Lifespans of T lymphocytes

Murine regulatory T cells induce death of effector T, B, and NK lymphocytes through a contact-independent mechanism involving telomerase suppression and telomere-associated senescence

Regulatory T cells (Tregs) suppress the activity of effector T, B and NK lymphocytes and sustain immunological tolerance, but the proliferative activity of suppressed cells remains unexplored. In the present study, we report that mouse Tregs can induce replicative senescence and the death of responder mouse CD4⁺CD25^- T cells, CD8⁺ T cells, B cells and NK cells in vitro and in vivo. Contact-independent in vitro co-cultivation with Tregs up-regulated endonuclease G (EndoG) expression and its translocation to the nucleus in responder cells. EndoG localization in the nucleus induced alternative mRNA splicing of the telomerase catalytic subunit Tert and telomerase inhibition. The lack of telomerase activity in proliferating cells led to telomere loss followed by the development of senescence and cell death. Injection of Tregs into mice resulted in EndoG-associated alternative splicing of Tert, telomerase inhibition, telomere loss, senescence development and increased cell death in vivo. The present study describes a novel contact-independent mechanism by which Tregs specify effector cell fate and provides new insights into cellular crosstalk related to immune suppression.

Influence of Inflammation in the Process of T Lymphocyte Differentiation: Proliferative, Metabolic, and Oxidative Changes

T lymphocytes, from their first encounter with their specific antigen as naïve cell until the last stages of their differentiation, in a replicative state of senescence, go through a series of phases. In several of these stages, T lymphocytes are subjected to exponential growth in successive encounters with the same antigen. This entire process occurs throughout the life of a human individual and, earlier, in patients with chronic infections/pathologies through inflammatory mediators, first acutely and later in a chronic form. This process plays a fundamental role in amplifying the activating signals on T lymphocytes and directing their clonal proliferation. The mechanisms that control cell growth are high levels of telomerase activity and maintenance of telomeric length that are far superior to other cell types, as well as metabolic adaptation and redox control. Large numbers of highly differentiated memory cells are accumulated in the immunological niches where they will contribute in a significant way to increase the levels of inflammatory mediators that will perpetuate the new state at the systemic level. These levels of inflammation greatly influence the process of T lymphocyte differentiation from naïve T lymphocyte, even before, until the arrival of exhaustion or cell death. The changes observed during lymphocyte differentiation are correlated with changes in cellular metabolism and these in turn are influenced by the inflammatory state of the environment where the cell is located. Reactive oxygen species (ROS) exert a dual action in the population of T lymphocytes. Exposure to high levels of ROS decreases the capacity of activation and T lymphocyte proliferation; however, intermediate levels of oxidation are necessary for the lymphocyte activation, differentiation, and effector functions. In conclusion, we can affirm that the inflammatory levels in the environment greatly influence the differentiation and activity of T lymphocyte populations. However, little is known about the mechanisms involved in these processes. The elucidation of these mechanisms would be of great help in the advance of improvements in pathologies with a large inflammatory base such as rheumatoid arthritis, intestinal inflammatory diseases, several infectious diseases and even, cancerous processes.

There are only four basic modes of cell death, although there are many ad-hoc variants adapted to different situations

There have been enough cell death modes delineated in the biomedical literature to befuddle all cell death researchers. Mulling over cell death from the viewpoints of the host tissue or organ and of the host animal, we construe that there should be only two physiological cell death modes, i.e. apoptosis and senescent death (SD), as well as two pathological modes, i.e. necrosis and stress-induced cell death (SICD). Other death modes described in the literature are ad-hoc variants or coalescences of some of these four basic ones in different physiological or pathological situations. SD, SICD and necrosis kill useful cells and will thus trigger regeneration, wound healing and probably also scar formation. SICD and necrosis will likely instigate inflammation as well. Apoptosis occurs as a mechanism to purge no-longer useful cells from a tissue via phagocytosis by cells with phagocytic ability that are collectively tagged by us as scavengers, including macrophages; therefore apoptosis is not followed by regeneration and inflammation. The answer for the question of “who dies” clearly differentiates apoptosis from SD, SICD and necrosis, despite other similarities and disparities among the four demise modes. Apoptosis cannot occur in cell lines in vitro, because cell lines are immortalized by reprogramming the death program of the parental cells, because in culture there lack scavengers and complex communications among different cell types, and because culture condition is a stress to the cells. Several issues of cell death that remain enigmatic to us are also described for peers to deliberate and debate.

Molecular mechanisms involved in the aging of the T-cell immune response

T-lymphocytes play a central role in the effector and regulatory mechanisms of the adaptive immune response. Upon exiting the thymus they begin to undergo a series of phenotypic and functional changes that continue throughout the lifetime and being most pronounced in the elderly. The reason postulated for this is that the dynamic processes of repeated interaction with cognate antigens lead to multiple division cycles involving a high degree of cell differentiation, senescence, restriction of the T-cell receptor (TCR) repertoire, and cell cycle arrest. This cell cycle arrest is associated with the loss of telomere sequences from the ends of chromosomes. Telomere length is reduced at each cell cycle, and critically short telomeres recruit components of the DNA repair machinery and trigger replicative senescence or apoptosis. Repetitively stimulated T-cells become refractory to telomerase induction, suffer telomere erosion and enter replicative senescence. The latter is characterized by the accumulation of highly differentiated T-cells with new acquired functional capabilities, which can be caused by aberrant expression of genes normally suppressed by epigenetic mechanisms in CD4+ or CD8+ T-cells. Age-dependent demethylation and overexpression of genes normally suppressed by DNA methylation have been demonstrated in senescent subsets of T-lymphocytes. Thus, T-cells, principally CD4+CD28^null T-cells, aberrantly express genes, including those of the KIR gene family and cytotoxic proteins such as perforin, and overexpress CD70, IFN-γ, LFA-1 and others. In summary, owing to a lifetime of exposure to and proliferation against a variety of pathogens, highly differentiated T-cells suffer molecular modifications that alter their cellular homeostasis mechanisms.

TLR agonists are highly effective at eliciting functional memory CTLs of effector memory phenotype in peptide immunization

Given the importance of memory cytotoxic T lymphocytes (CTLs) in eliminating altered self-cells, including virus-infected and tumor cells, devising effective vaccination strategies for generating memory CTLs is a priority in the field of immunology. Herein, we elaborate upon a novel boosting approach that utilizes synthetic peptides and Toll-like receptor (TLR) agonists as adjuvants to generate sufficient numbers of memory CTLs to protect against infection in mice. Peptide boosting with lipopolysaccharide (LPS), a TLR4-ligand, has been shown to progressively enhance memory CTLs. Whether this result is strictly dependent on activation of TLR4 or can be similarly achieved by signaling through other TLRs is of practical interest in vaccine development but is yet unknown. In this report, we present evidence that intravenous peptide boosting together with TLR3 and TLR9 agonists (Poly IC and CpG, respectively) is highly effective and induces large quantities of memory CTLs of effector memory phenotype after three boosts. Compared to LPS, CpG and Poly IC generate more robust immune responses after the first and second boosts, indicating that a protective level of CTLs might be achieved with fewer boosts when CpG or Poly IC is used. Lastly, the resultant memory CTLs from boosting with different TLR agonists as adjuvant are equally protective against pathogen challenge and are not immune senescent. Therefore, TLR agonists are effective adjuvants in intravenous peptide boosting for the generation of functional memory CTLs.

Telomere/telomerase dynamics within the human immune system: effect of chronic infection and stress

Aging of the immune system is a major factor responsible for the increased severity of infections, reduced responses to vaccines, and higher cancer incidence in the elderly. A major category of stressors that contribute to the alterations within the T lymphocyte compartment is the family of herpes viruses. These viruses, usually acquired early in life, persist for many decades and drive certain T cells to the end stage of replicative senescence, which is characterized by a variety of phenotypic and functional changes, including altered cytokine profile, resistance to apoptosis, and shortened telomeres. Indeed, high proportions of senescent CD8 (cytotoxic) T lymphocytess are associated with latent cytomegalovirus (CMV) infection in the elderly, and are part of a cluster of immune biomarkers that are associated with early mortality. Similar cells accumulate at younger ages in persons chronically infected with HIV-1. In addition to persistent viral infection, psychological stress as well as oxidative stress can also contribute to the generation of senescent dysfunctional T lymphocytes. Strategies such as cell culture manipulation of replicative senescence, as well as lifestyle and stress reduction techniques are discussed in terms of possible approaches to enhance immune function in older persons. This review highlights the importance of using humans in studies on immunosenescence and telomere/telomerase dynamics, since model organisms employed in other facets of aging research are not subject to the particular factors that cause the striking age-related reconfiguration of the human immune system.

The role of immunity in elderly cancer

The increased incidence of malignancies in elderly patients living in industrialized countries has led to both identify the causes that alter the normal homeostatic balance in elderly and designate the specific treatments. The progressive decline of the immune system (immunosenescence) involving cellular and molecular alterations impact both innate and adaptive immunity. The immunosenescence leads to increased incidence of infectious diseases morbidity and mortality as well as heightened rates of other immune disorders such as autoimmunity, cancer, and inflammatory conditions. Here, we summarize the knowledge on the major changes in the immune system associated with aging in primary lymphoid organs as well as a description of molecular mechanisms, and the impact on cancer development.

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.

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.

Engraftment of quiescent progenitors and conversion to full chimerism after nonmyelosuppressive conditioning and hematopoietic cell transplantation in miniature swine

Our laboratory has previously reported a nonmyelosuppressive preparative regimen for hematopoietic cell transplantation that leads to mixed chimerism and allograft tolerance in miniature swine across minor and major histocompatibility disparities. Stable chimerism persisted in most of these animals but was restricted to T cells and confined to peripheral blood. Because of the importance of myeloid and erythroid progenitors for the treatment of hematologic disorders, the objective of this study was to assess whether such cells existed in the bone marrow of these lymphoid chimeras as an indication of functional engraftment. Colony-formation assays were performed on donor inocula before infusion and on bone marrow cells harvested from the transplant recipients. Donor-origin myeloid/erythroid progenitor colonies were detected in bone marrow from 6 of 7 lymphoid chimeric recipients. A delayed donor leukocyte infusion successfully converted a stable lymphoid chimera to full multilineage chimerism within 2 weeks. Donor-origin myeloid/erythroid progenitors could be detected in the bone marrow of a host-matched recipient after myeloablation and adoptive transfer of mobilized cells from one of the engrafted lymphoid chimeras. These data suggest that even when only lymphoid chimerism is readily detected by flow cytometry, dormant myeloid/erythroid progenitors can exist and subsequent conversion to full donor chimerism can be achieved. The ability to establish multilineage engraftment and chimerism without significant toxicity may have important clinical implications for the management of nonmalignant hematopoietic disorders and hematologic malignancies.

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.

Telomeres in T and B cells

Telomeres are the structures at the ends of linear chromosomes. In mammalian cells, they consist of hexanucleotide (TTAGGG) repeats, together with many associated proteins. In the absence of a compensatory mechanism, dividing cells undergo gradual telomere erosion until a critical degree of shortening results in chromosomal abnormalities and cell death or senescence. For T and B cells, the ability to undergo extensive cell division and clonal expansion is crucial for effective immune function. This article describes our current understanding of telomere-length regulation in lymphocytes and its implications for immune function.

Long-term immunological memory against viruses

This review addresses a novel facet of human T cell biology that constitutes a fundamental problem for long-term maintenance of immunological memory against viruses. The finite proliferative capacity of human T lymphocytes is sufficiently great to accommodate the waves of clonal expansion associated with primary and even secondary immune responses. However, long-term memory to viruses that establish latency and to repeatedly encountered viruses such as influenza may be severely impaired by "replicative senescence", a genetically programmed process affecting most somatic cell types of human origin. Consistent with this idea, memory CD8+ T cells with hallmarks of replicative senescence have been identified in vivo. Such cells may contribute to compromised viral immunity and response to vaccines, and furthermore, their very presence may negatively influence homeostatic mechanisms that control the size of the memory T cell pool in elderly persons.

Resistance to apoptosis in human CD8+ T cells that reach replicative senescence after multiple rounds of antigen-specific proliferation

We have established an in vitro culture model of cellular aging in which antigen-specific T cells are stimulated repeatedly to divide until they reach the irreversible state of growth arrest known as "replicative senescence." T lymphocytes that reach replicative senescence in culture show complete loss of CD28 expression, shortened telomeres, undetectable telomerase, and reduced ability to produce heat shock proteins. We now document that in response to treatment with apoptotic stimuli, senescent CD8+ T-cell cultures show reduced apoptosis and diminished caspase 3 activity compared with quiescent early passage cultures from the same donor. Our results suggest that the progressive accumulation of T cells showing many of the hallmarks of replicative senescence during aging, chronic infection, and autoimmune disease may, in part, reflect the diminished capacity of such cells to undergo normal programmed cell death.

Activation of telomerase is induced by a natural antigen in allergen-specific memory T lymphocytes in bronchial asthma

The function of the immune system is known to be dependent on the cellular differentiation and clonal expansion of allergen-specific lymphocytes. Telomerase, a ribonucleoprotein enzyme, is believed to be essential for the indefinite proliferation of human cells. To clarify whether telomerase is involved in the pathogenesis of immune diseases as well as of malignancies, we investigated the upregulation of telomerase activity in allergen-specific T lymphocytes. Upregulation of telomerase in allergen-sensitized lymphocytes was induced not only by artificial mitogenic stimulations but also by the natural antigen, house dust mite, which causes allergic diseases. Moreover, the upregulation of telomerase activity in memory T cells activated during allergen-specific immune responses might be associated with the enduring allergen-specific atopic propensity in asthmatics.

Reconsideration of senescence, immortalization and telomere maintenance of Epstein-Barr virus-transformed human B-lymphoblastoid cell lines

We review recent data on senescence and immortalization of human B-lymphoblastoid cell lines (LCLs) transformed by the Epstein-Barr virus (EBV). Although EBV-transformed LCLs are generally believed to be immortalized, a series of recent studies, including ours, provided strong evidence that they are mostly mortal and have non-malignant properties, except for a small proportion of LCLs that are immortalized by developing a strong telomerase activity. A large proportion of mortal LCLs have exceptionally long lifespans. Some of them have a lifespan over 150 population-doubling levels, keeping a relatively constant telomere length in spite of the absence of a detectable telomerase activity, suggesting that they maintain telomeres by a pathway other than that using telomerase. Here we propose a model of an alternative pathway to maintain telomeres of such long-lived mortal LCLs by exploiting extra-chromosomal telomere repeat DNA, which was recently found by us.

How might replicative senescence contribute to human ageing?

Cell senescence is the limited ability of primary human cells to divide when cultured in vitro. This eventual cessation of division is accompanied by a specific set of changes in cell physiology, morphology, and gene expression. Such changes in phenotype have the potential to contribute to human ageing and age-related diseases. Until now, senescence has largely been studied as an in vitro phenomenon, but recent data have for the first time directly demonstrated the presence of senescent cells in aged human tissues. Although a direct causal link between the ageing of whole organisms and the senescence of cells in culture remains elusive, a large body of data is consistent with cell senescence contributing to a variety of pathological changes seen in the aged. This review considers the in vitro phenotype of cellular senescence and speculates on the various possible routes whereby the presence of senescent cells in old bodies may affect different tissue systems.

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.

Replicative senescence of T cells: does the Hayflick Limit lead to immune exhaustion?

Extensive in vitro research on fibroblasts has defined numerous genetic and phenotypic changes associated with replicative senescence. Identification of T-cell replicative senescence as a feature of human immunodeficiency virus (HIV) disease and ageing suggests this phenomenon merits more careful consideration by immunologists, especially with regard to chronic infection, memory and adoptive immunotherapy.