Humanized theta-defensins (retrocyclins) enhance macrophage performance and protect mice from experimental anthrax infections.

Retrocyclins are humanized versions of the -defensin peptides expressed by the leukocytes of several nonhuman primates. Previous studies, performed in serum-free media, determined that retrocyclins 1 (RC1) and RC2 could prevent successful germination of Bacillus anthracis spores, kill vegetative B. anthracis cells, and inactivate anthrax lethal factor. We now report that retrocyclins are extensively bound by components of native mouse, human, and fetal calf sera, that heat-inactivated sera show greatly enhanced retrocyclin binding, and that native and (especially) heat-inactivated sera greatly reduce the direct activities of retrocyclins against spores and vegetative cells of B. anthracis. Nevertheless, we also found that retrocyclins protected mice challenged in vivo by subcutaneous, intraperitoneal, or intranasal instillation of B. anthracis spores. Retrocyclin 1 bound extensively to B. anthracis spores and enhanced their phagocytosis and killing by murine RAW264.7 cells. Based on the assumption that spore-bound RC1 enters phagosomes by "piggyback phagocytosis," model calculations showed that the intraphagosomal concentration of RC1 would greatly exceed its extracellular concentration. Murine alveolar macrophages took up fluorescently labeled retrocyclin, suggesting that macrophages may also acquire extracellular RC1 directly. Overall, these data demonstrate that retrocyclins are effective in vivo against experimental murine anthrax infections and suggest that enhanced macrophage function contributes to this property.

The fungicidal mechanisms of human monocytes. I. Evidence for myeloperoxidase-linked and myeloperoxidase-independent candidacidal mechanisms.

We tested the ability of human peripheral blood monocytes to kill Candida albicans and Candida parapsilosis. Evidence that multiple fungicidal mechanisms operate in normla monocytes was found. Normal monocytes ingested and killed viable C. albicans, and could iodinate heat-killed C. albicans. Both functions were defective in monocytes from subjects with myeloperoxidase deficiency or chronic granulomatous disease. Methimazole, isoniazid, and aminotriazole inhibited iodination by normal monocytes without impairing their ability to kill C. albicans, indicating that iodination was not essential to the myeloperoxidase-hydrogen peroxide-mediated fungicidal system of the monocyte. C. parapsilosis, an organism killed with supranormal efficacy by monocytes from a patient with hereditary myeloperoxidase deficiency, was selected to examine the myeloperoxidase-independent fungicidal mechanisms of monocytes. Monocytes were obtained from the blood of normal or leukemic subjects and homogenized in 0.34 M sucrose to yield fractions rich in cytoplasmic granules. These fractions were extracted with 0.01 M citric acid and the soluble components were separated by micropreparative polyacrylamide electrophoresis. Monocytes were found to contain cationic proteins, other than myeloperoxidase, that kill C. parapsilosis in vitro.

Functional aspects of a second mechanism of candidacidal activity by human neutrophils.

We tested the ability of human neutrophils to kill five Candida species and the yeast Torulopsis glabrata. C. parapsilosis and C. pseudotropicalis were found to be killed readily by normal and myeloperoxidase-deficient neutrophils and were selected to probe the myeloperoxidase-independent fungicidal mechanisms of the neutrophil. These organisms were killed with relatively normal (C. parapsilosis) or moderately reduced (C. pseudotropicalis) effectiveness by neutrophils from two boys with X-linked chronic granulomatous disease. Azide (2 mm) and sulfadiazine (4 mm) exerted a relatively small inhibitory effect on the ability of normal neutrophils to kill C. parapsilosis. These compounds did not, however, inhibit the killing of C. parapsilosis by myeloperoxidase-deficient neutrophils, although they blocked their iodination of ingested Candida cells. Anaerobic incubation conditions inhibited the ability of normal neutrophils to kill C. parapsilosis slightly but did not impair this function in myeloperoxidase-deficient cells. All of the Candida species tested had catalase activity, yet their sensitivity to H(2)O(2) in cell-free systems varied considerably. Our C. parapsilosis strain was extraordinarily resistant to H(2)O(2) (LD(75): 0.14 m), as compared with C. pseudotropicalis or with our reference strain of C. albicans (LD(75): 2.3 x 10(-3)m and 3.4 x 10(-3)m, respectively). These data establish the existence in human neutrophils of a second mechanism that exerts microbicidal activity against certain Candida species; the mechanism is unrelated to myeloperoxidase, iodination, or to the direct effects of H(2)O(2) generated by the endogenous metabolic processes of the neutrophil. As yet unidentified, this mechanism appears to remain operative in the neutrophils of subjects with hereditary myeloperoxidase deficiency or chronic granulomatous disease.

Inhibition by sulfonamides of the candidacidal activity of human neutrophils.

Sulfonamides reduced substantially the ability of normal human neutrophils to kill strains of Candida albicans and Candida tropicalis, and impaired to a lesser extent their activity against Staphylococcus aureus 502A and Serratia marcescens. Sulfonamides also inhibited (a) iodination of Candida cells by normal neutrophils; (b) candidacidal activity in cell-free systems containing purified human myeloperoxidase, hydrogen peroxide, and potassium iodide; and (c) accumulation of molecular iodine in analogously constructed cell-free systems. In contrast to these effects on reactions catalyzed by myeloperoxidase, sulfonamides exerted relatively little effect on the levels of microbicidal activity manifested by human neutrophils that lacked myeloperoxidase. Sulfonamides appear to influence the function of human neutrophils predominantly by interfering with myeloperoxidase-mediated pathways. Certain basic and clinical implications of these data are discussed.

NADPH oxidase deficiency in X-linked chronic granulomatous disease.

We measured the cyanide-insensitive pyridine nucleotide oxidase activity of fractionated resting and phagocytic neutrophils from 11 normal donors, 1 patient with hereditary deficiency of myeloperoxidase, and 7 patients with X-linked chronic granulomatous disease (CGD). When measured under optimal conditions (at pH 5.5 and in the presence of 0.5 mM Mn++), NADPH oxidase activity increased fourfold with phagocytosis and was six-fold higher than with NADH. Phagocytic neutrophils from patients with CGD were markedly deficient in NADPH oxidase activity.

Receptor-mediated regulation of superoxide production in human neutrophils stimulated by phorbol myristate acetate.

Human neutrophils contain receptors for phorbol myristate acetate (PMA), a complex lipid that induces them to generate superoxide (O (2)). Binding of PMA to these receptors displays specificity, reversibility, and high affinity. The receptor's apparent KD was approximately 0.29 nM and multiple copies (approximately 2.1 +/- 0.6 x 10(5)) were present per neutrophil. We found that the timing and magnitude of the neutrophil's respiratory burst were set independently. The onset of O (2) production occurred after a lag that was inversely proportional to the initial concentration of added PMA. The extent (rate) of O (2) production was directly proportional to the fractional occupancy of the receptor by PMA. Dual regulatory controls, such as those we noted when neutrophils were stimulated by PMA, could afford metabolic stability in the face of transient or low intensity stimuli without compromising quick and powerful responses to larger disturbances.

NAD(P)H oxidase activity in human neutrophils stimulated by phorbol myristate acetate.

Phorbol myristate acetate activated in normal human neutrophils a single enzymatic entity that was dormant in unstimulated cells, optimally active at pH 7.0, and capable of oxidizing either NADH or NADPH, producing NAD(P)+ and superoxide (O27). Comparative fluorometric and spectrophotometric measurements supported the stoichiometry NAD(P)H + 20(2) leads to NAD(P)+ + 20(27) + H+. the seemingly considerable NAD(P)+ production at pH 5.5 and 6.0 was due largely to nonenzymatic oxidation of NAD(P)H by chain reactions initiated by HO27 (perhydroxyl radical), the conjugate acid of O27. This artifact, responsible for earlier erroneous assignments of an acid pH optimum for NAD(P)H oxidase, was prevented by including superoxide dismutase in fluorometric assays. NAD(P)H oxidase was more active towards NADPH (Km = 0.15 +/- 0.03 mM) than NADH (Km = 0.68 +/- 0.2 mM). No suggestion that oxidase activity was allosterically regulated by NAD(P)H was seen. Phorbol myristate acetate-induced O27 production was noted to be modulated by pH in intact neutrophils, suggesting that NAD(P)H oxidase is localized in the plasma membrane where its activity may be subject to (auto) regulation by local H+ concentrations.

Leukocyte myeloperoxidase deficiency and disseminated candidiasis: the role of myeloperoxidase in resistance to Candida infection.

The neutrophils and monocytes of a patient with disseminated candidiasis were found to lack detectable levels of the lysosomal enzyme myeloperoxidase (MPO), although they had normal levels of other granule-associated enzymes. Leukocytes from one of the patient's sisters also lacked detectable MPO; leukocytes from his four sons contained approximately one-third of mean normal peroxidase levels. Neither the patient nor his affected relatives had experienced frequent or unusual bacterial infections. The phagocytic activity of the patient's MPO-deficient neutrophils was intact, and the cells displayed normal morphologic and metabolic responses to phagocytosis. In contrast to normal leukocytes which killed 30.5+/-7.3% of ingested Candida albicans in 1 hr, however, the patient's neutrophils killed virtually none. His leukocytes also failed to kill the strain of C. albicans recovered from his lesions, as well as other Candida species. These MPO-deficient neutrophils killed Serratia marcescens and Staphylococens aureus 502A at an abnormally slow rate, requiring 3-4 hr to achieve the bactericidal effect attained by normal leukocytes after 45 min. No other abnormalities in his cellular or humoral immune responses were demonstrated. These findings suggest that hereditary MPO deficiency is transmitted as an autosomal recessive characteristic, that the homozygous state conveys enhanced susceptibility to disseminated candidiasis, and that MPO is necessary for candidacidal activity in human neutrophils. Although lending support to the suggested bactericidal role of MPO in leukocytes, the data indicate that alternative bactericidal mechanisms, effective in the absence of MPO, are functionally dominant in the human neutrophil.

Individual and synergistic effects of rabbit granulocyte proteins on Escherichia coli.

Affinity purification of crude acid extracts of rabbit polymorphonuclear leukocytes using Escherichia coli (J5) as adsorbent yields the bactericidal/permeability-increasing protein (BPI), two 15-kD species (p15s), and the two most potent (cationic) defensin species (neutrophil peptides [NP] -1 and -2). Tested in buffered isotonic medium, the relative antibacterial potency of these proteins against E. coli J5 is BPI (IC50 0.2 nM) > p15A (10 nM) > NP -1 (400 nM). Sublethal doses of p15A or NP-1 can synergize with BPI to decrease the dose required to inhibit the growth of E. coli by up to 50-fold. BPI and p15A display similar features of antibacterial action distinct from defensin NP-1, but NP-1 acts synergistically only with BPI and not with p15A. All aspects of the combined action of BPI and NP-1 resemble those observed with higher concentrations of BPI alone, implying that NP-1 enhances BPI potency. Neither NP-1 nor p15A alter the amount of BPI binding to E. coli but BPI enhances binding of p15A to E. coli, raising the possibility that synergy between these two proteins may occur at least partially at the level of binding. The potent synergistic actions of these proteins can also be demonstrated against serum-resistant clinical isolates of encapsulated E. coli tested in whole blood and plasma ex vivo, suggesting that such combined action may contribute to host defense in vivo.

Effects of cholera enterotoxin on adenosine 3',5'-monophosphate and neutrophil function. Comparison with other compounds which stimulate leukocyte adenyl cyclase.

Cholera enterotoxin caused a delayed accumulation of adenosine 3',5'-monophosphate (cyclic AMP) in human leukocytes, associated with an increase in leukocyte adenyl cyclase activity. The action of cholera enterotoxin contrasted with that of other agents which stimulate adenyl cyclase: (a) the effects of the toxin were delayed in onset, while prostaglandin-E(1) (PGE(1)) and isoproterenol acted rapidly; (b) removal of the soluble toxin from the extracellular medium did not abolish its effects on cyclic AMP and inhibition of antigenic histamine release, while removal of PGE(1) did prevent its effects; (c) PGE(1), but not cholera enterotoxin, stimulated adenyl cyclase activity when added directly to broken cell preparations. Binding of the toxin to leukocytes was rapid and irreversible, and was followed by a gradual increase in cyclic AMP which was not prevented by cycloheximide. Cholera enterotoxin caused accumulation of cyclic AMP in purified human neutrophils as well as mono-nuclear cells, but did not prevent the extrusion of lysosomal hydrolases from phagocytic cells. The toxin only slightly inhibited the ability of human neutrophils to kill Candida albicans. Thus these results with the toxin cast doubt on previous proposals that cyclic AMP regulates these two functions of neutrophils. The unique action of cholera enterotoxin on cyclic AMP production provides a potentially useful pharmacologic tool, in addition to methylxanthines and dibutyryl cyclic AMP, for testing hypotheses relating cyclic AMP to altered function of leukocytes and, perhaps, of other mammalian cells.

Cyclic 3',5'-adenosine monophosphate in the human lukocyte: synthesis, degradation, andeffects n neutrophil candidacidal activity.

Prostaglandins E(1) and E(2) (PGE(1) and PGE(2)) stimulate adenyl cyclase activity in broken cell preparations of normal human leukocytes, whereas prostaglandin F(1a) produces no effect. PGE(1) and PGE(2) also cause increased accumulation of cyclic 3',5'-adenosine monophosphate-(3)H ((3)H-labeled AMP) in intact leukocytes which have been preincubated with adenine-(3)H in vitro. Theophylline inhibits leukocyte phosphodiesterase activity and potentiates the stimulatory effect of the prostaglandins on intracellular accumulation of cyclic 3',5'-AMP-(3)H. The ability of human granulocytes in vitro to kill Candida albicans was consistently inhibited by PGE(1) and theophylline. This effect was reproduced by dibutyryl cyclic 3',5'-AMP, a lipid-soluble analogue of the endogenous nucleotide. The inhibition of candidacidal activity could not be accounted for by drug effects on phagocytosis, oxygen consumption, or hexose monophosphate shunt activity. These results are consistent with the hypothesis that increased intracellular concentrations of cyclic 3',5'-AMP impair the granulocyte's ability to kill C. albicans, but the precise mechanism of inhibition has not yet been defined.

Modulation of the in vitro candidacidal activity of human neutrophil defensins by target cell metabolism and divalent cations.

We tested the in vitro susceptibility of Candida albicans to three defensins from human neutrophilic granulocytes (HNP-1, 2, and 3), a homologous defensin from rabbit leukocytes (NP-1), and four unrelated cationic peptides. Although the primary amino acid sequences of HNP-1, 2, and 3 are identical except for a single amino-terminal amino acid alteration, HNP-1 and HNP-2 killed C. albicans but HNP-3 did not. C. albicans blastoconidia were protected from HNP-1 when incubations were performed in the absence of oxygen or in the presence of inhibitors that blocked both of its mitochondrial respiratory pathways. Neither anaerobiosis nor mitochondrial inhibitors substantially protected C. albicans exposed to NP-1, poly-L-arginine, poly-L-lysine, or mellitin. Human neutrophilic granulocyte defensin-mediated candidacidal activity was inhibited by both Mg2+ and Ca2+, and was unaffected by Fe2+. In contrast, Fe2+ inhibited the candidacidal activity of NP-1 and all of the model cationic peptides, whereas Mg2+ inhibited none of them. These data demonstrate that susceptibility of C. albicans to human defensins depends both on the ionic environment and on the metabolic state of the target cell. The latter finding suggests that leukocyte-mediated microbicidal mechanisms may manifest oxygen dependence for reasons unrelated to the production of reactive oxygen intermediates by the leukocyte.

Bactericidal properties of murine intestinal phospholipase A2.

We purified a molecule from the murine small intestine that killed both Escherichia coli and Listeria monocytogenes, and identified it as intestinal phospholipase A2 (iPLA2) by NH2-terminal sequencing and enzymatic measurements. The ability of iPLA2 to kill. L. monocytogenes was greatly enhanced by 5 mM calcium, inhibited by EGTA and abolished after reduction and alkylation, suggesting that enzymatic activity was required for iPLA2-mediated bactericidal activity. A mouse-avirulent phoP mutant, S. typhimurium 7953S, was 3.5-fold more susceptible to iPLA2 than its isogenic virulent parent, S. typhimurium 14028S (estimated minimal bactericidal concentrations 12.7 +/- 0.5 micrograms/ml vs. 43.9 +/- 4.5 micrograms/ml P < 0.001). Overall, these findings identify iPLA2 as part of the antimicrobial arsenal that equips Paneth cells to protect the small intestinal crypts from microbial invasion. Because iPLA2 is identical to Type 2 phospholipase A2 molecules found in other sites, including spleen, platelets and inflammatory exudate cells, this enzyme may also contribute to antibacterial defenses elsewhere in the body.

Defensins. Natural peptide antibiotics of human neutrophils.

We extracted a granule-rich sediment from normal human neutrophils and subjected it to chromatographic, electrophoretic, and functional analysis. The extract contained three small (molecular weight less than 3,500) antibiotic peptides that were named human neutrophil peptide (HNP)-1, HNP-2, and HNP-3, and which will be referred to as "defensins." HNP 1-3, a mixture of the three defensins, killed Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli effectively in vitro when tested in 10 mM phosphate buffer containing certain nutrients, but it had little or no bactericidal activity in nutrient-free buffer. In contrast, the nutrient-free buffer supported a high degree of activity by HNP 1-3 against Cryptococcus neoformans. In addition to its antibacterial and antifungal properties, HNP 1-3 directly inactivated herpes simplex virus, Type 1. Two of the individual purified defensins, HNP-1 and HNP-2, were as microbicidal as the mixture HNP 1-3. HNP-3 was less active than the other defensins against most but not all of the microbes tested. Immunoperoxidase stains revealed HNP 1-3 to have a granular localization in the neutrophil's cytoplasm by light microscopy. Frozen thin section immunogold transmission electron microscopy showed HNP 1-3 to be localized in azurophil granules. These studies define a broad-spectrum antimicrobial system in human neutrophils. The defensin system may operate in conjunction with or independently from oxygen-dependent microbicidal processes to enable human neutrophils to inactivate and destroy potential pathogens.

Primary structures of three human neutrophil defensins.

The primary structures of three human neutrophil antimicrobial peptides (HNP) were determined. The peptides, HNP-1, HNP-2, and HNP-3, which we have termed defensins, were rich in cystine, arginine, and aromatic residues, but were devoid of free sulfhydryl groups and carbohydrate moieties. They were 29-30 residues in length and identical in sequence in all but their amino terminal residues. The defensins were homologous in sequence to peptides of similar size and biological activity previously purified from rabbit polymorphonuclear leukocytes, but unrelated to other neutrophil proteins of known sequence. 11 amino acid residues of the human defensins, including all six cysteinyl residues, were invariantly conserved in the six rabbit members of this multigene peptide family. That similarly structured antimicrobial peptides are present in both rabbit and human leukocytes supports their purported role as cidal agents in phagocyte-mediated host defense.

Microbicidal/cytotoxic proteins of neutrophils are deficient in two disorders: Chediak-Higashi syndrome and "specific" granule deficiency.

Although several genetic defects are known to impair oxidative microbicidal/cytotoxic mechanisms in human PMN, no deficiencies of PMN granule components that mediate oxygen-independent microbicidal activity have yet been reported. We analyzed PMN from patients with various granulocyte disorders for their content of two azurophil granule constituents, defensins and cathepsin G, that exert microbicidal/cytotoxic activity in vitro, and one component, elastase, that has ancillary microbicidal/cytotoxic activity. PMN from two (of two) patients with specific granule deficiency (SGD) displayed an almost complete deficiency of defensins, which in normal cells constitute greater than 30% of the protein content of azurophil granules. The SGD PMN contained normal or mildly decreased amounts of cathepsin G and elastase. Conversely, the PMN of three (of three) patients with Chediak-Higashi syndrome (CHS) substantially lacked cathepsin G and elastase, but their defensin content was normal or mildly decreased. Both CHS and SGD patients suffer from frequent and severe bacterial infections, and CHS patients frequently develop an atypical lymphoproliferative syndrome. The profound deficiency of PMN components with microbicidal/cytotoxic activity in SGD and CHS may contribute to the clinical manifestations of these disorders.

Interaction of human defensins with Escherichia coli. Mechanism of bactericidal activity.

Defensins are small, cysteine-rich antimicrobial peptides that are abundant in human, rabbit, and guinea pig neutrophils (PMN). Three defensins (human neutrophil peptide defensin [HNP]-1, HNP-2, and HNP-3) constitute between 30 and 50% of the total protein in azurophil granules of human PMN. We examined the mechanism of HNP-mediated bactericidal activity against Escherichia coli ML-35 (i-, y-, z+) and its pBR322-transformed derivative, E. coli ML-35p. Under conditions that supported bactericidal activity, HNP-1 sequentially permeabilized the outer membrane (OM) and inner membrane (IM) of E. coli. Coincident with these events, bacterial synthesis of DNA, RNA, and protein ceased and the colony count fell. Although these events were closely coupled under standard assay conditions, OM permeabilization was partially dissociated from IM permeabilization when experiments were performed with E. coli that had been plasmolyzed by mannitol. Under such conditions, the rate and extent of bacterial death more closely paralled loss of IM integrity than OM permeabilization. Electron microscopy of E. coli that had been killed by defensins revealed the presence of striking electron-dense deposits in the periplasmic space and affixed to the OM. Overall, these studies show that HNP-mediated bactericidal activity against E. coli ML-35 is associated with sequential permeabilization of the OM and IM, and that inner membrane permeabilization appears to be the lethal event.

Defensins.

Defensins are widely distributed and abundant 3-4 kDa antimicrobial peptides that are variable cationic and contain six disulfide-paired cysteines. Three structurally distinct peptide families have been identified: 'classical' defensins, beta-defensins and insect defensins. In many animal species, defensin genes are found in clusters with substantial sequence variability outside the core disulfide-linked cysteines. Defensin peptides have been found in the granules of phagocytes and intestinal Paneth cells, on epithelial surfaces of the intestine and the trachea, and in the hemolymph of insects. They are produced from larger precursors by stepwise, tissue-specific, proteolytic processing, a production resembling that of peptide hormones. Microbes in the phagocytic vacuoles of granulocytes and certain macrophages encounter high concentrations of defensins. Increased transcription of defensin genes and stimulus-dependent release of pre-synthesized defensin-containing cytoplasmic granules contribute to the local antimicrobial response.

Antimicrobial peptides in mammalian and insect host defence.

During the past year, additional insights into systems that regulate antimicrobial peptide production in Drosophila were reported. Granulysin, a peptide stored in the cytoplasmic granules of human natural killer cells and cytolytic T cells, was shown to kill Mycobacterium tuberculosis. More data implicating antimicrobial peptides in the pathogenesis of bronchopulmonary infections in cystic fibrosis appeared. Studies that examined the potential contributions of antimicrobial peptides to regional innate immunity gained in prominence. Efforts to design peptide analogues to prevent or treat infections continued.

Antimicrobial peptides of vertebrates.

The past year brought several discoveries that focused attention on antimicrobial peptides on epithelial surfaces. The malfunction of these substances was implicated as a cause of airway infections in cystic fibrosis. Other highlights included new insights into the relative selectivity of antimicrobial peptides for microbial membranes, their primary site of action.