Myc transcriptional bursts promote B cell positive selection by T cells in germinal centers
Antibody affinity maturation occurs in secondary lymphoid organs within germinal centers (GCs) where B cells mutate their antibody-encoding genes in the dark zone (DZ) followed by a selection of the high-affinity variants in the light zone (LZ) by T cells. The amount of the T cell-derived selection signals is proportional to the BCR affinity and the magnitude of Myc expression. However, since the lifetimes of Myc mRNA and protein are extremely short, it is unclear how high levels of Myc are maintained in the GC B cell to allow positive selection. Here, by measuring Myc transcripts in situ at the single-cell level, we find that T cell help promotes the frequency of Myc transcriptional bursts. T cell help primarily increases the number of Myc-positive cells rather than the amount of Myc per cell. Measurement of Myc intronic and exonic transcripts in situ demonstrates that positive selection is mediated by Myc transcriptional bursts through a rapid increase in the active transcription sites rather than an elevation in transcription rate or gene accessibility. Thus, the amount of Myc transcriptional bursts over time in the LZ, rather than Myc abundance, dictates B cell selection in germinal centers. Overall design: RNA-seq of DZ and LZ GC B cells, after  in vivo anti CD40 treatment or isotype control