Ligands to nucleic acid-specific toll-like receptors and the onset of lupus nephritis.

Lupus nephritis develops from a combination of genetic and environmental factors such as microbial infection. A role for microbial nucleic acids (e.g., via nucleic acid-specific Toll-like receptors [TLR]) was hypothesized, in this context, because microbial nucleic acids can trigger multiple aspects of autoimmunity in vitro and in vivo. Eight-week-old MRL(lpr/lpr) and MRL wild-type mice received an injection of pI:C RNA (ligand to TLR-3), imiquimod (ligand to TLR-7), or CpG-DNA (ligand to TLR-9) on alternate days for 2 wk. Only CpG-DNA triggered the onset of lupus nephritis in MRL(lpr/lpr) mice, as defined by diffuse proliferative glomerulonephritis associated with glomerular IgG and complement C3 deposition, proteinuria, and glomerular macrophage infiltrates. None of the compounds caused DNA autoantibody production or glomerulonephritis in MRL wild-type mice. The role of CpG-DNA to trigger lupus nephritis in MRL(lpr/lpr) mice was found to relate to its potent immunostimulatory effects at multiple levels: B cell IL12p40 production, B cell proliferation, double-stranded DNA autoantibody secretion, and dendritic cell IFN-alpha production. The induction of lupus nephritis by CpG-DNA is motif specific and could be prevented by co-injection of inhibitory DNA. In summary, among the ligands tested, CpG-DNA triggers lupus nephritis in genetically predisposed hosts. These data support the concept that systemic lupus erythematosus is triggered by pathogens that release CG-rich DNA.

Expression and regulation of Toll-like receptors in lupus-like immune complex glomerulonephritis of MRL-Fas(lpr) mice.

BACKGROUND: How microbial infections exacerbate immune complex glomerulonephritis remains speculative. Toll-like receptors (TLRs) may be involved in this phenomenon, because TLRs have potent immunostimulatory functions when exposed to selected pathogen-associated molecules. METHODS: We addressed this issue by characterizing the expression of TLR1-9 in MRLlpr/lpr mice that spontaneously develop immune complex glomerulonephritis as part of a systemic lupus-like autoimmune syndrome. RESULTS: Five-week-old healthy MRLlpr/lpr mice expressed TLR3 mRNA in kidneys at comparable levels as in the spleen, while all other TLRs were expressed at low levels in the kidney. In 20-week-old nephritic MRLlpr/lpr mice, renal mRNA levels had increased for TLR1-9. Renal TLR mRNA originated at least in part from glomeruli as evidenced by real-time RT-PCR from laser capture microdissected glomeruli. Immunostaining for TLR3, TLR7 and TLR9 revealed their expression by F4/80-positive infiltrating macrophages in 20-week-old nephritic MRLlpr/lpr mice. In addition, TLR3 localized to glomerular mesangial cells. Cultured mesangial cells expressed TLR1-4 and TLR6, while murine macrophages expressed TLR1-9. TNF-alpha and IFN-gamma induced TLR2, TLR3 and TLR6 mRNA in mesangial cells, while they down-regulated TLR1-9 mRNA in macrophages. Stimulation of both cell types with ligands for TLR1-4, TLR5, TLR7 and TLR9 induced IL-6 production consistent with their respective TLR expression patterns. TNF-alpha and IFN-gamma enhanced ligand-induced IL-6 production in both cell types irrespective of their modulatory effect on respective TLR mRNA levels. CONCLUSION: Thus, cell-type-specific expression and regulation of TLRs may be involved in infection-associated exacerbation of immune complex glomerulonephritis of MRLlpr/lpr mice.