Characteristics of osteoclast precursor-like cells grown from mouse bone marrow

Cells resembling mononuclear osteoclast precursors (OCP-like) were grown from mouse bone marrow and subsequently studied for expression of macrophage-specific surface antigens (F4/80 and Mac-1), phagocytosis activity, and DNA synthesis. Expression of tartrate-resistant acid phosphatase (TRAcP) activity served as general marker for OCP-like cells. Mononuclear phagocytes, characterized by expression of tartrate-sensitive acid phosphatase (TSAcP) activity, were also grown from mouse bone marrow and were compared with OCP-like cells. TRAcP-positive OCP-like cells showed low values for F4/80 and Mac-1 expression, as well as low phagocytosis activity. Macrophages (TSAcP+) showed very high values for expression of both surface antigens and for expression of phagocytosis activity, while osteoclasts (TRAcP+), reported to lack Ag F4/80, were also negative for Mac-1. DNA synthesis in OCP-like cells was low and resembled the values found in macrophages. Chase experiments after nuclear pulse labeling indicated that TRAcP-positive OCP-like cells developed from immature proliferating progenitor cells which did not yet show TRAcP activity. These results show that TRAcP-positive OCP-like cells have characteristics of both mononuclear phagocytes and osteoclasts. We propose that OCP-like cells are relatively late stages in an osteoclast-specific cell lineage which diverges from the mononuclear phagocyte lineage.