Autoimmunity, histocompatibility, and aging

Immunostimulatory activity of prothymosin-alpha in senescence

In this study we evaluated the immunorestorative activity of prothymosin-alpha (ProT-alpha) in senescence. Aged rats were repeatedly injected with ProT-alpha after antigen challenge. Both humoral and cell-mediated immune reactions were tested. The results show that ProT-alpha exerted a marked immunopotentiating effect in aged rats. Moreover, ProT-alpha induced enhancement of PFC response, and skin hypersensitivity reactions were more pronounced in senescent rats than in young adult controls. ProT-alpha treatment increased thymus and spleen weights in aged rats, and induced qualitative and quantitative improvements of the cellular make-up of the thymus and thymus-dependent areas of the spleen. The results imply that "restoring" thymus endocrine function by exogenous administration of ProT-alpha may improve immune system homeostasis in senescence.

Effect of aging on the autoimmune response to rat male accessory glands: deficit of I-E-positive peritoneal cells capable of inducing suppression

The present report analyzes the ability to induce suppression to rat male accessory gland (RAG) autoantigens and the characteristics of T suppressor (Ts)-inducer peritoneal cells (PC) in old rats which show increased autoimmune responses. The injection of young rats with a purified fraction (FI) of RAG 10 and 3 days prior to immunization with chemically modified RAG (MRAG) markedly reduced the immune response to RAG autoantigens when compared with young rats which had only been immunized (controls), while the pretreatment of old rats did not block the delayed-type hypersensitivity reaction to MRAG when compared with control old rats. The study of cell surface markers on PC from rats injected i.p. 2 h previously with FI of RAG (FI-PC) showed an increase of OX-6 (I-A) and a decrease of OX-17 (I-E) in FI-PC of old rats with respect to FI-PC of young animals, which showed a selective increase of I-E+ Ts-inducer PC. The i.p. injection of FI-PC from old rats into young recipients, 10 and 3 days prior to immunization with MRAG in complete Freund's adjuvant, did not modify the autoimmune response when compared with controls. By contrast, the injection of young and old rats with FI-PC from young animals induced a significant suppression of the autoimmune response. The reduced percentage of I-E+ suppressor-inducer PC provides an explanation for the diminished ability to induce suppression to RAG autoantigens in old rats.

Macrophages but not B cells from aged mice are defective in stimulating autoreactive T cells in vitro

In the present study the effect of aging on the capacity of Ia+ cells to stimulate autoreactive T cells in the syngeneic mixed lymphocyte reaction (SMLR) was investigated. Using young CD4+ T cells as responders, it was observed that unseparated whole spleen cells from aged mice had normal stimulatory activity comparable to that of young spleen cells. Interestingly, however, when purified splenic adherent cells (SAC) enriched for macrophages or splenic B cells were used as stimulators, aged SAC but not aged B cells were found to be defective in stimulating autoreactive T cells. This defect in aged SAC was not due to decreased expression of Ia antigens since the percentage of Ia+ SAC and density of Ia antigen expression was similar in both young and old mice. Also, the B cells from aged mice expressed normal levels of Ia antigens. Aged SAC, when mixed with young SAC could also actively suppress the normal SMLR. However, this suppression was not due to increased prostaglandin production but was found to be associated with interleukin-1 (IL-1) regulation, inasmuch as addition of exogenous IL-1 could completely reconstitute the defective stimulatory activity of aged SAC and also abolished the suppressor activity of the SAC. Aged mice also demonstrated an intrinsic defect in the CD4+ T cells responding in the SMLR. Together, our studies on the SMLR demonstrate an age-related defect in responder autoreactive T cells and in stimulator splenic macrophages but not in the stimulatory activity of B cells.

Age-dependent LH and FSH effect on the proliferation of women's peripheral blood lymphocytes in vitro

Pituitary gonadotropic hormones (LH and FSH) were found to induce an age-dependent proliferation of women's peripheral blood lymphocytes (PBL). Results showed that PBL of elderly women gave a higher gonadotropic response than those of younger donors and that the number of responders to the mitogenic stimulus of the hormones was always more important in older than in younger women. A negative correlation between the mitogenic effect of FSH (10(-9) g/ml) and the level of plasma concentration of some steroid hormones (17-beta-estradiol or both 17-beta-estradiol and progesterone) was observed in younger donors. It was also found that physiological concentrations of LH and FSH can either increase or decrease the Concanavalin A (Con A)-induced proliferation in vitro of PBL taken from both young or postmenopausal women. In certain elderly women (4/9) a synergistic effect of Con A and LH, giving rise to high levels of thymidine incorporation similar to those achieved by Con A-stimulated PBL of young women, was observed. The possible physiological significance of these results is discussed.

A specific defect in the proliferative capacity of B cells from old mice stimulated with autoreactive T cells

B lymphocytes from aged mice were found to be defective in their ability to proliferate in response to stimulation with an autoreactive T cell clone D1.4. The differentiative response leading to antibody secretion was also impaired in the auto D1.4 T cell-stimulated B cells from old mice in comparison to similarly stimulated B cells from young mice. The B cells from old mice were competent in activating the autoreactive T cells such that the T cells were induced to proliferate. The B cell defect appears to be restricted to a certain phase of B cell activation, since old mouse B cells responded to the auto D1.4 T cells by increasing cell surface Ia as well as size, but failed to incorporate tritiated thymidine. The responsiveness to interleukin-4 was found to be similar between B cells from young and old mice. It appeared that the B cells from old mice are specifically defective in progressing from the G0 phase of cell cycle into the S phase when stimulated with the auto D1.4 T cells.

Impaired autoreactive T cell-induced T cell-T cell interaction in aged mice

Self-Ia-reactive (autoreactive) L3T4+ T cell clones have been shown earlier to stimulate the proliferation of syngeneic naive L3T4+ T cells and initiate a T cell-T cell (T-T) interaction leading to the generation of immunoregulatory circuits. Since aging has been shown to be associated with a decline of the immune responsiveness, age-related alterations in the T-T interaction was investigated in the present study. Using several I-Ed-specific autoreactive T cell clones isolated from 2- to 3-month-old (young) DBA/2 mice as stimulators, it was observed that L3T4+ T cells from 22- to 24-month-old (aged) DBA/2 mice, failed to demonstrate a significant response to the autoreactive T cells. In contrast, L3T4+ T cells from young mice responded strongly to the autoreactive T cell clones. The deficient T-T cell interaction in aged mice correlated with an impaired syngeneic mixed lymphocyte reaction in these mice, thereby suggesting that aging induces a defect both in the autoreactive T cells and in T cells which react with the autoreactive T cells. When exogenous recombinant interleukin 2 (rIL-2), recombinant interleukin 4 (rIL-4), or a combination of these was added to the interaction, it was observed that rIL-4 but not rIL-2 enhanced the T-T interaction in young mice. However, rIL-4 or a combination of rIL-2 and rIL-4 failed to correct the defective T-T interaction in aged mice. Since the T cell network is believed to play an important role in the maintenance of normal immune system homeostasis, the present study suggests that age-related alterations in T and B cell functions and increased susceptibility to autoimmune diseases with age may result from a defect in the T cell network regulation.

Dysdifferentiative nature of aging: age-dependent expression of mouse mammary tumor virus and casein genes in brain and liver tissues of the C57BL/6J mouse strain

Many aspects of the aging process could be the result of cells slowly drifting away from their proper state of differentiation. This possibility has been studied by searching for an age-dependent increase in the expression of specific genes in tissues where expression of these genes would not normally be expected. In these studies, cDNA probes of specific genes are used in a DNA X NA hybridization assay to detect possible complementary RNA sequences in tissues of different-aged animals. Using this technique in past experiments, a qualitative increase in the RNA sequence complexity of mouse leukemia virus (MuLV) and a quantitative increase in the amount of alpha- and beta-globin RNA were found with increasing age in the brain and liver of the C57BL/6J mouse strain. We report here a similar age-dependent qualitative increase in the RNA sequence complexity for mouse mammary tumor virus (MMTV) but no quantitative or qualitative age-dependent change in casein RNA sequences for the same tissues and mouse strain.

Effects of food restriction on aging: separation of food intake and adiposity

Restricted feeding of rodents increases longevity, but its mechanism of action is not understood. We studied the effects of life-long food restriction in genetically obese and normal mice of the same inbred strain in order to distinguish whether the reduction in food intake or the reduction in adiposity (percentage of fatty tissue) was the critical component in retarding the aging process. This was possible because food-restricted obese (ob/ob) mice maintained a high degree of adiposity. In addition to determining longevities, changes with age were measured in collagen, immune responses, and renal function. Genetically obese female mice highly congenic with the C57BL/6J inbred strain had substantially reduced longevities and increased rates of aging in tail tendon collagen and thymus-dependent immune responses, but not in urine-concentrating abilities. When their weight was held in a normal range by feeding restricted amounts, longevities were extended almost 50%, although these food-restricted ob/ob mice still had high levels of adiposity, with fat composing about half of their body weights. Their maximum longevities exceeded those of normal C57BL/6J mice and were similar to longevities of equally food-restricted normal mice that were much leaner. Food restricted ob/ob mice had greatly retarded rates of collagen aging, but the rapid losses with age in splenic immune responses were not mitigated. Thus, the extension of life-span by food restriction was inversely related to food consumption and corresponded to the aging rate of collagen. These results suggest that aging is a combination of independent processes; they show that reduced food consumption, not reduced adiposity, is the important component in extending longevity of genetically obese mice.

Studies of immune responses in mice prone to autoimmune disorders. I. Heterogeneity of the affinities of antihapten antibodies produced by NZB, NZW, and related strains of mice

Mice of the NZB and NZW strains and their F1 hybrid produce antihapten plaque-forming cell (PFC) responses to T-dependent antigens (trinitrophenylated bovine gamma globulin and dansylated keyhole limpet hemocyanin) which are of unusually restricted heterogeneity of affinity, are relatively lacking in low-affinity PFC, and are of relatively high average affinity. Since some low-affinity PFC are present in NZB mice early after immunization, the results suggest a particularly marked down-regulation of low-affinity antibody production by these strains. The non-autoimmune-prone F1 hybrid (NZB X CBA) produces a typical heterogeneous response containing a high proportion of low-affinity PFC. Thus, the tendency to down-regulate low-affinity PFC is not inherited as a simple Mendelian dominant trait. The response of NZB mice to T-independent antigens does not show the same restricted heterogeneity of affinity. In fact, late after injections of trinitrophenylated Ficoll, NZB mice tend to have more heterogeneous responses than nonautoimmune-prone BALB/c mice in which a marked down-regulation of high-affinity antibody-producing PFC is seen. The possible relationship between these unusual features of the immune response of NZB and some related strains and their tendency to develop autoimmune disease is discussed.

Normal mice express idiotypes related to autoantibody idiotypes of lupus mice

Spleen and fetal liver B cells of normal mice synthesized idiotypes shared by anti-DNA autoantibodies of genetically autoimmune mice. Some of the idiotypes were specific for DNA; the majority, however, were not. The findings indicate that the autoantibody idiotypes are related to a conserved family of antibody variable regions that are present in normal animals.

Immunoregulation in senescence: increased inducibility of antigen-specific suppressor T cells and loss of cell sensitivity to immunosuppression in aging mice

Azobenzenearsonate (ABA)-specific T cell-mediated suppression has been studied in aging mice. ABA-specific suppressor T cells were induced in young and old mice by injection of ABA conjugated to syngeneic spleen cells (ABA-SC). These suppressor cells were tested for their ability to suppress the in vitro anti-trinitrophenyl (TNP) antibody response of lymph node cells obtained from ABA-keyhole limpet hemocyanin (KLH)-primed young or old mice and cultured with TNP-ABA-KLH. Suppressor T cells were found to be more easily induced in old than in young mice but to suppress less efficiently the antibody response of cells from old than from young mice. The increased inducibility of antigen-specific suppressor T cells in old mice is compatible with the age-dependent decline of immune responsiveness to exogenous antigens. The loss of cell sensitivity to antigen-specific immunosuppression as well as the lack of evidence for increased nonspecific suppression in old mice is consistent with the age-related increase in autoimmune disorders. These findings provide a unifying explanation for the most relevant immunological phenomena of senescence.

Multigene families, histocompatibility systems, transformation, meiosis, stem cells, and DNA repair

Aging is probably not directly traceable to changes along the whole genome, but to a small portion thereof. The main histocompatibility complex appears to be one among the postulated sets of multigene families responsible. The immortality of transformed cells, the germ line, and possibly certain pluripotential stem cells may suggest common qualitative and/or quantitative differences in DNA repair mechanisms between these cell populations and committed, normal cell populations. A relationship between HLA and at least two diseases showing defective DNA-repair suggests that the same chromosome carrying the main histocompatibility complex may control some repair processes. The correspondence of variation in lifespans in different mouse strains with the DNA repair capabilities and degrees of autoimmune susceptibility of the same strains lends further support to the idea that DNA repair, immune dysfunction and aging in higher animals may be intimately related.