Extrathymic Aire-Expressing Cells Are a Distinct Bone Marrow-Derived Population that Induce Functional Inactivation of CD4+ T Cells

• eTACs are a distinct bone marrow-derived APC population
• Phenotypically equivalent eTACs are present in both mouse and humans
• eTACs induce CD4+ T cell tolerance through functional anergy
• eTAC-mediated tolerance is highly stable and resistant to perturbation

SummaryThe autoimmune regulator (Aire) is essential for prevention of autoimmunity; its role is best understood in the thymus, where it promotes self-tolerance through tissue-specific antigen (TSA) expression. Recently, extrathymic Aire-expressing cells (eTACs) have been described in murine secondary lymphoid organs, but the identity of such cells and their role in immune tolerance remains unclear. Here we have shown that eTACs are a discrete major histocompatibility complex class II (MHC II)hi, CD80lo, CD86lo, epithelial cell adhesion molecule (EpCAM)hi, CD45lo bone marrow-derived peripheral antigen-presenting cell (APC) population. We also have demonstrated that eTACs can functionally inactivate CD4+ T cells through a mechanism that does not require regulatory T cells (Treg) and is resistant to innate inflammatory stimuli. Together, these findings further define eTACs as a distinct tolerogenic cell population in secondary lymphoid organs.