Keeping track of pro-B cells: a new model for the effects of IL-7 during B cell development.

Trophic and instructive models have been invoked to explain how various cytokines influence the survival, proliferation and differentiation of lymphoid progenitors. There is evidence that IL-7 can act in either a trophic or an instructive manner during B and T cell development although its roles in B cell development in the fetus versus the adult mouse and in the mouse versus the human may vary. Here we outline the stages of B cell development and the conventional model of the role of IL-7 in the transitions from pro-B cells to pre-B cells and then to immature B cells. We also discuss the implications of recent data that have led to a new model for this process.