Heme augments proliferation and epigenetic programming during B cell differentiation

Abstract Heme is an iron-containing porphyrin ring that is a key part of the cytochrome proteins that function in the electron transport chain and facilitate OXPHOS metabolism. Recently, our group found that many iron-metabolism related genes are highly expressed in B cells, particularly in memory B cells (MBC) in both mice and humans. In ex vivo cultures, naïve (nB) and MBC respond to heme addition by augmenting the frequency of plasmablast formation. In this work, we further characterized the role of heme in augmenting B cell differentiation and function. To begin addressing the question of how heme modulates B cell differentiation, proliferation, or both, we first conducted ex vivo cultures of CellTraceYellow (CTY)-labeled B cells stimulated with CD40, IL-4 and IL-5, treated with and without heme. We found that the first division occurs within 28 hours post-stimulation for nB cells and nB and MBC that were treated with heme divide earlier than those that were treated with vehicle.. When the number of cells in each division was analyzed, we found more cells in later divisions in heme-treated than in vehicle-treated ones, suggesting that heme is important for B cell proliferation. To further characterize the molecular effects of heme on B cell differentiation we preformed RNA-seq and ATAC-seq on activated B cells and plasmablasts. The addition of heme augmented both the accessible chromatin and gene expression of hundreds of genes leading to differences surrounding key metabolic and differentiation regulators. These data further our understanding for how heme modulates B cell differentiation and promotes the formation of antibody secreting plasmablasts.