Platycodon grandiflorum and Glycyrrhiza uralensis extract attenuates cyclophosphamide-induced tracheal mucosal barrier injury in broilers by modulating gut microbiota and short-chain fatty acids.

Respiratory mucosal immunity is critical to sustaining respiratory health and production performance in poultry. Damage to this barrier increases susceptibility to respiratory diseases in poultry and causes economic losses. In traditional Chinese medicine, Platycodon grandiflorum and Glycyrrhiza uralensis are commonly combined for the treatment of respiratory diseases. This study hypothesized that Platycodon grandiflorum and Glycyrrhiza uralensis extract (PGL) may modulate the cecal microbiota structure and concentrations of short-chain fatty acids (SCFAs) in broilers, and potentially participate in alleviating cyclophosphamide (CTX)-induced tracheal mucus barrier injury in immunosuppressed broilers via the gut-trachea axis. Multiple indicators including growth performance, immune organ indices, intestinal and respiratory mucosal barrier function, cecal microbiota and SCFA levels were comprehensively evaluated. A total of 240 healthy 1-day-old AA broilers were randomly assigned to four groups (5 replicates per group, 12 broilers per replicate, 28 d): control group (CON), PGL supplementation group (PGL), CTX treatment group (CTX), and CTX combined with PGL group (CTX+PGL). Results showed that compared with the CTX group, the CTX+PGL group exhibited a significant increase (12.19%) in survival rate from 15 to 28 d. PGL alleviated CTX-induced tracheal inflammatory infiltration and goblet cell loss. It significantly restored the mRNA expression levels of tracheal tight junction proteins, FFAR2 (50.75%), sIgA (346.15%), NF-κB1 (35.16%), NLRP3 (31.82%) and mucins, while significantly downregulating the relative mRNA expression levels of IL-8 and IFN-γ (P < 0.05). In comparison with the CTX group, PGL ameliorated CTX-induced pathological changes, including alveolar wall thickening and alveolar space narrowing, and significantly increased the mRNA expression levels of tight junction proteins and mucins in lung tissue (P < 0.05). Quantitative immunofluorescence analysis showed that CTX+PGL treatment restored the protein expression levels of tracheal NF-κB p65 (43.28%), NLRP3 (81.13%), and MUC2 (43.47%) to near-normal levels in immunosuppressed broilers (P < 0.05). Cecal 16S rRNA sequencing analysis showed that the CTX+PGL group exhibited significant enrichment of Bacteroidota and Butyricimonas, accompanied by increased cecal SCFA concentrations (P < 0.05). The present findings provide correlative evidence supporting the potential involvement of the gut-trachea axis in the protective effects of PGL against respiratory mucosal immune injury.