A study of host defence peptide beta-defensin 3 in primates

A flow cytometric assay to monitor antimicrobial activity of defensins and cationic tissue extracts

To determine the antibacterial activity of defensins and other antimicrobial peptides in biopsy extracts, we evaluated a flow cytometric method with the membrane potential sensitive dye bis-(1,3-dibutylbarbituric acid) trimethine oxonol [DiBAC4(3)]. This assay enables us to discriminate intact non-fluorescent and depolarized fluorescent bacteria after exposure to antimicrobial peptides by measurement at the direct target, the cytoplasmic membrane and the membrane potential. The feasibility of the flow cytometric assay was evaluated with recombinant human beta-defensin 3 (HBD-3) against 25 bacterial strains representing 12 species. HBD-3 showed a broad-spectrum dose dependent activity and the minimal dose to cause depolarization ranged from 1.25 to >15 microg/ml HBD-3, depending on the species tested. The antibacterial effect was diminished with sodium chloride or dithiothreitol and could be abrogated with a HBD-3 antibody. Additionally, isolated cationic extracts from human intestinal biopsies showed a strong bactericidal effect against Escherichia coli K12, E. coli ATCC 25922 and Staphylococcus aureus ATCC 25923, which was diminished towards E. coli at 150 mM NaCl, whereas the activity towards S. aureus ATCC 25923 remained unaffected at physiological salt concentrations. DTT blocked the bactericidal effect of biopsy extracts completely.

Evolution of caprine and ovine β-defensin genes

Defensins comprise an important family of anti-microbial peptides. Among vertebrates, numerous defensin genes have been detected, but their evolutionary background is still discussed. We investigated the molecular evolution and variability of beta-defensins of Caprini via sequence analyses of defensin introns. Screening of several domestic and wild species of Caprini revealed a total of 13 discrete beta-defensin coding sequences, with three of them described before this study. Phylogenetic analyses revealed that the array of newly described defensin genes is of monophyletic origin and has arisen in numerous independent duplication events after separation of the ancestral defensins. As a result of that scenario, recent defensin genes are distributed in a species-specific manner. Values of synonymous and non-synonymous substitutions demonstrated that both modes of evolutionary pressure, positive as well as negative selection, have acted. In addition, conservation of some beta-defensin exons is demonstrated. Discrimination of certain beta-defensin genes was possible only due to intron-specific differences. Therefore, sequence analyses restricted to the exons would result in underestimation of the number of beta-defensin genes. Our study shows that for reconstruction of the phylogenetic history data of defensin introns are more appropriated. Comparisons among the amino acid sequences show moderate substitutions without changing the net charge of the mature peptides.

Mammalian defensins in the antimicrobial immune response

Defensins are peptidic components of the innate immune system of plants and animals. In mammals, defensins have evolved to have a central function in the host defense properties of granulocytic leukocytes, mucosal surfaces, skin and other epithelia. This review focuses on the biological functions of three structural subgroups of mammalian defensins and the evidence for their involvement as effectors of antimicrobial innate immunity.

The complexity of selection at the major primate beta-defensin locus

BACKGROUND

We have examined the evolution of the genes at the major human beta-defensin locus and the orthologous loci in a range of other primates and mouse. For the first time these data allow us to examine selective episodes in the more recent evolutionary history of this locus as well as the ancient past. We have used a combination of maximum likelihood based tests and a maximum parsimony based sliding window approach to give a detailed view of the varying modes of selection operating at this locus.

RESULTS

We provide evidence for strong positive selection soon after the duplication of these genes within an ancestral mammalian genome. Consequently variable selective pressures have acted on beta-defensin genes in different evolutionary lineages, with episodes both of negative, and more rarely positive selection, during the divergence of primates. Positive selection appears to have been more common in the rodent lineage, accompanying the birth of novel, rodent-specific beta-defensin genes. These observations allow a fuller understanding of the evolution of mammalian innate immunity. In both the rodent and primate lineages, sites in the second exon have been subject to positive selection and by implication are important in functional diversity. A small number of sites in the mature human peptides were found to have undergone repeated episodes of selection in different primate lineages. Particular sites were consistently implicated by multiple methods at positions throughout the mature peptides. These sites are clustered at positions predicted to be important for the specificity of the antimicrobial or chemoattractant properties of beta-defensins. Surprisingly, sites within the prepropeptide region were also implicated as being subject to significant positive selection, suggesting previously unappreciated functional significance for this region.

CONCLUSIONS

Identification of these putatively functional sites has important implications for our understanding of beta-defensin function and for novel antibiotic design.

Distribution of human β-defensin polymorphisms in various control and cystic fibrosis populations

Human beta defensins contribute to the first line of defense against infection of the lung. Polymorphisms in these genes are therefore potential modifiers of the severity of lung disease in cystic fibrosis. Polymorphisms were sought in the human beta-defensin genes DEFB1, DEFB4, DEFB103A, and DEFB104 in healthy individuals and cystic fibrosis (CF) patients living in various European countries. DEFB1, DEFB4, and DEFB104 were very polymorphic, but DEFB103A was not. Within Europe, differences between control populations were found for some of the frequent polymorphisms in DEFB1, with significant differences between South-Italian and Czech populations. Moreover, frequent polymorphisms located in DEFB4 and DEFB104 were not in Hardy Weinberg equilibrium in all populations studied, while those in DEFB1 were in Hardy Weinberg equilibrium. Sequencing of a monochromosomal chromosome 8 mouse-human hybrid cell line revealed signals for multiple alleles for some loci in DEFB4 and DEFB104, but not for DEFB1. This indicated that more than one DEFB4 and DEFB104 gene was present on this chromosome 8, in agreement with recent findings that DEFB4 and DEFB104 are part of a repeat region. Individual DEFB4 and DEFB104 PCR amplification products of various samples were cloned and sequenced. The results showed that one DNA sample could contain more than two haplotypes, indicating that the various repeats on one chromosome were not identical. Given the higher complexity found in the genomic organization of the DEFB4 and DEFB104 genes, association studies with CF lung disease severity were performed only for frequent polymorphisms located in DEFB1. No association with the age of first infection by Pseudomonas aeruginosa or with the FEV1 percentage at the age of 11-13 years could be found.

Parotid secretory protein is expressed and inducible in human gingival keratinocytes

BACKGROUND

Parotid secretory protein (PSP) is a major salivary protein that is thought to possess both antibacterial and anti-inflammatory activity. A major question is whether PSP expression can be regulated by humoral factors and bacteria. Periodontitis is an inflammatory lesion initiated by interaction between gingival keratinocytes and periodontopathogenic microorganisms such as the Gram-negative anaerobe Porphyromonas gingivalis. Cytokines and sex hormones have been implicated in the progression of various forms of periodontal diseases.

MATERIALS AND METHODS

We investigated the expression of PSP and its regulation in primary cultures of human gingival keratinocytes (HGK). HGK at the third or fourth passage were exposed to heat-killed P. gingivalis, tumor necrosis factor-alpha (TNF-alpha) and 17beta-estradiol. The PSP mRNA levels were examined by real-time polymerase chain reaction (PCR). The protein expression of PSP was confirmed by immunofluorescence.

RESULTS

Heat-killed P. gingivalis, TNF-alpha and 17beta-estradiol all resulted in increased HGK levels of mRNA for PSP as determined by real-time PCR analysis. Immunofluorescence demonstrated increased PSP localized within the cytoplasm of HGK following exposure to killed P. gingivalis.

CONCLUSION

The present study has demonstrated for the first time that PSP is expressed in keratinocytes and that it can be up-regulated by bacteria and humoral factors. Thus PSP may have a role in the innate defense system at the gingival epithelial surface.

Variability and evolution of bovine β-defensin genes

Defensins comprise an important family of antimicrobial peptides. Among vertebrates numerous defensin genes have been detected, but their evolutionary background is still discussed. We investigated the molecular evolution of bovine defensins via screening of different bovine species including the extinct ancestor of domestic cattle (Bos primigenius) for beta-defensin encoding genes. We detected a large variability of new defensin encoding sequences similar to previously published bovine neutrophil beta-defensin (bnbd), neutrophil beta-defensin 12 (nbd12), enteric beta-defensin (ebd), lingual antimicrobial peptide (lap), and tracheal antimicrobial peptide (tap). Our data suggest that variants of the same so-called subfamily (tap, lap, ebd, and nbd12) each share a common ancestry independent of their species origin, implicating several duplication events of tap, lap, ebd, and nbd12 before the different bovine lineages diverged. Variants of bovine neutrophil beta-defensins bnbd5 and bnbd9 were detected exclusively in domestic cattle and aurochs. Values of synonymous and nonsynonymous substitutions demonstrated lap, bnbd5, bnbd9 and nbd12 evolving under positive selection, whereas amino-acid altering substitutions among variants of ebd and tap are purified. Comparison of the amino-acid sequences with available structures of human and murine defensins suggested conservation of the typical secondary elements of defensins in the absence of high sequence similarity.

Mammalian defensins: structures and mechanism of antibiotic activity

Antibiotic peptides are important effector molecules in host-parasite interactions throughout the living world. In vertebrates, they function in first-line host defense by antagonizing a wide range of microbes including bacteria, fungi, and enveloped viruses. The antibiotic activity is thought to be based on their cationic, amphipathic nature, which enables the peptides to impair vital membrane functions. Molecular details for such activities have been elaborated with model membranes; however, there is increasing evidence that these models may not reflect the complex processes involved in the killing of microbes. For example, the overall killing activity of the bacterial peptide antibiotic nisin is composed of independent activities such as the formation of target-mediated pores, inhibition of cell-wall biosynthesis, formation of nontargeted pores, and induction of autolysis. We studied the molecular modes of action of human defense peptides and tried to determine whether they impair membrane functions primarily and whether additional antibiotic activities may be found. We compared killing kinetics, solute efflux kinetics, membrane-depolarization assays, and macromolecular biosynthesis assays and used several strains of Gram-positive cocci as test strains. We found that membrane depolarization contributes to rapid killing of a significant fraction of target cells within a bacterial culture. However, substantial subpopulations appear to survive the primary effects on the membrane. Depending on individual strains and species and peptide concentrations, such subpopulations may resume growth or be killed through additional activities of the peptides. Such activities can include the activation of cell-wall lytic enzymes, which appears of particular importance for killing of staphylococcal strains.

Characterization and phylogenetic epitope mapping of CD38 ADPR cyclase in the cynomolgus macaque

Background

The CD38 transmembrane glycoprotein is an ADP-ribosyl cyclase that moonlights as a receptor in cells of the immune system. Both functions are independently implicated in numerous areas related to human health. This study originated from an inherent interest in studying CD38 in the cynomolgus monkey (Macaca fascicularis), a species closely related to humans that also represents a cogent animal model for the biomedical analysis of CD38.

Results

A cDNA was isolated from cynomolgus macaque peripheral blood leukocytes and is predicted to encode a type II membrane protein of 301 amino acids with 92% identity to human CD38. Both RT-PCR-mediated cDNA cloning and genomic DNA PCR surveying were possible with heterologous human CD38 primers, demonstrating the striking conservation of CD38 in these primates. Transfection of the cDNA coincided with: (i) surface expression of cynomolgus macaque CD38 by immunofluorescence; (ii) detection of ~42 and 84 kDa proteins by Western blot and (iii) the appearance of ecto-enzymatic activity. Monoclonal antibodies were raised against the cynomolgus CD38 ectodomain and were either species-specific or cross-reactive with human CD38, in which case they were directed against a common disulfide-requiring conformational epitope that was mapped to the C-terminal disulfide loop.

Conclusion

This multi-faceted characterization of CD38 from cynomolgus macaque demonstrates its high genetic and biochemical similarities with human CD38 while the immunological comparison adds new insights into the dominant epitopes of the primate CD38 ectodomain. These results open new prospects for the biomedical and pharmacological investigations of this receptor-enzyme.

Effects of positively selected sequence variations in human and Macaca fascicularis beta-defensins 2 on antimicrobial activity

The evolution of orthologous genes coding for beta-defensin 2 (BD2) in primates has been subject to positive selection during the divergence of the platyrrhines from the catarrhines and of the Cercopithecidae from the Hylobatidae, great apes, and humans. Three peptides have been selected for a functional analysis of the effects of sequence variations on the direct antimicrobial activity: human BD2 (hBD2), Macaca fascicularis BD2 (mfaBD2), and a variant of the human peptide lacking Asp(4), (-D)hBD2, which is characteristic only of the human/great ape peptides. hBD2 and mfaBD2 showed a significant difference in specificity, the former being more active towards Escherichia coli and the later towards Staphylococcus aureus and Candida albicans. Asp(4) in the human peptide appears to be important, as (-D)hBD2 was less structured and had a markedly lower antimicrobial activity. The evolution of beta-defensin 2 in primates may thus have been driven, at least in part, by different environmental pressures so as to modulate antimicrobial activity.