TCR-dependent differentiation of thymic Foxp3+ cells is limited to small clonal sizes

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Abstract
Numerous studies have highlighted the importance of high-affinity interactions between T cell receptors (TCRs) and their ligands in the selection of Foxp3+ regulatory T cells (T reg cells). To determine the role of the TCR in directing T cells into the Foxp3+ lineage, we generated transgenic (Tg) mice expressing TCRs from Foxp3+ cells. Initial analyses of the TCR Tg mice crossed with RAG-deficient mice showed that the percentage of Foxp3+ cells was very low. However, intrathymic injection and bone marrow chimera experiments showed a saturable increase of the Foxp3+ population when T reg TCR Tg cells were present in low numbers. Furthermore, when analyzing whole thymi of T reg TCR Tg RAG-deficient mice, we found significantly more Foxp3+ cells than in conventional T cell TCR Tg mice. Our results indicate that although the TCR has an instructive role in determining Foxp3 expression, selection of Foxp3+ individual clones in the thymus is limited by a very small niche.