LL-37 is a 37-residue cationic alpha-helical peptide that is the C-terminal active fragment of the only cathelicidin in the human genome (hCAP18, encoded by the CAMP gene). The name reflects the two N-terminal leucines and the 37-residue length. The peptide is expressed by neutrophils, by epithelial cells across multiple tissues, and by certain other immune cell types, and it has both direct antimicrobial activity and substantial immunomodulatory effects.

What does 'cathelicidin' mean?

The cathelicidins are a family of cationic antimicrobial peptides characterized by a conserved N-terminal cathelin pro-region (homologous to the calf-cathepsin-L inhibitor 'cathelin' from which the family takes its name) followed by a variable C-terminal antimicrobial domain. The cathelin pro-region keeps the antimicrobial domain inactive until proteolytic processing releases the active C-terminal peptide. Cathelicidins are found in multiple mammalian species; humans have only one cathelicidin (the CAMP gene product) while many other mammals have multiple cathelicidin genes.

How does LL-37 kill bacteria?

LL-37 kills bacteria primarily through membrane-disruption mechanisms shared by many cationic amphipathic antimicrobial peptides. The peptide folds into an amphipathic α-helix upon encounter with anionic bacterial membrane surfaces, inserts into the membrane, and disrupts membrane integrity through carpet-model surface coverage, toroidal-pore formation, or intermediate disruption modes. The result is loss of membrane integrity and bacterial killing. The activity spectrum is broad and includes Gram-positive and Gram-negative bacteria, fungi, and certain enveloped viruses.

How is LL-37 produced from hCAP18?

hCAP18 (human cationic antimicrobial protein, 18 kDa) is the inactive precursor — a 170-residue protein consisting of a signal peptide, the conserved cathelin pro-region, and the C-terminal LL-37 antimicrobial domain. The processing to release the active LL-37 peptide is carried out by proteinase 3 in neutrophils (after release from secondary granules during neutrophil activation) and by kallikrein-5 (KLK5) and other tissue kallikreins in keratinocytes and other epithelial cells. The kallikrein-mediated processing in skin can also generate alternative LL-37 fragments with somewhat different biological profiles.

FPR2/ALX is formyl-peptide receptor 2 (also called lipoxin-A4 receptor for one of its other ligands). It is a G-protein-coupled receptor expressed on multiple immune cell types and is the principal characterized receptor through which LL-37 mediates its chemotactic and immunomodulatory effects on neutrophils, monocytes, T cells, and mast cells. The receptor is named for its activation by N-formylated bacterial peptides and has multiple other endogenous ligands beyond LL-37.

How is LL-37 expression regulated?

CAMP/hCAP18 expression is regulated by multiple inputs. Vitamin D signaling through the vitamin D receptor binding to a vitamin-D-response element in the CAMP gene promoter is one of the major characterized induction pathways. Inflammatory and infection-related stimuli induce expression in epithelial cells and other tissues. Various cytokine inputs and transcription factors modulate expression in different cellular contexts. The vitamin D regulation has been one of the more striking discoveries in the LL-37 literature and connects the peptide's biology to the broader vitamin D immunology field.

What is the connection between LL-37 and vitamin D?

The CAMP gene contains a vitamin-D-response element in its promoter, so 1,25-dihydroxyvitamin D3 signaling through the vitamin D receptor induces CAMP expression. This regulatory connection has been characterized in keratinocytes, in macrophages, and in various other cell types. The discovery has linked LL-37 biology to the broader research on vitamin D status and innate-immune function, including investigation of how vitamin D status affects host-defense capacity.

What is the connection to psoriasis?

Published research has characterized LL-37 as a contributing factor in psoriasis pathophysiology through a specific mechanism: LL-37 can bind self-DNA and self-RNA released from damaged cells in psoriatic skin, and the LL-37-nucleic-acid complexes activate plasmacytoid dendritic cells through Toll-like receptor 9 and other nucleic-acid-sensing pathways. The resulting type-I-interferon production contributes to the inflammatory cascade that drives psoriasis. This characterization has reframed LL-37 in the psoriasis context as a contributor to inflammation rather than purely a host-defense peptide, illustrating the complex context-dependent nature of LL-37 biology.

What is the connection to rosacea?

Published research, particularly from Richard Gallo's group at UCSD, has characterized the kallikrein-mediated processing of hCAP18 as relevant to rosacea pathophysiology. In rosacea, kallikrein-5 activity in skin is elevated, leading to increased processing of hCAP18 and to the generation of alternative LL-37 fragments with pro-inflammatory activity. The characterization has framed LL-37 processing as one of the molecular factors contributing to the inflammatory skin condition.

What is the molecular weight of LL-37?

LL-37 has a molecular formula of C205H340N60O53 and a molecular weight of approximately 4493 daltons. The peptide has 37 amino acid residues with no covalent modifications beyond the standard amide-bond backbone, no disulfide bonds, and a free C-terminal carboxylate. The molecular weight should be confirmed against the Certificate of Analysis from a reputable research-peptide supplier.

Is LL-37 approved as a medicine?

No. LL-37 is not approved as a medicine in the United States, the European Union, or other major regulatory jurisdictions at this writing. Development of antimicrobial peptides and peptidomimetics inspired by LL-37 and related cathelicidins has been pursued in various translational research programs, but no LL-37-based therapy has been advanced through pharmaceutical approval in major jurisdictions. The research-peptide form is supplied for laboratory and analytical use only.

What is the activity spectrum of LL-37?

Published research has characterized direct antimicrobial activity of LL-37 against a broad spectrum of pathogens: Gram-positive bacteria (including various staphylococci and streptococci), Gram-negative bacteria (including E. coli, Pseudomonas, and others), fungi (including Candida species), certain enveloped viruses, and some parasites. The activity is concentration-dependent and is reduced in the presence of physiological salt concentrations and of certain anionic matrix components, contributing to context-dependent activity in various biological fluids.

How is LL-37 different from defensins?

Both LL-37 and the defensins (α-defensins and β-defensins) are cationic antimicrobial peptides of mammalian innate immunity, but they are structurally distinct families. Defensins have disulfide-stabilized β-sheet architectures with three intramolecular disulfide bonds, while LL-37 is an unstructured-in-water peptide that adopts an amphipathic α-helical conformation upon membrane interaction. The two families also have somewhat different cellular-expression profiles and have been characterized for somewhat different roles in the broader innate-immune landscape, though there is substantial overlap.

How is LL-37 manufactured?

Research-grade LL-37 is produced by standard solid-phase peptide synthesis using Fmoc protecting-group chemistry. The 37-residue length is at the larger end of the practical range for SPPS but is achievable with optimized conditions. The crude peptide is purified by reversed-phase HPLC and characterized by mass spectrometry. Recombinant expression of the larger hCAP18 precursor (followed by in-vitro processing to release LL-37) is also used in some research-supply contexts. Reputable suppliers report purity in the 95-98% range for research-grade material.

What storage practices apply?

Lyophilized LL-37 stored sealed at -20 °C or below away from light and moisture is generally considered stable for extended periods. The peptide has no methionine and no cysteine residues — the most chemically vulnerable amino acid side chains — so it is relatively stable compared to many other research peptides. Reconstituted material in sterile aqueous solvent is typically held cold and used within weeks. The peptide can adsorb to plastic and to glass surfaces during dilution and handling; use of low-binding tubes and of carrier proteins (BSA) is common in research practice for low-concentration working solutions.

What is the role of LL-37 in wound healing?

Published research has characterized multiple effects of LL-37 on wound-healing processes including effects on keratinocyte migration (LL-37 promotes keratinocyte migration in in-vitro scratch assays and in-vivo wound-healing models), on keratinocyte proliferation, on angiogenesis (LL-37 has pro-angiogenic effects through FPR2-mediated signaling on endothelial cells), and on dermal-fibroblast biology. The wound-healing research context is one of the more actively developed areas of contemporary LL-37 research.

Is there only one cathelicidin in humans?

Yes — the CAMP gene encoding hCAP18/LL-37 is the only cathelicidin gene in the human genome. This is a striking finding given the multiple cathelicidins present in many other mammalian genomes (cattle have 11 cathelicidin genes, pigs have multiple, mice have one Camp gene encoding the CRAMP peptide that is the rodent counterpart of human LL-37). The single-cathelicidin status of the human genome places particular biological weight on hCAP18/LL-37 in human innate-immune function.

CRAMP (cathelicidin-related antimicrobial peptide) is the mouse counterpart of human LL-37, encoded by the single mouse Camp gene. CRAMP and LL-37 share the same precursor-processing architecture (cathelin pro-region followed by the C-terminal cationic antimicrobial domain) and have similar functional profiles, but they have different specific amino acid sequences. CRAMP is the relevant cathelicidin peptide in mouse-model studies of cathelicidin biology that inform the human LL-37 research field.

LL-37 (with Bacteriostatic Water) lyophilized vial with bacteriostatic water for research

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LL-37 is a naturally occurring human antimicrobial peptide belonging to the cathelicidin family. It is one of the most widely studied endogenous peptides in immunology, microbiology, and inflammation research due to its broad-spectrum antimicrobial activity and role in innate immune system signaling. This compound is manufactured to strict laboratory-grade standards and is intended strictly for qualified scientific and preclinical research use only.

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