Near-infrared dye conjugated polysaccharide nanogel-based contrast agents for M2 macrophage-targeting photoacoustic imaging

Cancer immunotherapy provides high anti-tumor effects with minimal side effects by harnessing the patient's immune systems. However, immunosuppressive tumors, called cold tumors, are unresponsive to immunotherapy, mainly owing to the high density of surrounding M2 macrophages, the exclusion of T cells, and the low expression of PD-L1 molecules. Thus, early identification of cold tumors through differences in the immunological environments of tumors would be promising to determine the optimal cancer treatment for each patient. Here, we developed a contrast agent targeting M2 macrophages to identify cold tumors by photoacoustic imaging. The polysaccharide-based contrast agent (denoted PMI) was synthesized by conjugating pullulan with mannose (M2 macrophage-targeting moiety) and IR-820 (near-infrared dye). In an aqueous solution, PMI formed a nanogel with a diameter of less than 100 nm via hydrophobic interactions involving IR-820 molecules. Owing to the IR-820 molecules, PMI nanogel absorbed near-infrared light, providing contrast for fluorescence and photoacoustic imaging. Moreover, M2 polarized RAW264.7 macrophage cells interacted with PMI nanogel but did not interact with IR-820 molecules and nanogels without mannose modification, indicating specific ligand–receptor interactions between mannose and CD206, a mannose receptor on M2 macrophages. Finally, PMI nanogel injected into colon26 tumor-bearing mice provided contrast for photoacoustic imaging of cold tumors. Therefore, PMI nanogel successfully targeted M2 macrophages to provide high contrast for photoacoustic imaging of cold tumors, demonstrating its potential as a platform for imaging and therapy of cold tumors. • A novel PMI (Pullulan-mannose-IR820) nanogel self-assembled via IR-820's hydrophobic interaction. • PMI nanogel absorbed near-infrared light and acted as a photoacoustic imaging contrast agent, same as IR-820. • PMI nanogel enhanced the interaction with M2 macrophages through mannose-receptor binding. • PMI nanogel improved the photoacoustic imaging contrasts of M2 macrophage cell pellet and colon26 tumor.