Enteric defensins: antibiotic peptide components of intestinal host defense

Five intestinal defensins, termed cryptdins 1-5, have been purified from mouse small bowel, sequenced, and localized to the epithelium by immunohistochemistry. Although identified as members of the defensin peptide family by peptide sequencing, enteric defensins are novel in that four cryptdins have amino termini which are three to six residues longer than those of leukocyte-derived defensins. A fifth cryptdin is the first defensin to diverge from the previously invariant spacing of cysteines in the peptide structure. The most abundant enteric defensin, cryptdin-1, had antimicrobial activity against an attenuated phoP mutant of Salmonella typhimurium but was not active against the virulent wild-type parent. Immunohistochemical localization demonstrated that cryptdin-1, and probably cryptdins 2 and 3, occur exclusively in Paneth cells, where the peptides appear to be associated with cytoplasmic granules. Biochemical and immunologic analysis of the luminal contents of the small intestine suggest that cryptdin peptides are secreted into the lumen, similar to Paneth cell secretion of lysozyme. The presence of several enteric defensins in the intestinal epithelium, evidence of their presence in the lumen, and the antibacterial activity of cryptdin-1 suggest that these peptides contribute to the antimicrobial barrier function of the small bowel mucosa.     

Activation of Paneth cell alpha-defensins in mouse small intestine.

Paneth cells in small intestine crypts secrete microbicidal alpha-defensins, termed cryptdins, as components of enteric innate immunity. The bactericidal activity of cryptdins requires proteolytic activation of precursors by matrix metalloproteinase-7 (MMP-7; matrilysin) (Wilson, C. L., Ouellette, A. J., Satchell, D. P., Ayabe, T., Lopez-Boado, Y. S., Stratman, J. L., Hultgren, S. J., Matrisian, L. M., and Parks, W. C. (1999) Science 286, 113-117). Here, we report on the intracellular processing of cryptdin proforms in mouse Paneth cells. Peptide sequencing of MMP-7 digests of purified natural procryptdins identified conserved cleavage sites in the proregion between Ser(43) and Val(44) as well as at the cryptdin peptide N terminus between Ser(58) and Leu(59). Immunostaining co-localized precursor prosegments and mature cryptdin peptides to Paneth cell granules, providing evidence of their secretion. Extensive MMP-7-dependent procryptdin processing occurs in Paneth cells, as shown by Western blot analyses of intestinal crypt proteins and proteins from granule-enriched subcellular fractions. The addition of soluble prosegments to in vitro antimicrobial peptide assays inhibited the bactericidal activities of cryptdin-3 and -4 in trans, suggesting possible cytoprotective effects by prosegments prior to secretion. Levels of activated cryptdins were normal in small bowel of germ-free mice and in sterile implants of fetal mouse small intestine grown subcutaneously. Thus, the initiation of procryptdin processing by MMP-7 does not require direct bacterial exposure, and the basal MMP-7 content of germ-free Paneth cells is sufficient to process and activate alpha-defensin precursors. MMP-7-dependent procryptdin activation in vivo provides mouse Paneth cells with functional peptides for apical secretion into the small intestine lumen.     

Crystal structures of human alpha-defensins HNP4, HD5, and HD6

Six alpha-defensins have been found in humans. These small arginine-rich peptides play important roles in various processes related to host defense, being the effectors and regulators of innate immunity as well as enhancers of adoptive immune responses. Four defensins, called neutrophil peptides 1 through 4, are stored primarily in polymorphonuclear leukocytes. Major sites of expression of defensins 5 and 6 are Paneth cells of human small intestine. So far, only one structure of human alpha-defensin (HNP3) has been reported, and the properties of the intestine defensins 5 and 6 are particularly poorly understood. In this report, we present the high-resolution X-ray structures of three human defensins, 4 through 6, supplemented with studies of their antimicrobial and chemotactic properties. Despite only modest amino acid sequence identity, all three defensins share their tertiary structures with other known alpha- and beta-defensins. Like HNP3 but in contrast to murine or rabbit alpha-defensins, human defensins 4-6 form characteristic dimers. Whereas antimicrobial and chemotactic activity of HNP4 is somewhat comparable to that of other human neutrophil defensins, neither of the intestinal defensins appears to be chemotactic, and for HD6 also an antimicrobial activity has yet to be observed. The unusual biological inactivity of HD6 may be associated with its structural properties, somewhat standing out when compared with other human alpha-defensins. The strongest cationic properties and unique distribution of charged residues on the molecular surface of HD5 may be associated with its highest bactericidal activity among human alpha-defensins.     

Chemoattraction of macrophages, T lymphocytes, and mast cells is evolutionarily conserved within the human alpha-defensin family

Human defensins are natural peptide antibiotics. On the basis of the position and bonding of six conserved cysteine residues, they are divided into two families, designated alpha- and beta-defensins. Human alpha-defensins are expressed predominantly in neutrophils (human neutrophil peptides (HNP) 1-4) or intestinal Paneth cells (human defensins (HD) 5 and 6). Although alpha-defensins have been implicated in the pathogenesis of inflammatory bowel disease, their immunomodulatory functions are poorly understood. In the present study, HNP-1, HNP-3, and HD5 were found to be potent chemotaxins for macrophages but not dendritic cells using Galphai proteins and MAPK as signal transducers. Alpha-defensins were also chemoattractive for the human mast cell line HMC-1 but lacked, in contrast to beta-defensins, the ability to induce intracellular calcium fluxes. Furthermore, HNP-1, HNP-3, and HD5 comparably mobilized naive as well as memory T lymphocytes. Using the protein kinase C (PKC) inhibitors GF109 and G?6976, we observed a PKC-independent functional desensitization to occur between human alpha-defensins, which suggests a common receptor for HNP-1, HNP-3, and HD5 on immune cells. This alpha-defensin receptor was subject to heterologous desensitization by the PKC activator PMA and to PKC-dependent cross-desensitization by human beta-defensins. Conversely, alpha-defensins desensitized beta-defensin-mediated migration of immune cells in a PKC-dependent manner, suggesting unique receptors for both defensin families. Taken together, our observations indicate that chemoattraction of macrophages, T lymphocytes, and mast cells represents an immunomodulatory function which is evolutionarily conserved within the human alpha-defensin family and tightly regulated by beta-defensins.     

Purification and primary structure of murine cryptdin-1, a Paneth cell defensin.

We have purified and determined the amino acid sequence of cryptdin-1, a murine Paneth cell defensin. The peptide corresponds to a previously characterized mRNA that accumulates to high abundance during postnatal ontogeny of the small bowel. Acid-extracted intestinal protein was fractionated by cation-exchange chromatography and fractions were assayed for antimicrobial activity. One peak of anti-Salmonella activity contained a putative defensin, based on its predicted electrophoretic migration in acid-urea PAGE. The peptide was purified to homogeneity by RP-HPLC and sequenced. These studies demonstrate defensin expression in non-myeloid tissue. The N-terminal extension of cryptdin-1 is a unique structural feature of this novel epithelial defensin.     

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.     

Events at the host-microbial interface of the gastrointestinal tract. V. Paneth cell alpha-defensins in intestinal host defense

Host defense of the small intestine is mediated, in part, by antimicrobial peptides, including alpha-defensins. In the small intestine, Paneth cells, specialized secretory epithelial cells located at the base of the crypt invaginations lining the intestinal wall, produce alpha-defensins. The alpha-defensins are cysteine-rich cationic peptides with antibiotic activity against a wide range of bacteria and other microbes. Studies of transgenic and knockout mice have supported a pivotal role of Paneth cell alpha-defensins in protection from bacterial pathogens. New data suggest that deficient expression of Paneth cell alpha-defensins may contribute to the pathophysiology of Crohn's disease, a chronic inflammatory bowel disease.     

Suppressive effects of natural and synthetic agents on dextran sulfate sodium-induced interleukin-1beta release from murine peritoneal macrophages

Interleukin (IL)-1beta, an anti-apoptotic and pro-inflammatory cytokine, plays an important role in the onset of inflammation-associated disease. We examined the suppressive effects of a total of 39 synthetic or natural compounds on dextran sulfate sodium-induced IL-1beta production in murine peritoneal macrophages. Several compounds, including alpha-tocopherol, gallic acid, (-)-catechin and rutin, were found to be highly effective for attenuating IL-1beta production, suggesting that they would be useful for anti-inflammatory application.     

Paneth cells of the human small intestine express an antimicrobial peptide gene.

Mucosal surfaces of several organ systems are important interfaces for host defense against microbes. Recent evidence suggests that antimicrobial peptides contribute to the defense of these surfaces. Defensins are one family of antimicrobial peptide, but their known distribution in humans has been limited to four members found in cells of myeloid origin. We sought to determine if the human defensin family was more complex. We found that the family of human defensins is diverse and is not restricted to expression in leukocytes. Southern blot and genomic clone analyses reveal that numerous defensin-related sequences are present in the human genome. A gene for a new human defensin family member was characterized. This gene, designated human defensin-5, is highly expressed in Paneth cells of the small intestine. This is the first example of an antimicrobial peptide gene expressed in an epithelial cell in humans. The data support the hypotheses that epithelial defensins equip the human small bowel with a previously unrecognized defensive capability which would augment other antimicrobial defenses.     

Defensin-driven viral evolution

Enteric alpha-defensins are potent effectors of innate immunity that are abundantly expressed in the small intestine. Certain enteric bacteria and viruses are resistant to defensins and even appropriate them to enhance infection despite neutralization of closely related microbes. We therefore hypothesized that defensins impose selective pressure during fecal-oral transmission. Upon passaging a defensin-sensitive serotype of adenovirus in the presence of a human defensin, mutations in the major capsid protein hexon accumulated. In contrast, prior studies identified the vertex proteins as important determinants of defensin antiviral activity. Infection and biochemical assays suggest that a balance between increased cell binding and a downstream block in intracellular trafficking mediated by defensin interactions with all of the major capsid proteins dictates the outcome of infection. These results extensively revise our understanding of the interplay between defensins and non-enveloped viruses. Furthermore, they provide a feasible rationale for defensins shaping viral evolution, resulting in differences in infection phenotypes of closely related viruses.     

The role of dendritic cells in the development of acute dextran sulfate sodium colitis

Dendritic cells (DCs) are essential mediators of the host immune response to surrounding microbes. In this study, we investigate the role of DCs in the pathogenesis of a widely used colitis model, dextran sulfate sodium-induced colitis. The effect of dextran sulfate sodium on the production of proinflammatory cytokines and chemokines by bone marrow-derived DCs (BM-DCs) was analyzed. BM-DCs were adoptively transferred into C57BL/6 mice or DCs were ablated using transgenic CD11c-DTR/GFP mice before treatment with 5% dextran sulfate sodium in drinking water. We found that dextran sulfate sodium induced production of proinflammatory cytokines (IL-12 and TNF-alpha) and chemokines (KC, MIP-1alpha, MIP-2, and MCP-1) by DCs. Adoptive transfer of BM-DCs exacerbated dextran sulfate sodium colitis while ablation of DCs attenuated the colitis. We conclude that DCs are critical in the development of acute dextran sulfate sodium colitis and may serve a key role in immune balance of the gut mucosa.     

Decay-accelerating factor deficiency increases susceptibility to dextran sulfate sodium-induced colitis: role for complement in inflammatory bowel disease

Decay-accelerating factor (DAF or CD55) is expressed on colonic epithelial cells but its function in the mucosa is unknown. In humans, a proportion of DAF-deficient (Cromer INAB) patients develop inflammatory bowel disease (IBD). To evaluate how DAF deficiency may contribute to gut inflammation and thus could play a role in IBD pathogenesis, we compared the severity of dextran sulfate sodium-induced colitis in Daf1 gene-targeted and control mice. Seven days after consuming 3% dextran sulfate sodium in their drinking water, Daf1(-/-) mice suffered markedly greater weight loss (-24.7 +/- 7.5% vs -14.2% +/- 4.9%), exhibited uniformly bloody diarrhea as compared with soft stool in control mice, developed shortened colons, and had larger spleens. Histological examination of distal colons showed massively increased neutrophilic and mononuclear cell infiltration, greater epithelial cell destruction, and increased ulcerations. Cytokine production in organ cultures of colonic explants showed increased levels of IL-12 and IL-6. Fourteen days after switching back to regular water, in contrast to the Daf1(+/+) controls which showed little stool abnormality, all Daf1(-/-) mice continued to have diarrhea. Organ culture cytokine measurements at this time point, i.e., the end of the recovery phase, showed markedly increased levels of IL-10 (6-fold), IL-12 (4-fold), and IL-6 (2-fold), as well as TNF-alpha (>10-fold) compared with the controls. Our findings argue that, as shown for IL-10 in IL-10(-/-) mice and IL-2 in IL-2(-/-) mice, DAF control of complement additionally is important in regulating gut homeostasis and consequently its activity may participate in protecting against IBD.     

PACAP provides colonic protection against dextran sodium sulfate induced colitis

Pituitary adenylate cyclase-activating polypeptide (PACAP) plays a crucial role in immunity and inflammation. Our aim was to obtain insight in the role of PACAP in experimental colitis in mice and thus its possible role in inflammatory bowel disease. PACAP-deficient (PACAP-/-) mice and wild-type control mice were challenged by colitis-inducing agent, dextran sulfate sodium (DSS). We monitored clinical symptoms, intestinal morphology, and difference of cytokine production in the proximal and distal colon. After DSS administration, mortality was more severe in PACAP-/- mice versus wild-type control mice. The histological score and the disease activity index of PACAP-/- mice were significantly higher than those of wild-type control mice. In proximal colon, production of IL-1beta and IL-6 in PACAP-/- mice were significantly upregulated on day 8 after DSS administration, compared to wild-type control mice. In distal colon, furthermore, production of IFNgamma, IL-1beta, IL-6, IL-12, and KC were significantly higher in PACAP-/- mice than in wild-type control mice on day 4. Our findings indicate that PACAP regulates the production of pro-inflammatory cytokine in the experimental colitis.     

α-Defensins from blood leukocytes of the monkey Papio hamadryas

Three antimicrobial peptides named PHD1-3 (Papio hamadryas defensin) have been isolated from hamadryas baboon blood leukocytes using preparative electrophoresis and reverse-phase HPLC. The primary structures of these peptides have been determined by automated Edman degradation and mass-spectrometry. The results suggest that the peptides belong to the alpha-defensin family. Structural homology analysis reveals that among alpha-defensins from other animal species, PHD3 is the most closely related to RMAD5 (rhesus macaque alpha-defensin) (90% homology) from rhesus macaque leukocytes and also highly similar to human alpha-defensin HD5 (60% homology), which is produced by intestinal Paneth cells. The homology of PHD3 with human neutrophil alpha-defensin HNP1 (human natural peptide) was 30%. The primary structures of PHD1 and PHD2 are most similar to RED1 (rhesus enteral defensin), one of six enteral alpha-defensins of rhesus monkeys. PHD1-3 have been shown to be active against the Gram-positive bacteria Listeria monocytogenes and Staphylococcus aureus, the Gram-negative bacterium Escherichia coli, and the fungus Candida albicans, similarly to the human HNP1 defensin.     

Mechanism of adenovirus neutralization by Human alpha-defensins

Defensins are naturally occurring antimicrobial peptides that disrupt bacterial membranes and prevent bacterial invasion of the host. Emerging studies indicate that certain defensins also block virus infection; however, the mechanism(s) involved are poorly understood. We demonstrate that human alpha-defensins inhibit adenovirus infection at low micromolar concentrations, and this requires direct association of the defensin with the virus. Moreover, defensins inhibit virus disassembly at the vertex region, thereby restricting the release of an internal capsid protein, pVI, which is required for endosomal membrane penetration during cell entry. As a consequence, defensins hamper the release of adenovirus particles from endocytic vesicles, resulting in virion accumulation in early endosomes and lysosomes. Thus, defensins possess remarkably distinct modes of activity against bacteria and viruses, and their function may provide insights for the development of new antiviral strategies.     

VPAC1 (vasoactive intestinal peptide (VIP) receptor type 1) G protein-coupled receptor mediation of VIP enhancement of murine experimental colitis

Distinct roles of the two T cell G protein-coupled receptors for vasoactive intestinal peptide (VIP), termed VPAC1 and VPAC2, in VIP regulation of autoimmune diseases were investigated in the dextran sodium sulfate (DSS)-induced murine acute colitis model for human inflammatory bowel diseases. In mice lacking VPAC2 (VPAC2-KO), DSS-induced colitis appeared more rapidly with greater weight loss and severe histopathology than in wild-type mice. In contrast, DSS-induced colitis in VPAC1-KO mice was milder than in wild-type mice and VPAC2-KO mice. Tissues affected by colitis showed significantly higher levels of myeloperoxidase, IL-6, IL-1β and MMP-9 in VPAC2-KO mice than wild-type mice, but there were no differences for IL-17, IFN-γ, IL-4, or CCR6. Suppression of VPAC1 signals in VPAC2-KO mice by PKA inhibitors reduced the clinical and histological severity of DSS-induced colitis, as well as tissue levels of IL-6, IL-1β and MMP-9. Thus VIP enhancement of the severity of DSS-induced colitis is mediated solely by VPAC1 receptors.     

A Gut Microbial Metabolite of Linoleic Acid, 10-Hydroxy-cis-12-octadecenoic Acid,Ameliorates Intestinal Epithelial Barrier Impairment Partially via GPR40-MEK-ERK Pathway

Gut microbial metabolites of polyunsaturated fatty acids have attracted much attention because of their various physiological properties. Dysfunction of tight junction (TJ) in the intestine contributes to the pathogenesis of many disorders such as inflammatory bowel disease. We evaluated the effects of five novel gut microbial metabolites on tumor necrosis factor (TNF)-α-induced barrier impairment in Caco-2 cells and dextran sulfate sodium-induced colitis in mice. 10-Hydroxy-cis-12-octadecenoic acid (HYA), a gut microbial metabolite of linoleic acid, suppressed TNF-α and dextran sulfate sodium-induced changes in the expression of TJ-related molecules, occludin, zonula occludens-1, and myosin light chain kinase. HYA also suppressed the expression of TNF receptor 2 (TNFR2) mRNA and protein expression in Caco-2 cells and colonic tissue. In addition, HYA suppressed the protein expression of TNFR2 in murine intestinal epithelial cells. Furthermore, HYA significantly up-regulated G protein-coupled receptor (GPR) 40 expression in Caco-2 cells. It also induced [Ca²⁺]_i responses in HEK293 cells expressing human GPR40 with higher sensitivity than linoleic acid, its metabolic precursor. The barrier-recovering effects of HYA were abrogated by a GPR40 antagonist and MEK inhibitor in Caco-2 cells. Conversely, 10-hydroxyoctadacanoic acid, which is a gut microbial metabolite of oleic acid and lacks a carbon-carbon double bond at Δ12 position, did not show these TJ-restoring activities and down-regulated GPR40 expression. Therefore, HYA modulates TNFR2 expression, at least partially, via the GPR40-MEK-ERK pathway and may be useful in the treatment of TJ-related disorders such as inflammatory bowel disease.     

Functional Role of Metaplastic Paneth Cell Defensins in Helicobacter pylori-Infected Stomach

Background and Aims:  Chronic gastritis is caused by Helicobacter pylori infection, and gastritis is classified as inflammation, atrophy, and intestinal metaplasia. Detailed pathologic studies have shown that H. pylori settles on the surface of gastric mucosa, and that it is eliminated from metaplastic mucosa. However, its mechanism of natural protection is not well known.
Methods:  Antimicrobial human enteric defensin expression was determined in the RNA and protein levels. Recombinant enteric defensins were produced with a bacterial expression system and their anti- H. pylori activities were assessed by bactericidal assay.
Results:  Human enteric defensin (HD)-5 and HD-6 were detected in Paneth cells, which are observed in the gastric metaplastic mucosa as well as small intestinal epithelia. HD-5 protein was coexpressed with trypsin, which is considered to be an activating enzyme of HD-5. Less H. pylori was observed in the intestinal metaplasia with HD-5 expressing Paneth cells. The recombinant defensins showed killing activity against H. pylori at a low concentration in vitro.
Conclusions:  The human defensins that are expressed in the metaplastic Paneth cells eliminate H. pylori . Metaplastic change may be a purposive development of the human stomach.     

Involvement of IL-17A in the pathogenesis of DSS-induced colitis in mice

To investigate the etiological implication of IL-17A in inflammatory bowel disease (IBD), dextran sodium sulfate (DSS) was administered to the mice deficient for the IL-17A gene. They showed only faint manifestations of colitis, as revealed by body weight loss, shrinkage in the colon length, serum haptoglobin concentration, and disease activity index. Although the mortality rate of WT mice reached approximately 60%, more than 90% of the IL-17A KO mice survived the DSS treatment. Histological change was also marginal in the IL-17A KO intestine, in which epithelial damage and inflammatory infiltrates were not obvious and the myeloperoxidase activity elevated only slightly. G-CSF and MCP-1 were abundantly produced in WT mouse intestine, whereas the production of these chemokines was drastically hampered in IL-17A-null intestine. The present results show that IL-17A plays a pivotal role in the pathogenesis of DSS-induced colitis, while MCP-1 and G-CSF may be crucially involved in the IL-17A-induced inflammation.     

Hes1-deficient mice show precocious differentiation of Paneth cells in the small intestine

We have previously shown that Hes1 is expressed both in putative epithelial stem cells just above Paneth cells and in the crypt base columnar cells between Paneth cells, while Hes1 is completely absent in Paneth cells. This study was undertaken to clarify the role of Hes1 in Paneth cell differentiation, using Hes1-knockout (KO) newborn (P0) mice. Electron microscopy revealed premature appearance of distinct cells containing cytoplasmic granules in the intervillous region in Hes1-KO P0 mice, whereas those cells were absent in wild-type (WT) P0 mice. In Hes1-KO P0 mice, the gene expressions of cryptdins, exclusively present in Paneth cells, were all enhanced compared with WT P0 mice. Immunohistochemistry demonstrated increased number of both lysozyme-positive and cryptdin-4-positive cells in the small intestinal epithelium of Hes1-KO P0 mice as compared to WT P0 mice. Thus, Hes1 appears to have an inhibitory role in Paneth cell differentiation in the small intestine.