Immune self-tolerance, the absence of an immune response against the body, is an active process involving several complementary mechanisms. The autoimmune regulator gene (Aire) plays a critical role in this process by inducing the expression of a diverse set of tissue-specific antigens (TSAs) in medullary thymic epithelial cells (mTECs). These TSAs are presented and help eliminate or control self-reactive thymocytes. The expression of Aire is highly restricted, occurring in adults only in mTECs and extrathymic Aire-expressing cells (eTACs), which are found in secondary lymphoid tissues. How this specific pattern of expression is achieved has remained unclear. Although RANK and non-canonical NF-κB signaling are known to be important for the development and maintenance of Aire-expressing mTECs, the cis-regulation of Aire is poorly understood. We aimed to discover distal cis-regulatory elements that control Aire. Here, we identify a conserved DNA element that is essential for Aire expression. We find enrichment of enhancer-associated histone marks near this element, which we call ACNS1 and show to be NF-κB-responsive. We demonstrate that a 4.3-kb region, which includes ACNS1 and the Aire promoter, is sufficient to recapitulate the cell-type specificity of Aire expression. We show that ACNS1 is essential for Aire expression in mTECs and eTACs in vivo and is required to prevent spontaneous autoimmunity. We also observe that Aire and Dnmt3l are co-expressed and that ACNS1 is necessary for thymic Dnmt3l expression. Finally, we present preliminary evidence for additional Aire cis-regulatory elements. Together, these findings further our understanding of the control of immune tolerance and may lead to improvements in the diagnosis and treatment of autoimmune disease.

Immune cells require numerous growth and survival factors during development in central lymphoid organs as well as in the periphery. Amongst these factors, interleukin-7 (IL-7) plays a central role and is required for T cell homeostasis and function in both health and disease (e.g., HIV disease). Understanding the cellular source(s) of such factors, the conditions controlling their regulated expression, and their mechanism of action on target cells is critical to therapeutic application in humans. Due to low expression levels and poor reagent availability, it has proven difficult to comprehensively describe the cellular sources of IL-7. We have accordingly generated mice in which eGFP is expressed from the IL-7 locus under the control of the endogenous promoter. We report that substantial Il-7 is produced by lymphatic endothelial cells (LECs) distributed throughout the systemic lymphatic vasculature, including the lymph node (LN) sinuses, and that STAT5 phosphorylation (pSTAT5) is increased in lymphocytes harvested from efferent lymphatics. We further show that Il-7 is increased in stromal cell subsets within the LN during acute lymphocyte blockade and that this effect is not driven by myeloid Il-7 expression. Our data support recent findings that lymphocyte homeostasis requires access to secondary lymphoid organs (SLOs) but introduce LECs as a significant in vivo source of potentially regulatable IL-7 that bathes trafficking immune cells under both resting and lymphopenic conditions. Finally, these data are not consistent with the concept of a static peripheral IL-7 pool and confirm the existence of a feedback loop that increases IL-7 availability during periods of lymphopenia. The Il-7 reporter model described here offers a new and exciting tool for understanding how this cytokine influences peripheral immune cell function.

Mutations of the Autoimmune Regulator (Aire) were first identified as the cause of a multi-organ autoimmune syndrome, APS-1, in 1997, and it has since become appreciated that Aire normally functions to prevent autoimmunity by facilitating self tolerance to peripheral antigens. Specifically, Aire is expressed at high levels in the medullary epithelial cells in the thymus, the site of T cell development, where it drives expression of otherwise silent, tissue-specific antigens (TSAs) like insulin, resulting in central tolerance towards TSAs. Here, we describe the pattern of Aire expression among medullary thymic epithelial cells (mTEC) and the potential of Aire+ mTECs to recover from targeted ablation. We demonstrate that Aire+ mTECs have a remarkable ability to quickly recover from targeted ablation in a manner that depends on signaling through the TNF superfamily member RANK. We also show that Aire+ mTECs harbor a previously unappreciated potential to reduce Aire and MHCII expression and migrate towards the center of the thymic medulla while maintaining TSA expression. In addition to its role in the thymus, Aire expression has also recently been identified in population of extrathymic Aire-expressing cells (eTACs) residing in the peripheral lymphoid tissues. We address the ability of eTACs to contribute to CD4+ T cell tolerance, and find that eTACs are capable of inducing anergy of autoreactive T through a signaling pathway that depends in part on low expression of costimulatory CD80 and CD86 molecules by eTACs. Finally, we identify eTACs as a non-conventional component of the classical dendritic cell lineage, suggesting that they can be efficiently expanded for future functional studies and potentially for therapeutic induction of tolerance.

Purpose of review

Recent findings

Summary