Reduction in the developmental potential of intrathymic T cell progenitors with age

Transient PU.1 low fetal progenitors generate lymphoid progeny that contribute to adult immunity

Hematopoietic stem cells and multipotential progenitors emerge in multiple, overlapping waves of fetal development. Some of these populations seed the bone marrow and sustain adult B- and T-cell development long-term after birth. However, others are present transiently, but whether they are vestigial or generate B and T cells that contribute to the adult immune system is not well understood. We now report that transient fetal progenitors distinguished by expression of low levels of the PU.1 transcription factor generated activated and memory T and B cells that colonized and were maintained in secondary lymphoid tissues. These included the small and large intestines, where they may contribute to the maintenance of gut homeostasis through at least middle age. At least some of the activated/memory cells may have been the progeny of B-1 and marginal zone B cells, as transient PU.1low fetal progenitors efficiently generated those populations. Taken together, our data demonstrate the potential of B- and T-cell progeny of transient PU.1low fetal progenitors to make an early and long-term contribution to the adult immune system.

An epigenetically distinct HSC subset supports thymic reconstitution

Hematopoietic stem cells (HSCs) with multilineage potential are critical for effective T cell reconstitution and restoration of the adaptive immune system after allogeneic Hematopoietic Cell Transplantation (allo-HCT). The Kit lo subset of HSCs is enriched for multipotential precursors, 1, 2 but their T-cell lineage potential has not been well-characterized. We therefore studied the thymic reconstituting and T-cell potential of Kit lo HSCs. Using a preclinical allo-HCT model, we demonstrate that Kit lo HSCs support better thymic recovery, and T-cell reconstitution resulting in improved T cell responses to infection post-HCT. Furthermore, Kit lo HSCs with augmented BM lymphopoiesis mitigate age-associated thymic alterations, thus enhancing T-cell recovery in middle-aged hosts. We find the frequency of the Kit lo subset declines with age, providing one explanation for the reduced frequency of T-competent HSCs and reduced T-lymphopoietic potential in BM precursors of aged mice. 3, 4, 5 Chromatin profiling revealed that Kit lo HSCs exhibit higher activity of lymphoid-specifying transcription factors (TFs), including Zbtb1 . Deletion of Zbtb1 in Kit lo HSCs diminished their T-cell potential, while reinstating Zbtb1 in megakaryocytic-biased Kit hi HSCs rescued T-cell potential, in vitro and in vivo . Finally, we discover an analogous Kit lo HSC subset with enhanced lymphoid potential in human bone marrow. Our results demonstrate that Kit lo HSCs with enhanced lymphoid potential have a distinct underlying epigenetic program.

Epigallocatechin-3-gallate ameliorates lipopolysaccharide-induced acute thymus involution in mice via AMPK/Sirt1 pathway

The thymus, a site to culture the naïve T lymphocytes, is susceptible to atrophy or involution due to aging, inflammation, and oxidation. Epigallocatechin-3-gallate (EGCG) has been proven to possess anti-inflammatory, antioxidant, and antitumor activity. Here, we investigate the effects of EGCG on thymic involution induced by lipopolysaccharide (LPS), an endotoxin derived from Gram-negative bacteria. The methodology included an in vivo experiment on female Kunming mice exposed to LPS and EGCG. Morphological assessment of thymic involution, immunohistochemical detection, and thymocyte subsets analysis by flow cytometry were further carried out to evaluate the potential role of EGCG on the thymus. As a result, we found that EGCG alleviated LPS-induced thymic atrophy, increased mitochondrial membrane potential and superoxide dismutase levels, and decreased malondialdehyde and reactive oxygen species levels. In addition, EGCG pre-supplement restored the ratio of thymocyte subsets, the expression of autoimmune regulator, sex-determining region Y-box 2, and Nanog homebox, and reduced the number of senescent cells and collagen fiber deposition. Western blotting results indicated that EGCG treatment elevated LPS-induced decrease in pAMPK, Sirt1 protein expression. Collectively, EGCG relieved thymus architecture and function damaged by LPS via regulation of AMPK/Sirt1 signaling pathway. Our findings may provide a new strategy on protection of thymus from involution caused by LPS by using EGCG. And EGCG might be considered as a potential agent for the prevention and treatment of thymic involution.

Effects of Deuterium Depletion on Age-Declining Thymopoiesis In Vivo

The thymus provides maturation and migration of T cells to peripheral organs of immunity, where they recognize diverse antigens and maintain immunological memory and self-tolerance. The thymus is known to be involved with age and in response to stress factors. Therefore, the search for approaches to the restoration of thymopoiesis is of great interest. The present investigation was aimed at evaluating how prolonged deuterium depletion affects morphogenetic processes and the physiological transition of the thymus to age-related involution. The study was performed on 60 male Wistar rats subjected to consumption of deuterium-depleted water with a 10 ppm deuterium content for 28 days. The control rats consumed distilled water with a normal deuterium content of 150 ppm. The examination found no significant differences in body weight gain or the amount of water consumed. The exposed rats exhibited similar to control dynamics of the thymus weight but significant changes in thymic cell maturation according to cytofluorimetric analysis of thymic subpopulations. Changes in T cell production were not monotonic and differentially engaged morphogenetic processes of cell proliferation, differentiation, and migration. The reactive response to deuterium depletion was a sharp increase in the number of progenitor CD4-CD8- cells and their differentiation into T cells. The compensatory reaction was inhibition of thymopoiesis with more pronounced suppression of differentiation of T-cytotoxic lymphocytes, followed by intensification of emigration of mature T cells to the bloodstream. This period lasts from 3 to 14 days, then differentiation of thymic lymphocytes is restored, later cell proliferation is activated, and finally the thymopoiesis rate exceeds the control values. The increase in the number of thymic progenitor cells after 3-4 weeks suggests consideration of deuterium elimination as a novel approach to prevent thymus involution.

Immune tolerance and the prevention of autoimmune diseases essentially depend on thymic tissue homeostasis

The intricate balance of immune reactions towards invading pathogens and immune tolerance towards self is pivotal in preventing autoimmune diseases, with the thymus playing a central role in establishing and maintaining this equilibrium. The induction of central immune tolerance in the thymus involves the elimination of self-reactive T cells, a mechanism essential for averting autoimmunity. Disruption of the thymic T cell selection mechanisms can lead to the development of autoimmune diseases. In the dynamic microenvironment of the thymus, T cell migration and interactions with thymic stromal cells are critical for the selection processes that ensure self-tolerance. Thymic epithelial cells are particularly significant in this context, presenting self-antigens and inducing the negative selection of autoreactive T cells. Further, the synergistic roles of thymic fibroblasts, B cells, and dendritic cells in antigen presentation, selection and the development of regulatory T cells are pivotal in maintaining immune responses tightly regulated. This review article collates these insights, offering a comprehensive examination of the multifaceted role of thymic tissue homeostasis in the establishment of immune tolerance and its implications in the prevention of autoimmune diseases. Additionally, the developmental pathways of the thymus are explored, highlighting how genetic aberrations can disrupt thymic architecture and function, leading to autoimmune conditions. The impact of infections on immune tolerance is another critical area, with pathogens potentially triggering autoimmunity by altering thymic homeostasis. Overall, this review underscores the integral role of thymic tissue homeostasis in the prevention of autoimmune diseases, discussing insights into potential therapeutic strategies and examining putative avenues for future research on developing thymic-based therapies in treating and preventing autoimmune conditions.

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 interferon gamma response signature links myocardial aging and immunosenescence

AIMS

Aging entails profound immunological transformations that can impact myocardial homeostasis and predispose to heart failure. However, preclinical research in the immune-cardiology field is mostly conducted in young healthy animals, which potentially weakens its translational relevance. Herein, we sought to investigate how the aging T-cell compartment associates with changes in myocardial cell biology in aged mice.

METHODS AND RESULTS

We phenotyped the antigen-experienced effector/memory T cells purified from heart-draining lymph nodes of 2-, 6-, 12-, and 18-month-old C57BL/6J mice using single-cell RNA/T cell receptor sequencing. Simultaneously, we profiled all non-cardiomyocyte cell subsets purified from 2- to 18-month-old hearts and integrated our data with publicly available cardiomyocyte single-cell sequencing datasets. Some of these findings were confirmed at the protein level by flow cytometry. With aging, the heart-draining lymph node and myocardial T cells underwent clonal expansion and exhibited an up-regulated pro-inflammatory transcription signature, marked by an increased interferon-γ (IFN-γ) production. In parallel, all major myocardial cell populations showed increased IFN-γ responsive signature with aging. In the aged cardiomyocytes, a stronger IFN-γ response signature was paralleled by the dampening of expression levels of transcripts related to most metabolic pathways, especially oxidative phosphorylation. Likewise, induced pluripotent stem cells-derived cardiomyocytes exposed to chronic, low grade IFN-γ treatment showed a similar inhibition of metabolic activity.

CONCLUSIONS

By investigating the paired age-related alterations in the T cells found in the heart and its draining lymph nodes, we provide evidence for increased myocardial IFN-γ signaling with age, which is associated with inflammatory and metabolic shifts typically seen in heart failure.

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.

An induced thymic epithelial cell-based high throughput screen for thymus extracellular matrix mimetics

Thymic epithelial cells (TECs) are key effectors of the thymic stroma and are critically required for T-cell development. TECs comprise a diverse set of related but functionally distinct cell types that are scarce and difficult to isolate and handle. This has precluded TEC-based screening assays. We previously described induced thymic epithelial cells (iTECs), an artificial cell type produced in vitro by direct reprogramming, raising the possibility that iTECs might provide the basis for functional screens related to TEC biology. Here, we present an iTEC-based three-stage medium/high-throughput in vitro assay for synthetic polymer mimics of thymic extracellular matrix (ECM). Using this assay, we identified, from a complex library, four polymers that bind iTEC as well as or better than gelatin but do not bind mesenchymal cells. We show that these four polymers also bind and maintain native mouse fetal TECs and native human fetal TECs. Finally, we show that the selected polymers do not interfere with iTEC function or T-cell development. Collectively, our data establish that iTECs can be used to screen for TEC-relevant compounds in at least some medium/high-throughput assays and identify synthetic polymer ECM mimics that can replace gelatin or ECM components in TEC culture protocols.

The potential role of the thymus in immunotherapies for acute myeloid leukemia

Understanding the factors which shape T-lymphocyte immunity is critical for the development and application of future immunotherapeutic strategies in treating hematological malignancies. The thymus, a specialized central lymphoid organ, plays important roles in generating a diverse T lymphocyte repertoire during the infantile and juvenile stages of humans. However, age-associated thymic involution and diseases or treatment associated injury result in a decline in its continuous role in the maintenance of T cell-mediated anti-tumor/virus immunity. Acute myeloid leukemia (AML) is an aggressive hematologic malignancy that mainly affects older adults, and the disease's progression is known to consist of an impaired immune surveillance including a reduction in naïve T cell output, a restriction in T cell receptor repertoire, and an increase in frequencies of regulatory T cells. As one of the most successful immunotherapies thus far developed for malignancy, T-cell-based adoptive cell therapies could be essential for the development of a durable effective treatment to eliminate residue leukemic cells (blasts) and prevent AML relapse. Thus, a detailed cellular and molecular landscape of how the adult thymus functions within the context of the AML microenvironment will provide new insights into both the immune-related pathogenesis and the regeneration of a functional immune system against leukemia in AML patients. Herein, we review the available evidence supporting the potential correlation between thymic dysfunction and T-lymphocyte impairment with the ontogeny of AML (II-VI). We then discuss how the thymus could impact current and future therapeutic approaches in AML (VII). Finally, we review various strategies to rejuvenate thymic function to improve the precision and efficacy of cancer immunotherapy (VIII).

Thymic changes as a contributing factor in the increased susceptibility of old Albino Oxford rats to EAE development

The study was aimed to examine putative contribution of thymic involution to ageing-associated increase in susceptibility of Albino Oxford (AO) rats to the development of clinical EAE, and vice versa influence of the disease on the progression of thymic involution. To this end we examined (i) the parameters of thymocyte negative selection efficacy, the thymic generation of CD4+CD25+Foxp3+ T regulatory cells (Tregs) and thymic capacity to instruct/predetermine IL-17-producing T-cell differentiation, and thymopietic efficacy-associated accumulation of "inflammescent" cytotoxic CD28- T cells in the periphery, and (ii) the key underlying mechanisms in young and old non-immunised AO rats and their counterparts immunised for EAE (on the 16^th day post-immunisation when the disease in old rats reached the plateau) using flow cytometry analysis and/or RT-qPCR. It was found that thymic involution impairs: (i) the efficacy of negative selection (by affecting thymocyte expression of CD90, negative regulator of selection threshold and the expression of thymic stromal cell integrity factors) and (ii) Treg generation (by diminishing expression of cytokines supporting their differentiation/maturation). Additionally, the results suggest that thymic involution facilitates CD8+ T-cell differentiation into IL-17-producing cells (previously linked to the development of clinical EAE in old AO rats). Furthermore, they confirmed that ageing-related decrease in thymic T-cell output (as indicated by diminished frequency of recent thymic emigrants in peripheral blood) resulted in the accumulation of CD28- T cells in peripheral blood and, upon immunisation, in the target organ. On the other hand, the development of EAE (most likely by increasing circulatory levels of proinflammatory cytokines) contributed to the decline in thymic output of T cells, including Tregs, and thereby to the progression/maintenance of clinical EAE. Thus, in AO rats thymic involution via multi-layered mechanisms may favour the development of clinically manifested autoimmunity, which, in turn, precipitates the thymus atrophy.

Age-related thymic involution: Mechanisms and functional impact

The thymus is the primary immune organ responsible for generating self‐tolerant and immunocompetent T cells. However, the thymus gradually involutes during early life resulting in declined naïve T‐cell production, a process known as age‐related thymic involution. Thymic involution has many negative impacts on immune function including reduced pathogen resistance, high autoimmunity incidence, and attenuated tumor immunosurveillance. Age‐related thymic involution leads to a gradual reduction in thymic cellularity and thymic stromal microenvironment disruption, including loss of definite cortical‐medullary junctions, reduction of cortical thymic epithelial cells and medullary thymic epithelial cells, fibroblast expansion, and an increase in perivascular space. The compromised thymic microenvironment in aged individuals substantially disturbs thymocyte development and differentiation. Age‐related thymic involution is regulated by many transcription factors, micro RNAs, growth factors, cytokines, and other factors. In this review, we summarize the current understanding of age‐related thymic involution mechanisms and effects.

T-Cell Progenitors As A New Immunotherapy to Bypass Hurdles of Allogeneic Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) is the treatment of preference for numerous malignant and non-malignant hemopathies. The outcome of this approach is significantly hampered by not only graft-versus-host disease (GvHD), but also infections and relapses that may occur because of persistent T-cell immunodeficiency following transplantation. Reconstitution of a functional T-cell repertoire can take more than 1 year. Thus, the major challenge in the management of allogeneic HSCT relies on the possibility of shortening the window of immune deficiency through the acceleration of T-cell recovery, with diverse, self-tolerant, and naïve T cells resulting from de novo thymopoiesis from the donor cells. In this context, adoptive transfer of cell populations that can give rise to mature T cells faster than HSCs while maintaining a safety profile compatible with clinical use is of major interest. In this review, we summarize current advances in the characterization of thymus seeding progenitors, and their ex vivo generated counterparts, T-cell progenitors. Transplantation of the latter has been identified as a worthwhile approach to shorten the period of immune deficiency in patients following allogeneic HSCT, and to fulfill the clinical objective of reducing morbimortality due to infections and relapses. We further discuss current opportunities for T-cell progenitor-based therapy manufacturing, including iPSC cell sources and off-the-shelf strategies. These opportunities will be analyzed in the light of results from ongoing clinical studies involving T-cell progenitors.

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.

Post-Irradiation Thymic Regeneration in B6C3F1 Mice Is Age Dependent and Modulated by Activation of the PI3K-AKT-mTOR Pathway

Simple Summary

Because children have a long life expectancy relative to adults and their tissues and organs are growing and developing rapidly, the risk of radiation carcinogenesis for children is considered higher than that for adults. However, the underlying mechanism(s) is unclear. To uncover the mechanism, we previously revealed that principal causative genes in mouse thymic lymphomas arising in irradiated infants or adults as Pten or Ikzf1, respectively, suggesting that cells with mutation in these genes might be the origin of lymphomas arising after irradiation depending on age at exposure. Here, we clarified the age-dependent differences in thymus-cell dynamics in mice during the initial post-irradiation period. Our results demonstrate that the dynamics of thymocytes during the post-irradiation period depends on the age at exposure. For irradiated infants in particular, the number of proliferating cells increase dramatically, and this correlate with activation of the PI3K-AKT-mTOR pathway. Thus, we conclude that the PI3K-AKT-mTOR pathway in infants contributed, at least in part, to thymus-cell dynamics through the modification of cell proliferation and survival after irradiation, which may be associated with the risk of Pten mutation-associated thymic lymphoma.

Abstract

The risk of radiation-induced carcinogenesis depends on age at exposure. We previously reported principal causative genes in lymphomas arising after infant or adult exposure to 4-fractionated irradiation as Pten or Ikzf1, respectively, suggesting that cells with mutation in these genes might be the origin of lymphomas arising after irradiation depending on age at exposure. Here, we clarified the age-dependent differences in thymus-cell dynamics in mice during the initial post-irradiation period. The thymocyte number initially decreased, followed by two regeneration phases. During the first regeneration, the proportion of phosphorylated-AKT-positive (p-AKT⁺) cells in cell-cycle phases S+G2/M of immature CD4⁻CD8⁻ and CD4⁺CD8⁺ thymocytes and in phases G0/G1 of mature CD4⁺CD8⁻ and CD4⁻CD8⁺ thymocytes was significantly greater in irradiated infants than in irradiated adults. During the second regeneration, the proportion of p-AKT⁺ thymocytes in phases G0/G1 increased in each of the three populations other than CD4⁻CD8⁻ thymocytes more so than during the first regeneration. Finally, PI3K-AKT-mTOR signaling in infants contributed, at least in part, to biphasic thymic regeneration through the modification of cell proliferation and survival after irradiation, which may be associated with the risk of Pten mutation-associated thymic lymphoma.

Interconnections between Inflammageing and Immunosenescence during Ageing

Acute inflammation is a physiological response to injury or infection, with a cascade of steps that ultimately lead to the recruitment of immune cells to clear invading pathogens and heal wounds. However, chronic inflammation arising from the continued presence of the initial trigger, or the dysfunction of signalling and/or effector pathways, is harmful to health. While successful ageing in older adults, including centenarians, is associated with low levels of inflammation, elevated inflammation increases the risk of poor health and death. Hence inflammation has been described as one of seven pillars of ageing. Age-associated sterile, chronic, and low-grade inflammation is commonly termed inflammageing-it is not simply a consequence of increasing chronological age, but is also a marker of biological ageing, multimorbidity, and mortality risk. While inflammageing was initially thought to be caused by "continuous antigenic load and stress", reports from the last two decades describe a much more complex phenomenon also involving cellular senescence and the ageing of the immune system. In this review, we explore some of the main sources and consequences of inflammageing in the context of immunosenescence and highlight potential interventions. In particular, we assess the contribution of cellular senescence to age-associated inflammation, identify patterns of pro- and anti-inflammatory markers characteristic of inflammageing, describe alterations in the ageing immune system that lead to elevated inflammation, and finally assess the ways that diet, exercise, and pharmacological interventions can reduce inflammageing and thus, improve later life health.

Ectopic cervical thymi and no thymic involution until midlife in naked mole rats

Immunosenescence is a hallmark of aging and manifests as increased susceptibility to infection, autoimmunity, and cancer in the elderly. One component of immunosenescence is thymic involution, age-associated shrinkage of the thymus, observed in all vertebrates studied to date. The naked mole rat (Heterocephalus glaber) has become an attractive animal model in aging research due to its extreme longevity and resistance to disease. Here, we show that naked mole rats display no thymic involution up to 11 years of age. Furthermore, we found large ectopic cervical thymi in addition to the canonical thoracic thymus, both being identical in their cell composition. The developmental landscape in naked mole rat thymi revealed overt differences from the murine T-cell compartment, most notably a decrease of CD4+ /CD8+ double-positive cells and lower abundance of cytotoxic effector T cells. Our observations suggest that naked mole rats display a delayed immunosenescence. Therapeutic interventions aimed at reversing thymic aging remain limited, underscoring the importance of understanding the cellular and molecular mechanisms behind a sustained immune function in the naked mole rat.

Recapitulation of Thymic Function by Tissue Engineering Strategies

The thymus is responsible for the development and selection of T lymphocytes, which in turn also participate in the maturation of thymic epithelial cells. These events occur through the close interactions between hematopoietic stem cells and developing thymocytes with the thymic stromal cells within an intricate 3D network. The complex thymic microenvironment and function, and the current therapies to induce thymic regeneration or to overcome the lack of a functional thymus are herein reviewed. The recapitulation of the thymic function using tissue engineering strategies has been explored as a way to control the body's tolerance to external grafts and to generate ex vivo T cells for transplantation. In this review, the main advances in the thymus tissue engineering field are disclosed, including both scaffold- and cell-based strategies. In light of the current gaps and limitations of the developed systems, the design of novel biomaterials for this purpose with unique features is also discussed.

Acute Myeloid Leukemia: Is It T Time?

Acute myeloid leukemia (AML) is a heterogeneous disease driven by impaired differentiation of hematopoietic primitive cells toward myeloid lineages (monocytes, granulocytes, red blood cells, platelets), leading to expansion and accumulation of "stem" and/or "progenitor"-like or differentiated leukemic cells in the bone marrow and blood. AML progression alters the bone marrow microenvironment and inhibits hematopoiesis' proper functioning, causing sustained cytopenia and immunodeficiency. This review describes how the AML microenvironment influences lymphoid lineages, particularly T lymphocytes that originate from the thymus and orchestrate adaptive immune response. We focus on the elderly population, which is mainly affected by this pathology. We discuss how a permissive AML microenvironment can alter and even worsen the thymic function, T cells' peripheral homeostasis, phenotype, and functions. Based on the recent findings on the mechanisms supporting that AML induces quantitative and qualitative changes in T cells, we suggest and summarize current immunotherapeutic strategies and challenges to overcome these anomalies to improve the anti-leukemic immune response and the clinical outcome of patients.

Vaccine-Induced CD8+ T Cell Responses in Children: A Review of Age-Specific Molecular Determinants Contributing to Antigen Cross-Presentation

Infections are most common and most severe at the extremes of age, the young and the elderly. Vaccination can be a key approach to enhance immunogenicity and protection against pathogens in these vulnerable populations, who have a functionally distinct immune system compared to other age groups. More than 50% of the vaccine market is for pediatric use, yet to date vaccine development is often empiric and not tailored to molecular distinctions in innate and adaptive immune activation in early life. With modern vaccine development shifting from whole-cell based vaccines to subunit vaccines also comes the need for formulations that can elicit a CD8⁺ T cell response when needed, for example, by promoting antigen cross-presentation. While our group and others have identified many cellular and molecular determinants of successful activation of antigen-presenting cells, B cells and CD4⁺ T cells in early life, much less is known about the ontogeny of CD8⁺ T cell induction. In this review, we summarize the literature pertaining to the frequency and phenotype of newborn and infant CD8⁺ T cells, and any evidence of induction of CD8⁺ T cells by currently licensed pediatric vaccine formulations. In addition, we review the molecular determinants of antigen cross-presentation on MHC I and successful CD8⁺ T cell induction and discuss potential distinctions that can be made in children. Finally, we discuss recent advances in development of novel adjuvants and provide future directions for basic and translational research in this area.

Thymus-Pineal Gland Axis: Revisiting Its Role in Human Life and Ageing

For years the thymus gland (TG) and the pineal gland (PG) have been subject of increasingly in-depth studies, but only recently a link that can associate the activities of the two organs has been identified. Considering, on the one hand, the well-known immune activity of thymus and, on the other, the increasingly emerging immunological roles of circadian oscillators and the rhythmically secreted main pineal product, melatonin, many studies aimed to analyse the possible existence of an interaction between these two systems. Moreover, data confirmed that the immune system is functionally associated with the nervous and endocrine systems determining an integrated dynamic network. In addition, recent researches showed a similar, characteristic involution process both in TG and PG. Since the second half of the 20th century, evidence led to the definition of an effectively interacting thymus-pineal axis (TG-PG axis), but much has to be done. In this sense, the aim of this review is to summarize what is actually known about this topic, focusing on the impact of the TG-PG axis on human life and ageing. We would like to give more emphasis to the implications of this dynamical interaction in a possible therapeutic strategy for human health. Moreover, we focused on all the products of TG and PG in order to collect what is known about the role of peptides other than melatonin. The results available today are often unclear and not linear. These peptides have not been well studied and defined over the years. In this review we hope to awake the interest of the scientific community in them and in their future pharmacological applications.

T cell regeneration after immunological injury

Following periods of haematopoietic cell stress, such as after chemotherapy, radiotherapy, infection and transplantation, patient outcomes are linked to the degree of immune reconstitution, specifically of T cells. Delayed or defective recovery of the T cell pool has significant clinical consequences, including prolonged immunosuppression, poor vaccine responses and increased risks of infections and malignancies. Thus, strategies that restore thymic function and enhance T cell reconstitution can provide considerable benefit to individuals whose immune system has been decimated in various settings. In this Review, we focus on the causes and consequences of impaired adaptive immunity and discuss therapeutic strategies that can recover immune function, with a particular emphasis on approaches that can promote a diverse repertoire of T cells through de novo T cell formation.

Resveratrol ameliorates thymus senescence changes in D-galactose induced mice

The thymic microenvironment plays an important role in the development of T cells. A decrease of thymic epithelial cells is the main cause of age-related thymic atrophy or degeneration. Resveratrol (RSV), a phytoalexin produced from plants, has been shown to inhibit the adverse effects of dietary obesity on the structure and function of the thymus. D-Galactose (D-gal) can induce accelerated aging in mice. In the present study, young mice (2 months old) were injected with D-gal (120 mg/kg/day) for 8 consecutive weeks to construct an accelerated aging model. Compared with normal control mice, the thymus epithelium of the D-gal treated mice had structural changes, the number of senescent cells increased, the number of CD4+ T cells decreased, and CD8+ T cells increased. After RSV administration by gavage for 6 weeks, it was found that RSV improved the surface phenotypes of D-gal treated mice, and recovered thymus function by maintaining the ratio of CD4+ to CD8+ cells. It also indicated that RSV enhanced the cell proliferation and inhibited cell senescence. Increased autoimmune regulator (Aire) expression was present in the RSV treated mice. The lymphotoxin-beta receptor (LTβR) expression also increased. These findings suggested that RSV intake could restore the alterations caused by D-gal treatment in the thymus via stimulation of Aire expression.

Thymic Engraftment by in vitro-Derived Progenitor T Cells in Young and Aged Mice

T cells play a critical role in mediating antigen-specific and long-term immunity against viral and bacterial pathogens, and their development relies on the highly specialized thymic microenvironment. T cell immunodeficiency can be acquired in the form of inborn errors, or can result from perturbations to the thymus due to aging or irradiation/chemotherapy required for cancer treatment. Hematopoietic stem cell transplant (HSCT) from compatible donors is a cornerstone for the treatment of hematological malignancies and immunodeficiency. Although it can restore a functional immune system, profound impairments exist in recovery of the T cell compartment. T cells remain absent or low in number for many months after HSCT, depending on a variety of factors including the age of the recipient. While younger patients have a shorter refractory period, the prolonged T cell recovery observed in older patients can lead to a higher risk of opportunistic infections and increased predisposition to relapse. Thus, strategies for enhancing T cell recovery in aged individuals are needed to counter thymic damage induced by radiation and chemotherapy toxicities, in addition to naturally occurring age-related thymic involution. Preclinical results have shown that robust and rapid long-term thymic reconstitution can be achieved when progenitor T cells, generated in vitro from HSCs, are co-administered during HSCT. Progenitor T cells appear to rely on lymphostromal crosstalk via receptor activator of NF-κB (RANK) and RANK-ligand (RANKL) interactions, creating chemokine-rich niches within the cortex and medulla that likely favor the recruitment of bone marrow-derived thymus seeding progenitors. Here, we employed preclinical mouse models to demonstrate that in vitro-generated progenitor T cells can effectively engraft involuted aged thymuses, which could potentially improve T cell recovery. The utility of progenitor T cells for aged recipients positions them as a promising cellular therapy for immune recovery and intrathymic repair following irradiation and chemotherapy, even in a post-involution thymus.

Conventional and Computational Flow Cytometry Analyses Reveal Sustained Human Intrathymic T Cell Development From Birth Until Puberty

The thymus is the organ where subsets of mature T cells are generated which subsequently egress to function as central mediators in the immune system. While continuously generating T cells even into adulthood, the thymus does undergo involution during life. This is characterized by an initial rapid decrease in thymic cellularity during early life and by a second age-dependent decline in adulthood. The thymic cellularity of neonates remains low during the first month after birth and the tissue reaches a maximum in cellularity at 6 months of age. In order to study the effect that this first phase of thymic involution has on thymic immune subset frequencies, we performed multi-color flow cytometry on thymic samples collected from birth to 14 years of age. In consideration of the inherent limitations posed by conventional flow cytometry analysis, we established a novel computational analysis pipeline that is adapted from single-cell transcriptome sequencing data analysis. This allowed us to overcome technical effects by batch correction, analyze multiple samples simultaneously, limit computational cost by subsampling, and to rely on KNN-graphs for graph-based clustering. As a result, we successfully identified rare, distinct and gradually developing immune subsets within the human thymus tissues. Although the thymus undergoes early involution from infanthood onwards, our data suggests that this does not affect human T-cell development as we did not observe significant alterations in the proportions of T-lineage developmental intermediates from birth to puberty. Thus, in addition to providing an interesting novel strategy to analyze conventional flow cytometry data for the thymus, our work shows that the early phase of human thymic involution mainly limits the overall T cell output since no obvious changes in thymocyte subsets could be observed.

Phosphate Transporter Profiles in Murine and Human Thymi Identify Thymocytes at Distinct Stages of Differentiation

Thymocyte differentiation is dependent on the availability and transport of metabolites in the thymus niche. As expression of metabolite transporters is a rate-limiting step in nutrient utilization, cell surface transporter levels generally reflect the cell's metabolic state. The GLUT1 glucose transporter is upregulated on actively dividing thymocytes, identifying thymocytes with an increased metabolism. However, it is not clear whether transporters of essential elements such as phosphate are modulated during thymocyte differentiation. While PiT1 and PiT2 are both phosphate transporters in the SLC20 family, we show here that they exhibit distinct expression profiles on both murine and human thymocytes. PiT2 expression distinguishes thymocytes with high metabolic activity, identifying immature murine double negative (CD4⁻CD8⁻) DN3b and DN4 thymocyte blasts as well as immature single positive (ISP) CD8 thymocytes. Notably, the absence of PiT2 expression on RAG2-deficient thymocytes, blocked at the DN3a stage, strongly suggests that high PiT2 expression is restricted to thymocytes having undergone a productive TCRβ rearrangement at the DN3a/DN3b transition. Similarly, in the human thymus, PiT2 was upregulated on early post-β selection CD4⁺ISP and TCRαβ⁻CD4^hiDP thymocytes co-expressing the CD71 transferrin receptor, a marker of metabolic activity. In marked contrast, expression of the PiT1 phosphate importer was detected on mature CD3⁺ murine and human thymocytes. Notably, PiT1 expression on CD3⁺DN thymocytes was identified as a biomarker of an aging thymus, increasing from 8.4 ± 1.5% to 42.4 ± 9.4% by 1 year of age (p < 0.0001). We identified these cells as TCRγδ and, most significantly, NKT, representing 77 ± 9% of PiT1⁺DN thymocytes by 1 year of age (p < 0.001). Thus, metabolic activity and thymic aging are associated with distinct expression profiles of the PiT1 and PiT2 phosphate transporters.

Impact of age-, cancer-, and treatment-driven inflammation on T cell function and immunotherapy

Many cancers are predominantly diagnosed in older individuals and chronic inflammation has a major impact on the overall health and immune function of older cancer patients. Chronic inflammation is a feature of aging, it can accelerate disease in many cancers and it is often exacerbated during conventional treatments for cancer. This review will provide an overview of the factors that lead to increased inflammation in older individuals and/or individuals with cancer, as well as those that result from conventional treatments for cancer, using ovarian cancer (OC) and multiple myeloma (MM) as key examples. We will also consider the impact of chronic inflammation on immune function, with a particular focus on T cells as they are key targets for novel cancer immunotherapies. Overall, this review aims to highlight specific pathways for potential interventions that may be able to mitigate the impact of chronic inflammation in older cancer patients.

Thymus aging in mice deficient in either EphB2 or EphB3, two master regulators of thymic epithelium development

BACKGROUND

The epithelial microenvironment is involved in thymus aging, but the possible role of EphB receptors that govern the thymic epithelium development has not been investigated. Herein, we study the changes undergone by the thymus of EphB-deficient mice throughout their life.

RESULTS

Immune alterations occurring throughout life were more severe in mutant than in wild-type (WT) mice. Mutant thymuses exhibit lower cellularity than WT ones, as well as lower proportions of early thymic progenitors cells and double-positive (CD4⁺ CD8⁺ ) thymocytes, but higher of double-negative (CD4^- CD8^- ) and single-positive (CD4⁺ CD8^- , CD4^- CD8⁺ ) cells. Throughout life, CD4⁺ naïve cells decreased particularly in mutant mice. In correlation, memory T cells, largely CD8⁺ cells, increased. Aged thymic epithelium undergoes changes including appearance of big epithelial free areas, decrease of K8⁺ K5^- areas, which, however, contain higher proportions of Ly51⁺ UEA1^- cortical epithelial cells, in correlation with reduced Aire⁺ medullary epithelial cells. Also, aged thymuses particularly those derived from mutant mice exhibited increased collagen IV, fat-storing cells, and connective cells.

CONCLUSIONS

The absence of EphB accelerates the alterations undergone throughout life by both thymic epithelium and thymocytes, and the proportions of peripheral naïve and memory T cells, all of which are hallmarks of immune aging.

Contributions of Age-Related Thymic Involution to Immunosenescence and Inflammaging

Immune system aging is characterized by the paradox of immunosenescence (insufficiency) and inflammaging (over-reaction), which incorporate two sides of the same coin, resulting in immune disorder. Immunosenescence refers to disruption in the structural architecture of immune organs and dysfunction in immune responses, resulting from both aged innate and adaptive immunity. Inflammaging, described as a chronic, sterile, systemic inflammatory condition associated with advanced age, is mainly attributed to somatic cellular senescence-associated secretory phenotype (SASP) and age-related autoimmune predisposition. However, the inability to reduce senescent somatic cells (SSCs), because of immunosenescence, exacerbates inflammaging. Age-related adaptive immune system deviations, particularly altered T cell function, are derived from age-related thymic atrophy or involution, a hallmark of thymic aging. Recently, there have been major developments in understanding how age-related thymic involution contributes to inflammaging and immunosenescence at the cellular and molecular levels, including genetic and epigenetic regulation, as well as developments of many potential rejuvenation strategies. Herein, we discuss the research progress uncovering how age-related thymic involution contributes to immunosenescence and inflammaging, as well as their intersection. We also describe how T cell adaptive immunity mediates inflammaging and plays a crucial role in the progression of age-related neurological and cardiovascular diseases, as well as cancer. We then briefly outline the underlying cellular and molecular mechanisms of age-related thymic involution, and finally summarize potential rejuvenation strategies to restore aged thymic function.

A Comprehensive Review of the Diagnosis and Pharmacological Management of Crohn's Disease in the Elderly Population

Crohn's disease (CD) in the elderly is rising in prevalence, which is related to an increase in its incidence and improving life expectancies. There are differences in the presentation, natural history, and treatment of CD between adult-onset patients who progress to older age and patients who are initially diagnosed at an older age. Presentation at an older age may also delay or make diagnosis challenging due to accumulating co-morbidities that mimic inflammatory bowel disease. Differences exist between adult- and older-onset disease, yet many guidelines do not specifically distinguish the management of these two distinct populations. Identifying patients at high risk for progression or aggressive disease is particularly important as elderly patients may respond differently to medical and surgical treatment, and may be at higher risk for adverse effects. Despite newer agents being approved for CD, the data regarding efficacy and safety in the elderly are currently limited. Balancing symptom management with risks of medical and surgical therapy is an ongoing challenge and requires special consideration in these two distinct populations.

Pulmonary Impairment after Respiratory Viral Infections Is Associated with High Mortality in Allogeneic Hematopoietic Cell Transplant Recipients

Pulmonary impairment predicts increased mortality in many settings, and respiratory viral infection (RVI) causes considerable morbidity and mortality in allogeneic hematopoietic cell transplant recipients (allo-HCT). We hypothesized that pulmonary impairment after RVI, defined as a decline of forced expiratory volume in 1 second values by ≥10%, may identify allo-HCT recipients at high risk for mortality. We studied all allo-HCT recipients at our institution who had RVI with respiratory syncytial virus, parainfluenza virus, or influenza from 2004 to 2013 and had pre-RVI and post-RVI pulmonary function tests. We used competing risk regression models to identify risk factors for 2-year nonrelapse mortality (NRM) as the primary outcome after RVI and relapse-related mortality as a competing risk. From 223 eligible patients, pulmonary impairment after RVI was associated with over a 3-fold increase in 2-year NRM (pulmonary impairment, 25.3%; no impairment, 7.4%; univariate subhazard ratio [SHR], 3.9; 95% confidence interval [CI], 1.9 to 8.1; P < .001). After adjusting for age and systemic steroid use, pulmonary impairment after RVI was still associated with increased 2-year NRM (SHR, 3.3 [95% CI, 1.6 to 6.9]; P = .002). After adjustment for race and graft-versus-host disease (GVHD) prophylaxis, chronic GVHD at the time of RVI (odds ratio [OR], 2.8 [95% CI, 1.4 to 5.4]; p = .003) and lymphopenia (OR, 2.2 [95% CI, 1.1 to 4.2]; P = .02) were associated with increased odds of pulmonary impairment, whereas use of nonmyeloablative conditioning was associated with reduced odds of pulmonary impairment (OR, .4 [95% CI, .2 to .8]; P = .006). In allo-HCT recipients with RVIs, pulmonary impairment after RVI is associated with high NRM at 2years.

Effects of Acute and Chronic Exercise on Immunological Parameters in the Elderly Aged: Can Physical Activity Counteract the Effects of Aging?

Immunosenescence is characterized by deterioration of the immune system caused by aging which induces changes to innate and adaptive immunity. Immunosenescence affects function and phenotype of immune cells, such as expression and function of receptors for immune cells which contributes to loss of immune function (chemotaxis, intracellular killing). Moreover, these alterations decrease the response to pathogens, which leads to several age-related diseases including cardiovascular disease, Alzheimer's disease, and diabetes in older individuals. Furthermore, increased risk of autoimmune disease and chronic infection is increased with an aging immune system, which is characterized by a pro-inflammatory environment, ultimately leading to accelerated biological aging. During the last century, sedentarism rose dramatically, with a concomitant increase in certain type of cancers (such as breast cancer, colon, or prostate cancer), and autoimmune disease. Numerous studies on physical activity and immunity, with focus on special populations (i.e., people with diabetes, HIV patients) demonstrate that chronic exercise enhances immunity. However, the majority of previous work has focused on either a pathological population or healthy young adults whilst research in elderly populations is scarce. Research conducted to date has primarily focused on aerobic and resistance exercise training and its effect on immunity. This review focuses on the potential for exercise training to affect the aging immune system. The concept is that some lifestyle strategies such as high-intensity exercise training may prevent disease through the attenuation of immunosenescence. In this context, we take a top-down approach and review the effect of exercise and training on immunological parameters in elderly at rest and during exercise in humans, and how they respond to different modes of training. We highlight the impact of these different exercise modes on immunological parameters, such as cytokine and lymphocyte concentration in elderly individuals.

Updates on Old and Weary Haematopoiesis

Blood formation, or haematopoiesis, originates from haematopoietic stem cells (HSCs), whose functions and maintenance are regulated in both cell- and cell non-autonomous ways. The surroundings of HSCs in the bone marrow create a specific niche or microenvironment where HSCs nest that allows them to retain their unique characteristics and respond rapidly to external stimuli. Ageing is accompanied by reduced regenerative capacity of the organism affecting all systems, due to the progressive decline of stem cell functions. This includes blood and HSCs, which contributes to age-related haematological disorders, anaemia, and immunosenescence, among others. Furthermore, chronological ageing is characterised by myeloid and platelet HSC skewing, inflammageing, and expanded clonal haematopoiesis, which may be the result of the accumulation of preleukaemic lesions in HSCs. Intriguingly, haematological malignancies such as acute myeloid leukaemia have a high incidence among elderly patients, yet not all individuals with clonal haematopoiesis develop leukaemias. Here, we discuss recent work on these aspects, their potential underlying molecular mechanisms, and the first cues linking age-related changes in the HSC niche to poor HSC maintenance. Future work is needed for a better understanding of haematopoiesis during ageing. This field may open new avenues for HSC rejuvenation and therapeutic strategies in the elderly.

Aging and the immune system: An overview

The world is witnessing a rapid demographic shift towards an older population, a trend with major medical, social, economic and political implications. Aging is a multifaceted process, involving numerous molecular and cellular mechanisms in the context of different organ systems. A crucial component of aging is a set of functional and structural alterations in the immune system that can manifest as a decreased ability to fight infection, diminished response to vaccination, increased incidence of cancer, higher prevalence of autoimmunity and constitutive low-grade inflammation, among others. In addition to cell-intrinsic changes in both innate and adaptive immune cells, alterations in the stromal microenvironment in primary and secondary lymphoid organs play an important role in age-associated immune dysfunction. This article will provide a broad overview of these phenomena and point out some of their clinical and therapeutic implications.

Major features of immunesenescence, including reduced thymic output, are ameliorated by high levels of physical activity in adulthood

It is widely accepted that aging is accompanied by remodelling of the immune system including thymic atrophy and increased frequency of senescent T cells, leading to immune compromise. However, physical activity, which influences immunity but declines dramatically with age, is not considered in this literature. We assessed immune profiles in 125 adults (55-79 years) who had maintained a high level of physical activity (cycling) for much of their adult lives, 75 age-matched older adults and 55 young adults not involved in regular exercise. The frequency of naïve T cells and recent thymic emigrants (RTE) were both higher in cyclists compared with inactive elders, and RTE frequency in cyclists was no different to young adults. Compared with their less active counterparts, the cyclists had significantly higher serum levels of the thymoprotective cytokine IL-7 and lower IL-6, which promotes thymic atrophy. Cyclists also showed additional evidence of reduced immunesenescence, namely lower Th17 polarization and higher B regulatory cell frequency than inactive elders. Physical activity did not protect against all aspects of immunesenescence: CD28^-ve CD57^+ve senescent CD8 T-cell frequency did not differ between cyclists and inactive elders. We conclude that many features of immunesenescence may be driven by reduced physical activity with age.

Differential Expression of PU.1 and Key T Lineage Transcription Factors Distinguishes Fetal and Adult T Cell Development

The PU.1 transcription factor plays a critical role in the regulation of T cell development, so a report that it is dispensable for fetal thymopoiesis is puzzling. To understand this paradox, we examined the requirement for PU.1, encoded by Spi1, during fetal, neonatal, and adult thymopoiesis in a PU.1 hypomorphic mouse generated by deletion of the Spi1 14-kb upstream regulatory element and by analysis of patterns of gene expression in fetal and adult T cell progenitors. Our data demonstrate that the initiation of thymopoiesis during early gestation is less dependent on PU.1 compared with T cell differentiation in adults and that fetal T cell progenitors express lower levels of Spi1 compared with their adult counterparts. We also show that expression of the core network of T lineage transcription factors regulated by PU.1 differs in fetal and adult T cell progenitors. In particular, PU.1-regulated genes that promote T cell differentiation are differentially expressed in fetal versus adult early T lineage progenitors. These results indicate that the transcriptional differences between the fetal and adult T cell developmental programs are driven in part by differential levels of PU.1 expression and that this likely underlies the differences in the properties of fetal and adult T cell progenitors.

Effects of age-related shifts in cellular function and local microenvironment upon the innate immune response to implants

The host macrophage response is now well recognized as a predictor of the success or failure of biomaterial implants following placement. More specifically, shifts from an "M1" pro-inflammatory towards a more "M2-like" anti-inflammatory macrophage polarization profile have been shown to result in enhanced material integration and/or tissue regeneration downstream. As a result, a number of biomaterials-based approaches to controlling macrophage polarization have been developed. However, the ability to promote such activity is predicated upon an in-depth, context-dependent understanding of the host response to biomaterials. Recent work has shown the impacts of both tissue location and tissue status (i.e. underlying pathology) upon the host innate immune response to implants, representing a departure from a focus upon implant material composition and form. Thus, the ideas of "biocompatibility," the host macrophage reaction, and ideal material requirements and modification strategies may need to be revisited on a patient, tissue, and disease basis. Immunosenescence, dysregulation of macrophage function, and delayed resolution of immune responses in aged individuals have all been demonstrated, suggesting that the host response to biomaterials in aged individuals should differ from that in younger individuals. However, despite the increasing usage of implantable medical devices in aged patients, few studies examining the effects of aging upon the host response to biomaterials and the implications of this response for long-term integration and function have been performed. The objective of the present manuscript is to review the putative effects of aging upon the host response to implanted materials and to advance the hypothesis that age-related changes in the local microenvrionement, with emphasis on the extracellular matrix, play a previously unrecognized role in determining the host response to implants.

Thymic Epithelial Cells

Intrathymic T cell development is a complex process that depends upon continuous guidance from thymus stromal cell microenvironments. The thymic epithelium within the thymic stroma comprises highly specialized cells with a high degree of anatomic, phenotypic, and functional heterogeneity. These properties are collectively required to bias thymocyte development toward production of self-tolerant and functionally competent T cells. The importance of thymic epithelial cells (TECs) is evidenced by clear links between their dysfunction and multiple diseases where autoimmunity and immunodeficiency are major components. Consequently, TECs are an attractive target for cell therapies to restore effective immune system function. The pathways and molecular regulators that control TEC development are becoming clearer, as are their influences on particular stages of T cell development. Here, we review both historical and the most recent advances in our understanding of the cellular and molecular mechanisms controlling TEC development, function, dysfunction, and regeneration.

Immunology of Gut-Bone Signaling

In recent years a link between the gastrointestinal tract and bone health has started to gain significant attention. Dysbiosis of the intestinal microbiota has been linked to the pathology of a number of diseases which are associated with bone loss. In addition modulation of the intestinal microbiota with probiotic bacteria has revealed to have both beneficial local and systemic effects. In the present chapter, we discuss the intestinal and bone immune systems, explore how intestinal disease affects the immune system, and examine how these pathologic changes could adversely impact bone health.

Lymphocyte generation and population homeostasis throughout life

Immune aging is a multi-faceted process that manifests as reduced competence to fight infections and malignant cells, as well as diminished tissue repair, unprovoked inflammation, and increased autoreactivity. The aging adaptive immune system, with its high complexity in functional cell subpopulations and diversity of B- and T-cell receptors, has to cope with the challenge of maintaining homeostasis while responding to exogenous stimuli and compensating for reduced generative capacity. With thymic involution, naïve T cells begin to function as quasi-stem cells and maintain the compartment through peripheral homeostatic proliferation that shapes the T-cell repertoire through peripheral selection and the activation of differentiation pathways. Similarly, reduced generation of early B-cell progenitors alters the composition of the peripheral B-cell compartment with the emergence of a unique, auto-inflammatory B-cell subset, termed age-associated B cells (ABCs). These changes in T- and B-cell composition and function are core manifestations of immune aging.

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.

Frontline Science: Tryptophan restriction arrests B cell development and enhances microbial diversity in WT and prematurely aging Ercc1-/Δ7 mice

With aging, tryptophan metabolism is affected. Tryptophan has a crucial role in the induction of immune tolerance and the maintenance of gut microbiota. We, therefore, studied the effect of dietary tryptophan restriction in young wild-type (WT) mice (118-wk life span) and in DNA-repair deficient, premature-aged (Ercc1^-/Δ7 ) mice (20-wk life span). First, we found that the effect of aging on the distribution of B and T cells in bone marrow (BM) and in the periphery of 16-wk-old Ercc1^-/Δ7 mice was comparable to that in 18-mo-old WT mice. Dietary tryptophan restriction caused an arrest of B cell development in the BM, accompanied by diminished B cell frequencies in the periphery. In general, old Ercc1^-/Δ7 mice showed similar responses to tryptophan restriction compared with young WT mice, indicative of age-independent effects. Dietary tryptophan restriction increased microbial diversity and made the gut microbiota composition of old Ercc1^-/Δ7 mice more similar to that of young WT mice. The decreased abundances of Alistipes and Akkermansia spp. after dietary tryptophan restriction correlated significantly with decreased B cell precursor numbers. In conclusion, we report that dietary tryptophan restriction arrests B cell development and concomitantly changes gut microbiota composition. Our study suggests a beneficial interplay between dietary tryptophan, B cell development, and gut microbial composition on several aspects of age-induced changes.

Thymus: the next (re)generation

As the primary site of T-cell development, the thymus plays a key role in the generation of a strong yet self-tolerant adaptive immune response, essential in the face of the potential threat from pathogens or neoplasia. As the importance of the role of the thymus has grown, so too has the understanding that it is extremely sensitive to both acute and chronic injury. The thymus undergoes rapid degeneration following a range of toxic insults, and also involutes as part of the aging process, albeit at a faster rate than many other tissues. The thymus is, however, capable of regenerating, restoring its function to a degree. Potential mechanisms for this endogenous thymic regeneration include keratinocyte growth factor (KGF) signaling, and a more recently described pathway in which innate lymphoid cells produce interleukin-22 (IL-22) in response to loss of double positive thymocytes and upregulation of IL-23 by dendritic cells. Endogenous repair is unable to fully restore the thymus, particularly in the aged population, and this paves the way toward the need for exogenous strategies to help regenerate or even replace thymic function. Therapies currently in clinical trials include KGF, use of the cytokines IL-7 and IL-22, and hormonal modulation including growth hormone administration and sex steroid inhibition. Further novel strategies are emerging in the preclinical setting, including the use of precursor T cells and thymus bioengineering. The use of such strategies offers hope that for many patients, the next regeneration of their thymus is a step closer.

Interleukin-21 administration to aged mice rejuvenates their peripheral T-cell pool by triggering de novo thymopoiesis

The vaccination efficacy in the elderly is significantly reduced compared to younger populations due to thymic involution and age‐related intrinsic changes affecting their naïve T‐cell compartment. Interleukin (IL)‐21 was recently shown to display thymostimulatory properties. Therefore, we hypothesized that its administration to ageing hosts may improve T‐cell output and thus restore a competent peripheral T‐cell compartment. Indeed, an increase in the production of recent thymic emigrants (RTEs) attributable to intrathymic expansion of early thymic progenitors (ETPs), double‐negative (DN), and double‐positive (DP) thymocytes as well as thymic epithelial cell (TEC) was observed in recombinant (r)IL‐21‐treated aged mice. In sharp contrast, no alterations in the frequency of bone marrow (BM)‐derived progenitors were detected following rIL‐21 administration. Enhanced production of naïve T cells improved the T‐cell receptor (TCR) repertoire diversity and re‐established a pool of T cells exhibiting higher levels of miR‐181a and diminished amounts of the TCR‐inhibiting phosphatases SHP‐2 and DUSP5/6. As a result, stimulation of T cells derived from rIL‐21‐treated aged mice displayed enhanced activation of Lck, ZAP‐70, and ERK, which ultimately boosted their IL‐2 production, CD25 expression, and proliferation capabilities in comparison with T cells derived from control aged mice. Consequently, aged rIL‐21‐treated mice vaccinated using a tyrosinase‐related protein 2 (Trp2)‐derived peptide exhibited a substantial delay in B16 tumor growth and improved survival. The results of this study highlight the immunorestorative function of rIL‐21 paving its use as a strategy for the re‐establishment of effective immunity in the elderly.

Sex steroid ablation: an immunoregenerative strategy for immunocompromised patients

Age-related decline in thymic function is a well-described process that results in reduced T-cell development and thymic output of new naïve T cells. Thymic involution leads to reduced response to vaccines and new pathogens in otherwise healthy individuals; however, reduced thymic function is particularly detrimental in clinical scenarios where the immune system is profoundly depleted such as after chemotherapy, radiotherapy, infection and shock. Poor thymic function and restoration of immune competence has been correlated with an increased risk of opportunistic infections, tumor relapse and autoimmunity. Apart from their primary role in sex dimorphism, sex steroid levels profoundly affect the immune system in general and, in fact, age-related thymic involution has been at least partially attributed to the increase in sex steroids at puberty. Subsequently it has been demonstrated that the removal of sex steroids, or sex steroid ablation (SSA), triggers physiologic changes that ultimately lead to thymic re-growth and improved T-cell reconstitution in settings of hematopoietic stem cell transplant (HSCT). Although the cellular and molecular process underlying these regenerative effects are still poorly understood, SSA clearly represents an attractive therapeutic approach to enhance thymic function and restore immune competence in immunodeficient individuals.

Prolongevity hormone FGF21 protects against immune senescence by delaying age-related thymic involution

Age-related thymic degeneration is associated with loss of naïve T cells, restriction of peripheral T-cell diversity, and reduced healthspan due to lower immune competence. The mechanistic basis of age-related thymic demise is unclear, but prior evidence suggests that caloric restriction (CR) can slow thymic aging by maintaining thymic epithelial cell integrity and reducing the generation of intrathymic lipid. Here we show that the prolongevity ketogenic hormone fibroblast growth factor 21 (FGF21), a member of the endocrine FGF subfamily, is expressed in thymic stromal cells along with FGF receptors and its obligate coreceptor, βKlotho. We found that FGF21 expression in thymus declines with age and is induced by CR. Genetic gain of FGF21 function in mice protects against age-related thymic involution with an increase in earliest thymocyte progenitors and cortical thymic epithelial cells. Importantly, FGF21 overexpression reduced intrathymic lipid, increased perithymic brown adipose tissue, and elevated thymic T-cell export and naïve T-cell frequencies in old mice. Conversely, loss of FGF21 function in middle-aged mice accelerated thymic aging, increased lethality, and delayed T-cell reconstitution postirradiation and hematopoietic stem cell transplantation (HSCT). Collectively, FGF21 integrates metabolic and immune systems to prevent thymic injury and may aid in the reestablishment of a diverse T-cell repertoire in cancer patients following HSCT.