Title: The life of the regulatory T cell repertoire
Description: Regulatory T cells (Tregs) are essential for maintaining peripheral tolerance in the immune system. It is unclear what factors shape the T cell receptor (TCR) repertoire of Tregs. To explore how that repertoire is formed in the thymus and utilized for suppression in the periphery, we employed single-cell sorting and high-throughput sequencing to compare the TCR repertoires of Tregs against their conventional T cell (Tconv) counterparts. Treg and Tconv repertoires were investigated within several contexts spanning thymic selection to peripheral, autoimmune suppression, in two experimental systems in which a restricted range of potential TCR diversity allowed meaningful comparisons. Treg and Tconv repertoires, were equivalently diverse and mainly distinct. TCR sequences were shared among Treg and Tconv populations at varying degrees, depending on the system examined, but each sequence typically exhibited a clear bias for one phenotype or the other. The CDR3αs of Treg TCRs exhibited an overall bias in charge, which appeared to complement the charge of the selecting peptide(s). Both Treg and Tconv populations experienced adaptation of their repertoires during their transition from thymus to periphery. Lastly, the conversion of Tconv cells to Tregs appeared to have little influence in shaping the peripheral Treg repertoire.

Repertoires were also compared in the context of autoimmunity, in dual-TCR cells where one TCR conferred anti-pancreas reactivity, while the second responded to additional cues. Tregs showed clear modulation of sequence representation within the repertoire between irrelevant LNs and the draining LN, presumably in response to exposed antigens. This repertoire was stable, however, when comparing sites of autoimmune antigen exposure. In contrast, Tconv cells, exhibited a constant repertoire in the thymus, LNs, draining LNs, and pancreas, suggesting that in the context of autoimmunity, the secondary TCR had a lesser role than the primary, self-reactive one. In spite of exposure to agonistic self-antigen, conversion of Tconv cells into Tregs made little or no contribution to the Treg repertoire. Therefore, throughout its life, the Treg repertoire is shaped by its encounter with self or environmental antigens.