Coccidiosis causes considerable economic losses in the poultry industry. At present, the pathology of coccidiosis is preventable with anticoccidials and vaccination, although at considerable cost to the international poultry industry. The purpose of the present study was to elucidate the relationship between Eimeria tenella development and host cell apoptosis in chickens, which provides a theoretical basis for further study of the injury mechanism of E. tenella and the prevention and treatment of coccidiosis. Cecal epithelial cells from chick embryo were used as host cells in vitro. In addition, flow cytometry, terminal deoxynucleotidyl transferase–mediated deoxyuridine triphosphate (dUTP) nick-end labeling, and histopathological assays were used to detect the dynamic changes in E. tenella infection rates, DNA injury rates, and apoptosis rates in groups treated with and without the caspase-9 inhibitor Z-LEHD-FMK. Following E. tenella infection, we demonstrated that untreated cells had less apoptosis at 4 h and, inversely, more apoptosis at 24 to 120 h compared with control cells. Furthermore, after the application of Z-LEHD-FMK, terminal deoxynucleotidyl transferase–mediated dUTP nick-end labeling assays, and translation of phosphatidyl serines to the host cell plasma membrane surface, the treated group chick embryo cecal epithelial cells exhibited decreased apoptosis and DNA injuries (P < 0.01) at 24 to 120 h. However, light microscopy showed that E. tenella infection rates of treated cells were higher (P < 0.01) than untreated cells during the whole experimental period. Together, these observations suggest that E. tenella can protect host cells from apoptosis at early stages of development but can promote apoptosis during the middle to late stages. In addition, the inhibition of host cell apoptosis can be beneficial to the intracellular growth and development of E. tenella.

Although the mitochondrial permeability transition pore (MPTP) is associated with cellular apoptosis and necrosis, its effect in host response to Eimeria infections is not well understood. In an effort to better understand the effect of MPTP on apoptosis in Eimeria tenella host cells, an MPTP inhibitor (cyclosporin A) was used to inhibit MPTP opening in vitro. Cecal epithelial cells from chick embryos, which were either treated or non-treated with cyclosporin A, were used as Eimeria tenella host cells. In addition, primary chick embryo cecum epithelial cell culture techniques and flow cytometry were used to detect the dynamic changes in MPTP opening, mitochondrial transmembrane potential, and cell apoptosis rate of Eimeria tenella host cells. Compared with the control group, cytometric techniques showed that untreated host cells exhibited a significantly higher (P < 0.01) degree of MPTP opening but lower (P < 0.01 or P < 0.05) mitochondrial transmembrane potential. Moreover, untreated group cells had less apoptosis (P < 0.01) at 4 h and more apoptosis (P < 0.05 or P < 0.01) at 24 to 120 h as compared with control group cells. After the application of cyclosporin A, the degree of MPTP opening in the treated group was significantly lower (P < 0.01) at 4 to 120 h compared to the untreated group, whereas the treated group had higher (P < 0.05 or P < 0.01) mitochondrial transmembrane potentials at 24 to 120 h. Flow cytometry assays also showed that there was less (P < 0.05 or P < 0.01) apoptosis after 24 h in the treated group than in the untreated group. Taken together, these observations indicate that MPTP is a key node that plays a predominant role in the mitochondrial apoptosis pathway in the host cell induced by Eimeria tenella.

The present study aimed to investigate the dynamic expression patterns of death-receptor adapter proteins TNF-receptor-associated death-domain protein (TRADD) and Fas-associated death-domain protein (FADD) in E. tenella-induced host-cell apoptosis. Culture techniques for primary chick embryo cecum epithelial cells, ELISA, hematoxylin-eosin staining, fluorescence quantitative PCR techniques, and Hoechst-Annexin V-PI apoptosis staining were used to detect the apoptosis rates and dynamic expression patterns of TRADD and FADD in E. tenella host cells at 4, 24, 48, 72, 96, and 120 h. The rates of early apoptosis, late apoptosis, and necrosis of E. tenella-infected group (group T0) were significantly lower (P < 0.05) or highly significantly lower (P < 0.01) than those of control group (group C) at 4 h, but higher (P < 0.05 or P < 0.01) at varying degrees than those of the same group at 24 to 120 h. Compared with group C, both the mRNA and protein expression levels of TRADD in the group T0 cells increased highly significantly (P < 0.01) at 4 and 24 h, and significantly (P < 0.05) at 48, 72, and 120 h. Compared with the mRNA expression in the group C cells, that of TRADD in the group T0 cells increased significantly (P < 0.05) at 96 h. Both the mRNA and protein expression of FADD in the group T0 cells displayed no significant difference (P > 0.05) from those in the group C cells at 4 h. However, FADD expression in the group T0 cells were significantly (P < 0.05) or highly significantly higher (P < 0.01) than those in the group C cells at 24 to 120 h. These observations indicate that in the early developmental stages of E. tenella, the host-cell apoptosis rate decreased, and TRADD expression increased. In the middle and later developmental stages of E. tenella, the host-cell apoptosis rate increased and expression of TRADD and FADD increased. The variation trends of TRADD and FADD expression were significantly positively correlated with the change rule of the host-cell apoptosis rate, respectively. These results indicate that TRADD and FADD may play an important role in E. tenella-induced host-cell apoptosis.

The present study aims to investigate the similarities and differences between the host cells apoptosis induced by virulent line of Eimeria tenella (