Characteristics of early murine B-lymphocyte precursors and their direct sensitivity to negative regulators

Recently, a collection of surface markers was exploited to isolate viable L in(-) TdT(+) cells from murine bone marrow. These early pro-B cells were en riched for B-lineage lymphocyte precursor activity measured by short-term c ulture and had little responsiveness to myeloid growth factors. Early precu rsors can be propagated with remarkably high cloning frequencies in stromal cell-free, serum-free cultures, permitting this analysis of direct regulat ory factors. Expression of the interleukin-7 receptor (IL-7R alpha) chain m arks functional precursors and IL-7 is necessary for progression beyond the CD45RA(+) CD19(-) stage. Efficient survival and differentiation were only observed when stem cell factor and Flt-3 ligand were also present. IL-7-res ponsive CD19(+) precursors are estrogen resistant. However, B-lineage diffe rentiation was selectively abrogated when highly purified Lin(-) precursors were treated with hormone in the absence of stromal cells. In addition, ea rly stages of B lymphopoiesis were arrested by limitin, a new interferon (I FN)-like cytokine as well as IFN-alpha, IFN-gamma, or transforming growth f actor beta (TGF-beta), but not by epidermal growth factor (EGF). Lin(-) TdT (+) early pro-B cells are shown here to be CD27(+) AA4.1(+/-)Ki-67(+) Ly-6C (-) Ly-6A/Sca-1(Lo/-)Thy-1(-)CD43(+) CD(4+/-)CD16/32(Lo/-)CD44(HI) and simi lar in some respects to the "common lymphoid progenitors" (CLP) identified by others. Although early pro-B cells have lost myeloid differentiation pot ential, transplantation experiments described here reveal that at least som e can generate T lymphocytes. Of particular importance is the demonstration that a pivotal early stage of lymphopoiesis is directly sensitive to negat ive regulation by hormones and cytokines. (Blood, 2001;97:2708-2715) (C) 20 01 by The American Society of Hematology.