Loss of PI3Kδ Activity Drives Autoimmune Colitis by Impairing Extrathymic Treg Differentiation

Peripherally derived regulatory T cells (pTregs) have a prominent role in maintaining intestinal immune homeostasis. In cases of phosphoinositide-3-kinase δ (PI3Kδ) inactivation, such as in patients receiving PI3Kδ inhibitor idelalisib as a cancer treatment, breakdown of intestinal immune tolerance occurs frequently in the form of diarrhea and colon inflammation. In a mouse model of systemic PI3Kδ inactivation, both enhancement of antitumor immunity and colitis have been described as a result of Treg impairment. However, in view of the critical role for Tregs in the prevention of systemic autoimmunity, the basis for such tissue-restricted breach of immune tolerance upon loss of PI3Kδ function is not yet understood. We report here that mice lacking PI3Kδ activity do not suffer a general defect in Treg immunosuppression, but specifically fail to develop Helios^- pTregs in the colon. We demonstrate reduced extrathymic Treg induction, in vitro and in vivo, from naïve CD4⁺ T cells with inactive PI3Kδ, along with dysregulation of a tissue-resident phenotype. These results suggest a nonredundant role for PI3Kδ-dependent pTreg differentiation in maintaining tolerance to commensal microbial antigens in the gut.