  The diversity of T-cell receptors recognizing foreign pathogens is generated
through a highly stochastic recombination process, making the independent
production of the same sequence rare. Yet unrelated individuals do share
receptors, which together constitute a "public" repertoire of abundant
clonotypes. The TCR repertoire is initially formed prenatally, when the enzyme
inserting random nucleotides is downregulated, producing a limited diversity
subset. By statistically analyzing deep sequencing T-cell repertoire data from
twins, unrelated individuals of various ages, and cord blood, we show that
T-cell clones generated before birth persist and maintain high abundances in
adult organisms for decades, slowly decaying with age. Our results suggest that
large, low-diversity public clones are created during pregnancy, and survive
over long periods, providing the basis of the public repertoire.
