LL-37 and Respiratory Research: Lung Innate Immunity, Antimicrobial Biology and Airway Inflammation UK 2026

Introduction: LL-37 in Pulmonary Biology

LL-37 — the sole human cathelicidin, processed from the 18 kDa precursor hCAP-18 by serine protease cleavage at the C-terminal cathelin domain — is an amphipathic α-helical 37-amino acid antimicrobial peptide (Leu-Leu-Gly-Asp-Phe-Phe-Arg-Lys-Ser-Lys-Glu-Lys-Ile-Gly-Lys-Glu-Phe-Lys-Arg-Ile-Val-Gln-Arg-Ile-Lys-Asp-Phe-Leu-Arg-Asn-Leu-Val-Pro-Arg-Thr-Glu-Ser). The lung is a primary site of LL-37 production and action: respiratory epithelial cells (airway surface liquid), alveolar macrophages, and neutrophils contribute to pulmonary LL-37 pools. LL-37 concentrations in bronchoalveolar lavage (BAL) fluid are markedly elevated during pneumonia, ARDS, and airway infection.

Pulmonary LL-37 biology spans antimicrobial activity against respiratory pathogens (bacteria, viruses, fungi), modulation of innate and adaptive airway immune responses, regulation of inflammatory mediator release from airway epithelial cells and macrophages, and complex dual roles in chronic airway inflammatory diseases (asthma, COPD, cystic fibrosis). This mechanistic complexity makes LL-37 a research tool of substantial interest for respiratory biology, though its concentration-dependent and context-dependent paradoxical effects require careful experimental design.

Antimicrobial Mechanisms Against Respiratory Pathogens

LL-37’s primary antimicrobial mechanism involves electrostatic interaction with anionic bacterial membranes (LPS in Gram-negative, lipoteichoic acid/peptidoglycan in Gram-positive), followed by amphipathic α-helix insertion into the lipid bilayer. Membrane disruption proceeds through: (1) the barrel-stave model (pore formation through oligomeric transmembrane helices); (2) the toroidal-pore model (lipid head groups lining the pore with incorporated peptide); or (3) the carpet model (peptide accumulation at the membrane surface producing micellisation and membrane solubilisation at critical concentration). Different bacterial membrane lipid compositions determine which mechanism predominates.

Clinically relevant respiratory pathogens for LL-37 antimicrobial research:

Staphylococcus aureus (including MRSA): Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) determination by broth microdilution (CLSI methodology) in Mueller-Hinton broth at physiological salt concentrations (note: LL-37 activity is strongly salt-sensitive; physiological 150 mM NaCl reduces activity compared to low-salt buffers). Biofilm formation inhibition (crystal violet biomass assay, confocal CLSM with live/dead staining) — critical given S. aureus biofilm role in chronic airway infection. Membrane permeability (SYTOX Green fluorescence uptake). Synergy testing with conventional antibiotics (checkerboard FICI — fractional inhibitory concentration index).

Pseudomonas aeruginosa: The primary chronic airway pathogen of cystic fibrosis — a context where LL-37 activity is of particular research interest, as airway surface liquid in CF may have altered salt concentration and proteolytic activity affecting LL-37 processing and activity. P. aeruginosa-LL-37 research uses both planktonic MIC assays and established biofilm models (MBEC — minimum biofilm eradication concentration; CDC biofilm reactor for mature biofilm formation; P. aeruginosa biofilm mucoid variant representative of chronic CF infection).

Influenza A Virus: LL-37 antiviral activity involves multiple mechanisms: direct virion membrane disruption (for enveloped viruses), competitive binding to host cell heparan sulphate (blocking viral attachment for heparan-binding viruses), and immunomodulatory antiviral signalling (Type I IFN induction through TLR7/9 or cGAS-STING pathway activation in plasmacytoid DCs exposed to LL-37-DNA complexes). Antiviral assays: plaque reduction neutralisation test (PRNT), viral yield reduction assay (TCID50 on MDCK cells), and qRT-PCR viral load quantification in LL-37-treated A549 or primary bronchial epithelial cell (PBEC) infection models.

Airway Epithelial Cell Biology

Airway surface epithelium — pseudostratified ciliated columnar epithelium in conducting airways, alveolar type I/II pneumocytes in the alveolar compartment — is both a site of LL-37 production and a responsive target for LL-37 signalling. LL-37 receptors on airway epithelial cells include: FPRL1/FPR2 (formyl peptide receptor 2, Gi-coupled), P2X7 (purinergic receptor, cation channel), EGF receptor (EGFR, transactivated by LL-37 through metalloprotease ADAM10/17-mediated HB-EGF release), and TMEM16A (calcium-activated chloride channel). These receptor engagements produce:

Wound repair and migration: LL-37 at low concentrations (0.5–2 µM) promotes airway epithelial migration and wound closure through EGFR-MAPK/ERK pathway activation and CXCL8 (IL-8) chemokine secretion. Scratch wound assay on confluent 16HBE or primary PBEC monolayers with LL-37 treatment quantifies migration rate (wound width at 0h, 8h, 24h by ImageJ analysis) and distinguishes migration from proliferation (mitomycin-C pre-treatment to arrest cell division).

Cytokine modulation: LL-37 concentration-dependently modulates LPS-induced cytokine production in bronchial epithelial cells — suppressing TNF-α/IL-6 at low concentrations through LPS neutralisation (LL-37:LPS electrostatic complex formation preventing TLR4 engagement) but potentially augmenting inflammatory responses at higher concentrations through FPR2 or EGFR-NF-κB pathway activation.

Barrier function: LL-37 effects on tight junction protein expression (ZO-1, occludin, claudin-4 — TEER measurement on air-liquid interface (ALI) cultures) and mucus secretion (MUC5AC/MUC5B ELISA on apical wash — goblet cell secretion) characterise LL-37’s effects on the physical airway barrier components.

Alveolar Macrophage Biology

Alveolar macrophages (AMs) — resident macrophages in the alveolar space — are primary sentinels against inhaled pathogens and are the dominant source of LL-37 in the distal lung (alongside type II alveolar epithelial cells). LL-37 from AMs acts in an autocrine manner through FPR2, modulating AM phagocytic capacity, reactive oxygen species production (NADPH oxidase Nox2 burst), and cytokine secretion profile in bacterial infection models.

LL-37 in AM biology research: primary human AMs from BAL of healthy volunteers or BALF-derived AMs from disease models; MH-S murine alveolar macrophage cell line as an accessible alternative. Endpoints: phagocytosis assay (FITC-labelled fluorescent bacteria or zymosan particle uptake by flow cytometry or confocal imaging); NADPH oxidase ROS burst (luminol-amplified chemiluminescence after PMA stimulation); TNF-α/IL-12/IL-10 cytokine polarisation; efferocytosis of apoptotic neutrophils (annexin-V+ Jurkat cells, FACS-measured uptake — resolution of inflammation readout); and LL-37-induced NETosis (NET formation by PMNs in BAL — citrullinated histone H3/MPO co-staining by immunofluorescence).

COPD and Chronic Airway Inflammation

COPD — characterised by progressive airflow limitation through emphysema (alveolar wall destruction) and chronic bronchitis (mucus hypersecretion) — involves sustained neutrophilic and macrophagic airway inflammation, oxidative stress, and proteolytic matrix destruction. LL-37 levels in COPD BAL and sputum are altered compared to healthy controls; the clinical significance remains contested, but preclinical research examines LL-37’s roles in:

Cigarette smoke-induced inflammation: Cigarette smoke extract (CSE, standardised by OD320 absorption or nicotine measurement) applied to 16HBE or PBEC cultures models the oxidative stress and inflammatory activation of COPD airway epithelium. LL-37 effects on CSE-treated cells examine whether cathelicidin modifies the NF-κB-driven cytokine cascade or protects against oxidative damage (Nrf2/HO-1/SOD upregulation).

Elastase-induced emphysema: Intratracheal porcine pancreatic elastase (PPE) instillation in mice produces emphysema-like alveolar destruction within 21 days (airspace enlargement quantified by mean linear intercept, Lm, on H&E sections). LL-37 co-treatment or post-instillation treatment characterises cathelicidin’s potential protective effects on alveolar architecture through modulation of MMP-12/MMP-9 (macrophage elastase) production and neutrophil recruitment (CXCL8/CXCL1-mediated).

Cystic Fibrosis Airway Research Context

Cystic fibrosis (CF) is caused by CFTR (CF transmembrane conductance regulator) mutations producing abnormal chloride transport, dehydrated mucus, and chronic airway infection. CF airway surface liquid (ASL) has elevated sodium concentration (reduced surface liquid osmolarity hypothesis) that was initially proposed to inhibit LL-37 antimicrobial activity — a hypothesis that has been subsequently debated and refined. Modern CF research examines LL-37 biology through:

CF bronchial epithelial cell cultures (CFBE41o- F508del-CFTR cell line, or primary nasal epithelial cells from CF patients) at ALI (air-liquid interface) — the physiologically relevant culture model for mucociliary biology. LL-37 concentration in CF vs non-CF ASL (immunoassay on apical wash); CFTR corrector/potentiator (VX-770/VX-809, elexacaftor/tezacaftor/ivacaftor) effects on LL-37 production (does CFTR restoration restore normal cathelicidin biology?); and Pseudomonas aeruginosa infection of CF ALI cultures with LL-37 treatment to characterise whether restored LL-37 concentrations improve bacterial killing in the CF mucus microenvironment.

COVID-19 and SARS-CoV-2 Research

LL-37 demonstrated antiviral activity against SARS-CoV-2 in early preclinical studies — reducing viral infectivity in Vero E6 and Calu-3 cell models through direct virion membrane disruption and potential ACE2 interaction competition. Low vitamin D (the primary inducer of cathelicidin/CAMP gene expression through VDR-RXRA response element) is associated epidemiologically with worse COVID-19 outcomes, suggesting LL-37 deficiency may be mechanistically relevant to COVID-19 severity. Research protocols for SARS-CoV-2-LL-37 biology require BSL-3 facility access for live virus work; pseudovirus systems (VSV-pseudotyped SARS-CoV-2 spike) provide a BSL-2 accessible alternative for spike-mediated entry inhibition assays.

Research Protocol Standards

LL-37 preparation for pulmonary research: RP-HPLC purified synthetic LL-37 (≥95% purity), ESI-MS MW confirmation (4493.3 Da), endotoxin testing (LAL ≤0.1 EU/µg — critical for respiratory cell biology where LPS contamination would confound inflammatory endpoints). Stock at 1 mg/mL in sterile 0.01% acetic acid; aliquot and store at –80°C. Note: LL-37 adsorbs to plastic at low concentrations — use siliconised tubes or add carrier protein (0.1% BSA) for concentrations below 1 µg/mL.

ALI culture standard: Primary PBEC or 16HBE on collagen-coated transwell inserts (0.4 µm pore, 12-well or 24-well format), submerged until confluence then air-exposed for 3–4 weeks to achieve pseudostratified mucociliary differentiation. TEER measurement (Millicell ERS-2 system) to verify barrier integrity before experimental use (TEER >300 Ω·cm² for PBEC ALI). LL-37 applied apically (mimicking luminal exposure) or basolaterally (mimicking tissue/submucosal exposure) at specified concentrations for defined time periods.

Summary

LL-37 respiratory research spans antimicrobial biology (MRSA, P. aeruginosa including biofilm, influenza virus — MIC/MBC/MBEC/PRNT endpoints), airway epithelial cell biology (EGFR-MAPK wound repair, LPS neutralisation, barrier function ALI TEER/MUC5AC), alveolar macrophage phagocytosis/efferocytosis/ROS burst biology, COPD smoke-induced inflammation and elastase emphysema models, cystic fibrosis CFTR-cathelicidin biology in ALI cultures, and SARS-CoV-2 antiviral mechanisms. LL-37’s concentration-dependent dual effects (anti-inflammatory at low concentrations through LPS neutralisation; pro-inflammatory at high concentrations through FPR2/EGFR) require careful dose-response characterisation in each experimental context. Rigorous controls — endotoxin-free LL-37 preparation, scrambled control peptide, FPR2 antagonist (WRW4) for receptor specificity — are essential for mechanistically interpretable pulmonary LL-37 research.

All information is for research and educational purposes only. LL-37 is not approved for human therapeutic use and must not be administered to humans outside of properly authorised clinical research settings.