A subset of CD8 memory T cells from old mice have high levels of CD28 and produce IFN-gamma

T-cell senescence: A crucial player in autoimmune diseases

Senescent T cells are proliferative disabled lymphocytes that lack antigen-specific responses. The development of T-cell senescence in autoimmune diseases contributes to immunological disorders and disease progression. Senescent T cells lack costimulatory markers with the reduction of T cell receptor repertoire and the uptake of natural killer cell receptors. Senescent T cells exert cytotoxic effects through the expression of perforin, granzymes, tumor necrosis factor, and other molecules without the antigen-presenting process. DNA damage accumulation, telomere damage, and limited DNA repair capacity are important features of senescent T cells. Impaired mitochondrial function and accumulation of reactive oxygen species contribute to T cell senescence. Alleviation of T-cell senescence could provide potential targets for the treatment of autoimmune diseases.

In or out of control: Modulating regulatory T cell homeostasis and function with immune checkpoint pathways

Regulatory T cells (Tregs) are the master regulators of immunity and they have been implicated in different disease states such as infection, autoimmunity and cancer. Since their discovery, many studies have focused on understanding Treg development, differentiation, and function. While there are many players in the generation and function of truly suppressive Tregs, the role of checkpoint pathways in these processes have been studied extensively. In this paper, we systematically review the role of different checkpoint pathways in Treg homeostasis and function. We describe how co-stimulatory and co-inhibitory pathways modulate Treg homeostasis and function and highlight data from mouse and human studies. Multiple checkpoint pathways are being targeted in cancer and autoimmunity; therefore, we share insights from the clinic and discuss the effect of experimental and approved therapeutics on Treg biology.

The role of late-differentiated T cells, a proxy for IFN-γ-production, in older adults' social networks

Interferon-γ (IFN-γ), an inflammatory biomarker that promotes antiviral immunity, may be a prerequisite for sociability. IFN-γ production in older adulthood is driven by late-differentiated CD8+ T cells, particularly CD28-and CD57+ subsets, which increase with age, reduce immune response, and increase chronic disease risk. The present study investigated the relationship between late-differentiated T cells (LDTC) and sociability in a longitudinal study of healthy aging. 139 older adults (Mage = 77.95, range 65-93; 58% female, 57% college educated, and 94% Caucasian) provided data at up to 10 occasions (M = 7). Social network size and diversity and cytomegalovirus (CMV) status were collected at every wave. Percentage of LDTC was measured at up to 4 waves and averaged for each participant. There were no significant main effects of LDTC or interactions between LDTC and time on social network size or diversity. Adjustment for baseline age, gender, and sensitivity analyses including CMV and imputed data did not change results. IFN-γ may not play a role in dictating social behavior in older adults. Alternately, LDTC may not have accurately represented circulating levels of IFN-γ. Future work should continue exploring IFN-γ and social behavior, particularly as it relates to age-related changes. The role of IFN-γ-producing, late-differentiated T cells in older adults' social networks.

T-cell senescence contributes to abnormal glucose homeostasis in humans and mice

Chronic inflammation is a driving force for the development of metabolic disease including diabetes and obesity. However, the functional characteristics of T-cell senescence in the abnormal glucose homeostasis are not fully understood. We studied the patients visiting a hospital for routine health check-ups, who were divided into two groups: normal controls and people with prediabetes. Gene expression profiling of peripheral blood mononuclear cells from normal controls and patients with type 2 diabetes was undertaken using microarray analysis. We also investigated the immunometabolic characteristics of peripheral and hepatic senescent T cells in the normal subjects and patients with prediabetes. Moreover, murine senescent T cells were tested functionally in the liver of normal or mice with metabolic deterioration caused by diet-induced obesity. Human senescent (CD28^-CD57⁺) CD8⁺ T cells are increased in the development of diabetes and proinflammatory cytokines and cytotoxic molecules are highly expressed in senescent T cells from patients with prediabetes. Moreover, we demonstrate that patients with prediabetes have higher concentrations of reactive oxygen species (ROS) in their senescent CD8⁺ T cells via enhancing capacity to use glycolysis. These functional properties of senescent CD8⁺ T cells contribute to the impairment of hepatic insulin sensitivity in humans. Furthermore, we found an increase of hepatic senescent T cells in mouse models of aging and diet-induced obesity. Adoptive transfer of senescent CD8⁺ T cells also led to a significant deterioration in systemic abnormal glucose homeostasis, which is improved by ROS scavengers in mice. This study defines a new clinically relevant concept of T-cell senescence-mediated inflammatory responses in the pathophysiology of abnormal glucose homeostasis. We also found that T-cell senescence is associated with systemic inflammation and alters hepatic glucose homeostasis. The rational modulation of T-cell senescence would be a promising avenue for the treatment or prevention of diabetes.

Unusual CD4+CD28- T Cells and Their Pathogenic Role in Chronic Inflammatory Disorders

CD28 is a primary co-stimulatory receptor that is essential for successful T cell activation, proliferation, and survival. While ubiquitously expressed on naive T cells, the level of CD28 expression on memory T cells is largely dependent on the T-cell differentiation stage in humans. Expansion of circulating T cells lacking CD28 was originally considered a hallmark of age-associated immunological changes in humans, with a progressive loss of CD28 following replicative senescence with advancing age. However, an increasing body of evidence has revealed that there is a significant age-inappropriate expansion of CD4⁺CD28⁻ T cells in patients with a variety of chronic inflammatory diseases, suggesting that these cells play a role in their pathogenesis. In fact, expanded CD4⁺CD28⁻ T cells can produce large amounts of proinflammatory cytokines such as IFN-γ and TNF-α and also have cytotoxic potential, which may cause tissue damage and development of pathogenesis in many inflammatory disorders. Here we review the characteristics of CD4⁺CD28⁻ T cells as well as the recent advances highlighting the contribution of these cells to several disease conditions.

Functionally Diverse NK-Like T Cells Are Effectors and Predictors of Successful Aging

The fundamental challenge of aging and long-term survivorship is maintenance of functional independence and compression of morbidity despite a life history of disease. Inasmuch as immunity is a determinant of individual health and fitness, unraveling novel mechanisms of immune homeostasis in late life is of paramount interest. Comparative studies of young and old persons have documented age-related atrophy of the thymus, the contraction of diversity of the T cell receptor (TCR) repertoire, and the intrinsic inefficiency of classical TCR signaling in aged T cells. However, the elderly have highly heterogeneous health phenotypes. Studies of defined populations of persons aged 75 and older have led to the recognition of successful aging, a distinct physiologic construct characterized by high physical and cognitive functioning without measurable disability. Significantly, successful agers have a unique T cell repertoire; namely, the dominance of highly oligoclonal αβT cells expressing a diverse array of receptors normally expressed by NK cells. Despite their properties of cell senescence, these unusual NK-like T cells are functionally active effectors that do not require engagement of their clonotypic TCR. Thus, NK-like T cells represent a beneficial remodeling of the immune repertoire with advancing age, consistent with the concept of immune plasticity. Significantly, certain subsets are predictors of physical/cognitive performance among older adults. Further understanding of the roles of these NK-like T cells to host defense, and how they integrate with other physiologic domains of function are new frontiers for investigation in Aging Biology. Such pursuits will require a research paradigm shift from the usual young-versus-old comparison to the analysis of defined elderly populations. These endeavors may also pave way to age-appropriate, group-targeted immune interventions for the growing elderly population.

CD57 in human natural killer cells and T-lymphocytes

The CD57 antigen (alternatively HNK-1, LEU-7, or L2) is routinely used to identify terminally differentiated 'senescent' cells with reduced proliferative capacity and altered functional properties. In this article, we review current understanding of the attributes of CD57-expressing T-cells and NK cells in both health and disease and discuss how this marker can inform researchers about their likely functions in human blood and tissues in vivo. While CD57 expression on human lymphocytes indicates an inability to proliferate, these cells also display high cytotoxic potential, and CD57(pos) NK cells exhibit both memory-like features and potent effector functions. Accordingly, frequencies of CD57-expressing cells in blood and tissues have been correlated with clinical prognosis in chronic infections or various cancers and with human aging. Functional modulation of senescent CD57(pos) T-cells and mature CD57(pos) NK cells may therefore represent innovative strategies for protection against human immunological aging and/or various chronic diseases.

Cell Type-Specific Regulation of Immunological Synapse Dynamics by B7 Ligand Recognition

B7 proteins CD80 (B7-1) and CD86 (B7-2) are expressed on most antigen-presenting cells and provide critical co-stimulatory or inhibitory input to T cells via their T-cell-expressed receptors: CD28 and CTLA-4. CD28 is expressed on effector T cells and regulatory T cells (Tregs), and CD28-dependent signals are required for optimum activation of effector T cell functions. CD28 ligation on effector T cells leads to formation of distinct molecular patterns and induction of cytoskeletal rearrangements at the immunological synapse (IS). CD28 plays a critical role in recruitment of protein kinase C (PKC)-θ to the effector T cell IS. CTLA-4 is constitutively expressed on the surface of Tregs, but it is expressed on effector T cells only after activation. As CTLA-4 binds to B7 proteins with significantly higher affinity than CD28, B7 ligand recognition by cells expressing both receptors leads to displacement of CD28 and PKC-θ from the IS. In Tregs, B7 ligand recognition leads to recruitment of CTLA-4 and PKC-η to the IS. CTLA-4 plays a role in regulation of T effector and Treg IS stability and cell motility. Due to their important roles in regulating T-cell-mediated responses, B7 receptors are emerging as important drug targets in oncology. In this review, we present an integrated summary of current knowledge about the role of B7 family receptor–ligand interactions in the regulation of spatial and temporal IS dynamics in effector and Tregs.

CD58/CD2 Is the Primary Costimulatory Pathway in Human CD28-CD8+ T Cells

A substantial proportion of CD8(+) T cells in adults lack the expression of the CD28 molecule, and the aging of the immune system is associated with a steady expansion of this T cell subset. CD28(-)CD8(+) T cells are characterized by potent effector functions but impaired responses to antigenic challenge. CD28 acts as the primary T cell costimulatory receptor, but there are numerous additional receptors that can costimulate the activation of T cells. In this study, we have examined such alternative costimulatory pathways regarding their functional role in CD28(-)CD8(+) T cells. Our study showed that most costimulatory molecules have a low capacity to activate CD28-deficient T cells, whereas the engagement of the CD2 molecule by its ligand CD58 clearly costimulated proliferation, cytokine production, and effector function in this T cell subset. CD58 is broadly expressed on APCs including dendritic cells. Blocking CD58 mAb greatly reduced the response of human CD28(-)CD8(+) T cells to allogeneic dendritic cells, as well as to viral Ags. Our results clearly identify the CD58/CD2 axis as the primary costimulatory pathway for CD8 T cells that lack CD28. Moreover, we show that engagement of CD2 amplifies TCR signals in CD28(-)CD8(+) T cells, demonstrating that the CD2-CD58 interaction has a genuine costimulatory effect on this T cell subset. CD2 signals might promote the control of viral infection by CD28(-)CD8(+) T cells, but they might also contribute to the continuous expansion of CD28(-)CD8(+) T cells during chronic stimulation by persistent Ag.

Accumulation of 4-1BBL+ B cells in the elderly induces the generation of granzyme-B+ CD8+ T cells with potential antitumor activity

Although the accumulation of highly-differentiated and granzyme B (GrB)-expressing CD8(+)CD28(-) T cells has been associated with aging, the mechanism for their enrichment and contribution to immune function remains poorly understood. Here we report a novel B-cell subset expressing 4-1BBL, which increases with age in humans, rhesus macaques, and mice, and with immune reconstitution after chemotherapy and autologous progenitor cell transplantation. These cells (termed 4BL cells) induce GrB(+)CD8(+) T cells by presenting endogenous antigens and using the 4-1BBL/4-1BB axis. We found that the 4BL cells increase antitumor responses in old mice, which may explain in part the paradox of retarded tumor growth in the elderly. 4BL cell accumulation and its capacity to evoke the generation of GrB(+)CD8(+) T cells can be eliminated by inducing reconstitution of B cells in old mice, suggesting that the age-associated skewed cellular immune responses are reversible. We propose that 4BL cells and the 4-1BBL signaling pathway are useful targets for improved effectiveness of natural antitumor defenses and therapeutic immune manipulations in the elderly.

The influence of intrinsic and extrinsic factors on immune system aging

Sex and age-matched wild-type and TCR transgenic mice were infected with cytomegalovirus (CMV) at 6 months of age and followed for 12 additional months to examine aging of the immune system. It was found that viral infection of C57Bl/6 mice resulted in accelerated aging of the immune system as shown by a loss of CD8(+)28(+) cells and an accumulation of KLRG1(+) T cells. CMV infection of OT-1 transgenic mice had no influence on immune aging of these mice which nonetheless demonstrated an accumulation of CD8(+)28(-) and KLRG1(+) T cells with time. CD4(+) T cells were unaffected in either strain of mice. Thus, immunological aging was found to be due to both cell-intrinsic and cell-extrinsic factors. Persistent viral infections may accelerate immunological aging but consideration must be given to individual variation in the aging process.

Elevated levels of interferon-γ production by memory T cells do not promote transplant tolerance resistance in aged recipients

Immunosenescence predisposes the elderly to infectious and autoimmune diseases and impairs the response to vaccination. We recently demonstrated that ageing also impedes development of transplantation tolerance. Unlike their young counterparts (8-12 weeks of age) aged male recipients (greater than 12 months of age) transplanted with a full MHC-mismatched heart are resistant to tolerance mediated by anti-CD45RB antibody. Surprisingly, either chemical or surgical castration restored tolerance induction to levels observed using young recipients. Based on the strong impact of endocrine modulation on transplant tolerance, we explored the impact of ageing and castration on the immune system. Here we report a significant increase in the percentage of T cells that produce interferon-γ (IFN-γ) in aged male versus young male animals and that the overall increase in IFN-γ production was due to an expansion of IFN-γ-producing memory T cells in aged animals. In contrast to IFN-γ production, we did not observe differences in IL-10 expression in young versus old male mice. We hypothesized that endocrine modulation would diminish the elevated levels of IFN-γ production in aged recipients, however, we observed no significant reduction in the percentage of IFN-γ+ T cells upon castration. Furthermore, we neutralized interferon-γ by antibody and did not observe an effect on graft survival. We conclude that while elevated levels of interferon-γ serves as a marker of tolerance resistance in aged mice, other as yet to be identified factors are responsible for its cause. Defining these factors may be relevant to design of tolerogenic strategies for aged recipients.

How T follicular helper cells and the germinal centre response change with age

Normal ageing is accompanied by a decline in the function of the immune system that causes an increased susceptibility to infections and an impaired response to vaccination in older individuals. This results in an increased disease burden in the aged population, even with good immunisation programmes in place. The decreased response to vaccination is partly due to the diminution of the germinal centre response with age, caused by impaired T-cell help to B cells. Within the germinal centre, T-cell help is provided by a specialised subset of CD4(+) T cells; T follicular helper (Tfh) cells. Tfh cells provide survival and selection signals to germinal centre B cells, allowing them to egress from the germinal centre and become long-live plasma cells or memory B cells, and provide life-long protection against subsequent infection. This review will discuss the cellular and molecular changes in both Tfh cells and germinal centre B cells that occur with advancing age, which result in a smaller germinal centre response and a less effective response to immunisation.

Reduced neointima formation after arterial injury in CD4-/- mice is mediated by CD8+CD28hi T cells

Background

CD8⁺ T‐cell activation, characterized by increased CD28 expression, reduces neointima formation after arterial injury in mice. The CD8⁺CD28^hi phenotype is associated with increased effector function. In this study, we used a mouse model that has CD8⁺ but no CD4⁺ T cells (CD4−/−) to assess the role of CD8⁺ T cells and test the function of CD8⁺CD28^hi T cells in modulating neointima formation after arterial injury.

Methods and Results

Neointima formation after pericarotid arterial cuff injury was significantly less in CD4−/− mice compared with wild‐type (WT) mice. Negligible baseline lytic activity by splenic CD8⁺ T cells from uninjured WT mice against target syngeneic smooth muscle cells was significantly increased 7 days after injury. Interestingly, CD8⁺ T cells from uninjured CD4−/− mice had significant lytic activity at baseline that remained elevated 7 days after injury. CD8⁺ T‐cell lytic activity was significantly reduced by depletion of CD28^hi cells. CD8⁺CD28^hi T cells adoptively transferred into recipient Rag‐1−/− mice significantly reduced neointima formation compared with CD8⁺CD28⁺ T‐cell recipient mice.

Conclusions

CD8⁺ T cells reduced neointima formation after arterial injury, attributed in part to increased function of the CD8⁺CD28^hi phenotype.

CD8+ CD28- and CD8+ CD57+ T cells and their role in health and disease

Chronic antigenic stimulation leads to gradual accumulation of late-differentiated, antigen-specific, oligoclonal T cells, particularly within the CD8(+) T-cell compartment. They are characterized by critically shortened telomeres, loss of CD28 and/or gain of CD57 expression and are defined as either CD8(+) CD28(-) or CD8(+) CD57(+) T lymphocytes. There is growing evidence that the CD8(+) CD28(-) (CD8(+) CD57(+)) T-cell population plays a significant role in various diseases or conditions, associated with chronic immune activation such as cancer, chronic intracellular infections, chronic alcoholism, some chronic pulmonary diseases, autoimmune diseases, allogeneic transplantation, as well as has a great influence on age-related changes in the immune system status. CD8(+) CD28(-) (CD8(+) CD57(+)) T-cell population is heterogeneous and composed of various functionally competing (cytotoxic and immunosuppressive) subsets thus the overall effect of CD8(+) CD28(-) (CD8(+) CD57(+)) T-cell-mediated immunity depends on the predominance of a particular subset. Many articles claim that CD8(+) CD28(-) (CD8(+) CD57(+)) T cells have lost their proliferative capacity during process of replicative senescence triggered by repeated antigenic stimulation. However recent data indicate that CD8(+) CD28(-) (CD8(+) CD57(+)) T cells can transiently up-regulate telomerase activity and proliferate under certain stimulation conditions. Similarly, conflicting data is provided regarding CD8(+) CD28(-) (CD8(+) CD57(+)) T-cell sensitivity to apoptosis, finally leading to the conclusion that this T-cell population is also heterogeneous in terms of its apoptotic potential. This review provides a comprehensive approach to the CD8(+) CD28(-) (CD8(+) CD57(+)) T-cell population: we describe in detail its origins, molecular and functional characteristics, subsets, role in various diseases or conditions, associated with persistent antigenic stimulation.

Reactive oxygen intermediate-induced pathomechanisms contribute to immunosenescence, chronic inflammation and autoimmunity

Deregulation of reactive oxygen intermediates (ROI) resulting in either too high or too low concentrations are commonly recognized to be at least in part responsible for many changes associated with aging. This article reviews ROI-dependent mechanisms critically contributing to the decline of immune function during physiologic - or premature - aging. While ROI serve important effector functions in cellular metabolism, signalling and host defence, their fine-tuned generation declines over time, and ROI-mediated damage to several cellular components and/or signalling deviations become increasingly prevalent. Although distinct ROI-associated pathomechanisms contribute to immunosenescence of the innate and adaptive immune system, mutual amplification of dysfunctions may often result in hyporesponsiveness and immunodeficiency, or in chronic inflammation with hyperresponsiveness/deregulation, or both. In this context, we point out how imbalanced ROI contribute ambiguously to driving immunosenescence, chronic inflammation and autoimmunity. Although ROI may offer a distinct potential for therapeutic targeting along with the charming opportunity to rescue from deleterious processes of aging and chronic inflammatory diseases, such modifications, owing to the complexity of metabolic interactions, may carry a marked risk of unforeseen side effects.

CD28(-) T cells: their role in the age-associated decline of immune function

The accumulation of CD28(-) T cells, particularly within the CD8 subset, is one of the most prominent changes during T-cell homeostasis and function associated with aging in humans. CD28, a major co-stimulatory receptor, is responsible for the optimal antigen-mediated T-cell activation, proliferation and survival of T cells. CD28(-) T cells exhibit reduced antigen receptor diversity, defective antigen-induced proliferation and a shorter replicative lifespan while showing enhanced cytotoxicity and regulatory functions. Gene expression analyses reveal profound changes of CD28(-) T cells in comparison to their CD28(+) counterparts and corroborate their functional differences. Here we review recent advances in our understanding of CD28(-) T cells and their role in the age-associated decline of immune function.

Age-associated T-cell Clonal Expansions (TCE) in vivo—Implications for Pathogen Resistance

Age-related T-cell clonal expansions (TCE) are an incompletely understood disturbance in T-cell homeostasis found frequently in old humans and experimental animals. These accumulations of CD8 T-cells have the potential to distort T-cell population balance and reduce T-cell repertoire diversity above and beyond the changes seen in the aging of T-cell pool in the absence of TCE. This chapter discusses our current knowledge of the role of these expansions in health and disease, with a special focus on their influence upon immune defense against infectious diseases.

Changes of CD4+CD25+Foxp3+ regulatory T cells in aged Balb/c mice

A progressive decline in the integrity of the immune system is one of the physiologic changes during aging. The frequency of autoimmune diseases or immune disorders increases in the aging population, but the state of regulatory T (Treg) cells in aged individuals has not been well determined. In the present study, we investigated the levels, phenotypes, and function of CD4(+)CD25(+) Treg cells in Balb/c mice, which were older than 20 months. Significantly enhanced percentages of CD4(+)CD25(+) Treg cells in the periphery (blood, spleen, and lymph nodes) of the aged mice were observed. These Treg cells showed modified Vbeta family distribution, reduced levels of CD45 receptor B and CD62 ligand molecules, as well as normal levels of forkhead box p3. However, when the inhibiting function of Treg cells was assayed in the in vitro assays and in a delayed-type hypersensitivity (DTH) model, CD4(+)CD25(+) Treg cells of aged mice displayed significantly lower inhibiting ability on alloantigen-induced DTH reaction or cytokine productions (IL-2 and IFN-gamma) but not cell proliferation of effector T cells, as compared with CD4(+)CD25(+) Treg cells of young mice. In addition, the percentages of CD4(+)CD8(-)CD25(+) Treg cells in the thymi of aged mice increased significantly, but their total cell numbers decreased markedly in these mice. Our present studies indicated collectively that the percentages, phenotypes, the size of TCR repertoire, and function of CD4(+)CD25(+) Treg cells were altered significantly with aging in mice. The functional defects of CD4(+)CD25(+) Treg cells may shed light on the role of CD4(+)CD25(+) Treg cells in the increased sensitivity to autoimmune diseases of aged populations.

Non-malignant clonal expansions of CD8+ memory T cells in aged individuals

CD8(+) T cells provide a major line of defense against intracellular pathogens. Upon encounter with antigen, CD8(+) T cells go through three distinct phases involving proliferation, contraction, and differentiation to become eventually long-lived CD8(+) memory T cells. CD8(+) memory T cells provide long-term protection against infection by intracellular pathogens. CD8(+) memory T-cell proliferation and survival are regulated by many factors, including cytokines, and CD8(+) memory T cells are stably maintained over a period of months to years. In aged humans and mice, however, there are significant alterations to the CD8(+) memory T-cell compartment with frequent development of monoclonal expansions of CD8(+) memory T cells in healthy individuals. Interestingly, CD8(+) clonal expansions are not malignant and do not progress to lymphomas, suggesting that these cells must still be under certain constraints. In this review, we discuss our current understanding of factors that contribute to and regulate these CD8(+) clonal expansions as well as the impact of CD8(+) clonal expansions on immune function of the aged. In addition, we discuss similarities and differences between CD8(+) clonal expansions observed in humans and mice, and we postulate that CD8(+) clonal expansions represent a spectrum of biological outcomes ranging from antigen-driven to antigen-independent phenomena.

The influence of age on immunity to infection with Mycobacterium tuberculosis

Changes within the immune system during aging lead to an elderly population that is both highly susceptible to infectious diseases and unresponsive to typical vaccine protocols. Using the murine model of tuberculosis, we have identified key differences in the generation of T-helper 1 cell immunity between old and young mice, and this information may be important for the design of new vaccines or post exposure therapies to protect the elderly against infectious diseases. In response to infection with Mycobacterium tuberculosis, it has been shown that the generation of antigen-specific CD4(+) T-cell immunity is impaired in old mice. In contrast, recent findings document that old mice display a transient enhanced resistance that occurs within the first 3 weeks of infection. Early resistance was associated with the presence of CD8(+) T cells and their ability to produce interferon-gamma (IFN-gamma) well before their young counterparts. Further investigation into the mechanism by which CD8(+) T cells are induced to secrete IFN-gammain vivo could provide an approach to enhance the effector function of these cells and subsequently protect elderly individuals from respiratory pathogens such as M. tuberculosis.

CD28 extinction in human T cells: altered functions and the program of T-cell senescence

The loss of CD28 expression on T cells is the most consistent biological indicator of aging in the human immune system, and the frequency of CD28(null) T cells is a key predictor of immune incompetence in the elderly. There is also mounting evidence for the high frequency of these unusual T cells among patients with inflammatory syndromes or with chronic infections disproportionate with their age. In these pathological states, CD28(null) T cells likely represent prematurely senescent lymphocytes due to persistent immune activation. Unlike the situation in CD28 gene knockout mice that have anergic CD28(0/0) T cells, human CD28(null) T cells are functionally active, long-lived, oligoclonal lymphocytes that lack or have limited proliferative capacity. Results of replicative senescence studies show that CD28(null) T cells are derived from CD28(+) precursors that have undergone repeated stimulation, indicating that CD28 silencing underlies the program of T-cell aging. Dissection of the machinery regulating CD28 expression is paving the way in elucidating the molecular events leading to immune senescence as well as providing clues into the functional rejuvenation of senescent T cells.

Age-related CD8 T cell clonal expansions constrict CD8 T cell repertoire and have the potential to impair immune defense

Peripheral T cell diversity is virtually constant in the young, but is invariably reduced in aged mice and humans. CD8+ T cell clonal expansions (TCE) are the most drastic manifestation of, and possible contributors to, this reduced diversity. We show that the presence of TCE results in reduced CD8+, but not CD4+, T cell diversity, and in functional inability to mobilize parts of the CD8+ T cell repertoire affected by TCE. In the model of herpes simplex virus (HSV)-1 infection of B6 mice, >90% of the responding CD8+ T cells use Vbeta10 or Vbeta8 and are directed against a single glycoprotein B (gB498-505) epitope, gB-8p. We found that old animals bearing CD8+ TCE within Vbeta10 or Vbeta8 families failed to mount an effective immune response against HSV-1, as judged by reduced numbers of peptide-major histocompatibility complex tetramer+ CD8 T cells and an absence of antiviral lytic function. Furthermore, Vbeta8 TCE experimentally introduced into young mice resulted in lower resistance to viral challenge, whereas Vbeta5+ TCE induced in a similar fashion did not impact viral resistance. These results demonstrate that age-related TCE functionally impair the efficacy of antiviral CD8+ T cell immunity in an antigen-specific manner, strongly suggesting that TCE are not the mere manifestation of, but are also a contributing factor to, the immunodeficiency of senescence.

T-cell senescence: a culprit of immune abnormalities in chronic inflammation and persistent infection

Long-lived clonal T cells deficient in CD28 expression are commonly found in patients with inflammatory syndromes and persistent infections. Considering that CD28 loss is the most consistent immunological marker of aging, we propose that, in pathological states, CD28(null) T cells represent prematurely senescent cells resulting from persistent immune activation. These unusual lymphocytes have aberrant functions that contribute to disease-related immune abnormalities, and the degree of accumulation of CD28(null) T cells predicts the severity of clinical manifestations. We suggest that understanding of the biological properties of T cells that have reached replicative senescence will influence the future management of certain diseases. Indeed, studies on the molecular basis for the loss of CD28 are already providing information on methods to functionally rescue senescent T cells.

Characterization of the immunoreactivity of anti-interferon alpha antibodies in myasthenia gravis patients. Epitope mapping

Cytokines, such as interferons (IFN), underlie many immunological functions and are increasingly implicated in disease-related symptoms and pathology. In order to study the potential roles of IFN alpha and its antagonists in autoimmune phenomena, the sera from 89 patients (aged 15-95 years, 65 females) diagnosed as having myasthenia gravis (MG) (2 months to 34 years duration) were tested for the presence of natural anti-IFN alpha-2b auto-antibodies. Sera were screened for anti-IFN alpha-2b by a sandwich-type enzyme immunoassay system. Ten (11.2%) and 6 (6.7%) sera were identified that contained positive-competing and non-competing anti-IFN alpha-2b auto-antibodies, respectively. The MG sera were further analyzed by immunobloting against reduced IFN alpha-2b and for neutralizing anti-IFN alpha activity in an antiviral assay cells system. From tested EIA positive-competing sera, 5 were shown to be positive by immunoblot and 6 sera were found to contain neutralizing anti-IFN alpha-2b. Four of the 6 neutralizing anti-IFN alpha-2b sera came from patients with thymoma-associated MG. The sera were studied for linear epitope recognition on the IFN alpha-2b molecule by a solid phase binding assay, in which overlapping peptides homologous with the entire IFN alpha-2b sequence were separately synthesized on a nitrocellulose sheet. Peptides number 2 (residues 8-21), 3 (15-28), 6 (33-46), 10 (63-76), 15 (98-112), and 21 (141-154) were immunoreactive. Peptide 21 was apparently associated with antiviral activity, although peptide 21 has not been previously described as an immunogenic determinant on the IFN alpha-2b molecule. These results indicate that neutralizing anti-IFN alpha-2b is often present in MG, particularly in cases of thymoma-associated MG, and recognize a variety of epitopes on the IFN alpha-2b molecule, including those involved in its biological activity. Two groups of IFN epitopes were described associated with patient's age but not with diseases evolution.

Antigen-independent expansion of CD28hi CD8 cells from aged mice: cytokine requirements and signal transduction pathways

Memory CD8+ T cells from old mice can proliferate in nonirradiated recipients. Transfer of labeled cells from aged donors into young recipients showed that proliferation of aged donor CD8 cells requires host cells that can both respond to interferon-gamma and produce interleukin-15. Reisolation of transferred CD8 cells from host mice showed that LAT (linker for activated T cells) translocation to the immunological synapse, and translocation of NF (nuclear factor)-kappaB to the nucleus were diminished in recovered CD8 T cells from old donors, whether they had divided in vivo or not. Cells able to proliferate in vivo could be isolated based on their unusually high levels of CD28 expression, but were found not to differ from other aged CD8 cells in their low levels of LAT and protein kinase C-theta (PKC-theta) translocation to the immunological synapse. Thus in vivo proliferation of CD28hi CD8 cells from aged mice cannot be attributed to retention of T-cell receptor signaling.

Phenotypic and functional T-cell aging in rhesus macaques (Macaca mulatta): differential behavior of CD4 and CD8 subsets

A prominent theory of immune senescence holds that repeated antigenic stimulation and decreased production of naive cells combine to progressively exhaust the reserve of lymphocytes available to fight new pathogens, culminating in an accumulation of lymphocytes that achieved replicative senescence. A well-defined primate model of immune senescence in vivo would greatly facilitate testing of this theory. Here, we investigated phenotypic and functional T-cell aging in the rhesus macaques (RMs), currently the dominant primate model of AIDS. Our results show that sharp differences exist between the CD8 and CD4 T-cell subsets in (1) cell-cycle programs (as assessed by both in vitro proliferation and in vivo turnover measurement); (2) CD28 regulation on cell-cycle entry; and (3) accumulation of immediate effector cells among the CD28- cells, believed to be close to or at replicative senescence. These results further suggest poor reliability of CD28 as a marker for senescence. We suggest that some of the T-cell aging phenomenology in RMs can be ascribed to accentuation over time of the inherent differences in activation programs in CD8 and CD4 T cells.