Notch induces human T-cell receptor γ δ+ thymocytes to differentiate along a parallel, highly proliferative and bipotent CD4 CD8 double-positive pathway

In wild-type mice, T-cell receptor (TCR) γδ+cells differentiate along a CD4 CD8 double-negative (DN) pathway whereas TCRαΒ+cells differentiate along the double-positive (DP) pathway. In the human postnatal thymus (PNT), DN, DP and single-positive (SP) TCRγδ+populations are present. Here, the precursor-progeny relationship of the various PNT TCRγδ+populations was studied and the role of the DP TCRγδ+population during T-cell differentiation was elucidated. We demonstrate that human TCRγδ+cells differentiate along two pathways downstream from an immature CD1+DN TCRγδ+precursor: a Notch-independent DN pathway generating mature DN and CD8αα SP TCRγδ+cells, and a Notch-dependent, highly proliferative DP pathway generating immature CD4 SP and subsequently DP TCRγδ+populations. DP TCRγδ+cells are actively rearranging the TCRα locus, and differentiate to TCR DP cells, to CD8αΒ SP TCRγδ+cells and to TCRαΒ+cells. Finally, we show that the γδ+subset of T-cell acute lymphoblastic leukemias (T-ALL) consists mainly of CD4 SP or DP phenotypes carrying significantly more activating Notch mutations than DN T-ALL. The latter suggests that activating Notch mutations in TCRγδ+thymocytes induce proliferation and differentiation along the DP pathway in vivo.