Crystallinity of dsRNA-Antimicrobial Peptide Complexes Modulates TLR3-Mediated Inflammation

Double-stranded RNA (dsRNA) induces production of pro-inflammatory cytokine signaling molecules in human keratinocytes by specific binding to endosomal Toll-like receptor-3 (TLR3). In the autoimmune disease psoriasis, it has recently been shown that hyperactivation of TLR3 in keratinocytes by dsRNA can occur in the presence of the human antimicrobial peptide (AMP) LL37. Here, we combine synchrotron X-ray scattering, microscopy, statistical mechanics, computer simulations, and measurements of cytokine production to elucidate a previously unanticipated form of specific molecular pattern recognition. LL37 and similar alpha-helical AMPs can form proinflammatory nanocrystalline complexes with dsRNA that are recognized by TLR3 differently than dsRNA alone. dsRNA complexes that activate IL-6 production in keratinocytes and those that do not are both able to enter cells and colocalize with TLR3. However, the crystallinity of these AMP-dsRNA complexes, specifically the geometric spacing between parallel dsRNA and the repeat number of ordered dsRNA, strongly influence the level of TLR3 activation. Crystalline complexes that present dsRNA at a spacing matching the steric size of TLR3 can recruit and engage multiple TLR3 receptors, driving receptor clustering and immune amplification.