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

In wild-type mice, T-cell receptor (TCR) gamma delta(+) cells differentiate along a CD4 CD8 double-negative (DN) pathway whereas TCR alpha beta(+) cells differentiate along the double-positive (DP) pathway. In the human postnatal thymus (PNT), DN, DP and single-positive (SP) TCR gamma delta(+) populations are present. Here, the precursor-progeny relationship of the various PNT TCR gamma delta(+) populations was studied and the role of the DP TCR gamma delta(+) population during T-cell differentiation was elucidated. We demonstrate that human TCR gamma delta(+) cells differentiate along two pathways downstream from an immature CD1(+) DN TCR gamma delta(+) precursor: a Notch-independent DN pathway generating mature DN and CD8 alpha alpha SP TCR gamma delta(+) cells, and a Notch-dependent, highly proliferative DP pathway generating immature CD4 SP and subsequently DP TCR gamma delta(+) populations. DP TCR gamma delta(+) cells are actively rearranging the TCR alpha locus, and differentiate to TCR- DP cells, to CD8 alpha beta SP TCR gamma delta(+) cells and to TCR alpha beta(+) cells. Finally, we show that the gamma delta 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 TCR gamma delta(+) thymocytes induce proliferation and differentiation along the DP pathway in vivo