A 37-amino-acid peptide and the only cathelicidin antimicrobial peptide in humans. The primary effector molecule of the innate immune system's first line of defense against bacterial, viral, and fungal pathogens. Also modulates adaptive immunity, wound healing, and inflammation.
LL-37 (named for its two N-terminal leucines and 37-residue length) is the only member of the cathelicidin antimicrobial peptide family in humans. It is produced by neutrophils, macrophages, and epithelial cells of the skin, gut, and respiratory tract as a first-line defense against microbial invasion.
LL-37 is cleaved from its precursor protein hCAP-18 (human cationic antimicrobial protein, 18 kDa) by proteinase 3 in neutrophils or by kallikreins in skin. Its expression is upregulated by vitamin D — which is why vitamin D deficiency is associated with increased susceptibility to infections like tuberculosis.
LL-37 is the only human cathelicidin — a class of antimicrobial peptides found across the animal kingdom as part of the innate immune system. It is produced by neutrophils, macrophages, epithelial cells, and other immune cells, and is one of the body's first-line defenses against bacterial, viral, and fungal infections. The name derives from its 37-amino-acid length and the two leucine residues at its N-terminus.
LL-37 kills pathogens primarily by disrupting their cell membranes — its amphipathic (part hydrophilic, part hydrophobic) helical structure allows it to insert into microbial membranes and create pores, causing cell lysis. This mechanism is fundamentally different from conventional antibiotics and is much harder for bacteria to develop resistance against, because resisting LL-37 would require bacteria to fundamentally alter their membrane structure. This has made LL-37 and other antimicrobial peptides a major research focus as potential alternatives to antibiotics in the era of antimicrobial resistance.
Beyond its direct antimicrobial activity, LL-37 functions as an immunomodulator — it recruits immune cells to infection sites, promotes wound healing, modulates inflammatory cytokine production, and can neutralize bacterial lipopolysaccharide (LPS), a potent trigger of sepsis. This dual role as both a direct antimicrobial and an immune coordinator makes LL-37 one of the most functionally versatile peptides in the innate immune system.
LL-37 adopts an amphipathic α-helical structure upon contact with bacterial membranes. Like melittin, its cationic charge (+6) attracts it to anionic bacterial membranes, and its hydrophobic face inserts into the lipid bilayer. However, LL-37 has better selectivity for bacterial vs human membranes than melittin, making it less toxic to host cells. Beyond direct killing, LL-37 signals through formyl peptide receptor-like 1 (FPRL1) and P2X7 receptors to recruit and activate immune cells.
| Pathway | Effect | Significance |
|---|---|---|
| Membrane disruption | Amphipathic helix inserts into bacterial membranes causing lysis | Broad-spectrum activity against Gram+, Gram−, fungi, and some viruses |
| Chemotaxis | Activates FPRL1 on neutrophils, monocytes, and T-cells | Recruits immune cells to infection sites |
| LPS neutralization | Binds and neutralizes bacterial endotoxin | Prevents septic shock signaling through TLR4 |
| Wound healing | Promotes keratinocyte migration and angiogenesis | Accelerates epithelial barrier repair |
| Vitamin D regulation | LL-37 gene (CAMP) is transcriptionally activated by vitamin D receptor | Explains the vitamin D-infection susceptibility connection |
| Study | Design | Findings | Level |
|---|---|---|---|
| Genetic studies | Human cohorts | LL-37 deficiency (Kostmann syndrome, specific granule deficiency) causes severe recurrent infections. Confirms essential role in immunity. | Level I (genetic) |
| Vitamin D connection | Observational + mechanistic | Vitamin D supplementation increases LL-37 expression. Low vitamin D → low LL-37 → increased TB susceptibility. | Level I-II |
| Wound healing | Preclinical + clinical pilot | Topical LL-37 promotes chronic wound healing (diabetic ulcers). Phase I/II studies ongoing. | Level II |
| Anti-biofilm | In vitro/in vivo | LL-37 disrupts bacterial biofilms, including MRSA and Pseudomonas biofilms | Preclinical |
| COVID-19 | Observational | Low LL-37 levels associated with more severe COVID-19 outcomes. Vitamin D supplementation trials ongoing. | Level II-III |
Endogenous peptide: LL-37 is naturally produced by the human body, which provides a theoretical safety advantage — the immune system is already adapted to its presence. However, exogenous administration at therapeutic doses may produce effects beyond physiological levels.
Inflammatory balance: While LL-37 is anti-infective, at high concentrations it can be pro-inflammatory, contributing to inflammatory conditions. Elevated LL-37 levels have been associated with psoriasis, rosacea, and atherosclerosis. This suggests a therapeutic window — too little increases infection susceptibility, but too much may drive inflammatory pathology.
Category 2 status: LL-37 was placed on the FDA Category 2 list with a notation about limited safety data. Its status under the February 2026 reclassification is uncertain — it may remain on Category 2 depending on the FDA's final determination.
Administration: In research settings, LL-37 is administered subcutaneously. Optimal dosing, frequency, and duration of treatment have not been established in human clinical trials.
| Jurisdiction | Status |
|---|---|
| FDA | Not approved as a drug. Clinical trials ongoing for wound healing. |
| Research | One of the most studied AMPs in the world. >5,000 publications. |
| Vitamin D connection | Public health implication: vitamin D supplementation as an indirect way to boost LL-37 levels |