A tumor necrosis factor-responsive long-term-culture-initiating cell is associated with the stromal layer of mouse long-term bone marrow cultures

Long-term bone marrow cultures provide a model for the study of hematopoiesis. Both an intact, adherent stromal layer and hematopoietic stem cells are necessary components in these cultures. Mycophenolic acid treatment of mouse long-term bone marrow cultures depletes them of all assayable hematopoietic precursors. The residual stromal cells are functional and support hematopoiesis if new progenitor cells are supplied. We now show that these mycophenolic acid-treated stromal cell cultures contain cells capable of hematopoietic differentiation without the addition of new progenitors. When treated with tumor necrosis factor α (20-200 units/ml), the apparently pure stromal cultures undergo an intense burst of hematopoietic activity. After 4 days such cultures contain ≈2 × 10⁶ hematopoietic cells and, by 1 week, they are indistinguishable from control long-term cultures that were not treated with mycophenolic acid. These results suggest that the stromal cultures either contain hematopoietic stem cells that are maintained quiescent and mycophenolic acid-resistant, perhaps by intimate contact with the stroma, or contain adherent cells that can be induced to differentiate into hematopoietic stem cells. These stem cells are primitive, in that they are capable of multilineage development in the long-term cultures, but are unable to form spleen colonies or myeloid colonies in semisolid medium. These data demonstrate that the adherent fraction of cultured bone marrow contains very primitive hematopoietic cells and that tumor necrosis factor α activates their proliferation and differentiation. They also suggest a strategy for obtaining the earliest progenitors free of other, more mature cell types.