T cell help transiently unlocks a high plasticity state in germinal center B cells during the humoral immune response [ActB_RNA-seq]

During cellular differentiation, phenotypic plasticity is gradually lost due to the strengthening of epigenetic barriers, although it may be re-acquired upon disease or injury. Here, we report an unprecedented physiological gain of epigenetic plasticity among mature B cells during the humoral immune response. Acquisition of plasticity was strictly dependent on T follicular helper (TFH) cells and restricted to germinal center (GC) but not activated B cells, indicating both non- and cell-autonomous contributions to this phenotype. Importantly, GC plasticity was not dependent on proliferation and could not be solely explained by the activation of MYC programs. Instead, GC plasticity was associated with the reciprocal weakening of B cell identity and the de- repression of progenitor and stem cell-like programs induced by NF?B and other signals, downstream of TFH signaling. Therefore, physiological acquisition of GC B cell plasticity is tightly controlled by the affinity maturation positive selection checkpoint. Notably, Histone 1 loss of function, which results in aberrant stem cell gene reactivation through chromatin decompaction, allowed GC B cells to bypass this gatekeeping mechanism. Finally, patients with Diffuse LargeB Cell Lymphoma (DLBCL) that enriched for GC plasticity signatures had significantly worse outcomes, implicating a role for this mechanism in immune regulation and fitness and potential relevance to lymphomagenesis. Overall design: Splenic B cells from mice immunized with SRBC for 9 days were pre-enriched for B220pos cells by negative magnetic enrichment. Naive and GC B cells were sorted from 3 x 2 individual 3-month-old male mice before culture with mIL4 and aCD40,CpG or LPS for 5 days. Cells were then collected for RNA extraction using the RNAeasy Qiagen kit according to the provider specifications. RNAs were QC using Agilent Bioanalyzer 2100 and Libraries were prepared using the NEB Ultra II Directional RNA Library Prep (plus Poly A isolation module), and clustered on an Illumina NovaSeq 6000