Defensins in granules of phagocytic and non-phagocytic cells

Paneth cells as the cornerstones of intestinal and organismal health: a primer

Paneth cells are versatile secretory cells located in the crypts of Lieberkühn of the small intestine. In normal conditions, they function as the cornerstones of intestinal health by preserving homeostasis. They perform this function by providing niche factors to the intestinal stem cell compartment, regulating the composition of the microbiome through the production and secretion of antimicrobial peptides, performing phagocytosis and efferocytosis, taking up heavy metals, and preserving barrier integrity. Disturbances in one or more of these functions can lead to intestinal as well as systemic inflammatory and infectious diseases. This review discusses the multiple functions of Paneth cells, and the mechanisms and consequences of Paneth cell dysfunction. It also provides an overview of the tools available for studying Paneth cells.

Human α-Defensin-5 Efficiently Neutralizes Clostridioides difficile Toxins TcdA, TcdB, and CDT

Infections with the pathogenic bacterium Clostridioides (C.) difficile are coming more into focus, in particular in hospitalized patients after antibiotic treatment. C. difficile produces the exotoxins TcdA and TcdB. Since some years, hypervirulent strains are described, which produce in addition the binary actin ADP-ribosylating toxin CDT. These strains are associated with more severe clinical presentations and increased morbidity and frequency. Once in the cytosol of their target cells, the catalytic domains of TcdA and TcdB glucosylate and thereby inactivate small Rho-GTPases whereas the enzyme subunit of CDT ADP-ribosylates G-actin. Thus, enzymatic activity of the toxins leads to destruction of the cytoskeleton and breakdown of the epidermal gut barrier integrity. This causes clinical symptoms ranging from mild diarrhea to life-threatening pseudomembranous colitis. Therefore, pharmacological inhibition of the secreted toxins is of peculiar medical interest. Here, we investigated the neutralizing effect of the human antimicrobial peptide α-defensin-5 toward TcdA, TcdB, and CDT in human cells. The toxin-neutralizing effects of α-defensin-5 toward TcdA, TcdB, and CDT as well as their medically relevant combination were demonstrated by analyzing toxins-induced changes in cell morphology, intracellular substrate modification, and decrease of trans-epithelial electrical resistance. For TcdA, the underlying mode of inhibition is most likely based on the formation of inactive toxin-defensin-aggregates whereas for CDT, the binding- and transport-component might be influenced. The application of α-defensin-5 delayed intoxication of cells in a time- and concentration-dependent manner. Due to its effect on the toxins, α-defensin-5 should be considered as a candidate to treat severe C. difficile-associated diseases.

Antimicrobial peptide ROAD-1 triggers phase change in local membrane environment to execute its activity

Emergence of antibiotic-resistant pathogens has paved way for development of newer class of drugs that would not be susceptible to resistance. Antimicrobial peptides such as defensins that target the microbial membrane are promising candidates. ROAD-1 is an alpha-defensin present in the oral cavity of rhesus macaque and shares very high sequence similarity to human enteric defensin 5. In this study we have performed microsecond long all atom molecular dynamic simulations to understand the mechanism of action of ROAD-1. We find that ROAD-1 is able to adopt an energetically stable conformation predominantly stabilized by electrostatic interactions only in presence of bacterial membranes. In mammalian membrane even though it gets absorbed onto the bilayer, it is unable to adopt an equilibrium conformation. Binding of ROAD-1 to bilayer induces clustering of POPG molecules up to 15 Å around the peptide. POPG molecules show higher order parameters than the neighboring POPE implying coexistence of different phases. Analysis of binding free energy of ROAD-1-membrane complex indicates Arg1, Arg2, Arg7, and Arg25 to play key role in its antimicrobial activity. Unlike its homolog HD5, ROAD-1 is not observed to form a dimer. Our study gives insight into the membrane-bound conformation of ROAD-1 and its mechanism of action that can aid in designing defensin-based therapeutics. Graphical abstract Antimicrobial peptide ROAD-1 adopts a different membrane-bound conformation as compared with HD5 even though they belong to the same family implying a different mechanism of action.

Review: Towards antibacterial strategies: studies on the mechanisms of interaction between antibacterial peptides and model membranes

Lipopolysaccharides (LPSs) play a dual role as inflammation-inducing and as membrane-forming molecules. The former role attracts significantly more attention from scientists, possibly because it is more closely related to sepsis and septic shock. This review aims to focus the reader's attention to the other role, the function of LPS as the major constituent of the outer layer of the outer membrane of Gram-negative bacteria, in particular those of enterobacterial strains. In this function, LPS is a necessary component of the cell envelope and guarantees survival of the bacterial organism. At the same time, it represents the first target for attacking molecules which may either be synthesized by the host's innate or adaptive immune system or administered to the human body. The interaction of these molecules with the outer membrane may not only directly cause the death of the bacterial organism, but may also lead to the release of LPS into the circulation. Here, we review membrane model systems and their application for the study of molecular mechanisms of interaction of peptides such as those of the human complement system, the bactericidal/permeability-increasing protein (BPI), cationic antibacterial peptide 18 kDa (CAP18) as an example of cathelicidins, defensins, and polymyxin B (PMB). Emphasis is on electrical measurements with a reconstitution system of the lipid matrix of the outer membrane which was established in the authors' laboratory as a planar asymmetric bilayer with one leaflet being composed solely of LPS and the other of the natural phospholipid mixture. The main conclusion, which can be drawn from these investigations, is that LPS and in general its negative charges are the dominant determinants for specific peptide—membrane interactions. However, the detailed mechanisms of interaction, which finally lead to bacterial killing, may involve further steps and differ for different antibacterial peptides.

Analysis of human breast milk cells: gene expression profiles during pregnancy, lactation, involution, and mastitic infection

The molecular processes underlying human milk production and the effects of mastitic infection are largely unknown because of limitations in obtaining tissue samples. Determination of gene expression in normal lactating women would be a significant step toward understanding why some women display poor lactation outcomes. Here, we demonstrate the utility of RNA obtained directly from human milk cells to detect mammary epithelial cell (MEC)-specific gene expression. Milk cell RNA was collected from five time points (24 h prepartum during the colostrum period, midlactation, two involutions, and during a bout of mastitis) in addition to an involution series comprising three time points. Gene expression profiles were determined by use of human Affymetrix arrays. Milk cells collected during milk production showed that the most highly expressed genes were involved in milk synthesis (e.g., CEL, OLAH, FOLR1, BTN1A1, and ARG2), while milk cells collected during involution showed a significant downregulation of milk synthesis genes and activation of involution associated genes (e.g., STAT3, NF-kB, IRF5, and IRF7). Milk cells collected during mastitic infection revealed regulation of a unique set of genes specific to this disease state, while maintaining regulation of milk synthesis genes. Use of conventional epithelial cell markers was used to determine the population of MECs within each sample. This paper is the first to describe the milk cell transcriptome across the human lactation cycle and during mastitic infection, providing valuable insight into gene expression of the human mammary gland.

Mucosal immunity in the female genital tract, HIV/AIDS

Mucosal immunity consists of innate and adaptive immune responses which can be influenced by systemic immunity. Despite having been the subject of intensive studies, it is not fully elucidated what exactly occurs after HIV contact with the female genital tract mucosa. The sexual route is the main route of HIV transmission, with an increased risk of infection in women compared to men. Several characteristics of the female genital tract make it suitable for inoculation, establishment of infection, and systemic spread of the virus, which causes local changes that may favor the development of infections by other pathogens, often called sexually transmitted diseases (STDs). The relationship of these STDs with HIV infection has been widely studied. Here we review the characteristics of mucosal immunity of the female genital tract, its alterations due to HIV/AIDS, and the characteristics of coinfections between HIV/AIDS and the most prevalent STDs.

Microbiome, sex hormones, and immune responses in the reproductive tract: challenges for vaccine development against sexually transmitted infections

The female and male reproductive tracts are complex eco-systems where immune cells, hormones, and microorganisms interact. The characteristics of the reproductive tract mucosa are distinct from other mucosal sites. Reproductive tract mucosal immune responses are compartmentalized, unique, and affected by resident bacterial communities and sex hormones. The female and male genital microbiomes are complex environments that fluctuate in response to external and host-associated stimuli. The female vaginal microbiota play an important role in preventing colonization by pathogenic organisms. Sex hormones and their duration of exposure affect the composition and stability of the microbiome as well as systemic and mucosal immune responses. In addition to the characteristics of the pathogen they are targeting, successful vaccines against sexually transmitted pathogens must take into account the differences between the systemic and mucosal immune responses, the compartmentalization of the mucosal immune responses, the unique characteristics of the reproductive tract mucosa, the role of the mucosal bacterial communities, the impact of sex hormones, and the interactions among all of these factors.

Endocrine control of mucosal immunity in the female reproductive tract: impact of environmental disruptors

The complexity of the human female reproductive tract (FRT) with its multiple levels of hormonally controlled immune protection has only begun to be understood. Dissecting the functions and roles of the immune system in the FRT is complicated by the differential hormonal regulation of its distinct anatomical structures that vary throughout the menstrual cycle. Although many fundamental mechanisms of steroid regulation of reproductive tract immune function have been determined, the effects of exogenous synthetic steroids or endocrine disruptors on immune function and disease susceptibility in the FRT have yet to be evaluated in detail. There is increasing evidence that environmental or synthetic molecules can alter normal immune function. This review provides an overview of the innate and adaptive immune systems, the current status of immune function in the FRT and the potential risks of environmental or pharmacological molecules that may perturb this system.

Paneth cell α-defensins in enteric innate immunity

Paneth cells at the base of small intestinal crypts of Lieberkühn secrete high levels of α-defensins in response to cholinergic and microbial stimuli. Paneth cell α-defensins are broad spectrum microbicides that function in the extracellular environment of the intestinal lumen, and they are responsible for the majority of secreted bactericidal peptide activity. Paneth cell α-defensins confer immunity to oral infection by Salmonella enterica serovar Typhimurium, and they are major determinants of the composition of the small intestinal microbiome. In addition to host defense molecules such as α-defensins, lysozyme, and Pla2g2a, Paneth cells also produce and release proinflammatory mediators as components of secretory granules. Disruption of Paneth cell homeostasis, with subsequent induction of endoplasmic reticulum stress, autophagy, or apoptosis, contributes to inflammation in diverse genetic and experimental mouse models.

Innate and adaptive immunity at mucosal surfaces of the female reproductive tract: stratification and integration of immune protection against the transmission of sexually transmitted infections

This review examines the multiple levels of pre-existing immunity in the upper and lower female reproductive tract. In addition, we highlight the need for further research of innate and adaptive immune protection of mucosal surfaces in the female reproductive tract. Innate mechanisms include the mucus lining, a tight epithelial barrier and the secretion of antimicrobial peptides and cytokines by epithelial and innate immune cells. Stimulation of the innate immune system also serves to bridge the adaptive arm resulting in the generation of pathogen-specific humoral and cell-mediated immunity. Less understood are the multiple components that act in a coordinated way to provide a network of ongoing protection. Innate and adaptive immunity in the human female reproductive tract are influenced by the stage of menstrual cycle and are directly regulated by the sex steroid hormones, progesterone and estradiol. Furthermore, the effect of hormones on immunity is mediated both directly on immune and epithelial cells and indirectly by stimulating growth factor secretion from stromal cells. The goal of this review is to focus on the diverse aspects of the innate and adaptive immune systems that contribute to a unique network of protection throughout the female reproductive tract.

Strain-specific polymorphisms in Paneth cell α-defensins of C57BL/6 mice and evidence of vestigial myeloid α-defensin pseudogenes

Paneth cells at the base of small intestinal crypts secrete microbicidal α-defensins, termed cryptdins (Crps) in mice, as mediators of innate immunity. Proteomic studies show that five abundant Paneth cell α-defensins in C57BL/6 mice are strain specific in that they have not been identified in other inbred strains of mice. Two C57BL/6-specific peptides are coded for by the Defcr20 and -21 genes evident in the NIH C57BL/6 genome but absent from the Celera mixed-strain assembly, which excludes C57BL/6 data and differs from the NIH build with respect to the organization of the α-defensin gene locus. Conversely, C57BL/6 mice lack the Crp1, -2, -4, and -6 peptides and their corresponding Defcr1, -2, -4, and -6 genes, which are common to several mouse strains, including those of the Celera assembly. In C57BL/6 mice, α-defensin gene diversification appears to have occurred by tandem duplication of a multigene cassette that was not found in the mixed-strain assembly. Both mouse genome assemblies contain conserved α-defensin pseudogenes that are closely related to functional myeloid α-defensin genes in the rat, suggesting that the neutrophil α-defensin defect in mice resulted from progressive gene loss. Given the role of α-defensins in shaping the composition of the enteric microflora, such polymorphisms may influence outcomes in mouse models of disease or infection.

Expression and purification of recombinant alpha-defensins and alpha-defensin precursors in Escherichia coli

Recombinant expression of alpha-defensins can be obtained at efficient levels in Escherichia coli. Amplified alpha-defensin or pro-alpha-defensin coding cDNA sequences are cloned directionally between EcoRI and SalI sites of the pET-28a expression vector and expressed in E. coli BL21 RIS cells. Cells growing exponentially in nutrient-rich liquid medium are induced to express the recombinant protein by addition of 50 mM isopropyl beta-D-1-thiogalactopyranoside for 3-6 h. After bacterial cells collected by centrifugation are lysed in 6 M guanidine-HCl under non-reducing conditions, the expressed defensin fused to its 6xHis-34 amino acid N-terminal fusion partner is purified by affinity chromatography on nickel-NTA columns. A Met codon introduced at the N terminus of expressed Met-free peptides provides a unique CNBr cleavage site, enabling release of the alpha-defensin free of ancillary residues by sequential C18 RP-HPLC. Molecular masses of C18 RP-HPLC purified peptides are confirmed by MALDI-TOF mass spectrometry, and peptide homogeneity is assessed using analytical RP-HPLC and acid-urea polyacrylamide gel electrophoresis. alpha-Defensins prepared in this manner are biochemically equivalent to the natural molecules.

Alpha-defensins in enteric innate immunity: functional Paneth cell alpha-defensins in mouse colonic lumen

Paneth cells are a secretory epithelial lineage that release dense core granules rich in host defense peptides and proteins from the base of small intestinal crypts. Enteric alpha-defensins, termed cryptdins (Crps) in mice, are highly abundant in Paneth cell secretions and inherently resistant to proteolysis. Accordingly, we tested the hypothesis that enteric alpha-defensins of Paneth cell origin persist in a functional state in the mouse large bowel lumen. To test this idea, putative Crps purified from mouse distal colonic lumen were characterized biochemically and assayed in vitro for bactericidal peptide activities. The peptides comigrated with cryptdin control peptides in acid-urea-PAGE and SDS-PAGE, providing identification as putative Crps. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry experiments showed that the molecular masses of the putative alpha-defensins matched those of the six most abundant known Crps, as well as N-terminally truncated forms of each, and that the peptides contain six Cys residues, consistent with identities as alpha-defensins. N-terminal sequencing definitively revealed peptides with N termini corresponding to full-length, (des-Leu)-truncated, and (des-Leu-Arg)-truncated N termini of Crps 1-4 and 6. Crps from mouse large bowel lumen were bactericidal in the low micromolar range. Thus, Paneth cell alpha-defensins secreted into the small intestinal lumen persist as intact and functional forms throughout the intestinal tract, suggesting that the peptides may mediate enteric innate immunity in the colonic lumen, far from their upstream point of secretion in small intestinal crypts.

SELDI-TOF-MS of gingival crevicular fluid--a methodological approach

Human neutrophil peptides (HNPs) and the human cathelicidin LL-37 are antimicrobial peptides secreted by neutrophils, which play a crucial role in innate immune responses. The aim of this study was to establish a new method for ProteinChip arrays in combination with surface enhanced laser desorption/ionization (SELDI) technology and time-of-flight mass spectrometry to analyze gingival crevicular fluid (GCF) samples. To optimize experimental conditions, four different ProteinChip arrays (NP20; CM10, pH 4; CM10, pH 7; IMAC) along with corresponding binding buffers were tested. GCF samples were collected from patients showing healthy periodontal sites and sites with early signs of inflammation (gingivitis), but with no pocket depth greater than 4 mm. For GCF analysis, NP20 arrays and CM10 (pH 4) arrays showed specific and reproducible profiles in the range of 2.5-30.0 kDa. Donors that demonstrated significantly higher intensity peaks corresponding to the mass of LL-37 (p=0.01) also tended to show greater intensity peaks corresponding to the masses of HNP-1 and HNP-2 in samples from inflamed compared to healthy periodontal sites. The findings indicate that analysis of GCF samples by SELDI-TOF mass spectrometry is a useful approach to simultaneously analyze multiple markers, such as antimicrobial peptides, which may be beneficial for determination of new periodontal risk factors.

Macrophage elastase kills bacteria within murine macrophages

Macrophages are aptly positioned to function as the primary line of defence against invading pathogens in many organs, including the lung and peritoneum. Their ability to phagocytose and clear microorganisms has been well documented. Macrophages possess several substances with which they can kill bacteria, including reactive oxygen species, nitric oxide, and antimicrobial proteins. We proposed that macrophage-derived proteinases may contribute to the antimicrobial properties of macrophages. Macrophage elastase (also known as matrix metalloproteinase 12 or MMP12) is an enzyme predominantly expressed in mature tissue macrophages and is implicated in several disease processes, including emphysema. Physiological functions for MMP12 have not been described. Here we show that Mmp12(-/-) mice exhibit impaired bacterial clearance and increased mortality when challenged with both gram-negative and gram-positive bacteria at macrophage-rich portals of entry, such as the peritoneum and lung. Intracellular stores of MMP12 are mobilized to macrophage phagolysosomes after the ingestion of bacterial pathogens. Once inside phagolysosomes, MMP12 adheres to bacterial cell walls where it disrupts cellular membranes resulting in bacterial death. The antimicrobial properties of MMP12 do not reside within its catalytic domain, but rather within the carboxy-terminal domain. This domain contains a unique four amino acid sequence on an exposed beta loop of the protein that is required for the observed antimicrobial activity. The present study represents, to our knowledge, the first report of direct antimicrobial activity by a matrix metallopeptidase, and describes a new antimicrobial peptide that is sequentially and structurally unique in nature.

Capsule polysaccharide is a bacterial decoy for antimicrobial peptides

Antimicrobial peptides (APs) are important host weapons against infections. Nearly all APs are cationic and their microbicidal action is initiated through interactions with the anionic bacterial surface. It is known that pathogens have developed countermeasures to resist these agents by reducing the negative charge of membranes, by active efflux and by proteolytic degradation. Here we uncover a new strategy of resistance based on the neutralization of the bactericidal activity of APs by anionic bacterial capsule polysaccharide (CPS). Purified CPSs from Klebsiella pneumoniae K2, Streptococcus pneumoniae serotype 3 and Pseudomonas aeruginosa increased the resistance to polymyxin B of an unencapsulated K. pneumoniae mutant. Furthermore, these CPSs increased the MICs of polymyxin B and human neutrophil alpha-defensin 1 (HNP-1) for unencapsulated K. pneumoniae, Escherichia coli and P. aeruginosa PAO1. Polymyxin B or HNP-1 released CPS from capsulated K. pneumoniae, S. pneumoniae serotype 3 and P. aeruginosa overexpressing CPS. Moreover, this material also reduced the bactericidal activity of APs. We postulate that APs may trigger in vivo the release of CPS, which in turn will protect bacteria against APs. We found that anionic CPSs, but not cationic or uncharged ones, blocked the bactericidal activity of APs by binding them, thereby reducing the amount of peptides reaching the bacterial surface. Supporting this, polycations inhibited such interaction and the bactericidal activity was restored. We postulate that trapping of APs by anionic CPSs is an additional selective virulence trait of these molecules, which could be considered as bacterial decoys for APs.

Isolation, synthesis, and antimicrobial activities of naturally occurring theta-defensin isoforms from baboon leukocytes

Theta-defensins are macrocyclic antimicrobial peptides that were previously isolated from leukocytes of a single species, the rhesus macaque. We now report the characterization of baboon theta-defensins (BTDs) expressed in bone marrow and peripheral blood leukocytes. Four cDNAs encoding theta-defensin precursors were characterized, allowing for the prediction of 10 theoretical theta-defensins (BTD-1 to BTD-10) produced by binary, head-to-tail splicing of nonapeptides excised from paired precursors. Five of the predicted theta-defensins were purified from baboon leukocytes, and synthetic versions of each were prepared. Anti-theta-defensin antibody localized the peptides in circulating neutrophils and monocytes and in immature and mature myeloid elements in bone marrow. Each of the BTDs possessed antimicrobial activity against bacterial and fungal test organisms in vitro. Peptide activities varied markedly despite a high degree of sequence conservation among the theta-defensins tested. Thus, baboons express numerous theta-defensins which appear to differentially contribute to host defense against diverse pathogens.

Structure-function studies of Bubalus bubalis lingual antimicrobial peptide analogs

Antimicrobial peptides expressed on different epithelial lining are major components of the innate immune system. Based on the deduced amino acid sequence of Bubalus bubalis lingual antimicrobial peptide (LAP) cDNA (Accession No. DQ458768), five overlapping peptides LAP(23-55), LAP(42-64), LAP(21-64), LAP(1-26) and LAP(1-64) were synthesized using solid phase fluorenylmethoxycarbonyl (Fmoc) chemistry. Circular Dichroism spectroscopy of synthesized peptides revealed predominantly beta-structure for LAP(23-55,) LAP(42-64) and LAP(21-64) with less alpha-helix in different solutions. Quantitation of secondary structure indicated the highest beta-structure for all these three peptides in membrane mimetic SDS solution. The helicogenic solvent TFE could not induce helix in LAP(23-55) however TFE induced helical propensity was observed in LAP(42-64) and LAP(21-64). The quantitation of secondary structure indicated the highest ordered structure for LAP(23-55) followed by LAP(42-64) and LAP(21-64). The antibacterial activity of LAP(23-55) was found to be more potent against Staphylococcus aureus, Listeria monocytogens, Escherichia coli and Salmonella typhimurium followed by LAP(42-64) and LAP(21-64). Minimum inhibitory concentration (MIC) also showed similar trend with lowest value for LAP(23-55) followed by LAP(42-64) and LAP(21-64). Haemolysis and cytotoxicity was observed above 3 fold for LAP(21-64,) above six fold for LAP(23-55) and LAP(42-64) of their MIC. The LAP(1-26) and LAP(1-64) could not produce any characteristic CD spectra and did not show any antimicrobial activity, indicating that N- terminal of the peptide negates the antimicrobial activity.

Molecular cloning and characterization of lingual antimicrobial peptide cDNA of Bubalus bubalis

Antimicrobial peptides form a crucial component of innate immune system, making it a highly effective first line of defense in animals. In the study, lingual antimicrobial peptide cDNA of Bubalus bubalis has been characterized. The characterized cDNA has complete ORF of 195 bases. The signal sequence of buffalo LAP comprised of N-terminal 1-20 amino acids and mature peptide from 23-64 amino acids. The percentage of similarity of buffalo LAP and buffalo EBD at nucleotide and amino acid level was 96.4% and 92.3% respectively. The identity of buffalo LAP with cattle LAP and TAP at nucleotide level was 92.8% and 90.3%. Both at nucleotide and amino acid level buffalo LAP is closer to buffalo EBD followed by cattle LAP and TAP. Phylogenetic tree at nucleotide and amino acid level also showed close relationship of buffalo LAP with buffalo EBD, cattle LAP and TAP. The synthesized LAP fragment had antibacterial activity.

The capsule sensitizes Streptococcus pneumoniae to alpha-defensins human neutrophil proteins 1 to 3

Streptococcus pneumoniae is a major cause of morbidity and mortality worldwide. Its polysaccharide capsule causes resistance to phagocytosis and interferes with the innate immune system's ability to clear infections at an early stage. Nevertheless, we found that encapsulated pneumococci are sensitive to killing by a human neutrophil granule extract. We fractionated the extract by high-performance liquid chromatography and identified alpha-defensins by mass spectrometry as the proteins responsible for killing pneumococci. Analysis of sensitivity to the commercial alpha-defensins human neutrophil proteins 1 to 3 (HNP1-3) confirmed these findings. We analyzed the sensitivities of different pneumococcal strains to HNP1-3 and found that encapsulated strains are efficiently killed at physiological concentrations (7.5 microg/ml). Surprisingly, nonencapsulated, nonvirulent pneumococci were significantly less sensitive to alpha-defensins. The proposed mechanisms of alpha-defensin resistance in nonencapsulated pneumococci is surface charge modification, e.g., by introduction of positive charge by D-alanylation of surface-exposed lipoteichoic acids. These mechanisms are surmounted by the presence of the capsule, which we hypothesize is masking these charge modifications. Hence, alpha-defensins in the phagolysosome of neutrophils possibly contribute to intracellular killing after antibody-mediated opsonophagocytosis of encapsulated pneumococci.

Study of the Interaction of Human Defensins with Cell Membrane Models: Relationships between Structure and Biological Activity

The HNP-1, HNP-2, and HNP-3 defensins are human antimicrobial peptides produced in response to microbial invasion. Their properties are distinct, with a more potent action for HNP-3. In this study, the relationship between their structural dissimilarities and their different microbial actions was evaluated by molecular dynamics simulation. Structural determinants related to their intra- and intermolecular interactions were defined for each HNP using a simplified membrane model consisting of a water/n-hexane interface. The hydrophobic portion of the HNPs promotes their diffusion to the interface with a concomitant, slight change in the structure induced by the intermolecular electrostatic interactions between the HPN molecules and the interface. As a consequence, different orientations are probably adopted by the HNPs at the interface, which may explain their different actions. The interaction of HNP-1 and HNP-2 with the surfaces was also studied using Langmuir monolayers as a biomimetic system. It was found that peptides adsorb rapidly at n-hexane/water interfaces as well as at phospholipid Langmuir monolayers but not at the air/liquid interface. This reveals that the presence of an organic phase is required for the exposure of the hydrophobic groups of the peptides. In addition, adsorption kinetics and surface pressure-area isotherms for Langmuir monolayers suggested that the lipid-peptide interaction is strongly influenced by the monolayer electrical charge and packing, depending also on the HPN structure. This study supports a model in which defensins, acting in a dimeric form, are able to disrupt membranes. The model also shows that the individual structures of the HNPs are responsible for their different actions on microbes.

Menstrual hemocidin HbB115-146 is an acidophilic antibacterial peptide potentiating the activity of human defensins, cathelicidin and lysozyme

PROBLEM

Our recent studies proved that menstrual discharge is exceptionally rich in bactericidal hemoglobin peptides (hemocidins). Of special interest is the behavior of hemocidins in low pH of the vagina, in different ionic strengths, and in the presence of other specialized antibacterial molecules acting in this organ.

METHODS OF STUDY

We studied the activity of a model representative of menstrual hemocidin: the peptide from human beta-globin, spanning residues 115-146 (HbB115-146). The bactericidal action of this peptide in different physicochemical conditions, as well as the evaluation of the synergistic effect of the peptide with mixtures of neutrophil alpha-defensin HNP-1, epithelial beta-defensin HBD-1, cathelicidin LL-37 and lysozyme were studied using the bacterial membrane permeability test.

RESULTS

The HbB115-146 is a salt-resistant antibiotic molecule strongly potentiating its activity in acidic conditions (pH 4.4-5.0), characteristic for the vagina. Hemocidin HbB115-146 is also an effective factor stimulating the activity of other antibacterial polypeptides present in the female urogenital tract. The observed synergistic effect is preserved or enhanced at lower pH and, with the exception of HBD-1, is observed also at physiological salt concentrations.

CONCLUSION

The results of this study clearly demonstrate that hemocidins are important polypeptide factors involved in maintaining vaginal immunity during normal menstrual bleedings.

Differentiation of Leishmania major is impaired by over-expression of pyroglutamyl peptidase I

Pyroglutamyl peptidases I (PPI) are cysteine peptidases of the clan CF, family C15, which hydrolyse N-terminal l-pyroglutamyl residues (l-pGlu). The l-pGlu modification is a post-transcriptional modification that confers relative aminopeptidase resistance and, in some cases, is essential to the modified peptides' biological activity. PPIs have been identified in a variety of organisms, although definitive biological functions have yet to be attributed to them. The L. major PPI was expressed in Escherichia coli as active recombinant enzyme, and shown to have biochemical properties more similar to mammalian than bacterial PPIs. The LmPPI active site catalytic triad of E101, C210, and H234 was confirmed by mutagenesis. PPI activity was detected in L. major promastigotes, and the enzyme localised to the parasite cytosol. No detectable phenotype could be observed for L. major PPI-deficient mutants, which retained infectivity to macrophages in vitro and mice. However, over-expression of the active PPI, but not inactive PPI(C210A), in L. major impaired differentiation from the procyclic promastigote to the infective metacyclic promastigote. Susceptibility to a natural l-pGlu-modified antimicrobial peptide, gomesin, was tested using the different cell lines, which were all equally susceptible. Whilst PPI is widespread through the eukaryotic kingdom, this study now suggests that the enzyme is not essential for normal eukaryotic cell function. However, PPI could be involved in regulating the action of l-pGlu-modified peptides required for differentiation of L. major.

Paneth cell alpha-defensin synthesis and function

Endogenous antimicrobial peptides (AMPs) mediate innate immunity in every species in which they have been investigated. Cathelicidins and defensins are the two major AMP families in mammals, and they are abundant components of phagocytic leukocytes and are released by epithelial cells at mucosal surfaces. In the small intestine, Paneth cells at the base of the crypts of Lieberkühn secrete alpha-defensins and additional AMPs at high levels in response to cholinergic stimulation and when exposed to bacterial antigens. Paneth cell alpha-defensins evolved to function in the extracellular environment with broad-spectrum antimicrobial activities, and they constitute the majority of bactericidal peptide activity secreted by Paneth cells. The release of Paneth cell products into the crypt lumen is inferred to protect mitotically active crypt cells from colonization by potential pathogens and confers protection from enteric infection, as is evident from the immunity of mice expressing a human Paneth cell alpha-defensin transgene to oral infection by Salmonella enterica serovar Typhimurium. alpha-Defensins in Paneth cell secretions also may interact with bacteria in the intestinal lumen above the crypt-villus boundary and influence the composition of the enteric microbial flora. Mutations that cause defects in the activation, secretion, dissolution, and bactericidal effects of Paneth cell AMPs may alter crypt innate immunity and contribute to immunopathology.

Antibacterial activity of human neutrophil defensin HNP-1 analogs without cysteines

The antibacterial activity of human neutrophil defensin HNP-1 analogs without cysteines has been investigated. A peptide corresponding to the HNP-1 sequence without the six cysteines (HNP-1deltaC) exhibited antibacterial activity toward gram-negative and gram-positive bacteria. Truncated analogs wherein the nine N-terminal residues of HNP-1 and the remaining three cysteines were deleted (HNP-1deltaC18) or the G was replaced with A (HNP-1deltaC18A) also exhibited antibacterial activity. Substantial activity was observed for HNP-1deltaC and HNP-1deltaC18 in the presence of 100 mM NaCl, except in the case of Pseudomonas aeruginosa. The linear peptides were active in the presence of carbonyl cyanide m-chlorophenylhydrazone (CCCP), indicating that proton motive force was not essential for killing of bacteria by the peptides. In fact, in the presence of CCCP, the peptides were active against P. aeruginosa even in the presence of 100 mM NaCl. The antibacterial activity of HNP-1deltaC, but not that of the shorter, 18-residue peptides, was attenuated in the presence of serum. The generation of defensins without cysteines would be easier than that of disulfide-linked defensins. Hence, linear defensins could have potential as therapeutic agents.

Effects of candidate vaginally-applied microbicide compounds on innate immune cells

Ideally, a vaginally-applied microbicide would be effective against a broad range of pathogens but would have minimal effects on the female genital tract. The aim of this study was to determine if representative candidate detergent-type and sulfated/sulfonated polymer-type microbicides altered the composition or function of innate immune cells normally found in the vaginal mucosa. The effect of microbicide on the composition of vaginal leukocytes was tested using a flow cytometric approach. Application of the detergent cholic acid, but not the sulfated polysaccharide lambda carrageenan, resulted in a significant increase in macrophages at the vaginal epithelial surface compared to control treatment (19.3% macrophages compared to 2.8%; p<0.0004). Phagocytosis of fluorochrome-labeled bacteria by macrophages was inhibited greater than 50% in the presence of 1.0mg/ml of the sulfonated polymer PRO 2000 but was not inhibited by the same concentration of lambda carrageenan. PRO 2000-pulsed macrophages regained phagocytic function after being washed free of the compound. Culture of macrophages with PRO 2000 also resulted in diminished detection of the surface proteins CD11b and CD18. After treated cells were washed free of PRO 2000, these proteins were detected at levels similar to control treated cells. In conclusion, application of a detergent-type microbicide, but not a sulfated polymer, resulted in the infiltration of inflammatory cells at the vaginal epithelial surface. Phagocytic function of macrophages was lost in the presence of 1mg/ml PRO 2000 which may have reflected masking of important cell surface proteins by the microbicide; however, there was no evidence of permanent loss of function upon removal of the compound.

Paneth cell α-defensins: peptide mediators of innate immunity in the small intestine

Epithelial cells contribute to innate immunity by releasing antimicrobial peptides (AMPs) onto mucosal surfaces. In the small bowel, Paneth cells at the base of the crypts of Lieberkühn secrete alpha-defensins and additional AMPs at high levels in response to cholinergic stimulation and when exposed to bacterial antigens. The release of Paneth cell products into the crypt lumen is inferred to protect mitotically active crypt cells that renew the epithelial cell monolayer from colonization by potentially pathogenic microbes and to confer protection from enteric infection. The most compelling evidence for a Paneth cell role in enteric resistance to infection is evident from studies of mice transgenic for a human Paneth cell alpha-defensin, HD-5, which are completely immune to infection and systemic disease from orally administered Salmonella enterica serovar typhimurium. Cystic fibrosis mice are subject to small bowel bacterial overgrowth that is associated with impaired dissolution of released Paneth cell granules in the crypt lumen. Mutations that cause defects in the activation, secretion, dissolution, and bactericidal effects of Paneth cell AMPs may alter crypt innate immunity and contribute to immunopathology.

Structural and functional differences of Litopenaeus vannamei crustins

Penaeid crustins were described in Litopenaeus vannamei and L. setiferus as proteins belonging to an antibacterial peptide family with similar sequences but different sizes. Six crustin-coding clones were isolated from a cDNA library from L. vannamei hemocytes, sequenced and compared. Two different isoforms (named I and P) were found, based on two nucleotide differences that produce one change in amino acid sequence (Ile/Pro). Other single differences in nucleotide sequences were also noted, but they did not change the translated product. The mRNA steady state levels of crustin I, but not of crustin P, were down regulated by Vibrio alginolyticus inoculation. Thus, the differences among penaeid crustins seem to be associated with one amino acid substitution, which affects their expression after bacterial inoculation. By structural similarity, shrimp crustins seem to belong to an antibacterial WAP-domain containing protein family.

alpha2-Macroglobulin-proteinase complexes protect Streptococcus pyogenes from killing by the antimicrobial peptide LL-37

The significant human bacterial pathogen Streptococcus pyogenes expresses GRAB, a surface protein that binds alpha(2)-macroglobulin (alpha(2)M), a major proteinase inhibitor of human plasma. alpha(2)M inhibits proteolysis by trapping the proteinase, which, however, still remains proteolytically active against smaller peptides that can penetrate the alpha(2)M-proteinase complex. Here we report that SpeB, a cysteine proteinase secreted by S. pyogenes, is trapped by alpha(2)M bound to protein GRAB. As a consequence, SpeB is retained at the bacterial surface and protects S. pyogenes against killing by the antibacterial peptide LL-37.

Pseudomonas aeruginosa-induced infection and degradation of human wound fluid and skin proteins ex vivo are eradicated by a synthetic cationic polymer

OBJECTIVES

Antimicrobial peptides are important effectors of innate immunity. Bacteria display multiple defence mechanisms against these peptides. For example, Pseudomonas aeruginosa releases potent proteinases that inactivate the human cathelicidin LL-37. Hence, in conditions characterized by persistent bacterial colonization, such as in P. aeruginosa-infected skin wounds, there is a need for efficient means of reducing bacterial load. Here, the effect of the cationic molecule polyhexamethylenebiguanide (PHMB) was evaluated.

METHODS

Infection models in human wound fluid and human skin were established. Radial diffusion methods, bacterial growth and bactericidal assays were used for determination of effects of PHMB on bacteria in the presence of plasma, wound fluid or human skin. At the protein and tissue levels, SDS-PAGE, light microscopy and scanning electron microscopy were used to study the effects of P. aeruginosa infection before and after addition of PHMB.

RESULTS

PHMB killed common ulcer-derived bacteria in the presence of human wound fluid. Furthermore, elastase-expressing P. aeruginosa completely degraded wound fluid proteins as well as human skin during infection ex vivo. The infection, and consequent protein degradation, was reversed by PHMB.

CONCLUSIONS

The ex vivo infection models presented here should be helpful in the screening of novel antimicrobials and constitute a prerequisite for future clinical studies.

Induction and antimicrobial activity of platelet basic protein derivatives in human monocytes

The antimicrobial activity of a number of chemokines has recently come into focus of research about innate immunity. We have previously shown that platelet basic protein (PBP), which gives rise to several antimicrobial peptides of platelets, is also expressed in human monocytes. In the present studies, we show that exposure of human monocytes to bacteria or microbial components (lipopolysaccharide and zymosan) induces a several-fold greater expression of derivates of PBP. Also, activation of proteinase-activated receptors (PARs) by thrombin or the synthetic peptide ligand SFLLRN of PAR-1 significantly increased PBP expression, presumably on the transcriptional level, as evidenced by higher mRNA levels. Derivates of PBP appeared to reach phago-lysosomes, as higher concentration was found in latex phagosomes isolated by a flotation method. By the gel-overlay technique, two bactericidal derivatives of PBP could be visualized, which were immunoreactive with anti-PBP antibody in Western blots. By matrix-assisted laser desorption/ionization time of flight and surface-enhanced laser desorption and ionization techniques, it was confirmed that the bands corresponded to PBP derivates. After immunofixation with a monoclonal antibody to PBP, the major peptide in zymosan-stimulated monocytes was identified to correspond by molecular weight to connective tissue-activating peptide III, which has been reported to be a major antimicrobial PBP derivate also in platelets. Our observations indicate that PBP and its derivates are constituents of the antimicrobial arsenal of human monocytes. Their increased expression after exposure to microorganisms allows a rapid host response to pathogens.

Ribosomally synthesized peptides with antimicrobial properties: biosynthesis, structure, function, and applications

Ribosomally synthesized peptides with antimicrobial properties (antimicrobial peptides-AMPs) are produced by eukaryotes and prokaryotes and represent crucial components of their defense systems against microorganisms. Although they differ in structure, they are nearly all cationic and very often amphiphilic, which reflects the fact that many of them attack their target cells by permeabilizing the cell membrane. They can be roughly categorized into those that have a high content of a certain amino acid, most often proline, those that contain intramolecular disulfide bridges, and those with an amphiphilic region in their molecule if they assume an alpha-helical structure. Most of the known ribosomally synthesized peptides with antimicrobial functions have been identified and studied during the last 20 years. As a result of these studies, new knowledge has been acquired into biology and biochemistry. It has become evident that these peptides may be developed into useful antimicrobial additives and drugs. The use of two-peptide antimicrobial peptides as replacement for clinical antibiotics is promising, though their applications in preservation of foods (safe and effective for use in meat, vegetables, and dairy products), in veterinary medicine, and in dentistry are more immediate. This review focuses on the current status of some of the main types of ribosomally synthesized AMPs produced by eucaryotes and procaryotes and discusses the novel antimicrobial functions, new developments, e.g. heterologous production of bacteriocins by lactic acid bacteria, or construction of multibacteriocinogenic strains, novel applications related to these peptides, and future research paradigms.

Paneth cell alpha-defensins from rhesus macaque small intestine

Antimicrobial peptides are secreted by small intestinal Paneth cells as components of innate immunity. To investigate the role of alpha-defensins in enteric host defenses in nonhuman primates, alpha-defensin cDNAs were isolated, alpha-defensin peptides were purified from rhesus macaque small bowel, and the bactericidal activities of the peptides were measured. Six rhesus enteric alpha-defensin (RED) cDNAs, RED-1 to RED-6, were identified in a jejunum cDNA library; the deduced RED peptides exhibited extensive diversity relative to the primary structures of rhesus myeloid alpha-defensins. RED-4 was purified from monkey jejunum, and N-terminal peptide sequencing of putative RED-4 peptides identified two N termini, RTCYCRTGR. and TCYCRTGRC.; these corresponded to alternative N termini for the RED-4 molecules, as deduced from their molecular masses and RED cDNAs. In situ hybridization experiments localized RED mRNAs exclusively to small intestinal Paneth cells. Recombinant RED-1 to RED-4 were purified to homogeneity and shown to be microbicidal in the low micromolar range (</=10 micro g/ml) against gram-positive and gram-negative bacteria, with individual peptides exhibiting variable target cell specificities. Thus, compared to myeloid alpha-defensins from rhesus macaques, enteric alpha-defensin peptides are highly variable in both primary structure and activity. These studies should facilitate further analyses of the role of alpha-defensins in primate enteric immunity.

Defensin-mediated innate immunity in the small intestine

Epithelial cells contribute to innate immunity by releasing antimicrobial peptides (AMPs) onto mucosal surfaces. In the small bowel, Paneth cells at the base of the crypts of Lieberkühn secrete alpha-defensins and additional AMPs at high levels in response to cholinergic stimulation and when exposed to bacterial antigens. The release of Paneth cell products into the crypt lumen is inferred to protect mitotically active crypt cells that renew the epithelial cell monolayer from colonization by potentially pathogenic microbes and to confer protection from enteric infection. The most compelling evidence for a Paneth cell role in enteric resistance to infection is evident from studies of mice transgenic for a human Paneth cell alpha-defensin, HD-5, which are completely immune to infection and systemic disease from orally administered Salmonella typhimurium. Alpha-defensins in Paneth cell secretions may also interact with bacteria in the intestinal lumen above the crypt-villus boundary and influence the composition of the enteric microbial flora, but that remains to be demonstrated.

Antimicrobial activities of heparin-binding peptides

Antimicrobial peptides are effector molecules of the innate immune system. We recently showed that the human antimicrobial peptides alpha-defensin and LL-37 bind to glycosaminoglycans (heparin and dermatan sulphate). Here we demonstrate the obverse, i.e. structural motifs associated with heparin affinity (cationicity, amphipaticity, and consensus regions) may confer antimicrobial properties to a given peptide. Thus, heparin-binding peptides derived from laminin isoforms, von Willebrand factor, vitronectin, protein C inhibitor, and fibronectin, exerted antimicrobial activities against Gram-positive and Gram-negative bacteria. Similar results were obtained using heparin-binding peptides derived from complement factor C3 as well as consensus sequences for heparin-binding (Cardin and Weintraub motifs). These sequence motifs, and additional peptides, also killed the fungus Candida albicans. These data will have implications for the search for novel antimicrobial peptides and utilization of heparin-protein interactions should be helpful in the identification and purification of novel antimicrobial peptides from complex biological mixtures. Finally, consensus regions may serve as templates for de novo synthesis of novel antimicrobial molecules.

SIC, a secreted protein of Streptococcus pyogenes that inactivates antibacterial peptides

Some isolates of the significant human pathogen Streptococcus pyogenes, including virulent strains of the M1 serotype, secrete protein SIC. This molecule, secreted in large quantities, interferes with complement function. As a result of natural selection, SIC shows a high degree of variation. Here we provide a plausible explanation for this variation and the fact that strains of the M1 serotype are the most frequent cause of severe invasive S. pyogenes infections. Thus, protein SIC was found to inactivate human neutrophil alpha-defensin and LL-37, two major antibacterial peptides involved in bacterial clearance. This inactivation protected S. pyogenes against the antibacterial effect of the peptides. Moreover, SIC isolated from S. pyogenes of the M1 serotype was more powerful in this respect than SIC variants from strains of M serotypes 12 and 55, serotypes rarely connected with invasive infections.

Proteinases of common pathogenic bacteria degrade and inactivate the antibacterial peptide LL-37

Effectors of the innate immune system, the anti-bacterial peptides, have pivotal roles in preventing infection at epithelial surfaces. Here we show that proteinases of the significant human pathogens Pseudomonas aeruginosa, Enterococcus faecalis, Proteus mirabilis and Streptococcus pyogenes, degrade the antibacterial peptide LL-37. Analysis by mass spectrometry of fragments generated by P. aeruginosa elastase in vitro revealed that the initial cleavages occurred at Asn-Leu and Asp-Phe, followed by two breaks at Arg-Ile, thus inactivating the peptide. Proteinases of the other pathogens also degraded LL-37 as determined by SDS-PAGE. Ex vivo, P. aeruginosa elastase induced LL-37 degradation in human wound fluid, leading to enhanced bacterial survival. The degradation was blocked by the metalloproteinase inhibitors GM6001 and 1, 10-phenantroline (both of which inhibited P. aeruginosa elastase, P. mirabilis proteinase, and E. faecalis gelatinase), or the inhibitor E64 (which inhibited S. pyogenes cysteine proteinase). Additional experiments demonstrated that dermatan sulphate and disaccharides of the structure [DeltaUA(2S)-GalNAc(4,6S)], or sucroseoctasulphate, inhibited the degradation of LL-37. The results indicate that proteolytic degradation of LL-37 is a common virulence mechanism and that molecules which block this degradation could have therapeutic potential.

Antibacterial activities and conformations of synthetic alpha-defensin HNP-1 and analogs with one, two and three disulfide bridges

Structure and biological activities of synthetic peptides corresponding to human alpha-defensin HNP-1, AC1YC2RIPAC3IAGERRYGTC4IYQGRLWAFC5C6 with the S-S connectivities: C1-C6, C2-C4, C3-C5, and its variants with one, two and three disulfide bridges were investigated. Oxidation of synthetic, reduced HNP-1 yielded a peptide with S-S connectivities C1-C3, C2-C4 and C5-C6, and not with the S-S linkages as in naturally occurring HNP-1. Selective protection of cysteine sulfhydryls was necessary for the formation of S-S bridges as in native HNP-1. Likewise, oxidation of peptide encompassing the segment from C2 to C5, resulted in the S-S linkages C2-C3 and C4-C5 instead of the expected linkage C2-C4 and C3-C5. Antibacterial activities were observed for all peptides, irrespective of how the S-S bridges were linked. Linear peptides without S-S bridges were inactive. Circular dichroism (CD) spectra suggest that peptides constrained by one and two S-S bridges do not form rigid beta-sheet structures in an aqueous environment. The spectrum of HNP-1 in an aqueous environment suggests the presence of a beta-hairpin conformation. In the presence of lipid vesicles, the S-S constrained peptides tend to adopt a beta-structure. Although the S-S connectivities observed in HNP-1 may be necessary for other physiological activities, such as chemotaxis, they are clearly not essential for antibacterial activity.

Antibacterial activities and conformations of bovine beta-defensin BNBD-12 and analogs:structural and disulfide bridge requirements for activity

Structure and biological activities of synthetic peptides corresponding to bovine neutrophil beta-defensin BNBD-12, GPLSC(1)GRNGGVC(2)IPIRC(3) PVPMRQIGTC(4) FGRPVKC(5) C(6)RSW with disulfide connectivities C(1)-C(5), C(2)-C(4) and C(3)-C(6) and its variants with one, two and three disulfide bridges have been investigated. Selective protection of cysteine thiols was necessary in the four and six cysteine containing peptides for the formation of disulfide connectivities as observed in BNBD-12. Circular dichroism (CD) spectra indicate that in aqueous medium, only a small fraction of molecules populate turn-like conformations. In the presence of micelles and lipid vesicles, the single, two and three disulfide containing peptides adopt beta-hairpin or beta-sheet structures. Antibacterial activity was observed for all the peptides, irrespective of the number of disulfide bridges or how they were connected. Our results suggest that a rigid beta-sheet structure or the presence of three disulfide bridges does not appear to be stringent requirements for antibacterial activity in beta-defensins.

The ability of endotoxin-stimulated enterocytes to produce bactericidal factors

OBJECTIVE

Bactericidal peptides, specifically defensins, are produced by polymorphonuclear cells. Intestinal epithelial cells also produce bactericidal peptides, perhaps as part of their barrier function, to the greatest load of endogenous bacteria present in the body. We sought to determine whether and under what conditions intestinal cell lines could produce bactericidal compounds.

DESIGN

Laboratory investigation.

SETTING

Children's burn hospital.

SUBJECTS

Caco-2, IEC-6, and HT-29 cell lines.

INTERVENTIONS

Three different enterocyte lines were cultured for 1 day +/- lipopolysaccharide (1 or 10 microg/mL), and their supernatants were tested for bactericidal activity. Also, reverse transcription-polymerase chain reaction of Caco-2 cells was performed to assess the expression of defensin-6 mRNA.

MEASUREMENTS AND MAIN RESULTS

After culture, enterocytes all were found to release one or more soluble factors with bactericidal activity (as measured fluorometrically by using a metabolizable dye) when stimulated by lipopolysaccharide (1 microg/mL). The bactericidal activity of these culture supernatants was saturated by increased bacterial load, additive to the effects of normal human peripheral blood polymorphonuclear cells, and was reduced by serial supernatant dilution. Enterocyte stimulation with larger amounts of lipopolysaccharide (10 microg/mL) resulted in greater bactericidal activity. After supernatant fractionation based on molecular weight, the bactericidal effect was best retained in the <10-kDa fraction. In addition, the expression of mRNA for defensin-6, a bactericidal peptide produced by neutrophils, was seen in Caco-2 cells.

CONCLUSION

Enterocytes are shown to produce a soluble, low molecular weight, bactericidal compound in response to endotoxin stimulation. The expression of defensin-6 mRNA in Caco-2 cells suggests that intestinal cells may release defensins as bactericidal peptides. This experimental system provides an in vitro model to study the activity and production of bactericidal factors by enterocytes.

Dermatan sulphate is released by proteinases of common pathogenic bacteria and inactivates antibacterial alpha-defensin

Defensins represent an evolutionarily conserved group of small peptides with potent antibacterial activities. We report here that extracellular proteinases secreted by the human pathogens Pseudomonas aeruginosa, Enterococcus faecalis and Streptococcus pyogenes release dermatan sulphate by degrading dermatan sulphate-containing proteoglycans, such as decorin. Dermatan sulphate was found to bind to neutrophil-derived alpha-defensin, and this binding completely neutralized its bactericidal activity. During infection, proteoglycan degradation and release of dermatan sulphate may therefore represent a previously unknown virulence mechanism, which could serve as a target for novel antibacterial strategies.

Characterization of luminal paneth cell alpha-defensins in mouse small intestine. Attenuated antimicrobial activities of peptides with truncated amino termini

Paneth cells at the base of small intestinal crypts secrete apical granules that contain antimicrobial peptides including alpha-defensins, termed cryptdins. Using an antibody specific for mouse cryptdin-1, -2, -3, and -6, immunogold-localization studies demonstrated that cryptdins are constituents of mouse Paneth cell secretory granules. Several cryptdin peptides have been purified from rinses of adult mouse small intestine by gel filtration and reverse-phase high performance liquid chromatography. Their primary structures were determined by peptide sequencing, and their antimicrobial activities were compared with those of the corresponding tissue forms. The isolated luminal cryptdins included peptides identical to the tissue forms of cryptdin-2, -4, and -6 as well as variants of cryptdin-1, -4, and -6 that have N termini truncated by one or two residues. In assays of antimicrobial activity against Staphylococcus aureus, Escherichia coli, and the defensin-sensitive Salmonella typhimurium phoP(-) mutant, full-length cryptdins had the same in vitro antibacterial activities whether isolated from tissue or from the lumen. In contrast, the N-terminal-truncated (des-Leu), (des-Leu-Arg)-cryptdin-6, and (des-Gly)-cryptdin-4 peptides were markedly less active. The microbicidal activities of recombinant cryptdin-4 and (des-Gly)-cryptdin-4 peptides against E. coli, and S. typhimurium showed that the N-terminal Gly residue or the length of the cryptdin-4 N terminus are determinants of microbicidal activity. Innate immunity in the crypt lumen may be modulated by aminopeptidase modification of alpha-defensins after peptide secretion.

Structural features and biological activities of the cathelicidin-derived antimicrobial peptides

Cathelicidins are a numerous group of mammalian proteins that carry diverse antimicrobial peptides at the C-terminus of a highly conserved preproregion. These peptides, which become active when released from the proregion, display a remarkable variety of sizes, sequences, and structures, and in fact comprise representatives of all the structural groups in which the known antimicrobial peptides have been classified. Most of the cathelicidin-derived peptides exert a broad spectrum and potent antimicrobial activity and also bind to lipopolysaccharide and neutralize its effects. In addition, some of them have recently been shown to exert other activities and might participate in host defense also by virtue of their ability to induce expression of molecules involved in a variety of biological processes. This review is aimed at providing a general overview of the cathelicidins and of the peptides derived therefrom, with emphasis on aspects such as structure, biological activities in vitro and in vivo, and structure/activity relationship studies.