Human T cell clone identifies a potentially protective 54-kDa protein antigen of Toxoplasma gondii cloned and expressed in Escherichia coli.

Protective immunity against Toxoplasma gondii is recognized to be cell mediated and IFN-gamma is considered to be the major mediator of resistance. Thus, protective Ag of the parasite must induce IFN-gamma-producing T cells. In order to identify such Ag, we have constructed a T. gondii cDNA library in the cloning/expression vector lambda gt11, screened this library with a pool of sera of immune donors, and further screened the set of selected recombinant Ag using, as probe, a T. gondii-reactive T-cell clone (TCC) derived from an infected/immune individual and producing a high level of IFN-gamma. One recombinant Ag was shown to induce TCC proliferation and was characterized. The corresponding mature T. gondii Ag has an apparent molecular mass of 54 kDa and the sequence of the cDNA clone suggests that it is membrane associated. The epitope defined by the TCC on this Ag was found to be present in three Toxoplasma strains independently of their phenotype (virulent or cyst forming). Recognition of this Ag by the TCC was shown to be restricted by HLA-DPw4, the most frequent allele in the Caucasian population (approximately 40%). The use of this Ag as a vaccine component is proposed.