OSTEOCLAST FORMATION FROM HUMAN CORD-BLOOD MONONUCLEAR-CELLS COCULTURED WITH MICE EMBRYONIC METATARSALS IN THE PRESENCE OF M-CSF

Investigating the potentiality of cord monocytes to differentiate towa rd osteoclast-like cells (OCL) in vitro, we previously reported that i n the presence of 1,25(OH)(2) vitamin D-3 (1,25-(OH)(2)D-3), multinucl eated-cells generated by cord monocyte cultures though displaying morp hological features of OCL failed to resorb devitalized bones. We thus hypothesized that full differentiation of cord monocytes toward bone-r esorbing cells may require the presence of factors released from and/o r direct interactions with living osteogenic cells. In the present stu dy, we tested these hypotheses using two culture systems supporting th e development of bone-resorbing cells in the presence of bone matrix. First, cord mononuclear cells were co-cultured with murine fetal metat arsals depleted of osteoclast progenitor cells (stripped metatarsals) in the presence of 1,25-(OH)(2)D-3. We found that cord mononuclear cel ls failed to differentiate toward OCL as indicated by the absence of t he release of Ca-45 previously incorporated in fetal bones and by the absence of formation of TRAP-positive (TRAP[+]) multinucleated cells w hich have invaded mineralized cartilage during the co-culture period. In the same model, we then investigated the, effect of some soluble fa ctors known as stimulators of osteoclast differentiation. Whereas exog enous rhIL6 and rhIL3 were ineffective in this assay, rhM-CSF consiste ntly increased both the number of TRAP(+) multinucleated cells inside the mineralized cartilage and the release of Ca-45 into the culture me dia. The effects of rhM-CSF were time-dependent reaching the maximum a fter 3 weeks of culture. In stripped metatarsal cultures treated with rhM-CSF but in the absence of cord mononuclear cells, no TRAP(+) cell was visible inside the mineralized area and the release of Ca-45 was u naffected. Finally, in the so-called pit formation assay in which cord mononuclear cells were cultured on slices of devitalized bones, we ob served that these cells did not form any resorption pit, even when cul tured in the presence of rhM-CSF. We suggest that M-CSF is required fo r full differentiation of cord mononuclear cells toward OCL, in the pr esence of living bone.