Systemic lupus erythematosus (SLE) is characterized by the production of autoantibodies directed against self-antigens including DNA- and RNA-associated cellular components. Recent work from our lab and several other groups has shown that the nucleic-acid sensing endosomal TLRs, TLR9 and TLR7, are required to break B cell self tolerance to these antigens. Nonetheless, a great deal remains unknown about the origin of autoantibodies in lupus. For example, it is not known to what extent autoantibodies arise from activation and differentiation of naive B cells, or whether a pool of resting autoreactive memory B cells is formed that can be reactivated over time. Recently a novel population of “age-associated B cells” (ABCs) was defined in mice and humans that are expanded in human SLE patients and in murine models of lupus, including MRL/lpr. These cells lack expression of B subset markers for follicular, marginal zone, transitional or B-1 lineages; express CD11c and CD11b; and express the transcription factor T-bet. Formation of ABCs is thought to require stimulation through the BCR, TLR7 or 9, and an appropriate cytokine milieu (IFNg or IL21). These cells have been postulated to contain autoreactive and antiviral memory B cells, based on the presence of somatic mutations, expression of memory markers, and other properties. In this proposal, we will use genetic tools available in the MRL/lpr mouse model of lupus to investigate key questions about ABCs in vivo and to test the hypothesis that these cells are autoimmune memory and plasmablast precursors that promote disease. We will determine the lineage relationship of ABCs relative to naive B cells and plasmablasts, and determine the kinetics of their formation and activation. We will determine how the gene signature of MRL/lpr ABCs is similar to or different from conventional memory B cells or ABCs generated in normal aging. Finally, using genetic systems in which ABCs can be specifically targeted for constitutive or inducible depletion, we will test the hypothesis that ABCs are necessary for the initiation and propagation of lupus disease and evaluate their potential as a novel target for therapeutic intervention.