A role for intrathymic B cells in the generation of natural regulatory T cells

B cells inhabit the normal human thymus, suggesting a role in T cell selection. In this study, we report that B cells can modulate thymic production of CD4+ Foxp3+ T cells (regulatory T cells [Tregs]). Mice with transgenic expression of BAFF (BAFF-Tg) harbor increased numbers of Helios+Foxp3+ thymic Tregs and, similar to some human autoimmune conditions, also exhibit increased numbers of B cells colonizing the thymus. Distinct intrathymic B cell subpopulations were identified, namely B220+, IgM+, CD23hi, CD21int cells; B220+, IgM+, CD23lo, CD21lo cells; and a population of B220+, IgM+, CD23lo, CD21hi cells. Anatomically, CD19+ B cells accumulated in the thymic medulla region juxtaposed to Foxp3+ T cells. These intrathymic B cells engender Tregs. Indeed, thymic Treg development was diminished in both B cell–deficient BAFF-Tg chimeras, but also B cell–deficient wild-type chimeras. B cell Ag capture and presentation are critical in vivo events for Treg development. In the absence of B cell surface MHC class II expression, thymic expansion of BAFF-Tg Tregs was lost. Further to this, expansion of Tregs did not occur in BAFF-Tg/Ig hen egg lysozyme BCR chimeras, demonstrating a requirement for Ag specificity. Thus, we present a mechanism whereby intrathymic B cells, through the provision of cognate help, contribute to the shaping of the Treg repertoire.