Impact of stage differentiation on diagnosis of toxoplasmosis

Within its intermediate host, such as man, the protozoan parasite Toxoplasma gondii interconverts between tachyzoites and bradyzoites. The replicative tachyzoite stage is thought to be responsible for acute/active infection and expresses immunodominant antigens thereby inducing a strong cellular immune response, which vice versa triggers the differentiation process into dormant and immunologically weak cyst stages. The immunodominance of tachyzoites is also responsible for the induction of a strong humoral immune response leading to the formation of antibodies specifically directed against tachyzoite antigens. In contrast, the bradyzoite stage which is associated with inactive/chronic infection, seems not to be a strong inducer of specific antibodies. However, since the humoral antibody response is also directed against antigens that are expressed in both stages, serodiagnosis cannot always adequately discriminate between active and inactive/chronic infection. This short review focuses on the impact of stage differentiation and discusses the potential of stage-specifically expressed antigens that might be useful in a recombinant form in order to improve future serodiagnostical approaches.