LL-37: The Only Human Cathelicidin
LL-37 Vitamin D
1 cathelicidin
Humans produce only a single cathelicidin, LL-37, yet this one peptide performs antimicrobial, immunomodulatory, wound healing, and anticancer functions across nearly every tissue.
Bucki et al., Archivum Immunologiae et Therapiae Experimentalis, 2010
Bucki et al., Archivum Immunologiae et Therapiae Experimentalis, 2010
View as imageMost mammals produce multiple cathelicidins. Pigs have at least five. Cattle have more than a dozen. Humans have exactly one: LL-37, a 37-amino-acid peptide cleaved from the precursor protein hCAP-18. This single peptide somehow manages to kill bacteria, neutralize viruses, recruit immune cells, promote wound healing, regulate intestinal barrier function, and influence cancer cell survival. Understanding why one peptide can do so much requires looking at its unusual structure, its expression pattern, and the multiple receptor systems it engages. For context on how vitamin D controls LL-37 production and why that matters clinically, see our pillar article.
Key Takeaways
- LL-37 is the only cathelicidin produced by humans, stored at high concentrations in neutrophil granules and secreted by epithelial cells across the skin, airways, gut, and urogenital tract (Bucki et al., 2010)
- The peptide's amphipathic alpha-helical structure allows it to interact with negatively charged bacterial membranes, host cell receptors, and viral particles through the same structural motif (Tjabringa et al., 2005)
- LL-37 selectively promotes apoptosis of Pseudomonas-infected airway cells while sparing uninfected neighbors, demonstrating a level of target discrimination unusual for antimicrobial peptides (Barlow et al., 2010)
- In cancer biology, LL-37 shows contradictory effects: it promotes ovarian, lung, and breast cancer growth but suppresses colon and gastric cancer, depending on which receptors it activates in each tissue (Piktel et al., 2016)
- LL-37 inhibited RSV replication, prevented virus-induced cell death, and reduced viral spread in epithelial cultures through both direct viral disruption and host cell modulation (Currie et al., 2013)
- Beyond killing pathogens, LL-37 acts as an alarmin that recruits neutrophils, monocytes, and T cells, stimulates angiogenesis, and promotes tissue repair (Bucki et al., 2010)
One Peptide, One Gene
The human cathelicidin gene (CAMP) is located on chromosome 3 and encodes a 170-amino-acid precursor protein called hCAP-18 (human cationic antimicrobial protein of 18 kDa). This precursor consists of a signal peptide, a conserved cathelin domain, and the C-terminal antimicrobial domain. Proteolytic cleavage by proteinase 3 (in neutrophils) or kallikrein-related peptidases (in skin) releases the active 37-amino-acid peptide, LL-37, named for its two N-terminal leucine residues and its length.[1]
Why humans have only one cathelicidin while other species have many is unclear. One hypothesis is that LL-37's structural versatility compensates for the lack of diversity. Its amphipathic alpha-helix can adopt different conformations depending on the local environment (membrane surface, aqueous solution, receptor interface), allowing a single sequence to perform multiple functions.[1]
LL-37 is constitutively expressed in bone marrow, testis, and the epithelial cells of skin, airways, gut, and urogenital tract. It is stored in large quantities in the specific (secondary) granules of neutrophils, ready for rapid release during infection. Monocytes, macrophages, mast cells, and natural killer cells also produce LL-37, though typically in response to inflammatory signals rather than constitutively.[1]
For comparison with cathelicidins from other species, see Cathelicidins Across Species: What Animal Versions Teach Us.
Direct Antimicrobial Activity
LL-37's antimicrobial mechanism centers on its amphipathic structure: one face of the alpha-helix is positively charged (cationic), the other is hydrophobic. The cationic face binds to negatively charged components of bacterial membranes (lipopolysaccharide in Gram-negative bacteria, lipoteichoic acid in Gram-positive bacteria). The hydrophobic face then inserts into the lipid bilayer, disrupting membrane integrity and killing the cell.[1]
This mechanism gives LL-37 broad-spectrum activity against Gram-positive and Gram-negative bacteria, fungi, and some parasites. Bucki et al. (2010) noted that LL-37 also neutralizes bacterial lipopolysaccharide (LPS), preventing it from triggering inflammatory cascades even after bacteria are killed.[1] This dual action, killing the pathogen and detoxifying its inflammatory debris, is unusual among antimicrobial peptides.
The peptide's antimicrobial potency is affected by salt concentration, pH, and the presence of serum components. Under physiological salt conditions, LL-37's bactericidal activity is reduced compared to low-salt laboratory conditions. This has raised questions about its effectiveness in vivo, though the high local concentrations released from neutrophil granules at infection sites likely exceed the threshold needed for activity. For detailed mechanisms, see How LL-37 Disrupts Bacterial Membranes and Biofilms.
Antiviral Properties
LL-37's antiviral activity extends beyond its ability to disrupt lipid-enveloped viruses (which share structural features with bacterial membranes). Currie et al. (2013) demonstrated that LL-37 inhibited respiratory syncytial virus (RSV) through multiple parallel mechanisms: it prevented virus-induced cell death, inhibited production of new infectious particles, and diminished viral spread between cells.[5]
The antiviral effects were directed at both the viral particles themselves (direct neutralization) and the host cells (enhanced antiviral defense). This two-pronged approach means LL-37 does not rely solely on physically destroying the virus; it also prepares cells to resist infection if some viral particles evade the initial defense.[5]
Subsequent studies have extended LL-37's antiviral repertoire to include influenza virus, HIV, herpes simplex virus, and SARS-CoV-2. The common thread is that LL-37 interferes with the initial stages of viral infection (attachment, entry, or uncoating) rather than inhibiting viral replication once the virus is established inside the cell.
Immunomodulation: The Alarmin Function
LL-37 is classified as an "alarmin," an endogenous molecule that signals danger and activates the immune system. Bucki et al. (2010) catalogued its immunomodulatory activities:[1]
Chemotaxis. LL-37 acts as a chemoattractant for neutrophils, monocytes, eosinophils, and T cells via formyl-peptide receptor-like 1 (FPRL1/FPR2). It recruits immune cells to the site of infection or tissue damage.
Anti-apoptotic effects on neutrophils. LL-37 inhibits neutrophil apoptosis, extending the lifespan of these short-lived cells at infection sites. This prolongs their antimicrobial activity but can contribute to tissue damage if inflammation becomes excessive.
Angiogenesis. LL-37 stimulates new blood vessel formation, which is essential for wound healing and tissue repair. It acts through FPRL1 on endothelial cells to promote proliferation and tube formation.
Cytokine modulation. LL-37 can both stimulate and suppress cytokine production, depending on context, cell type, and concentration. This dual capacity makes it a fine-tuner of inflammation rather than simply an activator or suppressor.
Tjabringa et al. (2005) reviewed how these immunomodulatory functions operate specifically in the lung, where LL-37 coordinates innate immunity, modulates dendritic cell maturation, and influences the transition from innate to adaptive immune responses.[3]
Selective Cell Killing: Not Just Pathogens
One of LL-37's most sophisticated functions is its ability to selectively kill compromised host cells. Barlow et al. (2010) demonstrated that LL-37 promoted apoptosis of Pseudomonas-infected airway epithelial cells while leaving uninfected neighboring cells unharmed.[4]
This selectivity is remarkable for an antimicrobial peptide. Most membrane-active peptides are relatively indiscriminate. The mechanism appears to involve differences in membrane composition between infected and uninfected cells. Bacterial infection alters the host cell membrane, exposing phosphatidylserine on the outer leaflet and changing the overall charge distribution. These changes make infected cells resemble the negatively charged bacterial membranes that LL-37 targets.[4]
The apoptosis is controlled (no release of inflammatory contents) rather than necrotic (cell rupture), limiting collateral damage to surrounding tissue. This positions LL-37 as a quality control mechanism: it removes cells that have been compromised by infection without amplifying the inflammatory response.
The Cancer Paradox
Piktel et al. (2016) reviewed the complex and contradictory relationship between LL-37 and cancer development.[2] The findings defy simple characterization:
LL-37 promotes certain cancers. Overexpression of LL-37 was found in ovarian cancer, lung cancer, and breast cancer tissues. In these contexts, LL-37 appeared to promote tumor growth by acting as a growth factor through receptor-mediated signaling (particularly through EGFR and FPRL1).[2]
LL-37 suppresses other cancers. In colon cancer and gastric cancer, LL-37 showed tumor-suppressive effects, including direct cytotoxicity against cancer cells and immune-mediated antitumor activity.[2]
The tissue-specific effects likely reflect differences in receptor expression, local microenvironment, and the balance between LL-37's membrane-disrupting activity (which would kill cancer cells) and its receptor-mediated signaling (which could stimulate growth). This duality means LL-37 cannot be simplistically classified as either a tumor promoter or suppressor. It is both, depending on context.
Barrier Protection
Otte et al. (2009) showed that LL-37 restores intestinal epithelial barrier integrity in bacteria-infected cells by reorganizing tight junction proteins, particularly ZO-1.[6] LL-37 also upregulated expression of claudin-1 and occludin, the transmembrane proteins that form the physical seal between cells. This barrier-protective function extends beyond the gut to the skin, airways, and urogenital epithelium.
The barrier effects are context-dependent. In infected or damaged tissue, LL-37 restores integrity. At supraphysiological concentrations in healthy tissue, it can transiently increase permeability.[6] This concentration-dependent duality mirrors LL-37's broader pattern: protective at physiological levels, potentially disruptive when overproduced.
Why Clinical Translation Is Hard
Despite decades of research and an impressive list of biological activities, LL-37 has not become a clinical therapeutic. Several properties make it difficult to develop as a drug:
Short half-life. LL-37 is rapidly degraded by serum proteases. Its plasma half-life is measured in minutes, making systemic administration impractical without significant chemical modification.
Concentration-dependent toxicity. At concentrations above the physiological range, LL-37 can damage host cell membranes (hemolysis) and trigger excessive inflammation. The therapeutic window between antimicrobial efficacy and host toxicity is narrow.
Salt sensitivity. LL-37's antimicrobial activity is reduced under physiological salt conditions. In cystic fibrosis airway fluid, high salt concentrations may inactivate the peptide, contributing to the recurrent infections characteristic of the disease.
Dual roles in cancer. The peptide's ability to both promote and suppress cancer growth makes it unsuitable as a systemic anticancer agent without tissue-specific targeting.
Cost of synthesis. A 37-amino-acid peptide is expensive to manufacture at pharmaceutical scale compared to small molecule drugs. Shorter derivatives and mimetics are under development but have not yet matched LL-37's full spectrum of activity.
The most promising clinical path may be indirect: boosting endogenous LL-37 production through vitamin D supplementation, or developing shorter synthetic analogs that retain specific activities while avoiding the challenges of the full-length peptide.
The Bottom Line
LL-37 is the sole human cathelicidin, yet it performs at least six distinct biological functions: direct antimicrobial killing, antiviral activity, immune cell recruitment, wound healing promotion, epithelial barrier maintenance, and cancer cell modulation. This versatility stems from its amphipathic alpha-helical structure, which allows it to interact with bacterial membranes, host cell receptors, and viral particles through conformation-dependent mechanisms. The same properties that make LL-37 biologically versatile, particularly its membrane-active nature and receptor promiscuity, also make it challenging to develop as a clinical therapeutic.