IL-7 does not prevent pro-B/pre-B cell maturation to the immature/slgM+ stage

IL-7 plays many fundamental roles during murine B lineage development. One reported function is to maintain progenitors in a developmentally immature state by preventing differentiation to the surface IgM (slgM)(+) stage. Withdrawal of IL-7 from cultures has been shown to lead to increases in mature traits such as RAG expression, IgL rearrangements and expression of sIgM. These observations have been interpreted as an inductive event promoting the differentiation of progenitor cells. In contrast to this, we reproducibly observe sIgM(+) cells that have differentiated in cultures containing IL-7. We find that sIgM(+) cells arise as a normal consequence when B lineage cells are cultured in the presence of IL-7. However, these cells are short-lived and are quickly replaced by newly emerging sIgM(+) cells that differentiate from sigM(-) progenitors. Withdrawal of IL-7 from these cultures only prevents the survival and proliferation of CD2(-)slgM(-) pro-B cells but does not change the number of cells that differentiate to the sIgM(+) stage. This changes the ratio of slgM(-)slgM(+) cells and results only in an apparent maturation of the culture as a whole. Therefore withdrawal of IL-7 from these cultures acts as a selection event, not an induction event, for populations that are normally present.