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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

12 Sep 2008

Maristela Martins de Camargo and Marcia Triunfol

Source: Diagnostic Microbiology and Infectious Disease (see original article)

Citation: Béla SR, Oliveira Silva DA, Cunha-Júnior JP, Pirovani CP, Chaves-Borges FA, Reis de Carvalho F, Carrijo de Oliveira T, Mineo JR (2008). 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. Diagn Microbiol Infect Dis.

Toxoplasmosis is caused by Toxoplasma gondii, a protozoan of the apicomplexa group, which infects a broad range of hosts, from humans to domestic and wild animals. Pregnant women and immunocompromised patients, such as people with AIDS, are the most seriously affected. Infection of the fetus, which occurs in the uterus, leads to congenital blindness and serious brain damage, with high mortality rates.

The diagnosis of toxoplasmosis requires demonstration that either specific IgM or IgG is found in the patient’s serum. At present, the serologic detection of antibodies in the sera of suspected patients requires the growth of massive amounts of parasites in tissue cultures. Alternatively, mice are experimentally infected for production of antigens. These procedures are expensive and labour intensive. To make things worse, the acute phase of Toxoplasma gondii infection resembles an ordinary infectious process, which makes the diagnosis of acute toxoplasmosis easy to miss, causing a delay in treatment until later phases, when the disease is more easily recognized.

The study by Mineo and collaborators compared two recombinant antigens against the currently used crude extract of parasites, to check which one presented higher sensitivity in immunoassays using sera from acute and chronic patients. The researchers compared a crude extract of T. gondii tachyzoites, a recombinant SAG2 antigen (SAG2, surface antigen 2), and a truncated form of SAG2 that lacked the C-terminal region of the protein. Patients were divided into three groups based on their serologic profile, obtained with conventional ELISA assays using T. gondii crude extracts: (i) uninfected, (ii) acute (presence of IgM, IgG and IgA antibodies against T. gondii), and (iii) presence of chronic infection (only IgG antibodies).

When exposed against the recombinant antigens, only 10% of those sera classified as positive reacted with the truncated antigen, suggesting that the C-terminal region of SAG2A is not antigenically dominant. More importantly, when crude extract and SAG2 were compared, SAG2 was more sensitive in detecting IgG in acute sera, indicating that this recombinant antigen could be used as a marker for acute toxoplasmosis in humans, which may improve the rates of diagnosing the acute infection. The authors claim that their findings are the first demonstration that a recombinant T. gondii antigen might be of use to discriminate between acute and chronic patients.

Note: This article is published in a journal which is not open access. To see the full article a subscription to Diagnostic Microbiology and Infectious Disease is therefore required. In some developing countries, readers who are based in institutions may be able to access it through the HINARI programme.

2008 Elsevier BV