Transcriptome profiling of Streptococcus uberis-induced mastitis reveals fundamental differences between immune gene expression in the mammary gland and in a primary cell culture model

Streptococcus uberis is a prevalent causative organism of mastitis and resides naturally in the environment of the dairy cow making prevention of the disease difficult. A bovine cDNA microarray comprising approximately 22,000 expressed sequence tags was used to evaluate the transcriptional changes that occur in the mammary gland after the onset of clinical Strep. uberis mastitis. Five lactating Friesian heifers were intramammary infused in an uninfected quarter with approximately 1,000 to 1,500 cfu of a wild-type strain of Strep. uberis. Microarray results showed that Strep. uberis mastitis led to the differential expression of more than 2,200 genes by greater than 1.5-fold compared with noninfected control quarters. The most highly upregulated genes were associated with the immune response, programmed cell death, and oxidative stress. Quantitative real-time reverse transcription PCR analysis confirmed the increase in mRNA expression of immune-related genes complement component 3, clusterin, IL-8, calgranulin C, IFN-gamma , IL-10, IL-1beta, IL-6, toll-like receptor-2, tumor necrosis factor-alpha, serum amyloid A3, lactoferrin, LPS-bonding protein, and oxidative stress-related genes metallothionein 1A and superoxide dimutase 2. In contrast, a decrease of mRNA levels was observed for the major milk protein genes. Bovine mammary epithelial cells in culture challenged with the same Strep. uberis strain used to induce clinical mastitis in the in vivo animal experiment did not cause a change in the mRNA levels of the immune-related genes. This suggests that the expression of immune-related genes by mammary epithelial cells may be initiated by host factors and not Strep. uberis. However, challenging epithelial cells with different Strep. uberis strains and Staphylococcus aureus resulted in an increase in the mRNA expression of a subset of the immune-related genes measured. In comparison, an Escherichia coli challenge caused an increase in the majority of immune-related genes measured. Results demonstrate the complexity of the bovine mammary gland immune response to an infecting pathogen and indicate that a coordinated response exists between the resident, recruited, and inducible immune factors.