Identification of suppressive components in "Haemophilus somnus" fractions which inhibit bovine polymorphonuclear leukocyte function

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.

Histophilus somni causes extracellular trap formation by bovine neutrophils and macrophages

Histophilus somni (formerly Haemophilus somnus) is a Gram-negative pleomorphic coccobacillus that causes respiratory, reproductive, cardiac and neuronal diseases in cattle. H. somni is a member of the bovine respiratory disease complex that causes severe bronchopneumonia in cattle. Previously, it has been reported that bovine neutrophils and macrophages have limited ability to phagocytose and kill H. somni. Recently, it was discovered that bovine neutrophils and macrophages produce extracellular traps in response to Mannheimia haemolytica, another member of the bovine respiratory disease complex. In this study, we demonstrate that H. somni also causes extracellular trap production by bovine neutrophils in a dose- and time-dependent manner, which did not coincide with the release of lactate dehydrogenase, a marker for necrosis. Neutrophil extracellular traps were produced in response to outer membrane vesicles, but not lipooligosacchride alone. Using scanning electron microscopy and confocal microscopy, we observed H. somni cells trapped within a web-like structure. Further analyses demonstrated that bovine neutrophils trapped and killed H. somni in a DNA-dependent manner. Treatment of DNA extracellular traps with DNase I freed H. somni cells and diminished bacterial death. Treatment of bovine monocyte-derived macrophages with H. somni cells also caused macrophage extracellular trap formation. These findings suggest that extracellular traps may play a role in the host response to H. somni infection in cattle.

Host response to bovine respiratory pathogens

Bovine respiratory disease (BRD) involves complex interactions amongst viral and bacterial pathogens that can lead to intense pulmonary inflammation (fibrinous pleuropneumonia). Viral infection greatly increases the susceptibility of cattle to secondary infection of the lung with bacterial pathogens likeMannheimia haemolyticaandHistophilus somni. The underlying reason for this viral/bacterial synergism, and the manner in which cattle respond to the virulence strategies of the bacterial pathogens, is incompletely understood. Bovine herpesvirus type 1 (BHV-1) infection of bronchial epithelial cellsin vitroenhances the binding ofM. haemolyticaand triggers release of inflammatory mediators that attract and enhance binding of neutrophils. An exotoxin (leukotoxin) released fromM. haemolyticafurther stimulates release of inflammatory mediators and causes leukocyte death. Cattle infected withH. somnifrequently display vasculitis. Exposure of bovine endothelial cells toH. somniior its lipooligosaccharide (LOS) increases endothelium permeability, and makes the surface of the endothelial cells pro-coagulant. These processes are amplified in the presence of platelets. The above findings demonstrate that bovine respiratory pathogens (BHV-1,M. haemolyticaandH. somni) interact with leukocytes and other cells (epithelial and endothelial cells) leading to the inflammation that characterizes BRD.

Complexities of the pathogenesis ofMannheimia haemolyticaandHaemophilus somnusinfections: challenges and potential opportunities for prevention?

Progress in producing improved vaccines against bacterial diseases of cattle is limited by an incomplete understanding of the pathogenesis of these agents. Our group has been involved in investigations of two members of the family Pasteurellaceae,Mannheimia haemolyticaandHaemophilus somnus, which illustrate some of the complexities that must be confronted. Susceptibility toM. haemolyticais greatly increased during active viral respiratory infection, resulting in rapid onset of a severe and even lethal pleuropneumonia. Despite years of investigation, understanding of the mechanisms underlying this viral–bacterial synergism is incomplete. We have investigated the hypothesis that active viral infection increases the susceptibility of bovine leukocytes to theM. haemolyticaleukotoxin by increasing the expression of or activating the β2integrin CD11a/CD18 (LFA-1) on the leukocyte surface.In vitroexposure to proinflammatory cytokines (i.e. interleukin-1β, tumor necrosis factor-α and interferon-γ) increases LFA-1 expression on bovine leukocytes, which in turn correlates with increased binding and responsiveness to the leukotoxin. Alveolar macrophages and peripheral blood leukocytes from cattle with active bovine herpesvirus-1 (BVH-1) infection are more susceptible to the lethal effects of the leukotoxinex vivothan leukocytes from uninfected cattle. Likewise,in vitroincubation of bovine leukocytes with bovine herpesvirus 1 (BHV-1) potentiates LFA-1 expression and makes the cells more responsive to leukotoxin. A striking characteristic ofH. somnusinfection is its propensity to cause vasculitis. We have shown thatH. somnusand its lipo-oligosaccharide (LOS) trigger caspase activation and apoptosis in bovine endothelial cellsin vitro. This effect is associated with the production of reactive oxygen and nitrogen intermediates, and is amplified in the presence of platelets. The adverse effects ofH. somnusLOS are mediated in part by activation of endothelial cell purinergic receptors such as P2X7. Further dissection of the pathways that lead to endothelial cell damage in response toH. somnusmight help in the development of new preventive or therapeutic regimens. A more thorough understanding ofM. haemolyticaandH. somnusvirulence factors and their interactions with the host might identify new targets for prevention of bovine respiratory disease.

Inhibition of bovine macrophage and polymorphonuclear leukocyte superoxide anion production by Haemophilus somnus

Virulent strains of the bovine opportunistic pathogen Haemophilus somnus (Histophilus somni) cause multi-systemic diseases in cattle. One of the reported virulence factors that H. somnus may use to persist in the host is resistance to intracellular killing. We report here that H. somnus significantly (P < 0.001) inhibited production of superoxide anion (O2-) by bovine mammary and alveolar macrophages as well as by polymorphonuclear leukocytes. Inhibition of O2- was time- and dose-dependent and did not occur after incubation with Escherichia coli, H. influenzae, or Brucella abortus. Non-viable H. somnus, purified lipooligosaccharide, or cell-free supernatant from mid-log phase cultures did not inhibit O2- production, indicating that O2- inhibition required contact with live H. somnus. Furthermore, preincubation of phagocytic cells with cytochalasin B to prevent phagocytosis did not decrease the ability of H. somnus to inhibit O2- production. Some H. somnus isolates from the prepuce of healthy bulls were less capable or incapable of inhibiting macrophage O2- production compared to isolates tested from disease sites. Our results suggest that inhibition of O2- may be an important virulence factor exploited by pathogenic strains of H. somnus to resist killing by professional phagocytic cells.

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.

Lochial secretions of Escherichia coli- or Arcanobacterium pyogenes-infected bovine uteri modulate the phenotype and the functional capacity of neutrophilic granulocytes

It has been suggested that in cases of puerperal endometritis of cattle infected with Escherichia coli and Arcanobacterium pyogenes, the neutrophils are compromised in their defense capacity or downregulated functionally. In addition to direct bacterial effects, contents of lochial secretions and secreted products of locally activated polymorphonuclear neutrophilic granulocytes (PMNs) may also account for changes in function of freshly immigrating neutrophils. In this study, lochial secretions were obtained from healthy cows and from cows infected by E. coli or A. pyogenes. Separated uterine PMN of infected cows displayed an altered phenotype and function which correlated with the degree of bacterial contamination. Concurrently tested circulating PMN showed no such changes. Infected lochial secretions sterilized by filtration also changed the phenotype of blood PMN. Lochial secretions of healthy cows displayed only minor effects. The effects on PMN function in infected cows varied: ingestion was less affected, whereas generation of reactive oxygen species (ROS) was severely depressed. Concurrently tested purified bacterial products (solubles and fragments) of E. coli and A. pyogenes did not induce the phenotypical and functional changes observed in blood PMN. Since infected lochia also contained high numbers of immigrated and probably activated PMN, the influence of supernatants from phorbol myristate acetate-activated PMN were tested on freshly isolated blood PMN. Such supernatants also increased the expression of certain surface molecules and inhibited the ROS generation. Thus, reduced function and altered phenotypes of PMN which immigrate into the uteri of cows with bacterial endometritis is due not only to interactions with bacteria or bacterial products, but is also to the uterine milieu.

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.

Effect of Haemophilus somnus on nitric oxide production and chemiluminescence response of bovine blood monocytes and alveolar macrophages

Haemophilus somnus is able to survive and multiply in bovine blood monocytes (BBM) and alveolar macrophages (BAM), but the mechanisms used by H. somnus to evade killing mechanisms of bovine mononuclear phagocytes are not completely understood. To study the bactericidal ability of bovine mononuclear phagocytes following interaction with H. somnus, in vitro assay systems were developed to detect the luminol-dependent chemiluminescence response (LDCL) and nitric oxide (NO) production of BBM and BAM. Live logarithmically growing or stationary phase H. somnus inhibited the LDCL of BBM and BAM costimulated with opsonized Staphylococcus aureus. Inhibition of the LDCL response of BBM and BAM was not mediated by live H. somnus opsonized with hyperimmune serum, or by killed bacteria. H. somnus stimulated both BBM and BAM to produce NO at levels comparable with Escherichia coli lipopolysaccharide. While NO was being produced, viable H. somnus could still be isolated from the cell cultures. The ability of H. somnus to inhibit LDCL of both BBM and BAM, and resistance to NO killing may be an important mechanism that contributes to survival of the organism following ingestion by bovine mononuclear phagocytes.

A rapid, direct assay to measure degranulation of bovine neutrophil primary granules

A direct, rapid, quantitative colorimetric assay to determine neutrophil primary granule degranulation was adapted for use with bovine neutrophils. The assay measures the exocytosis of myeloperoxidase (MPO) using 3,3',5,5'-tetramethylbenzidine as a substrate. The assay was validated by evaluating the effects of various stimulants and inhibitors of degranulation, the kinetics of primary granule exocytosis, and comparing the total myeloperoxidase content of neutrophils obtained from calves and adults. The results demonstrate that the assay is capable of detecting important differences that may occur in degranulation of bovine neutrophil primary granules and in total neutrophil myeloperoxidase content.

Evaluation of the influence of potential toxins on neutrophil function

Neutrophils undergo a complex series of events when functioning to control bacterial, viral, and fungal infections. Any factor that interferes with neutrophil production or suppresses neutrophil function will rapidly make the animal more susceptible to infectious diseases. In vitro assays are used to detect and characterize defects in neutrophil function that may be induced by exposure to potential toxins. This paper focuses on important considerations in designing experiments for the evaluation of neutrophil function and interpretation of data.

Effect of Haemophilus somnus on phagocytosis and hydrogen peroxide production by bovine polymorphonuclear leukocytes

The interactions between bovine polymorphonuclear leukocytes (PMNs) and the bacterium Haemophilus somnus are known to be complex. In this paper, we evaluated the effect of H. somnus on PMN function using a flow cytometric (FC) technique that simultaneously determined the extent of phagocytosis and hydrogen peroxide production by PMNs, as well as using conventional techniques, such as the nitroblue tetrazolium (NBT) and chemiluminescence assays, to analyse the PMN respiratory burst. Results from the FC and chemiluminescence assays demonstrated that in vitro exposure of PMNs to logarithmically growing H. somnus reduced the respiratory burst of PMNs obtained from healthy calves. However, this reduction was not detected by the NBT assay. A decrease in phagocytosis by PMNs could also be shown using the FC assay. In addition, PMNs from calves with acute Hemophilosis (i.e. exposed to H. somnus in vivo) showed reduced activity when compared to PMNs from healthy calves. These in vitro and in vivo observations indicate that the modulation of bovine PMN function by H. somnus may contribute significantly towards the pathogenesis of the disease.

A basis for resistance of Blastomyces dermatitidis killing by human neutrophils: inefficient generation of myeloperoxidase system products

The mechanism by which the yeast form of Blastomyces dermatitidis resists killing by human peripheral blood polymorphonuclear neutrophils (PMN) was investigated. The metabolic products of the oxidative burst generated during the interaction of PMN and B. dermatitidis or Candida albicans were detected by lucigenin- or luminol-enhanced chemiluminescence (CL). Interaction of PMN and C. albicans resulted in luminol-enhanced CL 100-fold greater than that generated by PMN and B. dermatitidis. This correlated with killing of C. albicans and resistance of B. dermatitidis. Since B. dermatitidis and PMN interactions resulted in significant lucigenin-enhanced CL, deficient luminol CL was not due to a lack of products from the NADPH oxidase system. Killed B. dermatitidis cells at 37 degrees C were more efficient than live cells in stimulating PMN for luminol-enhanced CL; however, only fragmented B. dermatitidis cells elicited luminol-enhanced CL equivalent to that of C. albicans. Since lysates of PMN were active in a cell-free hydrogen peroxide-peroxidase-halide system, resistance of B. dermatitidis to PMN was not due to a defect in PMN peroxidase. Taken together, these findings indicate that resistance of B. dermatitidis to killing by PMN results from inefficient generation of products from the peroxidase-dependent PMN microbicidal system.

Haemophilus somnus and reproductive disease in the cow: A review

Haemophilus somnus causes inflammatory disease in the genital tract of cows as reported in several field surveys and experimental trials. This organism can also innocuously colonize the healthy genital mucosa of the cow, which indicates its dual relationship with the host, that of pathogen and commensal. Experimental data indicate embryocidal capability of this pathogen suggesting a possible role in early embryonic death. Haemophilus somnus also causes sporadic abortions after a bacteremia in the dam. Retrograde infection of the pregnant uterus from the lower genital tract appears unlikely; however, this process can account for post-parturient endometritis. Detection of high homologous IgG(2) serum antibody titers using an ELISA test may be useful for the diagnosis of current or recent genital inflammation. Experimental laboratory data indicate that a proportion of genital strains of H. somnus are pathogenic and capable of causing thrombotic meningoencephalitis and perhaps pneumonia. In vivo testing of the pathogenicity of genital strains remains to be conducted.

Elimination of hydrogen peroxide by Haemophilus somnus, a catalase-negative pathogen of cattle

Haemophilus somnus is a catalase-negative, gram-negative pathogen of cattle which is refractory to killing by bovine neutrophils. In this report, we showed that H. somnus rapidly inhibited Luminol-dependent chemiluminescence of bovine neutrophils costimulated with opsonized zymosan or phorbol myristate acetate. We have postulated that this inhibition resulted in part from H. somnus preventing the accumulation of hydrogen peroxide (H2O2) during the oxidative burst. In support of this hypothesis, we have demonstrated that when stimulated with viable H. somnus, bovine neutrophils accumulate lower levels of H2O2 than did neutrophils stimulated with heat-killed H. somnus or opsonized zymosan. We have presented evidence that four separate strains of H. somnus, despite being catalase negative by conventional criteria, removed H2O2 from solution. Viable cells of H. somnus were required for the removal of H2O2 from solution; little or no activity was observed when suspensions of heat-killed, formalin-killed, or sonicated cells of H. somnus were incubated with H2O2. In addition, the elimination of H2O2 occurred only in the presence of carbon sources that could be utilized by H. somnus, indicating that elimination of H2O2 was an energy-dependent process. The amount of H2O2 that could be eliminated by 10(7) cells of H. somnus was greater than 10 nmol, an amount comparable to that produced by a similar number of stimulated bovine neutrophils. Thus, we suggest that the ability of H. somnus to remove H2O2 from solution may be an important virulence mechanism that contributes to the survival of the organism following ingestion by bovine neutrophils.

Characterization of a phosphomonoesterase from Brucella abortus

Brucellae are facultative intracellular bacterial pathogens that reside primarily in cells of the reticuloendothelial system. The high-speed supernatant obtained after centrifuging a suspension of Brucella abortus that had been frozen-thawed and sonicated contained abundant phosphomonoesterase activity, determined by using 4-methylumbelliferylphosphate as the substrate; this enzyme was purified 2,900-fold (yield, 570%) by chromatography on DE-52 cellulose and hydroxylapatite columns and high-performance liquid chromatography-gel filtration. The native enzyme had a molecular mass of 120,000 daltons (+/- 10,000 daltons), as determined by gel filtration chromatography, and resolved into two bands (60,000 and 66,000 daltons) when subjected to polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The B. abortus phosphomonoesterase had the following properties: pH optimum, 6.0 to 6.5; isoelectric point, 3.0; substrate specificity, 5'-AMP greater than 3'-AMP greater than 3'-GMP greater than 5'-GDP greater than 5'-CDP greater than 5'-CTP greater than 5'-UPT greater than phosphotyrosine greater than phosphoserine greater than phosphothreonine. The Km for 5'-AMP was 0.37 mM. Phosphatidylinositol 4,5-bisphosphate and myo-inositol 1,3,4-trisphosphate were poor substrates for the B. abortus enzyme. The phosphomonoesterase did not inhibit superoxide anion production by human neutrophils stimulated with formyl-methionyl-leucyl-phenylalanine. The phosphomonoesterase may be one of the bacterial enzymes in the pathway leading to the production of adenine, which is secreted by B. abortus and blocks the activation of neutrophils.

Model systems to study immunomodulation in domestic food animals

Development of immunomodulators for use in food producing animals is an active area of research. This research has generally incorporated aspects of immunosuppression in model systems. This methodology is appropriate because most of the research has been aimed at developing immunomodulators for certain economically significant diseases in which immunosuppression is believed to be an important component of their pathogenesis. The primary focus has been on stress-associated diseases (especially bovine respiratory disease), infectious diseases in young animals, and mastitis. The model systems used have limitations, but they have demonstrated that immunomodulators are capable of significantly increasing resistance to these important infectious disease syndromes. As our understanding of molecular immunology increases and as more potential immunomodulators become available, the use of relevant model systems should greatly aid advancement in the field of immunomodulation.

Immunoglobulin-binding activity among pathogenic and carrier isolates of Haemophilus somnus

Nonimmune binding of immunoglobulin to whole bacteria was quantitated for North American isolates of Haemophilus somnus recovered from cattle with pneumonia, reproductive failure (abortion), or thromboembolic meningoencephalitis or from the vagina or prepuce of carrier cattle. Quantitative binding activity covered a wide range, with most pathogenic and carrier isolates demonstrating significant immunoglobulin-Fc binding. Isolates for which Fc binding was not detectable were recovered only from the prepuces of asymptomatic bulls. Expression of Fc-binding activity correlated with the presence of the 41,000-molecular-weight protein (41K protein) and 270K protein. Isolates that lacked Fc-binding activity did not possess 41K or 270K protein. A 33K protein was detected in isolates that lacked Fc-binding activity but not in isolates that bound Fc.

Effects of in vitro and in vivo administration of recombinant bovine interferon-gamma on bovine neutrophil responses to Brucella abortus

The effects of in vitro and in vivo treatment of bovine polymorphonuclear leukocytes with recombinant bovine interferon-gamma on in vitro bovine polymorphonuclear leukocyte functions and the survival of Brucella abortus were determined. Activation of neutrophils in vitro with interferon-gamma resulted in enhanced production of O2- and myelopeoroxidase-H2O2-halide activity by neutrophils in the presence of B. abortus. The improved iodination responses were correlated with an enhanced ability to perform iodination in the presence of 5'-guanosine monophosphate and adenine which have previously been shown to contribute to inhibition of neutrophil myeloperoxidase-H2O2-halide activity by B. abortus. The ability of opsonized B. abortus to survive in the presence of neutrophils activated in vitro or in vivo was partially decreased by approximately 10% of control when compared to survival rates within control phagocytes. These results suggest that activation of neutrophils with recombinant interferon-gamma partially enhances their oxidative metabolic responses, resulting in a slightly enhanced ability to kill virulent B. abortus.

Virulence differences among three strains of Haemophilus somnus following intratracheal inoculation of calves

Pneumonia was induced in four month old Holstein calves by intratracheal inoculation of 1 x 10(9) colony forming units of Haemophilus somnus. Twenty calves were divided into four groups of five and challenged with a pneumonic strain (Group 1), an encephalitic strain (Group 2), a preputial strain (Group 3), or a placebo (Group 4). The clinical score, neutrophil count, respiratory rate, and temperature were significantly increased in group 1 by day 1 postinoculation (P less than 0.05) and maintained until day 6 postinoculation (P less than 0.05). The macroscopic pathological changes were significantly greater in group 1 (P less than 0.05). Haemophilus somnus was consistently isolated from pneumonic tissue of group 1 only. Groups 2 and 3 had mild transient increases in all parameters measured and macroscopically only small focal lesions were present. It is concluded that virulence differences exist between H. somnus strains following intratracheal challenge of bovine lungs.

"Haemophilus somnus," a facultative intracellular pathogen of bovine mononuclear phagocytes

We have reported previously that bovine neutrophils are unable to kill the bovine respiratory pathogen "Haemophilus somnus." In the present study we expanded our efforts and examined the interaction of bovine mononuclear phagocytes with this important veterinary pathogen. Bovine alveolar macrophages and blood monocytes ingested but did not kill opsonized "Haemophilus somnus" in vitro, whereas these same cells ingested and killed opsonized Escherichia coli. Because this suggested that "H. somnus" was a facultative intracellular pathogen, we developed an assay to monitor the intracellular fate of ingested "H. somnus" within bovine monocytes. Our results indicated that ingested "H. somnus" multiplied within bovine monocytes (1- to 2-log10 increase in 4 h); equivalent intracellular growth was noted for both a laboratory strain and a recent field isolate of "H. somnus." Bovine monocytes killed ingested E. coli (1- to 2-log10 decrease in 4 h) under the same assay conditions that were used to follow intracellular growth of "H. somnus," thus indicating that the assay conditions did not induce a generalized defect in monocyte antibacterial activity. Light and electron microscopic examination of "H. somnus"-infected monocytes confirmed that intracellular growth had occurred. We did not observe an obvious correlation between the release of superoxide anion from bovine mononuclear phagocytes that had ingested opsonized "H. somnus" and E. coli and the subsequent intracellular survival of the bacteria. The results of this study suggest that infected mononuclear phagocytes sustain "H. somnus" infections in cattle and thus contribute to the subacute to chronic clinical course that has been reported.