LL37 is the 37-amino-acid, C-terminal fragment of human cathelicidin antimicrobial protein (hCAP18), and it is the only cathelicidin-derived peptide identified in humans. It is studied in cell culture and animal models for its membrane-disrupting action on microbes, its binding of bacterial lipopolysaccharide and its signalling effects on immune and epithelial cells. Because it is cationic, amphipathic and sticky, handling and quality control of synthetic LL37 are unusually demanding. This article summarises its structure, mechanisms, published research areas and the analytical and storage standards needed for reproducible work. It is written for laboratory researchers and offers no guidance on human use.
What is LL-37?
LL-37 is classified as a host defence peptide, a term that has largely replaced “antimicrobial peptide” because these molecules do more than kill microbes. Its sequence is Leu-Leu-Gly-Asp-Phe-Phe-Arg-Lys-Ser-Lys-Glu-Lys-Ile-Gly-Lys-Glu-Phe-Lys-Arg-Ile-Val-Gln-Arg-Ile-Lys-Asp-Phe-Leu-Arg-Asn-Leu-Val-Pro-Arg-Thr-Glu-Ser, usually written LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES. The name comes from the two leucines at the N-terminus and the length of 37 residues. The molecular weight is approximately 4493 Da, and the peptide carries a net positive charge of about +6 at neutral pH, because it contains eleven basic residues against five acidic ones.
In solution the peptide is largely unstructured, but in the presence of membranes, detergents or certain anions it folds into an amphipathic alpha-helix, with one face of the helix carrying the charged residues and the other the hydrophobic ones. This dual character underlies its biology and also its tendency to aggregate and to adsorb to surfaces.
LL-37 was identified in 1995. Three groups reported the human cathelicidin around the same time: Agerberth and colleagues, working with Gudmundsson and Gudmundsson in Stockholm, Larrick and colleagues, who described it as a lipopolysaccharide-binding protein of 18 kDa from neutrophils, and Cowland and colleagues in Copenhagen, who cloned the gene. The 18 kDa precursor, hCAP18, is stored in neutrophil granules and is also made by epithelial cells. Cleavage by the serine protease proteinase 3 releases the active LL-37 from the C-terminus.
Regulatory context matters for any UK buyer. To the author’s understanding, LL-37 is not authorised as a medicine in the United Kingdom, and this should be checked against current Medicines and Healthcare products Regulatory Agency (MHRA) records. The MHRA has issued warnings about unlicensed products marketed for human use. The material is relevant here only as a laboratory research reagent.
Mechanism of action
The best-characterised activity of LL37 is direct action on membranes. Because bacterial membranes are rich in negatively charged phospholipids, the cationic peptide is attracted to them electrostatically, and the hydrophobic face of the helix then inserts into the lipid bilayer. Studies by Oren and colleagues in 1999 using biophysical methods supported a model in which the peptide accumulates on the membrane surface and, above a threshold concentration, disrupts it in a detergent-like manner, sometimes called the carpet mechanism. Other models propose toroidal pores, and the mechanism probably depends on lipid composition and peptide concentration.
The peptide also binds lipopolysaccharide (LPS) from Gram-negative bacteria, which neutralises some of its ability to activate immune cells. This property is the reason LL37 is relevant to endotoxin research, and also the reason that endotoxin contamination of the peptide itself must be considered.
A second class of mechanisms concerns signalling in host cells. LL37 is reported to act through formyl peptide receptor 2 (FPR2, also called FPRL1), a G protein-coupled receptor, to attract neutrophils, monocytes and T cells in chemotaxis assays. Other reports describe effects through the purinergic receptor P2X7, through transactivation of the epidermal growth factor receptor and through modulation of Toll-like receptor signalling. Several of these findings rest on specific cell systems, and the receptor pharmacology is less settled than for classical peptide hormones.
A third consideration is that the concentration matters in both directions. At the concentrations that disrupt bacterial membranes, LL37 can also be toxic to mammalian cells, and effects on host cells observed at lower concentrations may differ in kind. Serum, salts and polyanions such as DNA and glycosaminoglycans all reduce activity, so results from a low-salt buffer do not translate directly to culture medium. In cell-based work, “dosage” means molar concentration in the medium, and the medium must always be reported, since concentration, salt content and serum all alter outcome.
What the research shows
The literature on LL37 is large and spans microbiology, immunology and cell biology. Four areas are summarised below in general terms. Readers should consult the primary papers for methods, concentrations and statistics, because commercial summaries often overstate findings.
The first area is the original identification and characterisation. The 1995 papers described the human cathelicidin gene, the 18 kDa precursor and the processing that releases the 37-residue peptide, and they showed antibacterial activity in vitro. The foundational work can be located through a PubMed search for the identification of human cathelicidin LL-37.
The second area is the mechanism of membrane action. Biophysical studies, including the work by Oren and colleagues in 1999 in the Biochemical Journal, used spectroscopy and model membranes to show that the peptide adopts a helical, oligomeric form on bacterial-like membranes and permeabilises them. These are experiments with synthetic vesicles and bacterial cultures, and they establish the physical basis of the antimicrobial activity.
The third area is receptor-mediated signalling. Yang and colleagues reported in 2000 in the Journal of Experimental Medicine that LL-37 attracted human neutrophils, monocytes and T cells through FPRL1, which established that the peptide acts on host cells as well as on microbes. Later studies described effects on cytokine production and on epithelial cell migration in culture. These are cell-culture findings, described here only as part of the scientific record. They can be explored through a PubMed search on LL-37 and formyl peptide receptor-like 1 chemotaxis.
The fourth area is the interaction of LL37 with nucleic acids and the innate immune system. Lande and colleagues reported in 2007 in Nature that LL37 formed complexes with self-DNA that were taken up by plasmacytoid dendritic cells and activated Toll-like receptor 9, and they proposed that this could drive interferon production in a model of autoimmune skin inflammation. This is a study of immune signalling in cells from patients and healthy donors and in vitro systems. It carries no implication for use of the peptide in any setting, and it illustrates why the peptide is a standard tool in innate immunity research.
A recurring limitation runs through this body of work. The peptide’s activity depends so strongly on salt, serum, buffer and counterion that results from different laboratories are hard to compare, and the exact material used is not always characterised analytically. Reproducibility therefore depends on knowing exactly what was in the vial and in what medium it was used.
Research applications
In the laboratory, LL37 is used in several contexts. The most common is antimicrobial research, where it serves as a reference host defence peptide in minimum inhibitory concentration assays, time-kill assays and biofilm studies, and as a benchmark for novel antimicrobial peptides. Assay conditions strongly affect results, so low-binding plates and standardised media are used.
A second context is innate immunity and cell signalling. LL37 is applied to neutrophils, monocytes, dendritic cells, epithelial cells and keratinocytes to examine chemotaxis, cytokine release and receptor signalling, often with receptor antagonists or receptor-deficient cells to define specificity. Endotoxin control is essential, because the peptide binds LPS and these cells respond to trace lipopolysaccharide.
A third context is biophysics and structural biology. The peptide’s folding on membranes, its oligomerisation and its interaction with lipids and nucleic acids are studied with circular dichroism, NMR, fluorescence and atomic force microscopy, and it is a model for peptide-membrane interactions generally.
A fourth context is animal models of infection and inflammation, which require institutional ethical review and, in the United Kingdom, authorisation under the Animals (Scientific Procedures) Act 1986. A fifth is analytical, where the peptide is a useful test case for liquid chromatography and mass spectrometry methods for highly charged, adsorptive peptides.
Purity, storage and handling
A 37-residue peptide is among the longer sequences made by solid-phase synthesis, and LL37 is further complicated by its hydrophobic and cationic character. Across that many coupling steps, small inefficiencies accumulate, so the crude product contains truncated and deletion sequences, and the repeated arginine and lysine residues carry bulky protecting groups that can leave adducts. The tendency of the growing chain to aggregate on the resin can lower coupling efficiency. Reversed-phase high-performance liquid chromatography (HPLC) with UV detection at 214 or 220 nm is the standard purity method, and a main peak of at least 95 per cent, and ideally 98 per cent, is commonly expected. A shallow gradient is needed to resolve closely related impurities, and a stated purity is meaningful only if the method and chromatogram are available.
HPLC must be paired with mass spectrometry. The expected mass is about 4493 Da, and the highly basic peptide typically shows a series of multiply charged ions in electrospray ionisation. A mass 1 Da higher suggests deamidation at the glutamine or asparagine, and a species lacking one residue indicates a deletion product. A clean peak at the wrong mass is a clean peak of the wrong compound.
A batch-specific certificate of analysis matters because purity, peptide content and counterion content vary between synthesis runs. It should carry a lot number matching the vial, the HPLC trace or purity value, the observed and theoretical masses, the test date and ideally the salt form, net peptide content and endotoxin result. The counterion deserves special attention for this peptide. Trifluoroacetate is normally present as the counterion after HPLC purification, and with six or more positive charges per molecule, the TFA content of the powder can be substantial. TFA can alter the activity of antimicrobial peptides in assays, so acetate or hydrochloride salt forms are often preferred, and the salt form should be stated. Lyophilised powders also contain residual water, so the weighed mass is not the mass of peptide, and ignoring this introduces systematic errors into any concentration-response comparison.
Because endotoxin cannot be seen on a label, and because the peptide’s strong binding to LPS and plastic can mask or distort contamination, batch-level verification matters more here than with a plain linear peptide. UK laboratories sourcing LL37 should expect a batch-specific certificate of analysis that states the mass found, the salt form and an endotoxin result, with lyophilised storage at -20°C, and the same documentation standard applies when weighing any best peptides supplier uk claim, since a supplier’s analytical record rather than its search ranking shows whether the vial matches its stated specification.
Lyophilised LL37 is generally stored at -20°C in a sealed container with desiccant, and many laboratories use -80°C for long-term archives. Vials should reach room temperature before opening so that condensation does not form on the cold powder. After reconstitution, stability falls. Standard practice is to dissolve the peptide in sterile, endotoxin-free water or a dilute acidic buffer, divide the solution into single-use aliquots and freeze them to avoid repeated freeze-thaw cycles. Concentrated stocks are often prepared in water because the peptide aggregates in the presence of salts and phosphate, and are diluted into assay buffer immediately before use. Refrigerated storage at 2 to 8°C is sometimes used for solutions consumed within days, but the acceptable period should be validated for the assay concerned.
The peptide adsorbs strongly to glass and to many plastics, particularly at low concentrations, so low-bind polypropylene tubes and plates are strongly preferred, and some laboratories add a carrier protein where it does not interfere with the assay. Because serum and salts inhibit activity, buffer and medium composition should be recorded. Light sensitivity is a secondary concern, since the sequence has no tryptophan, tyrosine, methionine or cysteine and is therefore less vulnerable to oxidation than many peptides, but amber vials or dark storage remain good practice.
Laboratory safety follows ordinary chemical hygiene, and the peptide’s cytotoxicity to mammalian cells is a reason to treat powder and concentrated stocks with care. The safety data sheet should be read before use, gloves, a laboratory coat and eye protection are standard, lyophilised powder should be weighed in a ventilated enclosure, and institutional COSHH assessments should cover the material and procedure.
Labelling is a final indicator of quality. Clear naming, sequence, molecular weight, lot number, salt form and an unambiguous research-only statement suggest a supplier that understands its market, while any wording that implies use in humans is a warning sign and raises regulatory concerns in the United Kingdom.
Frequently asked questions
What is LL37 used for in laboratory research?
It is used as a reference host defence peptide in antimicrobial and biofilm assays, as a tool in innate immunity and cell signalling studies, in biophysical work on peptide-membrane interactions and in approved animal models. It is a research reagent and not an authorised medicine in the United Kingdom.
What is the sequence and molecular weight of LL-37?
LL-37 is a 37-residue peptide with the sequence LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES and a molecular weight of about 4493 Da. It is the C-terminal fragment of the human cathelicidin precursor hCAP18. The certificate of analysis should show a mass spectrometry result matching this value.
Why does the salt form matter for LL37?
LL-37 carries a strong positive charge, so the powder can contain a large amount of counterion. Trifluoroacetate from HPLC purification can alter antimicrobial and cell assay results, so acetate or hydrochloride forms are often preferred. The salt form should be stated on the certificate of analysis.
How should LL37 be stored and handled for research?
Keep lyophilised peptide at -20°C, or -80°C for long-term archiving, in a sealed, dry container. Allow vials to reach room temperature before opening. Dissolve in sterile, endotoxin-free water, aliquot and freeze, and use low-bind tubes because the peptide sticks strongly to plastic.
LL37 is supplied for in-vitro laboratory research only and is not intended for human or veterinary use.
