Bridging the gap between the immune response and mineralization during fracture healing

Jun 29, 2022, 2:20 PM
Room: S4 B

Room: S4 B


Lackington, William (Biointerfaces Lab, Empa )


Intro: Our broader understanding of the immune system's role in determining the success of intrinsic repair mechanisms has led to increased research focus on immunomodulatory therapies1. These therapies could be particularly effective in abnormal fracture healing, where bone tissue engineering has thus far failed to provide safe and reliable clinical therapeutics. While current developments rely heavily on endpoint assessment of bone formation in pre-clinical studies, the effect on key preceding processes, including hematoma formation, and the immune response to fracture, are seldomly taken into account2.
Aim: The aim of this study was to engineer an innovative in vitro model that facilitates studying the link between the immune response and mineralization during fracture healing.
Methods: Collagen-hydroxyapatite (CHA) scaffolds were incubated with human whole blood, mimicking the fracture hematoma formed when the scaffold is implanted in vivo. The effect of blood-biomaterial interactions on the microarchitecture of the scaffold, as well as the production of inflammatory mediators (IL-1β, IL-4, IL-6, IL-8, IL-10, MCP1 and VEGF), was assessed. The response of human bone progenitor cells (HBCs) to the blood-biomaterial interactions was evaluated in terms of cell infiltration, proliferation, and mineralization in the scaffold.

Results & discussion: The interaction between blood and CHA scaffolds led to the infiltration of erythrocytes, monocytes, and platelets, and the formation of a fibrin network. The porous microarchitecture of the scaffold was maintained in the presence of blood, while its support for HBC infiltration was limited by the presence of blood. The scaffold stimulated a limited production of pro-inflammatory factors (IL-6 and IL-8) by blood cells. However, the blood-biomaterial interactions significantly upregulated the production of IL-6 and IL-8 by HBCs by a factor of 10. The production of pro-inflammatory factors was temporally regulated, peaking between day 1 and 5, and tapered off by day 28. While blood-biomaterial interactions had no impact on the proliferation of HBCs over 28 days, blood impacted their capacity to mineralize, with a 28% reduction in calcium quantification, and a 50% reduction in intracellular alkaline phosphatase activity. Taken together, these data indicate that a transient hematoma-like pro-inflammatory matrix can be recapitulated in vitro, which can then be used to assess the effect of blood-biomaterial interactions on downstream processes of fracture healing, including mineralization. On-going experiments are assessing the capacity of blood-biomaterial interactions to alter the drug release kinetics of rhBMP-7, a commonly used inducer of bone formation, while the effect of rhBMP-7 on fibrin network formation and its ability to steer the immune response will be evaluated. Ultimately, the capacity of blood-biomaterial interactions to modulate the osteoinductive effects of rhBMP-7 will be determined.

Conclusion: In this study, a unique platform to bridge the knowledge gap between the immune response and mineralization during fracture healing was engineered, while also taking into account blood-biomaterial interactions, representing a significant advancement over current in vitro models of the fracture hematoma.

Acknowledgements: The authors would like to thank the Orthoregeneration Network & Orthopedic Research Society (Kick-Starter Grant) for providing financial support to this project.


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