Vaccination of mice against experimental Neospora caninum infection using NcSAG1- and NcSRS2-based recombinant antigens and DNA vaccines

NcSAG1 and NcSRS2, the two major immunodominant tachyzoite surface antigens of the apicomplexan parasite Neospora caninum, were investigated for their potential as vaccine candidates in mice. Recombinant recNcSRS2 and recNcSAG1 were expressed in Escherichia coli as poly-histidine-tagged fusion proteins. Separate groups of mice were immunized with purified recNcSAG1, recNcSRS2, or a combination of both, and were then challenged with N. caninum tachyzoites. Subsequent experiments included intramuscular vaccination of mice with the eukaryotic expression plasmid pcDNA3 containing either NcSRS2 or NcSAG1 cDNA inserts, followed by a single booster with the corresponding recombinant antigens. Immunization with a crude somatic antigen (NC1-extract) was included in the experiments. Following challenge, the presence of the parasite in the different organs was assessed by a N. caninum-specific PCR, while the parasite burden in infected brain tissue was assessed by quantitative real-time PCR. Immunization of mice employing individual recombinant antigens, or combined recNcSAG1/recNcSRS2, resulted in a lower degree of protection against cerebral infection, when compared to combined DNA/recombinant antigen vaccination. Serological analysis showed that this protective effect was associated with the occurrence of antibodies directed against native parasite antigens in those animals receiving combined DNA/recombinant antigen vaccination. Conversely, mice immunized with recombinant antigens alone generated antibodies recognizing only the recombinant antigens. Mice experiencing clinical signs such as walking disorders, rounded back, apathy and paralysis were observed only in the untreated positive control groups, but never in the vaccinated groups. Our results suggest that a combined DNA/recombinant antigen-vaccine, based on NcSAG1 and NcSRS2, respectively, exhibited a highly significant protective effect against experimentally induced cerebral neosporosis in mice.