Telomere fluorescence measurements in granulocytes and T lymphocyte subsets point to a high turnover of hematopoietic stem cells and memory T cells in early childhood

During ontogeny primitive (CD34+CD38−) hematopoietic cells show altered expression of a subset of genes associated with early cytokine and differentiation responses of their adult counterparts

Comparison of gene expression profiles in closely related subpopulations of primitive hematopoietic cells offers a powerful first step to elucidating the molecular basis of their different biologic properties. Here we present the results of a comparative quantitative analysis of transcript levels for various growth factor receptors, ligands, and transcription factor genes in CD34+CD38− and CD34+CD38+ cells purified from first trimester human fetal liver, cord blood, and adult bone marrow (BM). In addition, adult BM CD34+CD38− cells were examined after short-term exposure to various growth factors in vitro. Transcripts for 19 of the 24 genes analyzed were detected in unmanipulated adult BM CD34+CD38− cells. Moreover, the levels of transforming growth factor beta (TGF-β), gp130, c-fos, and c-jun transcripts in these cells were consistently and significantly different (higher) than in all other populations analyzed, including phenotypically similar but biologically different cells from fetal or neonatal sources, as well as adult BM CD34+ cells still in G0 after 2 days of growth factor stimulation. We have thus identified a subset of early response genes whose expression in primitive human hematopoietic cells is differently regulated during ontogeny and in a fashion that is recapitulated in growth factor-stimulated adult BM CD34+CD38− cells, before their cell cycle progression and independent of their subsequent differentiation response. These findings suggest a progressive alteration in the physiology of primitive hematopoietic cells during development such that these cells initially display a partially “activated” state, which is not maximally repressed until after birth.

During ontogeny primitive (CD34+CD38−) hematopoietic cells show altered expression of a subset of genes associated with early cytokine and differentiation responses of their adult counterparts

Comparison of gene expression profiles in closely related subpopulations of primitive hematopoietic cells offers a powerful first step to elucidating the molecular basis of their different biologic properties. Here we present the results of a comparative quantitative analysis of transcript levels for various growth factor receptors, ligands, and transcription factor genes in CD34+CD38− and CD34+CD38+ cells purified from first trimester human fetal liver, cord blood, and adult bone marrow (BM). In addition, adult BM CD34+CD38− cells were examined after short-term exposure to various growth factors in vitro. Transcripts for 19 of the 24 genes analyzed were detected in unmanipulated adult BM CD34+CD38− cells. Moreover, the levels of transforming growth factor beta (TGF-β), gp130, c-fos, and c-jun transcripts in these cells were consistently and significantly different (higher) than in all other populations analyzed, including phenotypically similar but biologically different cells from fetal or neonatal sources, as well as adult BM CD34+ cells still in G0 after 2 days of growth factor stimulation. We have thus identified a subset of early response genes whose expression in primitive human hematopoietic cells is differently regulated during ontogeny and in a fashion that is recapitulated in growth factor-stimulated adult BM CD34+CD38− cells, before their cell cycle progression and independent of their subsequent differentiation response. These findings suggest a progressive alteration in the physiology of primitive hematopoietic cells during development such that these cells initially display a partially “activated” state, which is not maximally repressed until after birth.

Polyclonal hematopoiesis with variable telomere shortening in human long-term allogeneic marrow graft recipients

Donor-derived hematopoiesis was assessed in 17 patients who received allogeneic marrow grafts from HLA-matched siblings between 1971 and 1980. Complete blood counts were normal or near normal in all patients except one. Chimerism analyses, using either dual-color XY-chromosome fluorescence in situ hybridization (FISH) or analysis of variable number tandem repeat loci, indicated that 15 out of 16 patients had greater than 97% donor-derived hematopoiesis, whereas 1 patient had indeterminate chimerism. All 12 recipients of grafts from female donors exhibited polyclonal hematopoiesis by X-linked clonal analysis with the use of molecular probes. Of the 17 recipients, 9 exhibited a less than 1.0-kilobase shortening of granulocyte telomere length compared with their respective donors, according to terminal restriction fragment analysis or flow-FISH with a fluorescein-labeled peptide nucleic acid probe. These data suggest that under standard transplantation conditions, the stem cell proliferative potential is not compromised during hematopoietic reconstitution.

Polyclonal hematopoiesis with variable telomere shortening in human long-term allogeneic marrow graft recipients

Donor-derived hematopoiesis was assessed in 17 patients who received allogeneic marrow grafts from HLA-matched siblings between 1971 and 1980. Complete blood counts were normal or near normal in all patients except one. Chimerism analyses, using either dual-color XY-chromosome fluorescence in situ hybridization (FISH) or analysis of variable number tandem repeat loci, indicated that 15 out of 16 patients had greater than 97% donor-derived hematopoiesis, whereas 1 patient had indeterminate chimerism. All 12 recipients of grafts from female donors exhibited polyclonal hematopoiesis by X-linked clonal analysis with the use of molecular probes. Of the 17 recipients, 9 exhibited a less than 1.0-kilobase shortening of granulocyte telomere length compared with their respective donors, according to terminal restriction fragment analysis or flow-FISH with a fluorescein-labeled peptide nucleic acid probe. These data suggest that under standard transplantation conditions, the stem cell proliferative potential is not compromised during hematopoietic reconstitution.

Ectopic human telomerase catalytic subunit expression maintains telomere length but is not sufficient for CD8+ T lymphocyte immortalization

Like most somatic human cells, T lymphocytes have a limited replicative life span. This phenomenon, called senescence, presents a serious barrier to clinical applications that require large numbers of Ag-specific T cells such as adoptive transfer therapy. Ectopic expression of hTERT, the human catalytic subunit of the enzyme telomerase, permits fibroblasts and endothelial cells to avoid senescence and to become immortal. In an attempt to immortalize normal human CD8(+) T lymphocytes, we infected bulk cultures or clones of these cells with a retrovirus transducing an hTERT cDNA clone. More than 90% of transduced cells expressed the transgene, and the cell populations contained high levels of telomerase activity. Measuring the content of total telomere repeats in individual cells (by flowFISH) we found that ectopic hTERT expression reversed the gradual loss of telomeric DNA observed in control populations during long term culture. Telomere length in transduced cells reached the levels observed in freshly isolated normal CD8(+) lymphocytes. Nevertheless, all hTERT-transduced populations stopped to divide at the same time as nontransduced or vector-transduced control cells. When kept in IL-2 the arrested cells remained alive. Our results indicate that hTERT may be required but is not sufficient to immortalize human T lymphocytes.

Short telomeres in patients with vascular dementia: an indicator of low antioxidative capacity and a possible risk factor?

Progressive cerebrovascular atherosclerosis and consecutive stroke are among the most common causes of dementia. However, specific risk factors for vascular dementia are still not known. Human telomeres shorten with each cell division in vitro and with donor age in vivo. In human fibroblasts in vitro, the telomere shortening rate decreased with increasing antioxidative capacity. There was a good intra-individual correlation between the age-corrected telomere lengths in fibroblasts and peripheral blood mononuclear cells. In 186 individuals including 149 geriatric patients (age range, 55-98 yr), leukocyte telomeres in patients with probable or possible vascular dementia were significantly shorter than in three age-matched control groups, namely in cognitively competent patients suffering from cerebrovascular or cardiovascular disease alone, in patients with probable Alzheimer's dementia, and in apparently healthy control subjects. No correlation was found to polymorphisms in the apolipoprotein E and glutathione-S-transferase genes. Telomere length may be an independent predictor for the risk of vascular dementia.

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.

Repair of telomeric DNA prior to replicative senescence

The average length of telomere repeats at the ends of chromosomes in most normal human somatic cells has been found to decrease by 50-200 base pairs with each cell division. The loss of telomere repeats has been causally linked to replicative senescence by the demonstration that overexpression of the enzyme telomerase can result in the elongation or maintenance of telomeres and immortalization of somatic cells with a diploid and apparently normal karyotype. Major questions that remain are related to the actual mechanism by which telomere shortening induces replicative senescence and the importance of telomere shortening and replicative senescence in the homeostasis of cells in renewal tissues and aging. This perspective is concerned with the consequences of telomere shortening at individual chromosomes in individual cells. Experimental evidence indicates that short telomeres accumulate prior to senescence and that replicative senescence is not triggered by the first telomere to reach a critical minimal threshold length. These observations are compatible with limited repair of short telomeres by telomerase-dependent or telomerase-independent DNA repair pathways. Deficiencies in telomere repair may result in accelerated senescence and aging as well as genetic instability that facilitates malignant transformation. Examples of molecules that may have a role in the repair of telomeric DNA prior to replicative senescence include ATM, p53, PARP, DNA-PK, Ku70/80, the human hRad50-hMre11-p95 complex, BRCA 1 and 2 and the helicases implicated in Bloom's and Werner's syndrome.

A ligand-receptor signaling threshold model of stem cell differentiation control: a biologically conserved mechanism applicable to hematopoiesis

A major limitation to the widespread use of hematopoietic stem cells (HSC) is the relatively crude level of our knowledge of how to maintain these cells in vitro without loss of the long-term multilineage growth and differentiation properties required for their clinical utility. An experimental and theoretical framework for predicting and controlling the outcome of HSC stimulation by exogenous cytokines would thus be useful. An emerging theme from recent HSC expansion studies is that a net gain in HSC numbers requires the maintenance of critical signaling ligand(s) above a threshold level. These ligand-receptor complex thresholds can be maintained, for example, by high concentrations of soluble cytokines or by extracellular matrix- or cell-bound cytokine presentation. According to such a model, when the relevant ligand-receptor interaction falls below a critical level, the probability of a differentiation response is increased; otherwise, self-renewal is favored. Thus, in addition to the identity of a particular receptor-ligand interaction being important to the regulation of stem cell responses, the quantitative nature of this interaction, as well as the dynamics of receptor expression, internalization, and signaling, may have a significant influence on stem cell fate decisions. This review uses examples from hematopoiesis and other tissue systems to examine existing evidence for a role of receptor activation thresholds in regulating hematopoietic stem cell self-renewal versus differentiation events.

Cytolytic mechanisms and expression of activation-regulating receptors on effector-type CD8+CD45RA+CD27- human T cells

Circulating CD8+ T cells with a CD45RA+CD27- phenotype resemble cytolytic effector cells because they express various cytolytic mediators and are able to execute cytotoxicity without prior stimulation in vitro. We here demonstrate that CD8+CD45RA+CD27- T cells can use both granule exocytosis and Fas/Fas ligand pathways to induce apoptosis in target cells. The availability of these cytolytic mechanisms in circulating T cells suggests that the activity of these cells must be carefully controlled to prevent unwanted tissue damage. For this reason, we analyzed the expression of surface receptors that either enhance or inhibit T cell function. Compared with memory-type cells, effector cells were found to express normal levels of CD3epsilon and TCRzeta and relatively high levels of CD8. CTLA-4 was absent from freshly isolated effector cells, whereas a limited number of unstimulated memory cells expressed this molecule. In line with recent findings on CD8+CD28- T cells, CD45RA+CD27- T cells were unique in the abundant expression of NK cell-inhibitory receptors, both of Ig superfamily and C-type lectin classes. Binding of NK cell-inhibitory receptors to classical and nonclassical MHC class I molecules may inhibit the activation of the cytolytic machinery induced by either Ag receptor-specific or nonspecific signals in CD8+CD45RA+CD27- T cells.

Cutting edge: naive T cells masquerading as memory cells

This study shows that naive CD8 T cells can acquire characteristics of memory T cells in the absence of stimulation with specific Ag simply by the process of homeostatic proliferation under lymphopenic conditions. This Ag-independent T cell differentiation pathway did not result in up-regulation of early activation markers (CD69, CD25, CD71), but expression of several memory markers (CD44, CD122, Ly6C) increased progressively with successive divisions. These markers were then stably expressed, and these cells also became more responsive functionally to specific Ag. Thus, all "memory" phenotype T cells in an individual may not be true Ag-experienced cells and may include naive cells masquerading as memory cells. These findings are specially relevant in cases of disease or treatment-induced lymphopenia such as in HIV-infected individuals or transplant recipients. In addition, this study may have implications for autoimmunity because homeostatic proliferation of naive T cells requires interaction with self peptide plus MHC molecules.

A ligand-receptor signaling threshold model of stem cell differentiation control: a biologically conserved mechanism applicable to hematopoiesis

A major limitation to the widespread use of hematopoietic stem cells (HSC) is the relatively crude level of our knowledge of how to maintain these cells in vitro without loss of the long-term multilineage growth and differentiation properties required for their clinical utility. An experimental and theoretical framework for predicting and controlling the outcome of HSC stimulation by exogenous cytokines would thus be useful. An emerging theme from recent HSC expansion studies is that a net gain in HSC numbers requires the maintenance of critical signaling ligand(s) above a threshold level. These ligand-receptor complex thresholds can be maintained, for example, by high concentrations of soluble cytokines or by extracellular matrix- or cell-bound cytokine presentation. According to such a model, when the relevant ligand-receptor interaction falls below a critical level, the probability of a differentiation response is increased; otherwise, self-renewal is favored. Thus, in addition to the identity of a particular receptor-ligand interaction being important to the regulation of stem cell responses, the quantitative nature of this interaction, as well as the dynamics of receptor expression, internalization, and signaling, may have a significant influence on stem cell fate decisions. This review uses examples from hematopoiesis and other tissue systems to examine existing evidence for a role of receptor activation thresholds in regulating hematopoietic stem cell self-renewal versus differentiation events.

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.

The role of telomerase expression and telomere length maintenance in human and mouse

The molecular regulation of telomere length has been well elucidated by a series of elegant studies over the past decade. More recently, experimental evidence has accrued that addresses the challenging question of if and how telomere length regulation may contribute to normal human aging or to human disease. Recent studies in mice have provided a mammalian precedent indicating that telomerase deficiency can lead to in vivo dysfunction, most probably as a consequence of progressive telomere shortening. In humans, the evidence that telomere shortening might lead to in vivo dysfunction is far less direct, although the recent description of telomerase deficiency and telomere shortening associated with the DKC syndrome is suggestive of such a link. Methodologies exist and continue to be developed that are increasingly capable of manipulating telomerase activity and telomere length in human cells. It remains to be determined whether scientifically rigorous and (equally important) medically ethical approaches will emerge to directly assess the ability of telomere length modulation to correct functional disorders of human cellular function ex vivo or more challenging still, in vivo.

Flow cytometric measurement of telomere length

The regulation of telomere length may be involved in the cellular physiology of senescence, reproduction, cancer, immune response to infection, and possibly immune deficiency. The measurement of telomere length, critical to research in this area, has traditionally been performed by Southern blot analysis, which is cumbersome and time consuming. Several alternative methods have been described in recent years. Some, such as pulsed-field electrophoresis, slot blots, and centromere-to-telomere ratio measurements are essentially improvements to the Southern blot technique. However, other methods such as fluorescent in situ hybridization on metaphase chromosome spreads and flow cytometry-based fluorescent in situ hybridization represent a completely new technical approach to the problem. In this review, we compare methods, with particular emphasis placed on flow cytometric techniques for measuring telomere length in situ and identifying potential areas where improvements may still be made.

Dynamics of telomere shortening in neutrophils and T lymphocytes during ageing and the relationship to skewed X chromosome inactivation patterns

Human haemopoiesis undergoes profound changes throughout life, resulting in compromised regenerative capacity of haemopoietic stem cells. It has been suggested that telomere shortening results in senescence of haemopoietic stem cell subsets and may influence the balance between stem cell renewal and proliferation. Telomere length and telomerase activity was measured in whole blood leucocytes, neutrophils and T cells from cord blood and individuals aged from 1 year to 96 years. Rapid telomere shortening [700 base pairs (bp)] was demonstrated in the first year of life, followed by a gradual decline of 31 bp/year. T cells were shown to have longer telomeres than neutrophils (mean difference 372 bp, P = < 0.001) but demonstrated similar rates of shortening (20 +/- 0.3 bp/year vs. 22 +/- 0.3 bp/year). Telomerase was detectable in T cells but not in neutrophils, suggesting that telomerase is not the rate-limiting step for regulation of telomere length in haemopoietic cells. Stem cell utilization as measured by X chromosome inactivation patterns was found to be independent of telomere length. This supports the concept that age-dependent skewed haemopoiesis is the result of random stem cell loss or X-allelic exclusion rather than telomeric senescence. These studies provide insight into the ageing process and a reference point for evaluating replicative stress in individuals of different age groups.

Biogerontological research in Canada

There is an ever increasing interest in the study of the aging process. This review is aimed to make an overview of the biological aging research in Canada. I will summarize, to the best of my knowledge, the biological aging research undertaken actually in Canadian institutions dealing with various aspects of this research using many different experimental approaches, models from animals to humans and a huge array of techniques. The biological aging research is developing continuously in Canada, however, it is very important that we assist in a near future to its huge explosion if we would respond to the needs of an ever increasing aging population. Initiatives recently proposed by the Canadian government concerning the creation of Canadian Institutes on Health Research will provide good opportunities to establish a performant, cost-effective, and innovative biological aging research.

Extension of cell life-span and telomere length in animals cloned from senescent somatic cells

The potential of cloning depends in part on whether the procedure can reverse cellular aging and restore somatic cells to a phenotypically youthful state. Here, we report the birth of six healthy cloned calves derived from populations of senescent donor somatic cells. Nuclear transfer extended the replicative life-span of senescent cells (zero to four population doublings remaining) to greater than 90 population doublings. Early population doubling level complementary DNA-1 (EPC-1, an age-dependent gene) expression in cells from the cloned animals was 3.5- to 5-fold higher than that in cells from age-matched (5 to 10 months old) controls. Southern blot and flow cytometric analyses indicated that the telomeres were also extended beyond those of newborn (<2 weeks old) and age-matched control animals. The ability to regenerate animals and cells may have important implications for medicine and the study of mammalian aging.

Accumulation of short telomeres in human fibroblasts prior to replicative senescence

The loss of telomere repeats has been causally linked to in vitro replicative senescence of human diploid fibroblasts (HDFs). In order to study the mechanism(s) by which telomere shortening signals cell senescence, we analyzed the telomere length at specific chromosome ends at cumulative population doublings in polyclonal and clonal HDFs by quantitative fluorescence in situ hybridization. The rate of telomere shortening at individual telomeres varied between 50 and 150 bp per population doubling and short telomeres with an estimated 1-2 kb of telomere repeats accumulated prior to senescence. The average telomere length in specific chromosome ends was remarkably similar between clones. However, some exceptions with individual telomeres measuring 0.5-1 kb were observed. In the fibroblast clones, the onset of replicative senescence was significantly correlated with the mean telomere fluorescence but, strikingly, not with chromosomes with the shortest telomere length. The accumulation of short telomeres in late passages of cultured HDFs is compatible with selection of cells on the basis of telomere length and limited recombination between telomeres prior to senescence.

Prognostic implications of differences in telomere length between normal and malignant cells from patients with chronic myeloid leukemia measured by flow cytometry

Chronic myeloid leukemia (CML) is a clonal, multilineage myeloproliferative disorder characterized by the Philadelphia chromosome (Ph) and a marked expansion of myeloid cells. Previous studies have indicated that the telomere length in blood cells may indicate their replicative history. However, the large variation in telomere length between individuals complicates the use of this parameter in CML and other hematologic disorders. To circumvent this problem, we compared the telomere length in peripheral blood or bone marrow cells with purified normal (Ph−) T lymphocytes from the same CML patient using fluorescence in situ hybridization and flow cytometry. Overall telomere fluorescence was significantly reduced in Ph+ cells from patients with CML compared to blood leukocytes from normal individuals (P &lt; 0.001) or normal (Ph−) T lymphocytes from the same individuals (n = 51, P &lt; 0.001). Cells from patients in accelerated phase or blast phase (AP/BP) showed significantly shorter average telomere length than cells from patients in chronic phase (CP,P = 0.02) or cytogenetic remission (CR,P = 0.03). Patients in CP who subsequently developed BP within 2 years had significantly shorter telomeres than those who did not develop BP for at least 2 years (P &lt; 0.05). Accelerated replication-dependent telomere shortening in Ph+ versus Ph− leukocytes supports previous evidence that Ph+ stem cells cycle more actively than their counterparts in normal individuals. Our data further suggest that telomere shortening may serve as a surrogate marker of disease progression in patients with CP CML, supporting a mechanistic link between CML stem cell turnover, genetic instability, and malignant evolution in this disease. (Blood. 2000;95:1883-1890)

Human immunosenescence: the prevailing of innate immunity, the failing of clonotypic immunity, and the filling of immunological space

According to the remodeling theory of aging we proposed several years ago, the current data on human immunosenescence depicts a complex scenario where clonotypical immunity deteriorates, while ancestral innate/natural immunity is largely conserved or even up-regulated with age. Under an evolutionary perspective, antigens are the cause of a persistent life-long antigenic stress, responsible for the accumulation of effector CD8+/CD28- T cells, the decrease of naive T cells (CD95-) and the marked shrinkage of T cell repertoire with age. Concomitantly, NK cytotoxicity, chemotaxis, phagocytosis and complement activities remain unaffected or negligibly affected, in comparison to clonotypical immunity. Thus, immunosenescence is not a random deteriorative phenomenon but appears to inversely recapitulate an evolutionary pattern. On the whole, immunosenescence can be envisaged as the result of the continuous challenge of the unavoidable exposure to a variety of potential antigens (viruses, bacteria, but also food and self molecules among others). From this perspective antigens are nothing else than a particular type of stressor and immunosenescence appears to be the price paid to immunological memory, i.e. one of the main characteristics of the most evolutionary recent and sophisticated type of immunity. Together with the age-related thymic involution, and the consequent age-related decrease of thymic output of new T cells, this situation leaves the body practically devoid of virgin T cells, and thus likely more prone to a variety of infectious and non infectious diseases.

Assessing the replicative history of human T cells

Upon encountering antigen, T cells clonally expand and differentiate into effector cells that directly or indirectly eliminate antigen-bearing pathogens. When renewed contact with the same pathogen occurs the immune response is mounted in a faster and more accurate way, a process that is referred to as immunological memory. The basis for T-cell memory is at least partially provided by an enhanced precursor frequency of antigen-specific T cells, and an increased responsiveness of primed T cells to activation signals. In contrast to B cells, which acquire mutations in the immunoglobulin genes after antigenic challenge, somatic markers are lacking that distinguish unprimed (or naive) from primed (encompassing memory and effector) T cells. Instead, differential expression of cell surface molecules on subsets of T cells and measures for replicative history can be used to obtain insight into the antigen-driven development of the T-cell compartment. Apart from fundamental issues addressing lineage relationships between naive, memory and effector T cells and the cellular basis for long-term T-cell memory, these types of studies have proved to be valuable in understanding T-cell reconstitution in situations of severe T-cell depletion, i.e., after chemotherapy, treatment with depleting CD4 monoclonal antibodies or during HIV infection.

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.

Accelerated telomere shortening in the human inactive X chromosome

Telomeres are nucleoprotein complexes at the end of eukaryotic chromosomes, with important roles in the maintenance of genomic stability and in chromosome segregation. Normal somatic cells lose telomeric repeats with each cell division both in vivo and in vitro. To address a potential role of nuclear architecture and epigenetic factors in telomere-length dynamics, the length of the telomeres of the X chromosomes and the autosomes was measured in metaphases from blood lymphocytes of human females of various ages, by quantitative FISH with a peptide nucleic-acid telomeric probe in combination with an X-chromosome centromere-specific probe. The activation status of the X chromosomes was simultaneously visualized with antibodies against acetylated histone H4. We observed an accelerated shortening of telomeric repeats in the inactive X chromosome, which suggests that epigenetic factors modulate not only the length but also the rate of age-associated telomere shortening in human cells in vivo. This is the first evidence to show a differential rate of telomere shortening between and within homologous chromosomes in any species. Our results are also consistent with a causative role of telomere shortening in the well-documented X-chromosome aneuploidy in aging humans.