In vivo protection studies of chimeric Burkholderia pseudomallei antigens presenting multiple epitopes on protein scaffolds and outer membrane vesicles
Led by Prof Gregory Bancroft (LSHTM), with Assistant Prof Louise Gourlay (University of Milan), Prof Martino Bolognesi (University of Milan), Prof Giorgio Colombo (University of Pavia), and Assistant Prof Ganjana Lertmemongkolchai (Khon Kaen University)
To develop vaccines against microbes that cause serious infection in humans, we must identify their protein (antigen) components, which activate the lymphocytes of the immune system. Many organisms have too many proteins to test individually; therefore efforts to identify immunoreactive molecules should be highly focused and based on sound scientific rationale. If we know how a protein antigen is structured in three-dimensions, and we know the location and conformation of the immunoreactive portion of the molecule, this information can be used to design an improved, more reactive antigen. This so called structural vaccinology (SV) strategy is proposed by experts in the field to drive the design of future vaccines targeting challenging diseases such as AIDS and cancer.
We applied a SV approach to protein antigens from the bacterial human pathogen Burkholderia pseudomallei that causes high mortality in tropical countries; there is no current vaccine to combat this infection, and antibiotic treatment is prolonged and often unsuccessful. In particular, we used antigen 3D-structure information and knowledge of the location of the immunoreactive portions (epitopes) to design new protein antigens. Each protein antigen was specifically engineered, using recombinant DNA technology, to present multiple epitopes from two known antigens that induce partial immune protection. The main aim is to evaluate the ability of these engineered antigens and peptide epitopes to protect mice against melioidosis, when presented on the surface of outer membrane vesicles (OMVs). Many bacteria naturally produce OMVs, which are naturally immunogenic and thus ideal vaccine delivery vessels. The immune response, by both engineered protein antigens and epitopes presented on OMVs, will be compared as a first step towards vaccine development.
The aim of the project was to evaluate the in vivo immune protection induced by immunisation with epitopes derived from the pathogenic bacterium Burkholderia pseudomallei when transplanted onto a second antigen (chimeric antigens), presented to the host as either protein-adjuvant mixtures or expressed on OMVs, in a murine model of melioidosis. These epitopes/constructs were selected based on structural biology/vaccinology principles and immunogenicity of these molecules in humans exposed to the organism in the endemic regions of Thailand. The antigens were synthesised and shipped to LSHTM for testing. Mice were immunised with each candidate, then challenged with virulent B. pseudomallei. No appreciable protection was observed for any candidate despite showing evidence of immunogenicity when immune serum was tested against the homologous epitope in vitro. We assume this lack of protection could be due to difficulty in the antibodies detecting the epitope of interest on the surface of intact bacteria in vivo and/or insufficiently high concentrations of antibodies achieved in the serum of vaccinated mice.