Telomere length, aging, and somatic cell turnover

Microarray analysis of E-box binding-related gene expression in young and replicatively senescent human fibroblasts

An E-box (CACGTG) designer microarray was developed to monitor a group of genes whose expressions share a particular regulatory mode. Sensitivity and specificity of microarray hybridization, as well as variability of microarray data, were evaluated. This designer microarray was used to generate expression profiles of E-box binding-related genes in WI-38 fibroblast cultures at three different growth states: low-passage replicating, low-passage contact-inhibited quiescent, and replicatively senescent. Microarray gene screening reveals that quiescent and senescent cells, in comparison with replicating ones, are characterized by downregulation of Pam, a protein associated with c-Myc, and upregulation of Mad family genes, Max dimerization proteins. Moreover, quiescence and senescence can be distinguished by increased expression of Irlb, c-Myc transcription factor, and Miz-1, c-Myc-interacting Zn finger protein 1, only in the former state. Senescence is characterized by downregulation of Id4, inhibitor of DNA binding 4, and Mitf, microphthalmia-associated transcription factor, in comparison with young replicating and quiescent states. Differential expression of genes detected by microarray hybridization was independently confirmed by reverse transcription polymerase chain reaction technique. Alterations in the expression of E-box-binding transcription factors and c-Myc-binding proteins demonstrate the importance of these genes in establishing the contact-inhibited quiescent or senescent phenotypes.

The genetics of exceptional human longevity

There is a substantial distinction to be made between the genetics of aging and the genetics of exceptional longevity. Twin studies suggest that the average set of genetic variations facilitates the average human's ability to live well into their octogenarian years. Other studies indicate that taking full advantage of this average set results in spending the majority of those years in good health. However, many people counteract such genetic endowment with poor health habits, resulting in a substantially lower average life expectancy and relatively more time spent in poor health. To live beyond the octogenarian years, life-span experiments in lower organisms and mammals and population and molecular genetic studies of centenarian sibships suggest that genetic factors play an important role in exceptional longevity. These factors are likely to influence basic mechanisms of aging, which in turn broadly influence susceptibility to age-related illnesses. Lacking genetic variations that predispose to disease, and having variations that confer disease resistance (longevity enabling genes), are probably both important to such a remarkable survival advantage. Recent studies indicate the likelihood that such factors will be elucidated in the near future.

Modelling perspectives on aging: can mathematics help us stay young?

We survey several types of mathematical models that keep track of age distributions in a population, or follow some aspects of aging, such as loss of replicative potential of stem cells. The properties of a class of linear models of this type are discussed and compared. We illustrate the applicability of such models with a simple example based on hypothetical stem cell dynamics developed to address age-related telomere loss in the human granulocyte pool. We then describe the contrasting behaviour of nonlinear systems. Examples are drawn from the class of "dynamical diseases" to illustrate some of the aspects of nonlinear systems. Applications of these, and other models to the problems of aging and replicative aging are discussed.

More than a sum of our cells

Cells in the body grow and die, cells in lab dishes grow and die, and individual organisms grow and die. The parallels seem maddeningly obvious, but scores of scientists still labor to draw the correct connections, to uncover the mechanisms that underlie aging in cell culture flasks and in whole animals. Do our cells stop growing, quit working, cease dividing, or start dying as we age? Do we die when our cells do, or are we somehow more than the sum of our cells? For decades, scientists have searched for evidence that links changes in cell growth, cell function, cell division, and cell death to the phenomenon we call aging. Although definitive proof eludes them, researchers continue to conduct experiments in tissue culture and in animal models, amassing information that points us toward a greater understanding of what aging is--and is not.

Neutrophil ageing and immunesenescence

As humans age, their morbidity and mortality from infection increases, their response to vaccination declines and they have an increased incidence of inflammatory diseases and cancer. The reasons for these effects are clearly complex, but reduced efficiency of the innate and adaptive immune system is likely to be important in the pathology of old age. Age-related changes in the adaptive immune system are well-documented and include alterations in T cell phenotype and effector functions and a reduced ability of B cells to produce high affinity antibody. In contrast, the innate immune system has been less well researched and the perception amongst many immunogerontologists is that this branch of the immune system is only moderately affected by age. However, it is becoming increasingly clear that the adaptive and innate immune systems co-operate at several levels to ensure the optimal immune response and any decline in adaptive immunity will impact upon the function of the innate immune system and vice-versa. Here, we review the literature concerning intrinsic age-related changes in neutrophil responses and consider how changes in lymphocyte function with age might further compromise efficiency of neutrophil function.

Presence of a population of CD20+, CD38- B lymphocytes with defective proliferative responsiveness in the synovial compartment of patients with rheumatoid arthritis

OBJECTIVE

To provide a comprehensive understanding of the humoral immune response that takes place at the site of inflammation in rheumatoid arthritis (RA), we studied the functional properties of synovial B cells. In particular, the response to various modes of mitogen stimulation was investigated.

METHODS

Purified synovial fluid (SF) B cells were cultured in the presence of CD40 ligand (CD40L)-expressing fibroblasts and cytokines, activated T cells, or phorbol myristate acetate (PMA)/ionomycin. Proliferation was determined by 3H-thymidine incorporation. Release of intracellular calcium was studied by flow cytometry.

RESULTS

The inflamed joints of RA patients contained a population of CD20+,CD38- B cells with dramatically impaired mitogen responsiveness. Although the Ig-producing capacity was intact, these cells failed to proliferate in response to (a) CD40 in the presence of interleukin-2 (IL-2) and IL-10, (b) activated T cells, or (c) stimulation via the B cell receptor. Moreover, SF CD20+,CD38- B cells revealed a defective B cell receptor-induced Ca2+ influx, reminiscent of anergic B cells. Release of intracellular Ca2+ by ionomycin in the presence of the protein kinase C activator PMA did not restore the proliferative capacity. These findings indicate blockades in the proximal and distal intermediates involved in mitogen signaling.

CONCLUSION

SF CD20+,CD38- B cells have functionally impaired proliferative responsiveness. The capacity of these cells to respond to activation by the production of Ig supports the notion that these cells might serve as Ig-producing effector cells and, as such, play a role in the pathophysiology of RA.

Modelling the human immune response: can mice be trusted? Commentary

The mouse is now the animal of choice for laboratory-based medical research. Although its contribution to advancing understanding of our inner workings is indisputable, we should acknowledge that mice and humans are tangibly different. This article highlights, and attempts some rationale for, discrepancies between the two species' immune systems.

From cells to organisms: can we learn about aging from cells in culture?

Can studying cultured cells inform us about the biology of aging? The idea that this may be was stimulated by the first formal description of replicative senescence. Replicative senescence limits the proliferation of normal human cells in culture, causing them to irreversibly arrest growth and adopt striking changes in cell function. We now know that telomere shortening, which occurs in most somatic cells as a consequence of DNA replication, drives replicative senescence in human cells. However, rodent cells also undergo replicative senescence, despite very long telomeres, and DNA damage, the action of certain oncogenes and changes in chromatin induce a phenotype similar to that of replicatively senescent cells. Thus, replicative senescence is an example of the more general process of cellular senescence, indicating that the telomere hypothesis of aging is a misnomer, Cellular senescence appears to be a response to potentially oncogenic insults, including oxidative stress. The growth arrest almost certainly suppresses tumorigenesis, at least in young organisms, whereas the functional changes may contribute to aging, although this has yet to be critically tested. Thus, cellular senescence may be an example of antagonistic pleiotropy. Cross-species comparisons suggest there is a relationship between the senescence of cells in culture and organismal life span, but the relationship is neither quantitative nor direct.

The flow cytometric analysis of telomere length in antigen-specific CD8+ T cells during acute Epstein-Barr virus infection

Acute infectious mononucleosis (AIM) induced by Epstein-Barr virus (EBV) infection is characterized by extensive expansion of antigen-specific CD8+ T cells. One potential consequence of this considerable proliferative activity is telomere shortening, which predisposes the EBV-specific cells to replicative senescence. To investigate this, a method was developed that enables the simultaneous identification of EBV specificity of the CD8+ T cells, using major histocompatibility complex (MHC) class I/peptide complexes, together with telomere length, which is determined by fluorescence in situ hybridization. Despite the considerable expansion, CD8+ EBV-specific T cells in patients with AIM maintain their telomere length relative to CD8+ T cells in normal individuals and relative to CD4+ T cells within the patients themselves and this is associated with the induction of the enzyme telomerase. In 4 patients who were studied up to 12 months after resolution of AIM, telomere lengths of EBV-specific CD8+ T cells were unchanged in 3 but shortened in one individual, who was studied only 5 months after initial onset of infection. Substantial telomere shortening in EBV-specific CD8+ T cells was observed in 3 patients who were studied between 15 months and 14 years after recovery from AIM. Thus, although telomerase activation may preserve the replicative potential of EBV-specific cells in AIM and after initial stages of disease resolution, the capacity of these cells to up-regulate this enzyme after restimulation by the persisting virus may dictate the extent of telomere maintenance in the memory CD8+ T-cell pool over time.

Functional disruption of the CD28 gene transcriptional initiator in senescent T cells

We recently reported that aging is accompanied by the emergence of CD4(+)CD28(null) T cells, a functionally aberrant lymphocyte subset rarely seen in individuals younger than 40 years. Here, we directly examined whether the lack of CD28 expression is due to a defect at the level of transcriptional initiation. Molecular studies reveal that CD28 gene transcription is controlled by two sequence motifs, sites alpha and beta. In vitro transcription assays using initiator-dependent DNA templates revealed that reversed polarity or the deletion of either motif inhibited transcription, indicating that alpha/beta sequences constitute a composite initiator. Moreover, nuclear extracts from CD28(null) cells failed to activate transcription of alphabeta-initiator DNA templates. Transcription of such templates was, however, restored with the addition of extracts from CD28(+) cells. Although previously described initiator elements have been defined by a consensus sequence, the alphabeta-initiator has no homology to such sequence. These studies demonstrate that initiators have functions other than positioning elements for the basal transcription complex. Rather, initiators can have a direct role in regulating the expression of specific genes. The gain or loss of initiator activity can be an important determinant of cell phenotypes.

Simultaneous flow cytometric analysis of cell surface markers and telomere length: analysis of human tonsilar B cells

Telomere Flow FISH is a recently developed method which allows the measurement of telomere length in purified subsets of cells using flow cytometry. However, the harsh conditions required for flow FISH have precluded its use with conventional cell surface staining, thus limiting its utility for large scale clinical studies. We have now developed a method which permits simultaneous analysis of cell surface markers along with telomere length estimation by flow cytometry. This new assay employs the covalent crosslinking of monoclonal antibodies conjugated with a heat stable fluorochrome to the cell surface prior to flow FISH. Using this technique we have confirmed that human germinal center B cells (IgD(-)/CD38(+)) have dramatically longer telomeres than pre-germinal center founder B cells (IgD(+)/CD38(+)). This approach simplifies the analysis of complex cell populations and will facilitate widespread investigation of telomere length in health and disease states.

Differential regulation of CD8+ T cell senescence in mice and men

The cytotoxic CD8+ T cell population expands considerably during acute immune infection with virus. Most of these cells are removed by apoptosis at the end of the immune response. However, a balance has to be attained between clearance and retention of a memory population of cells, which respond more rapidly and efficiently to secondary encounter with the antigen. In this article, the role of apoptosis and in particular the development of replicative senescence as mechanisms which control this homeostatic balance are discussed. Although similar mechanisms regulate apoptosis in both humans and rodents, the available data suggests that replicative senescence may be controlled differently in these species, suggesting the there may be different constraints in the regulation of CD8+ T cell memory between different species.

Immortalization of human CD8+ T cell clones by ectopic expression of telomerase reverse transcriptase

Replicative senescence of T cells is correlated with erosion of telomere ends. Telomerase plays a key role in maintaining telomere length. Therefore, it is thought that telomerase regulates the life span of T cells. To test this hypothesis, we have over-expressed human telomerase reverse transcriptase in human CD8(+) T cells. Ectopic expression of human telomerase reverse transcriptase led to immortalization of these T cells, without altering the phenotype and without loss of specificity or functionality. As the T cells remained dependent on cytokines and Ag stimulation for their in vitro expansion, we conclude that immortalization was achieved without malignant transformation.

Oligoclonal expansions in the CD8(+)CD28(-) T cells largely explain the shorter telomeres detected in this subset: analysis by flow FISH

We have previously reported that CD8(+)CD28(-) T cells have relatively shorter telomeres compared with CD8(+)CD28(+) T cells. Oligoclonal expansion is a common feature of CD8(+) T cells in human peripheral blood, and these expansions predominantly occur in the CD57(+)/CD28(-) population. We studied the telomere length in subsets of CD8(+) T cells using quantitative fluorescence in situ hybridization and flow cytometry (flow FISH). Our results confirm that CD8(+)CD28(-) T cells have shorter telomeres as compared with their CD28(+) counterpart cells. In addition, the oligoclonally expanded cells within the CD8(+)CD28(-) T cell subset generally have even shorter telomeres than the CD28(-) subset as a whole. We conclude that the presence of clonal expansions in the CD8(+)CD28(-) T cell population largely explain the shorter telomeres in this subset. These clonally expanded CD8(+)CD28(-) T cells generally have characteristics of terminally differentiated effector cells. Nevertheless, there is considerable individual variation in the degree of telomere shortening in these cells, which may reflect host genetic factors as well as the type and timing of the antigenic exposure.

Mammalian cloning: possibilities and threats

The cloning of mammals originated with the production of limited numbers of genetically identical offspring by blastomere separation or embryo splitting. In the past few years, remarkable progress has been reported in cloning by nuclear transfer (NT) with donor nuclei recovered from embryonic, fetal or adult cells. Factors that contribute to the successful reprogramming of the transferred nucleus and the normal term development of the newly reconstructed embryo include the cell cycle stage of both the donor nucleus and recipient cytoplast, the timing of fusion and cytoplast activation, and the source of donor nuclei. The possibility of producing live offspring by somatic cell NT carries potential applications in animal husbandry, biotechnology, transgenic and pharmaceutical production, biomedical research, and the preservation of endangered species. However, the low efficiencies of cloning by NT coupled with high embryonic, fetal and neonatal losses may restrict immediate commercial applications in agriculture. These limitations notwithstanding, the greatest benefits and practical implications of this new technology could be in transplantation medicine and therapeutic cloning.

Aging, chromatin, and food restriction--connecting the dots

Model organisms such as yeast have proved exceptionally valuable for revealing new information about the molecular pathways involved in the aging of cells. In her Perspective, Campisi comments on new work showing that caloric restriction increases longevity in yeast by activating the SIR2 gene, which alters the compactness of chromatin and thus regulates gene expression (Lin et al.).

Induction of cytotoxic T cell responses and tumor immunity against unrelated tumors using telomerase reverse transcriptase RNA transfected dendritic cells

The polypeptide component of telomerase (TERT) is an attractive candidate for a broadly expressed tumor rejection antigen because telomerase is silent in normal tissues but is reactivated in more than 85% of cancers. Here we show that immunization against TERT induces immunity against tumors of unrelated origin. Immunization of mice with TERT RNA-transfected dendritic cells (DC) stimulated cytotoxic T lymphocytes (CTL), which lysed melanoma and thymoma tumor cells and inhibited the growth of three unrelated tumors in mice of distinct genetic backgrounds. TERT RNA-transfected human DC stimulated TERT-specific CTL in vitro that lysed human tumor cells, including Epstein Barr virus (EBV)-transformed B cells as well as autologous tumor targets from patients with renal and prostate cancer. Tumor RNA-transfected DC were used as surrogate targets in the CTL assays, obviating the difficulties in obtaining tumor cells from cancer patients. In one instance, where a tumor cell line was successfully established in culture from a patient with renal cancer, the patient's tumor cells were efficiently lysed by the CTL. Immunization with tumor RNA was generally more effective than immunization with TERT RNA, suggesting that an optimal immunization protocol may have to include TERT as well as additional tumor antigens.

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

Age effects on telomere length and telomerase expression in peripheral blood lymphocytes were analyzed from 121 normal individuals age newborn to 94 years and revealed several new findings. 1) Telomere shortening was observed in CD4+ and CD8+ T and B cells with age. However, the rate of telomere loss was significantly different in these populations, 35 +/- 8, 26 +/- 7, and 19 +/- 7 bp/year for CD4+ and CD8+ T and B cells, respectively. In addition, CD4+ T cells had the longest average telomeres at all ages, followed by B cells, with CD8+ T cell telomeres the shortest, suggesting that these lymphocyte populations may have different replicative histories in vivo. 2) Telomerase activity in freshly isolated T and B cells was indistinguishably low to undetectable at all ages but was markedly increased after Ag and costimulatory receptors mediated stimulation in vitro. Furthermore, age did not alter the magnitude of telomerase activity induced after stimulation of T or B lymphocytes through Ag and costimulatory receptors or in response to PMA plus ionomycin treatment. 3) The levels of telomerase activity induced by in vitro stimulation varied among individual donors but were highly correlated with the outcome of telomere length change in CD4+ T cells after Ag receptor-mediated activation. Together, these results indicate that rates of age-associated loss of telomere length in vivo in peripheral blood lymphocytes is specific to T and B cell subsets and that age does not significantly alter the capacity for telomerase induction in lymphocytes.

Impaired germinal center reaction in mice with short telomeres

Reduction of germinal center reactivity is a landmark of immunosenescence and contributes to immunological dysfunction in the elderly. Germinal centers (GC) are characterized by extensive clonal expansion and selection of B lymphocytes to generate the pool of memory B cells. Telomere maintenance by telomerase has been proposed to allow the extensive proliferation undergone by B lymphocytes in the GC during the immune response. We show here that late generation mTR(-/-) mice, which lack the mouse telomerase RNA (mTR) and have short telomeres, present a dramatic reduction in GC number following antigen immunization. Upon immunization with an antigen, wild-type splenocyte telomeres are elongated and this is accompanied by a high expression of the telomerase catalytic subunit in the spleen GC. In contrast, telomerase-deficient mTR(-/-) splenocytes show telomere shortening after immunization, presumably due to cell proliferation in the absence of telomerase. All together, these results demonstrate the importance of telomere maintenance for antibody-mediated immune responses and support the notion that telomere elongation detected in wild-type spleens following immunization is mediated by telomerase.

Biomarkers of immunosenescence within an evolutionary perspective: the challenge of heterogeneity and the role of antigenic load

Under an evolutionary perspective, antigens can be considered nothing else than chronic stressors that constituted the major selective pressure for immune system emergence and evolution. In this review, recent data are discussed under the hypothesis that human immunosenescence is the consequence of the continuous attrition caused by chronic antigenic overload/stress. The advantage of this theoretical approach is that a unifying hypothesis is proposed, which tries to fill in the current gap between the conceptualizations concerning the mechanisms which counteract aging and favor longevity in invertebrates and vertebrates. The hypothesis is that the immune system is, at a higher level of biological organization and complexity, the counterpart of the anti-stress response network identified in invertebrates as the major determinant of survival. We argue that some of the most important characteristics of immunosenescence, i.e. the accumulation and the clonal expansion of memory and effector T cells, the reduction/exhaustion of naive T cells, and the shrinkage of T cell repertoire, are compatible with this assumption. Thus, immunosenescence can be envisaged as a global reduction of the "immunological space." Concomitantly, immunosenescence results in the progressive generation of cellular mosaicism which is the consequence of the heterogeneous replicative histories and telomere shortening of T and B cell subsets, as well as hemopoietic stem cells. Most of the parameters affected by immunosenescence appear to be under genetic control, and future research on biomarkers should address this point. On the whole, immunosenescence can be taken as a proof that the beneficial effects of the immune system, devoted to the neutralization of dangerous/harmful agents early in life and in adulthood, turn to be detrimental late in life, in a period largely not foreseen by evolution. This perspective fits with basic assumptions of evolutionary theories of aging, such as antagonistic pleiotropy.