LL-37

LL-37 is a 37-amino acid peptide that represents the sole cathelicidin antimicrobial peptide (AMP) produced in humans. It is derived from the 18-kDa precursor protein hCAP-18 (human cationic antimicrobial protein-18), encoded by the CAMP gene. Proteolytic cleavage of hCAP-18 by proteinase 3 in neutrophils — or by kallikreins in keratinocytes — releases the active LL-37 peptide. Its name reflects its N-terminal sequence: two leucine residues followed by 35 additional amino acids. LL-37 is expressed in a wide variety of tissues and cell types, including neutrophils, macrophages, epithelial cells of the skin, respiratory tract, gastrointestinal tract, and urogenital tract. It is constitutively present at epithelial barrier surfaces and is rapidly upregulated during infection and inflammation, representing a first line of defense in innate immunity. The peptide adopts an amphipathic alpha-helical structure in membrane-like environments, which is essential for its antimicrobial and immunomodulatory activities. Beyond its role as a direct antimicrobial agent, LL-37 has emerged as a multifunctional immune mediator with roles in chemotaxis, angiogenesis, wound healing, and modulation of adaptive immune responses. Dysregulation of LL-37 expression has been implicated in various pathological conditions including rosacea (overexpression), chronic wounds (deficiency), and susceptibility to infections. This broad functional repertoire has made LL-37 a subject of intense research interest across immunology, infectious disease, and regenerative medicine. All content is for educational and research purposes only.

LL-37 exerts its biological effects through a diverse set of mechanisms spanning direct antimicrobial activity and immune modulation: **Membrane Disruption**: The primary antimicrobial mechanism involves LL-37's amphipathic alpha-helical structure interacting with negatively charged microbial membranes. The peptide inserts into lipid bilayers via the "toroidal pore" or "carpet" model, creating transmembrane pores that disrupt membrane integrity and cause microbial cell lysis. Selectivity for microbial over mammalian membranes arises from the difference in membrane charge and composition. **Lipopolysaccharide (LPS) Neutralization**: LL-37 binds directly to LPS (endotoxin) from Gram-negative bacteria, neutralizing its ability to activate TLR4 signaling and trigger septic inflammatory cascades. This anti-endotoxin activity provides protection independent of direct bacterial killing. **Chemotaxis and Immune Cell Recruitment**: LL-37 acts as a chemoattractant for neutrophils, monocytes, and T cells through activation of formyl peptide receptor-like 1 (FPRL1/FPR2) and other receptors. This chemotactic activity bridges the initial antimicrobial response with recruitment of adaptive immune effectors. **Wound Healing and Angiogenesis**: The peptide promotes wound healing by stimulating keratinocyte and fibroblast migration and proliferation. It also induces angiogenesis through FPRL1-dependent signaling in endothelial cells, improving vascularization of healing tissues. **Modulation of Inflammatory Signaling**: LL-37 modulates inflammation in a context-dependent manner — enhancing protective inflammatory responses during infection while also suppressing excessive or pathological inflammation. It can alter TLR signaling, modulate cytokine release from macrophages, and influence dendritic cell maturation and function.

Research on LL-37 spans antimicrobial activity, immunomodulation, and therapeutic development: **Antimicrobial Spectrum**: In vitro studies demonstrate LL-37 activity against a broad range of pathogens including methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa, Escherichia coli, Candida albicans, and enveloped viruses including influenza and HIV. Minimum inhibitory concentrations typically range from 1–32 μM depending on the organism and assay conditions. **Anti-Biofilm Research**: LL-37 has shown significant anti-biofilm activity at sub-inhibitory concentrations. Research demonstrates that the peptide inhibits biofilm formation by Pseudomonas aeruginosa by downregulating biofilm-related genes, reducing surface attachment, and promoting twitching motility. This activity is particularly significant given the clinical challenge of biofilm-associated infections. **Wound Healing Studies**: In vivo wound healing studies demonstrate that topical LL-37 application accelerates closure of chronic wounds. A clinical trial in patients with venous leg ulcers showed that LL-37-treated wounds achieved significantly greater wound closure compared to placebo, validating the peptide's translational potential. **Immune Modulation in Disease**: Research in sepsis models shows that LL-37 reduces mortality through combined antimicrobial and anti-endotoxin activity. Studies in chronic inflammatory conditions demonstrate the peptide's ability to redirect immune responses from pathological to protective patterns. **Structural Optimization**: Extensive structure-activity relationship studies have generated LL-37 fragments and analogs with improved properties. The fragment FK-16 (residues 17–32) retains antimicrobial activity with improved protease stability, while D-amino acid substitutions and stapled peptide analogs offer enhanced in vivo half-life.

Research protocols for LL-37 vary based on application and formulation: **In Vitro Antimicrobial Studies**: Standard antimicrobial assays use concentrations of 1–64 μM, with most pathogens inhibited in the 2–16 μM range under standard test conditions. **Topical Wound Healing**: Clinical and preclinical wound healing studies have employed LL-37 in topical formulations at concentrations of 0.5–1.6 mg/mL applied directly to wound surfaces at intervals of 1–3 days. **Systemic Research**: Animal studies investigating systemic effects have used subcutaneous or intravenous doses of 1–5 mg/kg, though systemic administration is limited by the peptide's susceptibility to serum protease degradation. **Duration**: Topical protocols in wound healing studies typically extend for 4–8 weeks. Systemic research protocols are generally shorter (1–2 weeks) due to stability considerations. **Note**: Human clinical dosing is limited to the wound healing context. Optimal systemic dosing has not been established. LL-37's susceptibility to proteolytic degradation in vivo is a key challenge being addressed through analog development and formulation strategies. All information is for research reference purposes only.

The safety profile of LL-37 reflects both its endogenous nature and the challenges of therapeutic application: **Endogenous Context**: As a naturally produced human peptide, LL-37 is inherently biocompatible. Endogenous levels fluctuate in response to infection, inflammation, and vitamin D status without associated toxicity under normal physiological conditions. **Topical Application**: Clinical wound healing studies report good local tolerability. Mild transient erythema at application sites has been observed in a minority of subjects. No systemic adverse effects were reported with topical use. **Potential Concerns**: - At high concentrations (>25 μM), LL-37 can become cytotoxic to mammalian cells due to non-selective membrane disruption - Overexpression has been linked to rosacea pathogenesis, suggesting that supraphysiological concentrations may drive pathological inflammation in certain tissues - Mast cell degranulation at high concentrations may contribute to allergic-type reactions **Systemic Considerations**: Systemic administration at research doses in animal models has not revealed significant toxicity, but the rapid proteolytic degradation limits exposure and complicates safety assessment. **Research Limitations**: Clinical safety data are primarily from topical wound healing applications. Comprehensive systemic safety profiling in humans is not yet available. Standard precautions for investigational peptides should be observed.