Serodiagnosis of toxoplasmosis by using a recombinant form of the 54-kilodalton rhoptry antigen expressed in Escherichia coli

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.

Development of Human Toxo IgG ELISA Kit, and False-Positivity of Latex Agglutination Test for the Diagnosis of Toxoplasmosis

Toxoplasma gondii is an intracellular zoonotic parasite that causes infection in a wide range of warm-blooded animals and humans. The main aim of this study was to assess the diagnostic value of the recombinant SAG1 antigen (rSAG1) for T. gondii-IgG screening through the Human Toxo IgG ELISA Kit (K). The rSAG1 was expressed in E. coli (DE3), and it was purified through metal-affinity chromatography. The rSAG1 was confirmed by immunoblotting, and it had a band on 35 kDa. Total of 400 human sera were tested by LAT and K. One hundred and twenty-two (30.5%) sera were found positive by LAT and eighty-nine (22.25%) sera were found positive by K. Out of 400 samples, 80 were selected to evaluate the performance of K through commercial Toxoplasma gondii IgG ELISA Kit (C). Out of 80 human sera, 55 (68.75%) were found positive, 25 (31.25%) were found negative by K and C, respectively. The cut-off value for K was 0.398 and it was calculated through the receiver operator characteristic curve. The ELISA plates were coated at optimized concentration of rSAG1 = 0.125 µg/mL, and the test was performed by diluting the sera at 1:50. The sensitivity and specificity of K were observed to be 98.5% and 100%, respectively. The six sera (K⁻L⁺) were found positive through LAT and these human sera were later evaluated by Western blot analysis. These sera did not produce a band equivalent to 35 kDa on WB analysis thus, LAT produced false-positive results.

Review on the Current Trends of Toxoplasmosis Serodiagnosis in Humans

Toxoplasmosis is a widely distributed zoonotic infection caused by the obligate intracellular apicomplexan parasite Toxoplasma gondii. It is mainly transmitted through the ingestion of oocysts shed by an infected cat acting as its definitive host. The key to effective control and treatment of toxoplasmosis is prompt and accurate detection of T. gondii infection. Several laboratory diagnostic methods have been established, including the most commonly used serological assays such as the dye test (DT), direct or modified agglutination test (DAT/MAT), indirect hemagglutination test (IHA), latex agglutination test (LAT), indirect immunofluorescent test (IFAT), enzyme-linked immunosorbent assays (ELISA), immunochromatographic tests (ICT), and the western blot. Nonetheless, creating specific and reliable approaches for serodiagnosis of T. gondii infection, and differentiating between acute and chronic phases of infection remains a challenge. This review provides information on the current trends in the serodiagnosis of human toxoplasmosis. It highlights the advantages of the use of recombinant proteins for serological testing and provides insight into the possible future direction of these methods.

Antigens to detect the acute phase of toxoplasmosis in pregnant women: standardized comparison

A key element in any diagnostic technique is the antigen (Ag), a biomarker, but this is usually a protein that has a function to the parasite. Some biological aspects of the Ags and of the Toxoplasma gondii can influence the effectiveness of the diagnosis, as well as the antibody isotype and the characteristics of the assay. A large number of papers have assessed different proteins to distinguish the phases of infection, but the ‘indices of effectiveness’ differ among reports. This work presents for the first time a summary of all the Ags that have been evaluated, with standardized measurements of sensitivity and specificity. These values were calculated with information presented in the papers on Ag evaluations to differentiate the infection phases.

Advances in serological, imaging techniques and molecular diagnosis of Toxoplasma gondii infection

BACKGROUND

Toxoplasmosis is worldwide distributed zoonotic infection disease with medical importance in immunocompromised patients, pregnant women and congenitally infected newborns. Having basic information on the traditional and new developed methods is essential for general physicians and infectious disease specialists for choosing a suitable diagnostic approach for rapid and accurate diagnosis of the disease and, consequently, timely and effective treatment.

METHODS

We conducted English literature searches in PubMed from 1989 to 2016 using relevant keywords and summarized the recent advances in diagnosis of toxoplasmosis.

RESULTS

Enzyme-linked immunosorbent assay (ELISA) was most used method in past century. Recently advanced ELISA-based methods including chemiluminescence assays (CLIA), enzyme-linked fluorescence assay (ELFA), immunochromatographic test (ICT), serum IgG avidity test and immunosorbent agglutination assays (ISAGA) have shown high sensitivity and specificity. Recent studies using recombinant or chimeric antigens and multiepitope peptides method demonstrated very promising results to development of new strategies capable of discriminating recently acquired infections from chronic infection. Real-time PCR and loop-mediated isothermal amplification (LAMP) are two recently developed PCR-based methods with high sensitivity and specificity and could be useful to early diagnosis of infection. Computed tomography, magnetic resonance imaging, nuclear imaging and ultrasonography could be useful, although their results might be not specific alone.

CONCLUSION

This review provides a summary of recent developed methods and also attempts to improve their sensitivity for diagnosis of toxoplasmosis. Serology, molecular and imaging technologies each has their own advantages and limitations which can certainly achieve definitive diagnosis of toxoplasmosis by combining these diagnostic techniques.

Evaluation of a recombinant rhoptry protein 2 enzyme-linked immunoassay for the diagnosis of toxoplasmosis acquired during pregnancy

The aim of this study was to evaluate an enzyme-linked immunoassay with recombinant rhoptry protein 2 (ELISA-rROP2) for its ability to detect Toxoplasma gondii ROP2-specific IgG in samples from pregnant women. The study included 236 samples that were divided into groups according to serological screening profiles for toxoplasmosis: unexposed (n = 65), probable acute infection (n = 48), possible acute infection (n = 58) and exposed to the parasite (n = 65). When an indirect immunofluorescence assay forT. gondii-specific IgG was considered as a reference test, the ELISA-rROP2 had a sensitivity of 61.8%, specificity of 62.8%, predictive positive value of 76.6% and predictive negative value of 45.4% (p = 0.0002). The ELISA-rROP2 reacted with 62.5% of the samples from pregnant women with probable acute infection and 40% of the samples from pregnant women with previous exposure (p = 0.0180). Seropositivity was observed in 50/57 (87.7%) pregnant women with possible infection. The results underscored that T. gondii rROP2 is recognised by specific IgG antibodies in both the acute and chronic phases of toxoplasmosis acquired during pregnancy. However, the sensitivity of the ELISA-rROP2 was higher in the pregnant women with probable and possible acute infections and IgM reactivity.

Human toxoplasmosis: a comparative evaluation of the diagnostic potential of recombinant Toxoplasma gondii ROP5 and ROP18 antigens

Toxoplasmosis is one of the most common parasitic diseases worldwide and it poses a serious challenge regarding prevention, diagnosis and therapy. The commonly used diagnostic methods are mostly based on the detection of specific antibodies in sera. Since they are not always accurate enough and do not allow precise definition of the phase of the Toxoplasma gondii infection, there is an urgent need to find specific molecular markers of acute or chronic infection stages. This study provides a comparative assessment of recombinant ROP5 and ROP18 T. gondii proteins in the serodiagnosis of human toxoplasmosis. We found that both ROP5 and ROP18 proteins allowed the detection of specific IgM and IgG antibodies with a relatively low sensitivity; however, ROP18 IgM ELISA proved to be more sensitive than the SAG1 assay. This study also points to a relatively weak potential of the corresponding native ROP5 and ROP18 kinases in the generation of a strong antibody response in humans.

Recombinant dense granular protein (GRA5) for detection of human toxoplasmosis by Western blot

Toxoplasma gondii infects all warm-blooded animals, including humans, causing serious public health problems and great economic loss for the food industry. Commonly used serological tests require costly and hazardous preparation of whole Toxoplasma lysate antigens from tachyzoites. Here, we have evaluated an alternative method for antigen production, which involved a prokaryotic expression system. Specifically, we expressed T. gondii dense granular protein-5 (GRA5) in Escherichia coli and isolated it by affinity purification. The serodiagnostic potential of the purified recombinant GRA5 (rGRA5) was tested through Western blot analysis against 212 human patient serum samples. We found that rGRA5 protein was 100% specific for analysis of toxoplasmosis-negative human sera. Also, rGRA5 was able to detect acute and chronic T. gondii infections (sensitivities of 46.8% and 61.2%, resp.).

Toxoplasma gondii recombinant antigens as tools for serodiagnosis of human toxoplasmosis: current status of studies

Toxoplasma gondii is a parasitic protozoan which is the cause of toxoplasmosis. Although human toxoplasmosis in healthy adults is usually asymptomatic, serious disease can occur in the case of congenital infections and immunocompromised individuals. Furthermore, despite the exact recognition of its etiology, it still presents a diagnostic problem. Diagnosis of toxoplasmosis is mainly based on the results of serological tests detecting anti-T. gondii-specific antibodies in the patient's serum sample. The specificities and sensitivities of serology tests depend mostly on the diagnostic antigen(s) used. Most of the commercial serological kits currently available are based on Toxoplasma lysate antigens (TLAs). In recent years, many studies showed that recombinant antigenic proteins of T. gondii may be an alternative source of antigens which are very useful for the serodiagnosis of toxoplasmosis. This article presents a review of current studies on the application and usefulness of different T. gondii recombinant antigens in serological tests for the diagnosis of human toxoplasmosis.

rROP2(186-533): a novel peptide antigen for detection of IgM antibodies against Toxoplasma gondii

Toxoplasma gondii infections are prevalent in a wide range of mammalian hosts including humans. Infection in pregnant women may cause the transmission of parasite to the fetus that makes serious problems. IgM antibodies against Toxoplasma (Toxo-IgM) have been believed to be significant indicators for both recently acquired and congenital toxoplasmosis. So far, however, there has not been any recognized protein of T. gondii that specifically reacts to IgM antibodies. Here, an antigen exclusively for detection of IgM antibodies screened by two-dimensional electrophoresis and mass spectrometry has been reported. The study identified 13 Toxoplasma proteins probed by IgG antibodies and one (rhpotry protein 2 [ROP2]) by IgM antibodies with human sera of Toxo-IgM(-)-IgG(+) and -IgM(+)-IgG(-), respectively, which had been prescreened by Toxo-IgM and -IgG commercial kits from the suspected cases. Following cloning, expression, and purification of the fragment of ROP2(186-533), an enzyme-linked immunosorbent assay with rROP2(186-533) to measure IgM and IgG antibodies was developed. As a result, 100%(48/48) of sera with Toxo-IgM(+)-IgG(-)showed positive Toxo-IgM but none of them (0%) showed positive Toxo-IgG when rROP2(186-533) was used as antigen. Neither Toxo-IgG nor Toxo-IgM antibodies were found when tested with 59 sera of Toxo-IgM(-)-IgG(+). These results indicate that rROP2(186-533) could be used as an antigen that specifically capture Toxo-IgM antibodies and may have a high potential in the serological diagnosis of both acute acquired and congenital toxoplasmosis.

Evaluation of Pichia pastoris-expressed recombinant rhoptry protein 2 of Toxoplasma gondii for its application in diagnosis of toxoplasmosis

Rhoptry protein 2 (ROP2) of Toxoplasma gondii is a rhoptry-secreted protein that plays a critical role in parasitophorous vacuole membrane formation during invasion. In previous studies, ROP2 has been shown to be efficient in triggering humoral and cell-mediated responses. High immunogenicity of ROP2 makes it a potential candidate for diagnosis and vaccination against toxoplasmosis. In this study, the ROP2 gene was cloned into pPICZα A expression vector and extracellularly expressed in the yeast Pichia pastoris, which has numerous advantages over other expression systems for eukaryotic proteins expression. The effectiveness of the secreted recombinant ROP2 as a diagnosis agent was assessed by Western Blot with 200 human serum samples. Recombinant ROP2 reacted with toxoplasmosis-positive human serum samples and yielded an overall sensitivity of 90% and specificity of 95%. However, recombinant ROP2 is a better marker for detection of IgG (91.7%) rather than IgM (80%).

Expression, purification, and improved antigenic specificity of a truncated recombinant bp26 protein of Brucella melitensis M5-90: a potential antigen for differential serodiagnosis of brucellosis in sheep and goats

Antibodies produced in animals vaccinated using live attenuated vaccines against Brucella spp. are indistinguishable using current conventional serological tests from those produced in infected animals. One potential approach is to develop marker vaccines in which specific genes have been deleted from parental vaccine strains that show good immunogenicity and vaccine efficacy. Corresponding methods of detection for antibodies raised by the marker vaccine should also be developed. A specific fragment of the bp26 gene of Brucella melitensis M5-90 was cloned into vector pQE32 to construct the recombinant plasmid (pQE32-rΔbp26). It was used to transform Escherichia coli M15 (pREP4) host cells, which expressed the rΔbp26 protein. Subsequently, the recombinant protein was purified by immobilized metal affinity chromatography and size-exclusion chromatography. The results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the purified rΔbp26 protein was represented by only one band, with a molecular weight of 14 kDa, and it showed good antigenic specificity on western blot and enzyme-linked immunosorbent assay (ELISA). The purified rΔbp26 protein was intended to be used as an antigen to develop a novel ELISA to differentiate animals vaccinated with bp26 mutants of Brucella spp. from those infected naturally and those vaccinated with the parental vaccine strains.

Recombinant proteins in the diagnosis of toxoplasmosis

Toxoplasma gondii is an important human pathogen with a worldwide distribution. It is primarily of medical importance for pregnant women and immunocompromised patients. Primary infection of the former is often associated with fetal infection, which can lead to abortion or severe neonatal malformation. Immunocompromised patients are at risk of contracting the severe form of the disease that may be fatal. Thus, detection of T. gondii infection with high sensitivity and specificity is crucial in the management of the disease. Toxoplasmosis is generally diagnosed by demonstrating specific immunoglobulin M (IgM) and IgG antibodies to toxoplasma antigens in the patient's serum sample. Most of the commercially available tests use T. gondii native antigens and display wide variations in test accuracy. Recombinant antigens have great potential as diagnostic reagents for use in assays to detect toxoplasmosis. Thus in this review, we address recent advances in the use of Toxoplasma recombinant proteins for serodiagnosis of toxoplasmosis.

Serology using rROP2 antigen in the diagnostic of toxoplasmosis in pregnant women

Toxoplasma gondii causes severe fetal disease during acute infection in pregnant women, thus demanding early diagnosis for effective treatment and fetus preservation. Fetal tests are inefficient and risky, and diagnosis is based on maternal IgM serology, which had weak screening ability due to increased sensitivity, with alternative IgG avidity tests. Here, we performed ELISA and avidity assays using a recombinant T. gondii antigen, rROP2, in samples from 160 pregnant women screened from a large public hospital who were referred due to positive IgM assays. IgG serology and avidity assays were compared using whole T. gondii extract or rROP2. ELISA IgG detection with rROP2 showed good agreement with assays performed with T. gondii extract, but rROP2 IgG avidity assays were unrelated to whole extract antigen IgG avidity, regardless of the chaotrope used. These data show that avidity maturation is specific to individual antigen prevalence and immune response during infection. ELISA rROP2 IgG assays may be an alternative serological test for the diagnosis of toxoplasmosis during pregnancy, although our data do not support their use in avidity assays.

Toxoplasma gondii: serological characterization and immunogenicity of recombinant surface antigen 2 (SAG2) expressed in the yeast Pichia pastoris

The full length surface antigen 2 (SAG2) gene of the protozoan parasite Toxoplasma gondii was cloned and intracellularly expressed in the Pichia pastoris expression system. The molecular weight of the expressed recombinant SAG2 (36 kDa) was much larger than the native SAG2 (22 kDa). This discrepancy in size was due to hyperglycosylation, as deglycosylation assay reduced the size of the recombinant SAG2 to 22 kDa. Despite being hyperglycosylated, the recombinant SAG2 reacted strongly with pooled anti-Toxoplasma human serum, pooled anti-Toxoplasma mouse serum and a SAG2-specific monoclonal antibody. The glycosylated recombinant SAG2 was further evaluated in Western blot and in-house enzyme-linked immunosorbent assay (ELISA) using 80 human serum samples, including confirmed early acute (IgM positive, IgG negative; n=20), acute (IgM positive, IgG positive; n=20) and chronic (IgM negative, IgG positive; n=20) toxoplasmosis patients, and toxoplasmosis negative control patients (n=20). Results of the Western blot showed that the recombinant SAG2 reacted with all 60 samples of the toxoplasmosis cases but not with the Toxoplasma-negative samples. The sensitivity of in-house ELISA was 80%, 95% and 100% for early acute, acute and chronic patients' serum samples, respectively. Vaccination study showed that serum from mice immunised with the glycosylated recombinant SAG2 reacted specifically with the native SAG2 of T. gondii. The mice were significantly protected against lethal challenge with live T. gondii RH strain tachyzoites (P<0.01) and their survival time was increased compared to controls. Therefore, the present study shows that the P. pastoris-derived recombinant SAG2 was specific and suitable for use as antigen for detecting anti-Toxoplasma IgG and IgM antibodies. The vaccination study showed that recombinant SAG2 protein was immunoprotective in mice against lethal challenge.

GRA7 provides protective immunity in cocktail DNA vaccines against Toxoplasma gondii

In a previous study, single-gene vaccination with GRA1, GRA7 or ROP2 was shown to elicit partial protection against Toxoplasma gondii. In this study, the contribution of each antigen in the evoked humoral and cellular immune responses was evaluated after vaccination with plasmid mixtures containing GRA1, GRA7 and ROP2. Cocktail DNA vaccinated mice developed high antibody titers against the antigens from two-gene DNA vaccine cocktails, but lower titres when immunized with the three-gene cocktail. High numbers of IFN-gamma secreting splenocytes were generated predominantly against GRA7. Brain cyst burden was reduced by 81% in mice vaccinated with the three-gene mixture and they were completely protected against acute toxoplasmosis. Similar high levels of brain cyst reductions were obtained after vaccination with cocktails composed of GRA1 and GRA7 (89% reduction), or GRA7 and ROP2 (79% reduction), but not with the cocktail composed of GRA1 and ROP2. In low dose single-gene vaccinations, IFN-gamma and strong protection could only be elicited by GRA7. Hence, the presence of GRA7 in the DNA vaccine formulation was important for optimal protection and this was correlated with GRA7-specific IFN-gamma production. We propose GRA7 as a main component in cocktail DNA vaccines for vaccination against T. gondii.

Seroreactivity to and avidity for recombinant antigens in toxoplasmosis

To improve serodiagnostic methods for the diagnosis of acute toxoplasmosis during pregnancy, a new test system has been developed and evaluated based on the use of recombinant antigens. Five recombinant Toxoplasma gondii antigens (ROP1, MAG1, SAG1, GRA7, and GRA8) were cloned in Escherichia coli, purified, and applied directly onto nitrocellulose membranes in a line assay (recomLine Toxoplasma). A panel of 102 sera from 25 pregnant women with supposed recent toxoplasmosis and from two symptomatic children was compared to a panel of 71 sera from individuals with past infection. Both panels were analyzed using a recombinant line assay for immunoglobulin G (IgG), IgM, and IgA antibodies and a reference enzyme-linked immunosorbent assay. Within the IgM-positive samples, antibodies against ROP1 were predominant regardless of the infection state. In IgG analysis a characteristic antibody pattern was found for very recent infections. This pattern changed to a different one during the time course of infection: antibodies against GRA7 and GRA8 were characteristic for very early IgG, whereas antibodies against SAG1 and MAG1 appeared significantly later. These results were further confirmed by determination of the IgG antibody avidity for every single recombinant antigen. In the time course of infection, IgG antibodies against the early recognized antigens matured significantly earlier than those directed against the later antigens did. The IgA patterns did not give reliable information about the infection time points. The data revealed that the recombinant line assay provides valuable information on the actual state of infection, especially during the early infection time points.

Pleiotropic effect due to targeted depletion of secretory rhoptry protein ROP2 in Toxoplasma gondii

Long after their discovery, the function and biogenesis of rhoptries remain enigmatic. In Apicomplexan parasites, these organelles discharge and their contents are exocytosed at the time of host cell invasion, and are thus proposed to play an essential role in establishing the parasitophorous vacuole. In Toxoplasma gondii, ROP2 is suspected to serve as the molecular link between host cell mitochondria and parasitophorous vacuole membrane. In this study we addressed the function of ROP2. Targeted depletion of ROP2 using a ribozyme-modified antisense RNA strategy resulted in multiple effects on parasite morphology because of a disruption in the formation of mature rhoptries, and an arrest in cytokinesis. The association of host cell mitochondria with the parasitophorous vacuole membrane was abolished and the ROP2-deficient parasites had a reduced uptake of sterol from the host cell. Furthermore, these parasites invaded human fibroblasts poorly and had markedly attenuated virulence in mice. We conclude that rhoptry discharge, and in particular release of ROP2, are essential for parasite invasion, replication and host cell-parasite interaction.

High yield expression and single-step purification of Toxoplasma gondii SAG1, GRA1, and GRA7 antigens in Escherichia coli

This report describes a simple, highly efficient and reproducible method for obtaining large quantities of highly pure recombinant Toxoplasma gondii antigens, which can be used for diagnostic application. The obtained T. gondii SAG1, GRA1, and GRA7 antigens (as fusion proteins), expressed in Escherichia coli, contained polyhistidine tags at the N- and C-ends that allowed single-step isolation by metal-affinity chromatography on Ni(2+)-IDA-Sepharose columns. The immunoreactivity of the recombinant antigens was tested in an enzyme-linked immunosorbent assay (ELISA) format for potential application in the serodiagnosis of T. gondii infection.

High level of expression of the Toxoplasma gondii-recombinant Rop2 protein in Escherichia coli as a soluble form for optimal use in diagnosis

The rhoptry 2 protein (Rop2) is an interesting protein of Toxoplasma gondii that is involved in the parasite invasion of host cell, it has three T-cell epitopes and high antigenic value. However, the expression of Rop2 as a recombinant protein in Escherichia coli is not an easy task, showing low levels of expression or degradation and solubility problems. Using a recombinant Rop2(196-561) fused to 6 histidine residues, we showed high levels of expression in bacteria growing in terrific broth. rRop2(196-561) was purified mainly as a soluble product and in high concentrations (approx 1 mg/mL) under native conditions (40 mM imidazol in phosphate buffer). However, after a cycle of freezing-thawing rRop2(196-561) became insoluble. When glycerol was added to 26%, immediately after purification, the protein stayed soluble after cycles of freezing-thawing. Finally, it was demonstrated that under these conditions soluble rRop2(196-561) keeps its diagnostic value in contrast with the insoluble protein.

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.

Recombinant antigens to detect Toxoplasma gondii-specific immunoglobulin G and immunoglobulin M in human sera by enzyme immunoassay

We have evaluated the diagnostic utility of eleven Toxoplasma gondii recombinant antigens (P22 [SAG2], P24 [GRA1], P25, P28 [GRA2], P29 [GRA7], P30 [SAG1], P35, P41 [GRA4], P54 [ROP2], P66 [ROP1], and P68) in immunoglobulin G (IgG) and IgM recombinant enzyme-linked immunosorbent assays (Rec-ELISAs). Following an initial evaluation, six recombinant antigens (P29, P30, P35, P54, P66, and P68) were tested in the IgG and IgM Rec-ELISAs with four groups of samples which span the toxoplasmosis disease spectrum (negative, chronic infection, acute infection, and recent seroconversion). Our results suggest that the combination of P29, P30, and P35 in an IgG Rec-ELISA and the combination of P29, P35, and P66 in an IgM Rec-ELISA can replace the tachyzoite antigen in IgG and IgM serologic tests, respectively. The relative sensitivity, specificity, and agreement for the IgG P29-P30-P35 Rec-ELISA were 98.4, 95.7, and 97.2%, respectively. The resolved sensitivity, specificity, and agreement for the IgM P29-P35-P66 Rec-ELISA were 93.1, 95.0, and 94. 5%, respectively. Relative to the tachyzoite-based immunocapture IgM assay, the IgM P29-P35-P66 Rec-ELISA detects fewer samples that contain IgG antibodies with elevated avidity from individuals with an acute toxoplasmosis.

DNA vaccination with genes encoding Toxoplasma gondii antigens GRA1, GRA7, and ROP2 induces partially protective immunity against lethal challenge in mice

C57BL/6, C3H, and BALB/c mice were vaccinated with plasmids encoding Toxoplasma gondii antigens GRA1, GRA7, and ROP2, previously described as strong inducers of immunity. Seroconversion for the relevant antigen was obtained in the majority of the animals. T. gondii lysate stimulated specific T-cell proliferation and secretion of gamma interferon (IFN-gamma) in spleen cell cultures from vaccinated BALB/c and C3H mice but not in those from control mice. Although not proliferating, stimulated splenocytes from DNA-vaccinated C57BL/6 mice also produced IFN-gamma. No interleukin-4 was detected in the supernatants of lysate-stimulated splenocytes from DNA-vaccinated mice in any of the mouse strains evaluated. As in infected animals, a high ratio of specific immunoglobulin G2a (IgG2a) to IgG1 antibodies was found in DNA-vaccinated C3H mice, suggesting that a Th1-type response had been induced. For BALB/c mice, the isotype ratio of the antibody response to DNA vaccination was less polarized. The protective potential of DNA vaccination was demonstrated in C3H mice. C3H mice vaccinated with plasmid encoding GRA1, GRA7, or ROP2 were partially protected against a lethal oral challenge with cysts of two different T. gondii strains: survival rates increased from 10% in controls to at least 70% after vaccination in one case and from 50% to at least 90% in the other. In vaccinated C3H mice challenged with a nonlethal T. gondii dose, the number of brain cysts was significantly lower than in controls. DNA vaccination did not protect BALB/c or C57BL/6 mice. Our results demonstrate for the first time in an animal model a partially protective effect of DNA vaccination against T. gondii.

Expression of a recombinant Toxoplasma gondii ROP2 fragment as a fusion protein in bacteria circumvents insolubility and proteolytic degradation

A 268-amino-acid-residue carboxy-terminal antigenic fragment of the Toxoplasma gondii rhoptry protein ROP2 (recROP2(t), residues 196-464) was expressed in Escherichia coli. This recombinant fragment was produced at low concentration and in a highly insoluble form. By contrast, the level of recROP2(t) production was drastically greater when the same coding sequence was fused to the C-terminus of thioredoxin (TRX) or to the maltose-binding protein (MBP) gene. While both fusion proteins were found to be mainly insoluble, solubilization could be achieved without significant degradation. MBP was more efficient than TRX in increasing the recovery of soluble protein with more than 10% of total MBP-recROP2(t) being readily expressed in a soluble form. Moreover, the insoluble form of MBP-recROP2(t) could be correctly refolded with a recovery of more than 80%. Both forms of MBP-recROP2(t) were purified to homogeneity by amylose chromatography. In contrast, the refolding of TRX-recROP2(t) promoted aggregation of the protein, which was prevented by the use of zwitterionic detergent during the one-step purification by gel filtration. Subsequent proteolytic cleavages of purified TRX-recROP2(t) and of MBP-recROP2(t) led respectively to the complete degradation or to the truncation of the recROP2(t) moiety. However, recROP2(t), despite the presence of the fusion partners, adopted a suitable conformation recognized by human serum-derived antibodies from T. gondii-seropositive individuals. Finally, both fusion proteins were able to induce specific humoral and cell-mediated immune response to the ROP2 fragment. Such fusions could represent an alternative to study the immunogenicity of T. gondii proteins which are difficult to produce because of insolubility and degradation.

Evaluation of recombinant dense granule antigen 7 (GRA7) of Toxoplasma gondii for detection of immunoglobulin G antibodies and analysis of a major antigenic domain

Dense granule protein 7 (GRA7) of Toxoplasma gondii was expressed in Escherichia coli as a fusion protein. The leader peptide contained a 25-amino-acid mouse tumor necrosis factor fragment and six histidyl residues. After purification by metal chelate affinity chromatography, the antigen was evaluated in an enzyme-linked immunosorbent assay for detection of immunoglobulin G (IgG). For two sets of IgG-positive human serum samples, obtained from routine screening, an overall sensitivity of 81% was obtained. For chronic-phase sera, the sensitivity of detection was 79%, but chronic-phase sera with low titers were more difficult to detect (65% sensitivity for sera with immunofluorescence titer of 1/64). When GRA7 was combined with Tg34AR (rhoptry protein 2 C-terminal fragment), the sensitivity rose to 96%. For a set of acute-phase serum samples tested on GRA7, the sensitivity of detection was 94%, and high-titer IgM-positive sera were detected at an especially high rate. In contrast, when Tg34AR was used, the sensitivity was only 85% for this latter set of serum samples. Three truncated GRA7 fragments containing the same leader peptide as that of recombinant GRA7 were produced. The shortest fragment (97 N-terminal amino acids) was not reactive with human sera or with a specific anti-GRA7 monoclonal antibody, while the two larger fragments were reactive. The most important antigenic domain of GRA7 for human sera was localized between residues 97 and 146. The epitope for the specific monoclonal antibody could be further narrowed down by the use of synthetic peptides, but this epitope is not recognized by sera from T. gondii-infected humans. These results indicate that GRA7 may be considered as an additional tool for studying the immune response to T. gondii.

Detection of human Toxoplasma-specific immunoglobulins A, M, and G with a recombinant Toxoplasma gondii rop2 protein

The Toxoplasma gondii rhoptry protein Rop2 was expressed in Escherichia coli as a fusion protein containing 44 kDa of the 55-kDa mature Rop2, supplied with six histidyl residues at the N-terminal end (Rop2196-561). Humoral response during Toxoplasma infection of humans was analyzed by immunoglobulin G (IgG), IgA, and IgM enzyme-linked immunosorbent assay with Rop2196-561 as the antigen substrate. The analyzed sera were divided according to T. gondii-specific serological tests (IgG, IgA, or IgM indirect immunofluorescence and IgA or IgM immunosorbent agglutination assay) as group A (IgG+ IgA- IgM-; n = 35), group B (IgG+ IgA+ IgM+; n = 21), group C (IgG+ IgA+ IgM-; n = 5), and group D (IgG+ IgA- IgM+; n = 16). Twenty-six T. gondii-seronegative sera from individuals with other infections were also included (group E). Anti-Rop2 IgG antibodies were detected in 82.8% of group A sera and in 97.6% of the sera with acute-phase marker immunoglobulins (groups B, C, and D). The percentage of IgA antibody reactivity against Rop2196-561 was 17.1% in group A, 50% in group D, and 80.8% in groups B and C. The percentage of IgM antibody reactivity was 0% in groups A and C and 62% in groups B and D. Sera from group E failed to show IgA, IgM, or IgG antibody reactivity. Since T. gondii Rop2 elicits a strong humoral response from an early stage of infection, it is suggested that recombinant Rop2196-561 would be suitable for use in diagnostic systems, in combination with other T. gondii antigens, to detect specific IgG, IgA, and IgM antibodies.

Identification and heterologous expression of a new dense granule protein (GRA7) from Toxoplasma gondii

Immunoscreening of an expression library constructed with Toxoplasma gondii tachyzoite mRNA with sera from toxoplasmosis-positive humans has led to the identification of a new parasite antigen. Sequence analysis of the gene encoding this antigen allowed the calculation of the theoretical molecular mass (25,857 Da) and showed that the protein contains a putative signal sequence. The C-terminal region contains two hydrophobic regions, the last of which has the characteristics of a membrane-spanning domain. When the protein was heterologously expressed in E. coli and tested by Western blot, it reacted with the human sera originally used for screening. The new antigen also reacted with a monoclonal antibody raised against the entire parasite. Ultrastructural analysis showed that the protein is localized in the dense granules. After host cell invasion, the protein is secreted into the vacuolar network, the parasitophorous vacuole membrane, and into extensions protruding in the cytoplasm. Therefore, it is suggested to designate this new dense granule protein GRA7, following the established nomenclature for this protein family.

Humoral immune responses of Brucella-infected cattle, sheep, and goats to eight purified recombinant Brucella proteins in an indirect enzyme-linked immunosorbent assay

Brucellosis research is currently focused on the identification of nonlipopolysaccharide (LPS) antigens which could potentially be useful for the specific serologic diagnosis of brucellosis as well as for vaccinal prophylaxis. On the basis of previous reports, we selected eight Brucella proteins (OMP36, OMP25, OMP19, OMP16, OMP10, p17, p15, and p39) as candidate antigens to be further evaluated. The genes encoding these proteins were cloned, sequenced, and overexpressed in Escherichia coli. The recombinant proteins were purified with a polyhistidine tag and metal chelate affinity chromatography and evaluated in an indirect enzyme-linked immunosorbent assay (iELISA). The specificity of the iELISA was determined with sera from healthy cattle, sheep, and goats and ranged from 95 to 99%, depending on the recombinant antigen and the species tested. Sera from experimentally infected, and from naturally infected, animals were used to evaluate the sensitivity of the iELISA. The antiprotein antibody response was often delayed when compared to the anti-smooth LPS (S-LPS) response and was limited to animals which developed an active brucellosis infection (experimentally infected pregnant animals and sheep and goats from areas where brucellosis is still endemic). Among the recombinant antigens, the three cytoplasmic proteins (p17, p15, and p39) gave the most useful results. More than 80% of the animals positive in S-LPS serology were also positive with one of these cytoplasmic proteins alone or a combination of two of them. None of the recombinant antigens detected experimentally infected nonpregnant cows and sheep or naturally infected cattle. This study is a first step towards the development of a multiprotein diagnostic reagent for brucellosis.

Toxoplasma gondii recombinant antigens H4 and H11: use in ELISAs for detection of toxoplasmosis in swine

Toxoplasma gondii recombinant antigens H4 and H11 were assessed for their potential for use in ELISA for diagnosis of toxoplasmosis in swine. The antigens were evaluated with sera from young pigs experimentally infected with T. gondii. Results were compared with ELISAs based on a native T. gondii antigen extract. Although recombinant antigen ELISAs showed a sharp rise in response with some sera very early after infection, they were relatively non-reactive with late (chronic) infection sera.

Epitopes recognized by human T lymphocytes in the ROP2 protein antigen of Toxoplasma gondii

The ROP2 protein of Toxoplasma gondii possesses immunological and biological properties which have led to its proposal as a vaccine candidate. To identify epitopes recognized by human T cells in the ROP2 antigen, we submitted the sequence of this protein to three reported T-cell epitope prediction algorithms. Three sequences that were predicted by all three methods were selected (sequences 197 to 216, 393 to 410, and 501 to 524), and the corresponding peptides were synthesized. The peptides were first tested in a proliferation assay with a DPw4-restricted, ROP2-specific human T-cell clone, and the peptide corresponding to residues 197 to 216 was shown to stimulate the T-cell clone. The three peptides were further tested in proliferation assays with peripheral blood mononuclear cells from a panel of T. gondii-seropositive and -seronegative individuals. We found that cells from a high proportion of the seropositive donors (64%) recognized at least one of the three peptides. The most frequently recognized ones were peptides 197 to 216 (45%) and 501 to 524 (36%). None of the seronegative donors responded to any peptide. These results show that the ROP2 antigen of T. gondii contains T-cell epitopes recognized by a high percentage of the immune population and further strengthen its potential as a vaccine candidate.

Molecular characterization, occurrence, and immunogenicity in infected sheep and cattle of two minor outer membrane proteins of Brucella abortus

Screening of a Brucella abortus genomic library with two sets of monoclonal antibodies allowed the isolation of the genes corresponding to two minor outer membrane proteins (OMP10 and OMP19) found in this bacterial species. Sequence analysis of the omp10 gene revealed an open reading frame capable of encoding a protein of 126 amino acids. The nucleotide sequence of the insert producing the OMP19 protein contains two overlapping open reading frames, the largest of which (177 codons) was shown to encode the protein of interest. Analysis of the N-terminal sequences of both putative proteins revealed features of a bacterial signal peptide, and homology to the bacterial lipoprotein processing sequence was also observed. Immunoblotting with monoclonal antibodies specific for OMP10 or OMP19 showed that both proteins are present in the 34 Brucella strains tested, representing all six Brucella species and all their biovars. The OMP19 detected in the five Brucella ovis strains examined migrated at an apparent molecular weight that is slightly higher than those of the other Brucella species, confirming the divergence of B. ovis from these species. OMP10 and OMP19 were produced in recombinant Escherichia coli and purified to homogeneity for serological analysis. A large fraction of sera from sheep naturally infected with Brucella melitensis were reactive with these proteins in an enzyme-linked immunosorbent assay, whereas sera from B. abortus-infected cattle were almost completely unreactive in this assay.

Cellular and humoral immune responses to recombinant antigens in sheep infected with Toxoplasma gondii

Immune responses to recombinant fragments of the Toxoplasma gondii antigens ROP2 and GRA2 were investigated in sheep naturally and experimentally infected with T. gondii oocysts. Specific serum antibodies to C-terminal fragments were detected by ELISA. Cell-mediated responses in peripheral blood mononuclear cells were demonstrated by proliferation and interferon-gamma production following in vitro stimulation with the ROP2 fragment. This data indicates the presence of epitopes for sheep B cells in the recombinant GRA2 fragment and for both B and T cells in the ROP2 fragment.

Cloning and sequence analysis of a newly identified Brucella abortus gene and serological evaluation of the 17-kilodalton antigen that it encodes

A thus far unknown gene encoding a Brucella abortus protein has been isolated from a lambda gt11 expression library probed with sera from Brucella-infected sheep. Sequence analysis of the cloned gene revealed the presence of an open reading frame of 158 amino acids encoding a protein of 17.3 kDa (calculated molecular mass). The recombinant B. abortus protein, expressed in Escherichia coli, and the corresponding Brucella melitensis protein migrated at the same apparent molecular masses as shown by Western blotting (immunoblotting). Among a series of serum samples from B. melitensis- or B. abortus-infected sheep and cows, 51 and 39%, respectively, showed a signal at 17 kDa on Western blot analysis of total protein extract from Brucella bacteria. These figures amount to 70 and 61% for sheep and cattle, respectively, in a competitive enzyme-linked immunosorbent assay with a specific monoclonal antibody. These data indicate that the 17-kDa antigen may be useful for serological diagnosis of Brucella infection.

ELISAs based on recombinant antigens for seroepidemiological studies on Toxoplasma gondii infections in cats

Two recombinant Toxoplasma gondii polypeptides, H4 and H11, were tested as diagnostic antigens in enzyme-linked immunosorbent assays (ELISAs). The results obtained by ELISAs based on single H4 (H4-ELISA), on single H11 (H11-ELISA) and on a mixture of H4 and H11 (H4/H11-ELISA) were compared with results obtained by an ELISA based on traditional ELISA antigen (TEA-ELISA), an indirect fluorescent antibody test (IFAT), the Sabin-Feldman dye test (SFDT) and a direct agglutination test (DAT). A total of 306 cats from a suburban cat population were tested of which about 45% showed serological evidence of T. gondii infection. Infection rates varied from about 32% for cats kept indoors to about 55% for stray cats. Specificities > 99% were observed for all ELISAs based on the recombinant antigens (H4-ELISA, H11-ELISA and H4/H11-ELISA). The H4/H11-ELISA also reached a sensitivity of 95% which compared very favourably with those observed for the TEA-ELISA (98%) and for the IFAT (94%). Negative and positive predictive values for the H4/H11-ELISA were 96 and > 99%, respectively. Antibody titres measured by the H4/H11-ELISA also correlated well with those measured by the SFDT and the DAT. Hence, the H4/H11-ELISA appears to be a very suitable test for sero-epidemiological studies on T. gondii infections in cats.