Identification of the gene encoding BmpB, a 30 kDa outer envelope lipoprotein of Brachyspira (Serpulina) hyodysenteriae, and immunogenicity of recombinant BmpB in mice and pigs

Cytoplasmic expression of a model antigen with M Cell-Targeting moiety in lactic acid bacteria and implication of the mechanism as a mucosal vaccine via oral route

Lactic acid bacteria (LAB) have been widely studied as mucosal vaccine delivery carriers against many infectious diseases for heterologous expression of protein antigens. There are three antigen expression strategies for LAB: cytoplasmic expression (CE), cell surface display (SD), and extracellular secretion (ES). Despite the generally higher protein expression level and many observations of antigen-specific immunogenicity in CE, its application as a mucosal vaccine has been overlooked relative to SD and ES because of the antigens enclosed by the LAB cell wall. We hypothesized that the antigens in CE could be released from the LAB into the intestinal lumen before host bacterial access to gut-associated lymphoid tissue (GALT), which could contribute to antigen-specific immune responses after oral administration. To elucidate this hypothesis, three recombinant Lactobacillus plantarum (LP) strains were constructed to produce a model antigen, BmpB, with or without an M cell-targeting moiety, and their immunogenicities were analyzed comparatively as oral vaccines in mouse model. The data indicated that the recombinant LPs producing BmpBs with different conformations could induce mucosal immunity differentially. This suggests that the cytoplasmic antigens in LAB could be released into the intestinal lumen, subsequently translocated through M cells, and stimulate the GALT to generate antigen-specific immune responses. Therefore, the CE strategy has great potential, especially in the application of oral LAB vaccines as well as SD and ES strategies. This research provides a better understanding of the mechanism for recombinant oral LAB vaccines and gives insight to the future design of LAB vaccines and oral delivery applications for useful therapeutic proteins.

Vaccination of chickens with the 34 kDa carboxy-terminus of Bpmp72 reduces colonization with Brachyspira pilosicoli following experimental infection

The anaerobic intestinal spirochaete Brachyspira pilosicoli colonizes the large intestine of a variety of species of mammals and birds, and may result in colitis, diarrhoea and reductions in growth rate. Naturally occurring infections in chickens are largely confined to adult laying and breeding birds. In this study, the 34 kD carboxy-terminus of the prominent outer membrane protein Bmp72 of B. pilosicoli was expressed as a histidine-tagged recombinant protein and used to immunize two groups (B and C) of 15 individually housed layer chickens. Vaccination was with either 100 μg (B) or 1 mg (C) protein emulsified with Freund's incomplete adjuvant delivered into the pectoral muscles, followed three weeks later by 1 mg of protein in phosphate buffered saline delivered via crop tube. Two weeks later these and 15 non-vaccinated positive control birds (group A) housed in the same room were challenged via crop tube with B. pilosicoli avian strain CPS1. B. pilosicoli was detected in the faeces of all control birds and in 14 of the vaccinated birds in each vaccinated group at some point over the 30-day period following challenge. Colonization was delayed and the duration of excretion was significantly reduced (P = 0.0001) in both groups of vaccinated birds compared to the non-vaccinated control birds. Fewer immunized birds had abnormal caecal contents at post mortem examination compared to non-vaccinated birds, but the difference was not statistically significant. This study indicates that recombinant Bmp72 C-terminus has potential to be developed for use as a vaccine component to provide protection against B. pilosicoli infections. RESEARCH HIGHLIGHTS Laying chickens were immunized with recombinant Brachyspira pilosicoli membrane protein Bpmp72. Immunized birds had a highly significant reduction in the duration of colonization. Fewer immunized than control birds had abnormal caecal contents after infection. Bpmp72 showed potential for use as a novel vaccine component for B. pilosicoli.

In vitro attenuation of a virulent swine isolate of Brachyspira hampsonii

Brachyspira hampsonii causes dysentery-like disease in infected pigs. Serial passage of a virulent swine isolate (P13) one-hundred times in laboratory culture medium was conducted to produce an attenuated strain, and to identify genomic determinants of virulence through comparison of genome sequences of the original and passaged strains. The resulting strain, P113, did not differ from P13 in terms of diagnostic biochemical characteristics but had an enhanced growth rate in culture, indicating laboratory adaptation. Whole genome sequencing of P113 revealed several single-nucleotide changes including a T to C transition that results in an R to G amino acid change in a putative mannose-1-phosphate guanylytransferase that is implicated in production of lipo-oligosaccharide. P113 was partially attenuated in a mouse model of infection, indicated by significantly fewer observations of abnormal feces in mice infected with P113 relative to P13. No differences were detected in bacterial shedding in feces, demonstrating that the ability of the organism to colonize mice was not affected. Passage through a mouse did not further alter the virulence of P113. Results of this study provide insight into genomic determinants of virulence in B. hampsonii and a live attenuated vaccine candidate.

Swine Dysentery

Swine dysentery is a severe enteric disease in pigs, which is characterized by bloody to mucoid diarrhea and associated with reduced growth performance and variable mortality. This disease is most often observed in grower–finisher pigs, wherein susceptible pigs develop a significant mucohemorrhagic typhlocolitis following infection with strongly hemolytic spirochetes of the genus Brachyspira. While swine dysentery is endemic in many parts of the world, the disease had essentially disappeared in much of the United States by the mid-1990s as a result of industry consolidation and effective treatment, control, and elimination methods. However, since 2007, there has been a reported increase in laboratory diagnosis of swine dysentery in parts of North America along with the detection of novel pathogenic Brachyspira spp worldwide. Accordingly, there has been a renewed interest in swine dysentery and Brachyspira spp infections in pigs, particularly in areas where the disease was previously eliminated. This review provides an overview of knowledge on the etiology, pathogenesis, and diagnosis of swine dysentery, with insights into risk factors and control.

Soluble RANKL expression in Lactococcus lactis and investigation of its potential as an oral vaccine adjuvant

BACKGROUND

To initiate mucosal immune responses, antigens in the intestinal lumen must be transported into gut-associated lymphoid tissue through M cells. Recently, it has been increasingly recognized that receptor activator of NF-kB ligand (RANKL) controls M cell differentiation by interacting with RANK expressed on the sub-epithelium of Peyer's patches. In this study, we increased the number of M cells using soluble RANKL (sRANKL) as a potent mucosal adjuvant.

RESULTS

For efficient oral delivery of sRANKL, we constructed recombinant Lactococcus lactis (L. lactis) IL1403 secreting sRANKL (sRANKL-LAB). The biological activity of recombinant sRANKL was confirmed by observing RANK-RANKL signaling in vitro. M cell development in response to oral administration of recombinant L. lactis was determined by 1.51-fold higher immunohistochemical expression of M cell marker GP-2, compared to that of non-treatment group. In addition, an adjuvant effect of sRANKL was examined by immunization of mice with M-BmpB as a model antigen after treatment with sRANKL-LAB. Compared with the wild-type L. lactis group, the sRANKL-LAB group showed significantly increased systemic and mucosal immune responses specific to M-BmpB.

CONCLUSIONS

Our results show that the M cell development by sRANKL-LAB can increase the antigen transcytotic capability of follicle-associated epithelium, and thereby enhance the mucosal immune response, which implies that oral administration of sRANKL is a promising adjuvant strategy for efficient oral vaccination.

Evaluation of the use of recombinant Bhlp29.7 in immunoblotting with pig serum as a means to identify herds infected with Brachyspira hyodysenteriae

AIMS

Aim of the study is to evaluate the use of recombinant Bhlp29.7 in immunoblotting with sera as a means to detect pig herds infected with Brachyspira hyodysenteriae.

METHODS AND RESULTS

Sera samples from 789 sows and rectal swabs from 838 pigs of various categories on 22 farms of different size (median 450 animals), production type and history of swine dysentery (SD) were examined. Sera from 378 sows from farms with previous SD history were examined via immunoblotting. Specific antibodies were detected in 79 of these (20.9%). Examination of 411 serum samples from sows and gilts taken on 11 farms without previous history of SD detected specific antibodies in 13 sows and gilts (3.2%). These 13, however, had come from farms where the presence of B. hyodysenteriae was confirmed or SD status was not known. Seroprevalence in herds with previous SD history ranged from 2.5 to 35.7%. B. hyodysenteriae was confirmed on six (27.3%) of 22 monitored farms.

CONCLUSIONS

Immunoblotting using recombinant antigen Bhlp29.7 in conjunction with culturing B. hyodysenteriae proved to be a valuable tool for detecting swine herds latently infected with B. hyodysenteriae.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of immunoblotting with recombinant Bhlp29.7 should prove to be a useful adjunct to detecting herds with SD, and hence, it will assist in controlling this important disease.

Generation and characterization of thymidine/D-alanine auxotrophic recombinant Lactococcus lactis subsp. lactis IL1403 expressing BmpB

Genetic engineering of Lactococcus lactis to produce a heterologous protein may cause potential risks to the environment despite the industrial usefulness of engineered strains. To reduce the risks, we generated three auxotrophic recombinant L. lactis subsp. lactis IL1403 strains expressing a heterologous protein, BmpB, using thyA- and alr-targeting integration vectors: ITD (thyA(-)alr(+) bmpB(+)), IAD (thyA(+)alr(-)bmpB(+)), and ITDAD (thyA(-)alr(-) bmpB(+)). After construction of integration vectors, each vector was introduced into IL1403 genome. Integration of BmpB expression cassette, deletion of thyA, and inactivation of alr were verified by using PCR reaction. All heterologous DNA fragments except bmpB were eliminated from those recombinants during double crossover events. By using five selective agar plates, we also showed thymidine auxotrophy of ITD and ITDAD and D-alanine auxotrophy of IAD and ITDAD. In M17G and skim milk (SYG) media, the growth of the three recombinants was limited. In MRS media, the growth of IAD and ITDAD was limited, but ITD showed a normal growth pattern as compared with the wild-type strain (WT). All the recombinants showed maximal BmpB expression at an early stationary phase when they were cultivated in M17G supplemented with thymidine and D-alanine. These results suggest that auxotrophic recombinant L. lactis expressing a heterologous protein could be generated to reduce the ecological risks of a recombinant L. lactis.

Evaluation of recombinant Bhlp29.7 as an ELISA antigen for detecting pig herds with swine dysentery

Swine dysentery (SD) results from infection of the porcine large intestine with the anaerobic intestinal spirochaete Brachyspira hyodysenteriae. Diagnosis of SD traditionally has relied on detecting the spirochaete in the faeces of acutely affected pigs. To date simple and reliable serological assays that can be applied as a diagnostic tool at the herd level have not been available. In the current study a recombinant histidine tagged 29.7 kDa lipoprotein of B. hyodysenteriae (His6-Bhlp29.7) was used as an ELISA plate-coating antigen. Sera (n=1121) from slaughter-aged pigs on 19 farms were tested in this ELISA. Following optimization of the ELISA conditions using hyperimmune control sera, a set of 464 sera from slaughter-aged pigs from five herds where SD did not occur was tested. From these results a suitable cut-off value for herd negativity was defined as the mean optical density reading plus three standard deviations. Testing of 337 pig sera from six farms with SD then showed that the sensitivity of the test at the herd level was 100%, with all six farms having one or more serum samples exceeding the cut-off value for negativity. Finally, 320 sera from eight herds suspected of having SD were examined. Four of these herds were shown to have pigs with titres consistent with SD. The true health status of the other four herds that were serologically negative could not be confirmed. In conclusion, when used on sets of 40 sera from slaughter-aged pigs the His6-Bhlp29.7 ELISA as established proved to be a useful adjunct to the diagnosis of SD at the herd level.

Serologic detection ofBrachyspira (Serpulina) hyodysenteriaeinfections

Swine dysentery (SD) caused by the intestinal spirocheteBrachyspira hyodysenteriaeis an economically important disease in pig-producing countries throughout the world. To date, no specific serologic assay is commercially available for the diagnosis of pigs with SD. Several serologic techniques have been identified in the past; however, these tests have all used either whole-cell proteins or lipopolysaccharide (LPS) as the antigen. Whole-cell antigens are plagued with false-positive reactions due to cross-reactivity with common proteins shared with other spirochetes. LPS antigens produce fewer false-positives; however, false-negatives may result due to LPS components being serogroup-specific. Generally, these techniques are useful for detecting infected herds, but are unreliable for the detection of individual infected pigs. In order to develop improved serologic tests it will be necessary to identify suitable diagnostic antigens, in particular immunogenic cell-surface structures which are specific toB. hyodysenteriaebut common amongst different strains of the species. Recently, we identified and cloned a 30-kDa outer membrane lipoprotein (BmpB) which is specific toB. hyodysenteriaeand is recognized by experimentally and naturally infected pigs. In this review we summarize the available serologic tests for SD, and speculate on the use of recombinant BmpB as an antigen for future development of an improved serologic test for SD diagnosis.

The search forBrachyspiraouter membrane proteins that interact with the host

Little is known about the outer membrane structure ofBrachyspira hyodysenteriae and Brachyspira pilosicolior the role of outer membrane proteins (OMPs) in host colonization and the development of disease. The isolation of outer membrane vesicles fromB. hyodysenteriaehas confirmed that cholesterol is a significant outer membrane constituent and that it may impart unique characteristics to the lipid bilayer structure, including a reduced density. Unique proteins that have been identified in theB. hyodysenteriaeouter membrane include the variable surface proteins (Vsp) and lipoproteins such as SmpA and BmpB. While the function of these proteins remains to be determined, there is indirect evidence to suggest that they may be involved in immune evasion. These data may explain the ability of the organism to initiate chronic infection. OMPs may be responsible for the unique attachment ofB. pilosicolito colonic epithelial cells; however, the onlyB. pilosicoliOMPs that have been identified to date are involved in metabolism. In order to identify furtherB. pilosicoliOMPs we have isolated membrane vesicle fractions from porcine strain 95–1000 by osmotic lysis and isopycnic centrifugation. The fractions were free of contamination by cytoplasm and fla-gella and contained outer membrane. Inner membrane contamination was minimal but could not be completely excluded. An abundant 45-kDa, heat-modifiable protein was shown to have significant homology withB. hyodysenteriaeVsp, and monoclonal antibodies were produced that reacted with fiveB. pilosicoli-specificmembrane protein epitopes. The first of these proteins to be characterized is a unique surface-exposed lipoprotein.

SmpB: A novel outer membrane protein present in some Brachyspira hyodysenteriae strains

A novel outer membrane protein-encoding gene was identified in Brachyspira hyodysenteriae. The predicted protein, SmpB, was encoded by a gene that contains regions of identity with that encoding the previously identified lipoprotein SmpA. However, the majority of the reading frame encoding SmpA and SmpB share no detectable similarity. Analysis of several strains revealed that B. hyodysenteriae harbours either smpA or the newly identified gene smpB, but not both. smpB encodes for a slightly larger protein than smpA, 17.6 and 16.8 kDa, respectively. The predicted proteins share an identical leader sequence and the first 10 amino acids of the mature protein, however, the remainder of the predicted protein sequence shows no similarity. It is hypothesised that smpA and smpB are present on the same area of the chromosome. The proteins are antigenically unique, as antisera raised against a strain of B. hyodysenteriae that expresses SmpA cannot detect SmpB and vice versa. Although the presence of an identical leader peptide suggests identical localisation of SmpA and SmpB, it is not known if the two predicted proteins share similar function.

The distribution of bmpB, a gene encoding a 29.7kDa lipoprotein with homology to MetQ, in Brachyspira hyodysenteriae and related species

The distribution of the bmpB gene encoding BmpB, a 29.7 kDa outer membrane lipoprotein of the intestinal spirochaete Brachyspira hyodysenteriae, was investigated. Using PCR, the gene was detected in all the 48 strains of B. hyodysenteriae examined and in Brachyspira innocens strain B256T, but not in 11 other strains of B. innocens nor in 42 strains of other Brachyspira spp. The gene was sequenced from B. innocens strain B256T and from 11 strains of B. hyodysenteriae. The B. hyodysenteriae genes shared 97.9-100% nucleotide sequence similarity and had 97.5-99.5% similarity with the gene of B. innocens strain B256T. Southern hybridisation indicated that bmpB was present on a 1.9 kb HindIII fragment of the B. hyodysenteriae genome and on a 3.1 kb fragment of the B. innocens B256T genome. The B. innocens lipoprotein did not react in Western blots with monoclonal antibody BJL/SH1 that reacts with the B. hyodysenteriae lipoprotein. The difference in binding with the monoclonal antibody may reside in the replacement of a serine residue with a tyrosine residue at base position 210 in the lipoprotein from B. innocens B256T. Comparison of the BmpB amino acid sequence with sequences in the SWISS-PROT protein database indicated that it has 33.9-39.9% similarity with the d-methionine binding proteins (MetQ) of a number of pathogenic bacterial species. The bmpB gene was confirmed to be the same as a gene of B. hyodysenteriae that was recently designated "blpA".

Identification and cloning of the gene encoding BmpC: an outer-membrane lipoprotein associated with Brachyspira pilosicoli membrane vesicles

The intestinal spirochaeteBrachyspira pilosicolicauses colitis in a wide variety of host species. Little is known about the structure or protein constituents of theB. pilosicoliouter membrane (OM). To identify surface-exposed proteins in this species, membrane vesicles were isolated fromB. pilosicolistrain 95-1000 cells by osmotic lysis in dH2O followed by isopycnic centrifugation in sucrose density gradients. The membrane vesicles were separated into a high-density fraction (HDMV;ρ=1·18 g cm−3) and a low-density fraction (LDMV;ρ=1·12 g cm−3). Both fractions were free of flagella and soluble protein contamination. LDMV contained predominantly OM markers (lipo-oligosaccharide and a 29 kDaB. pilosicoliOM protein) and was used as a source of antigens to produce mAbs. FiveB. pilosicoli-specific mAbs reacting with proteins with molecular masses of 23, 24, 35, 61 and 79 kDa were characterized. The 23 kDa protein was only partially soluble in Triton X-114, whereas the 24 and 35 kDa proteins were enriched in the detergent phase, implying that they were integral membrane proteins or lipoproteins. All three proteins were localized to theB. pilosicoliOM by immunogold labelling using specific mAbs. The gene encoding the abundant, surface-exposed 23 kDa protein was identified by screening aB. pilosicoli95-1000 genome library with the mAb and was expressed inEscherichia coli. Sequence analysis showed that it encoded a unique lipoprotein, designated BmpC. Recombinant BmpC partitioned predominantly in the OM fraction ofE. colistrain SOLR. The mAb to BmpC was used to screen a collection of 13 genetically heterogeneous strains ofB. pilosicoliisolated from five different host species. Interestingly, only strain 95-1000 was reactive with the mAb, indicating that either the surface-exposed epitope on BmpC is variable between strains or that the protein is restricted in its distribution withinB. pilosicoli.

Characterization of a locus encoding four paralogous outer membrane lipoproteins of Brachyspira hyodysenteriae

The identification of Brachyspira hyodysenteriae outer membrane proteins (OMPs) that may stimulate immunity to swine dysentery is important for vaccine development. We report here the analysis of a novel locus, blpGFEA, encoding four tandem paralogous proteins of approximately 30 kDa from B. hyodysenteriae. The four proteins share 31-39% sequence identity with lipoproteins from several species of bacterial pathogens, but the locus possesses a unique genetic organization. Using antisera raised to recombinant versions of each of these proteins, only BlpA and BlpE were found to be immunologically cross-reactive with the other proteins encoded by the locus. Northern hybridization indicated that only blpA was expressed under in vitro growth conditions. In addition, convalescent swine serum recognized recombinant BlpA in immunoblotting experiments, demonstrating that it is also expressed during infection. Analysis of the translated sequences of each of the genes revealed atypical spirochetal signal peptidase II recognition sites, and BlpA was shown to be a lipoprotein by incorporation of tritiated palmitic acid. Native BlpA was completely extracted by Triton X-114 (TX-114) and partitioned exclusively into the detergent phase during extraction of whole B. hyodysenteriae cells, implicating it as a component of the brachyspiral outer membrane. Consistent with the transcriptional and immunological data, analysis of the brachyspiral outer membrane proteome also revealed expression of only BlpA. Notably, inactivation of blpA homologs in Haemophilus influenzae and Salmonella enteritidis resulted in attenuation of virulence.