Defensins: microbicidal and cytotoxic peptides of mammalian host defense cells

Serum β-Defensin 2, A Novel Biomarker for the Diagnosis of Acute Infections

BACKGROUND

Defensins are natural antimicrobial peptides that the human body secretes to protect itself from an infection. Thus, they are ideal molecules to serve as biomarkers for infection. This study was conducted to evaluate the levels of human β-defensins in patients with inflammation.

METHODS

CRP, hBD2 and procalcitonin were measured in 423 sera of 114 patients with inflammation and healthy individuals using nephelometry and commercial ELISA assays.

RESULTS

Levels of hBD2 in the serum of patients with an infection were markedly elevated compared to those of hBD2 in patients with inflammation of non-infectious etiology (p < 0.0001, t = 10.17) and healthy individuals. ROC analysis demonstrated that hBD2 showed the highest detection performance for infection (AUC 0.897; p < 0.001) followed by PCT (AUC 0.576; p = ns) and CRP (AUC 0.517; p = ns). In addition, analysis of hBD2 and CRP in patients' sera collected at different time points showed that hBD2 levels could help differentiate inflammation of infectious and non-infectious etiology during the first 5 days of hospitalization, while CRP levels could not.

CONCLUSIONS

hBD2 has the potential to serve as a diagnostic biomarker for infection. In addition, the levels of hBD2 may reflect the efficacy of antibiotic treatment.

Antimicrobial peptides with selective antitumor mechanisms: prospect for anticancer applications

In the last several decades, there have been significant advances in anticancer therapy. However, the development of resistance to cancer drugs and the lack of specificity related to actively dividing cells leading to toxic side effects have undermined these achievements. As a result, there is considerable interest in alternative drugs with novel antitumor mechanisms. In addition to the recent approach using immunotherapy, an effective but much cheaper therapeutic option of pharmaceutical drugs would still provide the best choice for cancer patients as the first line treatment. Ribosomally synthesized cationic antimicrobial peptides (AMPs) or host defense peptides (HDP) display broad-spectrum activity against bacteria based on electrostatic interactions with negatively charged lipids on the bacterial surface. Because of increased proportions of phosphatidylserine (negatively charged) on the surface of cancer cells compared to normal cells, cationic amphipathic peptides could be an effective source of anticancer agents that are both selective and refractory to current resistance mechanisms. We reviewed herein the prospect for AMP application to cancer treatment, with a focus on modes of action of cationic AMPs.

Characterization of Antimicrobial Peptides toward the Development of Novel Antibiotics

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

Rat recombinant β-defensin 22 is a heparin-binding protein with antimicrobial activity

Approximately 40-50 β-defensins are predominantly expressed in the male reproductive system of mammals. This selective expression raises the question as to the roles of these molecules in innate immunity and fertility in the male reproductive tract. Rat β-defensin 22 is an epididymis-specific β-defensin expressed in segments 12-14 of the epididymis. This protein contains both β-defensin and lectin signature sequences, yet its antimicrobial activity and carbohydrate-binding ability have not been shown. We herein demonstrated the antimicrobial activity of recombinant rat β-defensin 22 against Escherichia coli and Candida albicans. Its lectin-like activity was also investigated by demonstrating its binding ability with heparin beads. This heparin-binding activity implies some potential roles for this defensin in determining the fertilisation capabilities of sperm.

Fusobacterium nucleatum-associated beta-defensin inducer (FAD-I): identification, isolation, and functional evaluation

Human β-defensins (hBDs) are small, cationic antimicrobial peptides, secreted by mucosal epithelial cells that regulate adaptive immune functions. We previously reported that Fusobacterium nucleatum, a ubiquitous gram-negative bacterium of the human oral cavity, induces human β-defensin 2 (hBD2) upon contact with primary oral epithelial cells. We now report the isolation and characterization of an F. nucleatum (ATCC 25586)-associated defensin inducer (FAD-I). Biochemical approaches revealed a cell wall fraction containing four proteins that stimulated the production of hBD2 in human oral epithelial cells (HOECs). Cross-referencing of the N-terminal sequences of these proteins with the F. nucleatum genome revealed that the genes encoding the proteins were FadA, FN1527, FN1529, and FN1792. Quantitative PCR of HOEC monolayers challenged with Escherichia coli clones expressing the respective cell wall proteins revealed that FN1527 was most active in the induction of hBD2 and hence was termed FAD-I. We tagged FN1527 with a c-myc epitope on the C-terminal end to identify and purify it from the E. coli clone. Purified FN1527 (FAD-I) induced hBD2 mRNA and protein expression in HOEC monolayers. F. nucleatum cell wall and FAD-I induced hBD2 via TLR2. Porphorymonas gingivalis, an oral pathogen that does not induce hBD2 in HOECs, was able to significantly induce expression of hBD2 in HOECs only when transformed to express FAD-I. FAD-I or its derivates offer a potentially new paradigm in immunoregulatory therapeutics because they may one day be used to bolster the innate defenses of vulnerable mucosae.

Selective reciprocity in antimicrobial activity versus cytotoxicity of hBD-2 and crotamine

Recent discoveries suggest cysteine-stabilized toxins and antimicrobial peptides have structure-activity parallels derived by common ancestry. Here, human antimicrobial peptide hBD-2 and rattlesnake venom-toxin crotamine were compared in phylogeny, 3D structure, target cell specificity, and mechanisms of action. Results indicate a striking degree of structural and phylogenetic congruence. Importantly, these polypeptides also exhibited functional reciprocity: (i) they exerted highly similar antimicrobial pH optima and spectra; (ii) both altered membrane potential consistent with ion channel-perturbing activities; and (iii) both peptides induced phosphatidylserine accessibility in eukaryotic cells. However, the Na(v) channel-inhibitor tetrodotoxin antagonized hBD-2 mechanisms, but not those of crotamine. As crotamine targets eukaryotic ion channels, computational docking was used to compare hBD-2 versus crotamine interactions with prototypic bacterial, fungal, or mammalian Kv channels. Models support direct interactions of each peptide with Kv channels. However, while crotamine localized to occlude Kv channels in eukaryotic but not prokaryotic cells, hBD-2 interacted with prokaryotic and eukaryotic Kv channels but did not occlude either. Together, these results support the hypothesis that antimicrobial and cytotoxic polypeptides have ancestral structure-function homology, but evolved to preferentially target respective microbial versus mammalian ion channels via residue-specific interactions. These insights may accelerate development of anti-infective or therapeutic peptides that selectively target microbial or abnormal host cells.

The clinical value of neutrophil extracellular traps

Neutrophil extracellular traps (NETs) have recently been discovered as a central part of antimicrobial innate immunity. In the meanwhile, evidence accumulated that NETs are also generated upon non-infectious stimuli in various clinical settings. In acute or chronic inflammatory disorders aberrantly enhanced NET formation and/or decreased NET degradation seems to correlate with disease outcome. This review summarizes current knowledge about the relation of NETs in a broad spectrum of clinical settings. Specifically, we focus on the importance of NETs as a predictive marker in severely ill patients and further, we speculate about the potential pathophysiology of NETs.

Immunosenescence and influenza vaccine efficacy

A number of protective immune functions decline with age along with physiological and anatomical changes, contributing to the increased susceptibility of older adults to infectious diseases and suboptimal protective immune responses to vaccination. Influenza vaccination is the most cost-effective strategy to prevent complications from influenza viral infections; however, the immunogenicity and effectiveness of currently licensed vaccines in the United States is about 30-50% in preventing complications arising from influenza and preventing death from all causes during winter months in older adults. Hence, it is crucial to understand the molecular mechanisms that lead to immune dysfunction as a function of age so that appropriate strategies can be developed to enhance the disease resistance and immunogenicity of preventive vaccines, including influenza vaccines, for the older adult population.

Host defense peptides in wound healing

Host defense peptides are effector molecules of the innate immune system. They show broad antimicrobial action against gram-positive and -negative bacteria, and they likely play a key role in activating and mediating the innate as well as adaptive immune response in infection and inflammation. These features make them of high interest for wound healing research. Non-healing and infected wounds are a major problem in patient care and health care spending. Increasing infection rates, growing bacterial resistance to common antibiotics, and the lack of effective therapeutic options for the treatment of problematic wounds emphasize the need for new approaches in therapy and pathophysiologic understanding. This review focuses on the current knowledge of host defense peptides affecting wound healing and infection. We discuss the current data and highlight the potential future developments in this field of research.

Cloning and identification of a novel sperm binding protein, HEL-75, with antibacterial activity and expressed in the human epididymis

BACKGROUND

The HEL-75 protein is a beta-defensin that was identified by analyzing a human epididymis cDNA library. Studying its function may not only elucidate the mechanisms of host defense, but may also provide new alternatives for novel therapeutic drugs for reproductive tract infections.

METHODS

The HEL-75 gene was amplified by PCR, and its structure and function were predicted and analyzed with bioinformatics tools. Polyclonal serum was raised against recombinant HEL (rHEL)-75 protein. The gene expression pattern was analyzed with RT-PCR and immunofluorescent staining. Finally, the antimicrobial activity and function during fertilization of HEL-75 were analyzed using a colony-forming unit assay and IVF, respectively.

RESULTS

The human HEL-75 gene is located on chromosome 20p13 and encodes a 95 amino acid protein with a predicted N-terminal signal peptide of 22 amino acids. The protein has six conserved cysteine residues, characteristic of members of the beta-defensin superfamily, as well as several potential post-translational modification sites. At the transcriptional level, HEL-75 was expressed in the epididymis and lung, but only in the epididymis at the translational level. Immunofluorescent staining showed that HEL-75 protein bound spermatozoa in the epididymis. RHEL-75 protein could kill Escherichia coli in vitro in a dose- and time-dependent fashion. However, no effect was observed on sperm motility nor fertilization when spermatozoa were blocked with anti-rHEL-75 polyclonal serum.

CONCLUSION

HEL-75 is a new beta-defensin expressed in the epididymis and on sperm; it may play an important role in host defense.

Expression and purification of recombinant human alpha-defensins in Escherichia coli

Different strategies have been developed to produce small antimicrobial peptides (AMPs) using recombinant techniques. Up to now, all efforts to obtain larger quantities of active recombinant human alpha-defensins have been only moderately successful. Here we report an effective method of biosynthesis of human alpha-defensins (hNP-1 to hNP-3 and hD-5 and hD-6) in the Escherichia coli. All the peptides, expressed as insoluble fusions with the peptide encoded by a portion of E. coli tryptophan operon (trp DeltaLE 1413 polypeptide), were isolated from the inclusion bodies by immobilized metal affinity chromatography (IMAC) and separated from the fusion leader by chemical cleavage. Fully reduced peptides that were purified according to a straightforward protocol were subsequently folded, oxidized, and subjected to functional and structural analyses. With the exception of hD-6, all recombinant alpha-defensins exhibit expected anti-E. coli activity, as measured by the colony counting method. The method described in this report is a low-cost, efficient way of generating alpha-defensins in quantities ranging from milligrams to grams.

In human epidermis, beta-defensin 2 is packaged in lamellar bodies

The skin presents a mechanical, as well as an immunological barrier to infection, and displays considerable innate immune capacity. Recently, cultured human keratinocytes were described to produce and export a microbicidal peptide human beta-defensin 2 (HBD-2). Immunogold was used to label ultrathin cryosections of stimulated, cultured human epidermis. HBD-2 was found to be stored in the lamellar bodies (LBs) of the stimulated keratinocytes of the spinous layer of the epidermis. HBD-2 was also found in the intercellular space. These findings suggest that HBD-2 is released with the contents of the LBs. Along with other investigations, our findings indicate that the lipid "permeability" barrier of the skin contains antimicrobial substances.

Up-regulation of splenic prohormone convertases PC1 and PC2 in diabetic rats

Organisms respond to infection in a complex manner involving bidirectional interactions between the neuroendocrine and immune systems. Many of the bioactive endocrine/immune factors are synthesized in a precursor form and are expected to be activated by prohormone convertases (PCs). Since patients with both type 1 and type 2 diabetes have an increased incidence and severity of infections, we hypothesized that in a condition of hyperglycemia, these processing enzymes would be activated in an immune tissue, the spleen. To test this hypothesis, we treated rats with intraperitoneal streptozotocin (STZ) (50 mg/kg/day) daily for 5 days and measured splenic PC1 and PC2 mRNA by ribonuclease protection assay. We found that PC1 mRNA was increased 6.0+/-0.02-fold (P<0.05) and PC2 mRNA was increased 1.80+/-0.01-fold (P<0.005) in the spleen of rats that received STZ compared to rats that received vehicle. Western blot indicated that the 75-kDa form of PC1 was the only form of PC1 present in the spleen and that this form increased with STZ treatment. Immunohistochemistry revealed that PC1 was found in both the white pulp (T-lymphocytes) and red pulp (monocytes and macrophages) and that its increase in immunoreactivity occurred primarily in the white pulp. PC2 and pro-opiomelanocortin (POMC, a possible splenic substrate for PC1/PC2) immunoreactivity was found predominantly in the red pulp. STZ induced an increase in splenic PC1 and POMC, but not PC2 protein levels. We conclude that in the STZ model of diabetes, splenic PCs are induced, which could lead to an increased activation of many immune-derived hormones. We speculate that this up-regulation of prohormone converting enzymes may be related to the increased infections seen in patients with both type 1 and type 2 diabetes.

Role of YadA in resistance to killing of Yersinia enterocolitica by antimicrobial polypeptides of human granulocytes

The virulence plasmid pYVe of Yersinia enterocolitica codes for the production of the outer membrane protein YadA and the secretion of several proteins, called Yops, which may protect this bacterium against killing by human granulocytes. Granulocytes kill ingested microorganisms by oxygen-dependent and oxygen-independent mechanisms, the latter including antimicrobial polypeptides. The aim of this study was to determine whether virulent (pYVe+) Y. enterocolitica and plasmid-cured avirulent (pYVe-) Y. enterocolitica differ in susceptibility to antimicrobial polypeptides extracted from granules of human granulocytes. The acetic acid granule extract contained several polypeptides with antimicrobial activity against Y. enterocolitica as determined by gel overlay and radial diffusion assays. Two of these polypeptides were identified as lysozyme and defensins. pYVe+ Y. enterocolitica was less susceptible than pYVe- Y. enterocolitica to the antimicrobial activity of granule extract, lysozyme, and defensins as determined in a suspension assay, which indicated that the pYVe plasmid mediates a reduced susceptibility to these polypeptides. The role of YadA in the resistance to antimicrobial polypeptides was analyzed by using mutants of Y. enterocolitica that specifically lack or express YadA. The results demonstrated that YadA conferred resistance to the killing of Y. enterocolitica by the granule extract. Together, these results indicate that the plasmid-encoded factor YadA contributes to the resistance of Y. enterocolitica to the killing by antimicrobial polypeptides of human granulocytes.

Casocidin-I: a casein-alpha s2 derived peptide exhibits antibacterial activity

Here we report the isolation and characterization of an antibacterial peptide from bovine milk inhibiting the growth of Escherichia coli, and Staphylococcus carnosus. The primary structure of the peptide was revealed as a 39-amino-acid-containing fragment of bovine alpha s2-casein (position 165-203) by means of Edman amino acid sequencing and mass spectrometry. Since human milk does not contain any casein-alpha s2, these findings could explain the different influence of human and bovine milk on the gastrointestinal flora of the suckling.

Design and synthesis of amphipathic antimicrobial peptides

A large proportion of antimicrobial peptides share a common structural feature that is critical to their antimicrobial activity, i.e. amphipathic alpha-helices. The amphipathy of a polypeptide chain can be quantitated through the value of the hydrophobic moment. Generally, antimicrobial peptides are characterized by high hydrophobic moment and low hydrophobicity values. Using these criteria we have identified two short segments that possess hydrophobic moment properties associated with known antimicrobial peptides. Using in vitro assays the segment derived from the protein perforin displays no antifungal or antibacterial activity and, while showing no alpha-helicity in buffer or liposomes, exhibits a modest degree of alpha-helical structure in the presence of the alpha-helical inducer, 2,2,2-trifluoroethanol. However, rational modifications result in a derivative which assumes an alpha-helical conformation in the presence of liposomes, exhibits potent antifungal activity against plant fungal pathogens, has significant antibacterial activity, effects leakage of a fluorescent dye from acidic liposomes and is devoid of hemolytic activity. Results are also presented for a segment derived from the human immunodeficiency virus envelope protein. We suggest that the identification of putative amphipathic structures in proteins may provide a useful starting strategy in the design and synthesis of antimicrobial peptides.

Amoebapores, a family of membranolytic peptides from cytoplasmic granules of Entamoeba histolytica: isolation, primary structure, and pore formation in bacterial cytoplasmic membranes

Three peptides with pore-forming activity were isolated from the cytoplasmic granules of pathogenic Entamoeba histolytica by acidic extraction, gel filtration and reversed-phase high-performance liquid chromatography. Partial amino acid sequence analysis of the three active peptides revealed that the most abundant of them was amoebapore and the other two were isoforms thereof. Cloning and sequencing of genomic DNA resolved the amino acid sequence of the two newly recognized peptides. The three peptides designated amoebapores A, B and C were found to have the same molecular size but to differ markedly in their primary structure, although all six cysteine residues are conserved. Despite sequence divergence, structural implications predict for the three peptides a similar amphipathic alpha-helical conformation stabilized by disulphide bonds. All three isoforms exhibit pore-forming activity toward lipid vesicles, but they differ in their kinetics. They also are capable of perturbing the integrity of bacterial cytoplasmic membranes and thereby kill Gram-positive bacteria. The amoebapores represent a distinct family of membrane-active peptides that may function intracellularly as antimicrobial agents but may also confer cytolytic activity on the parasite.

Cytolytic and antibacterial activity of synthetic peptides derived from amoebapore, the pore-forming peptide of Entamoeba histolytica

The pore-forming peptide amoebapore is considered part of the cytolytic armament of pathogenic Entamoeba histolytica. Amoebapore is composed of 77 amino acid residues arranged in four alpha-helical domains. For structure-function analysis, synthetic peptides were constructed corresponding to these four domains: H1 (residues 1-22), H2 (25-39), H3 (40-64), and H4 (67-77). The peptides H1 and H3, representing two highly amphipathic alpha-helical regions of amoebapore, possessed pore-forming activity. Peptide H3 displayed cytolytic and antibacterial functions similar to those of natural amoebapore. The most potent antibacterial activity and the broadest activity spectrum were expressed by H1-Mel, a hybrid molecule composed of the N-terminal alpha-helix of amoebapore and the C-terminal hexapeptide of melittin from the venom of Apis mellifera.

The pore-forming peptide of Entamoeba histolytica, the protozoan parasite causing human amoebiasis

Amoebapore, the pore-forming peptide of E. histolytica has been isolated and its structure elucidated on the cDNA and protein level. The peptide is composed of 77 amino acid residues including six cysteine residues and has a molecular mass of 8244 Da. The primary translation product contains a signal sequence of 21 mostly hydrophobic amino acid residues. The active peptide has been located in the cytoplasmic granules of the amoebae. Circular dichroism spectroscopy revealed an all alpha-helical conformation and computer-aided secondary structure prediction yielded a structure of four helices. The helical conformation and three intramolecular disulfide bonds impart a highly compact and rigid structure upon the molecule. The activity of amoebapore, measured by a liposome depolarization assay, is resistant to heating at 100 degrees C in the absence of reducing agents. Synthetic peptides corresponding to the helices 1 and 3 exhibited pore-forming activity. Two minor, biologically active isoforms of amoebapore have amino acid sequence identity of 57% and 47%, respectively. Whereas amoebapore is a constituent of pathogenic E. histolytica isolates, nonpathogenic E. histolytica produce a structurally very similar peptide, the specific activity of which is approximately one third that of amoebapore. The biological significance of amoebapore for the pathogenicity of E. histolytica and specifically for its cytolytic activity remains to be determined.

Forms of lactoferrin: their antibacterial effect on enterotoxigenic Escherichia coli

The antibacterial effects of various forms of lactoferrin on enterotoxigenic strains of Escherichia coli were tested in vitro using a microassay for bacterial growth. Native and apo-lactoferrin exhibited variable activity against 19 strains, whereas holo-lactoferrin had no effect. At a concentration of .2 mg/ml of apo-lactoferrin, strains could be distinguished as either sensitive or resistant to inhibition. Zinc-saturated lactoferrin was as inhibitory as apo-lactoferrin when sensitive and resistant strains were tested over the concentration range .04 to 1.0 mg/ml of lactoferrin. A bactericidal effect was observed for native, apo-, and Zn-saturated lactoferrin against some sensitive strains. The antibacterial activity of apo-lactoferrin depended on bacterial inoculum size and was not enhanced by the addition of lysozyme. Addition of holo-lactoferrin or cytochrome c diminished the antibacterial effect of apo-lactoferrin, whereas addition of BSA had no effect. Resistance to inhibition by lactoferrin was not related to the production of bacterial siderophores.

Analytical methods for differentiating minor sequence variations in related peptides

A proline-rich peptide was isolated and purified to homogeneity from an extract of bovine neutrophil granules using semi-preparative RP-HPLC. The relative molecular mass of the peptide (called Bac-X) was determined by ionspray MS to be 5149 +/- 0.5. The amino acid composition of the peptide was characterized by its limited number of amino acid types, which included a high proline (43.3%) and arginine content (20.3%), and hydrophobic residues. Bac-X had similar characteristics to Bac-5, a previously characterised bactenecin of bovine neutrophil granules, with respect to its proline, arginine and hydrophobic amino acid content, molecular mass and antibacterial specificity. Tryptic and N-bromosuccinimide digestion of Bac-X produced fragments with masses (M(r) 785 and 4224 and 3100 respectively) consistent with those expected from a peptide with the reported sequence of Bac-5. Bac-X differed from Bac-5 in the number of amino acid residues (43 for Bac-X versus 42 for Bac-5) and contained glycine which Bac-5 did not. However, the calculated molecular mass of the peptide, based on the amino acid compositional data, did not match the experimental value. The purified peptide could not be sequenced by Edman degradation due to apparent blockage of the N-terminus. Partial sequence information, obtained by LC-MS and collision induced dissociation MS-MS analysis of a M(r) 785 tryptic fragment of Bac-X, showed that this peptide contained a six residue sequence (-RFPPIR-) not found in Bac-5 which, based on its reported sequence, contained a M(r) 785 tryptic fragment with the sequence -FRPPIR-. This difference in sequence of Bac-X compared with Bac-5 illustrates the application of electrospray (ionspray) MS techniques to the detection and identification of minor differences in related protein/peptide forms.

Comparison of pore-forming peptides from pathogenic and nonpathogenic Entamoeba histolytica

Similar to the findings obtained with pathogenic Entamoeba histolytica, nonpathogenic isolates were found to kill mammalian cells in vitro, and cell extract caused pore formation in liposome membranes. A pore-forming peptide termed APnp was isolated from a nonpathogenic isolate using the schedule developed for the purification of APp or amoebapore, the homologous peptide of the pathogenic isolate HM-1:IMSS. Compared to APp, the specific activity of APnp in pore formation was 60% lower. cDNA sequencing indicated 95% identity of the primary structures of APnp and APp, and secondary structure predictions revealed a high degree of similarity. Notably, a glutamic acid residue at position 2 of APp is in APnp replaced by proline, which shortens one of the two amphipathic alpha-helices considered crucial for the pore-forming function. This structural divergence of the two peptides might explain the difference in their pore-forming activities.