Background Recruitment and migration of phagocytes to the site of inflammation are key events in the onset of inflammation. Albeit crucial for pathogen elimination, tissue repair and restoration of tissue homeostasis, dysregulated phagocyte infiltration can also cause severe inflammatory disorders. Therefore, targeting and modulation of phagocyte infiltration represents a promising new approach to fight inflammatory disorders and diseases, such as rheumatoid arthritis. Additionally, non-invasive tracking of phagocyte migration to the site of inflammation could extend both scientific knowledge as well as the repertoire of diagnostic strategies in clinical use.

Objectives The aim of this study was to establish a fluorescence reflectance imaging (FRI) based system to visualize and analyze migration properties of different cell populations in inflammatory disease models, like experimental arthritis, in vivo.

Methods Immortalized murine myeloid progenitor ER-HoxB8 cells were differentiated to neutrophils or monocytes (1). Cells were labeled with the membrane-selective fluorescent dyes DIR (2) or DID, respectively. We analyzed viability and functionality of stained cells in vitro and investigated their ability to migrate to sites of inflammation in vivo in several mouse models - particularly in a collagen induced arthritis (CIA) mouse model - via fluorescence reflectance imaging (FRI). Using CRISPR-Cas9 technology we introduced targeted gene deletions for main adhesion molecules.

Results Differentiated ER-HoxB8 cells could effectively be labeled with DIR or DID. Labeling of monocytes or neutrophils did not affect cellular viability or functionality in vitro. Subsequent in vivo imaging experiments allowed the visualization of migrated labeled phagocytes in different murine disease models, thereby cells could be detected at sites of inflammation with high sensitivity and specificity. In a CIA mouse model the amount of immigrated cells could even be associated closely to disease score and disease severity. Thus, the detection of immigration of labeled cells might also give hints about new inflammatory spots that are about to settle up before they can be detected macroscopically. Furthermore, differential cell labeling allowed direct quantitative comparison of differences in migration rates of wildtype and CD18 or CD49d knockout cells in vivo.

Conclusions Specific and distinguishable labeling of diverse cell types allows in vivo tracking and subsequent quantification of migrated cells within the same animal. Targeted gene deletion allows analysis of molecular mechanisms relevant for leukocyte recruitment during different stages of arthritis. Correlation of the amount of immigrated cells to disease severity offers new opportunities to non-invasively detect and monitor inflammatory sites in vivo.

References

1. Wang GG, Calvo KR, Pasillas MP, Sykes DB, Häcker H, Kamps MP. Quantitative production of macrophages or neutrophils ex vivo using conditional Hoxb8. Nat Methods. 2006;3(4):287–93.

2. Eisenblätter M, Ehrchen J, Varga G, Sunderkötter C, Heindel W, Roth J, et al. In vivo optical imaging of cellular inflammatory response in granuloma formation using fluorescence-labeled macrophages. J Nucl Med. 2009;50:1676–82.

References

Disclosure of Interest None declared