Reversal of ageing changes in the thymus of rats by chemical or surgical castration

Immune-endocrine interactions in marsupials and monotremes

Interactions between the immune and endocrine systems are not well studied in marsupials and monotremes. One exception to this is the phenomenon of semelparity, which is well covered in the literature as this is an unusual reproductive strategy amongst mammals and is only observed in some dasyurid and didelphid marsupials. Thymus involution provides a direct link between the endocrine and immune systems and warrants further study in marsupials and monotremes. The thymus is a primary immune tissue which is essential for overall immune function. Whilst the organ is large in juvenile animals, it begins to involute around puberty due to the suppressive effects of sex steroids. Thymus involution has a significant effect on the immune system, as it signals the onset of immune aging and decline in function. The output of naïve T lymphocytes by the thymus decreases, increasing susceptibility of aged individuals to infection and cancers. Understanding the links between the immune and endocrine system in marsupials and monotremes may shed light on diseases such as devil facial tumour disease (DFTD) which threatens the future of the Tasmanian devil. We hypothesise that changes in sex hormones around puberty may drive changes in the immune system, such as thymus involution, which may make devils more susceptible to DFTD as they age. In addition, the Schwann cell origin of DFTD may enable tumours to respond to sex hormones, as occurs in similar cancers in humans.

Peripubertal ovariectomy influences thymic adrenergic network plasticity in adult rats

The study investigated the influence of peripubertal ovariectomy on the thymic noradrenaline (NA) concentration, and the thymocyte NA content and β2- and α1-adrenoceptor (AR) expression in adult 2- and 11-month-old rats. In control rats, the thymic NA concentration increased with age. This increase reflected rise in the density of catecholamine (CA)-containing fluorescent nerve fibers and cells and their CA content. Additionally, the average β2- and α1-AR thymocyte surface density changed in the opposite direction with age; the density of β2-AR decreased, whereas that of α1-AR increased. Ovariectomy diminished the thymic NA concentration in 2-month-old rats. This reflected the decrease in the density of fluorescent nerve fibers, and CA content in fluorescent nerve fibers and non-lymphoid cells, since the thymocyte NA content was increased in ovariectomized (Ox) rats. Estrogen supplementation prevented the ovariectomy-induced changes. In Ox rats, the density of CA-synthesizing nerve fibers and non-lymphoid cells diminished with age. To the contrary, NA content in thymocytes increased with age, but it did not exceed that in 11-month-old controls. Additionally, ovariectomy diminished the average thymocyte surface density of β2-ARs, but it increased that of α1-ARs in 2-month-old-rats (due to estrogen, and estrogen and progesterone deficiency, respectively). These changes, despite of the rise in circulating estrogen level post-ovariectomy, remained stable with age. This most likely reflected a decreased sensitivity to estrogen action, as a consequence of the hormone misprinting in peripubertal age. The analysis of thymocyte proliferation in culture suggested that age- and ovariectomy-induced alterations in thymocyte NA synthesis and AR expression altered NA autocrine/paracrine action on thymocytes. In conclusion, the study indicates that the ovarian hormone deficiency in peripubertal age affects ovarian steroid-dependent remodeling of thymic adrenergic regulatory network in adult rats.

Hormonal control of T-cell development in health and disease

The physiology of the thymus, the primary lymphoid organ in which T cells are generated, is controlled by hormones. Data from animal models indicate that several peptide and nonpeptide hormones act pleiotropically within the thymus to modulate the proliferation, differentiation, migration and death by apoptosis of developing thymocytes. For example, growth hormone and prolactin can enhance thymocyte proliferation and migration, whereas glucocorticoids lead to the apoptosis of these developing cells. The thymus undergoes progressive age-dependent atrophy with a loss of cells being generated and exported, therefore, hormone-based therapies are being developed as an alternative strategy to rejuvenate the organ, as well as to augment thymocyte proliferation and the export of mature T cells to peripheral lymphoid organs. Some hormones (such as growth hormone and progonadoliberin-1) are also being used as therapeutic agents to treat immunodeficiency disorders associated with thymic atrophy, such as HIV infection. In this Review, we discuss the accumulating data that shows the thymus gland is under complex and multifaceted hormonal control that affects the process of T-cell development in health and disease.

Ovarian hormone level alterations during rat post-reproductive life-span influence CD8 + T-cell homeostasis

The study examined the putative role of ovarian hormones in shaping of rat peripheral T-cell compartment during post-reproductive period. In 20-month-old rats ovariectomized (Ox) at the very end of reproductive period, thymic output, cellularity and composition of major TCRαβ + peripheral blood lymphocyte and splenocyte subsets were analyzed. Ovariectomy led to the enlargement of CD8 + peripheral blood lymphocyte and splenocyte subpopulations. This reflected: (i) a more efficient thymic generation of CD8 + cells as indicated by increased number of CD4+CD8 + double positive and the most mature CD4-CD8+TCRαβ(high) thymocytes and CD8 + recent thymic emigrants (RTEs) in peripheral blood, but not in the spleen of Ox rats, and (ii) the expansion of CD8 + memory/activated peripheral blood lymphocytes and splenocytes. The latter was consistent with a greater frequency of proliferating cells among freshly isolated memory/activated CD8 + peripheral blood lymphocytes and splenocytes and increased proliferative response of CD8 + splenocytes to stimulation with plate-bound anti-CD3 antibody. The former could be related to the rise in splenic IL-7 and IL-15 mRNA expression. Although ovariectomy affected the overall number of CD4 + T cells in none of the examined compartments, it increased CD4+FoxP3 + peripheral blood lymphocyte and splenocyte counts by enhancing their generation in periphery. Collectively, the results suggest that ovariectomy-induced long-lasting disturbances in ovarian hormone levels (mirrored in diminished progesterone serum level in 20-month-old rats) affects both thymic CD8 + cell generation and peripheral homeostasis and leads to the expansion of CD4+FoxP3 + cells in the periphery, thereby enhancing autoreactive cell control on account of immune system efficacy to combat infections and tumors.

Thymic rejuvenation and aging

The thymus is a vital organ for homeostatic maintenance of the peripheral immune system. It is within this mediastinal tissue that T cells develop and are extensively educated and exported to the periphery for establishment of a functional and effective immune system. A striking paradoxical feature of this critical lymphoid tissue is that it undergoes profound age-associated involution. Thymic decline is of minimal consequence to healthy individuals, but the reduced efficacy of the immune system with age has direct etiological linkages with an increase in diseases including opportunistic infections, autoimmunity, and incidence/burden of cancer. Furthermore the inability of adults to restore immune function following insult induced by chemotherapy, ionizing radiation exposure or therapy, and infections (e.g. HIV-1) leads to increased morbidity and often mortality in the elderly. For these reasons, it is important that investigators strive to translate their understanding of mechanisms that drive thymic involution, and develop safe and effective strategies to rejuvenate the thymus in settings of clinical need. In this review, we present a discussion of the current status of thymic rejuvenation efforts associated with: sex steroid ablation, cytokines, growth factors, and hormones.

The importance of the nurse cells and regulatory cells in the control of T lymphocyte responses

T lymphocytes from the immune system are bone marrow-derived cells whose development and activities are carefully supervised by two sets of accessory cells. In the thymus, the immature young T lymphocytes are engulfed by epithelial “nurse cells” and retained in vacuoles, where most of them (95%) are negatively selected and removed when they have an incomplete development or express high affinity autoreactive receptors. The mature T lymphocytes that survive to this selection process leave the thymus and are controlled in the periphery by another subpopulation of accessory cells called “regulatory cells,” which reduce any excessive immune response and the risk of collateral injuries to healthy tissues. By different times and procedures, nurse cells and regulatory cells control both the development and the functions of T lymphocyte subpopulations. Disorders in the T lymphocytes development and migration have been observed in some parasitic diseases, which disrupt the thymic microenvironment of nurse cells. In other cases, parasites stimulate rather than depress the functions of regulatory T cells decreasing T-mediated host damages. This paper is a short review regarding some features of these accessory cells and their main interactions with T immature and mature lymphocytes. The modulatory role that neurotransmitters and hormones play in these interactions is also revised.

Aerobic fitness is associated with lower proportions of senescent blood T-cells in man

Senescent T-cells accumulate with age, lowering the naïve T-cell repertoire and increasing host infection risk. As this response is likely to be influenced by certain lifestyle factors, we examined the association between aerobic fitness (VO(2max)) and the age-related accumulation of senescent T-cells. Blood lymphocytes from 102 healthy males (18-61 yr) were analyzed for KLRG1, CD57, CD28, CD45RA, CD45RO surface expression on CD4+ and CD8+ T-cells by 4-color flow cytometry. Advancing age (yr) was positively associated with the proportion (%) of senescent (KLRG1+/CD57+; KLRG1+/CD28-) CD4+ (B=1.00; 1.02) and CD8+ (B=0.429; 1.02) T-cells and inversely associated with naïve (KLRG1-/CD28+) CD4+ (B=-1.000) and CD8+ (B=-0.993) T-cells. VO(2max) was inversely associated with senescent CD4+ (B=-0.97) and CD8+ (B=-0.240). Strikingly, age was no longer associated with the proportions of senescent or naïve T-cells after adjusting for VO(2max), while the association between VO(2max) and these T-cell subsets withstood adjustment for age, BMI and percentage body fat. Ranking participants by age-adjusted VO(2max) revealed that the highest tertile had 17% more naïve CD8+ T-cells and 57% and 37% less senescent CD4+ and CD8+ T-cells, respectively, compared to the lowest tertile. VO(2max) was not associated with latent cytomegalovirus (CMV), Epstein-Barr virus (EBV) or herpes simplex virus-1 (HSV-1) infection, indicating that the moderating associations of VO(2max) were not confounded by persistent viral infections. This is the first study to show that aerobic fitness is associated with a lower age-related accumulation of senescent T-cells, highlighting the beneficial effects of maintaining a physically active lifestyle on the aging immune system.

Thymic involution: where endocrinology meets immunology

The decline in immune function with aging represents a major clinical challenge in many disease conditions. It is manifest in many parameters but is essentially linked to the adaptive immune responses. The prediction would be that abnormalities in both T and B lymphocytes underlie the loss of cellular and humoral capacity, respectively. Somewhat surprisingly, this is not reflected in numerical losses but more in alterations at the population and single cell levels. There is a major reduction in naïve T cells with a proportional increase in memory cells, and also a generally reduced function of these cells. While bone marrow function reduces with age, the most obvious reason for the T cell defects is the severe atrophy of the thymus. This is closely aligned with puberty, thereby implicating a major aetiological role for sex steroids in both thymus and immune system deterioration with age. Accordingly surgical or chemical castration (utilizing luteinizing hormone-releasing hormone) blocks sex steroids resulting in profound rejuvenation of the immune system.

Role of ovarian hormones in age-associated thymic involution revisited

A commonly held view that ovarian hormones are causally involved in age-associated thymic involution has been recently challenged. In particular, their relevance in the progression of thymic involution has been disputed. To reassess this issue 10-month-old rats with well advanced thymic involutive changes were ovariectomized (Ovx), and after 1 month thymic cellularity, thymocyte development and levels of recent thymic emigrants (RTEs) were examined in peripheral blood and spleen. In addition, the distribution of major conventional and regulatory T-cell subsets was analyzed in the same peripheral lymphocyte compartments. Ovariectomy increased thymic weight and cellularity above the levels in both 10-month-old and age-matched controls indicating that ovarian hormone ablation not only prevented further progression of thymic involution, but also reversed it. The increased thymic cellularity was accompanied by altered thymocyte differentiation/maturation culminating in increased thymic output of naïve T cells as indicated by elevated levels of both CD4+ and CD8+ RTEs in peripheral blood and spleen. The changes in T-cell development produced: (i) a disproportional increase in cellularity across thymocyte subsets, so that relative proportions of cells at all maturational stages preceding the CD4+CD8+ T cell receptor (TCR)alphabeta(low) stage were reduced; the relative numbers of CD4+CD8+ TCRalphabeta(low) cells entering positive selection and their immediate CD4+CD8+ TCRalphabeta(high) descendents were increased, while those of the most mature CD4+CD8- and CD4-CD8+ TCRalphabeta(high) cells remained unaltered; (ii) enhanced cell proliferation across all thymocyte subsets and (iii) reduced apoptosis of cells within the CD4+CD8+ thymocyte subset. The augmented thymic output of naïve T cells in Ovx rats most likely reflected an early disinhibition of thymocyte development followed by increased positive/reduced negative selection, at least partly, due to raised thymocyte surface Thy-1 expression. The greater number of CD4+CD25+Foxp3+ cells in both thymus and peripheral blood suggested augmented thymic production of these cells. In addition, an increased CD4+/CD8+ cell ratio was found in the spleen of Ovx rats. Thus, ovarian hormone ablation led not only to increased diversity of the T-cell repertoire, but also to a new balance among distinct T-cell subsets in the periphery.

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.

The immune risk profile is associated with age and gender: findings from three Swedish population studies of individuals 20-100 years of age

Earlier we identified an Immune Risk Profile (IRP) of very old individuals, 86-94 years of age, characterised by an inverted CD4/CD8 ratio and associated with persistent cytomegalovirus infection and an increase in the numbers of CD3+CD8+CD28- cells. In the present study we included data from a population-based sample in the age range of 20-79 years to examine the prevalence of individuals with an inverted CD4/CD8 ratio relative to age and gender across the entire adult lifespan. Immunological monitoring that was conducted included analysis of the numbers of T-cells in the subsets CD3+, CD3+CD4+, and CD3+CD8+ as well as CD3+CD8+CD28+, CD3+CD8+CD28-, and CD8+CD45RA+CCR7+. There was found to be a significant lowering of the numbers of CD3+, CD3+CD4+, and CD3+CD8+, and of the CD8+CD45RA+CCR7+ cells across the adult life-span. Notably, the prevalence of individuals with an inverted CD4/CD8 ratio increased from about 8% in the age range of 20-59 years to about 16% in the age range of 60-94 years. The mortality rate in individuals with an inverted CD4/CD8 ratio also increased significantly above the age of 60. Interestingly, the proportion of individuals with an inverted CD4/CD8 ratio was found to be significantly higher in men, whereas the numbers of CD3+CD4+ helper and CD8+CD45RA+CCR7+ naïve cells and the CD4/CD8 ratio were found to be significantly higher in women. These results highlight the importance of functioning of the thymus in the development of IRP and may partly account for the differences between sexes in terms of longevity.

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.

Clinical strategies to enhance T cell reconstitution

Strategies to enhance T cell recovery are of increasing clinical importance to overcome long lasting T cell deficiencies, which occur in association with infections, autoimmunity and chemo/radiotherapy as well as aging of the immune system. In this review we discuss those strategies that are close to or in the clinic. Interleukin-7, sex steroid modulation, keratinocyte growth factor, growth hormone and cellular therapies using ex vivo generated T-cell precursors are currently being tested in recipients of a hematopoietic stem cell transplantation and patients with malignancies or HIV/AIDS.

Immunosenescence: deficits in adaptive immunity in the elderly

Aging is associated clinically with increases in the frequency and severity of infectious diseases and an increased incidence of cancer, chronic inflammatory disorders and autoimmunity. These age-associated immune dysfunctions are the consequence of declines in both the generation of new naïve T and B lymphocytes and the functional competence of memory populations. These alterations collectively are termed immunosenescence.

Severe defect in thymic development in an insertional mutant mouse model

Transgenic mice were generated expressing NK1.1, an NK cell-associated receptor, under control of the human CD2 promoter. Unexpectedly, one of the founder lines, Tg66, showed a marked defect in thymic development characterized by disorganized architecture and small size. Mapping of the transgene insertion by fluorescence in situ hybridization revealed integration in chromosome 2, band G. Already from postnatal day 3, the thymic architecture was disturbed with a preferential loss of cortical thymic epithelial cells, a feature that became more pronounced over time. Compared with wild-type mice, total thymic cell numbers decreased dramatically between 10 and 20 days of age. Thymocytes isolated from adult Tg66 mice were predominantly immature double-negative cells, indicating a block in thymic development at an early stage of differentiation. Consequently, Tg66 mice had reduced numbers of peripheral CD4(+) and CD8(+) T cells. Bone marrow from Tg66 mice readily reconstituted thymi of irradiated wild-type as well as RAG-deficient mice. This indicates that the primary defect in Tg66 mice resided in nonhemopoietic stromal cells of the thymus. The phenotype is observed in mice heterozygous for the insertion and does not resemble any known mutations affecting thymic development. Preliminary studies in mice homozygous for transgene insertion reveal a more accelerated and pronounced phenotype suggesting a semidominant effect. The Tg66 mice may serve as a useful model to identify genes regulating thymic epithelial cell differentiation, thymic development, and function.

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.

Effects of Castration on Thymocyte Development in Two Different Models of Thymic Involution

Age-associated thymic involution is accompanied by decreased thymic output. This adversely affects general immune competence and T cell recovery following cytoreductive treatments such as chemotherapy. A causal link between increasing sex steroids and age-related thymic atrophy is well established. Although castration has been demonstrated to regenerate the atrophied thymus, little is known about how this is initiated or the kinetics of thymocyte regeneration. The present study shows that although castration impacts globally across thymocyte development in middle-aged mice, the regenerative effects are initiated in the immature triple-negative compartment and early T lineage progenitors (ETP). Specifically, there was a reduction in number of ETP with age, which was restored following castration. There was, however, no change in ETP reconstitution potential in ETP at this age or following castration. Furthermore, in a chemotherapy-induced model of thymic involution, we demonstrate castration enhances intrathymic proliferation and promotes differentiation through the triple-negative program. Clinically, reversible sex steroid ablation is achieved hormonally, and thus presents a means of ameliorating immune inadequacies, for example, following chemotherapy for bone marrow transplantation. By improving our understanding of the kinetics of thymic recovery, this study will allow more appropriate timing of therapy to achieve maximal reconstitution, especially in the elderly.

Insights into thymic aging and regeneration

The deterioration of the immune system with progressive aging is believed to contribute to morbidity and mortality in elderly humans due to the increased incidence of infection, autoimmunity, and cancer. Dysregulation of T-cell function is thought to play a critical part in these processes. One of the consequences of an aging immune system is the process termed thymic involution, where the thymus undergoes a progressive reduction in size due to profound changes in its anatomy associated with loss of thymic epithelial cells and a decrease in thymopoiesis. This decline in the output of newly developed T cells results in diminished numbers of circulating naive T cells and impaired cell-mediated immunity. A number of theories have been forwarded to explain this 'thymic menopause' including the possible loss of thymic progenitors or epithelial cells, a diminished capacity to rearrange T-cell receptor genes and alterations in the production of growth factors and hormones. Although to date no interventions fully restore thymic function in the aging host, systemic administration of various cytokines and hormones or bone marrow transplantation have resulted in increased thymic activity and T-cell output with age. In this review, we shall examine the current literature on thymic involution and discuss several interventional strategies currently being explored to restore thymic function in elderly subjects.

Vertebrate thymus and the neurotrophin system

An immunomodulary role has been proposed for growth factors included in the family of neurotrophins. This is supported by the presence of both neurotrophins and neurotrophin receptors in the immune organs and some immunocompetent cells, the in vitro and in vivo effects of the neurotrophins on the immune cells, and the structural changes of lymphoid organs in mice deficient in neurotrophins and their receptors. The current data strongly indicate that neurotrophins regulate the biology of thymic stromal cells and T cells, including survival, and are involved in the thymic organogenesis. This review compiles the available data about the occurrence and distribution of neurotrophins and their signaling receptors (Trk proteins and p75(NTR)) in the vertebrate thymus and the possible contribution of these molecules to the thymic microenvironment and, therefore, to the T cells differentiation.

Thymic generation and regeneration

The thymus is a complex epithelial organ in which thymocyte development is dependent upon the sequential contribution of morphologically and phenotypically distinct stromal cell compartments. It is these microenvironments that provide the unique combination of cellular interactions, cytokines, and chemokines to induce thymocyte precursors to undergo a differentiation program that leads to the generation of functional T cells. Despite the indispensable role of thymic epithelium in the generation of T cells, the mediators of this process and the differentiation pathway undertaken by the primordial thymic epithelial cells are not well defined. There is a lack of lineage-specific cell-surface-associated markers, which are needed to characterize putative thymic epithelial stem cell populations. This review explores the role of thymic stromal cells in T-cell development and thymic organogenesis, as well as the molecular signals that contribute to the growth and expansion of primordial thymic epithelial cells. It highlights recent advances in these areas, which have allowed for a lineage relationship amongst thymic epithelial cell subsets to be proposed. While many fundamental questions remain to be addressed, collectively these works have broadened our understanding of how the thymic epithelium becomes specialized in the ability to support thymocyte differentiation. They should also facilitate the development of novel, rationally based therapeutic strategies for the regeneration and manipulation of thymic function in the treatment of many clinical conditions in which defective T cells have an important etiological role.

One for all and all for one: thymic epithelial stem cells and regeneration

It has long been believed that the thymic epithelial microenvironment originates from both the endodermal and ectodermal germ cell layers. However, a growing body of evidence indicates that such a dual origin is not the case, and that the diverse thymic epithelial populations all develop from a common epithelial stem cell. This article explores these data, investigates the identity of such cells and the signals that might control their expansion and differentiation, and considers the possibility of stem cell transplantation for thymic regeneration.

Aging is a deprivation syndrome driven by a germ-soma conflict

Evolution through natural selection can be described as driven by a perpetual conflict of individuals competing for limited resources. Recently, I postulated that the shortage of resources godfathered the evolutionary achievements of the differentiation-apoptosis programming [Rev. Neurosci. 12 (2001) 217]. Unicellular deprivation-induced differentiation into germ cell-like spores can be regarded as the archaic reproduction events which were fueled by the remains of the fratricided cells of the apoptotic fruiting body. Evidence has been accumulated suggesting that conserved through the ages as the evolutionary legacy of the germ-soma conflict, the somatic loss of immortality during the ontogenetic segregation of primordial germ cells recapitulates the archaic fate of the fruiting body. In this heritage, somatic death is a germ cell-triggered event and has been established as evolutionary-fixed default state following asymmetric reproduction in a world of finite resources. Aging, on the other hand, is the stress resistance-dependent phenotype of the somatic resilience that counteracts the germ cell-inflicted death pathway. Thus, aging is a survival response and, in contrast to current beliefs, is antagonistically linked to death that is not imposed by group selection but enforced upon the soma by the selfish genes of the "enemy within". Environmental conditions shape the trade-off solutions as compromise between the conflicting germ-soma interests. Mechanistically, the neuroendocrine system, particularly those components that control energy balance, reproduction and stress responses, orchestrate these events. The reproductive phase is a self-limited process that moulds onset and progress of senescence with germ cell-dependent factors, e.g. gonadal hormones. These degenerate the regulatory pacemakers of the pineal-hypothalamic-pituitary network and its peripheral, e.g. thymic, gonadal and adrenal targets thereby eroding the trophic milieu. The ensuing cellular metabolic stress engenders adaptive adjustments of the glucose-fatty acid cycle, responses that are adequate and thus fitness-boosting under fuel shortage (e.g. during caloric restriction) but become detrimental under fuel abundance. In a Janus-faced capacity, the cellular stress response apparatus expresses both tolerogenic and mutagenic features of the social and asocial deprivation responses [Rev. Neurosci. 12 (2001) 217]. Mediated by the derangement of the energy-Ca(2+)-redox homeostatic triangle, a mosaic of dedifferentiation/apoptosis and mutagenic responses actuates the gradual exhaustion of functional reserves and eventually results in a multitude of aging-related diseases. This scenario reconciles programmed and stochastic features of aging and resolves the major inconsistencies of current theories by linking ultimate and proximate causes of aging. Reproduction, differentiation, apoptosis, stress response and metabolism are merged into a coherent regulatory network that stages aging as a naturally selected, germ cell-triggered and reproductive phase-modulated deprivation response.

Alterations in thymopoiesis in intact and peripubertally orchidectomized adult rats of different age

In adult rats 3, 6 and 9 months post-orchidectomy performed at the age of 30 days the thymus weight, thymocyte yield and relative proportions of thymocyte subsets (delineated by expression of CD4/CD8 molecules and TCRalphabeta) were analyzed in order to elucidate a putative role of male gonadal hormones in the shaping of thymus size and intrathymic T cell maturation. In 4-month-old control rats the thymus size and cellularity returned to the corresponding levels in 1-month-old rats. These levels were sustained during the following 6 months. In spite of that, the distribution of the main thymocyte subsets in these rats was subjected to significant changes, probably due to an age-associated diminishing thymus ability to provide efficient T cell differentiation. The results added further weight to a potential feedback regulatory role of CD4+8- cells in thymopoiesis. Furthermore, they revealed that the orchidectomy-induced (i) enlargement of the thymus size and enrichment of the thymic lymphoid cell content are of a limited duration; and (ii) alterations in the relative proportion of thymocytes become quantitatively more pronounced with duration of the gonadal deprivation. Thus, the study also indicates that the age-associated changes in gonadal hormones may be, at least partly, responsible for the age-related reshaping of the T cell maturation sequence, and hence for remodeling T cell dependent immune functions.

Effects of androgens on T and B lymphocyte development

The sexually dimorphic nature of normal immune responses and the remarkably higher incidence of autoimmune diseases in females have suggested a role for gonadal steroid hormones as modulators of immune system function. We have investigated the effects of androgens on the development of lymphocytes in the thymus and bone marrow. Expression of the androgen receptor, the ligand-activated transcription factor that mediates hormone actions, has been documented in lymphoid and nonlymphoid cells of thymus and bone marrow, but not in mature peripheral lymphocytes. This expression pattern suggests that the major impact of androgens must be on the developmental maturation of T and B lymphocytes rather than on the mature effector cells. Recent experiments have explored whether developing lymphoid precursors are the direct targets of androgen action or whether supporting cells, such as thymic epithelial cells and bone marrow stromal cells, are required for the receptor-mediated effects of androgens on lymphoid cell development. Bone marrow transplantation techniques using an androgen-resistant mouse strain permit the creation of chimeric mice with androgen receptor-defective lymphoid or epithelial/stromal cellular compartments. Hormonal manipulation experiments in these chimeric animals have suggested that thymic epithelial cells and bone marrow stromal cells are mediators of androgenic effects on immature lymphocytes. The long-range goal of these studies is to understand the basis for the disproportionate occurrence of autoimmune diseases in females.

Thymic activity in late-stage HIV-1 infected individuals receiving highly active antiretroviral therapy: potential effect of steroid therapy

OBJECTIVE

Our objective was to monitor the effect of steroid therapy on the thymic output and function of late-stage HIV-1-infected patients undergoing highly active antiretroviral therapy (HAART).

DESIGN

The indirect measurement of T cells that have recently emigrated from the thymus as a means of quantifying thymic output, and therefore thymic function, was achieved through use of the polymerase chain reaction-based signal joint T cell receptor rearrangement excision circles (sjTREC) assay. Proliferative capacity and interleukin (IL)-2 and IL-4 production by T cells after antigenic, mitogenic and IL-2 stimulation were also analysed.

METHOD

Measurements were made of sjTREC levels in peripheral blood mononuclear cell DNA samples from five HIV-1 infected patients (one on steroid therapy prior to and at the time of sample extraction) receiving HAART. IL-2 and IL-4 production and proliferative capacity were also measured in three patients, including the patient receiving steroids.

RESULT

The sjTREC assay gave an extremely weak result for the patient on steroids but, under the same assay conditions, provided clear, positive readings for the four patients not on steroids. Comparison of the patients' cytokine profiles revealed that IL-2 production was generally low or absent in all three patients tested but that IL-4 production was significantly higher in the patient given steroids. Functional potential as revealed by proliferation assays showed very low or absent cellular proliferation.

CONCLUSION

The thymic contribution to the restoration of T lymphocyte numbers, particularly during the treatment of HIV-1 infection, may become compromised if thymic inhibitory factors such as steroids are used. Furthermore, the use of steroids may also favour the development of a T helper 2 response, which could prove particularly undesirable during HIV-1 infection.

Both age and gender affect thymic output: more recent thymic migrants in females than males as they age

The thymus undergoes age-associated involution, with studies showing thymic size decreasing from birth at a rate of approximately 3% per year until middle age, and at a rate of 1% per year thereafter. The aim of this study was to determine the effect of thymic atrophy on T-lymphocyte production by the thymus, and to clarify the ongoing uncertainty regarding gender differences in thymic function. We quantified recent thymic emigrants (RTEs) in blood through the measurement of signal joint T-cell receptor rearrangement excision circles (sjTRECs), and showed that the decline in the number of RTEs in the blood with increasing age is gender-linked. Peripheral blood from females contained significantly higher levels of sjTRECs per CD3+ T cell than blood from males (P = 0.002), despite there being no significant gender difference in the absolute number of CD3+ T cells in the populations analysed (P > 0.10). Our findings suggest better thymic function in females compared with males, providing females with a higher number of recent thymic emigrants for longer periods of life. Such a finding provides a plausible explanation for the immunological gender differences observed in previous studies and possibly, for the general longer life expectancy in females compared with males.

Changes in the development of experimental autoimmune prostatitis (EAP) by castration in aged rats

During Experimental Autoimmune Prostatitis (EAP), 12-month-old rats show a higher cellular autoimmune response and lower humoral autoimmune response against prostatic components than 3-month-old rats subjected to the same antigen stimulus. We analyzed if thymus recovery by orchidectomy could affect the development of EAP in 12-month-old rats. Thirty days after gonadectomy, 12-month-old rats showed an increment in the thymic mass and in the thymocytes absolute number, with percentages of the four main cell subpopulations (defined by CD4-CD8 molecules expression) similar to the 3-month-old rats. The DTH response of castrated 12-month-old with EAP were diminished in comparison with sham-castrated 12-month-old rats with EAP, resembling the values observed in 3-month-old rats with EAP. The prostates of castrated 12-month-old rats with EAP did not show inflammatory mononuclear cell infiltration, as did control 3- and 12-month-old rats with EAP. Castration seems to modulate negatively EAP in 12-month-old rats, possibly through the regeneration of thymus after testosterone deprivation.

The role of the thymus in immune reconstitution in aging, bone marrow transplantation, and HIV-1 infection

The human thymus is a complex chimeric organ comprised of central (thymic epithelial space) and peripheral (perivascular space) components that functions well into adult life to produce naive T lymphocytes. Recent advances in identifying thymic emigrants and development of safe methods to study thymic function in vivo in adults have provided new opportunities to understand the role that the human thymus plays in immune reconstitution in aging, in bone marrow transplantation, and in HIV-1 infection. The emerging concept is that there are age-dependent contributions of thymic emigrants and proliferation of postthymic T cells to maintain the peripheral T cell pool and to contribute to T cell regeneration, with the thymus contributing more at younger ages and peripheral T cell expansion contributing more in older subjects. New studies have revealed a dynamic interplay between postnatal thymus output and peripheral T cell pool proliferation, which play important roles in determining the nature of immune reconstitution in congenital immunodeficiency diseases, in bone marrow transplantation, and in HIV-1 infection. In this paper, we review recent data on human postnatal thymus function that, taken together, support the notion that the human thymus is functional well into the sixth decade and plays a role throughout life to optimize human immune system function.

Thymic involution in aging

The size of the naive T-cell pool is governed by output from the thymus and not by replication. This pool contributes cells to the activated/memory T-cell pool whose size can be increased through cell multiplication; both pools together constitute the peripheral T-cell pool. Aging is associated with involution of the thymus leading to a reduction in its contribution to the naive T-cell pool; however, despite this diminished thymic output, there is no significant decline in the total number of T cells in the peripheral T-cell pool. There are, however, considerable shifts in the ratios of both pools of cells, with an increase in the number of activated/memory T cells and the accumulation in older individuals of cells that fail to respond to stimuli as efficiently as T cells from younger individuals. Aging is also associated with a greater susceptibility to some infections and some cancers. An understanding of the causal mechanism of thymic involution could lead to the design of a rational therapy to reverse the loss of thymic tissue, renew thymic function, increase thymic output, and potentially improve immune function in aged individuals.

Thymic atrophy in the mouse is a soluble problem of the thymic environment

Age related deterioration in the function of the immune system has been recognised in many species. The clinical presentations of such immune dysfunction are an age-related increased susceptibility to certain infections, and an increased incidence of autoimmune disease and certain cancers. Laboratory investigations reveal a reduced ability of the cells from older individuals, compared with younger individuals, to perform in functional in vitro assays. These manifestations are thought to be causally linked to an age associated involution of the thymus, which precedes the onset of immune dysfunction. Hypotheses to account for the age-related changes in the thymus include: (i) an age related decline in the supply of T cell progenitors from the bone marrow (ii) an intrinsic defect in the marrow progenitors, or (iii) problems with rearrangement of the TCR beta chain because of a defect in the environment provided by the thymus. We have analysed these possible options in normal mice and also in mice carrying a transgenic T cell receptor. The results from these studies reveal no age related decline either in the number of function of T cell progenitors in the thymus, but changes in the thymic environment in terms of the cytokines produced. We have shown that specific cytokine replacement therapy leads to an increase in thymopoiesis in old animals.

Leukemia inhibitory factor, oncostatin M, IL-6, and stem cell factor mRNA expression in human thymus increases with age and is associated with thymic atrophy

The roles that thymus cytokines might play in regulating thymic atrophy are not known. Reversing thymic atrophy is important for immune reconstitution in adults. We have studied cytokine mRNA steady-state levels in 45 normal human (aged 3 days to 78 years) and 34 myasthenia gravis thymuses (aged 4 to 75 years) during aging, and correlated cytokine mRNA levels with thymic signal joint (sj) TCR delta excision circle (TREC) levels, a molecular marker for active thymopoiesis. LIF, oncostatin M (OSM), IL-6, M-CSF, and stem cell factor (SCF) mRNA were elevated in normal and myasthenia gravis-aged thymuses, and correlated with decreased levels of thymopoiesis, as determined by either decreased keratin-positive thymic epithelial space or decreased thymic sjTRECs. IL-7 is a key cytokine required during the early stages of thymocyte development. Interestingly, IL-7 mRNA expression did not fall with aging in either normal or myasthenia gravis thymuses. In vivo administration of LIF, OSM, IL-6, or SCF, but not M-CSF, i.p. to mice over 3 days induced thymic atrophy with loss of CD4+, CD8+ cortical thymocytes. Taken together, these data suggest a role for thymic cytokines in the process of thymic atrophy.

Age- and disease-related decline in immune function: an opportunity for "thymus-boosting" therapies

The thymus is the site of production of mature T lymphocytes and thus is indispensable for the development and maintenance of the T cell-mediated arm of the immune system. Thymic production of mature T cells is critically dependent on an influx of bone marrow-derived progenitor T cells that undergo replication and selection within the thymus. Thymus cellularity and thymic hormone secretion reach a peak during the first year of life and then decline gradually until the age of 50-60 years, a process known as "thymic involution." A rapid reduction of thymus cellularity occurs in young patients following injuries, chemotherapy, and other forms of stress. The mechanisms underlying the involution process appear to be dependent on factors intrinsic to the thymic tissue, such as the local production of cytokines and chemoattractants, promoting the recruitment, growth, and differentiation of bone marrow-derived T cell progenitors in the thymus, as well as extrinsic factors, such as systemic levels of endocrine hormones and mediators released by intrathymic nerves of the autonomic nervous system. Knowledge of these factors provides a rational basis for the development of an approach based on tissue engineering that could be used to provide either temporary or permanent reconstitution of thymic function.

Failure of Rearranged TCR Transgenes to Prevent Age-Associated Thymic Involution

After puberty, the thymus undergoes a dramatic loss in volume, in weight and in the number of thymocytes, a phenomenon termed age-associated thymic involution. Recently, it was reported that age-associated thymic involution did not occur in mice expressing a rearranged transgenic (Tg) TCRαβ receptor. This finding implied that an age-associated defect in TCR rearrangement was the major, if not the only, cause for thymic involution. Here, we examined thymic involution in three other widely used MHC class I-restricted TCRαβ Tg mouse strains and compared it with that in non-Tg mice. In all three TCRαβ Tg strains, as in control mice, thymocyte numbers were reduced by ∼90% between 2 and 24 mo of age. The presence or absence of the selecting MHC molecules did not alter this age-associated cell loss. Our results indicate that the expression of a rearranged TCR alone cannot, by itself, prevent thymic involution. Consequently, other presently unknown factors must also contribute to this phenomenon.

Orchidectomy affects the innervation of the rat thymus

To assess a putative role of the neural pathways in transfer of information from the gonads to the thymus, adult AO rats were orchidectomized (ORX) or sham ORX (controls); sacrificed 1, 3, 7, or 15 days later and their thymi were analyzed for: (a) the concentrations of noradrenaline (NA), dopamine (DA) and serotonin (5-HT) and distribution of monoamine-containing nerve profiles and (b) the acetylcholinesterase (AChE) activity and distribution of AChE-containing nerve profiles. Three days after the castration, an elevation in the level of both catecholamines, reflecting an increase in the overall intensity of nerve fibers autofluorescence, was found. Seven days post castration neither NA nor DA concentration differed from the appropriate control values, while 15 days after the surgery the concentration of NA was lower than that in the controls, most likely, due to diminished density of noradrenergic nerve profiles. In both the rats sacrificed 7 and 15 days after orchidectomy the concentration of 5-HT was reduced as result of a decrease in the density of 5-HT-containing autofluorescent cells. The activity of AChE was depressed one day after the surgery; then increased, so that 3 days post castration its value was higher than that in the sham ORX. After this increase, AChE activity decreased being, at postoperative day 7 and 15, lower than that in the controls. It seems that this decrease in AChE activity reflected, not only a reduction in the density of AChE-containing nerve fibers, but also a decrease in the density of AChE positive cells. Thus, the results indicate that orchidectomy can evoke changes in the T-cell maturation altering modulatory influences on this process coming via neural route, as well as those coming from the mast cells and AChE positive epithelial cells which constitute important component of the thymus microenvironment.

Age-dependent immunolocalization of fibronectin and histological changes in the thymus of rats

Age-dependent variations in the immunolocalization of fibronectin (FN) in the thymus were investigated in 1-, 6-, 12- and 20-month-old male and female Swiss albino rats (Rattus rattus) at the light-microscopic level and the changes with ageing in the histological structure of the thymus were also studied. There were no significant differences in the age-dependent variations in the immunolocalization of fibronectin or in the histological structure of the thymus between male and female rats of the same age but there were increases with ageing in the fibronectin content of the thymic capsule, the connective tissue between the lobules, around blood vessels, and in the medulla and cortex of thymus. The connective-tissue content between lobules, fat cells, Hassall's corpuscles, the thickness of capsule and the ratio of the medulla to the cortex of the lobules showed age-dependent increases in the thymus of rats of both sexes. Decreases in the organ weight/body weight ratio were also observed with ageing.

Tamoxifen exerts testosterone-dependent and independent effects on thymic involution

Circulating testosterone concentrations and seminal vesicles weights, as well as thymus and spleen weights and histology were assessed in male Wistar rats from the infantile to post-pubertal period. The widely used anti-estrogenic agent tamoxifen was then administered in adult intact and castrated male rats and its long-term effects on thymic involution and splenic growth were examined. The results showed that: (1) age-related involution of the male thymus from the juvenile period through puberty to post-puberty depends on the rising testosterone levels and represents mainly a decrease of thymic lymphoid-cell elements; (2) tamoxifen administration reverses thymic involution in intact adult male rats and this effect is related to a dose-dependent, tamoxifen-induced castration and decrease of testosterone levels; (3) the changes of circulating testosterone levels, either resulting from maturity, or induced by tamoxifen or by castration, have a minimal effect on splenic growth and weight; and (4) in contrast to intact animals, administration of tamoxifen at pharmacological doses to adult castrated rats results in thymic regression. Underscoring the critical role of testosterone on thymic involution, these findings show that tamoxifen is able to reverse ageing changes in the thymus by suppressing testosterone production, while conversely, exerts thymolytic effects in the absence of androgens.

Effects of castration on the lymphocytes of the thymus, spleen and lymph nodes

It is well known that the thymus plays an important role in the development and maintenance of a competent immune system. The thymus atrophies with age, a process that is accelerated after puberty when there is elevation of serum sex steroid levels. We have used a panel of commercial monoclonal antibodies against various T and B cell surface markers to investigate the post-castration histological alterations in the thymus, spleen and lymph nodes of male Sprague-Dawley rats. Castration of 5-week-old male rats produced a significant increase in thymic weight (P < 0.05) compared to age-matched intact animals. The major observations from the immunohistochemical studies were post-castration elevations in staining for total T cells (MRC OX 19 and W 3/13), CD8 cells (MRC OX 8), B cells (MRC OX 12 and MARK-1) and cells bearing activation markers such as IL-2 receptor (MRC OX 39), transferrin receptor (MRC OX 26) and major histocompatibility class II antigen (MRC OX 6). These data suggest that following castration there is an increase in the ability of lymphocytes to respond to activation. As a result, there are elevated numbers of immature thymocytes within the thymus that undergo differentiation/maturation and consequently produce an increase in peripheral T and B cells.

Thymic microenvironment at the light microscopic level

The thymus is a primary lymphoid organ that serves the immune system by providing an optimal microenvironment for developing T cells to rearrange the genes encoding the T-cell receptor and to undergo positive and negative selection in shaping the peripheral T-cell repertoire. The microenvironment of the organ is peculiar among lymphoid organs, as the supporting stroma consists of reticular epithelial cells. Bone marrow-derived interdigitating cells and macrophages are the main accessory cell populations. The epithelium, interdigitating cells, and macrophages each contribute to the T-cell selection process. During the last decade knowledge has been gathered that these cell populations show a considerable heterogeneity, as documented for subcellular features and immunologic phenotype. This heterogeneity may reflect various stages in differentiation, but may otherwise be linked to the functional activity of the cells. The authors survey the major cell populations, i.e., epithelial cells and lymphocytes. Macrophages and interdigitating cells are briefly discussed. Emphasis is given to functional aspects of histologic/ cytologic features.

Heterogeneity of epithelial cells in the rat thymus

The morphological heterogeneity of the thymic epithelium has been well documented both at the light and electron microscopic level. Immunohistochemistry has revealed four broad classes of epithelial cells (EC): subcapsule/perivascular, cortical, medullary EC, and medullary Hassall's corpuscles. Ultrastructural analysis has revealed further heterogeneity. In the cortex, four EC subtypes have been described ultrastructurally: subcapsular/perivascular, "pale," "intermediate," and "dark" EC. These subtypes are also present in the medulla. Two additional EC subtypes are restricted to the medulla: an undifferentiated subtype, and a subtype displaying signs of high metabolic activity. Based on the morphological features of the epithelium, it has been hypothetized that the thymic EC subtypes represent a process of differentiation.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

Effect of chronic treatment with piracetam and tacrine on some changes caused by thymectomy in the rat brain

Thymectomized rats, 5 weeks after surgery, showed a significant impairment in learning and memory as shown by deficits in passive avoidance and in the Morris water maze test. The behaviour of the thymectomized rats in the "open field" apparatus was largely unchanged. Following treatment for 20 days with either piracetam (500 mg/kg) or tacrine (3.0 mg/kg), the deficit in passive avoidance learning was largely reversed. Chronic treatment with tacrine also reversed the deficit in the behaviour of the thymectomized rats in the Morris water maze. The effects of thymectomy on the biogenic amines and some of their metabolites in the amygdaloid cortex, hypothalamus, striatum and olfactory bulbs were also determined. Relative to the sham-operated controls, thymectomy resulted in a reduction in the noradrenaline concentration in the amygdala, hypothalamus, and olfactory bulbs. This effect was reversed by chronic piracetam and tacrine treatments. The concentration of dopamine was also reduced in the olfactory bulbs after thymectomy whereas in the striatum the concentration of 5-hydroxytryptamine (5-HT; serotonin) was increased. The concentration of gamma amino butyric acid (GABA) was determined in amygdaloid cortex and hippocampus only. The only significant change occurred following chronic treatment of thymectomized rats with tacrine, when a significant elevation of GABA was found. Neither piracetam nor tacrine produced any change in the amines of their metabolites in the sham-operated controls. Tacrine, however, elevated the dopamine and reduced the 5-HT content of the hypothalamus and increased the 3,4-dihydroxylphenylacetic acid concentration of the striatum of thymectomized rats. Examination of the differential white blood cell count of the thymectomized rats showed that the percentage of lymphocytes was decreased, and the percentage of neutrophils increased, relative to the sham-operated controls. Chronic lacrine, but not piracetam, treatment reversed the lesion-induced changes.

Involution of rat thymus: characterization of cytoplasmic glucocorticoid receptors, evidence of glucocorticoid resistant dexamethasone receptor-positive cells

In rats, thymic relative weight increased after birth reaching maximum values between days 15-30 and then decreased markedly in a similar way in both sexes, while the organ's absolute weight continued to increase until days 80-90 and declined slowly with apparent sex differences from day 30 onward. Scatchard analysis revealed that the [3H] dexamethasone (Dexa) receptor sites concentration showed a pattern comparable to that found in relative thymic weight, with no change in the apparent KD. The reduction of lymphocytes mitotic activity resulting in reduction of immature thymocytes production must be accompanied by a fall of the number of glycocorticoid receptors in ageing thymuses. Despite the profound decrease in the glucocorticoid receptor sites levels, the thymus sensitivity to Dexa remained unchanged during development. Indeed, in prepuberal and adult rats, the steroid administration was followed 4 days after by a transient thymic weight loss of about 70-80% which was mainly linked to the reduction in the cortical area. In contrast, the density of [3H] Dexa binding sites was reduced unexpectedly by 25% only after steroid treatment. These findings provided evidence that Dexa receptor-positive population cells in thymus was formed in a large part by relatively glucocorticoid resistant cells.

Histopathology of the immune system as a tool to assess immunotoxicity

Immunotoxicology studies the undesired effects of interactions between xenobiotics and the immune system, mainly in toxicity experiments in rodents. The histopathology of the lymphoid organs is a cornerstone in such studies. In this review we describe practical aspects of sampling lymphoid organs and subsequent tissue processing and application of conventional and advanced histologic techniques. Thereafter, some aspects of proper reading and interpretation of histopathology is discussed, in relation to modifying factors such as age, sex, strain of animals, housing conditions, and nutritional status. These factors can substantially confound the outcome and interpretation of experiments, due to the highly dynamic characteristics of the immune system. Immunotoxicity tests are normally performed in a tiered approach. We describe the screening tier in the rat species that has been developed in the National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands, and illustrate the value of histopathology by an example of immunotoxicity testing of pesticides. Subsequently, the tiered approach in the mouse species followed by the National Toxicology Program in the USA, is described. In the evaluation of chemicals with suspected immunotoxic potential using this approach, histopathology proved to be less sensitive in 'flagging' immunotoxicity. This may be related to the lower doses that are applied in this toxicity design, because at higher doses histopathology is a sensitive indicator of toxicity. A global description of pathologic alterations after toxic insult is given, followed by representative examples taken from immunosuppressive drugs--the cytostatic agent 5-fluorouracil, and drugs interfering with cytokine expression, namely, Cyclosporin A, FK-506, and Rapamycin.