PLGA/PLA micro- and nanoparticle formulations serve as antigen depots and induce elevated humoral responses after immunization of Atlantic salmon (Salmo salar L.)

Abstract

Novel vaccine delivery systems are highly needed to improve the salmon aquaculture industry. Although particles of biocompatible polymers such as poly(lactic-co-glycolic) acid (PLGA) have long been considered promising candidates for delivery of immunogenic compounds, few studies have addressed their use as vaccine carriers in Atlantic salmon (Salmo salar L.). Investigating their ability to retain/depot antigen and induce time and dosage dependent adaptive humoral responses to immunization, we here present a basic study of the adjuvantic properties PLGA and PLA particles may have in salmon vaccines. A model antigen (human gamma globulin, HGG) was co-encapsulated with β-glucan in nanoparticles (<1000nm) and microparticles (∼8μm) of different chemical compositions. Atlantic salmon were immunized with (a) PLGA or PLA particle entrapped antigen (12 different treatment groups), (b) antigen and β-glucan in PBS, (c) an oil-based formulation or (d) nanoparticles (NPs) or microparticles (MPs) combined with the oil-adjuvanted formulation. ELISA analysis showed that NPs and MPs were capable of inducing elevated antibody responses at day 60 and 75 post immunization, but the antibody levels were reduced at day 90 and 120. In contrast, oil-based formulations, either alone or in combination with NPs or MPs resulted in strong antibody responses at all sampling time points. Comparable dosage dependent increase in antibody responses was observed when administering antigen with β-glucan either in PBS, entrapped in NPs or MPs, or in an oil-adjuvanted formulation. However, as the antigen doses were increased, MPs and the oil-based formulation gave the strongest responses. Antigen presence in the blood, organ package/injection site, kidney, carcass and the whole body was quantified by radiotracing of I(125)-labelled HGG at day 7 and 36 post immunization. At both sampling time points, the highest radioactivity levels were measured from the whole-body and organ package/injection site in groups injected with MPs and oil-based formulations, indicating that these formulations resulted in superior antigen retention. Interestingly, NPs were found to accumulate in the kidney, a result that corroborated with in vitro uptake of NPs in a DC/Mφ-like cell line from Atlantic salmon.