The transferrin receptor of Actinobacillus pleuropneumoniae: quantitation of expression and structural characterization using a peptide-specific monoclonal antibody

Concurrent host-pathogen gene expression in the lungs of pigs challenged with Actinobacillus pleuropneumoniae

Background

Actinobacillus pleuropneumoniae causes pleuropneumonia in pigs, a disease which is associated with high morbidity and mortality, as well as impaired animal welfare. To obtain in-depth understanding of this infection, the interplay between virulence factors of the pathogen and defense mechanisms of the porcine host needs to be elucidated. However, research has traditionally focused on either bacteriology or immunology; an unbiased picture of the transcriptional responses can be obtained by investigating both organisms in the same biological sample.

Results

Host and pathogen responses in pigs experimentally infected with A. pleuropneumoniae were analyzed by high-throughput RT-qPCR. This approach allowed concurrent analysis of selected genes encoding proteins known or hypothesized to be important in the acute phase of this infection. The expression of 17 bacterial and 31 porcine genes was quantified in lung samples obtained within the first 48 hours of infection. This provided novel insight into the early time course of bacterial genes involved in synthesis of pathogen-associated molecular patterns (lipopolysaccharide, peptidoglycan, lipoprotein) and genes involved in pattern recognition (TLR4, CD14, MD2, LBP, MYD88) in response to A. pleuropneumoniae. Significant up-regulation of proinflammatory cytokines such as IL1B, IL6, and IL8 was observed, correlating with protein levels, infection status and histopathological findings. Host genes encoding proteins involved in iron metabolism, as well as bacterial genes encoding exotoxins, proteins involved in adhesion, and iron acquisition were found to be differentially expressed according to disease progression. By applying laser capture microdissection, porcine expression of selected genes could be confirmed in the immediate surroundings of the invading pathogen.

Conclusions

Microbial pathogenesis is the product of interactions between host and pathogen. Our results demonstrate the applicability of high-throughput RT-qPCR for the elucidation of dual-organism gene expression analysis during infection. We showed differential expression of 12 bacterial and 24 porcine genes during infection and significant correlation of porcine and bacterial gene expression. This is the first study investigating the concurrent transcriptional response of both bacteria and host at the site of infection during porcine respiratory infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1557-6) contains supplementary material, which is available to authorized users.

Cloning and expression of genes encoding transferrin-binding protein A and B from Actinobacillus pleuropneumoniae serotype 5

Actinobacillus pleuropneumoniae is an important primary pathogen in pigs, which causes a highly contagious pleuropneumonia. As an adaptation to the iron-restricted environment of the host, A. pleuropneumoniae possesses iron acquisition pathways mediated by surface receptors that specifically bind transferrin from the host. The receptor is composed of two receptor proteins, transferrin-binding protein A and B (TbpA and B), which are both capable of binding to transferrin. An impairment of iron uptake mechanisms is likely to reduce virulence. For this reason, these two proteins can be useful as a candidate target for A. pleuropneumoniae vaccination. To do this, genes encoding the TbpA and B from a serotype 5 isolate of A. pleuropneumoniae were amplified from genomic DNA template by PCR and cloned into a pRSET prokaryotic expression vector, generating the pRSET-A.pp-TbpA and B. Escherichia coli BL21(DE3)pLysS competent cells were transformed with each construct followed by the induction of protein expression by the addition of IPTG. Bands corresponding to the predicted sizes (110 and 60 kDa) were seen on the SDS-PAGE. Polyclonal antibodies raised against recombinant TbpA and B from mice were reacted with bacterial proteins. This result indicates that the recombinant proteins can induce immunological responses and might be useful as candidate targets for A. pleuropneumoniae vaccination.

Two TonB systems in Actinobacillus pleuropneumoniae: their roles in iron acquisition and virulence

Iron acquisition in vivo by Actinobacillus pleuropneumoniae depends upon a functional TonB system. Tonpitak et al. (W. Tonpitak, S. Thiede, W. Oswald, N. Baltes, and G.-F. Gerlach, Infect. Immun. 68:1164-1170, 2000) have described one such system, associated with tbpBA encoding the transferrin receptor, and here we report a second, termed tonB2. This gene cluster (exbB2-exbD2-tonB2) is highly homologous to those in other Pasteurellaceae, unlike the earlier system described (now termed tonB1), suggesting that it is the indigenous system for this organism. Both tonB2 and tonB1 are upregulated upon iron restriction. TonB2, but not TonB1, was found to be essential for growth in vitro when the sole source of iron was hemin, porcine hemoglobin, or ferrichrome. In the case of iron provided as iron-loaded porcine transferrin, neither tonB mutant was viable. The tonB1 phenotype could be explained by a polar effect of the mutation on transcription of downstream tbp genes. We propose that TonB2 is crucial for the acquisition of iron provided in this form, interacting with accessory proteins of the TonB1 system that have been demonstrated to be necessary by Tonpitak et al. TonB2 appears to play a much more important role in A. pleuropneumoniae virulence than TonB1. In an acute porcine infection model, the tonB2 mutant was found to be highly attenuated, while the tonB1 mutant was not. We hypothesize that acquisition of the tonB1-tbp gene cluster confers a biological advantage through its capacity to utilize transferrin-iron but that TonB1 itself plays little or no part in this process.