Autologous bone grafts are commonly used to repair defects in skeletal tissue, however, due to their limited supply there is a clinical need for alternatives. Synthetic ceramics present a promising option but currently lack biological activity to stimulate bone regeneration. One potential approach to address this limitation is the incorporation of immunomodulatory agents. In this study, we investigate the application of microbial stimuli to stimulate bone formation. Three different microbial stimuli were incorporated in a biphasic calcium phosphate (BCP) ceramic: Bacille Calmette-Guérin (BCG), gamma-irradiated Staphylococcus aureus (γi-S. aureus), or γi-Candida albicans (γi-C. Albicans). The constructs were then implanted in both a rabbit posterolateral spinal fusion (PLF) and an intramuscular implant model for 10 weeks and compared to a nonstimulated control construct. For the PLF model, the formation of a bony bridge was evaluated by manual palpation, micro computed tomography, and histology. While complete fusion was not observed, the BCG condition was most promising with higher manual stiffness and almost twice as much bone volume in the central fusion mass compared to the control (9 ± 4.4% bone area vs. 4.6 ± 2.3%, respectively). Conversely, the γi-S. aureus or γi-C. albicans appeared to inhibit bone formation (1.4 ± 1.4% and 1.2 ± 0.6% bone area). Bone induction was not observed in any of the intramuscular implants. This study indicates that incorporating immunomodulatory agents in ceramic bone substitutes can affect bone formation, which can be positive when selected carefully. The readily available and clinically approved BCG showed promising results, which warrants further research for clinical translation.
Keywords: bacteria; bone regenerative medicine; ceramic; fungi; mycobacterium; osteoimmunomodulation.