Neutralizing monoclonal antibodies to an extracellular Pseudomonas cepacia protease

Burkholderia cenocepacia ZmpB is a broad-specificity zinc metalloprotease involved in virulence

In previous studies we characterized the Burkholderia cenocepacia ZmpA zinc metalloprotease. In this study, we determined that B. cenocepacia has an additional metalloprotease, which we designated ZmpB. The zmpB gene is present in the same species as zmpA and was detected in B. cepacia, B. cenocepacia, B. stabilis, B. ambifaria, and B. pyrrocinia but was absent from B. multivorans, B. vietnamiensis, B. dolosa, and B. anthina. The zmpB gene was expressed, and ZmpB was purified from Escherichia coli by using the pPROEXHTa His(6) Tag expression system. ZmpB has a predicted preproenzyme structure typical of thermolysin-like proteases and is distantly related to Bacillus cereus bacillolysin. ZmpB was expressed as a 63-kDa preproenzyme precursor that was autocatalytically cleaved into mature ZmpB (35 kDa) and a 27-kDa prepropeptide. EDTA, 1,10-phenanthroline, and Zn(2+) cations inhibited ZmpB enzyme activity, indicating that it is a metalloprotease. ZmpB had proteolytic activity against alpha-1 proteinase inhibitor, alpha(2)-macrogobulin, type IV collagen, fibronectin, lactoferrin, transferrin, and immunoglobulins. B. cenocepacia zmpB and zmpA zmpB mutants had no proteolytic activity against casein and were less virulent in a rat agar bead chronic infection model, indicating that zmpB is involved in B. cenocepacia virulence. Expression of zmpB was regulated by both the CepIR and CciIR quorum-sensing systems.

Burkholderia cenocepacia utilizes ferritin as an iron source

Burkholderia cenocepacia is a member of the Burkholderia cepacia complex, a group of genetically similar species that inhabit a number of environmental niches, including the lungs of patients with cystic fibrosis (CF). To colonize the lung, this bacterium requires a source of iron to satisfy its nutritional requirements for this important metal. Because of the high potential for damage in lung tissue resulting from oxygen-iron interactions, this metal is sequestered by a number of mechanisms that render it potentially unavailable to invading micro-organisms. Such mechanisms include the intracellular and extracellular presence of the iron-binding protein ferritin. Ferritin has a highly stable macromolecular structure and may contain up to 4500 iron atoms per molecule. To date, there has been no known report of a pathogenic bacterial species that directly utilizes iron sequestered by this macromolecule. To examine the ability of ferritin to support growth of B. cenocepacia J2315, iron-deficient media were supplemented with different concentrations of ferritin and the growth kinetics characterized over a 40 h period. The results indicated that B. cenocepacia J2315 utilizes iron bound by ferritin. Further studies examining the mechanisms of iron uptake from ferritin indicated that iron utilization results from a proteolytic degradation of this otherwise stable macromolecular structure. Since it is known that the ferritin concentration is significantly higher in the CF lung than in healthy lungs, this novel iron-acquisition mechanism may contribute to infection by B. cenocepacia in people with CF.

Functional analysis of the Burkholderia cenocepacia ZmpA metalloprotease

Burkholderia cenocepacia ZmpA is expressed as a preproenzyme typical of thermolysin-like proteases such as Pseudomonas aeruginosa LasB and Bacillus thermoproteolyticus thermolysin. The zmpA gene was expressed using the pPRO-EXHTa His(6) tag expression system, which incorporates a six-His tag at the N-terminal end of the protein, and recombinant ZmpA was purified using Ni-nitrilotriacetic acid affinity chromatography. Upon refolding of the recombinant His(6)-pre-pro-ZmpA (62 kDa), the fusion protein was autoproteolytically cleaved into 36-kDa (mature ZmpA) and 27-kDa peptides. Site-directed mutagenesis was employed to infer the identity of the active site residues of ZmpA and to confirm that the enzyme undergoes autoproteolytic cleavage. Oligonucleotide mutagenesis was used to replace H(465) with G(465) or A(465), E(377) with A(377) or D(377), or H(380) with P(380) or A(380). Mutagenesis of H(465), E(377), or H(380) resulted in the loss of both autocatalytic activity and proteolytic activity. ZmpA with either substitution in H(380) was not detectable in B. cenocepacia cell extracts. The activity of the recombinant ZmpA was inhibited by EDTA and 1,10 phenanthroline, indicating that it is a zinc metalloprotease. ZmpA, however, was not inhibited by phosphoramidon, a classical inhibitor of the thermolysin-like proteases. The refolded mature ZmpA enzyme was proteolytically active against various substrates including hide powder azure, type IV collagen, fibronectin, neutrophil alpha-1 proteinase inhibitor, alpha(2)-macroglobulin, and gamma interferon, suggesting that B. cenocepacia ZmpA may cause direct tissue damage to the host or damage to host tissues through a modulation of the host's immune system.

Burkholderia cenocepaciaLipopolysaccharide, Lipid A, and Proinflammatory Activity

Organisms from the Burkholderia cepacia complex are important pathogens in cystic fibrosis and are associated with increased rates of sepsis and death. These organisms comprise nine closely related species known as genomovars. B. cenocepacia (genomovar III) is the most prevalent and appears the most virulent. We investigated the biological activity of a reference panel of strains using whole-cell lysates to induce septic-shock related cytokines from differentiated human monocytic cells. We found varying biological activity within and between genomovars, with B. cenocepacia strains possessing the greatest cytokine induction activity. This activity was CD-14 dependent, suggesting that LPS was responsible for the cytokine induction. Cytokine induction was not simply related to the expression of rough or smooth LPS. We purified LPS from two strains, B. cenocepacia LMG 12614 and B. multivorans LMG 14273, each possessing rough LPS. Divergence in biological activity of the two genomovars was preserved when human monocytic cells were stimulated with purified LPS. Lipid A purified from LMG 14273 and LMG 12614 were analyzed by matrix-assisted laser desorption ionization/time of flight mass spectrometry. Lipid A from the less effective cytokine inducer LMG 14273 was found to be missing a beta-hydroxymyristate (3-OH C14:0) relative to the lipid A of B. cenocepacia LMG 12614.

Putative virulence factors are released in association with membrane vesicles from Burkholderia cepacia

Like many other Gram-negative bacteria, Burkholderia cepacia naturally releases membrane vesicles (n-MVs) during normal growth. Through filtration and differential centrifugation, n-MVs from clinical isolates of the IIIa and V genomovars were isolated and their characteristics compared. Electron microscopy revealed that they were spherical, 30-220 nm in diameter, and bilayered. Virulence factors thought to play a role in pathogenicity (e.g., lipase, phospholipase-N, and protease, including a metalloprotease) were found associated with n-MVs, while peptidoglycan zymogram analysis also revealed 26, 28, 36, and 66 kDa peptidoglycan-degrading enzymes. n-MVs were often contaminated with flagella and pili when isolated by traditional methods, and a new strategy using a linear isopycnic sucrose gradient was utilized. For better characterization, this was applied to a representative genomovar IIIa strain (C5424) and showed that n-MVs consisted of a subset of specific outer membrane and periplasmic proteins as well as lipopoly saccharide possessing only a putative minor O-side chain polymer. This finding suggests that certain components are selected by B. cepacia during n-MV formation, and since some are putative virulence factors, this property could help deliver the factors to tissue, thereby aiding infection.

An extracellular zinc metalloprotease gene of Burkholderia cepacia

Burkholderia cepacia produces at least one extracellular zinc metalloprotease that may be involved in virulence. A B. cepacia zinc metalloprotease gene was cloned using a Burkholderia pseudomallei zinc metalloprotease gene as a probe. The predicted amino acid sequences of these B. cepacia and a B. pseudomallei extracellular zinc metalloproteases indicate that they are similar to the thermolysin-like family of metalloproteases (M4 family of metalloendopeptidases) and they are likely to be secreted via the general secretory pathway. zmpA isogenic mutants were constructed in B. cepacia genomovar III strains Pc715j and K56-2 by insertional inactivation of the zmpA genes. The zmpA mutants produced less protease than the parent strains. The B. cepacia strain K56-2 zmpA mutant was significantly less virulent than its parent strain in a chronic respiratory infection model; however, there was no difference between the virulence of B. cepacia strain Pc715j and a Pc715j zmpA mutant. The results indicate that this extracellular zinc metalloprotease may play a greater role in virulence in some strains of B. cepacia.

Detection of heme-binding proteins in epidemic strains of Burkholderia cepacia

A panel of 30 previously characterized strains representing five genomovars from the Burkholderia cepacia complex (E. Mahenthiralingam, T. Coenye, J. W. Chung, D. P. Speert, J. R. W. Govan, P. Taylor, and P. Vandamme, J. Clin. Microbiol. 38:910--913, 2000) were examined for their iron protoporphyrin IX-binding ability. These included B. cepacia genomovars I and III and B. stabilis (formerly B. cepacia genomovar IV), B. multivorans (formerly B. cepacia genomovar II), and B. vietnamiensis (formerly B. cepacia genomovar V). Cells were exposed to micro-oxo bisheme of iron protoporphyrin IX (micro-oxo dimers) and examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing, nondenaturing conditions for the presence of heme-binding proteins using tetramethylbenzidine-H(2)O(2) staining. Seven of the 30 strains, each belonging to B. cepacia genomovar III and designated epidemic (in possessing the B. cepacia epidemic strain marker), expressed a 96- to 100-kDa heme-binding protein which was located in the outer membrane. The heme-binding protein of B. cepacia genomovar III epidemic strain C5424 bound iron(III) protoporphyrin IX in both the monomeric and micro-oxo bisheme forms. Cells of all strains grown on Columbia agar bound iron protoporphyrin IX in the micro-oxo bisheme (dimeric) form. There were no statistical differences between the five genomovars, or those possessing the heme-binding protein, in their micro-oxo bisheme-binding ability. Possession of the outer membrane heme-binding protein may be a pathogenicity trait in enabling the bacterium to withstand oxidative stresses in inflammatory exudates in the lung and may aid identification of invasive epidemic strains of B. cepacia.

Quorum sensing in Burkholderia cepacia: identification of the LuxRI homologs CepRI

Burkholderia cepacia has emerged as an important pathogen in patients with cystic fibrosis. Many gram-negative pathogens regulate the production of extracellular virulence factors by a cell density-dependent mechanism termed quorum sensing, which involves production of diffusible N-acylated homoserine lactone signal molecules, called autoinducers. Transposon insertion mutants of B. cepacia K56-2 which hyperproduced siderophores on chrome azurol S agar were identified. One mutant, K56-R2, contained an insertion in a luxR homolog that was designated cepR. The flanking DNA region was used to clone the wild-type copy of cepR. Sequence analysis revealed the presence of cepI, a luxI homolog, located 727 bp upstream and divergently transcribed from cepR. A lux box-like sequence was identified upstream of cepI. CepR was 36% identical to Pseudomonas aeruginosa RhlR and 67% identical to SolR of Ralstonia solanacearum. CepI was 38% identical to RhlI and 64% identical to SolI. K56-R2 demonstrated a 67% increase in the production of the siderophore ornibactin, was protease negative on dialyzed brain heart infusion milk agar, and produced 45% less lipase activity in comparison to the parental strain. Complementation of a cepR mutation restored parental levels of ornibactin and protease but not lipase. An N-acylhomoserine lactone was purified from culture fluids and identified as N-octanoylhomoserine lactone. K56-I2, a cepI mutant, was created and shown not to produce N-octanoylhomoserine lactone. K56-I2 hyperproduced ornibactin and did not produce protease. These data suggest both a positive and negative role for cepIR in the regulation of extracellular virulence factor production by B. cepacia.

Identification of neutralizing epitopes on Pseudomonas aeruginosa elastase and effects of cross-reactions on other thermolysin-like proteases

Monoclonal antibodies (MAbs) to a Burkholderia (Pseudomonas) cepacia 36-kDa protease (PSCP) which neutralize PSCP and Pseudomonas aeruginosa elastase but not P. aeruginosa alkaline protease have been isolated (C. Kooi et al., Infect. Immun. 62:2811-2817, 1994). These MAbs, designated 36-6-6 and 36-6-8, react with N-chlorosuccinimide cleavage products of P. aeruginosa elastase, consistent with the recognition of a 13.9-kDa fragment which contains the active site. Overlapping 9-mer peptides that span this region were synthesized. Neutralizing MAbs to PSCP reacted strongly with two peptides (341HGFTEQNSG349 and 395RYM DQPSRD403). Peptide 341HGFTEQNSG349 overlaps the motif 337HEXXH341, which has been found in many zinc-dependent endopeptidases. Peptide 395RYMDQPSRD403 lies between E361, which binds a zinc atom, and H420, which acts as a proton donor at the active site. Polyclonal rabbit sera raised against these peptides reacted with elastase on Western immunoblots and by enzyme-linked immunosorbent assay. With hide powder azure as the substrate, antisera to either HGFTEQNG and RYMDQPSRD completely neutralized the activities of elastase, thermolysin, Vibrio cholerae hemagglutinin/protease, and PSCP but had no effect on P. aeruginosa alkaline protease or the Serratia marcescens major protease. These results suggest that the MAbs recognize two different epitopes on P. aeruginosa elastase and that antibodies raised against synthetic peptides corresponding to either of these epitopes neutralize proteolytic activity.

A genetic analysis system of Burkholderia cepacia: construction of mobilizable transposons and a cloning vector

A genetic analysis system of Burkholderia cepacia (Bc) was developed which included transposon mutagenesis and complementation of mutation with the cloned genes of interest. To deliver the transposon in this multidrug-resistant microorganism, two plasmids, pKN30 and pKN31, were constructed which contained Tn5 derivatives, Tn5-30Tp and Tn5-31Tp, respectively, carrying KmR and TpR genes. The plasmids have the origin of ColE1 replication and the mobilization gene of RP4. Tn5-31Tp was mobilized to Bc KF1, a strain isolated from a pneumonia patient, by the transfer system of RP4 integrated in the chromosome of Escherichia coli (Ec). Selection with trimethoprim resulted in generation of a number of transposants of Bc KF1. Fourteen protease-deficient mutants were isolated, all of which contained a single transposon marker in the chromosome. Thirteen protease-deficient mutants were also lipase deficient. An Ec-Bc shuttle plasmid, pTS1209, was constructed that consists of oriColE1, oripSa, ApR and CmR genes, and several unique restriction sites for cloning. Plasmid pTS1209 was successfully employed for cloning genes of Bc involved in protease production.

Invasion of respiratory epithelial cells by Burkholderia (Pseudomonas) cepacia

Pulmonary infections caused by Burkholderia (Pseudomonas) cepacia are an important cause of morbidity and mortality in cystic fibrosis (CF) patients. Several features suggestive of cellular invasion and intracellular sequestration of B. cepacia in CF are persistence of infection in the face of antibiotic therapy to which the organism demonstrates in vitro susceptibility and a propensity to cause bacteremic infections in patients with CF. Epithelial cell invasion was demonstrated in vitro in A549 cells by a modified gentamicin protection assay. The kinetics of invasion appear to be saturable. Electron microscopy of invaded monolayers showed intracytoplasmic bacteria enclosed by membrane-bound vacuoles. No lysosomal fusion with these vacuoles was observed. Intraepithelial cell replication was suggested by electron microscopy and confirmed by both a quantitative assay and a visual assay. Cytochalasin D, but not colchicine, inhibited invasion, suggesting a role for microfilaments but not microtubules. The invasion phenotype in B. cepacia may be an important virulence factor for CF infections.

The dsbB gene product is required for protease production by Burkholderia cepacia

Burkholderia cepacia KF1, isolated from a pneumonia patient, produces a 37-kDa extracellular metalloprotease. A protease-deficient and lipase-proficient mutant, KFT1007, was complemented by a clone having an open reading frame coding for a 170-amino-acid polypeptide which showed significant homology to Escherichia coli DsbB. KFT1007, a presumed dsbB mutant, also failed to show motility, and both protease secretion and motility were restored by the introduction of the cloned dsbB gene of B. cepacia. The mutant KFT1007 excreted a 43-kDa polypeptide that is immunologically related to the 37-kDa mature protease. These results suggested that the dsbB mutant secretes a premature and catalytically inactive form of protease and that disulfide formation is required for the production of extracellular protease by B. cepacia.

Effects of growth rate and nutrient limitation on virulence factor production in Burkholderia cepacia

The influence of growth rate and oxygen availability on siderophore, protease, and lipase production in Burkholderia cepacia was assessed for cells grown in a chemostat under iron limitation. Whereas siderophore and protease production increased with growth rate and oxygen yet decreased under oxygen depletion, lipase production demonstrated the opposite trend.