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 (DN) 3 stage, 0.3% of their DN3 cells expressed intermediate levels of γδ-TCR (further referred to as γδint) at their surface. Single-cell transcriptomics of LAT-deficient DN3 γδint 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 γδint 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.