Erythroid-specific deletion of the Lrf gene promotes aberrant lymphoid differentiation and HSC depletion. (A) Expression of the GFPCre fusion protein is limited to erythroid lineage cells. Representative FACS profile of BM cells from control and ErGFPCre⁺ mice are shown. (B) Dll4 is up-regulated in erythroblasts of Zbtb7a^Flox/FloxErGFPCre⁺ mice. Representative FACS profiles of BM (top) and spleen (bottom) cells are shown. (C) Graph shows MFI scores of Dll4 staining in erythroid (Ter119⁺CD71⁺) and nonerythroid (Ter119⁻CD71⁻) cells of control (white), Zbtb7a^Flox/+ErGFPCre⁺ (gray), and Zbtb7a^Flox/FloxErGFPCre⁺ (black) mice. Error bars: standard deviation. (D) Hematopoietic development in the BM was analyzed in 1-month-old mice. Representative FACS profiles for each group are shown. (E) Dot graph shows proportions of DP T and pre-B cell compartments in BM. Horizontal black bars: average value. Error bars: standard deviation. (F) Representative HSC/progenitor FACS profiles of control (Zbtb7a^Flox/Flox) and erythroid-specific Lrf conditional knockout mice (Zbtb7a^Flox/FloxErGFPCre⁺). Experiments were performed as described in Figure 1A. Aberrant T-cell progenitors within the Lin⁻ fraction are indicated by arrowheads. (G) Dot graphs of absolute counts (per 2 legs) of LT-HSCs and CLPs. Horizontal black bars: average value. Error bars: standard deviation. (H) Notch proteins are expressed in the most primitive CD34⁻ LT-HSCs. No or weak Notch1/Dll4-mediated signals are transmitted to CD34⁻ LT-HSCs under physiologic conditions and no premature T-cell differentiation occurs in the BM (left). In Lrf knockout mice, Dll4 is up-regulated (or de-repressed) in erythroblasts. When CD34⁻ LT-HSCs expressing high levels of Notch are stimulated by the high affinity Notch ligand Dll4, they receive T-instructive signals and differentiate into T cells in the BM, resulting in HSC depletion.