Lymphocyte recirculation is critical to maximize the efficiency of immunological surveillance and is an absolute requirement for the development of systemic memory, The consensus view of the lifespan of peripheral T cells holds that naive T cells are long-lived cells and most memory T cells are short-lived cells, although the question of the lifespan of peripheral T cells is not yet fully resolved, We have studied the lifespan of T cells circulating in efferent lymph draining lymph nodes (LN) in the immunologically naive sheep fetus and in postnatal lambs immediately following birth by examining the in vivo incorporation of [H-3]thymidine by newly formed T cells during continuous administration of [H-3]thymidine, We report that authentically naive fetal T cells are long-lived cells which continue to recirculate between blood and lymph during fetal life, At birth, however, a process is triggered whereby fetal T cells circulating through LN are rapidly lost from the peripheral T cell pool and are replaced by freshly arriving T cells which have been formed since birth. Our results indicate that by the end of the first week of postnatal life, around three-quarters of the T cells circulating through peripheral LN have been formed since birth.
