The pattern of T lymphocyte differentiation is altered during thymic involution

Neuroanatomy of lymphoid organs: Lessons learned from whole-tissue imaging studies

Decades of extensive research have documented the presence of neural innervations of sensory, sympathetic, or parasympathetic origin in primary and secondary lymphoid organs. Such neural inputs can release neurotransmitters and neuropeptides to directly modulate the functions of various immune cells, which represents one of the essential aspects of the body's neuroimmune network. Notably, recent studies empowered by state-of-the-art imaging techniques have comprehensively assessed neural distribution patterns in BM, thymus, spleen, and LNs of rodents and humans, helping clarify several controversies lingering in the field. In addition, it has become evident that neural innervations in lymphoid organs are not static but undergo alterations in pathophysiological contexts. This review aims to update the current information on the neuroanatomy of lymphoid organs obtained through whole-tissue 3D imaging and genetic approaches, focusing on anatomical features that may designate the functional modulation of immune responses. Moreover, we discuss several critical questions that call for future research, which will advance our in-depth understanding of the importance and complexity of neural control of lymphoid organs.

What's the role of thymus in diabetes mellitus?

Diabetes mellitus is considered as an autoimmune inflammatory and age-related disease. As an important immune organ, the thymus is involved in the immune response and inflammatory response process. Therefore, there may be a link between changes in thymus function and diabetes. Based on previous studies, we hypothesized that thymus dysfunction due to aging and other reasons leads to changes in the generation of various inflammatory-immune cells and inflammatory cytokines that regulate insulin resistance, and then participates in the development of diabetes and its complications. Therefore, thymus may be a key factor in diabetes and complications, and it may be a promising therapeutic strategy to improve the thymus function for patients with diabetes. The purpose of this review is to summarize and discuss recent advances in the influence of thymus function on diabetes, especially its potential mechanisms.

Thymus Reconstitution in Young and Aged Mice Is Facilitated by In Vitro-Generated Progenitor T Cells

The prolonged lag in T cell recovery seen in older patients undergoing hematopoietic stem cell transplant (HSCT), after chemo-/radiotherapy, can lead to immune dysfunction. As a result, recovering patients may experience a relapse in malignancies and opportunistic infections, leading to high mortality rates. The delay in T cell recovery is partly due to thymic involution, a natural collapse in the size and function of the thymus, as individuals age, and partly due to the damage sustained by the thymic stromal cells through exposure to chemo-/radiotherapy. There is a clear need for new strategies to accelerate intrathymic T cell reconstitution when treating aged patients to counter the effects of involution and cancer therapy regimens. Adoptive transfer of human progenitor T (proT) cells has been shown to accelerate T cell regeneration in radiation-treated young mice and to restore thymic architecture in immunodeficient mice. Here, we demonstrate that the adoptive transfer of in vitro-generated proT cells in aged mice (18-24 months) accelerated thymic reconstitution after treatment with chemotherapy and gamma irradiation compared to HSCT alone. We noted that aged mice appeared to have a more limited expansion of CD4-CD8- thymocytes and slower temporal kinetics in the development of donor proT cells into mature T cells, when compared to younger mice, despite following the same chemo/radiation regimen. This suggests a greater resilience of the young thymus compared to the aged thymus. Nevertheless, newly generated T cells from proT cell engrafted aged and young mice were readily present in the periphery accelerating the reappearance of new naïve T cells. Accelerated T cell recovery was also observed in both aged and young mice receiving both proT cells and HSCT. The strategy of transferring proT cells can potentially be used as an effective cellular therapy in aged patients to improve immune recovery and reduce the risk of opportunistic infections post-HSCT.

Phosphate Transporter Profiles in Murine and Human Thymi Identify Thymocytes at Distinct Stages of Differentiation

Thymocyte differentiation is dependent on the availability and transport of metabolites in the thymus niche. As expression of metabolite transporters is a rate-limiting step in nutrient utilization, cell surface transporter levels generally reflect the cell's metabolic state. The GLUT1 glucose transporter is upregulated on actively dividing thymocytes, identifying thymocytes with an increased metabolism. However, it is not clear whether transporters of essential elements such as phosphate are modulated during thymocyte differentiation. While PiT1 and PiT2 are both phosphate transporters in the SLC20 family, we show here that they exhibit distinct expression profiles on both murine and human thymocytes. PiT2 expression distinguishes thymocytes with high metabolic activity, identifying immature murine double negative (CD4⁻CD8⁻) DN3b and DN4 thymocyte blasts as well as immature single positive (ISP) CD8 thymocytes. Notably, the absence of PiT2 expression on RAG2-deficient thymocytes, blocked at the DN3a stage, strongly suggests that high PiT2 expression is restricted to thymocytes having undergone a productive TCRβ rearrangement at the DN3a/DN3b transition. Similarly, in the human thymus, PiT2 was upregulated on early post-β selection CD4⁺ISP and TCRαβ⁻CD4^hiDP thymocytes co-expressing the CD71 transferrin receptor, a marker of metabolic activity. In marked contrast, expression of the PiT1 phosphate importer was detected on mature CD3⁺ murine and human thymocytes. Notably, PiT1 expression on CD3⁺DN thymocytes was identified as a biomarker of an aging thymus, increasing from 8.4 ± 1.5% to 42.4 ± 9.4% by 1 year of age (p < 0.0001). We identified these cells as TCRγδ and, most significantly, NKT, representing 77 ± 9% of PiT1⁺DN thymocytes by 1 year of age (p < 0.001). Thus, metabolic activity and thymic aging are associated with distinct expression profiles of the PiT1 and PiT2 phosphate transporters.

Effects of melatonin on thymic and oxidative stress dysfunctions during Trypanosoma cruzi infection

Although the exact etiology of Chagas disease is not completely elucidated, thymic atrophy and oxidative stress are believed to be important contributors to the pathogenesis during acute Trypanosoma cruzi (T. cruzi) infection. We hypothesized that exogenous melatonin, administered by gavage (5 mg/kg, p.o., gavage) to young (5 weeks old) and middle-aged (18 months old) male Wistar rats, would modulate thymic oxidative damage and reverse the age-related thymus regression during T. cruzi acute infection. Increased levels of superoxide anion (O₂^- ) were detected in the thymus of infected animals, and treatment with melatonin reverted this response. We found reduced TBARS levels as well as a significant increase in superoxide dismutase (SOD) activity in the thymus of all middle-aged melatonin-treated animals, infected or not with T. cruzi. Furthermore, melatonin increased the thymic expression of SOD1 and SOD2 in middle-aged control animals. Nox2 expression was not affected by melatonin treatment in young or middle-aged animals. Melatonin reverted the age-related thymic regression as revealed by the increase in thymus weight, total number of thymocytes, and reduction in age-related accumulation of double-negative thymocytes. This is the first report to directly examine the effects of melatonin treatment on the thymic antioxidant/oxidant status and thymic changes during T. cruzi infection. Our results revealed new antioxidant features that turn melatonin a potentially useful compound for the treatment of Chagas disease, a condition in which an excessive oxidative damage occurs.

Strain differences in thymic atrophy in rats immunized for EAE correlate with the clinical outcome of immunization

An accumulating body of evidence suggests that development of autoimmune pathologies leads to thymic dysfunction and changes in peripheral T-cell compartment, which, in turn, perpetuate their pathogenesis. To test this hypothesis, thymocyte differentiation/maturation in rats susceptible (Dark Agouti, DA) and relatively resistant (Albino Oxford, AO) to experimental autoimmune encephalomyelitis (EAE) induction was examined. Irrespective of strain, immunization for EAE (i) increased the circulating levels of IL-6, a cytokine causally linked with thymic atrophy, and (ii) led to thymic atrophy reflecting partly enhanced thymocyte apoptosis associated with downregulated thymic IL-7 expression. Additionally, immunization diminished the expression of Thy-1, a negative regulator of TCRαβ-mediated signaling and activation thresholds, on CD4+CD8+ TCRαβ^lo/hi thymocytes undergoing selection and thereby impaired thymocyte selection/survival. This diminished the generation of mature CD4+ and CD8+ single positive TCRαβ^hi thymocytes and, consequently, CD4+ and CD8+ recent thymic emigrants. In immunized rats, thymic differentiation of natural regulatory CD4+Foxp3+CD25+ T cells (nTregs) was particularly affected reflecting a diminished expression of IL-7, IL-2 and IL-15. The decline in the overall thymic T-cell output and nTreg generation was more pronounced in DA than AO rats. Additionally, differently from immunized AO rats, in DA ones the frequency of CD28- cells secreting cytolytic enzymes within peripheral blood CD4+ T lymphocytes increased, as a consequence of thymic atrophy-related replicative stress (mirrored in CD4+ cell memory pool expansion and p16^INK4a accumulation). The higher circulating level of TNF-α in DA compared with AO rats could also contribute to this difference. Consistently, higher frequency of cytolytic CD4+ granzyme B+ cells (associated with greater tissue damage) was found in spinal cord of immunized DA rats compared with their AO counterparts. In conclusion, the study indicated that strain differences in immunization-induced changes in thymopoiesis and peripheral CD4+CD28- T-cell generation could contribute to rat strain-specific clinical outcomes of immunization for EAE.

Mechanisms in hypertension and target organ damage: Is the role of the thymus key? (Review)

A variety of cells and cytokines have been shown to be involved in the whole process of hypertension. Data from experimental and clinical studies on hypertension have confirmed the key roles of immune cells and inflammation in the process. Dysfunction of the thymus, which modulates the development and maturation of lymphocytes, has been shown to be associated with the severity of hypertension. Furthermore, gradual atrophy, functional decline or loss of the thymus has been revealed to be associated with aging. The restoration or enhancement of thymus function via upregulation in the expression of thymus transcription factors forkhead box N1 or thymus transplantation may provide an option to halt or reverse the pathological process of hypertension. Therefore, the thymus may be key in hypertension and associated target organ damage, and may provide a novel treatment strategy for the clinical management of patients with hypertension in addition to different commercial drugs. The purpose of this review is to summarize and discuss the advances in our understanding of the impact of thymus function on hypertension from data from animal and human studies, and the potential mechanisms.

The Pivotal Role of Thymus in Atherosclerosis Mediated by Immune and Inflammatory Response

Atherosclerosis is one kind of chronic inflammatory disease, in which multiple types of immune cells or factors are involved. Data from experimental and clinical studies on atherosclerosis have confirmed the key roles of immune cells and inflammation in such process. The thymus as a key organ in T lymphocyte ontogenesis has an important role in optimizing immune system function throughout the life, and dysfunction of thymus has been proved to be associated with severity of atherosclerosis. Based on previous research, we begin with the hypothesis that low density lipoprotein or cholesterol reduces the expression of the thymus transcription factor Foxn1 via low density lipoprotein receptors on the membrane surface and low density lipoprotein receptor related proteins on the cell surface, which cause the thymus function decline or degradation. The imbalance of T cell subgroups and the decrease of naive T cells due to thymus dysfunction cause the increase or decrease in the secretion of various inflammatory factors, which in turn aggravates or inhibits atherosclerosis progression and cardiovascular events. Hence, thymus may be the pivotal role in coronary heart disease mediated by atherosclerosis and cardiovascular events and it can imply a novel treatment strategy for the clinical management of patients with atherosclerosis in addition to different commercial drugs. Modulation of immune system by inducing thymus function may be a therapeutic approach for the prevention of atherosclerosis. Purpose of this review is to summarize and discuss the recent advances about the impact of thymus function on atherosclerosis by the data from animal or human studies and the potential mechanisms.

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.

Reduced regulatory T cell diversity in NOD mice is linked to early events in the thymus

The thymic natural regulatory T cell (Treg) compartment of NOD mice is unusual in having reduced TCR diversity despite normal cellularity. In this study, we show that this phenotype is attributable to perturbations in early and late stages of thymocyte development and is controlled, at least in part, by the NOD Idd9 region on chromosome 4. Progression from double negative 1 to double negative 2 stage thymocytes in NOD mice is inefficient; however, this defect is compensated by increased proliferation of natural Tregs (nTregs) within the single positive CD4 thymocyte compartment, accounting for recovery of cellularity accompanied by loss of TCR diversity. This region also underlies the known attenuation of ERK-MAPK signaling, which may preferentially disadvantage nTreg selection. Interestingly, the same genetic region also regulates the rate of thymic involution that is accelerated in NOD mice. These findings highlight further complexity in the control of nTreg repertoire diversity.

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.

Improved hepatitis C treatment response in younger patients: findings from the UK HCV National Register cohort study

In a cohort of 272 treatment-naive individuals with chronic hepatitis C infection acquired on a known date who were enrolled in the UK HCV National Register, a progressive improvement in response to treatment was found with the evolution of antiviral therapies from 20% (25/122) for interferon monotherapy to 63% (55/88) for pegylated interferon+ribavirin therapy. Multivariable analysis results showed increasing age to be associated with poorer response to therapy [odds ratio (OR) 0·84, 95% confidence interval (CI) 0·72-0·99, P=0·03] whereas time since infection was not associated with response (OR 0·93, 95% CI 0·44-1·98, P=0·85). Other factors significantly associated with a positive response were non-type 1 genotype (P<0·0001) and combination therapies (P<0·0001). During the first two decades of chronic HCV infection, treatment at a younger age was found to be more influential in achieving a sustained viral response than treating earlier in the course of infection.

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.

Morphogenesis and maintenance of the 3D thymic medulla and prevention of nude skin phenotype require FoxN1 in pre- and post-natal K14 epithelium

Expansion of thymic epithelial cysts represents disruption of an organized three-dimensional (3D) thymic epithelial cell (TEC) meshwork, which is crucial for T-lymphocyte development. Although the FoxN1-null mutant develops a rudimentary two-dimensional (2D) cystic thymus, 2D thymic cyst lining resulting from a dGUO culture was reported to be FoxN1-independent; thus, it is unclear whether loss of FoxN1 facilitates cyst formation and whether FoxN1 regulates the morphogenesis and maintenance of the 3D thymic microstructure. Using the loxP-floxed-FoxN1 mouse model, we demonstrated that specific deletion of FoxN1 in keratin (K)-14 promoter-driven TECs induced the loss of 3D thymic medullary structure by producing a large number of morphologic pulmonary alveolar-like 2D epithelial cysts, which increased with age. The cystic lining was positive for differential polarized keratins and had strong claudin-3,4, but reduced MHC-II, expression. However, an increased percentage of claudin-3,4(+) TECs, which are presumptive precursors of UEA-1(+) and Aire(+) mature medullary TECs, failed to promote the development of these mature descendants. Meanwhile, the K14Cre-mediated FoxN1 deletion alone was sufficient to induce a complete hair follicle defect, causing a nude phenotype in the skin, but was not sufficient to cause a complete loss of the thymus. All these changes to occur require deletion of FoxN1 in both prenatal (Cre-recombinase from parents during fertilization) and postnatal (Cre-recombinase from offspring themselves after fertilization) life. These findings provide new insights into FoxN1 regulation of 3D thymic epithelial morphogenesis and maintenance, the distinct impacts of FoxN1 in the K14 epithelial subset of the thymus and skin, and its postnatal requirement.

Lasting effects of an impairment of Th1-like immune response in γ-irradiated mice: A resemblance between irradiated mice and aged mice

Although one of the several chronic effects of ionizing radiation is aging, there are no experimental data on radiation-induced immunological aging. The most interesting change in aging was a helper T (Th) 1/Th2 imbalance. We investigated chronic effect on immune responses after ionizing radiation and its effects in irradiated mice were compared with those of aged mice. The 2-month-old mice received a whole-body irradiation of 5Gy. At 6months after irradiation, we compared the immune functions of the irradiated mice with those of normal mice of the same age and with those of older. Interferon (IFN)-γ and antigen-specific immunoglobulin (Ig)G2a level were lower in the irradiated mice than in normal mice of same age, showing similar levels to those of old normal mice. In contrast, interleukin (IL)-4 and IL-5 and antigen-specific IgG1 level were increased in irradiated mice when compared with the same aged-normal mice. Next, we investigated the low expression of IL-12p70, IL-12 receptors and IL-18 receptors in irradiated and old mice. Also, the decrease of natural killer cell activity was intensified in the irradiated mice, showing lower than values to those of old mice. Interestingly, in irradiated mice, the absolute numbers and the percentages of natural killer (NK) cells was extremely decreased. But the absolute numbers of Th cells and cytotoxic T (Tc) cells in old mice were significantly decreased. In conclusion, an immunological imbalance by the whole-body irradiation of 5Gy induces to persist in the long term, resulting in the similar results with aging. Our results suggest that the downregulation of the Th1-like immune response shown in old mice rapidly occurred through exposure of ionizing radiation.

Age-associated changes in miRNA expression profiles in thymopoiesis

During adult life, the thymus involutes and thymic output of mature T cells drastically declines. The molecular events underlying this process are not well understood. Here, we present evidence of the importance of miRNAs in regulating T cell differentiation in the aged. miRNAs are a wide-ranging regulatory element influencing gene expression throughout the lifetime of the organism. To establish whether they play a role in the age-specific thymic decline, the miRNA expression pattern was examined in TN subsets of young and aged mice. Fifty-two percent of the miRNAs exhibited elevated expression levels in the aged TN1 cells. This expression profile leads us to hypothesize that the large number of highly expressed miRNAs, indicative of rigidly controlled protein expression, limits the developmental potential of this population and results in the age-induced decline in thymopoiesis.

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.

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.

A reliable and simplified sj/beta-TREC ratio quantification method for human thymic output measurement

Current techniques to peripherally assess thymic function are: the signal-joint T-cell receptor excision circle (sj-TREC) level measurement and the naive T cell and CD31+ TREC-rich subset determination. However, all of them are indirect approaches and none could be considered a direct recent thymic emigrant (RTE) marker. To overcome their limitations, Dion et al. (2004) described the sj/beta-TREC ratio that allows the peripheral quantification of the double negative to double positive intrathymic proliferation step. Nevertheless, the protocol described is expensive, sample and time-consuming, thus, limiting its usefulness. In this study, we describe a simplified protocol that reduces from 33 to 9 the amount of PCR reaction needed but maintaining the sensitivity and reproducibility of the original technique. In addition, we corroborated the effectiveness of our technique as an accurate thymic output-related marker by correlating the peripheral sj/beta-TREC ratio with a direct measurement of thymic function as the percentage of double positive thymocytes (r=0.601, p<0.001).

Zinc supplementation increases zinc status and thymopoiesis in aged mice

The age-related decline in lymphocyte development and function coincides with impaired zinc status in the elderly. Thymic involution and reduced immune responsiveness are classic hallmarks of both aging and zinc deficiency, resulting in decreased host defense and an increased susceptibility to infections. Thus, compromised zinc status associated with aging may be an important contributing factor in reduced thymopoiesis and impaired immune functions. Our goal in this study was to understand how dietary zinc supplementation affects thymopoiesis in aged mice. We hypothesized that impaired zinc status associated with aging would mediate the decline in thymic function and output and that restoring plasma zinc concentrations via zinc supplementation would improve thymopoiesis and thymic functions. In this study, groups of young (8 wk) and aged (22 mo) mice were fed a zinc-adequate (30 mg/kg zinc) or zinc-supplemented diet (300 mg/kg) for 25 d. Aged mice had impaired zinc status, with zinc supplementation restoring plasma zinc to a concentration not different from those of young male C57Bl/6 mice. Zinc supplementation in aged mice improved thymopoiesis, as assessed by increased total thymocyte numbers. In addition, improved thymic output was mediated in part by reducing the age-related accumulation of immature CD4(-)CD8(-)CD44(+)CD25(-) thymocytes, as well as by decreasing the expression of stem cell factor, a thymosuppressive cytokine. Taken together, our results showed that in mice, zinc supplementation can reverse some age-related thymic defects and may be of considerable benefit in improving immune function and overall health in elderly populations.

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.

Foxn1 is required to maintain the postnatal thymic microenvironment in a dosage-sensitive manner

The postnatal thymus is the primary source of T cells in vertebrates, and many if not all stages of thymocyte development require interactions with thymic epithelial cells (TECs). The Foxn1 gene is a key regulator of TEC differentiation, and is required for multiple aspects of fetal TEC differentiation. Foxn1 is also expressed in the postnatal thymus, but its function after birth is unknown. We generated a Foxn1 allele with normal fetal expression and thymus development, but decreased expression in the postnatal thymus. This down-regulation causes rapid thymic compartment degeneration and reduced T-cell production. TEC subsets that express higher Foxn1 levels are most sensitive to its down-regulation, in particular MHCII(hi)UEA-1(hi) medullary TECs. The requirement for Foxn1 is extremely dosage sensitive, with small changes in Foxn1 levels having large effects on thymus phenotypes. Our results provide the first evidence that Foxn1 is required to maintain the postnatal thymus. Furthermore, the similarities of this phenotype to accelerated aging-related thymic involution support the possibility that changes in Foxn1 expression in TECs during aging contribute to the mechanism of 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.

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.

The effect of age on response to therapy with peginterferon alpha plus ribavirin in a cohort of patients with chronic HCV hepatitis including subjects older than 65 yr

OBJECTIVES

In many industrialized countries HCV infection is characterized by an increasing prevalence during ageing; however, data on the efficacy of treatment among older patients are scarce. This study was set up to evaluate the effect of age on the treatment of chronic HCV hepatitis with peginterferon alpha plus ribavirin.

METHODS

We retrospectively reviewed medical records of 153 adult patients with chronic HCV hepatitis treated with combination therapy; 30 of them (19.6%) were 65 years of age or older.

RESULTS

In multivariable analysis, age groups >/=40 years had similar odds of achieving sustained virologic response (P= 0.71) and significantly lower odds of sustained response compared with younger patients (odds ratio [OR] 0.16, 95% confidence interval [CI] 0.05-0.59, P= 0.006; OR 0.13, 95% CI 0.03-0.49, P= 0.002; OR 0.21, 95% CI 0.05-0.91, P= 0.037 for patients aged 40-49 years, 50-64 years, and older than 64 years, respectively). The effect of age was present in the 74 patients infected with genotype 1 or 4 (P= 0.04), while among the 79 patients with genotype 2 or 3 sustained virologic response rates were relatively uniform, with no statistically significant differences.

CONCLUSIONS

The probability of good response to combination treatment with peginterferon alpha plus ribavirin is decreased for patients aged more than 40 years infected with genotype 1 or 4, but patients aged more than 65 had a similar rate of response to those aged 40-64 years. Combination treatment may be safely extended to elderly patients with no major contraindications.

Changes of CD4+CD25+Foxp3+ regulatory T cells in aged Balb/c mice

A progressive decline in the integrity of the immune system is one of the physiologic changes during aging. The frequency of autoimmune diseases or immune disorders increases in the aging population, but the state of regulatory T (Treg) cells in aged individuals has not been well determined. In the present study, we investigated the levels, phenotypes, and function of CD4(+)CD25(+) Treg cells in Balb/c mice, which were older than 20 months. Significantly enhanced percentages of CD4(+)CD25(+) Treg cells in the periphery (blood, spleen, and lymph nodes) of the aged mice were observed. These Treg cells showed modified Vbeta family distribution, reduced levels of CD45 receptor B and CD62 ligand molecules, as well as normal levels of forkhead box p3. However, when the inhibiting function of Treg cells was assayed in the in vitro assays and in a delayed-type hypersensitivity (DTH) model, CD4(+)CD25(+) Treg cells of aged mice displayed significantly lower inhibiting ability on alloantigen-induced DTH reaction or cytokine productions (IL-2 and IFN-gamma) but not cell proliferation of effector T cells, as compared with CD4(+)CD25(+) Treg cells of young mice. In addition, the percentages of CD4(+)CD8(-)CD25(+) Treg cells in the thymi of aged mice increased significantly, but their total cell numbers decreased markedly in these mice. Our present studies indicated collectively that the percentages, phenotypes, the size of TCR repertoire, and function of CD4(+)CD25(+) Treg cells were altered significantly with aging in mice. The functional defects of CD4(+)CD25(+) Treg cells may shed light on the role of CD4(+)CD25(+) Treg cells in the increased sensitivity to autoimmune diseases of aged populations.

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

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

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.

Morphometrical Characteristics of Age-Associated Changes in the Thymus of Old Male Wistar Rats

In order to provide a morphometrical description of the changes in the aged rat thymus and to relate them to apoptotic and proliferative activity of thymocytes, the thymuses from 3- and 18-month-old male Wistar rats and the percentages of bromodeoxyuridine-incorporating and apoptotic cells in cultures of thymocytes were assessed by stereological analysis and flow cytometry, respectively. In old rats the volume of lymphoepithelial thymic tissue is markedly reduced, reflecting a sharp decrease in the total number of thymocytes. A reduction in the proliferative capacity of thymocytes and increase in their susceptibility to apoptosis are, most likely, primarily responsible for a 7-fold reduction in thymic cellularity in old animals. Furthermore, only the volume of cortical compartment was affected by aging, while that of medulla, despite of reduced cellularity, was not significantly altered. The loss of functional tissue in aged thymus is compensated by a substantial increase in the volume of inter-lobular connective and adipose tissue, so the thymic weight remained unaltered in old rats. These results suggest that thymus of aged Wistar rats exhibits morphological characteristics similar to those found in aged human thymus and thus may serve as an animal model for further investigations of thymus-related changes in immunological aging.

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.

Physiologic aspects of aging: impact on cancer management and decision making, part II

In this second article of our two-part review, we focus on age-associated physiologic changes involving the nervous, endocrine, hematologic, immune, and musculoskeletal systems, with close attention to the interconnected nature of these systems. There is a well-known connection between the neuroendocrine and immune systems via the hypothalamic-pituitary-adrenal axis and via interaction by means of cytokines, hormones, and neurotransmitters. These changes may lead to a loss of integration and resiliency with age, thus decreasing the ability of the elderly patient with cancer to adapt to stressful circumstances. Prominent changes include decline in memory and cognition, and increased susceptibility to peripheral neuropathy. Hematologic and immune changes like reduced bone marrow reserve and increased susceptibility to infections have far reaching implications for cancer care in the elderly. Gradual decline in hormone levels, and changes in muscle and body composition, can lead to functional decline and frailty. Use of the clinical interventions suggested in this article, along with an appreciation of the interplay of these age-related physiologic changes and their consequences, allows oncology professionals to customize therapy and minimize side effects in the geriatric oncology patient.

A Novel Approach to Thymic Rejuvenation in the Aged

With advancing age, the mammalian thymus undergoes involution, a progressive loss of architectural integrity and lymphoid cellularity that results in reduced T lymphopoiesis. Thymic involution also is associated with extreme malnutrition and states of immune deficiency, such as active HIV infection, after chemotherapy, or during pregnancy. Immune recovery appears to require restoration of normal thymopoiesis. Although several means are known to increase thymic cellularity in the aged, including systemic administration of hormones, androgen ablation, and thymic tissue transplantation, each suffers from specific limitations that prevent widespread application. This paper presents a novel approach to rejuvenate T cell differentiation in the aged that employs intrathymic implantation of engineered stromal cells. Two different proteins have been examined for their impact on thymopoiesis after delivery by somatic cell implantation. Intrathymic injection of IL-7-producing stromal cells enhances the earliest specification steps of T cell development, resulting in the increased representation of pro-T cells in the aged thymus. In contrast, increasing the intrathymic levels of sonic hedgehog diminishes this aspect of T cell poiesis.

Reevaluating current models of thymic involution

It is generally accepted that thymic involution commences, or at least accelerates, at puberty due to increases in sex steroid and declines in growth hormone production. As a result of these hormonal changes, the development of the most immature intrathymic progenitors is blocked. However, aspects of this model are now being questioned. The present chapter re-evaluates a number of findings on which traditional models of thymic involution are based and reviews new data that, taken together, indicate a need to revise current views of thymic involution.

Transforming growth factor-beta2 is involved in quantitative genetic variation in thymic involution

The mechanisms regulating thymic involution are unclear. In inbred mouse strains the rate of thymic involution and the function of the hematopoietic stem cell (HSC) compartment are subject to quantitative genetic variation. We have shown previously that transforming growth factor-beta2 (TGF-beta2) is a genetically determined positive regulator of HSCs. Here, we demonstrate that genetic variation in the rate of thymic involution correlates with genetic variation in the responsiveness of hematopoietic stem and progenitor cells to TGF-beta2. Corroborating these correlations, thymic cellularity and peripheral naive T-cell frequency were higher in old Tgfb2+/- mice than in wild-type littermates. The frequency of early T-cell precursors was increased in Tgfb2+/- mice, suggesting that TGF-beta2 affects the earliest stages of T-cell development in old mice. Reciprocal transplantation experiments indicated that TGF-beta2 expressed both in the (micro)environment and in the hematopoietic system can accelerate thymic involution; however, the age of the stem cells appeared irrelevant. Thus, although thymic involution is largely determined by the aged environment, TGF-beta2 plays a major modulatory role that is subject to genetic variation and is possibly mediated through its regulatory effects on early hematopoiesis.

CD1d-restricted NKT cells contribute to the age-associated decline of T cell immunity

NKT cells are known to regulate effector T cell immunity during tolerance, autoimmunity, and antitumor immunity. Whether age-related changes in NKT cell number or function occur remains unclear. Here, we investigated whether young vs aged (3 vs 22 mo old) mice had different numbers of CD1d-restricted NKT cells and whether activation of NKT cells by CD1d in vivo contributed to age-related suppression of T cell immunity. Flow cytometric analyses of spleen and LN cells revealed a 2- to 3-fold increase in the number of CD1d tetramer-positive NKT cells in aged mice. To determine whether NKT cells from aged mice differentially regulated T cell immunity, we first examined whether depletion of NK/NKT cells affected the proliferative capacity of splenic T cells. Compared with those from young mice, intact T cell preparations from aged mice had impaired proliferative responses whereas NK/NKT-depleted preparations did not. To examine the specific contribution of NKT cells to age-related T cell dysfunction, Ag-specific delayed-type hypersensitivity and T cell proliferation were examined in young vs aged mice given anti-CD1d mAb systemically. Compared with young mice, aged mice given control IgG exhibited impaired Ag-specific delayed-type hypersensitivity and T cell proliferation, which could be significantly prevented by systemic anti-CD1d mAb treatment. The age-related impairments in T cell immunity correlated with an increase in the production of the immunosuppressive cytokine IL-10 by splenocytes that was likewise prevented by anti-CD1d mAb treatment. Together, our results suggest that CD1d activation of NKT cells contributes to suppression of effector T cell immunity in aged mice.

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.

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.

IL-7 gene therapy in aging restores early thymopoiesis without reversing involution

Thymic involution begins early in life and continues throughout adulthood, resulting in a decreased population of naive T cells in the periphery and a reduced ability to fight off newly encountered infectious diseases. We have previously shown that the first step of thymopoiesis is specifically blocked in aging. This block at the DN1 to DN2 transition and the subsequent loss of thymic output in old age mirrors the changes seen in IL-7-deficient mice, and it is hypothesized that decreased intrathymic IL-7 is involved in age-related thymic involution. To separate the effect of IL-7 on thymic involution from its function as a peripheral lymphocyte growth cofactor, we injected IL-7-secreting stromal cells into the thymi of recipient mice. The increased local concentration of IL-7 maintained the first step of thymopoiesis at a level far higher than was seen in age-matched controls. However, despite this success, there was no decrease in thymic involution or increase in T cell output. The inability of IL-7 to prevent involution led us to the discovery of an additional age-sensitive step in thymopoiesis, proliferation of the DN4 population, which is unaffected by IL-7 expression.

Moderate exercise may attenuate some aspects of immunosenescence

BACKGROUND

Immunosenescence is related to the deterioration of many immune functions, which may be manifested in increased susceptibility to infection, cancer, and autoimmunity. Lifestyle factors, such as diet or physical activity, may influence the senescence of the immune system. It is widely accepted that moderate physical activity may cause beneficial effects for physical and psychological health as well as for the immune system activity in aged people.

METHODS

Thirty elderly women aged 62 to 86 were subjected to a two-years authorized physical activity program. Peripheral blood lymphocytes distribution and the production of cytokines involved in the immune response development and regulation (IL-2, IL-4 and IFN-gamma) were investigated. The same parameters were evaluated in two control groups of women: a sedentary group of 12 elderly women selected for the second round of the physical activity program and in a group of 20 sedentary young women. Flow cytometry methods were used for the examination of surface markers on peripheral blood lymphocytes and intracellular cytokines expression.

RESULTS

The distribution of the main lymphocytes subpopulations in the peripheral blood of elderly women did not show changes after long-term moderate physical training. The percentage of lymphocytes expressing intracellular IL-2 was higher in the group of women attending 2-years physical activity program than in the control group of elderly sedentary women, and it was similar to the value estimated in the group of young sedentary women. There was no difference in the intracellular expression of IL-4 and IFN-gamma between the active and elderly sedentary women.

CONCLUSIONS

Our results suggest that moderate, long-term physical activity in elderly women may increase the production of IL-2, an important regulator of the immune response. This may help ameliorate immunosenescence in these women.

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 differences in parameters of feline immune status

In order to assess age-related differences in feline immune status, 101 domestic short haired cats were assigned to two groups, adult (2-5 years, n=50) and senior (10-14 years, n=51). Analyses of leucocyte populations, lymphocyte subsets, complement activity, serum immunoglobulins and acute-phase proteins were undertaken and revealed significant differences between the two groups. The senior group had significantly lower WBC, lymphocyte and eosinophil counts than the adult group. Neutrophil, monocyte and basophil counts did not differ between the groups. Flow cytometry analysis, in combination with differential WBC data, revealed that the absolute values (cells/l) of T-cells, B-cells and natural killer (NK) cells were significantly lower in the older animals. While serum immunoglobulins IgA and IgM were higher in the senior group when compared with the adult group, no significant differences were observed in complement activity or in serum acute-phase proteins. Our findings suggest that age-related changes to parameters of immune status in the feline model are likely to follow a similar pattern to those observed in other long-lived mammalian species.

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.

IL-12 Inhibits Thymic Involution by Enhancing IL-7- and IL-2-Induced Thymocyte Proliferation

IL-12 has been reported to affect thymic T cell selection, but the role of IL-12 in thymic involution has not been studied. We found that in vivo, IL-12b knockout (IL-12b(-/-)) mice exhibited accelerated thymic involution compared with wild-type (WT) B6 mice. This is characterized by an increase in thymocytes with the early development stage phenotype of CD25(-)CD44(+)CD4(-)CD8(-) in aged IL-12b(-/-) mice. Histologically, there were accelerated degeneration of thymic extracellular matrix and blood vessels, a significantly decreased thymic cortex/medulla ratio, and increased apoptotic cells in aged IL-12b(-/-) mice compared with WT mice. There was, however, no apparent defect in thymic structure and thymocyte development in young IL-12(-/-) mice. These results suggest the importance of IL-12 in maintaining thymic integrity and function during the aging process. Surprisingly, in WT B6 mice, there was no age-related decrease in the levels of IL-12 produced from thymic dendritic cells. Stimulation of thymocytes with IL-12 alone also did not enhance the thymocyte proliferative response in vitro. IL-12, however, provided a strong synergistic effect to augment the IL-7 or IL-2 induced thymocyte proliferative response, especially in aged WT and IL-12b(-/-) mice. Our data strongly support the role of IL-12 as an enhancement cytokine, which acts through its interactions with other cytokines to maintain thymic T cell function and development during aging.