Construction of an isogenic leukotoxin deletion mutant of Pasteurella haemolytica serotype 1: characterization and virulence

Mucosal Immunization with Spore-Based Vaccines against Mannheimia haemolytica Enhances Antigen-Specific Immunity

BACKGROUND

Mannheimia haemolytica is a bovine respiratory pathogen commonly associated with bacterial bronchopneumonia. Current vaccine strategies have shown variable efficacy in feedlot cattle, and therefore novel vaccines are needed. Bacillus subtilis spores have been investigated as a mucosal vaccine platform, due to their ability to bind and present antigens to the mucosa and act as an adjuvant. The aim of this study was to develop two spore-based mucosal vaccines targeting M. haemolytica and evaluate their immunogenicity in mice.

METHODS

Two antigen constructs composed of cholera toxin B subunit, M. haemolytica leukotoxin, and either the M. haemolytica outer membrane protein PlpE (MhCP1) or GS60 (MhCP2) were synthesized, purified and then bound to spores as vaccines. In two separate mice trials, the spore-bound vaccines (Spore-MhCP1 and Spore-MhCP2) were administered to mice through intranasal and intragastric routes, while free antigens were administered intranasally and intramuscularly. Unbound spores were also evaluated intranasally. Antigen-specific serum IgG and mucosal IgA from bronchoalveolar lavage, feces, and saliva were measured after vaccination. Mice sera from all treatment groups were assessed for their bactericidal activity against M. haemolytica.

RESULTS

In both mice experiments, intramuscular immunization induced the strongest serum IgG antibody response. However, the intranasal administration of Spore-MhCP1 and Spore-MhCP2 elicited the greatest secretory IgA-specific response against leukotoxin, PlpE, and GS60 in bronchoalveolar lavage, saliva, and feces (p < 0.05). Compared to the intranasal administration of free antigen, spore-bound antigen groups showed greater bactericidal activity against M. haemolytica (p < 0.05).

CONCLUSIONS

Since intranasally delivered Spore-MhCP1 and Spore-MhCP2 elicited both systemic and mucosal immune responses in mice, these vaccines may have potential to mitigate lung infection in cattle by restricting M. haemolytica colonization and proliferation in the respiratory tract. The efficacy of these mucosal spore-based vaccines merits further assessment against M. haemolytica in cattle.

Insights from targeting transferrin receptors to develop vaccines for pathogens of humans and food production animals

While developing vaccines targeting surface transferrin receptor proteins in Gram-negative pathogens of humans and food production animals, the common features derived from their evolutionary origins has provided us with insights on how improvements could be implemented in the various stages of research and vaccine development. These pathogens are adapted to live exclusively on the mucosal surfaces of the upper respiratory or genitourinary tract of their host and rely on their receptors to acquire iron from transferrin for survival, indicating that there likely are common mechanisms for delivering transferrin to the mucosal surfaces that should be explored. The modern-day receptors are derived from those present in bacteria that lived over 320 million years ago. The pathogens represent the most host adapted members of their bacterial lineages and may possess factors that enable them to have strong association with the mucosal epithelial cells, thus likely reside in a different niche than the commensal members of the bacterial lineage. The bacterial pathogens normally lead a commensal lifestyle which presents challenges for development of relevant infection models as most infection models either exclude the early stages of colonization or subsequent disease development, and the immune mechanisms at the mucosal surface that would prevent disease are not evident. Development of infection models emulating natural horizontal disease transmission are also lacking. Our aim is to share our insights from the study of pathogens of humans and food production animals with individuals involved in vaccine development, maintaining health or regulation of products in the human and animal health sectors.

The trehalose glycolipid C18Brar promotes antibody and T-cell immune responses to Mannheimia haemolytica and Mycoplasma ovipneumoniae whole cell antigens in sheep

Bronchopneumonia is a common respiratory disease in livestock. Mannheimia haemolytica is considered the main causative pathogen leading to lung damage in sheep, with Mycoplasma ovipneumoniae and ParaInfluenza virus type 3, combined with adverse physical and physiological stress, being predisposing factors. A balance of humoral and cellular immunity is thought to be important for protection against developing respiratory disease. In the current study, we compared the ability of the trehalose glycolipid adjuvant C18Brar (C18-alkylated brartemicin analogue) and three commercially available adjuvant systems i.e., Quil-A, Emulsigen-D, and a combination of Quil-A and aluminium hydroxide gel, to stimulate antibody and cellular immune responses to antigens from inactivated whole cells of M. haemolytica and M. ovipneumoniae in sheep. C18Brar and Emulsigen-D induced the strongest antigen-specific antibody responses to both M. haemolytica and M. ovipneumoniae, while C18Brar and Quil-A promoted the strongest antigen-specific IL-17A responses. The expression of genes with known immune functions was determined in antigen-stimulated blood cultures using Nanostring nCounter technology. The expression levels of CD40, IL22, TGFB1, and IL2RA were upregulated in antigen-stimulated blood cultures from animals vaccinated with C18Brar, which is consistent with T-cell activation. Collectively, the results demonstrate that C18Brar can promote both antibody and cellular responses, notably Th17 immune responses in a ruminant species.

Protection against Mycoplasma bovis infection in calves following intranasal vaccination with modified-live Mannheimia haemolytica expressing Mycoplasma antigens

Novel live vaccine strains of Mannheimia haemolytica serotypes (St)1 and St6, expressing and secreting inactive yet immunogenic leukotoxin (leukotoxoid) fused to antigenic domains of Mycoplasma bovis Elongation Factor Tu (EFTu) and Heat shock protein (Hsp) 70 were constructed and tested for efficacy in cattle. Control calves were administered an intranasal mixture of M. haemolytica St1 and St6 mutants (ΔlktCAV4) expressing and secreting leukotoxoid while vaccinated calves were administered an intranasal mixture of like M. haemolytica St1 and St6 leukotoxoid mutants coupled to M. bovis antigens (EFTu-Hsp70-ΔlktCAV4). Both M. haemolytica strains were recovered from palatine tonsils up to 34 days post intranasal exposure. On day 35 all calves were exposed to bovine herpes virus-1, four days later lung challenged with virulent M. bovis, then euthanized up to 20 days post-challenge. Results showed all cattle produced systemic antibody responses against M. haemolytica. The vaccinates also produced systemic antibody responses to M. bovis antigen, and concurrent reductions in temperatures, middle ear infections, joint infection and lung lesions versus the control group. Notably, dramatically decreased lung loads of M. bovis were detected in the vaccinated cattle. These observations indicate that the attenuated M. haemolytica vaccine strains expressing Mycoplasma antigens can control M. bovis infection and disease symptoms in a controlled setting.

Immune Suppression Induced by Gallibacterium anatis GtxA During Interaction with Chicken Macrophage-Like HD11 Cells

The RTX toxin GtxA expressed by Gallibacterium anatis biovar haemolytica has been proposed a major virulence factor during disease manifestations in the natural host, the chicken. To better understand the role of GtxA in the pathogenesis of G. anatis, we compared the GtxA expressing wildtype strain with its isogenic ∆gtxA mutant that was unable to express GtxA during exposure to chicken macrophage-like HD11 cells. From adhesion and invasion assays, we showed that GtxA appears to promote adhesion and invasion of HD11 cells. By using quantitative RT-PCR, we also demonstrated that the G. anatis expressing GtxA induced a mainly anti-inflammatory (IL-10) host cell response as opposed to the pro-inflammatory (IL-1β, IL-6 and TNF-α) response induced by the GtxA deletion mutant. Interestingly, these results, at least partly, resemble recent responses observed from spleen tissue of chickens infected with the same two bacterial strains. The effect of the GtxA toxin on the type of cell death was less clear. While GtxA clearly induced cell death, our efforts to characterize whether this was due to primarily necrosis or apoptosis through expression analysis of a broad range of apoptosis genes did not reveal clear answers.

Identification of a reliable fixative solution to preserve the complex architecture of bacterial biofilms for scanning electron microscopy evaluation

Bacterial biofilms are organized sessile communities of bacteria enclosed in extracellular polymeric substances (EPS). To analyze organization of bacteria and EPS in high resolution and high magnification by scanning electron microscopy (SEM), it is important to preserve the complex architecture of biofilms. Therefore, fixation abilities of formalin, glutaraldehyde, and Methacarn (methanol/chloroform/acetic acid-6:3:1) fixatives were evaluated to identify which fixative would best preserve the complex structure of bacterial biofilms. Economically important Gram-negative Mannheimia haemolytica, the major pathogen associated with bovine respiratory disease complex, and Gram-positive Staphylococcus aureus, the major cause of chronic mastitis in cattle, bacteria were selected since both form biofilms on solid-liquid interface. For SEM analysis, round glass coverslips were placed into the wells of 24-well plates and diluted M. haemolytica or S. aureus cultures were added, and incubated at 37°C for 48–72 h under static growth conditions. Culture media were aspirated and biofilms were fixed with an individual fixative for 48 h. SEM examination revealed that all three fixatives were effective preserving the bacterial cell morphology, however only Methacarn fixative could consistently preserve the complex structure of biofilms. EPS layers were clearly visible on the top, in the middle, and in the bottom of the biofilms with Methacarn fixative. Biomass and three-dimensional structure of the biofilms were further confirmed spectrophotometrically following crystal violet staining and by confocal microscopy after viability staining. These findings demonstrate that Methacarn fixative solution is superior to the other fixatives evaluated to preserve the complex architecture of biofilms grown on glass coverslips for SEM evaluation.

GtxA is a virulence factor that promotes a Th2-like response during Gallibacterium anatis infection in laying hens

GtxA, a leukotoxic RTX-toxin, has been proposed a main virulence factor of Gallibacterium anatis. To evaluate the impact of GtxA during infection, we experimentally infected laying hens with a G. anatis wild-type (WT) strain and its isogenic gtxA deletion mutant (ΔgtxA), respectively, and monitored the birds during a 6 day period. Birds inoculated with ΔgtxA had significantly reduced gross lesions and microscopic changes compared to the birds inoculated with the WT strain. To assess the host response further, we quantified the expression of pro-inflammatory cytokines and apoptosis genes by RT-qPCR. In the ovarian tissue, the expression levels of IL-4 and TNF-α were significantly lower in the ΔgtxA group compared to the WT group, while IL-6 and IL-10 levels appeared similar in the two groups. In the spleen tissue of ΔgtxA infected chickens, IL-4 expression was also lower compared to the WT infected chickens. The results indicated that GtxA plays a key role in an acute cytokine-mediated Th2-like response against G. anatis infection in the ovary tissue. The pro-inflammatory response in the ovary tissue of birds inoculated with ΔgtxA mutant was thus significantly lower than the wild-type response. This was, at least partly, supported by the apoptosis gene expression levels, which were significantly higher in the ΔgtxA mutant compared to the wild-type infected chickens. In conclusion, GtxA clearly plays an important role in the pathogenesis of G. anatis infections in laying hens. Further investigations into the specific factors regulating the host response is however needed to provide a more complete understanding of the bacteria-host interaction.

RTX Toxins of Animal Pathogens and Their Role as Antigens in Vaccines and Diagnostics

Exotoxins play a central role in the pathologies caused by most major bacterial animal pathogens. The large variety of vertebrate and invertebrate hosts in the animal kingdom is reflected by a large variety of bacterial pathogens and toxins. The group of repeats in the structural toxin (RTX) toxins is particularly abundant among bacterial pathogens of animals. Many of these toxins are described as hemolysins due to their capacity to lyse erythrocytes in vitro. Hemolysis by RTX toxins is due to the formation of cation-selective pores in the cell membrane and serves as an important marker for virulence in bacterial diagnostics. However, their physiologic relevant targets are leukocytes expressing β2 integrins, which act as specific receptors for RTX toxins. For various RTX toxins, the binding to the CD18 moiety of β₂ integrins has been shown to be host specific, reflecting the molecular basis of the host range of RTX toxins expressed by bacterial pathogens. Due to the key role of RTX toxins in the pathogenesis of many bacteria, antibodies directed against specific RTX toxins protect against disease, hence, making RTX toxins valuable targets in vaccine research and development. Due to their specificity, several structural genes encoding for RTX toxins have proven to be essential in modern diagnostic applications in veterinary medicine.

Mannheimia haemolytica in bovine respiratory disease: immunogens, potential immunogens, and vaccines

Mannheimia haemolytica is the major cause of severe pneumonia in bovine respiratory disease (BRD). Early M. haemolytica bacterins were either ineffective or even enhanced disease in vaccinated cattle, which led to studies of the bacterium's virulence factors and potential immunogens to determine ways to improve vaccines. Studies have focused on the capsule, lipopolysaccharide, various adhesins, extracellular enzymes, outer membrane proteins, and leukotoxin (LKT) resulting in a strong database for understanding immune responses to the bacterium and production of more efficacious vaccines. The importance of immunity to LKT and to surface antigens in stimulating immunity led to studies of individual native or recombinant antigens, bacterial extracts, live-attenuated or mutant organisms, culture supernatants, combined bacterin-toxoids, outer membrane vesicles, and bacterial ghosts. Efficacy of several of these potential vaccines can be shown following experimental M. haemolytica challenge; however, efficacy in field trials is harder to determine due to the complexity of factors and etiologic agents involved in naturally occurring BRD. Studies of potential vaccines have led current commercial vaccines, which are composed primarily of culture supernatant, bacterin-toxoid, or live mutant bacteria. Several of those can be augmented experimentally by addition of recombinant LKT or outer membrane proteins.

Pathogenesis of co-infections of influenza D virus and Mannheimia haemolytica in cattle

Bovine respiratory disease (BRD) is economically significant, and influenza D virus (IDV) is commonly identified in cattle with BRD. Mannheimia haemolytica (MHA) is an opportunistic bacterial contributor to BRD; surveillance data suggest that MHA and IDV co-infection occurs in cattle. The objective of this study was to evaluate the synergistic pathogenesis in cattle co-infected with IDV and MHA. Sixteen dairy calves were randomly assigned to four groups of four calves. The IDV + MHA + group received D/bovine/C00046 N/Mississippi/2014 (D/46 N) intranasally at 0 days post-inoculation (DPI) and Mannheimia haemolytica D153 (MHA D153) intratracheally at 5 DPI. The IDV + MHA- group received only D/46 N at 0 DPI; the IDV-MHA + group received only MHA D153 at 5 DPI; and the IDV-MHA- group received neither agent. Clinical scores were calculated twice daily. At 10 DPI, IDV + MHA+, IDV-MHA+, and IDV-MHA- calves were euthanized and evaluated for pathologic lesions. The IDV + groups seroconverted to IDV by 10 DPI. Clinical scores were higher in IDV + groups than IDV- groups on 2-5 DPI (p =  0.001). After MHA challenge on 5 DPI, clinical scores (6-10 DPI) were slightly lower in IDV+MHA+ group than IDV-MHA+ group (p <  0.05) but not significantly different between MHA+ groups and MHA- groups. The average gross pathology score was higher for IDV-MHA+ group than groups IDV-MHA- and IDV+MHA+; however, no significant differences were identified among groups. Under the conditions of this study, infection with IDV before MHA enhance neither clinical disease nor lung pathology, relative to calves infected with MHA alone.

A bovine CD18 signal peptide variant with increased binding activity to Mannheimia hemolytica leukotoxin

Background: Mannheimia haemolytica is the major bacterial infectious agent of bovine respiratory disease complex and causes severe morbidity and mortality during lung infections. M. haemolytica secretes a protein leukotoxin (Lkt) that binds to the CD18 receptor on leukocytes, initiates lysis, induces inflammation, and causes acute fibrinous bronchopneumonia. Lkt binds the 22-amino acid CD18 signal peptide domain, which remains uncleaved in ruminant species. Our aim was to identify missense variation in the bovine CD18 signal peptide and measure the effects on Lkt binding.

Methods: Missense variants in the integrin beta 2 gene ( ITGB2) encoding CD18 were identified by whole genome sequencing of 96 cattle from 19 breeds, and targeted Sanger sequencing of 1238 cattle from 46 breeds. The ability of different CD18 signal peptide variants to bind Lkt was evaluated by preincubating the toxin with synthetic peptides and applying the mixture to susceptible bovine cell cultures in cytotoxicity-blocking assays.

Results: We identified 14 missense variants encoded on 15 predicted haplotypes, including a rare signal peptide variant with a cysteine at position 5 (C ₅) instead of arginine (R ₅). Preincubating Lkt with synthetic signal peptides with C ₅ blocked cytotoxicity significantly better than those with R ₅. The most potent synthetic peptide (C ₅PQLLLLAGLLA) had 30-fold more binding activity compared to that with R ₅.

Conclusions: The results suggest that missense variants in the CD18 signal peptide affect Lkt binding, and animals carrying the C ₅ allele may be more susceptible to the effects of Lkt. The results also identify a potent class of non-antibiotic Lkt inhibitors that could potentially protect cattle from cytotoxic effects during acute lung infections.

β-Hemolysis May Not Be a Reliable Indicator of Leukotoxicity of Mannheimia haemolytica Isolates

Mannheimia (Pasteurella) haemolytica causes bronchopneumonia in domestic and wild ruminants. Leukotoxin is the critical virulence factor of M. haemolytica. Since &beta;-hemolysis is caused by a large number of leukotoxin-positive M. haemolytica isolates, all &beta;-hemolytic M. haemolytica isolates are considered to be leukotoxic as well. However, conflicting reports exist in literature as to the leukotoxic and hemolytic properties of M. haemolytica. One group of researchers reported their leukotoxin-deletion mutants to be hemolytic while another reported their mutants to be non-hemolytic. The objective of this study was to determine whether &beta;-hemolysis is a reliable indicator of leukotoxicity of M. haemolytica isolates. Ninety-five isolates of M. haemolytica were first confirmed for presence of leukotoxin gene (lktA) by a leukotoxin-specific PCR assay. Culture supernatant fluids from these isolates were then tested for presence of leukotoxin protein by an ELISA, and for leukotoxic activity by a cytotoxicity assay. All isolates were tested for &beta;-hemolysis by culture on blood agar plates. Sixty-two isolates (65%) produced leukotoxin protein while 33 isolates (35%) did not. Surprisingly, 18 of the 33 isolates (55%), that did not produce leukotoxin protein, were hemolytic. Of the 62 isolates that produced leukotoxin, 55 (89%) were leukotoxic while 7 (11%) were not. All except one of the 55 leukotoxic isolates (98%) were also hemolytic. All seven isolates that were not leukotoxic were hemolytic. Taken together, these results suggest that &beta;-hemolysis may not be a reliable indicator of leukotoxicity of M. haemolytica isolates. Furthermore, all M. haemolytica isolates that possess lktA gene may not secrete active leukotoxin.

Differential Susceptibility of Bighorn Sheep (Ovis canadensis) and Domestic Sheep (Ovis aries) Neutrophils to Mannheimia haemolytica Leukotoxin is not due to Differential Expression of Cell Surface CD18

Bighornsheep ( Ovis canadensis ) are more susceptible to pneumonia caused by Mannheimia haemolytica than are domestic sheep ( Ovis aries ). Leukotoxin produced by M. haemolytica is the principal virulence factor involved in pneumonia pathogenesis. Although leukotoxin is cytolytic to all subsets of ruminant leukocytes, neutrophils are the most susceptible subset. Bighorn sheep neutrophils are four- to eightfold more susceptible to leukotoxin-induced cytolysis than are domestic sheep neutrophils. We hypothesized that the higher susceptibility of bighorn sheep neutrophils, in comparison to domestic sheep neutrophils, is due to higher expression of CD18, the receptor for leukotoxin on leukocytes. Our objective was to quantify CD18 expression on neutrophils of bighorn sheep and domestic sheep. Cell-surface CD18 expression on bighorn sheep and domestic sheep neutrophils was measured as antibody binding capacity of cells by flow cytometric analysis with two fluorochrome-conjugated anti-CD18 monoclonal antibodies (BAQ30A and HUH82A) and microspheres. Contrary to our expectations, CD18 expression was higher (P<0.0001) with monoclonal antibody BAQ30A and was higher (P<0.0002) as well with monoclonal antibody HUH80A on domestic sheep neutrophils in comparison to bighorn sheep neutrophils. These findings suggest that the higher in vitro susceptibility to leukotoxin of bighorn sheep neutrophils compared to domestic sheep neutrophils is not due to higher expression of the leukotoxin receptor CD18 on bighorn sheep neutrophils.

Precise gene editing paves the way for derivation of Mannheimia haemolytica leukotoxin-resistant cattle

Signal peptides of membrane proteins are cleaved by signal peptidase once the nascent proteins reach the endoplasmic reticulum. Previously, we reported that, contrary to the paradigm, the signal peptide of ruminant CD18, the β subunit of β₂ integrins, is not cleaved and hence remains intact on mature CD18 molecules expressed on the surface of ruminant leukocytes. Leukotoxin secreted by Mannheimia (Pasteurella) haemolytica binds to the intact signal peptide and causes cytolysis of ruminant leukocytes, resulting in acute inflammation and lung tissue damage. We also demonstrated that site-directed mutagenesis leading to substitution of cleavage-inhibiting glutamine (Q), at amino acid position 5 upstream of the signal peptide cleavage site, with cleavage-inducing glycine (G) results in the cleavage of the signal peptide and abrogation of leukotoxin-induced cytolysis of target cells. In this proof-of-principle study, we used precise gene editing to induce Q(‒5)G substitution in both alleles of CD18 in bovine fetal fibroblast cells. The gene-edited fibroblasts were used for somatic nuclear transfer and cloning to produce a bovine fetus homozygous for the Q(‒5)G substitution. The leukocyte population of this engineered ruminant expressed CD18 without the signal peptide. More importantly, these leukocytes were absolutely resistant to leukotoxin-induced cytolysis. This report demonstrates the feasibility of developing lines of cattle genetically resistant to M. haemolytica-caused pneumonia, which inflicts an economic loss of over $1 billion to the US cattle industry alone.

Acylation Enhances, but Is Not Required for, the Cytotoxic Activity of Mannheimia haemolytica Leukotoxin in Bighorn Sheep

Mannheimia haemolytica causes pneumonia in domestic and wild ruminants. Leukotoxin (Lkt) is the most important virulence factor of the bacterium. It is encoded within the four-gene lktCABD operon: lktA encodes the structural protoxin, and lktC encodes a trans-acylase that adds fatty acid chains to internal lysine residues in the protoxin, which is then secreted from the cell by a type 1 secretion system apparatus encoded by lktB and lktD. It has been reported that LktC-mediated acylation is necessary for the biological effects of the toxin. However, an LktC mutant that we developed previously was only partially attenuated in its virulence for cattle. The objective of this study was to elucidate the role of LktC-mediated acylation in Lkt-induced cytotoxicity. We performed this study in bighorn sheep (Ovis canadensis) (BHS), since they are highly susceptible to M. haemolytica infection. The LktC mutant caused fatal pneumonia in 40% of inoculated BHS. On necropsy, a large number of necrotic polymorphonuclear leukocytes (PMNs) were observed in the lungs. Lkt from the mutant was cytotoxic to BHS PMNs in an in vitro cytotoxicity assay. Flow cytometric analysis of mutant Lkt-treated PMNs revealed the induction of necrosis. Scanning electron microscopic analysis revealed the presence of pores and blebs on mutant-Lkt-treated PMNs. Mass spectrometric analysis confirmed that the mutant secreted an unacylated Lkt. Taken together, these results suggest that acylation is not necessary for the cytotoxic activity of M. haemolytica Lkt but that it enhances the potency of the toxin.

Bivalent vaccination against pneumonic pasteurellosis in domestic sheep and goats with modified-live in-frame lktA deletion mutants of Mannheimia haemolytica

A temperature-sensitive shuttle vector, pBB80C, was utilized to generate in-frame deletion mutants of the leukotoxin structural gene (lktA) of Mannheimia haemolytica serotypes 1, 2, 5, 6, 7, 8, 9, and 12. Culture supernatants from the mutants contained a truncated protein with an approximate molecular weight of 66 kDa which was reactive to anti-leukotoxin monoclonal antibody. No protein reactive to anti-LktA monoclonal antibody was detected at the molecular weight 100-105 kDa of native LktA. Sheep and goats vaccinated intramuscularly with a mixture of serotypes 5 and 6 mutants were resistant to virulent challenge with a mixture of the wild-type parent strains. These vaccinates responded serologically to both vaccine serotypes and exhibited markedly-reduced lung lesion volume and pulmonary infectious load compared to control animals. Control animals yielded a mixture of serotypes from lung lobes, but the proportion even within an individual animal varied widely from 95% serotype 5-95% serotype 6. Cultures recovered from liver were homogeneous, but two animals yielded serotype 5 and the other two yielded serotype 6 in pure culture.

Proximity-dependent inhibition of growth of Mannheimia haemolytica by Pasteurella multocida

Mannheimia haemolytica, Pasteurella multocida, and Bibersteinia trehalosi have been identified in the lungs of pneumonic bighorn sheep (BHS; Ovis canadensis). Of these pathogens, M. haemolytica has been shown to consistently cause fatal pneumonia in BHS under experimental conditions. However, M. haemolytica has been isolated by culture less frequently than the other bacteria. We hypothesized that the growth of M. haemolytica is inhibited by other bacteria in the lungs of BHS. The objective of this study was to determine whether P. multocida inhibits the growth of M. haemolytica. Although in monoculture both bacteria exhibited similar growth characteristics, in coculture with P. multocida there was a clear inhibition of growth of M. haemolytica. The inhibition was detected at mid-log phase and continued through the stationary phase. When cultured in the same medium, the growth of M. haemolytica was inhibited when both bacteria were separated by a membrane that allowed contact (pore size, 8.0 μm) but not when they were separated by a membrane that limited contact (pore size, 0.4 μm). Lytic bacteriophages or bactericidal compounds could not be detected in the culture supernatant fluid from monocultures of P. multocida or from P. multocida-M. haemolytica cocultures. These results indicate that P. multocida inhibits the growth of M. haemolytica by a contact- or proximity-dependent mechanism. If the inhibition of growth of M. haemolytica by P. multocida occurs in vivo as well, it could explain the inconsistent isolation of M. haemolytica from the lungs of pneumonic BHS.

MHC class II DR allelic diversity in bighorn sheep

We hypothesized that decreased diversity and/or unique polymorphisms in MHC class II alleles of bighorn sheep (BHS, Ovis canadensis) are responsible for lower titer of antibodies against Mannheimia haemolytica leukotoxin, in comparison to domestic sheep (DS, Ovis aries). To test this hypothesis, DRA and DRB transcripts from 24 captive BHS (Ovca-DRA and Ovca-DRB) were sequenced. Based on exon 2 (β1 domain) sequences, eight different Ovca-DRB cDNA sequences were identified in BHS. Six of them were 100% identical to previously reported Ovca-DRB genomic DNA sequences. The new alleles DRB*23 and DRB*24, were closely related to two other Ovca-DRB exon 2 genomic DNA sequences. Nineteen out of 24 BHS (79%) Ovca-DRB exon 3 (β2 domain) sequences were 100% identical to exon 3 sequence of DRB1 of DS (Ovar-DRB1). Ovca-DRA full length cDNA sequences exhibited >99% identity. Based upon exon 2 sequences, this BHS herd yielded higher Ovca-DRB allelic diversity than that reported in the previous study. Positively selected amino acid positions were identified in the peptide-binding groove of BHS and DS, but BHS showed more such sites. This highlights differing population histories, and may suggest differing needs for DR peptide-binding specificities. Presence of glutamine at position 52 (52Q) in some of the desert and captive BHS is predicted to alter the efficiency of DR dimerization, which may influence antigen presentation and T(h) cell activation. Functional assays with unique alleles should reveal whether the presentation of M. haemolytica leukotoxin peptides to T(h) cells by Ovca-DRB alleles is equivalent to that of Ovar-DRB1 alleles.

Mucosal and parenteral vaccination against pneumonic pasteurellosis in cattle with a modified-live in-frame lktA deletion mutant of Mannheimia haemolytica

A new temperature-conditional shuttle vector, pBB80C, was constructed and utilized to generate an in-frame deletion in the leukotoxin structural gene of Mannheimia haemolytica serotype 1. Culture supernatants from the mutant contained no detectable cytotoxicity to BL-3 lymphocyte targets, and contained a new protein with an approximate molecular weight of 66 kDa which was reactive to anti-leukotoxin monoclonal antibody. No protein reactive to anti-LktA monoclonal antibody was detected at the molecular weight 100-105 kDa of native LktA. Calves vaccinated mucosally by top-dressing the live mutant onto feed, or parenterally by subcutaneous injection, were resistant to virulent challenge with the parent strain. Serologic antibody response, reduction in lung lesion, and reduction in pulmonary infectious load were greater among calves mucosally vaccinated than those which were vaccinated by injection.

A multivalent Mannheimia-Bibersteinia vaccine protects bighorn sheep against Mannheimia haemolytica challenge

Bighorn sheep (BHS) are more susceptible than domestic sheep (DS) to Mannheimia haemolytica pneumonia. Although both species carry M. haemolytica as a commensal bacterium in the nasopharynx, DS carry mostly leukotoxin (Lkt)-positive strains while BHS carry Lkt-negative strains. Consequently, antibodies to surface antigens and Lkt are present at much higher titers in DS than in BHS. The objective of this study was to determine whether repeated immunization of BHS with multivalent Mannheimia-Bibersteinia vaccine will protect them upon M. haemolytica challenge. Four BHS were vaccinated with a culture supernatant vaccine prepared from M. haemolytica serotypes A1 and A2 and Bibersteinia trehalosi serotype T10 on days 0, 21, 35, 49, and 77. Four other BHS were used as nonvaccinated controls. On the day of challenge, 12 days after the last immunization, the mean serum titers of Lkt-neutralizing antibodies and antibodies to surface antigens against M. haemolytica were 1:160 and 1:4,000, respectively. Following intranasal challenge with M. haemolytica A2 (1 × 10(5) CFU), all four control BHS died within 48 h. Necropsy revealed acute fibrinonecrotic pneumonia characteristic of M. haemolytica infection. None of the vaccinated BHS died during the 8 weeks postchallenge observation period. Radiography at 3 weeks postchallenge revealed no lung lesions in two vaccinated BHS and mild lesions in the other two, which resolved by 8 weeks postchallenge. These results indicate that if BHS can be induced to develop high titers of Lkt-neutralizing antibodies and antibodies to surface antigens, they are likely to survive M. haemolytica challenge which is likely to reduce the BHS population decline due to pneumonia.

Cytokine expression by pulmonary leukocytes from calves challenged with wild-type and leukotoxin-deficient Mannheimia haemolytica

The objective of this study was to assess the role of leukotoxin (LKT) in modulating the pulmonary cytokine response of calves challenged with Mannheimia haemolytica. Thirty-six calves, seronegative to LKT and M. haemolytica whole cell antigen were divided into three groups (I, II and III). Calves in groups I and II were challenged by the intra-bronchial route with 25 mL of phosphate buffered saline (PBS) containing 0.44×10(9) cfu/mL of LKT deficient (lkt(-)) and 25 mL of PBS containing 0.31×10(9) cfu/mL of wild-type (wt) M. haemolytica serotype 1, respectively. Group III calves were challenged intra-bronchially with 25 mL of sterile PBS. Leukocytes were collected from broncho-alveolar lavage fluid (BALF) 4 days before and at 1, 3, and 6 days post-inoculation (p.i.). Expression of the following cytokines in the recovered leukocytes was measured using real-time PCR: interleukin (IL)-1β, -8, -10, -12 (p40) and TNF-α. The amount of TNF-α produced was also quantified by ELISA. Although a statistically significant difference in the expression of these cytokines was not observed between groups challenged with the wt and lkt(-) strains, the wt infected group (II) did exhibit higher mean clinical scores. Overall, there was considerable variation in the composition of the BALF between the groups and by day 7 p.i., both lkt(-)- and wt-challenged calves had seroconverted to M. haemolytica.

The role of RTX toxins in host specificity of animal pathogenic Pasteurellaceae

RTX toxins are bacterial pore-forming toxins that are particularly abundant among pathogenic species of Pasteurellaceae, in which they play a major role in virulence. RTX toxins of several primary pathogens of the family of Pasteurellaceae are directly involved in causing necrotic lesions in the target organs. Many RTX toxins are known as haemolysins because they lyse erythrocytes in vitro, an effect that is non-specific, but which serves as a useful marker in bacteriological identification and as an easily measurable signal in vitro in experimental studies. More recent studies have shown that the specific targets of most RTX toxins are leukocytes, with RTX toxins binding to the corresponding β-subunit (CD18) of β2 integrins and then exerting cytotoxic activity. After uptake by the target cell, at sub-lytic concentrations, some RTX toxins are transported to mitochondria and induce apoptosis. For several RTX toxins the binding to CD18 has been shown to be host specific and this seems to be the basis for the host range specificity of these RTX toxins. Observations on two very closely related species of the Pasteurellaceae family, Actinobacillus suis, a porcine pathogen particularly affecting suckling pigs, and Actinobacillus equuli subsp. haemolytica, which causes pyosepticaemia in new-born foals (sleepy foal disease), have revealed that they express different RTX toxins, named ApxI/II and Aqx, respectively. These RTX toxins are specifically cytotoxic for porcine and equine leukocytes, respectively. Furthermore, the ApxI and Aqx toxins of these species, when expressed in an isogenetic background in Escherichia coli, are specifically cytotoxic for leukocytes of their respective hosts. These data indicate the determinative role of RTX toxins in host specificity of pathogenic species of Pasteurellaceae.

Mannheimia haemolytica: bacterial-host interactions in bovine pneumonia

Mannheimia haemolytica serotype S1 is considered the predominant cause of bovine pneumonic pasteurellosis, or shipping fever. Various virulence factors allow M haemolytica to colonize the lungs and establish infection. These virulence factors include leukotoxin (LKT), lipopolysaccharide, adhesins, capsule, outer membrane proteins, and various proteases. The effects of LKT are species specific for ruminants, which stem from its unique interaction with the bovine β2 integrin receptor present on leukocytes. At low concentration, LKT can activate bovine leukocytes to undergo respiratory burst and degranulation and stimulate cytokine release from macrophages and histamine release from mast cells. At higher concentration, LKT induces formation of transmembrane pores and subsequent oncotic cell necrosis. The interaction of LKT with leukocytes is followed by activation of these leukocytes to undergo oxidative burst and release proinflammatory cytokines such as interleukins 1, 6, and 8 and tumor necrosis factor α. Tumor necrosis factor α and other proinflammatory cytokines contribute to the accumulation of leukocytes in the lung. Formation of transmembrane pores and subsequent cytolysis of activated leukocytes possibly cause leakage of products of respiratory burst and other inflammatory mediators into the surrounding pulmonary parenchyma and so give rise to fibrinous and necrotizing lobar pneumonia. The effects of LKT are enhanced by lipopolysaccharide, which is associated with the release of proinflammatory cytokines from the leukocytes, activation of complement and coagulation cascade, and cell cytolysis. Similarly, adhesins, capsule, outer membrane proteins, and proteases assist in pulmonary colonization, evasion of immune response, and establishment of the infection. This review focuses on the roles of these virulence factors in the pathogenesis of shipping fever.

Cytotoxicity and cytokine production by bovine alveolar macrophages challenged with wild type and leukotoxin-deficient Mannheimia haemolytica

Leukotoxin (LKT) is a virulence factor for Mannheimia haemolytica. In this study, bovine alveolar macrophages (BAMs) were challenged with wild type (wt) and LKT deficient (lkt(-)) M. haemolytica at a concentration of 1 bacterium/BAM and the cytokine response was quantified by ELISA and real-time reverse transcriptase-PCR. Significant increases in protein concentrations of tumor necrosis factor (TNF)-α and interleukin (IL)-10 were observed in supernatants obtained from BAMs challenged with the lkt(-) strain of M. haemolytica compared with wt challenged BAMs. There were no significant differences in mRNA expression of TNFα, IL-1ß, IL-6, IL-8 or IL-10 between BAMs challenged with the lkt(-) strain of M. haemolytica compared with wt challenged BAMs. BAMs challenged with the wt strain exhibited, on average, 43% more cytotoxicity than lkt(-) challenged BAMs (P<0.01).

Expression of Mycoplasma bovis variable surface membrane proteins in the respiratory tract of calves after experimental infection with a clonal variant of Mycoplasma bovis type strain PG45

The pathomorphological findings and the expression and distribution of variable surface protein antigens (Vsp) of Mycoplasma (M.) bovis were characterised immunohistochemically in lungs of eight calves following inoculation with a Vsp A-expressing clonal variant of M. bovis type strain PG45. Within 48 h post inoculation (p.i.) an innate immune response dominated by macrophages and neutrophils develops. The monoclonal antibodies (mAbs) 1A1 and 1E5 detected M. bovis Vsp antigens in paraffin tissue sections of seven calves. Vsp antigens were widely distributed and were already present at day two p.i. within macrophages and other lung compartments. Taken together, the results demonstrate that the bovine is unable to eliminate M. bovis during the time period examined. Based on the different immunohistochemical labelling patterns obtained with the mAbs, the results also support the speculation that the in vivo variability of Vsps together with immunological factors may contribute to the chronicity of pulmonary disease.

Molecular cloning of CD18 of bison, deer and elk, and comparison with that of other ruminants and non-ruminants

Pneumonia caused by Mannheimia haemolytica is an important disease of cattle, domestic sheep, bighorn sheep and goats. Leukotoxin (Lkt) produced by M. haemolytica is cytolytic to all leukocyte subsets of these species. Lkt utilizes CD18, the beta subunit of beta(2)-integrins, as its functional receptor on leukocytes of these species. Cytotoxicity assays revealed that leukocytes from bison, deer, and elk are also susceptible to Lkt-induced cytolysis. The availability of cDNA encoding CD18 of bison, deer and elk would facilitate the comparison of a greater number of ruminant CD18 cDNA with that of non-ruminants as a means of the elucidation of the molecular basis for the specificity of M. haemolytica Lkt for ruminant leukocytes. Herein, we report the cloning and characterization of bison, deer, and elk CD18. The full-length cDNA of bison and deer consists of 2310bp with an ORF encoding 769 amino acids while elk CD18 consists of 2313bp with an ORF encoding 770 amino acids. This gene is highly conserved among ruminants compared with non-ruminants. Phylogenetic analysis based on amino acid sequences showed that CD18 of bison is most closely related to that of cattle while CD18 of deer and elk are more closely related to each other.

Bibersteinia trehalosi inhibits the growth of Mannheimia haemolytica by a proximity-dependent mechanism

Mannheimia (Pasteurella) haemolytica is the only pathogen that consistently causes severe bronchopneumonia and rapid death of bighorn sheep (BHS; Ovis canadensis) under experimental conditions. Paradoxically, Bibersteinia (Pasteurella) trehalosi and Pasteurella multocida have been isolated from BHS pneumonic lungs much more frequently than M. haemolytica. These observations suggest that there may be an interaction between these bacteria, and we hypothesized that B. trehalosi overgrows or otherwise inhibits the growth of M. haemolytica. Growth curves (monoculture) demonstrated that B. trehalosi has a shorter doubling time ( approximately 10 min versus approximately 27 min) and consistently achieves 3-log higher cell density (CFU/ml) compared to M. haemolytica. During coculture M. haemolytica growth was inhibited when B. trehalosi entered stationary phase (6 h) resulting in a final cell density for M. haemolytica that was 6 to 9 logs lower than expected with growth in the absence of B. trehalosi. Coculture supernatant failed to inhibit M. haemolytica growth on agar or in broth, indicating no obvious involvement of lytic phages, bacteriocins, or quorum-sensing systems. This observation was confirmed by limited growth inhibition of M. haemolytica when both pathogens were cultured in the same media but separated by a filter (0.4-microm pore size) that limited contact between the two bacterial populations. There was significant growth inhibition of M. haemolytica when the populations were separated by membranes with a pore size of 8 mum that allowed free contact. These observations demonstrate that B. trehalosi can both outgrow and inhibit M. haemolytica growth with the latter related to a proximity- or contact-dependent mechanism.

GtxA from Gallibacterium anatis, a cytolytic RTX-toxin with a novel domain organisation

Gallibacterium anatis is a pathogen in chickens and other avian species where it is a significant cause of salpingitis and peritonitis. We found that bacterial cells and cell-free, filter-sterilised culture supernatant from the haemolytic G. anatis biovar haemolytica were highly cytotoxic towards avian-derived macrophage-like cells (HD11). We obtained the genome sequence of G. anatis 12656-12 and used a rational approach to identify a gene predicted to encode a 2026 amino acid RTX-toxin, which we named GtxA (Gallibacterium toxin). The construction of a gtxA knock-out mutant showed gtxA to be responsible for G. anatis’ haemolytic and leukotoxic activity. In addition, Escherichia coli expressing gtxA and an adjacent acyltransferase, gtxC, became cytolytic. GtxA was expressed during in vitro growth and was localised in the extracellular protein fraction in a growth phase dependent manner. GtxA had an unusual modular structure; the C-terminal 1000 amino acids of GtxA were homologous to the classical pore-forming RTX-toxins in other members of Pasteurellaceae. In contrast, the N-terminal approximately 950 amino acids had few significant matches in sequence databases. Expression of truncated GtxA proteins demonstrated that the C-terminal RTX-domain had a lower haemolytic activity than the full-length toxin, indicating that the N-terminal domain was required for maximal haemolytic activity. Cytotoxicity towards HD11 cells was not detected with the C-terminal alone, suggesting that the N-terminal domain plays a critical role for the leukotoxicity.

Intact signal peptide of CD18, the beta-subunit of beta2-integrins, renders ruminants susceptible to Mannheimia haemolytica leukotoxin

Signal peptides of membrane proteins are cleaved by endoplasmic reticulum-resident signal peptidase, and thus, are not present on mature membrane proteins. Here, we report that, contrary to the paradigm, the signal peptide of ruminant CD18, the beta-subunit of beta(2)-integrins, is not cleaved. Intriguingly, the intact signal peptide of CD18 is responsible for the susceptibility of ruminant leukocytes to Mannheimia (Pasteurella) haemolytica leukotoxin (Lkt). Inhibition of Lkt-induced cytolysis of ruminant leukocytes by CD18 peptide analogs revealed that the Lkt-binding site is formed by amino acids 5-17 of CD18, which, surprisingly, comprise most of the signal sequence. Flow cytometric analysis of ruminant leukocytes indicated the presence of the signal peptide on mature CD18 molecules expressed on the cell surface. Analysis of transfectants expressing CD18 containing the FLAG epitope at the putative cleavage site confirmed that the signal peptide of bovine CD18 is not cleaved. Analysis of the signal sequence of CD18 of eight ruminants and five nonruminants revealed that the signal sequence of CD18 of ruminants contains "cleavage-inhibiting" Q, whereas that of nonruminants contains "cleavage-conducive" G at position -5 relative to the cleavage site. Site-directed mutagenesis of Q to G at position -5 of the signal peptide of bovine CD18 resulted in the cleavage of the signal peptide and abrogation of cytolysis of transfectants expressing bovine CD18 carrying the Q(-5)G mutation. We propose that engineering cattle and other ruminants to contain this mutation would provide a novel technology to render them less susceptible to pneumonic pasteurellosis and concomitant economic losses.

Bovine herpesvirus type 1 infection of bovine bronchial epithelial cells increases neutrophil adhesion and activation

Respiratory infection of cattle with bovine herpesvirus type 1 (BHV-1) predisposes cattle to secondary pneumonia with Mannheimia haemolytica as part of the bovine respiratory disease complex (BRD). One cell type that has received limited investigation for its role in the inflammation that accompanies BRD is the respiratory epithelial cell. In the present study we investigated mechanisms by which BHV-1 infection of respiratory epithelial cells contributes to the recruitment and activation of bovine polymorphonuclear neutrophils (PMNs) in vitro. Primary cultures of bovine bronchial epithelial (BBE) cells were infected with BHV-1 and assessed for cytokine expression by real-time PCR. We found that BHV-1 infection elicits a rapid IL-1, IL-8 and TNF-alpha mRNA response by BBE cells. Bovine PMNs exhibited greater adherence to BHV-1 infected BBE cells than uninfected cells. The increased adherence was significantly reduced by the addition of an anti-IL-1beta antibody or human soluble TNF-alpha receptor (sTNF-alphaR). Pre-incubation of bovine PMNs with conditioned media from BHV-1 infected BBE cells increased PMN migration, which was inhibited by addition of an anti-IL-1beta antibody, sTNF-alphaR, or an IL-8 peptide inhibitor. Conditioned media from BHV-1 infected BBE cells activated bovine PMNs in vitro as demonstrated by PMN shape change, production of reactive oxygen species and degranulation. PMNs also exhibited increased LFA-1 expression and susceptibility to M. haemolytica LKT following incubation with BHV-1 infected BBE cell conditioned media. Our results suggest that BHV-1 infection of BBE cells triggers cytokine expression that contributes to the recruitment and activation of neutrophils, and amplifies the detrimental effects of M. haemolytica LKT.

Mannheimia haemolytica and bovine respiratory disease

Mannheimia haemolytica is the principal bacterium isolated from respiratory disease in feedlot cattle and is a significant component of enzootic pneumonia in all neonatal calves. A commensal of the nasopharynx, M. haemolytica is an opportunist, gaining access to the lungs when host defenses are compromised by stress or infection with respiratory viruses or mycoplasma. Although several serotypes act as commensals, A1 and A6 are the most frequent isolates from pneumonic lungs. Potential virulence factors include adhesin, capsular polysaccharide, fimbriae, iron-regulated outer membrane proteins, leukotoxin (Lkt), lipopolysaccharide (LPS), lipoproteins, neuraminidase, sialoglycoprotease and transferrin-binding proteins. Of these, Lkt is pivotal in induction of pneumonia. Lkt-mediated infiltration and destruction of neutrophils and other leukocytes impairs bacterial clearance and contributes to development of fibrinous pneumonia. LPS may act synergistically with Lkt, enhancing its effects and contributing endotoxic activity. Antibiotics are employed extensively in the feedlot industry, both prophylactically and therapeutically, but their efficacy varies because of inconsistencies in diagnosis and treatment regimes and development of antibiotic resistance. Vaccines have been used for many decades, even though traditional bacterins failed to demonstrate protection and their use often enhanced disease in vaccinated animals. Modern vaccines use culture supernatants containing Lkt and other soluble antigens, or bacterial extracts, alone or combined with bacterins. These vaccines have 50-70% efficacy in prevention of M. haemolytica pneumonia. Effective control of M. haemolytica pneumonia is likely to require a combination of more definitive diagnosis, efficacious vaccines, therapeutic intervention and improved management practices.

Immune evasion by pathogens of bovine respiratory disease complex

Bovine respiratory tract disease is a multi-factorial disease complex involving several viruses and bacteria. Viruses that play prominent roles in causing the bovine respiratory disease complex include bovine herpesvirus-1, bovine respiratory syncytial virus, bovine viral diarrhea virus and parinfluenza-3 virus. Bacteria that play prominent roles in this disease complex are Mannheimia haemolytica and Mycoplasma bovis. Other bacteria that infect the bovine respiratory tract of cattle are Histophilus (Haemophilus) somni and Pasteurella multocida. Frequently, severe respiratory tract disease in cattle is associated with concurrent infections of these pathogens. Like other pathogens, the viral and bacterial pathogens of this disease complex have co-evolved with their hosts over millions of years. As much as the hosts have diversified and fine-tuned the components of their immune system, the pathogens have also evolved diverse and sophisticated strategies to evade the host immune responses. These pathogens have developed intricate mechanisms to thwart both the innate and adaptive arms of the immune responses of their hosts. This review presents an overview of the strategies by which the pathogens suppress host immune responses, as well as the strategies by which the pathogens modify themselves or their locations in the host to evade host immune responses. These immune evasion strategies likely contribute to the failure of currently-available vaccines to provide complete protection to cattle against these pathogens.

Analysis of gene order data supports vertical inheritance of the leukotoxin operon and genome rearrangements in the 5' flanking region in genus Mannheimia

Background

The Mannheimia subclades belong to the same bacterial genus, but have taken divergent paths toward their distinct lifestyles. For example, M. haemolytica + M. glucosida are potential pathogens of the respiratory tract in the mammalian suborder Ruminantia, whereas M. ruminalis, the supposed sister group, lives as a commensal in the ovine rumen. We have tested the hypothesis that vertical inheritance of the leukotoxin (lktCABD) operon has occurred from the last common ancestor of genus Mannheimia to any ancestor of the diverging subclades by exploring gene order data.

Results

We examined the gene order in the 5' flanking region of the leukotoxin operon and found that the 5' flanking gene strings, hslVU-lapB-artJ-lktC and xylAB-lktC, are peculiar to M. haemolytica + M. glucosida and M. granulomatis, respectively, whereas the gene string hslVU-lapB-lktC is present in M. ruminalis, the supposed sister group of M. haemolytica + M. glucosida, and in the most ancient subclade M. varigena. In M. granulomatis, we found remnants of the gene string hslVU-lapB-lktC in the xylB-lktC intergenic region.

Conclusion

These observations indicate that the gene string hslVU-lapB-lktC is more ancient than the hslVU-lapB-artJ-lktC and xylAB-lktC gene strings. The presence of (remnants of) the ancient gene string hslVU-lapB-lktC among any subclades within genus Mannheimia supports that it has been vertically inherited from the last common ancestor of genus Mannheimia to any ancestor of the diverging subclades, thus reaffirming the hypothesis of vertical inheritance of the leukotoxin operon. The presence of individual 5' flanking regions in M. haemolytica + M. glucosida and M. granulomatis reflects later genome rearrangements within each subclade. The evolution of the novel 5' flanking region in M. haemolytica + M. glucosida resulted in transcriptional coupling between the divergently arranged artJ and lkt promoters. We propose that the chimeric promoter have led to high level expression of the leukotoxin operon which could explain the increased potential of certain M. haemolytica + M. glucosida strains to cause a particular type of infection.

Monomeric expression of bovine beta2-integrin subunits reveals their role in Mannheimia haemolytica leukotoxin-induced biological effects

The ruminant-specific leukotoxin (Lkt) of Mannheimia haemolytica is the key virulence factor contributing to the pathogenesis of lung injury in bovine pneumonic pasteurellosis. Previous studies by us and others indicate that M. haemolytica Lkt binds to CD18, the beta subunit of bovine beta(2)-integrins on leukocytes, and that the species specificity of Lkt-induced effects is resident in the beta subunit CD18 and not in the alpha subunit CD11. However, Lkt also binds to the CD11a subunit of LFA-1. Furthermore, antibodies specific for CD18 or CD11a inhibit signaling events leading to elevation of intracellular [Ca(2+)], tyrosine phosphorylation of the cytosolic domain of CD18, and cytolysis of bovine leukocytes. These observations underscore the need for further investigation to identify the precise subunit of bovine LFA-1 utilized by M. haemolytica Lkt as the functional receptor. For this purpose, monomeric bovine CD18 and CD11a and heterodimeric LFA-1 were expressed in the HEK-293 cell line by transfection, and the resulting transfectants were tested for susceptibility to Lkt-induced effects. All three transfectants effectively bound Lkt. However, Lkt-induced cytolysis was observed only with transfectants expressing monomeric bovine CD18 or LFA-1. Furthermore, intracellular [Ca(2+)] elevation following exposure to Lkt, which is a marker for postbinding signaling leading to cellular activation, was seen only with transfectants expressing monomeric bovine CD18 or LFA-1. These results clearly indicate that the bovine CD18 subunit of beta(2)-integrins is the functional receptor for M. haemolytica Lkt.

Evidence for Vertical Inheritance and Loss of the Leukotoxin Operon in Genus Mannheimia

The Mannheimia subclades belong to the same bacterial genus but have taken divergent paths toward their distinct lifestyles. M. haemolytica + M. glucosida are potential pathogens of the respiratory tract in the mammalian suborder Ruminantia, whereas M. ruminalis, the supposed sister group, lives as a commensal in the ovine rumen. We have tested the hypothesis that horizontal gene transfer of the leukotoxin operon has catalyzed pathogenic adaptation and speciation of M. haemolytica + M. glucosida, or other major subclades, by using a strategy that combines compositional and phylogenetic methods. We show that it has been vertically inherited from the last common ancestor of the diverging Mannheimia subclades, although several strains belonging to M. ruminalis have lost the operon. Our analyses support that divergence within M. ruminalis following colonization of the ovine rumen was very rapid and that functional decay of most of the leukotoxin operons occurred early when the adaptation to the rumen was fastest, suggesting that antagonistic pleiotropy was the main contributor to losses in the radiating lineages of M. ruminalis. To sum up, the scenario derived from these analyses reflects two aspects. On one hand, it opposes the hypothesis of horizontal gene transfer as a catalyst of pathogenic adaptation and speciation. On the other hand, it indicates that losses of the leukotoxin operons in the radiating lineages of M. ruminalis have catalyzed their adaptation to a commensal environment and reproductive isolation (speciation).

The response of Mannheimia haemolytica to iron limitation: implications for the acquisition of iron in the bovine lung

Mannheimia haemolytica is the major causative agent of shipping fever, a severe pneumonia in cattle causing high morbidity and mortality. A prerequisite of successful lung colonization by M. haemolytica is the necessity to adapt to the paucity of iron. The lack of genome information has precluded an assessment of the genetic repertoire available to M. haemolytica to adapt to low iron environments. To close this knowledge-gap, we have determined 90% of a virulent M. haemolytica serotype A1 genome sequence and produced a microarray in order to study gene expression under iron-limiting growth for 15, 30 and 60 min. M. haemolytica responded to iron limitation by the up-regulation of transcripts coding for receptors and ABC-type transporters of transferrin, haemoglobin, haem and siderophores. Real time PCR analysis of lung tissue from Mannheimia-infected calves demonstrated the in vivo transcription of two potential haemoglobin receptors, hmbR1 and hmbR2. The relative hmbR1 and hmbR2 transcript levels in the infected lung tissue were comparable to the induced levels observed under iron-limiting growth, demonstrating in vivo induction of receptor transcription in the context of an infection. When the iron response of M. haemolytica was compared to the iron response of Pasteurella multocida, another pathogen colonizing the bovine lung, only few homologous genes were induced in both organisms. These included the haemoglobin receptor hmbR2 and the periplasmic transport systems yfeABCD and fbpABC. The comparative analysis suggests that the two pathogens use different strategies to adapt to the iron-limiting environment in the bovine host.

Identification of an RTX determinant of Burkholderia cenocepacia J2315 by subtractive hybridization

This study utilized suppressive subtractive hybridization between the clinical isolate Burkholderia cenocepacia J2315 and the closely related environmental isolate Burkholderia cepacia ATCC 25416T to isolate DNA fragments specific to B. cenocepacia J2315. Analysis of the resulting pools of B. cenocepacia-specific DNAs identified several fragments that may be part of putative virulence factors. Further in silico analysis of a single fragment indicated that it was internal to a gene of which the predicted product had characteristics of repeat in toxin (RTX)-like proteins and high similarity to proteins in other human or plant pathogens. In conjunction with this finding, phenotypic traits associated with known RTX proteins were assessed. A haemagglutinating activity of B. cenocepacia J2315 was identified that was absent in B. cepacia ATCC 25416T. The expression of this activity appeared to be growth phase-dependent. Analysis of the gene presence and haemagglutinating activity across the species of the B. cepacia complex showed that both were common to the ET12 lineage of B. cenocepacia, but were absent in the other species examined. Haemagglutinating activity was limited to isolates with the RTX-like gene. Expression studies utilizing quantitative PCR demonstrated an association between onset of haemagglutinating activity and increased expression of the gene, which suggests that the putative RTX determinant encodes a haemagglutinating activity.

Incubation of bovine PMNs with conditioned medium from BHV-1 infected peripheral blood mononuclear cells increases their susceptibility to Mannheimia haemolytica leukotoxin

Active infection with bovine herpesvirus-1 (BHV-1) increases the susceptibility of cattle to secondary bacterial pneumonia with Mannheimia (Pasteurella) haemolytica A1. In the present study we found that bovine PMNs incubated with conditioned media from BHV-1 infected peripheral blood mononuclear cells (PBMCs) exhibited increased LFA-1 expression, enhanced LKT binding and increased LKT cytotoxicity. These effects were abrogated when the conditioned medium was pre-incubated with an anti-IL-1beta Mab before being added to the PMNs. These findings suggest that BHV-1 infection, and the resulting release of IL-1beta and perhaps other inflammatory cytokines, can stimulate activation of LFA-1 in bystander bovine PMNs, thus enhancing the binding and biological effects of LKT.

Potential involvement of gelatinases and their inhibitors in Mannheimia haemolytica pneumonia in cattle

Mannheimia haemolytica infection of the lower respiratory tract of cattle results in a bronchofibrinous pneumonia characterized by massive cellular influx and lung tissue remodeling and scarring. Since altered levels of gelatinases and their inhibitors have been detected in a variety of inflammatory conditions and are associated with tissue remodeling, we examined the presence of gelatinases in lesional and nonlesional lung tissue obtained from calves experimentally infected with M. haemolytica. Lesional tissue had elevated levels of progelatinase A and B and active gelatinase A and B when compared with nonlesional tissue obtained from the same lung lobe. In vitro, M. haemolytica products stimulated production of gelatinase B, but not its activation, by bovine monocytes. Alveolar macrophages showed constitutive production of gelatinase B but no change in response to M. haemolytica products. Bovine neutrophils exposed to M. haemolytica products also released gelatinase B, and there was a significant increase in the activated form of this enzyme. These effects were virtually identical when recombinant O-sialoglycoprotease was used to stimulate these cells. M. haemolytica products also enhanced the expression by bovine monocytes and alveolar macrophages of the tissue inhibitor of metalloproteinase 1. Our results provide evidence that matrix metalloproteinases are activated in lung lesions from cattle with shipping fever and that M. haemolytica virulence products induce production, release, and especially activation of gelatinase B by bovine inflammatory cells in vitro.

BHV-1 infection and inflammatory cytokines amplify the interaction of Mannheimia haemolytica leukotoxin with bovine peripheral blood mononuclear cells in vitro

Bovine herpesvirus-1 (BHV-1) has been reported to increase the susceptibility of cattle to respiratory disease caused by Mannheimia (Pasteurella) haemolytica A1. The principal virulence factor of M. haemolytica is a leukotoxin (LKT) that can specifically kill ruminant leukocytes following its binding to the beta2-integrin CD11a/CD18 (lymphocyte function-associated antigen 1 (LFA-1)). In this study, we investigated the effects of experimental infection of bovine peripheral blood mononuclear cells (MNCs) with BHV-1 in vitro, on the subsequent interaction of these cells with the M. haemolytica LKT. We found that BHV-1 infection increased LFA-1 expression (as assessed by flow cytometry), and subsequently enhanced LKT binding and cytotoxicity to bovine MNCs. We also found that BHV-1 infection increased CD18, IL-1beta, and IFN-gamma mRNA expression by MNCs. As previously reported for bovine polymorphonuclear neutrophils (PMNs), MNCs increased their expression of LFA-1, and their LKT binding and cytotoxicity, following exposure to IL-1beta, TNF-alpha, and IFN-gamma. These findings suggest that BHV-1 infection, and the resulting release of inflammatory cytokines, can stimulate expression of LFA-1 in bovine MNCs, thus enhancing the binding and biological effects of LKT. If such a mechanism occurs in vivo it might explain, in part, the increased susceptibility of BHV-1 infected cattle to bovine pasteurellosis.

Prior exposure to Mannheimia haemolytica leukotoxin or LPS enhances beta(2)-integrin expression by bovine neutrophils and augments LKT cytotoxicity

Mannheimia (Pasteurella) haemolytica serotype1 produces a variety of virulence factors that play an important role during the pathogenesis of bovine pneumonic pasteurellosis. Among these, a leukotoxin (LKT) and lipopolysaccharide (LPS) are thought to be the primary virulence factors that contribute to the characteristic pathology of pasteurellosis. Recent evidence suggests that M. haemolytica LKT binding to bovine leukocytes is mediated by the beta(2)-integrin CD11a/CD18 (LFA-1), which subsequently induces activation and death of these cells. Exposure of bovine peripheral blood neutrophils (PMNs) to LKT or LPS induces expression of inflammatory cytokines, which in turn can increase LFA-1 expression and conformational activation. In this study we demonstrated, by flow cytometry and Western blot, that bovine PMNs increased their LFA-1 expression following in vitro exposure to M. haemolytica LKT and LPS. Increased LFA-1 expression by PMNs exposed to LKT and LPS was associated with increased LKT binding and cell death. The results of this study suggest that M. haemolytica LKT and LPS might cooperatively increase LFA-1 expression, and by so doing amplify the lung inflammation that characterizes bovine pasteurellosis.

Biological effects of two genetically defined leukotoxin mutants of Mannheimia haemolytica

Mannheimia(Pasteurella)haemolytica serotype 1 is the primary causative agent responsible for bovine pneumonic mannheimiosis, also known as shipping fever in cattle. The bacterium produces a variety of virulence factors, foremost of which is the exotoxic leukotoxin. The leukotoxin is a calcium-dependent cytolysin that is a member of the RTX (repeats in toxin) family and exhibits a narrow cell-type and species specificity and has biological effects only on ruminant leukocytes and platelets. The genetic organization of the leukotoxin is comprised of four genes: lktC, lktA, lktB and lktD. The lktA structural gene encodes the protoxin (pro-LktA) and lktC encodes a transacylase that post-translationally modifies the inactive pro-LktA to a biologically active wild-type leukotoxin (LktA). The LktA has been implicated as the key factor that contributes to the pathogenesis of lung injury associated with the disease and considerable efforts have been employed in abrogating toxin function while retaining immunogenicity, with an eye towards design of attenuated vaccines. We hypothesized that the pro-LktA retains the ability to cause biological effects on target cells as has been reported in the case of the closely related RTX toxin alpha-hemolysin (HlyA). We also examined the biological effects of an amino-terminal truncation mutant leukotoxin DeltaLktA on target cells. Thus the objectives of our study were to investigate whether two different mutant leukotoxins, one a nonacylated pro-LktA, and the other lacking 344 amino acids at the N-terminal end of the LktA protein; DeltaLktA, are capable of (i). binding to the beta2-integrin leukotoxin receptor, (ii). inducing the elevation of second messenger intracellular calcium ([Ca(2+)](i)), and (iii). inducing inflammatory gene expression, reactive oxygen metabolites (ROMs) and cytolysis in target cells. Our results demonstrate that neither acylation nor the amino terminal 344 amino acids are required for LktA binding but are essential for LktA-induced [Ca(2+)](i) elevation, generation of ROM, generation of the inflammatory cytokine IL-8 and cytolysis in target cells.

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.

RTX toxins in Pasteurellaceae

RTX toxins (repeats in the structural toxin) are pore-forming protein toxins produced by a broad range of pathogenic Gram-negative bacteria. In vitro, RTX toxins mostly exhibit a cytotoxic and often also a hemolytic activity. They are particularly widespread in species of the family Pasteurellaceae which cause infectious diseases, most frequently in animals but also in humans. Most RTX toxins are proteins with a molecular mass of 100-200 kDa and are post-translationally activated by acylation via a specific activator protein. The repeated structure of RTX toxins, which gave them their name, is composed of iterative glycine-rich nonapeptides binding Ca2+ on the C-terminal half of the protein. Genetic analysis of RTX toxins of various species of Pasteurellaceae and of a few other Gram-negative bacteria gave evidence of horizontal transfer of genes encoding RTX toxins and led to speculations that RTX toxins might have originated from Pasteurellaceae. The toxic activities of RTX toxins in host cells may lead to necrosis and apoptosis and the underlying detailed mechanisms are currently under investigation. The impact of RTX toxins in pathogenicity and the immune responses of the host were described for several species of Pasteurellaceae. Neutralizing antibodies were shown to significantly reduce the cytotoxic activity of RTX toxins. They constitute a valuable strategy in the development of immuno-prophylactics against several animal diseases caused by pathogenic species of Pasteurellaceae. Although many RTX toxins possess cytotoxic and hemolytic activities toward a broad range of cells and erythrocytes, respectively, a few RTX toxins were shown to have cytotoxic activity only against cells of specific hosts and/or show cell-type specificity. Further evidence exists that RTX toxins play a potential role in host specificity of certain pathogens.

Inflammatory cytokines enhance the interaction of Mannheimia haemolytica leukotoxin with bovine peripheral blood neutrophils in vitro

Mannheimia (Pasteurella) haemolytica A1 produces several virulence factors that play an important role in the pathogenesis of bovine pneumonic pasteurellosis. Foremost among these is a leukotoxin (LKT) that specifically kills ruminant leukocytes. Recent evidence suggests that M. haemolytica LKT binding to bovine leukocytes is mediated by the beta(2)-integrin CD11a/CD18 (lymphocyte function-associated antigen 1 [LFA-1]), which subsequently induces activation and cytolysis of these cells. Inflammatory cytokines, which are released during viral and bacterial infection, are reported to increase LFA-1 expression and conformational activation. We investigated the effects of the inflammatory cytokines interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), and gamma interferon (IFN-gamma) on the interaction of M. haemolytica LKT with bovine peripheral blood neutrophils (PMNs). In this study we demonstrated, by flow cytometry, that bovine PMNs increased their binding to an anti-bovine LFA-1 monoclonal antibody (BAT75A) following in vitro incubation with IL-1beta, TNF-alpha, or IFN-gamma. Incubation with cytokines also increased CD18 expression, as assessed by real-time PCR and by Western blotting. Increased LFA-1 expression by PMNs exposed to cytokines was associated with increased LKT binding and cytotoxicity. The latter represented, at least in part, enhanced PMN apoptosis, as assessed by propidium iodine staining and caspase-3 activation. The results of this study suggest that inflammatory cytokines may play an important role in enhancing the biological response of bovine PMNs to M. haemolytica LKT.