The E3 ligase Hrd1 stabilizes Tregs by antagonizing inflammatory cytokine-induced ER stress response.

Treg differentiation, maintenance, and function are controlled by the transcription factor FoxP3, which can be destabilized under inflammatory or other pathological conditions. Tregs can be destabilized under inflammatory or other pathological conditions, but the underlying mechanisms are not fully defined. Herein, we show that inflammatory cytokines induce ER stress response, which destabilizes Tregs by suppressing FoxP3 expression, suggesting a critical role of the ER stress response in maintaining Treg stability. Indeed, genetic deletion of Hrd1, an E3 ligase critical in suppressing the ER stress response, leads to elevated expression of ER stress-responsive genes in Treg and largely diminishes Treg suppressive functions under inflammatory condition. Mice with Treg-specific ablation of Hrd1 displayed massive multiorgan lymphocyte infiltration, body weight loss, and the development of severe small intestine inflammation with aging. At the molecular level, the deletion of Hrd1 led to the activation of both the ER stress sensor IRE1α and its downstream MAPK p38. Pharmacological suppression of IRE1α kinase, but not its endoribonuclease activity, diminished the elevated p38 activation and fully rescued the stability of Hrd1-null Tregs. Taken together, our studies reveal ER stress response as a previously unappreciated mechanism underlying Treg instability and that Hrd1 is crucial for maintaining Treg stability and functions through suppressing the IRE1α-mediated ER stress response.