Analysis of Human Thymopoiesis Reveals Rapid Postnatal Remodeling of Thymocytes Within the First 3 Months of Life.

The thymus generates a diverse and self-tolerant T-cell repertoire. Its development and output undergo profound changes from fetal to early postnatal life, yet the dynamics of this transition remain incompletely understood. Here, we performed a comprehensive analysis of human thymopoiesis across the perinatal (birth to <10 days), early postnatal (10 days-3 months), and homeostasis (>3 months) periods using high-dimensional flow cytometry and signal joint T-cell receptor excision circle (sjTREC) quantification. Within the perinatal phase, the thymus exhibits the highest intrathymic production, with elevated frequencies of CD4+ T cells and unconventional T cell (UTC), including regulatory T cells (Tregs). After 10 days of age, the frequency of double-positive (DP) thymocytes increases and is associated with dynamic thymocyte phenotypic shifts that suggest enhanced progenitor commitment and proliferation, accompanied by reduced T-cell receptor (TCR) signaling strength. Early postnatal UTCs (birth to 3 months) display a distinct phenotype compared with those generated after 3 months. We notably identified a postnatal wave of effector Vδ2+ γδ T cells emerging between 10 days and 3 months of age. These findings suggest a transition in human thymopoiesis from an early effector-prone unconventional output to predominantly conventional T-cell production after 3 months, highlighting a critical window in immune system establishment.