LL-37

LL-37 Research

What is the research background of LL-37?

LL-37 was first discovered as a class of cationic small peptide substances in the pupae of the silkworm Samia cynthia ricini by the Swedish scientist Boman H G in 1980. LL-37 is the C-terminal peptide (CAMP, hCAP18) of human cathelicidin antibacterial peptide, which can increase the resistance to microbial invasion and plays important physiological functions in chemotaxis, promoting wound closure, and angiogenesis (Chen X, 2018). Antibacterial peptides are widely present in animals, plants, and a small amount of microorganisms, and they are an important part of the innate immunity of vertebrates. As a secretory protein, LL-37 is widely present in multiple organs and tissues of the human body. Various cells, including epithelial cells, keratinocytes, mast cells, neutrophils, macrophages, and monocytes, can secrete it. Most of this kind of antibiotic contains 37 - 39 "amino acid residues" and has 0 cysteine. Due to the strong basicity at the N-terminal position of the antibacterial peptide, it can form a stable "amphiphilic helical structure". The antibacterial peptide LL-37 also has an "amphiphilic α-helical structure". Because of its function of killing pathogenic bacteria, it is named an antibacterial peptide. The "37" in the name LL-37 may be related to the number of its amino acid residues. At the same time, it is also known as cathelicidin and ropocamptide.

What is the mechanism of action of LL-37 against antibiotic-resistant bacteria?

Disrupting the bacterial cell membrane: 

LL-37 can insert into the bacterial cell membrane, especially having a destructive effect on the cell membrane containing phosphatidylglycerol (DPPG). It will disrupt the structure of the bacterial cell membrane, thus exerting a bactericidal effect^[1]. For example, studies have found that LL-37 can insert into the cell membranes of both Gram-positive and Gram-negative bacteria, leading to an increase in the permeability of the cell membrane, the leakage of cell contents, and ultimately the death of bacteria.

Broad-spectrum antibacterial activity: 

LL-37 has antibacterial activity against a variety of antibiotic-resistant bacteria. It can act against Gram-positive bacteria (such as Staphylococcus aureus, Streptococcus, Enterococcus, etc.), Gram-negative bacteria (such as Pseudomonas aeruginosa, Escherichia coli, Salmonella, etc.), and other bacterial pathogens (such as Mycoplasma, Ureaplasma, Mycobacterium, etc.)^[2]. This broad-spectrum antibacterial activity makes LL-37 have potential application value in combating different types of antibiotic-resistant bacteria.

Destroying the formed biofilm: 

Bacterial biofilm is one of the important reasons for the drug resistance of pathogenic bacteria. The antibacterial peptide LL-37 can destroy the formed biofilm, thus reducing the drug resistance of bacteria. For example, in the prosthetic joint infection (PJI) after artificial joint replacement, the drug resistance of pathogenic bacteria caused by the bacterial biofilm makes the treatment difficult. However, LL-37 can play an effective antibacterial and bacteriostatic role by inhibiting the formation of the biofilm and destroying the formed biofilm.

Enhancing the antibacterial activity of antibiotics: 

Studies have shown that LL-37 has a synergistic effect with certain antibiotics. For example, when used in combination with amoxicillin clavulanic acid (AMC), LL-37 can strongly enhance the antibacterial activity of AMC.

Source:PubMed^[6]

What are the applications for LL-37?

Promoting bone regeneration: 

Some studies have shown that the antibacterial peptide LL-37 has a positive effect on bone regeneration. Some research has shown that human adipose-derived mesenchymal stem cells (hADSCs) were cultured with different concentrations of LL-37, and it was found that the concentration of LL-37 had an impact on the osteogenic ability of hADSCs, reaching a peak at 4μg/ml. In addition, the PSeD/hADSCs/LL-37 combination scaffold showed more superior osteogenic properties than the PSeD/hADSCs, PSeD, and control group scaffolds in the rat calvarial defect model, indicating a high potential in clinical bone regeneration.

Antibacterial effect:

Inhibition of multiple pathogenic bacteria: 

Some research used the micro-double dilution method to determine the minimum inhibitory concentration (MIC) of the antibacterial peptide LL-37 against Escherichia coli, Salmonella, and Staphylococcus aureus. The results showed that LL-37 had different degrees of inhibitory effects on these three pathogenic bacteria, with the minimum inhibitory concentrations being 3.12, 1.56, and 0.78μg/mL, respectively. The thermal stability test showed that the recombinant antibacterial peptide still had good activity at high temperatures. The acid-base stability test results showed that LL-37 had certain activity at a pH range of 2.0 to 12.0, with the best activity at a pH of 5.0 to 6.0, and -20°C being the best condition for long-term storage^[3].

Effect on antibiotic-resistant bacteria: 

Some people studied the antibacterial efficacy of the antibacterial peptide LL-37 and silver nanoparticles (AgNPs) against Staphylococcus aureus (S. aureus), a microorganism commonly found in biofilm-related infections. The results showed that LL-37 was the most effective antibacterial agent, with a reduction in colony count of more than 4 logarithms. In contrast, the effects of silver nanoparticles and conventional antibiotics were poorer, with a reduction in colony count of less than 1 logarithm. The antibacterial combination treatment with rifampicin significantly increased the logarithmic reduction of AgNPs and gentamicin, but it was still significantly lower than that of LL-37 used alone^[4].

Application in pulmonary infection: 

Studies have shown that Pseudomonas aeruginosa (PA) has become an urgent challenge for pulmonary infection and lung injury. The LL37 peptide is an effective antibacterial agent against PA strains, but its application is limited due to its rapid clearance in vivo, biosafety issues, and low bioavailability. Therefore, a reducing-sensitive albumin-based nanodrug delivery system has been developed to improve the performance of LL37 against PA in vivo by forming intermolecular disulfide bonds. Cationic LL37 can be effectively encapsulated through electrostatic interaction to exert an improved antibacterial effect. The LL37 peptide showed a sustained release of more than 48 hours from the LL37 peptide nanoparticles (LL37 PNP), and an extended antibacterial effect was noted with the increase in the incubation time. In a mouse model of acute PA pulmonary infection, LL37 PNP significantly reduced the expression of TNF-α and IL-1β and alleviated lung injury. It indicates that LL37 PNP can more effectively improve PA pulmonary infection and the subsequent inflammatory response than the free LL37 peptide^[3].

Activating the antibacterial function of platelets: 

Studies have pointed out that the antibacterial peptide LL-37 can activate the antibacterial function of human platelets. After platelets are treated with LL-37, the surface expression of receptors for recognizing microorganisms (Toll-like receptors (TLRs) 2 and -4, CD32, CD206, Dectin-1, CD35, LOX-1, CD41, CD62P, and αIIbβ3 integrin) and molecules related to presenting antigens to T lymphocytes (CD80, CD86, and HLA-ABC) is increased, and antibacterial molecules are secreted: bactericidal/permeability-increasing protein (BPI), azurocidin, human neutrophil peptide (HNP)-1, and myeloperoxidase. They also translate azurocidin and enhance the binding to Escherichia coli, Staphylococcus aureus, and Candida albicans. In addition, the supernatant of platelets treated with LL-37 can inhibit the growth of Escherichia coli, or platelets can use their LL-37 to inhibit microbial growth^[5].

Application in drug delivery systems

Studies have mentioned that antibacterial peptides (AMPs) are a new class of biomolecules with broad-spectrum antibacterial properties and have attracted attention due to the rapid increase in antibiotic resistance^[6]. LL37 is the only cathelicidin-derived antibacterial peptide found in humans. With in-depth research, LL37 has shown various biological functions, including regulating the inflammatory response, chemotaxis of immune cells, promoting wound healing, and osteogenesis, which have encouraged a variety of clinical applications. However, the clinical translation of LL37 is limited by its sensitivity to protease degradation, potential toxicity, poor bioavailability, etc. Various delivery systems, including metal nanoparticles, polymer materials, and lipid-based systems, have been introduced to achieve therapeutic applications.

In conclusion, as a multifunctional bioactive peptide, LL-37 has shown great potential in clinical applications. In terms of promoting bone regeneration, through the synergistic effects of multiple mechanisms such as promoting osteoblast differentiation and activity, antibacterial effects, immunomodulation, and promoting angiogenesis, it has brought new hope for bone injury repair. When dealing with antibiotic-resistant bacteria, by directly destroying the cell membrane, inhibiting the formation of biofilms, and synergizing with antibiotics, it is expected to become a powerful weapon to solve the problem of drug-resistant bacteria. In drug delivery systems, by designing and optimizing antibacterial peptide templates, constructing multiple drug delivery systems, and exploring the combined application of drugs, its clinical treatment effect can be further enhanced. In short, LL-37 has potential in multiple aspects of clinical applications, including promoting bone regeneration, antibacterial effects, activating the antibacterial function of platelets, and applications in drug delivery systems.

Relevant Citations

[1] Neville F, Cahuzac M, Konovalov O, et al. Lipid headgroup discrimination by antimicrobial peptide LL-37: Insight into mechanism of action[J]. Biophysical Journal, 2006,90(4):1275-1287.DOI:10.1529/biophysj.105.067595.

[2] Neshani A, Zare H, Eidgahi M R A, et al. LL-37: Review of antimicrobial profile against sensitive and antibiotic-resistant human bacterial pathogens[J]. Gene Reports, 2019,17:100519.DOI:10.1016/j.genrep.2019.100519.

[3] Li L, Peng Y, Yuan Q, et al. Cathelicidin LL37 Promotes Osteogenic Differentiation in vitro and Bone Regeneration in vivo[J]. Frontiers in Bioengineering and Biotechnology, 2021,9.DOI:10.3389/fbioe.2021.638494.

[4] Kang J, Dietz M J, Li B. Antimicrobial peptide LL-37 is bactericidal against Staphylococcus aureus biofilms[J]. Plos One, 2019,14(6).DOI:10.1371/journal.pone.0216676.

[5] Sanchez-Pena F J, Romero-Tlalolini M D L A, Torres-Aguilar H, et al. LL-37 Triggers Antimicrobial Activity in Human Platelets[J]. International Journal of Molecular Sciences, 2023,24(3).DOI:10.3390/ijms24032816.

[6] Lin X, Wang R, Mai S. Advances in delivery systems for the therapeutic application of LL37[J]. Journal of Drug Delivery Science and Technology, 2020,60.DOI:10.1016/j.jddst.2020.102016.