A mechanism for secretory IgA-mediated inhibition of the cell penetration and intracellular development of Eimeria tenella.

The ability of Eimeria tenella sporozoites to develop normally in cultured chick kidney cells was used as an indicator of the anticoccidial effects of sera and extracts of caecal contents or tissue. Pre-treatment of sporozoites with normal serum globulin enhanced the frequency of intracellular development but pre-treatment in balanced salt solution, without protein, damaged sporozoites so that most had lost the ability to differentiate, even when they were able to invade host cells. The same inhibitory effect was seen when sporozoites were incubated in extracts of caecal contents from non-immunized chickens, although parasitic development was unaffected when sporozoites were pretreated in similar extracts of mucosae. Extracts of immune caecal contents impaired both cell penetration and subsequent development. These results show that sporozoites can lose the ability to differentiate before the ability to penetrate cells and provide evidence of a possible synergism between non-specific factors and secretory antibodies in anticoccidial immunity.