Autoimmunity in thymic epithelial tumors: a not yet clarified pathologic paradigm associated with several unmet clinical needs

Thymic epithelial tumors (TETs) are rare mediastinal cancers originating from the thymus, classified in two main histotypes: thymoma and thymic carcinoma (TC). TETs affect a primary lymphoid organ playing a critical role in keeping T-cell homeostasis and ensuring an adequate immunological tolerance against "self". In particular, thymomas and not TC are frequently associated with autoimmune diseases (ADs), with Myasthenia Gravis being the most common AD present in 30% of patients with thymoma. This comorbidity, in addition to negatively affecting the quality and duration of patients' life, reduces the spectrum of the available therapeutic options. Indeed, the presence of autoimmunity represents an exclusion criteria for the administration of the newest immunotherapeutic treatments with checkpoint inhibitors. The pathophysiological correlation between TETs and autoimmunity remains a mystery. Several studies have demonstrated the presence of a residual and active thymopoiesis in adult patients affected by thymomas, especially in mixed and lymphocytic-rich thymomas, currently known as type AB and B thymomas. The aim of this review is to provide the state of art in regard to the histological features of the different TET histotype, to the role of the different immune cells infiltrating tumor microenvironments and their impact in the break of central immunologic thymic tolerance in thymomas. We discuss here both cellular and molecular immunologic mechanisms inducing the onset of autoimmunity in TETs, limiting the portfolio of therapeutic strategies against TETs and greatly impacting the prognosis of associated autoimmune diseases.

Primary and secondary defects of the thymus

The thymus is the primary site of T-cell development, enabling generation, and selection of a diverse repertoire of T cells that recognize non-self, whilst remaining tolerant to self- antigens. Severe congenital disorders of thymic development (athymia) can be fatal if left untreated due to infections, and thymic tissue implantation is the only cure. While newborn screening for severe combined immune deficiency has allowed improved detection at birth of congenital athymia, thymic disorders acquired later in life are still underrecognized and assessing the quality of thymic function in such conditions remains a challenge. The thymus is sensitive to injury elicited from a variety of endogenous and exogenous factors, and its self-renewal capacity decreases with age. Secondary and age-related forms of thymic dysfunction may lead to an increased risk of infections, malignancy, and autoimmunity. Promising results have been obtained in preclinical models and clinical trials upon administration of soluble factors promoting thymic regeneration, but to date no therapy is approved for clinical use. In this review we provide a background on thymus development, function, and age-related involution. We discuss disease mechanisms, diagnostic, and therapeutic approaches for primary and secondary thymic defects.

An integrative mechanistic model of thymocyte dynamics

Background

The thymus plays a central role in shaping human immune function. A mechanistic, quantitative description of immune cell dynamics and thymic output under homeostatic conditions and various patho-physiological scenarios are of particular interest in drug development applications, e.g., in the identification of potential therapeutic targets and selection of lead drug candidates against infectious diseases.

Methods

We here developed an integrative mathematical model of thymocyte dynamics in human. It incorporates mechanistic features of thymocyte homeostasis as well as spatial constraints of the thymus and considerations of age-dependent involution. All model parameter estimates were obtained based on published physiological data of thymocyte dynamics and thymus properties in mouse and human. We performed model sensitivity analyses to reveal potential therapeutic targets through an identification of processes critically affecting thymic function; we further explored differences in thymic function across healthy subjects, multiple sclerosis patients, and patients on fingolimod treatment.

Results

We found thymic function to be most impacted by the egress, proliferation, differentiation and death rates of those thymocytes which are most differentiated. Model predictions also showed that the clinically observed decrease in relapse risk with age, in multiple sclerosis patients who would have discontinued fingolimod therapy, can be explained mechanistically by decreased thymic output with age. Moreover, we quantified the effects of fingolimod treatment duration on thymic output.

Conclusions

In summary, the proposed model accurately describes, in mechanistic terms, thymic output as a function of age. It may be further used to perform predictive simulations of clinically relevant scenarios which combine specific patho-physiological conditions and pharmacological interventions of interest.

Origin, phenotype and autoimmune potential of T cells in human immune system mice receiving neonatal human thymus tissue

Robust human immune system (HIS) mice are created using human fetal thymus tissue and hematopoietic stem cells (HSCs). A HIS mouse model using neonatal human thymus tissue and umbilical cord blood (CB) HSCs (NeoHu) was recently described. We improved the model by removing the native murine thymus, which can also generate human T cells, and demonstrated definitively the capacity of human T cells to develop in a grafted neonatal human thymus. Human T cells derived from the neonatal thymus tissue appeared in peripheral blood early post-transplantation and CB-derived T cells appeared later. Naïve T cells were demonstrated in peripheral blood but effector memory and T peripheral helper phenotypes predominated later, in association with development of autoimmunity in some animals. Treatment of thymus grafts with 2-deoxyglucose (2-DG) increased the proportion of stem cells derived from injected HSCs, delayed onset of autoimmune disease, reduced early T cell reconstitution, and reduced effector/memory T cell conversion. Younger neonatal human thymus tissue was associated with improved T cell reconstitution. While the NeoHu model bypasses the need for fetal tissue, it has yet to demonstrate equivalent reconstitution to fetal tissue, though 2-DG can improve results by removing native thymocytes prior to transplantation.

Evaluation of thymopoiesis in healthy Turkish children aged 0-6 years

BACKGROUND

Early diagnosis and effective treatment serve as life-saving procedures for primary immunodeficiencies (PIDs) which are very common and a major public health problem in Turkey. Severe combined immunodeficiency (SCID) is constitutively a T-cell defect in which naïve T-cell development is defective due to the mutations in genes responsible for the T cell differentiation and insufficient thymopoiesis. So, assessment of thymopoiesis is very important in the diagnosis of SCID and several combined immune deficiencies (CIDs).

METHODS

The purpose of this study is to examine thymopoiesis in healthy children via measurement of recent thymic emigrants (RTE); T lymphocytes that express CD4, CD45RA and CD31 to establish the RTE reference values in Turkish children. RTE were measured in the peripheral blood (PB) of 120 healthy infants and children between 0-6 years including cord blood samples, by flow cytometry.

RESULTS

The absolute count of RTE cells and their relative ratios were found to be higher during the first year of life, being highest at the 6th month and tending to decrease significantly by age following birth (p=0.001). In the cord blood group, both values were lower than those in the 6-month-old group. The absolute lymphocyte count (ALC) varying by age, was found to reduce to 1850/mm³ in 4-years and after.

CONCLUSIONS

Here we evaluated normal thymopoiesis and established the normal reference levels of RTE cells in the peripheral blood of healthy children aged between 0-6 years. We believe that the collected data will contribute to early diagnosis and monitoring of immune reconstitution; serving as an additional fast and reliable marker for many PID patients especially for SCID including many other CIDs, especially in nations where newborn screening (NBS) via T cell receptor excision circles (TREC) has not yet become available.

A single-cell analysis of thymopoiesis and thymic iNKT cell development in pigs

Many aspects of the porcine immune system remain poorly characterized, which poses a barrier to improving swine health and utilizing pigs as preclinical models. Here, we employ single-cell RNA sequencing (scRNA-seq) to create a cell atlas of the early-adolescent pig thymus. Our data show conserved features as well as species-specific differences in cell states and cell types compared with human thymocytes. We also describe several unconventional T cell types with gene expression profiles associated with innate effector functions. This includes a cell census of more than 11,000 differentiating invariant natural killer T (iNKT) cells, which reveals that the functional diversity of pig iNKT cells differs substantially from the iNKT0/1/2/17 subset differentiation paradigm established in mice. Our data characterize key differentiation events in porcine thymopoiesis and iNKT cell maturation and provide important insights into pig T cell development.

Generation of Artificial Thymic Organoids from Human and Murine Hematopoietic Stem and Progenitor Cells

The generation of T cells is a complex, carefully orchestrated process that occurs in the thymus. The ability to mimic T cell differentiation in vitro has opened up avenues to better understand different stages of thymopoiesis but has also enabled the in vitro production of mature T cells suitable for immunotherapy. Among existing protocols, the artificial thymic organoid (ATO) system has been shown to be the most efficient at producing mature conventional T cells. In this serum-free model, human or murine hematopoietic stem and progenitor cells (HSPCs) are combined with a murine stromal cell line expressing a Notch ligand in a 3D cell aggregate. In ATOs, although only simple medium changes are required throughout the cultures, HSPCs differentiate into T cells with kinetics and phenotypes similar to those of endogenous thymopoiesis. This article describes protocols for the generation of ATOs from human and murine HSPCs. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Expansion and preparation of MS5-hDLL4 or MS5-mDLL4 cells Basic Protocol 2: Isolation of human hematopoietic stem and progenitor cells (HSPCs; CD34+ cells) Support Protocol 1: Transduction of human HSPCs (CD34+ cells) Basic Protocol 3: Production of thymic progenitors and mature T cells from human HSPCs in artificial thymic organoids (ATOs) Support Protocol 2: Phenotype analysis of human ATO cells by flow cytometry Basic Protocol 4: Isolation of murine HSPCs (Lin- Sca1+ cKit+; LSK) and hematopoietic stem cells (LSK CD150+ CD48-) Basic Protocol 5: Production of thymic progenitors and mature T cells from murine HSPCs in ATOs Support Protocol 3: Phenotype analysis of murine ATO cells by flow cytometry Alternate Protocol: Generation of ATOs from single HSPCs.

ETS2 repressor factor (ERF) is involved in T lymphocyte maturation acting as regulator of thymocyte lineage commitment

Thymocyte differentiation and lineage commitment is regulated by an extensive network of transcription factors and signaling molecules among which Erk plays a central role. However, Erk effectors as well as the molecular mechanisms underlying this network are not well understood. Erf is a ubiquitously expressed transcriptional repressor regulated by Erk-dependent phosphorylation. Here, we investigated the role of Erf in T cell maturation and lineage commitment, using a double-fluorescent Erf-floxed mouse to produce thymus-specific Erf knockouts. We observed significant accumulation of thymocytes in the CD4/CD8 DP stage, followed by a significant reduction in CD4SP cells, a trend for lower CD8SP cell frequency, and an elevated percentage of γδ expressing thymocytes in Erf-deficient mice. Also, an elevated number of CD69⁺ TCRβ⁺ cells indicates that thymocytes undergoing positive selection accumulate at this stage. The expression of transcription factors Gata3, ThPOK, and Socs1 that promote CD4⁺ cell commitment was significantly decreased in Erf-deficient mice. These findings suggest that Erf is involved in T cell maturation, acting as a positive regulator during CD4 and eventually CD8 lineage commitment, while negatively regulates the production of γδ T cells. In addition, Erf-deficient mice displayed decreased percentages of CD4⁺ and CD8⁺ splenocytes and elevated levels of IL-4 indicating that Erf may have an additional role in the homeostasis, differentiation, and immunologic response of helper and cytotoxic T cells in the periphery. Overall, our results show, for the first time, Erf's involvement in T cell biology suggesting that Erf acts as a potential regulator during thymocyte maturation and thymocyte lineage commitment, in γδ T cell generation, as well as in Th cell differentiation.

The Metabolic Landscape of Thymic T Cell Development In Vivo and In Vitro

Although metabolic pathways have been shown to control differentiation and activation in peripheral T cells, metabolic studies on thymic T cell development are still lacking, especially in human tissue. In this study, we use transcriptomics and extracellular flux analyses to investigate the metabolic profiles of primary thymic and in vitro-derived mouse and human thymocytes. Core metabolic pathways, specifically glycolysis and oxidative phosphorylation, undergo dramatic changes between the double-negative (DN), double-positive (DP), and mature single-positive (SP) stages in murine and human thymus. Remarkably, despite the absence of the complex multicellular thymic microenvironment, in vitro murine and human T cell development recapitulated the coordinated decrease in glycolytic and oxidative phosphorylation activity between the DN and DP stages seen in primary thymus. Moreover, by inducing in vitro T cell differentiation from Rag1^-/- mouse bone marrow, we show that reduced metabolic activity at the DP stage is independent of TCR rearrangement. Thus, our findings suggest that highly conserved metabolic transitions are critical for thymic T cell development.

Self-renewal of double-negative 3 early thymocytes enables thymus autonomy but compromises the β-selection checkpoint

T lymphocyte differentiation in the steady state is characterized by high cellular turnover whereby thymocytes do not self-renew. However, if deprived of competent progenitors, the thymus can temporarily maintain thymopoiesis autonomously. This bears a heavy cost, because prolongation of thymus autonomy causes leukemia. Here, we show that, at an early stage, thymus autonomy relies on double-negative 3 early (DN3e) thymocytes that acquire stem-cell-like properties. Following competent progenitor deprivation, DN3e thymocytes become long lived, are required for thymus autonomy, differentiate in vivo, and include DNA-label-retaining cells. At the single-cell level, the transcriptional programs of thymopoiesis in autonomy and the steady state are similar. However, a new cell population emerges in autonomy that expresses an aberrant Notch target gene signature and bypasses the β-selection checkpoint. In summary, DN3e thymocytes have the potential to self-renew and differentiate in vivo if cell competition is impaired, but this generates atypical cells, probably the precursors of leukemia.

Thymic activity in immune recovery after allogeneic hematopoietic stem cell transplantation in children

Thymic output was studied prospectively in 52 children who underwent allogeneic hematopoietic stem cell transplantation (allo-HSCT). Thymic activity was assessed by quantification of recent thymic emigrants (RTE) discriminated from the rest of naive T cells by immunophenotype CD3+/CD4+/CD31+/CD45RA+. Thymic output was analyzed in correlation with the kinetics of immune recovery and in relation to other potential risk factors that may influence thymopoiesis: underlying disease, type of HSCT, source of stem cells, age of recipient and donor, type of conditioning, implemented graft versus host disease (GvHD) prophylaxis, viral reactivations (herpes viruses cytomegalovirus - CMV, Epstein-Barr virus - EBV, adenovirus - ADV, BK virus - BKV), occurrence and grade of both acute and chronic graft versus host disease (aGvHD, cGvHD) and number of transplanted CD34 cells/kg. The absolute count of RTE in peripheral blood was evaluated at 6 time points: before the conditioning and on days +15, +30, +60 , +90 and +180 after HSCT. Occurrence of grade II-IV aGvHD was the most important factor associated with low RTE counts after HSCT. History of malignant disease, and transplantation from matched unrelated donor were risk factors for lower thymic output. We found a weak inverse correlation between the age of the recipient and thymic output on post-HSCT day +180. Source of stem cells, type of conditioning, viral reactivations, occurrence of chronic GvHD, age of the donor and the number of transplanted CD34 cells/kg did not affect thymopoiesis in our study group. These preliminary findings and identification of risk factors for deterioration of thymic activity may in the future help in selecting candidates for thymus rejuvenation strategies.

Thymic Epithelial Cells Contribute to Thymopoiesis and T Cell Development

The thymus is the primary lymphoid organ responsible for the generation and maturation of T cells. Thymic epithelial cells (TECs) account for the majority of thymic stromal components. They are further divided into cortical and medullary TECs based on their localization within the thymus and are involved in positive and negative selection, respectively. Establishment of self-tolerance in the thymus depends on promiscuous gene expression (pGE) of tissue-restricted antigens (TRAs) by TECs. Such pGE is co-controlled by the autoimmune regulator (Aire) and forebrain embryonic zinc fingerlike protein 2 (Fezf2). Over the past two decades, research has found that TECs contribute greatly to thymopoiesis and T cell development. In turn, signals from T cells regulate the differentiation and maturation of TECs. Several signaling pathways essential for the development and maturation of TECs have been discovered. New technology and animal models have provided important observations on TEC differentiation, development, and thymopoiesis. In this review, we will discuss recent advances in classification, development, and maintenance of TECs and mechanisms that control TEC functions during thymic involution and central tolerance.

Qualitative Changes in Cortical Thymic Epithelial Cells Drive Postpartum Thymic Regeneration

During gestation, sex hormones cause a significant thymic involution which enhances fertility. This thymic involution is rapidly corrected following parturition. As thymic epithelial cells (TECs) are responsible for the regulation of thymopoiesis, we analyzed the sequential phenotypic and transcriptomic changes in TECs during the postpartum period in order to identify mechanisms triggering postpartum thymic regeneration. In particular, we performed flow cytometry analyses and deep RNA-sequencing on purified TEC subsets at several time points before and after parturition. We report that pregnancy-induced involution is not caused by loss of TECs since their number does not change during or after pregnancy. However, during pregnancy, we observed a significant depletion of all thymocyte subsets downstream of the double-negative 1 (DN1) differentiation stage. Variations in thymocyte numbers correlated with conspicuous changes in the transcriptome of cortical TECs (cTECs). The transcriptomic changes affected predominantly cTEC expression of Foxn1, its targets and several genes that are essential for thymopoiesis. By contrast, medullary TECs (mTECs) showed very little transcriptomic changes in the early postpartum regenerative phase, but seemed to respond to the expansion of single-positive (SP) thymocytes in the late phase of regeneration. Together, these results show that postpartum thymic regeneration is orchestrated by variations in expression of a well-defined subset of cTEC genes, that occur very early after parturition.

Reduced positive selection of a human TCR in a swine thymus using a humanized mouse model for xenotolerance induction

BACKGROUND

Tolerance-inducing approaches to xenotransplantation would be optimal and may be necessary for long-term survival of transplanted pig organs in human patients. The ideal approach would generate donor-specific unresponsiveness to the pig organ without suppressing the patient's normal immune function. Porcine thymus transplantation has shown efficacy in promoting xenotolerance in humanized mice and large animal models. However, murine studies demonstrate that T cells selected in a swine thymus are positively selected only by swine thymic epithelial cells, and therefore, cells expressing human HLA-restricted TCRs may not be selected efficiently in a transplanted pig thymus. This may lead to suboptimal patient immune function.

METHODS

To assess human thymocyte selection in a pig thymus, we used a TCR transgenic humanized mouse model to study positive selection of cells expressing the MART1 TCR, a well-characterized human HLA-A2-restricted TCR, in a grafted pig thymus.

RESULTS

Positive selection of T cells expressing the MART1 TCR was inefficient in both a non-selecting human HLA-A2- or swine thymus compared with an HLA-A2+ thymus. Additionally, CD8 MART1 TCRbright T cells were detected in the spleens of mice transplanted with HLA-A2+ thymi but were significantly reduced in the spleens of mice transplanted with swine or HLA-A2- thymi. [Correction added on October 15, 2019, after first online publication: The missing superscript values +, -, and bright have been included in the Results section.] CONCLUSIONS: Positive selection of cells expressing a human-restricted TCR in a transplanted pig thymus is inefficient, suggesting that modifications to improve positive selection of cells expressing human-restricted TCRs in a pig thymus may be necessary to support development of a protective human T-cell pool in future patients.

Development of Immature CD4+CD8+T Cells Into Mature CD4+ T Cells Requires Alpha-1 Antitrypsin as Observed by Treatment in HIV-1 Infected and Uninfected Controls

Immune cells are, by default, migratory cells that traverse tissue for the purpose of carrying out recognition and recruitment in pathologic inflammation and infection. Members of the LDL receptor family (LDL-RFMs) interact with human leukocyte elastase on the cell surface (HLE-CS) in complex with the abundant blood protein α1proteinase inhibitor (α1PI, α1-antitrypsin, Alpha-1), a process that induces internalization of aggregated functionally-related receptors, including CD4 and the T cell antigen receptor, while simultaneously promoting cellular locomotion. We sought to determine whether augmenting α1PI blood concentration would promote the locomotion of immature T cells through the thymus and generate new CD4⁺ T cells. Two small clinical trials (NCT01370018, NCT01731691, https://clinicaltrials.gov) were conducted in which HIV-1 infected and uninfected individuals were augmented with α1PI and compared with placebo-treated subjects and untreated controls. Blood cell phenotypes were monitored weekly. We found that CD4/CD8 ratio was significantly increased by α1PI augmentation in both uninfected and HIV-1 infected individuals. We found that maturation of CD4⁺CD8⁺ T cells to become immunologically competent CD4⁺ T cells was regulated by α1PI. We propose a strategy targeting HLE-CS for treating secondary immunodeficiency for which there is currently no direct treatment. Treatment to directly elevate T cells in patients with secondary immunodeficiency, including HIV disease, can be provided by alpha-1 antitrypsin augmentation or small molecules that target HLE-CS. Because individuals infected with HIV-1 produce a monoclonal antibody, 3F5, which binds to and inactivates α1PI, a process that prevents α1PI from binding to HLE-CS, thereby blocking locomotion of immature T cells through the thymus to generate CD4⁺ T cells, we further propose that HIV-1 vaccination should include induction of an antibody that binds to and blocks 3F5 activity, thereby preventing AIDS in addition to the current vaccine strategy for preventing HIV-1 infection.

Generation of Tumor Antigen-Specific iPSC-Derived Thymic Emigrants Using a 3D Thymic Culture System

Induced pluripotent stem cell (iPSC)-derived T cells may provide future therapies for cancer patients, but those generated by current methods, such as the OP9/DLL1 system, have shown abnormalities that pose major barriers for clinical translation. Our data indicate that these iPSC-derived CD8 single-positive T cells are more like CD4⁺CD8⁺ double-positive T cells than mature naive T cells because they display phenotypic markers of developmental arrest and an innate-like phenotype after stimulation. We developed a 3D thymic culture system to avoid these aberrant developmental fates, generating a homogeneous subset of CD8αβ⁺ antigen-specific T cells, designated iPSC-derived thymic emigrants (iTEs). iTEs exhibit phenotypic and functional similarities to naive T cells both in vitro and in vivo, including the capacity for expansion, memory formation, and tumor suppression. These data illustrate the limitations of current methods and provide a tool to develop the next generation of iPSC-based antigen-specific immunotherapies.

Heterozygous FOXN1 Variants Cause Low TRECs and Severe T Cell Lymphopenia, Revealing a Crucial Role of FOXN1 in Supporting Early Thymopoiesis

FOXN1 is the master regulatory gene of thymic epithelium development. FOXN1 deficiency leads to thymic aplasia, alopecia, and nail dystrophy, accounting for the nude/severe combined immunodeficiency (nu/SCID) phenotype in humans and mice. We identified several newborns with low levels of T cell receptor excision circles (TRECs) and T cell lymphopenia at birth, who carried heterozygous loss-of-function FOXN1 variants. Longitudinal analysis showed persistent T cell lymphopenia during infancy, often associated with nail dystrophy. Adult individuals with heterozygous FOXN1 variants had in most cases normal CD4+ but lower than normal CD8+ cell counts. We hypothesized a FOXN1 gene dosage effect on the function of thymic epithelial cells (TECs) and thymopoiesis and postulated that these effects would be more prominent early in life. To test this hypothesis, we analyzed TEC subset frequency and phenotype, early thymic progenitor (ETP) cell count, and expression of FOXN1 target genes (Ccl25, Cxcl12, Dll4, Scf, Psmb11, Prss16, and Cd83) in Foxn1nu/+ (nu/+) mice and age-matched wild-type (+/+) littermate controls. Both the frequency and the absolute count of ETP were significantly reduced in nu/+ mice up to 3 weeks of age. Analysis of the TEC compartment showed reduced expression of FOXN1 target genes and delayed maturation of the medullary TEC compartment in nu/+ mice. These observations establish a FOXN1 gene dosage effect on thymic function and identify FOXN1 haploinsufficiency as an important genetic determinant of T cell lymphopenia at birth.

Zebrafish and Medaka: Two Teleost Models of T-Cell and Thymic Development

Over the past two decades, studies have demonstrated that several features of T-cell and thymic development are conserved from teleosts to mammals. In particular, works using zebrafish (Danio rerio) and medaka (Oryzias latipes) have shed light on the cellular and molecular mechanisms underlying these biological processes. In particular, the ease of noninvasive in vivo imaging of these species enables direct visualization of all events associated with these processes, which are, in mice, technically very demanding. In this review, we focus on defining the similarities and differences between zebrafish and medaka in T-cell development and thymus organogenesis; and highlight their advantages as two complementary model systems for T-cell immunobiology and modeling of human diseases.

Long-term survival and differentiation of human thymocytes in human thymus-grafted immunodeficient mice

Aim: Thymus transplants have produced encouraging clinical outcomes in achieving thymopoiesis and T-cell development. This study was aimed to investigate whether human thymus contains self-renewing lymphoid progenitors capable of maintaining long-term T-cell development. Materials & methods: Immunodeficient mice were transplanted with human thymic tissue along with autologous GFP-expressing or allogeneic CD34+ cells and followed for human thymopoiesis and T-cell development from the thymic progenitors versus CD34+ cells, which can be distinguished by GFP or HLA expression. Results: In both models, long-term thymopoiesis and T-cell development from the thymic grafts were detected. In these mice, human thymic progenitor-derived T cells including CD45RA+CD31+CD4+ new thymic emigrants were persistently present in the periphery throughout the observation period (32 weeks). Conclusion: The results indicate that human thymus contains long-lived lymphoid progenitors that can maintain durable thymopoiesis and T-cell development.

Long Term Outcome and Immune Function After Hematopoietic Stem Cell Transplantation for Primary Immunodeficiency

Transplantation techniques for patients with primary immunodeficiencies have improved so that survival from the procedure in many cases is >80%. However, long term complications may arise due to the use or not of conditioning agents. This may result in variable immune reconstitution, the long term effects of chemotherapy, particularly on fertility, and complications relating to the genetic disorder, unresolved by transplantation. For patients with severe combined immunodeficiency (SCID), long term T- and B-lymphocyte immune reconstitution is best achieved after pre-transplant chemotherapy. For patients who receive an unconditioned infusion of donor stem cells, the quality of immune reconstitution depends on the SCID genotype. Long term effects include chemotherapy-induced impaired fertility, and sequelae specific to the genotype. For patients with other primary immunodeficiencies, conditioning is required-sequelae related to direct effects of chemotherapy may be observed. Additional long term effects may be observed due to partial donor chimerism resulting in incomplete eradication of disease, and other geno-specific effects.

Thymopoiesis in Pre- and Post-Hematopoietic Stem Cell Transplantation

Hematopoietic stem cell transplantation (HSCT) is an important therapeutic option for some hematological diseases. However, patients who undergo HSCT acquire a state of immunodeficiency that causes significant mortality. Reconstitution of thymic function is needed to support the immune system. One way to measure thymic function is through T-cell receptor excision circle (TREC) quantification. TRECs are generated by T-cell receptor gene rearrangements during T-cell maturation in the thymus and represent a reliable marker for thymic output. In this study, we aimed to assess aging and malignant hematological diseases as two important factors that may influence thymic output before HSCT. We observed that patients before HSCT presented signal joint TREC (sjTREC) numbers lower than 606.55 copies/μg DNA (low values) compared with healthy individuals, with an odds ratio (OR) of 12.88 [95% confidence interval (CI): 5.26–31.53; p < 0.001]. Our results showed that a group of older individuals (≥50 years old), comprising both healthy individuals and patients, had an OR of 10.07 (95% CI: 2.80–36.20) for low sjTREC values compared with younger individuals (≤24 years old; p < 0.001). Multiple logistic regression analysis confirmed that both older age (≥50 years old) and malignant hematological diseases and their treatments were important and independent risk factors related to thymic function impairment (p < 0.001). The median sjTREC value for patients of all ages was significantly lower than the sjTREC median for the subgroup of older healthy individuals (≥50 years old; p < 0.001). These data suggested that patients before HSCT and healthy individuals exhibited age-dependent thymic impairment, and that prior treatment for hematological diseases may exacerbate aging-related deterioration of natural thymic function. Furthermore, we analyzed these patients 9 months post-HSCT and compared patients who underwent autologous HSCT with those who underwent allogeneic HSCT. Both groups of patients achieved sjTREC copy numbers similar to those of healthy individuals. We did not find a close relationship between impaired thymic function prior to HSCT and worse thymic recovery after HSCT.

Inhibition of RORγT Skews TCRα Gene Rearrangement and Limits T Cell Repertoire Diversity

Recent studies have elucidated the molecular mechanism of RORγT transcriptional regulation of Th17 differentiation and function. RORγT was initially identified as a transcription factor required for thymopoiesis by maintaining survival of CD4⁺CD8⁺ (DP) thymocytes. While RORγ antagonists are currently being developed to treat autoimmunity, it remains unclear how RORγT inhibition may impact thymocyte development. In this study, we show that in addition to regulating DP thymocytes survival, RORγT also controls genes that regulate thymocyte migration, proliferation, and T cell receptor (TCR)α selection. Strikingly, pharmacological inhibition of RORγ skews TCRα gene rearrangement, limits T cell repertoire diversity, and inhibits development of autoimmune encephalomyelitis. Thus, targeting RORγT not only inhibits Th17 cell development and function but also fundamentally alters thymic-emigrant recognition of self and foreign antigens. The analysis of RORγ inhibitors has allowed us to gain a broader perspective of the diverse function of RORγT and its impact on T cell biology.

Treatment of Pediatric Acute Graft-versus-Host Disease-Lessons from Primary Immunodeficiency?

Allogeneic hematopoietic stem cell transplant (HSCT) is used to treat increasing numbers of malignant and non-malignant disorders. Despite significant advances in improved human leukocyte antigens-typing techniques, less toxic conditioning regimens and better supportive care, resulting in improved clinical outcomes, acute graft-versus-host disease (aGvHD) continues to be a major obstacle and, although it principally involves the skin, gastrointestinal tract, and liver, the thymus is also a primary target. An important aim following HSCT is to achieve complete and durable immunoreconstitution with a diverse T-cell receptor (TCR) repertoire to recognize a broad range of pathogens providing adequate long-term adaptive T-lymphocyte immunity, essential to reduce the risk of infection, disease relapse, and secondary malignancies. Reconstitution of adaptive T-lymphocyte immunity is a lengthy and complex process which requires a functioning and structurally intact thymus responsible for the production of new naïve T-lymphocytes with a broad TCR repertoire. Damage to the thymic microenvironment, secondary to aGvHD and the effect of corticosteroid treatment, disturbs normal signaling required for thymocyte development, resulting in impaired T-lymphopoiesis and reduced thymic export. Primary immunodeficiencies, in which failure of central or peripheral tolerance is a major feature, because of intrinsic defects in hematopoietic stem cells leading to abnormal T-lymphocyte development, or defects in thymic stroma, can give insights into critical processes important for recovery from aGvHD. Extracorporeal photopheresis is a potential alternative therapy for aGvHD, which acts in an immunomodulatory fashion, through the generation of regulatory T-lymphocytes (Tregs), alteration of cytokine patterns and modulation of dendritic cells. Promoting normal central and peripheral immune tolerance, with selective downregulation of immune stimulation, could reduce aGvHD, and enable a reduction in other immunosuppression, facilitating thymic recovery, restoration of normal T-lymphocyte ontogeny, and complete immunoreconstitution with improved clinical outcome as the ability to fight infections improves and risk of secondary malignancy or relapse diminishes.

Antagonist effect of Interleukin 1 receptor on normal thymopoiesis and thymus toxicity of 5-azacytidine in mouse

Thymopoiesis is essential and significant for development and maintenance of the robust and healthy immune system. The acute suppression of thymopoiesis induced by 5-Azacytidine (5-Aza) is an intractable clinical problem complicating chemotherapy. Interleukin 1 receptor antagonist (IL-1Ra) is a cytokine that competitively blocks binding of interleukin 1 (IL-1) to its receptor. This study aims to investigate the effects of the IL-1Ra on the thymus toxicity of 5-Aza in mouse. In this study, we treated the mice with the 5-Aza (100 mg/kg per mouse). The GeneChip methodology developed by Affymetrix was used to monitor global gene expression during mouse thymus regeneration induced by a single injection of 5-Aza. The total thymocytes were counted using a hemocytometer. Cell cycle of samples were analyzed on a Becton Dickinson FACScan. Cells surfaces were labeled with anti-CD4, anti-CD8 and anti-CD45RA antibodies, and detected by flow cytometry. BrdU incorporation was detected by flow cytometry. The results indicated that administering exogenous IL-1Ra to normal mice inhibited cell cycle progress of thymocytes in a dosage-dependent manner. Proliferation of immature CD4(-)CD8(-) double negative (DN) and CD4(+)CD8(+) double positive (DP) thymocytes were both inhibited. The pretreatment of normal mice with exogenous IL-1Ra reduced acute toxicity on thymus and immune suppression induced by 5-Aza. Furthermore, thymus reconstitution after 5-Aza treatment was accelerated by IL-1Ra. In conclusion, interleukin 1 receptor antagonist could inhibit normal thymopoiesis and reduce thymus toxicity of 5-azacytidine in mouse. Pretreatment with IL-1Ra would offer a new and promising strategy to alleviate immunotoxicity of chemotherapy in clinical.

A Tale from TGF-β Superfamily for Thymus Ontogeny and Function

Multiple signaling pathways control every aspect of cell behavior, organ formation, and tissue homeostasis throughout the lifespan of any individual. This review takes an ontogenetic view focused on the large superfamily of TGF-β/bone morphogenetic protein ligands to address thymus morphogenesis and function in T cell differentiation. Recent findings on a role of GDF11 for reversing aging-related phenotypes are also discussed.

Cytoreductive conditioning for severe combined immunodeficiency--help or hindrance?

Use of chemotherapy-based conditioning-facilitated engraftment in patients with severe combined immunodeficiency (SCID) is contentious. In T- and NK lymphocyte-negative, B-lymphocyte-positive (T-B+NK+) and T-B-NK+ SCID, the osteo-medullary space is occupied by recipient hematopoietic stem cells and mature B-lymphocytes. The thymic niche is empty in T-B+NK+ SCID but fully occupied by developmentally arrested T-lymphocyte precursors in T-B-NK+ SCID. The outcome of infusion of donor stem cells differs and is dependent on genetic defect and the lymphocyte developmental arrest stage. At best, donor hematopoietic stem cell osteo-medullary engraftment induces normal B-lymphocyte function and long-term thymopoiesis; at worst, peripheral expansion of donor T-lymphocytes from the stem cell source results in a restricted T-lymphocyte receptor repertoire with possible B-lymphocyte failure. Conditioning improves immunoreconstitution but causes short- and long-term toxicities, and increased mortality. Newborn screening for SCID will propel the search for safe, effective methods of achieving donor cell engraftment and full immunoreconstitution without toxic sequalae.

Ovarian hormone withdrawal in prepubertal developmental stage does not prevent thymic involution in rats

The study was undertaken to assess the effects of ovarian hormone withdrawal in prepubertal age on thymopoiesis in 2- (young) and 11-month-old (middle-aged) rats. In ovariectomized (Ox) rats, irrespective of age, thymic weight and cellularity were greater than in age-matched controls, but the values of both parameters exhibited the age-related decline. In addition, although thymopoietic efficiency was increased in both groups of Ox rats when compared with age-matched controls, thymopoiesis exhibited the age-related decline mirrored in the lower numbers of both CD4+ and CD8+ recent thymic emigrants in peripheral blood. This reflected the prethymic changes affecting bone marrow progenitor generation/entry and the thymic alterations encompassing the impaired progenitor progression through early pre-T-cell receptor developmental stages (defined by CD45RC/CD2 expression) and, possibly, a more pronounced decrease in the proliferation of the most mature thymocytes. Apart from the changes at thymocyte level, in Ox rats the age-related alterations in thymic stroma (substantiated in a prominent loss of thymic epithelial cells) were registered. Ovariectomy-induced changes in thymic lymphoid and epithelial component, most probably, influenced each other leading to the increase in thymic expression of interleukin-6 and interleukin-7 mRNAs along with time after ovariectomy. Collectively, the study showed that the withdrawal of ovarian hormones in prepubertal age increases the efficiency of thymopoiesis in young adult rats, but does not prevent decline in thymopoiesis occurring with age.

Changes in primary lymphoid organs with aging

Aging is associated with decreased immune function that leads to increased morbidity and mortality in the elderly. Immune senescence is accompanied by age-related changes in two primary lymphoid organs, bone marrow and thymus, that result in decreased production and function of B and T lymphocytes. In bone marrow, hematopoietic stem cells exhibit reduced self-renewal potential, increased skewing toward myelopoiesis, and decreased production of lymphocytes with aging. These functional sequelae of aging are caused in part by increased oxidative stress, inflammation, adipocyte differentiation, and disruption of hypoxic osteoblastic niches. In thymus, aging is associated with tissue involution, exhibited by a disorganization of the thymic epithelial cell architecture and increased adiposity. This dysregulation correlates with a loss of stroma-thymocyte 'cross-talk', resulting in decreased export of naïve T cells. Mounting evidence argues that with aging, thymic inflammation, systemic stress, local Foxn1 and keratinocyte growth factor expression, and sex steroid levels play critical roles in actively driving thymic involution and overall adaptive immune senescence across the lifespan. With a better understanding of the complex mechanisms and pathways that mediate bone marrow and thymus involution with aging, potential increases for the development of safe and effective interventions to prevent or restore loss of immune function with aging.

Thymic microenvironment reconstitution after postnatal human thymus transplantation

A functional thymus develops after cultured thymus tissue is transplanted into subjects with complete DiGeorge anomaly. To gain insight into how the process occurs, 7 post-transplantation thymus biopsy tissues were evaluated. In 5 of 7 biopsies, the thymus appeared to be predominantly cortex with thymocytes expressing cortical markers. Unexpectedly, the epithelium expressed both cortical [cortical dendritic reticulum antigen 2 (CDR2)] and medullary [cytokeratin (CK) 14] markers. Early medullary development was suggested by epithelial cell adhesion molecule (EpCAM) reactivity in small areas of biopsies. Two other biopsies had distinct mature cortex and medulla with normal restriction of CK14 to the medulla and subcapsular cortex, and of CDR2 to cortex. These data are consistent with a model in which thymic epithelium contains CK14+ "progenitor epithelial cells". After transplantation these cells proliferate as CK14+CDR2+ thymic epithelial cells that are associated with cortical thymocytes. Later these cells differentiate into distinct cortical and medullary epithelia.

Leptin receptor is expressed in thymus medulla and leptin protects against thymic remodeling during endotoxemia-induced thymus involution

Leptin deficiency in mice results in chronic thymic atrophy, suppressed cell-mediated immunity, and decreased numbers of total lymphocytes, suggesting a key role for the metabolic hormone leptin in regulating thymopoiesis and overall immune homeostasis. Unfortunately, the thymus is highly susceptible to stress-induced acute involution. Prolonged thymus atrophy in stress situations can contribute to peripheral T cell deficiency or inhibit immune reconstitution. Little is known, however, about specific roles for leptin signaling in the thymus or the underlying mechanisms driving thymic involution or thymic recovery after acute stress. We report here that leptin receptor expression is restricted in thymus to medullary epithelial cells. Using a model of endotoxemia-induced acute thymic involution and recovery, we have demonstrated a role for supraphysiologic leptin in protection of thymic epithelial cells (TECs). We also present data in support of our hypothesis that leptin treatment decreases in vivo endotoxemia-induced apoptosis of double positive thymocytes and promotes proliferation of double negative thymocytes through a leptin receptor isoform b-specific mechanism. Furthermore, our studies have revealed that leptin treatment increases thymic expression of interleukin-7, an important soluble thymocyte growth factor produced by medullary TECs. Taken together, these studies support an intrathymic role for the metabolic hormone leptin in maintaining healthy thymic epithelium and promoting thymopoiesis, which is revealed when thymus homeostasis is perturbed by endotoxemia.

Adult thymus transplantation with allogeneic intra-bone marrow-bone marrow transplantation from same donor induces high thymopoiesis, mild graft-versus-host reaction and strong graft-versus-tumour effects

Although allogeneic bone marrow transplantation (BMT) plus donor lymphocyte infusion (DLI) is performed for solid tumours to enhance graft-versus-tumour (GVT) effects, a graft-versus-host reaction (GVHR) is also elicited. We carried out intra-bone marrow-bone marrow transplantation (IBM-BMT) plus adult thymus transplantation (ATT) from the same donor to supply alloreactive T cells continually. Normal mice treated with IBM-BMT + ATT survived for a long time with high donor-derived thymopoiesis and mild GVHR. The percentage of CD4(+) FoxP3(+) regulatory T cells in the spleen of the mice treated with IBM-BMT + ATT was lower than in normal B6 mice or mice treated with IBM-BMT alone, but higher than in mice treated with IBM-BMT + DLI; the mice treated with IBM-BMT + DLI showed severe GVHR. In tumour-bearing mice, tumour growth was more strongly inhibited by IBM-BMT + ATT than by IBM-BMT alone. Mice treated with IBM-BMT + a high dose of DLI also showed tumour regression comparable to that of mice treated with IBM-BMT + ATT but died early of GVHD. By contrast, mice treated with IBM-BMT + a low dose of DLI showed longer survival but less tumour regression than the mice treated with IBM-BMT + ATT. Histologically, significant numbers of CD8(+) T cells were found to have infiltrated the tumour in the mice treated with IBM-BMT + ATT. The number of terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labelling (TUNEL)-positive apoptotic tumour cells also significantly increased in the mice treated with IBM-BMT + ATT. Allogeneic IBM-BMT + ATT thus can induce high thymopoiesis, preserving strong GVT effects without severe GVHR.

FoxP3: a life beyond regulatory T cells

This review analyzes the current dogma that FoxP3 functions exclusively in the regulatory T cells (Treg) and that FoxP3(+) Treg is indispensable for survival of immune competent mice. We outline evidence that FoxP3 is expressed well beyond Treg and that the FoxP3 mutation in thymic stromal cells causes defective thymopoiesis, which in turn leads to increased homeostatic proliferation. We argue that the lethal autoimmune disease in mice with germline mutation of FoxP3 is due to both lack of Treg and enhanced homeostatic proliferation.

Thymic medullary epithelial cell differentiation, thymocyte emigration, and the control of autoimmunity require lympho-epithelial cross talk via LTbetaR

Thymocytes depend on the interaction with thymic epithelial cells for the generation of a diverse, nonautoreactive T cell repertoire. In turn, thymic epithelial cells acquire their three-dimensional cellular organization via instructive signals from developing thymocytes. The nature of these signals has been elusive so far. We show that thymocytes and medullary epithelial cells (MECs) communicate via the lymphotoxin beta receptor (LTbetaR) signaling axis. Normal differentiation of thymic MECs requires LTbetaR ligand on thymocytes and LTbetaR together with nuclear factor-kappaB-inducing kinase (Nik) in thymic epithelial cells. Impaired lympho-epithelial cross talk in the absence of the LTbetaR causes aberrant differentiation and reduced numbers of thymic MECs, leads to the retention of mature T lymphocytes, and is associated with autoimmune phenomena, suggesting an unexpected role for LTbetaR signaling in central tolerance induction.