Hydrogel-integrated osteogenic microtissues promote repair of infected intervertebral defects through sequential immunomodulation

Abstract

Intervertebral infection remains difficult to treat because the adult intervertebral disc is avascular, hypoxic, and poorly perfused, limiting drug penetration, immune-cell trafficking, and metabolic waste clearance. Solving this problem requires localized control of bacteria while reconstructing a microenvironment capable of supporting vascularized bone regeneration. We developed an injectable system that integrates active bone microtissues into a hydrogel with antibacterial and immune-regulatory functions, combining local microenvironment modulation with the delivery of active osteogenic units. Within the "soil-seed-nutrient" framework, the GA/SilMA matrix serves as the immunomodulatory "soil," the osteogenic bone microtissue at day 7 acts as the active "seed," and Zn2+ and BP function as "nutrient" providing antibacterial and regenerative effects. This composite hydrogel reduces bacterial growth and prevents biofilm formation, while triggering a time-dependent macrophage response characterized by an initial antibacterial inflammatory phase followed by a later pro-repair state. Zn2+ and black phosphorus additionally supported endothelial activity and osteogenic differentiation under inflammatory conditions. In a rat Co2/Co3 intervertebral defect model, after acute Staphylococcus aureus contamination and early implantation of the material, GA-Zn2+/BP/BO@SilMA demonstrated enhanced bone formation at 12 weeks and achieved more significant structural bridging as evidenced by X-ray and histological evaluations. Transcriptome analysis revealed putative associations with immune remodeling, extracellular matrix interactions, metabolic adaptation, and osteogenic signaling pathways. Collectively, this strategy integrates multiple synergistic functions, including antibacterial activity, macrophage modulation, angiogenesis promotion, and osteogenesis, offering potential as a local adjuvant for early intervention and regenerative repair of bacterial-contaminated intervertebral defects.