Normal bone marrow adherent cell-conditioned medium corrects the impaired differentiation of cultured mononuclear phagocytes from vitamin D-deficient rats.

There is increasing evidence that 1,25 dihydroxyvitamin D3 acts directly to differentiate and activate cells of mononuclear phagocyte lineage (MNP). However, it seems possible that in bone marrow this hormone may modulate the differentiation of MNP via the factor elaborated by the stromal cells or the macrophages. We tested this hypothesis using two separate culture systems of bone marrow cells from vitamin D-replete (D+) and -deficient (D-) rats. The animals were maintained from the third day of lactation on D+ or D- diet for up to 105 days. In vivo studies showed that the long-term D- state resulted in a depletion of acid phosphatase-positive macrophagelike cells and osteoclasts in bones. After a 14 day incubation period, cultures of D- bone marrow cells contained an increased number of fibroblastoid cell colonies and a reduced number of macrophages attached to fibroblastoid cells, compared with the controls. When D+ bone marrow cells were cultured for 4 days in the presence of supernatants obtained from 14 day cultures of D- bone marrow (marrow adherent cell-conditioned medium, MACCM), the differentiation of MNP into macrophages was inhibited. Impaired differentiation was almost complete in D- bone marrow cell cultures to which D- MACCM was added. These cultures consisted of only 6.6% macrophages compared with 53.9% in the control cultures. However, when D- bone marrow cells were cultured with D+ MACCM, differentiation was restored and 75.4% of the cells were macrophages. These results suggest that a paucity of macrophages and osteoclasts in D- rats is in part, if not entirely, a result of a failure of stromal cells and/or macrophages in the bone marrow to release a factor(s) necessary for differentiation of MNP and osteoclast precursors.