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Published online before print July 15, 2003

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(Journal of Leukocyte Biology. 2004;75:34-38.)
© 2004 by Society for Leukocyte Biology

Antimicrobial polypeptides

David Geffen School of Medicine, University of California, Los Angeles, CA 90095-1690

Correspondence: David Geffen School of Medicine, University of California, Los Angeles, CA. E-mail: tganz{at}mednet.ucla.edu

	ABSTRACT

TOP ABSTRACT Antimicrobial activity of... The composition of respiratory... Activities of antimicrobial... Epithelial secretions may be... Interventions to enhance... SUMMARY AND FUTURE PROSPECTS REFERENCES

 
The respiratory tract presents a large and potentially vulnerable surface to inhaled microbes. It is coated by a thin layer of secretions generated by airway epithelial cells, submucosal glands, resident and recruited phagocytes (neutrophils, eosinophils, monocytes, and macrophages) and alveolar epithelial cells, as well as substances that enter from blood plasma. More than 80 years ago, Alexander Fleming observed that respiratory secretions have microbicidal and microbistatic properties [¹ ]. He described the activity of lysozyme, one of the principal polypeptides of these secretions. Since then, a number of additional antimicrobial components have been identified, and there is increasing insight into their complex interactions. This review is an update of my previous summary of this area [² ].

Key Words: innate immunity • host defense • lysozyme • defensins • lactoferrin • secretory leukoprotease inhibitor

	Antimicrobial activity of respiratory secretions

TOP ABSTRACT Antimicrobial activity of... The composition of respiratory... Activities of antimicrobial... Epithelial secretions may be... Interventions to enhance... SUMMARY AND FUTURE PROSPECTS REFERENCES

 
Stimulated by efforts to understand the failure of respiratory host defenses in cystic fibrosis, several investigators reexamined the antimicrobial properties of airways secretions. In one study, fluids obtained by nasal or bronchoalveolar lavage, or as cystic fibrosis sputum, were tested under externally defined ionic or osmotic conditions against a test strain of Escherichia coli [³ ]. Under permissive (low ionic strength) conditions, both nasal and lung secretions were antimicrobial, as were their cationic polypeptide constituents lysozyme, lactoferrin, SLPI, and neutrophil and epithelial defensins. The antimicrobial activity of lavage fluids or their polypeptide constituents was inhibited by increasing ionic strength and was especially sensitive to divalent cations. This is consistent with the well-established ability of ionic constituents to inhibit the electrostatic attraction of cationic polypeptides to anionic microbial surfaces, and the more specific binding of divalent cations to outer membranes of Gram-negative bacteria, an interaction that stabilizes the outer membrane against penetration by cationic polypeptides. Mucin, an abundant polyanionic component of respiratory secretions, did not affect the antimicrobial activity of lavage fluids, lysozyme, or lactoferrin.

A complementary approach was taken by other investigators [⁴ , ⁵ ] who examined minimally manipulated nasal fluid collected with a suction catheter from the nasal mucosa of normal donors. In these experiments, the natural ionic concentrations and the concentrations of the polypeptide constituents were maintained and are reflective of mechanically stimulated nasal secretions. Nasal fluid from most donors killed or inhibited the growth of Staphyloccus aureus, Escherichia coli and Pseudomonas aeruginosa, but fluids from nasal carriers of S. aureus permitted the growth of this bacterium. Moreover, after 24 h of incubation, regrowth of P. aeruginosa was observed in some experiments, indicating that surviving bacteria could adapt to the injurious effects of nasal fluid. The inhibition or killing of bacteria by nasal fluid was not due to its intrinsic inability to provide nutrients for bacterial growth since brief heat treatment of the fluid at 100°C rendered it permissive for the growth of a CF isolate of P. aeruginosa. Nasal secretions from which cationic polypeptides were removed by treatment with cation exchangers were also made permissive for the growth of P. aeruginosa, E. coli, or Listeria monocytogenes, but the activity could be restored by adding back the cationic fraction [⁵ ]. Experiments in which individual cationic polypeptides were added back, lysozyme turned out to be an important contributor to the total activity of nasal fluid against a CF isolate of P. aeruginosa.

In another study, antimicrobial activity of BALF toward a sensitive test bacterium Bacillus subtilis was seen in both sarcoidosis patients and normal donors [⁶ ]. The activity was fractionated and attributed to lysozyme, neutrophil defensins, SLPI, the cathelicidin LL-37, and other as yet unidentified components.

These reports confirm the concept that respiratory secretions are broadly antimicrobial and that their activity is largely due to their cationic polypeptide components. They also indicate that the antimicrobial activity of respiratory fluid is surprisingly precarious and that it can be negated by alterations in the ionic milieu, by bacterial adaptation, and by as yet uncharacterized abnormalities in nasal carriers of S. aureus.

Studies of human airway epithelia maintained either in vitro or in nude mice demonstrate that respiratory epithelial cells generate antimicrobial activity even without forming differentiated glandular structures [⁷ , ⁸ ]. However, it has become clear that compared with respiratory secretions collected in vivo, the antimicrobial activity of cultured airways epithelia is much less potent, perhaps because these cell culture models secrete a low-protein fluid that contains scant amounts of the major substances found in airways secretions (lysozyme, lactoferrin, secretory leukoprotease inhibitor or defensins). Consequently, the contributions of submucosal glands are now receiving increased experimental attention.

	The composition of respiratory secretions

TOP ABSTRACT Antimicrobial activity of... The composition of respiratory... Activities of antimicrobial... Epithelial secretions may be... Interventions to enhance... SUMMARY AND FUTURE PROSPECTS REFERENCES

 
Since the epithelial fluid layer is very thin under resting conditions, the collection of sufficient volume of respiratory fluid from live human or animal donors either subjects the epithelium to mechanical or chemical stimulation, or depends on pathologic processes that increase and unavoidably modify secretions. Moreover, the commonly used bronchoalveolar lavage method admixes secretions from anatomically diverse locations and dilutes them to an unknown extent. Because of these obstacles, the ionic and polypeptide composition of resting respiratory fluid in specific anatomical locations is subject to substantial uncertainty. However, mixed samples of respiratory fluid obtained from the nose by mechanical or chemical stimulation [⁴ , ⁹ ], or as sputum from patients with chronic bronchitis [^{10 11 12} ], indicate that lysozyme and lactoferrin are the most abundant antimicrobial proteins of airway secretions, at 0.1–1 mg/ml. These estimates agree with bronchoalveolar lavage sampling [³ , ¹³ ] assuming that the lavage fluid represents about a 100-fold dilution of respiratory fluid, an approximate (and controversial) dilution factor based on measurements of urea concentrations in BALF [¹⁴ ]. Secretory leukoprotease inhibitor (SLPI) is about 10-fold less abundant [¹⁵ , ¹⁶ ]. The concentrations of neutrophil and epithelial defensins in respiratory fluid are highly dependent on acute inflammation. In CF sputum, the concentrations of neutrophil defensins ranged from 300 µg to >1 mg/ml [¹⁷ ], but concentrations in BALF of patients undergoing bronchoscopy without clinical signs of inflammation were only 100 ng/ml [¹⁸ ], corresponding to 10 µg/ml in respiratory fluid. Concentrations of epithelial defensins HBD-1 and HBD-2 in inflamed nasal fluid and BALF were about a thousand-fold lower than those of lysozyme, leading to an estimate of 1 µg/ml in the airway fluid [⁴ , ¹⁹ ]. Other antimicrobial components of respiratory fluid include the cathelicidin peptide LL-37 [⁶ , ²⁰ ]. Nasal fluid components with less well documented antimicrobial or host defense activity include ß-microseminoprotein, lipophilin C, ß2-microglobulin, calgranulin A and B, lipocalin-1 and PLUNC (palate, lung, and nasal epithelium clone).

Locally, at sites of infection, the concentrations of antimicrobial polypeptides in respiratory secretions are prominently modulated by inflammation. Inflammatory stimuli release chemoattractants that recruit neutrophils containing large amounts of lysozyme, lactoferrin, and neutrophil defensins. These stimuli also increase the synthesis of ß-defensin by epithelial cells [¹⁹ , ²¹ ], and when chronic, they induce the differentiation of respiratory epithelial cells into secretory cell types.

	Activities of antimicrobial polypeptides from respiratory secretions

TOP ABSTRACT Antimicrobial activity of... The composition of respiratory... Activities of antimicrobial... Epithelial secretions may be... Interventions to enhance... SUMMARY AND FUTURE PROSPECTS REFERENCES

 
Lysozyme is 14 kd enzyme directed against the ß 1 4 glycosidic bond between N-acetylglucosamine and N-acetylmuramic acid residues that make up peptidoglycan, the cell wall material that gives bacteria their shape. In addition to enzymatic lysis of bacterial cell walls, lysozyme can also kill bacteria by a nonenzymatic mechanism [²² ]. Although lysozyme is highly active against many Gram-positive species, for example, Bacillus megaterium, Micrococcus lysodeicticus, and many streptococci, it has been described as ineffective against Gram-negative bacteria [²³ ] unless potentiated by certain cofactors (lactoferrin, antibody-complement or hydrogen peroxide-ascorbic acid). These cofactors presumably disrupt the outer membrane of Gram-negative bacteria and allow lysozyme access to the sensitive peptidoglycan layer. Lysozyme is a component of both phagocytic and secretory granules of neutrophils and is also produced by monocytes, macrophages, and epithelial cells. Lysozyme is 10-fold more abundant in the initial "airway" aliquot than in subsequent aliquots of bronchoalveolar lavage [¹³ ], and its concentration correlates poorly with neutrophil concentrations suggesting that, on the average, airway epithelium and its glands are the major sources of lysozyme in airways secretions. Unlike humans who have one lysozyme gene, mice have two: lysozyme M, expressed in phagocytes and most epithelial secretions, and lysozyme P, found in Paneth cells. Mice overexpressing rat lysozyme under the control of the human surfactant protein C promoter have increased resistance to lung infection with both gp B Streptococcus and P. aeruginosa [²⁴ ]. Mice lacking lysozyme M compensate by newly expressing Paneth cell lysozyme (lysozyme P) in inflammatory macrophages [²⁵ ]. The predominant defect in these mice is greatly increased inflammatory response to subcutaneous injection of Micrococcus luteus, due to the inability of these mice to rapidly degrade peptidoglycan. Although there is a significant delay in the killing of M. luteus in the lysozyme M-deficient mice, both normal and deficient mice sterilize the subcutaneous abscess by day 5 after infection. These experiments suggest that lysozyme is involved in antimicrobial activity along with other cationic polypeptides, but its ability to degrade peptidoglycan and terminate its proinflammatory effects is nonredundant.

Lactoferrin, a close relative of the serum protein transferrin, is an 80 kd iron-binding protein that is highly abundant in the specific granules of human neutrophils and in epithelial secretions. Lactoferrin inhibits microbial growth by sequestering iron essential for microbial respiration [²⁶ ]. It can also be directly microbicidal [²⁷ ], an activity that is concentrated in its N-terminal cationic fragment "lactoferricin".

Secretory leukoprotease inhibitor is a 12 kd nonglycosylated protein consisting of two similar domains. The N-terminal domain has modest antimicrobial activity in vitro against both Gram-negative and Gram-positive bacteria [²⁸ ]. The C-terminal domain acts as an effective inhibitor of neutrophil elastase and may also be involved in intracellular regulation of responses to lipopolysaccharide [²⁹ ].

Human defensins [³⁰ ] are 3–5 kD peptides, members of a widely distributed family of microbicidal peptides with a characteristic three-dimensional fold and a 6-cysteine/3-disulfide pattern. Three closely related defensins, human neutrophil peptides HNP-1, HNP-2 and HNP-3, are major components of the dense azurophil granules of neutrophils, and a fourth, HNP-4, is found in the same location but is much less abundant. Two other human defensins [³¹ , ³² ], HD-5 and HD-6, are located in the lysozyme-rich secretory granules of Paneth cells, an epithelial cell type positioned at the bottom of small intestinal crypts and thought to be involved in local host defense. The four most recently characterized defensins, human ß-defensins HBD-1 [³³ , ³⁴ ], HBD-2 [³⁵ ], HBD-3 [^{36 37 38} ], and HBD-4 [³⁹ ] differ slightly from the classical ""defensins in the spacing and connectivity of their cysteines. Their mRNAs were expressed in epithelial organs, with HBD-1 most abundant in the kidney and HBD-2 and HBD-3 in inflamed skin, but all three are detectable in the respiratory tract as well [¹⁹ , ³⁷ , ⁴⁰ , ⁴¹ ]. Like the synthesis of the prototypic airway defensin, the bovine tracheal antimicrobial peptide [⁴² ], the synthesis and secretion of HBD-2 (and presumably HBD-3) is regulated by dual circuitry: 1) direct epithelial responses to LPS and other microbial stimuli, most likely mediated by epithelial CD-14/TLR/NF-B [²¹ ] and characterized by a high threshold, and 2) a lower-threshold, indirect, cytokine-mediated epithelial response triggered primarily by the encounter of microbes with local macrophages that then produce IL-1/ß and other cytokines [¹⁹ ], which, in turn, act on epithelial cytokine receptors to increase epithelial defensin synthesis. This scheme avoids promiscuous activation by low concentrations of inhaled noninvasive microbes that could lead to deleterious lung inflammation, while retaining the ability to activate local host defenses and inflammation in response to large boluses of microbes or epithelial penetration by fewer invasive microbes.

In vitro, under low-salt conditions (e.g., 10 mM sodium phosphate) defensins are microbicidal at µM (µg/ml) concentrations against many Gram-positive and Gram-negative bacteria, yeast and fungi, and certain enveloped viruses. Increasing salt concentrations competitively inhibit defensin activity with the inhibitory effect modulated by the properties of the target microbe. Defensins act preferentially on microbes by permeabilizing microbial membranes rich in anionic phospholipids, with relative sparing of the cholesterol- and neutral phospholipid-rich cell membranes of the host.

Estimates of defensin concentrations in the phagocytic vacuoles of neutrophils are in the milligram-to-milliliter range, a concentration that should be sufficient to overcome inhibition by extracellular ion concentrations. Similar considerations also apply to the activity of defensins in the narrow (5–10 µm diameter) intestinal crypts into which Paneth cells secrete their defensin-containing granules [⁴³ ]. The concentration of HBD-2 in desquamated inflamed skin is in the range of 10 µg/ml [³⁵ ], again sufficient to inhibit or kill many microbes. In the respiratory tract, the concentrations of neutrophil defensins HNP1–HNP4 and epithelial HBD-2 are greatly increased by inflammation [¹⁷ , ¹⁹ ] and could reach antimicrobial concentrations, but their specific role in antimicrobial events in the lung has not yet been documented. It should be noted that some defensins manifest additional in vitro activities that may contribute to inflammation and repair, including inhibition of ACTH-stimulated cortisol production, inhibition of fibrinolysis, mitogenic effects on fibroblasts [⁴⁴ ], and induction of IL-8 and SLPI secretion by bronchial epithelial cells [⁴⁵ , ⁴⁶ ]. The recently described chemoattractant effects on immature dendritic cells and memory T-cells [⁴⁷ , ⁴⁸ ] are likely to be important in linking the initial innate response to infection to the later development of acquired immunity.

Cathelicidins [⁴⁹ ] are members of a large family of mammalian microbicidal peptides with a conserved N-terminal precursor structure (containing typically 100 amino acid residues), but highly heterogeneous C-terminal peptides (10–40 amino acid residues). Most cathelicidins undergo extracellular proteolytic cleavage that frees the C-terminal peptide from the precursor and activates it. The sole known human cathelicidin has been named as hCAPl8 or FALL39/LL37 by the three groups that described its cDNA and gene and peptide forms [^{50 51 52} ]. Its abundance in neutrophil-specific granules appears to be about one third of that of lactoferrin or lysozyme, the two major proteins of specific granules. Exemplifying the overlap of phagocytic and epithelial host defenses, the mRNA for the human cathelicidin was also found in the testis, in inflamed human keratinocytes [⁵³ ], in other squamous epithelia [⁵⁴ ], and in human airway epithelia [²⁰ ]. In vitro, the human peptide LL-37 displayed both LPS binding [⁵⁰ ] and broad-spectrum microbicidal activities [²⁰ , ⁵⁵ ]. Overexpression of LL-37 in an airway xenograft model of cystic fibrosis restored the deficient antimicrobial activity [⁸ ], indicating that LL-37 retains activity in the complex milieu of the airway.

	Epithelial secretions may be a synergistic antimicrobial fluid

TOP ABSTRACT Antimicrobial activity of... The composition of respiratory... Activities of antimicrobial... Epithelial secretions may be... Interventions to enhance... SUMMARY AND FUTURE PROSPECTS REFERENCES

 
Antimicrobial proteins and peptides differ in their molecular targets (phospholipid membrane, peptidoglycan, iron, etc.) and in their mode of action on that target (enzymatic disruption, pore formation, etc.). Thus, they could act cooperatively by attacking multiple essential structures in bacteria or by unmasking structures vulnerable to other components of the mixture. The multiplicity of active ingredients may also broaden the effectiveness of the mixture against the many potential microbial targets and decrease the likelihood of acquired resistance. Indeed, synergistic interactions between the main polypeptide constituents (lysozyme, lactoferrin, and SLPI) of airway secretions have been reported [³ ].

	Interventions to enhance antimicrobial polypeptide-mediated host defense mechanisms

TOP ABSTRACT Antimicrobial activity of... The composition of respiratory... Activities of antimicrobial... Epithelial secretions may be... Interventions to enhance... SUMMARY AND FUTURE PROSPECTS REFERENCES

 
The selection of antimicrobial peptides and proteins as therapeutic candidates is governed by considerations of 1) biological activity, 2) toxicity profile and 3) costs of development, production and administration. Since the antimicrobial activities of these substances usually require at least micromolar concentrations, and the costs of production rise with the size of the polypeptide and its structural complexity, initial efforts have centered on relatively small peptides, or small fragments of larger proteins. The costs and toxicities increase with the volume of distribution in the body, favoring topical over systemic administration for initial trials of these new compounds. Among pulmonary applications, use of antimicrobial peptides as inhaled antibiotics is particularly attractive because only tobramycin and colistin are currently approved for this application, and there is increasing microbial resistance to them. The pathogenic role of Pseudomonas aeruginosa and similar bacteria in cystic fibrosis presents a unique opportunity to test the pharmacologic use of antimicrobial peptides that are variants of naturally occurring forms. Initial clinical trials of a protegrin (porcine neutrophil cathelicidin) derivative [⁵⁶ ] for this and other topical applications are ongoing.

Alternative strategies, at earlier stages of development, include the use of gene therapy technology to deliver functional antimicrobial peptide genes to the respiratory epithelium [⁸ ] or systemically [⁵⁷ ].

	SUMMARY AND FUTURE PROSPECTS

TOP ABSTRACT Antimicrobial activity of... The composition of respiratory... Activities of antimicrobial... Epithelial secretions may be... Interventions to enhance... SUMMARY AND FUTURE PROSPECTS REFERENCES

 
The increasing interest in mechanisms of innate immunity has refocused attention on the highly effective processes that keep the lower respiratory tract largely free of microbes. The picture that is emerging is far from the passive system depicted in a rudimentary form in most current immunology texts. Antimicrobial polypeptides, a part of the innate host defense system, are present constitutively and in high concentrations on respiratory epithelia but are also locally supplemented by secretion from recruited phagocytes or from activated epithelial cells. They selectively target vital microbial structures, taking advantage of structural and biochemical differences between the host and the microbes. Some antimicrobial polypeptides also function as signaling molecules, alerting the adaptive immune system to the presence of invaders. Several diseases characterized by persistent mucosal colonization or infection, including prominently cystic fibrosis [⁷ ] and nasal colonization with S. aureus, may turn out to be manifestations of defects on antimicrobial polypeptide-based defenses. For these and other airway infections, the use of inhaled or topical synthetic peptide antimicrobials is an exciting prospect.

Areas of ongoing scientific ferment are likely to include highly selective microsampling and analysis of the composition of respiratory secretions in specific locations in the respiratory system, including alveoli, small airways and the bronchi. Continued attention will be paid to the possible role of antimicrobial polypeptides and other innate host defense components in the early pathogenesis of cystic fibrosis in the lung. Much will be learned about antimicrobial polypeptides by knocking out genes or clusters of genes that encode antimicrobial polypeptides, especially when this becomes possible in larger animals whose respiratory systems are similar to those of humans. An alternative strategy to define the specific function of antimicrobial polypeptides, selective pharmaceutical inhibition of their post-translational proteolytic activation, may be productive in animals where the knockout technology is not yet available [⁵⁸ ]. Finally, we can also look forward to improved antimicrobial therapies that take advantage of potential synergies with the "natural antibiotics" already present at the site of infection.

	ACKNOWLEDGEMENTS

 
My laboratory has received support from the National Institutes of Health, The Will Rogers Fund and the Cystic Fibrosis Foundation. I would like to thank Robert Lehrer, Charles Bevins, and John Fiddes for discussions that stimulated some of the ideas in this manuscript. I apologize to those colleagues whose primary work could not be cited because of space limitations.

Received April 14, 2003; revised May 30, 2003; accepted May 30, 2003.

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TOP ABSTRACT Antimicrobial activity of... The composition of respiratory... Activities of antimicrobial... Epithelial secretions may be... Interventions to enhance... SUMMARY AND FUTURE PROSPECTS REFERENCES

 

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