The organization of mature T-cell pools.

To deal with exogenous pathogens the peripheral T-cell compartment requires diverse repertoires (as those of naive cells) and efficient responses, the latter dependent on the persistence of memory cells. In the present work we show that (i) naive and memory cells differ in the type of interactions required for survival and division; (ii) they are segregated into independent ecological niches; (iii) that the size of each niche is controlled by independent homeostatic mechanisms; and (iv) that naive T cells do not have intrinsic life spans, surviving in the absence of thymus output but being continuously substituted by thymus export. The independent homeostatic regulation of the naive and memory T-cell pools guarantees the maintenance of versatile and efficient repertoires throughout life as well as the persistence of the naive T-cell pool after the thymus atrophies at puberty.