Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 14, 2007
Publication Date: October 30, 2007
Citation: Fetterer, R.H., Jenkins, M.C., Miska, K.B., Barfield, R.C. 2007. Characterization of the antigen SO7 during development of Eimeria tenella. Journal of Parasitology. 93:1107-1113.
Interpretive Summary: Coccidiosis has significant economic impact on the poultry industry causing world-wide losses estimated to be over 800 million dollars annually. Losses are due to reduced weight gains and decreased feed efficiency caused by infection of birds. Current control strategies are based on prophylactic chemical treatment of birds with medicated feed. Alternative controls using vaccines show promise to reduce the need for use of drug-containing feeds. These vaccines consist of one or more defined proteins that stimulate immunity to coccidia infection in vaccinated birds and thus reduce the impact of the disease on the infected birds. Development of vaccines has been hampered by lack of knowledge of biological functions of proteins that may be useful antigenic components of a vaccine. In the present research, we investigate the function of a previously described antigen, SO7, that stimulates immunity to coccidia infections. SO7 appears to be a protein unique to coccidia but its function is unknown. This protein was localized to a specific organelle of the invasive stage, the refractile body. The protein was shown to be developmentally regulated since it was expressed in the early developmental stages but was absent in more advanced stages. During invasion of cell in culture the SO7 is present in intracellular stage of the parasite and also is secreted into the culture media. The abundance and relative stability of the SO7 suggest that it is a structural element of the refractile body.
The developmental expression of the antigen SO7, which has been previously shown to protect chickens against infection by several Eimeria species, was investigated. Using RT-PCR, mRNA for SO7 was found to be restricted primarily to unsporulated oocysts (0hr). Western blot (WB) analysis with an antibody to recombinant SO7 (rbSO7) revealed expression of the protein from 6 to 72 hr (fully sporulated) of sporulation and in sporozoites (SZ). SO7 was absent in host derived second stage merozoites (MZ) but was present in culture-derived first stage MZ but at a level of only 25% of that exhibited by SZ. During invasion of Madin Darby bovine kidney (MBDK) cells by SZ in vitro, the level of SO7 within cells, as determined by WB analysis, remained relatively constant until 48 hr of development and then decreased by about 40% by the next time point (72hr). The SO7 secreted into the culture media during in vitro development increased to a relative maximum at 48 hrs and then decrease to about 20% of maximum at 72 hr. Immunostaining with anti-rbSO7 indicates that SO7is highly concentrated in both refractile bodies (RB) of sporozoites (SZ) with some limited distribution in the apical complex. Anti-rbSO7 intensively stained the intracellular parasites and the first stage schizonts during in vitro development of E. tenella in MBDK cells. Upon release from the schizonts, the first stage merozoites stained with one or two bright spots typically at each end. The results suggest that SO7 is closely associated with the SZ RB and is developmentally regulated but may not play a direct role in cellular invasion