Notch Activation Converts B Cells into a T Cell Fate at the Earliest Stages of B Cell Committed Progenitors.

Notch activation has been suggested to promote T cell development at the expense of B cell commitment at the level of a common lymphoid progenitor prior to B cell commitment. Here, we explored the possibility that Notch activation might be able to switch the fate of already committed B cell progenitors towards T cell development upon Notch activation. To address this we overexpressed constitutively activated Notch-3 (N3IC) in B cell progenitors purified from transgenic mice in which human CD25 is expressed under control of the λ5 promoter. Strikingly, whereas untransduced and control transduced B220⁺λ5⁺CD3⁻ B cell progenitors gave rise exclusively to B cells, CD4⁺ and CD8⁺ T cells but no B cells were derived from N3IC-transduced cells when transplanted into sublethally irradiated NOD-SCID mice. Gene expression profiling demonstrated that untransduced B220⁺ λ5⁺CD3⁻ B cell progenitors expressed λ5 and CD19 but not the T cell specific genes GATA-3, lck and pTα, whereas CD3⁺ T cells derived from N3IC-transduced B220⁺λ5⁺CD3⁻cells failed to express λ5 and CD19, but were positive for GATA-3, lck and pTα expression as well as a and b T cell rearrangement. Furthermore, DJ rearrangements were detected at very low levels in CD3⁺ cells isolated from normal non-transduced BM, but were more abundant in the N3IC-transduced CD3⁺ BM cells. Noteworthy, N3IC-transduced B220⁺λ5⁺CD3⁻CD19⁺ proB cell progenitors failed to generate B as well as T cells, whereas N3IC-transduced B220⁺λ5⁺CD3⁻CD19⁻ pre-proB cells produced exclusively T cells, even when evaluated at low cell numbers. In conclusion Notch activation can switch committed B cell progenitors from a B cell to a T cell fate, but this plasticity is lost at the Pro-B cell stage, upon upregulation of CD19 expression.