Toll-Like Receptor 9 Deficiency Breaks Tolerance to RNA-Associated Antigens and Up-Regulates Toll-Like Receptor 7 Protein in Sle1 Mice

Objective: Toll-like receptors (TLRs) 7 and 9 are important innate signaling molecules with opposing roles in the development and progression of systemic lupus erythematosus (SLE). While multiple studies support the notion of a dependency on TLR-7 for disease development, genetic ablation of TLR-9 results in severe disease with glomerulonephritis (GN) by a largely unknown mechanism. This study was undertaken to examine the suppressive role of TLR-9 in the development of severe lupus in a mouse model. Methods: We crossed Sle1 lupus-prone mice with TLR-9–deficient mice to generate Sle1TLR-9^−/− mice. Mice ages 4.5–6.5 months were evaluated for severe autoimmunity by assessing splenomegaly, GN, immune cell populations, autoantibody and total Ig profiles, kidney dendritic cell (DC) function, and TLR-7 protein expression. Mice ages 8–10 weeks were used for functional B cell studies, Ig profiling, and determination of TLR-7 expression. Results: Sle1TLR-9^−/− mice developed severe disease similar to TLR-9–deficient MRL and Nba2 models. Sle1TLR-9^−/− mouse B cells produced more class-switched antibodies, and the autoantibody repertoire was skewed toward RNA-containing antigens. GN in these mice was associated with DC infiltration, and purified Sle1TLR-9^−/− mouse renal DCs were more efficient at TLR-7–dependent antigen presentation and expressed higher levels of TLR-7 protein. Importantly, this increase in TLR-7 expression occurred prior to disease development, indicating a role in the initiation stages of tissue destruction. Conclusion: The increase in TLR-7–reactive immune complexes, and the concomitant enhanced expression of their receptor, promotes inflammation and disease in Sle1TLR9^−/− mice.