Immune cells and their soluble factors regulate skeletal cells during normal bone regeneration and pathological bone formation. Bacterial infections can trigger immune responses that activate pro-osteogenic pathways, but these are usually overshadowed by osteolysis and concerns of systemic inflammation. The aim of this study was to determine whether the transient local inflammatory reaction to non-viable bacterial immune agonists could lead to favourable new bone formation. In a series of rabbit studies, as proof-of-concept, how tibial intramedullary injection of viable or killed bacterial species affected bone remodelling and new bone formation was determined. Application of killed bacteria led to considerable new bone formation after 4 weeks, without the prolonged systemic inflammation and exaggerated bone lysis seen with active infection. The osteo-immunomodulatory effects of various species of killed bacteria and the dose response relationship were subsequently screened in ectopically-implanted ceramic scaffolds. Histomorphometry after 8 weeks showed that a relatively low dose of killed bacteria enhanced ectopic bone induction. Moreover, lipoteichoic acid - the bacterial cell-wall derived toll-like-receptor (TLR)-2 activator - was identified as an osteo-stimulatory factor. Collectively, the data indicated that bacterial stimuli could be harnessed to stimulate osteogenesis, which occurs through a synergy with osteoinductive signals. This finding holds promise for the use of non-viable bacteria, bacterial antigens, or their simplified analogues as immuno-modulatory bone regenerating tools in bone biomaterials.

The main challenge in bone morphogenic protein 2 (BMP-2)-based application lies in finding strategies that prolong its effective period as it has a short biological half-life. Several BMP-2 release profiles have shown to enhance bone formation at various application sites. However, it remains to be determined which BMP-2 release profile best augments bone formation and whether this effect is location dependent. Therefore, the aim of this study was to investigate the effect of BMP-2 release from oligo[(polyethylene glycol) fumarate] bis(2-(methacryloyloxy)ethyl) phosphate (OPF-BP) composites on the osteoinductive efficacy at ectopic versus orthotopic application. By varying the BMP-2 loading method, three different OPF-BP composites were created with varied release profiles. The composites were compared with unloaded OPF-BP as negative control, and to the clinically used Infuse ^® absorbable collagen sponge (ACS) as positive control. Bone formation was assessed by microcomputed tomography after 9 weeks of subcutaneous implantation and 3, 6, and 9 weeks of orthotopic implantation in rats (n = 48). Whereas a BMP-2 burst release of >49% generated significantly more bone compared with sustained release (burst release <30%) at the subcutaneous implantation site, differential release did not affect bone formation at the orthotopic site. Furthermore, all BMP-2 containing OPF-BP composites showed significantly more bone formation compared with ACS in the orthotopic implantation site. In conclusion, this study clearly shows that the osteoinductive effect of different BMP-2 release profiles is location dependent. In addition, more bone formation in OPF-BP compared with ACS at both application sites emphasizes the role of biomaterials as a scaffold to achieve proper bone tissue formation. The main challenge in bone morphogenic protein 2 (BMP-2)-based application lies in finding strategies to prolong its biologic activity as it has a short biological half-life. The present study uses a phosphate-modified oligo[(polyethylene glycol) fumarate] hydrogel that can be tuned to achieve differential release profiles of biologically active BMP-2 release. We demonstrate that this platform outperforms Infuse ^® , currently used in the clinic and that the osteoinductive effect of BMP-2 is location dependent. Altogether, this study stresses the importance of evaluating efficacy of bone tissue engineering strategies at an orthotopic location rather than subcutaneously. Even more so, it emphasizes the role of biomaterials as a scaffold to achieve proper bone tissue engineering.

Pathologic conditions associated with bone formation can serve as models to identify bone-promoting mediators. The inflammatory response to bacterial infections generally leads to osteolysis and impaired bone healing, but
paradoxically, it can also have pro-osteogenic effects. As a potential model to investigate pro-osteogenic stimuli, this study characterizes the bone formation in an established rabbit tibia model of periprosthetic infection. Our hypothesis
was that the infection with Staphylococcus aureus (S. aureus) correlates with bone formation as a response to local inflammation. Fluorochromes showed excessive subperiosteal bone formation in infected tibiae, starting the first
week and continuing throughout the study period. Despite the observed cortical lysis on micro-CT after 28 days, infection resulted in a twofold higher bone volume in the proximal tibiae compared to uninfected controls. The
ipsilateral fibulae, nor the contralateral fibulae or tibiae were affected by infection. Next, we sought to confine the cause of stimulated bone formation to the isolated S. aureus cell wall. In absence of virulent bacterial infection, the S.
aureus cell wall extract induced bone in a more favorable way without cortical lysis. This suggests that the sterile inflammatory reaction to bacterial antigens may be harnessed for bone regenerative purposes. Future investigations
in this rabbit tibia model can lead to further identification of effective stimuli for clinical application.

Bone morphogenetic protein-2 (BMP-2) gene delivery has shown to induce bone formation in vivo in cell-based tissue engineering. In addition, the chemoattractant stromal cell-derived factor-1α (SDF-1α, also known as CXCL12) is known to recruit multipotent stromal cells towards its release site where it enhances vascularisation and possibly contributes to osteogenic differentiation. To investigate potential cooperative behaviour for bone formation, we investigated combined release of BMP-2 and SDF-1α on ectopic bone formation in mice. Multipotent stromal cellseeded and cell-free constructs with BMP-2 plasmid DNA and /or SDF-1α loaded onto gelatin microparticles, were implanted subcutaneously in mice for a period of 6 weeks. Histological analysis and histomorphometry revealed that the onset of bone formation and the formed bone volume were both enhanced by the combination of BMP-2 and SDF-1α compared to controls in cell-seeded constructs. Samples without seeded multipotent stromal cells failed to induce any bone formation. We conclude that the addition of stromal cellderived factor-1α to a cell-seeded alginate based bone morphogenetic protein-2 plasmid DNA construct has an additive effect on bone formation and can be considered a promising combination for bone regeneration.