Potential of PLG and LTB for oral delivery of antigens to salmonids
Biocompatible polymer poly(lactide-co-glycolide) (PLG) and Escherichia coli heat labile enterotoxin B-subunit (LTB) were investigated with regards to their potential for delivering antigens following oral delivery to salmonids. A commercial immersion vaccine against furunculosis was encapsulated within PLG microparticles and delivered to Atlantic salmon by oral intubation and intra-peritoneal injection. Following delivery, antibody responses and protection against lethal challenge with Aeromona salmonicida were minimal. SEM and protein studies of particles indicated that encapsulation of bacterial cells was sub-optimal. Adjuvant effect of PLG microparticles containing a model protein antigen (human gamma-globulin) was investigated following intra-peritoneal injection to rainbow trout. Release of protein from the microparticles was determined in-vitro. Specific antibodies were determined at set time points following injection by ELISA and no detectable immune response was measured in fish where protein encapsulated within PLG was delivered. This may be explained by the fact that in-vitro release of HGG from microparticles over the experimental period was minimal. The ability of Atlantic salmon to take up microparticles was investigated following anal and oral intubation of fluorescent microspheres of known sizes. Following anal intubation, microspheres in the range of 0.1 to 3mm, were detected within spleen and kidney sections. The route of uptake of microspheres was not determined. Following oral intubation, microspheres were detected within the hindgut lumen, although not to the same degree as following anal intubation, and no microspheres were detected within kidney or spleen sections. Incorporation of plasmid DNA vaccine into microparticles or adsorption onto positively charged microparticles were investigated. Both encapsulation within and adsorption onto charged PLG microparticles was demonstrated although technical difficulties were incurred in producing quantities sufficient for immunisation studies. The potential of LTB as an antigen carrier molecule was investigated using recombinant LTB produced within transgenic potato tubers.