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 w e show that (i) naive and memory cells differ in the type of interactions r equired for survival and division; (ii) they are segregated into independen t ecological niches; (iii) that the size of each niche is controlled by ind ependent homeostatic mechanisms; and (iv) that naive T cells do not have in trinsic life spans, surviving in the absence of thymus output but being con tinuously substituted by thymus export. The independent homeostatic regulat ion of the naive and memory T-cell pools guarantees the maintenance of vers atile and efficient repertoires throughout life as well as the persistence of the naive T-cell pool after the thymus atrophies at puberty.