PU.1, Ikaros, E2A, EBF, Pax5. Of these five, the activity or expression of three, E2A, EBF, and Pax5, is specifically regulated within the B lineage.
An Ets family member, is a hematopoietic transcription factor that is uniquely required for the generation of the lymphoid-myeloid system (B cells and macrophages).
PU.1 regulates the transcription of the M-CSF receptor (c-fms) gene and therefore the responsiveness of macrophage progenitors to M-CSF.
PU.1 may regulate the transcription of the IL-7R in B cell progenitors (IL-7R in developing T cell is likely to be regulated in a PU.1 independent manner since expression of the PU.1 gene is virtually undetectable in the thymus).
The Ikaros gene encodes a family of zinc finger transcription factors that are essential for the generation of fetal B and T cell progenitors. Ikaros gene activity remains essential for adult B lymphopoiesis. Although Ikaros proteins may function like PU.1, E2A, EBF, and Pax5 to positively regulate the early program of B cell-specific gene expresion, a novel mechanism has been proposed whereby Ikaros complexes function as repressors in lineage specification. In nuclei of B lineage cells, Ikaros complexes have been colocalized with inactive T cell-specific genes, such as CD4 and CD8, and heterochromatin foci. An implication of these results is that Ikaros proteins may target genes expressed in developmentally related lineages to heterochromatic nuclear domains for inactivation.
The E2A gene encodes two basic helix-loop-helix (bHLH) proteins, E12 and E47, which are generated by alternative splicing. Both proteins act in concert to promote B cell development.
EA2 functions both upstream of EBF by regulating its expression and also in concert with EBF to induce B cell-specific gene expresion (for example, transcription of the surrogate light chain genes VpreB and λ 5).
The Pax5 gene encoding the transcription factor BSAP appears to function later than PU.1, Ikaros, E2A, and EBF in B cell development.
In Pax5 mutant bone marrow B220+ B cell progenitors are generated but cannot differentiate in vitro in the presence of stromal cells and IL-7. They express wild-type levels of PU.1, Ikaros, E2A, and EBF transcripts. Furthermore expression of Rag1, Rag2, and the surrogate light chain genes is unimpaired. However, Pax5 mutant pro-B cells express low levels of mb-1 and do not induce CD19 expression. Pax5 directly regulates transcription of these two B-cell specific genes.
PU.1 → IL-7R → Pax5 → mb-1 → CD19
Pax5 recruits specific Ets family proteins (e.g., Ets1) to a composite element in the mb-1 gene promoter.
Intriguingly, Pax5 appears to negatively interact with the Ets factor PU.1 to modulate IgH gene transcription in B cells. IgH 3' enhancer contains functionally important binding sites for Pax5 and PU.1 separated by 44 bp. Pax5 was shown to repress enhancer activity stimulated by PU.1.
Pax5 is necessary for commitment to the B lineage. In its absence, PU.1, Ikaros, E2A and EBF generate B lineage progenitors that retain T lymphoid and macrophage developmental potentials.
All of these transcription factors functioning in distinct combinations could induce the early program of B cell gene expression including the genes mb-1, B29, λ 5, VpreB, CD19, Rag1 and Rag2.