GelMA Hydrogel Stiffness Modulates IL-6- and BMP-2-Induced Immune Dysregulation in Human Mesenchymal Stem Cells

Background: Fracture healing requires a coordinated inflammatory response, and its dysregulation, as seen in polytrauma, can impair bone regeneration. Human mesenchymal stem cells (hMSCs) play a central role in fracture repair through osteogenic differentiation and also via their secretome, which regulates local inflammation, angiogenesis, and tissue regeneration. Interleukin-6 (IL-6), an early pro-inflammatory cytokine, contributes to fracture healing by promoting MSC recruitment and osteogenic differentiation, whereas bone morphogenetic protein-2 (BMP-2) is a key osteoinductive factor that drives bone formation. However, the combined effects of IL-6 and BMP-2 on the hMSC secretome remain poorly understood. Methods: We cultured hMSCs in osteogenic media supplemented with recombinant IL-6 (1–20 ng/mL) alone or combined with recombinant BMP-2 (1 ng/mL) on tissue culture plastic (TCP) and within gelatin methacryloyl (GelMA) hydrogels of low (~3 kPa), medium (~15 kPa), and high (~30 kPa) stiffness. Osteogenic differentiation was assessed by alkaline phosphatase (ALP) activity and calcium deposition; cytokine profiling was performed using a multiplex antibody array. Results: When cultured on TCP, IL-6 suppressed ALP activity by day 21. Co-treatment with IL-6 and BMP-2 induced a dysregulated secretome with concurrent upregulation of pro-inflammatory markers (MIP-1α, TNF-α, and GM-CSF) and anti-inflammatory mediators (IL-10, TGF-β1, and VEGF). This hyperinflammatory response was attenuated when hMSCs were encapsulated in GelMA, with high-stiffness gels most effectively suppressing pro-inflammatory chemokines and medium-stiffness gels yielding the highest ALP activity. Conclusions: These findings suggest that mechanically tuned GelMA hydrogels modulate immune and osteogenic responses of hMSCs in vitro, warranting further investigation in the context of scaffold design for fracture care.