ICOS coreceptor signaling inactivates the transcription factor FOXO1 to promote Tfh cell differentiation

T follicular helper (Tfh) cells are essential in the induction of high-affinity, class-switched antibodies. The differentiation of Tfh cells is a multi-step process that depends upon the co-receptor ICOS and the activation of phosphoinositide-3 kinase leading to the expression of key Tfh cell genes. We report that ICOS signaling inactivates the transcription factor FOXO1, and a Foxo1 genetic deletion allowed for generation of Tfh cells with reduced dependence on ICOS ligand. Conversely, enforced nuclear localization of FOXO1 inhibited Tfh cell development even though ICOS was overexpressed. FOXO1 regulated Tfh cell differentiation through a broad program of gene expression exemplified by its negative regulation of Bcl6. Final differentiation to germinal center Tfh cells (GC-Tfh) was instead FOXO1 dependent as the Foxo1(-/-) GC-Tfh cell population was substantially reduced. We propose that ICOS signaling transiently inactivates FOXO1 to initiate a Tfh cell contingency that is completed in a FOXO1-dependent manner. Overall design: Performed ChIP-seq analysis to examine the role of foxo1 in the development of Tfh cells

滤泡辅助性T细胞（T follicular helper, Tfh）在高亲和力、类别转换抗体的诱导过程中发挥关键作用。Tfh细胞的分化是一个多步骤过程，依赖于共受体ICOS以及磷脂酰肌醇3-激酶（phosphoinositide-3 kinase）的激活，该激活事件可介导关键Tfh细胞基因的表达。本研究发现，ICOS信号可使转录因子FOXO1失活；而Foxo1基因敲除可使Tfh细胞的生成对ICOS配体的依赖程度显著降低。与之相反，即使ICOS过表达，强制FOXO1核定位仍会抑制Tfh细胞的发育。FOXO1通过广泛的基因表达程序调控Tfh细胞分化，其中以其对Bcl6的负调控为典型特征。而生发中心滤泡辅助性T细胞（germinal center Tfh, GC-Tfh）的最终分化则依赖于FOXO1：Foxo1敲除小鼠的GC-Tfh细胞群体大幅减少。本研究提出假说：ICOS信号可瞬时失活FOXO1以启动Tfh细胞分化程序，该程序的最终完成则依赖于FOXO1。总体实验设计：通过染色质免疫共沉淀测序（Chromatin Immunoprecipitation sequencing, ChIP-seq）分析foxo1在Tfh细胞发育中的调控作用。