Inactivation of the RB family prevents thymus involution and promotes thymic function by direct control of Foxn1 expression

Mechanisms underlying the direct programming of mouse embryonic fibroblasts to thymic epithelial cells by FOXN1

Thymic epithelial cells (TECs) are crucial to the ability of the thymus to generate T cells for the adaptive immune system in vertebrates. However, no in vitro system for studying TEC function exists. Overexpressing the transcription factor FOXN1 initiates transdifferentiation of fibroblasts into TEC-like cells (iTECs) that support T-cell differentiation in culture or after transplant. In this study, we have characterized iTEC programming at the cellular and molecular level in mouse to determine how it proceeds, and have identified mechanisms that can be targeted for improving this process. These data show that iTEC programming consists of discrete gene expression changes that differ early and late in the process, and that iTECs upregulate markers of both cortical and medullary TEC (cTEC and mTEC) lineages. We demonstrate that promoting proliferation enhances iTEC generation, and that Notch inhibition allows the induction of mTEC differentiation. Finally, we show that MHCII expression is the major difference between iTECs and fetal TECs. MHCII expression was improved by co-culturing iTECs with fetal double-positive T-cells. This study supports future efforts to improve iTEC generation for both research and translational uses.

Recombinant FOXN1 fusion protein increases T cell generation in old mice

T cell development in the thymus is dependent on the thymic microenvironment, in which thymic epithelial cells (TECs) are the major component. However, TECs undergo both a qualitative and quantitative loss during aging, which is believed to be the major factor responsible for age-dependent thymic atrophy. FOXN1 plays a critical role in TEC development and adult TECs maintenance. We have previously reported that intrathymic injection of a recombinant (r) protein containing murine FOXN1 and a protein transduction domain increases the number of TECs in mice, leading to enhanced thymopoiesis. However, intrathymic injection may not be an ideal choice for clinical applications. In this study, we produced a rFOXN1 fusion protein containing the N-terminal of CCR9, human FOXN1 and a protein transduction domain. When injected intravenously into 14-month-old mice, the rFOXN1 fusion protein enters the thymus and TECs, and enhances thymopoiesis, resulting in increased T cell generation in the thymus and increased number of T cells in peripheral lymphoid organ. Our results suggest that the rFOXN1 fusion protein has the potential to be used in preventing and treating T cell immunodeficiency in older adults.

FOXN Transcription Factors: Regulation and Significant Role in Cancer

A growing number of studies have demonstrated that cancer development is closely linked to abnormal gene expression, including alterations in the transcriptional activity of transcription factors. The Forkhead box class N (FOXN) proteins FOXN1-6 form a highly conserved class of transcription factors, which have been shown in recent years to be involved in the regulation of malignant progression in a variety of cancers. FOXNs mediate cell proliferation, cell-cycle progression, cell differentiation, metabolic homeostasis, embryonic development, DNA damage repair, tumor angiogenesis, and other critical biological processes. Therefore, transcriptional dysregulation of FOXNs can directly affect cellular physiology and promote cancer development. Numerous studies have demonstrated that the transcriptional activity of FOXNs is regulated by protein-protein interactions, microRNAs (miRNA), and posttranslational modifications (PTM). However, the mechanisms underlying the molecular regulation of FOXNs in cancer development are unclear. Here, we reviewed the molecular regulatory mechanisms of FOXNs expression and activity, their role in the malignant progression of tumors, and their value for clinical applications in cancer therapy. This review may help design experimental studies involving FOXN transcription factors, and enhance their therapeutic potential as antitumor targets.

The initial age-associated decline in early T-cell progenitors reflects fewer pre-thymic progenitors and altered signals in the bone marrow and thymus microenvironments

Age-related thymus involution results in decreased T-cell production, contributing to increased susceptibility to pathogens and reduced vaccine responsiveness. Elucidating mechanisms underlying thymus involution will inform strategies to restore thymopoiesis with age. The thymus is colonized by circulating bone marrow (BM)-derived thymus seeding progenitors (TSPs) that differentiate into early T-cell progenitors (ETPs). We find that ETP cellularity declines as early as 3 months (3MO) of age in mice. This initial ETP reduction could reflect changes in thymic stromal niches and/or pre-thymic progenitors. Using a multicongenic progenitor transfer approach, we demonstrate that the number of functional TSP/ETP niches does not diminish with age. Instead, the number of pre-thymic lymphoid progenitors in the BM and blood is substantially reduced by 3MO, although their intrinsic ability to seed and differentiate in the thymus is maintained. Additionally, Notch signaling in BM lymphoid progenitors and in ETPs diminishes by 3MO, suggesting reduced niche quality in the BM and thymus contribute to the early decline in ETPs. Together, these findings indicate that diminished BM lymphopoiesis and thymic stromal support contribute to an initial reduction in ETPs in young adulthood, setting the stage for progressive age-associated thymus involution.

Monitoring the Cell Cycle of Tumor Cells in Mouse Models of Human Cancer

Cell division is obligatory to tumor growth. However, both cancer cells and noncancer cells in tumors can be found in distinct stages of the cell cycle, which may inform the growth potential of these tumors, their propensity to metastasize, and their response to therapy. Hence, it is of utmost importance to monitor the cell cycle of tumor cells. Here we discuss well-established methods and new genetic advances to track the cell cycle of tumor cells in mouse models of human cancer. We also review recent genetic studies investigating the role of the cell-cycle machinery in the growth of tumors in vivo, with a focus on the machinery regulating the G1/S transition of the cell cycle.

Recombinant FOXN1 fusion protein increases T cell generation in aged mice

Background

Although the thymus continues to export T cells throughout life, it undergoes a profound involution/atrophy with age, resulting in decreased numbers of T cells in the older adult, which has direct etiological linkages with many diseases. T cell development in the thymus is dependent on the thymic microenvironment, in which thymic epithelial cells (TECs) are the major component. However, TECs undergo both a qualitative and quantitative loss during aging, which is believed to be the major factor responsible for age-dependent thymic atrophy. FOXN1 plays a critical role in TEC development and adult TECs maintenance. We have previously reported that intrathymic injection of a recombinant (r) protein containing FOXN1 and a protein transduction domain increases the number of TECs in mice, leading to enhanced thymopoiesis. However, intrathymic injection may not be an ideal choice for clinical applications. In this study, we produce a rFOXN1 fusion protein containing the N-terminal of CCR9, FOXN1 and a protein transduction domain.

Results

We show here that, when injected intravenously into aged mice, the rFOXN1 fusion protein migrates into the thymus and enhances thymopoiesis, resulting in increased T cell generation in the thymus and increased number of T cells in peripheral lymphoid organ.

Conclusions

Our results suggest that the rFOXN1 fusion protein has the potential to be used in preventing and treating T cell immunodeficiency in the older adult.

Effects of PAMK on lncRNA, miRNA, and mRNA expression profiles of thymic epithelial cells

Polysaccharides from Atractylodes macrocephala Koidz (PAMK) can promote the proliferation of thymocytes and improve the body's immunity. However, the effect of PAMK on thymic epithelial cells has not been reported. Studies have shown that miRNAs and lncRNAs are key factors in regulating cell proliferation. In this study, we found that PAMK could promote the proliferation of mouse medullary thymic epithelial cell line 1 (MTEC1) cells through CCK-8 and EdU experiments. To further explore its mechanism, we detected the effect of PAMK on the expression profiles of lncRNAs, miRNAs, and mRNAs in MTEC1 cells. The results showed that PAMK significantly affected the expression of 225 lncRNAs, 29 miRNAs, and 800 mRNAs. Functional analysis showed that these differentially expressed genes were significantly enriched in cell cycle, cell division, NF-kappaB signaling, apoptotic process, and MAPK signaling pathway. Finally, we used Cytoscape to visualize lncRNA-miRNA-mRNA(14 lncRNAs, 17 miRNAs, 171 mRNAs) networks based on ceRNA theory. These results suggest that lncRNAs and miRNAs may be involved in the effect of PAMK on the proliferation of MTEC1 cells, providing a new research direction for exploring the molecular mechanism of PAMK promoting the proliferation of thymic epithelial cells.

Transcriptional regulation of the thymus master regulator Foxn1

FOXN1 is a transcription factor critical for the development of both thymic epithelial cell (TEC) and hair follicle cell (HFC) compartments. However, mechanisms controlling its expression remain poorly understood. To address this question, we performed thorough analyses of the evolutionary conservation and chromatin status of the Foxn1 locus in different tissues and states and identified several putative cis-regulatory regions unique to TECs versus HFCs. Furthermore, experiments using genetically modified mice with specific deletions in the Foxn1 locus and additional bioinformatic analyses helped us identify key regions and transcription factors involved in either positive or negative regulation of Foxn1 in both TECs and HFCs. Specifically, we identified SIX1 and FOXN1 itself as key factors inducing Foxn1 expression in embryonic and neonatal TECs. Together, our data provide important mechanistic insights into the transcriptional regulation of the Foxn1 gene in TEC versus HFC and highlight the role of FOXN1 in its autoregulation.

Key Factors for Thymic Function and Development

The thymus is the organ responsible for T cell development and the formation of the adaptive immunity function. Its multicellular environment consists mainly of the different stromal cells and maturing T lymphocytes. Thymus-specific progenitors of epithelial, mesenchymal, and lymphoid cells with stem cell properties represent only minor populations. The thymic stromal structure predominantly determines the function of the thymus. The stromal components, mostly epithelial and mesenchymal cells, form this specialized area. They support the consistent developmental program of functionally distinct conventional T cell subpopulations. These include the MHC restricted single positive CD4+ CD8- and CD4- CD8+ cells, regulatory T lymphocytes (Foxp3+), innate natural killer T cells (iNKT), and γδT cells. Several physiological causes comprising stress and aging and medical treatments such as thymectomy and chemo/radiotherapy can harm the thymus function. The present review summarizes our knowledge of the development and function of the thymus with a focus on thymic epithelial cells as well as other stromal components and the signaling and transcriptional pathways underlying the thymic cell interaction. These critical thymus components are significant for T cell differentiation and restoring the thymic function after damage to reach the therapeutic benefits.

Role of thymus in health and disease

The thymus is a primary lymphoid organ, essential for the development of T-cells that will protect from invading pathogens, immune disorders, and cancer. The thymus decreases in size and cellularity with age referred to as thymus involution or atrophy. This involution causes decreased T-cell development and decreased naive T-cell emigration to the periphery, increased proportion of memory T cells, and a restricted, altered T-cell receptor (TCR) repertoire. The changes in composition and function of the circulating T cell pool as a result of thymic involution led to increased susceptibility to infectious diseases including the recent COVID and a higher risk for autoimmune disorders and cancers. Thymic involution consisting of both structural and functional loss of the thymus has a deleterious effect on T cell development, T cell selection, and tolerance. The mechanisms which act on the structural (cortex and medulla) matrix of the thymus, the gradual accumulation of genetic mutations, and altered gene expressions may lead to immunosenescence as a result of thymus involution. Understanding the molecular mechanisms behind thymic involution is critical for identifying diagnostic biomarkers and targets for treatment help to develop strategies to mitigate thymic involution-associated complications. This review is focused on the consequences of thymic involution in infections, immune disorders, and diseases, identifying potential checkpoints and potential approaches to sustain or restore the function of the thymus particularly in elderly and immune-compromised individuals.

miR-152-3p Represses the Proliferation of the Thymic Epithelial Cells by Targeting Smad2

MicroRNAs (miRNAs) control the proliferation of thymic epithelial cells (TECs) for thymic involution. Previous studies have shown that expression levels of miR-152-3p were significantly increased in the thymus and TECs during the involution of the mouse thymus. However, the possible function and potential molecular mechanism of miR-152-3p remains unclear. This study identified that the overexpression of miR-152-3p can inhibit, while the inhibition of miR-152-3p can promote, the proliferation of murine medullary thymic epithelial cell line 1 (MTEC1) cells. Moreover, miR-152-3p expression was quantitatively analyzed to negatively regulate Smad2, and the Smad2 gene was found to be a direct target of miR-152-3p, using the luciferase reporter assay. Importantly, silencing Smad2 was found to block the G1 phase of cells and inhibit the cell cycle, which was consistent with the overexpression of miR-152-3p. Furthermore, co-transfection studies of siRNA–Smad2 (siSmad2) and the miR-152-3p mimic further established that miR-152-3p inhibited the proliferation of MTEC1 cells by targeting Smad2 and reducing the expression of Smad2. Taken together, this study proved miR-152-3p to be an important molecule that regulates the proliferation of TECs and therefore provides a new reference for delaying thymus involution and thymus regeneration.

Redox regulation of age-associated defects in generation and maintenance of T cell self-tolerance and immunity to foreign antigens

Thymic atrophy reduces naive T cell production and contributes to increased susceptibility to viral infection with age. Expression of tissue-restricted antigen (TRA) genes also declines with age and has been thought to increase autoimmune disease susceptibility. We find that diminished expression of a model TRA gene in aged thymic stromal cells correlates with impaired clonal deletion of cognate T cells recognizing an autoantigen involved in atherosclerosis. Clonal deletion in the polyclonal thymocyte population is also perturbed. Distinct age-associated defects in the generation of antigen-specific T cells include a conspicuous decline in generation of T cells recognizing an immunodominant influenza epitope. Increased catalase activity delays thymic atrophy, and here, we show that it mitigates declining production of influenza-specific T cells and their frequency in lung after infection, but does not reverse declines in TRA expression or efficient negative selection. These results reveal important considerations for strategies to restore thymic function.

Transcriptional and epigenetic regulation in thymic epithelial cells

The thymus is required for the development of both adaptive and innate-like T cell subsets. There is keen interest in manipulating thymic function for therapeutic purposes in circumstances of autoimmunity, immunodeficiency, and for purposes of immunotherapy. Within the thymus, thymic epithelial cells play essential roles in directing T cell development. Several transcription factors are known to be essential for thymic epithelial cell development and function, and a few transcription factors have been studied in considerable detail. However, the role of many other transcription factors is less well understood. Further, it is likely that roles exist for other transcription factors not yet known to be important in thymic epithelial cells. Recent progress in understanding of thymic epithelial cell heterogeneity has provided some new insight into transcriptional requirements in subtypes of thymic epithelial cells. However, it is unknown whether progenitors of thymic epithelial cells exist in the adult thymus, and consequently, developmental relationships linking putative precursors with differentiated cell types are poorly understood. While we do not presently possess a clear understanding of stage-specific requirements for transcription factors in thymic epithelial cells, new single-cell transcriptomic and epigenomic technologies should enable rapid progress in this field. Here, we review our current knowledge of transcription factors involved in the development, maintenance, and function of thymic epithelial cells, and the mechanisms by which they act.

Age-Related Changes in Thymic Central Tolerance

Thymic epithelial cells (TECs) and hematopoietic antigen presenting cells (HAPCs) in the thymus microenvironment provide essential signals to self-reactive thymocytes that induce either negative selection or generation of regulatory T cells (Treg), both of which are required to establish and maintain central tolerance throughout life. HAPCs and TECs are comprised of multiple subsets that play distinct and overlapping roles in central tolerance. Changes that occur in the composition and function of TEC and HAPC subsets across the lifespan have potential consequences for central tolerance. In keeping with this possibility, there are age-associated changes in the cellular composition and function of T cells and Treg. This review summarizes changes in T cell and Treg function during the perinatal to adult transition and in the course of normal aging, and relates these changes to age-associated alterations in thymic HAPC and TEC subsets.

Germline genetic contribution to the immune landscape of cancer

Understanding the contribution of the host's genetic background to cancer immunity may lead to improved stratification for immunotherapy and to the identification of novel therapeutic targets. We investigated the effect of common and rare germline variants on 139 well-defined immune traits in ∼9000 cancer patients enrolled in TCGA. High heritability was observed for estimates of NK cell and T cell subset infiltration and for interferon signaling. Common variants of IFIH1, TMEM173 (STING1), and TMEM108 were associated with differential interferon signaling and variants mapping to RBL1 correlated with T cell subset abundance. Pathogenic or likely pathogenic variants in BRCA1 and in genes involved in telomere stabilization and Wnt-β-catenin also acted as immune modulators. Our findings provide evidence for the impact of germline genetics on the composition and functional orientation of the tumor immune microenvironment. The curated datasets, variants, and genes identified provide a resource toward further understanding of tumor-immune interactions.

miR-199b-5p enhances the proliferation of medullary thymic epithelial cells via regulating Wnt signaling by targeting Fzd6

Thymic epithelial cells (TECs) are essential regulators of T-cell development and selection. miRNAs play critical roles in regulating TEC proliferation during the process of thymic aging. Our previous studies revealed that miR-199b-5p was upregulated in TECs from 1- to 3-month-old mice. But its function and potential mechanism are not clear. We hypothesized that miR-199b-5p may play an important role in age-related thymus involution via targeting some genes. To confirm it, the murine thymic epithelial cell line 1 (MTEC1) cells were used. Our results showed that overexpression of miR-199b-5p can enhance MTEC1 cell proliferation. On the contrary, repression of miR-199b-5p can inhibit MTEC1 cell proliferation. Meanwhile, it was confirmed that frizzled receptor 6 (Fzd6) is the direct target gene of miR-199b-5p. Furthermore, overexpression of miR-199b-5p can upregulate the expressions of β-catenin, Tcf7, Wnt4, and C-myc to activate Wnt signaling and cell cycle signaling. Silence of Fzd6 and co-transfection with siFzd6 and miR-199b-5p mimic/inhibitor confirmed that the biological function of miR-199b-5p is indeed by targeting Fzd6 in medullary TECs. Overall, miR-199b-5p is an important regulator in medullary TEC proliferation through targeting Fzd6 to activate Wnt signaling and cell cycle signaling. Our data indicate that miR-199b-5p may block the process of thymic aging and be a potential therapeutic target for thymus involution.

Postnatal Involution and Counter-Involution of the Thymus

Thymus involution occurs in all vertebrates. It is thought to impact on immune responses in the aged, and in other clinical circumstances such as bone marrow transplantation. Determinants of thymus growth and size are beginning to be identified. Ectopic expression of factors like cyclin D1 and Myc in thymic epithelial cells (TEC)s results in considerable increase in thymus size. These models provide useful experimental tools that allow thymus function to be understood. In future, understanding TEC-specific controllers of growth will provide new approaches to thymus regeneration.

miR-205-5p inhibits thymic epithelial cell proliferation via FA2H-TFAP2A feedback regulation in age-associated thymus involution

Thymic epithelial cells (TECs) are essential regulators of T cell development and selection. microRNAs (miRNAs) play critical roles in regulating TECs proliferation during thymus involution. miR-205-5p is highly expressed in TECs and increases with age. However, the function and potential mechanism of miR-205-5p in TECs are not clear. miRNA expression was profiled using TECs from male and female mice at 1 and 3 months old. A total of 325 differentially expressed miRNAs (DEMs) were detected at different ages in two sexes. 24 of the DEMs had the same trend between males and females. Among them, miR-205-5p had the highest fold change. Our results showed that the expression of miR-205-5p was dramatically increased in TECs from 1 to 9 months old mice. miR-205-5p mimic inhibited TECs proliferation. Moreover, we confirmed that Fa2h was the direct target gene of miR-205-5p and FA2H was significantly decreased in TECs with increased expression of miR-205-5p. Silencing of Fa2h inhibited TECs proliferation. Furthermore, we found that the expression of Tfap2a could be promoted by FA2H and that TFAP2A could interact with miR-205-5p in TECs. Overall, miR-205-5p is an important regulator of TECs proliferation and regulates age-associated thymus involution via the miR-205-5p-FA2H-TFAP2A feedback regulatory circuit. miR-205-5p might act as a potential biomarker in TECs for age-related thymus involution.

Thymus Regeneration and Future Challenges

Thymus regenerative therapy implementation is severely obstructed by the limited number and expansion capacity in vitro of tissue-specific thymic epithelial stem cells (TESC). Current solutions are mostly based on growth factors that can drive differentiation of pluripotent stem cells toward tissue-specific TESC. Target-specific small chemical compounds represent an alternative solution that could induce and support the clonal expansion of TESC and reversibly block their differentiation into mature cells. These compounds could be used both in the composition of culture media designed for TESC expansion in vitro, and in drugs development for thymic regeneration in vivo. It should allow reaching the ultimate objective - autologous thymic tissue regeneration in paediatric patients who had their thymus removed in the course of cardiac surgery.

CD147 deficiency in T cells prevents thymic involution by inhibiting the EMT process in TECs in the presence of TGFβ

Thymic involution during aging is a major cause of decreased T-cell production and reduced immunity. Here, we show that the loss of CD147 on T cells prevents thymic senescence, resulting in slowed shrinkage of the thymus with age and increased production of naive T cells. This phenotype is the result of slowing of the epithelial–mesenchymal transition (EMT) process in thymic epithelial cells (TECs), which eventually leads to reduced adipocyte accumulation. In an in vitro coculture system, we found that TGFβ is an important factor in the EMT process in TECs and that it can reduce the expression of E-cadherin through p-Smad2/FoxC2 signaling. Moreover, CD147 on T cells can accelerate the decline in E-cadherin expression by interacting with Annexin A2 on TECs. In the presence of TGFβ, Annexin A2 and E-cadherin colocalize on TECs. However, CD147 on T cells competitively binds to Annexin A2 on TECs, leading to the isolation of E-cadherin. Then, the isolated E-cadherin is easily phosphorylated by phosphorylated Src kinase, the phosphorylation of which was induced by TGFβ, and finally, p-E-cadherin is degraded. Thus, in the thymus, the interaction between T cells and TECs contributes to thymic involution with age. In this study, we illuminate the mechanism underlying the triggering of the EMT process in TECs and show that inhibiting TGFβ and/or CD147 may serve as a strategy to hinder age-related thymic involution.

Myc controls a distinct transcriptional program in fetal thymic epithelial cells that determines thymus growth

Interactions between thymic epithelial cells (TEC) and developing thymocytes are essential for T cell development, but molecular insights on TEC and thymus homeostasis are still lacking. Here we identify distinct transcriptional programs of TEC that account for their age-specific properties, including proliferation rates, engraftability and function. Further analyses identify Myc as a regulator of fetal thymus development to support the rapid increase of thymus size during fetal life. Enforced Myc expression in TEC induces the prolonged maintenance of a fetal-specific transcriptional program, which in turn extends the growth phase of the thymus and enhances thymic output; meanwhile, inducible expression of Myc in adult TEC similarly promotes thymic growth. Mechanistically, this Myc function is associated with enhanced ribosomal biogenesis in TEC. Our study thus identifies age-specific transcriptional programs in TEC, and establishes that Myc controls thymus size.

Both fallopian tube and ovarian surface epithelium are cells-of-origin for high-grade serous ovarian carcinoma

The cell-of-origin of high grade serous ovarian carcinoma (HGSOC) remains controversial, with fallopian tube epithelium (FTE) and ovarian surface epithelium (OSE) both considered candidates. Here, by using genetically engineered mouse models and organoids, we assessed the tumor-forming properties of FTE and OSE harboring the same oncogenic abnormalities. Combined RB family inactivation and Tp53 mutation in Pax8 + FTE caused Serous Tubal Intraepithelial Carcinoma (STIC), which metastasized rapidly to the ovarian surface. These events were recapitulated by orthotopic injection of mutant FTE organoids. Engineering the same genetic lesions into Lgr5 + OSE or OSE-derived organoids also caused metastatic HGSOC, although with longer latency and lower penetrance. FTE- and OSE-derived tumors had distinct transcriptomes, and comparative transcriptomics and genomics suggest that human HGSOC arises from both cell types. Finally, FTE- and OSE-derived organoids exhibited differential chemosensitivity. Our results comport with a dualistic origin for HGSOC and suggest that the cell-of-origin might influence therapeutic response.

Improvement of glucocorticoid-impaired thymus function by dihydromyricetin via up-regulation of PPARγ-associated fatty acid metabolism

T lymphocytes produced by the thymus are essential mediators of immunity. Accelerated thymic atrophy appears in the patients with administration of glucocorticoids (GCs) which are commonly-used drugs to treat autoimmune and infectious diseases, leading to dysregulation of immunity with manifestation of progressive diminution of new T cell production. However, there is no ideal method to overcome such side effects of GCs. In the current study, we proposed a composition of dexamethasone (DEX) and dihydromyricetin (DMY) derived from a medicinal plant, which could protect from DEX-induced thymus damage and simultaneously enhance the anti-inflammatory effect of DEX. In the current study, we found that DEX-damaged thymic cellularity and architecture, reduced thymocyte numbers, induced thymocyte apoptosis and dropped CD4+ and CD8+ double positive T cell numbers in thymus which was effectively improved by co-treatment with DMY. Quantification of signal joint TCR delta excision circles (TRECs) and Vβ TCR spectratyping analysis were employed to determine the thymus function with indicated treatments. The results showed that DEX-impaired thymus output and decreased TCR cell diversity which was ameliorated by co-treatment with DMY. iTRAQ 2D LC-MS/MS was applied to analyze the proteomic profiling of thymus of mice treated with or without indicated agents, followed by informatics analysis to identify the correlated signaling pathway. After validated by Western blotting and Real-time PCR, we found that PPARγ-associated fatty acid metabolism was increased in the thymic tissues of the animals treated with DMY plus DEX than the animals treated with DEX alone. The agonist and antagonist of PPARγ were further employed to verify the role of PPARγ in the present study. Furthermore, DMY demonstrated a synergistic effect with co-administration of DEX on suppressing inflammation in vivo. Collectively, DMY relieved thymus function damaged by DEX via regulation of PPARγ-associated fatty acid metabolism. Our findings may provide a new strategy on protection of thymus from damage caused by GCs by using appropriate adjuvant natural agents through up-regulation of PPARγ-associated fatty acid metabolism.

Profiling analysis of 17β-estradiol-regulated lncRNAs in mouse thymic epithelial cells

Thymus is the primary organ for T cell differentiation and maturation. Many studies have demonstrated that estrogen plays a crucial role in thymic epithelial cell (TEC) proliferation during thymic involution. LncRNAs are involved in various biological processes; however, estrogen-mediated lncRNA expression in TECs has not been yet reported. To address this question, the mouse medullary thymic epithelial cell line 1 (MTEC1) was treated with 17β-estradiol (E2). By using CCK8 assay and flow cytometry, we found that E2 was able to inhibit viability and proliferation of MTEC1 cells. The expression profiles of lncRNAs in MTEC1 cells with or without E2 treatment were then measured by RNA-Seq, and a total of 962 lncRNAs and 2,469 mRNAs were shown to be differentially expressed. The reliability of RNA-Seq was confirmed by quantitative RT-PCR. Correlation analysis was conducted to investigate the potential function of lncRNAs. According to gene ontology (GO) analysis, differentially expressed lncRNAs were mainly related to cell proliferation, cell cycle and cell apoptosis. KEGG pathway analysis indicated that these lncRNAs were associated with several pathways, namely immunological activity, metabolism and cytokine-cytokine receptor interaction. In conclusion, our study provided a novel direction for studying the relationship between lncRNAs and E2 in the thymus.

Thymic stromal cells: Roles in atrophy and age-associated dysfunction of the thymus

Atrophy of the thymus, the primary site of T lymphocyte generation, is a hallmark of the aging immune system. Age-associated thymic atrophy results in diminished output of new, naïve T cells, with immune sequelae that include diminished responses to novel pathogenic challenge and vaccines, as well as diminished tumor surveillance. Although a variety of stimuli are known to regulate transient thymic atrophy, mechanisms governing progressive age-associated atrophy have been difficult to resolve. This has been due in part to the fact that one of the primary targets of age-associated thymic atrophy is a relatively rare population, thymic stromal cells. This review focuses on changes in thymic stromal cells during aging and on the contributions of periodic, stochastic, and progressive causes of thymic atrophy.

Twenty Years of AIRE

About two decades ago, cloning of the autoimmune regulator (AIRE) gene materialized one of the most important actors on the scene of self-tolerance. Thymic transcription of genes encoding tissue-specific antigens (ts-ags) is activated by AIRE protein and embodies the essence of thymic self-representation. Pathogenic AIRE variants cause the autoimmune polyglandular syndrome type 1, which is a rare and complex disease that is gaining attention in research on autoimmunity. The animal models of disease, although not identically reproducing the human picture, supply fundamental information on mechanisms and extent of AIRE action: thanks to its multidomain structure, AIRE localizes to chromatin enclosing the target genes, binds to histones, and offers an anchorage to multimolecular complexes involved in initiation and post-initiation events of gene transcription. In addition, AIRE enhances mRNA diversity by favoring alternative mRNA splicing. Once synthesized, ts-ags are presented to, and cause deletion of the self-reactive thymocyte clones. However, AIRE function is not restricted to the activation of gene transcription. AIRE would control presentation and transfer of self-antigens for thymic cellular interplay: such mechanism is aimed at increasing the likelihood of engagement of the thymocytes that carry the corresponding T-cell receptors. Another fundamental role of AIRE in promoting self-tolerance is related to the development of thymocyte anergy, as thymic self-representation shapes at the same time the repertoire of regulatory T cells. Finally, AIRE seems to replicate its action in the secondary lymphoid organs, albeit the cell lineage detaining such property has not been fully characterized. Delineation of AIRE functions adds interesting data to the knowledge of the mechanisms of self-tolerance and introduces exciting perspectives of therapeutic interventions against the related diseases.

MicroRNAs Regulate Thymic Epithelium in Age-Related Thymic Involution via Down- or Upregulation of Transcription Factors

Age-related thymic involution is primarily induced by defects in nonhematopoietic thymic epithelial cells (TECs). It is characterized by dysfunction of multiple transcription factors (TFs), such as p63 and FoxN1, and also involves other TEC-associated regulators, such as Aire. These TFs and regulators are controlled by complicated regulatory networks, in which microRNAs (miRNAs) act as a key player. miRNAs can either directly target the 3'-UTRs (untranslated regions) of the TFs to suppress TF expression or target TF inhibitors to reduce or increase TF inhibitor expression and thereby indirectly enhance or inhibit TF expression. Here, we review the current understanding and recent studies about how miRNAs are involved in age-related thymic involution via regulation of TEC-autonomous TFs. We also discuss potential strategies for targeting miRNAs to rejuvenate age-related declined thymic function.

Effect of cryopreservation on viability and growth efficiency of stromal-epithelial cells derived from neonatal human thymus

The thymus is the major site of T lymphocyte generation and so is critical for a functional adaptive immune system. Since, thymectomy is a component of neonatal surgery for congenital heart diseases, it provides great potential for collection and storage of thymic tissue for autologous transplantation. However, specific investigation into the optimum parameters for thymic tissue cryopreservation have not been conducted. In this research, we evaluated the effect of different cryoprotective media compositions, which included penetrating (Me₂SO, glycerol) and non-penetrating (dextran-40, sucrose, hydroxyethyl starch) components, on the viability and functionality of frozen-thawed human thymic samples to select an optimal cryoprotective medium suitable for long-term storage of thymic tissue and a stromal-epithelial enriched population. Our primary focus was on receiving, low-temperature storage, culturing and evaluation of thymic tissue samples from newborns and infants with congenital heart diseases, who had undergone thymectomy as a part of standard surgical procedure. Thus, this work builds the platform for autologous clinical intervention into the thymus-deficient patients with congenital heart diseases. From our data, we conclude that although there were no significant differences in efficiency of tested cryoprotective media compositions, the combination of Me₂SO and dextran-40 compounds was the most suitable for long-term storage both thymic cell suspensions and thymic fragments based on the viability of CD326⁺ epithelial cells and stromal-epithelial cell monolayer formation.

Acute Thymic Involution and Mechanisms for Recovery

Acute thymic involution (ATI) is usually regarded as a virulence trait. It is caused by several infectious agents (bacteria, viruses, parasites, fungi) and other factors, including stress, pregnancy, malnutrition and chemotherapy. However, the complex mechanisms that operate during ATI differ substantially from each other depending on the causative agent. For instance, a transient reduction in the size and weight of the thymus and depletion of populations of T cell subsets are hallmarks of ATI in many cases, whereas severe disruption of the anatomical structure of the organ is also associated with some factors, including fungal, parasitic and viral infections. However, growing evidence shows that ATI may be therapeutically halted or reversed. In this review, we highlight the current progress in this field with respect to numerous pathological factors and discuss the possible mechanisms. Moreover, these new observations also show that ATI can be mechanistically reversed.

RB inactivation in keratin 18 positive thymic epithelial cells promotes non-cell autonomous T cell hyperproliferation in genetically engineered mice

Thymic epithelial cells (TEC), as part of thymic stroma, provide essential growth factors/cytokines and self-antigens to support T cell development and selection. Deletion of Rb family proteins in adult thymic stroma leads to T cell hyperplasia in vivo. To determine whether deletion of Rb specifically in keratin (K) 18 positive TEC was sufficient for thymocyte hyperplasia, we conditionally inactivated Rb and its family members p107 and p130 in K18+ TEC in genetically engineered mice (TgK18GT₁₂₁; K18 mice). We found that thymocyte hyperproliferation was induced in mice with Rb inactivation in K18+ TEC, while normal T cell development was maintained; suggesting that inactivation of Rb specifically in K18+ TEC was sufficient and responsible for the phenotype. Transplantation of wild type bone marrow cells into mice with Rb inactivation in K18+ TEC resulted in donor T lymphocyte hyperplasia confirming the non-cell autonomous requirement for Rb proteins in K18+ TEC in regulating T cell proliferation. Our data suggests that thymic epithelial cells play an important role in regulating lymphoid proliferation and thymus size.

Immune senescence: significance of the stromal microenvironment

The immune system undergoes age-associated changes known as immunosenescence, resulting in increased susceptibility to infections, cancers and autoimmunity in the aged. The basis of our understanding of immunosenescence has been derived primarily from studies examining intrinsic defects within many of the cells of the immune system. While these studies have provided insight into the mechanisms of immunosenescence, a picture is now emerging that the stromal microenvironment within lymphoid organs also contributes significantly to the age-associated decline of immune function. These extrinsic defects appear to impact the functional activity of immune cells and may offer a potential target to recover immune activity. Indeed, rejuvenation studies which have targeted the stromal niche have restored immune function in aged successfully, highlighting the impact of the microenvironment towards the aetiology of immunosenescence.

FOXN1 in thymus organogenesis and development

Development of the primary T-cell repertoire takes place in the thymus. The linked processes of T-cell differentiation and T-cell repertoire selection each depend on interactions between thymocytes and thymic stromal cells; in particular, with the epithelial cells of the cortical and medullary thymic compartments (cortical and medullary thymic epithelial cells; cTECs and mTECs, respectively). The importance of the thymic epithelial cell lineage in these processes was revealed in part through analysis of nude (nu/nu) mice, which are congenitally hairless and athymic. The nude phenotype results from null mutation of the forkhead transcription factor FOXN1, which has emerged as a pivotal regulator both of thymus development and homeostasis. FOXN1 has been shown to play critical roles in thymus development, function, maintenance, and even regeneration, which positions it as a master regulator of thymic epithelial cell (TEC) differentiation. In this review, we discuss current understanding of the regulation and functions of FOXN1 throughout thymus ontogeny, from the earliest stages of organogenesis through homeostasis to age-related involution, contextualising its significance through reference to other members of the wider Forkhead family.

FOXN1 recombinant protein enhances T-cell regeneration after hematopoietic stem cell transplantation in mice

A prolonged period of T-cell recovery is the major challenge in hematopoietic stem cell transplantation (HSCT). Thymic epithelial cells (TECs) are the major component of the thymic microenvironment for T-cell generation. However, TECs undergo degeneration over time. FOXN1 plays a critical role in TEC development and is required to maintain adult TECs for thymopoiesis. To investigate the potential application of FOXN1, we have cloned and expressed recombinant FOXN1 protein (rFOXN1) that was fused with cell-penetrating peptides. We show here that the rFOXN1 protein can translocate from the cell surface into the cytoplasm and nucleus. Administration of rFOXN1 into both congenic and allogeneic HSCT recipient mice increased the number of TECs, resulting in enhanced thymopoiesis that led to an increased number of functional T cells in the periphery. The increased number of TECs is due to the enhanced survival and proliferation of TECs. Our results suggest that rFOXN1 has the potential to be used in enhancing T-cell regeneration in patients following HSCT.

Loss of Pten Disrupts the Thymic Epithelium and Alters Thymic Function

The thymus is the site of T cell development and selection. In addition to lymphocytes, the thymus is composed of several types of stromal cells that are exquisitely organized to create the appropriate environment and microenvironment to support the development and selection of maturing T cells. Thymic epithelial cells (TECs) are one of the more important cell types in the thymic stroma, and they play a critical role in selecting functional T cell clones and supporting their development. In this study, we used a mouse genetics approach to investigate the consequences of deleting the Pten tumor suppressor gene in the TEC compartment of the developing thymus. We found that PTEN deficiency in TECs results in a smaller thymus with significantly disordered architecture and histology. Accordingly, loss of PTEN function also results in decreased T cells with a shift in the distribution of T cell subtypes towards CD8+ T cells. These experiments demonstrate that PTEN is critically required for the development of a functional thymic epithelium in mice. This work may help better understand the effects that certain medical conditions or clinical interventions have upon the thymus and immune function.

Recruitment of Pontin/Reptin by E2f1 amplifies E2f transcriptional response during cancer progression

Changes in gene expression during tumorigenesis are often considered the consequence of de novo mutations occurring in the tumour. An alternative possibility is that the transcriptional response of oncogenic transcription factors evolves during tumorigenesis. Here we show that aberrant E2f activity, following inactivation of the Rb gene family in a mouse model of liver cancer, initially activates a robust gene expression programme associated with the cell cycle. Slowly accumulating E2f1 progressively recruits a Pontin/Reptin complex to open the chromatin conformation at E2f target genes and amplifies the E2f transcriptional response. This mechanism enhances the E2f-mediated transactivation of cell cycle genes and initiates the activation of low binding affinity E2f target genes that regulate non-cell-cycle functions, such as the Warburg effect. These data indicate that both the physiological and the oncogenic activities of E2f result in distinct transcriptional responses, which could be exploited to target E2f oncogenic activity for therapy.

E2F transcription factors are primarily known for the regulation of the cell cycle and are often dysregulated in cancer. Here, the authors show that during cancer progression E2F1 recruits a Pontin/Reptin complex to E2F target genes to open chromatin and increase E2F transcriptional response.

Young, proliferative thymic epithelial cells engraft and function in aging thymuses

The thymus reaches its maximum size early in life and then begins to shrink, producing fewer T cells with increasing age. This thymic decline is thought to contribute to age-related T cell lymphopenias and hinder T cell recovery after bone marrow transplantation. Although several cellular and molecular processes have been implicated in age-related thymic involution, their relative contributions are not known. Using heterochronic parabiosis, we observe that young circulating factors are not sufficient to drive regeneration of the aged thymus. In contrast, we find that resupplying young, engraftable thymic epithelial cells (TECs) to a middle-aged or defective thymus leads to thymic growth and increased T cell production. Intrathymic transplantation and in vitro colony-forming assays reveal that the engraftment and proliferative capacities of TECs diminish early in life, whereas the receptivity of the thymus to TEC engraftment remains relatively constant with age. These results support a model in which thymic growth and subsequent involution are driven by cell-intrinsic changes in the proliferative capacity of TECs, and further show that young TECs can engraft and directly drive the growth of involuted thymuses.

Unraveling the Link Between Ectodermal Disorders and Primary Immunodeficiencies

Primary immunodeficiencies (PIDs) include a heterogeneous group of mostly monogenic diseases characterized by functional/developmental alterations of the immune system. Skin and skin annexa abnormalities may be a warning sign of immunodeficiency, since both epidermal and thymic epithelium have ectodermal origin. In this review, we will focus on the most common immune disorders associated with ectodermal alterations. Elevated IgE levels represent the immunological hallmark of hyper-IgE syndrome, characterized by severe eczema and susceptibility to infections. Ectodermal dysplasia (ED) is a group of rare disorders that affect tissues of ectodermal origin. Hypoidrotic ED (HED), the most common form, is inherited as autosomal dominant, autosomal recessive or X-linked trait (XLHED). HED and XLHED are caused by mutations in NEMO and EDA-1 genes, respectively, and show similarities in the cutaneous involvement but differences in the susceptibility to infections and immunological phenotype. Alterations in the transcription factor FOXN1 gene, expressed in the mature thymic and skin epithelia, are responsible for human and murine athymia and prevent the development of the T-cell compartment associated to ectodermal abnormalities such as alopecia and nail dystrophy. The association between developmental abnormalities of the skin and immunodeficiencies suggest a role of the skin as a primary lymphoid organ. Recently, it has been demonstrated that a co-culture of human skin-derived keratinocytes and fibroblasts, in the absence of thymic components, can support the survival of human haematopoietic stem cells and their differentiation into T-lineage committed cells.

Developing stratified epithelia: lessons from the epidermis and thymus

Stratified squamous epithelial cells are found in a number of organs, including the skin epidermis and the thymus. The progenitor cells of the developing epidermis form a multi-layered epithelium and appendages, like the hair follicle, to generate an essential barrier to protect against water loss and invasion of foreign pathogens. In contrast, the thymic epithelium forms a three-dimensional mesh of keratinocytes that are essential for positive and negative selection of self-restricted T cells. While these distinct stratified epithelial tissues derive from distinct embryonic germ layers, both tissues instruct immunity, and the epithelial differentiation programs and molecular mechanisms that control their development are remarkably similar. In this review, we aim to highlight some of the similarities between the thymus and the skin epidermis and its appendages during developmental specification.

Global transcriptional profiling reveals distinct functions of thymic stromal subsets and age-related changes during thymic involution

Age-associated thymic involution results in diminished T cell output and function in aged individuals. However, molecular mediators contributing to the decline in thymic function during early thymic involution remain largely unknown. Here, we present transcriptional profiling of purified thymic stromal subsets from mice 1, 3, and 6 months of age spanning early thymic involution. The data implicate unanticipated biological functions for a subset of thymic epithelial cells. The predominant transcriptional signature of early thymic involution is decreased expression of cell-cycle-associated genes and E2F3 transcriptional targets in thymic epithelial subsets. Also, expression of proinflammatory genes increases with age in thymic dendritic cells. Many genes previously implicated in late involution are already deregulated by 3-6 months of age. We provide these thymic stromal data sets, along with thymocyte data sets, in a readily searchable web-based platform, as a resource for investigations into thymocyte:stromal interactions and mechanisms of thymic involution.

Organ size control is dominant over Rb family inactivation to restrict proliferation in vivo

In mammals, a cell's decision to divide is thought to be under the control of the Rb/E2F pathway. We previously found that inactivation of the Rb family of cell cycle inhibitors (Rb, p107, and p130) in quiescent liver progenitors leads to uncontrolled division and cancer initiation. Here, we show that, in contrast, deletion of the entire Rb gene family in mature hepatocytes is not sufficient for their long-term proliferation. The cell cycle block in Rb family mutant hepatocytes is independent of the Arf/p53/p21 checkpoint but can be abrogated upon decreasing liver size. At the molecular level, we identify YAP, a transcriptional regulator involved in organ size control, as a factor required for the sustained expression of cell cycle genes in hepatocytes. These experiments identify a higher level of regulation of the cell cycle in vivo in which signals regulating organ size are dominant regulators of the core cell cycle machinery.

Regeneration of the aged thymus by a single transcription factor

Thymic involution is central to the decline in immune system function that occurs with age. By regenerating the thymus, it may therefore be possible to improve the ability of the aged immune system to respond to novel antigens. Recently, diminished expression of the thymic epithelial cell (TEC)-specific transcription factor Forkhead box N1 (FOXN1) has been implicated as a component of the mechanism regulating age-related involution. The effects of upregulating FOXN1 function in the aged thymus are, however, unknown. Here, we show that forced, TEC-specific upregulation of FOXN1 in the fully involuted thymus of aged mice results in robust thymus regeneration characterized by increased thymopoiesis and increased naive T cell output. We demonstrate that the regenerated organ closely resembles the juvenile thymus in terms of architecture and gene expression profile, and further show that this FOXN1-mediated regeneration stems from an enlarged TEC compartment, rebuilt from progenitor TECs. Collectively, our data establish that upregulation of a single transcription factor can substantially reverse age-related thymic involution, identifying FOXN1 as a specific target for improving thymus function and, thus, immune competence in patients. More widely, they demonstrate that organ regeneration in an aged mammal can be directed by manipulation of a single transcription factor, providing a provocative paradigm that may be of broad impact for regenerative biology.

Immunosenescence: a product of the environment?

The major function of the immune system is to provide protection against pathogens, in order to prevent infections and potential death. However, with increasing age the immune system undergoes alterations culminating in a progressive deterioration in the ability to respond to infection and vaccination. The precise mechanisms associated with immunosenescence have not been fully elucidated although extensive analyses have suggested that intrinsic defects within immune cells are potentially involved. Despite the stromal niche playing a critical role in the development and activation of immune cells, the role of extrinsic factors within the microenvironment in immunosenescence is less well understood. Moreover, emerging evidence suggests that the aged microenvironment contributes significantly to the age-associated decline of immune function and additionally may offer a potential target for rejuvenating the immune system. Indeed, rejuvenation strategies which have targeted the thymic stromal microenvironment have proved to be successful in recovering thymic function in the aged.