AN IMMUNE-SYSTEM SWITCH IN T-CELL LIFE-SPAN AT BIRTH RESULTS IN EXTENSIVE LOSS OF NAIVE FETAL T-CELLS DURING THE FIRST WEEK OF POSTNATAL LIFE

Lymphocyte recirculation is critical to maximize the efficiency of imm unological surveillance and is an absolute requirement for the develop ment of systemic memory, The consensus view of the lifespan of periphe ral T cells holds that naive T cells are long-lived cells and most mem ory T cells are short-lived cells, although the question of the lifesp an of peripheral T cells is not yet fully resolved, We have studied th e lifespan of T cells circulating in efferent lymph draining lymph nod es (LN) in the immunologically naive sheep fetus and in postnatal lamb s immediately following birth by examining the in vivo incorporation o f [H-3]thymidine by newly formed T cells during continuous administrat ion of [H-3]thymidine, We report that authentically naive fetal T cell s are long-lived cells which continue to recirculate between blood and lymph during fetal life, At birth, however, a process is triggered wh ereby fetal T cells circulating through LN are rapidly lost from the p eripheral T cell pool and are replaced by freshly arriving T cells whi ch 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 ce lls circulating through peripheral LN have been formed since birth.