Influence of Different Death Receptor Signaling Pathways on Apoptosis of Eimeria tenella Host Cells

The aim of this study was to investigate the regulatory roles of distinct signaling cascades within the death receptor pathway in host cell apoptosis induced by Eimeria tenella (E. tenella); to this end, primary chicken embryo cecal epithelial cell culture, gene silencing, enzyme-linked immunosorbent assay (ELISA), Hoechst-Annexin V/PI apoptosis staining, hematoxylin-eosin (HE) staining, and quantitative real-time polymerase chain reaction (qRT-PCR) were employed. At 4, 24, 72, and 120 h post-inoculation (hpi) with E. tenella sporozoites, the proportion of apoptosis in six treatment groups [Group C, Group T0 (E. tenella infection group), Group T1 (E. tenella + Fas SiRNA), Group T2 (E. tenella + TRAIL SiRNA), Group T3 (E. tenella + TNFR1 SiRNA), and Group T4 (E. tenella + NC SiRNA)] and the dynamic changes in Fas cell surface death receptor (Fas), tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), tumor necrosis factor receptor 1 (TNFR1), TNF receptor-associated death domain (TRADD), Fas-associated death domain (FADD), and death domain-associated protein (Daxx) expression and caspase-8 activity in the cells were determined. The results demonstrated that, from 4 to 120 hpi, the Fas and TNFR1 mRNA expression levels in Group T0's host cells were significantly higher than those in Group C (p < 0.05 or p < 0.01). At 72 and 120 hpi, TRAIL mRNA expression in Group T0's host cells was significantly or highly significantly elevated compared to that in Group C (p < 0.05 or p < 0.01). From 24 to 120 hpi, the expression levels of the FADD and Daxx genes, caspase-8 activity, and apoptotic rates in Group T1's host cells were significantly lower than those in Group T4 (p < 0.05). At 72 and 120 hpi, the FADD expression, caspase-8 activity, and apoptotic rates in Group T2's host cells were significantly reduced relative to Group T4 (p < 0.05). Additionally, at 4 hpi, TRADD gene expression in Group T3's host cells was significantly lower than that in Group T4 (p < 0.05), while the apoptotic rate was significantly higher (p < 0.05). However, from 24 to 120 hpi, the TRADD expression, caspase-8 activity, and apoptotic rates in Group T3's host cells were significantly lower than those in Group T4 (p < 0.05). The results indicated that, in the early stages of E. tenella development, TNFR1 overexpression promoted TRADD mRNA expression, thereby inhibiting the apoptosis of E. tenella host cells. In the middle and late developmental stages of E. tenella, the Fas-FADD, Fas-Daxx, TRAIL-FADD, and TNFR1-TRADD apoptotic pathways were all activated, collectively facilitating host cell apoptosis. The pro-apoptotic effects of these pathways were ranked in descending order, as follows: Fas signaling pathway > TNFR1 signaling pathway > TRAIL signaling pathway.