Bone healing involves sequential and overlapping biological processes including inflammation, new bone
formation and remodeling. Most bone tissue engineering research to date focused on targeting bone-forming
cells such as stem cells. Emerging studies highlight the critical roles of immune cells in fracture healing.
Macrophages (M) is one of the first responders to bone defects and can be polarized into pro-inflammatory M1
and pro-regenerative M2 phenotype. Unsolved acute inflammatory phase and delayed M1 to M2 phenotype
switch often lead to long-term and chronic inflammation, resulting in delayed bone healing. To enhance bone
healing, there remains a lack of strategy that promotes desirable M polarization at appropriate timing.
Biomaterial scaffolds have been widely used for bone tissue engineering as carriers for cell and growth factors
delivery. Recent studies also demonstrate biomaterials compositions impact immune responses, with natural
extracellular derived materials promote more pro-regenerative immune response than synthetic materials.
However, several key gaps in knowledge remain. First, previous studies were done using soft-tissue defect
models only, and how varying biomaterials compositions impact bone healing remains unknown. Second,
macroporosity is critical for bone regeneration in vivo, whereas previous work on assessing immune response
to biomaterials is limited to conventional nanoporous hydrogels. Third, previous work on the role of T cells in
bone healing is limited to a non-critical size long bone fracture model, and no scaffolds were used. T-cell
response to scaffold implant in a critical-sized cranial defect model has never been studied before. Last, previous
work only studied individual cell type (i.e. immune cell only or stem cell only), yet how biomaterials composition
modulates cell-cell crosstalk and subsequent tissue regeneration remains largely unknown.
The goal of our original R01 was to assess the potential of µRB scaffolds for enhancing stem cell-based tissue
regeneration by focusing on stem cell differentiation. The goal of this renewal R01 application is to harness µRB
scaffolds to enhance bone formation through immunomodulation by tuning biomaterial composition, which has
never been investigated before. Specifically, we propose to (1) Assess the effect of varying µRB scaffold
composition on M polarization and osteogenic differentiation of mesenchymal stem cells (MSCs) in vitro; (2)
Investigate the effect of the varying µRB composition on MSC/M crosstalk, and further impacts on MSC-based
bone formation and T cell response using a 3D co-culture model in vitro; and (3) Evaluate the effect of the varying
µRB scaffold composition on immune responses and bone regeneration in vivo using a mouse critical-size cranial
defect model. By working at the interface of biomaterials, immunology, bone disease and biology, stem cells,
animal models, and high dimensional single cell analyses, the proposed work will fill in the critical gap of
knowledge on the divergent immune response to macroporous scaffolds with tunable compositions and guide
optimal scaffold design to enhance critical-size cranial bone repair through immunomodulation.