Quantitative analysis of bone marrow thymic progenitors in young and aged mice

RhoB deficiency in thymic medullary epithelium leads to early thymic atrophy

RhoB, a member of the Rho subfamily of small GTPases, mediates diverse cellular functions, including cytoskeletal organization, cell transformation and vesicle trafficking. The thymus undergoes progressive decline in its structure and function after puberty. We found that RhoB was expressed in thymic medullary epithelium. To investigate a role of RhoB in the regulation of thymic epithelial organization or thymocyte development, we analyzed the thymi of RhoB-deficient mice. RhoB-deficient mice were found to display earlier thymic atrophy. RhoB deficiency showed significant reductions in thymus weight and cellularity, beginning as early as 5 weeks of age. The enhanced expression of TGF-β receptor type II (TGFβRII) in thymic medullary epithelium was observed in RhoB-null mice. In addition, the expression of fibronectin, which is shown to be regulated by TGF-β signaling, was accordingly increased in the mutant thymic medulla. Since there is no age-related change of RhoB expression in the thymus, it is unlikely that RhoB in thymic epithelium directly contributes to age-related thymic involution. Nevertheless, our findings strongly support a physiological role of RhoB in regulation of thymus development and maintenance through the inhibition of TGF-β signaling in thymic medullary epithelium.

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.

A new mechanism for the aging of hematopoietic stem cells: aging changes the clonal composition of the stem cell compartment but not individual stem cells

Whether hematopoietic stem cells (HSCs) change with aging has been controversial. Previously, we showed that the HSC compartment in young mice consists of distinct subsets, each with predetermined self-renewal and differentiation behavior. Three classes of HSCs can be distinguished based on their differentiation programs: lymphoid biased, balanced, and myeloid biased. We now show that aging causes a marked shift in the representation of these HSC subsets. A clonal analysis of repopulating HSCs demonstrates that lymphoid-biased HSCs are lost and long-lived myeloid-biased HSCs accumulate in the aged. Myeloid-biased HSCs from young and aged sources behave similarly in all aspects tested. This indicates that aging does not change individual HSCs. Rather, aging changes the clonal composition of the HSC compartment. We show further that genetic factors contribute to the age-related changes of the HSC subsets. In comparison with B6 mice, aged D2 mice show a more pronounced shift toward myeloid-biased HSCs with a corresponding reduction in the number of both T- and B-cell precursors. This suggests that low levels of lymphocytes in the blood can be a marker for HSC aging. The loss of lymphoid-biased HSCs may contribute to the impaired immune response to infectious diseases and cancers in the aged.

Non-human primate models of T-cell reconstitution

Non-human primates (NHP) have become an indispensable model in studying the common and dangerous human chronic infections, including HIV/SIV, Hepatitis C virus, and tuberculosis. More recently, we and others have used aged NHP to model human immune aging. Chronic infections and aging are both characterized by a significant depletion of defined lymphocyte subsets and the compensatory attempts to regenerate the immune system. As the efficacious antiviral drugs and novel methods to improve and boost the immune system emerge, therapeutic immune regeneration has become a realistic goal in both the physiologic and pathologic settings. This article will summarize our current knowledge on this topic and will discuss future research directions as well as the potential and power of translational studies in non-human primate models of infection, aging and bone marrow transplantation.

Activation of Thymic Regeneration in Mice and Humans following Androgen Blockade

The thymus undergoes age-related atrophy, coincident with increased circulating sex steroids from puberty. The impact of thymic atrophy is most profound in clinical conditions that cause a severe loss in peripheral T cells with the ability to regenerate adequate numbers of naive CD4+ T cells indirectly correlating with patient age. The present study demonstrates that androgen ablation results in the complete regeneration of the aged male mouse thymus, restoration of peripheral T cell phenotype and function and enhanced thymus regeneration following bone marrow transplantation. Importantly, this technique is also applicable to humans, with analysis of elderly males undergoing sex steroid ablation therapy for prostatic carcinoma, demonstrating an increase in circulating T cell numbers, particularly naive (TREC+) T cells. Collectively these studies represent a fundamentally new approach to treating immunodeficiency states in humans.

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-associated loss of bone marrow hematopoietic cells is reversed by GH and accompanies thymic reconstitution

Deterioration of the thymus gland during aging is accompanied by a reduction in plasma GH. Here we report gross and microscopic results from 24-month-old Wistar-Furth rats treated with rat GH derived from syngeneic GH3 cells or with recombinant human GH. Histological evaluation of aged rats treated with either rat or human GH displayed clear morphologic evidence of thymic regeneration, reconstitution of hematopoietic cells in the bone marrow, and multiorgan extramedullary hematopoiesis. Quantitative evaluation of formalin-fixed, hematoxylin and eosin-stained sections of bone marrow from aged rats revealed at least a 50% reduction in the number hematopoietic bone marrow cells, compared with that of young 3-month-old rats. This age-associated decline in bone marrow leukocytes, as well as the increase in bone marrow adipocytes, was significantly reversed by in vivo treatment with GH. Restoration of bone marrow cellularity was caused primarily by erythrocytic and granulocytic cells, but all cell lineages were represented and their proportions were similar to those in aged control rats. On a per-cell basis, GH treatment in vivo significantly increased the number of in vitro myeloid colony forming units in both bone marrow and spleen. Morphological evidence of enhanced extramedullary hematopoiesis was observed in the spleen, liver, and adrenal glands from animals treated with GH. These results confirm that GH prevents thymic aging. Furthermore, these data significantly extend earlier findings by establishing that GH dramatically promotes reconstitution of another primary hematopoietic tissue by reversing the accumulation of bone marrow adipocytes and by restoring the number of bone marrow myeloid cells of both the erythrocytic and granulocytic lineages.

The roles of prolactin, growth hormone, insulin-like growth factor-I, and thyroid hormones in lymphocyte development and function: insights from genetic models of hormone and hormone receptor deficiency

An extensive literature suggesting that PRL, GH, IGF-I, and thyroid hormones play an important role in immunity has evolved. Because the use of one or more of these hormones as immunostimulants in humans is being considered, it is of critical importance to resolve their precise role in immunity. This review addresses new experimental evidence from analysis of lymphocyte development and function in mice with genetic defects in expression of these hormones or their receptors that calls into question the presumed role played by some of these hormones and reveals unexpected effects of others. These recent findings from the mutant mouse models are integrated and placed in context of the wider literature on endocrine-immune system interactions. The hypothesis that will be developed is that, with the exception of a role for thyroid hormones in B cell development, PRL, GH, and IGF-I are not obligate immunoregulators. Instead, they apparently act as anabolic and stress-modulating hormones in most cells, including those of the immune system.

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

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

Hematopoietic stem cells and aging

The question of whether hematopoietic stem cells are altered in aging has been the subject of considerable controversy for over two decades. The substantial advancement of knowledge on hematopoietic stem cells and developmental hematology in the last few years has reopened this issue for critical analysis. Dynamic changes have been noted regarding the anatomic site and the function of hematopoietic cells, from the early embryo to old age. Whereas basal hematopoietic potential is maintained in aging. the capacity for recovery from hematological stress and for stem cell self-renewal appears to decline gradually. A distinction is thus made between the steady-state hematopoiesis in aging and the developmental potential of stem cells. The establishment of proper tools to identify and to study purified stem cells and committed cell populations offers a direct approach to further elucidate aging across the axis from primitive stem cells to the mature blood cells. The present article represents a brief review of this area.

Insulin growth factor-I inhibits apoptosis in hematopoietic progenitor cells. Implications in thymic aging

A decline in plasma concentrations of both growth hormone and IGF-I occurs during aging of humans and rodents, and this is accompanied by involution of the thymus gland. Exogenous growth hormone induces the synthesis of IGF-I, which acts on bone marrow-derived hematopoietic progenitors of the myeloid and lymphoid lineages to promote their replication and survival. The increase in survival of these cells is caused by the ability of IGF-I to inhibit their apoptotic death. In contrast to the multipotential colony-stimulating-factor IL-3, inhibition of apoptosis by IGF-I requires the activation of the critical intracellular effector PI 3-kinase. These data establish that hematopoietic progenitors can use more than one intracellular signaling pathway in order to maintain their survival. The data also extend the original hypothesis that IGF-I shares with the colony-stimulating factors the properties of promoting DNA synthesis and inhibiting programmed cell death. Collectively, these data establish that hematopoietic progenitor cells are important targets for IGF-I, and this is likely to be important in understanding thymic aging.

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

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

Thymocytopoiesis in aging: the bone marrow-thymus axis

Manifestations of aging in the mature T lymphocyte compartment have been attributed, to a major extent, to effects of the involuted thymus, at the thymic microenvironment level. However, since generation of T lymphocytes starts from hemopoietic stem cells that settle in the thymus and differentiate there, aging effects on the stem cells, and as a consequence, on the bone marrow (BM)-thymus axis, may also have an impact on patterns of thymocytopoiesis and on age-related thymus remodeling. This communication reviews our studies designed to determine whether BM cells manifest any aging effects that become overt in the resulting thymocytes. The experiments were performed by seeding of BM cells onto lymphoid-depleted fetal thymus (FT) explants, to enable distinguishing between processes that occur in the BM and those that are caused by the aging thymic microenvironment. The data show changes in the developmental potential of BM-derived cells, as reflected from the kinetics of cell cycle and intermediate steps from stem cell settling in the thymus to an early stage at the transition from CD4(-)CD8(-), double negative (DN), to CD4(+)CD8(+), double positive (DP) thymocytes. In addition, we have demonstrated that these early developmental steps of thymocytopoiesis are subject to feedback regulation by mature T cells, and the extent of regulation may be altered in old age. The pattern of T lymphocyte generation in aging is thus a result of dynamic changes in thymic, as well as extrathymic functions, along the sequential developmental steps from the stem cell to the ultimate mature cell.

Feedback regulation of T cell development: manifestations in aging

Recent findings have indicated that mature T cells may regulate thymocytopoiesis in an age-related differential manner. The studies were based on T lymphocyte development in mouse fetal thymus stroma colonized with immature thymocytes and CD4+ T cells from young or old donors. In the present study, we used mathematical modeling and computer simulations in order to identify the thymocyte subsets that are targets for this type of regulation, and the processes affected by it. Our results suggest that thymocyte development is subject to regulation through 2 feedback loops: mature CD4+ cells exert a negative feedback on the double-negative to double-positive transition and on double-positive subset growth, and a positive feedback on the double-positive to CD4 single-positive transition. These effects may operate, in young mice, through a reduction in the rate of death of CD4+8- thymocytes, and a faster maturation of double-positive cells. In old mice, our simulations suggest that there may additionally be a reduction in double-positive proliferation rate. In some, but not all, of the simulations of old donor- derived thymocytes, we also had to assume a reduction in double-negative to double-positive differentiation, an increase in double-positive death rates, an increase of CD4+8- cell division rate, and a decrease of differentiation to the CD8 lineage.

Thymocyte progenitors in ageing

The effect of ageing on thymocyte progenitors in the bone marrow was studied in an in vitro experimental model that permits T lymphocyte development. The model is based on co-culture of BM cells from young and old mice with lymphoid depleted fetal thymus explants. We applied different strategies of thymic colonization, including competitive colonization by BM cells from different donor age groups and MHC backgrounds. Our data reveal intrinsic changes in the BM that lead to manifestation of immunosenescence in the T lymphocyte compartment.

A mathematical model of the effect of aging on bone marrow cells colonizing the thymus

The process of T cell generation in the thymus involves complex cell-cell interactions between the various types of thymic stromal cells, thymocyte progenitors, thymocytes at different stages of differentiation and external factors. We applied the tool of mathematical modelling to analyze hypotheses and direct experiments concerning mechanisms underlying the observed developmental inferiority of bone-marrow thymocyte progenitors from old mice. Previous experimental data showed that lower cell numbers were obtained from old bone marrow-derived thymocyte progenitors, compared to young bone marrow-derived progenitors, when colonizing simultaneously the same fetal thymus. In this study, simulations based on the mathematical model indicate that the developmental inferiority of old bone marrow-derived progenitors cannot be explained by a change in a single parameter, such as the observed differences in progenitor frequency, an increase in cell cycle duration, a reduction in the fraction of proliferating cells in old age, and/or an increase in the rate of cell death. We have performed experimental measurements of the fractions of cycling cells. No significant difference was found between these fractions in young and old bone marrow-derived thymocytes. The difference in developmental patterns of young and old bone marrow-derived thymocytes may be due to a combination of more than one mechanism, possibly including interactions between competing thymocytes of old and young bone marrow origin.

Aging in the T lymphocyte compartment. A developmental view

A decline in the capacity of bone marrow cells to differentiate to T lymphocytes was found when cells from young and old donors were seeded onto an alymphoid fetal thymus. A step-by-step analysis of cell-cell interactions of the lymphohemopoietic cells and the thymic stroma indicated an effect of age on a variety of cell differentiation parameters. These included a decrease in the affinity of bone marrow cells to the stroma, and in their capacity to compete with the thymic lymphoid resident cells on colonization of the thymus. There was a significant decrease in the ability of cells of old donors to replicate sequentially within the thymic microenvironment. There was a reduced capacity of bone marrow cells from aging mice to express a developmental preference after seeding onto a syngeneic fetal thymus in a mixture with cells from allogeneic donors. We addressed the question whether the aging thymus contains increased levels of immature cells that fail to differentiate in the involuted thymic microenvironment by seeding thymocytes from young and old donors onto the fetal thymic stroma. The values of T cells that developed from the old donor inoculum were lower under these conditions. Our studies suggest that at least some of the manifestations of aging in the T cell compartment are related to developmentally programmed events in the lymphohemopoietic cell compartment.

In vitro analysis of age-related changes in the developmental potential of bone marrow thymocyte progenitors

Mechanisms underlying the age-related decrease in the developmental capacity of thymocyte progenitors from the bone marrow (BM) were analyzed, focussing on interaction of these cells with the thymic microenvironment. We employed the experimental model in which mixtures of young and old mouse BM cells, congenic for the Thy-1 marker, were seeded onto fetal thymus (FT) explains depleted of self lymphocytes and the levels of Thy-1+ cells developing from each of the two donor types were measured. When cells from young and old BM donors were seeded simultaneously, in saturating quantities, a higher level of T cells developed from the young donors. To find out whether there were originally more thymocyte progenitors in the young BM, we carried out the competitive colonization under limiting dilution conditions and found that the advantage of the young had diminished under these conditions, thus suggesting that the age-related changes could not be related solely to quantitative differences. We then incubated the FT sequentially with old donor cells for 24 h, followed by young for an additional 48 h and found that the advantage of the young progenitors was eliminated. We thus established that the initial stage of colonization of the FT was important in determining the outcome of the subsequent development. The kinetics of simultaneous competition within the FT, however, revealed that the advantage of the young BM-derived cells became significant only from day 7 in organ culture, thus suggesting that sequential divisions of these cells were at a higher level than those of the old. Recolonization of FT explants by young or old BM-derived thymocytes obtained from the first colonization of the FT stroma showed a reduced, but still significant advantage for the young BM-derived cells over the old. Thus, we concluded that the old BM thymocyte progenitors manifested a qualitative disadvantage which became apparent during competitive colonization of the FT.