LL-37

LL-37 is the only cathelicidin-family antimicrobial peptide identified in humans. The encoding gene (FALL39 / CAMP) and the peptide were first described in the mid-1990s: Agerberth et al. (1995) identified the precursor while screening a human bone-marrow cDNA library and named the predicted product FALL-39, and Gudmundsson et al. (1996) characterized the human gene and the proteolytic processing of the cathelin precursor (hCAP-18) into the 37-residue peptide in granulocytes — the two leading leucine residues giving the peptide its "LL-37" name. Comprehensive reviews have since catalogued its biochemistry, expression and the broad set of in-vitro and preclinical model systems in which it is studied: Durr, Sudheendra & Ramamoorthy (2006) framed it as the single human cathelicidin from a membrane-biophysics standpoint, while Bucki et al. (2010) and Vandamme et al. (2012) surveyed its antimicrobial, chemotactic and immunomodulatory research literature.

A large structural literature examines how the peptide folds in membrane-mimicking environments. Porcelli et al. (2008), using solution NMR in dodecylphosphocholine micelles, described a helix-break-helix conformation in which hydrophobic residues partition into the micelle interior and hydrophilic residues face the solvent. Wang (2008) reported the structures of LL-37 and its minimal antibacterial fragment KR-12 (residues 18-29) in SDS and dodecylphosphocholine micelles, describing a curved amphipathic helix-bend-helix motif (residues ~2-31) followed by a disordered C-terminal tail. Sancho-Vaello et al. (2020) extended the structural picture with X-ray crystallography and planar-lipid-membrane electrophysiology, reporting tetrameric oligomerization and a defined transmembrane channel in the presence of membrane mimics. Across these studies the recurring methodological theme is how an amphipathic cationic helix is organized at the lipid-water interface.

Mechanistic work centers on how the peptide engages microbial membranes. Henzler-Wildman, Lee & Ramamoorthy (2003) used solid-state NMR of site-labeled peptide in oriented lipid bilayers to show that the amphipathic helix lies parallel to the bilayer surface in both anionic and zwitterionic membranes — a surface orientation that argued against a barrel-stave pore and supported carpet / toroidal-type models of bilayer disruption; companion work examined perturbation of the hydrophobic core of bilayers (Henzler-Wildman et al., 2004). Sevcsik et al. (2008) compared the peptide's behavior across model membranes of differing lipid composition. This body of biophysical research describes membrane association, not a benefit, and informs how cationic amphipathic peptides are thought to act on anionic microbial surfaces.

Beyond direct membrane activity, the peptide is studied as a host-defense signaling molecule. It binds lipopolysaccharide (LPS); Rosenfeld et al. (2006) examined endotoxin neutralization by innate-immunity host-defense peptides and the role of strong peptide-LPS binding and aggregate dissociation, and Coorens / Scott and colleagues (2017) evaluated LPS neutralization in vitro and ex vivo. On the receptor side, De Yang et al. (2000) identified the formyl peptide receptor FPRL1 (FPR2) as a receptor through which the peptide chemoattracts neutrophils, monocytes and T cells; Elssner et al. (2004) described P2X7-receptor-dependent IL-1beta processing and release in LPS-primed monocytes; and Mookherjee et al. (2009) reported GAPDH as an intracellular binding partner in monocytes. Koczulla et al. (2003) examined an FPRL1-linked angiogenic role in chick chorioallantoic-membrane and rabbit hind-limb-ischemia models.

A wide preclinical literature surveys the peptide's antimicrobial spectrum and its interaction with bacterial biofilms. Overhage et al. (2008) reported that the peptide inhibited Pseudomonas aeruginosa biofilm formation in vitro at concentrations well below those needed to inhibit planktonic growth, and affected pre-grown biofilms — work that seeded fragment-library studies identifying biofilm-active truncations of the sequence. Reviews place these antimicrobial and anti-biofilm observations alongside antifungal and antiviral assays within the broader innate-immunity research framework (Bucki et al., 2010; Vandamme et al., 2012). The recurring research question is how a single endogenous host-defense peptide engages diverse microbial targets and microbial-community structures.

Because the full-length peptide is large and its activity is sensitive to environment and salt, much research focuses on minimal active fragments and engineered analogs. The KR-12 fragment (residues 18-29) identified by Wang (2008) is the smallest reported antibacterial segment and a frequent scaffold for structure-activity studies; subsequent peptide-engineering work has examined lipidated and hybrid KR-12 analogs and other truncations to map the determinants of membrane activity and selectivity. This thread situates LL-37 as a template molecule in antimicrobial-peptide design research rather than as a finished agent.