OBF1 and Oct factors control the germinal center transcriptional program (ChIP-seq, CUT&RUN)

OBF1 is a specific coactivator of the POU family transcription factors OCT1 and OCT2. OBF1 and OCT2 are B cell-specific and indispensable for germinal center (GC) formation, but their mechanism of action is unclear. Here, we show by ChIP-seq that OBF1 extensively colocalizes with OCT1 and OCT2 and stabilizes their binding. We found that these factors also often colocalize with transcription factors of the ETS family, such as PU.1 and ETS1. Furthermore, we showed that OBF1, OCT2 and OCT1 bind widely to the promoters or enhancers of genes involved in GC formation, such as the master regulators Bcl6 and Foxo1. shRNA knockdown experiments demonstrated that OCT1, OCT2 and OBF1 are essential for proliferation of GC-derived lymphoma cell lines. OBF1 downregulation disrupts the GC transcriptional program: genes involved in GC maintenance -such as BCL6- are downregulated, while genes related to exit from the GC program are upregulated. Ectopic expression of BCL6 does not restore the proliferation of GC-derived Raji cells depleted of OBF1 unless IRF4 is also depleted, indicating that OBF1 controls a critical regulatory node in GC differentiation. mapping OBF1 and H3K27ac in GC-drived B lymphoma cell lines, primray murine and human germinal center B cells

OBF1是POU家族转录因子OCT1与OCT2的特异性辅激活因子。OBF1与OCT2均为B细胞特异性蛋白，且对于生发中心（germinal center, GC）的形成不可或缺，但其具体作用机制尚未明确。本研究通过染色质免疫沉淀测序（ChIP-seq）证实，OBF1可与OCT1、OCT2广泛共定位，并稳定二者的结合。研究发现，上述转录因子还常与ETS家族转录因子（如PU.1、ETS1）共定位。此外，本研究表明OBF1、OCT2与OCT1可广泛结合于参与生发中心形成的基因启动子或增强子区域，例如核心调控因子Bcl6与Foxo1。短发夹RNA（shRNA）敲低实验证实，OCT1、OCT2与OBF1对于生发中心来源的淋巴瘤细胞系的增殖至关重要。OBF1表达下调会破坏生发中心的转录程序：维持生发中心功能的基因（如BCL6）表达下调，而与脱离生发中心程序相关的基因表达上调。即使异位过表达BCL6，也无法恢复OBF1敲低的生发中心来源Raji细胞的增殖能力，除非同时敲低IRF4，这表明OBF1调控了生发中心分化过程中的关键调控节点。本研究在生发中心来源的B淋巴瘤细胞系、原代小鼠及人类生发中心B细胞中完成了OBF1与H3K27ac的定位分析。