The role of the IL-2R beta-mediated cytokine signaling in natural regulatory T cell development.

Abstract

Nature regulatory T cells (Tregs) are a group of CD4+Foxp3+ T cells that develop in the thymus, and they are crucial in maintaining the peripheral tolerance. Although it has been shown that IL-2/IL-2R mediated cytokine signaling is essential for Treg homeostasis, it is still controversial whether IL-2/IL-2R is required for Treg development, or how IL-2R-mediated signaling events govern Treg development. To answer these questions, I examined Foxp3+ Treg populations from 2 - 4 weeks old IL-2Rbeta-/- mice, and my data showed that Tregs were largely absent even in young and disease-free IL-2Rbeta-/- mice, indicating that IL-2Rbeta is required for Treg development. To investigate how IL-2Rbeta mediated cytokine signaling governs Treg development, I examined Treg development in STAT5abDeltaN/DeltaN and CD4Cre x STAT5fl/fl mice. My data suggested that STAT5 activation is the major signaling event downstream of IL-2beta that promotes Treg development. This conclusion was further corroborated by the observation that a restored Treg population was present in STAT5bCA x IL2Rbeta-/- mice. Furthermore, reconstitution of IL-2Rbeta-/- bone marrow with an IL2Rbeta-STAT5 mutant construct that selectively activates STAT5 restored CD4+Foxp3+ Treg development and prevented T cell activation. These data collectively suggests that STAT5 activation downstream of IL-2Rbeta is both necessary and sufficient to drive CD4+Foxp3+ Treg development. To study how cytokine signals cooperate with TCR signals in shaping Treg development, or any other factors involved in this process, I compared Treg progenitor (CD4+CD25+CD8-Foxp3-) and mature Treg populations (CD4+CD8-Foxp3+) in the thymus from several mouse strains that have either reduced or enhanced TCR signaling or STAT5-dependent signaling, including STAT5bCA, IL-2Rgamma-/-, and CD28-/- mice. Our data suggest that TCR/CD28 signaling is crucial in generating Treg progenitor cells, while cytokine signaling is essential in converting Treg progenitor T cells into mature Tregs in the thymus. My further studies also indicate that Carma1 is a major effector molecule downstream of TCR/CD28 signaling pathways in governing Treg progenitor cell development; likewise, STAT5 is a crucial factor downstream of IL-2Rbeta that converts Treg progenitor cells into mature Tregs, possibly by enhancing DNA methylation in the Foxp3 locus.