For example, MMP-9 can cleave proIL-1beta into an inactive 26 kDa fragment besides the classical active p17 fragment, while MMP-3 produces inactive 28 kDa as well as less active 14 kDa peptides. In addition, other papers – albeit not in a mammary gland context – describe that MMP-2 can cleave proIL-1beta into both a very low activity 16 kDa and an inactive 10 kDa fragment. At least some of these reported fragments should correspond to fragments from the complex pattern of lowmolecular weight bands found in the current study post-IMI with E. coli. Importantly however, they do not correspond with the molecular weight of those band found in the current study post-IMI with S. aureus. The suggestion that MMPs are induced during the hosts’ innate immune response against E. coli to inactivate IL-1beta is strengthened by our histological findings. Indeed, the epithelium post-IMI with E. coli was clearly damaged as seen on mammary gland sections, a deterioration which again was only mildly present post-IMI with S. aureus. Of relevance, a high NFkappa B activity during mammary gland infection increases caspase-3 mediated cell budding and shedding of epithelial cells. This form of accelerated involution is likely associated with MMPs. Finally, there was also one additional band with a MW between 17.5 kDa and 20 kDa that was selectively present postIMI with S. aureus and not post-IMI with E. coli. It is here hypothesized that the latter cleavage fragment might be the product of pathogen-associated proteases as previously described. Above mentioned arguments implicate the involvement of epithelial and neutrophilic proteases in the maturation of proIL-1beta. Surprisingly, in both these mammary cell types we could also detect NF-kappaB activity upon immunohistochemical evaluation. Furthermore, it should be highlighted that till date, the precise origin of the mammary proIL-1beta protein remains vague. Nevertheless, from the current study it is clear that the responsible transcription factor inducing the IL-1beta proform is certainly active prior to 12 h post-IMI with both pathogens. However, its maturation occurs only shortly before 12 h post-IMI with E. coli, while for S. aureus this process occurs about 12 h later i.e. around 24 h. Furthermore, our immunohistochemical data unequivocally demonstrated that for both bacteria the main subunit p65 of the transcription factor NFkappaB is translocated to the nucleus of the murine mammary epithelial cells. The latter translocation is an SCH772984 essential step for activation of this key inflammatory transcription factor. To evaluate the level of NF-kappaB activation, both pathogens were compared with in vivo imaging in the reporter model previously established by our group for E. coli. As described for E. coli in this latter paper, a fast and strong NF-kappaB activation was again observed in the current study. However an on average 3 times lower NF-kappaB activity was detected in the mammary gland for S. aureus. Remarkably, the transient enhancement of this NF-kappaB activity already peaked for both pathogens at 6 h post-IMI.