Reactivity of sera from sheep immunised with individual outer membrane proteins of Bacteroides nodosus against heterologous bacterial strains

Investigation of immunity in sheep following footrot infection and vaccination

Ovine footrot is a major disease affecting sheep welfare and production. The anaerobic Gram-negative bacterium Dichelobacter nodosus is the essential transmitting agent. Monovalent or bivalent vaccines induce high levels of D. nodosus antibodies and are the basis of several successful footrot treatment, control and eradication programs. Due to the rapid rate of disease transmission within a flock, the presence of therapeutic vaccination non-responders has major implications for a control program. The aim of this study was to assess the immunological basis of a therapeutic vaccination non-response. Sheep (n=120) were infected with D. nodosus in an artificial pen challenge. Once disease had established, animals were vaccinated with a serogroup specific D. nodosus fimbrial vaccine. Based on the response to therapeutic vaccination, animals were allocated into one of three groups: (i) TVNR where disease persisted despite vaccination (ii) non-diseased, where disease never established and (iii) TVR, where disease was established but resolved with vaccination. Factors related to both the innate and adaptive immune pathways were assessed. These included antigen-specific serum antibodies, interferon-γ, interleukin-10, proliferation of lymphocyte subsets and phagocytic activity of leukocytes. There was no significant difference between the three groups of sheep for any of these parameters. All three groups of sheep produced antibody in excess of a previously published minimum antibody titre required for protection. Opsonising activity in sera from the three groups of sheep was also not significantly different and phagocytic cells from sheep from all three groups were able to destroy D. nodosus intracellularly. These findings show that the measured systemic adaptive and innate immune responses were unlikely to be the cause of a therapeutic vaccination non-response. They also show that the accepted minimum protective titre may be incorrect and may need further examination.

Association between alleles of the ovine major histocompatibility complex and resistance to footrot

Variation in natural resistance to footrot may be genetically derived, implying that genetic markers for resistance may exist and allow selection of superior animals. In this study association between variation within the ovine MHC class II region and resistance to footrot was investigated in two trials. Half-sib progeny were subjected to a field challenge with footrot and their condition subsequently recorded. The animals were then typed at their MHC class II loci to investigate associations between inherited paternal haplotype and footrot status. In the first trial an association between MHC haplotype and footrot status was observed across all animals (P = 0.005), when the self-curing and resistant animals were combined (P = 0.002) and when the self-curing animals were excluded from the analysis (P = 0.001). No association was observed in the second trial, a result attributed to the dry weather conditions which led to poor disease transmission and unreliable disease classification.

The use of electroblotted antigens of Trichostrongylus colubriformis to induce proliferative responses in sensitized lymphocytes from sheep

Merino sheep were immunized against the intestinal nematode, T. colubriformis, by repeated infections, and proliferative responses of their peripheral blood lymphocytes (PBL) against parasite extracts and excretory-secretory (ES) antigens were monitored over 130 days. Maximal responses occurred 7-14 days after challenge. The ability of soluble proteins and parasite antigens to induce proliferation was compared with that of antigen-bearing particles obtained after antigen was adsorbed onto nitrocellulose. Blank particles increased c.p.m. two- to three-fold above that obtained in medium alone, and to elicit proliferative responses of comparable magnitude between 10 and 100 times more antigen was required when antigen-bearing particles were used instead of soluble extracts or defined proteins. Blood leucocytes as well as T-cell lines established by stimulation with parasite antigens in vitro reacted to moieties of from 5000 to 38,000 mol. wt in ES antigens on nitrocellulose particles. Direct comparisons of T-lymphocyte responses with antibody responses as assessed by immunoblots revealed different profiles of immunogenicity among ES proteins within individual sheep, but the 10,000, 30,000 and 75,000-90,000 mol. wt proteins were immunodominant. These proteins were also those consistently recognized by T-lymphocytes and sera from sheep immunized with ES proteins in adjuvant. Thus, this technique can be applied to identify parasite material which is immunogenic for T-lymphocytes, but the sensitivity of the procedure in sheep is less than reported in human studies.

An anamnestic serological test for ovine footrot

Following recovery from ovine footrot, a proportion of sheep in a flock may carry the causative organism and spread it to other sheep if environmental conditions are favourable. Footrot affected sheep have elevated levels of serum antibody against Bacteroides nodosus, but these levels decline rapidly after clinical recovery. When challenged by subcutaneous injection with 470 micrograms of protein extracted from the cell membrane of B. nodosus, without adjuvant, sheep that had recovered clinically from virulent footrot produced a marked increase in specific serum antibody within 7 d, while antibody levels in footrot-free sheep injected with the same antigen, and in saline injected controls, did not increase over a period of 25 d. Artificial stimulation and serological detection of immune memory may be useful in footrot eradication programs by identifying sheep that have had clinical footrot infection. This procedure may be applicable to other diseases where antibody responses are inconsistent or transient.

Serum antibody responses in sheep after natural infection with Bacteroides nodosus

Soluble outer membrane protein of Bacteroides nodosus extracted with potassium thiocyanate (KSCN) was employed as antigen in an enzyme linked immunosorbent assay (ELISA) to detect serum antibody in sheep naturally infected with a heterologous serogroup. Serum antibody responses in 55 sheep were monitored for 2 years and maximum levels were directly related to the severity of clinical foot lesions. Serum antibody levels rose 2 weeks after foot lesions developed and declined within several months of resolution of lesions. After the first footrot transmission period, antibody levels persisted significantly (P less than 0.001) longer in sheep that did not become affected in the next transmission period compared with sheep in which footrot recurred. Antibody response did not appear to result in resolution of foot lesions. ELISA using KSCN antigen gave similar results to whole cell ELISA where cells prepared from an homologous serogroup were used as antigen. Both these assays were more sensitive than ELISA in which heterologous whole cell antigen was used. Proteins extracted from the outer membrane of B. nodosus, which are known to be immunogenic in natural infection and common to different serogroups of B. nodosus, appear to be useful antigens for serological investigations of ovine footrot.

An analysis of cellular proliferation, and synthesis of lymphokines and specific antibody in vitro by leucocytes from immunized cattle

The relationships between the production of lymphokines, cellular proliferation and antibody synthesis by immune bovine PBL in vitro was examined to identify the cellular reactions responsible for differences in the titres of serum antibodies in calves from selected sire lines and MHC Class I phenotypes. Leucocytes from 22 calves immunized with ovalbumin and DNP-BSA proliferated specifically in vitro in the presence of 1-10 micrograms/ml ovalbumin 7-28 days after the second vaccination. Significant correlations between the production of IL-2, IFN-gamma and maximum proliferation were observed for the total group. The quantity of specific antibody produced when PBL were incubated alone or with 10(-1)-10(-2) micrograms/ml ovalbumin was also correlated significantly with the maximum proliferation and the serum antibody titre between 7 and 14 days. Anti-ovalbumin IgG was also synthesized in MLRs where the quantity of antibody was significantly correlated with the magnitude of proliferation. The responses in vitro to DNP-BSA were too low to provide meaningful comparisons. The results indicate that at intervals during the period of increasing serum titres, events in the bovine antibody response in vivo can be replicated in vitro. In addition, assays for proliferation, IL-2 or gamma-IFN, or specific antibody can be used to assess the magnitude of the immune response in vivo in experimental groups of cattle. Significant sire line differences in the serological responses to ovalbumin were observed but DNP-BSA was a poorer antigen and differences in the responses to this antigen were not significant. However, the sire line differences in vivo were not reflected in vitro in proliferative and lymphokine assays; only the production of antibody in vitro was significantly correlated with the in vivo serum titre.

The pathogenicity and cultural characteristics of virulent, intermediate and benign strains of Bacteroides nodosus causing ovine foot-rot

The relationship between the cultural and biochemical characteristics of 22 strains of Bacteroides nodosus and their virulence for sheep was examined. Virulent, intermediate and benign strains were recognised. Although there was some relationship between virulence and colony morphology on hoof medium with 4% agar, colonies of one virulent and 4 intermediate strains resembled those of benign strains. However, on hoof medium with 2% agar and on blood Euonagar, colonies of this virulent and one intermediate strain differed from each other and the other 3 intermediate strains, which in turn differed from the benign. The degree of piliation, as assessed by electron microscopy, was not a reliable indicator of virulence in strains not possessing a beaded colony type. Together, the results of colony morphology and proteolytic tests such as zymogram, degrading proteinase and elastin-agar tests allowed better discrimination of virulent and benign strains. Intermediate strains generally possessed virulent protease activity. In strains with benign zymogram patterns, activity bands 2 and 3 were more labile than in strains with virulent patterns. The addition of CaCl2 to the culture medium resulted in greater stability of proteolytic activity, particularly with benign strains, and prevented the disappearance of protease activity in the band 5 position in virulent, intermediate and benign strains during prolonged incubation. There were slight differences in the sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) patterns of outer membrane proteins of some benign strains but those of intermediate category resembled virulent strains. There was some relationship between the apparent Mr of the pilin monomer on SDS-PAGE gels and serogroup specificity.

Variation in the structural subunit and basal protein antigens of Bacteroides nodosus fimbriae

The fimbriae of Bacteroides nodosus play a major role in protective immunity against ovine footrot and are an important determinant in the serological classification system that divides field isolates into at least eight serogroups and 16 serotypes. Purified fimbriae contain two polypeptide antigens, the structural subunit of the fimbrial strand (molecular weight about 17,000) and a basal protein (molecular weight about 80,000), both of which exhibit structural variation. Fimbriae were prepared from all prototype strains, as well as from a number of other isolates representative of each of the B. nodosus serotypes, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Substantial variation was observed in the electrophoretic mobility of the fimbrial subunits from the prototypes of each of the eight serogroups. With the exception of serogroup H, which is an unusual case, the apparent molecular weights of the fimbrial subunits ranged from about 16,500 in serogroup D to 19,000 in serogroup F (serotype 1); in serogroup A, B, C and E, the apparent molecular weights were clustered in the range of 17,000 to 17,500, whereas serogroup G was about 18,500. Serogroup H fimbriae appeared to consist of two smaller polypeptides, which in the prototype (H1) had apparent molecular weights of about 6,000 and 10,000 and which seem to have arisen as a consequence of an internal proteolytic nick in the original subunit. Electrophoretic variation in the fimbrial subunit was also observed between different serotypes, although with the exceptions of serogroups F and H, this was not as pronounced as between the serogroups. Examination of a number of isolates classified within the same serotypes showed that some variation, although minor, also occurred at this level. The basal antigen exhibited significant variation at all levels of the serotypic hierarchy in a manner apparently unrelated to the classification system. Among the range of isolates examined, the apparent molecular weight of this antigen varied from about 77,000 to 88,000.

Cross-protection from Bacteroides nodosus vaccines and the interaction of pili and adjuvants

The effects of vaccination of Merino sheep with the purified pili or the whole cells of Bacteroides nodosus strain 198, either in oil or alum-oil adjuvant, on the severity of foot-rot induced with the homologous strain (198) and a heterologous strain (217) were determined in a field experiment, on flood irrigated pasture. The efficacy of the whole cell vaccines was comparable to that of purified pili vaccines, against homologous challenge, when both had a similar content of pilus antigen although the purified pili vaccines induced significantly greater homologous pilus agglutinating antibody titres than the whole cell vaccines. However, against heterologous challenge, the whole cell vaccines in oil (CO) or alum-oil (CAO) provided significantly greater protection than a purified pili-in-oil (PPO) vaccine, the number of severely affected feet in sheep vaccinated with PPO being similar to that of the unvaccinated group. The group vaccinated with purified pili in alum-oil (PPAO) was intermediate between these two extremes. The superior performance of the PPAO in comparison to the PPO vaccine, against heterologous challenge, was associated with significantly higher mean ELISA titres to the outer membrane complex. Western blot analyses implicated a role in cross-protection for outer membrane proteins, in particular a protein Mr 78,000. The PPO vaccine produced fewer, smaller and less persistent vaccination reactions at the inoculation sites than did the other vaccines. Bodyweight gains in the period prior to challenge were much lower for the groups vaccinated with CO and CAO than for the controls and those vaccinated with purified pili, due presumably to the larger vaccination reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the purification of the leucocidin of Fusobacterium necrophorum and its neutralization by specific antisera

Leucocidin from several strains of Fusobacterium necrophorum was partially purified by gel filtration on Fractogel HW55 (F), the majority of the activity being present in the 50 ml of filtrate collected after 1.1 void volumes had passed through the column (termed Fraction 1, or #1). The material also contained lipopolysaccharide in 12.5% SDS-PAGE gels run under reducing conditions, but the protein did not migrate into 7.5% PAGE gels run under non-reducing conditions. Rabbit and bovine antisera to the leucocidin possessed antibodies against antigens in concentrated, washed culture supernates from toxigenic F. necrophorum and neutralized the leucocidal activity of such supernates. Absorption of the antisera with homologous, washed F. necrophorum cells reduced ELISA antibody titres by greater than 50%, but decreased neutralization titres by 15%. Absorbed rabbit IgG anti-#1 precipitated a single rocket in crossed immunoelectrophoresis and identified two proteins, of molecular weights (M.W.) 14 000 and 13 000, and 1 protein of M.W. 13 500 in immunoblots from toxigenic and non-toxigenic strains, respectively. An additional protein of M.E. 103 000 was present after SDS-PAGE separation of supernates from toxigenic but not non-toxigenic F. necrophorum and was not present in whole cell components. It was considered that the leucocidin may be present in a dimeric form in culture supernates from toxigenic strains. Antisera to leucocidins from several strains of F. necrophorum exhibited variable neutralization titres against leucocidins from heterologous bacteria.