The ontogeny of T cell recirculation during foetal life

Studies on the ontogeny of the immune system in the sheep foetus, which remains immunologically naive until after birth, indicate that a large scale recirculation of T cells is just as much a feature of the foetal immune system as it is in animals after birth. An expensive recirculation of T cells and dendritic cells through peripheral tissues-including the gastrointestinal tract and skin-develops ear;ly in foetal life, although a population of gut-homing memory T cells does not develop until postnatal life. Current evidence suggests that two populations of thymic emigrants with distinct tissue-homing specificities to spleen and lymph node contribute to the development of the foetal peripheral T cell pool. CD8(+) thymic migrants mostly target spleen while CD4(+) thymic migrants home predominantly to lymph nodes. The lifespan of thymic emigrants is uncertain, although cells entering lymph are long-lived and form the basis of a diverse pre-immune repertoire of recirculating T cells. The life history and growth rates of non-circulating T cells in spleen and lymph nodes during foetal life are at present unknown.