Eimeria tenella: Local Antibodies and Interactions with the Sporozoite Surface

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Abstract
.Using fixed sporozoites in a 3‐layer immunofluorescence assay (TLIFA), class‐specific, parasite‐specific antibody responses in chicks to single‐pulse infection with Eimeria tenella have been studied in gut contents and bile as well as plasma and feces. After infection with 103 oocysts, IgA antibody was first detected in the duodenal lumen, then in bile, plasma, cecum, and the distal small intestine. The kinetics of the bile IgA response correlated with that in plasma and peaked 9 days post‐infection (d.p.i.); IgM was detected in gut contents and bile as well as plasma, and IgG was occasionally detected in gut contents, especially in the duodenum. In some experiments, IgA was detected in gut contents and bile to at least 21 d.p.i. Infection with 103 oocysts resulted in an earlier and increased response and relatively high IgG titers in cecal contents. Coproantibody was detected inconsistently and at low titer. When sporozoites that excysted in vitro were incubated in specific, antibody‐positive (9 d.p.i.) cecal contents, some complement‐mediated IgG‐associated anti‐sporozoite effects were observed; however, the major effect of cecal contents and the only effect of bile was a non‐lethal agglutination of living sporozoites. By fractionation of cecal contents and imtnunoblotting this was confirmed to be IgA mediated; IgA antibodies in cecal contents and bile after infection were shown to bind to sporozoite membrane antigens by surface fluorescence as well as agglutination. Agglutination detected anti‐sporozoite antibody in gut contents and bile up to 21 d.p.i., peaking between 7 and 13 d.p.i., corresponding with TLIFA results. The immunofluorescence studies reveal the extremely labile nature of the sporozoite surface antigens and support the hypothesis that naturally elaborated IgA antibodies are involved in the modulation of complexed sporozoite surface antigens.