LL-37 and Cancer Immunology: Antimicrobial Peptide Research, Tumour Biology and Immune Modulation

LL-37 is the sole member of the cathelicidin family expressed in humans, and among the most thoroughly studied host defence peptides in biomedical science. While its antimicrobial properties — direct membrane disruption of bacteria, fungi, and enveloped viruses — have been known for decades, a parallel body of literature has emerged exploring LL-37’s complex role in cancer biology. That role is paradoxical: in different tumour microenvironments and tissue contexts, LL-37 can suppress tumour growth or promote it, engage innate immune killing or enable immune escape, and trigger apoptosis or stimulate angiogenesis. This post examines the mechanistic basis of LL-37’s cancer biology for researchers investigating immunomodulatory peptides and host defence in oncological contexts. All research contexts discussed are Research Use Only (RUO).

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LL-37 Biology: Structure, Expression, and Core Mechanisms

LL-37 is processed from the 18 kDa proprotein hCAP-18 by serine proteases (proteinase 3 in neutrophils, kallikreins in epithelial tissues). The mature peptide is 37 amino acids with an amphipathic α-helical structure — a critical feature for membrane interaction. The two N-terminal leucines give it its name.

Expression is highest in:

• Neutrophils (stored in specific granules, released upon activation)
• Epithelial surfaces (skin, lung, gut, reproductive tract) — induced by inflammation, Toll-like receptor activation, and vitamin D
• Monocytes and macrophages
• NK cells and mast cells
• Spermatozoa and seminal plasma

In healthy tissue, LL-37 concentrations are low. They increase dramatically at sites of infection, tissue injury, and inflammation — positioning LL-37 as a damage-associated molecule at the interface between innate immune activation and tissue repair. Vitamin D3 (1,25-dihydroxyvitamin D3) is the best characterised transcriptional inducer of LL-37, acting through vitamin D response elements (VDREs) in the CAMP gene promoter.

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Tumour-Suppressive Effects of LL-37: Mechanisms

In several cancer types, LL-37 expression is reduced relative to normal adjacent tissue, and preclinical data suggest that exogenous LL-37 administration or CAMP gene upregulation has antitumour effects.

Direct Cytotoxicity

LL-37’s amphipathic α-helix preferentially disrupts cancer cell membranes over normal cells. The basis for selectivity is debated but likely involves:

• Phosphatidylserine externalisation: Cancer cells frequently expose phosphatidylserine on the outer leaflet of the plasma membrane (a feature normally restricted to apoptotic cells), creating an anionic surface that electrostatically attracts cationic LL-37
• Increased negative surface charge: Overexpression of heparan sulphate proteoglycans and O-glycosylated proteins on cancer cell surfaces increases anionic charge density
• Higher proliferative membrane fluidity: Rapidly dividing cells have more fluid membranes, potentially more susceptible to peptide insertion

Direct membrane disruption triggers calcium influx, mitochondrial membrane potential collapse, cytochrome c release, and caspase-mediated apoptosis. In vitro studies have demonstrated direct cytotoxicity against colon cancer cell lines (HCT-116, SW480), leukaemia (HL-60), and lung cancer cells at low micromolar concentrations.

Colon Cancer: LL-37 as Tumour Suppressor

Colon cancer represents the strongest evidence for a tumour-suppressive LL-37 function. Karin Herr’s group and subsequent studies found that LL-37/CAMP expression is significantly reduced in colorectal adenocarcinoma compared to normal colonic mucosa, suggesting epigenetic silencing or downregulation contributes to tumour progression. Mechanistically:

• LL-37 activates FPRL1/FPR2 (formyl peptide receptor-like 1) on colonocytes, triggering cAMP signalling and downstream anti-proliferative effects
• Vitamin D supplementation in colon cancer models upregulates LL-37 and partially mediates vitamin D’s chemopreventive effects
• LL-37 suppresses Wnt/β-catenin signalling, a key driver of colorectal tumour growth

Leukaemia and Lymphoma

LL-37 has demonstrated direct cytotoxicity against multiple leukaemia cell lines (HL-60, Jurkat, K562) through membrane disruption and mitochondria-mediated apoptosis. Importantly, selectivity for malignant over normal haematopoietic cells has been observed in some studies, though the therapeutic index in primary patient samples requires further characterisation.

Gastric Cancer

In H. pylori-associated gastric carcinogenesis, LL-37 plays a dual role. It directly kills H. pylori (particularly the virulent CagA+ strains), reducing a major gastric cancer risk factor. Additionally, LL-37 has been shown to inhibit gastric cancer cell migration and invasion, potentially acting on matrix metalloproteinase (MMP) expression downstream of receptor activation.

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Tumour-Promoting Effects of LL-37: The Paradox

The same antimicrobial peptide that kills cancer cells in colon tumours appears to promote tumour growth in breast, ovarian, and lung cancer contexts. Understanding why this paradox exists requires examining LL-37’s receptor-level biology and tumour microenvironment interactions.

EGFR Transactivation

LL-37 can transactivate epidermal growth factor receptor (EGFR) through receptor tyrosine kinase signalling. EGFR activation drives proliferation, survival, invasion, and angiogenesis. In lung cancer and breast cancer cell lines, LL-37-induced EGFR transactivation promotes tumour cell growth rather than cytotoxicity — demonstrating that the net effect depends on which receptors dominate the cellular signalling context.

Breast Cancer Promotion

Studies have found elevated LL-37 expression in breast cancer stroma (cancer-associated fibroblasts, tumour-associated macrophages) and correlating with worse prognosis in some ER-negative breast cancer subtypes. Proposed mechanisms include:

• Stimulation of Wnt/β-catenin and PI3K/Akt pathways in breast cancer stem cells
• EGFR transactivation as above
• MMP upregulation facilitating basement membrane degradation and metastasis
• Promotion of cancer stem cell phenotype (CD44+/CD24−) through FZD5/Wnt signalling

Ovarian Cancer

Ovarian cancer represents perhaps the strongest evidence for LL-37 as a tumour promoter. Elevated LL-37 in the ascitic fluid of ovarian cancer patients correlates with advanced disease stage. LL-37 stimulates ovarian cancer cell proliferation, migration, and invasion via EGFR/ERK and Akt/mTOR pathway activation. It also promotes angiogenesis in the tumour microenvironment through upregulation of VEGF and FGF-2.

Lung Cancer

Non-small cell lung cancer (NSCLC) research has identified LL-37 as potentially promoting tumour angiogenesis and metastasis through EGFR transactivation and MMP-9 upregulation. The paradox in lung cancer may relate to the predominantly stromal expression context: LL-37 from tumour-associated neutrophils or cancer-associated fibroblasts acts on the tumour epithelium in a paracrine manner, rather than autonomous tumour cell LL-37 expression.

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LL-37 and the Tumour Immune Microenvironment

Beyond direct tumour cell effects, LL-37 is a potent immunomodulator in the tumour microenvironment (TME):

NK Cell Activation

LL-37 has been shown to enhance NK cell cytotoxicity against tumour targets. This effect is likely mediated through direct activation of NK cell surface receptors and enhanced target recognition — potentially an important antitumour mechanism in haematological malignancies.

Dendritic Cell Maturation

LL-37 promotes the maturation of plasmacytoid dendritic cells (pDCs), the major producers of type I interferon. This is particularly important in antiviral and antitumour immunity — LL-37 complexes with self-nucleic acids (released from dying tumour cells) and facilitates their uptake by pDC endosomes, triggering TLR7/TLR9 signalling and IFN-α/β secretion. This mechanism may contribute to antitumour immune priming in immunogenic tumour types.

Neutrophil Extracellular Trap (NET) Biology

LL-37 is a major component of neutrophil extracellular traps (NETs). In cancer, NETs play a dual role: they can kill circulating tumour cells (CTCs) directly, but also create a pro-tumourigenic inflammatory scaffold that promotes metastatic seeding. The role of LL-37 within NETs in cancer represents an active research area, particularly in pancreatic and hepatocellular carcinoma where neutrophils are abundant in the TME.

Macrophage Polarisation

LL-37 influences macrophage polarisation toward M1 (pro-inflammatory, antitumour) phenotypes in some contexts through TLR activation, but in others may contribute to M2 (pro-tumourigenic) polarisation through alternative receptor pathways. The direction of macrophage polarisation depends on the local cytokine milieu, which LL-37 itself partially shapes — creating a context-dependent outcome that is difficult to predict without TME-specific data.

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LL-37 as Potential Cancer Biomarker

The context-dependence of LL-37’s tumour biology has sparked interest in using LL-37 levels as a biomarker — both prognostic and potentially predictive of immunotherapy response. Current evidence suggests:

• Colon cancer: Low LL-37 correlates with advanced stage — possible tumour suppressor marker
• Ovarian cancer: High ascitic LL-37 correlates with advanced disease and poor outcome
• Breast cancer: High stromal LL-37 may associate with triple-negative/basal-like subtype and worse prognosis in some cohorts
• Lung cancer: Data inconsistent; requires larger validation cohorts

Importantly, serum LL-37 measurement is confounded by neutrophil activation during blood collection (degranulation releases large amounts of LL-37 from specific granules). Standardised collection protocols (rapid processing, plasma versus serum, EDTA tubes) are essential for biomarker studies — a methodological detail that has undermined comparability across published datasets.

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LL-37 Analogues and Cancer Research

Given the paradoxical tumour biology, research has focused on developing LL-37-derived analogues that retain direct cytotoxicity and immune-activating properties while reducing EGFR transactivation and tumour-promoting receptor engagement:

• 17BIPHE2: A truncated analogue with reduced EGFR transactivation, retained anti-leukaemia cytotoxicity
• GF-17: LL-37 core region (residues 17–32), retaining membrane-disrupting activity with altered receptor selectivity
• P60.4Ac: Modified analogue with enhanced antimicrobial but reduced pro-inflammatory activity

Analogue engineering approaches are attempting to dissect LL-37’s pleiotropic biology to harness antitumour effects while minimising tumour-promoting signalling — a research direction that requires careful characterisation of full receptor pharmacology and TME context before any translational application.

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Vitamin D, LL-37, and Cancer Chemoprevention Research

The vitamin D–LL-37 axis has attracted considerable interest in chemoprevention research. Epidemiological data consistently associate low vitamin D status with increased colorectal, breast, and prostate cancer risk. Whether this association is partly mediated by reduced LL-37 expression in epithelial barriers (impairing immune surveillance and increasing cancer-promoting microbiome dysbiosis) is an active research question. In vitro models using 1,25(OH)₂D₃ to upregulate LL-37 in colon and lung epithelial cell lines and then assessing immune signalling and antimicrobial competence represent a tractable research system for investigating this hypothesis without direct cancer endpoint studies.