Age-related dysregulation in CD8 T cell homeostasis: kinetics of a diversity loss

Effect of cellular aging on memory T-cell homeostasis

The fact that T-cell numbers remain relatively stable throughout life, and that T-cell proliferation rates increase during lymphopenia, has led to the consensus that T-cell numbers are regulated in a density-dependent manner. Competition for resources among memory T cells has been proposed to underlie this ‘homeostatic’ regulation. We first review how two classic models of resource competition affect the T-cell receptor (TCR) diversity of the memory T-cell pool. First, ‘global’ competition for cytokines leads to a skewed repertoire that tends to be dominated by the very first immune response. Second, additional ‘cognate’ competition for specific antigens results in a very diverse and stable memory T-cell pool, allowing every antigen to be remembered, which we therefore define as the ‘gold-standard’. Because there is limited evidence that memory T cells of the same specificity compete more strongly with each other than with memory T cells of different specificities, i.e., for ‘cognate’ competition, we investigate whether cellular aging could account for a similar level of TCR diversity. We define cellular aging as a declining cellular fitness due to reduced proliferation. We find that the gradual erosion of previous T-cell memories due to cellular aging allows for better establishment of novel memories and for a much higher level of TCR diversity compared to global competition. A small continual source (either from stem-cell-like memory T-cells or from naive T-cells due to repeated antigen exposure) improves the diversity of the memory T-cell pool, but remarkably, only in the cellular aging model. We further show that the presence of a source keeps the inflation of chronic memory responses in check by maintaining the immune memories to non-chronic antigens. We conclude that cellular aging along with a small source provides a novel and immunologically realistic mechanism to achieve and maintain the ‘gold-standard’ level of TCR diversity in the memory T-cell pool.

CD8+ T cells specific for an immunodominant SARS-CoV-2 nucleocapsid epitope display high naive precursor frequency and TCR promiscuity

To better understand primary and recall T cell responses during coronavirus disease 2019 (COVID-19), it is important to examine unmanipulated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific T cells. By using peptide-human leukocyte antigen (HLA) tetramers for direct ex vivo analysis, we characterized CD8⁺ T cells specific for SARS-CoV-2 epitopes in COVID-19 patients and unexposed individuals. Unlike CD8⁺ T cells directed toward subdominant epitopes (B7/N₂₅₇, A2/S₂₆₉, and A24/S_1,208) CD8⁺ T cells specific for the immunodominant B7/N₁₀₅ epitope were detected at high frequencies in pre-pandemic samples and at increased frequencies during acute COVID-19 and convalescence. SARS-CoV-2-specific CD8⁺ T cells in pre-pandemic samples from children, adults, and elderly individuals predominantly displayed a naive phenotype, indicating a lack of previous cross-reactive exposures. T cell receptor (TCR) analyses revealed diverse TCRαβ repertoires and promiscuous αβ-TCR pairing within B7/N₁₀₅⁺CD8⁺ T cells. Our study demonstrates high naive precursor frequency and TCRαβ diversity within immunodominant B7/N₁₀₅-specific CD8⁺ T cells and provides insight into SARS-CoV-2-specific T cell origins and subsequent responses.

Why COVID-19 Transmission Is More Efficient and Aggressive Than Viral Transmission in Previous Coronavirus Epidemics?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is causing a pandemic of coronavirus disease 2019 (COVID-19). The worldwide transmission of COVID-19 from human to human is spreading like wildfire, affecting almost every country in the world. In the past 100 years, the globe did not face a microbial pandemic similar in scale to COVID-19. Taken together, both previous outbreaks of other members of the coronavirus family (severe acute respiratory syndrome (SARS-CoV) and middle east respiratory syndrome (MERS-CoV)) did not produce even 1% of the global harm already inflicted by COVID-19. There are also four other CoVs capable of infecting humans (HCoVs), which circulate continuously in the human population, but their phenotypes are generally mild, and these HCoVs received relatively little attention. These dramatic differences between infection with HCoVs, SARS-CoV, MERS-CoV, and SARS-CoV-2 raise many questions, such as: Why is COVID-19 transmitted so quickly? Is it due to some specific features of the viral structure? Are there some specific human (host) factors? Are there some environmental factors? The aim of this review is to collect and concisely summarize the possible and logical answers to these questions.

Functional recreation of age-related CD8 T cells in young mice identifies drivers of aging- and human-specific tissue pathology

Mitigating effects of aging on human health remains elusive because aging impacts multiple systems simultaneously, and because experimental animals exhibit critical aging differences relative to humans. Separation of aging into discrete processes may identify targetable drivers of pathology, particularly when applied to human-specific features. Gradual homeostatic expansion of CD8 T cells dominantly alters their function in aging humans but not in mice. Injecting T cells into athymic mice induces rapid homeostatic expansion, but its relevance to aging remains uncertain. We hypothesized that homeostatic expansion of T cells injected into T-deficient hosts models physiologically relevant CD8 T cell aging in young mice, and aimed to analyze age-related T cell phenotype and tissue pathology in such animals. Indeed, we found that such injection conferred uniform age-related phenotype, genotype, and function to mouse CD8 T cells, heightened age-associated tissue pathology in young athymic hosts, and humanized amyloidosis after brain injury in secondary wild-type recipients. This validates a model conferring a human-specific aging feature to mice that identifies targetable drivers of tissue pathology. Similar examination of independent aging features should promote systematic understanding of aging and identify additional targets to mitigate its effects on human health.

Thymic Epithelial Cell Support of Thymopoiesis Does Not Require Klotho

Age-related thymic involution is characterized by a decrease in thymic epithelial cell (TEC) number and function parallel to a disruption in their spatial organization, resulting in defective thymocyte development and proliferation as well as peripheral T cell dysfunction. Deficiency of Klotho, an antiaging gene and modifier of fibroblast growth factor signaling, causes premature aging. To investigate the role of Klotho in accelerated age-dependent thymic involution, we conducted a comprehensive analysis of thymopoiesis and peripheral T cell homeostasis using Klotho-deficient (Kl/Kl) mice. At 8 wk of age, Kl/Kl mice displayed a severe reduction in the number of thymocytes (10-100-fold reduction), especially CD4 and CD8 double-positive cells, and a reduction of both cortical and medullary TECs. To address a cell-autonomous role for Klotho in TEC biology, we implanted neonatal thymi from Klotho-deficient and -sufficient mice into athymic hosts. Kl/Kl thymus grafts supported thymopoiesis equivalently to Klotho-sufficient thymus transplants, indicating that Klotho is not intrinsically essential for TEC support of thymopoiesis. Moreover, lethally irradiated hosts given Kl/Kl or wild-type bone marrow had normal thymocyte development and comparably reconstituted T cells, indicating that Klotho is not inherently essential for peripheral T cell reconstitution. Because Kl/Kl mice have higher levels of serum phosphorus, calcium, and vitamin D, we evaluated thymus function in Kl/Kl mice fed with a vitamin D-deprived diet. We observed that a vitamin D-deprived diet abrogated thymic involution and T cell lymphopenia in 8-wk-old Kl/Kl mice. Taken together, our data suggest that Klotho deficiency causes thymic involution via systemic effects that include high active vitamin D levels.

Multi-color flow cytometry for evaluating age-related changes in memory lymphocyte subsets in dogs

While dogs are increasingly being utilized as large-animal models of disease, important features of age-related immunosenescence in the dog have yet to be evaluated due to the lack of defined naïve vs. memory T lymphocyte phenotypes. We therefore performed multi-color flow cytometry on peripheral blood mononuclear cells from young and aged beagles, and determined the differential cytokine production by proposed memory subsets. CD4+ and CD8+ T lymphocytes in aged dogs displayed increased cytokine production, and decreased proliferative capacity. Antibodies targeting CD45RA and CD62L, but less so CD28 or CD44, defined canine cells that consistently exhibited properties of naïve-, central memory-, effector memory-, and terminal effector-like CD4+ and CD8+ T lymphocyte subsets. Older dogs demonstrated decreased frequencies of naïve-like CD4+ and CD8+ T lymphocytes, and an increased frequency of terminal effector-like CD8+ T lymphocytes. Overall findings revealed that aged dogs displayed features of immunosenescence similar to those reported in other species.

Virtual memory cells make a major contribution to the response of aged influenza-naïve mice to influenza virus infection

Background

A diverse repertoire of naïve T cells is thought to be essential for a robust response to new infections. However, a key aspect of aging of the T cell compartment is a decline in numbers and diversity of peripheral naïve T cells. We have hypothesized that the age-related decline in naïve T cells forces the immune system to respond to new infections using cross-reactive memory T cells generated to previous infections that dominate the aged peripheral T cell repertoire.

Results

Here we confirm that the CD8 T cell response of aged, influenza-naïve mice to primary infection with influenza virus is dominated by T cells that derive from the memory T cell pool. These cells exhibit the phenotypic characteristics of virtual memory cells rather than true memory cells. Furthermore, we find that the repertoire of responding CD8 T cells is constrained compared with that of young mice, and differs significantly between individual aged mice. After infection, these virtual memory CD8 T cells effectively develop into granzyme-producing effector cells, and clear virus with kinetics comparable to naïve CD8 T cells from young mice.

Conclusions

The response of aged, influenza-naive mice to a new influenza infection is mediated largely by memory CD8 T cells. However, unexpectedly, they have the phenotype of VM cells. In response to de novo influenza virus infection, the VM cells develop into granzyme-producing effector cells and clear virus with comparable kinetics to young CD8 T cells.

Lifelong CMV infection improves immune defense in old mice by broadening the mobilized TCR repertoire against third-party infection

Lifelong interactions between host and the ubiquitous and persistent cytomegalovirus (CMV) have been proposed to contribute to the age-related decline in immunity. Prior work from us and others found some support for that idea, yet evidence that this led to increased vulnerability to other infections was not obtained. Moreover, evidence has accumulated that CMV infection can be beneficial to immune defense in young/adult mice and humans, dominantly via enhanced innate immunity. Here, we describe an unexpected impact of murine CMV (MCMV) upon the T cell response of old mice to Listeria monocytogenes expressing the model antigen, OVA (Lm-OVA). Single-cell sequencing of the OVA-specific CD8 T cell receptor β (TCRβ) repertoire of old mice demonstrated that old MCMV-infected mice recruited many diverse clonotypes that afforded broad and often more efficient recognition of antigenic peptide variants. This stood in contrast to old control mice, which exhibited strong narrowing and homogenization of the elicited repertoire. High-throughput sequencing of the total naïve CD8 TCRβ repertoire showed that many of these diverse OVA-specific clonotypes were present in the naïve CD8 repertoire of mice in all groups (adult, old control, and old MCMV⁺) yet were only recruited into the Lm-OVA response in MCMV⁺ old mice. These results have profound implications for our understanding of T cell immunity over a life span and suggest that our coevolution with CMV may include surprising, potentially positive impacts on adaptive heterologous immunity in late life.

Tuberculosis in the elderly: Why inflammation matters

Growing old is associated with an increase in the basal inflammatory state of an individual and susceptibility to many diseases, including infectious diseases. Evidence is growing to support the concept that inflammation and disease susceptibility in the elderly is linked. Our studies focus on the infectious disease tuberculosis (TB), which is caused by Mycobacterium tuberculosis (M.tb), a pathogen that infects approximately one fourth of the world's population. Aging is a major risk factor for developing TB, and inflammation has been strongly implicated. In this review we will discuss the relationship between inflammation in the lung and susceptibility to develop and succumb to TB in old age. Further understanding of the relationship between inflammation, age, and M.tb will lead to informed decisions about TB prevention and treatment strategies that are uniquely designed for the elderly.

Age-Associated B Cells: A T-bet-Dependent Effector with Roles in Protective and Pathogenic Immunity

A newly discovered B cell subset, age-associated B cells, expresses the transcription factor T-bet, has a unique surface phenotype, and accumulates progressively with age. Moreover, B cells with these general features are associated with viral infections and autoimmunity in both mice and humans. In this article, we review current understanding of the characteristics, origins, and functions of these cells. We also suggest that the protective versus pathogenic actions of these cells reflect appropriate versus aberrant engagement of regulatory mechanisms that control the Ab responses to nucleic acid-containing Ags.

Suppressive effects of androgens on the immune system

Sex-based disparities in immune responses are well known phenomena. The two most important factors accounting for the sex-bias in immunity are genetics and sex hormones. Effects of female sex hormones, estrogen and progesterone are well established, however the role of testosterone is not completely understood. Evidence from unrelated studies points to an immunosuppressive role of testosterone on different components of the immune system, but the underlying molecular mechanisms remains unknown. In this review we evaluate the effect of testosterone on key cellular components of innate and adaptive immunity. Specifically, we highlight the importance of testosterone in down-regulating the systemic immune response by cell type specific effects in the context of immunological disorders. Further studies are required to elucidate the molecular mechanisms of testosterone-induced immunosuppression, leading the way to the identification of novel therapeutic targets for immune disorders.

Aging of the T cell compartment in mice and humans: from no naive expectations to foggy memories

Until the mid-20th century, infectious diseases were the major cause of morbidity and mortality in humans. Massive vaccination campaigns, antibiotics, antivirals, and advanced public health measures drastically reduced sickness and death from infections in children and younger adults. However, older adults (>65 y of age) remain vulnerable to infections, and infectious diseases remain among the top 5-10 causes of death in this population. The aging of the immune system, often referred to as immune senescence, is the key phenomenon underlying this vulnerability. This review centers on age-related changes in T cells, which are dramatically and reproducibly altered with aging. I discuss changes in T cell production, maintenance, function, and response to latent persistent infection, particularly against CMV, which exerts a profound influence on the aging T cell pool, concluding with a brief list of measures to improve immune function in older adults.

Coordinated regulation of NK receptor expression in the maturing human immune system

NK cells are responsible for recognizing and killing transformed, stressed, and infected cells. They recognize a set of non-Ag-specific features termed "altered self" through combinatorial signals from activating and inhibitory receptors. These NKRs are also expressed on CD4(+) and CD8(+) T cells, B cells, and monocytes, although a comprehensive inventory of NKR expression patterns across leukocyte lineages has never been performed. Using mass cytometry, we found that NKR expression patterns distinguish cell lineages in human peripheral blood. In individuals with high levels of CD57, indicative of a mature immune repertoire, NKRs are more likely to be expressed on non-NK cells, especially CD8(+) T cells. Mature NK and CD8(+) T cell populations show increased diversity of NKR surface expression patterns, but with distinct determinants: mature NK cells acquire primarily inhibitory receptors, whereas CD8(+) T cells attain a specific subset of both activating and inhibitory receptors, potentially imbuing them with a distinct functional role. Concurrently, monocytes show decreased expression of the generalized inhibitory receptor leukocyte Ig-like receptor subfamily b member 1, consistent with an increased activation threshold. Therefore, NKR expression is coordinately regulated as the immune system matures, resulting in the transfer of "altered self" recognition potential among leukocyte lineages. This likely reduces Ag specificity in the mature human immune system, and implies that vaccines and therapeutics that engage both its innate and adaptive branches may be more effective in the settings of aging and chronic infection.

Ageing combines CD4 T cell lymphopenia in secondary lymphoid organs and T cell accumulation in gut associated lymphoid tissue

Background

CD4 T cell lymphopenia is an important T cell defect associated to ageing. Higher susceptibility to infections, cancer, or autoimmune pathologies described in aged individuals is thought to partly rely on T cell lymphopenia. We hypothesize that such diverse effects may reflect anatomical heterogeneity of age related T cell lymphopenia. Indeed, no data are currently available on the impact of ageing on T cell pool recovered from gut associated lymphoid tissue (GALT), a crucial site of CD4 T cell accumulation.

Results

Primary, secondary and tertiary lymphoid organs of C57BL/6 animals were analysed at three intervals of ages: 2 to 6 months (young), 10 to 14 months (middle-aged) and 22 to 26 months (old). We confirmed that ageing preferentially impacted CD4 T cell compartment in secondary lymphoid organs. Importantly, a different picture emerged from gut associated mucosal sites: during ageing, CD4 T cell accumulation was progressively developing in colon and small intestine lamina propria and Peyer’s patches. Similar trend was also observed in middle-aged SJL/B6 F1 mice. Interestingly, an inverse correlation was detected between CD4 T cell numbers in secondary lymphoid organs and colonic lamina propria of C57BL/6 mice whereas no increase in proliferation rate of GALT CD4 T cells was detected. In contrast to GALT, no CD4 T cell accumulation was detected in lungs and liver in middle-aged animals. Finally, the concomitant accumulation of CD4 T cell in GALT and depletion in secondary lymphoid organs during ageing was detected both in male and female animals.

Conclusions

Our data thus demonstrate that T cell lymphopenia in secondary lymphoid organs currently associated to ageing is not sustained in gut or lung mucosa associated lymphoid tissues or non-lymphoid sites such as the liver. The inverse correlation between CD4 T cell numbers in secondary lymphoid organs and colonic lamina propria and the absence of overt proliferation in GALT suggest that marked CD4 T cell decay in secondary lymphoid organs during ageing reflect redistribution of CD4 T cells rather than generalized CD4 T cell decay. Such anatomical heterogeneity may provide an important rationale for the diversity of immune defects observed during ageing.

Immunosenescence and age-related viral diseases

Immunosenescence is described as a decline in the normal functioning of the immune system associated with physiologic ageing. Immunosenescence contributes to reduced efficacy to vaccination and increased susceptibility to infectious diseases in the elderly. Extensive studies of laboratory animal models of ageing or donor lymphocyte analysis have identified changes in immunity caused by the ageing process. Most of these studies have identified phenotypic and functional changes in innate and adaptive immunity. However, it is unclear which of these defects are critical for impaired immune defense against infection. This review describes the changes that occur in innate and adaptive immunity with ageing and some age-related viral diseases where defects in a key component of immunity contribute to the high mortality rate in mouse models of ageing.

Use it or lose it: establishment and persistence of T cell memory

Pre-existing T cell memory provides substantial protection against viral, bacterial, and parasitic infections. The generation of protective T cell memory constitutes a primary goal for cell-mediated vaccines, thus understanding the mechanistic basis of memory development and maintenance are of major importance. The widely accepted idea that T cell memory pools are directly descended from the effector populations has been challenged by recent reports that provide evidence for the early establishment of T cell memory and suggest that the putative memory precursor T cells do not undergo full expansion to effector status. Moreover, it appears that once the memory T cells are established early in life, they can persist for the lifetime of an individual. This is in contrast to the reported waning of naïve T cell immunity with age. Thus, in the elderly, immune memory that was induced at an early age may be more robust than recently induced memory, despite the necessity for long persistence. The present review discusses the mechanisms underlying the early establishment of immunological memory and the subsequent persistence of memory T cell pools in animal models and humans.

The LCMV gp33-specific memory T cell repertoire narrows with age

Background

The memory response to LCMV in mice persists for months to years with only a small decrease in the number of epitope specific CD8 T cells. This long persistence is associated with resistance to lethal LCMV disease. In contrast to studies focused on the number and surface phenotype of the memory cells, relatively little attention has been paid to the diversity of TCR usage in these cells. CD8⁺ T cell responses with only a few clones of identical specificity are believed to be relatively ineffective, presumably due to the relative ease of virus escape. Thus, a broad polyclonal response is associated with an effective anti-viral CD8⁺ T cell response.

Results

In this paper we show that the primary CD8⁺ T cell response to the LCMV gp33-41 epitope is extremely diverse. Over time while the response remains robust in terms of the number of gp33-tetramer⁺ T cells, the diversity of the response becomes less so. Strikingly, by 26 months after infection the response is dominated by a small number TCRβ sequences. In addition, it is of note the gp33 specific CD8⁺ T cells sorted by high and low tetramer binding populations 15 and 22 months after infection. High and low tetramer binding cells had equivalent diversity and were dominated by a small number of clones regardless of the time tested. A similar restricted distribution was seen in NP396 specific CD8⁺ T cells 26 months after infection. The identical TCRVβ sequences were found in both the tetramer^hi and tetramer^lo binding populations. Finally, we saw no evidence of public clones in the gp33-specific response. No CDR3 sequences were found in more than one mouse.

Conclusions

These data show that following LCMV infection the CD8⁺ gp33-specific CD8 T cell response becomes highly restricted with enormous narrowing of the diversity. This narrowing of the repertoire could contribute to the progressively ineffective immune response seen in aging.

Early priming minimizes the age-related immune compromise of CD8⁺ T cell diversity and function

The elderly are particularly susceptible to influenza A virus infections, with increased occurrence, disease severity and reduced vaccine efficacy attributed to declining immunity. Experimentally, the age-dependent decline in influenza-specific CD8⁺ T cell responsiveness reflects both functional compromise and the emergence of ‘repertoire holes’ arising from the loss of low frequency clonotypes. In this study, we asked whether early priming limits the time-related attrition of immune competence. Though primary responses in aged mice were compromised, animals vaccinated at 6 weeks then challenged >20 months later had T-cell responses that were normal in magnitude. Both functional quality and the persistence of ‘preferred’ TCR clonotypes that expand in a characteristic immunodominance hierarchy were maintained following early priming. Similar to the early priming, vaccination at 22 months followed by challenge retained a response magnitude equivalent to young mice. However, late priming resulted in reduced TCRβ diversity in comparison with vaccination earlier in life. Thus, early priming was critical to maintaining individual and population-wide TCRβ diversity. In summary, early exposure leads to the long-term maintenance of memory T cells and thus preserves optimal, influenza-specific CD8⁺ T-cell responsiveness and protects against the age-related attrition of naïve T-cell precursors. Our study supports development of vaccines that prime CD8⁺ T-cells early in life to elicit the broadest possible spectrum of CD8⁺ T-cell memory and preserve the magnitude, functionality and TCR usage of responding populations. In addition, our study provides the most comprehensive analysis of the aged (primary, secondary primed-early and secondary primed-late) TCR repertoires published to date.

The elderly population is particularly susceptible to novel infections, especially the annual, seasonal epidemics caused by influenza viruses. Established T cell immunity directed at conserved viral regions provides some protection against influenza infection and promotes more rapid recovery, thus leading to better clinical outcomes. We asked whether priming early in life limits the time-related attrition of immune competence. We found that although influenza-specific T cell responses are compromised in the aged mice, vaccination with influenza early (but not late) in life ‘locks’ optimal T-cell responsiveness, maintains functional quality, persistence of preferred clones and a characteristic T cell hierarchy. Overall, our study supports development of vaccines that prime T cells early in life to elicit the broadest possible spectrum of pre-existing T cell memory and preserve the magnitude, functionality and clonal usage of responding populations for life-long immunity against influenza viruses.

CD8 T cell responses to influenza virus infection in aged mice

Influenza is one of the most common infectious diseases afflicting humans, particularly the elderly. The murine model has been widely employed for investigation of immunity to influenza virus infection. In this paper, we review the recent advances in understanding the diminished CD8 T cell immune response to influenza virus infection in aged mice. Possible mechanisms of impaired CD8 T cell responses with aging are addressed, including: (1) the role of dendritic cells (DCs); (2) the effect of age-associated changes in the T cell repertoire; and (3) the interactions with CD4 T cells, including T regulatory (Treg) cells and CD4 T helper cells. The aged murine model of the CD8 T cell response to influenza virus is helping to elucidate the mechanisms of immunosenescence which can lead to therapeutic improvements in the primary CD8 T cell response to new infections, as well as the development of new strategies for immunization to prevent influenza in the elderly.

Greater than the sum of their parts: combination strategies for immune regeneration following allogeneic hematopoietic stem cell transplantation

Cytoreductive conditioning regimes designed to allow for successful allogeneic hematopoietic stem cell transplantation (allo-HSCT) paradoxically are also detrimental to recovery of the immune system in general but lymphopoiesis in particular. Post-transplant immune depletion is particularly striking within the T cell compartment which is exquisitely sensitive to negative regulation, evidenced by the profound decline in thymic function with age. As a consequence, regeneration of the immune system remains a significant unmet clinical need. Over the past decade studies have revealed several promising therapeutic strategies to address ineffective lymphopoiesis and post-transplant immune deficiency. These include the use of cytokines such as IL-7, IL-12 and IL-15; growth factors and hormones like keratinocyte growth factor (KGF), insulin-like growth factor (IGF)-1 and growth hormone (GH); adoptive transfer of ex vivo-generated precursor T cells (pre-T) and sex steroid ablation (SSA). Moreover, recently several novel approaches have been proposed to generate whole thymii ex vivo using stem cell technologies and bioscaffolds. Increasingly, however, when transferred to the clinic, these strategies alone are not sufficient to restore thymopoiesis in all patients leading to the potential of combination strategies as a way to reign in non-responders. Synergistic enhancement in combination may be due to differential targets may therefore be effective in improving clinical outcomes in the transplant settings as well as in other lymphopenic states induced by high dose chemotherapy/radiation therapy or HIV, and may also be useful in improving responses to vaccination and augmenting anti-tumor immunotherapy.

The blood of healthy individuals exhibits CD8 T cells with a highly altered TCR Vb repertoire but with an unmodified phenotype

CD8 T cell clonal expansions (TCE) have been observed in elderly, healthy individuals as well in old mice, and have been associated with the ageing process. Both chronic latent and non-persistent viral infections have been proposed to drive the development of distinct non-functional and functional TCE respectively. Biases in TCR Vβ repertoire diversity are also recurrently observed in patients that have undergone strong immune challenge, and are preferentially observed in the CD8 compartment. Healthy adults can also exhibit CD8 T cells with strong alterations of their CDR3 length distribution. Surprisingly, no specific investigations have been conducted to analyze the CD8 T cell repertoire in normal adults, to determine if such alterations in TCR Vβ repertoire share the features of TCE. In this study, we characterized the phenotype and function of the CD8 population in healthy individuals of 25-52 years of age. All but one of the EBV-positive HLA-B8 healthy volunteers that were studied were CMV-negative. Using a specific unsupervised statistical method, we identified Vβ families with altered CDR3 length distribution and increased TCR Vβ/HPRT transcript ratios in all individuals tested. The increase in TCR Vβ/HPRT transcript ratio was more frequently associated with an increase in the percentage of the corresponding Vβ(+) T cells than with an absence of modification of their percentage. However, in contrast with the previously described TCE, these CD8(+) T cells were not preferentially found in the memory CD8 subset, they exhibited normal effector functions (cytokine secretion and cytotoxic molecule expression) and they were not reactive to a pool of EBV/CMV/Flu virus peptides. Taken together, the combined analysis of transcripts and proteins of the TCR Vβ repertoire led to the identification of different types of CD8(+) T cell clone expansion or contraction in healthy individuals, a situation that appears more complex than previously described in aged individuals.

Evolution of the antigen-specific CD8+ TCR repertoire across the life span: evidence for clonal homogenization of the old TCR repertoire

Defects in T cell responses against pathogens and reduced diversity of TCRs have been described at both extremes of the life span. Yet, we still lack information on how Ag-specific T cell populations are maintained and/or altered from birth to old age. In this study, for the first time to our knowledge, we provide insight into Ag-specific TCR repertoire changes over the life span at the single-cell level. We have examined the TCR diversity of the primary CD8(+) T cell response to the immunodominant HSV-1 epitope HSV glycoprotein B 495-502 (HSV gB(498-505); SSIEFARL) (gB-8p) in neonatal, adult, and old C57BL/6 mice. The global distinctive features of the gB-8p-specific TCR repertoire were preserved in mice of different ages. However, both old and especially neonatal mice exhibited significant decreases in TCR diversity compared with that of adult mice. Still, although the neonatal Ag-specific repertoire comprised expectedly shorter germline-biased CDR3β lengths, the repertoire was surprisingly complex, and only a minority of responding cells lacked random nucleotide additions. Changes with aging included increased use of the already dominant TCRVβ10 family, a trend for lower content of the TCR containing the germline WG motif in the CDR3, and a remarkable sharing of one dominant clonotype between individual old mice, implying operation of selective mechanisms. Implications for the rational design of vaccines for neonates and the elderly are discussed.

Nonmalignant clonal expansions of memory CD8+ T cells that arise with age vary in their capacity to mount recall responses to infection

Immune responsiveness declines with age in part due to the development of CD8(+) T cell clonal expansions (TCEs) that can dominate the peripheral T cell pool. Although some TCEs arise due to persistent Ag stimulation from chronic infections, others arise in the apparent absence of chronic infection. We have recently shown that this latter class of TCEs can arise over time from the memory CD8(+) T cell pool established by an acute viral infection. Unlike TCEs driven by chronic infections, these age-related TCEs do not display the phenotypic and in vitro functional characteristics of exhausted cells. However, the rate at which these age-related TCEs develop from the memory CD8(+) T cell pool, as well as their ability to mount a recall response to secondary pathogen challenge in vivo, is not known. In this study, we analyzed large cohorts of mice over time for the development of TCE following Sendai virus infection and found a progressive increase in the appearance of TCEs, such that most mice showed evidence of TCE within the memory T cell pool by 2 y postinfection. Using a dual adoptive transfer approach to address the recall potential of virus-specific TCEs, we also demonstrate that most TCEs examined are poorly responsive to a secondary infection. Therefore, we provide evidence that the development of TCE is a common occurrence due to the progressive dysregulation of the virus-specific memory T cell pool with age, but many TCEs are profoundly defective in their ability to mediate recall responses.

Immune memory and aging: an infinite or finite resource?

Recent developments in the field of immune memory research and the accumulating literature on age-related alterations in homeostasis, primary and memory T cell responses make it pertinent to address whether and how memory responses are affected by aging with regard to their generation, maintenance, and protective function. New knowledge of T cell repertoire maintenance over long periods of time, particularly when confronted with persistent pathogen challenge, is now enriched further by studies on whether recent immunological memory can 'overfill' and/or constrict prior memory responses. Along with studies on potentiation of memory responses by dietary/metabolic interventions and the recent advances on regulation of primary responses with aging, these findings provide a platform for new approaches to vaccination of older adults.

Loss of naive T cells and repertoire constriction predict poor response to vaccination in old primates

Aging is usually accompanied by diminished immune protection upon infection or vaccination. Although aging results in well-characterized changes in the T cell compartment of long-lived, outbred, and pathogen-exposed organisms, their relevance for primary Ag responses remain unclear. Therefore, it remains unclear whether and to what extent the loss of naive T cells, their partial replacement by oligoclonal memory populations, and the consequent constriction of TCR repertoire limit the Ag responses in aging primates. We show in this study that aging rhesus monkeys (Macaca mulatta) exhibit poor CD8 T cell and B cell responses in the blood and poor CD8 responses in the lungs upon vaccination with the modified vaccinia strain Ankara. The function of APCs appeared to be maintained in aging monkeys, suggesting that the poor response was likely intrinsic to lymphocytes. We found that the loss of naive CD4 and CD8 T cells, and the appearance of persisting T cell clonal expansions predicted poor CD8 responses in individual monkeys. There was strong correlation between early CD8 responses in the transitory CD28+ CD62L- CD8+ T cell compartment and the peak Ab titers upon boost in individual animals, as well as a correlation of both parameters of immune response to the frequency of naive CD8+ T cells in old but not in adult monkeys. Therefore, our results argue that T cell repertoire constriction and naive cell loss have prognostic value for global immune function in aging primates.

Sex steroid ablation enhances immune reconstitution following cytotoxic antineoplastic therapy in young mice

Cytotoxic antineoplastic therapy is used to treat malignant disease but results in long-term immunosuppression in postpubertal and adult individuals, leading to increased incidence and severity of opportunistic infections. We have previously shown that sex steroid ablation (SSA) reverses immunodeficiencies associated with age and hematopoietic stem cell transplantation in both autologous and allogeneic settings. In this study, we have assessed the effects of SSA by surgical castration on T cell recovery of young male mice following cyclophosphamide treatment as a model for the impact of chemotherapy. SSA increased thymic cellularity, involving all of the thymocyte subsets and early T lineage progenitors. It also induced early repair of damage to the thymic stromal microenvironment, which is crucial to the recovery of a fully functional T cell-based immune system. These functional changes in thymic stromal subsets included enhanced production of growth factors and chemokines important for thymopoiesis, which preceded increases in both thymocyte and stromal cellularity. These effects collectively translated to an increase in peripheral and splenic naive T cells. In conclusion, SSA enhances T cell recovery following cyclophosphamide treatment of mice, at the level of the thymocytes and their stromal niches. This provides a new approach to immune reconstitution following antineoplastic therapy.

Regeneration of dendritic cells in aged mice

Age-related thymic involution causes a decreased output of thymocytes from the thymus, thereby resulting in impairment of T cell-mediated immunity. While alterations in the T cell and non-haematopoietic stromal compartments have been described, the effects of thymic involution on thymic dendritic cells (DC) are not clearly known. Thymic DC play an essential role in shaping T cell-mediated immune responses by deleting self-reactive thymocytes to establish central tolerance and by inducing regulatory T-cell (Treg) development. It is therefore important to assess the prevalence of and alterations to thymic DC with age, as this may impact on their function. We assessed the numbers and proportions of the three distinct subsets of thymic DC in ageing mice, and showed that these subsets are differentially regulated. This is expected as thymic DC subsets have different origins of development. We further assessed the responses of thymic DC in a regenerative environment, such as that induced by sex-steroid ablation (SSA), and clearly showed that, consistent with global thymus regrowth, all three DC populations increased in numbers and regained their relative proportions to thymocytes after an initial lag period. These findings are important for the clinical translation of thymic regenerative approaches, and indicate that SSA facilitates the maintenance of critical processes such as negative selection and Treg induction through promoting thymic DC regeneration.

Sex steroid ablation enhances hematopoietic recovery following cytotoxic antineoplastic therapy in aged mice

Cytotoxic antineoplastic therapy is widely used in the clinic as a treatment for malignant diseases. The treatment itself, however, leads to long-term depletion of the adaptive immune system, which is more pronounced in older patients, predominantly due to thymic atrophy. We and others have previously shown that withdrawal of sex steroids is able to regenerate the aged thymus and enhance recovery from autologous and allogeneic hematopoietic stem cell transplant. In this study we have examined the effects of sex steroid ablation (SSA) on the recovery of lymphopoiesis in the bone marrow (BM) and thymus following treatment with the chemotherapeutic agent cyclophosphamide (Cy) in middle-aged and old mice. Furthermore, we have also examined the impact of this regeneration on peripheral immunity. SSA enhanced the recovery of BM resident hematopoietic stem cells and lymphoid progenitors and promoted lymphopoiesis. Interestingly, Cy alone caused a profound increase in the recently described common lymphoid progenitor 2 (CLP-2) population in the BM. In the thymus, SSA caused a profound increase in cellularity as well as all intrathymic T-lineage progenitors including early T-lineage progenitors (ETPs) and non-canonical T cell progenitors such as the CLP-2. We also found that these transferred into numerical increases in the periphery with enhanced B and T cell numbers. Furthermore, these lymphocytes were found to have an enhanced functional capacity with no perturbation of the TCR repertoire. Taken together, these results provide the basis for the use of SSA in the clinic to enhance treatment outcomes from cytotoxic antineoplastic therapy.

T-cell immunosenescence: lessons learned from mouse models of aging

It is well established that increasing age is associated with a decreased capacity of the immune system to mediate effective immune responses to vaccination and invading pathogens. Because of the inherent limitations of conducting experiments in humans, much of what we have learned is owed to the utility of experimental mouse models of aging. Recent studies performed in the mouse have demonstrated mechanisms responsible for age-related declines in the function of CD4(+) and CD8(+) cells. This review describes key findings regarding age-related defects in T-cell function and discusses the impact these defects have on vaccine efficacy and immunity.

Clonal expansions and loss of receptor diversity in the naive CD8 T cell repertoire of aged mice

There are well-characterized age-related changes in the peripheral repertoire of CD8 T cells characterized by reductions in the ratio of naive:memory T cells and the development of large clonal expansions in the memory pool. In addition, the TCR repertoire of naive T cells is reduced with aging. Because a diverse repertoire of naive T cells is essential for a vigorous response to new infections and vaccinations, there is much interest in understanding the mechanisms responsible for declining repertoire diversity. It has been proposed that one reason for declining repertoire diversity in the naive T cell pool is an increasing dependence on homeostatic proliferation in the absence of new thymic emigrants for maintenance of the naive peripheral pool. In this study, we have analyzed the naive CD8 T cell repertoire in young and aged mice by DNA spectratype and sequence analysis. Our data show that naive T cells from aged mice have perturbed spectratype profiles compared with the normally Gaussian spectratype profiles characteristic of naive CD8 T cells from young mice. In addition, DNA sequence analysis formally demonstrated a loss of diversity associated with skewed spectratype profiles. Unexpectedly, we found multiple repeats of the same sequence in naive T cells from aged but not young mice, consistent with clonal expansions previously described only in the memory T cell pool. Clonal expansions among naive T cells suggests dysregulation in the normal homeostatic proliferative mechanisms that operate in young mice to maintain diversity in the naive T cell repertoire.

Optimal window of caloric restriction onset limits its beneficial impact on T-cell senescence in primates

We have recently shown in non-human primates that caloric restriction (CR) initiated during adulthood can delay T-cell aging and preserve naïve CD8 and CD4 T cells into advanced age. An important question is whether CR can be initiated at any time in life, and whether age at the time of onset would modulate the beneficial effects of CR. In the current study, we evaluated the impact of CR started before puberty or during advanced age on T-cell senescence and compared it to the effects of CR started in early adulthood. Our data demonstrate that the beneficial effects of adult-onset CR on T-cell aging were lost by both early and late CR onset. In fact, some of our results suggest that inappropriate initiation of CR may be harmful to the maintenance of T-cell function. This suggests that there may be an optimal window during adulthood where CR can delay immune senescence and improve correlates of immunity in primates.

Identification of two major types of age-associated CD8 clonal expansions with highly divergent properties

CD8 memory T cells are tightly regulated in young, healthy individuals but are often perturbed in aged animals by the appearance of large CD8 T cell clones. These clones are associated with impaired immunity in the aged. The molecular basis of this phenomenon remains unclear. Here, it is shown that the issue is confused by the fact that the clones are heterogeneous. Some clones bear high, and others, low levels of integrin alpha(4) (itgalpha4). These subtypes differ by multiple criteria. They appear in mice of different ages, concentrate in different tissues, and have different stabilities in vivo and responses to stimulation in vitro. itgalpha4(high), but not itgalpha4(low), CD8 clonal expansions have several characteristics consistent with a chronically stimulated phenotype. These properties include lowered levels of CD8, decreased expression of some cytokine receptors, and elevated expression of various inhibitory receptors, including the programmed death-1 (PD1) receptor and the killer cell lectin-like receptor G1 (KLRG1). The characteristics of itgalpha4(high) clonal expansions suggest that they may arise from age-dependent alterations in antigen expression and tolerance. These data redefine CD8 clonal expansions into at least two distinct entities and indicate that there are multiple mechanisms that drive age-related alterations of CD8 T cell homeostasis.

Age-associated T-cell Clonal Expansions (TCE) in vivo—Implications for Pathogen Resistance

Age-related T-cell clonal expansions (TCE) are an incompletely understood disturbance in T-cell homeostasis found frequently in old humans and experimental animals. These accumulations of CD8 T-cells have the potential to distort T-cell population balance and reduce T-cell repertoire diversity above and beyond the changes seen in the aging of T-cell pool in the absence of TCE. This chapter discusses our current knowledge of the role of these expansions in health and disease, with a special focus on their influence upon immune defense against infectious diseases.

Ageing and life-long maintenance of T-cell subsets in the face of latent persistent infections

A decline in T-cell immunity is one of the most consistent and most profound deficiencies of the elderly. Therapeutic correction of this decline often restores immune responsiveness and immune defence.

T-cell immune decline in the elderly has at least two underpinnings: a drop in the responsiveness of naive T cells to stimulation (cell-autonomous defects) and a reduction in naive T-cell numbers and diversity that leads to a dominant memory T-cell pool (T-cell population imbalance).

This article discusses two key causes of age-related T-cell population imbalance: homeostatic cycling or proliferative expansion in the peripheral T-cell pool, and latent persistent infections, which repeatedly stimulate the T-cell pool over the lifetime of the individual.

The reduction in production of naive T cells by the thymus forces the ageing organism to rely on compensatory homeostatic mechanisms to maintain the balance between naive and memory T-cell pools. Although this may be initially successful, recent evidence suggests that late in life these mechanisms exhaust their usefulness and actually contribute to a further demise of the remaining naive T cells.

Latent persistent infections, particularly with herpesviruses, lead to life-long periodic restimulation of the immune system, here, evidence is presented for the role of viral reactivation in this restimulation using a mouse model of herpesvirus infection and ageing.

Relative roles and the interplay between the homeostatic and viral factors are discussed, with the former having a surprisingly prominent role. Finally, modes of immune rejuvenation and anti-ageing intervention are debated in light of these advances in our knowledge.

A decline in T-cell immunity is a major cause of morbidity and mortality from infectious diseases in the elderly. Janko Nikolich-Žugich weighs up the relative roles of and the interplay between homeostatic factors and persistent viruses in immune senescence.

A diverse and well-balanced repertoire of T cells is thought to be crucial for the efficacious defence against infection with new or re-emerging pathogens throughout life. In the last third of the mammalian lifespan, the maintenance of a balanced T-cell repertoire becomes highly challenging because of the changes in T-cell production and consumption. In this Review, I question whether latent persistent pathogens might be key factors that drive this imbalance and whether they determine the extent of age-associated immune deficiency.

Age-related dysregulation of CD8+ T cell memory specific for a persistent virus is independent of viral replication

The immune system devotes substantial resources to the lifelong control of persistent pathogens, which were hypothesized to play an important role in immune aging. Specifically, the presence of latent herpesviruses has been correlated with immune exhaustion and shorter lifespan in octogenarians. But neither the causality nor the mechanistic link(s) were established, and the relative roles of persistent antigenic stimulation and of virus-independent homeostatic disturbances in T cell aging remain unresolved. We longitudinally analyzed expansion, contraction, and long-term maintenance of CD8(+) T cells responding to localized infection with a latent virus, HSV-1. Young mice exhibited the expected expansion and contraction of HSV-1-specific cells and the stable maintenance of memory T cells into advanced adulthood. However, upon entry into senescence, many (>40%) animals exhibited an accumulation in Ag-specific cells (memory inflation) which in some animals was comparable to that observed in acute infection. Inflation occurred to the same extent in control mice and mice continuously treated with the anti-HSV drug famciclovir, which inhibits viral replication and was able to reduce expression of the glycoprotein B. Age-related inflation was also found long after infection with an acute virus. The inflating cells largely maintained Ag-specific function, and exhibited typical central memory phenotype, with no signs of Ag-specific activation. They exhibited increased expression of CD122 and CD127, akin to the Ag-independent T cell clonal expansions found in old specific pathogen-free laboratory mice. This collectively suggests that, in this model, the inflating cells may be selected for high responsiveness to environmental cytokines largely in an Ag-independent manner.

Age-associated decline in T cell repertoire diversity leads to holes in the repertoire and impaired immunity to influenza virus

A diverse T cell repertoire is essential for a vigorous immune response to new infections, and decreasing repertoire diversity has been implicated in the age-associated decline in CD8 T cell immunity. In this study, using the well-characterized mouse influenza virus model, we show that although comparable numbers of CD8 T cells are elicited in the lung and lung airways of young and aged mice after de novo infection, a majority of aged mice exhibit profound shifts in epitope immunodominance and restricted diversity in the TCR repertoire of responding cells. A preferential decline in reactivity to viral epitopes with a low naive precursor frequency was observed, in some cases leading to “holes” in the T cell repertoire. These effects were also seen in young thymectomized mice, consistent with the role of the thymus in maintaining naive repertoire diversity. Furthermore, a decline in repertoire diversity generally correlated with impaired responses to heterosubtypic challenge. This study formally demonstrates in a mouse infection model that naturally occurring contraction of the naive T cell repertoire can result in impaired CD8 T cell responses to known immunodominant epitopes and decline in heterosubtypic immunity. These observations have important implications for the design of vaccine strategies for the elderly.

Dramatic increase in naive T cell turnover is linked to loss of naive T cells from old primates

The loss of naïve T cells is a hallmark of immune aging. Although thymic involution is a primary driver of this naïve T cell loss, less is known about the contribution of other mechanisms to the depletion of naïve T cells in aging primates. We examined the role of homeostatic cycling and proliferative expansion in different T cell subsets of aging rhesus macaques (RM). BrdU incorporation and the expression of the G(1)-M marker Ki-67 were elevated in peripheral naïve CD4 and even more markedly in the naïve CD8 T cells of old, but not young adult, RM. Proliferating naïve cells did not accumulate in old animals. Rather, the relative size of the naïve CD8 T cell compartment correlated inversely to its proliferation rate. Likewise, T cell receptor diversity decreased in individuals with elevated naïve CD8 T cell proliferation. This apparent contradiction was explained by a significant increase in turnover concomitant with the naïve pool loss. The turnover increased exponentially when the naïve CD8 T cell pool decreased below 4% of total blood CD8 cells. These results link the shrinking naïve T cell pool with a dramatic increase in homeostatic turnover, which has the potential to exacerbate the progressive exhaustion of the naïve pool and constrict the T cell repertoire. Thus, homeostatic T cell proliferation exhibits temporal antagonistic pleiotropy, being beneficial to T cell maintenance in adulthood but detrimental to the long-term T cell maintenance in aging individuals.

Antigen-specific CD8+ T cell clonal expansions develop from memory T cell pools established by acute respiratory virus infections

Increasing age is associated with the development of CD8+ T cell clonal expansions (TCE) that can dominate the peripheral T cell repertoire and interfere with immune responses to infection and vaccination. Some TCE are driven by chronic infections, consistent with dysregulated outgrowth of T cell clones in response to persistent antigenic stimulation. However, a second class of TCE develops with age in the absence of chronic infections and is poorly understood in terms of origin or Ag dependence. In this study, we present evidence that Ag-specific TCE develop at high frequencies from conventional memory CD8+ T cell pools elicited by nonpersistent influenza and parainfluenza virus infections. Putative TCE occurred in both the central- and effector-memory CD8+ T cell populations and did not require Ag for their maintenance. In addition, they were similar to normal memory T cells in terms of phenotype and function, suggesting that they develop stochastically from the memory T cell pool. These data suggest that memory T cell pools become progressively dysregulated over time and this may have a significant impact on immune responsiveness in the aged.

Immunosenescence: deficits in adaptive immunity in the elderly

Aging is associated clinically with increases in the frequency and severity of infectious diseases and an increased incidence of cancer, chronic inflammatory disorders and autoimmunity. These age-associated immune dysfunctions are the consequence of declines in both the generation of new naïve T and B lymphocytes and the functional competence of memory populations. These alterations collectively are termed immunosenescence.

Enhanced immune reconstitution by sex steroid ablation following allogeneic hemopoietic stem cell transplantation

Delayed immune reconstitution in adult recipients of allogeneic hemopoietic stem cell transplantations (HSCT) is related to age-induced thymic atrophy. Overcoming this paucity of T cell function is a major goal of clinical research but in the context of allogeneic transplants, any strategy must not exacerbate graft-vs-host disease (GVHD) yet ideally retain graft-vs-tumor (GVT) effects. We have shown sex steroid ablation reverses thymic atrophy and enhances T cell recovery in aged animals and in congenic bone marrow (BM) transplant but the latter does not have the complications of allogeneic T cell reactivity. We have examined whether sex steroid ablation promoted hemopoietic and T cell recovery following allogeneic HSCT and whether this benefit was negated by enhanced GVHD. BM and thymic cell numbers were significantly increased at 14 and 28 days after HSCT in castrated mice compared with sham-castrated controls. In the thymus, the numbers of donor-derived thymocytes and dendritic cells were significantly increased after HSCT and castration; donor-derived BM precursors and developing B cells were also significantly increased. Importantly, despite restoring T cell function, sex steroid inhibition did not exacerbate the development of GVHD or ameliorate GVT activity. Finally, IL-7 treatment in combination with castration had an additive effect on thymic cellularity following HSCT. These results indicate that sex steroid ablation can profoundly enhance thymic and hemopoietic recovery following allogeneic HSCT without increasing GVHD and maintaining GVT.

CD8 T cell clonal expansions & aging: a heterogeneous phenomenon with a common outcome

A highly diverse CD8 T cell repertoire is thought to be critical for maintaining appropriate immune defenses against a variety of pathogens. However, in many aged individuals, the diversity of T cell receptors is significantly reduced by the presence of large, monoclonal expansions of CD8 memory T cells. While ongoing research is focused on understanding the molecular alterations in these expansions, one major hurdle to this goal is the apparent heterogeneity of CD8 clonal expansions, which is apparent even in reductionist systems. In this review, we discuss current evidence that CD8 clonal expansions are a heterogeneous phenomenon, and our evolving understanding of what this heterogeneity tells us about CD8 memory T cell homeostasis and how it is altered in aged individuals.

Immunizations in the elderly: do they live up to their promise?

In the 21st century, public health is not only challenged by newly emerging and re-emerging infectious diseases but also by demographic developments that are taking place in many countries. Importantly, infections in the elderly are more frequent, more severe and have distinct features with respect to clinical presentation and treatment. This is due to a decline in the functions of the immune system referred to as immunosenescence. The most important age-related changes affect the T cell system. Although this derogates the protective effect of some vaccines, vaccinations are still considered the most cost-effective medical procedure for preventing morbidity and mortality caused by infectious diseases. The present article aims at outlining the impact of infectious diseases on the elderly and summarizing the progress made in the field of vaccinations of the elderly and how age-related changes within the immune system contribute to the decreased efficacy of vaccines.

Age-related CD8+ T cell clonal expansions express elevated levels of CD122 and CD127 and display defects in perceiving homeostatic signals

Aging is accompanied by numerous changes in T cell biology. Among the most dramatic changes at the population level are the appearance and persistence of CD8+ T cell clonal expansions (TCE), whose frequency increases steadily with age, and whose biology is incompletely understood. In this study, we examined trafficking, phenotypic makeup, and homeostatic responsiveness of TCE, which arise spontaneously in specific pathogen-free mice. We show that these cells make up a specialized subset of central memory T cells with distinguishable phenotypic characteristics, most notably the higher expression of CD122 and CD127, molecules that make up IL-15R and IL-7R, respectively, than other memory T cells. We confirm that these cells proliferate at a continuous pace upon adoptive transfer into the eulymphoid recipient, unlike their non-TCE memory-phenotype counterparts, which remain undivided and die. However, upon transfer into lymphopenic recipients, TCE fail to rapidly expand, but rather resume their slow, continuous proliferation. The above results are discussed in light of possible mechanisms that afford selective survival advantage to TCE over other T cells in an aged T lymphocyte pool.

Effective control of chronic gamma-herpesvirus infection by unconventional MHC Class Ia-independent CD8 T cells

Control of virus infection is mediated in part by major histocompatibility complex (MHC) Class Ia presentation of viral peptides to conventional CD8 T cells. Although important, the absolute requirement for MHC Class Ia–dependent CD8 T cells for control of chronic virus infection has not been formally demonstrated. We show here that mice lacking MHC Class Ia molecules (K^b−/−xD^b−/− mice) effectively control chronic γ-herpesvirus 68 (γHV68) infection via a robust expansion of β₂-microglobulin (β₂-m)-dependent, but CD1d-independent, unconventional CD8 T cells. These unconventional CD8 T cells expressed: (1) CD8αβ and CD3, (2) cell surface molecules associated with conventional effector/memory CD8 T cells, (3) TCRαβ with a significant Vβ4, Vβ3, and Vβ10 bias, and (4) the key effector cytokine interferon-γ (IFNγ). Unconventional CD8 T cells utilized a diverse TCR repertoire, and CDR3 analysis suggests that some of that repertoire may be utilized even in the presence of conventional CD8 T cells. This is the first demonstration to our knowledge that β₂-m–dependent, but Class Ia–independent, unconventional CD8 T cells can efficiently control chronic virus infection, implicating a role for β₂-n–dependent non-classical MHC molecules in control of chronic viral infection. We speculate that similar unconventional CD8 T cells may be able to control of other chronic viral infections, especially when viruses evade immunity by inhibiting generation of Class Ia–restricted T cells.

In this paper the authors identify a β2-microglobulin–dependent but major histocompatibility complex (MHC) Class Ia– and CD1-independent class of CD8 T cells that effectively control chronic γ-herpesvirus infection in mice. The important point that should be of general interest to the readers of PLoS Pathogens is that an effective CD8 T cell response develops during chronic infection of mice lacking MHC Class Ia molecules. Enormous efforts have gone into characterizing the role of conventional CD8 T cells that recognize viral peptides together with MHC Class Ia molecules during chronic viral infection, and many vaccine approaches focus solely on this response. This paper shows that additional types of CD8 T cells can operate during chronic infection, and that indeed, conventional MHC Class Ia–restricted T cells may be dispensable for control of chronic herpesvirus infection. The authors believe this is a fundamentally important point because it raises the question of whether unconventional CD8 T cells are important for control of other chronic viral infections such as infection with HIV, Hepatitis C virus, Hepatitis B virus, or human herpesviruses.

Antibody-induced transplantation tolerance that is dependent on thymus-derived regulatory T cells

Targeting of the CD45RB isoform by mAb (anti-CD45RB) effectively induces donor-specific tolerance to allografts. The immunological mechanisms underlying the tolerant state remain unclear although some studies have suggested the involvement of regulatory T cells (T-regs). Although their generative pathway remains undefined, tolerance promoting T-regs induced by systemic anti-CD45RB treatment have been assumed to originate in the peripheral immune system. We demonstrate herein that separable effects on the peripheral and central immune compartments mediate graft survival induced by anti-CD45RB administration. In the absence of the thymus, anti-CD45RB therapy is not tolerogenic though it retains peripheral immunosuppressive activity. The thymus is required for anti-CD45RB to produce indefinite graft survival and donor-specific tolerance, and this effect is accomplished through thymic production of donor-specific T-regs. These data reveal for the first time an Ab-based tolerance regimen that relies on the central tolerance pathway.

CDR3 Spectratyping Analysis of the TCR Repertoire in Myasthenia Gravis

Because myasthenia gravis (MG) is an autoimmune disease mediated by Abs specific for the acetylcholine receptor, helper T cells play a role in Ab production. In this study, we have performed large-scale cross-sectional and longitudinal TCR studies by CDR3 spectratyping using PBL and thymus tissues from MG patients. We found that there was no preferential usage of any particular TCR beta-chains that was identical among MG patients. However, the longitudinal study clearly demonstrated that one or more TCR Vbeta expansions persisted frequently in MG patients. Importantly, persistent TCR expansions correlated with clinical severity and high anti-acetylcholine receptor Ab titer. Finally, examinations of T cells expressing CXCR5, i.e., follicular B-helper T cells, revealed that spectratype expansions in MG patients were detected mainly in the CD4+ CXCR5+ T cell populations, whereas CD8+ T cells were the major source of clonal expansion in healthy subjects. These findings suggest that persistent clonal expansions of T cells in MG patients are associated with the development and maintenance of MG. Close examination of pathogenic T cells in MG provides useful information to elucidate the pathogenesis and to estimate the disease status.

Physiologic aspects of aging: impact on cancer management and decision making, part II

In this second article of our two-part review, we focus on age-associated physiologic changes involving the nervous, endocrine, hematologic, immune, and musculoskeletal systems, with close attention to the interconnected nature of these systems. There is a well-known connection between the neuroendocrine and immune systems via the hypothalamic-pituitary-adrenal axis and via interaction by means of cytokines, hormones, and neurotransmitters. These changes may lead to a loss of integration and resiliency with age, thus decreasing the ability of the elderly patient with cancer to adapt to stressful circumstances. Prominent changes include decline in memory and cognition, and increased susceptibility to peripheral neuropathy. Hematologic and immune changes like reduced bone marrow reserve and increased susceptibility to infections have far reaching implications for cancer care in the elderly. Gradual decline in hormone levels, and changes in muscle and body composition, can lead to functional decline and frailty. Use of the clinical interventions suggested in this article, along with an appreciation of the interplay of these age-related physiologic changes and their consequences, allows oncology professionals to customize therapy and minimize side effects in the geriatric oncology patient.

Molecular, cellular, and antigen requirements for development of age-associated T cell clonal expansions in vivo

T cell aging manifests itself both at the cellular (cell-autonomous defects in signaling) and at the population (age-related dysregulation of T cell homeostasis) levels. A prominent contributor to the latter is the appearance of T cell clonal expansions (TCE), with a potential to impair immune defense. In this study, we investigated molecular, cellular, and Ag requirements for TCE development. Of the mutant mice tested, old animals lacking MHC class I exhibited 7-fold fewer TCE than controls, with a 7-fold reduction in TCE. By contrast, animals lacking only one of the MHC class I molecules (Kb or Db), or IL-7R, or devoid of T cell renewal via adult thymectomy, all exhibited significant increases in TCE incidence. This increase directly correlated to lymphopenia, increased CD8 T cell turnover and an accumulation of memory-phenotype T cells. These data suggested that homeostatic cell division in the CD8 compartment enhances the formation of TCE. Repeated immunization with peptide/adjuvant did not result in an increase in Ag-specific TCE; however, adjuvant alone increased TCE incidence. In these experiments, therefore, nonspecific and/or homeostatic proliferation was more efficient in generating TCE in mice than repeated Ag-driven stimulation, suggesting that many, if not most, TCE in specific pathogen-free laboratory mice may be Ag-independent.