HUMAN CORD-BLOOD MONOCYTES UNDERGO TERMINAL OSTEOCLAST DIFFERENTIATION IN-VITRO IN THE PRESENCE OF CULTURE-MEDIUM CONDITIONED BY GIANT-CELLTUMOR OF BONE

Osteoclasts (OCs), which form by fusion of hematopoietic precursor cel ls, are typically present in large numbers in giant cell tumors of bon e (GCTBs). These tumors may, therefore, contain cells which secrete fa ctors that stimulate recruitment and differentiation of OC precursors. Multinucleated cells resembling OCs also form in cultures of human co rd blood monocytes (CBMs) stimulated by 1.25 dihydroxyvitamin D-3, but these cells lack the ability to form bone resorption pits, the defini ng functional characteristic of mature OCs. CBMs may thus require addi tional stimulation to form OCs; we therefore investigated whether GCTB s are a source of such a stimulus. CBMs were stimulated in long term ( 21 day) culture by medium conditioned by explants of GCTBs; media coll ected within 15 days of explant (early-CM) and after 15 days (late-CM) were employed. We also cocultured CBMs with primary GCTB-derived stro mal cells as well as immortalized bone marrow stroma-derived cells. CB Ms stimulated by early-CM formed resorption pits on cortical bone slic es; however, stimulation by late-CM resulted in virtually no resorptio n. Both early-CM and late-CM increased CBM proliferation, but not the proportion of vitronectin receptor positive or multinucleated cells. C oculture of CBMs with stromal cells of GCTBs or bone marrow did not re sult in bone resorption, although these stromal cells (most expressing alkaline phosphatase but progressively losing parathyroid hormone rec eptor expression) expressed mRNA for cytokines involved in OC differen tiation, including macrophage-CSF, granulocyte-macrophage-CSF, IL-11, IL-6, and stem cell factor. Our results indicate that CBMs are capable of terminal OC differentiation in vitro, a process requiring 1,25 dih ydroxyvitamin D-3 as well as diffusible factor(s) which can be derived from GCTB. Stromal cells of GCTB may produce such factors in vivo, bu t do not support OC differentiation in vitro, possibly through phenoty pic instability in culture. (C) 1996 Wiley-Liss, Inc.