A clonal assay was used to study different stimuli involved in the progression of fetal liver B cell precursors to mature B lymphocytes. In this report we replaced fetal liver heterogenous feeder cells by a recombinant growth factor, interleukin 7 (IL 7), and a clonal stromal cell line, S17. Under those conditions we could clone 1 in 10 B220+ B cell precursors from fetal liver and the cells could differentiate to a mitogen-responsive, immunoglobulin-secreting stage. We found that IL 7 stimulates proliferation of B220+ precursors but is not sufficient to support maturation of those precursors to a stage of mitogen responsiveness. We show further that the cell line S17 does not produce IL 7 at functionally detectable level but provides support for B cell maturation. We conclude that this cell line supplies an exogenous stimulus required by B cell precursors to become mature lymphocytes. We describe therefore two stages in pre-B cell development: (a) IL 7-dependent proliferation and (b) S17-dependent maturation to mitogen reactivity. Further studies demonstrate that S17 has a profound effect on B cells by increasing the clonal efficiency of lipopolysaccharide-responsive cells to nearly 1:1 B cell in the spleen of adult C57BL/6 mice.
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Cumano et al. (1990) studied this question.
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