Induction of a protective antibody‐dependent response against toxoplasmosis by in vitro excreted/secreted antigens from tachyzoites of Toxoplasma gondii

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Abstract
Summary Toxoplasma gondii is a worldwide protozoan parasite which causes severe disease in congenitally infected children and in immunocompromised patients. Besides the well‐defined cytoplasmic and membrane antigens of tachyzoites, we felt that excreted/secreted antigens could play a major role in the immune response. We first report the development of a well‐controlled procedure for obtaining tachyzoite excreted/secreted antigens (E/SA) in cell‐free incubation media. The E/SA immunogenic in human, rat and mouse toxoplasmosis were then characterized. The major E/SA recognized by human sera from the chronic phase of toxoplasmosis had molecular weights of 108, 97, 86, 69,60, 57, 42, 39, 28.5, 27 and 26 kD. When injected into +/+ Fischer rats, E/SA elicited high antibody titres. In addition, passive transfer of these sera to highly susceptible nu/nu littermates induced a significant degree of protection towards the virulent RH strain of T. gondii. This work, which demonstrates the key role played by E/SA in the protective immune response, suggests that these antigens should be of value both for diagnostic purposes and for the development of new strategies for immunization against toxoplasmosis.