Expression of anti-apoptotic factors in cells parasitized by second-generation schizonts of Eimeria tenella and Eimeria necatrix.

Intracellular infections by parasites require a functional anti-apoptotic mechanism for parasite survival within the host cell. The intracellular cycle of Eimeria tenella and Eimeria necatrix in chicken intestinal cells involves the maturation of schizonts within the epithelial cells lining the crypt lumen of the ceca (E. tenella) and jejunum (E. necatrix). After invasion, these cells detach from the epithelial layer and migrate into the underlying connective tissue, where maturation of second-generation schizonts takes place. However, the detached epithelial cells that harbour the parasite and localize in the lamina propia do not undergo apoptosis despite the fact that they are parasitized cells and are located in an inappropriate microenvironment. In this study we consider the hypothesis that E. tenella and E. necatrix may inhibit the host cell apoptosis that accompanies parasite-mediated transformation during late schizogony. To that end, the expression of both NF-kappaB, a transcriptional factor that blocks parasite-induced apoptosis, and bcl-xL, an anti-apoptotic protein induced by NF-kappaB, were studied in the host cell during the maturation of second-generation schizonts. In addition, the expression of the phosphorylated inhibitor of NF-kappaB, p-IkBalpha, was also studied to further confirm NF-kappaB activation. Immunocytochemical techniques, flow cytometric and blott analysis were applied by using polyclonal antibodies that specifically react with bcl-xL, p-IkBalpha, and NF-kappaB to detect these anti-apoptotic proteins in the parasitized cell. Our results offer evidence that both these coccidial species first induce NF-kappaB activation to protect the transformed parasitized cells from apoptosis, allowing the second-generation schizonts to mature, and later, after complete schizonts maturation, cause NF-kappaB inhibition to trigger host cell apoptosis in order to facilitate the escape of merozoites. To determine whether inhibition of the NF-kappaB pathway would induce apoptosis of the host cell, a protease inhibitor (TPCK), which induces apoptosis by mediating inhibition of IkB phosphorylation, was administered to parasitized chickens.