Bone Abstracts

Background: Osteoclasts (OCs) are bone-degrading cells that differentiate from the monocyte/macrophage lineage. In human, three monocyte subsets have been identified: classical, intermediate and non-classical monocytes. We have previously demonstrated that comparable numbers of OCs can be generated from these subsets on plastic, but that the number of OCs significantly differs when the monocytes are cultured on bone. It is plausible that the observed differences are associated with initial attachment of the OC precursors to bone. However, the mechanism involved in attachment of OC precursors to bone is unknown.

Objective: To evaluate attachment of the three monocyte subsets to bone and their early differentiation towards OCs.

Methods: Monocytes were isolated from human peripheral blood and sorted with flow cytometry based on CD14 and CD16 expression. The subsets were seeded onto slices of human tibia bone. Differentiation was induced by osteoclastogenic medium containing 10 ng/ml M-CSF and 2 ng/ml RANKL. At baseline, and after 3, 18, 24 and 48 h of culture, monocyte attachment was visualized by staining the nuclei and surface expression of integrins β2, αM and αL was analysed with flow cytometry and confocal microscopy.

Results: Attachment to bone and early osteoclastogenesis of the three monocyte subsets was distinctly different. Classical monocytes attached readily and formed cell clusters shortly after seeding. Intermediate and non-classical monocytes attached less well to bone and no clusters were formed. Classical monocytes expressed high levels of integrin β2 and blocking of plasma membrane associated integrin β2 effectively inhibited monocyte attachment to bone.

Conclusion: Integrin β2 appears to be involved in the adhesion of monocytes to bone.

Research conducted within Euroclast, a Marie Curie FP7-People-2013-ITN: No. 607446.