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 Lin^- TdT⁺ cells from murine bone marrow. These early pro-B cells were enriched for B-lineage lymphocyte precursor activity measured by short-term culture and had little responsiveness to myeloid growth factors. Early precursors can be propagated with remarkably high cloning frequencies in stromal cell-free, serum-free cultures, permitting this analysis of direct regulatory factors. Expression of the interleukin-7 receptor (IL-7Rα) chain marks 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 Fit-3 ligand were also present. IL-7-responsive CD19⁺ precursors are estrogen resistant. However, B-lineage differentiation was selectively abrogated when highly purified Lin^- precursors were treated with hormone in the absence of stromal cells. In addition, early stages of B lymphopoiesis were arrested by limitin, a new interferon (IFN)-like cytokine as well as IFN-α, IFN-γ, or transforming growth factor β (TGF-β), 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⁺ CD4^+/-CD16/32^Lo/-CD44^Hi and similar in some respects to the "common lymphoid progenitors" (CLP) identified by others. Although early pro-B cells have lost myeloid differentiation potential, transplantation experiments described here reveal that at least some can generate T lymphocytes. Of particular importance is the demonstration that a pivotal early stage of lymphopoiesis is directly sensitive to negative regulation by hormones and cytokines.