LL-37 (Cathelicidin): Antimicrobial Peptide, Wound Healing, and Immune Research

LL-37 is the only cathelicidin expressed in humans, produced as hCAP18 by neutrophils, macrophages, NK cells, and epithelial cells. The mature 37-amino acid peptide (named for its two N-terminal leucines and total length) is released by serine protease cleavage and operates at the innate immune frontline.

Antimicrobial Mechanism

LL-37 is an amphipathic alpha-helical peptide that adopts a helical conformation near bacterial membranes, inserting into the lipid bilayer and forming transmembrane pores. This mechanism is effective against:

• Gram-positive bacteria: MRSA, *S. epidermidis* (MICs 2-8 μg/mL vs clinical MRSA isolates)
• Gram-negative bacteria: *E. coli*, *P. aeruginosa* (MICs 4-16 μg/mL)
• Fungi: *Candida albicans*
• Enveloped viruses: influenza A, HIV-1 in vitro

(Kanthawong S et al., *International Journal of Antimicrobial Agents*, 2012)

Mammalian cells are spared because their membranes lack negatively charged LPS, providing the structural selectivity that makes LL-37 a template for novel antimicrobial agents.

Wound Healing: EGFR Activation

At sub-microbicidal concentrations, LL-37 activates the epidermal growth factor receptor (EGFR) through extracellular domain phosphorylation, driving keratinocyte migration and re-epithelialization. Tokumaru S et al. (*Journal of Immunology*, 2005) demonstrated that LL-37 at 1-2 μg/mL accelerated keratinocyte monolayer closure 2.5-fold in scratch assays — blocked by EGFR kinase inhibitors.

In *db/db* diabetic mice — a model of impaired wound healing — local LL-37 application accelerated full closure by ~4 days (38% improvement) and increased new vessel density by 60% vs vehicle (Ramos R et al., *PLoS ONE*, 2011).

Immunomodulation: LPS Neutralisation

LL-37 directly binds and neutralises LPS (gram-negative bacterial endotoxin) — at equimolar concentrations reducing LPS-induced TNF-α production from macrophages by ~70% in vitro. This LPS-neutralising activity is relevant in gram-negative sepsis research models where Thymosin Alpha-1's immune-stimulatory effects may complement LL-37's endotoxin-buffering role.

Biofilm Disruption

LL-37 disrupts pre-formed *P. aeruginosa* and *S. aureus* biofilms at concentrations below its planktonic MIC — providing activity against chronic infections where antibiotics fail due to biofilm-conferred resistance.

LL-37 is a research peptide. No approved human therapeutic currently exists for injectable LL-37.

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Related Antimicrobial and Immune Peptides

LL-37 is the best-characterized human cathelicidin, but the broader antimicrobial peptide landscape includes several research-relevant compounds. Alpha-MSH has documented antimicrobial activity through membrane disruption and anti-biofilm properties in addition to its central anti-inflammatory actions. ARA-290 acts via innate repair receptors to suppress TLR-mediated inflammation, providing a complementary mechanism to LL-37's direct microbicidal effects. Humanin and SHLP-2 are mitochondrially derived peptides with cytoprotective roles in the tissue microenvironment where LL-37 is active. PNC-27, a p53-derived membrane-targeting peptide, shares structural and functional parallels with LL-37 in its ability to disrupt target cell membranes selectively. Vilon, though primarily a bioregulator, has anti-inflammatory properties studied in mucosal tissue contexts. Bronchogen and Taxorest address pulmonary epithelial tissue — the primary anatomical site for LL-37 secretion — in a tissue-bioregulator context.