What are the major age related changes in the immune system?

Immunosenescence

The aging of the immune system [immunosenescence] is a recent phenomenon related to the linear improvement in survival and life span not predicted by evolutionary force. Indeed, the immune system has probably been selected to serve individuals living until reproduction, as supported by the thymic ontogenesis and involution. Nowadays, the immune system supports the soma of individuals living 80–120 years; during this increased life span, elderly people have to cope with long-lasting antigenic burden [clinical and subclinical infection, as well as continuous exposure to other types of antigens], which causes a chronic activation of the immune system. In order to adapt to and counteract the effect of chronic stress, the immune system undergoes reshaping and remodeling, in which some parameters decrease with age while others increase or remain unchanged. The adaptive immunity is the most affected by aging, being characterized by expanded clones of memory and effector T cells, mostly prominent in CD8+ and specific for viral antigens. In particular, we showed that Epstein–Barr virus [EBV] and cytomegalovirus [CMV] infections are responsible for the expansion of memory CD8+ T cell clones specific for a limited number of viral peptide epitopes which accumulate in the immune system of older people, thus filling the ‘immunological space.’ We have shown that CD4+ T cells are also affected by viral chronic infections such as CMV. Indeed, when cells from CMV+ CD4+ T cell are stimulated in vitro with specific peptides of the CMV, they produce seven or eight times more inflammatory cytokines such as interferon-γ [IFN-γ] and TNF-α, thus contributing to the inflammatory status present in elderly people. Moreover, aging is associated with a shrinkage of T cell repertoire and a decrease in thymic emigrant T cells and virgin T cells. The result is the filling of the immunological space [the physical space in which all possible interactions among immune cells and/or cell subsets must occur] with indolent cells with memory markers which could exert a suppressor/negative effect on other bystander T cells, including the few naive [virgin or antigen-nonexperienced] cells. Considering the innate immunity, different studies have shown that age-associated alterations are also present in this branch of immunity, but overall it is well preserved. Our attention was particularly focused on NK cells, macrophages, and cytokine circuits. NK cells are large granular lymphocytes expressing surface markers such as CD16 and CD56 and lacking the CD3 T cell receptor. They display two alternative mechanisms of cytotoxicity activity: the MHC unrestricted cytotoxicity against neoplastic and virus-infected cells and antibody-dependent cell-mediated cytotoxicity. In elderly people, an increase in the absolute and relative number of NK cells, with a mature phenotype [CD56dim], is seen, with no variation in their killing activity as a whole, despite the presence of subtle impairment of killing activity on a per cell basis. Moreover, a feedback loop between NK cells and macrophages has been described. Since NK cells secrete factors such as IFN-γ which activate macrophages to eliminate pathogens, some of the age-associated macrophage dysfunctions may be secondary to alteration in cytokine production of NK cells. Additionally, an age-related reduction in hydrogen peroxide and nitric oxide synthesis by macrophage has been described, paralleled by an induction of suppressive substances [e.g., prostaglandin E] which have an inhibitory effect on dendritic cells, the main antigen presenting cells. In vitro studies on cytokine production by peripheral mononuclear cells from healthy young and aged subjects identified a complex scenario. Significant increases in levels of IL-6, TNF-α, and IL-1β were found in mitogen-stimulated cultures from aged donors despite a similar amount of cytokines in unstimulated cultures from young and aged subjects. Moreover, chemokines such as MCP-1 and RANTES, soluble TNF-α receptor I and II, and IL-1 receptor antagonist increased in the elderly. For one of the most important proinflammatory cytokines, IL-6, synthesis in macrophage cells is controlled by the CRH–DHEA circuit. Whereas production of cortisol remains constant or even increases with age, levels of DHEA decrease gradually from the third decade, reaching levels that are only 10–20% of their maximum by the eight decade; aged-related decrease of DHEA correlates with IL-6 overproduction. The elderly thus gradually approach a status of relative glucocorticoid excess with age, with a potential negative impact on baseline immune function and an exaggerated response to stressors. The correlation between DHEA and IL-6 has been reported in several studies. In particular, it has been demonstrated that in normal subjects, serum IL-6 levels increased with age, whereas serum DHEA concentrations decreased. Furthermore, serum IL-6 levels were inversely correlated with serum DHEA levels. In addition, in in vitro experiments, DHEA inhibited the lipopolysaccharide-stimulated IL-6 production from monocytes and peripheral blood mononucleated cells, even in a narrow concentration range.

Antioxidants

If oxidative damage in the immune system of aging/obese individuals plays an important role in prostate cancer, then antioxidants should inhibit the process. Selenium has been suggested to reduce prostate cancer progression. Clark et al., in examining the effects of selenium in skin cancer, reported an incidental 63% decrease in prostate cancer incidence using selenium supplements in the diet.72 β-Carotene and vitamin Ewere also evaluated for their cancer-inhibiting potential, but only vitamin E was associated with a decrease in the incidence of prostate cancer.73

Vitamin E supplementation was shown to improve immunocompetence in the elderly, therefore, requirements for vitamin E may be greater in this group.74 However, there is an inverse dose–response relationship. At high doses it antagonizes the effects of other fat-soluble vitamins, reduces liver storage of vitamin A, decreases bone mineralization and causes disorders of coagulation. Very high doses of vitamin E have been shown to decrease immunologic responses in the elderly.74

Daviglus et al. reported that overall survival of patients with prostate cancer was positively associated with the intake of β-carotene and vitamin C.75 This, however, does not prove that vitamin C alone improves survival.

26.4.2 Thymic Involution

One of the major features characterizing the aging of the immune system is the slow shrinking of the thymus with age or thymic involution, which significantly starts at the third decade of life and alters the daily number of naïve T-cells generated. This leads to a general decline in T-cell production and a loss of T-cell repertoire diversity [Aw and Palmer, 2011]. Similarly to the bone marrow, aging is associated with an increase in adipose tissue in the thymus which is associated with factors detrimental for thymic maintenance such as Leukemia Inhibitory Factor, Oncostatin M, IL-6 and sex hormones [Rega et al., 2007; Sempowski et al., 2000; Trayhurn and Wood, 2004]. The development of thymic adipocyte is also at the expense of thymic fibroblasts, which are key in the maintenance of thymic epithelial cells via the secretion of various mediators such as stem cell factor, fibroblast growth factors [FGF7, FGF10] and vascular endothelial growth factor [Yang et al., 2009]. It has also been proposed that the reduction in sex hormones occurring in the elderly is aggravating this process [Sutherland et al., 2005]. By acting on thymic epithelial cells expressing androgen/estrogen receptors, sex hormones are thought to be required for normal thymic development and ultimately thymic involution as their amounts are reduced during aging [Staples et al., 1999].

1.6 Aging and Inflammaging

Aging is associated with complex changes and dysregulation of the immune system, including its inflammatory component. The aging of the immune system, termed immunosenescence, has been suggested to be a consequence of continuous attrition caused by chronic antigenic overload and an inability of immune cell output, for example from the thymus, to keep up with the demand for naïve cells. Another phenomenon that accompanies the aging process is a low grade, chronic inflammatory state. The lifelong exposure to antigens and inflammatory stimuli accumulate and affect this phenomenon [28]. The elevated inflammatory state that occurs with aging can potentially trigger or facilitate the onset of the most important age-related diseases [29]. Many possible triggers of low-grade inflammation have been proposed, ranging from dysfunctional mitochondria to an imbalance in gut microbiota [termed dysbiosis]. Another important contributor to the onset and maintenance of low-grade inflammation is cellular senescence, defined as an irreversible block of the cell cycle. Actually, aging is accompanied by the accumulation of senescent cells in many organs and tissues. It is also hypothesized that failure of antiinflammatory and inflammation-resolving mechanisms to neutralize inflammatory processes plays a role in the development of chronic low-grade inflammation in the elderly. The terminal activators of the inflammatory response are the NF-κB pathway and the inflammasome platform [19]. Briefly, NF-κB is a multimeric transcription factor that modulates gene expression by binding to specific DNA sequences, known as κB response elements, in gene promoters and enhancers. NF-κB can be activated by over 150 different stimuli, including cytokines, ultraviolet irradiation, and bacterial or viral antigens. Moreover, it has a unique sensitivity to oxidative stress, as many of the agents activating NF-κB are either modulated by oxidative stress or are prooxidants themselves or are oxidized molecules, such as ox-LDL.

Inflammasomes are cytoplasmatic platforms that trigger the maturation and release of proinflammatory cytokines such as IL-1β. Inflammasome assembly mostly results from the oligomerization of a nucleotide-binding domain-like receptor [NLR] upon the recognition of different types of PAMPs from bacteria, viruses, or fungi, or danger associated molecular patterns [DAMPs], including ATP, nucleotides, cholesterol crystals, beta-amyloid, and hyaluronan. Other proteins such as absent in melanoma 2 [AIM2], retinoic acid-inducible gene I [RIG-I], and pyrin may be able to form inflammasome platforms. However, the NLR proteins are considered the main inflammasome [30,31]. They contain either a pyrin domain [PYD] or a caspase activation and recruitment domain [CARD]. Inflammasomes activate procaspase-1 to caspase, which in turn leads to the maturation of pro-IL-1β and pro-IL-18 to the respective mature forms [31]. Dysregulation of inflammasomes leads to well-recognized autoinflammatory diseases such as the cryopyrin-associated periodic syndrome [CAPS] for the NLRP3 inflammasome and the familial Mediterranean fever [FMF] and pyrin-associated autoinflammation with neutrophilic dermatosis [PAAND] for the pyrin inflammasome [32]. However, inflammasomes are involved in the pathophysiology of many other illnesses, including chronic inflammatory diseases, degenerative processes, fibrosis, or metabolic diseases. Age-related increase in low-grade inflammation is termed inflammaging and this is seen to contribute to many of the common declines in function, health, and well-being that accompany aging.

D Aging and Down’s Syndrome

In comparison with other etiologies of mental retardation, Down’s syndrome is the only one that has been widely studied in regard to age-related changes. As indicated earlier, in comparison with the general population and with other adults with mental retardation, research has indicated that persons with Down’s syndrome are more likely to experience premature aging in intellectual and sensory losses [Hawkins et al., 1991; Zigman, Seltzer, Adlin, & Silverman, 1991]. They have other age-related health problems, including hypothyroidism, premature aging of the immune system, and sleep apnea [Adlin, 1993]. Other characteristics of premature aging in this population may include changes in skin tone, graying or loss of hair, cataracts, hypogonadism, increased seizures, incidence of neoplasms, and degenerative vascular disease [Brown, 1985; Oliver & Holland, 1986].

Virtually all individuals with Down’s syndrome develop neuropathological changes similar to those seen in Alzheimer’s disease [reviewed in Zigman, Schupf, Zigman, & Silverman, 1993]. However, the age-specific risk for displaying symptoms of Alzheimer’s dementia in adults with Down’s syndrome is lower than would be expected given the Alzheimer’s neuropathology. In Zigman et al.’s re

view estimates of the prevalence rate of Alzheimer’s dementia across retrospective, cross-sectional, and longitudinal studies have ranged from 15% in adults over age 30 [Ropper & Williams, 1980] to 39% in adults over age 50 [Hewitt, Carter, & Jancar, 1985], with a 100% prevalence rate in a study limited to a neurological clinic population [Lott & Lai, 1982]. Cross-sectional studies have found increases in dementia with age [Thase, Liss, Smeltzer, & Maloon, 1982]. Given that most studies of Alzheimer’s in this population are cross-sectional and limited to institutional populations, the generalizability of these findings are limited [Zigman et al., 1994].

Also, diagnosis is complicated by the higher rate of depression among people with Down’s syndrome, which can also lead to lower adaptive and cognitive functioning [Sovner & Hurley, 1983]. Although conclusive results regarding the association between dementia and depression awaits further research, investigators in this area recommend the treatment for all persons with depression regardless of severity. Szymanski [1988] has concluded that the precipitous diagnosis of dementia may be disastrous and may lead to lack of treatment of persons who may actually have a treatable depression.

Hence, there is a need for studies of age-related changes that differentiate persons with Down’s syndrome from other persons with mental retardation. In order to obtain accurate diagnoses these studies need to have standard diagnostic criteria and multiple measures, including memory and cognitive loss, adaptive skill, and neuropsychological testing. Furthermore, there is a need for research on interventions aimed at improving the health and psychosocial well-being of older persons with Down’s syndrome.

Varicella Zoster Virus

Initial infection of VZV results in about four million cases of chickenpox a year in the United States [CDC, 2016a]. The infection becomes latent and may reactivate as shingles in old age. Most shingles cases are seen in older populations and other immunodeficient populations such as AIDS patients [CDC, 2016b]. Although two FDA-approved vaccinations exist, neither establishes long-term protective immunity in the aged [Haberthur and Messaoudi, 2013]. The unique pathology of VZV infection provides an opportunity to study the aging of the immune system. Therefore, it is important to reproduce a similar infection in animal models.

As with other viral infections, animal models of VZV should allow viral infection in the same target cells as humans, establish latency, and the virus should reactivate with age [Gilden et al., 2009]. An immunological analysis of simian varicella virus established intrabronchial inoculation of rhesus macaques is a strong model for human VZV infection. Similar initial pathologies, immune responses, and location of chronic infection were pointed out in the study [Messaoudi et al., 2009]. VZV cannot infect mouse cells. However, a clever severe-combined immunodeficient [SCID]–humanized mouse model has been developed in which human tissue [either fetal thymus, liver, skin, and/or dorsal root ganglia] is transplanted into SCID mice [Ku et al., 2005]. This model has demonstrated a role for T cells in transporting the virus from initial infection sites to the skin [Ku et al., 2004].

3.14 Altered intercellular communications and accumulation of inflammasomes

Another contributing factor for stem cell aging is the alteration in the intercellular communications between cells. Altered intercellular communication is indicative of stem cell aging, and some of the communications between the cells are mediated through gap junctions, and it has been found to be an important contributor to what is known as contagious aging [235]. Contagious aging is characterized by the induction of senescence through chronic inflammation. For this reason, lifespan manipulating techniques carried on one tissue can be found to influence the life span of other surrounding tissues [236].

Inflammatory markers such as an increase in cytokines like interleukin-6 [IL-6] and tumor necrosis factor alpha [TNFα] as well as C-reactive protein [CRP] have been linked to stem cell senescence. IL6 and TNFα, although frequently involved with acute-phase inflammatory responses, are indicative of chronic illnesses and monitoring of stem cell aging [231]. Aging of the immune system also known as immunosenescence is characterized by the aging of various tissues and niche deterioration. Other biomarkers related to hormonal changes and musculoskeletal changes have also been debated as potential markers of stem cell senescence but are generally known as predictors of the aging process in general. A marker of HSCs and CD34+ progenitor cells has been found to decline with age. A higher number of CD34+ cells for elderly ages of 80 years have been found to be better indicators of lifespan longevity than other markers [i.e., cardiovascular risk factors and inflammatory markers]. Interestingly, higher levels of adult stem cells have been observed among elderly population with a decline in their pluripotent capacity. This means that during aging process, progenitor cells give rise to more progeny as a way to compensate for the declining stem cell pool. A major difference in these progenitors from younger progenitors is the size of the clones, contributing to a finite capacity for division.

The systemic accumulation of inflammation factor in a chronic fashion with age is called inflammaging. This inflammaging is the driving force involved in the alteration of intercellular communications [11, 237]. Several hallmarks of stem cell aging, including loss of proteostasis and cell cycle arrest, trigger this immune response mainly due to the intracellular accumulation of misfolded proteins and senescence-associated secretory phenotypes [SASPs] [238]. The link between inflammatory factors and age-related changes has been reported in a large body of literature [239]. Inflammasome pathways, mainly by the proinflammatory cytokines such as IL-1b and IL-18, are reported to mediate this inflammatory signals to trigger the aging process [240]. The circulating levels of the proinflammatory cytokines in both the donor and recipient could serve as biomarker to predict the inflammaging-associated impact on stem cell function. In addition to inflammaging phenomenon, immunosenescence that includes quantitative and functional changes in various components of both the innate and adaptive immune systems contributes to the stem cell aging process [241]. Based on the expression of 57 immune response genes, an “IMM-AGE” score was derived by screening 135 healthy elderly individuals in a clinical study conducted by Alpert et al., [242], and this IMM-AGE scoring helped in drawing a trajectory of immunosenescence and their prognostic value.

T cells

The impaired function of the T cell system was one of the first observations of immunosenescence. The function of no other cell type is as strongly disturbed during aging as found for T cells. The resulting problems are often compared to HIV [human immunodeficiency virus] infections as a model of advanced aging of the immune system [Quiros-Roldan et al., 2020]. One common problem of aging and HIV infection is the chronic stimulation of T cells by latent herpes viruses, which continuously stimulate the reactive T cells clones and thereby decreases the remaining T cell repertoire, due to enhanced numbers of the herpes virus reactive T cell clones [Nikolich-Zugich, 2008]. The most prominent change during aging is the involution of the thymus [Henry, 1967]. Since all T cell populations mature in the thymus, the loss of thymic function results in a decrease in the number of T cells [Sansoni et al., 1993]. However, in contrast to the original discovery, it is now clear that the thymus is only partially replaced by fat tissue and that centenarians have a nearly functional thymus. Therefore, the thymic output, i.e., the number of newly produced naïve T cells [indicated by a TREC [T cell receptor rearrangement excision cycle]], is a marker of thymic activity and the immunological age [Pinti et al., 2010]. However, normally the thymus starts to degenerate from the third decade of life and is replaced by fat tissue [Elyahu and Monsonego, 2021]. The thymic involution is mainly induced by the male and female sex hormones, i.e., testosterone and estrogen, and by ROS [Bodey et al., 1997]. Some cytokines like IL-7 can slow down or even reverse the thymic involution, which is currently evaluated in clinical studies in a clinical experimental phase. The endogenous IL-7 production declines with age [Pangrazzi et al., 2017; Bradburn et al., 2018]. The decreasing thymic output is directly correlated to the low number of T cells in the blood. However, the functional disturbances are much more significant, since nonfunctional CD28− T cell clones accumulate with age, suggesting only a slight decrease in T cells [Grabstein et al., 1993; Posnett et al., 1994]. Furthermore, some original papers did not observe decreased T cell numbers, probably due to using CD2 as T cell marker instead of CD3. The latter is currently state of the art and CD2 is expressed by NK cells, which are increased with age [Xu et al., 1993].

Numbers of CD8+ cytotoxic T cells [CTL] decrease stronger in the elderly than the numbers of CD4+ T helper cells, which results in an increased CD4/CD8 ratio [Sansoni et al., 1993]. Since CTL are mostly needed to defend viral infections, viral infections increase with age. As pointed out before, the numbers of memory T cells [CD45R0+] increase with age, whereas the numbers of naïve T cells [CD45RA+] decrease [Xu et al., 1993]. This explains the weak response to new [neo] antigens, whereas the response to well-known antigens [recall antigens] remains quite normal.

In addition to the decline in T cell numbers, their function is disturbed. T cells of elderly show an impaired response to stimulation since they are already chronically pre-activated. This pre-activation becomes apparent by the higher numbers of CD25+ [high affinity IL-2 receptor] and HLA-DR+ T cells [Sansoni et al., 1993]. However, the pre-activation is the main reason for the overall reduced T cell response upon activation and the high clonal exhaustion, observed in the elderly. The poor activation of the T cells of elderly results in less proliferation of the T cells. Proliferation is a prerequisite for the adaptive immune response [Pawelec et al., 1999], since this depends on the clonal selection of the antigen-specific lymphocytes. Without proliferation of the T cells, the number of effector T cells will be too low for pathogen clearance. Furthermore, the activated T cells may be clonally exhausted, and a new memory cannot be established. The clonal exhaustion due to AICD [activation-induced cell death] is increased by the overall higher apoptosis rate of leukocytes in the elderly [McLeod, 2001]. The reduced activation is based on an impaired T cell receptor [TCR] signal transduction and on a lack of survival signals. The ζ-chain of the CD3 signal transduction complex of the TCR shows a reduced phosphorylation and the transcription factor c-fos is lowly expressed in the elderly [Song et al., 1992; McLeod, 2001]. Additionally, the balance between pro-apoptotic [e.g. Bax] and anti-apoptotic proteins [e.g., Bcl-2 and p53] in T cells is shifted towards the pro-apoptotic ones. Furthermore, the number of T cells expressing the death receptor CD95 and its ligand is increased. Elevated apoptosis is more prominent in the naïve T cells of elderly, which further contributes to the lower quotient of naïve to memory T cells [CD45RA+/CD45R0+].

As mentioned above, the number of CD28− T cell clones increases with age. These cells are anergic, which means that they cannot respond to their specific antigen. Naïve and resting T cells need at least two signals for their activation: First an antigen-specific signal through the TCR and second via the costimulatory molecule CD28. Consequently, these CD28− long-living T cell clones cannot be activated. So far it is unclear, why these unresponsive CD28− T cell clones have a much longer half-life than other T cells. In addition, these T cell clones lack CD40L [CD154], which is the ligand for CD40 expressed by B cells and other antigen presenting cells and necessary for T cell help [Weyand et al., 1998]. Most of the CD28− T cell clones are CD8+ and represent the T cell counterpart to the long-living B cell clones in MGUS [monoclonal gammopathy of undetermined significance]. The survival of the anergic T cell clones is maintained by signal transduction through the IL-2-receptor and CD2, which is not impaired in contrast to TCR-induced signaling via CD3 [Song et al., 1992]. However, the anergic T cell clones displace normal T cells in blood and lymphatic tissues, resulting in a higher functional loss than estimated due to reduced T cell counts.

The number of regulatory T cells [Treg] in the blood is decreased in elderly, while the number of TH17 cells is increased, leading to an increased TH17/Treg ratio. However, during stimulation this ratio turns around in the elderly, resulting in a reduced TH17/Treg ratio, which explains the disturbed antimicrobial response [Schmitt et al., 2013]. Furthermore, the number of memory Treg cells is increased in the elderly [van der Geest et al., 2014]. The age-related changes in the T cell system are summarized in Table 5. In conclusion, the capacity of CTL to kill virus infected cells is reduced in the elderly and the function of T helper cells is disturbed, leading to a dysbalanced immune response against other pathogens as well. Finally, the impaired proliferative capacity of T cells limit the strength and duration of the immune response

Table 5. Alterations in the T cell system in aging.

Significantly increased ↑, increased ↗, significantly decreased ↓, decreased ↘, normal/unchanged ↔, unknown ?

Table updated from Rink L and Dalhoff K [2004] Altersspezifische Veränderungen des Immunsystems und deren assoziierte Krankheitsbilder. In D Ganten, K Ruckpaul and A Ruiz-Torres [eds.], Molekularmedizinische Grundlagen von altersspezifischen Erkrankungen, 1st edn, pp. 429–464, Springer: Berlin. doi:10.1007/978-3-642-18741-4_16.

TELOMERASE AND T CELLS

It is generally accepted that the “clock” that keeps track of cell divisions and signals cell cycle arrest in human cells is the telomere, a region at the end of each chromosome that consists of multiple repeats of a specific DNA sequence [Campisi et al., 2001]. Due to the end-replication problem in copying the full length of the lagging DNA strand, normal somatic cells undergo progressive telomere shortening with cell division. Once the telomere reaches a certain critical length, the DNA damage signaling pathway is activated, with the concomitant up-regulation of cell cycle inhibitors. The process of replicative senescence is a stringent characteristic of human somatic cells, whereas in germ cells, in certain stem cells, and in tumor cells, telomere shortening and replicative senescence are prevented by the activity of an enzyme called telomerase, which uses its RNA template to synthesize the telomere sequence.

Human and mouse cells show fundamental differences with respect to telomere size and telomerase activity. Because of the significantly longer telomeres in laboratory strains of inbred Mus musculus, and the high levels of telomerase present in most mouse tissues, it is unlikely that mouse cells undergo telomere-based senescence [Akbar et al., 2000]. Indeed, the barrier to unlimited proliferation may actually be less stringent in murine cells, since, unlike human cells, mouse cells undergo frequent spontaneous immortalization in cell culture. Thus, there is a major difference between mice and humans with respect to this important facet of cell biology. In terms of the aging immune system as well, the life-long exposure to pathogens also differentiates elderly humans from aged mice housed in barrier facilities, further underscoring the value of using a human cell culture model for analysis of the role of T cell replicative senescence in human aging.

Our cell culture analysis of human T cells has documented that telomere length undergoes progressive shortening with increasing rounds of antigen-driven proliferation, reaching 5–7 kb at senescence [Vaziri et al., 1993] Although this telomere length was similar to that of other cell types that reach senescence in cell culture, we were somewhat perplexed by the observed telomere shortening in T cells, since lymphocytes are unique among human somatic cells in that they induce high levels of telomerase activity in concert with the activation process [Weng et al., 1996]. Indeed, the levels of telomerase activity in antigen or mitogen-stimulated T cells are comparable to those of tumor cells [Bodnar et al., 1996].

To address this issue, we performed a detailed kinetic analysis in cell culture of CD8+ T cell telomerase activity induced by activation. We showed that after mitogen or T cell receptor [TCR]-mediated activation, the telomerase activity peaks at 3–5 days, then undergoes a gradual decline, becoming undetectable at approximately 3 weeks. A second wave of telomerase activity can be induced by a subsequent exposure to the same antigen, and during the period of high telomerase activity, telomere length remains stable [Bodnar et al., 1996]. However, in CD8+ T cells, the antigen-induced upegulation of telomerase in response to stimulation with antigen is markedly reduced by the third stimulation, and is totally absent in all subsequent encounters with antigen [Valenzuela and Effros, 2002].

One of the unexpected findings in our cell culture studies was the significant difference between helper [CD4+] and cytotoxic [CD8+] T cell subsets with respect to telomerase. This observation was made by culturing CD4+ and CD8+ T cell subsets that were isolated from the same individual, using the identical stimulatory schedule. The initial stimulation by alloantigen elicited a 45-fold and 53-fold increase in telomerase activity in the CD8+ and CD4+ cell cultures, respectively. However, by the 4th antigenic stimulation, telomerase activity was undetectable in the CD8+ T cells, whereas CD4 T cells still showed robust telomerase activity even as late as the 10th round of antigenic stimulation.

The divergent patterns of telomerase activity between CD4+ and CD8+ T cells paralleled the pattern of CD28 expression changes. By the 7th antigenic stimulation, 90% of the cells in the CD8+ culture no longer expressed CD28, whereas at that same stage, the CD4+ cultures were still 75% CD28+ [Valenzuela et al., 2002]. The intimate relationship between telomerase activation and CD28 signaling was further demonstrated in experiments using antibodies to block CD28 binding to its ligand on antigen-presenting cells, which resulted in a significant reduction in telomerase activity. Finally, the distinct contribution of CD28 to telomerase activation was evident in experiments using Cyclosporin, a TCR signaling inhibitor. Cyclosporin was not able to inhibit telomerase in T cells activated by a combination of antibodies to CD3 and CD28, whereas potent inhibition was observed when only anti-CD3 was used for stimulation [Valenzuela et al., 2002].

HIV-Infected Patients

Modern antiretroviral therapy can completely suppress viral replication and prevent AIDS-related complications. However, HIV-infected patients age faster and prematurely develop age-associated diseases ranging from frailty to cardiovascular disease, central nervous system complications, and renal disease [Deeks et al., 2012; Rickabaugh et al., 2015].

Activation of immune cells and inflammation occur in both HIV infection and aging, and both conditions lead to immunosenescence [De Biasi et al., 2011; Fülöp et al., 2017]. In HIV patients, direct activation of adaptive and innate immune cells by infectious and noninfectious HIV antigens is present, while HIV disruption of the gastrointestinal barrier and subsequent microbial translocation lead to indirect induction of activation and inflammation [Fülöp et al., 2017]. The study of immunosenescence in HIV patients has gained interest now that HIV-infected patients can be treated with antiretroviral therapy and their overall prognosis shifted from years to decades. This signifies that the treatment transforms HIV infection from a chronic infection to a chronic inflammatory disease [Fülöp et al., 2017]. Moreover, immunosenescence has been proposed to be related to increased morbidity and mortality by all causes in HIV patients [Desai and Landay, 2010], as aging of the immune system continues to be accelerated after viral replication is suppressed, and not all HIV-associated immune defects can be reversed, even if lymphocyte counts recover in many patients following antiretroviral therapy [Dion et al., 2007; Douek et al., 1998].

Most people with HIV infection become aged, and older adults have been contracting HIV infection. Thus, there is a great interest to study HIV infection in relation to immunosenescence and inflammaging to determine whether immunosenescence contributes to HIV infection, or if HIV is causing immunosenescence and, as such, represents a premature immunosenescence and accelerated aging. Although there are many similarities in the immune and inflammatory changes and the occurrence of age-related chronic diseases between normal aging and HIV infection, the interaction between these processes is not well understood, and consequently the concept that HIV infection is an accelerated aging model is questioned.

Features of immunosenescence in treated HIV patients include impaired hematopoiesis, impaired T cell generation and homeostasis, defective responses to vaccination, and increased constitutive production of inflammatory mediators [Desai and Landay, 2010; Douek et al., 1998; Sauce et al., 2011]. Several of the T cell abnormalities associated with aging are similar to those observed in untreated HIV patients [Justice, 2010] and many of the T cell alterations that are current hallmarks of immunosenescence have been characterized earlier in the context of HIV infection [Kalayjian et al., 2003] Compared to age-matched controls, HIV-infected patients have reduced thymic output [Dion et al., 2004], decreased naïve T cells, reduced T cell telomere length, expansions of terminally differentiated effector T cells [CD28−], and a contracted T cell repertoire [Kalayjian et al., 2003; Dion et al., 2004; Appay et al., 2007; Effros, 2011; Effros et al., 2005].

HIV infection is associated with loss of CD4+ T helper cells, which occurs mostly in gut-associated lymphoid tissue [GALT] and leads to depletion of Th-17 cells, an essential part of the host defense mechanisms against bacterial infections [Korn et al., 2009]. Extensive depletion of Th-17 cell in the gut results in loss of barrier integrity, causing gut leakiness and translocation of microbes and microbial products into circulation [Cohen, 2008]. The mechanism of CD4+ T cell depletion is partially due to direct effects of viral replication, but results also from activation-induced cell death [AICD] [Desai and Landay, 2010]. Indeed, microbial translocation is associated with chronic immune activation in HIV-infected individuals [Brenchley et al., 2006]. An increased thymic output is an important factor to reduce the impact of aberrant activation caused both by HIV infection and the HIV-induced microbial translocation [Bourgeois et al., 2008]. The effects of aging on GALT are still poorly understood, but there is clinical evidence of increased infections in the elderly due to organisms in which gastrointestinal immunity is essentially involved [Redelings et al., 2007]. Thus, microbial translocation is an outcome of HIV pathology, and an aging immune system fails to control gut-derived antigens due to dysfunctional thymic output, suggesting that HIV infection is the major cause of microbial translocation and premature aging consequence.

Immunosenescence in HIV-infected patients is not caused by a single mechanism. HIV patients successfully treated must rebuild T cell repertoire from a thymus that is functionally compromised by aging and the infection [Teixeira et al., 2001]. As previously commented, cooccurrence with latent CMV infection is a confounding cofactor that accelerates immune aging in a host who has a compromised T cell regenerative system and a repertoire stability [Appay et al., 2011]. Even if normal lymphocyte numbers are being reached, T cell diversity and subset distribution may be compromised. Restoring thymic activity for reconstituting the T cell repertoire increases the replicative burden of the immune system more than it is necessary for normal immune aging, and it can benefit from therapeutic to improved thymic output [Hollander et al., 2010].