Repression of Flt3 by Pax5 is crucial for B-cell lineage commitment

Abstract

Early B-lymphopoiesis requires the growth-factor receptors, IL-7R and Flt3, and the activity of a number of transcription factors. One factor, Pax5, is required for commitment to the B-cell lineage, although the molecular mechanism by which this occurs is unknown. We demonstrate here that an important function of Pax5 is to repress Flt3 transcription in B-cell progenitors, as Pax5-deficient pro-B cells express abundant Flt3 that is rapidly silenced upon the reintroduction of Pax5, whereas enforced expression of Flt3 in wild-type progenitors significantly impairs B-cell development. These findings demonstrate that the repression of Flt3 by Pax5 is essential for normal B-lymphopoiesis. Hematopoietic stem cells (HSCs) have the dual capacity to self-renew and to give rise to cells of all the blood lineages. Although the identity and diversity of the intermediaries is still not completely understood, it is clear that an early differentiation step from the HSC gives rise to multipotent progenitors that have reduced capacity to self-renew. These progenitors up-regulate the tyrosine kinase receptor Flt3 (also termed flk2 and CD135) (Adolfsson et al. 2001) and efficiently reconstitute the lympho-myeloid but not the erythro-megakaryocytic lineages (Adolfsson et al. 2005). Flt3 is later expressed on a subset of common myeloid progenitors, with dendritic cell (DC) potential (D'Amico and Wu 2003), and common lymphoid progenitors (CLPs) (Sitnicka et al. 2002).Flt3 −/− mice have a severe deficiency in B-cell progenitors, and the mutant HSCs are impaired in their ability to reconstitute lymphoid and myeloid cells in recipient mice (Mackarehtschian et al. 1995). Similarly, Flt3L-deficient mice have significantly reduced numbers of CLPs, B-cell progenitors, DCs, and natural killer (NK) cells (McKenna et al. 2000). Flt3L expressed by bone marrow stroma (Lisovsky et al. 1996), is a weak growth stimulator of Flt3 + CLPs in vitro and synergizes with stem cell factor (SCF) and IL-7 to promote the proliferation of lymphocytes and, most potently, DCs (Ray et al. 1996;Saunders et al. 1996;Veiby et al. 1996). The ability of Flt3 to promote the expression of the IL-7 receptor on CLPs provides a mechanism by which this synergy occurs in early lymphopoiesis (Borge et al. 1999).B-cell specification is controlled by the coordinate activity of a number of transcription factors, including E2A and EBF, which regulate rearrangement of the IgH locus and pre-B-cell receptor expression (for review, see Busslinger 2004;Singh et al. 2005). These factors also induce Pax5, which is essential for the restriction of lymphoid progenitors to the B-cell fate ). In the absence of Pax5, B lymphopoiesis in the bone marrow is blocked at the early pro-B (or pre-BI)-cell stage (Urbanek et al. 1994;Nutt et al. 1997). Analysis of Pax5 −/− pro-B cells has demonstrated that Pax5 plays a dual role in B-cell commitment, activating lineage-specific genes such as CD19 and mb-1 while repressing lineage inappropriate genes such as M-CSR, and Notch1 (Nutt et al. 1998;Souabni ...

Positive intergenic feedback circuitry, involving EBF1 and FOXO1, orchestrates B-cell fate

“…In sum, we propose that, in the CLP compartment, the E2A proteins directly activate the expression of FOXO1, as well as IRF4 and IRF8 (8,(27)(28)(29). E2A and FOXO1, in turn, act in concert to directly activate the expression of EBF1 (30)(31)(32)(33).…”

Ebf1-mediated down-regulation of Id2 and Id3 is essential for specification of the B cell lineage

“…Alternatively, E2A activation alone may not be sufficient to induce Ebf1 without cooperation from other regulatory factors stimulated by IL-7 receptor signaling. Another attractive possibility is that Ebf1 is induced by removal of repressive signals like Flt3 (39) or TGF␤ signaling (34) that may restrain CLP from differentiation along the B cell pathway. Removal of repressive signals via daughter cell migration away from the CLP niche in the bone marrow may be sufficient to allow IL-7 receptor signaling to maximally activate Ebf1 expression.…”

Murine germinal center B cells require functional Fms-like tyrosine kinase 3 signaling for IgG1 class-switch recombination

“…Despite the block in early B-cell development, Flt3-and FL-targeted mice have normal numbers of peripheral B cells, antibody levels, and responses toward T-dependent immunization (16,24). Surface expression of Flt3 is lost when developing B cells acquire CD19 expression (25).…”