Nan-ping Weng

Accelerated Telomere Erosion Is Associated with a Declining Immune Function of Caregivers of Alzheimer’s Disease Patients

Caregivers of Alzheimer’s disease patients endure chronic stress associated with a decline of immune function. To assess the psychological and immunological changes of caregivers, we compared depressive symptoms, PBMC composition, in vitro activation-induced proliferation and cytokine production, and telomere length and telomerase activity of 82 individuals (41 caregivers and 41 age- and gender-matched controls). We found depressive symptoms were significantly higher in caregivers than in controls (p < 0.001). Correspondingly, caregivers had significantly lower T cell proliferation but higher production of immune-regulatory cytokines (TNF-α and IL-10) than controls in response to stimulation in vitro. We examined the impact of these changes on cellular replicative lifespan and found that caregivers had significantly shorter telomere lengths in PBMC than controls (6.2 and 6.4 kb, respectively, p < 0.05) with similar shortening in isolated T cells and monocytes and that this telomere attrition in caregivers was not due to an increase of shorter telomere possessing T cell subsets in PBMC. Finally, we showed that basal telomerase activity in PBMC and T cells was significantly higher in caregivers than in controls (p < 0.0001), pointing to an unsuccessful attempt of cells to compensate the excessive loss of telomeres in caregivers. These findings demonstrate that chronic stress is associated with altered T cell function and accelerated immune cell aging as suggested by excessive telomere loss.

Childhood Adversity Heightens the Impact of Later-Life Caregiving Stress on Telomere Length and Inflammation

Objective: To address the question of whether childhood abuse and other adversities have lasting, detectable consequences for inflammation and cell aging late in life, and whether the effects are large enough to be discernible beyond that of a major chronic stressor, dementia family caregiving. Previous research on the physical health consequences of childhood abuse and other adversities has been based on data from young or middle-aged adults. Method: In this community sample of 132 healthy older adults (mean age = 69.70 years; standard deviation = 10.14), including 58 dementia family caregivers and 74 noncaregivers, blood samples were analyzed for interleukin (IL)-6, tumor necrosis factor (TNF)-&agr;, and telomere length, a measure of cell aging. Depressive symptoms were assessed by the Center for Epidemiological Studies Depression Scale. Results: After controlling for age, caregiving status, gender, body mass index, exercise, and sleep, the presence of multiple childhood adversities was related to both heightened IL-6 (0.37 ± 0.03 log10 pg/mL versus 0.44 ± 0.03 log10 pg/mL) and shorter telomeres (6.51 ± 0.17 Kb versus 5.87 ± 0.20 Kb), compared with the absence of adversity; the telomere difference could translate into a 7- to 15-year difference in life span. Abuse was associated with heightened IL-6 and TNF-&agr; levels; for TNF-&agr;, this relationship was magnified in caregivers compared with controls. Moreover, abuse and caregiving status were associated significantly and independently with higher levels of depressive symptoms. Conclusions: Adverse childhood events are related to continued vulnerability among older adults, enhancing the impact of chronic stressors. Childhood adversities cast a very long shadow. BMI = body mass index; CES-D = Center for Epidemiological Studies Depression Scale; CRP = C-reactive protein; IL = interleukin; PBMCs = peripheral blood mononuclear cells; TNF = tumor necrosis factor.

Telomeres in T and B cells

Telomeres are the structures at the ends of linear chromosomes. In mammalian cells, they consist of hexanucleotide (TTAGGG) repeats, together with many associated proteins. In the absence of a compensatory mechanism, dividing cells undergo gradual telomere erosion until a critical degree of shortening results in chromosomal abnormalities and cell death or senescence. For T and B cells, the ability to undergo extensive cell division and clonal expansion is crucial for effective immune function. This article describes our current understanding of telomere-length regulation in lymphocytes and its implications for immune function.

Cutting edge: telomerase activation in human T lymphocytes does not require increase in telomerase reverse transcriptase (hTERT) protein but is associated with hTERT phosphorylation and nuclear translocation.

Capacity for cellular replication is critically important for lymphocyte function and can be regulated by telomerase-dependent maintenance of telomere length. In contrast to most normal human somatic cells that do not express telomerase due to the failure to transcribe telomerase reverse transcriptase (hTERT), lymphocytes express telomerase in a highly regulated fashion yet constitutively transcribe hTERT during development and activation. Here, we report that hTERT protein is present in both thymocytes and blood T cells at equivalent levels despite their substantial differences in telomerase activity, and that induction of telomerase activity in resting CD4+ T cells is not dependent on net hTERT protein increase. Moreover, hTERT is phosphorylated and translocated from cytoplasm to nucleus during CD4+ T cell activation. Thus, human T lymphocytes regulate telomerase function through novel events independent of hTERT protein levels, and hTERT phosphorylation and nuclear translocation may play a role in regulation of telomerase function in lymphocytes.

Tales of tails: regulation of telomere length and telomerase activity during lymphocyte development, differentiation, activation, and aging.

Summary: Telomerase activity and the regulation of telomere length are factors which have been implicated in the control of cellular replication. These variables have been examined during human lymphocyte development, differentiation, activation, and aging. It was found that telomere length of peripheral blood CD4+ T cells decreases with age as well as with differentiation from naive to memory cells in vivo, and decreases with cell division in vitro. These results provide evidence that telomere length correlates with lymphocyte replicative history and residual replicative potential. In contrast, telomere length appears to increase during tonsil B‐cell differentiation and germinal center (GC) formation in vivo. It was also found that telomerase activity is highly regulated during T‐cell development and B‐cell differentiation in vivo, with high levels of telomerase activity expressed in thymocytes and GC B cells, and low levels of telomerase activity in resting mature peripheral blood lymphocytes. Finally, resting lymphocytes retain the ability to upregulate telomerase activity upon activation, and this capacity does not appear to decline with age. Although the precise role of telomerase in lymphocyte function remains to be elucidated, telomerase may contribute to protection from telomere shortening in T and B lymphocytes, and may thus play a critical role in lymphocyte development, differentiation and activation. The future study of study telomerase and its regulation of telomere length may enhance our understanding of bow the replicative lifespan is regulated in lymphocytes.

Lineage-Specific Telomere Shortening and Unaltered Capacity for Telomerase Expression in Human T and B Lymphocytes with Age

Age effects on telomere length and telomerase expression in peripheral blood lymphocytes were analyzed from 121 normal individuals age newborn to 94 years and revealed several new findings. 1) Telomere shortening was observed in CD4+ and CD8+ T and B cells with age. However, the rate of telomere loss was significantly different in these populations, 35 ± 8, 26 ± 7, and 19 ± 7 bp/year for CD4+ and CD8+ T and B cells, respectively. In addition, CD4+ T cells had the longest average telomeres at all ages, followed by B cells, with CD8+ T cell telomeres the shortest, suggesting that these lymphocyte populations may have different replicative histories in vivo. 2) Telomerase activity in freshly isolated T and B cells was indistinguishably low to undetectable at all ages but was markedly increased after Ag and costimulatory receptors mediated stimulation in vitro. Furthermore, age did not alter the magnitude of telomerase activity induced after stimulation of T or B lymphocytes through Ag and costimulatory receptors or in response to PMA plus ionomycin treatment. 3) The levels of telomerase activity induced by in vitro stimulation varied among individual donors but were highly correlated with the outcome of telomere length change in CD4+ T cells after Ag receptor-mediated activation. Together, these results indicate that rates of age-associated loss of telomere length in vivo in peripheral blood lymphocytes is specific to T and B cell subsets and that age does not significantly alter the capacity for telomerase induction in lymphocytes.

The molecular basis of the memory T cell response: differential gene expression and its epigenetic regulation

How the immune system remembers a previous encounter with a pathogen and responds more efficiently to a subsequent encounter has been one of the central enigmas for immunologists for over a century. The identification of pathogen-specific memory lymphocytes that arise after an infection provided a cellular basis for immunological memory. But the molecular mechanisms of immunological memory remain only partially understood. The emerging evidence suggests that epigenetic changes have a key role in controlling the distinct transcriptional profiles of memory lymphocytes and thus in shaping their function. In this Review, we summarize the recent progress that has been made in assessing the differential gene expression and chromatin modifications in memory CD4+ and CD8+ T cells, and we present our current understanding of the molecular basis of memory T cell function.

Regulation of Telomere Length and Telomerase in T and B Cells: A Mechanism for Maintaining Replicative Potential

Recent studies have established that telomere length is altered during differentiation of both T and B lymphocytes. There exists a clear and strong correlation of replicative capacity with telomere length in normal somatic cells. In human T cells, this correlation extends to differences in telomere length and replicative capacity in subsets such as naive and memory CD4 cells, or CD28−positive and negative CD8 T cells. In addition, telomerase, a unique reverse transcriptase that is capable of extending telomeric length, is highly regulated during development and activation of both T and B lymphocytes. Together, these findings have provided a basis for hypotheses linking telomere length regulation to a functional role in sustaining the capacity for extensive clonal expansion in antigen-specific lymphocytes. In addition to providing insights into basic immune function, manipulation of telomere length has potential therapeutic applications as well. For example, the ability to extend the replicative capacity of cells such as hematopoietic stem cells or mature lymphocytes through telomerase induction by transfection could be critical to therapeutic approaches to adoptive cell transfer or reconstitution. In assessing the feasibility of such approaches, it will be critical not only to measure extension of the capacity for cell division but also to consider other possible consequences such as enhanced susceptibility to malignant transformation through dysregulated telomerase activity. Conversely, the proposed use of telomerase inhibition as a modality for anticancer therapy should consider the possible impact of such intervention on any telomerase-dependent aspects of immune function. The rapid pace of gene discovery and genetic engineering, in combination with a richness of available systems for studying immune cell biology, should allow vigorous pursuit of these remaining questions.‡To whom correspondence should be addressed (e-mail: Richard_Hodes@nih.gov).

Histone Acetylation Facilitates Rapid and Robust Memory CD8 T Cell Response through Differential Expression of Effector Molecules (Eomesodermin and Its Targets: Perforin and Granzyme B)

To understand the mechanism regulating the effector function of memory CD8 T cells, we examined expression and chromatin state of a key transcription factor (eomesodermin, EOMES) and two of its targets: perforin (PRF1) and granzyme B (GZMB). Accessible chromatin associated histone 3 lysine 9 acetylation (H3K9Ac) was found significantly higher at the proximal promoter and the first exon region of all three genes in memory CD8 T cells than in naive CD8 T cells. Correspondingly, EOMES and PRF1 were constitutively higher expressed in memory CD8 T cells than in naive CD8 T cells at resting and activated states. In contrast, higher expression of GZMB was induced in memory CD8 T cells than in naive CD8 T cells only after activation. Regardless of their constitutive or inducible expression, decreased H3K9Ac levels after treatment with a histone acetyltransferase inhibitor (Curcumin) led to decreased expression of all three genes in activated memory CD8 T cells. These findings suggest that H3K9Ac associated accessible chromatin state serves as a corner stone for the differentially high expression of these effector genes in memory CD8 T cells. Thus, epigenetic changes mediated via histone acetylation may provide a chromatin “memory” for the rapid and robust transcriptional response of memory CD8 T cells.

Rapid default transition of CD4 T cell effectors to functional memory cells

The majority of highly activated CD4 T cell effectors die after antigen clearance, but a small number revert to a resting state, becoming memory cells with unique functional attributes. It is currently unclear when after antigen clearance effectors return to rest and acquire important memory properties. We follow well-defined cohorts of CD4 T cells through the effector-to-memory transition by analyzing phenotype, important functional properties, and gene expression profiles. We find that the transition from effector to memory is rapid in that effectors rested for only 3 d closely resemble canonical memory cells rested for 60 d or longer in the absence of antigen. This is true for both Th1 and Th2 lineages, and occurs whether CD4 T cell effectors rest in vivo or in vitro, suggesting a default pathway. We find that the effector–memory transition at the level of gene expression occurs in two stages: a rapid loss of expression of a myriad of effector-associated genes, and a more gradual gain of expression of a cohort of genes uniquely associated with memory cells rested for extended periods.