RESEARCH SUMMARY Aged individuals, who are often at higher risk of fatality in life-threatening infectious diseases, do not form high- quality antibodies against new infections. Humoral immunity against infections depends on the germinal center (GC) differentiation process in the B cell follicles of lymph nodes. In GCs, naïve B cells rapidly proliferate in response to T cell-dependent antigens and somatically mutate into high-affinity antibody-secreting cells, i.e., plasma cells. B cells assume heterogeneous cell fates upon stimulation in young mice, with only a fraction differentiating into antibody-secreting cells (ASC). Notably, plasma cell differentiation is controlled by multiple cell division-coupled epigenetic programs. Chromatin accessibility changes correlate with gene expression and reveal the reprogramming of transcriptional networks and the genes they regulate at specific cell divisions. A subset of genes in naive B cells display accessible promoters in the absence of transcription and are marked by the histone modification H3 lysine 27 trimethylation (H3K27me3), a Polycomb protein Enhancer of zeste homolog 2 (EZH2) catalyzed repressive modification. Such genes encode regulators of cell division and metabolism and include the essential plasma cell transcription factor Blimp-1. Consequently, chemical inhibition of EZH2 results in enhanced plasma cell formation. A significant concern is that with aging, B cells exhibit a decreased expansion of B cells and GC reaction in response to antigen partly due to immune senescence and a defective follicular T helper cell (TFH) system. As a result, aged mice cannot generate sufficient GCs to provide insight into ASC fate and epigenomic remodeling of GC B cells, necessitating the development of a tissue-engineered model of an aged lymph node. Therefore, the long term goal of this R01 is to develop an ex vivo “aged B cell follicle” organoid technology capable of inducing early GC programming of aged B cells from both mice and humans and enabling the study of plasma cell fate and regulation of the epigenome of B cells to identify checkpoint targets that can be suppressed to boost GC response in aged B cells. The R01 brings together a multidisciplinary team of experts in GC organoids, vaccine, adjuvants, and lymphoid tissue engineering (Ankur Singh, PI, Georgia Tech) and GC immunology, ASC fate mapping and epigenomics (Jeremy Boss, Co-I, Emory Medicine), and microenvironment spatial omics (Ahmet Coskun, Co-I, Georgia Tech).