LL-37 5mg

Introduction & Research Disclaimer

LL-37 is the sole human cathelicidin antimicrobial peptide, a 37-residue, α-helical host defense peptide derived from the C-terminal domain of human cationic antimicrobial protein 18 (hCAP-18). Biosim Peptides supplies LL-37 (5 mg, lyophilized powder, ≥95% purity) exclusively for laboratory research purposes. This product is not for human or veterinary use, not a drug or dietary supplement, and is not intended for diagnostic or therapeutic applications. Researchers must comply with all applicable institutional, local, and national regulations governing peptide handling and disposal. By purchasing, the investigator certifies use within a qualified research program.

Molecular Overview

LL-37 (UniProt: P49913; gene: CAMP) is a linear, cationic peptide of 37 amino acids with the primary sequence LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES. The peptide has a calculated molecular weight of approximately 4,493 Da and a net charge of +6 at physiological pH, conferred by six basic residues (five lysines, one arginine) balanced against acidic residues. The N-terminal region (residues 1–12) adopts a predominantly hydrophobic character, while the C-terminal segment (residues 13–37) contains the cationic cluster critical for electrostatic interactions with negatively charged microbial membranes.

Structurally, LL-37 is unstructured in aqueous solution but adopts a pronounced amphipathic α-helical conformation upon contact with lipid bilayers or membrane-mimetic environments such as sodium dodecyl sulfate (SDS) micelles. The helical conformation spans roughly residues 2–31, with a flexible C-terminal tail. This amphipathic architecture — a hydrophilic face enriched in basic residues and a hydrophobic face — enables the peptide to insert into lipid bilayers and disrupt membrane integrity. NMR spectroscopy has resolved the three-dimensional structure of LL-37 in lipid micelles, revealing a curved α-helix spanning residues 2–31 followed by a disordered C-terminal segment (residues 32–37) that contributes to peptide solubility in aqueous media.

LL-37 is proteolytically liberated from its 18-kDa precursor protein hCAP-18 by serine proteases, including proteinase 3 and kallikrein-related peptidases, in neutrophils, epithelial cells, macrophages, and keratinocytes. Expression is regulated by the active form of vitamin D (1,25-dihydroxyvitamin D₃) via a vitamin D response element (VDRE) in the CAMP gene promoter, establishing a direct mechanistic link between vitamin D status and innate immune competence. LL-37 is stored at high concentrations in the specific granules of neutrophils and is released at sites of infection, inflammation, and tissue injury, where it reaches local concentrations estimated at 1–5 µg/mL or higher.

Mechanism of Action

LL-37 operates through at least three mechanistically distinct pathways: direct antimicrobial activity, immunomodulation, and promotion of wound repair. These activities are concentration-dependent and context-specific.

Direct Antimicrobial Activity. The peptide’s cationic amphipathic helix binds electrostatically to negatively charged components of bacterial membranes — lipopolysaccharide (LPS) in Gram-negative bacteria and lipoteichoic acid (LTA) in Gram-positive organisms. Membrane insertion proceeds via the “carpet” or “toroidal pore” model, wherein peptide accumulation on the membrane surface reaches a threshold concentration that triggers lipid displacement, membrane thinning, and ultimately permeabilization. LL-37 exhibits broad-spectrum activity against Gram-negative bacteria (including Pseudomonas aeruginosa, Escherichia coli, and Acinetobacter baumannii), Gram-positive species (Staphylococcus aureus, both methicillin-sensitive and methicillin-resistant strains), and certain fungi (Candida albicans). Beyond membrane lysis, LL-37 can penetrate bacterial cells and bind intracellular targets including nucleic acids, inhibiting DNA and protein synthesis.

Anti-Biofilm Activity. At subinhibitory concentrations, LL-37 suppresses biofilm formation by downregulating genes involved in quorum sensing and extracellular matrix production. This property is independent of its direct bactericidal activity and has been demonstrated at peptide concentrations as low as 0.5 µg/mL, well below the minimal inhibitory concentration (MIC) for planktonic bacteria. LL-37 also disperses pre-formed biofilms, enhancing bacterial susceptibility to conventional antibiotics — a finding with significant implications for research on chronic, biofilm-associated infections.

Immunomodulation. LL-37 is a multifunctional modulator of innate and adaptive immunity. It is chemotactic for neutrophils, monocytes, and CD4+ T lymphocytes via direct or indirect activation of formyl peptide receptor-like 1 (FPRL1, now designated FPR2). It attenuates TLR4-mediated inflammatory responses by binding and neutralizing LPS, reducing TNF-α and IL-6 production while preserving anti-inflammatory IL-10 secretion. LL-37 also promotes chemokine and cytokine production (IL-8, MCP-1) from epithelial cells and macrophages, shaping the local inflammatory milieu. These properties position LL-37 as a bridging molecule between innate and adaptive immune responses.

Wound Healing and Angiogenesis. LL-37 stimulates re-epithelialization of skin wounds through transactivation of the epidermal growth factor receptor (EGFR), inducing keratinocyte migration and proliferation. It promotes angiogenesis — new blood vessel formation — via direct activation of FPR2 on endothelial cells, stimulating endothelial proliferation and tube formation in in vitro models and neovascularization in in vivo angiogenesis assays. These activities are considered receptor-mediated and independent of its antimicrobial functions.

Research Applications

LL-37 is employed across a diverse spectrum of biomedical research domains:

• Antimicrobial Resistance Research: Investigating LL-37 as a model host defense peptide to elucidate structure-activity relationships that inform the design of novel antimicrobial agents resistant to conventional resistance mechanisms. Research examines peptide-membrane interactions, resistance evasion by pathogens, and synergistic effects with traditional antibiotics.
• Biofilm Studies: Using LL-37 in in vitro biofilm models to study quorum sensing inhibition, biofilm dispersal dynamics, and combinatorial approaches with antibiotics against biofilm-embedded bacteria.
• Innate Immunity & Inflammation: Elucidating signaling pathways downstream of LL-37–receptor interactions, especially FPR2-mediated chemotaxis, TLR antagonism, and cytokine network modulation. LL-37 serves as a tool to dissect the interface between innate and adaptive immune activation.
• Wound Healing & Tissue Regeneration: Investigating EGFR transactivation and FPR2-dependent angiogenic signaling. LL-37 is applied in scratch assays, organotypic skin models, and endothelial tube formation assays to examine re-epithelialization and neovascularization.
• Oncology Research: Studies have evaluated LL-37’s dual role in cancer biology — tumor-suppressive functions via immune surveillance enhancement and, in certain contexts, pro-tumorigenic effects mediated through receptor-dependent proliferation signals. This context-dependent behavior makes LL-37 a valuable probe for tumor microenvironment research.
• Vitamin D–Innate Immunity Axis: As expression of the CAMP gene is directly regulated by the vitamin D receptor, LL-37 is a key readout in studies of vitamin D-mediated innate immune responses, including in models of infectious and autoimmune disease.

Key Research Studies

The following peer-reviewed publications provide foundational insights into LL-37 biology and applications:

Agerberth et al. (1995) reported the discovery of FALL-39, the precursor to LL-37, as a cysteine-free peptide antibiotic expressed in human bone marrow and testis. This landmark study identified hCAP-18 as the pro-form and established the cathelicidin family in humans (Proc Natl Acad Sci USA, PMID: 7816819).

Scott et al. (2002) demonstrated that LL-37 is a multifunctional modulator of innate immune responses, chemoattracting neutrophils, monocytes, and T cells, while also neutralizing LPS and modifying cytokine secretion profiles. This work established that LL-37 functions far beyond direct antimicrobial killing, bridging innate immunity and inflammation (J Immunol, PMID: 12244186).

Heilborn et al. (2003) showed that LL-37 is strongly expressed in the epithelium of healing human skin wounds but is absent or drastically reduced in chronic, non-healing ulcer epithelium. LL-37 application stimulated keratinocyte migration and re-epithelialization in organotypic wound models, identifying the peptide as a critical factor in cutaneous wound closure (J Invest Dermatol, PMID: 12603850).

Gombart et al. (2005) identified a functional vitamin D response element in the CAMP gene promoter, demonstrating that 1,25-dihydroxyvitamin D₃ directly upregulates LL-37 expression in myeloid cells, keratinocytes, and epithelial cells. This provides the mechanistic basis for vitamin D-dependent innate immune activation (FASEB J, PMID: 15985530).

Dürr et al. (2006) published a comprehensive review of LL-37 structure, dynamics, and activity, consolidating biophysical data on membrane interactions, helical content in different solvent systems, and structure-activity relationships across the cathelicidin family (Biochim Biophys Acta, PMID: 16716248).

Overhage et al. (2008) demonstrated that LL-37 potently inhibits Pseudomonas aeruginosa biofilm formation at sub-MIC concentrations (as low as 0.5 µg/mL) by downregulating quorum-sensing genes (lasI, rhlI) and suppressing twitching motility. Pre-formed biofilms were also dispersed, revealing an anti-biofilm mechanism distinct from direct bactericidal activity (Infect Immun, PMID: 18591225).

Wang (2008) solved the high-resolution NMR structure of LL-37 in SDS micelles, revealing a continuous α-helix from residues 2–31 with a flexible C-terminal tail. The study also characterized KR-12, the minimal active antimicrobial fragment (residues 18–29), providing a structural template for designing peptide mimetics (J Biol Chem, PMID: 18818205).

Handling & Reconstitution Guidelines

LL-37 is supplied as a sterile, lyophilized powder (5 mg per vial, ≥95% purity by HPLC). For research use, the following handling practices are recommended:

• Storage of Lyophilized Powder: Store at -20°C in a desiccated environment, protected from light and moisture. Under these conditions, the lyophilized peptide is stable for the duration indicated on the certificate of analysis.
• Reconstitution: LL-37 is soluble in sterile, deionized water or aqueous buffers (e.g., phosphate-buffered saline, pH 7.4). The peptide is prone to aggregation at high concentrations; reconstitution at 1–2 mg/mL with gentle vortexing (avoid sonication, which may induce aggregation or degradation) is recommended. For long-term storage of stock solutions, aliquoting into single-use volumes minimizes freeze-thaw degradation.
• Solubility Notes: LL-37 solubility decreases in high-salt or low-pH conditions. If precipitation is observed, dilute the peptide in low-ionic-strength buffer and confirm solubility before use. The addition of 0.01–0.05% (v/v) acetic acid may improve solubility without compromising biological activity in certain assay systems.
• Handling Precautions: Always wear appropriate personal protective equipment (gloves, lab coat, eye protection). Use aseptic technique. Avoid repeated freeze-thaw cycles. Store reconstituted aliquots at -20°C or -80°C and use within the timeframe validated for the specific experimental system.
• Quantification: Peptide concentration in solution can be verified by UV absorbance at 214 nm (peptide bond absorbance) or by Bradford/BCA assay. LL-37 lacks tryptophan residues, so A280 measurement is not recommended; use 214 nm absorbance or colorimetric protein quantification methods.

Safety & Precautionary Information

LL-37 is a potent bioactive peptide. Researchers should observe the following safety measures:

• General Laboratory Safety: Handle in a certified biosafety cabinet or chemical fume hood. Wear nitrile gloves, a laboratory coat, and protective eyewear. LL-37 is for research use only and must not be administered to humans or animals outside an approved IACUC protocol.
• Inhalation/Aerosol Risk: Lyophilized peptides are lightweight. Open vials carefully to prevent aerosolization. Weigh and aliquot in a contained area.
• Skin & Eye Contact: In case of skin contact, wash immediately with soap and water. For eye exposure, rinse with copious water for at least 15 minutes and seek medical evaluation.
• Disposal: Dispose of unused peptide and contaminated materials according to institutional biohazard and chemical waste disposal guidelines. LL-37 is an antimicrobial peptide; environmental release may affect microbial ecosystems.
• Hemolytic Activity Caution: LL-37 exhibits hemolytic activity at high concentrations (>50 µM). Researchers should be aware of this when designing experiments involving blood or blood-derived cells and include appropriate concentration controls.
• Stability: LL-37 is susceptible to proteolytic degradation by endogenous serine proteases in biological matrices. Serum-containing assays should include protease inhibitor cocktails and stability time-course controls.

Frequently Asked Questions

Q: What is the difference between LL-37 and hCAP-18?
A: hCAP-18 (human cationic antimicrobial protein, 18 kDa) is the full-length precursor protein containing an N-terminal cathelin-like domain and the C-terminal LL-37 domain. LL-37 is the mature, biologically active 37-residue peptide liberated by proteolytic cleavage of hCAP-18, primarily by proteinase 3 in neutrophils and kallikreins in the skin.

Q: What concentration of LL-37 is typically used in antimicrobial assays?
A: Minimal inhibitory concentrations (MICs) vary by organism. Typical MIC values range from 1–16 µM for Gram-negative bacteria (e.g., E. coli, MIC ~2–8 µM; P. aeruginosa, MIC ~4–16 µM) and 1–32 µM for Gram-positive species. Anti-biofilm activity occurs at lower concentrations (0.1–1 µM). Researchers should perform pilot dose-response experiments to establish organism-specific and condition-specific effective concentrations.

Q: Does LL-37 work against antibiotic-resistant bacteria?
A: Yes. LL-37 retains activity against methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE), and multidrug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii. Because its primary mechanism (membrane disruption) differs fundamentally from conventional antibiotic targets, cross-resistance is less common — though some pathogens deploy LL-37 resistance strategies (proteolytic degradation, surface charge modification).

Q: Can LL-37 be used in cell culture experiments?
A: Yes, LL-37 is widely used in mammalian cell culture at concentrations typically ranging from 0.5–20 µg/mL (0.1–4.5 µM). Researchers should be aware that serum components and medium composition can influence peptide activity, and cytotoxicity should be assessed for each cell type and condition. LL-37 has been used in keratinocyte, fibroblast, endothelial, and immune cell cultures for wound healing, angiogenesis, and immunomodulation studies.

Q: How does vitamin D influence LL-37 research?
A: The CAMP gene encoding LL-37 contains a consensus vitamin D response element (VDRE) in its promoter. Treatment with 1,25-dihydroxyvitamin D₃ upregulates LL-37 mRNA and protein in multiple cell types, including monocytes/macrophages, keratinocytes, and bronchial epithelial cells. This provides a powerful experimental tool: vitamin D treatment can be used to induce endogenous LL-37 expression, enabling studies that distinguish between endogenous peptide function and exogenous peptide application.

References

1. Agerberth B, Gunne H, Odeberg J, Kogner P, Boman HG, Gudmundsson GH. FALL-39, a putative human peptide antibiotic, is cysteine-free and expressed in bone marrow and testis. Proc Natl Acad Sci U S A. 1995;92(1):195-199. PMID: 7816819
2. Scott MG, Davidson DJ, Gold MR, Bowdish D, Hancock REW. The human antimicrobial peptide LL-37 is a multifunctional modulator of innate immune responses. J Immunol. 2002;169(7):3883-3891. PMID: 12244186
3. Heilborn JD, Nilsson MF, Kratz G, et al. The cathelicidin anti-microbial peptide LL-37 is involved in re-epithelialization of human skin wounds and is lacking in chronic ulcer epithelium. J Invest Dermatol. 2003;120(3):379-389. PMID: 12603850
4. Gombart AF, Borregaard N, Koeffler HP. Human cathelicidin antimicrobial peptide (CAMP) gene is a direct target of the vitamin D receptor and is strongly up-regulated in myeloid cells by 1,25-dihydroxyvitamin D3. FASEB J. 2005;19(9):1067-1077. PMID: 15985530
5. Dürr UHN, Sudheendra US, Ramamoorthy A. LL-37, the only human member of the cathelicidin family of antimicrobial peptides. Biochim Biophys Acta. 2006;1758(9):1408-1425. PMID: 16716248
6. Overhage J, Campisano A, Bains M, Torfs ECW, Rehm BHA, Hancock REW. Human host defense peptide LL-37 prevents bacterial biofilm formation. Infect Immun. 2008;76(9):4176-4182. PMID: 18591225
7. Wang G. Structures of human host defense cathelicidin LL-37 and its smallest antimicrobial peptide KR-12 in lipid micelles. J Biol Chem. 2008;283(47):32637-32643. PMID: 18818205