Role of interleukin 2 on thymic Foxp3+CD4+ regulatory T (Treg) cell chromatin modeling

Interleukine 2 (IL-2) is still one of the most interesting cytokines in T cell biology with its ability to control immune homeostasis by maintaining the functional identity of Foxp3+ regulatory T (Treg) cells and the expansion of activated conventional T (Tconv) cells. Yet, how IL-2 exactly enables Treg cells to suppress autoreactive Tconv cells and to maintain their identity is unclear. Using a mouse model in which IL-2 signaling via its high affinity receptor CD25 is selectively impaired, the “Il2ramut/mut” mouse, we report that Treg cells that only receive low IL-2 signals keep Il2ramut/mut but not WT Tconv cells “in check”, suggesting equal IL-2 signals in Treg and Tconv cells is essential to safeguard immune homeostasis. Furthermore, the comparative analysis of gene expression and epigenetic landscape of Il2ramut/mut and WT Treg cells support a model in which IL-2 "locks in” Treg cell identity and functions in vivo by controlling their genome-wide chromatin accessibility. Thymuses from ~20 weeks old aged-matched females either Il2ramut/mut (4 mice) or WT (4 mice) Foxp3-RFP+ reporter mice were harvested and RFP+ Treg cells were enriched by negative selection (dynabeads) and flow-sorted (Aria III) before proceeding to extraction of nuclei, transposition and library preparation for sequencing (ATAC-seq).

白细胞介素2（Interleukine 2, IL-2）仍是T细胞生物学中最具研究价值的细胞因子之一，其可通过维持叉头框P3阳性调节性T（Foxp3+ regulatory T, Treg）细胞的功能特性，以及促进活化常规T（conventional T, Tconv）细胞的扩增来调控免疫稳态。然而，IL-2究竟如何介导Treg细胞抑制自身反应性Tconv细胞并维持其细胞特性，目前仍未阐明。本研究利用可选择性破坏IL-2与其高亲和力受体CD25信号通路的"Il2ramut/mut"小鼠模型，研究发现仅接收低水平IL-2信号的Treg细胞可抑制Il2ramut/mut小鼠体内的Tconv细胞，但无法抑制野生型（Wild Type, WT）小鼠的Tconv细胞，这提示Treg细胞与Tconv细胞接收均等的IL-2信号，对维护免疫稳态至关重要。此外，对Il2ramut/mut与WT小鼠的Treg细胞进行基因表达与表观遗传景观的比较分析，支持如下模型：IL-2可通过调控Treg细胞全基因组范围内的染色质开放性，在体内"锁定"其细胞特性与功能。本研究收集了约20周龄、年龄匹配的雌性Il2ramut/mut（4只）与WT（4只）Foxp3-红色荧光蛋白（Red Fluorescent Protein, RFP）阳性报告基因小鼠的胸腺，通过负向分选（磁珠Dynabeads富集）与流式分选（Aria III型）获得RFP阳性的Treg细胞，随后进行细胞核提取、转座酶处理与文库构建，以开展转座酶可及性染色质测序（Assay for Transposase-Accessible Chromatin using sequencing, ATAC-seq）。