Staphylococcus aureus Mastitis: A Time-Course Transcriptome of Immune Activation in Small-Tailed Han Sheep

Mastitis is a common mammary gland disease in mammals that severely impairs lactation function, with Staphylococcus aureus (S. aureus) being the primary pathogenic bacterium. However, the molecular mechanisms underlying S. aureus-induced mastitis in sheep remain incompletely elucidated. This study employed RNA sequencing (RNA-SEq) technology to systematically analyze the dynamic transcriptomic characteristics of mammary tissue in small-tailed sheep (SHT) after S. aureus infection, aiming to clarify the molecular regulatory mechanism of the host immune response and its relationship with the occurrence of mastitis. Twelve lactating STH were selected to establish an S. aureus-induced mastitis model. Blood, milk, and tissue samples were collected at 0, 24, 48, and 72 h post-infection (hpi). The infected sheep exhibited typical mastitis symptoms, including exacerbated breast swelling, reduced milk yield, elevated udder temperature, and darker, more viscous milk. Hematoxylin-eosin (HE) staining revealed significant pathological changes over time, such as stromal hyperplasia, extensive inflammatory cell infiltration, severe necrosis and sloughing of mammary epithelial cells, and compromised tissue integrity. RNA-Seq analysis identified 1299 differentially expressed genes (DEGs), among which 75 core genes maintained stable expression throughout the infection time (24 hpi, 48 hpi, and 72 hpi). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses indicated that these DEGs were associated with metabolic processes, protein binding, Toll-like receptor signaling, and the NF-κB pathway. The PPI network analysis identified core hub genes including PTK2B, STAT3, and JAK1/3, providing critical evidence for therapeutic target screening. Furthermore, qPCR verification indicated that the expressions of innate immune receptors TLR2, TLR4, TLR7, and TLR10, as well as pro-inflammatory factors IL-1β, IL-16, TNF-α, type I interferon (IFN-α), and nuclear transcription factor NF-κB were significantly upregulated in a time-dependent manner (p < 0.05). In conclusion, this study delineated the dynamic response of ovine mammary tissue to S. aureus infection, systematically elucidated temporal gene expression patterns, and revealed the molecular mechanisms underlying the tissue's initial defense against inflammatory challenges.