Dissociation of ¹⁹F and fluorescence signal upon cellular uptake of dual-contrast perfluorocarbon nanoemulsions

Perfluorocarbon nanoemulsions (PFCs) tagged with fluorescence dyes have been intensively used to confirm the in vivo ¹⁹F magnetic resonance imaging (MRI) localization of PFCs by post mortem histology or flow cytometry. However, only limited data are available on tagged PFCs and the potential dissociation of fluorescence and ¹⁹F label after cellular uptake over time.

PFCs were coupled to rhodamine (Rho) or carboxyfluorescein (Cfl) and their fate was analyzed after in vitro uptake by J774, RAW and CHO cells by flow cytometry and ¹⁹F MRI. In separate in vivo experiments, the dual-labelled emulsions were intravenously applied into mice and their distribution was monitored in spleen and liver over 24 h. In a final step, time course of fluorescence and ¹⁹F signals from injected emulsions were tracked in a local inflammation model making use of a subcutaneous matrigel depot doped with LPS (lipopolysaccharide).

Internalization of fluorescence-labelled PFCs was associated with a substantial whitening over 24 h in all macrophage cell lines while the ¹⁹F signal remained stable over time. In all experiments, ^CflPFCs were more susceptible to bleaching than ^RhoPFCs. After intravenous injection of ^RhoPFCs, the fluorescence signal in spleen and liver peaked after 30 min and 2 h, respectively, followed by a successive decrease over 24 h, whereas the ¹⁹F signal continuously increased during this observation period. Similar results were found in the matrigel/LPS model, where we observed increasing ¹⁹F signals in the inflammatory hot spot over time while the fluorescence signal of immune cells isolated from the matrigel depot 24 h after its implantation was only marginally elevated over background levels. This resulted in a massive underestimation of the true PFC deposition in the reticuloendothelial system and at inflammatory hot spots.

Cellular uptake of fluorescently tagged PFCs leads to a dissociation of the fluorescence and the ¹⁹F label signal over time, which critically impacts on interpretation of long-term experiments validated by histology or flow cytometry.