LL-37

Research Use Only: This page is for research and educational purposes only. It does not provide medical advice, treatment instructions, or guaranteed outcome claims.

What Is LL-37?

LL-37 is the only cathelicidin-family antimicrobial peptide identified in humans.[2][7] Its name reflects its structure: 37 amino acid residues beginning with two leucine (L) residues. The peptide is produced as the C-terminal fragment of human cationic antimicrobial peptide 18 (hCAP18), encoded by the CAMP gene (cathelicidin antimicrobial peptide). Proteolytic cleavage — primarily by proteinase 3 in neutrophils — releases the biologically active LL-37 form from the cathelin-domain precursor.[1][2]

Structurally, the ll-37 peptide adopts an amphipathic α-helical conformation in membrane-mimetic and physiological environments, with a molecular weight of approximately 4,493.3 g/mol. This helical structure creates distinct hydrophobic and hydrophilic faces, which is central to its membrane-active properties — allowing it to interact with both microbial and host cell membranes depending on concentration, lipid composition, and environmental context.[2][7]

Expression of LL 37 occurs constitutively in neutrophil specific granules and is inducible in macrophages, epithelial cells lining the respiratory and gastrointestinal tracts, and skin keratinocytes. The peptide is also found in body fluids including sweat, saliva, and wound fluid at concentrations ranging from low micromolar to high micromolar levels during active infection or inflammation. Critically, CAMP gene expression is directly regulated by vitamin D through the vitamin D receptor (VDR), creating a well-documented mechanistic link between vitamin D status and innate antimicrobial capacity — a connection explored further below.[5][1]

Compound Profile

Mechanism of Action

The mechanism of action of the ll-37 peptide operates through two principal pathways: direct antimicrobial killing and host immunomodulation — a combination that distinguishes it from most conventional antimicrobial agents.[1][2][7]

Direct antimicrobial activity. LL-37’s amphipathic α-helix inserts into microbial membranes, creating pores and disrupting membrane integrity to cause pathogen lysis. This carpet-model and toroidal-pore mechanism is effective against Gram-positive and Gram-negative bacteria, certain enveloped viruses, and fungi. Unlike conventional antibiotics targeting specific metabolic pathways, this physical membrane-disruption approach makes resistance development substantially more difficult for pathogens — a property driving considerable research interest given the global antimicrobial resistance crisis.[2][7]

Immunomodulatory signalling. Beyond direct killing, LL 37 acts as a versatile immune signalling molecule through multiple receptor interactions. It binds formyl peptide receptor-like 1 (FPRL1/FPR2) to promote chemotaxis of neutrophils, monocytes, and T cells to sites of infection or injury. It modulates NF-κB signalling, influences cytokine release profiles (both pro- and anti-inflammatory depending on context), and can neutralise bacterial lipopolysaccharide (LPS) to dampen excessive inflammatory responses.[1][6][7]

Additionally, LL-37 interacts with purinergic receptors (P2X7) on immune cells, triggering IL-1β release and inflammasome activation — mechanisms relevant to both antimicrobial defence and inflammatory disease pathology.[1][6]

The peptide also promotes angiogenesis through VEGF-related pathways and stimulates keratinocyte migration — properties that connect its antimicrobial function to wound-healing activity.[3][1] Research into LL-37’s neuroprotective potential has explored its capacity to modulate neuroinflammatory pathways through mTOR-dependent mitochondrial protection mechanisms.[8]

Antimicrobial Research

The ll-37 antimicrobial profile is notably broad-spectrum. In vitro and animal model studies demonstrate activity against clinically relevant pathogens including Staphylococcus aureus (including methicillin-resistant strains), Pseudomonas aeruginosa, Escherichia coli, Candida albicans, and mycobacterial species.[2][7]

Research has also explored LL-37’s capacity to disrupt bacterial biofilms — structured microbial communities embedded in extracellular matrix that are characteristically resistant to conventional antibiotic therapy. Biofilm disruption is considered mechanistically distinct from planktonic (free-floating) bacterial killing and represents an area of active investigation in chronic wound management and device-associated infection research. LL-37 appears to interfere with biofilm formation at sub-antimicrobial concentrations, suggesting potential utility as a biofilm-prevention strategy rather than solely a bactericidal agent.[2][7]

The antimicrobial peptide’s activity is concentration-dependent and influenced by ionic conditions, with physiological salt concentrations partially reducing its efficacy in some in vitro systems. This salt sensitivity — a common limitation among cationic antimicrobial peptides — has driven research into modified LL-37 analogues and truncated fragments with improved stability and activity profiles under physiological conditions.[2][7]

A distinct area of interest is LL-37’s potential synergy with conventional antibiotics. Some preclinical studies suggest that sub-inhibitory concentrations of the peptide can enhance the efficacy of standard antibiotics against resistant strains, though this remains an early-stage research observation rather than an established therapeutic strategy.[2][7]

It is important to note that most ll-37 antimicrobial data derives from in vitro assays and animal infection models. Translation to clinical antimicrobial applications remains at an early research stage, and LL-37 is not established as a therapeutic antimicrobial agent. The gap between promising in vitro activity and practical clinical utility represents the central challenge in this research area.

Wound Healing & Tissue Repair

Heilborn et al. (2003) provided foundational evidence that LL-37 is actively involved in human skin wound re-epithelialisation. Their study demonstrated that the cathelicidin peptide is upregulated in acute wound epithelium but notably absent in chronic, non-healing ulcers — suggesting a functional role in normal wound closure processes and identifying cathelicidin deficiency as a potential contributing factor in impaired wound healing.[3]

The wound-healing mechanisms attributed to LL 37 in preclinical research include:

• Keratinocyte migration and proliferation: LL-37 stimulates epithelial cell movement to wound sites through EGFR transactivation and supports the proliferative responses necessary for re-epithelialisation. This effect has been demonstrated in both scratch-wound assays and more complex organotypic skin models.[3][1]
• Angiogenesis promotion: the peptide promotes new blood vessel formation through VEGF-dependent and FPRL1-mediated pathways, supporting the nutrient and oxygen delivery critical to healing tissue. This pro-angiogenic activity connects LL-37’s wound-healing function to its involvement in rosacea pathology (discussed below).[1][7]
• Extracellular matrix remodelling: LL-37 influences matrix metalloproteinase (MMP) activity and tissue inhibitor of metalloproteinase (TIMP) balance, contributing to the structural reorganisation required during the proliferative and remodelling phases of tissue repair.[1]
• Inflammatory modulation at wound sites: by balancing pro- and anti-inflammatory signalling — including LPS neutralisation and selective cytokine suppression — LL-37 may help prevent the chronic inflammatory state that characterises non-healing wounds and is associated with delayed tissue repair.[1][6]

These tissue-repair properties overlap with the mechanisms studied in other peptides such as BPC-157 (gastric-derived repair signalling) and TB-500 (thymosin beta-4 fragment), though each operates through distinct molecular pathways and receptor systems. For broader context on tissue-repair peptide research, see the Injury & Tissue Support goal page.

Vitamin D & LL-37 Connection

One of the most significant discoveries in cathelicidin biology is the direct regulatory link between vitamin D and LL-37 expression — a finding that fundamentally connected nutritional immunology to antimicrobial peptide biology. Liu et al. (2006), publishing in Science, demonstrated that Toll-like receptor activation triggers a vitamin D-dependent antimicrobial pathway: TLR2/1 stimulation by mycobacterial lipopeptides upregulates both the vitamin D receptor (VDR) and the enzyme CYP27B1 (1α-hydroxylase), which converts circulating 25-hydroxyvitamin D to its active hormonal form (1,25-dihydroxyvitamin D). Active vitamin D then binds VDR to directly induce transcription of the CAMP gene encoding hCAP18/LL-37.[5]

This mechanism has substantial implications for understanding immune competence in relation to vitamin D status. Individuals with insufficient vitamin D levels (below approximately 30 ng/mL of 25-hydroxyvitamin D) show a reduced capacity to upregulate LL-37 in response to infection signals. The finding provided a compelling mechanistic explanation for the long-observed epidemiological association between vitamin D deficiency and increased susceptibility to infections, particularly tuberculosis — and offered a molecular rationale for the historical use of cod liver oil and sunlight in tuberculosis management.[5]

The vitamin D–LL 37 axis has been explored as a potential target for immune support strategies, particularly in populations with high prevalence of vitamin D insufficiency. Seasonal variation in vitamin D levels has been proposed as one contributing factor to winter infection susceptibility patterns, partly through reduced cathelicidin expression.[5] However, whether vitamin D supplementation reliably enhances LL-37-mediated antimicrobial responses in clinical settings remains an active area of investigation, with intervention trials producing mixed results to date.

LL-37 in Skin Conditions

The role of cathelicidin in skin health is complex and, in some conditions, paradoxical. While LL-37 contributes to normal skin barrier defence and wound repair, aberrant processing and overexpression are implicated in inflammatory skin conditions.[4]

Rosacea. Yamasaki et al. (2007) published landmark findings in Nature Medicine demonstrating that rosacea skin exhibits both elevated cathelicidin levels and abnormal proteolytic processing by kallikrein 5 (KLK5). In healthy skin, cathelicidin is processed to generate LL-37 with balanced antimicrobial and immunomodulatory activity. In rosacea, aberrant KLK5 overactivity generates different cathelicidin fragments with enhanced pro-inflammatory and pro-angiogenic activity compared to native LL-37, directly contributing to the erythema, persistent inflammation, and telangiectasia (visible blood vessel changes) characteristic of rosacea subtypes.[4]

Psoriasis. LL-37 has been identified as a potential autoantigen in psoriasis. The peptide can form complexes with self-DNA, activating plasmacytoid dendritic cells through TLR9 and driving the interferon-alpha response implicated in psoriatic inflammation.[6]

Atopic dermatitis. In contrast to rosacea and psoriasis, atopic dermatitis is associated with relative cathelicidin deficiency in lesional skin, which may contribute to the increased susceptibility to skin infections (particularly S. aureus colonisation) observed in this condition. This finding has prompted investigation into whether boosting LL-37 expression — for example through vitamin D pathways — might support barrier function in atopic skin.[1][5]

For broader skin-related peptide research, the GHK-Cu profile covers copper peptide signalling in skin remodelling contexts.

Immunomodulatory Properties

The ll-37 immune profile extends well beyond simple antimicrobial killing. The peptide functions as an endogenous immune modulator with the capacity to bridge innate and adaptive immune responses.[1][6]

Key immunomodulatory functions documented in preclinical research include:

• Chemotaxis: LL-37 recruits neutrophils, monocytes, and T cells to sites of infection or tissue damage through FPRL1/FPR2 receptor signalling.[1][2]
• Cytokine modulation: the peptide can both stimulate and suppress cytokine production depending on context — promoting pro-inflammatory responses during acute infection while dampening excessive inflammation in other settings through LPS neutralisation.[1][6]
• Dendritic cell activation: LL-37 influences dendritic cell maturation and antigen presentation, creating a functional bridge between innate pathogen detection and adaptive immune activation.[1][6]
• Mast cell degranulation: the peptide can trigger histamine release from mast cells, contributing to local inflammatory and vascular responses.[1]
• NF-κB pathway modulation: LL 37 modulates the NF-κB signalling cascade, a central regulatory pathway in inflammatory gene expression.[1][7]

Kahlenberg and Kaplan (2013) provided a comprehensive review of LL-37’s dual role in inflammation and autoimmunity, highlighting that the same immunomodulatory properties that provide protection against infection can, in certain contexts, contribute to autoimmune pathology — particularly in systemic lupus erythematosus (SLE) and psoriasis.[6]

Research into neuroprotective applications has explored LL-37’s capacity to modulate neuroinflammation, with preclinical evidence suggesting protective effects through mTOR-dependent mitochondrial mechanisms and reduction of neuronal apoptosis under inflammatory stress conditions.[8] These findings remain early-stage but contribute to the broader interest in antimicrobial peptides as multifunctional immune-neural mediators. For related neuroprotection research, see the Semax and Selank profiles, plus the Neuroprotection goal page for broader context.

Side Effects & Safety Profile

The ll-37 side effects profile is primarily characterised by concentration-dependent cytotoxicity and potential immunogenicity concerns documented in preclinical research.[2][7]

Cytotoxicity. At elevated concentrations, LL-37’s membrane-active mechanism does not discriminate perfectly between microbial and host cell membranes. In vitro studies demonstrate dose-dependent toxicity to mammalian cells, including red blood cells (haemolysis), at concentrations substantially above those associated with antimicrobial activity.[2][7]

Immunogenicity. As an endogenous human peptide, LL 37 is expected to have lower immunogenicity than foreign peptides. However, its capacity to form immunogenic complexes with self-nucleic acids (as demonstrated in psoriasis and lupus research) introduces considerations about potential autoimmune activation in susceptible individuals.[6]

Pro-inflammatory potential. The same immunomodulatory properties that provide defence can produce unwanted inflammation. Mast cell degranulation, excessive cytokine release, and pro-angiogenic activity represent potential adverse effects depending on context and concentration.[1][4]

Protease degradation. LL-37 is susceptible to rapid degradation by endogenous proteases, which limits its systemic exposure but also complicates delivery and dosing in research contexts.[2][7]

No standardised safety data from controlled human clinical trials exists for exogenous LL-37 administration. All safety inferences derive from in vitro studies, animal models, and observational data on endogenous LL-37 biology. The peptide is a research compound and is not approved for therapeutic use. Researchers investigating LL-37 analogues have focused on separating the beneficial antimicrobial activity from the host-cell cytotoxicity through structural modifications, with some success in preclinical models.[2][7]

Pharmacokinetics

The pharmacokinetic profile of LL-37 presents significant challenges for research applications. The peptide’s half-life has not been fully characterised in systematic pharmacokinetic studies, though it is understood to be subject to rapid proteolytic degradation in biological environments.[2][7]

Stability. LL 37 is susceptible to cleavage by multiple endogenous proteases, including those present in serum, at wound sites, and in the gastrointestinal tract. This protease susceptibility limits its persistence and bioavailability following administration and represents a major hurdle for any therapeutic development. Serum half-life estimates remain poorly defined, though degradation in plasma appears rapid relative to synthetic drug-like peptides.[2][7]

Salt sensitivity. The peptide’s antimicrobial activity is reduced under physiological ionic conditions (approximately 150 mM NaCl), which affects its functional efficacy in vivo. This has prompted research into modified analogues and D-amino acid substituted variants with improved salt tolerance and protease resistance.[2]

Delivery challenges. The combination of protease susceptibility, salt sensitivity, and molecular size (37 amino acids, ~4.5 kDa) creates delivery challenges that have driven investigation into nanoparticle encapsulation, liposomal formulations, modified analogues, and localised delivery approaches (including wound dressings and topical preparations) in preclinical research settings.[7]

Distribution. Endogenous LL-37 is found at highest concentrations at epithelial barrier surfaces (skin, respiratory tract, gastrointestinal tract) and in neutrophil-rich inflammatory exudates. Circulating plasma concentrations are generally lower, reflecting its primary role as a local defence molecule with localised immunomodulatory signalling rather than systemic pharmacological activity.[1][2]

FAQ

What is LL-37?

LL-37 (also written LL 37) is the only human cathelicidin antimicrobial peptide — a 37-amino-acid defence molecule produced by immune cells and epithelial tissues. It is the active fragment of the precursor protein hCAP18, encoded by the CAMP gene. Research interest centres on its dual antimicrobial and immunomodulatory properties.[1][2]

What are the researched benefits of LL-37?

Preclinical research into ll-37 benefits spans antimicrobial defence (broad-spectrum pathogen killing and biofilm disruption), wound healing support (keratinocyte migration, angiogenesis), immunomodulation (cytokine regulation, chemotaxis), and emerging neuroprotection investigations. All evidence derives primarily from in vitro and animal studies.[1][2][3][7]

How does vitamin D affect LL-37 levels?

Vitamin D directly regulates LL-37 production through the vitamin D receptor (VDR). Active vitamin D (1,25-dihydroxyvitamin D) binds VDR to induce CAMP gene transcription, increasing hCAP18/LL-37 expression. This mechanism links vitamin D status to innate antimicrobial capacity.[5]

Is LL-37 the same as cathelicidin?

LL-37 is the active peptide form of the human cathelicidin. Cathelicidins are a family of antimicrobial peptides found across vertebrate species, but LL 37 is the only member identified in humans. The precursor protein is called hCAP18; LL-37 is the 37-residue fragment released by proteolytic cleavage.[2][7]

What is the connection between LL-37 and rosacea?

Rosacea skin shows elevated levels of cathelicidin that is abnormally processed by the enzyme kallikrein 5 (KLK5), generating modified peptide fragments with enhanced pro-inflammatory and pro-angiogenic activity. This contributes directly to rosacea symptoms including redness, inflammation, and visible blood vessels.[4]

What are the side effects of LL-37?

The ll-37 side effects documented in preclinical research include concentration-dependent cytotoxicity to host cells, potential for excessive immune activation, mast cell degranulation, and theoretical autoimmune concerns related to self-nucleic acid complex formation. No controlled human safety data from clinical trials is available.[2][6][7]

Is LL-37 FDA approved?

No. LL-37 is not approved by the FDA or any regulatory agency for therapeutic use. It remains a research compound studied in preclinical contexts. All references to its properties on this page describe findings from laboratory and animal model investigations, not established clinical applications.

How does LL-37 differ from other antimicrobial peptides?

LL-37 is distinguished by being the only human cathelicidin, its amphipathic α-helical structure, and its unusually broad functional profile combining direct membrane-disrupting antimicrobial activity with extensive immunomodulatory, wound-healing, and potential neuroprotective properties. Most other antimicrobial peptides have narrower functional ranges.[2][7]

References

1. Vandamme D, Landuyt B, Luyten W, Schoofs L. A comprehensive summary of LL-37, the factotum human cathelicidin peptide. Cell Immunol. 2012;280(1):22-35. doi:10.1016/j.cellimm.2012.11.009. PMID: 23246832
2. Dürr UH, Sudheendra US, Ramamoorthy A. LL-37, the only human member of the cathelicidin family of antimicrobial peptides. Biochim Biophys Acta. 2006;1758(9):1408-1425. doi:10.1016/j.bbamem.2006.03.030. PMID: 16716248
3. Heilborn JD, Nilsson MF, Kratz G, et al. The cathelicidin anti-microbial peptide LL-37 is involved in re-epithelialization of human skin wounds and is lacking in chronic ulcer epithelium. J Invest Dermatol. 2003;120(3):379-389. doi:10.1046/j.1523-1747.2003.12069.x. PMID: 12603850
4. Yamasaki K, Di Nardo A, Bardan A, et al. Increased serine protease activity and cathelicidin promotes skin inflammation in rosacea. Nat Med. 2007;13(8):975-980. doi:10.1038/nm1616. PMID: 17676051
5. Liu PT, Stenger S, Li H, et al. Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response. Science. 2006;311(5768):1770-1773. doi:10.1126/science.1123933. PMID: 16497887
6. Kahlenberg JM, Kaplan MJ. Little peptide, big effects: the role of LL-37 in inflammation and autoimmune disease. J Immunol. 2013;191(10):4895-4901. doi:10.4049/jimmunol.1302005. PMID: 24185823
7. Xhindoli D, Pacor S, Benincasa M, Scocchi M, Gennaro R, Tossi A. The human cathelicidin LL-37 — A pore-forming antibacterial peptide and host-cell modulator. Biochim Biophys Acta. 2016;1858(3):546-566. doi:10.1016/j.bbamem.2015.11.003. PMID: 26556394
8. Sun W, Zheng Y, Lu Z, et al. LL-37 attenuates inflammatory impairment via mTOR signaling-dependent mitochondrial protection. Int J Biochem Cell Biol. 2014;54:26-35. doi:10.1016/j.biocel.2014.06.015. PMID: 24984264