CD4⁺ CD8⁺ T cells in young and elderly humans. Comment on Macchia I, Gauduin MC, Kaur A, Johnson RP. Expression of CD8α identifies a distinct subset of effector memory CD4⁺ T lymphocytes. Immunology 2006; 119:232–42

Abstract

Peripheral CD4⁺ CD8⁺ T cells have been described in animals as well as in humans. Two distinct populations can be distinguished, namely CD4^lo CD8^hi and CD4^hi CD8^lo T cells. We demonstrate here that the increase in the number of peripheral CD4⁺ CD8⁺ T cells in the elderly is the result of an increase of the CD4^lo CD8^hi T-cell population. While the phenotype of CD4^lo CD8^hi and CD4^hi CD8^lo T cells was very similar in young persons, CD4^hi CD8^lo, T cells from elderly subjects expressed a more differentiated phenotype and produced less interleukin-2 compared to CD4^lo CD8^hi T cells. In conclusion, our results suggest that aging leads to a phenotypic and functional difference between CD4⁺ CD8⁺ T-cell subsets. It may therefore be of relevance to distinguish between these subsets before assessing their functional significance in elderly humans.

In a recent article by Macchia et al.,¹ the frequency, phenotype and function of peripheral CD4⁺ CD8⁺ T cells in rhesus macaques were described. The authors demonstrate that the double-positive population is mainly CD4^hi CD8^lo and that 78% of this population exhibits an effector memory phenotype. They also state that the frequency of double-positive T cells does not change with age. No data are available about the characteristics of peripheral CD4^hi CD8^lo or CD4^lo CD8^hi T cells in young and elderly persons, so it is difficult to assess the significance of the data by Maccia et al.¹ for humans. We will therefore illustrate similarities and differences of peripheral CD4⁺ CD8⁺ T-cell subsets between rhesus macaques and humans and address the effect of age.

As demonstrated by Laux et al.,² we found a significant increase in the frequency of total peripheral CD4⁺ CD8⁺ T cells in the elderly compared to young persons (Table 1). Interestingly, CD4⁺ CD8⁺ T cells were enriched within the large cells of the lymphocyte population. Within the CD4⁺ CD8⁺ T-cell population we distinguished between CD4^hi CD8^lo, which expressed the CD8αα receptor and CD4^lo CD8^hi T cells which expressed the CD8αβ receptor (Fig. 1a). Immunofluorescence surface staining was performed as described elsewhere.³ Briefly, the antibodies used were CD3-peridinin chlorophyll protein, CD4-fluorescein isothiocyanate, CD8α-allophycocyanin and CD8β-phycoerythrin. Our data demonstrate that the number of CD4^lo CD8^hi T cells was increased in elderly compared to young persons, while the number of CD4^hi CD8^lo T cells was not affected by age (Table 1). In contrast to rhesus macaques, no dominance of the CD4^hi CD8^lo T cells was observed in either young or elderly persons, but a high variability in the frequency of CD4^hi CD8^lo T cells in both the young (0·06–1·15%) and the elderly (0·05–1·85%, Fig. 1a) was recorded.

Table 1.

Frequency of peripheral CD4⁺ CD8⁺, CD4^lo CD8^hi and CD4^hi CD8^lo T cells in young and elderly persons

	Young persons (n = 12)	Elderly persons (n = 16)
CD4+ CD8+	0·66 ± 0·11	1·20 ± 0·15*
CD4lo CD8hi	0·34 ± 0·05	0·75 ± 0·11**
CD4hi CD8lo	0·31 ± 0·09	0·45 ± 0·11

Results are expressed as percentages of CD3⁺ T cells (mean ± SEM) in young (mean age ± SD: 27·6 ± 5·1 years) and elderly persons (mean age ± SD: 70·5 ± 8·6 years)

^*P < 0·05

^**P < 0·01 compared to the corresponding T-cell population in young persons (Mann–Whitney U-test).

Figure 1.

Frequency, phenotype and cytokine profile of peripheral human CD4^lo CD8^hi and CD4^hi CD8^lo T cells. (a) Representative fluorescence-activated cell sorter pictures showing the frequency of peripheral CD4^lo CD8^hi and CD4^hi CD8^lo T cells in four young (YD) and four elderly (ED) blood donors. The percentages of CD4^lo CD8^hi and CD4^hi CD8^lo T cells within the CD3⁺ population are indicated. (b) Phenotype of CD4^lo CD8^hi (black bars) and CD4^hi CD8^lo T cells (white bars) from nine young (mean age ± SD: 27·6 ± 5·9 years) and 12 elderly persons (mean age ± SD: 68·8 ± 7·5 years). The percentage of cells with the indicated marker is shown for each subpopulation (mean ± SEM). (c) Cytokine profile of CD4^lo CD8^hi (black bars) and CD4^hi CD8^lo T cells (white bars) from six elderly persons (mean age ± SD: 67·3 ± 8·3 years). The percentage of cells with the indicated marker is shown for each subpopulation (mean ± SEM). *P < 0·05 and **P < 0·01 between CD4^lo CD8^hi and CD4^hi CD8^lo T cells (Mann–Whitney U-test).

Phenotypic analysis of CD4^lo CD8^hi and CD4^hi CD8^lo T cells from young persons (mean age ± SD: 27·6 ± 5·9 years, n = 9) did not reveal any differences,⁴ except for expression of natural killer group 2D (NKG2D), which was higher in the CD4^lo CD8^hi T-cell population (P = 0·004; Fig. 1b). In the elderly, however, CD4^hi CD8^lo T cells had a more differentiated phenotype compared to CD4^lo CD8^hi T cells, as demonstrated by a decreased expression of CD27 and an increased expression of CD56 and CD57 (Fig. 1b). The number of interleukin-2 (IL-2)-producing cells, as assessed by intracellular cytokine staining after stimulation with phorbol 12-myristate 13-acetate and ionomycin in the presence of brefeldin A,⁵ was also lower in CD4^hi CD8^lo than in CD4^lo CD8^hi T cells in elderly persons (Fig. 1c). No difference was observed in the number of cells producing IL-4, IL-10, interferon γ, tumour necrosis factor α and perforin between CD4^lo CD8^hi and CD4^hi CD8^lo T cells in both age groups. IL-2-producing CD25-expressing memory CD8⁺ T cells, which frequently coexpress CD4, have been shown to characteristically accumulate in a subgroup of healthy elderly persons who still have a good humoral response after influenza vaccination.^3,6 As a result of the coexpression of CD4, double-positive cells can enhance interaction with antigen-presenting cells by serving as both an adhesion and a costimulatory molecule and they may interact with major histocompatibility complex class II.⁷

In conclusion, our data highlight several differences regarding the frequency and the composition of the CD4⁺ CD8⁺ T-cell population between rhesus macaques and humans. They also demonstrate that the size of the CD4^lo CD8^hi subset increases with age. It may therefore be of relevance to distinguish between CD4⁺ CD8⁺ T-cell subsets before assessing their functional significance in elderly humans.

Glossary

Abbreviations

IL

interleukin

NKG2D

natural killer group 2D