Extracellular milieu grossly alters pathogen-specific immune response of mammary epithelial cells

Background: Considerably divergent data have been published from attempts to model the E. coli vs. S. aureus specific immune reaction of the udder using primary cultures of bovine mammary epithelial cells from cows (pbMEC). Some groups reported a swift, strong and transient inflammatory response against challenges with E. coli and only a weak and retarded response against S. aureus, in agreement with the respective reaction of the udder. Others found almost the reverse. Presence or absence of fetal calf serum distinguished the experimental setting between both groups. We examined here if this causes the divergent reaction of the pbMEC towards both pathogen species. We challenged pbMEC with proteins from heat killed E. coli or S. aureus pathogens or purified TLR2 and TLR4 ligands. The stimuli were applied in normal growth medium with (SM10) or without (SM0) 10% fetal calf serum, or in the basal medium supplemented with 10 mg/ml milk proteins (SM Milk). Results: Withdrawal of FCS slowed down and decreased the extent by which E. coli or LPS enhanced the expression of cyto- and chemokine encoding genes through impaired TLR4 signalling but enforced their expression during stimulation with S. aureus. SM Milk strongly quenched the induction of those genes. S. aureus strain specific differences in the reaction of the pbMEC could only be recorded in SM0. NF-B factors were activated by E. coli in all stimulation media, but only to a small extent by S. aureus, solely in SM0. Purified ligands for TLR2 stimulated expression of those genes and activated NF-B equally well in SM10 and SM0. The mRNA destabilizing factor tristetraproline was only induced by E. coli in SM10 and by purified PAMPs. Conclusions: Our data cross validate the correctness of previously published divergent data on the pathogen-specific induction of key immune genes in pbMEC. The differences are due to the presence of FCS, modulating signalling through TLR4 and TLR-unrelated pathogen receptors. S. aureus does not substantially activate any TLR signalling in MEC. Rather, receptors distinct from TLRs perceive the presence of S. aureus and control the immune response against this pathogen in MEC.