Enhanced efficacy of recombinant Brucella abortus RB51 vaccines against B. melitensis infection in mice

Registered Influenza Viral Vector Based Brucella abortus Vaccine for Cattle in Kazakhstan: Age-Wise Safety and Efficacy Studies

A novel influenza viral vector based Brucella abortus vaccine (Flu-BA) was introduced for use in cattle in Kazakhstan in 2019. In this study, the safety and efficacy of the vaccine was evaluated in male and female cattle at different ages, and during pregnancy as a part of its registration process. Our data demonstrated that the Flu-BA vaccine was safe after prime or booster vaccination in calves (5–7 months old male and female), heifers (15–17 months old) and cows (6–7 years old) and was not abortogenic in pregnant animals. A mild, localized granuloma was observed at the Flu-BA injection site. Vaccinated animals did not show signs of influenza infection or reduced milk production in dairy cows, and the influenza viral vector (IVV) was not recovered from nasal swabs or milk. Vaccinated animals in all age groups demonstrated increased IgG antibody responses against Brucella Omp16 and L7/L12 proteins with calves demonstrating the greatest increase in humoral responses. Following experimental challenge with B. abortus 544, vaccinates demonstrated greater protection and no signs of clinical disease, including abortion, were observed. The vaccine effectiveness against B. abortus 544 infection was 75, 60 and 60%, respectively, in calves, heifers and adult cows. Brucella were not isolated from calves of vaccinated cattle that were experimentally challenged during pregnancy. Our data suggests that the Flu-BA vaccine is safe and efficacious in cattle, including pregnant animals; and can therefore be administered to cattle of any age.

Immune Response to Mucosal Brucella Infection

Brucellosis is one of the most prevalent bacterial zoonosis of worldwide distribution. The disease is caused by Brucella spp., facultative intracellular pathogens. Brucellosis in animals results in abortion of fetuses, while in humans, it frequently manifests flu-like symptoms and a typical undulant fever, being osteoarthritis a common complication of the chronic infection. The two most common ways to acquire the infection in humans are through the ingestion of contaminated dairy products or by inhalation of contaminated aerosols. Brucella spp. enter the body mainly through the gastrointestinal and respiratory mucosa; however, most studies of immune response to Brucella spp. are performed analyzing models of systemic immunity. It is necessary to better understand the mucosal immune response induced by Brucella infection since this is the main entry site for the bacterium. In this review, some virulence factors and the mechanisms needed for pathogen invasion and persistence are discussed. Furthermore, some aspects of local immune responses induced during Brucella infection will be reviewed. With this knowledge, better vaccines can be designed focused on inducing protective mucosal immune response.

Overexpression of wbkF gene in Brucella abortus RB51WboA leads to increased O-polysaccharide expression and enhanced vaccine efficacy against B. abortus 2308, B. melitensis 16M, and B. suis 1330 in a murine brucellosis model

Brucella abortus RB51 is an attenuated, stable, spontaneous rough mutant derived in the laboratory from the virulent strain B. abortus 2308. Previous studies discovered that the wboA gene, which encodes a glycosyltransferase required for synthesis of the O-polysaccharide, is disrupted in strain RB51 by an IS711 element. However, complementation of strain RB51 with a functional wboA gene (strain RB51WboA) does not confer it a smooth phenotype but results in low levels of cytoplasmic O-polysaccharide synthesis. In this study, we asked if increasing the potential availability of bactoprenol priming precursors in strain RB51WboA would increase the levels of O-polysaccharide synthesis and enhance the protective efficacy against virulent Brucella challenge. To achieve this, we overexpressed the wbkF gene, which encodes a putative undecaprenyl-glycosyltransferase involved in bactoprenol priming for O-polysaccharide polymerization, in strain RB51WboA to generate strain RB51WboAKF. In comparison to strain RB51WboA, strain RB51WboAKF expressed higher levels of O-polysaccharide, but was still attenuated and remained phenotypically rough. Mice immunized with strain RB51WboAKF developed increased levels of smooth LPS-specific serum antibodies, primarily of IgG2a and IgG3 isotype. Splenocytes from mice vaccinated with strain RB51WboAKF secreted higher levels of antigen-specific IFN-γ and TNF-α and contained more numbers of antigen-specific IFN-γ secreting CD4⁺ and CD8⁺ T lymphocytes when compared to those of the RB51 or RB51WboA vaccinated groups. Immunization with strain RB51WboAKF conferred enhanced protection against virulent B. abortus 2308, B. melitensis 16M and B. suis 1330 challenge when compared to the currently used vaccine strains. Our results suggest that strain RB51WboAKF has the potential to be a more efficacious vaccine than its parent strain in natural hosts.

Immunization of BALB/c mice with a combination of four recombinant Brucella abortus proteins, AspC, Dps, InpB and Ndk, confers a marked protection against a virulent strain of Brucella abortus

In this study, we assessed the protective efficacy of single subunit vaccines, encoded by the B. abortus 544 genes aspC, dps, yaeC and inpB, against B. abortus infection in mice. First, immunization with these antigens, with the exception of the YaeC protein, was found to elicit both humoral and cellular immune responses with IgG2a being dominant over IgG1. In addition, a massive production of IFN-γ but lower degree of IL-10 was observed, suggesting that all three antigens were able to induce predominantly cell-mediated immunity in response to B. abortus infection. Further investigation of a combined subunit vaccine (CSV) consisting of purified AspC, Dps, InpB and Ndk proteins showed a superior protective effect in mice against brucellosis. The intraperitoneal injection of this combination was shown to induce a remarkable production of IFN-γ and IL-2, which occurred in conjunction with an increase of blood CD4⁺ and CD8⁺ T cell proportions. In addition, the higher titer of IgG2a compared to IgG1 elicited by this CSV was obtained, suggesting that this CSV induced a typical T-helper-1-dominated immune response in vivo. Furthermore, the protection level induced by this combination was significantly higher than that induced by single antigens and was not significantly different compared to a group immunized with a live attenuated vaccine (RB51). Altogether, our findings suggest that the combination of different immunogenic antigens could be a useful approach for the development of a new, effective and safe brucellosis vaccine that can replace current vaccine strains.

The Case for Live Attenuated Vaccines against the Neglected Zoonotic Diseases Brucellosis and Bovine Tuberculosis

Vaccination of humans and animals with live attenuated organisms has proven to be an effective means of combatting some important infectious diseases. In fact, the 20th century witnessed tremendous improvements in human and animal health worldwide as a consequence of large-scale vaccination programs with live attenuated vaccines (LAVs). Here, we use the neglected zoonotic diseases brucellosis and bovine tuberculosis (BTb) caused by Brucella spp. and Mycobacterium bovis (M. bovis), respectively, as comparative models to outline the merits of LAV platforms with emphasis on molecular strategies that have been pursued to generate LAVs with enhanced vaccine safety and efficacy profiles. Finally, we discuss the prospects of LAV platforms in the fight against brucellosis and BTb and outline new avenues for future research towards developing effective vaccines using LAV platforms.

Recent advances in Brucella abortus vaccines

Brucella abortus vaccines play a central role in bovine brucellosis control/eradication programs and have been successfully used worldwide for decades. Strain 19 and RB51 are the approved B. abortus vaccines strains most commonly used to protect cattle against infection and abortion. However, due to some drawbacks shown by these vaccines much effort has been undertaken for the development of new vaccines, safer and more effective, that could also be used in other susceptible species of animals. In this paper, we present a review of the main aspects of the vaccines that have been used in the brucellosis control over the years and the current research advances in the development of new B. abortus vaccines.

Overexpression of Brucella putative glycosyltransferase WbkA in B. abortus RB51 leads to production of exopolysaccharide

Brucella spp. are Gram-negative, facultative intracellular bacteria that cause brucellosis in mammals. Brucella strains containing the O-polysaccharide in their cell wall structure exhibit a smooth phenotype whereas the strains devoid of the polysaccharide show rough phenotype. B. abortus strain RB51 is a stable rough attenuated mutant which is used as a licensed live vaccine for bovine brucellosis. Previous studies have shown that the wboA gene, which encodes a glycosyltransferase required for the synthesis of O-polysaccharide, is disrupted in B. abortus RB51 by an IS711 element. Although complementation of strain RB51 with a functional wboA gene results in O-polysaccharide synthesis in the cytoplasm, it does not result in smooth phenotype. The aim of this study was to determine if overexpression of Brucella WbkA or WbkE, two additional putative glycosyltransferases essential for O-polysaccharide synthesis, in strain RB51 would result in the O-polysaccharide synthesis and smooth phenotype. Our results demonstrate that overexpression of wbkA or wbkE gene in RB51 does not result in O-polysaccharide expression as shown by Western blotting with specific antibodies. However, wbkA, but not wbkE, overexpression leads to the development of a clumping phenotype and the production of exopolysaccharide(s) containing mannose, galactose, N-acetylglucosamine, and N-acetylgalactosamine. Moreover, we found that the clumping recombinant strain displays increased adhesion to polystyrene plates. The recombinant strain was similar to strain RB51 in its attenuation characteristic and in its ability to induce protective immunity against virulent B. abortus challenge in mice.

Progress in Brucella vaccine development

Brucella spp. are zoonotic, facultative intracellular pathogens, which cause animal and human disease. Animal disease results in abortion of fetuses; in humans, it manifests flu-like symptoms with an undulant fever, with osteoarthritis as a common complication of infection. Antibiotic regimens for human brucellosis patients may last several months and are not always completely effective. While there are no vaccines for humans, several licensed live Brucella vaccines are available for use in livestock. The performance of these animal vaccines is dependent upon the host species, dose, and route of immunization. Newly engineered live vaccines, lacking well-defined virulence factors, retain low residual virulence, are highly protective, and may someday replace currently used animal vaccines. These also have possible human applications. Moreover, due to their enhanced safety and efficacy in animal models, subunit vaccines for brucellosis show great promise for their application in livestock and humans. This review summarizes the progress of brucellosis vaccine development and presents an overview of candidate vaccines.

Extended safety and efficacy studies of the attenuated Brucella vaccine candidates 16 M(Delta)vjbR and S19(Delta)vjbR in the immunocompromised IRF-1-/- mouse model

The global distribution of brucellosis and high incidence in certain areas of the world warrant the development of a safer and efficacious vaccine. For the past 10 years, we have focused our attention on the development of a safer, but still highly protective, live attenuated vaccine for human and animal use. We have demonstrated the safety and protective efficacy of the vaccine candidates 16 MΔvjbR and S19ΔvjbR against homologous and heterologous challenge in multiple immunocompetent animal models, including mice and deer. In the present study, we conducted a series of experiments to determine the safety of the vaccine candidates in interferon regulatory factor-1-knockout (IRF-1(-/-)) mice. IRF-1(-/-) mice infected with either wild-type Brucella melitensis 16 M or the vaccine strain Brucella abortus S19 succumb to the disease within the first 3 weeks of infection, which is characterized by a marked granulomatous and neutrophilic inflammatory response that principally targets the spleen and liver. In contrast, IRF-1(-/-) mice inoculated with either the 16 MΔvjbR or S19ΔvjbR vaccine do not show any clinical or major pathological changes associated with vaccination. Additionally, when 16 MΔvjbR- or S19ΔvjbR-vaccinated mice are challenged with wild-type Brucella melitensis 16M, the degree of colonization in multiple organs, along with associated pathological changes, is significantly reduced. These findings not only demonstrate the safety and protective efficacy of the vjbR mutant in an immunocompromised mouse model but also suggest the participation of lesser-known mechanisms in protective immunity against brucellosis.

Vaccination with Brucella abortus recombinant in vivo-induced antigens reduces bacterial load and promotes clearance in a mouse model for infection

Current vaccines used for the prevention of brucellosis are ineffective in inducing protective immunity in animals that are chronically infected with Brucella abortus, such as elk. Using a gene discovery approach, in vivo-induced antigen technology (IVIAT) on B. abortus, we previously identified ten loci that encode products up-regulated during infection in elk and consequently may play a role in virulence. In our present study, five of the loci (D15, 0187, VirJ, Mdh, AfuA) were selected for further characterization and compared with three additional antigens with virulence potential (Hia, PrpA, MltA). All eight genes were PCR-amplified from B. abortus and cloned into E. coli. The recombinant products were then expressed, purified, adjuvanted, and delivered subcutaneously to BALB/c mice. After primary immunization and two boosts, mice were challenged i.p. with 5 x 10⁴ CFU of B. abortus strain 19. Spleens from challenged animals were harvested and bacterial loads determined by colony count at various time points. While vaccination with four of the eight individual proteins appeared to have some effect on clearance kinetics, mice vaccinated with recombinant Mdh displayed the most significant reduction in bacterial colonization. Furthermore, mice immunized with Mdh maintained higher levels of IFN-γ in spleens compared to other treatment groups. Collectively, our in vivo data gathered from the S19 murine colonization model suggest that vaccination with at least three of the IVIAT antigens conferred an enhanced ability of the host to respond to infection, reinforcing the utility of this methodology for the identification of potential vaccine candidates against brucellosis. Mechanisms for immunity to one protein, Mdh, require further in vitro exploration and evaluation against wild-type B. abortus challenge in mice, as well as other hosts. Additional studies are being undertaken to clarify the role of Mdh and other IVI antigens in B. abortus virulence and induction of protective immunity.

Over-expression of homologous antigens in a leucine auxotroph of Brucella abortus strain RB51 protects mice against a virulent B. suis challenge

Infection by members of the Gram-negative bacterial genus Brucella causes brucellosis in a variety of mammals. Brucellosis in swine remains a challenge, as there is no vaccine in the USA approved for use in swine against brucellosis. Here, we developed an improved recombinant Brucella abortus vaccine strain RB51 that could afford protection against Brucella suis infection by over-expressing genes encoding homologous proteins: L7/L12 ribosomal protein, Cu/Zn superoxide dismutase [SOD] and glycosyl-transferase [WboA]. Using strain RB51leuB as a platform and an antibiotic-resistance marker free plasmid, strains RB51leuB/SOD, RB51leuB/SOD/L7/L12 and RB51leuB/SOD/WboA were constructed to over-express the antigens: SOD alone, SOD and ribosomal protein L7/L12 or SOD and glycosyl-transferase, respectively. The ability of these vaccine candidates to protect against a virulent B. suis challenge were evaluated in a mouse model. All vaccine groups protected mice significantly (P<0.05) when compared to the control group. Within the vaccine groups, the mice vaccinated with strain RB51leuB/SOD/WboA were significantly better protected than those that were vaccinated with either strain RB51leuB/SOD or RB51leuB/SOD/L7/L12. These results suggest that Brucella antigens can be over-expressed in strain RB51leuB and elicit protective immune responses against brucellosis. Since the plasmid over-expressing homologous antigens does not carry an antibiotic resistance gene, it complies with federal regulations and therefore could be used to develop safer multi-species vaccines for prevention of brucellosis caused by other species of Brucella.

B. melitensis rough strain B115 is protective against heterologous Brucella spp. infections

Brucellosis is one of the most serious zoonoses all over the world, with B. melitensis, B. abortus and B. suis being the most pathogenic species for humans. Vaccination of domesticated livestock still represents the most efficient way to prevent human infection. However, the available Brucella vaccines retain an important residual virulence and induce antibodies interfering with surveillance programs. Moreover, each vaccine shows different protective effects versus different Brucella species and different animal hosts. Nowadays, while B. melitensis and B. suis infections in cattle are emerging as a significant problem, there are no available vaccines to overcome such issue. B. melitensis strain B115, a natural, attenuated rough strain in our previous studies proved to be highly protective against B. melitensis and B. ovis infections in mice, without inducing interfering antibodies. In this study, we tested the efficiency of B115 as vaccine against B. abortus and B. suis. Vaccination of mice with 10(8) CFU/mouse of B. melitensis B115 conferred a satisfactory protection against B. abortus 2308. On the contrary, mice vaccinated once with 10(8) or 10(9) CFU/mouse of B115 were weakly protected against B. suis infection. Conversely, when mice were vaccinated twice with 10(9) CFU B115/mouse, the protective activity significantly increased. Unlike its rough phenotype, B115 showed an adequate persistence in mice accompanied to a solid humoral and cell-mediated immunity. All together, these findings suggest the potential usefulness of B115 to control brucellosis in animal hosts due to heterologous challenges.

Technical transformation of biodefense vaccines

Biodefense vaccines are developed against a diverse group of pathogens. Vaccines were developed for some of these pathogens a long time ago but they are facing new challenges to move beyond the old manufacturing technologies. New vaccines to be developed against other pathogens have to determine whether to follow traditional vaccination strategies or to seek new approaches. Advances in basic immunology and recombinant DNA technology have fundamentally transformed the process of formulating a vaccine concept, optimizing protective antigens, and selecting the most effective vaccine delivery approach for candidate biodefense vaccines.

Immune responses and protection against experimental challenge after vaccination of bison with Brucella abortus strain RB51 or RB51 overexpressing superoxide dismutase and glycosyltransferase genes

Vaccination is a tool that could be beneficial in managing the high prevalence of brucellosis in free-ranging bison in Yellowstone National Park. In this study, we characterized immunologic responses and protection against experimental challenge after vaccination of bison with Brucella abortus strain RB51 (RB51) or a recombinant RB51 strain overexpressing superoxide dismutase (sodC) and glycosyltransferase (wboA) genes (RB51+sodC,wboA). Bison were vaccinated with saline only or with 4.6 x 10(10) CFU of RB51 or 7.4 x 10(10) CFU of RB51+sodC,wboA (n = eight animals/treatment). Bison vaccinated with RB51 or RB51+sodC,wboA had greater (P < 0.05) antibody responses, proliferative responses, and production of gamma interferon to RB51 after vaccination than did nonvaccinates. However, bison vaccinated with RB51+sodC,wboA cleared the vaccine strain from draining lymph nodes faster than bison vaccinated with the parental RB51 strain. Immunologic responses of bison vaccinated with RB51+sodC,wboA were similar to responses of bison vaccinated with RB51. Pregnant bison were intraconjunctivally challenged in midgestation with 10(7) CFU of B. abortus strain 2308. Bison vaccinated with RB51, but not RB51+sodC,wboA vaccinates, had greater protection from abortion, fetal/uterine, mammary, or maternal infection than nonvaccinates. Our data suggest that the RB51+sodC,wboA strain is less efficacious as a calfhood vaccine for bison than the parental RB51 strain. Our data also suggest that the RB51 vaccine is a currently available management tool that could be utilized to help reduce brucellosis in free-ranging bison.

An Aerosolized Brucella spp. Challenge Model for Laboratory Animals

To characterize the optimal aerosol dosage of Brucella abortus strain 2308 (S2308) and B. melitensis (S16M) in a laboratory animal model of brucellosis, dosages of 10(3)-10(10) colony forming units (CFU) were nebulized to mice. Although tissue weights were minimally influenced, total CFU per tissues increased beginning at 10(6)-10(7) CFU dosages, with 10(9) CFU appearing to be an optimal dosage for S16M or S2308 aerosol delivery. At 12 weeks after vaccination with 10(7) CFU of B. abortus strain RB51 (SRB51) or saline (control), mice were challenged intraperitoneally (i.p.) (6.4 x 10(4) CFU) or via aerosol (1.76 x 10(9) CFU) with S2308. Mice vaccinated with SRB51 had reduced (P < 0.05) splenic, liver and lung colonization (total CFU and CFU/g) after i.p. challenge with S2308 as compared with control mice after i.p. S2308 challenge. Control and SRB51-vaccinated mice did not differ (P > 0.05) in splenic, liver or lung colonization after aerosol S2308 challenge. Failure to demonstrate vaccine protection was not because of a high aerosol challenge dosage as colonization of spleen and liver tissues was lower (P < 0.05) after aerosol challenge when compared with control mice after i.p. S2308 challenge.