Dynamic analysis of immune inflammation and bone destruction by intravital imaging.

Rapid development of fluorescent imaging techniques enables us to understand cellular dynamics in vivo. We have originally established an advanced imaging system for visualizing living bone tissues with intravital two-photon microscopy. By means of the system, we have recently succeeded in visualization of the in vivo behavior of living mature osteoclasts on the bone surface, and identified different functional subsets of osteoclasts in terms of their motility and function, i.e., 'static - bone resorptive' and 'moving - non resorptive'. Pathological conditions changed the composition of these populations as well as the total number of mature osteoclasts. We also found that RANKL-bearing Th17 cells could control bone resorption of mature osteoclasts, demonstrating novel actions of Th17. Furthermore, we have also developed the imaging system to visualize bone destruction by osteoclasts in arthritic joints using intravital two-photon microscopy. In this review, we summarize the latest data of intravital imaging of osteoclast dynamics, and also discuss its further application.