Characterization of immunodominant and potentially protective epitopes of Mannheimia haemolytica serotype 1 outer membrane lipoprotein PlpE

Development of a plant-based oral vaccine candidate against the bovine respiratory pathogen Mannheimia haemolytica

Bovine respiratory disease (BRD) affects feedlot cattle across North America, resulting in economic losses due to animal treatment and reduced performance. In an effort to develop a vaccine candidate targeting a primary bacterial agent contributing to BRD, we produced a tripartite antigen consisting of segments of the virulence factor Leukotoxin A (LktA) and lipoprotein PlpE from Mannheimia haemolytica, fused to a cholera toxin mucosal adjuvant (CTB). This recombinant subunit vaccine candidate was expressed in the leaves of Nicotiana benthamiana plants, with accumulation tested in five subcellular compartments. The recombinant protein was found to accumulate highest in the endoplasmic reticulum, but targeting to the chloroplast was employed for scaling up production due the absence of post-translational modification while still producing feasible levels. Leaves were freeze dried, then orally administered to mice to determine its immunogenicity. Sera from mice immunized with leaf tissue expressing the recombinant antigen contained IgG antibodies, specifically recognizing both LktA and PlpE. These mice also had a mucosal immune response to the CTB+LktA+PlpE protein as measured by the presence of LktA- and PlpE-specific IgA antibodies in lung and fecal material. Moreover, the antigen remained stable at room temperature with limited deterioration for up to one year when stored as lyophilized plant material. This study demonstrated that a recombinant antigen expressed in plant tissue elicited both humoral and mucosal immune responses when fed to mice, and warrants evaluation in cattle.

Combinations but Not a Single PlpE Epitope Induces Host Protective Immunity against Pasteurella multocida

Pasteurella multocida is the causative agent of a wide range of diseases (pasteurellosis) and a zoonotic pathogen in humans. Recombinant subunit vaccines are hot spots in recent pasteurellosis vaccine development. A chimeric vaccine is also constructed for rabbit hemorrhagic disease virus (RHDV) protective antigen VP60 chimeric with fragments of Pasteurella multocida protective antigen PlpE. The protective efficacy of the chimeric vaccine against P. multocida is not as high as that of PlpE, and the reason is not well known. In this study, we analyzed the linear B-cell epitopes of PlpE and then assessed the protective efficacy of these epitopes and their combinations. It was found that the immunodominant region of PlpE was mainly located in the region between the 21st to the 185th amino acids from the N terminus. Overlapping peptide scanning results demonstrated that this region contained six nonoverlapping epitopes, and epitope E was the predominant epitope. Chimeric protein antigens were constructed of single nonoverlapping PlpE epitopes or their combinations chimeric with the RHDV VP60 P domain. Immunization with recombinant antigen chimeric with a single PlpE epitope exhibited poor immunoprotection, whereas immunization with recombinant antigen chimeric with PlpE epitope combinations (epitopes A and E; epitopes C and E; epitopes A, C, and E; and epitopes B, D, and F) exhibited significant immunoprotection. In a word, P. multocida protective antigen PlpE contained six nonoverlapping linear B-cell epitopes, and combinations but not a single epitope induced host protective immunity. Our work will give help for future chimeric vaccine design.

Signal sequence contributes to the immunogenicity of Pasteurella multocida lipoprotein E

Recombinant Pasterurella multocida lipoprotein E (PlpE) has been shown to protect against fowl cholera. This study aimed to determine if the signal sequence may contribute to the antigenicity and protective efficacy of recombinant PlpE. A small antigenic domain of PlpE (termed truncated PlpE, tPlpE) was constructed with (SP-tPlpE) or without (tPlpE) the signal sequence and evaluated in vitro and in vivo. In vitro, the HEK-Bule hTLR2 Cells were used to evaluate the activation of NF-kB in the test associated with the stimulation of the SP-tPlpE and tPlpE proteins. When chickens were immunized, compared to the tPlpE vaccine group, the SP-tPlpE group showed higher antibody levels and enhanced CD4⁺ T cell response. In a challenge test, the SP-tPlpE group showed a survival rate of 87.5% (n = 8), compared to 25% for the tPlpE group. It is confirmed that the inclusion of the native signal sequence enhanced protective efficacy against fowl cholera and may act as a vaccine adjuvant. The short SP-tPlpE construct is amenable to further vaccine engineering and has potential to be developed as a fowl cholera vaccine.

Mannheimia haemolytica in bovine respiratory disease: immunogens, potential immunogens, and vaccines

Mannheimia haemolytica is the major cause of severe pneumonia in bovine respiratory disease (BRD). Early M. haemolytica bacterins were either ineffective or even enhanced disease in vaccinated cattle, which led to studies of the bacterium's virulence factors and potential immunogens to determine ways to improve vaccines. Studies have focused on the capsule, lipopolysaccharide, various adhesins, extracellular enzymes, outer membrane proteins, and leukotoxin (LKT) resulting in a strong database for understanding immune responses to the bacterium and production of more efficacious vaccines. The importance of immunity to LKT and to surface antigens in stimulating immunity led to studies of individual native or recombinant antigens, bacterial extracts, live-attenuated or mutant organisms, culture supernatants, combined bacterin-toxoids, outer membrane vesicles, and bacterial ghosts. Efficacy of several of these potential vaccines can be shown following experimental M. haemolytica challenge; however, efficacy in field trials is harder to determine due to the complexity of factors and etiologic agents involved in naturally occurring BRD. Studies of potential vaccines have led current commercial vaccines, which are composed primarily of culture supernatant, bacterin-toxoid, or live mutant bacteria. Several of those can be augmented experimentally by addition of recombinant LKT or outer membrane proteins.

A chimeric protein comprising the immunogenic domains of Mannheimia haemolytica leukotoxin and outer membrane protein PlpE induces antibodies against leukotoxin and PlpE

Mannheimia haemolytica is a very important pathogen of pneumonia in ruminants. Bighorn sheep (BHS, Ovis canadensis) are highly susceptible to M. haemolytica-caused pneumonia which has significantly contributed to the drastic decline of bighorn sheep population in North America. Pneumonia outbreaks in wild BHS can cause mortality as high as 90%. Leukotoxin is the critical virulence factor of M. haemolytica. In a 'proof of concept' study, an experimental vaccine containing leukotoxin and surface antigens of M. haemolytica developed by us induced 100% protection of BHS, but required multiple booster injections. Vaccination of wild BHS is difficult. But they can be vaccinated at the time of transplantation into a new habitat. Administration of booster doses, however, is impossible. Therefore, a vaccine that does not require booster doses is necessary to immunize BHS against M. haemolytica pneumonia. Herpesviruses are ideal vectors for development of such a vaccine because of their ability to undergo latency with subsequent reactivation. As the first step towards developing a herpesvirus-vectored vaccine, we constructed a chimeric protein comprising the leukotoxin-neutralizing epitopes and the immuno-dominant epitopes of the outer membrane protein PlpE. The chimeric protein was efficiently expressed in primary BHS lung cells. The immunogenicity of the chimeric protein was evaluated in mice before inoculating BHS. Mice immunized with the chimeric protein developed antibodies against M. haemolytica leukotoxin and PlpE. More importantly, the anti-leukotoxin antibodies effectively neutralized leukotoxin-induced cytotoxicity. Taken together, these results represent the successful completion of the first step towards developing a herpesvirus-vectored vaccine for controlling M. haemolytica pneumonia in BHS, and possibly other ruminants.

Expeditious screening of candidate proteins for microbial vaccines

Advancements in high-throughput "omics" technologies have revolutionized the way vaccine candidates are identified. Now every surface expressed protein that an organism produces can be identified in silico and possibly made available for the rapid development of recombinant/subunit vaccines. However, evaluating the antigenicity of a large number of candidate proteins is an immense challenge, typically requiring cloning of several hundred candidates followed by immunogenicity screening. Here we report the development of a rapid, high-throughput method for screening candidate proteins for vaccines. This method involves utilizing a coupled, cell-free transcription-translation system to screen tagged proteins that are captured at the C-termini using appropriate ligand coated wells in 96 well ELISA plates. The template DNA for the cell-free expression is generated by two sequential PCRs and includes gene coding sequences, promoter, terminator, other necessary cis-acting elements and appropriate tag sequences. The process generates expressible candidate proteins containing two different peptide tags at the N- and the C-termini of the protein molecules. Proteins are screened in parallel for their quantity and immunoreactivity with N-terminal tag antibodies and antisera raised against the pathogen of interest, respectively. Normalization against the total detectable bound protein in the control wells allows for the identification of highly immunoreactive candidates. For this study we selected 30 representatives of >300 potential candidate proteins from Mannheimia haemolytica, a bacterial agent of pneumonia in feedlot cattle for expression with N-terminal Strep-II and C-terminal His(x6)-tag and evaluated their relative immunoreactivities using Strep-tactin-HRP and rabbit antisera generated against M. haemolytica. Using this system we were able to swiftly and quantitatively analyze and rank the suitability of proteins to identify potentially viable vaccine candidates, with the majority of the high ranking candidates being associated with virulence and pathogenicity. The system is adaptable to any bacterial target and presents an alternative to conventional laborious cloning, expression and screening procedures.

Proteomic analysis and immunogenicity of Mannheimia haemolytica vesicles

Mannheimia haemolytica, a major causative agent in bovine respiratory disease, inflicts extensive losses each year on cattle producers. Commercially available vaccines are only partially efficacious. Immunity to M. haemolytica requires antibodies to secreted toxins and outer membrane proteins (OMPs) of the bacterium. Gram-negative bacteria produce membrane blebs or vesicles, the membrane components of which are primarily derived from OMPs. Accordingly, vesicles have been used as immunogens with various degrees of success. This study characterized components of M. haemolytica vesicles and determined their immunogenicity in mice and cattle. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of vesicles from this bacterium identified 226 proteins, of which 58 (25.6%) were OMPs and periplasmic and one (0.44%) was extracellular. Vesicles were used to vaccinate dairy calves and BALB/c mice. Analyses of sera from calves and mice by enzyme-linked immunosorbent assay (ELISA) showed that circulating antibodies against M. haemolytica whole cells and leukotoxin were significantly higher on days 21 and 28 (P < 0.05) than on day 0. For control calves and mice, there were no significant differences in serum anti-whole-cell and leukotoxin antibody levels from days 0 and 21 or 28, respectively. Lesion scores of lungs from vaccinated calves (15.95%) were significantly (P < 0.05) lower than those from nonvaccinated calves (42.65%). Sera from mice on day 28 and calves on day 21 showed 100% serum bactericidal activity. Sera from vesicle-vaccinated mice neutralized leukotoxin.

LKTA and PlpE small fragments fusion protein protect against Mannheimia haemolytica challenge

Bovine respiratory disease (BRD) complex is a major cause of economic losses for the cattle backgrounding and feedlot industries. Mannheimia haemolytica is considered the most important pathogen associated with this disease. Vaccines against M. haemolytica have been prepared and used for many decades, but traditional bacterins have failed to demonstrate effective protection and their use has often exacerbated disease in vaccinated animals. Thus, the BRD complex continues to exert a strong adverse effect on the health and wellbeing of stocker and feeder cattle. Therefore, generation of recombinant proteins has been helpful in formulating enhanced vaccines against M. haemolytica, which could confer better protection against BRD. In the present study, we formulated a vaccine preparation enriched with recombinant small fragments of leukotoxin A (LKTA) and outer-membrane lipoprotein (PlpE) proteins, and demonstrated its ability to generate high antibody titers in rabbits and sheep, which protected against M. haemolytica bacterial challenge in mice.

Immunogenicity of Mannheimia haemolytica recombinant outer membrane proteins serotype 1-specific antigen, OmpA, OmpP2, and OmpD15

We previously identified Mannheimia haemolytica outer membrane proteins (OMPs) that may be important immunogens by using immunoproteomic analyses. Genes for serotype 1-specific antigen (SSA-1), OmpA, OmpP2, and OmpD15 were cloned and expressed, and recombinant proteins were purified. Objective 1 of this study was to demonstrate immunogenicity of the four recombinant OMPs in mice and cattle. Objective 2 was to determine if the addition of individual recombinant OMPs or combinations of them would modify immune responsiveness of mice to the recombinant chimeric protein SAC89, containing the main epitope from M. haemolytica outer membrane lipoprotein PlpE and the neutralizing epitope of M. haemolytica leukotoxin. Mice vaccinated with recombinant OmpA (rOmpA), rSSA-1, rOmpD15, and rOmpP2 developed significant antibody responses to M. haemolytica outer membranes and to the homologous recombinant OMP. Cattle vaccinated with rOmpA and rSSA-1 developed significant antibodies to M. haemolytica outer membranes by day 28, whereas cattle vaccinated with rOmpD15 and rOmpP2 developed only minimal responses. Sera from cattle vaccinated with each of the recombinant proteins stimulated complement-mediated killing of the bacterium. Concurrent vaccination with SAC89 plus any of the four rOMPs singly resulted in increased endpoint anti-SAC89 titers, and for the SAC89/rSSA-1 vaccinees, the response was increased significantly. In contrast, the SAC89/P2/SSA-1 and SAC89/OmpA/P2/D15/SSA-1 combination vaccines resulted in significant decreases in anti-SAC89 antibodies compared to SAC89 vaccination alone. In conclusion, under the conditions of these experiments, vaccination of mice and cattle with rOmpA and rSSA-1 stimulated high antibody responses and may have protective vaccine potential.

Identification and immunogenicity of Mannheimia haemolytica S1 outer membrane lipoprotein PlpF

Immunity against Mannheimia haemolytica requires antibodies against leukotoxin (LKT) and bacterial cell surface antigens, most likely immunogenic outer membrane proteins (OMPs). Five immunogenic outer membrane lipoproteins identified and characterized in M. haemolytica were designated Pasteurella lipoproteins (Plp) A, -B, -C, -D and -E. Using immunoproteomics, we identified a heretofore-uncharacterized M. haemolytica immunogenic outer membrane lipoprotein that we designated PlpF, which was previously designated in the published sequence as a conserved hypothetical protein. We cloned and expressed rPlpF from two M. haemolytica serotype 1 strains (SAC159 and SAC160) and demonstrated a variable number of perfect (KKTEED) or imperfect (KKaEEa) repeats between residues 41 and 76 on the N-terminus. Antigenicity plots predicted the N-terminus repeat region to be highly antigenic. The plpF gene in multiple M. haemolytica S1, S2, and S6 isolates varied in the number of repeats from three to seven. C-terminal region was highly conserved. Immunization of mice with SAC159 or SAC160 demonstrated immunogenicity in a dose-response manner. Immunization of calves demonstrated an increase in antibodies to PlpF, and rPlpF antibodies stimulated complement-mediated killing of M. haemolytica. Because calves had pre-existing anti-M. haemolytica antibodies due to prior natural exposure, functionality of the anti-PlpF antibody responses were demonstrated by marked reduction of complement-mediated killing by blocking of anti-PlpF antibodies with rPlpF In conclusion, PlpF might have vaccination potential against M. haemolytica 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.

Intranasal vaccination of calves with Mannheimia haemolytica chimeric protein containing the major surface epitope of outer membrane lipoprotein PlpE, the neutralizing epitope of leukotoxin, and cholera toxin subunit B

This study was done to determine if intranasal vaccination of weaned beef calves with a chimeric protein containing the immunodominant surface epitope of Mannheimia haemolytica PlpE (R2) and the neutralizing epitope of leukotoxin (NLKT) covalently linked to truncated cholera toxin (CT) subunit B (CTB) could stimulate secretory and systemic antibodies against M. haemolytica while enhancing resistance of cattle against M. haemolytica intrabronchial challenge. Sixteen weaned beef calves were intranasally vaccinated with CTB-R2-NLKT chimeric (SAC102) or with R2-NLKT-R2-NLKT chimeric (SAC89) protein with or without native CT on days 0 and 14 and were challenged intrabronchially on day 28. In vitro, SAC102 bound the CT receptor molecule, GM(1)-ganglioside. Mean IgA antibodies to M. haemolytica whole cells (WC) and to LKT were high on day 0. A small, yet significant increase (p<0.05) was found in mean nasal antibodies to M. haemolytica WC for the SAC89+CT and SAC102 vaccinates after the second vaccination. SAC102 stimulated significant (p<0.05) mean serum antibody responses to all three antigens by day 28. Following challenge, mean antibodies to WC and LKT significantly increased (p<0.05) for the SAC102, SAC89 and SAC89+CT groups with the mean antibody responses to rPlpE stimulated by SAC102 vaccination being significantly higher (p<0.05) than for the other vaccinated and control groups. On day 1 after challenge, mean clinical score for the control group was significantly higher (p<0.05) than for the SAC102 and SAC89+CT vaccinates, and by day 2 after challenge, clinical score for the control group was significantly higher (p<0.05) than for all three chimeric vaccinated groups. Therefore, intranasal vaccination with CTB-R2-NLKT (SAC102) and R2-NLKT-R2-NLKT (SAC89) chimeric proteins enhanced resistance against intrabronchial challenge with the bacterium as well as stimulating antibody responses to M. haemolytica antigens.

Mannheimia haemolytica and bovine respiratory disease

Mannheimia haemolytica is the principal bacterium isolated from respiratory disease in feedlot cattle and is a significant component of enzootic pneumonia in all neonatal calves. A commensal of the nasopharynx, M. haemolytica is an opportunist, gaining access to the lungs when host defenses are compromised by stress or infection with respiratory viruses or mycoplasma. Although several serotypes act as commensals, A1 and A6 are the most frequent isolates from pneumonic lungs. Potential virulence factors include adhesin, capsular polysaccharide, fimbriae, iron-regulated outer membrane proteins, leukotoxin (Lkt), lipopolysaccharide (LPS), lipoproteins, neuraminidase, sialoglycoprotease and transferrin-binding proteins. Of these, Lkt is pivotal in induction of pneumonia. Lkt-mediated infiltration and destruction of neutrophils and other leukocytes impairs bacterial clearance and contributes to development of fibrinous pneumonia. LPS may act synergistically with Lkt, enhancing its effects and contributing endotoxic activity. Antibiotics are employed extensively in the feedlot industry, both prophylactically and therapeutically, but their efficacy varies because of inconsistencies in diagnosis and treatment regimes and development of antibiotic resistance. Vaccines have been used for many decades, even though traditional bacterins failed to demonstrate protection and their use often enhanced disease in vaccinated animals. Modern vaccines use culture supernatants containing Lkt and other soluble antigens, or bacterial extracts, alone or combined with bacterins. These vaccines have 50-70% efficacy in prevention of M. haemolytica pneumonia. Effective control of M. haemolytica pneumonia is likely to require a combination of more definitive diagnosis, efficacious vaccines, therapeutic intervention and improved management practices.

Protective immunity conferred by recombinant Pasteurella multocida lipoprotein E (PlpE)

The genes encoding Pasteurella multocida lipoprotein E (PlpE) and lipoprotein B (PlpB) were cloned from P. multocida strain X-73 (serotype A:1) and expressed in Escherichia coli. The protective immunity conferred by recombinant PlpE (r-PlpE) and PlpB (r-PlpB) on mice and chickens was evaluated. The results showed that mice immunized with 10microg of purified r-PlpE were protected (80-100% survival rate) against challenge infection with 10 or 20 LD(50) of P. multocida strains X-73 (serotype A:1), P-1059 (serotype A:3) and P-1662 (serotype A:4). In contrast, mice immunized with r-PlpB were not protected. Chickens immunized with 100microg of purified r-PlpE were protected (63-100% survival rate) against lethal challenge infection with strains X-73 and P-1662, whereas those immunized with r-PlpB were not. Sequence analyses showed that PlpE from different strains of P. multocida exhibited 90.8-100% sequence identity to each other, suggesting that PlpE might serve as a cross-protective antigen. This is the first report of a recombinant P. multocida antigen that confers cross protection on animals.

Recombinant Mannheimia haemolytica serotype 1 outer membrane protein PlpE enhances commercial M. haemolytica vaccine-induced resistance against serotype 6 challenge

Mannheimia haemolytica outer membrane protein PlpE, a major immunogenic outer membrane lipoprotein has identical sequences in serotypes 1 (S1) and S6. Recombinant outer membrane lipoprotein PlpE (rPLpE) from M. haemolytica S1 was added to commercial M. haemolytica S1 vaccines to determine if it would enhance vaccine-induced immunity against heterotypic M. haemolytica S6 challenge. Serum antibody responses to M. haemolytica whole cells, leukotoxin and rPlpE were measured. Experiment 1 consisted of four vaccine groups: controls, 100 microg rPlpE, M. haemolytica Bacterin-Toxoid (One Shot) and M. haemolytica Bacterin-Toxoid + 100 microg rPlpE. Vaccines were given on day 0. On day 21, calves were challenged transthoracically with M. haemolytica S6. Lung lesion scores and percentage lesion reduction were 6.3 +/- 2.0 for controls, 3.6 +/- 2.4 for rPlpE vaccinates (42.9% reduction), 3.4 +/- 1.5 for One Shot-vaccinates (46.0% reduction), and 2.4 +/- 1.4 for One Shot/rPlpE vaccinates (61.9% reduction). Experiment 2 consisted of four vaccine groups: controls, 100 microg rPlpE, M. haemolytica toxoid (Presponse), and M. haemolytica toxoid+100 microg rPlpE. On day 28, calves were challenged transthoracically with M. haemolytica S6. Lung lesion scores and percentage lesion reduction were 8.1 +/- 2.2 for controls, 4.4 +/- 4.7 for the rPlpE vaccinates (45.7% reduction), 4.8 +/- 2.2 for Presponse-vaccinates (40.7% reduction), and 2.0 +/- 1.2 for Presponse/rPlpE vaccinates (75.3% reduction). These results indicate that addition of rPlpE from M. haemolytica S1 can enhance commercial M. haemolytica vaccine-induced resistance against experimental challenge with M. haemolytica S6.