Genetic and immunological analyses of a 38 kDa surface-exposed lipoprotein of Pasteurella haemolytica A1

Microarray-based comparative genomic profiling of reference strains and selected Canadian field isolates of Actinobacillus pleuropneumoniae

Background

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, is a highly contagious respiratory pathogen that causes severe losses to the swine industry worldwide. Current commercially-available vaccines are of limited value because they do not induce cross-serovar immunity and do not prevent development of the carrier state. Microarray-based comparative genomic hybridizations (M-CGH) were used to estimate whole genomic diversity of representative Actinobacillus pleuropneumoniae strains. Our goal was to identify conserved genes, especially those predicted to encode outer membrane proteins and lipoproteins because of their potential for the development of more effective vaccines.

Results

Using hierarchical clustering, our M-CGH results showed that the majority of the genes in the genome of the serovar 5 A. pleuropneumoniae L20 strain were conserved in the reference strains of all 15 serovars and in representative field isolates. Fifty-eight conserved genes predicted to encode for outer membrane proteins or lipoproteins were identified. As well, there were several clusters of diverged or absent genes including those associated with capsule biosynthesis, toxin production as well as genes typically associated with mobile elements.

Conclusion

Although A. pleuropneumoniae strains are essentially clonal, M-CGH analysis of the reference strains of the fifteen serovars and representative field isolates revealed several classes of genes that were divergent or absent. Not surprisingly, these included genes associated with capsule biosynthesis as the capsule is associated with sero-specificity. Several of the conserved genes were identified as candidates for vaccine development, and we conclude that M-CGH is a valuable tool for reverse vaccinology.

Subcellular distribution of Plp-40, a lipoprotein in a serotype A strain of Pasteurella multocida

A 40-kDa lipoprotein (Plp-40) is expressed by serotype A strains of Pasteurella multocida in amounts which correlate with the amount of capsular material present. We hypothesized that Plp-40 is exposed at the outer surface of the outer membrane (OM) of the cell and is associated with the serotype A exopolysaccharide material. The objectives of the present study were to confirm the lipoprotein nature of Plp-40 and to determine its subcellular location. Plp-40 maturation was shown to be sensitive to globomycin, thereby confirming it to be a bacterial lipoprotein. Plp-40 was shown to be present in the OM fractions of P. multocida obtained by both sarkosyl extraction and sucrose density gradient centrifugation, as well as in capsule fractions obtained by either hyaluronidase treatment or warm buffer extraction. [(3)H]palmitic acid-labeled Plp-40 could be removed from the surface of whole cells by exposure to proteinase K. Autoradiography of (125)I-labeled cell surface proteins exhibited a 40-kDa band that was prominent in capsulated strains and greatly diminished in a noncapsulated strain. These results support the hypothesis that Plp-40 is a lipid-modified OM protein, which is exposed on the outer cell surface and is likely associated with serotype A extracellular polysaccharide.

Role of Mannheimia haemolytica leukotoxin in the pathogenesis of bovine pneumonic pasteurellosis

Bovine pneumonic pasteurellosis continues to be a major respiratory disease in feedlot cattle despite the recent advances in our understanding of the underlying complexities of causation. The etiological agent, Mannheimia haemolytica, possesses several virulence factors, including capsule, outer membrane proteins, adhesins, neuraminidase, endotoxin and exotoxic leukotoxin. Accumulating scientific evidence implicates leukotoxin as the primary factor contributing to clinical presentation and lung injury associated with this disease. Unlike other virulence factors, leukotoxin shows cell-type- and species-specific effects on bovine leukocytes. Recent investigations have delineated the mechanisms underlying the target-cell-specificity of leukotoxin and how this contributes to the pathogenesis of lung damage. This review summarizes current understanding of the secretion, regulation, mechanisms of action and evolutionary diversity of leukotoxin of M. haemolytica. Understanding the precise molecular mechanisms of leukotoxin is critical for the development of more effective prophylactic and therapeutic strategies to control this complex disease.

Characterization of, and immune responses of mice to, the purified OmpA-equivalent outer membrane protein of Pasteurella multocida serotype A:3 (Omp28)

Pasteurella multocida A:3 is a major cause of bovine pneumonia. A major antigenic heat-modifiable 28kDa outer membrane protein (Omp28) was previously identified. The purpose of this study was to purify and characterize Omp28 immunologically and structurally. Omp28 was extracted from N-lauroylsarcosine-insoluble protein preparations by a combination of detergent fractionation with Zwittergent 3-14 and chromatography. Partial N-terminal amino acid sequence confirmed Omp28 as a member of the OmpA-porin family. However, porin activity could not be demonstrated in a lipid-bilayer assay. Heat modifiability of purified Omp28 was demonstrated, and Omp28 was found in outer membrane fraction of P. multocida. Surface exposure of Omp28 was demonstrated by partial protease digestion of intact bacteria, by binding of anti-Omp28 polyclonal ascites fluid to the bacterial surface, and by partial inhibition of anti-outer membrane antiserum binding by previous incubation of the bacteria with anti-Omp28 serum. CD-1 mice vaccinated with purified Omp28 developed a significant antibody titer (P<0.05) compared to the control treatment group but were not protected from a homologous intraperitoneal bacterial challenge. By contrast, treatment groups vaccinated with P. multocida outer membrane, formalin-killed P. multocida or a commercial vaccine were significantly protected from challenge. In vitro complement-mediated killing of P. multocida was observed in post-vaccination sera of outer membrane, formalin-killed P. multocida, and commercial vaccine-treatment groups, but not with sera from the Omp28-treatment group. In conclusion, although Omp28 is surface exposed and antigenic, it may not be a desirable immunogen for stimulating immunity to P. multocida.

Identification of Actinobacillus pleuropneumoniae virulence genes using signature-tagged mutagenesis in a swine infection model

Actinobacillus pleuropneumoniae is a significant respiratory pathogen of swine causing a severe and often fatal fibrinous hemorrhagic bronchopneumonia with significant economic losses resulting from chronic as well as acute infections. This study describes the application of a signature-tagged mutagenesis (STM) system to identify in vivo critical genes of A. pleuropneumoniae. Twenty pools representing over 800 A. pleuropneumoniae mutants were screened in a natural-host porcine infection model and presumptive attenuated mutants were selected. The identity of the disrupted gene in each mutant was determined using an inverse PCR approach to amplify DNA sequences adjacent to the transposon insertion, followed by sequencing of the PCR product and comparison to bacterial databases. In vitro and in vivo competitive indices were determined for each unique mutant, and a total of 20 unique, attenuating gene disruptions were identified including insertions into homologues of genes involved in biosynthesis, virulence determinants, regulation, translation and unknown functions. Three of the genes required for virulence of A. pleuropneumoniae in this study were also identified in a previous STM study of Pasteurella multocida. Seven of the STM-derived mutants were also evaluated for their potential as live vaccine strains and provided good protection against homologous challenge.

Molecular cloning of the Pasteurella haemolytica pomA gene and identification of bovine antibodies against PomA surface domains

The gene (pomA) encoding PomA, an OmpA-like major outer membrane protein of the bovine respiratory pathogen Pasteurella haemolytica, was cloned, and its nucleotide sequence was determined. The deduced amino acid sequence of PomA has significant identity with the sequences of other OmpA family proteins. Absorption of three different bovine immune sera with whole P. haemolytica cells resulted in a reduction of bovine immunoglobulin G reactivity with recombinant PomA in Western immunoblots, suggesting the presence of antibodies against PomA surface domains.

Proteome analysis of the model microsymbiont Sinorhizobium meliloti: isolation and characterisation of novel proteins

Sinorhizobium meliloti is an agriculturally and ecologically important microbe due to its capacity to establish nitrogen-fixing symbiosis with plant legumes. Two-dimensional gel electrophoresis of total cellular protein was used to establish a proteome reference map for the model microsymbiont Sinorhizobium meliloti strain 1021. The extent of changes in the gene expression of cells grown in a defined medium at different growth phases was established. After examination of over 2000 resolved protein spots, a minimum of 52 reproducible changes in protein expression levels were detected when early exponential phase cells were compared to late exponential phase cells. In contrast, induction of nodulation gene expression by the addition of the flavonoid luteolin to cells did not result in detectable changes in protein expression at either early or late exponential phase. N-terminal microsequencing of eighteen unknown constitutive proteins plus four proteins, induced or up-regulated in late exponential phase cells, allowed the identification of proteins not previously described in rhizobia. These included an amide-binding protein, a putative hydrolase of the glyoxalase II protein family, a nucleoside diphosphate kinase, and a 5'-nucleotidase. N-terminal microsequencing was also valuable in revealing N-terminal post-translational processing and assigning a subcellular location to the analysed protein. Proteome analysis will provide a powerful analytical tool to complement the sequencing of the genome of strain 1021.

Identification of immunogenic, surface-exposed outer membrane proteins of Pasteurella haemolytica serotype 1

Pasteurella haemolytica serotype 1 (S1) is the bacterium most frequently recovered from the lungs of cattle that have succumbed to shipping fever pneumonia. P. haemolytica outer membrane proteins (OMPs) are important immunogens in the development of resistance to pneumonic pasteurellosis. The purpose of this study was to identify the repertoire of immunogenic, surface-exposed P. haemolytica (S1) OMPs, that could be important in the development of protective immunity. We determined surface exposure of OMPs by (1) their susceptibility to protease treatment and (2) their ability to adsorb out antibodies from bovine immune sera. For a comprehensive identification of immunogenic, surface-exposed OMPs, we used bovine antisera from calves that were resistant to experimental P. haemolytica challenge after (1) natural exposure to P. haemolytica, (2) vaccination with live P. haemolytica, or (3) vaccination with P. haemolytica OMPs. We identified 21 immunogenic, surface-exposed P. haemolytica OMPs. Most were recognized by all three immune sera. However, some were recognized by one or two of the three antisera. Our analyses identified surface-exposed, immunogenic proteins that were not identified in previous studies.

Genetic and immunologic analyses of PlpE, a lipoprotein important in complement-mediated killing of Pasteurella haemolytica serotype 1

Pasteurella haemolytica serotype 1 is the bacterium most commonly associated with bovine shipping fever. The presence of antibodies against P. haemolytica outer membrane proteins (OMPs) correlates statistically with resistance to experimental P. haemolytica challenge in cattle. Until now, specific P. haemolytica OMPs which elicit antibodies that function in host defense mechanisms have not been identified. In this study, we have cloned and sequenced the gene encoding one such protein, PlpE. Analysis of the deduced amino acid sequence revealed that PlpE is a lipoprotein and that it is similar to an Actinobacillus pleuropneumoniae lipoprotein, OmlA. Affinity-purified, anti-PlpE antibodies recognize a protein in all serotypes of P. haemolytica except serotype 11. We found that intact P. haemolytica and recombinant E. coli expressing PlpE are capable of absorbing anti-PlpE antibodies from bovine immune serum, indicating that PlpE is surface exposed in P. haemolytica and assumes a similar surface-exposed conformation in E. coli. In complement-mediated killing assays, we observed a significant reduction in killing of P. haemolytica when bovine immune serum that was depleted of anti-PlpE antibodies was used as the source of antibody. Our data suggest that PlpE is surface exposed and immunogenic in cattle and that antibodies against PlpE contribute to host defense against P. haemolytica.

Outer membrane proteins of bovine Pasteurella multocida serogroup A isolates

The outer membrane proteins (OMPs) of P. multocida serotypes A3 (7 isolates), A4 (2 isolates), A3,4 and A2 (one isolate each) obtained from pneumonic cattle (10 isolates) and from one pig isolate were investigated to identify potential immunogens. SDS-PAGE of P. multocida OM isolated by SDG centrifugation of spheroplasts revealed eight major OMPs. Outer membranes isolated by sarcosyl extraction or SDG had similar protein composition on Coomassie blue-stained SDS-PA gel and on immunoblots. Two major OMPs (M(r)s of 35 and 46 kDa at 100 degrees C) demonstrated heat modifiability with apparent M(r)s of 30 and 34 kDa at 37 degrees C, respectively. The N-terminal aa sequences of these heat modifiable proteins revealed homology with E. coli OmpA and Hib P1 proteins, respectively. Protease treatment of whole cells followed by western immunoblots using bovine convalescent sera identified several immunogenic, surface-exposed and conserved OMPs among the eleven P. multocida isolates examined. The whole organism SDS-PAGE profiles of the eleven P. multocida isolates differed such that six patterns were seen. These patterns could potentially be used as a typing system for P. multocida bovine isolates based on the molecular weights of whole cell proteins. The above observations have potentially important implications relative to the immunity to infection.

Purification and partial characterization of the OmpA family of proteins of Pasteurella haemolytica

This study was conducted to partially characterize and identify the purity of two major outer membrane proteins (OMPs) (with molecular weights of 32,000 and 35,000 [32K and 35K, respectively]) of Pasteurella haemolytica. The 35K and 32K major OMPs, designated Pasteurella outer membrane proteins A and B (PomA and PomB, respectively), were extracted from P. haemolytica by solubilization in N-octyl polyoxyl ethylene. The P. haemolytica strain used was a mutant serotype A1 from which the genes expressing the 30-kDa lipoproteins had been deleted. PomA and PomB were separated and partially purified by anion-exchange chromatography. PomA but not PomB was heat modifiable. The N-terminal amino acid sequences of the two proteins were determined and compared with reported sequences of other known proteins. PomA had significant N-terminal sequence homology with the OmpA protein of Escherichia coli and related proteins from other gram-negative bacteria. Moreover, polyclonal antiserum raised against the E. coli OmpA protein reacted with this protein. PomA was surface exposed, was conserved among P. haemolytica biotype A serotypes, and had porin activity in planar bilayers. No homology between the N-terminal amino acid sequence of PomB and those of other known bacterial proteins was found. Cattle vaccinated with live P. haemolytica developed a significant increase in serum antibodies to partially purified PomA, as shown by enzyme-linked immunosorbent assays, and to purified PomA and PomB, as detected on Western blots and by densitometry.