HUMAN OSTEOCLAST AND GIANT-CELL DIFFERENTIATION - THE APPARENT SWITCHFROM NONSPECIFIC ESTERASE TO TARTRATE-RESISTANT ACID-PHOSPHATASE-ACTIVITY COINCIDES WITH THE IN-SITU EXPRESSION OF OSTEOPONTIN MESSENGER-RNA

Animal model and in vitro cultures suggest that osteoclasts and cells of the mononuclear phagocyte system share a common precursor. However, the human osteoclast precursor has not been positively identified. We attempted to identify the precursor in situ by using a number of oste oclast- and macrophage-selective markers, together with the expression of osteopontin mRNA, previously shown to be abundant in human osteocl asts. Sections of osteophytic bone and a panel of inflammatory connect ive tissues were processed for in situ hybridization; serial sections were analyzed for tartrate-resistant acid phosphatase (TRAP) and nonsp ecific esterase (NSE) activity, selective cytochemical markers for the osteoclast and cells of the macrophage/monocyte lineage, respectively . The murine anti-human osteoclast monoclonal antibodies 23C6 (vitrone ctin receptor) and C35 (osteoclast-selective) were used to further ide ntify the osteoclast phenotype. We compared osteoclasts, giant cells, and their respective putative mononuclear precursors. At resorption si tes within osteophytic bone, osteopontin mRNA was expressed in osteocl asts and a distinct population of TRAP(+), NSE(-) mononuclear cells. A djacent clusters of mononuclear cells were TRAP(-) and NSE(+) or were active for both enzymes; these cells demonstrated variable expression of osteopontin mRNA. In the inflammatory connective tissues, abundant macrophage-like cells (NSE(+)/TRAP(-)) did not express osteopontin mRN A. However, TRAP(+) mononuclear cells observed among clusters of NSE() cells did express osteopontin mRNA. At these sites, dusters of putat ive macrophage polykaryons removing fragments of bone debris were obse rved. These giant cells and associated mononuclear cells were NSE(-) a nd distinctly TRAP(+), and expressed osteopontin mRNA, C35, and 23C6 ( human osteoclast) reactivity. Therefore, cells involved in the remodel ing (resorption) of bone or the removal of bone debris, together with their immediate precursors, switch from being NSE(+)/TRAP(-) to NSE(-) /TRAP(+) cells that express osteopontin mRNA. We propose that the dust ers of NSE(+)/TRAP(-) mononuclear cells represent the immature osteocl ast precursor. In support of this, TRAP(+)/NSE(+) cells were occasiona lly observed in both tissues, representing an intermediate stage in di fferentiation. These results further suggest that cells of the mononuc lear phagocyte lineage within bone and inflammatory connective tissue have the potential to differentiate into osteoclasts.