Lymphoid potential of primitive bone marrow progenitors evaluated in vitro.

Bone marrow contains a heterogeneous mixture of mature and maturing precursors of blood cells, progenitor cells for myeloid and lymphoid lineages, and hematopoietic and mesenchymal stem cells. The differentiation potential of these different stem, progenitor, and precursor populations can be evaluated by using transplantation and cell culture assays. In this study, we used a stromal cell co-culture system to evaluate the B and T lineage potential of different subsets of mouse bone marrow. We enriched hematopoietic stem (Lin(-)Sca-1(+)c-kit(+)Thy1.1(low) [Thy1.1(low)]) cells and lymphoid progenitor (Lin(-)Sca-1(+)c-kit(+)Thy1.1(-) [Thy1.1(-)]) cells from mouse bone marrow and co-cultured these populations with OP9 or OP9-DL1 stromal cell lines. Development of the B and T lineages was evaluated over time. Both populations gave rise to B and T cells but with different kinetics. Thy1.1(-) lymphoid progenitors gave rise to B and T lineage cells earlier than did Thy1.1(low) stem cells; and at any given time, percentages of differentiating B and T cells were higher in Thy1.1(-) cultures than in Thy1.1(low) cultures. We also compared the lineage potential of Thy-1.1(-) lymphoid progenitors with that of the recently described common lymphoid progenitor 2 (isolated as Lin(-)Sca-1(+)c-kit(-)Thy1.1(-)B220(+) cells [B220(+)]). B220(+) cells produced B lineage progeny in OP9 cultures more rapidly than did Thy1.1(-) cells and produced higher percentages of differentiating T cells in OP9-DL1 cultures. These studies demonstrate the utility of the OP9 and OP9-DL1 co-culture systems for evaluation of lymphoid lineage potential and for determining the relative position of specific bone marrow populations within the hematopoietic hierarchy.