What Is LL-37? Definition and What Research Reports

Plain definition

LL-37 is a small, naturally occurring human protein fragment — a peptide — that the body makes as part of its built-in defence system. It is found in white blood cells, skin, saliva, sweat, the lining of the airways and other surfaces that meet the outside world. Its name comes from its structure: the chain begins with two leucine residues ("LL") and is 37 amino acids long. In laboratory work it is usually described as an antimicrobial peptide, because it can disrupt the membranes of bacteria, but the published literature also describes it as a signalling molecule that changes how immune cells behave. Importantly, the same properties that let it damage microbial membranes can also damage human cells in culture, and researchers have reported this repeatedly.

What LL-37 is in biochemical terms

LL-37 is the mature, active form of the only cathelicidin identified in humans. The gene CAMP encodes a precursor protein called hCAP18 (human cationic antimicrobial protein, 18 kDa), which consists of a conserved cathelin-like domain plus the C-terminal peptide. Proteolytic cleavage — classically by proteinase 3 in neutrophils, and by kallikreins in skin — releases the 37-residue C-terminal fragment known as LL-37.

Structurally, LL-37 is cationic (net positive charge at physiological pH) and amphipathic: when it contacts a membrane it folds into an α-helix with charged residues on one face and hydrophobic residues on the other. That arrangement lets it bind preferentially to negatively charged bacterial membranes. The same amphipathic helix is the reason that peptide chemists have tried to redesign it: a 2023 synthesis paper described the design and characterisation of non-hemolytic antimicrobial peptides related to human cathelicidin LL-37, an effort that exists precisely because the parent sequence interacts with red blood cell membranes as well as microbial ones.

Expression of hCAP18/LL-37 is regulated rather than constant. A 2024 cell study reported that vitamin D triggered hCAP18/LL-37 production and, in the same work, that LL-37 exerted cytotoxic effects on human osteoblasts — a useful illustration that "more LL-37" is not automatically a favourable outcome in a tissue.

Regulatory status

LL-37 is a naturally occurring human peptide, not an approved drug product. No LL-37 medicine has been approved by the FDA or EMA, and synthetic LL-37 circulating in the peptide space is material labelled for research use only (RUO), which means it is not intended for human administration and is not manufactured to pharmaceutical standards. Describing a research-grade peptide as a "therapy" misrepresents its regulatory category. This page is for educational purposes only and is not medical advice; consult a licensed physician about any health question or medication.

How the term is used in peptide research

In the scientific literature the word "LL-37" usually appears in one of four contexts:

• As an antimicrobial agent tested against bacteria in vitro or in animal infection models.
• As an immunomodulator that alters receptor expression, cytokine output or interferon responses in host cells.
• As a biomarker — a measured concentration in saliva, serum or tissue that reflects innate immune activity or disease state.
• As a scaffold for engineered analogues, delivery systems and shortened derivatives.

Antimicrobial and wound-model work

A 2021 animal study evaluated the efficacy of cathelicidin LL-37 in an MRSA wound infection mouse model, examining the peptide against methicillin-resistant Staphylococcus aureus in infected wounds. Separately, researchers built a delivery approach around the sequence: a 2022 report described engineered exosomes containing cathelicidin/LL-37 that exhibited multiple biological functions, reflecting interest in carrying the peptide in a vesicle rather than as free peptide.

Immune signalling work

LL-37 does more than punch holes in membranes. A 2018 study reported that LL-37 affected surface and intracellular Toll-like receptor expression in tissue mast cells, placing it upstream of pattern-recognition signalling rather than only at the membrane. In airway biology, a 2025 study found that LL-37 increased rhinovirus-induced interferon β expression in human airway epithelial cells through a calcium-dependent mechanism, which the authors attributed to signalling rather than direct virucidal action.

Biomarker work

Because LL-37 can be measured in body fluids, several clinical studies treat it as a readout. A 2017 paper measured cathelicidin (LL-37) and human β2-defensin levels in children with post-infectious bronchiolitis obliterans, and a 2024 study examined salivary cathelicidin (LL-37) in children and adolescents living with HIV. Tissue-level expression has also been surveyed: researchers reported on cathelicidin LL-37 expression in human breast implant capsules. These are observational measurements of an endogenous peptide, not studies of administered peptide.

Where the term is misused

1. Treating "antimicrobial peptide" as "safe antibiotic." LL-37's membrane activity is not species-selective in the way a targeted antibiotic is. Hemolysis concerns are explicit in the analogue-design literature (PMID 37228679), and host-cell death has been documented in several models.
2. Equating LL-37 with vitamin D supplementation. Vitamin D signalling induces hCAP18/LL-37 transcription, and the 2024 osteoblast study described exactly that link alongside LL-37-induced cytotoxicity. Induction of a peptide is not the same as a clinical benefit.
3. Calling it purely anti-inflammatory. A 2021 dermatology study reported that LL-37 ignited primed NLRP3 inflammasomes in rosacea — a pro-inflammatory role in that disease context.
4. Assuming in vitro potency transfers to the body. Serum proteins, salt concentration and proteases all change peptide behaviour; most cited findings above are cell or animal work.
5. Confusing endogenous levels with administration. Biomarker papers (PMID 38287973) measure what the body already makes.

Related terms

Cytotoxicity and host-cell effects: What Studies Report

Several of the verified papers describe harm to human cells rather than benefit. The 2024 vitamin D paper reported LL-37-induced human osteoblast cytotoxicity. A 2022 immunology study reported that human cathelicidin peptide LL-37 induced cell death in autophagy-dysfunctional endothelial cells, indicating that a cell's stress-handling capacity influenced whether LL-37 was tolerated. In inflammatory skin disease, the study of rosacea described LL-37 activating primed NLRP3 inflammasomes, a mechanism associated with tissue inflammation. And the existence of a dedicated effort to make non-hemolytic LL-37-related peptides reflects the recognised interaction of the parent sequence with red blood cells. None of these papers describe safe human administration, and no adverse-event profile for administered LL-37 in people is established in the cited literature.

Emerging directions in the literature

Interest has widened beyond infection. A 2026 study reported that an LL-37–ApoB-100 interaction promoted LDL clearance and attenuated cholesterol accumulation in the liver, extending the peptide's described biology into lipid handling. Delivery engineering continues as well, as in the exosome work. These remain preclinical findings, and the literature as a whole is dominated by cell and animal models rather than controlled human trials of administered peptide.

Summary of the definition

LL-37 is best defined as the human cathelicidin peptide released from hCAP18: cationic, amphipathic, membrane-active and immunologically double-edged. Researchers have reported antimicrobial activity in a wound-infection model, interferon and receptor-level immune effects in epithelial and mast cells, measurable levels in saliva and tissue as a biomarker, and cytotoxicity toward osteoblasts and stressed endothelial cells. Any single description — "antimicrobial", "anti-inflammatory", "healing" — captures only part of what the studies show.

References

• Vitamin D triggers hCAP18/LL-37 production: Implications for LL-37-induced human osteoblast cytotoxicity (Biochemical and Biophysical Research Communications, 2024)
• Cathelicidin LL-37 Ignites Primed NLRP3 Inflammasomes in Rosacea (Journal of Investigative Dermatology, 2021)
• Design, synthesis, and characterization of non-hemolytic antimicrobial peptides related to human cathelicidin LL-37 (RSC Advances, 2023)
• Cathelicidin LL-37 Expression in Human Breast Implant Capsules (Plastic and Reconstructive Surgery, 2024)
• Cathelicidin (LL-37) and human β2-defensin levels of children with post-infectious bronchiolitis obliterans (The Clinical Respiratory Journal, 2017)
• Cathelicidin LL-37-ApoB-100 interaction promotes LDL clearance and attenuates cholesterol accumulation in the liver (Science China Life Sciences, 2026)
• Cathelicidin LL-37 Affects Surface and Intracellular Toll-Like Receptor Expression in Tissue Mast Cells (Journal of Immunology Research, 2018)
• Salivary Cathelicidin (LL-37) in Children and Adolescents Living with HIV (Biomedicine Hub, 2024)
• Human Cathelicidin Peptide LL-37 Induces Cell Death in Autophagy-Dysfunctional Endothelial Cells (Journal of Immunology, 2022)
• Antimicrobial peptide LL-37 increases rhinovirus-induced interferon β expression in human airway epithelial cells through a Ca2+-dependent mechanism (Biochemistry and Biophysics Reports, 2025)
• Engineered Exosomes Containing Cathelicidin/LL-37 Exhibit Multiple Biological Functions (Advanced Healthcare Materials, 2022)
• Efficacy of Cathelicidin LL-37 in an MRSA Wound Infection Mouse Model (Antibiotics, 2021)