A Rhodamine-Mannan Fluorescent Probe Targeting M₂ Macrophages for Polarization Imaging

Macrophage polarization into M₁ and M₂ phenotypes plays critical roles in homeostasis and disease pathogenesis. M₂ macrophages are characterized by high expression of the mannose receptor (CD206), which specifically binds and internalizes mannosylated ligands. In this study, we developed a novel fluorescent probe by conjugating Rhodamine B (RhB) to mannan isolated from Tremella fuciformis (RhB-mannan). The probe was thoroughly characterized by FTIR, NMR, and fluorescence spectroscopy. Using Raw 264.7 macrophages polarized to M₁ (LPS) or M₂ (IL-4) phenotypes, we demonstrated that the RhB-mannan probe selectively labels M₂ cells, correlating with CD206 expression confirmed by immunofluorescence. The specificity of the probe was further validated by competitive inhibition with free mannose, which abolished the fluorescent labeling. This probe provides a valuable tool for live-cell imaging of M₂ macrophages and studying their role in disease progression.