Single-cell characterization of transcriptomic heterogeneity in human tonsillar B-cell subsets

BFL-1 is an understudied anti-apoptotic protein that is upregulated in melanoma, certain forms of leukemias and lymphomas, and Epstein-Barr virus (EBV)-immortalized lymphoblastoid cell lines (LCLs). We have previously shown that BFL-1 is upregulated in LCLs through a viral-mediated chromatin conformation. However, BFL-1 is also highly expressed in uninfected B cells undergoing maturation in the germinal center. We therefore sought to determine the non-viral mechanisms underlying BFL-1 upregulation in B cells. Here, we characterized the chromatin landscape of maturing B cell subsets found in human tonsillar lymphoid tissue. While chromatin accessibility at the BFL-1 locus increases as naïve B cells enter the germinal center reaction, we found that BFL-1 expression during the transition from dark zone to light zone correlates with a significant increase in three-dimensional chromatin association between upstream enhancer regions and the BFL-1 transcriptional start site (TSS). Interestingly, both LCLs and germinal center light zone B cells shared similar chromatin architectures at the BFL-1 TSS, suggesting a conserved mechanism underlying BFL-1 upregulation. Increased BFL-1 in LCLs also protects against FasL and TRAIL-activated extrinsic apoptosis. In addition to BFL-1 expression, LCLs and germinal center light zone B cells share similar patterns of gene expression, suggesting strong evidence that EBV infection phenocopies various aspects of the germinal center reaction. From this, we infer that EBV-infected B cells co-opts strategies from germinal center light zone B cells to upregulate BFL-1 and enhance their survival in establishing long-lived latent infection. Overall design: Characterization of two human tonsils for heterogeneity in individual cell RNA levels. The gene expression profiles were used to identify cell subsets and also then to compare differential gene expression between these subsets. The purpose in the context of this study was primarily to corroborate the relationship between BFL-1 expression and the transition from dark zone to light zone of the germinal center. Furthermore, these samples provide biological references for phenotypes that arise in de novo B cell infection.