Haemopoietic stem cells and the problem of self-renewal

Haemopoiesis occurs in association with a complex stromal cell network in which all levels of haemopoietic cell development can be found. In order to understand the interaction between stromal cells and growth factors with the processes of self-renewal and differentiation, we have carried out a series of experiments attempting to define the circumstances in which self-renewal occurs in long-term marrow cultures. We have found that highly purified (FACS sorted) CFU-S do not undergo significant self-renewal in vitro when inoculated onto marrow stromal cells that can support self-renewal of unfractionated CFU-S. We have examined the effects of expression of the src oncogene on self-renewal of CFU-S. We have found that, following infection of long-term cultures with a retrovirus carrying the src oncogene, there is expression of src in certain of the stromal cells. There is also a selection for CFU-S that have an extended self-renewal capacity in vivo and in vitro. These CFU-S are non-leukaemic and can reconstitute haempoiesis in irradiated mice. Cells from src-infected cultures can also be induced to proliferate and form cell lines in vitro in the presence of interleukin 3 (IL-3). The cell lines produced are multipotential and non-leukaemic. From such data we conclude that expression of the src oncogene has (directly or indirectly) permanently altered the stem cells in such a way that they can undergo extensive self-renewal in situations that are unfavourable for growth and self-renewal of normal stem cells.