Human cathelicidin peptide LL-37 compacts nucleic acids and alters neutrophil extracellular trap structure.

The human cathelicidin host defense peptide LL-37 is expressed by many cell types, including neutrophils, macrophages, and epithelial cells, and forms complexes with nucleic acids that can have either beneficial or detrimental health effects. We suggest that these differential impacts are directly connected to the extent of nucleic acid binding by LL-37. Here, we use phage λ DNA and techniques such as high-resolution video microscopy, gel electrophoresis, circular dichroism, and displacement assays to show that LL-37 binds non-specifically to dsDNA, condensing it, followed by formation of progressively larger complexes from smaller domains, until "complete" complexation is attained at a (w/w) ratio of DNA/LL-37 of 1:1.7. The morphology of these complexes is concentration-dependent, with relatively low LL-37 amounts yielding loosely aggregated DNA structures and higher LL-37 concentrations leading to well-defined, disc-like complexes of about 150 nm in diameter. The condensation of nucleic acids, which causes a loss of the characteristic B-DNA features, results from interactions of the phosphodiester backbone with cationic amino acid side chains of the peptide at physiological pH, most likely in A-T rich sequences of the nucleic acid. Our results show that the α-helical structure of the peptide with its amphipathic and hydrophobic surfaces is essential. Finally, we show that LL-37 complexation alters the structure of neutrophil extracellular traps (NETs), causing a significant reduction in projected NET area at high LL-37 concentrations. Our data suggest that LL-37 helps prevent nucleic acid dispersal and condenses dsDNA, which may impact the biophysics of NET clearance.