Age-related change in the potential of bone marrow cells to repopulate the thymus and splenic T cells in mice

Considerations and Approaches for Cancer Immunotherapy in the Aging Host

Advances in cancer immunotherapy are improving treatment successes in many distinct cancer types. Nonetheless, most tumors fail to respond. Age is the biggest risk for most cancers, and the median population age is rising worldwide. Advancing age is associated with manifold alterations in immune cell types, abundance, and functions, rather than simple declines in these metrics, the consequences of which remain incompletely defined. Our understanding of the effects of host age on immunotherapy mechanisms, efficacy, and adverse events remains incomplete. A deeper understanding of age effects in all these areas is required. Most cancer immunotherapy preclinical studies examine young subjects and fail to assess age contributions, a remarkable deficit given the known importance of age effects on immune cells and factors mediating cancer immune surveillance and immunotherapy efficacy. Notably, some cancer immunotherapies are more effective in aged versus young hosts, while others fail despite efficacy in the young. Here, we review our current understanding of age effects on immunity and associated nonimmune cells, the tumor microenvironment, cancer immunotherapy, and related adverse effects. We highlight important knowledge gaps and suggest areas for deeper enquiries, including in cancer immune surveillance, treatment response, adverse event outcomes, and their mitigation.

Dynamics of thymus function and T cell receptor repertoire breadth in health and disease

T cell recognition of unknown antigens relies on the tremendous diversity of the T cell receptor (TCR) repertoire; generation of which can only occur in the thymus. TCR repertoire breadth is thus critical for not only coordinating the adaptive response against pathogens but also for mounting a response against malignancies. However, thymic function is exquisitely sensitive to negative stimuli, which can come in the form of acute insult, such as that caused by stress, infection, or common cancer therapies; or chronic damage such as the progressive decline in thymic function with age. Whether it be prolonged T cell deficiency after hematopoietic cell transplantation (HCT) or constriction in the breadth of the peripheral TCR repertoire with age; these insults result in poor adaptive immune responses. In this review, we will discuss the importance of thymic function for generation of the TCR repertoire and how acute and chronic thymic damage influences immune health. We will also discuss methods that are used to measure thymic function in patients and strategies that have been developed to boost thymic function.

Gene profiling reveals association between altered Wnt signaling and loss of T-cell potential with age in human hematopoietic stem cells

Functional decline of the hematopoietic system occurs during aging and contributes to clinical consequences, including reduced competence of adaptive immunity and increased incidence of myeloid diseases. This has been linked to aging of the hematopoietic stem cell (HSC) compartment and has implications for clinical hematopoietic cell transplantation as prolonged periods of T-cell deficiency follow transplantation of adult mobilized peripheral blood (PB), the primary transplant source. Here, we examined the gene expression profiles of young and aged HSCs from human cord blood and adult mobilized PB, respectively, and found that Wnt signaling genes are differentially expressed between young and aged human HSCs, with less activation of Wnt signaling in aged HSCs. Utilizing the OP9-DL1 in vitro co-culture system to promote T-cell development under stable Notch signaling conditions, we found that Wnt signaling activity is important for T-lineage differentiation. Examination of Wnt signaling components and target gene activation in young and aged human HSCs during T-lineage differentiation revealed an association between reduced Wnt signal transduction, increasing age, and impaired or delayed T-cell differentiation. This defect in Wnt signal activation of aged HSCs appeared to occur in the early T-progenitor cell subset derived during in vitro T-lineage differentiation. Our results reveal that reduced Wnt signaling activity may play a role in the age-related intrinsic defects of aged HSCs and early hematopoietic progenitors and suggest that manipulation of this pathway could contribute to the end goal of improving T-cell generation and immune reconstitution following clinical transplantation.

Stem cell aging: survival of the laziest?

The question whether stem cells age remains an enigma. Traditionally, aging was thought to change the properties of hematopoietic stem cells (HSC). We discuss here a new model of stem cell aging that challenges this view. It is now well-established that the HSC compartment is heterogeneous, consisting of epigenetically fixed subpopulations of HSC that differ in self-renewal and differentiation capacity. New data show that the representation of these HSC subsets changes during aging. HSC that generate lymphocyte-rich progeny are depleted, while myeloid-biased HSC are enriched in the aged HSC compartment. Myeloid-biased HSC, even when isolated from young donors, have most of the characteristics that had been attributed to aged HSC. Thus, the distinct behavior of the HSC isolated from aged hosts is due to the accumulation of myeloid-biased HSC. By extension this means that the properties of individual HSC are not substantially changed during the lifespan of the organism and that aged hosts do not contain many aged HSC. Myeloid-biased HSC give rise to mature cells slowly but contribute for a long time to peripheral hematopoiesis. We propose that such slow, "lazy" HSC are less likely to be transformed and therefore may safely sustain hematopoiesis for a long time.

Stem cell myths

Stem cells, although difficult to define, hold great promise as tools for understanding development and as therapeutic agents. However, as with any new field, uncritical enthusiasm can outstrip reality. In this review, we have listed nine common myths that we believe affect our approach to evaluating stem cells for therapy. We suggest that careful consideration needs to be given to each of these issues when evaluating a particular cell for its use in therapy. Data need to be collected and reported for failed as well as successful experiments and a rigorous scientific approach taken to evaluate the undeniable promise of stem cell biology.

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

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

Thymic involution: implications for self-tolerance

The thymus contributes to the regulation of tolerance and the prevention of autoimmunity at many levels. First, auto-reactive CD4+ and CD8+ T cells are clonally deleted during negative selection in the thymus, establishing central tolerance. The unique expression of the AIRE (autoimmune regulator) gene in medullary thymic epithelial cells results in expression of a broad array of tissue-specific antigens. Thymocytes bearing T-cell receptors that bind to these tissue-specific antigens are clonally deleted. This process removes self-reactive T cells from the repertoire before T cells are exported to the periphery. Second, CD4+CD25 bright regulatory T cells (Treg) develop in parallel with CD4+ and CD8+ effector T cells in the thymus. Unlike T effector cells, Treg fail to be deleted by exposure to tissue antigens during thymic maturation. After export to the periphery, Treg cells play a critical role in the prevention of autoimmunity, suppression of inflammatory responses, and the modulation of T-cell homeostasis. Finally, productive thymopoiesis, in and of itself, may be a factor deterring autoimmunity, The thymus continuously generates stable, resting populations of naive T cells that maintain the numbers and the diversity of the T-cell repertoire. Under conditions of lymphopenia prolonged by inadequate thymopoiesis, compensatory peripheral expansion of T cells occurs to maintain stable T-cell levels. Under circumstances in which the repertoire is limited, Homeostatic proliferation may increase the opportunity for T-cells reactive with self antigens to expand, leading to autoimmune disorders. In all of these respects, the thymus maintains immunologic tolerance to self. Given the importance of the thymus in control of autoimmunity, the gradual age-dependent decline in thymic cytoarchitecture and thymopoietic productivity may, therefore, contribute to the development of auto-reactivity and loss of self-tolerance.

Sustained thymopoiesis and improvement in functional immunity induced by exogenous KGF administration in murine models of aging

Age-related thymopoietic insufficiency has been proposed to be related to either defects in lymphohematopoietic progenitors or the thymic microenvironment. In this study, we examined whether keratinocyte growth factor (KGF), an epithelial cell-specific growth factor, could increase thymopoietic capacity in aged mice by restoration of the function of thymic epithelial cells (TECs). The thymic cellularity in KGF-treated aged mice increased about 4-fold compared to placebo-treated mice, resulting in an equivalent thymic cellularity to young mice. Enhanced thymopoiesis was maintained for about 2 months after a single course of KGF, and sustained improvement was achieved by administration of monthly courses of KGF. With the enhanced thymopoiesis after KGF treatment, the number of naive CD4 T cells in the periphery and T-cell-dependent antibody production improved in aged mice. KGF induced increased numbers of TECs and intrathymic interleukin-7 (IL-7) production and reorganization of cortical and medullary architecture. Furthermore, KGF enhanced thymopoiesis and normalized TEC organization in klotho (kl/kl) mice, a model of premature degeneration and aging, which displays thymopoietic defects. The result suggests that TEC damage is pathophysiologically important in thymic aging, and KGF therapy may be clinically useful in improving thymopoiesis and immune function in the elderly.

The effects of age, thymectomy, and HIV Infection on alpha and beta TCR excision circles in naive T cells

Due to homeostasis total naive T cell numbers remain fairly constant over life despite a gradual involution of the thymus. The contribution of the thymus to maintaining naive T cell pools is typically measured with TCR excision circles (TRECs) that are formed in thymocytes. The mechanisms underlying thymic involution are poorly understood. Some data suggest that thymocytes undergo fewer divisions in old (small) than young (large) thymi, and other data suggest that the number of TRECs per thymocyte is independent of age. If thymic involution were associated with a decreased number of divisions of the thymocytes, this would markedly complicate the interpretation of TREC data. To study this we develop a mathematical model in which the division rate of thymocytes decreases with increasing age. We describe the dilution of TRECs formed during the arrangement of both chains of the TCR by division of thymocytes, recent thymic emigrants, and mature naive T cells. The model behavior is complicated as TREC contents in naive T cells can increase with age due to decreased dilution in the thymus. Because our model is consistent with current data on the effects of age and thymectomy on TRECs in peripheral T cells, we conclude that aging may well affect thymocyte division, which markedly complicates the interpretation of TREC data. It is possible, but more difficult, to let the model be consistent with the rapid changes in alpha and beta TRECs observed shortly after HIV infection.

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.

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

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

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

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

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.

Age-dependent incidence, time course, and consequences of thymic renewal in adults

Homeostatic regulation of T cells involves an ongoing balance of new T cell generation, peripheral expansion, and turnover. The recovery of T cells when this balance is disrupted provides insight into the mechanisms that govern homeostasis. In a long-term, single cohort study, we assessed the role of thymic function after autologous transplant in adults, correlating serial computed tomography imaging of thymic size with concurrent measurements of peripheral CD4(+) T cell populations. We established the age-dependent incidence, time course, and duration of thymic enlargement in adults and demonstrated that these changes were correlated with peripheral recovery of naive CD45RA(+)CD62L(+) and signal-joint TCR rearrangement excision circle-bearing CD4(+) populations with broad TCR diversity. Furthermore, we demonstrated that renewed thymopoiesis was critical for the restoration of peripheral CD4(+) T cell populations. This recovery encompassed the recovery of normal CD4(+) T cell numbers, a low ratio of effector to central memory cells, and a broad repertoire of TCR Vbeta diversity among these memory cells. These data define the timeline and consequences of renewal of adult thymopoietic activity at levels able to quantitatively restore peripheral T cell populations. They further suggest that structural thymic regrowth serves as a basis for the regeneration of peripheral T cell populations.

Reduction in the developmental potential of intrathymic T cell progenitors with age

Current models of thymic involution propose that intrinsic developmental defects in intrathymic T cell precursors do not contribute to age-related declines in thymopoiesis. This premise was reassessed in a murine model in light of the recent definition of the early T lineage progenitor (ETP), which appears to be the earliest intrathymic precursor defined to date. The results demonstrate that the frequency of ETP declines with age and their potential to reconstitute the thymus is diminished. These findings are consistent with the fact that ETP from aged mice proliferate less and have a higher rate of apoptosis than their counterparts from young animals. Taken together, these data suggest that age-associated changes in T cell precursors should be considered when attempts to rejuvenate the involuted thymus are made.

Age-related changes in lymphocyte development and function

The effects of aging on the immune system are widespread and extend from hematopoietic stem cells and lymphoid progenitors in the bone marrow and thymus to mature lymphocytes in secondary lymphoid organs. These changes combine to result in a diminution of immune responsiveness in the elderly. This review aims to provide an overview of age-related changes in lymphocyte development and function and discusses current controversies in the field of aging research.

Age-dependent changes in thymic macrophages and dendritic cells

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

Long-lasting changes in the T-cell receptor V beta repertoires of CD4 memory T-cell populations in the peripheral blood of radiation-exposed people

To study the long-term effects of radiation-induced T-cell depletion on the T-cell receptor (TCR) Vbeta repertoires of human peripheral CD4 T-cell populations, we measured the percentages of CD4 T cells representing each of the full range of possible TCR Vbeta families in a cohort of atomic bomb survivors. We then estimated the extent to which the expression levels for individual TCR Vbeta families differed from the average expression level for that particular TCR Vbeta family across the entire cohort. We found no evidence of a systematic change in the TCR Vbeta repertoires of the naïve CD4 T-cell populations, but memory CD4 T-cell TCR Vbeta family expression levels diverged significantly from the population average for counterpart families, especially in individuals who had been exposed to higher doses and were at least 20 years of age at the time of the bombing. Comparisons of the TCR Vbeta family expression profiles in the naïve and memory CD4 T-cell pools of the same group of adult survivors revealed that differences in the TCR Vbeta repertoires of these two types of CD4 T-cell pool were larger in more heavily exposed survivors than in unexposed controls. These findings suggest that the memory CD4 T-cell pools of individuals who received significant radiation doses in adulthood may well have become (and could still be) dependent upon a much less diverse complement of TCR Vbeta families than would otherwise have been the case.

Radiation-induced-thymic lymphoma occurs in young, but not in old mice

Young mice exposed to fractionated whole-body irradiation develop thymic lymphoma. By using young and old mice, we examined the effect of age on the occurrence of radiation-induced thymic lymphoma in mice. In the first experiment, young and old mice were grafted with newborn thymus under kidney capsule and then treated with fractionated whole-body irradiation (FWI). In the second and third experiments, four combinations of bone marrow chimeras were constructed by transplanting bone marrow cells from young and old mice into young and old mice. Then these chimera mice were grafted with newborn thymus and treated with fractionated whole-body irradiation. The results in the present study indicate that the incidence of thymic lymphoma is influenced by age factors of thymic microenvironment, bone marrow, and host environment. If they are all young, the incidence of thymic lymphoma is high. If one of these is old, the incidence definitely decreases. Thymic lymphoma never occurred in old thymic environment even in the presence of young thymocytes. In conclusion, age advantage is present in the induction of thymic lymphoma after the treatment with FWI and the incidence definitely decreases in the presence of old factors.

Age-associated thymic atrophy is not associated with a deficiency in the CD44(+)CD25(-)CD3(-)CD4(-)CD8(-) thymocyte population

Age-associated thymic atrophy has been proposed to be due to changes in both the thymic microenvironment and in the intrinsic properties of the early T cell progenitors, the CD44(+)CD25(-)CD3(-)CD4(-)CD8(-) cells. We have purified these cells from the thymus of both old and young mice and demonstrate no age-associated defect in their ability to differentiate into their progeny in vitro when used to reconstitute fetal thymic organ cultures. We also demonstrate that in the presence of anti-IL-7, CD44(+)CD25(-)CD3(-)CD4(-)CD8(-) cells from young mice show reduced thymocyte development in fetal thymic organ cultures compared with controls. Finally we have shown that old mice treated with IL-7 show improved thymopoiesis compared with control groups. The increased thymopoiesis seen in the old animals occurs in the sequential manner which would be anticipated for an agent working directly on the early stages, including the CD44(+)CD25(-)CD3(-)CD4(-)CD8(-) cells.

Age-related changes of human bone marrow: a histometric estimation of proliferative cells, apoptotic cells, T cells, B cells and macrophages

We performed an immunohistological study using biopsy samples of bone marrow obtained from patients, ranging in age from a newborn baby to 100 years old. Those patients suffering from hematological diseases or diseases that would be capable of affecting hematopoiesis were not included in the present study. The cellularity of the bone marrow did not change significantly with age during the first and the eighth decades, while the oldest group, ranging in age from 80 to 100, revealed significantly low cellularity. Proliferative activity assessed by Ki-67-positive cells was high in the middle-aged group and declined slightly in the elderly group. It was of interest to note that the percentage of apoptosis was relatively low in the young and middle-aged group, but significantly increased in the elderly group. The percentage of T cells did not change greatly between the first and the fifth decades, peaked at the sixth decade and gradually decreased thereafter. The percentage of B cells was about 10% at the first decade, decreased thereafter until the third decade, then increased again showing a peak at the sixth decade, and decreased thereafter. The percentage of CD68-positive cells was high in young patients, and decreased in the adult and elderly patients. The data in the present study suggest that hypocellularity in the bone marrow of elderly people could be ascribed partly to the increase of apoptosis, and might possibly be related to a decrease in the number of lymphocytes and macrophages, which would constitute part of the bone marrow microenvironment supporting hematopoiesis.

Genetic regulation of primitive hematopoietic stem cell senescence

OBJECTIVE

To define effects of strain on PHSC (primitive hematopoietic stem cells) senescence (decline in function with age) in vivo, and to map a locus that regulates PHSC senescence.

MATERIALS AND METHODS

Long-term function and self-renewal were compared in bone marrow cells (BMC) from old and young mice of three strains: BALB/cBy (BALB), DBA/2 (D2) and C57BL/6 (B6), using competitive repopulation and serial transplantation in vivo. BMC from each old or young donor were mixed with standard doses of congenic, genetically marked BMC and transplanted into lethally recipients. Percentages of donor-type erythrocytes and lymphocytes in the recipients determined the functional ability of donor PHSC relative to the standard, where one repopulating unit (RU) of donor BMC equals the repopulating ability of 100,000 standard competitor BMC. Using similar techniques, repopulating abilities of old and young recombinant inbred (RI) donors of 12 strains derived from BALB and B6 were compared in NK-depleted BALBxB6 Fl recipients to map a locus that appears to have a major role in PHSC senescence.

RESULTS

PHSC function declined about 2 fold with age in BALB and D2 BMC, and increased more than 2-fold with age in B6 BMC, with all old/young strain differences significant, p<.01. Ten months after serial transplantation, young B6, BALB, and D2 PHSC had self-renewed 1.6-, 4.2-, and 3.2-fold better than old, with BALB and D2 old/young differences p<.01. Young B6 PHSC self-renewed 1.9- and 2.9-fold better than young BALB and D2 PHSC. The PHSC senescence phenotypes (old/young RU ratios) for 12 CXB RI strains suggested a genetic linkage to D12Nyul7 on Chromosome 12.

CONCLUSION

PHSC senescence is genetically regulated, and is much delayed in the B6 strain compared to the BALB and D2 strains. A locus on Chromosome 12 may regulate PHSC senescence.

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

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

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.

NZB mice exhibit a primary T cell defect in fetal thymic organ culture

Defects in T cell development have been suggested to be a factor in the development of systemic autoimmunity in NZB mice. However, the suggestion of a primary T cell defect has often been by extrapolation, and few direct observations of T cell precursors in NZB mice have been performed. Moreover, the capacity of NZB bone marrow T cell precursors to colonize the thymus and the ability of the NZB thymic microenvironment to support T lymphopoiesis have not been analyzed. To address this important issue, we employed the fetal thymic organ culture system to examine NZB T cell development. Our data demonstrated that NZB bone marrow cells were less efficient at colonizing fetal thymic lobes than those of control BALB/c or C57BL/6 mice. In addition, NZB bone marrow cells did not differentiate into mature T cells as efficiently as bone marrow cells from BALB/c or C57BL/6 mice. Further analysis revealed that this defect resulted from an intrinsic deficiency in the NZB Lin-Sca-1+c-kit+ bone marrow stem cell pool to differentiate into T cells in fetal thymic organ culture. Taken together, the data document heretofore unappreciated deficiencies in T cell development that may contribute to the development of the autoimmune phenotype in NZB mice.

High prevalence of thymic tissue in adults with human immunodeficiency virus-1 infection

The thymus in adults infected with the HIV-1 is generally thought to be inactive, both because of age-related involution and viral destruction. We have revisited the question of thymic function in adults, using chest-computed tomography (CT) to measure thymic tissue in HIV-1-seropositive (n = 99) or HIV-1-seronegative (n = 32) subjects, and correlating these results with the level of circulating CD4(+) and CD8(+) T cells that are phenotypically described as naive thymic emigrants. Abundant thymic tissue was detectable in many (47/99) HIV-1-seropositive adults, aged 20-59. Independent of age, radiographic demonstration of thymic tissue was significantly associated with both a higher CD4(+) T cell count (P = 0.02) and a higher percentage and absolute number of circulating naive (CD45RA+CD62L+) CD4(+) T cells (P < 0.04). The prevalence of an abundant thymus was especially high in younger HIV-1-seropositive adults (</= 39 yr) with CD4 counts in the range 300-500 cells/microl and in older subjects (> 40 yr) regardless of CD4 count (P = 0.03). These studies suggest that the thymus is functional in some but not all adults with HIV-1 disease.

Thymic aging and T-cell regeneration

Studies of T-cell regeneration using animal models have consistently shown the importance of the thymus for T-cell regeneration. In humans, recent studies have shown that declines in thymic T-cell regenerative capacity begins relatively early in life, resulting in a limited capacity for T-cell regeneration by young adulthood. As a result, adult humans who experience profound T-cell depletion regenerate T cells primarily via relatively inefficient thymic-independent pathways, resulting in prolonged CD4 depletion, CD4+ and CD8+ subset alterations, limited TCR repertoire diversity and a propensity for activation induced cell death. These limitations in T-cell regeneration have significant clinical implications in the setting of HIV infection and bone marrow transplantation and may also contribute to immunologic abnormalities associated with normal aging. While the mechanisms responsible for thymic aging are not well understood, current evidence suggests that changes within the thymus itself are primary, while age-related changes in marrow T-cell progenitors and inhibitory factors within the extrathymic host milieu contribute to a lesser extent. The development of therapies which can reverse thymic aging are critical for improving outcome in clinical settings of T-cell depletion, and could potentially improve immunologic function in normal aged hosts.

Effects of the aged microenvironment on CD4+ T cell maturation

Contributing to the functional alterations of the aged immune system is the accumulation of memory CD4+ T lymphocytes and decline in the proportion of naive cells occurring with advancing age. During attempts to alter the naive to memory ratio of CD4+ cells in aged mice, it was observed that regeneration of the peripheral T cell compartment resulted in a population which possessed the same memory cell-enriched characteristics as the unmanipulated age-matched controls. Thymopoiesis in aged mice does not appear to be altered in such a way as to give rise to emigrants with 'memory-like' characteristics. The aged peripheral microenvironment does, however, cause the accelerated maturation of mature, naive CD4+ T cells to the memory state.

Aging of the recipients but not of the bone marrow donors enhances autoimmunity in syngeneic radiation chimeras

Young and old mice have been lethally irradiated and injected with syngeneic bone marrow cells from young or old donors to investigate whether self reactivity in old mice results from age-related damage of the radioresistant stromal cells and/or of the bone marrow hematopoietic cells. Thymus and spleen cell repopulations and mitotic responses at 3 months after irradiation are lower in old than in young recipients, suggesting age-related accumulation of stromal cell damage in the thymus as well as in other central and peripheral lymphoid tissues. The same efficiency of bone marrow cells from young and old donors to repopulate the thymus and spleen in recipients of equal age rules out the detrimental effects of aging on stem cells as well as T and B cell precursors. The serum concentration of auto-antibody and glomerular lesions at 3 and 9 months after irradiation were more pronounced in old than in young recipients and displayed no difference in recipients of equal age, regardless of the age of the bone marrow cell donors. These findings support the possibility that age-related damage of stromal cells induces disregulation of the immune system leading to autoimmune phenomena.

Thymocyte development in vitro: implications for studies of ageing and thymic involution

Functional defects that accumulate in the T cell compartment are thought to be responsible for the pronounced immunodeficiency that develops during ageing, and reduced production of T cells by the thymus as it undergoes involution has been suggested to contribute to this phenomenon. Understanding the mechanisms responsible for thymic involution requires a thorough knowledge of how thymopoiesis is regulated. Obtaining such information is dependent upon the availability of defined experimental systems that permit analysis of thymopoiesis at the cellular and molecular levels. Recent advances have been made in the development of such human and murine in vitro systems, and their analysis has the potential to identify thymic microenvironmental signals that regulate T cell production. This information should, in turn, provide a basis for understanding changes in thymopoiesis that occur during ageing. The features of these culture systems are reviewed in this article, and their potential application to the study of T cell production during ageing is discussed.

Ultrastructural study of thymic microenvironment involution in aging mice

Aging involves morphological alterations of the thymus and deregulation of various immune response parameters. Altogether, these phenomena have been termed thymic involution. Using electron microscopy, we studied the morphological ultrastructure of the thymic microenvironment in aged mice. We observed cellular damages which progressively affected all the thymic stroma. At later stages (i.e., about 18-20 months old), a disappearance of the organ architecture with a drastic decrease in lymphocyte number was observed. The loss of cellular integrity of the microenvironment with lysis of cellular membranes and formation of a large and clear cytoplasmic layer engulfing a few remaining lymphocytes was noted. Extensive lipidic invasion surrounding the remaining epithelial cells grouped in nest formations and/or bordering cytics cavities was also present in these thymus from aged mice. Because the thymic microenvironment plays an important role in the "education" and functional maintenance of T cells and because the alteration of this cellular entity precedes a decline in certain immune functions, it can be suggested that membrane alterations, lack of cellular microenvironment integrity, and T cell dysfunction are correlated.

The pattern of T lymphocyte differentiation is altered during thymic involution

Thymic involution is likely to be a significant factor in the alteration of peripheral T lymphocyte function with age. The process of thymic involution involves the progressive loss of normal organ architecture and cellular composition, and a significant reduction in the output of mature T lymphocytes. The present study assesses the impact of thymic involution on the T cell differentiation process by quantitating the number and percent representation of various phenotypically distinguishable T cell developmental intermediates in C57BL/6 mice of various ages. The results suggest that several distinct sites in the developmental sequence are impacted by aging. By middle-age (14-17 months), significant perturbations in the frequencies of several CD4-CD8- (DN) subpopulations have occurred. These include a shift towards an increased percentage of Pgp-1+ IL-2R- DN cells, the earliest thymic progenitors, and a decreased percentage and total number of Pgp-1- IL-2R+ DN cells. Furthermore there is a threefold increase in the percentage of DN cells which express CD3 (from 16.6% to 45.5%) which occurs between 4 and 14 months of age. By 24-27 months of age, the percentage of the total DN population increases two- to threefold over that of young (2-3 months) animals, while the fraction of CD4+CD8+ (DP) is significantly reduced. These alterations are consistent with the possibility that thymic involution results in one or more 'developmental' blocks which limit key differentiative transitions within the DN population, and furthermore, the marked increase in the frequency of DN cells displaying CD3 argues that an alternative T cell differentiation pathway plays an increasingly significant role with advancing age.

Immunological alterations with aging—laying a stress on recent progress in Japan

Age-related decline of immune functions mainly occurs in the T-cell-dependent immune system and this is precede by physiological thymic involution starting after puberty. Age-related alterations of T cells are seen in their number, subsets and qualities. In relation to the qualitative change of aging T cells, disturbance of intracellular signal transduction is responsible for the impairment of proliferation and cytokine production upon antigenic stimulation. Thymus, but not bone marrow, is responsible for aging of the T-cell-dependent immune system. The thymic capacity to induce T cells starts to decline early in life. Older thymus induces different subsets of T cells which are functionally less active T cells than those of young thymus. In association with a decline in immune functions to exogenous antigens, autoimmune phenomena increase with advance of age, which are observed as production of autoantibodies and auto-reactive T cells. The possible site for controlling thymic function is discussed.

Antibody response in aged C57BL/6 mice: T helper cells are responsible for the decline of the primary antibody response to a bacterial antigen in aging

One of the most important effects of aging is the decline of immune effectiveness. A study is described here on the primary (IgM) antibody response to phosphorylcholine (PC), an antigen expressed on the surface of the Streptococcus pneumoniae R36a, in aged (18-22 mo.) C57BL/6 mice. Preliminary data showed a significant decrease of PC-specific response in aged mice as compared to young/adult (3-4 mo.) syngeneic mice. In vitro studies showed that B cells from aged donors, retained the ability to produce amounts of anti-PC antibody comparable to that of B cells from young donors. In addition, B cells from young and aged donors were cultured in the presence of L3T4 or Lyt2 T cells obtained from donors of different ages. "Aged" L3T4 cells failed to augment the anti-PC response, as compared to the young ones, while the Lyt 2 cells, either from young or aged donors, showed no effects on the anti-PC response. These observations were confirmed by in vivo selective depletion of T cells subpopulations in both young and aged C57BL/6 mice. L3T4 cell-depleted young mice displayed a markedly reduced antibody response; in contrast, age-depleted mice did not show any significant modifications of the response as compared to normal aged mice. Furthermore, we observed that the selective depletion of Lyt 2 cells in both young and aged C57BL/6 did not alter the anti-PC response in any way.

Aging and immunity

The function of the immune system peaks at around puberty and gradually declines thereafter with advance in age. The age-related decline of immunological function primarily occurs in the T cell-dependent immune system and is generally associated with increase in susceptibility to infections as well as in incidence of autoimmune phenomena in the elderly. The age-related change in T cell-dependent immune functions can be ascribed to the physiological thymic atrophy which starts in an early stage of life. Emigration of T cells from the thymus to the periphery mainly takes place in the late fetal and newborn stage, and dramatically declines after puberty. In other words, the thymic capacity to promote T cell differentiation starts to change in the early stage of life in terms of quantity and quality of T cells. Thus, the composition of T cell-subsets in the periphery gradually changes with age, resulting in the alteration of T cell functions in the elderly. The restoration of immunological functions of the aged individuals is possible and might be beneficial for them to cope with various diseases associated with aging. Physiological thymic atrophy is controlled by both extrathymic and intrathymic factors, and is not a totally irreversible process. The process of thymic atrophy might be explained by further understanding of the relationship between the neuroendocrine and the immune systems.

The effects of thymus supernatant on the thymus homing ability of aged murine bone marrow cells

This study examined the effect of thymus supernatant on the ability of bone marrow cells from senescent mice to home to the thymus. An in vitro and an in vivo assay were employed. The in vitro assay used a blind well migration of aged bone marrow cells to thymus supernatant across a membrane with 5-microns pores. The in vitro assay measured the ability of aged bone marrow to repopulate the thymus of an irradiated host. Our results support previous reports that the bone marrow from old mice has a greatly reduced ability to migrate to thymus supernatant and to repopulate the thymus of an irradiated host. Further, we found that a brief treatment of the old bone marrow with thymus supernatant significantly improved its thymus homing ability both in vitro and in vivo.

Bone marrow-thymus axis in senescence

This presentation offers a brief review of the bone marrow-thymus axis in senescence, a putative model for thymocyte differentiation, and recent results of our work on the status of pre-thymic stem cells in aged mice. The data presented here provide further evidence for a thymus endocrine influence on the bone marrow stem cells, specifically lymphocyte precursors. It has been postulated that the thymic hormones may act on lymphocyte precursors in the bone marrow and that the loss of thymic factors during senescence may be a contributing factor to the decreased cellular immune function. This study used Haar's in vitro model to investigate the bone marrow-thymus axis in aged mice. Erythroid-depleted bone-marrow cells from 3-month- and 24-month-old CBA (Thy 1.2) mice were placed in the upper half of a blind-well chamber with thymus supernatant in the lower half. Experimental cells were treated with thymus supernatant for 1 hr prior to migration. This study confirmed that pre-thymic stem cells in aged bone marrow are deficient in their ability to migrate to the thymus supernatant. It also revealed that treatment of the old bone marrow with thymus supernatant, made from neonatal thymus cultures, could dramatically improve the thymus migrating ability of the aged bone-marrow stem cells.

Aging and immune function

Since it is too difficult to study aging of the organism as a whole, most investigators try to focus on a specific physiological system that exhibits age-dependent functional changes, in the hopes that elucidation (in biochemical and developmental terms) of the mechanism of senescent change will provide insight into the aging process itself. The immune system is among the most maleable of such models, in that well-defined cell types will produce well-defined molecules with predictable functions in vitro and in vivo. The increasing power of basic immunological science should, in the next decade, permit an increasingly fine appreciation of how aging leads to immune decline. This expanding conceptual framework will then suggest new ideas about the role of immunosenescence in degenerative, infectious, and neoplastic illnesses and may also generate increasingly rational strategies for therapeutic intervention.

Transplantation studies in mice with congenital hemolytic anemia

Sphha/sphha anemic mice have an abnormality in the erythroid membrane protein, alpha spectrin, and exhibit multiple related clinical abnormalities, including spherocytosis, shortened red cell survival, chronic hemolysis, hemosiderosis, and extramedullary hematopoiesis. In addition, these mutant mice exhibit a granulocytosis and lymphocytosis, lymph node hyperplasia, elevated serum immunoglobulins, membranoproliferative glomerulonephritis, and decreased lifespan--abnormalities that are less clearly attributable to a spectrin defect. In order to further elucidate the mechanisms of disease in these animals, we undertook a series of bone marrow transplantation experiments. Transplantation of anemic marrow into lethally irradiated congenic +/+ mice resulted in chronic spherocytosis, hemolytic anemia, peripheral leukocytosis, and extramedullary hematopoiesis. Additionally, transplant recipients of anemic marrow which had received a higher radiation dose (12 Gy) had increased numbers of peripheral blood CD4+ and CD8+ lymphocytes, a hypocellcular thymus, and a severe pneumonitis characterized by nodular areas of consolidation and edema. Mice receiving congenic +/+ marrow and irradiated with the same radiation dose exhibited minimal pulmonary abnormalities. Anemic mice transplanted with congenic +/+ marrow usually died, but the survivors exhibited reversal of some clinicopathological changes. These results would suggest that the clinical abnormalities of sphha/sphha mice are in part attributable to abnormalities of hematopoietic stem cells but may also involve defects in other cell types. The pathogenesis of the accompanying lymphoid abnormalities observed in this mutant anemic mouse and any correlation with the erythroid spectrin defect are presently unknown. The pulmonary disease that develops in the transplant recipients of anemic marrow needs to be characterized further but may represent a unique model of lung injury.

Effect of aging on epidermal dendritic cell populations in C57BL/6J mice

The density and function of epidermal dendritic cell populations were investigated in aged C57BL/6J mice. The densities of both Langerhans cells (LC) and Thy-1+ dendritic epidermal cells were found to decrease with age. Epidermal cell suspensions from aged mice showed impaired immunologic function as assessed in vitro by the skin-lymphocyte reaction assay and by measuring the ability of epidermal cell suspensions to stimulate the proliferation of sensitized T cells in the presence of the sensitizing antigen. However, the capacity of LC to transport antigen from the skin to the draining lymph nodes was found in vivo to be comparable to that of young mice. Results of transplantation of bone marrow cells from young and old donors into irradiated recipients indicate that the decreased Langerhans cell density found in old mice may result from a deficiency in Langerhans cell bone marrow progenitors.