The C-terminal fragment of the internal 110-kilodalton passenger domain of the Hap protein of nontypeable Haemophilus influenzae is a potential vaccine candidate

Preparation and immunogenicity evaluation of C-HapS-P6 fusion protein vaccine against nontypeable Haemophilus influenzae in mice

Nontypeable Haemophilus influenzae (NTHi) is the dominant pathogen in several infectious diseases. Currently the use of antibiotics is the main intervention to prevent NTHi infections, however with the emergence of drug resistant strains, it has compromised the treatment of respiratory infections with antibiotics. Therefore there is an urgent need to develop a safe and effective vaccine to prevent NTHi infections. We investigate the potential of C-HapS-P6 fusion protein as a vaccine for treating NTHi in murine models. PGEX-6P2/C-HapS-P6 fusion gene was constructed using overlap extension polymerase chain reaction. The recombined plasmid was transformed into Escherichia coli for protein expression. The mice were subjected to intraperitoneal immunization using purified antigens. Immunoglobulin (Ig) G in serum samples and IgA in nasal and lung lavage fluids were analyzed using enzyme-linked immunosorbent assay. Cytokine release and proliferation capacity of splenic lymphocytes in response to antigens were measured in vitro. The protective effect of the C-HapS-P6 protein against NTHi infection was evaluated by NTHi count and histological examination. The data showed that the C-HapS-P6 fusion protein increased significantly the levels of serum IgG and nasal and lung IgA, and promoted the release of interleukin (IL)-2, interferon-ϒ, IL-4, IL-5, and IL-17 and the proliferation of splenic lymphocytes compared with C-HapS or P6 protein treatment alone. Moreover, C-HapS-P6 effectively reduced the NTHi colonization in the nasopharynx and lungs of mice. In conclusion, our results demonstrated that the C-HapS-P6 fusion protein vaccine can significantly enhance humoral and cell immune responses and effectively prevent against NTHi infection in the respiratory tract in murine models.

The autotransporter protein BatA is a protective antigen against lethal aerosol infection with Burkholderia mallei and Burkholderia pseudomallei

BACKGROUND

Burkholderia mallei and Burkholderia pseudomallei are the causative agents of glanders and melioidosis, respectively. There is no vaccine to protect against these highly-pathogenic and intrinsically antibiotic-resistant bacteria, and there is concern regarding their use as biological warfare agents. For these reasons, B. mallei and B. pseudomallei are classified as Tier 1 organisms by the U.S. Federal Select Agent Program and the availability of effective countermeasures represents a critical unmet need.

METHODS

Vaccines (subunit and vectored) containing the surface-exposed passenger domain of the conserved Burkholderia autotransporter protein BatA were administered to BALB/c mice and the vaccinated animals were challenged with lethal doses of wild-type B. mallei and B. pseudomallei strains via the aerosol route. Mice were monitored for signs of illness for a period of up to 40 days post-challenge and tissues from surviving animals were analyzed for bacterial burden at study end-points.

RESULTS

A single dose of recombinant Parainfluenza Virus 5 (PIV5) expressing BatA provided 74% and 60% survival in mice infected with B. mallei and B. pseudomallei, respectively. Vaccination with PIV5-BatA also resulted in complete bacterial clearance from the lungs and spleen of 78% and 44% of animals surviving lethal challenge with B. pseudomallei, respectively. In contrast, all control animals vaccinated with a PIV5 construct expressing an irrelevant antigen and infected with B. pseudomallei were colonized in those tissues.

CONCLUSION

Our study indicates that the autotransporter BatA is a valuable target for developing countermeasures against B. mallei and B. pseudomallei and demonstrates the utility of the PIV5 viral vaccine delivery platform to elicit cross-protective immunity against the organisms.

Recognition of conserved antigens by Th17 cells provides broad protection against pulmonary Haemophilus influenzae infection

Nontypeable Haemophilus influenzae (NTHi) is a major cause of community acquired pneumonia and exacerbation of chronic obstructive pulmonary disease. A current effort in NTHi vaccine development has focused on generating humoral responses and has been greatly impeded by antigenic variation among the numerous circulating NTHi strains. In this study, we showed that pulmonary immunization of mice with killed NTHi generated broad protection against lung infection by different strains. While passive transfer of immune antibodies protected only against the homologous strain, transfer of immune T cells conferred protection against both homologous and heterologous strains. Further characterization revealed a strong Th17 response that was cross-reactive with different NTHi strains. Responding Th17 cells recognized both cytosolic and membrane-associated antigens, while immune antibodies preferentially responded to surface antigens and were highly strain specific. We further identified several conserved proteins recognized by lung Th17 cells during NTHi infection. Two proteins yielding the strongest responses were tested as vaccine candidates by immunization of mice with purified proteins plus an adjuvant. Immunization induced antigen-specific Th17 cells that recognized different strains and, upon adoptive transfer, conferred protection. Furthermore, immunized mice were protected against challenge with not only NTHi strains but also a fully virulent, encapsulated strain. Together, these results show that the immune mechanism of cross-protection against pneumonia involves Th17 cells, which respond to a broad spectrum of antigens, including those that are highly conserved among NTHi strains. These mechanistic insights suggest that inclusion of Th17 antigens in subunit vaccines offers the advantage of inducing broad protection and complements the current antibody-based approaches.

Antibodies against In Vivo-Expressed Antigens Are Sufficient To Protect against Lethal Aerosol Infection with Burkholderia mallei and Burkholderia pseudomallei

Burkholderia mallei, a facultative intracellular bacterium and tier 1 biothreat, causes the fatal zoonotic disease glanders. The organism possesses multiple genes encoding autotransporter proteins, which represent important virulence factors and targets for developing countermeasures in pathogenic Gram-negative bacteria. In the present study, we investigated one of these autotransporters, BatA, and demonstrate that it displays lipolytic activity, aids in intracellular survival, is expressed in vivo, elicits production of antibodies during infection, and contributes to pathogenicity in a mouse aerosol challenge model. A mutation in the batA gene of wild-type strain ATCC 23344 was found to be particularly attenuating, as BALB/c mice infected with the equivalent of 80 median lethal doses cleared the organism. This finding prompted us to test the hypothesis that vaccination with the batA mutant strain elicits protective immunity against subsequent infection with wild-type bacteria. We discovered that not only does vaccination provide high levels of protection against lethal aerosol challenge with B. mallei ATCC 23344, it also protects against infection with multiple isolates of the closely related organism and causative agent of melioidosis, Burkholderia pseudomallei Passive-transfer experiments also revealed that the protective immunity afforded by vaccination with the batA mutant strain is predominantly mediated by IgG antibodies binding to antigens expressed exclusively in vivo Collectively, our data demonstrate that BatA is a target for developing medical countermeasures and that vaccination with a mutant lacking expression of the protein provides a platform to gain insights regarding mechanisms of protective immunity against B. mallei and B. pseudomallei, including antigen discovery.

Recombinant C-terminal 311 amino acids of HapS adhesin as a vaccine candidate for nontypeable Haemophilus influenzae: A study on immunoreactivity in Balb/C mouse

Hap, an auto-transporter protein, is an antigenically conserved adhesion protein which is present on both typeable and nontypeable Haemophilus influenzae. This protein has central role in bacterial attachment to respiratory tract epithelial cells. A 1000bp C-terminal fragment of Hap passenger domain (HapS) from nontypeable Haemophilus influenzae was cloned into a prokaryotic expression vector, pET-24a. BALB/c mice were immunized subcutaneously with purified rC-HapS. Serum IgG responses to purified rC-HapS, serum IgG subclasses were determined by ELISA and functional activity of antibodies was examined by Serum Bactericidal Assay. The output of rC-HapS was approximately 62% of the total bacterial proteins. Serum IgG responses were significantly increased in immunized group with rC-HapS mixed with Freund's adjuvant in comparison with control groups. Analysis of the serum IgG subclasses showed that the IgG1 subclass was predominant after subcutaneous immunization in BALB/c mice (IgG2a/IgG1 < 1). The sera from rC-HapS immunized animals were strongly bactericidal against nontypeable Haemophilus influenzae. These results suggest that rC-HapS may be a potential vaccine candidate for nontypeable Haemophilus influenzae.

Early-Life Intranasal Colonization with Nontypeable Haemophilus influenzae Exacerbates Juvenile Airway Disease in Mice

Accumulating evidence suggests a connection between asthma development and colonization with nontypeable Haemophilus influenzae (NTHi). Specifically, nasopharyngeal colonization of human infants with NTHi within 4 weeks of birth is associated with an increased risk of asthma development later in childhood. Monocytes derived from these infants have aberrant inflammatory responses to common upper respiratory bacterial antigens compared to those of cells derived from infants who were not colonized and do not go on to develop asthma symptoms in childhood. In this study, we hypothesized that early-life colonization with NTHi promotes immune system reprogramming and the development of atypical inflammatory responses. To address this hypothesis in a highly controlled model, we tested whether colonization of mice with NTHi on day of life 3 induced or exacerbated juvenile airway disease using an ovalbumin (OVA) allergy model of asthma. We found that animals that were colonized on day of life 3 and subjected to induction of allergy had exacerbated airway disease as juveniles, in which exacerbated airway disease was defined as increased cellular infiltration into the lung, increased amounts of inflammatory cytokines interleukin-5 (IL-5) and IL-13 in lung lavage fluid, decreased regulatory T cell-associated FOXP3 gene expression, and increased mucus production. We also found that colonization with NTHi amplified airway resistance in response to increasing doses of a bronchoconstrictor following OVA immunization and challenge. Together, the murine model provides evidence for early-life immune programming that precedes the development of juvenile airway disease and corroborates observations that have been made in human children.

Developing a vaccine to prevent otitis media caused by nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) is a predominant organism of the upper respiratory nasopharyngeal microbiota. Its disease spectrum includes otitis media, sinusitis, non-bacteremic pneumonia and invasive infections. Protein-based vaccines to prevent NTHi infections are needed to alleviate these infections in children and vulnerable populations such as the elderly and those with chronic obstructive pulmonary disease (COPD). One NTHi protein is included in a pneumococcal conjugate vaccine and has been shown to provide efficacy. Our lab has been interested in understanding the immunogenicity of NTHi vaccine candidates P6, protein D and OMP26 for preventing acute otitis media in young children. We expect that continued investigation and progress in the development of an efficacious protein based vaccine against NTHi infections is achievable in the near future.

The Autotransporter BpaB Contributes to the Virulence of Burkholderia mallei in an Aerosol Model of Infection

Burkholderia mallei is a highly pathogenic bacterium that causes the zoonosis glanders. Previous studies indicated that the genome of the organism contains eight genes specifying autotransporter proteins, which are important virulence factors of Gram-negative bacteria. In the present study, we report the characterization of one of these autotransporters, BpaB. Database searches identified the bpaB gene in ten B. mallei isolates and the predicted proteins were 99-100% identical. Comparative sequence analyses indicate that the gene product is a trimeric autotransporter of 1,090 amino acids with a predicted molecular weight of 105-kDa. Consistent with this finding, we discovered that recombinant bacteria expressing bpaB produce a protein of ≥300-kDa on their surface that is reactive with a BpaB-specific monoclonal antibody. Analysis of sera from mice infected with B. mallei indicated that animals produce antibodies against BpaB during the course of disease, thus establishing production of the autotransporter in vivo. To gain insight on its role in virulence, we inactivated the bpaB gene of B. mallei strain ATCC 23344 and determined the median lethal dose of the mutant in a mouse model of aerosol infection. These experiments revealed that the bpaB mutation attenuates virulence 8-14 fold. Using a crystal violet-based assay, we also discovered that constitutive production of BpaB on the surface of B. mallei promotes biofilm formation. To our knowledge, this is the first report of a biofilm factor for this organism.

Vaccines for Nontypeable Haemophilus influenzae: the Future Is Now

Infections due to nontypeable Haemophilus influenzae result in enormous global morbidity in two clinical settings: otitis media in children and respiratory tract infections in adults with chronic obstructive pulmonary disease (COPD). Recurrent otitis media affects up to 20% of children and results in hearing loss, delays in speech and language development and, in developing countries, chronic suppurative otitis media. Infections in people with COPD result in clinic and emergency room visits, hospital admissions, and respiratory failure. An effective vaccine would prevent morbidity, help control health care costs, and reduce antibiotic use, a major contributor to the global crisis in bacterial antibiotic resistance. The widespread use of the pneumococcal conjugate vaccines is causing a relative increase in H. influenzae otitis media. The partial protection against H. influenzae otitis media induced by the pneumococcal H. influenzae protein D conjugate vaccine represents a proof of principle of the feasibility of a vaccine for nontypeable H. influenzae. An ideal vaccine antigen should be conserved among strains, have abundant epitopes on the bacterial surface, be immunogenic, and induce protective immune responses. Several surface proteins of H. influenzae have been identified as potential vaccine candidates and are in various stages of development. With continued research, progress toward a broadly effective vaccine to prevent infections caused by nontypeable H. influenzae is expected over the next several years.

Characterization of an autotransporter adhesin protein shared by Burkholderia mallei and Burkholderia pseudomallei

Background

Autotransporters form a large family of outer membrane proteins specifying diverse biological traits of Gram-negative bacteria. In this study, we report the identification and characterization of a novel autotransporter gene product of Burkholderia mallei (locus tag BMA1027 in strain ATCC 23344).

Results

Database searches identified the gene in at least seven B. mallei isolates and the encoded proteins were found to be 84% identical. Inactivation of the gene encoding the autotransporter in the genome of strain ATCC 23344 substantially reduces adherence to monolayers of HEp-2 laryngeal cells and A549 type II pneumocytes, as well as to cultures of normal human bronchial epithelium (NHBE). Consistent with these findings, expression of the autotransporter on the surface of recombinant E. coli bacteria increases adherence to these cell types by 5–7 fold. The gene specifying the autotransporter was identified in the genome of 29 B. pseudomallei isolates and disruption of the gene in strain DD503 reduced adherence to NHBE cultures by 61%. Unlike B. mallei, the mutation did not impair binding of B. pseudomallei to A549 or HEp-2 cells. Analysis of sera from mice infected via the aerosol route with B. mallei and B. pseudomallei revealed that animals inoculated with as few as 10 organisms produce antibodies against the autotransporter, therefore indicating expression in vivo.

Conclusions

Our data demonstrate that we have identified an autotransporter protein common to the pathogenic species B. mallei and B. pseudomallei which mediates adherence to respiratory epithelial cells and is expressed in vivo during the course of aerosol infection.

Prospects for a vaccine against otitis media

Otitis media is a major cause of morbidity in 80% of all children less than 3 years of age and often goes undiagnosed in the general population. There is evidence to suggest that the incidence of otitis media is increasing. The major cause of otitis media is infection of the middle ear with microbes from the nasopharynx. The anatomical orientation of the eustachian tube, in association with a number of risk factors, predisposes infants and young children to the infection. Bacteria are responsible for approximately 70% of cases of acute otitis media, with Streptococcus pneumoniae, nontypeable Haemophilus influenzae and Moraxella catarrhalis predominating as the causative agents. The respiratory viruses, respiratory syncytial virus, rhinovirus, parainfluenza and influenza, account for 30% of acute otitis media cases. Over the past decade, there has been a profound increase in the reported resistance to antibiotics, which, with increased disease burden, has focussed attention on vaccine development for otitis media. A polymicrobial formulation containing antigens from all major pathogens would have the greatest potential to deliver a sustained reduction in the disease burden globally. The disappointing outcomes for otitis media seen with the polysaccharide pneumococcal conjugate vaccine have raised major challenges for the vaccination strategy. Clearly, more knowledge is required concerning immune mechanisms in the middle ear, as well as vaccine formulations containing antigens that are more representative of the polymicrobial nature of the disease. Antigens that have been extensively tested in animal models are now available for testing in human subjects.

Overexpression and Purification of C-terminal Fragment of the Passenger Domain of Hap Protein from Nontypeable Haemophilus influenzae in a Highly Optimized Escherichia coli Expression System

BACKGROUND

Nontypeable Haemophilus influenzae (NTHi) is a common cause of respiratory tract disease and initiates infection by colonization in nasopharynx. The Haemophilus influenzae (H. influenzae) Hap adhesin is an auto transporter protein that promotes initial interaction with human epithelial cells. Hap protein contains a 110 kDa internal passenger domain called "HapS" and a 45 kDa C-terminal translocator domain called "Hapβ". Hap adhesive activity has been recently reported to be connected to its Cell Binding Domain (CBD) which resides within the 311 C-terminal residues of the internal passenger domain of the protein. Furthermore, immunization with this CBD protein has been shown to prevent bacterial nasopharynx colonization in animal models.

METHODS

To provide enough amounts of pure HapS protein for vaccine studies, we sought to develop a highly optimized system to overexpress and purify the protein in large quantities. To this end, pET24a-cbd plasmid harboring cbd sequence from NTHi ATCC49766 was constructed and its expression was optimized by testing various expression parameters such as growth media, induction temperature, IPTG inducer concentration, induction stage and duration. SDS-PAGE and Western-blotting were used for protein analysis and confirmation and eventually the expressed protein was easily purified via immobilized metal affinity chromatography (IMAC) using Ni-NTA columns.

RESULTS

The highest expression level of target protein was achieved when CBD expressing E. coli BL21 (DE3) cells were grown at 37°C in 2xTY medium with 1.0 mM IPTG at mid-log phase (OD600 nm equal to 0.6) for 5 hrs. Amino acid sequence alignment of expressed CBD protein with 3 previously published CBD amino acid sequences were more than %97 identical and antigenicity plot analysis further revealed 9 antigenic domains which appeared to be well conserved among different analyzed CBD sequences.

CONCLUSION

Due to the presence of high similarity among CBD from NTHi ATCC49766 and other NTHi strains, CBD protein expressed here sounds to be theoretically ideal as a universal candidate for being used in vaccine studies against NTHi strains of various geographical areas. Further investigations to corroborate the potency of this protein as a vaccine candidate are under process.

Genomic and antigenic characterization of monomeric autotransporters of Haemophilus parasuis: an ongoing process of reductive evolution

The genome of the highly pathogenic Haemophilus parasuis Nagasaki strain (serovar 5) was sequenced to 99 % completion. A genomic comparison with two other pathogenic serovar 5 H. parasuis strains identified six genes per genome (bmaA1-bmaA6) encoding β-barrel monomeric autotransporters, bmaA2 and bmaA3 being pseudogenes in at least one strain. The remaining encoded proteins were predicted to belong to the subtilisin (BmaA1 and BmaA4) and cysteine (BmaA5 and BmaA6) protease families. Allelic polymorphism was detected in other H. parasuis strains by comparative genomic hybridization using microarrays. Recombination events were observed, some of them leading to gene disruption in one of the three strains, although synteny around bmaA genes was conserved. These results suggest that bmaA genes are undergoing a process of reductive evolution. To evaluate their use as potential vaccine antigens, the products of the passenger domains of bmaA1, bmaA4, bmaA5 and bmaA6 were produced in Escherichia coli as recombinant proteins. They were detected by immunoblotting using sera of colostrum-deprived piglets recovering from a sublethal infection with H. parasuis (Nagasaki). The existence of specific antibodies after infection with H. parasuis also demonstrated in vivo expression. Using proteomics, only BmaA6 was detected in the in vitro-grown Nagasaki strain. Interestingly, the translocator domain was found in the outer membrane, while the passenger domain was located in supernatants. These results indicate that BmaA proteins could be considered as immunogen candidates to improve H. parasuis vaccines. However, their capacity to confer protective immunity needs to be studied further.

Adhesion, invasion, and agglutination mediated by two trimeric autotransporters in the human uropathogen Proteus mirabilis

Fimbriae of the human uropathogen Proteus mirabilis are the only characterized surface proteins that contribute to its virulence by mediating adhesion and invasion of the uroepithelia. PMI2122 (AipA) and PMI2575 (TaaP) are annotated in the genome of strain HI4320 as trimeric autotransporters with "adhesin-like" and "agglutinating adhesin-like" properties, respectively. The C-terminal 62 amino acids (aa) in AipA and 76 aa in TaaP are homologous to the translocator domains of YadA from Yersinia enterocolitica and Hia from Haemophilus influenzae. Comparative protein modeling using the Hia three-dimensional structure as a template predicted that each of these domains would contain four antiparallel beta sheets and that they formed homotrimers. Recombinant AipA and TaaP were seen as ∼28 kDa and ∼78 kDa, respectively, in Escherichia coli, and each also formed high-molecular-weight homotrimers, thus supporting this model. E. coli synthesizing AipA or TaaP bound to extracellular matrix proteins with a 10- to 60-fold-higher level of affinity than the control strain. Inactivation of aipA in P. mirabilis strains significantly (P < 0.01) reduced the mutants' ability to adhere to or invade HEK293 cell monolayers, and the functions were restored upon complementation. A 51-aa-long invasin region in the AipA passenger domain was required for this function. E. coli expressing TaaP mediated autoagglutination, and a taaP mutant of P. mirabilis showed significantly (P < 0.05) more reduced aggregation than HI4320. Gly-247 in AipA and Gly-708 in TaaP were indispensable for trimerization and activity. AipA and TaaP individually offered advantages to P. mirabilis in a murine model. This is the first report characterizing trimeric autotransporters in P. mirabilis as afimbrial surface adhesins and autoagglutinins.

Current and Future Prospects for a Vaccine for Nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae is an important human respiratory tract pathogen that causes about 30% of otitis media in infants and children. This proportion is increasing as a result of pneumococcal conjugate vaccines. Because of the morbidity associated with otitis media, a strong rationale exists to develop strategies to prevent these infections. A challenge to developing a vaccine for nontypeable H. influenzae is the antigenic heterogeneity of several major surface antigens and the genetic heterogeneity among strains. Several research groups have identified conserved surface proteins and tested them as putative vaccines. A recent clinical trial with protein D, a conserved surface antigen, demonstrated partial efficacy in preventing H. influenzae otitis media. This important result provides a proof of principle for developing a vaccine to prevent otitis media caused by nontypeable H. influenzae. Several vaccine antigens for nontypeable H. influenzae are in development.

Nontypeable Haemophilus influenzae adhesin protein E: characterization and biological activity

The adhesin protein E (PE) of the human respiratory pathogen nontypeable Haemophilus influenzae (NTHi) exists in all clinical isolates. In the present study, NTHi adherence to epithelial cells of various origins was further analyzed. The number of intraepithelial PE-deficient NTHi was decreased compared with PE-expressing NTHi. Interestingly, PE-expressing NTHi or Escherichia coli transformants, in addition to soluble recombinant PE22-160 without a lipid moiety, induced a proinflammatory cell response. The adhesive PE domain was defined within PE84-108, and preincubation of epithelial cells with this peptide blocked adhesion of several clinical NTHi isolates. Mice immunized with PE84-108 cleared NTHi up to 8-fold more efficiently on pulmonary challenge than did mice immunized with a control peptide. Finally, anti-PE mouse antibodies from vaccinated mice prevented NTHi adhesion. Our data suggest that the ubiquitous adhesin PE plays an important role in the pathogenesis of NTHi infection.

Crystal structure of LipL32, the most abundant surface protein of pathogenic Leptospira spp

Spirochetes of the genus Leptospira cause leptospirosis in humans and animals worldwide. Proteins exposed on the bacterial cell surface are implicated in the pathogenesis of leptospirosis. However, the biological role of the majority of these proteins is unknown; this is principally due to the lack of genetic systems for investigating Leptospira and the absence of any structural information on leptospiral antigens. To address this, we have determined the 2.0-A-resolution structure of the lipoprotein LipL32, the most abundant outer-membrane and surface protein present exclusively in pathogenic Leptospira species. The extracellular domain of LipL32 revealed a compact, globular, "jelly-roll" fold from which projected an unusual extended beta-hairpin that served as a principal mediator of the observed crystallographic dimer. Two acid-rich patches were also identified as potential binding sites for positively charged ligands, such as laminin, to which LipL32 has a propensity to bind. Although LipL32 shared no significant sequence identity to any known protein, it possessed structural homology to the adhesins that bind components of the extracellular matrix, suggesting that LipL32 functions in an analogous manner. Moreover, the structure provides a framework for understanding the immunological role of this major surface lipoprotein.

Vaccination with proteus toxic agglutinin, a hemolysin-independent cytotoxin in vivo, protects against Proteus mirabilis urinary tract infection

Complicated urinary tract infections (UTI) caused by Proteus mirabilis are associated with severe pathology in the bladder and kidney. To investigate the roles of two established cytotoxins, the HpmA hemolysin, a secreted cytotoxin, and proteus toxic agglutinin (Pta), a surface-associated cytotoxin, mutant analysis was used in conjunction with a mouse model of ascending UTI. Inactivation of pta, but not inactivation of hpmA, resulted in significant decreases in the bacterial loads of the mutant in kidneys (P < 0.01) and spleens (P < 0.05) compared to the bacterial loads of the wild type; the 50% infective dose (ID(50)) of an isogenic pta mutant or hpmA pta double mutant was 100-fold higher (5 x 10(8) CFU) than the ID(50) of parent strain HI4320 (5 x 10(6) CFU). Colonization by the parent strain caused severe cystitis and interstitial nephritis as determined by histopathological examination. Mice infected with the same bacterial load of the hpmA pta double mutant showed significantly reduced pathology (P < 0.01), suggesting that the additive effect of these two cytotoxins is critical during Proteus infection. Since Pta is surface associated and important for the persistence of P. mirabilis in the host, it was selected as a vaccine candidate. Mice intranasally vaccinated with a site-directed (indicated by an asterisk) (S366A) mutant purified intact toxin (Pta*) or the passenger domain Pta-alpha*, each independently conjugated with cholera toxin (CT), had significantly lower bacterial counts in their kidneys ( P = 0.001) and spleens (P = 0.002) than mice that received CT alone. The serum immunoglobulin G levels correlated with protection (P = 0.03). This is the first report describing the in vivo cytotoxicity and antigenicity of an autotransporter in P. mirabilis and its use in vaccine development.

A novel autotransporter of uropathogenic Proteus mirabilis is both a cytotoxin and an agglutinin

One of the six predicted Proteus mirabilis autotransporters (ATs), ORF c2341, is predicted to contain a serine protease motif and was earlier identified as an immunogenic outer membrane protein in P. mirabilis. The 3.2 kb gene encodes a 117 kDa protein with a 58-amino-acid-long signal peptide, a 75-kDa-long N-terminal passenger domain and a 30-kDa-long C-terminal translocator. Affinity-purified 110 kDa AT exhibited chymotrypsin-like activity and hydrolysed N-Suc-Ala-Ala-Pro-Phe-pNa and N-Suc-Ala-Ala-Pro-Leu-pNa with a K(M) of 22 muM and 31 muM, respectively, under optimal pH of 8.5-9.0 in a Ca(2+)-dependent manner. Activity was inhibited by subtilase-specific inhibitors leupeptin and chymostatin. Both the cell-associated and purified form elicited cytopathic effects on cultured kidney and bladder epithelial cells. Substrate hydrolysis as well as cytotoxicity was associated with the passenger domain and was compromised upon mutation of any of the catalytic residues (Ser366, His147 and Asp533). At alkaline pH and optimal cell density, the AT also promoted autoaggregation of P. mirabilis and this function was independent of its protease activity. Cytotoxicity, autoaggregation and virulence were significantly reduced in an isogenic pta mutant of P. mirabilis. Proteus toxic agglutinin (Pta) represents a novel autotransported cytotoxin with no bacterial homologues that works optimally in the alkalinized urinary tract, a characteristic of urease-mediated urea hydrolysis during P. mirabilis infection.

Identification of a novel Haemophilus influenzae protein important for adhesion to epithelial cells

Non-typable Haemophilus influenzae (NTHi) is an important human-specific respiratory pathogen colonizing the mucosa of the upper respiratory tract. The bacterium is a common cause of acute otitis media in children and exacerbations in patients with chronic obstructive pulmonary disease (COPD). An immunoglobulin (Ig) D-lambda myeloma protein was found to detect a 16 kDa surface protein that we designated protein E (PE). The pe gene was cloned using an NTHi genomic DNA library, and a truncated PE-derived protein lacking the endogenous signal peptide (PE22-160) was synthesized and produced in large amounts in Escherichia coli. Interestingly, PE was expressed at the bacterial surface of NTHi as revealed by flow cytometry using the IgD-lambda myeloma protein or PE-specific polyclonal antibodies. A PE-deficient NTHi mutant was produced and lost 50% of its adhesive capacity as compared to the wild-type counterpart when analysed for adhesion to type II lung alveolar epithelial cells. In parallel, E. coli expressing full-length PE1-160 adhered significantly more efficiently to epithelial cells as compared to wild-type E. coli. Recombinant IgD that recognized the chemical dansyl-chloride did not interact with PE indicating that the IgD-lambda myeloma protein most likely was an antibody directed against the H. influenzae surface epitope. In conclusion, we have discovered a novel NTHi outer membrane protein with adhesive properties using an IgD-myeloma protein.

Regions important for the adhesin activity of Moraxella catarrhalis Hag

BACKGROUND

The Moraxella catarrhalis Hag protein, an Oca autotransporter adhesin, has previously been shown to be important for adherence of this respiratory tract pathogen to human middle ear and A549 lung cells.

RESULTS

The present study demonstrates that adherence of M. catarrhalis isogenic hag mutant strains to the human epithelial cell lines Chang (conjunctival) and NCIH292 (lung) is reduced by 50-93%. Furthermore, expressing Hag in a heterologous Escherichia coli background substantially increased the adherence of recombinant bacteria to NCIH292 cells and murine type IV collagen. Hag did not, however, increase the attachment of E. coli to Chang cells. These results indicate that Hag directly mediates adherence to NCIH292 lung cells and collagen, but is not sufficient to confer binding to conjunctival monolayers. Several in-frame deletions were engineered within the hag gene of M. catarrhalis strain O35E and the resulting proteins were tested for their ability to mediate binding to NCIH292 monolayers, middle ear cells, and type IV collagen. These experiments revealed that epithelial cell and collagen binding properties are separable, and that residues 385-705 of this ~2,000 amino acid protein are important for adherence to middle ear and NCIH292 cells. The region of O35E-Hag encompassing aa 706 to 1194 was also found to be required for adherence to collagen. In contrast, beta-roll repeats present in Hag, which are structural features conserved in several Oca adhesins and responsible for the adhesive properties of Yersinia enterocolitica YadA, are not important for Hag-mediated adherence.

CONCLUSION

Hag is a major adherence factor for human cells derived from various anatomical sites relevant to pathogenesis by M. catarrhalis and its structure-function relationships differ from those of other, closely-related autotransporter proteins.

Pathogen proteins eliciting antibodies do not share epitopes with host proteins: a bioinformatics approach

The best way to prevent diseases caused by pathogens is by the use of vaccines. The advent of genomics enables genome-wide searches of new vaccine candidates, called reverse vaccinology. The most common strategy to apply reverse vaccinology is by designing subunit recombinant vaccines, which usually generate an humoral immune response due to B-cell epitopes in proteins. A major problem for this strategy is the identification of protective immunogenic proteins from the surfome of the pathogen. Epitope mimicry may lead to auto-immune phenomena related to several human diseases. A sequence-based computational analysis has been carried out applying the BLASTP algorithm. Therefore, two huge databases have been created, one with the most complete and current linear B-cell epitopes, and the other one with the surface-protein sequences of the main human respiratory bacterial pathogens. We found that none of the 7353 linear B-cell epitopes analysed shares any sequence identity region with human proteins capable of generating antibodies, and that only 1% of the 2175 exposed proteins analysed contain a stretch of shared sequence with the human proteome. These findings suggest the existence of a mechanism to avoid autoimmunity. We also propose a strategy for corroborating or warning about the viability of a protein linear B-cell epitope as a putative vaccine candidate in a reverse vaccinology study; so, epitopes without any sequence identity with human proteins should be very good vaccine candidates, and the other way around.

VapC-1 of nontypeable Haemophilus influenzae is a ribonuclease

Nontypeable Haemophilus influenzae (NTHi) organisms are obligate parasites of the human upper respiratory tract that can exist as commensals or pathogens. Toxin-antitoxin (TA) loci are highly conserved gene pairs that encode both a toxin and antitoxin moiety. Seven TA gene families have been identified to date, and NTHi carries two alleles of the vapBC family. Here, we have characterized the function of one of the NTHi alleles, vapBC-1. The gene pair is transcribed as an operon in two NTHi clinical isolates, and promoter fusions display an inverse relationship to culture density. The antitoxin VapB-1 forms homomultimers both in vitro and in vivo. The expression of the toxin VapC-1 conferred growth inhibition to an Escherichia coli expression strain and was successfully purified only when cloned in tandem with its cognate antitoxin. Using total RNA isolated from both E. coli and NTHi, we show for the first time that VapC-1 is an RNase that is active on free RNA but does not degrade DNA in vitro. Preincubation of the purified toxin and antitoxin together results in the formation of a protein complex that abrogates the activity of the toxin. We conclude that the NTHi vapBC-1 gene pair functions as a classical TA locus and that the induction of VapC-1 RNase activity leads to growth inhibition via the mechanism of mRNA cleavage.

The role of outer membrane proteins of Ornithobacterium rhinotracheale in attachment to chicken tracheal epithelium

Ornithobacterium rhinotracheale (ORT) infections cause major losses to the poultry industry. In search for factors implicated in the pathogenesis of ORT infections, the role of outer membrane proteins (OMPs) in the interaction of ORT with chicken tracheal epithelium was investigated. For this purpose, immune sera were prepared against total extracted OMPs, whole cell bacteria and three major OMPs of 45, 53 and 70 kDa and used in bacterial adherence inhibition assay. The results showed antibodies against a 53 kDa OMP significantly (p < or = 0.05) inhibited the bacterial adherence to chicken tracheal epithelium up to 78%.

The Moraxella catarrhalis autotransporter McaP is a conserved surface protein that mediates adherence to human epithelial cells through its N-terminal passenger domain

The protein McaP was previously shown to be an adhesin expressed by the Moraxella catarrhalis strain O35E, which also displays esterase and phospholipase B activities (J. M. Timpe et al., Infect. Immun. 71:4341-4350, 2003). In the present study, sequence analysis suggests that McaP is a conventional autotransporter protein that contains a 12-stranded beta-barrel transporter module (amino acids [aa] 383 to 650) linked to a surface-exposed passenger domain exhibiting lipolytic activity (aa 62 to 330). An in-frame deletion removing most of this predicted N-terminal passenger domain was engineered, and Escherichia coli expressing the truncated McaP protein exhibited greatly reduced adherence to A549 human lung epithelial cells compared to E. coli expressing wild-type McaP. Site-directed mutagenesis of a serine residue at position 62 of McaP, predicted to be important for the lipolytic activity of the protein, resulted in loss of hydrolysis of p-nitrophenyl ester of caproate. E. coli expressing this mutated McaP, however, adhered to A549 monolayers at levels greater than recombinant bacteria expressing the wild-type adhesin. These results indicate that the predicted passenger domain of McaP is involved in both the binding and the lipolytic activity of the molecule and demonstrate that the adhesive properties of McaP do not require its lipolytic activity. Sequence analysis of mcaP from eight Moraxella catarrhalis strains revealed that the gene product is highly conserved at the amino acid level (98 to 100% identity), and Western blot analysis demonstrated that a panel of 16 isolates all express McaP. Flow cytometry experiments using antibodies raised against various portions of McaP indicated that its predicted passenger domain as well as transporter module contain surface-exposed epitopes. In addition to binding to the surface of intact bacteria, these antibodies were found to decrease adherence of M. catarrhalis to A549 human lung cells by up to 47% and to reduce binding of recombinant E. coli expressing McaP by 98%. These results suggest that McaP should be considered as a potential vaccine antigen.

Distribution of bacterial proteins in biofilms formed by non-typeable Haemophilus influenzae

The ability to preserve the fragile ultrastructural organization of bacterial biofilms using cryo-preparation methods for electron microscopy has enabled us to probe sections through non-typeable Haemophilus influenzae (NTHi) biofilms and determine the localization of NTHi-specific lipooligosaccharide (LOS) and proteins within these structures. Some of the proteins we examined are currently being considered as candidates for vaccine development, so it is important that their distribution and accessibility within the biofilms formed by NTHi be determined. We have localized LOS to the extracellular matrix (ECM) of the biofilm and the P6 outer membrane protein to the membrane of what appear to be viable bacteria within the biofilm. The Hap and HWM1/HMW2 adhesive proteins were associated with bacteria within the biofilm and were present in the biofilm ECM. The IgA1 protease is a secreted protein that was also associated with NTHi in the biofilm and was in the ECM, but was more concentrated in the top region of the biofilm, suggesting a role in protecting biofilm bacteria from antibody attack.

Defining the interaction of the Treponema pallidum adhesin Tp0751 with laminin

Various invasive pathogens attach to host tissues via the extracellular matrix component laminin, the major glycoprotein found within basement membranes. Previous investigations identified the laminin-binding adhesin Tp0751 within the spirochete bacterium Treponema pallidum. In the current study, Tp0751 was shown to attach to a variety of laminin isoforms that are widely distributed throughout the host, including laminins 1, 2, 4, 8, and 10. Such universal attachment is conducive for an adhesin present within a highly invasive pathogen that encounters a variety of tissue sites during the course of infection. Additional studies systematically identified the amino acid residues within Tp0751 that contribute to laminin binding using synthetic peptides designed from the mature protein sequence. The minimum laminin-binding region of the adhesin was localized to 10 amino acids; peptides containing these residues inhibited attachment of Tp0751 and T. pallidum to laminin. Further, Tp0751-specific antibodies inhibited attachment of T. pallidum to laminin. This study furthers our knowledge of the interaction of T. pallidum with laminin, an association that is proposed to facilitate bacterial traversal of basement membranes and subsequent entry into the circulation and tissue invasion. As such, these investigations will reveal new targets for possible prevention of bacterial dissemination and establishment of chronic infection.