The Inflammatory Immunity and Gut Microbiota are Associated with Fear Response Differences in Laying Hens

The fear response is a crucial adaptive mechanism for coping with environmental changes, and the individuals have different levels of fearfulness. The purpose of this study was to determine the status of the immune response and gut health in hens with different fear responses. A total of 80 healthy 75-week-old native Lindian chickens were individually housed in conventional cages and categorized into high (TH) and low (TL) levels of fearfulness using the tonic immobility (TI) test. The immunological status and intestinal health of the laying hens were assessed, and the intestinal microbial community was sequenced using 16S rRNA testing. The results showed that the immune-related genes of interleukin (IL)-1β, IL-4, IL-6, and IgG were significantly upregulated in the spleen of TH hens compared with hens in the TL group (P < 0.01). The inflammatory immune-related genes Toll-like receptor (TLR)2, TLR4, nuclear factor (NF)-κB, inducible nitric oxide synthase (iNOS), cyclooxygenase (COX)-2, IL-10, and IgG were significantly increased in the intestinal tract, whereas IL-4, IgA, and the intestinal barrier gene claudin-4 were significantly decreased in TH hens (P < 0.05). In addition, serum concentrations of IL-1β, IL-6, IL-10, interferon (IFN)-α and IgG were significantly higher in TH hens (P < 0.01). A high fear response also led to changes in gut microbial diversity, with a higher Simpson's index and lower β-diversity similarity than hens with a low-fear response (P < 0.05). The TH group showed an increase in eight genera, including Bacillaceae and Coprococcus, whereas the genus Anaerorhabdus decreased (P < 0.05). The gut microbiota has also been associated with gut barrier genes, and inflammatory cytokines. Bartonella stimulates IL-1β and IgG secretion, whereas Lactobacillus inhibits IL-6 secretion, and Coprococcus and Subdoligranulum are associated with the maintenance of intestinal barrier function. The results of this study suggest that laying hens with high fear response levels have a more sensitive immune response and a more enriched gut microbiota, which may have positive effects on adapting to a complex environment.