LAUSR

Given the current interest in the design of new T cell-driven immunotherapy, it is crucial to understand the principles that control T-cell development. As the generative organ of functionally diverse and self-tolerant T cells, the thymus is central for the establishment of immunity against cancer and pathogens and prevention of autoimmunity. Within the thymus, cortical (c) and medullary (m) thymic epithelial cells (TECs) play an essential role in T-cell development. While cTECs mediate T-cell lineage commitment and positive selection, mTECs regulate negative selection and T-regulatory cell differentiation. The capacity of mTECs to imposed self-tolerance is conferred by their capacity to promiscuously expressed tissue-restricted antigens, a process that is in part regulated by Aire and Fezf2. As a result, defects in TECs can lead to immunodeficiency or autoimmunity [1]. Thus, the identification of new regulators of TEC biology is fundamental to understand the foundations of immunity and develop new therapeutics to correct dysfunctional T-cell responses. In this issue of Cell Death and Differentiation, Jain et al. demonstrate that LUBAC (Linear Ubiquitin Chain Assembly Complex) is required for the maintenance of TEC microenvironments, and hence to sustain thymic function [2]. LUBAC is a heterotrimeric E3 ligase complex composed of HOIL-1, HOIP and SHARPIN. The complex is capable of linear ubiquitination members of the canonical nuclear factor kappa B (NF-κB) pathway, thereby regulating TNF receptor (TNFR) signaling [3]. Given the well-established role of NF-κB pathway in TEC differentiation, in particularly in mTECs [4], it is logical to interrogate the individual regulatory role of the distinct components of LUBAC in TEC homeostasis. To do so, the authors employed either mice with TEC-specific deletion of Hoil-1 and Hoip genes (HOIL-1 and HOIP cKO) or mice with a spontaneous germline mutation in the Sharpin gene (SHARPIN). They started by determining the TECintrinsic role for LUBAC in thymopoisesis in adult mice. Their first important finding was that deficiency in HOIL-1 and HOIP led to a severe thymic atrophy, which extended to a reduction in the number naive T cells [2]. The peripheral T-cell lymphopenia in cKO mice highlights the requirements of a regular thymic function to maintain a normal T-cell homeostasis [5]. Future studies may consider to examine the TCR repertoire of T cells that arise from mutant thymus and their capacity to mount protective immune responses. The severe phenotypes caused by HOIL-1 and HOIP-deficiency contrasted with a moderate impact in SHARPIN mice. Albeit the confounding effects of broad SHARPIN-deficiency in SHARPIN mice preclude the assessment of the specific function of SHARPIN in TECs [3], this study indicates that HOIL-1 and HOIP are essential in TECs to sustain thymic function in young and adult mice. The differentiation of cTECs and mTECs is a dynamic process that starts during early embryogenesis and continues throughout life, with the prototypical cortical-medullary compartmentalization achieved in the adulthood [6]. Further analysis on the cellautonomous role of HOIL-1 and HOIP in TEC differentiation in the corresponding cKO revealed a marked reduction in the number of mTECs, including mTEC, mTEC and Aire cells. The loss in mTECs appeared in the embryo and perpetuated during postnatal life. Yet, the aforementioned mTEC subsets developed in the absence of HOIL-1 and HOIP [2], suggesting that LUBAC guides the expansion and maintenance of mTEC subsets, but is dispensable for their differentiation. Although the establishment of the murine mTEC compartment is initiated in the embryo, it must be maintained through life. Particularly, mature AiremTECs turnover approximately every 7–10 days [7], implicating a regular replacement by their upstream precursors. Recent studies revealed a plethora of new dedicated mTEC precursors and how they contribute to the maintenance of mTEC niches [7]. A future in-depth analysis on the generation and maintenance of mTEC precursors in cKO mice may inform on specific development checkpoints controlled by HOIL-1 and HOIP. In contrast to mTECs, cTECs become mostly affected only in the adult thymus. As a result, the cortical-medullary regions of HOIL-1 and HOIP cKO thymi were severely disrupted. Together, their findings suggest that HOIL-1 and HOIP are relevant for mTEC survival and growth throughout life and cTEC maintenance in the adult period. Given that the deficiency in HOIL-1 phenocopies the thymic failures caused by the loss of HOIP, the authors focused on the first to examine the underlining molecular basis for the role of LUBAC in TECs. Transcriptional analysis of TEC subsets at 2 weeks of age, a time point that precedes the development of a severe thymic phenotype, revealed thousands of differentially expressed genes (DEG) in HOIL-1-deficient cTEC (~3000) and mTEC (~5700). Gene ontology analysis pointed to changes in cell adhesion, projection and morphology in cKO cTECs, and alterations in cell cycle and metabolism in mTEC counterparts [2]. These findings support that LUBAC-dependent signaling control a broad transcriptional program in TECs associated to basic processes in cell biology. It remains undetermined whether DEG include tissue-restricted antigen and direct or indirect NF-κB targets. These future analyses would enable to evaluate the extend of abnormal NF-κB activation in cKO TECs. Additionally, further studies may clarify whether the structural and transcriptional changes found in cKO cTECs are cellautonomous, and directly induced by HOIL-1and HOIP-deficiency. Alternatively, mTEC alterations might perturb the organization and