IF autologous iPSC-derived Plet1+/FOXN1+ thymic epithelial progenitors (iPSC-TEPs), generated via directed third pharyngeal pouch differentiation and subsequently pre-conditioned ex vivo with recombinant RANKL (200 ng/mL, 72 hours) prior to intrathymic injection (1×10⁶ cells per lobe), are administered to aged (20–24 month) C57BL/6J mice of both sexes,

THEN a significant restoration of the medullary TEC compartment — specifically a ≥2-fold increase in AIRE+/MHCII+ mTEC numbers, measurable by flow cytometry and immunohistochemistry, accompanied by increased polyclonal naïve CD4+ and CD8+ T cell output (measured by BrdU or Ki67 in peripheral blood) and restoration of TCR Vβ repertoire breadth (measured by spectratyping or single-cell TCR sequencing) — will be observed within 8 weeks of transplantation,

BECAUSE the causal chain is as follows:

1. Age-associated thymic involution results in an irreversible depletion of the AIRE+/MHCII+ medullary TEC pool, with loss of central tolerance enforcement capacity that cannot be corrected by FOXN1 upregulation alone, as demonstrated by the finding that K5.Foxn1 transgenic overexpression expands Plet1+ TEC progenitors and maintains cortico-medullary organization but fails to prevent aging-related involution or rescue thymus size in hypomorphic Foxn1 mice. (Foxn1 overexpression promotes TEC progenitor proliferation and mTEC maintenance but does not prevent thymic involution)[https://pubmed.ncbi.nlm.nih.gov/36975725/]

2. The critical mechanistic bottleneck identified by Li et al. is a context-dependent, separable dissociation between FOXN1's roles in proliferation versus differentiation: FOXN1 drives Plet1+ progenitor expansion but does not intrinsically resolve the transition to terminally differentiated AIRE+ mTECs without additional niche-derived instructive signals. (FOXN1 functions in promoting TEC proliferation and differentiation are separable and context dependent)[https://pubmed.ncbi.nlm.nih.gov/36975725/]

3. The missing instructive signal for Plet1+ → AIRE+ mTEC fate resolution is RANKL-RANK signaling, which is canonically required for mTEC maturation and AIRE induction downstream of FOXN1 transcriptional activity. Ex vivo pre-conditioning of iPSC-TEPs with RANKL before transplantation is predicted to epigenetically prime the RANK signaling axis, lowering the threshold for AIRE locus activation upon engraftment into the aged thymic niche. [SPECULATIVE — direct RANKL pre-conditioning of iPSC-TEPs has not been formally tested; rationale is drawn from canonical mTEC developmental biology]

4. Human iPSC-derived thymic epithelial progenitors generated via directed differentiation protocols express functional TEC markers (EpCAM, FOXN1, cytokeratins) and support thymopoiesis in co-culture, demonstrating that iPSC-to-TEP conversion is a feasible and scalable starting material for this strategy. (Generation of functional human thymic cells from induced pluripotent stem cells)[https://doi.org/10.1016/j...

SENS category: RepleniSENS

Key references: • doi.org/10.1016/j.jaci.2021.07.021] • doi.org/10.1172/jci.insight.140313] • doi.org/10.4049/jimmunol.1403158] • doi.org/10.1172/jci.insight.140313],