Aire controls the recirculation of murine Foxp3⁺ regulatory T‐cells back to the thymus

Abstract

In the thymus, medullary thymic epithelial cells (mTEC) determine the fate of newly selected CD4+ and CD8+ single positive (SP) thymocytes. For example, mTEC expression of Aire controls intrathymic self‐antigen availability for negative selection. Interestingly, alterations in both Foxp3+ Regulatory T‐cells (T‐Reg) and conventional SP thymocytes in Aire−/− mice suggest additional, yet poorly understood, roles for Aire during intrathymic T‐cell development. To examine this, we analysed thymocytes from Aire −/− mice using Rag2GFP and Foxp3 expression, and a recently described CD69/MHCI subset definition of post‐selection CD4+ conventional thymocytes. We show that while Aire is dispensable for de novo generation of conventional αβT‐cells, it plays a key role in controlling the intrathymic T‐Reg pool. Surprisingly, a decline in intrathymic T‐Reg in Aire−/− mice maps to a reduction in mature recirculating Rag2GFP− T‐Reg that express CCR6 and re‐enter the thymus from the periphery. Furthermore, we show mTEC expression of the CCR6 ligand CCL20 is reduced in Aire−/− mice, and that CCR6 is required for T‐Reg recirculation back to the thymus. Collectively, our study re‐defines requirements for late stage intrathymic αβT‐cell development, and demonstrates that Aire controls a CCR6‐CCL20 axis that determines the developmental makeup of the intrathymic T‐Reg pool.

Rethinking Thymic Tolerance: Lessons from Mice

“…Although earlier studies indicated that CXCR4 plays a role in the recruitment of peripheral Tregs to the thymus [53], analysis of Cd4 cre /Cxcr4 floxed mice showed no alterations in the intrathymic Treg pool [56]. Indeed, recent studies have shown that, in Ccr6 À/À Rag2GFP mice, thymic recirculating Treg numbers are reduced relative to wild-type controls, suggesting that CCR6 is an important regulator of peripheral Treg recruitment to the thymus [57]. Moreover, the medulla may be a key regulator of Treg recirculation, at least for CCR6-mediated thymic recirculation, because the CCR6 ligand CCL20 is detectable in mTECs.…”

“…Moreover, the medulla may be a key regulator of Treg recirculation, at least for CCR6-mediated thymic recirculation, because the CCR6 ligand CCL20 is detectable in mTECs. Thus, mTECs may generate a CCL20 chemokine gradient that recruits peripheral Tregs to the thymus and allows recirculating Tregs to specifically localize to the medulla [57]. In addition, CCL20 expression in mTEC from Aire À/À mice is reduced relative to wild-type mice, and Aire À/À Rag2GFP mice also exhibit reduced levels of Rag2GFP À recirculating Tregs (relative to wild-type controls); this supports a role for Aire in Treg recirculation to the thymus through mTEC secretion of CCL20 ( Figure 2) [57].…”

“…Thus, mTECs may generate a CCL20 chemokine gradient that recruits peripheral Tregs to the thymus and allows recirculating Tregs to specifically localize to the medulla [57]. In addition, CCL20 expression in mTEC from Aire À/À mice is reduced relative to wild-type mice, and Aire À/À Rag2GFP mice also exhibit reduced levels of Rag2GFP À recirculating Tregs (relative to wild-type controls); this supports a role for Aire in Treg recirculation to the thymus through mTEC secretion of CCL20 ( Figure 2) [57]. That Aire is involved in the regulation of a chemokine that can influence Treg migration in the thymus draws parallels with its role in controlling XCL1-mediated positioning of thymic DCs [39].…”

“…However, other findings might argue against this possibility. For example, in Aire À/À mice where recirculating thymic Tregs are reduced relative to wild-type mice, presumably less IL-2 competition would ensue because de novo Tregs would have fewer competitors for the cytokine, and de novo Treg numbers would increase; however, in Aire À/À mice, de novo Treg numbers were not increased [57]. Conversely, osteoprotegerin (OPG)-deficient mice exhibit increased thymic Treg recirculation relative to wild-type, which potentially provides more IL-2 competition; however, they do not show decreased de novo Treg development compared to wildtype control mice [59].…”

<p>Cancer Immunotherapies Targeting Tumor-Associated Regulatory T Cells</p>

“…Additionally, high CCR6 expression in tumor-associated Tregs is positively associated with tumor progression, including in patients with laryngeal squamous cell carcinoma, hepatocellular carcinoma and breast cancer. [55][56][57][58] Besides, CCL20 (a CCR6 ligand) has been detected in medullary thymic epithelial cells (mTECs) 59 and diverse cancer stem cells (CSCs) including cells from pancreatic, colorectal, gastric, lung, breast and head and neck cancer. [60][61][62] In the preclinical trial for advanced cutaneous T-cell lymphoma, knockdown of CCR6 by micro RNA-150 (miR-150) led to a distinct decrease in tumor metastasis and invasion.…”