The role of antimicrobial peptides in innate immunity

Production of antimicrobial peptides and proteins is an importantmeans of host defense in eukaryotes. The larger antimicrobialproteins, containing more than 100 amino acids, are often lyticenzymes, nutrient-binding proteins or contain sites that targetspecific microbial macromolecules. The smaller antimicrobialpeptides act largely by disrupting the structure or functionof microbial cell membranes. Hundreds of antimicrobial peptideshave been found in the epithelial layers, phagocytic cells andbody fluids of multicellular animals, from mollusks to humans.Some antimicrobial peptides are produced constitutively, othersare induced in response to infection or inflammation. Studiesof the regulation of antimicrobial peptide synthesis in Drosophilahave been particularly fruitful, and have provided a new paradigmfor the analysis of mammalian host defense responses. It nowappears that the general patterns of antimicrobial responsesof invertebrates have been preserved in vertebrates ("innateimmunity") where they contribute to host defense both independentlyand in complex interplay with adaptive immunity.     

Defensins: antimicrobial peptides of innate immunity

The production of natural antibiotic peptides has emerged as an important mechanism of innate immunity in plants and animals. Defensins are diverse members of a large family of antimicrobial peptides, contributing to the antimicrobial action of granulocytes, mucosal host defence in the small intestine and epithelial host defence in the skin and elsewhere. This review, inspired by a spate of recent studies of defensins in human diseases and animal models, focuses on the biological function of defensins.     

Original involvement of antimicrobial peptides in mussel innate immunity

Recently, the existence and extended diversity of antimicrobial peptides has been revealed in two mussel species. These molecules are classified into four groups according to common features of their primary structure: defensins, mytilins, myticins and mytimycin. In Mytilus galloprovincialis, gene structure reveals synthesis as precursors in circulating hemocytes. Synthesised even in absence of challenge, the precursors mature and the peptides are stored in granules as active forms. The different peptides are engaged in the destruction of bacteria inside phagocytes, before being released into hemolymph to participate in systemic responses. Such involvement in anti-infectious responses is unique, and apparently more related to those of mammalian phagocytes than to those of insects.     

Antimicrobial peptides from anurans skin secretions

This article is an overview of antimicrobial peptides found in anurans skin secretions. These molecules constitute an initial barrier against microbial infections because of their activity against a large array of microorganisms. These peptides hold remarkable pharmaceutical and technological interest since they selectively kill microorganisms and are unlikely to induce resistance in pathogens. Also, outstanding synergism occurs when these peptides are combined with classic antibiotics and other antimicrobial peptides.     

Antimicrobial peptides important in innate immunity

Antimicrobial peptides are present in all walks of life, from plants to animals, and they are considered to be endogenous antibiotics. In general, antimicrobial peptides are determinants of the composition of the microbiota and they function to fend off microbes and prevent infections. Antimicrobial peptides eliminate micro-organisms through disruption of their cell membranes. Their importance in human immunity, and in health as well as disease, has only recently been appreciated. The present review provides an introduction to the field of antimicrobial peptides in general and discusses two of the major classes of mammalian antimicrobial peptides: the defensins and the cathelicidins. The review focuses on their structures, their main modes of action and their regulation.     

Susceptibility of clinical Staphylococcus aureus isolates to innate defense antimicrobial peptides

Antimicrobial peptides (AMPs) are effector molecules of innate immunity. To determine whether AMP susceptibility of S. aureus varies according to different types of infection, 102 isolates from patients with S. aureus bacteremia or recurrent skin and soft tissue infection, and colonizing isolates were investigated. Using microbroth dilution assays we found a narrow range of MICs of human β-defensin-3, cathelicidin LL-37 and bovine indolicidin without significant differences between the groups. Colony-forming unit (CFU) assays revealed minor differences in bactericidal activity with slightly but not significantly higher CFU reduction in colonizing isolates. These data do not support a role for differential AMP susceptibility in vitro as a major determinant of S. aureus invasive infection.     

The contact system--a novel branch of innate immunity generating antibacterial peptides

Activation of the contact system has two classical consequences: initiation of the intrinsic pathway of coagulation, and cleavage of high molecular weight kininogen (HK) leading to the release of bradykinin, a potent proinflammatory peptide. In human plasma, activation of the contact system at the surface of significant bacterial pathogens was found to result in further HK processing and bacterial killing. A fragment comprising the D3 domain of HK is generated, and within this fragment a sequence of 26 amino acids is mainly responsible for the antibacterial activity. A synthetic peptide covering this sequence kills several bacterial species, also at physiological salt concentration, as effectively as the classical human antibacterial peptide LL-37. Moreover, in an animal model of infection, inhibition of the contact system promotes bacterial dissemination and growth. These data identify a novel and important role for the contact system in the defence against invasive bacterial infection.     

The role of Paneth cells and their antimicrobial peptides in innate host defense

The intestinal epithelium is the largest surface area that is exposed to various pathogens in the environment, however, in contrast to the colon the number of bacteria that colonize the small intestine is extremely low. Paneth cells, one of four major epithelial cell lineages in the small intestine, reside at the base of the crypts and have apically oriented secretory granules. These granules contain high levels of antimicrobial peptides that belong to the alpha-defensin family. Paneth cells secrete these microbicidal granules that contain alpha-defensins when exposed ex vivo to bacteria or their antigens, and recent evidence reveals that antimicrobial peptides, particularly alpha-defensins, that are present in Paneth cells contribute to intestinal innate host defense.     

Therapeutic antimicrobial peptides may compromise natural immunity

Antimicrobial peptides (AMPs) have been proposed as a promising new class of antimicrobials despite warnings that therapeutic use could drive the evolution of pathogens resistant to our own immunity peptides. Using experimental evolution, we demonstrate that Staphylococcus aureus rapidly evolved resistance to pexiganan, a drug-candidate for diabetic leg ulcer infections. Evolved resistance was costly in terms of impaired growth rate, but costs-of-resistance were completely ameliorated by compensatory adaptation. Crucially, we show that, in some populations, experimentally evolved resistance to pexiganan provided S. aureus with cross-resistance to human-neutrophil-defensin-1, a key component of the innate immune response to infection. This unintended consequence of therapeutic use could drastically undermine our innate immune system's ability to control and clear microbial infections. Our results therefore highlight grave potential risks of AMP therapies, with implications for their development.     

The contribution of the Toll-like/IL-1 receptor superfamily to innate and adaptive immunity to fungal pathogens in vivo

In vitro studies have indicated the importance of Toll-like receptor (TLR) signaling in response to the fungal pathogens Candida albicans and Aspergillus fumigatus. However, the functional consequences of the complex interplay between fungal morphogenesis and TLR signaling in vivo remain largely undefined. In this study we evaluate the impact of the IL-1R/TLR/myeloid differentiation primary response gene 88 (MyD88)-dependent signaling pathway on the innate and adaptive Th immunities to C. albicans and A. fumigatus in vivo. It was found that 1) the MyD88-dependent pathway is required for resistance to both fungi; 2) the involvement of the MyD88 adapter may occur through signaling by distinct members of the IL-1R/TLR superfamily, including IL-1R, TLR2, TLR4, and TLR9, with the proportional role of the individual receptors varying depending on fungal species, fungal morphotypes, and route of infection; 3) individual TLRs and IL-1R activate specialized antifungal effector functions on neutrophils, which correlates with susceptibility to infection; and 4) MyD88-dependent signaling on dendritic cells is crucial for priming antifungal Th1 responses. Thus, the finding that the innate and adaptive immunities to C. albicans and A. fumigatus require the coordinated action of distinct members of the IL-1R/TLR superfamily acting through MyD88 makes TLR manipulation amenable to the induction of host resistance to fungi.     

Two interdependent mechanisms of antimicrobial activity allow for efficient killing in nylon-3-based polymeric mimics of innate immunity peptides

Novel synthetic mimics of antimicrobial peptides have been developed to exhibit structural properties and antimicrobial activity similar to those of natural antimicrobial peptides (AMPs) of the innate immune system. These molecules have a number of potential advantages over conventional antibiotics, including reduced bacterial resistance, cost-effective preparation, and customizable designs. In this study, we investigate a family of nylon-3 polymer-based antimicrobials. By combining vesicle dye leakage, bacterial permeation, and bactericidal assays with small-angle X-ray scattering (SAXS), we find that these polymers are capable of two interdependent mechanisms of action: permeation of bacterial membranes and binding to intracellular targets such as DNA, with the latter necessarily dependent on the former. We systemically examine polymer-induced membrane deformation modes across a range of lipid compositions that mimic both bacteria and mammalian cell membranes. The results show that the polymers' ability to generate negative Gaussian curvature (NGC), a topological requirement for membrane permeation and cellular entry, in model Escherichia coli membranes correlates with their ability to permeate membranes without complete membrane disruption and kill E. coli cells. Our findings suggest that these polymers operate with a concentration-dependent mechanism of action: at low concentrations permeation and DNA binding occur without membrane disruption, while at high concentrations complete disruption of the membrane occurs. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.     

Antimicrobial peptides and proteins, exercise and innate mucosal immunity

This review examines the question of whether exercise can be used as an experimental model to further our understanding of innate antimicrobial peptides and proteins (AMPs) and their role in susceptibility to infection at mucosal surfaces. There is strong evidence to suggest that AMPs, in combination with cellular and physical factors, play an important role in preventing infection. Although AMPs act directly on microorganisms, there is increasing recognition that they also exert their protective effect via immunomodulatory mechanisms, especially in noninflammatory conditions. Further studies that manipulate physiologically relevant concentrations of AMPs are required to shed light on the role they play in reducing susceptibility to infection. Evidence shows that in various form prolonged and/or exhaustive exercise is a potent modulator of the immune system, which can either sharpen or blunt the immune response to pathogens. The intensity and duration of exercise can be readily controlled in experimental settings to manipulate the degree of physical stress. This would allow for an investigation into a potential dose-response effect between exercise and AMPs. In addition, the use of controlled exercise could provide an experimental model by which to examine whether changes in the concentration of AMPs alters susceptibility to illness.     

Cationic antimicrobial peptides as molecular immunity factors: multi-functionality

Cationic antimicrobial peptides (AMP) of mammals (defensins, cathelicidins, protegrins and many others) are regarded as important components of congenital immunity. AMP are multifunctional molecules, capable of killing microorganisms directly by acting as endogenic, natural antibiotics ("immediate immunity"); in addition, they may take part in congenital and adaptive immune reactions (immunoregulation) and function as signal molecules, involved into tissue reparation, inflammation (including sepsis), blood coagulation and other important processes in the body. The molecular mechanisms of the direct antimicrobial action of AMP are considered. In addition to antimicrobial and immunoregulating action, AMP have influence on immunoneuroendocrine interactions, taking part in the pathogenesis of stress reactions (corticostatic action), as well as play the role of regulatory peptides of adaptogenic action. The many-sided character of the action of AMP opens prospects to the creation of new medicinal remedies on their basis. Such requirements are met by the Russian preparation "Superlymph" (a complex of natural cytokines), containing protegrin-like AMP.     

Drosophila as a model system to unravel the layers of innate immunity to infection

Innate immunity relies entirely upon germ-line encoded receptors, signalling components and effector molecules for the recognition and elimination of invading pathogens. The fruit fly Drosophila melanogaster with its powerful collection of genetic and genomic tools has been the model of choice to develop ideas about innate immunity and host-pathogen interactions. Here, we review current research in the field, encompassing all layers of defence from the role of the microbiota to systemic immune activation, and attempt to speculate on future directions and open questions.     

Multivariate design and evaluation of a set of RGRPQ-derived innate immunity peptides

Oral commensal Streptococcus gordonii proteolytically cleave the salivary PRP-1 polypeptide into an RGRPQ innate peptide. The Arg and Gln termini are crucial for RGRPQ-mediated ammonia production and proliferation by S. gordonii SK12 and adhesion inhibition and desorption by Actinomyces naeslundii T14V, respectively. Here we have applied (i) a multivariate approach using RGRPQ-related peptides varied at amino acids 2, 3, and 4 simultaneously and (ii) size and N- and C-terminal modifications of RGRPQ to generate structure activity information. While the N-terminal arginine motif mediated ammonia production independent of peptide size, other responses required more or less full-length peptide motifs. The motifs for adhesion inhibition and desorption were the same. The adhesion and proliferation motifs required similarly a hydrophobic/low polarity amino acid 4 but differentially a hydrophilic or hydrophobic character of amino acids 2/3, respectively; polar peptides with small/hydrophilic and hydrophilic amino acids 2 and 3, respectively, had high adhesion inhibition/desorption activity, and lipophilic peptides with large/hydrophobic amino acids 2 and 3 had high proliferation activity. Accordingly, while RIWWQ had increased proliferation but abolished adhesion/desorption activity, peptides designed with hydrophilic amino acids 2 and 3 were predicted to behave in the opposite way. Moreover, a RGRPQ mimetic for all three responses should mimic small hydrophilic, large nitrogen-containing, and hydrophobic/low polarity amino acids 2, 3, and 4, respectively. Peptides fulfilling these criteria were 1-1.6-fold improved in all three responses. Thus, both mimetics and peptides with differential proliferation and adhesion activities may be generated for evaluation in biofilm models.