Demonstration of Toxoplasma antigen containing complexes in active toxoplasmosis.

With an enzyme-linked immunosorbent assay antigen, specific circulating immune complexes (CIC) were demonstrated in experimental and human toxoplasmosis. In experimentally infected mice, CIC became demonstrable as soon as antibodies appeared after fatal infection. When a nonvirulent strain of Toxoplasma was used CIC remained detectable for several weeks. This period was characterized by clinically healthy animals with increasing antibody titers and by cysts growing in the brains of the animals, indicating a subacute stage of the toxoplasma infection. In the human sera, a surprisingly high percentage of CIC was demonstrated. Both immunoglobulin G (IgG) and IgM were found in the CIC; however, IgG was seen in the majority. If the humans were grouped according to other serological results, such as a combination with IgM antibodies, circulating antigens, or both, or a positive complement fixation test, increasingly more CIC were observed. When sera were selected from patients with clinical symptoms generally associated with toxoplasmosis, more CIC were also again demonstrated. On the contrary, in healthy individuals (blood donors), CIC were also regularly observed, suggesting that exacerbations of latent infections or reinfections may regularly occur without leading to clinical signs. In conclusion, we propose that the interpretation of a positive CIC test requires great care but may provide useful information about the activity of a toxoplasma infection.