As direct effector cells for osteogenesis, osteoblastic lineage cells are commonly used to evaluate the in vitro osteogenic capacity of bone biomaterials. This strategy has achieved a degree of success in developing novel bone biomaterials;​ however, inconsistencies between in vitro and in vivo studies are common, indicating that the mechanisms that govern the material’s capacity to mediate osteogenesis are still not well understood. Osteoimmunology has revealed the important role of immune cells in the regulation of bone formation and remodelling. This has informed a paradigm shift in the development of bone biomaterials, from an inert to an osteoimmunomodulatory material, highlighting the important role of immune cells in materials-mediated osteogenesis. Neglecting the importance of the immune response can be a major shortcoming when assessing materials, and may explain the inconsistent results between in vitro and in vivo experiments. Therefore, this thesis aims to introduce the intellectual framework of osteoimmunology into the field of bone biomaterials to optimise development and assessment strategies of novel and clinically useful materials.

First, the involvement of immune cells (macrophages) in the bone biomaterials (βtricalcium phosphate, β-TCP)-mediated osteogenesis was demonstrated. β-TCP elicited significant effects on macrophages, resulting in the M2 phenotype switch, involving the inhibition of gene expression of pro-inflammatory cytokines. The osteogenic differentiation of bone marrow stroma cells (BMSCs) was enhanced by the macrophages/β-TCP conditioned medium compared with that of the β-TCP material extract, which can be correlated to the release of bone morphogenetic protein 2 (BMP2) from β-TCP stimulated macrophages. Together, these results indicate that the materials-mediated immune response actively participates in subsequent osteogenesis and must, therefore, be taken into account when developing and assessing bone biomaterials.

Next, the concept of osteoimmunomodulation was applied to refine the evaluation of the in vitro osteogenic potential of a biomaterial, by adding immune cells to the interaction between the biomaterial and bone cells. This approach altered the in vitro dynamics resulting in a significant different response of the bone cells compared to the control condition in which no immune cells were involved. CCP (cobalt incorporated tricalcium phosphate) extract by itself was sufficient to enhance osteogenic differentiation of BMSCs, whereas in the presence of macrophages this effect was negated. The results from the trilateral interaction between biomaterial, immune cells and bone cells better reflected subsequent in vivo outcomes. When the CCPs were implanted into defects in the femoral condyle, this manifested itself by an increase of inflammatory markers and bone destruction, coupled with fibrous encapsulation, as opposed to new bone formation. This trilateral system of biomaterial, immune cells and bone cells system is a sufficient in vitro assessment protocol that provides a more accurate picture of material-stimulated osteogenesis compared to the standard bilateral approach.

Finally, the concept of osteoimmunomodulation was also applied to the development of bone biomaterials with the result of more favourable osteoimmunomodulatory properties. For example, coating magnesium scaffolds with β-TCP resulted in these scaffolds having osteoimmunomodulatory properties that greatly enhanced in vitro osteogenesis. In another study, nutrient elements (silicon, strontium, and magnesium) were combined and coated on a titanium substrate. This novel coating material was found to elicit a beneficial osteoimmunomodulatory response, with high bonding strength between the coating material and the metal substrate.

In conclusion, the central concept of this thesis is the importance of osteoimmunomodulation as an integral component in both the development and assessment of bone biomaterials. It is the candidate’s ambition to highlight the need for an appreciation of fundamental principles pertaining to bone biology when seeking to develop novel bone biomaterials. Introducing the conceptual framework of osteoimmunology is set to radically alter the traditional methods for bone biomaterials assessment and contribute to the development of new bone biomaterials with osteoimmunomodulatory properties that enhance bone repair and regeneration.