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

Tissue-specific shaping of the TCR repertoire and antigen specificity of iNKT cells

Tissue homeostasis is critically dependent on the function of tissue-resident lymphocytes, including lipid-reactive invariant natural killer T (iNKT) cells. Yet, if and how the tissue environment shapes the antigen specificity of iNKT cells remains unknown. By analysing iNKT cells from lymphoid tissues of mice and humans we demonstrate that their T cell receptor (TCR) repertoire is highly diverse and is distinct for cells from various tissues resulting in differential lipid-antigen recognition. Within peripheral tissues iNKT cell recent thymic emigrants exhibit a different TCR repertoire than mature cells, suggesting that the iNKT population is shaped after arrival to the periphery. Consistent with this, iNKT cells from different organs show distinct basal activation, proliferation and clonal expansion. Moreover, the iNKT cell TCR repertoire changes following immunisation and is shaped by age and environmental changes. Thus, post-thymic modification of the TCR-repertoire underpins the distinct antigen specificity for iNKT cells in peripheral tissues

Diminished immune responses with aging predispose older adults to common and uncommon influenza complications

Influenza (flu) is a serious disease for older adults, with increased severity of infection and greater risk for hospitalization and death. Flu infection is limited to pulmonary epithelial cells, yet there are many systemic symptoms and older adults are more susceptible to flu-related complications. In older adults, flu rarely comes without additional complications and there is a perfect storm for enhanced disease due to multiple factors including existing co-morbidities, plus impaired lung function and dysregulated immune responses that occur with even healthy aging. Commonly, opportunistic secondary bacterial infections prosper in damaged lungs. Intensified systemic inflammation with aging can cause dysfunction in extra-pulmonary organs and tissues such as cardiovascular, musculoskeletal, neuropathologic, hepatic, and renal complications. Often overlooked is the underappreciated connections between many of these conditions, which exacerbate one another when in parallel. This review focuses on flu infection and the numerous complications in older adults associated with diminished immune responses.

Aging and influenza vaccine-induced immunity

Immunosenescence is defined as the progressive deterioration of the immune system with aging. Immunosenescence stifles the generation of protective B and T cell-mediated adaptive immunity in response to various pathogens, resulting in increased disease susceptibility and severity in the elderly population. In particular, immunosenescence has major impacts on the phenotype, function, and receptor repertoire of B and T cells in the elderly, hindering protective responses induced by seasonal influenza virus vaccination. In order to overcome the detrimental impacts of immunosenescence on protective immunity to influenza viruses, we review our current understanding of the effects of aging on adaptive immune responses to influenza and discuss current and future avenues of vaccine research for eliciting more potent anti-influenza immunity in the elderly.

Influenza Pathogenesis: The Effect of Host Factors on Severity of Disease

Influenza viruses continue to be a major global health threat. Severity and clinical outcome of influenza disease is determined by both viral and host factors. Viral factors have long been the subject of intense research and many molecular determinants have been identified. However, research into the host factors that protect or predispose to severe and fatal influenza A virus infections is lagging. The goal of this review is to highlight the recent insights into host determinants of influenza pathogenesis.

Mucosal CD8+ T cell responses induced by an MCMV based vaccine vector confer protection against influenza challenge

Cytomegalovirus (CMV) is a ubiquitous β-herpesvirus that establishes life-long latent infection in a high percentage of the population worldwide. CMV induces the strongest and most durable CD8⁺ T cell response known in human clinical medicine. Due to its unique properties, the virus represents a promising candidate vaccine vector for the induction of persistent cellular immunity. To take advantage of this, we constructed a recombinant murine CMV (MCMV) expressing an MHC-I restricted epitope from influenza A virus (IAV) H1N1 within the immediate early 2 (ie2) gene. Only mice that were immunized intranasally (i.n.) were capable of controlling IAV infection, despite the greater potency of the intraperitoneally (i.p.) vaccination in inducing a systemic IAV-specific CD8⁺ T cell response. The protective capacity of the i.n. immunization was associated with its ability to induce IAV-specific tissue-resident memory CD8⁺ T (CD8T_RM) cells in the lungs. Our data demonstrate that the protective effect exerted by the i.n. immunization was critically mediated by antigen-specific CD8⁺ T cells. CD8T_RM cells promoted the induction of IFNγ and chemokines that facilitate the recruitment of antigen-specific CD8⁺ T cells to the lungs. Overall, our results showed that locally applied MCMV vectors could induce mucosal immunity at sites of entry, providing superior immune protection against respiratory infections.

Vaccines against influenza typically induce immune responses based on antibodies, small molecules that recognize the virus particles outside of cells and neutralize them before they infect a cell. However, influenza rapidly evolves, escaping immune recognition, and the fastest evolution is seen in the part of the virus that is recognized by antibodies. Therefore, every year we are confronted with new flu strains that are not recognized by our antibodies against the strains from previous years. The other branch of the immune system is made of killer T cells, which recognize infected cells and target them for killing. Influenza does not rapidly evolve to escape T cell killing; thus, vaccines inducing T-cell responses to influenza might provide long-term protection. We introduced an antigen from influenza into the murine cytomegalovirus (MCMV) and used it as a vaccine vector inducing killer T-cell responses of unparalleled strength. Our vector controls influenza replication and provides relief to infected mice, but only if we administered it through the nose, to activate killer T cells that will persist in the lungs close to the airways. Therefore, our data show that the subset of lung-resident killer T cells is sufficient to protect against influenza.

Immunosenescence and human vaccine immune responses

The age-related dysregulation and decline of the immune system-collectively termed "immunosenescence"-has been generally associated with an increased susceptibility to infectious pathogens and poor vaccine responses in older adults. While numerous studies have reported on the clinical outcomes of infected or vaccinated individuals, our understanding of the mechanisms governing the onset of immunosenescence and its effects on adaptive immunity remains incomplete. Age-dependent differences in T and B lymphocyte populations and functions have been well-defined, yet studies that demonstrate direct associations between immune cell function and clinical outcomes in older individuals are lacking. Despite these knowledge gaps, research has progressed in the development of vaccine and adjuvant formulations tailored for older adults in order to boost protective immunity and overcome immunosenescence. In this review, we will discuss the development of vaccines for older adults in light of our current understanding-or lack thereof-of the aging immune system. We highlight the functional changes that are known to occur in the adaptive immune system with age, followed by a discussion of current, clinically relevant pathogens that disproportionately affect older adults and are the central focus of vaccine research efforts for the aging population. We conclude with an outlook on personalized vaccine development for older adults and areas in need of further study in order to improve our fundamental understanding of adaptive immunosenescence.

The impact of sex and age on T cell immunity and ischemic stroke outcomes

Sex differences are well-recognized in ischemic stroke, a disease mainly affecting the elderly. Stroke results in robust activation of central and peripheral immune responses which contributes to functional outcome. Aging is associated with increased low-grade chronic inflammation known as "inflammaging" that renders aged males and females more susceptible to poor outcomes after ischemic stroke. Despite the fact that sex differences are well-documented in immunity and inflammation, few studies have focused on sex differences in inflammatory responses after ischemic stroke and even fewer have been performed in the context of aging. The role of T cell responses in ischemic stroke have gained increasing attention over the past decade as data suggest a major role in the pathophysiology/recovery after ischemic injury. T cells offer an attractive therapeutic target due to their relatively delayed infiltration into the ischemic brain. This review will focus on T cell immune responses in ischemic stroke, highlighting studies examining the effects of aging and biological sex.

Impact of CMV upon immune aging: facts and fiction

Aging is accompanied by significant defects in immunity and compromised responses to new, previously unencountered microbial pathogens. Most humans carry several persistent or latent viruses as they age, interacting with the host immune systems for years. In that context maybe the most studied persistent virus is Cytomegalovirus, infamous for its ability to recruit very large T cell responses which increase with age and to simultaneously evade elimination by the immune system. Here we will address how lifelong CMV infection and the immunological burden of its control might affect immune reactivity and health of the host over time.

Immune system development and age-dependent maintenance in Klotho-hypomorphic mice

Circulating Klotho peptide hormone has anti-aging activity and affects tissue maintenance. Hypomorphic mutant Klotho [kl/kl] mice on C57BL/6xC3H, BALB/c and 129 genetic backgrounds, show decreased Klotho expression that correlate with accelerated aging including pre-mature death due to abnormally high levels of serum vitamin D. These mice also show multiple impairments in the immune system. However, it remains unresolved if the defects in the immune system stem from decreased Klotho expression or high vitamin D levels in the serum. Transfer of the kl/kl allele to pure C57BL/6 genetic background [B6-kl/kl] significantly reduced expression of Klotho at all ages. Surprisingly, B6-kl/kl mice showed normalized serum vitamin D levels, amelioration of severe aging-related phenotypes and normal lifespan. This paper reports a detailed analysis of the immune system in B6-kl/kl mice in the absence of detrimental levels of serum vitamin D. Remarkably, the data reveal that in the absence of overt systemic stress, such as abnormally high vitamin D levels, reduced expression of Klotho does not have a major effect on the generation and maintenance of the immune system.

A Mathematical Model of the Effects of Aging on Naive T Cell Populations and Diversity

The human adaptive immune response is known to weaken in advanced age, resulting in increased severity of pathogen-born illness, poor vaccine efficacy, and a higher prevalence of cancer in the elderly. Age-related erosion of the T cell compartment has been implicated as a likely cause, but the underlying mechanisms driving this immunosenescence have not been quantitatively modeled and systematically analyzed. T cell receptor diversity, or the extent of pathogen-derived antigen responsiveness of the T cell pool, is known to diminish with age, but inherent experimental difficulties preclude accurate analysis on the full organismal level. In this paper, we formulate a mechanistic mathematical model of T cell population dynamics on the immunoclonal subpopulation level, which provides quantitative estimates of diversity. We define different estimates for diversity that depend on the individual number of cells in a specific immunoclone. We show that diversity decreases with age primarily due to diminished thymic output of new T cells and the resulting overall loss of small immunoclones.

Allodepleted T-cell immunotherapy after haploidentical haematopoietic stem cell transplantation without severe acute graft-versus-host disease (GVHD) in the absence of GVHD prophylaxis

Graft‐versus‐host disease (GVHD) is a major cause of transplant‐related mortality (TRM) after allogeneic haematopoietic stem cell transplantation (HSCT) and presents a challenge in haploidentical HSCT. GVHD may be prevented by ex vivo graft T‐cell depletion or in vivo depletion of proliferating lymphocytes. However, both approaches pose significant risks, particularly infections and relapse, compromising survival. A photodepletion strategy to eliminate alloreactive T cells from mismatched donor lymphocyte infusions (enabling administration without immunosuppression), was used to develop ATIR101, an adjunctive therapy for use after haploidentical HSCT. In this phase I dose‐finding study, 19 adults (median age: 54 years) with high‐risk haematological malignancies were treated with T‐cell‐depleted human leucocyte antigen‐haploidentical myeloablative HSCT followed by ATIR101 at doses of 1 × 10⁴–5 × 10⁶ CD3⁺ cells/kg (median 31 days post‐transplant). No patient received post‐transplant immunosuppression or developed grade III/IV acute GVHD, demonstrating the feasibility of ATIR101 infusion for evaluation in two subsequent phase 2 studies. Additionally, we report long‐term follow ‐up of patients treated with ATIR101 in this study. At 1 year, all 9 patients receiving doses of 0·3–2 × 10⁶ CD3⁺ cells/kg ATIR101 remained free of serious infections and after more than 8 years, TRM was 0%, relapse‐related mortality was 33% and overall survival was 67% in these patients.

The Role of Pre-existing Cross-Reactive Central Memory CD4 T-Cells in Vaccination With Previously Unseen Influenza Strains

Influenza vaccination is a common approach to prevent seasonal and pandemic influenza. Pre-existing antibodies against close viral strains might impair antibody formation against previously unseen strains-a process called original antigenic sin. The role of this pre-existing cellular immunity in this process is, despite some hints from animal models, not clear. Here, we analyzed cellular and humoral immunity in healthy individuals before and after vaccination with seasonal influenza vaccine. Based on influenza-specific hemagglutination inhibiting (HI) titers, vaccinees were grouped into HI-negative and -positive cohorts followed by in-depth cytometric and TCR repertoire analysis. Both serological groups revealed cross-reactive T-cell memory to the vaccine strains at baseline that gave rise to the majority of vaccine-specific T-cells post vaccination. On the contrary, very limited number of vaccine-specific T-cell clones was recruited from the naive pool. Furthermore, baseline quantity of vaccine-specific central memory helper T-cells and clonotype richness of this population directly correlated with the vaccination efficacy. Our findings suggest that the deliberate recruitment of pre-existing cross-reactive cellular memory might help to improve vaccination outcome.

Tasmanian devils with contagious cancer exhibit a constricted T-cell repertoire diversity

The Tasmanian devil (Sarcophilus harrisii) is threatened by a contagious cancer, known as Devil Facial Tumour Disease (DFTD). A highly diverse T-cell receptor (TCR) repertoire is crucial for successful host defence against cancers. By investigating TCR beta chain diversity in devils of different ages, we show that the T-cell repertoire in devils constricts in their second year of life, which may explain the higher DFTD prevalence in older devils. Unexpectedly, we also observed a pronounced decline in TCR diversity and T cell clonal expansion in devils after DFTD infection. These findings overturned the previous assumption that DFTD did not directly impact host immunity.

Yuanyuan Cheng et al. showed that the T-cell repertoire diversity of Tasmanian devils diminishes during their second year of life which may explain the prevalence of devil facial tumor disease in older devils. Infection with this disease also impacts T-cell diversity highlighting a previously unknown effect of the devil facial tumor disease on host immunity.

Immune aging in diabetes and its implications in wound healing

Immune systems have evolved to recognize and eliminate pathogens and damaged cells. In humans, it is estimated to recognize 109 epitopes and natural selection ensures that clonally expanded cells replace unstimulated cells and overall immune cell numbers remain stationary. But, with age, it faces continuous repertoire restriction and concomitant accumulation of primed cells. Changes shaping the aging immune system have bitter consequences because, as inflammatory responses gain intensity and duration, tissue-damaging immunity and inflammatory disease arise. During inflammation, the glycolytic flux cannot cope with increasing ATP demands, limiting the immune response's extent. In diabetes, higher glucose availability stretches the glycolytic limit, dysregulating proteostasis and increasing T-cell expansion. Long-term hyperglycemia exerts an accumulating effect, leading to higher inflammatory cytokine levels and increased cytotoxic mediator secretion upon infection, a phenomenon known as diabetic chronic inflammation. Here we review the etiology of diabetic chronic inflammation and its consequences on wound healing.

Sex and sex steroids impact influenza pathogenesis across the life course

Males and females differ in the outcome of influenza A virus (IAV) infections, which depends significantly on age. During a typical seasonal influenza epidemic, young children (< 10 years of age) and aged adults (65+ years of age) are at greatest risk for severe disease, and among these age groups, males tend to suffer a worse outcome from IAV infection than females. Following infection with either pandemic or outbreak strains of IAVs, females of reproductive ages (i.e., 15-49 years of age) experience a worse outcome than their male counterparts. Among females of reproductive ages, pregnancy is one factor linked to an increased risk of severe outcome of influenza, although it is not the sole factor explaining the female-preponderance of severe disease. Small animal models of influenza virus infection illustrate that inflammatory immune responses and repair of damaged tissue following IAV infection also differ between the sexes and impact the outcome of infection. There also is growing evidence that sex steroid hormones, including estrogens, progesterone, and testosterone, directly impact immune responses during IAV infection to alter outcomes. Greater consideration of the combined effects of sex and age as biological variables in epidemiological, clinical, and animal studies of influenza pathogenesis is needed.

Sepsis-Induced T Cell Immunoparalysis: The Ins and Outs of Impaired T Cell Immunity

Sepsis results in a deluge of pro- and anti-inflammatory cytokines, leading to lymphopenia and chronic immunoparalysis. Sepsis-induced long-lasting immunoparalysis is defined, in part, by impaired CD4 and CD8 αβ T cell responses in the postseptic environment. The dysfunction in T cell immunity affects naive, effector, and memory T cells and is not restricted to classical αβ T cells. Although sepsis-induced severe and transient lymphopenia is a contributory factor to diminished T cell immunity, T cell-intrinsic and -extrinsic factors/mechanisms also contribute to impaired T cell function. In this review, we summarize the current knowledge of how sepsis quantitatively and qualitatively impairs CD4 and CD8 T cell immunity of classical and nonclassical T cell subsets and discuss current therapeutic approaches being developed to boost the recovery of T cell immunity postsepsis induction.

TCR repertoire evolution during maintenance of CMV-specific T-cell populations

During infections and cancer, the composition of the T-cell receptor (TCR) repertoire of antigen-specific CD8⁺ T cells changes over time. TCR avidity is thought to be a major driver of this process, thereby interacting with several additional regulators of T-cell responses to form a composite immune response architecture. Infections with latent viruses, such as cytomegalovirus (CMV), can lead to large T-cell responses characterized by an oligoclonal TCR repertoire. Here, we review the current status of experimental studies and theoretical models of TCR repertoire evolution during CMV infection. We will particularly discuss the degree to which this process may be determined through structural TCR avidity. As engineered TCR-redirected T cells have moved into the spotlight for providing more effective immunotherapies, it is essential to understand how the key features of a given TCR influence T-cell expansion and maintenance in settings of infection or malignancy. Deeper insights into these mechanisms will improve our basic understanding of T-cell immunology and help to identify optimal TCRs for immunotherapy.

Using T Cell Receptor Repertoires to Understand the Principles of Adaptive Immune Recognition

Adaptive immune recognition is mediated by antigen receptors on B and T cells generated by somatic recombination during lineage development. The high level of diversity resulting from this process posed technical limitations that previously limited the comprehensive analysis of adaptive immune recognition. Advances over the last ten years have produced data and approaches allowing insights into how T cells develop, evolutionary signatures of recombination and selection, and the features of T cell receptors that mediate epitope-specific binding and T cell activation. The size and complexity of these data have necessitated the generation of novel computational and analytical approaches, which are transforming how T cell immunology is conducted. Here we review the development and application of novel biological, theoretical, and computational methods for understanding T cell recognition and discuss the potential for improved models of receptor:antigen interactions.

Cytomegalovirus Exposure in the Elderly Does Not Reduce CD8 T Cell Repertoire Diversity

With age, the immune system becomes less effective, causing increased susceptibility to infection. Chronic CMV infection further impairs immune function and is associated with increased mortality in the elderly. CMV exposure elicits massive CD8⁺ T cell clonal expansions and diminishes the cytotoxic T cell response to subsequent infections, leading to the hypothesis that to maintain homeostasis, T cell clones are expelled from the repertoire, reducing T cell repertoire diversity and diminishing the ability to combat new infections. However, in humans, the impact of CMV infection on the structure and diversity of the underlying T cell repertoire remains uncharacterized. Using TCR β-chain immunosequencing, we observed that the proportion of the peripheral blood T cell repertoire composed of the most numerous 0.1% of clones is larger in the CMV seropositive and gradually increases with age. We found that the T cell repertoire in the elderly grows to accommodate CMV-driven clonal expansions while preserving its underlying diversity and clonal structure. Our observations suggest that the maintenance of large CMV-reactive T cell clones throughout life does not compromise the underlying repertoire. Alternatively, we propose that the diminished immunity in elderly individuals with CMV is due to alterations in cellular function rather than a reduction in CD8⁺ T cell repertoire diversity.

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.

Lymphopenia and risk of infection and infection-related death in 98,344 individuals from a prospective Danish population-based study

BACKGROUND

Neutropenia increases the risk of infection, but it is unknown if this also applies to lymphopenia. We therefore tested the hypotheses that lymphopenia is associated with increased risk of infection and infection-related death in the general population.

METHODS AND FINDINGS

Of the invited 220,424 individuals, 99,191 attended examination. We analyzed 98,344 individuals from the Copenhagen General Population Study (Denmark), examined from November 25, 2003, to July 9, 2013, and with available blood lymphocyte count at date of examination. During a median of 6 years of follow-up, they developed 8,401 infections and experienced 1,045 infection-related deaths. Due to the completeness of the Danish civil and health registries, none of the 98,344 individuals were lost to follow-up, and those emigrating (n = 385) or dying (n = 5,636) had their follow-up truncated at the day of emigration or death. At date of examination, mean age was 58 years, and 44,181 (44.9%) were men. Individuals with lymphopenia (lymphocyte count < 1.1 × 109/l, n = 2,352) compared to those with lymphocytes in the reference range (1.1-3.7 × 109/l, n = 93,538) had multivariable-adjusted hazard ratios of 1.41 (95% CI 1.28-1.56) for any infection, 1.31 (1.14-1.52) for pneumonia, 1.44 (1.15-1.79) for skin infection, 1.26 (1.02-1.56) for urinary tract infection, 1.51 (1.21-1.89) for sepsis, 1.38 (1.01-1.88) for diarrheal disease, 2.15 (1.16-3.98) for endocarditis, and 2.26 (1.21-4.24) for other infections. The corresponding hazard ratio for infection-related death was 1.70 (95% CI 1.37-2.10). Analyses were adjusted for age, sex, smoking status, cumulative smoking, alcohol intake, body mass index, plasma C-reactive protein, blood neutrophil count, recent infection, Charlson comorbidity index, autoimmune diseases, medication use, and immunodeficiency/hematologic disease. The findings were robust in all stratified analyses and also when including only events later than 2 years after first examination. However, due to the observational design, the study cannot address questions of causality, and our analyses might theoretically have been affected by residual confounding and reverse causation. In principle, fluctuating lymphocyte counts over time might also have influenced analyses, but lymphocyte counts in 5,181 individuals measured 10 years after first examination showed a regression dilution ratio of 0.68.

CONCLUSIONS

Lymphopenia was associated with increased risk of hospitalization with infection and increased risk of infection-related death in the general population. Notably, causality cannot be deduced from our data.

Age Correlates with Response to Anti-PD1, Reflecting Age-Related Differences in Intratumoral Effector and Regulatory T-Cell Populations

Purpose: We have shown that the aged microenvironment increases melanoma metastasis, and decreases response to targeted therapy, and here we queried response to anti-PD1.Experimental Design: We analyzed the relationship between age, response to anti-PD1, and prior therapy in 538 patients. We used mouse models of melanoma, to analyze the intratumoral immune microenvironment in young versus aged mice and confirmed our findings in human melanoma biopsies.Results: Patients over the age of 60 responded more efficiently to anti-PD-1, and likelihood of response to anti-PD-1 increased with age, even when we controlled for prior MAPKi therapy. Placing genetically identical tumors in aged mice (52 weeks) significantly increased their response to anti-PD1 as compared with the same tumors in young mice (8 weeks). These data suggest that this increased response in aged patients occurs even in the absence of a more complex mutational landscape. Next, we found that young mice had a significantly higher population of regulatory T cells (Tregs), skewing the CD8+:Treg ratio. FOXP3 staining of human melanoma biopsies revealed similar increases in Tregs in young patients. Depletion of Tregs using anti-CD25 increased the response to anti-PD1 in young mice.Conclusions: While there are obvious limitations to our study, including our inability to conduct a meta-analysis due to a lack of available data, and our inability to control for mutational burden, there is a remarkable consistency in these data from over 500 patients across 8 different institutes worldwide. These results stress the importance of considering age as a factor for immunotherapy response. Clin Cancer Res; 24(21); 5347-56. ©2018 AACR See related commentary by Pawelec, p. 5193.

Epstein-Barr virus-related diffuse large B-cell lymphoma in mogamulizumab-treated adult T-cell leukemia with incomplete T-cell reconstitution

Adult T-cell leukemia (ATL) is an aggressive mature T-cell malignancy with a poor prognosis. The anti-C-C motif chemokine receptor 4 (CCR4) antibody mogamulizumab (moga) reduces ATL cells and induces reconstitution of polyclonal T cells; however, ATL cases often remain resistant and moga sometimes causes fatal immunopathology. Epstein-Barr virus (EBV)-related B-cell lymphoma develops in severely immunocompromised subjects, and is particularly associated with impaired T-cell immunity. Here, we report an ATL patient who had received conventional chemotherapy plus moga, and subsequently developed EBV-related diffuse large B-cell lymphoma (DLBCL) of the central nervous system. Next-generation sequencing-based T-cell receptor repertoire analyses identified residual abnormal clones and revealed that reconstitution of polyclonal T cells was incomplete, even after moga treatment. Furthermore, a skin rash that developed after moga treatment was found to contain ATL clones. This case suggests that the limited therapeutic effects of moga and incomplete T-cell reconstitution are associated with severely impaired T-cell immunity and subsequent development of EBV-related DLBCL.

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.

Profiling of the TCRβ repertoire in non-model species using high-throughput sequencing

In recent years, immune repertoire profiling with high-throughput sequencing (HTS) has advanced our understanding of adaptive immunity. However, fast progress in the field applied mostly to human and mouse research, with only few studies devoted to other model vertebrates. We present the first in-depth characterization of the T-cell receptor (TCR) repertoire in a non-model mammal (bank vole, Myodes glareolus), widely used in ecological and evolutionary research. We used RNA from spleens, 5′RACE and HTS to describe V and J segments of TCRβ, qualitatively characterize preferential V–J segment usage and CDR3 length distribution. Overall orthology to murine genes was preserved, with 11 J and 37 V genes found in voles (although 3 V genes lacked a close orthologue). Further, we implemented unique molecular identifiers for quantitative analysis of CDR3 repertoire with stringent error correction. A conservative, lower bound estimation of the TCRβ repertoire was similar to that found for mice (1.7–2.3 × 10⁵ clonotypes). We hope that by providing an easy-to-follow molecular protocol and on-line bioinformatics tools that do not require reference sequences (AmpliTCR and AmpliCDR3), we will encourage HTS immune repertoire profiling in other non-model vertebrates, thus opening new research avenues in e.g. comparative immunology, ecology and evolutionary biology.

Is aging a "comorbidity" of asthma?

The aging population is growing at an unparalleled rate. Asthma is common in the elderly (age over 65 years) and can be more severe with little chance for remission. Asthma in older individuals is often under-diagnosed, misdiagnosed and frequently under-treated. Concomitant medical and psychosocial conditions are more prevalent in the elderly and can obfuscate the presentation of asthma and make it more difficult to assess and manage. While these comorbidities are important in understanding elderly asthma, aging itself can be considered a "comorbidity" since it impacts structural and functional changes in the lung. Structural changes of the aging lung may worsen physiologic function in asthma. The immune system also changes with age, with increased vulnerability to pathogens and differences in airway inflammation, leading to variability in how asthma manifests and responds to treatment. The fact that aging can influence the severity and presentation of asthma along with its diagnosis and management is important for the treating physician to understand. This article will discuss the multitude of factors that justify considering aging as a comorbidity of asthma.

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.

A disconnect between precursor frequency, expansion potential, and site-specific CD4+ T cell responses in aged mice

T cell recognition of peptides presented within self-major histocompatibility complex (pMHC) molecules is essential for long-lived protective immunity. As mice age the number of naïve CD4+ and CD8+ T cells declines. However, unlike for CD8+ T cells, there are more naïve and memory phenotype CD4+ T cells that bind foreign pMHCII in old mice (18-22 months) than adults (12-15 weeks), suggesting increased promiscuity of pMHCII recognition with aging. Here we asked if CD4+ T cell responses to immunization or infection increase with aging since the magnitude of a CD4+ T cell response to a foreign pMHCII is proportional to the size of the precursor population in adult mice. We observed no difference in the number of pMHCII-specific CD4+ T cells in adult versus old mice for pooled secondary lymphoid organs after immunization, bacterial infection, or viral infection, but we did observe diminished numbers of pMHCII-specific CD4+ T cells in both the draining lymph node and brain of old mice after West Nile virus infection. These data indicate that an increased precursor frequency does not translate into more robust responses upon immunization or infection in old mice.

Thymic stromal cells: Roles in atrophy and age-associated dysfunction of the thymus

Atrophy of the thymus, the primary site of T lymphocyte generation, is a hallmark of the aging immune system. Age-associated thymic atrophy results in diminished output of new, naïve T cells, with immune sequelae that include diminished responses to novel pathogenic challenge and vaccines, as well as diminished tumor surveillance. Although a variety of stimuli are known to regulate transient thymic atrophy, mechanisms governing progressive age-associated atrophy have been difficult to resolve. This has been due in part to the fact that one of the primary targets of age-associated thymic atrophy is a relatively rare population, thymic stromal cells. This review focuses on changes in thymic stromal cells during aging and on the contributions of periodic, stochastic, and progressive causes of thymic atrophy.

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.

Reinterpreting recent thymic emigrant function: defective or adaptive?

Recent thymic emigrants (RTEs) are those peripheral T cells that have most recently completed thymic development and egress. Over the past decade, significant advances have been made in understanding the cell-extrinsic and cell-intrinsic requirements for RTE maturation to mature naïve (MN) T cells and in detailing the functional differences that characterize these two T cell populations. Much of this work has suggested that RTEs are hypo-functional versions of more mature T cells. However, recent evidence has indicated that rather than being defective T cells, RTEs are exquisitely adapted to their cellular niche. In this review, we argue that RTEs are not flawed mature T cells but are adapted to fill an underpopulated T cell compartment, while maintaining self tolerance and possessing the capacity to mount robust immune responses.

Influenza vaccination in the elderly

Seasonal influenza is a prevalent and serious annual illness resulting in widespread morbidity and economic disruption throughout the population; the elderly and immunocompromised are particularly vulnerable to serious sequelae and mortality. The changing demographics worldwide to an aging society have important implications for public health policy and pharmaceutical innovations. For instance, primary prevention via immunization is effective in reducing the burden of influenza illness among the elderly. However, the elderly may be insufficiently protected by vaccination due to the immunosenescence which accompanies aging. In addition, vaccine hesitancy among the younger populations increases the likelihood of circulating infectious diseases, and thus concomitant exposure. While it is clear that the development of more immunogenic vaccines is an imperative and worthy endeavor, clinical trials continue to demonstrate that the current influenza vaccine formulation remains highly effective in reducing morbidity and mortality when well matched to circulating strains.

Perturbation of the T cell receptor repertoire occurs with increasing age in dogs

Immunosenescence is the gradual deterioration in immune system function associated with ageing. This decline is partly due to involution of the thymus, which leads to a reduction in the output of naive T cells into the circulating lymphocyte pool. Expansion of existing naive and memory T cell populations, to compensate for the reduction in thymic output, can lead to reduced diversity in the T cell repertoire with increasing age, resulting in impairment of immune responses to novel antigenic challenges, such as during infection and vaccination. Since associations between T cell repertoire and age have only been examined in a limited number of species, to gain further insights into this relationship, we have investigated age-related changes in the canine T cell receptor (TCR) repertoire. Blood samples were obtained from Labrador retriever dogs of varying ages and variation in the complementary determining region 3 (CDR3) of the T cell receptor beta (TCRB) chain was investigated. CDR3 size spectratyping was employed to evaluate clonal expansion/deletion in the T cell repertoire, allowing identification of profiles within individual variable (V) region families that skewed away from a Gaussian distribution. Older dogs (10-13 years) were found to have an increased number of TCRB V gene spectratypes that demonstrated a skewed distribution, compared with young dogs (≤3 years). Additionally, there was a reduction in the number of clonal peaks present in the spectratypes of old dogs, compared with those of young dogs. The study findings suggest that there is an age-associated disturbance in the diversity of the T cell receptor repertoire in dogs.

DNA demethylating agent decitabine broadens the peripheral T cell receptor repertoire

Purpose

Decitabine, a promising epi-immunotherapeutic agent has shown clinical responses in solid tumor patients, while the anti-tumor mechanisms were unclear. We aimed to investigate the immunomodulatory effect of decitabine in peripheral T cells.

Experimental design

We applied next-generation sequencing to investigate the complementarity-determining region 3 (CDR3) of the TCRβ gene, the diversity of which acts as the prerequisite for the host immune system to recognize the universal foreign antigens. We collected the peripheral blood mononuclear cells (PBMCs) from 4 patients, at baseline and after 2 cycles of low-dose decitabine therapy.

Results

An increase of the unique productive sequences of the CDR3 of TCRβ was observed in all of the 4 patients after decitabine treatment, which was characterized by a lower abundance of expanded clones and increased TCR diversity compared with before decitabine treatment. Further analysis showed a tendency for CD4 T cells with an increased CD4/CD8 ratio in response to decitabine therapy. In addition, the genome-wide expression alterations confirmed the effects of decitabine on immune reconstitution, and the increase of TCR excision circles (TRECs) was validated.

Conclusions

The low-dose DNMT inhibitor decitabine broadens the peripheral T cell repertoire, providing a novel role for the epigenetic modifying agent in anti-tumor immune enhancement.

Intrinsic and Extrinsic Thymic Adrenergic Networks: Sex Steroid-Dependent Plasticity

The thymus is sexually differentiated organ providing microenvironment for T-cell precursor differentiation/maturation in the major histocompatibility complex-restricted self-tolerant T cells. With increasing age, the thymus undergoes involution leading to the decline in efficacy of thymopoiesis. Noradrenaline from thymic nerve fibers and "(nor)adrenergic" cells is involved in the regulation of thymopoiesis. In rodents, noradrenaline concentration in thymus and adrenoceptor (AR) expression on thymic cells depend on sex and age. These differences are suggested to be implicated in the development of sexual diergism and the age-related decline in thymopoiesis. The programming of both thymic sexual differentiation and its involution occurs during the critical early perinatal period and may be reprogrammed during peripubertal development. The thymic (re)programming is critically dependent on circulating levels of gonadal steroids. Although the underlying molecular mechanisms have not yet been elucidated fully, it is assumed that the gonadal steroid action during the critical perinatal/peripubertal developmental periods leads to long-lasting changes in the efficacy of thymopoiesis partly through (re)programming of "(nor)adrenergic" cell networks and AR expression on thymic cells.

Direct measurement of T cell receptor affinity and sequence from naïve antiviral T cells

T cells recognize and kill a myriad of pathogen-infected or cancer cells using a diverse set of T cell receptors (TCRs). The affinity of TCR to cognate antigen is of high interest in adoptive T cell transfer immunotherapy and antigen-specific T cell repertoire immune profiling because it is widely known to correlate with downstream T cell responses. We introduce the in situ TCR affinity and sequence test (iTAST) for simultaneous measurement of TCR affinity and sequence from single primary CD8(+) T cells in human blood. We demonstrate that the repertoire of primary antigen-specific T cells from pathogen-inexperienced individuals has a surprisingly broad affinity range of 1000-fold composed of diverse TCR sequences. Within this range, samples from older individuals contained a reduced frequency of high-affinity T cells compared to young individuals, demonstrating an age-related effect of T cell attrition that could cause holes in the repertoire. iTAST should enable the rapid selection of high-affinity TCRs ex vivo for adoptive immunotherapy and measurement of T cell response for immune monitoring applications.

Immune cell type 'fingerprints' at the basis of outcome diversity of human infection

Despite the availability of antibiotics and immunization, infectious diseases remain a major cause of malignancy and death worldwide. Yet, it is well documented that for most infectious agents, clinical disease develops in only a small minority of infected individuals. There is, in fact, great heterogeneity in infection outcome, from complete clearance of the pathogen to severe illness. Understanding this variation remains elusive, despite its great potential to equip us with new tools for the treatment of infectious diseases. Here, we propose a novel perspective for studying this diversity in human infection outcome, one that utilizes single-cell analysis technologies. Recent advances in single-cell RNA-seq technologies allow the detection of rare subpopulations that play important roles in host-pathogen interactions. We propose that applying single-cell RNA-seq to the study of infection can provide a 'fingerprint' of the immune cell types that are associated with the ability of the host to clear a pathogen and, thereby, broaden our current understanding of variation in susceptibility to infection within the population.

Structural basis for clonal diversity of the human T-cell response to a dominant influenza virus epitope

Influenza A virus (IAV) causes an acute infection in humans that is normally eliminated by CD8⁺ cytotoxic T lymphocytes. Individuals expressing the MHC class I molecule HLA-A2 produce cytotoxic T lymphocytes bearing T-cell receptors (TCRs) that recognize the immunodominant IAV epitope GILGFVFTL (GIL). Most GIL-specific TCRs utilize α/β chain pairs encoded by the TRAV27/TRBV19 gene combination to recognize this relatively featureless peptide epitope (canonical TCRs). However, ∼40% of GIL-specific TCRs express a wide variety of other TRAV/TRBV combinations (non-canonical TCRs). To investigate the structural underpinnings of this remarkable diversity, we determined the crystal structure of a non-canonical GIL-specific TCR (F50) expressing the TRAV13-1/TRBV27 gene combination bound to GIL-HLA-A2 to 1.7 Å resolution. Comparison of the F50-GIL-HLA-A2 complex with the previously published complex formed by a canonical TCR (JM22) revealed that F50 and JM22 engage GIL-HLA-A2 in markedly different orientations. These orientations are distinguished by crossing angles of TCR to peptide-MHC of 29° for F50 versus 69° for JM22 and by a focus by F50 on the C terminus rather than the center of the MHC α1 helix for JM22. In addition, F50, unlike JM22, uses a tryptophan instead of an arginine to fill a critical notch between GIL and the HLA-A2 α2 helix. The F50-GIL-HLA-A2 complex shows that there are multiple structurally distinct solutions to recognizing an identical peptide-MHC ligand with sufficient affinity to elicit a broad anti-IAV response that protects against viral escape and T-cell clonal loss.

Successful and Maladaptive T Cell Aging

Throughout life, the T cell system adapts to shifting resources and demands, resulting in a fundamentally restructured immune system in older individuals. Here we review the cellular and molecular features of an aged immune system and discuss the trade-offs inherent to these adaptive mechanisms. Processes include homeostatic proliferation that maintains compartment size at the expense of partial loss in stemness and incomplete differentiation and the activation of negative regulatory programs, which constrain effector T cell expansion and prevent increasing oligoclonality but also interfere with memory cell generation. We propose that immune failure occurs when adaptive strategies developed by the aging T cell system fail and also discuss how, in some settings, the programs associated with T cell aging culminates in a maladaptive response that directly contributes to chronic inflammatory disease.

Age- and Sex-associated Differences in Phenotypic and Functional Characteristics of Peripheral Blood Lymphocytes in Chimpanzees (Pan troglodytes)

Chimpanzees are the closest phylogenetic relatives to humans, sharing more than 98% genetic sequence identity. These genetic similarities prompted the belief that chimpanzees can serve as an ideal model for human disease conditions and vaccine development. However, in light of the recent NIH decision to phase out biomedical research in chimpanzees and retire NIH-supported chimpanzees, data from the present study will continue to provide value for the care of aged and sick chimpanzees located in zoos, sanctuaries, and primate centers. Surprisingly little information has been published regarding the normal chimpanzee immune system, and most extant studies have been based on small numbers of animals. In the current study, we provide a better understanding of the chimpanzee immune system with regard to age and sex. We examined immune parameters of chimpanzees (n = 94; 51 female, 43 male; age, 6 to 47 y) by using flow cytometry, immune function analysis, and cytokine analysis. Because lymphocytes are key mediators of cellular immune responses, particularly to intracellular pathogens such as viruses, we surveyed the phenotypic and functional attributes of T and B lymphocytes in this healthy and age-stratified population of chimpanzees. We noted a significantly higher percentage of CD16+T cells in male compared with female chimpanzees but no significant changes in percentages of CD3+, CD4+, CD8+, or CD4+CD8+ T cells with age or sex. In addition, aging was associated with decreased proliferative responses to mitogens in both sexes. Sex-specific differences also were present in the percentage of NK cells but not in their cytotoxic activity and in circulating cytokine levels in plasma. Going forward, the data presented here regarding immune cell changes associated with aging in healthy chimpanzees will serve to enhance the care of geriatric and ill animals.

Cutting Edge: The Aging Immune System Reveals the Biological Impact of Direct Antigen Presentation on CD8 T Cell Responses

The vertebrate immune system uses multiple, sometimes redundant, mechanisms to contain pathogenic microorganisms that are always evolving to evade host defenses. Thus, the cowpox virus (CPXV) uses genes encoding CPXV12 and CPXV203 to prevent direct MHC class I presentation of viral peptides by infected cells. However, CD8 T cells are effectively primed against CPXV by cross-presentation of viral Ags in young mice. Old mice accumulate defects in both CD8 T cell activation and cross-presentation. Using a double-deletion mutant (∆12∆203) of CPXV, we show that direct priming of CD8 T cells in old mice yields superior recall responses, establishing a key contribution of this mechanism to host antipoxvirus responses and enhancing our fundamental understanding of how viral manipulation of direct presentation impacts pathogenesis. This also provides a proof of principle that suboptimal CD8 T cell in old organisms can be optimized by manipulating Ag presentation, with implications for vaccine design.

Aging-related changes in human T-cell repertoire over 20years delineated by deep sequencing of peripheral T-cell receptors

Recent deep sequencing studies on T-cell receptor (TCR) repertoire have provided robust data to characterize diversity of T-cell immune responsiveness to a wide variety of peptide antigens, including viral and tumor antigens. The human TCR repertoire declines with age, but this decline has not been fully investigated longitudinally in individuals. Using a deep sequencing approach, we analyzed TCRβ repertoires longitudinally over approximately 20years, with ages ranging from 23 to 50years at the start (23 to 65years overall), in peripheral-blood CD4 and CD8 T-cell populations that were collected and cryopreserved 3 times at intervals of approximately 10years from each of 6 healthy adults (3 men and 3 women). Sequence data at the hypervariable complementarity determining region 3 (CDR3) in the TCRB gene locus were evaluated by applying a random-coefficient statistical regression model. Two outcomes were analyzed: total number of distinct TCRB CDR3 sequences as a TCR diversity metric, and clonality of the T-cell populations. TCR repertoire diversity decreased (p<0.001) and frequencies of clonal populations increased (p=0.003) with age in CD8 T cells, whereas CD4 T cells retained fairly diverse TCR repertoires along with relatively low clonality. We also found that approximately 10-30% and 30-80% of read sequences in CD4 and CD8 T cells, respectively, overlapped at different ages within each individual, indicating long-term stable maintenance of T-cell clonal composition. Moreover, many of the most frequent TCRB CDR3 sequences (i.e., top T-cell clones) persisted over 20years, and some of them expanded and exerted a dominating influence on clonality of peripheral T-cell populations. It is thus possible that persistence or expansion of top T-cell clones is a driver of T-cell immunity aging, and therefore represents a potential interventional target.

Engaging Natural Killer T Cells as 'Universal Helpers' for Vaccination

Conventional vaccine adjuvants enhance peptide-specific T-cell and B-cell responses by modifying peptide stability or uptake or by binding to pattern-recognition receptors on antigen-presenting cells (APCs). This article discusses the application of a distinct mechanism of adjuvant activity: the activation of type I, or invariant, natural killer T (iNKT) cells to drive cellular and humoral immune responses. Using a semi-invariant T-cell receptor (TCR), iNKT cells recognize glycolipid antigens presented on cluster of differentiation (CD)-1d molecules. When their ligands are presented in concert with peptides, iNKT cells can provide T-cell help, 'licensing' APCs to augment peptide-specific T-cell and antibody responses. We discuss the potential benefits and limitations of exploiting iNKT cells as 'universal helpers' to enhance vaccine responses for the treatment and prevention of cancer and infectious diseases.

The emerging role of ECM crosslinking in T cell mobility as a hallmark of immunosenescence in humans

Immunosenescence is thought to result from cellular aging and to reflect exposure to environmental stressors and antigens, including cytomegalovirus (CMV). However, not all of the features of immunosenescence are consistent with this view, and this has led to the emergence of the sister theory of "inflammaging". The recently discovered diffuse tissue distribution of resident memory T cells (T_RM) which don't recirculate, calls these theories into question. These cells account for most T cells residing in barrier epithelia which sit in and travel through the extracellular matrix (ECM). With almost all studies to date carried out on peripheral blood, the age-related changes of the ECM and their consequences for T cell mobility, which is crucial for the function of these cells, have been largely ignored. We propose an update of the theoretical framework of immunosenescence, based on a novel hypothesis: the increasing stiffness and cross-linking of the senescent ECM lead to a progressive immunodeficiency due to an age-related decrease in T cell mobility and eventually the death of these cells. A key element of this mechanism is the mechanical stress to which the cell cytoplasm and nucleus are subjected during passage through the ECM. This hypothesis is based on an "evo-devo" perspective bringing together some major characteristics of aging, to create a single interpretive framework for immunosenescence.

IL-6 Production by TLR-Activated APC Broadly Enhances Aged Cognate CD4 Helper and B Cell Antibody Responses In Vivo

Naive CD4 T cell responses, especially their ability to help B cell responses, become compromised with aging. We find that using APC pretreated ex vivo with TLR agonists, polyinosinic-polycytidylic acid and CpG, to prime naive CD4 T cells in vivo, restores their ability to expand and become germinal center T follicular helpers and enhances B cell IgG Ab production. Enhanced helper responses are dependent on IL-6 production by the activated APC. Aged naive CD4 T cells respond suboptimally to IL-6 compared with young cells, such that higher doses are required to induce comparable signaling. Preactivating APC overcomes this deficiency. Responses of young CD4 T cells are also enhanced by preactivating APC with similar effects but with only partial IL-6 dependency. Strikingly, introducing just the activated APC into aged mice significantly enhances otherwise compromised Ab production to inactivated influenza vaccine. These findings reveal a central role for the production of IL-6 by APC during initial cognate interactions in the generation of effective CD4 T cell help, which becomes greater with age. Without APC activation, aging CD4 T cell responses shift toward IL-6-independent Th1 and CD4 cytotoxic Th cell responses. Thus, strategies that specifically activate and provide Ag to APC could potentially enhance Ab-mediated protection in vaccine responses.

Broad TCR repertoire and diverse structural solutions for recognition of an immunodominant CD8+ T cell epitope

A keystone of antiviral immunity is CD8 T-cell recognition of viral peptides bound to MHC-I proteins. The recognition mode of individual T cell receptors (TCRs) has been studied in some detail, but how TCR variation functions in providing a robust response to viral antigen is unclear. The influenza M1 epitope is an immunodominant target of CD8 T cells helping to control influenza in HLA-A2+ individuals. Here, we show that many distinct TCRs are used by CD8 T cells to recognize HLA-A2/M1, encoding different structural solutions to the problem of specifically recognizing a relatively featureless peptide antigen. The vast majority of responding TCRs target small clefts between peptide and MHC. These broad repertoires lead to plasticity in antigen recognition and protection against T cell clonal loss and viral escape.