Ageing of lymphocytes and lymphocytes in the aged

Identification of high caspase-3 mRNA expression as a unique signature profile for extremely old individuals

Apoptosis, or programmed cell death, is important for maintaining tissue homeostasis, as it permits the elimination of damaged, functionless or unwanted cells. As we age, our immune system undergoes constant remodeling, during which age-associated changes in immune parameters, including decreased naïve and increased memory T cells, have been reported. However, excessive immune cell loss, rendering the elderly more vulnerable to infections, and inappropriate deletion of damaged or functionless lymphocytes, can contribute to the development of age-associated diseases. As such, we studied the mRNA expression of cell death (specifically caspase) genes in nonagenarians and centenarians, successful models of ageing who have survived or avoided age-associated diseases, as well as in their younger counterparts and found that population composed of extremely old individuals shows a unique pattern of caspase mRNA expression, characterized by high levels of caspase-1 and -3, and low levels of caspase-8, mRNA while those composed of old individuals are characterize by high level of caspase-8 mRNA expression. Furthermore, we show that the described changes in caspases mRNA do not appear to results from age-related changes in PBMC composition, such as decreases in CD24. Therefore, we suggest that unique patterns of caspase mRNA results from the regulation of message abundance on a per cell basis, via a putative regulation of caspase genes at the transcription or RNA processing level, rather than changes in immune profiles.

Memory CD8+ T cells vary in differentiation phenotype in different persistent virus infections

The viruses HIV-1, Epstein-Barr virus (EBV), cytomegalovirus (CMV) and hepatitis C virus (HCV) are characterized by the establishment of lifelong infection in the human host, where their replication is thought to be tightly controlled by virus-specific CD8+ T cells. Here we present detailed studies of the differentiation phenotype of these cells, which can be separated into three distinct subsets based on expression of the costimulatory receptors CD28 and CD27. Whereas CD8+ T cells specific for HIV, EBV and HCV exhibit similar characteristics during primary infection, there are significant enrichments at different stages of cellular differentiation in the chronic phase of persistent infection according to the viral specificity, which suggests that distinct memory T-cell populations are established in different virus infections. These findings challenge the current definitions of memory and effector subsets in humans, and suggest that ascribing effector and memory functions to subsets with different differentiation phenotypes is no longer appropriate.

Clonal selection, clonal senescence, and clonal succession: the evolution of the T cell response to infection with a persistent virus

We have analyzed the CD8(+) T cell response to EBV and find that a larger primary burst size is associated with proportionally greater decay during the development of memory. Consequently, immunodominance and clonal dominance are less marked in memory than primary responses. An intuitive interpretation of this finding is that there is a limit to the number of cell divisions a T cell clone can undergo, and that the progeny of clones that have expanded massively during a primary immune response are more prone to die as a result of senescence. To test this hypothesis, we have derived a mathematical model of the response of different T cell clones of varying avidity for Ag in the primary and persistent phases of viral infection. When cellular survival and replication are linked to T cell avidity for Ag and Ag dose, then high-avidity T cells dominate both the primary and secondary responses. We then incorporated a limit in the number of cell divisions of individual T cell clones to test whether such a constraint could reproduce the observed association between cell division number and alterations in the contribution of clones to the response to persistent infection. Comparison of the model output with the experimental results obtained from primary and persistent EBV infection suggests that there is indeed a role for cellular senescence in shaping the immune response to persistent infection.

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.

Dynamics of T cell responses in HIV infection

Cytotoxic CD8(+) T cells play a major role in the immune response against viruses. However, the dynamics of CD8(+) T cell responses during the course of a human infection are not well understood. Using tetrameric complexes in combination with a range of intracellular and extracellular markers, we present a detailed analysis of the changes in activation and differentiation undergone by Ag-specific CD8(+) T cells, in relation to Ag-specific CD4(+) T cell responses, in the context of a human infection: HIV-1. During primary HIV-1 infection, the initial population of HIV-specific CD8(+) T cells is highly activated and prone to apoptosis. The Ag-specific cells differentiate rapidly from naive to cells at a perforin low intermediate stage of differentiation, later forming a stable pool of resting cells as viral load decreases during chronic infection. These observations have significant implications for our understanding of T cell responses in human viral infections in general and indicate that the definition of effector and memory subsets in humans may need revision.

Alterations in thymopoiesis in intact and peripubertally orchidectomized adult rats of different age

In adult rats 3, 6 and 9 months post-orchidectomy performed at the age of 30 days the thymus weight, thymocyte yield and relative proportions of thymocyte subsets (delineated by expression of CD4/CD8 molecules and TCRalphabeta) were analyzed in order to elucidate a putative role of male gonadal hormones in the shaping of thymus size and intrathymic T cell maturation. In 4-month-old control rats the thymus size and cellularity returned to the corresponding levels in 1-month-old rats. These levels were sustained during the following 6 months. In spite of that, the distribution of the main thymocyte subsets in these rats was subjected to significant changes, probably due to an age-associated diminishing thymus ability to provide efficient T cell differentiation. The results added further weight to a potential feedback regulatory role of CD4+8- cells in thymopoiesis. Furthermore, they revealed that the orchidectomy-induced (i) enlargement of the thymus size and enrichment of the thymic lymphoid cell content are of a limited duration; and (ii) alterations in the relative proportion of thymocytes become quantitatively more pronounced with duration of the gonadal deprivation. Thus, the study also indicates that the age-associated changes in gonadal hormones may be, at least partly, responsible for the age-related reshaping of the T cell maturation sequence, and hence for remodeling T cell dependent immune functions.

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.

Telomerase in cancer and aging

The telomere-telomerase hypothesis is the science of cellular aging (senescence) and cancer. The ends of chromosomes, telomeres, count the number of divisions a cell can undergo before entering permanent growth arrest. As divisions are being counted, events occur on the cellular and molecular level, which may either delay or hasten this arrest. As humans age, a particular concern is the accumulation of events that lead to the progression of cancer. Telomerase is a mechanism that most normal cells do not possess, but almost all cancer cells acquire, to overcome their mortality and extend their lifespan. This review aims to provide a comprehensive understanding of the role of telomerase in cancer development, progression, diagnosis, and in the future, treatment. The ultimate goal of telomerase research is to use our understanding to develop anti-telomerase therapies, an almost universal tumor target.

Hematopoietic cells and replicative senescence

Replicative senescence describes the finite cell replicative capacity in response to chronic proliferative stimulation. A key element in this process is the shortening of the telomeres, which to a major extent is caused by the lack of expression of telomerase. Whereas this situation has been well documented for a variety of somatic cell types, the question of whether stem cells "senesce" in the course of enforced chronic sequential divisions is as yet unresolved. This article examines several distinct features of hematopoietic cells (HC) in light of their similarity to certain aspects of memory T cells. It appears that although the capacity of HC for replication is not exhausted under normal physiological conditions in vivo, under certain experimental conditions and in specific in clinical situations HC do show signs of telomere shortening. This limited potential should be taken into account both with respect to aging in vivo, and also in terms of attempts to expand these cells ex vivo for therapeutic use.

A stochastic model for CD8(+)T cell dynamics in human immunosenescence: implications for survival and longevity

We propose here a stochastic model for the CD 8(+)T lymphocyte dynamics on the long time-scale of the human lifespan. Our purpose has been to test the hypothesis, recently proposed on the basis of our experimental data (Fagnoni et al., 2000), that the depletion of virgin CD8(+)T lymphocytes can be considered a reliable biomarker related to the risk of death. This hypothesis is embedded in a more general theory of immunosenescence according to which the accumulation of antigen experienced (AE) T cells and the concomitant exhaustion of antigen non-experienced (ANE) T cells with age, mostly due to the chronic lifelong exposure to antigens, is a major characteristic of the remodeling of the human immune system with age. In our model we considered a deterministic balance of ANE and AE T cell concentrations plus a stochastic forcing, which describes the chronic antigenic stress fluctuations, assuming a mean genetically determined capability of individuals to respond to antigens. The major results of our model is the validation of the above-mentioned hypothesis, since the model is capable of fitting the experimental data concerning the changes of ANE T cell concentration over age, and at the same time to reproduce survival curves similar to the demographic ones. Furthermore, the stochastic process results in being responsible for the peculiar shape of the survival curves.

Anatomic site and immune function correlate with relative cytokine mRNA expression levels in lymphoid tissues of normal rhesus macaques

Reverse transcriptase real-time polymerase chain reaction was used to determine pro-inflammatory, anti-viral and immunoregulatory cytokine mRNA expression levels in peripheral blood mononuclear cells (PBMC) of healthy juvenile, adolescent and adult rhesus macaques. Few age-related changes in cytokine mRNA expression levels were observed. Expression of interleukin 2 and Mx, a type I interferon-inducible gene, decreased with age, whereas interleukin 4 and macrophage inflammatory protein 1 (MIP-1) alpha and beta mRNA levels increased in older monkeys. Independent of age, the pro-inflammatory cytokines [tumour necrosis factor alpha (TNF-alpha) and chemokines] were expressed at higher mRNA levels in PBMC than the immunoregulatory cytokines (interleukins 2, 4, 12). Pro-inflammatory cytokine mRNA expression levels were highest in lymphoid tissues draining mucosal surfaces. Thus, a correlation exists between cytokine mRNA levels in lymphoid tissues and the anatomical site.

Value of immunological markers in predicting responsiveness to influenza vaccination in elderly individuals

Elderly individuals are at high risk for morbidity and mortality when infected with influenza virus. Vaccinations with inactivated virus are less effective in the elderly due to the declining competency of the aging immune system. We have explored whether immunological parameters predict poor anti-influenza virus vaccine responses and can be used as biological markers of immunosenescence. One hundred fifty-three residents of community-based retirement facilities aged 65 to 98 years received a trivalent influenza vaccine. Vaccine-induced antibody responses were determined by comparing hemagglutination inhibition titers before and 28 days after immunization. The composition of the T-cell compartment was analyzed by flow cytometry and the sizes of three T-cell subsets, CD4(+) CD45RO(+) cells, CD4(+) CD28(null) cells, and CD8(+) CD28(null) cells, were determined. Only 17% of the vaccine recipients were able to generate an increase in titers of antibody to all three vaccine components, and 46% of the immunized individuals failed to respond to any of the three hemagglutinins. The likelihood of successful vaccination declined with age and was independently correlated with the expansion of a particular T-cell subset, CD8(+) CD28(null) T cells. The sizes of the CD4(+) CD45RO(+) memory T-cell and CD4(+) CD28(null) T-cell subsets had no effect on the ability to mount anti-influenza virus antibody responses. Frequencies of CD8(+) CD28(null) T cells are useful biological markers of compromised immunocompetence, identifying individuals at risk for insufficient antibody responses.

The center for multidisciplinary research in aging (CMRA) at Ben Gurion University of the Negev in Israel

The Center for Multidisciplinary Research in Aging (CMRA) was established at Ben Gurion University of the Negev (BGU) in Beer Sheva in 2000, to promote research in the different disciplines of gerontology and geriatrics. It benefits from the special features of that university compared to other academic institutions in Israel and from the regional uniqueness of its location, in the southern part of Israel. CMRA serves as a comprehensive outreach unit for collaborative projects, as well as training programs and organization of professional meetings on aging.

Dangerous Liaisons

More often than not, cancer appears hand in hand with old age. Cancer is wedded to the progression of time through its need to accumulate multiple mutations. But some studies suggest a potentially deeper relation in which aging supplies cancer with a unique terrain where it can thrive. Although unresolved questions abound about the relation between cancer and aging, basic scientific insights are emerging, as are new ideas for keeping the lethal disease at bay.

TCR usage in naive and committed alloreactive cells: implications for the understanding of TCR biases in transplantation

The direct pathway of allorecognition is involved in acute allograft rejection and is characterised by TCR-mediated recognition of the MHC framework; this is thought to occur in a peptide-dependent but not peptide-specific manner. In contrast, the indirect pathway is restricted to the recipient's own MHC molecules and prevails in chronic rejection. In this pathway, the peptide has a major influence on the TCR recognition and selects alloreactive T cells with altered TCR Vbeta usage. However, qualitative analysis of Vbeta usage alone might limit our understanding of alloreactivity. The advantages of a combined quantitative assessment of Vbeta mRNA usage are discussed.

The unexpected contribution of immunosenescence to the leveling off of cancer incidence and mortality in the oldest old

In this paper the hypothesis that some features of immunosenescence might impact on the levelling off of cancer incidence and mortality in the oldest old will be considered. In fact, the term immunosenescence suggests that a progressive loss of immune system (IS) function occurs with aging. However, the age-related modifications of the IS can be more properly acknowledged as a 'remodeling' characterized by profound structural changes, which modify the functional properties of IS. We suggest that the expansion with age of natural killer cells (NK) and of T cells which progressively acquire phenotypes intermediate between T lymphocytes and NK cells, together with the age-related changes in the production of inflammatory/anti-inflammatory cytokines, such as INFgamma and IL-4, might create an environment unfavorable for neoplastic growth in the oldest old. In this perspective, studies on immunosenescence likely provide insights on mechanisms responsible for the individual capacity to escape from the life-threatening consequences of cancer outgrowth.

Third complementarity-determining region of mutated VH immunoglobulin genes contains shorter V, D, J, P, and N components than non-mutated genes

The third complementarity-determining region (CDR3) of immunoglobulin variable genes for the heavy chain (VH) has been shown to be shorter in length in hypermutated antibodies than in non-hypermutated antibodies. To determine which components of CDR3 contribute to the shorter length, and if there is an effect of age on the length, we analysed 235 cDNA clones from human peripheral blood of VH6 genes rearranged to immunoglobulin M (IgM) constant genes. There was similar use of diversity (D) and joining (JH) gene segments between clones from young and old donors, and there was similar use of D segments among the mutated and non-mutated heavy chains. However, in the mutated heavy chains, there was increased use of shorter JH4 segments and decreased use of longer JH6 segments compared to the non-mutated proteins. The overall length of CDR3 did not change with age within the mutated and non-mutated categories, but was significantly shorter by three amino acids in the mutated clones compared to the non-mutated clones. Analyses of the individual components that comprise CDR3 indicated that they were all shorter in the mutated clones. Thus, there were more nucleotides deleted from the ends of VH, D, and JH gene segments, and fewer P and N nucleotides added. The results suggest that B cells bearing immunoglobulin receptors with shorter CDR3s have been selected for binding to antigen. A smaller CDR3 may allow room in the antibody binding pocket for antigen to interact with CDRs 1 and 2 as well, so that as the VDJ gene undergoes hypermutation, substitutions in all three CDRs can further contribute to the binding energy.

Oligoclonal expansion of CD4(+)CD28(-) T lymphocytes in recipients of allogeneic hematopoietic cell grafts and identification of the same T cell clones within both CD4(+)CD28(+) and CD4(+)CD28(-) T cell subsets

Recipients of allogeneic bone marrow grafts have clonally expanded CD8(+)CD28(-) T lymphocytes during the early period after transplantation, which leads to skewing of T cell receptor (TCR) repertoires. Here, we have addressed the question of whether clonal expansion of CD28(-) T cells is also observed in CD4(+) T lymphocytes after human allogeneic hematopoietic cell transplantation. We found that the fraction of T cells lacking CD28 expression in the CD4(+) subset was increased after transplantation, and expanded CD4(+)CD28(-) T lymphocytes carrying certain TCRBV subfamilies showed limited TCR diversity. In order to further study the ontogeny of CD4(+)CD28(-) T cells, we analyzed the complementarity-determining region 3 (CDR3) of the TCR-beta chain of CD4(+)CD28(+) and CD4(+)CD28(-) cells. We identified the same T cell clones within both CD4(+)CD28(-) and CD4(+)CD28(+) T cell subsets. These results suggest that both subsets are phenotypic variants of the same T cell lineage.

The dynamics of CD4+ T-cell depletion in HIV disease

The size and composition of the CD4+ T-cell population is regulated by balanced proliferation of progenitor cells and death of mature progeny. After infection with the human immunodeficiency virus, this homeostasis is often disturbed and CD4+ T cells are instead depleted. Such depletion cannot result simply from accelerated destruction of mature CD4+ T cells - sources of T-cell production must also fail. Ironically, this failure may be precipitated by physiological mechanisms designed to maintain homeostasis in the face of accelerated T-cell loss.

Biogerontology research in Israel

The article describes the special features of gerontology research that has been expanding for five decades in Israel, and outlines the research in the biology of aging, covering a wide spectrum of areas and topics. A variety of associations, institutes and centers that have been established over the years play an important role in furthering the research and academic training.