IgG recognizing 21-24 kDa and 30-33 kDa tachyzoite antigens show maximum avidity maturation during natural and accidental human toxoplasmosis

Clinical utility of avidity assays

IgM may persist for months, presenting a risk of an erroneous diagnosis where serology is the only available tool. Indeed, IgM may be detected in secondary infection as a result of crossreactivity and/or nonspecific stimulation of the immune system. One test that can aid the serologist is IgG avidity testing, in that the avidity of IgG is low early in infection with the avidity of IgG antibodies increasing over time. Congenital toxoplasmosis can induce serious sequelae. Detectable IgM usually persists long after the acute infection. IgG avidity can be an important aid in diagnosis and assessing the risk to the fetus. Another infection that is of concern in pregnancy is cytomegalovirus (CMV). In pregnant women it is very important to differentiate primary from secondary infection, as primary infection presents the highest risk to the fetus. Serologic detection of IgM alone is not a specific marker of primary CMV infection. IgG avidity can have utility in identifying or excluding primary CMV infections during pregnancy. Outside of pregnancy, IgG avidity testing is increasingly recognized as a valuable tool. During the recent West Nile virus (WNV) epidemic in the US, it was recognized that WNV-specific IgM may persist for 6 - 12 months following exposure. Thus, a person presenting to their clinician with nonspecific symptoms may be tested and return a positive WNV IgM that may be the product of exposure during the previous period. In this environment, WNV IgG avidity testing is able to provide some assistance. IgG avidity testing should not be used alone and without an understanding of the limitations of the technique. Serology remains an important tool for the diagnosis and management of infectious disease. Classically, IgM is defined as a marker of acute infection and IgG, in the absence of clinical disease, is often considered a marker of past infection. However, the clinical reality can be quite different.

Spiramycin treatment of Toxoplasma gondii infection in pregnant women impairs the production and the avidity maturation of T. gondii-specific immunoglobulin G antibodies

The aim of the study was to evaluate the influence of treatment with spiramycin on the increase of immunoglobulin G (IgG) titers and IgG avidity indexes (AI) in pregnant women with seroconversion from the beginning of therapy until delivery and after delivery. This group was compared with adult patients with recently acquired untreated toxoplasmosis. One hundred four samples from 32 pregnant women with seroconversion for toxoplasmosis and/or very low IgG AI were followed from the beginning of therapy with spiramycin until delivery. Twenty-nine women were further followed some months after delivery and interruption of therapy. Thirty-eight samples from 16 untreated, nonpregnant patients were evaluated as the control group. The Toxoplasma gondii-specific IgG antibody and the T. gondii-specific IgG AI were significantly delayed in pregnant women receiving therapy compared to nonpregnant, untreated controls, and the findings were consistent with the results of assays from two different manufacturers. The T. gondii-specific IgG AI increased in pregnant women after they gave birth. Avidity maturation is delayed during pregnancy and treatment, and low-avidity antibodies in pregnant women within 3 to 4 months cannot be taken as a sign of infection.

Use of SAG2A recombinant Toxoplasma gondii surface antigen as a diagnostic marker for human acute toxoplasmosis: analysis of titers and avidity of IgG and IgG1 antibodies

We evaluated the reactivity of IgG and IgG1 antibodies by immunoassays in sera from patients with acute and chronic phases of toxoplasmosis against 2 recombinant antigens, SAG2A (full molecule) and SAG2ADelta (truncated molecule from the epitope recognized by A4D12 monoclonal antibody [mAb]), in comparison with soluble Toxoplasma antigen (STAg). Results demonstrated higher IgG reactivity in acute sera with both STAg and SAG2A than in chronic phase sera, and this difference was more evident for IgG1 antibodies to SAG2A. Low reactivity to SAG2ADelta was found in sera from both phases. ELISA-IgG-SAG2A showed high sensitivity (95%) and specificity (100%). ELISA-IgG1-SAG2A sensitivity was significantly higher (90%) for acute than for chronic (67%) phases. ELISA-IgG avidity using STAg demonstrated high performance for characterizing sera with high avidity (>60%), whereas the ELISA-IgG1 avidity-SAG2A immunoassay was the best to define chronic phase infection. It can be concluded that SAG2A is an antigen that may be used as a diagnostic tool to characterize the acute phase Toxoplasma gondii infection. Also, the epitope recognized by A4D12 mAb may be critical for the recognition of this molecule.

Use of MAG1 recombinant antigen for diagnosis of Toxoplasma gondii infection in humans

This paper describes the cloning, purification, and serological applications of matrix antigen MAG1 of Toxoplasma gondii. The expression system used allows the production of a large amount of T. gondii recombinant protein, which was assessed for its potential use in an enzyme-linked immunosorbent assay (ELISA) for detection of T. gondii infection in humans. Serum samples from 117 patients with different stages of infection, along with 10 serum samples from seronegative patients obtained for routine diagnostic tests, were used. The results were compared with those of an ELISA that uses a native T. gondii antigen extract. The MAG1 antigen detected antibodies more frequently from the acute stage (97.3%) than from the chronic stage (7.5%) of toxoplasmosis. Hence, this antigen may be used as a tool for detection of T. gondii immunoglobulin G antibodies in persons with acute toxoplasmosis.

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.

European multicenter study of the LIAISON automated diagnostic system for determination of Toxoplasma gondii-specific immunoglobulin G (IgG) and IgM and the IgG avidity index

The LIAISON system is a fully automated system based on chemiluminescence and antigen bound to magnetic microparticles. The system allows fast and precise measurement of Toxoplasma-specific immunoglobulin G (IgG) and IgM antibody levels and measurement of the IgG avidity index even at low levels of Toxoplasma-specific IgG antibodies in a single step without manual interference. Seven European centers participated in a multicenter evaluation of the LIAISON system. The sensitivity and specificity of the LIAISON system compared to the Sabin-Feldman dye test were 99.3 and 96.8%, respectively. In a comparison of the LIAISON Toxoplasma-specific IgM assay with an immunosorbent agglutination assay, the LIAISON assay had a sensitivity of 96.7% and a specificity of 95.4%. The LIAISON IgG assay showed agreements of 91, 100, and 100% with the AXSYM IgG (Abbott), VIDAS IgG (bioMerieux), and Platelia IgG (Bio-Rad) assays, respectively. The LIAISON IgM assay showed agreements of 95% with the AXSYM IgM and Platelia IgM assays, 96% with the ISAGA IgM assay (bioMerieux), and 97% with the VIDAS IgM assay. The coefficient of correlation between the LIAISON system and the VIDAS Toxoplasma-specific IgG avidity index was 0.81. By use of the Toxoplasma-specific IgG avidity index assay with specific IgM-positive samples, the diagnosis of infection with Toxoplasma gondii in early pregnancy has been improved significantly. The LIAISON avidity assay is a valuable assay for the exclusion of recently acquired infection with T. gondii (less than 4 months) in pregnant women, and it decreases significantly the necessity for follow-up testing.

Absence of avidity maturation of autoantibodies to the protein tyrosine phosphatase-like IA-2 molecule and glutamic acid decarboxylase (GAD65) during progression to type 1 diabetes

Immunoglobulin G avidity assays are used to distinguish between the acute and chronic phase of several infectious diseases, and there is evidence of autoantibody affinity maturation also in autoimmune diseases. To assess whether the analysis of the avidity of autoantibodies against the protein tyrosine phosphatase-like IA-2 molecule and glutamic acid decarboxylase (GAD65) could improve the accuracy of risk assessment of progression to clinical type 1 diabetes, we established methods for the determination of the autoantibody avidity based on our previously developed time-resolved fluorometric IA-2 and GAD65 autoantibody (IA-2A and GADA) assays. The avidity indices of sequential plasma samples from six IA-2A-positive and seven GADA-positive prediabetic children were analysed applying elution with urea and diethylamine (DEA). For comparison, corresponding avidity indices of control children, who have remained non-diabetic for at least 3 years after seroconversion to IA-2A and GADA positivity, were analysed. For most of the children, only a slight fluctuation in the avidity index values was observed over time, although the titres for IA-2A and GADA varied substantially in some cases. The avidity indices of the prediabetic children remained within the same range as those of the control group throughout the follow-up. Our results indicate that the analysis of the avidity index levels of IA-2A and GADA does not improve the accuracy of the prediction of type 1 diabetes based on autoantibody detection.