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Evidence from many lines of investigation supports the hypothesis that the release of gene-encoded antimicrobial peptides by epithelial cells contributes to innate mucosal immunity. In the small intestine, Paneth cells at the base of the crypts of Lieberkühn secrete defensins and other antimicrobial proteins in response to bacteria. It is likely that the Paneth cell products released into the narrow lumen of the crypt help to protect the epithelial stem cells from noxious microbes. The stem cells, which reside at the neck of the crypt, are responsible for continual renewal of epithelial cells, which line both the small intestinal villi and crypts. In addition to this protective function, Paneth cell secretions also may interact with bacteria that exist in the intestinal lumen and affect the composition of the enteric microbial flora. Mouse matrilysin, a metalloproteinase produced by Paneth cells, proteolytically processes prodefensin precursors to mature defensins. Disruption of the matrilysin gene in transgenic mice eliminates production of mature defensins and results in defective clearing of orally administered Escherichia coli and increased susceptibility to virulent Salmonella typhimurium. Further defining the mechanisms that regulate Paneth cell function should improve our understanding of the role of these cells, and their products, in sustaining normal small bowel function. The investigation of possible defects in Paneth cell biology in the pathophysiology of Crohn's disease and ulcerative colitis may prove to be a valuable area for future studies.

Host Defense At The Mucosa of The Small Intestine

  1. Top of page
  2. Host Defense At The Mucosa of The Small Intestine
  3. Antimicrobial Peptides As Agents of Host Defense
  4. Antimicrobial Peptides of Small Intestinal Paneth Cells
  5. Defensins: Cysteine-Rich Antimicrobial Peptides
  6. Paneth Cell Defensins
  7. Antimicrobial Activities of Paneth Cell α-Defensins
  8. Posttranslational Proteolytic Processing
  9. Secretion of α-Defensins By Paneth Cells
  10. Future Directions
  11. Acknowledgements
  12. References

The epithelium of the small bowel is the largest surface at which an organism interacts directly with the external environment, serving as a barrier between the luminal contents and portal circulation. This monolayer of epithelial cells must absorb vital nutrients while inhibiting invasive challenges by luminal microbes. Throughout the lifetime of mammals, this epithelium is replaced continually by a process involving stem cell proliferation, cellular differentiation, migration, apoptosis, and exfoliation. Stem cells that reside in the neck of the intestinal crypts replicate at a rate appropriate for maintaining a constant supply of new epithelial cells, which are crucial for the repopulation of crypts and villi (1). The interruption of this stem cell replication, or lesions introduced in the epithelium by various insults, including infection or inflammation, could generate portals of entry for luminal bacteria. Therefore, mechanisms that protect crypts against bacterial overgrowth and infection are vital to maintain epithelial monolayer integrity by preserving stem cell viability.

Remarkably, the bacterial load in the small intestine lumen is approximately 104 to 106-fold fewer microbes per gram of tissue than in the adjacent colon, but the responsible factors are not completely understood (2). Activities associated with digestion and the normal physiology of the gastrointestinal tract, including gastric acidity, digestive enzymes, bile salts, peristalsis, mucus, the resident commensal flora, and exfoliation of enterocytes during epithelial renewal are likely to help to control the small intestinal bacterial population (3). In addition to their protecting the crypt from microbial challenge, the secretion of gene-encoded antimicrobial peptides and proteins may also help reduce the numbers of bacteria in the small intestinal lumen (4,5).

Many lines of investigation have established that mammals also commit extensive resources to lymphocyte-mediated adaptive immune functions in the intestinal compartment. Prominent examples include humoral immunity via B-cell-mediated release of secretory IgA, which traverses the epithelium to the gut lumen (6–8), and cell-mediated immunity via intraepithelial T cells of villi (9). These processes are initiated by specific antigens. However, the kinetics of lymphocytic responses are not likely to be rapid enough to control acute bacterial challenges (10,11). Therefore, innate host defense mechanisms, including both physical and chemical factors, are thought to provide immediate protection against the threat of colonization and infection by deleterious microorganisms.

Antimicrobial Peptides As Agents of Host Defense

  1. Top of page
  2. Host Defense At The Mucosa of The Small Intestine
  3. Antimicrobial Peptides As Agents of Host Defense
  4. Antimicrobial Peptides of Small Intestinal Paneth Cells
  5. Defensins: Cysteine-Rich Antimicrobial Peptides
  6. Paneth Cell Defensins
  7. Antimicrobial Activities of Paneth Cell α-Defensins
  8. Posttranslational Proteolytic Processing
  9. Secretion of α-Defensins By Paneth Cells
  10. Future Directions
  11. Acknowledgements
  12. References

Originally characterized in plants and insects, gene-encoded antimicrobial peptides are now recognized as a widely deployed mechanism of biochemical defense against potential pathogens (12,13). Characteristically, antimicrobial peptides are ≤ 100 amino acids in size, cationic at neutral pH, and often have antibiotic activity against a wide array of microbes at micromolar concentrations, even though their primary structures may vary considerably (10). These peptides have been isolated from all phyla examined, and their production is observed in diverse host defense settings.

In mammalian neutrophils, antimicrobial peptides are stored in granules, where they are secreted extracellularly from specific granules or delivered to phagolysosomes to mediate nonoxidative killing of ingested microorganisms in azurophilic granules (14). Many mammalian epithelial cells release peptides onto mucosal surfaces, where they interface with microbial challenges from the external environment (15). For example, antimicrobial peptides are produced in the oral cavity (16–18), upper airway (19,20), and genitourinary tract (21,22). In the various contexts, these peptide antibiotics may be either inducible or synthesized continually.

The expression of antimicrobial peptides by gastrointestinal epithelial cells is observed across phyla. In the frog, magainins are the primary antimicrobial peptides of skin, and they are found in stomach glands and at the base of epithelial folds in the frog intestine (23). In insects, the midgut of Manduca sexta larvae contains cells with antimicrobial peptide-containing granules (24), and blood meal consumption induces midgut insect defensin production in mosquitoes and flesh flies (25,26). In mammals, evidence for production of antimicrobial peptides has been obtained throughout the gastrointestinal tract (27–29), but the molecules best characterized to date are the defensins of the small intestinal Paneth cells (4). The conserved utilization of antimicrobial peptides in gastrointestinal tissues suggests they are linked to an important mechanism of innate immunity.

Antimicrobial Peptides of Small Intestinal Paneth Cells

  1. Top of page
  2. Host Defense At The Mucosa of The Small Intestine
  3. Antimicrobial Peptides As Agents of Host Defense
  4. Antimicrobial Peptides of Small Intestinal Paneth Cells
  5. Defensins: Cysteine-Rich Antimicrobial Peptides
  6. Paneth Cell Defensins
  7. Antimicrobial Activities of Paneth Cell α-Defensins
  8. Posttranslational Proteolytic Processing
  9. Secretion of α-Defensins By Paneth Cells
  10. Future Directions
  11. Acknowledgements
  12. References

In most mammals, including humans and rodents, Paneth cells occupy the base of the crypts of Lieberkühn, a position from which they secrete large secretory granules apically into the crypt lumen. Paneth cells have characteristic and unusual morphology (1). They are intensely eosinophilic, with ultrastructural hallmarks of secretory cells, including an extensive endoplasmic reticulum and Golgi network (30). Studies in mice have shown by amplification of Paneth cell-specific cDNAs or by staining of crypt preparations with Amido black that every crypt contains Paneth cells (31). Paneth cells are found in increasing numbers from the duodenum to the ileum (32). Unlike the enterocytes, which are short lived and migrate upward to the villus tips, Paneth cells do not turn over rapidly and migrate to the base. Thus, Paneth cells have a 20-day average life span, compared to the villous enterocytes, which apoptose and exfoliate into the lumen 2–5 days after emergence from the stem cell zone. Paneth cell ontogeny does not require luminal bacteria, bacterial antigens, or dietary constituents, because normal Paneth cells develop prenatally during normal human ontogeny (33) and are present in mice reared under germ-free conditions (34).

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Figure 1. Defensins in human Paneth cells. A: Small intestinal crypts were prepared in vitro from adult outbred Swiss WT mouse by EDTA treatment and stained with 0.25% amido black for 2 minutes to label Paneth cell secretory granules (66). Arrow denotes stained Paneth cells at the base of a single crypt [Reprinted from Nature Immunology 2000;1:113–18, with permission]. B: Immunohistochemical detection of HD-5 in adult human small intestine as described (47). Intestinal sections were incubated with a primary rabbit polyclonal anti-HD-5 antibody for 2 hours at room temperature at a 1:100 dilution, washed, and treated with goat antirabbit IgG coupled to horseradish peroxidase (47). As previously reported, the enteric α-defensin HD-5 is localized to Paneth cells at the base of the crypts of Lieberkühn in adult small intestine [Reprinted from Infect Immun 1997;65:2389–95, with permission].

Figure 1. Continued

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Studies of Paneth cell proteins have provided key insights into the biological role of these cells. Several Paneth cell secretory products are identical, or homologous, to gene products found in phagocytic leukocytes, implicating these cells in enteric host defense. Specifically, Paneth cells of rodents and humans produce lysozyme, secretory phospholipase A2 (sPLA2), and α-defensins, well-established antimicrobial proteins, and peptides (35). In mice, these gene products, particularly α-defensins, are sensitive markers of postnatal crypt ontogeny (36,37). Human Paneth cells appear in the first trimester of gestation and begin to express α-defensins coincident with their morphological appearance (33,38).

Defensins: Cysteine-Rich Antimicrobial Peptides

  1. Top of page
  2. Host Defense At The Mucosa of The Small Intestine
  3. Antimicrobial Peptides As Agents of Host Defense
  4. Antimicrobial Peptides of Small Intestinal Paneth Cells
  5. Defensins: Cysteine-Rich Antimicrobial Peptides
  6. Paneth Cell Defensins
  7. Antimicrobial Activities of Paneth Cell α-Defensins
  8. Posttranslational Proteolytic Processing
  9. Secretion of α-Defensins By Paneth Cells
  10. Future Directions
  11. Acknowledgements
  12. References

Three defensin peptide subfamilies are known, the α-, β-, and θ-defensins. The α- andagr;-and β-defensins are cationic, 3–4 kDa peptides with six cysteine residues that are invariantly paired in three disulfide bonds, which characterize each of these peptide subfamilies (39). Although the α- andagr;-and β-defensin tridisulfide arrays differ in their Cys–Cys pairings, the peptides have remarkably similar folded conformations (40). The α-defensins were first recognized as granule components of phagocytic leukocytes (41) and later identified in mouse and human Paneth cells (42,43). The β-defensins were first isolated from trachea and neutrophils of cattle (20,44,45) and later in the mucosa of the airway, tongue, colon, kidney, skin, and gingiva in humans and in other species (19). The single known θ-defensin, RTD-1, is a 2 kDa macrocyclic peptide that is found in neutrophils and monocytes of the Rhesus macaque (46). RTD-1 consists of an 18-amino acid, covalently closed, circular polypeptide chain that is stabilized by three disulfide bonds. RTD-1 biosynthesis is unusual, because the peptide assembles from two distinct precursor molecules with each precursor contributing a nine-amino acid moiety to the final RTD-1 peptide by mechanisms that are not understood (46). The half-precursors are products of two different genes that contain three exons and resemble all known myeloid α-defensin genes, except that they are truncated by stop codons in exon 3.

Paneth Cell Defensins

  1. Top of page
  2. Host Defense At The Mucosa of The Small Intestine
  3. Antimicrobial Peptides As Agents of Host Defense
  4. Antimicrobial Peptides of Small Intestinal Paneth Cells
  5. Defensins: Cysteine-Rich Antimicrobial Peptides
  6. Paneth Cell Defensins
  7. Antimicrobial Activities of Paneth Cell α-Defensins
  8. Posttranslational Proteolytic Processing
  9. Secretion of α-Defensins By Paneth Cells
  10. Future Directions
  11. Acknowledgements
  12. References

α-Defensins are abundant constituents of mouse and human Paneth cell granules. Human Paneth cells code for two α-defensin peptides, HD-5 and HD-6, and mice express numerous Paneth cell α-defensins that have been termed “cryptdins” for crypt defensin (34). Immunohistochemical studies of small intestinal sections (1) have shown that antibodies to HD-5 and mouse cryptdin-1 react with Paneth cell secretory granules of the respective species (43,47,48), and cryptdins have been purified from tissue and rinses of adult mouse small intestine. Six cryptdins have been purified to homogeneity from adult mouse small intestine (35), and the relative abundance of individual isoforms differs, with levels of cryptdin-1,2, 5, and 6 judged to be equivalent in quantity and more abundant than cryptdins 3 and 4 (43,49). Variants of cryptdins 1, 4, and 6 with amino termini that are shorter than cognate tissue forms by one or two amino acids were detected in lumenal rinses (48). All of these defensins have microbicidal activities (see below). Interestingly, assays of antimicrobial activity against Staphylococcus aureus, E. coli, and S. typhimurium phoP- showed that N-terminally truncated variants of cryptdin-6 and cryptdin-4 peptides had attenuated activities relative to the full-length tissue forms. From this observation, it is probable that aminopeptidase modification of α-defensins after secretion may modulate peptide function in the crypt lumen.

In human specimens, there is an unexpected complexity in the N-terminally processed forms of HD-5 (50). By western blot analysis, studies of ileal tissue have demonstrated three isoforms. Two forms were chemically characterized and correspond to amino acids 23 to 94 and 29 to 94 (50). The detection of these larger forms suggests that human Paneth cells, in contrast to those of mice, may store defensins as propeptides. These propeptides are then likely to be processed to mature peptides either during or after exocytic secretion. One study of HD-5 in a special biological context has detected shorter extracellular forms of this peptide, consistent with this proposed processing. The surgical construction of neobladders from ileal tissue provided an opportunity to detect Paneth cell secretory products in the voided urine. From this neobladder urine, three major forms were chemically characterized, representing amino acids 36 to 94, 56 to 94, and 63 to 94 (50). Analysis of HD5 from small intestinal lumen and tissue will be of particular interest in exploring this proposed aspect of posttranslational processing.

Paneth cell α-defensins are coded by highly conserved genes that consist of two exons. The 5´-untranslated region, signal peptide, and prosequence are coded by exon 1, and exon 2 encodes the processed defensin peptide and the 3´-untranslated region. Paneth cell α-defensin genes map in proximity to the myeloid α-defensin and β-defensin genes at 8p23 in humans (51,52) and at an homologous locus on proximal chromosome 8 in mice (53–55). Overall levels of α-defensin mRNA and peptide appear to be approximately the same in human and mouse Paneth cells. Although genetic evidence has shown that mice express approximately 20 enteric α-defensin isoforms, the levels of cryptdin 7–19 mRNAs are judged to be much lower than those of cryptdins 1–6, based on the low cloning frequency of these cDNAs and the fact that cryptdins 7–19 have not been recovered as abundant peptide components of intestinal extracts.

Antimicrobial Activities of Paneth Cell α-Defensins

  1. Top of page
  2. Host Defense At The Mucosa of The Small Intestine
  3. Antimicrobial Peptides As Agents of Host Defense
  4. Antimicrobial Peptides of Small Intestinal Paneth Cells
  5. Defensins: Cysteine-Rich Antimicrobial Peptides
  6. Paneth Cell Defensins
  7. Antimicrobial Activities of Paneth Cell α-Defensins
  8. Posttranslational Proteolytic Processing
  9. Secretion of α-Defensins By Paneth Cells
  10. Future Directions
  11. Acknowledgements
  12. References

Mouse and human Paneth cell α-defensins are potent antimicrobial agents with selective activities against several varied microbial cell targets. For example, HD-5 is active against a variety of bacterial species, including Listeria monocytogenes, E. coli, S. typhimurium, as well as the yeast-like fungus Candida albicans (56). In vitro assays of cryptdins show that they are similarly microbicidal against E. coli ML35, S. aureus, and S. typhimurium (49).

Preliminary structure-function relationships in Paneth cell α-defensins have emerged from analysis of mouse cryptdins. For example, trophozoites of Giardia lamblia are highly sensitive to cryptdins 2 and 3, but cryptdins 1 and 6 have little effect on survival of this enteric pathogen as determined by Trypan blue exclusion (57). Peptide amino acid residue position 15 is implicated in this activity, because giardicidal cryptdins 2 and 3 contain Arg at position 15, but inactive cryptdins 1 and 6 contain a Gly at residue 15. By analogy with the crystal structure of neutrophil defensin HNP-3 (58), amino acid 15 is predicted to be on the peptide surface in a conserved turn and possibly important in interactions with eukaryotic cell envelopes (35).

Studies of neutrophilic α-defensins predict that Paneth cell defensin antimicrobial activity is likely to result from permeabilization of the target cell envelope leading to dissipation of electrochemical gradients. However, the details of Paneth cell defensin function have yet to be studied directly. Although both human and rabbit neutrophil α-defensins disrupt the target cell membrane, studies of model membrane vesicles suggest that the actual mechanisms between peptides may differ. The human neutrophil defensins are weakly basic and form dimers (59), which form large, ∼20 Å, stable multimeric pores after insertion into large unilamellar vesicle model membranes. In contrast, rabbit neutrophil α-defensins are more cationic and exist as monomers that create transient, graded defects in the large unilamellar vesicle model membranes. The differences in mechanisms for these human and rabbit defensins are attributed to their dimeric versus monomeric structures (40), but these are the only defensins that have been analyzed in this detail. These considerations suggest that a detailed understanding of the mechanisms by which the mouse and human Paneth cell α-defensins achieve microbial cell killing cannot be extrapolated without direct investigation of the specific peptides in model systems.

Posttranslational Proteolytic Processing

  1. Top of page
  2. Host Defense At The Mucosa of The Small Intestine
  3. Antimicrobial Peptides As Agents of Host Defense
  4. Antimicrobial Peptides of Small Intestinal Paneth Cells
  5. Defensins: Cysteine-Rich Antimicrobial Peptides
  6. Paneth Cell Defensins
  7. Antimicrobial Activities of Paneth Cell α-Defensins
  8. Posttranslational Proteolytic Processing
  9. Secretion of α-Defensins By Paneth Cells
  10. Future Directions
  11. Acknowledgements
  12. References

Biological activity of α-defensins requires posttranslational proteolytic activation. In human neutrophils, defensin peptides are synthesized as 90–100 amino acid prepropeptides that contain N-terminal endoplasmic reticulum targeting (signal) peptides, propeptide segments of approximately 40 amino acids, and a mature α-defensin peptide at the C-terminus. These myeloid defensins are processed in several cleavage steps prior to storage in the azurophilic granules as fully processed mature peptides (60–62). The anionic propeptide segments of some α-defensin precursors may be important for neutralization, processing, or folding of the cationic C-terminal defensin peptides (62,63). As noted above, in human Paneth cells, α-defensins appear to be stored as propeptides, which are likely to be processed to mature peptides either during or after exocytic secretion (5,50). Details of this proteolytic processing are areas of active investigation.

In mice, matrilysin (MMP-7) activity is essential for cryptdin peptides to function in mucosal immunity. MMP-7, a metalloproteinase expressed at high levels by Paneth cells of mouse small bowel (64), was identified as the enzyme responsible for posttranslational processing and activation of mouse cryptdin precursors (65). In vitro, MMP-7 processes procryptdins to activate 3.5 kDa α-defensins by cleaving precursors between residues Ser58 and Leu59, precisely at the junction of the propeptide and the mature cryptdin peptide amino terminus. Targeted disruption of the MMP-7 gene impairs enteric host defense in that MMP-7 null mice lack activated cryptdins, and they are less effective at clearing orally administered noninvasive E. coli, and they succumb more rapidly and to lower doses of virulent S. typhimurium (65). These findings create rationale for determining the molecular details of how procryptdins are recognized and cleaved by MMP-7 and for identifying the mechanisms of pro-α-defensin activation in human Paneth cells.

Secretion of α-Defensins By Paneth Cells

  1. Top of page
  2. Host Defense At The Mucosa of The Small Intestine
  3. Antimicrobial Peptides As Agents of Host Defense
  4. Antimicrobial Peptides of Small Intestinal Paneth Cells
  5. Defensins: Cysteine-Rich Antimicrobial Peptides
  6. Paneth Cell Defensins
  7. Antimicrobial Activities of Paneth Cell α-Defensins
  8. Posttranslational Proteolytic Processing
  9. Secretion of α-Defensins By Paneth Cells
  10. Future Directions
  11. Acknowledgements
  12. References

Studies in mice have shown that Paneth cells are responsive to their apical environment, releasing their secretory granules in response to bacteria and bacterial antigens (2). Paneth cell secretion may be stimulated by Gram-negative and Gram-positive bacteria and by lipopolysaccharide (LPS), lipoteichoic acid, lipid A, and muramyl dipeptide (66). The secretory response to bacteria and LPS is rapid and dose dependent, and it is selective for bacteria in that crypts do not release peptides when incubated with live fungi or protozoa. The secretion has microbicidal activity, and 70% of the secreted bactericidal peptide activity could be inhibited by a neutralizing antibody to mouse cryptdins 1, 2, 3, and 6. In response to the bacterial stimuli, Paneth cells in a single mouse crypt secrete ∼360 pg of cryptdin into a lumen of ∼3 pL per 100 mm of crypt height, corresponding to a local cryptdin concentration of ∼100 mg/ml at secretion (66). Given that the antimicrobial activity of cryptdins is observed at μg/ml concentrations, the elicited secretions generate considerable antimicrobial potential in the crypt lumen (2). These findings show that innate immunity in the crypt is an active process that responds effectively to environmental cues of infection.

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Figure 2. Small intestinal crypt architecture: a schematic diagram of the small intestinal villus-crypt axis (72). Stem cells residing at the neck of the small intestinal crypt divide, and their progeny migrate upward toward the villi or deeper towards the base of the crypt. Migration towards the villus tips is accompanied by cellular differentiation into absorptive enterocytes, goblet cells, or enteroendocrine cells. The life span of these villus cells from their origin in the crypt, through migration and differentiation, until apoptotic death and exfoliation into the lumen is approximately 2–5 days. Stem cell progeny that descend towards the crypt base differentiate into Paneth cells, which have life span of several weeks. Inset: Paneth cells release secretory vesicles into the narrow lumen of the crypt (72). These secretions contain α-defensins, lysozyme, and secretory phospholipase A2. This potent antimicrobial cocktail is proposed to defend the crypt from bacterial colonization and its vital stem cell population from microbial invasion. [Illustrated by D. Schumick, Department of Medical Illustration, Cleveland Clinic Foundation, ©2000, Cleveland Clinic Foundation; first published in Nature Immunol 2000;1:99–100 (72), reprinted with permission.]

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Paneth cell secretion also can be stimulated by cholinergic agonists (67). Pilocarpine, bethanechol, and the nonspecific G-protein activators NaF and AlCl3 induce massive Paneth cell secretion, and muscarinic receptor antagonists inhibit Paneth cell degranulation (68). In vitro, carbamyl choline interacts directly with isolated mouse ileal crypts to stimulate an increase in the cytosolic Ca++ concentration by mobilizing both intracellular and extracellular calcium stores (67). Interestingly, carbamyl choline affects calcium dynamics in Paneth cells but not other epithelial cell populations in the crypt. These pharmacological studies suggest that control of Paneth cell secretion in vivo may be linked to parasympathetic neural signalling as well as direct apical stimulation by bacterial antigens.

Conditions that disrupt normal crypt cell biology appear to recruit new cells to express Paneth cell α-defensin genes and accumulate the corresponding peptides. For example, in transgenic mice that express either attenuated diphtheria toxin A fragment or SV40 large T antigen under the control of a functional mouse cryptdin-2 gene promoter, crypts undergo a transient Paneth cell deficiency (69). At 4 weeks of age, the majority of small intestinal crypts of these transgenic mice lack apparent Paneth cells, but the morphology of most crypts is normal by 8 weeks. During the period of Paneth cell deficiency, numbers of intermediate cells and granule-containing goblet cells increase, and those cells accumulate electron-dense, secretory granules that contain elevated levels of cryptdin(s) and sPLA2 (69). Thus, Paneth cell deficiency appears to induce a compensatory response in crypt intermediate cells, altering their genetic repertoire to produce and secrete Paneth cell antimicrobial peptides. These observations suggest that there may be plasticity in cellular differentiation in the crypt under some circumstances.

Future Directions

  1. Top of page
  2. Host Defense At The Mucosa of The Small Intestine
  3. Antimicrobial Peptides As Agents of Host Defense
  4. Antimicrobial Peptides of Small Intestinal Paneth Cells
  5. Defensins: Cysteine-Rich Antimicrobial Peptides
  6. Paneth Cell Defensins
  7. Antimicrobial Activities of Paneth Cell α-Defensins
  8. Posttranslational Proteolytic Processing
  9. Secretion of α-Defensins By Paneth Cells
  10. Future Directions
  11. Acknowledgements
  12. References

Without excluding the possibility of additional physiologic roles, Paneth cell defensins are key to innate immunity of the small bowel. Paneth cells contribute actively to mucosal immunity by sensing bacteria and releasing microbicidal peptides at effective concentrations (2). As discussed above, two apparent functions of these antimicrobial peptides are to protect the crypt stem cells from deleterious effects of microbes and to influence the viability of bacteria in the intestinal lumen. The biosynthetic and granulogenic pathways, the receptors for pattern recognition, and the signalling pathway(s) associated with apical secretion will require further investigation to understand this axis more completely. These aspects of mucosal immunity will likely be important areas for future investigation and of general relevance to understanding regulated secretory pathways in epithelial cells.

Several lines of investigation have provided links between the inflammatory bowel diseases, Crohn's disease and ulcerative colitis, and altered mucosal barrier function and the intestinal microflora (70,71). This review has highlighted aspects of mucosal immunity ascribed to Paneth cell defensins. The notion that unrecognized Paneth cell dysfunction may be associated with enteric immunopathology in a segment of the population may be worthy of experimental analysis. First, although further investigation is warranted, available evidence suggests Paneth cell products affect the species distribution and numbers of bacteria that inhabit the small intestine. Changes in the bacterial flora related to altered Paneth cell function might, in principle, underlie the relative susceptibility to inflammatory bowel disease in some individuals. Second, intestinal epithelial monolayer integrity is dependent preserving stem cell viability. Aberrant stem cell function would be expected to have severe consequences on the maintenance of the normal barrier functions of the intestinal epithelium. Defensins and other antimicrobial factors of Paneth cells are likely to help protect crypts against bacterial overgrowth and infection. Defects in Paneth cell physiology might lead to defective barrier function as a result of disrupted crypt cell biology and consequent impairment of epithelial renewal. It should be noted that in vivo enteric immunity is impaired in mice that are cryptdin deficient secondary to genetic deletion of procryptdin activation (65), yet the Paneth cells in crypts of those animals are morphologically normal. It is possible that certain humans also may carry mutations that create defects in the pathways required for defensin synthesis and secretion, but such defects would escape detection by routine histochemical evaluation.

References

  1. Top of page
  2. Host Defense At The Mucosa of The Small Intestine
  3. Antimicrobial Peptides As Agents of Host Defense
  4. Antimicrobial Peptides of Small Intestinal Paneth Cells
  5. Defensins: Cysteine-Rich Antimicrobial Peptides
  6. Paneth Cell Defensins
  7. Antimicrobial Activities of Paneth Cell α-Defensins
  8. Posttranslational Proteolytic Processing
  9. Secretion of α-Defensins By Paneth Cells
  10. Future Directions
  11. Acknowledgements
  12. References