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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.

The efficiency of the adaptive immune system is dependent on the diversity of T- and B-cell receptors, which is created by random rearrangement of receptor gene segments. AmpliCot is an experimental technique that allows the measurement of the diversity of the T- and B-cell repertoire. This procedure has the advantage over other cloning and sequencing techniques of being time- and expense-effective. In previous studies, receptor diversity, measured with AmpliCot, has been inferred assuming a second-order kinetics model. The latter implies that the relation between diversity and concentration × time (Cot) values is linear. We show that a more detailed model, involving heteroduplex and transient-duplex formation, leads to significantly better fits of experimental data and to nonlinear diversity-Cot relations. We propose an alternative fitting procedure, which is straightforward to apply and which gives an improved description of the relationship between Cot values and diversity.

The age-related decline in immune system functions is responsible for the increased prevalence of infectious diseases and the low efficacy of vaccination in elderly individuals. In particular, the number of peripheral naive T-cells declines throughout life and they exhibit severe functional defects at advanced age. However, we have recently identified a non-regulatory CD8+CD45RO+CD25+ T-cell subset that occurs in a subgroup of healthy elderly individuals, who still exhibit an intact humoral immune response following influenza vaccination. Here, we demonstrate that CD8+CD45RO+CD25+ T-cells share phenotypic and functional characteristics with naive CD8+CD45RA+CD28+ T-cells from young individuals, despite their expression of CD45RO. CD8+CD45RO+CD25+ T-cells also have long telomeres and upon antigenic challenge, they efficiently expand in vitro and differentiate into functional effector cells. The expanded population also maintains a diverse T-cell receptor repertoire. In conclusion, CD8+CD45RO+CD25+ T-cells from elderly individuals compensate for the loss of functional naive T-cells and may therefore be used as a marker of immunological competence in old age.

We have recently described an IL-2/IL-4-producing CD8+CD25+ nonregulatory memory T cell population that occurs in a subgroup of healthy elderly persons who characteristically still have a good humoral response after vaccination. The present study addresses this specific T cell subset and investigates its origin, clonal composition, Ag specificity, and replicative history. We demonstrate that CD8+CD25+ memory T cells frequently exhibit a CD4+CD8+ double-positive phenotype. The expression of the CD8 αβ molecule and the occurrence of signal-joint TCR rearrangement excision circles suggest a thymic origin of these cells. They also have longer telomeres than their CD8+CD25− memory counterparts, thus indicating a shorter replicative history. CD8+CD25+ memory T cells display a polyclonal TCR repertoire and respond to IL-2 as well as to a panel of different Ags, whereas the CD8+CD25− memory T cell population has a more restricted TCR diversity, responds to fewer Ags, and does not proliferate in response to stimulation with IL-2. Molecular tracking of specific clones with clonotypic primers reveals that the same clones occur in CD8+CD25+ and CD8+CD25− memory T cell populations, demonstrating a lineage relationship between CD25+ and CD25− memory CD8+ T cells. Our results suggest that CD25-expressing memory T cells represent an early stage in the differentiation of CD8+ cells. Accumulation of these cells in elderly persons appears to be a prerequisite of intact immune responsiveness in the absence of naive T cells in old age.