Telomere erosion in memory T cells induced by telomerase inhibition at the site of antigenic challenge in vivo

Senescent T Cells in Age-Related Diseases

Age-induced alterations in human immunity are often considered deleterious and are referred to as immunosenescence. The immune system monitors the number of senescent cells in the body, while immunosenescence may represent the initiation of systemic aging. Immune cells, particularly T cells, are the most impacted and involved in age-related immune function deterioration, making older individuals more prone to different age-related diseases. T-cell senescence can impact the effectiveness of immunotherapies that rely on the immune system's function, including vaccines and adoptive T-cell therapies. The research and practice of using senescent T cells as therapeutic targets to intervene in age-related diseases are in their nascent stages. Therefore, in this review, we summarize recent related literature to investigate the characteristics of senescent T cells as well as their formation mechanisms, relationship with various aging-related diseases, and means of intervention. The primary objective of this article is to explore the prospects and possibilities of therapeutically targeting senescent T cells, serving as a valuable resource for the development of immunotherapy and treatment of age-related diseases.

Chronic inflammation and the hallmarks of aging

BACKGROUND

Recently, the hallmarks of aging were updated to include dysbiosis, disabled macroautophagy, and chronic inflammation. In particular, the low-grade chronic inflammation during aging, without overt infection, is defined as "inflammaging," which is associated with increased morbidity and mortality in the aging population. Emerging evidence suggests a bidirectional and cyclical relationship between chronic inflammation and the development of age-related conditions, such as cardiovascular diseases, neurodegeneration, cancer, and frailty. How the crosstalk between chronic inflammation and other hallmarks of aging underlies biological mechanisms of aging and age-related disease is thus of particular interest to the current geroscience research.

SCOPE OF REVIEW

This review integrates the cellular and molecular mechanisms of age-associated chronic inflammation with the other eleven hallmarks of aging. Extra discussion is dedicated to the hallmark of "altered nutrient sensing," given the scope of Molecular Metabolism. The deregulation of hallmark processes during aging disrupts the delicate balance between pro-inflammatory and anti-inflammatory signaling, leading to a persistent inflammatory state. The resultant chronic inflammation, in turn, further aggravates the dysfunction of each hallmark, thereby driving the progression of aging and age-related diseases.

MAIN CONCLUSIONS

The crosstalk between chronic inflammation and other hallmarks of aging results in a vicious cycle that exacerbates the decline in cellular functions and promotes aging. Understanding this complex interplay will provide new insights into the mechanisms of aging and the development of potential anti-aging interventions. Given their interconnectedness and ability to accentuate the primary elements of aging, drivers of chronic inflammation may be an ideal target with high translational potential to address the pathological conditions associated with aging.

The central role of DNA damage in immunosenescence

Ageing is the biggest risk factor for the development of multiple chronic diseases as well as increased infection susceptibility and severity of diseases such as influenza and COVID-19. This increased disease risk is linked to changes in immune function during ageing termed immunosenescence. Age-related loss of immune function, particularly in adaptive responses against pathogens and immunosurveillance against cancer, is accompanied by a paradoxical gain of function of some aspects of immunity such as elevated inflammation and increased incidence of autoimmunity. Of the many factors that contribute to immunosenescence, DNA damage is emerging as a key candidate. In this review, we discuss the evidence supporting the hypothesis that DNA damage may be a central driver of immunosenescence through senescence of both immune cells and cells of non-haematopoietic lineages. We explore why DNA damage accumulates during ageing in a major cell type, T cells, and how this may drive age-related immune dysfunction. We further propose that existing immunosenescence interventions may act, at least in part, by mitigating DNA damage and restoring DNA repair processes (which we term "genoprotection"). As such, we propose additional treatments on the basis of their evidence for genoprotection, and further suggest that this approach may provide a viable therapeutic strategy for improving immunity in older people.

A subset of gut leukocytes has telomerase-dependent "hyper-long" telomeres and require telomerase for function in zebrafish

BACKGROUND

Telomerase, the enzyme capable of elongating telomeres, is usually restricted in human somatic cells, which contributes to progressive telomere shortening with cell-division and ageing. T and B-cells cells are somatic cells that can break this rule and can modulate telomerase expression in a homeostatic manner. Whereas it seems intuitive that an immune cell type that depends on regular proliferation outbursts for function may have evolved to modulate telomerase expression it is less obvious why others may also do so, as has been suggested for macrophages and neutrophils in some chronic inflammation disease settings. The gut has been highlighted as a key modulator of systemic ageing and is a key tissue where inflammation must be carefully controlled to prevent dysfunction. How telomerase may play a role in innate immune subtypes in the context of natural ageing in the gut, however, remains to be determined.

RESULTS

Using the zebrafish model, we show that subsets of gut immune cells have telomerase-dependent"hyper-long" telomeres, which we identified as being predominantly macrophages and dendritics (mpeg1.1+ and cd45+mhcII+). Notably, mpeg1.1+ macrophages have much longer telomeres in the gut than in their haematopoietic tissue of origin, suggesting that there is modulation of telomerase in these cells, in the gut. Moreover, we show that a subset of gut mpeg1.1+ cells express telomerase (tert) in young WT zebrafish, but that the relative proportion of these cells decreases with ageing. Importantly, this is accompanied by telomere shortening and DNA damage responses with ageing and a telomerase-dependent decrease in expression of autophagy and immune activation markers. Finally, these telomerase-dependent molecular alterations are accompanied by impaired phagocytosis of E. coli and increased gut permeability in vivo.

CONCLUSIONS

Our data show that limiting levels of telomerase lead to alterations in gut immunity, impacting on the ability to clear pathogens in vivo. These are accompanied by increased gut permeability, which, together, are likely contributors to local and systemic tissue degeneration and increased susceptibility to infection with ageing.

Genes and Longevity of Lifespan

Aging is a complex process indicated by low energy levels, declined physiological activity, stress induced loss of homeostasis leading to the risk of diseases and mortality. Recent developments in medical sciences and an increased availability of nutritional requirements has significantly increased the average human lifespan worldwide. Several environmental and physiological factors contribute to the aging process. However, about 40% human life expectancy is inherited among generations, many lifespan associated genes, genetic mechanisms and pathways have been demonstrated during last decades. In the present review, we have evaluated many human genes and their non-human orthologs established for their role in the regulation of lifespan. The study has included more than fifty genes reported in the literature for their contributions to the longevity of life. Intact genomic DNA is essential for the life activities at the level of cell, tissue, and organ. Nucleic acids are vulnerable to oxidative stress, chemotherapies, and exposure to radiations. Efficient DNA repair mechanisms are essential for the maintenance of genomic integrity, damaged DNA is not replicated and transferred to next generations rather the presence of deleterious DNA initiates signaling cascades leading to the cell cycle arrest or apoptosis. DNA modifications, DNA methylation, histone methylation, histone acetylation and DNA damage can eventually lead towards apoptosis. The importance of calorie restriction therapy in the extension of lifespan has also been discussed. The role of pathways involved in the regulation of lifespan such as DAF-16/FOXO (forkhead box protein O1), TOR and JNK pathways has also been particularized. The study provides an updated account of genetic factors associated with the extended lifespan and their interactive contributory role with cellular pathways.

Flow-FISH as a Tool for Studying Bacteria, Fungi and Viruses

Many techniques are currently in use to study microbes. These can be aimed at detecting, identifying, and characterizing bacterial, fungal, and viral species. One technique that is suitable for high-throughput analysis is flow cytometry-based fluorescence in situ hybridization, or Flow-FISH. This technique employs (fluorescently labeled) probes directed against DNA or (m)RNA, for instance targeting a gene or microorganism of interest and provides information on a single-cell level. Furthermore, by combining Flow-FISH with antibody-based protein detection, proteins of interest can be measured simultaneously with genetic material. Additionally, depending on the type of Flow-FISH assay, Flow-FISH can also be multiplexed, allowing for the simultaneous measurement of multiple gene targets and/or microorganisms. Together, this allows for, e.g., single-cell gene expression analysis or identification of (sub)strains in mixed cultures. Flow-FISH has been used in mammalian cells but has also been extensively employed to study diverse microbial species. Here, the use of Flow-FISH for studying microorganisms is reviewed. Specifically, the detection of (intracellular) pathogens, studying microorganism biology and disease pathogenesis, and identification of bacterial, fungal, and viral strains in mixed cultures is discussed, with a particular focus on the viruses EBV, HIV-1, and SARS-CoV-2.

End-Stage Renal Disease-Related Accelerated Immune Senescence: Is Rejuvenation of the Immune System a Therapeutic Goal?

End-stage renal disease (ESRD) patients exhibit clinical features of premature ageing, including frailty, cardiovascular disease, and muscle wasting. Accelerated ageing also concerns the immune system. Patients with ESRD have both immune senescence and chronic inflammation that are resumed in the so-called inflammaging syndrome. Immune senescence is particularly characterised by premature loss of thymic function that is associated with hyporesponsiveness to vaccines, susceptibility to infections, and death. ESRD-related chronic inflammation has multiple causes and participates to accelerated cardiovascular disease. Although, both characterisation of immune senescence and its consequences are relatively well-known, mechanisms are more uncertain. However, prevention of immune senescence/inflammation or/and rejuvenation of the immune system are major goal to ameliorate clinical outcomes of ESRD patients.

Induction of T Cell Senescence by Cytokine Induced Bystander Activation

As people around the world continue to live longer, maintaining a good quality of life is of increasing importance. The COVID-19 pandemic revealed that the elderly are disproportionally vulnerable to infectious diseases and Immunosenescence plays a critical role in that. An ageing immune system influences the conventional activity of T cells which are at the forefront of eliminating harmful foreign antigens. With ageing, unconventional end-stage T cells, that exhibit a senescent phenotype, amass. These senescent T cells deviate from T cell receptor (TCR) signaling toward natural killer (NK) activity. The transition toward innate immune cell function from these adaptor T cells impacts antigen specificity, contributing to increased susceptibility of infection in the elderly. The mechanism by which senescent T cells arise remains largely unclear however in this review we investigate the part that bystander activation plays in driving the change in function of T cells with age. Cytokine-induced bystander activation may offer a plausible explanation for the induction of NK-like activity and senescence in T cells. Further understanding of these specific NK-like senescent T cells allows us to identify the benefits and detriments of these cells in health and disease which can be utilized or regulated, respectively. This review discusses the dynamic of senescent T cells in adopting NK-like T cells and the implications that has in an infectious disease context, predominately in the elderly.

Rethinking peripheral T cell tolerance: checkpoints across a T cell's journey

Following their exit from the thymus, T cells are endowed with potent effector functions but must spare host tissue from harm. The fate of these cells is dictated by a series of checkpoints that regulate the quality and magnitude of T cell-mediated immunity, known as tolerance checkpoints. In this Perspective, we discuss the mediators and networks that control the six main peripheral tolerance checkpoints throughout the life of a T cell: quiescence, ignorance, anergy, exhaustion, senescence and death. At the naive T cell stage, two intrinsic checkpoints that actively maintain tolerance are quiescence and ignorance. In the presence of co-stimulation-deficient T cell activation, anergy is a dominant hallmark that mandates T cell unresponsiveness. When T cells are successfully stimulated and reach the effector stage, exhaustion and senescence can limit excessive inflammation and prevent immunopathology. At every stage of the T cell's journey, cell death exists as a checkpoint to limit clonal expansion and to terminate unrestrained responses. Here, we compare and contrast the T cell tolerance checkpoints and discuss their specific roles, with the aim of providing an integrated view of T cell peripheral tolerance and fate regulation.

Serum anti-inflammatory and inflammatory markers have no causal impact on telomere length: a Mendelian randomization study

INTRODUCTION

The relationship between inflammatory and anti-inflammatory markers and telomere length (TL), a biological index of aging, is still poorly understood. By applying a 2-sample Mendelian randomization (MR), we investigated the causal associations between adiponectin, bilirubin, C-reactive protein (CRP), leptin, and serum uric acid (SUA) with TL.

MATERIAL AND METHODS

MR was implemented by using summary-level data from the largest ever genome-wide association studies (GWAS) conducted on our interested exposure and TL. Inverse variance weighted method (IVW), weighted median (WM)-based method, MR-Egger, MR-Robust Adjusted Profile Score (RAPS), and MR-Pleiotropy RESidual Sum and Outlier (PRESSO) were applied. Sensitivity analysis was conducted using the leave-one-out method.

RESULTS

With regard to adiponectin, CRP, leptin, and SUA levels, we found no effect on TL for all 4 types of tests (all p > 0.108). Results of the MR-Egger (p = 0.892) and IVW (p = 0.124) showed that bilirubin had no effect on telomere maintenance, whereas the results of the WM (p = 0.030) and RAPS (p = 0.022) were negative, with higher bilirubin concentrations linked to shorter TL. There was a low likelihood of heterogeneity for all the estimations, except for bilirubin (IVW p = 0.026, MR Egger p = 0.018). MR-PRESSO highlighted no outlier. For all the estimations, we observed negligible intercepts that were indicative of low likelihood of the pleiotropy (all p > 0.161). The results of leave-one-out method demonstrated that the links are not driven because of single nucleotide polymorphisms (SNPs).

CONCLUSIONS

Our results highlight that neither the anti-inflammatory nor pro-inflammatory markers tested have any significant causal effect on TL. The casual role of bilirubin on TL still needs to be investigated.

Both in vitro T cell proliferation and telomere length are decreased, but CD25 expression and IL-2 production are not affected in aged men

INTRODUCTION

Aging is a natural process involving dysfunction of multiple organs and is characterized by increased susceptibility to infections, cancer and autoimmune diseases. The functionality of the immune system depends on the capacity of lymphocytes to proliferate in response to antigenic challenges, and telomere length has an important role regulating the number of cell divisions. The aim of this study was to determine the possible relationship between telomere length, interleukin 2 (IL-2) production, CD25 expression and proliferation of peripheral blood mononuclear cells (PBMCs) in aged men.

MATERIAL AND METHODS

Telomere length was measured by RT-PCR in PBMCs from young and aged men. IL-2 production and CD25 expression were determined by ELISA and flow cytometry, respectively. Cell proliferation was measured by CFSE dilution assays upon in vitro stimulation with concanavalin A (Con A).

RESULTS

PBMCs from aged men showed a shorter telomere length and a reduced capacity to proliferate in vitro, compared to young men. In contrast, no significant differences in the level of CD25 expression on T lymphocytes, and in vitro production of IL-2 were detected in both groups. In addition, no significant correlation was detected between levels of CD25 expression, IL-2 production, cell proliferation, and telomere length in aged men.

CONCLUSIONS

In aged men the telomere length shortening and the reduced T cell proliferation are not related to the capacity of IL-2 production and CD25 expression on T lymphocytes.

The clock is ticking: the impact of ageing on T cell metabolism

It is now clear that access to specific metabolic programmes controls the survival and function of various immune cell populations, including T cells. Efficient naïve and memory T cell homoeostasis requires the use of specific metabolic pathways and differentiation requires rapid and dramatic metabolic remodelling. While we are beginning to appreciate the crucial role of metabolic programming during normal T cell physiology, many of the potential impacts of ageing on metabolic homoeostasis and remodelling in T cells remain unexplored. This review will outline our current understanding of T cell metabolism and explore age‐related metabolic changes that are postulated or have been demonstrated to impact T cell function.

Impact of Zostavax Vaccination on T-Cell Accumulation and Cutaneous Gene Expression in the Skin of Older Humans After Varicella Zoster Virus Antigen-Specific Challenge

Background

The live attenuated vaccine Zostavax was developed to prevent varicella zoster virus (VZV) reactivation that causes herpes zoster (shingles) in older humans. However, the impact of vaccination on the cutaneous response to VZV is not known.

Methods

We investigated the response to intradermal VZV antigen challenge before and after Zostavax vaccination in participants >70 years of age by immunohistological and transcriptomic analyses of skin biopsy specimens collected from the challenge site.

Results

Vaccination increased the proportion of VZV-specific CD4⁺ T cells in the blood and promoted the accumulation of both CD4⁺ and CD8⁺ T cells in the skin after VZV antigen challenge. However, Zostavax did not alter the proportion of resident memory T cells (CD4⁺ and CD8⁺) or CD4⁺Foxp3⁺ regulatory T cells in unchallenged skin. After vaccination, there was increased cutaneous T-cell proliferation at the challenge site and also increased recruitment of T cells from the blood, as indicated by an elevated T-cell migratory gene signature. CD8⁺ T-cell–associated functional genes were also highly induced in the skin after vaccination.

Conclusion

Zostavax vaccination does not alter the abundance of cutaneous resident memory T cells but instead increases the recruitment of VZV-specific T cells from the blood and enhances T-cell activation, particularly cells of the CD8⁺ subset, in the skin after VZV antigen challenge.

Telomeres, Telomerase and Ageing

Telomeres are specialised structures at the end of linear chromosomes. They consist of tandem repeats of the hexanucleotide sequence TTAGGG, as well as a protein complex called shelterin. Together, they form a protective loop structure against chromosome fusion and degradation. Shortening or damage to telomeres and opening of the loop induce an uncapped state that triggers a DNA damage response resulting in senescence or apoptosis.Average telomere length, usually measured in human blood lymphocytes, was thought to be a biomarker for ageing, survival and mortality. However, it becomes obvious that regulation of telomere length is very complex and involves multiple processes. For example, the "end replication problem" during DNA replication as well as oxidative stress are responsible for the shortening of telomeres. In contrast, telomerase activity can potentially counteract telomere shortening when it is able to access and interact with telomeres. However, while highly active during development and in cancer cells, the enzyme is down-regulated in most human somatic cells with a few exceptions such as human lymphocytes. In addition, telomeres can be transcribed, and the transcription products called TERRA are involved in telomere length regulation.Thus, telomere length and their integrity are regulated at many different levels, and we only start to understand this process under conditions of increased oxidative stress, inflammation and during diseases as well as the ageing process.This chapter aims to describe our current state of knowledge on telomeres and telomerase and their regulation in order to better understand their role for the ageing process.

Skin resident memory CD8+ T cells are phenotypically and functionally distinct from circulating populations and lack immediate cytotoxic function

The in-depth understanding of skin resident memory CD8+ T lymphocytes (TRM ) may help to uncover strategies for their manipulation during disease. We investigated isolated TRM from healthy human skin, which expressed the residence marker CD69, and compared them to circulating CD8+ T cell populations from the same donors. There were significantly increased proportions of CD8+ CD45RA- CD27- T cells in the skin that expressed low levels of killer cell lectin-like receptor G1 (KLRG1), CD57, perforin and granzyme B. The CD8+ TRM in skin were therefore phenotypically distinct from circulating CD8+ CD45RA- CD27- T cells that expressed high levels of all these molecules. Nevertheless, the activation of CD8+ TRM with T cell receptor (TCR)/CD28 or interleukin (IL)-2 or IL-15 in vitro induced the expression of granzyme B. Blocking signalling through the inhibitory receptor programmed cell death 1 (PD)-1 further boosted granzyme B expression. A unique feature of some CD8+ TRM cells was their ability to secrete high levels of tumour necrosis factor (TNF)-α and IL-2, a cytokine combination that was not seen frequently in circulating CD8+ T cells. The cutaneous CD8+ TRM are therefore diverse, and appear to be phenotypically and functionally distinct from circulating cells. Indeed, the surface receptors used to distinguish differentiation stages of blood T cells cannot be applied to T cells in the skin. Furthermore, the function of cutaneous TRM appears to be stringently controlled by environmental signals in situ.

A Suction Blister Protocol to Study Human T-cell Recall Responses In Vivo

Cutaneous antigen-recall models allow for studies of human memory responses in vivo. When combined with skin suction blister (SB) induction, this model offers accessibility to rare populations of antigen-specific T-cells representative of the cellular memory response as well as the cytokine microenvironment in situ.

This report describes the practical procedure of a cutaneous recall, an SB induction, and a harvest of antigen-specific T-cells. To exemplify the method, the tuberculin skin test is used for antigenic recall in individuals who, prior to this study, underwent a Bacillus Calmette-Guérin vaccination against an infection with Mycobacterium tuberculosis. Finally, examples of multiplex and flow cytometric analyses of SB specimens are provided, illustrating high fractions of antigen-specific polyfunctional CD4+ T-cells available by this sampling method compared with cells isolated from the blood.

The method described here is safe and minimally invasive, provides a unique opportunity to study both innate and adaptive immune responses in vivo, and may be beneficial to a broad community of researchers working with cell-mediated immunity and human memory responses, in the context of vaccine development.

ESRD-associated immune phenotype depends on dialysis modality and iron status: clinical implications

BACKGROUND

End-stage renal disease (ESRD) causes premature ageing of the immune system. However, it is not known whether hemodialysis (HD) and peritoneal dialysis (PD) similarly affect the T cell system.

METHODS

The aim of our study was to analyse whether dialysis modality may mitigate ESRD-induced immune senescence. We explored a large population of patients (675 ESRD patients) and both confirmed and refined the results in a second cohort (84 patients).

RESULTS

HD patients exhibited higher inflammatory monocytes counts (44/mm3 (1-520) vs 36/mm3 (1-161); p = 0.005). Patients on HD also had higher frequency of CD8 T cells (24% (7-61) vs 22% (8-42); p = 0.003) and reduced CD4/CD8 ratio. Such results were confirmed in the second cohort. Moreover, both CD4 + CD57 + CD28- (3.25% (0-38.2) vs 1.05% (0-28.5); p = 0.068) and CD8 + CD57 + CD28- (38.5% (3.6-76.8) vs 26.1 (2.1-46.9); p = 0.039) T cells frequencies were increased in HD patients. Telomere length did not differ according to dialysis modality, but was inversely related to ferritin levels (r = - 0.33; p = 0.003). There was a trend towards higher telomerase activity in PD patients (11 ± 13 vs 6 ± 11; p = 0.053). Thymic function was not different in PD and HD patients. Patients on PD before transplantation had a higher risk of acute rejection after kidney transplantation (HR, 1.61; 95%CI, 1.02 to 2.56; p = 0.041).

CONCLUSIONS

More pronounced inflammation with hemodialysis may induce premature aging of the immune system. This observation correlates with a lower risk of acute kidney rejection in patients previously on HD. Clinical consequences in patients maintained on dialysis should be determined.

TRIAL REGISTRATION

Trial registration: NCT02843867, registered July 8, 2016.

The involvement of plasmacytoid cells in HIV infection and pathogenesis

Plasmacytoid dendritic cells (pDCs) are a unique dendritic cell subset that are specialized in type I interferon (IFN) production. pDCs are key players in the antiviral immune response and serve as bridge between innate and adaptive immunity. Although pDCs do not represent the main reservoir of the Human Immunodeficiency Virus (HIV), they are a crucial subset in HIV infection as they influence viral transmission, target cell infection and antigen presentation. pDCs act as inflammatory and immunosuppressive cells, thus contributing to HIV disease progression. This review provides a state of art analysis of the interactions between HIV and pDCs and their potential roles in HIV transmission, chronic immune activation and immunosuppression. A thorough understanding of the roles of pDCs in HIV infection will help to improve therapeutic strategies to fight HIV infection, and will further increase our knowledge on this important immune cell subset.

Influence of Inflammation in the Process of T Lymphocyte Differentiation: Proliferative, Metabolic, and Oxidative Changes

T lymphocytes, from their first encounter with their specific antigen as naïve cell until the last stages of their differentiation, in a replicative state of senescence, go through a series of phases. In several of these stages, T lymphocytes are subjected to exponential growth in successive encounters with the same antigen. This entire process occurs throughout the life of a human individual and, earlier, in patients with chronic infections/pathologies through inflammatory mediators, first acutely and later in a chronic form. This process plays a fundamental role in amplifying the activating signals on T lymphocytes and directing their clonal proliferation. The mechanisms that control cell growth are high levels of telomerase activity and maintenance of telomeric length that are far superior to other cell types, as well as metabolic adaptation and redox control. Large numbers of highly differentiated memory cells are accumulated in the immunological niches where they will contribute in a significant way to increase the levels of inflammatory mediators that will perpetuate the new state at the systemic level. These levels of inflammation greatly influence the process of T lymphocyte differentiation from naïve T lymphocyte, even before, until the arrival of exhaustion or cell death. The changes observed during lymphocyte differentiation are correlated with changes in cellular metabolism and these in turn are influenced by the inflammatory state of the environment where the cell is located. Reactive oxygen species (ROS) exert a dual action in the population of T lymphocytes. Exposure to high levels of ROS decreases the capacity of activation and T lymphocyte proliferation; however, intermediate levels of oxidation are necessary for the lymphocyte activation, differentiation, and effector functions. In conclusion, we can affirm that the inflammatory levels in the environment greatly influence the differentiation and activity of T lymphocyte populations. However, little is known about the mechanisms involved in these processes. The elucidation of these mechanisms would be of great help in the advance of improvements in pathologies with a large inflammatory base such as rheumatoid arthritis, intestinal inflammatory diseases, several infectious diseases and even, cancerous processes.

Enhancement of cutaneous immunity during aging by blocking p38 mitogen-activated protein (MAP) kinase-induced inflammation

Background

Immunity decreases with age, which leads to reactivation of varicella zoster virus (VZV). In human subjects age-associated immune changes are usually measured in blood leukocytes; however, this might not reflect alterations in tissue-specific immunity.

Objectives

We used a VZV antigen challenge system in the skin to investigate changes in tissue-specific mechanisms involved in the decreased response to this virus during aging.

Methods

We assessed cutaneous immunity based on the extent of erythema and induration after intradermal VZV antigen injection. We also performed immune histology and transcriptomic analyses on skin biopsy specimens taken from the challenge site in young (<40 years) and old (>65 years) subjects.

Results

Old human subjects exhibited decreased erythema and induration, CD4⁺ and CD8⁺ T-cell infiltration, and attenuated global gene activation at the site of cutaneous VZV antigen challenge compared with young subjects. This was associated with increased sterile inflammation in the skin in the same subjects related to p38 mitogen-activated protein kinase–related proinflammatory cytokine production (P < .0007). We inhibited systemic inflammation in old subjects by means of pretreatment with an oral small-molecule p38 mitogen-activated protein kinase inhibitor (Losmapimod; GlaxoSmithKline, Brentford, United Kingdom), which reduced both serum C-reactive protein levels and peripheral blood monocyte secretion of IL-6 and TNF-α. In contrast, cutaneous responses to VZV antigen challenge were increased significantly in the same subjects (P < .0003).

Conclusion

Excessive inflammation in the skin early after antigen challenge retards antigen-specific immunity. However, this can be reversed by inhibition of inflammatory cytokine production that can be used to promote vaccine efficacy and the treatment of infections and malignancy during aging.

Telomere Dynamics in Immune Senescence and Exhaustion Triggered by Chronic Viral Infection

The progressive loss of immunological memory during aging correlates with a reduced proliferative capacity and shortened telomeres of T cells. Growing evidence suggests that this phenotype is recapitulated during chronic viral infection. The antigenic volume imposed by persistent and latent viruses exposes the immune system to unique challenges that lead to host T-cell exhaustion, characterized by impaired T-cell functions. These dysfunctional memory T cells lack telomerase, the protein capable of extending and stabilizing chromosome ends, imposing constraints on telomere dynamics. A deleterious consequence of this excessive telomere shortening is the premature induction of replicative senescence of viral-specific CD8+ memory T cells. While senescent cells are unable to expand, they can survive for extended periods of time and are more resistant to apoptotic signals. This review takes a closer look at T-cell exhaustion in chronic viruses known to cause human disease: Epstein–Barr virus (EBV), Hepatitis B/C/D virus (HBV/HCV/HDV), human herpesvirus 8 (HHV-8), human immunodeficiency virus (HIV), human T-cell leukemia virus type I (HTLV-I), human papillomavirus (HPV), herpes simplex virus-1/2 (HSV-1/2), and Varicella–Zoster virus (VZV). Current literature linking T-cell exhaustion with critical telomere lengths and immune senescence are discussed. The concept that enduring antigen stimulation leads to T-cell exhaustion that favors telomere attrition and a cell fate marked by enhanced T-cell senescence appears to be a common endpoint to chronic viral infections.

Modulation of CD4+ T Helper Cell Memory Responses in the Human Skin

Immunological memory is defined as the capacity to mount faster and more effective immune responses against antigenic challenges that have been previously encountered by the host. CD4+ T helper (Th) cells play central roles in the establishment of immunological memory as they assist the functions of other leukocytes. Th cells express polarized cytokine profiles and distinct migratory and seeding capacities, but also retain a certain functional plasticity that allows them to modulate their proliferation, activity, and homing behaviour upon need. Thus, in healthy individuals, T cell immunomodulation fulfils the task of eliciting protective immune responses where they are needed. At times, however, Th plasticity can lead to collateral tissue damage and progression to autoimmune diseases or, conversely, incapacity to reject malignant tissues and clear chronic infections. Furthermore, common immune players and molecular pathways of diseases can lead to different outcomes in different individuals. A mechanistic understanding of those pathways is therefore crucial for developing precise and curative medical interventions. Here, I focus on the skin microenvironment and comprehensively describe some of the cellular and molecular determinants of CD4+ T cell memory responses in homeostatic and pathological conditions. In discussing the cellular network orchestrating cutaneous immunity, I comprehensively describe the bidirectional interaction of skin antigen-presenting cells and mononuclear phagocytes with Th17 lymphocytes, and examine how the outcome of this interaction is influenced by endogenous skin molecules, including sodium salts and neuropeptides.

Early life infection, but not breastfeeding, predicts adult blood telomere lengths in the Philippines

OBJECTIVES

Telomeres are repetitive DNA at chromosomes ends that shorten with age due to cellular replication and oxidative stress. As telomeres shorten, this can eventually place limits on cell replication and contribute to senescence. Infections are common during early development and activate cellular immune responses that involve clonal expansion and oxidative stress. As such, a high infectious disease burden might shorten blood telomere length (BTL) and accelerate the pace of immune senescence.

METHODS

To test this, BTL measured in young adults (21.7 ± 0.3 years old) from the Philippines (N = 1,759) were linked to prospectively collected early life data on infectious burden.

RESULTS

As predicted, increased early life diarrheal prevalence was associated with shorter adult BTL. The association was most marked for infections experienced from 6 to 12 months, which corresponds with weaning and maximal diarrheal burden. A standard deviation increase in infections at 6-12 m predicts a 45 bp decrease in BTL, equivalent to 3.3 years of adult telomeric aging in this population. Contrary to expectations, breastfeeding duration was not associated with BTL, nor did effects vary by sex.

CONCLUSIONS

These findings show that infancy diarrheal disease predicts a marker of cellular aging in adult immune cells. These findings suggest that early life infectious burden may influence late life health, or alternatively, that short TL in early life increases infectious disease susceptibility.

Senescence of T Lymphocytes: Implications for Enhancing Human Immunity

As humans live longer, a central concern is to find ways to maintain their health as they age. Immunity declines during ageing, as shown by the increased susceptibility to infection by both previously encountered and new pathogens and by the decreased efficacy of vaccination. It is therefore crucial to understand the mechanisms responsible for this decrease in immunity and to develop new strategies to enhance immune function in older humans. We discuss here how the induction of senescence alters leukocyte, and specifically T cell, function. An emerging concept is that senescence and nutrient sensing-signalling pathways within T cells converge to regulate functional responses, and the manipulation of these pathways may offer new ways to enhance immunity during ageing.

Characterisation of the clinical and activated T cell response to repeat delayed-type hypersensitivity skin challenges in human subjects, with KLH and PPD, as a potential model to test T cell-targeted therapies

OBJECTIVE

To characterise the delayed-type hypersensitivity (DTH) skin reaction to repeated challenges of keyhole limpet hemocyanin (KLH) and tuberculin purified protein derivative (PPD) in healthy volunteers, as a potential model to test T cell-targeted investigational agents.

SUBJECTS, TREATMENT AND METHODS

Forty-nine subjects received either KLH, PPD, or PBS repeat skin challenges, and clinical assessments including induration, erythema and Laser Doppler Imaging. Skin biopsies or suction blisters were taken after challenge to investigate the cellular infiltrate of the challenge site, the T cell activation status, as determined by LAG-3 expression, and, specifically for the blister, the concentrations of inflammatory cytokines. Point estimates, estimates of variation and corresponding 95% confidence intervals were constructed for each type of challenge and timepoint.

RESULTS

The DTH response could be measured at 48 and 120 h post-KLH and PPD challenge with induration, erythema and Laser Doppler Imaging, with 48 h post-challenge demonstrating the peak of the response. PPD was well tolerated in subjects after multiple challenges, however, a significant number of KLH-treated subjects demonstrated an injection site reaction 6-7 days following the SC injection. PPD demonstrated a boost effect on the second challenge as measured by increased induration, where as this was not noted consistently for KLH. Compared to unchallenged and PBS control-injected skin, increased T cell numbers were detected in the challenge site by both the skin suction blister and biopsy technique, at either time point following KLH or PPD challenge. Use of the T cell activation marker LAG-3 demonstrated the activated phenotype of these cells. In skin blisters, higher numbers of LAG-3+ T cells were detected at 48 h post-challenge, whereas in the biopsies, similar numbers of LAG-3+ cells were observed at both 48 and 120 h. Analysis of blister T cell subpopulations revealed some differences in phenotypes between the time points and between the CD4 and CD8 T cells. Blister cytokine analysis revealed a pro-inflammatory dominated signature in PPD-challenged skin.

CONCLUSIONS

In summary, our data support the use of a repeat KLH and PPD DTH challenge in clinical trials and that the clinical measures of induration and to a lesser extent erythema are appropriate to monitor the clinical DTH response. Both the blister and biopsy can be utilised to assess and quantify activated T cells and at the dose used, PPD was better tolerated than KLH and hence may be optimal for future studies.

Topical steroid therapy induces pro-tolerogenic changes in Langerhans cells in human skin

We have investigated the efficacy of conditioning skin Langerhans cells (LCs) with agents to promote tolerance and reduce inflammation, with the goal of improving the outcomes of antigen-specific immunotherapy. Topical treatments were assessed ex vivo, using excised human breast skin maintained in organ bath cultures, and in vivo in healthy volunteers by analysing skin biopsies and epidermal blister roof samples. Following topical treatment with a corticosteroid, tumour necrosis factor-α levels were reduced in skin biopsy studies and blister fluid samples. Blister fluid concentrations of monocyte chemoattractant protein-1, macrophage inflammatory proteins -1α and 1β and interferon-γ inducible protein-10 were also reduced, while preserving levels of interleukin-1α (IL-1α), IL-6, IL-8 and IL-10. Steroid pre-treatment of the skin reduced the ability of LCs to induce proliferation, while supernatants showed an increase in the IL-10/interferon-γ ratio. Phenotypic changes following topical steroid treatment were also observed, including reduced expression of CD83 and CD86 in blister-derived LCs, but preservation of the tolerogenic signalling molecules immunoglobulin-like transcript 3 and programmed death-1. Reduced expression of HLA-DR, CD80 and CD86 were also apparent in LCs derived from excised human skin. Topical therapy with a vitamin D analogue (calcipotriol) and steroid, calcipotriol alone or vitamin A elicited no significant changes in the parameters studied. These experiments suggest that pre-conditioning the skin with topical corticosteroid can modulate LCs by blunting their pro-inflammatory signals and potentially enhancing tolerance. We suggest that such modulation before antigen-specific immunotherapy might provide an inexpensive and safe adjunct to current approaches to treat autoimmune diseases.

Multifunctional cytomegalovirus (CMV)-specific CD8(+) T cells are not restricted by telomere-related senescence in young or old adults

Antigen-specific multifunctional T cells that secrete interferon-γ, interleukin-2 and tumour necrosis factor-α simultaneously after activation are important for the control of many infections. It is unclear if these CD8⁺ T cells are at an early or late stage of differentiation and whether telomere erosion restricts their replicative capacity. We developed a multi-parameter flow cytometric method for investigating the relationship between differentiation (CD45RA and CD27 surface phenotype), function (cytokine production) and replicative capacity (telomere length) in individual cytomegalovirus (CMV) antigen-specific CD8⁺ T cells. This involves surface and intracellular cell staining coupled to fluorescence in situ hybridization to detect telomeres (flow-FISH). The end-stage/senescent CD8⁺ CD45RA⁺ CD27⁻ T-cell subset increases significantly during ageing and this is exaggerated in CMV immune-responsive subjects. However, these end-stage cells do not have the shortest telomeres, implicating additional non-telomere-related mechanisms in inducing their senescence. The telomere lengths in total and CMV (NLV)-specific CD8⁺ T cells in all four subsets defined by CD45RA and CD27 expression were significantly shorter in old compared with young individuals in both a Caucasian and an Asian cohort. Following stimulation by anti-CD3 or NLV peptide, similar proportions of triple-cytokine-producing cells are found in CD8⁺ T cells at all stages of differentiation in both age groups. Furthermore, these multi-functional cells had intermediate telomere lengths compared with cells producing only one or two cytokines after activation. Therefore, global and CMV (NLV)-specific CD8⁺ T cells that secrete interferon-γ, interleukin-2 and tumour necrosis factor-α are at an intermediate stage of differentiation and are not restricted by excessive telomere erosion.

Quantitative T cell subsets profile in peripheral blood from patients with idiopathic inflammatory myopathies: tilting the balance towards proinflammatory and pro-apoptotic subsets

The role of T cells in idiopathic inflammatory myopathies (IIM) is not yet clear. Some alterations in certain subsets have been reported in inflamed muscle cells. However, a broad quantitative assessment of peripheral T cell subsets has not been evaluated. The aim of this study was to address the quantitative profile of potential pathogenic T cell subsets, namely follicular helper T cells (Tfh), T helper type 17 (Th17), CD28(null) and regulatory T cells (Tregs ) in peripheral blood from IIM patients. Thirty IIM patients and 30 age- and gender-matched healthy donors were included. Peripheral blood mononuclear cells were isolated. T cell subsets were evaluated by flow cytometry, as follows: Tfh (CD4(+) CXCR5(+) ) and its subsets Tfh1 (CXCR3(+) CCR6(-) ), Tfh2 (CXCR3(-) CCR6(-) ), Tfh17 (CXCR3(-) CCR6(+) ), Th17 (CD4(+) IL17A(+) ), CD28(null) (CD4(+) CD28(-) CD244(+) ) and Tregs (CD4(+) CD25(high) forkhead box protein 3 (FoxP3(+) ); CD8(+) CD25(high) FoxP3(+) ). Percentage, absolute numbers and mean fluorescence intensity were analysed. We found increased numbers of total Tfh cells (28 ± 8.16 versus 6.64 ± 1.29, P=0.031) in IIM patients when compared to healthy controls. Moreover, this increment was dependent upon Tfh2 and Tfh17 (Tfh2:9.49 ± 2.19 versus 1.66 ± 0.46, P=0.005; Tfh17 9.48 ± 2.83 versus 1.18 ± 0.21, P=0.014). Also, IIM patients showed higher numbers of Th17 cells (30.25 ± 6.49 versus 13.46 ± 2.95, P=0.031) as well as decreased number of Tregs (5.98 ± 1.61 versus 30.82 ± 8.38, P=0.009). We also found an expansion of CD28(null) cells (162.88 ± 32.29 versus 64 ± 17.35, P=0.015). Our data suggest that IIM patients are characterized by an expansion of peripheral proinflammatory T cells, such as Tfh and Th17, as well as pro-apoptotic CD28 null cells and a deficiency of suppressor populations of Tregs (CD4(+) and CD8(+) ).

Multiplexing seven miRNA-Based shRNAs to suppress HIV replication

Multiplexed miRNA-based shRNAs (shRNA-miRs) could have wide potential to simultaneously suppress multiple genes. Here, we describe a simple strategy to express a large number of shRNA-miRs using minimal flanking sequences from multiple endogenous miRNAs. We found that a sequence of 30 nucleotides flanking the miRNA duplex was sufficient for efficient processing of shRNA-miRs. We inserted multiple shRNAs in tandem, each containing minimal flanking sequence from a different miRNA. Deep sequencing of transfected cells showed accurate processing of individual shRNA-miRs and that their expression did not decrease with the distance from the promoter. Moreover, each shRNA was as functionally competent as its singly expressed counterpart. We used this system to express one shRNA-miR targeting CCR5 and six shRNA-miRs targeting the HIV-1 genome. The lentiviral construct was pseudotyped with HIV-1 envelope to allow transduction of both resting and activated primary CD4 T cells. Unlike one shRNA-miR, the seven shRNA-miR transduced T cells nearly abrogated HIV-1 infection in vitro. Additionally, when PBMCs from HIV-1 seropositive individuals were transduced and transplanted into NOD/SCID/IL-2R γc(-/-) mice (Hu-PBL model) efficient suppression of endogenous HIV-1 replication with restoration of CD4 T cell counts was observed. Thus, our multiplexed shRNA appears to provide a promising gene therapeutic approach for HIV-1 infection.

Nef, the shuttling molecular adaptor of HIV, influences the cytokine network

Several viruses manipulate host innate immune responses to avoid immune recognition and improve viral replication and spreading. The viral protein Nef of Human Immunodeficiency Virus is mainly involved in this "hijacking" activity and is a well established virulence factor. In the last few years there have been remarkable advances in outlining a defined framework of its functions. In particular Nef appears to be a shuttling molecular adaptor able to exert its effects both on infected and non infected bystander cell. In addition it is emerging fact that it has an important impact on the chemo-cytokine network. Nef protein represents an interesting new target to develop therapeutic drugs for treatment of seropositive patients. In this review we have tried to provide a unifying view of the multiple functions of this viral protein on the basis of recently available experimental data.

The kinase p38 activated by the metabolic regulator AMPK and scaffold TAB1 drives the senescence of human T cells

In T lymphocytes, p38 MAP kinase (MAPK) regulates pleiotropic functions and is activated by canonical MAPK signaling or the alternative T cell receptor (TCR) activation pathway. Here we show that senescent human T cells lack the canonical and alternative pathways of p38 activation, but spontaneously engage the metabolic master regulator AMPK to trigger p38 recruitment to the scaffold TAB1 causing p38 auto-phosphorylation. Signaling via this pathway inhibits telomerase activity, T cell proliferation and expression of key components of the TCR signalosome. Our findings identify an unrecognized mode of p38 activation in T cells driven by intracellular changes such as low-nutrient and DNA-damage signaling (‘intra-sensory’ pathway). The proliferative defect of senescent T cells is reversed by blocking AMPK-TAB1-dependent p38 activation.

Human memory T cells: generation, compartmentalization and homeostasis

Memory T cells constitute the most abundant lymphocyte population in the body for the majority of a person's lifetime; however, our understanding of memory T cell generation, function and maintenance mainly derives from mouse studies, which cannot recapitulate the exposure to multiple pathogens that occurs over many decades in humans. In this Review, we discuss studies focused on human memory T cells that reveal key properties of these cells, including subset heterogeneity and diverse tissue residence in multiple mucosal and lymphoid tissue sites. We also review how the function and the adaptability of human memory T cells depend on spatial and temporal compartmentalization.

Dendritic cells in progression and pathology of HIV infection

Although the major targets of HIV infection are CD4⁺ T cells, dendritic cells (DCs) represent a crucial subset in HIV infection because they influence viral transmission and target cell infection and presentation of HIV antigens. DCs are potent antigen-presenting cells that can modulate antiviral immune responses. Through secretion of inflammatory cytokines and interferons (IFNs), DCs also alter T cell proliferation and differentiation, participating in the immune dysregulation characteristic of chronic HIV infection. Their wide distribution in close proximity with the mucosal epithelia makes them one of the first cell types to encounter HIV during sexual transmission. We discuss here the multiple roles that DCs play at different stages of HIV infection, emphasizing their relevance to HIV pathology and progression.

IFN-α inhibits telomerase in human CD8⁺ T cells by both hTERT downregulation and induction of p38 MAPK signaling

The cytokine IFN-α is secreted during viral infections and has been shown to inhibit telomerase activity and accelerate T cell differentiation in vivo. However, the mechanism for this inhibition is not clear. In this study, we show that IFN-α inhibits both the transcription and translation of human telomerase reverse transcriptase (hTERT), the catalytic component of telomerase, in activated CD8(+) T cells. This was associated with increased activity of the repressor of hTERT transcription E2 transcription factor and decreased activation of NF-κB that promotes hTERT transcription. However IFN-α did not affect the translocation of hTERT from the cytoplasm to the nucleus. IFN-α also inhibits AKT kinase activation but increases p38 MAPK activity, and both of these events have been shown previously to inhibit telomerase activity. Addition of BIRB796, an inhibitor of p38 activity, to IFN-α-treated cells reversed, in part, the inhibition of telomerase by this cytokine. Therefore, IFN-α can inhibit the enzyme telomerase in CD8(+) T cells by transcriptional and posttranslational mechanisms. Furthermore, the addition of IFN-α to CD8(+)CD27(+)CD28(+) T cells accelerates the loss of both these costimulatory molecules. This suggests that persistent viral infections may contribute to the accumulation of highly differentiated/senescent CD8(+)CD27(-)CD28(-) T cells during aging by promoting IFN-α secretion during repeated episodes of viral reactivation.

Telomere length dynamics in human memory T cells specific for viruses causing acute or latent infections

Background

Declining telomere length (TL) is associated with T cell senescence. While TL in naïve and memory T cells declines with increasing age, there is limited data on TL dynamics in virus-specific memory CD4⁺ T cells in healthy adults. We combined BrdU-labeling of virus-stimulated T cells followed with flow cytometry-fluorescent in situ hybridization for TL determination. We analyzed TL in T cells specific for several virus infections: non-recurring acute (vaccinia virus, VACV), recurring-acute (influenza A virus, IAV), and reactivating viruses (varicella-zoster virus, VZV, and cytomegalovirus, CMV) in 10 healthy subjects. Additionally, five subjects provided multiple blood samples separated by up to 10 years.

Results

VACV- and CMV-specific T cells had longer average TL than IAV-specific CD4⁺ T cells. Although most virus-specific cells were CD45RA^-, we observed a minor population of BrdU+ CD45RA⁺ T cells characterized by long telomeres. Longitudinal analysis demonstrated a slow decline in average TL in virus-specific T cells. However, in one subject, VZV reactivation led to an increase in average TL in VZV-specific memory T cells, suggesting a conversion of longer TL cells from the naïve T cell repertoire.

Conclusions

TLs in memory CD4⁺ T cells in otherwise healthy adults are heterogeneous and follow distinct virus-specific kinetics. These findings suggests that the distribution of TL and the creation and maintenance of long TL memory T cells could be important for the persistence of long-lived T cell memory.

Investigation of the cutaneous response to recall antigen in humans in vivo

In this paper we provide a detailed description of an experimental method for investigating the induction and resolution of recall immune response to antigen in humans in vivo. This involves the injection of tuberculin purified protein derivative (PPD) into the skin, followed by inducing suction blisters at the site of injection, from which leucocytes and cytokines that are involved in the response can be isolated and characterized. Using this technique we found that although the majority of CD4(+) T cells in the skin that are present early in the response express cutaneous lymphocyte antigen (CLA), the expression of this marker is reduced significantly in later phases. This may enable these cells to leave the skin during immune resolution. Furthermore, interleukin (IL)-2 production can be detected both in CD4(+) T cells and also in the blister fluid at the peak of the response at day 7, indicating that mediators found in the blister fluid are representative of the cytokine microenvironment in vivo. Finally, we found that older humans have defective ability to respond to cutaneous PPD challenge, but this does not reflect a global immune deficit as they have similar numbers of circulating functional PPD-specific CD4(+) T cells as young subjects. The use of the blister technology enables further characterization of the skin specific defect in older humans and also general mechanisms that govern immune regulation in vivo.

Molecular mechanisms involved in the aging of the T-cell immune response

T-lymphocytes play a central role in the effector and regulatory mechanisms of the adaptive immune response. Upon exiting the thymus they begin to undergo a series of phenotypic and functional changes that continue throughout the lifetime and being most pronounced in the elderly. The reason postulated for this is that the dynamic processes of repeated interaction with cognate antigens lead to multiple division cycles involving a high degree of cell differentiation, senescence, restriction of the T-cell receptor (TCR) repertoire, and cell cycle arrest. This cell cycle arrest is associated with the loss of telomere sequences from the ends of chromosomes. Telomere length is reduced at each cell cycle, and critically short telomeres recruit components of the DNA repair machinery and trigger replicative senescence or apoptosis. Repetitively stimulated T-cells become refractory to telomerase induction, suffer telomere erosion and enter replicative senescence. The latter is characterized by the accumulation of highly differentiated T-cells with new acquired functional capabilities, which can be caused by aberrant expression of genes normally suppressed by epigenetic mechanisms in CD4+ or CD8+ T-cells. Age-dependent demethylation and overexpression of genes normally suppressed by DNA methylation have been demonstrated in senescent subsets of T-lymphocytes. Thus, T-cells, principally CD4+CD28^null T-cells, aberrantly express genes, including those of the KIR gene family and cytotoxic proteins such as perforin, and overexpress CD70, IFN-γ, LFA-1 and others. In summary, owing to a lifetime of exposure to and proliferation against a variety of pathogens, highly differentiated T-cells suffer molecular modifications that alter their cellular homeostasis mechanisms.

The janus head of T cell aging - autoimmunity and immunodeficiency

Immune aging is best known for its immune defects that increase susceptibility to infections and reduce adaptive immune responses to vaccination. In parallel, the aged immune system is prone to autoimmune responses and many autoimmune diseases increase in incidence with age or are even preferentially encountered in the elderly. Why an immune system that suboptimally responds to exogenous antigen fails to maintain tolerance to self-antigens appears to be perplexing. In this review, we will discuss age-associated deviations in the immune repertoire and the regulation of signaling pathways that may shed light on this conundrum.

The role of the T cell in age-related inflammation

Ageing is accompanied by alterations to T-cell immunity and also by a low-grade chronic inflammatory state termed inflammaging. The significance of these phenomena is highlighted by their being predictors of earlier mortality. We have recently published that the proinflammatory cytokine TNFα is a strong inducer of CD4(+) T-cell senescence and T-cell differentiation, adding to the growing body of literature implicating proinflammatory molecules in mediating these critical age-related T-cell alterations. Moreover, the inflammatory process is also being increasingly implicated in the pathogenesis of many common and severe age-related diseases, including cancer, cardiovascular diseases and type 2 diabetes. Furthermore, major age-related risk factors for poor health, such as obesity, stress and smoking, are also associated with an upregulation in systemic inflammatory markers. We propose the idea that the ensuing inflammatory response to influenza infection propagates cardiovascular diseases and constitutes a major cause of influenza-related mortality. While inflammation is not a negative phenomenon per se, this age-related dysregulation of inflammatory responses may play crucial roles driving age-related pathologies, T-cell immunosenescence and CMV reactivation, thereby underpinning key features of the ageing process.

Varicella zoster-specific CD4+Foxp3+ T cells accumulate after cutaneous antigen challenge in humans

We investigated the relationship between varicella zoster virus (VZV)-specific memory CD4(+) T cells and CD4(+)Foxp3(+) regulatory T cells (Tregs) that accumulate after intradermal challenge with a VZV skin test Ag. VZV-specific CD4(+) T cells were identified with a MHC class II tetramer or by intracellular staining for either IFN-γ or IL-2 after Ag rechallenge in vitro. VZV-specific T cells, mainly of a central memory (CD45RA(-)CD27(+)) phenotype, accumulate at the site of skin challenge compared with the blood of the same individuals. This resulted in part from local proliferation because >50% of tetramer defined Ag-specific CD4(+) T cells in the skin expressed the cell cycle marker Ki67. CD4(+)Foxp3(+) T cells had the characteristic phenotype of Tregs, namely CD25(hi)CD127(lo)CD39(hi) in both unchallenged and VZV challenged skin and did not secrete IFN-γ or IL-2 after antigenic restimulation. The CD4(+)Foxp3(+) T cells from unchallenged skin had suppressive activity, because their removal led to an increase in cytokine secretion after activation. After VZV Ag injection, Foxp3(+)CD25(hi)CD127(lo)CD39(hi) T cells were also found within the VZV tetramer population. Their suppressive activity could not be directly assessed by CD25 depletion because activated T cells in the skin were also CD25(+). Nevertheless, there was an inverse correlation between decreased VZV skin responses and proportion of CD4(+)Foxp3(+) T cells present, indicating indirectly their inhibitory activity in vivo. These results suggest a linkage between the expansion of Ag-specific CD4(+) T cells and CD4(+) Tregs that may provide controlled responsiveness during Ag-specific stimulation in tissues.

Differing HLA types influence inhibitory receptor signalling in CMV-specific CD8+ T cells

The dysregulated immune response to CMV constitutes a major force driving T cell immunosenescence and growing evidence suggests that it is not a benign virus in old age. We show here that the PD-1/L pathway defines a reversible defect in CMV specific CD8(+) T cell proliferative responses in both young and old individuals. More specifically, highly differentiated CD45RA(+)CD27(-) CMV-specific CD8(+) T cells exhibit a proliferative deficit compared their central and effector memory counterparts, which is reversed following PD-L blockade. However, we also report that HLA-B(∗)07/TPR specific CD8(+) T cells express higher levels of PD-1 than HLA-A(∗)02/NLV specific cells and HLA-A(∗)02 individuals show a higher proliferative response to PD-L blockade, than HLA-B(∗)07 individuals, which we postulate may be due to the differing functional avidities for these two CMV-specific CD8(+) T cells populations. Nevertheless data presented here demonstrate that CMV-specific CD8(+) T cells can be functionally enhanced by perturbation of the PD-1/L signalling pathway, whose manipulation may provide a therapeutic modality to combat age-associated immune decline.

Analysis of stem-cell-like properties of human CD161++IL-18Rα+ memory CD8+ T cells

CD161(++)IL-18Rα(+)CD8(+) human T cells have recently been identified as a new subset of memory cells but their exact role remains unclear. CD161(++)IL-18Rα(+)CD8(+), mucosal-associated invariant T cells express a semi-invariant TCR Vα7.2-Jα33, which recognizes the MHC-related protein 1. On the basis of properties including the expression of the ABC-B1 transporter, cKit expression and survival after chemotherapy, CD161(++)IL-18Rα(+)CD8(+) T cells have been designated as 'stem' cells. Here we analyse location and functional properties of CD161(++)IL-18Rα(+) CD8(+) T cells and question whether they have other traits that would mark them as genuine 'stem' cells. CD161(++)IL-18Rα(+)CD8(+) T cells were found in peripheral blood, spleen and bone marrow but interestingly hardly at all in lymph nodes (LNs), which may possibly be explained by the finding that these cells express a specific set of chemokine receptors that allows migration to inflamed tissue rather than to LNs. In addition to TCR ligation and co-stimulation, CD161(++)IL-18Rα(+) CD8(+) T cells require cytokines for proliferation. The CD161(++)IL-18Rα(+) CD8(+) pool contains cells reactive towards peptides, derived from both persisting and cleared viruses. Although CD161(++)IL-18Rα(+) CD8(+) T cells express the ABC-B1 transporter, they have shorter telomeres and less telomerase activity and do not express aldehyde dehydrogenase. Finally, CD161(++)IL-18Rα(+) CD8(+) T cells show similarities to terminally differentiated T cells, expressing IFNγ, KLRG1 and the transcription factor Blimp-1. In conclusion, CD161(++)IL-18Rα(+) CD8(+) T cells lack many features of typical 'stem' cells, but appear rather to be a subset of effector-type cells.

A human challenge model for Mycobacterium tuberculosis using Mycobacterium bovis bacille Calmette-Guerin

(See the editorial commentary by Dockrell, on pages 1029–31.)

Background. There is currently no safe human challenge model of Mycobacterium tuberculosis infection to enable proof-of-concept efficacy evaluation of candidate vaccines against tuberculosis. In vivo antimycobacterial immunity could be assessed using intradermal Mycobacterium bovis bacille Calmette-Guérin (BCG) vaccination as a surrogate for M. tuberculosis infection.

Methods. Healthy BCG-naive and BCG-vaccinated volunteers were challenged with intradermal BCG. BCG load was quantified from skin biopsy specimens by polymerase chain reaction (PCR) and culture colony-forming units. Cellular infiltrate was isolated by suction blisters and examined by flow cytometry. Prechallenge immune readouts were correlated with BCG load after challenge.

Results. In BCG-naive volunteers, live BCG was detected at the challenge site for up to 4 weeks and peaked at 2 weeks. Infiltration of mainly CD15⁺ neutrophils was observed in blister fluid. In previously BCG-vaccinated individuals, PCR analysis of skin biopsy specimens reflected a degree of mycobacterial immunity. There was no significant correlation between BCG load after challenge and mycobacterial-specific memory T cells measured before challenge by cultured enzyme-linked immunospot assay.

Conclusions. This novel experimental human challenge model provides a platform for the identification of correlates of antimycobacterial immunity and will greatly facilitate the rational down-selection of candidate tuberculosis vaccines. Further evaluation of this model with BCG and new vaccine candidates is warranted.

Plasmacytoid dendritic cells in HIV infection

Plasmacytoid dendritic cells (pDCs) are innate immune cells that are specialized to produce interferon-alpha (IFNα) and participate in activating adaptive immune responses. Although IFNα inhibits HIV-1 (HIV) replication in vitro, pDCs may act as inflammatory and immunosuppressive dendritic cells (DCs) rather than classical antigen-presenting cells during chronic HIV infection in vivo, contributing more to HIV pathogenesis than to protection. Improved understanding of HIV-pDC interactions may yield potential new avenues of discovery to prevent HIV transmission, to blunt chronic immune activation and exhaustion, and to enhance beneficial adaptive immune responses. In this chapter we discuss pDC biology, including pDC development from progenitors, trafficking and localization of pDCs in the body, and signaling pathways involved in pDC activation. We focus on the role of pDCs in HIV transmission, chronic disease progression and immune activation, and immunosuppression through regulatory T cell development. Lastly, we discuss potential future directions for the field which are needed to strengthen our current understanding of the role of pDCs in HIV transmission and pathogenesis.

BCG vaccination induces different cytokine profiles following infant BCG vaccination in the UK and Malawi

Background. BCG vaccination of infants is thought to provide good protection in all settings. This study investigated whether Malawian infants made weaker responses across a cytokine panel after BCG vaccination, compared with UK infants.

Methods. Diluted whole-blood samples were cultured with Mycobacterium tuberculosis purified protein derivative for 6 days from BCG-vaccinated infants 3 months (n = 40 Malawi, 28 UK) and 12 months (n = 34 Malawi, 26 UK) after vaccination, and also from UK unvaccinated infants (n = 9 at 3 months, n = 10 at 12 months). Forty-two cytokines were measured in supernatants using a multiplex bead array assay. Principal component analysis was used to summarize the overall patterns in cytokine responses.

Results. We found differences in median responses in 27 of the 42 cytokines: 7 higher in the UK and 20 higher in Malawi. The cytokines with higher responses in the UK were all T helper 1 related. The cytokines with higher responses in Malawi included innate proinflammatory cytokines, regulatory cytokines, interleukin 17, T helper 2 cytokines, chemokines, and growth factors. Principal component analysis separated the BCG-vaccinated infants from Malawi from the UK vaccinated infants and from the unvaccinated infants.

Conclusions. Malawian infants make cytokine responses following BCG vaccination, but the cytokine profile is different from that in the UK. The different biosignatures following BCG vaccination in the 2 settings may indicate variability in the protective efficacy of infant BCG vaccination.

Transcriptional profiling of innate and adaptive human immune responses to mycobacteria in the tuberculin skin test

The tuberculin skin test (TST) is a model of integrated innate and adaptive human immune responses to Mycobacterium tuberculosis, but the component processes that are involved in this model have not previously been defined in vivo. We used transcriptional profiling to study these responses within the TST at molecular and system levels. Skin biopsies from TST injection sites were examined in subjects classified as TST⁺ or TST⁻ by clinical and histological criteria. Genome-wide expression arrays showed evolution of immune responses reflecting T-cell activation and recruitment with uniquely Th1-polarized responses and cytotoxic T cells (CTLs). In addition, distinct innate immune and IFN-γ-stimulated gene expression signatures were identified, under the regulation of NF-κB and STAT1 transcriptional control. These were highly enriched for chemokines and MHC class II molecules providing a potential mechanism for paracrine amplification of inflammatory responses in the TST, by supporting cellular recruitment and enhancing antigen presentation. The same repertoire of innate and adaptive immune responses was evident in TST⁺ and TST⁻ subjects alike, clinically positive TSTs being distinguished only by quantitatively greater differences. These data provide new insights into complex multifaceted responses within the TST, with much greater sensitivity than previous clinical or histological assessments.