LL-37

Vitamin D-Cathelicidin Axis and Tuberculosis

The landmark discovery linking vitamin D, cathelicidin, and antimicrobial immunity came from Liu et al. (2006), who showed that TLR2/1 activation on human macrophages induces VDR and CYP27B1, enabling local synthesis of 1,25(OH)₂D₃ from circulating 25(OH)D. This active vitamin D metabolite drives CAMP transcription and LL-37 production, which mediates intracellular killing of M. tuberculosis. Critically, this pathway requires sufficient circulating 25(OH)D — serum levels below 20 ng/mL result in inadequate LL-37 induction and impaired mycobacterial killing Liu et al. (2006). This finding provided the molecular explanation for the historical efficacy of sunlight and cod liver oil in tuberculosis treatment.

Skin Infections and Barrier Defense

Cathelicidin is a key component of skin antimicrobial defense. hCAP-18/LL-37 is constitutively expressed in keratinocytes and upregulated during infection, wounding, and UV exposure. Patients with atopic dermatitis have reduced cathelicidin expression, which contributes to their increased susceptibility to skin infections by S. aureus and eczema herpeticum Ong et al. (2002). Conversely, psoriatic skin overexpresses cathelicidin, but the LL-37 produced can form complexes with self-DNA that activate plasmacytoid dendritic cells, driving type I interferon production and autoimmune inflammation Lande et al. (2007).

Rosacea Connection

Yamasaki et al. (2007) demonstrated that rosacea skin contains abnormally high levels of cathelicidin and abnormally processed LL-37 fragments due to elevated KLK5 activity. These aberrant cathelicidin peptides are more potent inducers of inflammation and angiogenesis than native LL-37, directly contributing to the erythema, inflammation, and telangiectasia characteristic of rosacea. KLK5 inhibition reduces rosacea-like inflammation in mouse models, establishing cathelicidin processing as a therapeutic target Yamasaki et al. (2007).

Immunomodulation Beyond Antimicrobial Activity

The immunomodulatory functions of cathelicidin/LL-37 extend well beyond direct microbial killing. LL-37 neutralizes LPS and prevents endotoxin-induced septic shock. It modulates macrophage polarization, promotes autophagy, enhances neutrophil extracellular trap (NET) formation, and regulates apoptosis of immune cells. These pleiotropic effects position cathelicidin as a central mediator of the innate immune response rather than simply an antibiotic peptide Zanetti (2004).

Anti-Tumor Role in Colon Cancer

LL-37 expression is significantly reduced in colon cancer tissues compared to adjacent normal mucosa. Ren et al. (2012) demonstrated that exogenous LL-37 inhibits colon cancer cell proliferation (HCT116, HT-29, LoVo cell lines) in a dose-dependent manner, induces caspase-dependent apoptosis, and suppresses colony formation. The anti-tumor mechanism involves both direct membrane-mediated cytotoxicity and activation of intrinsic apoptotic pathways. In vivo, intratumoral injection of LL-37 reduced colon cancer xenograft growth in nude mice. The downregulation of LL-37 in colon cancer is attributed to epigenetic silencing of the CAMP promoter through DNA methylation Ren et al. (2012). These findings support the hypothesis that LL-37 functions as a tumor suppressor in the colonic epithelium and provide a mechanistic link between vitamin D's protective effect against colon cancer and cathelicidin expression.

Anti-Tumor Role in Gastric Cancer

Li et al. (2013) reported that LL-37 expression is downregulated in human gastric cancer specimens and that exogenous LL-37 inhibits gastric cancer cell proliferation, induces mitochondrial-mediated apoptosis, and suppresses tumor growth in xenograft models. The anti-tumor mechanism in gastric cancer is mediated through p53-dependent pathways, with LL-37 stabilizing p53 protein and promoting its nuclear translocation. Additionally, LL-37 inhibits proteasome activity in gastric cancer cells, leading to accumulation of pro-apoptotic proteins Li et al. (2013).

Wound Healing

LL-37 is highly expressed at wound margins and is critical for normal cutaneous repair. Heilborn et al. showed that chronic non-healing venous leg ulcers are deficient in LL-37 compared to normally healing acute wounds, implicating cathelicidin deficiency in impaired wound repair. LL-37 promotes healing through antimicrobial protection of the wound bed, keratinocyte migration via EGFR transactivation, and angiogenesis stimulation Heilborn et al. (2003).

Pro-Tumor Role in Ovarian Cancer

In stark contrast to colon cancer, LL-37 is significantly overexpressed in ovarian tumor tissues, with both cancer cells and tumor-infiltrating macrophages serving as sources. Coffelt et al. (2008, 2009) established that LL-37 functions as a growth factor in ovarian cancer, promoting proliferation through EGFR transactivation, enhancing migration and invasion through FPR2-MEK/ERK signaling, and increasing chemoresistance to cisplatin and paclitaxel. Critically, LL-37 recruits mesenchymal stem cells to the tumor microenvironment, which differentiate into tumor-associated fibroblasts that further support tumor growth through paracrine signaling Coffelt et al. (2009). Neutralizing antibodies against LL-37 inhibited ovarian tumor growth in mouse models, confirming the peptide's functional role in promoting tumor progression Coffelt et al. (2008).

Angiogenesis and Vascularization

Koczulla et al. (2003) identified LL-37 as a pro-angiogenic factor, demonstrating that the peptide promotes endothelial cell proliferation, migration, and neovascularization both in vitro and in vivo (chick chorioallantoic membrane and rabbit hindlimb ischemia models). The angiogenic effect is mediated through FPR2 on endothelial cells and involves activation of the MAPK/ERK pathway. In the context of cancer, this pro-angiogenic activity provides a mechanism by which LL-37-overexpressing tumors can promote their own vascularization, supporting nutrient delivery and metastatic spread Koczulla et al. (2003).

Immunomodulatory Effects in the Tumor Microenvironment

LL-37's immunomodulatory effects add another layer of complexity to its role in cancer. The peptide can recruit and activate multiple immune cell types: neutrophils and monocytes (via FPR2), immature dendritic cells (via FPR2 and CCR6-like mechanisms), and T cells. In some tumor contexts, LL-37 promotes anti-tumor immunity by enhancing NK cell cytotoxicity and dendritic cell maturation. In others, LL-37 promotes immunosuppression by driving macrophage polarization toward the M2 (tumor-associated) phenotype and expanding regulatory T cells. The net immunological effect depends on the local cytokine milieu, tumor type, and LL-37 concentration Kuroda et al. (2015).

Ongoing & Future Research

• OP-145 (P60.4Ac): Truncated LL-37 analog in Phase 2 clinical trials for chronic otitis media (NCT02022540)
• Development of D-amino acid LL-37 analogs resistant to protease degradation with improved therapeutic index
• LL-37-functionalized wound dressings and biomaterial coatings for medical device anti-infective applications
• Research into LL-37-based peptides for antibiotic-resistant infections (MRSA, carbapenem-resistant Enterobacteriaceae)
• Investigation of LL-37 analogs with reduced hemolytic activity and improved selectivity index
• LL-37 as a cancer therapeutic: ongoing research into analogs that retain anti-tumor activity without pro-tumorigenic EGFR effects
• Vitamin D supplementation trials using LL-37 levels as a biomarker for innate immune competence
• LL-37-derived peptides for anti-biofilm coatings on catheters and prosthetic implants (Zhang et al., DOI: 10.1016/j.peptides.2024.171156)

Antimicrobial Activity

LL-37 demonstrates broad-spectrum antimicrobial efficacy. It kills both gram-positive organisms (S. aureus, S. pyogenes) and gram-negative pathogens (E. coli, P. aeruginosa, K. pneumoniae) at micromolar concentrations. It is also effective against mycobacteria, including Mycobacterium tuberculosis, where vitamin D-induced LL-37 expression is a key mechanism of macrophage antimicrobial defense Liu et al. (2006). Antifungal activity against Candida albicans and antiviral effects against influenza, HIV, and herpes simplex virus have also been documented Barlow et al. (2011).

Anti-Biofilm Properties

Bacterial biofilms are a major clinical challenge due to their resistance to conventional antibiotics. LL-37 has shown the ability to prevent and disrupt biofilm formation by P. aeruginosa at concentrations well below its minimum inhibitory concentration (MIC) for planktonic bacteria. Overhage et al. demonstrated that LL-37 at 0.5 μg/mL inhibited P. aeruginosa biofilm formation by 40%, reduced pre-formed biofilm thickness, and stimulated twitching motility that disrupted biofilm architecture Overhage et al. (2008).

Wound Healing and Skin

LL-37 is upregulated at wound sites and plays a critical role in cutaneous repair. It is highly expressed in the wound edge epithelium and promotes keratinocyte migration through EGFR transactivation. Heilborn et al. showed that chronic non-healing ulcers are deficient in LL-37 compared to acute healing wounds, suggesting that LL-37 deficiency contributes to impaired wound healing Heilborn et al. (2003). In skin conditions, LL-37 overexpression is associated with rosacea, where aberrant processing of hCAP18 by kallikrein 5 generates pro-inflammatory LL-37 fragments Yamasaki et al. (2007).

Cancer Research

LL-37 has demonstrated context-dependent effects in cancer biology. It exhibits direct cytotoxic effects against certain cancer cell lines, including gastric cancer and hematological malignancies, through membrane disruption and apoptosis induction. However, in some cancer types (ovarian, breast, lung), LL-37 may promote tumor growth through EGFR activation and angiogenesis stimulation. This duality underscores the complexity of LL-37's role in oncology and the need for cancer type-specific investigation Wu et al. (2010).

Vitamin D Connection

The link between vitamin D and innate immunity is mediated largely through LL-37. Vitamin D receptor (VDR) activation directly upregulates CAMP gene transcription. Liu et al. demonstrated that TLR2 activation in macrophages upregulates VDR and CYP27B1 (the enzyme converting 25(OH)D to active 1,25(OH)₂D), leading to LL-37 induction and intracellular killing of M. tuberculosis. This landmark study established the molecular basis for the historical use of sunlight and cod liver oil in tuberculosis treatment Liu et al. (2006).

Pro-Tumor Role in Breast and Lung Cancer

LL-37 promotes breast cancer progression through EGFR transactivation and downstream activation of the HER2/ErbB2 signaling network. Weber et al. (2009) showed that LL-37 stimulates breast cancer cell migration and invasion through metalloproteinase-dependent release of EGFR ligands, and that LL-37 expression is elevated in breast tumor tissues Weber et al. (2009). In non-small cell lung cancer (NSCLC), von Haussen et al. (2008) demonstrated that LL-37 is overexpressed in tumor tissues and acts as an autocrine/paracrine growth factor, promoting NSCLC cell proliferation through FPRL1/FPR2 signaling and EGFR transactivation. LL-37 also enhances lung cancer cell migration and invasion, suggesting a role in metastatic progression von Haussen et al. (2008).

Neurological/Immunological Mechanisms

Membrane disruption mechanisms:

• Carpet model: LL-37 monomers align parallel to the membrane surface, covering it like a carpet until a critical concentration is reached, causing membrane disintegration
• Toroidal pore model: At higher concentrations, LL-37 inserts perpendicular to the membrane, forming transmembrane pores with lipid headgroups lining the pore interior
• The amphipathic alpha-helix is essential: hydrophobic face interacts with lipid acyl chains; hydrophilic/cationic face interacts with anionic lipid headgroups
• Selectivity for microbial membranes: bacterial membranes are enriched in anionic phospholipids (phosphatidylglycerol, cardiolipin) while mammalian membranes are enriched in zwitterionic phospholipids (phosphatidylcholine) and cholesterol

Immunomodulatory receptor signaling:

• FPRL1/FPR2 (formyl peptide receptor-like 1): Primary receptor for LL-37's immunomodulatory effects. Activation → Gi-coupled signaling → chemotaxis of neutrophils, monocytes, and T cells to infection sites (Scott et al., PMID: 11796220)
• P2X7 purinergic receptor: LL-37 activates P2X7 → IL-1β processing and release via NLRP3 inflammasome activation. This links LL-37 to inflammasome-mediated innate immune responses.
• EGFR transactivation: LL-37 → metalloproteinase activation → HB-EGF release → EGFR phosphorylation → keratinocyte migration and proliferation (wound healing) (Heilborn et al., PMID: 12932393)
• TLR9 (indirect): LL-37-DNA complexes activate endosomal TLR9 in plasmacytoid DCs → type I IFN production. This is a key pathogenic mechanism in psoriasis but also contributes to antiviral defense (Lande et al., PMID: 17676033)

Anti-biofilm mechanisms:

• At sub-MIC concentrations: downregulates biofilm-specific genes (lasI, lasR quorum sensing regulators in P. aeruginosa)
• Stimulates type IV pilus-dependent twitching motility → disrupts biofilm architecture (Overhage et al., PMID: 18227163)
• Prevents initial bacterial adhesion to surfaces
• Disrupts pre-formed biofilms through charge-mediated interactions with the biofilm exopolysaccharide matrix

Vitamin D-LL-37 axis (molecular detail):

• TLR2/1 activation on macrophages → upregulates VDR and CYP27B1
• CYP27B1 converts 25(OH)D → active 1,25(OH)₂D locally
• 1,25(OH)₂D binds VDR → VDR/RXR heterodimer binds VDRE (vitamin D response element) in CAMP gene promoter
• CAMP transcription → hCAP18 protein → proteolytic processing → mature LL-37 peptide
• This pathway requires sufficient circulating 25(OH)D substrate — explaining why vitamin D deficiency impairs antimicrobial immunity (Liu et al., PMID: 16497887)