Distinct osteoclastogenic potential of granulocyte-macrophage colony-stimulating factor-induced monocyte subsets.

Background: Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by synovial inflammation and progressive bone destruction. Although osteoclasts mediate bone resorption in RA, recent evidence suggests that inflammatory osteoclasts differ from physiological osteoclasts in various aspects, including their progenitor origins. This study aimed to compare the osteoclastogenic potential of monocyte-derived dendritic cells (moDCs) and other granulocyte-macrophage colony-stimulating factor (GM-CSF)-induced cells to identify potential progenitor populations involved in inflammatory bone damage. Methods: Classical monocytes were isolated from human peripheral blood mononuclear cells and cultured under four conditions: (i) M-CSF, (ii) GM-CSF, (iii) GM-CSF + interleukin (IL)-4, and (iv) GM-CSF + tumor necrosis factor-alpha (TNF-α). Subsequently, to induce osteoclast differentiation, the cells were cultured with M-CSF and receptor activator of NF-κB ligand (RANKL) with or without the presence of GM-CSF, IL-4, and TNF-α, followed by evaluations via tartrate-resistant acid phosphatase (TRAP) staining and pit formation assays. Results: Cells cultured with M-CSF, GM-CSF, and GM-CSF + TNF-α differentiated into TRAP-positive multinucleated osteoclasts with bone-resorbing activity. In contrast, moDCs (condition iii) exhibited minimal osteoclast differentiation without any bone-resorbing activity. Introduction of an intermediate M-CSF culture step induces adhesion of moDCs and partially induces osteoclastogenesis. However, their differentiation efficiency and bone resorption capacity remained inferior to those under other conditions. Notably, IL-4 and GM-CSF, but not TNF-α, suppressed osteoclast differentiation. Conclusions: moDCs exhibit limited potential as osteoclast precursors under inflammatory conditions. Comparatively, GM-CSF (+ TNF-α)-induced progenitors represent a more viable inflammatory osteoclast precursor population. Overall, our results provide insights into osteoclast heterogeneity and RA-associated bone destruction mechanisms.