While our groups and others have generated evidence suggesting that extrathymic AIRE and lymphoid stroma may play a role in ectopic TSA expression and immune tolerance, our respective approaches have thus far identified distinct cell populations. Lee et al.
] characterized a population of UEA1+
MHC class II+
LNSCs which ectopically expressed transgenic antigen, resulting in deletion of cognate T cells. In the same study, LNSCs were shown to ectopically express a number of TSAs relevant to autoimmunity, including those from the eye, thyroid, pancreas and central nervous system [50
]. These cells, which upregulated PD-L1 to prevent CD8 T cell activation during chronic viral infection [56
], are presumably part of the lymph node fibroblastic reticular cell (FRC) network. Far from a static scaffold, LN FRC form a matchmaking network for immune responses, expressing chemokines to attract and guide T cells and DC along processes rich in extracellular matrix, to meet in the paracortex [57
]. They also deliver cytokines, chemokines and soluble antigen from lymph along specialized conduits to the rest of the organ [59
]. Importantly, presentation of antigen via MHC on stromal cells in these studies was tolerogenic without contribution from dendritic cells. The mechanisms involved are under investigation, but certainly a blockade of PD-L1 rescued autoreactive CD8+
T cells, which became activated and caused disease [60
By comparison, a recent study by Gardner et al.
] using an Aire
reporter construct showed expression of both Aire
transcript and nuclear AIRE protein in a unique population of tolerogenic stromal cells, termed eTACs (extra-thymic Aire
-expressing cells). These cells, which express the epithelial marker EpCAM, are UEA1- and gp38-negative and phenotypically distinct from FRC populations. While expressing some markers reminiscent of mTECs (MHC class II, EpCAM), these cells appear to lack expression of canonical costimulatory molecules CD80 and CD86. This study also described weak expression of the Aire
reporter in a subset of CD11c+ dendritic cells, though AIRE protein was not specifically detected in this population, and deletional tolerance of cognate T cells did not depend on it. As in the thymus, comparison of eTACs from WT and Aire
-knockout mice demonstrated that Aire
regulates a set of TSAs that includes several important autoantigens, as well as genes important in antigen processing and presentation, suggesting that Aire
expression has broad transcriptional consequences for TSA expression in the periphery. Surprisingly, the genes regulated by AIRE in eTACs had virtually no overlap with AIRE-regulated genes in the thymus, suggesting a complementary role in the maintenance of self-tolerance (). In addition, this lack of overlap reinforces the idea that AIRE-regulation of transcription is complex and may vary between cell types because of changes in the array of transcriptional or epigenetic factors that differ between the two cell populations.
The relationship between these populations, and the physiologic relevance of extrathymic AIRE in a non-transgenic context, remain to be defined. The limited evidence available offers only a few clues. First, the immunologically relevant role of AIRE appears to be restricted to radioresistant populations [2
]. Second, the fact that transplantation of Aire
-deficient thymic stroma into Nude (Foxn1
knockout) hosts is sufficient to induce Aire
-like autoimmunity [2
] suggests that peripheral Aire
expression may not be sufficient to compensate for loss of thymic Aire
. This is neither surprising nor inconsistent with the idea that central and peripheral AIRE may play unique and complementary roles, and that multiple overlapping systems are required to maintain normal self-tolerance. Furthermore, because we know nothing about the Foxn1
-dependence of eTAC development, it is not clear whether the Nude mouse presents the best system for investigating these issues. Careful comparison of the relative spectra of autoimmune disease observed upon loss of central or peripheral Aire
, particularly in autoimmune-prone strains, remains to be done. Such experiments are eagerly anticipated, though the proper experimental approach to test this has been challenging, moreso given the recent report by Guerau-de-Arellano and colleagues that the critical time for Aire
expression may be early in perinatal development [61
The study by Guerau-de-Arellano et al.
also comments on Aire
mRNA expression in secondary lymphoid stroma and its relevance [61
]. Using a Tetracycline-responsive transgenic system designed to allow temporal control of Aire
expression in an otherwise Aire
-deficient mouse, the authors reported detecting transgene expression in lymph nodes and spleen, but argued that the absence of Tetracycline-responsiveness of this transgenic Aire
in lymph nodes, despite the observed autoimmunity in these mice, suggests that peripheral Aire
expression is not relevant to disease progression. However, factors such as overexpression of the transgene, its imprecise tissue-specificity (which, for example, is also expressed in cortical epithelium), and the fact that splenic Aire
levels do respond to Tetracycline administration, all cast doubt on the relevance of these conclusions to endogenous peripheral Aire
. Ultimately, peripheral Aire
must be assessed using experiments appropriate to the task.