Coactivation of Toll-like receptor-3 and -7 in immune complex glomerulonephritis

The molecular mechanisms of viral infection-induced glomerulonephritis are poorly understood. Toll-like receptor (TLR)-3 and TLR7 recognize viral RNA and their exposure to TLR3 or TLR7 can trigger the exacerbation of established immune complex disease in MRLlpr mice. Because coactivation of TLR3 and TLR7 was shown to synergistically activate dendritic cells in vitro, we hypothesized that simultaneous ligation of TLR3 and TLR7 would elicit additive effects on the exacerbation of glomerulonephritis in MRLlpr mice. Saline, 50 μg pI:C RNA, 25 μg of the TLR7 agonist imiquimod, or a combination of both were injected every other day to MRLlpr mice from week 16–18 of age. Coinjection of pI:C RNA and imiquimod had no synergistic effect on serum levels of IL-6 and IL12p70, dsDNA autoantibody levels, and glomerulonephritis. This was consistent with a lack of synergistic effects on cytokine release of TNF- and IFNγ-prestimulated monocytes in vitro. Furthermore, in glomerular mesangial cells a synergistic effect of pI:C RNA and imiquimod was generally absent due to the lack of TLR7 expression. We conclude that a number of mechanisms protect the host from additive effects of TLR3–TLR7 coactivation on renal pathology in vivo.