Maintenance and proliferation control of primitive hemopoietic progenitors in long-term cultures of human marrow cells

Primitive, high-proliferative potential hemopoietic progenitors can be routinely maintained for many weeks in long-term marrow cultures (LTC) in the absence of added hemopoietic growth factors. Nevertheless, these progenitors are clearly responsive to both positive and negative regulatory control mechanisms that operate within the adherent layer as evidenced by cyclic changes in their proliferative activity each time the medium is replaced. The key event appears to be the addition of a constituent of fresh horse serum that is not found in fetal calf serum. Analogous primitive neoplastic progenitor cell types from CML or PV patients are insensitive to the negative arm of this proliferation control mechanism both in vitro and in vivo. A model to explain the progenitor cell cycle changes normally observed in the LTC system is proposed. This model suggests that perturbations of nonhemopoietic mesenchymal cells determine the net positive or negative influence that these regulatory cells exert on adjacent primitive hemopoietic cells, possibly by a mechanism involving direct cell contact. Recently, we have identified a number of cytokines that can simulate the transient positive effect of fresh horse serum, as well as another cytokine, that is, tumor growth factor-beta (TGF-beta), that can mimic the negative but reversible effect exerted by mesenchymal cells. These studies demonstrating the effects of positive and negative regulatory cytokines on the control of hemopoiesis in the adherent layer of LTC suggest new approaches for analyzing the basis of both normal and abnormal stem cell regulation by marrow stromal elements.