Expression of the cell-adhesion molecule VCAM-1 by stromal cells is necessary for osteoclastogenesis

Osteoblastic cells have been shown to be involved in osteoclast formation through cell to cell contacts. This study was designed to examine the possible function of vascular cell adhesion molecule 1 (VCAM-1) during osteoclastogenesis. As a source for stromal cells we used the recently established mouse bone marrow stromal cell line mBMS-B1 which has the ability to support osteoclastogenesis when used in co-culture with a crude spleen cell suspension. mBMS-B1 cells express a single approximately 3.9 kb VCAM-1 mRNA species. Expression was low under basal culture conditions and a 5-10-fold increase was observed in the presence of 1,25(OH)2D3. Cell surface expression of VCAM-1 examined by FACS analysis was increased about 2-fold after 1,25(OH)2D3 treatment. Immunoprecipitation of cell surface expressed VCAM-1 or total VCAM-1 protein using the anti-VCAM-1 monoclonal antibody MK2.7 resulted in a single approximately 110 kDa protein on SDS-PAGE. Induction by 1,25(OH)2D3 was about 2-5-fold on day 3. The stromal cell-osteoclast precursor cell interaction was investigated in a co-culture of the mBMS-B1 and mouse spleen cells in the presence of 1,25(OH)2D3. The monoclonal antibody MK2.7 which is known to block hemopoietic-stromal cell recognition inhibited the formation of osteoclasts when added to the co-culture at day 2 but not day 4. These data suggest that VCAM-1 is involved in the interaction between stromal cells and osteoclastic precursor cells during osteoclastogenesis presumably most important during early stages of the formation of osteoclasts.