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LL-37 – Antimicrobial Peptide and Innate Immunity
LL-37 is an endogenous antimicrobial peptide of the cathelicidin family that plays a central role in innate immunity and possesses immunomodulatory and anti-inflammatory properties.
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LL-37 is an endogenous antimicrobial peptide of the cathelicidin family that plays a central role in innate immunity and possesses immunomodulatory and anti-inflammatory properties.
What is LL-37?
LL-37 is an antimicrobial peptide (AMP) and the only known human member of the cathelicidin family. It is derived from the precursor protein hCAP-18 (human cationic antimicrobial protein 18) and consists of 37 amino acids. The name LL-37 reflects its structure: the first two amino acids are both leucine (L), followed by 35 additional residues. LL-37 is produced by a variety of cell types, including neutrophils, keratinocytes, airway and intestinal epithelial cells, and macrophages.
Biological Functions
LL-37 serves multiple essential roles in the human body:
- Antimicrobial activity: LL-37 directly kills bacteria, viruses, fungi, and certain parasites. It binds to the negatively charged membranes of microorganisms, disrupts their membrane integrity, and causes cell death.
- Immunomodulation: The peptide regulates inflammatory responses by influencing the activation of immune cells such as monocytes, dendritic cells, and T cells.
- Wound healing: LL-37 promotes the migration and proliferation of epithelial cells, supporting tissue repair and wound closure.
- Angiogenesis: It stimulates the formation of new blood vessels, which is important for tissue regeneration.
- Endotoxin neutralization: LL-37 binds and neutralizes bacterial lipopolysaccharides (LPS), preventing excessive immune responses such as septic shock.
Production and Regulation
The production of LL-37 in the body is regulated by several factors. Vitamin D plays a particularly important role by stimulating the expression of the CAMP gene (Cathelicidin Antimicrobial Peptide), which encodes LL-37. Additional factors influencing LL-37 production include:
- Infections and inflammatory stimuli
- Certain fatty acids (e.g., short-chain fatty acids from the gut microbiome)
- Cytokines and growth factors
- Skin irritation and mechanical stimuli
LL-37 and Disease
Conditions Associated with Reduced LL-37 Activity
A deficiency or reduced activity of LL-37 is associated with increased susceptibility to infections and certain diseases:
- Atopic dermatitis (eczema): Patients with eczema often show reduced LL-37 levels in the skin, which helps explain their heightened susceptibility to bacterial superinfections, particularly with Staphylococcus aureus.
- Crohn's disease: Reduced LL-37 expression in the intestinal mucosa may contribute to impaired immune defense in the gut.
- Recurrent infections: Particularly of the respiratory tract and skin.
Conditions Associated with Elevated LL-37 Activity
Conversely, excessive LL-37 production can also be harmful:
- Psoriasis: Elevated LL-37 levels activate dendritic cells and promote the chronic skin inflammation characteristic of psoriasis.
- Systemic lupus erythematosus (SLE): LL-37 can combine with self-DNA to misdirect the immune system, contributing to autoimmune reactions.
- Rosacea: Increased LL-37 activity in the skin is implicated in the development of rosacea.
Therapeutic Potential
Due to its diverse properties, LL-37 is an intensively researched compound in biomedical science. Potential therapeutic applications include:
- Development of new antibiotics and anti-infective agents, especially against multidrug-resistant pathogens
- Improvement of wound healing, for example in chronic wounds and diabetic foot ulcers
- Treatment of skin conditions such as atopic dermatitis
- Cancer research: LL-37 exhibits both tumor-promoting and tumor-suppressive properties depending on the cancer type
- Use as an adjuvant in vaccines to enhance immune responses
Diagnostic Relevance
Measurement of LL-37 levels in blood, stool, saliva, or skin tissue can serve as a biomarker in research and clinical settings to assess inflammatory status, innate immune defense capacity, or therapeutic response. However, standardized routine clinical assays are still under development.
References
- Vandamme D, Landuyt B, Luyten W, Schoofs L. A comprehensive summary of LL-37, the factotum human cathelicidin peptide. Cellular Immunology, 2012; 280(1): 22-35.
- Mookherjee N, Anderson MA, Haagsman HP, Davidson DJ. Antimicrobial host defence peptides: functions and clinical potential. Nature Reviews Drug Discovery, 2020; 19(5): 311-332.
- Tecle T, Tripathi S, Hartshorn KL. Review: Defensins and cathelicidins in lung immunity. Innate Immunity, 2010; 16(3): 151-159.
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