Osteoclast precursors have not been well characterized because there are no known markers that can detect them. We have used osteoclast-like cells formed in vitro to develop a panel of specific antibodies that react with mature osteoclasts, osteoclast precursors, and other cells in the osteoclast lineage. Monoclonal antibody Kn22 reacted strongly with osteoclast-like multinucleated cells formed in long term marrow cultures and reacted very strongly with freshly isolated bonederived baboon osteoclasts. Using immune cell panning, Kn22 enriched precursors for osteoclasts. The majority of multinucleated cells (71%) formed from fresh marrow mononuclear cells adherent to Kn22 strongly reacted with a monoclonal antibody that recognizes mature osteoclasts (23c6) and responded appropriately to calcitonin. In contrast, only 23% of multinucleated cells formed from marrow mononuclear cells that were not bound by Kn22 formed osteoclast-like cells. The majority (77%) of these multinucleated cells did not strongly react with the osteoclast-specific monoclonal antibody 23c6 or respond to calcitonin. Thus, we have developed a panel of monoclonal antibodies that recognize cells in the osteoclast lineage. One of these antibodies, Kn22, is unique in that it identifies an osteoclast precursor. The 50K antigen detected by Kn22 appears to be a membrane protein present on osteoclast precursors and osteoclasts that has not been previously identified.
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