Pulmonary persistence of Haemophilus somnus in the presence of specific antibody

Strategies for Bovine Respiratory Disease (BRD) Diagnosis and Prognosis: A Comprehensive Overview

Despite significant advances in vaccination strategies and antibiotic therapy, bovine respiratory disease (BRD) continues to be the leading disease affecting the global cattle industry. The etiology of BRD is complex, often involving multiple microbial agents, which lead to intricate interactions between the host immune system and pathogens during various beef production stages. These interactions present environmental, social, and geographical challenges. Accurate diagnosis is essential for effective disease management. Nevertheless, correct identification of BRD cases remains a daunting challenge for animal health technicians in feedlots. In response to current regulations, there is a growing interest in refining clinical diagnoses of BRD to curb the overuse of antimicrobials. This shift marks a pivotal first step toward establishing a structured diagnostic framework for this disease. This review article provides an update on recent developments and future perspectives in clinical diagnostics and prognostic techniques for BRD, assessing their benefits and limitations. The methods discussed include the evaluation of clinical signs and animal behavior, biomarker analysis, molecular diagnostics, ultrasound imaging, and prognostic modeling. While some techniques show promise as standalone diagnostics, it is likely that a multifaceted approach-leveraging a combination of these methods-will yield the most accurate diagnosis of BRD.

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.

Effect of Histophilus somni on Heart and Brain Microvascular Endothelial Cells

Histophilus somni is a pathogenic gram-negative bacterium responsible for pneumonia and septicemia in cattle. Sequelae include infectious thrombotic meningoencephalitis (ITME), myocarditis, arthritis, and abortion. These syndromes are associated with widespread vasculitis and thrombosis, implicating a role for endothelium in pathogenesis. Histopathologic and immunohistochemical investigation of 10 natural cases of bovine H. somni myocarditis and 1 case of ITME revealed intravascular H. somni in large biofilm-like aggregates adherent to the luminal surface of microvascular endothelium. Ultrastructurally, bacterial communities were extracellular and closely associated with degenerating or contracted endothelial cells. Histophilus somni was identified by bacterial culture and/or immunohistochemistry. Western blots of the bacterial isolates revealed that they expressed the immunodominant protective 40 kDa OMP and immunoglobulin-binding protein A (IbpA) antigens. The latter is a large surface antigen and shed fibrillar antigen with multiple domains. The cytotoxic DR2Fic domain of IbpA was conserved as demonstrated by polymerase chain reaction. Treatment of endothelial cells in vitro with IbpA in crude culture supernatants or purified recombinant GST-IbpA DR2Fic (rDR2) cytotoxin induced retraction of cultured bovine brain microvascular endothelial cells. By contrast, no retraction of bovine endothelium was induced by mutant rDR2H/A with an inactive Fic motif or by a GST control, indicating that the cytotoxic DR2Fic motif plays an important role in endothelial cell retraction in vasculitis. The formation of biofilm-like aggregates by H. somni on bovine microvascular endothelium may be fundamental to its pathogenesis in heart and brain.

Evolving views on bovine respiratory disease: An appraisal of selected key pathogens - Part 1

Bovine respiratory disease (BRD) is one of the most commonly diagnosed causes of morbidity and mortality in cattle and interactions of factors associated with the animal, the pathogen and the environment are central to its pathogenesis. Emerging knowledge of a role for pathogens traditionally assumed to be minor players in the pathogenesis of BRD reflects an increasingly complex situation that will necessitate regular reappraisal of BRD pathogenesis and control. This review appraises the role of selected key pathogens implicated in BRD pathogenesis to assess how our understanding of their role has evolved in recent years.

Histophilosis as a Natural Disease

Histophilus somni is responsible for sporadic disease worldwide in cattle and, to a lesser extent, in small ruminants, bighorn sheep (Ovis canadensis), and North American bison (Bison bison). The importance of H. somni diseases can be attributed to improved clinical and laboratory recognition, combined with the growth in intensive management practices for cattle. Although outbreaks of bovine histophilosis can occur year-round, in northern and southern hemispheres, it is most frequent in late fall and early winter. Weather, stress, dietary changes, and comingling of cattle are likely to be major triggers for outbreaks. The most frequent clinical expressions of histophilosis include undifferentiated fever, fibrinosuppurative pneumonia, encephalitis-leptomeningitis, necrotizing myocarditis, and diffuse pleuritis. Neurological disease occurs either as thrombotic meningoencephalitis (TME) or as suppurative meningitis with ventriculitis. Acute myocarditis is characteristically necrotizing and generally involves one or both papillary muscles in the left ventricular myocardium. Biofilm-like aggregates of bacteria occur in capillaries and veins in myocardium, in the central nervous system, and on endocardial surfaces. H. somni is a component of bovine respiratory disease (BRD) complex. In our experience, it is most commonly diagnosed in subacute-to-chronic polymicrobial pulmonary infections in combination with Mannheimia haemolytica, Trueperella pyogenes, Pasteurella multocida, or Mycoplasma bovis. Other, less common forms of H. somni disease present as polyarthritis/tenosynovitis, abortion with placentitis and fetal septicemia, epididymitis-orchitis, and ocular infections. It is likely that H. somni is under-recognized clinically and diagnostically. Most state and provincial laboratories in North America rely on bacterial isolation to confirm infection. The use of more sensitive detection methods on field cases of histophilosis will help resolve the pathogenesis of H. somni in natural outbreaks, and whether the disease is as common elsewhere as it is in Canada.

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.

Detection of antibodies to the biofilm exopolysaccharide of Histophilus somni following infection in cattle by enzyme-linked immunosorbent assay

An enzyme-linked immunosorbent assay (ELISA) was developed to detect bovine antibodies to Histophilus somni exopolysaccharide (EPS), which is created during biofilm formation. When an index value of 0.268 was used, the sensitivity of the assay for infected calves was 90.5% at 3 weeks postinfection, but the number of positive animals increased by week 4. The specificity of the assay for healthy calves was 92.5%. The EPS ELISA may aid in identifying calves with H. somni diseases.

Bovine respiratory syncytial virus and Histophilus somni interaction at the alveolar barrier

Our previous studies showed that Histophilus somni and bovine respiratory syncytial virus (BRSV) act synergistically in vivo to cause more severe bovine respiratory disease than either agent alone causes. Since H. somni surface and secreted immunoglobulin binding protein A (IbpA) causes retraction of bovine alveolar type 2 (BAT2) cells and invasion between BAT2 cells in vitro, we investigated mechanisms of BRSV-plus-H. somni infection at the alveolar barrier. BRSV treatment of BAT2 cells prior to treatment with IbpA-rich H. somni concentrated culture supernatant (CCS) resulted in increased BAT2 cell rounding and retraction compared to those with either treatment alone. This mimicked the increased alveolar cell thickening in calves experimentally infected with BRSV followed by H. somni compared to that in calves infected with BRSV or H. somni alone. BRSV-plus-H. somni CCS treatment of BAT2 cells also enhanced paracellular migration. The effect of matrix metalloproteinases (MMPs) was investigated as well because microarray analysis revealed that treatment with BRSV plus H. somni synergistically upregulated BAT2 cell expression of mmp1 and mmp3 compared to that in cells treated with either agent alone. Enzyme-linked immunosorbent assay (ELISA) confirmed that MMP1 and MMP3 protein levels were similarly upregulated. In collagen I and collagen IV (targets for MMP1 and MMP3, respectively) substrate zymography, digestion was increased with supernatants from dually treated BAT2 cells compared with those from singly treated cells. Enhanced breakdown of collagen IV in the basal lamina and of fibrillar collagen I in the adjacent interstitium in the dual infection may facilitate dissemination of H. somni infection.

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.

Histophilus somni IbpA DR2/Fic in virulence and immunoprotection at the natural host alveolar epithelial barrier

Newly recognized Fic family virulence proteins may be important in many bacterial pathogens. To relate cellular mechanisms to pathogenesis and immune protection, we studied the cytotoxicity of the Histophilus somni immunoglobulin-binding protein A (IbpA) direct repeat 2 Fic domain (DR2/Fic) for natural host target cells. Live virulent IbpA-producing H. somni strain 2336, a cell-free culture supernatant (CCS) of this strain, or recombinant DR2/Fic (rDR2/Fic) caused dramatic retraction and rounding of bovine alveolar type 2 (BAT2) epithelial cells. IbpA-deficient H. somni strain 129Pt and a Fic motif His(298)Ala mutant rDR2/Fic protein were not cytotoxic. The cellular mechanism of DR2/Fic cytotoxicity was demonstrated by incubation of BAT2 cell lysates with strain 2336 CCS or rDR2/Fic in the presence of [alpha-(32)P]ATP, which resulted in adenylylation of Rho GTPases and cytoskeletal disruption. Since IbpA is not secreted by type III or type IV secretion systems, we determined whether DR2/Fic entered the host cytoplasm to access its Rho GTPase targets. Although H. somni did not invade BAT2 cells, DR2/Fic was internalized by cells treated with H. somni, CCS, or the rDR2/Fic protein, as shown by confocal immunomicroscopy. Transwell bacterial migration assays showed that large numbers of strain 2336 bacteria migrated between retracted BAT2 cells, but IbpA-deficient strain 129Pt did not cross a monolayer unless the monolayer was pretreated with strain 2336 CCS or rDR2/Fic protein. Antibody to rDR2/Fic or passively protective convalescent-phase serum blocked IbpA-mediated cytotoxicity and inhibited H. somni transmigration across BAT2 monolayers, confirming the role of DR2/Fic in pathogenesis and corresponding to the results for in vivo protection in previous animal studies.

Bovine respiratory syncytial virus infection: immunopathogenic mechanisms

Bovine respiratory syncytial virus (BRSV) causes severe respiratory disease in young cattle. Much like the human respiratory syncytial virus, BRSV induces immunomodulation in the infected host, favoring a Th2 response. Several groups have demonstrated IgE responses to BRSV proteins during infection and particularly in response to vaccination with formalin-inactivated vaccine in the field and experimentally. Newer vaccine modalities that favor a shift to Th1 cytokine production have provided promising results. Infection with BRSV is a major contributor to the multi-pathogen disease, bovine respiratory disease complex. This review stresses the unique immunomodulatory aspects of BRSV infection, vaccination and its interaction with the host's immune system.

Histophilus somni host-parasite relationships

Histophilus somni (Haemophilus somnus) is one of the key bacterial pathogens involved in the multifactorial etiology of the Bovine Respiratory Disease Complex. This Gram negative pleomorphic rod also causes bovine septicemia, thrombotic meningencephalitis, myocarditis, arthritis, abortion and infertility, as well as disease in sheep, bison and bighorn sheep. Virulence factors include lipooligosaccharide, immunoglobulin binding proteins (as a surface fibrillar network), a major outer membrane protein (MOMP), other outer membrane proteins (OMPs) and exopolysaccharide. Histamine production, biofilm formation and quorum sensing may also contribute to pathogenesis. Antibodies are very important in protection as shown in passive protection studies. The lack of long-term survival of the organism in macrophages, unlike facultative intracellular bacteria, also suggests that antibodies should be critical in protection. Of the immunoglobulin classes, IgG2 antibodies are most implicated in protection and IgE antibodies in immunopathogenesis. The immunodominant antigen recognized by IgE is the MOMP and by IgG2 is a 40 kDa OMP. Pathogenetic synergy of bovine respiratory syncytial virus (BRSV) and H. somni in calves can be attributed, in part at least, to the higher IgE anti-MOMP antibody responses in dually infected calves. Other antigens are probably involved in stimulating host defense or immunopathology as well.

Specificity of IgG and IgE antibody responses to Haemophilus somnus infection of calves

Haemophilus somnus is an important cause of bovine respiratory disease and septicemia with all it's sequelae. The role of immune responses in protection and immunopathogenesis is not well understood. We showed that infection with bovine respiratory syncytial virus (BRSV) 6 days before H. somnus increased clinical scores and levels of IgE antibody to H. somnus over that of infection with H. somnus alone. To determine whether antigenic specificity of IgE responses differed from IgG responses, Western blots were done with sera from the infected calves, at 0 time and at 21 days post infection. Thus each calf was its own control. IgG antibodies recognized primarily a 40 kDa outer membrane protein (OMP) in whole cell H. somnus preparations and a 270 kDa immunoglobulin binding protein (IgBPs) in culture supernatants but generally not the 41 kDa major OMP (MOMP). IgE antibodies recognized primarily the 41 kDa MOMP in whole cell pellet preparations. Results were consistent among calves. With culture supernatants, IgE antibodies recognized both the 270 kDa IgBPs and the MOMP. Since some H. somnus strains from asymptomatic carriers (including strain 129Pt), do not have IgBPs and express a truncated MOMP (33 kDa rather than 41 kDa), reaction of strain 129Pt cells with serum from calves infected with H. somnus or BRSV and H. somnus was studied. IgE did not react with the truncated MOMP even at much lower (1:100) dilutions than in Western blots with virulent strain 2336 (serum dilution of 1:500). Reactions of IgE with the 40 and 78 kDa antigens in strain 129Pt were noted but since the major reactivities with the IgBPs and the MOMP were not detected, this strain may be useful for inducing protective rather than immunopathogenic responses.

Structural analysis of the oligosaccharide of Histophilus somni (Haemophilus somnus) strain 2336 and identification of several lipooligosaccharide biosynthesis gene homologues

The structure of the core oligosaccharide from a pneumonic Histophilus somni (Haemophilus somnus) strain 2336 was elucidated. The lipooligosaccharide (LOS) was subjected to a variety of degradative procedures. The structures of the purified products were established by monosaccharide and methylation analyses, NMR spectroscopy and mass spectrometry. The following structure for the core oligosaccharide was determined on the basis of the combined data from these experiments: [formula-see text]. The structural elucidation was intriguing as it suggested several differences in the LOS structures between strain 2336 and the related strain 738. Strain 738 originated following passaging of strain 2336 through a calf. The differences between the two structures are a different linkage between Gal II and GlcNAc (1-->4 here; 1-->3 in 738), the absence of phosphocholine (PCho) from 2336 and the presence of two phosphoethanolamine (PEtn) residues and Gal III (at the 2-position) of Hep II in 2336. Although pulse-field gel electrophoresis data following digest with only one restriction enzyme showed identical profiles suggesting that strains 738 and 2336 are the same strain, the structural data does suggest that, if strain 738 is indeed a phase variant of strain 2336, considerable variation occurred on calf passaging and could therefore be an intriguing example of how broadly this bacterium can adapt itself in the host.

Caspase activation during Haemophilus somnus lipooligosaccharide-mediated apoptosis of bovine endothelial cells

Vasculitis is commonly seen during systemic Haemophilus somnus infections. Although, the mechanism of vascular damage is not completely understood, in a previous report we demonstrated that H. somnus and its lipooligosaccharide (LOS) induced apoptosis in bovine pulmonary artery endothelial cells in vitro. In the present study, we investigated the role of caspase activation in LOS-mediated apoptosis of bovine endothelial cells. Exposure to H. somnus LOS induced caspase-3 activation and chromatin condensation in endothelial cells. These responses were blocked by the addition of a pan-caspase inhibitor (z-VAD-fmk) or capase-3 inhibitor (DEVD-fmk). Incubation of endothelial cells with H. somnus LOS also induced activation of the initiator caspases, caspases-8 and 9, with the activity of the former increasing more rapidly than the latter. Addition of a caspase-8 inhibitor (IETD-fmk) significantly reduced LOS-mediated apoptosis, whereas, addition of a caspase-9 inhibitor (LEHD-fmk) had little effect. These data suggest that LOS-mediated activation of caspase-3 and apoptosis of endothelial cells is caspase-8 dependent.

Antibodies as effectors

Antibodies are critical in protection against extracellular microbial pathogens. Although antibodies also play a role in transplant/tumor rejection and in autoimmune disease, this paper focuses on defense against bovine infections. Effector mechanisms of different bovine isotypes, subisotypes and allotypes are discussed. The importance of antigen specificity is also stressed.

Incorporation of N-acetylneuraminic acid into Haemophilus somnus lipooligosaccharide (LOS): enhancement of resistance to serum and reduction of LOS antibody binding

Haemophilus somnus isolates from cases of thrombotic meningoencephalitis, pneumonia, and other disease sites are capable of undergoing a high rate of phase variation in the oligosaccharide component of their lipooligosaccharides (LOS). In contrast, the LOS of commensal strains isolated from the normal reproductive tract phase vary little or not at all. In addition, the LOS of H. somnus shares conserved epitopes with LOS from Neisseria gonorrhoeae, Haemophilus influenzae, and other species that can incorporate sialic acid into their LOS. We now report that growth of disease isolates of H. somnus with CMP-N-acetylneuraminic acid (CMP-NeuAc) or NeuAc added to the medium resulted in incorporation of NeuAc into the LOS. However, NeuAc was not incorporated into the LOS of commensal isolates and one disease isolate following growth in medium containing CMP-NeuAc or NeuAc. Sialylated LOS was detected by an increase in the molecular size or an increase in the amount of the largest-molecular-size LOS electrophoretic bands, which disappeared following treatment with neuraminidase. Sialylated LOS could also be detected by reactivity with Limax flavus agglutinin lectin, which is specific for sialylated species, by dot blot assay; this reactivity was also reversed by neuraminidase treatment. H. somnus strain 2336 LOS was found to contain some sialic acid when grown in medium lacking CMP-NeuAc or NeuAc, although supplementation enhanced NeuAc incorporation. In contrast strain 738, an LOS phase variant of strain 2336, was less extensively sialylated when the growth medium was supplemented with CMP-NeuAc or NeuAc, as determined by electrophoretic profiles and electrospray mass spectrometry. The sialyltransferase of H. somnus strain 738 was confirmed to preferentially sialylate the Gal(beta)-(1-3)-GlcNAc component of the lacto-N-tetraose structure by capillary electrophoresis assay. Enhanced sialylation of the strain 2336 LOS inhibited the binding of monoclonal antibodies to LOS by enzyme immunoassay and Western blotting. Furthermore, sialylation of the LOS enhanced the resistance of H. somnus to the bactericidal action of antiserum to LOS. Sialylation and increased resistance to killing by normal serum also occurred in a deletion mutant that was deficient in the terminal Gal-GlcNAc disaccharide. LOS sialylation may therefore be an important virulence mechanism to protect H. somnus against the host immune system.

Immunity to bovine reproductive infections

Protective immune responses in the genital tract are robust, as shown by convalescent and vaccine-induced immunity. Systemic immunity is crucial for systemic infections that result in reproductive failure (such as brucellosis, leptospirosis, and the systemic forms of C. fetus and H. somnus infection). Although IgA responses can protect against sexually transmitted or venereal infections, systemically induced IgG antibody responses also protect. IgA responses can be induced by immunization of the genital tract, where inductive sites develop after antigenic stimulation. The common mucosal immune system can also be used to induce a genital IgA response, as shown by intranasal vaccination. Lastly, it is necessary to determine which antigens of each infectious agent are protective and which types of immune responses protect best.

Haemophilus somnus induces apoptosis in bovine endothelial cells in vitro

Haemophilus somnus causes pneumonia, reproductive failure, infectious myocarditis, thrombotic meningoencephalitis, and other diseases in cattle. Although vasculitis is commonly seen as a result of systemic H. somnus infections, the pathogenesis of vascular damage is poorly characterized. In this study, we demonstrated that H. somnus (pathogenic isolates 649, 2336, and 8025 and asymptomatic carrier isolates 127P and 129Pt) induce apoptosis of bovine endothelial cells in a time- and dose-dependent manner, as determined by Hoechst 33342 staining, terminal deoxynucleotidyl transferase-mediated dUTP-FITC nick end labeling, DNA fragmentation, and transmission electron microscopy. H. somnus induced endothelial cell apoptosis in as little as 1 h of incubation and did not require extracellular growth of the bacteria. Viable H. somnus organisms induced greater endothelial cell apoptosis than heat-killed organisms. Since viable H. somnus cells release membrane fibrils and blebs, which contain lipooligosaccharide (LOS) and immunoglobulin binding proteins, we examined culture filtrates for their ability to induce endothelial cell apoptosis. Culture filtrates induced similar levels of endothelial cell apoptosis, as did viable H. somnus organisms. Heat inactivation of H. somnus culture filtrates partially reduced the apoptotic effect on endothelial cells, which suggested the presence of both heat-labile and heat-stable factors. We found that H. somnus LOS, which is heat stable, induced endothelial cell apoptosis in a time- and dose-dependent manner and was inhibited by the addition of polymyxin B. These data demonstrate that H. somnus and its LOS induce endothelial cell apoptosis, which may play a role in producing vasculitis in vivo.

Antigenic diversity of Haemophilus somnus lipooligosaccharide: phase-variable accessibility of the phosphorylcholine epitope

The lipooligosaccharide (LOS) of Haemophilus somnus undergoes antigenic phase variation, which may facilitate evasion from the bovine host immune response and/or colonization and dissemination. However, LOS antigenic diversity in H. somnus has not been adequately investigated. In this study, monoclonal antibodies (MAbs) specific to various LOS epitopes were used to investigate antigenic variation and stability in LOS from H. somnus strains and phase variants. Clinical isolates of H. somnus exhibited intrastrain, as well as interstrain, antigenic heterogeneity in LOS when probed with MAbs to outer core oligosaccharide epitopes in an enzyme-linked immunosorbent assay (ELISA). However, epitopes reactive with MAbs directed predominately to the inner core heptose region were highly conserved. At least one epitope, which was expressed in few strains, was identified. One LOS component affected by phase variation was identified as phosphorylcholine (PCho), which is linked to the primary glucose residue. Inhibition ELISA, immunoblotting, and electrospray-mass spectrometry were used to confirm that MAb 5F5.9 recognized PCho. LOS reactivity with MAb 5F5.9 was associated with loss of most of the outer core oligosaccharide, indicating that reactivity with PCho was affected by phase variation of the glucose residues in this region. Our results indicate that outer core epitopes of H. somnus LOS exhibit a high degree of random, phase-variable antigenic heterogeneity and that such heterogeneity must be considered in the design of vaccines and diagnostic tests.

Apoptosis: a possible tactic of Haemophilus somnus for evasion of killing by bovine neutrophils?

Haemophilus somnus is an important veterinary pathogen that causes respiratory disease, arthritis, septicaemia and abortion in cattle and sheep. In the present study we investigated the possibility that H. somnus resists killing by bovine neutrophils, by causing the latter to undergo morphological changes consistent with apoptosis. Both serum-sensitive and serum-resistant strains of H. somnus enhanced bovine neutrophil chromatin condensation and shape change (i.e. zeiosis) in vitro, suggesting that the cells were undergoing apoptosis. Heat-killed or formalin-killed H. somnus had less effect than viable H. somnus. Chromatin margination of neutrophils was greater whenH. somnus was opsonized with adult bovine serum, which facilitates phagocytosis of the bacteria. H. somnus culture filtrates did not cause bovine neutrophil chromatin condensation. These findings suggest that direct contact with H. somnus is required for the maximal effect on bovine neutrophils. Apoptosis was confirmed by flow cytometry, using propidium iodide staining to detect DNA fragmentation. These findings suggest that H. somnus can evade killing by bovine neutrophils, in part, by inducing these cells to undergo apoptosis.

Bovine IgG2a antibodies to Haemophilus somnus and allotype expression

Bovine IgG2a has been implicated in protection against pyogenic infections, including those caused by Haemophilus somnus. To further investigate the role of IgG2a in defense against H. somnus, IgG1 and IgG2a antibodies were purified from antiserum against an immunodominant 40 kDa outer membrane protein (p40) of H. somnus, which was previously shown to passively protect calves against H. somnus pneumonia. The passive protective capacity of anti-p40 IgG1 or IgG2a was evaluated in vivo in calves. Purified anti-p40 IgG1 or IgG2a was incubated with H. somnus for 15 min before intrabronchial inoculation of calves. Bacteria incubated with anti-p40 IgG1 or IgG2a were inoculated into one caudal lung lobe and bacteria incubated with IgG1 or IgG2a from the respective preimmunization serum were inoculated into the contralateral lobe. The volumes of pneumonia in the right and left lungs were determined 24 h later. The difference in volume of pneumonia with H. somnus preincubated in IgG1 pre- and postimmunization anti p40 was less (16 cm3, P = 0.298) than the difference in volume of pneumonia with H. somnus preincubated in IgG2a pre- and postimmunization anti p40 (30 cm3, P = 0.146). Although the differences in lesion size between pre- and postimmunization serum were not statistically significant, the trend suggests IgG2a may be more protective than IgG1. To examine this further, the peptide specificity of these IgG1 and IgG2a antibodies to p40 was examined. After limited proteolysis of p40, IgG2a antibodies reacted with 2 peptides not recognized by IgG1 antibodies. Other peptides were recognized by both isotypes. Since these studies suggested that IgG2a may be important in protection against infection, we then investigated some aspects of the role of the 2 IgG2a allotypes, A1 and A2. In retrospective studies of age differences in expression of IgG2a allotypes, no heterozygotes were detected in calves of 60 d old or less, and fewer heterozygotes were detected in calves 61-120 d old than in cattle older than 270 d (P < 0.01). In a subsequent prospective study of the time course of allotype expression, Holstein calves shown to be heterozygotes expressed the IgG2aA1 allotype early but the IgG2aA2 allotype was not usually detected until 3 to 4 mo of age. Thus, both the retrospective and the prospective studies showed age related differences in expression of the IgG2aA1 and A2 allotypes. This could have implication in protection.

Ovine Haemophilus somnus: experimental intracisternal infection and antigenic comparison with bovine Haemophilus somnus

Experimental infection was produced by two of four isolates of ovine Haemophilus somnus given by intracisternal inoculation into two to three-month-old lambs. Isolate 2041 (originally obtained from a septicemic lamb in Alberta) caused lethal infection in eight of nine lambs, isolate 67p from the prepuce of a normal lamb produced less acute disease in four of nine lambs, and the other two isolates (93p and 1190) caused no detectable disease. Significant lesions were limited to the brain and spinal cord. Purulent meningitis was characteristic but vasculitis or septicemia were not detected, perhaps due to the route of inoculation. Since a difference in virulence was noted among strains, we analyzed surface proteins thought to be virulence factors of bovine H. somnus. Protein profiles of bovine and ovine H. somnus done by sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed similar patterns for virulent bovine isolates and ovine septicemic isolates. Preputial isolates showed a lower molecular mass major outer membrane protein than septicemic isolates. Antigenic analysis revealed that outer membrane proteins p270, p78, p76, p40, and p39 were detected in both ovine and bovine isolates except for 1190, which was probably not a true H. somnus isolate. Thus the preputial and septicemic isolates of ovine H. somnus were similar to bovine H. somnus in pathogenicity and in surface antigens.

Adherence of Haemophilus somnus to tumor necrosis factor-alpha-stimulated bovine endothelial cells in culture

Vascular thrombosis and tissue infarction is a principal lesion in Haemophilus somnus septicemia known also as thrombotic meningoencephalitis. This study was undertaken to examine whether tumor necrosis factor-alpha (TNF-alpha) can influence the adherence of H. somnus to cultured bovine aortic endothelial cells (BAEC). Confluent BAEC were exposed to 0-100 nM of human recombinant TNF-alpha for 12-48 h. Suspensions of different strains of H. somnus (approximately 1.5-3 x 10(8) labelled with [methyl-3H]-thymidine, were added to BAEC and incubated for 1.5 h. Initial studies with one pathogenic (P) strain and one non-pathogenic (NP) strain revealed that both strains adhered to normal endothelial cells but minimally to subendothelial matrix remaining after removal of BAEC. Adherence to BAEC was reduced by an excess of unlabelled H. somnus of the same strain. Adherence was enhanced for both strains by exposure of BAEC to TNF-alpha in a manner that increased with TNF-alpha concentration and with duration of exposure to TNF-alpha prior to addition of bacteria. A survey of adherence of six live P strains and six NP strains demonstrated considerable variation but no difference in adherence between P and NP strains to normal or to TNF-alpha-stimulated BAEC. However, TNF-alpha consistently increased adhesion of each strain to BAEC. Both P and NP strains caused more severe cytotoxic changes in TNF-alpha-treated BAEC. Tumor necrosis factor-alpha also increased adhesion of formalin-killed bacteria of P and NP strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenotypic phase variation in Haemophilus somnus lipooligosaccharide during bovine pneumonia and after in vitro passage

A high rate of phenotypic variation in the lipooligosaccharide (LOS) electrophoretic profile of Haemophilus somnus occurred in most isolates obtained at approximately weekly intervals from three calves intrabronchially challenged with a cloned isolate of H. somnus 2336. Daily subculturing for 2 weeks resulted in at least one major alteration in the LOS electrophoretic profiles for strain 2336 and both additional disease isolates examined, but no change occurred in the LOS electrophoretic profiles for any of three commensal isolates examined. None of the LOSs from any of the postchallenge intrabronchial isolates reacted with rabbit antiserum to the challenge strain LOS in immunoblotting, but LOSs from two nasopharyngeal isolates did. Antigenic variation in the extracted LOSs of most of the isolates was supported by the results of an enzyme-linked immunosorbent assay. Preimmune serum from each of the calves did not react with any of the isolates or the challenge strain, whereas sera obtained 35 days after challenge reacted with the challenge strain and zero to five additional isolates and sera obtained 74 days after challenge reacted with two to six additional isolates. Recognition of LOSs from isolates obtained near the end of the 70-day experiment by day-74 sera was related to clearance of the bacteria from the lungs. Isolates demonstrating major electrophoretic changes showed variations in the composition of the oligosaccharide, but not lipid A, moiety of their LOSs. The oligosaccharide of the LOS of each isolate was composed predominantly of glucose but varied substantially in the contents of galactose, arabinose, xylose, mannose, and 3-deoxy-D-manno-octulosonic acid. Therefore, the LOS of H. somnus is capable of undergoing compositional and antigenic variations, which may act as an important virulence mechanism for evading host immune defense mechanisms.

Characterization of immunodominant surface antigens of Haemophilus somnus

An immunodominant Haemophilus somnus outer membrane protein with an apparent molecular mass of 40 kDa on Western blots (immunoblots) of gradient sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels was characterized because a monospecific antibody against this antigen was protective. This monospecific antibody was used for immunoaffinity purification of the antigen. The immunoaffinity-purified antigen reacted with a polyclonal antibody to the 40-kDa antigen but not with a monoclonal antibody (3G9) which reacted with the 40-kDa antigen in gradient gels. On 8 or 10% gels, the approximately 40-kDa antigen was resolved as two bands, a 40-kDa band which reacted with the protective monospecific polyclonal antibody (p40) and a band of lower molecular mass which reacted with monoclonal antibody 3G9. The latter antigen was designated p39. Both antigens were conserved in all H. somnus isolates tested. The specific antibodies were also used to detect cross-reacting antigens in other gram-negative bacteria. Antibody to p40 reacted with proteins of 55 to 28 kDa, with the greatest intensity shown among proteins from other members of the family Pasteurellaceae. Antibody to p40 was reduced by absorption with live H. somnus or other members of the family Pasteurellaceae, so the antigen appears to be surface exposed. Antibody to p39 only cross-reacted with a broad band (38 to 40 kDa) in Haemophilus agni. Since H. agni is not a bovine pathogen and since convalescent-phase serum from H. somnus-infected animals did recognize p39, the latter may be a good immunodiagnostic antigen, if the lack of cross-reactivity with antigens in other gram-negative bacteria is confirmed with a polyclonal antibody to p39. The cross-reactivity of antiserum to p40 with antigens of members of the family Pasteurellaceae and the ability of this antiserum to protect against H. somnus pneumonia indicate that p40 may be a useful vaccine antigen for H. somnus disease and perhaps even diseases caused by other members of the family Pasteurellaceae.

The development of protein A-gold electron microscopy for immunological studies of Haemophilus somnus

Two separate experiments were conducted using modifications of protein A-gold immunoelectron microscopy (PAGIEM) to evaluate the ability of sera from calves vaccinated against Haemophilus somnus to bind virulent organisms (experiment I) and to detect differences in the antibody accessible antigenic sites on the outer membrane of selected strains of H. somnus having different virulence attributes using a high IgG2 titre specific bovine hyperimmune serum (experiment II). The results of experiment I demonstrated that the direct opsonisation of H. somnus by specific antisera was related to its IgG2 titre. In experiment II, strain-dependent differences in the labelling of antigenic sites by specific IgG2 antibodies were observed. The virulent strains of both septicaemic and genital isolates of H. somnus showed higher protein A-gold labelling than their non-virulent counterparts. The results from a comparison of pathogenic and non-pathogenic respiratory isolates did not reveal the same difference in labelling intensity. The studies demonstrated the PAGIEM technique to be a sensitive, versatile and a reliable laboratory method to analyse antigen-antibody interactions of H. somnus.