The role of BRWD1 to establish epigenetic states for germinal center initiation, maintenance, and exit

PROJECT SUMMARY Germinal centers (GCs) drive adaptive humoral immunity by selecting for B cells with high affinity antibodies and producing memory B cells and plasma cells. We previously demonstrated that GC B cells can be subdivided into three subpopulations that are functionally, spatially, transcriptionally, and epigenetically distinct. Thus, we asked whether these unique epigenetic states are important for GC B cell function and differentiation. Our lab has also previously characterized how the epigenetic reader BRWD1 regulates epigenetic states and enhancer accessibility during the transition from large pre-B cells to small pre-B cells during B cell lymphopoiesis. Because BRWD1 is highly expressed in GC B cells, we hypothesized that BRWD1 regulates chromatin accessibility in GC B cells and is important for peripheral B cell differentiation. To study this, we generated Brwd1floxed mice to delete Brwd1 in different B cell populations. First, deletion of Brwd1 in follicular B cells inhibited GC responses with fewer GC B cells and smaller GCs observed by microscopy. Second, deletion of Brwd1 in GC B cells promoted proliferation of GC B cells and blocked differentiation of Brwd1-/- GC B cells into memory B cells without disrupting GC architecture. Furthermore, loss of Brwd1 caused an epigenetic collapse whereby differential chromatin accessibility between GC B cell subpopulations was lost. From this preliminary data, we propose a model where BRWD1 regulates chromatin accessibility at enhancers and transcription factor binding motifs in a manner critical for the cell fate decisions of peripheral B cells. To test this model, first we will further study how BRWD1 is important for B cell function and differentiation. In follicular B cells, we will characterize whether BRWD1 is necessary for either the initial differentiation or early expansion and proliferation of GC B cells (Aim 1). In GC B cells, we will study whether BRWD1 is important for somatic hypermutation and affinity maturation (Aim 2A). Furthermore, we will determine whether BRWD1 is necessary for development of pre-memory B cells or memory B cell subsets (Aim 2B). Finally, we will characterize how BRWD1 establishes epigenetic states between GC B cell subpopulations (Aim 3). We will characterize how BRWD1 binds at different sites and how the histone marks that recruit BRWD1 change between GC zones. We will identify active and repressed enhancers alongside our chromatin accessibility data to study how BRWD1 coordinates changes in enhancer accessibility between GC B cell subpopulations. Completion of this proposed project will provide insight into fundamental mechanisms central to the humoral immune response, autoimmunity, and lymphoma development within the GC.