Comparative study of mannose-containing polysaccharide nanogels for optimized M2 macrophage targeting and photoacoustic imaging

Immunotherapy efficacy is often hindered by the immunosuppressive “cold tumor” microenvironment, dominated by M2 macrophages. We developed pullulan–mannose–IR820 (PMI) nanogels as targeted photoacoustic (PA) contrast agents and performed a comparative study with mannan (MI) and fucoidan (FI) nanogels to establish structural design guidelines for M2 targeting. All the variants formed stable nanogels ( < 100 nm) driven by IR-820 self-association, providing robust PA signals. Despite the high intrinsic sugar content of natural polysaccharides, PMI nanogels exhibited significantly greater internalization by M2 macrophages. This superior efficiency was not due to electrostatic interactions, as all the variants possessed comparable negative ζ-potentials. Competitive inhibition assays revealed a moderate decrease in uptake ( ~ 10–15%) for all the nanogels, indicating that while nonspecific pathways are dominant in overall cell association, mannose receptors are partly involved. Remarkably, despite having a significantly lower mannose content than native mannan did, the inhibition rate of the PMI nanogel was comparable, suggesting that its unique pendant configuration offers superior spatial accessibility for efficient receptor interaction. Consequently, the PMI nanogels achieved the highest PA contrast in M2 macrophage imaging. These findings highlight that spatial ligand presentation, rather than absolute density, is a critical determinant in the design of next-generation drug delivery systems for targeting the immunosuppressive tumor microenvironment.