Density of the Notch ligand Delta1 determines generation of B and T cell precursors from hematopoietic stem cells

Notch signaling regulates multiple cell fate decisions by hematopoietic precursors. To address whether different amounts of Notch ligand influence lineage choices, we cultured murine bone marrow lin⁻Sca-1⁺c-kit⁺ cells with increasing densities of immobilized Delta1^ext-IgG consisting of the extracellular domain of Delta1 fused to the Fc domain of human IgG₁. We found that relatively lower densities of Delta1^ext-IgG enhanced the generation of Sca-1⁺c-kit⁺ cells, Thy1⁺CD25⁺ early T cell precursors, and B220⁺CD43^−/lo cells that, when cocultured with OP9 stroma cells, differentiated into CD19⁺ early B cell precursors. Higher densities of Delta1^ext-IgG also enhanced the generation of Sca-1⁺c-kit⁺ precursor cells and promoted the development of Thy1⁺CD25⁺ cells, but inhibited the development of B220⁺CD43^−/lo cells. Analyses of further isolated precursor populations suggested that the enhanced generation of T and B cell precursors resulted from the effects on multipotent rather than lymphoid-committed precursors. The results demonstrate the density-dependent effects of Delta1 on fate decisions of hematopoietic precursors at multiple maturational stages and substantiate the previously unrecognized ability of Delta1 to enhance the development of both early B and T precursor cells.