Increased nuclear Factor κB activity in milk cells of mastitis-affected cows

Bacterial mastitis is accompanied by a drastic increase in milk somatic cell count (SCC), with neutrophils being the predominant cell type found in the infected quarters. Accumulation and activation of neutrophils at the site of infection require local expression of many inflammatory genes encoding adhesion molecules, chemokines and cytokines. Most of the inflammatory genes contain binding sites for the nuclear factor kappaB (NF-kappaB) within their promoter and therefore partly depend on NF-kappaB for their expression. We thus hypothesized that an increase in NF-kappaB activity in the mammary gland could contribute to development of the neutrophilic inflammation that characterizes mastitis. In an attempt to verify this hypothesis, we first assessed milk cells from healthy and acute and chronic mastitis-affected cows for NF-kappaB activity using electrophoretic mobility shift assays. We next studied the relationships between the intensity of NF-kappaB activity in these cells and the degree of udder inflammation. Active NF-kappaB complexes were undetectable in milk cells from healthy cows, whereas high levels of NF-kappaB activity were always found in cells from cows with acute mastitis. In milk cells obtained from chronic mastitis-affected cows, NF-kappaB activity varied from low to high. Finally, the level of NF-kappaB activity measured in milk cells from chronic mastitis-affected cows was not correlated to SCC or to the proportion of neutrophils present in milk samples, but was highly correlated with the expression level of interleukin-8 and granulocyte/ macrophage colony-stimulating factor, two NF-kappaB-dependent cytokines crucially involved in initiation and perpetuation of neutrophilic inflammation. These results suggest that NF-kappaB might play a role in mastitis pathogenesis.