gms | German Medical Science

Objectives: Delayed healing or non-unions in bone fractures are relevant problems in clinics since more than 10% of outcomes are not satisfying for patients and clinicians. Highly important to a successful healing process appears to be its early phase and the interconnectivity with the inflammatory immune response. Hence, the immune system or certain immune cells, even though perfectly adapted to prevent pathogenic invasion could have a negative effect on regeneration. Indeed, cell subsets of the adaptive immune response, namely the CD8+ T effector cells were found to negatively influence bone regeneration. Aim of these studies was to determine the different effects of immune cells of the innate and adaptive immune system, respectively, onto fracture healing.

Methods: In a mouse fracture model healing was investigated for 7, 14, 21, and 28 days. One group (RAG) had no mature B and T cells and another group (MAC) was depleted of macrophages (via clodronate). Wild type (WT) mice were used as control. Bone mineralization was evaluated via µCT. Soft callus formation and collagen distribution was investigated via immune histochemistry. µ-arrays were used for gene expression analysis. For histomorphometric analysis calluses of in addition distinct T and B cell knockouts were investigated histologically and immunhistologically to determine whether effects seen in the RAG model where predominantly caused by B or by T cells.

Results and Conclusion: Depletion of immune cells from the innate immune system (MAC) resulted in non-unions. Furthermore, the collagen X pattern during the chondrogenic phase was especially diminished in this group. In contrast, knockout of cells of the adaptive immune system (RAG) resulted in an earlier bridging and solidification of the fractured bone when compared to WT animals. However, collagen I composition and deposition was found to be absent in the calcified callus regions of the RAG group. This was confirmed with asymmetric gene expression of the collagen I subunits. Specific knockout of immune cell subunits indicate that these effects are due to T cell activity and not caused by a lack of B cells.

In conclusion, depletion of distinct immune cell subsets of the innate and adaptive immune system respectively showed opposing healing outcomes. Depletion of macrophages resulted in a lack of bridging but increased intramembranous ossification. In contrast, lack of B and T cells changed matrix deposition assumedly via asymmetric regulation of collagen I subunits. These results clearly demonstrate, that immune cells are essential for matrix deposition during healing processes.