LL-37

What is LL-37?

LL-37 is a 37-amino-acid peptide that is the only cathelicidin-derived antimicrobial peptide found in humans. It is produced by immune cells, epithelial cells, and other tissues as part of the innate immune response. It has broad-spectrum antimicrobial activity against bacteria, viruses, and fungi, and also modulates inflammation and wound healing.

What LL-37 Is Investigated For

LL-37 is investigated for antimicrobial defense, immune modulation, wound healing (particularly chronic venous leg ulcers and diabetic foot ulcers), and gut barrier support — rooted in its well-characterized role as the only human cathelicidin antimicrobial peptide. The strongest human evidence is for topical use in chronic wounds, and the record there is split rather than uniformly positive: a 34-patient first-in-man venous-leg-ulcer trial and a small diabetic-foot-ulcer cream trial both reported faster healing, but the one adequately powered study — a 148-patient Phase IIb venous-leg-ulcer trial — found no significant benefit over placebo in its full study population, with improvement appearing only in a post hoc subgroup whose ulcers measured at least 10 cm². A small RCT of oral LL-37 in SARS-CoV-2 omicron infection sits alongside those. The most mechanistically secure finding is endogenous rather than exogenous: vitamin D directly upregulates LL-37 expression via a vitamin D response element in the CAMP gene. Whether supplementing a deficient person actually raises circulating LL-37 is a separate and far less settled question — of the seven vitamin D trials cited on this page, five found no clear effect on LL-37, including a 207-patient ICU trial where cholecalciferol raised 25(OH)D but not LL-37 and a 101-child atopic-dermatitis trial with no between-group change. Only a sepsis trial and a tuberculosis adjunct trial reported increases, and in the tuberculosis trial the LL-37 rise was seen in the phenylbutyrate arm rather than the vitamin-D-only arm. Part of the disagreement looks methodological: in the sepsis trial bioavailable 25(OH)D correlated with LL-37 (ρ = 0.44) while total 25(OH)D did not, so which vitamin D measure a study uses can decide whether it finds an effect at all. Systemic exogenous LL-37 via subcutaneous injection — as sold in research-chemical channels — has essentially no controlled human evidence base, and LL-37 has concentration-dependent and context-dependent effects: at higher concentrations it can be cytotoxic to host cells, and in rosacea, psoriasis, and lupus subsets LL-37 dysregulation drives pathology rather than preventing it. 'Natural antimicrobial' is not the same as 'safe to inject chronically' — this is one of the better-characterized biology stories in the peptide space and also one of the less justified self-administration stories.

History & Discovery

LL-37 was identified in the mid-1990s as the C-terminal antimicrobial fragment of human cathelicidin antimicrobial peptide-18 (hCAP-18), the only cathelicidin found in humans. The parent protein hCAP-18 is stored in the secondary granules of neutrophils and is also expressed by epithelial cells of the skin, gut, lung, and reproductive tract. Proteolytic cleavage by proteinase 3 in neutrophils — and by other proteases at epithelial surfaces — releases the active 37-amino-acid LL-37 peptide into local tissues and secretions. The biology of LL-37 has been one of the most heavily studied chapters of innate immunity over the last three decades. Key threads include its broad-spectrum antimicrobial activity through membrane disruption, its role as an immunomodulator beyond direct killing (chemotaxis, dendritic cell modulation, LPS neutralization), its regulation by vitamin D (the cathelicidin gene CAMP carries a vitamin D response element, providing a mechanistic link between vitamin D status and innate antimicrobial defense), and its complex involvement in chronic inflammatory conditions such as rosacea and psoriasis, where dysregulated LL-37 processing contributes to pathology. Despite this rich biology, LL-37 has not become an approved therapeutic. Several clinical trials have tested topical or oral LL-37 in chronic wounds (venous leg ulcers, diabetic foot ulcers) and in viral infection contexts (a small SARS-CoV-2 omicron-variant trial), with mixed and limited results. As of 2026 there is no FDA-approved LL-37 product. Synthetic LL-37 sold by research-chemical and compounding-adjacent suppliers for self-administered injection is not equivalent to the clinical-trial preparations and is not authorized for therapeutic use.

How It Works

LL-37 is part of your body's natural defense system. It can directly kill bacteria and viruses by disrupting their membranes, and it also signals immune cells to respond to threats. Think of it as both a weapon and a signal flare for your immune system.

LL-37 exerts antimicrobial effects through direct membrane disruption of pathogens via a carpet-like or toroidal pore mechanism. Beyond direct antimicrobial activity, it functions as an immune modulator: it is chemotactic for immune cells, promotes angiogenesis, modulates TLR signaling, influences dendritic cell differentiation, and can neutralize bacterial endotoxins (LPS). It also promotes wound healing through keratinocyte migration and proliferation. More recent work positions LL-37 within cardiovascular biology as well — circulating LL-37 binds ApoB-100 in atherogenic lipoproteins, and LL-37/ApoB-100 complexes have been reported as biomarkers associated with coronary artery disease, integrating innate-immunity and lipoprotein pathways in atherosclerosis (with a 2026 ATVB editorial framing the axis explicitly). Adaptive immunity to LL-37 has also emerged as a clinically informative dimension: LL-37 IgG levels are associated with clinical characteristics and the T follicular cell response in acute coronary syndrome (Physiol Rep 2026), where LL-37 stimulation of peripheral blood mononuclear cells significantly reduced CD8+CD25+CXCR5+ T follicular regulatory cells while CD8+CD137+CXCR5+ Tf-like cells stayed constant — a loss of follicular regulatory restraint rather than an expansion of helper cells — suggesting an autoreactive-antibody dimension to the LL-37/atherosclerosis story. Cathelicidin has additionally been linked to visceral-fat accumulation and CAD in 2026 clinical work. In the gut, a 2026 review by Liu, Xu, and Zhang (Immun Inflamm Dis) consolidates the intestinal-context evidence — LL-37 coordinates with microbial signals to regulate mucosal immunity and has conflicting roles in inflammatory bowel disease and colorectal cancer, with directional effects shifting according to peptide concentration, receptor binding, disease stage, and local microenvironment. This complements the broader theme that LL-37 is biologically active across multiple compartments and that 'more is not always better.'

Evidence Snapshot

Research Gaps & Open Questions

What the current literature has not yet settled about LL-37:

• 01Clinical efficacy of systemic exogenous LL-37 — most positive human trial signals come from topical wound applications; systemic injection has essentially no controlled human evidence base.
• 02Long-term safety — chronic immune modulation by exogenous LL-37 has not been studied beyond short windows.
• 03Selectivity for microbial versus host membranes at therapeutic concentrations — LL-37 can be cytotoxic to mammalian cells at higher exposures, and the safety margin in human systemic use is not well characterized.
• 04Role in chronic inflammatory disease — in some conditions (rosacea, psoriasis subsets) LL-37 dysregulation contributes to pathology; whether exogenous LL-37 helps or hurts varies by context, and predictive markers are lacking.
• 05Pharmacokinetics of synthetic exogenous LL-37 — absorption, distribution, half-life, and tissue uptake have not been characterized in humans for current research-chemical preparations.
• 06Quality and consistency of research-chemical supply — variation in purity, peptide identity, and endotoxin contamination across suppliers is not characterized for end users, and matters more for an immune-active peptide than for many others.
• 07Cardiovascular biology — LL-37/ApoB-100 complexes have been reported as biomarkers of coronary artery disease, and adaptive immunity to LL-37 as a self-antigen has been linked to atherosclerotic phenotypes. Whether exogenous LL-37 administration would alter atherogenic-lipoprotein binding, immune autoreactivity, or cardiovascular risk has not been studied.

Forms & Administration

LL-37 has been supplied as a subcutaneous injectable through compounding pharmacies and research-chemical sellers, but it is not on the FDA's 503A bulks list, so US compounding of it is not authorized. Protocols should be determined by a qualified clinician.

Dosing & Protocols

The ranges below reflect protocols commonly discussed in the literature and by clinicians — not a prescription. Actual dosing for any individual should be determined by a qualified healthcare provider who knows the patient.

LL-37 is not FDA-approved for any indication. The dosing description here reflects published trial parameters and online protocols, not a clinical recommendation.

Timeline of Effects

Common Questions

Who LL-37 Is NOT For

Drug & Supplement Interactions

There is no published human drug-interaction data for synthetic exogenous LL-37. The following are theoretical and based on mechanism. The most clinically important consideration is layering exogenous LL-37 onto a patient already taking immune-modulating therapy. Immunosuppressive medications (corticosteroids, calcineurin inhibitors, biologics targeting innate immune pathways including TLR signaling, IL-17, IL-23, TNF) may have unpredictable interactions with an exogenous antimicrobial peptide that is itself an immune modulator. Co-administration is not advised outside a controlled investigational setting. Because vitamin D upregulates endogenous LL-37 expression, vitamin D status is the most consistent practical interaction — though this is more about endogenous LL-37 biology than about the exogenous peptide. Patients optimizing innate antimicrobial defense are typically better served by vitamin D repletion than by injected synthetic peptide. LL-37 has been shown to neutralize bacterial endotoxin (LPS); whether this is clinically relevant to patients on therapies sensitive to inflammatory tone is not characterized. Antibiotic interactions are also theoretical — LL-37 is antimicrobial, and combining it with conventional antibiotics has shown synergy in some in vitro studies, but no clinical evidence supports specific combination strategies.

Safety Profile

Legal Status

Regulatory status changes over time. Verify current local rules with a qualified professional.

Myths & Misconceptions