Abstract
A thorough scientific review of LL-37, the sole human cathelicidin antimicrobial peptide, covering its discovery, structure, antimicrobial mechanisms, immunomodulatory functions, roles in innate immunity, wound healing, and cancer biology, and its therapeutic development status.
LL-37 is a 37-amino acid cationic antimicrobial peptide (AMP) that represents the only member of the cathelicidin family expressed in humans. Derived by proteolytic cleavage from its precursor protein hCAP-18 (human cationic antimicrobial protein of 18 kDa), LL-37 plays a central role in innate immune defense against infection while simultaneously modulating adaptive immune responses, promoting wound healing, and influencing cancer biology. The peptide was first identified and characterized by Zanetti, Gennaro, and colleagues in the early 1990s, with the human cathelicidin gene (CAMP) cloned and the mature LL-37 peptide sequence determined by 1995.
The name LL-37 derives from its 37-amino acid length and the two leucine residues at its N-terminus. The full amino acid sequence is LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES. The molecular weight is approximately 4,493 daltons, and the peptide carries a net positive charge of +6 at physiological pH, a characteristic feature of antimicrobial peptides that facilitates their interaction with negatively charged microbial membranes. In solution, LL-37 exists as a random coil at low concentrations but adopts an amphipathic alpha-helical conformation at higher concentrations and in the presence of membrane-mimetic environments such as lipid bilayers, detergent micelles, or trifluoroethanol. This helical conformation is essential for its membrane-disrupting antimicrobial activity: one face of the helix is hydrophobic and inserts into lipid bilayers, while the opposite face is positively charged and interacts with anionic membrane components.
The hCAP-18/LL-37 precursor is encoded by the CAMP gene located on chromosome 3p21.3. The precursor protein consists of a signal peptide, a cathelin-like domain, and the C-terminal LL-37 peptide. Processing of hCAP-18 to release mature LL-37 is performed by proteinase 3 in neutrophils, by kallikreins in the skin, and by gastricsin in the vaginal fluid. LL-37 is produced by multiple cell types: neutrophils (which store hCAP-18 in specific granules at concentrations up to 630 micrograms per milliliter), epithelial cells of the skin, respiratory tract, gastrointestinal tract, and genitourinary tract, monocytes, macrophages, dendritic cells, mast cells, and NK cells. Expression is constitutive in neutrophils but inducible in epithelial cells, where it can be upregulated by infection, inflammation, and vitamin D signaling.
The antimicrobial mechanism of LL-37 operates through multiple complementary pathways. The primary mechanism involves electrostatic binding to the negatively charged lipopolysaccharide (LPS) of Gram-negative bacteria or the lipoteichoic acid and anionic phospholipids of Gram-positive bacterial membranes. Following initial membrane binding, LL-37 inserts into the lipid bilayer and forms transmembrane pores or causes membrane thinning and disruption through the carpet model or toroidal pore model. The speed of membrane disruption (occurring within minutes) makes it difficult for bacteria to develop resistance through point mutations, as the target is the fundamental membrane structure rather than a specific protein. LL-37 demonstrates broad-spectrum antimicrobial activity against Gram-positive bacteria (including Staphylococcus aureus, Streptococcus species, and Enterococcus), Gram-negative bacteria (including Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae), mycobacteria, fungi (including Candida species), and enveloped viruses (including influenza, HIV, and herpes simplex virus).
Beyond direct killing, LL-37 neutralizes bacterial endotoxin (LPS) by binding to the lipid A moiety, preventing LPS from engaging TLR4 on immune cells and triggering inflammatory cytokine cascades. This anti-endotoxin activity is therapeutically significant because endotoxemia and the resulting septic cascade are major causes of morbidity and mortality in severe bacterial infections. LL-37 also disrupts bacterial biofilms, the structured microbial communities that are notoriously resistant to conventional antibiotics. The peptide prevents biofilm formation at sub-inhibitory concentrations by interfering with quorum sensing and surface attachment, and it can penetrate and disrupt established biofilms at higher concentrations.
The immunomodulatory functions of LL-37 extend far beyond direct antimicrobial activity and are increasingly recognized as the peptide's most important biological roles. LL-37 serves as a chemotactic agent for neutrophils, monocytes, mast cells, and T cells through activation of the formyl peptide receptor-like 1 (FPRL1/FPR2). This chemotactic activity recruits immune cells to sites of infection and injury, amplifying the innate immune response. LL-37 activates mast cells, promoting their degranulation and histamine release, which contributes to local vascular permeability and immune cell recruitment. The peptide modulates dendritic cell differentiation and function, promoting the development of Th1-polarizing dendritic cells that drive cell-mediated immunity.
A critically important immunological property of LL-37 is its ability to form complexes with self-DNA and self-RNA released from damaged or dying cells. LL-37-nucleic acid complexes are protected from extracellular nuclease degradation and are efficiently taken up by plasmacytoid dendritic cells, where they activate endosomal TLR9 (for LL-37-DNA complexes) and TLR7/8 (for LL-37-RNA complexes), triggering robust type I interferon production. This mechanism links LL-37 to the pathogenesis of autoimmune diseases, particularly psoriasis, where LL-37-self-DNA complexes drive the chronic type I interferon response that perpetuates skin inflammation. This discovery by Lande, Gilliet, and colleagues in 2007 was a landmark finding that fundamentally changed understanding of both cathelicidin biology and psoriasis pathogenesis.
In wound healing, LL-37 promotes re-epithelialization through stimulation of keratinocyte migration and proliferation via EGFR transactivation. The peptide activates matrix metalloproteinases (particularly MMP-9) that remodel the extracellular matrix to facilitate cell migration. LL-37 promotes angiogenesis by activating FPRL1 on endothelial cells and by upregulating VEGF expression. It also stimulates the proliferation and migration of fibroblasts, contributing to granulation tissue formation. These wound healing activities operate in concert with LL-37's antimicrobial function, providing simultaneous infection prevention and tissue repair at wound sites.
The regulation of LL-37 expression by vitamin D has become one of the most clinically significant aspects of cathelicidin biology. The CAMP gene promoter contains a vitamin D response element (VDRE), and 1,25-dihydroxyvitamin D3 (the active form of vitamin D) directly induces LL-37 expression in monocytes, macrophages, and epithelial cells. This mechanism was elucidated in a seminal 2006 study by Liu and colleagues that showed TLR2-mediated activation of monocytes by mycobacterial lipopeptides upregulates the vitamin D receptor and the CYP27B1 enzyme (which converts 25-hydroxyvitamin D to the active 1,25-dihydroxyvitamin D3), leading to vitamin D-dependent LL-37 induction and antimicrobial activity against Mycobacterium tuberculosis. This pathway provides a molecular explanation for the historical association between vitamin D deficiency and susceptibility to tuberculosis and other infections, and has provided a rational basis for vitamin D supplementation in infection prevention.
The role of LL-37 in cancer biology is complex and context-dependent. In some cancer types (ovarian cancer, breast cancer, lung cancer), LL-37 promotes tumor growth, angiogenesis, and metastasis through EGFR transactivation, FPRL1-mediated proliferative signaling, and immune evasion mechanisms. In other cancers (gastric cancer, colon cancer, hematological malignancies), LL-37 demonstrates anti-tumor activity through direct cytotoxicity against cancer cells, immune cell recruitment, and tumor microenvironment modulation. This dual nature reflects the peptide's pleiotropic biological activities and underscores the importance of understanding the specific tumor context before considering LL-37 in oncological applications.
Dysregulation of LL-37 expression is implicated in several disease states. Overexpression contributes to the pathogenesis of psoriasis (through self-nucleic acid complex formation), rosacea (where LL-37 fragments processed by kallikrein 5 drive inflammation and angiogenesis), and atherosclerosis (where LL-37 accumulates in plaques and may promote inflammatory cell recruitment). Deficiency of LL-37 is associated with increased susceptibility to infections, particularly in conditions such as chronic wounds, cystic fibrosis, and severe burns. Morbus Kostmann (severe congenital neutropenia) patients have near-complete deficiency of LL-37 and suffer from severe periodontal disease.
Therapeutic development of LL-37 and its analogs is being pursued along multiple avenues. OP-145 (P60.4Ac) is a synthetic 24-amino acid peptide derived from LL-37 that has been developed as a topical treatment for chronic otitis media and has completed phase I/II clinical trials. SAAP-148, another LL-37-derived peptide with enhanced antimicrobial potency, is in preclinical development for antibiotic-resistant infections. Brilacidin, a peptidomimetic that mimics the amphipathic structure of LL-37 without being a peptide, has been developed by Innovation Pharmaceuticals and has completed phase II clinical trials for acute bacterial skin and skin structure infections. These development programs validate the therapeutic potential of the cathelicidin-based antimicrobial approach while addressing the challenges of peptide stability and manufacturing cost.
In summary, LL-37 is a multifunctional peptide at the intersection of antimicrobial defense, immune regulation, wound healing, and cancer biology. Its discovery has fundamentally advanced understanding of innate immunity and opened therapeutic avenues for infections, inflammatory diseases, and wound healing. The vitamin D-LL-37 axis has provided molecular insights linking nutrition, immunity, and infectious disease susceptibility. As antimicrobial resistance continues to threaten global health, LL-37 and its derivatives represent a promising class of therapeutics that exploit fundamental differences between microbial and mammalian membrane composition.
