Osteoclast 121F antigen expression during osteoblast conditioned medium induction of osteoclast‐like cells in vitro: Relationship to calcitonin responsiveness, tartrate resistant acid phosphatase levels, and bone resorptive activity

Osteoclast differentiation from hematopoietic precursors into multinucleated cells uniquely capable of removing the organic and inorganic components of bone matrix occurs in a multistep process, during which osteoclasts acquire the specialized characteristics necessary for bone resorptive activity and physiological regulation. Among those traits is a novel plasma membrane glycoprotein, reactive with the anti‐osteoclast monoclonal antibody 121F, which is expressed during the course of osteoclast differentiation, shares structural and functional homologies with Mn²⁺/Fe²⁺ superoxide dismutase, and has been hypothesized to protect the osteoclast from the damaging effects of superoxide radicals generated during active bone resorption. We have reported previously that the expression of this membrane antigen is induced on multinucleated giant cells when the profusion marrow mononuclear cells are cultured in conditioned medium from avian calvaria. The studies reported here were designed to investigate the relationship between expression of the 121F antibody‐reactive osteoclast membrane antigen and tartrate resistant acid phosphatase levels, bone resorptive activity, calcitonin responsiveness, and ultrastructural features of avian bone marrow‐derived multinucleated giant cells formed either in the presence or absence of diffusible osteoblast secreted factors. Parallel analyses of in vivo formed osteoclasts isolated from the same animals were performed for direct comparisons. In this report we demonstrate: (1) that the 121F monoclonal antibody‐reactive osteoclast membrane antigen is stably induced in giant cells by soluble osteoblast‐derived factors in a species nonrestricted but concentration‐ and temporal‐dependent manner; (2) that osteoblast‐mediated antigen induction is reflected in both increased numbers of cells and elevated expression of individual cells that are reactive with the 121F antibody, as determined by ELISA and histomorphometry; (3) that osteoblast conditioned medium, in addition to inducing this antigen in bone marrow cells, also elevates other defining osteoclast characteristics in these avian giant cells including their TRAP activity, cell retraction from the bone surface in response to calcitonin, bone resorptive function, and expression of a series of additional osteoclast antigenic markers; and (4) that secreted osteoblast products alone do not raise the levels of these traits for in vitro formed marrow giant cells to the extent associated with in vivo formed osteoclasts. Therefore, osteoblast soluble factors alone appear unable to promote the full differentiation of bone marrow cells in vitro into mature bone‐resorbing osteoclasts. This inductive bone marrow model system, in conjunction with the ability to monitor an expanded profile of osteoclastic markers afforded by specific monoclonal antibodies, may therefore serve as a valuable tool for investigating intermediate stages of osteoclast cytodifferentiation and for identifying signals responsible for their partial or complete development into unique bone‐resorbing cells.