LL-37

Description

LL-37 is the only member of the cathelicidin family of antimicrobial peptides (AMPs) found in humans. It is a 37-amino-acid cationic peptide that begins with two leucine residues (Leu-Leu, hence "LL-37") and adopts an amphipathic α-helical structure in membrane-like environments — a configuration critical to its antimicrobial mechanism. LL-37 is produced as the C-terminal fragment of the 18 kDa precursor protein hCAP18 (human Cationic Antimicrobial Protein 18), which is stored in the specific granules of neutrophils and is also expressed by epithelial cells, macrophages, monocytes, mast cells, and keratinocytes. Cleavage of hCAP18 to release active LL-37 is mediated by proteinase 3 in neutrophils and by serine proteases in epithelial cells.

The antimicrobial mechanism of LL-37 is broad-spectrum and multi-modal. The cationic (+6 net charge at physiological pH) amphipathic helix is electrostatically attracted to the negatively charged phospholipid membranes of bacteria, fungi, and enveloped viruses (mammalian cell membranes have a neutral outer leaflet and are relatively resistant). Upon membrane contact, LL-37 inserts into the lipid bilayer through the "carpet model" or "toroidal pore" mechanism, disrupting membrane integrity and causing lysis. Beyond direct killing, LL-37 has potent anti-biofilm activity — it disrupts established biofilms and prevents biofilm formation, which is significant because biofilm-associated infections (chronic wounds, implant infections, cystic fibrosis lung disease) are notoriously resistant to conventional antibiotics. LL-37 also functions as an immunomodulator: it acts as a chemoattractant for neutrophils, monocytes, and T-cells (through FPR2/ALX receptor activation), modulates dendritic cell differentiation, promotes wound healing and angiogenesis, and neutralizes bacterial lipopolysaccharide (LPS) endotoxin.

A critical regulatory aspect of LL-37 production is its dependence on vitamin D. The gene encoding hCAP18/LL-37 (CAMP) contains a vitamin D response element (VDRE) in its promoter, and its transcription is directly upregulated by 1,25-dihydroxyvitamin D₃ (calcitriol). This provides the molecular basis for the well-established link between vitamin D deficiency and increased susceptibility to infections.

Clinical Context

LL-37 is at the forefront of antimicrobial peptide therapeutics — a field gaining urgency as antibiotic resistance becomes an increasingly critical global health threat. Unlike conventional antibiotics that target specific metabolic pathways (which bacteria can evolve resistance to through single mutations), LL-37's membrane-disrupting mechanism is much more difficult for bacteria to develop resistance against (they would need to fundamentally redesign their membrane architecture). The anti-biofilm activity is particularly valuable, as biofilm infections account for over 80% of chronic bacterial infections and are a major cause of treatment failure.