Regulations of gene expression in medullary thymic epithelial cells required for preventing the onset of autoimmune diseases

CD90 Marks a Mesenchymal Program in Human Thymic Epithelial Cells In Vitro and In Vivo

Thymic epithelium is critical for the structural integrity of the thymus and for T cell development. Within the fully formed thymus, large numbers of hematopoietic cells shape the thymic epithelium into a scaffold-like structure which bears little similarity to classical epithelial layers, such as those observed in the skin, intestine or pancreas. Here, we show that human thymic epithelial cells (TECs) possess an epithelial identity that also incorporates the expression of mesenchymal cell associated genes, whose expression levels vary between medullary and cortical TECs (m/cTECs). Using pluripotent stem cell (PSC) differentiation systems, we identified a unique population of cells that co-expressed the master TEC transcription factor FOXN1, as well as the epithelial associated marker EPCAM and the mesenchymal associated gene CD90. Using the same serum free culture conditions, we also observed co-expression of EPCAM and CD90 on cultured TECs derived from neonatal human thymus in vitro. Single cell RNA-sequencing revealed these cultured TECs possessed an immature mTEC phenotype and expressed epithelial and mesenchymal associated genes, such as EPCAM, CLDN4, CD90 and COL1A1. Importantly, flow cytometry and single cell RNA-sequencing analysis further confirmed the presence of an EPCAM+CD90+ population in the CD45- fraction of neonatal human thymic stromal cells in vivo. Using the human thymus cell atlas, we found that cTECs displayed more pronounced mesenchymal characteristics than mTECs during embryonic development. Collectively, these results suggest human TECs possess a hybrid gene expression program comprising both epithelial and mesenchymal elements, and provide a basis for the further exploration of thymus development from primary tissues and from the in vitro differentiation of PSCs.

Coxsackievirus B4 Transplacental Infection Severely Disturbs Central Tolerogenic Mechanisms in the Fetal Thymus

Thymus plays a fundamental role in central tolerance establishment, especially during fetal life, through the generation of self-tolerant T cells. This process consists in T cells education by presenting them tissue-restricted autoantigens promiscuously expressed by thymic epithelial cells (TECs), thus preventing autoimmunity. Thymus infection by Coxsackievirus B (CV-B) during fetal life is supposed to disturb thymic functions and, hence, to be an inducing or accelerating factor in the genesis of autoimmunity. To further investigate this hypothesis, in our current study, we analyzed thymic expression of autoantigens, at the transcriptional and protein level, following in utero infection by CV-B4. mRNA expression levels of Igf2 and Myo7, major autoantigens of pancreas and heart, respectively, were analyzed in whole thymus and in enriched TECs together along with both transcription factors, Aire and Fezf2, involved in autoantigens expression in the thymus. Results show that in utero infection by CV-B4 induces a significant decrease in Igf2 and Myo7 expression at both mRNA and protein level in whole thymus and in enriched TECs as well. Moreover, a correlation between viral load and autoantigens expression can be observed in the whole thymus, indicating a direct effect of in utero infection by CV-B4 on autoantigens expression. Together, these results indicate that an in utero infection of the thymus by CV-B4 may interfere with self-tolerance establishment in TECs by decreasing autoantigen expression at both mRNA and protein level and thereby increase the risk of autoimmunity onset.

Regulation of T cell-associated tissues and T cell activation by RANKL-RANK-OPG

The receptor activator of nuclear factor kappa-B ligand (RANKL)-RANK-osteoprotegerin (OPG) system is critical to bone homeostasis, but genetically deficient mouse models have revealed important roles in the immune system as well. RANKL-RANK-OPG is particularly important to T cell biology because of its organogenic control of thymic development and secondary lymphoid tissues influence central T cell tolerance and peripheral T cell function. RANKL-RANK-OPG cytokine-receptor interactions are often controlled by regulation of expression of RANKL on developing T cells, which interacts with RANK expressed on some lymphoid tissue cells to stimulate key downstream signaling pathways that affect critical tuning functions of the T cell compartment, like cell survival and antigen presentation. Activation of peripheral T cells is regulated by RANKL-enhanced dendritic cell survival, and dysregulation of the RANKL-RANK-OPG system in this context is associated with loss of T cell tolerance and autoimmune disease. Given its broader implications for immune homeostasis and osteoimmunology, it is critical to further understand how the RANKL-RANK-OPG system operates in T cell biology.

High frequency vaccination-induced immune stress reduces bone strength with the involvement of activated osteoclastogenesis in layer pullets

In poultry production, vaccination is an effective measure to protect chickens from diseases. Vaccination, however, is a stressor that may induce stress responses that interfere with the growth and development of chickens. The interaction between the skeletal and immune systems on bone quality has gained more attention. In the present study, the influence of high frequency vaccinations on the bone development of layer pullets was investigated. Thirty 35-day-old SPF White Leghorn layer pullets were obtained and randomly subjected to the following treatments: vaccinated against Newcastle disease (ND) with LoSota vaccine once at 35-day-old (V1, control); 4 times at 35, 49, 63, and 77 d of age (V4); and 7 times at 35, 42, 49, 56, 63, 70, and 77 d of age (V7). The body weight and organ index of the spleen, thymus, and tibia were recorded. The antibody titer and serum and the tibia calcium and phosphorus concentrations were measured. The transcription levels of the IL-6, IL-17, TNF-α, receptor activator of NF-κB ligand (RANKL), and osteoprotegerin (OPG) genes were determined in spleen, thymus, and the tibia. The results showed that V7 decreased body weight and increased the ND antibody titer, compared to V1-chickens. The expression levels of IL-6, IL-17, and TNF-α were upregulated in spleen, thymus, and the tibia of V7 chickens. In the tibia, RANKL was upregulated, while OPG was downregulated by V7 treatment. The results indicate that high frequency vaccination induces immune stress and impairs bone development. The results suggest that the augmented cytokine expression in immune organs and the tibia is associated with activation of the OPG/RANKL pathway, which, in turn, enhances osteoclastogenesis. The appropriate frequency of vaccination should support optimal bone development and full immunoprotection in layer pullets.

Cultured Human Thymic-Derived Cells Display Medullary Thymic Epithelial Cell Phenotype and Functionality

Thymic epithelial cells are one of the main components of the thymic microenvironment required for T-cell development. In this work, we describe an efficient method free of enzymatic and Facs-sorted methods to culture human medullary thymic epithelial cells without affecting the cell phenotypic, physiologic and functional features. Human medulla thymic epithelial cells (mTECs) are obtained by culturing thymic biopsies explants. After 7 days of primo-culture, mTECs keep their ability to express key molecules involved in immune tolerance processes such as autoimmune regulator, tissue-specific antigens, chemokines, and cytokines. In addition, the cells sensor their cultured environment and consequently adjust their gene expression network. Therefore, we describe and provide a human mTEC model that may be used to test the effect of various molecules on thymic epithelial cell homeostasis and physiology. This method should allow the investigations of the specificities and the knowledge of human mTECs in normal or pathological conditions and therefore discontinue the extrapolations done on the murine models.

Heterologous expression, purification and function of the extracellular domain of human RANK

Background

Receptor activator of NF-κB ligand (RANKL)/RANK signaling essentially functions within the skeletal system, particularly participating in osteoclastogenesis and bone resorption. In addition, this signaling pathway has also been shown to influence tumor progression as well as the development and function of the immune system. Therefore, blocking the interaction between RANKL and RANK is a new therapeutic approach to prevent bone-related diseases and cancer.

Results

The coding sequence encoding the extracellular domain of human RANK (RANK-N) was codon optimized for Pichia pastoris and cloned into the pPIC9K vector, and the recombinant plasmid was then transformed into P. pastoris. The expression of RANK-N protein was confirmed using SDS-PAGE with Coomassie Brilliant Blue stain and western blotting. Recombinant RANK-N protein was purified by a multistep process including ultrafiltration (UF), Sephadex G-50 size-exclusion chromatography and Q-Sepharose Fast Flow ion exchange chromatography, which resulted in a purity >95%. We found that the RANK-N protein can block RANKL-RANK signaling both in vitro and in vivo. Furthermore, using a patient-derived xenograft of human colon cancer, we found that the recombinant RANK-N protein can inhibit the growth of colorectal cancer.

Conclusions

The results show that a simple system to express and purify functional RANK-N protein has been developed. This work has thus laid a foundation for further research and clinical applications of RANK-N protein in treating bone-related diseases or even colorectal cancer.

Electronic supplementary material

The online version of this article (10.1186/s12896-017-0405-y) contains supplementary material, which is available to authorized users.

Association between RANK, RANKL and OPG polymorphisms with ACPA and erosions in rheumatoid arthritis: results from a meta-analysis involving three French cohorts

OBJECTIVES

The RANK/RANKL/osteoprotegerin (OPG) system plays a central role in the pathogenesis of bone erosions in rheumatoid arthritis (RA). The aim of this study was to test the association between 11 single-nucleotide polymorphisms (SNPs) located on RANK, RANKL and OPG genes and anticitrullinated peptide antibody (ACPA) presence or erosions in RA.

METHODS

PATIENTS

This work was performed on three independent samples of French patients with RA: the Etude de Suivi des PolyArthrites Indifférenciées Récentes (ESPOIR) (n=632), Rangueil Midi-Pyrénées (RMP) (n=249) and French Rheumatoid Arthritis Genetic Consortium (FRAGC) (n=590) cohorts. Genotyping: the genotyping of 11 SNPs located on RANK, RANKL and OPG were performed by PCR.

STATISTICAL ANALYSES

The association between the genotypes with ACPA or erosions was first tested in the ESPOIR cohort using a χ(2) test and, in the case of significant association, replicated in the RMP and FRACG cohorts. A meta-analysis on the three cohorts was performed using the Mantel-Haenszel method.

RESULTS

One SNP on RANK (rs8086340) and three SNPs on RANKL (rs7984870, rs7325635, rs1054016) were significantly associated with ACPA presence, while one SNP on OPG (rs2073618) and one SNP on RANKL (rs7325635) were significantly associated with erosions in the ESPOIR cohort. Following meta-analysis performed on the three samples, the SNP on RANK and the GGG haplotype of the three SNPs located on RANKL were both significantly associated with ACPA presence, while only the SNP on OPG remained significantly associated with erosions.

CONCLUSIONS

This study identified one SNP located on RANK, one haplotype on RANKL associated with ACPA presence, and one SNP located on OPG associated with erosions in three different samples of French patients with RA.

RANK-RANKL signalling in cancer

Oncogenic events combined with a favourable environment are the two main factors in the oncological process. The tumour microenvironment is composed of a complex, interconnected network of protagonists, including soluble factors such as cytokines, extracellular matrix components, interacting with fibroblasts, endothelial cells, immune cells and various specific cell types depending on the location of the cancer cells (e.g. pulmonary epithelium, osteoblasts). This diversity defines specific "niches" (e.g. vascular, immune, bone niches) involved in tumour growth and the metastatic process. These actors communicate together by direct intercellular communications and/or in an autocrine/paracrine/endocrine manner involving cytokines and growth factors. Among these glycoproteins, RANKL (receptor activator nuclear factor-κB ligand) and its receptor RANK (receptor activator nuclear factor), members of the TNF and TNFR superfamilies, have stimulated the interest of the scientific community. RANK is frequently expressed by cancer cells in contrast with RANKL which is frequently detected in the tumour microenvironment and together they participate in every step in cancer development. Their activities are markedly regulated by osteoprotegerin (OPG, a soluble decoy receptor) and its ligands, and by LGR4, a membrane receptor able to bind RANKL. The aim of the present review is to provide an overview of the functional implication of the RANK/RANKL system in cancer development, and to underline the most recent clinical studies.

Identification of embryonic precursor cells that differentiate into thymic epithelial cells expressing autoimmune regulator

Medullary thymic epithelial cells (mTECs) expressing autoimmune regulator (Aire) are critical for preventing the onset of autoimmunity. However, the differentiation program of Aire-expressing mTECs (Aire(+) mTECs) is unclear. Here, we describe novel embryonic precursors of Aire(+) mTECs. We found the candidate precursors of Aire(+) mTECs (pMECs) by monitoring the expression of receptor activator of nuclear factor-κB (RANK), which is required for Aire(+) mTEC differentiation. pMECs unexpectedly expressed cortical TEC molecules in addition to the mTEC markers UEA-1 ligand and RANK and differentiated into mTECs in reaggregation thymic organ culture. Introduction of pMECs in the embryonic thymus permitted long-term maintenance of Aire(+) mTECs and efficiently suppressed the onset of autoimmunity induced by Aire(+) mTEC deficiency. Mechanistically, pMECs differentiated into Aire(+) mTECs by tumor necrosis factor receptor-associated factor 6-dependent RANK signaling. Moreover, nonclassical nuclear factor-κB activation triggered by RANK and lymphotoxin-β receptor signaling promoted pMEC induction from progenitors exhibiting lower RANK expression and higher CD24 expression. Thus, our findings identified two novel stages in the differentiation program of Aire(+) mTECs.

Positive and Negative Regulatory Mechanisms for Fine-Tuning Cellularity and Functions of Medullary Thymic Epithelial Cells

Self-tolerant T cells and regulatory T cells develop in the thymus. A wide variety of cell-cell interactions in the thymus is required for the differentiation, proliferation, and repertoire selection of T cells. Various secreted and cell surface molecules expressed in thymic epithelial cells (TECs) mediate these processes. Moreover, cytokines expressed by cells of hematopoietic origin regulate the cellularity of TECs. Tumor necrosis factor (TNF) family RANK ligand, lymphotoxin, and CD40 ligand, expressed in T cells and innate lymphoid cells (ILCs), promote the differentiation and proliferation of medullary TECs (mTECs) that play critical roles in the induction of immune tolerance. A recent study suggests that interleukin-22 (IL-22) produced by ILCs promotes regeneration of TECs after irradiation. Intriguingly, tumor growth factor-β and osteoprotegerin limit cellularity of mTECs, thereby attenuating regulatory T cell generation. We will review recent insights into the molecular basis for cell-cell interactions regulating differentiation and proliferation of mTECs and also discuss about a perspective on use of mathematical models for understanding this complicated system.

Thymic Crosstalk Coordinates Medulla Organization and T-Cell Tolerance Induction

The thymus ensures the generation of a functional and highly diverse T-cell repertoire. The thymic medulla, which is mainly composed of medullary thymic epithelial cells (mTECs) and dendritic cells (DCs), provides a specialized microenvironment dedicated to the establishment of T-cell tolerance. mTECs play a privileged role in this pivotal process by their unique capacity to express a broad range of peripheral self-antigens that are presented to developing T cells. Reciprocally, developing T cells control mTEC differentiation and organization. These bidirectional interactions are commonly referred to as thymic crosstalk. This review focuses on the relative contributions of mTEC and DC subsets to the deletion of autoreactive T cells and the generation of natural regulatory T cells. We also summarize current knowledge regarding how hematopoietic cells conversely control the composition and complex three-dimensional organization of the thymic medulla.

Natural Th17 cells are critically regulated by functional medullary thymic microenvironments

The thymic medulla is critical for the enforcement of central tolerance. In addition to deletion of auto-reactive T-cells, the thymic medulla supports the maturation of heterogeneous natural αβT-cells linked to tolerance mechanisms. Natural IL-17-secreting CD4(+)αβT-cells (nTh17) represent recently described natural αβT-cells that mature and undergo functional priming intrathymically. Despite a proposed potential to impact upon either protective or pathological inflammatory responses, the intrathymic mechanisms regulating the balance of nTh17 development are unclear. Here we compare the development of distinct natural αβT-cells in the thymus. We reveal that thymic stromal MHC class II expression and RelB-dependent medullary thymic epithelial cells (mTEC), including Aire(+) mTEC, are an essential requirement for nTh17 development. nTh17 demonstrate a partial, non-redundant requirement for both ICOS-ligand and CD80/86 costimulation, with a dispensable role for CD80/86 expression by thymic epithelial cells. Although mTEC constitutively expressed inducible nitric oxide synthase (iNOS), a critical negative regulator of conventional Th17 differentiation, iNOS was not essential to constrain thymic nTh17. These findings highlight the critical role of the thymic medulla in the differential regulation of novel natural αβT-cell subsets, and reveal additional layers of thymic medullary regulation of T-cell driven autoimmunity and inflammation.

The thymic cortical epithelium determines the TCR repertoire of IL-17-producing γδT cells

The thymus provides a specialized microenvironment in which distinct subsets of thymic epithelial cells (TECs) support T-cell development. Here, we describe the significance of cortical TECs (cTECs) in T-cell development, using a newly established mouse model of cTEC deficiency. The deficiency of mature cTECs caused a massive loss of thymic cellularity and impaired the development of αβT cells and invariant natural killer T cells. Unexpectedly, the differentiation of certain γδT-cell subpopulations-interleukin-17-producing Vγ4 and Vγ6 cells-was strongly dysregulated, resulting in the perturbation of γδT-mediated inflammatory responses in peripheral tissues. These findings show that cTECs contribute to the shaping of the TCR repertoire, not only of "conventional" αβT cells but also of inflammatory "innate" γδT cells.

Biology of the RANKL-RANK-OPG System in Immunity, Bone, and Beyond

Discovery and characterization of the cytokine receptor-cytokine-decoy receptor triad formed by receptor activator of nuclear factor kappa-B ligand (RANKL)–receptor activator of NF-κB (RANK)–osteoprotegerin (OPG) have led not only to immense advances in understanding the biology of bone homeostasis, but have also crystalized appreciation of the critical regulatory relationship that exists between bone and immunity, resulting in the emergence of the burgeoning field of osteoimmunology. RANKL–RANK–OPG are members of the tumor necrosis factor (TNF) and TNF receptor superfamilies, and share signaling characteristics common to many members of each. Developmentally regulated and cell-type specific expression patterns of each of these factors have revealed key regulatory functions for RANKL–RANK–OPG in bone homeostasis, organogenesis, immune tolerance, and cancer. Successful efforts at designing and developing therapeutic agents targeting RANKL–RANK–OPG have been undertaken for osteoporosis, and additional efforts are underway for other conditions. In this review, we will summarize the basic biology of the RANKL–RANK–OPG system, relate its cell-type specific functions to system-wide mechanisms of development and homeostasis, and highlight emerging areas of interest for this cytokine group.

Autoimmune regulator expression in thymomas with or without autoimmune disease

BACKGROUND

The autoimmune regulator (AIRE) regulates autoimmunity and self-antigen expression, such as acetylcholine receptor (AchR), in the thymus. Regulatory T cells (Tregs) can down-regulate autoimmunity, but also promote tumor growth. The objective of this study was to examine the levels of AIRE, AchR, and Foxp3 expression in thymomas.

METHODS

The relative levels of AIRE, AchR, and Foxp3 mRNA transcripts and the frequency of AIRE+, AchR+, and Foxp3+ cells were determined by quantitative RT-PCR and immunohistochemistry in 79 thymoma tissue samples from 21 patients with simple thymoma (the Tm group), 39 patients with myasthenia gravis (the MG group) and 19 patients with myasthenia gravis and one other autoimmune disease (the AD group). The numbers of peripheral blood CD4+CD25+Foxp3+ Tregs were determined by flow cytometry analysis.

RESULTS

The relative levels of AIRE and AchR mRNA transcripts in the MG group were significantly lower than that in the Tm group (p=0.04, p=0.03), but higher than that in the AD group (p=0.03, p=0.04). The relative levels of Foxp3 mRNA transcripts in the Tm group were significantly higher than that in the MG and AD groups (p=0.03 for both). A similar pattern of the percentages of AIRE+, AchR+, and Foxp3+ cells in the thymoma tissues and the numbers of peripheral blood Tregs were detected in these patients. The levels of AIRE mRNA transcripts were correlated positively with that of the AchR and Foxp3 in this population. The levels of AIRE and AchR mRNA transcripts in the A/AB/B1 types of thymomas were significantly higher than that in the B2/B3/C types of thymomas in this population.

CONCLUSION

Significantly lower levels of AIRE, AchR, and Foxp3 expression are associated with the development of thymoma-related autoimmune diseases.

The IL-2/IL-2R system: from basic science to therapeutic applications to enhance immune regulation

IL-2 plays a critical role in the normal function of the immune system. A trophic factor for lymphocytes, IL-2 is required for mounting and sustaining adaptive T cell responses; however, IL-2 is also critical for immune regulation via its effects on regulatory T cells (Treg cells). Over the years, we have contributed to the understanding of the biology of IL-2 and its signaling through the IL-2 receptor and helped define the key role played by IL-2 in Treg development and function. Our data show that Treg cells have a heightened sensitivity to IL-2, which may create a therapeutic window to promote immune regulation by selective stimulation of Treg cells. We are now developing new efforts to translate this knowledge to the clinical arena, through our focused interest in Type 1 diabetes as a prototypic autoimmune disease. Specifically, we aim at developing IL-2-based therapeutic regimens and incorporate means to enhance antigen-specific Treg responses, for improved and more selective regulation of islet autoimmunity. In parallel, we are pursuing studies in preclinical models of autoimmunity and transplantation to define critical factors for successful adoptive Treg therapy and develop clinically applicable therapeutic protocols.