LL-37: The Human Cathelicidin Peptide Vitamin D Turns On

LL-37 is a 37-amino-acid cationic peptide cleaved from the C-terminus of human cathelicidin, hCAP-18. It is the only cathelicidin humans make. Peptide catalogs sell it as an “immune” or “antimicrobial” research compound. The vitamin D biology is real, the tuberculosis macrophage papers are famous, and the same molecule is also a driver of psoriasis and rosacea when it shows up in the wrong form, in the wrong place.

TL;DR

• LL-37 is residues of hCAP-18 released by proteinase 3 after neutrophils dump the precursor.
• The CAMP gene (cathelicidin antimicrobial peptide, not the second messenger cAMP) is a direct vitamin D receptor target in primates.
• In human macrophages, TLR2/1 plus vitamin D induces cathelicidin that helps kill intracellular Mycobacterium tuberculosis.
• Excess or misprocessed LL-37 promotes rosacea inflammation and, complexed with self-DNA, turns on plasmacytoid dendritic cells in psoriasis.
• Topical trial product helped some small venous-ulcer cohorts. The larger phase IIb study did not beat placebo overall.

Educational only. Not medical advice, not a protocol, and not a reason to treat a research vial as innate-immunity therapy.

What is LL-37, and how is it made?

Humans encode one cathelicidin gene, CAMP. The protein product is hCAP-18, an 18 kDa precursor stored in neutrophil granules and also made by keratinocytes and other epithelia.

The antimicrobial fragment is not the whole protein. After the cell releases hCAP-18, proteinase 3 cuts it extracellularly to free the C-terminal 37-mer that starts with two leucines, hence the name LL-37 (Sorensen et al., Blood, 2001).

That processing step matters. In healthy skin the cut is controlled. In rosacea, extra serine-protease activity produces abnormal peptide forms.

LL-37 is amphipathic, one face hydrophobic and the other cationic. That shape lets it disrupt microbial membranes and also bind DNA and RNA, which is how a host-defense peptide becomes an autoinflammatory trigger. For the chemistry primer, see Peptides 101.

Why does vitamin D turn on the CAMP gene?

In 2005, Gombart, Borregaard, and Koeffler showed that 1,25-dihydroxyvitamin D3 strongly induces CAMP in human myeloid cells, keratinocytes, and colon lines. Induction is direct. A consensus vitamin D response element (VDRE) in the promoter binds the vitamin D receptor (VDR).

The VDRE sits in a primate-specific SINE repeat. Mice, rats, and dogs do not have it. That is why vitamin D does not drive cathelicidin the same way in standard lab rodents (Gombart et al., FASEB Journal, 2005).

Liu and colleagues then put the circuit into an infection story. TLR2/1 activation on human macrophages raises VDR and CYP27B1, the enzyme that converts circulating 25-hydroxyvitamin D into the active hormone, which then induces cathelicidin.

That pathway needed human serum, which carries more vitamin D precursor than typical culture media. Sera with lower 25-hydroxyvitamin D induced less cathelicidin, and adding vitamin D restored the response that helped kill intracellular M. tuberculosis (Liu et al., Science, 2006).

That is the honest vitamin D link. It is a transcriptional switch for an endogenous peptide, not a reason to inject a catalog vial because your 25-OH D is low.

Why can more LL-37 make skin worse?

Host defense and autoinflammation share the same peptide.

Rosacea. Yamasaki’s group found abnormally high cathelicidin in facial skin, in peptide forms that differed from healthy skin, with elevated stratum corneum tryptic enzyme activity. Injecting those peptides, or raising protease activity, produced mouse skin inflammation. Deleting the cathelicidin gene blocked it (Yamasaki et al., Nature Medicine, 2007).

Psoriasis. Free self-DNA is usually ignored. Complexed with cationic LL-37, it reaches endosomal TLR9 in plasmacytoid dendritic cells and drives interferon-alpha, a proposed early event in plaques (Lande et al., Nature, 2007).

The marketing one-liner (“immune peptide”) is half a sentence. In the right compartment, LL-37 is antimicrobial. In lesional skin, extra peptide plus nucleic acid is a DAMP.

What did the wound-healing trials actually measure?

Endogenous LL-37 is abundant in acute wounds and often missing in chronic ulcers. That observation, plus animal and ex vivo work, led to topical synthetic LL-37 as an investigational drug, not a research chemical.

The first-in-human study used a 3-week placebo run-in, then 4 weeks of twice-weekly topical LL-37 at 0.5, 1.6, or 3.2 mg/mL. The two lower concentrations moved healing-rate constants. The highest dose did not (Gronberg et al., Wound Repair and Regeneration, 2014).

HEAL LL-37 tested 13 weeks of twice-weekly 0.5 or 1.6 mg/mL plus compression. Median ulcer duration was about 20 months. The full population did not heal better than placebo (Mahlapuu et al., 2021).

An inverted dose-response in the small study, then a negative primary analysis in the larger one, is not “LL-37 heals wounds.” It is early clinical data with a real mechanism and an unresolved efficacy question.

How is a research vial different from the trial product?

The trial drug was a sterile topical concentrate of synthetic human LL-37 made for a regulated study. Catalog vials are research chemicals. Identity, salt form, and endotoxin are COA questions, not label promises.

There is no FDA-approved LL-37 drug. Licensed peptides live in pharmacy and telehealth channels, which is what the provider directory and price index compare. Handling education lives on the reconstitution guide and the calculator.

Bottom line: LL-37 is a real human host-defense peptide with a clean vitamin D transcriptional story and mixed clinical data. It is also a documented driver of rosacea and psoriatic inflammation. A catalog vial is not that biology in a bottle.

Educational only. Not medical advice. Not a recommendation to use LL-37, cathelicidin fragments, or any research-grade antimicrobial peptide.