Age influences serum immune indices and gut microbiota composition in adult broilers

While the gut microbiota is recognized as a key modulator of host immunity, its age-related dynamics in adult-to-senescent broilers remain poorly characterized. This study aimed to comprehensively analyze age- and sex-related variations in serum immune indices and gut microbiota composition in yellow-feathered broilers aged 1 to 4 years (n = 40). Peripheral blood and fecal samples were gathered for immune profiling using ELISA (STP, IgA, IgG, IL-4, IFN-γ) and 16S rDNA sequencing. The results indicated that serum STP levels peaked at 1 year (p < 0.01), whereas IgG levels exhibited an age-related increase (p < 0.05). Although no significant compositional separation was detected among age groups by PERMANOVA (Adonis p = 0.14), within-group beta dispersion differed significantly across ages (p < 0.05). Firmicutes and Proteobacteria dominated the fecal microbiota at the phylum level. Exploratory Spearman correlation analysis (based on nominal p-values) identified potential associations, such as a positive nominal correlation between Limosilactobacillus abundance and IgG levels (p < 0.01), and a negative nominal correlation between Achromobacter and IL-4 (p < 0.01); however, these findings require further validation due to the lack of correction for multiple testing. Notably, one-year-old hens exhibited the highest abundance of beneficial taxa, such as Lactobacillus, which corresponded with their elevated STP levels. These findings highlight age- and sex-specific interactions between the gut microbiota and immune responses. From a theoretical perspective, one-year-old hens display a gut microbiota and immune profile that could be favorable for microbiota transplantation; however, direct functional validation (e.g., FMT experiments in recipient models) is required to support this hypothesis. This study offers novel insights into avian aging models and microbiota-mediated immune regulation, presenting potential strategies to enhance poultry health in intensive farming systems.