T-cell development is accompanied by epigenetic changes that ensure the silencing of stem cell-related genes and the activation of lymphocyte-specific programmes. How transcription factors influence these changes remains unclear. We show that the Ikaros transcription factor forms a complex with Polycomb repressive complex 2 (PRC2) in CD4−CD8− thymocytes and allows its binding to more than 500 developmentally regulated loci, including those normally activated in haematopoietic stem cells and others induced by the Notch pathway. Loss of Ikaros in CD4−CD8− cells leads to reduced histone H3 lysine 27 trimethylation and ectopic gene expression. Furthermore, Ikaros binding triggers PRC2 recruitment and Ikaros interacts with PRC2 independently of the nucleosome remodelling and deacetylation complex. Our results identify Ikaros as a fundamental regulator of PRC2 function in developing T cells.

The Ikaros transcription factor is a tumor suppressor that is also important for lymphocyte development. How post-translational modifications influence Ikaros function remains partially understood. We show that Ikaros undergoes sumoylation in developing T cells that correspond to mono-, bi- or poly-sumoylation by SUMO1 and/or SUMO2/3 on three lysine residues (K58, K240 and K425). Sumoylation occurs in the nucleus and requires DNA binding by Ikaros. Sumoylated Ikaros is less effective than unsumoylated forms at inhibiting the expansion of murine leukemic cells, and Ikaros sumoylation is abundant in human B-cell acute lymphoblastic leukemic cells, but not in healthy peripheral blood leukocytes. Our results suggest that sumoylation may be important in modulating the tumor suppressor function of Ikaros.

Murine peripheral lymph node TCR gd T cells have been divided into type 1 and type 17 functional categories based on phenotypic and functional markers. Localized in the gut epithelial barrier, intestinal intraepithelial lymphocytes (iIEL) gd T cells constitute a peculiar subset of T lymphocytes involved in intestinal homeostasis. However, whether iIEL gd T cells obey the type 1/type 17 dichotomy is unclear. Using both global transcriptional signatures and expression of cell surface markers, we reveal that murine iIEL gd T cells compose a distinct population, expressing ~1000 specific genes, in particular genes that are responsible for cytotoxicity and regulatory functions. The expression of the transcription factor Helios is a feature of iIEL gd T cells, distinguishing them from the other TCR gd T subsets, including those present in the epithelia of other tissues. The marked expression of Helios is also shared by the other iIELs, TCRabCD8aa lymphocytes present within the intestinal epithelium. Finally, we show that Helios expression depends in part on TGF-b signaling but not on the microbiota. Thus, our study proposes iIEL gd T cells as a distinct subset and identifies novel markers to differentiate them from their peripheral counterparts. ImmunoHorizons, 2022, 6: 515-527.