Assessment of epithelial cells' immune and inflammatory response to Staphylococcus aureus when exposed to a macrolide

Non-specific (innate) immune response plays a major role in defending the udder from bacterial invasion. Moreover, recent investigations suggest that mammary gland epithelial cells (MGEC) could have a large and important role as a source of soluble components of immune defences. Despite many attempts to find other ways to control/prevent mastitis (i.e. vaccine) antimicrobial therapy is still the most used and effective means of curing clinical and subclinical mastitis. However, drug concentrations and therapy durations are far from the optimal in order to reduce costs. Therefore, efficacy of antimicrobial therapy is dependent not only on the substance activity but also on the positive interactions with the host innate immune response. Surprisingly, information on these interactions is rather scarce in the mastitis field. A simple experimental model was developed based on BME-UV cell line, Staphylococcus aureus as a challenge and a macrolide as an antimicrobial to assess the interactions among epithelial cells, Staph. aureus and the potential effects of antimicrobials on the immune system. The results of this study confirmed that tylosin has good antimicrobial activity against both intracellular and extracellular Staph. aureus in bovine MGEC without affecting cell functions. In this study, a significant down-regulation of IL-1 and IL-6 was observed, while TNF and IL-8 expression rate numerically increased, but differences were not significant. To our knowledge, this is the first paper assessing the concentration of two lysosomal enzymes, lysozyme and N-acetyl-β-d-glucosaminidase (NAGase), in Staph. aureus-stimulated MGEC. The results of this study confirmed that tylosin could have a significant effect on the release of these enzymes. Moreover, even if both enzymes have a similar substrate as a target, the results suggest different secretion mechanisms and an influence of antimicrobial treatment on these mechanisms. Successful mastitis cure is the result of achieving the optimal efficiency of both innate immune defences and therapeutical activities, by means of killing bacteria without eliciting an excessive inflammatory response. Therefore, antimicrobials for mastitis therapy should be selected not only on bacterial sensitivity, but also for their positive interactions with the innate immune response of the mammary gland. This study showed that an in-vitro model based on Staph. aureus challenge on MGEC could be helpful in assessing both the intracellular and extracellular activity of antimicrobials and their influence on epithelial cell immune and inflammatory response.