FoxN1-dependent thymic epithelial cells promote T-cell leukemia development.

T-cell acute lymphoblastic leukemia (T-ALL) and T-lymphoblastic lymphomas (T-LBL) are aggressive malignancies of thymocytes. The role of thymic microenvironmental cells and stromal factors in thymocyte malignant transformation and T-ALL development remains little explored. Here, using the TEL-JAK2 transgenic (TJ2-Tg) mouse model of T-ALL/LBL, which is driven by constitutive JAK/STAT signaling and characterized by the acquisition of Notch1 mutations, we sought to identify stromal cell alterations associated with thymic leukemogenesis. Immunofluorescence analyses showed that thymic lymphomas presented epithelial areas characterized by keratin (Krt) 5 and Krt8 expression, adjacently to epithelial-free areas negative for Krt expression. Both areas contained abundant laminin (extracellular matrix) and ER-TR7+ (fibroblasts) CD31+ (endothelial) and CD11c+ (dendritic) cells. Besides Krt5, Krt-positive areas harbored medullary thymic epithelial cells (TECs) labeled by Ulex europaeus agglutinin-1. By performing flow cytometry and RNA sequencing analyses of thymic lymphomas, we observed an enrichment in medullary TEC markers in detriment of cortical TEC markers. To assess whether TECs are important for T-ALL/LBL development, we generated TJ2-Tg mice heterozygous for the FoxN1 transcription factor nude null mutation (Foxn1+/nu). Strikingly, in TJ2-Tg;Foxn1+/nu compound mice, both emergence of malignant cells in preleukemic thymi and overt T-ALL onset were significantly delayed. Moreover, in transplantation assays, leukemic cell expansion within the thymus of recipient Foxn1+/nu mice was reduced as compared with control littermates. Since thymopoesis is largely normal in Foxn1+/nu mice, these results indicate that FoxN1 haploinsufficiency in TECs has a more profound impact in thymic leukemogenesis.