Cellular immunosenescence in F344 rats: decline in responsiveness to phytohemagglutinin involves changes in both T cells and macrophages

Dietary vitamin E and T cell-mediated function in the elderly: effectiveness and mechanism of action

One of the most dramatic and consequence-bearing age-related phenomena is the decline of the immune function with old age. Age-related T cell-mediated immunity dysfunction has been implicated in the etiology of many of the chronic degenerative diseases of the elderly, including arthritis, cancer, autoimmune diseases and increased susceptibility to infectious diseases. T cells from aged individuals are impaired in their response to mitogens and in their cytokine production. In recent years, several studies have emphasized the importance of intracellular anti-oxidant levels for preserving the immune function. Recent progress in understanding the mechanisms of action of anti-oxidants on cellular metabolism, have shown that anti-oxidants may modulate signal transduction and gene expression in immune cells. Vitamin E is widely recognized as a major lipid-soluble chain-breaking anti-oxidant in the biological membrane, where it scavenges free radicals, inhibiting the initiation and chain propagation of lipid peroxidation and protecting cellular structures against oxidative stress damage. Experimental studies have provided evidences for a role of vitamin E in protecting the immune system of elderly subjects. This article reviews the studies concerning the effect of both vitamin E deficiency and supplementation on T cell-mediated immune function in aging. Following a chronological pathway, the present article will also discuss the knowledge regarding the underlying mechanism of action of vitamin E.

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.

A proposed role for nerve growth factor in the etiology of multiple sclerosis

It is proposed that, in addition to genetic factors involved in immune attack on myelin, higher concentrations of nerve growth factor in certain tissues during development determine susceptibility to multiple sclerosis. High early nerve growth factor in some vasculature of spontaneously hypertensive rats increases sympathetic innervation and catecholamine production in these vessels. They become more sensitive than controls to noradrenaline after chemical sympathectomy. Continued exposure to high noradrenaline can result in sympathectomy-like effects, heightening sensitivity to constricting neurotransmitters. Vasoresponses of spontaneously hypertensive rats are impaired with submaximal but not maximal hypoxia. Such a situation in multiple sclerosis patients could result in insufficient blood flow by vasoconstriction until it becomes maximal. Glutamate increase by ischemia and hyperemic release of free radicals could injure neurons, prompting an immune response to myelin proteins in susceptible people. Developmental adaptation to situations requiring lower sympathetic activity might help counteract these effects.

Macrophage prostaglandin production contributes to the age-associated decrease in T cell function which is reversed by the dietary antioxidant vitamin E

The aging process is associated with a decline in T cell-mediated immunity, including decreased interleukin (IL)-2 production and mitogen-induced T cell proliferation. Because macrophages (M phi) from old mice have higher production of prostaglandin (PG) E2 than young mice, and PGE2 has been shown to suppress T cell-mediated function, we hypothesized that increased production of PGE2 would contribute to decreased T cell function with aging and that decrease in PGE2 production by dietary antioxidants would enhance T cell-mediated function. Experiments were conducted in which combinations of purified M phi and T cells (> 95% pure) from young or old C57BL/6N1A mice were cultured together. Co-cultures containing T cells and M phi from old mice had reduced ConA-stimulated proliferation and IL-2 secretion than those consisting of T cells and M phi from young mice. Addition of M phi from old mice suppressed proliferation and IL-2 secretion by T cells from young mice. Likewise, T cells from old mice secreted more IL-2 when cultured with M phi from young mice compared to those cultured with M phi from old mice. Addition of PGE2, at concentrations produced by old M phi, decreased proliferation and IL-2 production by young but not old T cells. Neither addition of H2O2 at physiological levels, nor catalase changed the response of cultures from young or old mice. However, addition of indomethacin and the antioxidant nutrient vitamin E, both of which decreased PGE2 production, improved T cell proliferation and IL-2 production. These experiments demonstrate that increased production of PGE2 by M phi contributes to the age-associated decline in T cell function. Vitamin E improves T cell responsiveness in old mice mostly by reducing M phi PGE2 production, although a direct effect of vitamin E on T cells was also observed.

Protective effect of vitamin E on lymphocyte growth capacity during incubation in vitro

Vitamin E, an essential lipid-soluble micronutrient, plays an important role in the immune system and serves as an immunostimulant in geriatric subjects. Using an in vitro incubation to simulate aging processes, we find that vitamin E mitigates loss of growth capacity in lymphocytes. Vitamin E presence during in vitro incubation did not affect significantly the level of peroxidation, the effects of exogenous IL-2, PGE2, or indomethacin, nor levels of IL-2 production. Thus, the preservative effect on lymphocyte growth by vitamin E is not due primarily to its antioxidant function or to interleukin-2 or prostaglandin effects. The decreased growth capacity generated by in vitro incubation is accompanied by a variety of cellular alterations, including decreased CD5 surface antigen, enhanced suppression by adherent cells, and impaired communication between lymphocytes and adherent cells. The decrease in CD5 surface antigen correlates inversely with the cell density required for maximal cell proliferation, and the diminished CD5 levels were unaltered by vitamin E presence during the aging process. In contrast, protection of T-cell proliferative capacity by vitamin E in vitro correlates with diminished suppression by adherent cells and normalized interaction between lymphocytes and adherent cells.

Human lymphocytes incubated in vitro share multiple characteristics with geriatric-derived lymphocytes: a potential in vitro model for aging?

Aging involves a complicated set of parallel reactions that result in multiple cellular and organismic changes and may eventuate in chronic illness. In the immune system, several alterations that correlate with age have been established. In the present study, we report the results of incubating lymphocytes in vitro in whole blood and, employing measures known to be age-dependent, compare these cells 'aged' in vitro with cells from geriatric patients aged in vivo. Cells from blood aged in vivo and incubated in vitro share a number of common characteristics that include decreased growth capacity, shifted growth patterns, increased suppression by adherent cells, decrease in CD5 surface antigen, similar responses to addition of exogenous IL-1, IL-2, PGE2, or indomethacin, and similar production of PGE2. Differences found between in vivo aging and in vitro incubation are IL-2 plasma levels and IL-2 production by activated cells monitored in conditioned minimal medium. Based on these observations, this in vitro system provides a simple method to generate cells that exhibit a very significant subset, but not all, of the characteristics associated with in vivo aging in lymphocytes.

Effects of subchronic d-fenfluramine on splenic immune functions in young and old male and female Fischer 344 rats

The present study was designed to demonstrate age- and sex-related differences in immune functions, and to determine whether subchronic elevations in serotonin (5-HT) availability in vivo would alter immune functions assessed subsequently in vitro. Male and female F344 rats (5 and 21 months of age) were administered the 5-HT releaser and reuptake inhibitor, d-fenfluramine (d-Fen), in their drinking water for 30-38 days then killed. The young animals received a higher dose (1.8 mg/kg/day) of d-Fen than the old rats (0.6 mg/kg/day) in order to compensate for age-related decreases in drug biotransformation and clearance. Brain and spleen d-Fen and metabolite concentrations, however, were considerably higher in the young than in the old rats. d-Fen treatment did not affect body weight or fluid intake. Although substantial sex differences in immune function were not discerned, age-related decreases were observed in absolute splenic cellularity, recombinant interleukin-2 (rIL-2) stimulated natural killer (NK) cytotoxicity, LPS stimulated B-cell mitogenesis, and in the level of Ox19 (CD5) positive cells. d-Fen caused an increase in absolute spleen weight and a decrease in absolute splenic cellularity only in the old rats of both sexes. Spleen cells from young male and old female rats receiving d-Fen had relatively more large granular lymphocytes and enhanced baseline and rIL-2 activated killing of YAC-1 cells than their vehicle matched or opposite sex counterparts. The drug also increased Con A-induced T-cell proliferation in young males and LPS induced B-cell proliferation in old females. d-Fen decreased Ox39 (CD25) levels by 19%, but did not affect any of the other phenotypes examined. The results suggest that 5-HT has a selective stimulatory effect on young male and old female NK activity, and that old female rats are more sensitive to the immunological effects of d-Fen than old male rats.

Vitamin E supplementation suppresses prostaglandin E1(2) synthesis and enhances the immune response of aged mice

The potential for vitamin E to modulate prostaglandin metabolism and alter immune response in aged mice was studied. Semi-purified diets containing 30 ppm or 500 ppm dl-alpha-tocopheryl acetate (VitE) were fed for 6 weeks to young (3 months) and old (24 months) C57BL/6J mice. Delayed hypersensitivity skin test to DNFB and the proliferative response of splenocytes to T- and B-cell mitogens were assessed. Ex-vivo synthesis of Prostaglandin E2 (PGE2) was measured in spleen homogenates and serum vitamin E was measured by HPLC. Vitamin E supplementation of aged mice enhanced percent ear swelling to DNFB as well as the mitogenic response of splenocytes to Con A and LPS (P less than 0.05). Furthermore, spleen homogenates from old mice fed 30 ppm VitE had a significantly higher PGE2 level than young mice fed 30 ppm VitE and old mice fed 500 ppm VitE (3.20 +/- 0.07 micrograms/g vs. 2.60 +/- 0.08 and 2.3 +/- 0.10, respectively). Thus, the vitamin E enhanced immune response of aged mice appears to be mediated by decreased prostaglandin synthesis.

Studies on the age-related decline in the response of lymphoid cells to mitogens: measurements of concanavalin A binding and stimulation of calcium and sugar uptake in thymocytes from rats of varying ages

We have previously demonstrated that mitogenic response of rat thymocytes to concanavalin A (Con A) declines with age (Segal, Troen and Ingbar, Thymus, in press). To elucidate the mechanism underlying this process, we have examined the effect of age on Con A binding and stimulation of calcium and sugar uptake in thymocytes from rats varying in age from 10 to 360 days. Binding of Con A by thymocytes remained unchanged with advancing age. Basal uptake of the glucose analogue 2-[3H]deoxyglucose (2-DG) by rat thymocytes declined with age, becoming significantly lower than maximal values (26 days) at 4 months of age. While the proportionate increase in thymocyte 2-DG uptake produced by Con A remained essentially unchanged. However, because of the decline in basal 2-DG uptake, total uptake of 2-DG in the presence of Con A decreased with age becoming significantly lower than maximal values at 4 months. Basal calcium-45 uptake by thymocytes was practically the same in all the age-groups studied, except at 21 days, where, as with basal 2-DG uptake, it was markedly smaller, But the stimulatory effect of Con A on 45Ca uptake declined progressively with age and was nil at 360 days. From these observations I suggest that the age-related decline in the responsiveness of rat thymocytes to Con A does not result from a change in the binding of Con A by the lymphoid cell, but from, at least in part, a decrease in its cellular stimulation of calcium and sugar uptake.

Age-related changes in glucose metabolizing enzymes in spleen, thymus, and pulmonary lavage cells from F344 rats

Healthy male Fischer 344 rats were sampled at 6, 12, 18, and 24 months of age. There was no gross pathological evidence or deviations in body weight, hematology, or clinical chemistry that were indicative of disease. Mixed populations of thymus, spleen, and pulmonary cells were obtained for enzymatic analyses. Key enzymes from the hexose monophosphate shunt, glycolysis and the tricarboxylic acid cycle were evaluated to determine if there were tissue-specific or pathway-specific changes that occurred during aging. The enzyme responses among the tissues were not consistent during the aging process. Generally the activities of the glucose metabolizing enzymes in thymus and pulmonary lavage cells decreased with age whereas they increased in the spleen cells. Between 18 and 24 months enzymes representative of all three glucose metabolic pathways decreased in pulmonary lavage cells, whereas the decreases in thymic cells were mainly restricted to glycolytic enzymes. By contrast there were two- to ten-fold increases during aging in all of the splenic enzymes measured except malate dehydrogenase. The alterations in tissue enzyme activities probably reflected the changing cellular populations during aging, and in the thymic and pulmonary lavage cellular environment resulted in a loss of energy production by glucose oxidation, compared to the vigorous activity maintained in spleen.

Neuromodulation of lymphocyte reactivity in aged rats

Numerous reports indicate that both central nervous system (CNS) and immune system functions decline with age. We have previously shown that the CNS can modulate both mitogen-induced spleen cell proliferation and NK activity in young Fischer 344 rats. In the present study we have determined the effects of AHT lesions on the lymphocyte reactivity of aged Fischer 344 rats. These data show that lesions in the AHT of aged rats cannot modulate splenic mitogen responsiveness, however, NK activity is impaired. This differential effect may be due to multiple factors including enhanced splenic suppressor cell activity, the inability of the brain to send modulatory signals following lesioning, or the failure of the immune system to receive a neural signal and react to it.

Role of spontaneous interstitial cell testicular tumors on the mitogen reactivity of spleen cells from aged male Fischer-344 rats

Virtually all aged, male, Fischer-344 rats have testicular tumors. The influence of this tumor on lymphocyte reactivity from aged Fischer-344 rats is unknown. In this report we demonstrate that neither the presence of this tumor nor the serum concentration of luteinizing hormone has an effect on the splenic mitogen reactivity of old animals.

Cellular immunosenescence in F344 rats: decreased natural killer (NK) cell activity involves changes in regulatory interactions between NK cells, interferon, prostaglandin and macrophages

Natural killer (NK) activity of F344 rat spleen cells remained constant between 1 and 18 months of age under specific pathogen-free (SPF) conditions. Between 18 and 24 months of age, however, there was a dramatic decline in activity which remained at a low baseline throughout the normal lifespan. Removal of adherent cells on G-10 Sephadex columns revealed age-related changes in adherent cell regulation of NK activity. Young (4-6 week) NK activity was consistently decreased by adherent cell removal while old (24-30 month) NK activity was slightly but reproducibly increased. Moreover, splenic macrophages from old rats purified by adherence to microexudate-coated surfaces were highly suppressive to young nonadherent NK activity. A role for endogenous prostaglandin (PG) in suppressed old rat NK activity was suggested by the effectiveness of anti-PGE2 in vivo to boost old NK activity. Although old rat NK activity was boosted to a relatively greater extent by interferon (IFN) in vitro than was young NK activity, IFN-boosted NK activity of old rats was much more sensitive to PGE2 inhibition than was IFN-boosted young rat NK activity. IFN treatment in vitro or poly(I:C) treatment in vivo induced protection against PGE2 inhibition of NK activity in young rats, while no resistance to PGE2 inhibition was induced in old rat NK cells by similar treatments. In vivo, the same protocol of IFN administration which boosted young rat NK activity further suppressed old rat activity. These results support the hypothesis that immunosuppression related to aging, which supersedes the boosting effect of IFN, involves the combined effects of suppressor macrophages (via PGE2) and intrinsic changes in effector (NK) cells which render them more sensitive to PGE2 inhibition.