Bovine TLR2 and TLR4 properly transduce signals from Staphylococcus aureus and E. coli, but S. aureus fails to both activate NF-κB in mammary epithelial cells and to quickly induce TNFα and interleukin-8 (CXCL8) expression in the udder

Staphylococcus aureus, but not E. coli pathogens frequently cause subclinical, chronic infections of the mammary gland. We examined here, if inadequate activation of the bovine TLR2 and TLR4 pathogen receptors by ligands derived from S. aureus pathogens might contribute to molecular mechanisms underpinning the escape strategies from mammary immune defence of this pathogen. We show that infections with live E. coli, but not S. aureus pathogens induce strongly IL-8 and TNFα gene expression in the udders. Yet, preparations of heat-killed bacteria from both pathogens activate equally well bovine TLR2 and TLR4 receptors to induce NF-κB activation, as shown in the HEK293 reconstitution system of TLR-signal transduction. LTA prepared from the S. aureus strain used to infect the cows activates the bovine TLR2 as strongly as the entire, heat-killed pathogen. Both pathogens induce in primary bovine mammary epithelial cells (pbMEC) IL-8 and TNFα gene expression, but S. aureus to less than 5% of the degree caused by E. coli. This impaired proinflammatory activation is paralleled by a complete lack of NF-κB activation in pbMEC by S. aureus or LTA. In contrast, E. coli and LPS activate strongly NF-κB in these cells. A large proportion of this activation is attributable to TLR-mediated signalling, since a dual transdominant negative DN-MyD88-DN-TRIF factor blocks >80% of the pathogen-related NF-κB activation in pbMEC. Our results prove that impaired binding of TLR-ligands from the pathogenic S. aureus strain are not the cause for the inadequate mammary immune response elicited by this pathogen. Rather, the pathogen causing subclinical mastitis impairs NF-κB activation in MEC thereby severely weakening the immune response in the udder.