Lineage-specific telomere shortening and unaltered capacity for telomerase expression in human T and B lymphocytes with age

Age-dependent alterations of the T cell repertoire and functional diversity of T cells of the aged

The aging immune system is characterized by the contraction of T cell receptor (TCR) diversity and the de novo expression of NKrelated receptors (NKR) on oligoclonal T cells. NKR+ T cells likely represent a secondary immune diversification as a biological adaptation of aging to ensure host defense despite shrinkage of the TCR repertoire. NKRs are expressed in various combinations even among TCR-identical cells, and are capable of triggering effector pathways in either TCR-independent or TCR-dependent fashion. Understanding the biology of NKR+ T cells will be pivotal to the development of strategies to enhance immunity in the elderly.

Free radicals in aging: causal complexity and its biomedical implications

Superoxide generated adventitiously by the mitochondrial respiratory chain can give rise to much more reactive radicals, resulting in random oxidation of all classes of macromolecules. Harman's 1956 suggestion that this process might drive aging has been a leading strand of biogerontological thinking since the discovery of superoxide dismutase. However, it has become apparent that the many downstream consequences of free radical damage can also be caused by processes not involving oxidation. Moreover, free radicals have been put to use by evolution to such an extent that their wholesale elimination would certainly be fatal. This multiplicity of parallel pathways and side-effects illustrates why attempts to postpone aging by "cleaning up" metabolism will surely fail for the foreseeable future: we simply understand metabolism too poorly. This has led me to pursue the alternative, "repair and maintenance" approach that sidesteps our ignorance of metabolism and may be feasible relatively soon.

Prolonged exposure of naïve CD8+ T cells to interleukin-7 or interleukin-15 stimulates proliferation without differentiation or loss of telomere length

Interleukin (IL)-7 and IL-15 are cytokines implicated in homeostatic control of the peripheral CD8 T-cell pool. We compared the effects of IL-7 and IL-15 on survival and proliferation of purified human CD8+ T-cell subsets. Low concentrations of either cytokine reduced the spontaneous apoptosis of all subsets, and enhancement of survival corresponded to the extent of Bcl-2 up-regulation. Surprisingly, although minimal proliferation of naïve CD8+ T cells was observed during the first week of culture with cytokines, a marked expansion of these cells occurred at later time points, particularly in response to IL-15. This occurred largely without phenotypic change or acquisition of effector function, indicating a dissociation of differentiation from proliferation. Notably, progression of naïve CD8+ T cells through several cell divisions resulted in up-regulation of telomerase and the maintenance of telomere length. These data show that IL-7 and IL-15 induce cell proliferation and rescue from apoptosis in a concentration, time and subset-dependent manner, and have implications for the homeostatic expansion of the naïve CD8+ T-cell pool.

Human telomere biology: pitfalls of moving from the laboratory to epidemiology

Remarkable progress has been made during the last 2 decades in understanding telomere biology at the molecular and cellular levels. Clinical epidemiology research of human telomeres, in contrast, is a discipline just coming into its own. The most important observation in studying human telomere biology is that telomere length is highly variable among humans. Here we explain some of the reasons for this variability and propose several principles that should be considered in conducting epidemiological telomere research. Ignoring these principles could lead to misleading conclusions.

Aging of the immune system: how much can the adaptive immune system adapt?

The competency of the adaptive immune function decreases with age, primarily because of the decline in production of naive lymphocytes in the bone marrow and thymus as well as the expansion of incompetent memory lymphocytes. Here I discuss the recent progress on age-associated changes in lymphocytes and their effect on the adaptive immune system.

Mechanisms of chromosome instability in cancers

Most tumours arise through clonal selection and waves of expansion of a somatic cell that has acquired genetic alterations in essential genes either controlling cell death or cell proliferation. Furthermore, stability of the genome in cancer cells becomes precarious and compromised because several cancer-predisposing mutations affect genes that are responsible for maintaining the integrity and number of chromosomes during cell division. Consequently, the archetypical transformation in tumour cells results in aneuploidy. Indeed, almost all tumour cells display a host of karyotype alterations, showing translocations, gains or losses of entire or large parts of chromosomes. Cancers do not necessarily have a higher mutation rate than normal tissue at the nucleotide level, unless they have gained a mutator phenotype through exposure to environmental stress, but rather exhibit gross chromosomal changes. Therefore, it appears that the main mechanism of tumour progression stems from chromosome instability. Chromosomal instability prevailing in tumour cells arises through several different pathways and is probably controlled by hundreds of genes. Therefore, this review describes the main factors that control chromosome stability through telomere maintenance, mechanisms of cell division, and the mitotic checkpoints that govern centrosome duplication and correct chromosome segregation.

Flow cytometric analysis of fluorescence in situ hybridization with dye dilution and DNA staining (flow-FISH-DDD) to determine telomere length dynamics in proliferating cells

BACKGROUND

Telomeres shorten during DNA replication; extensive erosion of telomeres likely promotes replicative senescence and chromosomal instability. Telomere length in individual cells has been quantified by flow cytometric analysis of fluorescence in situ hybridization (flow-FISH). To determine the rate of telomere attrition (telomere erosion per cell division), we combined flow-FISH with dye dilution and DNA staining (flow-FISH-DDD) and measured telomere-specific fluorescence in proliferating cells identified by cell generation and cell cycle phase.

METHODS

Peripheral blood mononuclear cells (PBMC) were stained with the cell division tracking dye carboxyfluorescein diacetate succinimidyl ester (CFSE), stimulated with phytohemagglutinin (PHA), grown for 5-6 days, hybridized with a telomere sequence-specific peptide nucleic acid fluorescent probe (PNA-Cy5), counterstained with DAPI, and analyzed by flow cytometry. The cell cycle distribution and cell division generations were respectively identified by analysis of DAPI emission and deconvolution of CFSE emission, and Cy5 emission was used to determine telomere-specific fluorescence, an indicator of telomere length, in each cell.

RESULTS

In stimulated PBMC, in each cell cycle phase, the telomere-specific fluorescence diminished with increasing cell generation. The rate of decline of the telomere-specific fluorescence per cell generation did not significantly differ between cell cycle phases.

CONCLUSIONS

Application of flow-FISH-DDD to measure mean telomere length and the rate of telomere attrition in proliferating cells may find use in studies of ageing and disease, the effects of telomere-modifying agents, and variability between individuals.

Age-associated deficiency in activation-induced up-regulation of telomerase activity in CD4+ T cells

For lymphocytes, the ability to undergo clonal expansion is crucial for effective immune function. Telomerase activity compensates for telomere erosion during cell division and contributes to the capability of lymphocytes to maintain cellular proliferation. In addition, telomerase activity may have a fundamental role in cell growth and survival. To determine whether age-related immune dysfunction is associated with an abnormality in telomerase activity, we investigated telomerase activity in T cell populations from young adult and aged mice. Our data show that the ability of T cells from aged mice to up-regulate telomerase activity after activation was significantly diminished. This age-related deficiency in telomerase induction is restricted to CD4(+) T cells, as CD8(+) T cells retain the capability to up-regulate telomerase activity. These findings reinforce the notion that age-related immune dysfunction results mainly from impairment of helper T cells, and may have important implications for designing novel means to improve immune responses in aged individuals by enhancing CD8(+) T cell functions, which are crucial in both viral and tumour immunity.

Impaired activation-induced telomerase activity in PBMC of early but not chronic rheumatoid arthritis patients

Although telomerase activity is important in normal immune function, it is unclear whether telomerase or telomerase (dys)regulation plays a role in the pathogenic immune response in autoimmune diseases like rheumatoid arthritis (RA). In this study, we evaluated the dynamics of the activation-induced human telomerase reverse transcriptase (hTERT) response in RA patients and non-RA controls. The expression of the catalytic subunit of telomerase, hTERT, was measured in peripheral blood mononuclear cells (PBMC) of RA patients and controls after in vitro stimulation with anti-CD3 monoclonal antibody (mAb) using real-time PCR. Anti-CD3 mAb stimulation induced activation and proliferation of the T cells in all populations studied. In early RA patients with a disease duration of less than 1 year, the activation-induced hTERT mRNA levels were found to be reduced as compared to healthy controls (HC). Chronic RA patients, with a disease duration of more than 1 year, did not show these impaired hTERT mRNA levels after stimulation with anti-CD3 mAb. Decreased hTERT mRNA levels were also found in multiple sclerosis patients and patients suffering from flu-like symptoms, indicating that these deviations are not disease-specific. The impaired activation-induced hTERT response in PBMC may be a general response of the immune cells in cases of acute or chronic immune activation, presumably to control unwanted clonal expansions and to maintain the diversity of the TCR repertoire. Our results also indicate that clonal T cell expansions, described in RA, are probably not mediated by an elevated potency to express hTERT.

IL-15 Activates Telomerase and Minimizes Telomere Loss and May Preserve the Replicative Life Span of Memory CD8+T Cells In Vitro

The preservation of the replicative life span of memory CD8(+) T cells is vital for long-term immune protection. Although IL-15 plays a key role in the homeostasis of memory CD8(+) T cells, it is unknown whether IL-15 regulates the replicative life span of memory CD8(+) T cells. In this study, we report an analysis of telomerase expression and telomere length in human memory phenotype CD8(+) T cells maintained by IL-15 in vitro. We demonstrate that IL-15 is capable of activating telomerase in memory CD8(+) T cells via Jak3 and PI3K signaling pathways. Furthermore, IL-15 induces a sustained level of telomerase activity over long periods of time, and in turn minimizes telomere loss in memory CD8(+) T cells after substantial cell divisions. These findings suggest that IL-15 activates stable telomerase expression and compensates telomere loss in memory phenotype CD8(+) T cells, and that telomerase may play an important role in memory CD8(+) T cell homeostasis.

Impaired innate immunity predicts frailty in old age. The Leiden 85-plus study

Aging is associated with an impaired capacity of the immune system to respond properly to danger signals, such as infection and cancer. Here, we provide evidence that an impaired innate immune response, as measured by a low production capacity of pro- and anti-inflammatory cytokines upon ex vivo standardized danger signalling with bacterial LPS, is predictive for frailty in elderly people: participants who at age 85-year produced low levels of LPS-induced IL-1beta, IL-6, TNF-alpha and IL-1Ra and IL-10, were found to have a more than 2-fold elevated overall mortality risk, independent of chronic illnesses (relative risk is 2.21, 95% confidence interval 1.27-3.82, P = 0.005), compared to peers with a higher production of any of the pro- and/or anti-inflammatory cytokines. A significant genetic association with the IL-10 promoter gene was found, indicating that people who are genetically predisposed low cytokine producers are at a higher risk of losing the capacity to respond properly to danger signals with aging. We conclude that a malfunctioning innate immune response predicts frailty in old age and is under specific (immuno-) genetic control.

Lymphocyte telomere dynamics and telomerase activity in inflammatory bowel disease: effect of drugs and smoking

BACKGROUND

The chromosome instability observed in peripheral blood lymphocytes in ulcerative colitis could be a biomarker of cancer susceptibility.

AIM

To determine whether accelerated telomere shortening could explain chromosome instability and assess the effect of drugs and smoking on telomere dynamics in these cells.

METHODS

Peripheral blood lymphocytes were isolated from ulcerative colitis, Crohn's disease and non-inflammatory bowel disease control patients. Telomere lengths were measured by quantitative real-time polymerase chain reaction. After activation and cell separation, telomerase activity and human telomerase reverse transcriptase messenger ribonucleic acid were measured by telomerase repeat amplification protocol enzyme-linked immunosorbent serological assay and quantitative real-time polymerase chain reaction, respectively.

RESULTS

Age-related telomere loss in peripheral blood lymphocytes was similar in ulcerative colitis, Crohn's disease and control patients. Telomerase activity decreased with age in all groups and correlated positively with telomere length (r = 0.489, P = 0.006). Among Crohn's disease patients, azathioprine was associated with decreased telomerase activity (0.66 vs. 1.54, P = 0.026, P < 0.05) and smoking was associated with decreased human telomerase reverse transcriptase mRNA expression (10.5 vs. 33.3, P = 0.036, P < 0.05).

CONCLUSIONS

Telomere shortening is not accelerated and therefore cannot be the cause of the chromosome instability observed in ulcerative colitis peripheral blood lymphocytes. Azathioprine and cigarette smoking modify telomerase expression in these cells.

Decrease of CD4 + and B-Lymphocyte Populations Is Not Associated with Severe Infectious Complications in Children with Acute Lymphoblastic Leukemia during Maintenance

The suppression of lymphopoiesis and immune competence during the maintenance phase in children with acute lymphoblastic leukemia (ALL) and the occurrence of infectious complications remain an unexplored area. In this study we assessed lymphocyte subpopulation disturbances during maintenance for childhood ALL along with the incidence, type, and severity of infections that occur during that period in the absence of neutropenia. Twenty-eight children (13 boys, 15 girls) with ALL aged 3-14 years (median 7 years) and treated according to the ALL-BFM 90/95 protocol were studied during maintenance for ALL. Complete white blood cell (WBC) counts and peripheral blood lymphocyte (PBL) analyses were performed. Major lymphocyte subsets (CD19+, CD3+CD4+, CD3-CD8+, CD3-CD16+CD56+, CD45RA-, CD45RO+) and markers of T-cell activation (CD25, CD38, CD69, HLA-DR) were analyzed with flow cytometry. Serum immunoglobulin G (IgG), IgA, and IgM levels were measured by a nephelometric assay. All infectious episodes during the study period were recorded in detail. Additionally, 41 age-matched immunocompetent children were used as controls. Absolute WBC counts (median, 3627/microL) and PBL counts (median, 1206/microL) were significantly below the age-adjusted control values (7400/microL and 2673/microL, respectively; P < .0001). B-lymphocyte, total CD4+, and memory CD4+ (CD4+CD45RO+) subsets were also significantly decreased (33/microL versus 377/microL [P < .0001], 531/microL versus 1045/microL [P < .01], and 80/microL versus 299/microL [P < .001], respectively). Significantly lower immunoglobulin levels were found in all patients. Twenty-two of the 28 patients presented with 74 episodes of a variety o minor infections (mostly respiratory viral [39], skin [7], and gastrointestinal [3]), none demanding prolonged hospital treat ment. Our findings demonstrate a profound immunosuppression throughout maintenance therapy in children with ALL tha has no major clinical impact in terms of increased incidence or severity of systemic infections.

K+ channel expression during B cell differentiation: implications for immunomodulation and autoimmunity

Using whole-cell patch-clamp, fluorescence microscopy and flow cytometry, we demonstrate a switch in potassium channel expression during differentiation of human B cells from naive to memory cells. Naive and IgD(+)CD27(+) memory B cells express small numbers of the voltage-gated Kv1.3 and the Ca(2+)-activated intermediate-conductance IKCa1 channel when quiescent, and increase IKCa1 expression 45-fold upon activation with no change in Kv1.3 levels. In contrast, quiescent class-switched memory B cells express high levels of Kv1.3 ( approximately 2000 channels/cell) and maintain their Kv1.3(high) expression after activation. Consistent with their channel phenotypes, proliferation of naive and IgD(+)CD27(+) memory B cells is suppressed by the specific IKCa1 inhibitor TRAM-34 but not by the potent Kv1.3 blocker Stichodactyla helianthus toxin, whereas the proliferation of class-switched memory B cells is suppressed by Stichodactyla helianthus toxin but not TRAM-34. These changes parallel those reported for T cells. Therefore, specific Kv1.3 and IKCa1 inhibitors may have use in therapeutic manipulation of selective lymphocyte subsets in immunological disorders.

Central memory and effector memory T cell subsets: function, generation, and maintenance

The memory T cell pool functions as a dynamic repository of antigen-experienced T lymphocytes that accumulate over the lifetime of the individual. Recent studies indicate that memory T lymphocytes contain distinct populations of central memory (TCM) and effector memory (TEM) cells characterized by distinct homing capacity and effector function. This review addresses the heterogeneity of TCM and TEM, their differentiation stages, and the current models for their generation and maintenance in humans and mice.

Direct Measurement of T Cell Subset Kinetics In Vivo in Elderly Men and Women

The age-associated decline in immunocompetence is paralleled by changes in the proportions of PBL subpopulations. In turn, the size and composition of the peripheral lymphocyte pool is determined by input from the thymus and bone marrow and by the balance of proliferation and death in each lymphocyte subpopulation. We compared the kinetics of lymphocyte subtypes in young (seven of eight CMV seronegative) and healthy elderly human subjects (six of seven CMV seropositive), using deuterated glucose DNA labeling in vivo to measure rates of T cell proliferation and disappearance. For CD45RO(+) cells of both CD4(+) and CD8(+) subtypes and for CD4(+)CD45RA(+) cells the kinetics of proliferation and disappearance were remarkably similar between elderly and young subjects. In the young, the kinetics of CD8(+)CD45RA(+) cells with a naive phenotype resembled those of CD4(+)CD45RA(+) cells. However, CD8(+)CD45RA(+) T cells from the elderly exhibited a predominantly primed phenotype, and for this subset, although the proliferation rate was similar to that of other CD45RA(+) cells, the disappearance rate of labeled cells was greatly decreased compared with that of all other T cell subsets. Our data provide a direct demonstration that there are no substantial changes in in vivo kinetics for most T cell populations in healthy elderly compared with young subjects. However, primed CD8(+)CD45RA(+) cells show unusual kinetic properties, indicating the persistence of these cells in the blood and dissociation of proliferation from disappearance.

Telomere length and telomerase activity: variations with advancing age and potential role in childhood malignancies

Telomeres, representing the chromosome nucleoprotein tails, shorten during each cell division due to the inability of conventional DNA polymerases to completely replicate the chromosome termini. When telomeres become critically short, cells are directed to exit from the cell division cycle (replicative senescence). Telomerase is a reverse transcriptase that synthesizes telomeric sequences, thereby prolonging the lifespan of cells. Telomere length and telomerase activity expression vary significantly in different normal somatic tissues and age groups. In many childhood malignancies (ie, acute leukemias and solid tumors), telomere length and telomerase activity of the malignant cell population may be correlated with the disease outcome and thus may be promising tools in evaluating prognosis and monitoring treatment progress. Finally, telomerase inhibition by using several strategies (ie, antisense oligonucleotides) represents a potentially valuable target for antitumor therapy in the near future.

In vitro senescence of immune cells

Immune cells are eminently suitable model systems in which to address the possible role of replicative senescence during in vivo aging. Since there are more than 10(8) unique antigen specificities present within the total T lymphocyte population of each individual, the immune response to any single antigen requires massive clonal expansion of the small proportion of T cells whose receptors recognize that antigen. The Hayflick Limit may, therefore, constitute a barrier to effective immune function, at least for those T cells that encounter their specific antigen more than once over the life course. Application of the fibroblast replicative senescence model to the so-called cytotoxic or CD8 T cell, the class of T cells that controls viral infection and cancer, has revealed certain features in common with other cell types as well as several characteristics that are unique to T cells. One senescence-associated change that is T cell-specific is the complete loss of expression of the activation signaling surface molecule, CD28, an alteration that enabled the documentation of high proportions of senescent T cells in vivo. The T cell model has also provided the unique opportunity to analyze telomere dynamics in a cell type that has the ability to upregulate telomerase yet nevertheless undergoes senescence. The intimate involvement of the immune system in the control of pathogens and cancer as well as in modulation of bone homeostasis suggests that more extensive analysis of the full range of characteristics of senescent T cells may help elucidate a broad spectrum of age-associated physiological changes.

Telomere length and telomerase activity delineate distinctive replicative features of the B-CLL subgroups defined by immunoglobulin V gene mutations

Patients with B-cell chronic lymphocytic leukemia (B-CLL) segregate into subgroups with very different survival times. Because clinical observations suggest that leukemic cells accumulate at different rates, we measured telomere length and telomerase activity in B-CLL cells to distinguish differences in cellular replication. Our data indicate that the telomeres of B-CLL cells are shorter than telomeres of B cells from healthy subjects, indicating that the leukemic cells have a prolonged proliferative history. Leukemic cells of the immunoglobulin V gene mutation subgroups differ in telomere length and telomerase activity. B lymphocytes from the subgroup with poor outcome and with limited IgV gene mutations have uniformly shorter telomeres and more telomerase activity than those from the subgroup with better outcome and with considerable mutations. Differences in telomere length appear to largely reflect the proliferative histories of precursors of the leukemic cells, although differences in cell division, masked by the action of telomerase, cannot be excluded. These results may provide insight into the stages of maturation and the activation pathways of the cells that give rise to B-CLL. In addition, they reinforce the concept that B-CLL is not simply an accumulative disease of slowly dividing B lymphocytes but possibly one of B cells with extensive proliferative histories.

Telomere maintenance and disease

The proliferative capacity of human cells is regulated by telomerase, an enzyme uniquely specialised for telomeric DNA synthesis. The critical role of telomerase activation in tumour progression and tumour maintenance has been well established in studies of cancer and of oncogenic transformation in cell culture. New evidence suggests that telomerase activation has an important role in normal somatic cells, and that failure to activate sufficient telomerase also promotes disease. We review the evidence for premature telomere attrition in proliferative deficiencies of the human haemopoietic system, and discuss the potential use of telomerase activation in telomere-restorative gene therapy.

Telomerase levels control the lifespan of human T lymphocytes

The loss of telomeric DNA with each cell division contributes to the limited replicative lifespan of human T lymphocytes. Although telomerase is transiently expressed in T lymphocytes upon activation, it is insufficient to confer immortality. We have previously shown that immortalization of human CD8+ T lymphocytes can be achieved by ectopic expression of the human telomerase reverse transcriptase (hTERT) gene, which encodes for the catalytic component of the telomerase complex. To study the role of endogenous hTERT in the lifespan of human T cells, we blocked endogenous hTERT expression by ectopic expression of dominant-negative (DN) hTERT. Cells expressing DN-hTERT had a decreased lifespan and showed cytogenetic abnormalities, including chromosome ends without detectable telomeric DNA as well as chromosome fusions. These results indicate that while endogenous hTERT cannot prevent overall telomere shortening, it has a major influence on the longevity of human T cells. Furthermore, we show that up-regulation of hTERT in T cells upon activation decreases over time in culture. Long-term-cultured T cells also show a decreased expression of c-myc upon activation, resulting in less c-myc-induced transcription of hTERT. Moreover, memory T cells, which have expanded in vivo upon antigen encounter, expressed a lower level of hTERT upon activation than naive cells from the same donor. The observed inverse correlation between telomerase levels and replicative history suggests that telomerase levels in T cells are limiting and increasingly insufficient to sustain their proliferation.

Immunosenescence, autoimmunity, and rheumatoid arthritis

Current disease models of autoimmune syndromes, such as rheumatoid arthritis, propose that chronic inflammation is caused by 'forbidden T-cell clones' that recognize disease-inducing antigens and drive tissue-injurious immune reactions. Reappraisal of disease incidence data, however, emphasizes that rheumatoid arthritis is a syndrome of the elderly that occurs with highest likelihood in individuals in whom the processes of T-cell generation and T-cell repertoire formation are compromised. Thymic T-cell production declines rapidly with advancing age. Multiple mechanisms, including antigen-driven clonal expansion and homeostasis-driven autoproliferation of post-thymic T cells, impose replicative stress on T cells and induce the biological program of cellular senescence. T-cell immunosenescence is associated with profound changes in T-cell functional profile and leads to accumulation of CD4+ T cells that have lost CD28 but have gained killer immunoglobulin-like receptors and cytolytic capability and produce large amounts of interferon-gamma. In patients with rheumatoid arthritis, T-cell immunosenescence occurs prematurely, probably due to a deficiency in the ability to generate sufficient numbers of novel T cells. We propose that autoimmunity in rheumatoid arthritis is a consequence of immunodegeneration that is associated with age-inappropriate remodeling of the T-cell pool.

Telomerase maintains telomere structure in normal human cells

In normal human cells, telomeres shorten with successive rounds of cell division, and immortalization correlates with stabilization of telomere length. These observations suggest that human cancer cells achieve immortalization in large part through the illegitimate activation of telomerase expression. Here, we demonstrate that the rate-limiting telomerase catalytic subunit hTERT is expressed in cycling primary presenescent human fibroblasts, previously believed to lack hTERT expression and telomerase activity. Disruption of telomerase activity in normal human cells slows cell proliferation, restricts cell lifespan, and alters the maintenance of the 3' single-stranded telomeric overhang without changing the rate of overall telomere shortening. Together, these observations support the view that telomerase and telomere structure are dynamically regulated in normal human cells and that telomere length alone is unlikely to trigger entry into replicative senescence.

Ectopic hTERT expression extends the life span of human CD4+ helper and regulatory T-cell clones and confers resistance to oxidative stress-induced apoptosis

Human somatic cells have a limited life span in vitro. Upon aging and with each cell division, shortening of telomeres occurs, which eventually will lead to cell cycle arrest. Ectopic hTERT expression has been shown to extend the life span of human T cells by preventing this telomere erosion. In the present study, we have shown that ectopic hTERT expression extends the life span of CD4+ T helper type 1 or 2 and regulatory T-cell clones and affected neither the in vitro cytokine production profile nor their specificity for antigen. In mixed cell cultures, ectopic hTERT-expressing clones were found to expand in greater numbers than untransduced cells of the same replicative age. This ectopic hTERT-induced growth advantage was not due to an enhanced cell division rate or number of divisions following T-cell receptor-mediated activation, as determined in carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeling experiments. Moreover, the susceptibility to activation-induced cell death of both cell types was similar. However, cultures of resting hTERT-transduced T cells contained higher frequencies of Bcl-2-expressing cells and lower active caspase-3-expressing cells, compared with wild-type cells. Furthermore, hTERT-transduced cells were more resistant to oxidative stress, which causes preferential DNA damage in telomeres. Taken together, these results show that ectopic hTERT expression not only protects proliferating T cells from replicative senescence but also confers resistance to apoptosis induced by oxidative stress.

Duration of senescent cell survival in vitro as a characteristic of organism longevity, an additional to the proliferative potential of fibroblasts

More than 40 years have passed since the original publication by Hayflick and Moorhead led to the concept of the 'Hayflick limit' of the maximum number of divisions which somatic cells undergo in vitro. This concept is still regarded as a fundamental characteristic of species longevity. Here we want to emphasize another characteristic of somatic cells, namely, the duration of their survival in vitro in the non-dividing state after cessation of proliferation. This is suggested on the basis of results of recent experiments with so-called Japanese accelerated senescent mice. Results of these experiments reveal a good correlation between the longevity of the mice, the number of duplications of their fibroblasts in vitro, and the survival time of these cells in the non-dividing state. In routine culture conditions, cell survival time may be very long, as much as a few years. However, when the cells are grown under conditions of oxidative stress, cellular longevity is markedly shortened. This new test may serve as an additional marker of organismic longevity. The comparative value of both tests, the classical 'Hayflick limit' and the new test, is discussed.

Telomere length and telomerase activity: effect of ageing on human NK cells

Telomeres are repeats of TTAGGG sequences located at the end of eukaryotic chromosomes. They are essential for stabilisation and protection of chromosomal ends and for the regulation of cell replicative capacity. Due to the end-replication defect of DNA polymerase, telomeres shorten progressively with each cell division and telomere length may be an indicator of the replicative history of a cell. Compensatory mechanisms for the telomere loss have been identified. The most widely studied one is mediated by telomerase a ribonuclear protein-enzyme complex that synthesise telomeric repeats. In this study we have investigated whether NK cells, derived from a group of old healthy subjects, underwent the modifications of telomere length and telomerase activity observed in other sub-populations of lymphocytes with advancing age. We demonstrated that: (a) telomere shortening occurred and telomerase activity decreased in human NK cells with ageing; (b) the rate of telomere loss was different under and over 80 years of age; (c) similarly to telomere shortening, the modification of telomerase activity was particularly evident in octogenarians; (d) subjects with the most evident modifications of telomeres and telomerase were the oldest and those with increased NK cell numbers.

Stable telomere length and telomerase expression from naïve to memory B-lymphocyte differentiation

Telomere length and telomerase activity play important roles in regulating replicative lifespan of cells. The length of telomeres also serves as a marker for the replicative history and for the remaining replicative potential of cells. Differential telomere length has been reported in human naïve and memory T cells but not in naïve versus memory B-lymphocytes. We report here an analysis of telomere length and induced telomerase expression in naïve (CD27(-)) and memory (CD27(+)) B cells from normal adults. Although both naïve and memory B cells lose telomere repeats with age, there is no consistent difference in telomere length between these two B cell subsets. Furthermore, both naïve and memory B cells are capable of inducing telomerase activity at similar levels after in vitro stimulation independent of donor's age. Finally, there is a slow increase of memory B cells in peripheral blood with age. Together, these findings suggest that B cells are capable of maintaining telomere length during differentiation from naïve to memory B cells and this ability is maintained through age.

Ageing and telomeres: a study into organ- and gender-specific telomere shortening

Telomeres, the non-coding sequences at the ends of chromosomes, in the absence of telomerase, progressively shorten with each cell division. Shortening of telomeres can induce cell cycle arrest and apoptosis. The aim of this study was to investigate age- and gender-related changes in telomere length in the rat and to detect possible tissue- specific rates of telomere shortening. Changes with age in telomere lengths were assessed by Southern blotting in the kidney, pancreas, liver, lung and brain of male and female rats. We determined the percentage of telomeres in various molecular size regions rather than measuring the average telomere length. The latter was unable to detect telomere shortening in the tissues. The percentage of short telomeres increased with age in the kidney, liver, pancreas and lung of both males and females, but not in the brain. Males had shorter telomeres than females in all organs analysed except the brain, where the lengths were similar. These findings indicate that telomeres shorten in the rat kidney, liver, pancreas and the lung in an age-dependent manner. These data also provide a novel mechanism for the gender-related differences in lifespan and suggest a tissue-specific regulation of telomere length during development and ageing in the rat.

Telomere dynamics in childhood leukemia and solid tumors: a follow-up study

Telomeres of hematopoietic cells shorten with age, possibly contributing to the aging-associated hematopoietic pathology (immunosenescence, malignant transformation). Accelerated telomere shortening is seen with replicative stress, such as during administration of serial chemotherapy cycles for the treatment of childhood cancer. To define the long-term consequences of pediatric cancer treatment on hematopoietic cell telomere length, we undertook a prospective 4-year follow-up study of a 61-patient cohort of pediatric malignancies in a community-based setting. We found that mononuclear cells (MNC) and granulocytes of children with standard-risk acute lymphoblastic leukemia (ALL) suffered minimal telomere shortening throughout therapy (less than 1 kbp; average follow-up, 20 months), while those of children with solid tumors showed greater and more heterogenous telomere attrition (0.5-2.8 kbp, average follow-up, 9 months). In addition, we evaluated the role of telomerase, the enzyme commonly up-regulated in pediatric leukemic and solid tumor cells for telomere length maintenance, as a disease marker. Serial determinations of telomerase in MNC were useful to confirm disease remission in leukemia, but play no role in the follow-up of children with solid tumors.

Telomerase activation and rejuvenation of telomere length in stimulated T cells derived from serially transplanted hematopoietic stem cells

Telomeres shorten in hematopoietic cells, including hematopoietic stem cells (HSCs), during aging and after transplantation, despite the presence of readily detectable levels of telomerase in these cells. In T cells, antigenic stimulation has been shown to result in a marked increase in the level of telomerase activity. We now show that stimulation of T cells derived from serially transplanted HSC results in a telomerase-dependent elongation of telomere length to a size similar to that observed in T cells isolated directly from young mice. Southern analysis of telomere length in resting and anti-CD3/CD28 stimulated donor-derived splenic T cells revealed an increase in telomere size by approximately 7 kb for the population as a whole. Stimulation of donor-derived T cells from recipients of HSCs from telomerase-deficient mice did not result in regeneration of telomere length, demonstrating a dependence on telomerase. Furthermore, clonal anti-CD3/CD28 stimulation of donor-derived T cells followed by fluorescent in situ hybridization (FISH) analysis of telomeric signal intensity showed that telomeres had increased in size by approximately 50% for all clonal expansions. Together, these results imply that one role for telomerase in T cells may be to renew or extend replicative potential via the rejuvenation of telomere length.

Telomere maintenance in human B lymphocytes

Telomere shortening has been causally linked to replicative senescence in human cells. To characterize telomere-length heterogeneity in peripheral blood cells of normal individuals, we analysed the mean length of telomeric repeat sequences in subpopulations of peripheral blood leucocytes, using fluorescence in situ hybridization and flow cytometry (flow-FISH). Although the telomere length of most haematopoietic subsets was within the same range, the mean telomere length was found to be 15% higher in B compared with T lymphocytes in adult peripheral blood. Whereas telomere loss with ageing corresponded to 33 base pairs (bp) per year in T cells, telomere shortening was slower in B cells, corresponding to 15 bp per year. Separation of adult B-lymphocyte subpopulations based on CD27 expression revealed that telomere length was almost 2 kb longer in CD19+CD27+ (memory) compared with CD19+CD27- (naive) cells. Furthermore, peripheral blood B cells were activated in vitro. Whereas B-cell activation with Staphylococcus aureus Cowan strain (SAC) did not increase telomere length, a striking telomere elongation was observed when cells were stimulated with SAC and interleukin 2 to induce plasma cell differentiation. Our observations support the concept that telomere dynamics in B cells are distinct from other haematopoietic cell lineages and that telomere elongation may play an essential role in the generation of long-term B memory cells.

Comparative analysis of telomere lengths and erosion with age in human epidermis and lingual epithelium

We investigated progressive telomere shortening in normal human epidermis and lingual epithelium during aging, and attempted, in particular, to ascertain whether the telomere shortening that accompanies aging occurs at the same rate in different tissues. We studied telomeric DNA integrity, and estimated annual telomere loss, in 52 specimens of epidermis and 48 specimens of lingual epithelium collected at autopsy from subjects who had died at ages between 0 and 101 y. Most of the DNA samples were measured twice by southern blot hybridization. In addition, the correlation between telomere lengths in the two types of tissues was examined. The telomere reduction rates in epidermis and lingual epithelium were 36 bp and 30 bp per y, respectively, and these were significantly different. The rates obtained by the second measurements in epidermis and lingual epithelium were 39 and 32 bp per y, respectively, and these were also significantly different. The mean telomere lengths in the epidermis of eight neonates and the lingual epithelium of seven neonates were 13.2+/-1.0 and 13.8+/-1.0 kb, respectively. Comparison of telomere lengths in the two tissues for 41 paired samples showed that the mean telomere length in the epidermis (10.7+/-2.3 kb) was less than that in the lingual epithelium (12.4+/-2.5 kb); however, statistical analysis revealed a very significant relationship between epidermal and lingual epithelial telomere length (r=0.842, p<0.0001). These results indicate that the telomeres in epidermis and lingual epithelium are characterized by tissue-specific loss rates.

Simultaneous flow cytometric analysis of two cell surface markers, telomere length, and DNA content

BACKGROUND

Various protocols for estimation of telomere length in individual cells by flow cytometry using fluorescence in situ hybridization of fluorescently labeled peptide nucleic acid (PNA) probes (Flow-FISH) have been described. Combined analysis of telomere length and cell phenotype, however, remains difficult because few fluorochromes with suitable emission spectra tolerate the harsh conditions needed for DNA denaturation during hybridization of the telomere-specific PNA probe. We overcame these problems and developed a method for measuring telomere length in cell subsets characterized by the expression of two surface antigens.

METHODS

Alexa Fluor 488 and Alexa Fluor 546 were used for cell surface staining. Antigen-antibody complexes were covalently cross-linked onto the cell membrane before Flow-FISH. Cells were hybridized with a PNA probe conjugated to cyanine 5 (Cy5). Hoechst 33342 (HO342) was added for determination of cellular DNA content. For assay standardization, we added an aliquot of a single batch of 1,301 cells to each sample as an internal control before hybridization with the PNA probe. Samples were prepared in duplicate and analyzed on a standard three-laser BD LSR flow cytometer. For assay validation, the same samples were analyzed in parallel to correlate the percentage of telomere length of the sample versus 1,301 control cells to the mean size of terminal restriction fragments (TRFs) of DNA as determined by Southern gel analysis.

RESULTS

The method permitted clear identification of lymphocyte subsets in samples hybridized for Flow-FISH, with subset frequencies comparable to those of untreated samples. At a concentration of 10 nM, the Cy5-labeled telomere-specific PNA probe produced a bright fluorescence signal well separated from background. Addition of HO342 in low concentration did not interfere with Cy5 telomere fluorescence, produced adequate DNA histograms, and permitted clear identification of cell phenotype. The probe concentration of 10 nM also proved optimal for inclusion of 1,301 control cells for assay standardization. Telomere length estimations by the current method correlated highly with TRF calculations by Southern gel hybridization (r(2)= 0.9, P = 0.0003). Application of our protocol to the analysis of human CD8CD28 lymphocyte subsets showed that CD8(+bright)CD28(-) lymphocytes generally exhibit shorter telomeres than CD8(+bright)CD28(+) cells. These data concurred with previous results of telomere shortening in CD8(+)CD28(-) T cells that were obtained by using different techniques.

CONCLUSIONS

The multiparameter Flow-FISH protocol permitted rapid determination of differences in telomere length in subpopulations characterized by two surface markers without prior cell separation.

Telomeres in T and B cells

Telomeres are the structures at the ends of linear chromosomes. In mammalian cells, they consist of hexanucleotide (TTAGGG) repeats, together with many associated proteins. In the absence of a compensatory mechanism, dividing cells undergo gradual telomere erosion until a critical degree of shortening results in chromosomal abnormalities and cell death or senescence. For T and B cells, the ability to undergo extensive cell division and clonal expansion is crucial for effective immune function. This article describes our current understanding of telomere-length regulation in lymphocytes and its implications for immune function.

Rejection and recipient age

In transplantation the risk of acute rejection decreases with recipient age. This is clearly illustrated in transplantation of a non-vascularised tissue, such as the cornea. In vascularised transplants, such as kidneys, acute rejection decreases with recipient age, but the phenomenon is obscured by the fact that chronic allograft nephropathy increases with age, and is further confounded by increased death from infectious disease and drug-related causes. The underlying cellular mechanisms responsible for this weakening of rejection are discussed, as is defective signal transduction leading to decreased activation of cells and decreased resistance to immunosuppressive drugs. This supports a view that less intensive immunosuppressive drug therapy is desirable in elderly recipients. Although pharmacokinetic studies are documented, there are no routine assays to measure efficacy of these drugs in individual patients. In summary, the decline in acute rejection with increasing recipient age may be due both to immunosenescence and decreased resistance to immunosuppressive drugs. Future assays to test these mechanisms could be used to tailor therapy to individual needs.

Aging-dependent exclusion of antigen-inexperienced cells from the peripheral B cell repertoire

Aging is accompanied by greatly reduced B cell production in the bone marrow, yet peripheral B cell numbers do not decline. We hypothesize that this may reflect filling of the peripheral pool with B cells that are long-lived as a consequence of specificity for, and chronic stimulation by, environmental Ags. To begin to explore this possibility, we analyzed the effects of aging on B cell population dynamics in the anti-H2(k/b) 3-83 mu-delta Ig-transgenic mouse. We predicted that, because they presumably do not bind environmental Ags, B cells bearing the transgenic receptor may be lost in aged animals. As seen in nontransgenic animals, total splenic B cell numbers remained constant with age in the Ig-transgenic animals despite reduced B cell production. Importantly, although the few newly produced B cells in the bone marrow of aged mice are 3-83 positive, the peripheral compartment of these mice is dominated by B cells that express endogenous Ig genes rather than the transgenes. This population includes large numbers of marginal zone-like and CD21(low/-)CD23(low/-)IgM(low) B cells, as well as elevated numbers of CD5+ B cells. Many of these cells express only non-B220 CD45 isoforms, suggesting that they may be memory cells. A significant proportion of aged transgenic animals produce autoantibodies that are reactive with ssDNA, dsDNA, or histones. Results support the hypothesis that, in the face of severely reduced production with age, B cells are selected based on reactivity to environmental Ags, accumulate, and display activated phenotypes. Cells bearing 3-83-transgenic receptors are excluded from this population due to their specificity. Beyond their importance in aging, these findings define a novel form of receptor revision in which B cells are selected rather than deleted based on Ag reactivity.

Telomere lengths are characteristic in each human individual

BACKGROUND

A great deal of attention has been focused on telomeres in relation to cellular aging, immortality, and cancer. However, there is no simple link between telomeres and tissue turnover. We recently proposed a hypothesis that telomere shortening with aging and telomere lengths in different organs are characteristic for human individuals.

METHODS

To test this, telomere lengths were measured using DNA from cerebral cortex, myocardium, liver, renal cortex and spleen tissues obtained from human subjects ranging in age from neonates to centenarians.

RESULTS

Regression analyses demonstrated telomere reduction rates of 29-60 base pair (bp) per year in the liver, renal cortex and spleen, but no such decrease in the cerebral cortex and myocardium. Significant correlation was found between tissues within individuals, such as cerebral cortex versus (vs) myocardium, cerebral cortex vs liver, cerebral cortex vs renal cortex, myocardium vs liver, myocardium vs renal cortex, and liver vs renal cortex. In most cases, the longest telomeres were observed in the myocardium and the shortest in the liver or renal cortex.

CONCLUSIONS

Telomere lengths did not show clear correlation with tissue renewal times in vivo, but rather were characteristic for individuals.

Age-related telomere length dynamics in peripheral blood mononuclear cells of healthy cynomolgus monkeys measured by Flow FISH

Telomere length is a good biomarker to study the cellular senescence as well as aging of an organism, because it regulates the replicative capacity of vertebrate somatic cells. To demonstrate age-related telomere length dynamics in the peripheral blood mononuclear cells (PBMC) of the cynomolgus monkey, we introduced a novel method of measuring telomere length by fluorescence in situ hybridization with a Peptide Nucleic Acid (PNA) labelled probe and flow cytometry (Flow FISH). A highly significant correlation was observed between the intensity of telomere-specific fluorescence by Flow FISH and telomere length by Southern blot analysis (R = 0.923, n = 22). The intensity of telomere fluorescence in PBMC significantly decreased with age in 55 monkeys aged from 0 to 34 years and this decrease corresponded to the loss of 62.7 base pairs per year (R = - 0.52, P < 0.00004). We also analysed the expression of naive cell-associated markers, CD28, CD62L and CD45RA/CD62L in T lymphocytes of 47 cynomolgus monkeys. An age-related increase in the CD28- subset was observed in CD8+ T lymphocytes in monkeys less than 11 years old and in CD4+ T lymphocytes in monkeys over 23 years old, respectively. The percentage of CD62L+ subsets was significantly decreased with age in both CD4+ (R = - 0.55) and CD8+ T lymphocytes (R = - 0.73). From the comparison of telomere length among PBMC, CD62L+ and CD62L- T lymphocytes, it was clearly evident that loss of naive subsets results in the shortening of telomere length in vivo. These results show that this method can be applicable to studying the turnover and precursor-progeny of PBMC in cynomolgus monkeys as an animal model of aging.

CD8+CD28- T cells: certainties and uncertainties of a prevalent human T-cell subset

Human peripheral blood CD8+ T cells comprise cells that are in different states of differentiation and under the control of complex homeostatic processes. In a number of situations ranging from chronic inflammatory conditions and infectious diseases to ageing, immunodeficiency, iron overload and heavy alcohol intake, major phenotypic changes, usually associated with an increase in CD8+ T cells lacking CD28 expression, take place. CD8+CD28- T cells are characterized by a low proliferative capacity to conventional stimulation in vitro and by morphological and functional features of activated/memory T cells. Although the nature of the signals that give origin to this T-cell subset is uncertain, growing evidence argues for the existence of an interplay between epithelial cells, molecules with the MHC-class I fold and CD8+ T cells. The possibility that the generation of CD8+CD28- T cells is the combination of TCR/CD3zeta- and regulatory factor-mediated signals as a result of the sensing of modifications of the internal environment is discussed.

Shortened telomere length is demonstrated in T-cell subsets together with a pronounced increased telomerase activity in CD4 positive T cells from blood of patients with mycosis fungoides and parapsoriasis

We have recently demonstrated that telomerase activity is increased and telomere length shortened in lymphocytes from peripheral blood of patients with cutaneous T-cell lymphoma. In order to determine which cell type has increased telomerase activity and shortened telomere length, CD4+, CD8+, CLA+ CD3+ and CLA- CD3+ T cells were isolated from peripheral blood of 25 patients, including 15 patients with mycosis fungoides and 10 patients with parapsoriasis. Eleven healthy individuals were used as controls; CD19+ B cells were separated from each individual as an internal control. The results showed that the increased telomerase activity was significantly predominating in the CD4+ T-cell subset. Significantly shortened telomere length was found in CD4+ and CD8+ T-cell subsets from the patients compared with the same cell subsets obtained from healthy individuals. However, no difference was observed between the subsets; CD19+ B cells collected from patients and healthy control individuals had similar telomerase activity and telomere length which was significantly different from the values found in T cells. The telomere length was significantly shorter in CLA+ CD3+ subset than in CLA- CD3+ subset. Interestingly, increased telomerase activity and shortened telomere length was also detected in CD4+ T cells from patients with parapsoriasis indicating that alteration of telomerase activity and telomere length in CD4+ T cells is an early event in the pathogenesis of cutaneous T-cell lymphoma. Thus, the results indicate that a significant high level of telomerase activity and shortened telomere length frequently occur in T cells of patients with CTCL and may reflect tumorigenesis.

Age-related telomere shortening occurs in lens epithelium from old rats and is slowed by caloric restriction

We have investigated whether the average relative telomere length of lens epithelial cells (LECs) from brown Norway rats decreases with the age of the donor animal, and whether chronic caloric restriction (CR) of the rats delays the telomere shortening. Our previous studies have demonstrated that clonal proliferative potential of rodent LECs as well as the in vivo rate of DNA synthesis decreases with age and that this decrease is slowed by chronic lifelong caloric restriction (CR). In order to determine if telomeric shortening might be involved in this loss of proliferative potential, we examined relative telomeric lengths in young, old ad lib fed (AL), and old calorically restricted (CR) brown Norway rats. We used fluorescence in situ hybridization with a peptide nucleic acid probe (PNA) complementary to the telomeric repeat sequence to quantitate relative telomere lengths in LECs in lens sections (TELO-FISH). Control experiments demonstrated that the PNA probe binding was restricted almost entirely to the terminal portions of the rat chromosomes with less than 5% bound at interstitial sites in typical metaphase spreads. The relative telomere lengths of interphase human fibroblast standards, as determined by TELO-FISH, were in good agreement with terminal restriction fragment analyses of the same standards and with literature values for rat cells. The average telomere lengths of interphase nuclei in the old AL rat LECs were found to be 21% shorter than paired young AL controls (P < 0.01 by Wilcoxian signed rank test). The calorically restricted old rats had less telomere erosion (12%) than the old AL group (P < 0.05). Although it is not clear whether such moderate telomeric erosion can limit cell division in rodent LECs, the telomeric shortening correlated well with previous studies demonstrating reduced clonal, replicative potential, and reduced rates of in vivo DNA replication in LECs from old rodents and a delay in this attenuation in animals on chronic CR.

Telomere shortening and decreased replicative potential, contrasted by continued proliferation of telomerase-positive CD8+CD28(lo) T cells in patients with systemic lupus erythematosus

To evaluate whether the immune system of systemic lupus erythematosus (SLE) patients shows features of premature aging, we compared telomere length and proliferative potential of SLE peripheral blood mononuclear cells (PBMC) (N = 90) to those of controls (N = 64). SLE samples showed accelerated loss of telomeric DNA (P = 0.00008) and higher levels of senescent (< or =5 kb) telomeric DNA (P = 0.00003). Viability cell counts and CFSE tracking in 6-week-old cell cultures indicated that SLE PBMC (CD8+ and CD4+ T cells) underwent fewer mitotic cycles and had shorter telomeres than controls (P = 0.04). However, a CD8(+)CD28(lo) T cell subset expanded preferentially in SLE-derived bulk cultures (P = 0.0009), preserved telomeric DNA (P = 0.01 vs entire CD8+), and displayed telomerase activity [2.1 telomerase arbitrary units (TAU) vs 0.5 TAU in CD8+CD28(hi) cells and 0.3 TAU in bulk PBMC; P = 0.05]. These T cell anomalies could be due to chronic in vivo stimulation of the immune system and may contribute to the immune dysregulation found in SLE.

Hypothesis: pulse pressure and human longevity

In exploration of the association between pulse pressure and longevity in humans, 3 hypotheses are briefly discussed: the fetal origin hypothesis, antagonistic pleiotropy, and the telomere hypothesis of cellular aging. The implications of these hypotheses serve to draw a critical distinction between biologic age (aging) and chronological age and, thereby, offer an answer to a question that presently matters most in the field of hypertension: Why has it been so difficult to disentangle the genetic components of essential hypertension and to identify the variant genes responsible for elevated blood pressure in a large segment of the human population?