22q11.2 Deletion Syndrome Causes a Thymus Hypoplasia Corrected by Mesenchymal Cell Replacement

Thymus hypoplasia occurs in several clinical conditions including 22q11.2 deletion syndrome (22q11.2DS). 22q11.2DS is the most common human microdeletion disorder, affecting 1/4000. Thymuses from 60–70% 22q11.2DS patients are small and produce fewer T cells than normal. For individuals with a severe hypoplasia, an allogenic thymus tissue graft is needed to restore thymopoiesis. To determine the molecular mechanisms contributing to a small thymus in 22q11.2DS, we compared the development of the thymus in embryos from various 22q11.DS mouse models, normal controls and Foxn1 mutant mice. Reaggregate fetal thymic organ culture assays reveal that replacing mesenchymal cells from 22q11.2del hypoplastic lobes with normal ones restores tissue expansion and thymopoiesis. Thymic epithelial cells used as substitutes cannot. This is distinct from the Foxn1 mutant mice, wherein defective thymic epithelial cell functions lead to thymus hypoplasia/aplasia. Single cell RNA sequencing of normal and hypoplastic thymus lobes revealed differential expression of transcripts that primarily impacted 5 distinct mesenchymal cell subsets in 22q11.2DS. These transcripts are involved in cell-cell interactions, collagen deposition and growth. Elevated levels of collagen are present in the hypoplastic thymus tissues, suggesting a structural restriction. Mesenchymal and epithelial cell differentiation/expansion assays reveal a selective reduction in mesenchymal tissue expansion due to 22q11.2DS Supported by grants from the National Institutes of Health (R01AI114523, R21AI144140) and the Jeffrey Modell Foundation (MdlM, CAW)