RECOGNITION OF TISSUE CYST-SPECIFIC ANTIGENS IN REACTIVATING TOXOPLASMOSIS

Current serological tests do not discriminate between asymptomatic lat ent Toxoplasma gondii infection and reactivating toxoplasmosis, but ti mely therapeutic intervention before the development of symptoms would lead to major reductions in morbidity and permanent disability. This study developed a new enzyme-linked immunosorbent assay (ELISA) for an tibody to T. gondii tissue cyst antigens and screened tissue cyst anti gens by Western blot analysis to test the hypothesis that antibody rec ognition of T. gondii tissue cyst-derived antigen is a good indicator of reactivation disease. A total of 187 sera was tested by Sabin-Feldm an dye test and tissue cyst ELISA. AIDS patients and patients with ocu lar disease were considered separately as the exposure to parasite ant igens may be different in these two groups. The dye test did not discr iminate between immunocompetent and immunocompromised T. gondii seropo sitive patients or between active and quiescent toxoplasmosis. Tissue cyst ELISA demonstrated a raised specific antibody response in immunoc ompetent T. gondii seropositive patients and in quiescent HIV positive sera. These data support the view that the tissue cyst population is in a state of dynamic equilibrium. It is proposed that, in the immunoc ompetent host, tissue cyst development and rupture are under some degr ee of immune control, but that in the immunocompromised host this equi librium is disturbed and reactivation disease results. Data from patie nts with reactivating ocular toxoplasmosis demonstrate that tissue cys t-specific antibody levels are not different in active and quiescent d isease and indeed they are not significantly different from immunocomp etent T. gondii seronegative sera. In the Western blot analysis of 57 HIV positive patient sera, eight antigens (65, 57, 49, 47, 36, 28, 26 and 18 kDa) were consistently recognised by one third or more of the s era tested, but no single antigen was diagnostic of quiescent or activ e toxoplasmosis. It is concluded that tissue cyst-derived antigens are not a reliable serological marker of reactivating toxoplasmosis.