Epigenetic regulation of early B cell differentiation - Humoral immunity provides protection from pathogenic viral, bacterial, and parasitic infections and is mediated by antibodies that are produced following the differentiation of B cells into antibody secreting cells (ASC). Both T cell independent and dependent processes contribute to the formation of ASC, as well as memory B cells (MBC); each of which can provide lifelong immunity. Although major transcription factor networks and gene expression changes that occur when B cells differentiate to ASC have been described, the transcriptional and epigenetic mechanisms by which B cells adopt an early heritable cell fate program that determines ASC fate versus one of memory and how initial and recall immune responses are programmed is still largely unknown. As evidenced by the absolute need to create vaccines and other immune-based therapies to treat emerging diseases, such as that caused by SARS-CoV2, it is essential that we have a full understanding of the pathways that control the cell fate choices made by B cells. When stimulated in vivo, mouse naïve B cells undergo eight cellular divisions as they differentiate to ASC. Each of these divisions is associated with a unique epigenetic program that orchestrates a distinct transcriptional program providing the structure and machinery to regulate proliferation, metabolism, stress responses, and pathways that are necessary to form an ASC. We recently observed that instead of a single pathway arising from in vivo B cell activation, two branches emerged during early cell divisions. One leads to ASC; whereas the other (termed non-ASC branch) expresses pre-MBC genes, suggesting that it is the differentiation path to an MBC. Each cell fate path or branch express a unique set of transcription factors that influence how a B cell will respond to stimulation. Aim 1 will determine which of these differentially expressed factors contribute to this early decision and determine their regulatory targets that drive cell fate choice. Aim 1 will also seek to understand the roles played by three epigenetic modifiers (EZH2, UTX and LSD1) that alter ASC formation and determine how their absence alters B cell fate choices. Little is known about the cell intrinsic mechanisms (epigenetic and transcriptional programming) that allow MBC to respond to antigen more quickly. Our data show that MBC have a unique chromatin accessibility profile that suggests that they can respond more quickly because they are epigenetically preprogrammed to differentiate. Thus, Aim 2 will determine: how memory B cells respond more quickly to antigen rechallenge, rely on the above epigenetic modifiers to form ASC, and whether they follow the same fate determining choices as naïve cells. This will be tested in vivo using a series of reporter mice and Cre recombinase-drivers, conditional knockouts for the above epigenetic modifiers, and epigenomic/transcriptomic assays. Results from our study will provide a molecular understanding of how B cells respond to antigen and differentiate through multiple pathways leading to immunity and will provide new clues to how to better develop immune therapeutics and vaccines.
