Evaluation of immunoblotting for the detection of Toxoplasma gondii immunoglobulin M antibodies

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.

Improvement of immunoglobulin M capture immunoassay specificity: toxoplasma antibody detection method as a model

In the Toxoplasma gondii immunoglobulin M (IgM) capture fluorometric enzyme immunoassay used as a model, nonspecific responses due to the binding of human IgM to horseradish peroxidase (HRP) conjugates were observed despite the removal of the Fc portion of the immunoglobulin. This interaction may be mediated through the binding of human IgM to the HRP moiety of the conjugate. Addition of polymerized HRP into the reaction mixture reduced nonspecific signals in the majority of low false-positive serum reactions. Other plausible sites of interaction are conserved epitopes of mouse immunoglobulins presenting antigenic similarities with the allotopes of other species. Fragmentation of mouse antimicrobial IgG to Fab' and selection of proper conjugation procedure improved assay specificity.

Analysis of Toxoplasma gondii antigens with sera from toxoplasmosis patients

Some proteins of the Toxoplasma gondii are recognized by IgG, IgM and IgA antibodies in patients with acute and chronic toxoplasmosis, depending on the strain and stage of the Toxoplasma. Sixty-nine sera from immunocompetent individuals were studied through the Western-Blot Test: 20 has an acute infection, 29 has a chronic toxoplasmosis infection and 20 were healthy (seronegatives). The protein analysis revealed by IgG and IgM antibodies were performed through the immunoplot method in order to know their recognition frequency (f) and be valued as infection markers. In the acute phase, the IgM antibodies showed a recognition frequency (f = 0.60) for the 60 kDa protein, and in the chronic phase the IgG antibodies showed a recognition frequency (f = 0.68) for the 12 kDa protein. Seronegatives revealed no type of band. The protein of 12 kDa can be a diagnostic marker of the chronic phase while protein 60 kDa of the acute phase of toxoplasmosis.

Detection of IgA and low-avidity IgG antibodies for the diagnosis of recent active toxoplasmosis

OBJECTIVE: To determine the clinical value of testing IgA and the avidity of IgG (by two commercial systems) for the detection of recent active toxoplasmosis (RAT), and to study the IgG avidity during the course of infection. METHODS: The IgA was tested by a capture ELISA (Pasteur, France) and the avidity of IgG was determined by two modified commercial indirect ELISA methods (Sorin, Italy; Behringwerke, Germany) in 12 patients who were not immunosuppressed (group I) and 57 healthy subjects with a past infection by Toxoplasma gondii (group II). RESULTS: IgA was present in 75% of patients from group I and 21% of subjects from group II. The reliability for diagnosis of RAT was: sensitivity 75%, specificity 84%, positive predictive value 52.9% and negative predictive value 93.3%. In group I, 91.7% of patients had more than 50% low-avidity IgG, by both methods; in group II, 21% of subjects had low-avidity IgG at levels from 40% to 50%, by both methods. The diagnostic reliability of the two methods for the detection of low-avidity IgG in the first samples of RAT was similar when a breakpoint of 50% was used, with values of: sensitivity 91.7%, specificity 100%, positive predictive value 100% and negative predictive value 98%. CONCLUSIONS: The study of IgA is not on its own adequate for diagnosis of RAT. However, testing the avidity of IgG is more reliable for the diagnosis of RAT, in studies of one serum sample or sequential samples.

Improved serological diagnosis of Toxoplasma gondii infection by detection of immunoglobulin A (IgA) and IgM antibodies against P30 by using the immunoblot technique

Immunoglobulin M (IgM) and IgA antibodies against the major surface protein of Toxoplasma gondii were determined in a total of 195 human sera and five human cerebrospinal fluids by using a P30 membrane extract and the immunoblot technique. By using two different T. gondii strains (RH and BK) simultaneously as antigens, we were able to demonstrate diagnostically important strain-specific human antibody responses in 4.5% of the samples tested. A comparison of the immunoblot technique with an IgM immunocapture enzyme-linked immunosorbent assay demonstrated that the IgM and IgA immunoblot seems to be of advantage in the diagnosis of acute toxoplasmosis in certain groups of patients, especially in the diagnosis of cerebral toxoplasmosis in patients with AIDS. The immunoblot technique described is easy to perform and might be useful as an additional serological assay for routine diagnosis of T. gondii infections.