Background Toll like receptor (TLR) 9 has been shown to play a crucial role in the pathogenesis of systemic lupus erythematosus (SLE) in animal models.Its pathogenic role in human SLE,however,was poorly elucidated.This study was performed to investigate the role of TLR9 involved in the aberrant signaling pathway and its correlation with disease activity in SLE.Methods mRNA level of TLR9 and interferon (IFN) regulatory factor 5 (IRF5) in peripheral blood mononuclear cells (PBMCs) were determined by real-time polymerase chain reaction (PCR).IFN-α expression was measured in the serum of the SLE patients by enzyme-linked immunosorbent assay (ELISA).Results TLR9 expression was significantly higher in SLE patients than that in health controls (P=-0.011).SLE patients with positive anti-dsDNA antibody had significantly higher expression of TLR9 than that with negative anti-dsDNA antibody (P=0.001).TLR9 expression was positively correlated with fever (P=0.017),alopecia (P=0.046),safety of estrogens in lupus erythematosus national assessment SLE disease activity index (SELENA-SLEDAI) score (rs=0.385,P=0.003),and the level of IRF5 (rs=0.35,P=0.027) and IFN-α (rs=0.627,P=0.001) in SLE patients.Conclusion TLR9 is associated with SLE disease activity and might be involved in the IFN-α pathway of SLE.