Cell surface markers, inositol phosphate levels and membrane potential of lymphocytes from young and old human patients

Modulation of human lymphocyte proliferative response with aging

Previously, we have demonstrated age-associated alterations in transmembrane signaling. One of the most reproducible alterations found in the immune response with aging is the decrease of lymphocyte proliferation on stimulation with various different mitogens. Here, we confirm that proliferative responses to stimulation with phytohaemagglutin (PHA), recombinant human IL-2, or anti-CD3 monoclonal antibody are all greater in the young (20-25 years) than old (60-87 years) population. We attempted to modulate the proliferative response using various agents acting at different levels of transmembrane signaling (pertussis toxin, cholera toxin, isoproterenol, PMA, Ca ionophore A23187), as well as at the level of the lymphocyte plasma membrane (methyl-beta-cyclodextrin, MBCD), or by using antioxidant vitamins (Vitamin E or C). None of these agents was able to restore effectively the proliferative response of lymphocytes from the aged to the level of young subjects. Even the combination of A23187 and PMA acting directly on calcium metabolism and protein kinase C activity was insufficient to restore the decreased mitogenic capacity of T cells from elderly subjects. Cyclodextrin, which decreases the cholesterol content of the membrane, increased the proliferative response of lymphocytes of elderly subjects, but not to the level of the young. Vitamin E had a very strong inhibitory effect on lymphocyte stimulation in both the age groups, except in combination with MBCD in T cells of the elderly, while Vitamin C had no significant modulatory effect. MAPK ERK and p38 activation was found to be decreased with aging in T cells after anti-CD3 mAb stimulation. Vitamin E but not Vitamin C strongly inhibited MAPK ERK or p38 activation. The direct activation of certain molecules or the modulation of the cholesterol content of the membrane seems to be effective immunomodulatory interventions with aging.

Experimental gerontology in Hungary

Gerontological research has some past and sporadically also some highlights in Hungary, but its present state can be easily deduced from the following data. During the last 12 years and more, well over 10,000 Hungarian scientific papers have been published in well-recognized national or international journals. Altogether approximately 1% of them have been classified as gerontological publications from Hungary. This low figure shows that gerontology has low priority and--unfortunately low support--in Hungary. This statement does not intend to downgrade Hungarian gerontologists, however points out that the Hungarian trends are not far from those of European or world wide interest in aging. Despite the recognition that we have to accept the inevitable fact that industrial societies will have (they already have) an aging population with all the social and medical problems arising, the focus of interest is wide from this significant and interesting (sub)population, which is neglected (sometimes even despised); yet everybody is absolutely eager to join this club. The average of the Hungarian research achievements and publication activities are among the better European achievements. There are some highlights and new trends even initiated by some outstanding Hungarian scientists, yet the overall weight of gerontology research is still an orphan in the Hungarian scientific life. We deal in this short and far from complete summary almost exclusively with experimental gerontology. We have to apologize if we have not included everybody, who also contributed even significantly to this field because the time for the preparation of this overview was short.

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-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.

Decreased association of p56lck with CD4 may account for lowered tyrosine kinase activity in mitogen-activated human T lymphocytes during aging

Treatment of T cells from young donors with PHA rapidly induces tyrosine phosphorylation of several intracellular substrates. In contrast, T cells from elderly donors treated in a similar manner showed far fewer tyrosine-phosphorylated proteins. To understand the basis of this age-associated difference in T cells, we examined the in vitro catalytic activity of kinase(s) following activation. We demonstrate both lowered overall in vitro kinase activity as well as a significant decrease in the activity of lymphocyte-specific tyrosine kinase p56lck in activated T cells from the elderly. Our results demonstrate for the first time an altered association of p56lck with coreceptors such as CD4 and CD45 in the elderly. These results suggest that alterations in p56lck tyrosine kinase and its association with CD4 and CD45 may underlie lowered T cell function during aging.

Impairment of signal transduction in T cells from old mice

T cells from old mice showed impaired proliferative response to antigenic stimulation. To understand the mechanism underlying the age-related impairment of T cell functions, the signal transduction pathway was examined and compared between T cells from young and old mice, and between T cell clones established from a young and old mouse. The age-related changes in T cells were as follows: (1) reduction in the expression and the activation of protein tyrosine kinases associated with T cell receptor (TCR) after antigenic stimulation; (2) reduced phosphorylation of phospholipase C gamma 1 (PLC gamma 1); (3) reduced production of second messengers such as inositoltrisphosphate (IP3) and diacylglycerol (DAG); and (4) reduced influx of Ca2+ ion. Thus, a T cell clone established from an old mouse showed impaired proliferation by stimulation with anti-CD3 antibody, but was fully activated to the level of a T cell clone from a young mouse by stimulation with phorbol acetate myristate (PMA) plus ionomycin (INM). However, splenic T cells freshly prepared from old mice did not show full recovery by the same treatment. The results indicate that one major blockade in the signal transduction of T cells from old mice is present in the pathway just after TCR, but besides this, the blockade is also present in multiple sites down-stream, which can not be bypassed by stimulation with PMA plus INM.

Age-associated decrease in inositol 1,4,5-trisphosphate and diacylglycerol generation by 1,25(OH)2-vitamin D3 in rat intestine

The hormonal form of vitamin D3, 1,25(OH)2-vitamin D3(1,25[OH]2D3), stimulates the breakdown of membrane phosphoinositides, generating inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) in a variety of cell systems. Several studies suggest that alterations in the receptor-mediated phosphoinositide cascade are involved in the pathophysiology of aging. Therefore, the formation of IP3 and DAG were determined under basal conditions and after stimulation with physiological concentrations of 1,25(OH)2D3 in duodenum from young (3-mo-old) and aged (24-mo-old) rats. The hormone induced a transient and biphasic formation of IP3 and DAG. Values obtained in young rats peaking at 15 s (51% and 42% above basal levels for IP3 and DAG, respectively) and at 3 min (90% and 74% above basal levels for IP3 and DAG, respectively) were significantly decreased in duodenum from senescent animals (IP3: +20% and DAG: +18% above basal level at 15 s; and IP3: +18% and DAG: +29% above basal level at 3 min). The 1,25(OH)2D3-induced generation of DAG in both young and aged duodenum was effectively inhibited in the presence of neomycin, a phospholipase C (PLC) inhibitor, and was dependent on extracellular Ca2+. After the biphasic response, the levels of DAG generated by the hormone (10 min stimulation) remained elevated; the elevation occurred in the absence of IP3 production; and the elevated levels were not abolished by neomycin, implying that phospholipids other than phosphoinositides are the source of DAG. This 1,25(OH)2D3-dependent late phase of DAG generation was also diminished in aged animals. The precise molecular basis and the physiological significance of decreased liberation of IP3 and DAG by 1,25(OH)2D3 in the aged rat duodenum remains to be determined.

Cellular distribution of protein kinase C isozymes in CD3-mediated stimulation of human T lymphocytes with aging

Protein kinase C (PKC) is involved in a variety of cellular responses, such as the expression and secretion of IL-2, the regulation of cytotoxic killing and cell proliferation. It is known that these immune functions are altered with aging. Here, we show that anti-CD3-triggered T cell proliferation is significantly decreased with aging and that H7, an inhibitor of PKC, impairs the anti-CD3-induced T cell proliferation in a differential manner, lymphocytes of healthy young subjects being more sensitive to the PKC inhibitor than those of elderly subjects. We examined (Western blot) the presence and the cellular distribution of PKC isozymes in T lymphocytes of healthy young and elderly subjects in the resting state and after anti-CD3 mAb stimulation using antibodies directed against PKC alpha, beta, delta, epsilon and zeta isoforms in the cytosol and the plasma membrane fractions. These five PKC isotypes were present in human T cells of young and elderly subjects. However, their distribution between the cytosolic and membrane fractions varied according to the isozymes and the age of the subjects. In resting lymphocytes of young subjects, all the PKC isozymes were found in the cytosolic fraction, except PKC-zeta. In resting lymphocytes of elderly subjects PKC-zeta and -epsilon were almost equally distributed between the cytosolic and the membrane fractions, whereas PKC-alpha and -zeta were mainly found in the membrane fraction and PKC-beta was almost exclusively located in the cytosolic fraction. The translocation of PKC-alpha, -beta, -delta and -epsilon could be observed under anti-CD3 mAb stimulation in lymphocytes of young subjects, while in the case of elderly subjects only the PKC beta isoform was translocated. Our results suggest tha the decreased availability of cytosolic PKC may contribute to the diminished PKC-dependent responses to CD3-triggered stimulation of human T lymphocytes with aging.

Cell mediated immunity changes in ageing, relative importance of cell subpopulation switches and of nutritional factors

Decreased T-cell functions with ageing have been extensively described. This review focuses on recent data on changes in T-cell subpopulations related to ageing and their consequences on T-cell proliferation. Increase of immature T cells CD2+ CD3- is an ageing phenomenon related to T-cell declining proliferation. Recently it was shown that increase of immature T cells was due to an increase in different subtypes of the CD2+ CD3- population, double-negative CD2+ CD4- CD8- and double-positive CD2+ CD4+ CD8+ subpopulations, the former being associated with nutritional deficit, the latter with associated diseases. Other authors have focused on decreases of native T cells with parallel increase of memory T cells; such a switch is also relevant to declining T-cell proliferation. This review focuses on two major factors which influence immune ageing; nutritional parameters and antigen exposure.

Signal transduction changes in granulocytes and lymphocytes with ageing

It is well known that the immune response declines with ageing. However, the exact cause of this decline is still unknown. In recent years signal transduction events leading to the transmission of a signal from the cell surface to the nucleus have been extensively studied in various cell systems. These studies have indicated that an alteration in signal transduction occurs with ageing. It is not possible to identify a single age-sensitive step in this sequence, but rather a series of deficiencies contributing to the decline in competency of aged lymphocytes and granulocytes to undergo normal activation. Thus, signal transduction events such as calcium mobilization, phosphatidylinositol breakdown, accumulation of proto-oncogene transcripts, expression of activation markers, and synthesis of new proteins are deficient in the aged. Other events in signal transduction have been much less studied such as protein tyrosine kinase activity and G-protein functions. alterations in these various intracellular signalling events may fundamentally influence the functional activity of lymphocytes and granulocytes in the aged, as suggested by several investigations performed in recent years and reviewed in the subsequent sections. Future study on the signal transduction pathways using well-defined experimental models and healthy individuals should help to elucidate the molecular basis of immunosenescence and to develop effective approaches for reducing age-associated deficits and thereby reducing the incidence of age-associated diseases.

Age-dependent changes in transmembrane signalling: identification of G proteins in human lymphocytes and polymorphonuclear leukocytes

In human neutrophils (PMNLs) we found that in the elderly IP3 formation was significantly decreased compared to that of young subjects. For FMLP receptor binding affinity and number no measurable differences occurred upon ageing, studying both the low or the high affinity receptors. The amount of ADP-ribosylated G proteins, catalysed by pertussis toxin (PT) or cholera toxin (CT), was significantly increased in PMNLs of the elderly. In lymphocytes, the PT-catalysed ADP ribosylation of G proteins was also increased with ageing, while the CT-catalysed ribosylation was decreased. The autoradiogram of [32P]ADP-ribosylated proteins by CT in lymphocytes of young individuals showed a major polypeptide of 40,000 M(r). In contrast, in lymphocytes of the elderly, the major polypeptide was 45,000 M(r). In PMNLs, CT labelled quite strongly the 45,000 M(r) band, mainly in the elderly. When PT was used, no age-related pattern changes could be demonstrated, while differences could be observed between the two types of cells. The use of antiserum P680 (G alpha common) showed no age-related pattern changes, while the intensity of the labelled proteins varies with age and cell type. The antiserum U46 (Go alpha) could identify in lymphocytes of young subjects two polypeptides 68,000 and 41,000 M(r). The prominent polypeptide in lymphocytes of the elderly was the 70,000 M(r) and no other polypeptides could be recognized. In PMNLs of young subjects the U46 and serum identified a range of species. In PMNLs of the elderly all these bands were weakly labelled. The present data indicate changes in the pattern and the quantity of G proteins in lymphocytes and PMNLs of elderly subjects.

Transmembrane signaling changes with aging

Altered immune response and transmembrane signaling with aging has previously been demonstrated. The aim of the present study was to characterize PMNLs and lymphocyte G proteins and to determine whether their relative amounts are altered with aging. First we studied the effects of FMLP on PMNLs IP3 formation. It was found that in any group of elderly the PMNLs IP3 formation was significantly decreased compared to that of young subjects. In FMLP receptor binding affinity no measurable difference exists in either low- or high-affinity FMLP receptors. The autoradiogram of 32P-ADP-ribosylated proteins by CT in lymphocytes of young individuals showed a major polypeptide of 40 kDa, and two much less prevalent components of 52 and 45 kDa. In contrast, in lymphocytes of elderly subjects the major polypeptide was 45 kDa, and the two others were very weakly labeled. In PMNLs, CT labeled the 45-kDa band quite strongly, mainly in the elderly, and the 52- and 40-kDa bands were very weakly labeled, mainly in young subjects. When PT was used, no age-related pattern changes could be demonstrated, while differences could be observed between the two types of cells.

Molecular mechanisms mediating the effects of L-alpha-glycerylphosphorylcholine, a new cognition-enhancing drug, on behavioral and biochemical parameters in young and aged rats

The behavioral effects of the acute and subchronic administration of L-alpha-glycerylphosphorylcholine (alpha-GPC) on passive and active avoidance behavioral tasks were investigated. When administered IP after training together with scopolamine 2 h before retest, alpha-GPC reverses the scopolamine-induced amnesia in the passive avoidance conditioning in young and old rats. Furthermore, the subchronic treatment with alpha-GPC positively and significantly influences the performance of both young and old animals in the active avoidance test. Moreover, in in vitro/ex vivo experiments alpha-GPC potentiates receptor-stimulated phosphatidylinositol hydrolysis in cortical synaptoneurosomes derived from young and old animals. In young but not old animals, alpha-GPC significantly potentiates potassium (40 mM)-stimulated intrasynaptosomal calcium oscillations in purified synaptosomes derived from the hippocampus. These results show that alpha-GPC improves the performance of animals in both active and passive conditioning tasks. Furthermore, subchronic treatment with the compound enhances in young and restores in aged animals the transduction of the signal, namely, the receptor-mediated production of inositol phosphate and the potassium-induced calcium mobilization. These modifications may represent at least part of the molecular mechanism of action of the compound.

Phytohemagglutinin induced changes of membrane lipid packing, c-myc and c-myb encoded protein expression in human lymphocytes during aging

Three parameters which signal different stages of cell activation were analyzed in lymphocytes from young and old subjects. Merocyanine 540 (MC-540) incorporation into the membrane lipid phase was used as a very early marker of activation and was measured after 1 h of phytohemagglutinin (PHA) stimulation. The proteins coded by c-myc and c-myb protooncogenes were determined by appropriate antibodies and were taken as markers of the G0/G1 and G1/S phase transition, respectively. The number of cells which increased the uptake of MC-540 following PHA stimulation did not differ when comparing young and old individuals. Both the number of the responding cells and the size of the response were decreased during aging when the presence of the c-myc protein was taken into account. A consistent decrease of the percentage of lymphocytes able to express the c-myb protein was observed in the cells from old donors as compared to those from the young ones, but the amount of detectable protein per cell remained unchanged. Our data suggest that the deficiency of responsiveness which accompanies aging is due to impairments at different points of the cell cycle. The very low number of cells expressing the c-myb protein is likely the result of step by step elimination of those cells not able to fulfill the requirements to progress along the cell cycle.

Age- and strain-related differences in murine spleen cell responses to different activation signals

The effect of age on the response of splenocytes to activation with anti-CD3 mAb and a combination of anti-CD3 mAb and TPA, as evidenced by interleukin-2 (IL-2) and interleukin-4 (IL-4) production and cell proliferation, was examined in the C57BL/6 and DBA/2 murine strains. Depending on the mode of activation, there were age and strain differences in IL-2 and IL-4 production. With all modes of activation, cells from the old C57BL/6 mice produced less IL-2 than their young counterparts. In DBA/2 mice there was no age-related difference in IL-2 production with anti-CD3 mAb activation alone, whereas when the same cell population was activated with anti-CD3 mAb and TPA an age-associated decrease in IL-2 production occurred. In both strains, there was an age-related increase in IL-4 production with anti-CD3 mAb activation. After addition of TPA, however, there was an age-related decrease in IL-4 production. An age-related decline in the proliferation occurred with all modes of activation in both mouse strains. There were also strain-related differences in proliferation after the addition of forskolin, an inhibitor of Th1-cell function. While forskolin inhibited the proliferation of cells from the young C57BL/6 mice only, in the DBA/2 mice proliferation of cells was inhibited in both age groups. There were no strain-related differences in inhibition by anti-transferrin receptor (TrfR) mAb, although cells from the old mice were slightly more sensitive to this inhibition.

Aging impairs membrane potential responsiveness as well as opening of voltage and ligand gated Na+ channels in human lymphocytes

Depolarizing effects of increasing concentrations of extracellular K(+), as well as the repolarizing effect of bretylium tosylate (BT) were evaluated in human lymphocytes from young and elderly volunteers. Cells from elderly volunteers were less responsive to depolarization induced by increased extracellular potassium concentrations than those from young volunteers. Upon a near complete depolarization induced by 140 mM K(+) in the extracellular space, a significant amount of non-responding cells were found in samples from elderly volunteers. BT, which opens the otherwise silent Na(+) channels of partially depolarized cells, with subsequent activation of the Na(+)-K(+) pump (Pieri et al., 1989). repolarized both young and old lymphocytes. However, the degree of the repolarization was only 40% in the case of lymphocytes from elderly volunteers than from that of the young. It is suggested that an increase of membrane microviscosity, characteristic of old cells, may be at least partially responsible for the decreased responsiveness of plasma membrane functions which were observed.

Electroimmunology: membrane potential, ion-channel activities, and stimulatory signal transduction in human T lymphocytes from young and elderly

There are conflicting data on the functional role and direction of the changes in membrane potential and ion currents accompanying lymphocyte stimulation. Recently, we discovered that a known sodium channel opener, bretylium tosylate (BT), may influence the stimulatory processes of lymphocyte activation at more than one site. Parallel flow cytometric and electrophysiological measurements with patch clamp techniques showed that BT quickly repolarizes previously slightly depolarized human peripheral blood as well as splenic murine lymphocytes. The repolarization occurred through opening ligand- and voltage-gated, hitherto unknown sodium channels, and the sodium influx activated Na(+)-K(+)-dependent, electrogenic ATP-ase activity. A comparison of the flexible responsiveness of the membrane potential was carried out between lymphocytes from young and elderly using the above mechanism and a number of combinations of channel blockers and ionophores in order to obtain information on the alleged changes in immunological behavior. A significant difference has been found between lymphocytes from human young and elderly volunteers in the readiness to respond to channel-activating perturbations. An explanation is offered, based upon known physicochemical changes in the plasma membrane during aging.

Parameters to monitor aging with a possible perspective for intervention — an immunological approach

Reliable aging markers are very rare, which are better than the chronological age or those symptoms which have such great individual variability that their scientific value is questionable. The effect of aging on immunological behavior of human (and animal) individuals is reasonably well established. In this communication an attempt is made to find an immunological marker of aging at the level of cell surface phenomena. It was observed that ion-channel activities, having a complex regulation, loose their flexible responsiveness in lymphocyte membranes during aging. A recently discovered voltage regulation of the calcium-activated potassium channels showed a distinct change with aging of human lymphocytes. A possibility to find a better marker system in complex regulatory processes is also discussed.