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

The gene regulatory basis of bystander activation in CD8+ T cells

CD8+ T cells are classically recognized as adaptive lymphocytes based on their ability to recognize specific foreign antigens and mount memory responses. However, recent studies indicate that some antigen-inexperienced CD8+ T cells can respond to innate cytokines alone in the absence of cognate T cell receptor stimulation, a phenomenon referred to as bystander activation. Here, we demonstrate that neonatal CD8+ T cells undergo a robust and diverse program of bystander activation, which corresponds to enhanced innate-like protection against unrelated pathogens. Using a multi-omics approach, we found that the ability of neonatal CD8+ T cells to respond to innate cytokines derives from their capacity to undergo rapid chromatin remodeling, resulting in the usage of a distinct set of enhancers and transcription factors typically found in innate-like T cells. We observed that the switch between innate and adaptive functions in the CD8+ T cell compartment is mediated by changes in the abundance of distinct subsets of cells. The innate CD8+ T cell subset that predominates in early life was also present in adult mice and humans. Our findings provide support for the layered immune hypothesis and indicate that the CD8+ T cell compartment is more functionally diverse than previously thought.

Surfing on the waves of the human γδ T cell ontogenic sea

While γδ T cells are present virtually in all vertebrates, there is a remarkable lack of conservation of the TRG and TRD loci underlying the generation of the γδ T cell receptor (TCR), which is associated with the generation of species-specific γδ T cells. A prominent example is the human phosphoantigen-reactive Vγ9Vδ2 T cell subset that is absent in mice. Murine γδ thymocyte cells were among the first immune cells identified to follow a wave-based layered development during embryonic and early life, and since this initial observation, in-depth insight has been obtained in their thymic ontogeny. By contrast, less is known about the development of human γδ T cells, especially regarding the generation of γδ thymocyte waves. Here, after providing an overview of thymic γδ wave generation in several vertebrate classes, we review the evidence for γδ waves in the human fetal thymus, where single-cell technologies have allowed the breakdown of human γδ thymocytes into functional waves with important TCR associations. Finally, we discuss the possible mechanisms contributing to the generation of waves of γδ thymocytes and their possible significance in the periphery.

Quantitative restoration of immune defense in old animals determined by naive antigen-specific CD8 T-cell numbers

Older humans and animals often exhibit reduced immune responses to infection and vaccination, and this often directly correlates to the numbers and frequency of naive T (Tn) cells. We found such a correlation between reduced numbers of blood CD8+ Tn cells and severe clinical outcomes of West Nile virus (WNV) in both humans naturally exposed to, and mice experimentally infected with, WNV. To examine possible causality, we sought to increase the number of CD8 Tn cells by treating C57BL/6 mice with IL-7 complexes (IL-7C, anti-IL-7 mAb bound to IL-7), shown previously to efficiently increase peripheral T-cell numbers by homeostatic proliferation. T cells underwent robust expansion following IL-7C administration to old mice increasing the number of total T cells (>fourfold) and NS4b:H-2D^b -restricted antigen-specific CD8 T cells (twofold). This improved the numbers of NS4b-specific CD8 T cells detected at the peak of the response against WNV, but not survival of WNV challenge. IL-7C-treated old animals also showed no improvement in WNV-specific effector immunity (neutralizing antibody and in vivo T-cell cytotoxicity). To test quantitative limits to which CD8 Tn cell restoration could improve protective immunity, we transferred graded doses of Ag-specific precursors into old mice and showed that injection of 5400 (but not of 1800 or 600) adult naive WNV-specific CD8 T cells significantly increased survival after WNV. These results set quantitative limits to the level of Tn reconstitution necessary to improve immune defense in older organisms and are discussed in light of targets of immune reconstitution.

MicroRNA-29 specifies age-related differences in the CD8+ T cell immune response

MicroRNAs (miRNAs) have emerged as critical regulators of cell fate in the CD8+ T cell response to infection. Although there are several examples of miRNAs acting on effector CD8+ T cells after infection, it is unclear whether differential expression of one or more miRNAs in the naive state is consequential in altering their long-term trajectory. To answer this question, we examine the role of miR-29 in neonatal and adult CD8+ T cells, which express different amounts of miR-29 only prior to infection and adopt profoundly different fates after immune challenge. We find that manipulation of miR-29 expression in the naive state is sufficient for age-adjusting the phenotype and function of CD8+ T cells, including their regulatory landscapes and long-term differentiation trajectories after infection. Thus, miR-29 acts as a developmental switch by controlling the balance between a rapid effector response in neonates and the generation of long-lived memory in adults.

Age-Related Changes in Thymic Central Tolerance

Thymic epithelial cells (TECs) and hematopoietic antigen presenting cells (HAPCs) in the thymus microenvironment provide essential signals to self-reactive thymocytes that induce either negative selection or generation of regulatory T cells (Treg), both of which are required to establish and maintain central tolerance throughout life. HAPCs and TECs are comprised of multiple subsets that play distinct and overlapping roles in central tolerance. Changes that occur in the composition and function of TEC and HAPC subsets across the lifespan have potential consequences for central tolerance. In keeping with this possibility, there are age-associated changes in the cellular composition and function of T cells and Treg. This review summarizes changes in T cell and Treg function during the perinatal to adult transition and in the course of normal aging, and relates these changes to age-associated alterations in thymic HAPC and TEC subsets.

Aging, cancer, and antitumor immunity

Aging leads to numerous changes that affect many components of the immune system, called "immunosenescence". Indeed, elderly individuals exhibit dysregulated immune responses against pathogens, poor responses to vaccination, and increased susceptibility to many diseases including cancer, autoimmune disorders, and other chronic inflammatory diseases. Despite progressed understanding of immunosenescence, its detailed mechanisms are still not fully understood. With advances in medicine, the population of older cancer patients is expected to rapidly increase in the coming years. Cancer immunotherapies, including immune checkpoint inhibitors (ICIs), have been shown to be effective for multiple cancer types, whereas to date, few specific data for elderly individuals have been published. Some systemic reviews have demonstrated that ICIs exhibit similar efficacy in older cancer patients, but they seem to be less effective in very old patients. In addition, toxicities might be more frequently observed in such patients. Here, we provide a summary to better understand immunosenescence and an overview of its relationship with cancer and antitumor immunity, including the efficacy and toxicity of ICIs.

T cell Tolerance in Early Life

T cell-mediated immune tolerance is a state of unresponsiveness of T cells towards specific self or non-self antigens. This is particularly essential during prenatal/neonatal period when T cells are exposed to dramatically changing environment and required to avoid rejection of maternal antigens, limit autoimmune responses, tolerate inert environmental and food antigens and antigens from non-harmful commensal microorganisms, promote maturation of mucosal barrier function, yet mount an appropriate response to pathogenic microorganisms. The cell-intrinsic and cell extrinsic mechanisms promote the generation of prenatal/neonatal T cells with distinct features to meet the complex and dynamic need of tolerance during this period. Reduced exposure or impaired tolerance in early life may have significant impact on allergic or autoimmune diseases in adult life. The uniqueness of conventional and regulatory T cells in human umbilical cord blood (UCB) may also provide certain advantages in UCB transplantation for hematological disorders.

How age and infection history shape the antigen-specific CD8+ T-cell repertoire: Implications for vaccination strategies in older adults

Older adults often show signs of impaired CD8⁺ T‐cell immunity, reflected by weaker responses against new infections and vaccinations, and decreased protection against reinfection. This immune impairment is in part thought to be the consequence of a decrease in both T‐cell numbers and repertoire diversity. If this is indeed the case, a strategy to prevent infectious diseases in older adults could be the induction of protective memory responses through vaccination at a younger age. However, this requires that the induced immune responses are maintained until old age. It is therefore important to obtain insights into the long‐term maintenance of the antigen‐specific T‐cell repertoire. Here, we review the literature on the maintenance of antigen‐experienced CD8⁺ T‐cell repertoires against acute and chronic infections. We describe the complex interactions that play a role in shaping the memory T‐cell repertoire, and the effects of age, infection history, and T‐cell avidity. We discuss the implications of these findings for the development of new vaccination strategies to protect older adults.

Cytomegalovirus-Mediated T Cell Receptor Repertoire Perturbation Is Present in Early Life

Human cytomegalovirus (CMV) is a highly prevalent herpesvirus, particularly in sub-Saharan Africa, where it is endemic from infancy. The T cell response against CMV is important in keeping the virus in check, with CD8 T cells playing a major role in the control of CMV viraemia. Human leukocyte antigen (HLA) B*44:03-positive individuals raise a robust response against the NEGVKAAW (NW8) epitope, derived from the immediate-early-2 (IE-2) protein. We previously showed that the T cell receptor (TCR) repertoire raised against the NW8-HLA-B*44:03 complex was oligoclonal and characterised by superdominant clones, which were shared amongst unrelated individuals (i.e., "public"). Here, we address the question of how stable the CMV-specific TCR repertoire is over the course of infection, and whether substantial differences are evident in TCR repertoires in children, compared with adults. We present a longitudinal study of four HIV/CMV co-infected mother-child pairs, who in each case express HLA-B*44:03 and make responses to the NW8 epitope, and analyse their TCR repertoire over a period spanning more than 10 years. Using high-throughput sequencing, the paediatric CMV-specific repertoire was found to be highly diverse. In addition, paediatric repertoires were remarkably similar to adults, with public TCR responses being shared amongst children and adults alike. The CMV-specific repertoire in both adults and children displayed strong fluctuations in TCR clonality and repertoire architecture over time. Previously characterised superdominant clonotypes were readily identifiable in the children at high frequency, suggesting that the distortion of the CMV-specific repertoire is incurred as a direct result of CMV infection rather than a product of age-related "memory inflation." Early distortion of the TCR repertoire was particularly apparent in the case of the TCR-β chain, where oligoclonality was low in children and positively correlated with age, a feature we did not observe for TCR-α. This discrepancy between TCR-α and -β chain repertoire may reflect differential contribution to NW8 recognition. Altogether, the results of the present study provide insight into the formation of the TCR repertoire in early life and pave the way to better understanding of CD8 T cell responses to CMV at the molecular level.

Building a T cell compartment: how immune cell development shapes function

We are just beginning to understand the diversity of the peripheral T cell compartment, which arises from the specialization of different T cell subsets and the plasticity of individual naive T cells to adopt different fates. Although the progeny of a single T cell can differentiate into many phenotypes following infection, individual T cells are biased towards particular phenotypes. These biases are typically ascribed to random factors that occur during and after antigenic stimulation. However, the T cell compartment does not remain static with age, and shifting immune challenges during ontogeny give rise to T cells with distinct functional properties. Here, we argue that the developmental history of naive T cells creates a 'hidden layer' of diversity that persists into adulthood. Insight into this diversity can provide a new perspective on immunity and immunotherapy across the lifespan.

Neonatal T Cells: A Reinterpretation

Neonatal CD4⁺ and CD8⁺ T cells have historically been characterized as immature or defective. However, recent studies prompt a reinterpretation of the functions of neonatal T cells. Rather than a population of cells always falling short of expectations set by their adult counterparts, neonatal T cells are gaining recognition as a distinct population of lymphocytes well suited for the rapidly changing environment in early life. In this review, I will highlight new evidence indicating that neonatal T cells are not inert or less potent versions of adult T cells but instead are a broadly reactive layer of T cells poised to quickly develop into regulatory or effector cells, depending on the needs of the host. In this way, neonatal T cells are well adapted to provide fast-acting immune protection against foreign pathogens, while also sustaining tolerance to self-antigens.

Differential Sensitivity to IL-12 Drives Sex-Specific Differences in the CD8+ T Cell Response to Infection

It is well known that males and females respond differently to intracellular pathogens. Females mount a more robust immune response than males, which decreases their susceptibility to infection but comes at the cost of increasing immunopathology. However, the underlying basis for sex-specific differences in the CD8⁺ T cell response to infection remains poorly understood. In this study, we show that female CD8⁺ T cells have an intrinsic propensity to become short-lived effectors, whereas male CD8⁺ T cells give rise to more memory precursor effector cells after murine infection with either a virus (vaccinia virus) or bacteria (Listeria monocytogenes). Interestingly, we found that the propensity of female CD8⁺ T cells to form short-lived effectors is not because they respond to lower amounts of cognate Ag but rather because they have an enhanced capacity to respond to IL-12, which facilitates more effector cell differentiation at each round of cell division. Our findings provide key insights into the sex-based immunological differences that underlie variations in the susceptibility to infection in males and females. ImmunoHorizons, 2019, 3: 121–132.

Role of Cell-Intrinsic and Environmental Age-Related Changes in Altered Maintenance of Murine T Cells in Lymphoid Organs

Age-related changes in primary lymphoid organs are well described. Less is known about age-related changes affecting peripheral lymphoid organs, although defects in old peripheral lymph nodes (pLNs) were recently described in both steady state and during viral infection. To address whether such pLN defects were intrinsic to old T cells or extrinsic (due to aging microenvironment), we employed heterochronic parabiosis. We found no age-related intrinsic or extrinsic barriers to T cell circulation and seeding of pLN, spleen, and bone marrow. However, heterochronic parabiosis failed to improve cellularity of old pLN, suggesting an environment-based limit on pLN cellularity. Furthermore, upon parabiosis, pLN of the adult partner exhibited reduced, old-like stromal and T cell cellularity, which was restored following separation of parabionts. Decay measurement of adult and old T cell subsets following separation of heterochronic parabionts delineated both T cell-intrinsic and environmental changes in T cell maintenance. Moreover, parabiotic separation revealed differences between CD4 and CD8 T cell subset maintenance with aging, the basis of which will require further investigation. Reasons for this asymmetric and subset-specific pattern of differential maintenance are discussed in light of possible age-related changes in lymph nodes as the key sites for peripheral T cell maintenance.

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.

Dissecting the defects in the neonatal CD8+ T-cell response

The neonatal period presents a complex scenario where the threshold of reactivity toward colonizing microbiota, maternal antigens, autoantigens, and pathogens must be carefully moderated and balanced. CD8⁺ T cells are critical for the response against intracellular bacteria and viruses, but this immune compartment maintains altered function relative to adult counterparts because of the unique challenges which infants face. Here, we review our current understanding of the factors which may promote the attenuation and altered function of the neonatal CD8⁺ T-cell response and potential avenues for future study. Specifically, we have focused on the neonatal CD8⁺ T-cell ontogeny, memory formation, TCR structure and repertoire, TCR inhibitory receptors, and the clinical implications of altered neonatal CD8⁺ T-cell function. Special emphasis has been placed on examining the response of preterm neonates relative to term neonates and adults.

The expanding role of systems immunology in decoding the T cell receptor repertoire

T cells play a crucial role in the immune system's defense against many infectious diseases, including persistent infections for which no effective vaccines currently exist. The T cell component of the adaptive immune system is highly complex involving a constantly evolving landscape of various inter-related T cell populations. These T cell populations are characterized by their phenotypic and functional properties as well as the collection, or repertoire, of T cell receptors (TCR) that mediate T cell recognition of antigenic peptides derived from pathogens. Understanding the various processes and factors that impact the development and evolution of the broader T cell repertoire available to recognize and respond to pathogens and the characteristics of antigen-experienced T cell repertoires associated with effective immune control of pathogens is critical to the rational design of T cell-based vaccines and therapies. In this article we discuss, using examples of recent research, the promise that systems immunology approaches, involving quantitative analysis and mathematical and computational modeling of immunological data, hold for decoding the complex TCR repertoire system in the current era of advancing technologies.

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.

Developmental Origin Governs CD8+ T Cell Fate Decisions during Infection

Heterogeneity is a hallmark feature of the adaptive immune system in vertebrates. Following infection, naive T cells differentiate into various subsets of effector and memory T cells, which help to eliminate pathogens and maintain long-term immunity. The current model suggests there is a single lineage of naive T cells that give rise to different populations of effector and memory T cells depending on the type and amounts of stimulation they encounter during infection. Here, we have discovered that multiple sub-populations of cells exist in the naive CD8⁺ T cell pool that are distinguished by their developmental origin, unique transcriptional profiles, distinct chromatin landscapes, and different kinetics and phenotypes after microbial challenge. These data demonstrate that the naive CD8⁺ T cell pool is not as homogeneous as previously thought and offers a new framework for explaining the remarkable heterogeneity in the effector and memory T cell subsets that arise after infection.

The twilight of immunity: emerging concepts in aging of the immune system

Immunosenescence is a series of age-related changes that affect the immune system and, with time, lead to increased vulnerability to infectious diseases. This Review addresses recent developments in the understanding of age-related changes that affect key components of immunity, including the effect of aging on cells of the (mostly adaptive) immune system, on soluble molecules that guide the maintenance and function of the immune system and on lymphoid organs that coordinate both the maintenance of lymphocytes and the initiation of immune responses. I further address the effect of the metagenome and exposome as key modifiers of immune-system aging and discuss a conceptual framework in which age-related changes in immunity might also affect the basic rules by which the immune system operates.

The molecular signature of murine T cell homeostatic proliferation reveals both inflammatory and immune inhibition patterns

T lymphocyte homeostatic proliferation, driven by the engagement of T cell antigen receptor with self-peptide/major histocompatibility complexes, and signaling through the common γ-chain-containing cytokine receptors, is critical for the maintenance of the T cell compartment and is regulated by the Fas death receptor (Fas, CD95). In the absence of Fas, Fas-deficient lymphoproliferation spontaneous mutation (lpr) mice accumulate homeostatically expanded T cells. The functional consequences of sequential rounds of homeostatic expansion are not well defined. We thus examined the gene expression profiles of murine wild-type and Fas-deficient lpr CD8+ T cell subsets that have undergone different amounts of homeostatic proliferation as defined by their level of CD44 expression, and the CD4-CD8-TCRαβ+ T cell subset that results from extensive homeostatic expansion of CD8+ T cells. Our studies show that recurrent T cell homeostatic proliferation results in global gene expression changes, including the progressive upregulation of both cytolytic proteins such as Fas-Ligand and granzyme B as well as inhibitory proteins such as programmed cell death protein 1 (PD-1) and lymphocyte activating 3 (Lag3). These findings provide an explanation for how augmented T cell homeostatic expansion could lead to the frequently observed clinical paradox of simultaneous autoinflammatory and immunodeficiency syndromes and provide further insight into the regulatory programs that control chronically stimulated T cells.

Quantifiable predictive features define epitope-specific T cell receptor repertoires

T cells are defined by a heterodimeric surface receptor, the T cell receptor (TCR), that mediates recognition of pathogen-associated epitopes through interactions with peptide and major histocompatibility complexes (pMHCs). TCRs are generated by genomic rearrangement of the germline TCR locus, a process termed V(D)J recombination, that has the potential to generate marked diversity of TCRs (estimated to range from 10¹⁵ (ref. 1) to as high as 10⁶¹ (ref. 2) possible receptors). Despite this potential diversity, TCRs from T cells that recognize the same pMHC epitope often share conserved sequence features, suggesting that it may be possible to predictively model epitope specificity. Here we report the in-depth characterization of ten epitope-specific TCR repertoires of CD8⁺ T cells from mice and humans, representing over 4,600 in-frame single-cell-derived TCRαβ sequence pairs from 110 subjects. We developed analytical tools to characterize these epitope-specific repertoires: a distance measure on the space of TCRs that permits clustering and visualization, a robust repertoire diversity metric that accommodates the low number of paired public receptors observed when compared to single-chain analyses, and a distance-based classifier that can assign previously unobserved TCRs to characterized repertoires with robust sensitivity and specificity. Our analyses demonstrate that each epitope-specific repertoire contains a clustered group of receptors that share core sequence similarities, together with a dispersed set of diverse 'outlier' sequences. By identifying shared motifs in core sequences, we were able to highlight key conserved residues driving essential elements of TCR recognition. These analyses provide insights into the generalizable, underlying features of epitope-specific repertoires and adaptive immune recognition.

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.

Dysregulated TGF-β Production Underlies the Age-Related Vulnerability to Chikungunya Virus

Chikungunya virus (CHIKV) is a re-emerging global pathogen with pandemic potential, which causes fever, rash and debilitating arthralgia. Older adults over 65 years are particularly susceptible to severe and chronic CHIKV disease (CHIKVD), accounting for >90% of all CHIKV-related deaths. There are currently no approved vaccines or antiviral treatments available to limit chronic CHIKVD. Here we show that in old mice excessive, dysregulated TGFβ production during acute infection leads to a reduced immune response and subsequent chronic disease. Humans suffering from CHIKV infection also exhibited high TGFβ levels and a pronounced age-related defect in neutralizing anti-CHIKV antibody production. In vivo reduction of TGFβ levels minimized acute joint swelling, restored neutralizing antibody production and diminished chronic joint pathology in old mice. This study identifies increased and dysregulated TGFβ secretion as one key mechanism contributing to the age-related loss of protective anti-CHIKV-immunity leading to chronic CHIKVD.

Modeling the dynamics of neonatal CD8+ T-cell responses

Neonates are particularly susceptible to a number of infections, and the neonatal CD8+ T cell response demonstrates differences in both the phenotype and magnitude of responses to infection compared with adults. However, the underlying basis for these differences is unclear. We have used a mathematical modeling approach to analyze the dynamics of neonatal and adult CD8+ T cell responses following in vitro stimulation and in vivo infection, which allows us to dissect key cell-intrinsic differences in expansion, differentiation and memory formation. We found that neonatal cells started dividing 8 hrs earlier and proliferated at a faster rate (0.077 day⁻¹ vs 0.105 day⁻¹) than adult cells in vitro. In addition, neonatal cells also differentiated more rapidly, as measured by the loss in CD62L and Ly6C expression. We extended our mathematical modeling to analysis of neonatal and adult CD8+ T cells responding in vivo and demonstrated that neonatal cells divide more slowly than adult cells after day 4 post-infection. However, neonatal cells differentiate more rapidly, up-regulating more KLRG-1 per division than adult cells (20% vs. 5%). The dynamics of memory formation were also found to be different, with neonatal effector cells showing increased death (1.0 day⁻¹ vs. 2.45 day⁻¹). Comparison of the division of human cord blood and adult naïve cells stimulated in vitro showed more division in cord blood derived cells, consistent with the observations in mice. This work highlights differences of the cell-intrinsic division and differentiation program in neonatal CD8+ T cells.

Beta-catenin signaling drives differentiation and proinflammatory function of IRF8-dependent dendritic cells

Beta-catenin signaling has recently been tied to the emergence of tolerogenic dendritic cells (DCs). In this article, we demonstrate a novel role for beta-catenin in directing DC subset development through IFN regulatory factor 8 (IRF8) activation. We found that splenic DC precursors express beta-catenin, and DCs from mice with CD11c-specific constitutive beta-catenin activation upregulated IRF8 through targeting of the Irf8 promoter, leading to in vivo expansion of IRF8-dependent CD8a+, plasmacytoid, and CD103+ CD11b2 DCs. beta-catenin–stabilized CD8a+ DCs secreted elevated IL-12 upon in vitro microbial stimulation, and pharmacological beta-catenin inhibition blocked this response in wild-type cells. Upon infections with Toxoplasma gondii and vaccinia virus, mice with stabilized DC beta-catenin displayed abnormally high Th1 and CD8+ T lymphocyte responses, respectively. Collectively, these results reveal a novel and unexpected function for beta-catenin in programming DC differentiation toward subsets that orchestrate proinflammatory immunity to infection.

Rapid Evolution of the CD8+ TCR Repertoire in Neonatal Mice

Currently, there is little consensus regarding the most appropriate animal model to study acute infection and the virus-specific CD8(+) T cell (CTL) responses in neonates. TCRβ high-throughput sequencing in naive CTL of differently aged neonatal mice was performed, which demonstrated differential Vβ family gene usage. Using an acute influenza infection model, we examined the TCR repertoire of the CTL response in neonatal and adult mice infected with influenza type A virus. Three-day-old mice mounted a greatly reduced primary NP(366-374)-specific CTL response when compared with 7-d-old and adult mice, whereas secondary CTL responses were normal. Analysis of NP(366-374)-specific CTL TCR repertoire revealed different Vβ gene usage and greatly reduced public clonotypes in 3-d-old neonates. This could underlie the impaired CTL response in these neonates. To directly test this, we examined whether controlling the TCR would restore neonatal CTL responses. We performed adoptive transfers of both nontransgenic and TCR-transgenic OVA(257-264)-specific (OT-I) CD8(+) T cells into influenza-infected hosts, which revealed that naive neonatal and adult OT-I cells expand equally well in neonatal and adult hosts. In contrast, nontransgenic neonatal CD8(+) T cells when transferred into adults failed to expand. We further demonstrate that differences in TCR avidity may contribute to decreased expansion of the endogenous neonatal CTL. These studies highlight the rapid evolution of the neonatal TCR repertoire during the first week of life and show that impaired neonatal CTL immunity results from an immature TCR repertoire, rather than intrinsic signaling defects or a suppressive environment.

The Neonatal CD8+ T Cell Repertoire Rapidly Diversifies during Persistent Viral Infection

CMV is the most common congenital infection in the United States. The major target of congenital CMV is the brain, with clinical manifestations including mental retardation, vision impairment, and sensorineural hearing loss. Previous reports have shown that CD8(+) T cells are required to control viral replication and significant numbers of CMV-specific CD8(+) T cells persist in the brain even after the initial infection has been cleared. However, the dynamics of CD8(+) T cells in the brain during latency remain largely undefined. In this report, we used TCR sequencing to track the development and maintenance of neonatal clonotypes in the brain and spleen of mice during chronic infection. Given the discontinuous nature of tissue-resident memory CD8(+) T cells, we hypothesized that neonatal TCR clonotypes would be locked in the brain and persist into adulthood. Surprisingly, we found that the Ag-specific T cell repertoire in neonatal-infected mice diversified during persistent infection in both the brain and spleen, while maintaining substantial similarity between the CD8(+) T cell populations in the brain and spleen in both early and late infection. However, despite the diversification of, and potential interchange between, the spleen and brain Ag-specific T cell repertoires, we observed that germline-encoded TCR clonotypes, characteristic of neonatal infection, persisted in the brain, albeit sometimes in low abundance. These results provide valuable insights into the evolution of CD8(+) T cell repertoires following neonatal CMV infection and thus have important implications for the development of therapeutic strategies to control CMV in early life.

DAMPs and influenza virus infection in ageing

Influenza A virus (IAV) is a serious global health problem worldwide due to frequent and severe outbreaks. IAV causes significant morbidity and mortality in the elderly population, due to the ineffectiveness of the vaccine and the alteration of T cell immunity with ageing. The cellular and molecular link between ageing and virus infection is unclear and it is possible that damage associated molecular patterns (DAMPs) may play a role in the raised severity and susceptibility of virus infections in the elderly. DAMPs which are released from damaged cells following activation, injury or cell death can activate the immune response through the stimulation of the inflammasome through several types of receptors found on the plasma membrane, inside endosomes after endocytosis as well as in the cytosol. In this review, the detriment in the immune system during ageing and the links between influenza virus infection and ageing will be discussed. In addition, the role of DAMPs such as HMGB1 and S100/Annexin in ageing, and the enhanced morbidity and mortality to severe influenza infection in ageing will be highlighted.

MicroRNAs and Their Targets Are Differentially Regulated in Adult and Neonatal Mouse CD8+ T Cells

Immunological memory, which protects organisms from re-infection, is a hallmark of the mammalian adaptive immune system and the underlying principle of vaccination. In early life, however, mice and other mammals are deficient at generating memory CD8+ T cells, which protect organisms from intracellular pathogens. The molecular basis that differentiates adult and neonatal CD8+ T cells is unknown. MicroRNAs (miRNAs) are both developmentally regulated and required for normal adult CD8+ T cell functions. We used next-generation sequencing to identify mouse miRNAs that are differentially regulated in adult and neonatal CD8+ T cells, which may contribute to the impaired development of neonatal memory cells. The miRNA profiles of adult and neonatal cells were surprisingly similar during infection; however, we observed large differences prior to infection. In particular, miR-29 and miR-130 have significant differential expression between adult and neonatal cells before infection. Importantly, using RNA-Seq, we detected reciprocal changes in expression of messenger RNA targets for both miR-29 and miR-130. Moreover, targets that we validated include Eomes and Tbx21, key genes that regulate the formation of memory CD8+ T cells. Notably, age-dependent changes in miR-29 and miR-130 are conserved in human CD8+ T cells, further suggesting that these developmental differences are biologically relevant. Together, these results demonstrate that miR-29 and miR-130 are likely important regulators of memory CD8+ T cell formation and suggest that neonatal cells are committed to a short-lived effector cell fate prior to infection.

Vaccination and heterologous immunity: educating the immune system

This review discusses three inter-related topics: (1) the immaturity of the neonatal and infant immune response; (2) heterologous immunity, where prior infection history with unrelated pathogens alters disease outcome resulting in either enhanced protective immunity or increased immunopathology to new infections, and (3) epidemiological human vaccine studies that demonstrate vaccines can have beneficial or detrimental effects on subsequent unrelated infections. The results from the epidemiological and heterologous immunity studies suggest that the immune system has tremendous plasticity and that each new infection or vaccine that an individual is exposed to during a lifetime will potentially alter the dynamics of their immune system. It also suggests that each new infection or vaccine that an infant receives is not only perturbing the immune system but is educating the immune system and laying down the foundation for all subsequent responses. This leads to the question, is there an optimum way to educate the immune system? Should this be taken into consideration in our vaccination protocols?

αβ T cell receptors as predictors of health and disease

The diversity of antigen receptors and the specificity it underlies are the hallmarks of the cellular arm of the adaptive immune system. T and B lymphocytes are indeed truly unique in their ability to generate receptors capable of recognizing virtually any pathogen. It has been known for several decades that T lymphocytes recognize short peptides derived from degraded proteins presented by major histocompatibility complex (MHC) molecules at the cell surface. Interaction between peptide-MHC (pMHC) and the T cell receptor (TCR) is central to both thymic selection and peripheral antigen recognition. It is widely assumed that TCR diversity is required, or at least highly desirable, to provide sufficient immune coverage. However, a number of immune responses are associated with the selection of predictable, narrow, or skewed repertoires and public TCR chains. Here, we summarize the current knowledge on the formation of the TCR repertoire and its maintenance in health and disease. We also outline the various molecular mechanisms that govern the composition of the pre-selection, naive and antigen-specific TCR repertoires. Finally, we suggest that with the development of high-throughput sequencing, common TCR 'signatures' raised against specific antigens could provide important diagnostic biomarkers and surrogate predictors of disease onset, progression and outcome.

Narrowing of human influenza A virus-specific T cell receptor α and β repertoires with increasing age

Alterations in memory CD8 T cell responses may contribute to the high morbidity and mortality caused by seasonal influenza A virus (IAV) infections in older individuals. We questioned whether memory CD8 responses to this nonpersistent virus, to which recurrent exposure with new strains is common, changed over time with increasing age. Here, we show a direct correlation between increasing age and narrowing of the HLA-A2-restricted IAV Vα and Vβ T cell repertoires specific to M1 residues 58 to 66 (M158-66), which simultaneously lead to oligoclonal expansions, including the usage of a single identical VA12-JA29 clonotype in all eight older donors. The Vα repertoire of older individuals also had longer CDR3 regions with increased usage of G/A runs, whose molecular flexibility may enhance T cell receptor (TCR) promiscuity. Collectively, these results suggest that CD8 memory T cell responses to nonpersistent viruses like IAV in humans are dynamic, and with aging there is a reduced diversity but a preferential retention of T cell repertoires with features of enhanced cross-reactivity.

IMPORTANCE

With increasing age, the immune system undergoes drastic changes, and older individuals have declined resistance to infections. Vaccinations become less effective, and infection with influenza A virus in older individuals is associated with higher morbidity and mortality. Here, we questioned whether T cell responses directed against the highly conserved HLA-A2-restricted M158-66 peptide of IAV evolves with increasing age. Specifically, we postulated that CD8 T cell repertoires narrow with recurrent exposure and may thus be less efficient in response to new infections with new strains of IAV. Detailed analyses of the VA and VB TCR repertoires simultaneously showed a direct correlation between increasing age and narrowing of the TCR repertoire. Features of the TCRs indicated potentially enhanced cross-reactivity in all older donors. In summary, T cell repertoire analysis in older individuals may be useful as one of the predictors of protection after vaccination.

A population biological approach to understanding the maintenance and loss of the T-cell repertoire during aging

The adaptive immune system requires a diverse T-cell repertoire to be able to respond to a wide variety of pathogens. Worryingly, the repertoire diversity declines dramatically in old age. As thymic output generates novel T cells, the conventional view holds that a decrease in this output with age is responsible for the loss in the repertoire. However, many additional factors affect the repertoire such as homeostatic turnover and antigen-dependent expansion in response to infection. Mathematical models taking a population biology perspective are important tools for understanding how the interplay between these factors affects the immune repertoire. These models suggest that thymic decline is not a major factor but rather that some combination of virus-induced proliferation and T-cell-intrinsic genetic or epigenetic changes gives rise to the oligoclonal expansions that cause the decline in T-cell diversity. We also discuss consequences for strategies to rejuvenate the immune repertoire in old age.

Rapid proliferation and differentiation impairs the development of memory CD8+ T cells in early life

Neonates often generate incomplete immunity against intracellular pathogens, although the mechanism of this defect is poorly understood. An important question is whether the impaired development of memory CD8+ T cells in neonates is due to an immature priming environment or lymphocyte-intrinsic defects. In this article, we show that neonatal and adult CD8+ T cells adopted different fates when responding to equal amounts of stimulation in the same host. Whereas adult CD8+ T cells differentiated into a heterogeneous pool of effector and memory cells, neonatal CD8+ T cells preferentially gave rise to short-lived effector cells and exhibited a distinct gene expression profile. Surprisingly, impaired neonatal memory formation was not due to a lack of responsiveness, but instead because neonatal CD8+ T cells expanded more rapidly than adult cells and quickly became terminally differentiated. Collectively, these findings demonstrate that neonatal CD8+ T cells exhibit an imbalance in effector and memory CD8+ T cell differentiation, which impairs the formation of memory CD8+ T cells in early life.

Impact of ageing on the response and repertoire of influenza virus-specific CD4 T cells

BACKGROUND

Ageing has been shown to reduce CD8 T cell repertoire diversity and immune responses against influenza virus infection in mice. In contrast, less is known about the impact of ageing on CD4 T cell repertoire diversity and immune response to influenza virus infection.

RESULTS

The CD4 T cell response was followed after infection of young and aged C57BL/6 mice with influenza virus using a tetramer specific for an immunodominant MHC class II epitope of the influenza virus nucleoprotein. The appearance of virus-specific CD4 T cells in the lung airways of aged mice was delayed compared to young mice, but the overall peak number and cytokine secretion profile of responding CD4 T cells was not greatly perturbed. In addition, the T cell repertoire of responding cells, determined using T cell receptor Vβ analysis, failed to show the profound effect of age we previously described for CD8 T cells. The reduced impact of age on influenza-specific CD4 T cells was consistent with a reduced effect of age on the overall CD4 compared with the CD8 T cell repertoire in specific pathogen free mice. Aged mice that were thymectomized as young adults showed an enhanced loss of the epitope-specific CD4 T cell response after influenza virus infection compared with age-matched sham-thymectomized mice, suggesting that a reduced repertoire can contribute to impaired responsiveness.

CONCLUSIONS

The diversity of the CD4 T cell repertoire and response to influenza virus is not as profoundly impaired by ageing in C57BL/6 mice as previously shown for CD8 T cells. However, adult thymectomy enhanced the impact of ageing on the response. Understanding the impact of ageing on CD4 T cell responses to influenza virus infection is an important prerequisite for developing better vaccines for the elderly.

Acute neonatal infections 'lock-in' a suboptimal CD8+ T cell repertoire with impaired recall responses

Microbial infection during various stages of human development produces widely different clinical outcomes, yet the links between age-related changes in the immune compartment and functional immunity remain unclear. The ability of the immune system to respond to specific antigens and mediate protection in early life is closely correlated with the level of diversification of lymphocyte antigen receptors. We have previously shown that the neonatal primary CD8+ T cell response to replication competent virus is significantly constricted compared to the adult response. In the present study, we have analyzed the subsequent formation of neonatal memory CD8+ T cells and their response to secondary infectious challenge. In particular, we asked whether the less diverse CD8+ T cell clonotypes that are elicited by neonatal vaccination with replication competent virus are ‘locked-in’ to the adult memory T cell, and thus may compromise the strength of adult immunity. Here we report that neonatal memory CD8+ T cells mediate poor recall responses compared to adults and are comprised of a repertoire of lower avidity T cells. During a later infectious challenge the neonatal memory CD8+ T cells compete poorly with the fully diverse repertoire of naïve adult CD8+ T cells and are outgrown by the adult primary response. This has important implications for the timing of vaccination in early life.

Newborns typically have a heightened sensitivity to infectious diseases, the reasons for which are not yet well understood. One contributing factor is the limited diversity of lymphocyte receptors early in life to recognize antigen and control infection. We have previously shown that antigen-specific CD8+ T cell repertoires are significantly constricted in neonates compared with adults. In this study, we addressed the question of whether the developmental stage of the host at the time of vaccination influences the composition of the memory CD8⁺ T cell repertoire and its ability to mount a robust response to subsequent infections. We observed that the antigen-specific T cell repertoires elicited in the context of an acute neonatal infection, that are less diverse and comprised of lower-avidity T cells, are partially ‘locked-in’ to the adult memory T cell repertoire. However, in the face of a secondary infectious challenge, naïve adult T cells outcompete the lower avidity neonatal memory T cells and raise the diversity of the overall CD8⁺ T cell response. These results have potential implications for the design of vaccines to be administered in early life.

Specificity, promiscuity, and precursor frequency in immunoreceptors

The immune system is comprised of various immune cell populations that utilize a spectrum of immunoreceptors characterized by different levels of specificity, diversity, and prevalence within a host and across a population. These range from the universal receptors employed by both innate cells and innate-like cells, such as NKT and MAIT cells, through to receptors expressed on T cells with sporadic incidence. Here we review recent advances in understanding the molecular mechanisms that drive the observed spectra of T cell receptors in vivo.

Peripheral selection rather than thymic involution explains sudden contraction in naive CD4 T-cell diversity with age

A diverse array of T cells is required for defense against pathogens. The naive CD4 T-cell repertoire reaches its peak diversity by early human adulthood and is maintained until older age. Surprisingly, around age 70, this diversity appears to plummet abruptly. A similar qualitative pattern holds for the CD4 T memory-cell population. We used mathematical models to explore different hypotheses for how such a loss of diversity might occur. The prevailing hypotheses suggest that the loss of diversity is due to a decline in emigration of cells from the thymus or a contraction in total number of cells. Our models reject these mechanisms because they yield only a gradual and minimal decline in the repertoire instead of the observed sudden and profound decrease later in life. We propose that an abrupt decline in the repertoire could be caused by the accumulation of mutations (defined here as any cell-intrinsic heritable event) that provide a short-term fitness advantage to a small number of T-cell clones (e.g., by an increased division rate or decreased death rate), with the person as a whole incurring the long-term cost of a decreased ability to fight infections.

Lifelong persistent viral infection alters the naive T cell pool, impairing CD8 T cell immunity in late life

Persistent CMV infection has been associated with immune senescence. To address the causal impact of lifelong persistent viral infection on immune homeostasis and defense, we infected young mice systemically with HSV-1, murine CMV, or both viruses and studied their T cell homeostasis and function. Herpesvirus(+) mice exhibited increased all-cause mortality compared with controls. Upon Listeria-OVA infection, 23-mo-old animals that had experienced lifelong herpesvirus infections showed impaired bacterial control and CD8 T cell function, along with distinct alterations in the T cell repertoire both before and after Listeria challenge, compared with age-matched, herpesvirus-free controls. Herpesvirus infection was associated with reduced naive CD8 T cell precursors above the loss attributable to aging. Moreover, the OVA-specific CD8 T cell repertoire recruited after Listeria challenge was entirely nonoverlapping between control and herpesvirus(+) mice. To our knowledge, this study for the first time causally links lifelong herpesvirus infection to all-cause mortality in mice and to disturbances in the T cell repertoire, which themselves correspond to impaired immunity to a new infection in aging.

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.

Extralymphocytic flexible immune recognition: a new angle on inflammation and aging

Longstanding immunological dogma holds that flexible immune recognition, which forms the mechanistic basis of adaptive immunity, is strictly confined to the lymphocyte lineage. In higher vertebrates, flexible immune recognition is represented by recombinatorial antigen receptors of enormous diversity known as immunoglobulins, expressed by B lymphocytes, and the T cell receptor (TCR), expressed by T lymphocytes. The recent discovery of recombinatorial immune receptors that are structurally based on the TCR (referred to as TCR-like immunoreceptors, "TCRL") in myeloid phagocytes such as neutrophils and monocytes/macrophages now challenges the lymphocentric paradigm of flexible immunity. Here, we introduce the emerging concept of "extralymphocytic flexible immune recognition" and discuss its implications for inflammation and aging.

On the horizon: flexible immune recognition outside lymphocytes

Since decades there is consensus among immunologists that in jawless and jawed vertebrates flexible immune recognition is strictly confined to the lymphoid lineage. In jawed vertebrates the adaptive immune system is represented by two lineages of lymphocytes, B cells and T cells that express recombinatorial antigen receptors of enormous diversity known as immunoglobulins and the T cell receptor (TCR). The recent identification of recombined immune receptors that are structurally based on the TCR in subpopulations of neutrophils and eosinophils (referred to here as TCR-like immunoreceptors, "TCRL") provides unexpected evidence for the existence of flexible host defense mechanisms beyond the realm of lymphocytes. Consistent with this, subpopulations of monocytes and macrophages from humans and mice now have also been shown to constitutively express recombined TCR-like immunoreceptors. Available in vitro evidence suggests that the TCRL in macrophages may exert functions as facilitators of phagocytosis and self-recruitment. More importantly, our recent findings that the macrophage-TCRL is implicated in granuloma formation in tuberculosis and the neutrophil-TCRL is associated with autoimmune hemolytic anemia establish for the first time a link between myeloid recombinatorial immune receptors and clinical disease. The discovery of recombined TCR-like immune receptors in granulocytes and macrophages extends the principle of combinatorial immune recognition to phagocytic cells. Conceptually, this unifies the two hitherto disparate cardinal features of innate and adaptive immunity, phagocytic capacity and recombinatorial immune recognition on a common cellular platform. Moreover, it strongly suggests that flexible host defense in vertebrates may operate on a broader cellular basis than currently thought.

Age-related changes in CD8 T cell homeostasis and immunity to infection

Studies of CD8 T cell responses to vaccination or infection with various pathogens in both animal models and human subjects have revealed a markedly consistent array of age-related defects. In general, recent work shows that aged CD8 T cell responses are decreased in magnitude, and show poor differentiation into effector cells, with a reduced arsenal of effector functions. Here we review potential mechanisms underlying these defects. We specifically address phenotypic and numeric changes to the naïve CD8 T cell precursor pool, the impact of persistent viral infection(s) and inflammation, and contributions of the aging environment in which these cells are activated.

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 quiescence revisited

Naïve T cells are typically considered to be in a default state of quiescence, whereas memory T cells undergo basal proliferation and quickly exhibit effector responses when stimulated. Over the past few years, however, a more complex picture has emerged, with evidence that naïve T cell quiescence is actively enforced, and that heterogeneity among naïve T cells influences their capacity to escape quiescence in response to homeostatic cues. Furthermore, the active state of memory T cells may also be instructed, requiring contact with dendritic cells to avoid reversion to quiescence. Here, we discuss these new findings and propose that there is much more flexibility in the quiescent state of naïve and memory T cells than previously thought.

Derivation and maintenance of virtual memory CD8 T cells

Memory CD8(+) T cells are an important component of the adaptive immune response against many infections, and understanding how Ag-specific memory CD8(+) T cells are generated and maintained is crucial for the development of vaccines. We recently reported the existence of memory-phenotype, Ag-specific CD8(+) T cells in unimmunized mice (virtual memory or VM cells). However, it was not clear when and where these cells are generated during normal development, nor the factors required for their production and maintenance. This issue is especially pertinent given recent data showing that memory-like CD8 T cells can be generated in the thymus, in a bystander response to IL-4. In this study, we show that the size of the VM population is reduced in IL-4R-deficient animals. However, the VM population appears first in the periphery and not the thymus of normal animals, suggesting this role of IL-4 is manifest following thymic egress. We also show that the VM pool is durable, showing basal proliferation and long-term maintenance in normal animals, and also being retained during responses to unrelated infection.

Immunosenescence and Challenges of Vaccination against Influenza in the Aging Population

Influenza is an important contributor to morbidity and mortality worldwide. Accumulation of genetic mutations termed antigenic drift, allows influenza viruses to inflict yearly epidemics that may result in 250,000 to 500,000 deaths annually. Over 90% of influenza-related deaths occur in the older adult population. This is at least in part a result of increasing dysregulation of the immune system with age, termed immunosenescence. This dysregulation results in reduced capacity to cope with infections and decreased responsiveness to vaccination. The older adult population is in dire need of improved vaccines capable of eliciting protective responses in the face of a waning immune system. This review focuses on the status of immunity, responses to influenza vaccination, and strategies that are currently being explored to elicit enhanced immune responses in this high risk population.

Mechanisms of immunosenescence: lessons from models of accelerated immune aging

With increasing age, the ability of the adaptive immune system to respond to vaccines and to protect from infection declines. In parallel, the production of inflammatory mediators increases. While cross-sectional studies have been successful in defining age-dependent immunological phenotypes, studies of accelerated immune aging in human subpopulations have been instrumental in obtaining mechanistic insights. The immune system depends on its regenerative capacity; however, the T cell repertoire, once established, is relatively robust to aging and only decompensates when additionally stressed. Such stressors include chronic infections such as CMV and HIV, even when viral replication is controlled, and autoimmune diseases. Reduced regenerative capacity, chronic immune activation in the absence of cell exhaustion, T cell memory inflation, and accumulation of highly potent effector T cells in these patients synergize to develop an immune phenotype that is characteristic of the elderly. Studies of accelerated immune aging in autoimmune diseases have identified an unexpected link to chronic DNA damage responses that are known to be important in aging, but so far had not been implicated in immune aging.