Major histocompatibility complex control of immunity elicited by genetically engineered Eimeria tenella (Apicomplexa) antigen in chickens.

The immunogenicity of a recombinant Eimeria tenella coccidial antigen was studied in 6(1).B congenic chickens derived from B2B2 and B5B5 parents segregating for haplotypes B2 and B5. Five-week-old chickens were immunized with 2.4 micrograms of recombinant protein (designated 5401) in Freund complete adjuvant and challenged with 75,000 oocysts at 28 days postimmunization (DPI) to determine the degree of elicited protective immunity. Serum samples were collected weekly for 5 weeks postimmunization for analysis by enzyme-linked immunosorbent assay, immunofluorescence assay, and Western blotting. Lesion scores following oocyst challenge were significantly reduced in B5B5 chickens compared with those in B2B2 chickens. Immunization induced a sporozoite-specific immunoglobulin G (IgG) titer in serum detected by the enzyme-linked immunosorbent assay that peaked at 28 DPI, the day of challenge, in B5B5 chickens and at 42 DPI in B2B2 chickens. After challenge, this titer declined for each genotype. Anti-sporozoite IgG detected by the immunofluorescence assay attained a peak titer at 21 DPI in B2B2 chickens and 28 DPI in B5B5 chickens. Serum from immunized B5B5 chickens reacted strongly in Western blots with several high-molecular-weight (greater than 100,000), soluble proteins prepared from sporozoites. Serum from B2B2 chickens reacted with similar proteins as well as with a 51- to 53-kilodalton protein that was not labeled by serum from B5B5 chickens. These results demonstrate further the role of host genetics on anticoccidial immunity and suggest that a peak anti-sporozoite IgG titer in B5B5 chickens on the day of challenge may signal a state of immunocompetence to that challenge.