Ultrastructural study of thymic microenvironment involution in aging mice

Establishment of transplantation tolerance via minimal conditioning in aged recipients

Mixed hematopoietic chimerism is a powerful means of generating donor-specific tolerance, allowing long-term graft acceptance without lifelong dependence on immunosuppressive drugs. To avoid the need for whole body irradiation and associated side effects, we utilized a radiation-free minimal conditioning regime to induce long-term tolerance across major histocompatibility barriers. We found that low-dose busulfan, in combination with host T cell depletion and short-term sirolimus-based immunosuppression, facilitated efficient donor engraftment. Tolerance was achieved when mice were transplanted with whole or T cell-depleted bone marrow, or purified progenitor cells. Tolerance induction was associated with an expansion in regulatory T cells and was not abrogated in the absence of a thymus, suggesting a dominant or compensatory peripheral mode of tolerance. Importantly, we were able to generate durable chimerism and tolerance to donor skin grafts in both young and aged mice, despite age-related thymic atrophy and immune senescence. Clinically, this is especially relevant as the majority of transplant recipients are older patients whose immune recovery might be dangerously slow and would benefit from radiation-free minimal conditioning regimes that allow efficient donor engraftment without fully ablating the recipient immune system.

Thymus and aging: morphological, radiological, and functional overview

Aging is a continuous process that induces many alterations in the cytoarchitecture of different organs and systems both in humans and animals. Moreover, it is associated with increased susceptibility to infectious, autoimmune, and neoplastic processes. The thymus is a primary lymphoid organ responsible for the production of immunocompetent T cells and, with aging, it atrophies and declines in functions. Universality of thymic involution in all species possessing thymus, including human, indicates it as a long-standing evolutionary event. Although it is accepted that many factors contribute to age-associated thymic involution, little is known about the mechanisms involved in the process. The exact time point of the initiation is not well defined. To address the issue, we report the exact age of thymus throughout the review so that readers can have a nicely pictured synoptic view of the process. Focusing our attention on the different stages of the development of the thymus gland (natal, postnatal, adult, and old), we describe chronologically the morphological changes of the gland. We report that the thymic morphology and cell types are evolutionarily preserved in several vertebrate species. This finding is important in understanding the similar problems caused by senescence and other diseases. Another point that we considered very important is to indicate the assessment of the thymus through radiological images to highlight its variability in shape, size, and anatomical conformation.

Interpreting Stress Responses during Routine Toxicity Studies

Stress often occurs during toxicity studies. The perception of sensory stimuli as stressful primarily results in catecholamine release and activation of the hypothalamic–pituitary–adrenal (HPA) axis to increase serum glucocorticoid concentrations. Downstream effects of these neuroendocrine signals may include decreased total body weights or body weight gain; food consumption and activity; altered organ weights (e.g., thymus, spleen, adrenal); lymphocyte depletion in thymus and spleen; altered circulating leukocyte counts (e.g., increased neutrophils with decreased lymphocytes and eosinophils); and altered reproductive functions. Typically, only some of these findings occur in a given study. Stress responses should be interpreted as secondary (indirect) rather than primary (direct) test article–related findings. Determining whether effects are the result of stress requires a weight-of-evidence approach. The evaluation and interpretation of routinely collected data (standard in-life, clinical pathology, and anatomic pathology endpoints) are appropriate and generally sufficient to assess whether or not changes are secondary to stress. The impact of possible stress-induced effects on data interpretation can partially be mitigated by toxicity study designs that use appropriate control groups (e.g., cohorts treated with vehicle and subjected to the same procedures as those dosed with test article), housing that minimizes isolation and offers environmental enrichment, and experimental procedures that minimize stress and sampling and analytical bias.

This article is a comprehensive overview of the biological aspects of the stress response, beginning with a Summary (Section 1) and an Introduction (Section 2) that describes the historical and conventional methods used to characterize acute and chronic stress responses. These sections are followed by reviews of the primary systems and parameters that regulate and/or are influenced by stress, with an emphasis on parameters evaluated in toxicity studies: In-life Procedures (Section 3), Nervous System (Section 4), Endocrine System (Section 5), Reproductive System (Section 6), Clinical Pathology (Section 7), and Immune System (Section 8). The paper concludes (Section 9) with a brief discussion on Minimizing Stress-Related Effects (9.1.), and a final section explaining why Parameters routinely measured are appropriate for assessing the role of stress in toxicology studies (9.2.).

Influence of surgical and chemical orchidectomy on weight and distribution of AChE-nerve fibres in thymuses of adult rats

The thymus is a crossroad between the immune and neuroendocrine systems. As such, it is innervated by acetylcholinesterase (AChE)-positive fibres of the vagus, the recurrent laryngeal and the phrenic nerves. It is well know, that the innervations density of the thymus increases with age. In our study, adult rats were orchidectomized (surgically and chemically by the application of a luteinizing hormone-releasing hormone). The density of AChE-positive nerve fibres in thymuses, as well as the weight of thymuses was examined. The authors found that both surgical and chemical orchidectomy result in macroscopic and microscopic regeneration of the atrophied thymuses. In regenerated rat’s thymuses after orchidectomy the density of AChE-positive nerve fibres was markedly higher in comparison with the control animals. The distribution, as well as the density of AChE-positive nerve fibres in regenerated thymuses after orchidectomy evokes the images of its innervations like in young animals before age-related involution. The authors also found a markedly higher weight of thymuses of orchidectomized rats in comparison with the control groups. In recent study the authors proved that after 8 weeks surgical orchidectomy leads to the regeneration of thymic AChE-positive innervation and chemical orchidectomy by administration of luteinizing hormone-releasing hormone after 4 weeks of adult rats.

Impact of aging on dendritic cell functions in humans

Aging is a paradox of reduced immunity and chronic inflammation. Dendritic cells are central orchestrators of the immune response with a key role in the generation of immunity and maintenance of tolerance. The functions of DCs are compromised with age. There is no major effect on the numbers and phenotype of DC subsets in aged subjects; nevertheless, their capacity to phagocytose antigens and migrate is impaired with age. There is aberrant cytokine secretion by various DC subsets with CDCs secreting increased basal level of pro-inflammatory cytokines but the response on stimulation to foreign antigens is decreased. In contrast, the response to self-antigens is increased suggesting erosion of peripheral self tolerance. PDC subset also secretes reduced IFN-α in response to viruses. The capacity of DCs to prime T cell responses is also affected. Aging thus has a profound affect on DC functions. Present review summarizes the effect of advancing age on DC functions in humans in the context of both immunity and tolerance.

Sex steroid ablation enhances immune reconstitution following cytotoxic antineoplastic therapy in young mice

Cytotoxic antineoplastic therapy is used to treat malignant disease but results in long-term immunosuppression in postpubertal and adult individuals, leading to increased incidence and severity of opportunistic infections. We have previously shown that sex steroid ablation (SSA) reverses immunodeficiencies associated with age and hematopoietic stem cell transplantation in both autologous and allogeneic settings. In this study, we have assessed the effects of SSA by surgical castration on T cell recovery of young male mice following cyclophosphamide treatment as a model for the impact of chemotherapy. SSA increased thymic cellularity, involving all of the thymocyte subsets and early T lineage progenitors. It also induced early repair of damage to the thymic stromal microenvironment, which is crucial to the recovery of a fully functional T cell-based immune system. These functional changes in thymic stromal subsets included enhanced production of growth factors and chemokines important for thymopoiesis, which preceded increases in both thymocyte and stromal cellularity. These effects collectively translated to an increase in peripheral and splenic naive T cells. In conclusion, SSA enhances T cell recovery following cyclophosphamide treatment of mice, at the level of the thymocytes and their stromal niches. This provides a new approach to immune reconstitution following antineoplastic therapy.

Regeneration of dendritic cells in aged mice

Age-related thymic involution causes a decreased output of thymocytes from the thymus, thereby resulting in impairment of T cell-mediated immunity. While alterations in the T cell and non-haematopoietic stromal compartments have been described, the effects of thymic involution on thymic dendritic cells (DC) are not clearly known. Thymic DC play an essential role in shaping T cell-mediated immune responses by deleting self-reactive thymocytes to establish central tolerance and by inducing regulatory T-cell (Treg) development. It is therefore important to assess the prevalence of and alterations to thymic DC with age, as this may impact on their function. We assessed the numbers and proportions of the three distinct subsets of thymic DC in ageing mice, and showed that these subsets are differentially regulated. This is expected as thymic DC subsets have different origins of development. We further assessed the responses of thymic DC in a regenerative environment, such as that induced by sex-steroid ablation (SSA), and clearly showed that, consistent with global thymus regrowth, all three DC populations increased in numbers and regained their relative proportions to thymocytes after an initial lag period. These findings are important for the clinical translation of thymic regenerative approaches, and indicate that SSA facilitates the maintenance of critical processes such as negative selection and Treg induction through promoting thymic DC regeneration.

Luteinizing hormone-releasing hormone enhances T cell recovery following allogeneic bone marrow transplantation

Posttransplant immunodeficiency, specifically a lack of T cell reconstitution, is a major complication of allogeneic bone marrow transplantation. This immunosuppression results in an increase in morbidity and mortality from infections and very likely contributes to relapse. In this study, we demonstrate that sex steroid ablation using leuprolide acetate, a luteinizing hormone-releasing hormone agonist (LHRHa), increases the number of lymphoid and myeloid progenitor cells in the bone marrow and developing thymocytes in the thymus. Although few differences are observed in the peripheral myeloid compartments, the enhanced thymic reconstitution following LHRHa treatment and allogeneic bone marrow transplantation leads to enhanced peripheral T cell recovery, predominantly in the naive T cell compartment. This results in an increase in T cell function in vivo and in vitro. Graft-versus-host-disease is not exacerbated by LHRHa treatment and graft-versus-tumor activity is maintained. Because LHRHa allows for reversible (and temporary) sex steroid ablation, has a strong safety profile, and has been clinically approved for diseases such as prostate and breast cancer, this drug treatment represents a novel therapeutic approach to reversal of thymic atrophy and enhancement of immunity following immunosuppression.

Strategies for reconstituting and boosting T cell-based immunity following haematopoietic stem cell transplantation: pre-clinical and clinical approaches

Poor immune recovery is characteristic of bone marrow transplantation and leads to high levels of morbidity and mortality. The primary underlying cause is a compromised thymic function, resulting from age-induced atrophy and further compounded by the damaging effects of cytoablative conditioning regimes on thymic epithelial cells (TEC). Several strategies have been proposed to enhance T cell reconstitution. Some, such as the use of single biological agents, are currently being tested in clinical trials. However, a more rational approach to immune restoration will be to leverage the evolving repertoire of new technologies. Specifically, the combined targeting of TEC, thymocytes and peripheral T cells, together with the bone marrow niches, promises a more strategic clinical therapeutic platform.

Deletion of the developmentally essential gene ATR in adult mice leads to age-related phenotypes and stem cell loss

Developmental abnormalities, cancer, and premature aging each have been linked to defects in the DNA damage response (DDR). Mutations in the ATR checkpoint regulator cause developmental defects in mice (pregastrulation lethality) and humans (Seckel syndrome). Here we show that eliminating ATR in adult mice leads to defects in tissue homeostasis and the rapid appearance of age-related phenotypes, such as hair graying, alopecia, kyphosis, osteoporosis, thymic involution, fibrosis, and other abnormalities. Histological and genetic analyses indicate that ATR deletion causes acute cellular loss in tissues in which continuous cell proliferation is required for maintenance. Importantly, thymic involution, alopecia, and hair graying in ATR knockout mice were associated with dramatic reductions in tissue-specific stem and progenitor cells and exhaustion of tissue renewal and homeostatic capacity. In aggregate, these studies suggest that reduced regenerative capacity in adults via deletion of a developmentally essential DDR gene is sufficient to cause the premature appearance of age-related phenotypes.

The role of sex steroids and gonadectomy in the control of thymic involution

A major underlying cause for aging of the immune system is the structural and functional atrophy of the thymus, and associated decline in T cell genesis. This loss of naïve T cells reduces adaptive immunity to new stimuli and precipitates a peripheral bias to memory cells against prior antigens. Whilst multiple mechanisms may contribute to this process, the temporal alliance of thymic decline with puberty has implicated a causative role for sex steroids. Accordingly ablation of sex steroids induces profound thymic rejuvenation. Although the thymus retains some, albeit highly limited, function in healthy adults, this is insufficient for resurrecting the T cell pool following cytoablative treatments such as chemo- and radiation-therapy and AIDS. Increased risk of opportunistic infections and cancer relapse or appearance, are a direct consequence. Temporary sex steroid ablation may thus provide a clinically effective means to regenerate the thymus and immune system in immunodeficiency states.

Thymic generation and regeneration: a new paradigm for establishing clinical tolerance of stem cell-based therapies

Tolerance to tissue-engineering products is a major obstacle hindering the clinical application of this rapidly advancing technology. Manipulation of central tolerance, by establishing thymus chimerism of both donor and host-derived haemopoietic cells (haemopoietic stem cell transplant--HSCT), should purge any T cells reactive to potential donor organ or tissue transplant. A functional thymus, however, is required to induce chimerism and repopulate the peripheral T cell pool, but age-related thymic atrophy and damage caused by ablative conditioning regimes significantly reduce thymic function and increase incident of infection-dependent morbidity and mortality. Thus rejuvenation of the thymus alongside HSCT may potentiate the use of this strategy in the clinic. In addition, the use of thymic epithelial progenitor cell technology may allow growth of ex vivo thymic tissue for use in clinical situations of immunodeficiency as well as in establishing tolerance to tissue/organ products derived from the same source.

Multiple prethymic defects underlie age-related loss of T progenitor competence

Aging in mice and humans is characterized by declining T-lymphocyte production in the thymus, yet it is unclear whether aging impacts the T-lineage potential of hematopoietic progenitors. Although alterations in the lymphoid progenitor content of aged mouse bone marrow (BM) have been described, irradiation-reconstitution experiments have failed to reveal defects in T-lineage potential of BM hematopoietic progenitors or purified hematopoietic stem cells (HSCs) from aged mice. Here, we assessed T-progenitor potential in unmanipulated recipient mice without conditioning irradiation. T-progenitor potential was reduced in aged BM compared with young BM, and this reduction was apparent at the earliest stages of intrathymic differentiation. Further, enriched populations of aged HSCs or multipotent progenitors (MPPs) gave rise to fewer T-lineage cells than their young counterparts. Whereas the T-precursor frequency within the MPP pool was unchanged, there was a 4-fold decline in T-precursor frequency within the HSC pool. In addition, among the T-competent HSC clones, there were fewer highly proliferative clones in the aged HSC pool than in the young HSC pool. These results identify T-compromised aged HSCs and define the nature and cellular sites of prethymic, age-related defects in T-lineage differentiation potential.

Enhanced immune reconstitution by sex steroid ablation following allogeneic hemopoietic stem cell transplantation

Delayed immune reconstitution in adult recipients of allogeneic hemopoietic stem cell transplantations (HSCT) is related to age-induced thymic atrophy. Overcoming this paucity of T cell function is a major goal of clinical research but in the context of allogeneic transplants, any strategy must not exacerbate graft-vs-host disease (GVHD) yet ideally retain graft-vs-tumor (GVT) effects. We have shown sex steroid ablation reverses thymic atrophy and enhances T cell recovery in aged animals and in congenic bone marrow (BM) transplant but the latter does not have the complications of allogeneic T cell reactivity. We have examined whether sex steroid ablation promoted hemopoietic and T cell recovery following allogeneic HSCT and whether this benefit was negated by enhanced GVHD. BM and thymic cell numbers were significantly increased at 14 and 28 days after HSCT in castrated mice compared with sham-castrated controls. In the thymus, the numbers of donor-derived thymocytes and dendritic cells were significantly increased after HSCT and castration; donor-derived BM precursors and developing B cells were also significantly increased. Importantly, despite restoring T cell function, sex steroid inhibition did not exacerbate the development of GVHD or ameliorate GVT activity. Finally, IL-7 treatment in combination with castration had an additive effect on thymic cellularity following HSCT. These results indicate that sex steroid ablation can profoundly enhance thymic and hemopoietic recovery following allogeneic HSCT without increasing GVHD and maintaining GVT.

Aging and the dendritic cell system: implications for cancer

The immune system shows a decline in responsiveness to antigens both with aging, as well as in the presence of tumors. The malfunction of the immune system with age can be attributed to developmental and functional alterations in several cell populations. Previous studies have shown defects in humoral responses and abnormalities in T cell function in aged individuals, but have not distinguished between abnormalities in antigen presentation and intrinsic T cell or B cell defects in aged individuals. Dendritic cells (DC) play a pivotal role in regulating immune responses by presenting antigens to naïve T lymphocytes, modulating Th1/Th2/Th3/Treg balance, producing numerous regulatory cytokines and chemokines, and modifying survival of immune effectors. DC are receiving increased attention due to their involvement in the immunobiology of tolerance and autoimmunity, as well as their potential role as biological adjuvants in tumor vaccines. Recent advances in the molecular and cell biology of different DC populations allow for addressing the issue of DC and aging both in rodents and humans. Since DC play a crucial role in initiating and regulating immune responses, it is reasonable to hypothesize that they are directly involved in altered antitumor immunity in aging. However, the results of studies focusing on DC in the elderly are conflicting. The present review summarizes the available human and experimental animal data on quantitative and qualitative alterations of DC in aging and discusses the potential role of the DC system in the increased incidence of cancer in the elderly.

Morphometrical Characteristics of Age-Associated Changes in the Thymus of Old Male Wistar Rats

In order to provide a morphometrical description of the changes in the aged rat thymus and to relate them to apoptotic and proliferative activity of thymocytes, the thymuses from 3- and 18-month-old male Wistar rats and the percentages of bromodeoxyuridine-incorporating and apoptotic cells in cultures of thymocytes were assessed by stereological analysis and flow cytometry, respectively. In old rats the volume of lymphoepithelial thymic tissue is markedly reduced, reflecting a sharp decrease in the total number of thymocytes. A reduction in the proliferative capacity of thymocytes and increase in their susceptibility to apoptosis are, most likely, primarily responsible for a 7-fold reduction in thymic cellularity in old animals. Furthermore, only the volume of cortical compartment was affected by aging, while that of medulla, despite of reduced cellularity, was not significantly altered. The loss of functional tissue in aged thymus is compensated by a substantial increase in the volume of inter-lobular connective and adipose tissue, so the thymic weight remained unaltered in old rats. These results suggest that thymus of aged Wistar rats exhibits morphological characteristics similar to those found in aged human thymus and thus may serve as an animal model for further investigations of thymus-related changes in immunological aging.

Thymic function in severely immunodeficient HIV type 1-infected patients receiving stable and effective antiretroviral therapy

The role of the thymus in long-term immune reconstitution has not been addressed in HIV patients who were severely immunodeficient prior to successful treatment with combination antiretroviral therapy (ART). Adult HIV-1 patients (n = 78) with nadir CD4+ T cell counts <100 T cells/microl, at least 12 months on ART and 6 months of complete viral suppression (<50 HIV RNA copies/ml) were selected from a patient database. The cohort was divided according to current CD4+ T cell counts and patients from the lowest (n = 15) and highest (n = 12) tertiles were studied. Thymic volume was assessed by spiral computed tomography. Naive (CD45RA+CD62L+) and replicating (Ki67+) T cells were quantitated by flow cytometry, T cell receptor excision circles (TREC) were assessed by real-time PCR, and serum IL-7 and testosterone by immunoassay. Patients with low CD4+ T cell counts had smaller thymuses [0(0-5.3) vs. 3.5(0-15.6) cm(3), p = 0.04] and were more likely to have no detectable thymus. They had similar proportions of replicating cells, but fewer naive CD4+ and CD8+ T cells and less TREC in CD4+ and CD8+ T cells/ml of blood than patients with high CD4+ T cell counts. However, some patients with no detectable thymus had high numbers of naive and TREC-bearing T cells. Thus, the recovery of CD4+ T cells in severely immunodeficient HIV patients with a virological response to ART is probably limited by thymic function. However, the data are consistent with extrathymic T cell production contributing to the naive T cell pool in some patients.

Sex Steroid Ablation Enhances Lymphoid Recovery Following Autologous Hematopoietic Stem Cell Transplantation

BACKGROUND

Autologous hematopoietic stem cell transplantation (auto-HSCT) patients experience long-term immunosuppression, which increases susceptibility to infection and relapse rates due to minimal residual disease (MRD). Sex steroid (SS) ablation is known to reverse age-related thymic atrophy and decline in B-cell production

METHODS

This study used a congenic HSCT mouse model to analyze the effects of SS ablation (through surgical castration) on immune reconstitution and growth factor production following auto-HSCT. Bone marrow (BM) and thymic stromal cell (TSCs) populations were analyzed using RT-PCR and were tested for the production of growth factors previously implicated in immune reconstitution or age-relate immune degeneration

RESULTS

Castration increased bone marrow (BM), thymic, and splenic cellularity following auto-HSCT. HSC number and common lymphoid precursor (CLP) frequency and number were increased in castrated mice. B cell precursor numbers were also significantly increased in the BM of these mice. Triple negative, double positive and single positive thymocytes were increased following HSCT and castration, as were thymic dendritic cells and natural killer T (NKT) cells. This enhanced lymphoid reconstitution of the primary immune organs leads to a significant increase in splenic T and B cells 42 days after HSCT. The molecular mechanisms behind the enhanced reconstitution were also studied. TGF-beta1 was decreased in castrated mice compared to sham-castrated controls in TSCs and BM cells. TSC production of IL-6 was also decreased in castrated mice

CONCLUSIONS

These data suggest that sex steroid ablation significantly enhances lymphopoiesis following auto-HSCT providing a new strategy for posttransplant immune reconstitution.

Aging and T cell development: interplay between progenitors and their environment

Thymic involution is the hallmark of hematopoietic aging. Because T cell differentiation is a multistep process that occurs non-cell autonomously, aging defects can occur at multiple points along the developmental pathway, both in the T progenitors themselves and in the thymic stromal cells that support their development. Here we review the evidence for age-related thymopoiesis defects at key steps in the production of naïve mature T cells, highlighting the importance of the interaction between stromal aging and progenitor aging.

Keratinocyte growth factor (KGF) is required for postnatal thymic regeneration

Keratinocyte growth factor (KGF) is a member of the fibroblast growth factor family that mediates epithelial cell proliferation and differentiation in a variety of tissues, including the thymus. We studied the role of KGF in T-cell development with KGF-/- mice and demonstrated that thymic cellularity and the distribution of thymocyte subsets among KGF-/-, wildtype (WT), and KGF+/- mice were similar. However, KGF-/- mice are more vulnerable to sublethal irradiation (450 cGy), and a significant decrease was found in thymic cellularity after irradiation. Defective thymopoiesis and peripheral T-cell reconstitution were found in KGF-/- recipients of syngeneic or allogeneic bone marrow transplant, but using KGF-/- mice as a donor did not affect T-cell development after transplantation. Despite causing an early developmental block in the thymus, administration of KGF to young and old mice enhanced thymopoiesis. Exogenous KGF also accelerated thymic recovery after irradiation, cyclophosphamide, and dexamethasone treatment. Finally, we found that administering KGF before bone marrow transplantation (BMT) resulted in enhanced thymopoiesis and peripheral T-cell numbers in middle-aged recipients of an allogeneic BM transplant. We conclude that KGF plays a critical role in postnatal thymic regeneration and may be useful in treating immune deficiency conditions.

Strategies to enhance T-cell reconstitution in immunocompromised patients

Immune deficiency, together with its associated risks such as infections, is becoming an increasingly important clinical problem owing to the ageing of the general population and the increasing number of patients with HIV/AIDS, malignancies (especially those treated with intensive chemotherapy or radiotherapy) or transplants (of either solid organs or haematopoietic stem cells). Of all immune cells, T cells are the most often affected, leading to a prolonged deficiency of T cells, which has important clinical consequences. Accordingly, strategies to improve the recovery and function of T cells, as we discuss here, should have a direct impact on reducing the morbidity and mortality of many patients and should increase the efficacy of therapeutic and prophylactic vaccinations against microbial pathogens or tumours.

Thymic epithelial cells in age-dependent involution

Aging involves morphological and functional alterations within the microenvironment of the thymus where heterogenous populations of thymic epithelial cells (TEC) play the main roles. The studies performed to date on thymic involution signalize a disturbed interaction between individual thymic compartments that disrupt thymocyte-TEC interactions and, as a sequele, disturb differentiation of both TEC and thymocytes. The process of aging affects the various subsets of TEC at different periods of life. Changes in different subsets of TEC are documented on the basis of their phenotypical characteristics, involving morphological analysis and immunocytochemistry. The character and kinetics of changes in TEC are typical for individual subsets and probably sex-dependent. In the course of life, the involutionary changes, expressed by disorganised thymic structure and function, are accompanied by changes in medullary TEC, manifested by alterations in the differentiation process of the cells. In parallel, at the same stage of individual life, the aging process induces increased proliferative and secretory activity of subseptal TEC, which seem to functionally replace medullary TEC. Structural and phenotypic modifications of TEC are locally controlled by complex sets of different factors and seem to represent a morphological adaptation of the gland to the process of aging. Microsc. Res. Tech. 62:488-500, 2003.

Involvement of growth factors in thymic involution

The thymus undergoes an age-dependent degenerative process which is mainly characterized by a progressive loss of lymphoid tissue. Thymic involution is particularly important in relation to immunosenescence and its various associated diseases; this fact has prompted many studies aimed at understanding the causes and mechanisms of thymic degeneration which may, ultimately, lead to the possibility of manipulating it. In this sense, one of the aspects which has deserved most attention is the thymic microenvironment, and more precisely, the many growth factors to which the cells present in the organ are exposed. Thus, the levels of several of such factors have been reported to undergo age-dependent changes in the thymus, which may point at an influence on the regression of the organ. In this article we consider which growth factors and growth factor receptors occur in the vertebrate thymus. Then, focusing on those whose influences are better documented, i.e., neurotrophins, cytokines and IGFs, we discuss their potential role in the organ and the possibility of their being involved in thymic involution.

Age-dependent changes in thymic macrophages and dendritic cells

Aging is characterized by the decline and deregulation of several physiological systems, especially the immune system. The involution of the thymus gland has been identified as one of the key events that precedes the age-related decline in immune function. Whereas the decrease in thymocyte numbers and in the thymic output during thymus atrophy has been analyzed by various authors, very little information is available about the age-associated modifications in thymic macrophages and dendritic cells. Here we present evidence that these thymic stromal cell components are only slightly affected by age.

Cellular mechanism of thymic involution

Involution of the thymus and alterations in the development of thymocytes are the most prominent features of age-related immune senescence. We have carried out a comparative analysis of thymocyte and stroma in rapid thymic involution DBA/2 (D2) strain of mice compared with slow involution C57BL/6 (B6) strain of mice. Analysis of mice at 15 months of age suggested an age-related decrease in the thymocyte cell count, a block in the development of T cells and cortical involution in D2 mice compared with 3-month-old mice. TUNEL (terminal-deoxynucleotidyl-transferase-mediated dUTP-digoxigenin nick end labelling) staining and fluorescence-activated cell sorter (FACS) analysis showed that there was a significant increase in apoptotic cells in the cortex region of thymus in 15-month-old D2 mice compared with the same aged B6 mice. The thymocyte proliferation rate, as assessed by bromodeoxyuridine (BrdU) staining and [3H]-thymidine incorporation assay, was lower in 3-month-old D2 mice compared with the same age B6 mice. Immunohistochemical staining showed that the arrangement of MTS (mouse thymus stromal)-10+ epithelial cells and MTS-16+ connective tissue staining pattern had become disorganized in 15-month-old D2 mice but remained intact in B6 mice of the same age. These results suggest that, in D2 mice, both the thymocytes and stromal cells exhibit age-related defects, and that the genetic background of mice plays an important role in determining age-related alterations in thymic involution.

Age-related change in thymic T-cell development is associated with genetic loci on mouse chromosomes 1, 3, and 11

Age-related decline in thymic T-cell development in 22-month-old C57BL/6J X DBA/2J (BXD) recombinant inbred strains of mice was functionally and phenotypically analyzed and genetically mapped. There was a positive correlation of the concanavalin A (Con A)-induced thymocyte proliferative response with the capability of thymocytes to mature to the CD4(+)CD8(+) stage. The accumulation of CD4(-)CD8(-) stage of thymocytes in 22-month-old BXD mice was further identified to be associated with a developmental block between the CD25(-)CD44(+) and the CD25(+)CD44(+) stages. The quantitative trait loci regulating the Con A-induced thymocyte proliferative response were mapped to mouse chromosome 1, 3, and 11, nearest to 32.1 centimorgan (cM), 5.6 cM, and 18.0 cM, respectively. Our results suggest that several genetic loci regulate the intra-thymic T-cell maturation process and play an important role in determining age-related decline in thymic T-cell development.

Ultrastructural study of the FVB/N mouse thymus: presence of an immature epithelial cell in the medulla and premature involution

FVB/N inbred mice have been widely used to generate a variety of transgenic lines, but their physiology and especially their immunological characteristics are poorly documented. We therefore studied the ultrastructure of the thymus and the distribution of thymocyte subpopulations in FVB/N mice at several ages. In young FVB/N mice the stromal microenvironment exhibits the three types of epithelial cells and the two types of bone-marrow derived cells (macrophages and interdigitated cells) previously described in other strains of mice. Moreover, in the thymic medulla of young FVB/N mice, a fourth cell type with the morphological characteristics of an immature epithelial cell was present in relatively high number. Furthermore, thymocyte subpopulations distribution shows an earlier thymocyte maturation than in other strains. Finally, changes associated with thymic involution were observed about 5 months earlier than in many other mouse strains. Our results demonstrated that the FVB/N strain has a specific immunological status.

Altered aging-related thymic involution in T cell receptor transgenic, MHC-deficient, and CD4-deficient mice

During aging in mice and humans, a gradual decline in thymus integrity and function occurs (thymic involution). To determine whether T cell reactivity or development affects thymic involution, we compared the thymic phenotype in old (12 months) and young (2 months) mice transgenic for rearranged alphabeta or beta 2B4 T cell receptor (TCR) genes, mice made deficient for CD4 by gene targetting (CD4(-/-)), mice made deficient for major histocompatibility complex (MHC) class I (beta2M-/-) or class II genes (A(beta)(b-/-) on C57Bl/6 background) or both. The expected aging-related reductions in thymic weights were observed for all strains except those bearing disruption of both class I and class II MHC genes. Therefore, disruption of MHC class I and class II appeared to reverse or delay aging-related thymic atrophy at 12 months. Immunohistochemical analysis of aging-associated alterations in thymic morphology revealed that TCR alphabeta transgenes, CD4 disruption, and MHC class II disruption all reduced or eliminated these changes. All strains examined at 12 months showed alterations in the distribution of immature thymocyte populations relative to young controls. These results show that aging-associated thymic structural alterations, size reductions, and thymocyte developmental delays can be separated and are therefore causally unrelated. Furthermore, these results suggest that the T cell repertoire and/or its development play a role in aging-related thymic involution.

Immunoreactivity of neural crest-derived cells in thymic tissue developing under the rat kidney capsule

In order to study the functional development of a thymus in an experimental model, small pieces of adult rat thymic tissue were cultured for 9 days and implanted under the kidney capsule of littermates. The tissues were examined with a panel of antibodies raised against thymic and neural factors and neural crest cells at intervals from 5 to 13 days. At 5 days post-implantation, there were groups of L1+ cells within the implants that reacted with antibodies raised against neural and neural crest cell markers. L1+ cells were highly mitotic, rounded cells measuring 8.7 +/- 0.6 micrometer in diameter. Double immunostaining with different combinations of antibodies showed that 94% of the L1+ cells were also TH+, and many were HNK-1/NCAM+, PGP 9.5+, NGF+, chromogranin A+, VIP+, S100+, CGRP+, GAD+, and A2B5+. A few were also pan-cytokeratin+. These results indicate that these cells are derived from neural crest derived cells and belong to the neuroepithelial line of development. The L1+ cells were most numerous before nerves appeared (about Day 9) and reduced in number and extent as the thymus differentiated. The neural crest cells occasionally had long cytoplasmic extensions, but it was not possible to decide if they formed the nerves that appeared in the implants. Adult thymuses also contained a population of L1+ and HNK-1/NCAM+ cells, mainly in the subcapsular cortex, the septa, and the medulla. These cells could be a source of neural crest cells able to repopulate the implant. The adult thymus may always contain a reservoir of cells potentially capable of producing neuropeptides and transmitter factors required for thymic growth and regeneration.

Age-dependent remodeling of rat thymus. Morphological and cytofluorimetric analysis from birth up to one year of age

Structural and phenotypic modifications of rat thymocytes from birth up to one year of age, i.e. during maturation and at the beginning of the involutive process of the thymus are described. Since the biological significance and the mechanisms of thymic involution are still a matter of debate, this study aims at clarifying the complexity of the compensatory events occurring during this relatively neglected period of time. Thymuses from Sprague-Dawley rats were analyzed morphologically and morphometrically by light and electron microscopy. At the same time, thymocyte subsets, isolated from the same animals, were characterized by flow cytometry according to physical parameters and phenotypic markers. Results indicate that major changes occur during the first month from birth and from six months onward. In particular, already during the first weeks after birth, thymocytes undergo a slight reduction of mitoses associated with an increased number of apoptoses. Moreover, during the same period of time, flow cytometry revealed an expansion of small thymocytes and changes in thymocyte subsets such as increase of CD4+CD8+ and CD5+alpha(beta)TCR- and a decrease of CD4-CD8-, CD4-CD8+ cells. The thymus of adult rats was characterized by time-dependent decrease of both mitoses and apoptoses, progressive physical disconnection among cells, increase of necrotic areas and fibrosis. Around one year of age tissue changes were associated with a dramatic reduction of the population of large thymocytes and the rise of numerous small thymocytes that were unexpectedly negative for all tested markers. By contrast, medium-size thymocytes exhibited a marked decrease of CD4+CD8+ and CD5+alpha(beta)TCR- subsets. In conclusion, our data indicate that thymus undergoes, with time, a complex remodeling and suggest that thymic involution is not only a simple shrinkage of the organ but rather the result of a series of compensatory mechanisms among different cell populations in a setting of progressive involution.

Premature thymic involution, observed at the ultrastructural level, in two lineages of human-SOD-1 transgenic mice

The human Cu/Zn superoxide dismutase (hSOD-1) gene, catalyses the dismutation of O2 to H2O2 and O2. It is located on chromosome 21 in q22.1 and is overexpressed in Down's syndrome (DS) patients. These patients present various abnormalities including mental retardation, congenital heart disease, immunological deficits and premature aging. In order to explore the potential role of SOD-1 overexpression in DS, we have generated two lineages of transgenic mice for the hSOD-1 gene and studied, at the ultrastructural level, the effect of hSOD-1 overexpression on the thymic microenvironment. Modification of the cellular architecture and morphology associated with a lipidic invasion, signs of a premature involution of the thymus, were observed in both lineages. A rupture of the filamentous network in the extracellular and probably also in the intracellular matrix was first observed. These results correlate the thymic alterations visualized in light microscopy, on the thymus from DS patients, and raise the question of the relationship between the SOD-1 overexpression and the different morphological alterations associated with the premature thymic involution observed in SOD-1 transgenic mice. They suggest that thymic and immunological impairments present in DS patients may be related to the SOD-1 gene dosage effect.