Is thymocyte development functional in the aged?

Abstract

T cells are an integral part of a functional immune system with the majority being produced in the thymus. Of all the changes related to immunosenescence, regression of the thymus is considered one of the most universally recognised alterations. Despite the reduction of thymic size, there is evidence to suggest that T cell output is still present into old age, albeit much diminished; leading to the assumption that thymocyte development is normal. However, current data suggests that recent thymic emigrant from the aged thymus are functionally less responsive, giving rise to the possibility that the generation of naïve T cell may be intrinsically impaired in the elderly. In light of these findings we discuss the evidence that suggest aged T cells may be flawed even before exiting to the periphery and could contribute to the age-associated decline in immune function.

Keywords: T cells; aging; immunity; thymocyte; thymus.

Figure 1.. CD3 expression on DN thymocytes shows an age-dependent increase.

Thymocytes from different aged mice were stained with anti-CD3, anti-CD4 and anti-CD8 mAb, analysed by flow cytometry and CD3 on DN cells was determined gating the appropriate population. This study revealed that the proportion of CD3⁺ DN thymocytes showed an age-dependent increase. (One month n=5; six months n=5; 12 months n=8; 18 months n=4). *P<0.05; **P<0.01; ***P<0.001.

Figure 2.. CD3 expression is altered on aged thymocytes.

Thymocytes from different aged mice were stained with anti-CD3 mAb and analysed by flow cytometry. The top histogram shows the percentage of CD3+ cells positive and the bottom shows mean fluorescent intensity (MFI) of CD3 expression for one month old, six month old, 12 month old and an 18 month old animals. MFI was obtained by gating on the entire population. Although there were no age-related changes in the proportion of CD3+ thymocytes, a significant decrease in the number of CD3 molecules on thymocytes associated with age was observed. (One month n=5; six months n=5; 12 months n=8; 18 months n=4). **P<0.01; ***P<0.001.

Figure 3.. Cell cycle analysis on stimulated thymocytes from young and old mice.

The various stages of the cell cycle in thymocytes from young and old mice following treatment with ConA and IL-2 after 24 hours was determined by flow cytometry. Data is expressed as fold increase compared to time zero. It was observed that there was a significant increase in the proportion of thymocytes from young mice at the G₂-M phase compared to thymocytes from older animals. One month n=4; 18 months n=4. *P<0.05.

Figure 4.. Aged thymocytes have increased resistance to spontaneous and dexamethasone-induced apoptosis.

Spontaneous (A) and dexamethasone (dex)-induced (B) apoptosis at 0.5nM was assessed by flow cytometry. Graphs show the percentage of viable thymocytes defined as Annexin V^- 7AAD^- (top graphs)and those undergoing early apoptosis as Annexin V⁺ 7AAD^- (bottom graphs). Closed square/circle with dotted line symbolise young thymocytes cultured in media or with the addition of 0.5nM dex respectively. Whereas, open square/circle with solid line signify thymocytes from 18 month old mice cultured in media or with the addition of 0.5nM dex respectively. The data revealed that there is an age-associated increased resistance to spontaneous and dex-induced apoptosis with a higher percentage of viable thymocytes from older mice compared to younger mice and delayed kinetic of older thymocytes to initiate apoptosis. Data representative of four experiments. *P<0.05; **P<0.01.