Identification and characterization of a 14-kilodalton Brucella abortus protein reactive with antibodies from naturally and experimentally infected hosts and T lymphocytes from experimentally infected BALB/c mice

Pathogen contingency loci and the evolution of host specificity: Simple sequence repeats mediate Bartonella adaptation to a wild rodent host

Parasites, including pathogens, can adapt to better exploit their hosts on many scales, ranging from within an infection of a single individual to series of infections spanning multiple host species. However, little is known about how the genomes of parasites in natural communities evolve when they face diverse hosts. We investigated how Bartonella bacteria that circulate in rodent communities in the dunes of the Negev Desert in Israel adapt to different species of rodent hosts. We propagated 15 Bartonella populations through infections of either a single host species (Gerbillus andersoni or Gerbillus pyramidum) or alternating between the two. After 20 rodent passages, strains with de novo mutations replaced the ancestor in most populations. Mutations in two mononucleotide simple sequence repeats (SSRs) that caused frameshifts in the same adhesin gene dominated the evolutionary dynamics. They appeared exclusively in populations that encountered G. andersoni and altered the dynamics of infections of this host. Similar SSRs in other genes are conserved and exhibit ON/OFF variation in Bartonella isolates from the Negev Desert dunes. Our results suggest that SSR-based contingency loci could be important not only for rapidly and reversibly generating antigenic variation to escape immune responses but that they may also mediate the evolution of host specificity.

Exposure of Agrobacterium tumefaciens to agroinfiltration medium demonstrates cellular remodelling and may promote enhanced adaptability for molecular pharming

Agroinfiltration is used to treat plants with modified strains of Agrobacterium tumefaciens for the purpose of transient in planta expression of genes transferred from the bacterium. These genes encode valuable recombinant proteins for therapeutic or industrial applications. Treatment of large quantities of plants for industrial-scale protein production exposes bacteria (harboring genes of interest) to agroinfiltration medium that is devoid of nutrients and carbon sources for prolonged periods of time (possibly upwards of 24 h). Such conditions may negatively influence bacterial viability, infectivity of plant cells, and target protein production. Here, we explored the role of timing in bacterial culture preparation for agroinfiltration using mass spectrometry-based proteomics to define changes in cellular processes. We observed distinct profiles associated with bacterial treatment conditions and exposure timing, including significant changes in proteins involved in pathogenesis, motility, and nutrient acquisition systems as the bacteria adapt to the new environment. These data suggest a progression towards increased cellular remodelling over time. In addition, we described changes in growth- and environment-specific processes over time, underscoring the interconnectivity of pathogenesis and chemotaxis-associated proteins with transport and metabolism. Overall, our results have important implications for the production of transiently expressed target protein products, as prolonged exposure to agroinfiltration medium suggests remodelling of the bacterial proteins towards enhanced infection of plant cells.

Identification of Cross-Protective Potential Antigens against Pathogenic Brucella spp. through Combining Pan-Genome Analysis with Reverse Vaccinology

Brucellosis is a zoonotic infectious disease caused by bacteria of the genus Brucella. Brucella melitensis, Brucella abortus, and Brucella suis are the most pathogenic species of this genus causing the majority of human and domestic animal brucellosis. There is a need to develop a safe and potent subunit vaccine to overcome the serious drawbacks of the live attenuated Brucella vaccines. The aim of this work was to discover antigen candidates conserved among the three pathogenic species. In this study, we employed a reverse vaccinology strategy to compute the core proteome of 90 completed genomes: 55 B. melitensis, 17 B. abortus, and 18 B. suis. The core proteome was analyzed by a metasubcellular localization prediction pipeline to identify surface-associated proteins. The identified proteins were thoroughly analyzed using various in silico tools to obtain the most potential protective antigens. The number of core proteins obtained from analyzing the 90 proteomes was 1939 proteins. The surface-associated proteins were 177. The number of potential antigens was 87; those with adhesion score ≥ 0.5 were considered antigen with "high potential," while those with a score of 0.4-0.5 were considered antigens with "intermediate potential." According to a cumulative score derived from protein antigenicity, density of MHC-I and MHC-II epitopes, MHC allele coverage, and B-cell epitope density scores, a final list of 34 potential antigens was obtained. Remarkably, most of the 34 proteins are associated with bacterial adhesion, invasion, evasion, and adaptation to the hostile intracellular environment of macrophages which is adjusted to deprive Brucella of required nutrients. Our results provide a manageable list of potential protective antigens for developing a potent vaccine against brucellosis. Moreover, our elaborated analysis can provide further insights into novel Brucella virulence factors. Our next step is to test some of these antigens using an appropriate antigen delivery system.

The Brucella abortus virulence regulator, LovhK, is a sensor kinase in the general stress response signalling pathway

In the intracellular pathogen Brucella abortus, the general stress response (GSR) signalling system determines survival under acute stress conditions in vitro, and is required for long-term residence in a mammalian host. To date, the identity of the Brucella sensor kinase(s) that function to perceive stress and directly activate GSR signalling have remained undefined. We demonstrate that the flavin-binding sensor histidine kinase, LovhK (bab2_0652), functions as a primary B. abortus GSR sensor. LovhK rapidly and specifically phosphorylates the central GSR regulator, PhyR, and activates transcription of a set of genes that closely overlaps the known B. abortus GSR regulon. Deletion of lovhK severely compromises cell survival under defined oxidative and acid stress conditions. We further show that lovhK is required for cell survival during the early phase of mammalian cell infection and for establishment of long-term residence in a mouse infection model. Finally, we present evidence that particular regions of primary structure within the two N-terminal PAS domains of LovhK have distinct sensory roles under specific environmental conditions. This study elucidates new molecular components of a conserved signalling pathway that regulates B. abortus stress physiology and infection biology.

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.

Cloning, expression and characterization of immunogenic aminopeptidase N from Brucella melitensis

A 97-kDa purified aminopeptidase N (PepN) of Brucella melitensis was previously identified to be immunogenic in humans. The B. melitensis pepN gene was cloned, expressed in Escherichia coli and purified by affinity chromatography. The recombinant PepN (rPepN) exhibited the same biochemical properties, specificity and susceptibility to inhibitors as the native PepN. rPepN was evaluated as a diagnostic antigen in an indirect enzyme-linked immunosorbent assay (ELISA) using sera from patients with acute and chronic brucellosis. The specificity of the ELISA was determined with sera from healthy donors. The ELISA had a cutoff value of 0.156 with 100% specificity and 100% sensitivity. Higher sensitivity was obtained using rPepN compared with crude extract from B. melitensis. Anti-PepN sera did not exhibit serological cross-reaction to crude extracts from Rhizobium tropici, Ochrobactrum anthropi, Yersinia enterocolitica 09 or E. coli O157H7.

Role in virulence of a Brucella abortus protein exhibiting lectin-like activity

Brucella abortus is a facultative, intracellular zoonotic pathogen which can cause undulant fever in humans and abortions in cattle. A 14-kDa protein of B. abortus was previously identified to be immunogenic in animals infected with Brucella spp. In this study, we discovered that the 14-kDa protein possessed immunoglobulin binding and hemagglutination properties that appeared to be based on the protein's lectin-like properties. Hemagglutination inhibition experiments suggested that the 14-kDa protein has affinity towards mannose. Disruption of the gene encoding the 14-kDa protein in virulent B. abortus strain 2308 induced a rough-like phenotype with an altered smooth lipopolysaccharide (LPS) immunoblot profile and a significant reduction in the bacterium's ability to replicate in mouse spleens. However, the mutant strain was stably maintained in mouse spleens at 2.0 to 2.6 log(10) CFU/spleen from day 1 to week 6 after intraperitoneal inoculation with 4.65 log(10) CFU. In contrast to the case for the smooth virulent strain 2308, in the rough attenuated strain RB51 disruption of the 14-kDa protein's gene had no effect on the mouse clearance pattern. These findings indicate that the 14-kDa protein of B. abortus possesses lectin-like properties and is essential for the virulence of the species, probably because of its direct or indirect role in the synthesis of smooth LPS.

Correlation of antigenic expression with progress in antibiotic therapy of acute human brucellosis

Human brucellosis is a zoonotic disease which is endemic in Saudi Arabia. The aim of this study was to investigate the humoral immune responses and identify the target antigens that persist at different stages in human brucellosis during antibiotic therapy. To do this, an acute case of accidental nosocomial infection was studied experimentally. Blood was collected from the patient at the time of diagnosis, and at weekly intervals during therapy until remission. IgG and IgM immunoblotting was used to characterize specific antigenic determinants, and ELISA antibody titration was performed to quantify the circulating antibodies. Results indicated that protein bands of 12-13.5 kDa bound IgG in the patient's sera but did not bind IgM on immunoblots and are probably not specific for, or important in, early stage infections. However, an 18 kDa band persisted during infection through remission. The pivotal and most important findings were that the number of protein bands seen on immunoblots, the magnitude of ELISA antibody titres and the concomitant changes in the intensity of the polypeptide bands of 42-43 kDa were positively correlated with the stage of infection. High numbers of anti-IgG and -IgM immunoblot bands coupled with high ELISA antibody titres and a concomitant increase in intensity of the 42-43 kDa bands were positively correlated with acute and severe infection. Conversely, a reduction in the number of polypeptide bands as well as a decrease in the intensity, until the complete disappearance of the 42-43 kDa bands, coupled with low (baseline) ELISA antibody titration values indicated successful treatment and remission. The routine use of the methods described here to ascertain the stage of the disease, assess the progress of antimicrobial therapy and monitor cases of relapse in human brucellosis is suggested.

Molecular host-pathogen interaction in brucellosis: current understanding and future approaches to vaccine development for mice and humans

Brucellosis caused by Brucella spp. is a major zoonotic disease. Control of brucellosis in agricultural animals is a prerequisite for the prevention of this disease in human beings. Recently, Brucella melitensis was declared by the Centers for Disease Control and Prevention to be one of three major bioterrorist agents due to the expense required for the treatment of human brucellosis patients. Also, the economic agricultural loss due to bovine brucellosis emphasizes the financial impact of brucellosis in society. Thus, vaccination might efficiently solve this disease. Currently, B. abortus RB51 and B. melitensis REV.1 are used to immunize cattle and to immunize goats and sheep, respectively, in many countries. However, these genetically undefined strains still induce abortion and persistent infection, raising questions of safety and efficiency. In fact, the REV.1 vaccine is quite virulent and apparently unstable, creating the need for improved vaccines for B. melitensis. In addition, Brucella spp. may or may not provide cross-protection against infection by heterologous Brucella species, hampering the acceleration of vaccine development. This review provides our current understanding of Brucella pathogenesis and host immunity for the development of genetically defined efficient vaccine strains. Additionally, conditions required for an effective Brucella vaccine strain as well as the future research direction needed to investigate Brucella pathogenesis and host immunity are postulated.

Molecular and cellular interactions between Brucella abortus antigens and host immune responses

Host protection against Brucella abortus, is thought to be mediated primarily by a Th1 type immune response. Unfortunately, only few specific bacterial antigens involved in stimulating protective cellular immunity against Brucella are known. Therefore, identifying bacterial proteins that induce a T-lymphocyte mediated response is critical to determine Brucella immunity. Several library screening methods are discussed that have been used to identify Brucella proteins that stimulate T lymphocytes including cellular immunoblotting, Escherichia coli expressed Brucella proteins, green fluorescence reporter systems, and signature tagged mutagenesis. Future studies would likely examine how bacterial proteins expressed within host cells aid pathogen survival and/or induce host responses. Some of these newly identified bacterial gene products may serve as antigens to activate a protective host immune response. Also, identifying Brucella proteins expressed at particular times during infection will also yield insights into Brucella pathogenesis.