LL-37

Overview

LL-37 is a 37-amino-acid peptide and the sole cathelicidin-derived antimicrobial peptide (AMP) identified in humans. It is cleaved from the C-terminal end of the 18-kDa precursor protein hCAP18 (human cationic antimicrobial protein 18) by the serine protease proteinase 3 in neutrophils, or by kallikreins in keratinocytes. The designation "LL-37" reflects its two N-terminal leucine residues and 37-residue length.

LL-37 is produced by a wide range of cell types including neutrophils, monocytes, macrophages, mast cells, natural killer cells, keratinocytes, and epithelial cells of the respiratory, urinary, and gastrointestinal tracts. Its gene, CAMP (cathelicidin antimicrobial peptide), is located on chromosome 3 and is transcriptionally upregulated by vitamin D receptor activation — a finding that linked vitamin D status to innate immune competence.

First characterized by Gudmundsson et al. in 1996, LL-37 has since become one of the most extensively studied antimicrobial peptides in human biology. Its significance extends well beyond direct microbial killing: LL-37 functions as a multifaceted immunomodulator that bridges innate and adaptive immunity, promotes wound healing, disrupts biofilms, and modulates inflammatory responses. As of 2026, LL-37 and its derivatives are under investigation for applications ranging from chronic wound management to antibiotic-resistant infection control.

Structure and Sequence

Sequence: LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES

• Molecular formula: C₂₀₅H₃₄₀N₆₀O₅₃
• Molecular weight: approximately 4,493.3 g/mol
• Net charge: +6 at physiological pH
• Structure: Adopts an amphipathic alpha-helical conformation in membrane-mimetic environments and in the presence of lipopolysaccharide (LPS). In aqueous solution at low ionic strength, LL-37 is largely unstructured
• Gene: CAMP (cathelicidin antimicrobial peptide), chromosome 3p21.31

The amphipathic helix — with hydrophobic residues aligned on one face and cationic residues on the other — is critical to LL-37's membrane-disrupting activity. This structural arrangement allows the peptide to interact with and insert into negatively charged bacterial membranes while showing lower affinity for the more neutrally charged membranes of mammalian cells.

Mechanism of Action

Direct Antimicrobial Activity

LL-37 exhibits broad-spectrum antimicrobial activity against Gram-positive bacteria, Gram-negative bacteria, fungi, and enveloped viruses. The primary mechanism involves:

1. Electrostatic attraction — The cationic peptide is drawn to the negatively charged surfaces of microbial membranes (lipopolysaccharide in Gram-negatives, lipoteichoic acid in Gram-positives)
2. Membrane insertion — The amphipathic helix inserts into the lipid bilayer
3. Membrane disruption — Through carpet model, toroidal pore, or barrel-stave mechanisms, LL-37 disrupts membrane integrity, leading to ion leakage and cell death
4. Intracellular targeting — At sub-lytic concentrations, LL-37 can translocate across membranes and interfere with intracellular processes including DNA and protein synthesis

Biofilm Disruption

LL-37 has demonstrated significant anti-biofilm activity, a property of particular clinical relevance given that biofilm-associated infections are notoriously resistant to conventional antibiotics. Mechanisms include:

• Inhibition of initial bacterial attachment to surfaces
• Disruption of quorum-sensing signaling pathways
• Degradation of established biofilm architecture at concentrations below those required for planktonic killing
• Stimulation of twitching motility in Pseudomonas aeruginosa, promoting biofilm dispersal

Overhage et al. (2008) demonstrated that LL-37 at concentrations as low as 0.5 micrograms/mL could inhibit Pseudomonas aeruginosa biofilm formation — well below its minimum inhibitory concentration for planktonic bacteria.

Immunomodulation

Beyond direct antimicrobial effects, LL-37 functions as a potent immunomodulator:

• Chemotaxis — Recruits neutrophils, monocytes, mast cells, and T cells to sites of infection through direct chemoattractant activity and formyl peptide receptor-like 1 (FPRL1) activation
• Cytokine modulation — Modulates the production of pro-inflammatory and anti-inflammatory cytokines in a context-dependent manner
• LPS neutralization — Binds and neutralizes lipopolysaccharide, reducing endotoxin-mediated inflammation
• Dendritic cell activation — Promotes dendritic cell maturation and antigen presentation, linking innate and adaptive immunity via toll-like receptor signaling
• Mast cell degranulation — Induces mast cell degranulation and histamine release, contributing to local inflammatory responses

Wound Healing Promotion

LL-37 promotes wound healing through multiple mechanisms:

• Stimulation of keratinocyte and fibroblast migration and proliferation
• Promotion of angiogenesis through VEGF-dependent and VEGF-independent pathways
• Transactivation of the epidermal growth factor receptor (EGFR)
• Re-epithelialization acceleration in both in vitro and in vivo wound models

Vitamin D Connection

The CAMP gene contains a vitamin D response element (VDRE) in its promoter region. 1,25-dihydroxyvitamin D3 (calcitriol) directly upregulates LL-37 transcription, providing a molecular mechanism for the long-observed connection between vitamin D status and immune function. This finding by Liu et al. (2006) was a landmark discovery in innate immunity research.

Research Summary

Pharmacokinetics

Pharmacokinetic data for exogenous LL-37 administration is limited, with most studies focusing on endogenous expression levels:

• Endogenous plasma concentration: Approximately 1.2–1.8 micrograms/mL in healthy adults; significantly elevated during infection and inflammation
• Half-life: Estimated at under 1 hour in plasma due to susceptibility to serine proteases; local tissue persistence may be longer
• Protein binding: Binds to apolipoprotein A-I in plasma, which may serve as both a carrier mechanism and a means of modulating activity
• Proteolytic susceptibility: Degraded by bacterial and host proteases, leading to research into protease-resistant analogs (e.g., D-LL-37 composed of D-amino acids)
• Salt sensitivity: Antimicrobial activity is reduced at physiological salt concentrations (150 mM NaCl), though immunomodulatory functions are retained

The development of delivery systems — including nanoparticle formulations, hydrogels, and electrospun wound dressings — represents an active area of research aimed at overcoming LL-37's pharmacokinetic limitations for therapeutic applications.

Dosing Protocols

The following dosing information is compiled from published research and community discussion for educational purposes only. No FDA-approved human dosing guidelines exist for most research peptides. Always consult a qualified healthcare professional.

Reconstitution

Dosing Schedule

Syringe Measurements (U-100 insulin syringe)

Cycle Guidelines

• Cycle length: 8-12 weeks (up to 16 weeks)
• Route: Subcutaneous injection
• Alternative schedule: 5 days on, 2 days off
• Titration: Increase by ~50 mcg per week as tolerated
• Injection sites: Rotate between abdomen, thighs, and upper arms
• Tip: For small doses (3-10 units), use 30- or 50-unit syringes for improved accuracy

Common Discussion Topics

1. Vitamin D optimization — The direct transcriptional link between vitamin D and LL-37 production drives discussion of vitamin D supplementation as a strategy to support innate immunity
2. Chronic wound management — LL-37 deficiency in chronic non-healing wounds has generated interest in topical LL-37 applications
3. Antibiotic resistance — LL-37's mechanism of action (membrane disruption) is less susceptible to conventional resistance mechanisms, positioning it as a candidate for combating multidrug-resistant infections
4. Biofilm-related infections — Applications in device-related infections, dental biofilms, and chronic wound biofilms
5. Psoriasis and autoimmunity — The dual role of LL-37 as both immune defender and autoimmune trigger (in psoriasis) illustrates the complexity of antimicrobial peptide biology
6. Synthetic analogs — Development of shorter, more stable, and more potent LL-37 derivatives (e.g., FK-16, GF-17, SAAP-148) for therapeutic use

Related Compounds

• KPV — an anti-inflammatory tripeptide with complementary immunomodulatory properties
• BPC-157 — a tissue-repair peptide with distinct but potentially complementary wound-healing mechanisms
• Thymosin Alpha-1 — an immunomodulatory peptide that also bridges innate and adaptive immunity
• Defensins — another major family of human antimicrobial peptides (alpha-defensins, beta-defensins)
• Lactoferricin — an antimicrobial peptide derived from lactoferrin with similar membrane-disrupting activity