PreTCR and TCR gamma delta Signal Initiation in Thymocyte Progenitors Does Not Require Domains Implicated in Receptor Oligomerization

Whether thymocytes adopt an αβ or a γδ T cell fate in the thymus is determined at a checkpoint (β-selection) by the relatively weak or strong signals that are delivered by either the pre-T cell receptor (preTCR) or the γδ TCR, respectively. However, how these signals are initiated, and how different signal strengths are generated, remains unclear. Although binding of thymic agonist ligand would predict strong signaling, the preTCR and TCRγδ appear to be capable of ligand-independent signaling. Some reports have suggested that receptor oligomerization, which is thought to be mediated by either the immunoglobulin (Ig)-like domain of the preTCR α-chain (pTα) or the variable domain of TCRδ, is a unifying mechanism that initiates signaling in early CD4(−) CD8(−) double negative (DN) thymocyte progenitors. Here, we demonstrate that the extracellular regions of pTα and TCRd that were implicated in mediating receptor oligomerization were not required for signal initiation from the preTCR or TCRγδ. Indeed, a truncated TCRγδ that lacked all of its extracellular Ig-like domains still formed a signaling-competent TCR that drove cells through the β-selection checkpoint. These observations suggest that signal initiation in DN thymocytes is simply a consequence of the surface-pairing of TCR chains, with signal strength being a function of the abundances of surface TCR. Thus, processes that regulate the surface abundances of TCR complexes in DN cells, such as oligomerization-induced endocytosis, would be predicted to have a major influence in determining whether cells adopt an αβ versus γδ T cell fate

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