Background: Various biologic agents, which inhibit TNFα, IL-6 receptor or T cell co-stimulation, are now clinically used for rheumatoid arthritis therapy, and it is known that all of them equally suppress joint inflammation as well as bone erosion. However, little is known about the difference in mode of action among these agents to date. We previously succeed in visualization of mature osteoclasts in living mouse calvaria bone with two-photon microscopy, and identified different populations of living mature osteoclasts in terms of their motility and function, i.e., ‘static - bone resorptive’ and ‘moving - non resorptive’. In this study, by means of this intravital imaging system, we analyzed the cellular dynamics of mature osteoclasts and their precursors in LPS-induced inflammatory environments, and evaluated the effect of biologic agents on these cells in vivo.
Methods: LPS (20 mg/kg) was injected into periosteum of calvarial bone of mice in which mature osteoclasts or their precursors were fluorescently labeled with GFP. Anti-TNFα mAb, anti--IL-6 receptor mAb, CTLA4-Ig or PBS was administered intraperitoneally, concomitantly with LPS. Five days later, mature osteoclasts or their precursors in calvaria were observed using intravital two-photon microscopy.
Results: The motility of mature osteoclasts was increased in mice treated with anti-TNFα mAb or anti-IL-6 receptor mAb, which indicates that the proportion of non-resorptive osteoclasts was increased. On the other hand, CTLA4-Ig did not change the motility of mature osteoclast, but significantly increased the tracking velocity of osteoclast precursors, compared to other agents, attenuating osteoclastgenesis elicited by LPS-induced inflammation.
Conclusion: Our results suggest that biologic agents have different modes of action on osteoclast lineage cells in inflammatory bone destruction model; anti-TNFα and anti-IL-6 receptor antibody act on mature osteoclasts and CTLA4-Ig on osteoclast precursors, respectively.