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

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

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

Cytokine modulation, oxidative stress and thymic dysfunctions: Role of age-related changes in the experimental Trypanosoma cruzi infection: Age-related thymic dysfunctions and Trypanosoma cruzi infection

Aging is linked with a thymic oxidative damage and some infectious diseases such as Chagas' disease may aggravate this process. The aim of this study was to evaluate the production of distinct cytokines as well as the antioxidant/oxidant status of the thymus and thymocytes populations during Trypanosoma cruzi (T. cruzi) infection. Young (5 weeks old) and aged (18 weeks old) male Wistar rats were inoculated with blood trypomastigotes forms of the Y strain of T. cruzi. On the 16th day after T. cruzi infection, increased concentrations of transforming growth factor β (TGF-β), interleukin (IL)-12, IL-17 were detected in aged infected subjects as compared to young infected ones. Interestingly, a reduction in the production of tumor necrose factor (TNF)-α was observed in aged infected rats when compared to young infected subjects. Aged-infected rats presented increased O₂^- levels, compared to young counterparts. Significant raise in the generation of O₂^- in aged infected animals, as compared to uninfected counterparts was observed. Up-regulated expression of Nox2 in the thymus of young and aged infected animals was observed. An increased SOD2 expression was detected in the thymus of young animals infected with T. cruzi, when compared to uninfected young rats. Aged animals showed reduced thymus weight and the number of thymocytes. Decreased percentages of SPCD4⁺ and SPCD8⁺T cells were detected in aged and control groups when compared to young counterparts. In summary, this is the first data to directly examine the influence of aging on age-related dysfunctions during the acute phase of experimental Chagas disease. Concerning to oxidative stress, it is clear from our analysis that aged infected rats suffer a more intense oxidative damage when compared to young and infected ones. Age and infection triggered a dynamic interplay of cytokines, oxidative stress and thymic dysfunctions which led to impaired response from aged and infected rats. Such findings may have significant functional relevance in therapeutic strategies in order to reestablish the thymic immunological function which occurs in aged and T. cruzi infected subjects.

Thymic homing of activated CD4+ T cells induces degeneration of the thymic epithelium through excessive RANK signaling

Activated T cells have been shown to be able to recirculate into the thymus from the periphery. The present study was aimed to elucidate the functional consequences of thymic homing of activated T cells upon developing thymocytes and thymic epithelial cells (TEC). In the presence of activated T cells, especially CD4⁺ T cells, T cell development was found to be inhibited in thymic organ cultures with markedly reduced cellularity. Thymic transplantation demonstrated that the inhibitory effect was most likely due to a defective microenvironment. As the major component of the thymic stroma, the TEC compartment was severely disturbed after prolonged exposure to the activated T cells. In addition to reduced cell proliferation, TEC differentiation was heavily skewed to the mTEC lineage. Furthermore, we demonstrated that RANKL highly expressed by activated CD4⁺ T cells was primarily responsible for the detrimental effects. Presumably, excessive RANK signaling drove overproduction of mTECs and possibly exhaustion of epithelial progenitors, thereby facilitating the deterioration of the epithelial structures. These findings not only reveal a novel activity of activated T cells re-entering the thymus, but also provide a new perspective for understanding the mechanism underlying thymic involution.

The effect of age on thymic function

Age-related regression of the thymus is associated with a decline in naïve T cell output. This is thought to contribute to the reduction in T cell diversity seen in older individuals and linked with increased susceptibility to infection, autoimmune disease, and cancer. Thymic involution is one of the most dramatic and ubiquitous changes seen in the aging immune system, but the mechanisms which underlying this process are poorly understood. However, a picture is emerging, implicating the involvement of both extrinsic and intrinsic factors. In this review we assess the role of the thymic microenvironment as a potential target that regulates thymic involution, question whether thymocyte development in the aged thymus is functionally impaired, and explore the kinetics of thymic involution.

T cells affect thymic involution during puberty by inducing regression of the adrenal reticularis

The thymus involutes after puberty, although the mechanism by which this process occurs remains poorly understood. The profile of thymic involution, which is inversely correlated with an increase in peripheral T cells, may indicate that the accumulation of T cells in the periphery is related to thymic atrophy. In this study, it was shown that the prevention of T cell generation delayed the initiation of thymic involution. The activation of T cells increased the serum concentration of glucocorticoid (GC) and thymic involution, which was completely prevented by adrenalectomy. In the adrenals of growing mice, the activity of the zona fasciculata, which produces GC, increased and plateaued by the weaning period; however, the zona reticularis (ZR), which produces dehydroepiandrosterone (DHEA) that has anti-GC actions, started to decline just before puberty. Thymic atrophy was preceded by the infiltration of activated T cells into the ZR and by the loss of ZR cells. Thus, T cells are involved in thymic involution, a process which was retarded by DHEA administration, through an increase in GC activity due to ZR cell-killing.

Is thymocyte development functional in the aged?

T cells are an integral part of a functional immune system with the majority being produced in the thymus. Of all the changes related to immunosenescence, regression of the thymus is considered one of the most universally recognised alterations. Despite the reduction of thymic size, there is evidence to suggest that T cell output is still present into old age, albeit much diminished; leading to the assumption that thymocyte development is normal. However, current data suggests that recent thymic emigrant from the aged thymus are functionally less responsive, giving rise to the possibility that the generation of naïve T cell may be intrinsically impaired in the elderly. In light of these findings we discuss the evidence that suggest aged T cells may be flawed even before exiting to the periphery and could contribute to the age-associated decline in immune function.

Long-term beta-adrenergic receptor blockade increases levels of the most mature thymocyte subsets in aged rats

Age-related increase in the density of thymic noradrenergic fibres and noradrenaline (NA) concentration is proposed to be associated with thymic involution and altered thymopoiesis. To test this hypothesis thymocyte differentiation/maturation and thymic structure were studied in 18-month-old male Wistar rats subjected to 14-day-long propranolol (P) blockade of beta-adrenoceptors (beta-ARs). The treatment primarily resulted in changes in the T-cell receptor (TCR)-dependent stages of thymopoiesis, which led to an increase in both the relative and absolute numbers of the most mature single positive (SP) CD4(+)CD8(-) (including cells with the CD4(+)CD25(+) regulatory phenotype) and CD4(-)CD8(+) TCRalphabeta(high) thymocytes. Accordingly, in the thymi of these rats an increase in both numerical density and absolute number of medullary thymocytes encompassing mainly the most mature SP cells was found. These findings, together with an increase in the thymocyte surface expression of the regulatory molecule Thy-1 (CD90) (implicated in negative regulation of TCRalphabeta-dependent thymocyte selection thresholds) in the same rats, may suggest increased positive/reduced negative thymocyte selection. Collectively, the results indicate that a decline in thymic efficiency in generating both conventional and regulatory T cells, and consequently in immune function, in aged rats may be, at least partly, attenuated by long-term blockade of beta-ARs with P.

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.

Age-associated changes in CD90 expression on thymocytes and in TCR-dependent stages of thymocyte maturation in male rats

To elucidate the effects of ageing on T-cell-maturation, in 3- and 18-month-old rats, we analysed the expression of: (i) CD4/CD8/TCRalphabeta and (ii) Thy-1, which is supposed to be a regulator of TCRalphabeta signalling, and thereby the thymocyte selection thresholds. Since an essential role for TCRalphabeta signalling in the development of CD4+25+T(reg)-cells was suggested, the frequency of these cells was also quantified. We demonstrated that, as for mice, early thymocyte differentiational steps within the CD4-8- double negative (DN) developmental stage are age-sensitive. Furthermore, we revealed that TCRalphabeta-dependent stages of T-cell development are affected by ageing, most likely due to an impaired expression of Thy-1 on TCRalphabeta(low) thymocytes entering selection processes. The diminished frequency of the post-selection CD4+8+ double positive (DP) cells in aged rats, together with an overrepresentation of mature single positive (SP) cells, most probably suggests more efficient differentiational transition from the DP TCRalphabeta(high) to the SP TCRalphabeta(high) developmental stage, which is followed by an increase in pre-migration proliferation of the mature SP cells. Moreover, the study indicated impaired intrathymic generation of CD4+25+T(reg)-cells in aged rats, thus providing a possible explanation for the increased frequency of autoimmune diseases in ageing.

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.

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.

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.

Mutant and genetically modified mice as models for studying the relationship between aging and carcinogenesis

Increased interest is emerging in using mouse models to assess the genetics of aging and age-related diseases, including cancer. However, only limited information is available regarding the relationship between aging and spontaneous tumor development in genetically modified mice. Analysis of various transgenic and knockout rodent models with either a shortened or an extended life span, provides a unique opportunity to evaluate interactions of genes involved in the aging process and carcinogenesis. There are only a few models which show life span extension. Ames dwarf mutant mice, p66(-/-) knockout mice, alpha MUPA and MGMT transgenic mice live longer than wild-type strains. The incidence of spontaneous tumors in these mutant mice was usually similar to those in controls, whereas the latent period of tumor development was increased. Practically all models of accelerated aging showed increased incidence and shorter latency of tumors. This phenomenon has been observed in animals which display a phenotype that more closely resembles natural aging, and in animals which manifest only some features of the normal aging process. These observations are in agreement with an earlier established positive correlation between tumor incidence and the rate of tumor incidence increase associated with aging and the aging rate in a population. Thus, genetically modified animals are a valuable tool in unravelling mechanisms underlying aging and cancer. Systemic evaluation of newly generated models should include onco-gerontological studies.