Toll-like receptors (TLRs) have a crucial role in sensing microbial products and triggering immune responses. Recent reports have indicated that TLR7 and TLR9 have an important role in activating autoreactive B cells. In addition to TLR7 and TLR9, mouse B cells express TLR2, TLR4 and structurally related Radioprotective105 (RP105). We have previously shown that RP105 works in concert with TLR2/4 in antibody response to TLR2/4 ligands. We here report that B cells are constitutively activated by TLR2/4 and RP105. Such B cell activation was revealed by the gamma3 germ line transcript and serum IgG3 production, both of which were impaired by the lack of RP105 or TLR2/4. Serum IgG3 was not altered in germ-free or antibiotics-treated mice, suggesting that the microbial flora hardly contributes to the continuous activation of B cells. The lack of RP105-dependent B cell activation ameliorated disease progression in lupus-prone MRL/lpr mice. RP105(-/-) MRL/lpr mice showed less lymphoadenopathy/splenomegaly and longer survival than MRL/lpr mice. Whereas glomerulonephritis and auto-antibody production were not altered, improvement in blood urea nitrogen and lower incidence of renal arteritis indicated that renal function was ameliorated in the absence of RP105. Our results suggest that RP105-dependent tonic B cell activation has a pathogenic role in MRL/lpr mice.