Runx/Cbfβ regulates the development of tolerogenic Thetis cells

Establishing immune tolerance to gut microbiota and food antigens upon first exposures during early life is essential to prevent inflammatory bowel diseases and food allergy and depends on induction of peripherally induced Rorγt expressing regulatory T (Rorγt ⁺ pTreg) cells ^{1, 2, 3, 4, 5, 6} . Recent studies have identified a critical role for Rorγt expressing antigen-presenting cells (APC), Thetis cells (TCs), in peripheral regulatory T (pTreg) cell differentiation and tolerance to food and commensal microbes ^{7, 8, 9, 10, 11} . TCs encompass four distinct subsets, and a subset of TCs, TC IV induces pTreg differentiation, but the transcription factors that control their differentiation are not fully known. Here, using orthogonal genetic approaches to impair Runx/Cbfβ activity, we show that development of specific TCs subsets is regulated by Runx/Cbfβ transcriptional factor complexes. While attenuated Runx3 by germline mutations resulted in a severe reduction of all Rorγt ⁺ APCs, mice lacking one of two Cbfβ splicing variants, Cbfβ2, exhibited a loss of TC II, III and TC IV subsets with associated loss of Rorγt ⁺ pTreg cells. Conditional inactivation of Runx1 and Runx3 genes by CD11c-Cre led to a specific loss of TC III and TC IV subsets. Strikingly, CD11c-Cre driven transgenic Runx expression, particularly Runx1, led to enhanced TC IV differentiation and thus Rorγt ⁺ pTreg cells. Furthermore, transgenic Cbfβ2 by CD11c-Cre recovered only TC IV subset with restoration of Rorγt ⁺ pTreg in Cbfβ2-deficient mice. Collectively, our findings establish a critical pathway for TC IV differentiation and provide new insights into therapeutic interventions to promote Rorγt ⁺ pTreg induction in autoimmune and inflammatory diseases.