Characterization of inter-animal variation in the innate immune response of the bovine and its relation to S. aureus mastitis.

Mastitis represents one of the major economical and animal welfare concerns within the dairy industry. Animals affected with this disease can experience a range of clinical symptoms from mild discomfort and swelling of the udder to a severe systemic inflammatory response that could result in the death of the animal. This range of responses is due to differences in pathogen, environment, and inter-animal differences in their innate immune response. A dermal fibroblast model was used to predict the magnitude of an animal’s innate immune response towards an intra-mammary S. aureus challenge. Animals whose fibroblasts exhibited a low response phenotype, characterized by lower levels of IL-8 following in vitro immune stimulation, suffered less mammary tissue damage and a less severe reduction in milk quality following the in vivo S. aureus challenge as compared to animals classified as high responders. Furthermore, the heightened inflammatory response of the high responders offered no advantage in bacterial clearance. For a S. aureus infection, the lower response phenotype is preferred. To further explore inter-animal variation in the innate immune response, fibroblast cultures were established and challenged with LPS from two breeds of cattle, Holsteins, a dairy breed and Angus, a beef breed. Cultures from Holstein animals exhibited a higher responding phenotype than cultures from Angus animals. As these two breeds undergo selection for different traits and are reared differently as calves, whole transcriptome analysis (RNA-Seq) and DNA methylation analysis (Methylated CpG Island Recovery Assay; MIRA-Seq) of their fibroblasts was completed to examine the genetic and epigenetic basis for the contrasting responses. RNA-Seq revealed several immune associated genes that were expressed at higher levels in Holstein cultures compared to Angus cultures, including TLR4, IL-8, CCL5, and TNF-α, both basally and following LPS exposure. Although MIRA-Seq analysis revealed 49 regions with differential methylation between the Holstein and Angus cultures, overall, the methylation of the fibroblast genome was similar between these breeds. A combination of genetic and epigenetic factors seems to contribute to the breed-dependent differences observed between Holstein and Angus fibroblasts. Early life exposure to bacterial compounds or inflammatory mediators can have long-term effects on the magnitude of an animal’s innate immune response, and may contribute to inter-animal variation in this response. To determine if an early life exposure to LPS would modify the response to a subsequent LPS challenge in dairy animals, neonatal Holstein calves were treated with LPS or saline at 7 days of age and subsequently challenged with LPS 25 days later. Calves that received LPS at 7 days of age had greatly elevated levels of plasma IL-6 and TNF-α compared to calves that received saline, indicating a substantial inflammatory response. However, following the subsequent LPS challenge completed on all calves, there were no differences in plasma IL-6 and TNF-α between the LPSand salinetreated calves. Alternative exposure strategies in calves may generate the long-term effects observed in other model systems. There is a wide range in the responses observed in the innate immune response of the bovine. Animals with a lower innate immune response effectively clear the infection, but avoid the collateral tissue damage from excessive inflammation. Therefore, it seems that a reduced innate immune response would be more beneficial to the dairy cow.