Aims and objectives: To assess the hypothesis of premetastatic tissue priming and the role of immune cells and associated chemokines during tumour cell migration and seeding. Methodology A syngeneic mouse model for breast cancer metastasis will be established using the cell line 4T1, known to preferentially metastasise to the lungs. As control, non- and low-metastatic 4T1 clones will be used, all equipped with a fusion protein allowing for both, fluorescence and radionuclide mediated imaging. A labelled antibody will serve for parallel visualisation of monocyte activity indicating S100A9 expression. For visualisation of cell interactions and the influence of S100A9 on premetastatic niche (PN) formation, intravital and ex-vivo microscopy (incl. FRET, FLIM) will be performed. This will inter alia allow for the first in-vivo visualisation of the aggregation of S100A8 and A9 to form the active S100A8/A9 heterodimer and evaluation of the influence of mon- and dimers on the malignant microenvironment. PN tissue will be identified by signs of inflammation (S100A9) without tumour cell associated signals in whole-body imaging and further examined ("optical biopsy"). PN-associated macrophages will be characterised and transferred to transcriptome profiling, screening for potential target genes for intervention. Scientific and medical opportunities This study will result in a model system for further study of the metastatic process in general. As a paradigm, the impact and regulation of S100A8/A9, discussed as a major promoter of the PN, will be evaluated. This and the identification of PN-associated macrophages with regards to their specific makeup and role within this process might reveal novel opportunities to impair tumour metastasis.