Certain site-directed, nonenzymatically active mutants of the Haemophilus influenzae P4 lipoprotein are able to elicit bactericidal antibodies

Microbial 5'-nucleotidases: their characteristics, roles in cellular metabolism, and possible practical applications

5′-Nucleotidases (EC 3.1.3.5) are enzymes that catalyze the hydrolytic dephosphorylation of 5′-ribonucleotides and 5′-deoxyribonucleotides to their respective nucleosides and phosphate. Most 5′-nucleotidases have broad substrate specificity and are multifunctional enzymes capable of cleaving phosphorus from not only mononucleotide phosphate molecules but also a variety of other phosphorylated metabolites. 5′-Nucleotidases are widely distributed throughout all kingdoms of life and found in different cellular locations. The well-studied vertebrate 5′-nucleotidases play an important role in cellular metabolism. These enzymes are involved in purine and pyrimidine salvage pathways, nucleic acid repair, cell-to-cell communication, signal transduction, control of the ribo- and deoxyribonucleotide pools, etc. Although the first evidence of microbial 5′-nucleotidases was obtained almost 60 years ago, active studies of genetic control and the functions of microbial 5′-nucleotidases started relatively recently. The present review summarizes the current knowledge about microbial 5′-nucleotidases with a focus on their diversity, cellular localizations, molecular structures, mechanisms of catalysis, physiological roles, and activity regulation and approaches to identify new 5′-nucleotidases. The possible applications of these enzymes in biotechnology are also discussed.

Key points

• Microbial 5′-nucleotidases differ in molecular structure, hydrolytic mechanism, and cellular localization.

• 5′-Nucleotidases play important and multifaceted roles in microbial cells.

• Microbial 5′-nucleotidases have wide range of practical applications.

The adhesins of non-typeable Haemophilus influenzae

INTRODUCTION

Nontypeable Haemophilus influenzae (NTHi) is an opportunistic pathogen of the respiratory tract and the greatest contributor to invasive Haemophilus disease. Additionally, in children, NTHi is responsible for the majority of otitis media (OM) which can lead to chronic infection and hearing loss. In adults, NTHi infection in the lungs is responsible for the onset of acute exacerbations in chronic obstructive pulmonary disease (COPD). Unfortunately, there is currently no vaccine available to protect against NTHi infections. Areas covered: NTHi uses an arsenal of adhesins to colonise the respiratory epithelium. The adhesins also have secondary roles that aid in the virulence of NTHi, including mechanisms that avoid immune clearance, adjust pore size to avoid antimicrobial destruction, form micro-colonies and invoke phase variation for protein mediation. Bacterial adhesins can also be ideal antigens for subunit vaccine design due to surface exposure and immunogenic capabilities. Expert commentary: The host-pathogen interactions of the NTHi adhesins are not fully investigated. The relationship between adhesins and the extracellular matrix (ECM) play a part in the success of NTHi colonisation and virulence by immune evasion, migration and biofilm development. Further research into these immunogenic proteins would further our understanding and enable a basis for better combatting NTHi disease.

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.

Expeditious screening of candidate proteins for microbial vaccines

Advancements in high-throughput "omics" technologies have revolutionized the way vaccine candidates are identified. Now every surface expressed protein that an organism produces can be identified in silico and possibly made available for the rapid development of recombinant/subunit vaccines. However, evaluating the antigenicity of a large number of candidate proteins is an immense challenge, typically requiring cloning of several hundred candidates followed by immunogenicity screening. Here we report the development of a rapid, high-throughput method for screening candidate proteins for vaccines. This method involves utilizing a coupled, cell-free transcription-translation system to screen tagged proteins that are captured at the C-termini using appropriate ligand coated wells in 96 well ELISA plates. The template DNA for the cell-free expression is generated by two sequential PCRs and includes gene coding sequences, promoter, terminator, other necessary cis-acting elements and appropriate tag sequences. The process generates expressible candidate proteins containing two different peptide tags at the N- and the C-termini of the protein molecules. Proteins are screened in parallel for their quantity and immunoreactivity with N-terminal tag antibodies and antisera raised against the pathogen of interest, respectively. Normalization against the total detectable bound protein in the control wells allows for the identification of highly immunoreactive candidates. For this study we selected 30 representatives of >300 potential candidate proteins from Mannheimia haemolytica, a bacterial agent of pneumonia in feedlot cattle for expression with N-terminal Strep-II and C-terminal His(x6)-tag and evaluated their relative immunoreactivities using Strep-tactin-HRP and rabbit antisera generated against M. haemolytica. Using this system we were able to swiftly and quantitatively analyze and rank the suitability of proteins to identify potentially viable vaccine candidates, with the majority of the high ranking candidates being associated with virulence and pathogenicity. The system is adaptable to any bacterial target and presents an alternative to conventional laborious cloning, expression and screening procedures.

Outer membrane protein P4 is not required for virulence in the human challenge model of Haemophilus ducreyi infection

Background

Bacterial lipoproteins often play important roles in pathogenesis and can stimulate protective immune responses. Such lipoproteins are viable vaccine candidates. Haemophilus ducreyi, which causes the sexually transmitted disease chancroid, expresses a number of lipoproteins during human infection. One such lipoprotein, OmpP4, is homologous to the outer membrane lipoprotein e (P4) of H. influenzae. In H. influenzae, e (P4) stimulates production of bactericidal and protective antibodies and contributes to pathogenesis by facilitating acquisition of the essential nutrients heme and nicotinamide adenine dinucleotide (NAD). Here, we tested the hypothesis that, like its homolog, H. ducreyi OmpP4 contributes to virulence and stimulates production of bactericidal antibodies.

Results

We determined that OmpP4 is broadly conserved among clinical isolates of H. ducreyi. We next constructed and characterized an isogenic ompP4 mutant, designated 35000HPompP4, in H. ducreyi strain 35000HP. To test whether OmpP4 was necessary for virulence in humans, eight healthy adults were experimentally infected. Each subject was inoculated with a fixed dose of 35000HP on one arm and three doses of 35000HPompP4 on the other arm. The overall parent and mutant pustule formation rates were 52.4% and 47.6%, respectively (P = 0.74). These results indicate that expression of OmpP4 in not necessary for H. ducreyi to initiate disease or progress to pustule formation in humans. Hyperimmune mouse serum raised against purified, recombinant OmpP4 did not promote bactericidal killing of 35000HP or phagocytosis by J774A.1 mouse macrophages in serum bactericidal and phagocytosis assays, respectively.

Conclusions

Our data suggest that, unlike e (P4), H. ducreyi OmpP4 is not a suitable vaccine candidate. OmpP4 may be dispensable for virulence because of redundant mechanisms in H. ducreyi for heme acquisition and NAD utilization.

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.

Crystal structure and immunogenicity of the class C acid phosphatase from Pasteurella multocida

Pasteurella multocida is a pathogen of veterinary and medical importance. Here, we report the 1.85Å resolution crystal structure of the class C acid phosphatase from this organism (denoted rPmCCAP). The structure shows that rPmCCAP exhibits the same haloacid dehalogenase fold and dimeric assembly as the class C enzyme from Haemophilus influenzae. Formation of the dimer in solution is demonstrated using analytical ultracentrifugation. The active site is devoid of a magnesium ion due to the presence of citrate in the crystallization buffer. Absence of the metal ion minimally perturbs the active site structure, which suggests that the main role of the ion is to balance the negative charge of the substrate rather than stabilize the active site structure. The crystal lattice displays unusual crystal packing involving the C-terminal polyhistidine tag mimicking the substrate. Steady-state kinetic constants are determined for the substrates NMN, 5'-AMP, 3'-AMP, 2'-AMP, and p-nitrophenyl phosphate. The highest catalytic efficiency is observed with NMN. The production of polyclonal anti-rPmCCAP antibodies is demonstrated, and these antibodies are shown to cross-react with the H. influenzae class C phosphatase. The antibodies are used to detect PmCCAP in clinical P. multocida and Mannheimia haemolytica strains cultured from infected animals.

Recognition of nucleoside monophosphate substrates by Haemophilus influenzae class C acid phosphatase

The e (P4) phosphatase from Haemophilus influenzae functions in a vestigial NAD(+) utilization pathway by dephosphorylating nicotinamide mononucleotide to nicotinamide riboside. P4 is also the prototype of class C acid phosphatases (CCAPs), which are nonspecific 5',3'-nucleotidases localized to the bacterial outer membrane. To understand substrate recognition by P4 and other class C phosphatases, we have determined the crystal structures of a substrate-trapping mutant P4 enzyme complexed with nicotinamide mononucleotide, 5'-AMP, 3'-AMP, and 2'-AMP. The structures reveal an anchor-shaped substrate-binding cavity comprising a conserved hydrophobic box that clamps the nucleotide base, a buried phosphoryl binding site, and three solvent-filled pockets that contact the ribose and the hydrogen-bonding edge of the base. The span between the hydrophobic box and the phosphoryl site is optimal for recognizing nucleoside monophosphates, explaining the general preference for this class of substrate. The base makes no hydrogen bonds with the enzyme, consistent with an observed lack of base specificity. Two solvent-filled pockets flanking the ribose are key to the dual recognition of 5'-nucleotides and 3'-nucleotides. These pockets minimize the enzyme's direct interactions with the ribose and provide sufficient space to accommodate 5' substrates in an anti conformation and 3' substrates in a syn conformation. Finally, the structures suggest that class B acid phosphatases and CCAPs share a common strategy for nucleotide recognition.

Characterization of a unique class C acid phosphatase from Clostridium perfringens

Clostridium perfringens is a gram-positive anaerobe and a pathogen of medical importance. The detection of acid phosphatase activity is a powerful diagnostic indicator of the presence of C. perfringens among anaerobic isolates; however, characterization of the enzyme has not previously been reported. Provided here are details of the characterization of a soluble recombinant form of this cell-associated enzyme. The denatured enzyme was approximately 31 kDa and a homodimer in solution. It catalyzed the hydrolysis of several substrates, including para-nitrophenyl phosphate, 4-methylumbelliferyl phosphate, and 3' and 5' nucleoside monophosphates at pH 6. Calculated K(m)s ranged from 0.2 to 0.6 mM with maximum velocity ranging from 0.8 to 1.6 micromol of P(i)/s/mg. Activity was enhanced in the presence of some divalent cations but diminished in the presence of others. Wild-type enzyme was detected in all clinical C. perfringens isolates tested and found to be cell associated. The described enzyme belongs to nonspecific acid phosphatase class C but is devoid of lipid modification commonly attributed to this class.

Expression, purification and crystallization of class C acid phosphatases from Francisella tularensis and Pasteurella multocida

Class C nonspecific acid phosphatases are bacterial enzymes that are secreted across the cytoplasmic membrane and hydrolyze a variety of phosphomonoesters at acidic pH. These enzymes are of interest for the development of improved vaccines and clinical diagnostic methods. In one case, the category A pathogen Francisella tularensis, the class C phosphatase plays a role in bacterial fitness. Here, the cloning, expression, purification and crystallization methods for the class C acid phosphatases from F. tularensis and Pasteurella multocida are reported. Crystals of the F. tularensis enzyme diffracted to 2.0 A resolution and belonged to space group C222(1), with one enzyme molecule in the asymmetric unit. Crystals of the P. multocida enzyme diffracted to 1.85 A resolution and belonged to space group C2, with three molecules in the asymmetric unit. Diffraction patterns from crystals of the P. multocida enzyme exhibited multiple interpenetrating reciprocal-space lattices, indicating epitaxial twinning. Despite this aberrance, autoindexing was robust and the data could be satisfactorily processed to 1.85 A resolution using MOSFLM and SCALA.

Physico-chemical characterisation and immunogenicity of a multi-valent candidate vaccine against non-typeable Haemophilus influenzae and Moraxella catarrhalis

The physico-chemical characteristics and immunogenicity of a candidate vaccine against otitis media, prepared from recombinant lipidated outer membrane proteins (rLP4 and rLP6) from non-typeable Haemophilus influenzae (NTHi) and of the ubiquitous cell surface protein UspA2 from Moraxella catarrhalis, were evaluated. Optical spectroscopy, size exclusion chromatography and gel electrophoresis were used to characterise the purified protein components and assess their purity and molecular sizes. The results showed that the three proteins were highly purified. Possible dimers in rLP4, dimers and multimers in rLP6 and UspA2 were detected. Small amounts of rLP4 and rLP6 dimers and most of UspA2 complexes remained tightly bound even after SDS treatment under reducing conditions. Immunogenicity studies showed that all proteins induced substantial antibody responses in mice immunised with AlPO4-adsorbed rLP4, rLP6 or UspA2 or a combination of these proteins. However, combination of these proteins resulted in a reduced response to rLP4 and rLP6, but not to UspA2, suggesting interference between these proteins which should be taken into consideration during the development and evaluation of this vaccine.

Cloning, purification and crystallization of Bacillus anthracis class C acid phosphatase

Cloning, expression, purification and crystallization studies of a recombinant class C acid phosphatase from the Category A pathogen Bacillus anthracis are reported. Large diffraction-quality crystals were grown in the presence of HEPES and Jeffamine ED-2001 at pH 7.0. The crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 53.4, b = 90.1, c = 104.2 angstroms. The asymmetric unit is predicted to contain two protein molecules with a solvent content of 38%. Two native data sets were collected from the same crystal before and after flash-annealing. The first data set had a mosaicity of 1.6 degrees and a high-resolution limit of 1.8 angstroms. After flash-annealing, the apparent mosaicity decreased to 0.9 degrees and the high-resolution limit of usable data increased to 1.6 angstroms. This crystal form is currently being used to determine the structure of B. anthracis class C acid phosphatase with experimental phasing techniques.

Crystallization of recombinant Haemophilus influenzae e (P4) acid phosphatase

Haemophilus influenzae infects the upper respiratory tract of humans and can cause infections of the middle ear, sinuses and bronchi. The virulence of the pathogen is thought to involve a group of surface-localized macromolecular components that mediate interactions at the host-pathogen interface. One of these components is lipoprotein e (P4), which is a class C acid phosphatase and a potential vaccine candidate for nontypeable H. influenzae infections. This paper reports the crystallization of recombinant e (P4) and the acquisition of a 1.7 angstroms resolution native X-ray diffraction data set. The space group is P4(2)2(1)2, with unit-cell parameters a = 65.6, c = 101.4 angstroms, one protein molecule per asymmetric unit and 37% solvent content. This is the first report of the crystallization of a class C acid phosphatase.