Developing thymocytes can differentiate into either αβ or γδ T cells, but how do they decide which path to take? Two papers in Immunity provide evidence that the key determinant in the αβ/γδ lineage-fate decision is the strength of the signal transmitted by the T-cell receptor (TCR).

Thymocytes at the CD4−CD8− (double negative, DN) stage of development can express either the γδ-TCR or the pre-TCR (which consists of a complex of the TCR β-chain and the invariant pre-TCR α-chain). Although it is clear that the TCR is important in αβ/γδ lineage commitment, its exact role in this process remains controversial. Recent data show that both the αβ-TCR and the γδ-TCR can promote cross lineage development, which is inconsistent with models stating that the specific TCR isoform (pre-TCR or γδ-TCR) only directs development or promotes survival along its respective lineage. This, together with speculation that the pre-TCR provides a weaker signal than the γδ-TCR, led both groups to ask whether TCR signal strength is an important factor.

To manipulate TCR signalling potential, both groups used γδ-TCR-transgenic mice crossed with mice that are deficient in various molecules involved in either ligand recognition or TCR signal transduction. In γδ-TCR-transgenic mice, thymocyte development results not only in γδ T-lineage cells, which remain DN, but also in αβ T-lineage cells, which can be detected as CD4+CD8+ (double positive, DP) cells. Haks et al. used mice transgenic for the KN6 γδ-TCR crossed onto the recombination-activating gene (RAG)-deficient background to prevent expression of other TCR isoforms (the pre-TCR or αβ-TCR). In these mice, almost all thymocytes expressed the γδ-TCR. They then showed that backcrossing these mice onto the β2-microglobulin-deficient background (which lacks the KN6 ligand) or the tyrosine kinase LCK-deficient background markedly reduced the number of mature γδ T cells but allowed the generation of a substantial population of DP thymocytes with the hallmarks of αβ T-lineage cells. This indicates that interfering with ligand binding or limiting downstream signalling enables a γδ-TCR to efficiently promote the development of αβ, rather than γδ, T-lineage cells.

In the model used by Hayes et al., reducing γδ-TCR cell-surface expression resulted in a significant increase in the number of DP thymocytes and a corresponding decrease in γδ-TCR+ thymocyte numbers. Because TCR signalling is coupled to ITAMs (immunoreceptor tyrosine-based activation motifs) in the TCR-associated CD3 ζ-chain, genetic reconstitution of γδ-TCR-transgenic, CD3ζ-deficient mice with transgenes encoding CD3ζ molecules that had between zero and three ITAMs enabled the authors to show that attenuation of the γδ-TCR signal favoured αβ T-cell development. Conversely, increasing γδ-TCR cell-surface expression or removing a negative regulator of signalling, CD5, favoured γδ T-cell development. So, in these experiments, reducing the signal strength resulted in more αβ T-lineage cells at the expense of γδ T-lineage cells, whereas increasing the signal strength had the converse effect.

Importantly, both groups showed, in non-transgenic mice, that γδ-TCR+ thymocytes have higher levels of activated intracellular-signalling molecules than do pre-TCR+ thymocytes. Taken together, these data are consistent with a model in which γδ-TCR+ thymocytes that receive a 'strong' signal develop into γδ T-lineage cells, whereas γδ-TCR+ or pre-TCR+ thymocytes that receive a 'weak' signal develop into αβ T-lineage cells.