BCL6 is required for the thymic development of TCRaß+CD8aa+ intraepithelial lymphocyte lineage

TCRaß+CD8aa+ intraepithelial lymphocytes (CD8aa+ aß IELs), a specialized subset of T cells in the gut epithelium, develop from thymic agonist-selected IEL precursors (IELps). The molecular mechanisms underlying the selection and differentiation of this T cell type in the thymus are largely unknown. Here, we found that Bcl6 deficiency in aß T cells resulted in nearly the absence of CD8aa+ aß IELs. BCL6 was expressed by approximately 50% of CD8aa+ aß IELs but the majority thymic PD1+ IELps post agonist selection; its deficiency blocked early IELp generation in the thymus. Moreover, BCL6 expression in IELps was induced by thymic TCR signaling in an ERK-dependent manner. As a result of Bcl6 deficiency, the precursors of IELps among CD4+CD8+ double positive (DP) thymocytes exhibited increased apoptosis during agonist selection, and impaired IELp differentiation and maturation. Taken together, our results elucidate BCL6 as a crucial transcription factor during the thymic development of CD8aa+ aß IELs. Overall design: For in vitro samle: sorted CD4+CD8+ DP thymocytes and in vitro cultured with plate-bound a-CD3(8µg/ml)+a-CD28(8µg/ml) for 24h, then sorted CD4-CD8- DN cells for in vitro bulk RNAseq. For in vivo sample: sorted CD4-CD8-CD5+TCRb+ thymocytes (agonist-p) from Bcl6-wt and Bcl6-cko mice for RNAseq