LL-37 (CAP-18): The Sole Human Cathelicidin and Its Role in Innate Immunity

Research summary. LL-37 is the only known human cathelicidin-derived antimicrobial peptide, generated by C-terminal cleavage of the precursor protein hCAP-18. It is studied across an unusually broad range of biological contexts — direct antimicrobial activity against gram-positive and gram-negative bacteria, immunomodulation in inflammatory conditions, epithelial wound repair, and pro-angiogenic signalling. Its activity profile is highly context-dependent: research has reported that LL-37 can either dampen or amplify inflammatory signalling depending on the local milieu and the activation state of the immune cells present.

Molecular profile

• Sequence: LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES (37 residues)
• Molecular formula: C₂₀₅H₃₄₀N₆₀O₅₃
• Molecular weight: ~4493.3 g/mol
• PubChem CID: 16198951
• CAS: 154947-66-7
• Precursor: hCAP-18 (cathelicidin antimicrobial peptide, 18 kDa); LL-37 is the active C-terminal fragment

Mechanism of action

LL-37 is a cationic, amphipathic α-helical peptide. Its primary direct antimicrobial mechanism involves electrostatic association with negatively charged microbial membranes — including binding to lipopolysaccharide (LPS) on gram-negative bacteria — followed by insertion into the lipid bilayer and disruption of membrane integrity. Beyond direct membrane activity, research has reported a range of receptor-mediated effects: signalling through formyl peptide receptor-like 1 (FPRL1, also called FPR2) on leukocytes, modulation of Toll-like receptor 4 (TLR4) signalling, and binding of extracellular nucleic acids to alter endosomal TLR responses (TLR3, TLR7, TLR9). LL-37 is also produced and stored in neutrophil secondary granules and in macrophages, with rapid release at sites of microbial challenge.

Preclinical research highlights

Broad-spectrum antimicrobial activity. Preclinical studies have characterised activity against a wide range of gram-positive and gram-negative bacteria and against some fungi and enveloped viruses. The peptide's interaction with bacterial LPS is one of the most studied aspects of its activity profile and has motivated interest in LL-37 as a model for antimicrobial peptide design.

Context-dependent immunomodulation. Research in cell culture has reported that LL-37 modulates neutrophil and eosinophil chemotaxis, alters keratinocyte apoptosis, modulates IFN-α and IL-18 production, and produces opposing effects on T-cell inflammatory output depending on prior activation state. This has driven interest in LL-37 as a homeostatic regulator rather than a unidirectional pro- or anti-inflammatory agent.

Epithelial wound repair. Preclinical models have reported that LL-37 promotes epithelial cell migration and proliferation in airway and intestinal epithelium and accelerates wound closure in skin models. Research has also reported a role in maintaining the intestinal epithelial barrier under inflammatory stress.

Angiogenesis. Endothelial cell studies have reported that LL-37 induces synthesis of prostaglandin E₂ and stimulates blood vessel formation, with implications studied across wound healing, ischaemia, and tumour biology research models.

Disease-association studies. LL-37 expression has been studied in psoriasis, rheumatoid arthritis, atherosclerosis, inflammatory bowel disease, and cystic fibrosis lung disease research models. The interpretation of elevated LL-37 in these contexts (driver of pathology versus compensatory homeostatic response) remains an active research question and the answer appears to differ across conditions.

Current research status

LL-37 is an investigational research peptide. It is not an FDA-approved drug. Several LL-37 derivatives and analogues have been advanced into early clinical development as topical antimicrobials and as candidate therapeutics for infectious and inflammatory conditions, but no human cathelicidin-derived product has cleared late-stage approval at the time of writing. The peptide's dual-edged immunomodulatory profile means that translational interpretation requires careful matching of model system, dose range, and inflammatory context.

For research-supplier contexts, LL-37 is supplied as a research-grade investigational peptide and is not intended for self-administration.

Key takeaways for researchers

• LL-37 is the only human cathelicidin-derived antimicrobial peptide and is produced by neutrophils, macrophages, and several epithelial tissues.
• Direct activity is membrane-disruptive and is driven by cationic, amphipathic α-helical character; receptor-mediated effects include FPR2 signalling and TLR modulation.
• Preclinical research has reported broad antimicrobial activity, context-dependent immunomodulation, epithelial wound repair, and pro-angiogenic effects.
• Effects are highly context-dependent — research model, dose, and inflammatory state all materially change the observed direction of activity.
• LL-37 is not an FDA-approved drug; analogues are at varying stages of investigational development.

References

1. 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.
2. 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.

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This article is provided for educational and research purposes only. LL-37 is not an FDA-approved drug. It is not intended for self-administration, and any research-context handling should be conducted by qualified personnel within an appropriate institutional setting and in compliance with applicable regulatory requirements. Nothing in this article constitutes medical advice or a recommendation for use.