Differential Requirement for IL-2 and IL-15 during Bifurcated Development of Thymic Regulatory T Cells

Abstract

The developmental pathways of regulatory T cells (Treg) generation in the thymus are not fully understood. In this study, we reconstituted thymic development of Zap70-deficient thymocytes with a tetracycline-inducible Zap70 transgene to allow temporal dissection of Treg development. We find that Treg develop with distinctive kinetics, first appearing by day 4 among CD4 single-positive (SP) thymocytes. Accepted models of CD25+Foxp3+ Treg selection suggest development via CD25+Foxp3− CD4 SP precursors. In contrast, our kinetic analysis revealed the presence of abundant CD25−Foxp3+ cells that are highly efficient at maturing to CD25+Foxp3+ cells in response to IL-2. CD25−Foxp3+ cells more closely resembled mature Treg both with respect to kinetics of development and avidity for self-peptide MHC. These population also exhibited distinct requirements for cytokines during their development. CD25–Foxp3+ cells were IL-15 dependent, whereas generation of CD25+Foxp3+ specifically required IL-2. Finally, we found that IL-2 and IL-15 arose from distinct sources in vivo. IL-15 was of stromal origin, whereas IL-2 was of exclusively from hemopoetic cells that depended on intact CD4 lineage development but not either Ag-experienced or NKT cells.