Single-cell multiomics defines tolerogenic extrathymic Aire-expressing populations with unique homology to thymic epithelium

The Autoimmune Regulator ( Aire ) gene, well defined for its role in medullary thymic epithelial cells (mTECs) and immune self-tolerance, is also expressed in extrathymic Aire -expressing cells (eTACs) in the secondary lymphoid organs. eTACs have been shown to be hematopoietic antigen presenting cells (APCs) and potent inducers of immune tolerance ([1][1]–[3][2]). However, the precise identity and function of these cells remain unclear. Here, we use high-dimensional single-cell multiomics and functional approaches to define eTACs at the transcriptional, genomic, and proteomic level. We find that eTACs consist of two similar cell types: CCR7+ Aire-expressing migratory dendritic cells (AmDCs) and a unique Aire-hi population co-expressing Aire and RAR-related orphan receptor gamma-t (RORγt). The latter, which have significant transcriptional and genomic homology to migratory dendritic cells (migDCs) and mTECs, we term Janus cells (JCs). All eTACs, and JCs in particular, have a highly accessible chromatin structure and high levels of broad gene expression, including tissue-specific antigens, as well as remarkable transcriptional and genomic homology to thymic medullary epithelium. As in the thymus, Aire expression in eTACs is also dependent on RANK-RANK-ligand interactions. Furthermore, lineage-tracing shows that JCs are not precursors to the majority of AmDCs. Finally, self-antigen expression by eTACs is sufficient to mediate negative selection of T cells escaping thymic selection and can prevent autoimmune diabetes in non-obese diabetic mice. This transcriptional, genomic, and functional symmetry between a hematopoietic Aire-expressing population in the periphery and an epithelial Aire-expressing population in the thymus suggests that a core biological program may influence self-tolerance and self-representation across the spectrum of immune development. ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-1 [2]: #ref-3