Virulence attributes of Histophilus somni with a deletion mutation in the ibpA gene

Monitoring ventral tail base surface temperature for fever detection in calves

In this study, we investigated whether monitoring the ventral tail base surface temperature (ST) using a wearable wireless sensor could be effective for fever detection in calves with experimentally induced pneumonia after inoculation with Histophilus somni strain 2336. We found a significant difference in the changes in ST values between the control and H. somni-inoculated groups after 24 h of inoculation and detected fever; however, the rectal temperature showed a significant difference between the groups after 12 h of inoculation. When a significant difference in the ST between the two groups was observed, serum haptoglobin concentration and exacerbation of clinical score increased in the H. somni-inoculated group compared with those in the control group. Pneumonia was observed in the H. somni-inoculated group at necropsy, indicating that the changes in ST may reflect fever with inflammation caused by H. somni infection. Our results demonstrated that monitoring ST using a sensor attached to the ventral tail base can detect fever in calves and may be a useful and labor-saving tool for the health management of calves.

The role of uspE in virulence and biofilm formation by Histophilus somni

UspE is a global regulator in Escherichia coli. To study the function of Histophilus somni uspE, strain 2336::TnuspE was identified from a bank of mutants generated with EZ::Tn5™<KAN-2> Tnp Transposome™ that were biofilm deficient. The 2336::TnuspE mutant was highly attenuated in mice, the electrophoretic profile of its lipooligosaccharide (LOS) indicated the LOS was truncated, and the mutant was significantly more serum-sensitive compared to the wildtype strain. In addition to forming a deficient biofilm, exopolysaccharide (EPS) production was also compromised, but the electrophoretic profile of outer membrane proteins was not altered. RNA sequence analysis revealed that the transcription levels of some stress response chaperones, transport proteins, and a large number of ribosomal protein genes in 2336::TnuspE were significantly differentially regulated compared to strain 2336. Therefore, uspE may differentially function in direct and indirect expression of H. somni genes, but its attenuation may be linked to poor biofilm formation and rapid clearance of the bacteria resulting from a compromised LOS structure. Our results support that uspE is a global stress regulatory gene in H. somni.

The Role of luxS in Histophilus somni Virulence and Biofilm Formation

S-Ribosylhomocysteinase (LuxS) is required for the synthesis of the autoinducer-2 (AI-2) quorum-sensing signaling molecule in many Gram-negative bacteria. The bovine (and ovine) opportunistic pathogen Histophilus somni contains luxS and forms a biofilm containing an exopolysaccharide (EPS) in the matrix. Since biofilm formation is regulated by quorum sensing in many bacteria, the roles of luxS in H. somni virulence and biofilm formation were investigated. Although culture supernatants from H. somni were ineffective at inducing bioluminescence in the Vibrio harveyi reporter strain BB170, H. somni luxS complemented the biosynthesis of AI-2 in the luxS-deficient Escherichia coli strain DH5α. H. somni strain 2336 luxS was inactivated by transposon mutagenesis. RNA expression profiles revealed that many genes were significantly differentially expressed in the luxS mutant compared to that in the wild-type, whether the bacteria were grown planktonically or in a biofilm. Furthermore, the luxS mutant had a truncated and asialylated lipooligosaccharide (LOS) and was substantially more serum sensitive than the wild-type. Not surprisingly, the luxS mutant was attenuated in a mouse model for H. somni virulence, and some of the altered phenotypes were partially restored after the mutation was complemented with a functional luxS However, no major differences were observed between the wild-type and the luxS mutant in regard to outer membrane protein profiles, biofilm formation, EPS production, or intracellular survival. These results indicate that luxS plays a role in H. somni virulence in the context of LOS biosynthesis but not biofilm formation or other phenotypic properties examined.

Histophilus somni Survives in Bovine Macrophages by Interfering with Phagosome-Lysosome Fusion but Requires IbpA for Optimal Serum Resistance

Histophilus somni is capable of intracellular survival within professional phagocytic cells, but the mechanism of survival is not understood. The Fic motif within the direct repeat (DR1)/DR2 domains of the IbpA fibrillary network protein of H. somni is cytotoxic to epithelial and phagocytic cells, which may interfere with the bactericidal activity of these cells. To determine the contribution of IbpA and Fic to resistance to host defenses, H. somni strains and mutants that lacked all or a region of ibpA (including the DR1/DR2 regions) were tested for survival in bovine monocytic cells and for serum susceptibility. An H. somni mutant lacking IbpA, but not the DR1/DR2 region within ibpA, was more susceptible to killing by antiserum than the parent, indicating that the entire protein was associated with serum resistance. H. somni strains expressing IbpA replicated in bovine monocytes for at least 72 h and were toxic for these cells. Virulent strain 2336 mutants lacking the entire ibpA gene or both DR1 and DR2 were not toxic to the monocytes but still survived within the monocytes for at least 72 h. Monitoring of intracellular trafficking of H. somni with monoclonal antibodies to phagosomal markers indicated that the early phagosomal marker early endosome antigen 1 colocalized with all isolates tested, but only strains that could survive intracellularly did not colocalize with the late lysosomal marker lysosome-associated membrane protein 2 and prevented the acidification of phagosomes. These results indicated that virulent isolates of H. somni were capable of surviving within phagocytic cells through interference in phagosome-lysosome maturation. Therefore, H. somni may be considered a permissive intracellular pathogen.

A Predominant Clonal Thromboembolic Meningoencephalitis Group of Histophilus somni Assigned by Major Outer Membrane Protein Gene Sequencing and Pulsed-Field Gel Electrophoresis

Histophilus somni, a member of the family Pasteurellaceae, causes a variety of diseases, including thromboembolic meningoencephalitis (TEME) and respiratory diseases, which result in considerable economic losses to the cattle and sheep industries. In this study, 132 chronologically diverse isolates from cattle in Japan and 68 isolates from other countries comprising 49 from cattle and 19 from sheep were characterized using major outer membrane protein (MOMP) gene sequence and pulsed-field gel electrophoresis (PFGE) analyses. The H. somni isolates formed nine MOMP genetic clades (clade Ia, Ib, and II-VIII) and 10 PFGE clusters (HS1-HS10). Except for two (1.0%), all isolates fell into one of the nine MOMP genetic clades, while 62 (31.0%) isolates belonged to no PFGE cluster. MOMP genetic clade Ia and PFGE cluster HS1 were the major groups, and all HS1 isolates possessed the clade Ia MOMP gene. Isolates from TEME cases were significantly associated with these major groups (chi-square test, p < 0.0001), as 88.2% of the TEME isolates belonged to MOMP genetic clade Ia and PFGE cluster HS1, which formed the most predominant clonal group. After an inactivated vaccine using an HS1 strain with the clade Ia MOMP gene was introduced in Japan in late 1989, the number of TEME cases and isolates assigned into the clonal group decreased simultaneously. However, the proportions of clade Ia and cluster HS1 isolates from TEME cases remained high after 1990. These results suggest a close association of TEME with PFGE cluster HS1 and MOMP genetic clade Ia, and imply the presence of factors or characteristics commonly possessed by those strains that contribute to the development of TEME.

A recombinant subunit vaccine for bovine RSV and Histophilus somni protects calves against dual pathogen challenge

Bovine respiratory syncytial virus (BRSV) and Histophilus somni synergize to cause respiratory disease in cattle. These pathogens cause enhanced disease during dual-infection and an IgE response to antigens of H. somni in dual-infected but not singly infected calves. Vaccines containing whole inactivated BRSV or H. somni have been associated with IgE responses A vaccine strategy that avoids stimulation of IgE antibodies would provide superior protection from dual infection. We hypothesized that a subunit vaccine consisting of the nucleoprotein (NP) from BRSV and the recombinant antigen IbpA DR2 (a surface antigen of H. somni with two toxic fic motifs) in Quil A adjuvant would elicit protection without disease enhancement. Three groups of calves were vaccinated twice with either: Formalin inactivated BRSV (FI) plus Somnivac®, NP & IbpA DR2 plus Quil A or Quil A alone, followed by BRSV and H. somni challenge. Clinical scores and antibody levels (to whole pathogens and to the subunits) were evaluated. Lungs were examined at necropsy on day 23 after infection. Clinical scores were significantly greatest for the FI & Somnivac® group and both clinical scores and lung pathology were lowest for the subunit group. All calves shed BRSV in nasal secretions. FI & Somnivac® induced IgE antibodies to H. somni and BRSV, but not to NP or DR2. The subunit vaccine did not induce an IgE antibody response to IbpA DR2 antigen and induced little IgE to H. somni. It did not induce an IgG antibody response to BRSV and H. somni, but stimulated production of IgG antibodies against the subunits. In summary, the subunit vaccine, consisting of the BRSV NP and H. somni IbpA DR2 in Quil A, protected against severe clinical signs and decreased lung pathology but did not prevent viral shedding. Importantly it prevented synergistic disease expression in response to dual infection.

Cattle Immunized with a Recombinant Subunit Vaccine Formulation Exhibits a Trend towards Protection against Histophilus somni Bacterial Challenge

Histophilosis, a mucosal and septicemic infection of cattle is caused by the Gram negative pathogen Histophilus somni (H. somni). As existing vaccines against H. somni infection have shown to be of limited efficacy, we used a reverse vaccinology approach to identify new vaccine candidates. Three groups (B, C, D) of cattle were immunized with subunit vaccines and a control group (group A) was vaccinated with adjuvant alone. All four groups were challenged with H. somni. The results demonstrate that there was no significant difference in clinical signs, joint lesions, weight change or rectal temperature between any of the vaccinated groups (B,C,D) vs the control group A. However, the trend to protection was greatest for group C vaccinates. The group C vaccine was a pool of six recombinant proteins. Serum antibody responses determined using ELISA showed significantly higher titers for group C, with P values ranging from < 0.0148 to < 0.0002, than group A. Even though serum antibody titers in group B (5 out of 6 antigens) and group D were significantly higher compared to group A, they exerted less of a trend towards protection. In conclusion, the vaccine used in group C exhibits a trend towards protective immunity in cattle and would be a good candidate for further analysis to determine which proteins were responsible for the trend towards protection.

Histophilus somni Stimulates Expression of Antiviral Proteins and Inhibits BRSV Replication in Bovine Respiratory Epithelial Cells

Our previous studies showed that bovine respiratory syncytial virus (BRSV) followed by Histophilus somni causes more severe bovine respiratory disease and a more permeable alveolar barrier in vitro than either agent alone. However, microarray analysis revealed the treatment of bovine alveolar type 2 (BAT2) epithelial cells with H. somni concentrated culture supernatant (CCS) stimulated up-regulation of four antiviral protein genes as compared with BRSV infection or dual treatment. This suggested that inhibition of viral infection, rather than synergy, may occur if the bacterial infection occurred before the viral infection. Viperin (or radical S-adenosyl methionine domain containing 2—RSAD2) and ISG15 (IFN-stimulated gene 15—ubiquitin-like modifier) were most up-regulated. CCS dose and time course for up-regulation of viperin protein levels were determined in treated bovine turbinate (BT) upper respiratory cells and BAT2 lower respiratory cells by Western blotting. Treatment of BAT2 cells with H. somni culture supernatant before BRSV infection dramatically reduced viral replication as determined by qRT PCR, supporting the hypothesis that the bacterial infection may inhibit viral infection. Studies of the role of the two known H. somni cytotoxins showed that viperin protein expression was induced by endotoxin (lipooligosaccharide) but not by IbpA, which mediates alveolar permeability and H. somni invasion. A naturally occurring IbpA negative asymptomatic carrier strain of H. somni (129Pt) does not cause BAT2 cell retraction or permeability of alveolar cell monolayers, so lacks virulence in vitro. To investigate initial steps of pathogenesis, we showed that strain 129Pt attached to BT cells and induced a strong viperin response in vitro. Thus colonization of the bovine upper respiratory tract with an asymptomatic carrier strain lacking virulence may decrease viral infection and the subsequent enhancement of bacterial respiratory infection in vivo.

Reverse vaccinology as an approach for developing Histophilus somni vaccine candidates

Histophilosis of cattle is caused by the Gram negative bacterial pathogen Histophilus somni (H. somni) which is also associated with the bovine respiratory disease (BRD) complex. Existing vaccines for H. somni include either killed cells or bacteria-free outer membrane proteins from the organism which have proven to be moderately successful. In this study, reverse vaccinology was used to predict potential H. somni vaccine candidates from genome sequences. In turn, these may protect animals against new strains circulating in the field. Whole genome sequencing of six recent clinical H. somni isolates was performed using an Illumina MiSeq and compared to six genomes from the 1980's. De novo assembly of crude whole genomes was completed using Geneious 6.1.7. Protein coding regions was predicted using Glimmer3. Scores from multiple web-based programs were utilized to evaluate the antigenicity of these predicted proteins which were finally ranked based on their surface exposure scores. A single new strain was selected for future vaccine development based on conservation of the protein candidates among all 12 isolates. A positive signal with convalescent serum for these antigens in western blots indicates in vivo recognition. In order to test the protective capacity of these antigens bovine animal trials are ongoing.

Natural competence in Histophilus somni strain 2336

Histophilus somni is an etiologic agent of shipping fever pneumonia, myocarditis, and other systemic diseases of bovines. Virulence factors that have been identified in H. somni include biofilm formation, lipooligosaccharide phase variation, immunoglobulin binding proteins, survival in phagocytic cells, and many others. However, to identify the genes responsible for virulence, an efficient mutagenesis system is needed. Mutagenesis of H. somni using allelic exchange is difficult, likely due to its tight restriction modification system. Mutagenesis by natural transformation in Haemophilus influenzae is well established and shows a strong bias for fragments containing specific uptake signal sequences (USS) within the genome. We hypothesized that natural transformation may also be possible in H. somni strain 2336 because its genome is over-represented with H. influenzae USS (5'-AAGTGCGGT-3') and contains most of the genes necessary for competence. H. somni strain 2336 was successfully transformed and mutated with genomic linear DNA from an H. somni mutant (738Δlob2a), which contains a kanamycin-resistance (Kan(R)) gene and the USS within lob2A. Although most of the competence genes found in H. influenzae were present in H. somni, comD and the 5' portion of comE were absent, which may account for the low transformation efficiency. The transformation efficiency of strain 2336 was greatest during mid-log growth phase and when cyclic adenosine monophosphate was added to the transformation medium. However, mutants were not isolated when strain 2336 was transformed with genomic DNA containing the same Kan(R) gene from H. somni luxS or uspE mutants, which lack the USS in these specific genes. Shuttle vector pNS3K was also naturally transformed into strain 2336, though at a lower efficiency. However, natural transformation with either H. somni linear DNA (2336Δlob2A) or pNS3K was unsuccessful with H. somni commensal strain 129Pt and several other disease isolates.

Failure of respiratory defenses in the pathogenesis of bacterial pneumonia of cattle

The respiratory system is well defended against inhaled bacteria by a dynamic system of interacting layers, including mucociliary clearance, host defense factors including antimicrobial peptides in the epithelial lining fluid, proinflammatory responses of the respiratory epithelium, resident alveolar macrophages, and recruited neutrophils and monocytes. Nevertheless, these manifold defenses are susceptible to failure as a result of stress, glucocorticoids, viral infections, abrupt exposure to cold air, and poor air quality. When some of these defenses fail, the lung can be colonized by bacterial pathogens that are equipped to evade the remaining defenses, resulting in the development of pneumonia. This review considers the mechanisms by which these predisposing factors compromise the defenses of the lung, with a focus on the development of bacterial pneumonia in cattle and supplemented with advances based on mouse models and the study of human disease. Deepening our understanding of how the respiratory defenses fail is expected to lead to interventions that restore these dynamic immune responses and prevent disease.

Antibody responses of calves to Histophilus somni recombinant IbpA subunits

Histophilus somni causes bovine pneumonia and septicemia, but protective immune responses are not well understood and immunodiagnostic methods are not well defined. We previously showed that antibody to a new virulence factor, IbpA, neutralizes cytotoxicity and immunization with a recombinant IbpA domain protects calves against experimental H. somni pneumonia. To further define immune responses to IbpA, we determined isotypic serum antibody responses to three IbpA domains (IbpA3, an N-terminal coiled coil region; IbpA5, a central region of 200 bp repeats and IbpA DR2, a C-terminal cytotoxic domain). ELISA was used to quantitate IgG1 or IgG2 antibodies to each of the IbpA subunits as well as H. somni whole cells (WCs) or culture supernatant (SUP). Calves experimentally infected with H. somni and monitored for up to 10 weeks had the least "0 time" (background) antibody levels to IbpA5, as well as the earliest and highest responses of greatest duration to the IbpA5 subunit. Responses of these calves were high to WC or SUP antigens but with higher "0 time" (background) antibody levels. We concluded that IbpA5 may be a useful immunodiagnostic antigen. Calves immunized with H. somni WC vaccine had antibody responses to WC antigens, but not to IbpA subunits before challenge. After challenge with H. somni, vaccinated calves had slight anamnestic responses to IbpA3 and IbpA5, but not to IbpA DR2. Since IbpA DR2 is a protective antigen, the data suggest the IbpA DR2 would be a useful addition to H. somni vaccines.

IbpA DR2 subunit immunization protects calves against Histophilus somni pneumonia

Histophilus somni is a prevalent cause of pneumonia and septicemia in cattle. Yet evidence for protection against pneumonia by current vaccines is controversial. We have identified a new H. somni virulence factor, IbpA. Previous studies implicated three likely protective subunits or domains in IbpA (A3, A5, and DR2), which were expressed as recombinant GST fusion proteins and purified for systemic vaccination of calves. After two subcutaneous immunizations, calves were challenged intrabronchially with virulent H. somni strain 2336 and clinical signs were monitored for four days before necropsy. Serum samples were collected throughout. At necropsy, the area of gross pneumonia was estimated, bronchial lavage fluid was collected, lesions were cultured and tissue samples were fixed for histopathology. Results showed that calves immunized with IbpA DR2 had a statistically lower percentage of lung with gross lesions than controls, fewer histologic abnormalities in affected areas and no H. somni isolated from residual pneumonic lesions. Calves immunized with the control GST vaccine, IbpA3 or IbpA5 had larger H. somni positive pneumonic lesions. ELISA results for serum antibodies showed that calves immunized with the IbpA DR2 antigen had high IgG1 and IgG2 and lowest IgE responses to the immunizing antigen. Specific IgG responses were also high in the bronchial lavage fluid. High specific serum IgE responses were previously shown to be associated with more severe pneumonia, but high IgG specific anti-IbpA DR2 responses seem to be critically related to protection. Since the IbpA DR2 Fic motif has been shown to cause bovine alveolar cells to retract, we tested the neutralizing ability of pooled serum from the IbpA DR2 immunized group. This pooled serum reduced cytotoxicity by 75-80%, suggesting that the protection was due to antibody neutralization of IbpA cytotoxicity, at least in part. Therefore, IbpA DR2 appears to be an important protective antigen of H. somni. The study shows, for the first time, that immunization with a purified Fic protein protects against disease in a natural host.

Structural basis of Fic-mediated adenylylation

The Fic family of adenylyltransferases, defined by a core HPFx(D/E)GN(G/K)R motif, consists of over 2,700 proteins found in organisms from bacteria to humans. The immunoglobulin-binding protein A (IbpA) from the bacterial pathogen Histophilus somni contains two Fic domains that adenylylate the switch1 tyrosine residue of Rho-family GTPases, allowing the bacteria to subvert host defenses. Here we present the structure of the second Fic domain of IbpA (IbpAFic2) in complex with its substrate, Cdc42. IbpAFic2-bound Cdc42 mimics the GDI-bound state of Rho GTPases, with both its switch1 and switch2 regions gripped by IbpAFic2. Mutations disrupting the IbpAFic2-Cdc42 interface impair adenylylation and cytotoxicity. Notably, the switch1 tyrosine of Cdc42 is adenylylated in the structure, providing the first structural view for this post-translational modification. We also show that the nucleotide-binding mechanism is conserved among Fic proteins and propose a catalytic mechanism for this recently discovered family of enzymes.

Update on bacterial pathogenesis in BRD

Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, Mycoplasma bovis and Arcanobacterium pyogenes are all frequently implicated in bovine respiratory disease (BRD). M. haemolytica is considered the most important of the group. These bacteria are commensals in the nasopharynx and establish infection in the lungs of cattle that are subjected to a variety of stresses. Factors that permit adherence to and proliferation in the lungs and factors that cause tissue destruction and inflammation have been identified as having major roles in pathogenesis. These virulence factors include protein adhesins, capsular polysaccharide, outer membrane proteins, iron-binding proteins, lipopolysacharide or lipooligosaccharide, enzymes and toxins. These bacterial products function to evade the immune system, damage the immune system and induce a severe inflammatory response.