LL-37: Cathelicidin Antimicrobial Peptide Research

LL-37 is a synthetic cathelicidin-derived antimicrobial peptide studied in laboratory settings for its role in innate immune signalling, antimicrobial membrane disruption mechanisms, immunomodulatory pathway studies, and wound healing biology. It is the only cathelicidin in humans, processed from the C-terminal domain of the hCAP18 precursor protein.

Chemical and Molecular Data

Structure and Amphipathic Helical Design

LL-37 adopts an alpha-helical conformation in membrane-mimicking environments and in solution at physiological pH. This amphipathic helix is fundamental to its mechanism of action: the helix positions hydrophobic residues on one face and cationic residues on the other, enabling simultaneous insertion into negatively charged bacterial membranes and disruption of membrane integrity.

The cationic nature of LL-37 (net charge approximately +6 at physiological pH) facilitates initial electrostatic attraction to the negatively charged surfaces of bacterial membranes (phosphatidylglycerol and cardiolipin-rich outer leaflets) while mammalian cell membranes, which are predominantly zwitterionic on the outer leaflet (phosphatidylcholine and sphingomyelin), are less susceptible.

Antimicrobial Membrane Biology Research

Laboratory research has examined LL-37's mechanism of bacterial membrane disruption through several proposed models. The toroidal pore model proposes that LL-37 monomers insert into the membrane alongside lipid head groups, forming water-filled pores. The carpet model suggests that LL-37 accumulates on the membrane surface at high concentrations, disrupting membrane integrity through a detergent-like mechanism. Research using membrane model systems (lipid vesicles, black lipid membranes) has investigated which model predominates under different peptide-to-lipid ratios.

Immunomodulatory Research

Beyond direct antimicrobial activity, LL-37 has been studied extensively as an immunomodulatory molecule. Laboratory research has examined its role as an endogenous ligand for several receptors including FPRL1 (formyl peptide receptor-like 1) and P2X7 receptors, and its effects on dendritic cell maturation, T cell polarisation, and macrophage activation in cell culture models.

Research has also examined LL-37 in the context of wound healing, investigating its effects on keratinocyte and fibroblast migration and proliferation — connecting it to BPC-157 and TB-500 research in cellular migration biology.

Cathelicidin Comparison Across Species

Membrane Disruption Mechanisms in Detail

Two primary structural models have been proposed for LL-37 membrane disruption and are actively investigated in laboratory research:

Toroidal pore model. LL-37 monomers insert perpendicular to the membrane surface alongside lipid head groups, forming water-filled pores that span the bilayer. The lipid-peptide pores cause ion leakage and ultimately membrane disruption. This model predicts concentration-dependent pore formation above a threshold peptide-to-lipid ratio.

Carpet model. At high peptide concentrations, LL-37 accumulates on the outer leaflet of the membrane, orientated parallel to the surface. Above a critical coverage density, the membrane undergoes detergent-like solubilisation and disruption. This model does not require specific pore structures.

Research distinguishing between these models uses solid-state NMR to determine peptide orientation in model membranes, oriented circular dichroism to examine membrane-bound secondary structure, and single-channel electrophysiology to characterise pore conductance.

Frequently Asked Questions

Why is LL-37 named as it is?
LL-37 is named for its N-terminal sequence (Leu-Leu, giving the double-L prefix) and its total length of 37 amino acids. It is cleaved from the C-terminal domain of the precursor protein hCAP18 (human cationic antimicrobial protein of 18 kDa). The hCAP18 precursor is stored in granules of neutrophils and other immune cells and is processed to the active LL-37 fragment by extracellular proteases — primarily proteinase 3.

Does LL-37 selectively target bacteria over mammalian cells?
LL-37 shows preferential activity against bacterial over mammalian cell membranes due to compositional differences. Bacterial membranes are rich in negatively charged phospholipids (phosphatidylglycerol, cardiolipin) that attract the cationic LL-37. Mammalian outer membrane leaflets are predominantly zwitterionic (phosphatidylcholine, sphingomyelin) — less attractive to cationic peptides. However, this selectivity is concentration-dependent: at higher concentrations, LL-37 can disrupt mammalian membranes, which is a consideration in cytotoxicity controls for laboratory research.

For laboratory and analytical research purposes only. Not for human or veterinary use. No dosage or administration guidance is provided or implied.

Related research peptides: BPC-157 | TB-500 | KPV

View LL-37 product page

LL-37 Receptor Pharmacology: Multiple Receptor Systems

Unlike many research peptides that target a single well-defined receptor, LL-37 interacts with multiple cell surface receptors depending on the cell type and concentration:

FPRL1 (Formyl Peptide Receptor-Like 1, FPR2/ALX) is a Gi-coupled GPCR expressed on neutrophils, monocytes, and epithelial cells. LL-37 activation of FPRL1 promotes chemotaxis, calcium mobilisation, and phosphoinositide hydrolysis. FPRL1 is also the receptor for lipoxin A4 and other pro-resolving lipid mediators, connecting LL-37's immunomodulatory effects to the resolution phase of inflammation.

P2X7 receptor is an ATP-gated ion channel expressed on macrophages, dendritic cells, and microglia. LL-37 can activate P2X7 at higher concentrations, triggering calcium influx, NLRP3 inflammasome activation, and IL-1beta processing. This provides a mechanistic connection between antimicrobial peptide release and inflammasome-mediated inflammatory amplification.

EGFR (EGF receptor) activation by LL-37 has been reported in epithelial cell research, linking cathelicidin release to wound healing through EGFR/MAPK/ERK-mediated proliferation and migration signals.

Direct membrane interaction (not receptor-mediated) occurs at higher LL-37 concentrations and is the primary antimicrobial mechanism — disrupting bacterial membranes through toroidal pore formation or carpet-model membrane solubilisation.

LL-37 in Wound Healing Research

The wound healing research applications of LL-37 extend beyond its antimicrobial activity. Published studies have examined LL-37 in:

Keratinocyte migration: LL-37 promotes keratinocyte migration in scratch assay models through EGFR transactivation and MAPK/ERK signalling. This migration-promoting activity is relevant to the re-epithelialisation phase of wound healing.

Angiogenesis: LL-37 has been shown to promote tube formation in endothelial cell Matrigel assays and to increase VEGF expression in keratinocytes and fibroblasts — connecting cathelicidin release in wounds to angiogenic responses.

Fibroblast proliferation: Published research has examined LL-37's effects on dermal fibroblast proliferation and collagen synthesis, with proposed connections to the TGF-beta and EGF signalling systems.

These wound healing research directions make LL-37 a relevant companion tool to BPC-157, TB-500, and GHK-Cu research — all of which are studied in tissue biology and wound healing contexts through distinct mechanisms.

Frequently Asked Questions

What makes LL-37 selectively toxic to bacteria over human cells?
LL-37's selectivity for bacterial over mammalian cell membranes is based on membrane charge differences. Bacterial membranes are rich in negatively charged phospholipids (phosphatidylglycerol, cardiolipin) that attract the cationic LL-37 peptide electrostatically. Mammalian outer membrane leaflets are predominantly composed of zwitterionic phospholipids (phosphatidylcholine, sphingomyelin) that are much less attractive to cationic peptides. This selectivity is concentration-dependent — at high concentrations, LL-37 can disrupt mammalian cell membranes, which defines the therapeutic window for antimicrobial peptide research.

How is LL-37 generated from hCAP18 in vivo?
hCAP18 (human cationic antimicrobial protein of 18 kDa) is the precursor protein stored in neutrophil secondary granules and secreted by epithelial cells. The C-terminal LL-37 domain is released from hCAP18 by proteinase 3 (from neutrophil azurophilic granules) upon degranulation, and by kallikreins (KLK5, KLK7) in skin. Different proteases generate slightly different N-terminal variants of the LL-37 sequence with different antimicrobial and immunomodulatory activities — a consideration when comparing published data using different LL-37 preparations.

Browse all Signal Labs research peptides | Peptide storage guide | Reconstitution guide