Although the steps involved in the selection of αβ T cells in the thymus have been elucidated in superb detail, the proposed thymic development of γδ T cells has, until now, remained a black box. Robert Tigelaar and colleagues now provide direct evidence that the development of a subset of skin-resident intraepithelial lymphocytes (IELs) expressing an oligoclonal γδ T-cell receptor (TCR) depends on positive selection in the thymus.

IELs are well known to have very limited antigen-receptor diversity. One such IEL population in the skin consists of dendritic epidermal T cells (DETCs), which express an oligoclonal γδ TCR (Vγ5Vδ1) detectable with the monoclonal antibody 17D1. Indeed, in the skin of most mouse strains, 17D1 binds 95% of DETCs. However, the authors have identified a mouse substrain, named FVB-Tac, that has a heterogeneous repertoire of DETCs, with very few cells expressing the Vγ5Vδ1 TCR. Nevertheless, these mice have normal γδ TCR repertoires of IELs in other tissues, including the intestine and reproductive tract. This indicates that FVB-Tac mice lack a crucial factor that is specifically involved in generating the usually homogeneous DETC repertoire.

An important functional role for the presence of a homogeneous 17D1-reactive DETC repertoire was illustrated by the fact that FVB-Tac mice developed spontaneous ear inflammation, as well as more severe dermatitis following contact with an irritant than control animals.

Further analysis of the defect in FVB-Tac mice showed that the maturation of DETC precursors in the fetal thymus at embryonic days 15 to 17 was defective. That is, the precursors failed express appropriate levels of maturation markers such as CD45RB and CD122 and chemokine receptors that are required for thymic emigration. Moreover, this maturation defect was concomitant with increased apoptosis of the DETC precursors, indicating an association between maturation and selection.

To determine whether the defect in FVB-Tac mice lies in the haematopoietic (CD45+) or stromal (CD45−) components of the fetal thymus, the authors used reaggregated thymic organ cultures (RTOCs). Notably, RTOCs generated with CD45+ cells from FVB-Tac mice and CD45− cells from control mice showed normal DETC maturation, as evidenced by co-expression of Vγ5Vδ1 and CD45RB. By contrast, RTOCs generated with FVB-Tac CD45− cells and wild-type CD45+ cells contained very few CD45RBhiVγ5Vδ1+ cells, indicating that the defect in FVB-Tac mice is intrinsic to stromal cells.

Last, the authors confirmed that the delivery of signals through the Vγ5Vδ1 TCR by thymic stromal cells was important for the maturation of DETC precursors, as the maturation defect could be rescued by the presence of 17D1 in the RTOCs.

This study highlights the importance of thymic development for a subpopulation of IELs, as well as an important and often overlooked role for these cells in physiological responses.