Temporal and Sequential Expression of EBF1 and PAX5 Restricts the Non-B Cell Fate In Early Lymphopoiesis

Abstract 3867

The generation of B cells from multipotent hematopoietic stem cells involves activation of B cell specific and repression of alternative lineage programs. This process to a large extent is under the control of transcription factors such as EBF1, E2A, IKAROS and PAX5. Inactivation of PAX5 and EBF1, by point mutations or deletions can be found in 30% of the acute lymphocytic leukemia (ALL) cases, probably due to a disturbance of differentiation in early B cell development. Conditional deletion of PAX5 and EBF1 leads to a complete block in B cell production and subsequent accumulation of early immature B cell progenitors. Although the roles of these transcription factors have been established in B cell development, the presence of a feedback loop between PAX5 and EBF1 as well as lack of well defined early B cell developmental events and cell populations, lead to some discrepancies in the current literature regarding the role of these factors in restricting non-B cell lineages and the establishment of B cell fate. By using a combination of RAG1/EBF1 reporter mice and newly identified surface markers i.e Ly6D, we have shown the exact point of myeloid (M), natural killer (NK) and T cells restriction within conventional common lymphoid progenitors (CLP) population. The Lin-IL7r+flt3+kit^loSca1^lo(CLP)Ly6D-λ5- population retains the residue of myeloid potential together with dendritic (D), Nk, T and B potentials. Upon expression of Ly6D, these cells lose the residual myeloid, NK and D potentials, and eventually the expression of λ5, associated with loss of T cell potential, marks the B-cell committed cells. Based on this new model of lymphoid development we have revisited the functional roles of EBF1 and PAX5 in B-cell commitment. This work suggests that EBF1 restricts the myeloid, NK and D potentials in the transition from CLPLy6D- to CLPLy6D+ stage while the expression of PAX5 in the transition from CLPLy6D+λ5- to CLPLy6D+λ5+ restricts the T cell potential by possibly by counteracting Notch1 signaling in the bone marrow.

Considering the high prevalence of PAX5 and EBF1 mutations in ALL and diffuse large B cell lymphoma (DLBL) cases and the fact that inactivation of PAX5 and EBF1 leads to accumulation of B cell progenitors in mice models, understanding the temporal and spatial expression of EBF1 and PAX5 in regard to restriction of non-B cell fates in early stages of lymphoid development will allow us to identify the leukemia-initiating cells and the mechanisms of leukemogenesis in these diseases.