title: LL37 and cationic peptides enhance TLR3 signaling by viral double-stranded RNAs.
abstract: BACKGROUND
  Toll-like Receptor 3 (TLR3) detects viral dsRNA during viral infection. However, most natural viral dsRNAs are poor activators of TLR3 in cell-based systems, leading us to hypothesize that TLR3 needs additional factors to be activated by viral dsRNAs. The anti-microbial peptide LL37 is the only known human member of the cathelicidin family of anti-microbial peptides. LL37 complexes with bacterial lipopolysaccharide (LPS) to prevent activation of TLR4, binds to ssDNA to modulate TLR9 and ssRNA to modulate TLR7 and 8. It synergizes with TLR2/1, TLR3 and TLR5 agonists to increase IL8 and IL6 production. This work seeks to determine whether LL37 enhances viral dsRNA recognition by TLR3.
  
  
  RESULTS
  Using a human bronchial epithelial cell line (BEAS2B) and human embryonic kidney cells (HEK 293T) transiently transfected with TLR3, we found that LL37 enhanced poly(I:C)-induced TLR3 signaling and enabled the recognition of viral dsRNAs by TLR3. The presence of LL37 also increased the cytokine response to rhinovirus infection in BEAS2B cells and in activated human peripheral blood mononuclear cells. Confocal microscopy determined that LL37 could co-localize with TLR3. Electron microscopy showed that LL37 and poly(I:C) individually formed globular structures, but a complex of the two formed filamentous structures. To separate the effects of LL37 on TLR3 and TLR4, other peptides that bind RNA and transport the complex into cells were tested and found to activate TLR3 signaling in response to dsRNAs, but had no effect on TLR4 signaling. This is the first demonstration that LL37 and other RNA-binding peptides with cell penetrating motifs can activate TLR3 signaling and facilitate the recognition of viral ligands.
  
  
  CONCLUSIONS
  LL37 and several cell-penetrating peptides can enhance signaling by TLR3 and enable TLR3 to respond to viral dsRNA.
authors: Department of Molecular and Cellular Biochemistry, Indiana University, Bloomington, Indiana, United States of America. yylai@indiana.edu
authors: Yvonne
authors: Y
authors: Lai
authors: Sreedevi
authors: S
authors: Adhikarakunnathu
authors: Kanchan
authors: K
authors: Bhardwaj
authors: C T
authors: CT
authors: Ranjith-Kumar
authors: Yahong
authors: Y
authors: Wen
authors: Jarrat L
authors: JL
authors: Jordan
authors: Linda H
authors: LH
authors: Wu
authors: Bogdan
authors: B
authors: Dragnea
authors: Lani
authors: L
authors: San Mateo
authors: C Cheng
authors: CC
authors: Kao
mesh_terms: D000595:Amino Acid Sequence; D023181:Antimicrobial Cationic Peptides / Q000502:physiology*; D054804:Cathelicidins / Q000737:chemistry / Q000502:physiology*; D002460:Cell Line; D006801:Humans; D018613:Microscopy, Confocal; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D011070:Poly I-C / Q000737:chemistry; D012330:RNA, Double-Stranded / Q000378:metabolism*; D012229:Rhinovirus / Q000235:genetics*; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction / Q000502:physiology*; D051196:Toll-Like Receptor 3 / Q000378:metabolism*
chemical_list: D023181:Antimicrobial Cationic Peptides; D054804:Cathelicidins; D012330:RNA, Double-Stranded; C495344:TLR3 protein, human; D051196:Toll-Like Receptor 3; D011070:Poly I-C