Susceptibility to apoptosis of T lymphocytes from elderly humans is associated with increased in vivo expression of functional Fas receptors

Senescent T Cells in Age-Related Diseases

Age-induced alterations in human immunity are often considered deleterious and are referred to as immunosenescence. The immune system monitors the number of senescent cells in the body, while immunosenescence may represent the initiation of systemic aging. Immune cells, particularly T cells, are the most impacted and involved in age-related immune function deterioration, making older individuals more prone to different age-related diseases. T-cell senescence can impact the effectiveness of immunotherapies that rely on the immune system's function, including vaccines and adoptive T-cell therapies. The research and practice of using senescent T cells as therapeutic targets to intervene in age-related diseases are in their nascent stages. Therefore, in this review, we summarize recent related literature to investigate the characteristics of senescent T cells as well as their formation mechanisms, relationship with various aging-related diseases, and means of intervention. The primary objective of this article is to explore the prospects and possibilities of therapeutically targeting senescent T cells, serving as a valuable resource for the development of immunotherapy and treatment of age-related diseases.

Evaluation of quantitative biomarkers of aging in human PBMCs

Functional decline with age contributes significantly to the burden of disease in developed countries. There is growing interest in the development of therapeutic interventions which slow or even reverse aging. Time and cost constraints prohibit the testing of a large number of interventions for health and lifespan extension in model organisms. Cell-based models of aging could enable high throughput testing of potential interventions. Despite extensive reports in the literature of cell properties that correlate with donor age, few are robustly observed across different laboratories. This casts doubt on the extent that aging signatures are captured in cultured cells. We tested molecular changes previously reported to correlate with donor age in peripheral blood mononuclear cells (PBMCs) and evaluated their suitability for inclusion in a panel of functional aging measures. The tested measures spanned several pathways implicated in aging including epigenetic changes, apoptosis, proteostasis, and intracellular communication. Surprisingly, only two markers correlated with donor age. DNA methylation age accurately predicted donor age confirming this is a robust aging biomarker. Additionally, the apoptotic marker CD95 correlated with donor age but only within subsets of PBMCs. To demonstrate cellular rejuvenation in response to a treatment will require integration of multiple read-outs of cell function. However, building a panel of measures to detect aging in cells is challenging and further research is needed to identify robust predictors of age in humans.

Increased CD95 (Fas) and PD-1 expression in peripheral blood T lymphocytes in COVID-19 patients

A low count of CD4⁺ and CD8⁺ lymphocytes is a hallmark laboratory finding in the coronavirus disease 2019 (COVID‐19). Using flow cytometry, we observed significantly higher CD95 (Fas) and PD‐1 expression on both CD4⁺ T and CD8⁺ T cells in 42 COVID‐19 patients when compared to controls. Higher CD95 expression in CD4⁺ cells correlated with lower CD4⁺ counts. A higher expression of CD95 in CD4⁺ and CD8⁺ lymphocytes correlated with a lower percentage of naive events. Our results might suggest a shift to antigen‐activated T cells, expressing molecules increasing their propensity to apoptosis and exhaustion during COVID‐19 infection.

CD70 contributes to age-associated T cell defects and overwhelming inflammatory responses

Aging is associated with immune dysregulation, especially T cell disorders, which result in increased susceptibility to various diseases. Previous studies have shown that loss of co-stimulatory receptors or accumulation of co-inhibitory molecules play important roles in T cell aging. In the present study, CD70, which was generally regarded as a costimulatory molecule, was found to be upregulated on CD4⁺ and CD8⁺ T cells of elderly individuals. Aged CD70⁺ T cells displayed a phenotype of over-activation, and expressed enhanced levels of numerous inhibitory receptors including PD-1, 2B4 and LAG-3. CD70⁺ T cells from elderly individuals exhibited increased susceptibility to apoptosis and high levels of inflammatory cytokines. Importantly, the functional dysregulation of CD70⁺ T cells associated with aging was reversed by blocking CD70. Collectively, this study demonstrated CD70 as a prominent regulator involved in immunosenescence, which led to defects and overwhelming inflammatory responses of T cells during aging. These findings provide a strong rationale for targeting CD70 to prevent dysregulation related to immunosenescence.

Increased susceptibility of CD4+ T cells from elderly individuals to HIV-1 infection and apoptosis is associated with reduced CD4 and enhanced CXCR4 and FAS surface expression levels

Background

Elderly HIV-1 infected individuals progress to AIDS more frequently and rapidly than people becoming infected at a young age. To identify possible reasons for these differences in clinical progression, we performed comprehensive phenotypic analyses of CD4+ T cells from uninfected young and elderly individuals, and examined their susceptibility to HIV-1 infection and programmed death.

Results

Peripheral blood mononuclear cells (PBMCs) from older people contain an increased percentage of central memory and Th17 CD4+ T cells that are main target cells of HIV-1 and strongly reduced proportions of naïve T cells that are poorly susceptible to HIV-1. Unstimulated T cells from elderly individuals expressed higher levels of activation markers, death receptors, and the viral CXCR4 co-receptor than those from young individuals but responded poorly to stimulation. CD4+ T cells from older individuals were highly susceptible to CXCR4- and CCR5-tropic HIV-1 infection but produced significantly lower quantities of infectious virus than cells from young individuals because they were highly prone to apoptosis and thus presumably had a very short life span. The increased susceptibility of T cells from the elderly to HIV-1 infection correlated directly with CXCR4 and inversely with CD4 expression. The levels of apoptosis correlated with the cell surface expression of FAS but not with the expression of programmed death receptor 1 (PD1) or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL).

Conclusions

Increased levels of activated and highly susceptible HIV-1 target cells, reduced CD4 and enhanced CXCR4 cell surface expression, together with the high susceptibility to FAS-induced programmed cell death may contribute to the rapid CD4+ T cell depletion and accelerated clinical course of infection in elderly HIV-1-infected individuals.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0213-1) contains supplementary material, which is available to authorized users.

Activation-induced and damage-induced cell death in aging human T cells

In multicellular organisms the proper system functionality is ensured by the balance between cell division, differentiation, senescence and death. This balance is changed during aging. Immunosenescence plays a crucial role in aging and leads to the shrinkage of T cell repertoire and the propensity to apoptosis. The elimination of expanded T cells at the end of immune response is crucial to maintain homeostasis and avoid any uncontrolled inflammation. Resting mature T lymphocytes, when activated via their antigen-specific receptor (TCR) and CD28 co-receptor, start to proliferate and then undergo the so called activation induced cell death (AICD), which mechanistically is triggered by the death receptor and leads to apoptosis. T lymphocytes, like other cells, are also exposed to damage, which can trigger the so called damage-induced cell death (DICD). It was hypothesized that oxidative stress and chronic antigenic load increasing with age reduced lymphocyte susceptibility to DICD and enhanced a proinflamatory status leading to increased AICD. However, data collected so far are inconsistent and does not support this assumption. Systematic and comprehensive studies are still needed for conclusive elucidation of the role of AICD and DICD in human immunosenescence, including the role of autophagy and necroptosis in the processes.

Advanced glycation end products induce T cell apoptosis: Involvement of oxidative stress, caspase and the mitochondrial pathway

Accumulation of advanced glycation end products (AGEs) is a hallmark in aged people. T cells play important roles in maintaining homeostasis of immune function. This study investigated the effects of AGEs-bovine serum albumin (AGEs) in human T cells. Incubation of Jurkat and several immortalized T cell lines with AGEs resulted in cell death dose-dependently. AGEs-induced cell death was partially but significantly blocked by neutralizing antibodies recognizing receptor of AGEs. In addition to detecting DNA nick, simultaneous stainings of annexin V with 7-amino-actinomycin D further confirmed the apoptotic nature of cell death. AGEs also caused apoptosis in purified T cells. Although AGEs-induced apoptosis could be blocked by the pan-caspase inhibitor, Ala-Asp-fluomethyl ketone (Z-VAD-fmk), there was no activation of caspase-3, -5, -8 and -9. AGEs caused mitochondrial outer membrane permeabilization and this process was prevented by an antioxidant or Z-VAD-fmk. Furthermore, AGEs treatment led to translocation of apoptosis inducing factor (AIF) from the mitochondria into the nucleus. Altogether, this report demonstrated that AGEs induced T cell apoptosis in an oxidative stress-associated and caspase-dependent manner with involvement of the mitochondrial pathway. It is likely that AGEs-induced T cell apoptosis may play a role in T cell homeostasis in ageing.

Mitochondrial function, mitochondrial DNA and ageing: a reappraisal

The impressive performance of the research in biology of mitochondrion has greatly improved our knowledge on the functions of this organelle and highlighted the influence its functioning has on numerous human phenotypes. In particular, many studies have focused on the involvement of mitochondrion function (and dysfunction) in human ageing. To date, the literature in this specific field of mitochondrial biology is so vast that it is often difficult to properly put new data and new findings in the right context. The present paper aims to review the findings of the last few years in order to outline a general framework to understand how mitochondria can affect ageing and how ageing affects mitochondria.

The role of sex steroids and gonadectomy in the control of thymic involution

A major underlying cause for aging of the immune system is the structural and functional atrophy of the thymus, and associated decline in T cell genesis. This loss of naïve T cells reduces adaptive immunity to new stimuli and precipitates a peripheral bias to memory cells against prior antigens. Whilst multiple mechanisms may contribute to this process, the temporal alliance of thymic decline with puberty has implicated a causative role for sex steroids. Accordingly ablation of sex steroids induces profound thymic rejuvenation. Although the thymus retains some, albeit highly limited, function in healthy adults, this is insufficient for resurrecting the T cell pool following cytoablative treatments such as chemo- and radiation-therapy and AIDS. Increased risk of opportunistic infections and cancer relapse or appearance, are a direct consequence. Temporary sex steroid ablation may thus provide a clinically effective means to regenerate the thymus and immune system in immunodeficiency states.

CD95-mediated apoptosis in naïve, central and effector memory subsets of CD4+ and CD8+ T cells in aged humans

Aging is associated with a decrease in naïve (T(N)) and central memory (T(CM)), and an accumulation of effector memory (T(EM) and T(EMRA)) T cell subsets. Previously, we have demonstrated an increased sensitivity of T(N) and T(CM) CD4+ and CD8+ T cells in aging to TNF-alpha-induced apoptosis. In this investigation, we examined whether similar differential sensitivity is applicable to CD95-mediated apoptosis. We show that T(N) and T(CM) CD4+ and CD8+ T cells from aged subjects are significantly more sensitive to CD95-mediated apoptosis. Increased apoptosis is associated with increased activation of caspase-8 and caspase-3. Both caspase-8 and caspase-3 inhibitors blocked CD95-mediated apoptosis and activation of caspase-8 and caspase-3 in T(N) and T(CM) CD4+ and CD8+ T cells. No significant difference was observed in apoptosis or in activation of caspase-8 and caspase-3 in T(EM) and T(EMRA) CD4+ and CD8+ T cells between young and aged subjects; both populations were relatively and comparably resistant to CD95-mediated apoptosis and caspase activation. No correlation was observed between the sensitivity/resistance of any of the subsets of CD4+ or CD8+T cells to CD95-mediated apoptosis and the expression of CD95. Our data suggest that increased CD95-mediated apoptosis of T(N) and T(CM) CD8+ and CD4+ T cells may play a role in their decline in human aging.

Neuronal FasL induces cell death of encephalitogenic T lymphocytes

Apoptosis of inflammatory cells plays a crucial role in the recovery from autoimmune CNS disease. However, the underlying mechanisms of apoptosis induction are as yet ill-defined. Here we report on the neuronal expression of FasL and its potential function in inducing T-cell apoptosis. Using a combination of facial nerve axotomy and passive transfer encephalomyelitis, the fate of CD4+ encephalitogenic T cells engineered to express the gene for green fluorescent protein was followed. FasL gene transcripts and FasL protein were detected in neurons by in sit-hybridization and immunohistochemistry. T cells infiltrating preferentially the injured brain parenchyma were found in the immediate vicinity of FasL expressing neurons and even inside their perikarya. In contrast to neurons, T cells rapidly underwent apoptosis. In co-cultures of hippocampal nerve cells and CD4 T lymphocytes, we confirmed expression of FasL in neurons and concomitant induction of T-cell death. Antibodies blocking neuronal FasL were shown to have a protective effect on T-cell survival. Thus, FasL expression by neurons in neuroinflammatory diseases may constitute a pivotal mechanism underlying apoptosis of encephalitogenic T cells.

Iron dysregulation combined with aging prevents sepsis-induced apoptosis

BACKGROUND

Sepsis, iron loading, and aging cause independent increases in gut epithelial and splenic apoptosis. It is unknown how their combination will affect apoptosis and systemic cytokine levels.

MATERIALS AND METHODS

Hfe-/- mice (a murine homologue of hemochromatosis) abnormally accumulate iron in their tissues. Aged (24-26 months) or mature (16-18 months) Hfe-/- mice and wild type (WT) littermates were subjected to cecal ligation and puncture (CLP) or sham laparotomy. Intestine, spleen, and blood were harvested 24 h later and assessed for apoptosis and cytokine levels.

RESULTS

Gut epithelial and splenic apoptosis were low in both aged septic and sham Hfe-/- mice, regardless of the amount of iron in their diet. Mature septic WT mice had increased apoptosis compared to age-matched sham WT mice. Mature septic Hfe-/- mice had similar levels of intestinal cell death to age-matched septic WT mice but higher levels of splenic apoptosis. Apoptosis was significantly lower in septic aged Hfe-/- mice than septic mature Hfe-/- animals. Interleukin-6 was elevated in septic aged Hfe-/- mice compared to sham mice.

CONCLUSIONS

Although sepsis, chronic iron dysregulation, and aging each increase gut and splenic apoptosis, their combination yields cell death levels similar to sham animals despite the fact that aged Hfe-/- mice are able to mount an inflammatory response following CLP and mature Hfe-/- mice have elevated sepsis-induced apoptosis. Combining sepsis with two risk factors that ordinarily increase cell death and increase mortality in CLP yields an apoptotic response that could not have been predicted based upon each element in isolation.

Life and death of lymphocytes: a role in immunesenescence

Human aging is associated with progressive decline in immune functions, increased frequency of infections. Among immune functions, a decline in T cell functions during aging predominates. In this review, we will discuss the molecular signaling in two major pathways of apoptosis, namely death receptor pathway and mitochondrial pathway, and their alterations in both T and B lymphocytes in human aging with a special emphasis on naïve and different memory subsets of CD8+ T cells. We will also discuss a possible role of lymphocyte apoptosis in immune senescence.

Molecular mechanisms of apoptosis in the cells of the immune system in human aging

Aging is associated with progressive decline in immune functions and increased frequency of infections, autoimmunity, and cancer. Among immune functions, a decline in T-cell functions during aging predominates. In this review, I discuss the molecular signaling of three distinct pathways of apoptosis, namely the death receptor pathway, the mitochondrial pathway, and the most recently described endoplasmic reticulum stress pathway, and the relative sensitivity of naive, central memory, and effector memory CD8(+) T-cell subsets to apoptosis. In addition, I review apoptosis, especially via death receptor pathway, in naive and various memory subsets of CD4(+) and CD8(+) T cells (with primary emphasis on CD8(+) naive and memory subsets) in human aging and discuss the role of apoptosis in immune senescence.

Impaired apoptosis and immune senescence - cause or effect?

Aged animals and humans exhibit a decreased T-cell activation response although they also exhibit increased susceptibility to responses to self-antigens and a loss of self-tolerance. The age-related alteration in T-cell reactivity, polyclonal expansion of T cells, and enhanced production of autoantibodies may reflect the numerous age-associated alterations in the T-cell arm of the immune system that have been revealed in numerous studies. These studies suggest that subpopulations of T cells are not deleted appropriately in older animals. They further suggest that an age-related impairment of Fas/Fas ligand (FasL)-mediated apoptosis - which plays a major role in activation-induced cell death (AICD) of T cells - may contribute to compromised regulation of the immune system. The likely mechanisms that may lead to impaired induction of FasL in AICD senescent T cells include an age-related shift from the apoptosis-sensitive T-helper 1 cell (Th1) response to the AICD-resistant Th2 response, aberrant T-cell receptor/CD3 downstream-signaling pathways, and altered CD28/B7-mediated T-cell costimulatory signals. Pathologically, accumulation of AICD-senescent T cells is associated with a defective cytotoxic T lymphocyte response and generation of autoreactive T cells. Based on the accumulating evidence, we propose that the emergence of the FasL(lo) AICD-senescent T cells is not only an effect of immune aging but also an important cause of T-cell proliferative senescence in both humans and mice.

Age disproportionately increases sepsis-induced apoptosis in the spleen and gut epithelium

Both aging and sepsis independently increase splenic and gut epithelial apoptosis. Sepsis-induced apoptosis in either cell type is also associated with increased mortality in young mice. We sought to determine whether age alters sepsis-induced splenic and gut epithelial cell death. Young (2 months) and aged (22 months) male ND4 mice were subjected to either single-puncture cecal ligation and puncture (CLP) with a 23-gauge needle or sham laparotomy. Apoptosis was assessed 24 hours later in the spleen and gut epithelium by active caspase 3 and hematoxylin and eosin staining. Aged septic mice had increased splenic apoptosis compared with either young septic animals or aged sham animals (15 vs. 7 vs. 5 apoptotic cells/high-powered field, P < 0.05). Similarly, aged septic animals had an elevation in gut epithelial cell death compared with either young septic or aged sham mice (33 vs. 16 vs. 6 apoptotic cells/100 contiguous crypts, P < 0.05). Elevated intestinal cell death was not associated with changes in either gut proliferation or cell division. To verify that the increase in splenic apoptosis seen in septic aged animals was not strain specific, double-puncture CLP with a 25-gauge needle or sham laparotomy was performed on young (4 months) or aged (24 months) C57BL/6 male mice. Similar to results seen in outbred animals, aged septic animals in this inbred strain had increased splenic apoptosis compared with either young septic animals or aged sham animals (23 vs. 7 vs. 4 apoptotic cells/ high powered field, P < 0.05). These results indicate that although infection and aging each independently cause an increase in splenic and gut epithelial apoptosis, their combination leads to a disproportionate increase in cell death in these rapidly dividing cell populations,and potentially plays a role in the marked increase in mortality seen with aging in sepsis.

Impairment of gamma/delta T lymphocytes in elderly: implications for immunosenescence

Gamma/delta T lymphocytes cells recognize the antigen in a non-classical way and are considered the third branch of the immune system devoted to defend the integrity of the body. Ageing is characterized by an impairment of the main way of protection (the adaptive branch) but, successfully aged people show compensatory mechanisms of defense such as proneness to inflammation. Moreover, very old subjects show an increased number of NK cells. We have previously demonstrated that gamma delta T lymphocytes are reduced in elderly. In the present paper we have studied some characteristics of these cells to evaluate the possibility that these cells might balance the decreased action of the adaptive branch in successfully aged people. Cytofluorimetric analysis of cells collected from young, old and centenarian subjects has been used to evaluate the ability of these cells to expand in vitro. Here we demonstrate that gamma delta T cells are impaired in the ability to proliferate to different stimuli such as isopentenyl pyrophoshate, that select gamma delta T lymphocytes bearing delta 2 chain, other than to phytohemagglutinin and anti-CD3 that are polyclonal activators. Moreover, we demonstrate that gamma delta T cells in aged and centenarians show an enhanced sensitivity to undergo apoptosis induced both by alpha-Fas and TNF-alpha. All together these data suggest that gamma delta T lymphocytes are impaired in elderly and suggest that the reduced ability to proliferate and the reduced number of circulating gamma delta T lymphocytes is due to the proneness to apoptosis. Finally on the basis of these data, we conclude that gamma delta T lymphocytes, do not participate in the remodeling of the immune system due to the reduction of classical T cell response and replacement by NK cells in elderly.

Role of NF-κB signaling pathway in increased tumor necrosis factor-α-induced apoptosis of lymphocytes in aged humans

In human aging, lymphocytes display increased sensitivity to tumor necrosis factor-alpha (TNF-alpha)-induced apoptosis. TNF-alpha induces both survival and apoptotic signals. The survival signal is mediated by the activation of NF-kappaB. Although a role of certain proapoptotic molecules in aging has been reported, a role of altered NF-kappaB signaling pathway has not been explored in detail. In this study, we have compared TNF-alpha-induced activation of NF-kappaB, phosphorylation of IkappaBalpha, and the expression of IKKbeta between lymphocytes from young and aged humans. Furthermore, we have explored a role of IKKbeta in increased susceptibility of lymphocytes from aged humans to TNF-alpha-induced apoptosis. Lymphocytes from aged humans displayed decreased activation of NF-kappaB, reduced phosphorylation of IkappaBalpha, and decreased expression of IKKbeta. In addition, overexpression of IKKbeta in lymphocytes from aged humans normalized TNF-alpha-induced apoptosis to the level of young subjects. These data suggest a deficiency of NF-kappaB signaling pathway and a role of IKKbeta, at least in part, for increased sensitivity of lymphocytes from aged humans to TNF-alpha-induced apoptosis.

A role of fas-associated death domain (FADD) in increased apoptosis in aged humans

Fas-associated death domain (FADD) is an adapter protein that is recruited to the death-inducing signaling complex (DISC) during signaling via death receptors. FADD is a common conduit in both CD95-mediated and TNF-R-mediated apoptosis. In aging, T cells show increased susceptibility to death receptor-induced apoptosis. Therefore, in this investigation, we have examined the role of FADD in increased TNF-alpha-induced apoptosis of lymphocytes from aged humans. Our study shows that FADD expression is increased in lymphocytes from aged humans as compared to those from young subjects. Furthermore, transfection of aged lymphocytes with FADD dominant negative (FADD dn) plasmid resulted in the reduction of TNF-induced apoptosis in aged lymphocytes to a level comparable to that of young subjects. In addition, overexpression of FADD by transfection of lymphocytes from young subjects with wild-type FADD (FADD wt) resulted in increased apoptosis of young lymphocytes to a level comparable to that of young subjects. These data suggest that FADD plays an important role in increased apoptosis in aged lymphocytes signaled via TNF-R.

Balanced Regulation of mRNA Production for Fas and Fas Ligand in Lymphocytes From Centenarians

Background— The functionality of the immune system during aging is crucial for protection against the most common age-related diseases. Apoptosis plays a central role in the senescence of the immune system, as evidenced by the increased plasma membrane expression of a key molecule like Fas protein. We analyzed the mRNA levels of different forms of Fas (total [tFas] and membrane [mFas]) and of its ligand (FasL) in peripheral blood lymphocytes from centenarians, the best example of successful aging, who were compared with young and middle-aged donors.

Methods and Results— Using real-time polymerase chain reaction, we quantified mRNA for different forms of Fas and for FasL. In resting lymphocytes, mRNA for tFas, but not for mFas, significantly increases with age, whereas FasL mRNA significantly decreases. In vitro production of Fas/FasL mRNA after different stimuli was similar in cells from the 3 groups. Even if the percentage of Fas + cells was higher than in the other groups, peripheral blood lymphocytes from centenarians had normal Fas-induced apoptosis, as revealed by flow cytometry. By ELISA, we observed that cells from centenarians showed normal in vitro production of the soluble form of Fas (sFas) and that plasma levels of such molecule were significantly higher in centenarians than in the other groups.

Conclusions— Lymphocytes from centenarians are able to balance the production of proapoptotic (mFas and FasL) and antiapoptotic (sFas) molecules, whose proportions are likely crucial for the well-preserved immune functionality at the extreme limits of human life.

Decreased proliferative capability of CD4(+) cells of elderly people is associated with faster loss of activation-related antigens and accumulation of regulatory T cells

Decreased proliferation of CD4(+) lymphocytes of elderly people is at least in part due to lowered proportion of cells that are capable of dividing and producing viable progeny (effective precursors). We show that age-dependent reduction in effective precursor numbers depends on the one hand, extensive, activation-dependent apoptosis occurring from the very onset of stimulation and, on the other, the accumulation of non-dividing, regulatory (suppressor) CD4(lo)CD25(+)CD28(lo) T cells. In addition, analysis of changes in surface expression of activation-related antigens, including CD25, CD69, and PCNA in consecutive generations of dividing CD4(+) cells traced by carboxyfluorescein diacetate succinimidyl ester staining showed variable patterns of these changes that may relate to various aspect of impaired division of these cells in elderly.

Increased intensities of fas expression on peripheral T-cell subsets in severe autoimmune thyroid disease

Fas (CD95)-Fas ligand (FasL; CD178)-induced apoptosis is necessary for the maintenance of self-tolerance. To clarify whether or not any abnormalities in the Fas-FasL system exist in patients with autoimmune thyroid disease (AITD), we examined the expression of Fas and FasL on peripheral T lymphocytes by three-color flow cytometry in 113 patients with AITD and 49 healthy controls. The intensities of Fas expression in both CD4(+) and CD8(+) T cells decreased in thyrotoxic patients with Graves' disease (GD), but increased in both patients with severe Hashimoto's disease (HD) undergoing treatment and seriously intractable patients with GD continuously positive for thyrotropin (TSH) receptor antibody despite treatment with antithyroid drugs for more than 5 years. The proportion of Fas expression was increased in CD4(+) T cells from patients with untreated GD, and in CD8(+) T cells from patients with severe HD. The proportion of CD8(+) T cells decreased in patients with severe HD. FasL were not expressed on T cells in controls and patients with AITD. These results indicate that (1). the intensities of Fas expression on peripheral T cells increase in severe autoimmune thyroid diseases and (2). both the intensity and the proportion of Fas expression may be important for the induction of apoptosis.

Involvement of Rel/NF-kappa B transcription factors in senescence

Senescence is now established as a genetically controlled phenomenon that alters different cell functions, including proliferation, apoptosis, resistance to stress, and energetic metabolism. Underlying changes in gene expression are governed by some transcription factors, whose expression or activity must change with senescence as well. Transcription factors of the Rel/NF-kappa B family are good candidates to participate in the establishment of senescence. Arguments range from correlation between cell functions controlled by these factors and cell functions altered during senescence, to phenotypes resulting from in vitro manipulations of Rel/NF-kappa B activity.

Evidence for autoantibody-induced CD4 depletion mediated by apoptotic and non-apoptotic mechanisms in HIV-positive long-term surviving haemophilia patients

It is believed that autoimmune phenomena and apoptosis contribute to CD4 depletion. We investigated 11 long-term (>20 years) HIV-infected haemophilia patients and 10 healthy controls. Using four-colour-fluorescence flow cytometry, we studied the proportions of CD3+CD4+ and CD3+CD4- blood lymphocytes that were CD95+, CD95L+, immune complex+ (IC+, consisting of IgM, IgG, C3d and/or gp120), and were viable or non-viable (propidium iodide+ = PI+). In addition, we studied viability of CD4+IgG+ patient lymphocytes using the apoptosis marker annexin and the permeability indicator 7-amino actinomycin D (7-AAD). HIV+ patients had a higher proportion of CD3+CD4+IgG+PI+ lymphocytes than healthy controls (median: 3.7%versus 0.3%; P = 0.00001). These non-viable IgG-coated lymphocytes might have been killed in vivo by ADCC or complement lysis; 9.1% of the circulating CD3+CD4+ blood lymphocytes were IgG+PI- (controls: 2.5%; P = 0.001). These viable IgG-coated lymphocytes might be targets for phagocytosis or anti-CD95 autoantibody-mediated apoptosis. Because HIV+ patients and healthy controls had similar proportions of PI+ or PI- CD3+CD4+ lymphocytes that carried CD95L on the surface, and because CD3+CD4+CD95L+ cells that were IgG+, C3d+ and/or gp120- were increased in HIV+ patients, the role of CD95L-induced apoptosis in long-term HIV-infected haemophilia patients remains unclear. The findings that HIV+ patients had higher proportions of CD3+CD4+CD95+ (PI+: 6.5%versus 1.4%; P = 0.00002; PI-: 55.8%versus 44.4%; P = 0.04) blood lymphocytes and that the proportion of CD4+IgG+Annexin+7-AAD- blood lymphocytes was associated inversely with peripheral CD4 counts (r = -0.636; P < 0.05) suggest that attachment of IgG to CD4+ blood lymphocytes (anti-CD95?) induces in some lymphocytes apoptosis with subsequent depletion of these IgG-coated apoptotic CD4+ lymphocytes from the circulation. We found supporting evidence for the contention that autoantibody-induced apoptotic and non-apoptotic mechanisms contribute to CD4 depletion in long-term HIV-infected haemophilia patients.

Ageing–apoptosis relation in murine spleen

Relatively few studies have been published with regard to modification of apoptosis in normal tissues as a function of ageing. The majority of these studies demonstrated an increase in programmed cell death (PCD) with age. However, opposite results, namely loss of apoptotic control with age, have also been reported. In the present study, we examined proliferation and apoptotic cell death in spleens of C57/BL mice of different ages. A tendency towards decrease in cell proliferative capacity was seen with age. By contrast, apoptosis was increased in spleens from aged animals. Moreover, the proliferative cell/apoptotic cell ratio decreased in function of age. Ladder type DNA degradation was much more pronounced in DNA derived from splenocytes of old mice. These results were supported by a decrease of Bcl-2 and an increase in Fas receptor expression as well as by increased activation of caspases 8, 3 and 9 in splenocytes from aged animals. In addition, cell surface molecular markers recognizable by macrophages in apoptotic cells, namely decreased sialic acid concomitant with increased unmasking of galactose residues, were more pronounced on splenocytes from old mice than on those from young animals. In addition to the experimental evidence which supports a role of apoptotic cell death in ageing, a series of theoretical reasoning, which could also favor this possibility, are discussed.

Age-dependent changes in the susceptibility to apoptosis of peripheral blood CD4+ and CD8+ T lymphocytes with virgin or memory phenotype

Susceptibility to apoptosis changes with age and most of the available data on lymphocytes refer to mitogen stimulated cells. We studied this susceptibility in quiescent, purified CD4+ or CD8+ T cells from a group of Italian old people compared with a group of young people. We found that an apoptotic agent such as 2-deoxy-D-ribose (dRib), which acts via glutathione depletion and oxidative stress, was more effective in CD4+ T cells from young donors, while no difference was found in CD8+ T cells. On the contrary, another agent such as TNF-alpha, which acts via receptor engagement, was more effective in CD8+ T cells from old subjects, and no difference was found in CD4+ T cells. When marker of activation-memory were investigated, no difference between young and old subjects was found when dRib was used. Differently, when TNF-alpha was used, memory and activated CD4+ T cells from old donors were less sensitive than younger counterparts, while memory CD8+ T cells from old donors were more sensitive than younger counterparts. This suggests that age-related changes in susceptibility to apoptosis of resting T cells largely depend on the type of the apoptotic stimulus which is used as well as on the memory phenotype of the cells. These results may also account, at least in part, for the deep remodelling of T cell repertoire that occurs during ageing.

Origin of late-onset autoimmune disease

Autoimmune disease in the elderly is hypothesized to be caused by an imbalance in T-cell expansion and deletion after an encounter with self-antigens. A decrease in thymic output leads to a decreased pool of naive T cells in the periphery and to increased oligoclonal expansion of T cells. This expansion may be caused by stimulation with autoantigens that drive high-affinity interactions with self-antigens. Accumulation of presenescent, apoptosis-resistant, and proinflammatory T cells results in the growth of these autoreactive T cells. A decreased T-cell activation response that occurs with age leads to several defects that diminish the immune response.

The effect of aging on immune responses

Although vaccine manufacturers make no specific recommendations regarding the vaccination of older horses and ponies, the similarities in age-induced immunologic changes between human beings and equids suggests that similar vaccination recommendations should be followed. The need for vaccination of the older horse depends, of course, on the relative risk of exposure for the individual horse. Particular care should be taken when using attenuated vaccine products because these live agents may pose a unique risk to the older individual. Immunization with inactivated agent vaccines is likely to be safer. In general, annual vaccination against equine influenza virus infection, tetanus, rabies, and encephalomyelitis viruses is warranted.

Inhibition of H2O2-induced apoptosis of lymphocytes by calorie restriction during aging

Calorie restriction (CR) is known to delay the aging process in rodents and is postulated to act by decreasing free radical generation and increasing antioxidant enzyme activity. The present study was designed to investigate the effect of CR and age on oxidative stress-induced apoptosis and associated changes in the levels of TNF-alpha, and Bcl-2 in splenic T lymphocytes. Ad libitum (AL)- or CR-fed C57BL/6J mice were sacrificed either at 6 (young) or 18 (old) months and splenic lymphocytes were incubated with or without 25 micro M H2O2 to induce apoptosis. Apoptosis increased with age in cells of AL-fed mice incubated with H2O2. CR prevented this rise in apoptosis in total splenic lymphocytes and in CD4(+) and CD8(+) T lymphocyte subsets either with or without H2O2. Free radicals increased and mitochondrial membrane potential decreased in aged mice. CR prevented these changes and also prevented the age-associated increase in TNF-alpha and loss of Bcl-2 in total splenic lymphocytes and in CD4(+) and CD8(+) lymphocyte subsets. In summary, lymphocytes in aged AL-fed mice were much more susceptible to oxidative stress-induced apoptosis whereas CR normalized apoptosis by preventing the increase in TNF-alpha and the decrease in Bcl-2 associated with aging.

Apoptosis and reduced influenza A virus specific CD8+ T cells in aging mice

Some studies have reported increased apoptosis in CD8(+) T cells from aged mice. We previously demonstrated diminished virus-specific CD8(+) cytotoxic T lymphocyte (CTL) activity in aged mice in comparison to young mice. The present study investigated the role of apoptosis in age-related influenza virus-specific CD8(+) CTL deficiency. Splenocytes from influenza-primed aged and young mice were stimulated in vitro with virus. The CD8(+) T cell/total lymphocyte ratios correlated with CTL activity and were significantly decreased and increased in aged and young mice, respectively. Fas, FasL, TNF-alpha and TNFR-p55 expression, measured by flow cytometry, ELISA and/or RT-PCR, were significantly elevated in aged mice. Apoptotic CD8(+) T cells (Annexin V binding) were also elevated in aged mice. IL-12 treatment increased CD8(+) CTL activity and IFN-gamma production but did not affect apoptosis. Thus, apoptosis may contribute to reduced influenza virus-specific CD8(+) T cell frequency, CTL deficiency and increased influenza disease in aging.

Tumor necrosis factor-alpha-induced apoptosis in T cells from aged humans: a role of TNFR-I and downstream signaling molecules

Tumor necrosis factor-alpha (TNF-alpha) induces apoptosis predominantly via TNF-receptor I (TNF-RI). We have examined the molecular and biochemical pathways of TNF-alpha-induced apoptosis in T cells from aged and young subjects. Aged subjects show absolute lymphopenia and decreased numbers of both CD4+ and CD8+ T cells. T cells from aged subjects show increased sensitivity to TNF-alpha-induced apoptosis that is associated with increased expression of TNF-RI and decreased expression of TNF-RII in both CD4+ and CD8+ T cells. Agonistic TNF-RI also induced greater apoptosis in T cells from aged subjects as compared to young subjects, suggesting that increased TNF-alpha-induced apoptosis in aging is predominantly mediated via TNF-RI. There was an increased expression of FADD and increased activation of caspase 8 and caspase 3 in lymphocytes from aged humans as compared to young subjects. A role of impaired TNF-RII-mediated signaling in increased apoptosis in aged subjects is also discussed.

Age associated decline in CD25 and CD28 expression correlate with an increased susceptibility to CD95 mediated apoptosis in T cells

Immunosenescence is believed to contribute to increase susceptibility to infectious diseases and cancer in the elderly, and is caused mainly by changes in the T cell compartment. Longitudinal studies were undertaken to examine T cell surface receptor expression and apoptotic susceptibility using Staphylococcal enterotoxin B (SEB) activated human T cells as an in vitro model of an ageing T cell culture. An intracellular stain Carboxyfluorescein diacetate succinimidyl ester (CFSE) was used to assess the number of population divisions (PD) occurring in the ageing T cell culture. One major biomarker of aged T cells is a decrease in expression of CD28 and since this is an essential co-stimulatory molecule, its decreasing expression with age could compromise their activation and apoptotic capacity. Activation of T cells resulted in initial up-regulation of CD25, CD95 and CD28, although expression of CD25 and CD28 subsequently decreased with increasing PD. CD4 and CD8 T cells expressed similar CD25 profiles although CD28 expression was unique in each subset. CD4+ cells expressed the highest CD28 levels, and showed a gradual decline in expression with increasing PD, whereas CD8+ cells were low CD28 expressers, but did not appear to lose their expression as they aged. To determine T cell susceptibility to apoptosis via CD95/CD95-L interactions with increasing age, cells were challenged with CD95-L transfected CHO cells at various PD. Increased death was observed as they aged, which correlated with the decreased expression of activation markers CD25 and CD28.

Neutrophil ageing and immunesenescence

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

Apoptotic capability in ageing T cells

The ageing immune system shows a gradual decline in responsiveness to antigens and tumours due to the emergence of immunosenescence. The main functions of T cells are activation, anergy and apoptosis and these are all affected during ageing. Apoptosis is vital in controlling cell numbers, deleting self-reactive T cells and maintaining immune surveillance. One of the principle instigators of death involves the CD95:CD95-ligand interaction and as T cells age both receptor and ligand levels increase. This view will describe the current knowledge of the apoptotic susceptibility of ageing T cells and evaluate the factors that may affect the apoptotic capability of immunosenescent T cells.

Molecular steps of cell suicide: an insight into immune senescence

The cellular and molecular basis of immune senescence is unclear. A number of mechanisms have been proposed. In this issue of the Journal of Clinical Immunology, some of the mechanisms for various immunologic abnormalities in aging are presented. In this article, various molecular steps of both death receptor and mitochondrial pathways of apoptosis in general are reviewed. In particular, the role of apoptosis in T-cell immune senescence is discussed.

Alterations in splenic architecture and the localization of anti-double-stranded DNA B cells in aged mice

Aging is characterized by a decline in humoral immunity and a concommitant increased incidence of anti-DNA and other autoantibodies. To define how the regulation of autoreactive B cells is altered with age, we have used BALB/c mice with an Ig heavy H chain transgene to track the fate of anti-double-stranded (ds) DNA B cells in vivo. In young adult mice, anti-dsDNA B cells are developmentally arrested and excluded from the splenic B cell follicle, whereas in most aged mice they are mature and localize within the B cell follicle. Furthermore, we have detailed global changes in lymphoid architecture that accompany aging: CD4(+) T cells are found not only in the periarteriolar lymphoid sheath, but also in the B cell follicles. Strikingly, these disruptions are similar to those that precede serum anti-dsDNA antibody expression in autoimmune MRL-lpr/lpr mice.

Fas/APO-1 protein is increased in spaceflown lymphocytes (Jurkat)

Human lymphocytes flown on the Space Shuttle respond poorly to mitogen stimulation and populations of the lymphoblastoid T cell line, Jurkat, manifest growth arrest, increase in apoptosis and time- and microgravity-dependent increases in the soluble form of the cell death factor, Fas/APO-1 (sFas). The potential role of apoptosis in population dynamics of space-flown lymphocytes has not been investigated previously. We flew Jurkat cells on Space Transportation System (STS)-80 and STS-95 to determine whether apoptosis and the apparent microgravity-related release of sFas are characteristic of lymphocytes in microgravity. The effects of spaceflight and ground-based tests simulating spaceflight experimental conditions, including high cell density and low serum concentration, were assessed. Immunofluorescence microscopy showed increased cell associated Fas in flown cells. Results of STS-80 and STS-95 confirmed increase in apoptosis during spaceflight and the release of sFas as a repeatable, time-dependent and microgravity-related response. Ground-based tests showed that holding cells at 1.5 million/ml in medium containing 2% serum before launch did not increase sFas. Reports of increased Fas in cells of the elderly and the increases in spaceflown cells suggest possible similarities between aging and spaceflight effects on lymphocytes.

Shortage of circulating naive CD8+ T cells provides new insights on immunodeficiency in aging

Clinical observations indicate that elderly people are prone to severe, often lethal infectious diseases induced by novel pathogens. Since the ability to mount primary immune responses relies on the availability of naive T cells, the circulating naive T-cell reservoir was evaluated throughout the human life span. Naive T cells were identified as CD95− T lymphocytes for their phenotypic and functional features. Indeed, the lack of CD95 marker is sufficient to identify a population of naive T cells, as defined by coincidence with previously characterized CD45RA+ CD62L+ T cells. Naive CD95− T cells, as expected, require a costimulatory signal, such as CD28, to optimally proliferate after anti-CD3 stimulation. Cytofluorimetric analysis of circulating T lymphocytes from 120 healthy subjects ranging in age from 18 to 105 years revealed that naive T cells decreased sharply with age. The younger subjects had a naive T-lymphocyte count of 825 ± 48 cells/μL, and the centenarians had a naive T-lymphocyte count of 177 ± 28 cells/μL. Surprisingly, the naive T-cell count was lower in CD8+than in CD4+ subsets at any age, and the oldest individuals were almost completely depleted of circulating naive CD8+ T cells (13 ± 4 cells/μL). Concomitantly, a progressive expansion of CD28− T cells occurs with age, which can be interpreted as a compensatory mechanism. These data provide new insights into age-related T-cell–mediated immunodeficiency and reveal some analogies of T-cell dynamics between advanced aging and human immunodeficiency virus (HIV) infection. In conclusion, the exhaustion of the naive CD8+ T-cell reservoir, which has never been reported before, suggests that this T-cell pool is a major target of the aging process and may define a parameter possibly related to the life span of humans.

Molecular and biochemical pathways of apoptosis in lymphocytes from aged humans

In this study, we investigated a role of apoptosis in lymphopenia and progressive cell-mediated immunodeficiency associated with aging. We examined two major signaling pathways of apoptosis in lymphocytes from aged humans and compared them with lymphocytes from young subjects. Both CD4+ and CD8+ T cell subsets from aged subjects demonstrated increased sensitivity to TNFR-mediated and Fas-mediated apoptosis that was associated with overexpression of death receptors and adapter molecules associated with death signaling. An increased expression and activity of both initiator (caspase 8) and effector (caspase 3) caspases was observed in lymphocytes from aged subjects as compared to young individuals. Furthermore, an increased expression of Bax and decreased expression of Bcl-2 (both at the protein and mRNA level) was found in lymphocytes from aged subjects. These data suggest that increased sensitivity of lymphocytes from aged subjects to death signals may play an important role in the pathogenesis of lymphopenia and T cell deficiency associated with the aging process.

In vitro senescence models for human T lymphocytes

Immunosenescence is an age-associated dysregulation of immune function which may contribute to the increased susceptibility of the elderly to infectious disease. Although age-associated changes are measurable in the innate immune system, it is the adaptive arm of the immune system which is particularly susceptible to the deleterious effects of ageing, especially the T cell compartment. In this review, the characteristics of longitudinal ageing in cultured monoclonal human T cell populations will be summarized. It will be argued that parallels between this in vitro model and T cell senescence in vivo suggest the use of such models to screen for interventions ameliorating immunosenescence in vivo.

Age-related changes in the expression of CD95 (APO1/FAS) on blood lymphocytes

Aging is associated with alterations of the immune system, thought to be related to an increased susceptibility to infectious diseases, and possibly to cancer and autoimmunity in the elderly. In the present paper we report data obtained on freshly collected blood from 148 healthy subjects of different ages (from cord blood to 102 years old). The subjects were divided into seven age classes (cord blood, 3-11 years, 15-39 years, 41-60 years, 61-74 years, 75-84 years, 85-102 years) and their lymphocyte subsets and the expression of the apoptosis-related molecule CD95 were evaluated. In respect of lymphocyte subsets, the major differences were found in the cord-blood samples compared with the oldest old groups. In the cord-blood group, the absolute number of all the lymphocyte subsets was enhanced, but in the oldest group, an increase of CD16+ lymphocytes was observed, whereas CD19+ lymphocytes, which progressively decrease with age, continue to decrease further in the very old. The data show that the expression of CD95 increases until age 74 years, whereas in the oldest old it tends to decrease again. The trend of CD95 expression seems to be related to the change of expression of CD95 on CD4+ lymphocytes, because the CD8+/CD95+ population rose steadily throughout the entire age range. The evaluation of CD95+/CD45R0+ lymphocytes shows similar results to those observed analyzing CD95 on total lymphocytes. Furthermore, a constant increase of CD95+/CD28+ and a related decline of CD28+ lymphocytes was observed in all age groups. These data suggest that the expression of CD95 on the different subsets of lymphocytes can be considered a good marker for studies of immunosenescence, because it may be predictive of successful aging, and can partially explain the change in lymphocytes subsets in elderly.

Increased activity of caspase 3 and caspase 8 in anti-Fas-induced apoptosis in lymphocytes from ageing humans

We have previously shown an increased susceptibility of T cell subsets to anti-Fas-induced apoptosis in human ageing [1]. In this study, we have examined the role of downstream mediators, including caspases, in Fas-mediated apoptosis in lymphocytes from ageing humans. The cleavage activity of caspase-8 and caspase-3 was compared between ageing and young subjects at different times following anti-Fas treatment, using colorimetric detection analysis. The expression of Fas-associated death domain (FADD), caspase-8, and caspase-3 in lymphocytes was compared at the protein level using Western blotting, and at the mRNA level by Northern blot analysis. In lymphocytes from ageing subjects, there was an early increase in the cleavage activity of caspase-8 and caspase-3 compared with young controls. Furthermore, increased protein expression of FADD, caspase-8 and caspase-3 at the basal level was observed in lymphocytes from ageing humans. Our results suggest that the altered expression and activity of molecules in the Fas/FasL signalling pathway may play a role in increased Fas-induced apoptosis and T cell deficiency in ageing humans.

T cell immunosenescence in vitro and in vivo

The term "immunosenescence" refers to an age-associated dysregulation of immune function which contributes to the increased susceptibility of the elderly to infectious disease. Although there are age-associated changes measurable in the innate immune system (Pawelec et al., 1998c), it is the adaptive, particularly T cell, system which is most susceptible to the deleterious effects of aging. In this minireview, characteristics of aging in long-term human T cell cultures will be summarized, and the parallels between the in vitro model and in vivo immunosenescence will be documented. The use of culture models to screen for ways of manipulating immunosenescence in vitro may provide a basis for intervention to ameliorate immunosenescence in vivo.

Increased TNF-α-Induced Apoptosis in Lymphocytes from Aged Humans: Changes in TNF-α Receptor Expression and Activation of Caspases

Aging is characterized by increased T cell lymphopenia, T cell dysfunction, and increased serum TNF levels. In this study, we have examined the role of TNF-induced apoptosis in T cell deficiency in lymphocytes from aged humans. The constitutive expression of TNF receptors (TNFRI and TNFRII) and the adapter molecules, including TNFR-associated death domain protein (TRADD), TNFR-associated factor 2 (TRAF-2), and receptor interacting protein (RIP), were analyzed both at the protein level by flow cytometry or Western blotting, and at the mRNA level using quantitative PCR or Northern blotting in lymphocytes from aged and young subjects. The susceptibility of T cells to undergo TNF-induced apoptosis was analyzed using terminal deoxynucleotidyltransferase-mediated UTP-end-labeling (TUNEL) and DNA ladder assays. Caspase (caspase-8 and caspase-3) activation was compared between aged and young subjects using Western blotting and colorimetric assays. In lymphocytes from aged humans, there was an increased susceptibility of CD4+ and CD8+ T cells to undergo TNF-α-induced apoptosis, as observed by TUNEL assay and DNA fragmentation ladder assay. Increased TNF-α-induced apoptosis was also observed in both CD45RA+ and CD45RO+ T cells from aging subjects. An increased constitutive expression of TNFRI and TRADD and decreased expression of TNFRII and TRAF-2 were observed in lymphocytes from aged as compared with young controls. In addition, there was an early and increased activation of caspases (caspase-8 and caspase-3) involved in TNFR/TNF signaling pathway, as evident by early cleavage of caspase-8, poly(ADP-ribose) polymerase (PARP), and caspase-3 substrate DEVD-p-nitroamilide NA. These data suggest that an increased TNF-α-induced apoptosis may play a role in T cell deficiency associated with human aging.

Aging increases CD8 T cell apoptosis induced by hyperstimulation but decreases apoptosis induced by agonist withdrawal in mice

Apoptosis of T cells is thought to play a critical role in intrathymic T cell selection, in controlling the strength of the immune response to antigens, and in peripheral modulation of the T cell repertoire by influencing memory cell formation and survival. Peripheral T lymphocyte apoptosis or activation-induced cell death can be induced in vitro by repeated stimulation through the T cell receptor (TCR), and several groups have reported that aging increases the susceptibility of T cells to hyperstimulation-induced cell death in mice and humans. Alternately, apoptosis can also be induced in T cells by withdrawal of TCR stimulation from T cell blasts late in the activation process. This agonist withdrawal cell death, unlike apoptosis induced by repeated stimulation, is Fas- and TNFalpha-independent but is modulated by CD30 ligation. We show here that aging leads to an increase in susceptibility to apoptosis induced by repeated stimulation, but also to a decline in mouse CD8 T cell sensitivity to apoptosis induced by agonist withdrawal. Cell mixture experiments show that intercellular signals are required for the induction of apoptosis after agonist withdrawal and that the CD8 cells from aged mice can respond to these death-inducing signals but cannot produce them. A defect in this form of apoptosis after cessation of TCR signaling might contribute to the accumulation of functionally ineffective CD8 cells in aging mice.

Aging and apoptosis control

The phenomenon of aging is distinct from processes associated with advanced age known to increase risk of diseases, such as cancer. Furthermore, the process of aging is not necessarily related to phenomena such as in vitro replicative senescence; however, any unifying hypothesis of aging must account for all age-dependent phenomena, including senescence. It is proposed that apoptosis forms the ultimate protective process for preservation of phenotypic fidelity in multicellular organisms since it is the process by which the organism detects damage and replaces the defective cell. Time-dependent degeneration of apoptosis control is the rate-limiting step in the process of aging.