Technical Abstract: Interferon-gamma (IFN-gamma) response is essential to the development of a host protective immunity in response to infections by intracellular parasites. The induction of IFN-gamma is critical to the control the acute phase of many parasitic infections. Neosporosis, an infection caused by the intracellular protozoan parasite Neospora caninum (Nc), is fatal when there is a complete lack of IFN-gamma in the infected host. However, the mechanism by which IFN-gamma is elicited by the invading parasite is unclear. This study has identified a microbial protein in the Nc tachyzoite, Nc cyclophilin (NcCyP), as a major component of the parasite responsible for the induction of IFN-gamma production by bovine peripheral blood mononuclear cells (PBMC) and antigen-specific CD4 T cells. The NcCyP has high sequence homology (86%) with the T. gondii (Tg) 18 kDa CyP (C-18) with a calculated molecular weight (Mr.) of 19.4 kDa. The NcCyP is a secretory protein with a predicted signal peptide of 17 amino acids. Abundant NcCyP was detected in Nc tachyzoite whole lysate (NcAg) and Nc tachyzoite culture supernatant with apparent Mr. of 19, 22, and 24 kDa. NcAg stimulated high levels of IFN-gamma production by PBMC and CD4 T cells. IFN-gamma-inducing effect of NcAg was blocked by cyclosporin A (CsA), a specific ligant for CyP, in a dose-dependent manner. Furthermore, CsA abolished IFN-gamma production by PBMC from naïve cows as well as PBMC and CD4 T cells from infected/immunized cows. These results indicate that the Nc tachyzoite naturally produces a potent IFN-gamma-inducing protein, NcCyP, which may be important for parasite survival as well as host protection.