LL-37 is the only cathelicidin-derived antimicrobial peptide found in humans — a 37-residue, cationic, amphipathic alpha-helix released from the precursor protein hCAP-18. It is studied in vitro for two broadly separate activities: direct disruption of microbial membranes, and receptor-mediated signaling in host cells that touches chemotaxis, angiogenesis, and epithelial migration. As of 2026 it remains one of the most heavily characterized host-defense peptides in the literature, with review coverage continuing into 2025 (Kondej et al., International Journal of Molecular Sciences, 2025).
Research Background
Cathelicidins are a family of host-defense peptides stored as inactive precursors in the granules of neutrophils and in epithelial tissue. Humans express a single cathelicidin gene, CAMP, whose product is the 18 kDa precursor hCAP-18. The mature peptide is liberated by extracellular proteolysis: Sørensen and colleagues showed in Blood (2001) that of the three known serine proteases in neutrophil azurophil granules, proteinase 3 was solely responsible for cleaving exocytosed hCAP-18 into LL-37. The name derives from its sequence — two leading leucines followed by 37 total residues (LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES).
A parallel processing route exists outside neutrophils. In seminal plasma, hCAP-18 is cleaved by the aspartic protease gastricsin at low pH to yield a distinct 38-residue fragment, ALL-38 (Sørensen et al., Journal of Biological Chemistry, 2003) — a useful reminder for researchers that the fragment generated depends on the protease and compartment being modeled.
LL-37 has a molecular weight of approximately 4,493 g/mol and CAS number 154947-66-7. Its net positive charge at physiological pH (roughly +6) and its segregation of hydrophobic and cationic faces when helical are the two structural features that most of its in-vitro behavior is attributed to.
Mechanism of Action
Membrane interaction
In solution LL-37 is largely unstructured; it folds into an amphipathic alpha-helix upon contact with anionic lipids or micelles. Wang's NMR work in dodecylphosphocholine micelles (Journal of Biological Chemistry, 2008) resolved this micelle-bound helical structure and identified the core helical region responsible for membrane engagement. The cationic face is electrostatically drawn to the negatively charged phospholipids and lipopolysaccharide of bacterial envelopes; the hydrophobic face then inserts into the bilayer. Dürr and colleagues, and later reviewers, have characterized the resulting permeabilization as consistent with a carpet or toroidal-pore model rather than a discrete barrel-stave channel (Biochimica et Biophysica Acta, 2006; see also Xhindoli et al., BBA Biomembranes, 2016).
This mechanism is not selective for bacteria in an absolute sense. Because the same amphipathicity drives interaction with eukaryotic membranes, LL-37 shows concentration-dependent cytotoxicity toward mammalian cells in culture. Cholesterol content and the lower anionic charge density of mammalian membranes are the usual explanations for the observed selectivity window, and that window is narrow enough that dose-response characterization is a standard first step in any in-vitro design.
Receptor-mediated signaling
Separate from membrane lysis, LL-37 acts on host cells through G-protein-coupled receptors. De Yang and colleagues demonstrated in the Journal of Experimental Medicine (2000) that LL-37 uses formyl peptide receptor-like 1 (FPRL1, now FPR2) to chemoattract human peripheral blood neutrophils, monocytes, and T cells, and to induce calcium mobilization in FPRL1-transfected HEK293 cells. Tjabringa et al. later extended the chemoattractant profile to eosinophils via formyl-peptide receptors (International Archives of Allergy and Immunology, 2006). This receptor engagement is the basis for describing LL-37 as an "alarmin" — a molecule that both kills microbes and recruits the cells that follow up.
A third mechanism appears in epithelial models: Tokumaru and colleagues showed that LL-37 induces keratinocyte migration through transactivation of the epidermal growth factor receptor (Journal of Immunology, 2005), linking the peptide to re-epithelialization pathways independent of its antimicrobial function.
Immunomodulation
LL-37 binds and neutralizes lipopolysaccharide, which is one route by which it dampens rather than amplifies inflammatory signaling in some systems. Hu et al. reported dual-mechanism inhibition of LPS/ATP-induced macrophage pyroptosis in vitro (Journal of Immunology, 2014). The direction of the inflammatory effect is context-dependent — pro-inflammatory in some assays, anti-inflammatory in others — which is a recurring theme in the literature and a reason results are difficult to generalize across cell types.
Published Research Overview
Most primary literature clusters into four areas.
Antimicrobial characterization. Minimum inhibitory concentration work against Gram-negative and Gram-positive organisms, antibiofilm assays, and studies of activity loss in high-salt or serum-containing media. Salt sensitivity is a well-documented limitation: physiological ionic strength substantially reduces measured antibacterial potency in many assay formats, which is why buffer composition is reported carefully in this field.
Wound and epithelial biology. Carretero et al. reported wound-healing-promoting activity in vitro and in vivo models (Journal of Investigative Dermatology, 2008), with reported induction of VEGFa and IL-6 and accelerated re-epithelialization in the animal models used. Related work has examined LL-37 fragments in HaCaT keratinocyte lines alongside antibiofilm activity against Staphylococcus epidermidis (Ridyard et al., 2018).
Antiviral studies. Bergman and colleagues reported inhibition of HIV-1 replication by LL-37 in cell culture (Current HIV Research, 2007). Antiviral work in this area is entirely in-vitro or model-system based.
Oncology-adjacent cell biology. Several reviews cover the peptide's reported context-dependent effects on tumor cell lines — cytotoxic in some, proliferative in others — including Chen et al. (Frontiers in Immunology, 2018). These are cell-culture observations, not clinical findings.
Across all four areas, the practical caveats reported most often are salt and serum sensitivity, protease susceptibility, aggregation at high concentration, and cytotoxicity toward host cells above a threshold that varies by cell line. Any of these can confound an assay if not controlled for.
Storage & Handling
LL-37 (99%+) is supplied as a lyophilized powder. Store at -20°C, protected from light and moisture. Because the peptide is highly cationic, it adsorbs readily to glass and to some plastics; low-binding tubes and pipette tips are commonly used to avoid unexplained concentration loss.
Reconstitute immediately prior to use following established laboratory protocols. Sterile or bacteriostatic water is typical; some protocols use dilute acetic acid to improve solubility before dilution into working buffer. Avoid repeated freeze-thaw cycles, which degrade peptide integrity. Aliquot the reconstituted stock into single-use volumes. Given the peptide's salt and serum sensitivity, record buffer composition alongside every result — replication failures in this literature are frequently traced to assay medium rather than to the peptide itself.
Frequently Asked Questions
What is LL-37 derived from?
LL-37 is the mature, active fragment of hCAP-18, the only cathelicidin precursor encoded in the human genome. Proteinase 3 cleaves hCAP-18 extracellularly after neutrophil degranulation to release it.
How does LL-37 differ from other antimicrobial peptides?
Its distinguishing feature is dual functionality within a single short helix: direct membrane disruption plus receptor-mediated host-cell signaling through FPR2/FPRL1 and EGFR transactivation. Many antimicrobial peptides do one or the other; LL-37 is studied for both, which is why it appears in immunology literature as often as in microbiology literature.
Why do LL-37 antimicrobial results vary between studies?
Activity is strongly affected by ionic strength, serum proteins, and pH. Elevated salt concentrations reduce measured potency in many assay formats, and serum components can sequester the peptide. Comparing MIC values across papers without matching buffer conditions is generally not meaningful.
Is LL-37 cytotoxic to mammalian cells in culture?
Yes, in a concentration-dependent manner. The same amphipathic structure that permeabilizes bacterial membranes interacts with eukaryotic membranes at higher concentrations. Researchers typically establish a cytotoxicity curve in their specific cell line before interpreting functional results.
What solvent is used to reconstitute LL-37?
Sterile or bacteriostatic water is standard; some protocols use dilute acetic acid to aid initial dissolution before dilution into the working buffer. Solvent choice should be matched to the downstream assay and its compatibility with other reagents.
Conclusion
LL-37 occupies an unusual position in peptide research: it is simultaneously a model antimicrobial peptide and a model immunomodulatory signal, and the two roles are hard to disentangle experimentally. Its membrane mechanism is reasonably well described, its receptor pharmacology at FPR2 is established, and its epithelial effects via EGFR transactivation are documented — but effects remain strongly context-dependent across cell types and buffer conditions, and 2025 reviews continue to flag standardization as the field's open problem.
This product is intended for laboratory research use only. It is not a drug, food, or cosmetic, and is not intended for human or animal consumption, diagnostic, or therapeutic use.