Insulin-like growth factor 2 (IGF2) as a driving force of exponential expansion in neonatal thymus

Like all organs, the thymus grows in size and function rapidly during development, which comes to a halt after birth. However, the molecular mechanisms behind such a transition in the thymus remain obscure. Using single-cell RNA sequencing (scRNA-seq) of the murine thymic stroma, we identified that major transcriptomic changes occur in the endothelium and mesenchyme across the transition to homeostasis. Differentially expressed gene and intercellular network analyses of temporally resolved scRNA-seq data revealed fibroblast-derived insulin-like growth factor 2 (IGF2) as a candidate driving neonatal thymic expansion. We demonstrated IGF2 activity promotes a cortical TEC-specific proliferation and is tightly regulated at the thymic growth transition. Bulk RNA-seq of human thymi across the transition also revealed IGF2 to drive thymic expansion, suggesting an evolutionarily conserved role. Our study highlights the role of a fibroblast-derived IGF2 in promoting cTEC proliferation, resulting in early thymic expansion that is followed by down-regulation to establish homeostasis. For our time-series single-cell RNA sequencing, we used 3 thymi per library. Our merged scRNAseq dataset contains 3-day-old (3 Foxn1+/+ males, 3 Foxn1lacZ/lacZ males, 2 Foxn1+/+ females, 2 Foxn1lacZ/lacZ females), 7-day-old (2 Foxn1+/+ males, 3 Foxn1lacZ/lacZ males), 14-day-old (2 Foxn1+/+ males, 2 Foxn1lacZ/lacZ males, 2 Foxn1+/+ females, 2 Foxn1lacZ/lacZ females). 30-day-old (2 Foxn1+/+ males, 3 Foxn1lacZ/lacZ males) datasets are available in a separate manuscript, Xiao*, Kang* et al. (2025). GSE264402.与所有器官类似，胸腺在发育过程中会经历尺寸与功能的快速增长，该进程于出生后终止。然而，胸腺这一发育转变背后的分子机制仍不甚明确。本研究通过对小鼠胸腺基质开展单细胞RNA测序（single-cell RNA sequencing, scRNA-seq），发现在向稳态过渡的过程中，内皮细胞与间充质细胞发生了显著的转录组变化。通过对时序性scRNA-seq数据进行差异表达基因与细胞间互作网络分析，本研究鉴定出成纤维细胞来源的胰岛素样生长因子2（insulin-like growth factor 2, IGF2）是驱动新生胸腺扩张的候选调控因子。本研究证实，IGF2可促进皮质胸腺上皮细胞（cortical thymic epithelial cell, cTEC）的特异性增殖，并在胸腺生长转变过程中受到严格调控。对不同发育阶段的人胸腺进行批量RNA测序（bulk RNA-seq）分析，同样证实IGF2可驱动胸腺扩张，提示该调控作用在进化过程中具有保守性。本研究阐明了成纤维细胞来源的IGF2在促进cTEC增殖中的关键作用：该因子介导早期胸腺扩张，随后其表达被下调以建立胸腺稳态。本研究的时序性单细胞RNA测序实验中，每个文库使用3个胸腺样本。整合后的scRNA-seq数据集包含：3日龄样本（3只Foxn1+/+雄性小鼠、3只Foxn1lacZ/lacZ雄性小鼠、2只Foxn1+/+雌性小鼠、2只Foxn1lacZ/lacZ雌性小鼠）、7日龄样本（2只Foxn1+/+雄性小鼠、3只Foxn1lacZ/lacZ雄性小鼠）、14日龄样本（2只Foxn1+/+雄性小鼠、2只Foxn1lacZ/lacZ雄性小鼠、2只Foxn1+/+雌性小鼠、2只Foxn1lacZ/lacZ雌性小鼠）。30日龄样本数据集（2只Foxn1+/+雄性小鼠、3只Foxn1lacZ/lacZ雄性小鼠）已在Xiao*、Kang*等人2025年发表的另一篇论文中公开，登录号为GSE264402。