Multiple B-cell epitopes in a recombinant GRA2 secreted antigen of Toxoplasma gondii

Two cDNA clones encoding the GRA2 (G.P.28.5) secreted antigen of Toxoplasma gondii were expressed in Escherichia coli as glutathione-S-transferase fusion polypeptides. A high level of expression was obtained for the first clone expressing the 59C-terminal amino acids of GRA2. The other one was an open-reading-frame of 212 amino acids containing the entire GRA2 cDNA. By ELISA, IgG antibodies directed against the 59aa recombinant polypeptide were detected in 33/44 (75%) sera from patients chronically infected with T. gondii and in 19/23 (82.6%) sera derived from patients with acute, primary toxoplasmosis. 10 of the 11 "chronic" sera which were negative by the 59aa ELISA were tested in a immunoblot against the 212aa open-reading-frame of GRA2: 8/10 were positive. A peptide representing the 15 C-terminal amino acids of GRA2 has been shown to contain the epitope recognized by a mouse monoclonal antibody (TG17-179). The reactivity of human sera with the 59aa recombinant polypeptide was inhibited to varying degrees when the sera were co-incubated with this peptide. Twelve chronic sera showed a range of inhibition from 8 to 100% and twelve acute sera an inhibition range of 15 to 90%. This suggests that the 15aa C-terminal peptide contains an epitope recognized in both the acute and chronic phases of infection and that other major epitope(s) exist in the 59aa C-terminal region of GRA2. As a conclusion, the recombinant GRA2 protein appears to contain at least three B-cell epitopes.