Production of a d-glycero-d-manno-heptosyltransferase mutant of Mannheimia haemolytica displaying a veterinary pathogen specific conserved LPS structure; development and functionality of antibodies to this LPS structure

Characterization of Two Novel Lipopolysaccharide Phosphoethanolamine Transferases in Pasteurella multocida and Their Role in Resistance to Cathelicidin-2

The lipopolysaccharide (LPS) produced by the Gram-negative bacterial pathogen Pasteurella multocida has phosphoethanolamine (PEtn) residues attached to lipid A, 3-deoxy-d-manno-octulosonic acid (Kdo), heptose, and galactose. In this report, we show that PEtn is transferred to lipid A by the P. multocida EptA homologue, PetL, and is transferred to galactose by a novel PEtn transferase that is unique to P. multocida called PetG. Transcriptomic analyses indicated that petL expression was positively regulated by the global regulator Fis and negatively regulated by an Hfq-dependent small RNA. Importantly, we have identified a novel PEtn transferase called PetK that is responsible for PEtn addition to the single Kdo molecule (Kdo1), directly linked to lipid A in the P. multocida glycoform A LPS. In vitro assays showed that the presence of a functional petL and petK, and therefore the presence of PEtn on lipid A and Kdo1, was essential for resistance to the cationic, antimicrobial peptide cathelicidin-2. The importance of PEtn on Kdo1 and the identification of the transferase responsible for this addition have not previously been shown. Phylogenetic analysis revealed that PetK is the first representative of a new family of predicted PEtn transferases. The PetK family consists of uncharacterized proteins from a range of Gram-negative bacteria that produce LPS glycoforms with only one Kdo molecule, including pathogenic species within the genera Vibrio, Bordetella, and Haemophilus We predict that many of these bacteria will require the addition of PEtn to Kdo for maximum protection against host antimicrobial peptides.

Characterization of the lipopolysaccharide from Pasteurella multocida Heddleston serovar 9: identification of a proposed bi-functional dTDP-3-acetamido-3,6-dideoxy-α-D-glucose biosynthesis enzyme

Pasteurella multocida strains are classified into 16 different lipopolysaccharide (LPS) serovars using the Heddleston serotyping scheme. Ongoing studies in our laboratories on the LPS aim to determine the core oligosaccharide (OS) structures expressed by each of the Heddleston type strains and identify the genes and transferases required for the biosynthesis of the serovar-specific OSs. In this study, we have determined the core OS of the LPS expressed by the Heddleston serovar 9 type strain, P2095. Structural information was established by a combination of monosaccharide and methylation analyses, nuclear magnetic resonance spectroscopy and mass spectrometry revealing the following structure: . The serovar 9 OS contains an inner core that is conserved among P. multocida strains with an elaborate outer core extension containing rhamnose (Rha), a D-glycero-D-manno isomer of heptose, and the unusual deoxyamino sugar, 3-acetamido-3,6-dideoxy-α-D-glucose (Qui3NAc). Genetic analyses of the LPS outer core biosynthesis locus revealed that in addition to the glycosyltransferases predicted to transfer the sugars to the nascent LPS molecule, the locus also contained the complete set of genes required for the biosynthesis of the nucleotide sugar donors dTDP-Rha and dTDP-Qui3NAc. One of the genes identified as part of the dTDP-Qui3NAc biosynthesis pathway, qdtD, encodes a proposed bi-functional enzyme with N-terminal amino acid identity to dTDP-4-oxo-6-deoxy-D-glucose-3,4-oxoisomerase and C-terminal amino acid identity to dTDP-3-oxo-6-deoxy-α-D-glucose transacetylase.

Natural selection in the chicken host identifies 3-deoxy-D-manno-octulosonic acid kinase residues essential for phosphorylation of Pasteurella multocida lipopolysaccharide

Pasteurella multocida is the causative agent of a number of diseases in animals, including fowl cholera. P. multocida strains simultaneously express two lipopolysaccharide (LPS) glycoforms (glycoforms A and B) that differ only in their inner core structure. Glycoform A contains a single 3-deoxy-d-manno-octulosonic acid (Kdo) residue that is phosphorylated by the Kdo kinase, KdkA, whereas glycoform B contains two unphosphorylated Kdo residues. We have previously shown that P. multocida mutants lacking the heptosyltransferase, HptA, produce full-length glycoform B LPS and a large amount of truncated glycoform A LPS, as they cannot add heptose to the glycoform A inner core. These hptA mutants were attenuated in chickens because the truncated LPS made them vulnerable to host defense mechanisms, including antimicrobial peptides. However, here we show that birds inoculated with high doses of the hptA mutant developed fowl cholera and the P. multocida isolates recovered from diseased birds no longer expressed truncated LPS. Sequencing analysis revealed that the in vivo-derived isolates had mutations in kdkA, thereby suppressing the production of glycoform A LPS. Interestingly, a number of the spontaneous KdkA mutant strains produced KdkA with a single amino acid substitution (A112V, R123P, H168Y, or D193N). LPS structural analysis showed that complementation of a P. multocida kdkA mutant with wild-type kdkA restored expression of glycoform A to wild-type levels, whereas complementation with any of the mutated kdkA genes did not. We conclude that in P. multocida KdkA, the amino acids A112, R123, H168, and D193 are critical for Kdo kinase function and therefore for glycoform A LPS assembly.

Functional characterization of Lpt3 and Lpt6, the inner-core lipooligosaccharide phosphoethanolamine transferases from Neisseria meningitidis

The lipooligosaccharide (LOS) of Neisseria meningitidis contains heptose (Hep) residues that are modified with phosphoethanolamine (PEtn) at the 3 (3-PEtn) and/or 6 (6-PEtn) position. The lpt3 (NMB2010) and lpt6 (NMA0408) genes of N. meningitidis, which are proposed to encode the required HepII 3- and 6-PEtn transferases, respectively, were cloned and overexpressed as C-terminally polyhistidine-tagged fusion proteins in Escherichia coli and found to localize to the inner membrane, based on sucrose density gradient centrifugation. Lpt3-His(6) and Lpt6-His(6) were purified from Triton X-100-solubilized membranes by nickel chelation chromatography, and dot blot analysis of enzymatic reactions with 3-PEtn- and 6-PEtn-specific monoclonal antibodies demonstrated conclusively that Lpt3 and Lpt6 are phosphatidylethanolamine-dependent LOS HepII 3- and 6-PEtn transferases, respectively, and that both enzymes are capable of transferring PEtn to both fully acylated LOS and de-O-acylated (de-O-Ac) LOS. Further enzymatic studies using capillary electrophoresis-mass spectrometry (MS) demonstrated that both Lpt3 and Lpt6 are capable of transferring PEtn to de-O-Ac LOS molecules already containing PEtn at the 6 and 3 positions of HepII, respectively, demonstrating that there is no obligate order of PEtn addition in the generation of 3,6-di-PEtn LOS moieties in vitro.

Structural and genetic basis for the serological differentiation of Pasteurella multocida Heddleston serotypes 2 and 5

Pasteurella multocida is classified into 16 serotypes according to the Heddleston typing scheme. As part of a comprehensive study to define the structural and genetic basis of this scheme, we have determined the structure of the lipopolysaccharide (LPS) produced by P. multocida strains M1404 (B:2) and P1702 (E:5), the type strains for serotypes 2 and 5, respectively. The only difference between the LPS structures made by these two strains was the absence of a phosphoethanolamine (PEtn) moiety at the 3 position of the second heptose (Hep II) in M1404. Analysis of the lpt-3 gene, required for the addition of this PEtn residue, revealed that the gene was intact in P1702 but contained a nonsense mutation in M1404. Expression of an intact copy of lpt-3 in M1404 resulted in the attachment of a PEtn residue to the 3 position of the Hep II residue, generating an LPS structure identical to that produced by P1702. We identified and characterized each of the glycosyltransferase genes required for assembly of the serotype 2 and 5 LPS outer core. Monoclonal antibodies raised against serotype 2 LPS recognized the serotype 2/5-specific outer core LPS structure, but recognition of this structure was inhibited by the PEtn residue on Hep II. These data indicate that the serological classification of strains into Heddleston serotypes 2 and 5 is dependent on the presence or absence of PEtn on Hep II.

Identification of novel glycosyltransferases required for assembly of the Pasteurella multocida A:1 lipopolysaccharide and their involvement in virulence

We previously determined the structure of the Pasteurella multocida Heddleston type 1 lipopolysaccharide (LPS) molecule and characterized some of the transferases essential for LPS biosynthesis. We also showed that P. multocida strains expressing truncated LPS display reduced virulence. Here, we have identified all of the remaining glycosyltransferases required for synthesis of the oligosaccharide extension of the P. multocida Heddleston type 1 LPS, including a novel alpha-1,6 glucosyltransferase, a beta-1,4 glucosyltransferase, a putative bifunctional galactosyltransferase, and two heptosyltransferases. In addition, we identified a novel oligosaccharide extension expressed only in a heptosyltransferase (hptE) mutant background. All of the analyzed mutants expressing LPS with a truncated main oligosaccharide extension displayed reduced virulence, but those expressing LPS with an intact heptose side chain were able to persist for long periods in muscle tissue. The hptC mutant, which expressed LPS with the shortest oligosaccharide extension and no heptose side chain, was unable to persist on the muscle or cause any disease. Furthermore, all of the mutants displayed increased sensitivity to the chicken antimicrobial peptide fowlicidin 1, with mutants expressing highly truncated LPS being the most sensitive.

Carbohydrate Moieties as Vaccine Candidates: meeting summary

In September 2007, a meeting entitled 'Carbohydrate Moieties as Vaccine Candidates' was held at the National Institutes of Health (Bethesda, MD). This meeting brought together scientists from a number of disciplines to address issues concerning carbohydrate moieties as targets for vaccines for a variety of pathogens and tumors. In addition, the meeting participants addressed fundamental topics of glycoimmunology including the recognition of glycotopes by B and T lymphocytes, the ontogeny of anti-carbohydrate immune responses, peptide mimicry, carbohydrate antigen processing pathways and adjuvants. One session reported progress in the development of new tools such as computational algorithms, glycan arrays and oligosaccharide synthesis and their application to carbohydrate vaccine research. The session titles were: (1) immune response to bacterial carbohydrate antigens; (2) immune response to glycolipids; (3) immune response to carbohydrate antigens on other microbes and on tumors; (4) novel vaccine approaches; (5) novel tools in carbohydrate vaccine research; (6) bench to bedside: carbohydrate moieties as vaccine immunopotentiators.

Pasteurella multocida expresses two lipopolysaccharide glycoforms simultaneously, but only a single form is required for virulence: identification of two acceptor-specific heptosyl I transferases

Lipopolysaccharide (LPS) is a critical virulence determinant in Pasteurella multocida and a major antigen responsible for host protective immunity. In other mucosal pathogens, variation in LPS or lipooligosaccharide structure typically occurs in the outer core oligosaccharide regions due to phase variation. P. multocida elaborates a conserved oligosaccharide extension attached to two different, simultaneously expressed inner core structures, one containing a single phosphorylated 3-deoxy-D-manno-octulosonic acid (Kdo) residue and the other containing two Kdo residues. We demonstrate that two heptosyltransferases, HptA and HptB, add the first heptose molecule to the Kdo(1) residue and that each exclusively recognizes different acceptor molecules. HptA is specific for the glycoform containing a single, phosphorylated Kdo residue (glycoform A), while HptB is specific for the glycoform containing two Kdo residues (glycoform B). In addition, KdkA was identified as a Kdo kinase, required for phosphorylation of the first Kdo molecule. Importantly, virulence data obtained from infected chickens showed that while wild-type P. multocida expresses both LPS glycoforms in vivo, bacterial mutants that produced only glycoform B were fully virulent, demonstrating for the first time that expression of a single LPS form is sufficient for P. multocida survival in vivo. We conclude that the ability of P. multocida to elaborate alternative inner core LPS structures is due to the simultaneous expression of two different heptosyltransferases that add the first heptose residue to the nascent LPS molecule and to the expression of both a bifunctional Kdo transferase and a Kdo kinase, which results in the initial assembly of two inner core structures.