Generation of human osteoclasts in stromal cell-free and stromal cell-richcultures: differences in osteoclast CD11c/CD18 integrin expression

Osteoclasts form in the presence of macrophage colony-stimulating factor (M -CSF) and receptor activator of Nf kappab ligand (RANKL), a membrane-bound differentiation factor that is now available as a soluble recombinant molec ule. Acquisition of the osteoclast phenotype [the alpha (v)beta (3) subunit of the vitronectin receptor (VNR)-, calcitonin receptor (CTR)- and F-actin ring-positive cells] is associated with loss of monocyte/macrophage-associ ated integrins, specifically CD11b, CD11c and CD18. We hypothesized that di fferences in the osteoclast integrin adhesion molecule profile may exist in osteoclasts generated in stromal cell-rich and in stromal-free conditions. Unlike osteoclasts generated in vivo, F-actin ring-positive (resorbing) os teoclasts formed in soluble RANKL in vitro, in the absence of stromal cells , and co-expressed CD11c and CD18. However, when osteoclasts were generated from peripheral blood mononuclear cells (PBMNCs) in co-cultures with the m urine bone marrow stromal cell line 218 (which does not produce membrane-bo und RANKL) in the presence of soluble RANKL, CD11c and CD18 were not expres sed by osteoclasts. These findings indicate that the persistent expression of CD11c and CD18 is not accounted for by RANKL being presented in a solubl e form and that membrane-bound RANKL is not required for the normal integri n expression in resorbing osteoclasts. This study demonstrates that potenti ally misleading information may arise by using data obtained from osteoclas ts generated in the absence of stromal cells as they do not completely refl ect the situation in vivo.