LL-37: Cathelicidin Antimicrobial Peptide

LL-37 is a 37-amino acid peptide (LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES) and the only cathelicidin antimicrobial peptide found in humans. It is derived from the C-terminal cleavage of the 18 kDa precursor protein hCAP18 (human cationic antimicrobial protein 18) by the protease proteinase 3.

LL-37 is expressed by neutrophils, monocytes, macrophages, epithelial cells, and other cell types as part of the innate immune response. It is found in mucosal surfaces, wound fluid, sweat, and plasma, where it serves as a first-line defense against microbial invasion.

Beyond direct antimicrobial activity, LL-37 functions as an immunomodulatory peptide — it influences chemotaxis, cytokine production, wound closure, and angiogenesis in preclinical models. This multifunctionality has made it one of the most extensively studied host defense peptides, with over 2,000 publications in the PubMed database.

LL-37 has a molecular weight of approximately 4,493.33 Da and adopts an alpha-helical conformation in membrane-like environments, which is critical for its antimicrobial mechanism.

LL-37's antimicrobial activity relies on direct membrane disruption of microbial cells.

**Membrane interaction:** As a cationic (+6 net charge at physiological pH) amphipathic peptide, LL-37 is electrostatically attracted to the negatively charged outer membranes of bacteria (which contain lipopolysaccharide in Gram-negative species or teichoic acids in Gram-positive species). Upon contact, LL-37 inserts into the lipid bilayer in its alpha-helical form, creating pores or disrupting membrane integrity through a "carpet" or "toroidal pore" mechanism.

**Broad-spectrum activity:** LL-37 demonstrates activity against Gram-positive bacteria (S. aureus, including MRSA), Gram-negative bacteria (E. coli, P. aeruginosa, K. pneumoniae), fungi (C. albicans), and enveloped viruses in vitro. Minimum inhibitory concentrations (MICs) typically range from 1-32 micromolar depending on the organism and assay conditions.

**Biofilm disruption:** A particularly significant property of LL-37 is its ability to inhibit biofilm formation and disrupt pre-formed biofilms at sub-MIC concentrations. In P. aeruginosa models, LL-37 at 0.5 micromolar reduced biofilm biomass by more than 50% without killing planktonic cells at that concentration. This anti-biofilm activity operates through interference with quorum-sensing pathways and attachment to surfaces.

**Salt sensitivity:** LL-37's antimicrobial activity is partially inhibited at physiological salt concentrations (150 mM NaCl), which reduces electrostatic attraction to microbial membranes. This is an important consideration for in-vitro assay design.

LL-37's role extends beyond direct microbial killing into broader immune regulation.

LL-37 requires careful handling due to its amphipathic nature and tendency to aggregate.

**Reconstitution:** Dissolve in sterile water or dilute acetic acid (0.01% acetic acid is commonly used). LL-37 can be difficult to dissolve at high concentrations due to its amphipathic character. Prepare stock solutions at 1-5 mg/mL and dilute for working concentrations. Do not vortex vigorously — gentle swirling is sufficient.

**Aggregation:** At concentrations above 10 micromolar in aqueous buffers, LL-37 can form oligomeric structures that may affect its biological activity. For assays requiring defined monomeric peptide, use freshly prepared dilutions from concentrated stocks and verify the oligomeric state by dynamic light scattering if needed.

**Storage:** Lyophilized LL-37 is stable at -20°C for up to 12 months. Reconstituted solutions should be stored at 2-8°C and used within 7 days due to the peptide's susceptibility to aggregation and adsorption to container surfaces. Aliquot immediately after reconstitution and freeze at -20°C for longer storage.

**Container binding:** LL-37 adsorbs to polypropylene and polystyrene surfaces. Use low-binding tubes and plates for handling and storage. Adding 0.1% BSA to diluent buffers can reduce surface adsorption in assay plates, but ensure BSA does not interfere with your specific readout.

*All materials are for research use only.*

LL-37 is used across multiple research disciplines:

**Antimicrobial susceptibility testing:** Standard MIC and MBC assays using broth microdilution in Mueller-Hinton broth. Note that LL-37 activity is salt-sensitive — use low-ionic-strength media for optimal antimicrobial detection, and physiological-salt media to assess activity under conditions closer to the in-vivo environment.

**Biofilm assays:** The Calgary Biofilm Device (MBEC assay) and crystal violet biofilm quantification are used to study LL-37's anti-biofilm activity. Sub-MIC concentrations (0.5-4 micromolar) are typically used for biofilm prevention studies, while higher concentrations are needed for disruption of established biofilms.

**Immunomodulation studies:** Monocyte/macrophage cell lines (THP-1, RAW 264.7) treated with LPS in the presence or absence of LL-37 allow assessment of cytokine modulation. Measure TNF-alpha, IL-6, IL-8, and IL-10 by ELISA at 4-24 hours post-treatment.

**Wound models:** Scratch assays on keratinocyte (HaCaT) or epithelial monolayers with LL-37 at 1-5 micromolar. Image at 0, 6, 12, and 24 hours to quantify closure rate.

**Concentration ranges:** Antimicrobial assays: 1-64 micromolar. Biofilm assays: 0.5-8 micromolar. Immunomodulation: 1-20 micromolar. Wound closure: 1-10 micromolar.