Hematopoiesis in the presence of macrophages in long-term bone marrow cultures

We have studied the role of macrophages as stromal elements in long-term cultures of murine bone marrow cells. Normal ectopic macrophages were collected from the lung and co-cultured in various ratios with bone marrow cells in Dexter suspension cultures. At weekly intervals, various parameters of the cultures were evaluated and compared with controls. Increased numbers of macrophages in the bone marrow caused a delay in the formation of the confluent stroma, with distinct differences in the pattern of hematopoietic foci in the stroma. Cytologic analysis of nucleated cells in the nonadhering fraction showed that the monocytic macrophages were the eventual line of differentiation in all cultures; however, this phenomenon was accelerated in cultures containing various fractions of macrophages. There was a dose-dependent reduction in the number of myeloid progenitors in the nonadhering and adhering fractions of macrophage-enriched cultures with respect to controls. This effect was mediated through both humoral factors released in the culture and direct interaction between macrophages and bone marrow cells. The result indicate that elevation of the number of macrophages in the bone marrow could result in an impaired process of hematopoiesis.