CHARACTERIZATION OF HEMATOPOIETIC PROGENITOR CELLS THAT EXPRESS THE TRANSCRIPTION FACTOR SCL, USING A LACZ KNOCK-IN STRATEGY

Gene targeting experiments have demonstrated that the transcription fa ctor SCL is essential for primitive and definitive hematopoiesis in th e mouse. To study the functional properties of hematopoietic cells exp ressing SCL, we have generated mutant mice (SCLlacZ/w) in which the Es cherichia coli lacZ reporter gene has been ''knocked in'' to the SCL l ocus, thereby linking beta-galactosidase expression to transcription f rom the SCL promoter. Bone marrow cells from heterozygous SCLlacZ/w mi ce were sorted into fractions expressing high, intermediate and low le vels of beta-galactosidase (designated lacZ(high), lacZ(int), and lacZ (neg)). Cells that were lacZ(high) or lacZ(int) were enriched for day 12 spleen colony-forming units and myeloid and erythroid colony-formin g cells (CFCs). These fractions included >99% of the erythroid and >90 % of the myeloid CFCs, Culture of sorted bone marrow populations on st romal cells secreting interleukin-7 or in fetal thymic organ cultures showed that B and T lymphoid progenitors were also present in the lacZ (high) and lacZ(int) fractions. These data provide a functional correl ation between SCL expression and colony-forming ability in immature he matopoietic cells. Our data also suggested that expression of SCL was transient and confined to hematopoietic stem and/or progenitor cells, because the differentiated progeny of most lineages (except the erythr oid) were beta-galactosidase-negative.