Liquid-crystalline ordering of antimicrobial peptide–DNA complexes controls TLR9 activation

Liquid-crystalline arrangements of complexes of DNA and antimicrobial peptides can lead to multivalent electrostatic interactions that drastically amplify TLR9-mediated immune responses. Double-stranded DNA (dsDNA) can trigger the production of type I interferon (IFN) in plasmacytoid dendritic cells (pDCs) by binding to endosomal Toll-like receptor-9 (TLR9; refs 1 , 2 , 3 , 4 , 5 ). It is also known that the formation of DNA–antimicrobial peptide complexes can lead to autoimmune diseases via amplification of pDC activation 1 , 2 . Here, by combining X-ray scattering, computer simulations, microscopy and measurements of pDC IFN production, we demonstrate that a broad range of antimicrobial peptides and other cationic molecules cause similar effects, and elucidate the criteria for amplification. TLR9 activation depends on both the inter-DNA spacing and the multiplicity of parallel DNA ligands in the self-assembled liquid-crystalline complex. Complexes with a grill-like arrangement of DNA at the optimum spacing can interlock with multiple TLR9 like a zipper, leading to multivalent electrostatic interactions that drastically amplify binding and thereby the immune response. Our results suggest that TLR9 activation and thus TLR9-mediated immune responses can be modulated deterministically.