Heat stress aggravates intestinal inflammation through TLR4-NF-κB signaling pathway in Ma chickens infected with Escherichia coli O157:H7

Heat stress can decrease poultry performance indices, immune function, and intestinal development, which can reduce birds' innate protective mechanisms and may be more susceptible for pathogens. Ma chickens heat-stressed with 41°C for 12 h and recovered for 7 d had extremely low immunity. In this study, a susceptible chicken model induced by heat stress and then infected with Escherichia coli O157:H7 was established to explore the mechanisms of birds' intestinal immune function changes. Ma chickens in heat stress + E. coli (HS + E. coli) group were stressed at 41°C for 12 h and recovered for 7 d, then chickens in E. coli group and HS + E. coli group were orally administered with 1 mL E. coli O157:H7 (1 × 10⁹ cfu/mL). Chickens were sacrificed at the fourth day after E. coli administration. Results showed that the HS + E. coli group had increased intestinal length and weight, had higher E. coli counts in cecum contents than the E. coli group. Heat stress also enhanced serum diamine oxidase and decreased IgA level in chickens infected by E. coli. Heat stress had protective effects in small intestinal morphology except for duodenum by using hematoxylin and eosin staining. Compared with the E. coli group birds, IL-1β, TNF-α, and caspase-1 protein levels in the duodenum and ileum were significantly increased. Heat stress also can significantly enhance the gene and protein expression of Hsp70, TLR4, and NF-κB in the duodenum and ileum, respectively. The gene expression of Hsp70, TLR4, and NF-κB in the jejunum was not influenced, but the protein expression of Hsp70 and NF-κB was inhibited by heat stress. The results indicated heat stress can amplify the effect of E. coli on intestinal inflammatory injury of Ma chickens through increasing TLR4-NF-κB signaling pathway.