Early immune responses in Atlantic salmon (Salmo salar L.) after immunization with PLGA nanoparticles loaded with a model antigen and β-glucan.

Polymeric nanoparticles (NPs) of poly (lactic-co-glycolic) acid (PLGA) possess adjuvant properties. To date, there are few studies exploring their application as antigen carriers for vaccination of fish. This study presents a preclinical assessment of the early innate and adaptive immune responses in Atlantic salmon following immunization with PLGA NPs. A model antigen (TNP-LPH) and an immunostimulant (β-glucan) were entrapped in NPs of 300-400nm either alone or in combination. Both the antigen and the β-glucan were efficiently entrapped (>50%) in particles and an antigen release study indicated particle stability up to 50 days at 8°C. Spleen and head kidney were analyzed for pro-inflammatory markers (TNF-α, IL-1β, IL-8, C3a) and T cell cytokines, effector molecules and transcription factors (IFN-γ, T-bet, GATA-3, granzyme A, IL-10, Foxp3) at mRNA transcription levels 2, 4 and 8 days post i.p. immunization. NPs alone were able to moderately up-regulate pro-inflammatory immune responses. Addition of immunogenic cargo, either an antigen or β-glucan generally increased the gene expression of pro-inflammatory markers, while administering both resulted in the highest gene expression. These findings were also reflected by concurrently increased levels of IL-10. Comparing the treatment groups injected with antigen and β-glucan co-administered either in NPs or FCA demonstrated that the magnitude of the acute pro-inflammatory responses was equal between the treatments or highest in the NP injected group. Although elevated expression of granzyme A in the NP injected groups (carrying antigen and/or β-glucan) was observed, PLGA NPs were unable to induce T cell differentiation on mRNA gene expression levels, as increased levels of the indicating cytokines and transcriptions factors failed to occur. In conclusion, this study demonstrates that PLGA NPs have potential as an adjuvant in salmon vaccines as they enhance the early pro-inflammatory responses to immunization.