Activated CD8(+) T cells from aged mice exhibit decreased activation-induced cell death

TNFSF15 and MIA Variant Associated with Immunotherapy and Prognostic Evaluation in Esophageal Cancer

Background

Esophageal cancer (ESCA) is a common gastrointestinal tumor, and China is one of the regions with a high incidence. Tumor immune-related cells play important roles in the tumorigenesis and development of ESCA. However, the role of tumor immune-related genes in the development of ESCA has not been established.

Methods

In this study, weighted gene coexpression network analysis (WGCNA) was used to analyze ESCA gene expression using data from The Cancer Genome Atlas (TCGA) database. Gene expression was associated with clinical traits, and modules related to CD8+T cells, dendritic cells, and regulatory T cells (Tregs) were obtained.

Results

The GO analysis showed that inflammatory chemotaxis networks were activated by cell chemotaxis, chemokine activity, and chemokine binding receptor. Three hub genes (IL17C, TNFSF15, and MIA) related to tumor immunity and metastasis were identified by WGCNA, and the abnormal expression of each hub gene in ESCA has a poor prognosis, especially in patients with high expression (P < 0.05). The risk assessment analysis also showed that tumor stage was positively correlated with tumor risk in ESCA (P < 0.05). Therefore, more than 50 pairs of tumor tissues from the T1-T3 stages with different degrees of differentiation and paracancerous tissues were selected to confirm the expression of the three genes using RT-qPCR and immunofluorescence (IF). The infiltration of CD8+ T cells in tumor tissues was lower than that in normal tissues. According to the RT-qPCR, the expressions of IL17 C, TNFSF15, and MIA in moderately and poorly differentiated tissues were significantly higher than those in normal tissues (P < 0.05). In contrast, their expressions were decreased in high differentiated tissues (P < 0.05). Furthermore, IL17C, TNFSF15, and MIA were all positively correlated with immune checkpoint PD-1; TNFSF15 and MIA were also positively correlated with CTLA4, TIGIT, and CD96.

Conclusion

In summary, IL17C, TNFSF15, and MIA may act as biomarkers for prognosis in moderately and poorly differentiated ESCAs, and they may be used as predictive genes of immunotherapy associated with CD8+ T cell and Tregs invasion in ESCAs.

Aging-related changes in human T-cell repertoire over 20years delineated by deep sequencing of peripheral T-cell receptors

Recent deep sequencing studies on T-cell receptor (TCR) repertoire have provided robust data to characterize diversity of T-cell immune responsiveness to a wide variety of peptide antigens, including viral and tumor antigens. The human TCR repertoire declines with age, but this decline has not been fully investigated longitudinally in individuals. Using a deep sequencing approach, we analyzed TCRβ repertoires longitudinally over approximately 20years, with ages ranging from 23 to 50years at the start (23 to 65years overall), in peripheral-blood CD4 and CD8 T-cell populations that were collected and cryopreserved 3 times at intervals of approximately 10years from each of 6 healthy adults (3 men and 3 women). Sequence data at the hypervariable complementarity determining region 3 (CDR3) in the TCRB gene locus were evaluated by applying a random-coefficient statistical regression model. Two outcomes were analyzed: total number of distinct TCRB CDR3 sequences as a TCR diversity metric, and clonality of the T-cell populations. TCR repertoire diversity decreased (p<0.001) and frequencies of clonal populations increased (p=0.003) with age in CD8 T cells, whereas CD4 T cells retained fairly diverse TCR repertoires along with relatively low clonality. We also found that approximately 10-30% and 30-80% of read sequences in CD4 and CD8 T cells, respectively, overlapped at different ages within each individual, indicating long-term stable maintenance of T-cell clonal composition. Moreover, many of the most frequent TCRB CDR3 sequences (i.e., top T-cell clones) persisted over 20years, and some of them expanded and exerted a dominating influence on clonality of peripheral T-cell populations. It is thus possible that persistence or expansion of top T-cell clones is a driver of T-cell immunity aging, and therefore represents a potential interventional target.

Persistent inflammation and T cell exhaustion in severe sepsis in the elderly

Introduction

Sepsis is known as a complex immunological response with hyperinflammation in the acute phase followed by immunosuppression. Although aging is crucial in sepsis, the impact of aging on inflammation and immunosuppression is still unclear. The purpose of this study was to investigate the relationship between inflammation and immunosuppression in aged patients and mice after sepsis.

Methods

Fifty-five patients with severe sepsis and 30 healthy donors were prospectively enrolled, and 90-day survival was compared between elderly (≥65 years) and adult (18–64 years) septic patients with serial measurement of serum interleukin (IL)-6. Within 24 h after diagnosis of severe sepsis, peripheral blood mononuclear cells were stimulated ex vivo to measure expression of the activation maker CD25 in T cells, IL-2 levels in the supernatant, and proliferation. In the mouse study, young (6–8 weeks) and aged (20–22 months) C57/B6 mice were subjected to cecal ligation and puncture (CLP), and survival was compared after 7 days with serial measurement of serum IL-6. Expression of the negative co-stimulatory molecules, CD25, and IL-2 in CD4+ T cells was measured.

Results

The survival rate in elderly sepsis patients and aged septic mice was significantly lower than that in adult patients and young septic mice (60% vs. 93% in septic patients, 0% vs. 63% in septic mice, P < 0.05). Serum IL-6 levels in elderly sepsis patients and aged septic mice were persistently higher than those in adult patients and young septic mice. Expression of negative co-stimulatory molecules in CD4+ T cells in the spleen, lymph nodes, and peripheral blood was significantly higher in aged mice than in young mice (P < 0.01). Ex vivo stimulation decreased CD25 expression, IL-2 production, and proliferation to a greater extent in CD4+ T cells from elderly patients and aged septic mice than in those from adult patients and young septic mice. Elderly patients demonstrated increased detection of gram-negative bacteria at days 14–16 and 28–32 after sepsis (P < 0.05).

Conclusions

Persistent inflammation and T cell exhaustion may be associated with decreased survival in elderly patients and mice after sepsis.

Intrinsic defects in CD8 T cells with aging contribute to impaired primary antiviral responses

Aging is associated with altered immune responses, particularly with a diminished CD8 T cell response. Although both intrinsic and extrinsic factors are hypothesized to impact this decreased T cell response, the direct evidence of an intrinsic deficiency in virus-specific CD8 T cells is limited. In this study, a TCR transgenic (Tg) P14 mouse model was utilized to compare the activation and proliferation of the Tg CD8 T cells of young and aged P14 mice upon stimulation with antigen or infection with virus. The proliferation of purified Tg CD8 T cells of aged mice was significantly lower than that of young mice when cultured in vitro with both the LCMV specific peptide and antigen presenting cells from young wild type mice. In addition, expression of the activation markers, CD69, CD25, and CD44, was delayed on Tg T cells of aged mice after stimulation. Importantly, while adoptive transfer of purified Tg CD8 T cells of young or aged mice into young wild type mice resulted in expansion of the Tg CD8 T cells of both ages after LCMV infection, the expansion of the Tg T cells from aged mice was significantly decreased compared with that of the Tg T cells from young mice. However, while the number of IFN-γ secreting Tg CD8 T cells from aged mice was significantly decreased compared to that of young mice, the percentages of Tg CD8 T cells producing IFN-γ were similar in young and aged mice, demonstrating that proliferation, but not function, of the Tg CD8 T cells of aged mice was impaired. Importantly, chronological age alone was not sufficient to predict an altered proliferative response; rather, expression of high levels of CD44 on CD8 T cells of aged mice reflected a decreased proliferative response. These results reveal that alterations intrinsic to CD8 T cells can contribute to the age-associated defects in the primary CD8 T cell response during viral infection.

Increased vitamin D is associated with decline of naïve, but accumulation of effector, CD8 T cells during early aging

Given the protective roles of 25-hydroxyvitamin D (25[OH]D or vitamin D) in musculoskeletal health and the potential beneficial effects of vitamin D supplementation in reducing the risk of various chronic diseases, intensive repletion of vitamin D has been widely advocated. Of note, CD8 T cells have the highest levels of the vitamin D receptor compared with other major immune cells. The effects of vitamin D on CD8 T cells during aging, however, remain unclear. This study determined the relationship between vitamin D levels and CD8 T-cell status in 34 healthy female subjects (all >60 years old). The CD8 T cell phenotype was defined by the surface expression of CD28 and CD95. The low-25(OH)D serum groups (≤30 ng/ml) had higher percentages of CD28⁺CD95^-CD8⁺ (naïve) T cells and lower percentages of CD28⁺CD95⁺CD8⁺ (effector) T cells. By contrast, subjects with high levels of 25(OH)D had very low percentages of naïve CD8 T cells but very high percentages of effector CD8 T cells. There was a significant inverse correlation between 25(OH)D levels and the frequency of naïve CD8 T cells. The results show that higher levels of vitamin D are correlated with decreased frequencies of naïve CD8 T cells during early aging, suggesting that higher levels of 25(OH)D accelerate CD8 T-cell senescence. These results warrant the further evaluation of the effects of vitamin D supplementation in immune aging.

Metabolic syndrome, hormones, and maintenance of T cells during aging

Although the phenotype of T-cell senescence has been extensively investigated, few studies have analyzed the factors that promote the generation and maintenance of naïve and memory T cells that exist throughout the lifespan of the individuals. Unlike senescent T cells, naïve and memory T cells are able to participate in useful immune responses as well as respond to new activation. Hormones such as leptin, ghrelin, insulin-like growth factor 1, IGFBP3, and cytokines, including IL-7, regulate both thymopoiesis and maintenance of naïve T cells in the periphery. Although chronic viruses such as cytomegalovirus (CMV) are thought to drive T-cell senescence, other microbes may be important for the maintenance of nonsenescent T cells. Microbiota of the gut can induce metabolic syndrome as well as modulate T-cell development into specific subpopulations of effector cells. Finally, T-cell generation, maintenance, and apoptosis depend upon pathways of energy utilization within the T cells, which parallel those that regulate overall metabolism. Therefore, better understanding of metabolic syndrome, T-cell metabolism, hormones, and microbiota may lead to new insights into the maintenance of proper immune responses in old age.

Naive CD4 T cells from aged mice show enhanced death upon primary activation

Poor T cell immunity is one of the many defects seen in elderly humans and aged (Ad) mice. We report that naive CD4 T cells from aged mice (ANCD4 cells) showed greater apoptosis upon primary activation than those from young (Yg) mice, with loss of mitochondrial membrane potential, poor activation of Rel family transcription factors and increased DNA damage. Their ability to enhance glycolysis, produce lactate and induce autophagy following activation was also compromised. ANCD4 cells remained susceptible to death beyond first cell division. Activated ANCD4 cells also showed poor transition to a 'central memory' (CM) CD44(high), CD62L(high) phenotype in vitro. This correlated with low proportions of CM cells in Ad mice in vivo. Functionally, too, IFN-gamma responses recalled from T cells of immunized Ad mice, poor to begin with, worsened with time as compared with Yg mice. Thus, ANCD4 cells handle activation-associated stress very poorly due to multiple defects, possibly contributing to poor formation of long-lasting memory.

Enhancement of cytotoxic T-lymphocyte response in aged mice by a novel treatment with recombinant AdIL-12 and wild-type adenovirus in rapid succession

A decrease in the expression of Th1 cytokines has been associated with age-related decrease in cytotoxic T-lymphocyte (CTL) function. We utilized an E1-deleted adenovirus (Ad) vector to deliver the murine interleukin-12 (IL-12) gene in order to enhance the antivirus CTL response. Wild-type (WT) Ad was administered 3 days after AdIL-12 treatment, when IL-12 production was at its peak and the anti-Ad antibody response had not yet begun to develop. Before receiving AdIL-12 treatment, aged (18 month old) mice exhibited a 58% decrease in the number of virus-specific CTLs, and a 30% decrease in in vivo CTL activity as compared to young (2 month old) mice. After AdIL-12 treatment, aged mice displayed a greater increase in IL-12 expression and endogenous production of interferon-gamma than observed in young mice. When infected with WT Ad, these AdIL-12-treated aged mice exhibited an increased in vivo CTL response and an in vitro proliferative response that was similar to those in young mice. The frequencies of occurrence of D(b)-E1Bp(+)CD8(+) T cells in the spleen, liver, and lung in aged mice were higher than the corresponding values in young mice. These results indicate that IL-12 treatment significantly promotes the virus-specific CTL response in aged mice and, more importantly, specifically targets the virally infected organs, such as the liver and lung, promoting enhanced CTL activity against the virus.

Immunosupportive therapies in aging

The primary role of the immune system is to protect the organism against pathogens, but age-associated alterations to immunity increase the susceptibility of the elderly to infectious disease. The exact nature of these changes is still controversial, but the use of screening procedures, such as the SENIEUR protocol to exclude underlying illness, helped to better characterize the changes actually related to physiological aging rather than pathology. It is generally agreed that the most marked changes occur in the cellular immune response reflecting profound alterations in T cells. Much of this is due to thymic involution as well as changes in the proportions of T cell subpopulations resulting from antigen exposure, and altered T cell activation pathways. However, a body of data indicates that innate immune responses, including the critical bridge between innate and adaptive immunity, and antigen presenting capacity are not completely resistant to senescence processes. The consequences of all these alterations are an increased incidence of infections, as well as possibly cancers, autoimmune disorders, and chronic inflammatory diseases. The leading question is what, if anything, can we do to prevent these deleterious changes without dangerously dysregulating the precarious balance of productive immunity versus immunopathology? There are many potential new therapeutic means now available to modulate immunosenescence and many others are expected to be available shortly. One main problem in applying these experimental therapies is ethical: there is a common feeling that as ageing is not a disease; the elderly are not sick and therefore do not require adventurous therapies with unpredictable side-effects in mostly frail individuals. Animal models are not helpful in this context. In this chapter we will first briefly review what we think we know about human immunosenescence and its consequences for the health status of elderly individuals. We will then discuss possible interventions that might one day become applicable in an appropriate ethical environment.

Age increases vulnerability to bacterial endotoxin-induced behavioral decrements

Peripheral lipopolysaccharide (LPS) or proinflammatory cytokines produce alterations in learning, memory, and other behaviors. Additionally, research has demonstrated that factors such as dose, route of administration, species, strain, gender, and age are important modulatory factors in the effects of endotoxin exposure. Previous research from our laboratory and others indicate that LPS-induced behavioral deficits are greater in older subjects. The current study examined avoidance learning in a negatively reinforced operant procedure (i.e., two-way active avoidance conditioning) following single or repeated intraperitoneal LPS injections in 2- and 12-month-old male C57BL/6J mice. LPS-treated subjects show impaired acquisition of the task regardless of the age of the subject, as these animals performed significantly fewer avoidance responses than controls. However, the effects of LPS administration were more pronounced in the 12-month-old animals, particularly for the subjects given repeated LPS injections. These results support the hypothesis that endotoxin exposure is capable of altering performance in this task in a way that may reflect deficits in learning, and provide evidence that increased age may exacerbate these deleterious behavioral effects.

Aging affects initiation and continuation of T cell proliferation

Aging is associated with a decline in immune responses, particularly within the T cell compartment. While the expansion of specific T cells in response to virus infections is consistently decreased in aged mice, the differences in T cell activation between young and aged mice as demonstrated in each round of proliferation remain poorly defined. In the present study, we utilized the T cell mitogen, ConA, to explore if fewer T cells of aged mice initiate proliferation upon mitogen stimulation or if similar numbers of T cells of aged mice begin proliferation but undergo fewer rounds of division. We also examined whether these age-associated changes in proliferation are reflected by differences in T cell activation by comparing activation markers (CD25, CD69, CD44, and CD62L) on T cells of young and aged mice at each round of proliferation. Not only was the kinetics of the expression of these markers greatly different between young and aged mice on the entire CD8 T cell population, but also at each round of proliferation. Our results demonstrate that a larger percentage of CD8 T cells of aged mice do not proliferate at all upon stimulation. Of the CD8 T cells of aged mice that do proliferate, a larger percentage start later and stop sooner. These results suggest that multiple levels of alteration may need to be considered when trying to maximize the immune response of aged individuals.

A membrane form of TNF-alpha presented by exosomes delays T cell activation-induced cell death

In common with many other cell types, synovial fibroblasts produce exosomes. In this study, we show that the exosomes produced by synovial fibroblasts obtained from individuals with rheumatoid arthritis (RASF), but not exosomes produced by synovial fibroblasts obtained from individuals with osteoarthritis, contain a membrane bound form of TNF-alpha as demonstrated by colloidal gold immunostaining of TNF-alpha and confirmed by both Western blot and mass spectrometry. The RASF-derived exosomes, but not exosomes derived from fibroblasts obtained from individuals with osteoarthritis, are cytotoxic for the L929 cell, a TNF-alpha-sensitive cell line, and stimulate activation of NF-kappaB and induction of collagenase-1 in RASF. These effects are blocked by addition of soluble TNFR1 (sTNFbp), suggesting that a TNF-alpha-signaling pathway mediates these biological activities. sTNFbp also reduced the production of exosomes by RASF, suggesting the interruption of a positive amplification loop. Exosomes can transmit signals between cells, and RASF exosomes, effectively taken up by anti-CD3-activated T cells, activated AKT and NF-kappaB and rendered these activated T cells resistant to apoptosis. Neutralization of exosomal membrane TNF-alpha by sTNFbp partially reversed this resistance, suggesting that not only TNF-alpha but also additional exosomal proteins may contribute to the development of apoptosis resistance.

B cells from aged mice exhibit reduced apoptosis upon B-cell antigen receptor stimulation and differential ability to up-regulate survival signals

During ageing, autoimmune disorders and the higher susceptibility to infectious have been associated with alterations in the humoral immune response. We report that splenic B lymphocytes from aged mice exhibit lower level of apoptosis induced by B-cell antigen receptor (BCR) ligation in vitro. Respect to B cells from young mice the anti-mu stimulated aged B cells show similar Bcl-2 and Bcl-xL expression but differential kinetic of A1 degradation and a higher level of cFLIP and FAIM. Even though B cells from aged mice show minor Fas expression they exhibit the same susceptibility to anti-Fas induced apoptosis. Aged B cells also present upon BCR stimulation, a higher proliferative response and similar level of activation markers expression than B cells from young mice. These data agree with the observation that aged mice exhibit an increment of T2 and mature B cell subset which rapidly enters cell cycle upon BCR engagement. The diminished apoptosis after activation in aged mice could compromise homeostatic mechanism allowing the persistence of self and non-self antigen specific B cells.

Impaired apoptosis and immune senescence - cause or effect?

Aged animals and humans exhibit a decreased T-cell activation response although they also exhibit increased susceptibility to responses to self-antigens and a loss of self-tolerance. The age-related alteration in T-cell reactivity, polyclonal expansion of T cells, and enhanced production of autoantibodies may reflect the numerous age-associated alterations in the T-cell arm of the immune system that have been revealed in numerous studies. These studies suggest that subpopulations of T cells are not deleted appropriately in older animals. They further suggest that an age-related impairment of Fas/Fas ligand (FasL)-mediated apoptosis - which plays a major role in activation-induced cell death (AICD) of T cells - may contribute to compromised regulation of the immune system. The likely mechanisms that may lead to impaired induction of FasL in AICD senescent T cells include an age-related shift from the apoptosis-sensitive T-helper 1 cell (Th1) response to the AICD-resistant Th2 response, aberrant T-cell receptor/CD3 downstream-signaling pathways, and altered CD28/B7-mediated T-cell costimulatory signals. Pathologically, accumulation of AICD-senescent T cells is associated with a defective cytotoxic T lymphocyte response and generation of autoreactive T cells. Based on the accumulating evidence, we propose that the emergence of the FasL(lo) AICD-senescent T cells is not only an effect of immune aging but also an important cause of T-cell proliferative senescence in both humans and mice.

Genetic determinants influencing the response to injury, inflammation, and sepsis

The genetic background has recently been recognized as an important element in the response to injury, contributing to the variability in the clinical outcome of critically ill patients. The traditional approach to studying the genetic contribution requires the availability of families with multiple members who have experienced similar disease conditions, a situation that is nearly impossible to find in the case of trauma. Association studies looking at unrelated individuals across populations require large economic and labor-intensive efforts. Thus, a candidate gene approach has been the sole methodology used to correlate genetic variability with clinical outcome. However, this approach cannot provide a comprehensive description of a multigenic condition. Animal models are an alternative for studying the genetic contributions to variability in the response to injury. A murine model is ideal because a large set of inbred strains are available; congenic, consomic, transgenic, and recombinant strains can also be used. Employing this paradigm, we have demonstrated that the response to several stressors, such as injection of E. coli lipopolysaccharide (LPS) and polymicrobial sepsis induced by cecal ligation and puncture (CLP), is modified by the genetic background. The inflammatory response in mice has also been shown to be affected by sex, age, and other, nongenetic components such as diet. We have exploited the differences in response among various inbred mouse strains to map loci contributing to the inflammatory response. Fine mapping strategies allow the refinement of sets of candidate genes, which can be identified by positional cloning. Detection of genetic variation affecting the inflammatory response in murine models provides a basis for determining whether polymorphisms in orthologous human genes correlate with particular clinical outcomes from injury. Thus, discovery of these genes could impact patient care by acting as markers of a specific predisposition in humans.

Age and caloric restriction diets are confounding factors that modify the response to lipopolysaccharide by peritoneal macrophages in C57BL/6 mice

Aging is the result of several detrimental changes that lead to a decrease in homeostasis, an increase in the incidence of degenerative diseases, and death. A caloric-restricted diet (CR), which consists of a significant reduction in calorie intake (40%) without malnutrition, has been shown to delay the onset of age-related diseases and pathologies and to extend life span. The aims of this study were to assess the effects of aging and CR on lipopolysaccharide (LPS)-dependant cytokine production by peritoneal macrophages (PMphis). Resident naïve PMphis were isolated from 2- to 24-month-old male C57BL/6 mice and were stimulated with Escherichia coli LPS (100 ng/mL) for 1 to 5 h in culture conditions. A linear decrease in the production of LPS-induced tumor necrosis factor alpha (TNF-alpha) and interleukin (IL) 10 was observed with age. LPS-induced IL-6 and IL-1beta levels were also reduced with age, but in a nonlinear fashion. Expression of CD14, the major receptor for LPS, on the PMphi surface was also observed to decline with age. Moreover, TNF-alpha production by PMphis was reduced in mice undergoing the two different CR diets of limited daily feeding and intermittent fasting, as compared with ad libitum-fed mice. The results of this study add the new variables age and diet to the paradigm proposing that the response to LPS is modulated by multiple components, including genetic background and sex.

Impaired antigen-induced CD8+ T cell clonal expansion in aging is due to defects in antigen presenting cell function

CD8+ T cell activation depends on interaction with antigen-presenting cells (APCs) and this interaction leads to the expansion of T cells with the capacity to control infection. Using professional APCs, we demonstrate that with age, the duration of APC-T cell contact time required to achieve clonal expansion increases. Naïve CD8+ T cells from aged mice showed no defect in antigen-induced proliferation when stimulated with APC from young mice. In contrast, CD8+ T cells from young mice exhibited reduced clonal expansion and secreted significantly lower amounts of IFN-gamma when stimulated by APCs from aged mice. The aged APCs were defective in costimulatory molecule expression and cytokine and chemokine secretion. These data indicate that defects in APC function lead to poor T cell clonal expansion and function in aging.

No intrinsic deficiencies in CD8+ T cell-mediated antitumor immunity with aging

Aging is associated with a decline in immune function, particularly within the T cell compartment. Because CD8(+) T cells are critical mediators of protective immunity against cancer, which arises more frequently with advancing age, it is important to understand how aging affects T cell-based antitumor responses. We used our DUC18 T cell/CMS5 tumor model system to examine the ability of both aged APCs and aged, tumor-specific CD8(+) T cells to mount protective responses to tumors in vivo. An assessment of aged DUC18 T cells in vitro showed a naive phenotype, but impaired proliferation in response to anti-CD3 and anti-CD28 stimulation. We found that DCs from young and old recipient mice are comparable phenotypically, and endogenous APCs in these mice are equally able to prime adoptively transferred young DUC18 T cells. Even when aged DUC18 T cells are transferred into aged CMS5-challenged mice, Ag-specific proliferation and CD25 expression are similar to those found when young DUC18 T cells are transferred into young mice. Although trafficking to tumor sites appears unequal, old and young DUC18 T cells reject primary CMS5 challenges to the same degree and with similar kinetics. Overall, we found no loss of endogenous APC function or intrinsic defects in CD8(+) DUC18 T cells with advanced age. Therefore, when young and old tumor-specific T cell populations are equivalently sized, CD8(+) T cell-mediated antitumor immunity in our system is not impaired by age, a finding that has positive implications for T cell-based immunotherapies.

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.

Cuttine edge: T cells from aged mice are resistant to depletion early during virus infection

Aging is associated with decreased expansion of T cells upon stimulation. In young mice, infection induces a transient T cell depletion followed by the development of an Ag-specific T cell response that controls the infection. We found that T cells were depleted early after infection with E55 + murine leukemia retrovirus in young, but not aged, mice. Adoptive transfer experiments showed donor T cells of young, but not aged, mice were depleted due to apoptosis in various tissues of young recipients. However, T cells of neither young nor aged donors were depleted in aged recipients. These results indicate that both environmental and intrinsic cellular properties limit depletion of T cells of aged mice and suggest a novel explanation for the decreased T cell response associated with aging.

An age-related increase in the number of CD8+ T cells carrying receptors for an immunodominant Epstein-Barr virus (EBV) epitope is counteracted by a decreased frequency of their antigen-specific responsiveness

The aim of this study was to provide a basis for investigating the effects of one very common environmental factor, Epstein-Barr virus (EBV), on age-related changes in the immune system. To this end, the frequency of CD8(+) T cells carrying receptors for an immunodominant EBV lytic epitope was assessed by direct staining with HLA-peptide tetrameric complexes in 19 very old (>87 years) and 12 young (20-40 years) EBV carriers. The frequency of EBV-tetramer-positive cells within the CD8(+) subset was significantly greater in the old compared to the young group (P=0.001). However, the frequency of EBV antigen-specific IFN-gamma producing T cells, as determined by ELISPOT, was significantly lower in the old (P=0.001). Therefore, the absolute number of functional EBV-specific T cells in the elderly and the young was probably similar. These data suggest CD8 clonal expansions in the elderly, resulting in an accumulation of dysfunctional EBV-specific cells which possibly fill the 'immunological space' and could lead to a shrinking of the T cell repertoire for other novel antigens. This may help to explain the increased incidence and case-fatality caused by viruses and intracellular pathogens in the elderly.

Pathways to a robust immune response in the elderly

Circumstantial evidence suggests that infectious disease is the major cause of morbidity and mortality in the elderly, and immune-system dysfunction may contribute to this finding. Because innate and humoral immunity seem to be relatively unaffected by aging and because the T-cell compartment shows marked age-associated alterations, this article focuses on the association between T cells and aging. Longitudinal studies suggest that immune parameters, which predominantly are related to T cells, can be clustered to yield an IRP that is predictive of mortality in the elderly. Determining the IRP also may be helpful in younger individuals, particularly those under chronic antigenic stress (eg, patients with cancer or chronic infections) who experience premature aging of the immune system. Some changes in T cells can be modeled in clonal cultures in vitro to discover new biomarkers of immune aging. These biomarkers, which need to be validated in vivo, could be used to refine IRP. Interventions to selectively target changes that are identified as part of IRP may improve the health and quality of life of the elderly, reduce healthcare costs, and avoid potential unwanted side effects of global intervention approaches, such as triggering or exacerbating autoimmunity and inflammation.

Origin of late-onset autoimmune disease

Autoimmune disease in the elderly is hypothesized to be caused by an imbalance in T-cell expansion and deletion after an encounter with self-antigens. A decrease in thymic output leads to a decreased pool of naive T cells in the periphery and to increased oligoclonal expansion of T cells. This expansion may be caused by stimulation with autoantigens that drive high-affinity interactions with self-antigens. Accumulation of presenescent, apoptosis-resistant, and proinflammatory T cells results in the growth of these autoreactive T cells. A decreased T-cell activation response that occurs with age leads to several defects that diminish the immune response.

Cutting edge: impaired Toll-like receptor expression and function in aging

Toll-like receptors (TLR) are pattern recognition receptors that recognize conserved molecular patterns on microbes and link innate and adaptive immune systems. We investigated whether the enhanced susceptibility to bacterial, yeast, and viral infections and poor adaptive immune responses in aging are a result of diminished expression and function of TLRs. We examined the expression and function of all murine TLRs on macrophages from young and aged mice. Both splenic and activated peritoneal macrophages from aged mice expressed significantly lower levels of all TLRs. Furthermore, macrophages from aged mice secreted significantly lower levels of IL-6 and TNF-alpha when stimulated with known ligands for TLR1 and 2, 2 and 6,TLR3, TLR4, TLR5, and TLR9 when compared with those from young mice. These results support the concept that increased susceptibility to infections and poor adaptive immune responses in aging may be due to the decline in TLR expression and function.

Beneficial influences of systemic cooperation and sociological behavior on longevity

During his long research career in the field of aging, Dr Bernard Strehler developed a series of theories concerning the identity of genes that can promote longevity and their role in natural selection. As a tribute to Dr Strehler, we have taken this opportunity to summarize a selection of these theories and to illustrate how these insights have influenced our search for longevity genes within the immune system. The identification of longevity genes has proven difficult. We believe that, at least in part, this reflects the emphasis on the concept of survival of the 'physically' fittest. We have used the immune system as a model to demonstrate that, over and above the self-evident advantage of those genes that contribute the attributes commonly associated with survival of the 'physically' fittest, those genes that lead to a predisposition to cooperate also confer a competitive survival advantage. As the acquisition of cooperativity in a society is linked to support mechanisms provided by older individuals, the search for longevity genes should not be limited to those genes that are associated with extended expression of a youthful phenotype. Rather these studies should be expanded to include identification of those genes that regulate physiologic parameters that affect individual longevity, even if they do not correspond with the traditional view of reproductive competitiveness. At the societal level, longevity genes may encode attributes that regulate sociologic or psychological parameters that may contribute to a tendency to non-aggressive or cooperative behavior that leads to achievement of common goals necessary for the survival of the species. This view of the selection for longevity impacts the analysis of longevity genes and aging at the organismal level. Dr Strehler viewed organismal aging as an integrated functional state, in which he conceived the outcome as reflecting the net balance of functional decrementers and evolved compensatory features. We propose that, in more evolved species, the longevity genes will be those genes, or sets of genes, that counterbalance of age-related functional decrementers with the age-related manifestation of evolved compensatory features. Thus, as illustrated here through analysis of the immune system, the longevity genes may well be those genes that promote overall systemic cooperation and compensation within the immune system and associated systems, rather than the genes that prevent age-related alterations in only one or a limited number of pathways.