Abstract
Bone defects present significant challenges in clinical contexts, particularly among the elderly, and are often linked to altered innate immune responses; however, underlying mechanisms remain to be understood. This study investigates immune changes in early bone healing in aged mice, emphasizing the effects of zinc in modulating inflammatory processes. By exploring the role of zinc and NETosis in this process, we seek to develop novel therapeutic strategies that could improve bone repair in aging populations. Critical-sized calvarial bone defects were induced in young (8-week-old) and aged (18-month-old) mice, with RNA sequencing analysis. Zinc oxide nanoparticle-infused polycaprolactone (ZnPCL) scaffolds were then fabricated using electrospinning, and their effects on intracellular zinc levels, NETosis, M2 polarization, and bone formation were assessed through in vitro and in vivo experiments. In aged mice, bone healing was delayed, inflammation was prolonged, and NETosis was excessive. RNA sequencing identified alterations in zinc ion transport genes, alongside excessive NETosis. Aged mouse neutrophils exhibited low intracellular zinc levels. ZnPCL fibers effectively reduced NETosis and inflammation, promoted M2 macrophage polarization, and enhanced new bone formation, thereby improving bone healing in aged mice. This study demonstrates that ZnO nanoparticle-infused biomaterials, ZnPCL, effectively deliver zinc to neutrophils, reduce NETosis, promote M2 polarization, and enhance bone healing in aged mice.