Immunogenicity of plant-produced African horse sickness virus-like particles: implications for a novel vaccine

African horse sickness (AHS) is a debilitating and often fatal viral disease affecting horses in much of Africa, caused by the dsRNA orbivirus African horse sickness virus (AHSV). Vaccination remains the single most effective weapon in combatting AHS, as there is no treatment for the disease apart from good animal husbandry. However, the only commercially available vaccine is a live-attenuated version of the virus (LAV). The threat of outbreaks of the disease outside its endemic region and the fact that the LAV is not licensed for use elsewhere in the world, have spurred attempts to develop an alternative safer, yet cost-effective recombinant vaccine. Here, we report the plant-based production of a virus-like particle (VLP) AHSV serotype five candidate vaccine by Agrobacterium tumefaciens-mediated transient expression of all four capsid proteins in Nicotiana benthamiana using the cowpea mosaic virus-based HyperTrans (CPMV-HT) and associated pEAQ plant expression vector system. The production process is fast and simple, scalable, economically viable, and most importantly, guinea pig antiserum raised against the vaccine was shown to neutralize live virus in cell-based assays. To our knowledge, this is the first report of AHSV VLPs produced in plants, which has important implications for the containment of, and fight against the spread of, this deadly disease.

The phylogenetically-related pattern recognition receptors EFR and XA21 recruit similar immune signaling components in monocots and dicots

During plant immunity, surface-localized pattern recognition receptors (PRRs) recognize pathogen-associated molecular patterns (PAMPs). The transfer of PRRs between plant species is a promising strategy for engineering broad-spectrum disease resistance. Thus, there is a great interest in understanding the mechanisms of PRR-mediated resistance across different plant species. Two well-characterized plant PRRs are the leucine-rich repeat receptor kinases (LRR-RKs) EFR and XA21 from Arabidopsis thaliana (Arabidopsis) and rice, respectively. Interestingly, despite being evolutionary distant, EFR and XA21 are phylogenetically closely related and are both members of the sub-family XII of LRR-RKs that contains numerous potential PRRs. Here, we compared the ability of these related PRRs to engage immune signaling across the monocots-dicots taxonomic divide. Using chimera between Arabidopsis EFR and rice XA21, we show that the kinase domain of the rice XA21 is functional in triggering elf18-induced signaling and quantitative immunity to the bacteria Pseudomonas syringae pv. tomato (Pto) DC3000 and Agrobacterium tumefaciens in Arabidopsis. Furthermore, the EFR:XA21 chimera associates dynamically in a ligand-dependent manner with known components of the EFR complex. Conversely, EFR associates with Arabidopsis orthologues of rice XA21-interacting proteins, which appear to be involved in EFR-mediated signaling and immunity in Arabidopsis. Our work indicates the overall functional conservation of immune components acting downstream of distinct LRR-RK-type PRRs between monocots and dicots.

Pests and diseases cause significant agricultural losses. Plants recognize pathogen-derived molecules via plasma membrane-localized immune receptors (called pattern recognition receptors or PRRs), resulting in pathogen resistance. In recent years, the transfer of PRRs across plant species has emerged as a promising biotechnological approach to improve crop disease resistance. Successful transfers of PRRs suggest that immune signaling components are conserved across plant species. In this study, we demonstrate that the PRR XA21 from the monocot plant rice is functional in the dicot plant Arabidopsis thaliana (Arabidopsis) and that it confers quantitatively enhanced resistance to bacteria. Furthermore, we show that the rice XA21 and the Arabidopsis EFR, which are evolutionary-distant but phylogenetically closely related, recruit similar signaling components for their function, revealing an overall conservation of immune pathways across monocots and dicots. These findings demonstrate evolutionary conservation of downstream signaling from PRRs and indicate that transfer of PRRs is possible between different plant families, but also between monocots and dicots.

Partial protection against Brucella infection in mice by immunization with nonpathogenic alphaproteobacteria

Previous findings indicate that Brucella antigens and those from nonpathogenic alphaproteobacteria (NPAP) are cross-recognized by the immune system. We hypothesized that immunization with NPAP would protect mice from Brucella infection. Mice were immunized subcutaneously with heat-killed Ochrobactrum anthropi, Sinorhizobium meliloti, Mesorhizobium loti, Agrobacterium tumefaciens, or Brucella melitensis H38 (standard positive control) before intravenous challenge with Brucella abortus 2308. Cross-reacting serum antibodies against Brucella antigens were detected at the moment of challenge in all NPAP-immunized mice. Thirty days after B. abortus challenge, splenic CFU counts were significantly lower in mice immunized with O. anthropi, M. loti, and B. melitensis H38 than in the phosphate-buffered saline controls (protection levels were 0.80, 0.66, and 1.99 log units, respectively). In mice immunized intraperitoneally with cytosoluble extracts from NPAP or Brucella abortus, protection levels were 1.58 for the latter, 0.63 for O. anthropi, and 0.40 for M. loti. To test whether the use of live NPAP would increase protection further, mice were both immunized and challenged by the oral route. Immunization with NPAP induced a significant increase in serum immunoglobulin G (IgG), but not serum or fecal IgA, against Brucella antigens. After challenge, anti-Brucella IgA increased significantly in the sera and feces of mice orally immunized with O. anthropi. For all NPAP, protection levels were higher than those obtained with systemic immunizations but were lower than those obtained by oral immunization with heat-killed B. abortus. These results show that immunization with NPAP, especially O. anthropi, confers partial protection against Brucella challenge. However, such protection is lower than that conferred by immunization with whole Brucella or its cytosoluble fraction.

The Brucella suis type IV secretion system assembles in the cell envelope of the heterologous host Agrobacterium tumefaciens and increases IncQ plasmid pLS1 recipient competence

Pathogenic Brucella species replicate within mammalian cells, and their type IV secretion system is essential for intracellular survival and replication. The options for biochemical studies on the Brucella secretion system are limited due to the rigidity of the cells and biosafety concerns, which preclude large-scale cell culture and fractionation. To overcome these problems, we heterologously expressed the Brucella suis virB operon in the closely related alpha(2)-proteobacterium Agrobacterium tumefaciens and showed that the VirB proteins assembled into a complex. Eight of the twelve VirB proteins were detected in the membranes of the heterologous host with specific antisera. Cross-linking indicated protein-protein interactions similar to those in other type IV secretion systems, and the results of immunofluorescence analysis supported the formation of VirB protein complexes in the cell envelope. Production of a subset of the B. suis VirB proteins (VirB3-VirB12) in A. tumefaciens strongly increased its ability to receive IncQ plasmid pLS1 in conjugation experiments, and production of VirB1 further enhanced the conjugation efficiency. Plasmid recipient competence correlated with periplasmic leakage and the detergent sensitivity of A. tumefaciens, suggesting a weakening of the cell envelope. Heterologous expression thus permits biochemical characterization of B. suis type IV secretion system assembly.

[Production of miniantibodies to Agrobacterium tumefaciens VirE2 virulence protein by the method of phage display]

The scFv miniantibodies to the recombinant protein VirE2 from Agrobacterium tumefaciens were obtained by the method of phage display. The miniantibodies were purified and tested using timmunodot method for binding to a recombinant protein from Escherichia coli and to the native protein VirE2 from A. tumefaciens. The functional activity of the miniantibodies was comparable to the activity of mouse polyclonal antibodies against the VirE2 protein.

An integral approach towards a practical application for a plant-made monoclonal antibody in vaccine purification

The use of transgenic plants for the production of pharmaceutical compounds has received increasing attention in the last few years. However, many technological and regulatory issues regarding the practical exploitation of this alternative system of production remain to be solved; a situation that explains the lack of commercial products derived from such a system. This paper reports the expression in transgenic plants and cells of a single-chain antibody variable-region fragment (scFv) and a mouse monoclonal antibody to the hepatitis B virus surface antigen (HBsAg). The large-scale purification of the scFv from plants and its use for immunopurification of HBsAg are also described, together with elements concerning regulatory issues and technologies for compliance with good manufacturing and agricultural practices.

A cross-reactive polyol-responsive monoclonal antibody useful for isolation of core RNA polymerase from many bacterial species

The use of antibodies for protein purification is a powerful technique but the release of the target protein in its active form is often difficult. So called "polyol-responsive" monoclonal antibodies (PR-MAbs) have a feature that allows elution of the antigen under very gentle conditions, so that even multi-subunit proteins can be released in their active form. In this work a PR-MAb, 8RB13, was isolated that can purify RNA polymerase (RNAP) from many different bacterial species. High specificity towards RNAP with a broad species cross-reactivity was achieved by immunization with RNAP from Escherichia coli and screening with Bacillus subtilis RNA polymerase. The isolated MAb could detect the beta-subunit of RNA polymerase from 10 out of 12 species tested on a Western blot indicating its potential for purification of core RNAP from these organisms. Representatively, four of these species E. coli, B. subtilis, Pseudomonas aeruginosa, and Streptomyces coelicolor were subjected to immunoaffinity purification yielding RNA polymerases that were active in in vitro transcription and seemed to be primarily core polymerase, lacking sigma-subunits.

Characterization of an Agrobacterium tumefaciens lectin

An Agrobacterium tumefaciens suspension induces a strong agglutination of aldehyde-fixed pig erythrocytes at pH 5.0. The agglutination is inhibited by some polysaccharides, such as fucoidin, and also when the pH is raised to 7.0. Lectins (sugar-binding proteins) associated with the bacterial cell wall of A. tumefaciens strain 84.5 were directly evidenced by spectrofluorimetry using fluoresceinylated neoglycoproteins. The specific binding of the fluorescein-labelled neoglycoprotein bearing alpha-L-fucoside residues was also optimal at pH 5.0. A lectin was purified by affinity chromatography on agarose substituted with alpha-L-fucopyranoside. Furthermore, the haemagglutination activity of this lectin was inhibited by polysaccharides isolated from poplar leaves.