Background: Osteoclasts are bone resorbing cells and treatment of bone diseases like rheumatoid arthritis have targeted this cell type. It is reported that distinct osteoclast precursor subsets like early blasts, myeloid blasts and monocytes respond differently to the osteoclastogenesis inducing cytokines M-CSF and RANKL. Whether these subsets respond also differently to the osteoclast stimulating inflammatory cytokine IL-1β is not known. Our objective was to investigate the effect of IL-1β on osteoclastogenesis by early blasts, myeloid blasts and monocytes.
Methods: Bone marrow cells were isolated from 6 weeks old male mice (approved by the Animal Welfare Committee of the VU University Amsterdam). Early blasts, myeloid blasts and monocytes were sorted using flow cytometry. Cells were cultured in the presence of M-CSF and RANKL for 4-6 days, without or with IL-1β (0.1-10 ng/ml), on plastic and bone, followed by TRACP staining to identify osteoclasts and Coomassie Brilliant Blue staining to visualise bone resorption. Gene expression was quantified using qPCR.
Results: Osteoclast formation on plastic was stimulated in early blasts and myeloid blasts whereas monocytes responded less to IL-1β. Very large osteoclasts (>20 nuclei) were formed in myeloid blast cultures whereas smaller (3-5 nuclei) osteoclasts were formed from early blasts. IL-1β caused a significantly increased bone resorption by osteoclasts generated from monocytes. TRACP and DC-STAMP were highly expressed by myeloid blasts, and IL-1RI is initially high expressed while IL-RII is low expressed in early blasts.
Conclusion: IL-1β differently affects osteoclastogenesis. Osteoclast formation is stimulated in early precursors like early blasts and myeloid blasts both on plastic and on bone. Monocytes hardly respond to IL-1β on plastic, but they did when seeded on bone. We propose that early blasts and myeloid blasts are the osteoclast precursors recruited during inflammation.
Disclosure: The authors declared no competing interests. This work was supported by Euroclast, a Marie Curie programme (FP7-People-2013-ITN; # 607446).