Background/Purpose:

Pathogenesis of osteoarthritis (OA) is accompanied by chronic inflammation evidenced by macrophage infiltration into the joint. CD14, is expressed by monocyte/macrophage lineage cells, and is a co-receptor that facilitates Toll-like receptor (TLR) signaling. Upon binding, the CD14/TLR complex can activate cytokine production and promote chronic inflammation. Given that CD14 is expressed by osteoclast precursors, and TLR signaling influences osteoclast development and activation, we hypothesized that CD14 may play a role in bone remodeling after joint injury in a murine post-traumatic knee OA model.

Methods:

10-12 week old male mice from CD14^-/- and congenic C57BL/6 (WT) controls were subjected to destabilization of medial meniscus (DMM), sham surgery or left un-operated. Mice were sacrificed at 6 and 19 weeks post-surgery, knees isolated, and cartilage and bone histopathology evaluated with the modified OARSI score. Micro-CT was used to measure subchondral bone mineral density (BMD) and trabecular thickness (TbTh).  In separate experiments, bone marrow was isolated from tibias and fibulas of WT and CD14^-/- mice. Non-adherent cells were isolated after 24-hour initial culture, and then media containing M-CSF (35 ng/ml) and RANKL (100 ng/ml) was added to promote osteoclast differentiation. After 8 days, TRAP+ (tartrate-resistant acid phosphatase) staining was assessed, and mRNA expression of calcitonin receptor and cathepsin K were measured using quantitative PCR.

Results:

6 weeks after DMM surgery cartilage histology was similar in the two strains, but at 19 weeks degeneration was significantly less in CD14^-/- mice compared to WT (7.125 vs 16.22, p=0.0002). Micro-CT analysis showed age-related increases in BMD and TbTh in WT mice (12.8% increase at 19 weeks compared to baseline, p=0.006) but not in CD14^-/- mice. CD14^-/- mice were also protected from surgery-related increases in BMD, which were observed in WT mice 6 week post-DMM: (7.3% increase in WT BMD DMM vs. naïve, p=0.04; 2.0% increase in CD14^-/- BMD DMM vs. naïve, p=ns). In response to M-CSF and RANKL, cells from CD14^-/- mice showed less TRAP staining, reduced numbers of multi-nucleated cells, and lower expression of calcitonin receptor and cathepsin K compared to WT mice (Fig 1).

Conclusion:

CD14 deficiency protects mice from subchondral bone changes after joint injury, and reduces the capacity of precursors to differentiate into osteoclasts. Bone remodeling is dependent on both bone resorption and anabolism, therefore it is possible that the defect in osteoclastogenesis in this strain is related to the ability of subchondral bone to remodel in response to age and joint injury. Additional studies are underway to confirm this.  This study brings attention to pathologic bone remodeling in osteoarthritis models, and these results suggest a possible new therapeutic target to explore.

Figure 1