Characterization and differentiation of an early murine yolk sac-derived IL-7-independent pre-pro-B cell line.

We describe a unique, stable pre-pro-B cell line (YS-PPB) derived from AA4.1+ yolk sac cells from day 10 mouse embryos. This cell line, discovered fortuitously during the course of studies of in vitro B cell differentiation, is independent of IL-7 supplementation for long term expansion in vitro. YS-PPB cells as well as clonal sublines expressed AA4.1, CD43, B220, Sca-1, CD19, heat stable antigen, MHC class I, IL-7R, and FcyR, but did not express cytoplasmic mu-chain, surface IgM (sIgM), or MHC class II molecules. PCR analysis showed that the cells expressed TdT, lambda5, and RAG-1 genes, but that their Ig genes were still in germline configuration. The cell line was dependent on direct contact with S17 stromal cells for growth, but, in contrast to bone marrow stem cells, required no additional growth factors for maintenance and expansion. When stimulated with IL-7 and LPS, YS-PPB cells and cells from all tested clonal sublines differentiated into sIgM+ B cells in vitro. Irradiated mice reconstituted with YS-PPB cells yielded spleens containing 38% sIgM+ donor-derived B cells, demonstrating that YS-PPB cells, although stably arrested in development at the boundary between pre-pro-B and pro-B stages of B cell differentiation, still retain their competence to differentiate into mature, Ig-producing B cells when transferred to a normal host environment. Thus, this new cell line can provide a reproducible source of B cell precursors arrested at that critical time in B cell differentiation when the machinery for Ig gene rearrangement is in place but rearrangement has not yet occurred.