A follicular helper-like program enables T cell tolerance to ingested antigen 2309672

Abstract Introduction Extrathymic T cell tolerance towards innocuous antigen is mediated by 3 complementary mechanisms: deletion, anergy and peripheral regulatory T cell (pTreg) induction. Recent studies suggest that a follicular helper T cell (Tfh)-like program underlies T cell tolerance; yet the mechanisms by which it influences anergy and pTreg differentiation remain poorly understood. Methods Key elements of Tfh program (PD-1 and BCL6) are knocked out in donor mice. We use a mouse model of antigen-specific oral tolerance whereby naïve donor chicken ovalbumin (Ova) reactive CD4+ T cells are adoptively transferred into secondary hosts which are then fed Ova ad libidum in drinking water. 7 to 21 days later, donor T cell responses are analyzed in mesenteric lymph nodes by flow cytometry and scRNA-seq. Results We demonstrate that CD4+ T cell anergy and pTreg are sequentially engaged, suggesting a two-stage process. First, T cells enter a multipotent ‘proto-anergic’ state, characterized by Tfh-ike features, most notably induction of BCL6 and PD-1 (Stage 1). Then, they proceed to terminal tolerogenic states, becoming either terminally anergic or pTreg (Stage 2). Strikingly, we found that tolerance is broken in the absence of PD-1 or BCL6; T cells fail to become anergic and proceed to effector differentiation. PD-1 is also required for pTreg induction, suggesting its key role in extrathymic T cell tolerance. We also learned that BCL6 promotes survival and stemness of T cells responding to ingested antigen, ultimately enabling them to take on terminal tolerogenic fates. Moreover, pTreg generated in the absence of BCL6 appear prone to losing FOXP3, indicating that BCL6 is required for lineage stability. Conclusion These data establish that cells responding to innocuous antigen acquire Tfh-like features, specifically expression of PD-1 and BCL6, which combine to mediate T cell anergy and pTreg development, thereby driving transition from Stage 1 to Stage 2 tolerance and, in turn, preventing deleterious effector responses. Funding Source n/a Topic Categories Immune Response Regulation: Cellular Mechanisms (IRC)