The paired domain transcription factor Pax-5, has been demonstrated to play a crucial role in development and final commitment of the B cell lineage. In fact, otherwise committed B cell progenitors acquire multipotentiality (myelo-lymphoid) potential upon targeted deletion of Pax-5 expression (

Nutt et al.
Nature
.
1999
;
401
:
556
–562
). Thus, in addition, to promoting B cell development through acting as an activator of transcription of B cell specific genes such as CD19, Pax-5 is also thought to act as a suppressor of transcription of genes involved in determination of other blood cell lineages. However, it remains unclear how Pax-5 might repress myeloid development. Thus, we investigated the effect of Pax-5 expression on lympho-myeloid differentiation by overexpressing human (h)Pax-5 (through retroviral transduction) in adult murine bone marrow Linlo/−Sca-1+c-kit+ (LSK) cells. When compared to cells transduced with a control vector, LSK cells ectopically expressing hPax-5 very efficiently developed into hPax-5+B220+CD19+ pro-B cells in response to flt3 ligand and interleukin-7 in vitro. In contrast, when hPax-5+ LSK cells were cultured under myeloid conditions, we consistently observed development of a highly proliferative and immortalised bi-phenotypic (B-myeloid) hPax-5+B220+Gr-1+Mac-1+ population that predominantly consisted of immature myeloblasts but also maturated granulocytes and monocytes. Global gene expression analysis by micro-array, and confirmation by RT-PCR, demonstrated that hPax-5+B220+Gr-1+Mac-1+ cells also possessed a bi-phenotypic gene expression pattern, characteristic for B-cell as well as myeloid lineages including the Pax-5 target B cell genes mb-1 and BLNK as well as GM-CSFRα and low levels of C/EBPα. Similar findings were observed when targeting committed myeloid progenitors. These findings suggest that in addition to promoting B cell development, Pax-5 is capable of inhibiting/blocking myeloid differentiation and inducing immortalization as well as expression of B cell specific genes in otherwise myeloid committed progenitors. These findings motivate a careful investigation of the potential involvement of Pax-5 also in myeloid leukemias.

Topics:
cd19 antigens, flt3 ligand, genes, interleukin-7, leukemia, myeloid, macrophage-1 antigen, microbiology procedures, proto-oncogene protein c-kit, reverse transcriptase polymerase chain reaction, transcription factor

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2005, The American Society of Hematology
2004
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