Abstract: Objective  To express the recombinant BSR4 protein of Toxoplasma gondii Prugniaud (PRU) strain and study its immunologic characteristics.  Methods  The recombinant plasmid pET28a(+)-BSR4 was transformed into E. coli BL21, followed by expression of BSR4 induced by IPTG and its purification. The immunoreactivity of the recombinant protein BSR4 was analyzed by Western blotting with the sera from mice infected by PRU strain of T. gondii or normal mice as the first antibody. The 3H-TdR incorporation assay was performed to determine the proliferation of splenocytes in mice infected by PRU strain stimulated by BSR4, and the stimulation index (SI) was calculated. ELISA was used to evaluate the immunoreactivity of BSR4 protein, in which 20 sera from each of acute (anti-T. gondii IgG^－IgM⁺) and chronic (IgG⁺IgM^－) toxoplasmosis patients, and healthy people were tested.  Results  The recombinant BSR4 was induced and expressed. After denaturation, renaturation and purification, the soluble protein (M_r 45 000) was obtained and detected by anti-T. gondii serum with Western blotting. BSR4 in 1, 5, and 25 μg/ml induced the proliferation of splenocytes in mice infected by PRU strain with higher SI (1.13, 0.88, and 1.17) than that of control (0.46, 0.24, and 0.49, respectively, P<0.01). ELISA showed that the recombinant BSR4 specifically reacted with the sera of toxoplasmosis patients (IgG⁺IgM^－, 20/20), not with that of the acute cases (IgG^－IgM⁺, 0/20).  Conclusion  The recombinant BSR4 shows specific immunogenicity and immunoreactivity.

Key words: Toxoplasma gondii, BSR4, Recombinant expression, Immunogenicity, Immunoreactivity