Telomeres and immunological diseases of aging

Understanding immunosenescence to improve responses to vaccines

In the older adult, the benefits of vaccination to prevent infectious disease are limited, mainly because of the adaptive immune system's inability to generate protective immunity. The age-dependent decrease in immunological competence, often referred to as 'immunosenescence', results from the progressive deterioration of innate and adaptive immune responses. Most insights into mechanisms of immunological aging have been derived from studies of mouse models. In this Review, we explore how well such models are applicable to understanding the aging process throughout the 80-100 years of human life and discuss recent advances in identifying and characterizing the mechanisms that underlie age-associated defective adaptive immunity in humans.

What is causing my arthritis, doctor? A glimpse beyond the usual suspects in the pathogenesis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a common, but heterogeneous, disease. Usually, when it comes to the pathogenesis of RA the physician faces a complex network of cytokines and cells of the immune system-the so-called effector level. However, is this network 'the cause' of the disease? Or is this rather the level most physicians are somewhat familiar with, as modern anti-rheumatic medications are having their targets there? In this review, we are looking beyond the usual culprits from the physician's perspective and discuss how other factors, such as genes, epigenetics, environmental factors, local joint characteristics or processes of aging might influence the clinical phenomenon RA.

The role of telomeres in musculoskeletal diseases

The telomeric region of repetitive DNA sequences at the end of chromosomes prevents end-to-end fusion of chromosome terminals and deterioration of the doublestrand free ends. Because of the 'end-replication problem', telomeres shorten with each round of cell division, resulting in cell senescence. The enzyme telomerase compensates for telomere shortening by elongating telomeric sequences, thereby prolonging the lifespan of the cell. Studies of articular cartilage and bone tissues have indicated that telomere shortening limits normal cell function and proliferation, while the telomere maintenance mechanisms of osteosarcoma cells facilitate escape from cell death and promote immortality. This article reviews the literature on this topic and provides an extensive discussion of the basic molecular biology and roles of telomeres and telomerase in musculoskeletal diseases such as osteoarthritis, osteoporosis and osteosarcoma. Findings to date suggest that telomeres and telomerase may become novel therapeutic targets for the diagnosis, treatment and prevention of musculoskeletal disorders.

Chronic kidney disease: a clinical model of premature aging

Premature aging is a process associated with a progressive accumulation of deleterious changes over time, an impairment of physiologic functions, and an increase in the risk of disease and death. Regardless of genetic background, aging can be accelerated by the lifestyle choices and environmental conditions to which our genes are exposed. Chronic kidney disease is a common condition that promotes cellular senescence and premature aging through toxic alterations in the internal milieu. This occurs through several mechanisms, including DNA and mitochondria damage, increased reactive oxygen species generation, persistent inflammation, stem cell exhaustion, phosphate toxicity, decreased klotho expression, and telomere attrition. Because recent evidence suggests that both increased local signaling of growth factors (through the nutrient-sensing mammalian target of rapamycin) and decreased klotho expression are important modulators of aging, interventions that target these should be tested in this prematurely aged population.

Telomeres and early-life stress: an overview

The long-term sequelae of adverse early-life experiences have long been a focus in psychiatry, with a historic neurobiological emphasis on physiological systems that are demonstrably stress-responsive, such as the hypothalamic-pituitary-adrenal axis and neuroimmune function. However, there has been increasing recognition in the general medical literature that such sequelae might encompass more pervasive alterations in health status and physiology. Recent findings in telomere biology have suggested a new avenue for exploring the adverse health effects of childhood maltreatment. Telomere length in proliferative tissues declines with cell replication and the effect can be accelerated by such factors as inflammation, oxidative stress, radiation, and toxins. Reduced telomere length, as a proxy for cellular aging, has been associated with numerous chronic somatic diseases that are generally considered to be diseases of aging, such as diabetes, cancer, and heart disease. More recently, shorter telomeres have been demonstrated in several psychiatric conditions, particularly depression. Sustained psychosocial stress of a variety of types in adulthood appears to be associated with shorter telomeres. Now, emerging work suggests a robust, and perhaps dose-dependent, relationship with early-life stress. These findings present new opportunities to reconceptualize the complex relationships between experience, physical and psychiatric disease, and aging.

The risk of infections associated with rheumatoid arthritis, with its comorbidity and treatment

RA is known to be associated with an increased risk of serious infection. Even more than 50 years ago, observational studies showed a greater than 2-fold increased risk of serious infection in RA. This was reinforced by various subsequent cohort studies. The elevated susceptibility of patients with RA can be explained by the pathobiology of the disease itself, the impact of chronic comorbid conditions, as well as sequelae of immunosuppressive treatment. It has been suggested that premature ageing of the immune system in RA contributes to weakened protection against infectious organisms. In addition, chronic comorbid conditions such as diabetes or chronic lung or kidney disease, disease-related functional disability, as well as lifestyle factors such as smoking, increase the risk in individual patients. For a long time glucocorticoids (GCs) have been used as potent immunosuppressive drugs in RA. There is evidence that they increase the risk of serious infections up to 4-fold in a dose-dependent manner. TNF-α inhibitors increase the serious infection risk up to 2-fold. They have, however, the potential to outweigh their risk when higher GC doses can be tapered down. If patients need higher dosages of GCs in addition to treatment with biologic agents, their risk of infection is substantial. This combination should be used carefully and, if possible, avoided in patients with additional risk factors such as older age or comorbid conditions.

Analysis of sperm telomere length in men with idiopathic infertility

PURPOSE

Telomeres are multifunctional nucleoprotein domains with hexanucleotide tandem repeat (5' TTAGGG 3') sequences, which cap the chromosome ends. However, the role of telomere and its length in sperm with regard to fertility remains unknown.

METHODS

In this pilot study, we analyzed 32 idiopathic infertile men and 25 controls for sperm telomere length by quantitative polymerase chain reaction (Q-PCR), and correlated it with sperm DNA fragmentation index (DFI) and reactive oxygen species (ROS) levels.

RESULTS

The relative sperm mean telomere length (T/S) of infertile men was found to be significantly lower (p < 0.005) when compared to controls (0.674 ± 0.028 vs. 0.699 ± 0.030). None of the sperm parameters such as sperm count, forward motility, morphology, ROS, and DFI were found to correlate with the sperm telomere length.

CONCLUSION

Shorter telomeres in sperm may be one of the causative factors responsible for male infertility, but further detailed studies are needed to confirm these findings.

HIV and aging: state of knowledge and areas of critical need for research. A report to the NIH Office of AIDS Research by the HIV and Aging Working Group

HIV risk behaviors, susceptibility to HIV acquisition, progression of disease after infection, and response to antiretroviral therapy all vary by age. In those living with HIV, current effective treatment has increased the median life expectancy to >70 years of age. Biologic, medical, individual, social, and societal issues change as one ages with HIV infection, but there has been only a small amount of research in this field. Therefore, the Office of AIDS Research of the National Institutes of Health commissioned a working group to develop an outline of the current state of knowledge and areas of critical need for research in HIV and Aging; the working groups' findings and recommendations are summarized in this report. Key overarching themes identified by the group included the following: multimorbidity, polypharmacy, and the need to emphasize maintenance of function; the complexity of assessing HIV versus treatment effects versus aging versus concurrent disease; the inter-related mechanisms of immune senescence, inflammation, and hypercoagulability; the utility of multivariable indices for predicting outcomes; a need to emphasize human studies to account for complexity; and a required focus on issues of community support, caregivers, and systems infrastructure. Critical resources are needed to enact this research agenda and include expanded review panel expertise in aging, functional measures, and multimorbidity, and facilitated use and continued funding to allow long-term follow-up of cohorts aging with HIV.

Telomeres and immune competency

Telomeres are essential for the integrity of chromosomes and for cellular replication. Attrition of telomeres occurs during DNA replication owing to the inability of conventional DNA polymerase to replicate chromosomal termini and the insufficient compensation for telomere loss by telomerase, an enzyme that synthesizes telomeric DNA. A number of genetic defects have been described in humans and in animal models that cause accelerated telomere attrition, in turn leading to severe phenotypes of hematopoietic and other proliferating cells. Telomere length, most frequently measured as an average value in heterogeneous peripheral blood leukocyte populations in humans, has been associated with a wide range of health conditions and diseases of immune and non-immune cells. Here, I review recent studies of telomere length dynamics with particular relevance to immune function.

Changes in stress, eating, and metabolic factors are related to changes in telomerase activity in a randomized mindfulness intervention pilot study

BACKGROUND

Psychological distress and metabolic dysregulation are associated with markers of accelerated cellular aging, including reduced telomerase activity and shortened telomere length. We examined whether participation in a mindfulness-based intervention, and, secondarily, improvements in psychological distress, eating behavior, and metabolic factors are associated with increases in telomerase activity in peripheral blood mononuclear cells (PBMCs).

METHODS

We enrolled 47 overweight/obese women in a randomized waitlist-controlled pilot trial (n=47) of a mindfulness-based intervention for stress eating and examined changes in telomerase activity from pre- to post-intervention. In secondary analyses, changes in telomerase activity across the sample were examined in relation to pre- to post-intervention changes in psychological distress, eating behavior, and metabolic factors (weight, serum cortisol, fasting glucose and insulin, and insulin resistance).

RESULTS

Both groups increased in mean telomerase activity over 4 months in intent-to-treat and treatment efficacy analyses (p<0.001). Nonsignificant trends showed that greater attendance was associated with increases in telomerase, and telomerase increases were 18% higher among 'as treated' participants compared to controls. Across groups, changes in chronic stress, anxiety, dietary restraint, dietary fat intake, cortisol, and glucose were negatively correlated with changes in telomerase activity. In exploratory analyses, decreases in dietary fat intake partially mediated the association between dietary restraint and telomerase activity with marginal significance.

CONCLUSIONS

While there was no clear effect of the intervention on telomerase activity, there was a striking pattern of correlations between improvements in psychological distress, eating behavior, and metabolic health and increases in telomerase activity. These findings suggest that telomerase activity may be in part regulated by levels of both psychological and metabolic stress.

mRNA decay factor AUF1 maintains normal aging, telomere maintenance, and suppression of senescence by activation of telomerase transcription

Inflammation is associated with DNA damage, cellular senescence, and aging. Cessation of the inflammatory cytokine response is mediated in part through cytokine mRNA degradation facilitated by RNA-binding proteins, including AUF1. We report a major function of AUF1-it activates telomerase expression, suppresses cellular senescence, and maintains normal aging. AUF1-deficient mice undergo striking telomere erosion, markedly increased DNA damage responses at telomere ends, pronounced cellular senescence, and rapid premature aging that increases with successive generations, which can be rescued in AUF1 knockout mice and their cultured cells by resupplying AUF1 expression. AUF1 binds and strongly activates the transcription promoter for telomerase catalytic subunit Tert. In addition to directing inflammatory cytokine mRNA decay, AUF1 destabilizes cell-cycle checkpoint mRNAs, preventing cellular senescence. Thus, a single gene, AUF1, links maintenance of telomere length and normal aging to attenuation of inflammatory cytokine expression and inhibition of cellular senescence.

Signaling pathways in aged T cells - a reflection of T cell differentiation, cell senescence and host environment

With increasing age, the ability of the immune system to protect against new antigenic challenges or to control chronic infections erodes. Decline in thymic function and cumulating antigenic experiences of acute and chronic infections threaten T cell homeostasis, but insufficiently explain the failing immune competence and the increased susceptibility for autoimmunity. Alterations in signaling pathways in the aging T cells account for many of the age-related defects. Signaling threshold calibrations seen with aging frequently built on mechanisms that are operational in T cell development and T cell differentiation or are adaptations to the changing environment in the aging host. Age-related changes in transcription of receptors and signaling molecules shift the balance towards inhibitory pathways, most dominantly seen in CD8 T cells and to a lesser degree in CD4 T cells. Prominent examples are the expression of negative regulatory receptors of the CD28 and the TNF receptor superfamilies as well the expression of various cytoplasmic and nuclear dual-specific phosphatases.

IL-12 upregulates TIM-3 expression and induces T cell exhaustion in patients with follicular B cell non-Hodgkin lymphoma

The cytokine IL-12 induces IFN-γ production by T and NK cells. In preclinical models, it contributes to antitumor immunity. However, in clinical testing, it has shown limited benefit in patients with any one of a variety of malignancies. Moreover, in a clinical trial testing a combination of IL-12 and rituximab in patients with follicular B cell non-Hodgkin lymphoma (FL), those treated with IL-12 showed a lower response rate, suggesting that IL-12 actually plays a detrimental role. Here, we investigated whether the failure of IL-12 treatment for FL was due to T cell exhaustion, a condition characterized by reduced T cell differentiation, proliferation, and function, which has been observed in chronic viral infection. We found that extended exposure to IL-12 induced T cell exhaustion and contributed to the poor prognosis in FL patients. Long-term exposure of freshly isolated human CD4+ T cells to IL-12 in vitro caused T cell dysfunction and induced expression of TIM-3, a T cell immunoglobulin and mucin domain protein with a known role in T cell exhaustion, via an IFN-γ-independent mechanism. TIM-3 was required for the negative effect of IL-12 on T cell function. Importantly, TIM-3 also was highly expressed on intratumoral T cells that displayed marked functional impairment. Our findings identify IL-12- and TIM-3-mediated exhaustion of T cells as a mechanism for poor clinical outcome when IL-12 is administered to FL patients.

DNA repair: the link between primary immunodeficiency and cancer

The adaptive component of the immune system depends greatly on the generation of genetic diversity provided by lymphocyte-specific genomic rearrangements. V(D)J recombination, class switch recombination (CSR), and somatic hypermutation (SHM) constitute complex and vulnerable processes that are orchestrated by a multitude of DNA repair pathways. When inherited defects in certain DNA repair proteins are present, lymphocyte development can be compromised and, consequently, patients can develop primary immunodeficiencies (PIDs). PID patients often have a strong predisposition for cancer development as a result of genomic instability generated from defective DNA repair mechanisms. Tumors of lymphoid origin are one of the most common PID-associated cancers, likely due to DNA lesions resulting from defective V(D)J, CSR, and SHM. In this review, we describe PID syndromes that confer an increased risk for cancer development. Furthermore, we discuss the role of the affected proteins in tumorigenesis/lymphomagenesis.

Mechanisms of immunosenescence: lessons from models of accelerated immune aging

With increasing age, the ability of the adaptive immune system to respond to vaccines and to protect from infection declines. In parallel, the production of inflammatory mediators increases. While cross-sectional studies have been successful in defining age-dependent immunological phenotypes, studies of accelerated immune aging in human subpopulations have been instrumental in obtaining mechanistic insights. The immune system depends on its regenerative capacity; however, the T cell repertoire, once established, is relatively robust to aging and only decompensates when additionally stressed. Such stressors include chronic infections such as CMV and HIV, even when viral replication is controlled, and autoimmune diseases. Reduced regenerative capacity, chronic immune activation in the absence of cell exhaustion, T cell memory inflation, and accumulation of highly potent effector T cells in these patients synergize to develop an immune phenotype that is characteristic of the elderly. Studies of accelerated immune aging in autoimmune diseases have identified an unexpected link to chronic DNA damage responses that are known to be important in aging, but so far had not been implicated in immune aging.

Telomere dysfunction, autoimmunity and aging

Immune aging is associated with loss of critical immune functions, such as host protection from infection and malignancy. Unexpectedly, immunosenescence also renders the host susceptible to inflammation, which may translate into tissue-damaging disease as the senescent immune system loses its ability to maximize inflammatory protection while minimizing inflammatory injury. On the other hand, chronic inflammation associated with immune-mediated disease represents a profound stress factor for the immune system, affecting cellular turn-over, replication and exhaustion. Immune cell longevity is tightly connected to the functional integrity of telomeres which are regulated by cell multiplication, exposure to oxidative stress and DNA repair mechanisms. Lymphocytes are amongst the few cell types that can actively elongate telomeres through the action of telomerase. In patients with the autoimmune disease rheumatoid arthritis (RA), telomerase deficiency is associated with prematurity of immune aging. Patients with RA have other defects in DNA repair mechanisms, including the kinase Ataxia telangiectasia mutated (ATM), critically involved in the repair of DNA double strand breaks. ATM deficiency in RA shortens lymphocyte survival. Dynamics of telomeric length and structure are beginning to be understood and have distinct patterns in different autoimmune diseases, suggesting a multitude of molecular mechanisms defining the interface between chronic immune stimulation and progressive aging of the immune system.

Down syndrome: is it really characterized by precocious immunosenescence?

The immune system declines with aging, leading to an increased susceptibility to infections and higher incidence and progression of autoimmune phenomena and neoplasia. Down syndrome prematurely shows clinical manifestations that are normally seen with aging. This review provides a concise overview of abnormalities in the adaptive immune system of Down syndrome in comparison to normal and precocious (Progeria syndromes) aging. Clinical signs and immunological changes are reviewed. We challenge the hypothesis that the immunological abnormalities in Down syndrome should be interpreted as precocious immunosenescence.

The catalytic subunit of human telomerase is a unique caspase-6 and caspase-7 substrate

Telomerase is a ribonucleoprotein complex that is essential for persistent cellular proliferation. The catalytic subunit of human telomerase, hTERT, functions as a reverse transcriptase and promotes vitality by maintaining telomeric DNA length. hTERT is tightly regulated with complex but poorly understood positive and negative regulation at several levels including transcription, protein-protein interactions, and post-translation modifications. Because evidence implicates hTERT as an apoptosis inhibitor and because telomerase activity tends to decrease during apoptosis, we hypothesized that hTERT is a caspase substrate leading to down regulation during apoptosis. Caspases are proteases that initiate and execute apoptosis by cleaving target proteins. Indeed, we found that caspases-6 and -7 cleave hTERT during apoptosis in cultured cells. Caspase-6 cleaves at residues D129 and D637, and caspase-7 cleaves at E286 and D628. Three of the caspase cleavage sites are unique motifs. All four caspase motifs appear conserved in TERTs from Old World monkeys and apes, and the caspase-6 sites appear conserved in all primates. The caspase site that cleaves at D129 appears conserved in amniotes. hTERT fragments generated by cleavage were remarkably persistent, lasting hours after caspase activation. These results reveal a new biologically relevant mechanism for telomerase down regulation through caspase-mediated cleavage of hTERT and expand the list of known caspase motifs.

Aging and immune function: molecular mechanisms to interventions

Abstract The immune system of an organism is an essential component of the defense mechanism aimed at combating pathogenic stress. Age-associated immune dysfunction, also dubbed "immune senescence," manifests as increased susceptibility to infections, increased onset and progression of autoimmune diseases, and onset of neoplasia. Over the years, extensive research has generated consensus in terms of the phenotypic and functional defects within the immune system in various organisms, including humans. Indeed, age-associated alterations such as thymic involution, T cell repertoire skewing, decreased ability to activate naïve T cells and to generate robust memory responses, have been shown to have a causative role in immune decline. Further, understanding the molecular mechanisms underlying the generation of proteotoxic stress, DNA damage response, modulation of ubiquitin proteasome pathway, and regulation of transcription factor NFκB activation, in immune decline, have paved the way to delineating signaling pathways that cross-talk and impact immune senescence. Given the role of the immune system in combating infections, its effectiveness with age may well be a marker of health and a predictor of longevity. It is therefore believed that a better understanding of the mechanisms underlying immune senescence will lead to an effective interventional strategy aimed at improving the health span of individuals. Antioxid. Redox Signal. 14, 1551-1585.

Telomeres and telomerase in renal health

The role of telomeres and telomerase in human biology has been studied since the early 1990s because telomere attrition is implicated in various diseases including cardiovascular dysfunction, carcinogenesis, and the progression of acute kidney injury. Telomeric length is a reliable indicator of intrinsic biologic age and a surrogate for the mitotic clock. Because the prevalence of chronic kidney disease increases with age, telomere length and telomerase activity may play a role in its progression.

Immune aging and rheumatoid arthritis

Immunologic models of rheumatoid arthritis (RA) have to take into account that the disease occurs at an age when immunocompetence is declining and in a host whose immune system shows evidence of accelerated immune aging. By several immune aging biomarkers, the immune system in patients with RA is prematurely aged by more than 20 years. One major pathogenetic mechanism is a defect in telomere maintenance and DNA repair that causes accelerated cell death. These findings in RA are reminiscent of murine autoimmunity models, in which lymphopenia was identified as a major risk factor for autoimmunity. Progress in the understanding of how accelerated immune aging is pathogenetically involved in RA may allow development of new therapeutic approaches that go beyond the use of anti-inflammatory agents and eventually could open new avenues for preventive intervention.