Background/Purpose: Osteoporosis and peri-articular erosions in patients with inflammatory arthritis are characterized by increased osteoclast resorptive activity.  Although a great deal is known about how osteoclasts differentiate from precursors and resorb bone, the identity of the osteoclast precursor (OCP) in vivo remains elusive. Using ex vivo assays of osteoclast differentiation, we previously demonstrated that the majority of bone marrow OCP activity resides in the CD11b^-/lo Ly6C^hi cKit⁺ population. 

Methods: Fluorescence activated cell sorting was used to examine multiple markers on OCP and to isolate OCP from mT/mG cathepsin K cre+ mice.  mT/mg cathepsin K cre donor OCP are RFP+ at baseline and GFP+ in the presence of cre recombinase in mature osteoclasts.  Donor OCP were transfered by intramedulary injection into NFATc1 fl/fl Mx1-Cre osteoclast deficient mice and donor derived osteoclasts were detected by IHC.  Alternatively, donor OCP were injected intravenously into C57BL/6 mice 24 hrs after calvarial injection of 20mg/kg LPS.  Ex vivo osteoclast, dendritic cell, and macrophage cutures and assays were performed according to standard techniques.

Results: Here we show that the CD11b^-/lo Ly6C^hi cKit⁺ OCP population can be distinguished from the recently described quiescent osteoclast precursor by their proliferative capacity and absence of RANK (receptor activator of nuclear factor kappa B) expression. Similar to other myeloid precursors, OCP retain plasticity in vitro, differentiating into dendritic cells or phagocytic macrophages when stimulated with GM-CSF or MCSF, respectively. Adoptive transfer of this OCP population into osteoclast deficient hosts resulted in the formation of donor-derived mature, multinucleated TRAP⁺osteoclasts in vivo.  By combining adoptive transfer with an inflammatory stimulus (lipopolysaccharide injection into the calvaria) we further show that these OCP can be recruited to and differentiate at sites of inflammatory osteolysis.  Thus, we demonstrate that bone marrow OCP can migrate to stimuli known to promote osteoclastic bone resorption.

Conclusion: We demonstrate that the CD11b^-/lo Ly6C^hi cKit⁺ bone marrow population is a bona fide OCP.  Furthermore, this work provides a model system to identify the chemokine and cytokine requirements for recruiting osteoclasts to sites of inflammatory bone loss in diseases such as rheumatoid and psoriatic arthritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-mouse-myeloid-precursor-with-osteoclastogenic-function-in-vivo/