Abstract Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by autoantibody production and immune complex mediated organ damage. One of the more profound features of SLE is females having a 9:1 prevalence of disease over men. The cause of this gender difference in SLE is multifactorial, including the sex hormones themselves and their receptors. Estrogen acts primarily via its receptors, estrogen receptor alpha and beta (ER/ER). Estrogen can also act, however, through non-receptor mediated mechanisms and, in kind, the ERs mediate physiologic functions independent of estrogen. In the previous funding period, we derived ER and ER knockout lupus prone MRL/lpr and NZM2410 mice. In both strains, the female ER KOs developed significantly less proteinuria and pathologic renal disease and had significantly prolonged survival, despite increased serum levels of autoantibodies. These findings led us to postulate that the primary impact of ER deficiency in lupus nephritis was on the response of the kidney to inflammation. We derived ER KO and ER KO mesangial cells from B6 mice and found that ER KO mesangial cells had a marked blunted response to TLR 2, 3 and 7 ligands. ER expression had no effect on mesangial cell responses as tested. Based on these findings, we hypothesize that the lack of ER is renal protective in female lupus mice by blunting the response of mesangial cells to TLR3/7 induced inflammation. We believe this ER protective effect is estrogen independent and mediated via TLR3/7 induced phosphorylation of ER. To test this hypothesis, we propose the following Specific Aims: 1. Determine in vivo the effects of ER deficiency on known mechanisms of lupus pathogenesis using sle1, sle3 and sle1/3 congenic mice and bone marrow transplantation of ER KO and ER WT mice. 2. Define the in vitro molecular mechanisms underlying TLR/ER interactions that impact the inflammatory response in mesangial cells assessing the impact of ER on the TLR3/7 activation pathways and TLR3/7 on ER expression. 3. Define in vivo and in vitro the mechanisms by which ER impacts TLR signaling utilizing mutant ER knockin strains that affect specific ER functions allowing delineation of specific ER functions on the immune response. These studies will provide novel insight into the mechanisms by which ER deficiency impacts lupus disease expression and further delineate the interaction between ER and TLR induced inflammation that may partially underlie the female predominance in lupus.