Immunoageing - the cause or effect of morbidity

T cell aging and exhaustion: Mechanisms and clinical implications

T cell senescence and exhaustion represent critical aspects of adaptive immune system dysfunction, with profound implications for health and the development of disease prevention and therapeutic strategies. These processes, though distinct, are interconnected at the molecular level, leading to impaired effector functions and reduced proliferative capacity of T cells. Such impairments increase susceptibility to diseases and diminish the efficacy of vaccines and treatments. Importantly, T cell senescence and exhaustion can dynamically influence each other, particularly in the context of chronic diseases. A deeper understanding of the molecular mechanisms underlying T cell senescence and exhaustion, as well as their interplay, is essential for elucidating the pathogenesis of related diseases and restoring dysfunctional immune responses. This knowledge will pave the way for the development of targeted therapeutic interventions and strategies to enhance immune competence. This review aims to summarize the characteristics, mechanisms, and disease associations of T cell senescence and exhaustion, while also delineating the distinctions and intersections between these two states to enhance our comprehension.

Cellular Senescence in Hepatocellular Carcinoma: The Passenger or the Driver?

With the high morbidity and mortality, hepatocellular carcinoma (HCC) represents a major yet growing burden for our global community. The relapse-prone nature and drug resistance of HCC are regarded as the consequence of varying intracellular processes and extracellular interplay, which actively participate in tumor microenvironment remodeling. Amongst them, cellular senescence is regarded as a fail-safe program, leading to double-sword effects of both cell growth inhibition and tissue repair promotion. Particularly, cellular senescence serves a pivotal role in the progression of chronic inflammatory liver diseases, ultimately leading to carcinogenesis. Given the current challenges in improving the clinical management and outcome of HCC, senescence may exert striking potential in affecting anti-cancer strategies. In recent years, an increasing number of studies have emerged to investigate senescence-associated hepatocarcinogenesis and its derived therapies. In this review, we intend to provide an up-to-date understanding of liver cell senescence and its impacts on treatment modalities of HCC.

T-cell Immunoglobulin and ITIM Domain Contributes to CD8+ T-cell Immunosenescence

Aging is associated with immune dysfunction, especially T-cell defects, which result in increased susceptibility to various diseases. Previous studies showed that T cells from aged mice express multiple inhibitory receptors, providing evidence of the relationship between T-cell exhaustion and T-cell senescence. In this study, we showed that T-cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif (ITIM) domain (TIGIT), a novel co-inhibitory receptor, was upregulated in CD8+ T cells of elderly adults. Aged TIGIT+ CD8+ T cells expressed high levels of other inhibitory receptors including PD-1 and exhibited features of exhaustion such as downregulation of the key costimulatory receptor CD28, representative intrinsic transcriptional regulation, low production of cytokines, and high susceptibility to apoptosis. Importantly, their functional defects associated with aging were reversed by TIGIT knockdown. CD226 downregulation on aged TIGIT+ CD8+ T cells is likely involved in TIGIT-mediated negative immune suppression. Collectively, our findings indicated that TIGIT acts as a critical immune regulator during aging, providing a strong rationale for targeting TIGIT to improve dysfunction related to immune system aging.

Exhaustion of T lymphocytes in the tumor microenvironment: Significance and effective mechanisms

T lymphocytes play crucial roles in adaptive immune responses to tumors. However, due to different tolerance mechanisms and inhibitory effects of the tumor microenvironment (TME) on T cells, responses to tumors are insufficient. In fact, cellular and molecular suppressive mechanisms repress T cell responses in the TME, resulting in senescent, anergic and exhausted lymphocytes. Exhaustion is a poor responsive status of T cells, with up-regulated expression of inhibitory receptors, decreased production of effective cytokines, and reduced cytotoxic activity. Low immunogenicity of tumor antigens and inadequate presentation of tumor-specific antigens results in inappropriate activation of naive T lymphocytes against tumor antigens. Moreover, when effector cytotoxic T cells enter TME, they encounter a complicated network of cells and cytokines that suppress their effectiveness and turn them into exhausted T cells. Thus, the mechanism of T cell exhaustion in cancer is different from that in chronic infections. In this review we will discuss the main components such as inhibitory receptors, inflammatory cells, stromal cells, cytokine milieu as well as environmental and metabolic conditions in TME which play role in development of exhaustion. Furthermore, recent therapeutic methods available to overcome exhaustion will be discussed.

Differential Effect of Cytomegalovirus Infection with Age on the Expression of CD57, CD300a, and CD161 on T-Cell Subpopulations

Immunosenescence is a progressive deterioration of the immune system with aging. It affects both innate and adaptive immunity limiting the response to pathogens and to vaccines. As chronic cytomegalovirus (CMV) infection is probably one of the major driving forces of immunosenescence, and its persistent infection results in functional and phenotypic changes to the T-cell repertoire, the aim of this study was to analyze the effect of CMV-seropositivity and aging on the expression of CD300a and CD161 inhibitory receptors, along with the expression of CD57 marker on CD4⁺, CD8⁺, CD8⁺CD56⁺ (NKT-Like) and CD4⁻CD8⁻ (DN) T-cell subsets. Our results showed that, regardless of the T-cell subset, CD57⁻CD161⁻CD300a⁺ T-cells expand with age in CMV-seropositive individuals, whereas CD57⁻CD161⁺CD300a⁺ T-cells decrease. Similarly, CD57⁺CD161⁻CD300a⁺ T-cells expand with age in CMV-seropositive individuals in all subsets except in DN cells and CD57⁻CD161⁺CD300a⁻ T-cells decrease in all T-cell subsets except in CD4⁺ T-cells. Besides, in young individuals, CMV latent infection associates with the expansion of CD57⁺CD161⁻CD300a⁺CD4⁺, CD57⁻CD161⁻CD300a⁺CD4⁺, CD57⁺CD161⁻CD300a⁺CD8⁺, CD57⁻CD161⁻CD300a⁺CD8⁺, CD57⁺CD161⁻CD300a⁺NKT-like, and CD57⁺CD161⁻CD300a⁺DN T-cells. Moreover, in young individuals, CD161 expression on T-cells is not affected by CMV infection. Changes of CD161 expression were only associated with age in the context of CMV latent infection. Besides, CD300a⁺CD57⁺CD161⁺ and CD300a⁻CD57⁺CD161⁺ phenotypes were not found in any of the T-cell subsets studied except in the DN subpopulation, indicating that in the majority of T-cells, CD161 and CD57 do not co-express. Thus, our results show that CMV latent infection impact on the immune system depends on the age of the individual, highlighting the importance of including CMV serology in any study regarding immunosenescence.

Lifewide profile of cytokine production by innate and adaptive immune cells from Brazilian individuals

Background

Immunosenescence is associated with several changes in adaptive and innate immune cells. Altered cytokine production is among the most prominent of these changes. The impact of age-related alterations on cytokine global profiles produced by distinct populations of leukocytes from healthy Brazilian individuals was studied. We analysed frequencies of cytokine-producing lymphocytes and innate immune cells from individuals at several ages spanning a lifetime period (0–85 years).

Results

Healthy adult individuals presented a balanced profile suggestive of a mature immune system with equal contributions of both innate and adaptive immunity and of both categories of cytokines (inflammatory and regulatory). In healthy newborns and elderly, innate immune cells, especially neutrophils and NK-cells, contributed the most to a balanced profile of cytokines.

Conclusions

Our results support the hypothesis that ageing is not associated with a progressive pro-inflammatory cytokine production by all leukocytes but rather with distinct fluctuations in the frequency of cytokine-producing cells throughout life.

Electronic supplementary material

The online version of this article (doi:10.1186/s12979-017-0084-5) contains supplementary material, which is available to authorized users.

Cellular energy metabolism in T-lymphocytes

Energy homeostasis is a hallmark of cell survival and maintenance of cell function. Here we focus on the impact of cellular energy metabolism on T-lymphocyte differentiation, activation, and function in health and disease. We describe the role of transcriptional and posttranscriptional regulation of lymphocyte metabolism on immune functions of T cells. We also summarize the current knowledge about T-lymphocyte adaptations to inflammation and hypoxia, and the impact on T-cell behavior of pathophysiological hypoxia (as found in tumor tissue, chronically inflamed joints in rheumatoid arthritis and during bone regeneration). A better understanding of the underlying mechanisms that control immune cell metabolism and immune response may provide therapeutic opportunities to alter the immune response under conditions of either immunosuppression or inflammation, potentially targeting infections, vaccine response, tumor surveillance, autoimmunity, and inflammatory disorders.

T cell anergy, exhaustion, senescence, and stemness in the tumor microenvironment

Human tumors progress despite the presence of tumor associated antigen (TAA)-specific T cells. Many different molecular and cellular mechanisms contribute to the failure of T cells to eradicate the tumor. These include immune suppressive networks that impair ongoing T cell function and enable tumor escape. Recent studies have started to reveal the nature of effector T cells in the tumor microenvironment. In this article we discuss T cell anergy, exhaustion, senescence, and stemness, and review the phenotype of dysfunctional T cell subsets and the underlying molecular mechanisms in the tumor microenvironments. We suggest that targeting T cell dysfunctional mechanisms and introducing/promoting T cell stemness are important approaches to treat patients with cancer.

Aging and immune response in chronic human schistosomiasis

There has been no systematic study of the immune response of individuals over 60 residing in areas endemic for Schistosoma mansoni infection although senescence is reportedly associated with susceptibility to infection and progressive decline in immune function. We have shown previously, in two endemic areas in Minas Gerais, Brazil, that the frequency of individuals over 60 with chronic schistosomiasis is no longer negligible. Several aging-related immunological alterations are already described in medical literature, mostly in the T-cell compartment. Since aging is associated with a decline in T-cell function, it is not surprising that individuals over 60 would be more susceptible to infection. However, not all aged individuals in endemic areas have high intensity of infection; some of them display a negative stool-screening test for the presence of schistosome eggs and low levels of serum antibodies reactive with S. mansoni antigens indicating they are not infected. Non-infected, negative individuals may develop compensatory mechanisms to cope with immune dysfunction and to generate protective responses against the constant threat of infection in these areas. Herein, we reviewed previous reports from our group showing that two mechanisms contribute to distinguish between infected and egg negative aged individuals. First, egg negative aged individuals develop innate immune responses to replace the decline in T-cell function that is observed with aging. Second, chronically activated regulatory T cells, that may impair protective immune responses, are more vigorous in infected aged individuals. We propose that egg negative individuals may be considered as an example of healthy aging in areas endemic for infectious disease.

Decreased frequency and proliferative response of invariant Valpha24Vbeta11 natural killer T (iNKT) cells in healthy elderly

Invariant natural killer T (iNKT) cells represent a well-established T cell lineage characterised in humans by TCR consisting of an invariant alpha chain encoded by Valpha24-JalphaQ genes, paired preferentially with a Vbeta11 chain. iNKT cells also share some characteristics with NK cells, such as the expression of the NK-associated receptor CD161 in humans. The T cell immune response is the most dramatically affected by ageing, although age-associated alterations in the phenotype and function of other cells of the immune system have been demonstrated. Despite the importance of iNKT cells in the regulation of the immune response, there are a limited number of studies on the effect of ageing on peripheral blood iNKT cells. Thus, in this work we analyse the effect of ageing on peripheral blood Valpha24(+)Vbeta11(+) iNKT cells by studying their frequency, phenotype and proliferative function in elderly individuals fulfilling the SENIEUR criteria of healthy ageing compared with healthy young donors. Our results demonstrated a significant decrease of the percentage of Valpha24(+)Vbeta11(+) iNKT cells in elderly donors. No significant differences were found in the expression of CD27, CD28, CD45RO, CD45RA(bright), CD161, CD94 and NKG2D on iNKT cells from young and elderly individuals. Proliferation of Valpha24(+)Vbeta11(+) iNKT cells in response to alpha-GalCer and IL2 was analysed by calculating the cumulative population doubling (PD) after 14 days of culture. The PD levels were lower in the elderly indicating that Valpha24(+)Vbeta11(+) iNKT cells from healthy elderly subjects had an impaired proliferative capacity. These results indicate that ageing associates with a significant decline in the percentage and proliferative response of peripheral blood iNKT cells. Given the important immunoregulatory role of iNKT cells, these alterations in their number and function could contribute to the deleterious immune response in the elderly.

Ageing and Toll-like receptor expression by innate immune cells in chronic human schistosomiasis

There has been no systematic study of the immune response of individuals aged over 60 years living in Schistosomiasis mansoni-endemic areas, although senescence is reportedly associated with susceptibility to infection and progressive decline in immune function. We have shown previously, in two endemic areas in Minas Gerais, Brazil, that the frequency of individuals aged over 60 years with chronic schistosomiasis is no longer negligible. Moreover, several elderly individuals who have always lived in these endemic areas stay protected from infection. An important question for studies of ageing and disease control in developing countries is which differences in the immunological profile of these negatively tested (non-infected) individuals can account for their resistance to either infection or reinfection. We show, in the present study, that non-infected (negative) elderly individuals develop innate immune mechanisms of protection that replace the age-associated decline in T cell function. Non-infected elderly individuals from endemic areas of schistosome infection present an increase in the frequency of the natural killer (NK) CD56(low) subset of NK cells expressing Toll-like receptors (TLR)-1, -2, -3 and -4 as determined by flow cytometry analysis. In addition, the proportion of dendritic cells expressing TLR-1 is elevated as well as the frequency of monocytes expressing TLR-1 and -4. These results suggest that TLR expression by cells of the innate immune system may be related to the negative status of infection in some elderly individuals who are constantly exposed to S. mansoni. Developing mechanisms of protection from infection may represent a biomarker for healthy ageing in this population.

Longitudinal studies of clonally expanded CD8 T cells reveal a repertoire shrinkage predicting mortality and an increased number of dysfunctional cytomegalovirus-specific T cells in the very elderly

The age-associated decrease in functionality of the human immune system is thought to have a negative impact on the capacity to provide protection against infection, in turn leading to increased incidence of mortality. In a previous longitudinal study of octogenarians, we identified an immune risk phenotype (IRP) in the very elderly defined by an inverted CD4/CD8 ratio, which was associated with increased mortality and persistent CMV infection. In this study, we analyzed the CD8 clonal composition of nonagenarians and middle-aged individuals. An increased number of CD8 T cell clones was observed in the nonagenarians, and was associated with CMV-seropositivity. Surprisingly, CMV-seropositive nonagenarians with the IRP had a significantly lower number of clones compared with non-IRP individuals. The decrease in clone numbers in IRP individuals was associated with shorter survival time. MHC/peptide multimer staining indicated that the frequency of CMV-specific T cells was higher in nonagenarians than in the middle-aged, but the ratio of functionally intact cells was significantly lower. The lowest ratio of functional CMV-specific T cells was found in an IRP individual. A thorough longitudinal analysis of the CMV-specific T cells in nonagenarians showed a stable pattern with respect to frequency, phenotype, and clonal composition. We hypothesize that the number of different CD8 T cell clonal expansions increases as the individual ages, possibly, as a compensatory mechanism to control latent infections, e.g., CMV, but eventually a point is reached where clonal exhaustion leads to shrinkage of the CD8 clonal repertoire, associated with decreased survival.

Human cytomegalovirus infection and T cell immunosenescence: a mini review

The mammalian immune system defends the organism against pathogens, and possibly cancer, but is known to become dysregulated with increasing age. This results in greater morbidity and mortality due to infectious disease in old people. The most important changes occur in T cell immunity, manifested sometimes dramatically as altered clonal expansions of cells of limited antigen specificity and a marked shrinkage of the T cell antigen receptor repertoire. At the same time, it was independently reported that CMV seropositivity was associated with many of the same T cell changes that were being identified as biomarkers of immune ageing. It has now become clear that CMV is commonly the driving force behind the oligoclonal expansions and altered phenotypes and functions of CD8 cells seen in most old people. These changes are much less obvious in centenarians and most extreme in people whom longitudinal studies have shown to possess an "immune risk profile". This is a cluster of immunological parameters of which CMV seropositivity is one component and which predicts incipient mortality in an elderly population. Taken together, these findings suggest the hypothesis that persistence of CMV as a chronic antigenic stressor is a major contributor to immunosenescence and associated mortality.

Melatonin, immune function and aging

Aging is associated with a decline in immune function (immunosenescence), a situation known to correlate with increased incidence of cancer, infectious and degenerative diseases. Innate, cellular and humoral immunity all exhibit increased deterioration with age. A decrease in functional competence of individual natural killer (NK) cells is found with advancing age. Macrophages and granulocytes show functional decline in aging as evidenced by their diminished phagocytic activity and impairment of superoxide generation. There is also marked shift in cytokine profile as age advances, e.g., CD3+ and CD4+ cells decline in number whereas CD8+ cells increase in elderly individuals. A decline in organ specific antibodies occurs causing reduced humoral responsiveness. Circulating melatonin decreases with age and in recent years much interest has been focused on its immunomodulatory effect. Melatonin stimulates the production of progenitor cells for granulocytes-macrophages. It also stimulates the production of NK cells and CD4+ cells and inhibits CD8+ cells. The production and release of various cytokines from NK cells and T-helper lymphocytes also are enhanced by melatonin. Melatonin presumably regulates immune function by acting on the immune-opioid network, by affecting G protein-cAMP signal pathway and by regulating intracellular glutathione levels. Melatonin has the potential therapeutic value to enhance immune function in aged individuals and in patients in an immunocompromised state.

Human immunosenescence: is it infectious?

Morbidity and mortality due to infectious disease is greater in the elderly than in the young, at least partly because of age-associated decreased immune competence, which renders individuals more susceptible to pathogens. This susceptibility is particularly evident for novel infectious agents such as in severe acute respiratory syndrome but is also all too apparent for common pathogens such as influenza. Many years ago, it was noted that the elderly possessed oligoclonal expansions of T cells, especially of CD8(+) cells. At the same time, it was established that cytomegalovirus (CMV) seropositivity was associated with many of the same phenotypic and functional alterations to T-cell immunity that were being reported as biomarkers associated with aging. It was discovered that CMV was the prime driving force behind most of the oligoclonal expansions and altered phenotypes and functions of CD8 cells. Independently, longitudinal studies of a free-living population of the very old in Sweden over the past decade have led to the emerging concept of an 'immune risk phenotype' (IRP), predicting mortality, which was itself found to be associated with CMV seropositivity. These findings support our hypothesis that the manner in which CMV and the host immune system interact is critical in determining the IRP and hence is predictive of mortality. In this sense, then, we suggest that immunosenescence is contagious.

Production of interferon-gamma by natural killer cells and aging in chronic human schistosomiasis

BACKGROUND

Aging is associated with several alterations in the phenotype, repertoire and activation status of lymphocytes as well as in the cytokine profile produced by these cells. As a lifelong condition, chronic parasitic diseases such as human schistosomiasis overlaps with the aging process and no systematic study has yet addressed the changes in immune response during infection with Schistosoma mansoni in older individuals.

AIM

Herein we study the influence of immunological alterations brought about by senescence in the course of schistosomiasis.

MATERIALS AND METHODS

Individuals 10-95 years of age, from both sexes, from an endemic area for S. mansoni infection were matched by intensity of infection as measured by egg counts. We analyzed, as a parameter, cytokine expression by lymphocytes and natural killer cells after in vitro stimulation with soluble egg antigen and soluble worm antigen using flow cytometry.

RESULTS

We demonstrated that the frequency of CD16+ interferon-gamma (IFN-gamma)+ natural killer cells in negative individuals over the age of 70 years is significantly higher than in positive individuals after in vitro stimulation with S. mansoni antigen extracts. The frequency of these cells is increased in all individuals over the age of 50 years and only declines in positive individuals after 70 years of age. Analysis of either CD4? or CD8? cells after antigen stimulation show no significant increase in frequency of IFN-gamma in negative or in positive individuals of this age group, suggesting that the effect on CD16+ cells is not T-cell dependent.

CONCLUSION

Since production of IFN-gamma has been related to resistance to schistosome infection, our data suggest that age-associated changes in CD16+ cells may play a role in controlling infection intensity in the elderly in S. mansoni endemic areas of Brazil.

Determinants of HIV-specific CD8 T-cell responses in HIV-infected pediatric patients and enhancement of HIV-gag-specific responses with exogenous IL-15

Cellular immune responses play a central role in controlling HIV-1 infection. HIV-specific IFN-gamma production by CD8 T cells was evaluated in 17 HLA-A2+ HIV-infected pediatric patients (age range 1 month to 16 years) in an ELISPOT assay. Most patients (15/17) exhibited responses to HIV-gag, followed by responses to envelope gp120, gp41, and V3 loop. Only 7 patients responded to all four antigenic peptides. Treatment-related immune reconstitution of CD4 T cells was associated with increase in gag-specific responses, but these declined with prolonged viral suppression. Exogenous IL-15 resulted in augmentation of HIV-gag-specific response in 71% of patients, while IL-2 and IL-7 had variable effects, augmenting responses in 25% patients. Thus, HIV-specific CD8 T-cell responses are dependent on both CD4 T-cell help and antigenic stimulation. The cytokine IL-15 may be a useful modality as adjunctive therapy to augment HIV-specific memory CD8 T cells.

Aging is a deprivation syndrome driven by a germ-soma conflict

Evolution through natural selection can be described as driven by a perpetual conflict of individuals competing for limited resources. Recently, I postulated that the shortage of resources godfathered the evolutionary achievements of the differentiation-apoptosis programming [Rev. Neurosci. 12 (2001) 217]. Unicellular deprivation-induced differentiation into germ cell-like spores can be regarded as the archaic reproduction events which were fueled by the remains of the fratricided cells of the apoptotic fruiting body. Evidence has been accumulated suggesting that conserved through the ages as the evolutionary legacy of the germ-soma conflict, the somatic loss of immortality during the ontogenetic segregation of primordial germ cells recapitulates the archaic fate of the fruiting body. In this heritage, somatic death is a germ cell-triggered event and has been established as evolutionary-fixed default state following asymmetric reproduction in a world of finite resources. Aging, on the other hand, is the stress resistance-dependent phenotype of the somatic resilience that counteracts the germ cell-inflicted death pathway. Thus, aging is a survival response and, in contrast to current beliefs, is antagonistically linked to death that is not imposed by group selection but enforced upon the soma by the selfish genes of the "enemy within". Environmental conditions shape the trade-off solutions as compromise between the conflicting germ-soma interests. Mechanistically, the neuroendocrine system, particularly those components that control energy balance, reproduction and stress responses, orchestrate these events. The reproductive phase is a self-limited process that moulds onset and progress of senescence with germ cell-dependent factors, e.g. gonadal hormones. These degenerate the regulatory pacemakers of the pineal-hypothalamic-pituitary network and its peripheral, e.g. thymic, gonadal and adrenal targets thereby eroding the trophic milieu. The ensuing cellular metabolic stress engenders adaptive adjustments of the glucose-fatty acid cycle, responses that are adequate and thus fitness-boosting under fuel shortage (e.g. during caloric restriction) but become detrimental under fuel abundance. In a Janus-faced capacity, the cellular stress response apparatus expresses both tolerogenic and mutagenic features of the social and asocial deprivation responses [Rev. Neurosci. 12 (2001) 217]. Mediated by the derangement of the energy-Ca(2+)-redox homeostatic triangle, a mosaic of dedifferentiation/apoptosis and mutagenic responses actuates the gradual exhaustion of functional reserves and eventually results in a multitude of aging-related diseases. This scenario reconciles programmed and stochastic features of aging and resolves the major inconsistencies of current theories by linking ultimate and proximate causes of aging. Reproduction, differentiation, apoptosis, stress response and metabolism are merged into a coherent regulatory network that stages aging as a naturally selected, germ cell-triggered and reproductive phase-modulated deprivation response.