Frontline Science: Targeting the TLR7 signalosome assembly

TLRs sense a broad range of microbial molecules and initiate antimicrobial immune response. The members of the TLR family use cytoplasmic Toll/interleukin-1R homology (TIR) domain to initiate intracellular signaling. The activated TLRs dimerize their TIRs and recruit adapter proteins to the dimer, through multiple interactions of receptor and adapter TIR domains. Although TLRs play an essential role in innate immunity, the aberrant TLR signaling may cause pathogenic inflammation. This study has screened a library of cell-permeable decoy peptides (CPDPs) derived from the TLR7 TIR for interference with TLR7 signaling and identified new CPDPs that target the TLR7 signalosome assembly. Peptides 7R1, 7R6, 7R9, and 7R11 inhibited the TLR7-induced signaling in murine and human macrophages. The most potent inhibitory peptide of the four, 7R11, significantly reduced the systemic cytokine levels elicited by administration of a TLR7 agonist to mice. TLR7 TIR surface regions that correspond to inhibitory peptides generally corresponded to four TIR sites that mediate signalosome assembly for other TLRs. The cell-based Förster resonance energy transfer/fluorescence lifetime imaging confirmed that 7R9 and 7R11 interact with adapter TIRs. These findings clarify the molecular mechanisms that trigger the adapter recruitment to activated TLR7 and suggest that 7R9 and 7R11 have a significant translational potential as candidate or lead therapeutics for treatment of TLR7-related inflammatory diseases.