Visualizing and modulating lysosomal pH in macrophages via internalization and degradation of polyglycolic acid particles

Macrophages, key players in the immune response, undergo polarization into pro-inflammatory (M1) or anti-inflammatory (M2) phenotypes upon receiving external stimuli. Alterations in lysosomal pH values within macrophages regulate the polarization process. The internalization of degradable polyester particles may interfere with the lysosomal pH of macrophages. This study investigated how internalized poly(glycolic acid) (PGA) particles with different degradation rates induced lysosomal acidification in murine macrophages (RAW 264.7) and modulated their phenotypic polarization. Lysosomal pH changes and particle internalization were monitored using a pH-sensitive fluorescent dye and PGA particles tagged with fluorescein isothiocyanate (FITC). Real-time imaging and quantitative measurements of pH alterations following PGA internalization were conducted. The findings revealed that culturing/internalizing 20 PGA particles per cell with a fast acidification rate (from pristine porous foam) induced pro-inflammatory responses in M0 and M1 macrophages, characterized by increased M1 cytokine secretion and biomarker expression. In contrast, culturing/internalizing 20 PGA particles per cell with slower acidification rates (from partially degraded foam) promoted the polarization of M0 macrophages to M2 macrophages, as evidenced by increased M2 marker expression and cytokine secretion. V-ATPase, a key regulator of lysosomal pH, was inhibited in M1 macrophages by internalization of PGA particles, whereas it was activated in M2 macrophages, thereby further supporting the anti-inflammatory effects. These results confirm that the acidification of PGA particles and their acidification rate can modulate macrophage polarization intracellularly and further affect inflammatory responses. This study highlights the potential of degradable biomaterials in regulating immune responses and provides a foundation for designing biomaterials with targeted immunomodulatory effects.