T cell help transiently unlocks a high plasticity state in germinal center B cells during the humoral immune response [Foxo1_RNA-seq]

Germinal Center (GC) B cellsconstitutekey effectorsofthehumoral immune responseandundergoiterativerounds ofproliferation, somatic hypermutationof their immunoglobulin genesand eventuallycompete for T follicular helper (TFH) cellshelpin order togenerate cells with high affinity B cell receptor (BCR).While selected cells passing the critical TFHfitness checkpointcan differentiate toplasma cells,memory B cellsor go through additional rounds of proliferation and mutagenesis, the mechanisms enabling fully differentiated GCs to re-acquire plasticity–usually lost upon cell fate specification–arestill largely unknown.Herein,we report that matureGCsundergo a physiologicaltransientstate characterized byanincreased functional plasticity andinduced pluripotent stem cell (iPSC)reprogramming capacity.This transition state isreminiscent of anaplasisdue to theconcomitantpartial activation of embryonic stem cellandattenuation of the B cell programs.We furthermore demonstrate thatGC anaplasisis dependent on TFHcells sincein vivointerference of TFH:GC B cells communication or lymphoma driver mutationsaffectingGCsresponse to TFHcell help were shown to modulate their reprogramming efficiency accordingly. Importantly, GC stem-like gene signatures, as revealed by single-cell Multiome analysis of thistransient anaplastic state, were often enriched inDiffuse Large B cell lymphoma (DLBCL)patients and associate with poor outcomes, suggesting that this rare preexisting plastic state might be hijacked and further enhancefitnessduring lymphomagenesis. Overall design: Splenic B cells from mice immunized with SRBC for 9 days were pre-enriched for B220pos cells by negative magnetic enrichment and stained for naive B cell markers before flow sorting. Naive B cells were sorted from 10 individual 3-month-old male mice from 2 independent experiments and pooled before culture with mIL4 and aCD40 for 5 days. On day 3, cells were either treated with DMSO or with 1uM of the Foxo1 inhibitor (Cpd10) for 48h. 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