Endotoxin (Lipopolysaccharide) Neutralization by Innate Immunity Host-Defense Peptides

Functional Roles of Aromatic Residues and Helices of Papiliocin in its Antimicrobial and Anti-inflammatory Activities

A cecropin-like peptide, papiliocin, isolated from the swallowtail butterfly Papilio xuthus, possesses high selectivity against gram-negative bacteria. Since Trp(2) and Phe(5) are highly conserved residues in cecropin-like peptides, we investigated the role of Trp(2) and Phe(5) in antibacterial activity. Substitution of Trp(2) and Phe(5) in papiliocin with Ala (papiliocin-2A and papiliocin-5A) revealed that Trp(2) is a key residue in its antibacterial activities. In order to understand the structural requirements for papiliocin function and to design shorter, but more potent, peptide antibiotics, we designed papiliocin constructs, PapN (residues Arg(1)-Ala(22) from the N-terminal amphipathic helix). PapN exhibited significant broad-spectrum antibacterial activities without cytotoxicity. Bactericidal kinetics of peptides against E.coli showed that papiliocin completely and rapidly killed E.coli in less than 10 minutes at 2× MIC concentration, while papiliocin-2A and papiliocin-5A killed four times more slowly than papiliocin. The PapN series peptides permeabilized bacterial membranes less effectively than papiliocin, showing no antibacterial activities in an hour. The results imply that the Trp(2) and Phe(5) in the amphipathic N-terminal helix are important in the rapid permeabilization of the gram-negative bacterial membrane. The hydrophobic C-terminal residues permeabilize the hydrophobic bacterial cell membrane synergistically with these aromatic residues, providing selectivity against gram-negative bacteria.

Anti-Inflammatory Action of an Antimicrobial Model Peptide That Suppresses the TRIF-Dependent Signaling Pathway via Inhibition of Toll-Like Receptor 4 Endocytosis in Lipopolysaccharide-Stimulated Macrophages

Antimicrobial peptides (AMPs), also called host defense peptides, particularly those with amphipathic helical structures, are emerging as target molecules for therapeutic development due to their immunomodulatory properties. Although the antimicrobial activity of AMPs is known to be exerted primarily by permeation of the bacterial membrane, the mechanism underlying its anti-inflammatory activity remains to be elucidated. We report potent anti-inflammatory activity of WALK11.3, an antimicrobial model peptide with an amphipathic helical conformation, in lipopolysaccharide (LPS)-stimulated RAW264.7 cells. This peptide inhibited the expression of inflammatory mediators, including nitric oxide, COX-2, IL-1β, IL-6, INF-β, and TNF-α. Although WALK11.3 did not exert a major effect on all downstream signaling in the MyD88-dependent pathway, toll-like receptor 4 (TLR4)- mediated pro-inflammatory signals were markedly attenuated in the TRIF-dependent pathway due to inhibition of the phosphorylation of STAT1 by attenuation of IRF3 phosphorylation. WALK11.3 specifically inhibited the endocytosis of TLR4, which is essential for triggering TRIF-mediated signaling in macrophage cells. Hence, we suggest that specific interference with TLR4 endocytosis could be one of the major modes of the anti-inflammatory action of AMPs. Our designed WALK11 peptides, which possess both antimicrobial and anti-inflammatory activities, may be promising molecules for the development of therapies for infectious inflammation.

CXCL14 displays antimicrobial activity against respiratory tract bacteria and contributes to clearance of Streptococcus pneumoniae pulmonary infection

CXCL14 is a chemokine with an atypical, yet highly conserved, primary structure characterized by a short N terminus and high sequence identity between human and mouse. Although it induces chemotaxis of monocytic cells at high concentrations, its physiological role in leukocyte trafficking remains elusive. In contrast, several studies have demonstrated that CXCL14 is a broad-spectrum antimicrobial peptide that is expressed abundantly and constitutively in epithelial tissues. In this study, we further explored the antimicrobial properties of CXCL14 against respiratory pathogens in vitro and in vivo. We found that CXCL14 potently killed Pseudomonas aeruginosa, Streptococcus mitis, and Streptococcus pneumoniae in a dose-dependent manner in part through membrane depolarization and rupture. By performing structure-activity studies, we found that the activity against Gram-negative bacteria was largely associated with the N-terminal peptide CXCL141-13. Interestingly, the central part of the molecule representing the β-sheet also maintained ∼62% killing activity and was sufficient to induce chemotaxis of THP-1 cells. The C-terminal α-helix of CXCL14 had neither antimicrobial nor chemotactic effect. To investigate a physiological function for CXCL14 in innate immunity in vivo, we infected CXCL14-deficient mice with lung pathogens and we found that CXCL14 contributed to enhanced clearance of Streptococcus pneumoniae, but not Pseudomonas aeruginosa. Our comprehensive studies reflect the complex bactericidal mechanisms of CXCL14, and we propose that different structural features are relevant for the killing of Gram-negative and Gram-positive bacteria. Taken together, our studies show that evolutionary-conserved features of CXCL14 are important for constitutive antimicrobial defenses against pneumonia.

Mucosal physical and chemical innate barriers: Lessons from microbial evasion strategies

The innate immune system has evolved since millions of years under a selective pressure. Among the different host mechanisms selected and conserved as a first line of defense, the gastrointestinal mucus layer constitutes an efficient physical and chemical barrier against invading microbes. Mucin glycoproteins and antimicrobial peptides are the major components of the mucus barrier, and evidences prove that they form an effective protection against most microbes. However, successful pathogens have evolved evasion strategies to circumvent this defense barrier. Here, we discuss the interactions between pathogens, mucins, and antimicrobial peptides, and the mechanisms that pathogens have developed to evade the innate defense systems of the intestinal mucosal barrier.

Lactoferrin: A Roadmap to the Borderland between Caries and Periodontal Disease

Lactoferrin is one of a number of multifunctional proteins that are present in or on all mucosal surfaces throughout the body. Levels of lactoferrin are consistently elevated in inflammatory diseases such as arthritis, inflammatory bowel diseases, corneal disease, and periodontitis. Single-nucleotide polymorphisms (SNPs) in lactoferrin have been shown to be present in individuals susceptible to Escherichia coli-induced travelers' diarrhea and in tear fluid derived from virally associated corneal disease. Here, we review data showing a lactoferrin SNP in amino acid position 29 in the antimicrobial region of lactoferrin that acts against caries associated bacteria. This SNP was initially discovered in African American subjects with localized aggressive periodontitis (LAP) who had proximal bone loss but minimal proximal caries. Results were confirmed in a genetic association study of children from Brazil with this same SNP who showed a reduced level of caries. In vitro data indicate that lactoferrin from whole saliva derived from subjects with this SNP, recombinant human lactoferrin containing this SNP, or an 11-mer peptide designed for this SNP kills mutans streptococci associated with caries by >1 log. In contrast, the SNP has minimal effect on Gram-negative species associated with periodontitis. Moreover, periodontally healthy subjects homozygous for this lysine (K) SNP have lactoferrin in their saliva that kills mutans streptococci and have reduced proximal decay. The review summarizes data supporting the ecologic plaque hypothesis and suggests that a genetic variant in lactoferrin with K in position 29 when found in saliva and crevice fluid can influence community biofilm composition. We propose that, for caries, this SNP is ethnicity independent and protective by directly killing caries-provoking bacteria (reducing proximal decay). However, the clinical effect of this SNP in LAP is ethnicity dependent, destructive (increases LAP incidence), and complex with mechanisms still to be determined.

Green tea polyphenol epigallocatechin-3-gallate and cranberry proanthocyanidins act in synergy with cathelicidin (LL-37) to reduce the LPS-induced inflammatory response in a three-dimensional co-culture model of gingival epithelial cells and fibroblasts

OBJECTIVES

The human antimicrobial peptide cathelicidin (LL-37) possesses anti-inflammatory properties that may contribute to attenuating the inflammatory process associated with chronic periodontitis. Plant polyphenols, including those from cranberry and green tea, have been reported to reduce inflammatory cytokine secretion by host cells. In the present study, we hypothesized that A-type cranberry proanthocyanidins (AC-PACs) and green tea epigallocatechin-3-gallate (EGCG) act in synergy with LL-37 to reduce the secretion of inflammatory mediators by oral mucosal cells.

METHODS

A three-dimensional (3D) co-culture model of gingival epithelial cells and fibroblasts treated with non-cytotoxic concentrations of AC-PACs (25 and 50 μg/ml), EGCG (1 and 5 μg/ml), and LL-37 (0.1 and 0.2 μM) individually and in combination (AC-PACs+LL-37 and EGCG+LL-37) were stimulated with Aggregatibacter actinomycetemcomitans lipopolysaccharide (LPS). Multiplex ELISA assays were used to quantify the secretion of 54 host factors, including chemokines, cytokines, growth factors, matrix metalloproteinases (MMPs), and tissue inhibitors of metalloproteinases (TIMPs).

RESULTS

LL-37, AC-PACs, and EGCG, individually or in combination, had no effect on the regulation of MMP and TIMP secretion but inhibited the secretion of several cytokines. AC-PACs and LL-37 acted in synergy to reduce the secretion of CXC-chemokine ligand 1 (GRO-α), granulocyte colony-stimulating factor (G-CSF), and interleukin-6 (IL-6), and had an additive effect on reducing the secretion of interleukin-8 (IL-8), interferon-γ inducible protein 10 (IP-10), and monocyte chemoattractant protein-1 (MCP-1) in response to LPS stimulation. EGCG and LL-37 acted in synergy to reduce the secretion of GRO-α, G-CSF, IL-6, IL-8, and IP-10, and had an additive effect on MCP-1 secretion.

CONCLUSION

The combination of LL-37 and natural polyphenols from cranberry and green tea acted in synergy to reduce the secretion of several cytokines by an LPS-stimulated 3D co-culture model of oral mucosal cells. Such combinations show promising results as potential adjunctive therapies for treating inflammatory periodontitis.

Endogenous intracellular cathelicidin enhances TLR9 activation in dendritic cells and macrophages

Cathelicidins are a gene family best known for their antimicrobial action, but the diverse mature peptides they encode also have other host defense functions. The human cathelicidin peptide LL-37 enhances recognition of nucleic acids, an action whose significance is seen in human diseases such as psoriasis where it is associated with increased type 1 IFN production. This function has been attributed to the extracellular action of the peptide to facilitate uptake of nucleic acids. In this study, we demonstrate that the murine mature cathelicidin peptide (CRAMP), encoded by the mouse gene (Camp), is functionally distinct from the human mature peptide (LL-37), as it does not facilitate CpG entry. However, mouse cathelicidin does influence recognition of CpG as bone marrow-derived dendritic cells from Camp(-/-) mice have impaired CpG responses and Camp(-/-) mice had impaired response to CpG given i.v. or s.c. We show that cathelicidin concentrates in Lamp1 positive compartments, is colocalized with CpG in the endolysosome, can be immunoprecipitated with TLR9, and binds to CpG intracellulary. Collectively, these results indicate that the functions of cathelicidin in control of TLR9 activation may include both intracellular and extracellular effects.

Asp residues of βDELSEED-motif are required for peptide binding in the Escherichia coli ATP synthase

This study demonstrates the requirement of Asp-380 and Asp-386 in the βDELSEED-motif of Escherichia coli ATP synthase for peptide binding and inhibition. We studied the inhibition profiles of wild-type and mutant E. coli ATP synthase in presence of c-terminal amide bound melittin and melittin related peptide. Melittin and melittin related peptide inhibited wild-type ATPase almost completely while only partial inhibition was observed in single mutations with replacement of Asp to Ala, Gln, or Arg. Additionally, very little or no inhibition occurred among double mutants βD380A/βD386A, βD380Q/βD386Q, or βD380R/βD386R signifying that removal of one Asp residue allows limited peptide binding. Partial or substantial loss of oxidative phosphorylation among double mutants demonstrates the functional requirement of βD380 and βD386 Asp residues. Moreover, abrogation of wild-type E. coli cell growth and normal growth of mutant cells in presence of peptides provides strong evidence for the requirement of βDELSEED-motif Asp residues for peptide binding. It is concluded that while presence of one Asp residue may allow partial peptide binding, both Asp residues, βD380 and βD386, are essential for proper peptide binding and inhibition of ATP synthase.

Anti-inflammatory activities of cecropin A and its mechanism of action

Cecropin A is a novel 37-residue cecropin-like antimicrobial peptide isolated from the cecropia moth, Hyalophora cecropia. We have demonstrated that cecropin A is an antibacterial agent and have investigated its mode of action. In this study, we show that cecropin A has potent antimicrobial activity against 2 multidrug resistant organisms-Acinetobacter baumanii and-Pseudomonas aeruginosa. Interactions between cecropin A and membrane phospholipids were studied using tryptophan blue shift experiments. Cecropin A has a strong interaction with bacterial cell mimetic membranes. These results imply that cecropin A has selectivity for bacterial cells. To address the potential the rapeutic efficacy of cecropin A, its anti-inflammatory activities and mode of action in mouse macrophage-derived RAW264.7 cells stimulated with lipopolysaccharide (LPS) were examined. Cecropin A suppressed nitrite production, mTNF-α, mIL-1β, mMIP-1, and mMIP-2 cytokine release in LPS-stimulated RAW264.7 cells. Furthermore, cecropin A inhibited intracellular cell signaling via the ERK, JNK, and p38 MAPK pathway, leading to the prevention of COX-2 expression in LPS-stimulated RAW264.7 cells. These results strongly suggest that cecropin A should be investigated as a potential agent for the prevention and treatment of inflammatory diseases.

Interaction of the core fragments of the LL-37 host defense peptide with actin

Host defense peptides are effector molecules of the innate immunity that possess antimicrobial and health-promoting properties. Due to their potential therapeutic activities, host defense peptides are being explored as alternatives for antibiotics. The human LL-37 and its shorter, cost-effective, bactericidal core peptide derivates have been suggested for their therapeutic potential. Bacteria evade host defense peptides by proteolytic inactivation. Actin released from necrotized cells and abundant in infected sites was shown to bind and protect LL-37 from microbial proteolytic degradation, and to enable the peptide's antimicrobial action despite the presence of the proteases. Here, we characterized the interactions of the 10-13 residues long LL-37 core peptides with actin. We show that the LL-37 core peptides associate with actin with a lower affinity than that of LL-37. Their association with actin, which is very ionic strength sensitive, is mainly based on electrostatic interactions. Likewise, the antimicrobial activity against Escherichia coli of the minimal antimicrobial peptide KR-12 but not FK-13 nor LL-37 is also very sensitive to salts. In addition, the antimicrobial activity of the FK-13 core peptide is protected by actin against the tested bacterial proteases in a similar manner to that of LL-37, supporting its potential for therapeutic use.

Role of phenylalanine and valine10 residues in the antimicrobial activity and cytotoxicity of piscidin-1

Piscidin-1 (Pis-1) is a linear antibacterial peptide derived from mast cells of aquacultured hybrid striped bass that comprises 22 amino acids with a phenylalanine-rich amino-terminus. Pis-1 exhibits potent antibacterial activity against pathogens but is not selective for distinguishing between bacterial and mammalian cells. To determine the key residues for its antibacterial activity and those for its cytotoxicity, we investigated the role of each Phe residue near the N-terminus as well as the Val¹⁰ residue located near the boundary of the hydrophobic and hydrophilic sectors of the helical wheel diagram. Fluorescence dye leakage and tryptophan fluorescence experiments were used to study peptide-lipid interactions, showing comparable depths of insertion of substituted peptides in different membranes. Phe² was found to be the most deeply inserted phenylalanine in both bacterial- and mammalian-mimic membranes. Each Phe was substituted with Ala or Lys to investigate its functional role. Phe² plays key roles in the cytotoxicity as well as the antibacterial activities of Pis-1, and Phe⁶ is essential for the antibacterial activities of Pis-1. We also designed and synthesized a piscidin analog, Pis-V10K, in which Lys was substituted for Val¹⁰, resulting in an elevated amphipathic α-helical structure. Pis-V10K showed similar antibacterial activity (average minimum inhibitory concentration (MIC)  = 1.6 µM) to Pis-1 (average MIC  = 1.5 µM). However, it exhibited much lower cytotoxicity than Pis-1. Lys¹⁰-substituted analogs, Pis-F1K/V10K, Pis-F2K/V10K, and Pis-F6K/V10K in which Lys was substituted for Phe retained antibacterial activity toward standard and drug-resistant bacterial strains with novel bacterial cell selectivity. They exert anti-inflammatory activities via inhibition of nitric oxide production, TNF-α secretion, and MIP-1 and MIP-2 production. They may disrupt the binding of LPS to toll-like receptors, eventually suppressing MAPKs-mediated signaling pathways. These peptides may be good candidates for the development of peptide antibiotics with potent antibacterial activity but without cytotoxicity.

(Lipo)polysaccharide interactions of antimicrobial peptides

Due to rapidly increasing resistance development against conventional antibiotics, as well as problems associated with diseases either triggered or deteriorated by infection, antimicrobial and anti-inflammatory peptides have attracted considerable interest during the last few years. While there is an emerging understanding of the direct antimicrobial function of such peptides through bacterial membrane destabilization, the mechanisms of their anti-inflammatory function are less clear. We here summarize some recent results obtained from our own research on anti-inflammatory peptides, with focus on peptide-(lipo)polysaccharide interactions.

Antimicrobial peptides and the enteric mucus layer act in concert to protect the intestinal mucosa

The intestinal mucosa squares the circle by allowing efficient nutrient absorption while generating a firm barrier toward the enteric microbiota, enteropathogenic microorganisms and high luminal concentrations of potent immunostimulatory molecules. The mucus layer together with local antimicrobial and anti-inflammatory peptides significantly contribute to this ability. Here we summarize the recent progress made to better understand the critical importance of this dynamic, complex and highly structured anti-inflammatory and antimicrobial barrier.

Antimicrobial effects of helix D-derived peptides of human antithrombin III

Antithrombin III (ATIII) is a key antiproteinase involved in blood coagulation. Previous investigations have shown that ATIII is degraded by Staphylococcus aureus V8 protease, leading to release of heparin binding fragments derived from its D helix. As heparin binding and antimicrobial activity of peptides frequently overlap, we here set out to explore possible antibacterial effects of intact and degraded ATIII. In contrast to intact ATIII, the results showed that extensive degradation of the molecule yielded fragments with antimicrobial activity. Correspondingly, the heparin-binding, helix D-derived, peptide FFFAKLNCRLYRKANKSSKLV (FFF21) of human ATIII, was found to be antimicrobial against particularly the Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa. Fluorescence microscopy and electron microscopy studies demonstrated that FFF21 binds to and permeabilizes bacterial membranes. Analogously, FFF21 was found to induce membrane leakage of model anionic liposomes. In vivo, FFF21 significantly reduced P. aeruginosa infection in mice. Additionally, FFF21 displayed anti-endotoxic effects in vitro. Taken together, our results suggest novel roles for ATIII-derived peptide fragments in host defense.

Synergistic anti-inflammatory activity of the antimicrobial peptides human beta-defensin-3 (hBD-3) and cathelicidin (LL-37) in a three-dimensional co-culture model of gingival epithelial cells and fibroblasts

Given the spread of antibiotic resistance in bacterial pathogens, antimicrobial peptides that can also modulate the immune response may be a novel approach for effectively controlling periodontal infections. In the present study, we used a three-dimensional (3D) co-culture model of gingival epithelial cells and fibroblasts stimulated with Aggregatibacter actinomycetemcomitans lipopolysaccharide (LPS) to investigate the anti-inflammatory properties of human beta-defensin-3 (hBD-3) and cathelicidin (LL-37) and to determine whether these antimicrobial peptides can act in synergy. The 3D co-culture model composed of gingival fibroblasts embedded in a collagen matrix overlaid with gingival epithelial cells had a synergistic effect with respect to the secretion of IL-6 and IL-8 in response to LPS stimulation compared to fibroblasts and epithelial cells alone. The 3D co-culture model was stimulated with non-cytotoxic concentrations of hBD-3 (10 and 20 µM) and LL-37 (0.1 and 0.2 µM) individually and in combination in the presence of A. actinomycetemcomitans LPS. A multiplex ELISA assay was used to quantify the secretion of 41 different cytokines. hBD-3 and LL-37 acted in synergy to reduce the secretion of GRO-alpha, G-CSF, IP-10, IL-6, and MCP-1, but only had an additive effect on reducing the secretion of IL-8 in response to A. actinomycetemcomitans LPS stimulation. The present study showed that hBD-3 acted in synergy with LL-37 to reduce the secretion of cytokines by an LPS-stimulated 3D model of gingival mucosa. This combination of antimicrobial peptides thus shows promising potential as an adjunctive therapy for treating inflammatory periodontitis.

Structural and functional characterization of a TGFβ molecule from amphioxus reveals an ancient origin of both immune-enhancing and -inhibitory functions

Transforming growth factor beta (TGFβ) is a pleiotropic cytokine with important roles in mediating inflammatory response. TGFβ has been shown to be widely present in invertebrates, but little is known about its functions in immune and inflammatory responses. Moreover, structural and functional insights into TGFβ molecules in invertebrates remain completely lacking. Here we demonstrate the presence of a single TGFβ-like gene in the amphioxus Branchiostoma japonicum, Bjtgfβ, which represents the archetype of vertebrate TGFβ proteins, and displays a higher expression in the hind-gut, hepatic caecum, ovary, and gill. We also show that amphioxus TGFβ exerts both enhancing and suppressing effects on the migration of macrophages like RAW264.7, and the motif WSTD is important for TGFβ in inducing or inhibiting the migration of macrophages. Altogether, these data suggest that amphioxus TGFβ is phylogenetically and functionally similar to vertebrate TGFβ, suggesting an ancient origin of bipolar function of TGFβ proteins.

Effects of antimicrobial peptide L-K6, a temporin-1CEb analog on oral pathogen growth, Streptococcus mutans biofilm formation, and anti-inflammatory activity

Dental caries and periodontitis are common bacterial mouth infections. As a potentially attractive substitute for conventional antibiotics, antimicrobial peptides have been widely tested and used for controlling bacterial infections. In this study, we tested the efficacy of the peptides from the skin secretions of Rana chensinensis for killing several major cariogenic and periodontic pathogens as well as Candida albicans. L-K6, a temporin-1CEb analog, exhibited high antimicrobial activity against the tested oral pathogens and was able to inhibit Streptococcus mutans biofilm formation and reduce 1-day-old S. mutans biofilms with a minimum biofilm inhibitory concentration and reducing concentration of 3.13 and 6.25 μM, respectively. The results of confocal laser scanning microscopy demonstrated that the peptide significantly reduced cell viability within oral biofilms. Furthermore, as little as 5 μM L-K6 significantly inhibited lipopolysaccharide (LPS)- and interleukin-1β-induced productions of interleukin-8 and tumor necrosis factor-α from THP-1 monocytic cells. This anti-inflammatory activity is associated with the binding of L-K6 to LPS and neutralizing LPS-induced proinflammatory responses in THP-1 cells, as well as dissociating LPS aggregates. Our results suggest that L-K6 may have potential clinical applications in treating dental caries by killing S. mutans within dental plaque and acting as anti-inflammatory agents in infected tissues.

Cysteine deleted protegrin-1 (CDP-1): anti-bacterial activity, outer-membrane disruption and selectivity

BACKGROUND

Protegin-1 (PG-1: RGGRLCYCRRRFCVCVGR-amide) assumes a rigid β-hairpin like structure that is stabilized by two disulfide bridges between Cys6-Cys15 and Cys8-Cys13. Previous studies, employing linear analogs of PG-1, with Cys to Ala mutations or modified Cys, have demonstrated that the disulfide bridges are critical for the broad spectrum and salt resistant antimicrobial activity of PG-1.

METHODS

In order to understand structural and functional roles of disulfide bonds in protegrins, we have synthesized a Cys deleted variant of PG-1 or CDP-1, RGGRLYRRRFVVGR-amide, and two of its analogs, RR11, RLYRRRFVVGR-amide, and LR10, LYRRRFVVGR-amide, containing deletion of residues at the N-terminus. These peptides have been characterized for bactericidal activity and mode of action in lipopolysaccharide (LPS) using optical spectroscopy, ITC and NMR.

RESULTS

Antibacterial activity, against Gram-negative and Gram-positive strains, of the three peptides follows the order: CDP-1>RR11>LR10. LR10 displays only limited activity toward Gram-negative strains. CDP-1 demonstrates efficient membrane permeabilization and high-affinity interactions with LPS. CDP-1 and RR11 both assume β-hairpin like compact structures in complex with LPS, whereas LR10 adopts an extended conformation in LPS. In zwitterionic DPC micelles CDP-1 and the truncated analog peptides do not adopt folded conformations.

MAJOR CONCLUSIONS

Despite the absence of stabilizing disulfide bridges CDP-1 shows broad-spectrum antibacterial activity and assumes β-hairpin like structure in complex with LPS. The β-hairpin structure may be essential for outer membrane permeabilization and cell killing.

Little peptide, big effects: the role of LL-37 in inflammation and autoimmune disease

The innate immune system utilizes many approaches for defense against invading microorganisms, including complement-mediated lysis, engulfment, formation of neutrophil extracellular traps, and release of antimicrobial peptides. Although classically thought to be driven by adaptive immunity, the development of autoimmune diseases such as rheumatoid arthritis and systemic lupus erythematosus is increasingly associated with dysregulated innate immune pathways. An emerging theme within this literature is the contribution of antimicrobial peptides to the development of autoimmune disorders. This is best exemplified in atopic dermatitis and psoriasis where the defensins and the single human cathelicidin, LL-37, may contribute to disease. Furthermore, in the past few years, a role for LL-37 has emerged in the pathogenesis of systemic lupus erythematosus, rheumatoid arthritis, atherosclerosis, and possibly other diseases. In this review, we discuss the role of LL-37 and its murine ortholog, mCRAMP, in the modulation of immune and inflammatory pathways and their effects on autoimmune and inflammatory diseases.

Antiviral potential of cathelicidins

ABSTRACT: The global burden of morbidity and mortality arising from viral infections is high; however, the development of effective therapeutics has been slow. As our understanding of innate immunity has expanded over recent years, knowledge of natural host defenses against viral infections has started to offer potential for novel therapeutic strategies. An area of current research interest is in understanding the roles played by naturally occurring cationic host defense peptides, such as the cathelicidins, in these innate antiviral host defenses across different species. This research also has the potential to inform the design of novel synthetic antiviral peptide analogs and/or provide rationale for therapies aimed at boosting the natural production of these peptides. In this review, we will discuss our knowledge of the antiviral activities of cathelicidins, an important family of cationic host defense peptides, and consider the implications for novel antiviral therapeutic approaches.

Avian host defense peptides

Host defense peptides (HDPs) are important effector molecules of the innate immune system of vertebrates. These antimicrobial peptides are also present in invertebrates, plants and fungi. HDPs display broad-spectrum antimicrobial activities and fulfill an important role in the first line of defense of many organisms. It is becoming increasingly clear that in the animal kingdom the functions of HDPs are not confined to direct antimicrobial actions. Research in mammals has indicated that HDPs have many immunomodulatory functions and are also involved in other physiological processes ranging from development to wound healing. During the past five years our knowledge about avian HDPs has increased considerably. This review addresses our current knowledge on the evolution, regulation and biological functions of HDPs of birds.

A helix-PXXP-helix peptide with antibacterial activity without cytotoxicity against MDRPA-infected mice

In response to the growing problem of multidrug-resistant pathogenic microbes, much attention is being paid to naturally occurring and synthetic antimicrobial peptides (AMPs) and the effects of their structural modification. Among these modifications, amino acid substitution is a simple approach to enhancing biological activity and reducing cytotoxicity. An earlier study indicated that HPA3, an analog of HP (2-20) derived from the N-terminus of Helicobacter pylori ribosomal protein L1, forms large pores and shows considerable cytotoxicity. However, HPA3P, in which a proline (Pro) is substituted for glutamic acid (Glu) at position 9 of HPA3, shows markedly less cytotoxicity. This may be attributable to the presence of a Pro-kink into middle of the HPA3P structure within the membrane environment. Unfortunately, HPA3P is not an effective antibacterial agent in vivo. We therefore designed a helix-PXXP-helix structure (HPA3P2), in which Pro was substituted for the Glu and phenylalanine (Phe) at positions 9 and 12 of HPA3, yielding a molecule with a flexible central hinge. As compared to HPA3P, HPA3P3 exhibited dramatically increased antibacterial activity in vivo. ICR mice infected with clinically isolated multidrug-resistant Pseudomonas aeruginosa showed 100% survival when administered one 0.5-mg/kg dose of HPA3P2 or three 0.1-mg/kg doses of HPA3P2. Moreover, in a mouse model of septic shock induced by P. aeruginosa LPS, HPA3P2 reduced production of pro-inflammatory mediators and correspondingly reduced lung (alveolar) and liver tissue damage. The changes in HPA3 behavior with the introduction of Pro likely reflects alterations of the mechanism of action: i) HPA3 forms pores in the bacterial cell membranes, ii) HPA3P permeates the cell membranes and binds to intracellular RNA and DNA, and iii) HPA3P2 acts on the outer cellular membrane component LPS. Collectively, these results suggest HPA3P2 has the potential to be an effective antibiotic for use against multidrug-resistant bacterial strains.

Host defense peptides: general overview and an update on their activity against Chlamydia spp

Chlamydiae are obligate intracellular bacteria that cause serious diseases in a wide range of hosts. Chlamydia trachomatis is one of the leading sexually transmitted pathogens in the world. Because vaccines are not currently available, effective drugs are essential. In both animals and humans, chlamydial infections are often treated with tetracycline or its derivatives. A stable tetracycline-resistant phenotype was described in Chlamydia suis strains from pigs in the USA and in Europe. In humans, there are reports of tetracycline treatment failure and the in vitro adaptability of C. trachomatis to evolve to antibiotic resistance has been described, suggesting the pressing need to search for alternative and effective classes of antimicrobial drugs. Host defense peptides (HDPs) are known as direct antimicrobial agents as well as innate immune modulators. Being active against multidrug-resistant bacteria, HDPs are attractive candidates as templates for new drugs. A number of studies evaluated the activity of natural and synthetic HDPs against Chlamydia spp., showing C. trachomatis to be the most sensitive among chlamydia species tested. Protegrins and α-helical peptides were the most active among the HDPs assessed.

Defensins and LL-37: a review of function in the gingival epithelium

Antimicrobial peptides represent an important aspect of the innate defense system that contributes to the control of bacterial colonization and infection. As studies have progressed it has become clear that antimicrobial peptides manifest other functions in addition to their antimicrobial effects. These functions include chemotaxis of numerous types of host cells involved in both the innate and adaptive immune responses. In this review, the antimicrobial activity, the regulation and the contribution to host homeostasis of alpha-defensins and LL-37, as well as of beta-defensins, are discussed in the context of their specific tissue locations in the junctional epithelium and oral epithelium, respectively.

Short KR-12 analogs designed from human cathelicidin LL-37 possessing both antimicrobial and antiendotoxic activities without mammalian cell toxicity

KR-12 (residues 18-29 of LL-37) was known to be the smallest peptide of human cathelicidin LL-37 possessing antimicrobial activity. In order to optimize α-helical short antimicrobial peptides having both antimicrobial and antiendotoxic activities without mammalian cell toxicity, we designed and synthesized a series of KR-12 analogs. Highest hydrophobic analogs KR-12-a5 and KR-12-a6 displayed greater inhibition of lipopolysaccharide (LPS)-stimulated tumor necrosis factor-α production and higher LPS-binding activity. We have observed that antimicrobial activity is independent of charge, but LPS neutralization requires a balance of hydrophobicity and net positive charge. Among KR-12 analogs, KR-12-a2, KR-12-a3 and KR-12-a4 showed much higher cell specificity for bacteria over erythrocytes and retained antiendotoxic activity, relative to parental LL-37. KR-12-a5 displayed the strongest antiendotoxic activity but almost similar cell specificity as compared with LL-37. Also, these KR-12 analogs (KR-12-a2, KR-12-a3, KR-12-a4 and KR-12-a5) exhibited potent antimicrobial activity (minimal inhibitory concentration: 4 μM) against methicillin-resistant Staphylococcus aureus. Taken together, these KR-12 analogs have the potential for future development as a novel class of antimicrobial and anti-inflammatory therapeutic agents.

Lipopolysaccharide neutralization by a novel peptide derived from phosvitin

Lipopolysaccharide (LPS), also known as endotoxin, is the primary trigger of sepsis, which is associated with high mortality in patients. No therapeutic agents are currently efficacious enough to protect patients from sepsis characterized by LPS-mediated tissue damage and organ failure. Previously, a phosvitin-derived peptide, Pt5, which consists of the C-terminal 55 residues of zebrafish phosvitin, has been shown to function as an antibacterial agent. In this study, we have generated six mutants by site-directed mutagenesis based on the sequence of Pt5, and found that one of the six mutants, Pt5e, showed the strongest bactericidal activities against Escherichia coli and Staphylococcus aureus. We then demonstrated that Pt5e was able to bind to LPS and lipoteichoic acid (LTA). More importantly, we showed that Pt5e significantly inhibited LPS-induced tumor-necrosis factor (TNF)-α and interleukin (IL)-1β release from murine RAW264.7 cells and considerably reduced serum TNF-α and IL-1β levels in mice. Additionally, Pt5e protected the liver from damage by LPS, and remarkably promoted the survival rate of the endotoxemia mice. Furthermore, Pt5e displayed no cytotoxicity to murine RAW264.7 macrophages and no hemolytic activity toward human red blood cells. These data together indicate that Pt5e is an endotoxin-neutralizing agent with a therapeutic potential in clinical treatment of LPS-induced sepsis.

Characterization of Antimicrobial Peptides toward the Development of Novel Antibiotics

Antimicrobial agents have eradicated many infectious diseases and significantly improved our living environment. However, abuse of antimicrobial agents has accelerated the emergence of multidrug-resistant microorganisms, and there is an urgent need for novel antibiotics. Antimicrobial peptides (AMPs) have attracted attention as a novel class of antimicrobial agents because AMPs efficiently kill a wide range of species, including bacteria, fungi, and viruses, via a novel mechanism of action. In addition, they are effective against pathogens that are resistant to almost all conventional antibiotics. AMPs have promising properties; they directly disrupt the functions of cellular membranes and nucleic acids, and the rate of appearance of AMP-resistant strains is very low. However, as pharmaceuticals, AMPs exhibit unfavorable properties, such as instability, hemolytic activity, high cost of production, salt sensitivity, and a broad spectrum of activity. Therefore, it is vital to improve these properties to develop novel AMP treatments. Here, we have reviewed the basic biochemical properties of AMPs and the recent strategies used to modulate these properties of AMPs to enhance their safety.

Fortifying the barrier: the impact of lipid A remodelling on bacterial pathogenesis

Gram-negative bacteria decorate their outermost surface structure, lipopolysaccharide, with elaborate chemical moieties, which effectively disguises them from immune surveillance and protects them from the onslaught of host defences. Many of these changes occur on the lipid A moiety of lipopolysaccharide, a component that is crucial for host recognition of Gram-negative infection. In this Review, we describe the regulatory mechanisms controlling lipid A modification and discuss the impact of modifications on pathogenesis, bacterial physiology and bacterial interactions with the host immune system.

Identification of cell-penetrating peptides that are bactericidal to Neisseria meningitidis and prevent inflammatory responses upon infection

Meningococcal disease is characterized by a fast progression and a high mortality rate. Cell-penetrating peptides (CPPs), developed as vectors for cargo delivery into eukaryotic cells, share structural features with antimicrobial peptides. A screen identified two CPPs, transportan-10 (TP10) and model amphipathic peptide (MAP), with bactericidal action against Neisseria meningitidis. Both peptides were active in human whole blood at micromolar concentrations, while hemolysis remained negligible. Additionally, TP10 exhibited significant antibacterial activity in vivo. Uptake of SYTOX green into live meningococci was observed within minutes after TP10 treatment, suggesting that TP10 may act by membrane permeabilization. Apart from its bactericidal activity, TP10 suppressed inflammatory cytokine release from macrophages infected with N. meningitidis as well as from macrophages stimulated with enterobacterial and meningococcal lipopolysaccharide (LPS). Finally, incubation with TP10 reduced the binding of LPS to macrophages. This novel endotoxin-inhibiting property of TP10, together with its antimicrobial activity in vivo, indicates the possibility to design peptide-based therapies for infectious diseases.

Assembly of the TLR2/6 transmembrane domains is essential for activation and is a target for prevention of sepsis

TLR2, together with TLR1 and TLR6, is essential for detecting lipopeptides and bacterial cell wall components such as lipoteichoic acid from Gram-positive bacteria. In this study, we report that transmembrane domain (TMD)-derived peptides from TLR2 and TLR6 specifically inhibit TLR2 activation. Secretion of the cytokines TNF-α and IL-6 by cultured macrophages (RAW264.7 cell line) was inhibited by these peptides in response to TLR2 activation by lipoteichoic acid (TLR2/6 activator) or palmitoyl (3)-Cys-Ser-Lys(4)-OH (TLR2/1 activator) but not by LPS (TLR4 activator). Extensive biophysical and biochemical assays, combined with GALLEX experiments, show that these peptides heterodimerize with their complementary TMDs on their reciprocal protein. These results suggest that TLR2/6/1 TMD assembly is essential for activating this complex. Importantly, when administered to mice inflicted by TLR2, but not TLR4-driven lethal inflammation, a selected peptide rescued 60% of these septic mice, showing potent in vivo inhibition of TNF-α and IL-6 secretion. Furthermore, this peptide also showed high protection in a whole bacteria model. Owing to the importance of TLR2 regulation under a variety of pathological conditions, compounds that can fine-tune this activity are of great importance.

Human antimicrobial peptide LL-37 modulates proinflammatory responses induced by cytokine milieus and double-stranded RNA in human keratinocytes

Epidermal keratinocytes produce proinflammatory cytokines/chemokines upon stimulation with cytokine milieus and Toll-like receptor ligands, which are considered to reflect epidermal environments in inflamed skin. The human antimicrobial peptide LL-37, besides having microbicidal functions, plays multiple roles as a "host defense peptide" in the immune system. Here, we examined the effect of LL-37 on proinflammatory responses induced by double-stranded RNA (dsRNA) and cytokines in primary human keratinocytes. LL-37 inhibited dsRNA-induced production of thymic stromal lymphopoietin (TSLP), CCL5/RANTES, CXCL10/IP-10, and CXCL8/IL-8, which was attributable to interaction between LL-37 and dsRNA, although LL-37 upregulated CXCL8 expression at an earlier time point (8 h). LL-37 inhibited the increase of CXCL10 and CCL5 induced by TNF-α- and/or IFN-γ but enhanced that of CXCL8. LL-37 and Th17 cytokines (IL-17 and IL-22) synergistically upregulated the expression of CXCL8 and IL-6. LL-37 showed the effects above at a high concentration (25 μg/ml, 5.6 μM). We also examined effects of a peptide with a scrambled LL-37 sequence, which has been frequently used as a negative control, and those of another peptide with the reversed LL-37 sequence, activities of which have not been well investigated. Interestingly, the reversed LL-37 had effects similar to LL-37 but the scrambled LL-37 did not. The modulation by LL-37 of the keratinocyte proinflammatory responses induced by cytokine milieus and dsRNA suggests novel roles for LL-37 in skin inflammation such as the promotion of IL17/IL-22/IL-6-associated psoriasis and suppression of TSLP-associated atopic dermatitis.

Introduction of a lysine residue promotes aggregation of temporin L in lipopolysaccharides and augmentation of its antiendotoxin property

Temporin L (TempL) is a 13-residue frog antimicrobial peptide that shows moderate bactericidal activity and antiendotoxin properties in macrophages. We envisioned that, due to its very hydrophobic nature, the peptide might fail to show its desired biological properties. It was predicted by employing the available algorithms that the replacement of a glutamine by lysine at position 3 could appreciably reduce its aggregation propensity in an aqueous environment. In order to investigate the structural, functional, and biological consequences of replacement of glutamine by lysine at its third position, TempL and the corresponding analog, Q3K-TempL, was synthesized and characterized. Introduction of the lysine residue significantly promoted the self-assembly and oligomeric state of TempL in lipopolysaccharide (LPS). Q3K-TempL exhibited augmented binding to LPS and also dissociated LPS aggregates with greater efficacy than TempL. Further, Q3K-TempL inhibited the LPS-induced proinflammatory cytokines in rat primary macrophages in vitro and in vivo in BALB/c mice with greater efficacy than TempL. The results showed that a simple amino acid substitution in a short hydrophobic antimicrobial peptide, TempL, enhanced its antiendotoxin properties and illustrate a plausible correlation between its aggregation properties in LPS and LPS detoxification activity.

Design, structural and functional characterization of a Temporin-1b analog active against Gram-negative bacteria

BACKGROUND

Temporins are small antimicrobial peptides secreted by the Rana temporaria showing mainly activity against Gram-positive bacteria. However, different members of the temporin family, such as Temporin B, act in synergy also against Gram-negative bacteria. With the aim to develop a peptide with a wide spectrum of antimicrobial activity we designed and analyzed a series of Temporin B analogs.

METHODS

Peptides were initially obtained by Ala scanning on Temporin B sequence; antimicrobial activity tests allowed to identify the TB_G6A sequence, which was further optimized by increasing the peptide positive charge (TB_KKG6A). Interactions of this active peptide with the LPS of E. coli were investigated by CD, fluorescence and NMR.

RESULTS

TB_KKG6A is active against Gram-positive and Gram-negative bacteria at low concentrations. The peptide strongly interacts with the LPS of Gram-negative bacteria and folds upon interaction into a kinked helix.

CONCLUSION

Our results show that it is possible to widen the activity spectrum of an antimicrobial peptide by subtle changes of the primary structure. TB_KKG6A, having a simple composition, a broad spectrum of antimicrobial activity and a very low hemolytic activity, is a promising candidate for the design of novel antimicrobial peptides.

GENERAL SIGNIFICANCE

The activity of antimicrobial peptides is strongly related to the ability of the peptide to interact and break the bacterial membrane. Our studies on TB_KKG6A indicate that efficient interactions with LPS can be achieved when the peptide is not perfectly amphipathic, since this feature seems to help the toroidal pore formation process.

Modulation of proinflammatory activity by the engineered cationic antimicrobial peptide WLBU-2

BACKGROUND

Host-derived (LL-37) and synthetic (WLBU-2) cationic antimicrobial peptides (CAPs) are known for their membrane-active bactericidal properties. LL-37 is an important mediator for immunomodulation, while the mechanism of action of WLBU-2 remains unclear.

OBJECTIVE

To determine if WLBU-2 induces an early proinflammatory response that facilitates bacterial clearance in cystic fibrosis (CF).

METHODS

C57BL6 mice were given intranasal or intraperitoneal 1×10 (6) cfu/mL Pseudomonas aeruginosa (PA) and observed for 2h, followed by instillation of LL-37 or WLBU-2 (2-4mg/kg) with subsequent tissue collection at 24h for determination of bacterial colony counts and quantitative RT-PCR measurement of cytokine transcripts. CF airway epithelial cells (IB3-1, ΔF508/W1282X) were cultured in appropriate media with supplements. WLBU-2 (25μM) was added to the media with RT-PCR measurement of TNF-α and IL-1β transcripts after 20, 30, and 60min. Flow cytometry was used to determine if WLBU-2 assists in cellular uptake of Alexa 488-labeled LPS.

RESULTS

In murine lung exposed to intranasal or intraperitoneal WLBU-2, there was a reduction in the number of surviving PA colonies compared to controls. Murine lung exposed to intraperitoneal WLBU-2 showed fewer PA colonies compared to LL-37. After 24h WLBU-2 exposure, PA-induced IL-1β transcripts from lungs showed a twofold decrease (p<0.05), while TNF-α levels were unchanged. LL-37 did not significantly change transcript levels. In IB3-1 cells, WLBU-2 exposure resulted in increased TNF-α and IL-1β transcripts that decreased by 60min. WLBU-2 treatment of IB3-1 cells displayed increased LPS uptake, suggesting a potential role for CAPs in inducing protective proinflammatory responses. Taken together, the cytokine response, LPS uptake, and established antimicrobial activity of WLBU-2 demonstrate its ability to modulate proinflammatory signaling as a protective mechanism to clear infection.

CONCLUSIONS

The immunomodulatory properties of WLBU-2 reveal a potential mechanism of its broad-spectrum antibacterial activity and warrant further preclinical evaluation to study bacterial clearance and rescue of chronic inflammation.

Characterization of antimicrobial, cytotoxic, and antiendotoxin properties of short peptides with different hydrophobic amino acids at "a" and "d" positions of a heptad repeat sequence

To understand the influence of different hydrophobic amino acids at "a" and "d" positions of a heptad repeat sequence on antimicrobial, cytotoxic, and antiendotoxin properties, four 15-residue peptides with leucine (LRP), phenylalanine (FRP), valine (VRP), and alanine (ARP) residues at these positions were designed, synthesized, and characterized. Although valine is similarly hydrophobic to leucine and phenylalanine, VRP showed significantly lesser cytotoxicity than LRP and FRP; further, the replacement of leucines with valines at "a" and "d" positions of melittin-heptads drastically reduced its cytotoxicity. However, all four peptides exhibited significant antimicrobial activities that correlate well with their interactions with mammalian and bacterial cell membranes and the corresponding lipid vesicles. LRP most efficiently neutralized the LPS-induced pro-inflammatory mediators like NO, TNF-α, and IL-6 in macrophages followed by FRP, VRP, and ARP. The results could be useful for designing short antimicrobial and antiendotoxin peptides with understanding the basis of their activity.