Cathelicidins such as the human 37-amino
acid peptide (LL-37) are
peptides that not only potently kill microbes but also trigger
inflammation by enabling immune recognition of endogenous
nucleic acids. Here, a detailed structure-function analysis of LL-37 was performed to understand the details of this process.
Alanine scanning of 34-amino
acid peptide (LL-34) showed that some variants displayed increased antimicrobial activity against Staphylococcus aureus and group A Streptococcus. In contrast, different substitutions clustered on the hydrophobic face of the LL-34 alpha helix inhibited the ability of those variants to promote type 1
interferon expression in response to
U1 RNA or to present U1 to the
scavenger receptor (SR) B1 on the keratinocyte cell surface. Small-angle X-ray scattering experiments of the LL-34 variants LL-34, F5A, I24A, and L31A demonstrated that these
peptides form cognate supramolecular structures with U1 characterized by inter-dsRNA spacings of approximately 3.5 nm, a range that has been previously shown to activate
toll-like receptor 3 by the parent
peptide LL-37. Therefore, while
alanine substitutions on the hydrophobic face of LL-34 led to loss of binding to SRs and the complete loss of autoinflammatory responses in epithelial and endothelial cells, they did not inhibit the ability to organize with
U1 RNA in
solution to associate with
toll-like receptor 3. These observations advance our understanding of how
cathelicidin mediates the process of innate immune self-recognition to enable inert
nucleic acids to trigger
inflammation. We introduce the term "innate immune vetting" to describe the capacity of
peptides such as LL-37 to enable certain
nucleic acids to become an inflammatory stimulus through SR binding prior to cell internalization.