PROJECT ABSTRACT Genome-wide association studies (GWAS) have identified immune pathways linked to the pathogenesis of systemic lupus erythematosus (SLE). However, despite these insights, our understanding of how individual genetic variants promote autoimmunity remains poor. Loss-of-function mutations in genes of the phagocytic NADPH oxidase complex (NOX2), including NCF1 and NCF2, have been linked with the pathogenesis of SLE and other humoral autoimmune diseases. The current model for how reduced NOX2 activity promotes lupus development focuses on defects in the clearance of apoptotic material by phagocytic myeloid lineages. While myeloid defects likely contribute to disease risk, we hypothesize that a parallel B cell-intrinsic mechanism underlies the profound increase risk of SLE in human carriers of NCF1 and NCF2 variants. In addition to the production of pathogenic autoantibodies, recent studies have demonstrated that B cells can promote lupus pathogenesis by initiating immune tolerance breaks and facilitating the generation of spontaneous germinal centers (GC). The activation of autoreactive B cells in SLE requires B cell-intrinsic expression of the endosomal toll-like receptors TLR7 and TLR9, and our published and preliminary data show that reduced NOX2 activity results in dysregulated endosomal TLR signaling by impacting non-canonical autophagy pathways. Based on these data, we hypothesize that a B cell-specific reduction in NOX2 activity will result in enhanced TLR- dependent GC formation and the development of humoral autoimmunity. We will test this idea via parallel in vivo and in vitro mechanistic studies. In Aim 1, we will test whether B cell-intrinsic deletion NOX2 component genes results in enhanced TLR-dependent GC responses using a well-characterized model of viral infection. In Aim 2, we will study whether reduced NOX2 activity promotes autoantibody production and humoral autoimmunity in murine SLE. Finally, in Aim 3, we will use biochemical and cell imaging approaches to test whether NAPDH oxidase activity impacts B cell TLR signaling and non-canonical autophagy pathways using both murine genetic models and gene-edited human B cells. This supplemental research proposal will extend these proposed Specific Aims and test a new hypothesis that loss of Wiskott-Aldrich syndrome protein similarly impacts B cell endolysosomal trafficking, resulting in enhanced TLR signaling. Together, these studies promise to advance our understanding of B cell biology and mechanisms underlying the pathogenesis of humoral autoimmunity, allowing the candidate to receive clinically-relevant scientific training in support of their long-term career goals.