gms | German Medical Science

Objectives: Skeletal structure is determined by two well-coordinated biological processes, modeling and remodeling, in which the actions of bone-resorbing osteoclasts and bone-forming osteoblasts are coupled during development and fracture healing. Understanding the complex and diverse mechanisms involved in the coupling process and how each contributes to the sequence and balance of remodeling is of great importance. The programmed cell death ligand 1 (PD-L1) and its receptor, programmed cell death 1 (PD-1), provide inhibitory signals to regulate immune tolerance in immune-mediated diseases. However, the effect of PD-L1 on bone health and mesenchymal stem cell (MSC) differentiation towards the osteoblastic lineage and osteoclast differentiation remains unknown. The aims of the current study were to investigate the bone architecture and osteogenic potential of MSCs and osteoclast differentiation in PD-L1 KO compared to wild-type (WT) mice.

Methods: Micro-computed tomography (mCT) was used to evaluate the three-dimensional structure of trabecular and cortical bone in age-matched WT and PD-L1 KO mice. Mouse bone marrow cells were harvested from the tibia and femur. MSCs were expanded under standard culture condition medium and differentiated towards the osteogenic lineage with osteogenic induction growth factors. Osteoblast mineralization was assessed by Alizarin Red S staining followed by extraction and photometric measurement. Osteoclast differentiation was induced by MCSF and RANKL and evaluated by TRAP staining and RT-PCR. Osteoclast functional activity was assessed by a bone resorption assay.

Results: PD-L1 KO mice showed decreased bone length compared to age-matched WT mice. Microarchitectural analysis revealed that PD-L1 KO mice had reduced trabecular bone volume, trabeculae quantity and thickness and increased inter-trabecular separation. Interestingly, when induced towards the osteoblastic lineage, MSCs from PD-L1 KO mice displayed a 3-fold decrease in their osteoblast differentiation and mineralizing potential compared to MSCs from WT mice. In addition, PD-L1 KO showed significantly more TRAP-positive cells and increased resorption pits, indicating higher osteoclast differentiation and function compared to the WT control.

Conclusions: In PD-L1 KO mice, bone density and trabecular structure are impaired. This effect is supported by increased osteoclastogenesis and reduced osteogenic differentiation, indicating that PD-L1 plays an important role in maintaining bone homeostasis. Further research is needed to determine the role of PD-L1 in bone health.