Early B-lymphocyte precursors and their regulation by sex steroids.

This review describes an improved characterization of early B-lymphocyte precursors in mice and the remarkable sensitivity of the same cells to hormones. The nuclear enzyme terminal deoxynucleotidyl transferase (TdT) was used as a marker to image and characterize bone marrow cells lacking all lineage-associated markers. Most early TdT+ precursors have a distinctive density of c-kit and express the interleukin-7Ralpha chain, as well as flt-3/flk2, but lack CD34. An understanding of those cell surface properties made it possible to obtain highly enriched, viable cells with the potential to give rise to CD19+ lymphocytes in culture. A series of other flow cytometry and culture experiments suggested a possible differentiation sequence for these early pro-B cells. This new model was used to advantage in our studies of sex steroids. It appears that early precursors represent a hormone-sensitive control point for determining numbers of new B lymphocytes that are produced within bone marrow. We also compare and contrast these findings with B lymphopoiesis in humans.