A novel hematopoietic multilineage clone, Myl-D-7, is stromal cell- dependent and supported by an alternative mechanism(s) independent of stem cell factor/c-kit interaction

A strictly stroma-dependent hematopoietic clone, Myl-D-7, with lympho- myeloid potential has been isolated. A subset of cells expresses myeloid-macrophage (Mac-1 and Gr-1), erythroid (TER119), and lymphoid (Thy-1 and B220) lineage markers. Spontaneous differentiation to the myeloid-macrophage, erythroid, or lymphoid pathway can be seen by morphologic criteria, detection of beta major globin synthesis, or expression of the early lymphoid specific transcription factor, Ikaros. By sorting lineage marker (Mac-1, Gr-1, B220, and TER119)-negative (LIN- ) cells, we showed that the LIN- population actively self-renews on top of MS-5 stromal cells, and differentiates to LIN+ cells. Removal of stroma induces apoptosis and none of the growth factors tested can prevent apoptosis. Granulocyte-macrophage colony-stimulating factor accelerates the differentiation towards the myeloid-macrophage lineage. Using this clone, we show that (1) contact with stroma induces expression of bcl-2, (2) stromal cells derived from SI/SI homozygous fetuses can support long-term growth, and (3) conditioned media of specific stromal cells contains an activity that supports proliferation and self-renewal of the clone. Myl-D-7 can thus be used as an indicator cell for unknown factors that may provide stromal cell support.