Aged mice exhibit in vivo defective peripheral clonal deletion of D(b)/H-Y reactive CD8(+) T cells

Immune response of macrophages from young and aged mice to the oral pathogenic bacterium Porphyromonas gingivalis

Periodontal disease is a chronic inflammatory gum disease that in severe cases leads to tooth loss. Porphyromonas gingivalis (Pg) is a bacterium closely associated with generalized forms of periodontal disease. Clinical onset of generalized periodontal disease commonly presents in individuals over the age of 40. Little is known regarding the effect of aging on inflammation associated with periodontal disease. In the present study we examined the immune response of bone marrow derived macrophages (BMM) from young (2-months) and aged (1-year and 2-years) mice to Pg strain 381. Pg induced robust expression of cytokines; tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-10, chemokines; neutrophil chemoattractant protein (KC), macrophage colony stimulating factor (MCP)-1, macrophage inflammatory protein (MIP)-1α and regulated upon activation normal T cell expressed and secreted (RANTES), as well as nitric oxide (NO, measured as nitrite), and prostaglandin E2 (PGE2) from BMM of young mice. BMM from the 2-year age group produced significantly less TNF-α, IL-6 and NO in response to Pg as compared with BMM from 2-months and 1-year of age. We did not observe any difference in the levels of IL-1β, IL-10 and PGE2 produced by BMM in response to Pg. BMM from 2-months and 1-year of age produced similar levels of all chemokines measured with the exception of MCP-1, which was reduced in BMM from 1-year of age. BMM from the 2-year group produced significantly less MCP-1 and MIP-1α compared with 2-months and 1-year age groups. No difference in RANTES production was observed between age groups. Employing a Pg attenuated mutant, deficient in major fimbriae (Pg DPG3), we observed reduced ability of the mutant to stimulate inflammatory mediator expression from BMMs as compared to Pg 381, irrespective of age. Taken together these results support senescence as an important facet of the reduced immunological response observed by BMM of aged host to the periodontal pathogen Pg.

Metabolic syndrome, hormones, and maintenance of T cells during aging

Although the phenotype of T-cell senescence has been extensively investigated, few studies have analyzed the factors that promote the generation and maintenance of naïve and memory T cells that exist throughout the lifespan of the individuals. Unlike senescent T cells, naïve and memory T cells are able to participate in useful immune responses as well as respond to new activation. Hormones such as leptin, ghrelin, insulin-like growth factor 1, IGFBP3, and cytokines, including IL-7, regulate both thymopoiesis and maintenance of naïve T cells in the periphery. Although chronic viruses such as cytomegalovirus (CMV) are thought to drive T-cell senescence, other microbes may be important for the maintenance of nonsenescent T cells. Microbiota of the gut can induce metabolic syndrome as well as modulate T-cell development into specific subpopulations of effector cells. Finally, T-cell generation, maintenance, and apoptosis depend upon pathways of energy utilization within the T cells, which parallel those that regulate overall metabolism. Therefore, better understanding of metabolic syndrome, T-cell metabolism, hormones, and microbiota may lead to new insights into the maintenance of proper immune responses in old age.

Innate and adaptive immunosenescence

OBJECTIVE

To review the effect of increasing age on the immune system and some of its clinical implications.

DATA SOURCES

MEDLINE and PubMed searches were performed cross-referencing the keywords immunosenescence, aging, and immunity. Articles were reviewed for additional citations.

STUDY SELECTION

Articles were reviewed and selected based on relevance to subject matter.

RESULTS

The study of immunosenescence is complex and not completely understood. Aging affects both the innate and adaptive arms of the immune response. With increased age, there may be a decrease in phagocytosis, alteration of cellular migration, changes in cell populations and numbers, and a decreased ability to produce specific antibodies. Clinically, these changes potentially increase morbidity and mortality in elderly individuals through an increased rate of infections, malignancy, and autoimmunity.

CONCLUSIONS

The process of aging is accompanied by diverse changes in immunity. Several therapeutic approaches are under investigation, including cytokine therapy, hormonal replacement, antioxidant supplementation, and caloric restriction, to attenuate or potentially reverse immunosenescence.

Is thymocyte development functional in the aged?

T cells are an integral part of a functional immune system with the majority being produced in the thymus. Of all the changes related to immunosenescence, regression of the thymus is considered one of the most universally recognised alterations. Despite the reduction of thymic size, there is evidence to suggest that T cell output is still present into old age, albeit much diminished; leading to the assumption that thymocyte development is normal. However, current data suggests that recent thymic emigrant from the aged thymus are functionally less responsive, giving rise to the possibility that the generation of naïve T cell may be intrinsically impaired in the elderly. In light of these findings we discuss the evidence that suggest aged T cells may be flawed even before exiting to the periphery and could contribute to the age-associated decline in immune function.

Complexity of Anti-immunosenescence Strategies in Humans

Immunosenescence is characterized by three main aspects: (i) the shrinkage of the T cell repertoire and the accumulation of oligoclonal expansions (megaclones) of memory/effector cells directed toward ubiquitary infectious agents; (ii) the involution of the thymus and the exhaustion of naïve T cells; and (iii) a chronic inflammatory status called inflamm-aging. We present here possible strategies to counteract these main aspects of immunosenescence in humans with particular attention to the reduction of antigenic load by pathogens, such as CMV, and the normalization of intestinal microflora, the possible utilization of IL-7 to reverse thymic involution, the purging of megaclones, the forced expression of CD28 on T lymphocytes, the reduction of inflamm-aging and the administration of nutrients such as vitamin D. Possible drawbacks of all these strategies are discussed. Finally, the complexity of a rejuvenation approach is stressed, with particular attention to the inhibitory role played by the "old microenvironment" on the performance of progenitor cells, the best candidate to counteract the decline in regenerative potential characteristic of organs and tissues from old organisms.

Impaired apoptosis and immune senescence - cause or effect?

Aged animals and humans exhibit a decreased T-cell activation response although they also exhibit increased susceptibility to responses to self-antigens and a loss of self-tolerance. The age-related alteration in T-cell reactivity, polyclonal expansion of T cells, and enhanced production of autoantibodies may reflect the numerous age-associated alterations in the T-cell arm of the immune system that have been revealed in numerous studies. These studies suggest that subpopulations of T cells are not deleted appropriately in older animals. They further suggest that an age-related impairment of Fas/Fas ligand (FasL)-mediated apoptosis - which plays a major role in activation-induced cell death (AICD) of T cells - may contribute to compromised regulation of the immune system. The likely mechanisms that may lead to impaired induction of FasL in AICD senescent T cells include an age-related shift from the apoptosis-sensitive T-helper 1 cell (Th1) response to the AICD-resistant Th2 response, aberrant T-cell receptor/CD3 downstream-signaling pathways, and altered CD28/B7-mediated T-cell costimulatory signals. Pathologically, accumulation of AICD-senescent T cells is associated with a defective cytotoxic T lymphocyte response and generation of autoreactive T cells. Based on the accumulating evidence, we propose that the emergence of the FasL(lo) AICD-senescent T cells is not only an effect of immune aging but also an important cause of T-cell proliferative senescence in both humans and mice.

Origin of late-onset autoimmune disease

Autoimmune disease in the elderly is hypothesized to be caused by an imbalance in T-cell expansion and deletion after an encounter with self-antigens. A decrease in thymic output leads to a decreased pool of naive T cells in the periphery and to increased oligoclonal expansion of T cells. This expansion may be caused by stimulation with autoantigens that drive high-affinity interactions with self-antigens. Accumulation of presenescent, apoptosis-resistant, and proinflammatory T cells results in the growth of these autoreactive T cells. A decreased T-cell activation response that occurs with age leads to several defects that diminish the immune response.

Ceramide mediates age-associated increase in macrophage cyclooxygenase-2 expression

Previously, we showed that macrophages (MØ) from old mice have significantly higher levels of lipopolysaccharide (LPS)-induced prostaglandin E(2) (PGE(2)) production than young mice, due to increased cyclooxygenase-2 (COX-2) mRNA levels. The aim of the current study was to determine the underlying mechanisms of age-associated increase in COX-2 gene expression. The results demonstrate that increased COX-2 mRNA expression in the old mice is due to a higher rate of transcription rather than increased stability of COX-2 mRNA. Furthermore, the results show that LPS-induced ceramide levels from the old mice are significantly higher than those of young mice, whereas there is no age-related difference in concentration of its down stream metabolite, sphingosine. The addition of ceramide in the presence or absence of LPS resulted in a significant increase in PGE(2) production in a dose- and time-dependent manner. Inhibition of ceramide conversion to sphingosine had no effect on this ceramide-induced effect. The ceramide-induced up-regulation in PGE(2) production was mediated through increase in COX activity and transcriptional up-regulation of COX-2 mRNA. Collectively, these data suggest that the age-associated increase in MØ COX-2 mRNA is due to transcriptional up-regulation. Furthermore, this increase in transcription is mediated by higher cellular ceramide concentration in old MØ compared with that of young MØ.

Hepatic DR5 induces apoptosis and limits adenovirus gene therapy product expression in the liver

A major limitation of adenovirus (Ad) gene therapy product expression in the liver is subsequent elimination of the hepatocytes expressing the gene therapy product. This elimination is caused by both necrosis and apoptosis related to the innate and cell-mediated immune response to the Ad. Apoptosis of hepatocytes can be induced by the innate immune response by signaling through death domain receptors on hepatocytes including the tumor necrosis factor alpha (TNF-alpha) receptor (TNFR), Fas, and death domain receptors DR4 and DR5. We have previously shown that blocking signaling through TNFR enhances and prolongs gene therapy product expression in the liver. In the present study, we constructed an Ad that produces a soluble DR5-Fc (AdsDR5), which is capable of neutralizing TNF-related apoptosis-inducing ligand (TRAIL). AdsDR5 prevents TRAIL-mediated apoptosis of CD3-activated T cells and decreases hepatocyte apoptosis after AdCMVLacZ administration and enhances the level and duration of lacZ transgene expression in the liver. In addition to blocking TRAIL and directly inhibiting apoptosis, AdsDR5 decreases production of gamma interferon (IFN-gamma) and TNF-alpha and decreases NK cell activation, all of which limit Ad-mediated transgene expression in the liver. These results indicate that (i) AdsDR5 produces a DR5-Fc capable of neutralizing TRAIL, (ii) AdsDR5 can reduce activation of NK cells and reduce induction of IFN-gamma and TNF-alpha after Ad administration, and (iii) administration of AdsDR5 can enhance Ad gene therapy in the liver.

Immunological memory and late onset autoimmunity

This review will address a paradox that has long fascinated scientists studying the effects of aging on the immune system. Although it has been clearly documented that B and T lymphocytes lose the ability to respond to antigenic or mitogenic stimulation with age, it has nonetheless been noted that the frequency of autoreactive antibodies is higher in older individuals. Given that the majority of the age-associated defects in immune regulation target the naïve T and B lymphocyte subsets, it has been presumed that this increase in antibodies specific for self antigens was due to changes in the B cell repertoire and/or to differences in the mechanisms responsible for generating immune tolerance in primary responses. However, in this review, we will address an alternative possibility that memory immune responses, first generated when the individual was young, may play a critical role in the appearance of serum autoantibodies by reactivation later in life (recall memory). It has recently been shown, in several different systems, that memory immunity can be maintained over the lifetime of the animal. Thus, memory B cells which are self-reactive may be harbored within an organism as it ages and the potential exists that they become re-activated at a later time, resulting in a vigorous autoreactive recall response. This may occur preferentially in older individuals due to several factors, including deficiencies in immune tolerance with age, progressive age-associated loss of tissue integrity yielding neo-self antigens, and possible re-exposure to an infectious agent which induces an autoimmune memory response through molecular mimicry. Thus, we propose that some of the autoantibodies seen in elderly patients and in older animals may have been produced by memory lymphocytes originally generated against antigens encountered during one's youth, but maintained in a tolerant (non reactive) state until a subsequent triggering event occurs. Possible implications of this model will be discussed.

Beneficial influences of systemic cooperation and sociological behavior on longevity

During his long research career in the field of aging, Dr Bernard Strehler developed a series of theories concerning the identity of genes that can promote longevity and their role in natural selection. As a tribute to Dr Strehler, we have taken this opportunity to summarize a selection of these theories and to illustrate how these insights have influenced our search for longevity genes within the immune system. The identification of longevity genes has proven difficult. We believe that, at least in part, this reflects the emphasis on the concept of survival of the 'physically' fittest. We have used the immune system as a model to demonstrate that, over and above the self-evident advantage of those genes that contribute the attributes commonly associated with survival of the 'physically' fittest, those genes that lead to a predisposition to cooperate also confer a competitive survival advantage. As the acquisition of cooperativity in a society is linked to support mechanisms provided by older individuals, the search for longevity genes should not be limited to those genes that are associated with extended expression of a youthful phenotype. Rather these studies should be expanded to include identification of those genes that regulate physiologic parameters that affect individual longevity, even if they do not correspond with the traditional view of reproductive competitiveness. At the societal level, longevity genes may encode attributes that regulate sociologic or psychological parameters that may contribute to a tendency to non-aggressive or cooperative behavior that leads to achievement of common goals necessary for the survival of the species. This view of the selection for longevity impacts the analysis of longevity genes and aging at the organismal level. Dr Strehler viewed organismal aging as an integrated functional state, in which he conceived the outcome as reflecting the net balance of functional decrementers and evolved compensatory features. We propose that, in more evolved species, the longevity genes will be those genes, or sets of genes, that counterbalance of age-related functional decrementers with the age-related manifestation of evolved compensatory features. Thus, as illustrated here through analysis of the immune system, the longevity genes may well be those genes that promote overall systemic cooperation and compensation within the immune system and associated systems, rather than the genes that prevent age-related alterations in only one or a limited number of pathways.

The aging of lympho-hematopoietic stem cells

The extensive self-renewal capacity of hematopoietic stem cells (HSCs) implies that this cell population may not age and thus may provide undiminished replenishment of blood cells throughout the lifespan of an organism. In contrast, accumulating experimental evidence supports the premise that HSCs show signs of aging and may have a limited functional lifespan. We summarize here the evidence for HSC aging, discuss the possible molecular mechanisms that may be involved and show evidence of a genetic connection between the effects of age on blood-forming cells and the longevity of mice. We speculate that age-related functional decline in adult tissue HSCs limits longevity in mammals.

Activated CD8(+) T cells from aged mice exhibit decreased activation-induced cell death

To uncouple the defects of activation and apoptosis of T cells from aged mice, we used anti-CD3 plus IL-2 stimulation to induce an activation response and analyzed the subsequent activation-induced cell death (AICD) response of T cells from 16-month-old mice. The results herein demonstrate that T cells from 16-month-old mice could be activated by anti-CD3-induced activation signals but exhibited distinct phenotypic and functional features compared to young (2-month-old) mice. These include a decrease in AICD, a delayed entry into the cell cycle, and a decreased telomerase activity. The decreased AICD of T cells from 16-month-old mice is associated with a decreased expression of Fas and Fas ligand (FasL), decreased susceptibility to anti-Fas-induced apoptosis, and an increased expansion of a CD8(+) T-cell population. Prior to activation, these T cells exhibit a phenotype that is CD44(hi)CD62L(hi). After stimulation, these T cells produced high levels of the pro-inflammatory cytokine, IFN-gamma, and developed an increased population of IFN-gamma(+)IFN-gamma R(-) T cells. Our results suggest that there is a dysregulation in T-cell homeostasis in aged mice associated with a decrease in AICD of CD8(+) T cells.