In vivo imaging of NF‐κB activity during Escherichia coli‐induced mammary gland infection

In mammary gland infections, the contribution of NF‐κB is not well defined, and was therefore investigated following intramammary inoculation of Escherichia coli. Non‐invasive real‐time in vivo imaging of the transcription factor activation was performed in mammary glands of transgenic mice expressing luciferase under the control of NF‐κB. Bacterial inoculation resulted in a major increase in luminescence as compared with control glands. This activation was confirmed by immunohistochemical nuclear staining of the NF‐κB p65 subunit in mammary epithelium of infected glands, while nuclear p50 was not detected. The systemic response to the intramammary inoculation of Escherichia coli was also studied. NF‐κB activation in the liver increased over time, and a relatively mild but longer‐lasting response was observed as compared with the acute hepatic response of mice receiving lipopolysaccharide. This systemic reaction was confirmed by increased circulating levels of the acute phase protein serum amyloid A, tumour necrosis factor‐α and interleukin‐6. In addition, high concentrations of both cytokines in the mammary gland inoculated with bacteria showed that the infection was also well established at the local level. These results indicate that in vivo monitoring of NF‐κB activation is an attractive novel approach to study mammary gland inflammation, and that this transcription factor is imperative in the early stages of the host immune response towards coliform intramammary infections, both at the local and systemic level.