Variation in fibroblast expression of toll-like receptor 4 and lipopolysaccharide-induced cytokine production between animals predicts control of bacterial growth but not severity of Escherichia coli mastitis

Abstract

Mastitis caused by environmental pathogens such as Escherichia coli is highly problematic to the dairy industry because it incurs substantial cost and tends to be difficult to manage. An effective innate immune response by the host is key to controlling infection, but it should also limit collateral damage to the mammary gland. Between-animal differences in mastitis severity have been attributed to variability in the innate response. In the current study, we used primary dermal fibroblast as a model to rank animals based on composite expression of the toll-like receptor 4 gene (TLR4) and lipopolysaccharide (LPS)-induced IL-8 and IL-6 protein production. Animals ranked as high and low responders (HR and LR, respectively) were then infected with the P4 strain of E. coli to determine how difference in rank would affect response to mastitis. All animals developed an acute response to the infection with varying degrees in severity; however, HR animals had an elevated somatic cell count and fever response at 12 h post-infection and greater production of milk IL-8 at 24 h post-infection. The HR animals were also significantly more capable of limiting bacterial growth. No differences in post-infection milk production or concentrations of milk BSA were measured. The current study indicates that HR animals have an early upregulation in their innate response that is beneficial for bacterial clearance; however, they are equally susceptible to tissue damage caused by an exuberant response to the infection. The dermal fibroblast may be used in conjunction with other cell types to determine how the innate response is regulated to mitigate unnecessary injury to the mammary gland while still effectively clearing the pathogen.

Keywords: dairy cow; experimental mastitis; innate immunity; lipopolysaccharide.