Non-regulatory CD8(+)CD45RO(+)CD25(+) T-lymphocytes may compensate for the loss of antigen-inexperienced CD8(+)CD45RA(+) T-cells in old

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.