NOD2 Acts As a Master Regulator of Endogenous Thymic Regeneration
Authors: Kinsella Sinéad , Cooper Kirsten , DeRoos Paul , Jain Reema , Iovino Lorenzo , Dudakov Jarrod A 
Published: October 2019

Abstract
Endogenous thymic regeneration is a crucial function that allows for renewal of immune competence following immunodepletion caused by common cancer therapies such as cytoreductive chemotherapy or radiation; however, the mechanisms governing this regeneration remain poorly understood. Despite this capacity, prolonged T cell deficiency is a major clinical hurdle in recipients of hematopoietic stem cell transplantation (HSCT) and can precipitate high morbidity and mortality from opportunistic infections, and may even facilitate malignant relapse. Our recent studies have revealed that innate lymphoid cells (ILCs) and endothelial cells (ECs), through their production of the regeneration-associated factors (RAFs) IL-22 and BMP4, respectively, have profound reparative effects in the thymus after acute injury; and can be utilized individually as therapeutic strategies of immune regeneration (Dudakov 2012 Science 336:91; Dudakov 2017 Blood 130:933; Wertheimer 2018 Sci Immunol 3:19). These two pathways act by stimulating thymic epithelial cells (TECs), a heterogeneous population of stromal cell in the thymus critical for thymopoiesis. However, the regulation of these endogenous regenerative responses is still poorly understood. Here we reveal an unexpected role for the pattern recognition receptor Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) in governing multiple pathways of thymic regeneration.
Analysis of thymic recovery following acute injury in mice deficient for NOD2 revealed increased intrathymic BMP4 and IL-23 (Fig. 1a), a key regulator of IL-22 production, and commensurate improved ability to regenerate (Fig. 1b). Although NOD2 is expressed ubiquitously across all populations within the thymus, in regeneration-initiating DCs and ECs, but not thymocytes (damage-targets), we identified a specific reduction in the expression of miR29c after damage, which previous reports suggest could mediate NOD2-induced suppression of IL-23 (Brain 2013 Immunity 39:521). Consistent with these findings, miR29c expression was decreased in the thymus of NOD2-deficient mice (Fig. 1d), and overexpression of miR29c in either ECs or DCs reduced their expression of Bmp4 or Il23, respectively (Fig. 1e). Canonical ligands for NOD2 are peptidoglycans found in the cell wall of bacteria; however, these are unlikely to serve as a NOD2 activator in the thymus since it is typically thought of as a sterile organ. One recently described alternate function of NOD2 is as a cytosolic sensor of activated Rho GTPases. The Rho GTPase family is responsible for a wide range of physiological processes, including the intrathymic regulation of b-selection and positive selection, and inhibition of Rho GTPase signaling is of considerable clinical interest. Consistent with a role in suppressing regeneration, unbiased transcriptome analysis revealed significant downregulation of many members of the RhoGTPase family after damage, corresponding to the increase in production of RAFs. Importantly, suggestive of a potential clinical application, pharmacological suppression of RhoGTPase in vitro significantly induced the expression of Bmp4 in ECs, and Il23 in DCs (Fig. 1f), as well as suppressing the expression of miR29c (Fig. 1g).
Although several pathways have been described as contributing to endogenous thymic regeneration, the specific mechanisms regulating their induction has been poorly understood. Here we reveal a common mechanism triggering production of multiple distinct regeneration pathways such as those centered on production of BMP4 and IL-22. Therefore, the mechanistic and pre-clinical studies described not only define an important regulatory mechanism governing endogenous tissue regeneration, but could also offer an innovative therapeutic strategy to boost thymic function and T cell reconstitution in recipients of allo-HSCT, as well as for individuals with T cell deficiencies due to aging, autoimmune diseases, genetic causes, infectious disease, shock, radiation injury (nuclear accident, terrorism) and common cancer treatments such as chemo- and radiation-therapy.
Figure 1. Figure 1.

Disclosures
No relevant conflicts of interest to declare.