The findings of this study support the emerging notion that chronic psychological stress has a negative impact on immune cell function and may accelerate their aging. By parallel analyses of composition and function of immune cells and their telomere length and telomerase activity, we provide compelling evidence that chronic stress influences at least three aspects of T cell functions: 1) hyperproduction of the immune-regulatory cytokine TNF-α and inhibitory cytokine IL-10; 2) decreased activation-induced proliferation without reduction of growth-related cytokines such as IL-2 and IL-4; and 3) impaired telomere length maintenance, even with up-regulated telomerase. However, the mechanistic pathways of chronic stress that lead to the impairment of these immune cell functions remain to be elucidated.
Accumulating evidence suggests that chronic stress is associated with decline of immune function and may accelerate aging (4
). In this study, we found a reduction of T cell proliferation and increased production of TNF-α
and IL-10 in response to in vitro stimulation in caregivers, but normal production of other cytokines such as IL-2, IL-4, IL-6, IL-8, GM-CSF, and IFN-γ
(). It is interesting to note that IL-10 has antioxidant function (21
) and the increased expression of IL-10 in the caregivers may reflect a feedback response of T cells in reaction to increased oxidative stress. Our analysis of the inducible NO synthase also showed a correlation of increased expression of inducible NO synthase and depressive symptoms (our unpublished data). Additional experiments will be needed to elucidate the changes of oxidative stress in response to caregiving. Previous studies have reported several age-associated changes in T cells (22
). However, we did not observe significant changes in the number or proportion of CD28−
T cells or other age-related subsets in caregivers as compared with controls. Considering the average caregiving time of our subjects was only 5 years, it may take a longer time to observe the age-associated gross changes of T cell compositions. Together, these findings suggest, in the caregiver population studied here, there are significant functional defects of T cells but no detectable accelerated age-associated changes in T cell subpopulations.
Accelerated loss of telomeres has been reported in conditions associated with defective telomerase (23
). Dyskeratosis congenita, a human genetic disorder which is marked by defects of telomerase RNA directly or indirectly via different genetic abnormalities, displays a progressive bone marrow failure syndrome affecting several tissues and organs (24
). The common feature of these affected tissues and organs is that they are highly regenerative, requiring a high rate of cell proliferation for their function. Without sufficient telomerase to compensate for telomere loss, a high rate of proliferation leads to shortened telomere length in these cells. Similar findings were also reported in telomerase-deficient mice (23
). Epel et al. (14
) reported telomere shortening in the peripheral blood leukocytes of women who were caregivers for chronically ill children. However, it was not clear whether shortened telomeres of PBMC in the report were due to an increase of shorter telomere possessing T cells in PBMC or across the board telomere shortening in all types of cells. Our findings here demonstrate that the loss of telomeres in caregivers was not due to the increase of shorter telomere cells in PBMC. Based on the reported rates of telomere attrition in PBMC (ranging from 31 bp/year to 67 bp/year) (27
), the differences of 240 bp shorter could account for ~4–8 years of shortened lifetime and could be even greater if it was calculated based on the rate of 19 bp/year of this study.
Epel et al. (14
) also reported a reduced basal level of telomerase activity in caregivers compared with controls and concluded that defects of telomerase activity in the caregivers may contribute to the loss of telomere. In contrast, we found here a significant increase in basal telomerase activity but not in the activation-induced levels of telomerase activity in caregivers of AD patients. It is not clear what the reason is behind the difference of these two findings. The increased basal telomerase activity in caregivers of AD patients may reflect an attempt of immune cells to compensate for excessive loss of telomeres. In this regard, caregivers in this study may be at the relative early phase of stress-induced impairment of telomere maintenance as compared with the mothers of sick children. Further studies are needed to elucidate the kinetic relationship of cell proliferation and telomerase activity in the maintenance of telomere length.
Telomerase activity is strictly regulated in human cells during development and differentiation (29
), and can be positively or negatively regulated by cytokines and hormones. IL-2, IL-7, and IL-15 are among cytokines that are capable of inducing telomerase in T cells (19
). In contrast, IFN-α
, and dexamethasone are capable of reducing telomerase activity in different types of cells (31
). Although it is known that chronic stress can alter the balance of the production of hormones and cytokines, the specific hormones and/or cytokines that are responsible for regulation of telomerase in immune cells is unknown. In this study, we found that TNF-α
levels were significantly higher in supernatants of activated T cells and serum from caregivers than from controls. However, the role of TNF-α
in regulation of telomerase is still controversial. Akiyama et al. (34
) showed that TNF-α
can induce activation and nuclear translocation of telomerase in the first hour following stimulation of PBMC. But the long-term effects of TNF-α
on telomerase activity are unclear, as is the identity of cell types in PBMC that are responsible for such changes. In contrast, Beyne-Rauzy et al. (35
) recently reported that TNF-α
inhibits human telomerase reverse transcriptase expression in myeloid cells through activation of a JNK pathway. It remains to be determined what the role of elevated TNF-α
may be in regulation of telomerase activity in caregivers. We also found that IL-10 levels were significantly higher in supernatants of activated T cells from caregivers than from controls. Previous caregiver studies have found a higher expression of IL-10 in response to stress experienced by caregivers (36
). However, it remains to be determined whether an increase in the immuno-suppressive cytokine, IL-10, is a counter measurement for an increase in proinflammatory cytokines or other unidentified changes in caregivers of AD patients.
It is now evident that individuals experiencing chronic stress are associated with shortened telomere length in their PBMC. However, the rate of telomere attrition in these individuals is not known. As telomere length is influenced by genetic factors and exhibits considerable polymorphisms within the population, a longitudinal analysis will be required to determine the rate of telomere attrition, changes of telomerase activity, and decline of immune function in association with the levels and duration of chronic stress of these caregivers and their controls. It is equally important to determine the physiological impact of telomerase activity and shortened telomeres on the overall function of immune cells. Further determination of how the psychological stress signals translate and influence cellular functions will bridge the mechanistic gap linking these two arenas.