Defensins enable macrophages to inhibit the intracellular proliferation of Listeria monocytogenes

Cutting Edge: Cytosolic Receptor AIM2 Is Induced by Peroxisome Proliferator-activated Receptor γ following Mycobacterium tuberculosis Infection of Human Macrophages but Does Not Contribute to IL-1β Release

AIM2 (absent in melanoma 2), an inflammasome component, mediates IL-1β release in murine macrophages and cell lines. AIM2 and IL-1β contribute to murine control of Mycobacterium tuberculosis (M.tb) infection, but AIM2's impact in human macrophages, the primary niche for M.tb, remains unclear. We show that M.tb, Mycobacterium bovis bacillus Calmette-Guérin (BCG), and M. smegmatis induce AIM2 expression in primary human macrophages. M.tb-induced AIM2 expression is peroxisome proliferator-activated receptor γ (PPARγ)-dependent and M.tb ESX-1-independent, whereas BCG- and M. smegmatis-induced AIM2 expression is PPARγ-independent. PPARγ and NLRP3, but not AIM2, are important for IL-1β release in response to M.tb, and NLRP3 colocalizes with M.tb. This is in contrast to the role for AIM2 in inflammasome activation in mice and peritoneal macrophages. Altogether, we show that mycobacteria induce AIM2 expression in primary human macrophages, but AIM2 does not contribute to IL-1β release during M.tb infection, providing further evidence that AIM2 expression and function are regulated in a cell- and/or species-specific manner.

Current Advances in Japanese Encephalitis Virus Drug Development

Japanese encephalitis virus (JEV) belongs to the Flaviviridae family and is a representative mosquito-borne flavivirus responsible for acute encephalitis and meningitis in humans. Despite the availability of vaccines, JEV remains a major public health threat with the potential to spread globally. According to the World Health Organization (WHO), there are an estimated 69,000 cases of JE each year, and this figure is probably an underestimate. The majority of JE victims are children in endemic areas, and almost half of the surviving patients have motor or cognitive sequelae. Thus, the absence of a clinically approved drug for the treatment of JE defines an urgent medical need. Recently, several promising and potential drug candidates were reported through drug repurposing studies, high-throughput drug library screening, and de novo design. This review focuses on the historical aspects of JEV, the biology of JEV replication, targets for therapeutic strategies, a target product profile, and drug development initiatives.

Defensins: A novel weapon against Mycobacterium tuberculosis?

Tuberculosis (TB) is a serious airborne communicable disease caused by organisms of the Mycobacterium tuberculosis (Mtb) complex. Although the standard treatment antimicrobials, including isoniazid, rifampicin, pyrazinamide, and ethambutol, have made great progress in the treatment of TB, problems including the rising incidence of multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB), the severe toxicity and side effects of antimicrobials, and the low immunity of TB patients have become the bottlenecks of the current TB treatments. Therefore, both safe and effective new strategies to prevent and treat TB have become a top priority. As a subfamily of cationic antimicrobial peptides, defensins are rich in cysteine and play a vital role in resisting the invasion of microorganisms and regulating the immune response. Inspired by studies on the roles of defensins in host defence, we describe their research history and then review their structural features and antimicrobial mechanisms, specifically for fighting Mtb in detail. Finally, we discuss the clinical relevance, therapeutic potential, and potential challenges of defensins in anti-TB therapy. We further debate the possible solutions of the current application of defensins to provide new insights for eliminating Mtb.

Combination of MCL-1 and BCL-2 inhibitors is a promising approach for a host-directed therapy for tuberculosis

Tuberculosis (TB) accounts for 1.6 million deaths annually and over 25% of deaths due to antimicrobial resistance. Mycobacterium tuberculosis (M.tb) drives MCL-1 expression (family member of anti-apoptotic BCL-2 proteins) to limit apoptosis and grow intracellularly in human macrophages. The feasibility of re-purposing specific MCL-1 and BCL-2 inhibitors to limit M.tb growth, using inhibitors that are in clinical trials and FDA-approved for cancer treatment has not be tested previously. We show that specifically inhibiting MCL-1 and BCL-2 induces apoptosis of M.tb-infected macrophages, and markedly reduces M.tb growth in human and murine macrophages, and in a pre-clinical model of human granulomas. MCL-1 and BCL-2 inhibitors limit growth of drug resistant and susceptible M.tb in macrophages and act in additive fashion with the antibiotics isoniazid and rifampicin. This exciting work uncovers targeting the intrinsic apoptosis pathway as a promising approach for TB host-directed therapy. Since safety and activity studies are underway in cancer clinics for MCL-1 and BCL-2 inhibitors, we expect that re-purposing them for TB treatment should translate more readily and rapidly to the clinic. Thus, the work supports further development of this host-directed therapy approach to augment current TB treatment.

Inhibition of Pertussis Toxin by Human α-Defensins-1 and -5: Differential Mechanisms of Action

Whooping cough is a severe childhood disease, caused by the bacterium Bordetella pertussis, which releases pertussis toxin (PT) as a major virulence factor. Previously, we identified the human antimicrobial peptides α-defensin-1 and -5 as inhibitors of PT and demonstrated their capacity to inhibit the activity of the PT enzyme subunit PTS1. Here, the underlying mechanism of toxin inhibition was investigated in more detail, which is essential for developing the therapeutic potential of these peptides. Flow cytometry and immunocytochemistry revealed that α-defensin-5 strongly reduced PT binding to, and uptake into cells, whereas α-defensin-1 caused only a mild reduction. Conversely, α-defensin-1, but not α-defensin-5 was taken up into different cell lines and interacted with PTS1 inside cells, based on proximity ligation assay. In-silico modeling revealed specific interaction interfaces for α-defensin-1 with PTS1 and vice versa, unlike α-defensin-5. Dot blot experiments showed that α-defensin-1 binds to PTS1 and even stronger to its substrate protein Gαi in vitro. NADase activity of PTS1 in vitro was not inhibited by α-defensin-1 in the absence of Gαi. Taken together, these results suggest that α-defensin-1 inhibits PT mainly by inhibiting enzyme activity of PTS1, whereas α-defensin-5 mainly inhibits cellular uptake of PT. These findings will pave the way for optimization of α-defensins as novel therapeutics against whooping cough.

Cell-Penetrating Antimicrobial Peptides with Anti-Infective Activity against Intracellular Pathogens

Cell-penetrating peptides (CPPs) are natural or engineered peptide sequences with the intrinsic ability to internalize into a diversity of cell types and simultaneously transport hydrophilic molecules and nanomaterials, of which the cellular uptake is often limited. In addition to this primordial activity of cell penetration without membrane disruption, multivalent antimicrobial activity accompanies some CPPs. Antimicrobial peptides (AMPs) with cell-penetrability exert their effect intracellularly, and they are of great interest. CPPs with antimicrobial activity (CPAPs) comprise a particular class of bioactive peptides that arise as promising agents against difficult-to-treat intracellular infections. This short review aims to present the antibacterial, antiparasitic, and antiviral effects of various cell-penetrating antimicrobial peptides currently documented. Examples include the antimicrobial effects of different CPAPs against bacteria that can propagate intracellularly, like Staphylococcus sp., Streptococcus sp., Chlamydia trachomatis, Escherichia coli, Mycobacterium sp., Listeria sp., Salmonella sp. among others. CPAPs with antiviral effects that interfere with the intracellular replication of HIV, hepatitis B, HPV, and herpes virus. Additionally, CPAPs with activity against protozoa of the genera Leishmania, Trypanosoma, and Plasmodium, the etiological agents of Leishmaniasis, Chagas' Disease, and Malaria, respectively. The information provided in this review emphasizes the potential of multivalent CPAPs, with anti-infective properties for application against various intracellular infections. So far, CPAPs bear a promise of druggability for the translational medical use of CPPs alone or in combination with chemotherapeutics. Moreover, CPAPs could be an exciting alternative for pharmaceutical design and treating intracellular infectious diseases.

Antimicrobial peptides: Defending the mucosal epithelial barrier

The recent epidemic caused by aerosolized SARS-CoV-2 virus illustrates the importance and vulnerability of the mucosal epithelial barrier against infection. Antimicrobial proteins and peptides (AMPs) are key to the epithelial barrier, providing immunity against microbes. In primitive life forms, AMPs protect the integument and the gut against pathogenic microbes. AMPs have also evolved in humans and other mammals to enhance newer, complex innate and adaptive immunity to favor the persistence of commensals over pathogenic microbes. The canonical AMPs are helictical peptides that form lethal pores in microbial membranes. In higher life forms, this type of AMP is exemplified by the defensin family of AMPs. In epithelial tissues, defensins, and calprotectin (complex of S100A8 and S100A9) have evolved to work cooperatively. The mechanisms of action differ. Unlike defensins, calprotectin sequesters essential trace metals from microbes, which inhibits growth. This review focuses on defensins and calprotectin as AMPs that appear to work cooperatively to fortify the epithelial barrier against infection. The antimicrobial spectrum is broad with overlap between the two AMPs. In mice, experimental models highlight the contribution of both AMPs to candidiasis as a fungal infection and periodontitis resulting from bacterial dysbiosis. These AMPs appear to contribute to innate immunity in humans, protecting the commensal microflora and restricting the emergence of pathobionts and pathogens. A striking example in human innate immunity is that elevated serum calprotectin protects against neonatal sepsis. Calprotectin is also remarkable because of functional differences when localized in epithelial and neutrophil cytoplasm or released into the extracellular environment. In the cytoplasm, calprotectin appears to protect against invasive pathogens. Extracellularly, calprotectin can engage pathogen-recognition receptors to activate innate immune and proinflammatory mechanisms. In inflamed epithelial and other tissue spaces, calprotectin, DNA, and histones are released from degranulated neutrophils to form insoluble antimicrobial barriers termed neutrophil extracellular traps. Hence, calprotectin and other AMPs use several strategies to provide microbial control and stimulate innate immunity.

Amentoflavone attenuates Listeria monocytogenes pathogenicity through an LLO-dependent mechanism

Background and purpose L. monocytogenes remain a leading cause of foodborne infection. Listeriolysin O (LLO), an indispensable virulence determinant involved in diverse pathogenic mechanisms of L. monocytogenes infection, represents a promising therapeutic target. In this study, we sought to identify an effective inhibitor of LLO pore formation and its mechanism of action in the treatment of L. monocytogenes infection. Experimental approach Hemolysis assay was carried out to screen an effective LLO inhibitor. The interaction between candidate and LLO was investigated using SPR assay and MD. The effect of candidate on LLO-mediated cytotoxicity, barrier disruption and immune response were investigated. Finally, the in vivo effect of candidate on mice challenged with L. monocytogenes was examined. Key results Amentoflavone, a natural flavone broadly present in traditional Chinese herbs, effectively inhibited LLO pore formation by engaging the residues Lys93, Asp416, Tyr469 and Lys505 in LLO. Amentoflavone dose-dependently reduced L. monocytogenes-induced cell injury in an LLO-dependent manner. In the Caco-2 monolayer model, amentoflavone maintained the integrity of the epithelial barrier exposed to LLO. Additionally, amentoflavone inhibited the inflammatory response evoked by L. monocytogenes in an LLO-dependent manner, and such inhibition was attributed to its ability to block perforation-associated K⁺ efflux and Ca²⁺ influx. In mice infection model, amentoflavone treatment significantly reduced bacterial burdens and pathological lesions in target organs, with a significant increase in the survival rate. Conclusions and implications Amentoflavone reduced the pathogenicity of L. monocytogenes by specifically inhibiting LLO pore formation, and this natural compound may present a potential candidate treatment for L. monocytogenes infection.

Defensins: The natural peptide antibiotic

Defensins are a family of cationic antimicrobial peptides active against a broad range of infectious microbes including bacteria, viruses and fungi, playing important roles as innate effectors and immune modulators in immunological control of microbial infection. Their antibacterial properties and unique mechanisms of action have garnered considerable interest in developing defensins into a novel class of natural antibiotic peptides to fend off pathogenic infection by bacteria, particularly those resistant to conventional antibiotics. However, serious pharmacological and technical obstacles, some of which are unique to defensins and others are common to peptide drugs in general, have hindered the development and clinical translation of defensins as anti-infective therapeutics. To overcome them, several technologies have been developed, aiming for improved functionality, prolonged circulation time, enhanced proteolytic stability and bioavailability, and efficient and controlled delivery and release of defensins to the site of infection. Additional challenges include the alleviation of potential toxicity of defensins and their cost-effective manufacturing. In this review, we briefly introduce defensin biology, focus on various transforming strategies and practical techniques developed for defensins and their derivatives as antibacterial therapeutics, and conclude with a summation of future challenges and possible solutions.

Mechanism through Which Retrocyclin Targets Flavivirus Multiplication

Currently, there are no approved drugs for the treatment of flavivirus infection. Accordingly, we tested the inhibitory effects of the novel θ-defensin retrocyclin-101 (RC-101) against flavivirus infection and investigated the mechanism underlying the potential inhibitory effects. First, RC-101 robustly inhibited both Japanese encephalitis virus (JEV) and Zika virus (ZIKV) infections. RC-101 exerted inhibitory effects on the entry and replication stages. Results also indicated that the nonstructural protein NS2B-NS3 serine protease might serve as a potential viral target. Furthermore, RC-101 inhibited protease activity at the micromolar level. We also demonstrated that with respect to the glycoprotein E protein of flavivirus, the DE loop of domain III (DIII), which is the receptor-binding domain of the E protein, might serve as another viral target of RC-101. Moreover, a JEV DE mutant exhibited resistance to RC-101, which was associated with deceased binding affinity of RC-101 to DIII. These findings provide a basis for the development of RC-101 as a potential candidate for the treatment of flavivirus infection. IMPORTANCE Retrocyclin is an artificially humanized circular θ-defensin peptide, containing 18 residues, previously reported to possess broad antimicrobial activity. In this study, we found that retrocyclin-101 inhibited flavivirus (ZIKV and JEV) infections. Retrocyclin-101 inhibited NS2B-NS3 serine protease activity, suggesting that the catalytic triad of the protease is the target. Moreover, retrocyclin-101 bound to the DE loop of the E protein of flavivirus, which prevented its entry.

Antilisterial Potential of Lactic Acid Bacteria in Eliminating Listeria monocytogenes in Host and Ready-to-Eat Food Application

Listeriosis is a severe food borne disease with a mortality rate of up to 30% caused by pathogenic Listeria monocytogenes via the production of several virulence factors including listeriolysin O (LLO), transcriptional activator (PrfA), actin (Act), internalin (Int), etc. It is a foodborne disease predominantly causing infections through consumption of contaminated food and is often associated with ready-to-eat food (RTE) and dairy products. Common medication for listeriosis such as antibiotics might cause an eagle effect and antibiotic resistance if it is overused. Therefore, exploration of the use of lactic acid bacteria (LAB) with probiotic characteristics and multiple antimicrobial properties is increasingly getting attention for their capability to treat listeriosis, vaccine development, and hurdle technologies. The antilisterial gene, a gene coding to produce antimicrobial peptide (AMP), one of the inhibitory substances found in LAB, is one of the potential key factors in listeriosis treatment, coupled with the vast array of functions and strategies; this review summarizes the various strategies by LAB against L. monocytogenes and the prospect in development of a ‘generally regarded as safe’ LAB for treatment of listeriosis.

Antimicrobial Peptides and Cell-Penetrating Peptides for Treating Intracellular Bacterial Infections

Bacterial infections caused by intracellular pathogens are difficult to control. Conventional antibiotic therapies are often ineffective, as high doses are needed to increase the number of antibiotics that will cross the host cell membrane to act on the intracellular bacterium. Moreover, higher doses of antibiotics may lead to elevated severe toxic effects against host cells. In this context, antimicrobial peptides (AMPs) and cell-penetrating peptides (CPPs) have shown great potential to treat such infections by acting directly on the intracellular pathogenic bacterium or performing the delivery of cargos with antibacterial activities. Therefore, in this mini-review, we cover the main AMPs and CPPs described to date, aiming at intracellular bacterial infection treatment. Moreover, we discuss some of the proposed mechanisms of action for these peptide classes and their conjugation with other antimicrobials.

Defensins: A Double-Edged Sword in Host Immunity

Defensins are a major family of host defense peptides expressed predominantly in neutrophils and epithelial cells. Their broad antimicrobial activities and multifaceted immunomodulatory functions have been extensively studied, cementing their role in innate immunity as a core host-protective component against bacterial, viral and fungal infections. More recent studies, however, paint defensins in a bad light such that they are "alleged" to promote viral and bacterial infections in certain biological settings. This mini review summarizes the latest findings on the potential pathogenic properties of defensins against the backdrop of their protective roles in antiviral and antibacterial immunity. Further, a succinct description of both tumor-proliferative and -suppressive activities of defensins is also given to highlight their functional and mechanistic complexity in antitumor immunity. We posit that given an enabling environment defensins, widely heralded as the "Swiss army knife," can function as a "double-edged sword" in host immunity.

Innate and Adaptive Immune Responses during Listeria monocytogenes Infection

It could be argued that we understand the immune response to infection with Listeria monocytogenes better than the immunity elicited by any other bacteria. L. monocytogenes are Gram-positive bacteria that are genetically tractable and easy to cultivate in vitro, and the mouse model of intravenous (i.v.) inoculation is highly reproducible. For these reasons, immunologists frequently use the mouse model of systemic listeriosis to dissect the mechanisms used by mammalian hosts to recognize and respond to infection. This article provides an overview of what we have learned over the past few decades and is divided into three sections: "Innate Immunity" describes how the host initially detects the presence of L. monocytogenes and characterizes the soluble and cellular responses that occur during the first few days postinfection; "Adaptive Immunity" discusses the exquisitely specific T cell response that mediates complete clearance of infection and immunological memory; "Use of Attenuated Listeria as a Vaccine Vector" highlights the ways that investigators have exploited our extensive knowledge of anti-Listeria immunity to develop cancer therapeutics.

Human Enteric Defensin 5 Promotes Shigella Infection of Macrophages

Human α-defensins are 3- to 5-kDa disulfide-bridged peptides with a multitude of antimicrobial activities and immunomodulatory functions. Recent studies show that human enteric α-defensin 5 (HD5), a host defense peptide important for intestinal homeostasis and innate immunity, aids the highly infectious enteropathogen Shigella in breaching the intestinal epithelium in vitro and in vivo Whether and how HD5 influences Shigella infection of resident macrophages following its invasion of the intestinal epithelium remain poorly understood. Here, we report that HD5 greatly promoted phagocytosis of Shigella by macrophages by targeting the bacteria to enhance bacterium-to-cell contacts in a structure- and sequence-dependent fashion. Subsequent intracellular multiplication of phagocytosed Shigella led to massive necrotic cell death and release of the bacteria. HD5-promoted phagocytosis of Shigella was independent of the status of the type 3 secretion system. Furthermore, HD5 neither inhibited nor enhanced phagosomal escape of Shigella Collectively, these findings confirm a potential pathogenic role of HD5 in Shigella infection of not only epithelial cells but also macrophages, illuminating how an enteropathogen exploits a host protective factor for virulence and infection.

Distinctive Roles and Mechanisms of Human Neutrophil Peptides in Experimental Sepsis and Acute Respiratory Distress Syndrome

OBJECTIVES

To examine the effects and mechanisms of human neutrophil peptides in systemic infection and noninfectious inflammatory lung injury.

DESIGN

Prospective experimental study.

SETTING

University hospital-based research laboratory.

SUBJECTS

In vitro human cells and in vivo mouse models.

INTERVENTIONS

Wild-type (Friend virus B-type) and conditional leukocyte human neutrophil peptides transgenic mice were subjected to either sepsis induced by cecal ligation and puncture or acute lung injury by intratracheal instillation of hydrochloric acid followed by mechanical ventilation. Using human neutrophil peptides as bait, the basal cell adhesion molecule (CD239) and the purinergic P2Y purinoceptor 6 receptor were identified as the putative human neutrophil peptides receptor complex in human lung epithelial cells.

MEASUREMENTS AND MAIN RESULTS

In the cecal ligation and puncture sepsis model, Friend virus B-type mice exhibited higher systemic bacterial load, cytokine production, and lung injury than human neutrophil peptides transgenic mice. Conversely, an increased lung cytokine production was seen in Friend virus B-type mice, which was further enhanced in human neutrophil peptides transgenic mice in response to two-hit lung injury induced by hydrochloric acid and mechanical ventilation. The human neutrophil peptides-mediated inflammatory response was mediated through the basal cell adhesion molecule-P2Y purinoceptor 6 receptor signal pathway in human lung epithelial cells.

CONCLUSIONS

Human neutrophil peptides are critical in host defense against infectious sepsis by their cationic antimicrobial properties but may exacerbate tissue injury when neutrophil-mediated inflammatory responses are excessive in noninfectious lung injury. Targeting the basal cell adhesion molecule/P2Y purinoceptor 6 signaling pathway may serve as a novel approach to attenuate the neutrophil-mediated inflammatory responses and injury while maintaining the antimicrobial function of human neutrophil peptides in critical illness.

Human Antimicrobial RNases Inhibit Intracellular Bacterial Growth and Induce Autophagy in Mycobacteria-Infected Macrophages

The development of novel treatment against tuberculosis is a priority global health challenge. Antimicrobial proteins and peptides offer a multifaceted mechanism suitable to fight bacterial resistance. Within the RNaseA superfamily there is a group of highly cationic proteins secreted by innate immune cells with anti-infective and immune-regulatory properties. In this work, we have tested the human canonical members of the RNase family using a spot-culture growth inhibition assay based mycobacteria-infected macrophage model for evaluating their anti-tubercular properties. Out of the seven tested recombinant human RNases, we have identified two members, RNase3 and RNase6, which were highly effective against Mycobacterium aurum extra- and intracellularly and induced an autophagy process. We observed the proteins internalization within macrophages and their capacity to eradicate the intracellular mycobacterial infection at a low micro-molar range. Contribution of the enzymatic activity was discarded by site-directed mutagenesis at the RNase catalytic site. The protein induction of autophagy was analyzed by RT-qPCR, western blot, immunofluorescence, and electron microscopy. Specific blockage of auto-phagosome formation and maturation reduced the protein's ability to eradicate the infection. In addition, we found that the M. aurum infection of human THP1 macrophages modulates the expression of endogenous RNase3 and RNase6, suggesting a function in vivo. Overall, our data anticipate a biological role for human antimicrobial RNases in host response to mycobacterial infections and set the basis for the design of novel anti-tubercular drugs.

Increased gene copy number of DEFA1/DEFA3 worsens sepsis by inducing endothelial pyroptosis

Sepsis claims an estimated 30 million episodes and 6 million deaths per year, and treatment options are rather limited. Human neutrophil peptides 1-3 (HNP1-3) are the most abundant neutrophil granule proteins but their neutrophil content varies because of unusually extensive gene copy number polymorphism. A genetic association study found that increased copy number of the HNP-encoding gene DEFA1/DEFA3 is a risk factor for organ dysfunction during sepsis development. However, direct experimental evidence demonstrating that these risk alleles are pathogenic for sepsis is lacking because the genes are present only in some primates and humans. Here, we generate DEFA1/DEFA3 transgenic mice with neutrophil-specific expression of the peptides. We show that mice with high copy number of DEFA1/DEFA3 genes have more severe sepsis-related vital organ damage and mortality than mice with low copy number of DEFA1/DEFA3 or wild-type mice, resulting from more severe endothelial barrier dysfunction and endothelial cell pyroptosis after sepsis challenge. Mechanistically, HNP-1 induces endothelial cell pyroptosis via P2X7 receptor-mediating canonical caspase-1 activation in a NLRP3 inflammasome-dependent manner. Based on these findings, we engineered a monoclonal antibody against HNP-1 to block the interaction with P2X7 and found that the blocking antibody protected mice carrying high copy number of DEFA1/DEFA3 from lethal sepsis. We thus demonstrate that DEFA1/DEFA3 copy number variation strongly modulates sepsis development in vivo and explore a paradigm for the precision treatment of sepsis tailored by individual genetic information.

Plasma levels of alarmin HNPs 1-3 associate with lung dysfunction after cardiac surgery in children

Background

Early onset of lung injury is considerable common after cardiac surgery and is associated with increasing in morbidity and mortality, but current clinical predictors for the occurrence of this complication always have limited positive warning value. This study aimed to evaluate whether elevated plasma levels of human neutrophil peptides (HNPs) 1–3 herald impaired lung function in infants and young children after cardiac surgery necessitating cardiopulmonary bypass (CPB).

Methods

Consecutive children younger than 3 years old who underwent cardiac surgery were prospectively enrolled. Plasma concentrations of HNPs 1–3 and inflammatory cytokines were measured before, and immediately after CPB, as well as at 1 h, 12 h, and 24 h after CPB.

Results

Thirty patients were enrolled, 18 (60%) of whom were infants. Plasma levels of HNPs 1–3 and the pro-inflammatory cytokine interleukin-6 (IL-6) significantly increased immediately after CPB (P < 0.001), while IL-8 increased 1 h after the CPB operation (P = 0.002). The anti-inflammatory cytokine IL-10 levels were also significantly elevated immediately after CPB compared with the baseline (P < 0.001). The stepwise multiple linear regression analysis showed that the plasma HNPs 1–3 levels immediately after CPB was independent correlated with the declined lung function, as reflected by the PaO₂/FiO₂ ratio on the first 2 days after operation (for the first day: OR, −1.067, 95% CI, −0.548 to −1.574; P < 0.001; for the second day: OR, −0.667, 95% CI, −0.183 to −1.148; P = 0.009) and prolonged mechanical ventilation time (OR, 0.039, 95% CI, 0.005 to 0.056; P = 0.011). Plasma levels of HNPs 1–3 and IL-10 returned to the baseline values, while IL-6 and IL-8 levels remained significantly higher than baseline 24 h after CPB (P ≤ 0.01).

Conclusions

Elevated HNPs 1–3 levels immediately after CPB correlate with impaired lung function, and HNPs 1–3 could serve as a quantifiable early alarmin biomarker for onset of lung injury in infants and young children undergoing cardiac surgery with CPB.

Electronic supplementary material

The online version of this article (10.1186/s12890-017-0558-4) contains supplementary material, which is available to authorized users.

Targeting and inactivation of bacterial toxins by human defensins

Defensins, as a prominent family of antimicrobial peptides (AMP), are major effectors of the innate immunity with a broad range of immune modulatory and antimicrobial activities. In particular, defensins are the only recognized fast-response molecules that can neutralize a broad range of bacterial toxins, many of which are among the deadliest compounds on the planet. For a decade, the mystery of how a small and structurally conserved group of peptides can neutralize a heterogeneous group of toxins with little to no sequential and structural similarity remained unresolved. Recently, it was found that defensins recognize and target structural plasticity/thermodynamic instability, fundamental physicochemical properties that unite many bacterial toxins and distinguish them from the majority of host proteins. Binding of human defensins promotes local unfolding of the affected toxins, destabilizes their secondary and tertiary structures, increases susceptibility to proteolysis, and leads to their precipitation. While the details of toxin destabilization by defensins remain obscure, here we briefly review properties and activities of bacterial toxins known to be affected by or resilient to defensins, and discuss how recognized features of defensins correlate with the observed inactivation.

Immuno-Stimulatory Peptides as a Potential Adjunct Therapy against Intra-Macrophagic Pathogens

The treatment of infectious diseases is increasingly prone to failure due to the rapid spread of antibiotic-resistant pathogens. Antimicrobial peptides (AMPs) are natural components of the innate immune system of most living organisms. Their capacity to kill microbes through multiple mechanisms makes the development of bacterial resistance less likely. Additionally, AMPs have important immunomodulatory effects, which critically contribute to their role in host defense. In this paper, we review the most recent evidence for the importance of AMPs in host defense against intracellular pathogens, particularly intra-macrophagic pathogens, such as mycobacteria. Cathelicidins and defensins are reviewed in more detail, due to the abundance of studies on these molecules. The cell-intrinsic as well as the systemic immune-related effects of the different AMPs are discussed. In the face of the strong potential emerging from the reviewed studies, the prospects for future use of AMPs as part of the therapeutic armamentarium against infectious diseases are presented.

The θ-defensin retrocyclin 101 inhibits TLR4- and TLR2-dependent signaling and protects mice against influenza infection

A member of the θ‐defensin family protects mice during infection with influenza, suggesting a new strategy for viral therapy in humans.

Despite widespread use of annual influenza vaccines, seasonal influenza‐associated deaths number in the thousands each year, in part because of exacerbating bacterial superinfections. Therefore, discovering additional therapeutic options would be a valuable aid to public health. Recently, TLR4 inhibition has emerged as a possible mechanism for protection against influenza‐associated lethality and acute lung injury. Based on recent data showing that rhesus macaque θ‐defensins could inhibit TLR4‐dependent gene expression, we tested the hypothesis that a novel θ‐defensin, retrocyclin (RC)‐101, could disrupt TLR4‐dependent signaling and protect against viral infection. In this study, RC‐101, a variant of the humanized θ‐defensin RC‐1, blocked TLR4‐mediated gene expression in mouse and human macrophages in response to LPS, targeting both MyD88‐ and TRIF‐dependent pathways. In a cell‐free assay, RC‐101 neutralized the biologic activity of LPS at doses ranging from 0.5 to 50 EU/ml, consistent with data showing that RC‐101 binds biotinylated LPS. The action of RC‐101 was not limited to the TLR4 pathway because RC‐101 treatment of macrophages also inhibited gene expression in response to a TLR2 agonist, Pam3CSK4, but failed to bind that biotinylated agonist. Mouse macrophages infected in vitro with mouse‐adapted A/PR/8/34 influenza A virus (PR8) also produced lower levels of proinflammatory cytokine gene products in a TLR4‐independent fashion when treated with RC‐101. Finally, RC‐101 decreased both the lethality and clinical severity associated with PR8 infection in mice. Cumulatively, our data demonstrate that RC‐101 exhibits therapeutic potential for the mitigation of influenza‐related morbidity and mortality, potentially acting through TLR‐dependent and TLR‐independent mechanisms.

Interactions of fungal pathogens with phagocytes

The surveillance and elimination of fungal pathogens rely heavily on the sentinel behaviour of phagocytic cells of the innate immune system, especially macrophages and neutrophils. The efficiency by which these cells recognize, uptake and kill fungal pathogens depends on the size, shape and composition of the fungal cells and the success or failure of various fungal mechanisms of immune evasion. In this Review, we describe how fungi, particularly Candida albicans, interact with phagocytic cells and discuss the many factors that contribute to fungal immune evasion and prevent host elimination of these pathogenic microorganisms.

Retrocyclins neutralize bacterial toxins by potentiating their unfolding

Defensins are a class of immune peptides with a broad range of activities against bacterial, fungal and viral pathogens. Besides exerting direct anti-microbial activity via dis-organization of bacterial membranes, defensins are also able to neutralize various unrelated bacterial toxins. Recently, we have demonstrated that in the case of human α- and β-defensins, this later ability is achieved through exploiting toxins' marginal thermodynamic stability, i.e. defensins act as molecular anti-chaperones unfolding toxin molecules and exposing their hydrophobic regions and thus promoting toxin precipitation and inactivation [Kudryashova et al. (2014) Immunity 41, 709-721]. Retrocyclins (RCs) are humanized synthetic θ-defensin peptides that possess unique cyclic structure, differentiating them from α- and β-defensins. Importantly, RCs are more potent against some bacterial and viral pathogens and more stable than their linear counterparts. However, the mechanism of bacterial toxin inactivation by RCs is not known. In the present study, we demonstrate that RCs facilitate unfolding of bacterial toxins. Using differential scanning fluorimetry (DSF), limited proteolysis and collisional quenching of internal tryptophan fluorescence, we show that hydrophobic regions of toxins normally buried in the molecule interior become more exposed to solvents and accessible to proteolytic cleavage in the presence of RCs. The RC-induced unfolding of toxins led to their precipitation and abrogated activity. Toxin inactivation by RCs was strongly diminished under reducing conditions, but preserved at physiological salt and serum concentrations. Therefore, despite significant structural diversity, α-, β- and θ-defensins employ similar mechanisms of toxin inactivation, which may be shared by anti-microbial peptides from other families.

Human defensins facilitate local unfolding of thermodynamically unstable regions of bacterial protein toxins

Defensins are short cationic, amphiphilic, cysteine-rich peptides that constitute the front-line immune defense against various pathogens. In addition to exerting direct antibacterial activities, defensins inactivate several classes of unrelated bacterial exotoxins. To date, no coherent mechanism has been proposed to explain defensins' enigmatic efficiency toward various toxins. In this study, we showed that binding of neutrophil ?-defensin HNP1 to affected bacterial toxins caused their local unfolding, potentiated their thermal melting and precipitation, exposed new regions for proteolysis, and increased susceptibility to collisional quenchers without causing similar effects on tested mammalian structural and enzymatic proteins. Enteric ?-defensin HD5 and ?-defensin hBD2 shared similar toxin-unfolding effects with HNP1, albeit to different degrees. We propose that protein susceptibility to inactivation by defensins is contingent to their thermolability and conformational plasticity and that defensin-induced unfolding is a key element in the general mechanism of toxin inactivation by human defensins.

Human cathelicidin LL-37 and its derivative IG-19 regulate interleukin-32-induced inflammation

Human cathelicidin LL-37 protects against infections and endotoxin-induced inflammation. In a recent study we have shown that IG-19, an LL-37-derived peptide, protects in a murine model of arthritis. Cytokine interleukin-32 (IL-32) is elevated and directly associated with the disease severity of inflammatory arthritis. Therefore, in this study we examined the effects of LL-37 and IG-19 on IL-32-induced responses in human peripheral blood-derived mononuclear cells (PBMC) and macrophages. We showed that CD14(+) monocytes are the primary cells that produce pro-inflammatory tumour necrosis factor-α (TNF-α) following stimulation of PBMC with IL-32. We demonstrated that LL-37 and IG-19 significantly suppress IL-32-induced production of pro-inflammatory cytokines, e.g. TNF-α and IL-1β, without altering chemokine production. In contrast, LL-37 and IG-19 enhance the production of the anti-inflammatory cytokine IL-1RA. Further mechanistic studies revealed that LL-37 and IG-19 suppress IL-32-mediated phosphorylation of Fyn (Y420) Src kinase. In contrast, IL-32-mediated phosphorylation of AKT-1 (T308) and MKP-1 (S359) is not suppressed by the peptides. LL-37 and IG-19 alone induce the phosphorylation of MKP-1 (S359), which is a known negative regulator of inflammation. Furthermore, the peptides induce the activity of p44/42 mitogen-activated protein kinase, which is known to phosphorylate MKP-1 (S359). This is the first study to demonstrate the regulation of IL-32-induced inflammation by LL-37 and its derivative peptide IG-19. The mechanistic results from this study suggest that regulation of immune-mediated inflammation by these peptides may be controlled by the dual phosphatase MKP-1. We speculate that LL-37 and its derivatives may contribute to the control of immune-mediated inflammatory diseases.

The chemistry and biology of theta defensins

Cyclic peptides are found in a diverse range of organisms and are characterized by their stability and role in defense. Why is only one class of cyclic peptides found in mammals? Possibly we have not looked hard enough for them, or the technologies needed to identify them are not fully developed. We also do not yet understand their intriguing biosynthesis from two separate gene products. Addressing these challenges will require the application of chemical tools and insights from other classes of cyclic peptides. Herein, we highlight recent developments in the characterization of theta defensins and describe the important role that chemistry has played in delineating their modes of action. Furthermore, we emphasize the potential of theta defensins as antimicrobial agents and scaffolds for peptide drug design.

LysPGS formation in Listeria monocytogenes has broad roles in maintaining membrane integrity beyond antimicrobial peptide resistance

Listeria monocytogenes is an intracellular, foodborne gastrointestinal pathogen that is primarily responsible for causing listeriosis or food poisoning in otherwise healthy individuals. Infections that arise during pregnancy or within immune compromised individuals are much more serious resulting in the risk of fetal termination or fetal fatality postpartum in the former and septicemia or meningitis with a 20% fatality rate in the latter. While the roles of internalin proteins and listeriolysin-O in the infection process are well characterized, the specific roles of lysine-modified phospholipids in the membrane of L. monocytogenes are not. Investigation into the lipid bilayer composition of L. monocytogenes indicated that the overall proportions of lipids, including lysylcardiolipin and lysylphosphatidylglycerol (LysPG), vary with growth temperature and growth phase. In addition, we demonstrate that LysPG formation is essential for L. monocytogenes survival in the presence of increased osmolytic stress but has no effect on bacterial adherence, invasion or survival in the presence of physiologically relevant concentrations of human neutrophil peptide (HNP-1). In the absence of LysPG synthesis, L. monocytogenes unexpectedly retained flagellum-mediated motility at 37 °C. Taken together, these findings show that LysPG formation in L. monocytogenes has broader functions in virulence and survival beyond its known role in the modification of membrane potential previously observed in other bacteria.

Antimicrobial proteins in intestine and inflammatory bowel diseases

Mucosal surface of the intestinal tract is continuously exposed to a large number of microorganisms. To manage the substantial microbial exposure, epithelial surfaces produce a diverse arsenal of antimicrobial proteins (AMPs) that directly kill or inhibit the growth of microorganisms. Thus, AMPs are important components of innate immunity in the gut mucosa. They are frequently expressed in response to colonic inflammation and infection. Expression of many AMPs, including human β-defensin 2-4 and cathelicidin, is induced in response to invasion of pathogens or enteric microbiota into the mucosal barrier. In contrast, some AMPs, including human α-defensin 5-6 and human β-defensin 1, are constitutively expressed without microbial contact or invasion. In addition, specific AMPs are reported to be associated with inflammatory bowel disease (IBD) due to altered expression of AMPs or development of autoantibodies against AMPs. The advanced knowledge for AMPs expression in IBD can lead to its potential use as biomarkers for disease activity. Although the administration of exogenous AMPs as therapeutic strategies against IBD is still at an early stage of development, augmented induction of endogenous AMPs may be another interesting future research direction for the protective and therapeutic purposes. This review discusses new advances in our understanding of how intestinal AMPs protect against pathogens and contribute to pathophysiology of IBD.

Multifaceted activity of listeriolysin O, the cholesterol-dependent cytolysin of Listeria monocytogenes

The cholesterol-dependent cytolysins (CDCs) are a large family of pore-forming toxins that are produced by numerous Gram-positive bacterial pathogens. These toxins are released in the extracellular environment as water-soluble monomers or dimers that bind to cholesterol-rich membranes and assemble into large pore complexes. Depending upon their concentration, the nature of the host cell and membrane (cytoplasmic or intracellular) they target, the CDCs can elicit many different cellular responses. Among the CDCs, listeriolysin O (LLO), which is a major virulence factor of the facultative intracellular pathogen Listeria monocytogenes, is involved in several stages of the intracellular lifecycle of the bacterium and displays unique characteristics. It has long been known that following L. monocytogenes internalization into host cells, LLO disrupts the internalization vacuole, enabling the bacterium to replicate into the host cell cytosol. LLO is then used by cytosolic bacteria to spread from cell to cell, avoiding bacterial exposure to the extracellular environment. Although LLO is continuously produced during the intracellular lifecycle of L. monocytogenes, several processes limit its toxicity to ensure the survival of infected cells. It was previously thought that LLO activity was limited to mediating vacuolar escape during bacterial entry and cell to cell spreading. This concept has been challenged by compelling evidence suggesting that LLO secreted by extracellular L. monocytogenes perforates the host cell plasma membrane, triggering important host cell responses. This chapter provides an overview of the well-established intracellular activity of LLO and the multiple roles attributed to LLO secreted by extracellular L. monocytogenes.

The pore-forming toxin listeriolysin O is degraded by neutrophil metalloproteinase-8 and fails to mediate Listeria monocytogenes intracellular survival in neutrophils

The pore-forming toxin listeriolysin O (LLO) is a major virulence factor secreted by the facultative intracellular pathogen Listeria monocytogenes. This toxin facilitates L. monocytogenes intracellular survival in macrophages and diverse nonphagocytic cells by disrupting the internalization vesicle, releasing the bacterium into its replicative niche, the cytosol. Neutrophils are innate immune cells that play an important role in the control of infections, yet it was unknown if LLO could confer a survival advantage to L. monocytogenes in neutrophils. We report that LLO can enhance the phagocytic efficiency of human neutrophils and is unable to protect L. monocytogenes from intracellular killing. To explain the absence of L. monocytogenes survival in neutrophils, we hypothesized that neutrophil degranulation leads to the release of LLO-neutralizing molecules in the forming phagosome. In support of this, L. monocytogenes is a potent inducer of neutrophil degranulation, since its virulence factors, such as LLO, facilitate granule exocytosis. Within the first few minutes of interaction with L. monocytogenes, granules can fuse with the plasma membrane at the bacterial interaction site before closure of the phagosome. Furthermore, granule products directly degrade LLO, irreversibly inhibiting its activity. The matrix metalloproteinase-8, stored in secondary granules, was identified as an endoprotease that degrades LLO, and blocking neutrophil proteases increased L. monocytogenes intracellular survival. In conclusion, we propose that LLO degradation by matrix metalloproteinase-8 during phagocytosis protects neutrophil membranes from perforation and contributes to maintaining L. monocytogenes in a bactericidal phagosome from which it cannot escape.

Human neutrophil peptide-1 (HNP-1): a new anti-leishmanial drug candidate

The toxicity of available drugs for treatment of leishmaniasis, coupled with emerging drug resistance, make it urgent to find new therapies. Antimicrobial peptides (AMPs) have a strong broad-spectrum antimicrobial activity with distinctive modes of action and are considered as promising therapeutic agents. The defensins, members of the large family of AMPs, are immunomodulatory molecules and important components of innate immune system. Human neutrophil peptide-1 (HNP-1), which is produced by neutrophils, is one of the most potent defensins. In this study, we described anti-parasitic activity of recombinant HNP-1 (rHNP-1) against Leishmania major promastigotes and amastigotes. Furthermore, we evaluated the immunomodulatory effect of rHNP-1 on parasite-infected neutrophils and how neutrophil apoptosis was affected. Our result showed that neutrophils isolated from healthy individuals were significantly delayed in the onset of apoptosis following rHNP-1 treatment. Moreover, there was a noteworthy increase in dying cells in rHNP-1- and/or CpG–treated neutrophils in comparison with untreated cells. There is a considerable increase in TNF-α production from rHNP-1-treated neutrophils and decreased level of TGF-β concentration, a response that should potentiate the immune system against parasite invasion. In addition, by using real-time polymerase chain reaction (real-time PCR), we showed that in vitro infectivity of Leishmania into neutrophils is significantly reduced following rHNP-1 treatment compared to untreated cells.

In Iran, cutaneous leishmaniasis (CL) is a widespread and highly endemic disease in young individuals. To date, treatment strategy is based on chemotherapy accompanied with high incidence of toxicity and drug resistance. Distinctive mode of action of defensins (members of antimicrobial peptides) with low susceptibility to resistance and low toxicity to mammalian cells makes them suitable candidates for anti-leishmanial agents. The most active human defensin is human neutrophil peptide-1 (HNP-1) produced by neutrophils; the first effector cells during Leishmania infection. In this work, we used recombinant HNP-1 (rHNP-1) against both the promastigote and amastigote forms of Leishmania (L.) major. Furthermore, immunomodulatory effect of rHNP-1 on Leishmania-infected neutrophils was investigated. Our result showed that rHNP-1 has anti-parasitic effect against L. major promastigotes and amastigotes and also reduces infectivity rate of Leishmania-infected neutrophils. Moreover, assessment of cytokine production from Leishmania-infected neutrophils reveals an increase in TNF-α and a decrease in TGF-β production after rHNP-1 treatment; a cytokine pattern anticipated to facilitate control of parasites. The immunomodulatory effect of rHNP-1 on cytokine production from parasite-infected neutrophils besides its direct effect on free parasites is considered as promising step towards developing new anti-leishmanial agents.

Human milk hyaluronan enhances innate defense of the intestinal epithelium

Breast-feeding is associated with enhanced protection from gastrointestinal disease in infants, mediated in part by an array of bioactive glycan components in milk that act through molecular mechanisms to inhibit enteric pathogen infection. Human milk contains hyaluronan (HA), a glycosaminoglycan polymer found in virtually all mammalian tissues. We have shown that synthetic HA of a specific size range promotes expression of antimicrobial peptides in intestinal epithelium. We hypothesize that hyaluronan from human milk also enhances innate antimicrobial defense. Here we define the concentration of HA in human milk during the first 6 months postpartum. Importantly, HA isolated from milk has a biological function. Treatment of HT-29 colonic epithelial cells with human milk HA at physiologic concentrations results in time- and dose-dependent induction of the antimicrobial peptide human β-defensin 2 and is abrogated by digestion of milk HA with a specific hyaluronidase. Milk HA induction of human β-defensin 2 expression is also reduced in the presence of a CD44-blocking antibody and is associated with a specific increase in ERK1/2 phosphorylation, suggesting a role for the HA receptor CD44. Furthermore, oral administration of human milk-derived HA to adult, wild-type mice results in induction of the murine Hβ D2 ortholog in intestinal mucosa and is dependent upon both TLR4 and CD44 in vivo. Finally, treatment of cultured colonic epithelial cells with human milk HA enhances resistance to infection by the enteric pathogen Salmonella typhimurium. Together, our observations suggest that maternally provided HA stimulates protective antimicrobial defense in the newborn.

Antimicrobial peptides and colitis

Antimicrobial peptides (AMPs) are important components of innate immunity. They are often expressed in response to colonic inflammation and infection. Over the last several years, the roles of several antimicrobial peptides have been explored. Gene expression of many AMPs (beta defensin HBD2-4 and cathelicidin) is induced in response to invasion of gut microbes into the mucosal barrier. Some AMPs are expressed in a constitutive manner (alpha defensin HD 5-6 and beta defensin HBD1), while others (defensin and bactericidal/ permeability increasing protein BPI) are particularly associated with Inflammatory Bowel Disease (IBD) due to altered defensin expression or development of autoantibodies against Bactericidal/permeability increasing protein (BPI). Various AMPs have different spectrum and strength of antimicrobial effects. Some may play important roles in modulating the colitis (cathelicidin) while others (lactoferrin, hepcidin) may represent biomarkers of disease activity. The use of AMPs for therapeutic purposes is still at an early stage of development. A few natural AMPs were shown to be able to modulate colitis when delivered intravenously or intracolonically (cathelicidin, elafin and SLPI) in mouse colitis models. New AMPs (synthetic or artificial non-human peptides) are being developed and may represent new therapeutic approaches against colitis. This review discusses the latest research developments in the AMP field with emphasis in innate immunity and pathophysiology of colitis.

Specific-sized hyaluronan fragments promote expression of human β-defensin 2 in intestinal epithelium

Hyaluronan (HA) is a glycosaminoglycan polymer found in the extracellular matrix of virtually all mammalian tissues. Recent work has suggested a role for small, fragmented HA polymers in initiating innate defense responses in immune cells, endothelium, and epidermis through interaction with innate molecular pattern recognition receptors, such as TLR4. Despite these advances, little is known regarding the effect of fragmented HA at the intestinal epithelium, where numerous pattern recognition receptors act as sentinels of an innate defense response that maintains epithelial barrier integrity in the presence of abundant and diverse microbial challenges. Here we report that HA fragments promote expression of the innate antimicrobial peptide human β-defensin 2 (HβD2) in intestinal epithelial cells. Treatment of HT-29 colonic epithelial cells with HA fragment preparations resulted in time- and dose-dependent up-regulated expression of HβD2 protein in a fragment size-specific manner, with 35-kDa HA fragment preparations emerging as the most potent inducers of intracellular HβD2. Furthermore, oral administration of specific-sized HA fragments promotes the expression of an HβD2 ortholog in the colonic epithelium of both wild-type and CD44-deficient mice but not in TLR4-deficient mice. Together, our observations suggest that a highly size-specific, TLR4-dependent, innate defense response to fragmented HA contributes to intestinal epithelium barrier defense through the induction of intracellular HβD2 protein.

θ-Defensins: cyclic peptides with endless potential

θ-Defensins, the only cyclic peptides of animal origin, have been isolated from the leukocytes of rhesus macaques and baboons. Their biogenesis is unusual because each peptide is an 18-residue chimera formed by the head-to-tail splicing of nonapeptides derived from two separate precursors. θ-Defensins have multiple arginines and a ladder-like tridisulfide array spanning their two antiparallel β-strands. Human θ-defensin genes contain a premature stop codon that prevents effective translation of the needed precursors; consequently, these peptides are not present in human leukocytes. Synthetic θ-defensins with sequences that correspond to those encoded within the human pseudogenes are called retrocyclins. Retrocyclin-1 inhibits the cellular entry of HIV-1, HSV, and influenza A virus. The rhesus θ-defensin RTD-1 protects mice from an experimental severe acute respiratory syndrome coronavirus infection, and retrocyclin-1 protects mice from infection by Bacillus anthracis spores. The small size, unique structure, and multiple host defense activities of θ-defensins make them intriguing potential therapeutic agents.

Functional determinants of human enteric α-defensin HD5: crucial role for hydrophobicity at dimer interface

Human α-defensins are cationic peptides that self-associate into dimers and higher-order oligomers. They bind protein toxins, such as anthrax lethal factor (LF), and kill bacteria, including Escherichia coli and Staphylococcus aureus, among other functions. There are six members of the human α-defensin family: four human neutrophil peptides, including HNP1, and two enteric human defensins, including HD5. We subjected HD5 to comprehensive alanine scanning mutagenesis. We then assayed LF binding by surface plasmon resonance, LF activity by enzyme kinetic inhibition, and antibacterial activity by the virtual colony count assay. Most mutations could be tolerated, resulting in activity comparable with that of wild type HD5. However, the L29A mutation decimated LF binding and bactericidal activity against Escherichia coli and Staphylococcus aureus. A series of unnatural aliphatic and aromatic substitutions at position 29, including aminobutyric acid (Abu) and norleucine (Nle) correlated hydrophobicity with HD5 function. The crystal structure of L29Abu-HD5 depicted decreased hydrophobic contacts at the dimer interface, whereas the Nle-29-HD5 crystal structure depicted a novel mode of dimerization with parallel β strands. The effect of mutating Leu(29) is similar to that of a C-terminal hydrophobic residue of HNP1, Trp(26). In addition, in order to further clarify the role of dimerization in HD5 function, an obligate monomer was generated by N-methylation of the Glu(21) residue, decreasing LF binding and antibacterial activity against S. aureus. These results further characterize the dimer interface of the α-defensins, revealing a crucial role of hydrophobicity-mediated dimerization.

α-Defensins in human innate immunity

Defensins are small, multifunctional cationic peptides. They typically contain six conserved cysteines whose three intramolecular disulfides stabilize a largely β-sheet structure. This review of human α-defensins begins by describing their evolution, including their likely relationship to the Big Defensins of invertebrates, and their kinship to the β-defensin peptides of many if not all vertebrates, and the θ-defensins found in certain non-human primates. We provide a short history of the search for leukocyte-derived microbicidal molecules, emphasizing the roles played by luck (good), preconceived notions (mostly bad), and proper timing (essential). The antimicrobial, antiviral, antitoxic, and binding properties of human α-defensins are summarized. The structural features of α-defensins are described extensively and their functional contributions are assessed. The properties of HD6, an enigmatic Paneth cell α-defensin, are contrasted with those of the four myeloid α-defensins (HNP1-4) and of HD5, the other α-defensin of human Paneth cells. The review ends with a decalogue that may assist researchers or students interested in α-defensins and related aspects of neutrophil function.

Cationic host defence peptides: multifaceted role in immune modulation and inflammation

Host defence peptides (HDPs) are innate immune effector molecules found in diverse species. HDPs exhibit a wide range of functions ranging from direct antimicrobial properties to immunomodulatory effects. Research in the last decade has demonstrated that HDPs are critical effectors of both innate and adaptive immunity. Various studies have hypothesized that the antimicrobial property of certain HDPs may be largely due to their immunomodulatory functions. Mechanistic studies revealed that the role of HDPs in immunity is very complex and involves various receptors, signalling pathways and transcription factors. This review will focus on the multiple functions of HDPs in immunity and inflammation, with special reference to cathelicidins, e.g. LL-37, certain defensins and novel synthetic innate defence regulator peptides. We also discuss emerging concepts of specific HDPs in immune-mediated inflammatory diseases, including the potential use of cationic peptides as therapeutics for immune-mediated inflammatory disorders.

Biosynthesis and antimicrobial evaluation of backbone-cyclized α-defensins

Defensins are antimicrobial peptides that are important in the innate immune defense of mammals. Upon stimulation by bacterial antigens, enteric α-defensins are secreted into the intestinal lumen where they have potent microbicidal activities. Cryptdin-4 (Crp4) is an α-defensin expressed in Paneth cells of the mouse small intestine and the most bactericidal of the known cryptdin isoforms. The structure of Crp4 consists of a triple-stranded antiparallel β-sheet but lacks three amino acids between the fourth and fifth cysteine residues, making them distinct from other α-defensins. The structure also reveals that the α-amino and C-terminal carboxylic groups are in the proximity of each other (d ≈ 3 Å) in the folded structure. We present here the biosynthesis of backbone-cyclized Crp4 using a modified protein splicing unit or intein. Our data show that cyclized Crp4 can be biosynthesized by using this approach both in vitro and in vivo, although the expression yield was significantly lower when the protein was produced inside the cell. The resulting cyclic defensins retained the native α-defensin fold and showed equivalent or better microbicidal activities against several Gram-positive and Gram-negative bacteria when compared to native Crp4. No detectable hemolytic activity against human red blood cells was observed for either native Crp4 or its cyclized variants. Moreover, both forms of Crp4 also showed high stability to degradation when incubated with human serum. Altogether, these results indicate the potential for backbone-cyclized defensins in the development of novel peptide-based antimicrobial compounds.

The pore-forming toxin listeriolysin O mediates a novel entry pathway of L. monocytogenes into human hepatocytes

Intracellular pathogens have evolved diverse strategies to invade and survive within host cells. Among the most studied facultative intracellular pathogens, Listeria monocytogenes is known to express two invasins-InlA and InlB-that induce bacterial internalization into nonphagocytic cells. The pore-forming toxin listeriolysin O (LLO) facilitates bacterial escape from the internalization vesicle into the cytoplasm, where bacteria divide and undergo cell-to-cell spreading via actin-based motility. In the present study we demonstrate that in addition to InlA and InlB, LLO is required for efficient internalization of L. monocytogenes into human hepatocytes (HepG2). Surprisingly, LLO is an invasion factor sufficient to induce the internalization of noninvasive Listeria innocua or polystyrene beads into host cells in a dose-dependent fashion and at the concentrations produced by L. monocytogenes. To elucidate the mechanisms underlying LLO-induced bacterial entry, we constructed novel LLO derivatives locked at different stages of the toxin assembly on host membranes. We found that LLO-induced bacterial or bead entry only occurs upon LLO pore formation. Scanning electron and fluorescence microscopy studies show that LLO-coated beads stimulate the formation of membrane extensions that ingest the beads into an early endosomal compartment. This LLO-induced internalization pathway is dynamin-and F-actin-dependent, and clathrin-independent. Interestingly, further linking pore formation to bacteria/bead uptake, LLO induces F-actin polymerization in a tyrosine kinase-and pore-dependent fashion. In conclusion, we demonstrate for the first time that a bacterial pathogen perforates the host cell plasma membrane as a strategy to activate the endocytic machinery and gain entry into the host cell.

Humanized theta-defensins (retrocyclins) enhance macrophage performance and protect mice from experimental anthrax infections

Retrocyclins are humanized versions of the -defensin peptides expressed by the leukocytes of several nonhuman primates. Previous studies, performed in serum-free media, determined that retrocyclins 1 (RC1) and RC2 could prevent successful germination of Bacillus anthracis spores, kill vegetative B. anthracis cells, and inactivate anthrax lethal factor. We now report that retrocyclins are extensively bound by components of native mouse, human, and fetal calf sera, that heat-inactivated sera show greatly enhanced retrocyclin binding, and that native and (especially) heat-inactivated sera greatly reduce the direct activities of retrocyclins against spores and vegetative cells of B. anthracis. Nevertheless, we also found that retrocyclins protected mice challenged in vivo by subcutaneous, intraperitoneal, or intranasal instillation of B. anthracis spores. Retrocyclin 1 bound extensively to B. anthracis spores and enhanced their phagocytosis and killing by murine RAW264.7 cells. Based on the assumption that spore-bound RC1 enters phagosomes by "piggyback phagocytosis," model calculations showed that the intraphagosomal concentration of RC1 would greatly exceed its extracellular concentration. Murine alveolar macrophages took up fluorescently labeled retrocyclin, suggesting that macrophages may also acquire extracellular RC1 directly. Overall, these data demonstrate that retrocyclins are effective in vivo against experimental murine anthrax infections and suggest that enhanced macrophage function contributes to this property.

In vitro and in vivo synergistic effects of cryptdin 2 and ampicillin against Salmonella

In view of the emergence of multidrug-resistant Salmonella strains, there is a need for therapeutic alternatives. To reduce the dose of antibiotic required in order to decrease the associated side effects, the present study was aimed at evaluating the synergism between cryptdin 2 (a Paneth cell antimicrobial peptide) and ampicillin (Amp) against Salmonella enterica serovar Typhimurium. The synergy was evaluated in terms of the fractional bactericidal concentration (FBC) index, time-kill assay results (in vitro), macrophage functions, i.e., intracellular killing, lipid peroxidation, superoxide dismutase activity, and generation of nitrite (ex vivo), and decreases in CFU of salmonellae in livers, spleens, and small intestines of infected mice treated with cryptdin 2 and/or Amp (in vivo). In vitro synergism between the two agents was observed on the basis of the FBC index and time-kill assays. When the agents were used in combination, ex vivo studies revealed an enhanced effect on macrophage functions, particularly exhibiting a synergetic effect in terms of SOD levels. In vivo synergy was indicated by larger log unit decreases in all target organs of mice treated with the combination than those for the drugs used alone. These results point toward the possible use of cryptdin 2 as an adjunct to ampicillin and may help in developing alternate strategies to combat Salmonella infections.