OCT2 pre-positioning facilitates cell fate transition and chromatin architecture changes in humoral immunity

During the germinal center (GC) reaction, B cells undergo profound transcriptional, epigenetic and genomic architectural changes. How such changes are established remains unknown. Mapping chromatin accessibility during the humoral immune response, we show that OCT2 was the dominant transcription factor linked to differential accessibility of GC regulatory elements. Silent chromatin regions destined to become GC-specific super-enhancers (SEs) contained pre-positioned OCT2-binding sites in naive B cells (NBs). These preloaded SE 'seeds' featured spatial clustering of regulatory elements enriched in OCT2 DNA-binding motifs that became heavily loaded with OCT2 and its GC-specific coactivator OCAB in GC B cells (GCBs). SEs with high abundance of pre-positioned OCT2 binding preferentially formed long-range chromatin contacts in GCs, to support expression of GC-specifying factors. Gain in accessibility and architectural interactivity of these regions were dependent on recruitment of OCAB. Pre-positioning key regulators at SEs may represent a broadly used strategy for facilitating rapid cell fate transitions. Elemento, Melnick and colleagues examine the chromatin and transcriptional changes that occur during differentiation of human primary B cells into antibody-secreting cells. In naive B cells, the transcription factor OCT2 is preloaded at high-affinity super-enhancer sites present in repressed 'silent' chromatin; upon activation, OCAB is recruited to these regions, where it facilitates arrays of OCT2 binding to lower-affinity octamer motifs, leading to active formation of germinal center B cell-specific super-enhancers.

Automated summary

The study reveals that OCT2 plays a key role in regulating the accessibility of GC regulatory elements during the humoral immune response, with pre-positioned key regulators at SEs potentially representing a widely used strategy for facilitating rapid cell fate transitions.