BCL6 is required for the thymic development of TCRαβ+CD8αα+ intraepithelial lymphocyte lineage.

TCRαβ⁺CD8αα⁺ intraepithelial lymphocytes (CD8αα⁺ αβ IELs) are a specialized subset of T cells in the gut epithelium that 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 αβ T cells resulted in the near absence of CD8αα⁺ αβ IELs. BCL6 was expressed by approximately 50% of CD8αα⁺ αβ IELs and by the majority of thymic PD1⁺ IELps after agonist selection. Bcl6 deficiency blocked early IELp generation in the thymus, and its 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 thymocytes exhibited increased apoptosis during agonist selection and impaired IELp differentiation and maturation. Together, our results elucidate BCL6 as a crucial transcription factor during the thymic development of CD8αα⁺ αβ IELs. Editor's summary: Intraepithelial lymphocytes (IELs) reside within the intestinal epithelium, where they regulate immune tolerance. Unconventional CD8αα⁺ αβ IELs are selected from thymic IEL precursors (IELps) after strong TCR signaling; however, the transcription factors governing this process of agonist selection are not well defined. Xing et al. found that mice lacking the transcription factor BCL6 in αβ T cells almost did not develop CD8αα⁺ αβ IELs. Thymic IELps up-regulated BCL6 after TCR stimulation, and BCL6 expression was required for their survival, differentiation, and maturation. These findings identify that BCL6 is required for the development of unconventional CD8αα⁺ αβ IELs during agonist selection. —Hannah Isles [ABSTRACT FROM AUTHOR]