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

When the Going Gets Rough: The Significance of Brucella Lipopolysaccharide Phenotype in Host-Pathogen Interactions

Brucella is a facultatively intracellular bacterial pathogen and the cause of worldwide zoonotic infections, infamous for its ability to evade the immune system and persist chronically within host cells. Despite the frequent association with attenuation in other Gram-negative bacteria, a rough lipopolysaccharide phenotype is retained by Brucella canis and Brucella ovis, which remain fully virulent in their natural canine and ovine hosts, respectively. While these natural rough strains lack the O-polysaccharide they, like their smooth counterparts, are able to evade and manipulate the host immune system by exhibiting low endotoxic activity, resisting destruction by complement and antimicrobial peptides, entering and trafficking within host cells along a similar pathway, and interfering with MHC-II antigen presentation. B. canis and B. ovis appear to have compensated for their roughness by alterations to their outer membrane, especially in regards to outer membrane proteins. B. canis, in particular, also shows evidence of being less proinflammatory in vivo, suggesting that the rough phenotype may be associated with an enhanced level of stealth that could allow these pathogens to persist for longer periods of time undetected. Nevertheless, much additional work is required to understand the correlates of immune protection against the natural rough Brucella spp., a critical step toward development of much-needed vaccines. This review will highlight the significance of rough lipopolysaccharide in the context of both natural disease and host–pathogen interactions with an emphasis on natural rough Brucella spp. and the implications for vaccine development.

Whole-genome sequencing of rough Brucella melitensis in China provides insights into its genetic features

Brucella spp. can cause the zoonosis brucellosis, which affects public health and safety and even economic development. B. melitensis has a smooth phenotype, while 28 B. melitensis isolates had a rough phenotype in 2018. In this study, rough phenotype detection and whole genome sequencing methods were used to analyze the genetic features of rough B. melitensis. A drug susceptibility test was also performed. The results showed that the rough B. melitensis strains originated from strains isolated in China rather than from foreign strains. Furthermore, an MS tree showed that 9 complexes to be epidemic in China. For the rough B. melitensis strains, expression of the metabolic function genes varied in the earlier stages of evolution compared to the cellular process and signalling function genes. Expression of some transcriptional regulatory factors also varied in the later stages of evolution, and compared to MFS transporter genes, ABC transporter genes varied in the earlier stages. Moreover, as there was no significant difference in rifampicin, doxycycline and streptomycin susceptibility between the smooth and rough B. melitensis strains, treatment of brucellosis was not affected by strain type. This study provided important information for understanding the genetics and evolution of rough B. melitensis in China.

Meta-Analysis and Advancement of Brucellosis Vaccinology

Background/Objectives

In spite of all the research effort for developing new vaccines against brucellosis, it remains unclear whether these new vaccine technologies will in fact become widely used. The goal of this study was to perform a meta-analysis to identify parameters that influence vaccine efficacy as well as a descriptive analysis on how the field of Brucella vaccinology is advancing concerning type of vaccine, improvement of protection on animal models over time, and factors that may affect protection in the mouse model.

Methods

A total of 117 publications that met the criteria were selected for inclusion in this study, with a total of 782 individual experiments analyzed.

Results

Attenuated (n = 221), inactivated (n = 66) and mutant (n = 102) vaccines provided median protection index above 2, whereas subunit (n = 287), DNA (n = 68), and vectored (n = 38) vaccines provided protection indexes lower than 2. When all categories of experimental vaccines are analyzed together, the trend line clearly demonstrates that there was no improvement of the protection indexes over the past 30 years, with a low negative and non significant linear coefficient. A meta-regression model was developed including all vaccine categories (attenuated, DNA, inactivated, mutant, subunit, and vectored) considering the protection index as a dependent variable and the other parameters (mouse strain, route of vaccination, number of vaccinations, use of adjuvant, challenge Brucella species) as independent variables. Some of these variables influenced the expected protection index of experimental vaccines against Brucella spp. in the mouse model.

Conclusion

In spite of the large number of publication over the past 30 years, our results indicate that there is not clear trend to improve the protective potential of these experimental vaccines.

Evaluation of a Brucella melitensis mutant deficient in O-polysaccharide export system ATP-binding protein as a rough vaccine candidate

Rough Brucella mutants have been sought as vaccine candidates that do not interfere with the conventional serological diagnosis of brucellosis. In this study, a rough mutant of Brucella melitensis was generated by the disruption of the wzt gene, which encodes the O-polysaccharide (O-PS) export system ATP-binding protein. In vivo, the mutant 16MΔwzt was attenuated and conferred a level of protection against B. melitensis 16M challenge similar to that conferred by the vaccine strain B. melitensis M5 in mice. In pregnant sheep, the mutant 16MΔwzt did not induce abortion. In vitro, 16MΔwzt was more susceptible to polymyxin B and complement-mediated killing than B. melitensis 16M was. Most importantly, although 16MΔwzt had a rough phenotype, it was able to synthesize O-PS and did not induce detectable specific antibodies in sheep. These results suggested that 16MΔwzt deserved to further systematic evaluation as a vaccine for target animal hosts due to its promising features.

Evaluation of the immunogenicity and safety of Brucella melitensis B115 vaccination in pregnant sheep

In spite of its limitations, Rev.1 is currently recognized as the most suitable vaccine against Brucella melitensis (the causative agent of ovine and caprine brucellosis). However, its use is limited to young animals when test-and-slaughter programs are in place because of the occurrence of false positive-reactions due to Rev.1 vaccination. The B. melitensis B115 rough strain has demonstrated its efficacy against B. melitensis virulent strains in the mouse model, but there is a lack of information regarding its potential use in small ruminants for brucellosis control. Here, the safety and immune response elicited by B115 strain inoculation were evaluated in pregnant ewes vaccinated at their midpregnancy. Vaccinated (n=8) and non-vaccinated (n=3) sheep were periodically sampled and analyzed for the 108 days following inoculations using tests designed for the detection of the response elicited by the B115 strain and routine serological tests for brucellosis [Rose Bengal Test (RBT), Complement Fixation Test (CFT) and blocking ELISA (ELISAb)]. Five out of the 8 vaccinated animals aborted, indicating a significant abortifacient effect of B115 inoculation at midpregnancy. In addition, a smooth strain was recovered from one vaccinated animal, suggesting the occurrence of an in vivo reversion phenomenon. Only one animal was positive in both RBT and CFT simultaneously (91 days after vaccination) confirming the lack of induction of cross-reacting antibody responses interfering with routine brucellosis diagnostic tests in most B115-vaccinated animals.

In vivo differences in the virulence, pathogenicity, and induced protective immunity of wboA mutants from genetically different parent Brucella spp

To explore the effects of the genetic background on the characteristics of wboA gene deletion rough mutants generated from different parent Brucella sp. strains, we constructed the rough-mutant strains Brucella melitensis 16 M-MB6, B. abortus 2308-SB6, B. abortus S19-RB6, and B. melitensis NI-NB6 and evaluated their survival, pathogenicity, and induced protective immunity in mice and sheep. In mice, the survival times of the four mutants were very different in the virulence assay, from less than 6 weeks for B. abortus S19-RB6 to 11 weeks for B. abortus 2308-SB6 and B. melitensis NI-NB6. However, B. abortus S19-RB6 and B. melitensis 16 M-MB6, with a shorter survival time in mice, offered better protection against challenges with B. abortus 2308 in protection tests than B. abortus 2308-SB6 and B. melitensis NI-NB6. It seems that the induced protective immunity of each mutant might not be associated with its survival time in vivo. In the cross-protection assay, both B. melitensis 16 M-MB6 and B. abortus S19-RB6 induced greater protection against homologous challenges than heterologous challenges. When pregnant sheep were inoculated with B. abortus S19-RB6 and B. melitensis 16 M-MB6, B. abortus S19-RB6 did not induce abortion, whereas B. melitensis 16 M-MB6 did. These results demonstrated the differences in virulence, pathogenicity, and protective immunity in vivo in the wboA deletion mutants from genetically different parent Brucella spp. and also indicated that future rough vaccine strain development could be promising if suitable parent Brucella strains and/or genes were selected.

Acellular vaccines for ovine brucellosis: a safer alternative against a worldwide disease

Ovine brucellosis is a very contagious zoonotic disease distributed worldwide and constitutes a very important zoosanitary and economic problem. The control of the disease includes animal vaccination and slaughter of infected flocks. However, the commercially available vaccine in most countries is based on the attenuated strain Brucella melitensis Rev 1, which presents important safety drawbacks. This review is focused on the most recent and promising acellular vaccine proposals.

Protective role of antibodies induced by Brucella melitensis B115 against B. melitensis and Brucella abortus infections in mice

It has been demonstrated that antibodies specific for O-PS antigen of Brucella smooth strains are involved in the protective immunity of brucellosis. Since the rough strain Brucella melitensis B115 was able to protect mice against wild Brucella strains brucellosis despite the lack of anti-OPS antibodies, in this study we evaluated the biological significance of antibodies induced by this strain, directed to antigens other than O-PS, passively tranferred to untreated mice prior to infection with Brucella abortus 2308 and B. melitensis 16M virulent strains. The protective ability of specific antisera collected from mice vaccinated with B. melitensis B115, B. abortus RB51 and B. abortus S19 strains was compared. The results indicated that antibodies induced by B115 were able to confer a satisfactory protection, especially against B. abortus 2308, similar to that conferred by the antiserum S19, while the RB51 antiserum was ineffective. These findings suggest that antibodies induced by B115 could act as opsonins as well as antibodies anti-O-PS, thus triggering more efficient internalization and degradation of bacteria within phagocytes. This is the first study assessing the efficacy of antibodies directed to antigens other than O-PS in the course of brucellosis infection.

Antigenic, immunologic and genetic characterization of rough strains B. abortus RB51, B. melitensis B115 and B. melitensis B18

The lipopolysaccharide (LPS) is considered the major virulent factor in Brucella spp. Several genes have been identified involved in the synthesis of the three LPS components: lipid A, core and O-PS. Usually, Brucella strains devoid of O-PS (rough mutants) are less virulent than the wild type and do not induce undesirable interfering antibodies. Such of them proved to be protective against brucellosis in mice. Because of these favorable features, rough strains have been considered potential brucellosis vaccines. In this study, we evaluated the antigenic, immunologic and genetic characteristics of rough strains B.abortus RB51, B.melitensis B115 and B.melitensis B18. RB51 derived from B.abortus 2308 virulent strain and B115 is a natural rough strain in which the O-PS is present in the cytoplasm. B18 is a rough rifampin-resistan mutant isolated in our laboratory.

The surface antigenicity of RB51, B115 and B18 was evaluated by testing their ability to bind antibodies induced by rough or smooth Brucella strains. The antibody response induced by each strain was evaluated in rabbits. Twenty-one genes, involved in the LPS-synthesis, were sequenced and compared with the B.melitensis 16M strain.

The results indicated that RB51, B115 and B18 have differences in antigenicity, immunologic and genetic properties. Particularly, in B115 a nonsense mutation was detected in wzm gene, which could explain the intracellular localization of O-PS in this strain.

Complementation studies to evaluate the precise role of each mutation in affecting Brucella morphology and its virulence, could provide useful information for the assessment of new, attenuated vaccines for brucellosis.