YTHDF2 suppresses the plasmablast genetic program and promotes germinal center formation

Antibody-mediated immunity is initiated by B cell interactions with cognate antigen, followed by differentiation into multiple cell subsets, including plasmablast, memory, and germinal center (GC) cells. B cell differentiation trajectories are determined by specific transcription factors, yet very few mechanisms that determine B cell fate at the early stage of the response have been described. Here, we report an epigenetic mechanism that specifically suppresses the plasmablast genetic program and promotes GC B cell fate commitment. Single-cell RNA-seq analysis of antigen-specific B cells revealed activated B cell precursors at the pre-GC stage that express high levels of RNA binding proteins, including YTHDF2, which enhances the decay of methylated transcripts. Ythdf2-deficient B cells exhibited intact proliferation and upregulation of early activation markers in response to antigenic stimulation, whereas differentiation into GC B cells was blocked. Mechanistically, B cells required YTHDF2 to attenuate the plasmablast genetic program during GC seeding, and key transcripts of plasmablast-related genes, including Irf4 and Xbp1, were methylated and bound by YTHDF2. Modulation of YTHDF2-dependent gene expression by YTHDF1 and YTHDF3 was less pronounced, and no functional redundancy of these paralogs in GC seeding was observed. Collectively, this study reveals a novel epigenetic mechanism that specifically directs appropriate B cell fate commitment through post-transcriptional suppression of gene expression. Bulk and single-cell RNAseq of WT/CD23cre and Ythdf2 cKO B18-hi B cells, five days after immunization