The overall goal of this project is to determine if the IKAROS transcription factor regulates a program that maintains IL-4 receptor (IL-4R)-mediated B-cell quiescence to ribonuclear protein (RNP) autoantigens (autoAgs). We propose that B cell activation through the type I interferon (IFN)/TLR7 activation pathway is inhibited by this B-cell quiescence program. We also propose that failure to maintain B cell quiescence is associated with the development of RNP autoantibody in systemic lupus erythematosus (SLE). Our preliminary data suggest that in both lupus mice and in human SLE, multiple immune disease phenotypes can be initiated by loss of quiescence at the earliest stage of B cell development; that is, the transitional B cell. The key forces that promote quiescence is an IKAROS (or IKZF1)-based program that is sustained through IL-4R signaling on IgD+CD23+ B cells. The major opposing program that promotes loss of quiescence is mediated through the type I interferon receptor (IFNAR) signaling, assembly and signaling competency of the TLR7 pathway, and development of B cells that can produce autoantibodies. We hypothesize that early-stage loss of quiescence leads to a series of B cell development defects after the transitional stage involving progression from a resting naïve to an activated naïve predominance and then to development of the pathogenic CD11c+Tbet+IgD−CD27− double negative 2 (DN2) or germinal center (GC) B cells that ultimately lead to the development of RNP autoantibody producing plasmablasts and plasma cells (PB/PC). In Aim 1, we will use several knockout and knock-in mouse strategies in lupus prone mice to determine if type I IFN and IL-4 act at the Tr B cells to regulate autoreactive B cell development and survival. The specific effects of Ikaros in modulating type I IFN and IL-4 signaling-mediated RNP-reactive B-cell quiescence at the Tr and naïve stage will also be analyzed. In Aim 2, we will determine if SLE patients exhibit a loss of IL-4R/IKZF1-mediated B cell quiescence program starting at the Tr stage of B-cell development. We will determine if the loss of this pathway disrupts B-cell tolerance to type I IFN and TLR7 stimulation, leading to the development of RNP- reactive DN2 B cells and PB/PC. The innovative scientific basis of this proposal will be its ability to interpret the broad spectrum of immune and disease characteristics of SLE by understanding the key initiating events of loss of B cell quiescence at the transitional stage. B cells from mouse models of lupus and from SLE subjects will be characterized by high dimension flow cytometry and transcriptomics analyses for well-established cell surface protein antigen markers that define B cell development at all major developmental stages. We have established a team of coordinators at the BVAMC to facilitate recruitment and have acquired numbers of SLE subjects necessary for definitive and statistically meaningful results. A state-of-the-art immunologic laboratory ...