Regulation of T cell homeostasis during fetal and early postnatal life

Before parturition the fetal lamb develops a large pool of long-lived recir culating T cells which provides a large population of naive T cells with a diverse TcR repertoire. After birth and concomitant with exposure to enviro nment antigens, fetal T cells are rapidly replaced by short-lived cells for med postnatally. The majority of thymic emigrants homing to spleen in postn atal lambs are short-lived, in contrast to emigrants targeting lymph nodes where a population appears to be long-lived. The lifespan of thymic emigran ts in the fetus is unknown as in the relative importance of antigen-driven processes versus developmental programming in regulating T cell homeostasis in early postnatal life. (C) 1999 Elsevier Science B.V. All rights reserve d.