Modern dairy and beef cattle efficiently produce large quantities of high value products, milk and meat respectively, for human consumption. In order to meet the energy demands of production, cattle are fed diets consisting of rapidly fermentable carbohydrates. As a result, ruminal acidosis may occur. The acidotic conditions in the ruminal fluid can result in compromised barrier function and, potentially, translocation of microbes and microbe-associated molecular patterns, (MAMP) from the lumen across the ruminal epithelium. Translocation of microbes or MAMP may lead to an interaction with the ruminal epithelial cells (REC), thus inducing a local, pro-inflammatory response. However, little is known about the capability of REC to initiate such a response as well as the effects of inflammation on the physiological functions of the ruminal epithelium. The objective of this research was to assess the inflammatory response of the ruminal epithelium and to investigate the potential effects of inflammation on nutrient uptake and metabolism using in vivo, ex vivo and primary cell culture models.
In Chapter 3, ruminal papillae biopsies were collected from beef heifers following induction of subacute ruminal acidosis (SARA). The papillae were used to evaluate differential gene expression and toll-like receptor (TLR) 4 quantification. Despite an increase in ruminal fluid concentration of LPS, gene expression of inflammatory molecules and immunohistofluorescent analysis of TLR4 protein expression indicated an anti-inflammatory response 2 d following the SARA challenge.
The Ussing chamber model was used in Chapter 4 to explore the effects of LPS exposure on the inflammatory response and the potential effects on butyrate flux and metabolism. Analysis of gene expression suggested that the pro-inflammatory response to LPS may have been suppressed or prevented by the epithelial barrier. In tissue exposed to LPS, butyrate flux tended to increase linearly (P = 0.063); however, production of β-hydroxybutyrate (BHB) was not affected (P = 0.21), suggesting that the impact of LPS exposure on metabolism of the ruminal epithelium was minimal.
To further evaluate REC responses to LPS exposure, a cell culture model was established (Chapter 5). Using that model in Chapter 6, I evaluated the effects of dose, duration, and timing of LPS exposure on viability and gene expression of pattern recognition receptors (PRR), pro-inflammatory cytokines, chemokines, and other immunomodulatory molecules in cultured primary REC. There was no indication that LPS negatively impacted cell viability, but exposure to LPS increased TLR2 and TLR4 expression and induced a pro-inflammatory response. Results suggested that the REC response was influenced by LPS dose and duration of exposure, and that gene expression may have been regulated to prevent an excessive pro-inflammatory response and potential damage to the cells.
In Chapter 7, cultured REC were exposed to LPS when grown with or without the addition of short-chain fatty acids (SCFA) to the cell culture media in order to evaluate the effects of the inflammatory response on metabolic function. Results showed that LPS exposure tended to increase glucose utilization compared to control REC (31.8 versus 28.7 ± 2.7%; P = 0.072). Analysis of gene and protein expression further indicated that nutrient transport and metabolism in the cells may have been moderately altered by the inflammatory response.
Overall, the results of this research indicate that exposure of the REC to LPS activates an inflammatory response, the nature of which is dependent on the dose, duration and timing of exposure. However, the response may differ between whole tissue and cultured REC. The data suggested that although metabolism of the REC may have been altered, the changes observed were moderate, indicating that key functions of the ruminal epithelium are resilient to local inflammatory responses
