Single-cell transcriptomics uncovers an instructive T-cell receptor role in adult γδ T-cell lineage commitment

After entering the adult thymus, bipotent T-cell progenitors give rise to αβ or γδ T cells. To determine whether the γδ T-cell receptor (TCR) has an instructive role in γδ T-cell lineage commitment or only "confirms" a pre-established γδ Τ-cell lineage state, we exploited mice lacking expression of LAT, an adaptor required for γδ TCR signaling. Although these mice showed a T-cell development block at the CD4^- CD8^- double-negative third (DN3) stage, 0.3% of their DN3 cells expressed intermediate levels of γδ TCR (further referred to as γδ^int ) at their surface. Single-cell transcriptomics of LAT-deficient DN3 γδ^int cells demonstrated no sign of commitment to the γδ T-cell lineage, apart from γδ TCR expression. Although the lack of LAT is thought to tightly block DN3 cell development, we unexpectedly found that 25% of LAT-deficient DN3 γδ^int cells were actively proliferating and progressed up to the DN4 stage. However, even those cells failed to turn on the transcriptional program associated with the γδ T-cell lineage. Therefore, the γδ TCR-LAT signaling axis builds upon a γδ T-cell uncommitted lineage state to fully instruct adult γδ T-cell lineage specification.