Cross-protection against four species of chicken coccidia with a single recombinant antigen.

A cDNA clone, SO7', from an Eimeria tenella cDNA library was inserted into the high-expression vector pJC264 and was expressed in Escherichia coli as a fusion protein, CheY-SO7', with a molecular mass of approximately 36 kDa. By using the purified recombinant antigen to immunize young chicks, it was demonstrated that a single dose, without adjuvant, not only protected against severe coccidiosis induced by infection with E. tenella but also protected chicks challenged with the heterologous species Eimeria acervulina, E. maxima, and E. necatrix. By using rabbit antiserum raised against recombinant CheY-SO7', Western blot (immunoblot) analysis of sporulated oocysts of all seven major species of chicken coccidia showed that all species tested contained proteins characteristic of the B class of antigens, of which CheY-SO7' is representative. It seems likely that a single B antigen could protect chickens against severe coccidiosis caused by infection with any of these Eimeria species. Although chicks exposed to prolonged, natural infection develop antibodies to B antigen, active immunization of young chicks with a protective dose of CheY-SO7' does not elicit a humoral antibody response, suggesting that the partial protection results from cell-mediated effector mechanisms. In addition, the cross-protective nature of the immunity indicates that the response to B antigen is different from that induced by natural infection, which elicits a species-specific immunity. To date, the protection induced by B antigen immunization, although remarkable for a single recombinant protein, is not sufficient to compete with prophylactic chemotherapy.

Parasite exposure elicits a preferential T-cell response involved in protective immunity against Eimeria species in chickens primed by an internal-image anti-idiotypic antibody.

Polyclonal anti-idiotype 1073 (anti-Id 1073), raised against a monoclonal antibody specific for the protective epitope(s) of Eimeria tenella sporozoites, induced cell-mediated immune (CMI) responses in bursectomized chickens. Whereas alhydrogel-adsorbed anti-Id 1073 was sufficient to engender the CMI response at 4 h after injection, induction of the CMI response at 24 h required both alhydrogel and muramyl dipeptide sterol. Exposure of immunized chickens to live parasites prompted a dichotomous effect on the CMI response engendered by anti-Id in that the 4-h CMI response was preferentially stimulated and the 24-h CMI response was down regulated. Both types of CMI response were transferable to naive chickens by T cells from anti-Id 1073 immune donors or by parasite-specific T cells from clones 21 and 27. These T-cell clones were generated from chickens immunized by repeated infections with E. tenella and showed in vitro proliferative responses to anti-Id 1073. The abilities of T cells from clone 21 to selectively transfer the 4-h CMI response and to generate gamma interferon to activate macrophages for their cytotoxic effects on Eimeria sporozoites correlate with the preferential stimulation by parasites of the 4-h CMI response in chickens immunized with anti-Id 1073. These data show that anti-Id 1073 mimics the protective epitope(s) of the parasite and primes chickens for protective CMI responses. Cytotoxic T cells, equivalent to the mammalian T-cell subset of the Lyt2+ phenotype, appear to be the primary effector T cells in the CMI response engendered by anti-Id 1073 against Eimeria parasites.