Signaling circuits in early B-cell development.

Early B-cell development is an ordered and highly regulated process with alternating phases of cell proliferation and differentiation leading to B cells with the ability to recognize an extraordinarily large repertoire of different antigens. Here, we discuss what is currently known about the receptors in B-cell progenitors and how their signaling pathways influence immunoglobulin (Ig) gene rearrangement and the transcriptional program of early B cells. In particular, we address the interplay of the interleukin-7 receptor and the pre-B-cell receptor (preBCR) in shaping the survival, proliferation, and differentiation of early B cells. Each receptor addresses a unique set of signaling components but they also share signaling pathways, most prominently the MAPK/Erk and phosphoinositide-3 kinase pathways. The latter pathway regulates transcription factors of the FoxO family that play a central role in the proliferation to differentiation switch of pre-B cells. Interestingly, these two alternative cellular programs (proliferation and differentiation) are both controlled by the preBCR. Finally, we discuss how mutations or alterations of these pathways result in deregulated pre-B-cell expansion and leukemia.