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 vac cine candidate against toxoplasmosis. In this work we characterize the immu ne 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 rev els of susceptibility to toxoplasmosis and compare it with that obtained by vaccination with the live attenuated ts-4 strain of T. gondii. The ROP2 ge ne was cloned in the eukaryotic expression vector pcDNA3 and the resulting plasmid, named pcDNA3/ROP2, was used to immunize mice. After three immuniza tions with the plasmid, mice developed antibodies that could be detected by ELISA using a recombinant truncated form of ROP2; and these antibodies als o recognized the natural protein by Western blot. Plasmid immunization gene rated antibodies against the ROP2 of both of the IgG(1) and IgG(2a) isotype s in CBA/J and BALB/c mice and both of the IgG(1) and IgG(2a)cisotypes in C 57BL/6 mice. However, animals vaccinated with the ts-4 strain generated onl y IgG(2a) (in CBA/J and BALB/c mice) or IgC(2c) (in C57BL/6 mice) against R OP2. Kinetic studies of the generation of isotypes indicated that both isot ypes were generated at the same time. Mice immunized with the plasmid DNA d id not resist a challenge with the virulent RH strain of T. gondii, while m ice vaccinated with the ts-4 strain resisted the same challenge. However, i n pcDNA3/ROP2-immunized BALB/c mice, death was significantly delayed with r espect to the pcDNA3-immunized control group. These results suggest that pl asmid immunization using the ROP2 gene generates a mixed T-H1/T-H2 response against ROP2, which is different from that obtained by vaccination with li ve tachyzoites of the ts-4 strain (T-H1 response) and is not protective aga inst the highly virulent RH strain of the parasite.