PreTCR and TCRγδ Signal Initiation in Thymocyte Progenitors Does Not Require Domains Implicated in Receptor Oligomerization

Abstract

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 ligandindependent 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.

T cell receptor signalling in γδ cell development: strength isn’t everything

“…Thymic ligand-independent signalling was similarly proposed for TCRgd, possibly mediated by oligomerisation of the variable region of TCRd [7]. However, a recent study has suggested that TCRgd complexes that lack variable domains, or that lack both variable and constant Ig-like domains, can still initiate signals that drive Recombination activating gene 2 (RAG-2)-deficient thymocytes toward a gd-like fate [6]. This implies that appropriate surface pairing of TCRg and TCRd chains that possibly bring CD3e-containing signalling modules into close proximity of available lymphocyte-specific protein tyrosine kinase (Lck) is sufficient for TCRgd signal initiation in DN thymocytes.…”

“…This implies that appropriate surface pairing of TCRg and TCRd chains that possibly bring CD3e-containing signalling modules into close proximity of available lymphocyte-specific protein tyrosine kinase (Lck) is sufficient for TCRgd signal initiation in DN thymocytes. Thus, strong TCRgd signalling may not only be a consequence of ligand engagement; additionally, efficiently paired TCRg/TCRd chains that are expressed at the cell surface above a certain critical threshold will also commit DN progenitors to a gd cell fate [6].…”

“…Moreover, ligandmediated positive and negative selection through TCRgd remain poorly understood, as too does the correlation between thymic TCRgd signalling and subsequent gd effector fates. Here, we build on recent studies that have assessed the initiation and consequences of TCRgd signalling in immature thymocytes [5][6][7], to propose that thymic TCRgd-ligand engagement versus ligand-independent TCRgd signalling may differentially impose innate-like versus adaptive-like features on developing gd cells.…”

γδTCR recruits the Syk/PI3K axis to drive proinflammatory differentiation program

“…Self-oligomerization of γδTCR at the cell surface of precursor thymocytes induces their differentiation into mature γδT cells (12,13). It has been proposed that γδTCR-ligand interaction determines the effector function of γδT cells (14).…”

Temporal Predisposition to αβ and γδ T Cell Fates in the Thymus

“…Notably, TCRgd can also mediate bselection-like differentiation to the DP stage. This is most evident in animals unable to express a functional pre-TCR, in TCRgd Tg mice in vivo (8,10,(19)(20)(21)(22)(23), and in cultures in which TCRgdexpressing progenitor cells are allowed to develop in vitro (24)(25)(26). Like b-selected DP cells, gd-selected DP cells silence TCRg gene expression and undergo TCRa rearrangements, indicating that they are bona fide ab-lineage cells (21).…”