Accelerated telomere erosion is associated with a declining immune function of caregivers of Alzheimer's disease patients

Abstract

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-alpha 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.

FIGURE 1

Increased psychological stress in caregivers of AD patients. The levels of depression were measured in caregivers and controls. Data are presented as mean and SEM (n = 41). Depression scores were significantly higher in caregivers than in controls as analyzed by paired Student’s t test.

FIGURE 2

Reduced proliferation and increased proinflammatory cytokine production of T cells in caregivers. A, Activation-induced T cell proliferation is lower in caregivers than in controls. PBMC were stimulated with anti-CD3 and anti-CD28 Abs for 72 h in the presence of 1 μCi [³H]thymidine for the last 20 h. [³H]Thymidine incorporation was quantified using a liquid scintillation counter and normalized based on the number of T cells. Data are presented as mean (×10³) and SEM (n = 38). B, Production of TNF-α and IL-10 are higher in caregivers than in controls. Culture supernatants of PBMC were collected after 72 h stimulation by anti-CD3 and anti-CD28 Abs, and TNF-α and IL-10 were quantified with the BioPlex protein array system. The concentrations of cytokines are presented as nanograms per milliliter. C, Higher levels of TNF-α in serum of caregivers than that of controls. Sera of 41 pairs of subsets were collected and TNF-α concentration was determined by ELISA (4). Data are presented as mean and SEM (n = 37). All statistical analyses were done by paired Student’s t test; *, p < 0.05 by Students’ t test.

FIGURE 3

Shortened telomere length of PBMC, T cells, and monocytes in caregivers. A, Telomere length of PBMC decreases with age in caregivers and controls. Mean TRF (mTRF) length for caregivers and controls is plotted against their age. Caregivers: solid circle and dotted line; controls: open circle and solid line. B, Caregivers have shorter telomere length than controls. The mean TRF was calculated and compared between caregivers and controls. Data are presented as mean and SEM (n = 41). C, Caregivers have shorter T cell and monocyte telomere length than controls. The mean TRF was calculated and compared between caregivers and controls. Data are presented as mean and SEM (n = 21 and n = 11 in T cells and monocytes, respectively); *, p < 0.05 by Students’ t test.

FIGURE 4

Increased basal level but no difference in activation-induced telomerase activity of PBMC and T cells in caregivers. A, Basal PBMC telomerase activity was higher in caregivers compared with controls. Telomerase activity was measured by TRAP assay and normalized to total lymphocytes. B, Basal T cell telomerase activity was higher in caregivers compared with controls. Telomerase activity was measured by TRAP assay. The activity was presented at mean and SEM (n = 12). C, Activation-induced telomerase activity was not significantly different between caregivers and controls. PBMC were stimulated in vitro with anti-CD3 plus anti-CD28 Abs for 72 h and harvested for measurement of telomerase activity. Data are presented as mean and SEM (n = 38); *, p < 0.0001 by Students’ t test.