Interactions between primitive hematopoietic cells and bone marrow stroma

The longterm culture (LTC) system has proven a useful in vitro model of the bone marrow microenvironment because it reproduces many of the features of stromal cell-mediated regulation of primitive normal and leukemic cell behaviour in vivo. This includes effects on the viability, proliferation, self-renewal and differentiation of transplantable stern cells and LTC-IC, as well as interactions that can reversibly promote or arrest the cell cycle progression of normal, high proliferative potential colonyforming cells (HPP-CFC) of both erythroid and granulopoietic types. We have focussed on an analysis of the latter mechanism because it is one that CML HPP-CFC are able to evade both in vivo and in the LTC system. Its delineation might, therefore, help explain how the CML clone acquires a selective and deregulated growth advantage in vivo at the level of the CFC compartment. Our initial studies demonstrated an ability of normal LTC adherent layer cells to exert a strong localized anti-proliferative effect an normal (but not CML) HPP-CFC which could be manipulated in 3 important ways: (1) by the addition of agents that activate endogenous production of various hematopoietic growth factors (or inhibitors), (2) by the addition of candidate direct-acting mediators, and (3) by the addition of antibodies or antagonists of endogenous mediators. From such studies we have identified 2 types of endogenous inhibitors that cooperate in forcing normal HPP-CFC to enter G0 in the LTC model. These are certain β (-CC-) chernokines (ie, MIP-lcc and MCP-1) which are active on normal but not CML HPP-CFC, and TGF-β, which is active on both normal and CML HPP-CFC. Other β(RANTES, MCP-2, MCP-3 and MIP-lβ) and a (-CXC-) chernokines (IL-8 and IP-10) were found to be inactive even on normal HPP-CFC, Interestingly, thus far MCP-1 is the only active chemokine found to contribute to the endogenous cycling control of normal HPP-CFC in LTC. We have also demonstrated a marked difference in the cycling activity of normal and CML LTC-IC in vivo. However, studies of normal LTC-IC cycling in the LTC system suggest that the inhibition of these more primitive cells, like their activation may involve different types or concentrations of factors than those that regulate HPP-CFC proliferation.