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

Detection of HHV-5 HHV-6a HHV-6b and HHV-7 in the urine: potential use as a non-invasive diagnostic tool for immune profiling

Decline in immune function with age has been studied extensively, but approaches to immune restoration have been hampered by the lack of simple methods of identifying individuals whose immune system is in decline. Our approach has been to identify individuals whose immune decline has led to a loss of control of common latent viral infections and their consequent reactivation. Viruses excreted in urine were detected and quantified and we believe this approach could provide a 'surrogate marker' for identifying immune compromised individuals. Here we report the detection of human herpes virus (HHV) 5, 6a, 6b and 7 in the urine of healthy individuals over a wide age range and their correlation with T cell receptor excision circle (TREC) data. The results did not show a clear correlation between TREC values and the detection of individual specific viruses or viral load values when measured singly. However, a correlation was found between low TREC values and the detection of several different human herpes viruses in the urine in males. We present evidence suggesting that for males, the detection of three or more different human herpes viruses in the urine could identify individuals with declining immune function as evidenced by their significantly lower TREC levels.

Short-chain chlorinated paraffins may induce thymic aging in mice by activating PERK-CHOP

Humans indirectly consume approximately 0.02 mg/kg/day of short-chained chlorinated paraffins (SCCPs) through the environment; however, the thymic senescence/damage induced by SCCPs has not been assessed. In this study, 16 female mice (4-week-old) per group were orally administered 0, 0.01, 0.1, and 1 mg/kg/day of SCCPs for 21 days, and the phenotypes and levels of superoxide dismutase (SOD), malondialdehyde (MDA), Tβ4, αβ TCR, SA-β-Gal, GRP78, PERK/CHOP, P53/P21, and CASPASE-1 of the thymus were assessed as indicators. Another group comprising 16 mice was killed at 4-week-old and these indicators were assessed. Thereafter, the thymuses cultured in vitro were exposed to 0, 14, 140, and 1400 μg/L SCCPs, respectively, and the above indicators were measured after 7-day. Based on the results, the oral administration of ≥0.01 mg/kg/day SCCPs to mice and ≥14 μg/L of SCCPs in medium caused thymic aging features, such as a decrease in the ratio of cortex to medulla, gradual blurring of the boundary between the cortex and medulla, dose-dependent oxidative stress (decreased SOD and increased MDA), and decreased levels of Tβ4 and αβ TCRs in the thymus. The oral administration of ≥1 mg/kg/day of SCCPs also impeded the growth and development of female mice and their thymuses. Exposure to the low levels of SCCPs activated PERK-CHOP in the mouse thymus, which modulated increases in SA-β-Gal, IL-1β, P53, and CASPASE-1 in vivo and in vitro. Overall, environmental levels and human blood concentrations (14.8-1400 μg/L) of SCCPs may induce mouse thymus senescence by activating PERK-CHOP in vivo and in vitro, respectively.

Thymic atrophy induced by Plasmodium berghei ANKA and Plasmodium yoelii 17XL infection

The thymus is the anatomical site where T cells undergo a complex process of differentiation, proliferation, selection, and elimination of autorreactive cells which involves molecular signals in different intrathymic environment. However, the immunological functions of the thymus can be compromised upon exposure to different infections, affecting thymocyte populations. In this work, we investigated the impact of malaria parasites on the thymus by using C57BL/6 mice infected with Plasmodium berghei ANKA and Plasmodium yoelii 17XL; these lethal infection models represent the most severe complications, cerebral malaria, and anemia respectively. Data showed a reduction in the thymic weight and cellularity involving different T cell maturation stages, mainly CD4-CD8- and CD4+CD8+ thymocytes, as well as an increased presence of apoptotic cells, leading to significant thymic cortex reduction. Thymus atrophy showed no association with elevated serum cytokines levels, although increased glucocorticoid levels did. The severity of thymic damage in both models reached the same extend although it occurs at different stages of infection, showing that thymic atrophy does not depend on parasitemia level but on the specific host-parasite interaction.

Age-associated remodeling of T cell immunity and metabolism

Aging results in remodeling of T cell immunity and is associated with poor clinical outcomes in age-related diseases such as cancer. Among the hallmarks of aging, changes in host and cellular metabolism critically affect the development, maintenance, and function of T cells. Although metabolic perturbations impact anti-tumor T cell responses, the link between age-associated metabolic dysfunction and anti-tumor immunity remains unclear. In this review, we summarize recent advances in our understanding of aged T cell metabolism, with a focus on the bioenergetic and immunologic features of T cell subsets unique to the aging process. We also survey insights into mechanisms of metabolic T cell dysfunction in aging and discuss the impacts of aging on the efficacy of cancer immunotherapy. As the average life expectancy continues to increase, understanding the interplay between age-related metabolic reprogramming and maladaptive T cell immunity will be instrumental for the development of therapeutic strategies for older patients.

The thymus regulates skeletal muscle regeneration by directly promoting satellite cell expansion

The thymus is the central immune organ, but it is known to progressively degenerate with age. As thymus degeneration is paralleled by the wasting of aging skeletal muscle, we speculated that the thymus may play a role in muscle wasting. Here, using thymectomized mice, we show that the thymus is necessary for skeletal muscle regeneration, a process tightly associated with muscle aging. Compared to control mice, the thymectomized mice displayed comparable growth of muscle mass, but decreased muscle regeneration in response to injury, as evidenced by small and sparse regenerative myofibers along with inhibited expression of regeneration-associated genes myh3, myod and myogenin. Using Pax7 immunofluorescence staining and BrdU incorporation assay, we determined that the decreased regeneration capacity was caused by a limited satellite cell pool. Interestingly, the conditioned culture medium of isolated thymocytes (TCMs) had a potent capacity to directly stimulate satellite cell expansion in vitro. These expanded cells were enriched in subpopulations of quiescent satellite cells (Pax7^highMyoD^lowEdU^pos) and activated satellite cells (Pax7^highMyoD^highEdU^pos), which were efficiently incorporated into the regenerative myofibers. We thus propose that the thymus plays an essential role in muscle regeneration by directly promoting satellite cell expansion and may function profoundly in the muscle aging process.

Thymic Aging May Be Associated with COVID-19 Pathophysiology in the Elderly

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) caused the global pandemic of coronavirus disease 2019 (COVID-19) and particularly exhibits severe symptoms and mortality in elderly individuals. Mounting evidence shows that the characteristics of the age-related clinical severity of COVID-19 are attributed to insufficient antiviral immune function and excessive self-damaging immune reaction, involving T cell immunity and associated with pre-existing basal inflammation in the elderly. Age-related changes to T cell immunosenescence is characterized by not only restricted T cell receptor (TCR) repertoire diversity, accumulation of exhausted and/or senescent memory T cells, but also by increased self-reactive T cell- and innate immune cell-induced chronic inflammation, and accumulated and functionally enhanced polyclonal regulatory T (Treg) cells. Many of these changes can be traced back to age-related thymic involution/degeneration. How these changes contribute to differences in COVID-19 disease severity between young and aged patients is an urgent area of investigation. Therefore, we attempt to connect various clues in this field by reviewing and discussing recent research on the role of the thymus and T cells in COVID-19 immunity during aging (a synergistic effect of diminished responses to pathogens and enhanced responses to self) impacting age-related clinical severity of COVID-19. We also address potential combinational strategies to rejuvenate multiple aging-impacted immune system checkpoints by revival of aged thymic function, boosting peripheral T cell responses, and alleviating chronic, basal inflammation to improve the efficiency of anti-SARS-CoV-2 immunity and vaccination in the elderly.

miR-205-5p inhibits thymic epithelial cell proliferation via FA2H-TFAP2A feedback regulation in age-associated thymus involution

Thymic epithelial cells (TECs) are essential regulators of T cell development and selection. microRNAs (miRNAs) play critical roles in regulating TECs proliferation during thymus involution. miR-205-5p is highly expressed in TECs and increases with age. However, the function and potential mechanism of miR-205-5p in TECs are not clear. miRNA expression was profiled using TECs from male and female mice at 1 and 3 months old. A total of 325 differentially expressed miRNAs (DEMs) were detected at different ages in two sexes. 24 of the DEMs had the same trend between males and females. Among them, miR-205-5p had the highest fold change. Our results showed that the expression of miR-205-5p was dramatically increased in TECs from 1 to 9 months old mice. miR-205-5p mimic inhibited TECs proliferation. Moreover, we confirmed that Fa2h was the direct target gene of miR-205-5p and FA2H was significantly decreased in TECs with increased expression of miR-205-5p. Silencing of Fa2h inhibited TECs proliferation. Furthermore, we found that the expression of Tfap2a could be promoted by FA2H and that TFAP2A could interact with miR-205-5p in TECs. Overall, miR-205-5p is an important regulator of TECs proliferation and regulates age-associated thymus involution via the miR-205-5p-FA2H-TFAP2A feedback regulatory circuit. miR-205-5p might act as a potential biomarker in TECs for age-related thymus involution.

Contributions of Age-Related Thymic Involution to Immunosenescence and Inflammaging

Immune system aging is characterized by the paradox of immunosenescence (insufficiency) and inflammaging (over-reaction), which incorporate two sides of the same coin, resulting in immune disorder. Immunosenescence refers to disruption in the structural architecture of immune organs and dysfunction in immune responses, resulting from both aged innate and adaptive immunity. Inflammaging, described as a chronic, sterile, systemic inflammatory condition associated with advanced age, is mainly attributed to somatic cellular senescence-associated secretory phenotype (SASP) and age-related autoimmune predisposition. However, the inability to reduce senescent somatic cells (SSCs), because of immunosenescence, exacerbates inflammaging. Age-related adaptive immune system deviations, particularly altered T cell function, are derived from age-related thymic atrophy or involution, a hallmark of thymic aging. Recently, there have been major developments in understanding how age-related thymic involution contributes to inflammaging and immunosenescence at the cellular and molecular levels, including genetic and epigenetic regulation, as well as developments of many potential rejuvenation strategies. Herein, we discuss the research progress uncovering how age-related thymic involution contributes to immunosenescence and inflammaging, as well as their intersection. We also describe how T cell adaptive immunity mediates inflammaging and plays a crucial role in the progression of age-related neurological and cardiovascular diseases, as well as cancer. We then briefly outline the underlying cellular and molecular mechanisms of age-related thymic involution, and finally summarize potential rejuvenation strategies to restore aged thymic function.

Assessment of industry data on pulmonary and immunosuppressive effects of IQOS

Introduction

Heated tobacco products are being touted as novel reduced-harm tobacco products by tobacco companies. In the USA, Philip Morris International submitted a modified risk tobacco product (MRTP) application to the US Food and Drug Administration in 2016 in which it purports that its heated tobacco product, I-Quit-Ordinary-Smoking (IQOS), is associated with reduced harm compared with conventional cigarettes.

Methods

We reviewed Philip Morris International’s MRTP application to assess the pulmonary and immune toxicities associated with IQOS use in both animal and human studies.

Results

Among rats exposed to IQOS, there was evidence of pulmonary inflammation and immunomodulation. In human users, there was no evidence of improvement in pulmonary inflammation or pulmonary function in cigarette smokers who were switched to IQOS.

Conclusion

IQOS is associated with significant pulmonary and immunomodulatory toxicities with no detectable differences between conventional cigarette smokers and those who were switched to IQOS in Philip Morris International’s studies. Philip Morris International also failed to consider how dual use and secondhand aerosol exposure may further impact, and likely increase, the harms associated with these products.

Soluble antigens from the neurotropic pathogen Angiostrongylus cantonensis directly induce thymus atrophy in a mouse model

The nematode Angiostrongylus cantonensis (A.C.) is a neurotropic pathogen; stage-III larva invade the human (non-permissive host) central nervous system (CNS) to cause eosinophilic meningitis or meningoencephalitis accompanied by immunosuppression. In an A.C.-infectedmouse (another non-permissive host) model, CNS damage-associated T cell immune deficiency and severe inflammation were proposed to result from activation of the hypothalamic-pituitary-adrenal (HPA) axis. However, glucocorticoids are anti-inflammatory agents. Additionally, while defects in thymic stromal/epithelial cells (TECs) are the major reason for thymic atrophy, TECs do not express the glucocorticoid receptor. Therefore, activation of the HPA axis cannot fully explain the thymic atrophy and inflammation. Using an A.C.-infected mouse model, we found that A.C.-infected mice developed severe thymic atrophy with dramatic impairments in thymocytes and TECs, particularly cortical TECs, which harbor CD4⁺CD8⁺ double-positive thymocytes. The impairments resulted from soluble antigens (sAgs) from A.C. in the thymuses of infected mice, as intrathymic injection of these sAgs into live mice and the addition of these sAgs to thymic cell culture resulted in thymic atrophy and cellular apoptosis, respectively. Therefore, in addition to an indirect effect on thymocytes through the HPA axis, our study reveals a novel mechanism by which A.C. infection in non-permissive hosts directly induces defects in both thymocytes and TECs via soluble antigens.

Acute Thymic Involution and Mechanisms for Recovery

Acute thymic involution (ATI) is usually regarded as a virulence trait. It is caused by several infectious agents (bacteria, viruses, parasites, fungi) and other factors, including stress, pregnancy, malnutrition and chemotherapy. However, the complex mechanisms that operate during ATI differ substantially from each other depending on the causative agent. For instance, a transient reduction in the size and weight of the thymus and depletion of populations of T cell subsets are hallmarks of ATI in many cases, whereas severe disruption of the anatomical structure of the organ is also associated with some factors, including fungal, parasitic and viral infections. However, growing evidence shows that ATI may be therapeutically halted or reversed. In this review, we highlight the current progress in this field with respect to numerous pathological factors and discuss the possible mechanisms. Moreover, these new observations also show that ATI can be mechanistically reversed.

The effects of aging and maternal protein restriction during lactation on thymic involution and peripheral immunosenescence in adult mice

Environmental factors such as nutrition during early life can influence long-term health, a concept termed developmental programming. Initial research was focused towards the effects on metabolic health but more recent studies have demonstrated effects on parameters such as lifespan and immunity. In this study we report that maternal protein restriction during lactation in mice, that is known to prolong lifespan, slows aging of the central and peripheral immune systems. Offspring of dams fed a postnatal low-protein (PLP) diet during lactation had a significant increase in thymic cellularity and T cell numbers across their lifespan compared to controls, and a less marked age-associated decrease in thymocyte cluster of differentiation (CD) 3 expression. PLP animals also demonstrated increased relative splenic cellularity, increased naïve: memory CD4⁺ and CD8⁺ T cell ratios, increased staining and density of germinal centres, and decreased gene expression of p16 in the spleen, a robust biomarker of aging. A slower rate of splenic aging in PLP animals would be expected to result in decreased susceptibility to infection and neoplasia. In conclusion nutritionally-induced slow postnatal growth leads to delayed aging of the adaptive immune system, which may contribute towards the extended lifespan observed in these animals.

MicroRNA-181a-5p enhances cell proliferation in medullary thymic epithelial cells via regulating TGF-β signaling

The expression profiles of miRNAs in thymus tissues from mice of different age have been demonstrated in our previous study. After an integrated analysis of the miRNA expression profiles, we demonstrated that the expression of miR-181a-5p was significantly decreased in thymic epithelial cells (TECs) from 10- to 19-month-old mice when compared with that in TECs from 1-month-old mice by quantitative reverse transcriptase polymerase chain reaction. We hypothesized that miR-181a-5p in TECs might be associated with the age-related thymus involution through regulating some genes or signaling pathway. To test this hypothesis, the mouse medullary thymic epithelial cells (MTEC1) were used. Transfection with miR-181a-5p mimic promoted the proliferation of MTEC1 cells, but did not affect apoptosis. The effect was reversed when the expression of miR-181a-5p was suppressed in MTEC1 cells. Furthermore, the transforming growth factor beta receptor I (Tgfbr1) was confirmed as a direct target of miR-181a-5p by luciferase assay. Moreover, it was found that overexpression of miR-181a-5p down-regulated the phosphorylation of Smad3 and blocked the activation of the transforming growth factor beta signaling. Nevertheless, an inversely correlation was observed between the expression of Tgfbr1 and miR-181a-5p in TECs derived from mice of different age. Collectively, we provide evidence that miR-181a-5p may be an important endogenous regulator in the proliferation of TECs, and the expression levels of miR-181a-5p in TECs may be associated with the age-related thymus involution.

Thymus: the next (re)generation

As the primary site of T-cell development, the thymus plays a key role in the generation of a strong yet self-tolerant adaptive immune response, essential in the face of the potential threat from pathogens or neoplasia. As the importance of the role of the thymus has grown, so too has the understanding that it is extremely sensitive to both acute and chronic injury. The thymus undergoes rapid degeneration following a range of toxic insults, and also involutes as part of the aging process, albeit at a faster rate than many other tissues. The thymus is, however, capable of regenerating, restoring its function to a degree. Potential mechanisms for this endogenous thymic regeneration include keratinocyte growth factor (KGF) signaling, and a more recently described pathway in which innate lymphoid cells produce interleukin-22 (IL-22) in response to loss of double positive thymocytes and upregulation of IL-23 by dendritic cells. Endogenous repair is unable to fully restore the thymus, particularly in the aged population, and this paves the way toward the need for exogenous strategies to help regenerate or even replace thymic function. Therapies currently in clinical trials include KGF, use of the cytokines IL-7 and IL-22, and hormonal modulation including growth hormone administration and sex steroid inhibition. Further novel strategies are emerging in the preclinical setting, including the use of precursor T cells and thymus bioengineering. The use of such strategies offers hope that for many patients, the next regeneration of their thymus is a step closer.

Insights into thymic involution in tumor-bearing mice

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

Vaccine responsiveness in the elderly: best practice for the clinic

The success of vaccines developed since the beginning of the 20th century, has enabled the conquest of several childhood diseases preventing death and or disability for millions of children. But, globally, the number of children will soon be surpassed by the number of adults over the age of 65. The active lifestyle of these older individuals, coupled with a degree of immune deficiency recognised within this population will lead to a change in the profile of diseases affecting the elderly. The challenge for policy makers and also those involved in primary healthcare is how to protect this population from communicable diseases and keep them healthy, autonomous and independent when vaccines in the main have been developed for use on children and young adults.

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.

Genetic regulation of thymocyte progenitor aging

The number of T cell progenitors is significantly reduced in the involuted thymus, and the growth and developmental potential of the few cells that are present is severely attenuated. This review provides an overview of how aging affects T cell precursors before and following entry into the thymus and discusses the age-related genetic changes that may occur in them. Finally, interventions that rejuvenate thymopoiesis in the elderly by targeting T cell progenitors are discussed.

Chronic resveratrol intake reverses pro-inflammatory cytokine profile and oxidative DNA damage in ageing hybrid mice

Thymic involution and shrinkage of secondary lymphoid organs are leading causes of the deterioration of the T-cell compartment with age. Inflamm-aging, a sustained inflammatory status, has been associated with chronic diseases and shortened longevity. This is the first study to investigate the effect of treating aging hybrid mice with long-term, low-dose resveratrol (RSV) in drinking water by assessing multiple immunological markers and profiles in the immune system. We found that hybrid mice exhibited marked age-related changes in the CD3+CD4+, C3+CD8+, CD4+CD25+, CD4M and CD8M surface markers. RSV reversed surface phenotypes of old mice to that of young mice by maintaining the CD4+ and CD8+ population in splenocytes as well as reducing CD8+CD44+ (CD8M) cells in the aged. RSV also enhanced the CD4+CD25+ population in old mice. Interestingly, pro-inflammatory status in young mice was transiently elevated by RSV but it consequently mitigated the age-dependent increased pro-inflammatory cytokine profile while preserving the anti-inflammatory cytokine condition in the old mice. Age-dependent increase in 8OHdG, an oxidative DNA damage marker was ameliorated by RSV. Immunological-focused microarray gene expression analysis showed that only the CD72 gene was significantly downregulated in the 12-month RSV-treated mice compared to age-matched controls. Our study indicates that RSV even at low physiological relevant levels is able to affect the immune system without causing marked gene expression changes.

Expression of p16(INK4a) prevents cancer and promotes aging in lymphocytes

Previous authors have suggested that tumor suppressor expression promotes aging while preventing cancer, but direct experimental support for this cancer-aging hypothesis has been elusive. Here, by using somatic, tissue-specific inactivation of the p16(INK4a) tumor suppressor in murine T- or B-lymphoid progenitors, we report that ablation of p16(INK4a) can either rescue aging or promote cancer in a lineage-specific manner. Deletion of p16(INK4a) in the T lineage ameliorated several aging phenotypes, including thymic involution, decreased production of naive T cells, reduction in homeostatic T-cell proliferation, and attenuation of antigen-specific immune responses. Increased T-cell neoplasia was not observed with somatic p16(INK4a) inactivation in T cells. In contrast, B lineage-specific ablation of p16(INK4a) was associated with a markedly increased incidence of systemic, high-grade B-cell neoplasms, which limited studies of the effects of somatic p16(INK4a) ablation on B-cell aging. Together, these data show that expression of p16(INK4a) can promote aging and prevent cancer in related lymphoid progeny of a common stem cell.

Reversal of age-related thymic involution by an LHRH agonist in miniature swine

BACKGROUND AND AIMS OF STUDY: We have previously demonstrated a requirement for the presence of a juvenile thymus for the induction of transplantation tolerance to renal allografts by a short-course of calcineurin inhibition in miniature swine. We have also shown that aged, involuted thymi can be rejuvenated when transplanted as vascularized thymic lobes into juvenile swine recipients. The present studies were aimed at elucidating the extrinsic factors facilitating this restoration of function in the aged thymus. In particular, we tested the impact of sex steroid blockade by Luteinizing Hormone-Releasing Hormone (LHRH).

MATERIALS AND METHODS

30 naive animals (25 males and 5 females) were used for measurement of serum testosterone levels. 3 mature male pigs (aged at 22, 22 and 29 months old) were used to test the effects of Lupron (LHRH analog) injection at 45 mg (per 70-80 kg body weight) as a 3-month depot on testosterone levels and thymic rejuvenation. Thymic rejuvenation was assessed by histology, flow cytometric analysis, morphometric analysis and TREC assays.

RESULTS

Hormonal alterations were induced by Lupron and resulted in macroscopic and histologic regeneration of the thymus of aged animals within 2 months, as evidenced by restoration of juvenile thymus architecture and increased cellularity. Two animals that were evaluated for TREC both showed increased levels in the periphery following Lupron treatment.

CONCLUSION

Treatment of aged animals with Lupron leads to thymic rejuventaion in adult miniature swine. This result could expand the applicability of thymus-dependent tolerance-inducing regimens to adult recipients.

Innate and adaptive immunosenescence

OBJECTIVE

To review the effect of increasing age on the immune system and some of its clinical implications.

DATA SOURCES

MEDLINE and PubMed searches were performed cross-referencing the keywords immunosenescence, aging, and immunity. Articles were reviewed for additional citations.

STUDY SELECTION

Articles were reviewed and selected based on relevance to subject matter.

RESULTS

The study of immunosenescence is complex and not completely understood. Aging affects both the innate and adaptive arms of the immune response. With increased age, there may be a decrease in phagocytosis, alteration of cellular migration, changes in cell populations and numbers, and a decreased ability to produce specific antibodies. Clinically, these changes potentially increase morbidity and mortality in elderly individuals through an increased rate of infections, malignancy, and autoimmunity.

CONCLUSIONS

The process of aging is accompanied by diverse changes in immunity. Several therapeutic approaches are under investigation, including cytokine therapy, hormonal replacement, antioxidant supplementation, and caloric restriction, to attenuate or potentially reverse immunosenescence.

Declining expression of a single epithelial cell-autonomous gene accelerates age-related thymic involution

Age-related thymic involution may be triggered by gene expression changes in lymphohematopoietic and/or nonhematopoietic thymic epithelial cells (TECs). The role of epithelial cell-autonomous gene FoxN1 may be involved in the process, but it is still a puzzle because of the shortage of evidence from gradual loss-of-function and exogenous gain-of-function studies. Using our recently generated loxP-floxed-FoxN1(fx) mouse carrying the ubiquitous CreER(T) (uCreER(T)) transgene with a low dose of spontaneous activation, which causes gradual FoxN1 deletion with age, we found that the uCreER(T)-fx/fx mice showed an accelerated age-related thymic involution owing to progressive loss of FoxN1(+) TECs. The thymic aging phenotypes were clearly observable as early as at 3-6 months of age, resembling the naturally aged (18-22-month-old) murine thymus. By intrathymically supplying aged wild-type mice with exogenous FoxN1-cDNA, thymic involution and defective peripheral CD4(+) T-cell function could be partially rescued. The results support the notion that decline of a single epithelial cell-autonomous gene FoxN1 levels with age causes primary deterioration in TECs followed by impairment of the total postnatal thymic microenvironment, and potentially triggers age-related thymic involution in mice.

Sex steroid ablation enhances immune reconstitution following cytotoxic antineoplastic therapy in young mice

Cytotoxic antineoplastic therapy is used to treat malignant disease but results in long-term immunosuppression in postpubertal and adult individuals, leading to increased incidence and severity of opportunistic infections. We have previously shown that sex steroid ablation (SSA) reverses immunodeficiencies associated with age and hematopoietic stem cell transplantation in both autologous and allogeneic settings. In this study, we have assessed the effects of SSA by surgical castration on T cell recovery of young male mice following cyclophosphamide treatment as a model for the impact of chemotherapy. SSA increased thymic cellularity, involving all of the thymocyte subsets and early T lineage progenitors. It also induced early repair of damage to the thymic stromal microenvironment, which is crucial to the recovery of a fully functional T cell-based immune system. These functional changes in thymic stromal subsets included enhanced production of growth factors and chemokines important for thymopoiesis, which preceded increases in both thymocyte and stromal cellularity. These effects collectively translated to an increase in peripheral and splenic naive T cells. In conclusion, SSA enhances T cell recovery following cyclophosphamide treatment of mice, at the level of the thymocytes and their stromal niches. This provides a new approach to immune reconstitution following antineoplastic therapy.

Early-life nutrition influences thymic growth in male mice that may be related to the regulation of longevity

Nutrition and growth rate during early life can influence later health and lifespan. We have demonstrated previously that low birthweight, resulting from maternal protein restriction during pregnancy followed by catch-up growth in rodents, was associated with shortened lifespan, whereas protein restriction and slow growth during lactation increased lifespan. The underlying mechanisms by which these differences arise are unknown. In the present study, we report that maternal protein restriction in mice influences thymic growth in early adult life. Offspring of dams fed a low-protein diet during lactation (PLP offspring) had significant thymic growth from 21 days to 12 weeks of age, whereas this was not observed in control mice or offspring of dams fed a low-protein diet during pregnancy (recuperated offspring). PCNA (proliferating-cell nuclear antigen) and SIRT1 (silent information regulator 1) protein levels at 21 days of age were significantly higher in the thymus from both PLP mice (P<0.001 and P<0.05 respectively) and recuperated mice (P<0.001 and P<0.01 respectively) compared with controls. At 12 weeks, PLP mice maintained a higher SIRT1 level, whereas PCNA was decreased in the thymus from recuperated offspring. This suggests that mitotic activity was initially enhanced in the thymus from both PLP and recuperated offspring, but remained sustained into adulthood only in PLP mice. The differential mitotic activity in the thymus from PLP and recuperated mice appeared to be influenced by changes in sex hormone concentrations and the expression of p53, p16, the androgen receptor, IL-7 (interleukin-7) and the IL-7 receptor. In conclusion, differential thymic growth may contribute to the regulation of longevity by maternal diet.

The effect of age on the phenotype and function of developing thymocytes

The immune system declines with age leading to a progressive deterioration in the ability to respond to infection and vaccination. Age-associated thymic involution is one of the most recognized changes in the ageing immune system and is believed to be a major contributor towards immunosenescence; however, the precise mechanisms involved in age-associated thymic involution remain unclear. In order to gain further insight into the effect of ageing on T-cell development, steady-state thymopoiesis was studied in mice ranging from 1 to 18 months of age. There was a decrease in thymic cellularity with age, but the most dramatic loss occurred early in life. Although there were no alterations in the proportion of the major thymocyte subsets, there was a significant decline in the expression of other key molecules including CD3 and CD24. There was a decline in the ability of thymocytes from older mice to respond to mitogens, which was demonstrated by a failure to up-regulate expression of the activation marker CD69 and to enter the G(2)--M phase of the cell cycle. This was concurrent with an increased resistance to apoptosis in thymocytes from aged animals. Together, these results suggest that T cells may be flawed even before exiting to the periphery and that this could contribute to the age-associated decline in immune function.

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

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

An evolutionary perspective on the mechanisms of immunosenescence

There is an accumulating body of evidence that a decline in immune function with age is common to most if not all vertebrates. For instance, age-associated thymic involution seems to occur in all species that possess a thymus, indicating that this process is evolutionary ancient and conserved. The precise mechanisms regulating immunosenescence remain to be resolved, but much of what we do know is consistent with modern evolutionary theory. In this review, we assess our current knowledge from an evolutionary perspective on the occurrence of immunosenescence, we show that life history trade-offs play a key role and we highlight the possible advantages of the age-related decline in thymic function.

The role of stress factors during aging of the immune system

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

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.

Strategies for reconstituting and boosting T cell-based immunity following haematopoietic stem cell transplantation: pre-clinical and clinical approaches

Poor immune recovery is characteristic of bone marrow transplantation and leads to high levels of morbidity and mortality. The primary underlying cause is a compromised thymic function, resulting from age-induced atrophy and further compounded by the damaging effects of cytoablative conditioning regimes on thymic epithelial cells (TEC). Several strategies have been proposed to enhance T cell reconstitution. Some, such as the use of single biological agents, are currently being tested in clinical trials. However, a more rational approach to immune restoration will be to leverage the evolving repertoire of new technologies. Specifically, the combined targeting of TEC, thymocytes and peripheral T cells, together with the bone marrow niches, promises a more strategic clinical therapeutic platform.

Phenotypical and morphological changes in the thymic microenvironment from ageing mice

The thymus is crucial for T-cell output and the age-associated involution of this organ, is thought to have a major impact in the decline in immunity that is seen in later life. The mechanism that underlines thymic involution is not known, however, we have evidence to suggest that this is may be due to changes in the thymic microenvironment. To further test this hypothesis, we quantified the in situ changes to markers that identify cortical and medullary thymic epithelial cells. This analysis revealed an age-dependent decline in cortical and medullary markers together with an increase in Notch and Delta expression, in older mice, as judged by immunohistochemistry. This was accompanied by alterations of the archetypal staining patterns and three dimensional analysis revealed changes in the morphology of the thymic microenvironment. These studies suggest that there are age-associated alterations in the thymic microenvironment, which may therefore play a role in thymic involution.

Overexpression of lymphotoxin in T cells induces fulminant thymic involution

Activated lymphocytes and lymphoid-tissue inducer cells express lymphotoxins (LTs), which are essential for the organogenesis and maintenance of lymphoreticular microenvironments. Here we describe that T-cell-restricted overexpression of LT induces fulminant thymic involution. This phenotype was prevented by ablation of the LT receptors tumor necrosis factor receptor (TNFR) 1 or LT beta receptor (LTbetaR), representing two non-redundant pathways. Multiple lines of transgenic Ltalphabeta and Ltalpha mice show such a phenotype, which was not observed on overexpression of LTbeta alone. Reciprocal bone marrow transfers between LT-overexpressing and receptor-ablated mice show that involution was not due to a T cell-autonomous defect but was triggered by TNFR1 and LTbetaR signaling to radioresistant stromal cells. Thymic involution was partially prevented by the removal of one allele of LTbetaR but not of TNFR1, establishing a hierarchy in these signaling events. Infection with the lymphocytic choriomeningitis virus triggered a similar thymic pathology in wt, but not in Tnfr1(-/-) mice. These mice displayed elevated TNFalpha in both thymus and plasma, as well as increased LTs on both CD8(+) and CD4(-)CD8(-) thymocytes. These findings suggest that enhanced T cell-derived LT expression helps to control the physiological size of the thymic stroma and accelerates its involution via TNFR1/LTbetaR signaling in pathological conditions and possibly also in normal aging.

Immune activation and inflammation in HIV-1 infection: causes and consequences

Thorough research on HIV is progressively enabling us to understand the intricate mechanisms that link HIV-1 infection to the onset of immunodeficiency. The infection and depletion of CD4(+) T cells represent the most fundamental events in HIV-1 infection. However, in recent years, the role played by chronic immune activation and inflammation in HIV pathogenesis has become increasingly apparent: quite paradoxically, immune activation levels are directly associated with HIV-1 disease progression. In addition, HIV-1-infected patients present intriguing similarities with individuals of old age: their immune systems are characterized by a loss of regenerative capacity and an accumulation of ageing T cells. In this review, we discuss the potential reasons for the establishment of sustained immune activation and inflammation from the early stages of HIV-1 infection, as well as the long-term consequences of this process on the host immune system and health. A simplified model of HIV pathogenesis is proposed, which links together the three major facets of HIV-1 infection: the massive depletion of CD4(+) T cells, the paradoxical immune activation and the exhaustion of regenerative capacity.

Architectural changes in the thymus of aging mice

Age-associated thymic involution is one of the most dramatic and ubiquitous changes in the immune system, although the precise mechanisms involved still remain obscured. Several hypotheses have been proposed incorporating extrinsic and intrinsic factors, however, changes in the thymic microenvironment itself is one of the least investigated. We therefore decided to undertake a detailed histological examination of the aging thymus in order to elucidate possible mechanisms of thymic atrophy. This investigation provides insight into the changes within the murine thymus with age, demonstrating a new approach to quantify protein expressional differences while preserving the thymic architecture. There is a decline in expression of thymic epithelial cell-specific makers and an increase in fibroblast content in the aging mouse thymus. This is concurrent with a disorganization of the thymic compartments, a morphological transformation within the epithelial cells and alterations of their archetypal staining patterns. Furthermore, this is linked to a rise in apoptotic cells and the novel finding of increased senescence in the thymus of older mice that appears to be colocalized in the epithelial compartment. These changes within the thymic epithelial cells may be in part accountable for thymic atrophy and responsible for the decline in T-cell output.

Diminished response to tick-borne encephalitis vaccination in thymectomized children

In order to analyze the clinical impact of immunological alterations in thymectomized children after exposure to a new antigen (tick-borne encephalitis virus (TBEV) vaccine), 17 thymectomized children completed a three-dose immunization regimen. Thymectomized children showed significantly lower TBEV IgG antibody levels after the second vaccination when compared to healthy age-matched controls (n=30) (p=0.03), but a normal response after the third vaccination. Age at thymectomy correlated significantly with the TBEV IgG antibody levels (p=0.04). Thymectomized children also showed significantly lower total counts and percentages for naïve T cells correlating with time after thymectomy (p=0.02), than observed for controls. These changes in T cell subsets and the decreased ability to respond to new antigens in thymectomized children, as observed here, may precede more striking effects such as higher infection rates or autoimmune conditions as they age.

Lymphohematopoietic progenitors do not have a synchronized defect with age-related thymic involution

It has been speculated that aging lymphohematopoietic progenitor cells (LPC) including hematopoietic stem cells (HSC) and early T-cell progenitors (ETP) have intrinsic defects that trigger age-related thymic involution. However, using a different approach, we suggest that that is not the case. We provided a young thymic microenvironment to aged mice by transplanting a fetal thymus into the kidney capsule of aged animals, and demonstrated that old mouse-derived LPCs could re-establish normal thymic lymphopoiesis and all thymocyte subpopulations, including ETPs, double negative subsets, double positive, and CD4(+) and CD8(+) single positive T cells. LPCs derived from aged mice could turn over young RAG(-/-) thymic architecture by interactions, as well as elevate percentage of peripheral CD4(+)IL-2(+) T cells in response to costimulator in aged mice. Conversely, intrathymic injection of ETPs sorted from young animals into old mice did not restore normal thymic lymphopoiesis, implying that a shortage and/or defect of ETPs in aged thymus do not account for age-related thymic involution. Together, our findings suggest that the underlying cause of age-related thymic involution results primarily from changes in the thymic microenvironment, causing extrinsic, rather than intrinsic, defects in T-lymphocyte progenitors.

The aged thymus shows normal recruitment of lymphohematopoietic progenitors but has defects in thymic epithelial cells

Aging is associated with reduced numbers of all thymocyte sub-populations, including early T-cell progenitors. However, it is unclear if this is due to inadequate recruitment of lymphohematopoietic progenitor cells (LPCs) to the aged thymus or to abnormal development of T cells within the thymus. We found that LPCs from young mice were recruited equally well to the thymi of young or aged mice and that thymic stromal cells (TSCs) from young and old mice expressed similar levels of P-selectin and CCL25, which are believed to mediate recruitment of LPCs to the adult thymus. However, the number of recruited thymocytes in old thymus was markedly reduced after two weeks, indicating that T-cell development or proliferation is defective in the aged thymus. We also found that LPCs from aged and young mice have similar capacities to seed a fetal thymus that was transplanted under the kidney capsule. Thymic epithelial cells (TECs) in aged mice had lower proliferative capacity and higher rate of apoptosis, compared with findings in young animals. In addition, immunofluorescence staining with antibodies to cortical and medullary TECs revealed that aged thymi had a disorganized thymic stromal architecture, combined with reduced cellularity of the medulla, and apoptosis of thymocyte sub-populations in the medullary microenvironment was increased, compared with that in young mice. We conclude that aging does not impair recruitment of LPCs to the thymus, but is characterized by abnormalities in thymic epithelial architecture, especially medullary TEC function that may provide sub-optimal support for thymic development of LPCs.

Enhanced immune reconstitution by sex steroid ablation following allogeneic hemopoietic stem cell transplantation

Delayed immune reconstitution in adult recipients of allogeneic hemopoietic stem cell transplantations (HSCT) is related to age-induced thymic atrophy. Overcoming this paucity of T cell function is a major goal of clinical research but in the context of allogeneic transplants, any strategy must not exacerbate graft-vs-host disease (GVHD) yet ideally retain graft-vs-tumor (GVT) effects. We have shown sex steroid ablation reverses thymic atrophy and enhances T cell recovery in aged animals and in congenic bone marrow (BM) transplant but the latter does not have the complications of allogeneic T cell reactivity. We have examined whether sex steroid ablation promoted hemopoietic and T cell recovery following allogeneic HSCT and whether this benefit was negated by enhanced GVHD. BM and thymic cell numbers were significantly increased at 14 and 28 days after HSCT in castrated mice compared with sham-castrated controls. In the thymus, the numbers of donor-derived thymocytes and dendritic cells were significantly increased after HSCT and castration; donor-derived BM precursors and developing B cells were also significantly increased. Importantly, despite restoring T cell function, sex steroid inhibition did not exacerbate the development of GVHD or ameliorate GVT activity. Finally, IL-7 treatment in combination with castration had an additive effect on thymic cellularity following HSCT. These results indicate that sex steroid ablation can profoundly enhance thymic and hemopoietic recovery following allogeneic HSCT without increasing GVHD and maintaining GVT.

Immunosenescence: emerging challenges for an ageing population

It is now becoming apparent that the immune system undergoes age-associated alterations, which accumulate to produce a progressive deterioration in the ability to respond to infections and to develop immunity after vaccination, both of which are associated with a higher mortality rate in the elderly. Immunosenescence, defined as the changes in the immune system associated with age, has been gathering interest in the scientific and health-care sectors alike. The rise in its recognition is both pertinent and timely given the increasing average age and the corresponding failure to increase healthy life expectancy. This review attempts to highlight the age-dependent defects in the innate and adaptive immune systems. While discussing the mechanisms that contribute to immunosenescence, with emphasis on the extrinsic factors, particular attention will be focused on thymic involution. Finally, we illuminate potential therapies that could be employed to help us live a longer, fuller and healthier life.

Thymic rejuvenation and the induction of tolerance by adult thymic grafts

The thymus, the site of origin of T cell immunity, shapes the repertoire of T cell reactivity through positive selection of developing T cells and prevents autoimmunity through negative selection of autoreactive T cells. Previous studies have demonstrated an important role for the thymus not only in central deletional tolerance, but also in the induction of peripheral tolerance by vascularized renal allografts in juvenile miniature swine recipients. The same protocol did not induce tolerance in thymectomized recipients nor in recipients beyond the age of thymic involution. We subsequently reported that vascularized thymic lobe grafts from juvenile donors were capable of inducing tolerance in thymectomized juvenile hosts. However, the important question remained whether aged, involuted thymus could also induce tolerance if transplanted into thymectomized hosts, which, if true, would imply that thymic involution is not an intrinsic property of thymic tissue but is rather determined by host factors extrinsic to the thymus. We report here that aged, involuted thymus transplanted as a vascularized graft into juvenile recipients leads to rejuvenation of both thymic structure and function, suggesting that factors extrinsic to the thymus are capable of restoring juvenile thymic function to aged recipients. We show furthermore that rejuvenated aged thymus has the ability to induce transplant tolerance across class I MHC barriers. These findings indicate that it may be possible to manipulate thymic function in adults to induce transplantation tolerance after the age of thymic involution.

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.

Phenotype of genetically regulated thymic involution in young BXD RI strains of mice

Age-related thymic involution is a multifactorial process related to age-related changes in intrathymic T-cell development and cytokines. In contrast, early thymic involution, because of genetic differences that cause rapid or slow thymic involution at younger age, is less well characterized. Here, we analysed three representative rapid-involuting strains of mice, BXD 8, 18 and 32, compared with three representative slow-involuting strains, BXD 9, 19 and 29, all at 2 months of age. In rapid-involuting strains compared with slow involution strains, thymocyte production, as indicated by CD4+ and CD8+ T-cell receptor recombination excision circle (TREC), were decreased. Rapid-involution strains of mice exhibited a developmental block at the DN1 to DN2 and CD4-CD8- (DN) to CD4+CD8+ (double positive, DP) transition stages. There was also increased susceptibility to H2O2-induced apoptosis, decreased thymic expression of IL-7, decreased expression of an IL-7 downstream anti-apoptosis gene, Bcl-2, and increased expression of a pro-apoptotic gene, Bad. In contrast, IL-7R expression was higher on DN thymocytes of rapid-involution strains. The increased expression of IL-7R was associated with an increased thymocyte proliferation in response to anti-CD3 + IL-7 or anti-CD3 + IL-12 + IL-7. These findings indicate that, even at young age, genetic differences of IL-7/IL-7R regulation pathway in BXD strains of mice can lead to characteristic phenotypic changes that have been previously associated with age-related thymic involution.

Sex Steroid Ablation Enhances Lymphoid Recovery Following Autologous Hematopoietic Stem Cell Transplantation

BACKGROUND

Autologous hematopoietic stem cell transplantation (auto-HSCT) patients experience long-term immunosuppression, which increases susceptibility to infection and relapse rates due to minimal residual disease (MRD). Sex steroid (SS) ablation is known to reverse age-related thymic atrophy and decline in B-cell production

METHODS

This study used a congenic HSCT mouse model to analyze the effects of SS ablation (through surgical castration) on immune reconstitution and growth factor production following auto-HSCT. Bone marrow (BM) and thymic stromal cell (TSCs) populations were analyzed using RT-PCR and were tested for the production of growth factors previously implicated in immune reconstitution or age-relate immune degeneration

RESULTS

Castration increased bone marrow (BM), thymic, and splenic cellularity following auto-HSCT. HSC number and common lymphoid precursor (CLP) frequency and number were increased in castrated mice. B cell precursor numbers were also significantly increased in the BM of these mice. Triple negative, double positive and single positive thymocytes were increased following HSCT and castration, as were thymic dendritic cells and natural killer T (NKT) cells. This enhanced lymphoid reconstitution of the primary immune organs leads to a significant increase in splenic T and B cells 42 days after HSCT. The molecular mechanisms behind the enhanced reconstitution were also studied. TGF-beta1 was decreased in castrated mice compared to sham-castrated controls in TSCs and BM cells. TSC production of IL-6 was also decreased in castrated mice

CONCLUSIONS

These data suggest that sex steroid ablation significantly enhances lymphopoiesis following auto-HSCT providing a new strategy for posttransplant immune reconstitution.

Effects of aging on early B- and T-cell development

Lymphocyte production in the bone marrow and the thymus is reduced during aging, but why this decline occurs has not been fully elucidated. The ability to isolate hematopoietic stem and progenitor cells using sophisticated flow cytometric strategies and to manipulate them in vitro and in vivo has provided insights into the effects of aging on primary lymphopoiesis. These analyses have showed that intrinsic changes in hematopoietic precursors that affect their proliferative potential are one factor that contributes to the age-related decline in B- and T-cell production. This and other age-related defects may be exacerbated by changes in the lymphopoietic support potential of the bone marrow and thymic microenvironments as well as by age-induced fluctuations in the production of various endocrine hormones. Particular attention with regard to the latter point has focused on changes in the production of sex steroids, growth hormone, and insulin-like growth factor-I. The present review summarizes recent studies of how age-related perturbations affect primary lymphopoiesis and highlights how the data necessitate the reevaluation of a number of existing paradigms.

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.

Strategies to enhance T-cell reconstitution in immunocompromised patients

Immune deficiency, together with its associated risks such as infections, is becoming an increasingly important clinical problem owing to the ageing of the general population and the increasing number of patients with HIV/AIDS, malignancies (especially those treated with intensive chemotherapy or radiotherapy) or transplants (of either solid organs or haematopoietic stem cells). Of all immune cells, T cells are the most often affected, leading to a prolonged deficiency of T cells, which has important clinical consequences. Accordingly, strategies to improve the recovery and function of T cells, as we discuss here, should have a direct impact on reducing the morbidity and mortality of many patients and should increase the efficacy of therapeutic and prophylactic vaccinations against microbial pathogens or tumours.