SUCCESSFUL DETECTION OF ACTIVE OSTEOCLASTS IN-SITU BY SYSTEMIC ADMINISTRATION OF AN OSTEOCLAST-SPECIFIC MONOCLONAL-ANTIBODY

Cell-surface proteins preferentially expressed on osteoclasts are thou ght to play important roles in the functional modulation of the osteoc lasts. Recently, we found a novel cell-surface antigen designated Kat1 -antigen (Kat1-Ag) specifically expressed on rat osteoclasts. It would be useful to regulate the functional activity of the osteoclasts dire ctly via an osteoclast-specific antigen expressed on the cell surface of the osteoclasts. In order to establish the basis of such an applica tion, in the present study we established a method for the direct dete ction of osteoclasts in situ by a systemic administration of the anti- Kat1-Ag monoclonal antibody (mAb Kat1) to rats, and we successfully de tected functional osteoclasts in situ. Prior to performing in vivo exp eriments, we examined the reactivity of the mAb Kat1 to the isolated r at osteoclasts. Approximately 40-80% of the osteoclasts were reactive with mAb Kat1, suggesting that this mAb recognizes osteoclasts in a sp ecific differentiation or functional state. Calcitonin treatment of os teoclast-like cells formed in vitro from bone marrow cells resulted in a conversion of Kat1-positive osteoclast-like cells into Kat1-negativ e multinucleated cells, showing the positive correlation between the K at1-Ag expression and the potential bone-resorbing activity of osteocl asts, Administration of this lineage-specific mAb to the peritoneal ca vity of newborn rats resulted in a successful recruitment of mAb Kat1 to the newly formed osteoclasts and functional osteoclasts in a highly specific manner. Detailed analysis by immunoelectron microscopy revea led that this mAb specifically bound to the basolateral side of the ac tive osteoclasts, which were identified by their typical ruffled borde r and clear zone, whereas the mAb did not react to postfunctional oste oclasts. These findings demonstrate a high potential utility of mAb Ka t1 in osteoclast-targeted regulation of bone remodeling.