E. coli P4 invades mammary epithelium of IL1R −/− mice 48 h after intramammary infection. Paraffin embedded sections stained with H&E (A) or Sytox orange (red) combined with DAPI (blue) (B–C). Numerous bacteria are visible in the alveolar space and in the alveolar epithelial cells (yellow arrow). Inflammation is characterized by massive neutrophil recruitment into the alveolar and tubular spaces (black arrows in A) which is better seen in the enlarged alveolus in B (white arrows). Congestion and accumulation of neutrophils in the interstitial spaces is also seen in IL1R −/− mice (white arrows in A and C), which is also reflected by significantly higher relative MPO activity than in wt mice (D). Data are mean and S.E., n ≥ 6/group and p value compared means between IL1R −/− and wt mice using Student t-test. Scale bars 200 μm (A), 20 μm (B) and 50 μm (C). All images are representative of the entire sample and the histological morphology and pathology results were very similar for each gland in a given mouse and between mice. (For a color version of this figure please consult www.vetres.org.)