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A hallmark of SLE is the production of high-titer, high-affinity, isotype-switched IgG autoantibodies directed against nucleic acid–associated antigens. Several studies have established a role for both type I IFN (IFN-I) and the activation of TLRs by nucleic acid–associated autoantigens in the pathogenesis of this disease. Here, we demonstrate that 2 IFN-I signaling molecules, IFN regulatory factor 9 (IRF9) and STAT1, were required for the production of IgG autoantibodies in the pristane-induced mouse model of SLE. In addition, levels of IgM autoantibodies were increased in pristane-treated Irf9^–/– mice, suggesting that IRF9 plays a role in isotype switching in response to self antigens. Upregulation of TLR7 by IFN-α was greatly reduced in Irf9^–/– and Stat1^–/– B cells. Irf9^–/– B cells were incapable of being activated through TLR7, and Stat1^–/– B cells were impaired in activation through both TLR7 and TLR9. These …