Konnikova, L.; Strine, M. S.; Kalida, G.; Phan, L.; Gu, W.; Wang, W.; Yuan, H.; Llivichuzhca, D.; St Denis, K.; Gonzalez Santiago, E.; Liu, J.; Zeve, D.; Breault, D.; Tseng, G.

Although early-life immunity was once considered immature, the human fetal immune system is dynamic and compartmentalized by the second trimester. By 21 weeks of gestation, T lymphocytes become a major immune population in the fetal small intestine (SI), yet their functional roles within this tissue remain largely undefined. To explore their unique contributions to intestinal development, we established an ex vivo co-culture system in which mucosal T cells isolated from fetal, neonatal, or adult SI donors were cultured with tissue-derived 3D SI organoids derived from various ages. Homeostatic early-life (fetal and neonatal) SI T cells uniquely promoted organoid generation, a metric of stem cell renewal, by upregulating cell cycle-associated gene programs. These early-life T cells also directed intestinal stem cell differentiation toward the secretory lineage in both growth and differentiation phase, highlighting that T cells poise stem cells to adopt secretory fates. T cells from infants with necrotizing enterocolitis (NEC), an inflammatory intestinal disease affecting predominantly preterm infants, failed to activate these same programs, suggesting a pathologic role for T cells in NEC. T cells from the adult SI similarly failed to support organoid growth or differentiation, revealing developmentally specialized, nonimmune functions for early-life T cells in the intestine. Similarly, T cells derived from cord blood did not enhance organoid generation, indicating that this function is not necessarily a generalized feature of early-life T cells but rather is restricted to mucosal T cells. Organoids derived from adults or NEC, however, could re-enter regenerative states when co-cultured with fetal T cells, indicating that fetal T cells can restore stem cell self-renewal across developmentally and disease-imposed states. We further identified that T cell-derived soluble factors alone were insufficient to modulate intestinal stem cell fate, implying the need for physical interactions. Concordant with this finding, we report that T cells heavily localize to the stem cell niche during prenatal development, where they express factors involved in Notch, Wnt, and growth factor signaling to support fetal stem cell function. Collectively, these findings reveal a coordinated developmental program in which fetal SI T cells balance stem cell self-renewal and differentiation, identifying a developmental immune-epithelial axis that can be harnessed to restore intestinal regeneration.