  The adaptive immune system responds to pathogens by selecting clones of cells
with specific receptors. While clonal selection in response to particular
antigens has been studied in detail, it is unknown how a lifetime of exposures
to many antigens collectively shape the immune repertoire. Here, through
mathematical modeling and statistical analyses of T cell receptor sequencing
data we demonstrate that clonal expansions during a perinatal time window leave
a long-lasting imprint on the human T cell repertoire. We demonstrate how the
empirical scaling law relating the rank of the largest clones to their size can
emerge from clonal growth during repertoire formation. We statistically
identify early founded clones and find that they are indeed highly enriched
among the largest clones. This enrichment persists even after decades of human
aging, in a way that is quantitatively predicted by a model of fluctuating
clonal selection. Our work presents a quantitative theory of human T cell
dynamics compatible with the statistical laws of repertoire organization and
provides a mechanism for how early clonal dynamics imprint the hierarchy of T
cell clone sizes with implications for pathogen defense and autoimmunity.
