T cell help awakens embryonic stem-like programs in germinal center B-cells

True stem-like phenotypic plasticity is gradually lost during cellular differentiation due to the strengthening of epigenetic barriers. Here, we describe an unprecedented physiological gain of stem-like plasticity in germinal center B cells, that we termed “anaplasis”. Anaplasis was strictly dependent on help from T follicular helper cells and restricted to germinal center (GC) but not activated B cells, indicating both non- and cell-autonomous contributions to this phenotype. Anaplasis involved the transient reactivation of embryonic stem cell programs by T cell-responsive germinal center B cell transcription factors, suggesting an immunological functional checkpoint, that we show can be hijacked and even bypassed by specific lymphoma mutations. Anaplasis signatures were associated with unfavorable outcomes in B cell lymphomas, suggesting their contribution to tumor fitness. Altogether, we document a physiological re-acquisition of stem-like plasticity in germinal center B cells, which co-evolved with non cell-autonomous checkpoints linked to immune selection processes. 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 Naïve (B220+ IgD+) and GC (B220+ GL7+ Fas+) B markers pooled from 2 individual male mice, before processing for scRNA-seq using Chromium Next GEM Single Cell 3' Reagent Kits v2 (10X Genomics #N-120267) according to the provider specifications. cDNA Indexed libraries were quantified using Qubit and QC using Agilent Bioanalyzer 2100. Libraries were diluted to 2 nM and clustered on an Illumina Hiseq4000. Additionally, GC and post-GC (YFP+ IgD-) from 3 individual mice were isolated similarly before processing for Single Cell 5' gene expression + B Cell Repertoire.