Immune function in aged mice. I. T-cell responsiveness using phytohaemagglutinin as a functional probe

Age-associated remodeling of T cell immunity and metabolism

Aging results in remodeling of T cell immunity and is associated with poor clinical outcomes in age-related diseases such as cancer. Among the hallmarks of aging, changes in host and cellular metabolism critically affect the development, maintenance, and function of T cells. Although metabolic perturbations impact anti-tumor T cell responses, the link between age-associated metabolic dysfunction and anti-tumor immunity remains unclear. In this review, we summarize recent advances in our understanding of aged T cell metabolism, with a focus on the bioenergetic and immunologic features of T cell subsets unique to the aging process. We also survey insights into mechanisms of metabolic T cell dysfunction in aging and discuss the impacts of aging on the efficacy of cancer immunotherapy. As the average life expectancy continues to increase, understanding the interplay between age-related metabolic reprogramming and maladaptive T cell immunity will be instrumental for the development of therapeutic strategies for older patients.

Humoral immune response and B-cell functions including immunoglobulin class switch are downregulated in aged mice and humans

Vaccinations are powerful tools for combating infections. Because of the age-related impairment in immune functions, the currently available vaccines are protecting only a small proportion of the elderly population. We, here, provide an overview of age-related changes in innate and adaptive immunity with particular emphasis to changes in antibody production with aging. We also summarize our results showing that the E2A-encoded transcription factor E47, which regulates many B cell functions including class switch recombination (CSR) and somatic hypermutation (SHM), is downregulated in splenic B cells from old mice. This leads to a reduction in the activation-induced cytidine deaminase (AID), which directly induces CSR and SHM, and, in turn, to reduced amounts of switched antibodies produced by splenic activated B cells. Our preliminary results in humans indicate similar reductions: we show herein that the expression of E2A and AID progressively decline with age. Our results provide a possible molecular basis for a decrease in the humoral immune response in aging mice and humans.

B cells, E2A, and aging

Both mouse and human exhibit deficiencies in humoral immunity during 'old age'. While alterations in phenotype and function have been well documented, the molecular mechanisms that result in immune senescence remain undefined. B lymphopoiesis is suppressed in senescent mice, which may result from deficits at the pre-B-cell stage or earlier (e.g. pro-B cells). This deficit contrasts with the maintenance of the normal number of total peripheral B lymphocytes in senescent mice. However, mature peripheral B cells in aged mice can exhibit reduced efficiencies of both immunoglobulin isotype switching and somatic hypermutation. The basic helix-loop-helix transcription factor E2A is crucial at several stages of B-lymphocyte differentiation, including the development of pro-B and pre-B cells within the bone marrow and in isotype switch and somatic hypermutation among peripheral B cells. Therefore, we have focused on the regulation of E2A expression and function during both B lymphopoiesis and isotype class switching in senescent mice. These studies show that E2A expression is normally under complex control at both post-transcriptional and post-translational levels. Alterations in E2A expression at both the B-cell precursor and mature B-cell developmental stages are hypothesized to contribute to defects in humoral immunity during senescence.

Reduced Ig Class Switch in Aged Mice Correlates with Decreased E47 and Activation-Induced Cytidine Deaminase

The capacity to class switch the IgH chain is critical to the effectiveness of humoral immune responses. We show that in vitro-stimulated splenic B cells from senescent mice are deficient in production of multiple class switch isotypes (IgG1, G2a, G3, and E), class switch recombination (CSR), and induction of the E2A-encoded transcription factor E47. E47 has previously been shown to be required for CSR, at least in part via expression of the activation-induced cytidine deaminase. Our studies show that impaired induction of E47, and subsequently activation-induced cytidine deaminase, contribute to poor CSR and production of secondary isotypes in senescence.

Effects of aging on proliferation and E47 transcription factor activity induced by different stimuli in murine splenic B cells

In the present paper, we have investigated the effects of aging on the expression and function of the E2A-encoded transcription factor E47 in splenic B lymphocytes, unactivated or activated with different stimuli (LPS, anti-CD40, anti-IgM, alone or in combination with IL-4). Results indicate that unstimulated splenic B cells show very low E47 protein levels as well as E47 DNA-binding activity and that, upon B cell activation, E47 expression and DNA-binding activity are strongly induced in young and, to a significantly lesser extent, in old mice. The level of E47 protein expression in stimulated splenic B cells was found significantly higher in young than in old mice, suggesting that DNA-binding activity correlates with protein expression. These results altogether suggest that the reduced expression of the transcriptional regulator E47 could help explain the reduced B cell functions in aging mice.

The effect of taurine on the incorporation of thymidine by chick B cells

1. This study was conducted to determine the effect of extracellular taurine on chick B cell proliferation as measured by the incorporation of [3H] thymidine. 2. B cells were cultured in RPMI-1640 media containing bacterial lipopolysaccharide and either 0 or 100 microM taurine. 3. Thymidine incorporation was significantly increased in the presence of 100 microM extracellular taurine. 4. These results suggest a role for taurine in the chick B cell proliferation process.

The effect of age, sex and breeding on graft versus host reactivity of spleen cells from C57BL mice

Spleen cell graft versus host (GvH) reactivity was determined in male and female, either virgin or breeder, C57BL mice from 3 to 24 months of age. The GvH reaction was assessed by a local popliteal lymph node assay and by a splenomegaly test for a systemic reaction. Although the GvH reactivity declines progressively with age in both sexes the virgin female response was greater than that of males throughout the period of 6-18 months of age. Two-year-old mice of both sexes were practically unable to mount a GvH reaction. No differences were evident in aging female reactivity after one or two syngeneic pregnancies. On the other hand, 3 or more consecutive pregnancies resulted in enhanced GvH reactivity of 12-24-month-old females, which responded comparably to young virgin mice. This long-lasting immunopotentiating effect of multiparity was similar after 3-4 and 8-9 pregnancies. The possible role of developing foetuses on the maintenance of high GvH reactivity in breeder females is suggested.

Lymphoproliferative responses in diet-restricted and aging Sprague-Dawley rats

Immunological effects of aging and dietary restriction (i.e., chronic underfeeding without malnutrition) were investigated in male CFY Sprague-Dawley rats. From weaning, diet-restricted animals were given the amount of food that maintained their body weights at approximately 50% of age-matched ad libitum controls. Cells from the spleen, Peyer's patches, brachial/axillary lymph nodes, and mesenteric nodes of 12- and 20-month-old rats were tested in vitro against the T-cell mitogens phytohaemagglutinin and concanavalin A, and the B-cell mitogen Salmonella minnesota lipopolysaccharide (with dextran sulphate). No clear boosting effect of dietary restriction nor age-related decline in T- or B-cell response was observed when a standard foetal bovine serum supplement was used. In general however, serum from diet-restricted rats supported better proliferative responses than serum from age-matched controls, suggesting that dietary restriction may promote lymphocyte proliferation by an indirect mechanism. A feature of this study was the variation in phenotype and immune responsiveness among closely related animals. Thus in an outbred population, any beneficial effects of dietary restriction upon immune responses could be outweighed by variation among individuals.

The effect of taurine on the age-related decline of the immune response in mice: the restorative effect on the T cell proliferative response to costimulation with ionomycin and phorbol myristate acetate

1. Proliferative responses to the costimulation with phorbol-12-myristate-13-acetate (PMA) and suboptimal doses of ionomycin in the purified T and B cells from old mice were lower than those from young mice. 2. The degree of the age-related decline was more significant in T cells than in B cells. 3. Taurine, a sulfur containing amino acid, augmented the proliferative responses of T cells from both young and old mice. 4. The augmentation of the proliferative response by taurine was more marked in old T cells than in young T cells. 5. The concentration of intracellular free calcium ion ([Ca2+]i) was significantly lower in old T cells when stimulated with PMA and ionomycin than observed in young T cells. 6. In the presence of taurine, the concentration of [Ca2+]i in the old T cells significantly increased under stimulation by PMA and ionomycin. 7. The results indicate that taurine improved the proliferative response in old T cells by restoration of the increment of the concentration of [Ca2+]i under the stimulation by PMA and ionomycin.

A decreased functional capacity of CD4+ T cells underlies the impaired DTH reactivity in old mice

The data presented in this paper show that the in vivo delayed-type-hypersensitivity (DTH) reaction to both H-2 and non-H-2 alloantigens declines with increasing age. It is also shown that cells generated in vitro are capable to transfer DTH to young naive syngeneic recipients. Using this in vitro system it could be demonstrated that cells from old CBA/Rij mice induced lower DTH responses than cells from young CBA/Rij mice. Depletion experiments with the effector T cell population showed that the DTH effector phase is mediated by CD4+ T cells. Lower responses in old mice were not due to increased CD8+ suppressor T cell activity, since after removal of the CD8+ T cells old CD4+ cells were still less effective in the generation of DTH effector T cells than young CD4+ cells. Addition of IL-2 containing supernatant to in vitro cultures did not improve the subsequent DTH response. From these data it can be concluded that the reduced DTH responses in old mice are not solely due to CD8+ suppressor cell activity and/or lack of IL-2, but that rather intrinsic defects of the CD4+ T cell population appear to play a major role in the impaired DTH reactivity during ageing.

Differences in the pattern of proliferative response with age in thymocytes undergoing spontaneous and induced proliferation

The proliferative capacity of thymocytes from C3H/HeJ mice decrease as the animals attain maturity. The proliferative response of thymocytes from 24- to 28-week-old mice to stimulation with concanavalin A (Con A) is only 20% of that observed at 4 weeks of age. The decreased proliferative capacity of thymocytes in response to Con A stimulation observed between 4 and 24 weeks of age closely correlates to the drop in thymic weight and cellularity observed during this period. In contrast, the spontaneous proliferative capacity of thymocytes, as well as proliferation of thymocytes in response to stimulation with phorbol myristate acetate (PMA) and ionomycin, drops only slightly during this period, as proliferation under these condition in thymocytes from 24- to 28-week-old mice is approximately 65-70% of that observed in 4-week-old animals. We have previously shown that cytoplasmic extracts from proliferating lymphoid cells contain a factor, termed the activator of DNA replication (ADR), which is capable of inducing DNA synthesis in isolated, quiescent nuclei. We show in this study that the decreased proliferative capacity of thymocytes during whole organism maturation and thymic involution is associated with decreased endogenous levels of ADR, while nuclear sensitivity of thymocyte to ADR was retained during these process. The diminution of ADR activity during thymic involution was quantitatively greater than the loss in proliferative capacity.

Decline in the production of interleukin-3 with age in mice

Previously, we and others have found that the ability to produce interleukin-1 (IL-1) and interleukin-2 (IL-2) declines with age in mice. The purpose of this study was to determine the influence of age on the capacity of mice to produce interleukin-3 (IL-3). Splenic cells (5 X 10(6)/ml) from young (3-4 months) and old (24-32 months) C57BL/6 mice were first assessed for their IL-3-producing capacities in response to varying doses of concanavalin A (Con A; 2-20 micrograms/ml) in a time-dependent manner. The results showed that the production of IL-3 by both young and old C57BL/6 mice was maximal on Days 3 and 4 in response to 20 micrograms/ml of Con A, and that of IL-2 was minimal (activity was less than 0.1 unit) on Day 4. Consequently, Day 4, was selected to assess the effect of age on IL-3 production by splenic cells. The results showed a twofold reduction in IL-3 production with age (P less than 0.05). Young-old splenic cell mixture experiments at ratios of 1:0, 3:1, 1:1, 1:3, and 0:1 indicated that the decrease in IL-3 production with age was not due to an increase in suppressor cell activity. Experiments based on mixtures of nylon wool-enriched splenic T-cell and adherent cells and on anti-MAC-1 plus complement-treated spleen cells indicated that (a) adherent cells are not required for T-cell production of IL-3, unlike IL-2 production, and (b) the decrease in IL-3 production with age is due solely to alteration in IL-3-producing T cells. Finally, a strong correlation was demonstrated between the production of IL-2 and IL-3 by spleen cells of individual young and old mice (r = 0.92, P less than 0.01). That production of both IL-2 and IL-3 is affected in a similar manner by age would suggest that a single class of helper T cells may be responsible for production of both lymphokines.

Phytohemagglutinin induced proliferation by aged lymphocytes: reduced expression of high affinity interleukin-2 receptors and interleukin-2 secretion

Human lymphocytes from elderly and young donors were cultured with phytohemagglutinin. Cultures from two groups of aged donors, recruited respectively from our ambulatory clinic and a nursing home, incorporated less tritiated thymidine (3H-TdR) and secreted less interleukin-2 than did young donors. Furthermore, as determined for the first time by a radioligand binding receptor assay, the aged lymphoblasts possessed significantly fewer high affinity IL-2 receptors per cell. Despite a decrease in the number of high affinity receptor cells the dissociation constant (Kd) was comparable for the three groups. It was also shown that the amounts of soluble IL-2 receptors that were released into the supernatants by mitogen stimulated cells did not differ for the aged and young donors. These data suggest that defects in IL-2 production and high affinity IL-2 receptor generation may both be responsible for immune deficiency in the elderly.

Biomarkers of aging: immunological events

Certain immune functions decline in a gradual way over the life span. To the extent that these changes are well characterized and are phylogenetically observed, they may be considered excellent biomarkers of aging. Many of these age-related changes, however, are significantly influenced by age-associated phenomena such as certain diseases or psychosocial factors. Immunologic factors, therefore, should be considered as markers of physiologic aging only under very careful experimental conditions.

Immunoregulatory defects in murine aged Peyer's patches

Aging effect on immunoregulatory processes in Peyer's patches (PP) from BALB/c mice of 3-4 months and 24-28 months was studied in vitro. The magnitude of isotype-specific Ig production by aged or young PP B cells in co-culture with young or old fresh PP T cells strongly suggested that the suppressor activity of PP T cells was impaired in aged mice. Further studies on in vitro induction of concanavalin A-activated T helper cells, T suppressor-inducer cells and T suppressor cells from aged PP indicated that the generation of T suppressor cells was largely impaired, in contrast to a minor defect(s) in that of T helper cells. The findings obtained here at least suggest that in aged PP, a T suppressor-inducer cell subset appears to be more selectively impaired in the aging process than the other lymphocyte subpopulations, which possess minor intrinsic functional defects. Thus, these abnormal T and B cell responses in PP could be responsible for the senescence-associated gut mucosal immunologic dysfunction.

Effects of tobacco glycoprotein (TGP) on the immune system. III. The effect of aging on the mitogenic response of human peripheral blood lymphocytes to TGP

We have shown that the mitogenic response of human peripheral blood lymphocytes (PBL) to tobacco glycoprotein (TGP), a glycoprotein rich in rutin or rutinlike polyphenol moieties, which is isolated from cured tobacco leaves, does not decrease with age. In contrast, the proliferative response to lipopolysaccharide (LPS) tends to decline with age, and a significant decrease is observed in the mitogenic response to rutin-bovine serum albumin (R-BSA). LPS and R-BSA are similar in some aspects to TGP, the former in that TGP and LPS are both T-independent B-cell mitogens for mice and are both highly negatively charged, and the latter in that covalently bound polyphenol groups are present on both R-BSA and TGP. Although the kinetics of the mitogenic responses to these three mitogens are similar (T. Francus, R. F. Klein, L. Staiano-Coico, G. W. Siskind, and C. G. Becker, Effects of tobacco glycoprotein (TGP) on the immune system. II. TGP is a mitogen for human peripheral blood lymphocytes. Submitted for publication.), multiple regression analyses show no correlation in the mitogenic responses of PBL from young donors to TGP and LPS, to TGP and R-BSA, or to LPS and R-BSA. In contrast, there are significant correlations between the proliferative responses to these mitogens by PBL from old donors. The results suggest that only a small subpopulation of the cells which are stimulated by R-BSA and LPS are not altered with age, and these are most likely the cells that are stimulated by the three mitogens studied.

Expression of intracellular biochemical defects of lymphocytes in aging: proposal of a general aging mechanism which is not cell-specific

There is a decline in immune capacity with age which is expressed on the organismic level by association with numerous immune-related diseases, on the cellular level by impaired mitogenesis, on the biochemical level by impaired metabolic pathways, and on the molecular level by decreased protein synthesis and degradation. Defects in various cofactors such as calcium and several nucleotides also occur and may be related to the impaired enzyme function during mitogenesis in the aged. The central cause for decreased mitogenesis in the aged could be a decrease in protein synthesis which appears to cause impaired enzyme induction. This impaired enzyme induction accounts in part for the decreased glycolytic flux and DNA synthesis in these cells. Decreased protein synthesis also has been associated with a decreased synthesis of lymphokines which help these cells to proliferate. Numerous other intracellular age-related defects of lymphocytes also occur which may collectively play important interdependent roles in the impaired lymphocyte function of the aged. A potential general underlying mechanism of cellular senescence is proposed based on a genetic "slowing-cycle" effect of transcription, translation, and enzyme induction with immunosenescence presented as an example of an expression of these basic defects.

Interleukin-2 production and activity in aged humans

Peripheral blood mononuclear cells (PBMC) from aged humans were found impaired in their ability to mount T mitogen induced proliferative responses and NK cell activity. The production of IL-2 in response to PHA has also been found decreased in the aged subjects. When the data from IL-2 production and cellular responsiveness of aged and young groups were pooled for regression analysis, a significant correlation was found both for mitogenic responses and NK activity. The reduced proliferative responses and NK cell activity can be enhanced by exposing them to exogenous IL-2. However, they were not fully reconstituted to the levels achieved in PBMC of the young subjects supplemented in vitro with IL-2. This would indicate that the reduced cellular responsiveness in aging may be related both to a defect of IL-2 receptor expression on T cells of aged subjects and to an impairment in the endogenous IL-2 synthesis in the aged individuals.

Carbohydrate metabolism in transforming lymphocytes from the aged

There is an age-related decline in immune capacity which has been linked to a decreased response of lymphocytes to mitogens in vitro. During transformation, lymphocytes require a marked increase in energy production and biosynthesis which is supplied primarily by glycolysis. In the elderly, the glycolytic enzymes increase significantly in transforming lymphocytes at least 24 hr later than in the young and then at significantly reduced levels. Glucose utilization is also impaired in stimulated lymphocytes from the elderly but follows the impairment of glycolysis. In stimulated cells from the young, increases in glycolytic enzyme activity levels accompany sharp increases in blastogenesis while a delayed increase in glycolytic enzyme activity in the elderly is accompanied by a delay in blastogenesis. Maximal glycolytic enzyme activity levels are significantly reduced in transformed lymphocytes from the elderly though the number of transformed cells is also significantly reduced. However, glycolytic enzyme activity levels are significantly lower in the elderly than in the young even on a per transformed cell basis. Thus, this reduction cannot be attributed to the lower number of transformed cells that are present in the elderly. This defect in the increase of glycolysis in stimulated cells from the elderly suggests an intracellular mechanism which could be related to the impaired lymphocyte stimulation in vitro in the aged.

Acquired immunological tolerance in aged mice. II. The cellular basis of the loss of tolerance sensitivity

Aged C57Bl/6 mice (12 and 24 month) become resistant to tolerance induction with deaggregated human IgG (DHGG). The cellular basis for this tolerance resistance was investigated using adoptive transfer systems and challenged with endotoxin and antigen in the form of heat aggregated HGG. DHGG induced antigen specific suppressor cells in the spleens of aged mice although not in young mice and aged normal splenocytes were more sensitive to suppression. Low dose cyclophosphamide treatment to ablate suppressor cells did not curtail tolerance induction in aged animals. These observations coupled with the transient nature of B cell tolerance in aged mice suggest that the tolerance defect of senescence lies in the B cell compartment.

Changes in the macrophage function with aging

Changes in the adherence, opsonization, phagocytosis, nitroblue tetrazolium reduction and antibody-dependent cell cytotoxicity of peritoneal macrophages were observed with aging in BALB/C mice. A decrease in all the parameters studied, except for the reduction capacity of nitroblue tetrazolium, which increased with aging, was found in the long-lived mice. The role of the macrophage in immune hyporesponsiveness with aging is discussed.

Interleukin-1 synthesis and activity in aged mice

Recent studies have provided evidence that deficient interleukin-2 (IL-2) production by helper T cells contributes to the impaired T-cell-mediated functions observed in aged mice. Since most of these responses depend upon the presence of macrophages, a deficit in the functional capacity or in cell cooperation of macrophages may result in a decrease in immune reactivity. We found in the present study, that in vitro the cytostatic activity of macrophages from aged C57BL/6 (B6) mice is affected only slightly, but that in vivo their number increases with age. The synthesis of IL-1 is reduced when macrophages from aged mice are stimulated in vitro by lipopolysaccharide, but addition of exogenous IL-1 apparently does not restore either the mixed lymphocyte reaction or cytotoxic T lymphocyte generation. Co-cultures of young splenic macrophages with aged T lymphocytes do not restore to normal level the impaired proliferative response to T mitogens of aged B6 mice, but aged splenic macrophages provide a full accessory help for mitogenesis of young T cells. Thus, absorption of IL-1 by phytohemagglutinin-activated T cells is slightly altered in aged mice. IL-2 responsive T cells are not altered since exogenous IL-2 supply in vitro completely reconstitutes cytotoxic T lymphocyte generation after an allogeneic stimulation. Moreover, the number of Lyt 1+ cells is not modified in aged B6 mice. These results suggest that the impaired capacity of macrophages to release IL-1 and of blast T cells to bind IL-1 may contribute to the depression of cell-mediated immune reactivity associated with aging but also that the main defect is a functional lesion of IL-2 production by Lyt 1+ helper T cells.

Effect of levamisole on cytotoxic T-cell-mediated immune resistance to L1210 murine leukemia in hyperimmune mice

The effect of levamisole (LMS) on T-cell-mediated antitumor immunity was examined in adult and aged mice hyperimmune to L1210 leukemia. The immune resistance of aged mice was depressed compared with that of adult mice, which almost completely rejected 5 X 10(7) L1210 cells inoculated IP. A significant level of tumor-specific cytotoxicity was detected in the spleen cells of adult hyperimmune mice by the 51Cr-release assay after in vitro sensitization with mitomycin C-treated L1210 cells. This was mediated by cytotoxic T cells, since in vivo administration of antithymocyte serum or in vitro treatment of the spleen cells with anti-Thy 1.2 antibody and complement abrogated the cytotoxicity completely. In aged mice, however, cytotoxic T-cell activity was lower although the animals were immune to L1210. Administration of LMS (0.38 mg/kg) restored the depressed cytotoxicity of aged mice to the level seen in adult mice. Furthermore, in adult hyperimmune mice LMS augmented T-cell-mediated cytotoxic activity and restored the reduced cytotoxicity caused by in vivo administration of antithymocyte serum. These results indicate that LMS was effective in augmenting T-cell-mediated tumor immunity in immunologically competent or deficient hosts.

Enhancement of the response of ageing mouse lymphocytes by in vitro treatment with lecithin

The effect of treatment with phosphatidylcholine (lecithin) has been examined on splenocytes from young (6 months) and aged (greater than 20 months) (C3H/eB X C57BL/6)F1 male mice, by probing their responsiveness to proliferative signals of mitogens and mixed lymphocyte reaction. This study was initiated since old mice have been known to manifest an increased ratio of cholesterol: phospholipids (C/PL) in their plasma membranes. Unlike the case in young mice, enhancement in proliferative responses was demonstrated on cells from the old after incubation with lecithin. This enhancement was achieved using a variety of methods by which lymphocytes were exposed to the lipids, and which were all designed to reduce the C/PL ratio in the plasma membrane. The observed differences in response to lecithin treatment between young and old did not stem from different extents of lipid incorporation, and seems unlikely to be a result of modified binding of the mitogens after treatment with the lipid. The data suggest that the normal functional performance of specific reactions, which decline with age, may be restored artificially at an appropriate age, if other complementary functions have not been damaged.

Age-associated impairment of murine natural killer activity

Natural cell-mediated cytotoxicity may be critical in the resistance displayed by animals and humans to tumors and various pathogenic microorganisms. Because the frequency of tumors and infections increases markedly in aging populations, we have compared the natural killer (NK) competence of lymphoid tissues (spleen, bone marrow) and of peritoneal cells of young adult and aged mice. Spontaneous NK activity was much lower, and the rate of target cell lysis was much less, in aged mice. The level of NK activity was only modestly increased in old, compared to young, mice when they were exposed to Trypanosoma musculi, an organism that provides strong stimulation of NK activity. Restricted NK activity of aged mice was not attributable to suppressor cells. The NK effector cells in old mice were characterized as being nonadherent to plastic, completely susceptible to lysis by complement plus an antiserum against specificity NK 1.2, and only slightly affected by treatment with antiserum against specificity Thy-1.2. Two indirect methods were employed to assess the relative frequency of splenic NK cells at the time of maximal stimulation by T. musculi: a cytotoxicity assay with antiserum against NK 1.2 and a binding assay involving monolayers of YAC-1 tumor target cells. Similar results were obtained in both assays, indicating that at maximal stimulation about 10% of the total spleen cells of both young and old mice were NK cells. We conclude (cautiously) that the functional efficiency of aged NK cells is impaired and that this defect may account, in part, for reduced ability of aged individuals to resist certain types of cancer and certain pathogenic microorganisms.

Production of auto-antiidiotypic antibody during the normal immune response. VII. Analysis of the cellular basis for the increased auto-antiidiotype antibody production by aged mice

We have previously shown that old mice produce more hapten-augmentable plaque-forming cells (PFC) than do young animals, suggesting a greater auto-antiidiotype antibody (auto anti-Id) component in their immune response. In the present studies this is confirmed serologically. The marked auto-anti-Id response of aged mice can be transferred to lethally irradiated young recipients with spleen but not bone marrow cells from old donors, suggesting that it is an intrinsic property of their peripheral B cell population and that the distribution of Id arising from the bone marrow of old and young mice is similar. In contrast with young mice the auto-anti-Id response of old animals is relatively T cell-independent and old donors do not show an increase in their ability to transfer an auto-anti-Id response after priming with TNP-F. These observations suggest that old mice behave as if already primed for auto-anti-Id production. Irradiated mice reconstituted with bone marrow cells from either young or old donors together with splenic T cells from old donors generate a relatively large auto-anti-Id response, whereas mice reconstituted with bone marrow from either young or old donors together with splenic T cells from young donors produce few hapten-augmentable PFC. It is suggested that differences in Id expression and auto-anti-Id production are the consequences of the interaction of Id (and anti-Id) arising from the marrow with anti-Id (and Id) present in the peripheral T cell population which serves as a repository of information about shifts in Id distribution, resulting from lifelong interactions with environmental and self-antigens.

Aging of natural and acquired immunity of mice. II. Decreased T cell responses to syngeneic tumor cells and parental-strain spleen cells

Cytolytic T lymphocyte (CTL) functions were compared in mice between the ages of 2 and 30 months. The stimulator cells were H-2 allogeneic spleen or tumor cells, parental-strain spleen cells, or syngeneic tumor cells. Effector cells capable of lysing syngeneic tumor cells were shown to be T cells and not NK cells. The cell-mediated lympholysis (CML) responses by spleen cells of aged mice were near normal against H-2 allogeneic spleen or tumor cells but were defective against syngeneic tumor cells or parental-strain spleen cells. The defective syngeneic tumor CML response was observed at various responder:stimulator ratios and at various days of incubation. The defect was in the nonadherent, and not in the adherent, fraction of spleen cells. Suppressor cells were detected in spleens of 30 month, but not of 18 month old mice. Aged mice were more susceptible than young mice to small inocula of syngeneic C57BL EL-4 lymphoma cells. The immunogenicity of irradiated spleen cells of old mice had not changed for the F1 antiparent CML response. Splenic CML responses of young mice treated with 89Sr demonstrated a similar pattern, i.e., good responses to H-2 allogeneic stimulator cells but poor responses to syngeneic tumor cells or to parental-strain spleen cells. This loss of certain CTL functions influenced by marrow dependent cells can partially explain the increased susceptibility of old animals to tumors.

Biochemical events associated with lymphocyte activation in aging. K+ transport and sensitivity of the Na+-K+ pump to digoxine

The present investigation was directed toward answering the question of whether some age-related changes of membrane dependent triggering mechanisms during lymphocyte activation could account for the depressed T cell response to mitogens in aging. For this purpose, the K+ movements were analyzed in PHA-stimulated peripheral blood lymphocytes (PHA-PBL) from old humans (O) compared to adult (A). Indeed, plasma membrane Na+, K+, ATPase activation plays an essential role in cell proliferation and results from direct interaction between the loaded mitogen receptor and the enzyme. No difference could be found in the magnitude and the timing of the PHA-induced increase of K+ fluxes between PHA-PBL from O and A despite a higher K+ inflow in unstimulated but 20-hour preincubated PBL from O. Further experiments showed that the lectin-induced triggering mechanism of cation transport resulted from digoxine (DGX: a glycosid cardiotonic) sensitive ATPase. Moreover, whereas PBL from O exhibited a decreased PHA-induced DNA synthesis, DGX depressed the thymidine incorporation by 72-hour cultured PHA-PBL within the same inhibitory dose-related pattern in both O and A. We conclude that the triggering mechanism of Na+, K+-ATPase induced by PHA occurs adequately in early stimulated PBL from old subjects. In addition, digoxine sensitive structures work freely during PHA-induced lymphocyte proliferation in aging, thereby supporting further arguments for adequate Na+, K+-ATPase activity.

Antibody-specific immunoregulation and the immunodeficiency of aging

Experiments were carried out to assess the role of naturally acquired antibody-specific immunoregulation in the immunodeficiency of aged individuals. It was found that greater than 50% of the primary dinitrophenyl-specific BALB/c B cells did not respond in carrier-primed 2-yr-old BALB/c adoptive hosts as compared with similarly primed younger recipients. Similar suppression was observed in carrier-primed younger BALB/c mice that had received 4 x 10(7) spleen cells from 2-yr-old BALB/c mice, as opposed to those that had received 4 x 10(7) spleen cells from younger mice. This diminution in responsiveness was noted only for syngeneic BALB/c B cells because B cells of strains differing from BALB/c in the heavy chain allotype-idiotype locus were not suppressed. These findings indicate that old, but not young, mice had developed the capacity to suppress primary B cells bearing receptors expressing much of the syngeneic antibody repertoire. This suppression may play an important causative role in the relatively poor humoral immune responsiveness of aged individuals.

Age-related changes of mitogen responsiveness in different lymphoid organs from outbred NMRI mice

Lymphocytes from spleen, thymus and lymph nodes from individual young adult (3--4 months) and aged (26--30 months) NMRI mice were stimulated with the mitogens Con A, PHA and LPS. 24 hours later, the number of cell with increased RNA-content (G1 cells) was determined by cytofluorometry. In parallel the 3H-thymidine incorporation after 48 hours was measured for the same cell samples. Aged animals in average produced less G1 cells and incorporated less 3H-thymidine as compared to young adults. By calculating the 3H-thymidine incorporation per G1 cell, the proliferative capacity of mitogen-induced G1 cells can be estimated. These ratios are lower in aged mice as compared to young adult, suggesting that in these animals not only less cells can be activated as measured by cytofluorometry, but also from these activated cells again fewer continue the cell cycle by initiating DNA-synthesis. In response to Con A and PHA, aged mice in average produce less G1 cells in all of the three lymphoid organs tested. In response to LPS, however, the young adult produced only few G1 cells in lymph nodes and practically none in thymus, whereas in aged animals a considerable number of G1 cells was found in both organs. Corresponding results were found for the 3H-thymidine incorporation. These results indicate that in addition to the reduction of the mitogen-response an age-related change in the distribution of mitogen-responsive cells in the different lymphoid organs takes place.

Immunological studies of aging. Decreased production of and response to T cell growth factor by lymphocytes from aged humans

Human lymphocytes from elderly and young donors were cultured with phytohemagglutinin (PHA) or concanavalin A. Cultures from old donors produced less T cell growth factor (TCGF) and incorporated less tritiated thymidine (3H-Tdr) than did similar cultures from young donors in the presence of either mitogen. Furthermore, the response of lymphocytes from elderly donors to TCGF was impaired. Thus, PHA-activated T cells from aged donors showed no increase tritiated thymidine incorporation when incubated with exogenous human TCGF. In contrast, addition of exogenous human TCGF to PHA-activated peripheral blood leukocytes from younger individuals increased tritiated thymidine incorporation by 30-50%. The impaired response to TCGF was associated with decreased binding of TCGF by PHA-activated cells from old donors. TCGF production or responsiveness was not associated with the presence of "suppressor" activity in elderly T cell preparations. These studies suggest a possible molecular mechanism for the impaired proliferative response of elderly human T cells. These data lend support to the hypothesis that defects in the capacity to either produce or respond to TCGF may be a fundamental cause of immune deficiency.

Production of auto-anti-idiotypic antibody during the normal immune response: changes in the auto-anti-idiotypic antibody response and the idiotype repertoire associated with aging

Hapten-augmentable plaque-forming cells (PFC) are cells whose secretion of antibody is specifically inhibited by surface-bound auto-anti-iodotype antibody that can be displaced by hapten. This study showed that the percentage of hapten-augmentable PFC present in mice during the primary response to trinitrophenylated Ficoll (TNP-F) increases with age. The data suggest that there is a relative increase in the auto-anti-idiotypic antibody response with age and therefore a greater down-regulation of antibody production. The effect of age on idiotype expression was also studied. Hapten-reversible inhibition of plaque formation was used as an assay for anti-inhibition of plaque formation was used as an assay for anti-idiotype antibody and idiotype-bearing antibody-secreting cells. Sera from aged (21- to 22-month-old) C57BL/6 mice immunized with TNP-F significantly inhibited plaque formation, in a hapten-reversible manner, by spleen cells from 81% of TNP-F-immunized aged mice. However, these sera inhibited plaque formation by cells from only 50% of similarly immunized young adult (6- to 8-week-old) mice and 20% of immature (3- to 4-week-old) syngeneic mice. Similarly, sera from TNP-F-immunized young adult or immature mice inhibited formation of plaques by cells from immunized donors of the same age as the mice from whom the serum was obtained, but only rarely inhibited plaque formation by cells from mice of other age groups. The data thus suggest that the repertoire of TNP-specific idiotypes that are produced in response to TNP-F varies with age in syngeneic mice.