The impact of telomere erosion on memory CD8+ T cells in patients with X-linked lymphoproliferative syndrome

Multiple outcomes of the germline p16INK4a mutation affecting senescence and immunity in human skin

The integrated behaviour of multiple senescent cell types within a single human tissue leading to the development of malignancy is unclear. Patients with Familial Melanoma Syndrome (FMS) have heterozygous germline defects in the CDKN2A gene coding for the cyclin inhibitor p16INK4a. Melanocytes within skin biopsies from FMS patients express significantly less p16INK4a but express higher levels of the DNA-damage protein 𝛾H2AX a than fibroblastic cells. However, patient fibroblasts also exhibit defects since senescent cells do not increase in the skin during ageing and fibroblasts isolated from the skin of patients have increased replicative capacity compared to control fibroblasts in vitro, culminating in abnormal nuclear morphology. Patient derived fibroblasts also secreted less SASP than control cells. Predisposition of FMS patients to melanoma may therefore result from integrated dysregulation of senescence in multiple cell types in vivo. The inherently greater levels of DNA damage and the overdependence of melanocytes on p16 for cell cycle inhibition after DNA damage makes them exquisitely susceptible to malignant transformation. This may be accentuated by senescence-related defects in fibroblasts, in particular reduced SASP secretion that hinders recruitment of T cells in the steady state and thus reduces cutaneous immunosurveillance in vivo.

CD28null T cells in aging and diseases: From biology to assessment and intervention

CD28null T cells, an atypical subset characterized by the loss of CD28 costimulatory molecule expression, exhibit functional variants and progressively expand with age. Moreover, T cells with these phenotypes are found in both typical and atypical humoral immune responses. Consequently, they accumulate during infectious diseases, autoimmune disorders, cardiovascular conditions, and neurodegenerative ailments. To provide an in-depth review of the current knowledge regarding CD28null T cells, we specifically focus on their phenotypic and functional characteristics as well as their physiological roles in aging and diseases. While uncertainties regarding the clinical utility remains, we will review the following two crucial research perspectives to explore clinical translational applications of the research on this specific T cell subset: 1) addressing the potential utility of CD28null T cells as immunological markers for prognosis and adverse outcomes in both aging and disease, and 2) speculating on the potential of targeting CD28null T cells as an interventional strategy for preventing or delaying immune aging processes and disease progression.

Implications of Senescent T Cells for Cancer Immunotherapy

T-cell senescence is thought to result from the age-related loss of the ability to mount effective responses to pathogens and tumor cells. In addition to aging, T-cell senescence is caused by repeated antigenic stimulation and chronic inflammation. Moreover, we demonstrated that T-cell senescence was induced by treatment with DNA-damaging chemotherapeutic agents. The characteristics of therapy-induced senescent T (TIS-T) cells and general senescent T cells are largely similar. Senescent T cells demonstrate an increase in the senescence-associated beta-galactosidase-positive population, cell cycle arrest, secretion of senescence-associated secretory phenotypic factors, and metabolic reprogramming. Furthermore, senescent T cells downregulate the expression of the co-stimulatory molecules CD27 and CD28 and upregulate natural killer cell-related molecules. Moreover, TIS-T cells showed increased PD-1 expression. However, the loss of proliferative capacity and decreased expression of co-stimulatory molecules associated with T-cell senescence cause a decrease in T-cell immunocompetence. In this review, we discuss the characteristics of senescent T-cells, including therapy-induced senescent T cells.

Sub-lethal doses of chemotherapeutic agents induce senescence in T cells and upregulation of PD-1 expression

Cellular senescence is a stable cell cycle arrest, usually in response to internal and/or external stress, including telomere dysfunction, abnormal cellular growth, and DNA damage. Several chemotherapeutic drugs, such as melphalan (MEL) and doxorubicin (DXR), induce cellular senescence in cancer cells. However, it is not clear whether these drugs induce senescence in immune cells. We evaluated the induction of cellular senescence in T cells were derived from human peripheral blood mononuclear cells (PBMNCs) in healthy donors using sub-lethal doses of chemotherapeutic agents. The PBMNCs were kept overnight in RPMI 1640 medium with 2% phytohemagglutinin and 10% fetal bovine serum and then cultured in RPMI 1640 with 20 ng/mL IL-2 and sub-lethal doses of chemotherapeutic drugs (2 μM MEL and 50 nM DXR) for 48 h. Sub-lethal doses of chemotherapeutic agents induced phenotypes associated with senescence, such as the formation of γH2AX nuclear foci, cell proliferation arrest, and induction of senescence-associated beta-galactosidase (SA-β-Gal) activity, (control vs. MEL, DXR; median mean fluorescence intensity (MFI) 1883 (1130-2163) vs. 2233 (1385-2254), 2406.5 (1377-3119), respectively) in T cells. IL6 and SPP1 mRNA, which are senescence-associated secretory phenotype (SASP) factors, were significantly upregulated by sublethal doses of MEL and DXR compared to the control (P = 0.043 and 0.018, respectively). Moreover, sub-lethal doses of chemotherapeutic agents significantly enhanced the expression of programmed death 1 (PD-1) on CD3 + CD4 + and CD3 + CD8 + T cells compared to the control (CD4 + T cells; P = 0.043, 0.043, and 0.043, respectively, CD8 + T cells; P = 0.043, 0.043, and 0.043, respectively). Our results suggest that sub-lethal doses of chemotherapeutic agents induce senescence in T cells and tumor immunosuppression by upregulating PD-1 expression on T cells.

T cell memory revisited using single telomere length analysis

The fundamental basis of T cell memory remains elusive. It is established that antigen stimulation drives clonal proliferation and differentiation, but the relationship between cellular phenotype, replicative history, and longevity, which is likely essential for durable memory, has proven difficult to elucidate. To address these issues, we used conventional markers of differentiation to identify and isolate various subsets of CD8+ memory T cells and measured telomere lengths in these phenotypically defined populations using the most sensitive technique developed to date, namely single telomere length analysis (STELA). Naive cells were excluded on the basis of dual expression of CCR7 and CD45RA. Memory subsets were sorted as CD27+CD45RA+, CD27intCD45RA+, CD27-CD45RA+, CD27+CD45RAint, CD27-CD45RAint, CD27+CD45RA-, and CD27-CD45RA- at >98% purity. The shortest median telomere lengths were detected among subsets that lacked expression of CD45RA, and the longest median telomere lengths were detected among subsets that expressed CD45RA. Longer median telomere lengths were also a feature of subsets that expressed CD27 in compartments defined by the absence or presence of CD45RA. Collectively, these data suggested a disconnect between replicative history and CD8+ memory T cell differentiation, which is classically thought to be a linear process that culminates with revertant expression of CD45RA.

Senescent T cells: Beneficial and detrimental roles

As the thymus involutes during aging, the T-cell pool has to be maintained by the periodic expansion of preexisting T cells during adulthood. A conundrum is that repeated episodes of activation and proliferation drive the differentiation of T cells toward replicative senescence, due to telomere erosion. This review discusses mechanisms that regulate the end-stage differentiation (senescence) of T cells. Although these cells, within both CD4 and CD8 compartments, lose proliferative activity after antigen-specific challenge, they acquire innate-like immune function. While this may confer broad immune protection during aging, these senescent T cells may also cause immunopathology, especially in the context of excessive inflammation in tissue microenvironments.

Idiopathic Pulmonary Fibrosis and Telomeres

Idiopathic pulmonary fibrosis is an interstitial lung disease of unknown etiology with a highly compromised prognosis and a significant mortality rate within a few years of diagnosis. Despite being idiopathic, it has been shown that telomeric shortening could play an important role in its etiopathogenesis. Mutations in telomere-related genes have been identified, but they are not always present despite telomere shortening. On the other hand, this telomeric shortening has been linked to a worse prognosis of the disease independently of other clinical factors, implying it may serve as a biomarker.

Cellular Senescence in Immunity against Infections

Cellular senescence is characterized by irreversible cell cycle arrest in response to different triggers and an inflammatory secretome. Although originally described in fibroblasts and cell types of solid organs, cellular senescence affects most tissues with advancing age, including the lymphoid tissue, causing chronic inflammation and dysregulation of both innate and adaptive immune functions. Besides its normal occurrence, persistent microbial challenge or pathogenic microorganisms might also accelerate the activation of cellular aging, inducing the premature senescence of immune cells. Therapeutic strategies counteracting the detrimental effects of cellular senescence are being developed. Their application to target immune cells might have the potential to improve immune dysfunctions during aging and reduce the age-dependent susceptibility to infections. In this review, we discuss how immune senescence influences the host's ability to resolve more common infections in the elderly and detail the different markers proposed to identify such senescent cells; the mechanisms by which infectious agents increase the extent of immune senescence are also reviewed. Finally, available senescence therapeutics are discussed in the context of their effects on immunity and against infections.

Tumour burden and efficacy of immune-checkpoint inhibitors

Accumulating evidence suggests that a high tumour burden has a negative effect on anticancer immunity. The concept of tumour burden, simply defined as the total amount of cancer in the body, in contrast to molecular tumour burden, is often poorly understood by the wider medical community; nonetheless, a possible role exists in defining the optimal treatment strategy for many patients. Historically, tumour burden has been assessed using imaging. In particular, CT scans have been used to evaluate both the number and size of metastases as well as the number of organs involved. These methods are now often complemented by metabolic tumour burden, measured using the more recently developed 2-deoxy-2-[¹⁸F]-fluoro-D-glucose (FDG)-PET/CT. Serum-based biomarkers, such as lactate dehydrogenase, can also reflect tumour burden and are often also correlated with a poor response to immune-checkpoint inhibitors. Other circulating markers (such as circulating free tumour DNA and/or circulating tumour cells) are also attracting research interest as surrogate markers of tumour burden. In this Review, we summarize evidence supporting the utility of tumour burden as a biomarker to guide the use of immune-checkpoint inhibitors. We also describe data and provide perspective on the various tools used for tumour burden assessment, with a particular emphasis on future therapeutic strategies that might address the issue of inferior outcomes among patients with cancer with a high tumour burden.

hTERT Transduction Extends the Lifespan of Primary Pediatric Low-Grade Glioma Cells While Preserving the Biological Response to NGF

The neurotrophin nerve growth factor (NGF) modulates the growth of human gliomas and is able to induce cell differentiation through the engagement of tropomyosin receptor kinase A (TrkA) receptor, although the role played in controlling glioma survival has proved controversial. Unfortunately, the slow growth rate of low-grade gliomas (LGG) has made it difficult to investigate NGF effects on these tumors in preclinical models. In fact, patient-derived low-grade human astrocytoma cells duplicate only a limited number of times in culture before undergoing senescence. Nevertheless, replicative senescence can be counteracted by overexpression of hTERT, the catalytic subunit of telomerase, which potentially increases the proliferative potential of human cells without inducing cancer-associated changes. We have extended, by hTERT transduction, the proliferative in vitro potential of a human LGG cell line derived from a pediatric pilocytic astrocytoma (PA) surgical sample. Remarkably, the hTERT-transduced LGG cells showed a behavior similar to that of the parental line in terms of biological responses to NGF treatment, including molecular events associated with induction of NGF-related differentiation. Therefore, transduction of LGG cells with hTERT can provide a valid approach to increase the in vitro life-span of patient-derived astrocytoma primary cultures, characterized by a finite proliferative potential.

T cell markers recount the course of immunosenescence in healthy individuals and chronic kidney disease

Aging results in substantial changes in almost all cellular subpopulations within the immune system, including functional and phenotypic alterations. T lymphocytes, as the main representative population of cellular immunity, have been extensively studied in terms of modifications and adjustments during aging. Phenotypic alterations are attributed to three main mechanisms; a reduction of naïve T cell population with a shift to more differentiated forms, a subsequent oligoclonal expansion of naïve T cells characterized by repertoire restriction, and replicative insufficiency after repetitive activation. These changes and the subsequent phenotypic disorders are comprised in the term "immunosenescence". Similar changes seem to occur in chronic kidney disease, with T cells of young patients resembling those of healthy older individuals. A broad range of surface markers can be utilized to identify immunosenescent T cells. In this review, we will discuss the most important senescence markers and their potential connection with impaired renal function.

FOXO1 constrains activation and regulates senescence in CD8 T cells

Naive and memory T cells are maintained in a quiescent state, yet capable of rapid response and differentiation to antigen challenge via molecular mechanisms that are not fully understood. In naive cells, the deletion of Foxo1 following thymic development results in the increased expression of multiple AP-1 family members, rendering T cells less able to respond to antigenic challenge. Similarly, in the absence of FOXO1, post-infection memory T cells exhibit the characteristics of extended activation and senescence. Age-based analysis of human peripheral T cells reveals that levels of FOXO1 and its downstream target, TCF7, are inversely related to host age, whereas the opposite is found for AP-1 factors. These characteristics of aging also correlate with the formation of T cells manifesting features of cellular senescence. Our work illustrates a role for FOXO1 in the active maintenance of stem-like properties in T cells at the timescales of acute infection and organismal life span.

Senescence in the Development and Response to Cancer with Immunotherapy: A Double-Edged Sword

Cellular senescence was first described as a physiological tumor cell suppressor mechanism that leads to cell growth arrest with production of the senescence-associated secretory phenotype known as SASP. The main role of SASP in physiological conditions is to attract immune cells to clear senescent cells avoiding tumor development. However, senescence can be damage-associated and, depending on the nature of these stimuli, additional types of senescence have been described. In the context of cancer, damage-associated senescence has been described as a consequence of chemotherapy treatments that were initially thought of as a tumor suppressor mechanism. However, in certain contexts, senescence after chemotherapy can promote cancer progression, especially when immune cells become senescent and cannot clear senescent tumor cells. Moreover, aging itself leads to continuous inflammaging and immunosenescence which are responsible for rewiring immune cells to become defective in their functionality. Here, we define different types of senescence, pathways that activate them, and functions of SASP in these events. Additionally, we describe the role of senescence in cancer and its treatments, including how aging and chemotherapy contribute to senescence in tumor cells, before focusing on immune cell senescence and its role in cancer. Finally, we discuss potential therapeutic interventions to reverse cell senescence.

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.

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

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

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.

Contact-independent suppressive activity of regulatory T cells is associated with telomerase inhibition, telomere shortening and target lymphocyte apoptosis

Regulatory T cells (Tregs) play a fundamental role in the maintenance of immunological tolerance by suppressing effector target T, B and NK lymphocytes. Contact-dependent suppression mechanisms have been well-studied, though contact-independent Treg activity is not fully understood. In the present study, we showed that human native Tregs, as well as induced ex vivo Tregs, can cause in vitro telomere-dependent senescence in target T, B and NK cells in a contact-independent manner. The co-cultivation of target cells with Tregs separated through porous membranes induced alternative splicing of the telomerase catalytic subunit hTERT (human Telomerase Reverse Transcriptase), which suppressed telomerase activity. Induction of the hTERT splicing variant was associated with increased expression of the apoptotic endonuclease EndoG, a splicing regulator. Inhibited telomerase in target cells co-cultivated with Tregs for a long period of time led to a decrease in their telomere lengths, cell cycle arrest, conversion of the target cells to replicative senescence and apoptotic death. Induced Tregs showed the ability to up-regulate EndoG expression, TERT alternative splicing and telomerase inhibition in mouse T, B and NK cells after in vivo administration. The results of the present study describe a novel mechanism of contact-independent Treg cell suppression that induces telomerase inhibition through the EndoG-provoked alternative splicing of hTERT and converts cells to senescence and apoptosis phenotypes.

Murine regulatory T cells induce death of effector T, B, and NK lymphocytes through a contact-independent mechanism involving telomerase suppression and telomere-associated senescence

Regulatory T cells (Tregs) suppress the activity of effector T, B and NK lymphocytes and sustain immunological tolerance, but the proliferative activity of suppressed cells remains unexplored. In the present study, we report that mouse Tregs can induce replicative senescence and the death of responder mouse CD4⁺CD25^- T cells, CD8⁺ T cells, B cells and NK cells in vitro and in vivo. Contact-independent in vitro co-cultivation with Tregs up-regulated endonuclease G (EndoG) expression and its translocation to the nucleus in responder cells. EndoG localization in the nucleus induced alternative mRNA splicing of the telomerase catalytic subunit Tert and telomerase inhibition. The lack of telomerase activity in proliferating cells led to telomere loss followed by the development of senescence and cell death. Injection of Tregs into mice resulted in EndoG-associated alternative splicing of Tert, telomerase inhibition, telomere loss, senescence development and increased cell death in vivo. The present study describes a novel contact-independent mechanism by which Tregs specify effector cell fate and provides new insights into cellular crosstalk related to immune suppression.

T-cell Dysfunction in Glioblastoma: Applying a New Framework

A functional, replete T-cell repertoire is an integral component to adequate immune surveillance and to the initiation and maintenance of productive antitumor immune responses. Glioblastoma (GBM), however, is particularly adept at sabotaging antitumor immunity, eliciting severe T-cell dysfunction that is both qualitative and quantitative. Understanding and countering such dysfunction are among the keys to harnessing the otherwise stark potential of anticancer immune-based therapies. Although T-cell dysfunction in GBM has been long described, newer immunologic frameworks now exist for reclassifying T-cell deficits in a manner that better permits their study and reversal. Herein, we divide and discuss the various T-cell deficits elicited by GBM within the context of the five relevant categories: senescence, tolerance, anergy, exhaustion, and ignorance. Categorization is appropriately made according to the molecular bases of dysfunction. Likewise, we review the mechanisms by which GBM elicits each mode of T-cell dysfunction and discuss the emerging immunotherapeutic strategies designed to overcome them. Clin Cancer Res; 24(16); 3792-802. ©2018 AACR.

Beneficial and Detrimental Effects of Antiretroviral Therapy on HIV-Associated Immunosenescence

Since the introduction of highly active antiretroviral therapy more than 2 decades ago, HIV-related deaths have dramatically decreased and HIV infection has become a chronic disease. Due to the inability of antiretroviral drugs to eradicate the virus, treatment of HIV infection requires a systemic lifelong therapy. However, even when successfully treated, HIV patients still show increased incidence of age-associated co-morbidities compared with uninfected individuals. Virus- induced immunosenescence, a process characterized by a progressive decline of immune system function, contributes to the premature ageing observed in HIV patients. Although antiretroviral therapy has significantly improved both the quality and length of patient lives, the life expectancy of treated patients is still shorter compared with that of uninfected individuals. In particular, while antiretroviral therapy can contrast some features of HIV-associated immunosenescence, several anti-HIV agents may themselves contribute to other aspects of immune ageing. Moreover, older HIV patients tend to have a worse immunological response to the antiviral therapy. In this review we will examine the available evidence on the role of antiretroviral therapy in the control of the main features regulating immunosenescence.

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.

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.

Dedicator of cytokinesis 8-deficient CD4+ T cells are biased to a TH2 effector fate at the expense of TH1 and TH17 cells

BACKGROUND

Dedicator of cytokinesis 8 (DOCK8) deficiency is a combined immunodeficiency caused by autosomal recessive loss-of-function mutations in DOCK8. This disorder is characterized by recurrent cutaneous infections, increased serum IgE levels, and severe atopic disease, including food-induced anaphylaxis. However, the contribution of defects in CD4+ T cells to disease pathogenesis in these patients has not been thoroughly investigated.

OBJECTIVE

We sought to investigate the phenotype and function of DOCK8-deficient CD4+ T cells to determine (1) intrinsic and extrinsic CD4+ T-cell defects and (2) how defects account for the clinical features of DOCK8 deficiency.

METHODS

We performed in-depth analysis of the CD4+ T-cell compartment of DOCK8-deficient patients. We enumerated subsets of CD4+ T helper cells and assessed cytokine production and transcription factor expression. Finally, we determined the levels of IgE specific for staple foods and house dust mite allergens in DOCK8-deficient patients and healthy control subjects.

RESULTS

DOCK8-deficient memory CD4+ T cells were biased toward a TH2 type, and this was at the expense of TH1 and TH17 cells. In vitro polarization of DOCK8-deficient naive CD4+ T cells revealed the TH2 bias and TH17 defect to be T-cell intrinsic. Examination of allergen-specific IgE revealed plasma IgE from DOCK8-deficient patients is directed against staple food antigens but not house dust mites.

CONCLUSION

Investigations into the DOCK8-deficient CD4+ T cells provided an explanation for some of the clinical features of this disorder: the TH2 bias is likely to contribute to atopic disease, whereas defects in TH1 and TH17 cells compromise antiviral and antifungal immunity, respectively, explaining the infectious susceptibility of DOCK8-deficient patients.

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.

Polyfunctional Melan-A-specific tumor-reactive CD8(+) T cells elicited by dacarbazine treatment before peptide-vaccination depends on AKT activation sustained by ICOS

The identification of activation pathways linked to antitumor T-cell polyfunctionality in long surviving patients is of great relevance in the new era of immunotherapy. We have recently reported that dacarbazine (DTIC) injected one day before peptide-vaccination plus IFN-α improves the antitumor lytic activity and enlarges the repertoire of Melan-A-specific T-cell clones, as compared with vaccination alone, impacting the overall survival of melanoma patients. To identify the mechanisms responsible for this improvement of the immune response, we have analyzed the endogenous and treatment-induced antigen (Ag)-specific response in a panel of Melan-A-specific CD8⁺ T-cell clones in terms of differentiation phenotype, inhibitory receptor profile, polyfunctionality and AKT activation. Here, we show that Melan-A-specific CD8⁺ T cells isolated from patients treated with chemoimmunotherapy possess a late differentiated phenotype as defined by the absence of CD28 and CD27 co-stimulatory molecules and high levels of LAG-3, TIM-3 and PD-1 inhibitory receptors. Nevertheless, they show higher proliferative potential and an improved antitumor polyfunctional effector profile in terms of co-production of TNF-α, IFNγ and Granzyme-B (GrB) compared with cells derived from patients treated with vaccination alone. Polyfunctionality is dependent on an active AKT signaling related to the engagement of the co-stimulatory molecule ICOS. We suggest that this phenotypic and functional signature is dictated by a fine-tuned balance between TCR triggering, AKT activation, co-stimulatory and inhibitory signals induced by chemoimmunotherapy and may be associated with antitumor T cells able to protect patients from tumor recurrence.

Telomere length in interstitial lung diseases

BACKGROUND

Interstitial lung disease (ILD) is a heterogeneous group of rare diseases that primarily affect the pulmonary interstitium. Studies have implicated a role for telomere length (TL) maintenance in ILD, particularly in idiopathic interstitial pneumonia (IIP). Here, we measure TL in a wide spectrum of sporadic and familial cohorts of ILD and compare TL between patient cohorts and control subjects.

METHODS

A multiplex quantitative polymerase chain reaction method was used to measure TL in 173 healthy subjects and 359 patients with various ILDs, including familial interstitial pneumonia (FIP). The FIP cohort was divided into patients carrying TERT mutations, patients carrying SFTPA2 or SFTPC mutations, and patients without a proven mutation (FIP-no mutation).

RESULTS

TL in all cases of ILD was significantly shorter compared with those of control subjects (P range: .038 to < .0001). Furthermore, TL in patients with idiopathic pulmonary fibrosis (IPF) was significantly shorter than in patients with other IIPs (P = .002) and in patients with sarcoidosis (P < .0001). Within the FIP cohort, patients in the FIP-telomerase reverse transcriptase (TERT) group had the shortest telomeres (P < .0001), and those in the FIP-no mutation group had TL comparable to that of patients with IPF (P = .049). Remarkably, TL of patients with FIP-surfactant protein (SFTP) was significantly longer than in patients with IPF, but similar to that observed in patients with other sporadic IIPs.

CONCLUSIONS

The results show telomere shortening across all ILD diagnoses. The difference in TL between the FIP-TERT and FIP-SFTP groups indicates the distinction between acquired and innate telomere shortening. Short TL in the IPF and FIP-no mutation groups is indicative of an innate telomere-biology defect, while a stress-induced, acquired telomere shortening might be the underlying process for the other ILD diagnoses.

Ageing and the telomere connection: An intimate relationship with inflammation

Telomeres are the heterochromatic repeat regions at the ends of eukaryotic chromosomes, whose length is considered to be a determinant of biological ageing. Normal ageing itself is associated with telomere shortening. Here, critically short telomeres trigger senescence and eventually cell death. This shortening rate may be further increased by inflammation and oxidative stress and thus affect the ageing process. Apart from shortened or dysfunctional telomeres, cells undergoing senescence are also associated with hyperactivity of the transcription factor NF-κB and overexpression of inflammatory cytokines such as TNF-α, IL-6, and IFN-γ in circulating macrophages. Interestingly, telomerase, a reverse transcriptase that elongates telomeres, is involved in modulating NF-κB activity. Furthermore, inflammation and oxidative stress are implicated as pre-disease mechanisms for chronic diseases of ageing such as neurodegenerative diseases, cardiovascular disease, and cancer. To date, inflammation and telomere shortening have mostly been studied individually in terms of ageing and the associated disease phenotype. However, the interdependent nature of the two demands a more synergistic approach in understanding the ageing process itself and for developing new therapeutic approaches. In this review, we aim to summarize the intricate association between the various inflammatory molecules and telomeres that together contribute to the ageing process and related diseases.

Genetic and epigenetic trends in telomere research: a novel way in immunoepigenetics

Telomeres are protective heterochromatic structures that cap the end of linear chromosomes and play a key role in preserving genomic stability. Telomere length represents a balance between processes that shorten telomeres during cell divisions with incomplete DNA replication and the ones that lengthen telomeres by the action of telomerase, an RNA-protein complex with reverse transcriptase activity which adds telomeric repeats to DNA molecule ends. Telomerase activity and telomere length have a crucial role in cellular ageing and in the pathobiology of several human diseases attracting intense research. The last few decades have witnessed remarkable advances in our understanding about telomeres, telomere-associated proteins, and the biogenesis and regulation of the telomerase holoenzyme complex, as well as about telomerase activation and the telomere-independent functions of telomerase. Emerging data have revealed that telomere length can be modified by genetic and epigenetic factors, sex hormones, reactive oxygen species and inflammatory reactions. It has become clear that, in order to find out more about the factors influencing the rate of telomere attrition in vivo, it is crucial to explore both genetic and epigenetic mechanisms. Since the telomere/telomerase assembly is under the control of multiple epigenetic influences, the unique design of twin studies could help disentangle genetic and environmental factors in the functioning of the telomere/telomerase system. It is surprising that the literature on twin studies investigating this topic is rather scarce. This review aims to provide an overview of some important immune response- and epigenetics-related aspects of the telomere/telomerase system demanding more research, while presenting the available twin data published in connection with telomere research so far. By emphasising what we know and what we still do not know in these areas, another purpose of this review is to urge more twin studies in telomere research.

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.

Phenotypic and functional characterization of herpes simplex virus glycoprotein B epitope-specific effector and memory CD8+ T cells from symptomatic and asymptomatic individuals with ocular herpes

Herpes simplex virus 1 (HSV-1) glycoprotein B (gB)-specific CD8(+) T cells protect mice from herpes infection and disease. However, whether and which HSV-1 gB-specific CD8(+) T cells play a key role in the "natural" protection seen in HSV-1-seropositive healthy asymptomatic (ASYMP) individuals (who have never had clinical herpes disease) remain to be determined. In this study, we have dissected the phenotypes and the functions of HSV-1 gB-specific CD8(+) T cells from HLA-A*02:01 positive, HSV-1 seropositive ASYMP and symptomatic (SYMP) individuals (with a history of numerous episodes of recurrent ocular herpes disease). We found the following. (i) Healthy ASYMP individuals maintained a significantly higher proportion of differentiated HSV-1 gB-specific effector memory CD8(+) T cells (TEM cells) (CD45RA(low) CCR7(low) CD44(high) CD62L(low)). In contrast, SYMP patients had frequent less-differentiated central memory CD8(+) T cells (TCM cells) (CD45RA(low) CCR7(high) CD44(low) CD62L(high)). (ii) ASYMP individuals had significantly higher proportions of multifunctional effector CD8(+) T cells which responded mainly to gB342-350 and gB561-569 "ASYMP" epitopes, and simultaneously produced IFN-γ, CD107(a/b), granzyme B, and perforin. In contrast, effector CD8(+) T cells from SYMP individuals were mostly monofunctional and were directed mainly against nonoverlapping gB17-25 and gB183-191 "SYMP" epitopes. (iii) Immunization of an HLA-A*02:01 transgenic mouse model of ocular herpes with "ASYMP" CD8(+) TEM cell epitopes, but not with "SYMP" CD8(+) TCM cell epitopes, induced a strong CD8(+) T cell-dependent protective immunity against ocular herpes infection and disease. Our findings provide insights into the role of HSV-specific CD8(+) TEM cells in protection against herpes and should be considered in the development of an effective vaccine.

IMPORTANCE

A significantly higher proportion of differentiated and multifunctional HSV-1 gB-specific effector memory CD8(+) T cells (TEM cells) (CD45RA(low) CCR7(low) CD44(high) CD62L(low)) were found in healthy ASYMP individuals who are seropositive for HSV-1 but never had any recurrent herpetic disease, while there were frequent less-differentiated and monofunctional central memory CD8(+) T cells (TCM cells) (CD45RA(low) CCR7(high) CD44(low) CD62L(high)) in SYMP patients. Immunization with "ASYMP" CD8(+) TEM cell epitopes, but not with "SYMP" CD8(+) TCM cell epitopes, induced a strong protective HSV-specific CD8(+) T cell response in HLA-A*02:01 transgenic mice. These findings are important for the development of a safe and effective T cell-based herpes vaccine.

High efficiency ex vivo cloning of antigen-specific human effector T cells

While cloned T cells are valuable tools for the exploration of immune responses against viruses and tumours, current cloning methods do not allow inferences to be made about the function and phenotype of a clone's in vivo precursor, nor can precise cloning efficiencies be calculated. Additionally, there is currently no general method for cloning antigen-specific effector T cells directly from peripheral blood mononuclear cells, without the need for prior expansion in vitro. Here we describe an efficient method for cloning effector T cells ex vivo. Functional T cells are detected using optimised interferon gamma capture following stimulation with viral or tumour cell-derived antigen. In combination with multiple phenotypic markers, single effector T cells are sorted using a flow cytometer directly into multi-well plates, and cloned using standard, non antigen-specific expansion methods. We provide examples of this novel technology to generate antigen-reactive clones from healthy donors using Epstein-Barr virus and cytomegalovirus as representative viral antigen sources, and from two melanoma patients using autologous melanoma cells. Cloning efficiency, clonality, and retention/loss of function are described. Ex vivo effector cell cloning provides a rapid and effective method of deriving antigen-specific T cells clones with traceable in vivo precursor function and phenotype.

The Many Unknowns Concerning the Bioenergetics of Exhaustion and Senescence during Chronic Viral Infection

The immune system cannot be continuously reactivated throughout the lifetime of an organism; there is a finite point at which repeated antigenic challenge leads to the loss of lymphocyte function or the cells themselves. Antigen-specific T cells can be compromised in two ways through the distinct processes of replicative senescence and exhaustion. Senescence is initiated by a DNA damage response whereas exhaustion triggers inhibitory receptors to dampen the immune response. These two distinct pathways not only differ in their initiation but also growing evidence suggests that their biogenergetics is also different. Here, we review recent findings uncovering the metabolism of these unique states.

p38 signaling inhibits mTORC1-independent autophagy in senescent human CD8⁺ T cells

T cell senescence is thought to contribute to immune function decline, but the pathways that mediate senescence in these cells are not clear. Here, we evaluated T cell populations from healthy volunteers and determined that human CD8+ effector memory T cells that reexpress the naive T cell marker CD45RA have many characteristics of cellular senescence, including decreased proliferation, defective mitochondrial function, and elevated levels of both ROS and p38 MAPK. Despite their apparent senescent state, we determined that these cells secreted high levels of both TNF-α and IFN-γ and showed potent cytotoxic activity. We found that the senescent CD45RA-expressing population engaged anaerobic glycolysis to generate energy for effector functions. Furthermore, inhibition of p38 MAPK signaling in senescent CD8+ T cells increased their proliferation, telomerase activity, mitochondrial biogenesis, and fitness; however, the extra energy required for these processes did not arise from increased glucose uptake or oxidative phosphorylation. Instead, p38 MAPK blockade in these senescent cells induced an increase in autophagy through enhanced interactions between p38 interacting protein (p38IP) and autophagy protein 9 (ATG9) in an mTOR-independent manner. Together, our findings describe fundamental metabolic requirements of senescent primary human CD8+ T cells and demonstrate that p38 MAPK blockade reverses senescence via an mTOR-independent pathway.

Stem cell function and maintenance - ends that matter: role of telomeres and telomerase

Stem cell research holds a promise to treat and prevent age-related degenerative changes in humans. Literature is replete with studies showing that stem cell function declines with aging, especially in highly proliferative tissues/ organs. Among others, telomerase and telomere damage is one of the intrinsic physical instigators that drive agerelated degenerative changes. In this review we provide brief overview of telomerase-deficient aging affects in diverse stem cells populations. Furthermore, potential disease phenotypes associated with telomerase dysregulation in a specific stem cell population is also discussed in this review. Additionally, the role of telomerase in stem cell driven cancer is also briefly touched upon.

Asymptomatic memory CD8+ T cells: from development and regulation to consideration for human vaccines and immunotherapeutics

Generation and maintenance of high quantity and quality memory CD8(+) T cells determine the level of protection from viral, bacterial, and parasitic re-infections, and hence constitutes a primary goal for T cell epitope-based human vaccines and immunotherapeutics. Phenotypically and functionally characterizing memory CD8(+) T cells that provide protection against herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2) infections, which cause blinding ocular herpes, genital herpes, and oro-facial herpes, is critical for better vaccine design. We have recently categorized 2 new major sub-populations of memory symptomatic and asymptomatic CD8(+) T cells based on their phenotype, protective vs. pathogenic function, and anatomical locations. In this report we are discussing a new direction in developing T cell-based human herpes vaccines and immunotherapeutics based on the emerging new concept of "symptomatic and asymptomatic memory CD8(+) T cells."

Stemness of T cells and the hematopoietic stem cells: fate, memory, niche, cytokines

Stem cells are able to generate both cells that differentiate and cells that remain undifferentiated but potentially have the same developmental program. The prolonged duration of the protective immune memory for infectious diseases such as polio, small pox, and measles, suggested that memory T cells may have stem cell properties. Understanding the molecular basis for the life-long persistence of memory T cells may be useful to project targeted therapies for immune deficiencies and infectious diseases and to formulate vaccines. In the last decade evidence from different laboratories shows that memory T cells may share self-renewal pathways with bone marrow hematopoietic stem cells. In stem cells the intrinsic self-renewal activity, which depends on gene expression, is known to be modulated by extrinsic signals from the environment that may be tissue specific. These extrinsic signals for stemness of memory T cells include cytokines such as IL-7 and IL-15 and there are other cytokine signals for maintaining the cytokine signature (TH1, TH2, etc.) of memory T cells. Intrinsic and extrinsic pathways that might be common to bone marrow hematopoietic stem cells and memory T lymphocytes are discussed and related to self-renewal functions.

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.

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.

Functional T cell immunodeficiencies (with T cells present)

Severe combined immunodeficiency (SCID) comprises a group of disorders that are fatal owing to genetic defects that abrogate T cell development. Numerous related defects have recently been identified that allow T cell development but that compromise T cell function by affecting proximal or distal steps in intracellular signaling. These functional T cell immunodeficiencies are characterized by immune dysregulation and increased risk of malignancies, in addition to infections. The study of patients with these rare conditions, and of corresponding animal models, illustrates the importance of intracellular signaling to maintain T cell homeostasis.

Properties of end-stage human T cells defined by CD45RA re-expression

Persistent viral infections, inflammatory syndromes and ageing all induce the accumulation of highly differentiated CD45RA re-expressing memory T cells. These cells increase during ageing, especially in individuals who are infected with cytomegalovirus (CMV). These cells have decreased proliferative capacity, increased activation of senescence signalling pathways and greater susceptibility to apoptosis in vitro. However these cells are capable of multiple effector functions and thus bear all the hallmarks of short-lived effector T cells. This indicates that senescence signalling may govern the unique characteristics of effector T cells. In this article, we address the functional and migratory properties of these T cells and mechanisms that are involved in their generation. Finally we assess the potential for manipulation of their activity and whether this may improve immune function during ageing.

Expansion of somatically reverted memory CD8+ T cells in patients with X-linked lymphoproliferative disease caused by selective pressure from Epstein-Barr virus

In patients with XLP, a primary immunodeficiency caused by mutations in SH2D1A, EBV infection can lead to somatic reversion of the disease-causing mutation selectively in effector memory CD8 T cells; reverted CD8 cells are better able to respond to and kill EBV-infected cells.

Patients with the primary immunodeficiency X-linked lymphoproliferative disease (XLP), which is caused by mutations in SH2D1A, are highly susceptible to Epstein-Barr virus (EBV) infection. Nonetheless, some XLP patients demonstrate less severe clinical manifestations after primary infection. SH2D1A encodes the adaptor molecule SLAM-associated protein (SAP), which is expressed in T and natural killer cells and is required for cytotoxicity against B cells, the reservoir for EBV. It is not known why the clinical presentation of XLP is so variable. In this study, we report for the first time the occurrence of somatic reversion in XLP. Reverted SAP-expressing cells resided exclusively within the CD8⁺ T cell subset, displayed a CD45RA⁻CCR7⁻ effector memory phenotype, and were maintained at a stable level over time. Importantly, revertant CD8⁺ SAP⁺ T cells, but not SAP⁻ cells, proliferated in response to EBV and killed EBV-infected B cells. As somatic reversion correlated with EBV infection, we propose that the virus exerts a selective pressure on the reverted cells, resulting in their expansion in vivo and host protection against ongoing infection.

Molecular pathogenesis of EBV susceptibility in XLP as revealed by analysis of female carriers with heterozygous expression of SAP

X-linked lymphoproliferative disease (XLP) is a primary immunodeficiency caused by mutations in SH2D1A which encodes SAP. SAP functions in signalling pathways elicited by the SLAM family of leukocyte receptors. A defining feature of XLP is exquisite sensitivity to infection with EBV, a B-lymphotropic virus, but not other viruses. Although previous studies have identified defects in lymphocytes from XLP patients, the unique role of SAP in controlling EBV infection remains unresolved. We describe a novel approach to this question using female XLP carriers who, due to random X-inactivation, contain both SAP(+) and SAP(-) cells. This represents the human equivalent of a mixed bone marrow chimera in mice. While memory CD8(+) T cells specific for CMV and influenza were distributed across SAP(+) and SAP(-) populations, EBV-specific cells were exclusively SAP(+). The preferential recruitment of SAP(+) cells by EBV reflected the tropism of EBV for B cells, and the requirement for SAP expression in CD8(+) T cells for them to respond to Ag-presentation by B cells, but not other cell types. The inability of SAP(-) clones to respond to Ag-presenting B cells was overcome by blocking the SLAM receptors NTB-A and 2B4, while ectopic expression of NTB-A on fibroblasts inhibited cytotoxicity of SAP(-) CD8(+) T cells, thereby demonstrating that SLAM receptors acquire inhibitory function in the absence of SAP. The innovative XLP carrier model allowed us to unravel the mechanisms underlying the unique susceptibility of XLP patients to EBV infection in the absence of a relevant animal model. We found that this reflected the nature of the Ag-presenting cell, rather than EBV itself. Our data also identified a pathological signalling pathway that could be targeted to treat patients with severe EBV infection. This system may allow the study of other human diseases where heterozygous gene expression from random X-chromosome inactivation can be exploited.

DOCK8 deficiency impairs CD8 T cell survival and function in humans and mice

In humans, DOCK8 immunodeficiency syndrome is characterized by severe cutaneous viral infections. Thus, CD8 T cell function may be compromised in the absence of DOCK8. In this study, by analyzing mutant mice and humans, we demonstrate a critical, intrinsic role for DOCK8 in peripheral CD8 T cell survival and function. DOCK8 mutation selectively diminished the abundance of circulating naive CD8 T cells in both species, and in DOCK8-deficient humans, most CD8 T cells displayed an exhausted CD45RA(+)CCR7(-) phenotype. Analyses in mice revealed the CD8 T cell abnormalities to be cell autonomous and primarily postthymic. DOCK8 mutant naive CD8 T cells had a shorter lifespan and, upon encounter with antigen on dendritic cells, exhibited poor LFA-1 synaptic polarization and a delay in the first cell division. Although DOCK8 mutant T cells underwent near-normal primary clonal expansion after primary infection with recombinant influenza virus in vivo, they showed greatly reduced memory cell persistence and recall. These findings highlight a key role for DOCK8 in the survival and function of human and mouse CD8 T cells.

Are senescence and exhaustion intertwined or unrelated processes that compromise immunity?

Can the immune system be reactivated continuously throughout the lifetime of an organism or is there a finite point at which repeated antigenic challenge leads to the loss of lymphocyte function or the cells themselves or both? Replicative senescence and exhaustion are processes that control T cell proliferative activity and function; however, there is considerable confusion over the relationship between these two intrinsic cellular control mechanisms. In this Opinion article, we compare the molecular regulation of senescence and exhaustion in T cells. Available data suggest that both processes are regulated independently of each other and that it may be safer to block exhaustion than senescence to enhance immunity.

Aging and immune function: molecular mechanisms to interventions

Abstract The immune system of an organism is an essential component of the defense mechanism aimed at combating pathogenic stress. Age-associated immune dysfunction, also dubbed "immune senescence," manifests as increased susceptibility to infections, increased onset and progression of autoimmune diseases, and onset of neoplasia. Over the years, extensive research has generated consensus in terms of the phenotypic and functional defects within the immune system in various organisms, including humans. Indeed, age-associated alterations such as thymic involution, T cell repertoire skewing, decreased ability to activate naïve T cells and to generate robust memory responses, have been shown to have a causative role in immune decline. Further, understanding the molecular mechanisms underlying the generation of proteotoxic stress, DNA damage response, modulation of ubiquitin proteasome pathway, and regulation of transcription factor NFκB activation, in immune decline, have paved the way to delineating signaling pathways that cross-talk and impact immune senescence. Given the role of the immune system in combating infections, its effectiveness with age may well be a marker of health and a predictor of longevity. It is therefore believed that a better understanding of the mechanisms underlying immune senescence will lead to an effective interventional strategy aimed at improving the health span of individuals. Antioxid. Redox Signal. 14, 1551-1585.

Impaired Epstein-Barr virus–specific CD8+ T-cell function in X-linked lymphoproliferative disease is restricted to SLAM family–positive B-cell targets

X-linked lymphoproliferative disease (XLP) is a condition associated with mutations in the signaling lymphocytic activation molecule (SLAM)–associated protein (SAP; SH2D1A). SAP functions as an adaptor, binding to and recruiting signaling molecules to SLAM family receptors expressed on T and natural killer cells. XLP is associated with extreme sensitivity to primary Epstein-Barr virus (EBV) infection, often leading to a lethal infectious mononucleosis. To investigate EBV-specific immunity in XLP patients, we studied 5 individuals who had survived EBV infection and found CD8+ T-cell responses numerically comparable with healthy donors. However, further investigation of in vitro–derived CD8+ T-cell clones established from 2 of these donors showed they efficiently recognized SLAM ligand–negative target cells expressing EBV antigens, but showed impaired recognition of EBV-transformed, SLAM ligand–positive, lymphoblastoid cell lines (LCLs). Importantly, LCL recognition was restored when interactions between the SLAM receptors CD244 and natural killer–, T-, and B-cell antigen (NTBA) and their ligands on LCLs were blocked. We propose that XLP patients' particular sensitivity to EBV, and not to other viruses, reflects at least in part EBV's strict tropism for B lymphocytes and the often inability of the CD8+ T-cell response to contain the primary infection of SLAM ligand–expressing target cells.

Serine protease inhibitors and cytotoxic T lymphocytes

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmoregulation, tissue remodeling, and immunity to infection. Cytotoxic T lymphocytes (CTLs) are required for adaptive cell-mediated immunity to intracellular pathogens by killing infected cells and through the development of memory T cells. Serine proteases not only allow a CTL to kill but also impose homeostatic control on CTL number. Serine protease inhibitors (serpins) are the physiological regulators of serine proteases' activity. In this review, I discuss the role of serpins in controlling the recognition of antigen, effector function, and homeostatic control of CTLs through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This view is discussed in the context of the T-lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. Given the important role serpins play in CTL immunity, I discuss the potential for developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.