LL-37 is the only cathelicidin the human body produces. It is a 37 amino acid, cationic, amphiphilic alpha-helical peptide cleaved from its CAP-18 precursor by proteinase 3. Most production occurs in the large bowel, with additional expression in the lung and other tissues, where it serves as a natural broad-spectrum antimicrobial.

This lesson examines LL-37 across innate immunity, wound healing, and emerging therapeutic potential. Its pleiotropic profile means it works through many parallel pathways at once, producing antimicrobial, immunomodulatory, wound-healing, antibiofilm, and anticancer effects.

How it works

LL-37 acts through three primary mechanisms. It disrupts bacterial membranes by interacting with LPS and forming a pore in unsaturated phospholipids, driving cell death. It modulates immunity by neutralizing toll-like receptor four signaling, suppressing pro-inflammatory cytokine activity, and activating dendritic cells. It supports wound healing by promoting keratinocyte migration and angiogenesis through the FPRL1 and FPR2 pathway, with stimulation of VEGF, transforming growth factor beta, and fibroblast growth factor expression.

Receptor engagement depends on tissue context, peptide concentration, and which receptors a cell expresses. The vitamin D axis is central: toll-like receptor activation upregulates the vitamin D receptor in macrophages, and vitamin D binding enhances CAP18/LL-37 transcription. Vitamin D status directly influences LL-37-mediated immunity, which is why optimizing vitamin D comes first.

Clinical considerations

LL-37 shows broad-spectrum activity against gram-positive, gram-negative, fungal, and viral organisms, including resistant isolates. Antibiofilm effects appear at sub-MIC concentrations, and the peptide can show synergy with conventional antibiotics. Animal sepsis models reduced endotoxin and TNF levels comparable to polymyxin B.

Anticancer effects are context-dependent. In some tumor types LL-37 can support antitumor activity, while in lung, breast, and ovarian cancer it may be protumorigenic through Wnt/beta-catenin and NF-kB. This profile calls for careful risk-benefit assessment. Red flags include psoriasis, lupus, and active malignancy, with limited data in pregnancy, lactation, and immunocompromised states. LL-37 remains investigational, with phase one intratumoral trials in melanoma.

Key clinical points

• LL-37 is the only human cathelicidin, with antimicrobial, antibiofilm, immunomodulatory, wound-healing, and context-dependent anticancer activity.
• Optimize vitamin D status first; LL-37 expression and function depend on it, with a target range Dr. Seeds describes around 60 to 80.
• Antibiofilm activity occurs at very low concentrations, and resistance development potential is low.
• Screen for autoimmune conditions and cancer history; avoid use in psoriasis, lupus, and active malignancy.
• Baseline labs include vitamin D, white count, and inflammatory markers (TNF alpha, IL-6, CRP, ESR), with cycles generally held to six to twelve weeks.