Evaluation of Protective Immune Responses Induced in BALB/c Mice and Goats by the Neospora caninum Surface SRS Proteins and Interleukin-18

Simple Summary Neospora caninum is an apicomplexan parasite with worldwide distribution, infecting cattle, goats and other kinds of warm-blooded animals. However, specific medicines for the prevention or treatment of Neospora caninum are still limited. This study is aimed at selecting a potential immunoprotective antigen of N. caninum and evaluating the adjuvant function of interleukin (IL)-18 in mice or goats; NcSRS2, NcSRS17 and NcSRS52 from the SRS superfamily were considered to be targets. The results reveal that the recombinant proteins NcSRS2, NcSRS17 and NcSRS52 can all induce humoral and cellular immune responses, and NcSRS17 showed itself to be a promising new antigen candidate. IL-18 had a certain immunoenhancing effect in both experimental animal mice and intermediate host goats and could be used as an adjuvant. Neosporosis is caused by Neospora caninum (N. caninum), which mainly infects cattle and goats and severely threatens the animal industry. In this study, the inhibitory effects of polyclonal antiserum anti-NcSRS17, NcSRS2 and NcSRS52 were explored. Cytokines in mice or goat serum were detected after immunization. After infection, the survival of mice was recorded. The pathological changes and parasite loads were observed and detected in tissues. The results showed that anti-NcSRS2, NcSRS17 and NcSRS52 antibodies all inhibit the invasion and proliferation of N. caninum. The IFN-gamma level in the NcSRS17 group was higher than that in the NcSRS2 and NcSRS52 groups, and higher in the NcSRS2-mIL-18 group than in the NcSRS2 group. The survival rates of mice were 16% in the positive control group, 67% in the SRS52 group, 83% in the SRS2 and mIL-18 groups and 100% in the SRS17 and SRS2-mIL-18 groups. Goats immunized with NcSRS17-gIL-18 developed high levels of IL-4, IL-12 and IFN-gamma compared with those immunized with NcSRS-17. Parasite loads in the brains of animals in the NcSRS17 and NcSRS17-gIL-18 groups were significantly reduced, and were significantly lower in the NcSRS17-gIL-18 group (p <= 0.01). This study indicates that SRS17 may be an antigen candidate for vaccine development against neosporosis, and IL-18 can enhance the immune protective efficiency of antigen candidates.