LL-37 (Cathelicidin): A Six-Module Literature Course

This page is for educational purposes only and is not medical advice; consult a licensed physician about any health decision. Nothing here describes a protocol, and no product is offered or recommended. The aim is narrower: to show what published studies of cathelicidin LL-37 actually examined, what they measured, and where the record stops.

How this course is organised

Six modules move from definition to regulation. Module 1 covers what LL-37 is and the kinds of studies that exist. Module 2 summarises mechanism as described by researchers. Module 3 lists reported outcomes by study, with model and endpoint attached to each result. Module 4 collects adverse findings as published. Module 5 addresses pharmacokinetics, where such data exist. Module 6 states regulatory status factually. Every module ends with the limits of its own evidence, and the page closes with what the cited studies did not test.

Module 1 — What LL-37 Is and How It Has Been Studied

Definition and class

LL-37 is a cationic host-defence peptide of the cathelicidin family. In humans, a single cathelicidin gene (CAMP) produces a precursor protein called hCAP18; proteolytic processing releases the mature C-terminal peptide named for its two leading leucine residues and its 37-residue length, the parent sequence used as a template in a 2023 analog-design study (RSC Advances, 2023). The peptide is usually described as amphipathic and helical in membrane-like environments, which is the structural feature most often linked in the literature to both its antimicrobial and its cell-damaging behaviour.

Origin and regulation

Expression of the precursor is not static. A 2024 laboratory study reported that vitamin D triggers hCAP18/LL-37 production and examined the consequences of that induction for human osteoblasts (Biochemical and Biophysical Research Communications, 2024). That paper is a useful entry point because it links an upstream regulator to a downstream cellular effect inside one experimental system.

Forms that appear in the literature

• Endogenous LL-37 measured as a biomarker in human samples — for example, salivary LL-37 quantified in children and adolescents living with HIV (Biomedicine Hub, 2024), and LL-37 alongside human β2-defensin in children with post-infectious bronchiolitis obliterans (The Clinical Respiratory Journal, 2017).
• Tissue expression assessed histologically, as in an analysis of cathelicidin LL-37 expression in human breast implant capsules (Plastic and Reconstructive Surgery, 2024).
• Synthetic LL-37 applied to cultured cells or animal models, such as the mouse MRSA wound infection model used to test efficacy (Antibiotics, 2021).
• Engineered analogs and delivery systems, including non-hemolytic peptides related to LL-37 (RSC Advances, 2023) and exosomes engineered to contain cathelicidin/LL-37 (Advanced Healthcare Materials, 2022).

Limits of the evidence (Module 1)

The literature summarised here is dominated by cell-culture work, small animal models and observational measurement of endogenous peptide levels. Observational studies describe how much LL-37 was present in a sample; they do not establish that the peptide caused the clinical state being observed, and they say nothing about administered peptide.

Module 2 — Mechanism as Described in the Literature

Membrane activity and antimicrobial behaviour

The most commonly described mechanism is direct interaction between the positively charged peptide and negatively charged microbial membranes. The same physicochemical property is implicated in damage to host cells, which is why researchers designing derivatives have explicitly sought sequences that retain antimicrobial activity while avoiding hemolysis (RSC Advances, 2023).

Immune signalling

Beyond membrane disruption, several papers describe LL-37 as a modulator of innate immune signalling. One study reported that LL-37 affects surface and intracellular Toll-like receptor expression in tissue mast cells (Journal of Immunology Research, 2018). In airway epithelium, researchers reported that LL-37 increased rhinovirus-induced interferon β expression through a calcium-dependent mechanism (Biochemistry and Biophysics Reports, 2025). A separate line of work reported that LL-37 ignites primed NLRP3 inflammasomes in rosacea, connecting the peptide to an inflammatory pathway rather than an antimicrobial one (Journal of Investigative Dermatology, 2021).

Interaction with host proteins and cells

A 2026 study described an interaction between LL-37 and apolipoprotein B-100 that promoted LDL clearance and attenuated cholesterol accumulation in the liver in a preclinical setting (Science China Life Sciences, 2026). Another reported that LL-37 induced cell death in endothelial cells whose autophagy machinery was dysfunctional, indicating that the cellular context determines whether exposure is tolerated (Journal of Immunology, 2022).

Limits of the evidence (Module 2)

Mechanistic descriptions come from controlled systems in which peptide concentration, cell type and exposure time were set by the investigators. Mechanisms shown in isolated cells do not automatically operate at the same scale in an intact organism, and the same molecule appears in these papers as both a defence signal and a damage signal depending on the model used.

Module 3 — Reported Outcomes, Study by Study

The table lists the model, the endpoint and the reported direction of the result for each cited paper. No study in this set reported a human therapeutic dose, so none is given.

Reading the outcome record

Three patterns stand out. First, the infection work is animal and in vitro: the mouse MRSA wound study assessed LL-37 against a defined pathogen in a defined wound (Antibiotics, 2021), and the exosome study packaged the peptide to explore delivery-related functions (Advanced Healthcare Materials, 2022). Second, the metabolic finding is a single preclinical report of an LL-37–ApoB-100 interaction affecting LDL clearance (Science China Life Sciences, 2026), not a replicated clinical outcome. Third, the human data in this set are measurements of endogenous peptide, not trials of administered peptide (Biomedicine Hub, 2024).

Limits of the evidence (Module 3)

None of these papers constitutes a randomised controlled trial of LL-37 in people. Endpoints differ so widely — bacterial clearance, interferon transcription, receptor expression, lipid handling, tissue staining — that they cannot be pooled into a single efficacy statement, and single reports await independent replication.

Module 4 — LL-37 Side Effects: What Studies Report

Adverse findings in this literature are cellular and preclinical. They are reported as experimental observations, not as clinical adverse-event rates, because no cited study administered the peptide to human volunteers.

• Hemolysis. The concern that LL-37 damages red blood cells is explicit in a 2023 design study whose stated purpose was to produce non-hemolytic antimicrobial peptides related to human cathelicidin LL-37 (RSC Advances, 2023).
• Cytotoxicity to bone cells. Researchers reported LL-37-induced human osteoblast cytotoxicity in the context of vitamin D-triggered hCAP18/LL-37 production (BBRC, 2024).
• Endothelial cell death. The study of autophagy-dysfunctional endothelial cells reported that LL-37 induced cell death when the autophagy pathway was impaired (Journal of Immunology, 2022).
• Pro-inflammatory activation. In rosacea, LL-37 was reported to ignite primed NLRP3 inflammasomes, a pathway associated with inflammatory skin pathology rather than resolution (Journal of Investigative Dermatology, 2021).
• Immune-receptor shifts. Altered surface and intracellular Toll-like receptor expression in mast cells was reported as a measurable change, with consequences that the study did not translate into clinical terms (Journal of Immunology Research, 2018).

Limits of the evidence (Module 4)

Cell-level toxicity depends on concentration, exposure duration and cell type, all of which investigators controlled. There is no systematic safety database, no long-term animal toxicology summarised here, and no human adverse-event reporting in the cited set. Absence of reported harm in a paper that did not look for harm is not evidence of safety.

Module 5 — Pharmacokinetics: Where Data Exist

The pharmacokinetic record in this verified set is essentially empty. None of the cited papers reported human absorption, distribution, metabolism, elimination, plasma half-life or bioavailability for administered LL-37, and no dosing schedule can be stated from them.

What the literature does supply is indirect. Endogenous concentrations have been quantified in accessible fluids — salivary LL-37 in children and adolescents living with HIV (Biomedicine Hub, 2024) and LL-37 with human β2-defensin in children with post-infectious bronchiolitis obliterans (The Clinical Respiratory Journal, 2017) — showing that the peptide is detectable and variable between individuals and conditions. Delivery, rather than systemic exposure, is the focus of the engineered-exosome study, in which LL-37 was packaged into exosomes and the resulting particles were reported to exhibit multiple biological functions (Advanced Healthcare Materials, 2022). Interest in delivery vehicles is itself a signal that free peptide stability is treated as a practical obstacle by researchers.

Limits of the evidence (Module 5)

Biomarker concentrations in saliva or serum describe endogenous biology; they cannot be converted into exposure estimates for an administered peptide. Without human PK data, no statement about duration of action, accumulation or route comparison can be supported from these papers.

Module 6 — Regulatory Status

The following is a factual description of regulatory categories and is not legal advice.

• No approved LL-37 drug product. As of this writing there is no US Food and Drug Administration-approved finished drug product whose active ingredient is LL-37 or a cathelicidin peptide. The studies cited above are preclinical or observational and were not submitted as pivotal evidence for an approval.
• Research-use-only material. Synthetic LL-37 sold to laboratories is typically labelled "research use only," meaning it is not manufactured, tested or released as a medicine for human administration. RUO labelling is a distribution category, not a quality or safety endorsement for human use.
• Compounding. Under section 503A of the Federal Food, Drug, and Cosmetic Act, bulk drug substances used in compounding must be the subject of a USP monograph, be a component of an approved drug, or appear on FDA's published list of substances permitted for compounding. FDA has evaluated nominated peptide substances and assigned LL-37 to the category of substances it identified as raising significant safety risks, which places it outside routine 503A compounding. Outsourcing facilities operating under section 503B are governed by a separate bulks list.
• Jurisdictional variation. Regulatory classification differs by country, and sports-governing bodies maintain their own prohibited-substance rules that are independent of drug approval status.

Limits of the evidence (Module 6)

Regulatory lists change, and a category assignment reflects an agency's risk evaluation at a point in time rather than a final scientific verdict. Regulatory status also carries no information about efficacy: a substance can be both unapproved and under active investigation, which is the situation for LL-37.

What the Studies Did Not Test

Reading the set as a whole, the gaps are as instructive as the findings.

1. No human efficacy trials. The infection evidence rests on a mouse MRSA wound model (Antibiotics, 2021); no cited study randomised patients to receive LL-37.
2. No human dosing data. No route, quantity or schedule for administration in people appears anywhere in these papers, which is why none is stated on this page.
3. No long-term follow-up. Cellular endpoints such as osteoblast cytotoxicity (BBRC, 2024) and endothelial cell death (Journal of Immunology, 2022) were measured over short experimental windows.
4. No causal link from biomarker to disease. Studies measuring salivary or tissue LL-37 (Biomedicine Hub, 2024) and (Plastic and Reconstructive Surgery, 2024) described associations, not cause and effect.
5. No comparison across analogs in vivo. The non-hemolytic derivatives were characterised chemically and microbiologically rather than compared with native LL-37 in a clinical setting (RSC Advances, 2023).
6. No interaction or special-population data. Pregnancy, paediatric administration, hepatic or renal impairment and drug interactions were not examined in the cited work.

Readers comparing these modules with promotional claims found elsewhere will notice the distance between an in vitro interferon response (Biochemistry and Biophysics Reports, 2025) and any statement about outcomes in people. This page is for educational purposes only and is not medical advice; consult a licensed physician regarding any medical question.

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 (The 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)