Effect of age on response to T- and B-cell mitogens in mice congenic at theH-2 locus

Genetics of life span in mice

Thymic involution that occurs earlier in some individuals than others may be the result of complex interactions between genetic factors and the environment. Such interactions may produce defects of thymus-dependent immune regulation associated with susceptibility to developing autoimmune diseases, malignancy, and an increased number of infections associated with aging. The major histocompatibility complex may be important in determining profiles of cause of death and length of life in mice. Genetic influences on life span involve interactions between loci and allelic interactions during life which may change following viral infections or exposure to other environmental factors. We have used different experimental protocols to study the influence of H-2 on life span and found that interactions between genetic regions, are inconsistent, particularly when comparing mice infected or not infected with Sendai virus. Genes important for life span need to be studied against many genetic backgrounds and under differing environmental conditions because of the complexity of the genetics of life span. Several genetic models were used to demonstrate that the MHC is a marker of life span in backcross and intercross male mice of the H-2d and H-2b genotypes in B10 congenic mice. Females lived longer than males in backcross and intercross mice, while males lived longer than females in B10 congenics. H-2d was at a disadvantage for life span in backcross mice of the dilute brown and brown males exposed to Sendai infection, but intercross mice not exposed to Sendai virus of the same genotype were not at a disadvantage. H-2d mice were not disadvantaged when compared to H-2b in B10 congenics that had not been exposed to Sendai virus infection but the reverse was true when they were exposed. Overall, all our studies suggest that genetic influences in life span may involve interactions between loci and many allelic interactions in growing animals or humans. These genetic influences on life span may vary after they are exposed to infections or other environmental conditions. This paper emphasizes the need to use several genetic models, especially animals that have been monitored for infections, to study the genetics of life span.

Longevity and heredity in humans. Association with the human leucocyte antigen phenotype

Several arguments support the idea of a link between longevity and heredity, both in experimental animals and in the human species. In mice, genes in the major histocompatibility complex (MHC) are associated with a significant effect on life span. Results of analogous studies in man are confusing and contradictory. We have therefore investigated the question of an association of the human leucocyte antigen (HLA) and longevity in a large and ethnically homogeneous population. Our study population consisted of all 964 available inhabitants aged 85 years and over in the Dutch community of Leiden (pop. 104,000). Our control group comprised 2444 young inhabitants, aged 20-35 years, with an identical ethnic and demographic background. In addition, control groups of different age-brackets from the same region were used. Two antigens differed in frequency: HLA-B40 was lower and HLA-DR5 was higher in the group of 85 years and over, as compared to the control group, aged 20-35 years. Both differences were more evident in females. No major disease associations with HLA-B40 or HLA-DR5 have been reported. It is unlikely that these results are a chance observation: the overall similarity of the HLA pattern of the old and young age groups is a confirmation of their identical ethnic and demographic background and the changes as observed in the different age-groups were gradual. The biological meaning of these results is still unclear.

Aging-associated intrinsic defects in IgA production by murine Peyer's patch B cells stimulated by autoreactive Peyer's patch T cell hybridoma-derived B cell stimulatory factors (BSF)

Immune functions deteriorate with age, primarily as a result of alterations in the number and subpopulations of T cells of the immune system. In contrast, the B cell component of the immune system is generally affected by senescence only to a minor extent. In the present report, we stimulated murine Peyer's patch (PP) B cells by nonspecific multifunctional B cell stimulatory factors (BSF) secreted by one of several autoreactive (self-major histocompatibility complex (MHC)-class II antigen-responsive) T cell hybridoma clones derived from PP of syngeneic mature adult mice, and then determined in vitro whether aging-associated intrinsic defects could be demonstrated in the proliferation of, and the synthesis and secretion of mucosal IgA by, the BSF-activated B cells. This approach could be a useful new in vitro method for assessing the effect of senescence on B cell Ig production, especially that of IgA, in the gut-associated lymphoid tissue (GALT). Aged PP B cells stimulated by the autoreactive PP T cell-derived BSF proliferated more (P less than 0.05), contained larger amounts of IgA (nearly 10 times) and also secreted considerably more IgA (nearly 4.5 times) than did mature adult PP B cells. However, the ratio of intracellular dimeric (d) IgA to total IgA in the aged B cell lysates was significantly reduced (by approx. 44%) as was also the secreted dIgA (by approximately 50%). The augumentation of not only the proliferation, but also the synthesis and secretion of IgA in vitro along with reduced dIgA/total IgA ratios of BSF-stimulated aged PP B cells appears to be due to aging-related intrinsic defects. Alterations in intracellular regulatory mechanisms of B cells, mediated by B cell receptors for autoreactive T cell-derived BSF, could be largely responsible for the observed polyclonal B cell hyperreactivity, associated with senescence.

Influence of major histocompatibility complex region genes on human longevity among Okinawan-Japanese centenarians and nonagenarians

The frequencies of 80 HLA antigen phenotypes in 82 centenarians and 20 nonagenarians in Okinawa, Japan, were compared with those in other healthy adults in various age-brackets. Subjects aged over 90 had an extremely low frequency of HLA-DRw9 and an increased frequency of DR1. In this age-group the relative risk of corrected (for number of antigens) p value for HLA-DRw9 were 5.2 and 0.0001, respectively; those for HLA-DR1 were 13.3 and 0.0367, respectively. Since a high frequency of DRw9 and a low frequency of DR1 are associated with autoimmune or immune deficiency diseases, the genetic protection against these disorders may contribute to longevity.

Environmental and genetic factors that influence immunity and longevity in mice

Many different theoretical approaches may be taken toward understanding the association between aging and immunologic malfunction. The leading theory is based on the natural phenomenon of thymic involution and argues that the T-dependent lymphoid system is genetically programmed to decline in effectiveness, possibly through altered endocrine and central nervous system controls. The "thymic time clock" theory of aging is strongly supported by the consistent finding of defective cellular immunity functions in aged humans and animals and an associated development of the age-related diseases. In several animal models, including autoimmune-prone strains, high spontaneous tumor incidence strains, and normal long-lived strains, it has been possible to forestall the development of the major diseases of aging and extend longevity by restricting diet. The predominant effect of dietary restriction is prolongation of immunologic vigor and retardation of the immunologic dysfunction that normally occurs with age. Studies on environmental factors affecting longevity such as these and others which demonstrate a complex interaction between genes influencing longevity underscore the complexity and challenge of aging research.

Food intake reduction and immunologic alterations in mice fed dehydroepiandrosterone

A diet containing 0.4% DHEA was fed to male mice of a long-lived strain from 3 weeks until 18 weeks of age. These mice were compared with others fed a control diet ad libitum and with mice pair-fed the control diet in amounts approximating the intake of the DHEA-fed group. Mice fed the DHEA diet failed to eat all of the food presented to them whereas the pair-fed mice ate all of their food. All mice were studied at 18 weeks of age for two age-sensitive immune parameters (spleen lymphocyte proliferation induced by T-cell mitogens [PHA or ConA] and natural killer cell lysis of an allogeneic tumor). DHEA feeding led to: 1) a decrease in food intake (approximately 30% less than for mice fed the control diet ad libitum), 2) a lower body weight at 18 weeks of age (approximately 40% lower than for ad libitum controls) due to a decrease in the body weight gained from 3 weeks through 18 weeks of age (approximately 55% lower than controls), 3) a lower spleen weight (approximately 30% lower than controls) but without lower numbers of nucleated cells per spleen, 4) an increase in PHA-induced proliferation by spleen lymphocytes (approximately 100% higher than for controls) and, 5) no influence on splenic natural killer cell activity. The inhibition of body weight gain for mice fed DHEA appeared due to both a reduction in food intake and a metabolic effect since mice eating DHEA gained less body weight per gram of food eaten than did mice in either group eating the control diet.

HLA and T-lymphocyte function in old age

The role of the major histocompatibility complex in the genetic control of reactivity of peripheral blood mononuclear cells (T lymphocytes) to lectins and allogeneic cells as a function of age was investigated. In randomly selected aged subjects the frequencies of HLA-A, B, and some C locus alleles did not differ significantly from those in the control group. However, some tendencies of haplotype frequency differences between young and aged subjects were found. Significant associations of impaired or preserved T-lymphocyte function could be detected in connection with some HLA-A (A3, A11) antigens only. The tendency of some phenotypic HLA-A and B or C and B antigen associations to be in correlation with impaired or preserved T-lymphocyte reactivity in old age seemed to be independent of their age-related frequency differences. In family studies of a partially inbred Hungarian population, differences were found in the rate of diminution of allogeneic reactivity in groups sharing different HLA haplotypes. Based on statistical analysis of these data, a genetic factor segregating with the MHC and taking part in the regulation of the age-dependent decline of T-lymphocyte reactivity can be postulated.

Acquired immunological tolerance in aged mice. I. The dose-response relationship

Immunoresponsiveness declines with advancing age. In addition, there is an increase in specific and nonspecific suppressor cell activity. In view of these observations it might be predicted that aged mice would be easier to render immunologically tolerant. However, we show here that aged C57B1/6 mice are markedly more resistant to tolerance induction so that 100 times more tolerogen is required to induce unresponsiveness in 24-month-old than in 2-month-old animals. With age the variability between individuals for spleen size and sensitivity to antigen and tolerogen increases in C57B1/6 and Balb/c mice. On the other hand, the duration of tolerance is very little affected by age. The relationships between tolerance, aging and autoimmunity are discussed.

Correction of immunodeficiency in aged mice by levamisole and bestatin administration

An attempt to correct the impaired immune functions of aged mice was made by injecting repeatedly (over a 6-month period) two chemically defined immunostimulating agents, levamisole and bestatin, into 12- to 16-month-old hybrid mice. Continuous treatment with levamisole restored T-cell-dependent functions (delayed-type hypersensitivity reaction and antibody response to T-dependent antigens) and prevented the appearance of suppressor cells induced by aging. In aged animals, this treatment led to macrophage activation and to a significant reduction of ADCC activity near the baseline value of young animals. Weekly injections of bestatin resulted in varying effects, depending on the dose administered. Small doses (10 microgram/injection) were more effective in restoring humoral response to SRBC rather than delayed-type hypersensitivity reaction, whereas large doses (100 microgram/injection) had the opposite effect. Macrophage activation was obtained only after the administration of the high dose of bestatin. Continuous treatment with bestatin did not eliminate suppressor cell activity, but decreased the ADCC normally elevated in aged animals. A significant reduction of spontaneous tumors and prolongation of median survival was observed in mice given repeated injections of levamisole and of 100 microgram bestatin, compared with untreated aged mice and with mice given low doses of bestatin.

Cellular immunity in aging

Our study of the aging process in human beings and in mice is complicated by our need to know whether we are observing diseases of aging or natural nondisease state processes. Results from studies on inbred strains of mice and retrospective studies on HLA types in aging human populations suggest that genetic effects play a significant role in predetermining the life span of an individual. It is clear that in such mouse strains genetic defects that affect cell regulatory mechanisms result in the production of autoimmune reactivity, tumor development, and a shortened life span. In human beings, although results are less clear-cut, strong associations exist between some disease states and the HLA type. Also, the disappearance of HLA-B8 from older women suggests that this HLA type does not confer longevity. Cellular immune reactivity declines with age in all populations studied to date, and cell cooperative or regulatory mechanisms function less well. We need to characterize the specific nature of the cells directly responsible for these alterations and to attempt to correct deficiencies by dietary manipulation or transfer techniques.

The thymus: clock for immunologic aging?

A brief overview of the effects of age on T cell function is presented. Normal immune functions can begin to decline shortly after an individual reaches sexual maturity. Foremost among the cellular changes are those in the stem cells as reflected in their growth properties and the availability of precursor T cells, and in the T cells, in which a shift in subpopulations may be occurring. Present evidence indicates that thymic involution precedes, and therefore may be responsible for, the age-dependent decline in the ability of the immune system to generate functional T cells. It now appears that the primary effect of thymic involution is on a T cell differentiation pathway; the more mature T cells are affected first, the less mature T cells only later. Thus, the thymus may be the aging clock for the immune system. Current studies are centered on processes regulating growth and atrophy of the thymus, and methods for restoring the impaired immune function of elderly individuals.

Autoimmunity, histocompatibility, and aging

The immunologic theory of aging proposes that the normal process of aging in man and all animals is pathogenetically related to faulty immunological processes and may be analogous to a type of autoimmune phenomena ultimately involving all body tissues. It may be said that the sharply increased incidence in elderly humans of the autoimmune and immunodeficiency "diseases of age" are thought to be greatly potentiated by the age-related decline in immune surveillance mechanisms particularly involving self/non-self discriminatory abilities. The major histocompatibility complex has emerged as a complex of "supergenes" coding for antigens whose ultimate biological function may be to serve as recognition units allowing lymphocytes to recognize self from non-self on an immunological basis. Also, recent data are consistent with our supposition that differences in age-specific peaks of various immune functional parameters in genetically homozygous mice may be influenced by genes linked to the major histocompatibility complex. These differences may account, at least in part, for the highly strain-dependent, age-specific incidence of certain diseases, including autoimmune and malignant diseases in the mouse. Heightened susceptibility to develop a particular disease in a susceptible animal occurs when a certain balance is reached between the interplay of immune functional parameters which mature, differentiate, or decline at different rates in the same animal. The age-specificity of this balance may be under partial control of H-2 or HLA-linked genes.

Control of immunologic abnormalities associated with aging

In an attempt to encourage more intensive studies on the control of immunologic abnormalities associated with aging, the six different approaches which have been attempted are reviewed briefly. They are as follows: (a) internal body temperature control, (b) tissue ablation, (c) dietary manipulation, (d) genetic manipulation, (d) genetic manipulation (e) cell therapy and (f) chemical therapy. The first four are preventive and the latter two are restorative in approach. Many of these studies are very preliminary, but overall, the findings are most encouraging. These studies should resolve the extent to which decline in immunologic vigor with age influences the disease pattern of aging individuals and on their life expectancy.

An overview of immune aging

A brief overview of the effect of age on the function of cells of the immune system is presented. Normal immune functions can begin to decline shortly after an individual reaches sexual maturity. Foremost among the cellular changes are those in the stem cells as reflected in their growth properties and the availability of precursor T cells, and in the T cell where a shift in subpopulations may be occurring. Present evidence indicates that thymic involution precedes and therefore may be responsible for the age dependent decline in the ability of the immune system to generate functional T cells. It now appears that the primary effect of thymic involution is on a T cell differentiation pathway affecting the more mature T cells first and only later the less mature T cells. Thus, the thymus may be the aging clock for the immune system. Future studies should be centered around processes regulating growth and atrophy of the thymus.

Cyclic nucleotide levels in resting and mitogen-stimulated spleen cell suspensions from young and old mice

The levels of cyclic adenosine 3', 5'-monophosphate (cAMP) in suspensions of unstimulated spleen cells from tumor-free 30-month old (C57BL/10Sn X C3H/HeDiSn)F1 hybrid mice averaged only 14% of that of 6-month old mice. By contrast, the level of cyclic guanosine 3', 5'-monophosphate (cGMP) in spleen cell suspensions from old mice was about 270% that of young mice. The cAMP/cGMP ratio for the unstimulated (resting) state showed a decline by 30 months to about 5% of its 6-month value. Cyclic nucleotide levels were also measured in cell suspensions from old and young mice at intervals over a two hour period following in vitro stimulation with the plant mitogens phytohemagglutinin, concanavalin-A and pokeweed mitogen. Quantitative and in some instances qualitative differences in responses were noted. These results might conceivably reflect either age-related changes in the splenic lymphoid cell subpopulations or intrinsic cellular alterations or both. It is unlikely that changes of this degree could be wholly explained by population shifts. An imbalance in cyclic nucleotide levels in both resting and stimulated lymphoid cells in older animals might contribute to the immune dysfunction known to occur with normal aging.