Protective Immunity Induced In Mice By Multiantigenic DNA Vaccine With Genes Encoding SAG1 And MIC8 Of Toxoplasma Gondii

Toxoplasma gondii (T. gondii) is an intestinal coccidium, it parasitizes members of the cat family as definitive hosts and has a wide range of intermediate hosts, including humans. In most cases T. gondii parasitizes both definitive and intermediate hosts without producing clinical signs. In humans, severe disease such as pneumonitis, myocarditis, chorioretinitis, and multisystem organ failure is usually observed in immunosuppressed individuals, including patients with acquired immune deficiency syndrome (AIDS). Current primary control measures depend on chemotherapy. However, the chemotherapeutic agents used presently are inadequate, expensive, and often toxic. Thus the development of an effective vaccine against T. gondii is a promising alternative and useful in human medicine, and a vaccine which will provide immunity against T. gondii is urgently required.Monovalent vaccines are usually weak immunogenically. To circumvent these limitations, we choose two vaccine candidates: SAG1 (GenBank Accession No. S76248) and microneme protein 8 (MIC8, GenBank Accession No. AF353165). SAG1 is the prototypic member of a superfamily of surface antigens and called SRS (SAG1-related sequence). Recent studies have demonstrated that it is also involved in host cell adhesion and invasion. It constitutes the most abundant and predominant antigen. Being highly immunogenic and immuno-protective, it was considered as an important candidate for effective diagnostic reagents and vaccine development. MIC8 is a single-copy gene and contains no introns, and is expressed in tachyzoites and bradyzoites. As an escorter for MIC3, it has been shown that MIC8 is capable of binding to the host cell upon dimerisation. Studies have demonstrated that it is a new essential invasion factor in T. gondii, upon dimerisation, involved in a signaling cascade that ultimately leads to rhoptry secretion.In the present study, we constructed the multiantigenic SAG1-MIC8 DNA vaccine against Toxoplasma gondii infection and observed the protective immune responses induced in C57BL/6J mice. The sequences of genes encoding SAG1 and MIC8 protein were inserted into the eukaryotic expression vector pcDNA3.1 and the multiantigenic recombinant plasmid pcDNA3.1-SAG1-MIC8 was constructed. Mice were immunized intra-muscularly with the recombinant plasmid. Serum IgG antibodies and cytokines IFN-γand IL-4 were demonstrated by ELISA and the T lymphocyte subsets were assayed and the survival times of the lethal challenged mice were observed. Low levels of IL-4 were evident in supernatants from spleen cells. The productions of IgG antibodies and IFN-γwere more obvious in mice immunized with pcDNA3.1-SAG1-MIC8 multiantigenic DNA vaccine than those in mice immunized with pcDNA3.1-SAG1 or pcDNA3.1-MIC8 monovalent vaccine. The survival time of mice immunized with the multiantigenic DNA vaccine was considerably prolonged in comparison with that of mice immunized with monovalent DNA vaccine. It is evident from the present study that the multiantigenic DNA vaccine elicit a better immuno-protection in mice than the monovalent DNA vaccine.