Oral delivery of a dsRNA-Phytoglycogen nanoparticle complex enhances both local and systemic innate immune responses in rainbow trout

Salmonids are one of the most farmed fish species worldwide. These aquatic vertebrates rely heavily on their innate immune responses as the first line of defense to defend themselves against invading pathogens. Although commercial vaccines are available against some viral and bacterial pathogens affecting salmonids, their protective efficacy varies. Using a prophylactic inducer of local and systemic innate immune responses to limit infection could have significant implications in salmonid aquaculture. A potent inducer of innate immune responses in fish is double-stranded RNA (dsRNA), a molecule that all viruses make during their replicative cycle. Polyinosinic: polycytidylic acid (polyI:C) is a synthetic dsRNA commonly used to induce type I interferons (IFNs), interferon stimulated genes (ISGs) as well as an antiviral state in vertebrate species. Based on in vitro data it was hypothesized that both local and systemic innate immune responses, in salmonids, would be enhanced by orally delivering high molecular weight polyI:C (HMW polyI:C) using cationic phytoglycogen nanoparticles (NPs) as a delivery method. The present study investigates this hypothesis using two feed delivery methods. In the first in vivo study, to ensure an equal distribution of dose, individual rainbow trout (Oncorhynchus mykiss) were orally gavaged with feed moistened with a solution containing HMW-NP (polyI:C complexed with cationic phytoglycogen nanoparticles) or HMW polyI:C alone. In a second in vivo experiment, to better mimic a more realistic feeding scenario, rainbow trout were fed feed pellets to which HMW, or HMW-NP was added. The expression of IFN1 and ISGs (vig-3, Mx1) were quantified using real-time PCR in the intestine (local response) and head kidney (systemic response). The results of these studies indicate that HMW-NP induced a higher level of IFN1 and ISG expression in the intestine and head kidney compared to the HMW fed fish. The results of this study could lead to new advances in therapeutics for the aquaculture industry by utilizing the innate immune response against invading pathogens using an orally delivered stimulant.