Dissection of mammary cell landscape in ruminal dysbiosis-induced mastitis by single-cell RNA sequencing

Growing evidence has underscored the contribution of gastrointestinal dysbiosis to the onset of mastitis, however, the local cellular changes responsible for the pathological processes of ruminal dysbiosis-induced mastitis (RDIM) are still unclear. Here, we profiled mammary single-cell transcriptomes in goats with RDIM, complemented by ruminal metagenomic and untargeted metabolomic analyses of rumen fluid and serum. Our results indicated that compromised lactation and barrier integrity in LumSec were linked to RDIM. Increased inflammatory macrophages and DCs, γδT and CD4⁺ T_H cell populations, along with reduced T_ex/T_reg and B cells were implicated in RDIM. Fibroblasts exhibited increased gene expression related to the extracellular matrix, while lymphatic endothelial cells and Vas-venous structures displayed elevated inflammatory gene expression. Tight junction integrity and apelin signaling pathways were compromised in Vas-capillary during RDIM. Notably, metagenomic analysis indicated that RDIM correlated with reduced ruminal microbial diversity and shifts in microbial community composition. Key metabolic pathways including microbial tryptophan metabolism, secondary bile acid biosynthesis, and vitamin metabolism were significantly diminished during RDIM. Furthermore, tryptophan-induced AHR signaling and secondary bile acid receptor GPBAR1, primarily expressed in vascular endothelial cells and macrophages, respectively, which were reduced during RDIM. Collectively, our study provides a comprehensive atlas of mammary cell landscapes in RDIM, which may enhance the understanding of mastitis pathogenesis.