What Is LL-37? Human Cathelicidin Peptide Profile

Cover illustration: AI-generated editorial artwork. Conceptual only; not an exact molecular structure, actual laboratory result or evidence of clinical effects.

LL-37 is the mature form of the only cathelicidin humans have. It is 37 residues long, named for its two N-terminal leucines, and it is released by proteolytic processing from a precursor called hCAP18. In the literature it belongs to two fields at once: antimicrobial peptide research and immunology.

For laboratory research use only. The research materials supplied by HealthyWelness are not approved medicines, foods or cosmetics and are not for human or veterinary use. This article summarises published research and is not medical advice.

Key points

Length: 37 amino acids; cationic and amphipathic.
Precursor: hCAP18, cleaved to release the active peptide.
Uniqueness: the single human cathelicidin — other species have several.
Two literatures: direct antimicrobial activity, and host immunomodulation.
Described as pleiotropic in reviews, which is accurate and also a warning about interpretation.

How the antimicrobial mechanism works

LL-37 carries a net positive charge and folds into an amphipathic helix — hydrophobic residues on one face, charged residues on the other. Bacterial membranes carry more negative surface charge than mammalian plasma membranes, so the peptide associates preferentially with them, inserts, and compromises membrane integrity.

Two consequences follow from the mechanism being physical:

Broad activity. There is no single molecular target to lose, so the spectrum is wide and classical resistance by target mutation does not apply straightforwardly.
Condition dependence. Activity is sensitive to ionic strength, pH and the presence of divalent cations and serum proteins, all of which affect the electrostatic interaction. This is the single most common reason antimicrobial peptide results differ between laboratories.

That second point is a practical warning. A minimum inhibitory concentration measured in low-salt buffer may be markedly lower than one measured in physiological salt. When comparing values across papers, check the assay medium before concluding anything about potency.

The immunological half of the literature

The 2016 review in Pharmacological Reports uses the word pleiotropic, which is the right term. Beyond killing microbes, LL-37 has been reported to act on host cells: chemotactic effects on immune cells, modulation of inflammatory signalling, interaction with nucleic acids and with pattern recognition receptors, and effects on wound repair processes.

Source	Focus	Theme
Fabisiak et al., 2016	General review	Pleiotropic activity across antimicrobial and immune functions
Nagaoka et al., 2020	Murine sepsis model	Evaluated in a systemic inflammatory setting
Miao et al., 2024	Heart disease	Multifunctional involvement in cardiac contexts

Published work has also examined cathelicidin expression in the context of severe inflammatory responses and microthrombosis. The breadth is genuine, but breadth of this kind is also where interpretive discipline matters most: a peptide that disrupts membranes and signals to immune cells has many ways to produce an observable effect, and distinguishing them requires deliberate controls.

The control that separates mechanism from charge

For any cationic amphipathic peptide, the essential control is a sequence-scrambled peptide of identical composition. It has the same charge and the same hydrophobicity but not the same sequence. If an effect is driven by charge and amphipathicity alone, the scrambled control reproduces it; if the effect depends on sequence-specific interaction, it does not.

Omitting this control is common, and it is why some reported LL-37 activities are better supported than others. When reading a paper, look for the scrambled or all-D control before accepting a sequence-specific mechanism.

Handling in the lab

Consideration	Practice
Storage of powder	Sealed, −20 °C, protected from light and moisture
Solubility	Cationic and generally water-soluble; avoid high-salt diluents for stock solutions
Surface adsorption	Substantial — use low-binding tubes and plates; losses at low concentration are a real source of variability
Assay medium	Record salt, pH and serum content; activity depends on all three
Essential control	Scrambled-sequence peptide of matched composition
Identity confirmation	MS plus HPLC; at 37 residues, synthesis-related impurities are worth examining

A further caution: LL-37 is a substrate for proteases, and in a sample containing proteolytic activity it will be degraded, sometimes into fragments with their own reported activities. A result obtained in such a system may partly reflect fragments rather than the intact peptide. Concentration maths: peptide reconstitution calculator.

Our LL-37 research material is supplied for laboratory research use. Certificates are available for listed products — contact us for the latest report.

Open questions

Which reported immunomodulatory effects survive a scrambled-peptide control?
How much of the activity observed in biological samples is attributable to proteolytic fragments?
Can antimicrobial and immunomodulatory activities be separated structurally?

Frequently asked questions

What is LL-37?

LL-37 is the mature, active form of the only human cathelicidin, released by proteolytic cleavage from the precursor protein hCAP18. It is 37 amino acids long and begins with two leucine residues, which is the origin of the name.

What does the name LL-37 mean?

The two L letters refer to the two N-terminal leucine residues, and 37 is the peptide length. The naming convention is descriptive of the sequence rather than of any function.

Is LL-37 only an antimicrobial peptide?

No. It has documented direct antimicrobial activity, but a large body of work describes immunomodulatory roles including chemotaxis, effects on inflammatory signalling, and interaction with host cells. Reviews describe it as pleiotropic.

How does LL-37 act on bacteria?

The classical account is membrane disruption: the peptide is cationic and amphipathic, so it associates with negatively charged bacterial membranes and compromises their integrity. This mechanism is physical rather than target-specific.

Why is LL-37 studied in non-infectious conditions?

Because of its immunomodulatory activity. Published work covers sepsis models, cardiovascular disease and inflammatory conditions, where the interest is in its signalling effects on host cells rather than direct killing of microbes.

References

Fabisiak A, et al. LL-37: Cathelicidin-related antimicrobial peptide with pleiotropic activity. Pharmacol Rep. 2016. PMID: 27117377 https://pubmed.ncbi.nlm.nih.gov/27117377/
Nagaoka I, et al. Therapeutic Potential of Cathelicidin Peptide LL-37, an Antimicrobial Agent, in a Murine Sepsis Model. Int J Mol Sci. 2020. PMID: 32825174 https://pubmed.ncbi.nlm.nih.gov/32825174/
Miao S, et al. Cathelicidin peptide LL-37: A multifunctional peptide involved in heart disease. Pharmacol Res. 2024. PMID: 39615616 https://pubmed.ncbi.nlm.nih.gov/39615616/