PURPOSE: To investigate the antigen specificity of the intraocular anti–Toxoplasma gondii antibody response in patients with ocular toxoplasmosis.

METHODS: Paired ocular fluid and serum samples were collected from 13 patients with active ocular toxoplasmosis. Serum IgM anti–T. gondii antibodies were tested to distinguish recently-acquired from chronic infection. Anti–T. gondii IgG specificity was analyzed by immunoblotting using a crude T. gondii extract.

RESULTS: Two of the 13 patients tested were IgM positive and considered to have acquired ocular toxoplasmosis. The antibody specificity in ocular fluid and serum of these two patients was similar, whereas in the patients with presumed chronic disease, marked differences could be observed. Most ocular fluid samples contained antibodies that stained a 28-kD antigen more intensely than did antibodies from paired serum samples. Using absorption and elution experiments, we demonstrated that this 28-kD protein was identical to the GRA-2 antigen, which is expressed in both the tachyzoite and the bradyzoite stages of the parasite.

CONCLUSIONS: Our results show that the intraocular T. gondii antibody response of patients with recurrent ocular toxoplasmosis differs from the systemic response. This finding may have implications for our understanding of the immunopathogenesis of ocular toxoplasmosis and could be employed to improve diagnosis of the disease.