Brain-Thymus Connections in Chagas Disease.

BACKGROUND: The brain and the immune systems represent the two primary adaptive systems within the body. Both are involved in a dynamic process of communication, vital for the preservation of mammalian homeostasis. This interplay involves two major pathways: the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system. SUMMARY: The establishment of infection can affect immunoneuroendocrine interactions, with functional consequences for immune organs, particularly the thymus. Interestingly, the physiology of this primary organ is not only under the control of the central nervous system (CNS) but also exhibits autocrine/paracrine regulatory circuitries mediated by hormones and neuropeptides that can be altered in situations of infectious stress or chronic inflammation. In particular, Chagas disease, caused by the protozoan parasite Trypanosoma cruzi (T. cruzi), impacts upon immunoneuroendocrine circuits disrupting thymus physiology. Here, we discuss the most relevant findings reported in relation to brain-thymic connections during T. cruzi infection, as well as their possible implications for the immunopathology of human Chagas disease. KEY MESSAGES: During T. cruzi infection, the CNS influences thymus physiology through an intricate network involving hormones, neuropeptides, and pro-inflammatory cytokines. Despite some uncertainties in the mechanisms and the fact that the link between these abnormalities and chronic Chagasic cardiomyopathy is still unknown, it is evident that the precise control exerted by the brain over the thymus is markedly disrupted throughout the course of T. cruzi infection.

PI3K/AKT pathway modulation and cold acclimation alleviation concerning apoptosis and necroptosis in broiler thymus.

Moderate cold stimulation regulates the thymus's growth and function and facilitates cold acclimatization in broilers. However, the underlying mechanism remains unknown. To explore the possible mechanism of the thymus in cold-acclimated broilers against cold stress, 240 one-day-old Arbor Acres (AA) broilers were assigned to 2 groups randomly. The control group (C) was housed at conventional temperatures. The temperature during the first week was 33°C to 34°C. Between the ages of 8 and 32 d, the temperature was lowered by 1°C every 2 d, i.e., gradually from 32°C to 20°C, and then maintained at 20°C until 42 d of age. The cold-acclimated group (C-3) was housed at the same temperature as C from 1 to 7 d after birth. Between 8 and 42 d, the temperature of C-3 was 3°C colder than C. After 24 h exposure to acute cold stress (ACS) at 42 d, C and C-3 were named as S and S-3. The results showed that ACS was able to induce oxidation stress, modulate PI3K/AKT signal, and cause necroptosis and apoptosis in broiler thymus. By contrast, cold acclimation could alleviate apoptosis and necroptosis induced by cold stress via alleviating oxidative stress, efficiently activating the PI3K/AKT signal, as well as decreasing apoptotic and necrotic genes' levels. This study offers a novel theoretical basis for cold acclimation to improve the body's cold tolerance.

Epithelial-mesenchymal transition in age-associated thymic involution: Mechanisms and therapeutic implications.

The thymus is a critical immune organ with endocrine and immune functions that plays important roles in the physiological and pathological processes of the body. However, with aging, the thymus undergoes degenerative changes leading to decreased production and output of naive T cells and the secretion of thymic hormones and related cytokines, thereby promoting the occurrence and development of various age-associated diseases. Therefore, identifying essential processes that regulate age-associated thymic involution is crucial for long-term control of thymic involution and age-associated disease progression. Epithelial-mesenchymal transition (EMT) is a well-established process involved in organ aging and functional impairment through tissue fibrosis in several organs, such as the heart and kidney. In the thymus, EMT promotes fibrosis and potentially adipogenesis, leading to thymic involution. This review focuses on the factors involved in thymic involution, including oxidative stress, inflammation, and hormones, from the perspective of EMT. Furthermore, current interventions for reversing age-associated thymic involution by targeting EMT-associated processes are summarized. Understanding the key mechanisms of thymic involution through EMT as an entry point may promote the development of new therapies and clinical agents to reverse thymic involution and age-associated disease.

Thymus aging and immune reconstitution, progresses and challenges.

Thymus is a primary lymphoid organ essential for the development of T lymphocytes. Age-related thymic involution is a prominent feature of immune senescence. The thymus undergoes rapid growth during fetal and neonatal development, peaks in size before puberty and then begins to undergo a decrease in cellularity with age. Dramatic changes occur with age-associated thymic involution. The most prominent features of thymic involution include: (i) epithelial structure disruption, (ii) adipogenesis, and (iii) thymocyte development arrest. There is a sex disparity in thymus aging. It is a multifactorial process controlled and regulated by a series of molecules, including the transcription factor FOXN1, fibroblast and keratinocyte growth factors (FGF and KGF, respectively), sex steroids, Notch signaling, WNT signaling, and microRNAs. Nevertheless, there is still no satisfactory evolutionary or physiological explanation for age-associated thymic involution, and understanding the precise mechanism(s) for thymus aging remains challenging. Sustained thymic regeneration has yet to be achieved by sex steroid ablation. Recent preclinical studies indicate that long-term thymic reconstitution can be achieved via adoptive transfer of in vitro-generated progenitor T (proT) cells, and improvements in the methods for the generation of human proT cells make this an attractive approach. Future clinical applications may rely on new applications integrating proT cells, cytokine support and sex-steroid inhibition treatments.

Disorganization of secondary lymphoid organs and dyscoordination of chemokine secretion as key contributors to immune aging.

Aging is characterized by progressive loss of organ and tissue function, and the immune system is no exception to that inevitable principle. Of all the age-related changes in the body, reduction of the size of, and naïve T (Tn) cell output from, the thymus occurs earliest, being prominent already before or by the time of puberty. Therefore, to preserve immunity against new infections, over much of their lives, vertebrates dominantly rely on peripheral maintenance of the Tn cell pool in the secondary lymphoid organs (SLO). However, SLO structure and function subsequently also deteriorate with aging. Several recent studies have made a convincing case that this deterioration is of major importance to the erosion of protective immunity in the last third of life. Specifically, the SLO were found to accumulate multiple degenerative changes with aging. Importantly, the results from adoptive transfer and parabiosis studies teach us that the old microenvironment is the limiting factor for protective immunity in old mice. In this review, we discuss the extent, mechanisms, and potential role of stromal cell aging in the age-related alteration of T cell homeostatic maintenance and immune function decline. We use that discussion to frame the potential strategies to correct the SLO stromal aging defects - in the context of other immune rejuvenation approaches, - to improve functional immune responses and protective immunity in older adults.

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.

Light-Induced Functional Pinealectomy. Effect on the Thymus of C57BL/6 Mice.

Light-induced functional pinealectomy was simulated in C57BL/6 mice by 14-day exposure to constant lighting. Immunophenotyping of CD3hi and CD3low thymocytes was performed by staining with CD3-APC antibodies followed by flow cytofluorometry. To study the cell cycle distribution of thymus cells, the content of intracellular DNA was measured by the level PI inclusion. In animals with light-induced functional pinealectomy, blood leukocyte content, the relative number of CD3low and CD3hi T cells in the thymus, and the ratio of CD3low/CD3hi thymocytes decreased. The number of G0/G1-phase thymus cells (non-dividing cells) increased and the content of S-phase cells (division phase) decreased. Continuous lighting stimulated the development of thymocyte apoptosis. The results obtained indicate that prolonged 24-h illumination inhibits differentiation and maturation of young CD3low thymocytes into mature CD3hi forms and leads to the development of T-cell apoptosis in the thymus and, as a consequence, to leukopenia.

Aging is accompanied by T-cell stiffening and reduced interstitial migration through dysfunctional nuclear organization.

Age-associated changes in T-cell function play a central role in immunosenescence. The role of aging in the decreased T-cell repertoire, primarily because of thymic involution, has been extensively studied. However, increasing evidence indicates that aging also modulates the mechanical properties of cells and the internal ordering of diverse cell components. Cellular functions are generally dictated by the biophysical phenotype of cells, which itself is also tightly regulated at the molecular level. Based on previous evidence suggesting that the relative nuclear size contributes to variations of T-cell stiffness, here we examined whether age-associated changes in T-cell migration are dictated by biophysical parameters, in part through nuclear cytoskeleton organization and cell deformability. In this study, we first performed longitudinal analyses of a repertoire of 111 functional, biophysical and biomolecular features of the nucleus and cytoskeleton of mice CD4+ and CD8+ T cells, in both naive and memory state. Focusing on the pairwise correlations, we found that age-related changes in nuclear architecture and internal ordering were correlated with T-cell stiffening and declined interstitial migration. A similarity analysis confirmed that cell-to-cell variation was a direct result of the aging process and we applied regression models to identify biomarkers that can accurately estimate individuals' age. Finally, we propose a biophysical model for a comprehensive understanding of the results: aging involves an evolution of the relative nuclear size, in part through DNA-hypomethylation and nuclear lamin B1, which implies an increased cell stiffness, thus inducing a decline in cell migration.

Generation of functional human thymic cells from induced pluripotent stem cells.

BACKGROUND: The thymus is a glandular organ that is essential for the formation of the adaptive immune system by educating developing T cells. The thymus is most active during childhood and involutes around the time of adolescence, resulting in a severe reduction or absence of naive T-cell output. The ability to generate a patient-derived human thymus would provide an attractive research platform and enable the development of novel cell therapies. OBJECTIVES: This study sought to systematically evaluate signaling pathways to develop a refined direct differentiation protocol that generates patient-derived thymic epithelial progenitor cells from multiple induced pluripotent stem cells (iPSCs) that can further differentiate into functional patient-derived thymic epithelial cells on transplantation into athymic nude mice. METHODS: Directed differentiation of iPSC generated TEPs that were transplanted into nude mice. Between 14 and 19 weeks posttransplantation, grafts were removed and analyzed by flow cytometry, quantitative PCR, bulk RNA sequencing, and single-cell RNA sequencing for markers of thymic-cell and T-cell development. RESULTS: A direct differentiation protocol that allows the generation of patient-derived thymic epithelial progenitor cells from multiple iPSC lines is described. On transplantation into athymic nude mice, patient-derived thymic epithelial progenitor cells further differentiate into functional patient-derived thymic epithelial cells that can facilitate the development of T cells. Single-cell RNA sequencing analysis of iPSC-derived grafts shows characteristic thymic subpopulations and patient-derived thymic epithelial cell populations that are indistinguishable from TECs present in primary neonatal thymus tissue. CONCLUSIONS: These findings provide important insights and resources for researchers focusing on human thymus biology.

Attenuation of apoptotic cell detection triggers thymic regeneration after damage.

The thymus, which is the primary site of T cell development, is particularly sensitive to insult but also has a remarkable capacity for repair. However, the mechanisms orchestrating regeneration are poorly understood, and delayed repair is common after cytoreductive therapies. Here, we demonstrate a trigger of thymic regeneration, centered on detecting the loss of dying thymocytes that are abundant during steady-state T cell development. Specifically, apoptotic thymocytes suppressed production of the regenerative factors IL-23 and BMP4 via TAM receptor signaling and activation of the Rho-GTPase Rac1, the intracellular pattern recognition receptor NOD2, and micro-RNA-29c. However, after damage, when profound thymocyte depletion occurs, this TAM-Rac1-NOD2-miR29c pathway is attenuated, increasing production of IL-23 and BMP4. Notably, pharmacological inhibition of Rac1-GTPase enhanced thymic function after acute damage. These findings identify a complex trigger of tissue regeneration and offer a regenerative strategy for restoring immune competence in patients whose thymic function has been compromised.

Thymus Degeneration and Regeneration.

The immune system's ability to resist the invasion of foreign pathogens and the tolerance to self-antigens are primarily centered on the efficient functions of the various subsets of T lymphocytes. As the primary organ of thymopoiesis, the thymus performs a crucial role in generating a self-tolerant but diverse repertoire of T cell receptors and peripheral T cell pool, with the capacity to recognize a wide variety of antigens and for the surveillance of malignancies. However, cells in the thymus are fragile and sensitive to changes in the external environment and acute insults such as infections, chemo- and radiation-therapy, resulting in thymic injury and degeneration. Though the thymus has the capacity to self-regenerate, it is often insufficient to reconstitute an intact thymic function. Thymic dysfunction leads to an increased risk of opportunistic infections, tumor relapse, autoimmunity, and adverse clinical outcome. Thus, exploiting the mechanism of thymic regeneration would provide new therapeutic options for these settings. This review summarizes the thymus's development, factors causing thymic injury, and the strategies for improving thymus regeneration.

Aire regulates chromatin looping by evicting CTCF from domain boundaries and favoring accumulation of cohesin on superenhancers.

Aire controls immunological tolerance by driving promiscuous expression of a large swath of the genome in medullary thymic epithelial cells (mTECs). Its molecular mechanism remains enigmatic. High-resolution chromosome-conformation capture (Hi-C) experiments on ex vivo mTECs revealed Aire to have a widespread impact on higher-order chromatin structure, disfavoring architectural loops while favoring transcriptional loops. In the presence of Aire, cohesin complexes concentrated on superenhancers together with mediator complexes, while the CCCTC-binding factor (CTCF) was relatively depleted from structural domain boundaries. In particular, Aire associated with the cohesin loader, NIPBL, strengthening this factor's affiliation with cohesin's enzymatic subunits. mTEC transcripts up-regulated in the presence of Aire corresponded closely to those down-regulated in the absence of one of the cohesin subunits, SA-2. A mechanistic model incorporating these findings explains many of the unusual features of Aire's impact on mTEC transcription, providing molecular insight into tolerance induction.

The Early Postnatal Life: A Dynamic Period in Thymic Epithelial Cell Differentiation.

The microenvironments formed by cortical (c) and medullary (m) thymic epithelial cells (TECs) play a non-redundant role in the generation of functionally diverse and self-tolerant T cells. The role of TECs during the first weeks of the murine postnatal life is particularly challenging due to the significant augment in T cell production. Here, we critically review recent studies centered on the timely coordination between the expansion and maturation of TECs during this period and their specialized role in T cell development and selection. We further discuss how aging impacts on the pool of TEC progenitors and maintenance of functionally thymic epithelial microenvironments, and the implications of these chances in the capacity of the thymus to sustain regular thymopoiesis throughout life.

Lymphotoxin: from the physiology to the regeneration of the thymic function.

The members of the Tumor Necrosis Factor (TNF) superfamily, the ligand lymphotoxin α1ß2 (LTα1ß2) and its unique receptor lymphotoxin ß receptor (LTßR), play a pivotal role in the establishment and regulation of the immune system by allowing a tight communication between lymphocytes and stromal cells. Recent advances using transgenic mice harboring a specific deletion of the Ltbr gene in distinct stromal cells have revealed important roles for LTßR signaling in the thymic function that ensures the generation of a diverse and self-tolerant T-cell repertoire. In this review, we summarize our current knowledge on this signaling axis in the thymic homing of lymphoid progenitors and peripheral antigen-presenting cells, the trafficking and egress of thymocytes, the differentiation of medullary thymic epithelial cells, and the establishment of central tolerance. We also highlight the importance of LTα1ß2/LTßR axis in controlling the recovery of the thymic function after myeloablative conditioning regimen, opening novel perspectives in regenerative medicine.

Recent Advancements in Regenerative Approaches for Thymus Rejuvenation.

The thymus plays a key role in adaptive immunity by generating a diverse population of T cells that defend the body against pathogens. Various factors from disease and toxic insults contribute to the degeneration of the thymus resulting in a fewer output of T cells. Consequently, the body is prone to a wide host of diseases and infections. In this review, first, the relevance of the thymus is discussed, followed by thymic embryological organogenesis and anatomy as well as the development and functionality of T cells. Attempts to regenerate the thymus include in vitro methods, such as forming thymic organoids aided by biofabrication techniques that are transplantable. Ex vivo methods that have shown promise in enhancing thymic regeneration are also discussed. Current regenerative technologies have not yet matched the complexity and functionality of the thymus. Therefore, emerging techniques that have shown promise and the challenges that lie ahead are explored.

The pioneer transcription factors Foxa1 and Foxa2 regulate alternative RNA splicing during thymocyte positive selection.

During positive selection at the transition from CD4+CD8+ double-positive (DP) to single-positive (SP) thymocyte, TCR signalling results in appropriate MHC restriction and signals for survival and progression. We show that the pioneer transcription factors Foxa1 and Foxa2 are required to regulate RNA splicing during positive selection of mouse T cells and that Foxa1 and Foxa2 have overlapping/compensatory roles. Conditional deletion of both Foxa1 and Foxa2 from DP thymocytes reduced positive selection and development of CD4SP, CD8SP and peripheral naïve CD4+ T cells. Foxa1 and Foxa2 regulated the expression of many genes encoding splicing factors and regulators, including Mbnl1, H1f0, Sf3b1, Hnrnpa1, Rnpc3, Prpf4b, Prpf40b and Snrpd3. Within the positively selecting CD69+DP cells, alternative RNA splicing was dysregulated in the double Foxa1/Foxa2 conditional knockout, leading to >850 differentially used exons. Many genes important for this stage of T-cell development (Ikzf1-3, Ptprc, Stat5a, Stat5b, Cd28, Tcf7) and splicing factors (Hnrnpab, Hnrnpa2b1, Hnrnpu, Hnrnpul1, Prpf8) showed multiple differentially used exons. Thus, Foxa1 and Foxa2 are required during positive selection to regulate alternative splicing of genes essential for T-cell development, and, by also regulating splicing of splicing factors, they exert widespread control of alternative splicing.

Indispensable epigenetic control of thymic epithelial cell development and function by polycomb repressive complex 2.

Thymic T cell development and T cell receptor repertoire selection are dependent on essential molecular cues provided by thymic epithelial cells (TEC). TEC development and function are regulated by their epigenetic landscape, in which the repressive H3K27me3 epigenetic marks are catalyzed by polycomb repressive complex 2 (PRC2). Here we show that a TEC-targeted deficiency of PRC2 function results in a hypoplastic thymus with reduced ability to express antigens and select a normal repertoire of T cells. The absence of PRC2 activity reveals a transcriptomically distinct medullary TEC lineage that incompletely off-sets the shortage of canonically-derived medullary TEC whereas cortical TEC numbers remain unchanged. This alternative TEC development is associated with the generation of reduced TCR diversity. Hence, normal PRC2 activity and placement of H3K27me3 marks are required for TEC lineage differentiation and function and, in their absence, the thymus is unable to compensate for the loss of a normal TEC scaffold.

ILC-You in the Thymus: A Fresh Look at Innate Lymphoid Cell Development.

The discovery of innate lymphoid cells (ILCs) has revolutionized our understanding of innate immunity and immune cell interactions at epithelial barrier sites. Their presence and maintenance are critical for modulating immune homeostasis, responding to injury or infection, and repairing damaged tissues. To date, ILCs have been defined by a set of transcription factors, surface antigens and cytokines, and their functions resemble those of three major classes of helper T cell subsets, Th1, Th2 and Th17. Despite this, the lack of antigen-specific surface receptors and the notion that ILCs can develop in the absence of the thymic niche have clearly set them apart from the T-cell lineage and promulgated a dogma that ILCs develop directly from progenitors in the bone marrow. Interestingly however, emerging studies have challenged the BM-centric view of adult ILC development and suggest that ILCs could arise neonatally from developing T cell progenitors. In this review, we discuss ILC development in parallel to T-cell development and summarize key findings that support a T-cell-centric view of ILC ontogeny.

Radiation inducible MafB gene is required for thymic regeneration.

The thymus facilitates mature T cell production by providing a suitable stromal microenvironment. This microenvironment is impaired by radiation and aging which lead to immune system disturbances known as thymic involution. Young adult thymus shows thymic recovery after such involution. Although various genes have been reported for thymocytes and thymic epithelial cells in such processes, the roles of stromal transcription factors in these remain incompletely understood. MafB (v-maf musculoaponeurotic fibrosarcoma oncogene homolog B) is a transcription factor expressed in thymic stroma and its expression was induced a day after radiation exposure. Hence, the roles of mesenchymal MafB in the process of thymic regeneration offers an intriguing research topic also for radiation biology. The current study investigated whether MafB plays roles in the adult thymus. MafB/green fluorescent protein knock-in mutant (MafB+/GFP) mice showed impaired thymic regeneration after the sublethal irradiation, judged by reduced thymus size, total thymocyte number and medullary complexity. Furthermore, IL4 was induced after irradiation and such induction was reduced in mutant mice. The mutants also displayed signs of accelerated age-related thymic involution. Altogether, these results suggest possible functions of MafB in the processes of thymic recovery after irradiation, and maintenance during aging.