Intramolecular inhibition of human defensin HNP-1 by its propiece.

We examined mechanisms that protect host defense cells from their cytotoxic effector molecules. Human neutrophil peptides (HNP) 1-3 are microbicidal and cytotoxic defensins, initially synthesized as 94-amino acid preproHNP(1-94), cotranslationally proteolyzed to proHNP(20-94), then converted by removal of the anionic propiece to mature HNP(65-94)(HNP-1 and -3) and HNP(66-94) (HNP-2). We hypothesized that during synthesis and subcellular sorting the anionic propiece inhibits the cytotoxicity of the cationic defensin. We expressed preproHNP-1 cDNA in recombinant baculovirus-infected insect cells that secreted the normally transient proHNP-1(20-94) into the medium. Cyanogen bromide cleaved proHNP-1(20-94) at the fortuitously located Met64 to yield mature recombinant HNP-1(65-94) and unlinked propiece. Recombinant and native HNP-1 purified from PMN were identical as judged by mass spectrometry, retention time in reverse-phase high performance liquid chromatography, migration on acid-urea polyacrylamide gels, and reaction with a conformation-specific antibody. Recombinant and native HNP-1 had comparable microbicidal activity towards Listeria monocytogenes and were similarly potent in permeabilizing K562 leukemia cells, but proHNP-1(20-94) was virtually inactive in both assays. Addition of unlinked propiece (proHNP-1(20-64) with Met64-->homoserine) inhibited the bactericidal and cell-permeabilizing activity of mature HNP-1 in a dose-dependent manner. Linked, and to a lesser extent unlinked, propiece interfered with the binding of HNP-1 to target cells. The propiece thus acts as an efficient intramolecular inhibitor of defensin HNP-1 cytotoxicity.

Human beta-defensin-1: an antimicrobial peptide of urogenital tissues.

Antimicrobial peptides are widely distributed mediators of innate host defense in animals and plants. A 36 amino acid antimicrobial peptide belonging to the defensin family, and named human beta-defensin-1 (HBD-1), was purified recently from hemodialysate fluid, but its tissue sources were not identified. By Northern blotting, we found the highest concentrations of HBD-1 mRNA in the kidney and the female reproductive tract. In situ hybridization localized the HBD-1 mRNA in the epithelial layers of the loops of Henle, distal tubules, and the collecting ducts of the kidney and the epithelial layers of the vagina, ectocervix, endocervix, uterus, and fallopian tubes in the female reproductive tract. Using a novel technique designed to detect cationic peptides in urine, we recovered several forms of HBD-1 ranging in length from 36 to 47 amino acid (aa) residues and differing from each other by amino terminal truncation. The total concentration of HBD-1 forms in voided urine was estimated at 10-100 microg/liter, with individual variations in the total amount of HBD-1 peptides and the relative proportion of HBD-1 forms. Multiple forms of HBD-1 (size 36-47 aa) were also found in the blood plasma, bound to carrier macromolecules that released the peptide under acid conditions, and in vaginal mucosal secretions (39, 40, and 44 aa). By immunostaining, HBD-1 was located in the kidney within the lumen of the loops of Henle, but no intracellular storage sites were identified in renal or female reproductive tissues. Recombinant HBD-1 forms (36, 39, and 42 aa) and natural HBD-1 forms were antimicrobial to laboratory and clinical strains of Escherichia coli at micromolar concentrations. HBD-1 activity was not changed appreciably by low pH, but was inhibited by high salt conditions. Some of the HBD-1 peptides retained their activity against E. coli in unconcentrated (low conductance) urine, and the 36 aa form was microbicidal even in normal (high conductance) urine. Production of HBD-1 in the urogenital tract could contribute to local antimicrobial defense.

Posttranslational processing of defensins in immature human myeloid cells.

Human neutrophil promyelocytes synthesize, process, and package several microbicidal proteins, including the highly abundant defensins, into their azurophil granules. As deduced from their cDNA sequences, defensins are initially synthesized as 94 amino acid (aa) precursors that must undergo extensive processing. We performed metabolic labeling studies of defensin synthesis in the human promyelocytic cell line HL-60 and in chronic myeloid leukemia cells, and showed that preprodefensins are processed to mature 29 to 30 aa defensins over 4 to 24 hours via two major intermediates: a 75 aa prodefensin generated by the cleavage of the signal sequence, and a 56 aa prodefensin that results from a subsequent preaspartate proteolytic cleavage. Almost all of the 75 aa form was found in the cytoplasmic/microsomal fraction, whereas the 56 aa prodefensin and mature defensins predominated in the granule-enriched fraction. The 75 aa prodefensin was also selectively released into the culture supernatant. Treatment of HL-60 cells with monensin, chloroquine, or ammonium chloride, substances that neutralize acidic subcellular compartments, partially blocked conversion of the 75 aa prodefensin into 56 aa prodefensin, but did not increase the extracellular release of the 75 aa form. Further studies will be required to determine the role of this processing pathway in subcellular targeting to azurophil granules and avoidance of autocytotoxicity.

Posttranslational processing and targeting of transgenic human defensin in murine granulocyte, macrophage, fibroblast, and pituitary adenoma cell lines.

Human defensins are 29 to 30 amino acid (aa) antimicrobial peptides that are among the principal constituents of the neutrophil's azurophil granules. To determine the tissue specificity of posttranslational processing and subcellular targeting of defensins, the cDNA for a 94 aa human preprodefensin was transduced into murine cell lines (NIH 3T3 embryonic fibroblasts, AtT-20 pituitary adenoma, J774.1 and RAW 264.7 macrophages, and 32D and 32D cl3 granulocytes) using retroviral vectors. All transduced cell types expressed and to a variable extent constitutively secreted a 75 aa prodefensin formed by the removal of the amino terminal signal sequence. In AtT-20 cells, the 75 aa form accumulated intracellularly in granules and was releasable by secretagogues. Proteolytic processing to mature defensins was seen only in myeloid cells (J774.1, RAW 264.7, 32D, and 32D cl3). Newly formed mature defensin was rapidly degraded in J774.1 and RAW 264.7 macrophages, but accumulated stably in multivesicular bodies in 32D cells and in cytoplasmic granules of 32D cl3 cells. Our data suggest that the enzymatic and transport machinery required to process preprodefensin to mature defensin and to store it in cytoplasmic granules is a specialized feature of cells of granulocytic lineage.

Hepcidin, a urinary antimicrobial peptide synthesized in the liver.

Cysteine-rich antimicrobial peptides are abundant in animal and plant tissues involved in host defense. In insects, most are synthesized in the fat body, an organ analogous to the liver of vertebrates. From human urine, we characterized a cysteine-rich peptide with three forms differing by amino-terminal truncation, and we named it hepcidin (Hepc) because of its origin in the liver and its antimicrobial properties. Two predominant forms, Hepc20 and Hepc25, contained 20 and 25 amino acid residues with all 8 cysteines connected by intramolecular disulfide bonds. Reverse translation and search of the data bases found homologous liver cDNAs in species from fish to human and a corresponding human genomic sequence on human chromosome 19. The full cDNA by 5' rapid amplification of cDNA ends was 0.4 kilobase pair, in agreement with hepcidin mRNA size on Northern blots. The liver was the predominant site of mRNA expression. The encoded prepropeptide contains 84 amino acids, but only the 20-25-amino acid processed forms were found in urine. Hepcidins exhibited antifungal activity against Candida albicans, Aspergillus fumigatus, and Aspergillus niger and antibacterial activity against Escherichia coli, Staphylococcus aureus, Staphylococcus epidermidis, and group B Streptococcus. Hepcidin may be a vertebrate counterpart of cysteine-rich antimicrobial peptides produced in the fat body of insects.

The structure of the rabbit macrophage defensin genes and their organ-specific expression.

Defensins are a family of microbicidal and cytotoxic peptides abundant in the lysosomal granules of mammalian phagocytes. We present the cDNA and genomic sequences of two rabbit defensins, macrophage cationic peptides MCP-1 and MCP-2. Their cDNA and genomic sequences are highly homologous, reflecting the homology between the two defensins (32 of 33 amino acids). The MCP genes are closely linked (within 13 kb) suggesting that they evolved by a recent tandem gene duplication. Their cDNA sequences indicate that the peptides are synthesized as 95 amino acid prepro-MCPs, consistent with their lysosomal location. The MCP genes are separated into three exons encoding distinct domains: the 5' untranslated region, the prepropeptide domain, and the mature defensin sequence. Fully developed polymorphonuclear leukocytes, short-lived phagocytes with limited capacity for protein and nucleic acid synthesis, contained MCPs but lacked MCP mRNA. MCP mRNA was found in bone marrow and spleen, organs which contained immature polymorphonuclear leukocytes. MCP and MCP mRNA were detected in lung macrophages, but not in macrophages from other organs, nor in monocytes, the putative macrophage precursors. In macrophages, the expression of MCPs appears to be a marker of lung-specific differentiation.

Broad-spectrum antimicrobial activity of human intestinal defensin 5.

Defensins are antibiotic peptides expressed in human and animal myeloid and epithelial cells. Due to the limited availability of natural peptides, the properties of human epithelial defensins have not been studied. We assayed the microbicidal activity of recombinant human intestinal defensin 5 (rHD-5) in the presence of salt (O to 150 mM NaCl) with varied pH (pH 5.5 to pH 8.5) and trypsin (25 and 250 microg/ml). rHD-5 exhibits microbicidal activity against Listeria monocytogenes, Escherichia coli, and Candida albicans. In contrast to cryptdins, the mouse intestinal defensins, rHD-5 is active against both mouse-virulent wild-type Salmonella typhimurium and its isogenic, mouse-avirulent phoP mutant. In the presence of salt, rHD-5 activity was reduced, and at 100 mM NaCl, activity against S. typhimurium was abolished. However, at all salt concentrations tested, rHD-5 remained bactericidal to L. monocytogenes. Activity against L. monocytogenes was not pH dependent but was diminished at pH 5.5 against wild-type S. typhimurium. This acid-induced resistance may have been mediated by the virulence gene regulator phoP, since the phoP mutant was equally sensitive at pH 5.5 and pH 7.4. In the presence of trypsin, rHD-5 was partially cleaved, but even then, rHD-5 at 100 microg/ml decreased the number of CFU of wild-type S. typhimurium by more than 99%. The persistence of microbicidal activity of rHD-5 under these conditions supports the notion that naturally occurring human intestinal defensin is an effective arm of mucosal host defense.

Beta-defensin expression in human mammary gland epithelia.

Milk of mammalian species contains a wide spectrum of anti-infectious factors, some of which are heat stable. Focusing on recently discovered heat-stable antibacterial peptides called defensins, which are expressed in epithelial tissues such as airway, skin, and kidney, we hypothesized that mammary gland epithelia produce and secrete defensins onto the epithelial surface and into milk. Using a reverse-transcription PCR assay, we identified the human beta-defensin-1 (hBD-1) gene transcript in a human mammary gland epithelial cell line, MCF-12A, and in mammary glandular tissue of nine nonlactating women. Epithelial cells harvested from milk of lactating women also expressed hBD-1 mRNA. Presence of hBD-1 peptide in mammary epithelia was confirmed by immunostaining with an hBD-1 antibody. In contrast, expression of human beta-defensin-2 was not apparent both at mRNA and protein levels. Our findings suggest a biologic role of hBD-1 in the human mammary gland.

Localized antimicrobial peptide expression in human gingiva.

The stratified epithelia of the oral cavity are continually exposed to bacterial challenge that is initially resisted by innate epithelial factors and by the recruitment of neutrophils. Antimicrobial peptides from phagocytes and epithelia contribute to this antimicrobial barrier. Using antibodies and in situ hybridization, we explored antimicrobial peptide expression in the varied epithelia of the periodontium and in cultured gingival epithelial cells. In gingival tissue, mRNA for the beta-defensins, human beta-defensin 1 (hBD-1) and human beta-defensin 2 (hBD-2) was predominately localized in suprabasal stratified epithelium and the peptides were detected in upper epithelial layers consistent with the formation of the stratified epithelial barrier. In cultured epithelial cells, both hBD-1 and -2 peptides were detected only in differentiating, involucrin-positive epithelial cells, although hBD-2 required stimulation by proinflammatory mediators or bacterial products for expression. Beta-defensins were not detected in junctional epithelium (JE) that serves as the attachment to the tooth surface. In contrast, alpha-defensins and cathelicidin family member LL-37 were detected in polymorphonuclear neutrophils (PMNs) that migrate through the JE, a localization that persists during inflammation, when the JE and surrounding tissue are highly infiltrated with PMNs. Thus, the undifferentiated JE contains exogenously expressed alpha-defensins and LL-37, and the stratified epithelium contains endogenously expressed beta-defensins. These findings show that defensins and other antimicrobial peptides are localized in specific sites in the gingiva, are synthesized in different cell types, and are likely to serve different roles in various regions of the periodontium.

Production of beta-defensins by human airway epithelia.

Human beta-defensins (HBDs) are antimicrobial peptides that may play a role in mucosal defense. Diminished activity of these peptides has been implicated in the pathogenesis of cystic fibrosis (CF) lung disease. We show that HBD-1 and HBD-2 mRNAs are expressed in excised surface and submucosal gland epithelia from non-CF and CF patients. The pro-inflammatory cytokine interleukin-1beta stimulated the expression of HBD-2 but not HBD-1 mRNA and peptide in primary cultures of airway epithelia. HBD-1 was found in bronchoalveolar lavage (BAL) fluid from normal volunteers, CF patients, and patients with inflammatory lung diseases, whereas HBD-2 was detected in BAL fluid from patients with CF or inflammatory lung diseases, but not in normal volunteers. Both HBD-1 and HBD-2 were found in BAL fluid in concentrations of several ng/ml, and both recombinant peptides showed salt-sensitive bactericidal activity. These data suggest that in the lung HBD-2 expression is induced by inflammation, whereas HBD-1 may serve as a defense in the absence of inflammation.