LL-37 (Human Cathelicidin), Research Reference

LL-37 is the only known human cathelicidin, a 37-amino acid antimicrobial peptide derived from the precursor protein hCAP18 (human cationic antimicrobial protein, 18 kDa), which is encoded by the CAMP gene. The name LL-37 derives directly from its structure: two N-terminal leucines (“LL”) and a total length of 37 amino acids. Research has investigated LL-37 for antimicrobial activity against bacteria, fungi, and some viruses, as well as for its roles in wound healing, angiogenesis, and innate immune modulation.

Quick Reference

Overview

LL-37 is an endogenous antimicrobial peptide produced principally by neutrophils, epithelial cells of the skin, lung, and gastrointestinal tract, as well as macrophages, mast cells, and natural killer (NK) cells. It is released in response to infection, tissue injury, and inflammatory stimuli, and serves as a component of the first line of innate immune defence.

The primary characterised mechanism involves the formation of an amphipathic alpha-helical structure in membrane environments. This helix inserts into and disrupts bacterial and fungal cell membranes through a direct lytic action, a mechanism distinct from antibiotic compounds that target specific intracellular processes. This membrane-disrupting activity has been characterised against gram-positive bacteria, gram-negative bacteria, fungi, and, in vitro, certain viruses including HIV and influenza strains.

Beyond direct antimicrobial activity, research has characterised LL-37 as a modulator of innate immune signalling, acting as both an agonist and an antagonist of Toll-like receptor 4 (TLR4) and Toll-like receptor 9 (TLR9) depending on the cellular context and concentration. This dual TLR modulation contributes to its complex immunological profile, which includes both pro-inflammatory and anti-inflammatory effects across different biological settings.

Research has investigated LL-37 for potential roles in:

• Direct antimicrobial activity against gram-positive and gram-negative bacteria, fungi, and select viruses
• Wound healing promotion through keratinocyte migration, proliferation, and re-epithelialisation
• Angiogenesis, via stimulation of vascular endothelial growth factor (VEGF) signalling
• Chemotaxis of neutrophils, monocytes, and mast cells to sites of infection or injury
• Anti-biofilm activity, disrupting bacterial biofilms that are resistant to conventional antibiotics
• Gut microbiome modulation and intestinal barrier function
• Antiviral properties, including in vitro activity against HIV-1 and influenza A

LL-37 is not approved by the United States Food and Drug Administration (FDA) or the European Medicines Agency (EMA) for any therapeutic application and is classified as a research chemical in most jurisdictions.

Reported Protocols

The following information represents commonly reported research ranges drawn from anecdotal accounts and available research literature. These are not medical recommendations.

Subcutaneous Protocol

Subcutaneous injection is the most commonly reported administration route for LL-37 in anecdotal research accounts. Commonly reported doses range from 100 mcg to 200 mcg, administered 2 to 3 times weekly. Pharmacokinetic data for LL-37 in humans are limited; the estimated plasma half-life of approximately 3 to 4 hours is derived from preclinical models and early clinical investigations.

• Dosing frequency: 2 to 3 times weekly, with doses spaced evenly across the week
• Cycle duration: Anecdotal research accounts describe cycles of 4 to 8 weeks, followed by a period off before resuming
• Loading phase: A formal loading phase is not commonly described for LL-37 in the research literature; the standard dosing schedule is applied from the outset

Dosing Schedule

A commonly reported schedule in anecdotal accounts involves 100 to 200 mcg administered on non-consecutive days, for example Monday, Wednesday, and Friday, to allow recovery between doses. Some accounts describe starting at the lower end of the range (100 mcg) for the first week before moving to 200 mcg, though this approach has no formal clinical basis and reflects individual preference rather than established protocol.

Injection Site Selection

Subcutaneous injection is typically performed in the abdominal region, with site rotation between each dose. The thigh is reported as an alternative site. Rotation between sites is described in most anecdotal accounts to minimise localised tissue response and to distribute any injection site reactions across different areas.

Reported Effects

The following effects have been reported in preclinical research, clinical investigations, and anecdotal accounts. This list reflects the research landscape and does not constitute confirmed clinical outcomes for any specific individual.

Antimicrobial Activity

Research has most consistently characterised LL-37 for direct antimicrobial properties:

• Disruption of gram-positive bacterial membranes, including strains of Staphylococcus aureus and Streptococcus species
• Disruption of gram-negative bacterial membranes, including Escherichia coli and Pseudomonas aeruginosa
• Antifungal activity against Candida species in vitro
• In vitro antiviral activity against HIV-1, influenza A, herpes simplex virus, and respiratory syncytial virus (RSV), though human clinical evidence for antiviral applications is limited
• Anti-biofilm activity, with research demonstrating disruption of established biofilms that are typically resistant to conventional antibiotics

Wound Healing and Angiogenesis

Research has investigated LL-37 in the context of cutaneous wound healing:

• Promotion of keratinocyte migration and proliferation, supporting the re-epithelialisation phase of wound healing
• Stimulation of VEGF (vascular endothelial growth factor) signalling, contributing to angiogenesis at wound sites
• Chemotaxis of monocytes, neutrophils, and mast cells to sites of tissue injury
• Clinical investigations for chronic venous leg ulcers have examined intradermal LL-37 administration, with some trials reporting improved healing outcomes compared to placebo

Immunomodulation

Research has characterised LL-37 as a modulator of both innate and, indirectly, adaptive immune responses:

• Modulation of TLR4 and TLR9 signalling: LL-37 can act as an antagonist of LPS-induced TLR4 signalling (potentially reducing excessive inflammatory responses to bacterial endotoxin) while also forming complexes with nucleic acids that can activate TLR9
• Promotion of dendritic cell maturation and modulation of cytokine production
• Induction of IL-18 and other cytokines that bridge innate and adaptive immune responses

Skin and Gut Research Contexts

Anecdotal research accounts and observational research have described interest in LL-37 in two specific tissue contexts:

• Skin: LL-37 is naturally expressed in skin epithelium and is elevated in psoriatic lesions. Research has noted that the peptide’s role in skin is complex: while it is antimicrobial and wound-healing in normal concentrations, overexpression in psoriasis is associated with pro-inflammatory activity and may contribute to the pathology of the condition. This dual role is an important consideration in the research context.
• Gut: Research has investigated LL-37 expression in the gastrointestinal epithelium and its potential role in gut microbiome modulation, intestinal barrier integrity, and response to gut pathogens. Vitamin D signalling is a known regulator of CAMP gene expression in intestinal epithelial cells, creating a documented link between vitamin D status and LL-37 availability in the gut.

Reported Side Effects

Reported side effects in research and anecdotal accounts include the following. This list does not constitute a comprehensive safety profile and should not be interpreted as predictive of individual outcomes.

LL-37 is characterised as a cationic peptide with membrane-active properties, which means that at sufficient concentrations it can interact with host cell membranes as well as pathogen membranes. Haemolytic activity at high concentrations has been documented in vitro; anecdotal research accounts at the doses typically described (100-200 mcg subcutaneous) do not commonly report haemolytic effects, but this property of the peptide class is noted in the research literature.

The psoriasis overexpression context warrants specific mention: individuals with autoimmune skin conditions, particularly psoriasis, should be aware that LL-37 is implicated in the pathophysiology of psoriatic skin lesions. In psoriasis, LL-37 overexpression drives inflammatory signalling by forming immunostimulatory complexes with self-DNA, contributing to the chronic inflammatory cycle. This is a distinct context from the antimicrobial and wound healing applications that are the focus of most research interest.

The compound is not reported to produce hormonal suppression or organ toxicity at the doses described in anecdotal research accounts. Human safety data outside of wound healing clinical trials is limited, as most LL-37 research to date has been conducted in vitro or in preclinical models.

Storage & Handling

Lyophilized Powder (Unreconstituted)

• Refrigerator (2-8°C): Preferred storage condition; commonly reported as stable for 12 months or more when stored properly and protected from light
• Room temperature: Not recommended for extended periods; refrigeration is consistently described as the baseline condition in research supplier guidance
• Freezer: Acceptable for long-term storage of the dry powder; avoid repeated freeze-thaw cycles, which can degrade peptide integrity
• Light sensitivity: Store in an opaque or amber vial away from direct light exposure

Reconstituted Solution

• Refrigerator (2-8°C): Use within 24 to 48 hours of reconstitution; anecdotal accounts suggest LL-37 in solution is moderately stable over this window when kept cold
• Do not freeze a reconstituted solution; freezing can degrade the peptide structure and reduce activity
• Sterile water is commonly reported as the diluent of choice for single-use preparations; bacteriostatic water may be used for multi-use vials where multiple doses will be drawn over a short period
• Discard if the solution becomes cloudy, discoloured, or shows particulate matter

Reconstitution

Add the chosen diluent slowly to the lyophilized vial, directing the liquid along the inside wall rather than directly onto the peptide powder. Swirl gently; do not shake. Allow several minutes for complete dissolution. The stability window begins at the point of reconstitution. See the Reconstitution Guide for step-by-step instructions.

Frequently Asked Questions

What makes LL-37 the only human cathelicidin? Cathelicidins are a family of antimicrobial peptides found across vertebrates, but humans express only a single gene in this family: the CAMP gene (cathelicidin antimicrobial peptide). CAMP encodes the precursor protein hCAP18 (human cationic antimicrobial protein, 18 kDa), which is proteolytically cleaved by serine proteases to release the 37-amino acid active fragment known as LL-37. Other mammals express multiple cathelicidin genes; the single human cathelicidin gene appears to have arisen through evolutionary gene reduction. LL-37 is consequently the sole endogenous human representative of this antimicrobial peptide class.

How does LL-37 differ from Thymosin Alpha-1 in immune research? Thymosin Alpha-1 and LL-37 are both investigated for immune-related properties but act through distinct mechanisms and at different levels of the immune system. Thymosin Alpha-1 is a 28-amino acid peptide derived from prothymosin alpha and is primarily characterised for its effects on adaptive immunity: T-lymphocyte differentiation, NK cell activation, and dendritic cell maturation. LL-37, by contrast, is a 37-amino acid amphipathic alpha-helical peptide that acts primarily within the innate immune system. Its direct membrane-disrupting mechanism targets bacterial, fungal, and some viral membranes, and it also modulates TLR4 and TLR9 signalling, promotes wound healing, and supports keratinocyte migration and angiogenesis. The two peptides occupy complementary roles: Thymosin Alpha-1 in adaptive immune enhancement and LL-37 in innate defence and tissue repair contexts.

What is the regulatory and sourcing status of LL-37? LL-37 is not approved as a therapeutic agent by the United States Food and Drug Administration (FDA), the European Medicines Agency (EMA), or most other major regulatory bodies. It is available from peptide research suppliers as a research chemical, primarily in lyophilized form. The peptide has been the subject of academic and clinical investigations, including wound healing studies and early-phase trials for conditions such as chronic leg ulcers, but no regulatory approval for human therapeutic use has been granted to date. Sourcing and possession regulations vary by jurisdiction; individuals should verify applicable local regulations before acquisition.

Can LL-37 be used topically as well as subcutaneously? Research has investigated LL-37 via multiple administration routes, including topical application, in addition to subcutaneous injection. Clinical investigations for wound healing, particularly in chronic venous leg ulcers, have examined intradermal and topical formulations. Topical application faces challenges related to peptide stability, skin penetration, and degradation by proteases present in wound exudate. Anecdotal research accounts focus predominantly on subcutaneous injection for systemic research applications, while topical use is more commonly discussed in wound-healing research contexts. Neither route has established regulatory approval for human therapeutic application.

Related Pages

Goals: Immune Support | Recovery & Healing

Class: Antimicrobial Peptides

Comparisons: Thymosin Alpha-1 vs LL-37

Also see: Thymosin Alpha-1 (adaptive immune modulation, T-cell differentiation) | BPC-157 (tissue repair and anti-inflammatory properties)

References & Further Reading

• Hancock REW, Sahl HG. (2006). Antimicrobial and host-defence peptides as new anti-infective therapeutic strategies. Nature Biotechnology, 24(12), 1551-1557. PubMed →
• Sorensen OE, Borregaard N. (2016). Neutrophil extracellular traps: the dark side of neutrophil-derived host defense. Nature Reviews Immunology, 16(12), 749-759. PubMed →
• Vandamme D, Landuyt B, Luyten W, Schoofs L. (2012). A comprehensive summary of LL-37, the factotum human cathelicidin peptide. Cellular and Molecular Life Sciences, 69(9), 1555-1569. PubMed →
• Ramos R, et al. (2011). Wound healing activity of the human antimicrobial peptide LL37. Peptides, 32(7), 1469-1476. PubMed →
• Heilborn JD, et al. (2003). The cathelicidin anti-microbial peptide LL-37 is involved in re-epithelialization of human skin wounds and is lacking in chronic ulcer epithelium. Journal of Investigative Dermatology, 120(3), 379-389. PubMed →
• Elias M, et al. (2014). Microbiome-related mechanisms in the pathogenesis of psoriasis: role of LL-37 as a key antimicrobial peptide. Immunology Letters, 161(2), 201-207. PubMed →
• Tjabringa GS, et al. (2006). The antimicrobial peptide LL-37 activates innate immunity at the airway epithelial surface by transactivation of the epidermal growth factor receptor. Journal of Immunology, 177(6), 4064-4070. PubMed →