T cell cultures and the Hayflick limit

Obesity Increases Gene Expression of Markers Associated With Immunosenescence in Obese Middle-Aged Individuals

Recently, it has been argued that obesity leads to a chronic pro-inflammatory state that can accelerate immunosenescence, predisposing to the early acquisition of an immune risk profile and health problems related to immunity in adulthood. In this sense, the present study aimed to verify, in circulating leukocytes, the gene expression of markers related to early immunosenescence associated with obesity and its possible relationships with the physical fitness in obese adults with type 2 diabetes or without associated comorbidities. The sample consisted of middle-aged obese individuals (body mass index (BMI) between 30-35 kg/m²) with type 2 diabetes mellitus (OBD; n = 17) or without associated comorbidity (OB; n = 18), and a control group of eutrophic healthy individuals (BMI: 20 - 25 kg/m²) of same ages (E; n = 18). All groups (OBD, OB and E) performed the functional analyses [muscle strength (1RM) and cardiorespiratory fitness (VO_2max)], anthropometry, body composition (Air Displacement Plethysmograph), blood collections for biochemical (anti-CMV) and molecular (gene expression of leptin, IL-2, IL-4, IL-6, IL-10, TNF-α, PD-1, P16^ink4a, CCR7, CD28 and CD27) analyses of markers related to immunosenescence. Increased gene expression of leptin, IL-2, IL-4, IL-10, TNF-α, PD-1, P16^ink4a, CCR7 and CD27 was found for the OBD and OB groups compared to the E group. Moreover, VO_2max for the OBD and OB groups was significantly lower compared to E. In conclusion, obesity, regardless of associated disease, induces increased gene expression of markers associated with inflammation and immunosenescence in circulating leukocytes in obese middle-aged individuals compared to a eutrophic group of the same age. Additionally, increased adipose tissue and markers of chronic inflammation and immunosenescence were associated to impairments in the cardiorespiratory capacity of obese middle-aged individuals.

Methods for Characterization of Senescent Circulating and Tumor-Infiltrating T-Cells: An Overview from Multicolor Flow Cytometry to Single-Cell RNA Sequencing

Immunosenescence is the general term used to describe the aging-associated decline of immunological function that explains the higher susceptibility to infectious diseases and cancer, increased autoimmunity, or the reduced effectiveness of vaccinations. Senescence of CD8+ T-cells has been described in all these conditions.The most important classical markers of T senescent cells are the cell cycle inhibitors p16ink4a, p21, and p53, together with positivity for SA-βgal expression and the acquirement of a peculiar IFNγ -based secretory phenotype commonly defined SASP (Senescence Associated Secretory Phenotype). Other surface markers are the CD28 and CD27 loss together with gain of expression of CD45RA, CD57, TIGIT, and/or KLRG1. However, this characterization could not be sufficient to distinguish from truly senescent cells and exhausted T-cells. Furthermore, more complexity is added by the wide heterogeneity of T-cells subset in aged individuals or in the tumor microenvironment. A combined analysis by multicolor flow cytometry for surface and intracellular markers integrated with gene-expression arrays and single-cell RNA sequencing is required to develop effective interventions for therapeutic modulation of specific T-cell subsets. The RNASeq offers the great possibility to reveal at single-cell resolution the exact molecular hallmarks of senescent CD8+ T-cells without the limitations of bulk analysis. Furthermore, the comprehensive integration of multidimensional approaches (genomics, epigenomics, proteomics, metabolomics) will increase our global understanding of how immunosenescence of T-cells is interlinked to human aging.

Chemokine receptors on peripheral blood T lymphocytes in children on peritoneal dialysis

BACKGROUND

Immune cell dysfunction is listed among complications resulting from chronic kidney disease (CKD). It could be associated with T-cells, which play a role in the lymphocytic migration and infiltration. However, the data on chemokine receptors expression on T-cells in patients with CKD particularly treated with peritoneal dialysis (PD) are still limited.

METHODS

The study aimed at multiparameter flow-cytometric analysis of the absolute numbers and percentage of T-cell subsets with surface chemokine receptors (CCR4, CCR5, CCR7, CXCR3, and CXCR4) or receptors' combinations in 47 children treated with PD.

RESULTS

We found lower absolute numbers of total T lymphocytes, lymphocytes with surface CCR5, CXCR4⁺CCR5, CXCR3⁺CCR5 antigens and T-cells with CCR4, CCR4⁺CD4, CXCR3, CXCR3⁺CD4, and CD8 receptors. Lymphocytes T with CD4, CCR7, CD28⁺CCR7, CXCR3⁺CD8 antigens showed higher percentage in children on PD as compared to healthy children and opposite percentage values of CCR4⁺, CCR4⁺CD4⁺, CXCR3⁺ T lymphocytes were diminished. Mean fluorescent intensity for CCR7⁺, CCR7⁺CD45RO⁺, CCR7⁺CD28⁺, CXCR4⁺CD4⁺, CCR5⁺CD4⁺, CCR4⁺, CCR4⁺CD4⁺ T-cells was lower in the PD group than in healthy children. The analysis of correlation between T lymphocyte subpopulations with chemokine receptors and other parameters revealed positive correlation of CCR7⁺ and CCR7⁺CD28⁺ T-cells and weekly creatinine clearance, negative correlation between the percentage of CD45RO⁺CCR7 antigen positive T-cells and KT/V_urea.

SUMMARY

In conclusion, we could not confirm the phenomenon of earlier senescence of T-cells in children with CKD on PD treatment. This still requires further investigation. The higher percentage of T-cells with CCR7 surface receptor could be responsible for the increase of proliferation activity in this group of children.

Influenza vaccination in the elderly

Seasonal influenza is a prevalent and serious annual illness resulting in widespread morbidity and economic disruption throughout the population; the elderly and immunocompromised are particularly vulnerable to serious sequelae and mortality. The changing demographics worldwide to an aging society have important implications for public health policy and pharmaceutical innovations. For instance, primary prevention via immunization is effective in reducing the burden of influenza illness among the elderly. However, the elderly may be insufficiently protected by vaccination due to the immunosenescence which accompanies aging. In addition, vaccine hesitancy among the younger populations increases the likelihood of circulating infectious diseases, and thus concomitant exposure. While it is clear that the development of more immunogenic vaccines is an imperative and worthy endeavor, clinical trials continue to demonstrate that the current influenza vaccine formulation remains highly effective in reducing morbidity and mortality when well matched to circulating strains.

Effects of bioactive compounds on senescence and components of senescence associated secretory phenotypes in vitro

Senescence is a permanent cell cycle arrest that is accompanied by changes in cell morphology and physiology occurringin vitroandin vivo.

Structure and function of the telomeric CST complex

Telomeres comprise the ends of eukaryotic chromosomes and are essential for cell proliferation and genome maintenance. Telomeres are replicated by telomerase, a ribonucleoprotein (RNP) reverse transcriptase, and are maintained primarily by nucleoprotein complexes such as shelterin (TRF1, TRF2, TIN2, RAP1, POT1, TPP1) and CST (Cdc13/Ctc1, Stn1, Ten1). The focus of this review is on the CST complex and its role in telomere maintenance. Although initially thought to be unique to yeast, it is now evident that the CST complex is present in a diverse range of organisms where it contributes to genome maintenance. The CST accomplishes these tasks via telomere capping and by regulating telomerase and DNA polymerase alpha-primase (polα-primase) access to telomeres, a process closely coordinated with the shelterin complex in most organisms. The goal of this review is to provide a brief but comprehensive account of the diverse, and in some cases organism-dependent, functions of the CST complex and how it contributes to telomere maintenance and cell proliferation.

The emerging role of senescent cells in tissue homeostasis and pathophysiology

Cellular senescence is a state of permanent growth arrest and is thought to play a pivotal role in tumor suppression. Cellular senescence may play an important role in tumor suppression, wound healing, and protection against tissue fibrosis in physiological conditions in vivo. However, accumulating evidence that senescent cells may have harmful effects in vivo and may contribute to tissue remodeling, organismal aging, and many age-related diseases also exists. Cellular senescence can be induced by various intrinsic and extrinsic factors. Both p53/p21 and p16/RB pathways are important for irreversible growth arrest in senescent cells. Senescent cells secret numerous biologically active factors. This specific secretion phenotype by senescent cells may largely contribute to physiological and pathological consequences in organisms. Here I review the molecular basis of cell cycle arrest and the specific secretion phenotype in cellular senescence. I also summarize the current knowledge of the role of cellular senescence in vivo in physiological and pathological settings.

Mechanistic insights on immunosenescence and chronic immune activation in HIV-tuberculosis co-infection

Immunosenescence is marked by accelerated degradation of host immune responses leading to the onset of opportunistic infections, where senescent T cells show remarkably higher ontogenic defects as compared to healthy T cells. The mechanistic association between T-cell immunosenescence and human immunodeficiency virus (HIV) disease progression, and functional T-cell responses in HIV-tuberculosis (HIV-TB) co-infection remains to be elaborately discussed. Here, we discussed the association of immunosenescence and chronic immune activation in HIV-TB co-infection and reviewed the role played by mediators of immune deterioration in HIV-TB co-infection necessitating the importance of designing therapeutic strategies against HIV disease progression and pathogenesis.

From "truly naïve" to "exhausted senescent" T cells: when markers predict functionality

The study of T cell biology has been accelerated by substantial progress at the technological level, particularly through the continuing advancement of flow cytometry. The conventional approach of observing T cells as either T helper or T cytotoxic is overly simplistic and does not allow investigators to clearly identify immune mechanisms or alterations in physiological processes that impact on clinical outcomes. The complexity of T cell sub-populations, as we understand them today, combined with the immunological and functional diversity of these subsets represent significant complications for the study of T cell biology. In this article, we review the use of classical markers in delineating T cell sub-populations, from "truly naïve" T cells (recent thymic emigrants with no proliferative history) to "exhausted senescent" T cells (poorly proliferative cells that display severe functional abnormalities) wherein the different phenotypes of these populations reflect their disparate functionalities. In addition, since persistent infections and chronological aging have been shown to be associated with significant alterations in human T cell distribution and function, we also discuss age-associated and cytomegalovirus-driven alterations in the expression of key subset markers.

T cell replicative senescence in human aging

The decline of the immune system appears to be an intractable consequence of aging, leading to increased susceptibility to infections, reduced effectiveness of vaccination and higher incidences of many diseases including osteoporosis and cancer in the elderly. These outcomes can be attributed, at least in part, to a phenomenon known as T cell replicative senescence, a terminal state characterized by dysregulated immune function, loss of the CD28 costimulatory molecule, shortened telomeres and elevated production of proinflammatory cytokines. Senescent CD8 T cells, which accumulate in the elderly, have been shown to frequently bear antigen specificity against cytomegalovirus (CMV), suggesting that this common and persistent infection may drive immune senescence and result in functional and phenotypic changes to the T cell repertoire. Senescent T cells have also been identified in patients with certain cancers, autoimmune diseases and chronic infections, such as HIV. This review discusses the in vivo and in vitro evidence for the contribution of CD8 T cell replicative senescence to a plethora of age-related pathologies and a few possible therapeutic avenues to delay or prevent this differentiative end-state in T cells. The age-associated remodeling of the immune system, through accumulation of senescent T cells has farreaching consequences on the individual and society alike, for the current healthcare system needs to meet the urgent demands of the increasing proportions of the elderly in the US and abroad.

T cell replicative senescence in human aging

The decline of the immune system appears to be an intractable consequence of aging, leading to increased susceptibility to infections, reduced effectiveness of vaccination and higher incidences of many diseases including osteoporosis and cancer in the elderly. These outcomes can be attributed, at least in part, to a phenomenon known as T cell replicative senescence, a terminal state characterized by dysregulated immune function, loss of the CD28 costimulatory molecule, shortened telomeres and elevated production of proinflammatory cytokines. Senescent CD8 T cells, which accumulate in the elderly, have been shown to frequently bear antigen specificity against cytomegalovirus (CMV), suggesting that this common and persistent infection may drive immune senescence and result in functional and phenotypic changes to the T cell repertoire. Senescent T cells have also been identified in patients with certain cancers, autoimmune diseases and chronic infections, such as HIV. This review discusses the in vivo and in vitro evidence for the contribution of CD8 T cell replicative senescence to a plethora of age-related pathologies and a few possible therapeutic avenues to delay or prevent this differentiative end-state in T cells. The age-associated remodeling of the immune system, through accumulation of senescent T cells has farreaching consequences on the individual and society alike, for the current healthcare system needs to meet the urgent demands of the increasing proportions of the elderly in the US and abroad.

Persistent viral infections and immune aging

Immunosenescence comprises a set of dynamic changes occurring to both, the innate as well as the adaptive immune system that accompany human aging and result in complex manifestations of still poorly defined deficiencies in the elderly population. One of the most prominent alterations during aging is the continuous involution of the thymus gland which is almost complete by the age of 50. Consequently, the output of naïve T cells is greatly diminished in elderly individuals which puts pressure on homeostatic forces to maintain a steady T cell pool for most of adulthood. In a great proportion of the human population, this fragile balance is challenged by persistent viral infections, especially Cytomegalovirus (CMV), that oblige certain T cell clones to monoclonally expand repeatedly over a lifetime which then occupy space within the T cell pool. Eventually, these inflated memory T cell clones become exhausted and their extensive accumulation accelerates the age-dependent decline of the diversity of the T cell pool. As a consequence, infectious diseases are more frequent and severe in elderly persons and immunological protection following vaccination is reduced. This review therefore aims to shed light on how various types of persistent viral infections, especially CMV, influence the aging of the immune system and highlight potential measures to prevent the age-related decline in immune function.

Regulation of the p21Sdi1/Cip1/Waf1DNA Synthesis Inhibitor in Senescent Human Diploid Fibroblasts

A large body of evidence has demonstrated that normal human fibroblasts have a limited division potential in culture and underwent senescence, a process whereby cells became arrested in the G1 phase of the cell cycle and overexpressed a DNA synthesis inhibitor(s). Cyclin-dependent kinase two (Cdk2) is required for the promotion of the Gi-to-S phase transition in human cells. Senescent fibroblasts contain intact cyclin-Cdk2 complexes but cannot induce Cdk2 protein kinase activity in response to mitogen stimulation. Recently, we cloned p21Sdi1, a potent inhibitor of DNA synthesis and Cdk2 kinase activity, from a senescent cell cDNA library and demonstrated that it was expressed at significantly higher levels in senescent cells than actively proliferating cells. In contrast to actively dividing cells, mitogen-stimulated senescent cells do not down-regulate the expression of p21Sdi1and do not express late G1 phase gene products that are required for entry into S phase. We suggest that the inability of mitogen-stimulated senescent cells to down-regulate p21Sdi1levels contributes to the resulting lack of late Gi gene expression and failure to traverse the G1/S phase boundary.

The role of CD8+ T-cell replicative senescence in human aging

The strict limit in proliferative potential of normal human somatic cells - a process known as replicative senescence - is highly relevant to the immune system, because clonal expansion is fundamental to adaptive immunity. CD8(+) T cells that undergo extensive rounds of antigen-driven proliferation in cell culture invariably reach the end stage of replicative senescence, characterized by irreversible cell-cycle arrest and a critically short telomere length. Cultures of senescent CD8(+) T cells also show resistance to apoptosis, permanent loss of CD28 expression, altered cytokine profiles, reduced ability to respond to stress, and various functional changes. Cells with similar characteristics accumulate during normal aging as well as in younger persons infected with human immunodeficiency virus, suggesting that the process of replicative senescence is not an artifact of cell culture but is also occurring in vivo. Interestingly, in elderly persons, the presence of high proportions of CD8(+) T cells with characteristics of replicative senescence is correlated with reduced antibody responses to vaccines as well as with osteoporotic fractures. CD8(+)CD28(-) T cells also accumulate in patients with certain types of cancer. The emerging picture is that senescent CD8(+) T cells may modulate both immune and non-immune functions, contributing not only to reduced anti-viral immunity but also to diverse age-related pathologies.

Roy Walford and the immunologic theory of aging

Roy Walford died on April 27, 2004, at the age of 79. His contributions to gerontological research in such diverse areas as caloric restriction, genetics of lifespan, immunosenescence, DNA repair and replicative senescence were truly remarkable in their depth and innovation. Significantly, most of the areas that he pioneered during his illustrious research career remain the "hot" areas of current gerontological research. In this sense, he has achieved the most important type of immortality. His death was a major personal and professional loss to numerous scientists within the gerontological community. In launching this new journal on Immunity and Ageing, it is highly fitting, therefore, to remember him on the anniversary of his death by briefly reviewing the contributions of Roy Walford to this important facet of gerontology. Indeed, it was Roy who actually first coined the commonly used term "immunosenescence".

From Hayflick to Walford: the role of T cell replicative senescence in human aging

The immunologic theory of aging, proposed more than 40 years ago by Roy Walford, suggests that the normal process of aging in man and in animals is pathogenetically related to faulty immunological processes. Since that time, research on immunological aging has undergone extraordinary expansion, leading to new information in areas spanning from molecular biology and cell signaling to large-scale clinical studies. Investigation in this area has also provided unexpected insights into HIV disease, many aspects of which represent accelerated immunological aging. This article describes the initial insights and vision of Roy Walford into one particular facet of human immunological aging, namely, the potential relevance of the well-studied human fibroblast replicative senescence model, initially developed by Leonard Hayflick, to cells of the immune system. Extensive research on T cell senescence in cell culture has now documented changes in vitro that closely mirror alterations occurring during in vivo aging in humans, underscoring the biological significance of T cell replicative senescence. Moreover, the inclusion of high proportions of putatively senescent T cells in the 'immune risk phenotype' that is associated with early mortality in octogenarians provides initial clinical confirmation of both the immunologic theory of aging and the role of the T cell Hayflick Limit in human aging, two areas of gerontological research pioneered by Roy Walford.

Human CD4+ T cell clone longevity in tissue culture: lack of influence of donor age or cell origin

CD4+ human T cell clones were derived from activated peripheral blood lymphocytes of healthy young adults to establish cloning efficiencies (CE) and clonal longevities. These results were compared with those obtained using cells from the very elderly, also in excellent health. CE and both maximal and average longevities under appropriate culture conditions were very similar in the two groups. Moreover, CE of CD34+ hematopoietic progenitor cells and longevities of clones derived from them were also similar. Finally, CE and longevities of clones derived from a patient with chronic myelogenous leukaemia were found to be comparable as well. Hence, T cells with absolutely no antigenic exposure in vivo prior to cloning (i.e. CD34-derived) and those potentially exposed to chronic antigenic stimulation (CML-derived) and those from young or old donors all had similar cloning and propagation properties in vitro. These results imply that the longevity of T cells in culture is more likely to be dictated by cloning conditions than any intrinsic differences between the cells studied.

CD4+ CD8+ (thymocyte-like) T lymphocytes present in blood and skin from patients with atopic dermatitis suggest immune dysregulation

BACKGROUND

Atopic dermatitis (AD) is a chronic inflammatory skin disease expressed early in life. Disease development is primarily determined by as yet unknown genetic factors, leading to the accumulation of activated T lymphocytes in the skin.

OBJECTIVES

To investigate the nature of these T cells.

METHODS

T-cell lines could be established from AD skin biopsies, but not from normal skin or AD peripheral blood, when placed in RPMI 1640 medium with 10% human AB serum, antibiotics, and the T-lymphocyte growth factors interleukins 2 and 4. The cell lines were subjected to phenotypic analysis using a fluorescence-activated cell sorter and compared with lymphocytes from AD and normal control peripheral blood.

RESULTS

T-cell lines from 22 of 24 consecutive skin biopsies taken from 24 adult patients with AD were established. All cells were T lymphocytes expressing several activation markers. A significant proportion of the lymphocytes had stable expression of a CD4+ CD8+ phenotype (26% +/- 6%; mean +/- SEM). Such double-positive T lymphocytes are normally only seen in the thymus and not in the peripheral immune system. CD4+ CD8+ cells in peripheral blood of the patients (12.5% +/- 3.3%) were also detected.

CONCLUSIONS

We suggest that a basic pathophysiological change in AD may be a faulty maturation of the T-lymphocyte system, leading to skin inflammation with CD4+ CD8+ T lymphocytes resembling immature T cells. This is likely to lead to skewing of many immune reactions in the patients.

Molecular aspects of the relationship between cancer and aging: tumor suppressor activity during cellular senescence

Normal cells cultured in vitro lose their proliferative potential after a finite number of doublings in a process termed replicative cellular senescence (Hayflick, 1965). The roles that growth inhibitory tumor suppressors play in the establishment and maintainence of cellular senescence have been reported in many different systems. The Rb and p53 tumor suppressors are examples of growth inhibitors that lose the ability to be regulated and are constantly activated during senescence. Other proteins that inhibit the initiation of DNA synthesis in early passage fibroblasts and that link the action of tumor suppressors with the cell cycle machinery, are also expressed at higher levels in senescent cells. For example, the increased expression of the cyclin-dependent kinase inhibitor p16 may contribute to arresting the growth of senescent cells. Identification and characterization of additional genes encoding growth inhibitors that are upregulated in senescent cells, such as the recently isolated p33ING1 protein, should provide a better understanding of the "aging program" that ceases to operate in the generation of immortal cancer cells.

Human T-cell clones in long-term culture as a model of immunosenescence

We have consistently observed that like other normal somatic tissue cells, human T lymphocytes manifest a finite proliferative capacity in culture in vitro. When measured in population doublings (PD), this averages about 35 PD for T-cell clones (TCC) derived from mature peripheral T cells of young adults and about 20 PD more for TCC derived from T-cell precursors in their bone marrow. We believe that alterations in surface marker phenotypes and corresponding functional changes observed in these human TCC as they progress through their finite lifespans in vitro can provide valuable information on processes of T-cell immunosenescence in vivo. They may also provide a model system for studying ways of modulating the ageing process to delay or prevent immunosenescence in the elderly and the chronically infected or possibly to accelerate immunosenescence in organ transplantation.

Extension of the replicative life span of human diploid fibroblasts by inhibition of the p33ING1 candidate tumor suppressor

Previous studies suggest that tumor suppressors may play significant roles in blocking the growth of cells during cellular senescence. We therefore studied the potential involvement of a novel growth inhibitor and candidate tumor suppressor gene called ING1, which we have cloned recently (I. Garkavtsev, A. Kazarov, A. Gudkov, and K. Riabowol, Nat. Genet. 14:415-420, 1996), in the process of cellular senescence. Our results show that the RNA and protein levels of ING1 were 8- to 10-fold higher in senescent cells than in young, proliferation-competent human diploid fibroblasts. Expression of the nuclear p33ING1 protein was regulated during the cell cycle, reaching maximal levels during DNA synthesis. Chronic expression of antisense ING1 RNA reproducibly resulted in extension of the proliferative life span of normal human fibroblasts by approximately seven population doublings.

Thymocytopoiesis in aging: the bone marrow-thymus axis

Manifestations of aging in the mature T lymphocyte compartment have been attributed, to a major extent, to effects of the involuted thymus, at the thymic microenvironment level. However, since generation of T lymphocytes starts from hemopoietic stem cells that settle in the thymus and differentiate there, aging effects on the stem cells, and as a consequence, on the bone marrow (BM)-thymus axis, may also have an impact on patterns of thymocytopoiesis and on age-related thymus remodeling. This communication reviews our studies designed to determine whether BM cells manifest any aging effects that become overt in the resulting thymocytes. The experiments were performed by seeding of BM cells onto lymphoid-depleted fetal thymus (FT) explants, to enable distinguishing between processes that occur in the BM and those that are caused by the aging thymic microenvironment. The data show changes in the developmental potential of BM-derived cells, as reflected from the kinetics of cell cycle and intermediate steps from stem cell settling in the thymus to an early stage at the transition from CD4(-)CD8(-), double negative (DN), to CD4(+)CD8(+), double positive (DP) thymocytes. In addition, we have demonstrated that these early developmental steps of thymocytopoiesis are subject to feedback regulation by mature T cells, and the extent of regulation may be altered in old age. The pattern of T lymphocyte generation in aging is thus a result of dynamic changes in thymic, as well as extrathymic functions, along the sequential developmental steps from the stem cell to the ultimate mature cell.

Decreased proliferative capacity and increased susceptibility to activation-induced cell death in late-passage human CD4+ TCR2+ cultured T cell clones

The growth characteristics in vitro of interleukin 2 (IL 2)-dependent human CD4+ alpha beta-T cell receptor-positive helper T cell clones (TCC) were studied in relation to alterations in surface phenotype, cytokine responsiveness, and susceptibility to activation-induced cell death (AICD). TCC derived from peripheral blood T cells had finite lifespans averaging 33 population doublings (PD) with a recorded maximum lifespan of 80 PD (n = 208). First analyses of the TCC were undertaken at ca. 25 PD, at which time all cells of all TCC expressed high intensity CD45RO and low intensity CD45RA, as well as high intensity CD95 (fas) and MHC class II antigens. The expression of these molecules remained elevated throughout the proliferative lifespan of the clones, but for those TCC which were initially CD28+ (the majority), the density of expression of the latter was diminished in most late-passage clones. Concomitant with this, late-passage cells showed reduced responsiveness to CD28-mediated costimulation by CHO transfectants expressing human CD80 compared to early-passage cells. Additionally, the level of expression of IL 2R gamma c and IL 7R chains was commonly reduced, as was the response to IL 2 and IL 7. Despite unchanged levels of fas expression on TCC with time, late-passage cells were more susceptible to AICD than early, passage cells. These observations further document functional and phenotypic alterations in long-term cultured human T helper cells, which may be considered as biomarkers of immunosenescence. This may contribute to an improved understanding of the mechanisms underlying depressed T cell function in old age.

Replicative senescence: implications for in vivo aging and tumor suppression

Normal cells have limited proliferative potential in culture, a fact that has been the basis of their use as a model for replicative senescence for many years. Recent molecular analyses have identified numerous changes in gene expression that occur as cells become senescent, and the results indicate that multiple levels of control contribute to the irreversible growth arrest. These include repression of growth stimulatory genes, overexpression of growth inhibitory genes, and interference with downstream pathways. Studies with cell types other than fibroblasts will better define the role of cell senescence in the aging process and in tumorigenesis.

Studies demonstrating the complexity of regulation and action of the growth inhibitory gene SDI1

The identification of the DNA synthesis inhibitory gene SDI1 by investigators studying cell senescence, tumor suppression, cell cycle control and differentiation suggest a key regulatory role for this gene. To better understand the growth regulatory activity of this gene we proceeded with the experiments described here. The data demonstrate that SDI1 is an important downstream effector of p53, but here we report that it can also cause inhibition of DNA synthesis in several immortal human cell lines, independent of p53 or Rb status. Levels of SDI1 mRNA expression in immortal cells are consistently much lower than that of normal cells, indicating that immortalization is not compatible with high expression of SDI1. These results highlight the complex nature of regulation of this gene and its mode of action.

Insights on immunological aging derived from the T lymphocyte cellular senescence model

A human T cell culture system has been developed to study cellular senescence in a manner analogous to that used for fibroblasts. Using this system, several markers of replicative senescence have been identified. Evidence is provided to show that new insights into the immune deficiency of aging as well as HIV infection have emerged from the in vitro model. The culture model is now being exploited to dissect the basic mechanisms underlying the senescence program in T cells.

Long-term culture of monoclonal human T lymphocytes: models for immunosenescence?

Human monoclonal T lymphocyte populations maintained in long-term culture by intermittent reactivation via the antigen receptor and supplied with exogenous interleukin 2 manifest finite proliferative lifespans. T lymphocytes cloned from mature peripheral T cells of adult donors were constantly lost from the time point of their first isolation up to an estimated maximum of 80 population doublings (PD) for the longest lived. T lymphocytes cloned from T cell progenitors in bone marrow, on the other hand, survived for a maximum of ca. 100 PD. One facet of the functional capacity of cells derived from these two different sources was assessed by measuring their autocrine proliferation after mitogenic stimulation. For a majority of T cell clones (TCC), autocrine proliferative capacity decreased as a function of culture age, becoming absent by 50 PD for adult-derived-TCC and by 70 PD for bone marrow-derived TCC, thereby clearly occurring prior to the end of the proliferative life spans of the clones. Limiting dilution frequency analysis showed that the number of autocrine proliferative precursors within these monoclonal populations declined with age, paralleling loss of autocrine proliferative capacity in the 'bulk' clones. Of a variety of surface structures monitored during culture ageing of TCC, the density of expression of the coreceptor molecule CD28 was found to correlate with decreasing autocrine proliferative capacity in two-thirds of the clones. Thus, at least for a fraction of monoclonal human T lymphocytes, decreasing autocrine proliferative capacity, a measure of clonal expansion, may correlate with decreasing numbers of CD28 molecules expressed on the surface and therefore presumably with the strength of costimulatory signal delivered via this important coreceptor.

The in vitro senescence of human T lymphocytes: failure to divide is not associated with a loss of cytolytic activity or memory T cell phenotype

Normal human T lymphocytes, activated in vitro and cultured in the continuous presence of the growth factor interleukin 2 (IL2), have a limited proliferative potential. Senescent T cell cultures will not proliferate, even if restimulated by the original allogeneic stimulator cells. However, we have now observed that such restimulation induces an increase in the percentage of cells expressing the 55 kDa chain of the IL2 receptor (IL2R alpha, CD25) without any associated increase in cell number. A younger culture, which showed a comparable increase in CD25, underwent two population doublings in the same time period after restimulation. The senescent cultures, (primarily of the CD8+, cytotoxic/suppressor, phenotype), were also found to be highly potent and specific effector cells in a 51chromium release assay for cytolytic activity. Furthermore, senescent cultures maintain the surface phenotype of memory T cells. These findings demonstrate that while senescent T cells are unable to proliferate in response to restimulation or to IL2, they are able to recognize the foreign stimulator cells and to initiate an otherwise normal T cell response. Our results lend support to the hypothesis that in vitro senescence is not associated with a generalized decline in functional activity in a differentiated cell type, but with a specific event which limits cell division. Thus, the long term T lymphocyte culture system will be useful for studying the mechanism by which proliferation is blocked in these, apparently, post-mitotic cells.

The biochemistry of mammalian senescence

Senescence is a process which, until quite recently, has been the subject of little scientific investigation. Even the word "senescence" is difficult to define, and complex methodological pitfalls have impeded progress. In the past few years, there have been exciting advances in understanding the physiological, cell biological, biochemical, and molecular biological nature of senescence. Changes in membrane function, protein synthesis, DNA structure (including glycosylation, altered tertiary structure, free-radical effects, and loss of telomeric DNA), and changes in gene regulation with age are reviewed. Recent work on changes in responses to transcriptional regulatory proteins and cellular senescence factors, some of which have been identified, is particularly promising and leads to the conclusion that senescence, at least in part, is a programmed process. Despite these advances, the fundamental cause of senescence remains elusive but might, as in the case of other biological processes which are phylogenetically widespread, turn out to be quite simple, and perhaps, even modifiable.

Common senescent cell-specific antibody epitopes on fibronectin in species and cells of varied origin

The phenomenon of in vitro cellular senescence has been demonstrated in cultured cells derived from humans and various other species. We have previously shown that monoclonal antibodies SEN-1, SEN-2, and SEN-3 react to epitopes on fibronectin that are exposed when human diploid fibroblasts become senescent. We here present results demonstrating that exposure of these epitopes is specific to senescence for a variety of human cells: epidermal keratinocytes, mammary epithelial cells, as well as fibroblasts. Fibronectin from 11 additional species was also analyzed by Western immunoblot for ability to bind the SEN antibodies. SEN-1 bound only human and gorilla fibronectin, whereas SEN-2 and SEN-3 bound fibronectin from those two species as well as the horse, cow, sheep, goat, dog, and chick. None of the antibodies reacted with fibronectin from the rabbit, rat, or mouse. These data indicated a correlation between the ability of the SEN antibodies to bind fibronectin from a particular species and the ability of cells from that species to exhibit a stable senescent phenotype in vitro. Therefore, exposure of this region of fibronectin may be important in the establishment and maintenance of cellular senescence. In addition, the ability of the SEN antibodies to react with fibronectin from a variety of senescent cells emphasizes their usefulness as markers for cellular senescence.

Molecular genetic approaches to the study of cellular senescence

Normal cells in culture exhibit limited division potential, which is used as a model for cellular aging. In contrast, tumor-derived, carcinogen- or virus-transformed cells are capable of dividing indefinitely (immortal). Fusion of normal with immortal human cells yielded hybrids having limited life span, indicating that cellular senescence is a dominant phenotype and that immortality is recessive. Fusions of various immortal human cell lines with each other led to the identification of four complementation groups for indefinite division. In order to identify the chromosomes and genes involved in growth regulation, that had been modified in immortal cells, we used the technique of microcell fusion to introduce either a normal human chromosome 11 or 4 into cell lines representative of the different complementation groups. Chromosome 11 had no effect on the in vitro life span of the different immortal human tumor lines. However, when a normal human chromosome 4 was introduced into cell lines assigned to complementation group B, the cells lost the immortal phenotype. No effect on the proliferation potential of cell lines representative of the other complementation groups was observed. These results suggest that a gene(s) on human chromosome 4 has been modified in immortal cell lines assigned to complementation group B, to allow escape from senescence. They also provide evidence for a genetic basis for cellular aging.

Hybrids from fusion of normal human T lymphocytes with immortal human cells exhibit limited life span

A number of normal human cell types have been shown to exhibit cellular senescence in vitro. We and others had found that fusion of normal human fibroblasts with immortal human cells yielded hybrids having limited lifespan. This indicated that the phenotype of cellular senescence is dominant and that immortality results from recessive changes in genes involved in growth control. They also supported the hypothesis that senescence results from genetic mechanisms rather than random damage. Since T lymphocytes are a highly differentiated cell type, in contrast to fibroblasts, it was of interest to determine whether similar mechanisms caused senescence in the T cells. We therefore fused normal human T lymphocytes with an immortal human cell line to determine whether they could restore the senescent, nondividing phenotype in hybrids, as do normal human fibroblasts. Eleven of fifteen hybrid clones studied exhibited limited proliferative potential after achieving a range of population doubling similar to that observed in the cell fusion studies involving normal fibroblasts. These results provide evidence that cellular senescence in T lymphocytes occurs via genetic mechanisms.

Clonotypic chromosomal aberrations in long-term lines of myelin-specific rat T lymphocytes

A panel of 16 long-term rat T lymphocyte lines and clones were screened for cytogenetical abnormalities using chromosomal banding techniques. All T lines were CD4+, recognizing the relevant antigen in the molecular context of major histocompatibility complex (MHC) class II determinants. With one exception (an ovalbumin-specific line), all lines were specific for myelin proteins, and apart of one BS rat-derived T line and its clones, all lines were selected from the Lewis strain of rat. After in vitro culture of more than 1 year, all lines and clones exhibited subtle but definite chromosomal aberrations, which included deletions, enlargement, translocations and formation of isochromosomes. All lines were near diploid, structural chromosomal changes being more frequent than numerical abnormalities. Each T line investigated had an individual pattern of chromosomal changes. In our analysis, 16 of the 22 different chromosomes had changes in at least one line. Chromosome 9 and the X chromosome appeared to have an enhanced susceptibility of alterations. In two cases, chromosomal markers could be traced through different stages of in vitro culture of the T lines.

Human T lymphocytes possess a limited in vitro life span

The T lymphocyte offers certain theoretical advantages over other available cell types for the study of aging. Immunosenescence is a well-established part of, and may be directly relevant to, mammalian aging, and the T lymphocyte is well-characterized as to function, cell-surface antigen make-up, and other factors. However, prior efforts at studying in vitro aging of T cells have been hampered by poor reproducibility in doubling potential and the occurrence of a peculiar type of crisis. We have improved the culture conditions for long-term in vitro propagation of normal human T lymphocytes so that previously described variability between identically manipulated cultures and the crisis period have been eliminated. Analysis of the growth patterns of 109 individual cultures revealed a limited proliferative life span, with the number of cumulative population doublings corresponding to that reported for adult human fibroblasts. This accord between the in vitro life spans of two vastly different cell types lends further support to the concept of the Hayflick Limit as a general biological phenomenon.

A routine method for storing lymphocytes for repeated isolation of DNA

A protocol for easy storage and later expansion of lymphocyte populations is given. Compared with methods using transformed cell lines, the method has a number of advantages for repeated production of cells for the isolation of DNA in amounts sufficient for use in diagnostic DNA technology.

Neonatal T cells as a model system to study the possible in vitro senescence of lymphocytes

In this study long-term neonatal T-lymphocyte cultures were initiated from cord blood following alloantigenic stimulation. Growth curves and population doublings were measured for replicate cultures, functional and phenotypic analyses performed, and cells were cloned. Thus, newborn T cells were shown to constitute a potentially excellent model for the analysis of possible in vitro senescence of immunologically relevant cells. Certain problems of the system centering around "crisis" periods and reproducibility, were additionally explored.

Isolation of cell lines possessing functional and serological properties resembling those of thymocyte precursors

Thymocytes develop from a committed haematopoietic progenitor, referred to as a prothymocyte. They are uniquely capable of migrating to and restoring the thymus of a lethally irradiated host, a property which has been exploited as a specific assay for these cells. Like other committed haematopoietic progenitors, prothymocytes are found only in small numbers in even the richest sources (0.05-1.0% of the nucleated cells in bone marrow). Purification has proved difficult both in terms of finding a suitable starting material and in the degree of enrichment achieved. We now report the isolation of cloned lines of cells with some of the serological and functional properties of prothymocytes. One of these lines has been in continuous culture for almost 2 years. When injected into irradiated recipients, cells from this line migrate to the thymus and there develop into cells which resemble normal cortical thymocytes.

Characterization of a tumorigenic murine T-lymphoid-cell line spontaneously derived from an IL-2-dependent T-cell line

The establishment of IL-2-independent T-cell lines spontaneously derived from long-term IL-2-dependent cytotoxic T-cell lines is described. Two lines (cloned and uncloned) studied in detail have shown the following characteristics: (1) Permanent loss of IL-2 dependence. (2) Partial or complete loss of both cytotoxic activity and the IL-2 receptor. (3) Increased expression of T-cell membrane markers (Thy1.2, Lyt1.2) compared with the parental line. (4) Lower level of DNA methylation than in freshly obtained lymphoid cells. (5) Different karyotypic pattern from the parental IL-2-dependent line, with a mean number of 39-40 chromosomes and a resemblance to T leukemic lines. (6) Leukemia caused in normal syngeneic C57BL/6 mice by the uncloned line, in contrast to the cloned IL-2-independent line or the parental dependent line. Unlike established leukemic lines, however, the independent line gave rise to tumors which regressed in some mice within a few days of their appearance. These findings suggest that T-cell lines maintained with IL-2 for prolonged periods of time (greater than 3 months) can undergo transformation and, therefore, should not be utilized for immunotherapeutic purposes.

Longevity of human allospecific TLCs: mycoplasma infection as a cause of in vitro "suppression" of MLC

It has been suggested that allospecific T-cell clones lose specific reactivity after approximately 30 cell doublings and subsequently acquire suppressor and NK-like characteristics. We have tested this hypothesis by assaying paired functional and nonfunctional TLCs for suppressor activity in PLT and MLC cocultures. Two sets of clones were initially studied: the first pair consisted of clone S5.2B, a functional TLC, and S5.14A, a nonfunctional TLC; the second pair of clones tested was comprised of two different expansions of the same clone S5.5A (nonfunctional) and S5.5B (functional). These experiments yielded no evidence for suppressive activity by nonfunctional clones toward functional clones, furthermore, the addition of nonfunctional clones to primary MLC assays had no effect on the level of responsiveness. Eight clones were subcloned and 89 subclones were retested for function after approximately 50 cell doublings. Generally, the subclones failed to suppress MLC proliferation. A minority of TLCs could suppress MLC responses, but this "suppression" was reversible with the addition of 2% exogenous TCGF. However, eight subclones and two parental TLC lines did suppress MLC responses in the presence or absence of TCGF, but the suppressive effects in such cocultures were reversible in the presence of tylocine, an anti-mycoplasma antibiotic. Therefore, human T-cells, cultured for extended periods, do not inexorably and universally lose specific alloreactivity and gain suppressive characteristics due to some presumed differentiative event.

Immunocompetence of chimeric rabbits. III. Serial passage and persistence of B-lymphocyte memory

In a model consisting of noninbred rabbits matched for major histocompatibility antigens and mismatched for immunoglobulin allotypes, using cell donors and recipients unrelated to each other, B-cell memory has been demonstrated to persist through three successive transfers for a period approaching 2 years. Memory cells from the original donor are shown to dominate specific antibody responses of the primary and secondary recipients. Vigorous antibody responses by donor-derived cells are obtained even when antigenic stimulation is delayed by several months. The data suggest that B memory cells may be particularly efficient in the colonization of recipients, and the potential significance of these findings for adoptive immunization of human bone marrow recipients is discussed.