Recombinant vaccines against ovine footrot

Genomic heterogeneity of Dichelobacter nodosus within and between UK sheep flocks and between age groups within a flock

Background

Footrot and interdigital dermatitis are endemic infectious diseases in all sheep farming regions, impairing welfare and production. The development of efficacious vaccines against the primary causative pathogen has been hampered by the extensive antigenic diversity of Dichelobacter nodosus. Understanding the heterogeneity of the pathogen within and between flocks is essential if the feasibility of bespoke vaccine production is to be assessed for use in the U.K.

Results

In this study 56 ewe and lamb isolates from 9 flocks were compared by D. nodosus serogroup and Multi Locus Sequence Type which provides significantly enhanced discriminatory power for molecular epidemiology. Serogroup heterogeneity between flocks ranged from two to five unique serogroups per flock. Three flocks contained isolates of two serogroups, two flocks contained isolates of three serogroups and one flock included isolates of five serogroups. Analysis of 25 isolates from one flock with high prevalence of lameness, identified that serogroup and sequence type was significantly correlated with age. Significantly higher proportion of lambs were infected with serogroup B (principally ST85) as opposed to serogroup H (principally ST86), which predominated amongst adult sheep.

Conclusions

Genomic heterogeneity of the pathogen was significantly lower within flock compared to heterogenicity observed between flocks. Furthermore, this study indicates that within a flock, the host-pathogen dynamics and susceptibility to particular D. nodosus strains may be age dependent.

Constituting a glutathione S-transferase-cocktail vaccine against tick infestation

Cocktail vaccines are proposed as an attractive way to increase protection efficacy against specific tick species. Furthermore, such vaccines made with different tick antigens have the potential of cross-protecting against a broad range of tick species. However, there are still limitations to the selection of immunogen candidates. Acknowledging that glutathione S-transferases (GSTs) have been exploited as vaccines against ticks and other parasites, this study aimed to analyze a GST-cocktail vaccine as a potential broad-spectrum tick vaccine. To constitute the GST-cocktail vaccine, five tick species of economic importance for livestock industry were studied (Rhipicephalus appendiculatus, Rhipicephalus decoloratus, Rhipicephalus microplus, Amblyomma variegatum, and Haemaphysalis longicornis). Tick GST ORF sequences were cloned, and the recombinant GSTs were produced in Escherichia coli. rGSTs were purified and inoculated into rabbits, and the immunological response was characterized. The humoral response against rGST-Rd and rGST-Av showed a stronger cross-reactivity against heterologous rGSTs compared to rGST-Hl, rGST-Ra, and rGST-Rm. Therefore, rGST-Rd and rGST-Av were selected for constituting an experimental rGST-cocktail vaccine. Vaccination experiment in rabbits showed that rGST-cocktail caused 35% reduction in female numbers in a Rhipicephalus sanguineus infestation. This study brings forward an approach to selecting immunogens for cocktail vaccines, and the results highlight rGST-Rd and rGST-Av as potentially useful tools for the development of a broad-spectrum tick vaccine.

Melatonin enhances responsiveness to Dichelobacter nodosus vaccine in sheep and increases peripheral blood CD4 T lymphocytes and IgG-expressing B lymphocytes

The immunomodulatory functions mediated by melatonin support its use as vaccine adjuvant. Previously, we have demonstrated that melatonin enhances antibody responses in sheep vaccinated against Dichelobacter nodosus. Here, we analyze the effect of melatonin on T and B lymphocyte subsets in peripheral blood of sheep vaccinated against D. nodosus. We also compare the use of melatonin in implants and in injections. Melatonin administration either as implants or by injection produced higher antibody titers against A1 and C serotypes compared to those animals that received only the vaccine. These results support the use of melatonin as an adjuvant in vaccination against D. nodosus. Firstly, melatonin induces higher antibody titer than the vaccine alone, secondly, melatonin increase IgG+ B lymphocytes and CD4+ T lymphocytes in vaccinated sheep. These results suggest that melatonin enhances T CD4 cell activation and subsequently secondary humoral immune responses. Further studies are required to determine the mechanism underlining the immunomodulatory role of melatonin in the context of vaccination.

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.

Glycosylation substrate specificity of Pseudomonas aeruginosa 1244 pilin

The beta-carbon of the Pseudomonas aeruginosa 1244 pilin C-terminal Ser is a site of glycosylation. The present study was conducted to determine the pilin structures necessary for glycosylation. It was found that although Thr could be tolerated at the pilin C terminus, the blocking of the Ser carboxyl group with the addition of an Ala prevented glycosylation. Pilin from strain PA103 was not glycosylated by P. aeruginosa 1244, even when the C-terminal residue was converted to Ser. Substituting the disulfide loop region of strain PA103 pilin with that of strain 1244 allowed glycosylation to take place. Neither conversion of 1244 pilin disulfide loop Cys residues to Ala nor the deletion of segments of this structure prevented glycosylation. It was noted that the PA103 pilin disulfide loop environment was electronegative, whereas that of strain 1244 pilin had an overall positive charge. Insertion of a positive charge into the PA103 pilin disulfide loop of a mutant containing Ser at the C terminus allowed glycosylation to take place. Extending the "tail" region of the PA103 mutant pilin containing Ser at its terminus resulted in robust glycosylation. These results suggest that the terminal Ser is the major pilin glycosylation recognition feature and that this residue cannot be substituted at its carboxyl group. Although no other specific recognition features are present, the pilin surface must be compatible with the reaction apparatus for glycosylation to occur.

Eradication of footrot of lesser clinical severity (intermediate footrot)

OBJECTIVE

To determine if intermediate footrot (IFR) can be eradicated from a flock of sheep by inspection and culling of cases during a non-transmission period and if prior antibiotic treatment or vaccination increases the likelihood of eradication.

PROCEDURE

A replicated field experiment that compared the three eradication strategies was followed by an observational study of the best of these applied in a commercial flock of 3000 sheep.

RESULTS

In the replicated experiment, IFR was eradicated either by inspection and culling alone, or when combined with vaccination. Eradication failed when the sheep were treated with parenteral antibiotics before inspection and culling during the non-transmission period. In the whole-flock program, eradication by repeated inspection and culling of footrot cases during the non-transmission period was successful and the flock remained free of infection 3 years later.

CONCLUSIONS

IFR can be eradicated by inspection and culling but latent infections, which may persist undetected for at least 34 weeks, require surveillance inspections to be repeated during the non-transmission phase of the program. The use of parenteral antibiotics as an aid to the eradication of IFR is contraindicated.

Transformation-mediated serogroup conversion of Dichelobacter nodosus

Dichelobacter nodosus is the essential causative agent of footrot in sheep. The type IV fimbriae of D. nodosus are required for virulence, are highly immunogenic and immunoprotective, and can be divided into 10 major serogroups. Fimbrial variation has been postulated to have arisen because of genetic recombination within the fimbrial gene region perhaps as a means of evading the immune response invoked by infection. To show that antigenic variation in these fimbriae could occur after natural transformation and subsequent homologous recombination, a suicide plasmid containing the fimbrial subunit gene, fimA, of a serogroup G strain was used to convert a serogroup I strain to serogroup G. The resultant mutants were shown by Western blotting and slide agglutination to produce serogroup G fimbriae, but by two independent methods to still have the genotype of the parent type I strain. These data have significant implications for the use of fimbrial vaccines for the control of ovine footrot and suggest that benign strains of D. nodosus could play an important role as a reservoir of alternative fimbrial antigens.

Ovine periodontitis as a potential model for periodontal studies. Cross-sectional analysis of clinical, microbiological, and serum immunological parameters

OBJECTIVES

: To investigate infection and host immunity patterns in sheep with naturally occurring "broken-mouth" periodontitis.

MATERIALS AND METHODS

: Eight periodontally healthy (HS) and eight periodontally diseased ewes (PDS) were selected. Subgingival plaque and sera were collected and examined for evidence of human periodontitis-associated pathogens. Serum IgG titers were measured by ELISA to multiple strains of Porphyromonas gingivalis, Bacteroides forsythus, Dichelobacter nodosus, Actinobacillus actinomycetemcomitans, Prevotella intermedia, and Fusobacterium nucleatum as well as several purified antigens (cysteine proteases, LPS, K, and fimbriae).

RESULTS

: Neither the organism Aa nor antigens to Aa were found in any animal. Most animals were positive for Pg, Bf, and Pi, but DNA probes detected no difference between HS and PDS relative to amounts of pathogens in subgingival plaque. PDS had significantly higher serum IgG titers to all Pg strains, to 50% of Bf strains, to the Pi and Fn strains, and to fimbriae and the two cysteine proteases (p-values ranging from 0.05 to 0.001). Regression analysis demonstrated a significant association between number of teeth lost and serum IgG antibody titers to whole-cell sonicate antigens of P. gingivalis strains (p<0.01) and body weight (p<0.01).

CONCLUSIONS

: The presence of pathogens associated with periodontitis was reflected in differences in serum IgG titers between healthy and diseased sheep. This may have influenced animal body weight and might have systemic health and economic consequences. The data suggest that susceptible and non-susceptible sheep can be identified for periodontal research.

Eradication of virulent footrot from sheep and goats in an endemic area of Nepal and an evaluation of specific vaccination

Programmes based on the identification and treatment of cases and the culling of animals refractory to treatment had failed to eradicate virulent footrot from two districts in the western region of Nepal. From 1993 to 1996 vaccination against two endemic virulent strains of Dichelobacter nodosus was tested for its potential to contribute to the eradication of footrot from the region. Only sheep and goats which had been free of signs of footrot at three inspections at monthly intervals before their annual migration to alpine pastures were eligible for inclusion. From November 1992, the treatment of cases identified during inspections included the injection of specific vaccine. Successfully treated cases migrated with their flocks but were excluded from the vaccine trial. Non-responding cases were culled. Forty combined flocks of sheep and goats (approximately 9500 animals) were used initially to compare three vaccination regimens. Eleven flocks (sheep and goats) were treated with two doses of specific vaccine (group A), nine (sheep and goats) were treated with commercial vaccine followed by specific vaccine (group B) and 10 (sheep and goats) were treated with two doses of commercial vaccine (group C) in March to April 1993 before the annual migration; 10 flocks (sheep and goats) remained unvaccinated (group D). Only sheep and goats free of signs of footrot were allowed to migrate. Nevertheless, virulent footrot recurred in many flocks three months later. However, its prevalence was significantly lower in group A than in the other three groups combined. Groups A, B and C then received the specific vaccine before their migrations in 1994 to 1996; group D remained unvaccinated. The annual programme of inspection and identification and treatment of cases continued for seven years, but the vaccinations ceased after four years. There was no recurrence of virulent footrot after November 1993. After the first season the virulent strains of D nodosus used in the specific vaccine could no longer be isolated, although antigenically distinct, benign strains of the organism persisted in cases of benign footrot.

Rapid and accurate typing of Dichelobacter nodosus using PCR amplification and reverse dot-blot hybridisation

Here we describe an approach to genotyping D. nodosus, based on variation in the fimbrial subunit gene (fimA), which uses polymerase chain reaction (PCR) amplification and hybridisation to immobilised oligonucleotides (PCR/oligotyping). The variable region of D. nodosus fimA, amplified and labelled with digoxigenin (DIG) in a single multiplex PCR amplification, was hybridised to a panel of group- and type-specific poly-dT tailed oligonucleotides that were immobilised on a nylon membrane strip. A mixture of positive control poly-dT tailed oligonucleotides was also included on the membrane. After hybridisation the membrane was washed to a defined specificity, and DIG-labelled fragments hybridising were detected with nitroblue tetrazolium (NBT) and 5-bromo-4-chloro-3-indolyl phosphate (SCIP). The specificity of the oligonucleotides was verified by the lack of cross-reactivity with D. nodosus fimA sequences that had a single base difference. DNA from 14 footrot samples previously genotyped by PCR-SSCP/sequencing [Vet. Microbiol. 71 (2000) 113], was assayed using the PCR/oligotyping technique. All types of D. nodosus which had been detected previously with a PCR-SSCP/sequencing method were detected by this procedure. However, for three of the 14 footrot samples, PCR/oligotyping detected additional types of D. nodosus. Further PCR amplification using type-specific primers, confirmed that these types of the bacterium were present in the footrot samples. These results indicate that PCR/oligotyping is a specific, accurate, and useful tool for typing footrot samples. In combination with a rapid DNA extraction protocol, D. nodosus strains present in a footrot sample can be accurately identified in less than 2 days.

Dichelobacter nodosus serotype M fimbrial subunit gene: implications for serological classification

Dichelobacter nodosus fimbrial subunit gene (fimA) from a serotype M strain (M-SPAHL) was investigated in this study. A primer set targeting the relatively conserved fimA regions and based on the published sequence from Nepalese serogroup M isolates (Nepalese M), failed to amplify the fimA of M-SPAHL. However, when the downstream primer was substituted with a primer that is specific for other serogroups of D. nodosus, the fimA was successfully amplified. Cloning followed by DNA sequencing, revealed that the M-SPAHL fimA was different to the Nepalese M fimA. The predicted amino acid sequence of the M-SPAHL fimA did not show homology to any known serogroups or serotypes. The most similar sequence was from serotype F1, and not Nepalese M. The consequences of serological relatedness and sequence dissimilarity are discussed.

Vaccination against ovine footrot using a live bacterial vector to deliver basic protease antigen

A strain of Corynebacterium pseudotuberculosis, designated Toxminus, that has been rationally attenuated by deletion of the phospholipase D gene, is being developed as a live vaccine vector for the delivery of veterinary vaccine antigens. In the present study a recombinant form of the basic protease gene of the ovine footrot causative bacterium, Dichelobacter nodosus, was introduced into the vector strain using the high copy number plasmid pEP2. This strain secreted the basic protease protein. Vaccination trials in sheep with the recombinant strain demonstrated that, although an IgG immune response was elicited, the animals were not protected from footrot following artificial challenge under pen conditions. Although the sheep were not protected there was evidence that the progression of the disease was slowed.

Novel fimbrial subunit genes of Dichelobacter nodosus: recombination in vivo or in vitro?

Polymerase chain reaction (PCR) was used to amplify the variable region of the fimbrial subunit encoding gene (fimA) of Dichelobacter nodosus from sheep and goats infected with footrot. Two amplimers (designated X and Y) generated single-strand conformation polymorphism (SSCP) patterns different to those of previously identified serogroups and serotypes. DNA sequencing revealed that these two fragments were novel. The upstream of X (nt 1-183) was identical to serotype M1 while its downstream (nt 223-414) was identical to serotype F1; the upstream of Y (nt 1-116) was identical to serotype E1, whereas its downstream (nt 148-423) was identical to serotype F1. A 14-mer sequence consisting of two partially overlapping Chi-like sequences, 5'-GCTGGTGCTGGTGA-3', was also found in these fragments. Two primer sets with the downstream primer specific for serotype F1 and the upstream primer specific for serotype M or E1, generated PCR products of the expected sizes from the footrot samples from which fragments X and Y were isolated, respectively. These primer sets did not appear to amplify artificially mixed genomic DNA from serotypes M and F1 or E1 and F1. However, when the reactions were re-amplified, PCR recombination artifacts were observed, suggesting that PCR recombination does occur, but at a low frequency. It, therefore, seems more likely that fragments X and Y reflect genuine fimA genes of D. nodosus which have resulted from in vivo DNA recombination rather than from a PCR recombination artifact.

Extensive diversity in New Zealand Dichelobacter nodosus strains from infected sheep and goats

Footrot is a contagious bacterial disease of ruminants spread by the Gram-negative, anaerobic organism, Dichelobacter nodosus. It is endemic in New Zealand and throughout sheep and goat farming regions of the world. Using the polymerase chain reaction (PCR) to amplify fragments of the fimbrial gene (fimA), D. nodosus was detected in 14 hoof scrapings, sampled from six farming regions within New Zealand. DNA sequencing revealed 15 strains covering eight serogroups on the New Zealand farms. The predominant serogroup was B which contained six strains, followed by serogroups F, H and G. No strains from serogroups D and I were detected in this investigation. Eleven out of the 15 D. nodosus strains had fimbriae sequences different to those previously reported and the presence of multiple strains on a single hoof was common (86% samples). Individual sheep from the same farm, or the same paddock, were often infected by a different range of strains, which suggests a host role in mediating footrot infection.

Biodegradable implants for the delivery of veterinary vaccines: design, manufacture and antibody responses in sheep

Biodegradable implants made from cholesterol and lecithin (C:L) were used to deliver a recombinant antigen (recombinant Dichelobacter nodosus pili) and adjuvant (Quil A) to sheep. Implants (5.5- x 1.8-mm) were placed subcutaneously and compared to a conventional vaccination regime (2 injections, 4 weeks apart) for antibody responses and tissue compatibility. Release profiles of antigen and adjuvant were also studied in vitro and in vivo. The presence of Quil A in vaccine implants had a marked effect on the rate at which antigen was released with 29 and 44% being released in the first 24 h from implants containing pili alone and pili with Quil A, respectively. Sheep produced significant levels of antibody when immunized with implants, however the response was short-lived and of significantly lower intensity than the response stimulated by two injections of antigen with Quil A (P < 0.01). A second implant system was developed where implants coated with C:L to delay antigen release, were used in combination with uncoated implants to deliver a priming dose and boosting dose of antigen. Antibody titres stimulated by the 4 double implant system were equivalent to those stimulated by a conventional regime of two injections (four weeks apart) for the first six weeks of the experiment. After this time, antibody levels in the groups which received implants dropped significantly. In vitro studies revealed that some of the implant coatings had caused a delay in the release of antigen (the rate of release peaked at 72 h), however this was not long enough to provide a significant boosting effect. In all cases, implants were well tolerated by sheep and caused less local reaction than injected vaccines.

Effect of adjuvants on antibody responses of sheep immunised with recombinant pili from Dichelobacter nodosus

OBJECTIVE

To compare the effects of two oil emulsion adjuvants (incomplete Freunds adjuvant and a proprietary oil adjuvant), DEAE-dextran, L-tyrosine particles and Quil A on the humoral immune responses of sheep immunised with recombinant pili of Dichelobacter Nodosus (strain A).

PROCEDURE

Antibody titres were studied for up to 32 weeks and were measured by bacterial agglutination and ELISA. The relative avidity of antibodies for pili was determined and the incidence and severity of adverse reactions at the site of injection of vaccines were recorded.

RESULTS

The oil emulsion adjuvants and Quil A were more effective than either DEAE-dextran or L-tyrosine at stimulating antibodies in sheep. The incidence and severity of adverse reactions was lower in sheep which received vaccines containing either Quil A or DEAE-dextran than in sheep which received vaccines containing oil emulsion adjuvants. L-tyrosine had no adverse effects.

CONCLUSION

Quil A was as effective as oil adjuvants at stimulating high levels of antibodies against recombinant pili in sheep and had the significant advantage of being less irritant after subcutaneous injection.

Immunological parameters associated with antigenic competition in a multivalent footrot vaccine

A murine model for antigenic competition with multivalent D. nodosus pili vaccine has been established that parallels the phenomenon observed in sheep where levels of antibody, specific for any particular serogroup of pili, are significantly lower following vaccination in the presence of multiple serogroups of pili than with that serogroup alone. This competition was observed in both high and low responder strains of mice and was not dependent on the multiplicity of the antigens in the multivalent vaccine but could be observed with a large excess of a single heterologous serogroup. Competition was manifest by a reduction in the number of serogroup-specific antibody secreting cells elicited in response to vaccination. The antibody response to a single serogroup of pili reached a plateau at high doses and it was at these doses that antigenic competition was most pronounced, under conditions where both B- and T-cell responses were limiting. The limit in T-cell responsiveness was not imposed at the level of presentation of antigen. Pili-specific T cells were largely cross-reactive for different serogroups, and under conditions of limiting T-cell stimulation within a lymph node the available T cells would have to be shared between B cells specific for each serogroup of pili, which may in turn result in the decrease of serogroup-specific antibody induced following inoculation with the multivalent vaccine.