Decline in excision circles requires homeostatic renewal or homeostatic death of naive T cells

When the TCR is formed in the thymus, fragments of DNA are excised from the T cell progenitor chromosome. These TCR rearrangement excision circles (TRECs) are stable, are not replicated in cell division and are therefore most frequent in naive T cells that have recently left the thymus. During life, the average TREC content of peripheral naive T cells decreases between one and two orders of magnitude in humans. It is generally believed that the age-dependent decrease in the production of naive T cells by the thymus is sufficient to explain the decrease in the TREC content. Here, we demonstrate that this decrease in thymic production is required, but it is not sufficient to explain the TREC data. Only if the decrease in thymic output is compensated by homeostasis can one explain the decrease in the TREC content. The homeostatic response can take two forms: when the total number of naive T cells declines, there could be an increase in the renewal rate or an increase of the average cellular lifespan.