Figure 1. Model for basis of age-related changes in CD4 T cells.

In young animals thymic output is high, resulting in a rapid replacement of naive T cell populations in the periphery. As animals age and thymic involution occurs, thymic output of new naive T cells declines. To maintain the homeostasis of peripheral T cell numbers, the lifespan of naive T cells in the periphery is increased. This increase in longevity is accompanied by a decline the naive T cell function. Induction of new naive CD4 T cell production in the aged environment can generate T cells with robust function.