The same self-peptide selects conventional and regulatory CD4⁺ T cells with identical antigen receptors.

The role of the T-cell receptor (TCR) in commitment of thymocytes to regulatory CD4(+)Foxp3(+) and conventional CD4(+)Foxp3(-) T-cell lineages remains controversial. According to the prevailing view, commitment to the former lineage, in contrast to the latter, requires that high affinity TCRs bind rare class II MHC/peptide complexes presented in 'thymic niches', which could explain differences between their TCR repertoires. Here we challenge this view and show that the binding of identical TCRs to the same ubiquitously expressed MHC/peptide complex often directs thymocytes to both CD4(+) lineages, indicating that the TCR affinity does not play the instructive role, and that restricted presentation of peptides in 'thymic niches' is not necessary for selection of CD4(+)Foxp3(+) T cells.

Thymus-derived regulatory T cells contribute to tolerance to commensal microbiota.

Peripheral mechanisms preventing autoimmunity and maintaining tolerance to commensal microbiota involve CD4(+) Foxp3(+) regulatory T (Treg) cells generated in the thymus or extrathymically by induction of naive CD4(+) Foxp3(-) T cells. Previous studies suggested that the T-cell receptor repertoires of thymic Treg cells and induced Treg cells are biased towards self and non-self antigens, respectively, but their relative contribution in controlling immunopathology, such as colitis and other untoward inflammatory responses triggered by different types of antigens, remains unresolved. The intestine, and especially the colon, is a particularly suitable organ to study this question, given the variety of self-, microbiota- and food-derived antigens to which Treg cells and other T-cell populations are exposed.