Reversal of thymic atrophy

Studies on the immune status of calves with chronic inflammation and thymus atrophy

The thymus is a primary lymphoid organ where the primary T cell repertoire is generated. Thymus atrophy is induced by various conditions, including infectious diseases, glucocorticoid treatment, and poor breeding management. Cattle with thymus atrophy tend to exhibit weak calf syndrome, a condition in which approximately half of neonates die shortly after birth. Calves with thymus atrophy that survive the first month typically contract chronic inflammatory diseases. In this study, we analyzed the populations of the peripheral blood mononuclear cells and thymocytes in calves with thymus atrophy. In addition, we evaluated polarization of master gene and cytokine mRNA expression in peripheral blood CD4⁺ cells in the calves. The population of CD4⁺CD8⁺ cells in thymus of the calves with thymus atrophy was lower than that of control calves. IL10 mRNA expression in peripheral blood CD4⁺ cells of calves with thymus atrophy was significantly lower than that of control calves. TBX21 mRNA expression in peripheral CD4⁺ cells of thymus atrophy calves was tended to be higher than that of the control group. In addition, FOXP3 mRNA expression in peripheral CD4⁺ cells of the thymus atrophy calves was tended to be lower than that of the control calves. Thymus atrophy calves exhibited chronic inflammatory disease leading, in severe situations, to conditions such as pneumonia with caseous necrosis. These severe inflammatory responses likely are due to decreases in IL10 mRNA expression, impairing control of macrophages, one of the main cell fractions of natural immunity.

Evaluation of bovine thymic function by measurement of signal joint T-cell receptor excision circles

A signal joint T-cell receptor excision circle (sjTREC) is a circular DNA produced by T-cell receptor α gene rearrangement in the thymus. Measurements of sjTREC values have been used to evaluate thymic function. We recently established a quantitative PCR (QPCR) assay of bovine sjTREC. In the present study, we used this QPCR assay to measure the sjTREC value in bovine peripheral blood mononuclear cells and we then evaluated the relationships between sjTREC values and peripheral blood T-cell number, growth stage, gender, and meteorological season. The sjTREC value was highest at the neonatal stage, and its value subsequently decreased with age. On the other hand, the peripheral T-cell number increased with age. The sjTREC value in calves up to 50-days old was significantly higher for males than for females, suggesting that thymic function might differ by gender. In addition, the sjTREC value and the peripheral T-cell number were significantly higher in calves in the summer season than in calves in the winter season. These data suggest that bovine thymic function is highly variable and varies according to the growth stage, gender, and environmental factors such as air temperature or the UV index.

A molecular mucosal adjuvant to enhance immunity against pneumococcal infection in the elderly

Streptococcus pneumoniae (the pneumococcus) causes a major upper respiratory tract infection often leading to severe illness and death in the elderly. Thus, it is important to induce safe and effective mucosal immunity against this pathogen in order to prevent pnuemocaccal infection. However, this is a very difficult task to elicit protective mucosal IgA antibody responses in older individuals. A combind nasal adjuvant consisting of a plasmid encoding the Flt3 ligand cDNA (pFL) and CpG oligonucleotide (CpG ODN) successfully enhanced S. pneumoniae-specific mucosal immunity in aged mice. In particular, a pneumococcal surface protein A-based nasal vaccine given with pFL and CpG ODN induced complete protection from S. pneumoniae infection. These results show that nasal delivery of a combined DNA adjuvant offers an attractive potential for protection against the pneumococcus in the elderly.

Insights into thymic involution in tumor-bearing mice

The thymus is a central lymphoid organ critical for the development and maintenance of an effective peripheral T-cell repertoire. Most important, it provides a specialized environment for the selection of rearranged clones that will function appropriately in the adaptive immune response. Thymic involution has been observed in several model systems; including graft-versus-host disease, aging, viral infection, and tumor development, however, the precise mechanisms involved in this phenomenon remain poorly defined. Here, we review some of our results related to the studies of the cell-mediated immunity in a mammary tumor model; more specifically, those related to the tumor-induced impaired T-cell development and thymic involution. Collectively, the understanding of the mechanisms and pathways associated with the tumor-induced thymic involution is essential for the development of innovative and safe therapies to fight against the immune suppression caused by the tumor development.

Immunosenescence and immune response in organ transplantation

The immune system undergoes a complex and continuous remodeling with aging. Immunosenescence results into both quantitative and qualitative changes of specific cellular subpopulations that have major impact on allorecognition and alloresponse, and consequently on graft rejection and tolerance. Here, we are going to review the immunological changes associated with the aging process relevant for transplantation. Interventions to selectively target changes associated with the senescence process seem promising therapeutic strategies to improve transplantation outcome.

The immune system in the aging human

With the improvement of medical care and hygienic conditions, there has been a tremendous increment in human lifespan. However, many of the elderly (>65 years) display chronic illnesses, and a majority requires frequent and longer hospitalization. The robustness of the immune system to eliminate or control infections is often eroded with advancing age. Nevertheless, some elderly individuals do cope better than others. The origin of these inter-individual differences may come from genetic, lifestyle conditions (nutrition, socio-economic parameters), as well as the type, number and recurrence of pathogens encountered during life. The theory we are supporting is that chronic infections, through life, will induce profound changes in the immune system probably due to unbalanced inflammatory profiles. Persistent viruses such a cytomegalovirus are not eliminated and are a driven force to immune exhaustion. Because of their age, elderly individuals may have seen more of these chronic stimulators and have experienced more reactivation episodes ultimately leading to shrinkage of their repertoire and overall immune robustness. This review integrates updates on immunity with advancing age and its impact on associated clinical conditions.

Thymectomy in early childhood: a model for premature T cell immunosenescence?

The thymus is the main source of recent thymic emigrants (RTE) and naïve T cells. The aging of the immune system (immunosenescence) is characterized by loss of thymic function, decreased numbers of RTE, peripheral proliferation of mature T cells, and oligoclonal expansions of specific T cell subpopulations. As shown in several studies, thymectomized patients demonstrate signs of premature immunosenescence reminiscent of aged people, such as decreased proportions of naïve T cells and RTE, a compensatory increase of mature T cell subpopulations with increased proliferation rates, restriction of the T cell receptor repertoire, and a delayed response to new antigens and vaccinations. This review demonstrates that, despite some limitations, childhood thymectomy may serve as an useful model for premature immunosenescence, mimicking changes expected after physiological thymus involution in the elderly. Thus, it may prove an insightful tool for obtaining better understanding of human naïve T cell development, thymic function, and maintenance of the naïve T cell pool.

Reactive oxygen intermediate-induced pathomechanisms contribute to immunosenescence, chronic inflammation and autoimmunity

Deregulation of reactive oxygen intermediates (ROI) resulting in either too high or too low concentrations are commonly recognized to be at least in part responsible for many changes associated with aging. This article reviews ROI-dependent mechanisms critically contributing to the decline of immune function during physiologic - or premature - aging. While ROI serve important effector functions in cellular metabolism, signalling and host defence, their fine-tuned generation declines over time, and ROI-mediated damage to several cellular components and/or signalling deviations become increasingly prevalent. Although distinct ROI-associated pathomechanisms contribute to immunosenescence of the innate and adaptive immune system, mutual amplification of dysfunctions may often result in hyporesponsiveness and immunodeficiency, or in chronic inflammation with hyperresponsiveness/deregulation, or both. In this context, we point out how imbalanced ROI contribute ambiguously to driving immunosenescence, chronic inflammation and autoimmunity. Although ROI may offer a distinct potential for therapeutic targeting along with the charming opportunity to rescue from deleterious processes of aging and chronic inflammatory diseases, such modifications, owing to the complexity of metabolic interactions, may carry a marked risk of unforeseen side effects.

Ex vivo expansion of memory CD8 T cells from lymph nodes or spleen through in vitro culture with interleukin-7

Interleukin-7 (IL-7) increases lymphocyte numbers, a critical feature of immune reconstitution, through mechanisms that are still poorly understood. Part of the problem is that IL-7 is produced in limited amounts by non-lymphoid cells, making in vivo studies of the cytokine's activity a challenge. To overcome this, we developed an in vitro system by which lymphocytes from secondary immune organs could be cultured to produce IL-7 responsive cells. Using this method, we showed that CD8(hi)CD44(hi) T cells accumulate in culture with IL-7 from a population of lymph node or splenic cells. These results were validated when a similar lymphocyte subset was found in mice expressing a constitutively active form of STAT5b, a key transducer of IL-7 signals. Interestingly, IL-7-expanded cells also up regulated the activation marker, CD69. The IL-7-derived CD44(hi)CD69(hi) cells were not generated from naïve cells, but expanded from an existing population, since culture in IL-7 of naïve lymphocytes from OT-1/Rag1(-/-) mice did not produce CD44(hi)CD69(hi) cells. Using the in vitro culture system to study lymphocytes from mice deficient in the apoptotic protein, BIM, we were able to attribute the expansion of CD8(hi)CD44(hi)CD69(hi) T cells to the proliferative and not survival activity of IL-7. The in vitro culture system provides an important new methodology to examine the activities of this essential as well as immunotherapeutic cytokine.

The role of stress factors during aging of the immune system

This manuscript reviews current evidence suggesting that aging of the immune system (immunosenescence) may be closely related to chronic stress and stress factors. Healthy aging has been associated with emotional distress in parallel to increased cortisol to dehydroepiandrosterone (DHEA) ratio. The impaired DHEA secretion together with the increase of cortisol results in an enhanced exposure of lymphoid cells to deleterious glucocorticoid actions. The lack of appropriated growth hormone signaling during immunosenescence is also discussed. It follows that altered neuroendocrine functions could be underlying several immunosenescence features. Indeed, changes in both innate and adaptive immune responses during aging are also similarly reported during chronic glucocorticoid exposure. In addition, chronically stressed elderly subjects may be particularly at risk of stress-related pathology because of further alterations in both neuroendocrine and immune systems. The accelerated senescent features induced by chronic stress include higher oxidative stress, reduced telomere length, chronic glucocorticoid exposure, thymic involution, changes in cellular trafficking, reduced cell-mediated immunity, steroid resistance, and chronic low-grade inflammation. These senescent features are related to increased morbidity and mortality among chronically stressed elderly people. Overall, these data suggest that chronic stress leads to premature aging of key allostatic systems involved in the adaptation of the organisms to environmental changes. Stress management and psychosocial support may thus promote a better quality of life for elderly people and at the same time reduce hospitalization costs.

Exclusive expression of proteasome subunit {beta}5t in the human thymic cortex

The ubiquitin-proteasome pathway, which degrades intracellular proteins, is involved in numerous cellular processes, including the supply of immunocompetent peptides to the antigen presenting machinery. Proteolysis by proteasomes is conducted by three beta subunits, beta1, beta2, and beta5, of the 20S proteasome. Recently, a novel beta subunit expressed exclusively in cortical thymic epithelial cells was discovered in mice. This subunit, designated beta5t, is a component of the thymoproteasome, a specialized type of proteasomes implicated in thymic positive selection. In this study, we show that, like its mouse counterpart, human beta5t is expressed exclusively in the thymic cortex. Human beta5t was expressed in approximately 80% of cortical thymic epithelial cells and some cortical dendritic cells. Human beta5t was incorporated into proteasomes with two other catalytically active beta subunits beta1i and beta2i, forming 20S proteasomes with subunit compositions characteristic of thymoproteasomes. The present study demonstrates, for the first time, the existence of thymoproteasomes in the human thymic cortex, indicating that thymoproteasome function is likely conserved between humans and mice.

Restoration of Thymic Homeostasis in a Tumor-Bearing Host by in Vivo Administration of Medicinal Herb Tinospora Cordifolia

In vivo administration of alcoholic extract of medicinal plant Tinospora cordifolia (TC) to mice bearing a spontaneous T cell lymphoma designated as Dalton's lymphoma prevented tumor growth-dependent regression of thymus. TC was found to augment proliferation of thymocytes with a concomitant decrease in thymocyte apoptosis. It also resulted in a decrease in the number of Hassal's corpuscles. Restoration of thymus homeostasis was caused by TC-dependent augmentation in production of thymocyte growth promoting cytokines Interleukin-2 and Interferon-gamma from thymocytes. TC was found to downregulate thymocyte apoptosis by modulation of Caspase pathway. TC administration retarded tumor growth and prolonged survival of tumor-bearing mice. The possible mechanisms are discussed.

Histochemical and molecular overview of the thymus as site for T-cells development

The thymus represents the primary site for T cell lymphopoiesis, providing a coordinated set for critical factors to induce and support lineage commitment, differentiation and survival of thymus-seeding cells. One irrefutable fact is that the presence of non-lymphoid cells through the thymic parenchyma serves to provide coordinated migration and differentiation of T lymphocytes. Moreover, the link between foetal development and normal anatomy has been stressed in this review. Regarding thymic embryology, its epithelium is derived from the embryonic endodermal layer, with possible contributions from the ectoderm. A series of differentiating steps is essential, each of which must be completed in order to provide the optimum environment for thymic development and function. The second part of this article is focused on thymic T-cell development and differentiation, which is a stepwise process, mediated by a variety of stromal cells in different regions of the organ. It depends strongly on the thymic microenvironment, a cellular network formed by epithelial cells, macrophages, dendritic cells and fibroblasts, that provide the combination of cellular interactions, cytokines and chemokines to induce thymocyte precursors for the generation of functional T cells. The mediators of this process are not well defined but it has been demonstrated that some interactions are under neuroendocrine control. Moreover, some studies pointed out that reciprocal signals from developing T cells also are essential for establishment and maintenance of the thymic microenvironment. Finally, we have also highlighted the heterogeneity of the lymphoid, non-lymphoid components and the multi-phasic steps of thymic differentiation. In conclusion, this review contributes to an understanding of the complex mechanisms in which the foetal and postnatal thymus is involved. This could be a prerequisite for developing new therapies specifically aimed to overcome immunological defects, linked or not-linked to aging.

Long-term effects of perinatal nutrition on T lymphocyte kinetics in young Gambian men

BACKGROUND

Nutritional status is highly dependent on season in countries such as The Gambia. In a rural Gambian setting, individuals born during periods of seasonal nutritional deprivation ("hungry seasons") are susceptible to mortality from infectious diseases in adult life.

OBJECTIVE

We investigated the hypothesis that impaired immunocompetence in those born in the hungry season results from an underlying defect in immunologic memory, similar to the immunosenescence of old age, which is likely to be reflected in the phenotype and kinetics of T lymphocytes in young adults.

DESIGN

T cell phenotype in terms of CD3, CD4, CD8, CD45RA, and CD45R0 expression and in vivo dynamics measured by stable isotope labeling of T cell subsets combined with gas chromatography-mass spectrometry and frequency of T cell receptor excision circles were measured in 25 young (18-24-y-old) Gambian men. Thirteen of these 25 men were exposed to perinatal malnutrition as defined by birth season and birth weight.

RESULTS

In persons born in the hungry season with low birth weight, no differences in the proportions of memory or naive T cells were found. Kinetic analysis showed higher proliferation rates in memory (CD45R0(+)) subsets of T cells than in naïve (CD45R0(-)) cells, which is consistent with previous studies, but no evidence was found for an effect of birth weight or season on T lymphocyte proliferation and disappearance rates. No significant correlations were found between in vivo T cell kinetics and frequency of T cell receptor excision circles. Only absolute numbers of granulocytes were elevated in those born in the nutritionally deprived season.

CONCLUSION

In healthy young Gambian men, T lymphocyte homeostasis is extremely robust regardless of perinatal nutritional compromise.

Recent thymic output function in patients with hematological malignancy

Thymic function is important for the generation of T-cell diversity in the periphery of both children and adults during both health and disease. Until recently, thymic function could not be monitored, as a consequence of the absence of adequate technology to differentiate recent thymic emigrants (RTEs) from naïve T cells. The generation of TCR diversity occurs in the thymus through recombination of gene segments encoding the variable parts of the TCR alpha and beta chains. During these processes, by-products of the rearrangements are generated in the form of signal joint T-cell receptor excision circles (sjTRECs). As sjTRECs are stable extrachromosomal DNA fragments, they are not replicated during mitosis and thus diluted with each round of cell division, and are therefore most frequent in naïve T cells that have recently left the thymus, their quantification is actually considered as a valuable tool to estimate thymic function. Therefore, quantitative sjTRECs content have been recently used to assess thymic output during both health and disease. In this review, we summarize recent data on the recent thymic output function feature in patients with hematological malignancy and the immune reconstitution after stem cell transplantation and we also characterize factors that may improve the thymic output function.

Aging of the immune system: a risk factor for autoimmunity?

Aging of the immune system, or immunosenescence, is characterized by changes in T cell subsets, cellular and molecular level alterations and thymic atrophy, resulting in a decline of T and B cell function. These alterations have been shown to be accompanied by a loss of ability to recognize "self" and "foreign" antigens. Therefore the development of autoimmune responses like production of autoantibodies has been hypothesized to be secondary to thymus involution with a decline of naïve T cells and accumulation of clonal T cells with activation due to "neoantigens" during the aging process. Altered apoptosis and altered T cell homeostasis have been emphasized to promote a chronic inflammatory state and lead to the concept of a immune-risk phenotype. However, it has to be proven which kinds of mechanisms turn the immune system to manifest autoimmune disease and how speculated defects in T cell differentiation and interaction leading to premature aging of the immune system may contribute to the development of autoimmune diseases.

T cell development and receptor diversity during aging

The T cell system is a complex and highly dynamic system that is amazingly robust over many decades of human life. Its functional competence is determined not only by its size but also by its diversity. Homeostatic control mechanisms have to secure sufficient T cell replenishment while preventing loss of clonal diversity. Major homeostatic challenges include profound expansion and shrinkage of T cell clonotypes upon antigenic triggering and, more importantly, age-related changes in T cell regeneration. The ability of the thymus to rebuild a diverse repertoire ceases in the fifth decade of life. Emerging data suggest that the end of the 7th decade of life defines a critical time period when T cell homeostasis is no longer guaranteed and diversity of the naïve T-cell repertoire collapses. Thus, failure of T cell homeostasis appears to result from cumulative defects of T cell generation. Elucidation of the underlying mechanisms will allow for extending this turning point to later in life; ultimately, interventions have to aim at restoring thymic function and complementary modes of T cell reconstitution.

Stress, glucocorticoids and ageing of the immune system

Ageing has been associated with immunological changes (immunosenescence) that resemble those observed following chronic stress or glucocorticoid (GC) treatment. These changes include thymic involution, lower number of naïve T cells, reduced cell-mediated immunity, and poor vaccination response to new antigens. It follows that immunosenescence could be associated with changes of peripheral GC levels. Indeed, when compared with young subjects, healthy elders are more stressed and show activation of the hypothalamus-pituitary-adrenal (HPA) axis. However, both beneficial and undesirable effects of GCs ultimately depend on the target tissue sensitivity to these steroids. Recent data indicate that peripheral lymphocytes from elders respond poorly to GC treatment in vitro. The present review summarizes recent findings which suggest that immunosenescence may be closely related to both psychological distress and stress hormones. Furthermore, chronically stressed elderly subjects may be particularly at risk of stress-related pathology because of further alterations in GC-immune signalling. Finally, the neuroendocrine hypothesis of immunosenescence is finally reconsidered in which the age-related increase in the cortisol/DHEA ratio is major determinant of immunological changes observed during ageing.

Thermodynamics and Information in Aging: Why Aging Is Not a Mystery and How We Will Be Able to Make Rational Interventions

Currently, the aging research field lacks consensus in its focus and methodology. Foundational principles, such as the evolutionary origins and physiological definition of aging, remain controversial. The aim of this paper is to resolve these issues. By applying the concepts of thermodynamics and information in an evolutionary context, the aging phenotype can be derived from first principles. Life uses information storage to maintain its distance from thermodynamic equilibrium. Since it is impossible to make any process 100% efficient, a selective force (i.e., natural selection) is needed to maintain the information's viability. Natural selection operates upon generations, and for reasons discussed subsequently, the somatic body cannot implement an analogous selective process. The aging phenotype we see can be derived from this model along with a number of insights that will enhance our ability to make intelligent and rational interventions.

Will telomere erosion lead to a loss of T-cell memory?

Evidence is accumulating that elderly individuals are more susceptible to infection with organisms to which they were previously immune. This indicates that there might be a limit to the persistence of immune memory. This fact is particularly disturbing because the average life expectancy of humans has almost doubled in the past 200 years and is still increasing. We discuss mechanisms that might constrain the persistence of memory T cells and consider whether humans will suffer from memory T-cell exhaustion as life expectancy increases.