Assembling a Gene Regulatory Network for Specification of the B Cell Fate

Abstract

The generation of B lymphocyte precursors is dependent on the combinatorial action of the transcription factors PU.1, Ikaros, E2A, EBF, and Pax-5. Loss of PU.1 results in a severe reduction in Flk2+, IL-7R+ lymphoid progenitors as well as impaired expression of EBF and Pax-5. Restoration of EBF expression facilitates rapid generation of pro-B cells from PU.1-/- progenitors. Molecular analysis suggests that PU.1 directly participates in regulation of the EBF gene. Although PU.1 is dispensable for expression of most early B lineage genes, it is required for CD45R/B220. Using EBF-/- mutant progenitors, we show that EBF induces Pax-5 and the early program of B lineage gene expression. Importantly, Pax-5 does not rescue B cell development from either PU.1-/- or EBF-/- progenitors. Pax-5 expression and function are contingent on EBF. Based on these results, we propose a hierarchical regulatory network for specification and commitment to the B cell fate.

Spi-C Has Opposing Effects to PU.1 on Gene Expression in Progenitor B Cells

“…The increased frequencies of B cells in cultures of Spi-C-rescued cells are most likely due to the severely reduced numbers of macrophages because B cell populations grow continuously under IL-7 and stromal cell conditions. Once a B cell progenitor is generated, it can proliferate and express IL-7R␣ without a requirement for either PU.1 or Spi-B (25). Spi-B is also less efficient than PU.1 at rescuing both B cells and macrophages.…”

“…First, there are PU.1-binding sites in many B cell-specific genes (15)(16)(17)(18)(19), including all of the Ig loci, such as the IgH intronic enhancer and 3Ј regulatory regions, the Ig 3Ј enhancer, and the Ig 2-4 enhancer (20 -23). Second, PU.1 is required for the generation of lymphoid progenitors, and therefore no B cells are detected in PU.1 Ϫ/Ϫ fetal livers (24,25). However, conditional knockout studies indicate that PU.1 is not required for the continued survival and differentiation of B cells once they are generated (24,26,27).…”

The role of PU.1 and GATA-1 transcription factors during normal and leukemogenic hematopoiesis

“…36,38,39 Conditional deletion of the PU.1 gene in adult bone marrow progenitors showed its requirement for both monocyte and lymphoid progeny whereas granulocyticlike cells were increased indicating that PU.1 may inhibit adult in vivo granulopoiesis at latter stages of development. 34,40 Similarly, PU.1 has only a minor function in advanced differentiation states of B cells 25,34,35,[41][42][43] and is not required in mature T cells. [44][45][46] Collectively the above-mentioned facts reveal an important and crucial role of PU.1 as a primary transcriptional determinant of hematopoietic cell fate with current evidence showing its substantial role in the cell fate control of HSC and progenitor populations rather than the maturation of terminally differentiated cells.…”

PU.1, a Shared Transcriptional Regulator of Innate and Adaptive Immune Cell Fates

“…These experiments provided the first evidence that physiologically graded levels of a transcription factor were used to specify distinct cell fates in the immune and hematopoietic system. Recent analyses from my laboratory have revealed PU.1 to be a pivotal component of distinct gene regulatory networks that orchestrate macrophage and B cell development, thereby helping to fulfill the molecular vision with which the article by R. Maki and his colleagues concluded (31,32).…”