T cell subset-specific susceptibility to aging

Abstract

With increasing age, the competence of the immune system to fight infections and tumors declines. Age-dependent changes have been mostly described for human CD8 T cells, raising the question of whether the response patterns for CD4 T cells are different. Gene expression arrays of memory CD4 T cells yielded a similar age-induced fingerprint as has been described for CD8 T cells. In cross-sectional studies, the phenotypic changes were not qualitatively different for CD4 and CD8 T cells, but occurred much more frequently in CD8 T cells. Homeostatic stability partially explained this lesser age sensitivity of CD4 T cells. With aging, naïve and central memory CD8 T cells were lost at the expense of phenotypically distinct CD8 effector T cells, while effector CD4 T cells did not accumulate. However, phenotypic shifts on central memory T cells were also more pronounced in CD8 T cells. This distinct stability in cell surface marker expression can be reproduced in vitro. The data show that CD8 T cells are age sensitive by at least two partially independent mechanisms: fragile homeostatic control and gene expression instability in a large set of regulatory cell surface molecules.

Figure 1. Influence of age on CD28 loss within CD4⁺ and CD8⁺ T cells

The frequency of CD28 loss in CD4 (left panel) and CD8 (right panel) T-cell subpopulations was determined by FACS in healthy individuals of different ages. Results are shown as box blots with medians, 25^th and 75^th percentiles as boxes and 10^th and 90^th percentile as whiskers for different age strata. Although decline was significant for both CD4 (p<0.001) and CD8 T cells (p<0.001), loss was much more dramatic and earlier in life in the CD8 population and was already significant for middle-aged individuals (p<0.001).

Figure 2. Age-dependent gain of CD57, CD85j, and CD158b/j expression in CD8⁺ T cells

(A) Frequencies of CD85j, CD158b/j, and CD57 cells within CD4 (left panel) and CD8 (right panel) T-cells were determined by FACS. Results are shown as box plots with medians, 25^th and 75^th percentiles as boxes, and 10^th and 90^th percentiles as whiskers for healthy individuals representing different age strata. (B) In both CD4 and CD8 T cells, CD57 and CD85j were preferentially expressed on CD28⁻ (shaded areas) and less on CD28⁺ (dark lines).

Figure 3. Influence of age on functional T-cell subset distribution

PBMCs were isolated from healthy young (n=31; 27.9± 5.5years) and elderly (n=26; 73.1± 4.2years) individuals and stained with anti-CD3, -CD4, -CD8, -CD45RA, and -CCR7 mAbs to determine frequencies of T-cell subsets. Results are shown for CD45RA⁺ CCR7⁺ naïve CD4 (A) and CD8 (B), CD45RA⁻ CCR7⁺ central memory (CM), CD45RA⁻ CCR7⁻ effector memory (EM) and CD45RA⁺ CCR7-effector (CD45RA EM) CD4 (C) and CD8 cells (D). Expression of CD57, CD85j and CD158b/j in the elderly was highest in effector CD8 T cells, but CD57 and CD85j was also present in a central memory T cells. Representative histograms are shown (E).

Figure 4. Expansion of CMV-specific T cells does not account for the preference of age-dependent phenotypic changes for the CD8 compartment

PBMCs were isolated from HLA-A2⁺ individuals and stained with HLA-A2-CMV peptide (NLVPMVATV) (A2/NLV) pentamer, anti-CD3, -CD8, -CD28, and -CD85j mAbs. (A) Frequencies of CMV peptide-specific CD8 T cells are shown in correlation to age. (B) CMV-peptide-specific (solid boxes) and total CD8 T cells (open boxes) from young and elderly individuals who had <0.1% of CMV-peptide specific cells, were compared. (C) Individuals who had (solid bars) or did not have expanded CMV-peptide specific T cells (open bars) were compared for the frequency of CD85j and CD28 expression on CD8 T cells.

Figure 5. CD4⁺ and CD8⁺ T cells differ in CD26 expression with age

Results of CD26 expression within CD4 (left panel) and CD8 (right panel) T-cell populations in relation to age in cross-sectional studies are shown as box plots (A). Representative histograms of CD26 expression in CD28⁺ and CD28⁻ CD4 and CD8 cells from a 27 (top row) and 73 year-old (bottom row) individual are shown in (B).

Figure 6. Increased expression of HLA-DR on CD4 and CD8 T-cells with age

(A) The frequencies of HLA-DR⁺ cells within CD4 (left panel) and CD8 (right panel) T-cells are shown as box plots for different age groups. HLA-DR in elderly individuals is expressed on CD28⁺ as well as CD28⁻ T cells (B) and on central as well as effector memory CD8 T cells (C). Representative histograms from a 27 (top row) and 70 year-old (bottom row) individual are shown.

Figure 7. Phenotypic changes of the surface molecules: CD28, CD85j, HLA-DR in CD8 and CD4 subsets caused by long-term culture of CD28 positive T cells

CD28 positive cells were sorted, labeled with CFSE, and stimulated with irradiated, EBV transformed PBMC and OKT3. FACS analysis was carried out every seventh day beginning from the first stimulation. CD28, HLA-DR, and CD85j molecules surface expression changes over time and is much more pronounced in CD8 T cells compared to the CD4 subset, suggesting that the phenotypic instability is intrinsic to CD8 T cells.