Genetic immunization with plasmid DNA coding for the ROP2 protein of Toxoplasma gondii

The ROP2 protein of Toxoplasma gondii has previously been proposed as a vaccine candidate against toxoplasmosis. In this work we characterize the immune response induced by injection of plasmid DNA coding for this protein in three strains of mice (BALB/c, C57BL/6, and CBA/J) displaying different levels of susceptibility to toxoplasmosis and compare it with that obtained by vaccination with the live attenuated ts-4 strain of T. gondii. The ROP2 gene was cloned in the eukaryotic expression vector pcDNA3 and the resulting plasmid, named pcDNA3/ROP2, was used to immunize mice. After three immunizations with the plasmid, mice developed antibodies that could be detected by ELISA using a recombinant truncated form of ROP2; and these antibodies also recognized the natural protein by Western blot. Plasmid immunization generated antibodies against the ROP2 of both of the IgG1 and IgG2a isotypes in CBA/J and BALB/c mice and both of the IgG1 and IgG2c isotypes in C57BL/6 mice. However, animals vaccinated with the ts-4 strain generated only IgG2a (in CBA/J and BALB/c mice) or IgG2c (in C57BL/6 mice) against ROP2. Kinetic studies of the generation of isotypes indicated that both isotypes were generated at the same time. Mice immunized with the plasmid DNA did not resist a challenge with the virulent RH strain of T. gondii, while mice vaccinated with the ts-4 strain resisted the same challenge. However, in pcDNA3/ROP2-immunized BALB/c mice, death was significantly delayed with respect to the pcDNA3-immunized control group. These results suggest that plasmid immunization using the ROP2 gene generates a mixed T(H1)/T(H2) response against ROP2, which is different from that obtained by vaccination with live tachyzoites of the ts-4 strain (T(H1) response) and is not protective against the highly virulent RH strain of the parasite.