LL-37 is catalogued as an antimicrobial peptide, but at the concentrations human tissue actually reaches, the published record describes its signaling activity as the more consequential one. The 37 amino acid cationic fragment released from the inactive hCAP-18 precursor behaves in the extracellular space less like an antibiotic than like an alarm announcing a breached epithelial surface, and the reframing carries a caveat that belongs up front: nearly all of this mechanism rests on cell culture and animal work, not human clinical evidence.
LL-37 is a 37 amino acid cationic fragment of the hCAP-18 precursor whose documented signaling functions, chemotaxis through FPR2, lipid A sequestration, and nucleic acid complexing that triggers TLR9 and TLR7/8, are drawn almost entirely from cell culture and animal models rather than from human clinical trials.
FPR2, a Gi-coupled formyl peptide receptor also written FPRL1, carries most of the chemotactic signal. Foundational work published in 2000 reported that LL-37 drew neutrophils, monocytes, and T cells, that pertussis toxin almost completely inhibited the response, and that transfection of FPR2 alone reproduced it. Confidence drops sharply past that receptor, and the drop is the difference between a mechanism a clinician can rely on and one that is still a hypothesis.
The chemotactic activity of LL-37, unlike its antimicrobial action, is not significantly inhibited by 10 percent human serum, and that serum resistance rather than any concentration gap is a large part of why the signaling role is considered physiologically plausible while the killing role is debated.
Neutralization begins as charge chemistry: LL-37 carries a net charge near +6 at physiological pH, lipid A is anionic, and reported binding affinities run from nanomolar to low micromolar. The block acts at several points in the chain rather than one, which is why the suppression is more thorough than a receptor antagonist would achieve. What matters downstream is that the suppression is selective, steering the response toward recruitment and away from the cytokine surge that drives tissue injury and shock.
Endotoxin neutralization by LL-37 is typically demonstrated in vitro at 10 to 50 µg/mL, above the roughly 1 to 2 µg/mL measured in plasma, and cathelicidin-derived peptides pursued as sepsis therapeutics on this rationale have produced no convincing clinical benefit to date.
This is the mechanism that moved LL-37 from antimicrobial curiosity to a central player in autoimmunity. Extracellular self-DNA is normally invisible to the immune system, degraded by nucleases and, being anionic, taken up poorly into the endosomal compartment where TLR9 waits, and the dense positive charge of LL-37 solves both problems at once. Work published in Nature in 2007 reported that plasmacytoid dendritic cells exposed to LL-37/self-DNA complexes mount a strong TLR9-dependent interferon-alpha response that free self-DNA does not elicit at all.
| Criteria | LL-37/self-DNA complexes | LL-37/self-RNA complexes |
|---|---|---|
| Sensor engaged | TLR9 | TLR7 and TLR8 |
| Responding cells | Plasmacytoid dendritic cells | Plasmacytoid DCs (TLR7), myeloid DCs (TLR8) |
| Dominant output | Type I interferon | TNF-alpha and IL-6 |
| Nuclease resistance | Condensation shields DNA from DNase digestion | Less characterized |
LL-37 condenses self-DNA into aggregated, near-crystalline particles whose internal periodicity sustains TLR9 engagement, producing an interferon-alpha response from plasmacytoid dendritic cells that free self-DNA does not elicit at all.
The repair effects were documented clinically before they were explained. Wounded skin shows a sharp rise in hCAP-18 expression in the migrating epithelial tongue within hours of injury, while chronic venous leg ulcers that fail to re-epithelialize show conspicuously low LL-37 at the wound edge. The relationship is not linear, though, and that is the part most often glossed over when this literature gets summarized.
Synthetic LL-37 evaluated in a small early-phase trial in hard-to-heal venous leg ulcers showed encouraging effects at lower doses and less benefit at the highest one, and no LL-37 based wound product has established efficacy in a large controlled trial.
Once the microbial threat is handled the question shifts from killing to cleanup, and the record has LL-37 pulling in two directions at once. It delays constitutive neutrophil apoptosis while also improving macrophage disposal of the apoptotic neutrophils that resolution depends on. Which face dominates is decided by context, and in chronic inflammatory disease the context is unfavorable.
LL-37 delays constitutive neutrophil apoptosis dose-dependently across 0.01 to 5 µg/mL through FPRL1 and P2X7, while no consistent M1 or M2 macrophage polarization assignment is defensible from the current literature.
The inflammasome connection runs mainly through P2X7, an ATP-gated ion channel on monocytes and macrophages whose activation causes the potassium efflux that triggers NLRP3 assembly with ASC and caspase-1, and LL-37 engages this axis without ATP. Which signal the peptide actually supplies is genuinely disputed, and the dispute is not a data artifact: the same molecule has been reported to suppress inflammasome output in other systems, so the net effect depends entirely on what else is present and in what order.
| Criteria | Reported pro-inflammasome role | Reported suppressive role |
|---|---|---|
| Mechanism | Direct P2X7 activation; assisted IL-1 beta exit | Neutralizes the priming LPS; scavenges extracellular ATP and DNA |
| Required context | Pro-IL-1 beta already primed, though some work reports NF-kB dependent priming activity as well | Peptide present before the priming stimulus lands |
| Typical concentration | 5 to 25 µg/mL | Overlapping range, system dependent |
Most P2X7-dependent inflammasome effects of LL-37 are demonstrated at 5 to 25 µg/mL, plausible at an inflamed epithelium but not in circulation, and in no human setting, including psoriatic skin, cystic fibrosis, or COPD, has LL-37 been shown to be the necessary driver of the IL-1 beta observed.
CAMP, the single human cathelicidin gene, sits on chromosome 3p21 and carries a vitamin D response element in its promoter, so 1,25-dihydroxyvitamin D3 bound to the vitamin D receptor with RXR drives transcription directly. The evolutionary detail is the consequential one: that response element arrived through an Alu short interspersed element insertion in the primate lineage, so the vitamin D to cathelicidin link exists in humans and simply does not exist in mice. Regulation also does not stop at transcription, which is where one gene starts yielding functionally different products.
The vitamin D response element in the human CAMP promoter arrived through an Alu insertion in the primate lineage and is absent in mice, so no mouse model reproduces the vitamin D to cathelicidin pathway at all.
For most conditions in which LL-37 is elevated, the elevation is equally consistent with the peptide being a consequence of inflammation as with it being a cause, and that distinction is the one that gets lost when a mechanism story travels. Psoriasis and rosacea are the two settings where mechanistic and interventional data push toward causation. The reason one molecule both helps and harms is that its functions carry no intrinsic direction: recruiting leukocytes, making nucleic acid visible, and prolonging neutrophil life are appropriate responses to a real breach and pathological when the peptide persists without one.
T cells specific for LL-37 were identified in roughly two thirds of patients with moderate to severe psoriasis in a 2014 report, are enriched in lesions, produce IL-17, and track disease activity, which makes LL-37 one of the few well-characterized autoantigens in the disease.
Nearly all early LL-37 work ran in low ionic strength media, and adding physiological 150 mM sodium chloride, divalent cations, or serum abolishes much of the direct antimicrobial killing, which is what pushed the field toward the signaling functions in the first place. The two activities are not equally salt sensitive, so the salt problem argues for the signaling role rather than against it. Dose is the harder issue, and it is where most published mechanism sits at or above the range human tissue actually reaches.
Many published mechanistic effects of LL-37 use 10 to 50 µg/mL, at or above the top of the physiological envelope of 1 to 2 µg/mL in plasma and 5 to 30 µg/mL in inflamed airway surface liquid, and in the upper part of that range close to concentrations cytotoxic to host cells.
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