Unraveling devitalization: its impact on immune response and ectopic bone remodeling from autologous and allogeneic callus mimics

Endochondral bone regeneration is a promising approach in regenerative medicine. Callus mimics (CMs) are engineered and remodeled into bone tissue upon implantation. The long-term objective is to fabricate a sustainable off-the-shelf treatment option for patients. Devitalization was introduced to facilitate storage and using allogeneic (donor) cells would further propel the off-the-shelf approach. However, allogeneic CMs for bone regeneration pose a potential antigenicity concern. Here, we explored the impact of devitalization on antigenicity and osteoinductive bone formation when implanting syngeneic or allogeneic CM in a vital or devitalized state. For this, we implanted chondrogenically differentiated rat-derived mesenchymal stromal cells using an allogeneic immunocompetent ectopic rat model. Vital syngeneic CMs demonstrated the highest bone formation, and vital allogeneic CMs showed the lowest bone formation, while both devitalized CMs showed comparable intermediate levels of bone formation. Preceding bone formation, the level of tartrate-resistant acid phosphatase staining at 7 and 14 days was proportional to the level of eventual bone formation. No differences were observed for local innate immune responses at any time point before or after bone formation. In contrast, allogeneic CMs elicit a mild adaptive immune response, which still permits bone formation in an immunocompetent environment, albeit at a reduced rate compared to the autologous living counterpart. Overall, devitalization delays bone formation when autologous CMs are implanted, whereas it accelerates bone formation in allogeneic CMs, highlighting the potential of this approach for achieving off-the-shelf treatment.