Impact of Aging, Cytomegalovirus Infection, and Long-Term Treatment for Human Immunodeficiency Virus on CD8+ T-Cell Subsets

Abstract

Both healthy aging and human immunodeficiency virus (HIV) infection lead to a progressive decline in naive CD8⁺ T-cell numbers and expansion of the CD8⁺ T-cell memory and effector compartments. HIV infection is therefore often considered a condition of premature aging. Total CD8⁺ T-cell numbers of HIV-infected individuals typically stay increased even after long-term (LT) combination antiretroviral treatment (cART), which is associated with an increased risk of non-AIDS morbidity and mortality. The causes of these persistent changes in the CD8⁺ T-cell pool remain debated. Here, we studied the impact of age, CMV infection, and LT successful cART on absolute cell numbers in different CD8⁺ T-cell subsets. While naïve CD8⁺ T-cell numbers in cART-treated individuals (N = 38) increased to healthy levels, central memory (CM), effector memory (EM), and effector CD8⁺ T-cell numbers remained higher than in (unselected) age-matched healthy controls (N = 107). Longitudinal analysis in a subset of patients showed that cART did result in a loss of memory CD8⁺ T-cells, mainly during the first year of cART, after which memory cell numbers remained relatively stable. As CMV infection is known to increase CD8⁺ T-cell numbers in healthy individuals, we studied whether any of the persistent changes in the CD8⁺ T-cell pools of cART-treated patients could be a direct reflection of the high CMV prevalence among HIV-infected individuals. We found that EM and effector CD8⁺ T-cell numbers in CMV⁺ healthy individuals (N = 87) were significantly higher than in CMV^- (N = 170) healthy individuals. As a result, EM and effector CD8⁺ T-cell numbers in successfully cART-treated HIV-infected individuals did not deviate significantly from those of age-matched CMV⁺ healthy controls (N = 39). By contrast, CM T-cell numbers were quite similar in CMV⁺ and CMV^- healthy individuals across all ages. The LT expansion of the CM CD8⁺ T-cell pool in cART-treated individuals could thus not be attributed directly to CMV and was also not related to residual HIV RNA or to the presence of HIV-specific CM T-cells. It remains to be investigated why the CM CD8⁺ T-cell subset shows seemingly irreversible changes despite years of effective treatment.

Keywords: CD8+ T-cells; combination antiretroviral treatment; cytomegalovirus; healthy aging; human immunodeficiency virus infection.

Figure 1

Potential of the CD8⁺ T-cell pool to normalize on long-term (LT) combination antiretroviral treatment (cART). (A) Total CD8⁺ T-cell numbers and (B) naïve (CD27⁺CD45RO⁻), (C) central memory (CM) (CD27⁺CD45RO⁺), (D) effector memory (EM) (CD27⁻CD45RO⁺), and (E) effector (CD27⁻CD45RO⁺) CD8⁺ T-cell numbers of human immunodeficiency virus (HIV)-infected individuals at a moment just before the start of cART (N = 13), during LT (i.e., at least 7 years of) cART (N = 38), and of [a mixed group of cytomegalovirus (CMV)⁺ and CMV⁻] age-matched healthy controls (N = 107). Bars represent median values of all individuals in a group, and data from the same individual are connected by lines. Comparisons between cross-sectional data from HIV-infected individuals and healthy individuals were based on a Mann–Whitney U-test. A Wilcoxon matched-pair signed rank test was used to study the significance of longitudinal changes in cell numbers from HIV-infected individuals. All (uncorrected) P-values of <0.05 are provided in the figure.

Figure 2

Early and late effects of combination anti-retroviral treatment (cART) on CD8⁺ T-cell numbers. Longitudinal analysis of cell numbers in four CD8⁺ T-cell subsets [(A) naïve, (B) central memory (CM), (C) effector memory (EM), and (D) effector] of nine HIV-infected individuals, during the first year of cART until at least 7 years of cART. Because of low sample sizes, we refrained from testing these changes statistically.

Figure 3

CD8⁺ T-cell numbers in cytomegalovirus (CMV)⁺ and CMV⁻ healthy individuals. Changes in absolute (A) naïve, (B) central memory (CM), (C) effector memory (EM), and (D) effector CD8⁺ T-cell numbers in CMV⁺ (black circles) and CMV⁻ (grey circles) healthy individuals with age. Solid curves represent linear regression lines through the data from adults and children, which were analyzed separately. Naïve cell numbers declined significantly with age in both CMV⁺ and CMV⁻ children, while EM and effector cell numbers declined significantly in CMV⁺ children only. CM, EM, and effector cell numbers increased significantly with age in CMV⁻ adults only.

Figure 4

CD8⁺ T-cell numbers in cytomegalovirus (CMV)⁺ healthy individuals and human immunodeficiency virus (HIV) patients on long-term (LT) combination anti-retroviral treatment (cART). Comparison of (A) total, (B) naïve, (C) central memory (CM), (D) effector memory (EM), and (E) effector CD8⁺ T-cell numbers between HIV-infected individuals on LT cART (N = 38) and CMV⁺ age-matched healthy controls (N = 39). Bars represent median values. Comparisons between data from HIV-infected individuals and healthy individuals were based on a Mann–Whitney U-test. All (uncorrected) P-values of <0.05 are provided in the figure.

Figure 5

Neither low-level human immunodeficiency virus (HIV) viremia nor the presence of HIV-specific T-cells explain increased central memory (CM) CD8⁺ T-cell numbers during long-term (LT) combination anti-retroviral treatment (cART). CM CD8⁺ T-cell numbers in HIV-infected individuals on LT cART were neither associated with (A) levels of plasma HIV RNA load (grouped by the number of viral RNA copies/ml plasma determined using the Roche Cobas Taqman v2.0 assay), nor by (B) the percentage of HIV-specific CM CD8⁺ T-cells, measured using tetramers of the three immuno-dominant HIV-1 peptides: HLA-A2-SLYNTVATL (SLY), HLA-B8-FLKEKGGL (FLK), and HLA-B8-EIYKRWII (EIY). Depicted are individual values (symbols), medians (bars in panel A), and a linear regression line for the results of HLA-A2-SLYNTVATL (P = 0.01).

Figure 6

T-cell proliferation, senescence, and apoptosis. Percentages of (A) proliferating (Ki67⁺), (B) senescent (CD28⁻CD57⁺), and (C) apoptotic (AnnexinV⁺7AAD⁻) CD8⁺ T-cells in the different subsets in human immunodeficiency virus (HIV)-infected individuals on long-term combination anti-retroviral treatment (cART) (filled circles) compared to cytomegalovirus (CMV)⁺ healthy age-matched controls (open diamonds). Bars depict medians. Comparisons between data from HIV-infected individuals and healthy individuals were based on a Mann–Whitney U-test. All (uncorrected) P-values of <0.05 are provided in the figure.