Diagnostic implications of kinetics of immunoglobulin M and A antibody responses to Toxoplasma gondii

Anti-Toxoplasma gondii IgM Long Persistence: What Are the Underlying Mechanisms?

Diagnosis of Toxoplasma gondii acute infection was first attempted by detection of specific IgM antibodies, as for other infectious diseases. However, it was noted that this immunoglobulin declines slowly and may last for months or even years. Apart from the diagnostic problem imposed on clinical management, this phenomenon called our attention due to the underlying phenomena that may be causing it. We performed a systematic comparison of reports studying IgM antibody kinetics, and the data from the papers were used to construct comparative plots and other graph types. It became clear that this phenomenon is quite generalized, and it may also occur in animals. Moreover, this is not a technical issue, although some tests make more evident the prolonged IgM decay than others. We further investigated biological reasons for its occurrence, i.e., infection dynamics (micro-reactivation–encystment, reinfection and reactivation), parasite strain relevance, as well as host innate, natural B cell responses and Ig class-switch problems inflicted by the parasite. The outcomes of these inquiries are presented and discussed herein.

IgG Avidity Test as a Tool for Discrimination between Recent and Distant Toxoplasma gondii Infection—Current Status of Studies

Toxoplasma gondii, an obligate intracellular protozoan parasite, is the causative agent of one of the most prevalent zoonoses worldwide. T. gondii infection is extremely important from a medical point of view, especially for pregnant women, newborns with congenital infections, and immunocompromised individuals. Thus, an accurate and proper diagnosis of this infection is essential. Among the available diagnostic tests, serology is commonly used. However, traditional serological techniques have certain limitations in evaluating the duration of T. gondii infection, which is problematic, especially for pregnant women. Avidity of T. gondii-specific IgG antibodies seems to be a significant tool for discrimination between recent and distant infections. This article describes the problem of diagnosis of T. gondii infection, with regard to IgG avidity tests. The IgG avidity test is a useful serological indicator of toxoplasmosis, which in many cases can confirm or exclude the active form of the disease. IgG antibodies produced in the recent primary T. gondii infection are of low avidity while IgG antibodies with high avidity are detected in the chronic phase of infection. Furthermore, this paper presents important topics of current research that concern the usage of parasite recombinant antigens that may improve the performance of IgG avidity tests.

Role of Toxoplasma IgA as Part of a Reference Panel for the Diagnosis of Acute Toxoplasmosis during Pregnancy

This study evaluated the usefulness of adding the Toxoplasma gondii IgA antibody enzyme-linked immunosorbent assay (ELISA) to the serologic panel of tests done for the diagnosis of acute toxoplasmosis in pregnant women in a reference laboratory in the United States. We conducted a retrospective study of 690 consecutive pregnant women with positive T. gondii IgG antibody test results who also had T. gondii IgA and IgM antibody tests performed.

This study evaluated the usefulness of adding the Toxoplasma gondii IgA antibody enzyme-linked immunosorbent assay (ELISA) to the serologic panel of tests done for the diagnosis of acute toxoplasmosis in pregnant women in a reference laboratory in the United States. We conducted a retrospective study of 690 consecutive pregnant women with positive T. gondii IgG antibody test results who also had T. gondii IgA and IgM antibody tests performed. Patients were defined as acutely or chronically infected with T. gondii based on a panel of serologic tests performed at the Palo Alto Medical Foundation Toxoplasma Serology Laboratory (PAMF-TSL). Among the 81 women who were positive by T. gondii IgA antibody ELISA testing, 61 (75.3%) were acutely infected with T. gondii, while of the 547 who were negative by IgA testing, only 24 (4.4%) were acutely infected (P < 0.001). Among the 71 women who were positive by both IgA and IgM antibody tests, 61 (85.9%) were acutely infected, whereas 24 (19.2%) of the 125 women who were positive by only the IgM ELISA were acutely infected (P < 0.001). These results demonstrate that pregnant women with T. gondii IgA antibodies are more likely than pregnant women without T. gondii IgA antibodies to have had a recent infection with T. gondii. Toxoplasma IgA antibody testing can therefore improve the accuracy of a serologic panel for the diagnosis of acute toxoplasmosis during pregnancy. Physicians who ordered testing only for T. gondii IgG and IgM should also request additional testing for IgA and IgG avidity, if both IgG and IgM are positive. This further testing should, ideally, be performed in a reference laboratory.

Diagnosis of toxoplasmosis and typing of Toxoplasma gondii

Toxoplasmosis, caused by the obligate intracellular protozoan Toxoplasma gondii, is an important zoonosis with medical and veterinary importance worldwide. The disease is mainly contracted by ingesting undercooked or raw meat containing viable tissue cysts, or by ingesting food or water contaminated with oocysts. The diagnosis and genetic characterization of T. gondii infection is crucial for the surveillance, prevention and control of toxoplasmosis. Traditional approaches for the diagnosis of toxoplasmosis include etiological, immunological and imaging techniques. Diagnosis of toxoplasmosis has been improved by the emergence of molecular technologies to amplify parasite nucleic acids. Among these, polymerase chain reaction (PCR)-based molecular techniques have been useful for the genetic characterization of T. gondii. Serotyping methods based on polymorphic polypeptides have the potential to become the choice for typing T. gondii in humans and animals. In this review, we summarize conventional non-DNA-based diagnostic methods, and the DNA-based molecular techniques for the diagnosis and genetic characterization of T. gondii. These techniques have provided foundations for further development of more effective and accurate detection of T. gondii infection. These advances will contribute to an improved understanding of the epidemiology, prevention and control of toxoplasmosis.

Identification of Toxoplasma gondii antigens associated with different types of infection by serum antibody profiling

Acquisition of acute toxoplasmosis during the first trimester of pregnancy can have catastrophic consequences for the foetus. Diagnosis is routinely based on the detection of maternal Toxoplasma gondii--antibodies using whole parasite extracts as detection antigen. While such assays are sensitive, they show no specificity for the stage of infection. We hypothesized diagnosis might be improved using recombinant antigens for detection, particularly if antibodies to certain antigen(s) were associated with early or late stages of infection. To address this, protein microarrays comprising 1513 T. gondii exon products were probed with well-characterized sera from seronegative ('N') controls, and acute ('A'), chronic/IgM-persisting ('C/M') and chronic ('C') toxoplasmosis cases from Turkey. Three reactive exon products recognized preferentially in A infections, and three recognized preferentially in C/M infections, were expressed in Escherichia coli and tested for discrimination in IgG ELISAs. The best discriminators were exon 1 of TGME49_086450 (GRA5) which detected C/M infections with 70.6% sensitivity and 81.8% specificity, and exon 6 of TGME49_095700 (ubiquitin transferase domain-containing protein) which detected A infections with 84.8% sensitivity and 82.4% specificity. Overall, the data support a recombinant protein approach for the development of improved serodiagnostic tests for toxoplasmosis.

Identification of potential serodiagnostic and subunit vaccine antigens by antibody profiling of toxoplasmosis cases in Turkey

Toxoplasmosis, caused by infection of the protozoan parasite Toxoplasma gondii, is associated with mild disease in healthy individuals, whereas individuals with depressed immunity may develop encephalitis, neurologic disorders, and other organ diseases. Women who develop acute toxoplasmosis during pregnancy are at risk of transmitting the infection to the fetus, which may lead to fetal damage. A diagnosis is usually confirmed by measuring IgG, or IgM where it is important to determine the onset of infection. A negative IgM result essentially excludes acute infection, whereas a positive IgM test is largely uninterpretable because IgM can persist for up to 18 months after infection. To identify antigens for improved diagnosis of acute infection, we probed protein microarrays displaying the polypeptide products of 1357 Toxoplasma exons with well-characterized sera from Turkey. The sera were classified according to conventional assays into (1) seronegative individuals with no history of T. gondii infection; (2) acute infections defined by clinical symptoms, high IgM titers, and low avidity IgG; (3) chronic/convalescent cases with high avidity IgG but persisting IgM; (iv) true chronic infections, defined by high avidity IgG and no IgM. We have identified 38 IgG target antigens and 108 IgM target antigens that can discriminate infected patients from healthy controls, one or more of which could form the basis of a 'tier-1' test to determine current or previous exposure. Of these, three IgG antigens and five IgM antigens have the potential to discriminate chronic/IgM persisting or true chronics from recent acutely infected patients (a 'tier-2' test). Our analysis of the antigens revealed several enriched features relative to the whole proteome, which include transmembrane domains, signal peptides, or predicted localization at the outer membrane. This is the first protein microarray survey of the antibody response to T. gondii, and will help in the development of improved serodiagnostics and vaccines.

Seroprevalence of Toxoplasma gondii antibodies in North Indian blood donors: implications for transfusion transmissible toxoplasmosis

INTRODUCTION

Transfusion transmitted Toxoplasma gondii (T. gondii) can result in significant clinical consequences in immunocompromised and multiply transfused patients, pregnant women and fetus etc. Anti-T. gondii seroprevalence, specifically IgM antibodies reflect the risk of transfusion transmission.

METHODS

Four hundred and ninety-three blood donors in a tertiary care hospital in North India were screened for IgG and IgM anti-T. gondii antibodies by enzyme linked immunosorbent assay (ELISA).

RESULTS

The prevalence of IgG and IgM anti-T. gondii antibodies was 51.8% and 5% respectively. The prevalence was higher in females (M=51.6%, F=89.2%) and in replacement donors (replacement donors=63.2%, voluntary donors=33.5%).

CONCLUSION

The donor population constitutes a significant risk of transfusion transmitted toxoplasmosis. Effective strategies are required to prevent transfusion transmitted toxoplasmosis.

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.

Assessment of the value of detecting specific IgA antibodies for the diagnosis of a recently acquired primary Toxoplasma infection

OBJECTIVE

To assess the value of detecting IgA antibodies for the diagnosis of a recently acquired primary Toxoplasma infection.

METHODS

IgA antibodies were screened in sera from 87 women with different serological profiles of Toxoplasma gondii IgM and IgG antibodies and Toxoplasma-specific IgG avidity. The IgM and IgG antibodies and the IgG avidity were measured with an automated Vitek Immuno Diagnostic Assay System (VIDAS). Anti-T.gondii IgA was measured with Platelia Toxo IgA TMB kits.

RESULTS

All 12 sera obtained from women with clinical and/or serological evidence of a recently acquired Toxoplasma infection were positive for IgA. In 42 serum samples obtained more than 6 months after T. gondii infection from women with no clinical evidence of infection, but who had a positive IgM test and a high IgG avidity index, the IgA-enzyme linked immunosorbent assay (ELISA) test results were positive, negative, and doubtful in 16 (38.1%), 23 (54.8%), and 3 (7.1%) sera, respectively. In eight women, IgA was detected in sera collected more than 9 months after the onset of infection. The IgA test result was also positive in 11 of 12 sera (91.7%) obtained from women with no clinical evidence of toxoplasmosis, but who had a positive IgM test and a borderline IgG avidity index. The IgA-ELISA was negative in 21 sera obtained more than 2 years after the onset of T. gondii infection from women with no clinical evidence of toxoplasmosis, but who had a negative IgM test and a positive IgG test.

CONCLUSION

These results show that IgA is not a dependable marker for a recently acquired primary Toxoplasma infection.

Delayed maturation of immunoglobulin G avidity: implication for the diagnosis of toxoplasmosis in pregnant women

The low avidity of immunoglobulin G has been reported to be a useful marker of recent infection with Toxoplasma. Several investigators, however, have published discrepant result on the maturation of avidity over time. The aim of this study was to analyse persistent low avidity of immunoglobulin G in immunocompetent individuals and in pregnant women and how it could interfere in the flowchart of antenatal diagnosis of toxoplasmosis in the latter group. An international literature search was conducted together with a retrospective study of a hospital database. Eleven publications that met the inclusion criteria reported delayed maturation of avidity at a frequency ranging from 0 to 66.6% of the patients. Examination of those publications demonstrated an important heterogeneity in the type of assay used, the calculation of avidity, the cutoff above which avidity was considered to be elevated, and the delay since infection after which indices are expected to be high. In the hospital database, persistent low avidity was found even after a median follow-up period of 6 years. Different factors could interfere with maturation of avidity, such as variations between individuals, the assay system used, and, possibly, the treatment administered. The results of this study clearly demonstrate that, in a pregnant woman, an acute infection cannot be reliably diagnosed solely on the basis of low avidity of immunoglobulin G. Further investigations and standardization of assays are urgently needed. Estimation of the time of infection remains difficult, especially in cases in which the samples are drawn late in pregnancy; the final estimate must be based on several tests repeated at intervals of weeks.

Detection of immunoglobulin M antibodies specific for Toxoplasma gondii with increased selectivity for recently acquired infections

Toxoplasma gondii infections can cause serious complications in pregnant women, leading to miscarriage, stillbirth, and birth defects. Definitive diagnosis of T. gondii acute infection is therefore critical for the clinical management of a mother and her fetus. Positive immunoglobulin M (IgM) results are not sufficient as evidence of recent infection, as these antibodies are often present for many months. Further, IgG avidity and differential agglutination tests, two tests used by reference laboratories to distinguish between recent and past infections, are not always in agreement, and both methods yield a significant number of indeterminate results. We report the development of a new toxoplasma IgM immunoassay that is performed by using a bead-based immunoassay on an automated analyzer (BioPlex 2200). Initial validation included 204 samples from pregnant women and 198 samples from asymptomatic healthy adults. An overall specificity of 99% was observed. Further, 100% sensitivity for acute infections was observed for 10 well-characterized seroconversion panels. We then examined 50 samples from pregnant women, all of which were IgM positive by ELISA, which had been fully evaluated in a reference laboratory. Of the 50 samples, 34 (68%) tested positive in the BioPlex 2200 toxoplasma IgM assay, of which 32 of 34 (94%) exhibited an acute or equivocal pattern by differential agglutination. Of the 16 negative samples, 15 (94%) showed high-IgG-avidity antibodies. Collectively, these results suggest that this new toxoplasma assay shows a preferential response to IgM antibodies produced by recent infections, reducing the number of positive results for pregnant women that will require extensive additional clinical evaluation.

Aqueous humor and serum immunoblotting for immunoglobulin types G, A, M, and E in cases of human ocular toxoplasmosis

The purpose of this study was to compare the local and systemic Toxoplasma-specific humoral immune responses in individuals with ocular toxoplasmosis (OT). To this end, paired aqueous humor and serum samples from 46 individuals with active OT and from 30 individuals without inflammatory eye disease (controls) were analyzed by immunoblotting for anti-Toxoplasma immunoglobulin G (IgG), IgA, IgM, and IgE directed against 20- to 120-kDa antigens. The presence in the aqueous humor of a unique band, or of at least three bands that were at least three times more intense in aqueous humor than in serum, was taken as evidence of local antibody production. IgG bands were detected in 98% of the aqueous humor samples, while IgA bands were detected in 76%, IgM bands were detected in 8%, and IgE bands were not detected in any. Evidence of local production of specific antibodies was found in 32 cases (70%) (IgG in 23 [50%]; IgA in 16 [35%]). In 10 instances (22%), routine laboratory tests were not indicative of OT. In 14 cases (30%), no local antibody production was detected by immunoblotting; 3 of these cases yielded evidence of local antibody production according to the Goldmann-Witmer coefficient. Local antibody production was revealed for 7 of the 30 controls (23%). Hence, the sensitivity of immunoblotting for IgG and IgA is 70%, and the specificity is 77%. We conclude that immunoblotting for local specific IgG and IgA supports the clinical diagnosis of OT in 70% of cases. In 22% of these, the diagnosis is not confirmed by other laboratory tests. Hence, immunoblotting increases the sensitivity of routine laboratory tests and should be considered for samples that register negative by such tests.

Clinical value of specific immunoglobulin E detection by enzyme-linked immunosorbent assay in cases of acquired and congenital toxoplasmosis

The clinical value of immunoenzymatic (enzyme-linked immunosorbent assay) detection of anti-Toxoplasma immunoglobulin E (IgE) was assessed by studying 2,036 sera from 792 subjects, comprising seronegative controls and subjects with acute, active, reactivated, or congenital toxoplasmosis. Included were nonimmunized adults; pregnant women with recently acquired infection (acute toxoplasmosis); immunocompetent subjects with recently acquired severe infection (active toxoplasmosis) expressed as fever, adenopathies, splenomegaly, pneumonia, meningitis, or disseminated infection; subjects-some of them immunocompromised-whose previously moderate IgG antibody levels rose, suggesting a reactivation of quiescent toxoplasmosis; and infants born to seroconverted mothers and evaluated for diagnosis of congenital infection and therapeutic management. Specific IgE antibodies were never detected in seronegative subjects. They were present in 85.7% of asymptomatic seroconverters and in 100% of seroconverters with overt toxoplasmosis, following two different kinetics: in the former, the specific IgE titer generally presented a brief peak 2 to 3 months postinfection and then fell rapidly, whereas specific IgE persisted at a very high titer for several months in the latter. IgE emerged concomitantly with the increase in IgG during toxoplasmic reactivation. For neonatal diagnosis of congenital toxoplasmosis, IgE was less informative than IgM and IgA (sensitivities, 59.5, 64.3, and 76.2%, respectively) and had a specificity of 91.9%. Nevertheless, simultaneous measurement of the three isotypes at birth improved the diagnostic yield to 81% relative to the combination of IgA and IgM. Emergence of specific IgE during postnatal treatment for congenital toxoplasmosis is a sign of poor adherence or inadequate dosing.

Igm recognition of recombinant Toxoplasma gondii antigens by sera of acutely or latently infected humans

Clinical non-relevant (CNR) IgM specific for Toxoplasma gondii is responsible for false-positive results in commercially available IgM assays. Using IgM immunoblotting, it is possible to distinguish between IgM in sera of acutely infected (AI) patients and CNR IgM. Especially the combination of staining of a 55 and 30 kD antigen in T.gondii lysate proved useful in this respect. The 55 kD antigen was identified as Rop1, while the 30 kD antigen was confirmed to be Sag1. However, the use of recombinant antigens instead of lysates for diagnostic assays would improve reproducibility. IgM recognized recombinant Rop1, but most CNR sera also had low anti-Rop1 titers. Although purified native Sag1 separated AI and CNR sera very well on immunoblot, IgM did not recognize recombinant Sag1 at all. Clearly, it is difficult to produce a recombinant Sag1 that can be recognized by IgM. Recombinant Rop1 might be suitable as one of the recombinant antigens in an IgM immunoblot assay, but has to be combined with at least one other immunogenic antigen.

Dissecting the IgM antibody response during the acute and latent phase of toxoplasmosis

A major problem in anti-toxoplasma IgM serology is the occurrence of clinically non-relevant (CNR) IgM in sera of latently infected (LI) individuals. The susceptibility for CNR IgM of the Toxo ISAGA IgM, Platelia Toxo IgM and Vidas Toxo IgM assays was determined using sera of LI individuals with an IgM titer in the Abbott IMx Toxo IgM assay. The specificity of CNR-IgM and IgM antibodies in sera of acutely infected (AI) individuals was compared by immunoblotting.Seven/19 samples with CNR IgM antibodies were found positive in the ISAGA IgM, compared with 16/19 and 17/19 with the Vidas and Platelia Toxo IgM assays, respectively. In contrast, immunoblotting allowed a distinction between CNR IgM and AI IgM antibodies of the 30 sera studied.Clearly, IgM assays are susceptible to CNR IgM antibodies. In cases of doubt, immunoblotting is of value as confirmation method. Because CNR IgM antibodies are specific for toxoplasma, it will be difficult to improve IgM ELISAs in such a way that CNR IgM antibodies will not interfere with the analysis.

Purification of a recombinant protein expressed in yeast: optimization of analytical and preparative chromatography

The industrial production of recombinant proteins requires control of both fermentation and purification steps. For the serodiagnosis of toxoplasmosis, the main antigen is a membrane protein of 30 kDa (P30). The P30 gene was cloned and expressed in Schizosaccharomyces pombe at 0.7 microg/ml in culture medium. Batch fermentation was optimized by the specific choice of peptones, which enabled optimum growth and protein expression without reducing the efficacy of the purification step. Analytical purification was then carried out using cation-exchange chromatography. For larger volumes, scaling up was performed on expanded mode by using a Streamline system (Pharmacia). This purification step allowed us to obtain a 67.5% recovery with a purification factor greater than 27-fold. Expanded bed adsorption technology is a convenient and effective technique for protein capture directly from feedstock, and the eluted fraction is ready for a second affinity chromatography step. This second step is performed with a yield of 40% and provides a final purification factor of 2000-fold.

IgM and IgA antibody responses in 12 cases of human acquired toxoplasmosis

The persistence, in some subjects, of specific IgM antibodies to Toxoplasma gondii for several months after the acute phase of infection has complicated the interpretation of serological test results for toxoplasmosis. Several reports have emphasized the value of the detection of Toxoplasma-specific IgA antibodies for the diagnosis of acute toxoplasmosis. In this article, we report the follow-up profiles of Toxoplasma-specific IgM and IgA antibodies in serum samples obtained from 12 patients at various intervals after the onset of the clinical manifestations of infection. IgM antibodies were detected by the indirect immunofluorescence (IIF) test, antibody capture enzyme-linked immunosorbent assay (cELISA) and enzyme-mediated chemilluminescent technique (CmL). IgA antibodies were quantified by the direct ELISA (dELISA) and cELISA procedures. As defined by the manufacturer of the cELISA test for IgA used, most patients with acute toxoplasmosis have antibody levels > 40 arbritary units per ml (AU/ml). At values > 40 AU/ml, the cELISA for IgA detected significant antibody levels for a shorter time than the other techniques used for IgM and IgA detection. However, IgA levels < or = 40 AU/ml do not exclude the possibility of acute toxoplasmosis since such levels can be reached very soon after infection with T. gondii. The results obtained in the present study show that the serological diagnosis of acute toxoplasmosis may not be such an easy task. Our data suggest that use of the IgA-cELISA concomitantly with IgM antibody screening could permit, in some circumstances, a more efficient diagnosis of acute acquired toxoplasmosis.

False-positive results in immunoglobulin M (IgM) toxoplasma antibody tests and importance of confirmatory testing: the Platelia Toxo IgM test

Although tests for detection of immunoglobulin M (IgM) toxoplasma antibodies have been reported to have a high degree of accuracy, it is well recognized by investigators in the United States and Europe that false-positive results may occur with many of these tests, at times to an alarming degree. Unfortunately, this information is not well documented in the literature. Studies on various toxoplasma IgM test kits are frequently flawed. The investigators often use reference tests which have not previously been carefully evaluated as well as sera that were not appropriate to answer the question of how often false-positive results might occur. We recently had the unique opportunity to evaluate the accuracy of the Platelia Toxo IgM test in 575 serum samples obtained during an outbreak of toxoplasmosis which occurred in 1995 in the Capital Regional District of British Columbia, Canada. When compared with results obtained in a reference IgM enzyme-linked immunosorbent assay (ELISA), the Platelia Toxo IgM test had a sensitivity of 99.4%, specificity of 49.2%, positive predictive value of 51.9%, negative predictive value of 99.3%, and an overall agreement of 67.0%. In an attempt to resolve discrepancies between these two tests, a serological profile (Sabin-Feldman dye test, IgA and IgE antibody tests, differential agglutination [AC/HS] test, and IgG avidity method) was performed. Of 153 serum samples that were positive in the Platelia Toxo IgM test and negative in the IgM ELISA, 71 (46.4%) were negative in the Sabin-Feldman dye test. Of the serum samples that were positive in the dye test, 77 (93.9%) had a serological profile most compatible with an infection acquired in the distant past. These results reveal high numbers of false-positive results in the Platelia Toxo IgM test and highlight the importance of appropriate evaluation of commercial tests that are currently being marked. Our results also emphasize the importance of confirmatory testing to determine whether the results of an IgM antibody test reflect the likelihood of a recently acquired infection.