Rapid tyrosine phosphorylation of Grb2 and Shc in T cells exposed to anti-CD3, anti-CD4, and anti-CD45 stimuli: differential effects of aging

Aging and Senescence of Dental Pulp and Hard Tissues of the Tooth

The ability to consume a meal using one's own teeth influences an individual's quality of life. In today's global aging society, studying the biological changes in aging teeth is important to address this issue. A tooth includes three hard tissues (enamel, dentin, and cementum) and a soft tissue (dental pulp). With advancing age, these tissues become senescent; each tissue exhibits a unique senescent pattern. This review discusses the structural alterations of hard tissues, as well as the molecular and physiological changes in dental pulp cells and dental pulp stem cells during human aging. The significance of senescence in these cells remains unclear. Thus, there is a need to define the regulatory mechanisms of aging and senescence in these cells to aid in preservation of dental health.

Engagement of CD45 alters early signaling events in human T cells co-stimulated through TCR + CD28

Previously our lab has shown that co-stimulation of human T cells through different co-stimulatory molecules tune differentiation to different phenotypes. An open question is where in the signaling pathways induced by the co-stimulation do differences occur that contribute to outcome of differentiation. In this project, we investigate the early signaling process by comparing events that follow engagement of CD45 alone or in association with a co-stimulatory molecule: CD28. CD45 plays a crucial role to initiate T cell signaling by dephosphorylating a negatively regulatory tyrosine residue in Src family kinases such as Lck. First, we observed that engagement of CD45 alone induced signaling in T cells. We observed that TCR/CD3 stimulation with CD45 promoted prolonged Lck association with TCR/CD3 complex and Lck remained associated during TCR/CD3 + CD28 + CD45 stimulation as well. We concluded that Lck association is dependent on TCR/CD3 and CD45 engagement. Hence, CD45 altered early signaling events in T cells.

Fucoidan⁻Fucoxanthin Ameliorated Cardiac Function via IRS1/GRB2/ SOS1, GSK3β/CREB Pathways and Metabolic Pathways in Senescent Mice

The effects of low molecular weight fucoidan (LMWF) in combination with high-stability fucoxanthin (HSFUCO) on cardiac function and the metabolic pathways of aging mice (Mus musculus) were investigated. We demonstrated that LMWF and HSFUCO could improve cardiac function in aging mice. Aging mice were treated with LMWF and HSFUCO, either on their own or in combination, on 28 consecutive days. Electrocardiography and whole-cell patch-clamp were used to measure QT interval and action potential duration (APD) of the subjects. Cardiac tissue morphology, reactive oxygen species, and Western blot were also applied. Ultra-high-performance liquid chromatography⁻quadrupole time-of-flight (UPLC-QTOF) mass spectrometry was used for investigating metabolic alterations. The use of LMWF and HSFUCO resulted in improvements in both ventricular rhythms (QT and APD). Treatment with fucoidan and fucoxanthin reduced the expression levels of SOS1 and GRB2 while increasing GSK3β, CREB and IRS1 proteins expression in the aging process. Three main metabolic pathways, namely the TCA cycle, glycolysis, and steroid hormone biosynthesis, were highly enriched in the pathway enrichment analysis. When taken together, the LMWF and HSFUCO treatment improved both the ventricular rhythm and the muscular function of aging subjects by interfering with the metabolism and gene function.

Grb2 monomer-dimer equilibrium determines normal versus oncogenic function

The adaptor protein growth factor receptor-bound protein 2 (Grb2) is ubiquitously expressed in eukaryotic cells and involved in a multitude of intracellular protein interactions. Grb2 plays a pivotal role in tyrosine kinase-mediated signal transduction including linking receptor tyrosine kinases to the Ras/mitogen-activated protein (MAP) kinase pathway, which is implicated in oncogenic outcome. Grb2 exists in a constitutive equilibrium between monomeric and dimeric states. Here we show that only monomeric Grb2 is capable of binding to SOS and upregulating MAP kinase signalling and that the dimeric state is inhibitory to this process. Phosphorylation of tyrosine 160 (Y160) on Grb2, or binding of a tyrosylphosphate-containing ligand to the SH2 domain of Grb2, results in dimer dissociation. Phosphorylation of Y160 on Grb2 is readily detectable in the malignant forms of human prostate, colon and breast cancers. The self-association/dissociation of Grb2 represents a switch that regulates MAP kinase activity and hence controls cancer progression.

Defective B cell ontogeny and humoral immune response in mice prematurely expressing human complement receptor 2 (CR2, CD21) is similar to that seen in aging wild type mice

Mice prematurely expressing human CR2 (hCR2) in the B cell lineage have a defective B cell ontogeny and humoral immune response. We have previously determined altered tyrosine phosphorylation patterns within hCR2 transgenic mice, suggesting that irreversible changes in B cell signaling pathways had occurred, which could explain the B cell unresponsiveness associated with hCR2 transgene expression. In support of that assertion, we found that increasing antigen dose or addition of adjuvant had a minimal impact on the ability of B cells to respond to antigen. However, analysis of aged hCR2(high) mice (1 year plus) revealed that both B cell numbers, B cell sub-population distribution including expansion of a newly described B regulatory cell subset, and immune responses were comparable with age-matched hCR2 negative mice. Finally, we established that B cell unresponsiveness to antigen in aging wild type mice (1 year plus) was equivalent to that noted in 3-month-old hCR2(high) mice. This data provides evidence that 3-month-old hCR2(high) mice have a humoral immune system resembling aged mice and suggests that further examination of the precise molecular and cellular parallels between aged wild type mice and 3-month-old hCR2(high) mice could provide an important insight into the mechanisms which lead to B cell unresponsiveness in the aging immune system.

Co-aggregation of FcgammaRII with FcepsilonRI on human mast cells inhibits antigen-induced secretion and involves SHIP-Grb2-Dok complexes

Signaling through the high affinity IgE receptor FcepsilonRI on human basophils and rodent mast cells is decreased by co-aggregating these receptors to the low affinity IgG receptor FcgammaRII. We used a recently described fusion protein, GE2, which is composed of key portions of the human gamma1 and the human epsilon heavy chains, to dissect the mechanisms that lead to human mast cell and basophil inhibition through co-aggregation of FcgammaRII and FcepsilonRI. Unstimulated human mast cells derived from umbilical cord blood express the immunoreceptor tyrosine-based inhibitory motif-containing receptor FcgammaRII but not FcgammaRI or FcgammaRIII. Interaction of the mast cells with GE2 alone did not cause degranulation. Co-aggregating FcepsilonRI and FcgammaRII with GE2 1) significantly inhibited IgE-mediated histamine release, cytokine production, and Ca(2+) mobilization, 2) reduced the antigen-induced morphological changes associated with mast cell degranulation, 3) reduced the tyrosine phosphorylation of several cellular substrates, and 4) increased the tyrosine phosphorylation of the adapter protein downstream of kinase 1 (p62(dok); Dok), growth factor receptor-bound protein 2 (Grb2), and SH2 domain containing inositol 5-phosphatase (SHIP). Tyrosine phosphorylation of Dok was associated with increased binding to Grb2. Surprisingly, in non-stimulated cells, there were complexes of phosphorylated SHIP-Grb2-Dok that were lost upon IgE receptor activation but retained under conditions of Fcepsilon-Fcgamma co-aggregation. Finally, studies using mast cells from Dok-1 knock-out mice showed that IgE alone triggers degranulation supporting an inhibitory role for Dok degranulation. Our results demonstrate how human FcepsilonRI-mediated responses can be inhibited by co-aggregation with FcgammaRIIB and implicate Dok, SHIP, and Grb2 as key intermediates in regulating antigen-induced mediator release.

Ageing, autoimmunity and arthritis: Perturbations of TCR signal transduction pathways with ageing - a biochemical paradigm for the ageing immune system

It is widely accepted that cell-mediated immune functions decline with age, rendering an individual more susceptible to infection and possibly cancer, as well as to age-associated autoimmune diseases. The exact causes of T-cell functional decline are not known. One possible cause could be the development of defects in the transduction of mitogenic signals following TCR stimulation. This T-cell hyporesponsiveness due to defects of signalling through the TCR either from healthy elderly subjects or from individuals with autoimmune diseases such as rheumatoid arthritis or systemic lupus erythematosus results in an impaired ability to mount efficient immune responses and to maintain responsiveness to foreign antigens. This implies that a high proportion of autoreactive T cells might accumulate either intrathymically or in the periphery. T-cell anergy and differential TCR signalling could thus also be key players in the disruption of tolerance and the onset of autoimmune diseases. The increasing number of the elderly may lead to an increase of clinically important autoimmune diseases. We will review the signal transduction changes through the TCR-CD3 complex in T lymphocytes from healthy elderly subjects, which result in a modification of the activation of transcription factors involved in IL-2 gene expression leading to decreased IL-2 production. The putative contribution of altered T-cell signalling with ageing in the development of autoimmune diseases will be also discussed.

Defect in ERK2 and p54(JNK) activation in aging mouse splenocytes

We have previously reported on a defect in both extracellular signal-regulated protein kinase (ERK) and c-jun N-terminal kinase (JNK) mitogen-activated protein kinase (MAPK) activation in splenocytes obtained from old rats. In order to investigate whether these effects are conserved across species, we have now used mouse splenocytes to measure the effect of aging on the activation of the same two MAPK families: ERK and JNK. Our results demonstrate that, as in rats, both MAPK signal transduction pathways are affected by aging in mice, indicating the existence of a further defect located downstream of the receptor-proximal events. Whereas ERK1 and p46(JNK) activation were not significantly modified, the kinetics of both ERK2 and p54(JNK) activation and inactivation were affected in splenocytes from old animals. Specifically, by analyzing the kinetics of activation and inactivation of these enzymes, we found a nearly 50% decrease in the fold of activation of both ERK2 and p54(JNK). These defects result in an overall diminution of enzyme activities without changes in the steady-state levels of relevant proteins. The impaired activity of these two MAPK pathways is likely to play a role in the reduced expression of interleukin-2 and diminished lymphoproliferation observed in old animals.

Effect of age and oxidative stress on tyrosine phosphorylation of ZAP-70

Oxidative stress is believed to be one of the leading contributors to the aging process and loss of cellular function with aging. Although, several age-associated phenomena have been correlated with reactive oxygen species, the role of oxidative stress in the age-related decline of T-cell activity is not yet clear. The present study was carried out to determine the effect of reactive oxygen species on tyrosine phosphorylation of ZAP-70, a key enzyme in the T-cell signal transduction pathway. Our previous as well as present studies have demonstrated an age-related down-regulation of tyrosine phosphorylation of ZAP-70 following activation of freshly isolated T cells with various stimulating agents. Currently, we observe that under mild and chronic oxidative stress, tyrosine phosphorylation of ZAP-70 is impaired following stimulation of the T cells with anti-CD3 antibody. Interestingly, in contrast to our observation using freshly isolated T cells, there is no age-associated impairment of tyrosine phosphorylation of ZAP-70 when T cells, cultured for 2 days in a serum-free medium in the presence or in the absence of oxidative stress, are stimulated with anti-CD3 antibody. The current observation suggests that the age-associated down-regulation of tyrosine phosphorylation of ZAP-70 is not an intrinsic defect inherent of the T cells due to aging and is reversible. We also observed that under oxidative stress in vitro, there was no significant difference in inhibition of tyrosine phosphorylation of ZAP-70 in T cells isolated from elderly and young donors. Finally, it was also noted that the down-regulation of tyrosine phosphorylation of ZAP-70 under oxidative stress in young and elderly donors was not due to impairment in the net expression of ZAP-70 under oxidative stress thereby suggesting that dysregulation in the balance of kinase-phosphatase activities under oxidative stress might have been implicated in the observed phenomenon.

Tyrosine phosphorylation of Grb2 by Bcr/Abl and epidermal growth factor receptor: a novel regulatory mechanism for tyrosine kinase signaling

Growth factor receptor-binding protein-2 (Grb2) plays a key role in signal transduction initiated by Bcr/Abl oncoproteins and growth factors, functioning as an adaptor protein through its Src homology 2 and 3 (SH2 and SH3) domains. We found that Grb2 was tyrosine-phosphorylated in cells expressing BCR/ABL and in A431 cells stimulated with epidermal growth factor (EGF). Phosphorylation of Grb2 by Bcr/Abl or EGF receptor reduced its SH3-dependent binding to Sos in vivo, but not its SH2-dependent binding to Bcr/Abl. Tyr209 within the C-terminal SH3 domain of Grb2 was identified as one of the tyrosine phosphorylation sites, and phosphorylation of Tyr209 abolished the binding of the SH3 domain to a proline-rich Sos peptide in vitro. In vivo expression of a Grb2 mutant where Tyr209 was changed to phenylalanine enhanced BCR/ABL-induced ERK activation and fibroblast transformation, and potentiated and prolonged Grb2-mediated activation of Ras, mitogen-activated protein kinase and c-Jun N-terminal kinase in response to EGF stimulation. These results suggest that tyrosine phosphorylation of Grb2 is a novel mechanism of down-regulation of tyrosine kinase signaling.

Cyclodextrin modulation of T lymphocyte signal transduction with aging

There is an alteration of the immune response in aging that leads to the increased incidence of infections, cancers and autoimmune disorders. The aim of the present study was to investigate whether there exists changes in signal transduction under the IL-2 receptor stimulation and the role of plasma membrane cholesterol in the activation of T cells with aging. We report age-related changes in the JAK-STAT signalling pathway that results in decreased tyrosine phosphorylation of STAT5. We present evidence for the importance of cholesterol content in regulating signalling pathways in T cells and in modulating their proliferation by using the plasma membrane cholesterol-depleting agent methyl-beta-cyclodexrin (MBCD). MBCD treatment (0.5 mM) induced a significant decrease in the cholesterol content of T cells of elderly subjects whereas it was increased in T cells of young subjects. MBCD induced changes in the phosphorylation of p56(lck), especially in T cells of elderly subjects. The proliferation of MBCD-treated T cells decreased in lymphocytes of young subjects but did not change in T cells of elderly subjects. These results suggest a role for plasma membrane cholesterol in the regulation of the TcR signalling pathways with differential effects related to aging. However, the data suggest that modulation of the plasma membrane cholesterol content alone may not be enough to restore signal transduction changes with aging.

Single-cell analyses reveal two defects in peptide-specific activation of naive T cells from aged mice

Confocal fluorescent microscopy was used to study redistribution of membrane-associated proteins in naive T cells from young and old mice from a transgenic stock whose T cells express a TCR specific for a peptide derived from pigeon cytochrome C. About 50% of the T cells from young mice that formed conjugates with peptide-pulsed APC were found to form complexes, at the site of binding to the APC, containing CD3epsilon, linker for activation of T cells (LAT), and Zap-70 in a central area and c-Cbl, p95(vav), Grb-2, PLC gamma, Fyn, and Lck distributed more uniformly across the interface area. Two-color staining showed that those cells that were able to relocalize c-Cbl, LAT, CD3epsilon, or PLC gamma typically relocalized all four of these components of the activation complex. About 75% of conjugates that rearranged LAT, c-Cbl, or PLC gamma also exhibited cytoplasmic NF-AT migration to the T cell nucleus. Aging had two effects. First, it led to a diminution of approximately 2-fold in the proportion of T cell/APC conjugates that could relocalize any of the nine tested proteins to the immune synapse. Second, aging diminished by approximately 2-fold the frequency of cytoplasmic NF-AT migration among cells that could generate immune synapses containing LAT, c-Cbl, or PLC gamma. Thus naive CD4 T cells from old mice exhibit at least two separable defects in the earliest stages of activation induced by peptide/MHC complexes.

Age-dependent alterations in the assembly of signal transduction complexes at the site of T cell/APC interaction

TCR interaction with peptide-MHC complexes triggers migration of protein kinases, actin-binding proteins, and other accessory molecules to the T cell/APC synapse. We used confocal immunofluorescence methods to show that the adapter protein LAT (linker for activation of T cells) and the guanine nucleotide exchange factor Vav also move to the APC interface in mouse CD4 T cells conjugated to anti-CD3 hybridoma cells, and in TCR-transgenic CD4 cells conjugated to APC bearing agonist (but not closely related nonagonist) peptides. The proportion of CD4+ T cells able to relocalize LAT or Vav, or to relocate cytoplasmic NT-AT (NF-ATc) from cytoplasm to nucleus, declines about 2-fold in aged mice. The decline in LAT relocalization is accompanied by a similar decline in tyrosine phosphorylation of LAT in CD4 cells stimulated by CD3/CD4 cross-linking. Two-color experiments show that LAT redistribution is strongly associated with relocalization of both NF-ATc and protein kinase C-theta among individual cells. LAT migration to the immunological synapse depends on actin polymerization as well as on activity of Src family kinases, but aging leads to only a small change in the percentage of CD4 cells that redistribute F-actin to the site of APC contact. These results suggest that defects in the ability of T cells from aged donors to move kinase substrates and coupling factors, including LAT and Vav, into the T cell/APC contact region may contribute to the decline with age in NF-ATc-dependent gene expression, and thus to defects in T cell clonal expansion.

Effect of aging on T lymphocyte activation

Studies of the early stages of T-cell activation reveal that T cells from aged mice show multiple abnormalities within the first few minutes after stimulation, including decline in the activation of the Raf-1/MEK/ERK kinases and in JNK protein kinase. Zap-70 kinase associated with the CD3zeta chain shows a 2-fold increase with age in resting CD4 T cells, despite a three-fold decline with age in the levels of tyrosine phosphorylation of CD3zeta; nonetheless, there is no effect of aging on Zap-70 kinase function in activated T cells as measured by in vitro kinase methods. Age-related impairment of the translocation of PKCθ from cytoplasm to the site of T-cell interaction with antigen-presenting cells may underlie downstream defects in the activation cascade.

Impaired signal transduction in mitogen activated rat splenic lymphocytes during aging

Mitogen activated protein kinases (MAPK) are activated by a wide variety of signals leading to cell proliferation and differentiation in different cell types. With aging, there is a marked decrease in proliferation of T-lymphocytes in response to a variety of mitogens. Several age-related changes in the activation of MAPK pathways in T-lymphocytes activated via the T-cell receptor (TCR) have been described in different species. This way, some TCR proximal defects in tyrosine kinase activity have been delineated. In this study, we have used rat splenic lymphocytes to measure the effect of aging on the activation of two MAP kinase families: ERK and JNK. In order to bypass the receptor-proximal age-dependent defects previously described, we used phorbol ester (PMA) and Ca2+ ionophore (A23187) as co-mitogens. Our results demonstrate that splenic lymphocytes from old rats have a disturbance in the activation of the ERK and JNK MAPK signal transduction pathways, that are located downstream of the receptor-proximal events. At least part of the age-related defect leading to decreased ERK activity appears to be located upstream of ERK itself, since activation of MEK is also impaired. On the other hand, the observed defects in MAPK activation do result in decreased activation of downstream events, such as c-Jun phosphorylation. Thus, we conclude that aging of splenic lymphocytes results in a functional decline in signal transduction, and at least some of these defects are located downstream of the receptor-proximal events previously described by others. The impaired activity of these two MAP kinase pathways is likely to play a role in the diminished lymphoproliferation observed in old individuals.

Aging impairs induction of cyclin-dependent kinases and down-regulation of p27 in mouse CD4(+) cells

To define the link between the early activation defects and the impaired proliferation response of cells from old mice, we characterized the influence of age on expression and activity of proteins that participate in cell-cycle regulation. We found that aging led to significant declines in the ability of mouse CD4(+) T cells to respond to CD3 and CD28 stimuli by induction of the cyclin-dependent kinases CDK2, CDK4, and CDK6, whether the defect was assessed by protein level or functional activity. Induction of CDK2 activity was also impaired in cells from old mice that were activated with PMA plus ionomycin, stimuli that bypass the TCR/CD3 complex, or by CD3/CD28 in the presence of IL-2, indicating that the age-related changes lie, at least in part, downstream of the enzymes activated by these stimuli. We also noted an impairment in the ability of CD4(+) cells from old mice to down-regulate the CDK inhibitor p27 after activation, but we found no change in induction of p21, an inhibitor of CDK that may also play other roles in cell-cycle control. Altered CDK activation is likely to mediate the age-related decline in T cell proliferation to polyclonal stimulation.

Age-related decline in activation of JNK by TCR- and CD28-mediated signals in murine T-lymphocytes

c-Jun N-terminal kinase (JNK) is activated when T-lymphocytes are stimulated jointly through the T-cell receptor (TCR) and CD28, and it contributes to T-cell activation and IL-2 production through phosphorylation of transcription factors, including c-Jun. We performed in vitro kinase assays on JNK in CD4(+) T-cells, from young and old mice, activated by antibodies to CD3, CD4, and CD28, and found a approximately 2-fold decline in JNK activity at the peak of activation, but no significant change in the kinetics of stimulation or in the steady-state expression of JNK. We found a similar decline in c-Jun phosphorylation in stimulated CD4(+) T-cells from old mice, suggesting that JNK activation also declined with age in intact cells. Aging does not, however, alter the level of Ras activation by anti-CD3/CD4 +/- anti-CD28 or change the level of Ras protein in CD4(+) cells, suggesting that the JNK defect is due to changes in the regulation of other upstream regulators. Our results suggest that a decline with age in JNK responses may contribute to the decline in proliferation and IL-2 production seen in CD4(+) T-cells from old mice.

Nutrition and immunity in the elderly

Immune function declines with age, leading to increased infection and cancer rates in aged individuals. In fact, recent progress in the study of immune ageing has introduced the idea that rather than a general decline in the functions of the immune system with age, immune ageing is mainly characterized by a progressive appearance of immune dysregulation throughout life. Changes appear earlier in life for cell-mediated immunity than for humoral immunity. Thus, age-related modifications in cell-mediated immunity, i.e. changes in naive : memory T-cells, mature : immature T-cells, T-helper 1 : T-helper 2 cells are more important in the elderly than changes in humoral immunity, i.e. CD5 : CD5+ cells or length of antibody responses. Such evolution of the immune system has been linked to declining thymus function and to accumulative antigenic influence over the lifespan. In contrast, innate immunity (macrophage functions) is preserved or even increased during the ageing process. This finding shows that the 'primitive' immune system is less affected by the ageing process than the sophisticated specific immune system. The present review focuses on innate and cell-mediated immune changes with ageing. It provides evidence that primary changes (intrinsic modifications in the immune system) and secondary changes (resulting from environmental influences during the lifespan) exert different influences on the immune system. Primary changes, occurring in healthy individuals, seem less important nowadays than they were considered to be previously. For example, interleukin 2 secretion in some very healthy aged individuals is comparable with that in younger adults. Primary immune changes may not explain the increased incidence and severity of infections observed in the elderly population. Secondary immunological changes are far more frequent and are certainly responsible for most of the immune modifications observed in the elderly population. Environmental factors leading to secondary immune dysfunctions include not only antigenic influence, which is a reflection of diseases experienced over the lifespan, but also many other factors such as drug intake, physical activity and diet; factors for which important changes occur in the elderly population. Nutritional factors play a major role in the immune responses of aged individuals and the present review shows that nutritional influences on immune responses are of great consequence in aged individuals, even in the very healthy elderly.

Aging and T-cell-mediated immunity

Changes in the T-lymphocyte compartment represent the most critical component of immunological aging. Recent studies have demonstrated that the age-related decline in T-cell-mediated immunity is a multifactorial phenomenon affecting T-cell subset composition as well as several proximal events such as protein tyrosine phosphorylation, generation of second messengers, calcium mobilization and translocation of protein kinase C, and distal events such as lymphocyte proliferation and cytokine production of the T-cell activation pathway. Age-related T-cell immune deficiency is preceded by thymic involution and is influenced by several intrinsic as well as extrinsic factors. Further, the role of monocytes and macrophages in T-cell activation changes with advancing age. This brief review will summarize the current knowledge of the cellular as well as molecular aspects of immunodeficiency of T cells due to aging, some of the paradoxes of aging as related to T-cell-mediated immunity, and possible factors which contribute to this paradox. Finally, experimental approaches will be suggested that might resolve these controversies and that might provide insights into the diverse and complex mechanisms that contribute to immunodeficiency of T cells. Ultimately these studies may suggest possible therapeutic interventions to enhance immune function in the elderly.

Age-dependent decline in mitogenic stimulation of hepatocytes. Reduced association between Shc and the epidermal growth factor receptor is coupled to decreased activation of Raf and extracellular signal-regulated kinases

The proliferative potential of the liver has been well documented to decline with age. However, the molecular mechanism of this phenomenon is not well understood. Cellular proliferation is the result of growth factor-receptor binding and activation of cellular signaling pathways to regulate specific gene transcription. To determine the mechanism of the age-related difference in proliferation, we evaluated extracellular signal-regulated kinase-mitogen-activated protein kinase activation and events upstream in the signaling pathway in epidermal growth factor (EGF)-stimulated hepatocytes isolated from young and old rats. We confirm the age-associated decrease in extracellular signal-regulated kinase-mitogen-activated protein kinase activation in response to EGF that has been previously reported. We also find that the activity of the upstream kinase, Raf kinase, is decreased in hepatocytes from old compared with young rats. An early age-related difference in the EGF-stimulated pathway is shown to be the decreased ability of the adapter protein, Shc, to associate with the EGF receptor through the Shc phosphotyrosine binding domain. To address the mechanism of decreased Shc/EGF receptor interaction, we examined the phosphorylation of the EGF receptor at tyrosine 1173, a site recognized by the Shc phosphotyrosine binding domain. Tyrosine 1173 of the EGF receptor is underphosphorylated in the hepatocytes from old animals compared with young in a Western blot analysis using a phosphospecific antibody that recognizes phosphotyrosine 1173 of the EGF receptor. These data suggest that a molecular mechanism underlying the age-associated decrease in hepatocyte proliferation involves an age-dependent regulation of site-specific tyrosine residue phosphorylation on the EGF receptor.

Age-related impairment of p56lck and ZAP-70 activities in human T lymphocytes activated through the TcR/CD3 complex

Cellular immune responses decrease with aging. Lymphocytes of aged individuals do not perform as well as cells from young subjects in a number of in vitro assays including cell proliferation, cytokine production, and protection against apoptosis. Here, we have tested the hypothesis that a decrease in T cell responses in tymphocytes from elderly subjects could parallel a decrease in the activity of protein tyrosine kinases (PTK) associated with signal transduction in T lymphocytes. We report that anti-CD3-triggered T lymphocyte proliferation was significantly decreased in T lymphocytes from elderly subjects, but the decrease was not due to an alteration of the percentage or mean fluorescence intensities of CD3, CD4, and CD45. Of significance, the activities of p56lck and ZAP-70 in vitro were significantly decreased in T lymphocytes from elderly subjects compared to young individuals. However, the level of expression of the two kinases did not change with aging. The activity of p59fyn did not show changes with aging, suggesting that p59fyn did not compensate for the decreased activity of p56lck. We also found that the extent of tyrosine phosphorylation of the adaptor protein p95vav was similar in activated T lymphocytes from elderly and young subjects. Our results suggest that the altered cellular immune responses observed in T lymphocytes with aging may be the result, at least in part, of an alteration in early events associated with signal transduction through the TcR/CD3 complex that translates into decreased activities of p56lck and ZAP-70. Impairment in the activities of these twokey components of T cell signaling may contribute to reduced immune functions associated with aging.

Differential signalling by variant ligands of the T cell receptor and the kinetic model of T cell activation

The structural basis of T cell activation through the T cell receptor is still a major unresolved issue in T cell biology. The wealth of information on the generation and structure of T cell receptor ligands and the biochemistry of signal transduction from this receptor have been useful in the initial approach to explain how T cell activation occurs. More recently, the generation of variant T cell receptor ligands with partial agonist or antagonist properties, the determination of crystal structures for unengaged and engaged T cell receptors, and the kinetics of T cell receptor interactions with peptide:MHC molecule complexes have provided new insights on T cell receptor function. The common theme arising from these experiments is that the T cell receptor is a versatile signalling machine, with an inherent flexibility for ligand recognition that translates in different signalling patterns. Here, I will review the data on differential signalling from the T cell receptor upon recognition of partial agonist and antagonist ligands and how these data impact on a more general kinetic model of T cell receptor-mediated activation.

Increased Zap-70 association with CD3zeta in CD4 T cells from old mice

Aging diminishes the amount of phosphotyrosine in the CD3zeta chains of resting and activated mouse CD4 T cells by about threefold and might therefore be expected to a corresponding decline in Zap-70 association with CD3zeta and in Zap-70 kinase function in CD3zeta complexes. We show here that aging leads, unexpectedly, to an approximately twofold increase in the amount of Zap-70 associated with CD3zeta in resting CD4 T cells. There is, however, no effect of age on total intracellular Zap-70 content. Cross-linking CD3 to CD4 leads to an increase of only 50% in the functional activity of Zap-70 in CD3zeta complexes from freshly isolated CD4 T cells of young donors. Compared to Jurkat and HT-2 cells, fresh T cells show both higher baseline levels and lower induced levels of Zap-70 function in CD3zeta complexes. CD4 T cells from old mice have baseline levels of Zap-70 activity similar to those seen in activated T cells from young mice, and these levels do not increase after CD3/CD4 cross-linking. Tyrosine-specific phosphorylation of Zap-70 is also higher at rest in old T cells than in young T cells and inducible only in cells from young donors. These data suggest that age-related defects in T cell activation are not likely to be attributable simply to a decline in Zap-70 association with CD3zeta or to diminished Zap-70 phosphorylation. The increase with age in CD3zeta-Zap association, despite the loss with age in CD3zeta tyrosine phosphorylation, suggests that the pattern of tyrosine phosphate groups among CD3zeta ITAM groups may be different in T cells from young and old donors or that access to ITAM regions within CD3zeta may be blocked by inter- or intramolecular steric hindrance in young CD4 T cells.

Analysis of Raf-1 activation in response to TCR activation and costimulation in murine T-lymphocytes: effect of age

Stimulation of the ERK (MAPK) pathway in T-lymphocytes contributes to cell activation and IL-2 production. The ERK pathway is initiated by the activation of the serine/threonine kinase Raf-1 in a Ras-dependent manner. Raf-1 activates the dual-specific kinase MEK, which in turn activates ERK. To see if aging leads to an alteration of Raf-1 kinase activity we performed in vitro kinase assays on Raf-1 isolated from CD4(+) T-cells from young and old mice. We found an age-related impairment in the kinase activity of Raf-1 in T-cells stimulated by a combination of antibodies to the CD3epsilon chain of the T-cell receptor and CD4. Aging led to a two- to fourfold decline in Raf-1 activity (depending on the stimulation time) without a change in the kinetics of enzyme activation. We also found that Raf-1 activation by CD3/CD4 costimulation is lower in memory cells than in naïve cells from mice of the same age. However, aging also leads to a decline in Raf-1 activity in the naïve subset of CD4(+) T-cells, suggesting that two mechanisms lead to the age related decline in Raf-1 function. Finally, we found that antibodies to the costimulatory molecule CD28 trigger Raf-1 activation and enhance anti-CD3-mediated Raf-1 activation but cannot restore Raf-1 activation levels from old T-cells to those seen in young mice. Our data suggest that age-dependent declines in T-cell ERK function are caused by alterations in the signals that activate Raf-1 and that age-dependent defects in T-cell cytokine production and proliferation may be caused at least in part by defects in signals that activate Raf-1.

Phosphorylation and coupling of zeta-chains to activated T-cell receptor (TCR)/CD3 complexes from peripheral blood T-cells of elderly humans

Aging is often accompanied by altered T-cell signaling and functions. Signals mediated through the T-cell receptor (TCR)/CD3 complex are associated with tyrosine phosphorylations of zeta-chains by the regulated activities of protein tyrosine kinases p56(lck) and p59(fyn) as well as protein tyrosine phosphatases. In the present investigation, the coupling and phosphorylation of zeta-chains to TCR/CD3 immunocomplexes were examined in peripheral blood T-cells from 13 elderly and young humans stimulated by ligation of the TCR/CD3 with cross-linked anti-CD3epsilon monoclonal antibody OKT3. Western blots analyzing the non-covalent coupling of zeta-chains to TCR/CD3 immunocomplexes from Brij-96 detergent lysates of anti-CD3 ligated T-cells showed that the levels of zeta-chains within TCR/CD3 immunocomplexes from T-cells of elderly and young subjects did not significantly differ. By contrast, the levels of phosphorylated zeta-chains generated during in vitro phosphorylations of TCR/CD3 immunocomplexes from elderly subjects were significantly reduced and averaged 44% of those observed for anti-CD3epsilon ligated T-cells from young subjects. Analyses of the levels of zeta-chain coupling and phosphorylations in T-cells from each of the 13 elderly individuals also showed that the reductions in zeta-chain phosphorylations were heterogeneous and unrelated to modest reductions in coupling. Furthermore, the age-related decreases in zeta-chain phosphorylations were not due to diminished frequencies of CD3epsilon+ cells or densities of CD3epsilon surface receptors and could be observed without reductions in epsilon-chain phosphorylations. These results suggest that aberrancies of zeta-chain phosphorylations can occur in T-cells of elderly humans independent from any uncoupling of zeta-chains to activated TCR/CD3 complexes.

Effect of age on mitogen induced protein tyrosine phosphorylation in human T cell and its subsets: down-regulation of tyrosine phosphorylation of ZAP-70

Several events of T cell activation have been reported to decline in humans with age. Since protein tyrosine phosphorylation is an early critical event of T cell activation, we performed a systematic analysis of the age-associated changes in the mitogen induced protein tyrosine phosphorylation of human T lymphocytes using SDS-PAGE and Western blotting techniques. Following stimulation with Con A and PHA, an identical pattern of protein tyrosine phosphorylation was observed in the lysates of T cells prepared from seven healthy young adults and eight healthy elderly human subjects. Five different high molecular mass proteins (75, 115, 120, 140 and 170 kDa) were consistently tyrosine phosphorylated in all of the donors from both age groups and peaked between 3 and 10 min. Tyrosine phosphorylation of the above substrates was observed in both CD4 and CD8 subsets. When compared for individual donors from both age groups, variations in the T cell response with regard to net tyrosine phosphorylation for all the substrates was observed. However, the mitogen induced level of tyrosine phosphorylation of only p75 was found to be significantly lower in unfractionated T cells as well as CD4 and CD8 subsets of older subjects than that of young subjects. Using immunoblotting, p75 was identified as ZAP-70, a member of the syk family of protein tyrosine kinases. Understanding of the biochemical basis of the reduced level of tyrosine phosphorylation of ZAP-70 will be helpful in delineating the molecular basis of age-associated impairment of T cell activation.

Reductions in the activation of ERK and JNK are associated with decreased IL-2 production in T cells from elderly humans stimulated by the TCR/CD3 complex and costimulatory signals

T cells from elderly humans often display impaired IL-2 production, but the mechanisms are unknown. Because the activities of extracellular signal-regulated kinases (ERK) and c-Jun NH2-terminal kinases (JNK) are important for IL-2 production, the current study evaluated if aberrancies in the expression and activation of ERK2 or JNK might underlie decreased IL-2 production by human T cells during aging. The present results show that diminished ERK2 and JNK catalytic activities were commonly detected in T cells from elderly humans stimulated with anti-CD3 mAb OKT3 plus PMA. These reductions did not represent temporal shifts in activation or altered expression of ERK2 or JNK. In addition, the reductions of ERK2 activation in stimulated T cells from elderly individuals were accompanied by decreased Raf-1 kinase activation and could be observed without coexisting impairments in JNK activation. Stimulation of ERK2 activation in elderly T cells correlated with IL-2 production and decreased ERK2 activation was consistently associated with reduced IL-2 production. Although the age-related decreases in JNK activation were accompanied by reduced IL-2 production, substantial impairments of JNK activation were observed with diminished ERK2 activation. Moreover, anti-CD3/PMA-stimulated T cells from elderly individuals that displayed normal JNK activation and impaired ERK2 activation continued to demonstrate reduced IL-2 production. These findings show that impairments in the activation of ERK2 and JNK can accompany decreased IL-2 production by T cells from elderly humans and further suggest that aberrancies in TCR/CD3-dependent activation of the Raf-1/MEK/ERK2 cascade may be rate-limiting for the full induction of IL-2.

Early activation defects in T lymphocytes from aged mice

Aging affects both calcium signals and protein kinase cascades in mouse T lymphocytes. The decline in calcium signal development largely represents differences between naive and memory T cells; the latter are resistant to increases in calcium concentration, and are more common in aged mice. Aging leads to declines in phosphorylation of a wide range of substrates in T cells stimulated by either anti-CD3 antibodies or by substances, such as phorbol myristate acetate (PMA) or ionomycin, that act at intracellular sites, but some phosphoproteins respond only in old T cells, and others respond regardless of age. Tyrosine phosphorylation of the CD3 zeta chain declines with age, both in resting T cells and after activation, but the proportion of Zap-70 that is bound to CD3 zeta increases in T cells from old mice. Zap-70 function and phosphorylation of CD3 zeta-associated Zap-70 change only slightly after stimulation of T cells by anti-CD3 and anti-CD4, and are at similar levels in activated old and young T cells. Nonetheless, induction of Raf-1, MEK, and ERK kinase activity declines with age in CD4 T cells. The effect of aging on T-cell activation is not simply an overall decline in signal intensity, but a set of qualitative changes that differ among subsets and depend at least partly on the nature of the stimulus.

Primary and secondary alterations of immune reactivity in the elderly: impact of dietary factors and disease

The function of the immune system declines with age. It is the aim of the present review to demonstrate that it makes sense to distinguish between primary and secondary alterations of immune reactivity in the elderly. Primary changes occur as the result of an age-dependent intrinsic decline of immune responsiveness. They also occur in healthy persons, i.e. persons selected according to the criteria of the SENIEUR protocol of the European Community's Concerted Action Program on Aging (EURAGE). T lymphocytes are hereby more severely affected than B cells or antigen presenting cells, possibly due to the involution of the thymus, which is almost complete at the age of 60. Secondary immunological changes occur as the result of environmental factors including diet, drug intake, physical activity etc. or are alternatively due to underlying diseases. In this article, the effects of high lipid intake as well as the impact of diseases, such as for instance Alzheimer's disease and atherosclerosis, will be addressed. The results underline the complexity of immunological alterations to be expected in old age. Changes in the aging immune system represent an opportunity for increased frequency and severity of disease and endanger the protective effect of vaccination.

Expression and catalytic activities of protein tyrosine kinases (PTKs) Fyn and Lck in peripheral blood T cells from elderly humans stimulated through the T cell receptor (TCR)/CD3 complex

Optimal signal transduction through the T cell receptor (TCR)/CD3 complex requires the coordinated activities of protein tyrosine kinases (PTKs) Fyn and Lck in addition to protein tyrosine phosphatases (PTPases) such as CD45. Although T cells stimulated with anti-CD3 monoclonal antibodies (mAb) exhibit age-related reductions in tyrosine phosphorylations of cellular proteins, it is unknown if the reduction represent abnormalities in PTKs or PTPases. In the current studies, immune complex kinase assays showed that the stimulation of peripheral blood T (PBT) cells from young humans with cross-linked anti-CD3 epsilon mAb OKT3 induced increased Fyn catalytic activity while anti-CD3 stimulation failed to induce significant increases in Lck activation. By contrast, Fyn activation in anti-CD3 stimulated PBT cells from a substantial proportion of elderly humans was reduced compared to anti-CD3 stimulated PBT cells from young humans. Also, we failed to find any increase in anti-CD3 stimulation of Lck activity in PBT cells from elderly subjects that could compensate for the decline in Fyn activity. However, no age-related alterations were detected in PBT cell expression of Fyn or Lck that might contribute to the changes in enzymatic activity. The results of other experiments demonstrated that the functional activities of PTPases in PBT cells from elderly subjects were equivalent to PBT cells from young subjects. These observations suggest that aberrant regulation of TCR/CD3 coupled PTKs may contribute to the age-related defects in signaling cascades and immune responsiveness of human T cells.

The role of CD45 and CD45-associated molecules in T cell activation

CD45 (lymphocyte common antigen) is a receptor-linked protein tyrosine phosphatase that is expressed on all leucocytes, and which plays a crucial role in the function of these cells. On T cells the extracellular domain of CD45 is expressed in several different isoforms, and the particular isoform(s) expressed depends on the particular subpopulation of cell, their state of maturation, and whether or not they have previously been exposed to antigen. It has been established that the expression of CD45 is essential for the activation of T cells via the TCR, and that different CD45 isoforms display a different ability to support T cell activation. Although the tyrosine phosphatase activity of the intracellular region of CD45 has been shown to be crucial for supporting signal transduction from the TCR, the nature of the ligands for the different isoforms of CD45 have been elusive. Moreover, the precise mechanism by which potential ligands may regulate CD45 function is unclear. Interestingly, in T cells CD45 has been shown to associate with numerous molecules, both membrane associated and intracellular; these include components of the TCR-CD3 complex and CD4/CD8. In addition, CD45 is reported to associate with several intracellular protein tyrosine kinases including p56lck and p59fyn of the src family, and ZAP-70 of the Syk family, and with numerous proteins of 29-34 kDa. These CD45-associated molecules may play an important role in regulating CD45 tyrosine phosphatase activity and function. However, although the role of some of the CD45-associated molecules (e.g. CD45-AP and LPAP) has become better understood in recent years, the role of others still remains obscure. This review aims to summarize recent findings on the role of CD45 and CD45-associated molecules in T cell activation, and to highlight issues that seem relevant to ongoing research in this area.

Increased apoptosis of CD45RO- T cells with aging

Increasing susceptibility to infectious and autoimmune phenomena have long been recognized to accompany advancing age in otherwise healthy individuals. Recently animal models of aging have suggested that age-associated immune dysfunction may correlate with defects in T cell apoptosis. We have examined activation-induced apoptosis defects in human peripheral T cells from young individuals (mean age = 31 +/- 3 years old) compared to aged individuals (mean age = 67 +/- 8 years old). Following in vitro activation of T cells with PHA and IL-2, apoptosis was measured in T cell subsets using 7-amino actinomycin D (7-AAD) staining and analysis via three colour flow cytometry. There was no significant difference in apoptosis of the total CD3+ T cell population at early and late time points. Interestingly, increased apoptosis in the CD3+ CD45RO- T cell population of older adults was observed by culture day 6. While the total number of CD3+ CD45RO- cells was not different between young (< 33 years) and old (> 65 years) individuals, 32% of these cells did not undergo apoptosis in younger individuals while only 10% of these cells avoided this fate in older individuals. These results suggest that accumulation of CD45RO+ T cells may occur in aged subjects due in part to preferential elimination of CD45RO- cells with activation. Furthermore, as new or continued immune response requires differentiation of CD45RO+ T cells to CD45RO+ T cells after activation, increased apoptosis instead of survival in aged individuals could lead to observed T cell immune deficiency.

Diminished activation of the MAP kinase pathway in CD3-stimulated T lymphocytes from old mice

Stimulation of the ERK family of protein kinases ('extracellular signal regulated kinases', also known as MAP kinases) plays an important role in the activation of many cell types, including T lymphocytes. ERKs are activated when they are phosphorylated by an upstream activator, the dual-specific protein kinase MEK. To see if aging leads to an impairment of MEK activation in mouse T cells, we used a mobility shift assay in which activation of MEK leads to phosphorylation and altered mobility of ERK-2 kinase. Similarly, we monitored mobility of pp90rsk, a known ERK substrate, as an indication of ERK function. We found an age-related decline in the ability of mouse T cells to activate both MEK and ERK function in response to stimulation by antibodies to the CD3 chain of the T cell receptor. Aging did not alter the kinetics of enzyme activation, but did diminish (by about 2-fold) the maximal level of substrate converted into the slower migrating form. Naive and memory CD4 T cells from young mice were equally able to convert ERK2 to its slower migrating form, suggesting that the decline in MEK function is not likely to be attributable to the shift, with age, from naive to memory T cell predominance. Our data suggest that age-dependent declines in gene activation, including genes for key cytokines like IL-2, may be due to declines in the upstream signals that lead to activation of the MEK/ERK protein kinase cascade.

The aging immune system: primer and prospectus

Changes in T lymphocyte populations underlie much of the age-related decline in the protective immune response. Aging leads to the replacement of virgin T cells by memory T cells and to the accumulation of cells with signal transduction defects. Studies of antibody gene assembly, accessory cell function, post-thymic T cell development, skewed selection of T cell receptor repertoire, and the clinical concomitants of immune senescence will shed new light on the causes and consequences of age-dependent immune failure.

Calcium signals in T lymphocytes from old mice

Mitogen-induced increases in free calcium ion concentration ([Ca]i) are a key element of the process by which T lymphocytes are induced to proliferate and differentiate into effector cells. T cells from old mice exhibit lower average rises in calcium concentration than T cells from young donors when stimulated with either mitogenic lectins or antibodies to the CD3 chains of the antigen receptor. The decline with age in calcium signal generation is largely attributable to a shift from naïve to memory T cells, in that memory T cells, from mice of any age, are more resistant to mitogen-induced changes in [Ca]i. The decline in calcium signal generation is likely to be functionally significant, since T cells isolated on the basis of poor calcium signals show diminished ability to produce and to respond to the growth factor IL-2. Con A induces a transient increase in uptake of radiolabeled calcium from extracellular sources, and the extent of this increase declines with age. Alterations in production of inositol tris-phosphate (IP3) seem not to contribute to age-related changes in calcium signal generation. T cells from old mice, and memory T cells from mice of any age, are relatively resistant to increases in [Ca]i even when these are induced by receptor-independent stimuli such as the calcium ionophore ionomycin. The ionomycin-resistance of memory T cells suggests that these cells may have an augmented ability to buffer changes in [Ca]i, perhaps by increased activity of the ATP-dependent plasma membrane calcium pump. It seems likely that age-related declines in calcium signal generation contribute to the functional immunodeficiency of old age.