing a multi-epitope anti-T. gondii DNA vaccine.

A new perspective on and re-assessment of SAG2 locus as the tool for genetic analysis of Toxoplasma gondii isolates

SAG2 locus, the coding gene of the P22 protein, has been widely used for the molecular epidemiology of Toxoplasma gondii and characterization of the parasite isolates with two separate polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) processes. To re-assess the resolution power and suitability of this genetic marker for molecular characterization of the parasite isolates, a number of 27 Toxoplasma strains from different zymodeme patterns were used in the present study. Both codon and non-codon regions of the SAG2 locus of all 27 strains were amplified and subjected to sequencing and nucleotide alignment. Nucleotide variations clustered the three major genotypes (I, II and III). Some minor genotypes, unidentifiable by SAG2-RFLP, could be identified by sequence comparison. However, there were other genotypes that could not be differentiated from the major types due to having identical sequences. This suggests that a remarkable number of field isolates representing several minor types will be miss-clustered with the major types by using the traditional SAG2-PCR-RFLP method. It was concluded that this technique seems not to be suitable for Toxoplasma population study. Thus, the utilization of more variable markers and other discriminatory methods are also recommended.

Recent trends in molecular diagnostics for Toxoplasma gondii infections

Toxoplasmosis is an important parasitic infection of man and animals. It is well-known that the progression and severity of disease depend on the immunological status of the host, but recent studies suggest that the genetics of the parasite can also play a role. Diagnosis based on clinical appearance and serology is not always easy. However, molecular methods do not depend on an immune response, and allow direct detection of the parasite in biological samples. Thus they can be used to establish a diagnosis when serological tests are not definitive. Multicopy sequences specific for Toxoplasma gondii, e.g., the B1 gene or the 529-bp sequence, are especially useful in molecular tests. Real-time PCR is very sensitive and is a promising technique that is capable of providing a quantitative result. Molecular methods are also used for genotypic characterisation of T. gondii isolates. Analysis of polymorphic sequences determines the precise strain. The choice of sequence is critical when undertaking studies on the correlation between clinical signs and symptoms of disease and the T. gondii genotype. Further studies involving direct genotyping of T. gondii from clinical samples are needed.

Speculation on possible life cycles for the clonal lineages in the genus toxoplasma

Recent evidence suggests that the strains currently classified in the genus Toxoplasma, ie. within the species Toxoplasma gondii, may actually comprise at least two clonal lineages correlated with their virulence in mice. Here, Alan Johnson reviews these data in the context of evolution and speciation within the genus, and raises hypotheses on how the virulent lineage may undergo an asexual life cycle in nature, similar to that found for the very closely related coccidian, Neospora camnum. The putative vertical transmission life cycle of this mouse virulent lineage of T. gondii could involve passage to the foetus late in pregnancy, or transmission in milk to the neonate after birth.

Analysis of the SAG5 locus reveals a distinct genomic organisation in virulent and avirulent strains of Toxoplasma gondii

We have recently characterised, in the virulent strain RH of Toxoplasma gondii, three glycosylphosphatidylinositol-anchored surface antigens related to SAG1 (p30) and encoded by highly homologous, tandemly arrayed genes named SAG5A, SAG5B and SAG5C. In the present study, we compared the genomic organisation of the SAG5 locus in strains belonging to the three major genotypes of T. gondii. Southern blot analysis using a SAG5-specific probe produced two related but distinct hybridisation patterns, one exclusive of genotype I virulent strains, the other shared by avirulent strains of either genotype II or genotype III. To understand the molecular bases of this intergenotypic heterogeneity, we cloned and sequenced the SAG5 locus in the genotype II strain Me49. We found that in this isolate the SAG5B gene is missing, with SAG5A and SAG5C laying contiguously. This genomic arrangement explains the hybridisation profiles observed for all the avirulent strains examined and indicates that the presence of SAG5B is a distinctive trait of genotype I. Furthermore, we identified two novel SAG1-related genes, SAG5D and SAG5E, mapping respectively 1.8 and 4.0 kb upstream of SAG5A. SAG5D is transcribed in tachyzoites and encodes a polypeptide of 362 amino acids sharing 50% identity with SAG5A-C, whereas SAG5E is a transcribed pseudogene. We also evaluated polymorphisms at the SAG5 locus by comparing the coding regions of SAG5A-E from strains representative of the three archetypal genotypes. In agreement with the strict allelic dimorphism of T. gondii, we identified two alleles for SAG5D, whereas SAG5A, SAG5C and SAG5E were found to be three distinct nucleotide variants. The higher intergenotypic polymorphism of SAG5A, SAG5C and SAG5E suggests that these genes underwent a more rapid genetic drift than the other members of the SAG1 family. Finally, we developed a new PCR-restriction fragment length polymorphism method based on the SAG5C gene that is able to discriminate between strains of genotype I, II and III by a single endonuclease digestion.

Recent origins among ancient parasites

Toxoplasma gondii is a wide spread protozoan parasite belonging to the phylum Apicomplexa. Members of this group of parasites cause economically and medically important diseases in a variety of animals, including humans. T. gondii is notable among this group for the fact that it readily infects virtually all warm-blooded vertebrates, while most apicomplexans are typically restricted in their host range. Individual strains of T. gondii are genetically quite similar and molecular studies indicate that the vast majority (>95%) of strains can be grouped into one of three distinct clonal lineages. Remarkably, these three lineages are comprised of different combinations of two parental alleles, indicating they arose from a single genetic cross. The unusual population structure of T. gondii originated within the last 10,000 years, while the genus itself is approximately 10 million years old. The remarkable success of the three lineages following their recent origin is likely attributable to a dramatic change in the life cycle that greatly facilitated transmission. Prior to the cross that gave rise to the clonal lineages, and in all closely related parasites, transmission occurred by an obligatory two-host life cycle that alternated between definitive (carnivorous) and intermediate (herbivorous) hosts. The reassortment of genes that occurred in the cross allowed direct oral transmission between many different intermediate hosts. These findings illustrate that complex biological life cycles can change rapidly and dramatically, thus presenting a constant source of new zoonotic infections.

The relationship between nucleoside triphosphate hydrolase (NTPase) isoform and Toxoplasma strain virulence in rat and human toxoplasmosis

Avirulent strains of Toxoplasma gondii possess only the nucleoside triphosphate hydrolase II (NTPaseII) isoform, whilst virulent strains possess both NTPaseI and NTPaseII. To determine if it is possible to identify the infective strain type (virulent or avirulent) in T. gondii infections by serological methods, we developed isoform-specific peptide ELISAs from the NTPaseI and NTPaseII antigens of T. gondii. When rats were immunized with either recombinant NTPaseI or NTPaseII, the ELISA could differentially identify antibody reactivity to each NTPase isoform. This ELISA was then used to test six groups of rats that were infected with either one of three virulent (RH, P or Ent) or three avirulent (Me49, C or TPR) strains of T. gondii. No differential antibody reactivity was detected by either whole recNTPase ELISA or peptide ELISA in the sera of rats, whether infected by virulent or avirulent strains of T. gondii. We also studied a panel of human sera from patients infected with known laboratory strains of T. gondii or naturally infected patients where the parasite was isolated and its virulence determined in mice. Differential reactivity to whole recNTPase isoforms was detected in some human sera, but this reactivity was not detected by the isoform-specific peptide ELISAs. Although the NTPase peptides do exhibit differential antibody reactivity, this is not correlated with the virulence status of the infecting strain.

Recent expansion of Toxoplasma through enhanced oral transmission

The global predominance of three clonal Toxoplasma gondii lineages suggests that they are endowed with an exceptional trait responsible for their current parasitism of nearly all warm-blooded vertebrates. Genetic polymorphism analyses indicate that these clonal lineages emerged within the last 10,000 years after a single genetic cross. Comparison with ancient strains (approximately 1 million years) suggests that the success of the clonal lineages resulted from the concurrent acquisition of direct oral infectivity. This key adaptation circumvented sexual recombination, simultaneously promoting transmission through successive hosts, hence leading to clonal expansion. Thus, changes in complex life cycles can occur rapidly and can profoundly influence pathogenicity.

Microsatellite analysis of Toxoplasma gondii shows considerable polymorphism structured into two main clonal groups

Previous studies on Toxoplasma gondii population structure, based essentially on multilocus restriction fragment length polymorphism analysis or on multilocus enzyme electrophoresis, indicated that T. gondii comprises three clonal lineages. These studies showed a weak polymorphism of the markers (2-4 alleles by locus). In this study, we used eight microsatellite markers to type 84 independent isolates from humans and animals. Two microsatellite markers were present in the introns of two genes, one coding for beta-tubulin and the other for myosin A, and six were found in expressed sequence tags. With 3-16 alleles detected, these markers can be considered as the most discriminating multilocus single-copy markers available for typing T. gondii isolates. This high discriminatory power of microsatellites made it possible to detect mixed infections and epidemiologically related isolates. Evolutionary genetic analyses of diversity show that the T. gondii population structure consists of only two clonal lineages that can be equated to discrete typing units, but there is some evidence of occasional genetic exchange that could explain why one of these discrete typing units is less clearly individualised than the other.

Success and virulence in Toxoplasma as the result of sexual recombination between two distinct ancestries

Toxoplasma gondii is a common human pathogen causing serious, even fatal, disease in the developing fetus and in immunocompromised patients. Despite its ability to reproduce sexually and its broad geographic and host range, Toxoplasma has a clonal population structure comprised principally of three lines. We have analyzed 15 polymorphic loci in the archetypal type I, II, and III strains and found that polymorphism was limited to, at most, two rather than three allelic classes and no polymorphism was detected between alleles in strains of a given type. Multilocus analysis of 10 nonarchetypal isolates likewise clustered the vast majority of alleles into the same two distinct ancestries. These data strongly suggest that the currently predominant genotypes exist as a pandemic outbreak from a genetic mixing of two discrete ancestral lines. To determine if such mixing could lead to the extreme virulence observed for some strains, we examined the F(1) progeny of a cross between a type II and III strain, both of which are relatively avirulent in mice. Among the progeny were recombinants that were at least 3 logs more virulent than either parent. Thus, sexual recombination, by combining polymorphisms in two distinct and competing clonal lines, can be a powerful force driving the natural evolution of virulence in this highly successful pathogen.

Attenuation of mouse-virulent Toxoplasma gondii parasites is associated with a decrease in interleukin-12-inducing tachyzoite activity and reduced expression of actin, catalase and excretory proteins

Determinants of Toxoplasma gondii virulence are still unknown, although genetic markers associated with T. gondii pathogenicity or host susceptibility to infection have been identified. To define indicator proteins of mouse virulence, type I strain parasites were attenuated by continuous passage in fibroblast culture and compared with the parental strain passaged in mice. The loss of acute virulence, evident by a 1000-fold higher pathogen dose causing 100% lethality in mice correlated with a less efficient infection of inflammatory cells at the site of inoculation, while parasite proliferation and invasiveness in vitro proved unimpaired. Infection with the attenuated parasites elicited earlier local interleukin-12 and strong interferon-gamma responses in vivo, although the activity that triggers interleukin-12 secretion in macrophages is reduced in the attenuated compared to the virulent strain variant. The interleukin-12-inducing T. gondii stimulus was identified as a protein(s) present in tachyzoite excretory products. Comparative proteome analysis combined with immunodetection and quantitation of a variety of T. gondii antigens indicated that the steady-state levels of actin, catalase, microneme protein 5, as well as dense granule proteins 1, 2, 3, 4, 5, 7, 8 and nucleoside triphosphate hydrolase 1 are decreased in the attenuated phenotype, whereas the surface antigen 1 and rhoptry protein 1 are produced at a similar level by virulent and attenuated parasites. In conclusion, these findings reveal a correlation between the efficient establishment of T. gondii infection in vivo and parasite synthesis of actin, catalase and several excretory proteins, and thus postulate a role for these molecules in acute virulence.

Intergenic spacer (IGS) polymorphism: a new genetic marker for differentiation of Toxoplasma gondii strains and Neospora caninum

The region between the 28S and 18S rRNA genes, including the intergenic spacer (IGS) region and the 5S rRNA gene, from 32 strains of Toxoplasma gondii and the NC1 strain of Neospora caninum was amplified and used for DNA sequencing and/or restriction fragment length polymorphism (RFLP) analysis. The 5S rDNA sequences from 20 strains of T. gondii were identical. The IGS region between the 5S and 18S rRNA genes (nontranscribed spacer 2 or NTS 2) showed 10 nucleotide variations. Six of the 10 variant positions correlated with the murine virulence of the strains. Intraspecific polymorphisms distinguished the virulent strains of zymodemes 5, 6, and 8 from other virulent strains (in zymodeme 1). RFLP methods (IGS-RFLP) were developed and used to characterize the virulent and avirulent patterns among 29 T. gondii strains. Sequence diversity of 19.8% was found between T. gondii and N. caninum when comparing a region of 919 bp at the 3' end of NTS 2. The sequence variation in ribosomal IGS could therefore be a useful marker for Toxoplasma strain identification and for distinguishing N. caninum from T. gondii.

Enzyme-linked immunosorbent assays using the recombinant dense granule antigens GRA6 and GRA1 of Toxoplasma gondii for detection of immunoglobulin G antibodies

The potential of the dense granule antigens GRA1 and GRA6 of Toxoplasma gondii to be used as diagnosis reagents in a recombinant form was evaluated. Both proteins were expressed in Escherichia coli as glutathione-S-transferase (GST) fusions. The GST-GRA1 fusion comprises the entire GRA1 sequence devoid of its N-terminal signal peptide. Separate expression of the two N- and C-terminal hydrophilic regions of GRA6 showed that only the N-terminal hydrophilic part of the protein was recognized by a pool of positive human sera in an immunoblot. One hundred T. gondii-positive and 98 negative human sera were tested in two separate immunoglobulin G (IgG)-direct enzyme-linked immunosorbent assays (ELISAs) using either GST-GRA1 or GST-GRA6-Nt recombinant protein. Whereas the sensitivity of the GST-GRA1 IgG ELISA was low (68%), the GST-GRA6-Nt IgG ELISA reached a sensitivity of 96%. The reactivity to GRA6-Nt was shown to be high even with human sera of low IgG titers. In addition, comparison of the optical density values for each serum revealed that GRA1 may complement GRA6-Nt to reach an overall sensitivity of 98%. Therefore, the GST-GRA6-Nt ELISA could be used together with another antigen like GRA1 for the development of a recombinant antigen-based test for serodiagnosis of toxoplasmosis.

Molecular typing of Toxoplasma gondii strains by GRA6 gene sequence analysis

The utility of sequence polymorphisms in the dense granule antigen GRA6 gene as typing markers for Toxoplasma gondii was investigated. The coding region of GRA6 was amplified, sequenced and compared for 30 Toxoplasma strains from eight different zymodemes (Z1-Z8). Sequence alignment identified nucleotide polymorphisms at 24 positions out of 690 bp, which correlated with murine-virulence. Types I, II, and III could be distinguished from each other on the basis of three, 10, and six variable positions, respectively. Two deletions of 15 bp and 3 bp existed in the avirulent (type II) strains. With one exception, all polymorphic positions resulted in amino acid substitutions, and the two gaps of 15 bp and 3 bp caused the deletion of six amino acids in type II strains. Intra-specific polymorphisms were also found in the virulent group. A high degree of sequence polymorphism correlating with the phenotypes of T. gondii strains points to the GRA6 gene being a good marker for strain characterisation and typing of the isolates of this apicomplexan. The large variety of amino acid changes supports the view that the GRA6 protein plays an important role in the antigenicity and pathogenicity of T. gondii. The existence of polymorphic restriction sites for endonuclease MseI was used to develop a PCR-RFLP method which could simply differentiate the three different groups (types I, II, III) of T. gondii.

Differential recognition of Toxoplasma gondii recombinant nucleoside triphosphate hydrolase isoforms by naturally infected human sera

Toxoplasma gondii possesses a highly active nucleoside triphosphate hydrolase, which has been shown to be an immunodominant antigen in mice and humans. Two isoforms (I and II) which exhibit different activities with respect to hydrolysis of ATP exist. Past studies suggest that all strains of T. gondii contain the less active nucleoside triphosphate hydrolase II, whilst only virulent strains contain the nucleoside triphosphate hydrolase I isoform. In order to further investigate the correlation between nucleoside triphosphate hydrolase isoform and biological significance, we cloned and expressed as glutathione S-transferase fusion proteins the full-length nucleoside triphosphate hydrolase I and II isoforms and two truncations of the nucleoside triphosphate hydrolase I isoform in Escherichia coli. We then used ELISAs with the full-length recombinant nucleoside triphosphate hydrolases as antigens to examine 188 naturally infected T. gondii-positive sera and 83 T. gondii-negative sera for antibody reactivity. All positive sera reacted to T. gondii whole tachyzoite lysate antigen, 31 sera reacted to both nucleoside triphosphate hydrolase isoforms, three sera reacted specifically to nucleoside triphosphate hydrolase I and two sera reacted to only nucleoside triphosphate hydrolase II. Immunoblot analysis of the five sera reacting to either nucleoside triphosphate hydrolase I or II revealed both quantitative and qualitative differences in reactivity to the two isoforms. Comparative immunoblot analysis using the truncations of the nucleoside triphosphate hydrolase I isoform, and one of these positive sera identified a presumptive differential epitope between the nucleoside triphosphate hydrolase I and II isoforms within an 81 amino acid region (amino acids 445-526) at the C-terminus of the nucleoside triphosphate hydrolase I isoform. This differential reactivity was further localised to the 12-residue region of greatest variability between the two isoforms (residues 488-499) using synthetic peptides. This is the first report where naturally infected human sera have been used to identify a differential epitope. Because this region is essential for substrate binding, an antibody response to this region may play some role in inhibition of this highly active enzyme.

A polymorphism in a DNA polymerase alpha gene intron differentiates between murine virulent and avirulent strains of Toxoplasma gondii

The IC intron, found within the DNA polymerase alpha gene of Toxoplasma gondii, was used to evaluate the genetic relationship among 10 strains of T. gondii. Sequence comparison detected polymorphisms within this 652 bp intron which correlated with murine virulence. The results reported here suggest that T. gondii contains two lineages, corresponding with their virulence, evolving independently following their separation. The extensive homology of the IC sequences within the virulent and avirulent groups affirms the close relationship of the strains within the group, as reflected by the identical nucleotide substitutions and dinucleotide insertions/deletions observed. In addition, the presence of the Nde I restriction enzyme site within the IC intron of avirulent strains allows definition of a T. gondii strain as murine virulent or avirulent without needing to test it in vivo.

Microsatellite in the beta-tubulin gene of Toxoplasma gondii as a new genetic marker for use in direct screening of amniotic fluids

To examine the correlation between Toxoplasma gondii genotype and congenital human toxoplasmosis, the polymorphism of the microsatellite consisting of a dinucleotide (TG) repeat in the intron of the beta-tubulin gene was investigated by PCR. Thirty-four reference strains were studied, including 7 strains virulent in mice and 27 strains avirulent in mice. The seven virulent strains had a (TG)8 microsatellite, and the avirulent strains had a (TG)7 microsatellite. This confirms the dichotomy already observed for virulent and avirulent strains. Additionally, 37 samples of amniotic fluid from infected fetuses were tested. All of them had the (TG)7 microsatellite marker. This result confirms that most of the human cases of congenital toxoplasmosis are due to strains avirulent in mice. Nevertheless, their virulence in human fetuses was obvious, as numerous abnormalities were observed on ultrasonic examination. The new genetic marker is the first one directly used for typing T. gondii isolates without any bias due to cultivation of the parasite. This microsatellite marker is not sufficient to type the strains which are avirulent in mice; however, seeking more polymorphic microsatellites should be worthwhile to obtain new genetic markers for direct screening of biological samples.

Detection of a novel 40,000 MW excretory Toxoplasma gondii antigen by murine Th1 clone which induces toxoplasmacidal activity when exposed to infected macrophages

To analyse target molecules of the CD4+ T-cell response to toxoplasma infection, a panel of Toxoplasma gondii-specific murine CD4+ T-cell clones has been established. Clone 3Tx15, belonging to the T helper 1 (Th1) subtype, abolished intracellular parasite growth when co-cultured with macrophages and live toxoplasma at a ratio of 2:2:1. This effect results from macrophage toxoplasmicidal activity induced upon parasite-dependent cellular interaction, an irrelevant Th1 clone failed in this three-party system. Clone 3Tx15 detects its corresponding antigen in the supernatant of infected cells and also reacts with a host cell-free preparation of T. gondii-excreted/secreted antigens. T-cell blot analysis of two-dimensionally separated toxoplasma lysate revealed a molecular weight of about 40,000 for the fractions stimulating clone 3Tx15. As checked in parallel enzyme-linked immunosorbent assay, the 40,000 MW T-cell antigen co-migrates with the excretory protein GRA4, the sole 40,000 MW T. gondii antigen hitherto known to be recognized by T lymphocytes. Nevertheless, neither recombinant GRA4 nor immunoaffinity-purified natural GRA4 was stimulatory for clone 3Tx15. Our findings thus demonstrate that Th1 clone 3Tx15 which induces toxoplasmicidal activity during antigenic interaction with infected macrophages defines a new 40,000 MW excretory T. gondii antigen.