Characterization of a large transferrin-binding protein from Actinobacillus pleuropneumoniae serotype 7

Virulence factors of Actinobacillus pleuropneumoniae involved in colonization, persistence and induction of lesions in its porcine host

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia. The virulence factors of this microorganism involved in colonization and the induction of lung lesions have been thoroughly studied and some have been well characterized. A. pleuropneumoniae binds preferentially to cells of the lower respiratory tract in a process involving different adhesins and probably biofilm formation. Apx toxins and lipopolysaccharides exert pathogenic effects on several host cells, resulting in typical lung lesions. Lysis of host cells is essential for the bacterium to obtain nutrients from the environment and A. pleuropneumoniae has developed several uptake mechanisms for these nutrients. In addition to persistence in lung lesions, colonization of the upper respiratory tract – and of the tonsils in particular – may also be important for long-term persistent asymptomatic infection. Information on virulence factors involved in tonsillar and nasal cavity colonization and persistence is scarce, but it can be speculated that similar features as demonstrated for the lung may play a role.

Use of an Actinobacillus pleuropneumoniae multiple mutant as a vaccine that allows differentiation of vaccinated and infected animals

Vaccination against Actinobacillus pleuropneumoniae is hampered by the lack of vaccines inducing reliable cross-serotype protection. In contrast, pigs surviving natural infection are at least partially protected from clinical symptoms upon reinfection with any serotype. Thus, we set out to construct an attenuated A. pleuropneumoniae live vaccine allowing the differentiation of vaccinated from infected animals (the DIVA concept) by successively deleting virulence-associated genes. Based on an A. pleuropneumoniae serotype 2 prototype live negative marker vaccine (W. Tonpitak, N. Baltes, I. Hennig-Pauka, and G.-F. Gerlach, Infect. Immun. 70:7120-7125, 2002), genes encoding three enzymes involved in anaerobic respiration and the ferric uptake regulator Fur were deleted, resulting in a highly attenuated sixfold mutant; this mutant was still able to colonize the lower respiratory tract and induced a detectable immune response. Upon a single aerosol application, this mutant provided significant protection from clinical symptoms upon heterologous infection with an antigenically distinct A. pleuropneumoniae serotype 9 challenge strain and allowed the serological discrimination between infected and vaccinated groups.

Effect of endobronchial challenge with Actinobacillus pleuropneumoniae serotype 9 of pigs vaccinated with a vaccine containing Apx toxins and transferrin-binding proteins

The efficacy of a subunit vaccine containing the Apx toxins of Actinobacillus pleuropneumoniae and transferrin-binding proteins was determined. Ten pigs were vaccinated twice with the vaccine. Eight control animals were injected twice with a saline solution. Three weeks after the second vaccination, all pigs were endobronchially inoculated with 10(6.5) colony-forming units (CFU) of an A. pleuropneumoniae serotype 9 strain. In the vaccine group, none of the pigs died after inoculation. Only one pig of the control group survived challenge. Surviving pigs were killed at 7 days after challenge. The mean percentage of affected lung tissue was 64% in the control group and 17% in the vaccine group. Actinobacillus pleuropneumoniae was isolated from the lungs of all animals. The mean bacterial titres of the caudal lung lobes were 5.0 x 10(8) CFU/g in the control group and 3.0 x 10(6) CFU/g in the vaccine group. It was concluded that the vaccine induced partial protection against severe challenge.

Actinobacillus pleuropneumoniae iron transport and urease activity: effects on bacterial virulence and host immune response

Actinobacillus pleuropneumoniae, a porcine respiratory tract pathogen, has been shown to express transferrin-binding proteins and urease during infection. Both activities have been associated with virulence; however, their functional role for infection has not yet been elucidated. We used two isogenic A. pleuropneumoniae single mutants (DeltaexbB and DeltaureC) and a newly constructed A. pleuropneumoniae double (DeltaureC DeltaexbB) mutant in aerosol infection experiments. Neither the A. pleuropneumoniae DeltaexbB mutant nor the double DeltaureC DeltaexbB mutant was able to colonize sufficiently long to initiate a detectable humoral immune response. These results imply that the ability to utilize transferrin-bound iron is required for multiplication and persistence of A. pleuropneumoniae in the porcine respiratory tract. The A. pleuropneumoniae DeltaureC mutant and the parent strain both caused infections that were indistinguishable from one another in the acute phase of disease; however, 3 weeks postinfection the A. pleuropneumoniae DeltaureC mutant, in contrast to the parent strain, could not be isolated from healthy lung tissue. In addition, the local immune response-as assessed by fluorescence-activated cell sorter and enzyme-linked immunosorbent spot analyses-revealed a significantly higher number of A. pleuropneumoniae-specific B cells in the bronchoalveolar lavage fluid (BALF) of pigs infected with the A. pleuropneumoniae DeltaureC mutant than in the BALF of those infected with the parent strain. These results imply that A. pleuropneumoniae urease activity may cause sufficient impairment of the local immune response to slightly improve the persistence of the urease-positive A. pleuropneumoniae parent strain.

Actinobacillus pleuropneumoniae iron transport: a set of exbBD genes is transcriptionally linked to the tbpB gene and required for utilization of transferrin-bound iron

Upon iron restriction, Actinobacillus pleuropneumoniae has been shown to express the transferrin-binding proteins TbpB and TbpA, both of which have been implied to be important virulence factors. In order to identify additional iron-regulated proteins, we cloned and analyzed the region upstream of the transferrin-binding protein genes in an A. pleuropneumoniae serotype 7 strain. We located immediately upstream of the tbpB gene two open reading frames which were 43% homologous to the neisserial ExbBD protein genes. By raising specific antibodies, we showed that ExbB is expressed under iron-limiting growth conditions only, and RT-PCR analysis revealed that the exbBD genes and the tbpB gene are transcribed on a single polycistronic mRNA. By constructing an isogenic and nonpolar exbBD mutant, we showed that the exbBD genes are required by A. pleuropneumoniae for utilization of transferrin-bound iron. Using PCR and Western blotting, we showed that the genetic organization found in A. pleuropneumoniae serotype 7 is similar in all 12 A. pleuropneumoniae serotype reference strains.

First chromosomal restriction map of Actinobacillus pleuropneumoniae and localization of putative virulence-associated genes

Combined physical and genetic maps of the genomes of Actinobacillus pleuropneumoniae AP76 (serotype 7 clinical isolate) and of A. pleuropneumoniae ATCC 27088 (serotype 1 reference strain) were constructed by using the restriction endonucleases ApaI, AscI, NotI, and SalI. The chromosome sizes as determined by the addition of estimated fragment sizes were 2.4 Mbp, and both maps had a resolution of approximately 100 kbp. The linkages between the ApaI, AscI, NotI, and SalI fragments and their relative positions were determined by (i) fragment excision and redigestion and (ii) partial digests of defined fragments and Southern blot using end-standing probes. The single SalI site within the chromosome of strain A. pleuropneumoniae AP76 was defined as position 1 of the map; for the map of A. pleuropneumoniae ATCC 27088, the corresponding SalI site was chosen. Putative virulence-associated genes (apx, omlA, sodA, tbpBA, ureC, and a repeat element) and housekeeping genes (glyA, metJ, recA, and rhoAP) were positioned on the physical maps and located on the ApaI and NotI fragments of A. pleuropneumoniae serotype reference strains.