PwPepwise

LL-37 (Cathelicidin)

Immune Support

a.k.a. hCAP-18

Antimicrobial peptide

LL-37, also known as cathelicidin or hCAP-18, is the only human member of the cathelicidin family of host defense peptides.

§Dosing at a glance

3 protocols · from the research
What it's forDoseHow oftenHowFor how long
Topical Administration (Wound Healing / Diabetic Foot Ulcers)0.5 mg/mLWeeklyTopicalApplied on the skin.4 wks
Vitamin D-Mediated Endogenous Induction (Indirect Upregulation of LL-37)50,000 IUWeeklyOralTaken by mouth.5 wks
Preclinical / In Vitro Reference Concentrations1 µg/mLWeekly——

Approximate values pulled from the research — double-check before dosing.

§01Summary

LL-37, also known as cathelicidin or hCAP-18, is the only human member of the cathelicidin family of host defense peptides. Produced naturally by immune cells, skin cells, and the lining of the lungs and urinary tract, it acts as a frontline defender against bacterial, viral, and fungal threats while simultaneously coordinating the body's broader immune response. In healthy skin and mucosal tissues, LL-37 helps kill invading pathogens directly and recruits immune cells such as neutrophils, monocytes, and T cells to sites of infection2. It also appears to play an important role in wound healing, promoting the regrowth of skin cells and supporting the formation of new blood vessels18,20. A particularly well-studied connection links LL-37 production to vitamin D levels — the body appears to rely on adequate vitamin D to sustain LL-37 output in immune cells responding to infection, including against tuberculosis3,6,11. Research has explored LL-37's potential in skin infections1, urinary tract protection12, and wound repair18, with early human studies emerging for conditions like diabetic foot ulcers16. Its biological roles span antimicrobial defense, immune modulation, and tissue repair, making it one of the most functionally diverse peptides identified in human innate immunity10.

This is the layperson summary. Mechanism, dosing, the evidence base, and the published literature are in the sections below — every claim links to its source.

§02In depth

LL-37 is a 37-amino acid cationic, amphipathic alpha-helical peptide derived from the C-terminal domain of the 18 kDa precursor protein hCAP-18, encoded by the CAMP gene on human chromosome 3. Processing to the active form occurs extracellularly: upon exocytosis of neutrophil azurophil granules, the serine protease proteinase 3 cleaves hCAP-18 at a species-specific cleavage site to generate LL-37, a mechanism distinct from elastase-mediated processing seen in bovine and porcine cathelicidins4. This extracellular-only activation is proposed to function as a regulatory safeguard against intracellular cytotoxicity, given LL-37's established cytotoxic properties4.

LL-37 exerts direct membrane-disrupting antimicrobial activity through its cationic charge and amphipathic helical conformation, enabling insertion into and destabilization of bacterial phospholipid membranes. Broad-spectrum activity has been demonstrated against Gram-negative organisms including Pseudomonas aeruginosa as well as Gram-positive bacteria, fungi, and viruses, though efficacy is attenuated at elevated NaCl concentrations relevant to cystic fibrosis airway surface liquid7. Beyond direct microbicidal action, LL-37 functions as a chemoattractant for human neutrophils, monocytes, and T lymphocytes through engagement of the G protein-coupled receptor formyl peptide receptor-like 1 (FPRL1), inducing intracellular Ca²⁺ mobilization confirmed by cross-desensitization studies2. The same FPRL1 receptor on vascular endothelial cells mediates LL-37's pro-angiogenic activity, stimulating endothelial proliferation, tube formation, and neovascularization in vivo20.

At the transcriptional level, the human CAMP gene harbors a functional vitamin D response element (VDRE) embedded within a primate-specific SINE retrotransposon in its promoter, enabling direct transcriptional upregulation by the vitamin D receptor (VDR) upon binding of 1,25-dihydroxyvitamin D33. This regulatory pathway is absent in murine, rat, and canine genomes, making it a uniquely human/primate mechanism3. Downstream of VDR activation, cathelicidin acts as an intermediary inducing transcription of autophagy genes Beclin-1 and Atg5 in human macrophages, promoting phagosome-autophagosome colocalization and antimycobacterial killing6. IFN-gamma–induced antimicrobial activity against M. tuberculosis in human macrophages similarly requires vitamin D-sufficient substrate availability for CYP27B1-mediated conversion of 25OHD to active 1,25D3, with cathelicidin serving as a downstream effector bridging acquired T cell immunity and innate killing mechanisms11.

LL-37 also modulates TLR-mediated inflammatory signaling at physiological concentrations (≤1 µg/mL), selectively suppressing NF-κB subunit nuclear translocation (p50 and p65 reduced by ≥50%) and attenuating proinflammatory gene expression — including NFκB1 and TNFAIP2 — while leaving anti-inflammatory regulatory genes such as TNFAIP3 and NFκBIA intact17. This selective immunomodulatory profile operates upstream of TNF-α and IL-1β signaling, specifically targeting TLR2/4 and TLR9 pathway activation. In keratinocytes, LL-37 neutralizes cytosolic DNA to suppress AIM2 inflammasome activation and IL-1β processing13, while in wound contexts, local TGF-beta1–induced CYP27B1 upregulation generates active 1,25D3 within the wound microenvironment to drive LL-37 and TLR2/CD14 co-expression9 — illustrating an autocrine/paracrine regulatory circuit governing cutaneous innate immunity.

§04Evidence & efficacy

Evidence base
319Studies
157Human
23Animal

LL-37 demonstrates a broad and replicated mechanistic profile across multiple functional domains in human and animal research, with human interventional efficacy data actively emerging.

Antimicrobial Activity:
Endogenous LL-37 appears essential for cutaneous and mucosal innate defense. Cathelicidin-deficient mice showed markedly increased bacterial burden and necrotic lesion formation following Group A Streptococcus skin infection compared to wild-type controls1. In the urinary tract, CRAMP-deficient mice demonstrated significantly impaired bacterial clearance, and clinical E. coli strains with greater LL-37 resistance caused more severe urinary tract infections in human patients — a direct translational correlation12. LL-37 has been reported to exhibit broad-spectrum antimicrobial activity against Gram-positive and Gram-negative bacteria including Pseudomonas aeruginosa in airway surface fluid models, though activity appears sensitive to elevated NaCl concentrations7.

Wound Healing and Angiogenesis:
hCAP18/LL-37 is strongly upregulated in acute human skin wounds and is markedly reduced in chronic ulcer epithelium. Ex vivo antibody inhibition of LL-37 suppressed re-epithelialization and epithelial cell proliferation in a concentration-dependent manner18, suggesting a functional role in wound closure. LL-37 may promote angiogenesis via FPRL1 receptors on endothelial cells, with in vivo neovascularization observed in CAM and rabbit ischemia models20. A registered human RCT is evaluating topical LL-37 cream in diabetic foot ulcers, with human efficacy data emerging16.

Immunomodulation:
LL-37 recruits neutrophils, monocytes, and T lymphocytes via the FPRL1 receptor with confirmed calcium flux and receptor specificity2. At physiological concentrations (≤1 µg/mL), LL-37 may suppress TLR-mediated proinflammatory signaling, reducing NF-κB nuclear translocation by ≥50% and substantially attenuating TNF-α and cytokine release from human PBMCs stimulated with LPS and TLR2/4 and TLR9 agonists17.

Vitamin D–Mediated Antimycobacterial Activity:
Vitamin D supplementation in vitro appears to restore IFN-gamma–induced LL-37 production, autophagy, phagosome-lysosome fusion, and antimycobacterial killing of M. tuberculosis in macrophages from vitamin D-deficient donors11. Cathelicidin has been identified as a transcriptional inducer of autophagy genes Beclin-1 and Atg5 in human macrophages6, with the vitamin D–cathelicidin–autophagy axis supporting antimycobacterial defense.

§05Safety

The safety profile of endogenously expressed LL-37 is well-characterized at the mechanistic level, while formal human clinical safety data from exogenous administration trials remains an emerging area of research.

From a biological standpoint, LL-37 exhibits cytotoxic properties in addition to its antimicrobial activity, and the human body appears to regulate this through spatial and temporal control of activation — notably, the precursor hCAP-18 is cleaved to active LL-37 only extracellularly by proteinase 3 after exocytosis, rather than intracellularly, a mechanism proposed to limit host cell damage during immune responses4. This suggests evolved safety regulation around the peptide's cytotoxic potential.

At the tissue and disease level, endogenous LL-37 is associated with inflammatory pathology when dysregulated. In rosacea, abnormally processed cathelicidin peptide fragments drive chronic skin inflammation, with aberrant SCTE-mediated cleavage producing distinct peptide forms responsible for inflammatory responses in mouse models5. In psoriasis, LL-37 has a dual role: it can complex with self-DNA to activate plasmacytoid dendritic cells via TLR9, contributing to autoinflammatory signaling, while simultaneously suppressing AIM2 inflammasome activation in keratinocytes13. These context-dependent inflammatory associations are observed for endogenous, dysregulated LL-37 and are part of the active research characterization of exogenous therapeutic use.

No human clinical adverse event data from exogenous LL-37 administration has been published. The one registered RCT evaluating topical LL-37 cream (0.5 mg/mL) in diabetic foot ulcers had no reported safety outcomes in its available protocol documentation16.

§06History

LL-37 was first characterized in 1995 as the active antimicrobial peptide derived from hCAP-18, the sole human cathelicidin, and takes its name from its two N-terminal leucines and 37-amino acid length. Early research established its expression in neutrophil granules and positioned it within the broader cathelicidin family of innate immune peptides conserved across mammals. A landmark 1998 study confirmed its expression in human airway epithelium and demonstrated broad-spectrum antimicrobial activity in pulmonary surface fluid7, establishing LL-37 as a key mucosal defense molecule. The identification of proteinase 3 as the sole processing enzyme for hCAP-18 cleavage in 2001 resolved a key mechanistic question about how the active peptide is generated in inflammatory contexts4.

A pivotal 2001 Nature study using cathelicidin-deficient mice formally demonstrated in vivo protective function against invasive Group A Streptococcus skin infection, addressing prior skepticism about whether antimicrobial peptides were functionally active in physiological conditions1. Subsequent years brought foundational discoveries: identification of FPRL1 as a chemotactic receptor2, characterization of its angiogenic role20, and its wound-healing function18. The 2004–2009 period produced seminal findings connecting vitamin D signaling directly to CAMP gene transcription via a primate-specific VDRE3,6,11, with significant implications for tuberculosis susceptibility and innate immune regulation. Research in the 2007–2012 period expanded understanding of LL-37's complex roles in inflammatory skin diseases including rosacea5 and psoriasis13. Human clinical investigation is actively developing, with registered trials evaluating topical LL-37 in wound healing16 and ongoing research across infectious, inflammatory, and oncological indications10.

§07References