Instructor Univ. of Texas Southwestern Med. Ctr. Dallas, Texas, United States
Disclosure(s):
Pratibha Bhalla, PhD: No financial relationships to disclose
Introduction/Rationale: Thymus organogenesis depends on coordinated communication between stromal and hematopoietic cells. We found that thymic mesenchyme contains a specialized stromal subset (CD45⁻Ter119⁻PDGFRA⁺PDGFRB⁺CD90⁺) distinct from mesenchyme of the pharyngeal, submandibular, tongue, and lung regions, which uniquely signals to thymic epithelial cells.
Methods: We compared stromal competence across organs using flow cytometry, single-cell transcriptomics, and recombinant reaggregate thymic organoid culture (ReRTOC). Pharmacologic reprogramming was tested in Tbx1neo2/neo2 embryos (~36% normal Tbx1), a model of 22q11.2 deletion syndrome, using the FDA-approved drug Minoxidil.
Results: ReRTOC assays showed that pharyngeal and submandibular mesenchyme retain partial ability (~60% thymic growth) to support epithelial expansion and thymopoiesis, revealing inherent plasticity. In Tbx1neo2/neo2 mutants, mesenchyme exhibited a shift from Bmp4 to Bmp5–Sox9 signaling, driving chondrogenic differentiation (Col2a1, Col9a1, Col11a1), loss of stromal organization, and reduced vascular branching with a lower SMA/Cx40 ratio. Minoxidil restored Bmp4 activity, suppressed Sox9/Bmp5 expression, and normalized thymic size and vascular complexity. CellChat analysis of wild-type thymic stroma revealed perivascular mesenchymal subsets acting as dominant signaling hubs, communicating with endothelial cells through the LAMININ, SPP1, BSP, and WNT pathways, which maintain basement membrane stability and endothelial anchoring.
Conclusion: Our results demonstrate that mesenchymal–endothelial communication is essential for thymic vascular patterning and that targeted mesenchymal reprogramming—through pharmacological or growth factor cues—can restore thymic vascularization and function, even during aging.
