A Transgenic Mouse Model to Track MRC1-High Macrophages Using In Vivo Optical Imaging

Macrophages play a crucial role in health and disease. Currently, reporter mice for tracking alternatively activated macrophages in vivo are lacking. We designed a transgenic mouse model in which luminescence and fluorescence proteins, click beetle red luciferase (CBRED2) and mKate2, report on the expression of the Mrc1/Cd206 promoter, active in the monocyte/macrophage population. The mouse line was named B6Mrc1-mKate2-CBRED2. Using this novel mouse model, we were able to develop in vitro assays to validate transgenic macrophage polarization and test them with compounds of repolarization potency. Furthermore, in the in vivo assays, we exploited the migratory and infiltrative potency of macrophages for detecting tumor locations via optical imaging. In fact, macrophages can act as universal cancer markers, as they infiltrate primary and secondary tumors, stimulating or suppressing tumor growth. We first characterized transgenic mice for reporter expression ex vivo, followed by the generation of luminescence-based assays to reflect the polarity of differentiated macrophages, and lastly, we visualized reporter macrophages accumulating and infiltrating the tumor microenvironment (TME) of murine pancreatic ductal adenocarcinoma (PDAC) at multiple time points. We found that the extent of macrophage recruitment and retention was dependent on the infiltrative T-cell and dendritic cell populations present in the TME, reflecting the immunologically hot or cold nature of the PDAC clones, respectively. In conclusion, the ability to optically detect light-emitting macrophages can be applied not only for cancer studies but also in the context of inflammatory diseases.