Immunization with the Haemophilus ducreyi hemoglobin receptor HgbA protects against infection in the swine model of chancroid

The etiologic agent of chancroid is Haemophilus ducreyi. To fulfill its obligate requirement for heme, H. ducreyi uses two TonB-dependent receptors: the hemoglobin receptor (HgbA) and a receptor for free heme (TdhA). Expression of HgbA is necessary for H. ducreyi to survive and initiate disease in a human model of chancroid. In this study, we used a swine model of H. ducreyi infection to demonstrate that an experimental HgbA vaccine efficiently prevents chancroid, as determined by several parameters. Histological sections of immunized animals lacked typical microscopic features of chancroid. All inoculated sites from mock-immunized pigs yielded viable H. ducreyi cells, whereas no viable H. ducreyi cells were recovered from inoculated sites of HgbA-immunized pigs. Antibodies from sera of HgbA-immunized animals bound to and initiated antibody-dependent bactericidal activity against homologous H. ducreyi strain 35000HP and heterologous strain CIP542 ATCC; however, an isogenic hgbA mutant of 35000HP was not killed, proving specificity. Anti-HgbA immunoglobulin G blocked hemoglobin binding to the HgbA receptor, suggesting a novel mechanism of protection through the limitation of heme/iron acquisition by H. ducreyi. Such a vaccine strategy might be applied to other bacterial pathogens with strict heme/iron requirements. Taken together, these data suggest continuing the development of an HgbA subunit vaccine to prevent chancroid.     

Identification and purification of a conserved heme-regulated hemoglobin-binding outer membrane protein from Haemophilus ducreyi.

A hemoglobin-binding protein (HgbA) from Haemophilus ducreyi was identified and purified. The 100-kDa HgbA was detected in all strains of H. ducreyi tested, and a somewhat larger hemoglobin-binding protein was found in one strain of Haemophilus influenzae. HgbA was purified and the amino acid sequence of the N terminus of HgbA revealed no significant homologies with known proteins. Two different antisera to HgbA from H. ducreyi 35000 recognized HgbA proteins from all tested H. ducreyi strains; they did not recognize proteins from the H. influenzae strain. Expression of HgbA was regulated by the level of heme but not by iron present in the medium. Animal species of hemoglobin competed with iodinated human hemoglobin for binding to whole cells of H. ducreyi and supported the growth of H. ducreyi. The lack of immunological cross-reactivity and the differences in hemoglobin specificities between the H. ducreyi and the H. influenzae hemoglobin-binding proteins suggest that they are unrelated.     

Involvement of HxuC outer membrane protein in utilization of hemoglobin by Haemophilus influenzae

Haemophilus influenzae can utilize different protein-bound forms of heme for growth in vitro. A previous study from this laboratory indicated that nontypeable Haemophilus influenzae (NTHI) strain N182 expressed three outer membrane proteins, designated HgbA, HgbB, and HgbC, that bound hemoglobin or hemoglobin-haptoglobin and were encoded by open reading frames (ORFs) that contained a CCAA nucleotide repeat. Testing of mutants expressing the HgbA, HgbB, and HgbC proteins individually revealed that expression of any one of these proteins was sufficient to allow wild-type growth with hemoglobin. In contrast, mutants that expressed only HgbA or HgbC grew significantly better with hemoglobin-haptoglobin than did a mutant expressing only HgbB. Construction of an isogenic hgbA hgbB hgbC mutant revealed that the absence of these three gene products did not affect the ability of NTHI N182 to utilize hemoglobin as a source of heme, although this mutant was severely impaired in its ability to utilize hemoglobin-haptoglobin. The introduction of a tonB mutation into this triple mutant eliminated its ability to utilize hemoglobin, indicating that the pathway for hemoglobin utilization in the absence of HgbA, HgbB, and HgbC involved a TonB-dependent process. Inactivation in this triple mutant of the hxuC gene, which encodes a predicted TonB-dependent outer membrane protein previously shown to be involved in the utilization of free heme, resulted in loss of the ability to utilize hemoglobin. The results of this study reinforce the redundant nature of the heme acquisition systems expressed by H. influenzae.     

Outer membrane protein DsrA is the major fibronectin-binding determinant of Haemophilus ducreyi

The ability to bind extracellular matrix proteins is a critical virulence determinant for skin pathogens. Haemophilus ducreyi, the etiological agent of the genital ulcer disease chancroid, binds extracellular matrix components, including fibronectin (FN). We investigated H. ducreyi FN binding and report several important findings about this interaction. First, FN binding by H. ducreyi was greatly increased in bacteria grown on heme and almost completely inhibited by hemoglobin. Second, wild-type strain 35000HP bound significantly more FN than did a dsrA mutant in two different FN binding assays. Third, the expression of dsrA in the dsrA mutant restored FN binding and conferred the ability to bind FN to a non-FN-binding Haemophilus influenzae strain. Fourth, an anti-DsrA monoclonal antibody partially blocked FN binding by H. ducreyi. The hemoglobin receptor, the collagen-binding protein, the H. ducreyi lectin, the fine-tangle pili, and the outer membrane protein OmpA2 were not involved in H. ducreyi FN binding, since single mutants bound FN as well as the parent strain did. However, the major outer membrane protein may have a minor role in FN binding by H. ducreyi, since a double dsrA momp mutant bound less FN than did the single dsrA mutant. Finally, despite major sequence differences, DsrA proteins from both class I and class II H. ducreyi strains mediated FN and vitronectin binding. We concluded that DsrA is the major factor involved in FN binding by both classes of H. ducreyi strains.     

Detection of phase variation in expression of proteins involved in hemoglobin and hemoglobin-haptoglobin binding by nontypeable Haemophilus influenzae

Haemophilus influenzae can utilize different protein-bound forms of heme for growth in vitro. A previous study (I. Maciver, J. L. Latimer, H. H. Liem, U. Muller-Eberhard, Z. Hrkal, and E. J. Hansen. Infect. Immun. 64:3703-3712, 1996) indicated that nontypeable H. influenzae (NTHI) strain TN106 expressed a protein that bound hemoglobin-haptoglobin and was encoded by an open reading frame (ORF) that contained a CCAA nucleotide repeat. Southern blot analysis revealed that several NTHI strains contained between three and five chromosomal DNA fragments that bound an oligonucleotide probe for CCAA repeats. Three ORFs containing CCAA repeats were identified in NTHI strain N182; two of these ORFs were arranged in tandem. The use of translational fusions involving these three ORFs and the beta-lactamase gene from pBR322 revealed that these three ORFs, designated hgbA, hgbB, and hgbC, encoded proteins that could bind hemoglobin, hemoglobin-haptoglobin, or both compounds. Monoclonal antibodies (MAbs) specific for the HgbA, HgbB, and HgbC proteins were produced by immunizing mice with synthetic peptides unique to each protein. Both HgbA and HgbB were readily detected by Western blot analysis in N182 cells grown in the presence of hemoglobin as the sole source of heme, whereas expression of HgbC was found to be much less abundant than that of HgbA and HgbB. The use of these MAbs in a colony blot radioimmunoassay analysis revealed that expression of both HgbA and HgbB was subject to phase variation. PCR and nucleotide sequence analysis were used in conjunction with Western blot analyses to demonstrate that this phase variation involved the CCAA repeats in the hgbA and hgbB ORFs.     

Identification of an outer membrane protein involved in utilization of hemoglobin-haptoglobin complexes by nontypeable Haemophilus influenzae.

A recombinant plasmid containing a 6.5-kb fragment of nontypeable Haemophilus influenzae (NTHI) chromosomal DNA was shown to confer a hemoglobin-haptoglobin-binding phenotype on Escherichia coli. Use of a mini-Tn10kan transposon for random insertion mutagenesis of this recombinant plasmid allowed localization of the NTHI DNA responsible for this hemoglobin-haptoglobin-binding phenotype to a 3.5-kb PstI-XhoI fragment within the 6.5-kb NTHI DNA insert. When this mutagenized NTHI DNA fragment was used to transform the wild-type NTHI strain, the resultant kanamycin-resistant mutant exhibited significantly decreased abilities to bind hemoglobin-haptoglobin and utilize it as a source of heme for aerobic growth in vitro. This mutant also lacked expression of a 115-kDa outer membrane protein that was present in the wild-type parent strain. Transformation of this mutant with wild-type NTHI chromosomal DNA restored the abilities to bind and utilize hemoglobin-haptoglobin and to express the 115-kDa outer membrane protein. Nucleotide sequence analysis of the relevant NTHI DNA revealed the presence of a gene, designated hhuA, that encoded a predicted 117,145-Da protein. The HhuA protein exhibited features typical of a TonB-dependent outer membrane receptor and had significant identity with the hemoglobin receptors of both Haemophilus ducreyi and Neisseria meningitidis.     

Passive immunization with a polyclonal antiserum to the hemoglobin receptor of Haemophilus ducreyi confers protection against a homologous challenge in the experimental swine model of chancroid

Haemophilus ducreyi, the etiologic agent of chancroid, has an obligate requirement for heme. Heme is acquired by H. ducreyi from its human host via TonB-dependent transporters expressed at its bacterial surface. Of 3 TonB-dependent transporters encoded in the genome of H. ducreyi, only the hemoglobin receptor, HgbA, is required to establish infection during the early stages of the experimental human model of chancroid. Active immunization with a native preparation of HgbA (nHgbA) confers complete protection in the experimental swine model of chancroid, using either Freund's or monophosphoryl lipid A as adjuvants. To determine if transfer of anti-nHgbA serum is sufficient to confer protection, a passive immunization experiment using pooled nHgbA antiserum was conducted in the experimental swine model of chancroid. Pigs receiving this pooled nHgbA antiserum were protected from a homologous, but not a heterologous, challenge. Passively transferred polyclonal antibodies elicited to nHgbA bound the surface of H. ducreyi and partially blocked hemoglobin binding by nHgbA, but were not bactericidal. Taken together, these data suggest that the humoral immune response to the HgbA vaccine is protective against an H. ducreyi infection, possibly by preventing acquisition of the essential nutrient heme.     

A hemoglobin-binding outer membrane protein is involved in virulence expression by Haemophilus ducreyi in an animal model.

Haemophilus ducreyi exhibits a requirement for exogenously supplied heme for aerobic growth in vitro. Nine of ten wild-type isolates of H. ducreyi were shown to contain a readily detectable hemoglobin-binding activity. Spontaneous hemoglobin-binding-negative mutants of two of these wild-type isolates lost the ability to express an outer membrane protein with an apparent molecular mass of approximately 100 kDa. Similarly, the single wild-type isolate that lacked the ability to bind hemoglobin also appeared to lack expression of this same 100-kDa protein. A monoclonal antibody (5A9) to this 100-kDa protein was used to identify a recombinant clone which possessed an H. ducreyi chromosomal fragment containing the gene encoding the 100-kDa protein; this protein was designated hemoglobin utilization protein A (HupA). Nucleotide sequence analysis of the hupA gene revealed that the predicted protein, with a calculated molecular mass of 108 kDa, was similar to TonB-dependent outer membrane proteins of other bacteria. Increasing the concentration of heme in the growth medium resulted in decreased expression of the HupA protein. Mutant analysis was used to prove that the HupA protein was essential for the utilization by H. ducreyi of both hemoglobin and hemoglobin-haptoglobin as sources of heme in vitro. In addition, it was found that an isogenic hupA mutant was less virulent than the wild-type parent strain in the temperature-dependent rabbit model for dermal lesion production by H. ducreyi.     

Development of a rapid immunodiagnostic test for Haemophilus ducreyi

Haemophilus ducreyi is the etiologic agent of chancroid, a sexually transmitted disease that increases the rate of transmission of human immunodeficiency virus. Chancroid ulcerations are difficult to distinguish from those produced by syphilis and herpes. Diagnosis based solely on clinical grounds is inaccurate, and culture is insensitive. Highly sensitive PCR has largely superseded culture as the preferred method of laboratory diagnosis; however, neither culture nor PCR is feasible where chancroid is endemic. We developed a rapid (15-min) diagnostic test based on monoclonal antibodies (MAbs) to the hemoglobin receptor of H. ducreyi, HgbA. This outer membrane protein is conserved in all strains of H. ducreyi tested and is required for the establishment of experimental human infection. MAbs to HgbA were generated and tested for cross-reactivity against a panel of geographically diverse strains. Three MAbs were found to be unique and noncompetitive and bound to all strains of H. ducreyi tested. Using an immunochromatography format, we evaluated the sensitivity and specificity of the test using geographically diverse strains of H. ducreyi, other Haemophilus strains, and other bacteria known to superinfect genital ulcers. All H. ducreyi strains were positive, and all other bacteria were negative, resulting in a specificity of 100%. The minimum number of CFU of H. ducreyi detected was 2 x 10(6) CFU, and the minimum amount of purified HgbA protein detected was 8.5 ng. Although this level of sensitivity may not be sufficient to detect H. ducreyi in all clinical specimens, further work to increase the sensitivity could potentially make this a valuable bedside tool in areas where chancroid is endemic.     

Oral immunization using HgbA in a recombinant chancroid vaccine delivered by attenuated Salmonella typhimurium SL3261 in the temperature-dependent rabbit model

Chancroid, a sexually transmitted genital ulcer disease caused by the Gram-negative bacterium Haemophilus ducreyi, facilitates the acquisition and transmission of HIV. An effective vaccine against chancroid has not been developed. In this preliminary study, the gene encoding the H. ducreyi outer membrane hemoglobin receptor HgbA was cloned into the plasmid pTETnir15. The recombinant construct was introduced into the attenuated Salmonella typhimurium SL3261 strain and stable expression was induced in vitro under anaerobic conditions. The vaccine strain was delivered into the temperature-dependent rabbit model of chancroid by intragastric immunization as a single dose, or as three doses administered at two-weekly intervals. No specific antibody to HgbA was elicited after either dose schedule. Although the plasmid vector survived in vivo passage for up to 15 days following single oral challenge, HgbA expression was restricted to plasmid isolates recovered one day after immunization. Rabbits inoculated with the 3-dose booster regimen achieved no protective immunity from homologous challenge. These results emphasize that refinements in plasmid design to enhance a durable heterologous protein expression are necessary for the development of a live oral vaccine against chancroid.     

Protein sources of heme for Haemophilus influenzae.

Although Haemophilus influenzae requires heme for growth, the source of heme during invasive infections is not known. We compared heme, lactoperoxidase, catalase, cytochrome c, myoglobin, and hemoglobin as sources of heme for growth in defined media. The minimum concentration of heme permitting unrestricted growth of strain E1a, an H. influenzae type b isolate from cerebrospinal fluid, was 0.02 micrograms/ml. Using molar equivalents of heme as lactoperoxidase, catalase, cytochrome c, myoglobin, and hemoglobin, we determined that myoglobin and hemoglobin permitted unrestricted growth at this concentration. To determine the ability of host defenses to sequester heme from H. influenzae, we used affinity chromatography to purify human haptoglobin and hemopexin, serum proteins which bind hemoglobin and heme. Plate assays revealed that 12 strains of H. influenzae acquired heme from hemoglobin, hemoglobin-haptoglobin, heme-hemopexin, and heme-albumin. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of outer membrane proteins of strain E1a grown in heme-replete and heme-restricted conditions revealed a heme-repressible outer membrane protein with an apparent molecular mass of 38 kilodaltons. These results demonstrated that, unlike Escherichia coli, H. influenzae may acquire heme from hemoglobin-haptoglobin. H. influenzae also may acquire heme from hemopexin and albumin, which have not been previously investigated. The role of outer membrane proteins in the acquisition of heme is not yet clear.     

Role of the Haemophilus ducreyi Ton System in Internalization of Heme from Hemoglobin

By cloning into Escherichia coli and construction of isogenic mutants of Haemophilus ducreyi, we showed that the hemoglobin receptor (HgbA) is TonB dependent. An E. coli hemA tonB mutant expressing H. ducreyi hgbA grew on low levels of hemoglobin as a source of heme only when an intact H. ducreyi Ton system plasmid was present. In contrast, growth on heme by the E. coli hemA tonB mutant expressing hgbA was observed only at high concentrations of heme, was TonB independent, and demonstrated that H. ducreyi HgbA was not sufficient to function as a typical TonB-dependent heme receptor in E. coli. Allelic replacement of the wild-type H. ducreyi exbB, exbD, and tonB loci with the exbB, exbD, and tonB deletion resulted in an H. ducreyi isogenic mutant unable to utilize hemoglobin but able to utilize hemin at the same levels as the parent strain to fulfill its heme requirement. This finding confirms the TonB dependence of HgbA-mediated hemoglobin utilization and suggests that uptake of hemin in H. ducreyi is TonB independent. Additionally, the H. ducreyi Ton system mutant synthesized increased amounts of HgbA and other heme-regulated outer membrane proteins, consistent with derepression of these proteins due to lower intracellular heme and/or iron concentrations in the mutant. Sequencing of the Ton system genes revealed that the arrangement of the genes was exbB exbD tonB. The proximity and structure of these genes suggested that they are transcribed as an operon. This arrangement, as well as the DNA and deduced amino acid sequences of these H. ducreyi genes, was most similar to those from other pasteurellae.

Haemophilus ducreyi is the etiologic agent of chancroid, a genital ulcer disease transmitted by sexual contact (reviewed in references 2 and 66). Prevalent in Africa, Asia, and certain developing nations, chancroid has recently gained importance as an independent risk factor for the heterosexual transmission of human immunodeficiency (67). There is accumulating evidence suggesting that the rapid spread of human immunodeficiency virus type 1 in eastern and southern Africa has been due, at least in part, to the presence of chancroid and other genital ulcer diseases (13, 51). At present no vaccine or practical field diagnostic test exists for H. ducreyi.H. ducreyi is a fastidious slow-growing, gram-negative rod with an optimum growth temperature of 33°C. An important feature of H. ducreyi is its requirement for heme, which it is unable to synthesize. This obligate requirement for heme by H. ducreyi is in contrast with H. influenzae, which is able to use its enzyme ferrochelatase to ferrate protoporphyrin IX, bypassing the need for heme (24, 53). In H. ducreyi, heme is acquired from its only known host, humans, in several possible forms, including hemoglobin, hemoglobin complexed to haptoglobin, catalase, free hemin, or heme bound to carrier proteins such as albumin (3, 39, 64). Some or all of these heme sources could be released from host cells by the action of the H. ducreyi hemolysin (45, 46, 63) or cytotoxin (16, 37, 49).Host heme compounds represent important sources of iron (44). Host iron is sequestered by several mechanisms, and invading bacteria must gain access to these iron sources to survive and initiate disease. Some bacteria utilize iron-scavenging siderophore systems; others directly bind host iron or heme/iron-containing compounds such as transferrin, lactoferrin, heme, or hemoglobin (19, 44). No siderophores have been found in H. ducreyi (39). Since H. ducreyi requires both heme and iron, differentiation between the requirement for both of these can be problematic.Siderophore receptors and most receptors for host iron-binding compounds belong to a family of outer membrane receptors designated TonB dependent because they require the cytoplasmic membrane protein TonB for activity (7, 11, 57). TonB-dependent receptors are related structurally and functionally; where studied, they have been shown to form specific pores for the entry of low-molecular-weight molecules (12, 40). TonB-dependent receptors for host-iron complexes such as transferrin or lactoferrin have been shown to internalize the iron only (41), whereas receptors for siderophores internalize the iron-siderophore complex (43). TonB-dependent receptors are also related at the amino acid level by virtue of several regions of distinct homology, including an N-terminal sequence termed the TonB Box (18, 28, 35, 54). In Escherichia coli, the TonB protein has been shown to directly interact with TonB-dependent receptors FepA (5, 55). TonB and its accessory proteins ExbB and ExbD comprise a system for the transfer of energy from the cytoplasmic membrane to outer membrane receptors (10). Null mutations in either tonB, exbB, or exbD can variably affect the ability of TonB-dependent receptors to internalize or utilize their cognate ligand (48). The effect of these null mutations can vary from partial to total inhibition of uptake of ligands.Previously, we identified and purified an outer membrane hemoglobin receptor from H. ducreyi termed HgbA (22) and showed that the HgbA protein is functionally and immunologically conserved between geographically diverse isolates. Further analysis was performed by using molecular techniques (23, 56). The hgbA gene was cloned and sequenced; its deduced amino acid sequence is similar to the TonB-dependent family of outer membrane receptors (23, 56). An hgbA isogenic mutant of H. ducreyi cannot bind or utilize hemoglobin as a source of heme but can utilize free hemin, implying that the utilization of hemin does not require HgbA. It has been shown by Stevens et al. (56) that an isogenic mutant of hgbA (hup) expressed reduced virulence in an animal model of H. ducreyi infection.In the course of these functional studies, we found that an E. coli hemA mutant expressing hgbA binds hemoglobin yet does not grow on hemoglobin as a porphyrin source (23). This result suggested that additional components are necessary for utilization of hemoglobin after the binding step. The objective of the present study was to identify the additional components necessary for removal of heme from hemoglobin and for the transport of heme across the outer membrane.     

Cloning and Characterization of tdhA, a Locus Encoding a TonB-Dependent Heme Receptor from Haemophilus ducreyi

Haemophilus ducreyi is unable to synthesize heme and must acquire it from its only known host, humans. We cloned and sequenced a gene encoding an outer membrane receptor for heme. It was designated tdhA (for TonB-dependent heme receptor A) since it was related by sequence homology to the family of TonB-dependent receptors. TdhA was strikingly similar to open reading frame HI0113 from the genome of Haemophilus influenzae Rd and also shared homology with five other heme receptors, including HxuC, HemR, HmuR, ChuA, and ShuA, from gram-negative bacteria. An Escherichia coli hemA tonB mutant strongly expressing H. ducreyi tdhA grew on low levels of heme as a source of heme only when an intact H. ducreyi Ton system plasmid was present, formally demonstrating functional TonB dependence. tdhA was expressed poorly in vitro by H. ducreyi and only under conditions of heme limitation. A survey of H. ducreyi revealed that all tested strains but one synthesized small amounts of TdhA in vitro under heme-limiting conditions. Surprisingly, an isogenic mutant of tdhA as well as its parent, 35000, both required the same high levels of heme for growth (50 μg/ml [77 μM] on agar medium). This result, together with previous findings, suggests that in vitro, the uptake of heme by H. ducreyi is mediated by a TonB- and TdhA-independent mechanism, possibly diffusion.     

Characterization of a novel lipoprotein expressed by Haemophilus ducreyi.

Pooled sera from patients with chancroid contain antibodies to a Haemophilus ducreyi antigen with an approximate molecular weight of 28,000 (28K). Rabbit polyclonal serum that reacts to a 28K protein can be used to detect H. ducreyi in clinical samples. A monoclonal antibody, designated 5C9, bound to a 28K outer membrane protein and to 35 of 35 H. ducreyi isolates with diverse geographic origins and did not bind to many species of the families Pasteurellaceae, Neisseriaceae, and Enterobacteriaceae or to Corynebacterium and Candida species strains. A 5C9-reactive phage was recovered from a genomic library, and the gene encoding the 28K protein was localized to a 626-bp open reading frame, designated hlp, for H. ducreyi lipoprotein. Translation of hlp predicted a 23K polypeptide that contained a lipoprotein processing site. Escherichia coli transformed with a plasmid containing hlp expressed a novel, membrane-associated protein that could be labeled with [3H]palmitic acid. In H. ducreyi, processing of Hlp was inhibited by globomycin. Database searches found no homologies to hlp or to the predicted Hlp amino acid sequence. Restriction enzyme analysis indicated that hlp was conserved among H. ducreyi isolates. Serum samples from patients with chancroid and other genital ulcer diseases and from normal subjects contained antibodies that bound to purified, recombinant Hlp. Although monoclonal antibody 5C9 recognizes a species-specific epitope of a unique H. ducreyi lipoprotein, the presence of serum antibodies to Hlp may not indicate previous infection with H. ducreyi.     

Iron acquisition by Actinobacillus suis: identification and characterization of a single-component haemoglobin receptor and encoding gene

Actinobacillus suis is an important swine pathogen. As with other pathogens, the ability of A. suis to acquire iron within the host is crucial for virulence. Here, we investigated the ability of seven strains of A. suis to acquire iron from haemoglobins. In growth assays, all strains could use porcine, bovine and human haemoglobins as iron sources for growth. Using solid phase binding assays, membranes derived from all strains, grown under iron-restricted conditions, were shown to bind all three haemoglobins. Competition binding assays indicated that these haemoglobins were bound by the same receptor and an affinity procedure allowed the isolation and identification of an iron-repressible, haemoglobin-binding polypeptide (approximately 105 kDa) from all strains. Nucleotide sequence analyses revealed that A. suis possesses a gene (hgbA) that encodes a homologue of the Actinobacillus pleuropneumoniae haemoglobin-binding protein, HgbA. hgbA, encoding a mature protein of 105 kDa, was shown to be preceded by a hugZ homologue; putative promoter sequences and a putative Fur box were located upstream of hugZ and RT-PCR revealed that hugZ and hgbA are co-transcribed and iron-repressible. It is concluded that the acquisition of haemoglobin-bound iron by A. suis involves a single-component receptor that is up-regulated in response to iron restriction.     

Cloning, overexpression, purification, and immunobiology of an 85-kilodalton outer membrane protein from Haemophilus ducreyi

We have identified an 85-kDa outer membrane protein that is expressed by all tested strains of Haemophilus ducreyi. Studies of related proteins from other pathogenic bacteria, including Haemophilus influenzae, Pasteurella multocida, Neisseria gonorrhoeae, Neisseria meningitidis, and Shigella dysenteriae, suggested a role for these proteins in pathogenesis and immunity. In keeping with the first such described protein from Haemophilus influenzae type B, we termed the H. ducreyi protein D15. The gene encoding the H. ducreyi D15 protein was cloned and sequenced, and the deduced amino acid sequence was found to be most similar to sequences of the D15-related proteins from other Pasteurella spp. The arrangement of the flanking genes was similar to that of H. influenzae Rd and suggested that D15 was part of a multigene operon. Attempts to make a null mutation of the D15 gene were unsuccessful, paralleling results in other D15 gene studies. Overexpression of H. ducreyi D15 in Escherichia coli resulted in a source of recombinant D15 (rD15) from which it was readily purified. rD15 was immunogenic, and it was found that immunization of rabbits with an rD15 vaccine preparation conferred partial protection against a virulent challenge infection. Antisera to an N-terminal peptide recognized all tested strains of H. ducreyi.     

Identification and purification of a hemoglobin-binding outer membrane protein from Neisseria gonorrhoeae.

The majority of in vitro-grown Neisseria gonorrhoeae strains were unable to use hemoglobin as the sole source of iron for growth (Hgb-), but a minor population was able to do so (Hgb+). The ability of Hgb+ gonococci to utilize hemoglobin as the iron source was associated with the expression of an iron-repressible 89-kDa hemoglobin-binding protein in the outer membrane. The N-terminal amino acid sequence of this protein revealed amino acids, from positions 2 to 16, identical to those of HpuB, an 85 kDa iron-regulated hemoglobin-haptoglobin utilization outer membrane protein of Neisseria meningitidis. Isogenic mutants constructed by allelic replacement with a meningococcal hpu::mini-Tn3erm construct no longer expressed the 89-kDa protein. Mutants could not utilize hemoglobin to support growth but still grew on heme. Thus, the gonococcal HpuB homolog is a functional hemoglobin receptor and is essential for growth with hemoglobin.     

Immunization with the Haemophilus ducreyi Hemoglobin Receptor HgbA with Adjuvant Monophosphoryl Lipid A Protects Swine from a Homologous but Not a Heterologous Challenge

Haemophilus ducreyi, the etiological agent of chancroid, has a strict requirement for heme, which it acquires from its only natural host, humans. Previously, we showed that a vaccine preparation containing the native hemoglobin receptor HgbA purified from H. ducreyi class I strain 35000HP (nHgbA_I) and administered with Freund''s adjuvant provided complete protection against a homologous challenge. In the current study, we investigated whether nHgbA_I dispensed with monophosphoryl lipid A (MPL), an adjuvant approved for use in humans, offered protection against a challenge with H. ducreyi strain 35000HP expressing either class I or class II HgbA (35000HPhgbA_I and 35000HPhgbA_II, respectively). Pigs immunized with the nHgbA_I/MPL vaccine were protected against a challenge from homologous H. ducreyi strain 35000HPhgbA_I but not heterologous strain 35000HPhgbA_II, as evidenced by the isolation of only strain 35000HPhgbA_II from nHgbA_I-immunized pigs. Furthermore, histological analysis of the lesions showed striking differences between mock-immunized and nHgbA_I-immunized animals challenged with strains 35000HPhgbA_I but not those challenged with strain 35000HPhgbA_II. Mock-immunized pigs were not protected from a challenge by either strain. The enzyme-linked immunosorbent assay (ELISA) activity of the nHgbA_I/MPL antiserum was lower than the activity of antiserum from animals immunized with the nHgbA_I/Freund''s vaccine; however, anti-nHgbA_I from both studies bound whole cells of 35000HPhgbA_I better than 35000HPhgbA_II and partially blocked hemoglobin binding to nHgbA_I. In conclusion, despite eliciting lower antibody ELISA activity than the nHgbA_I/Freund''s, the nHgbA_I/MPL vaccine provided protection against a challenge with homologous but not heterologous H. ducreyi, suggesting that a bivalent HgbA vaccine may be needed.Chancroid is one of the genital ulcer diseases and is transmitted through sexual contact. Lesions caused by chancroid initially appear as papules, which evolve within several days into pustules. If left untreated, chancroid pustules develop into painful, bleeding ulcers with soft, irregular borders. Chancroid is prevalent in certain developing countries but is rarely found in the United States (47, 62). Several studies have shown that chancroid serves as an important independent cofactor in the heterosexual transmission of HIV where both diseases are endemic (29, 35, 48, 51, 66). Commercial sex workers serve as the reservoir of chancroid, and control of disease in this population strikingly reduces the number of cases of chancroid in their male clients (58). Thus, one possible approach to control chancroid is to implement a limited vaccination program to control infection in this reservoir; however, no vaccine for chancroid currently exists.Haemophilus ducreyi, the etiologic agent of chancroid, is a fastidious Gram-negative bacterium and a strict human pathogen. An interesting biologic feature of H. ducreyi is its obligate requirement for heme. Heme (for H. ducreyi) and iron are critical nutrients required for most pathogenic bacteria. Many Gram-negative bacteria obtain heme/Fe through systems that include TonB-dependent outer membrane receptors specific for heme/Fe compounds. The relatively small genome of prototypical H. ducreyi strain 35000HP encodes only three TonB-dependent receptors; in comparison, other bacterial genomes can encode more than 30 (14). Using isogenic mutants, the Spinola and Elkins laboratories surveyed the ability of H. ducreyi TonB-dependent receptor mutants to initiate infection in the human experimental model of chancroid. An H. ducreyi mutant that does not express the gene encoding the hemoglobin (Hb) receptor, hgbA, did not establish human infection (3). In contrast, an isogenic double mutant lacking the genes encoding the two other TonB-dependent receptors of H. ducreyi, tdhA and tdX, was fully virulent, indicating that HgbA is the only TonB-dependent receptor of H. ducreyi to be a virulence factor in the human experimental model of chancroid (38). Since HgbA is required for the utilization of heme from Hb by H. ducreyi (17), these data suggest that Hb is the most important source of heme in the early stages of the human experimental model of chancroid and that HgbA is a potential vaccine candidate.HgbA is a large, 100-kDa outer membrane protein that has a complex structure similar to that of other TonB-dependent receptors whose structure has been solved (10, 12, 20, 21, 40). HgbA is believed to contain 22 transmembrane beta sheets and 11 putatively surface-exposed loops. A recent study in our laboratory using H. ducreyi mutants expressing single loop deletions in HgbA provided evidence for surface exposure of loops 4, 5, 6, and 7 (44). Moreover, deletions of loops 5 and 7 but not of the other 9 loops of HgbA abrogated the binding of human Hb to HgbA. We also found that IgG from pigs immunized with native HgbA (nHgbA) bound loops 4, 5, and 7 and that antibodies directed at loops 4 and 5 partially blocked Hb binding to HgbA in vitro. Thus, a central domain of the primary sequence of HgbA is immunogenic, required for binding Hb, and surface exposed.H. ducreyi strains exist in two groups, designated class I and class II, based on striking primary sequence differences in certain outer membrane proteins, such as DsrA (ducreyi serum resistance A) and NcaA (necessary for collagen adhesion A), and on their lipooligosaccharide (LOS) structures (49, 50, 53, 67). In contrast, the HgbA proteins of different classes of H. ducreyi strains are more than 95% identical. Prototypical H. ducreyi strain 35000HP, a class I isolate, is the strain used for most studies, including isogenic mutant construction and the experimental human model of chancroid. 35000HP is the only H. ducreyi strain whose genome has been sequenced.Previously, we showed that immunization of swine with native HgbA from class I strain 35000HP (nHgbA_I) in Freund''s adjuvant provided complete protection from a homologous challenge infection with H. ducreyi strain 35000HP (1). The antibodies elicited by nHgbA_I/Freund''s showed modest bactericidal activity, bound to the cell surface of both class I and class II H. ducreyi strains, and partially blocked Hb binding to nHgbA (1). nHgbA_I antisera did not recognize the surface of, nor did they show bactericidal activity against the isogenic hgbA mutant, demonstrating specificity of the humoral response to HgbA.In the current study, we pursued two objectives. First, we investigated the effectiveness of monophosphoryl lipid A (MPL), an adjuvant approved for use in humans, to elicit an immune response to the nHgbA_I vaccine that is protective against an H. ducreyi challenge in the experimental swine model of chancroid. Second, we examined the ability of this vaccine to protect swine from a challenge infection with H. ducreyi strain 35000HP expressing either class I hgbA (hgbA_I) or class II hgbA (hgbA_II) from H. ducreyi strain DMC111 (homologous versus heterologous challenge, respectively).     

Identification and characterization of an iron-regulated hemopexin receptor in Haemophilus influenzae type b.

Heme can serve Haemophilus influenzae as a source of both essential porphyrin and iron. In extracellular mammalian body fluids neither free heme nor free iron is available, since they are tightly bound to hemopexin and transferrin, respectively. Since H. influenzae grows in the presence of iron-transferrin and heme-hemopexin and is known to express a saturable receptor for transferrin, we investigated the process by which this pathogen acquired heme from hemopexin for use as an iron source. The ability of human and rabbit hemopexin to donate heme as a source of iron to H. influenzae type b strains was demonstrated by plate bioassays. With a dot enzyme assay with biotinylated hemopexin as ligand, H. influenzae bound heme-hemopexin and apo-hemopexin following growth in iron-restricted, but not in iron-sufficient, medium. Competitive binding studies with heme-hemopexin and apo-hemopexin demonstrated saturability of binding. Neither heme, protoporphyrin IX, hemoglobin, nor transferrin blocked the binding of hemopexin to whole cells, demonstrating the specificity of binding. Treatment of whole H. influenzae cells with trypsin abolished binding. Taken together, these observations suggest that H. influenzae type b expresses an outer membrane protein(s) which acts as a receptor for hemopexin and which is regulated by the availability of iron in the growth medium. In iron-restricted media, H. influenzae 706705 and DL42 did not express the 100-kDa hemopexin-binding protein previously reported (M.S. Hanson, S.E. Pelzel, J. Latimer, U. Muller-Eberhard, and E.J. Hansen, Proc. Natl. Acad. Sci. USA 89:1973-1977, 1992). The putative iron-regulated hemopexin receptor was solubilized from cell envelopes of H. influenzae 706705, DL42, and Eagan with the detergent CHAPS (3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate) and isolated by affinity chromatography on heme-hemopexin-Sepharose 4B. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the proteins bound to the affinity resin revealed three proteins of 29, 38, and 57 kDa, of which the 57- and 29-kDa proteins bound hemopexin after Western blotting (immunoblotting). A monoclonal antibody to the 57-kDa hemopexin-binding protein of 706705 recognized a 57-kDa protein on Western blots of the cell envelope proteins of 706705, DL42, and Eagan; no reaction was observed with the 100-kDa hemopexin-binding protein of DL42. These data suggest that some H. influenzae strains possess at least two hemopexin receptors, the expression of which is determined by the prevailing growth environment.