Hypothesis: loss of telomerase inducibility and subsequent replicative senescence in cultured human T cells is a result of altered costimulation

Deciphering the impact of TERT/telomerase on immunosenescence and T cell revitalization

Immunosenescence impacts both the innate and adaptive immune systems, predominantly affecting certain immune cell types. A notable manifestation of immunosenescence is the diminished efficacy of adaptive immunity. The excessive senescence of immune cells, particularly T cells, leads to marked immune deficiency, consequently escalating the risk of infections, tumors, and age-associated disorders. Lymphocytes, especially T cells, are subject to both replicative and premature senescence. Telomerase reverse transcriptase (TERT) and telomerase have multifaceted roles in regulating cellular behavior, possessing the ability to counteract both replicative and premature senescence in lymphocytes. This review encapsulates recent advancements in understanding immunosenescence, with a focus on T cell senescence, and the regulatory mechanisms involving TERT/telomerase. Additionally, it comprehensively discusses strategies aimed at inhibiting immunosenescence by augmenting TERT/telomerase activity.

A silver bullet for ageing medicine?: clinical relevance of T-cell checkpoint receptors in normal human ageing

Immunosenescence describes dysregulation of the immune system with ageing manifested in both the innate and adaptive immunity, including changes in T-cell checkpoint signaling. Through complex and nuanced process, T-cells lose excitatory signaling pathways and upregulate their inhibitory signaling, leading to ineffective immune responses that contribute to the formation of the ageing phenotype. Here we expand on the expression, function, and clinical potential of targeting the T-cell checkpoint signaling in age and highlight interventions offering the most benefits to older adults' health. Notably, modifications in vaccination such as with mTOR inhibitors show immediate clinical relevance and good tolerability. Other proposed treatments, including therapies with monoclonal antibodies fail to show clinical efficacy or tolerability needed for implementation at present. Although T-cell co-signaling fits a valuable niche for translational scientists to manage immunosenescence, future study would benefit from the inclusion of older adults with multiple long-term conditions and polypharmacy, ensuring better applicability to actual patients seen in clinical settings.

Impact of age on T cell signaling: a general defect or specific alterations?

Decreased immune responsiveness associated with aging is generally termed "immunosenescence". Several theories have been proposed to explain age-related declines in immune responses. Here, we will focus on and describe potential defects in T cell signal transduction from the membrane to the nucleus, leading to changes in the type, intensity and duration of the response as a major factor contributing to immunosenescence. We will first detail T cell signaling through the T cell receptor (TCR), CD28 and IL-2 receptor (IL-2R) and then discuss the observed age-related alterations to these signaling pathways. The role of membrane rafts in T cell signaling and T cell aging will be described. These factors will be considered in the context of the notion that age-related changes to T cell signaling may be attributed to changes in the functionality of the T cells due to shifts in T cell subpopulations with age. For this reason, we conclude by highlighting the application of multiparametric signaling analysis in leukocyte subsets using flow cytometry as a means to obtain a clearer picture with respect to age-related changes to immune signaling.

Rhadinovirus vector-derived human telomerase reverse transcriptase expression in primary T cells

The rhadinovirus herpesvirus saimiri (HVS) as a gene delivery vector allows large DNA insertions and long-termed gene expression. In the case of T-cell transduction, such vectors use the viral transformation-associated genes of HVS C488 for T-cell amplification. In this report, we investigated whether the gene for the catalytic telomerase subunit human telomerase reverse transcriptase (hTERT) can substitute for the transformation-associated genes in rhadinoviral T-cell transduction and amplification. By using virus mutants generated by en passant mutagenesis from bacterial artificial chromosomes, we observed a very early and functional transgene expression even by virus mutants without transformation-associated genes. The markers of T-cell transformation by HVS, namely CD2 hyperreactivity, overexpression of interleukin-26, and of the tyrosine kinase Lyn could neither be induced nor enhanced by ectopic hTERT expression. When the viral transformation-associated genes were replaced by the hTERT gene, it was not sufficient for growth transformation, although hTERT was efficiently transduced and functionally expressed by the rhadinovirus vector. Thus, the transformation-associated proteins StpC and Tip are responsible for the T-cell phenotype after transduction by HVS and, additionally, modulate telomerase activity independently of hTERT expression.

Does structural commonality of metal complex formation by PAC-1 (anticancer), DHBNH (anti-HIV), AHL (autoinducer), and UCS1025A (anticancer) denote mechanistic similarity? Signal transduction and medical aspects

There is an urgent need of novel approaches to drugs in the cancer, HIV, and bacterial areas. Increasing resistance to conventional therapies is observed. This minireview provides novel insights for drugs in these three areas. The agents PAC-1 (anticancer), DHBNH (anti-HIV), AHL (autoinducer), and UCS1025A (anticancer) have recently attracted attention due to considerable potential based on new approaches. PAC-1 activates procaspase-3 to caspase-3, resulting in induction of apoptosis in tumor cells. DHBNH binds to a newly revealed site on HIV reverse transcriptase. The drug mainly inhibits RNase H (RNA-cleaving). AHLs comprise an important class that participates in bacterial cell communication. UCS1025A is a fungus-derived inhibitor of the enzyme telomerase, present in cancer cells, which is crucially involved in tumor cell immortality. All four agents possess chelating sites for metal binding, which has not been appreciated. In PAC-1 and DHBNH, the coordinating portion is similar to salicylaldehyde semicarbazone. For AHL and UCS1025A, the metal-binding moiety is a beta -ketoamide. Metal complexes of heavier metals are well-known electron transfer (ET) functionalities that can generate reactive oxygen species. Hence, it is reasonable to hypothesize a commonality in mechanism based on metal ET. Differences in receptor binding can result, in part, in diverse physiological responses. There is considerable literature that addresses involvement of signal transduction with the various physiologically active agents discussed herein. Thus, cell communication appears to play an important role in the biochemistry of these endogenous and exogenous substances. Details of cell signaling are presented for complexes of metals (Fe, Cu, Ni, and As), telomerase, caspase-3, and RNase. In addition, practical medical aspects are discussed.

Immunosupportive therapies in aging

The primary role of the immune system is to protect the organism against pathogens, but age-associated alterations to immunity increase the susceptibility of the elderly to infectious disease. The exact nature of these changes is still controversial, but the use of screening procedures, such as the SENIEUR protocol to exclude underlying illness, helped to better characterize the changes actually related to physiological aging rather than pathology. It is generally agreed that the most marked changes occur in the cellular immune response reflecting profound alterations in T cells. Much of this is due to thymic involution as well as changes in the proportions of T cell subpopulations resulting from antigen exposure, and altered T cell activation pathways. However, a body of data indicates that innate immune responses, including the critical bridge between innate and adaptive immunity, and antigen presenting capacity are not completely resistant to senescence processes. The consequences of all these alterations are an increased incidence of infections, as well as possibly cancers, autoimmune disorders, and chronic inflammatory diseases. The leading question is what, if anything, can we do to prevent these deleterious changes without dangerously dysregulating the precarious balance of productive immunity versus immunopathology? There are many potential new therapeutic means now available to modulate immunosenescence and many others are expected to be available shortly. One main problem in applying these experimental therapies is ethical: there is a common feeling that as ageing is not a disease; the elderly are not sick and therefore do not require adventurous therapies with unpredictable side-effects in mostly frail individuals. Animal models are not helpful in this context. In this chapter we will first briefly review what we think we know about human immunosenescence and its consequences for the health status of elderly individuals. We will then discuss possible interventions that might one day become applicable in an appropriate ethical environment.

The unmet need in the elderly: Designing new influenza vaccines for older adults

Influenza is a serious illness and probably the single most important cause of excess disability and mortality during the winter months. In spite of limited efficacy in older adults, influenza vaccination is nevertheless a cost-saving medical intervention since it does reduce hospitalisation and death rates due to pneumonia, exacerbations of heart failure and, surprisingly, heart attacks and strokes. Yet hospitalisation and death rates for acute respiratory illnesses continue to rise in spite of widespread vaccination programs. As a person ages, the immune response to antigenic stimulation with the influenza virus shifts toward T helper type 2 cytokine production. This is associated with a relative reduction in cytotoxic T-cell activity and a reduced capacity to destroy infected host cells and clear the virus from infected lung tissue. Breakthrough strategies to improve the current influenza vaccines are required to avoid a crisis in health care. A targeted approach will develop vaccines that can reverse these age-related changes in T-cell responses, particularly the functions of cytotoxic T lymphocytes.

Large numbers of dysfunctional CD8+ T lymphocytes bearing receptors for a single dominant CMV epitope in the very old

Longitudinal studies suggest that a set of immune parameters including high percentages of peripheral CD8+, CD28-, CD57+ T lymphocytes, low CD4 and B cell counts, and poor T cell proliferative responses to mitogens is associated with decreased remaining longevity in the free-living very elderly (> 85 years). This combination of immune parameters was also significantly associated with an inverted CD4/CD8 ratio and cytomegalovirus seropositivity. Here, using tetramer technology, we show markedly increased numbers of CD8+ T cells bearing receptors for one single CMV epitope in the very elderly. Moreover, the fraction of these tetramer-reactive cells secreting interferon-gamma after specific antigenic stimulation was significantly lower in the old than in the young, as was the percentage of CD28-positive cells in this population. Therefore, we conclude that marked expansions of CMV-specific CD8+ T cells have occurred and that the obsession of a large fraction of the entire CD8+ T cell subset with one single viral epitope may contribute to the increased incidence of infectious disease in the elderly by shrinking the T cell repertoire available for responses to other antigens.

Influence of ex vivo expansion and retrovirus-mediated gene transfer on primary T lymphocyte phenotype and functions

To modulate alloreactivity after hematopoietic stem cell (HSC) transplantation, suicide gene-expressing donor T cells can be administered with an allogeneic T cell-depleted HSC graft. Immune competence of such cells is a critical issue. We have examined the impact of our ex vivo gene transfer protocol (12-day culture period including CD3/IL-2 activation, retrovirus-mediated gene transfer, and G418-based selection) on the phenotype and functional properties of gene-modified cells (GMC). GMC were compared with control cells that had been cultured in parallel with GMC, but nontransduced and nonselected, as well as with peripheral blood mononuclear cells (PBMC). Our data show that phenotypical modifications are similar in control cells and GMC, demonstrating that alterations result from the 12-day culture rather than from the transduction and/or selection process itself. Such modifications include a reversal of CD4/CD8 ratio, activated phenotype (increased expression of CD45RO, CD95, and HLA-DR), and acquisition or increased expression of co-stimulatory molecules (CD80, CD86, and CD40). This led to an enhanced allostimulating potential of GMC, as compared with resting T cells, when used as stimulating cells in mixed lymphocyte reactions. Conversely, when using them as responder cells in mixed lymphocyte reactions, GMC exhibited a rapid loss of alloreactivity that resulted both from culture-dependent and from transduction and/or selection-dependent events. In conclusion, the retrovirus-mediated gene transfer can be associated with major phenotypical and functional alterations that could have strong clinical implications (increased immunogenicity, reduced anti-leukemic effect). Thus, future T cell expansion protocols should try to improve not only cell expansion or gene transfer efficiency, but also T cell functions.

CD8+CD28- T cells: certainties and uncertainties of a prevalent human T-cell subset

Human peripheral blood CD8+ T cells comprise cells that are in different states of differentiation and under the control of complex homeostatic processes. In a number of situations ranging from chronic inflammatory conditions and infectious diseases to ageing, immunodeficiency, iron overload and heavy alcohol intake, major phenotypic changes, usually associated with an increase in CD8+ T cells lacking CD28 expression, take place. CD8+CD28- T cells are characterized by a low proliferative capacity to conventional stimulation in vitro and by morphological and functional features of activated/memory T cells. Although the nature of the signals that give origin to this T-cell subset is uncertain, growing evidence argues for the existence of an interplay between epithelial cells, molecules with the MHC-class I fold and CD8+ T cells. The possibility that the generation of CD8+CD28- T cells is the combination of TCR/CD3zeta- and regulatory factor-mediated signals as a result of the sensing of modifications of the internal environment is discussed.