LL-37 (human cathelicidin antimicrobial peptide)

Discovery and biological context

LL-37 is the only cathelicidin family member identified in humans. The CAMP gene (originally described as FALL39) on chromosome 3p21 encodes a preproprotein that gives rise to hCAP18 (the 18-kDa human cathelicidin proprotein). Cloning and characterization of the gene and its processing were carried out in the mid-1990s, when the C-terminal fragment was shown to be released in granulocytes as an antibacterial peptide named for its two N-terminal leucines and 37-residue length (LL-37) ^[1]. Unlike rodents and ruminants, which possess many cathelicidins, humans have only one, making LL-37 a central effector of innate immunity ^[2][3].

The peptide is expressed by neutrophils (stored in secondary/specific granules), keratinocytes, and the epithelia of the respiratory, gastrointestinal and urogenital tracts, as well as by monocytes and mast cells. As part of the hCAP18 proprotein it circulates bound to plasma lipoproteins and in seminal plasma; mature LL-37 is released by extracellular proteolysis at sites of inflammation ^[2][3].

Structure and physicochemical properties

LL-37 is a linear cationic peptide with the sequence LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES (37 residues, ≈4493 Da). Its net charge at physiological pH is about +6 owing to numerous Arg and Lys residues. In aqueous solution at low ionic strength the peptide is largely disordered, but in the presence of anionic lipids, detergent micelles, high salt or anion concentrations it folds into an amphipathic α-helix ^[3][4]. The solution structures of LL-37 and its minimal active core KR-12 in lipid micelles, determined by NMR, reveal a curved helix with clear segregation of the hydrophobic and cationic faces. This amphipathicity governs membrane interaction ^[4].

A key feature is the conformation dependence of activity: the coil-to-helix transition correlates directly with bactericidal potency, and factors that stabilize the helix (anions, increased ionic strength) enhance activity ^[5]. The peptide is thermostable and retains activity across a broad pH range, but it aggregates and partly loses activity at high divalent-cation concentrations.

Molecular mechanism: membrane activity

The primary antimicrobial action of LL-37 is the electrostatic attraction of the cationic peptide to anionic components of the microbial surface (lipopolysaccharide, lipoteichoic acid, anionic phospholipids), followed by insertion of the helix's hydrophobic face into the lipid bilayer. LL-37 acts mainly via a carpet mechanism: the peptide accumulates parallel to the membrane surface until it reaches a threshold density, then destabilizes the bilayer, causing thinning, formation of transient toroidal pores and ultimately detergent-like disruption ^[6]. Orientation studies in phospholipid membranes show an in-plane orientation of the helix, consistent with a non-channel, non-selective mechanism, and explain host-cell cytotoxicity at high concentrations ^[6].

LL-37 activity depends strongly on the ionic environment: physiological NaCl and divalent-cation concentrations attenuate direct bactericidal action, and the ionic composition of tissue fluid largely dictates microbial susceptibility to the peptide ^[7]. This salt sensitivity is directly relevant to interpreting in vitro results across different media compositions.

Signaling pathways and immunomodulation

Beyond direct lysis, LL-37 is a potent immunomodulator acting through specific receptors. It chemoattracts neutrophils, monocytes and T cells using the formyl peptide receptor FPR2 (FPRL1), a G-protein-coupled receptor that triggers Ca²⁺ mobilization and migration ^[8]. LL-37 also modulates inflammasome activation and cytokine release via the P2X7 receptor and interacts with epidermal growth factor receptors, influencing epithelial proliferation.

A key immunological property is LPS neutralization: by binding lipopolysaccharide, LL-37 dampens the TLR4-mediated hyperinflammatory macrophage response. At the same time the peptide is double-edged. By forming complexes with self DNA/RNA released during tissue damage, LL-37 protects nucleic acids from nucleases and delivers them to endosomal Toll-like receptors (TLR9/TLR7) in plasmacytoid dendritic cells, driving strong type I interferon production ^[9]. This mechanism links LL-37 to the pathogenesis of psoriasis and other autoimmune conditions and makes the peptide an important model for studying innate recognition of self material.

Angiogenesis and tissue repair

LL-37 displays pro-angiogenic properties: it stimulates endothelial cell proliferation and migration and promotes vessel formation, acting in part through FPR2. In preclinical implantation and ischemia models the peptide enhanced neovascularization, linking it to wound healing and tissue remodeling ^[10]. These effects are the subject of active research interest as a mechanistic bridge between innate immunity and repair.

Preclinical research

A broad body of in vitro work has documented LL-37 activity against Gram-negative and Gram-positive bacteria, enveloped viruses, fungi and biofilms, as well as synergy with lysozyme, lactoferrin and human β-defensins. Cell-culture and rodent models have examined the peptide's role in skin and mucosal barrier defense, anti-biofilm action, endotoxin neutralization and inflammation modulation. This work is conducted exclusively in cell and animal models and belongs to the published research record ^[3].

Clinical / human data as literature

In human physiology, LL-37 levels are associated with infectious, inflammatory and dermatological conditions: reduced cathelicidin expression is described in atopic dermatitis, whereas excess LL-37 activity is documented in psoriasis and rosacea, where the peptide acts as an autoantigen and an amplifier of the interferon response ^[9]. Vitamin D induction of CAMP links vitamin D status to the antimicrobial potential of macrophages, an observation described as a biological fact in the literature ^[12]. These data are literature observations of the endogenous peptide and do not constitute use instructions.

Processing, pharmacokinetics and metabolism

Mature LL-37 is not translated directly: it is generated by extracellular proteolytic cleavage of the C-terminal domain from the hCAP18 proprotein. In neutrophils the key protease is proteinase 3, which cleaves hCAP18 to LL-37 after degranulation ^[11]. In other tissues, notably at the skin surface, serine proteases of the kallikrein family (KLK5/KLK7) generate LL-37 as well as shorter functional fragments (e.g., RK-31, KS-30) that broaden the antimicrobial repertoire. This diversity of post-secretory processing products defines the local spectrum of action.

In biological fluids LL-37 is susceptible to further proteolytic degradation by serum and microbial proteases, and binding to plasma lipoproteins and apolipoproteins modulates its availability and toxicity. Consequently, no stable pharmacokinetic half-life has been established for the exogenous peptide, and researchers account for rapid inactivation in the presence of serum when designing experiments.

Regulation of expression

The CAMP gene promoter contains a functional vitamin D response element (VDRE): 1,25-dihydroxyvitamin D₃, acting through the VDR receptor, is a direct inducer of CAMP transcription in myeloid and epithelial cells ^[12]. Expression is also modulated by butyrate, short-chain fatty acids, hypoxia and inflammatory stimuli, making LL-37 a convenient model for studying regulation of innate immune genes.

Related compounds and analogues

The most important derivative fragment is KR-12 (residues 18–29), the minimal segment that retains antibacterial activity with reduced host-cell cytotoxicity ^[4]. Other natural processing products include LL-23, RK-31 and KS-30, as well as the full-length hCAP18 proprotein. Orthologues in other species include mouse CRAMP, porcine protegrins and PR-39, and bovine Bac5/Bac7, all members of the cathelicidin family ^[2]. This family serves as a benchmark for comparative structure-function studies of host defense peptides.

Analytical characterization

The identity and purity of synthetic LL-37 are confirmed by mass spectrometry (ESI-MS or MALDI-TOF; expected average mass ≈4493 Da) and analytical RP-HPLC (typically an acetonitrile/water gradient with 0.1% TFA on a C18 column), where research-grade purity is usually ≥95%. Secondary structure and its environment dependence are assessed by circular dichroism: in aqueous buffer the spectrum reflects a disordered state, whereas in the presence of TFE, SDS micelles or anionic vesicles the characteristic α-helix minima at 208 and 222 nm appear ^[5]. Sequencing (Edman or tandem MS) and amino acid analysis confirm the sequence; residual TFA content and endotoxin level are controlled for cell-culture experiments.

Handling, reconstitution and storage

Lyophilized LL-37 is hygroscopic; it is stored sealed at −20 °C or below, protected from moisture. Before opening, the vial is equilibrated to room temperature to avoid condensation. Because of its high cationicity the peptide dissolves readily in water or dilute aqueous buffers; phosphate buffers with high divalent-cation concentrations that promote aggregation are avoided. Stock aliquots are prepared to minimize freeze-thaw cycles that reduce activity. Adsorption of the cationic peptide to plastic and glass should be considered (low-protein-binding labware may be used). All work is carried out within RUO research and not applied to humans or animals outside protocols.

Research applications and model systems

LL-37 is widely used as a tool in membrane biophysics models (liposomes, supported bilayers, solid-state NMR), in studies of innate immune mechanisms (LPS neutralization, TLR activation, inflammasome), in autoimmunity research (complexes of peptide with nucleic acid, psoriatic model), and as a reference antimicrobial peptide for screening new host defense compounds and anti-biofilm agents. The peptide serves as a positive control in comparative cytotoxicity and membrane-permeabilization assays, and KR-12 is a model scaffold for engineering shorter analogues with improved therapeutic index ^[3][4].